INDUSTRIALTRIBOLOGY
TRIBOLOGY SERIES
Advisory Editor: DOUGLAS SCOTT Editorial Board W.J. Bartz (Germany) R. Bassani (Italy) C.A. Brockley (Canada) H. Czichos (Germany) D. Dowson (UK) N.Gane (Australia) W.A. Glaeser (USA)
M. Goder (France) H.E. Hinterman (Switzerland) I.V. Kragelskii(USSR) K.C. Ludema (USA) G.W. Rowe (UK) T. Sakurai (Japan) J.P. Sharma (India)
Vol. 1 Tribology - A Systems Approach to the Science and Technology of Friction, Lubricationand Wear (Czichos) Vol. 2 Impact Wear of Materials (Engel) Vol. 3 Tribology of Natural and Artificial Joints (Dumbleton) Vol. 4 Tribology of Thin Layers (Iliuc) Vol. 5 Surface Effects in Adhesion, Friction, Wear and Lubrication (Buckley) Vol. 6 Frictionand Wear of Polymers (Bartenev and Lavrentev) Vol. 7 Microscopic Aspects of Adhesion and Lubrication (Georges, Editor) Vol. 8 IndustrialTribology- The PracticalAspects of Friction, Lubrication and Wear (Jones and Scott, Editors) Vol. 9 Mechanics and Chemistry in Lubrication (Dorinson and Ludema) Vol. 10 Microstructure and Wear of Materials (Zum Gahr) Vol. 11 Fluid Film Lubrication- Osborne Reynolds Centenary (Dowson et al., Editors) Vol. 12 Interface Dynamics (Dowson et al., Editors) Vol. 13 Tribology of Miniature Systems (Rymuza) Vol. 14 Tribological Design of Machine Elements (Dowson et al., Editors) Vol. 15 Encyclopediaof Tribology (Kajdas et al.) Vol. 16 Tribology of Plastic Materials (Yamaguchi) Vol. 17 Mechanics of Coatings (Dowson et al., Editors)
TRIBOLOGY SERIES, 8
INDUSTRIAL TRIBOLOGY The PracticalAspects of Friction, Lubrication and Wear
edited by
MERVIN H. JONES Department of Mechanical Engineering, University College of Swansea, Swansea, U.K.
and
DOUGLAS SCOTT Consultant, Editor of "Wear", Secretary of The Institution of Engineers and Shipbuilders in Scotland, Glasgow, U.K.
ELSEVIE R SCl ENTl F IC PUBLISHING COMPANY AMSTERDAM -OXFORD -NEW YORK
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat25 P.O. Box 21 1,1000 AE Amsterdam, The Netherlands Distributorsfor the USA and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas New York, NY 10010, USA
Library of Congress Cataloging in Publication Data
Main e n t r y under t i t l e : Industrial tribology.
(Tribology s e r i e s ; 8 ) Includes b i b l i o g r a p h i c a l r e f e r e n c e s and indexes. 1. Tribology. I. J o n e s , Mervin H., 193911. S c o t t , Douglas, 1916111. S e r i e s . TJ1075.148 1983 621.8’9 82-24248 ISBN 0-444-42161-0 (U. S. )
.
ISBN 0444421614 (Vol. 8 ) ISBN 044441677-3 (Series)
First edition 1983 Second impression 1991
0 Elsevier Scientific Publishing Company, 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 21 1,1000 AE Amsterdam, The Netherlands.
Special regulations for readers in the USA. This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, MA 01970, USA. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products. instructions or ideas contained in the material herein. Printed in The Netherlands
V
CONTENTS
ACKNOWLEDGEMENT
XXI
FOREWORD
XXIII
CHAPTER 1.
TRIBOLDGY IN PERSPECTIVE, D.
SCOTT, CONSULTANT,
EDITOR OF WEAR
1
1.1
Introduction
1
1.2
Impact o f T r i b o l o g y
1
1.3
Economic a s p e c t s o f t r i b o l o g y
2
1.4
Mechanisms o f wear
1.5
Surface stu d ie s
3 4
1.6
Lubrication
5
1.7
Lubricants
1.8
Materials
5 6
1.9
Surface treatments
1.10 Computer a i d e d d e s i g n
7 8
1.11 Machinery c o n d i t i o n m o n i t o r i n g
8
3
1.12 C o n c l u s i o n s
10
References
CHAPTER 2.
WEAR,
D.
SCOTT, CONSULTANT, EDITOR OF WEAR
12 12
2.1
Introduction
2.2
The wear p r o c e s s
14
2.3
Scuffing
14
2.4
A b r a s i v e wear
2.5
Fretting
18
2.6
F l u i d and c a v i t a t i o n e r o s i o n
19
2.7
R o l l i n g contact f a t i g u e
20
2.8
Wear d e t e c t i o n and assessment
24
2.9
Conclusions
29
References
I
17
VI CHAPTER 3.
SELECTION OF BEARINGS, M.J.
NEALE, MICHAEL NEALE
AND ASSOCIATES LTD.
3.1
Introduction
31 31
3.2
Bearing types
31
3.3
Performance of v a r i o u s t y p e s o f b e a r i n g
33
3.3.1
Rubbing b e a r i n g s
33
3.3.2
R o l l e r bearings
35
3.3.3
F l u i d f i l m bearings
35
3.3.4
F l e x i b l e members
36
Selection o f a s u i t a b l e bearing
37
3.4
3.4.1
Applications w i t h u n i d i r e c t i o n a l load
37
and c o n t i n u o u s movement
3.4.2
Applications with o s c i l l a t i n g
40
movernent
3.4.3
Applications with multidirectional
42
l o a d and c o n t i n u o u s movement
CHAPTER 4.
DESIGN OF PLAIN BEARINGS,
D E S I G N CHARTS, F.A. GLACIER METAL CO.
4.1 4.2
LTD.
45 45
4.1.1
Journal b e a rin g design l i m i t s
45 46
4.2.1
Notation
Limits o f operation
46
4.2.1.1
Thin o i l f i l m l i m i t
4.2.1.2
High b e a rin g temperature
47 48
4.2.1.3
O i l oxidation l i m i t
50
4.2.1.4
O i l f i l m instabil
50 51
Region o f s a f e o p e r a t i o n
C a l c u l a t i o n and d e s i g n p r o c e d u r e s
-
4.3.1
Dimensionless d a t a
4.3.2
Design p r o c e d u r e s w i t h hea
4.3.3 4.4
GARNER, THE
Introduction
4.2.2 4.3
USE OF BEARING DATA
MARTIN AND D.R.
52
Stage 1
52 ba 1 ance
Stage 1 1
Improvement i n d e s i g n a i d s
57
-
Stage 111
57
Design p r o c e d u r e f o r c y l i n d r i c a l b o r e b e a r i n g s
58
4.4.1
Method o f approach
4.4.2
Guidance f o r s a f e o p e r a t i o n
58 60
VII
4.4.2.1
O i l f i l m t h i c k n e s s and
4.4.2.2 4.4.3
4.5
4.6
temperatures
60
O i l f i l m instability
64
Performance p r e d i c t i o n
65
4.4.3.1
Minimum o i l f i l m t h i c k n e s s
67
4.4.3.2
Misalignment
67
4.4.3.3
Power l o s s and o i l f l o w
67
4.4.3.4
Temperatures
69
High speed a p p l i c a t i o n s
72
4.5.1
Non l a m i n a r o p e r a t i o n
72
4.5.2
P r o f i l e bore bearings
72
Example o f use o f design a i d s
75
4.6.1
75
Problem
4.6.2
76
Procedure
4.6.2.1
Diametral c l e a r a n c e
76
4.6.2.2
Maximum s p e c i f i c l o a d r a t i n g
76
4.6.2.3
Region o f s a f e o p e r a t i o n
76
4.6.2.4
P r e d i c t i o n o f minimum o i l
77
f i l m thickness
4.6.2.5
Check for l a m i n a r o p e r a t i o n
77
4.6.2.6
P r e d i c t i o n o f power loss
77
4.6.2.7
Prediction o f o i l flow
77
4.6.2.8
P r e d i c t i o n o f maximum b e a r i n g
78
temperature
4.6.2.9
Prediction o f o i l o u t l e t
78
temperature
78
Re f e r ence s
CHAPTER 5 .
THE D I A G N O S I S OF PLAIN BEARING FAILURES, R.W. AND E.B.
SHONE,
SHELL RESEARCH CENTRE.
WILSON
80
5.1
Introduction
80
5.2
Properties required o f bearing m a t e r i a l s
80
5.2.1
Fatigue resistance
81
5.2.2
Compressive s t r e n g t h
81
5.2.3
Conformability
81
5.2.4
Embeddability
81
5.2.5
S t r e n g t h a t e l e v a t e d temperatures
81
5.2.6
Compatibility
81
5.2.7
Corrosion resistance
81
5.2.8
Cost
82
VIII 5.3
Type, c o n s t r u c t i o n and c h a r a c t e r i s t i c s o f p l a i n bearing materials
5.4
82
5.3.1
White m e t a l s ( B a b b i t t s )
82
5.3.2
Copper l e a d a l l o y s
86
5.3.3
Lead bronzes
86 87
5.3.4
Aluminium a l l o y s
5.3.5
Phosphor and s i l i c o n bronzes
88
5.3.6
Silver
88
5.3.7
Porous and s e l f l u b r i c a t i n g b e a r i n g s
88
5.3.8
Discussion o f m e t a l l i c b e a r i n g m a t e r i a l s
88
Bearing f a i l u r e s
89
5.4.1
89
M e t a l l u r g i c a l d e f e c t s i n new b e a r i n g s 5.4.1.1
Bad bonding
89
5.4.1.2
Gas c a v i t i e s
81
5.4.1.3
Oversize cuboids
91
5.4.1.4
Excessive l e a d c o n t e n t i n t i n based w h i t e m e t a l s
5.4.1.5
copper- l e a d
5.4.2
91
Uneven l e a d d i s t r i b u t i o n i n and lead-bronze a I I o y s
o r o p e r a t i n g environment
96
5.4.2.1
96
Bad f i t t i n g
5.4.2.2
Extraneous p a r t i c l e s
5.4.2.3
Corrosion
102 117
97
5.4.2.4
C a v i t a t i o n and e r o s i o n
5.4.2.5
E l e c t r i c a l d i scha r g e damage
119
5.4.2.6
Wire wool f a i l u r e s
122 123
5.4.2.7
Fatigue
6.4.2.8
Thermal c y c l i n g damage
125
5.4.2.9
A l l o y i n g i n service
125
5.4.2.10
Inadequate v i s c o s i t y and l a c k of lubricant
5.5
126
Conclusions
127
Ref e rence s
CHAPTER 6.
ROLLING ELEMENT BEARINGS, SKF (UK)
91
F a i l u r e s a s s o c i a t e d w i t h design, f i t t i n g
130
D.G.
HJERTZEN AND R.A.
JARVIS,
LTD.
132
6.1
Introduction
132
6.2
Bearing s e l e c t i o n
132
IX 6.3
Bearing types
133
6.3.1
S i n g l e row deep groove b a l l b e a r i n g
133
6.3.2
Self-a1 i g n i n g b a l l b e a r i n g s
133
6.3.3 6.3.4 6.3.5
Angular c o n t a c t b a l l b e a r i n g
134 134
6.3.6 6.3.7
Double row c y l i n d r i c a l r o l l e r b e a r i n g s
135 136
Needle r o l l e r b e a r i n g s
136
6.3.8
Taper r o l l e r b e a r i n g s
137
6.3.9 6.3.10
Double row s p h e r i c a l r o l l e r b e a r i n g s Spherical r o l l e r t h r u s t bearings
137 138
6.3.11
B a l l t h r u s t bearings
138
6.3.12
Bearings w i t h s p h e r i c a l o u t s i d e surface
Double row a n g u l a r c o n t a c t b a l l b e a r i n g C y l i n d r i c a l r o l l e r bearing
and extended i n n e r r i n g w i d t h
6.4
6.5 6.6 6.7 6.8 6.9
F a t i g u e l i f e and l o a d c a r r y i n g c a p a c i t y
6.4.1 6.4.2
Hertzian contact
6.4.3
F u r t h e r development o f t h e l i f e e q u a t i o n
R e l a t i o n s h i p between l o a d and l i f e
139 139 140 141
Usage
143 144 144
Speed l i m i t s
145
F r ic t i o n
150
Lubrication
151 152
Boundary dimensions
6.9.1
Greases
6.9.1.1 6.9.2
Temperature range
O i l lubrication
6.10 Seals 6.11 Noise 6.12 A n t i c i p a t i n g b e a r i n g damage 6.13 D e t e c t i o n o f b e a r i n g damage by shock p u l s e mea su rement
6.14 F i t s ( s h a f t and housing) 6.15 C o n d i t i o n s o f r o t a t i o n 6.16 I n f l u e n c e o f l o a d and temperature
152 155 157 159 162 163 163 165
6.17 Bearing a p p l i c a t i o n s
165 166
6.18 Bearing c a r e
167
6.19 Bearing mounting 6.20 D im o u n t i n g b e a r i n g s
167 168
6.21 Mounting and dismounting by o i l i n j e c t i o n
169 171
6.22 Cleaning o f b e a r i n g s
X 6.23 R e c o g n i t i o n o f b e a r i n g f a i l u r e s 6.23.1 6.23.2
Wear
172
I n c o r r e c t mounting
6.23.3 6.23.4
Cage f a i l u r e s
174 175 177
6.23.5
Vibrations
6.23.6
Rust and o t h e r t y p e s o f c o r r o s i o n
6.23.7
Smearing
181
PRACTICAL GEAR TRIBOLOGY, T . I .
FOWLE, CONSULTANT
184 184 184
7.1
Introduction
7.2
Alignment
7.3
Tooth a c t i o n
187
Tooth s u r f a c e d i s t r e s s
189
7.4.1
Pitting
189
7.4.2 7.4.3 7.4.4
Scuffing A b r a s i v e wear
191 194
Other forms o f gear wear
195
7.4
7.5
Problems i n l u b r i c a t i o n systems
References Gear problems : causes and remedies
CHAPTER 8.
178 181
Passage o f e l e c t r i c c u r r e n t through bearings
CHAPTER 7.
172
195 196 197
MATERIALS FOR TRIBOLOGICAL APPLICATIONS, 0. SCOTT,
8.1 8.2
Introduction
205 205
Types o f m a t e r i a l s
205
8.3
Materials f o r specific applications
CONSULTANT, EDITOR OF WEAR
8.3.1 8.3.2
P l a i n bearings
207 7-07
Gears
21 0
8.3.3 8.3.4
R o l l i n g bearings
21 1
Wear r e s i s t a n t m a t e r i a l s
21 3
8.3.5
Tools
214
8.3.6 8.3.7
Cutting tools
8.3.8
Cams and t a p p e t s
215 21 5 216
8.3.9
F r i c t i o n materials
217
8.3.10
P l a s t i c bearings
21 7
Piston rings
XI 8.4
8.5
Surface t r e a t m e n t s and c o a t i n g s
218
Conclusions
22 1 221
References
CHAPTER 9.
SELECTION OF LUBRICANTS, A.R.
LANSDOWN, SWANSEA 223
TRIBOLOGY CENTRE 9.1
Introduction
223
9.2
Selecting the l u b r i c a n t type
224
9.3
Properties o f mineral o i l s
226
9.3.1
Viscosity
226
9.3.2
Viscosity
9.3.3
Viscosity
9.3.4
A n t i - w e a r , extreme p r e s s u r e and a n t i -
-
temperature r e l a t i o n s h i p
229
pressure r e l a t i o n s h i p
23 1
f r i c t i o n properties
9.4
CHAPTER 10.
9.3.5
Stability
9.3.6
Con tami nan t s
Choice o f base o i l 9.4.1
L i m i t a t i o n s o f mineral o i l s
9.4.2
High temperatures
9.4.3
Flammability
9.4.4
Compatibil i t y
9.5
Greases
9.6
Sol i d l u b r i c a n t s
9.7
Gas l u b r i c a t i o n
LUBRICANT ADDITIVES, THEIR APPLICATION, PERFORMANCE AN0 LIMITATIONS, D.M.
SOUL, LUBRIZOL INTERNATIONAL 242
LABORATORIES. Introduction
242
10.2
Basic p r o p e r t i e s of l u b r i c a n t s
242
10.3
Lubricant a d d i t i v e s
243
10.1
10.3.1
Detergent and d i s p e r s a n t a d d i t i v e s
243
10.3.1.1
Detergent t y p e s
243
.1 Sulphonates
245
.2 Phosphonates and/or thiophosphonates
245
.3 Phenates
248
.4 A l k y l s u b s t i t u t e d sal i c y l a t e s
249
XI1 10.3.1.2
Mode o f a c t i o n o f d e t e r g e n t add i t i v e s
10.3.1.3
10.3.1.4
249
Di spe rsant a d d i t i v e s
249
. 1 Cop0 1 yme r s
250
.2 S u b s t i t u t e d Succinimides
250
.3 Amides
250
.4 Other chemicals
251
Mode o f a c t i o n o f d i s p e r s a n t additives
10.3.2
251
A n t i o x i d a n t s and b e a r i n g c o r r o s i o n inhibitors
10.3.3
251
10.3.2.1
Di t h i op ho sphates
252
10.3.2.2
Hindered phenol
252
10.3.2.3
N i t r o g e n bases
252
10.3.2.4
S u l ph uri sed p o l y o l e f i n s
252
10.3.2.5
Mode o f a c t i o n o f a n t i o x i d a n t s
252
Corro si on i n h i b i t o r s
253
10.3.3.1
Di t h i op ho sphates
253
10.3.3.2
Di t h i carb amates
253
10.3.3.3
Sulphur p r o d u c t s
254
10.3.3.4
Phosphorus
10.3.3.5
T r i a z o l e s and c h e l a t i n g agents
10.3.3.6
-
Sulphur p r o d u c t s
2 54
Mode o f a c t i o n o f c o r r o s i o n inhibitors
10.3.4
Rust i n h i b i t o r s
10.3.5
V i s c o s i t y improvers 10.3.5.1
10.3.6
254 254
255
Mode o f a c t i o n o f V . I .
improvers
Pour p o i n t depressants 10.3.6.1
2 56 257
Mode o f a c t i o n o f pour p o i n t d ep ressa nts
10.3.7
254
Dime rcap t o t h iad i o z o l e derivatives
10.3.3.7
254
257
258
Extreme p ressu re a d d i t i v e s 10.3.7.1
Comrnerical E.P.
t h e i r application
258
10.3.7.2
Automotive E.P.
259
10.3.7.3
An t i wea r a d d i t i v e s
260
10.3.7.4
E.P.
a d d i t i v e s f o r t u r b i n e o Is
262
10.3.7.5
E.P.
a d d i t i v e s f o r c u t t i n g o Is
262
10.3.7.6
Mode o f a c t i o n o f E.P.
a d d i t i v e s and
gear o i l s
a d d i t ve s
263
XIII
10.4
10.3.8
Emulsifiers
264
10.3.9
Friction modifiers
264 265
Conclusions
266
References
CHAPTER 11. CONSUMPTION AND CONSERVATION OF LUBRICANTS,
A.R.
LANSDOWN, SWANSEA TRIBOLOGY
CENTRE
26 7
11.1
Consumption
26 7
11.2
Reducing consumption
270
11.3
Reclamation and r e - r e f i n i n g
271
11.4
Economics
273
CHAPTER 12. HEALTH AND SAFETY ASPECTS OF LUBRICANTS, A.R.
EYRES, MOBIL EUROPE INC.,
275
12.1
Introduction
275
12.2
Composition o f l u b r i c a n t s
275
12.3
M i n e r a l base o i l f a c t o r s
2 76
12.3.1
Acute t o x i c i t y
276
12.3.2
Dermatitis
277
12.3.3
O i l mist
277
12.3.4
O i l vapours
278
12.3.5
Skin cancer
278
12.3.6
Eye i r r i t a t i o n
280
12.4
Additive factors
280
12.4.1
Lead compounds
280
12.4.2
Orthophosphates
281
12.4.3
C h l o r i n a t e d Naphthalenes
12.4.4
Sodium N i t r i t e and Amines
12.4.5
Sodium Mercaptobenzothiazole
282
12.4.6
Trichloroethylene
282
281 28 1
282
12.5
B a c t e r i a and b i o c i d e s
12.6
Synthetic l u b r i c a n t s
283
12.7
Used and r e c l a i m e d o r r e - r e f i n e d o i l s
283
12.8
H e a l t h and s a f e t y p r e c a u t i o n s
284 284
12.8.1
Suppliers r e s p o n s i b i l i t i e s
12.8.2
Skin p r o t e c t i o n
285
12.8.3
O i l m i s t and vapour
287
12.8.4
S k i n cancer
288
XIV 12.8.5 12.9
B a c t e r i a and b i o c i d e s
Conclusions
289 289
References
289
CHAPTER 13. EFFECTIVE CONTAMINATION CONTROL I N FLUID POWER
SYSTEMS, J.B.
SPENCER,
13.1
Introduction
13.2
D i r t Input
13.1.1
291
A syst e mat i c approach t o f i l t r a t i o n
-
291
t yp es and sources o f c o n t a m i n a t i o n
292 294
13.2. I
New o i 1
294
13.2.2
B u i l t i n contamination
294
13.2.3
Environmental Co nt amination
294
13.2.4
E n t r y p o i n t s f o r e nvironmental
13.2.5 13.3
SPERRY VICKERS
contamination
295
13.2.4.1
A i r breathers
295
13.2.4.2
Power u n i t access p l a t e s
295
13.2.4.3
Cylinder seals
295
Generated c o n t a m i n a t i o n
296
E f f e c t s o f t ype s and s i z e s o f p a r t i c l e s
296
13.3.1
Catastrophic f a i l u r e
296
13.3.2
Intermittent f a i l u r e
296
13.3.3
Deg rad at i on f a i l u r e
297
13.3.4
Pumps
297
13.3.5
Motors
300
13.3.6
D i r e c t i o n a l valves
30 1
13.3.7
Pressure c o n t r o l s
302
13.3.8
Flow c o n t r o l s
302
13.3.9
Summary
303
13.4
S p e c i f y i n g con t a mi na t i o n l e v e l s
305
13.5
Selecting the f i l t e r
305
13.5.1
Nominal r a t i n g
305
13.5.2
A b sol u t e r a t i n g
306
13.5.3
The b ub bl e t e s t
306
13.5.4
Mean f i l t r a t i n g r a t i n g
306
13.5.5
Multipass f i l t e r test
307
13.5.6
Beta r a t i o
307
13.5.7
Practical c l a s s i f i c a t i o n of f i l t e r performance
307
13.5.7.1
Pulsating flow
308
13.5.7.2
Bypass i n g
308
xv 13.5.8 13.6
Filter sizing
Locating the f i l t e r
13.6.1
318
Pump i n l e t f i l t r a t i o n
318
Pressure 1 i n e f i l t r a t i o n
320
13.6.3
Return 1 i n e f i 1 t r a t i o n
322
13.6.4
O f f line filtration
324
13.6.2
13.7
312
327
Summary
CHAPTER 14. SEALS FOR FLUID POWER EQUIPMENT, PART ONE,
B.D.
HALLIGAN, JAMES WALKER 6 CO. LTD.
330
14.1
Introduction
330
14.2
Materials
331
14.3
Seal designs
338
14.4
Tribological considerations
339
14.4.1
Film conditions
339
14.4.2
Surface f i n i s h
343
14.4.3
Seal f r i c t i o n
344
14.4.4
Type o f f l u i d
346
14.4.5
Filtration
347
A i r entrainment
347
14.4.6 14.5 14.6
Selection
348
Storage
350
14.6.1
14.7
Recommendations
350
14.6.1.1
Temperature
350
14.6.1.2
Humidity
351
14.6.1.3
Light
351
14.6.1.4
Oxygen and ozone
351
14.6.1.5
Deformation
351 351
Assembly
CHAPTER 15. SEALS FOR FLUID POWER EQUIPMENT, PART TWO, ROTARY SHAFT L I P SEALS, B.D.
HALLIGAN, JAMES WALKER E CO. LTD.
353
15.1
Introduction
353
15.2
Design
354
15.3
Material
355
15.4
Seal l u b r i c a t i o n
356
15.5
Shaft surfaces
357
15.6
Friction
358
15.7
Speeds
359
XVI 15.9
Pressure
359 360
15.10 Care and h a n d l i n g
360
15.10.1
Storage
15.10.2
Handling
361
15.10.3
Fitting
36 1 362
15.11 S e r v i c e problems and t h e i r s o l u t i o n s 15.11.1
362
Unacceptable leakage
CHAPTER 16. SEALS FOR FLUID POWER EQUIPMENT, PART THREE, COMPRESSION PACKINGS, 16.1
Appendix 1
B.D. HALLIGAN, JAMES WALKER t CO. LTD.
370
The packed g l a n d
370
16.1.1
Pumps
371
16.1.2
Valves
371
16.2
Operating p r i n c i p l e s
373
16.3
Gland design
374
16.4
Packing c o n s t r u c t i o n and m a t e r i a l s
376
16.4.1
Fibre material
3 76
16.4.2
Lubricants
376
16.4.3
Construct ion
3 76
16.4.4
Additional material
379
16.4.5
Selection
383
16.4.6
Fitting
388
16.4.7
Fault finding
390
16.4.8
Standardization
391
16.4.9
The f u t u r e
392 393
Safety o f asbestos g l a n d packings and gaskets
CHAPTER 17. CENTRALISED LUBRICATION SYSTEMS DESIGN, J.G.
MERRETT,
ENGINEERING AN0 GENERAL EQUIPMENT LTD.
395
17.1
Introduction
395
17.2
Points of l u b r i c a t i o n
396
17.3
Selecting the l u b r i c a n t 17.3.1
-
o i l or grease
the other 17.3.2
397
L u b r i c a t i n g grease
-
t h e t y p e s and how
t o s e l e c t them 17.3.3
396
O i l o r grease7 when t o use one and when
Lubricating o i l s s e l e c t them
398
-
types and how t o 400
XVII
17.3.3.1 17.3.3.2
The p r o p e r t i e s o f t h e o i l Specialist lubricants
40 1 402
17.4
Pipe diameter vs f l o w c h a r a c t e r i s t i c s
402 402
17.5
L u b r i c a t i o n requirements f o r p l a i n b e a r i n g s
402
17.6
S e l e c t i n g t h e r i g h t t y p e o f l u b r i c a t i o n system Grease l u b r i c a t i o n systems
403 403
17.6.1.1
D i r e c t f e e d i n g systems
403
17.6.1.2
I n d i r e c t o r l i n e systems
405
17.6.1.3
Comparison o f p a r a l l e l
17.3.4
17.6.1
Summarising l u b r i c a n t s e l e c t i o n
-
systems
s i n g l e l i n e and
dual l i n e
17.6.1.4
412
systems
17.6.1 - 5
Lubrication o f p l a i n o r sleeve bearings
17.6.1.6
O i l l u b r i c a t i o n systems
17.6.2.1
T o t a l l o s s systems
17.6.2.2
Systems designed w i t h small amount o f h e a t removal
17.6.2.3
17.6.4 17.7
412 413 413 416
Systems designed f o r l u b r i c a t i o n
418
c o o l ing
17.6.3
41 2
Lubrication o f a n t i f r i c t i o n ( b a l l and r o l l e r ) b e a r i n g s
17.6.2
412
S e l e c t i n g grease l u b r i c a t i o n
M i c r o - f o g l u b r i c a t i o n systems
41 9
17.6.3.1 17.6.3.2
Working p r i n c i p l e s O i l quality
419 421
17.6.3.3
Compressed a i r
421
17.6.3.4
Systems d e s i g n c o n s i d e r a t i o n s
421
17.6.3.5
Some t y p i c a l a p p l i c a t i o n s
421 423 424
Check l i s t
Summary
425
References
CHAPTER 18. ON CONDITION MAINTENANCE, R.A.
COLLACOTT, FAULT
DIAGNOSIS CENTRE
427
18.1
Introduction
18.2
Background
18.3 18.4
Management o f cond i t i o n mon i t o r ing
427 427 428 428
F a i l u r e modes and e f f e c t s a n a l y s i s
XVIII 18.5
Monitoring technique s e l e c t i o n
432
18.5.1
V i b r a t i o n monitoring
433
18.5.1.1
Proximity probes
433
18.5.1.2
Seismic v e l o c i t y transducers, accelerometers,
vibration
meters, spectrum analysers
18.5.1.3
18.6
Waveform a n a l y s i s
434
4 36
18.5.2
Inspection, i n t e g r i t y s u r v e i l l a n c e
436
18.5.3
Contaminant a n a l y s i s
4 38
18.5.4
Trends a n a l y s i s
444 444
Deterioration l i m i t s
445
References
CHAPTER 19. THE TRIBOLOGY OF METAL CUTTING, E.M.
TRENT, U N I V E R S I T Y
OF BIRMINGHAM
446
19.1
Introduction
446
19.2
Metal c u t t i n g phenomena
19.3
Conditions a t the tool-work
19.3.1
19.4
19.5
446 interface
Tool forces and stresses
448
19.3.2
C u t t i n g speed
451
19.3.3
Heat i n metal c u t t i n g
45 1
19.3.4
Seizure o f the tool-work i n t e r f a c e
452
19.3.5
C u t t i n g t o o l temperatures
456
19.3.6
SI i d i n g a t the tool-work i n t e r f a c e
4 58
C u t t i n g t o o l wear
459
19.4.1
Abrasion
46 1
19.4.2
Surface shearing
46 1
19.4.3
D i f f u s i o n and i n t e r a c t i o n
463
19.4.4
Attrition
465 467
Coolants and l u b r i c a n t s
470
References
CHAPTER 20. ROLE OF LABORATORY TEST MACHINES, F.T.
M.H.
448
JONES, UNIVERSITY COLLEGE
BARWELL AND
O F SWANSEA
471
20.1
Experimental method
471
20.2
Lubricant type approval t e s t i n g machines
4 72
20.2.1
Engine t e s t s
472
20.2.2
Gear t e s t s
474
20.2.3
Simulation o f i n d u s t r i a l s i t u a t i o n s
479
XIX 20.3
20.4
A n a l y s i s i n t o system elements o f t h e b a s i s f o r selection o f laboratory tests
480
Equipment f o r b a s i c re sea rch
488
Ref e r enc e s
492
GLOSSARY
493
AUTHOR INDEX
502
SUBJECT INDEX
506
This Page Intentionally Left Blank
XXI
ACKNOWLEDGEMENT
The e d i t o r s g r a t e f u l l y acknowledge t h e a s s i s t a n c e o f M r . G. W i l l i a m s and Mr.
J . Thomas o f t h e l u b r i c a t i o n department o f B r i t i s h Steel C orporation,
Po r t T a l b o t and D r . G. Thomas o f t he E x t r a Mural Department, U n i v e r s i t y Colle g e , Swansea, who have been a c t i v e l y a s s o c i a t e d w i t h t h e annual Seminars on i n d u s t r i a l T r i b o l o g y s i n c e t h e i r i n c e p t i o n .
Acknowledgement i s a l s o due t o v i s i t i n g l e c t u r e r s who generously gave t h e i r t i m e i n d e v e lo p in g t h e Seminar t o i t s p r e s e n t successful f o r m a t . i n c l u d e R. Gronbech P.
Gadd
-
-
Davey Un i t e d, J. Bathgate
NAML Gosport, N.W.
International, Dr.
E.T.
Morris
Jagger
-
-
-
F arva l ube Ltd.,
Angus Seals Ltd.,
These
David Brown Gears Ltd.,
D. H a t t o n
and D r .
D.J.
-
Shell
Haines
-
B r i t i s h A i r c r a f t Corp ora t i o n.
The e d i t o r s e x te nd t h e i r g r a t i t u d e t o t h e t y p i n g e x p e r t i s e o f Mrs. M.A. W i l l i a m s and Mrs. P.T. ready copy.
Hancock who have so e x p e r t l y produced t h i s camera
This Page Intentionally Left Blank
XXIII
FOREWORD Some e le v e n ye ars ago M r .
Ronald Dale, B r i t i s h Steel C orporation,
P o r t Talbot,
v i s i t e d t h e C o lleg e t o propose t h e i n t r o d u c t i o n o f a course i n T r i b o l o g y f o r Steelworks Design S t a f f .
T h i s p rop osa l was r e a d i l y accepted because prepar-
a t i o n s f o r t h e e s t a b l i s h m e n t o f t h e T r i b o l o g y Centre were a l r e a d y w e l l advanced. The f i r s t c o urse was r a t h e r a m b i t i o u s i n s o f a r as i t was o f two weeks duration,
t h e f i r s t week b e i n g devoted t o f a i r l y heavy t h e o r y w h i l s t more
p r a c t i c a l m a t e r i a l was i n t r o d u c e d d u r i n g t h e second week.
A particular feature
was t h e i n c l u s i o n o f p r o j e c t s based on contemporary problems a t t h e steelw orks. These p r o j e c t s were a l l o c a t e d t o small teams and, on t h e f i n a l day,
the leader
o f each team had t o r e p o r t t o t h e whole co urse on t h e s o l u t i o n a r r i v e d a t by h i s team. The demand f o r re pe at courses was s t r o n g and they have been r u n a t annual i n t e r v a l s e v e r s i nce .
However, t h e p a t t e r n o f i n s t r u c t i o n has been m o d i f i e d from
y e a r t o y e a r as t h e r e s u l t o f q u e s t i o n n a i r e s completed by course members and i n response t o r e p r e s e n t a t i o n s by employers.
Thus t h e p r e s e n t course i s o f s h o r t e r
d u r a t i o n and i s o r i e n t a t e d towards p r a c t i c e from t h e o u t s e t , t h e o r y o n l y b e i n g i n t r o d u c e d when r e q u i r e d t o p o i n t t h e way towards t h e s o l u t i o n s o f p r a c t i c a l problems. One o f t h e o b j e c t s o f t h e f i r s t course,
t o c o n t r i v e t h e maximum degree o f
i n te r c h a n g e o f i n f o r m a t i o n between p a r t i c i p a n t s , h a s
been r e t a i n e d and developed,
p a r t i c u l a r l y s i n c e t h e course i s now r e c r u i t e d from a number o f i n d u s t r i e s be s id e s t h e s t e e l i n d u s t r y and indeed f rom several c o u n t r i e s . The t e r m ' T r i b o l o g y ' , as f o l l o w s : -
d e f i n e d i n t h e Chambers T w e n t i e t h Century D i c t i o n a r y
a sci e nce and t e chn ol o gy embracing a l l s u b j e c t s i n v o l v e d when s u r -
fa c e s i n c o n t a c t move i n r e l a t i o n t o each o t h e r (Greek t r i b e i n , t o rub, and logos, speech, a d i s c o u r s e ) had then been o n l y r e c e n t l y i n t r o d u c e d t o emphasize the m u l t i - d i s c i p l i n a r y
n a t u r e o f t h e study o f b e a r i n g system.
The term 'Lub-
r i c a t i o n ' which had been used p r e v i o u s l y was considered t o be inadequate because i t focussed a t t e n t i o n on one element o n l y o f t h e b e a r i n g system
-
n o t a b l y the
l u b r i c a n t t o t h e e x c l u s i o n o f o t h e r f a c t o r s such as t h e m a t e r i a l o f c o n s t r u c t i o n o f t h e i n t e r a c t i n g elements. The need f o r t h e i n t r o d u c t i o n o f t h e new term was n o t n e c e s s i t a t e d by any f a i l u r e t o d e v e l o p t h e s u b j e c t f rom t h e re search and development p o i n t o f view b u t r a t h e r t o draw a t t e n t i o n t o a body o f knowledge w hich was n o t thought t o be s u f f i c i e n t l y a p p l i e d w i t h i n i n d u s t r y a t t h a t p a r t i c u l a r time.
The development
o f T r i b o l o g y has always been r e l a t e d t o advances i n t h e S t a t e o f t h e A r t o f
XXIV Engineering.
Thus, d u r i n g t h e n i n e t e e n t h c e n t u r y , progress was dominated by
the r a i l w a y and t h e steamship.
The hydrodynamic t h e o r y o f l u b r i c a t i o n was
developed t o e x p l a i n Beachamp Towers experiments on b e a r i n g s used on t h e Metrop o l i t a n RaiIway,and M i c h e l l ' s t i l t i n g pad t h r u s t b e a r i n g s were considered t o be very s u i t a b l e for s h i p s ' p r o p e l l e r s h a f t s .
The t h i r t i e s t h i s c e n t u r y were
devoted t o t h e a i r c r a f t and a ut omo bi l e ;
t h e f o r t i e s t o the gas t u r b i n e .
o f th e e a r l y work o f N.E.L.
Cl e veland was d i r e c t e d t o the problems
o f high-speed b e ari ng s. institution,
and N.A.S.A.
Then came at omi c energy.
Much
The o r i g i n o f o u r s i s t e r
The N a t i o n a l T r i b o l o g y L a b o r a t o r y a t R i s l e y , can be a t t r i b u t e d t o
t h i s need and, more re cen t l y, spa ce
technology has i n t r o d u c e d a whole range o f
new problems p a r t i c u l a r l y r e l a t e d t o o p e r a t i o n i n r a r i f i e d atmospheres. I f one has t o d eci d e on t o d a y ' s s p e c i a l c h a r a c t e r i t i s t h e urge f o r g r e a t e r p r o d u c t i v i t y m a i n l y achieved by au t o mat i c and o t h e r w i s e c a p i t a l - i n t e n s i v e equipment.
T h i s has focussed a t t e n t i o n on t h e importance o f r e l i a b i l i t y and o f
reduced maintenance,which c a l l f o r t h e utmost refinement o f t r i b o l o g i c a l design. Means f o r m o n i t o r i n g t h e c o n d i t i o n o f machines so as t o a n t i c i p a t e f a i l u r e and t o r a t i o n a l i s e maintenance have now assumed t h e utmost importance. The t r i b o l o g y o f t h e m a n u fa c t u ri ng i n d u s t r y w i l l p r o b a b l y be t h e most important grow th area o f o u r s u b j e c t d u r i n g t h e n e x t decade. N o t w i t h s t a n d i n g t h e o u t p u t o f p r e s e n t day research schools i n T r i b o l o g y , o f which t h e r e a r e a growing number, t h e body o f knowledge which has been b u i l t up d u r i n g t h e p a s t c e n t u r y p r o v i d e s p owerf ul t o o l s f o r any engineer who wishes t o improve a p r o d u c t o r p r a c t i c e and i t i s hoped t h a t t h e f o l l o w i n g volume w i l l p o i n t t h e way t o t h e a p p l i c a t i o n o f sound t r i b o l o g y i n many i n d u s t r i e s .
F.T. Barw ell U n i v e r s i t y C ollege o f Swansea
1
1 TRIBOLOGY
IN PERSPECTIVE
D. SCOTT, C o n s u l t a n t , E d i t o r o f Wear 1.1
INTRODUCTION As o u r t e c h n o l o g i c a l c i v i l i z a t i o n expands, m a t e r i a l and energy c o n s e r v a t i o n
i s becoming i n c r e a s i n g l y i mp ort an t .
Wear i s a major cause o f m a t e r i a l wastage,
so any r e d u c t i o n o f wear can e f f e c t c o n s i d e r a b l e savings.
Friction is a
p r i n c i p a l cause o f energy d i s s i p a t i o n and c o n s i d e r a b l e savinas a r e p o s s i b l e by improved f r i c t i o n c o n t r o l .
L u b r i c a t i o n i s t h e most e f f e c t i v e means o f c o n t r o l -
l i n g wear and r e du ci ng f r i c t i o n .
Thus t r i b o l o g y , which i s t h e science and tech-
nolo g y o f f r i c t i o n l u b r i c a t i o n and wear,
i s o f c o n s i d e r a b l e importance i n
The h i s t o r y o f t h i s r e l a t i v e l y new science
m a t e r i a l and energy c o n s e r v a t i o n .
which i s concerned w i t h problems t h a t have always presented man w i t h a c h a l l e n g e has been r e c o r d e d 111, and t h e fundamentals reviewed [ 2 ] .
1.2
IMPACT OF TRIBOLOGY Since t h e p u b l i c a t i o n o f t h e L u b r i c a t i o n Report [ 3 ] t h e r e has been an i n -
c r e a s i n g awareness t hro ug ho ut i n d u s t r y o f t h e s u b j e c t o f t r i b o l o g y .
I n t h e UK
t h e N a t i o n a l Centre f o r T r i b o l o g y and I n d u s t r i a l U n i t s o f T r i b o l o g y have been s e t up t o p r o v i d e a d v i c e t o i n d u s t r y on t he u t i l i s a t i o n o f e x i s t i n g knowledge. These a r e now v i a b l e e s t a b l i s h m e n t s o p e r a t i n g as c o n t r a c t research o r g a n i s a t i o n s s e l l i n g t h e i r s e r v i c e s a t commercial r a t e s .
Over t h i r t y u n i v e r s i t i e s , p o l y -
t e c h n i c s and t e c h n i c a l c o l l e g e s have i n c o r p o r a t e d courses on v a r i o u s aspects o f t r i b o l o g y i n t o t h e i r syllabuses.
A b a s i c t r i b o l o g y module
mechanical e n g i n e e r i n g courses has been drawn up. j e c t f o r t h e h i g h e r n a t i o n a l c e r t i f i c a t e (H.N.C.)
[41 f o r undergraduate
T r i b o l o g y i s an e l e c t i v e subi n e n g i n e e r i n g i n the U n i t e d
Kingdom and a t r i b o l o g y c o n t e n t i s i n c l u d e d i n some committee f o r n a t i o n a l academic awa ds (C.N.N.A.)
courses.
Post-graduate research i n t r i b o l o g y ,
l e a d i n g t o h gher degrees i s c a r r i e d o u t a t several u n i v e r s i t i e s ; c h a i r s i n t r b o log y.
t h r e e have
Various courses and t r a i n i n g programmes a r e a l s o a v a i l a b e
to i n d u s t r y .
T r i b o l o g y i s now reco gn i zed u n i v e r s a l l y and P r e s i d e n t C a r t e r o f U.S.A.
151
d e c l a r e d i t t o be a g e n e r i c t ech no l og y u n d e r l y i n g many i n d u s t r i a l s e c t i o n s and t h e p r o s p e c tu s f o r an I n d u s t r i a l T r i b o l o g y I n s t i t u t e a t Rensselaer Technolooy Center has been p rese nt ed [ 6 ] . Numerous papers on t r i b o l o g y a r e p u b l i s h e d a n n u a l l y and many r e p o r t research d i r e c t e d towards a b e t t e r under.standing o f t h e fundamental p r i n c i p l e s governinq
2 i n t e r a c t i n g surfaces.
U n f o r t u n a t e l y , most o f t h e i n f o r m a t i o n p r o v i d e d i s n o t
s u i t a b l e f o r d i r e c t use by d esi g ne rs and en gi neers as research workers g e n e r a l l y f i n d i t more c o n ve ni e nt t o express r e s u l t s i n terms o f non-dimensional parameters r a t h e r than as t h e s p e c i f i c da t a r e q u i r e d f o r design purposes. bology handbook
A tri-
[7] has been produced w i t h t h e o b j e c t o f p r o v i d i n g i n f o r m a t i o n
t o i n d u s t r y i n a f o rm t h a t i s r e a d i l y a c c e s s i b l e and understood by e n g i n e e r i n g de s ig n e r s , draughtsmen and works e ng i ne ers.
A synoptic j o u r n a l
[8] has been
i ntr o d u c e d t o reduce t i me spent i n l i t e r a t u r e p e r u s a l . There i s a steady g rowt h i n t h e f o r m a t i o n o f T r i b o l o g y S o c i e t i e s on an i n t e r n a t i o n a l s c a l e.
1.3
ECONOMIC ASPECTS OF TRIBOLOGY
The L u b r i c a t i o n Report [ 3 ] est i mat ed , w i t h i n an e r r o r o f t w e n t y - f i v e p e r c e n t , t h a t an amount exceeding f i v e hundred m i l l i o n pounds per annum can be saved i n t h e c i v i l i a n s e c t o r o f t h e UK economy by improvements i n e d u c a t i o n and res e a r c h i n t r i b o l o g y .
Such improvements a r e s i g n i f i c a n t , n o t merely i n c o s t
s a v in g s , b u t a r e c r u c i a l t o t e c h n o l o g i c a l pro gress and have doubly s i g n i f i c a n t i m p l i c a t i o n s f o r t he economic w e l l - b e i n g o f t he n a t i o n and t h e r e p u t a t i o n o f i t s e n g i n e e r i n g p r o d uct s. The ASME Research Committee on L u b r i c a t i o n i n t h e i r " Strategy Conservation t h r ou gh T r i b o l o g y "
f o r Energy
[ 9 ] r e p o r t e d t h e magnitudes o f energy conserv-
a t i o n t h a t can p o t e n t i a l l y be o b t a i n e d i n t h e f o u r major areas o f road t r a n s p o r t a t i o n , power g e n e r a t i o n , t u r b o machinery and i n d u s t r i a l processes through progres s i n t r i b o l o g y .
The e s t i m a t e d 1 1 p er c e n t t o t a l savings i n annual US energy
consumption i s e q u i v a l e n t t o some s i x t e e n b i l l i o n US d o l l a r s by an e x p e n d i t u r e i n r e s e a r c h and development o f an e s t i m a t e d t w e n t y - f o u r m i l l i o n d o l l a r s . A techno-economic
st ud y [ l o ] concluded t h a t t h e a p p l i c a t i o n o f t r i b o l o g i c a l
p r i n c i p l e s and p r a c t i c e s can e f f e c t n a t i o n a l energy savings o f c o n s i d e r a b l e magnitude i n t h e U n i t e d Kingdom,
i n t he areas covered which comprise t h e major
p a r t s o f 87% o f energy consumption.
These savings a r e e s t i m a t e d a t €468 t o
f 7 0 0 m i l l io n p e r annum. E r o s io n can be expensive and i t has been r e p o r t e d [ l l ] t h a t t h e i n g e s t i o n o f du s t c l o u d s can reduce t h e l i v e s o f h e l i c o p t e r engines by as much as 90 per c en t;
l o c a l s t a l l can be caused by removal o f as l i t t l e as 0 . 0 5 mm o f m a t e r i a l
from t h e l e a d i n g edges o f compressor blades.
I n pneumatic t r a n s p o r t a t i o n o f
m a t e r i a l t h r o u g h p i pe s, t h e e r o s i v e wear a t bends can be up t o f i f t y times more than t h a t i n s t r a i g h t s e c t i o n s .
Even wood c h i p s can cause such wear [ 1 2 ] .
Analyses o f t h e f a i l u r e o f b o i l e r tubes i n d i c a t e t h a t about one t h i r d o f a l l occ u r r e n c e s were due t o e r o s i o n [131.
3 Although a b r a s i v e wear i s u s e f u l t o shape and p o l i s h e n g i n e e r i n g components, i t s unwanted occurrence i s p r o b a b l y t h e most s e r i o u s i n d u s t r i a l wear problem. In the a g r i c u l t u r a l
i n d u s t r y as many as f o r t y p e r c e n t o f t h e components
r e p l a c e d on equipment have f a i l e d by a b r a s i v e wear [ 1 4 ] . The wear o f t o o l s used f o r c u t t i n g m e t a l s i s o f c o n s i d e r a b l e importance t o t h e economics o f t h e e n g i n e e r i n g i n d u s t r y ,
I t was e s t i m a t e d i n 1971 [ 1 5 ] t h a t
f o r t y b i l l i o n d o l l a r s was spent i n t h e USA on t h e machining o f metal p a r t s .
In
t h e UK about twenty m i l l i o n c a r b i d e c u t t i n g t o o l s a r e used p e r year a t a c o s t o f f i f t y m i l l i o n pounds. Several e s t i m a t e s have been made on t h e c o s t o f f r i c t i o n and wear.
Jost [16]
s t a t e d t h a t f r i c t i o n and wear i n t h e USA accounted f o r an e x p e n d i t u r e o f one hundred b i l l i o n d o l l a r s p e r annum. Technology of F.R.G.
A Committee o f t h e M i n i s t r y o f Research and
[ 1 7 ] e s t i m a t e d t h a t f r i c t i o n and wear caused a n a t i o n a l
economic waste o f t e n b i l l i o n OM p e r annum o f which about f i f t y p e r c e n t i s due t o a b r a s i v e wear.
Rabinowicz [18] has e s t i m a t e d t h a t about t e n p e r c e n t o f a l l
energy generated by man i s d i s s i p a t e d i n f r i c t i o n processes. T r i b o l o g i c a l f a i l u r e s a r e i n v a r i a b l y a s s o c i a t e d w i t h b e a r i n g s and t o i l l u s t r a t e t h e c o s t s which can be i n v o l v e d i t has been r e p o r t e d [19] t h a t a simple b e a r i n g f a i l u r e i n a f u l l y i n t e g r a t e d s t e e l m i l l can l e a d t o a t o t a l shut down which a t f u l l o u t p u t r a t e may c o s t one hundred and f i f t y t o t h r e e hundred pounds p e r minute.
A s i m i l a r b e a r i n g f a i l u r e on a modern g e n e r a t o r s e t c o u l d i n v o l v e
t h e C e n t r a l E l e c t r i c i t y Generating Board i n a l o s s o f one t o twenty pounds s t e r l i n g p e r m i n u t e t i l l t h e s e t was a g a i n o p e r a t i o n a l .
A s i m i l a r bearing
f a i l u r e i n t h e USA has been quoted t o c o s t t w e n t y - f i v e thousand d o l l a r s p e r day [ZO].
I t has been r e p o r t e d [ Z l ]
t h a t t h e t o t a l c o s t o f wear f o r a US naval
a i r c r a f t amounted t o two hundred and f o r t y t h r e e d o l l a r s p e r f l i g h t hour.
1.4
MECHANISMS OF WEAR Progress i n wear c o n t r o l can be a i d e d by a b e t t e r u n d e r s t a n d i n g o f t h e
mechanisms by which i t occurs.
Research workers have tended t o i s o l a t e and
study s p e c i f i c wear mechanisms such as adhesion, a b r a s i o n , e r o s i o n and f a t i g u e . Such r e s e a r c h has g e n e r a l l y been d i r e c t e d towards t h e study o f s u r f a c e s i n r e l a t i v e m o t i o n , t h e changes brought about by t h e i r i n t e r a c t i o n and t h e e f f e c t s o f t h e l u b r i c a n t and t h e environment p r e s e n t .
L i t t l e a t t e n t i o n has been g i v e n
t o t h e p r o d u c t s o f wear, t h a t i s t o t h e d e b r i s generated.
Recently, p a r t i c l e
t r i b o l o g y [ 2 2 ] has a l l o w e d p o s t u l a t i o n o f t h e mechanisms o f t h e i r f o r m a t i o n which t o g e t h e r w i t h r e f i n e d techniques o f s u r f a c e i n v e s t i g a t i o n and t h e study of sub-surface changes a i d s t h e e l u c i d a t i o n of t h e wear process. Advances i n u n d e r s t a n d i n g emerge o n l y f r o m a w i l l i n g n e s s t o q u e s t i o n accepted t h e o r i e s .
Q u e s t i o n i n g o f t h e t h e o r i e s o f wear a i d e d by r e f i n e d
4 i n v e s t i g a t i o n t e ch ni q ue s a r e now s t i m u l a t i n g the p i o n e e r i n g s p i r i t . d e l a m i n a t i o n t h e o r y o f wear
[23]
Suh's
i s t h e t y p i c a l example o f r e c e n t progress.
Surface e x a m in at i on and wear p a r t i c l e a n a l y s i s has l e d t o t h e h y p o t h e s i s [24] t h a t i n t e r a c t i o n p o l i s h e s t h e surf ace s and c r e a t e s a shear mix l a y e r o f s h o r t c r y s t a l l i n e o r d e r o f a l most s u p e r d u c t i l e m a t e r i a l which spreads over t h e s u r f a c e as f i r s t proposed by B e i l b y
[25].
Repeated r u b b i n g c o n t a c t causes t h e shear
mix l a y e r t o f a t i g u e and c h a r a c t e r i s t i c p a r t i c l e s f l a k e o f f . F u r t h e r work i s r e q u i r e d t o p r o v i d e a more complete d e s c r i p t i o n o f t h e s u r fac e b e h a v io u r o f m a t e r i a l s and t h e wi d er a p p l i c a t i o n o f new t h e o r i e s must a w a i t th e a d d i t i o n a l evidence.
I t may thus be p o s s i b l e t o p r e d i c t t h e wear r a t e s o f
m a t e r i a l s based on f i r s t p r i n c i p l e s and fundamental p r o p e r t i e s . The a p p l i c a t i o n o f a system a n a l y s i s t o wear problems i s r e c e i v i n g c o n s i d e r a b l e a t t e n t i o n [26]. The complex n a t u r e o f wear has delayed i t s i n v e s t i g a t i o n b u t i t now appears t h a t t he e r a when wear was considered a branch o f s t u d i e s i n f r i c t i o n and l u b r i c a t i o n i s coming t o an end.
The success o f t h e f i r s t I n t -
e r n a t i o n a l Conference on Wear o f M a t e r i a l s 1271 e s t a b l i s h e d wear as a s u b j e c t o f i n t e r n a t i o n a l importance i n i t s own r i g h t . i o n a l Conference
[29]
A second [ 2 8 ] and a t h i r d I n t e r n a t -
have been h e l d and a f o u r t h i s planned.
Microscopic
[30] and c a l c u l a t i o n methods f o r [31]. A s t a t e o f the a r t r e v i e w o f
asp e c ts o f wear a r e r e c e i v i n g a t t e n t i o n f r i c t i o n and wear have been reviewed available
1.5
wear i s
[32].
SURFACE STUDIES The f r i c t i o n a l and wear b eh avi o ur o f m a t e r i a l s i s g r e a t l y dependent upon t h e
s u r f a c e m a t e r i a l and i t s topography.
Surface i n t e r a c t i o n causes changes i n
these p r o p e r t i e s b u t d e t a i l e d knowledge o f happenings i n t h e i n t e r f a c e when wear i s occurring i s d i f f i c u l t t o acquire.
I t has been usual t o study surfaces a t
v a r i o u s stages o f wear t o p o s t u l a t e t h e sequence o f events.
Besides t h i s p r o -
cedure g r e a t e r a t t e n t i o n i s now b e i n g g i v e n t o the s i z e , morphology and s t r u c t u r e o f wear p a r t i c l e s as w e l l as t o t h e l o c a l i s e d n a t u r e o f damage t o surface,
i n t e r f a c e and su bsu rf a ce m a t e r i a l .
Several new t o o l s a r e a v a i l a b l e f o r
t h e s tu d y o f s u r fa ces a t a t o mi c l e v e l , n o t a b l y Auger e l e c t r o n spectroscopy, x - r a y photon e l e c t r o n spectroscopy,
scanning ion spectroscopy and i o n s c a t t e r i n g
spectroscopy which w i t h complementary i n f o r m a t i o n from x - r a y energy a n a l y s i s i n t h e scanning e l e c t r o n microscope and micro-probe a n a l y s i s a i d t h e t r i b o l o g i c a l e l u c i d a t i o n o f s u r f a c e phenomena
[33,34].
Advances have been made i n t h e a p p l i c a t i o n o f s t a t i s t i c a l techniques t o the c h a r a c t e r i s a t i o n o f rough s u r f a c e s
[35,36].
The e n t i r e s t a t i s t i c a l m i c r o -
geometry o f c e r t a i n rough s u r f a c e s can now be c o m p l e t e l y described i n terms o f t h e number o f peaks and mean l i n e c r o s s i n g s counted on a s i n g l e p r o f i l e .
These
5 te c h n iq u e s a r e now b e i n g a p p l i e d i n t r i b o l o g y and i t appears t h a t i n instrume n t a t i o n t h r e e - d imen si on al mapping i s now w e l l e s t a b l i s h e d [ 3 7 ] .
For t h e
measurement, assessment and c h a r a c t e r i s a t i o n o f v e r y f i n e surfaces, a l a s e r beam te c h n iq u e o f l i g h t s c a t t e r i n g appears p o t e n t i a l l y a t t r a c t i v e f o r q u a l i t y c o n t r o l purposes [ 3 8 ] .
1.6
LUBRICATION Since Reynolds [ 3 9] produced h i s e q u a t i o n f o l l o w i n g t h e p i o n e e r i n g work o f
Tower,
(see [ 4 0 ] )
t h e mathematical e x p r e s s i o n o f t h e process o f f i l m f o r m a t i o n
between r e l a t i v e l y moving s u r f a c e s has been fundamental t o a l l l u b r i c a t i o n theory.
Equations have been d e r i v e d and a p p l i e d t o t h e s t u d y o f the v a r i o u s
s u r f a c e c o n f i g u r a t i o n s used i n p r a c t i c e , and t h e i n t r o d u c t i o n o f t h e high-speed d i g i t a l computer a l l o w e d t h e si mu l t a ne ou s s o l u t i o n o f Reynolds e q u a t i o n t o g e t h e r w i t h e q u a t i o n s f o r t h e e l a s t i c d ef orma t i o n o f t h e surfaces. el a s to h y d r o d y n a mi c l u b r i c a t i o n (EHL) f i l m s ,
Optical studies o f
i n f r a - r e d temperature measurements
and t h e e l u c i d a t i o n o f t h e response o f vi sco us l i q u i d s t o h i g h frequency shear have g r e a t l y improved t h e u nd erst an di n g o f e l astohydrodynamic c o n t a c t s .
It i s
perhaps b e t t e r t o d e s c r i b e t h e l u b r i c a n t i n a h i g h l y loaded EHL c o n t a c t as an e l a s t o - p l a s t i c s o l i d r a t h e r t ha n a s a vi scou s f l u i d .
Based on t h e new under-
s ta n d in g , a t h e o r y o f EHL t r a c t i o n has been advanced [ 4 1 ] which may be a p p l i e d t o e n g i n e e r i n g components such as r o l l i n g b e a r i n g s and v a r i a b l e - s p e e d d r i v e s .
The e la s t o h y d r o d yn ami cs o f e l l i p t i c a l c o n t a c t s has been a p p l i e d t o b a l l and r o l l e r bearing l u b r i c a t i o n [42]. Progress i n hydrodynamic l u b r i c a t i o n appears t o be c e n t r e d on d e t a i l e d developments r a t h e r t h an improved fundamental understanding.
Work on boundary
l u b r i c a t i o n seems t o be o r i e n t e d towards s p e c i f i c problem areas such as e l e v a t e d temperatures and h o s t i l e environments.
Two c e n t u r i e s o f study have
f a i l e d t o u n r a v e l c o m p l e t e l y t he m y s t e r i e s o f l u b r i c a t i o n problems most i m p o r t a n t t o mankind, t he mechanism o f human j o i n t s .
Following the t e n t a t i v e
pr o p o s a l o f squeeze f i l m s [43] and t h e emphasis on t h e p r o t e c t i v e motion o f t h e s im p le squeeze f i l m [44]
i t i s co nsi d ere d t h a t t h e prospect o f EHL i s good
b u t t h a t t h e p r o m i s i n g mode i s squeeze f i l m and n o t r o l l i n g s l i d i n g [451. C o l l e c t i o n s o f i n f o r m a t i o n a r e a v a i l a b l e on t h e t r i b o l o g y o f n a t u r a l and a r t i f i c i a l j o i n t s [46] and t h e mechanical p r o p e r t i e s o f b i o m a t e r i a l s [ 4 7 ] .
1.7
LUBRICANTS When f a i l u r e i n s e r v i c e o ccurs i t i s co mmercially more a c c e p t a b l e t o change
t h e l u b r i c a n t r a t h e r t ha n t h e d esi g n.
Thus research and development work i s
c o n t i n u o u s l y d i r e c t e d towards improved l u b r i c a n t s , a d d i t i v e s t o impart o r r e i n f o r c e d e s i r a b l e p r o p e r t i e s and s y n t h e t i c l u b r i c a n t s w i t h unique p r o p e r t i e s . The more r e c e n t ma j or developments i n l u b r i c a n t f o r m u l a t i o n appear t o have
6 been on c u t t i n g f l u i d s , f i r e - r e s i s t a n t carbon f l u i d s .
h y d r a u l i c f l u i d s and s y n t h e s i s e d h y d r o -
Although t h e l a t t e r may c o s t more than m i n e r a l - o i l based
p r o d u c t s , e x p e r i e n c e i n d i c a t e s t h a t t h e y may g i v e an o v e r a l l c o s t s a v i n g . Complications caused by l u b r i c a n t s l e a d t o c o n s i d e r a t i o n o f w e a r - r e s i s t a n t m a t e r i a l s w i t h good f r i c t i o n a l p r o p e r t i e s which can o p e r a t e w i t h o u t l u b r i c a t i o n . A n t i - p o l l u t i o n and c o n s e r v a t i o n i s p l a c i n g emphasis on sealed, l u b r i c a t e d - f o r l i f e machinery u s i n g s o l i d l u b r i c a n t s and s u r f a c e t r e a t m e n t s which l u b r i c a t e . Under such c o n d i t i o n s i n t e r f a c i a l c o n d i t i o n s become i m p o r t a n t .
Plastics
m a t e r i a l s a r e r e c e i v i n g increased a t t e n t i o n e s p e c i a l l y where chemical and therma 1 i n e r t n e s s a r e requ i red.
.
Pol y t e t r a f 1 u o r o e t h y 1 ene ( P .T. F. E ) has become
the standard s o l i d l u b r i c a n t i n cryogenic applications. f l o w has been c o n t r o l l e d by s u i t a b l e r e i n f o r c e m e n t .
I t s tendency t o c o l d
Newer polymers a r e b e i n g
i n c r e a s i n g l y used where h i g h thermal s t a b i l i t y i s r e q u i r e d .
Metal f i l m l u b r i c -
a n t s a r e now f i n d i n g use and p o t e n t i a l developments i n s o l i d l u b r i c a n t technol o g y may a r i s e f r o m composite s o l i d - l i q u i d l u b r i c a n t s t o use t h e s p e c i f i c p r o p e r t i e s o f each.
1.8
MATERIALS The emergence o f new design concepts i s a m a j o r i n c e n t i v e f o r t h e development
o f w e a r - r e s i s t a n t m a t e r i a l s and t h e a c q u i s i t i o n o f m a t e r i a l s data. and s t r e s s problems a s s o c i a t e d w i t h advanced t r i b o - e n g i n e e r i n g strength,
light-weight materials.
The thermal
require high-
Conventional m a t e r i a l s have been improved by
o r t h o d o x methods almost t o t h e l i m i t o f t h e i r p o t e n t i a l mechanical p r o p e r t i e s so t h a t new t y p e s o f m a t e r i a l s such as composites,
s y n t h e t i c diamond and s a p p h i r e ,
new g r a p h i t e 5 and c a r b i d e s , metal b o r i d e s and n i t r i d e s which approach t h e hardness o f n a t u r a l diamond a r e b e i n g developed.
To u t i l i z e t h e i r s p e c i f i c p r o -
p e r t i e s new d e s i g n concepts a r e r e q u i r e d as t h e s u b s t i t u t i o n o f such m a t e r i a l s i n e x i s t i n g designs can l e a d t o problems and f a i l u r e s i n s e r v i c e .
Besides
r e p l a c i n g m e t a l s , ceramics may be used as c o a t i n g s t o complement d e s i r a b l e metal characteristics with refractory properties,
i n s u l a t i n g and e r o s i o n , wear,
o x i d a t i o n and c o r r o s i o n r e s i s t a n c e . I n t h e f i e l d o f p l a i n b e a r i n g s n o major development o f s o f t metal b e a r i n g s appears l i k e l y i n t h e immediate f u t u r e as t h e p o s s i b l e a l l o y s o f a l l commercially f e a s i b l e s o f t e r metals have been f u l l y e x p l o i t e d ,
A v a i l a b l e m a t e r i a l s come
c l o s e to u t i l i s i n g f u l l y t h e p o t e n t i a l i t y o f p l a i n b e a r i n g s o f c u r r e n t designs and l u b r i c a t i o n systems.
The development o f p l a s t i c s b e a r i n g m a t e r i a l s capable
o f b e i n g manufactured t o and m a i n t a i n i n g t h e c l o s e t o l e r a n c e s o f m e t a l s c o u l d cause something o f a r e v o l u t i o n i n t h e b e a r i n g f i e l d . P l a s t i c s and t h e i r composites dominate t h e d r y b e a r i n g scene m a i n l y due t o t h e a v a i l a b i l i t y o f design and performance d a t a [48,49].
A s i g n i f i c a n t advance
i n fundamental understandinn o f t h e wear o f p l a s t i c s composites has been t h e
7 r e c o g n i t i o n o f t h e dominant r o l e o f t h e c o u n t e r f a c e metal [50]. d e p o s i t i o n techniques such as s p u t t e r i n g , chemical vapour d e p o s i t i o n (C.V.D)
ion-implantation,
Vacuum
i o n - p l a t i n g and
appear p o t e n t i a l l y a t t r a c t i v e f o r s o l i d f i l m
l u b r i c a n t s o l u t i o n s t o a wide range o f d r y b e a r i n g problems. I n the f i e l d o f r o l l i n g b e a r i n g m a t e r i a l s , methods such as r e d u c t i o n o f gas c o n t e n t [ 5 1 ] and d e l e t e r i o u s c a r b i d e s e g r e g a t i o n [ 5 2 ] a r e b e i n g developed t o improve r o l l i n g c o n t a c t f a t i g u e r e s i s t a n c e .
Developments i n h i g h speed t o o l
s t e e l b e a r i n g s have c e n t r e d around a f i n e r d i s p e r s i o n o f c a r b i d e s [ 5 3 ] and weight s a v i n g [ 5 4 ] .
The use o f h i g h e r than normal a d d i t i o n s o f a l l o y i n g
elements t o p r o v i d e m a r g i n a l l y improved p r o p e r t i e s may n o t j u s t i f y t h e increased cost.
There appears t o be a steady b u t u n s p e c t a c u l a r development o f ceramic
m a t e r i a l s such as s i l i c o n n i t r i d e [ 5 5 ] . Brakes and c l u t c h e s r e q u i r e t o d i s s i p a t e c o n t i n u a l l y g r e a t e r e n e r g i e s due t o loads and speeds g e n e r a l l y i n c r e a s i n g and improved m a t e r i a l s a r e c o n s t a n t l y demanded t o contend w i t h more arduous d u t i e s and h i g h e r temperatures.
Owing t o
t h e p o s s i b l e h e a l t h hazard t h e r e i s c o n s i d e r a b l e p r e s s u r e t o r e p l a c e asbestos, t h e most e f f e c t i v e f i l l e r m a t e r i a l f o r p h e n o l i c r e s i n s due t o i t s f i b r o u s n a t u r e and h e a t r e s i s t a n c e .
S i n t e r e d metal m a t r i c e s a r e now used f o r severe d u t y
a p p l i c a t i o n s b u t a t t e m p t s t o i n t r o d u c e o t h e r o r g a n i c and i n o r g a n i c m a t e r i a l s have n o t y e t succeeded i n d i s p l a c i n g c o n v e n t i o n a l m a t e r i a l s except i n h i g h l y specialised f i e l d s .
Concorde uses carbon composites a g a i n s t themselves and
these m a t e r i a l s w i t h cheaper f i b r e s and f i l l e r s may be a p r o m i s i n g method o f approach t o t h e replacement o f a s b e s t o s - f i l l e d p h e n o l i c r e s i n s .
1.9
SURFACE TREATMENTS The m a t e r i a l o f e n g i n e e r i n g components must have s t r u c t u r a l c h a r a c t e r i s t i c s
t o s a t i s f y t h e design requirements and s u r f a c e c h a r a c t e r i s t i c s t o contend w i t h wear, f a t i g u e and environmental e f f e c t s . promise t o these requirements.
Surface c o a t i n g s o f f e r t h e b e s t com-
Surface t r e a t m e n t s a r e a l s o a t t r a c t i v e as an
a l t e r n a t i v e t o design o r l u b r i c a n t changes t o combat wear i n s e r v i c e [56,571. Besides t h e c o n v e n t i o n a l t r e a t m e n t s r e c e n t l y developed, s u r f a c e t r e a t m e n t s i n v o l v i n g t h i n s u r f a c e f i l m s w i t h s p e c i f i c p r o p e r t i e s a r e now f i n d i n g i n c r e a s i n g use and p r o v i n g t o be advantageous as w e a r - r e s i s t a n t
coatings.
i n c l u d e p h y s i c a l and chemical vapour d e p o s i t i o n processes.
The t r e a t m e n t s
The u s e o f T i c
c o a t i n g s on s i n t e r e d c a r b i d e c u t t i n g t o o l s i s a t y p i c a l example o f r e d u c i n g t o o l wear and c u t t i n g c o s t s [58].
Low temperature CVD processes and c o n t r o l l e d
n u c l e a r thermo-chemical d e p o s i t i o n a r e b e i n g developed t o produce equiaxed g r a i n m a t e r i a l o f e x c e p t i o n a l hardness.
Ion p l a t i n g and i o n i m p l a n t a t i o n a l s o
appear t o be f i n d i n g use f o r s p e c i f i c a p p l i c a t i o n s .
8
1.10
COMPUTER-AIDED D E S I G N
I t i s o n l y by t he co mbi n at i on o f improved s c i e n t i f i c undetstanding and i t s speedy i n d u s t r i a l u t i l i s a t i o n t h a t r a p i d t e c h n o l o g i c a l progress can be achieved. I n the p a s t, a major d i f f i c u l t y has been t h e d e l a y o r l a c k o f feedback from i n d u s t r y and t h u s t h e de l ay i n t h e t i m e t ake n f r o m t h e i n c e p t i o n o f a good idea t o i t s f r u i t f u l application.
To e l i m i n a t e such delays i n c r e a s i n g use i s b e i n g
made o f t h e computer i n de si gn t o en ab l e al most instantaneous feedback.
In
the f i e l d o f p l a i n b e a r i n g s , u s i n g a s u i t a b l e programme t h e designer need o n l y tra n s m it i n f o r m a t i o n on b e a r i n g d esi g n t o be informed o f performance c h a r a c t e r istics.
I n t h i s way he can have i n s t a n t feedback and make use o f t h e l a t e s t
research r e s u l t s w i t h o u t b e i n g an e x p e r t i n t h e f i e l d s o f t r i b o l o g y , o r programming.
In effect,
computation
he has a t hand what amounts t o a u n i v e r s a l t e s t i n g
machine i n which he can p l a n h i s design,
t e s t i t s c h a r a c t e r i s t i c s , modify the
design and a g a i n measure i t s c h a r a c t e r i s t i c s , c o n t i n u i n g t h e process u n t i l s a t i s f i e d t h a t he has t h e optimum de si gn b e f o r e c o m m i t t i n g h i m s e l f t o f u l l - s c a l e t e s t , production o r service.
M a t e r i a l s s e l e c t i o n f o r optimum performance by
computer i s now approaching r a p i d l y .
1.11
MACHINERY CONDITION MONITORING
Economic p r e s sure s a r e ca usi n g t h e p r a c t i c e o f w i t h d r a w i n g equipment from s e r v i c e a t p e r i o d i c i n t e r v a l s f o r i n s p e c t i o n and maintenance t o be r e p l a c e d by f a i l u r e p r e v e n t i o n maintenance.
Thus, means have been developed t o determine
th e c o n d i t i o n o f machinery w h i l s t i n s e r v i c e and t o d e t e c t any d e t e r i o r a t i o n o f performance so t h a t remedial a c t i o n can be t a ken b e f o r e t h e breakdown p o i n t i s reached.
The m o n i t o r i n g t ech ni q ue chosen depends upon the s p e c i f i c i n f o r m a t i o n
r e q u i r e d and t h e c o s t o f a c q u i r i n g t h e i n f o r m a t i o n compared w i t h t h e savings such i n f o r m a t i o n can e f f e c t . There has been a g rad ua l acceptance o f v i b r a t i o n a n a l y s i s a l t h o u g h t h i s has proved t o be n e i t h e r t h e s i m p l e s t n o r t h e most e f f e c t i v e method t o use.
The
problem o f d a t a i n t e r p r e t a t i o n u s u a l l y c r e a t e s t h e need f o r expensive t r e n d a n a l y s i s f r o m a massive b u i l d - u p o f da t a . The h i s t o r y o f a wear process i s reco rde d i n t h e wear d e b r i s produced and magnetic p l u g s and s p e c t r o g r a p h i c o i l a n a l y s i s (SOAP) a r e now e x t e n s i v e l y used t o d e t e c t abnormal wear.
The US Defense Department spends f o r t y m i l l i o n d o l l a r s
pe r y e a r on o i l a n a l y s i s [ZO] t o p r e d i c t o n l y c e r t a i n types o f f a i l u r e i n one power system, th e a i r c r a f t gas t u r b i n e , d i r e c t r e p a i r costs.
t o save t w i c e t h i s f i g u r e i n terms o f
A l t ho ug h t h ese t ech ni q ues have proved e f f e c t i v e i n p r o -
v i d i n g wa r n in g o f changes i n a system, t he y have some disadvantages.
SOAP
p r o v i d e s a knowledge o n l y o f t h e q u a n t i t y o f metal i n t h e l u b r i c a n t b u t no i n f o r m a t i o n on t he s i z e o r shape o f t he wear p a r t i c l e s .
Some damage has u s u a l l y
9
o c c u r r e d when t h e magnetic p l u g p i c k s up d e b r i s l a r g e enough f o r o b s e r v a t i o n . Ferrography [24,59]
a convenient method f o r t h e i s o l a t i o n and a n a l y s i s o f
wear p a r t i c l e s has opened up a new dimension i n wear d e t e c t i o n and assessment i n the form o f p a r t i c l e t r i b o l o g y ,
[22].
N o n - m e t a l l i c p a r t i c l e s can a l s o be
i s o l a t e d f r o m l u b r i c a n t s so t h a t l u b r i c a n t degeneration p r o d u c t s can be i d e n t i f i e d t o assess c o n d i t i o n and performance.
Recent developments [ 6 0 ] have
enabled t h e a d o p t i o n o f f e r r o g r a p h y t o b i o - e n g i n e e r i n g f o r t h e study o f p r o s t h e s i s j o i n t s , which should a s s i s t t h e development o f improved m a t e r i a l s and design o f a r t i f i c i a l i m p l a n t s .
As a r t h r i t i c j o i n t s a r e s u b j e c t e d t o wear t h e
a n a l y s i s o f a s p i r a t e d s y n o v i a l f l u i d appears p o t e n t i a l l y a t t r a c t i v e f o r t h e s t u d y o f wear r a t e s , mechanisms and b i o l o g i c a l responses t o wear i n human j o i n t s . F e r r o g r a p h i c s y n o v i a l f l u i d a n a l y s i s should augment u n d e r s t a n d i n g o f t h e e t i o l o g y and pathogenesis o f d e g e n e r a t i v e a r t h r i t i s and p r o v i d e a method f o r t h e d i a g n o s i s , documentation,
p r o g n o s t i c a t i o n and t r e a t m e n t o f t h e disease.
f i f t e e n m i l l i o n USA c i t i z e n s a r e a f f l i c t e d w i t h o s t e o a r t h r i t i s [ 2 0 ] .
Some
The f i r s t
I n t e r n a t i o n a l Conference on Ferrography has been planned [611.
1 .12
CONCLUS I ONS
I n t h e s h o r t t i m e s i n c e t r i b o l o g y was launched as a concept on i t s own,
it
has been d e s c r i b e d as t h e w o r l d ' s f a s t e s t growing a p p l i e d science, as s t i l l i n i t s i n f a n c y [ 6 2 ] and as a means o f n a t i o n a l w e a l t h c r e a t i o n w i t h o u t commensurate c a p i t a l investment [ 6 3 ] .
So t h a t t r i b o l o g y may q u i c k l y achieve m a t u r i t y s t i l l
g r e a t e r use must be made o f e x i s t i n g knowledge.
Most i n d u s t r i a l t r i b o l o g i c a l
problems can be s o l v e d s a t i s f a c t o r i l y by a l o g i c a l s y s t e m a t i c i n v e s t i g a t i o n o f t h e problem and t h e a p p l i c a t i o n o f e x i s t i n g knowledge.
I t t h u s appears t h a t
increased e f f o r t i s r e q u i r e d t o d i s s e m i n a t e knowledge i n a r e a d i l y understood f o r m t o e f f e c t g r e a t e r energy, m a t e r i a l s and manpower savings a t a minimum c o s t . I t i s hoped t h a t t h e subsequent c h a p t e r s go some way towards doing t h i s .
F u t u r e t r e n d s may be t o experiment l e s s b u t t o measure and i n t e r p r e t more. Research may be j u s t i f i e d o n l y i f i t can p r o v i d e i n f o r m a t i o n t o a l l o w i n d u s t r y t o s o l v e i t s immediate problems o r can produce s i g n i f i c a n t advances i n technol o g i c a l progress.
The e n v i r o n m e n t a l i s t s may i n f l u e n c e t r i b o l o g i s t s by
demanding reduced n o i s e l e v e l s o f mechanisms and t h e e l i m i n a t i o n o f p o l l u t i o n and t o x i c i t y f r o m l u b r i c a n t s .
L u b r i c a n t s g e n e r a l l y do n o t wear o u t b u t become
contaminated and s o , f r o m t h e p o i n t o f view o f t h e environment and c o n s e r v a t i o n , r e c l a m a t i o n , which i s p r e s e n t l y o n l y p r a c t i s e d i f economical, w i l l tend t o become o f major importance as we approach an e r a i n which q u a l i t y o f l i f e , s a f e t y and a c l e a n environment may w e l l be t h e m o t i v a t i n g f o r c e behind t e c h n o l o g i c a l innovation.
10 REFERENCES Dowson,D. H i s t o r y o f T r i b o l o g y , 1979, Longmans, London. Suh,N.P. and Saka,N. (Ed.), Fundaments o f T r i b o l o g y , 1980, M.I.T. Press, Cambridge, Mass., U.S.A. 3 L u b r i c a t i o n ( T r i b o l o g y ) - Education and Research. A Report on t h e Present P o s i t i o n and I n d u s t r y ' s Needs, 1966, HM S t a t i o n e r y O f f i c e , London. 4 A Basic T r i b o l o g y Module, 1973, Dept. Trade and I n d u s t r y , London. 5 White House Fact Sheet - The P r e s i d e n t ' s I n d u s t r i a l I n n o v a t i o n A c t i v i t i e s , Oct. 1979, White House Press S e c r e t a r y , U.S.A. 6 I n d u s t r i a l T r i b o l o g y I n s t i t u t e - Prospectus, Sept. 1981, Rensselaer P o l y t e c h n i c , Troy, N.Y. 7 Neale,M.J. (Ed.), T r i b o l o g y Handbook, 1973, B u t t e r w o r t h s , London. 8 Synoptic J o u r n a l , I n s t . Mech. Engrs., London. 9 S t r a t e g y f o r Energy Conservation Through T r i b o l o g y , 1978, ASME, N.Y. 10 Jost,H.P. and S c h o f i e l d , J . Energy Saving Through T r i b o l o g y - The James C l a y t o n L e c t u r e , Feb. 1981. I . Mech. Engrs., London. 1 1 Tilly,G.P., 8 t h I C A S Congress, Amsterdam, 1972. 12 Lehrke,W.D. and Nonnen,F.A., 1st I n t . Conf. P r o t e c t i o n o f Pipes, Durham, 1975, Paper G2, BHRA, C r a n f i e l d . 13 Raask,E., Wear, 1968, '3, 301. 14 Richardson,R.C.D., Jone5,M.P. and Attwood,D.G., Proc. A g r i c . Eng. Symp., 1967, Div. 2, Paper 26, I n s t . A g r i c . Enors., London. 15 Z l a t i n , L . , 1st I n t . Cemented Carbide Conf., Chicago, 1971, Paper 1071-918. 16 Jost,H.P. i n H a l l i n g , J . (Ed.), P r i n c i p l e s o f T r i b o l o g y , 1975, X I I , Macmillan, N.Y. 17 T r i b o l o g i e Res. Rep., T76-35, 1976, M i n i s t r y o f Research and Technology, Z e n t r a l s t e l l e f u r L u f t und Raumfahrtdokumentation und I n f o r m a t i o n , Munich. 18 Rabinowicz,E., I n Chynoweth,A. and Walsh,Wm. (Eds). , M a t e r i a l s Technology, 1976, p. 165 (Amer. I n s t . Phys. Conf. Proc. No.32, N.Y.). 19 Braithwaite,E.R., I n d u s t r i a l L u b r i c a t i o n , 1969, 21, 241. 20 Ling,F.F., Proc. o f t h e T r i b o l o g y Workshop, 1974, N a t i o n a l Science Foundation, U.S.A. 21 Devine,M.J. (Ed.), Proc. o f a Workshop on Wear C o n t r o l t o A l l o w Product D u r a b i l i t y , 1977, Naval A i r Development Centre, Warminster, PA. 22 Scott,D., Proc. I n s t . Mech. Engrs., 1975, ,&l 623. 1. 23 Suh,N.P. Wear, 1977, 24 Scott,D., Seifert,W.W. and Westcott,V.C., S c i . Amer., 1974, 230, 88. 25 Beilby,G., Aggregation and Flow i n S o l i d s , 1921, Macmillan, London. 26 Czichos,H., T r i b o l o n y - A Systematic Approach t o t h e Science and Technology o f F r i c t i o n , L u b r i c a t i o n and Wear, 1979, E l s e v i e r , Amsterdam. 27 Ludema,K. (Ed.), Wear o f M a t e r i a l s , 1977, ASME, N.Y. 28 Ludema,K. (Ed.), Wear o f M a t e r i a l s , 1979, ASME, N.Y. 29 Ludema,K. (Ed.), Wear o f M a t e r i a l s , 1971, ASME, N.Y. 30 Georges,J.M. (Ed.), M i c r o s c o p i c Aspects o f Adhesion and L u b r i c a t i o n , 1982, E l s e v i e r , T r i b o l o g y S e r i e s 7, Amsterdam. 31 Kragelsky,I.V., Dobychin,M.N. and Kombalov,V.S., F r i c t i o n and Wear C a l c u l a t i o n Methods, 1982, Pergamon, Oxford. 32 Scott,D. (Ed.), T r e a t i s e on M a t e r i a l s Science and Technology, Vol.13, Wear, 1979, Academic Press, N.Y. 19. 33 Buckley,D.H., Wear, 1978, 34 Buckley,D.H., Surface E f f e c t s i n Adhesion, F r i c t i o n and Wear, 1981, E l s e v i e r , T r i b o l o g y S e r i e s 5, Amsterdam. 35 P r o p e r t i e s and Metrology o f Surfaces, Proc. I n s t . Mech. Engrs., 1967/68, 182, 3 K , London. 36 Thoma5,T.R. and King,M., Surface Topography i n Engineering - A S t a t e o f t h e A r t Review and B i b l i o g r a p h y , 1977, BHRA, C r a n f i e l d . 37 Thomas,T.R., (Ed.), Rough Surfaces, 1982, Longman, London. 38 F o r s y t h , l . , and Scott,D. C h a r a c t e r i s a t i o n o f Micro-machined M i r r o r Surfaces, Wear, 1982, I n Press.
1 2
32,
44,
5,
11
177,
157. Reynolds,O., P h i l . Trans. Roy. Soc., 1886, Cameron,A. Proc. I n s t . Mech. Engrs., 1979, 193, P r e p r i n t No.25, London. Proc. Roy. SOC. A., 1977, 356, 215. 41 Johnson,K.L. and Tevazwark,J.L., 42 Hancock,B.T., and Dowson,D., B a l l Bearing L u b r i c a t i o n - The E l a s t o hydrodynamics o f E l l i p t i c a l Contacts, 1981, W i l e y & Sons, N.Y. 43 Fein,R.S., Proc. I n s t . Mech. Engrs., London, 1967, 181, ( 3 J ) , 125. ( 3 J ) , 45. 44 Dowson,D., Proc. I n s t . Mech. Engrs., London, 1967, P r e p r i n t 33/77. 45 Higginson,G., Proc. I n s t . Mech. Engrs., London, 1977, 46 Dumbleton,J.H., (Ed.), The T r i b o l o g y o f N a t u r a l and A r t i f i c a l J o i n t s , 1980, E l s e v i e r , T r i b o l o g y S e r i e s , 3, Amsterdam. Mechanical P r o p e r t i e s o f B i o m a t e r i a l s , 47 Hastings,G.W., and Williams,D.F. 1980, W i l e y & Sons, N.Y. 48 A Guide t o t h e Design and S e l e c t i o n o f Dry Rubbing Bearings, 1976, E.S.D.U. Data Item, 76029. T r i b o l o g y I n t . , 1973, 6, 219. 49 Lancaster,J.K., and Lavrentev,V.V., F r i c t i o n and Wear o f Polymers, 50 Bartenev,G.M., 1981, E l s e v i e r , T r i b o l o g y S e r i e s , 6 , Amsterdam. 339. 51 Scott,D. and McCullagh,P.J., Wear, 1973, 52. Scott,D. and B l a c k w e l l ,J., Wear, 1975, 46, 273. 149. 53 Scott,D. and B l a c k w e l l , J . , Wear, 1978, 54 Scott,D., T r i b o l o g y I n t . , 1976, 2, 261. 55 Scott,D., Wear, 1977, 43, 71. Proc. 1 s t T u r o . T r i b o l o g y Congress, 1975, p.165, I . Mech. 56 Wilson,R.W., Engrs., London. 283. 57 Scott,D., Wear, 1978, Wear, 1978, 407. 58 Hintermann,H.E., Tribology Int., 59 Bowen,R., Scott,D., Seifert,W.W. and Westcott,V.C., 1976, 2, 261. Proc. I n s t . Mech. Engrs., London, 1978, 60 Scott,D. and Westcott,V.C., P r e p r i n t C42/78, 123. 61 F i r s t I n t e r n a t i o n a l Conf. on Ferrography, 1982 (Sept.), Univ. Swansea ( I n Press). 62 E u r o t r i b , ‘7 7, 1977, Bundesrepublik, Deutschland, D u s s e l d o r f . T r i b o l o g y , 1978, l J , 34. 63 Jost,H.P.,
39
40
T L
191,
3,
3,
5,
5,
12
9 D.
2.1
WEAR
SCOTT, Consultant, E d i t o r of Wear
INTRODUCTION Wear may be d e f i n e d as the undesired displacement o r removal o f surface mat-
e r i a l , although under some circumstances,
the i n i t i a l stages o f wear o r m i l d
wear which tends t o smooth surfaces, may be b e n e f i c i a l f o r the running-in o f mechanisms.
The economic i m p l i c a t i o n s o f wear cause concern i n industry, as a
reasonable l i f e i s r e q u i r e d o f mechanical equipment t o cover c a p i t a l and maintenance costs.
Whilst,
i n many instances, wear may n o t place an absolute l i m i t
on the l i f e o f an investment,
i t c e r t a i n l y causes a great deal o f expenditure
on maintenance t h a t must take place;
such maintenance i s c o s t l y i n i t s e l f , b u t
a l s o c o s t l y i n l o s t p r o d u c t i v i t y w h i l s t i t i s being c a r r i e d o u t . Although wear has f o r long been a subject o f p r a c t i c a l i n t e r e s t , fundamental knowledge o f wear i s sparse.
This i s due probably t o the i n t e r d i s c i p l i n a r y
nature o f wear making i t d i f f i c u l t t o e l u c i d a t e and the f a c t t h a t wear has been accepted as i n e v i t a b l e and unavoidable and so mechanical p a r t replacement technology has dominated wear c o n t r o l technology. Progress i n wear c o n t r o l and prevention can be made o n l y a f t e r a b e t t e r understanding o f the mechanisms by which i t occurs and o f the c o n t r o l l i n g f a c t o r s has been acquired.
2.2
THE WEAR PROCESS Wear may take many forms depending upon surface topography, c o n t a c t c o n d i t i o n s
and environment b u t , g e n e r a l l y , t h e r e are two main types, mechanical and chemical. Mechanical wear involves processes which may be associated w i t h f r i c t i o n , abrasion, erosion and f a t i g u e .
Chemical wear a r i s e s from surface a t t a c k by
r e a c t i v e compounds and the subsequent rubbing o r breaking away o f the r e a c t i o n products by mechanical a c t i o n .
The d i f f e r e n t types o f wear may occur s i n g l y ,
s e q u e n t i a l l y o r simultaneously, b u t a l l wear phenomena centre on a common chara c e r t i s t i c , an o v e r s t r e s s i n g o f the surface 111. When two surfaces a r e i n contact,
the r e a l area o f contact i s considerably
less than the apparent area o f c o n t a c t being confined t o a number o f small areas where opposing h i g h spots touch.
Pressure i n these areas w i l l be h i g h and the
surface m a t e r i a l deformed by the a p p l i e d load u n t i l the c o n t a c t area becomes s u f f i c i e n t l y l a r g e t o support the load.
According t o Bowden and Tabor 121, i n
13 the absence o f an e f f e c t i v e s e p a r a t i n g f i l m a j u n c t i o n may be formed between th e s u r fa c e s , and r e l a t i v e mo t i o n w i l l cause t h e j u n c t i o n t o be broken, r e s u l t i n g i n t h e removal o f m a t e r i a l f ro m one o r t h e o t h e r o f t h e surfaces. Ming Feng [ 3 ] c o n s i d e r s t h a t p l a s t i c f l o w o c c u r s a t c o n t a c t i n g a s p e r i t i e s so t h a t m a t i n g s u r f a c e s conform i n a p a t t e r n o f r i d g e s and grooves and g i v i n g r i s e t o s t r a i n hardening.
Mechanical i n t e r l o c k i n g p r e v e n t s s l i p a t t h e i n t e r f a c e
d u r i n g r e l a t i v e mo t i o n and shear o ccurs i n t h e s o f t e r subsurface m a t e r i a l una f f e c t e d by s t r a i n hardening.
The sheared o f f m a t e r i a l may be i n t h e form o f
d e b r i s o r i f t h e she ari n g process produces a s u f f i c i e n t r i s e i n temperature, t h e wear p r o d u c t s may become a t t a c h e d t o one o f t h e surfaces. The a d h e s iv e wear t h e o r y as d e s c r i b e d by Archard [4,5]
postulates the
f o r m a t i o n o f wear p a r t i c l e s a t c o n t a c t i n g a s p e r i t i e s which a r e hemispherical i n shape.
Rabinowicz [ 6 ] c o n s i d e r s t h a t t he r a t i o o f s u r f a c e energy t o m a t e r i a l
hardness i s an i m p o r t a n t f a c t o r i n wear and may have some e f f e c t on wear p a r t i c l e size.
Al t ho ug h wear eq ua t i o ns [4,6]
c o n s i s t e n t w i t h e xpe ri men t a l r e s u l t s ,
d e r i v e d f r o m these t h e o r i e s a r e
t he y do n o t account f o r t h e b a s i c
m e t a l l u r g y o f t h e m a t e r i a l s and a r e based on s e v e r a l a r b i t r a r y assumptions. S e i f e r t and We s tco t t [7,8]
have demonstrated t h a t r u b b i n g wear p a r t i c l e s take
t h e f o r m o f t h i n f l a k e s o f metal w i t h h i g h l y p o l i s h e d s u r f a c e s and a r e n o t hemis p h e r i c a l fragments g e n e r a l l y proposed by adhesion t h e o r y . [9,10]
Suh and o t h e r s
have proposed a d e l a m i n a t i o n t h e o r y o f wear based on the behaviour o f
d i s l o c a t i o n s a t t h e r u b b i n g su rf a ce,
su bsu rf a ce v o i d and c r a c k f o r m a t i o n and
t h e subsequent j o i n i n g o f c r a c k s by shear d e f o r m a t i o n o f t h e surface.
The de-
l a m i n a t i o n t h e o r y p r e d i c t s t h a t t h e wear p a r t i c l e shape i s t h i n f l a k e - l i k e sheets as opposed t o t h e he mi sph eri ca l shape proposed by t h e adhesion t h e o r y and t h a t t h e s u r f a c e l a y e r can undergo l a r g e p l a s t i c deformation.
Experimental
r e s u l t s showing t h e process o f wear sheet f o r m a t i o n by d e l a m i n a t i o n a r e claimed t o s u b s t a n t i a t e t h e t h e o r y [ll]. Suh has r e p o r t e d t h a t b u l k m a t e r i a l hardness i t s e l f i s n o t t h e c o n t r o l l i n g f a c t o r on wear and t h a t t h e d e l a m i n a t i o n t h e o r y s a t i s f i e s t h e thermodynamic req ui re men t s o f t h e f r i c t i o n a l and wear behaviour o f m e t a l s [ 1 2 ].
W e st co t t and o t h e r s [8,13]
suggest t h a t s u r f a c e i n t e r a c t i o n
p o l i s h e s t h e s u r f a c e s and c r e a t e s a shear mi x l a y e r of s h o r t c r y s t a l l i n e o r d e r o f a lm o s t s u p e r d u c t i l e m a t e r i a l which spreads o v e r t h e s u r f a c e t o e f f e c t the smooth n a t u r e o f r u n - i n s u r f a c e s as f i r s t proposed by B e i l b y [14].
Repeated
r u b b i n g causes t h e shear mi x l a y e r t o become f a t i g u e d and r u b b i n g wear p a r t i c l e s f l a k e o f f [ 1 3 ].
Such a s t a t e o f e q u i l i b r i u m m a i n t a i n s benign wear, b u t d i s -
r u p t i o n o f t h i s s t a t e can cause i n i t i a t i o n o f a more severe mode o f wear [13]. A r h e o l o g i c a l mechanism of p e n e t r a t i v e wear [ 1 5 ] has been proposed f o r the
formation o f p l a t e - l i k e debris.
The st ud y o f subsurface e f f e c t s d u r i n g the
s l i d i n g o f m e t a l s su pp ort s t h e assumption o f d e l a m i n a t i o n 1161.
Study o f the
14 wear behaviour o f u l t r a h i g h m o l e c u l a r we i gh t (UHMW) p o l y t h e n e r e v e a l e d a p l a s t i c a l l y f lo wed smooth s u r f a c e and sub surf ace c r a c k i n g which separated sheets o f polymer f rom t h e wear t r a c k i n t h e form o f smooth s u r f a c e d p l a t e l e t s [17].
H i r s t [18],
K r a g e l s k i i [19] and S c o t t [20] have reviewed t h e s u b j e c t o f
Bickerman 1211 has c r i t i c a l l y reviewed t h e t h e o r i e s o f adhesion and
wear.
f r i c t i o n a l phenomena i n s l i d i n g c o n t a c t and Barw ell [22] has reviewed t h e t h e o r i e s o f wear and t h e i r s i g n i f i c a n c e f o r e n g i n e e r i n g p r a c t i c e .
2.3
SCUFFING Under s l i d i n g c o n d i t i o n s ,
the c h i e f ta sk o f a l u b r i c a n t i s t o a l l o w r e l a t i v e
m o t io n between s urf ace s, w i t h low f r i c t i o n and no damage.
T h i s can be achieved
i f th e l u b r i c a n t f i l m i s t h i c k enough t o keep t h e surfaces a p a r t and hydrodynamic c o n d i t i o n s p r e v a i l .
I f however, i d e a l c o n d i t i o n s cannot be maintained,
t h e s u r fa c e s w i l l come i n t o c o n t a c t and wear o r damage i n the form o f s c u f f i n g
w i l l o c c u r , Fig.1.
The m e t a l l o g r a p h i c a l l y changed, s c u f f e d m a t e r i a l , Fig.2,
i s c o n s i d e r a b l y h ard er ( v a r y i n g f r o m 300 t o 850HV) than the o r i g i n a l carbon
Fig.2. (H= x110, V= ~ 1 1 0 0 ) Taper s e c t i o n through a s c u f f e d surface.
Fig.1. ( ~ 7 5 )S c uf f e d s t e e l surface
.
s t e e l (180HV). [23,241
Electron microscopical
i n v e s t i g a t i o n o f the s c u f f e d m a t e r i a l
r e v e a l s t h a t i t i s m a r t e n s i t e and tempered m a r t e n s i t e o r t r o o t s t i t e ,
i n d i c a t i n g t h a t t h e m a t e r i a l has been heated t o above t h e a u s t e n i t i s i n g tempe r a t u r e and r a p i d l y coo l ed .
Hydrocarbon l u b r i c a n t breakdown i n t h e c o n t a c t
15 zone [ 1 5 ] can l e a d t o i n crea se o f carbon c o n t e n t and hardness due t o d i f f u s i o n o f carbon and gases i n t o t h e heated deformed m a t e r i a l . The complex mechanism o f s c u f f i n g i s d i f f i c u l t t o e l u c i d a t e as t h e process, by c u m u la t iv e a c t i o n , d e s t r o y s evidence o f i t s i n i t i a l stages.
To study
s c u f f i n g and t o f o l l o w t h e development o f s u r f a c e f a i l u r e a cros,sed c y l i n d e r machine has been used [26] i n whi ch one c y l i n d e r i s r o t a t e d and a mating c y l i n d e r , a t r i g h t an gl e s t o i t , i s so t r a v e r s e d t h a t t h e area o f c o n t a c t moves Examination o f t h e h e l i c a l t r a c k round a
al o n g t h e s u r f a c e o f b o t h c y l i n d e r s .
t e s t e d c y l i n d e r r e v e a l s how s u r f a c e damage b u i l d s up w i t h t e s t d u r a t i o n and l o a d 1271.
The b e a r i n g t r a c k s on l u b r i c a t e d s t e e l c y l i n d e r s a r e d i s t i n g u i s h e d
by t h e in c r e a s e d o p t i c a l r e f l e c t i v i t y . i n i t i a t e d by s h o r t f i n e marks,
As t h e l o a d increases f a i l u r e i s
f o l l o w e d by i n c i p i e n t s c u f f i n g and continuous
f a i 1 ure. Study o f t a p e r s e c t i o n s has h el p ed c o n s i d e r a b l y i n t h e e l u c i d a t i o n o f t h e e f f e c t s o f rubbing a c t i o n ;
t h e i n i t i a l p o l i s h i n g seems t o be achieved by
smoothing o f t h e g r i n d i n g a s p e r i t i e s by p l a s t i c deformation as suggested by Westcott Fig.3.
[7,81 and Suh 191, Fig.3.
Subsurface metal l o g r a p h i c changes occur,
Heat produced by ru bb i ng a c t i o n appears t o temper t h e hard s t e e l .
Surface hardness may be pre serve d by r a p i d quenching from above the austeni t i s i n g t e m p e r a t ure by l u b r i c a n t o r b u l k m a t e r i a l . surface m a t e r i a l u s u a l l y contains cracks.
The w h i t e - e t c h i n g ,
hard
Thermal s o f t e n i n g may occur by con-
d u c t i o n o f h e a t i n t he subsurface are a away from l u b r i c a n t quenching a c t i o n . Local h e a t i n g , and subsequent quenching by l u b r i c a n t o r c o l d b u l k metal may be s u f f i c i e n t t o m e t a l l o g r a p h i c a l l y change and harden t h e s u r f a c e o f s o f t s t e e l , so t h a t b o t h h a r d and s o f t s t e e l when s c u f f e d ,
and t h e s c u f f e d m a t e r i a l ,
develop a s i m i l a r m e t a l l o g r a p h i c s t r u c t u r e and hardness.
The e l e c t r o n micro-
scope has r e v e a l e d t h e f i n e m e t a l l o g r a p h i c s t r u c t u r e o f rubbed m a t e r i a l , the n a t u r e o f t h e o r i g i n a l ground s u r f a c e and t he smooth r u n - i n s u r f a c e and has rev e a le d t h a t th e i n i t i a t i o n o f wear on a sub-microscopic s c a l e develops i n a s i m i l a r manner t o t h a t observed on a l a r g e r scale.
With l i g h t loads, t h e sur-
fa c e s a r e smoothed by p l a s t i c d ef orma t i o n o f t h e a s p e r i t i e s , m e t a l l u r g i c a l t r a n s f o r m a t i o n s occu r and s c u f f i n g appears t o i n i t i a t e from small scores. W i t h i n t h e scores on h a r d s t e e l , f e a t u r e s a r e v i s i b l e s u p p o r t i n g t h e Bowden and Tabor ( 2 ) mechanism o f f a i l u r e ,
Fig.4a.
P l a s t i c deformation and roughening o f
s o f t s t e e l i n d i c a t e o v e r - s t r e s s i n g o f t h e s u r f a c e m a t e r i a l which i s n o t incons i s t e n t w i t h t h e ideas o f B l o k [ l ] and Ming-Feng pre s s u r e (e.p.1
131, Fig.4b.
Extreme
a d d i t i v e s a r e used t o p r e v e n t metal t o metal c o n t a c t between
h e a v i l y loaded moving surf ace s.
D e t a i l e d e xamination o f surfaces by t h e
e l e c t r o n microscope has shown t h e b u i l d up o f p r o t e c t i v e f i l m s and p r o v i d e s exp e r im e n ta l e v ide nce f o r t h e g e n e r a l l y agreed a c t i o n o f these a d d i t i v e s ; r e a c t i o n w i t h s u r f a c e metal t o form adherent s u r f a c e f i l m s w i t h good boundary p r o p e r t i e s [27].
16
Fig.3a.
(x80)
F i n e l y qround
surface adjacent to run-in surface.
F i g . 3 ~ . ( ~ 7 5 0 0 ) Run-in s u r f a c e .
( ~ 7 5 0 0 ) F i n e l y ground
Fig.3b. surface.
Fig.3d.
(H= x 1 0 0 , V= ~ 1 1 0 0 )
Taper s e c t i o n t h r o u g h a s c u f f e d surface.
17
Fig.4b.
( ~ 7 5 0 0 ) I n i t i a t i o n of
Fig.4a.
(~7500) Initiation of
s l i d i n g wear on a ha rd s t e e l
s l i d i n g wear on a s o f t s t e e l
s u r fa c e .
surface,
2.4
ABRASIVE WEAR From an economic p o i n t o f view, a b r a s i v e wear caused by p l o u g h i n g o r gouging
o f a h a r d s u r f a c e , h ard p a r t i c l e s o r d e b r i s , a g a i n s t a r e l a t i v e l y s o f t e r mating s u r f a c e i s p r o b a b l y t he most s e r i o u s s i n g l e cause o f wear i n e n g i n e e r i n g practice.
There a r e i n d i c a t i o n s t h a t a b r a s i o n , p r i m a r i l y a crude machining
process,
i s r e l a t e d t o i n d e n t a t i o n hardness and hence t o s t a t i c y i e l d s t r e s s
[28,31].
Evidence o f e x t e n s i v e damage o r i g i n a t i n g from a small p a r t i c l e o f
d e b r i s has been r e p o r t e d [32,34],
Fig.5.
A p a r t i c l e o f hard b r i t t l e m a t e r i a l
may cause damage i n a s i n g l e pass t hro ug h t he area o f minimum f i l m t h i c k n e s s o f a b e a r in g .
However,
i n so passing,
i t may be rendered i n e f f e c t i v e due e i t h e r
t o breakdown i n t o s m a l l e r p a r t i c l e s o f dimensions s m a l l e r than t h e minimum o i l f i l m t h i c k n e s s o r by b ei n g comp l et el y embedded i n the s o f t e r o f t h e mating materials.
On t h e o t h e r hand a s o f t m a t e r i a l p a r t i c l e may work harden on
passage between r e l a t i v e l y moving su rf a ces.
I n gouging t h e s o f t e r b e a r i n g
m a t e r i a l s u r f a c e , t h e p a r t i c l e may, i f t h e b e a r i n g m a t e r i a l a l s o work hardens, be o n l y p a r t i a l l y embedded i n an e q u a l l y ha rd s u r f a c e area and become a source o f f u r t h e r damage t o t h e mat i ng su rf a ce.
18
a.
Large b e a r i n g machined
b.
by hard p a r t i c l e Fig.5.
Wire-wool
( ~ 1 5 0 ) S e c t i o n through t h e p a r t i a l l y embedded h a r d p a r t i c l e
type b e a r i n g f a i l u r e .
I f a b r a s i v e p a r t i c l e s a r e conveyed by a f l u i d stream t h e impact o f t h e
abr a s iv e p a r t i c l e s u r fa c e [35].
laden f l u i d w i l l g i v e r i s e t o e r o s i v e wear o f any i n t e r p o s e d
The e x t e n t and t ype o f wear depends upon t h e impinging angle of
t h e p a r t i c l e s and t h e d u c t i l i t y o f t h e su rf a ce.
2.5
FRETTING F r e t t i n g i s a s p e c i f i c f o rm o f wear wh i ch occurs when t h e r e i s s l i g h t
v i b r a t o r y movement between loaded su rf a ces i n c o n t a c t and which m a n i f e s t s i t s e l f by p i t t i n g o f t h e su rf a ces and t h e accu mul a t i on o f o x i d i s e d d e b r i s ,
Fig.6.
An
e l e c t r o n m ic r o s c o pi ca l st ud y of t h e i n i t i a t i o n o f f r e t t i n g [23] has p r o v i d e d s u p p o r t f o r t h e suggestion t h a t a t t h e o u t s e t i t i s no d i f f e r e n t from o t h e r forms o f wear, b u t t h a t t h e f i n e d e b r i s produced by t h e i n i t i a l damage due t o metal t o metal c o n t a c t and r e l a t i v e mo t i o n p r o v i d e s t h e s t a r t i n g p o i n t f o r a c u m u l a t i v e a b r a s i v e a c t i o n [36,38].
The d e b r i s , b e i n g l a r g e l y t h e o x i d e o f t h e
m e t a l s in v o lv e d , occupies a g r e a t e r volume t h an t h a t o f t h e metal destroyed and i n a l i m i t e d space,
t h i s can l e a d t o a p ressu re b u i l d up and seizure.
The form
and e x t e n t o f f r e t t i n g damage depends on t h e chemical n a t u r e o f t h e environment and on whether or n o t t he d e b r i s can escape or i s b u i l t up between t h e surfaces.
19
I t has been suggested t h a t t h e i n i t i a l d e b r i s has a p l a t e i k e form produced by a process of d e l a m i n a t i o n
[39].
The a c t u a l r a t e o f wear may slow down i f t h e d e b r i s ac s as a b u f f e r between t h e two s u r fa c e s .
Thus a p roce ss which i n i t i a t e s as adhesive wear may change
t o a b r a s i o n and t he n t h e wear r a t e may sl o w down due t o d e b r i s keeping t h e s u r f a c e s a p a r t . The f i n a l f a i l u r e may t he n be by f a t i g u e f r a c t u r e , c r a c k i n i t i a t i o n b e i n g e f f e c t e d by t h e s t r e s s r a i s i n g r o l e o f f r e t t i n g p i t s .
(~7500) I n i t i a t i o n
Fig.6a.
o f f r e t t i n g on h a r d s t e e l .
2.6
Fig.6b.
(x10,ODO)
Fretting
damage on a t i t a n i u m implant.
FLUID AND CAVITATION EROSION These wear mechanisms a r i s e f rom t h e impact o f f l u i d s a t h i g h v e l o c i t i e s .
F l u i d e r o s i o n damage caused by smal l drops o f l i q u i d can occur i n steam t u r b i n e s and f a s t f l y i n g a i r c r a f t t hro ug h t h e impact o f w a t e r d r o p l e t s causing p l a s t i c d e p r e s s ions i n t h e su rf a ce.
As t h e f l u i d f l o w s f r o m t h e deformed zone
i t can cause shear d e f o r m a t i o n i n p e r i p h e r a l areas and repeated deformation
causes a f a t i g u e t y p e o f damage by p i t t i n g and roughening o f t h e surface. C a v i t a t i o n e r o s i o n damage is caused by impact from t h e c o l l a p s e o f vapour or gas bubbles formed i n c o n t a c t w i t h a r a p i d l y moving o r v i b r a t i n g surface.
The
p h y s i c a l damage t o m e t a l s i s c h a r a c t e r i s e d by p i t t i n g s u g g e s t i v e o f a f a t i g u e origin.
The u l t i m a t e r e s i l i e n c e o f m a t e r i a l , measured as t h e energy t h a t can be
d i s s i p a t e d b e f o r e a p p r e c i a b l e de f o rmat i on and c r a c k i n g occurs, appears t o be an
20 i mp o r ta n t p r o p e r t y o f met al s i n c a v i t a t i o n r e s i s t a n c e .
2.7
ROLLING CONTACT FATIGUE The u s e f u l l i f e o f r o l l i n g elements i s l i m i t e d by s u r f a c e d i s i n t e g r a t i o n
p i t s o r f r a c t u r e b ei n g caused by a f a t i g u e process dependent upon t h e p r o perties o f the material,
[40,42].
t h e n a t u r e o f t h e l u b r i c a n t and t h e environment
The phenomenon i s c h a r a c t e r i s e d by t h e sudden removal o f s u r f a c e
m a t e r i a l o r f r a c t u r e due t o repeated a l t e r n a t i n g stresses.
The process has
t h r e e phases, p r e c o n d i t i o n i n g o f t he m a t e r i a l p r i o r t o c r a c k i n i t i a t i o n , c r a c k i n i t i a t i o n and c r a c k pro pa ga t i o n. R o l l i n g c o n t a c t f a t i g u e c r a c k s i n i t i a t e e i t h e r a t t h e s u r f a c e and propagate i n t o t h e m a t e r i a l , o r s t a r t below t h e s u r f a c e i n t h e area o f c a l c u l a t e d maximum H e r t z i a n s t r e s s and propagate towards t h e s u r f a c e depending upon p r e v a i l i n g circumstances.
The p r o p a g a t i o n o f s u r f a c e c r a c k s seems t o be c o n t r o l l e d by t h e
n a t u r e o f t h e l u b r i c a n t and t h e environment th e r o l l i n g d i r e c t i o n , Fig.7,
Fig.7.
[43].
The cracks, t r a n s v e r s e t o
propagate s t e a d i l y i n t o t h e m a t e r i a l a t an a c u t e
( ~ 1 5 , 0 0 0 ) R o l l ing
Fig.8.
Single f a i l u r e p t i n
contact f a t i g u e cracks i n
a mineral o i l l u b r i c a t e d
En31 b a l l b e a r i n g s t e e l .
bearing b a l l .
an g le t o t h e r o l l i n g d i r e c t i o n ,
t he n i n f l u e n c e d by t h e maximum sheai ng
s t r e s s e s , propagate p a r a l l e l t o t h e s u r f a c e t o detach s u r f a c e m a t e r i a l and form a p i t , Fig.8.
I f t h e environment i s d e l e t e r i o u s ,
hydrogen e m b r i t t l e m e n t m a t e r i a l , Fig.9,
f o r example,
i f i t leads t o
[ 4 4 , 4 5 ] t h e c r a c k s may propagate r a p i d l y , deep i n t o t h e
so t h a t f r a c t u r e ensues,
Fig.10.
21
Fig.9.
( ~ 7 5 ) Rapid crack
Fig.10.
Fractured non-
propagation i n a non-flammable
flammable f l u i d l u b r i c a t e d
f l u i d l u b r i c a t e d bearing b a l l .
bearing b a l l .
Subsurface cracks i n i t i a t e a t depths associated w i t h the region o f c a l c u l ated maximum H e r t r i a n s t r e s s and propagate p a r a l l e l t o the surface t o remove surface m a t e r i a l , Fig.11.
Crack i n i t i a t i o n may be f a c i l i t a t e d by b r i t t l e , non-
m e t a l l i c i n c l u s i o n s i n the stressed region which crack, break the m e t a l l i c c o n t i n u i t y and a c t as s t r e s s r a i s e r s . Owing t o r o l l i n g and s l i d i n g a c t i o n , mechanical and metallographic changes occur i n the stressed surface and immediate subsurface m a t e r i a l o f r o l l i n g elements [41,42,46].
The s t r u c t u r e o f conventional En 31 b a l l bearing s t e e l
c o n s i s t s o f f i n e l y dispersed carbide spheroids i n martensite, Fig.12.
The
m e t a l l o g r a p h i c a l l y changed m a t e r i a l o f the surface l a y e r i s devoid o f carbides as a r e s u l t o f h i g h c o n t a c t stresses and l o c a l h i g h temperature f l a s h e s causing s o l u t i o n o f the carbides f o l l o w e d by r a p i d quenching under pressure,
Fig.13.
Absorption o f gases from l u b r i c a n t breakdown may c o n t r i b u t e t o surface hardening and crack i n i t i a t i o n .
S t r i n g e r type carbides may form i n the subsurface area
o f c o n t a c t due t o a n n i h i l a t i o n o f the coarse carbides by p l a s t i c deformation, Fig.14.
Sections transverse t o the r o l l i n g d i r e c t i o n may reveal the presence
o f l o c a l i s e d areas o f tempered martensite; Fig.15.
cracks develop i n such areas,
There i s a t h r e s h o l d s t r e s s l e v e l above which metallographic change
22
Fig.11.
( ~ 3 7 5 ) Subsurface c r a c k
i n an En31 s t e e l b e a r i n g b a l l .
Fig.13.
(~12,000)
Deformed m e t a l -
Fig.12.
(~8000) S t r u c t u r e o f
En31 b a l l b e a r i n g s t e e l .
Fig.14.
(~15,000)
S t r i n g e r type
l o g r a p h i c a l l y changed, s p h e r i c a l
c a r b i d e s i n subsurface m e t a l l o g r a p h i c -
carbide free, surface material o f a
a l l y changed b a l l b e a r i n g s t e e l .
used b e a r i n g b a l l .
23 oc c u r s , s u g g e s t ing t h a t t h e changes may be due t o a y i e l d i n g o r p l a s t i c f l o w phenomenon r a t h e r than a tempering e f f e c t o r be i n d i c a t i v e o f o v e r l o a d o r a l o n g duration o f stressing. S i m i l a r s u b s u rf a ce changes and a s s o c i a t e d c r a c k s a r e found i n s e c t i o n s o f r o l l i n g elements p a r a l l e l t o t h e r o l l i n g d i r e c t i o n t o g e t h e r w i t h a s s o c i a t e d e l o n g a t e d w h i t e e t c h i n g a rea s o f i n crea sed hardness,
Fig.16.
The extreme hard-
ness o f t h e w h i t e e t c h i n g m a t e r i a l may be due t o a f i n e c e l l s i z e o r the almost c o l l o i d a l d i s p e r s i o n o f v e r y f i n e c a r b i d e s formed p o s s i b l y by s t r a i n induced p r e c i p i t a t i o n f o l l o w i n g s o l u t i o n o f coa rse c a r b i d e s , Fig.17.
F i g 15.
( ~ 5 7 5 ) Tempered m a r t e n s i t e i n
t h e s u b s u r f a c e a re a o f r o l l i n g c o n t a c t .
Fig.16
( ~ 1 1 0 0 ) Subsurface c r a c k s and
associated white etching m a t e r i a l .
t appears t h a t seve ral d i f f e r e n t modes o f r o l l i n g c o n t a c t f a t i g u e can cause
c r a ks t o n u c l e a t e and p rop ag at e i n de pe nd en t ly a t v a r i o u s r a t e s ;
thus the
phenomenon i s g r e a t l y i n f l u e n c e d by h i g h l y l o c a l i s e d c o n d i t i o n s .
Whilst the
ge n e r a l p r o p e r t i e s o f t h e b u l k m a t e r i a l a r e i mportant, s p e c i f i c aspects such as t h e s t e e lm a k in g process, gas c o n t e n t and c l e a n l i n e s s a r e a l s o e q u a l l y important [48,52].
The n a t u r e o f t h e l u b r i c a n t and t h e environment can have a dominant
e f f e c t on f a i l u r e . M a t e r i a l c o m bi n at i on [53] and m a t e r i a l l u b r i c a n t combination r e q u i r e c a r e f u l c o n s i d e r a t i o n t o ensure s a t i s f a c t o r y performance [54,55]. I n r o l l i n g c o n t a c t i n t h e absence o f a l u b r i c a n t , f a i l u r e occurs, n o t by the usual f a i l u r e mechanism b u t by e xcessi ve wear l i m i t i n g t h e u s e f u l l i f e due t o v i b r a t i o n and n o i s e tS6,571.
24
Fig.17.
( ~ 7 0 0 0 ) F i ne s t r u c t u r e i n
subsurface w h i t e e t c h i n g m a t e r i a l o f En31 s t e e l .
2.8
WEAR DETECTION AND ASSESSMENT One o f t h e most d i f f i c u l t problems i n e n g i n e e r i n g design i s t h e p r e d i c t i o n
and assessment o f p o s s i b l e wear.
An e q u a l l y i m p o r t a n t and d i f f i c u l t t a s k i s t h e
d e t e c t i o n o f wear d u r i n g t h e o p e r a t i o n o f machines.
Simple methods o f wear
measurement such as t h e d e t e r m i n a t i o n o f changes i n s u r f a c e topography by s t y l u s measurement and t h e d e t e r m i n a t i o n of w e i g h t loss have disadvantages. Machinery must be d i s m a n t l e d f o r t h e measurements t o be made and i n a c c u r a c i e s i n weig h t may a r i s e due t o o x i d a t i o n and a b s o r p t i o n o f l u b r i c a n t . As t h e h i s t o r y o f t h e wear process i s re corded i n t h e wear d e b r i s produced
[581 an a t t r a c t i v e method o f wear d e t e c t i o n and assessment i s contaminant a n a l y s i s o f t h e l u b r i c a n t used.
L u b r i c a n t s i n o p e r a t i n g mechanisms may be con-
v e n i e n t l y checked f o r t h e d e t e c t i o n o f wear by s p e c t r o g r a p h i c a n a l y s i s a l t h o u g h t h e method has some disadvantages [59] b e i n g i n some i n s t a n c e s r e l a t i v e l y b l i n d t o large particles.
A si mpl e d e v i c e such as a magnetic p l u g i n an o i l sump can
c o l l e c t f e r r o u s d e b r i s and i n d i c a t e wear o f a moving p a r t 1601 b u t some s e r i o u s damage may have o c c u r r e d b e f o r e d e b r i s l a r g e enough t o be d e t e c t e d has been collected. Wear p a r t i c l e s a r e unique, h a v i n g i n d i v i d u a l c h a r a c t e r i s t i c s which bear evidence o f t h e c o n d i t i o n s under which t he y were formed [8,58,61].
Careful
25 ex a m in a tio n o f t he morphology and d e t e r m i n a t i o n o f the composition o f wear p a r t i c l e s can t h u s y i e l d s p e c i f i c i n f o r m a t i o n concerning t h e surfaces from which they were produced, t h e mechanism o f t h e i r f o r m a t i o n and t h e o p e r a t i v e wear mode i n t h e system f r o m whi ch t he y were e x t r a c t e d . Ferrography [7,8,62,63]
i s a t e chn i qu e developed t o separate wear d e b r i s and
contaminant p a r t i c l e s c o n v e n i e n t l y f ro m a l u b r i c a n t f o r examination and a n a l y s i s . The duplex Fe r r o gra ph a n a l y s e r c o n s i s t s o f two p a r t i c l e separators, a standard a n a l y s e r and a d i r e c t r e a d i n g (DR) Ferrograph.
The DR Ferrograph i s a simple
i ns tr u m e n t used t o d et ermi n e t he amount and s i z e d i s t r i b u t i o n o f wear p a r t i c l e s i n a l u b r i c a n t sample f ro m which s i g n i f i c a n t numerical d a t a can be d e r i v e d [ 6 2 , 641.
When s u c c e ssi ve l u b r i c a n t samples y i e l d c o n s t a n t d e n s i t y readings i t may
be concluded t h a t t h e machine i s o p e r a t i n g n o r m a l l y and producing benign wear p a r t i c l e s a t a steady r a t e .
A r a p i d i ncre ase i n t h e q u a n t i t y o f p a r t i c l e s and
i n p a r t i c u l a r i n t h e r a t i o o f l a r g e t o small p a r t i c l e s i n d i c a t e s t h e i n i t i a t i o n o f a more severe wear process.
The use o f a simple e q u a t i o n p r o v i d e s a s i n g l e
f i g u r e f o r a c o mpa rat i ve s e v e r i t y o f wear index. u s i n g t h e b i c h r o m a t i c microscope [62,63],
F u l l Ferrographic a n a l y s i s
e l e c t r o n microscopy [62,651 and
h e a t i n g te c h n iq u es [66] may be used t o supplement t h e i n f o r m a t i o n from p a r t i c l e s p r e c i p i t a t e d a c c o r d i n g t o s i z e on a p rep are d s u b s t r a t e by the a n a l y t i c a l Ferrograph. P a r t i c l e s generated by d i f f e r e n t wear mechanisms have c h a r a c t e r i s t i c s which may be i d e n t i f i e d w i t h t h e s p e c i f i c wear mechanism [8,61,62,67]. t i c l e a t l a s has been p rep are d [68].
A wear par-
Rubbing wear p a r t i c l e s found i n t h e lub-
r i c a n t o f most machines have t h e f o r m o f p l a t e l e t s and i n d i c a t e normal permissi b l e ' w e a r , Fig.18.
C u t t i n g wear p a r t i c l e s t a k e t h e form o f m i n i a t u r e s p i r a l s ,
l oo p s and b e n t w i r e s s i m i l a r t o c u t t i n g s f r o m a machining o p e r a t i o n ,
Fig.19.
A c o n c e n t r a t i o n o f such p a r t i c l e s i s i n d i c a t i v e o f a severe a b r a s i v e wear process. The o p e r a t i v e regimes o f s l i d i n g wear can be c l a s s i f i e d by t h e p a r t i c l e s produced.
S i x regimes wh i ch ge ne rat e c h a r a c t e r i s t i c p a r t i c l e s have been
i d e n t i f i e d [13].
Free metal p a r t i c l e s a r e produced i n regimes 1,2 and 3 and
thes e regimes may be re cog ni se d by p a r t i c l e s i z e ,
r a n g i n g f r o m t h e small p a r -
t i c l e s o f regime 2 a s s o c i a t e d w i t h hydrodynamic l u b r i c a t i o n , m e t a l l i c p a r t i c l e s o f regime
3,
Fig.20,
Fig.18,to
large
which may v a r y i n s i z e up t o 2 5 0 ~ 1
when t h e shear mixed s u r f a c e l a y e r becomes u n s t a b l e and l o c a l i s e d adhesion oc cu r s . Three d i s t i n c t p a r t i c l e types, w i t h r o l l i n g bearing fatigue.
l ami n ar, s p h e r i c a l and chunks a r e a s s o c i a t e d
Laminar p a r t i c l e s a r e t h i n metal p a r t i c l e s up t o
50um i n major dimension c o n t a i n i n g h o l e s formed i n passage through the r o l l i n g contact.
Such p a r t i c l e s a r e generated t hro ug hout t h e l i f e o f t h e bearing.
T h e i r c o n c e n t r a t i o n increases w i t h t h e onset o f s p a l l i n g .
Spherical p a r t i c l e s ,
26
Fig.18.
( ~ 7 5 0 ) Rubbing wear particles and friction polymer.
( ~ 7 5 0 ) Optical micrograph Fig.19.
( ~ 4 5 0 ) Scanning electron micrograph
Cutting wear particles.
21
Fig.20.
a.
( ~ 7 5 0 )O p t i c a l
( ~ 4 0 0 ) Large m e t a l l i c wear p a r t i c l e .
micrograph Fig.21.
Fig.21,
b.
(x3,000) Scanning e l e c t r o n m ic r o g raph
Steel spherical p a r t i c l e s .
a r e generated w i t h i n a p r o p a g a t i n g f a t i g u e c r a c k and t h e i r d e t e c t i o n
g i v e s warning o f impending f a t i g u e f a i l u r e [ 6 9 ] .
X-ray energy a n a l y s i s i n t h e
scanning e l e c t r o n microscope can e s t a b l i s h t h a t t h e y a r e composed o f r o l l i n g bearing material.
F a t i g u e chunks c o n s t i t u t e t h e m a t e r i a l removed by s u r f a c e
d i s i n t e g r a t i o n and p i t f o r m a t i o n .
28 Fatigue p a r t i c l e s from a gear t o o t h although o f s i m i l a r dimensions d i f f e r from those from r o l l i n g bearings, a r e g e n e r a l l y o f i r r e g u l a r shape, f r e e from holes and have a smooth surface. f a t i g u e process progresses.
The number o f p a r t i c l e s increases as the
Larger f a t i g u e chunks o f gear m a t e r i a l a r e ind-
i c a t i v e o f surface d e t e r i o r a t i o n by p i t t i n g . S c u f f i n g o f gears causes an increase i n the number o f wear p a r t i c l e s which tend t o have a rough surface and an i r r e g u l a r shape.
As the s e v e r i t y of
s c u f f i n g increases, the l a r g e r p a r t i c l e s produced have surface s t r i a t i o n s indicative o f s l i d i n g action.
Owing t o the thermal e f f e c t s o f s c u f f i n g , p a r t i c l e s
may be p a r t i a l l y o x i d i s e d w i t h a range o f temper colours. As both a r t h r i t i c and a r t i f i c i a l j o i n t s a r e subjected t o wear, Ferrographic a n a l y s i s of synovial f l u i d i s a p o t e n t i a l l y a t t r a c t i v e method o f studying the mechanisms and b i o l o g i c a l responses t o wear i n human j o i n t s
2.9
1701.
CONCLUSIONS I n the present and foreseeable f u t u r e world economic s i t u a t i o n , m a t e r i a l and
energy conservation i s becoming i n c r e a s i n g l y important.
As wear i s a major
cause o f m a t e r i a l wastage, any reduction i n wear can e f f e c t considerable savings i n m a t e r i a l and the energy necessary f o r t h e i r production.
Thus increasing
emphasis w i l l be given t o methods o f wear c o n t r o l and prevention. The complex mechanisms o f wear, however, a r e n o t e a s i l y e l u c i d a t e d as the process by cumulative a c t i o n o b l i t e r a t e s evidence o f the important i n i t i a l stages o f damage.
Being an i n t e r d i s c i p l i n a r y subject, a m u l t i d i s c i p l i n a r y app-
roach i s r e q u i r e d f o r the i n v e s t i g a t i o n o f wear and the c o n t r o l l i n g f a c t o r s t o enable the m s t s u i t a b l e design t o be chosen embodying the best m a t e r i a l and the c o r r e c t l u b r i c a t i o n t o ensure minimum wear and s a t i s f a c t o r y s e r v i c e performance from moving mechanisms.
REFERENCES
1 Blok,H.,
Engineering, London,
1952, 173(4502)594.
and Tabor,D., "The F r i c t i o n and L u b r i c a t i o n o f Sol ds", Clarendon, Oxford. Ming-Feng,l., J.Appl.Phys., 1952, 23(9)1011-1019. Archard,J.F., J.Appl.Phys., 1953, 24(8)981-988. Archard,J.F., Research, 1952, 5(8)395-396. London. Rabinowicz,E., " F r i c t i o n and Wear o f Materials", 1966, J.Wiley Seifert,W.W. and Westcott,V.C., Wear, 1972, 21,2742. Scott,D., Seifert,W.W. and Westcott,V.C., Scient.Amer., 1974,
2 Bowden,F.P.
1950,
3 4 5 6 7 8
230(5)88-97. . .. 9
Suh,N.P.
and co-workers,
Wear,
1977, 44,l-162.
10 Suh,N.P., Wear, 1973, 25,111-124. 1 1 Jahanmir,S., Suh,N.P. and Abrahamson,E.P.,
Wear, 1974, 28, 235-249. 12 Suh,N.P. and Sridharan,P., Wear, 1975, 34,291-299. 13 Reda,A.A., Bowen,E.R. and Westcott,V.C., Wear, 1975, 34,261-273. 14 Beilby,G., "Aggregation and Flow o f Solids", Macmillan, London, 1921. Ludema,K.C. and Brainard,W.A., Wear, 1974, 30,365-375. 15 Bates,T.R.,
29
16 17 18 19 20
21 22
23
24 25
26 27 28 29 30
31 32
33
34 35 36 37 38 39 40 41 42
Kirk,J.A. and Swanson,T.D., Wear, 1975, 35, 63-67. Dumbleton,J.H. and Shen,C., Wear, 1976, 37, 279-289. Hirst,W., M e t a l l . Rev., 1965, 10, 145-172, I . M e t a l s , London. Kragelskii,I.V., " F r i c t i o n and Wear", 1955, B u t t e r w o r t h s , London. Scott,D. (Ed.), " T r e a t i s e on M a t e r i a l s Science and Technology, 13, "Wear", 1979, Academic Press, N.Y. Bickerman,J.J., Wear, 1976, 39, 1-14. B a r w e l l ,F.T., i n " T r e a t i s e on M a t e r i a l s Science and Technology, 13, "Wear", ( S c o t t , D.Ed.) 1979, 1-83, Academic Press, N.Y. Scott,D. and Scott,H.M., Proc. Conf. L u b r i c a t i o n and Wear, 1957, 609-612, I n s t . Mech. Engrs., London. Scott,D., Proc. I n s t . k c h . Engrs., 1967, 181 ( 3 L ) 39-51. Welsh,N.C., J . I n s t . M e t a l s , 1959, 88, 103-111. Barwel1,F.T. and Milne,A.A., B r . Pat. No. 732, 447, 1955. Milne,A.A., Scott,D. and Macdonald,D., Proc. Conf. L u b r i c a t i o n and Wear, 1957, 735-741, I n s t . Mech. Engrs., London. Wright,K.H.R., E n g i n e e r i n g , London, 1961, 191 (4956) 546-547. Kruschov,M.M. and Babichev, , Akad. Nauk. SSR, 1960, 66-76. NEL T r a n s l a t i o n 893, N a t i o n a l E n g i n e e r i n g L a b o r a t o r y , East K i l b r i d e , Glasgow. Kruschov,M.M., Wear, 1974, 28, 69-88. Moore,M.A., Wear, 1974, 28, 59-68. Barwel1,F.T. and Scott,D., Proc. 4 t h L u b r i c a t i o n and Wear Convention, 1966, 277-297, I n s t . Mech. Engrs., London. Hother-Lushington,S., Proc. 4 t h L u b r i c a t i o n and Wear Convention, 1966, 243-252, I n s t . Mech. Engrs., London. Dawson,P.H. and F i d l e r , F . , Proc. I n s t . Mech. Engrs., 1965/66, 180, 513-530. Engel ,P.A., "Impact Wear o f M a t e r i a l s " , 1976, E l s e v i e r , Amsterdam. Wright,K.H.R., Proc. I n s t . Mech. Engrs., 1952/53, lB(l1) 556-574. Godfrey,D. and Bailey,J.M., L u b r i c . Engng., 1954, 10, 155. Waterhouse,R.B., " F r e t t i n g Corrosion", 1972, Pergamon, Oxford. Waterhouse,R.B,, i n "Wear o f Metals", 1977, 55, ASME, N.Y. Scott,D., i n " F a t i g u e i n R o l l i n g Contact", 1963, 103-115, I n s t . Mech. Engrs., London. Scott,D., i n "Low A l l o y Steels", 1965, 203-209, I.S. I., London. Scott,D., R o l l i n g Contact F a t i g u e i n Wear, i n (Sc0tt.D. Ed.) " T r e a t i s e on M a t e r i a l s Science and Technology", 1979, 13, 321-361, Academic Press,
N.Y.
43
Scott,D.,
Proc.
I n s t . Mech. Engrs. Conf. L u b r i c a t i o n and Wear,
1957,
463-468. 44 45 46
Grunberg,L. and Scott,D., J. I n s t . P e t r o l . , 1958, 44 (419), 406-410. Grunberg,L., Scott,D. and Jamieson,D.T., P h i l . Mag. 1963, 8(93) 1553-1568. Scott,D., Loy,B. and MilIs,G.H., Proc. l n s t . Mech. Engrs., 1967,
47 48
Scott,D. Scott,D.
181 (31) 94-106. and McCullagh,P.J. Wear, 1973, 24, 235-242. and B l a c k w e l l , J . , Proc. I n s t . Mech. Engrs.,
1964, 178, (3N)
81-89. 49 50
Scott,D. Scott,D.
and B l a c k w e l l ,J., Wear, and B l a c k w e l l , J . , Proc.
1978, 46, 273-279. I n s t . Mech. Engrs.,
1968, 182(3N)
239-242. 51 52 53 54 55 56 57 58 59
Scott,D. and McCullagh,P.J., Wear, 1975, 34, 222-237. Scott,D. and McCullagh,P.J., Wear, 1973, 25, 339-344. Scott,D., i n " R o l l i n g Contact F a t i g u e and Performance o f L u b r i c a n t s " , (Tourret,R. and Wright,E.P. Eds.), 1977, 3-17, Heydon and Sons, London. Scott,D. and Blackwell,J., Proc. I n s t . Mech. Engrs., 1966, 180, (3K) 32-37. Scott,D., Proc. I n s t . Mech. Engrs., 1968, 182(3J) 116-123. Scott,D., Proc. I n s t . Mech. Engrs., 1967, 182(3A) 325-341. Scott,D. and Blackwell,J., NEL Report 278, 1967, N a t i o n a l Engineering L a b o r a t o r y , East K i 1 b r ide. Scott,D., Proc. I n s t . Mech. Engrs., 1976, 189/75, 623-633. Seifert,W.W. and Westcott,V.C., Wear, 1973, 23, 239-249.
30
60 61 62
Col l a c o t t , R . A . , "Mechanical F a u l t Diagnos s and C o n d i t i o n M o n i t o r i n g " , 1977, Chapman and H a l l , London. Scott,D., Wear, 1975, 34, 15-22. Tribology I n t . , Bowen,E.R., Scott,D., Seifert,W.W. and Ve t c o t t , V . C . ,
1976, 9 ( 3 ) 109-115. 63 64 65 66 67 68 69 70
Scott,O. and Westcott,V.C., Proc. E u r o t r i b 77, 1977, Band 1, paper 70, 1-6 Westcott,V.C., Naval Research Reviews, 1977 (March) 1-8, O f f i c e o f Naval Research, Wash i n g t o n . Scott,D. and Mills,G.H., i n "Scanning E l e c t r o n Microscopy", 1974, P a r t I V , 838-888, I.I.T., Chicago. B a r w e l l , F . T . , Bowen,E.R. and Westcott,V.C., Wear, 1977, 44, 163-171. Ruff,W. Wear, 1977, 42, 49-62. Bowen,E.R. and Westcott,V.C., "Wear P a r t i c l e A t l a s " , 1976, Foxboro/Transo n i c s I n c . , Mass, U . S . A . Scott,D. and Mills,G.H., Wear, 1973, 24, 235-242. Mears,D.C., Wear, 1978, 50, 115-126.
31
'{
SELECTION OF BEARINGS
M.J.
NEALE
Michael Neale and A s s o c i a t e s L t d .
3.1
INTRODUCTION The s e l e c t i o n o f an a p p r o p r i a t e t y p e o f b e a r i n g , f o r use i n a p a r t i c u l a r
a p p l i c a t i o n , i s a d e c i s i o n t h a t i s u s u a l l y made v e r y e a r l y i n a design process. A t t h a t stage, v e r y d e t a i l e d i n f o r m a t i o n on b e a r i n g performance i s n o t u s u a l l y necessary, and what i s r e a l l y r e q u i r e d i s broad guidance on t h e i m p o r t a n t _ L . .
a c t e r i s t i c s o f t h e v a r i o u s types.
The i n f o r m a t i o n presented i n t h i s s e c t i o n
ntended t o meet t h i s requirement.
3.2
B E A R I N G TYPES
The b a s i c f u n c t i o n o f b e a r i n g s i s t o a l l o w a l o a d t o be t r a n s m i t t e d between two s u r f a c e s which a r e i n r e l a t i v e motion.
There a r e t h r e e main t y p e s o f
b e a r i n g as shown i n F i g u r e 1, and these a r e p l a i n b e a r i n g s , r o l l i n g b e a r i n g s and f l e x u r e s .
In p l a i n bearings the load i s transmitted over a considerable
area, w h i l e i n r o l l i n g b e a r i n g s t h e area a c t u a l l y i n c o n t a c t , and t r a n s m i t t i n g t h e load,
i s very small.
The t h i r d t y p e depends on t h e use o f f l e x i b l e com-
ponents and i s o n l y s u i t a b l e f o r o s c i l l a t o r y movement. I t can be seen f r o m F i g u r e 1 t h a t t h e r e a r e f i v e b a s i c p r i n c i p l e s b e h i n d t h e o p e r a t i o n o f t h e v a r i o u s types and these a r e : ( i ) To p e r m i t t h e two s u r f a c e s t o r u b t o g e t h e r and t o a r r a n g e t h e s u r f a c e p r o p e r t i e s so t h a t s e i z u r e o r excess f r i c t i o n does n o t o c c u r and so t h a t an a c c e p t a b l e r a t e o f wear i s o b t a i n e d .
In practice t h i s i s usually
achieved by t h e c h o i c e o f m a t e r i a l s w i t h s u i t a b l e b u l k p r o p e r t i e s o r by t h e use of some f o r m o f s u r f a c e c o a t i n g , which may e i t h e r be a p p l i e d i n advance o r a l l o w e d t o form i n s i t u . ( i i ) To keep t h e s u r f a c e s separated by a f i l m o f f l u i d , so t h a t t h e r e l a t i v e movement can o c c u r w i t h i n t h e f i l m .
To do t h i s t h e f l u i d must be
maintained a t a s u f f i c i e n t pressure t o h o l d the surfaces apart against
32
L J Z h
1
I
The basic problem
To transmit load between two surfaces in relative motior!
Plain rubbing bearings
The surfaces are allowed to rub together
' J(
( (((),
, ,j ]
s-4
tn 0
.-C G
Plain fluid film bearings
Pressure In the film keeps the surfaces apart
Magnetic or electrostatic bearings
Surfaces held apart by mutual repulsion
Rolling element bearings
Surfaces separated by rolling elements of circular cross section
.-r
Rocker pods
The pivot point moves as the bearing rocks
8 a
Knife edges
The sharp edge gives the minimum pivot point movement
Rubber bearings
Movement by elastic deflection of the rubber
Crossed strip
Rotation about the crossover point by strip deflection
.-Q
L
E
tn
m L
m
Q
.--E
I
I
tn
!3 !! X
Q)
h Cables or torsion rods
Fig.1
Movement permitted by elastic deflection
Various types o f b e a rin g
the a p p l i e d load.
T h i s p r e s s u r e may be o b t a i n e d by f e e d i n g i n f r o m an
e x t e r n a l h i g h p r e s s u r e source, o r may be g e n e r a t e d w i t h i n t h e f i l m by r e l a t i v e movement o f s u i t a b l y shaped s u r f a c e s f l o o d e d w i t h a v i s c o u s fluid.
33 ( i i i ) One s u r f a c e may be a l l o w e d t o r o l l on t h e o t h e r .
T h i s , however,
produces an i n t e r a c t i o n between t h e a n g u l a r and t r a n s l a t i o n a l movements o f t h e a d j a c e n t components, which i s d e t e r m i n e d by t h e shape o f t h e r o l l i n g s u r f a c e s , and i n p r a c t i c e may n o t always a l l o w t h e r e q u i r e d degrees o f freedom.
To overcome t h i s p r o b l e m t h e r o l l i n g s u r f a c e s a r e
o f t e n p e r m i t t e d t o s l i d e a s w e l l as r o l l , such as i n g e a r t e e t h , o r t h e p r o b l e m may be overcome by i n s e r t i n g a t h i r d element between t h e s u r f a c e s o f t h e two o r i g i n a l components as i n a r o l l i n g element b e a r i n g . ( i v ) To produce a r e p u l s i v e f o r c e between t h e s u r f a c e s by magnetic o r e l e c t r o s t a t i c means. ( v ) To p o s i t i o n a f l e x i b l e member between t h e two components which can d e f l e c t t o a l l o w a r e l a t i v e o s c i l l a t i n g movement t o o c c u r between them. S u i t a b l e members can be formed by f i l l i n g most o f t h e space between t h e s u r f a c e s w i t h an adequate t h i c k n e s s o f e l a s t o m e r i c m a t e r i a l o r by u s i n g t h i n connecting ligaments o f higher strength materials.
3.3
PERFORMANCE OF V A R I O U S TYPES OF BEARING Since t h e a b i l i t y t o t r a n s m i t a l o a d w i t h r e l a t i v e movement i s t h e b a s i c
f u n c t i o n o f a b e a r i n g , a s t u d y o f t h e r e l a t i o n s h i p between t h e a l l o w a b l e l o a d and speed f o r b e a r i n g s o f v a r i o u s s i z e s and t y p e s s h o u l d p r o v i d e a c o n v e n i e n t s t a n d a r d f o r t h e comparison o f t h e i r performance. For t h e d e t e r m i n a t i o n o f l o a d and speed c h a r a c t e r i s t i c s o f t h e v a r i o u s b e a r i n g t y p e s , t h e f i r s t i n f o r m a t i o n t h a t r e q u i r e s t o be e s t a b l i s h e d i s t h e , a p p r o x i m a t e shape o f t h e r e l a t i o n s h i p , so t h a t a g e n e r a l comparison can be made between t h e v a r i o u s t y p e s .
The b a s i c forms o f t h e s e r e l a t i o n s h i p s can be
d e r i v e d f r o m t h e p h y s i c a l p r i n c i p l e s w h i c h govern t h e o p e r a t i o n o f t h e v a r i o u s b e aring types.
T h i s i s d i s c u s s e d below and t h e r e s u l t i n g performance c u r v e s
a p p l i e d t o j o u r n a l b e a r i n g s compared i n F i g . 2 .
3.3.1
Rubbing B e a r i n g s
I n a b e a r i n g w h i c h o p e r a t e s by p e r m i t t i n g t h e two s u r f a c e s t o r u b t o g e t h e r , t h e p h y s i c a l l i m i t a t i o n s on performance a r e t h e r i s k o f o v e r h e a t i n g and s e i z u r e and t h e p o s s i b i l i t y o f e x c e s s i v e wear. The g e n e r a t i o n o f h e a t a t t h e r u b b i n g s u r f a c e s a r i s e s f r o m t h e movement a g a i n s t t h e f r i c t i o n a l r e s i s t a n c e o f t h e c o n t a c t , and t h i s h e a t has t o be cond u c t e d away a l o n g h e a t - f l o w p a t h s , o f which t h e a r e a w i l l be a p p r o x i m a t e l y p r o p o r t i o n a l t o t h e p r o j e c t e d b e a r i n g a r e a A. The r i s k o f o v e r h e a t i n g a t a s l i d i n g speed V i s t h e r e f o r e a p p r o x i m a t e l y proportional t o
'r
-o r
'PV
34
Pressure limit
PV limit
RPM
Fatigue limit
N
RPM
(al Rubbing bearings
/-Available
N
(bl Rolling bearings
pressure limits Fall due ta effect of
3
Fall d k t o effect
of speed on vixosity
V
8
_1
Film thickness limit RPM
RPM
N
fcl Externally pressurized film
I
N
( d l Pressure fed film
r
Pressure limit
i0)i
Film thickness limit
(rPV limit
8 limit RPM
N
(el Self- contained film
FIGURE 2
RPM
N
( f l Porous metal
Performance of various types of journal bearing flog W plotted against log N )
35 When two s u r f a c e s a r e i n r u b b i n g c o n t a c t , t h e volume o f m a t e r i a l worn f r o m the rubbing surfaces a f t e r s l i d i n g a distance x w i t h a load W i s approximately p r o p o r t i o n a l t o Wx. The wear volume i s , however, o f l i t t l e s i g n i f i c a n c e i n t h e performance o f a b e a r i n g , and t h e depth wear r a t e i s much more r e l e v a n t as a design f a c t o r , s i n c e t h i s i s a measure o f t h e r a t e a t which slackness i n t h e assembly i s l i k e l y t o be produced. The d e p t h wear r a t e w i l l be a p p r o x i m a t e l y p r o p o r t i o n a l t o wx
o r PV At Thus b o t h t h e p h y s i c a l l i m i t a t i o n s t o t h e performance o f r u b b i n g b e a r i n g s i n d i c a t e t h a t ' t h e s e v e r i t y o f t h e b e a r i n g o p e r a t i o n i s r e l a t e d t o PV ( b e a r i n g p r e s s u r e x s l i d i n g speed).
Fig.2a
a rubbing type o f journal bearing.
shows t h e shape o f t h e load-speed c u r v e f o r T h i s i s a composite c h a r a c t e r i s t i c i n c l u d i n g
a PV l i m i t and a l s o a maximum a l l o w a b l e p r e s s u r e l i m i t a s s o c i a t e d w i t h t h e r i s k o f f a t i g u e o r e x t r u s i o n o f t h e b e a r i n g m a t e r i a l , which i s f r e q u e n t l y nonmetal 1 i c .
3.3.2
R o l l e r Bearings
Owing t o t h e c o n c e n t r a t e d c o n t a c t s t r e s s e s i n t h i s t y p e o f b e a r i n g t h e u l t i m a t e l i m i t on performance a r i s e s from t h e p o s s i b l e f a t i g u e o f t h e r o l '"9 elements o r races, a l t h o u g h i n p r a c t i c e r o l l i n g b e a r i n g s o f t e n f a i l as we 1 from l a c k o f p r o p e r l u b r i c a t i o n , d i r t c o n t a m i n a t i o n and u n s u i t a b l e l o a d i n g .
f
f a t i g u e i s taken as t h e performance l i m i t i t i s reasonable t o assume t h a t the l i f e o f a r o l l i n g c o n t a c t b e a r i n g measured i n r e v o l u t i o n s i s p r o p o r t i o n a l t o (l/W)3.
Consequently f o r a g i v e n l i f e W
shown i n Fig.2b.
a
N-1'3
and t h i s c h a r a c t e r i s t i c i s
As b e f o r e , t h e a c t u a l maximum l o a d i s a l s o l i m i t e d by t h e cap-
a c i t y o f t h e b e a r i n g t o c a r r y s t a t i c l o a d which i n t h i s case i s l i m i t e d by t h e r e s i s t a n c e o f t h e races t o b r i n e l l i n g by t h e r o l l i n g elements.
3.3.3
F l u i d F i l m Bearings
I f these a r e o f t h e e x t e r n a l l y p r e s s u r i z e d o r h y d r o s t a t i c t y p e , t h e l o a d c a p a c i t y i s p r i m a r i l y dependent on t h e a v a i l a b l e supply p r e s s u r e , and r u n n i n g speed has v e r y l i t t l e e f f e c t , a l t h o u g h w i t h l i q u i d l u b r i c a t i o n a s l i g h t f a l l i n l o a d c a p a c i t y may o c c u r a t h i g h e r speeds owing t o t h e r e d u c t i o n i n l u b r i c a n t v i s c o s i t y caused by h i g h e r temperatures.
The r e s u l t i n g load-speed c h a r a c t e r i s t i c
i s t h e r e f o r e as shown i n Fig.2c. With t h e hydrodynamic t y p e o f f l u i d f i l m b e a r i n g , t h e l o a d c a p a c i t y increases w i t h speed p r o v i d e d t h a t t h e f i l m i s k e p t adequately s u p p l i e d w i t h l u b r i c a n t , a l t h o u g h t h e r e i s n o r m a l l y a tendency w i t h l i q u i d l u b r i c a n t s f o r t h e l o a d capa c i t y t o f a l l away a t t h e h i g h e s t speeds owing t o t h e h e a t i n g o f t h e l u b r i c a n t .
36 T h i s g i v e s a c h a r a c t e r i s t i c o f t h e g en era l shape shown i n Fig.2d. I n o r d e r t o keep t h e f i l m f u l l y s u p p l i e d w i t h l u b r i c a n t ,
t h i s has t o be f e d
under p r e s s u r e , which i n t u r n r e q u i r e s some f o r m o f l u b r i c a n t supply system. I t i s n o t always co nve ni e nt o r economical t o h a v e t o i n c l u d e such a supply system and c o n s eq ue nt l y many hydrodynamic f l u i d f i l m b e a r i n g s a r e used w i t h self-contained l u b r i c a n t supplies.
I n the l a r g e r sizes, the o i l i s g e n e r a l l y
c o n t a in e d i n a sump below t h e s h a f t and l i f t e d by a r i n g o r d i s c , w h i l e i n t h e s m a l l e r s i z e s i t may be f e d f r o m an o i l - s o a k e d pad o r r e t a i n e d i n t h e b e a r i n g s t r u c t u r e by making i t porous.
These methods o f o i l f e e d i n g have a lower
e f f i c i e n c y than a p ressu re f e d arrangement and r e s u l t i n t h e b e a r i n g o p e r a t i n g w i t h a l o a d - c a r r y i n g f i l m o f reduced c i r c u m f e r e n t i a l l e n g t h and t h e r e f o r e o f reduced l o a d c a p a c i t y .
I t i s a l s o u s u a l l y found w i t h these systems t h a t t h e
volume o f o i l d e l i v e r e d p e r r e v o l u t i o n of t h e s h a f t decreases w i t h speed and consequently t h e l o a d - c a r r y i n g c a p a c i t y a l s o decreases w i t h speed. I n a d d i t i o n t o t h i s e f f e c t , t h e absence o f a l u b r i c a t i o n system means t h a t a l l t h e h e a t g e ne rat ed has t o be d i s s i p a t e d d i r e c t l y t o t h e surroundings, and w i t h a maximum a l l o w a b l e t e mpe rat ure f o r a l ong l i f e o f m i n e r a l o i l l u b r i c a n t s , p l a c e s a l i m i t on t h e maximum a l l o w a b l e speed. T,hese v a r i o u s e f f e c t s r e s u l t i n load-speed c h a r a c t e r i s t i c s f o r b e a r i n g s o f t h i s t y p e which a r e o f t h e ge ne ral shape shown i n Figs.2e and 2 f .
3.3.4
F l e x i b l e Members
B e a r in g s o f t h i s t ype , wh i ch use e i t h e r el astomers i n shear o r h i g h t e n s i l e l i ga m e n t s i n bending t o a l l o w an o s c i l l a t i n g motion t o occur, a r e i n many cases p h y s i c a l l y l i m i t e d i n performance by t h e f a t i g u e s t r e n g t h o f t h e m a t e r i a l o r i t s bonding on t o th e a d j a c e n t components.
The s t r e s s e s a r i s e b o t h from t h e
d e f l e c t i o n and f r o m t h e a p p l i e d l o ad and a r e u s u a l l y a d d i t i v e a t some c r i t i c a l r e g i o n o f t h e assembly, w i t h t h e r e s u l t t h a t t h e a l l o w a b l e loads tend t o decrease w i t h t h e p e r m i t t e d a n g l e o f movement.
Blocks o f e l a s t o m e r i c m a t e r i a l used
i n compression do n o t have t h i s a d d i t i v e s t r e s s c o n d i t i o n , b u t i n t h i s case t h e a l l o w a b l e d e f l e c t i o n i s l i m i t e d by t h e b l o c k t h i c k n e s s , which i n t u r n l i m i t s t h e maximum l o a d i f t h e assembly i s t o be s t a b l e a g a i n s t excessive b u l g i n g o r buckling.
T h i s can be overcome t o some e x t e n t by t h e use o f s t i f f e n i n g p l a t e s
i n t h e e la s t o m e r a rran ge d t o be p a r a l l e l t o t h e r e q u i r e d d i r e c t i o n o f motion, b u t even when these a r e i n c o r p o r a t e d t h e maximum d e f l e c t i o n i s s t i l l l i m i t e d by t h e amount t h a t t h e l o a d can be p e r m i t t e d t o be o f f s e t . I t can t h e r e f o r e be g e n e r a l l y assumed t h a t t h e l o a d - d e f l e c t i o n c h a r a c t e r i s t i c
o f f l e x i b l e b e a r in gs w i l l g e n e r a l l y show a decrease i n a l l o w a b l e l o a d as t h e d e f l e c t i o n i s increased.
31
3.4
SELECTION OF A SUITABLE BEARING The c h a r a c t e r i s t i c r e l a t i o n s h i p s between l o a d and'movement f o r t h e v a r i o u s
typ e s o f b e a r i n g s can be compared w i t h t h e b e a r i n g performance r e q u i r e d i n v a r i o u s a p p l i c a t i o n s and used as a g ui d e t o t h e s e l e c t i o n o f a s u i t a b l e t y p e o f bea r in g . Any p o s s i b l e a p p l i c a t i o n f o r a b e a r i n g i n a machine o r s t r u c t u r e w i l l have some f o r m o f c h a r a c t e r i s t i c r e l a t i o n s h i p between t h e t y p e o f l o a d t o be c a r r i e d and t h e movement t o be a l l owed .
I n p r a c t i c e an important f e a t u r e i s whether
t h e l o a d and movement a r e n o m i n a l l y steady o r whether they v a r y i n some c y c l i c manner.
T h i s can g i v e r i s e t o f o u r p o s s i b l e combinations o f l o a d and movement
as i n d i c a t e d w i t h examples i n Table 3.1. P r o b a b ly t h e g r e a t e s t number o f b e a r i n g a p p l i c a t i o n s a r e o f t h e u n i d i r e c t i o n a l l o a d and c o n t i n u o u s movement t y p e and i n t h i s c a t e g o r y t h e way i n which t h e b e a r i n g l o a d v a r i e s w i t h r o t a t i o n a l speed, d u r i n g t h e o p e r a t i o n o f t h e machine, i s an i m p o r t a n t f a c t o r i n b e a r i n g s e l e c t i o n and design. u n i d i r e c t i o n a l load s have been su b-d i vi de d
For t h i s reason t h e
i n Table 3.1 t o draw a t t e n t i o n t o
t h i s situation. Ta b le 3.1
Examples o f v a r i o u s t ype s o f l o a d and movement p a t t e r n s
Type o f lo a d Unidirectional Cons t a n t
Type o f movement
Examples
Continuous
T urbine j o u r n a l b e a r i n g s
Rising w i t h speed
Continuous
Marine gearbox p i n i o n b e a r i n g s
Falling with speed
Continuous
H y d r a u l i c motor b e a r i n g s
Un i d ir e c t iona 1
Osc i 1 l a t o r y
B r i d g e support b e a r i n g s G r i n d i n g machine t a b l e s
M u l t i d i r e c t i o n a l Continuous
Pi st on-engine c r a n k s h a f t b e a r i n g s
Multidirectional Oscillatory
Linkage b e a r i n g s
3.4.1
A p p l i c a t i o n s w i t h U n i d i r e c t i o n a l Load and Continuous Movement
T h i s i s t h e f i r s t c a t e g o r y o f b e a r i n g a p p l i c a t i o n s l i s t e d i n Table 3.1 and t h e v a r i o u s t y p e s o f b e a r i n g which c o u l d be used a r e t h e rubbing, r o l l i n g element and f l u i d - f i l m types.
F i g . 2 shows t h a t these v a r i o u s types o f b e a r i n g
have q u i t e d i f f e r e n t load-speed c h a r a c t e r i s t i c s . I n f a c t , examples a r e g i v e n i n Table
3.1 o f v a r i o u s a p p l i c a t i o n s w hich a l s o
have s e v e r a l forms o f r e l a t i o n s h i p between l o a d and speed, and i d e a l l y a b e a r i n g s h o u ld be s e l e c t e d w i t h a matching c h a r a c t e r i s t i c .
T h i s k i n d o f app-
roach t o t h e problem i n d i c a t e s t h a t t u r b i n e j o u r n a l b e a r i n g s should i d e a l l y be o f
38 the e x t e r n a l l y p r e s s u r i z e d o r h y d r o s t a t i c t ype, w h i l e an a p p l i c a t i o n such as marine gearbox p i n i o n b e a r i n g i s p a r t i c u l a r l y w e l l s u i t e d t o t h e simple type o f hydrodynamic b e a ri ng . practice,
W h i l e t h i s l a t t e r example i s i n l i n e w i t h c u r r e n t
t u r b i n e s a r e n o t a t p r e s e n t u s i n g t he h y d r o s t a t i c type o f b e a r i n g
except i n those used t o d r i v e high-speed d e n t a l d r i l l s .
T h i s may be an example
o f a design s i t u a t i o n where a p a r t i c u l a r t yp e o f machine i s developed on the b a s i s o f a workable b u t n o t i d e a l f orm o f b e a r i n g design and i t then r e q u i r e s c o n s i d e r a b l e commercial courage t o make t h e necessary change on a v i t a l component i n a machine o f such h i g h c a p i t a l val u e.
From t h e economic p o i n t o f
view, however, h y d r o s t a t i c b e a r i n g s a r e p a r t i c u l a r l y a p p l i c a b l e i n s i t u a t i o n s where a source of h i g h p r e s s u r e f l u i d i s a l r e a d y a v a i l a b l e , and t h i s i s i n f a c t t h e case w i t h a steam t u r b i n e .
Problems o f e r o s i o n may occur i f t h e steam i s
al lo we d t o condense i n t h e b e a r i n g c l e a r a n c e b u t t h i s problem should n o t be
in supe r a b 1 e
.
H y d r a u l i c motors were suggested as an example o f an a p p l i c a t i o n i n w hich t h e l oa d decreased w i t h i n c r e a s i n g speed, a l t h o u g h t h i s c h a r a c t e r i s t i c tends t o be common t o any machine d r i v e n by a source o f a p p r o x i m a t e l y c o n s t a n t power.
For
such an a p p l i c a t i o n , a r o l l i n g c o n t a c t b e a r i n g would appear t o be i d e a l i n t h a t i t has a matching c h a r a c t e r i s t i c ,
as w e l l as a low s t a r t i n g f r i c t i o n which i s
u s u a l l y a l s o r e q u i r e d i n t h i s p a r t i c u l a r t yp e o f a p p l i c a t i o n .
A rubbing type o f
b e a r i n g would also appear t o be p o s s i b l e , b u t i n f a c t t h e loads and speeds a l l o w a b l e w i t h t h i s t y p e o f b e a r i n g a r e c o n s i d e r a b l y below t h e corresponding v a lu e s f o r t h e r o l l i n g - c o n t a c t
t ype .
A lt h o u g h t h i s t ech ni q ue o f matching t h e f o r m o f t h e load-speed r e l a t i o n s h i p o f t h e a p p l i c a t i o n w i t h t h a t o f v a r i o u s b e a r i n g s i s a u s e f u l guide i n b e a r i n g selection,
i t i s s t i l l necessary t o c o n s i d e r t h e a c t u a l values o f l o a d and
speed which can be c a r r i e d by d i f f e r e n t s i z e s o f b e a r i n g s o f t h e v a r i o u s types, i n o r d e r t o be c e r t a i n t h a t t h ey a r e s u i t a b l e .
A c o n v e n ie n t way o f d oi n g t h i s i s t o p l o t t h e performance o f t h e v a r i o u s ty p e s o f b e a r i n g on one diagram so t h a t comparisons can be made, and i f t h i s i s done on l o g a r i t h m i c axes, t h e whole span o f e n g i n e e r i n g loads and speeds can be covered. T h i s t e c h n iq u e can be used t o show t h e performance o f s t e a d i l y loaded j o u r n a l b e a r i n g s w i t h c o n t i n u o u s l y r o t a t i n g s h a f t s , and Fig.3 shows t h e t y p e o f diagram which r e s u l t s . Fig.3
i s o n l y i nt en de d t o g i v e b roa d guidance b u t i t does show t h e general
trends q u i t e c l e a r l y .
I t indicates,
f o r example, t h a t f l u i d - f i l m p l a i n b e a r i n g s
a r e the b e s t ty p e t o use a t h i g h speeds and t h a t t h i s i s p a r t i c u l a r l y t r u e i n th e case o f a l l s h a f t s g r e a t e r t h an about 100 mm diameter.
This f i g u r e a l s o
i n d i c a t e s t h a t t he r u b b i n g t ype o f b e a r i n g i s o n l y r e a l l y s u i t a b l e f o r low
0.01
0.1
1
10
100
1000
10000
100000
Shaft speed ,n ( rev/s)
FIGURE 3
Indication of the performance of common types of journal bearings for shafts of various diameters
40 r o t a t i o n a l speeds, and a t a n y t h i n g more than one o r two hundred r e v o l u t i o n s p e r minute a r o l l ing bearing
3.4.2
s usually a better solution.
A p p l i c a t i o n s w i t h O s c i l l a t i n g Movement
The r u b b i n g t y p e o f b e a r i n g i s , however, p a r t i c u l a r l y good f o r a p p l i c a t i o n s w i t h o s c i l l a t i n g movement, s i n c e i t does n o t need t o b u i l d up an o i l f i l m each time i t s t a r t s t o move, and w i t h small a n g l e s o f o s c i l l a t i o n i t cannot b r i n e l l i n t h e same way as a r o l l i n g b e a r i n g can do under these circumstances. I t i s t h e r e f o r e u s e f u l t o c o n s i d e r t h e p r o b a b l e performance o f t h e v a r i o u s
types o f r u b b i n g b e a r i n g under c o n d i t i o n s o f o s c i l l a t i n g movement.
As i n t h e
case o f c o n t i n u o u s r o t a t i o n , t h e performance w i l l be l i m i t e d by t h e maximum a l l o w a b l e l o a d p r e s s u r e and by t h e wear which w i l l occur as a r e s u l t o f t h e rubbing movement.
A convenient method o f showing a comparison between t h e
v a r i o u s types i s t o p l o t t h e i r a l l o w a b l e b e a r i n g p r e s s u r e and s l i d i n g speed and t h i s been done i n Fig.4.
The values g i v e n h e r e a r e o n l y approximate b u t should
h e l p t o g i v e an i n d i c a t i o n o f t h e v a r i o u s types o f m a t e r i a l which can be used i n any p a r t i c u l a r a p p l i c a t i o n . The f l e x i b l e member type o f b e a r i n g i s a l s o s u i t a b l e f o r a p p l i c a t i o n w i t h o s c i l l a t i n g motion, and Fig.5 g i v e s an i n d i c a t i o n o f t h e a l l o w a b l e maximum l o a d i n g on f l e x i b l e b e a r i n g s which can be f i t t e d w i t h i n a g i v e n space.
These
b e a r i n g s a r e commonly used i n v a r i o u s t y p e s o f l i n k a g e and t h e c r o s s - s e c t i o n a l area a v a i l a b l e a t t h e end o f a l e v e r which r e q u i r e s t o be connected t o a n o t h e r component i s g e n e r a l l y t h e l i m i t i n g f a c t o r on s i z e r a t h e r than t h e diameter of any c o n n e c t i n g p i n which c o u l d be regarded as t h e e q u i v a l e n t o f t h e s h a f t i n t h e a p p l i c a t i o n s examined p r e v i o u s l y .
For t h i s reason t h e l o a d i n F i g . 5 i s
p l o t t e d i n terms o f t h e a l l o w a b l e b e a r i n g p r e s s u r e on t h e o u t s i d e diameter o f rubber bushes and e q u i v a l e n t shaped crossed f l e x u r e p i v o t s .
T h i s diagram i n -
d i c a t e s t h a t f l e x i b l e member b e a r i n g s a r e o n l y usable up t o an a b s o l u t e maximum e q u i v a l e n t p r e s s u r e o f 14 MN/m*
2000 I b / i n 2 w i t h v e r y low a n g u l a r movements,
and w i t h much lower loads up t o a maximum p o s s i b l e a n g u l a r movement o f 30 degrees. Outside these c o n d i t i o n s , pin-and-bush
i t i s necessary t o r e v e r t t o t h e more c o n v e n t i o n a l
t y p e o f b e a r i n g design, f o r which t h e m a t e r i a l can then be s e l e c t e d
as i n d i c a t e d by Fig.4.
I t may appear from Fig.5 t h a t rubber bushes a r e always l i k e l y t o be s u p e r i o r t o f l e x u r a l ligament b e a r i n g s .
While t h i s i s t r u e i n terms o f compressive l o a d
and a l l o w a b l e d e f l e c t i o n , i t must be remembered t h a t f l e x u r a l l i g a m e n t s can be designed t o have a much lower s t i f f n e s s f o r a g i v e n l o a d c a p a c i t y compared t o a rubber bush, and they w i l l a l s o accept a much w i d e r range o f environmental conditions.
The d a t a presented f o r f l e x u r a l l i g a m e n t s a r e based on crossed
f l e x u r e p i v o t s and i t i s i m p o r t a n t t o remember t h a t t h e design can be s i m p l i f i e d t o a much s i m p l e r s i n g l e l i g a m e n t , u s u a l l y i n t e n s i o n ,
i f the r o t a t i o n a l centre
41
I
I
Hard steel on ,hard steel
Q, c
E!
.-
X
0.01
0 01
a
0.1
1
10
100
Approximate maximum rubbing speed (m/sl
Fig.4
Indication of the performance of dry rubbing bearing with oscillating motion
42
.+ .C 3 . I -
0
al
0 m
+
3 0
C 0
Maximum deflection each way (degrees)
Fig.5
I n d i c a t i o n o f t h e performance o f f l e x i b l e member b e a r i n g s w i t h o s c i l l a t i n g motion
o f m o t i o n does n o t have t o be k e p t under c l o s e c o n t r o l .
3.4.3
A p p l i c a t i o n s w i t h M u l t i d i r e c t i o n a l Load and Continuous Movement
I n a p p l i c a t i o n s o f t h i s k i n d , o f which t y p i c a l examples a r e t h e c r a n k s h a f t b e a r i n g s o f p i s t o n engines and r e c i p r o c a t i n g compressors, t h e b e a r i n g p r e s s u r e s used i n p r a c t i c e a r e c o n s i d e r a b l y g r e a t e r than those a p p l i e d t o u n i d i r e c t i o n a l l y loaded b e a r i n g s .
T h i s i s p o s s i b l e because
( i ) t h e l o a d frequency changes i n d i r e c t i o n as w e l l as i n magnitude, and t h e s h a f t does n o t have t i m e t o squeeze t h e o i l f r o m t h e f i l m t o a s u f f i c i e n t e x t e n t t o make metal-to-metal
c o n t a c t b e f o r e t h e l o a d r e v e r s e s and l i f t s
t h e s h a f t away again. ( i i ) s i n c e v a r i o u s p o s i t i o n s o f t h e b e a r i n g and s h a f t s u r f a c e c a r r y t h e minimum f i l m r e g i o n i n t u r n ,
t h e l o c a l thermal c o n d i t i o n s a r e n o t as
severe as i n a s t e a d i l y loaded b e a r i n g where one r e g i o n o f t h e b e a r i n ?
43 metal i s c o n t i n u o u s l y s u b j e c t e d t o l o c a l h e a t i n p u t .
As a r e s u l t lower
v a l u e s o f f i l m t h i c k n e s s may be p e r m i t t e d f o r a g i v e n r e v o l u t i o n speed and s h a f t s i z e , p r o v i d e d t h a t s h a f t s u r f a c e f i n i s h i s smooth enough. The methods o f c a l c u l a t i n g t h e performance o f d y n a m i c a l l y loaded b e a r i n g s a r e e s s e n t i a l l y r a t h e r complex and t h e most comprehensive method o f assessing t h e i r p r o b a b l e performance i s t o c a l c u l a t e t h e p r o b a b l e l o c u s o f t h e s h a f t w i t h a computer
.
From t h e p o i n t of view o f v e r y g e n e r a l design guidance, however,
i t can
p r o b a b l y be a p p r e c i a t e d t h a t s i n c e much g r e a t e r b e a r i n g p r e s s u r e s a r e a l l o w a b l e , t h e s t r e n g t h o f t h e b e a r i n g m e t a l s becomes an i m p o r t a n t f a c t o r i n b e a r i n g s e l e c t i o n , t o g e t h e r w i t h t h e method o f f e e d i n g o i l t o t h e b e a r i n g s i n o r d e r t o ensure t h a t t h e b e a r i n g c l e a r a n c e s a r e k e p t as f u l l o f o i l as p o s s i b l e i n readiness f o r c a r r y i n g t h e dynamic loads. I t i s d i f f i c u l t t o quote p r e c i s e data f o r t h e a l l o w a b l e loads on v a r i o u s b e a r i n g m a t e r i a l s s i n c e f a c t o r s such as s h a f t d e f l e c t i o n , e f f i c i e n c y o f l u b r i c a t i o n and t h e t y p e o f l o a d i n g p a t t e r n have a c o n s i d e r a b l e e f f e c t .
Table 3.2
however, g i v e s some broad guidance on t h e t y p e o f m a t e r i a l s t h a t a r e l i k e l y t o be s u i t a b l e . Table 3.2
P o s s i b l e m a t e r i a l s f o r p l a i n b e a r i n g s i n r e c i p r o c a t i n g machinery
Maximum b e a r i n g p r e s s u r e lb/in2 MN/m2
Possible bearing material
Below 2500
Whitemetal
17
2500- 4000
17
-
28
Copper leads
2500- 5000
17
-
34
Aluminium t i n s
4000- 6000
28
-
41
Low t i n l e a d bronzes
6000-10000
41
High t i n l e a d bronzes
70
-
70
10000-15000
100
Phosphor bronzes
The s t r o n g e r m a t e r i a l s a r e h a r d e r t h a n t h e weaker ones and i t i s t h e r e f o r e d e s i r a b l e t o p i c k a m a t e r i a l which i s o n l y j u s t adequately s t r o n g enough i n o r d e r t o have t h e maximum p o s s i b l e c o n f o r m a b i l i t y w i t h t h e s h a f t and embedda b i l i t y for d i r t . h a r d shaft,
With h a r d e r b e a r i n g m a t e r i a l s i t i s a l s o necessary t o u s e a
p a r t l y t o m a i n t a i n an adequate hardness d i f f e r e n t i a l w i t h t h e
b e a r i n g m a t e r i a l and p a r t l y because t h e h i g h e r b e a r i n g pressures which have c a l l e d f o r t h e h a r d e r m a t e r i a l a r e a l s o l i k e l y t o r e s u l t i n lower f i l m t h i c k nesses and t h e r e f o r e g r e a t e r d i r t s e n s i t i v i t y . As a v e r y g e n e r a l guide,
loads below 24 MN/m2
i t i s n o t u s u a l l y necessary t o harden t h e s h a f t a t
(3500 l b / i n Z ) b u t g e n e r a l l y d e s i r a b l e t o do so a t loads
44 above 31 MN/m2
(4.500 l b / i n 2 ) .
R o l l i n g element b e a r i n g s can a l s o be used f o r p i s t o n engine c r a n k s h a f t be a r in g s , a l t h o u g h i n most cases they o f f e r no b e t t e r performance than a f l u i d -
f , i l m b e a r i n g , a t t h e expense i n l a r g e r en gi n es o f a r a t h e r more c o m p l i c a t e d design.
R o l l i n g b e a r i n g s do have t h e p a r t i c u l a r m e r i t o f r e q u i r i n g much l e s s
o i l supply than f l u i d - f i l m b e a r i n g s and t h i s makes them a f i r s t c h o i c e f o r twos t r o k e engines o f t h e p e t r o i l type, and f o r o t h e r small cheap engines where t h e o p p o r t u n i t y t he y o f f e r o f e l i m i n a t i n g t h e o i l supply system r e s u l t s i n a more commercial design.
F i g u r e s were r e p r i n t e d by p e r m i s s i o n o f t h e C ouncil o f t h e I n s t i t u t i o n o f Mechanical Engineers f r o m t h e Proceedings o f 1967 I n t e r n a t i o n a l Conference on L u b r i c a t i o n and \!ear.
45
4
DESIGN OF PLAIN BEARINGS Use of Bearing Data Design Charts
MARTIN and D.R.
F.A.
GARNER
Research 8 Development O r g a n i s a t i o n The G l a c i e r Metal Company L i m i t e d .
4.1
INTRODUCTION The d e s ig n o f hydrodynamic b e a r i n g s can be a d a u n t i n g t a s k f o r t h e average
machinery d e s ig ner.
W i t h r o l l i n g element b e a r i n g s t h e design process i s very
much b i a s e d towards b e a r i n g s e l e c t i o n d at a, where m a n u f a c t u r e r s ' catalogues g i v e t a b u l a t e d i n f o r m a t i o n on s i z e ,
load, speed and l i f e .
i t i s n o t so easy t o f i n d such ' p o t t e d '
information,
For p l a i n bearings
and i n t h e p a s t t h e
de s ig n e r had e i t h e r t o become i n v o l v e d i n t h e mathematical c o m p l e x i t i e s o f the hydrodynamics, o r r e l y on t h e v a g a r i e s o f ru l e-of-thumb guides.
With the con-
t i n u a l u p r a t i n g o f machinery, t h i s l a t t e r approach i s seldom adequate, but i t can be d i f f i c u l t t o o b t a i n more d e t a i l e d i n f o r m a t i o n i n a r e a d i l y d i g e s t i b l e form. V a r io u s t e c h ni qu es f o r a n a l y s i n g and ch ecking t h e performance o f p l a i n j o u r n a l b e a r i n g s a r e p rese nt ed here, s o t h a t t h e designer has t h e means t o appr e c i a t e t h e e f f e c t s o f t h e v a r i o u s parameters on b e a r i n g performance and design, w i t h o u t t he need f o r e x t e n s i v e mathematical m a n i p u l a t i o n .
4.1.1 b 'd
Notat ion = a x i a l b e a r i n g l e n g t h (m) = b e a r i n g d i a m e t r a l c l e a r a n c e (m)
C
= b e a r i n g r a d i a l cl e ara nce (m)
d
= b e a r i n g d i ame t e r (m)
e
= d i s t a n c e between b e a r i n g and s h a f t c e n t r e s (m)
hmin
= minimum o i l f i l m t h i c k n e s s (m)
H
= power loss (\I)
M
= e f f e c t i v e r o t o r mass a t b e a r i n g (kg)
N
= s h a f t r o t a t i o n a l speed ( r e v / s )
46 = r o t a t i o n a l speed o f l o a d v e c t o r ( r e v / s )
NL P
= b e a r i n g s p e c i f i c l o a d W/bd (Pa o r N/m2)
Q
= o i l f l o w req ui re men t (m3/s)
W
= a p p l i e d load
6
= j o u r n a l mi sa l i gn men t o ver b e a r i n g l e n g t h (m)
I-
= e f f e c t i v e f i l m v i s c o s i t y (Ns/m2)
ee
(N)
= e f f e c t i v e t emp era t u re ("C)
emax
= maximum t emp era t u re ( " C )
p
= o i l d e n s i t y (kg/m3)
w
= a n g u la r v e l o c t y ( r a d / s )
Dimensionless Terms ( i n any c o n s i s t e n t s e t o f u n i t s eg. as g i v e n above)
W'
load v a r i a b l e
& [?I2
=
P
=
e/cr
d2N2p
turbulence v a r i a b l e
eccentricity r a t i o
E
di m e n s io n le s s c r i t i c a l mass c
4.2
=
[t]
= 1
- hmin/cr
w 2 M/W
JOURNAL BEARING D E S I G N LIMITS The t o t a l d e si gn p roce ss f o r most mechanical components,
be a r in g s ,
including p l a i n
i n v o l v e s many stages, w i t h f u n c t i o n a l , economic and perhaps a e s t h e t i c
asp e c ts a l l needing t o be considered.
I t i s t h e f i r s t o f these w hich n o r m a l l y
i n v o l v e s t h e d e si g ne r i n most e f f o r t and whi ch i s t o be considered i n d e t a i l here.
For convenience we w i l l s p l i t t h i s f u n c t i o n a l design stage i n t o two:( i ) e n s u r i n g t h a t t h e b e a r i n g i s cap ab l e o f o p e r a t i n g s a t i s f a c t o r i l y under
th e imposed c o n d i t i o n s , and t h a t i t i s n e i t h e r t o o c l o s e t o i t s l i m i t s o f o p e r a t i o n t o endanger r e l i a b i l i t y n o r so f a r away t h a t i t has p e n a l t i e s i n over-design, ( i i ) p r e d i c t i n g t h e performance o f t h e component as i t a f f e c t s the design o f i t s a s s o c i a t e d p a r t s o r t he o v e r a l l system. i n i t i a l l y , therefore,
4.2.1
t h e l i m i t s o f o p e r a t i o n o f b e a r i n g s must be d e f i n e d .
L i m i t s o f Ope rat i on
Consider a b e a r i n g o f some g i v e n s i z e and geometry, w i t h a d e f i n e d l u b r i c a n t grade and f e e d i n g c o n d i t i o n s .
The l i m i t i n g c o n d i t i o n s o f load and speed w hich
t h i s b e a r i n g can s u c c e s s f u l l y a ccep t a r e shown i n Fig.1; be c o n s id e r e d i n more d e t a i l .
these l i m i t s w i l l now
47 Thin O i l F i l m L i m i t
4.2.1.1
The danqer h e r e i s o f met al -t o-me t a l
c o n t a c t o f t h e surfaces, w i t h con-
sequent severe wear (and perhaps o v e r h e a t i n g ) l e a d i n g t o a breakdown i n b e a r i n g op e r a t io n .
There i s e vi de nce t o show t h a t t h i s c o n t a c t o c c u r s a t a p r e d i c t e d
f i l m t h i c k n e s s which i s a f u n c t i o n o f t h e s u r f a c e roughness [l],w hich i n t u r n T h i s has been considered
i s dependent upon machining process and b e a r i n g s i z e .
i n some d e t a i l i n r e f e r e n c e [ 2 ] , and t h e c o n c l u s i o n reached t h a t a r e a l i s t i c ' f a i l u r e ' v a l u e o f t h e f i l m t h i c k n e s s i s g i v e n by t h e p e a k - t o - v a l l e y
surface
f i n i s h (Rmax) o f t h e j o u r n a l , assuming t h a t t h e s u r f a c e f i n i s h o f t h e b e a r i n g i s o f t h e same o r d e r . An a p p r o x im a te c o r r e l a t i o n between Ra and Rmax values i s a l s o g i v e n i n r e f e r e n c e [ 2 ] , e n a b l i n g t h e i n f o r m a t i o n g i v e n i n Fig.2
t o be presented.
The
s u r f a c e f i n i s h e s shown h ere a r e r e p r e s e n t a t i v e o f those w hich can be o b t a i n e d by normal m a n u f a c t u r i n g methods. f a i l u r e values,
Since t h e Rmax values have been thought o f as
some a d d i t i o n a l f a c t o r must be a p p l i e d i f we a r e t o s p e c i f y
s a f e v a lu e s o f o i l f i l m t h i ckne ss.
T h i s f a c t o r has t o a l l o w f o r s l i g h t u n i n -
t e n t i o n a l m is a lignme nt which may t ake p l a c e between s h a f t and bearing, and f o r d i r t c o n t a m i n a t i o n i n t h e o i l supply. i n Fig.2
The a l l o w a b l e f i l m t h i c k n e s s values shown
a r e a f a c t o r o f t h r e e above t h e f a i l u r e values, and have been found by
exp e r ie n c e t o be acce pt ab l e.
For v e r y h i g h standards o f b u i l d and o p e r a t i o n
a f a c t o r o f two can be s a t i s f a c t o r y ,
BEARIN( LOAD
THIN OIL FILM LIMIT danger of metal -to - metal contact if load lies above this line
O :/ F
b u t i t i s considered t h a t t h e values shown
HIGH BEARING TEMPERATURE LIMIT danger of lining material wiping if load lies above this h e
/..
SAFE OPERATION
)
OIL OXIDATION LIMIT danger of excessive oil oxidation if speed lies beyond OILthis FILM lineWHIRL LIMIT
I
danger of unacceptable vibration if speed lies beyond this line
JOURNAL SPEED Fig.1
L i m i t s o f s a f e o p e r a t i o n f o r hydrodynamic j o u r n a l b e a r i n g s
48 i n Fig.2
should be used as a general g u i d e t o a v o i d w o r k i n g t o o c l o s e t o
failure limits.
I t must be emphasised t h a t t h e r e l a t i o n s h i p between Ra and
Rmax i s o n l y approximate,
as evidenced by t h e "spread o f r e s u l t s " band on t h e
f i g u r e , b u t i t i s adequate t o show t h e o r d e r o f r e d u c t i o n i n t h e a l l o w a b l e f i l m t h i c k n e s s t h a t can be p e r m i t t e d by improving s u r f a c e f i n i s h .
To increase t h e
o p e r a t i n g o i l f i l m t h i c k n e s s , f o r g i v e n c o n d i t i o n s o f l o a d and speed, t h e b e a r i n g s i z e must be increased, o r a t h i c k e r l u b r i c a n t and/or a reduced i n l e t temperature used (change i n o i l supply p r e s s u r e w i l l u s u a l l y have l i t t l e e f f e c t ) . An increase i n c l e a r a n c e may e i t h e r increase o r reduce t h e f i l m t h i c k n e s s , depending upon t h e p r e c i s e o p e r a t i n g c o n d i t i o n s .
1
2
I
6 810
20
LO
1inI
JOURNAL DIAMETER
Fig.2
4.2.1.2
Guidance on s h a f t s u r f a c e f i n i s h and a l l o w a b l e o i l f i l m t h i c k n e s s
High Bearing Temperature
The shearing which t a k e s p l a c e i n t h e b e a r i n g c l e a r a n c e space i s l s e e n ' e x t e r n a l l y as power d i s s i p a t i o n ,
and a l s o as an i n c r e a s e i n t h e temperature o f
s h a f t and b e a r i n g s u r f a c e s and o f t h e l u b r i c a n t . The l i m i t i n g a c c e p t a b l e temperature o b v i o u s l y depends upon t h e l i n i n g m a t e r i a l used, b u t whatever t h e m a t e r i a l t h e temperature must always be k e p t w e l l below i t s theoretical melting point.
For example,
i n t i n based w h i t e metal t h e
m e l t i n g temperature o f 232OC cannot even be c l o s e l y approached due t o t h e s o f t e n i n g and subsequent p l a s t i c f l o w o f t h e m a t e r i a l which o c c u r s a t temper-
49 atures w e l l below 200°C under the i n f l u e n c e o f hydrodynamic pressure. et al.
[3]
Booser
observed l i m i t i n g whitemetal temperatures i n j o u r n a l bearings as low
as 130°C;
however, since o n l y c a l c u l a t e d temperatures are a v a i l a b l e a t the
design stage, and c u r r e n t methods o f e s t i m a t i o n are known t o be f a r from i t i s s a f e r t o lower the l i m i t t o about 120'C.
accurate,
I n machines i n which
some dynamic loading can occur on top o f the normal steady load, and where bearing f a t i g u e i s t h e r e f o r e a p o s s i b l e problem, keep the temperature below 100°C.
i t i s customary t o t r y and
I f higher temperatures have t o be accomm-
odated then the use o f an aluminium t i n o r a copper lead m a t e r i a l must be considered.
The former o f these, when c o n t a i n i n g about 40% t i n , can be used a t
temperatures up t o 15O-16O0C, and i s almost equal t o whitemetal t o withstand seizure c o n d i t i o n s and d i r t contamination.
less f o r g i v i n g i n t h i s respect, and i d e a l l y should have a s o f t , p l a t e t o a i d i n bedding-in,
in i t s ability
Copper lead i s much t h i n overlay
but i t can be used a t temperatures o f 200°C plus.
I t r e q u i r e s a hardened s h a f t (about 300 HV) and very good l u b r i c a n t f i l t r a t i o n .
I t i s worth n o t i n g t h a t i f a h i g h temperature c o n d i t i o n i s present i n a bearing t h e r e i s u s u a l l y n o t h i n g t h a t can be done t o the o i l feeding c o n d i t i o n s t o improve the s i t u a t i o n , a p a r t from changing the o i l grade.
The oft-used
' p a l l i a t i v e s ' o f increasing o i l feed pressure o r grooving area i n order t o f o r c e more o i l through the bearing a r e u s u a l l y n o t successful since they o n l y reduce the b u l k temperature o f the o i l passing through the bearing (see section 4.2.1.3).The
bearing m a t e r i a l temperature i s c o n t r o l l e d by the amount o f lub-
r i c a n t passing through the a c t i v e p a r t o f the o i l f i l m , s e n s i t i v e t o feeding conditions, starved.
and t h i s i s very i n -
provided t h a t the bearing i s n o t grossly
As a rough approximation,
the maximum bearing temperature w i l l be
dropped by about a h a l f o f any decrease i n o i l i n l e t temperature,
i e a 10°C
reduction i n i n l e t temperature may be expected t o decrease the maximum bearing temperature by about 5°C.
The bearing temperature may a l s o g e n e r a l l y be de-
creased by increasing the bearing s i z e o r clearance, o r by using a t h i n n e r grade o f o i l . One f u r t h e r p o i n t on bearing temperatures which i s sometimes ignored i s the n e c e s s i t y o f considering the influence o f machine temperatures on the bearings.
I f t h e r e can be appreciable heat soak along the s h a f t , o r ( l e s s o f t e n ) through the bearing housing, then the design must a l l o w f o r adequate o i l f l o w t o deal w i t h it.
Additionally,
i n some machinery, the worst temperature c o n d i t i o n s a t
t h e bearing can occur a f t e r shut down when heat soak r a i s e s temperatures w e l l above peak running values, and the choice o f m a t e r i a l should then be d i c t a t e d by c o n d i t i o n s which a r e o f t e n n o t drawn t o the a t t e n t i o n o f the bearing designer.
50 4.2.1.3
O i l Oxidation L i m i t
S t r a i g h t mineral o i l s i n a normal (oxygen c o n t a i n i n g ) atmosphere can be r a p i d l y o x i d i s e d a t the order o f temperatures t h a t we have been discussing above.
There i s no p r e c i s e 'go/no-go'
l i m i t f o r t h i s process, r a t h e r the r a t e
o f degradation i s a f u n c t i o n o f temperature [4].
I n d u s t r i a l mineral o i l s
u s u a l l y c o n t a i n a n t i - o x i d a n t s which r e t a r d t h i s process, b u t f o r comnonly used t u r b i n e o i l s , f o r l i v e s i n the order o f thousands o f hours,
i t i s necessary t o
r e s t r i c t b u l k temperatures o f o i l i n tanks,
t o about 75-80°C.
r e s e r v o i r s etc.
Thus a b u l k d r a i n temperature from a bearing a t a higher value than t h i s i s u n l i k e l y t o be acceptable.
As mentioned above, t h e d r a i n temperature can u s u a l l y be reduced by i n creasing o i l supply pressure o r p r o v i d i n g bleed grooves i n the bore, and thereby h e l p i n g t o avoid the o x i d a t i o n l i m i t .
I t i s u n f o r t u n a t e t h a t the b u l k o u t l e t
temperature i s o f t e n used t o judge bearing performance, presumably because i t i s the e a s i e s t temperature t o measure, since t h i s j u g g l i n g w i t h supply cond i t i o n s can r a d i c a l l y a l t e r d r a i n temperatures w i t h o u t appreciably a f f e c t i n g m a t e r i a l temperatures.
The o u t l e t temperature can be used as a long-term
monitoring device ( i e a temperature which has been steady a t 70-75'C should n o t suddenly r i s e t o 80-85°C) misleading,
4.2.1.4
f o r months
b u t i t i s n o t s u i t a b l e , and may indeed be
i f used for ' s e t t i n g up' o i l feed c o n d i t i o n s on a new machine.
O i l F i l m Instability
Under c e r t a i n c o n d i t i o n s , normally a t low load and/or h i g h s h a f t speeds, a s e l f e x c i t e d and s e l f s u s t a i n i n g motion can occur i n which the s h a f t c e n t r e precesses around the bearing c e n t r e a t something s l i g h t l y l e s s than h a l f s h a f t speed, t y p i c a l l y 0.42-0.47
o f s h a f t speed.
Under these c o n d i t i o n s the hydro-
dynamic a c t i o n o f the bearing i s a l l but l o s t , and metal-to-metal contact can occur;
i n practice,
i f l e f t f o r long periods, a f a t i g u e type damage i s pro-
duced due t o t h e h i g h temperatures generated.
There can a l s o be a large, and
perhaps unacceptable, v i b r a t i o n t r a n s m i t t e d through the machine.
Guidance on
the l i k e l i h o o d o f i n s t a b i l i t y i n c y l i n d r i c a l bore bearings i s given i n Fig.3. This c h a r t i s s t r i c t l y o n l y a p p l i c a b l e t o r i g i d , simply supported s h a f t s [ 5 ] , but experience has shown i t t o g i v e a f a i r guide f o r o t h e r systems, f o r example overhung rotors.
I t can be seen t h a t i f the o p e r a t i n g e c c e n t r i c i t y r a t i o i s
g r e a t e r than about 0.8
then the bearing i s stable.
This i s the reason why
- adding e x t r a grooving i n the loaded region, using a grade o f o i l etc. - can sometimes cure h a l f speed w h i r l problems.
d e r a t i n g a bearing thinner
A d d i t i o n a l l y , whatever t h e o p e r a t i n g e c c e n t r i c i t y r a t i o there i s some value o f the dimensionless c r i t i c a l mass below which the bearing w i l l always be stable. I n h e a v i l y loaded a p p l i c a t i o n s ,
such as gearboxes,
the a p p l i e d load i s normally
51 o f a much higher magnitude than the s h a f t mass and i n s t a b i l i t y , a t any r e a l i s t i c speed,
i s n o t a problem.
However there may well. be some p a r t load
c o n d i t i o n , f o r example a s p i n t e s t , a t which s t a b i l i t y needs t o be c a r e f u l l y checked.
1
1
Cr radial clearance shaft angular velocity M effective rotor mass at bearing
0
W bearing load
- 100 -80
-
With consistent units e.g. m, radls, kg. N.
60
4 0VL3 1 ~
-20
-I
-10 - 8
UNSTABLE 0.1 0.5
. . 1.0
t
~ 0
-6
4: g
3 2
w 4
-4
1.5
I
2 4
I
length diamotrr
v)
z W
STABLE
E - 2
'
" 0.2
"
'
0-4
" 0.6
' 0-8
-
1.0
ECCENTRICITY RATIO Fig.3
O i l f i l m w h i r l i n s t a b i l i t y o f c y l i n d r i c a l bore j o u r n a l bearings L i m i t i n g dimensionless mass
4.2.2
-
Region o f Safe Operation
The l i m i t s j u s t defined serve t o bound a region o f safe operation.
The
simplest approach t o design i s merely t o c a l c u l a t e such o p e r a t i n g parameters as f i l m thickness and temperatures and t o compare them against the defined limits.
If b o t h the l i m i t s and the design p o i n t a r e p l o t t e d , i n a form s i m i l a r
t o Fig.1,
then judgements on the degree o f s a f e t y can e a s i l y be made.
There a r e various methods o f t a c k l i n g t h i s , and the most comnon w i l l now be described;
the f i n a l design c h a r t s presented enable very r a p i d and accurate
estimates o f bearing o p e r a t i o n t o be made w i t h minimal e f f o r t .
52 4.3
CALCULATION AND D E S I G N PROCEDURES The a n a l y s i s o f o i l f i l m c o n d i t i o n s w i t h i n a b e a r i n g , desc ibed by t h e
Reynolds e q u a tio n,
i s a v e r y i n v o l v e d mathematical e x e r c i s e .
The e q u a t i o n i s
d e c e p t i v e l y s im p l e, b u t an a n a l y t i c a l s o l u t i o n cannot be o b t a ned f o r any b u t th e s i m p l e s t o f arrangements.
I nst ea d re cou rse must be made t o numerical
techniques, u s u a l l y i n v o l v i n g computer s o l u t i o n s .
A f u r t h e r c o m p l i c a t i o n i n o i l l u b r i c a t e d b e a r i n g s i s t h e need t o make a r e a l i s t i c estimation o f the o i l v i s c o s i t y w i t h i n the f i l m . normal m i n e r a l o i l s is v e r y dependent on temperature,
The v i s c o s i t y of
and a r e l a t i v e l y a c c u r a t e
assessment o f t h e l a t t e r must be made f o r r e a l i s t i c design.
One method o f
do in g t h i s i s t o s o l v e t h e Reynolds and energy equations simultaneously.
This
c o n s i d e r a b l y in c rea ses t h e comp ut at i on al e f f o r t and t h e r e s u l t i n g s o l u t i o n s a r e somewhat r e s t r i c t e d i n t h e i r g e n e r a l i t y . I n s te a d i t i s u s u a l l y more co nve ni e nt t o s o l v e t h e Reynolds e q u a t i o n assuming a s i n g l e , g l o b a l , e f f e c t i v e be determined a t a l a t e r stage.
v i s c o s i t y value,
which then can, and must,
T h i s i s done by e q u a t i n g t h e work done i n
s h e a r in g t h e o i l w i t h i n t h e b e a r i n g c l e a r a n c e space t o t h e amount o f h e a t c a r r i e d away, b o t h by t h e o i l ( t h e predominant mechanism) o r through t h e b e a r i n g and housing. It i s , unfortunately,
balance i s o m i t t e d ,
n o t uncommon t o see design methods i n w hich t h i s h e a t
t h ere by assuming t h a t t r e n d s i n dimensionless groups acc-
u r a t e l y p o r t r a y t r e n d s i n r e a l performance. clusions;
T h i s can l e a d t o i n c o r r e c t con-
f o r example an i n crea sed c l e a r a n c e may a p p a r e n t l y reduce f i l m
thickness i f the v i s c o s i t y i s a r t i f i c i a l l y h e l d constant, b u t i n p r a c t i c e the o p e r a t i n g v i s c o s i t y may i ncre ase and p o s s i b l y r e v e r s e t h e trend. Whatever c a l c u l a t i o n method i s chosen, t h e r e s u l t s must then be compared a g a i n s t t h e e s t i m a t e d o r known l i m i t s o f s a t i s f a c t o r y o p e r a t i o n . Fig.4 Stage
i l l u s t r a t e s t h e v a r i o u s stages i n p r oducing design data:-
1
i s t h e s o l u t i o n of t h e Reynolds e q u a t i o n t o g i v e dimensionless data.
Stage I I
ta k e s t h i s d a t a and d et ermi n es a r e a l i s t i c o i l v i s c o s i t y t o g i v e s p e c i f i c answers.
Stage I l l
uses t h e r e s u l t s f r o m t h e p r e v i o u s two stages t o produce 'easy-to-use'
4.3.1
d esi g n c h a r t s .
Dimensionless Data
- Stage
I
Dimensionless d esi g n d a t a have been p u b l i s h e d f o r t h e most commonly used b e a r i n g c o n f i g u r a t i o n s [6,7,8,9]
and Fig.5
shows a t y p i c a l c h a r t r e l a t i n g a
l o a d v a r i a b l e t o a f i l m t h i c k n e s s r a t i o f o r a c y l i n d r i c a l bore j o u r n a l b e a r i n g w i t h a steady ( o r p ure r o t a t i n g ) load.
T h i s may be used f o r b e a r i n g s w i t h two
a x i a l o i l f e e d grooves (l oa de d midway between them), w i t h a s i n g l e f e e d groove
53
1
I
I Fundamental theory
program e.g. solution to
StageI- Dimensionless groups StagelI-Specific answer for selected input Stagem- Infinite number of answers (certain restrictions usually necessary)
Fig.4
Stages in design
I
4
I
54 i n th e unloaded h a l f o r w i t h a c e n t r a l c i r c u m f e r e n t i a l groove;
w i t h the l a t t e r
the l e n g t h i s t h a t of one l an d and t h e l o a d i s h a l f t h e t o t a l a p p l i e d load. The c e n t r a l c i r c u m f e r e n t i a l l y grooved b e a r i n g i s g e n e r a l l y used i n a p p l i c a t i o n s where th e l o a d i s r o t a t i n g o r where an unknown l o a d d i r e c t i o n has t o be c a t e r e d for.
When a b e a r i n g has t o c a r r y a r o t a t i n g l o a d t h e l o a d c a p a c i t y i s p r o -
p o r t i o n a l t o ( 1 - 2 N ~ / N l , as shown i n Fig.5 where NL i s t h e a n g u l a r r o t a t i o n a l speed o f t h e l o a d measured i n t h e same d i r e c t i o n as s h a f t r o t a t i o n (N). i s f u r t h e r i l l u s t r a t e d i n Fig.6,
which shows d i a g r a m n a t i c a l l y t h e o i l f i l m
forma t ion.
FILM THICKNESS RATIO
Fig.5
This
hmin/cr
B e a r in g l o a d c a p a c i t y for steady and p u r e r o t a t i n g l o a d cases
55
An examination of dimensionless data can also be instructive where gross effects on performance are apparent.
For instance the position of oil feed
grooves in a steadily loaded bearing is very important, since they can seriously derate the load-carrying capability of the bearing.
Such a derating effect is
clearly demonstrated in Fig.7. Case (a) shows a n uninterrupted coverging oil film which generates a substantial hydrodynamic pressure to support the load ( W ) . Case (b) shows the same converging oil film a s (a) but interrupted by an oil groove.
This will only support a very reduced load because of the
smaller integrated pressure. Case (c) shows an uninterrupted film with the same load a s (a) which needs a smaller oil film thickness to generate sufficient pressure.
+
ongulor velocity of load ongulor velocity of shaft
hmin position oheod of pressure region
I
I
N
mL
hmin position logs behind pressure region
Analogous
-
Fig.6
DRAGGING OIL
SQUEEZING
PUSHING OIL
Film formation and relative load capacity under rotating load conditions.
56
E;
I
c
Cr
~
groove
me min
min smaller load
t
I
(a)
I
W
Fig.7
(b)
The derating effect of grooving positioned in a converging oil film
- 0.1
Fig.8
Variation o f load capacity with direction of load for a bearing with two axial grooves
67 The d im e n s io n le s s l o a d and f i l m t h i c k n e s s c h a r t , Fig.8,
shows more p r e c i s e l y
th e v a r i a t i o n i n l o a d c a p a c i t y w i t h any l o a d d i r e c t i o n r e l a t i v e t o t h e grooves. In t h i s f i g u r e a b e a r i n g w i t h a l e n g t h equal t o h a l f the diameter i s cons id e r e d , h a v i n g two a x i a l grooves d i s p l a c e d 180" a p a r t , each groove e x t e n d i n g
30"
around t h e b e a r i n g .
These a r e t y p i c a l groove arrangements.
Wit h t h e l o a d d i r e c t i o n e x a c t l y between two grooves t h e l o a d c a p a c i t y shown i n Fig.8
i s t h e same as t h a t i n F i g . 5 and v a r i a t i o n s i n t h e angle w i t h i n
f 20" cause l i t t l e d i f f e r e n c e .
As t h e l oa d l i n e g e t s near t o a groove p o s i t i o n
th e lo a d c a p a c i t y d r a s t i c a l l y reduces as seen by the r e d u c t i o n i n t h e m o d i f i e d load v a r i a b l e ( W ' / T )
f o r t h e same f i l m t h i c k n e s s .
I t can be seen t h a t t h e load
v a r i a b l e i s h a l v e d when g o i n g f rom p o i n t (a) t o p o i n t (b) i n Fig.8, p o i n t s (a),
(b) and ( c ) can be i d e n t i f i e d w i t h cases (a),
where
(b) and ( c ) i n Fig.7.
S i m i l a r l y f o r t h e same v a l u e o f l o a d parameter t h e f i l m t h i c k n e s s f o r case (c) i s much s m a l l e r t ha n case ( a ) . I t s h o u ld be n ot ed t h a t t h e c o n d i t i o n o f l o a d i n g d i r e c t l y i n t o a groove does n o t n o r m a l l y produce t h e l owest o i l f i l m t h i c k n e s s ; t h e l o a d i s d i r e c t e d j u s t b e f o r e t h e groove.
i n s t e a d t h i s occurs when
T h i s i s because t h e peak o i l f i l m
pre s s u r e o c c u r s j u s t beyond t h e l o a d l i n e ( f o r a steady l o a d c o n d i t i o n ) , and a groove i n t h i s p o s i t i o n t h e r e f o r e more s e v e r e l y d e r a t e s t h e b e a r i n g .
4.3.2
Design Procedures w i t h Heat Balance
-
Stage I I
The h e a t b a lan ce procedure can be c a r r i e d o u t by hand c a l c u l a t i o n i n a r e l a t i v e l y s t r a i g h t f o r w a r d manner, b u t i s i d e a l l y s u i t e d t o simple computer techniques.
A p a r t i c u l a r l y co nve ni e nt and we1 1 documented procedure f o r
[9] which [9] enables t h e
c y l i n d r i c a l bore b e a r i n g s i s g i v e n i n r e f e r e n c e
can form t h e b a s i s o f
a computer programme [ l o ] .
influence o f a very
The procedure
wide range o f v a r i a b l e s t o be co nsi d ere d and i s an extremely v a l u a b l e design t o o l , e s p e c i a l l y when i n t h e f orma t o f a c o n v e r s a t i o n a l type computer programme.
4 . 3 . 3 Improvement i n Design A i d s
-
Stage I l l
W h i l s t t h e above arrangement i s s u i t a b l e f o r p r e d i c t i n g b e a r i n g performance w i t h ' s i n g l e s h o t ' answers,
i t does n o t p r o v i d e a ' f e e l '
for how near a
p a r t i c u l a r d e s ig n i s t o t h e v a r i o u s s a f e o p e r a t i n g l i m i t s .
Therefore, t h e r e i s
a need f o r s t a g e I l l where t h e 'computed answers' f o r b e a r i n g cases a r e repres e n t e d i n d e s ign c h a r t form.
These e na bl e t h e designer t o answer such
q u e s t i o n s as:-
- W i l l t h i s bearing operate s a f e l y ? - How n e a r i s i t t o t h e l i m i t s of o p e r a t i o n ? -
What changes can be made t o improve t h e design?
58 When assessing safe l i m i t s o f o p e r a t i o n many f a c t o r s a r e involved, f o r example the bearing data considered must include diameter, length, o i l grade, load and speed.
The various permutati.ons,for a range o f these f a c t o r s r e s u l t s
i n many thousands o f cases t o be considered.
I f such data were represented i n
conventional graphical form, a t h i c k volume o f unmanageable design a i d s would result.
The ' s l i d e c h a r t ' obviates the need f o r t h i s and gives a d i r e c t and
i n s t a n t f e e l f o r the e f f e c t o f changing any o f t h e v a r i a b l e s , thus b r i n g i n g the power o f the computer t o the designers' desk top.
4.4
DESIGN PROCEDURE
4.4.1
FOR CYLINDRICAL BORE BEARINGS
Method o f Approach
The procedure d e t a i l e d here presents design a i d s which a l l o w f o r l i m i t i n g c o n d i t i o n s o f o p e r a t i o n and t h e p r e d i c t i o n o f bearing performance i n c y l i n d r i c a l bore j o u r n a l bearings which have two a x i a l grooves spaced 180" a p a r t . type o f bearing, w i t h a steady l o a d midway between the grooves, most common i n use.
This
i s one o f the
A l l the tedious heat balance c a l c u l a t i o n s have already
been c a r r i e d o u t by computer, based on the design procedure given i n ESDU Data Item
66023 [ g ] ,
and a r e consequently inherent i n the design aids.
Comparison of experimental work w i t h r e s u l t s from t h i s theory i n d i c a t e d t h a t w h i l s t c o r r e l a t i o n was good . f o r most v a r i a b l e s , the p r e d i c t e d maximum tempe r a t u r e could,
i n many circumstances, be considerably i n e r r o r .
A new method
was evolved based on considerable experimental evidence which r e l a t e s the maximum bearing temperature t o the c a l c u l a t e d e f f e c t i v e temperature [ l l ] . The r e s u l t i n g procedure was used t o c a l c u l a t e bearing performance over a wide range o f o p e r a t i n g c o n d i t i o n s .
However, t o keep the problem t o a manage-
able s i z e i t was decided t h a t c e r t a i n v a r i a b l e s should be f i x e d throughout, values commonly found i n c u r r e n t p r a c t i c e .
These were:-
(i) o i l groove dimensions: a x i a l l e n g t h = 0.8 o f bearing l e n g t h c i r c u m f e r e n t i a l width = 0.25 o f bearing diameter. ( i e 150" between the edges o f grooves, i n top and bottom o f bearing). Grooves w i t h 'square' ends
(ii) o i l feed c o n d i t i o n s a t the bearing: pressure 0.1 MPa (approx 15 I b f / i n Z ) temperature 50°C
to
59 ( i i i ) o i l grades: i n o r d e r t h a t t h e o i l grade c o u l d be d e f i n e d s i m p l y , a t r e n d i n viscosity-temperature characteristics typical o f industrial m i n e r a l o i l s was assumed t o a p p l y throughout.
The d e f i n i t i o n o f
an o i l grade can t h en be made by q u o t i n g one v i s c o s i t y a t a certain (arbitrary)
temperature;
40°C was used ,here t o d e f i n e
t h e o i l grade, t h i s b e i n g t h e same b a s i c r e f e r e n c e as f o r IS0 v i s c o s i t y grades.
Fig.9 shows t h e v i s c o s i t y t r e n d s considered.
I n a d e s ig n problem t h e b e a r i n g d i ame t e r and o p e r a t i n g c o n d i t i o n s o f speed and l o a d a r e g e n e r a l l y determined p r i o r t o t h e b e a r i n g design stage, s i n c e they u s u a l l y f o r m p a r t o f an o v e r a l l system.
For s i m i l a r reasons t h e l u b r i c a t i n g
o i l grade t o be used i s o f t e n a l s o imposed l e a v i n g c l e a r a n c e and b e a r i n g l e n g t h as t h e two dimensions s t i l l t o be determined.
Fig.9
Viscosity
%
Temperature c h a r a c t e r i s t i c s f o r t h e range o f o i l s co nsi d ere d i n t h e design c h a r t s
60
Several authors have considered optimising on clearance [12,13] to give the largest possible oil film thickness in the bearing or the lowest power loss. Unfortunately these two optima do not coincide and a compromise solution is usually required.
Indeed when the problem is treated purely as a mathematical
exercise there is a danger of having excessively slack clearances for optimum power loss and, in some instances, very tight clearances for optimum film thickness, especially if the effect of change in clearance on viscosity is ignored.
4.4.2 Guidance for Safe Operation 4.4.2.1 Oil Film Thickness and Temperatures It is appropriate to consider the tight clearance end of the tolerance range
when examining conditions at h i g h temperature.
These tight clearances may also
be used when making judgements on safe operation relating to small oil films since the variation of clearance throughout m s t practical tolerance ranges generally has little effect on oil film thickness.
The design aids, Figs.lO(a),
(b) and (c), associated with both temperature and film thickness limits, have been developed using the minimum clearance values shown in Fig.11.
These
values relate to an extreme of the tolerance range; average manufactured clearance will be larger.
These clearances are based on many years' expe.rience
with much 'feed-back' relating to the performance of practical designs. With clearances known it is possible, for any particular values of diameter, length and lubricant grade, to plot limiting lines of load against speed, representing film thickness limit and temperature limits (as in Fig.1).
The design
aids, in 'slide chart' form, enable the designer to 'plot' these limiting lines in a matter of seconds, not just for a discrete number of variable combinations, b u t for an infinite number of cases.
In fact the designer merely has to move
an already drawn curve to its correct position relative to the axes, the bearing conditions dictating exactly where this position is. Fig.lO(a) is used to 'plot' the film thickness limit lines, and Fig.lO(b) is used to 'plot' the temperature limit lines, both on the grid of load and speed given in Fig.lO(c).
A transparent version of the chart in question,
Fig.lO(a) or (b), should be placed on the backing sheet, Fig.lO(c) using the guide lines on the top (transparent) sheet to keep the two sheets square.
The
transparency should then be moved to a position where a point in its grid, defined by the diameter and oil grade values, is coincident with the cross on the backing sheet.
The relevant length/diameter line then shows the limit of
load against speed for the conditions considered. By using the two transparencies the relative position of a design point
(defined by its speed and load values) to the limiting lines can be seen.
If
this point is within the limits then the bearing, under reasonable environmental
61
Fig.lO(a)
Load c a p a c i t y s l i d e c h a r t :
Thin o i l f i l m l i m i t (transparency)
62
This chart may not be applicable at high speeds if non laminar conditions prevail.
-
U C
4
LC
z
5 eE 2 C
a
Y
I
a
.-C a
K .C
\
lest at LO'C I
Ensure that the design point of speed and load lies to the left of the appropriate guidance curve, otherwise full lines -there is a danger of EXCESSIVE OIL OXIDATION broken lines - the BEARING MATERIAL may be TOO HOT
03'
) 0%
guide line -keep square with backing sheet
Fig.lO(b)
Load c a p a c i t y s l i d e c h a r t :
High temperature 1 i m i t s (transparency)
63
such that appropriate DIAMETER and OIL GRADE are coincident with this point Bearing load
length diameter
Ices
Journal weed Irev/minl
Fig.lO(c)
Load c a p a c i t y s l i d e c h a r t :
Load-speed g r i d (backing s h e e t )
64 c o n d i t i o n s , should o pe rat e s a t i s f a c t o r i l y .
I f i t i s c l o s e t o o r outside the
l i m i t s t h e n t h e s l i d e c h a r t s p r o v i d e a q u i c k method o f d e t e r m i n i n g which v a r i a b l e o r v a r i a b l e s can be changed t o improve t h e design.
For i n s t a n c e
increasing the bearing length w i l l r a i s e the l i m i t l i n e f o r f i l m thickness, b u t may worsen a h i g h temperature s i t u a t i o n . e x p e r i m e n t a l l y by Brown and Newman [ 1 4 ] .
The l a t t e r e f f e c t was n o t e d
A t these h i g h speeds the c h a r t s dem-
o n s t r a t e and q u a n t i f y t he advantage o f t h i n n e r o i l s .
I f t h e use o f t h i c k o i l s
i s u n a v o id a b le t he n an i n crea sed cl e ara nce , above t h e v a l u e s g i v e n by Fig.11, may h e l p a t h i g h speeds.
The s l i d e c h a r t s a r e then n o t a p p l i c a b l e , b u t a
s u i t a b l e d e s ig n method i s d e t a i l e d i n s e c t i o n 4.4.3. Finally,
i t i s advisable,
l i m i t i n g lines;
wherever p o s s i b l e , t o work away from t h e a c t u a l
t h i s a l l o w s an e x t r a s a f e t y margin f o r u n i n t e n t i o n a l adverse
c o n d i t i o n s such as small mi sa l i gn men t (see s e c t i o n
4 . 4 . 3 1 , contaminated o i l , etc.
I t i s a l s o a d v i s a b l e t o work w e l l w i t h i n t h e l i m i t i n g l i n e s where t h e r e i s a
chance o f h a v in g two modes o f f a i l u r e a t t he same t i m e ( i e . a t t h e d o t t e d apex i n Fig.1). C u r r e n t p r a c t i c e i s t o q u a n t i f y t h i s a d d i t i o n a l l i m i t i n terms o f s p e c i f i c l oa d ( l o a d d i v i d e d by p r o j e c t e d area, W/bd).
For hydrodynamic b e a r i n g s i t i s
usual t o keep t h e s p e c i f i c l o a d below about 4 MPa, s t i l l checking t h a t f i l m t h i c k n e s s and t e mp era t u res are-acceptable.
However,
t h e l o a d on a b e a r i n g a t
s t a r t - u p o r r u n down must be c o n s i d e r a b l y lower, no more than 1 t o 1.3 MPa o t h e r w i s e h i g h p re ssure j a c k i n g o i l must be s u p p l i e d . Fig.12 has been developed t o g i v e t he maximum s p e c i f i c l o a d r a t i n g based on the t h i n o i l f i l m l i m i t ;
t h i s assumes t h e same geometry and o i l feed c o n d i t i o n s
as c o n s id e r e d i n t h e s l i d e c h a r t .
4.4.2.2
O i l Film I n s t a b i l i t y
T h i s i s o n l y l i k e l y t o be a problem i n l i g h t l y loaded b e a r i n g s a t h i g h speeds. As a f i r s t check c a l c u l a t e t h e v a l u e o f t h e dimensionless c r i t i c a l mass parameter shown i n Fig.3.
I f t h i s i s l e s s t ha n the minimum v a l u e on t h e p a r t i c u l a r
b/d c u r v e , t h e n t h e b e a r i n g sh ou l d be s t a b l e .
I f n o t , then s t a b l e o p e r a t i o n
may s t i l l be p r e d i c t e d p r o v i d e d t h a t t h e e c c e n t r i c i t y r a t i o (l-hmin/cr) l a r g e enough;
s e c t i o n 4.4.3
g i v e s a method for o b t a i n i n g t h i s value.
is The
,
r e s i s t a n c e t o i n s t a b i l i t y i s r a i s e d by any e x t e r n a l damping w i t h i n the system, f o r example gear meshing, so t h a t t h e l i m i t s can be unduly p e s s i m i s t i c .
I f i n s t a b i l i t y i s a problem t h e b ore p r o f i l i n g can o f t e n p r o v i d e t h e s o l u t i o n ; s e c t i o n 5. c o n s i d e r s t h i s i n some d e t a i l .
65
MINIMUM DIAMETRAL CLEARANCE
MINIMUM
(in.]
0.02
0.016
0.012
0.008
0
-
SPEED I rcv/min 1
Fig. 11
4.4.3
Recommended minimum c l e a r a n c e
Performance P r e d i c t i o n
Having determined t h a t a g i v e n b e a r i n g i s l i k e l y t o o p e r a t e s a t i s f a c t o r i l y , t h e d e s ig n e r t h e n o f t e n needs t o know t h e power loss and o i l f l o w requirement o f t h e b e a r in g .
As w e l l as b e i n g o f i n t e r e s t as f a r as t h e o v e r a l l e f f i c i e n c y
o f a machine i s concerned, these q u a n t i t i e s have a d i r e c t i n f l u e n c e on the l u b r i c a n t s u p p ly system coolers etc.
-
t h e s i z e o f o i l pump and supply l i n e s , t h e need f o r
T h i s system must be ca pa bl e o f adequately s u p p l y i n g s u f f i c i e n t
c o o l e d o i l (heat i s t h e t a n g i b l e f o rm o f b e a r i n g power l o s s ) f o r any b e a r i n g w i t h i n t h e spread o f t h e man uf act uri n g t o l e r a n c e on clearance.
When d e t e r m i n i n g
t h e o i l pump c a p a c i t y , t h e b e a r i n g f l o w a t maximum p o s s i b l e c l e a r a n c e ( w i t h i n t h e t o l e r a n c e range) sho ul d be co nsi d ere d s i n c e t h i s has the maximum f l o w req u ir e m e n t .
Maximum power l o s s on t h e o t h e r hand,
i m p o r t a n t when c o n s i d e r i n g
t h e h e a t d i s s i p a t i o n from t h e o v e r a l l system. can occur anywhere w i t h i n t h e t o l e r a n c e range o f cl e ara nce .
The cl e ara nce has t h e r e f o r e been l e f t as a
v a r i a b l e i n t h e p r e d i c t i o n c h a r t s f o r power l o s s , o i l f l o w and temperature. However, t h e c l e a r a n c e range sh ou l d p r e f e r a b l y s t i l l have a minimum v a l u e c o r r e s p o n d in g w i t h Fig.11,
s i n c e t h e s l i d e c h a r t s (which i n h e r e n t l y c o n t a i n
these c l e a r a n c e s ) can t h en be used t o check f o r safe o p e r a t i o n p r i o r t o p r e d i c t i n g b e a r i n g performance.
66
Fig.12
Maximum specific load rating for a two axial groove bearing based on thin oil film limit
67 Under some c r i t i c a l c o n d i t i o n s t h e need t o c o n t r o l t h e v a r i a t i o n s i n f i l m t h i c k n e s s , power loss o r o i l f l o w m i g h t r e g u l a t e t h e range o f t h e c l e a r a n c e t o l e r a n c e s , b u t u s u a l l y p r e d e f i n e d m a n u f a c t u r i n g d e t a i l s w i l l impose a g i v e n c l e a r a n c e range which t h e d e s i g n e r must c a t e r f o r adequately.
The s i z e o f t h i s
t o l e r a n c e range i s m a i n l y a f u n c t i o n o f economic c o n s i d e r a t i o n s , and may t h e r e f o r e vary considerably.
However, a p r a c t i c a l g u i d e i s t h a t t h e t o l e r a n c e (mm)
on d i a m e t r a l c l e a r a n c e l i e s w i t h i n t h e range (2.10 -6 x d i a ) l l 3 t o (5.10 -6 x d i a ) 1/3 depending on t h e m a n u f a c t u r i n g process;
' d i a ' i s t h e nominal s h a f t diameter i n
mn. The minimum f i l m t h i c k n e s s r a t i o (hmin/cr)
i s a s i g n i f i c a n t term when p r e -
d i c t i n g t h e power loss, o i l f l o w and maximum b e a r i n g temperature.
Using t h i s
r a t i o , p r e d i c t i o n c h a r t s i n nomograph f o r m have been d e v i s e d which p e r m i t q u i c k and a c c u r a t e d e t e r m i n a t i o n o f t h e v a r i o u s parameters w i t h v e r y l i t t l e calculation.
4.4.3.1
Minimum O i l F i l m Thickness
The f i l m t h i c k n e s s r a t i o may be o b t a i n e d f r o m Fig.13 f o r known o p e r a t i n g conditions.
B a s i c a l l y t h r e e g r i d s a r e used t o d e f i n e t h e problem, and by
l i n k i n g these a l o n g t h e a p p r o p r i a t e g u i d e l i n e s , as i n d i c a t e d by t h e arrows on the chart, a p o i n t i n the fourth,
'answer' g r i d i s o b t a i n e d .
T h i s c h a r t i s a l s o u s e f u l f o r p r e d i c t i n g t h e minimum f i l m t h i c k n e s s i n a b e a r i n g which has a d i f f e r e n t c l e a r a n c e t o t h e minimum v a l u e shown i n Fig.11, i e a b e a r i n g which cannot be c o n s i d e r e d on t h e s l i d e c h a r t s .
The a c c e p t a b i l i t y
o f any f i l m t h i c k n e s s can be checked on Fig.2.
4.4.3.2
Misalignment
Good a l i g n m e n t between s h a f t and b e a r i n g can be c r i t i c a l l y i m p o r t a n t because o f t h e d r a m a t i c r e d u c t i o n i n o i l f i l m t h i c k n e s s which misalignment causes. There a r e many ways i n which misalignment can o c c u r , f r o m poor b u i l d o f t h e machine t o mechanical d i s t o r t i o n s due t o l o a d o r temperature, and each o f them produces d i f f e r e n t c o n d i t i o n s w i t h i n t h e o i l f i l m .
However, a g e n e r a l guide t o
t h e d e r a t i n g e f f e c t o f misalignment i s g i v e n i n Fig.14.
T h i s shows t h e r e -
d u c t i o n i n o i l f i l m t h i c k n e s s from t h e p e r f e c t l y a l i g n e d case (Fig.13) g i v e n misalignment a c r o s s t h e l e n g t h o f t h e b e a r i n g (6).
for a
The r e s u l t i n g minimum
f i l m t h i c k n e s s , which o c c u r s a t one a x i a l end o f t h e b e a r i n g , s h o u l d be checked
on Fig.2 f o r a c c e p t a b i l i t y .
4.4.3.3
Power Loss and O i l Flow
Power l o s s and o i l f l o w may be determined f r o m Figs.15 and 16.
The method
o f use i s shown on each c h a r t and i s f u r t h e r i l l u s t r a t e d by t h e example i n s e c t i o n 4.6.The
f l o w g i v e n i s t h e b e a r i n g requirement f l o w , and should be
68
Fig.13
P r e d i c t i o n o f minimum o i l f i l m thickness
69 9’0
so E 7’0
5 L.
E‘O 2’0 L‘O
The d e r a t i n g e f f e c t o f mi sa l i gn men t on f i l m t h i c k n e s s
Fia.14
increased when d e t e r m i n i n g pump c a p a c i t y t o a l l o w f o r b e a r i n g wear, t o m u l t i - s u p p l i e d bearings etc.
unequal f l o w
T y p i c a l l y t h e f i g u r e s sho u ld be incre a se d by
20-25%, and a pump chosen which can a de qu at el y su ppl y t h i s amount.
4.4.3.4
Temperatures
The v a l u e o f t h e o i l o u t l e t temp era t u re may be e s t i m a t e d f r o m t h e power l o s s and o i l f l o w v al ues, as read f r o m t h e c h a r t s , u s i n g t h e e q u a t i o n : O u t l e t tem perature = i n l e t temperat ure
-
50
+
t e mpera t ure r i s e
+
A H/Q ( " C )
where A = 0 . 0 0 0 5 f o r H kW and Q m3/s
A = 5
f o r H hp and Q gal / mi n.
(A = 6 f o r hp and US gal /mi n) The maximum b e a r i n g temperature may be determined f r o m Fig.17;
ag a in an
example o f use i s shown on t h e c h a r t and i s f u r t h e r a m p l i f i e d i n s e c t i o n 4.6. I t s houl d be noted t h a t t h i s temp era t u re o c c u r s a t t h e s u r f a c e o f t h e l i n i n g m a t e r i a l and a t some c i r c u m f e r e n t i a l p o s i t i o n w hi ch i s n o t w e l l d e f i n e d ; r o u g h l y c o i n c i d e s w i t h t h e p o s i t i o n o f t h e minimum o i l f i l m t h i c k n e s s .
it
I0
POWER LOSS
UUD IN1 1 U 1
Fig.15
P r e d i c t i o n of b e a r i n g power loss
L FLOW REQUIREMENTS 70 :
1w: 200. OM (mn 500 400 3w
2w /
7000 10000 20000 30000
Fig.16
P r e d i c t i o n o f b e a r i n g o i l f l o w requirement
71
I
Fig.17
here
-
I
P r e d i c t i o n of maximum b e a r i n g temperature
72 Unless a great deal o f thought and care i s taken i n determining the p o s i t i o n o f temperature instrumentation then the recorded temperature w i l l be less than the a c t u a l maximum value.
4.5
H I G H SPEED APPLICATIONS The general t r e n d towards l a r g e r sizes and f a s t e r o p e r a t i n g speeds i n equip-
ment such as compressors and t u r b i n e s has caused d i f f i c u l t i e s w i t h both the design and the o p e r a t i o n o f conventional c y l i n d r i c a l bore bearings.
The design
d i f f i c u l t y occurs when the o i l f l o w w i t h i n the bearing clearance space becomes non-laminar,
and the information presented so f a r ceases t o be v a l i d .
The
operational d i f f i c u l t i e s concern p o s s i b l e i n s t a b i l i t y o f e i t h e r the o i l f i l m (see Sect ion 4.2.1.4)
4.5.1
or of the complete r o t o r system.
Non-Laminar Operation
C o l l o q u i a l l y i t i s common t o r e f e r t o e i t h e r laminar o r t u r b u l e n t o p e r a t i o n , but i n p r a c t i c e there i s a wide o p e r a t i n g band separating the two regimes i n which o t h e r forms o f f l u i d motion, eg Taylor v o r t i c e s , occur,
Since the design
methods d e t a i l e d i n the previous sections a r e f o r laminar operation o n l y i t i s necessary t o have some check on whether laminar o r non-laminar c o n d i t i o n s apply. Fig.18 i s a quick way o f performing t h i s check;
note t h a t i t uses the value o f
f i l m thickness r a t i o (hmin/cr) obtained from the laminar c h a r t Fig.13.
I f non-
laminar operation i s p r e d i c t e d then recourse must be made t o a l t e r n a t i v e design methods and procedures [ 1 5 ] . I t should be emphasised t h a t turbulence per se i s n o t a problem as f a r as safe bearing operation i s concerned, merely a d i f f i c u l t y ( a l b e i t a r e l a t i v e l y major one) w i t h the design process. l a r g e clearances o r t h i n o i l s .
Turbulence i s increased by h i g h speeds,
Bearings o p e r a t i n g i n the t u r b u l e n t regime can
have s i g n i f i c a n t l y higher power losses and temperatures than p r e d i c t e d by
I ami nar theory.
4.5.2
P r o f i l e Bore Bearings
A c y l i n d r i c a l bore bearing has a s i n g l e converging clearance space (which may i n some circumstances be i n t e r r u p t e d by a groove) i n which o i l f i l m pressure i s generated t o support the e x t e r n a l load.
I n c o n t r a s t the bore o f a p r o f i l e
bearing i s arranged so t h a t two o r more separate converging regions a r e present under normal operation, each developing f i l m pressures which a c t i n various d i r e c t i o n s around the bearing.
The s t a b i l i t y o f such a bearing i s b e t t e r than a
c y l i n d r i c a l bore, b u t t h e r e a r e u s u a l l y p e n a l t i e s t o be p a i d i n terms o f reduced load c a p a c i t y o r increased power loss and l u b r i c a n t f l o w requirement.
73
Fig.18
Guidance on t h e 1 i k e l ihood o f l a m i n a r o r non-laminar o p e r a t i o n
The range o f poss b l e p r o f i l e s a r e shown i n Fig.19,
i s almost i n f i n i t e b u t t h e more common forms
t o g e t h e r w i t h an i n d i c a t i o n o f t h e i r r e l a t i v e m e r i t s i n
terms o f v a r i o u s o p e r a t i n g parameters.
O f n e c e s s i t y these comparisons a r e
approximate o n l y , s i n c e minor changes t o e i t h e r o f t h e two c l e a r a n c e s i n v o l v e d i n p r o f i l e b o r e b e a r i n g s ( t h e so c a l l e d 'shake c l e a r a n c e ' which i s a measure o f p o s s i b l e s h a f t movement and t h e hydrodynamic c l e a r a n c e which i s t h e d i f f e r e n c e between r a d i i o f formance.
i n d i v i d u a l lobes and t h e s h a f t ) can s i g n i f i c a n t l y a l t e r p e r -
Experience has shown t h a t as a general r u l e t h e 'shake c l e a r a n c e ' ,
as a r a t i o ( i e c d / d ) ,
should be no t i g h t e r than 0.001 mm/mrn.
T h i s then a v o i d s
p o s s i b l e problems w i t h l o s s o f c l e a r a n c e on s t a r t - u p due t o d i f f e r e n t i a l
74 thermal expansion o f t h e s h a f t r e l a t i v e I n f o r m a t i o n on s t i f f n e s s and damping
o t h e b e a r i n g and housing.
s i m p o r t a n t when c o n s i d e r i n g t h e
dynamics o f t h e machine as a whole, w i t h s t i f f n e s s i n f l u e n c i n g t h e c r i t i c a l speeds o f t h e r o t o r , and damping c o n t r o l i n g t h e v i b r a t i o n a m p l i t u d e s when running through t h e c r i t i c a l s . In o r d e r t o g i v e a f e e l f o r i n s t a b i l i t y t h r e s h o l d speeds f o r r e a l i s t i c
clearances,
Fig.20,
has been prepared.
T h i s shows t h e minimum speed a t which
o i l f i l m i n s t a b i l i t y can o c c u r f o r a h o r i z o n t a l , w e i g h t loaded, r o t o r system ( i e where t h e b e a r i n g l o a d i s p u r e l y due t o t h e mass).
Fig.19
Comparison o f s t a t i c and dynamic c h a r a c t e r i s t i c s o f commonly used journal bearing types
15
Comprehensive and d e t a i l e d p r o c e d u r e s f o r t h e d e s i g n o f p r o f i l e b o r e b e a r i n g s have n o t been p u b l i s h e d , and c u r r e n t l y t h e d e s i g n e r must e i t h e r r e s o r t t o a f u n d a m e n t a l t h e o r e t i c a l s t u d y o r seek a d v i c e , u s u a l l y on a s p e c i f i c c a s e basis,
f r o m s p e c i a l i s t s [16].
IS0 VG L6 @, 50' INLET 1 MEDIUM TURBINE GRADE 1 b/d : 0.5
OIL:
LOO.
-
CLEARANCE VARIED TO MAINTAIN :
300-
DRAIN TEMPERATURE < 75'C MATERIAL TEMPERATURE < 100°C
W
LL
0
m
4LL 9m
200-
100-
LEMON BORE
0
F 9.20
4.6
10000 20000 30000 JOURNAL SPEED I revlmin 1
O i l f i l m whirl
-
LOO00
i n s t a b i l i t y of various types of journal bearings L i m i t i n g speeds f o r h o r i z o n t a l r o t o r s
EXAMPLE OF USE OF DESIGN AIDS
4.6.
Problem
A g e a r b o x b e a r i n g 200 mm d i a m e t e r and 50 mm l o n g h a s t o c a r r y a l o a d o f 20 kN a t 3600 r e v / m i n .
bearing length,
The b e a r i n g has t w o a x i a l g r o o v e s e x t e n d i n g 0.8 o f t h e
s i t u a t e d a t ? 90" t o t h e l o a d l i n e .
The o i l
i s w i t h i n t h e IS0
VG 32 s p e c i f i c a t i o n ( a c t u a l t y p i c a l v i s c o s i t y v a l u e s a r e g i v e n a s 30 c S t a t 40°C and 5.2 c S t a t l O O " C , temperature
t h e o i l d e n s i t y b e i n g 880 kg/m3).
The o i l f e e d
i s 50°C and t h e o i l f e e d p r e s s u r e 1 b a r (0.1 MPa).
Check t h a t t h i s b e a r i n g w i l l o p e r a t e s a f e l y and e s t i m a t e o i l f i l m t h i c k n e s s , power l o s s , o i l f l o w r e q u i r e m e n t , maximum b e a r i n g t e m p e r a t u r e and o i l o u t l e t temperature.
76 4.6.2
Procedure
T h i s p a r t i c u l a r e x a m p l e h a s been i n c o r p o r a t e d o n t h e d e s i g n a i d s a s dashed
1 ines.
4.6.2.1
Diametral Clearance t h e minimum d i a m e t r a l c l e a r a n c e , f o r a 200 mm d i a m e t e r b e a r i n g
From F i g . 1 1 ,
o p e r a t i n g a t 3600 r e v / m i n ,
4.6.2.2
Maximum S p e c i f i c Load R a t i n g t h e maximum s p e c i f i c l o a d r a t i n g ( b a s e d o n t h i n o i l f i l m
From F i g . 1 2 , limit),
i s 0.26 mm.
i s o b t a i n e d by f o l l o w i n g t h e g u i d e l i n e s , through t h e r e l e v a n t o i l grade,
l e n g t h , speed and d i a m e t e r .
The a l l o w a b l e l i m i t i s seen t o be 4.3
MPa, w e l l
above t h e a c t u a l s p e c i f i c l o a d o f 2 MPa.
4.6.2.3
Region o f Safe O p e r a t i o n
Use F i g . l O ( a ) , on g r i d ,
transparent copy, w i t h Fig.lO(b),
(200 mm).
Place transparency on backing sheet,
l i n i n g u p marked p o i n t w i t h
c a n t h e n e a s i l y be i n t e r p o l a t e d .
parency, F i g . l O ( b ) ,
Mark p o i n t
and d i a m e t e r
The l i m i t i n g l i n e f o r s a f e o p e r a t i o n , w i t h b / d
datum p o i n t ( s e e F i g . 2 l ( a ) ) . equal t o 0.25,
backing sheet.
c o r r e s p o n d i n g t o o i l g r a d e (30 c S t a t 40°C)
Fig.lO(a),
The t e m p e r a t u r e l i m i t t r a n s -
i s p o s i t i o n e d i n a n i d e n t i c a l manner ( s e e F i g . 2 1 ( b ) ) .
BACKING SHEET TRANSPARE,NT OVERLAY r - - - - - - --1
I
I I
I
I
I
I
I
I I
I I< 1;
b" /
o'/\ /'
'
THIN FILM LIMIT
i __----SPEED
I I
I I
I I J
I $1
HIGH
A\
I I I
SI
J
( a1
Fig.21
I
TEMPERATURE LIMITS
SPEED
Example use o f s l i d e c h a r t s
DESIGN POINT IS WITHIN LIMITS \
I
I
0
a
1 0
3600 rei/rnin
I SPEED
(C)
These t w o c o m b i n e d g i v e g u i d a n c e t o a r e g i o n o f s a f e o p e r a t i o n o n a l o a d , speed f r a m e w o r k a s shown i n F i g . Z l ( c ) . well within t h i s l i m i t i n g region,
4.6.2.4
The a c t u a l o p e r a t i n g c o n d i t i o n s a r e
i n d i c a t i n g a r e l i a b l e design.
P r e d i c t i o n o f Minimum O i l F i l m T h i c k n e s s
F i g . 1 3 p r e d i c t s minimum o i l f i l m t h i c k n e s s a n d i s v e r y e a s y t o u s e , a l t h o u g h looking f a i r l y formidable.
F i r s t mark p o i n t s on t h e f o l l o w i n g t h r e e g r i d s .
1) C l e a r a n c e r a t i o and speed (0.0013 and 3600 r e v / m i n )
2 ) l e n g t h t o d i a m e t e r r a t i o and o i l ( 0 . 2 5 a n d 30 c S t a t 40°C)
( 2 MPa a n d 30 c S t a t 40°C)
3) s p e c i f i c l o a d a n d o i l
J o i n t h e s e p o i n t s a s shown, f o l l o w i n g t h e g u i d e l i n e s , and c o n t i n u e t o t h e answer g r i d i n t h e t o p r i g h t hand c o r n e r ; ness r a t i o ( h m i n / c r )
Minimum o i l f i l m t h i c k n e s s hmin
4.2.6.5
a t b/d equal t o 0.25 t h e f i l m t h i c k -
i s r e a d o f f d i r e c t l y a s 0.16. =
0.16
x 0 . 1 3 = 0.021
mm
Check f o r L a m i n a r O p e r a t i o n
Fig.18 g i v e s guidance on whether l a m i n a r , as p r e s e n t e d h e r e , o r non-laminar d e s i g n methods [ l 5 ] s h o u l d be used.
In consistent units P = 2 . 1 0 6 N/m2
d
=
0.2rn,
N
I n F i g . 1 8 mark p o i n t where Y
=
-
and c i d = G . 0 0 1 3 d =
60 rev/s,
ti
=
880 kg/m3,
f r o m w h i c h Y = 48.74.
g=
1.76 and b / d
= 0.25
(on lower g r i d ) .
P r o j e c t v e r t i c a l l y upwards a s i n d i c a t e d b y dashed l i n e t o f i l m t h i c k n e s s r a t i o
o f 0.16.
The r e s u l t i n g p o i n t a p p e a r s i n t h e l a m i n a r r e g i o n ,
therefore the
design a i d s presented here a r e s u i t a b l e .
4.6.2.6
P r e d i c t i o n o f Power L o s s
On F i g . 1 5 ,
using the value of hmin/cr a l r e a d y obtained (0.16),
j o i n up t h e
a p p r o p r i a t e p o i n t s on t h e s c a l e s a s shown. Power l o s s = 5.3
4.6.2.7
kW
P r e d i c t i o n o f O i l Flow
On F i g . 1 6 ,
s t a r t i n g a t t h e s p e e d - c l e a r a n c e r a t i o g r i d , j o i n up t h e a p p r o p -
r i a t e p o i n t s a s shown.
O i l f l o w r e q u i r e m e n t = 0.00021 m 3 / s
(0.21
1/51
78 4.6.2.8
P r e d i c t i o n o f Maximum Bearing Temperature
From Fig.17,
t h e maximum b e a r i n g temperature i s o b t a i n e d by f o l l o w i n g t h e
rd
numbered steps s h o w n ~ i n t h e sketch i n t h e lower l e f t hand c o r n e r . Ca 1 c u 1a t e
t
lo3
]
(=o.oao47)
M u l t i p l y t h i s by t h e m o d i f i e d s p e c i f i c l o a d P
(=0.136)
S t a r t i n g a t hmin/cr equal t o 0.16 f o l l o w t h e g u i d e l i n e s as shown, through values o f b/d r a t i o , o i l grade, and t h e two terms c a l c u l a t e d above. e f f e c t i v e temperature ( O e ) , value o f
emax/ee
(=1.55)
A v a l u e of
can be o b t a i n e d a t t h e end o f s t e p 3 (=65"C), and a
f r o m s t e p 5 (which completes t h e pentagon).
Maximum temperature = l 0 l ' C
4.6.2.9
P r e d i c t i o n o f O i l O u t l e t Temperature
From e q u a t i o n (1)
-
Section
4 4.1.4.
O i l o u t l e t temperature = 50 + 0.0005
= 50
-HQ
+ 0.0005 x 5.3 0.00021
= 63°C
REFERENCES 1 2
3 4 5 6
7 8
9
10
11
Elwel1,R.C. and Booser,E.R., 'Low Speed L i m i t o f L u b r i c a t i o n P a r t 1, What i s a "Too Slow" B e a r i n g ? ' , Machine Design, 15 June, 1972. Martin,F.A., 'Minimum A l l o w a b l e O i l F i l m Thickness i n S t e a d i l y Loaded Journal B e a r i n g s ' , Proc. L u b r i c a t i o n and Wear Convention 1964 ( I n s t . Mech. Engrs. London), V01.178, P t . 3 N pp. 161-167. Booser,E.R., Ryman,F.D. and Linkinhoker,C.L., 'Maximum Temperature f o r Hydrodynamic Bearings under Steady L o a d ' , ASLE Trans. 1970, Vol.26, No.7. T r i b o l o g y Handbook, 1973, S e c t i o n B1, B u t t e r w o r t h s . Lund,J.W., ' S e l f - e x c i t e d S t a t i o n a r y W h i r l O r b i t s o f a Journal i n a Sleeve B e a r i n g ' , Thesis, 1966, Rensselaer P o l y t e c h n i c I n s t i t u t e , N Y . Pinkus,O. and S t e r n l i c h t , B . , 'Theory o f Hydrodynamic L u b r i c a t i o n ' , 1961, Chapter 4, McGraw-Hi 11. Cameron ,A. ' P r i n c i p l e s o f L u b r i c a t i o n I , 1966, Chapter 13, Longmans. Lund,J.W. and Thomsen,K.K., 'A C a l c u l a t i o n Method and Data f o r t h e Dynamic C o e f f i c i e n t s o f O i l - L u b r i c a t e d J o u r n a l B e a r i n a s ' f r o m 'TODiCS i n F l u i d F i l m Bearings and Rotor B e a r i n g System Designland O p t i m i z a t i o n ' , 1978, ASME. ' C a l c u l a t i o n Methods f o r S t e a d i l y Loaded Pressure Fed Hydrodynamic Journal B e a r i n g s ' , Engineering Sciences Data U n i t Item 66023, Sept. 1966 ( I n s t . Mech. Engrs., London). 'Computer S e r v i c e f o r P r e d i c t i o n o f Performance o f S t e a d i l y Loaded Pressure Fed Hydrodynamic Journal B e a r i n g s ' , Engineering Sciences Data U n i t , Item 69002, Sept. 1972, Amendment B. Martin,F.A. and Garner,D.R., ' P l a i n Journal Bearings under Steady Loads Design Guidance f o r Safe O p e r a t i o n ' , F i r s t European T r i b o l o g y Congress, 1973 ( I n s t . Mech. Engrs., London) paper C313/73.
79 12
13 14 15
16
Blok,H., D i s c u s s i o n t o Conference. Proc. Conf. L u b r i c a t i o n and \.!ear, 1957 ( I n s t . Mech. Engrs., London), pp. 745-747. Moes,H. and Bosma,R., ' D e s i g n C h a r t s f o r Optimum B e a r i n g C o n f i g u r a t i o n 1 . The F u l l J o u r n a l B e a r i n g ' A p r i l 1971, ASME Trans. Vol.93, S e r i e s F, No.2, pp. 302-306. Brown,T.W.F. and Newman,A.O., 'High-speed H i g h l y Loaded B e a r i n g s and T h e i r Development', Proc. Conference on L u b r i c a t i o n and Wear, 1957 ( I n s t . Mech. E n g r s ) , pp. 20-27. Garner,D.R., Jone5,G.J. and Martin,F.A. ' T u r b u l e n t J o u r n a l B e a r i n g s Design C h a r t s f o r Performance P r e d i c t i o n ' , J u l y 1977, ASLE Trans. V01.20, No.3, pp. 221-232. Garner,O.R., Lee,C.S, and Martin,F.A., ' S t a b i l i t y o f P r o f i l e Bore B e a r i n g s : I n f l u e n c e o f B e a r i n g Type S e l e c t i o n ' , October 1980, T r i b o l o g y I n t e r n a t i o n a l , pp. 204-210.
80
5 R.W.
THE DIAGNOSIS
WILSON and E . B .
S h e l l Research L t d . ,
5.1
OF PLAIN BEARING FAILURES
SHONE Thornton Research Centre, P.O.
Box 1, C h e s t e r CH1 3SH
I NTRODUCT I ON
A f e w y e a r s ago t h e c a s u a l o b s e r v e r m i a h t have c o n c l u d e d t h a t b o t h t h e m e t a l l u r g y o f b e a r i n g s a n d t h e u n d e r s t a n d i n g o f t h e i r f a i l u r e mechanisms h a d r e a c h e d a s e t t l e d s t a g e i n w h i c h b e a r i n g m a t e r i a l s were a v a i l a b l e t o meet a l m o s t every requirement and t h a t c h a r a c t e r i s t i c f a i l u r e s were well-documented.
Thus,
most b e a r i n g f a i l u r e s c o u l d be d e s c r i b e d q u i t e a d e q u a t e l y f r o m a s t r i c t l y metallurgical o r s t r i c t l y engineerins viewpoint. only i n passing;
The l u b r i c a n t was m e n t i o n e d
a s l o n g a s i t had t h e r i q h t v i s c o s i t y , p r o v i d e d a d e q u a t e
c o o l i n g and d i d n o t c o r r o d e t h e b e a r i n o a l l o y ,
i t gave l i t t l e c a u s e f o r c o n c e r n .
O p e r a t o r s f r e q u e n t l y blamed l u b r i c a n t s f o r b e a r i n q f a i l u r e s , b u t b e a r i n g manu f a c t u r e r s a c k n o w l e d g e d t h a t v e r y f e w f a i l u r e s c o u l d be a t t r i b u t e d t o i n f e r i o r lubricants.
T h e r e was n o s p e c i a l p r e s s u r e o n l u b r i c a t i o n t e c h n o l o q i s t s t o
become e x p e r t s o n b e a r i n g f a i l u r e s ;
i n any case, b e a r i n g manufacturers g e n e r a l l y
provided e x c e l l e n t t e c h n i c a l s e r v i c e t o t h e i r customers.
However, e n g i n e e r i n g
d e v e l o p m e n t s i n r e c e n t y e a r s have made i n c r e a s i n g l y s e v e r e demands o n p l a i n b e a r i n g s and have g i v e n r i s e t o r a t h e r more c o m p l i c a t e d f a i l u r e mechanisms. Many o f t h e s e f a i l u r e s c a n be d e s c r i b e d a s t r i b o l o g i c a l f a i l u r e s ;
they cannot
be a s c r i b e d t o a p a r t i c u l a r d e f e c t i n t h e d e s i g n o r i n t h e m e t a l l u r g y o f t h e bearing or t o a shortcoming i n t h e l u b r i c a n t
-
t h e y a r e t h e consequence of
the
i n t e r a c t i o n o f a number o f f a c t o r s . Before i t i s p o s s i b l e t o t a l k about p l a i n bearing f a i l u r e s
it i s essential t o
have an u n d e r s t a n d i n g o f t h e m a t e r i a l s u s e d , how t h e y a r e made a n d t h e r e q u i r e ments t h a t must be met.
T h e r e a r e a number o f v e r y good p u b l i c a t i o n s d e s c r i b i n g
t h e c o n s t r u c t i o n and p r o p e r t i e s of p l a i n b e a r i n g s and we i n t e n d t o b r i e f l y summarise t h e s e .
5.2
PROPERTIES R E Q U I R E D OF BEARING MATERIALS
I t i s e v i d e n t t h a t a b e a r i n g must w i t h s t a n d a v a r i e t y o f imposed c o n d i t i o n s . No b e a r i n g i s e q u a l l y good w i t h r e s p e c t t o a l l r e q u i r e m e n t s ,
and t h e s e l e c t i o n
o f t h e most s u i t a b l e b e a r i n g m a t e r i a l f o r a p a r t i c u l a r s e t o f circumstances r e q u i r e s a c a r e f u l e v a l u a t i o n o f t h e most c r i t i c a l o p e r a t i n g f a c t o r s . t h e f a c t o r s t h a t h a v e t o be c o n s i d e r e d a r e o u t l i n e d i n S e c t i o n s 5.2.1
Some o f
-
5.2.8.
81 5.2.1
Fatigue Resistance
This is the most important property in those applications where the load varies.
However, fatigue failure in bearings is frequently associated with some
other factor, such a s corrosion, which reduces the strength o f the bearing, or wear, which reduces the load-carrying area.
The fatigue strength of a fund-
amentally weak bearing alloy can be increased by making the bearing alloy thin and bonding i t firmly to a bronze or steel backing. 5.2.2
Compressive Strength
This is the steady load that the bearing alloy can support without extruding. There is little correlation between compressive strength and fatigue resistance when a single class o f bearing material is considered.
5.2.3
Conformability
This is the ability t o compensate for misalignment that occurs a s a consequence o f bad design or manufacture or that may develop in service.
5.2.4
Embeddabil ity
This i s the ability to tolerate and absorb foreign particles, thereby avoiding scoring or wear.
5.2.5
Strength at Elevated Temperatures
High sliding speeds and heavy loads can generate considerable heat, even when a bearing is operating hydrodynamically.
One of the main functions of the lub-
ricant and the bearing alloy i s to conduct heat away from the sliding surfaces; even so, bearings are often required to operate at elevated temperatures, and a lack o f high-temperature strength may result in extrusion of the bearing alloy and/or fatigue failure.
5.2.6 Compa t i b i 1 i ty All bearing assemblies experience some metal-to-metal contact at some stage i n their lives;
the resistance of the bearing metal/journal combination to
seizure is therefore important.
However some of the harder bearing alloys do
not function satisfactorily against unhardened steel journals.
5.2.7
Corrosion Resistance
Bearings may be exposed to weak organic acids formed a s a result of the oxidation of lubricating oils in service.
They may also be subject to corrosion
by weak organic acids and strong mineral acids derived from fuel combustion products.
Sometimes the operating environment is corrosive
chemical plants and marine installations.
-
for example. on many
a2
cost
5.2.8
I t i s e s s e n t i a l t h a t b e a r i n g a l l o y s a r e cheap and easy t o manufacture.
TYPE, CONSTRUCTION AN0 CHARACTERISTICS OF PLAIN BEARING MATERIALS
5.3
Some o f t h e more w i d e l y used b e a r i n g a l l o y s a r e l i s t e d i n Table 5.1;
the
more commonly used o v e r l a y s a r e d e s c r i b e d i n Table 5.2.
-
Sections 5.3.1
5.3.7
o u t l i n e t h e methods by which b e a r i n g s a r e u s u a l l y
manufactured f r o m these a l l o y s and d e s c r i b e t h e c h a r a c t e r i s t i c s o f t h e v a r i o u s bearing materials.
The c h a r a c t e r i s t i c s o f t h e v a r i o u s b e a r i n g m a t e r i a l s a r e
summarised i n Table 5.3 and t h e g e n e r a l p r o p e r t i e s b r i e f l y discussed i n Section 5.3.8.
5.3.1
White M e t a l s ( B a b b i t s )
These a r e t i n - b a s e d o r lead-based a l l o y s , t h e o r i g i n a l t i n - b a s e d w h i t e metal being invented by Isaac B a b b i t t i n 1839. t h e manufacture o f b o t h t h i c k ,
large,
T h i s a l l o y i s w i d e l y used today f o r
i n d i v i d u a l l y manufactured b e a r i n g s and
t h i n - w a l l e d b e a r i n g s made f r o m c o n t i n u o u s s t r i p . The coated s t r i p i s c u t i n t o s e c t i o n s o f a s u i t a b l e s i z e which a r e p r e s s Newer m a n u f a c t u r i n g techniques p e r m i t t h e use of b e a r i n g
formed t o shape.
metal combinations and c o n t r o l l e d m i c r o s t r u c t u r e s t h a t c o u l d n o t o t h e r w i s e be produced. Another i n n o v a t i o n i s t h e three-component
(or tri-metal)
bearing,
i n which
one o f t h e harder b e a r i n g m a t e r i a l s such as bronze, a l r e a d y bonded t o a s t e e l backing,
i s covered w i t h a t h i n (20-200 urn), e l e c t r o d e p o s i t e d o r c a s t o v e r l a y
o f white metal. The t i n - b a s e d a l l o y c o n s i s t s of a t i n - r i c h m a t r i x , w i t h some antimony and copper i n s o l i d s o l u t i o n ; needles o f Cu6Sn5.
d i s p e r s e d i n t h e m a t r i x a r e cuboids o f SbSn and
T h i s m i c r o s t r u c t u r e l e d t o t h e erroneous b e l i e f t h a t
b e a r i n g a l l o y s must c o n s i s t o f h a r d c r y s t a l s d i s p e r s e d i n a s o f t m a t r i x .
In
f a c t , t h e i n t e r m e t a l l i c compounds i n w h i t e m e t a l s appear t o serve no purpose o t h e r than t o s t r e n g t h e n t h e a l l o y s . White m e t a l s a r e o u t s t a n d i n g l y good b e a r i n g a l l o y s i n many r e s p e c t s , t h e i r main d e f e c t b e i n g l a c k o f l o a d - c a r r y i n g c a p a c i t y , p a r t i c u l a r l y a t e l e v a t e d temperatures.
The c o r r o s i o n r e s i s t a n c e o f lead-based w h i t e m e t a l s i s i n f e r i o r
t o t h a t o f t i n - b a s e d w h i t e metals;
nevertheless,
w i d e l y used, p a r t i c u l a r l y i n t h e U.S.A.
lead-based a l l o y s a r e q u i t e
TABLE 5.1 Some l u b r i c a t e d b e a r i n g m a t e r i a l s Bearing m a t e r i a l
Major a l l o y i n a elements
1. Lead-based w h i t e metal ( lead-Babb i t t )
8-16% antimony 5-11% t i n
Lead
2. Tin-based w h i t e metal ( t i n Babbitt)
7-14% a n t imony 3- 9% copper
Tin
3. S i n t e r e d c o p p e r - l e a d 4. Cast c o p p e r - l e a d
20-50% l e a d
Copper
20-30% l e a d
Copper
5. Lead-bronze
20-30% l e a d 3- 5% t i n
Copper
6. Aluminium-low t i n
7. Aluminium-high t i n 8. A l uminium-si 1 i c o n
6% t i n , 1.5% s i l i c o n 20% t i n ,
1.0% copper
11% s i l i c o n ,
1 % copper
A l umi n i um
A l umi n i um
A l umin i um
(overlay plated) 9. A l umin i um-Babb i t t
10% lead, 2% t i n
A l umi n ium
8-12% t i n , 0.2-1.0% phosphorous
Copper
11. S i l icon-bronze
1.5-4% s i l i c o n
Copper
12. Lead-bronzes ( g e n e r a l l y s t e e l -backed)
5-10% t i n 8-12% l e a d
Copper
10. Phosphor-bronze ( c a s t )
13. S i l v e r
14. Porous bronze
8-12% t i n
15. B r o n z e - g r a p h i t e composites
5-40% g r a p h i t e
16. Laminated r e s i n s
F i b r e - r e i n f o r c e d phenol i c s o r epoxies
17. PTFE and f i 1 l e r i n
PTFE and l e a d
Metal
Bronze and g r a p h i t e
PTFE
fillers 19. Metal-backed thermoplastics
Nylon o r p o l y a c e t a l
G e n e r a l l y used w i t h 90% lead-10% t i n o r 96% lead-4% i n d i u m e l e c t r o d e p o s i t e d o v e r l a y , 25 urn t h i c k ; l e a d bronzes can a l s o have t h i n - B a b b i t t o v e r l a y s
1 1
Hardened j o u r n a l s p r e f e r r e d
S t i l l under development
P r i ma r i 1 y bush i ng mate r ia 1
E l e c t r o p l a t e d , 2 5 0 - 5 0 0 ~ 1t h i c k , o f t e n used w i t h l e a d - i n d i u m o v e r l a y Self-lubricating,
T i n , bronze
impregnated w i t h o i l
Can be used d r y Usually water-lubricated
metal m a t r i x
18. PTFE m a t r i x and metal
Spec i a 1 f e a t u r e s
Remainder
Good under c o n d i t i o n s o f m a r g i n a l lubrication
W
TABLE 5.2 Commonly used overlays Overlay type
Special features
Major alloying elements
~
~~
A
lead-tin, electroplated
8-12% tin
Simultaneously precision-plated, about inch thick (25 urn)
0.001
B
C
D
Lead-tin-copper, electroplated Lead-indium, electroplated
Lead-Babbitt, cast Tin-Babbitt, cast
E Satco alloy, cast
8 - 1 2 % tin 1% copper
4-8% indium
Antimony-tin Antimony-copper Lead, with 2% tin and 0.5% calcium
Simultaneously precision-plated, about inch thick (25 urn)
0.001
Precision-plated, first lead then indium, then alloyed by diffusion; 0.001 inch thick (25 urn) 0.005-0.015
inch thick (125-375
about thick)
TABLE 5 . 3 C h a r a c t e r i s t i c s o f b e a r i n g rnater i a l s M a t e r i a l (see Table 1)
Relative load-carrying capacity MN/m2 Ib/in2
Embeddabil i t y S e i z u r e resistance (tolerance for dirt)
Maximum operating temp. "C
Tolerance f o r Corrosion r e s i s t a n c e m i s a l i g n m e n t Organic a c i d s M i n e r a l a c i d s
1
2000
14
Excellent
Very good
130
Very good
Moderate
Fair
2
2000
14
Excellent
Very good
130
Very good
Excel l e n t
Very good
3
3000-5000
20-35
4
4000-6000
28-41
5 6
5000-8000
35-55
7 8
Moderate with-Moderate w i t h out overlay; out overlay; good w i t h good w i t h o v e r Iay o v e r I ay
150
Good
Fair
160
Moderate
Poor
170
Fair
Fa i r
Poor
Fair
180
Poor
Good
Poor Fa i r
8000
55
5000
35
Poor
Fa i r
170
Fa i r
Good
Fa i r
8000
55
Poor
Moderate
180
Fa i r
Good
Moderate
9
5000
35
Poor ?
Moderate
160
Fa i r
Poor ?
Fair ?
10
8000+
55+
Poor
Moderate
220
Fa i r
Fa i r
Fa i r
11
8000+
55+
Poor
Fa i r
220
Poor
Moderate
Fa i r
12
6000
41
Moderate
Moderate
180
Moderate
Fa i r
Moderate
13
8000+
55+
Poor
Poor
180
Poor
Good e x c e p t f o r sulphur
Moderate
130
Fa i r
Poor
Fa i r
Withstands Fair occasional very h i g h temp.
Fa i r
Fa i r
14
2000-4000
15
Very v a r i a b l e
16
High
14-28
Moderate
Good Good
Moderate
Good
100-300
Moderate
Good
Good Good
Good Good
Good Good Good
Good
Good
Very Good
depending on resin
17 18 19
Comparat i v e l y poor; depends on o p e r a t i n g conditions
Fair
Very Good
Fa i r
Very Good
Fair
Good
250 250 100-350 depending on
Spec i a 1 features
1
Resistance t o organic acids g r e a t 1 y improved by o v e r l a y
Excellent for high loads a t low speeds
I
Some p l a s t i c s a r e damaged by t h e s o l v e n t a c t i o n of 1 ubricants
86 5.3.2
Copper-lead A l l o y s
These can be regarded as t h e f i r s t o f t h e modern a l l o y s .
They a r e blanked
from c o n t i n u o u s l y c o a t e d s t e e l s t r i p , t h e b e a r i n g a l l o y b e i n g a p p l i e d e i t h e r by c a s t i n g or by s i n t e r i n g . the l i q u i d s t a t e , and,
Lead and copper a r e i m m i s c i b l e i n b o t h t h e s o l i d and
i n o r d e r t o a v o i d complete s e g r e g a t i o n when c a s t i n g , t h e
moving s t e e l s t r i p i s c h i l l e d immediately a f t e r c a s t i n g by a w a t e r spray on t h e underside.
T h i s r e s u l t s i n a pronounced d e n d r i t i c s t r u c t u r e , w i t h c r y s t a l s o f
copper and l e a d normal t o t h e s t e e l backing and t h e b e a r i n g s u r f a c e . I n t h e s i n t e r i n g process t h e copper-lead powder i s compacted a f t e r a p p l i c a t i o n and then s i n t e r e d .
T h i s method o f manufacture g i v e s good c o n t r o l o f l e a d d i s t -
r i b u t i o n and r e s u l t s i n a homogeneous and e q u i a x i a l s t r u c t u r e . Both types o f copper-lead b e a r i n g a r e s a i d t o r e l y on a t h i n , e x t r u d e d s u r f a c e f i l m o f l e a d f o r s a t i s f a c t o r y performance.
I n many ways they a r e
s i m i l a r , b u t t h e d i f f e r e n t methods o f manufacture g i v e r i s e t o i m p o r t a n t d i f f erences i n performance i n c e r t a i n r e s p e c t s .
I n t h e c a s t a l l o y t h e l o n g copper
d e n d r i t e s enhance t h e l o a d - c a r r y i n g c a p a c i t y and f a c i l i t a t e h e a t f l o w away from t h e b e a r i n g surface. distribution.
I t i s , however, d i f f i c u l t t o o b t a i n a s a t i s f a c t o r y l e a d
On t h e o t h e r hand,
i t i s much e a s i e r t o ensure a u n i f o r m l e a d
d i s t r i b u t i o n by s i n t e r i n g techniques, and t h e s i n t e r e d a l l o y i s t h e r e f o r e less susceptible t o corrosion. Copper-lead b e a r i n g s have g r e a t e r s t r e n g t h and b e t t e r h i g h - t e m p e r a t u r e performance than w h i t e m e t a l s , b u t i n most o t h e r r e s p e c t s t h e y a r e i n f e r i o r . I n p a r t i c u l a r , they a r e s u s c e p t i b l e t o c o r r o s i o n , cannot t o l e r a t e as much d i r t and g e n e r a l l y r e q u i r e hardened j o u r n a l s .
I n o r d e r t o overcome these d i s a d -
vantages, copper-lead b e a r i n g s a r e g e n e r a l l y o v e r l a y - p l a t e d , e l e c t r o d e p o s i t o f a 90% l e a d
-
applied t o the bearing surface. plated;
1 0 % t i n o r a 95% l e a d
-
25 urn)
t h e a l l o y i s co-
t h e l e a d i s p l a t e d f i r s t and then t h e
indium, which i s f i n a l l y d i f f u s e d i n t o t h e lead. o v e r l a y i s merely a r u n n i n g - i n a i d .
(%
5% indium a l l o y b e i n g
I n t h e case o f l e a d - t i n ,
i n t h e case o f lead-indium,
a thin
It i s often stated that t h i s
T h i s i s n o t t h e case;
expected t o l a s t t h e l i f e o f t h e b e a r i n g .
the overlay i s
It provides a seizure-resistant
surface, a l l o w s s o f t s h a f t s t o be used, increases t h e a b i l i t y o f t h e b e a r i n g t o absorb d i r t and combats c o r r o s i o n o f t h e pure l e a d i n t h e u n d e r l y i n g copperlead.
Since i t i s so t h i n i t d e r i v e s c o n s i d e r a b l e s u p p o r t f r o m t h e u n d e r l y i n g
b e a r i n g a l l o y , and t h e f a t i g u e s t r e n g t h o f t h e composite i s h a r d l y impaired.
5.3.3
Lead Bronzes
The h i g h - t e m p e r a t u r e performance and l o a d - c a r r y i n g c a p a c i t y o f copper-lead a l l o y s can be improved by t i n a d d i t i o n s . copper, t h e r e b y s t r e n g t h e n i n g t h e b e a r i n g ; subject t o corrosion.
The t i n d i s s o l v e s c o m p l e t e l y i n t h e t h e l e a d remains u n a l l o y e d and
L i k e copper-lead b e a r i n g s , lead-bronze b e a r i n g s can be
c a s t o r s i n t e r e d , and show t h e same disadvantages.
L a r g e r - s i z e b e a r i n g s may have a t h i n l a y e r (100-200 pin)
usually overlay-plated.
o f tin-based w h i t e metal c a s t on them; micro-Babbitt o r tri-metal This m i c r o - B a b b i t t
For t h i s reason they a r e
such b e a r i n g s a r e sometimes c a l l e d
bearings.
l a y e r should always be separated from t h e lead-bronze by
a d i f f u s i o n b a r r i e r , o t h e r w i s e t h e bronze w i l l a l l o y w i t h t h e t i n - r i c h B a b b i t t t o form a hard,
i n t e r m e t a l l i c phase which can damage t h e s h a f t .
Lead-bronzes a r e o f t e n used f o r l i t t l e - e n d bushes i n p i s t o n s f o r t u r b o charger bearings.
5.3.4
Aluminium A l l o y s
Two a l l o y s a r e i n widespread use, one c o n t a i n i n g about 6% t i n and t h e o t h e r 20% t i n .
The 6% t i n a l l o y may be used i n massive f o r m o r bonded t o s t e e l , b u t ,
unless i t i s o v e r l a y - p l a t e d ,
i t r e q u i r e s hardened j o u r n a l s i f wear i s t o be
kept w i t h i n a c c e p t a b l e l i m i t s .
The 20% t i n a l l o y i s a more r e c e n t development
and i t s method o f manufacture p r o v i d e s a good example o f t h e advanced t e c h n o l ogy a p p l i e d t o b e a r i n g manufacture. H i g h - t i n aluminium a l l o y s , as c a s t , have v e r y poor mechanical p r o p e r t i e s , because t h e t i n forms a c o n t i n u o u s network e n c l o s i n g t h e p r i m a r y aluminium crystals.
By c o l d - w o r k i n g and low-temperature h e a t t r e a t m e n t t h i s continuous
phase can be broken up t o produce an i n t e r l o c k i n g network s t r u c t u r e ,
t h e so-
c a l l e d r e t i c u l a r s t r u c t u r e which has g r e a t l y improved mechanical p r o p e r t i e s . However, t h e a l l o y i s s t i l l i n s t r i p form and must be bonded t o a t h i n , s t e e l s t r i p backing.
T h i s i s achieved by a c o n t i n u o u s p r e s s u r e - w e l d i n g o p e r a t i o n
c a r r i e d o u t between r o l l e r s , t h e bonding between t h e two s t r i p s b e i n g promoted by a v e r y t h i n sheet o f p u r e aluminium. T h i s t y p e o f b e a r i n g has a h i g h e r l o a d - c a r r y i n g c a p a c i t y than copper-lead a l l o y s and y e t can be used i n c o n j u n c t i o n w i t h s o f t j o u r n a l s .
I t s tolerance
f o r d i r t i s n o t good and i t i s o c c a s i o n a l l y o v e r l a y - p l a t e d . The l a t e s t development i n aluminium b e a r i n g s i s t h e a l u m i n i u m - B a b b i t t a l l o y . A t l e a s t t h r e e s u p p l i e r s a r e known t o be a c t i v e i n t h i s f i e l d , each u s i n g a
d i s t i n c t l y d i f f e r e n t m a n u f a c t u r i n g technique.
I n one case t h e method o f manuf-
a c t u r e i s n o t u n l i k e t h a t used f o r t h e 20% t i n - 8 0 % aluminium b e a r i n g s . another, a c o n t i n u o u s s i n t e r i n g process i s used.
In
The B a b b i t t a l l o y i s about
90% lead-10% t i n , and s i n c e t i n i s o n l y s p a r i n g l y s o l u b l e i n aluminium, t h e t i n remains i n s o l u t i o n i n t h e lead.
T h i s means t h a t t h e l e a d - r i c h phase ( t h e
B a b b i t t ) should remain c o r r o s i o n - r e s i s t a n t copper-lead and lead-bronze a l l o y s .
-
i n contrast t o the s i t u a t i o n w i t h
88
5.3.5
Phosphor and S i l i c o n Bronzes
These a l l o y s a r e c a s t as e i t h e r i n d i v i d u a l b e a r i n g s o r tubes from which be a r in g s can be machined.
I n r e c e n t yea rs, c e n t r i f u g a l and semi-continuous
c a s t i n g procedures have been used.
M a t e r i a l s o f t h i s t y p e a r e used m a i n l y f o r
bushes, p a r t i c u l a r l y l i t t l e - e n d bushes, where heavy loads and h i g h temperatures a r e encountered.
The presence o f phosphorus a t about t h e 0.5% l e v e l has a
marked e f f e c t on r e s i s t a n c e t o pounding,
i.e.
the a l l o y has o u t s t a n d i n g r e s -
i s t a n c e t o wear when s u b j e c t t o heavy l oa ds a t low s l i d i n g speeds.
5.3.6
Silver
S i l v e r b e a r in gs, sometimes p l a i n , sometimes w i t h a t h i n o v e r l a y o f leadindium, have been adopted by t h e a i r c r a f t i n d u s t r y and a r e used on one w e l l known make o f d i e s e l engine. and i s about 0.5 mm t h i c k .
The s i l v e r i s e l e c t r o d e p o s i t e d on a s t e e l backing E l e c t r o d e p o s i t e d s i l v e r i s much harder than c a s t
s i l v e r , and s i l v e r b e a r i n g s a r e un eq ua l l ed w i t h respect t o l o a d - c a r r y i n g capa c i t y and f a t i g u e r e s i s t a n c e .
U n f o r t u n a t e l y , they a r e prone t o s e i z u r e and a r e
v e r y s e n s i t i v e t o t h e n a t u r e o f t h e l u b r i c a n t and t o c e r t a i n l u b r i c a t i n g - o i l add i t ives .
5.3.7
Porous and S e l f - L u b r i c a t i n g Bearings
Most porous b e a r i n g s c o n s i s t o f s i n t e r e d bronze o r i r o n powders w i t h i n t e r c on n e c t in g pores. operation,
These p ore s may t ake up 10-30% o f t h e t o t a l volume and, i n
l u b r i c a t i n g o i l i s s t o r e d i n them and i s subsequently f e d t o t h e
bearing surface. i l l a r y action.
Any o i l e scap i ng f ro m t h e loaded zone i s reabsorbed by cap-
A t y p i c a l bronze c o n t a i n s 90% copper and 10% t i n ;
may be added t o t h e mi x t o enhance t h e s e l f - l u b r i c a t i n g p r o p e r t i e s . p o r o s i t y and h i g h l u b r i c a t i n g - o i l
1-4% g r a p h i t e H igh
c o n t e n t a r e r e q u i r e d f o r high-speed,
light-
l oa d a p p l i c a t i o n s , whereas a l o w - p o r o s i t y m a t e r i a l w i t h a h i g h g r a p h i t e c o n t e n t i s b e t t e r f o r o s c i l l a t o r y and r e c i p r o c a t i n g movement, where i t i s d i f f i c u l t t o e s t a b l i s h an o i l f i l m .
5.3.8
Dis c u s s io n on M e t a l l i c Be ari n g M a t e r i a l s
No b e a r i n g m a t e r i a l combines a l l t he d e s i r a b l e p r o p e r t i e s .
However, t h e
i m p o r ta n t p r o p e r t i e s o f b e a r i n g s can be grouped i n two main c a t e g o r i e s . includes surface c h a r a c t e r i s t i c s ,
One
such as wear r e s i s t a n c e , j o u r n a l c o m p a t a b i l i t y ,
c o n f o r m a b i l i t y and e m b e d d a b i l i t y , t h e o t h e r i n c l u d e s mechanical p r o p e r t i e s , such a f a t i g u e s t r e n g t h and l o a d - c a r r y i n g c a p a c i t y . are divergent,
Since these two c a t e g o r i e s
t o have optimum s u r f a c e c h a r a c t e r i s t i c s s t r e n g t h must be sac-
r i f i c e d and v i c e versa.
However, by u s i n g modern manufacturing methods,
it i s
g e n e r a l l y p o s s i b l e t o a r r i v e a t a good compromise s o l u t i o n as, f o r example, the case o f t r i - m e t a l
b ea ri ng s.
in
89 There i s n o c l e a r u nd erst an di n g o f t h e way i n which b e a r i n g a l l o y s f u n c t i o n , and general t h e o r i e s r e g a r d i n g t h e i r b eh avi o ur can almost always be d i s c r e d i t e d by r e f e r e n c e t o some p a r t i c u l a r b ea ri ng . p r a c t i c e has always been ahead o f t he ory.
In the formulation o f bearing a l l o y s , T h i s i s n o t t o say t h a t b e a r i n g
design and manufacture i s a backward i n d u s t r y ;
i n f a c t , t h e manufacturing
techniques d e s c r i b e d p rove t h e o p p o s i t e , and b e a r i n g manufacturers can p r o v i d e s a t i s f a c t o r y s o l u t i o n s t o a l most any b e a r i n g problem.
N evertheless, many
be a r in g s s t i l l f a i l pre mat ure l y i n s e r v i c e and t h e r e i s a g r e a t need t o t e l l designers about t h e b e a r i n g m a t e r i a l s a l r e a d y a v a i l a b l e o r under development.
5.4
BEARING FAILURES
-
S e c tio n s 5.1
5.3 have p r o v i d e d t h e reader w i t h a b r i e f i n t r o d u c t i o n t o the
s u b je c t, and i t i s now p o s s i b l e t o d e s c r i b e and d i s c u s s some o f t h e l i k e l y causes o f b e a r i n g f a i l u r e s , g i v i n g p a r t i c u l a r emphasis t o t h e i d e n t i f i c a t i o n o f factors that give r i s e t o failures.
5.4.1
M e t a l l u r g i c a l Def ect s i n New Be ari n gs
Modern p l a i n b e a r i n g s can be v e r y complex c o n s t r u c t i o n s and much may go wrong d u r i n g t h e i r manufacture.
Neve rt h el e ss, v e r y few d e f e c t i v e b e a r i n g s e n t e r
s e r v i c e , s i n c e t h e ma j or b e a r i n g man uf act ure r s m a i n t a i n a h i g h standard o f quality control. bea r in g s , e.g.
Def ect s a r e more l i k e l y t o occur on i n d i v i d u a l l y manufactured
l a r g e , whi t e-me t a l b e a r i n g s , t han on mass-produced bearings o f
the c o p p e r - le a d o r a l u m i n i u m - t i n described i n S e ct i on s 5.4.1.1
5.4.1.1
-
varieties.
D efects t h a t may be encountered a r e
5.4.1.5.
Bad Bonding
Bonding i s s t i l l a ma j or problem on l a r g e , w hite-metal bearings, a l t h o u g h poor bonds a r e e xt re mel y r a r e on o t h e r t ype s o f bearing. white-metal
Bearing s h e l l s f o r
b e a r i n g s must be t i n n e d b e f o r e t h e w h i t e metal i s c a s t i n place,
and t h e te m p e r a tu re o f b o t h t h e s h e l l and t h e w h i t e metal must be c a r e f u l l y controlled.
A m e t a l l u r g i c a l bond between t h e b e a r i n g and t h e s h e l l strengthens
the w h i t e metal and f a c i l i t a t e s t he f l o w o f heat away from the b e a r i n g surface. In t h e p a s t , some "wh i t e m e t a l l e r s " have argued t h a t i f they machined d o v e t a i l grooves i n t h e s h e l l , these h e l d t h e w h i t e metal i n p l a c e and t h e r e was no need t o i n s i s t on good bonding. sighted p o l i c y ;
Experience p rove s t h a t t h i s i s an extremely s h o r t -
d o v e t a i l grooves a r e no s u b s t i t u t e f o r good bonding and,
in
a d d i t i o n , t h e sharp edges a t t h e sh ou l de rs o f t h e grooves a c t as l o c a l i s e d s t r e s s r a i s e r s and can i n i t i a t e c r a c k s ( F i g . 1 ) .
A general v ie w o f a cop pe r-l ea d b e a r i n g which f a i l e d due t o bad bonding, t h e copper-lead b e a r i n g a l l o y h a v i n g separated c l e a n l y from t h e s t e e l s h e l l , shown i n Fig.2.
is
90
Fig.1
Dovetail in white-metal thrust pad bearing.
Fig.2
Bad bonding:
copper-lead bearing.
91 Bad bonding on new b e a r i n g s can be d e t e c t e d r e a d i l y u s i n g u l t r a s o n i c o r e l e c t r i c a l r e s i s t a n c e measuring techniques,
a l l y a v a i l a b l e t e s t i nst rume nt s.
and t h e r e a r e a number o f commerci-
On a f a i l e d bearing, bad bonding can be
d i s t i n g u i s h e d f r om f a t i g u e f a i l u r e because w i t h t h e former, t h e b e a r i n g metal detaches c l e a n l y f ro m t h e s h e l l (Fig.3).
5.4.1.2
Gas C a v i t i e s
I n l a r g e , wh ite -met al b e a r i n g s , t o o l ow a c a s t i n g temperature o r i n s u f f i c i e n t p r e h e a t i n g o f t h e s h e l l can g i v e r i s e t o gas c a v i t i e s near t h e s h e l l s u r f a c e . Such c a v i t i e s weaken t h e b e a r i n g metal and a d v e r s e l y a f f e c t heat f l o w ;
some-
times, t h e h y d r o s t a t i c pre ssure above t h e c a v i t i e s may cause t h e i r c o l l a p s e (F ig s . 4 and 5). Cast c o p p e r - le ad a l l o y s sometimes e x h i b i t c a s t i n g p o r o s i t y , w hich i n t h i s i n s ta n c e i s g e n e r a l l y assumed t o be due t o hydrogen e v o l u t i o n from t h e copper. This p o r o s i t y due t o o u t - g a s s i n g must be d i s t i n g u i s h e d from c o r r o s i o n o f t h e lead phase by o i l - o x i d a t i o n pro du ct s.
Cast i ng p o r o s i t y i s sub-surface,
whereas
w i t h corrosion, the lead i n the surface l a y e r s i s p r e f e r e n t i a l l y dissolved (F ig .6 ) .
5.4.1.3
Ov e r s ize Cuboids
T h i s p r o b le m i s s p e c i f i c t o wh i t e -met al b earings.
I n l a r g e r b e a r i n g s , slow
c o o l i n g t h r o u g h t h e s o l i d i f i c a t i o n range can g i v e r i s e t o o v e r s i z e (> 0 . 1 mm) t i n - a n t i m o n y c u b o i ds (F i g. 7).
T h i s i n t e r m e t a l l i c compound i s b r i t t l e and the
l a r g e c u b o id s can c r a c k and break-up i n s e r v i c e , causing s c o r i n g o f t h e j o u r n a l s and damage elsewhere i n t h e o i l system ( F i g . 8 ) .
I n general,
i f cuboids
a r e r e a d i l y v i s i b l e t o t h e naked eye t h e y a r e t o o b i g .
5.4.1.4
Excessive Lead Content \ i n Tin-Based White Metals
The le a d c o n t e n t o f t i n-b ase d w h i t e me t a l s should be below 0.5% t o prevent th e f o r m a t i o n o f a l o w - m e l t i n g ,
l e a d - t i n e u t e c t i c i n t h e g r a i n boundaries.
The e u t e c t i c weakens t h e a l l o y and makes i t more s u s c e p t i b l e t o w i p i s g . s i m i l a r reasons,
For
i t i s bad p r a c t i c e t o mix lead-based and t i n - b a s e d w hite-metal
h a l f - b e a r i n g s , as i s sometimes done t o save i n i t i a l c o s t s , t i n b e i n g used on t h e loaded h a l f and l e a d on t h e unloaded h a l f .
Carry-over o f l e a d from one
h a l f - b e a r i n g t o t h e o t h e r can r e s u l t i n t h e f o r m a t i o n of t h e l o w - m e l t i n g - p o i n t eutect ic.
5.4.1.5
Uneven Lead D i s t r i b u t i o n i n Copper-Lead and Lead-Bronze A l l o y s
Copper-lead and lead-bronze b e a r i n g a l l o y s can be manufactured by m e l t i n g and c a s t i n g o r by s i n t e r i n g techniques.
Lead d i s t r i b u t i o n i s more r e a d i l y
c o n t r o l l e d by p o wde r-met al l u rgy t e chn i qu es, so uneven d i s t r i b u t i o n i s m a i n l y
92
Fig.3
Micro-sections.
93
Fiq.4
Cross-section o f White-metals Showing Gas Cavities
Fig.5
Collapsed surface o f white-metal thrust pad bearing.
94
Fig.6
C a s t i n g p o r o s i t y i n c a s t copper-lead big-end b e a r i n g .
Fiq.7
O v e r s i z e t i n - a n t i m o n y cuboids i n w h i t e - m e t a l steam t u r b i n e
t h r u s t pad from
95
Fig.8
S i n g l e cuboids showing c r a c k s .
Fig.9
Bad l e a d d i s t r i b u t i o n .
96 encountered on c a s t b e a r i n g s .
Copper and l e a d a r e i m m i s c i b l e and good l e a d
d i s t r i b u t i o n i n c a s t a l l o y s depends on s p e c i a l m a n u f a c t u r i n g techniques, p a r t i c u l a r on r a p i d c o o l i n g o f t h e a l l o y a f t e r c a s t i n g .
in
This r a p i d cooling
r e s u l t s i n a h i g h l y o r i e n t e d s t r u c t u r e w i t h l o n g copper and l e a d d e n d r i t e s normal t o t h e s u r f a c e o f t h e b e a r i n g s h e l l . laxed, t h e l e a d can be unevenly d i s t r i b u t e d ;
If manufacturing c o n t r o l s are ref o r example,
i t may be p r e s e n t i n
l a r g e s l u g s , normal o r p a r a l l e l t o t h e s u r f a c e ( F i g . 9 ) .
5.4.2
F a i l u r e s A s s o c i a t e d w i t h Design, F i t t i n g o r O p e r a t i n g Environment
The f i n a l m a n i f e s t a t i o n o f f a i l u r e r e s u l t i n g f r o m adverse o p e r a t i n g c o n d i t i o n s o f t h i s type i s generally fatigue.
The l o a d - c a r r y i n g c a p a c i t i e s and tem-
p e r a t u r e l i m i t a t i o n s o f t y p i c a l b e a r i n g a l l o y s a r e i n d i c a t e d i n Table 5 . 2 . However, these depend on a number o f f a c t o r s , such as t h e t h i c k n e s s o f t h e b e a r i n g a l l o y and t h e support i t g e t s f r o m i t s s u b s t r a t e o r s h e l l .
Also, the
s t r e n g t h o f b e a r i n g a l l o y s decreases w i t h i n c r e a s i n g temperature, and b e a r i n g s u r f a c e temperatures a r e g e n e r a l l y a t l e a s t 2OoC i n excess o f measured o i l temperatures. The design o f b e a r i n g s , p a r t i c u l a r l y t h e l o c a t i o n o f o i l feeds and o i l grooves,
i s o u t s i d e t h e scope o f t h i s paper.
N e v e r t h e l e s s , i t should be em-
phasised t h a t l o a d - c a r r y i n g c a p a c i t y i s r a r e l y enhanced by a p r o l i f e r a t i o n of o i l grooves.
5.4.2.1
Bad F i t t i n g
Modern b e a r i n g s a r e manufactured t o v e r y c l o s e t o l e r a n c e s and s h o u l d be assembled w i t h g r e a t care.
I f t h e y a r e n o t a c c u r a t e l y a l i g n e d , t h e l o a d i n g w i l l be
uneven and premature f a i l u r e due t o f a t i g u e may o c c u r .
I f t h e b e a r i n g does n o t
f i t p r o p e r l y i n i t s housing, s l i g h t movement may o c c u r , which can r e s u l t i n
severe f r e t t i n g on t h e back o f t h e b e a r i n g .
Loose f i t t i n g and/or f r e t t i n g can
a d v e r s e l y a f f e c t h e a t t r a n s f e r and can g i v e r i s e t o e x c e s s i v e b e a r i n g - s u r f a c e temperature (Fig.10). D i r t p a r t i c l e s trapped between a b e a r i n g and i t s housing c o n s t i t u t e a n o t h e r
cause o f poor heat t r a n s f e r .
Such p a r t i c l e s can a l s o cause h i g h spots on t h e
b e a r i n g s u r f a c e , which may g i v e r i s e t o l o c a l i s e d w i p i n g and f a t i g u e (Fig.11). I t w i l l be c l e a r f r o m t h e above t h a t e x a m i n a t i o n o f t h e back o f a f a i l e d
b e a r i n g can p r o v i d e i m p o r t a n t i n f o r m a t i o n r e g a r d i n g i t s mode o f f a i l u r e . comment has been made t h a t t h e b e s t way t o e v a l u a t e "experts"
The
on b e a r i n g
f a t i g u e s i s t o n o t e whether t h e y examine t h e backs o f b e a r i n g s w i t h as much c a r e as t h e sl i d i n g surface.
91
Fig.10
5.4.2.2
F r e t t i n g and d i s t o r t i o n due t o bad f i t t i n g .
Extraneous P a r t i c l e s
T h i s i s p r o b a b l y t h e m s t common cause o f b e a r i n g f a i l u r e .
The more e x p e r t
we become a t d e t e c t i n g and i d e n t i f y i n g f o r e i g n m a t e r i a l , t h e more obvious i t i s t h a t c l e a n l i n e s s on assembly and good l u b r i c a n t f i l t r a t i o n i n s e r v i c e a r e e s s e n t i a l f o r lo ng, t r o u b l e - f r e e s e r v i c e l i v e s .
A l l too frequently, engineering
components a r e assembled w i t h o u t adequate c l e a n i n g i n d i r t y environments, w i t h the r e s u l t t h a t machining swarf , moulding sand and o t h e r k i n d s o f d e b r i c c i r c u l a t e w i t h the l u b r i c a t i n g o i l .
Large p a r t i c l e s a r e g e n e r a l l y removed by t h e
o i l f i l t e r s , b u t medium-sized p a r t i c l e s may embed i n t h e b e a r i n g s and v e r y small p a r t i c l e s c o n t i n u e t o c i r c u l a t e w i t h t he o i l .
F o r e i g n p a r t i c l e s and wear
d e b r i s can a l s o accumulate d u r i n g s e r v i c e , and v e r y l o n g o i l - c h a n g e p e r i o d s , now much i n f a v o u r , ag gra vat e t h e s i t u a t i o n .
98
Fig.12
Shaft wear due t o embedded p a r t i c l e s i n bearinq.
99 B e a r in g a l l o y s , p a r t i c u l a r l y w h i t e met al s, have a c o n s i d e r a b l e c a p a c i t y f o r abrasive materials.
When t h i s c a p a c i t y i s exceeded, t h e b e a r i n g s u r f a c e can a c t
as a l a p , a b r a d i n g away t h e j o u r n a l s u r f a c e , t h e r e b y i n c r e a s i n g clearances and leading t o f a t i g u e f a i l u r e .
T h i s l a p p i n g a c t i o n by embedded p a r t i c l e s i s t h e
prime cause o f j o u r n a l wear (F i g. 12 ).
In most in s t a nces, t h e b e a r i n g s u r f a c e w i l l be i n r e l a t i v e l y good c o n d i t i o n (unless complete f a i l u r e has o c c u r r e d ) , a l t h o u g h t h e o v e r a l l t h i c k n e s s o f t h e be a r in g can a c t u a l l y i n crea se owing t o t h e amount o f embedded m a t e r i a l . P a r t i c l e s embedded i n b e a r i n g s u r f a c e s d i s p l a c e metal and t h e r e f o r e a r e o f t e n surrounded by a r a i s e d , b urn i she d h a l o (F i g. 13).
I f t h e c o m p o si t i o n o f embedded p a r t i c l e s can be determined, t h e i r o r i g i n may be e s t a b l i s h e d and t h e a p p r o p r i a t e s t e p s taken t o e l i m i n a t e them.
A v a r i e t y o f i d e n t i f i c a t i o n t ech ni q ue s a r e now a v a i l a b l e .
For example, t h e
p a r t i c l e s may be e x t r a c t e d c h e m i c a l l y o r m e c h a n i c a l l y f o r i d e n t i f i c a t i o n by X-ray d i f f r a c t i o n o r X-ray sp ect rome t ry.
They can be examined i n s i t u u s i n g an
el e c tr o n - p r o b e .
Alternatively,
chemical e x t r a c t i o n techniques have much t o
recommend them;
t h e amount o f d e b r i s g e n e r a l l y o b t a i n e d i s enough t o h o r r i f y
most p l a n t o p e r a t o r s (F i g . 1 4). P a r t i c l e s i n e l e c t r o d e p o s i t e d l e a d o v e r l a y s can be e x t r a c t e d w i t h an a c e t i c acid/hydrogen p e r o x i d e m i x t u r e , which d i s s o l v e s t h e o v e r l a y ; - t i n b e a r i n g s can be e x t r a c t e d w i t h c a u s t i c soda.
those i n aluminium
Both m i x t u r e s can loosen
p a r t i c l e s i n t i n - b a s e d w h i t e met al s. Embedded f e r r o u s p a r t i c l e s ( t h e l a r g e s t s i n g l e group) can be q u i c k l y i d e n t i f i e d by " i r o n p r i n t i n g " (Fig.15). T h i s i n v o l v e s so aki n g an unglazed paper i n a 5 % s~o l u t i o n o f potassium f e r r i cyanide t o wh ic h a few drops of h y d r o c h l o r i c a c i d and w e t t i n g agent have been added.
The s u r p l u s l i q u i d i s d r a i n e d o f f t h e paper, w hich i s then p l a c e d i n con-
t a c t w i t h t h e degreased surf ace o f t h e b e a r i n g f o r about 30 seconds.
The paper
i s then removed, and b l u e sp ot s i n d i c a t e t h e presence o f f e r r o u s p a r t i c l e s . p a r t i c l e s t h a t remain i n t h e b e a r i n g s u r f a c e a r e a l s o s t a i n e d b l u e .
The
Similar
techniques a r e a v a i l a b l e f o r t h e i d e n t i f i c a t i o n o f p a r t i c l e s o f most common nonf e r r o u s m e t a ls . Since f e r r o u s p a r t i c l e s c o n s t i t u t e t h e most common t y p e o f a b r a s i v e wear p a r t i c l e , t h e use o f magnetic f i l t e r s ,
i n a d d i t i o n t o o r d i n a r y f i l t e r s , has much t o
recommend i t. Fin e p a r t i c l e s , s m a l l e r t ha n normal b e a r i n g clearances, can c i r c u l a t e w i t h the l u b r i c a n t and erode t h e b e a r i n g s u i f a c e .
Hard p a r t i c l e s erode deep, w e l l -
d e f i n e d channels, w h i l e s o f t p a r t i c l e s g i v e r i s e t o more general e r o s i o n (Fig.16), p a r t i c u l a r l y on s o f t e l e c t r o d e p o s i t e d o v e r l a y s . E r o s io n by f i n e p a r t i c l e s i s most p r e v a l e n t on high-speed b e a r i n g s and may be associated w i t h c a v i t a t i o n erosion.
100
Fig.13
P a r t i c l e s embedded i n b e a r i n g s u r f a c e .
Fig.14
Steel p a r t i c l e s e x t r a c t e d from b e a r i n g .
101
Fig.16
Erosion damage.
102
5.4.2.3
Cor r o s i o n
L u b r i c a n t s d e t e r i o r a t e i n s e r v i c e i n two ways
-
they become contaminated and
th e y undergo p h y s i c a l and chemical changes due t o o x i d a t i o n .
I n engines,
the
common contaminants a r e a i r b o r n e du st and wear products, unburnt f u e l , f u e l combustion p r o d u c t s and wat er. e r i a l s and a s p h a l t e ne s. sludges and lacquers.
The o x i d a t i o n p r o d u c t s a r e m a i n l y a c i d i c mat-
Asphaltenes i n a s s o c i a t i o n w i t h f u e l contaminants form The a c i d i c m a t e r i a l s - r e s u l t i n g f r o m t h e o x i d a t i o n o f
l u b r i c a n t s a r e g e n e r a l l y weak o r g a n i c a c i d s , a l t h o u g h i n extreme cases s t r o n g m i n e r a l a c i d s may be produced.
However, a l most a l l s t r o n g m i n e r a l a c i d contami
n a t i o n i n l u b r i c a n t s i s d e r i v e d f rom f u e l combustion p r o d u c t s , s u l p h u r i c a c i d b e i n g a major contaminant i n d i e s e l e ng i ne s and halogen a c i d s i n g a s o l i n e eng ines. Since b e a r i n g s a r e o f complex c o n s t r u c t i o n ,
the way i n w hich t h e i r s t r u c t u r e
and c o m p o s i t i o n a f f e c t s t h e i r c o r r o s i o n r e s i s t a n c e w i l l be discussed w i t h r e f erence t o v a r i o u s c o r r o s i o n mechanisms. I t s h o u ld be emphasised t h a t b e a r i n g f a i l u r e s t h a t a r e p r i m a r i l y due t o any form o f c o r r o s i o n a r e c o m p a r a t i v e l y r a r e and t h a t b e a r i n g f a i l u r e s caused by i n h e r e n t d e f i c i e n c i e s on t h e p a r t o f t h e l u b r i c a n t a r e extremely r a r e .
Never-
t h e l e s s , c e r t a i n s p e c i f i c forms o f c o r r o s i o n , such as t i n o x i d e f o r m a t i o n on ti n- b a s e d white-metal
b e a r i n g s and sul p hu r c o r r o s i o n o f phosphor-bronze a l l o y s ,
have a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n .
5.4.2.3
A l s o , as w i l l be apparent i n S e c t i o n
( c o r r o s i o n by weak o r g a n i c a c i d s ) and 5 . 4 . 2 . 4
i n c r e a s i n g l y severe o p e r a t i n g c o n d i t i o n s ,
( c a v i t a t i o n and e r o s i o n ) ,
such as longer-oil-change
periods,
v e r y h i g h o p e r a t i n g temperatures and i n crea sed speeds, can g i v e r i s e t o p a r t i c u l a r problems.
These a r e problems i n which c o r r o s i o n p l a y s a p a r t , b u t i n
which o t h e r f a c t o r s a r e also o p e r a t i v e . ( i ) Co r r o s io n by weak o r g a n i c a c i d s Weak o r g a n i c a c i d s a r i s e e i t h e r f r o m prolonged exposure o f t h e l u b r i c a n t a t e l e v a t e d temperatures o r by c o n t a m i n a t i o n o f t h e l u b r i c a n t w i t h p a r t i a l l y b u r n t combustion p rod uct s.
These a c i d s a t t a c k l e a d f a r more
r e a d i l y t h an o t h e r met al s, and can d i s s o l v e t h e lead phase i n copper-lead and lead-bronze b e a r i n g s (F i g . 1 7). Some e n g in e l u b r i c a t i n g o i l s p e c i f i c a t i o n s i n c l u d e t e s t s t o determine t h e c o r r o s i v i t y o f l u b r i c a t i n g o i l s towards copper-lead bearings.
The loss
o f le a d i s assessed b y we i gh i ng t h e b e a r i n g s h e l l s b e f o r e and a f t e r t e s t . I n s e r v i c e f a i l u r e s , where t h e wei g ht s o f new b e a r i n g s a r e n o t a v a i l a b l e , le a d c o r r o s i o n can be d e t e c t e d by m e t a l l u r g i c a l s e c t i o n i n g (Fig.18). Copper-lead and lead-bronze b e a r i n g s may be manufactured by s i n t e r i n g o r casting.
I n s i n t e r e d a l l o y s i t i s p o s s i b l e t o ensure t h a t t h e lead
103
Fig.17
Fig.18
Corroded copper-lead b e a r i n g
Corroded s i n t e r e d copper-lead b e a r i n g
104 phase i s w e l l d i s t r i b u t e d and d i s c o n t i n u o u s , s o t h e c o r r o s i o n w i l l be r e s t r i c t e d t o the surface layers.
On t h e o t h e r hand, w i t h c a s t a l l o y s ,
s a t i s f a c t o r y l e a d d i s t r i b u t i o n depends on h i g h l y s p e c i a l i s e d manufactur i n g techniques and r a p i d c o o l i n g o f t h e a l l o y a f t e r c a s t i n g .
This
rapid cooling gives r i s e t o a h i g h l y o r i e n t e d structure, w i t h long copper and l e a d d e n d r i t e s normal t o t h e s u r f a c e o f t h e b e a r i n g s h e l l . The l o n g l e a d d e n d r i t e s p r o v i d e an easy p a t h f o r t h e p e n e t r a t i o n o f a corrosive lubricant.
I n these circumstances i t i s p o s s i b l e f o r almost
a l l o f t h e l e a d phase t o be leached o u t o f a c a s t copper-lead a l l o y (Fig.19).
Fig.19
Corroded c a s t copper-lead b e a r i n g .
Most copper-lead b e a r i n g s now have a p r e c i s i o n e l e c t r o d e p o s i t e d o v e r l a y o f a l e a d - t i n o r lead-indium a l l o y . the c o r r o s i o n r e s i s t a n c e o f t h e lead.
The indium o r t i n a d d i t i o n s improve We have e s t a b l i s h e d t h a t about
3%
t i n o r 4 . 8 % indium i s r e q u i r e d t o render t h e o v e r l a y c o m p l e t e l y r e s i s t a n t t o c o r r o s i o n (Fig.20). Bearing manufacturers g e n e r a l l y p r o v i d e o v e r l a y s w i t h about 5% indium o r 10% t i n .
However, a t t h e h i g h temperatures t h a t b e a r i n g s can e x p e r i e n c e
i n s e r v i c e , t h e indium o r t i n d i f f u s e s q u i t e r a p i d l y and m i g r a t e s i n t o
105
Average w t loss m 9.
200Indium alloys 150-
100
Tin alloys
-
A Lead-tin alloys X Lead- indium alloys
50
1.0
0
Fig.20
2.0
3.0 4.0 Percentage alloying element
- 6.0
5.0
Corrosion of l e a d - t i n and lead-indium a l l o y s .
the u n d e r l y i n g copper-lead.
This d i f f u s i o n can r e a d i l y be observed both
on bearings removed from s e r v i c e and on bearings t e s t e d i n the laboratory. I n the case o f t i n , the t i n a l l o y s w i t h the copper t o form c o p p e r - t i n i n t e r m e t a l l i c compounds.
I n the case of indium, enrichment occurs ad-
j a c e n t t o t h e copper, but we have no evidence o f compound formation. S i m i l a r e f f e c t s can be observed on s i l v e r bearings w i t h l e a d - r i c h e l e c t r o deposited overlays. This m i g r a t i o n o f indium and t i n from the surface l a y e r s o f the o v e r l a y leaves them s u s c e p t i b l e t o c o r r o s i o n (Figs.21-23).
I t i s s a i d t h a t one
way t o prevent t h i s d i f f u s i o n i s t o interpose a dam between the overlay and the u n d e r l y i n g copper-lead Very t h i n n i c k e l ,
(Fig.24).
i r o n o r brass l a y e r s have been used as dams.
However,
many bearings users a r e u n w i l l i n g t o face the increased cost associated w i t h the use o f dams and,
i n any case, t h e i r e f f e c t i v e n e s s i s being
questioned. There i s another way i n which the indium i n lead-indium overlays may be depleted.
I f the indium content f a l l s t o below about 3%, i n t e r n a l
o x i d a t i o n o f the indium can occur i n the g r a i n boundaries (Fig.25).
The
indium oxide formed i n t h i s way e m b r i t t l e s the a l l o y and renders i t very susceptible t o f a t i g u e f a i l u r e as w e l l as t o c o r r o s i v e a t t a c k (Fig.26).
106
Fig.21
Corrosion o f b e a r i n g o v e r l a y .
Fig.22
E l e c t r o n probe micrographs showing indium d i s t r i b u t i o n .
107
A f t e r 13,000 h r s . Fig.23
Fig.24
A f t e r 200 h r s . a t f u l l l o a d .
l n t e r m e t a l 1 i c compound f o r m a t i o n i n o v e r l a y b e a r i n g s ( c o p p e r - l e a d ) .
O v e r l a y b e a r i n g w i t h dam.
108
Fig.25
Internal oxidation o f lead indium overlay.
Fig.26
Corrosion o f lead indium overlay.
109 Another consequence o f t h e d i f f u s i o n o f t i n f r o m l e a d - r i c h o v e r l a y s i s t h e roughening o f t h e o v e r l a y s u r f a c e (Fig.27).
Although copper
has a much h i g h e r a f f i n i t y f o r t i n t h a n has lead, t i n d i f f u s e s much more r a p i d l y i n l e a d than i n copper.
T h i s means t h a t where t h e l e a d
phase i n copper-lead a l l o y i s continuous w i t h t h e o v e r l a y ,
the t i n
can p e n e t r a t e deeply i n t o t h e a l l o y i n t h e l e a d phase (Fig.28) cause s i n k i n g o f t h e o v e r l a y .
and
T h i s movement o f t i n away f r o m t h e
o v e r l a y can g i v e t h e o v e r l a y a roughened appearance, which may be mistaken f o r c o r r o s i o n . ( i i ) C o r r o s i o n by s t r o n g m i n e r a l a c i d s The main source o f s t r o n g m i n e r a l a c i d s i n l u b r i c a n t s i s c o n t a m i n a t i o n by f u e l combustion p r o d u c t s .
Diesel fuels,
p a r t i c u l a r l y marine
diesel f u e l s , contain s i g n i f i c a n t q u a n t i t i e s o f sulphur;
t h i s sulphur
i s t h e source o f s u l p h u r i c a c i d , which may f i n d i t s way i n t o t h e lubricant.
Gasolines, on t h e o t h e r hand, c o n t a i n v e r y l i t t l e s u l p h u r ,
b u t do c o n t a i n c h l o r i n e and bromide compounds which a r e added t o scavenge t h e l e a d a n t i k n o c k compounds.
Thus, l e a d h a l i d e compounds
may accumulate i n t h e l u b r i c a t i n g o i l i n g a s o l i n e engines and,
in
c e r t a i n circumstances, halogen a c i d s can form. The s t r o n g a c i d s g e n e r a l l y a t t a c k bare s t e e l s u r f a c e s r a t h e r than bearing a l l o y s .
T h i s u s u a l l y r e s u l t s i n j o u r n a l s b e i n g roughened by
c o r r o s i o n , and b e a r i n g s t h e n f a i l e i t h e r because o f t h i s o r as a r e s u l t o f damage by c o r r o s i o n p r o d u c t s ( r u s t ) . I n t h e presence o f aluminium and m o i s t u r e , l e a d h a l i d e s can d e l i q u e s c e , g i v i n g r i s e t o halogen a c i d s , which c o r r o d e aluminium.
This corrosion
r e a c t i o n was f i r s t observed on a l u m i n i u m - a l l o y p i s t o n s and can r e a d i l y be d u p l i c a t e d i n t h e l a b o r a t o r y .
F o r t u n a t e l y , examples o f t h i s t y p e
o f a t t a c k on a l u m i n i u m - t i n b e a r i n g s i n engines a r e e x t r e m e l y r a r e , a l t h o u g h b e a r i n g s a r e s u s c e p t i b l e t o h a l i d e c o r r o s i o n a f t e r removal f r o m engines. Halogen a t t a c k has a l s o been observed on l e a d o v e r l a y b e a r i n g s o p e r a t e d a t v e r y h i g h engine temperatures, when mixed b r o m i d e / c h l o r i d e l a y e r s can f o r m on t h e o v e r l a y s u r f a c e and cause b l a c k e n i n g ( F i g s .
29 and 30).
E l e c t r o n - p r o b e s t u d i e s o f t h e r e a c t i o n l a y e r show t h a t
i t c o n t a i n s indium a t t h e same c o n c e n t r a t i o n as i n t h e b e a r i n g , i n d i c a t i n g t h a t t h e l e a d h a l i d e s have been formed i n s i t u as a r e s u l t o f c o r r o s i o n and have n o t been d e r i v e d d i r e c t l y f r o m t h e f u e l . ( i i i ) Sulphur c o r r o s i o n T h i s i s a general d e s c r i p t i o n which i s a p p l i e d t o most forms of c o r r o s i o n encountered on s i l v e r - r i c h o r c o p p e r - r i c h b e a r i n g a l l o y s .
110
Fig.27
Roughening o f overlay due t o d i f f u s i o n .
Fig.28
' S i n k i n g ' o f overlay.
111
Fig.29
Blackened b i g - e n d b e a r i n g s .
Fig.30
Lead h a l i d e l a y e r .
112 There i s no doubt t h a t sulphur compounds i n l u b r i c a t i n g o i l s can promote the corrosion of these p a r t i c u l a r a l l o y s .
On the o t h e r hand, b o t h
n a t u r a l l y o c c u r r i n g sulphur compounds i n l u b r i c a n t s and sulphur-containing a d d i t i v e s (e. g. z i n c d i a l k y l d it h iophosphates) confer b e n e f i c i a l p r o p e r t i e s on l u b r i c a n t s .
Thus, the dithiophosphates show considerable a,ntioxidant
and anti-wear e f f e c t s .
With regard t o n a t u r a l l y o c c u r r i n g sulphur com-
pounds, modern r e f i n i n g techniques can remove them completely, b u t experience shows t h a t t h i s i s most unwise, since some o f these compounds p l a y a l a r g e p a r t i n i n h i b i t i n g the c o r r o s i o n o f many metals, p a r t i c u l a r l y lead. On s i l v e r bearings the problem,
i n theory,
i s f a i r l y simple.
S i l v e r has
a h i g h a f f i n i t y f o r sulphur, and dithiophosphate-type a d d i t i v e s cannot be used.
However, a s m a l l amount o f e.p.
(extreme-pressure) a c t i v i t y on the
p a r t o f the l u b r i c a n t i s necessary, and the n a t u r a l sulphur can sometimes meet t h i s requirement. be used.
A l t e r n a t i v e l y , a c h l o r i n e - c o n t a i n i n g a d d i t i v e may
A s i l v e r bearing a f t e r s e r v i c e would be expected t o show a
c e r t a i n amount o f d i s c o l o r a t i o n due t o the formation o f s i l v e r sulphide o r s i l v e r c h l o r i d e , which have a b e n e f i c i a l e f f e c t on the f r i c t i o n a l characteristics. corrosion.
However, too much r e a c t i v e sulphur can cause severe
I t can be concluded t h a t the f o r m u l a t i o n o f o i l s t o l u b r i c a t e
s i l v e r bearings r e q u i r e s t h e e x p e r t i s e o f an o i l chemist. The c o r r o s i o n problems o f copper a l l o y s a r e more complex because the a l l o y s themselves a r e complex. phase MY
On simple copper-lead a l l o y s the copper
be attacked by sulphur, but t h i s i s a comparatively r a r e
occurrence, the problem o f sulphur c o r r o s i o n being much more acute on phosphor-bronze a1 loys (Fig.31).
This i s because phosphor-bronze a1 loys,
which a r e very popular f o r l i t t l e - e n d bushes i n d i e s e l engines, a r e expected t o operate a t considerably higher temperatures than copper-lead bearings.
I n any case, t h e r e a r e very few bare copper-lead bearings i n
use today. There i s no general agreement about the c o r r o s i o n mechanism.
Some
engine manufacturers and users h o l d dithiophosphate a d d i t i v e s e n t i r e l y responsible, but t h i s o p i n i o n cannot be substantiated, f o r severe c o r r o s i o n can occur when o i l s c o n t a i n i n g o n l y n a t u r a l sulphur compounds a r e used. TWO important f a c t o r s i n f l u e n c i n g the s e v e r i t y o f c o r r o s i o n a r e the
amount o f a l l o y i n g element i n s o l u t i o n i n the copper-rich phase and the p o r o s i t y o f the a l l o y .
I n phosphor bronzes,
i f the amount o f t i n i n
s o l u t i o n can be increased by special c a s t i n g techniques, such as continuous c a s t i n g , o r by s o l u t i o n treatment a f t e r manufacture, the resistance t o c o r r o s i o n i s g r e a t l y increased.
The presence o f z i n c
113 and/or s i l i c o n as a l l o y i n g elements i n copper a l l o y s a l s o increases t h e r e s i s t a n c e o f these a l l o y s t o su l ph ur c o r r o s i o n .
However,
it is d i f f i -
c u l t t o make sound s i l i c o n - b r o n z e c a s t i n g s .
I f t h e a l l o y i s porous, t h e l u b r i c a n t i s drawn i n t o the pores, where i t s ta g n a t e s and, i f o p e r a t i n g temperatures a r e h i g h , can become very c o r r o s i v e (F i g. 32 ).
The p a r t i c u l a r temperature a t which c o r r o s i o n be-
comes severe depends on t h e t ype o f d i t h i o p h o s p h a t e used;
very a c t i v e
v a r i e t i e s can s t a r t t o co rrod e a t about 1 3 O o C , whereas o t h e r v a r i e t i e s may be c o mp ara t i ve l y s t a b l e up t o 180°C. l i t t l e t r o u b l e below 1 7 O O C .
N a t u r a l sulphur compounds g i v e
Most phosphor-bronze c a s t i n g s a r e micro-
porous, and t h e g r e a t e r t h e p o r o s i t y t he g r e a t e r the r i s k o f c o r r o s i o n . Cases a r e known where o n l y c e r t a i n bushes corroded i n a p a r t i c u l a r engine, and m e t a l l u r g i c a l e xami n at i on showed t h a t the bushes which c o r roded were porous.
Co nt i nu ou s-cast i ng techniques g i v e sounder a l l o y s
t h a n o t h e r c a s t i n g t ech ni q ue s and,
i n addition, a greater proportion o f
t i n s t a y s i n s o l u t i o n , t h ere by improving t h e i n t r i n s i c c o r r o s i o n resistance.
However,
i f t h e problem i s t o be c o n t r o l l e d , t h e o n l y s a t -
i s f a c t o r y s o l u t i o n i s t o use an a l l o y t h a t i s r e s i s t a n t t o sulphur corrosion.
L ab ora t o ry t e s t s f o l l o w e d by e x t e n s i v e f i e l d experience ex-
t e n d i n g o v e r t e n yea rs have shown t h a t a l l o y s o f t h e gun-metal
type,
i.e.
c o p p e r - t i n a l l o y s w i t h 2-4% z i n c , a r e completely immune f r m sulphur corrosion.
These gun-metal a l l o y s a r e r a t h e r s o f t e r than t h e t r a d i t i o n a l
phosphor bronzes, and n i c k e l may a l s o be added t o compensate f o r the reduced hardness. The importance o f m i c r o s t r u c t u r e i n t h i s type o f c o r r o s i o n was r e c e n t l y demonstrated by some phosphor-bronze bushes which corroded i n r e g u l a r l y spaced bands, d e s p i t e t h e f a c t t h a t t h ey were f r e e from p o r o s i t y and had been o p e r at ed on a l u b r i c a n t c o n t a i n i n g o n l y a small amount o f a very s t a b l e d it hi o ph osp ha t e .
M e t a l l u r g i c a l examination showed t h a t they had
been manufactured by a semi-continuous c a s t i n g process, which gave r i s e t o marked s e g r e g a t i o n i n t h e a l l o y , making i t v e r y s u s c e p t i b l e t o c o r r o s i o n i n c e r t a i n are as (F i g. 33 ). Re c e n t ly some cases o f what i s cl a i med t o be sulphur c o r r o s i o n have been encountered on t i n - b a s e d whi t e-me t a l b e a r i n g a l l o y s .
I t appears t h a t very
a c t i v e s u l p hu r compounds can s e l e c t i v e l y a t t a c k t h e c o p p e r - r i c h ,
copper-
t i n i n t e r m e t a l l i c compounds i n t h e w h i t e metal, and t h a t t h e r e s u l t i n g c o r r o s i o n p r o d u c t s , r i c h i n Cu2S, can spread over t h e b e a r i n g surface. The d a r k e ni n g caused by these c o r r o s i o n products should n o t be confused w i t h t h e d arke ni n g due t o t i n o x i d e f o r m a t i o n .
114
Fig.31
Corrosion o f phosphor bronze.
Fig.32
Subsurface a t t a c k on phosphor-bronze.
115 ( i v ) C o r r o s io n o f t i n - b a s e d w h i t e m e t a l s :
t i n oxide formation
I n r e c e n t ye ars t h e f o r m a t i o n o f hard, b l a c k l a y e r s on t h e s u r f a c e o f t i n based wh ite -met al b e a r i n g s i n s h i p s ' t u r b i n e s has a t t r a c t e d a l o t o f attention.
Yet t h i s i s by no means a new problem;
i t has been encoun-
t e r e d i n t h e main d i e s e l e ng i ne s o f s h i p s f o r many years.
Some t u r b i n e
b u i l d e r s and s h i p owners have blamed t h e c o r r o s i o n on t h e i n c r e a s i n g use o f a d d i t i v e s i n l u b r i c a t i n g o i l s , b u t t h e f a c t t h a t d i e s e l engine bearings o p e r a t i n g on base o i l s have s u f f e r e d f r o m t h i s c o r r o s i o n f o r many years throws some doubt on t h e i r assumptions. The c h a r a c t e r i s t i c f e a t u r e s o f t h e c o r r o s i o n a r e as f o l l o w s .
A hard,
b l a c k l a y e r forms on t h e s u r f a c e o f t h e bearings, sometimes o n l y on the wo r k in g s u r f a c e s b u t g e n e r a l l y on a l l t h e exposed s u r f a c e s (Fig.34). The presence o f t h i s l a y e r i s e xt re mel y harmful f o r two reasons: i t i s v e r y h a r d and may damage t h e a d j a c e n t s t e e l surfaces,
i t s f o r m a t i o n decreases normal b e a r i n g clearances.
firstly,
and secondly,
M e t a l l u r g i c a l sections
f r o m f a i l e d b e a r i n g s show t h a t t h e h a r d l a y e r i s formed from t h e t i n - r i c h m a t r i x o f t h e b e a r i n g a l l o y (F i g . 3 5).
The c o p p e r - t i n and a n t i m o n y - t i n
i n t e r m e t a l l i c compounds a r e c o m p l e t e l y unattacked.
Microhardness t e s t s
show t h a t t h e hardness o f t h e b l a c k m a t r i x i s between 200 and 600 DPN, and u s u a l l y between 400 and 500 DPN.
The hardness o f t h e untransformed
m a t r i x i s about 25 OPN. X-ray d i f f r a c t i o n e xami n at i on shows t h a t t h e b l a c k l a y e r c o n s i s t s m a i n l y o f c a s s i t e r i t e , Sn02, u s u a l l y w i t h a l i t t l e stannous o x i d e , SnO, associated with it.
Dn t h r u s t pads, where b e a r i n g clearances a r e l a r g e , the
b l a c k l a y e r can grow t o a c o n s i d e r a b l e t h i c k n e s s , b u t e v e n t u a l l y i t d i s i n t e g r a t e s and t h e h a r d d e b r i s c i r c u l a t e s w i t h t h e l u b r i c a n t . Some y e a r s ago, Bryce and Roehner d i scussed t h i s problem i n d e t a i l , b u t f a i l e d t o a r r i v e a t a s a t i s f a c t o r y explanation.
However,
i t has been
e s t a b l i s h e d t h a t t h e c o r r o s i o n o ccurs o n l y when aqueous e l e c t r o l y t e s a r e p r e s e n t i n t h e l u b r i c a n t , which suggests t h a t t h e t i n d i o x i d e i s formed by an e l e c t r o c h e m i c a l mechanism.
Some i n v e s t i g a t o r s c l a i m t o have r e -
produced t h e c o r r o s i o n i n t h e l a b o r a t o r y by making a p i e c e o f w h i t e metal t h e anode i n sea-water b u t , a l t h o u g h t h i s produces a b l a c k l a y e r , the l a y e r i s s o f t and amorphous.
We have found t h a t t h e n a t u r a l c u r r e n t
between p i e c e s o f w h i t e metal ( t i n - b a s e d t o B.S.S.
3332/2) and copper,
p a r t i a l l y immersed i n f u l l - s t r e n g t h o r d i l u t e d s y n t h e t i c sea-water under o i l a t 60"C, w i l l cause t h e w h i t e me t a l t o corrode, f o r m i n g t h i c k , hard, adherent sta nn ic oxide. The b l a c k l a y e r s formed i n t h i s way have the c r y s t a l l o g r a p h i c and metall o g r a p h i c c h a r a c t e r i s t i c s o f t h e l a y e r s found i n engines. s i m p l e set-up
With t h i s
i t has been p o s s i b l e t o i n v e s t i g a t e t h e c o r r o s i o n mechanism
116
Fig.33
Effect o f micro-structure on corrosion.
fig.34
T i n oxide on t h r u s t bearing
117 and t o s t u d y t h e i n f l u e n c e o f v a r i o u s o i l a d d i t i v e s on t h e c o r r o s i o n r a t e . Some a d d i t i v e s reduced o r p r e v e n t e d t i n d i o x i d e f o r m a t i o n i n t h e t e s t cells;
u n f o r t u n a t e l y , almost a l l t h e a d d i t i v e s t h a t behaved i n t h i s way
would have an adverse e f f e c t on o t h e r components i n engines.
For example,
one a d d i t i v e t h a t c o m p l e t e l y prevented c o r r o s i o n o f t h e t i n was c o r r o s i v e t o copper a l l o y s . The mechanism o f t i n o x i d e f o r m a t i o n on b e a r i n g s i s s t i l l n o t f u l l y understood.
However, i t has been d e f i n i t e l y e s t a b l i s h e d t h a t c o r r o s i o n
occurs o n l y when e l e c t r o l y t e s a r e p r e s e n t i n t h e l u b r i c a n t and when t h e r e i s a r e s t r i c t e d supply o f oxygen.
The f a c t t h a t b e a r i n g s on bronze
supports appear t o e x p e r i e n c e more t r o u b l e than b e a r i n g s on s t e e l supports suggests t h a t t h i s i s p r i m a r i l y g a l v a n i c c o r r o s i o n .
T h i s i s n o t a problem
t h a t can be e a s i l y overcome by changes i n l u b r i c a t i n g o i l c o m p o s i t i o n o r changes i n a l l o y composition.
The b e s t remedy i s t o keep e l e c t r o l y t e s o u t
o f t h e system. ( v ) C o r r o s i o n o f c o p p e r - l e a d a l l o y s by water p r e s e n t i n t h e o i l T h i s f o r m o f c o r r o s i o n i n uncommon and i n d i c a t e s t h a t c o n s i d e r a b l e q u a n t i t i e s o f water a r e p r e s e n t i n t h e l u b r i c a t i n g o i l system.
Analysis
may i n d i c a t e t h a t t h e o i l i s i n e x c e l l e n t c o n d i t i o n and t h a t t h e water c o n t e n t i s n o t abnormal.
However, t h e water c o n t e n t o f a sample taken f o r
a n a l y s i s can depend v e r y much on when and where i t was taken.
I f sub-
s t a n t i a l amounts o f water a r e p r e s e n t i t c o u l d be e x e m p l i f i e d by t h e need t o change f i l t e r s f r e q u e n t l y
-
as t h e y s w e l l and b l o c k when i n c o n t a c t
w i t h water. The damage a s s o c i a t e d w i t h t h e presence o f water i n t h e o i l takes t h e f o r m o f removal o f t h e lead-based o v e r l a y m a t e r i a l and severe l o c a l i s e d c o r r o s i o n o f t h e l e a d phase i n t h e c o p p e r - l e a d b e a r i n g a l l o y (Figs. and 3 7 ) .
36
The o v e r l a y i s even removed f r o m t h e unloaded shoulders o f t h e
b e a r i n g , and t h e i n t e n s i v e l o c a l i s e d n a t u r e o f t h e a t t a c k i s i n d i c a t i v e o f galvanic corrosion, attacked
5.4.2.4
t h e l e s s - n o b l e ( a n o d i c ) l e a d being p r e f e r e n t i a l l y
.
C a v i t a t i o n and E r o s i o n
C a v i t a t i o n and e r o s i o n a r e i n c r e a s i n g l y a cause o f f a i l u r e on b e a r i n g s , owing t o i n c r e a s i n g l y h i g h and v a r i a b l e loads and speeds. these terms.
I t i s important t o define
Two t y p e s o f c a v i t i e s can form i n l u b r i c a n t s , vaporous c a v i t i e s
and gaseous c a v i t i e s .
Gaseous c a v i t i e s , which a r e formed by o u t g a s s i n g o f t h e
l u b r i c a n t , can f o r m and c o l l a p s e o n l y s l o w l y and cannot cause any mechanical damage d i r e c t l y .
However, t h e i r presence reduces t h e l o a d - c a r r y i n g c a p a c i t y o f
the l u b r i c a n t f i l m ,
t h e r e b y p r o m o t i n g o t h e r k i n d s o f f a i l u r e , such as f a t i g u e .
118
Tig.35
Fig.36
General o x i d a t i o n o f white-metal.
Damage associated w i t h the presence o f water i n o i l .
119 Vaporous c a v i t i e s , which c o n t a i n vapour o f t h e
i q u i d concerned, and l i t t l e o r
no gas, can f o r m and c o l l a p s e e x t r e m e l y r a p i d l y
and t h e v e r y h i g h p r e s s u r e
a s s o c i a t e d w i t h t h e i r c o l l a p s e can cause mechan c a l ( i m p a c t ) damage on metal surfaces.
Both t y p e s o f c a v i t y can be generate
by t h e p r e s s u r e f l u c t u a t i o n s
a s s o c i a t e d w i t h t h e f l o w o f l u b r i c a n t through a b e a r i n g and t h e f l u c t u a t i n g loads imposed on i t .
Vaporous c a v i t a t i o n can a l s o be caused by t h e v i b r a t i o n
o f a metal s u r f a c e i n c o n t a c t w i t h a l u b r i c a n t , as i n an u l t r a s o n i c c l e a n e r . The damage a s s o c i a t e d w i t h b o t h types o f c a v i t a t i o n may be aggravated by t h e presence o f f i n e p a r t i c l e s i n t h e l u b r i c a n t ;
i t i s i n such circumstances t h a t
i t may be j u s t i f i e d t o t a l k o f e r o s i o n damage and c a v i t a t i o n e r o s i o n .
However,
i t should be emphasised t h a t vaporous c a v i t a t i o n can cause severe damage even i n
the absence o f sol i d p a r t i c l e s . U n l i k e o t h e r types o f damage, vaporous c a v i t a t i o n damage i s g e n e r a l l y encount e r e d on t h e unloaded areas o f b e a r i n g s , where o i l - f i l m pressures a r e low, and t h i s p r o v i d e s a u s e f u l means o f i d e n t i f i c a t i o n ( F i g . 3 8 ) .
Microsections o f
damaged areas show s i g n s o f l o c a l work-hardening and f a t i g u e c r a c k i n g .
When t h e
damage i s due s o l e l y t o c a v i t a t i o n , t h e t e x t u r e i n t h e damaged areas i s rough (Fig.39);
when p a r t i c l e s a r e p r e s e n t ( c a v i t a t i o n e r o s i o n ) , t h e damaged surfaces
a r e smooth (Fig.40). Vaporous c a v i t a t i o n can remove p r o t e c t i v e f i l m s , and i n i t i a t e c o r r o s i o n .
such as o x i d e s , from m e t a l s
I n a d d i t i o n , t h e v e r y h i g h l o c a l pressures and temp-
e r a t u r e s a s s o c i a t e d w i t h t h e f i n a l stage o f c a v i t y c o l l a p s e can induce chemical r e a c t i o n s which would n o t n o r m a l l y t a k e p l a c e .
There i s some evidence t h a t
c e r t a i n o i l a d d i t i v e s a r e u n s t a b l e under c a v i t a t i n g c o n d i t i o n s and t h a t t h e decomposition p r o d u c t s can be c o r r o s i v e .
I n such circumstances, b e a r i n g s u r -
faces can be s u b j e c t e d t o t h e combined e f f e c t s o f c a v i t a t i o n and c o r r o s i o n . Work c a r r i e d o u t a t Thornton Research Centre i n d i c a t e s t h a t t h e r e i s v e r y l i t t l e one can do t o a commercial l u b r i c a n t t o e l i m i n a t e t h e e f f e c t s o f c a v i t a t i o n , and c a v i t a t i o n must be regarded p r i m a r i l y as a d e s i g n problem.
5.4.2.5
E l e c t r i c a l Discharge Damage
On e l e c t r i c a l machinery, and o c c a s i o n a l l y on o t h e r types o f machinery, p o t e n t i a l d i f f e r e n c e s can be b u i l t up and e l e c t r i c a l discharges may o c c u r across the b e a r i n g s u r f a c e s . Each d i s c h a r g e g i v e s r i s e t o a small p i t , and a l a r g e number o f discharges can e v e n t u a l l y cause damage o f t h e t y p e i l l u s t r a t e d i n Fig.41.
This type o f
damage i s c h a r a c t e r i s e d by t h e f a c t t h a t t h e p i t t i n g o c c u r s on b o t h b e a r i n g and j o u r n a l surfaces.
120
Fig.37
Section showing the i n t e n s i v e l o c a l i s e d n a t u r e o f the a t t a c k .
Fig.38
C a v i t a t i o n damage.
121
Fig.39
C a v i t a t i o n of w h i t e m e t a l .
Fig.40
Cavitation erosion o f white metal.
122
Fig.41
5.4.2.6
E l e c t r i c a l p i t t i n g i n l e a d base b a b b i t t b e a r i n g .
Wire-wool
Failures
T h i s i s th e name g i v e n t o a c a t a s t r o p h i c t y p e o f f a i l u r e t h a t has been enc ou n t e r e d d u r i n g t h e l a s t 10 ye ars on t i n-b ased j o u r n a l and t h r u s t b e a r i n g s on b o t h land-based and mari n e t u r b i n e i n s t a l l a t i o n s .
A hard, b l a c k scab (Fig.421,
which i n some cases has been shown t o be an amorphous m i x t u r e o f i r o n and t i n c o n t a i n i n g d i s p e r s e d i r o n c a r b i d e s and p o s s i b l y n i t r i d e s , forms on t h e surfaces o f t h e t i n - b a s e d whi t e-me t a l b e a r i n g s and proceeds t o machine-away t h e a d j a c e n t s t e e l s u r fa c e s .
S t e e l s l i v e r s a r e u s u a l l y found i n and around t h e b e a r i n g ,
sometimes i n s u f f i c i e n t q u a n t i t y t o g i v e t h e appearance o f w ire-w ool. f a i l u r e s a lm o s t always occu r w i t h i n a few hours o f s t a r t - u p ,
The
e i t h e r f r o m new o r
a f t e r an o v e r h a ul , and a r e c h a r a c t e r i s e d by a v e r y h i g h wear r a t e (sometimes c e n t i m e t r e s o f s t e e l may be l o s t ) w i t h o u t h i g h c o e f f i c i e n t s o f f r i c t i o n o r much temperature r i s e . ium-steel
The f a i l u r e s have been a t t r i b u t e d b o t h t o t h e use o f chrom-
r o t o r s h a f t s and t o e . ~ . o i l s .
There i s no c l e a r understanding o f t h e
sequence o f e v e n t s l e a d i n g t o f a i l u r e , and i n p a r t i c u l a r o f t h e r o l e s p l a y e d by metal s u r fa c e s , t h e base o i l and t h e a d d i t i v e s p r e s e n t i n t h e base o i l . can be s a i d i s t t i a t t h i s i s indeed a t r i b o l o g i c a l f a i l u r e .
A l l that
However, v a r i o u s
i n v e s t i g a t o r s have e s t a b l i s h e d t h a t t h e t r i g g e r i n g agent r e s p o n s i b l e f o r w i r e -
wool f a i l u r e s i s a d i r t p a r t i c l e , p a r t i a l l y embedded i n a w hite-metal b e a r i n g . I t i s n o t c e r t a i n what causes p a r t i c u l a r p a r t i c l e s t o t r i g g e r t h e sequence o f events l e a d i n g t o f a i l u r e , b u t whether o r n o t a b l a c k scab "machine t o o l " develops depends v e r y much on t h e n a t u r e o f t h e l u b r i c a n t and the composition o f
123 the r o t o r s t e e l .
O i l a d d i t i v e s (e.g.
sulphur-containing
e.p.
additives) that
can p r e v e n t b l a c k scab f o r m a t i o n w i t h ch romi um-containing-steel
r o t o r s may
a c t u a l l y promote scab f o r m a t i o n when used w i t h o t h e r r o t o r s t e e l s such as m i l d s t e e l and
4% molybdenum
that chlorine-containing
With these l a t t e r s t e e l s i t has been claimed
steel. e.p.
a d d i t i v e s a l s o increase t h e s u s c e p t i b i l i t y t o
failure,
b u t t h i s i s n o t supported by o u r i n v e s t i g a t i o n s a t Thornton Research
Centre.
What i s more pro ba bl e i s t h a t some l u b r i c a n t s a r e more l i k e l y t o a l l o w
f a i l u r e when t h e i r s p e c i f i c a n t i - r u s t / a n t i - w e a r The b e s t way t o a v o i d wire-wool
agent becomes depleted.
f a i l u r e s i s t o avoid d i r t .
t h e o u t s e t must be as c l e a n as p o s s i b l e .
The system from
The o i l should be c o n t i n u o u s l y f i l t e r e d
and t h e oil p r e ssu re i n t he t h r u s t ho usi n g under r u n n i n g c o n d i t i o n s should always be a t l e a s t 7 I b f / i n 2 (48 kN/m2). Now t h a t b l a c k scab f a i l u r e s have been w i d e l y p u b l i c i s e d , may have been encountered p r e v i o u s l y b u t passed unrecognised.
i t appears t h a t they
For example,
severe wear o f n i t r i d e d s t a i n l e s s - s t e e l s h a f t j o u r n a l s on power-recovery t u r b i n e s o f a i r c r a f t p i s t o n engines has been observed from t i m e t o t i m e and these appear t o have s u f f e r e d severe ma chi n i ng -t yp e wear.
It i s p a r t i c u l a r l y s i g n i f -
i c a n t t h a t t h e b e a r i n g s i n c o n t a c t w i t h these f a i l e d j o u r n a l s a r e i n g e n e r a l l y q u i t e good c o n d i t i o n , showing much l e s s damage than would be expected from the s t a t e o f the j o u r n a l s .
T h i s i s a l s o a c h a r a c t e r i s t i c o f b l a c k scab f a i l u r e s .
However, t h e a i r c r a f t b e a r i n g s were s i l v e r w i t h lead-indium o v e r l a y s .
I f these
f a i l u r e s a r e o f t h e b l a c k scab t yp e, then b l a c k scab i s n o t p e c u l i a r t o w h i t e metal b e a r in g s .
5.4.2.7
Fatigue
A p l a i n b e a r i n g may f a i l by f a t i g u e when i t has achieved i t s designed l i f e expectancy;
however,
i f f a i l u r e occu rs p rematurely, t h i s w i l l be because e i t h e r
an i n c o r r e c t b e a r i n g m a t e r i a l has been used o r t h e b e a r i n g has been i n c o r r e c t l y fitted. I n f a t i g u e f a i l u r e s t h e c r a c k s s t a r t a t t h e b e a r i n g s u r f a c e , propagate n o r mal t o t h e s u r f a c e u n t i l t he y approach t h e s h e l l , then t u r n through 90" and exte n d p a r a l l e l t o t h e bond between t h e b e a r i n g metal and the s h e l l l e a v i n g a t h i n l a y e r o f b e a r i n g metal a t t a c h e d t o t h e s h e l l (Fig.43). Bad bonding and f a t i g u e a r e s u p e r f i c i a l l y
similar.
However, w i t h bad bonding
th e b e a r i n g m e ta l separates c l e a n l y f r o m t h e s h e l l , whereas w i t h f a t i g u e the c r a c k s s t a r t a t t h e b e a r i n g s u r f a c e , propagate normal t o t h e b e a r i n g s u r f a c e u n t i l t h e y approach t h e s h e l l and t h en ext en d p a r a l l e l t o the bond between t h e b e a r i n g m e ta l and t h e s h e l l , always l e a v i n g a l a y e r o f b e a r i n g metal a t t a c h e d t o the s h e l l .
For a comparison o f t h e two t ypes o f f a i l u r e see Fig.44.
124
Fig.42
Black scab formation on a thrust bearing.
Fi9.43
F a t i g u e of lead b r o n z e bearing.
125
Fig.44
5.4.2.8
White met al b e a r i n g s
Thermal C y c l i n g Damage
T h i s i s o n l y a problem on t i n - r i c h b e a r i n g a l l o y s . o f t i n crystals i s anisotropic,
i.e.
The thermal expansion
t h e l i n e a r c o e f f i c i e n t o f thermal expansion
i s d i f f e r e n t a l o n g t h e t h r e e p r i n c i p a l c r y s t a l l o g r a p h i c axes. ti n - b a s e d w h ite - met al
Consequently,
b e a r i n g s exposed t o c o n s i d e r a b l e thermal c y c l i n g can ex-
p e r i e n c e g r a in - b o un da ry d i s t o r t i o n and c r a c k i n g (Fig.45).
Bad bonding f a c i l -
i t a t e s t h i s type o f f a i l u r e .
5.4.2.9
A l l o y i n g i n Service
T h i s can be a problem on e l e c t r o p l a t e d l e a d - a l l o y o v e r l a y b e a r i n g s and on micro-Babbitt
b eari n gs.
I n b o t h cases t h e t i n i n t h e s u r f a c e l a y e r m i g r a t e s
towards and a l l o y s w i t h t h e c o p p e r - r i c h phase o r s i l v e r phase i n t h e underlay. Hard,
i n t e r m e t a l l i c compounds a r e formed which,
the j o u r n a l surfaces.
i f they a r e exposed, can score
T h i s p rob l em i s most a c u t e on m i c r o - B a b b i t t bearings,
where complete a l l o y i n g o f t h e 200-pm t h i c k , t i n - r i c h s u r f a c e has been observed i n s e r v i c e (Fig.46).
Ob vi ou sl y, t h e a l l o y i n g r e a c t i o n i s dependent on b o t h
ti me and temperature;
e xpe ri men t s show t h a t i t can proceed v e r y r a p i d l y a t
150°C ( Fig .4 7 ) .
126
Fig.45
5.4.2.10
D i s t o r t i o n s i n t i n due t o thermal c y c l i n g .
Inadequate V i s c o s i t y and Lack o f L u b r i c a n t
Should t h e s up pl y o f l u b r i c a n t t o a b e a r i n g be i n t e r r u p t e d , even f o r a m a t t e r o f seconds, c a t a s t r o p h i c f a i l u r e can occu r.
The damage u s u a l l y takes t h e form
o f complete s e i z u r e and e x t e n s i v e m e l t i n g o f t h e b e a r i n g a l l o y .
I f the f l o w o f
a l u b r i c a n t i s l i a b l e t o i n t e r r u p t i o n , some degree o f temporary p r o t e c t i o n can
be p r o v i d e d by t h e use o f s p e c i a l s u r f a c e t r e a t m e n t s on t h e s t e e l surfaces, by using p l a s t i c o r plastic-impregnated
b e a r i n g s and by t h e use o f g r a p h i t e o r
molybdenum d i s u l p h i d e i f t h e l a t t e r i s c o r r e c t l y a p p l i e d . Some f a i l u r e s due t o o i l s t a r v a t i o n a r i s e because o f l a c k o f l u b r i c a n t on start-up,
and on many items o f heavy machinery i t i s e s s e n t i a l t o have a means
of c i r c u l a t i n g t he o i l p r i o r t o s t a r t - u p t o p r o v i d e l u b r i c a t i o n and c o o l i n g .
127
I n t e r m e t a l l i c compound f o r m a t i o n on m i c r o - B a b b i t t b e a r i n g .
Fig.46
D i l u t i o n o f t h e l u b r i c a n t by f u e l can o c c u r i n b o t h g a s o l i n e and d i e s e l engines.
I t i s most f r e q u e n t l y encountered on l i g h t l y loaded, c o o l - r u n n i n g
engines, e s p e c i a l l y those w i t h poor combustion c h a r a c t e r i s t i c s . fuel
i n t h e l u b r i c a n t lowers i t s v i s c o s i t y ,
capacity.
The presence o f
thereby r e d u c i n g i t s l o a d - c a r r y i n g
I f t h e r e d u c t i o n i n v i s c o s i t y i s m a r g i n a l , premature f a i l u r e by
f a t i g u e may o c c u r ;
however, i f t h e r e d u c t i o n i s severe,
then sudden, c a t a s t -
r o p h i c f a i l u r e may t a k e p l a c e .
5.5
CONCLUS IONS I n t h e p r e c e d i n g pages we have attempted t o d e s c r i b e t h e m e t a l l u r g i c a l
f e a t u r e s o f some o f t h e b e a r i n g f a i l u r e s t h a t we have i n v e s t i g a t e d i n r e c e n t years.
Bearing f a i l u r e mechanisms a r e becoming i n c r e a s i n g l y complex and i t i s
no l o n g e r always p o s s i b l e t o g i v e r e l i a b l e , on-the-spot
diagnosis.
Metallo-
graphic examination o f sections from f a i l e d bearings w i l l o f t e n provide the necessary i n f o r m a t i o n t o e s t a b l i s h t h e cause o f t h e t r o u b l e .
For t h e more
d i f f i c u l t and complex cases t h e r e i s a wide v a r i e t y o f i n v e s t i g a t i o n a l t e c h niques t h a t can be a p p l i e d t o t h e problem.
To a s s i s t o t h e r s i n t h e d i a g n o s i s
o f p l a i n b e a r i n g f a i l u r e s we have compiled Table 5.4, v a r i o u s f a i l u r e s a r e described.
i n which t h e f e a t u r e s o f
W h i l s t t h i s i s n o t a s u b s t i t u t e f o r experience,
we b e l i e v e t h a t , c o u p l e d w i t h t h e accompanying b i b l i o g r a p h y , i t may be an a i d t o e s t a b l i s h i n g t h e cause o f f a i l u r e s .
128
Electro deposited t i n lead overlay
on
A f t e r 2 hrs. a t 170°C
Fig.47
25pm
After 2 h r s at 130'C
After 2 hrs.at
Electron-probe micrograph showing rate of diffusion o f tin.
150°C
129 TABLE 5.4 Cause o f f a i 1 ure
Typical f e a t u r e s
Extraneous matter c i r c u l a t i n g i n 1 ubr icant
S o f t p a r t i c l e s , e.g. carbonaceous matter, can erode w h i t e metals and overlays. Hard p a r t i c l e s , e.g. m e t a l l i c wear d e b r i s c u t s w e l l - d e f i n e d channels.
Extraneous matter embedded i n bearing
Burnished spots round embedded p a r t i c l e s . Wear o f j o u r n a l - can lead t o f a t igue.
Fatigue
Cracks, i n i t i a t e d a t bearing surface, propagate normal t o surface; then, near backing, t u r n and run p a r a l l e l w i t h bearing surface.
Bad bonding
Bearing a l l o y l i f t s c l e a r l y away from no evidence o f a l l o y i n g . backing Comnon on l a r g e , white-metal bearings.
The rma 1 cyc 1 ing
Can cause roughening o f tin-based whitemetal bearing surfaces
Surface p i t t i n g o f tin-based w h i t e metals
Cracking o f o v e r s i z e tin-antimony cuboids
Hard, black surface l a y e r s a l l over tin-based white-metal bearings
T i n - r i c h m a t r i x o f a l l o y transformed t o hard t i n oxides. Antimony-tin and c o p p e r - t i n phases unchanged.
Sudden c a t a s t r o p h i c wear o f a s i n g l e t u r b i n e j o u r n a l o r t h r u s t bearing shortly after start-up
Black scab formation i n white-metal bearing, which "machines" s h a f t , producing wire-wool.
Electrical p i t t i n g
Fine p i t s on both bearing surface and journal.
Cavitation i n lubricant f i l m
Local ised metal removal, g e n e r a l l y i n unloaded areas o f bearing.
Corrosion by weak o r g a n i c a c i d s
Surface roughening and f i l l i n g . I n copper-lead and lead-bronze, pure lead phase i s leached o u t and surface may d i s i n t e g r a t e . Lead overlays d e f i c i e n t i n t i n o r indium may be corroded.
Corrosion by s t r o n g a c i d s
Takes a v a r i e t y o f forms; bearing metal o r j o u r n a l may be attached. See t e x t .
"Sulphur" a t t a c k
D i s c o l o r a t i o n and c o r r o s i o n o f copper and s i l v e r - r i c h a l l o y s . Can be very severe i n phosphor bronzes.
Corrosion by water i n the o i l
Takes t h e form o f removal o f the o v e r l a y and l o c a l i s e d i n t e n s i v e a t t a c k o f the lead phase o f copper-lead bearing a l l o y s .
Excessive o p e r a t i n g temperatures
\ t i p i n g o f surface layers. Fatigue f a i l u r e .
Inadequate v i s c o s i t y o f l u b r i c a n t
If marginal, may g i v e r i s e t o premature f a i l u r e by f a t i g u e . I f severe, f o r example due t o excessive f u e l d i l u t i o n , can g i v e sudden c a t a s t r o p h i c f a i l u r e .
Lack o f l u b r i c a n t
Complete seizure. Extensive m e l t i n g o f bearing a l l o y .
Bad f i t t i n g
F r e t t i n g o f backs o f bearing. Lack o f thermal c o n t a c t w i t h housing may cause bearing t o bow.
Misalignment
Uneven c o n t a c t and wear across bearing surface.
Manufacturing defects, e.g. poor lead d i s t r i b u t i o n i n copper-lead alloys
Requires expert. m e t a l l u r g i c a l examinat ion.
Surface hardening o f t i n B a b b i t t overlay
Cracking o f o v e r l a y , wear of j o u r n a l .
I n t e r n a l o x i dat i o n o f lead- i n d i um over I ays
Corrosion and f a t i g u e o f o v e r l a y .
-
130 REFERENCES
1
2 3
Sleeve Bearing M a t e r i a l s , 1949, ASM Cleveland, Ohio. Metals Handbook, 1961, 8 t h e d i t i o n , Vol.1, p.843-863, ASM Cleveland, Ohio. ( 3 ) , p.29, "The Modern Bus and Truck - Fuels and L u b r i c a t i o n , 1953, Lubricants". ( 5 ) , p.57, "Some Problems A s s o c i a t e d w i t h L u b r i c a t i o n L u b r i c a t i o n , 1953, o f Large Engines". L u b r i c a t i o n , 1953, 8, I ( l l ) , p.45, "Automotive Engine Bearings". L u b r i c a t i o n , 1955, 10, ( 4 ) , p.37, " I n d u s t r i a l B e a r i n g L u b r i c a t i o n " . L u b r i c a t i o n , 1957, ( 8 ) , p.85, "Abrasives and Wear". L u b r i c a t i o n , 1958, 13, ( 9 ) , p.245, "Petroleum L a b o r a t o r y I n v e s t i g a t i o n s " . ( 6 ) , p.81, "Diesel Power Plants". L u b r i c a t i o n , 1963, ( 7 ) , p.77, " P l a i n Bearing F a i l u r e s " . L u b r i c a t i o n , 1964, F o r r e s t e r , P.G., Modern M a t e r i a l s , 1964, 4, p.173, Academic Press, New York and London, " M a t e r i a l s f o r P l a i n BeaTings". " E l e c t r o - g r a p h i c Methods o f Hunter,M.S., C h u r c h i l l , J.R. and Mear, R.B., Surface A n a l y s i s " , Metal Progress 1942, 42, p.1070. Crooks, C.S. and Eastham, D.R. " P l a t i n g z r Bearing A p p l i c a t i o n s " . Trans. I n s t . o f Metal F i n i s h i n g 1982 v01.60. Rafique, S.O., I n s t . Mech. Eng. L u b r i c a t i o n and Wear, Second Convention " F a i l u r e s o f P l a i n Bearings and t h e i r Causes". 1964, p.180. Love,P.P., Forrester,P.G. and Burke,A.E., I n s t . Mech. Eng. Auto. D i v . p.29. "Function o f M a t e r i a l s i n Bearing Operation". Proc., 1953-54, L u b r i c a n t s and L u b r i c a t i o n ( e d i t o r B r a i t h w a t e , E.R.) E l s e v i e r , London,
8, 8,
E,
4 5 6
7 8 9
5, z,
2,
1967. M o r r i s , J.A., Ch.7. p.310, " M e t a l l i c Bearing M a t e r i a l s " . P r a t t , G.C., Ch.8, p.377, " P l a s t i c - b a s e d Bearings". 10
Pratt,G.C. and Perkins,C.A., " S i l i c o n Aluminium Bearings f o r High-speed Diesels", D i e s e l and Gas T u r b i n e Worldwide Vol. X I I I , No.10, p.76-78. 1 1 Davis,T.A., " P l a i n b e a r i n g wear i n IC Engines", Automotive Engineer, Aug./Sept. 1981. 12 P r i n c i p l e s of L u b r i c a t i o n ( e d i t o r A. Cameron) Longmans Green and Co., London, 1966. Holligan,P.T., Ch.25, p.511. " P l a i n Bearings - Bearing M a t e r i a l s and Diagnosis o f Bearing Fa i 1 ures". 1 3 Engineering, 1967, 20, p.260. "Bond S t r e n g t h o f White M e t a l l i n g " . 14 Rose, A. Trans. l n s t . Mar. Eng. 1967, 79, p.233, "Marine Bearings". J o i n t Course on T r i b o l o g y , I n s t i t u t i o n o f 15 Wilson,R.W. and Shone,E.B., M e t a l l u r g i s t s , London, 1968, Paper 4, " M e t a l l u r g i c a l S t u d i e s o f Bearing Failures". 16 Wilson,R.W. and Shone,E.B., A n t i - C o r r o s i o n Methods and M a t e r i a l s , 1970, 17, p.9. "The C o r r o s i o n o f Lead Overlay Bearings". "The Science o f T r i b o l o g y 17 Quayle, J.P., Copper, 1969, 3, ( 5 ) , p.12. P a r t 3Il. 18 Bryce,J.B. and Roehner,T.G., Trans. I n s t . Mar. Engs., 1961, 73, p.377, "The C o r r o s i o n o f Tin-Base B a b b i t t Bearings i n Marine Steam Turbines". I n s t . Mech. Engs., T r i b o l o g y Convention 1969, 19 Lloyd,K.A. and Wilson,R.W., Paper 10, p.76. "Formation o f T i n Oxides on White Metal Bearinqs". "Corrosion o f T i n Base B a b b i t t Bearings t o f o r m T i n Oxides", 20 H i l e y , R.W., Trans l n s t . Mar. Eng., 1979, 91, (2) p. 52-66. 21 Dawson, P.H. and F i d l e r , F., I n s t . Mech. Engs. ( L u b r i c a t i o n and Wear F i f t h Convention), 1967, 1811, p. 207, "Wire-wool Type F a i l u r e s ; The e f f e c t o f S t e e l Composition, S t r u c t u r e and Hardness". 22 Dowson, D., Godet, M. and T a y l o r , C.M., " C a v i t a t i o n and R e l a t e d Phenomena i n L u b r i c a t i o n " , U n i v e r s i t y o f Leeds, Yorks, England. Leeds-Lyon Symposium on T r i b o l o g y , l s t , Prog. Pap and Discuss., U n i v e r s i t y o f Leeds, Yorks, England, Sept. 1974, Publ. by Mech. Eng. Publ. f o r I n s t i t u t e o f T r i b o l o g y , Publ. Ltd., New York, NY, 1974, 248. Leeds U n i v e r s i t y , Yorks, England.
131
23 24 25 26 27 28 29
James, R.D., " C a v i t a t i o n Damage i n P l a i n Bearings". T r i b o l o g y I n s t . Feb. 1978, 1 1 , (1) p. 22-23. Garner, D.R., James, R.D. and W a r r i n e r , J.F., " C a v i t a t i o n E r o s i o n i n Engine B e a r i n g s - Theory and P r a c t i c e " , 1 3 t h CIMAC Conf. Vienna 1979. F o r r e s t e r , P.G., " B e a r i n g M a t e r i a l s " , M e t a l l u r g i c a l Reviews, 1960, 5, p. 507. F o r r e s t e r , P.G., " E l e c t r o d e p o s i t i o n i n P l a i n B e a r i n g Manufacture", Trans. I n s t . Met. F i n i s h i n g , 1961, 38, p.52. Booser, E.R., " P l a i n B e a r i n g M a t e r i a l s " , Machine Design, 1970, 42, p. 14. Standard Handbook o f L u b r i c a t i o n E n g i n e e r i n g , ASLE, M c G r a w - H i l l , 1968. Chapter 18, "S1 i d i n g Bearings". T r i b o l o g y Handbook ( e d i t o r M.J. N e a l e ) , B u t t e r w o r t h s , London 1973.
132
(;
ROLLING ELEMENT BEARINGS
D.G.
HJERTZEN and
6.1
INTRODUCTION
R.A.
JARVIS,
SKF (U.K.)LTD.
The p r e s e n t development o f r o l l i n g b e a r i n g s i s c h a r a c t e r i s e d by numerous a p p a r e n t l y s m a l l i n t e r n a l improvements i n t h e b e a r i n g s and t o m a t e r i a l q u a l i t y r a t h e r than t h e i n t r o d u c t i o n o f r a d i c a l new designs.
There i s c o n s i d e r a b l e
t e c h n i c a l r e s e a r c h i n t o r o l l i n g b e a r i n g technology and i m p o r t a n t progress i s b e i n g made i n m a n u f a c t u r i n g and i n s p e c t i o n methods, improved l u b r i c a n t s and l u b r i c a t i o n equipment, e t c . I t i s possibly s u r p r i s i n g , but true,
t h a t although r o l l i n g bearings are
e x t e n s i v e l y used, t h e r e a r e many misconceptions and t h e methods o f s e l e c t i n g t h e most s u i t a b l e b e a r i n g s a r e n o t always f u l l y understood o r a p p l i e d .
It
has been known f o r some u n f o r t u n a t e e x p e r i e n c e w i t h a c e r t a i n b e a r i n g t y p e t o r e s u l t i n l o s s of confidence; cular
6.2
consequently,
the c h a r a c t e r i s t i c s o f t h a t p a r t i -
bearing are o f t e n n o t f u l l y u t i l i s e d .
BEARING SELECTION The major f u n c t i o n o f b e a r i n g s i s t o t r a n s m i t loads between machine p a r t s i n
r e l a t i v e motion, b u t a d d i t i o n a l l y t h e r e may be s p e c i a l performance o r e n v i r o n ment requirements a f f e c t i n g c h o i c e o f b e a r i n g s . I t i s e s s e n t i a l f o r t h e d e s i g n e r t o s e l e c t from t h e wide range o f b e a r i n g s a v a i l a b l e t h e b e a r i n g design s u i t a b l e t o c a r r y t h e loads i n v o l v e d under t h e v a r i o u s o p e r a t i n g c o n d i t i o n s and t o s a t i s f y t h e requirements o f r o t a t i o n a l speed, temperature v a r i a t i o n s , b e a r i n g housing misalignment, and r i g i d i t y e t c . I t i s a l s o i m p o r t a n t t o ensure t h a t c o r r e c t f i t s a r e used between t h e b e a r i n g i n n e r r i n g t o s h a f t and o u t e r r i n g t o housing.
The c h o i c e o f f i t depends
e s s e n t i a l l y on t h e w o r k i n g c o n d i t i o n s , b u t t h e r e a r e o t h e r c o n s i d e r a t i o n s such as b e a r i n g t y p e and s i z e , b e a r i n g i n t e r n a l c l e a r a n c e and method o f assembly, etc.
133 6.3
BEARING TYPES
6.3.1
S i n g l e Row Deep Groove B a l l B e ari n g ( Fig.1)
The depth o f t h e b a l l t r a c k s cou pl e d w i t h a r e l a t i v e l y l a r g e b a l l s i z e and h i g h degree of c o n f o r m i t y ( r a t i o o f t r a c k r a d i u s t o b a l l r a d i u s ) g i v e s t h i s bearing considerable a x i a l - c a r r y i n g capacity i n a d d i t i o n t o r a d i a l capacity, even a t h i g h speeds.
These b e a r i n g s n o r m a l l y have a c y l i n d r i c a l bore and a r e
mounted d i r e c t o n t o t h e s h a f t .
For l o c a t i o n purposes, a groove can be p r o v i d e d
i n t h e o u t e r r i n g f o r a snap r i n g wh i ch can be used f o r a x i a l l o c a t i o n . These b e a r i n g s can be f i t t e d w i t h s h i e l d s o r s e a l s .
The s h i e l d s a r e intended
as grease r e t a i n e r s and t o keep o u t a c e r t a i n amount o f f o r e i g n m a t t e r , b u t t h e s e a l s a r e a c t u a l l y r u b b i n g s e a l s and designed f o r dusty environments,
etc.
Be a r in g s w i t h two s h i e l d s o r s e a l s a r e i n i t i a l l y charged w i t h t h e c o r r e c t quant i t y o f grease and, co nse qu en t l y, do n o t r e q u i r e r e l u b r i c a t i o n .
These bearings
a r e f r e q u e n t l y c a l l e d l u b r i c a t e d - f o r - l i f e bearings.
Fig.1
6.3.2
S i n g l e Row Deep Groove B a l l B e a r ing .
F ig.2
Self-A igning
B a l l Bearing.
S e l f - a l i g n i n g B a l l B e ari n gs (F i g. 2)
These have two rows o f b a l l s each i n i t s own groove on the i n n e r r i n g , b u t i n a common sphered t r a c k i n t h e o u t e r r i n g .
The i n n e r r i n g and b a l l s form a
u n i t wh ic h can a l i g n f r e e l y a bo ut t h e b e a r i n g c e n t r e .
T h i s f e a t u r e i s an advan-
tage i n cases where i t i s d i f f i c u l t t o o b t a i n a c c u r a t e p a r a l l e l i s m between t h e s h a f t and h o u s in g bo re, o r where t h e r e i s a d e f l e c t i o n o f t h e s h a f t .
Due t o
the sphered o u t e r r i n g t r a c k , t h e b e a r i n g does n o t have h i g h l o a d - c a r r y i n g
134 c a p a c i t y and t h e a x i a l - c a r r y i n g c a p a c i t y i s l i m i t e d . The degree o f mis-alignment o f t h e s h a f t o r housing i s l i m i t e d by t h e b a l l s c o n t a c t i n g t h e edges o f t h e o u t e r r i n g and p e r m i s s i b l e m i s - a l i g n m e n t n o r m a l l y v a r i e s from 2'
-
3'.
These b e a r i n g s a r e manufactured w i t h c y l i n d r i c a l o r t a p e r b o r e and t h e l a t t e r i s u s u a l l y mounted on a s p l i t s l e e v e .
6.3.3
Angular Contact B a l l B e a r i n g [ F i g . 3 )
The d i r e c t i o n o f l o a d through t h e b a l l s i s a t an a n g l e t o t h e b e a r i n g a x i s which makes these b e a r i n g s p a r t i c u l a r l y s u i t a b l e f o r c a r r y i n g combined r a d i a l and a x i a l loads.
A r a d i a l l o a d imposed on a s i n g l e row a n g u l a r c o n t a c t b a l l
b e a r i n g g i v e s r i s e t o an induced a x i a l l o a d which must be c o u n t e r a c t e d ; t h e r e f o r e these b e a r i n g s need t o be arranged so t h a t t h e y can be a d j u s t e d a g a i n s t a second b e a r i n g .
Fig.3
Angular Contact B a l l B e a r i n g .
Fig.4
Double Row Angular Contact B a l l Bearing.
These b e a r i n g s a r e f r e q u e n t l y mounted i n p a i r s i n f a c e - t o - f a c e ,
back-to-back
o r tandem f o r m a t i o n , and i n o r d e r t o achieve t h i s t h e s i d e faces o f t h e b e a r i n g r i n g s a r e ground t o achieve t h e c o r r e c t r e s i d u a l i n t e r n a l c l e a r a n c e . The c o n t a c t a n g l e can v a r y between 15' 6.3.4
- 25'
-
30'
-
40'.
Double Row Angular Contact B a l l Bearing ( F i g . 4 )
T h i s b e a r i n g has s i m i l a r c h a r a c t e r i s t i c s t o two s i n g l e row a n g u l a r c o n t a c t b a l l b e a r i n g s mounted back-to-back;
consequently, t h e l i n e s o f p r e s s u r e o r
135 c o n t a c t through t h e b a l l s a r e d i r e c t e d outwards, rigidity.
thereby g i v i n g increased
These b e a r i n g s have v e r y l i t t l e a x i a l c l e a r a n c e , g i v i n g c l o s e a x i a l
l o c a t i o n o f t h e s h a f t and e l i m i n a t i n g t h e n e c e s s i t y
f o r a x i a l adjustment.
These b e a r i n g s can be s u p p l i e d t o g i v e a p r e - l o a d c o n d i t i o n when mounted so t h a t even when s u b j e c t e d t o a x i a l load, t h e a x i a l displacement o f t h e s h a f t i s very small.
6.3.5
C y l i n d r i c a l R o l l e r Bearings ( F i g . 5 )
The r o l l e r s i n these b e a r i n g s a r e g u i d e d between i n t e g r a l f l a n g e s on the o u t e r o r i n n e r r i n g , thereby a l l o w i n g t h e r i n g s t o move a x i a l l y r e l a t i v e t o each o t h e r , which i s an advantage when t h e s h a f t expansion i s g r e a t e r than t h e housing expansion i n an a x i a l plane.
The f l a n g e d r i n g and r o l l e r s a r e h e l d
t o g e t h e r by a cage t o form an assembly which can be removed from t h e o t h e r r i n g . T h i s s e p a r a b l e f e a t u r e i s o f t e n u t i l i s e d t o ease assembly problems.
The bear-
i n g has h i g h r a d i a l l o a d - c a r r y i n g c a p a c i t y w i t h a c c u r a t e g u i d i n g o f t h e r o l l e r s , r e s u l t i n g i n a c l o s e approach t o t r u e r o l l i n g .
Consequently,
t h e low f r i c t i o n
p e r m i t s h i g h speeds.
Fig.5
C y l i n d r i c a l R o l l e r Bearing.
Fig.6
Double Row C y l i n d r i c a l R o l l e r Bearing.
V a r i o u s designs a r e made w i t h d i f f e r e n t f l a n g e arrangements and a d d i t i o n a l
loose f l a n g e s can be i n c o r p o r a t e d t o a x i a l l y l o c a t e t h e r o l l e r s i n b o t h d i r e c tions.
The r o l l e r ends and r i n g f l a n g e s a r e capable o f s u s t a i n i n g r e l a t i v e l y
h i g h a x i a l loads.
136 6.3.6
Double Row C y l i n d r i c a l R o l l e r B e ari n gs ( F i g . 6 )
These b e a r in g s a r e o f sep ara bl e d esi g n which a l l o w s a f l a n g e d r i n g w i t h r o l l e r and cage assembly and f l a n g e l e s s r i n g t o be f i t t e d independently, thus f a c i l i t a t i n g mounting and d i smou nt i ng . Low c r o s s - s e c t i o n a l
h e i g h t and h i g h l o a d - c a r r y i n g c a p a c i t y g i v e r i g i d bear-
i n g arrangements f o r t h e accommodation o f heavy r a d i a l loads.
They a r e , there-
f o r e , m a i n l y used f o r machine t o o l and r o l l i n g m i l l a p p l i c a t i o n s . r i n g n o r m a l l y has a t ap ere d
The i n n e r
b ore t o e na bl e t h e r i n g t o be d r i v e n up a tapered
j o u r n a l t o a c h ie ve a g i v e n r a d i a l cl e ara nce o r even p r e - l o a d .
They can be
supplied i n special o r u l t r a - p r e c i s i o n execution.
6.3.7
Needle R o l l e r Be ari n gs (F i g. 7)
These a r e s i m i l a r t o c y l i n d r i c a l r o l l e r b e a r i n g s and have h i g h l o a d - c a r r y i n g capacity.
They a r e made i n v a r i o u s d esi g ns and a r e p a r t i c u l a r l y s u i t a b l e f o r
a p p l i c a t i o n s i n c o n f i n e d spaces.
These b e a r i n g s a r e used i n gudgeon p i n s and
u n i v e r s a l j o i n t s , b u t they a r e n o t recomnended where t h e r e i s l i k e l y t o be misal ig n m e n t o r s h a f t bending o r any c o n d i t i o n where t h e r o l l e r s can be s u b j e c t e d t o t i l t i n g f o r c e s whi ch can cause a c l u t c h a c t i o n .
Fig.7
Needle R o l l e r Bearing.
Fig.8
Taper R o l l e r Bearing.
The d ia m e t e r s o f ne ed l e r o l l e r s a r e s m a l l , u s u a l l y 1 . 5 t o 5 mm and t h e r o l l e r l e n g t h i s a p p r o xi ma t e l y 2 . 5 t i mes t h e d i ame t er.
137
6.3.8
Taper R o l l e r Bearings ( F i g . 8 )
The r o l l i n g elements i n these b e a r i n g s a r e t r u n c a t e d cones and t h e t r a c k o f b o t h t h e i n n e r r i n g (cone) and t h e o u t e r r i n g (cup) a r e tapered.
The t a p e r s of
th e r o l l e r s and t h e i n n e r r i n g have a common apex on t h e b e a r i n g a x i s and t h e greater the i n c l i n a t i o n o f the r o l l e r s , the greater the a x i a l carrying capacity. As w i t h s i n g l e row a n g u l a r c o n t a c t b a l l be ari ngs, t a p e r r o l l e r bearings must always be mounted i n p a i r s o r a d j u s t e d towards another b e a r i n g capable o f dealing w i t h a x i a l forces a c t i n g i n the opposite d i r e c t i o n .
The t a p e r b e a r i n g can
a l s o be s u p p l i e d as a p a i r e d u n i t w i t h a common i n n e r o r o u t e r r i n g .
This
b e a r i n g i s used e x t e n s i v e l y i n t h e Automobile I n d u s t r y .
6.3.9
Double Row S p h e r i c a l R o l l e r B e ari n gs ( F i g . 9 )
Due t o t h e s i z e , shape, and number o f r o l l e r s , t o g e t h e r w i t h the accuracy w i t h wh ic h t h e r o l l e r s a r e guided, t h i s b e a r i n g has e x c e l l e n t l o a d - c a r r y i n g c a p a c i t y i n b o t h a r a d i a l and a x i a l d i r e c t i o n .
Since t h e b e a r i n g i s s e l f -
a l i g n i n g , a n g u l a r mi sa l i gn men t between s h a f t and housing has no d e t r i m e n t a l e f f e c t and f u l l c a p a c i t y i s always a v a i l a b l e .
Accurate r o l l e r guidance i n the
normal ' C ' d e s ig n i s by means of a l oo se r e l a t i v e l y narrow guide r i n g of t h i c k r a d i a l s e c t i o n between t h e rows o f r o l l e r s and by t h e cage.
Fig.9
Double Row S p h e r i c a l R o l l e r B e a r in g
Fig.10
Spherical R o l l e r Thrust Bearing
S p h e r i c a l r o l l e r b e a r i n g s a r e manufactured w i t h c y l i n d r i c a l o r tapered bores. They can be mounted on t ap ere d sleeves and a r e used i n Railway and Heavy Engineering.
These b e a r i n g s a r e regarded as t h e heavy-weight o f t h e standard b e a r i n g
138 types and a r e used e x t e n s i v e l y i n plummer b l o c k s .
6.3.10
S p h e r i c a l R o l l e r T h r u s t Bearings (Fig.10)
U n l i k e most o t h e r t ype s o f t h r u s t be ari n g,
t h e l i n e o f a c t i o n o f t h e load
a t t h e c o n t a c t s between t h e t r a c k s and t h e r o l l e r s forms an a n g l e w i t h t h e b e a r i n g a x i s wh ich makes these be ari n gs s u i t a b l e f o r c a r r y i n g a r a d i a l load up t o 55% o f t h e s i m u l t a n e o u s l y a c t i n g a x i a l l o ad.
The sphered t r a c k o f t h e hous-
i n g washer p r o v i d e s s e l f - a l i g n i n g p r o p e r t i e s w hich p e r m i t a c e r t a i n angular displacement o f t h e s h a f t r e l a t i v e t o t h e housing, due t o mounting e r r o r s o r shaft deflection. I n o r d e r t o p r e v e n t u na ccep t a bl e s l i d i n g a t t h e r o l l e r - t o - t r a c k c o n t a c t s under t h e i n f l u e n c e o f c e n t r i f u g a l f o r c e and g y r a t o r y moments,
i t i s necessary
t o a p p l y a c e r t a i n minimum a x i a l l o a d t o t h e bearing. The s h a f t washer,
r o l l e r s and cage f orm an assembly w hich i s separable from
the housing washer, a l l o w i n g t h e washers t o be mounted independently.
These
be a r in g s a r e s u i t a b l e f o r heavy l o ad s and f o r r e l a t i v e l y h i g h speeds.
6.3.11
B a l l T h r u s t Bearings (Fig.11)
These c o n s i s t o f a row o f b a l l s , r e t a i n e d i n p o s i t i o n by a cage and two washers known as t h e s h a f t and h ou si ng washer, each w i t h a s h a l l o w b a l l t r a c k groove.
The s h a f t washer has a s m a l l e r bo re than t h e o t h e r washer and
l o c a t e d by th e s h a f t .
5
The ho usi n g washer has a l a r g e r o u t s i d e diameter than
the s h a f t washer f o r l o c a t i o n i n a housing. These b e a r in g s a r e o n l y capable o f c a r r y i n g a x i a l l o a d i n one d i r e c t on.
Fig.11
B a l l T hru st B e ari n g
Fig.12
B e a r i n g w i t h S p h e r i c a l Outside Surf a c e and Extended I n n e r R ing Width.
139 6.3.12
Bearings w i t h S p h e r i c a l Ou t si d e Su rf ace and Extended I n n e r Ri n g Width (Fig.12)
These b e a r i n g s a r e e s s e n t i a l l y a v a r i a t i o n o f a deep groove b a l l b e a r i n g and are n o r m a l l y used i n c o n j u n c t i o n w i t h a range o f c a s t i r o n o r pressed s t e e l housings.
They
have a sphered o u t e r s u r f a c e and t h i s , when f i t t e d i n t o a
sphered s e a t i n g i n t h e housing, a l l o w s t h e u n i t t o accommodate any i n i t i a l misalignment o f t h e s h a f t whi ch may o c c u r d u r i n g mounting.
These bearings normally
have an extended i n n e r r i n g w i t h some f o r m o f l o c k i n g d e v i c e w hich i s used t o l o c k t h e b e a r i n g and, hence, t h e u n i t t o t h e s h a f t . made t o t h e same degree o f accuracy as
Although the b e a r i n g i s
deep groove b a l l b e a r i n g s , t h e method
o f l o c k i n g t h e b e a r i n g t o t h e s h a f t does n o t g i v e t h e same c e n t e r i n g accuracy as when b e a r i n g s a r e mounted w i t h an i n t e r f e r e n c e f i t on t h e s h a f t .
These
be a r in g s a r e used e x t e n s i v e l y i n machinery where t h e r o t a t i o n a l accuracy r e q u i r e ments a r e n o t t o o s t r i n g e n t , such as a g r i c u l t u r a l machinery and conveyors, e t c . where t h e y o f f e r a s i m p l e and economical s o l u t i o n .
6.4.
FATIGUE LIFE AND LOAD-CARRYING CAPACITY The concept o f a r o l l i n g b e a r i n g i s t h a t l o a d between t h e s t a t i o n a r y and
r o t a t i n g machine components i s t r a n s f e r r e d t hrough t h e b e a r i n g by means of i nte r p o s e d r o l l i n g elements.
Pure r o l l i n g seldom occurs and t h e n e t e f f e c t
u s u a l l y a c o m b in at i o n o f r o l l i n g and s l i d i n g . f u n c t i o n o f t h e b e a r i n g t ype , b u t
is
The exact p r o p o r t i o n s a r e a
90-99% o f t h e l o a d i s r e l a t e d t o r o l l i n g motion
and 1-10% t o s l i d i n g .
I f a b e a r i n g i s p r o p e r l y handled, c o r r e c t l y mounted, l u b r i c a t e d , and p r o t e c ted, a l l causes o f f a i l u r e a r e e l i m i n a t e d except one
-
fatigue o f the material.
The l i f e o f a r o l l i n g b e a r i n g as d e f i n e d by IS0 ( I n t e r n a t i o n a l Standards Organi s a t i o n ) i s t h e number o f r e v o l u t i o n s ( o r number o f o p e r a t i n g hours a t a g i v e n c o n s t a n t speed) whi ch t h e b e a r i n g i s cap ab l e o f e n d u r i n g b e f o r e f a t i g u e occurs on one o f i t s r i n g s o r r o l l i n g elements.
Repeated t e s t s have v e r i f i e d t h a t
when a group o f a p p a r e n t l y i d e n t i c a l be ari n gs a r e run under t h e same c o n d i t i o n s o f l o a d and speed t h e y have d i f f e r e n t l i v e s . "life"
A c l e a r d e f i n i t i o n o f t h e term
i s , therefore, essential f o r the c a l c u l a t i o n o f bearing s i z e .
Dynamic
l oa d r a t i n g s g i v e n i n b e a r i n g ma nu f a ct u rers catalogues a r e based on t h e l i f e t h a t 90% o f a s u f f i c i e n t l y l a r g e group o f a p p a r e n t l y i d e n t i c a l bearings can be expected t o a t t a i n o r exceed.
T h i s i s c a l l e d t h e b a s i c r a t i n g l i f e ( o r the
nominal l i f e ) and agrees w i t h t h e IS0 d e f i n i t i o n .
A t y p i c a l l i f e d i s p e r s i o n cu rve
is shown i n Fig.13, and i t can be seen t h a t
h a i f t h e b e a r in g s a c h i e v e an average l i f e f i v e times g r e a t e r than the nominal l i f e on which t h e c a l c u l a t i o n s a r e based.
140
F%
The classic curve of hearing life shows the life distribution for identical bearings run under idcntical conditions of load and speed. S is the perccntagc of still serviceable brarings, F the percentagc showing signs of fatigue, and L, the life of an individual bearing, given in t e r n of I.,, as explained in the text
S,%
Fig.13
6.4.1
A T y p i c a l B e a r i n g L i f e D i s p e r s i o n Curve.
The H e r t z i a n Contact
W h i l s t t h e H e r t z t h e o r y r e l a t i n g t o t h e c o n t a c t between s o l i d bodies i s s t i l l of i n t e r e s t i n c o n n e c t i o n w i t h c a l c u l a t i n g c o n t a c t pressures and deforma t i o n s i n r o l l i n g b e a r i n g s , t h e g r e a t e s t i n t e r e s t i s i n t h e shear s t r e s s e s beneath th e c o n t a c t s u r f a c e as t he se a r e con sidered t o be t h e cause o f b o t h p l a s t i c d e f o m a t i o n s and f a t i g u e c r a c k s i n t h e c o n t a c t zone. F i g u r e 14 shows t he s t r e s s d i s t r i b u t i o n i n a H e r t z i a n c o n t a c t and from a f a t i g u e aspect t h e shear s t r e s s e s a t t h e edge o f t h e c o n t a c t zone a r e t h e most dangerous ones.
They a r e p a r a l l e l t o t h e c o n t a c t s u r f a c e and a r e s i t u a t e d
beneath th e s u r f a c e . age o f t h e r o l l i n g
As these shear s t r e s s e s change d i r e c t i o n w i t h t h e pass-
body, m a t e r i a l f a t i g u e w i l l o c c u r i f any weak p o i n t such
as a s l a g i n c l u s i o n comes w i t h i n t h e su bsu rf ace zone where these shear s t r e s s e s a r e c o n s id e r a b le . s t i t u t e weak
I t has been proved t h a t even t i n y s l a g i n c l u s i o n s can con-
p o i n t s o f t h i s k i n d and t h a t under t h e i n f l u e n c e o f the a l t e r n a -
t i n g shear s t r e s s e s these i n c l u s i o n s r e s u l t i n m i c r o c r a c k s w hich subsequently i nc r e a s e i n s i z e , f i n d t h e i r way up t o t h e s u r f a c e , and l e a d t o s u r f a c e f a t i g u e f a i l u r e (see F ig. 15 ).
141
Fig.14 T
max
Shear Stresses i n t h e Contact Zone.
z g r e a t e s t shear s t r e s s i n t h e c o n t a c t zone and occurs a t a depth z
below t h e c o n t a c t s u r f a c e f o r m i n g an a n g l e o f the contact surface. and zmax z Tzy
max
0.786
max w i t h the plane o f
45'
For l i n e c o n t a c t o f i n f i n i t e l e n g t h
b where ' b '
T~~~~
0.30Uo
i s h a l f t h e w i d t h o f t h e c o n t a c t zone.
g r e a t e s t shear s t r e s s p a r a l l e l t o t h e p l a n e o f t h e c o n t a c t s u r f a c e and o c c u r s at a depth"zo" t a c t o f i n f i n i t e length
beneath T~~
max
=
the contact surface.
0.25Uo and zo
=
For l i n e con-
0.50 b.
R e l a t i o n s h i p between Load and L i f e
6.4.2
The r e l a t i o n s h i p between b e a r i n g l o a d and l i f e shown i n F i g u r e 16 has been o b t a i n e d by t e s t i n g b e a r i n g s under loads o f d i f f e r e n t magnitude. s e r i e s were r u n by t h e ings
under loads P 1 , P 2 and P3.
d o t s on t h e h o r i z o n t a l l i n e s . i n each t e s t s e r i e s ,
by t h e numerals 1, 2 and
3.
i.e.
The l i v e s o b t a i n e d by 90% o f t h e bear-
t h e nominal b e a r i n g l i f e , has been i n d i c a t e d
I t w i l l be seen i n t h e graph t h a t these t h r e e
p o i n t s l i e on an a p p r o x i m a t e l y s t r a i g h t l i n e .
T h i s means t h a t i n v i e w o f t h e
f a c t t h a t t h e s c a l e s o f b o t h axes a r e l o g a r i t h m i c ,
t h e l i f e can be expressed
as a power o f t h e load, hence:
where
L = nominal b e a r i n g l i f e i n m i l l i o n s o f r e v o l u t i o n s ; P
C p
=
Three t e s t
The l i v e s o b t a i n e d a r e i n d i c a t e d
e q u i v a l e n t b e a r i n g l o a d i n Newtons b a s i c dynamic load r a t i n g o f t h e b e a r i n g i n Newtons 3 f o r b a l l b e a r i n g s and 10/3 f o r r o l l e r b e a r i n g s .
142
Fig.15
Normal F a t i g u e F a i l u r e
The b a s i c dynamic l o a d r a t i n g o f t h e b e a r i n g i s d e f i n e d as t h e load t h a t gives a
nominal b e a r i n g l i f e o f one m i l l i o n r e v o l u t i o n s .
On t h e graph,
the
l o a d a t t h e p o i n t a t t h e i n t e r s e c t i o n o f t h e o r d i n a t e f o r t h e l i f e one m i l l i o n r e v o l u t i o n s w i t h t h e l i n e r e p r e s e n t i n g t h e r e l a t i o n s h i p between l o a d and l i f e i s t h e b a s i c dynamic l o a d r a t i n g " C " .
T h i s i s t h e l o a d r a t i n g g i v e n i n SKF
catalogues and i s used f o r t h e c a l c u l a t i o n o f t h e b e a r i n g l i f e .
143
Fig.16 6.4.3
R e l a t i o n s h i p between Load and L i f e .
F u r t h e r Development o f t h e L i f e E q ua tion
With t h e advancement o f technology,
g r e a t e r r e l i a b i l i t y i s r e q u i r e d from
r o l l i n g b e a r i n g s and o f t he c a l c u l a t i o n methods used.
I t i s necessary t o con-
s i d e r f a c t o r s n o t i n c l u d e d i n t h e b a s i c l i f e e q u a t i o n even though t h e r e s u l t s g i v e n by t h e b a s i c l i f e e q u a t i o n a r e s a t i s f a c t o r y i n the m a j o r i t y o f cases.
IS0 has suggested t h a t t h e l i f e e q u a t i o n should be as f o l l o w s : -
al
=
r e l i a b i l i t y f a c t o r , which enables t h e b e a r i n g l i f e t o be c a l c u l a t e d f o r any g i v e n p r o b a b i l i t y o f f a t i g u e (al
a
=
m a t e r i a l f a c t o r (a
::
1 f o r 90% p r o b a b i l i t y ) .
1 f o r a r o l l i n g b e a r i n g o f good q u a l i t y s t e e l w i t h
normal hardness and s t r u c t u r e s . a
l u b r i c a t i o n f a c t o r (a, = 1 f o r normal l u b r i c a t i o n ) . A lt h o u g h t h e new l i f e e q u a t i o n i s a development o f t h e o l d e q u a t i o n , a m u l t i -
p l i c a t i v e c o m b inat i on o f the t h r e e m o d i f i e r s r e s u l t i n an improvement. i ns ta n c e , material.
is
a 1 , az, and a
does n o t always
The n e g a t i v e e f f e c t o f inadequate l u b r i c a t i o n , f o r
n o t compensated f o r by u s i n g say a vacuum remelted s t e e l b e a r i n g
144 6.5
BOUNDARY D I HENS I ONS AN0 I NTERNAL CONTROLS
Every s t a n d a r d b e a r i n g w i t h m e t r i c boundary dimensions belongs t o a "dimens i o n a l s e r i e s " which forms p a r t o f g e n e r a l p l a n s p r e p a r e d by t h e I n t e r n a t i o n a l Organisation f o r Standardisation. d i f f e r e n t outside
For any g i v e n b o r e t h e r e a r e a s e r i e s o f
d i a m e t e r s and w i t h i n each d i a m e t e r s e r i e s t h e r e a r e b e a r i n g s
o f d i f f e r e n t w i d t h s , as shown i n F i g . 1 7 .
Each s t a n d a r d b e a r i n g b e l o n g i n g t o
a dimension s e r i e s i s d e s i g n a t e d by a t w o - d i g i t number.
The l a s t two d i g i t s i n
a f o u r o r f i v e d i g i t number a r e 1/5 o f t h e b o r e when t h i s i s i n m i l l i m e t r e s ; i . e . b e a r i n g 6004 has a b o r e o f 20 mm.
Supplementary symbols f a l l i n t o v a r i o u s groups r e l a t i n g t o i n t e r n a l d e s i g n f e a t u r e s such as cage, m a t e r i a l , s e a l i n g ,
i n t e r n a l r a d i a l c l e a r a n c e and p r e -
cision etc. The r a d i a l c l e a r a n c e f o r s i n g l e row deep groove b a l l b e a r i n g s has been agreed b o t h i n t e r n a t i o n a l l y and by B r i t i s h Standards O r g a n i s a t i o n s .
A general
summary i s a s f o l l o w s : -
SKF
I.S.O.
Designation c2 Norma I
c3 c4
& B.S.
A l s o r e f e r r e d t o as
Group 2
0
Norma 1
00
3 4
000 OD00
SKF a l s o manufacture c e r t a i n b e a r i n g s w i t h s m a l l e r r a d i a l c l e a r a n c e s ( C I ) and l a r g e r r a d i a l c l e a r a n c e s (C5).
6.6
USAGE C1
-
used on machine t o o l s p i n d l e s where minimum movement and maximum r i g i d i t y are required.
145 C2
-
s u i t a b l e f o r f r a c t i o n a l h orse power motors f o r domestic appliances p a r t i c u l a r l y where s i l e n t run ni n g i s r e q u i r e d .
Normal c le a r a nce ture,
-
C3
-
used f o r normal a p p l i c a t i o n s where t h e r e a r e no tempera-
speed, o r i n t e r f e r e n c e f i t problems.
o f t e n used on l a r g e e l e c t r i c motors, p a r t i c u l a r l y where the i n n e r r i n g temperature exceeds t h a t o f t h e o u t e r r i n g , o r when t h e r i n g s a r e mounted w i t h heavy i n t e r f e r e n c e f i t s .
C4 -
used f r e q u e n t l y on
a p p l i c a t i o n s such as l a r g e t r a c t i o n motors f o r
d i e s e l e l e c t r i c and e l e c t r i c l o como t ives where t h e r e a r e temperature d i f f e r e n t i a l s between the i n n e r and o u t e r r i n g and t h e i n t e r f e r e n c e f i t s a r e much g r e a t e r than those used f o r C3 bearings.
-
C5
t h i s c o n t r o l i s o f t e n used f o r b ea ri ngs i n furnace t r u c k s where t h e r e a r e l a r g e t emp era t u re d i f f e r e n t i a l s between t h e i n n e r and o u t e r r i n g s c o u p le d w i t h heavy i n t e r f e r e n c e f i t s .
6.7
SPEED LIMITS
Due t o t h e many f a c t o r s combining t o de t e rmine t h e maximum speed l i m i t s f o r r o l l i n g b e a r i n g s such as b e a r i n g t yp e, s i z e , t i o n s , method o f l u b r i c a t i o n ,
r a d i a l clearance, c o o l i n g condi-
l oa d, degree of p r e c i s i o n and environment e t c . ,
it
i s d i f f i c u l t t o g i v e p r e c i s e l i m i t s and a l l attempts t o g i v e l i m i t i n g speeds must be t o p r o v i d e an approximate general g ui de. For comparison purposes between speed and b e a r i n g s i z e , r o l l i n g b e a r i n g speeds a r e u s u a l l y expressed i n terms o f "ndm"
and
and n
JbFi,
ndm = A x f l x f 2 f o r r a d i a l b e a r i n g s
where n
/% = n = dm
d
= =
A x f
x f f o r t h r u s t bearings 1 2 speed, rpm
mean d i ame t e r o f b e a r i n g , 0.5(d+D)mm b e a r i n g bo re, rnm
D = b e a r i n g o u t s i d e d i ame t e r, mm H
=
f
h e i g h t o f s i n g l e t h r u s t be ari n g, mm c o r r e c t i o n f a c t o r f o r b e a r i n g s i z e (Fig.18)
1 f2
=
c o r r e c t i o n f a c t o r f o r b e a r i n g l oad (Fig.19)
A
=
c o n s t a n t , whi ch i s a f u n c t i o n o f t h e l u b r i c a n t and l u b r c a t i o n method (Fig.20).
A lt h o u g h t h e speed l i m i t f ormu l a i s based on p r a c t i c a l experience re s e a r c h i n t o more a c c u r a t e d e t e r m i n a t i o n o f t h e f a c t o r s
A,
fl
extensive
and f 2 has
r e s u l t e d i n l i m i t i n g speeds i n c l o s e ap pro xi mation t o t h e c o n d i t i o n s found i n practice. Approximate speed l i m i t s f o r a l o ad g i v i n g t h e b e a r i n g a minimum l i f e Lh
=
100,000 h o u rs ( f 2 = 1) a r e shown on F i gs.21 and 22.
I f t h e load i s g r e a t e r
146
1
0.5
50
100
200
300
500
1000
dm Correction factor for bcaring size (d,,,=mean diameter o bearing, mm)
(1
Fig.18
Correction Factor f o r Bearing S i z e .
fi 1
0.5
I ,
50
, , , , f
100
I
Mo
I
300
I
500
J d?n
lo00
Correction factor f* for bearing load, expressed as the resulting life L b in hours (d,=mcm diameter o f bearing, mm)
Fig.19
Correction Factor f o r Bearing Load
147
Bearing type Radial bearing^: dn = . f , f a A Deep groove ball bearings
"
Factor A
Remarks
Normally Maximum
500'000 1 000 000
Pressed-steel cages Solid brass cages
Self-aligning ball bearings
Vormally Maximum
500 000 800 000
Pressed-steel cages Solid brass cages
Single row angular contact ball bearings
Normally Maximum
500OOO
Pressed-steel cages Solid brass cages
Double row angular contact hall bearings
Normally Maximum
200000 400000
Cylindrical roller bearings
Normally Maximum
400000 Presstd-steel cages 600 000 Solid brass cages
Spherical roller radial bearings
Normally Maximum
200000 400OOO
Taper roller bearings
Normally Maximum
200 OOO In the case of a predominant 400000 thrust load 20-40% lowei
%wt
600000
limit values apply, depending o n the working conditions
bearitigJ:
" I/DH=f,f, A Ball thrust bearings Spherical roller thrust bearings
Normally Maximum Normally Maximum
1OOOOO
200 000 200000 Good, natural cooling gene rally sufficient. 300000 Effective cooling necessary.
At high speeds the bearing slackness must usually be greater than normal Thrust bearings operating at high speeds must carry a certain minimum load Fa n i r , as shown in the graph in Fig.
F i g . 20
148 th e l i m i t values a r e reduced by m o d i f y i n g t h e f a c t o r f 2 .
F i g u r e s 21 and 22
show two speed l i m i t s (normal and maximum) f o r each b e a r i n g type.
Bearings
o p e r a t i n g up tomorma1 l i m i t s can be f i t t e d w i t h standard cages and, as a general r u l e , grease l u b r i c a t i o n may be used.
I t should be n o t e d t h a t B a l l
Th r u s t and S p h e r i c a l R o l l e r T h r u s t b e a r i n g s shown i n Fig.22 should be mounted f r e e f r o m s la c k n ess and c a r r y a c e r t a i n minimum a x i a l load, o t h e r w i s e t h e t r a c k s may be damaged (due t o smearing) a s a r e s u l t o f t h e g y r a t o r y f o r c e s a c t i n g on th e r o l l i n g elements.
--__-I_
UBAlllNGS
M-
d-trr (1 ) far d u m r r mrs: -I_
u
2
-_ 23
3
4
I8
zu
24
25
2934
26 31
21 34
3240 36 46 44 53
43 41 54
46 54
51 58 65
1 I
zn
M
22
n
60
n
25
60 68 75
60 65
65 70
73
13
18
0
7B
85
83 93 9 0 %
95 113 10
I05 I13 I25
1 5 1 1113 0 3 I 110 U8 I 118
18 25 33
140 141
I5
I25
2U 25
133 14U
40 48 58
1% 168 175
13 54
148 155 168
65 75
183 195
65 75 n8
180 195 210
05
235
35
265
I5
225 240 250
65 ml
4u 55 Ml
265
95 I0 40
325 3 u
Y O 370 511 3 w
70 OU
410
all
60
w5
95
535
411
In Jo
uy) 3IU
UI W
03
m : 90
%
I33
9O 215
20 1 25U
w r)5 m 310
)80
4w
YI I 4 7 5
w
420 4 4w P
50
505
55
7u
51) 5uJ
u 10 u615 o
1x1
UI
570
UI5
h&d kariuEs. Approximate s p e d limits for a load (predorninurtly rcrdi.1 in the o f roller bcarinp) givin the bearings a minimum life L, of IOOOM) houn. Lower limits apply to axially loadtd d i a l roller bearings See adjoining table for thc values d. case
m 5w
845 87U
tMJ
1111
615 645 675 718
$55 755 IM 135 i75
795 M! 8%
us
94,
-
Fig.21
Approximate speed l i m i t s .
Rad i al Bearings.
149
n
.C.
m
BALL
Ill
-
113
- 114
15
31
Y
u
36
m
(0
U 51
45
51
(P
63 M 16
11 24
a
61
81
64 11
m
P 30
95
3a
n n
101
34
U 45 46
26
f
86
107 It4
P
111
m
n im
IW IU
44
U
I43
62 11 15
121 1%
im
52 56
m
141 151 115
n
141 1U
I(0
m
U
IN
6(
I14
IU
12
Ed
IW
M
16
no
51
s
79
16
P
M
96
00
I39
1%
IS3
191
111
in
40
m
M
!I4
I35
M
up
16
92
w
!43 !59
36
QI
62
m
I3
I24
59
id0
m
m
b9
Fig.22
11 21 XI
54 61
41
m -
15 I6 I1
31 35
UI
-
14
16 19 2a
P
LI
'8.
130
rn
ef
Tkvrr bariag~. Approximate 8 Umiu for a Land giving the bearings a pinimum lice l&MO houn. Scc adjoining uble for the values P H . It rhould be noted a y y t ofb m + d that the value 0.001 C, C p s u t i c carrying c it often grater than tte numerial value o F. obtam from the graph. The former value i8 then t a k a . It should .Is0 h noted chat the F . i lina have diffuent n l u a for ball mnd mUu baring$
I1 I3
16
25
E N. -
-
>
L78
w
112 I35
-
Approximate speed 1imits.Thrust bearings.
.I.
150 I n the top p a r t o f Fig.21 the speed curves w i t h A = 1500000 and A = 3000000 have been drawn i n and the broken l i n e shows the extreme values which have been achieved w i t h bearing types i n a few known cases.
Such speeds, however, do
r e q u i r e experience i n mounting and the gr a t e s t care must be exercised. FRICTION
6.8
The extensive use o f r o l l i n g bearings than s l i d i n g f r i c t i o n .
s due t o r o l l i n g f r i c t i o n being less
F r i c t i o n a l losses i n r o l l ing bearings are u s u a l l y very
low, hence the term a n t i f r i c t i o n bearings. The f r i c t i o n a l resistance o f a r o l l i n g bearing i s dependent on several f a c t o r s such as bearing load, speed o f r o t a t i o n , and the p r o p e r t i e s o f the l u b r i cant.
Under c e r t a i n c o n d i t i o n s (bearing load P
0.1C.
adequate l u b r i c a t i o n
and normal o p e r a t i n g c o n d i t i o n s ) i t i s p o s s i b l e t o c a l c u l a t e the f r i c t i o n a l resistance w i t h s u f f i c i e n t accuracy using t h e c o e f f i c i e n t s o f f r i c t i o n given i n Table 6.1.
Where rubbing seals are used t h e i r f r i c t i o n a l resistance, which
can be g r e a t e r than t h a t i n the bearing, must a l s o be taken i n t o account. f r i c t i o n torque "M"
M =
x F x
d 2
o f a bearing i s obtained from the equation:
(Nmn)
where IJ = c o e f f i c i e n t o f f r i c t i o n f o r the bearing (see Table 6.1)
F = bearing load,
N
d = bearing bore, mm.
TABLE 6.1
Coefficient o f f r i c t i o n
Bearing type
IJ
S e l f a1 i g n i n g B a l l Bearings
.001
C y l i n d r i c a l R o l l e r Bearings
.0011
Thrust B a l l Bearings
.0013
Deep Groove B a l l Bearings
.0015
Spherical R o l l e r Bearings
.0018
Taper R o l l e r Bearings
.0018
Spherical R o l l e r Thrust Bearings
.0018
Angular Contact B a l l Bearings Single Row
.002
Double Row
.0024
Needle R o l l e r Bearings
.0025
C y l i n d r i c a l R o l l e r Thrust Bearings
.004
Needle R o l l e r Thrust Bearings
.004
The
151 Higher c o e f f i c i e n t o f f r i c t i o n values than those given i n Table 6.1 a r e obtained w i t h new bearings and t h i s a p p l i e s p a r t i c u l a r l y t o r o l l e r bearings which have n o t been run i n . and using
Higher values w i l l a l s o be achieved when s t a r t i n g
excessive q u a n t i t i e s o f l u b r i c a n t .
The f r i c t i o n loss i s :
wF =
9.8 x
M x n (Watts)
-103 where
M
=
n
6.9
f r i c t i o n moment, Nmm speed, rpm.
LUBRICATION R o l l i n g bearings must be l u b r i c a t e d t o prevent m e t a l l i c contact between the
r o l l i n g elements, t r a c k s and cage and a l s o t o p r o t e c t the bearing from corrosion and wear.
The most favourable running temperature f o r a r o l l i n g bearing i s
achieved when the minimum o f l u b r i c a n t necessary t o ensure r e l i a b l e l u b r i c a t i o n i s used. L u b r i c a t i n g p r o p e r t i e s d e t e r i o r a t e due t o ageing and mechanical working and a l l l u b r i c a n t s become contaminated i n s e r v i c e and must, t h e r e f o r e be replenished or
changed
periodically.
R o l l i n g bearings may be l u b r i c a t e d w i t h grease o r o i l , o r i n special cases with a s o l i d lubricant.
When considering l u b r i c a t i o n f o r bearings the choice
is between o i l and grease and various aspects need t o be considered.
Grease
l u b r i c a t i o n has c e r t a i n advantages which are:-
1. Costs involved i n mounting a r e lower than w i t h o i l . 2 . Less maintenance i s required and i t i s n o t necessary t o
ncorpora t e
p i p i n g o r pumping equipment.
3.
Constant o i l l e v e l devices n o t required.
4. Easier t o c o n t a i n grease i n housing than o i l . 5. Cleaner t o use grease as there i s no splashing as w i t h o 1 .
6. Cheaper t o seal f o r grease than f o r o i l . 7. Grease a s s i s t s i n s e a l i n g a g a i n s t the e n t r y o f moisture and o t h e r impur i t ies Tests
.
have shown t h a t o n l y small amounts o f grease adhere t o the surfaces
o f the bearing.
Reservoirs o f grease form on the cage and a g a i n s t the side
faces o f the bearing.
The b u l k o f t h e grease c o l l e c t s o u t s i d e the bearing and
i n the grease c a v i t i e s of the housing and, as a r e s u l t , t h i s i s u s u a l l y i n a c t i v e . I t can be argued
t h a t t h i s reserve o f grease helps t o m a i n t a i n an o i l bleed t o
the bearings, b u t experience suggests t h a t although t h i s reserve may be i n reasonable q u a n t i t i e s i n the c a v i t i e s ,
i t i s s t i l l p o s s i b l e f o r the bearing to
152 f a i l due t o i n s u f f i c i e n t l u b r i c a n t .
The b a s i c r u l e f o r a normal b e a r i n g arrange-
ment i s t h a t th e b e a r i n g s h o u l d be w e l l packed w i t h grease w i t h t h e housing no I f t h e space round t h e b e a r i n g i s e x c e s s i v e l y f i l l e d w i t h
more than h a l f f u l l .
grease then c h u r n i n g o f t h e grease i n t h e b e a r i n g can o c c u r w hich c o u l d l e a d t o a r a p i d breakdown o f t h e grease s t r u c t u r e due t o o v e r h e a t i n g .
I n such cases
the grease s o f t e n s and t h e o i l i n t h e grease tends t o b l e e d from t h e soaps. The s t i f f n e s s o r hardness o f a grease i s c a l l e d c o n s i s t e n c y and i s u s u a l l y quoted i n terms o f t h e N a t i o n a l L u b r i c a t i n g Grease I n s t i t u t e (NLGI) s c a l e and Consistencies
2 o r 3 a r e c o m p l e t e l y s a t i s f a c t o r y i n normal a p p l i c a t i o n s f o r b a l l and r o l l e r bear i ng 1 u b r i c a t i o n .
A ' 3 ' c o n s is te n cy grease would be used i n an a p p l i c a t i o n such as an axlebox o r t r a c t i o n motor where t h e r e i s c o n s i d e r a b l e v i b r a t i o n and a r i s k o f t h e grease slumping.
6.9.1
Greases
L u b r i c a t i n g greases a r e t h i c k e n e d m i n e r a l o i l s o r s y n t h e t i c f l u i d s . soaps a r e m a i n l y used
as t he t h i c k e n i n g agent.
depends m o s t l y on t h e t ype and q u a n t i t y o f t h e t h i c k e n i n g agent used. s e l e c t i n g a grease,
i t s con si st e ncy,
Metal
The c o n s i s t e n c y o f t h e grease When
t emp era t ure range and r u s t - i n h i b i t i n g p r o -
p e r t i e s a r e th e most i m p o r t a n t f a c t o r s t o be considered. 6.9.1.1
Temperature Range Sodium Base Greases.
(i)
-30 t o +8OoC,
These greases may be used a t temperatures between
a l t h o u g h some s p e c i a l v e r s i o n s may be used up t o +120°C
base greases a r e w a t e r - s o l u b l e ,
Sodium
i . e . they a b s o r b w a t e r t o a c e r t a i n e x t e n t and
form a r u s t - i n h i b i t i n g emulsion wi ch ou t t h e i r l u b r i c a t i n g p r o p e r t i e s b e i n g impaired.
These greases w i l l p r o t e c t t h e be arings s u f f i c i e n t l y a g a i n s t r u s t
p r o v i d i n g t h a t w a t e r cannot e n t e r t h e b e a r i n g arrangement.
Where w ater c2n
e n t e r , such greases a r e e a s i l y washed o u t o f t h e b e a r i n g housing. Calcium Base Greases.
(ii)
1 t o 3% w a t e r .
Most c a l c i u m based greases a r e s t a b i l i s e d w i t h
W i t h i ncre ase d t emp era t u re t h e w ater evaporates and s e p a r a t i o n
o f t h e grease i n t o m i n e r a l o i l and soap occurs.
The upper temperature l i m i t
f o r t h e s e greases i s t h e r e f o r e ap pro xi mat el y +6OoC. base
Some h e a t - s t a b l e c a l c i u m
greases a r e a v a i l a b l e wh i ch p e r m i t o p e r a t i n g temperatures up t o +120°C.
( i ii )
L i t h i u m Base Greases.
temperatures
These greases a r e g e n e r a l l y s u i t a b l e f o r use a t
between -30 t o +llO°C,
b u t a few greases o f t h i s t y p e a r e s u i t a b l e
f o r wo r k in g temperatures up t o +150°C. L i t h i u m and c a l c i u m base greases a r e v i r t u a l l y i n s o l u b l e i n w a t e r and do n o t therefore give p r o t e c t i o n against corrosion.
Such greases should t h e r e f o r e
never be used u n l e s s t he y c o n t a i n a r u s t - i n h i b i t o r .
153 For heavily-loaded r o l l i n g bearings, e.g.
r o l l i n g - m i l l bearings, greases
c o n t a i n i n g EP a d d i t i v e s are used since these increase the load-carrying a b i l i t y o f the l u b r i c a n t f i l m .
Such greases are a l s o g e n e r a l l y recommended f o r the
l u b r i c a t i o n o f medium and l a r g e sized r o l l e r bearings.
The r u s t i n h i b i t i n g
p r o p e r t i e s o f calcium and I i thium base greases c o n t a i n i n g EP a d d i t i v e s (mainly lead compounds) a r e good.
These greases adhere w e l l t o the bearing surfaces as
w e l l as being i n s o l u b l e i n water.
They are,
therefore, p a r t i c u l a r l y s u i t a b l e
f o r a p p l i c a t i o n s where water can penetrate the bearing arrangement, e.g.
paper-
making machines o r r o l l i n g - m i l l s . Creases c o n t a i n i n g inorganic thickeners instead o f metal soaps, e.g.
clay or
s i l i c a , may be used f o r s h o r t periods a t higher temperatures than l i t h i u m base greases.
Synthetic greases, e.g.
those made from d i e s t e r or s i l i c o n e f l u i d s ,
may be used a t both higher and lower temperatures than greases made from mineral
oils. Grease r e - l u b r i c a t i o n i n t e r v a l s as recomnended by SKF a r e given i n graphs i n these i n t e r v a l s are conservative and are known t o g i v e a wide s a f e t y
Fig.23; margin
.
Relubrication interval hours 01 operation
t b
20000
300 200 160 100
a Radial wl bearings b Cylindrical roller bearings,needc roller bearings c Sphericd mler bearings. taper roller bearings, tnrust ball bearings
Fig.23
Re-lubrication Intervals
-
n r/min
164 To p r e v e n t t h e p o s s i b i l i t y o f m i x i n g i n c o m p a t i b l e greases w i t h i t s i n h e r e n t problems, i t i s a d v i s a b l e t o ensure t h a t o n l y greases h a v i n g t h e same t h i c k e n e r and w i t h a s i m i l a r
base o i l a r e used when r e - l u b r i c a t i n g .
There i s o f t e n doubt as t o t h e q u a n t i t y o f grease t o be used i n a b e a r i n g and w h i l s t i t i s d i f f i c u l t t o be e x a c t , t h e f o l l o w i n g g u i d e can be used:
0.005 x D x B
G
where
G
=
grease q u a n t i t y , grams
D
=
b e a r i n g o u t s i d e di a met er, m
B
b e a r i n g w i d t h , mm.
For high-speed b e a r i n g s n e c e s s i t a t i n g f r e q u e n t r e - l u b r i c a t i o n i n t e r v a l s i t i s e s s e n t i a l t o a v o i d o v e r - f i l l i n g t h e housings, s i n c e t o o much grease causes the grease t o churn,
r e s u l t i n g i n an e x c e s s i v e r i s e i n temperature.
Churning can
l ea d t o a breakdown i n t h e l u b r i c a t i n g p r o p e r t i e s o f t h e grease w i t h a f u r t h e r r i s e i n t e m p e r a t ure and t he b e a r i n g s o p e r a t i n g i n a p r e - l o a d c o n d i t i o n .
This
problem can be a voi d ed by u t i l i s i n g a grease escape v a l v e arrangement, as shown i n Fig.24.
The v a l v e c o n s i s t s o f a d i s c which r o t a t e s w i t h t h e s h a f t and,
c o n j u n c t i o n w i t h a ho usi n g end cove r, forms a narrow r a d i a l gap.
in
Excess grease
Cnum d v m for mnd-fnmm typ. m l e r t c motor
Fig.24
Grease Escape Valve Arrangement
i s thrown o u t by t he d i s c i n t o an a n g u l a r c a v i t y and e j e c t e d through an opening on t h e u n d e r s id e o f t h e end cover.
T h i s system o f escape v a l v e i s extremely
u s e f u l f r o m a maintenance a spe ct as e xami n at i on o f t h e e j e c t e d grease can g i v e v a l u a b l e e v id e n c e r e g a r d i n g t h e c o n d i t i o n o f t h e bearing,
i.e.
i f the bearing
i s f i t t e d w i t h a b rass cage and t h i s i s b e g i n n i n g t o wear t h e grease w i l l become
165 F i g u r e 25 shows t h e
d i s c o l o u r e d , and t h i s can be e a s i l y seen and recognized.
temperature e f f e c t o f o p e r a t i n g b e a r i n g s a t h i g h speeds w i t h and w i t h o u t grease v a lv e s .
Experiments i n t h e machine t o o l i n d u s t r y where b e a r i n g s a r e r u n w i t h
o n l y a l i g h t smear o f grease on t h e t r a c k s i n d i c a t e d t h a t i n i t i a l l u b r i c a t i o n c o u l d be as l i t t l e as 1 cm3 p e r 50 mm mean b e a r i n g diameter.
The use o f such
small q u a n t i t i e s i n 9Omm bo re c y l i n d r i c a l r o l l e r b e a r i n g s i n j i g b o r e r s p r e loaded 0.0025 mm
and o p e r a t i n g a t a speed o f 2500 rpm has reduced t h e temper-
a t u r e r i s e t o a slow as
Al t ho ug h l o n g - r u n n i n g p e r i o d s have a l s o been
8OC.
a c h ie v e d u s i n g t h i s technique,
t h e method i s extremely d e l i c a t e and n e c e s s i t a t e s
special t r a i n i n g f o r the f i t t e r s .
-u Y
b 60 10
L
a
" 0
2
0
24
48
72
L
0
96 h
120
144
1
188 l.54
2
1
1
1
1
1
1
24
48
72
96
120
144
I
1
168 192
h
Y
a
E
a
a
a
A
h
A1 n
C
Y I-
I
0
46
& ,
96
h
6.9.2
t
'
k
L
I
-
d
I
I
h
Typical comparison between lubication with a grease valve and ordinary grease lubication for a high-speed bearing in a cloaed housing. The steady high temperature in diagram (b) indicates that the grease has deteriorated during the first high temperature running period. Grease has seeped out through the seals and less grease remains than in the housing with grease valve. The temperature peaks in diagram (a) normally occur at each lubrication and a r e permissible with soda-base greases.
Fig.25
50-
P-
(a) Roller bearing 22328M running at 1100rev/min in conjunction with grease valve (b) Same type of roller bearing without grease valve (c) Ball bearing 6310 MA running at 8000rev/min in conjunction with grease valve (d) Same type of bearing running at 12400rev/min in conjunction with grease valve x = relubrication point
E f f e c t on Temperature o f I n c o r p o r a t i n g Grease Valve.
O i l Lubrication
O i l has s e v e r a l
advantages compared t o grease such as ease o f d r a i n i n g and
r e p l e n i s h i n g when necessary and p a r t i c u l a r l y when t h e r e l u b r i c a t i o n i n t e r v a l f o r grease i s v e r y s h o r t . o r operating
O i l l u b r i c a t i o n i s g e n e r a l l y used when h i g h speeds
temperatures p r o h i b i t t h e use o f grease and i s u s e f u l when i t i s
necessary t o d i s s i p a t e f r i c t i o n a l o r . a p p l i e d h e a t f r o m t h e b e a r i n g . t i o n o f a l u b r i c a t i n g o i l i s e a s i e r t ha n t h e c h o i c e o f a grease.
The selec-
O i l s a r e more
u n i f o r m i n t h e i r c h a r a c t e r i s t i c s and i f r e s i s t a n t t o o x i d a t i o n , gumming, and e v a p o r a t i o n can be s e l e c t e d on t h e b a s i s o f a s u i t a b l e v i s c o s i t y (Fig.26).
156
Graph for selection of oil. d = bore of bearing. mlp., spccd, I. m. Example : spheric11 roller thxust b r i n g 29469 : d = &O mm, I 500 r.p.m. UK an oil which has a minimwm viscosity of 13.5 cSt at working temperature. When the working I C I n p c n N r C is known the ap mximarc riscosit of the oil required at 50' C. can b;obuinef; with the aid of t i e thin. oblique lines. At a working tcmpcnturc of 70" C. the oil in the cnmplc should hare a viscosit of appmx. 27 cSr at 50" C. Select an oil from those avaiible who= this d u e . for cmmplc. Ckwylc viscosity is thc ~ r a ( i / DTE Oil Medium (27.3 cSt at 50" C.) n
-
P
Figure 26.
Selection o f Oil.
Due t o t h e f a c t t h a t he at -ge ne rat ed i n r o l l i n g b e a r i n g s increases w i t h v i s c osity,
i t i s necessary to s e l e c t a t h i n o i l f o r high-speed o p e r a t i o n , o t h e r w i s e
t h e b e a r i n g t e m pera t u re would be t o o h i g h . such
For v e r y slow speeds, a p p l i c a t i o n s
as s p h e r i c a l r o l l e r t h r u s t b e a r i n g s i n cranes, an extremely t h i c k o i l
(minimum v i s c o s i t y 400 c S t a t 5D°C) oil film.
For normal ambient temperatures and w o r k i n g c o n d i t i o n s , an o i l w i t h
a v i s c o s i t y o f 12-22c S t a t 5OoC oil is
i s used t o ensure a s u f f i c i e n t l y s t r o n g
i s suitable.
A t moderate speeds, no s p e c i a l
r e q u i r e d for b e a r i n g s i n gear-boxes s i n c e they can be l u b r i c a t e d by t h e
gear o i l p r o v i d e d t h e b e a r i n g s a r e a de qu at el y p r o t e c t e d a g a i n s t wear p a r t i c l e s from th e gears e n t e r i n g t h e be ari n gs. bea r in g s
I f t h i s cannot be prevented,
must be s e p a r a t e l y l u b r i c a t e d , u s u a l l y w i t h
oil.
then t h e
Grease can be used,
b u t t h e s e a l s must have t h e a b i l i t y t o p r e v e n t t h e gear o i l f l u s h i n g t h e grease o u t o f the bearing.
I t must be mentioned t h a t ,
as w i t h grease, an e x c e s s i v e
q u a n t i t y o f o i l can cause c h u r n i n g and c o n s i d e r a b l e heat;
therefore,
for normal
r e s e r v o i r systems i t i s e s s e n t i a l f o r t h e maximum o i l l e v e l t o be no h i g h e r th a n t h e c e n t r e o f t h e l owest r o l l i n g element.
I f t h e o i l l e v e l exceeds t h i s
t h e r e c o u l d be a t e mpe rat ure r i s e due t o c h u r n i n g .
F i g u r e 27 shows t h e e f f e c t
on t e m p e r a t u r e r i s e and f r i c t i o n t o r q u e o f i n c r e a s i n g t h e q u a n t i t y o f o i l . When the q u a n t i t y
o f o i l reaches a minimum l e v e l ,
i.e.
t h e d o t t e d zone, m e t a l - t o -
metal c o n t a c t o ccu rs, r e s u l t i n g i n r a p i d t emperature r i s e and p o s s i b l e b e a r i n g seizure.
B e a r i n g f r i c t i o n t o r q u e i s a l s o a f u n c t i o n o f o i l q u a n t i t y and i t can
be seen t h a t t h e t o r q u e i ncre ase s w i t h t h e q u a n t i t y o f o i l . rep r e s e n t
As t h i s c o u l d
c o n s i d e r a b l e power loss i t i s e s s e n t i a l t o ensure t h a t the o i l l e v e l
does nmt exceed a l e v e l c o m p a t i b l e w i t h adequate l u b r i c a t i o n .
O i l lubrication
167 can be by c i r c u l a t i o n , d r i p feed, w i c k feed, o r o i l m i s t .
In a circulatory
system t h e o u t l e t must be g r e a t e r t ha n t h e i n l e t t o p r e v e n t t h e p o s s i b i l i t y o f exc e s s iv e o i l i n t h e b ea ri ng .
For high-speed a p p l i c a t i o n s such as g r i n d i n g
s p i n d l e s , o i l m i s t l u b r i c a t i o n i s o f t e n used.
I n t h i s system a m i s t o f o i l and
a i r i s t r a n s p o r t e d t h rou gh p i p e s t o t h e b ea rings.
Condensing n i p p l e s immediately
b e f o r e each b e a r i n g pos t i o n cause t h e o i l t o be s u p p l i e d t o the b e a r i n g i n d r o p l e t form.
The smal
q u a n t i t i e s o f o i l can be a c c u r a t e l y r e g u l a t e d , conse-
quently the l u b r i c a t i o n f r i c t i o n i s n e g l i g b l e .
F i g u r e 28 shows a t y p i c a l o i l
mist u n i t .
The dependence of burlng friction on quintlcy of 011 (in It+/hr) Lower curve: frlction torque, Ib. In. Upper curve: outer rln; tempenwre. 'C
Fig.27
6.10
E f f e c t on B e ari n q F r i c t i o n o f V a ryi ng a u a n t i t y o f O i l
SEALS
Selecting the c o r r e c t bearings f o r a p a r t i c u l a r a p p l i c a t i o n necessitates more t h a n a s c e r t a i n i n g c o r r e c t t y p e and s i z e . satisfactorily,
If bearings are t o f u n c t i o n
t h e y must be p r o p e r l y l u b r i c a t e d and p r o t e c t e d f r o m t h e oper-
a t i n g environment by means o f c o r r e c t l y designed seals. Seals a r e n o r m a l l y i n t e nd ed t o p r e v e n t f o r e i g n m a t e r i a l e n t e r i n g t h e b e a r i n g and i n c e r t a i n cases t o p r e v e n t t h e i n g r e s s o f m o i s t u r e and c o r r o s i v e media. The f r i c t i o n developed by a r u b b i n g se al must be considered r e l a t i v e t o t h e power i n p u t .
The seal o r s i d e p l a t e must a l s o r e t a i n t h e l u b r i c a n t i n t h e
b e a r i n g o r housing.
R o t a t i o n a l speed o r r u b b i n g s e a l s must a l s o be considered
t o ensure e x c e s si ve h e a t i s n o t developed.
158
Pipeline layout for condensed oil mist lubricating system
1 I 3 4
5 Mstribution box 6-8 Condenstng nipples 9 Branch pipelines
Compressed air cut-off valve OU mint lubricator
Pressure control valve
10 Spray nomle
Mein pipeline
Fig.28
O i l M i s t L u b r i c a t i o n System
Simple gap seals a r e e f f i c i e n t using grease l u b r i c a t i o n , the purpose o f the gap seals being t o keep o u t r e l a t i v e l y small a m u n t s o f f o r e i g n m a t e r i a l and t o r e t a i n the grease w i t h o u t excessive temperature problems. long a x i a l l y and as small as p o s s i b l e r a d i a l l y . gap should be 0.1
t o 0 . 3 mm.
A gap seal should be
For normal a p p l i c a t i o n s the
Gap seals can be used w i t h o i l l u b r i c a t i o n , but
i t i s advisable t o incorporate a groove i n the s h a f t o r , a l t e r n a t i v e l y ,
fit a
r i n g , both arrangements causing the o i l t o f l o w back i n t o the housing.
A fur-
t h e r a l t e r n a t i v e would be t o machine a small groove i n the s h a f t adjacent t o the gap seal (which has a d r a i n hole) thereby encouraging the o i l t o f l o w back i n t o the housing. L a b y r i n t h seals a r e e x t e n s i v e l y used w i t h grease l u b r i c a t i o n i n d i r t y and wet c o n d i t i o n s .
The l a b y r i n t h c o n s i s t s o f a number o f r a d i a l l y separated
tongues w i t h a small r a d i a l clearance and can be considered as e l a b o r a t e grooved gap seals.
The grooves a r e o f t e n f i l l e d w i t h grease t o prevent the ingress o f
169 d i r t and a t y p i c a l l a b y r i n t h system f o r a t r a c t i o n motor bearing i s shown i n Fig.29.
lkaring srbcmc /or rrartion motor armatwe
Fig.29
6.11
Bearing Scheme f o r T r a c t i o n Hotor Armature
NOISE
Noise o r i g i n a t e s from an o s c i l l a t i n g body which produces a moving l o n g i t u d i n a l wave c o n s i s t i n g o f v a r i a t i o n i n pressure.
I n order t o reduce noise i r r i t a t i o n t o
a minimum t h e r e i s a constant demand f o r s i l e n t running bearings p a r t i c u l a r l y i n domestic and o f f i c e equipment such as vacuum cleaners, f l o o r p o l i s h e r s , c i r c u l a t o r y pump motors f o r c e n t r a l heating and fans e t c .
A clean h i g h q u a l i t y r o l l i n g bearing runs w i t h an even p u r r i n g sound and depending on the bearing arrangement, the n o i s e may be so low as t o be i n a u d i b l e o r i t can be unpleasant and d i s t u r b i n g .
Bearing noise i s a f u n c t i o n o f the l e v e l
o f v i b r a t i o n i n the bearing and the q u i e t e r the bearing the lower the l e v e l o f vibration.
V i b r a t i o n s i n a bearing depend on many f a c t o r s such as surface
f i n i s h , speed, load and accuracy o f geometric form e t c . Clearance i n a bearing i s a f u r t h e r c o n t r i b u t o r y f a c t o r t o n o i s e and i t i s necessary t o s e l e c t bearings and f i t s t o achieve zero clearance i n the bearing under o p e r a t i n g c o n d i t i o n s . bearing clearances.
Fig.30
shows the e f f e c t on noise of d i f f e r e n t
Freedom from clearance i n mounted b a l l bearings can a l s o be
160 achieved by a d j u s t i n g t h e b e a r i n g a g a i n s t each o t h e r by means o f d i s c s p r i n g s as shown i n Fig.31.
T h i s method ensures p r a c t i c a l l y z e r o c l e a r a n c e under a l l
working c o n d i t i o n s .
The t o t a l s p r i n g p r e s s u r e should be a p p r o x i m a t e l y 5
Newtons per mm o f s h a f t diameter i . e .
100 Newton p r e s s u r e f o r a 20 mm s h a f t .
Clearance in mounted bearing The dependence of motor noise on bearing clearance. Measurement of vibration of electric motor over the frequency range 400--800 c/s with progressive variation of bearing clearance A=clearance range obtained with C2 bearings and normal fits B=clearance range obtained with normal bearings and normal fits
Fig.30
E f f e c t o f Noise on D i f f e r e n t Bearing C 1 earances
For b a l l and c y l i n d r i c a l r o l l e r b e a r i n g s o p e r a t i n g a t h i g h speeds (ndm 500000 and 400000 r e s p e c t i v e l y ) t h e temperature d i f f e r e n t i a l between t h e i n n e r and o u t e r r i n g s i s g r e a t e r than a t normal speeds and t h e e f f e c t o f t h i s must be taken i n t o account.
Any r e d u c t i o n i n c l e a r a n c e due t o temperature d i f f e r e n t i a l
must be compensated f o r by u s i n g b e a r i n g s w i t h g r e a t e r i n i t i a l r a d i a l clearance. W h i l s t v i b r a t i o n l e v e l s i n b e a r i n g s can be reduced, i t i s e q u a l l y i m p o r t a n t t o ensure t h a t t h e o t h e r components i n t h e machine a r e a l s o manufactured t o s i m i l a r accuracy t o ensure t h e y a r e n o t t h e cause o f v i b r a t i o n , o t h e r w i s e t h e q u i e t running p r o p e r t i e s o f t h e b e a r i n g w i l l n o t be u t i l i s e d .
From a n o i s e
aspect t h e o v a l i t y and t a p e r o f t h e s h a f t and housing s e a t i n g s should be accura t e and l i e w i t h i n h a l f t h e t o l e r a n c e range f o r grades I T 5 and IT6 f o r s h a f t and housing r e s p e c t i v e l y .
B e a r i n g a l i g n m e n t must a l s o be c o n s i d e r e d , a s mis-
a l i g n m e n t can a l s o be a source o f n o i s e .
F i g u r e 32 shows t h e h i g h and c o r r e s -
ponding d i a m e t r i c a l l y opposed low s p o t s on a lobed t y p e o f out-of-roundness, and Fig.33
shows t h e e f f e c t o f a l i g n m e n t e r r o r s on n o i s e i n t e n s i t y .
There a r e
161
Fig.31
Spring preloading of deep groove ball bearings. 1 with spring washers, 2 with helical springs.
Fig.32
Lobing. For every high spot there is a corresponding low spot diametrically opposite. There are always an odd number of lobes.
162
Noise intensity
dR
I
Fig.33
Errors of alignment
-
E f f e c t o f Alignment E r r o r s on Noise
many a p p l i c a t i o n s where n o i s e must be t o l e r a t e d i n o r d e r t o a c h i e v e a s a t i s f a c t o r y bearing l i f e . o f railway vehicles,
I n e l e c t r i c t r a c t i o n motors geared t o t h e d r i v i n g a x l e s t h e o p e r a t i n g c o n d i t i o n s d i c t a t e heavy f i t s w hich i n t u r n
n e c e s s i t a t e b e a r i n g s w i t h l a r g e i n i t i a l r a d i a l clearance.
The increased r a d i a l
c l ea r a n c e r e s u l t s i n combined r o l l e r and cage d r o p as they come o u t o f t h e load zone which can cause increased b e a r i n g no i se, a l t h o u g h i t has no e f f e c t on t h e l i f e o f t h e b e a r i n g and cannot be heard when t h e motor is f i t t e d under t h e l oc o m o tiv e .
A h i g h p i t c h e d whine can o c c a s i o n a l l y o c c u r a t c e r t a i n speed ranges
w i t h c y l i n d r i c a l r o l l e r be ari n gs, d e s p i t e adequate l u b r i c a t i o n .
T h i s has been
a t t r i b u t e d t o resonance caused by c e r t a i n f r e q u e n c i e s o f the r o l l i n g elements c o i n c i d i n g w i t h t h e n a t u r a l frequency of t h e b e a r i n g end frame and, i n c e r t a i n cases, whine has been s i g n i f i c a n t l y reduced by m o d i f y i n g t h e mass d i s t r i b u t i o n o f t h e housing.
6.12
ANTICIPATING BEARING DAMAGE
The r u n n in g performance o f r o l l i n g b e a r i n g s from a n o i s e aspect can be checked w i t h reasonable accuracy u s i n g a wooden l i s t e n i n g s t i c k and l i s t e n i n g t h r o u g h t h e h a n d le t o t h e t r a n s m i t t e d n oi se .
A wooden s t i c k i s p a r t i c u l a r l y u s e f u l i n
t h a t i t t r a n s m i t s n o i s e r e l a t i n g t o t h e c o n d i t i o n o f t h e b e a r i n g and c u t s o u t most o f t h e extraneous n o i s e from o t h e r machine components w hich can cause p r o blems f o r t h e more s e n s i t i v e stethoscope.
With experience, an o p e r a t o r becomes
tuned t o c e r t a i n n o i s e s and can a s c e r t a i n t h a t b e a r i n g examination should be c a r r i e d o u t a t t h e n e x t shutdown p e r i o d .
Naturally,
if say a c o n s i s t e n t knocking
develops w i t h t h e normal n o i s e l e v e l , t h en an immediate i n s p e c t i o n o f t h e bearing
163 should be made.
Damaged t r a c k s caused by c a r e l e s s mounting produce pronounced
low p i t c h n o i s e and inadequate r a d i a l c l e a r a n c e produces m e t a l l i c n o i s e .
6.13
DETECTION OF BEARING DAMAGE BY SHOCK PULSE MEASUREMENT
When a r o l l i n g b e a r i n g s u f f e r s f a t i g u e f a i l u r e , element s u r f a c e s o r i n t h e t r a c k s .
f l a k i n g occurs i n t h e r o l l i n g
A b e a r i n g component coming i n t o c o n t a c t
w i t h t h e damaged zone causes a mechanical shock, causing t r a n s i e n t v i b r a t i o n s which a r e t r a n s m i t t e d t o t h e b e a r i n g housing.
These v i b r a t i o n s g e n e r a t e e l e c -
t r i c v o l t a g e s i n an acce l ero met er and t h e i r a m p l i t u d e i s determined by a shock p u l s e meter.
The c o n d i t i o n o f t h e b e a r i n g i s monitored, t a k i n g readings a t
suitable intervals.
I n c i p i e n t damage can, t h e r e f o r e be d e t e c t e d a t an e a r l y
s ta g e and b e a r i n g replacement planned a c c o r d i n g l y . Earphones can be used t o l i s t e n t o t h e rhythm o f t h e shocks and, p r o v i d e d t h e damage i s n o t t o o complex,
t h e rhythm w i l l suggest w hich component o f t h e
b e a r i n g i s damaged. Temperature i s a f u r t h e r method f o r gauging b e a r i n g c o n d i t i o n , and b e a r i n g f a i l u r e s a r e sometimes preceded by a d r o p i n temperature f o l l o w e d by a r a p i d i nc r e a s e i n temperature,
u s u a l l y caused by metal-to-metal
quent f a t i g u e f a i l u r e o r even s e i z u r e .
c o n t a c t w i t h subse-
The o l d method o f checkina t h e b e a r i n g
te m p e r a t u r e by f e e l i n g t h e h ou si ng i s n o t s a t i s f a c t o r y ,
and i n a p p l i c a t i o n s
where breakdowns cause maintenance problems i t i s usual t o use thermal-couples p o s i t i o n e d as c l o s e t o t h e b e a r i n g o u t e r r i n g as p o s s i b l e .
When u s i n g t h e
te m p e r a t u r e method o f ch ecki n g b e a r i n g c o n d i t i o n i t must be noted t h a t t h e r e
w i l l always be a t emp era t u re r i s e w i t h new o r freshly-greased
bearings.
This
o n l y a p p l i e s d u r i n g i n i t i a l r u n n i n g and once t h e new l u b r i c a n t has d i s t r i b u t e d itself,
6.14
t h e te m p era t u re w i l l s t a r t t o f a l l .
FITS (SHAFT AND HOUSING)
To le r a n c e s f o r t h e bo re and o u t s i d e d i ame ter o f m e t r i c r o l l i n g b e a r i n g s a r e i n t e r n a t i o n a l l y s t a n d a r d i s e d and t h e r e q u i r e d f i t s a r e achieved by s e l e c t i n g s h a f t and h o u s in g t o l e r a n c e s u s i n g t h e IS0 t o l e r a n c e system ( i n c o r p o r a t e d i n BS 4 5 0 0 : P a r t 1:1969).
Only a sma l l s e l e c t i o n o f t h e IS0 t o l e r a n c e zones need
t o be c o n s id e r e d f o r r o l l i n g b e a r i n g , and Fig.34 shows these r e l a t i v e t o t h e b e a r i n g b o r e ( a ) and b e a r i n g o u t s i d e d i ame t e r ( b ) . The s h a f t t o l e r a n c e i s i n d i c a t e d by a sma ll l e t t e r and a number and the
A t y p i c a l s h a f t and housing
housing b o r e by a c a p i t a l l e t t e r and a number. bore t o l e r a n c e c omb i na t i o n would be w r i t t e n j 6 obtained from t ole ran ce tab le s.
-
J7, t h e values f o r each being
164
b
Fig.34
Shaft and Housing F i t s .
The various symbol gradings a r e as f o l l o w s : Shafts Clearance
Bearings always have clearance f i t i f the s h a f t t o l e r a n c e
Transition
Bearings can be clearance o r i n t e r f e r e n c e f i t depending on
i sl l f l l
the a c t u a l dimensions, i f t h e shaft t o l e r a n c e is w i t h i n t h e range 'lg Interference
- j".
Bearings w i l l always have an i n t e r f e r e n c e f i t i f t h e s h a f t t o l e r a n c e range i s w i t h i n "k
-
r'l.
Housings C I earance
Bearings w i l l always have clearance i f t h e housing tolerance
Transition
Bearings w i l l have a clearance or an i n t e r f e r e n c e f i t depend-
range i s I'G
- H".
i n g on the a c t u a l dimensions i f the housing tolerance i s w i t h i n the range "J Interference
-
N".
Bearings w i l l have i n t e r f e r e n c e f i t i f the housing tolerance i sl l p l l
The most important f a c t o r s t o consider when s e l e c t i n g f i t s are;-
1 . Conditions o f r o t a t i o n 2 . Magnitude o f the load
3 . Temperature c o n d i t i o n s
165 6.15
C O N D I T I O N S OF ROTATION
When a bearing r i n g i s t o r o t a t e r e l a t i v e t o t h e d i r e c t i o n of the r a d i a l load,
i t must be mounted w i t h s u f f i c i e n t i n t e r f e r e n c e on the s h a f t o r i n the
housing t o prevent " r o l l "
o r "creept'.
I f creep occurs between heavily-loaded
d r y surfaces, t h e surfaces undergo r a p i d wear. the f i t used i s t i g h t enough t o
It i s therefore essential that
ensure t h a t no clearance e x i s t s and none can
I f the load i s always d i r e c t e d towards
develop due t o the a c t i o n o f the load.
the same p o i n t on the r i n g , however, no creep o'ccurs and a clearance f i t i s g e n e r a l l y permissible. Various loading c o n d i t i o n s can be c l a s s i f i e d as f o l l o w s ; (i)
R o t a t i n g inner r i n g load.
The s h a f t r o t a t e s r e l a t i v e t o the d i r e c t i o n
o f the load so t h a t a l l p o i n t s on the inner r i n g t r a c k a r e subjected t o load d u r i n g one r e v o l u t i o n . Example
-
Shaft w i t h b e l t d r i v e .
( i i ) S t a t i o n a r y inner r i n g load.
The s h a f t i s s t a t i o n a r y r e l a t i v e t o the
d i r e c t i o n o f loading so t h a t the load i s always towards the same sector o f the inner r i n g . Example
- Automobile
f r o n t hub.
( i i i ) S t a t i o n a r y o u t e r r i n g load.
The bearing housing remains s t a t i o n a r y
r e l a t i v e t o t h e d i r e c t i o n o f loading so t h a t
t h e load i s always d i r e c t e d
towards t h e same sector o f the o u t e r r i n g . Example
-
Shaft w i t h b e l t d r i v e .
( i v ) Rotating o u t e r r i n g load.
The bearing housing r o t a t e s r e l a t i v e t o the
d i r e c t i o n o f loading so t h a t a l l p o i n t s on t h e o u t e r r i n g t r a c k come under load d u r i n g one r e v o l u t i o n . Example
- Automobile
f r o n t hub.
I n many a p p l i c a t i o n s o p e r a t i n g c o n d i t i o n s cannot be r e l a t e d t o any o f these simple loading cases and v a r i a b l e e x t e r n a l loads o r out-of-balance
forces
i n f l u e n c e t h e d i r e c t i o n o f loading, an a p p r o p r i a t e f i t being c l a s s i f i e d as " d i r e c t i o n o f loading indeterminate".
I n t h i s instance, bearing r i n g "creep"
can o n l y be prevented by using an i n t e r f e r e n c e f i t f o r both r i n g s and i n such cases bearings w i t h increased r a d i a l clearance a r e u s u a l l y necessary.
6.16
INFLUENCE OF LOAD AND TEMPERATURE
The load compresses t h e inner r i n g i n a r a d i a l d i r e c t i o n which s t r e t c h e s the r i n g i n a c i r c u m f e r e n t i a l d i r e c t i o n and compresses t h e s h a f t , thereby loosening the f i t .
S i m i l a r l y , as the bearing inner r i n g t r a c k is warmer d u r i n g o p e r a t i o n
than the bearing bore, t h i s a l s o has the e f f e c t o f loosening the f i t o f the
166 b e a r i n g i n n e r r i n g on i t s j o u r n a l . I n t h e m a j o r i t y o f a p p l i c a t i o n s i t i s unnecessary t o c a l c u l a t e t h e r e q u i r e d
i n t e r f e r e n c e s i n c e e xpe ri en ce i s a good g u i d e i n s e l e c t i n g s u i t a b l e f i t s f o r d i f f e r e n t o p e r a t i n g c o n d i t i o n s and o n l y i n p a r t i c u l a r l y d i f f i c u l t or unusual c o n d i t i o n s i s i t necessary t o r e s o r t t o s p e c i a l c a l c u l a t i o n s .
6.17
BEARING A P P L I C A T I O N S
When d e s i g n i n g b e a r i n g arrangements t h e r e a r e c e r t a i n b a s i c r u l e s w hich should be f o l l o w e d . l o c a t i o n purposes.
Wherever p o s s i b l e o n l y one b e a r i n g should be used f o r T h i s means t h a t t h e b ea ring o u t e r r i n g should be h e l d
a x i a l l y i n i t s hou si ng w i t h t h e i n n e r r i n g l o c a t e d on t h e s h a f t i n a s i m i l a r manner.
All o t h e r be ari n gs on t h e same s h a f t should be a x i a l l y f r e e , e i t h e r
on t h e s h a f t o r i n t h e housing, as shown on Fig.35.
I f t h i s basic r u l e i s
ignored and two a x i a l l y l o c a t e d b e a r i n g s a r e used, any s h a f t expansion o c c u r r i n g due t o generated o r e x t e r n a l h e a t c o u l d cause severed l o c k i n g ( p r e l o a d i n g ) acr o s s t h e b e a r in gs, r e s u l t i n g i n premature b e a r i n g f a i l u r e .
In certain appli-
c a t i o n s u s i n g a ng ul a r c o n t a c t b a l l o r t a pe r r o l l e r bearings i t i s n o t p o s s i b l e t o use only one b ea ri ng f o r l o c a t i o n and t h e bearings must be a d j u s t e d by end covers.
FIG.
35
FTG.
36
W it h these b e a r ing s g r e a t c a r e must be t ake n t o ensure t h a t t h ? bearings can accommodate a x i a l v a r i a t i o n s and i n sane a p p l i c a t i o n s a small gap i s l e f t between t h e b e a r i n g o u t e r r i n g and t h e abutment cover, as shown i n Fig.36. The a x i a l expansion problem can be r e s o l v e d by i n c o r p o r a t i n g a c y l i n d r i c a l r o l l e r b e a r in g a t t h e no n-l o cat ed p o s i t i o n , t h i s arrangement being used extens i v e l y on e l e c t r i c motors where a b a l l b ea ri ng i s used a t t h e commutator end and a r o l l e r b e a ri ng a t t h e d r i v e end.
I n a d d i t i o n t o r e s o l v i n g t h e thermal
expansion problem such an arrangement has an added advantage i n t h a t a c y l i n d r i c a l r o l l e r b e ari n g has s i g n i f i c a n t l y h i g h e r I o a d - c a r r y i n g c a p a c i t y (compared t o a d i m e n s i o n a l l y e q u i v a l e n t b a l l be ari n g) which makes i t p a r t i c u l a r l y s u i t a b l e f o r r e a c t i n g heavy d r i v e f o r c e s .
167 6.18
BEARING CARE
Cleanliness i s o f paramount importance when handling and mounting bearings, but i n s p i t e o f the f a c t t h a t bearings a r e p r e c i s i o n made, one has o n l y t o look around the average workshop t o see open bearings l e f t on benches o r i n dusty environments.
I t should be noted t h a t i f f o r e i g n matter i s allowed t o enter a
bearing the r o l l i n g elements i n passing over i t d u r i n g s e r v i c e can cause indenta t i o n s leading t o f a t i g u e f a i l u r e i n t h e r o l l i n g elements o r t r a c k s which can shorten the l i f e o f the bearing considerably.
As a general r u l e , i n order t o
ensure the bearings remain f r e e from i m p u r i t i e s , they should n o t be removed from t h e i r o r i g i n a l packing u n t i l they a r e required f o r i n s t a l l a t i o n .
Before
packing bearings a r e u l t r a - s o n i c a l l y washed and coated w i t h a bearing preservat i v e which mixes r e a d i l y w i t h most l u b r i c a n t s , a p a r t from c e r t a i n c l a y o r synthetic-based greases,
i n which case i t i s advisable t o contact the bearing manu-
facturers. Although i n i t i a l washing o f bearings by t h e user i s n o t recommended due t o the p o s s i b i l i t y o f the washing f l u i d n o t completely evaporating,
i f washing i s
necessary then the bearings should be washed w i t h c l e a n w h i t e s p i r i t a f t e r which the bearings must be thoroughly d r i e d before adding l u b r i c a n t .
6.19
BEARING MOUNTING
I f the bearing inner r i n g i s t o be a t i g h t f i t on t h e s h a f t , i t can be d r i v e n onto the s h a f t j o u r n a l by means o f a t u b u l a r d r i f t which should bear evenly
I f the bearing outer
a g a i n s t t h e f a c e o f t h e inner r i n g , as shown i n Fig.37.
r i n g i s t o be a t i g h t f i t i n i t s housing then the r i n g should be d r i v e n i n t o the housing by a p p l y i n g f o r c e t o the outer r i n g . Hollow drift for bearings with an interference fit on the shaft
I
FIG-
37
Hollow drift for bearings with an interference fit both in the housing and on the shaft
I FIG.
38
Pressure must n e v e r be a p p l i e d t o the inner o r outer bearing r i n g i n order t o mount the o t h e r r i n g which has a t i g h t f i t , otherwise t h e r e i s a danger o f the t r a c k s being indented and the bearing would probably f a i l i n s e r v i c e a f t e r
168 a s h o r t t im e .
I f b o t h t he i n n e r and o u t e r r i n g s a r e a t i g h t f i t , then a t o o l
o f t h e t y p e shown i n F i g . 3 8 must be used whi ch c o n t a c t s b o t h s i d e f a c e s o f t h e be a r in g r i n g s . Care must be t ake n t o e nsu re t h a t t h e b e a r i n g r i n g b e i n g pressed on i s c o r r e c t l y a l i g n e d , p a r t i c u l a r l y i n t h e case o f small r i n g s .
Excessive m i s a l i g n -
ment p u t s severe s t r e s s on t h e cage, wh i ch can r e s u l t i n premature b e a r i n g failure. C y l i n d r i c a l r o l l e r b ea ri ng s sometimes g i v e assembly problems due t o t h e r o l l e r s being scored as t h e y a r e f e d o v e r t h e t r a c k s .
T h i s problem can be r e -
s o lv e d by u s i n g a s p e c i a l mounting s l e e v e as shown on Fig.39.
Mounting sleeves simplify the mounting of cylindrical roller bearings and prevent damage to the tracks
FTG.
39
For l a r g e r o l l i n g b e a r i n g s w i t h c y l i n d r i c a l bores where t h e i n n e r r i n g i s a
t i g h t f i t , mounting can be s i m p l i f i e d by h e a t i n g t h e complete b e a r i n g i n an o i l ba th a t a t e m p e ra t u re o f 70°
-
8OoC above ambient.
I f t h e b e a r i n g needs t o be
t r a n s p o r t e d some d i s t a n c e i t can be heated t o a h i g h e r temperature t o compens a t e f o r c o o l i n g down d u r i n g t r a n s i t .
Bearings can be heated up t o 12OoC i n
t h i s way, b u t t h i s t e mpe rat ure sh ou l d n o t be exceeded as, above 120°C,
there i s
a danger o f r e d uci n g t h e b e a r i n g m a t e r i a l hardness.
6.20
DISMOUNTING BEARINGS
I f an i n t e r f e r e n c e f i t has been used t o l o c a t e a b e a r i n g on a s h a f t i t i s e s s e n t i a l t o use a s u i t a b l y designed wi t h dra wal t o o l t o remove t h e b e a r i n g . The b a s i c p r i n c i p l e s for mounting a b e a r i n g a p p l y a l s o t o dismounting.
The d i s -
mounting f o r c e must be a p p l i e d t o t h e b e a r i n g r i n g having t h e i n t e r f e r e n c e f i t and n o t t o t h e loose r i n g and t hro ug h t o t h e o t h e r r i n g by means o f t h e r o l l i n g
169 elements.
Claw-type p u l l e t s a r e o f t e n used t o remove b a l l bearings,
but design-
e r s must t a k e t h i s i n t o c o d s i d e r a t i o n d u r i n g t h e d e s i g n s t a g e by i n c o r p o r a t i n g s u i t a b l e grooves i n t h e abutment shoulder a d j a c e n t t o t h e b e a r i n g f a c e t o enable the b e a r i n g i n n e r r i n g t o be g r i p p e d by t h e p u l l e r c l a w s .
N a t u r a l l y , the p u l l e r
f o r c e must be a p p l i e d t o t h e i n n e r r i n g and n o t t r a n s m i t t e d through t h e r o l l i n g elements.
I f t h e i n n e r r i n g i s i n a c c e s s i b l e i t i s necessary t o p u l l on t h e
o u t e r r i n g , b u t b e a r i n g damage can be avoided by l o c k i n g t h e p u l l e r c e n t r e screw w i t h a spanner and r o t a t i n g t h e claws.
The o u t e r r i n g w i l l then r o t a t e
d u r i n g t h e w i t h d r a w l process, d i s t r i b u t i n g t h e load over t h e t r a c k s and thereby preventing the p o s s i b i l i t y o f indentations.
The b e a r i n g i s t h e n p u l l e d o f f
s u f f i c i e n t l y t o enable t h e i n n e r r i n g t o be g r i p p e d w i t h t h e p u l l e r ( F i g . 4 0 ) .
Extractor
U FIG. 40 6.21
MOUNTING AND DISMOUNTING B Y O I L INJECTION
I n o r d e r t o overcome many o f t h e mounting and dismounting problems SKF, i n 1940, i n t r o d u c e d an o i l - i n j e c t i o n t e c h n i q u e which i s now w i d e l y used i n many branches o f e n g i n e e r i n g .
O i l under h i g h p r e s s u r e i s i n j e c t e d between t h e bear-
ing i n n e r r i n g and s h a f t j o u r n a l d u r i n g mounting o r dismounting.
An o i l f i l m
i s formed which b o t h separates and l u b r i c a t e s t h e c o n t a c t s u r f a c e s .
The o i l
separates t h e c o n t a c t s u r f a c e s except f o r a narrow zone a t each end o f t h e r i n g . The s u r f a c e p r e s s u r e i s g r e a t e r i n these zones due t o t h e i n f l u e n c e o f t h e s h a f t m a t e r i a l beyond t h e end o f t h e r i n g and t h e zones a c t a s an o i l l o c k which r e t a i n s t h e o i l between t h e c o n t a c t s u r f a c e s .
When t h e o i l p r e s s u r e i s released
t h e o i l i s f o r c e d a u t o m a t i c a l l y back through t h e s u p p l y d u c t s , thereby r e s t o r i n g the o r i g i n a l f r i c t i o n . The advantage o f u s i n g o i l - i n j e c t i o n i s t h a t t h e f o r c e r e q u i r e d t o move t h e component i s s i g n i f i c a n t l y reduced and t h a t by e l i m i n a t i n g d i r e c t c o n t a c t and t h e r e s u l t i n g f r i c t i o n between t h e c o n t a c t s u r f a c e s , t h e p o s s i b i l i t y o f damage t o t h e s u r f a c e s o c c u r r i n g d u r i n g t h e mounting o r dismounting process i s minimised.
170 A f u r t h e r advantage i s t h a t
components can be dismounted o r a d j u s t e d w i t h o u t
t h e r i s k o f the f i t d e t e r i o r a t i n g . I f t h e b e a r i n g r i n g i s mounted on a t a p e r e d j o u r n a l w i t h a s e l f - r e l e a s i n g taper, the
r i n g w i l l be e j e c t e d w i t h some f o r c e when t h e o i l i s i n t r o d u c e d and
some f o r o f a x i a l r e s t r a i n t such as a n u t w i l l be necessary.
I f , however, a
l o c k i n g t a p e r i s used then an e x t e r n a l f o r c e a d d i t i o n a l t o t h e o i l i n j e c t i o n f o r c e w i l l be r e q u i r e d .
F i g u r e s 41, 42 and 43 show t y p i c a l arrangements.
Position of distribution groove in a tapered and a cylindrical seating for a rolling bearing
u
Cylindrical seating
FIG.
41
Tapered seating
FIG.
42
The f i r s t two show a b e a r i n g on a p a r a l l e l s h a f t and a b e a r i n g on a tapered shaft.
The t h i r d shows o i l i n j e c t i o n b e i n g a p p l i e d t o a gear wheel mounted on
a parallel shaft. bearing r i n g i s
I t should be n o t e d t h a t , w i t h a p a r a l l e l s h a f t , once t h e
p a s t t h e o i l e n t r y h o l e t h e r e i s no l o n g e r any o i l p r e s s u r e
and t h e r i n g c o u l d
lock.
I t has been found, however, t h a t i f t h e r i n g i s w i t h -
drawn r a p i d l y then i t w i l l t r a v e l o v e r t h e remaining area w i t h o u t t o o much d i f f iculty
Large r o l l i n g b e a r i n g s w i t h a t a p e r b o r e and mounted on a d a p t o r o r w i t h -
d rawa 1 sleeves can be e a s i l y mounted o r dismounted u s i n g h y d r a u l i c n u t s designed by SKF
The h y d r a u l i c n u t c o n s i s t s o f an i n t e r n a l l y threaded s t e e l r i n g w i t h a
groove i n one f a c e i n t o which i s f i t t e d an a n n u l a r p i s t o n sealed w i t h O - r i n g s . When o 1 i s pumped i n t o t h e a n n u l a r space behind t h e p i s t o n i t i s f o r c e d o u t wards,
thereby f o r c i n g t h e b e a r i n g on o r p u l l i n g t h e b e a r i n g o f f t h e s l e e v e o r
shaft.
F i g u r e 4 4 shows a h y d r a u l i c n u t b e i n g used t o mount a s p h e r i c a l r o l l e r
b e a r i n g on an adaptor s l e e v e and Fig.45 shows an arrangement f o r d i s m o u n t i n g a b e a r i n g on an adaptor s l e e v e .
An arrangement f o r dismounting a b e a r i n g from a
withdrawal sleeve i s shown on Fig.46.
171
H M V nut for driving up a bearing on an adapter sleeve
Cylindrical mating surface having two distribution grooves
FIG.
43
H M V nut used to free a withdrawal sleeve
H M V nut and stop rings in position to press loose an adapter sleeve
FIG.
6.22
46
C L E A N I N G OF B E A R I N G S
A s p r e v i o u s l y mentioned,
i t i s n o t necessary t o c l e a n b e a r i n g s taken d i r e c t
from t h e i r packages and any a t t e m p t t o do so c o u l d be d e t r i m e n t a l from b o t h a c l e a n l i n e s s and f u t u r e l u b r i c a t i o n aspect. bearings need t o
There a r e occasions, however, when
be dismounted, cleaned, and inspected i n s p i t e o f the f a c t
t h a t unnecessary dismounting may cause d e t e r i o r a t i o n o f t h e s h a f t and housing f i t s as w e l l as p o s s i b l e damage t o t h e t r a c k s .
172 A f t e r d is m o u n t i n g f o r c l e a n i n g purposes a l l used grease should f i r s t be caref u l l y removed. ing f l u i d
Small b e a r i n g s sh ou l d be immersed i n w h i t e s p i r i t o r o t h e r clean-
and s w i r l e d round, t he r e s i d u a l grease and d i r t b e i n g removed u s i n g a
good q u a l i t y b r ush . tr a p p e d between
Care must be taken t o ensure t h a t none o f the b r i s t l e s a r e
t h e cage and r o l l i n g elements.
A f t e r t h i s p r e l i m i n a r y washing t he b e a r i n g s should be washed i n one o r two additional b a t h s o f white s p i r i t .
A warm (lOO°C maximum) l i g h t m i n e r a l o i l
s h o u ld then be f l u s h e d t h rou gh t h e b e a r i n g as i t i s s l o w l y r o t a t e d .
Under no
circumstances s ho ul d t h e b e a r i n g be r o t a t e d u n t i l t h e o i l passes through i t , o t h e r w i s e any f o r e i g n p a r t i c l e s i n t h e b e a r i n g w i l l be r o l l e d i n t o t h e t r a c k s c a u s in g i n d e n t a t i o n s . they s h o u ld
I f t h e b e a r i n g s a r e n o t intended f o r immediate mounting
be l u b r i c a t e d w i t h a good q u a l i t y o i l c o n t a i n i n g a w a t e r - r e p e l l a n t
additive.
6.23
RECOGNITION OF B E A R I N G FAILURES
Ob v io u s ly premature b e a r i n g f a i l u r e s o c c u r , b u t i n t h e m a j o r i t y o f cases i f the symptoms a r e reco gn i sed t h e cause o f t h e f a i l u r e s can be e l i m i n a t e d .
If
r o l l i n g b e a r in g s a r e p r o p e r l y mounted, sea l ed, l u b r i c a t e d and maintained, they w i l l run u n t i l f a t i g u e f a i l u r e ( f l a k i n g ) occurs on t h e b e a r i n g i n n e r o r o u t e r r i n g o r r o l l i n g elements. f a i l u r e can
As a l r e a d y mentioned, b e a r i n g l i f e based on f a t i g u e
be p r e d i c t e d as a f u n c t i o n o f t h e l i f e d i s p e r s i o n curve, and i f a
b e a r i n g f a i l s w e l l s h o r t o f i t s normal L,o
l i f e i t i s important to a s c e r t a i n
th e cause o f the f a i l u r e b e f o r e f i t t i n g new b e a r i n g s .
6.23.1
Wear
I f a b e a r i n g housing i s i na de qu at el y se al ed, a b r a s i v e p a r t i c l e s can e n t e r the bearing
c a u s in g wear i n t h e t r a c k s ,
be a r in g s wear i n t h e g u i d e f l a n g e s .
r o l l i n g elements and cage, and i n c e r t a i n The worn s u r f a c e s a r e d u l l i n appearance
and an example o f worn t r a c k s i s shown i n Fig.47.
Wear can a l s o o c c u r between
th e i n n e r r i n g bo re and i t s j o u r n a l o r between t h e o u t e r r i n g o u t s i d e diameter and h o u s in g
bore due t o creep.
T h i s i s u s u a l l y the r e s u l t
o f excessive c l e a r -
ance and i s p a r t i c u l a r l y s e r i o u s because n o t o n l y the b e a r i n g b u t a l s o t h e s h a f t o r h o u s in g c o u l d be damaged and r e q u i r e r e p l a c i n g . faces
Creep between mating s u r -
causes f r e t t i n g c o r r o s i o n and wear, and p a r t i c l e s o f r u s t from t h e c o r r o -
ded areas may a l s o e n t e r t h e b e a r i n g cau si ng wear i n t h e t r a c k s and r o l l i n g e l e ments.
Wear can a l s o
be caused
having a h i g h l y planished f i n i s h . failure.
by inadequate l u b r i c a t i o n ,
t h e worn surfaces
T h i s t y p e o f wear develops i n t o f a t i g u e
173
Fig.47
Worn T r a c k s due t o Abrasive P a r t i c l e s .
174 Incorrect
6.23.2
Mounting
F a u l t y mounting o f t e n r e s u l t s i n t h e b e a r i n g b e i n g s u b j e c t e d t o h i g h loads which can
cause m a t e r i a l f a t i g u e i n t h e b e a r i n g r i n g s o r r o l l i n g elements w e l l
b e f o r e t h e nominal c a l c u l a t e d l i f e o f t h e b e a r i n g has been reached.
Indenta-
t i o n s w i t h t h e same p i t c h as t h e r o l l i n g elements can be caused by t h e mounting f o r c e b e i n g a p p l i e d through t h e r o l l i n g elements.
D u r i n g s e r v i c e an o v e r l o a d
occurs each t i m e a r o l l i n g element passes o v e r an i n d e n t a t i o n , and a f t e r a r e l a t i v e l y s h o r t t i m e small fragments o f b e a r i n g m a t e r i a l break away, t h e c o n d i t i o n b e i n g g e n e r a l l y r e f e r r e d t o as " f l a k i n g " .
F i g u r e 48 shows t h e i n n e r r i n g o f a
deep groove b a l l b e a r i n g w i t h advanced areas o f f l a k i n g on t h e r i g h t - h a n d s i d e o f the track.
Fig.48
F l a k i n g caused by F a u l t y Mounting
F i g u r e 49 shows t h e i n n e r and o u t e r r i n g o f a b a l l b e a r i n g w i t h a f i l l i n g s l o t . The f l a k e d wear on t h e r i g h t - h a n d s i d e o f t h e i n n e r r i n g and on t h e l e f t - h a n d side
o f t h e o u t e r r i n g show t h a t f a i l u r e has been caused by a heavy t h r u s t l o a d
due t o c r o s s - l o c a t i o n .
F a t i g u e f a i l u r e o f t h i s t y p e begins a t a p o i n t below t h e
s u r f a c e o f t h e t r a c k o r r o l l i n g elements. work up t o t h e s u r f a c e where,under
Small c r a c k s develop, which g r a d u a l l y
repeated o v e r l o a d a t such weak p o i n t s , even-
t u a l l y cause fragments t o break away, t h e s e fragments b e i n g r o l l e d i n t o o t h e r p a r t s o f t h e t r a c k thereby i n i t i a t i n g f u r t h e r areas o f weakness.
175
Fig.49
F a i l u r e caused by a Heavy T h r u s t Load due t o Cross L o c a t i o n .
6.23.3
Cage F a i l u r e s
With an adequately l u b r i c a t e d b e a r i n g o p e r a t i n g i n normal c o n d i t i o n s , t h e
I f t h e l u b r i c a t i o n i s inadequate,
cage i s t h e most l i g h t l y - l o a d e d component.
wear w i l l o c c u r where t h e cage makes c o n t a c t w i t h t h e r i n g s and r o l l i n g elements and e v e n t u a l l y t h e cage may f r a c t u r e .
Highly planished surfaces are u s u a l l y
a s s o c i a t e d w i t h inadequate cage l u b r i c a t i o n .
Excessive misalignment o f t h e
i n n e r and o u t e r b e a r i n g r i n g r e l a t i v e t o each o t h e r i s a n o t h e r cause o f cage f a i l u r e and severe misalignment can r e s u l t i n t h e cage, o r even t h e b e a r i n g rings, cracking.
Wear can a l s o be caused by f o r e i g n p a r t i c l e s e n t e r i n g t h e
bearing, t h e p a r t i c l e s jamming o r wedging between t h e cage and r o l l i n g elements. F i g u r e 50 shows a s p h e r i c a l r o l l e r b e a r i n g case w i t h worn r o l l e r prongs and wear i n t h e bottom o f t h e r o l l e r pockets. where t h e prongs have a c t u a l l y sheared.
F i g u r e 51 shows a more advanced case
176
Fig.50
Spherical R o l l e r Bearing Cage w i t h Worn R o l l e r Prongs and Wear i n the Bottom of the R o l l e r Pockets.
Fig.51
More Advanced Case than F i q . 5 0 , w i t h Prongs Sheared.
6.23.4
Smearing
Smearing
i s a t r a n s f e r e n c e o f m a t e r i a l from one body t o another when two
inadequately l u b r i c a t e d
surfaces s l i d e a g a i n s t each o t h e r .
Local s t r e s s con-
c e n t r a t i o n s a r e induced by smearing, and these produce cracks i n t h e s u r f a c e l a y e r s and subsequent f l a k i n g . which
F i g u r e 52 shows a smeared c y l i n d r i c a l r o l l e r
has been caused by r a p i d a c c e l e r a t i o n o f t h e r o l l e r i n t h e loaded zone
coupled w i t h inadequate o r i n c o r r e c t l u b r i c a t i o n . subjected
I f c y l i n d r i c a l r o l l e r s are
t o severe t h r u s t f o r c e s combined w i t h inadequate o r u n s u i t a b l e l u b r i -
c a t i o n , smearing develops a t t h e ends o f t h e r o l l e r and on t h e g u i d e f l a n g e s
Fig.52
Smeared C y l i n d r i c a l R o l l e r caused by Rapid A c c e l e r a t i o n o f t h e R o l l e r i n t h e Loaded Zone coupled w i t h Inadequate o r Incorrect Lubrication.
o f b e a r i n g r i n g s , and Fig.53 shows smearing on t h e end o f a r o l l e r .
Smearing
can a l s o o c c u r on l i g h t l y - l o a d e d high-speed b a l l t h r u s t b e a r i n g s where t h e g y r a t o r y moment may f o r c e t h e b a l l s t o s l i d e t a n g e n t i a l l y t o t h e d i r e c t i o n o f
178
Fig.53
Smearing Caused by Severe Thrust Forces combined with Inadequate o r Unsuitable Lubrication.
rolling, causing spiral-shaped smearing.
Lightly-loaded high-speed ball thrust
bearing must therefore always have a small pre-load or spring load when mounted as given on Fig.22.
6.23.5
Vibrations
A phenomenon known as 'False Brinelling' can occur in rolling bearings fitted to stationary machines, due to vibrations from adjacent machinery.
Bearings in
ships ancillary equipment subject t o vibrations from the ship's machinery are particularly prone to this damage and this can apply to bearings in machines being transported by sea.
179
Fi9.54
Fig.55
Vibration Damage.
Vibration Oamage.
180
Rotor clamp for preventing damage due t o vibration during transport
FIG.
Fig.57
56
Corrosion a t the Contact Surfaces due t o Water.
181 A c h a r a c t e r i s t i c f e a t u r e o f v i b r a t i o n damage i s t h a t t h e damaged areas a r e spaced a t t h e same p i t c h as t h e r o l l i n g elements.
The v i b r a t i o n s induce small
movements a t t h e c o n t a c t surf ace s between t h e r o l l i n g elements and t h e r i n g s , and t h e r e s u l t i n g wear causes mi cro scop i c p a r t i c l e s o f m a t e r i a l t o break away. These p a r t i c l e s o x i d i s e and, on m i x i n g w i t h t h e l u b r i c a n t , cause a l a p p i n g a c t i o n , and hence an i n crea se i n t h e o x i d a t i o n r a t e .
The h i g h e r t h e frequency
o f t h e v i b r a t i o n a l movement a t t h e c o n t a c t s u r f a c e s , t h e more r a p i d t h e damage, each r o l l i n g element g r a d u a l l y f o r m i n g a cavity’. C y l i n d r i c a l r o l l e r and n ee dl e r o l l e r b e a r i n g s a r e more prone t o t h i s t y p e o f damage t h a n b a l l b e a r i n g s and s p h e r i c a l r o l l e r b e a r i n g s , and one method o f overcoming t h e problem i s t o use spri n g-l o ad ed b a l l bearings.
The p r e - l o a d
s h o u ld be a p p r o x i m a t e l y equal t o 20 Newtons per m i l l i m e t e r o f s h a f t diameter. Be fo r e p r e - l o a d i n g ,
however,
i t i s necessary t o check t h a t t h e combination o f
e x t e r n a l f o r c e s and a d d i t i o n a l p r e - l o a d does n o t o v e r l o a d t h e bearing. T y p i c a l examples o f v i b r a t i o n damage t o b e a r i n g s d u r i n g t r a n s p o r t a t i o n due t o r e l a t i v e movement between t h e s h a f t and housing, whether by sea, r a i l , o r road, can be r e s o l v e d by d r i v i n g a wooden wedge between t h e s h a f t o r c o u p l i n g and a r o b u s t p a r t o f t h e c r a t e .
Alternatively,
t h e s h a f t can be locked r e -
l a t i v e t o t h e h o usi n g by means o f a clamp, as shown i n Fig.56.
6.23.6
Rust and Other Types o f C o r r o s i o n
R o l l i n g b e a r i n g s w i l l r u s t i f wat er o r m o i s t u r e i s p r e s e n t and t h e bearings a r e n o t p r o t e c t e d by a l u b r i c a n t c o n t a i n i n g a r u s t - i n h i b i t o r .
P i t t i n g develops
i n t h e c o r r o d e d are as, r e l e a s i n g small p a r t i c l e s o f r u s t , and i f t h e r e a r e such areas i n t h e t r a c k s t h e r u s t mixes w i t h grease and a c t s as a l a p p i n g agent. Fig.57 shows c o r r o s i o n on a s p h e r i c a l r o l l e r b e a r i n g and i n t h i s case t h e grease was u n a b l e t o p r o t e c t t h e b e a r i n g f rom w ater.
E l e c t r o l y t i c a c t i o n due
t o t h e wa t e r has r e s u l t e d i n c o r r o s i o n a t t h e c o n t a c t s u r f a c e s between t h e r o l l e r s and r i n g s .
6.23.7
Passage o f E l e c t r i c Cu rren t Through Bearings
E l e c t r i c c u r r e n t p assi n g t hro ug h a r o l l i n g b e a r i n g causes damage t o t h e t r a c k s o r r o l l i n g elements, which can r e s u l t i n premature f a i l u r e o f t h e bearing. A l t e r n a t i n g and d i r e c t c u r r e n t have a s i m i l a r e f f e c t and where t h e r e i s a r i s k o f e l e c t r i c a l leakage, o f current.
r o l l i n g b e a r i n g s must be p r o t e c t e d a g a i n s t t h e passage
I f a c o n t i n u o u s c u r r e n t passes through a r o t a t i n g b e a r i n g a dark
c o l o u r e d f i l m i s produced on t h e t r a c k s and r o l l i n g elements which g r a d u a l l y develop i n t o a washboard f o r m a t i o n as shown i n Fig.58. The b a l l s i n b a l l b e a r i n g s s u b j e c t e d t o e l e c t r i c c u r r e n t do n o t u s u a l l y develop t h e washboard surf ace , b u t become u n i f o r m l y dark-coloured over the whole surface.
T h i s i s due t o t he b a l l s s p i n n i n g when t h e b e a r i n g r o t a t e s .
The
182
Fig.58
\,/ashboardEffect Caused by Continuous Passage of Electric Current.
rings have fluting similar to that on rollers, but the bands are much narrower. On applications such a s traction motors, craters and burns occur instead of the washboard formation and a typical burn is shown on Fig.59. Generally pitting from electrical leakage does not mean rapid failure and on railway applications, it is known that pitted bearings have run for very long periods before requiring replacing. The main problem is removing the debris released during the formation of the craters, and provided this is carried out and fresh lubricant added, preferably by means of a grease escape valve, the bearings should function successfully for a further period. There are many problems relating to ball and roller bearings, but providing the basic principles governing bearing selection and a knowledge of lubrication is developed, then the calculated nominal bearing life can be achieved.
183
Fig.59
Typical Burn Caused by E l e c t r i c Current Leakage i n A Traction Motor.
184
I-
4
PRACTICAL GEAR TRIBOLOGY
T.I.
FOWLE, C o n su l t a nt , Tenterden
7.1
INTRODUCTION The i d e n t i f i c a t i o n o f t h e causes o f t h e v a r i o u s forms o f d i s t r e s s appearing
on gear t e e t h i s seldom an easy m a t t e r because o f t h e i r g r e a t v a r i e t y and because few e n g i ne ers have t h e o p p o r t u n i t y t o see even a m i n o r i t y o f them a t f i r s t hand.
The t rou bd esh oo t i n g c h a r t s g i v e n a t t h e end o f & i s
chapter a r e
i n te n d e d t o s i m p l i f y t h e i d e n t i f i c a t i o n o f p o s s i b l e causes and t h e s e l e c t i o n o f a p p r o p r i a t e remedies f r o m t h e observed symptoms.
Systematic c o n s i d e r a t i o n o f
the v a r i o u s p o s s i b i l i t i e s should a t l e a s t narrow down t h e number and suggest t e s t s which m ig h t be a p p l i e d t o c o n f i r m t h e f i n a l i d e n t i f i c a t i o n .
The f o l l o w i n g
not e s a r e g i v e n t o a m p l i f y and e x p l a i n t h e reasons f o r t h e most i m p o r t a n t effects.
7.2
ALIGNMENT There a r e two a spe ct s o f gear a l i g n m e n t : e x t e r n a l and i n t e r n a l .
e x t e r n a l a lig n m e nt ,
t h a t i s a l i gn men t w i t h t h e connected
loads on t h e b e ari n gs and c o u p l i n g s ,
Errors i n
machines, p l a c e over-
r i s k i n g f a i l u r e o r a t l e a s t n o i s e which
c o u ld be e r r o n e o u s l y a t t r i b u t e d t o t h e gears themselves.
Errors i n internal
alig n m e n t cause uneven d i s t r i b u t i o n o f t h e l oad a l o n g t h e gear t e e t w i t h consequent r i s k o f damage and n o i s y r u n n i n g and a r e o f p a r t i c u l a r concern i n t h i s c h a p te r . With p a r a l l e l - s h a f t gears t h e r e a r e b a s i c a l l y two e r r o r s o f i n t e r n a l a l i g n ment:
t h e s h a f t s may n o t be p a r a l l e l , and they may n o t be i n t h e same p l a n e .
Bo t h these e r r o r s may, o f course, be p r e s e n t t o g e t h e r .
The v a r i o u s combinations
produce p a t t e r n s of c o n t a c t , o r o f damage such as p i t t i n g o r s c u f f i n g , as shown i n F i g u r e 1.
Only i n a few cases a r e gears p r o v i d e d w i t h means for a d j u s t i n g
t h e p a r a l l e l i s m o f t h e two s h a f t s , and indeed, w i t h modern machining i t i s u n l i k e l y t h a t t h e s h a f t s w i l l n o t be p a r a l l e l . eve n ly supported on i t s f e e t by t h e f ou nd at i ons,
However,
i f t h e gearcase i s n o t
t h e s h a f t s w i l l n o t be i n t h e
same p la n e . Checking t h a t t h e two s h a f t s a r e i n t h e same p l a n e i s b e s t c a r r i e d o u t by removing t h e t o p o f t h e gear c a s i n g and, o r b e a r i n g keeps.
i f necessary, t h e top halves o f bearings
A s t r a i g h t edge w i t h a spacer b l o c k t o a l l o w f o r any d i f f e r -
ence i n diameter s u p p o r t i n g a p r e c i s i o n s p i r i t l e v e l can then be placed across
185 b o t h ends o f t h e s h a f t s i n t u r n t o check t h a t t h e ends a r e i n t h e same plane. An accuracy o f 1 p e r 60 000 i s s a t i s f a c t o r y .
The t h i c k n e s s o f the shims r e q u i r e d
can then be e a s i l y determined.
Single helical and spur
€2
Double helical
Gear axes parailei but not
co-planar
Single helical and spur
€3
Q
Gear axes co-pianar but not parailei Single helical and spur
iii ozzl
Gear axes neither co-planar nor parallel
Fig.1
I n t e r n a l misalignment p a t t e r n s on p a r a l l e l s h a f t gears
I n t e r n a l a l i g n m e n t can a l s o be checked by d e t e r m i n i n g t h e e x t e n t o f t o o t h c o n t a c t w i t h marking b l u e .
A f t e r e n s u r i n g t h a t t h e j o u r n a l s cannot l i f t o u t o f
t h e i r b e a r i n g s and t h a t t h e j o u r n a l s a r e l u b r i c a t e d , a t h i n c o a t i n g o f marking b l u e i s a p p l i e d i n a t h i n a x i a l band on one o f the gears.
The gears a r e then
t u r n e d so t h a t t h e marking i s t r a n s f e r r e d o n t o t h e o t h e r gear. then be taken by means of a S e l l o t a p e impression. depends on t h e c o n d i t i o n s
A r e c o r d may
The degree o f c o n t a c t required
o f s e r v i c e as i n d i c a t e d i n F i g u r e 2 ( a ) t o (d) which
i s based on BS 1807 f o r t u r b i n e gears and s i m i l a r d r i v e s .
' S p l i t Marking' as
186 shown i n F i g u r e 2 ( e ) , which i s due t o t h e hob n o t h a v i n g been c o n c e n t r i c w i t h i t s mandrel,
i s u n d e s i r a b l e because i t p r e v e n t s t h e most f a v o u r a b l e p a r t o f t h e
tooth p r o f i l e ,
i.e.
t h a t w i t h t h e l e a s t s l i d i n g , from s u p p o r t i n g t h e l o a d and
transfers i t instead t o less favourable p a r t s .
25%
25%
50%
la)
T
I
I
I
I
(6)
I
I
I
I
Fig.2
Contact area
( a ) , (b) and ( c ) r e p r e s e n t t h e minimum c o n t a c t areas r e q u i r e d by BS 1 8 0 7 f o r t u r b i n e gears and s i m i l a r d r i v e s , c l a s s e s B, A 2 and A1 r e s p e c t i v e l y . The s p e c i f i c a t i o n does n o t s t i p u l a t e t h e d i s t r i b u t i o n o f t h e c o n t a c t areas, o n l y t h e i r depth and t o t a l l e n g t h . Classes A 2 and A1 a r e f o r gears w i t h p i t c h l i n e speeds o v e r 50 m / s , A 2 b e i n g suggested f o r 50-100 m/s and A1 f o r 50-150 m / s . ( d ) r e p r e s e n t s t h e requirements o f some a u t h o r i t i e s f o r p r e c i s i o n gears. ( e l S p l i t marking.
187 W i t h n o n - p a r a l l e l s h a f t gears,
such as b e v e l g e a r s and worm gears, a l l o w a n c e
has t o be made f o r t h e i n e v i t a b l e d i s t o r t i o n under l o a d , as shown i n F i g u r e s and 4.
3
I n a l l cases c o n t a c t s h o u l d n o t e x t e n d t o t h e ends o f t h e t e e t h as l o a d s
No load Fig.3
'
Full load C o n t a c t marks on b e v e l
gears
.+
No load
Full load
Fig.4
C o n t a c t marks on worm gears
t h e r e would be h i g h l y c o n c e n t r a t e d .
I n t h e case o f worm g e a r s t h e r e must a l s o
be c l e a r a n c e where t h e worm e n t e r s t h e c o n t a c t , even a t t h e h i g h e s t l o a d s , so t h a t t h e o i l on i t s s u r f a c e i s n o t scraped o f f by t h e edge o f t h e t o o t h .
I f
t h i s c l e a r a n c e i s n o t a v a i l a b l e t h e f r i c t i o n and wear a r e g r e a t l y i n c r e a s e d and t h e t r a n s m i t t e d power 1 i m i t e d .
7.3
TOOTH ACTION An u n d e r s t a n d i n g o f t o o t h a c t i o n h e l p s t o e x p l a i n many a s p e c t s o f g e a r t r i b o -
logy.
I n s p u r , h e l i c a l and b e v e l g e a r t o o t h a c t i o n i s as r e p r e s e n t e d i n F i g . 5 .
The p o i n t o f c o n t a c t moves c o n t i n u o u s l y o v e r b o t h t e e t h and i n t h i s sense t h e y r o l l o v e r one a n o t h e r . p r e s s u r e s t o be c a r r i e d .
The t r a n s i e n t n a t u r e o f t h e c o n t a c t e n a b l e s v e r y heavy
A t t h e p i t c h p o i n t b o t h t o o t h s u r f a c e s a r e moving i n
t h e same d i r e c t i o n a t t h e same speed without sliding.
and so m o m e n t a r i l y r o l l o v e r one a n o t h e r
A t a l l o t h e r p o i n t s t h e s u r f a c e s a r e moving a t d i f f e r e n t
speeds so t h a t t h e r e i s s l i d i n g as w e l l as r o l l i n g .
188
Opposing surface slides in these directions Line of action Dr
Fig.5
Contact c o n d i t i o n i n Spur, H e l i c a l and Bevel t e e t h . The h a t c h i n g i n d i c a t e s t h e d i r e c t i o n o f d i s t o r t i o n o f the surface layers.
On t h e s u r f a c e o f t h e d r i v i n g gear t h e s l i d i n g i s always away from t h e p i t c h l i n e , w h i l e on t h e d r i v e n t o o t h i t i s always towards t h e p i t c h l i n e . lubrication i s
When
inadequate t h e s l i d i n g shear f o r c e s thus tend t o draw m a t e r i a l
away from the p i t c h l i n e o f t h e d r i v e r and t o p i l e i t up a t t h e p i t c h l i n e o f the d r i v e n tooth.
T h i s produces t h e c o n d i t i o n known as g r o o v i n g and r i d g i n g
( r e f e r symptom 1 1 o f t h e T r o u b l e - s h o o t i n g c h a r t ) .
Another e f f e c t o f t h i s system
of s l i d i n g i s t h a t on t h e dedendum s u r f a c e o f b o t h gears any cracks a r e p u l l e d open i n advance o f t h e c o n t a c t . favourable d i r e c t i o n ,
The cra cks b e i n g a l s o i n c l i n e d i n t h e most
t h e o i l r e a d i l y e n t e r s t h e crack.
F u r t h e r movement o f
t h e c o n t a c t t h e n s e a l s t h e mouth o f t h e c r a c k and compresses t h e o i l i n i t t o exte n d t h e c r a c k s t i l l f u r t h e r .
I n c o n t r a s t , any cracks on t h e addendum surfaces
s l o p e away from t h e on-coming c o n t a c t which,
furthermore,
tends t o push t h e
s i d e s o f t h e c r a c k t o g e t h e r i n advance o f c o n t a c t so t h a t o i l i s n o t encouraged t o e n t e r t h e c r a ck.
The r e s u l t i s t h a t f a t i g u e p i t t i n g tends t o occur almost
e x c l u s i v e l y on t h e dedendum s u r f a c e o f gear t e e t h ,
b o t h d r i v e r and d r i v e n .
An i m p o r t a n t f e a t u r e o f spur and h e l i c a l gears i s t h a t t h e r e i s no s l i d i n g al o n g t h e l i n e o f c o n t a c t , whi ch i s a l s o v i r t u a l l y t h e case w i t h bevel gears. I n h y p o id gears and worm gears, however, t h e r e i s a c o n s i d e r a b l e component o f s l i d i n g a l o n g t h e l i n e o f c o n t a c t whi ch makes f o r g r e a t e r d i f f i c u l t y i n
189 lubrication,
because any a s p e r i t y on one s u r f a c e i s i n c o n t a c t w i t h t h e o t h e r
f o r a g r e a t e r t ime and d i s t a n c e .
I n worm gears p a r t i c u l a r l y ,
s i d e r a b l y in c r e a se s f r i c t i o n and t e mpe rat ure r i s e .
t h i s s l i d i n g con-
Much o f t h e f r i c t i o n , more-
over, m e r e ly serves t o d i s t o r t t h e wheel towards c r i t i c a l c o n d i t i o n s o f c o n t a c t (see symptom 20).
7.4
TOOTH SURFACE DISTRESS
O f t h e t h r e e main forms o f s u r f a c e d i s t r e s s : p i t t i n g , a b r a s i v e wear and s c u f f i n g o r a d h e si ve wear, t h e f i r s t tends t o o c c u r a t t h e lower end o f t h e speed range w h i l e s c u f f i n g tends t o o ccur a t t h e h i g h e r end as i n d i c a t e d i n Figure 6 .
S c u f f i n g may f o low p i t t i n g , b u t p i t t i n g does n o t occur where t h e r e
i s s c u f f i n g o r where t h e r e i s a b r a s i v e wear.
The i d e a l c o n d i t i o n o f f u l l - f i l m
o r elastohydrodynamic l u b r c a t i o n (EHL) occu rs a t low loads and h i g h speeds.
Bending fatigue failure, impact f radure
I
'tr
U
0
4
Partial E.H.L.
Speed F i g. 6
-
Zones o f gear t o o t h d i s t r e s s .
The d i f f e r e n t slopes o f l i m i t s f o r p i t t i n g i n h a r d and i n s o f t gears i l l u s t r a t e s e p a r a t e l y t h e two e f f e c t s o f speed on f i l m t h i c k n e s s and t o o t h e r r o r s .
7.4.1
Pitting
About t h e most common f orm o f s u r f a c e d i s t r e s s i n s o f t o r through-hardened gears i s p i t t i n g , wh i ch i s a t y p e o f f a t i g u e f a i l u r e . p i t t i n g i s r a t h e r unusual i n surface-hardened g e a r i n g .
As i n d i c a t e d i n F i g u r e Various sub-types and
6,
190 causes a r e l i s t e d a g a i n s t sympton 2, and an example i s shown i n F i g u r e 7. t h i s Figure the t y p i c a l oyster-shell
In
shaped c r a t e r s produced by t h e o i l p r i s i n g
t h e fragments l o o s e can be seen.
Fig.7
Surface f a t i g u e p i t t i n g .
Note c h a r a c t e r i s t i c o y s t e r - s h e l l shape on t h e l e f t . Note a l s o t h e e f f e c t s o f ' s p l i t m a r k i n g ' . (Photo c o u r t e s y o f S h e l l I n t e r n a t i o n a l Co.Ltd.) A s well as t h e s t r e n g t h o f t h e m a t e r i a l , t h e o i l f i l m t h i c k n e s s , o r r a t h e r t h e r a t i o o f t h e o r e t i c a l o i l f i l m t h i c k n e s s t o t h e s u r f a c e roughness o f t h e harder t o o t h , i s a most i m p o r t a n t f a c t o r , and t h i c k f i l m s o r f i n e f i n i s h e s o r b o t h a r e d e s i r a b l e t o reduce t h e i n c i d e n c e o f p i t t i n g . t o be avoided as much stresses.
Shock-loading i s a l s o
as p o s s i b l e as i t c o n s i d e r a b l y increases t h e s u r f a c e
T h i s i s i n d i c a t e d by t h e n e g a t i v e s l o p e o f t h e p i t t i n g l i m i t f o r s o f t
gears i n F i g u r e 6. P i t t i n g may be e i t h e r o f t h e i n i t i a l o r t h e p r o g r e s s i v e t y p e .
With the
former t h e removal o f a s p e r i t i e s and prominent areas by p i t t i n g increases t h e a c t u a l area o f c o n t a c t and reduces t h e s t r e s s e s s o t h a t i t a r r e s t s i t s e l f . W i t h overloaded s u r f a c e s , however, p i t t i n g c o n t i n u a l l y reduces t h e a r e a o f
191 a c t u a l c o n t a c t and promotes f u r t h e r p i t t i n g .
But i f one gear i s a b l e t o r e s i s t
p i t t i n g and can m a i n t a i n i t s shape, p i t t i n g w i l l progress less r a p i d l y on t h e o t h e r (see sympton 2 ( f ) ) . The s i z e o f t h e p i t s i s g e n e r a l l y r e l a t e d t o t h e s i z e
o f the gears, b u t there
may be wid e v a r i a t i o n s due, perhaps, t o s u r f a c e a s p e r i t i e s and o t h e r d e t a i l s . I n p a r t i c u l a r , where t h e o i l f i l m t h i c k n e s s i s r e l a t i v e l y v e r y t h i n t h e s u r f a c e t r a c t i o n can be v e r y h i g h and produce v e r y l a r g e s h a l l o w p i t s (see 2 ( j ) ) . The e f f e c t o f extreme p r e s s u r e (EP) a d d i t i v e o i l s i s v a r i e d .
I n laboratory
t e s t s u s i n g v e r y a c c u r a t e gears o r d i s c s , EP o i l s a r e g e n e r a l l y found t o a c c e l e r a t e p i t t i n g , b u t where p r a c t i c a l gears h a v i n g s u r f a c e u n d u l a t i o n s o r s i m i l a r i n a c c u r a c i e s a r e concerned, EP o i l s a r e o f t e n a b l e t o d e l a y o r suppress t h e problem.
A p p a r e nt l y,
a t l e a s t some EP ag en t s can prevent t h e c r e s t s o f such
u n d u l a t i o n s from work-hardening so t h e c o n t a c t zones can deform t o increase t h e i r area and thus reduce s t r e s s e s and temperatures.
7.4.2
Scuffing
Severe a d h e s ive wear t ake s seve ral d i f f e r e n t forms i n gear t e e t h according t o c o n d i t i o n s and i s g i v e n even more d i f f e r e n t names. s c o r i n g , g a l l i n g and p l u c k i n g .
For example, s c u f f i n g ,
The nomenclature i s even f u r t h e r confused by t h e
f a c t t h a t what i s c a l l e d s c u f f i n g i n t h e UK i s known as s c o r i n g i n t h e USA.
In
l a b o r a t o r y t e s t r i g s t h e v a r i o u s forms a r e n o t t o o d i f f i c u l t t o recognise and a r e reasonably w e l l d e f i n e d , as f o r example s c u f f i n g and s c o r i n g i n I P 166 f o r the I A E Gear Rig, b u t i n n on -st a nd ard i sed t e s t s d i s t i n c t i o n s a r e sometimes made between high-speed and low-speed
s c u f f i n g ( l e s s than about 4 m/s p i t c h l i n e
speed a c c o r d i n g t o some a u t h o r i t i e s ) and between s e l f - p r o p a g a t i n g and s e l f h e a l i n g forms i n b o t h cases. I n p r a c t i c a l gears appearances may v a r y t o an even g r e a t e r e x t e n t , b u t a r e d i f f e r e n c e s i n degree, n o t o f k i n d .
I n comparison w i t h l a b o r a t o r y t e s t gears i t
should be borne i n mind t h a t i n t h e l a t t e r a l a r g e p a r t o f t h e power i s used t o overcome t o o t h f r i c t i o n i n t h e t e s t gears so t h a t when l u b r i c a t i o n f a i l s and s c u f f i n g o c c u r s , t h e r e i s a c o n s i d e r a b l e i n c r e a s e i n t h e power r e q u i r e d .
There
i s thus an immediate f a l l i n speed wh i ch f u r t h e r increases f r i c t i o n and noise, as w e l l as p r o d u ci n g
smoke and sometimes sp arks.
Under these c o n d i t i o n s
s c u f f i n g o f t e n does n o t l a s t f o r more than a f r a c t i o n o f a minute and, FZG r i g , f o r example,
never more than 15 mi n utes.
i n the
I n p r a c t i c e , however, the
e x t r a power absorbed when gear t e e t h s c u f f i s o n l y a v e r y small f r a c t i o n o f t h e power b e in g t r a n s m i t t e d so t h a t t h e o nse t o f t h e damage i s u s u a l l y unnoticed and may c o n t i n u e f o r many hours.
Under such c o n d i t i o n s t h e v e r y severe type known
as ' g a l l i n g ' a r i s e s . Except p o s s i b l y a t low speeds,
s c u f f i n g appears t o be an e s s e n t i a l l y thermal
phenomenon due t o t o o t h s l i d i n g f r i c t i o n .
The speed of s l i d i n g depends mainly on
192
S c u f f e d p i n i o n t o o t h f r o m FZG t e s t . (Photo c o u r t e s y o f S h e l l I n t e r n a t i o n a l Co. L t d . )
Fig.8
t h e p e r i p h e r a l speed o f t h e gears and t h e s i z e o f t h e t e e t h .
And s i n c e l a r g e
t e e t h can be more h e a v i l y loaded than small t e e t h , t o o t h s i z e tends tohave a preponderant i n f l u e n c e on t h e i n c i d e n c e o f s c u f f i n g , as i n d i c a t e d i n Table 7.1 below, which summarises general e x p e r i e n c e , a l s o i n s e c t i o n 3 ( f ) . Table 7.1
Danger o f s c u f f i n g i n spur, h e l i c a l and bevel gears.
Module p i t c h (mm) 1.25
Danger o f s c u f f i n g None
2.5
Only a t v e r y h i g h speeds w i t h t h i n o i l .
5
A t moderate speeds even w i t h medium o i l .
10
A t low speeds even w i t h heavy o i l .
193 A s i n d i c a t e d i n t h e above T a b l e , and a l s o i n symptom 3 ( d ) , t h e h i g h e r t h e
v i s c o s i t y grade o f t h e o i l t h e g r e a t e r t h e p r o t e c t i o n i t a f f o r d s a g a i n s t s c u f f ing.
I f t h e grade i s t o o v i s c o u s ,
however, t h e r e c o u l d be d i f f i c u l t i e s i n
s t a r t i n g up f r o m c o l d and t h e power loss and t e m p e r a t u r e r i s e i n high-speed b e a r i n g s m i g h t be e x c e s s i v e .
Compromises have t o be made, t h e r e f o r e , and l i g h t e r
grades have t o be used f o r high-speed gears which,
i n any case, a r e more a c c u r -
a t e l y made and have s m a l l e r t e e t h and c o r r e s p o n d i n g l y s m a l l e r l o a d s .
Viscosity
grade recommendations f o r s p u r , h e l i c a l and bevel gears a r e , t h e r e f o r e , o f t e n g i v e n i n terms o f speed o n l y , as i n T a b l e 7.2 below, which i s f o r gears operat i n g a t ambient temperatures between about 10°C and 25OC. Table 7.2
O i l v i s c o s i t y grades f o r spur, h e l i c a l and bevel gears P i t c h l i n e speed m/s
Range o f I S 0 V i s c o s i t y Grades c S t a t 4OoC
ft/min
0.5
100
460
1.3
250
320
2.5
500
220
5
1000
150
12.5
2500
100
25
5000
68
10000
46
50
-
1000
680 460 320
220 150 100
H i g h e r v i s c o s i t y grades may be needed where t h e ambient t e m p e r a t u r e exceeds 2S0C, where t h e gears a r e s u b j e c t t o shock loads, o r where b o t h gears a r e made o f through-hardened n i c k e l - c h r o m e s t e e l s .
Lower v i s c o s i t y grades may be used
where t h e ambient t e m p e r a t u r e i s below 10°C,
o r where t h e t e e t h have been g i v e n
a scuff-resistant coating t o a s s i s t running-in. For d o u b l e - r e d u c t i o n g e a r i n g t h e low v i s c o s i t y end of t h e range f o r t h e low speed t r a i n s h o u l d be taken, and f o r m u l t i p l e t r a i n s t h e mean f o r t h e two slowest trains. Worm gears need t o be t r e a t e d r a t h e r more g e n e r o u s l y because t o o t h f r i c t i o n i s o f much g r e a t e r importance.
To m i n i m i s e t o o t h f r i c t i o n H V I o i l s a r e p r e -
f e r r e d t o o t h e r t y p e s and v i s c o s i t y grades a r e h i g h e r than f o r o t h e r types o f g e a r i n g , a s i n d i c a t e d i n T a b l e 7 . 3 below. For d e s i g n e r s , t h e r i s k o f s c u f f i n g can be assessed by t h e use o f B l o k ' s C r i t i c a l Contact Temperature t h e o r y o r t h e Niemann and S e i t z i n g e r b u l k t o o t h temperature c r i t e r i o n o f s t r a i g h t o r EP o i l s . a t i o n o f T a b l e 7.1
[l]and a d e c i s i o n made on whether t o recommend t h e use
For f i e l d use s i m p l e r guidance i s needed and c o n s i d e r -
i s recommended.
EP o i l s should,
f u r t h e r m o r e , be c o n s i d e r e d
f o r p a r a l l e l - s h a f t gears where t h e t o o t h c o n t a c t markings do n o t reach t h e e x t e n t
194 r e q u i r e d by t h e speed as i n d i c a t e d i n F i g u r e 2, where t h e gears a r e s u b j e c t t o dynamic o v e r lo a d s, and where t h e gears s t e p up t h e speed. T a bl e 7.3 IS0 o i l v i s c o s i t y grades r e q u i r e d f o r enclosed worm gears Output o r wormwheel r.p.m.
Centre d i s t a n c e ( in c h e s ) (mm) 2.5
800
50 and under
64
HVI 1000
HVI 460
150 and o v e r
HVI 320
4
100
460
320
320
10
250
3 20
220
220
20
500
320
2 20
220
EP o i l s s h o u ld g e n e r a l l y be used f o r h y p o i d gears because o f t h e i r h i g h com-
ponent o f s l i d i n g a l o n g t h e l i n e o f c o n t a c t , t h e i r l a r g e t e e t h and t h e i r l i a b i l i t y t o c o n s i d e r a b l e dynamic o verl o ad s
itl
a ut omotive s e r v i c e .
They should n o t ,
however, be used f o r worm gears u n l e s s t h e o i l temperature i s c o n s i s t e n t l y below about 6OoC because o f t h e r i s k o f e xcessi ve c o r r o s i v e wear o f t h e bronze. Since s c u f f i n g i s a thermal phenomenon, o v e r h e a t i n g can be a cause, and i t s courses and t h e a p p r o p r i a t e counter-measures d e t a i l e d under symptom 23 should be c o n s i dered.
7.4.3
A b r a s i v e Wear
There a r e t w o k i n d s of a b r a s i v e wear.
One, as i n symptom 4, where a rough,
hard s u r f a c e rubs a g a i n s t a s o f t e r one, which i s known as 'two-body'
abrasion;
t h e o t h e r , as i n symptom 5, where a b r a s i v e d i r t a c t s between two r u b b i n g surfaces, which i s known as ' t h r e e - b o d y '
a bra si on .
The f i r s t k i n d o ccurs where a rough surface-hardened p i n i o n runs a g a i n s t a s o f t s t e e l o r p l a s t i c wheel, and a l s o where a rough surface-hardened worm runs a g a i n s t a bronze wheel.
I n t h e l a t t e r case t h e a s s o c i a t e d h i g h f r i c t i o n may so
d i s t o r t t h e gears t h a t c o n t a c t i s b rou gh t o n t o t h e i n l e t edge o f t h e wheel t e e t h and f r i c t i o n f u r t h e r i n crea ses t o t h e e x t e n t t h a t power t r a n s m i s s i o n may be l i m i t e d (see symptom 20). smoother f i n i s h , e.g.
The remedy i s t o stone o r l a p t h e harder member t o a
0 . 5 t o 0.2 microns Ra.
T y p i c a l contaminants cau si ng three-body a b r a s i o n a r e sand and m i l l s c a l e . F i l t r a t i o n down t o 20 mi cro ns (nominal)
i s n o r m a l l y t h e best p r a c t i c a b l e s o l u t i o n ,
though s m a l l e r p a r t i c l e s can s t i l l cause a b r a s i o n .
The maximum amount o f conta-
minant t o l e r a b l e i n t h e o i l depends on i t s hardness r e l a t i v e t o t h a t o f t h e gears concerned.
For example,
0.3% w by D I N 51 592
,
i n steel m i l l practice a typical l i m i t f o r millscale i s
i . e.
r e t a i n e d on a 0.45 micron m i l l i p o r e f i l t e r ) .
c o r r e s p o n d in g l i m i t f o r sand would be 0.1% w.
The
Greases a r e p a r t i c u l a r l y l i a b l e
196 t o p e r m i t a b r a s i o n s i n c e t h e y t e n d t o keep t h e a b r a s i v e c o n t a m i n a n t s and t h e wear p r o d u c t s i n t h e v i c i n i t y o f t h e mesh.
Fig.9
Three-body a b r a s i o n o f a g e a r t o o t h by sand i n t h e o i l Note t h e sh o rt l e n g t h o f t h e scars. (Photo c o u r t e s y o f S h e l l I n t e r n a t i o n a l Co.Ltd.1
7.4.4
O t h e r forms o f gear wear
The o t h e r forms o f g e a r wear l i s t e d under symptoms 6 t o 19 a r e r e l a t i v e l y r a r e and space does n o t p e r m i t s p e c i a l d i s c u s s i o n h e r e .
7.5
PROBLEMS I N LUBRICATION SYSTEMS The main problems a r i s i n g i n s u p p l y i n g and c o n t r o l l i n g t h e f l o w o f o i l t o
and f r o m t h e t o o t h mesh a r e :
o v e r h e a t i n g , windage ( i n v e r y h i g h speed g e a r s ) ,
196 e x c e s s i v e foaming ( i n b a t h l u b r i c a t e d systems) and e x c e s s i v e a e r a t i o n ( i n l a r g e c i r c u l a t i o n systems). One f r e q u e n t source o f o v e r h e a t i n g i n b a t h systems i s when one gear d i p s t o o deeply i n t o t h e b a t h (see symptom 2 3 ( a ) ,
(b) and ( c ) ) .
T h i s can sometimes be
avoided by s u r r o u n d i n g t h e lower p a r t o f t h e d i p p i n g gear by a s p e c i a l t r o u g h . When t h e gear i s a t r e s t t h e t r o u g h f i l l s w i t h o i l , b u t d u r i n g o p e r a t i o n t h e excess i s thrown o u t and t h e meshing t e e t h r e c e i v e s u f f i c i e n t o i l f o r l u b r i c a t i o n and c o o l i n g from h o l e s i n t h e bottom o f t h e trough. Above about 15 m/s p i t c h l i n e speed even t h i s s p e c i a l f o r m o f b a t h l u b r i c a t i o n tends t o cause e x c e s s i v e power loss and temperature r i s e so t h a t spray l u b r i c a t i o n f r o m c i r c u l a t i o n systems i s n o r m a l l y adopted. i n t h e range 0.3 t o 1.5 b a r .
Pump p r e s s u r e s a r e u s u a l l y
Pressures below 0.3 b a r may be s a t i s f a c t o r y , b u t
whenever t h e gauges read below 1 b a r a v i s u a l check should be made t o ensure t h a t the o i l i s reaching t h e p a r t s required.
(Upper l i m i t s t o o i l p r e s s u r e may
be d i c t a t e d by t h e s a f e t y l i m i t s f o r pumps, c o o l e r s , f i l t e r s , e t c . ) .
A t speeds above about 50 m/s windage e f f e c t s become n o t i c e a b l e and a t around 100 m/s and o v e r t h e y need t o be s p e c i a l l y c a t e r e d for i n t h e d e s i g n by arrangi n g f o r p l e n t y o f space between t h e gears and t h e housing, by a r r a n g i n g comple t e l y s e p a r a t e o r even dual d r a i n l i n e s w i t h s h i e l d s , o t h e r w i s e t h e c a s i n g may become choked w i t h o i l . The l o c a t i o n o f t h e o i l s p r a y e r s needs s p e c i a l c o n s i d e r a t i o n where p i t c h l i n e speeds a r e above 50 m/s.
Above t h a t speed t h e sprayers should n o t be d i r e c t e d
s t r a i g h t i n t o t h e mesh, b u t a l i t t l e i n advance.
In t h i s way a l l t h e o i l
serves t o c o o l t h e gears, t h e excess above t h a t r e q u i r e d f o r l u b r i c a t i o n i s thrown o f f b e f o r e t h e gears mesh. 'down-going'
gears.
Above
T h i s arrangement i s e s p e c i a l l y r e q u i r e d f o r
a p p r o x i m a t e l y 75 m/s,
about 80-90% o f t h e f l o w
should be d i r e c t e d o n t o t h e o u t g o i n g s i d e f o r c o o l i n g , w i t h t h e remainder b e i n g sprayed s l i g h t l y i n advance o f mesh f o r l u b r i c a t i o n . Excessive foaming and a e r a t i o n o f t h e o i l i s due e i t h e r t o a d e t e r i o r a t i o n o f the properties o f t h e o i l from contamination o r t o excessive ingress o f a i r i n t o the o i l .
The former may be r e a d i l y checked by c a r r y i n g o u t t h e a p p r o p r i a t e
t e s t s on t h e o i l , e.g. 24(a),
I P 146 and I P 313, t h e l a t t e r by r e f e r e n c e t o symptom
(b) and ( c ) .
REFERENCES
1 Fowle, T. I . , Lubn. Engg.,
1976, 32 N O . 1,
17.
GEAR PROBLEMS : CAUSES AND REMEDIES Symptom
1. Broken t e e t h
P o s s i b l e Causes (a) F a t i g u e ( i ) Load unevenly d i s t r i b u t e d . ( i i ) Sharp c o r n e r i n t o o t h r o o t . ( i i i ) Notches i n t o o t h r o o t caused by improper f i l i n g o r g r i n d i n g . ( i v ) Overload. (v) Torsional v i b r a t i o n s . ( v i ) Bruises on t e e t h , e.g. caused by dropping. ( v i i ) Coarse m a r t e n s i t i c s t r u c t u r e i n hardened s t e e l . (viii)Blow-holes i n roots o f cast teeth. ( i x ) Surface f a t i g u e p i t s a d j a c e n t t o root o f tooth.
Remedies Temporary c o r r e c t i o n s may be made by c u t t i n g o u t broken t e e t h and cracked s e c t i o n s u n t i l o n l y sound t e e t h a r e l e f t , and r u n n i n g a t p r o p o r t i o n a l l y reduced load; by i n s e r t i n g pegs and b u i l d i n g up new t e e t h on them by w e l d i n g , f o l l o w e d by reshaping
.
Apart from e l i m i n a t i n g overloads t h e o n l y permanent c o r r e c t i o n i s , however, t o procure gears w i t h o u t t h e f a u l t s l i s t e d , t o have them c o r r e c t l y l i n e d - u p and run w i t h any shock loads l i m i t e d by shock absorbing c o u p l i n g s .
(b) F r a c t u r e ( i ) Repeated heavy loads. ( i i ) Hard f o r e i g n o b j e c t s jammed i n gear mesh. 2. Pitting (a) Small w i d e l y s c a t t e r e d p i t s on worki n g s u r f a c e s .
(a ) F a t i g u e a t s u r f a c e a s p e r i t i e s d u r i n g i n i t i a l running.
(a) None: t h e p i t t i n g w i l l cease t o spread and w i l l be o f no consequence.
(b) P i t s c o n ce nt rat ed a t one o f tooth o r helix.
(b) C o n c e n t r a t i o n o f load due t o s l i g h t mi sa l i gnment.
(b) Check alignment o f s h a f t s and c a s i n g f o r d i s t o r t i o n . The p i t t i n g may cease t o spread.
(c) Concentration o f p i t s i n r e g u l a r ba nd s a l o n g tooth width.
( c ) C o n c e n t r a t i o n o f load on s u r f a c e undulations.
( c ) The p i t t i n g may cease t o spread, b u t lapping o r stoning advisable i n severe cases.
Remed ies
P o s s i b l e Causes
SvmDtom 2. P i t t i n g ( c o n t d . ) ( d ) P i t s c o n ce nt rat ed a l on g pitch line.
(d ) ( i ) F a u l t y p r o f i l e . ( i i ) Excessive wear o r s c u f f i n g has produced a r i d g e a l o n g t h e p i t c h l i n e w hich becomes overloaded.
(d) ( i ) As above. ( i i ) Use o i l o f h i g h e r v i s c o s i t y o r o f h i g h e r EP a c t i v i t y .
( e ) P i t s c o n ce nt rat ed on dedendum su rf a ces o f one gear.
(e) ( i ) I n s u f f i c i e n t hardness o f s o f t e r gear o r excessive f r e e - f e r r i t e i n rnicros t r u c t u r e r e l a t i v e t o loading. ( i i ) E xcessive amounts o f hard nonm e t a l l i c inclusions i n the metal. ( i i i ) I n i t i a l s u r f a c e f i n i s h t o o rough and o i l v i s c o s i t y t o o low. ( i v ) Overload, e s p e c i a l l y by shock, t o r s i o n a l o s c i l l a t i o n s and h i g h s t a r t i n g torque. ( f ) I n s u f f i c i e n t hardness o f b o t h gears r e l a t i v e t o the loading.
(e) The p i t t i n g w i l l cease t o spread i f opposing gear u n a f f e c t e d . An i n c r e a s e i n o i l v i s c o s i t y c o u l d be b e n e f i c i a l
(g) P i t t i n g i n casehardened g ea rs.
(g) Teeth t o o s o f t e i t h e r from a c c i d e n t a l d e c a r b u r i s a t i o n o r inadequate quenching.
(9) Reduce load.
(h) M i c r o - p i t t i n g o r ' f r o s t i n g ' i n s u r f a c e hardened gears ( c l o s e l y spaced p i t s s m a l l e r t ha n 0.1mm across).
(h) ( i )
(h) ( i )
( f ) P i t s on dedendum s u r faces o f b o t h gears, often with d i s t i n c t step a t the p i t c h l i n e .
!ii)
N i t r i d e d gears: t h i n s u r f a c e l a y e r o f b r i t t l e super-rich n i t r i d e s (white l a y e r ) . Inadequate o i l f i l m .
F i t shock absorbing c o u p l i n g .
( f ) Reduce l o a d i n g i n s e r t shock absorbing t y p e o f coupling,between source o f shock and gear. I f possible, increase operating v i s c o s i t y o f o i l . Change t o EP o i l , m a i n l y t o p r e v e n t subsequent s c u f f i n g o f remaining c o n t a c t areas. I f p o s s i b l e , l a p o r stone t e e t h t o improve s u r f a c e and r u b i n d r y MoS2 powder.
A 25 micron l a y e r o r l e s s i s gene r a l l y harmless b u t removal o f t h i c k e r l a y e r s by l a p p i n g o r g r i n d i n g should be considered. ( i i ) Increase o p e r a t i n g v i s c o s i t y o f the o i l .
Symptom
P o s s i b l e Causes
Remed i es
2 . P i t t i n g (contd.) ( i ) Very l a r g e oyster-shaped p i t s e x te nd i ng ove r p r a c t i c a l l y t h e whole o f t h e a c t i v e f l a n k o f casehardened t e e t h .
3. S c u f f i n g
( i ) Hi gh t o o t h f r i c t i o n due t o d i r e c t c o n t a c t between t h e t e e t h ; t h e o r e t i c a l o i l f i l m thickness l e s s t han 0.5 micron.
( i ) Increase o p e r a t i n g v i s c o s i t y o f t h e o i l . NB. P i t t i n g o f s o f t and through-hardened s t e e l gears i s so common t h a t i t cannot be counted as a f a i l u r e . But, i f p o s s i b l e , i t i s prudent t o c o r r e c t as d e t a i l e d above. In some cases, a f r e s h s t a r t can be made by t u r n i n g round one o r b o t h gears and l a p p i n g them t o g e t h e r as a p p r o p r i a t e . I n severe cases where n o i s e i s e x c e s s i v e o r reduced areas o f c o n t a c t cannot be prevented from s c u f f i n g , o r t h e r e i s a danger o f s u r f a c e p i t t i n g i n i t i a t i n g t o o t h breakage, i t may be necessary t o r e p l a c e gears.
(a) Tooth l o a d i n g t o o h i g h .
(a) Reduce l o a d i n g
(b) I n s u f f ic ien t 1 ub r i c a t i o n ( i ) O i l - b a t h l e v e l t o o low. ( i i ) O i l sprayer nozzles choked.
(b) ( i )
( i i i ) Pump s u c t i o n s t r a i n e r choked. ( i v ) Bearings r o b o i l from sp rayer n o z z l e s . ( v ) Windage i n h i g h speed gears d e f l e c t s o i l spray. ( v i ) L u b r i c a n t channels a t low s t a r t i n g temperatures. ( v i i ) Clearance between gears and c a s i n g t o o s m a l l : o i l cannot f l o w back t o bath.
Raise o i l l e v e l so t h a t gear d i p s 1-3 t o o t h h e i g h t s when running. ( i i ) Check o i l nozzles, c l e a r as necessary, f i l t e r o r change o i l . ( i i i ) C h e c k s u c t i o n s t r a i n e r , c l e a n as necessary, f i l t e r o r change o i l . ( i v ) R e s t r i c t o i l supply t o bearings. Increase o i 1 pressure. Reposition sprayer nozzles. ( v i ) Use l u b r i c a n t w i t h lower channel p o i n t .
(v)
( v i i ) I f p o s s i b l e use lower v i s c o s i t y o i l . Increase clearance.
( c ) Ope rat ing temperatures excessive.
( c ) Reduce temperatures (see Symptom 23)
(d ) O i l v i s c o s i t y t o o low ( p a r t i c u l a r l y s o f t o r through-hardened gears).
(d) Use h i g h e r v i s c o s i t y grade;
improve c o o l i n g .
N
Symptom
3. S c u f f i n g ( c o n t d . )
0
(e) L u b r i c a n t has i n s u f f i c i e n t EP a c t i v i t y ( p a r t i c u l a r l y hardened g e a r s ) .
(e) Use more a c t i v e EP o i l .
( f ) T e e t h have e x c e s s i v e addendum h e i g h t f o r t h e speed.
( f ) Check d e s i g n . Use more a c t i v e EP o i l .
(9) Teeth do n o t have s u f f i c i e n t c o n t a c t because o f u n d u l a t i o n s , s p l i t markings o r m i s a l i g n m e n t .
(g) Reduce u n d u l a t i o n s by s t o n i n g , l a p p i n g o r shaving. Check a l i g n m e n t . Check t h a t c a s i n g has n o t d i s t o r t e d f r o m uneven s e t t l i n g o f f o u n d a t i o n s . Check t h a t b e a r i n g s a r e riot worn.
(h) T e e t h a r e n o t a d e q u a t e l y r e l i e v e d .
(h) A p p l y t i p - r e l i e f
( i ) F u l l - l o a d a p p l i e d b e f o r e gears have been a d e q u a t e l y r u n - i n .
( i ) Use a c t i v e EP o i l t o p r e v e n t f u r t h e r s c u f f i n g w h i l e c o n t i n u e d r u n n i n g makes s u r f a c e s smoother.
( j ) B o t h gears have h i g h n i c k e l - c h r o m i u m c o n t e n t and a r e n o t case-hardened.
4. Wear o f s o f t e r member
0
Remed i es
P o s s i b l e Cause
by s h a v i n g o r s t o n i n g .
(j) Use a c t i v e EP o i l o r h i g h e r v i s c o s i t y grade. NB. P r o v i d e d s c u f f i n g has n o t roughened t h e surfaces unduly, e s p e c i a l l y w i t h s p i r a l b e v e l and h e l i c a l g e a r s , once t h e b a s i c cause has been e l i m i n a t e d f u r t h e r r u n n i n g , p r e f e r a b l y w i t h an EP o i l , w i l l c o r r e c t t h e problem.
S u r f a c e f i n i s h o f h a r d e r member t o o coarse.
Stone o r l a p t e e t h t o a f i n e r f i n i s h .
A b r a s i v e d i r t i n l u b r i c a n t becoming embedded i n s o f t e r inember.
Change o i l o r pass i t t h r o u g h a f i n e f i l t e r . F i t f i l t e r s on a i r v e n t s .
6. Wear a t low speed.
Lubricant f i l m too t h i n .
Use h i g h e r v i s c o s i t y l u b r i c a n t .
7. Wear a t h i g h speed.
E x c e s s i v e f r i c t i o n caused by o v e r l o a d , overspeed, loss o f b a c k l a s h , o r f a u l t y lubrication.
Reduce o v e r l o a d s o r overspeed, and d i s t r i b u t i o n .
o n l y o f gear p a i r .
5. Wear o f h a r d e r member i s greater.
improve o i l f l o w
Symptom
Remed ies
P o s s i b l e Causes
8. Wear o f worm wheel t e e t h .
M a t e r i a l combination may be u n s u i t a b l e . (See a l s o No.20).
I f p o s s i b l e use case-hardened s t e e l worm and c e n t r i f u g a l l y cast phosphor bronze.
9 . T e e th t i p s rounded and
I n t e r f e r e n c e : gears n o t p r o p e r l y matched o r c e n t r e d i s t a n c e t o o s m a l l .
Check design. distance.
(a ) Combination o f s o f t m a t e r i a l and re pe at ed shock loads l e a d i n g t o separation o f t e e t h w i t h re-contact i n s u f f i c i e n t l y damped by o i l f i l m (peening)
(a) Reduce shock loads, use h i g h e r v i s c o s i t y o i l , reduce backlash, change t o harder gear m a t e r i a l .
(b) Very heavy steady loads and s o f t materials ( r o l l i n g ) .
(b) Reduce loads, i n c r e a s e gear s u r f a c e hardness.
11. Groove a l o n g p i t c h l i n e o f d r i v i n g t e e t h and r i d g e a lo n g p i t c h l i n e o f d r i v e n teeth.
When a s s o c i a t e d w i t h s c u f f i n g o f t h e r e s t o f t h e t e e t h i t may be due t o complete f a i l u r e o f t h e l u b r i c a n t su pp l y. When n o t , o i l f i l m may be t o o thin.
Check o i l supply, e.g. t h a t o i l b a t h l e v e l i s c o r r e c t when gears a r e running, t h a t o i l supply p i p e s , f i l t e r s and sprayer nozzles a r e n o t choked.
12. Grooves a l o n g p i t c h l i n e o f both d r i v i n g and d r i v e n teeth.
E r o s i o n by spark discharge.
E s t a b l i s h source o f s t r a y e l e c t r i c c u r r e n t s and lead t o e a r t h by (a) e a r t h i n g brushes o f generous s i z e , and (b) s t o p p i n g o t h e r p o s s i b l e paths through t h e gear mesh by i n s u l a t i n g pads under p e d e s t a l s and i n s u l a t i n g bushes f o r holding-down b o l t s .
13. Rippling.
E xce ssi ve s u r f a c e f r i c t i o n a t low speeds.
Use hypoid t y p e f u l l EP o i l .
14. B u lk p l a s t i c d e f o r m a t i o n o f
Very severe o v e r h e a t i n g due t o f a i l u r e o f o i l su pply.
Repair t e e t h o r renew gears. E l i m i n a t e cause o f o i l supply f a i l u r e . I n s t a l l alarms so t h a t u n i t can be stopped q u i c k l y i n event o f o i l supply f a i l u r e .
dedendum s u r f aces gouged.
10. P l a s t i c f l o w o f t o o t h s u r faces w i t h pronounced f i n at tips.
I f p o s s i b l e extend c e n t r e
.
t e e t h e s p e c i a l l y a t middle o f tooth width.
N
0
System
Remed i es
P o s s i b l e Causes
15. I n d e n t a t ions.
Hard p a r t i c l e s i n system, o f t e n s w a r f , o c c a s i o n a l l y from EP o i l carbonised on h i g h l y r a t e d h e a t e r s i n system.
Thoroughly c l e a n system, check h e a t e r surfaces, c l e a n and reduce s u r f a c e temperature as necessary .
16. Cracks i n s u r f a c e of
Overheating d u r i n g g r i n d i n g , h e a t t r e a t m e n t s , o r both.
incorrect
Check w i t h m a n u f a c t u r e r .
17. L o n g i t u d i n a l c r a c k i n g and f l a k i n g i n casehardened gears .
Case i s t o o t h i n and c o r e t o o s o f t so t h a t s u r f a c e has c o l l a p s e d under l o a d .
Check w i t h m a n u f a c t u r e r .
18. Red-brown spots on s u r -
A t t a c k by c o r r o s i v e substances such as s a l t s from hardening process.
Clean spots w i t h emery s t i c k . Clean, f l u s h and r e f i l l l u b r i c a t i o n system.
F r e t t i n g due t o v i b r a t i o n w h i l e under s t a t i o n a r y load.
Arrange f o r t h e gears t o be f l u s h e d w i t h o i l and s l o w l y r o t a t e d .
(a) I n s u f f i c i e n t allowance f o r d i s t o r t i o n under load has brought c o n t a c t o n t o e n t r y s i d e o f wheel t e e t h .
(a) A d j u s t p o s i t i o n o f wheel so t h a t even under f u l l l o a d c o n t a c t i s n o t on e n t r y s i d e o f wheel t e e t h and o i l can be drawn i n t o contact.
(b) Excessive t o o t h f r i c t i o n , due e i t h e r t o rough worm s u r f a c e o r t o u n s u i t a b l e combination o f gear m a t e r i a l s , causing e x c e s s i v e d i s t o r t i o n .
(b) Improve s u r f a c e f i n i s h o f worm, reduce o i l temperature o r use h i g h e r v i s c o s i t y grade. Best m a t e r i a l combination i s casehardened s t e e l worm and c e n t r i f u g a l l y c a s t phosphor bronze wheel.
hardened gears, o f t e n i n net-like pattern.
faces o f case-hardened gears.
19. Red-brown c o n t a c t marks on t e e t h and r e d - t - o w n powder i n n o m i n a l l y s t a t i o n a r y gears. 20. Worm gear f a i l s t o transmi t f u l l t o r q u e ,
N
Symptom
21.
22.
Vibration
Unusual n oi se .
P o s s i b l e Causes
Remed i es
(a) D e f e c t i v e b e a r i n g s o r coup1 i n g s .
(a) Check bearings and c o u p l i n g and r e p l a c e as necessary.
(b) S h a f t s m i s a l i g n e d .
(b) Check t h a t c a s i n g i s not d i s t o r t e d , r e a l i g n shafts.
( c ) Check and rebalance as necessary.
( c ) Check and rebalance as necessary.
(a) D e f e c t i v e r o l l i n g element bearing. (b) D e f e c t i v e o i l pump.
Check these p a r t s and r e p l a c e as necessary.
(c) Defective coupling.
23.
Overheating.
(d) Tooth s u r f a c e s e x c e s s i v e l y p i t t e d o r roughened.
(d) Stone o r l a p t e e t h and rub i n MoS2 powder.
(e) Continuous t o o t h double h e l i c a l gears r u n n i n g 'down-going' w i t h apex t r a i l i n g , s q u i r t i n g o i l o u t o f mesh.
(e) Reduce excessive o i l supply t o mesh by reducing immersion i n b a t h o r reducing f l o w t o sprayers and d i r e c t i n g spray i n advance o f mesh.
(a ) O i l l e v e l t o o h i g h i n bath.
(a) A d j u s t l e v e l when gear r u n n i n g t o d i p 1-3 t o o t h heights.
(b) O i l v i s c o s i t y too high.
(b) Change t o lower v i s c o s i t y grade.
( c ) Speed t o o h i g h f o r b a t h l u b r i c a t i o n .
( c ) Change t o spray l u b r i c a t i o n system.
(d) TOO much o i l sprayed t o o c l o s e t o i n g o i n g mesh o f high-speed gears.
(d) D i r e c t o i l spray f u r t h e r i n advance o f mesh; r e s t r i c t amount o f o i l t o sprayers.
(e)
(e) Improve drainage o r r e s t r i c t amount o f o i l t o sprayers.
Inadequate drainage from housing.
( f ) Clogged c o o l e r .
( f ) Check o i l and w ater s i d e s and c l e a n as necessary
(9) Cooler inadequate.
(9) Change c o o l e r f o r l a r g e r s i z e o r s w i t c h t o c o o l e r w ater supply.
(h) Heat r a d i a t e d from surroundings.
(h) I n t e r p o s e r a d i a t i o n s h i e l d s . PJ
0
w
tu Symptom
2 3 . Overheating (con t d . )
24. Excessive foaming and aeration.
Remed ies
P o s s i b l e Causes
( i ) Inadequate a i r f l o w o v e r gearbox.
( i ) Increase v e n t i l a t i o n o f surrounding a i r space.
( j ) D i r t accu mulations on casing.
( j ) Clean d i r t away.
( k ) (Worm g e a r s ) , u n s u i t a b l e l u b r i c a n t .
( k ) Change t o HVI m i n e r a l o i l o r , p r e f e r a b l y , p o l y g l y c o l type s y n t h e t i c o i l .
(1) E xce ssi ve power l o s s i n p l a i n bearings.
(1) Check b e a r i n g design.
(a ) Gear d i p s t o o deeply i n t o o i l bath.
(a) A d j u s t l e v e l when gear r u n n i n g t o d i p 1 - 3 tooth heights.
(b) A i r l e a k s on s u c t i o n s i d e o f c i r c u l a t i n g system.
(b) Remake s u c t i o n - s i d e j o i n t s i n c l u d i n g pump gland.
(c) O i l cascades down v e r t i c a l r e t u r n pipes i n t o r e s e r v o i r .
( c ) Rearrange r e t u r n 1 ines t o a1 low smooth f l o w i n t o tank below o i l l e v e l .
(d) O i l contaminated by grease, j o i n t i n g compound, another, and i n c o m p a t i b l e o i l etc.
(d) Renew o i l charge.
0 P
205
MATERIALS FOR TRIBOLOGICAL APPLICATIONS
D. SCOTT, Consultant,
8.1
E d i t o r o f Wear
I NTRODUCT I O N Engineering design i s t h e c r e a t i o n o f i n s t r u c t i o n s f o r making an a r t i c l e t o
s a t i s f y a s p e c i f i c requirement.
From a t r i b o l o g i c a l p o i n t o f view, t h e mat-
e r i a l s o f c o n s t r u c t i o n and the
u b r i c a n t a r e i m p o r t a n t f a c t o r s i n such s p e c i f -
ications.
For t r i b o l o g i c a l app i c a t i o n s t h e i m p o r t a n t p r o p e r t i e s of m a t e r i a l s
a r e those p r o p e r t i e s which must be taken i n t o account i n d e s i g n i n g a component t o w i t h s t a n d t h e mechanical and thermal s t r e s s e s t o which i t w i l l be exposed and the e f f e c t s o f t h e environment
n which i t has t o f u n c t i o n [ 1 , 2 ] .
There i s a
continuous demand f o r m a t e r i a l s o f improved p r o p e r t i e s and w i t h b e t t e r s t r e n g t h t o weight r a t i o s .
Mechanisms operaLing under arduous c o n d i t i o n s o f h i g h speed,
heavy l o a d o r extremes o f environment r e q u i r e m a t e r i a l s o f h i g h s t r e n g t h .
If
s u b j e c t e d t o r e l a t i v e m o t i o n they may r e q u i r e m a t e r i a l s o f g r e a t hardness, wear and c o r r o s i o n r e s i s t a n c e and s t r u c t u r a l and dimensional s t a b i l i t y
[31.
Newer
m a t e r i a l s [ 4 ] may meet s t r i n g e n t design requirements beyond t h e c a p a b i l i t i e s o f the more commonly used m a t e r i a l s b u t a v a i l a b i l i t y and c o s t make c o n v e n t i o n a l m a t e r i a l s more a t t r a c t i v e commercially and encourage i n n o v a t i o n . The d e s i g n e r has a v a s t range o f m a t e r i a l s f r o m which t o s e l e c t . erence book
[5] gives
35000 p r o p r i e t a r y m a t e r i a l c o m p o s i t i o n s .
however, besides s e a r c h i n g f o r
One r e f -
The d e s i g n e r
improved m a t e r i a l s must o f t e n seek t h e cheapest
m a t e r i a l t o s a t i s f y h i s requirements and sometimes t h e more r e a d i l y a v a i l a b l e indiginous materials.
A d e t e r m i n i n g f a c t o r i n t h e e x t e n s i v e use o f many mat-
e r i a l s i s t h e a m e n a b i l i t y o f t h e m a t e r i a l to m a n i p u l a t i o n and t h e e x t e n t t o which t h e d e s i g n e r can c o n t r o l and v a r y p r o p e r t i e s such as s t r e n g t h , hardness and d u c t i l i t y w i t h i n t h e range o f s p e c i f i c e n g i n e e r i n g requirements. o f m a t e r i a l s i s o f t e n r e s t r i c t e d by t h e m a n u f a c t u r i n g f a c i l i t i e s
8.2
dt
The c h o i c e h i s disposal.
TYPES OF MATERIALS M a t e r i a l s may be c o n v e n i e n t l y d i v i d e d i n t o f o u r p r i n c i p a l t y p e s : -
n o n - f e r r o u s , n o n - m e t a l l i c and composite m a t e r i a l s .
ferrous,
The abundance o f i r o n and
i t s a l l o y s c o m p r i s i n g t h e b u l k o f m e t a l s made, t h e i r f a v o u r a b l e economics and d i v e r s e p r o p e r t i e s make f e r r o u s m a t e r i a l s t h e d e s i r a b l e c h o i c e f o r t r i b o l o g i c a l applications.
Modern c a s t i r o n s and s t e e l s f i n d e x t e n s i v e use i n t r i b o -
206 e n g i n e e r i n g as a l l o y i n g and h e a t - t r e a t m e n t e nables them t o be t a i l o r e d t o specific applications.
The p r i n c i p a l methods o f s t r e n g t h e n i n g s t e e l i n c l u d e
work-hardening,
d ecre asi n g t h e g r a i n s i z e , s o l i d s o l u t i o n and d i s p e r s i o n
s t r e n g th e n in g .
I n c u r r e n t l y used s t e e l s ,
the martensite transformation pro-
duces th e b e s t c o mbi n at i on o f s t r e n g t h and d u c t i l i t y b u t as hardness increases, d u c t i l i t y decreases and a t t h e h i g h e s t s t r e n g t h l e v e l s produced by c o n v e n t i o n a l heat t r e a t m e n t procedures, d u c t i l i t y i s d i m i n i s h e d t o l e v e l s considered a t pre s e n t unacceptable f o r most e n g i n e e r i n g a p p l i c a t i o n s [ 6 ] .
I f a thermo-
mechanical t r e a t men t i s used whereby a u s t e n i t e i s s t r a i n hardened b e f o r e t r a n s f o r m a t i o n t o m a r t e n s i t e unusual d u c t i l i t y , f a t i g u e and impact p r o p e r t i e s a r e ob ta in e d .
Ausforming may t hu s a l l o w increased s t r e n g t h s above t h e p r e s e n t
usa b le l i m i t s w i t h o u t s a c r i f i c e of d u c t i l i t y
[7].
With m a t e r i a l s g e n e r a l l y , h i g h hardness i s u s u a l l y a s s o c i a t e d w i t h a h i g h melting point.
Powder m e t a l l u r g y has widened t h e f i e l d o f a v a i l a b l e hard
m e t a l l i c m a t e r i a l s by making p o s s i b l e metal combinations u n o b t a i n a b l e by conv e n t i o n a l m e l t i n g and c a s t i n g t ech ni q ue s.
Hard s i n t e r e d c a r b i d e s may be used t o
advantage i n many a p p l i c a t i o n s r e q u i r i n g a h i g h degree o f wear r e s i s t a n c e . However, such m a t e r i a l s a r e u s u a l l y expensive t o manufacture and d i f f i c u l t t o form and s u r f a c e t r e a t m e n t s and c o a t i n g s on o r d i n a r y m a t e r i a l s may be used t o i nc r e a s e s t r e n g t h and improve wear r e s i s t a n c e . For use a t e l e v a t e d temperatures m e t a l s must form a dense, tough,
impervious
o x i d e l a y e r which r e s i s t s c r a c k i n g under l o ad and p r e v e n t s a t t a c k o f t h e metal by h o s t i l e environments.
The e s t a b l i s h e d n o n - f e r r o u s metal a l l o y systems i n
c u r r e n t use a r e based on n i c k e l r i c h and c o b a l t r i c h a l l o y s .
I n b o t h cases t h e
necessary r e s i s t a n c e t o o x i d a t i o n and c o r r o s i o n i s c o n f e r r e d by the i n t r o d u c t i o n o f chromium.
The n i c k e l based a l l o y s a r e s t i f f e n e d p r i n c i p a l l y by the a d d i t i o n
o f t i t a n i u m and aluminium.
I n c o b a l t a l l o y s s t i f f e n i n g i s e f f e c t e d by complex
c a r b i d e s o f molybdenum, n i o b i u m and t an t a l um. High speed t o o l s t e e l s and s i m i l a r s p e c i a l s t e e l s a r e a l s o used f o r e l e v a t e d temperature s e r v i c e .
Corrosion resistance requires c a r e f u l m a t e r i a l s e l e c t i o n
and t h e use o f s t a i n l e s s s t e e l s , chromium,
n o n - f e r r o u s m e t a l s such as aluminium, n i c k e l ,
t i t a n i u m and t h e i r a l l o y s o r n o n - m e t a l l i c m a t e r i a l s such as p l a s t i c s
o r e la s to m e r s .
To r e s i s t severe a b r a s i v e wear, cemented c a r b i d e s , cermets or
even diamond may be r e q u i r e d . As c o n v e n t i o n a l m a t e r i a l s have been improved by o r t h o d o x methods almost t o t h e l i m i t o f t h e i r p o t e n t i a l mechanical p r o p e r t i e s , new types o f m a t e r i a l a r e bein g developed.
Composites whi ch combine m a t e r i a l s o f d i s s i m i l a r mechanical
and p h y s i c a l p r o p e r t i e s , can have p r o p e r t i e s s u p e r i o r t o one o r b o t h o f t h e i r constituents.
There a r e two p r i n c i p a l t ype s.
In one, a m a t r i x may be r e i n -
fo r c e d w i t h f i b r e s o r p a r t i c l e s t o improve i t s p r o p e r t i e s .
I n the o t h e r the
207 the m a t r i x i s e s s e n t i a l l y a g l u e t o h o l d t o g e t h e r f i b r e s o r p a r t i c l e s which have d e s i r a b l e p r o p e r t i e s b u t which by themselves cannot be used as e n g i n e e r i n g materials.
C e r t a i n d i f f i c u l t i e s r e q u i r e t o be surmounted b e f o r e composites
achieve t h e i r f u l l p o t e n t i a l .
Whiskers and f i b r e s a r e expensive and have prob-
lems w i t h s t r e s s c o n c e n t r a t i o n a t t h e i r ends which can i n f l u e n c e c r a c k initiation.
Conventional methods a r e n o t s u i t a b l e f o r t h e manufacture o f com-
ponents from composites n o r f o r t h e f o r m a t i o n and j o i n i n g o f f i b r e r e i n f o r c i n g materials.
By u s i n g r e i n f o r c e m e n t s o f o x i d e and n o n - m e t a l l i c w h i s k e r s which
approach t h e t h e o r e t i c a l s t r e n g t h , v e r y h i g h u l t i m a t e s t r e n g t h s i n composites are p o s s i b l e . metals.
Glass, carbon,
s i l i c o n n i t r i d e and alumina a r e a t t r a c t i v e non-
Besides r e p l a c i n g m e t a l s , ceramics may be used as c o a t i n g s t o comple-
ment d e s i r a b l e metal c h a r a c t e r i s t i c s by a d d i n g r e f r a c t o r y p r o p e r t i e s ,
insulation,
and e r o s i o n , wear, o x i d a t i o n and c o r r o s i o n r e s i s t a n c e . The s t r e n g t h e n i n g o f m e t a l s f o r use a t h i g h temperatures can be achieved by d i s p e r s i n g n o n - m e t a l l i c p a r t i c l e s i n them t o m a i n t a i n u s e f u l p r o p e r t i e s t o w i t h i n 5O-10O0C o f t h e m e l t i n g p o i n t o f t h e m a t r i x m e t a l .
Only small amounts o f
the d i s p e r s o i d a r e r e q u i r e d and n i c k e l a l l o y s w i t h t h o r i u m , TD n i c k e l , a r e commercially a v a i l a b l e .
Other newer m a t e r i a l s f o r arduous k o n d i t i o n s i n c l u d e
s y n t h e t i c diamond and sapphire, new g r a p h i t e s and m a t e r i a l s such as t h e carbides, borides and n i t r i d e s o f c e r t a i n m e t a l s which approach t h e hardness o f diamond.
8.3
MATERIALS FOR SPECIFIC APPLICATIONS
Adequate m a t e r i a l p r o p e r t i e s f o r design a r e u s u a l l y ensured by i n d i r e c t means mainly by t h e d e s i g n e r s p e c i f y i n g chemical a n a l y s i s , h e a t - t r e a t m e n t and mecha n i c a l p r o p e r t i e s a l t h o u g h such s p e c i f i e d p r o p e r t i e s may n o t be d i r e c t l y r e p r e s entative i n service.
For instance,
t h e most i m p o r t a n t m a t e r i a l p r o p e r t y may be
r e s i s t a n c e t o a b r a s i o n o r r e s i s t a n c e t o s c u f f i n g and s e i z u r e , o r t o r o l l i n g contact f a t i g u e o r t o l u b r i c a n t a t t a c k o r corrosion.
A p r o p e r t y such as
dimensional s t a b i l i t y may c o m p l e t e l y determine t h e s e r v i c e l i f e .
As t h e u l t -
imate assessment o f a m a t e r i a l i s performance i n p r a c t i c e , f u l l s c a l e t e s t i n g and s e r v i c e s i m u l a t i o n t e s t i n g a r e u s u a l l y r e s o r t e d t o as a means o f m a t e r i a l selection. T r i b o - e n g i n e e r i n g depends i n many i n s t a n c e s upon b e a r i n g s , components which a l l o w r e l a t i v e m o t i o n between members o f a mechanism w h i l s t t r a n s f e r r i n g load. Bearings may t a k e many forms b u t t h e most w i d e l y used types a r e p l a i n b e a r i n g s , gears and r o l l i n g b e a r i n g s .
8.3.1
P l a i n Bearings
I n p l a i n b e a r i n g s t h e l o a d i s t r a n s m i t t e d between moving p a r t s by s l i d i n g c o n t a c t and t h e c r i t e r i o n o f s a t i s f a c t o r y b e a r i n g performance i s minimum wear o f the components t o g e t h e r w i t h freedom from s e i z u r e and freedom f r o m mechanical
f a i l u r e by deformation o r f a t i g u e .
To c a r r y a h a r d s t e e l s h a f t , u s u a l l y spec-
i f i e d f o r i t s mechanical p r o p e r t i e s , a b e a r i n g m a t e r i a l must be c o m p a r a t i v e l y s o f t t o a v o i d wear o f t h e harder m a t e r i a l y e t s t r o n g enough t o w i t h s t a n d heavy loads w i t h o u t d i s t o r t i o n and w i t h o u t s u f f e r i n g f a t i g u e .
Soft bearing m a t e r i a l s
a l s o a l l o w a b r a s i v e p a r t i c l e s t o become embedded and thus reduce a b r a s i v e wear. As a low hardness i s u s u a l l y a s s o c i a t e d w i t h a low m e l t i n g p o i n t , h i g h s p o t s o f s o f t b e a r i n g s a r e removed by s l i d i n g c o n t a c t w i t h o u t damage t o t h e m a t i n g s u r face and w i t h o u t t h e r i s k o f s e i z u r e .
However, low hardness i s u s u a l l y assoc-
i a t e d w i t h low f a t i g u e s t r e n g t h and, as s t r e s s l e v e l s a r e r a i s e d , t h e demand i s f o r harder b e a r i n g m a t e r i a l s t o improve t h e l o a d c a r r y i n g c a p a c i t y b u t w i t h t h e minimum loss o f f r i c t i o n and wear p r o p e r t i e s .
As a general r u l e i t i s a d v i s a b l e
t o use t h e s o f t e s t b e a r i n g m a t e r i a l p o s s i b l e . White m e t a l , a w i d e l y used p l a i n b e a r i n g m a t e r i a l i s based on t i n o r l e a d o r t h e i r intermediate a l l o y s .
A t y p i c a l t i n based a l l o y c o n t a i n s 7-10% Sb and
3-5% Cu, t h e p r i n c i p a l c o n s t i t u e n t s b e i n g SbSn, CueSns and a t e r n a r y p e r i t e c t i c complex, Fig.1. composition [ 8 ] .
Hardness and mechanical p r o p e r t i e s a r e l i t t l e a f f e c t e d by A t lOO"C,
t h e hardness ranges f r o m 11-16 HV and t h e f a t i g u e
s t r e n g t h f o r l o 7 c y c l e s from 1.6-1.9
MN/m2.
Lead based a l l o y s c o n t a i n i n g Sb and Sn and Cu i n t h e f o r m o f i n t e r m e t a l l i c SbSn and CusSns may be cheaper than s i m i l a r t i n based a l l o y s b u t a r e s l i g h t l y i n f e r i o r r e g a r d i n g wear and f a t i g u e p r o p e r t i e s .
Intermediate a l l o y s o f h i g h
lead and t i n c o n t e n t a r e w i d e l y used b u t appear t o have no advantages o v e r t h e o t h e r w h i t e metals.
The success o f w h i t e m e t a l s i s g e n e r a l l y regarded as b e i n g
due t o t h e c o r r e c t compromise between s o f t n e s s t o a v o i d wear and s t r e n g t h t o resist fatigue. Copper-based a l l o y s , s t r o n g e r b e a r i n g m a t e r i a l s than t h e w h i t e m e t a l s a t o p e r a t i n g temperatures,
range from t h e phosphor bronzes (10% Sn, 0.5% P) through
the leaded bronzes t o t h e copper l e a d a l l o y s o f up t o 50% Pb, Fig.2. p r o p e r t i e s o f leaded bronze a r e b e t t e r than those o f w h i t e m e t a l . o f copper-lead v a r i e s a c c o r d i n g t o composition from 30-70 H.V.
The wear
The hardness
J o u r n a l wear
increases w i t h increase i n hardness b u t f a t i g u e s t r e n g t h increases r o u g h l y i n t h e same p r o p o r t i o n as j o u r n a l wear. minimize wear.
Increased j o u r n a l hardness can h e l p t o
A d i s a i v a n t a g e o f copper-lead a l l o y s i s t h e i r s u s c e p t i b i l i t y t o
l u b r i c a n t c o r r o s i o n of t h e l e a d phase. A compromise between a l l o y s s o f t enough t o a v o i d wear, those h a r d enough t o
r e s i s t f a t i g u e and those a b l e t o r e s i s t c o r r o s i o n has e v o l v e d by t h e use o f o v e r l a y b e a r i n g s i n which a s t r o n g m e t a l , such as copper-based m e t a l , has a s o f t metal o v e r l a y .
For economic reasons, t h e c o p p e r - l e a d may be used as an
i n t e r l a y between a s t e e l base and t h e o v e r l a y ,
Fig.3.
To a v o i d f a t i g u e under
the a p p l i e d loads, t h e o v e r l a y i s u s u a l l y t h i n n e r than 5 urn.
The o v e r l a y p l a t e d
209
Fig.1
(x75) S t r u c t u r e o f g r a v i t y c a s t t i n based w h i t e m e t a l .
Fig.2
(x75) S t r u c t u r e o f copper-lead on s t e e l base.
copper-lead b e a r i n g i s w i d e l y used f o r h i g h d u t y e n g i n e b e a r i n g s b u t t h e continuous search i s f o r s u p e r i o r r e a d i l y a v a i l a b l e m a t e r i a l s .
Fig.3
(x75) S o f t o v e r l a y on copper lead bearing.
Fig.4
(x120) S t r u c t u r e o f aluminium 20% t i n b e a r i n g .
210 Aluminium, a c o m p a r a t i v e l y cheap m a t e r i a l i n abundant supply, has met w i t h some success by t h e c o n v e n t i o n a l approach o f u s i n g hard metal compounds i n the aluminium m a t r i x t o produce a s t r u c t u r e analogous t o t h a t of w h i t e m e t a l .
The
use o f a n o t h e r s o f t met al , t i n , w i t h a l umi n i um has produced good r e s u l t s .
By
s u i t a b l e c o l d wo rki ng f o l l o w e d by h e a t - t r e a t m e n t and r e c r y s t a l l i z a t i o n the d i s t r i b u t i o n o f about 20% t i n produces a r e t i c u l a r s t r u c t u r e , Fig.4, adequate b e a r i n g p r o p e r t i e s .
with
The a d d i t i o n o f a small amount o f hardener such as
copper i s b e n e f i c i a l and seems t o o f f e r t h e b e s t combination of l o a d c a r r y i n g c a p a c i t y , wear and c o r r o s i o n r e s i s t i n g p r o p e r t i e s c u r r e n t l y a v a i l a b l e . o v e r l a y s f o r aluminium b e a r i n g s a i d r u n n i n g - i n , r e d u c t i o n o f t i n c o n t e n t t o about 9%.
Suitable
minimise j o u r n a l wear and a l l o w
S i m i l a r aluminium-lead b e a r i n g s e f f e c t
f u r t h e r economies. For j o u r n a l s , t h e cheapest p o s s i b l e m a t e r i a l i s u s u a l l y chosen. i s used f o r l e s s arduous a p p l i c a t i o n s ,
Mild steel
and p l a i n carbon s t e e l s can be heat-
.
t r e a t e d t o meet most c o n v e n t i o n a l a p p l i c a t i o n s , t h e p r o p e r t i e s improving w i t h i n c r e a s e o f carbon c o n t e n t .
Medium-carbon s t e e l s used f o r s m a l l e r s i z e s o f
j o u r n a l s and e n gi n e c r a n k s h a f t s have low h a r d e n a b i l i t y .
For more massive p a r t s ,
which a r e r e q u i r e d i n t h e hardened and tempered c o n d i t i o n , l o w - a l l o y s t e e l s a r e needed t o e n a b le s a t i s f a c t o r y p r o p e r t i e s t o be obtained.
Depending upon t h e
s p e c i f i c p r o p e r t i e s r e q u i r e d , manganese, n i c k e l , chromium, molybdenum and vanadium,
used s e p a r a t e l y o r i n v a r i o u s co mbinations, p r o v i d e a wide range o f
m a t e r i a l s f o r h i g h l y s t r e s s e d t r a n s m i s s i o n components and t h e more s t r i n g e n t applications.
Surface-hardening
t ech ni q ue s, such as c a r b u r i z i n g and n i t r i d i n g ,
a r e b e n e f i c i a l f o r p r o v i d i n g an e x t r e m e l y hard, w e a r - r e s i s t a n t s u r f a c e u s u a l l y on s p e c i a l l y manufactured low-carbon manganese o r tough a l l o y s t e e l s c o n t a i n i n g small amounts o f manganese, chromium, n i c k e l and molybdenum.
Nickel steels are
p a r t i c u l a r l y s u i t a b l e f o r case h ard en i ng a s such s t e e l s p r o v i d e a strong, wear-resistant
tough,
case w i t h a d u c t i l e c o r e w h i l s t t h e h a r d e s t n i t r i d e d cases a r e
o b t a i n e d w i t h a lu mi ni u m-con t a i ni n g
steels.
C o m p a t a b i l i t y o f m e t a l s i n s l i d i n g c o n t a c t i s a dominant f a c t o r i n b e a r i n g performance and b o t h t h e b e a r i n g m a t e r i a l and t h e h a r d e r s h a f t r e q u i r e c a r e f u l selection.
M a t e r i a l s f o r h i g h t emp era t u re b e a r i n g s and s l i d i n g a p p l i c a t i o n s
have been surveyed [ 9 ] .
8.3.2
Gears
Gears i n s e r v i c e a r e s u b j e c t e d t o r o l l i n g , s l i d i n g , a b r a s i v e , chemical, v i b r a t o r y and s h ock-l o ad i ng a c t i o n . scuffing,
pitting, fretting,
T h e i r u s e f u l l i f e may be t e r m i n a t e d by
a b r a s i o n , c o r r o s i o n and f r a c t u r e .
must be chosen t o r e s i s t t he se phenomena. a r e t h e carbon-manganese s t e e l s ;
Gear m a t e r i a l s
The most e x t e n s i v e l y used gear s t e e l s
manganese c o n t r i b u t e s markedly t o s t r e n g t h
211 and hardness b u t i t s e f f e c t depends upon t h e carbon c o n t e n t . hardenability, strength.
I t a l s o enhances
and f i n e - g r a i n e d manganese s t e e l s a t t a i n unusual toughness and
For more s t r i n g e n t gear a p p l i c a t i o n s , a l l o y s t e e l s , h e a t - t r e a t e d t o
p r o v i d e t h e optimum p r o p e r t i e s , a r e used.
Nickel provides solid-solution
s t r e n g t h e n i n g and i ncre ase s toughness and r e s i s t a n c e t o impact, p a r t i c u l a r l y a t low temperatures,
lessens d i s t o r t i o n i n quenching,
ance and a l l o w s more l a t i t u d e i n h ea t t rea t ment.
improves c o r r o s i o n r e s i s t Chromium increases harden-
a b i l i t y and has a s t r o n g tendency t o f orm s t a b l e c a r b i d e s which hamper g r a i n growth and p r o v i d e f i n e - g r a i n e d ,
tough s t e e l s .
Vanadium forms s t a b l e c a r b i d e s
t h a t do n o t r e a d i l y go i n t o s o l u t i o n and which a r e n o t prone t o agglomeration by tempering.
I t i n h i b i t s g r a i n growth, t h u s i m p a r t i n g s t r e n g t h and toughness Molybdenum and vanadium a r e g e n e r a l l y used i n combin-
t o heat-treated steels.
a t i o n w i t h o t h e r a l l o y i n g elements. f a s t e r machining r a t e s ,
Lead may be added t o gear s t e e l s t o a t t a i n
i n crea sed p r o d u c t i o n and l o n g e r t o o l l i f e .
S u r fa c e h a r d e ni n g t o reduce wear i s e x t e n s i v e l y a p p l i e d t o gear s t e e l s without s a c r i f i c i n g desirable core properties. l i q u i d , gas o r pack c a r b u r i z e d . inium-containing steels.
Carbon and a l l o y s t e e l s can be
N i t r i d i n g i s u s u a l l y a p p l i e d t o s p e c i a l alum-
Flame and i n d u c t i o n hardening methods a r e a l s o used.
Other s u r f a c e t r e a t m e n t s such as S u l ph i nu z, phosphating and s o f t n i t r i d i n g which reduce f r i c t i o n and a i d l u b r i c a t i o n , can be b e n e f i c i a l .
8.3.3
R o l l i n g Be ari n gs
A lth o u g h b a l l and r o l l e r b e a r i n g s a r e b a s i c a l l y r o l l i n g elements,
i n oper-
a t i n g mechanisms t h e y a r e a l s o s u b j e c t e d t o some wear by s l i d i n g and t o chemical a t t a c k by l u b r i c a n t and environment.
Their useful l i f e i s usually
l i m i t e d by s u r f a c e d i s i n t e g r a t i o n , p i t s b e i n g formed by a f a t i g u e process dependent upon t h e p r o p e r t i e s o f t h e m a t e r i a l , t h e n a t u r e o f t h e l u b r i c a n t and the environment [10,11,12].
The p r i n c i p a l q u a l i t i e s o f b a l l - b e a r i n g m a t e r i a l s
a r e dimensional s t a b i l i t y , h i g h hardness t o r e s i s t wear,
high e l a s t i c l i m i t
t o a v o i d p l a s t i c d e f o r m a t i o n under load, and good f a t i g u e r e s i s t a n c e t o contend w i t h h i g h a l t e r n a t i n g stresses.
A h i gh -carb on s t e e l s a t i s f i e s these r e q u i r e -
ments i f a c a r b i d e - f o r m i n g element i s i n c o r p o r a t e d t o increase hardness, g i v e h a r d e n a b i l i t y and a l l o w o i l quenching t o mi n imize d i s t o r t i o n d u r i n g heattre a t m e n t.
EN 31, 535A99 o r SEA 52100 (1.0% C, 1.5% Cr) through hardening
s t e e l i s used f o r c o n v e n t i o n a l b e a r i n g s , Fig.5.
Vacuum degassed, vacuum r e -
melted a i d e l e c t r o - s l a g r e f i n e d m a t e r i a l o f improved mechanical p r o p e r t i e s may be used f o r in c r e ase d r o l l i n g c o n t a c t f a t i g u e r e s i s t a n c e .
For convenience,
i n t h e manufacture o f t h e l a r g e r s i z e s o f r o l l e r b e a r i n g s case-hardening s t e e l s c o n t a i n i n g chromium, n i c k e l and molybdenum a c c o r d i n g t o t h e degree o f hardena b i l i t y , shock r e s i s t a n c e and c o r e hardness r e q u i r e d a r e used.
For use i n a
212 c o r r o s i v e environment m a r t e n s i t i c s t a i n l e s s s t e e l s a r e used w i t h some loss of f a t i g u e resistance. For use a t e l e v a t e d temperatures,
conventional r o l l i n g bearing s t e e l s a r e not
s a t i s f a c t o r y owing t o loss o f hardness and f a t i g u e r e s i s t a n c e and h i g h speed t o o l s t e e l s w i t h h i g h tempering temperatures a r e used, Fig.6.
H igh speed t o o l
s t e e l s c o n t a i n i n g p r i n c i p a l l y t un gst en , molybdenum and vanadium a r e a l s o l e s s prone t o d e l e t e r i o u s l u b r i c a n t e f f e c t s t h an EN 31 s t e e l [ l l ] .
M a t e r i a l com-
b i n a t i o n and m a t e r i a l l u b r i c a n t co mbi n at i on a r e i m p o r t a n t t o ensure adequate r o l l i n g c o n t a c t f a t i g u e l i f e [13,14,15].
Fi g. 5
( ~ 4 0 0 0 ) S t r u c t u r e o f EN 31 b a l l bearing s t e e l .
F i g. 6 ( ~ 6 , 5 0 0 ) S t r u c t u r e o f 18%W h i g h speed t o o l s t e e l .
Under c o n d i t i o n s o f u n l u b r i c a t e d r o l l i n g c o n t a c t , f a i l u r e occurs n o t by t h e usu a l f a t i g u e mechanism b u t by exce ssi ve wear l i m i t i n g u s e f u l l i f e , owing t o v i b r a t i o n and rough, n o i s y run ni n g.
Va ri ou s s u p e r a l l o y s w i t h base composition
o f chromium, molybdenum and c o b a l t and c o n t a i n i n g s i g n i f i c a n t amounts o f n i c k e l , tu n g s t e n , vanadium and o t h e r a l l o y i n g elements, a l s o cermets and ceramics, a r e p o t e n t i a l l y s u i t a b l e and have been used under arduous t e s t conditions.
Under c e r t a i n t e s t c o n d i t i o n s
1131,
tungsten c a r b i d e was t h e b e s t o f
t h e m a t e r i a l s t r i e d , g i v i n g t h e l owest wear r a t e and being r e l a t i v e l y u n a f f e c t e d by temperature.
The b e s t r e s u l t s were o b t a i n e d w i t h t h e s m a l l e s t c a r b i d e s i z e
and t h e lo w e s t percentage o f m a t r i x m a t e r i a l , Fig.7. r i d e was a l s o s u i t a b l e .
Hot pressed s i l i c o n n i t -
213
F i n e grained Fig.7
8.3.4
Coarse grained
( ~ 5 0 0 )S t r u c t u r e o f 6% Co tungsten c a r b i d e .
Wear R e s i s t a n t M a t e r i a l s
Wear r e s i s t a n t m a t e r i a l s r e q u i r e t h e c o r r e c t combination o f hardness t o r e s i s t a b r a s i o n and d u c t i l i t y t o contend w i t h shock loads and c y c l i c s t r e s s i n g .
AS these a r e c o n f l i c t i n g requirements, s u i t a b l e m a t e r i a l s e l e c t i o n i n v o l v e s compromise.
Other f a c t o r s such as t h e types o f wear,
t h e n a t u r e o f any
abrasive, t h e o p e r a t i n g temperature and t h e environment a f f e c t t h e c h o i c e o f material. For c o n d i t i o n s o f h i g h s t r e s s and impact, t h e toughness and work hardening
p r o p e r t i e s o f a u s t e n i t i c manganese s t e e l a r e u s u a l l y r e q u i r e d .
For lower s t r e s s
s l i d i n g c o n d i t i o n s where toughness i s l e s s i m p o r t a n t , depending upon t h e c o s t f a c t o r , hardened s t e e l s , a l l o y c a s t i r o n s , h a r d f a c i n g m a t e r i a l s , c o n c r e t e o r ceramics may be used.
Where maximum wear r e s i s t a n c e i s r e q u i r e d and c o s t i s
immaterial, cemented c a r b i d e s may be used.
Corrosion resistance requires the
use of s t a i n l e s s m e t a l s , s u i t a b l e rubbers o r p l a s t i c s .
Suitably reinforced
polymer m a t e r i a l may be used where a low c o e f f i c i e n t o f f r i c t i o n as w e l l as wear resistance i s required.
High chromium a l l o y s o f i r o n and s t e e l o f f e r t h e b e s t
wear r e s i s t a n c e t o e l e v a t e d temperature problems o f c r a c k i n g , s p a l l i n g and thermal shock. The wear r e s i s t a n c e o f a metal v a r i e s w i t h d i f f e r e n t a b r a s i v e s and t h e e f f e c t i v e hardness o f an a b r a s i v e has been d e f i n e d as t h e maximum v a l u e o f hardness o f a metal t h a t can be abraded by i t .
T h i s i s o f importance i n m a t e r i a l
s e l e c t i o n where a known a b r a s i v e i s t o be encountered i n p r a c t i c e . o f importance i n a b r a s i v e s e l e c t i o n f o r m a t e r i a l removal.
It i s also
Next t o diamond, t h e
hardest a b r a s i v e a v a i l a b l e , s i l i c o n c a r b i d e i s t h e most i m p o r t a n t a b r a s i v e f o r lapping and g r i n d i n g and f o r g r i n d i n g wheels.
Diamond can be s y n t h e s i z e d from
214 carbon and i n a s i m i l a r manner, c u b i c c r y s t a l l i n e boron n i t r i d e o f s i m i l a r hardness can be produced.
I t s extreme hardness and d u r a b i l i t y make t h i s mat-
e r i a l o f economic s i g n i f i c a n c e even though t h e p r e s e n t i n i t i a l c o s t i s h i g h .
8.3.5
To o ls
The e v o l u t i o n o f modern p r o d u c t i o n methods has been l a r g e l y dependent on the development o f tool s t e e l s cap ab l e of o p e r a t i n g e f f i c i e n t l y under i n c r e a s i n g High hardness i s a r e q u i s i t e o f almost every t o o l s t e e l
arduous c o n d i t i o n s .
b u t t h e optimum hardness depends upon t h e a p p l i c a t i o n and, as hardness and toughness v a r y i n v e r s e l y , maximum hardness can o n l y be used i n t h e absence of shock l o a d i n g .
Hardness a t e l e v a t e d t e mpe rature i s sometimes e s s e n t i a l as w e l l
as r e s i s t a n c e t o a b r a s i o n and d i s t o r t i o n .
The c h o i c e o f s t e e l i s u s u a l l y based
on a c o n s i d e r a t i o n o f t h e r e l a t i v e importance of t h e p r o p e r t i e s r e q u i r e d .
As
no s i n g l e s t e e l possesses a l l d e s i r a b l e p r o p e r t i e s , a compromise i s g e n e r a l l y necessary.
Economic a spe ct s can r a r e l y be ignored.
The s i m p l e s t t o o l s t e e l s a r e p l a i n carbon (0.6 t o 1.4% C) types, which by wa t e r quenching, d eve l op a h a r d case.
Low a l l o y v a r i e t i e s c o n t a i n up t o 0.25%
V f o r c a r b i d e s t a b i l i z a t i o n and t o r e t a r d g r a i n growth, and up t o 0.5% C r t o i n c r e a s e h a r d e n a b i l i t y and case depth.
O i l - h a r d e n i n g manganese s t e e l s (1.0 t o
2.0% Mn) p r o v i d e h i g h hardness w i t h minimum d i s t o r t i o n .
Chromium may be added
t o h e l p h a r d e n a b i l i t y w h i l s t a d d i t i o n s of t u ngsten and vanadium improve wear r e s i s t a n c e and c o n t r o l g r a i n growth. high-carbon
(2.0% C),
For t he more arduous a p p l i c a t i o n s t h e
high-chromium (14% C r )
s t e e l s a r e used t o r e s i s t wear and
c o r r o s i o n and t o m a i n t a i n h i g h d i men si on al s t a b i l i t y .
They m a i n t a i n a good
c u t t i n g edge f o r c o l d b l a n k i n g t o o l s .
I f shock r e s i s t a n c e i s r e q u i r e d , s t e e l s o f lower carbon c o n t e n t (0.4
to
0.5% C) a r e u s u a l l y r e q u i r e d and a l l o y i n g elements such as s i l i c o n , tungsten, vanadium and chromium may be added t o improve a b r a s i o n r e s i s t a n c e .
For h o t -
w o r k in g , high-speed t o o l o r h o t d i e s t e e l s which r e t a i n h i g h hardness and a good c u t t i n g edge a t e l e v a t e d temperatures a r e needed. t y p e i s t h e 18-4-1
( t u n g s t e n , chromium, vanadium)
The most w i d e l y used
type and h o t hardness may be
increased by r a i s i n g t h e vanadium c o n t e n t o r by a d d i t i o n o f c o b a l t .
It i s
p o s s i b l e t o p a r t l y r e p l a c e t u ng st e n w i t h molybdenum as i s t h e case w i t h , f o r example, M.2,
6.5.4.2
(W,
Mo, C r ,
V) s t e e l .
Tungsten, chromium and molybdenum
chromium h o t d i e s t e e l s may be regarded as l e a n type high-speed t o o l s t e e l s i n which improved toughness can be developed i f t h e carbon c o n t e n t i s reduced t o below 0.4% C.
N i c k e l , chromium, molybdenum s t e e l s g e n e r a l l y used as s t r u c t u r a l
s t e e l s , may by s u i t a b l e heat t rea t me nt be used as t o o l s t e e l s .
They do n o t
possess t h e h i g h hardness o r a b r a s i o n r e s i s t a n c e o f high-speed t o o l s t e e l s b u t advantage can be taken o f t h e i r s u p e r i o r toughness.
215
8.3.6
C u t t i n g Tools
From an economic p o i n t o f view, one o f t h e most i m p o r t a n t f a c t o r s i n machining i s t h e r a t e a t which c u t t i n g t o o l s wear 1161. most w e a r - r e s i s t a n t ,
Generally the hardest,
t o o l m a t e r i a l which r e s i s t s b r i t t l e f r a c t u r e should be
used. High-speed s t e e l s have good shock r e s i s t a n c e and can be r e a d i l y shaped by f o r g i n g and machining, so a r e t h e p r e f e r r e d c u t t i n g t o o l m a t e r i a l s f o r a l a r g e range o f a p p l i c a t i o n s .
Surface t r e a t m e n t s which increase hardness and minimize
adhesion t o t h e workpiece a r e b e n e f i c i a l .
Cast cobalt-chromium-tungsten a l l o y s ,
the S t e l l i t e s a r e s u i t a b l e f o r a p p l i c a t i o n s i n v o l v i n g h i g h temperatures, where c o o l i n g i s i m p r a c t i c a b l e b u t where impact i s absent.
Sintered carbide t o o l s
may be s u c c e s s f u l l y used f o r most m e t a l - c u t t i n g o p e r a t i o n s and t h e i r e x c e p t ional performance r e s u l t s from h i g h hardness and compressive s t r e n g t h s .
The
s t r a i g h t t u n g s t e n c a r b i d e s have t h e h i g h e s t hardness and a r e used f o r general applications,
b u t grades c o n t a i n i n g t i t a n i u m and t a n t a l u m c a r b i d e a r e more
r e s i s t a n t t o c r a t e r i n g and used g e n e r a l l y f o r machining s t e e l .
The use o f t h i n
CVD c o a t i n g s o f T i c improves t o o l l i f e c o n s i d e r a b l y . The i n t r o d u c t i o n o f new m a t e r i a l s which a r e d i f f i c u l t t o machine and t h e focus on h i g h p r o d u c t i v i t y , has accentuated t h e need f o r harder c u t t i n g t o o l s . Ceramic and cermet t o o l s a r e now i n use.
The h e a t - r e s i s t i n g p r o p e r t i e s o f
ceramic t o o l s enable them t o be used a t speeds u n a t t a i n a b l e by c a r b i d e t o o l s w i t h consequently improved s t o c k removal r a t e s .
Their high r i g i d i t y prevents
c h a t t e r i n g and a l l o w s smoother c u t s and s u p e r i o r work f i n i s h .
T h e i r h i g h hard-
ness and w e a r - r e s i s t a n t p r o p e r t i e s a l l o w c l o s e maintenance o f dimensions and enable t h e machining o f h i g h hardness m a t e r i a l s .
Being n o n - m e t a l l i c ,
ceramics
do n o t weld t o t h e m a t e r i a l b e i n g c u t and t h e i r extreme r e f r a c t o r i n e s s obv i a t e s t h e need f o r c o o l a n t s .
Diamond t o o l s a r e used f o r s p e c i a l purpose
c u t t i n g where t h e h i g h c o s t can be j u s t i f i e d ;
t o o l l i f e , however,
i s superior
t o t h a t o f carbide.
8.3.7
P i s t o n Rings
Apart f r o m speed, temperature and load, t h e p r i n c i p a l f a c t o r s which i n f l u e n c e wear o f p i s t o n r i n g s a r e c o r r o s i o n , frequency o f use and d i r t .
Whilst high
temperatures may cause wear by a d v e r s e l y a f f e c t i n g l u b r i c a t i o n , l o w tempera t u r e s cause c y l i n d e r and r i n g wear due t o c o r r o s i o n by t h e condensate o f combustion p r o d u c t s , t h i s i s p a r t i c u l a r l y so w i t h i n f r e q u e n t l y used engines. A i r b o r n e d u s t can be a s e r i o u s problem. Grey c a s t i r o n i s p r o b a b l y t h e most w i d e l y used m a t e r i a l f o r compression and
o i l r i n g s b u t h a r d e r m a t e r i a l s , such as carbon s t e e l o r even En31 b a l l b e a r i n g s t e e l , f i n d use.
For g r e a t e r hardness and wear r e s i s t a n c e , chromium p l a t i n g
i n s u f f i c i e n t t h i c k n e s s (0.005
inch) can be used on r i n g s and g i v e s reduced
216 wear r a t e s f o r b o t h c y l i n d e r and r i n g s .
Chromium p l a t i n g does n o t r u n s a t i s -
f a c t o r i l y a g a i n s t i t s e l f , so o n l y one mat i ng s u r f a c e i s p l a t e d .
Although occ-
a s i o n a l l y used f o r l i g h t e r s e r v i c e on passenger c a r engines, chromium p l a t i n g i s g e n e r a l l y used on compression r i n g s on heavy duty engines; d i e s e l and n a t u r a l gas engines.
i t i s w i d e l y used i n
As chromium p l a t i n g reduces f a t i g u e s t r e n g t h
i t i s usual t o p l a t e m a t e r i a l s o f h i g h f a t i g u e s t r e n g t h such as h i g h carbon
Molybdenum c o a t i n g s o f hardness o ver 1000 HV and a p o r o s i t y o f 15-25%
steels.
may now be f a v o ure d i n p r e f e r e n c e t o chromium p l a t i n g . r i n g s a r e f i n d i n g i n c r e a s i n g use.
Carbon-graphite,
p i s t o n r i n g m a t e r i a l f o r u n l u b r i c a t e d compressors, b u t i t has l i t t l e i n h e r e n t f l e x i b i l i t y ,
Sintered iron piston
used f o r many years as a i s a satisfactory material
needs support and depends f o r i t s low
wear p r o p e r t i e s on t he presence o f condensed w ater [ 1 7 ] .
Non-metallic p i s t o n
r i n g s , p a r t i c u l a r l y f a b r i c r e i n f o r c e d PF r e s i n s , have been used where t h e r e a r e doubts about l u b r i c a t i o n so t h a t ,
i n t h e e vent o f l u b r i c a t i o n f a i l u r e , t h e s o f t
r i n g s w i l l n o t damage t h e c y l i n d e r w a l l . Dry r u n n in g PTFE p i s t o n r i n g s a r e su ccessful
i n medium p r e s s u r e oxygen com-
p r e s s o r s as t h e y possess low f r i c t i o n a l c h a r a c t e r i s t i c s , dimensional s t a b i l i t y a t o p e r a t i n g temperatures,
good wear r e s i s t a n c e ,
c a r r y i n g a b i l i t y and chemical i n e r t n e s s . pr e s s u r e a p p l i c a t i o n s , i s reinforced.
h i g h s t r e n g t h and load-
However, f o r n o n - l u b r i c a t e d h i g h -
dimensional s t a b i l i t y can o n l y be r e t a i n e d i f t h e PTFE
G l a s s - f i b r e r e i n f o r c e m e n t appears t o be p o t e n t i a l l y more s u i t -
able than s t e e l reinforcement.
Other f i l l e r s commonly used a r e carbon, powd-
e r e d m e t a ls , MoS2, ce rami cs and carbon f i b r e s .
The concept o f avoidance o f
damage t o t h e c y l i n d e r w a l l and i n c r e a s i n g e xperience w i t h PTFE r i n g s on unlubr i c a t e d compressors have l e d t o t h e idea o f r e p l a c i n g m e t a l l i c r i n g s on l u b r i c a t e d compressors w i t h PTFE r i n g s t o a l l o w reduced l u b r i c a n t feed r a t e s . o f t h e a r t r e v i e w o f t h e i r use has been pre sented [ I S ] .
A state
A l i t e r a t u r e survey o f
m a t e r i a l and m e t a l l u r g i c a l asp ect s o f p i s t o n r i n g s c u f f i n g has been c a r r i e d out [lg].
8.3.8
Cams and Tappets
I n modern high-speed,
high-output,
small automtoive u n i t s d i s t r e s s o f cams
and t a p p e ts can be a major problem due t o a c c e l e r a t e d normal wear, p i t t i n g , s c u f f i n g and b u r n i s h i n g . camshaft m a t e r i a l .
Hardenable g rey c a s t i r o n i s t h e most w i d e l y used
Water-quenched hi g h-ca rbo n o r oil-quenched a l l o y s t e e l s o f
h i g h carbon c o n t e n t a r e used i n t h e through-hardened c o n d i t i o n .
Carburizing
s t e e l s may a l s o be used w i t h s e l e c t i v e f l a me o r i n d u c t i o n hardening o f s u r f a c e areas.
For a u t o mot i ve en gi n e cams, chromium and molybdenum c o n t a i n i n g i r o n s a r e
g e n e r a l l y used w i t h i n d i v i d u a l cams s u r f a c e hardened.
Tappet m a t e r i a l s a r e
u s u a l l y through-hardened h i g h carbon, chromium o r molybdenum types o f c a r b u r i z e d low-alloy steels.
The most commn t a p p e t m a t e r i a l s i n automotive a p p l i c a t i o n s
217 are grey hardenable c a s t i r o n c o n t a i n i n g chromium, molybdenum and n i c k e l o r c h i l l e d cast iron.
Oxide c o a t i n g s on hardened s t e e l tappet faces improve
f r i c t i o n a l q u a l i t i e s and a c c e l e r a t e w e a r i n g - i n . t o ensure adequate l i f e ;
Surface f i n i s h e s a r e important
s t r e s s - r e l i e v i n g can be b e n e f i c i a l .
Salt-bath
n i t r i d i n g t r e a t m ent can be b e n e f i c i a l t o cam f o l l o w e r s .
8.3.9
F r i c t i o n Materials
Te c h n o lo g ic a l p rog ress i n t h e aeronaut i c a ? and automobile i n d u s t r i e s makes demands on f r i c t i o n m a t e r i a l s i n c r e a s i n g l y more severe. m a t e r i a l s must have h i g h c o e f f i c i e n t s o f f r i c t i o n , temperatures,
High q u a l i t y brake
s t a b i l i t y a t a l l operating
good wear r e s i s t a n c e and s t r e n g t h , h i g h thermal c o n d u c t i v i t y and
corrosion s t a b i l i t y .
For arduous a p p l i c a t i o n s where s u r f a c e temperatures a r e
hi gh , o r g a n i c f r i c t i o n m a t e r i a l
i s no l o ng er s u i t a b l e .
A t o p e r a t i n g temper-
a t u r e s above 350"C, wear o f o r g a n i c m a t e r i a l becomes e x t r e m e l y severe.
Copper-
base f a c i n g s can o p e r a t e up t o lO0O'C b u t new m a t e r i a l s a r e under development f o r a e r o n a u t i c a l a p p l i c a t i o n s where r u b b i n g s u r f a c e s may w e l l reach temperatures w e l l above t h i s .
S i n t e r e d m a t e r i a l s o f f e r a w ide range o f f r i c t i o n a l c h a r a c t e r -
i s t i c s by t h e d i s p o s a l o f c a r e f u l l y graded n o n - m e t a l l i c p a r t i c l e s i n a m e t a l l i c matrix; level.
t h e c o m p o s i t i o n can be a d j u s t e d t o minimise wear, s e i z u r e and n o i s e Cermet and carbon f r i c t i o n m a t e r i a l s a r e b e i n g s u c c e s s f u l l y used i n
many a p p l i c a t i o n s .
8.3.10
P l a s t i c Be ari n gs
Since t h e f i r s t appearance on t h e market i n t h e 1 9 3 0 ' s o f b e a r i n g s based on t h e r m o s e t t i n g r e s i n s , t h e r e has been a con t i n uous increase i n t h e u t i l i z a t i o n o f polymers and polymer based composites i n t h e b e a r i n g f i e l d ,
[20,21].
Nylon
was t h e f i r s t o f t h e t h e r m o p l a s t i c m a t e r i a l s used f o l l o w e d by PTFE and more recently the polyacetals.
P l a s t i c s o f f e r a number o f advantages over metals.
T h e i r p h y s i c a l and mechanical p r o p e r t i e s can be v a r i e d over a wide range by s u i t a b l e c h o i c e o f polymer t ype , f i l l e r and r e i n f o r c e m e n t ; easy t o shape.
some a r e cheap and
Many polymers a r e r e s i s t a n t t o chemical a t t a c k and e x h i b i t low
c o e f f i c i e n t s o f f r i c t i o n during unlubricated sliding.
T h e i r wear r a t e s s l i d i n g
a g a i n s t smooth metal c o u n t e r f a c e s a r e low and they do n o t n o r m a l l y e x h i b i t s c u f f i n g o r seizure.
L u b r i c a t i o n by f l u i d s can o f t e n be dispensed w i t h , b u t
when l u b r i c a t e d , polymers a l l o w elastohydrodynamic metals.
They have disadvantages compared w i t h metals, e s p e c i a l l y r e g a r d i n g
u l t i m a t e strengths, pansion.
l u b r i c a t i o n more r e a d i l y than
e l a s t i c mo du l i , cre ep r e s i s t a n c e and c o e f f i c i e n t s o f ex-
Polymers may a l s o r e a d i l y absorb f l u i d s r e s u l t i n g i n dimensional
instability.
T h e i r low thermal c o n d u c t i v i t i e s can cause problems w i t h d i s s -
i p a t i o n o f f r i c t i o n a l h ea t .
They a l s o have t emperature l i m i t a t i o n s r e g a r d i n g
218 softening,
m e l t i n g and thermal d eg rad at i on .
The p r im e v i r t u e s o f t h e r m o - s e t t i n g reinforced,
r e s i n based bearings, c l o t h o r f i b r e
i s t h e i r h i g h s t r e n g t h and . e x c e l l e n t performance under c o n d i t i o n s
o f wa t e r l u b r i c a t i o n .
A t y p i c a l use i s r o l l neck b e a r i n g s o f s t e e l r o l l i n g
m i l l s where heat removal by wat er i s e s s e n t i a l .
On a s m a l l e r scale,
reinforced
r e s i n b e a r in g s a r e used i n pumps f o r wat er c i r c u l a t i o n and w i t h o i l and grease l u b r i c a t i o n i n a ut o mot i ve a p p l i c a t i o n s . Nylon and p o l y a c e t a l bushes p r o v i d e good d r y b e a r i n g l i v e s i f t h e PV f a c t o r i s k e p t low.
These m a t e r i a l s s i n t e r e d t o p r o v i d e c o n t r o l l e d p o r o s i t y a r e used
f o r o i l impregnated b e a r i n g s a t much g r e a t e r PV values w i t h low wear.
Thermo-
p l a s t i c b e a r i n g s f i n d e x t e n s i v e use i n f r a c t i o n a l HP e l e c t r i c motors, automotive a p p l i c a t i o n s and washing machines.
P o l y e t h y l e n e and h i g h d e n s i t y p o l y e t h y l e n e
a r e used i n p r o s t h e s i s human j o i n t s . PTFE based b e a r i n g s a r e now used a l most anywhere where c l e a n l i n e s s , resistance,
heat
freedom f ro m s e i z u r e and l u b r i c a n t e l i m i n a t i o n i s r e q u i r e d .
Thin
f i l m s o f PTFE f i n d e f f e c t i v e use i n many a p p l i c a t i o n s , p a r t i c u l a r l y t o a v o i d fretting.
Depending upon t h e a p p l i c a t i o n and the p r o p e r t i e s r e q u i r e d , PTFE
may be f i l l e d w i t h g l a s s , asbestos, carbon f i b r e s , MoS2, g r a p h i t e , bronze, n i c k e l and i r o n o x i d e .
A p r i n c i p a l a p p l i c a t i o n o f PTFE i s f o r gas l u b r i c a t e d
b e a r i n g s t o contend w i t h s t o p p i n g and s t a r t i n g .
8.4
SURFACE TREATMENTS AND COATINGS M a t e r i a l s f o r t r i b o l o g i c a l a p p l i c a t i o n s must f u l f i l two i m p o r t a n t f u n c t i o n s .
They must have s t r u c t u r a l p r o p e r t i e s f o r l o a d c a r r y i n g and s u r f a c e c h a r a c t e r i s t i c s t o a l l o w r e l a t i v e mot i on w i t h low f r i c t i o n and t h e minimum amount o f wear damage. demands.
Surface t re at men t s and c o a t i n g s a l l o w design scope t o meet these A ls o , w i t h wear problems i t i s o f t e n d i f f i c u l t t o make w o r t h w h i l e
i n n o v a t i o n s i n d e si gn , l u b r i c a t i o n o r m a t e r i a l s s e l e c t i o n ,
b u t many s u r f a c e
t r e a t m e n t s and c o a t i n g s a r e a v a i l a b l e which can e f f e c t wear r e s i s t a n c e and improved f r i c t i o n a l p r o p e r t i e s r Z Z , Z 3 ] .
Hard wear r e s i s t a n t m a t e r i a l s a r e
u s u a l l y expensive t o manufacture and d i f f i c u l t t o shape, and f o r economic reasons, w e a r - r e s i s t a n t
s u r f a c e t r e a t m e n t s may be a p p l i e d t o more common,
cheaper t o produce and more e a s i l y shaped m a t e r i a l s . Many s u r f a c e t r e a t m e n t s a r e a v a i l a b l e t o combat wear, and t h e c h o i c e depends l a r g e l y upon t h e t y p e and s e v e r i t y o f wear i n v o l v e d .
Some treatments
a r e s h o r t l i v e d b u t t h e more common s u r f a c e t r e a t m e n t s a r e those which a r e expected t o l a s t t h e l i f e o f t h e component.
Both types o f treatment may be com-
b i n e d t o e f f e c t p r o t e c t i o n d u r i n g arduous c o n d i t i o n s o f r u n n i n g i n and l o n g service effectiveness.
Treatments expected t o l a s t t h e l i f e o f a component
i n v o l v e s u r f a c e ha rde ni n g w i t h o u t changing t h e composition, s u r f a c e hardening by d i f f u s i o n t r e a t m ent s and by t h e a p p l i c a t i o n o f s u r f a c e c o a t i n g s .
Carbon and
219 alloy steels of a hardenable composition and cast iron may be surface hardened by flame or induction methods.
Shot peening work hardens the surface o f metals.
An extensively used method o f surface hardening to improve wear resistance depends upon diffusing specific elements into the surfaces of metal by such treatments a s carburizing, carbo-nitriding, nitriding, chromosing, boronising and siliconising.
Care must be taken to ensure a sufficient depth of case of
the correct structure and hardness and a satisfactory transition zone, Fig.8. Electro-deposition o f hard metal such a s chromiumprovides hard surface coatings and alloy coatings may be used to improve wear resistance.
Composite
coatings may be produced by the co-deposition o f hard particles and electrodeposited metal, Fig.9.
Coatings o f silicon carbide in nickel, tungsten carbide
in nickel and cobalt and silicon nitride in nickel have been used effectively. Diamond containing coatings have been developed for specific applications 1241. Hard surfacing o r facing finds wide use in many applications to provide specific wear resistant alloy o r ceramic coatings.
Almost any metal o r alloy
which can be cast may be used as a welding rod to apply a coating.
With plasma
spraying even the most refractory materials can be deposited with good surface bonding
Fig.8
.
(x75) Micro hardness survey o f a case hardened gear tooth with superimposed HV.
Fig.9
( ~ 1 5 0 ) Co-deposited Fe-b%Ni and chromium carbide.
220 Some s u r f a c e t r e a t m e n t s , besides e f f e c t i n g wear r e s i s t a n c e , may a l s o a i d lubrication.
Thus Sulphinuz [25] t r e a t m e n t can n i t r o g e n harden a s u r f a c e w i t h
t h e a s s o c i a t e d compressive s t r e s s e s b e n e f i c i a l t o f a t i g u e r e s i s t a n c e , as w e l l as produce a s u l p h u r - r i c h s u r f a c e l a y e r w i t h good l u b r i c a n t p r o p e r t i e s under boundary c o n d i t i o n s . an oxygen-ion r i c h ,
S o f t n i t r i d i n g c a r r i e d o u t i n an o x i d i s i n g b a t h produces low f r i c t i o n s u r f a c e b e n e f i c i a l i n r e d u c i n g wear and a i d i n g
l u b r i c a t i o n under arduous c o n d i t i o n s . may s o f t e n h a r d m a t e r i a l s .
Sulphinuz and s o f t n i t r i d i n g t r e a t m e n t s
T h i s can be avoided w i t h t h e Noskuff process [25]
which i n c o r p o r a t e s a quenching t r e a t m e n t o f t h e S u f t BT process [25] which p r o duces a s u l p h u r r i c h l a y e r , by an e l e c t r o l y t i c t r e a t m e n t i n a low temperature s a l t bath.
Various t y p e s o f phosphating t r e a t m e n t s p r o v i d e a t h i n , porous
c r y s t a l l i n e l u b r i c a t i n g s u r f a c e f i l m o f i n s o l u b l e phosphate, Fig.10,
which can
a l s o r e t a i n l u b r i c a n t o r p r o v i d e an e f f e c t i v e base f o r a s o l i d l u b r i c a n t . S o l i d l u b r i c a n t f i l m s such as PTFE and MoS2 can a l s o c o n t r o l wear,
reduce
f r i c t i o n and a i d l u b r i c a t i o n .
Fig.10
Phosphate c o a t i n g .
A v e r y hard, s t r o n g l y adherent, homogeneous t h i n f i l m u s u a l l y o f h a r d c a r b i d e such as TIC may be a p p l i e d t o metal by chemical vapour d e p o s i t i o n (CVD) [26,27].
R e f r a c t o r y c o a t i n g s such as alumina and s i l i c o n n i t r i d e may a l s o be
chemical vapour deposited. P h y s i c a l vapour d e p o s i t i o n techniques a r e a v i a b l e approach t o t h e p r o d u c t i o n o f s u r f a c e f i l m s o f extreme v e r s a t i l i t y i n d e p o s i t c o m p o s i t i o n .
V i r t u a l l y any
m e t a l , a l l o y , r e f r a c t o r y o r i n t e r m e t a l l i c compound, some p o l y m e r i c t y p e m a t e r i a l s and t h e i r m i x t u r e s can be d e p o s i t e d [ 2 8 ] .
Metal f i l m s d e p o s i t e d by
221 i o n - p l a t i n g a r e s t r o n g l y bonded t o t h e s u r f a c e as t h e f i l m i s d e p o s i t e d on a surface cleaned by s p l u t t e r e t c h i n g
[29,30].
S o f t metal l u b r i c a n t f i l m s may
a l s o be bonded t o a metal s u r f a c e by i o n - p l a t i n g
[31].
I o n - n i t r i d i n g speeds up
the n i t r i d i n g o p e r a t i o n .
8.5
CONCLUSIONS Mechanisms can o n l y p e r f o r m s a t i s f a c t o r i l y i f t h e d e s i g n and t h e m a t e r i a l s
o f c o n s t r u c t i o n a r e c o r r e c t l y chosen t o contend w i t h t h e o p e r a t i n g c o n d i t i o n s and t h e environment i n which t h e y a r e r e q u i r e d .
I t i s o n l y by e f f i c i e n t s e l -
e c t i o n and c a r e f u l s p e c i f i c a t i o n t h a t t h e most e f f e c t i v e use o f m a t e r i a l s can be accomplished t o e f f e c t m a t e r i a l c p n s e r v a t i o n and energy s a v i n g i n t h e i r p r o duct ion. Economics i n m a t e r i a l s and m a n u f a c t u r i n g c o s t s can o f t e n be made by j u d i c i o u s use o f s u r f a c e c o a t i n g s and s u r f a c e t r e a t m e n t s .
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1 2
3 4
Scott,O., T r i b o l o g y , 1968, 1, 14. Scott,D, I n T r i b o l o g y - Proc. J. R e s i d e n t i a l Course, 1968, Paper 1 , I n s t . M e t a l l u r g i s t s , London. Scott,D., Proc. I n s t . Mech. Engrs., London I n t . Conf. on L u b r i c a t i o n and Wear, 1967, ( 3 A ) , 325. New E n g i n e e r i n g M a t e r i a l s , Proc. I n s t . Mech. Engrs, London, 1965/66,
182,
180, ( 3 D ) .
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13 14 15
16 17 18 19 20
21 22
mdman,N.E., E n g i n e e r i n g A l l o y s , 1962, Chapman & H a l l , London. Special Zakay,V.F. and Justusson,W.M., I n High S t r e n g t h S t e e l s - I . S . I . Report 76, 1962, 14. I r o n & S t e e l l n s t . , London. Ouckworth,W.E., Leak,D.A. and P h i l l i p s , R . , I n High S t r e n g t h S t e e l s , I.S.I. Special Report 76, 1962, 22, I r o n & S t e e l l n s t . , London. Forrester,P.G., Metal1 Rev., 1960, (ZO), 507. Amateau,M.F., Nicholson,D.W. and Glaeser,W.A., 1961, O.T.S. PB 171625, O f f i c e o f T e c h n i c a l Services, Washington D.C. Scott,O., I n F a t i g u e i n R o l l i n g Contact, 1963, 103, I n s t . Mech. Engrs. London. Scott,D., I n Low A l l o y S t e e l s , 1968, 203, I r o n and S t e e l I n s t . London. Scott,D., I n R o l l i n g Contact F a t i g u e , (Ed. Tourret,R., and Wright,E.P.), 1977, 3, Heyden, London. Scott,D., Wear, 1977, 71. Scott,D., Proc. I n s t . Mech. Engrs., 1969, 183, (3L), 9. Scott,D., R o l l i n g Contact F a t i g u e , I n , Wear, (Scott,D., Ed.), T r e a t i s e on M a t e r i a l s Science and Technology, 1978, 13, 321, Academic Press, NY. Trent,E.M. Metal C u t t i n g , 1977, B u t t e r w o x h s , London. Summers-Smith,D., Wear, 1966, 425. Summers-Smith,D., Proc. T r i b o l o g y Conf., 1971, Paper C93/71, I n s t . Mech. Engrs. Scott,D., Smith,A.I., T a i t , J . and Tremain,G.R., Wear, 1975, 2, 293. Pratt,G.C., P l a s t i c Based Bearings i n " L u b r i c a t i o n and L u b r i c a n t s " , (E.R. B r a i t h w a i t e - Ed.), 1967, 377, E l s e v i e r , Amsterdam. Evan5,F.C. and Lancaster,J.K., The Wear o f Polymers, i n Wear, (Scott,D.), T r e a t i s e on M a t e r i a l s Science and Technology, 1978, 13, 85, Academic Press, NY. Scott,D., Wear, 1978, 283.
5,
9,
9,
48,
222
23 Wilson,R.W., Proc. 1st Euro, Tribology Congr., 1973, 165, Inst. Mech. Engrs., London. 24 Sharp,W.F., Wear, 1975, 32, 315. 25 Gregory,J.C., Tribology, 1970, 3, 73. 26 Gass,H. and Hintermann,H.E., SwTss Patent 452.205, 1968. 27 Hintermann,H.E. and Aubert,F., Proc. 1 s t Euro Tribology Congr., 1973, 207, Inst. Mech. Engrs., London. 28 Bunshah,R.F. and Juntz,R.S., J. Vac. Sci. Technol., 1972, 9, 1389. 29 Spalvins,T., Przbyszewski,J.S. and Buckley,D.H., NASA Tech. Note TN D - 3707, 1966. 30 Teer,D.G., Tribology, 1975, 1, 245. 211. 31 Sherbiney,M.A. and Halling,J., Wear, 1977,
3,
223
!)
SELECTION OF LUBRICANTS
A.R.
LANSDOWN, D i r e c t o r , Swansea T r i b o l o g y Centre, U.K.
9.1
I NTRODUCT I ON The v a r i e t y o f a v a i l a b l e l u b r i c a n t s i s enormous.
I f we simply c o n s i d e r
broad, b a s i c t y p es t h e r e a r e p r o b a b l y many hundreds:
i f we take i n t o account
minor d i f f e r e n c e s i n comp osi t i on and t h e v a r i o u s commercial brands, t h e r e a r e probably t e n s o f thousands.
To t h e n o n - s p e c i a l i s t
t h e problem o f proper l u b r i -
cant s e l e c t i o n can t h e r e f o r e seem v e r y c o n f u s i n g . For many a p p l i c a t i o n s t h e s e l e c t i o n o f a l u b r i c a n t i s i n f a c t n o t c r i t i c a l , and a wide v a r i e t y o f l u b r i c a n t s c o u l d work q u i t e s a t i s f a c t o r i l y .
For such
a p p l i c a t i o n s t h e o b j e c t o f l u b r i c a n t s e l e c t i o n should p r o b a b l y be t o ensure t h e lowest o v e r a l l l i f e c o s t f o r t h e system as a whole. For some a p p l i c a t i o n s , however, t h e s e l e c t i o n o f l u b r i c a n t may be v e r y c r i t i cal indeed, and t h e r e may be v e r y few o r even no l u b r i c a n t s capable o f e n s u r i n g satisfactory operation. Because t h e r e a r e thousands o f d i f f e r e n t l u b r i c a n t s a v a i l a b l e , and because many a p p l i c a t i o n s a r e n o t c r i t i c a l , t h e r e i s o f t e n a tendency t o leave l u b r i c a n t s e l e c t i o n t o a v e r y l a t e st ag e i n t h e de si gn process.
The r e s u l t may be t h a t a
design i s completed and a machine c o n s t r u c t e d f o r which no s u i t a b l e l u b r i c a n t e x i s t s , and t h e r e have been cases where v e r y expensive m o d i f i c a t i o n has been necessary t o r e s o l v e t h e problem o f l u b r i c a t i o n . One im p o r t a n t p r i n c i p l e i s t h e r e f o r e t h a t l u b r i c a n t requirements should always be c o n s id ere d a t an e a r l y st a ge i n d esign.
In o r d e r t o approach l u b r i c a n t s e l e c t i o n r e a I i s t i c a l l y , we should be c l e a r as t o what a r e t h e o b j e c t s o f l u b r i c a t i o n . The p r i m a r y o b j e c t o f l u b r i c a t i o n i s t o reduce f r i c t i o n o r wear, o r u s u a l l y both f r i c t i o n and wear. cant
.
There a r e i n (i)
T h i s i s t h e f a c t o r w hich d e f i n e s a m a t e r i a l as a l u b r i -
a d d i t i o n t h r e e secondary f u n c t i o n s o f a l u b r i c a n t : -
To a c t as a c o o l a n t .
I n some systems t h i s w i l l be a v i t a l f u n c t i o n o f
t h e l u b r i c a n t because f r i c t i o n a l o r process heat must be removed and no a l t e r n a t i v e c o o l i n g f l u i d can be used. (ii)
To remove wear d e b r i s o r o t h e r contaminants, o r t o p r e v e n t o t h e r contamin a n ts from e n t e r i n g t h e system.
( i i i ) To p r o t e c t m e t a l s a g a i n s t c o r r o s i o n .
There i s no good reason why a
224 l u b r i c a n t sho ul d be expected t o p r o v i d e such p r o t e c t i o n , b u t because mine r a l o i l s are very e f f e c t i v e corrosion preventives,
many designers have
come t o expect t he same o f a l l l u b r i c a n t s . The l u b r i c a n t w i l l o f t e n s a t i s f a c t o r i l y f u l f i l a l l these tasks, b u t where the a v a i l a b l e choice o f l u b r i c a n t s i s l i m i t e d ,
i t may be necessary t o choose t h e
l u b r i c a n t o n l y t o meet t h e f r i c t i o n and wear requirements, and t o use o t h e r techniques t o s o l v e t h e c o o l i n g , c o n t a m i n a t i o n and c o r r o s i o n problems.
9.2
SELECTING THE LUBRICANT TYPE L u b r i c a t i o n systems as such a r e o u t s i d e t h e scope of t h i s chapter, b u t the
problem o f l u b r i c a n t s e l e c t i o n cannot e n t i r e l y be separated from t h a t o f select i n g t h e l u b r i c a t i o n system. The most s t r a i g h t f o r w a r d way t o s e l e c t b o t h l u b r i c a n t and l u b r i c a t i o n system i s p r o b a b ly t o s t a r t w i t h t h e s i m p l e s t t e chn i que and t o progress from t h a t o n l y
a s f a r as i s necessary t o ensure s a t i s f a c t o r y o p e r a t i o n .
The s i m p l e s t technique
w i l l u s u a l l y have t h e l owest i n i t i a l c o s t , and w i l l o f t e n also be t h e most re1 i a b l e . The s i m p l e s t system c o n s i s t s o f a sma l l q u a n t i t y o f p l a i n m i n e r a l o i l i n p l a c e i n t h e l u b r i c a t e d component, w i t h o u t any f a c i l i t y f o r r e - l u b r i c a t i o n . Such a system w i l l cope w i t h a s u r p r i s i n g l y wide v a r i e t y o f mechanisms, i n c l u d i n g watches and c l o c k s and many o t h e r p r e c i s i o n instruments, d o o r - l o c k s and h in g e s , sewing-machines,
t y p e w r i t e r s , b i c y c l e s , r o l l e r skates, skate-boards,
and so on. I t ceases t o cope when t h e r e i s t o o much l oad o r speed o r heat o r d e b r i s , o r when t h e l i f e r e q u i r e d i s so l on g t h a t t h e o i l o x i d i s e s . evaporates, o r creeps away f r o m th e b e a r i n g surf ace s.
I t i s then necessary t o use a more s o p h i s t i c a t e d
o i l , a grease, a s o l i d l u b r i c a n t , o r sometimes even a gas l u b r i c a n t , o r t o use a more complex l u b r i c a t i o n system. T a b le 9.1 shows some o f t h e p o s s i b l e cho i ces o f a l t e r n a t i v e systems when t h e s i m p l e s t system i s no l on ge r adequate.
Some o f these choices a r e concerned w i t h
t h e l u b r i c a t i o n system r a t h e r than t h e c h o i c e o f l u b r i c a n t , b u t whatever l u b r i c a t i o n system i s used, a c h o i c e o f l u b r i c a n t i s needed. As t h e demands on a b e a r i n g i ncre ase , a p o i n t i s e v e n t u a l l y reached where a p l a i n m i n e r a l o i l i s no l o ng er adequate, and i t i s a t t h i s p o i n t t h a t t h e p r o blem o f l u b r i c a n t s e l e c t i o n b eg i ns.
A c o n v e n ie n t approach t o l u b r i c a n t s e l e c t i o n w i l l be t o c o n s i d e r f i r s t t h e p r o p e r t i e s which i n f l u e n c e t h e s e l e c t i o n o f d i f f e r e n t m i n e r a l o i l s , then t h e v a r i o u s o t h e r o i l s which can be used, and f i n a l l y t h e a l t e r n a t i v e s t o l u b r i c a t i n g o i l s , namely greases, s o l i d l u b r i c a n t s and gases. I t may be u s e f u l t o r e f e r t o F i g u r e 1, whi ch i n d i c a t e s broad l i m i t s o f speed
and l o a d w i t h i n w hi ch d i f f e r e n t c l a s s e s o f l u b r i c a n t can be used.
SPEED ( f t / m i n ]
1,ooo.000
2
10
boo0
100
10.000
100.000
100.000
2.
t
3.
- looeooo
For 1 iqiiid Iiihricnnts t h c rmlnmcntnl pronerty is v i s c o s i t y , :ind t h i s f i m i r c L I S S I U ~ C St h a t t h e corrert v i s c o s i t y h;is hecn chosen. 1:xtcrnalIy nressiiriscd hcnrinps can he uscd ovcr a wide rnnp,c of snecd :ind lo:1d.
10.000
>
.c _
E
a
t
-
N
N
\
10.000 1,000
Y
-ea
1,000 100
100
10
100
1.000
10.000
rw,ooo
14.5
r,ooo,ooo
SPEED AT BEARING CONTACT lrnm/s) --C
Fig.1
Speed/load limitations f o r d i f f e r e n t types o f lubricant. N
tu cn
226 Table 9.1 P o s s i b l e ch oi ce s when a small q u a n t i t y o f p l a i n m i n e r a l o i l i n p l a c e i s no lonqer adequate. Too much l oa d
More viscous o i l Grease Extreme p r e s s u r e o i l Extreme p r e s s u r e grease Externally pressurised l u b r i c a t i o n Sol i d l u b r i c a n t
Too much speed (which may l ea d t o t o o much he at )
Greater q u a n t i t y o f o i l Less viscous o i l O i l c i r c u l a t i o n system Gas l u b r i c a t i o n
Too much heat
O i l w i t h anti-oxidant More viscous o i l Greater q u a n t i t y o f o i l O i 1 c i r c u l a t i o n system Synthetic o i l Sol i d l u b r i c a n t
Too much d e b r i s
Greater q u a n t i t y o f o i l C i r c u l a t i o n system w i t h f i l t r a t i o n
Contamination
O i l c i r c u l a t i o n system Grease Solid lubricant
Long 1 i f e r e q u i r e d
O i l with anti-oxidant More viscous o i l Great q u a n t i t y o f o i l R e l u b r i c a t i o n system Synthetic o i l Grease Sol i d l u b r i c a n t
PROPERTIES OF MINERAL OILS
9.3
9.3.1
Viscosity
V i s c o s i t y i s t h e most i m p o r t a n t s i n g l e p r o p e r t y o f a l u b r i c a t i n g o i l , s i n c e i t i s th e s o l e p r o p e r t y which determines whether t h e o i l i n a b e a r i n g a t a
s p e c i f i c lo a d and speed w i l l g i v e f u l l f l u i d f i l m s e p a r a t i o n o f t h e b e a r i n g s u r fa c e s . V i s c o s i t y may be expressed i n any one o f a number o f d i f f e r e n t u n i t s .
Pro-
ba b ly t h e most commonly used i n E n gl i sh -spe aking c o u n t r i e s i s the c e n t i s t o k e ( c S t ) which p r o p e r l y d e s c r i b e s K i n ema t i c V i s c o s i t y , and i s d i r e c t l y measured by t h e v a r i o u s s t a n dard I n s t i t u t e o f Petroleum, ASTM, B r i t i s h Standard and IS0 t e s t methods.
The A b sol u t e V i s c o s i t y i s commonly expressed i n terms o f t h e
c e n t i p o i s e (cP) and i s t h e v i s c o s i t y used i n t h e v a r i o u s e n g i n e e r i n g c a l c u l a t i o n s such as e qu at i on s ( 1 ) t o ( 3 ) .
227 Each o f these u n i t s has i t s S t e q u i v a l e n t , b u t t h e i n t r o d u c t i o n o f t h e S I U n i t s has been t he s u b j e c t of c o n s i d e r a b l e d i s p u t e . c e n t i s t o k e i s t h e m i l l i m e t r e z pe r second (mm2 s - ' )
The SI e q u i v a l e n t o f t h e and t h e S I e q u i v a l e n t o f t h e
c e n t i p o i s e i s t h e m i l l inewton second p e r metre2 o r m i l 1 i p a s c a l second ( m N s / m Z ) . There a r e a l s o t h r e e o t h e r u n i t s wh i ch a r e s t i l l w i d e l y used, a l t h o u g h c u r r e n t p o l i c y i s t o d i s c o n t i n u e them i n f a v o u r o f k i n e m a t i c v i s c o s i t y i n c e n t i stokes.
They a r e Redwood Seconds, used i n B r i t a i n , Saybolt Seconds (SUS)
i n the
United S ta t e s , and degrees E n gl e r, used i n Germany and o t h e r p a r t s o f c o n t i n e n t a l Europe. There a r e s e v era l e q u a t i o n s which r e l a t e t h e o i l f i T m t h i c k n e s s t o t h e o i l For g en era l hydrodynamic l u b r i c a t i o n t h e Reynolds Equation [ l ]
viscosity. applies, ax
*
[
h3
ax1
+
aY
I
h3
3
aY1
f o r elastohydrodynamic
=
6pU
ah
+ 6ph
+
12pV
(1)
l u b r i c a t i o n w i t h c y l i n d r i c a l elements, as i n r o l l e r
bearings and many gear c o n f i g u r a t i o n s , an e q u a t i o n o f t h e Dowson-Higginson type,
121 a p p l i e s ,
"0.7 hmin/R
=
2.65
G0.54
w0.03
w h i l e f o r e la s t o hyd rod yna mi c l u b r i c a t i o n w i t h s p h e r i c a l elements, as i n b a l l bearings, t h e Archard-Cowking Equation
[3]
applies,
where U i s p r o p o r t i o n a l t o v i s c o s i t y .
10-1
z P
I-
uLL
u.
lL 0
Iz W -
2
U W
8
10-2
10.~
10-8 PV/ p
Fig.2
T y p i c a l S t r i b e c k curve
DIMENSIONLESS
228 F i g u r e 2 , sometimes c a l l e d a S t r i b e c k curve, shows how t h e v i s c o s i t y o f a l u b r i c a n t a f f e c t s t h e l u b r i c a t i o n mode and t h e c o e f f i c i e n t o f f r i c t i o n i n a bearing . By t h e use of t h e a p p r o p r i a t e e q u a t i o n t h e o i l v i s c o s i t y r e q u i r e d f o r any b e a r i n g can be c a l c u l a t e d , b u t i f t h i s i s done f o r every b e a r i n g i n every machi n e i n a f a c t o r y t h e r e s u l t w i l l p r o b a b l y be a l i s t of several hundred v i s c o s i ties.
The number o f a v a i l a b l e m i n e r a l o i l s i s so g r e a t t h a t i t m i g h t w e l l be
p o s s i b l e t o o b t a i n o i l s w i t h eve ry v i s c o s i t y r e q u i r e d , b u t t h e r e a r e some very good reasons f o r n o t d o i n g so.
I f a l l t h e b e a r i n g s i n a complex machine can use t h e same
(i)
u b r i c a n t , then
a s i n g l e o i l r e s e r v o i r and c i r c u l a t i o n system can be used, and t h i s w i l l g e n e r a l l y be cheaper and more r e l i a b l e . (ii)
A l a r g e number o f d i f f e r e n t l u b r i c a n t s causes s t o r a g e prob ems.
( i i i ) The g r e a t e r t h e number o f l u b r i c a n t s i n use i n one p l a n t , t h e g r e a t e r i s t h e p r o b a b i l i t y o f t h e wrong one b e i n g used. (iv)
Most b e a r i n g s a r e r e q u i r e d t o o p e r a t e over a range o f temperature,
so the
v i s c o s i t y o f t h e o i l w i l l o n l y be a t t h e optimum v a l u e f o r a p a r t o f the o p e r a t i n g time. F o r t u n a t e l y t h e v i s c o s i t y req ui re men t i s o f t e n n o t c r i t i c a l .
A s l o n g as the
v i s c o s i t y i s h i g h enough t o ensure t h e r e q u i r e d o i l f i l m t h i c k n e s s , harm i n g o in g t o a s l i g h t l y h i g h e r v i s c o s i t y . s i t y i s p a r t l y s e l f -co mpe nsa t i n g.
t h e r e i s no
I n any case an increase i n v i s c o -
The e f f e c t of i n c r e a s i n g v i s c o s i t y i s t o
i n c r e a s e v is c o u s f r i c t i o n , whi ch i n crea ses t h e power d i s s i p a t e d i n t h e bearing. The excess power i s con vert ed t o h ea t , t h e h eat r a i s e s t h e o i l temperature, and t h e h i g h e r t e m p erat ure reduces t h e o i l v i s c o s i t y .
So t h e end r e s u l t of u s i n g a
more v is c o u s o i l i s t h a t t h e power consumption and t h e temperature s t a b i l i s e a t a s l i g h t l y higher level. I t i s t h e r e f o r e p o s s i b l e t o use a sma l l number o f v i s c o s i t y grades t o f i l l a wide range o f o i l requirements,
and i n f a c t many n a t i o n a l and i n t e r n a t i o n a l
standards ( i n c l u d i n g B r i t i s h Standard 4231 " V i s c o s i t y t r i a l L i q u i d Lubricants")
t h e v i s c o s i t y o f each o i l i n c e n t i s t o k e s a t 4OoC pre c e d in g grade.
C l a s s i f i c a t i o n f o r Indus-
r e s t r i c t t h e number o f l u b r i c a n t s i n such a way t h a t i s 50% h i g h e r than t h a t o f t h e
T h i s g i v e s o n l y e i g h t e e n grades t o cover t h e whole range from
2 c S t t o 1500 c S t . So t h e f i r s t t h i n g t o do i n m i n e r a l o i l s e l e c t i o n i s t o c a l c u l a t e t h e lowest
v i s c o s i t y which w i l l ensure f u l l f l u i d f i l m l u b r i c a t i o n o f a l l t h e b e a r i n g s i n t h e system, and t he n t o i d e n t i f y t h e n e x t h i g h e r standard v i s c o s i t y grade. The v i s c o s i t y o f an o i l i s always quoted a t a s t a t e d temperature, u s u a l l y O°F,
7OoF, 100°F
o r 130°F,
b u t i n c r e a s i n g l y 4OoC.
These temperatures w i l l
229 r a r e l y be t h o s e a t wh i ch yo ur b e a r i n g s a r e i n tended t o operate, so t h e second i mp o r t a n t p r o p e r t y t o c o n s i d e r i s t h e v a r i a t i o n o f v i s c o s i t y w i t h temperature.
9.3.2
V i scos i ty/Tempe r a t u r e Re1a t ionsh i p
For a l l l i q u i d s t h e v i s c o s i t y decreases as t h e temperature increases, b u t t h e r a t e o f decrease v a r i e s c o n s i d e r a b l y f ro m
one l i q u i d t o a n o t h e r .
I t follows
t h a t i f we p l o t graphs o f o i l v i s c o s i t y a g a i n s t temperature, w hich g i v e s somet h i n g approaching s t r a i g h t l i n e s , t h e s l o p e o f t h e l i n e s i s d i f f e r e n t f o r d i f f erent o i l s .
F i g u r e 3 shows some t y p i c a l v i s c o s i t y / t e m p e r a t u r e
relationships
for different oils. Standard c u r v es a r e r e a d i l y a v a i l a b l e showing the v i s c o s i t y / t e m p e r a
ur e
r e l a t i o n s h i p f o r d i f f e r e n t o i l s , so t h a t we can c o n v e r t o u r r e q u i r e d v scos i t y t o t h e v i s c o s i t y a t a st an da rd r e f e r e n c e t e mperature, p r o v i d e d t h a t we know which l i n e ( i . e .
whi ch sl o pe ) r e l a t e s t o t h e o i l under c o n s i d e r a t i o n .
The p r o p e r t y whi ch i s most w i d e l y used t o d e s c r i b e t h e v i s c o s i t y / t e m p e r a t u r e beh a v io u r o f an o i l i s t h e V i s c o s i t y Index (V.1).
-
T h i s index r e l a t e s t h e
change o f v i s c o s i t y w i t h t emp era t u re t o two a r b i t r a r y o i l s , one based on a Pe n n s y lv a n ia Grude o i l (V . I .
0) and one on a G u l f Coast o i l (V.1.
-
100).
The v i s c o s i t y index o f an unknown o i l can be c a l c u l a t e d from t h e measured v i s c o s i t i e s a t 4OoC and 100°C by means of e q u a t i o n
V.I.
100 ( L - u ) / ( L - H )
where U i s t h e v i s c o s i t y a t 4OoC o f t h e o i l sample i n c e n t i s t o k e s , L i s t h e v i s c o s i t y a t 4OoC o f an o i l o f 0 v i s c o s i t y index having t h e same v i s c o s i t y a t 100°C as t h e o i l sample, and H i s t h e v i s c o s i t y a t 4OoC o f an o i l of 100 v i s c o s i t y in d e x h a v i n g t h e same v i s c o s i t y a t 100°C as t h e o i l sample.
Tables o f
v al u e s f o r L and H a r e c o n t a i n e d i n t h e st an dard I n s t i t u t e o f Petroleum and ASTM t e s t methods. Briefly,
t h e V i s c o s i t y Index w i l l be low i f t h e v i s c o s i t y changes r a p i d l y
w i t h temperature, and h i g h i f v i s c o s i t y i s l e s s a f f e c t e d by temperature. Standard D.S.4231 The h i g h e s t V . I . synthetic o i l s
British
r e f e r s t o m i n e r a l o i l s h avi ng V i s c o s i t y I n d i c e s of 0, 50 o r 95. f o r a s i m p l e m i n e r a l o i l i s s l i g h t l y over 100, w h i l e c e r t a i n
have V . I .
Ge n e r a lly , a V . I .
g r e a t e r t ha n 200.
h i g h e r t h an 100 i s o n l y o b t a i n e d w i t h a m i n e r a l o i l by t h e
use o f a p o l y m e r i c a d d i t i v e c a l l e d a V i s c o s i t y Index Improver.
V.I.Improvers
a r e used i n m u l t i - g r a d e e ng i ne o i l s and i n h y d r a u l i c f l u i d s w hich a r e r e q u i r e d t o o p e r a t e o v e r a wi d e t emp era t u re range. Most b e a r i n g s have t o o p e r a t e o v e r a range of temperatures, w i d t h o f t h i s range which determines what V . I .
i s required.
and i t i s t h e
A V.I.
o f 0 may be
a c c e p t a b l e i f t h e t emp era t u re range i s v e r y narrow, o r i f a l a r g e change i n v i s c o s i t y i s acceptable.
On t h e o t h e r hand, a V . I .
o f 160 o r more i s s p e c i f i e d
230
Fig.3
Viscosity/temperature characteristics of various oils
231 f o r an a i r c r a f t h y d r a u l i c f l u i d which i s r e q u i r e d t o o p e r a t e f r o m -4OOC t o +15OoC. The e f f e c t i v e n e s s o f a V.I.Improver
i s a f f e c t e d by t h e shear r a t e , t h e r a t e
a t which t h e o i l has t o pass through c o n f i n e d spaces, and a t h i g h shear r a t e i n a high-speed b e a r i n g t h e e f f e c t i v e v i s c o s i t y o f a V . I .
improved o i l may be
l i t t l e o r no d i f f e r e n t from t h a t o f t h e base o 1 . Having chosen t h e r e q u i r e d V i s c o s i t y Index, t h e v i s c o s i t y o f t h e s e l e c t e d o i l a t the r e f e r e n c e temperature can be worked o u t by means o f a graph such as Figure 3 o r a c h a r t such as t h e one i n B.S.423
9.3.3
Viscosity/Pressure Relationship
Before l e a v i n g t h e s u b j e c t o f v i s c o s i t y ,
i t i s perhaps d e s i r a b l e f o r complete-
ness t o mention t h e v a r i a t i o n o f v i s c o s i t y w i t h p r e s s u r e . l u b r i c a t i n g o i l s increases as t h e p r e s s u r e i s increased.
The v i s c o s i t y o f a l l I n p r a c t i c e the r a t e
o f increase i s v e r y low, and i n p l a i n hydrodynamic o r e x t e r n a l l y p r e s s u r i s e d bearings i t can s a f e l y be n e g l e c t e d . I n elastohydrodynamic l u b r i c a t i o n , however, t h e p r e s s u r e generated i n t h e l u b r i c a n t can be s u b s t a n t i a l .
The i n c r e a s e i n l u b r i c a n t v i s c o s i t y i s c o r r e s -
pondingly h i g h , and i s a major f a c t o r i n e n s u r i n g s u c c e s s f u l l u b r i c a t i o n o f gears and r o l l i n g b e a r i n g s . I t i s rare t o specify o r quote a pressure c o e f f i c i e n t o f v i s c o s i t y f o r a
l u b r i c a t i n g o i l , and t h i s i s n o r m a l l y o n l y done f o r v e r y c r i t i c a l s i t u a t i o n s such as i n c e r t a i n s p a c e c r a f t a p p l i c a t i o n s .
9.3.4
A n t i -Wear,
Extreme Pressure and A n t i -Fr ic t i o n P r o p e r t i e s
I n t h e o r y , from t h e p o i n t o f view o f p u r e p h y s i c a l l u b r i c a t i o n , we have completely s p e c i f i e d t h e o i l when we have s p e c i f i e d t h e v i s c o s i t y , t h e V . I . perhaps t h e p r e s s u r e c o e f f i c i e n t o f v i s c o s i t y .
and
I n p r a c t i c e t h i s i s n o t always
so, and i t i s o f t e n necessary t o s p e c i f y t h e a n t i - w e a r ,
extreme p r e s s u r e o r
a n t i - f r i c t i o n properties o f the o i l . R e f e r r i n g a g a i n t o F i g u r e 2, t h e boundary and mixed l u b r i c a t i o n r e g i o n s represent s i t u a t i o n s i n which t h e b e a r i n g s u r f a c e s a r e n o t c o m p l e t e l y separated by a f i l m o f l u b r i c a n t , b u t e x p e r i e n c e some degree o f s o l i d - t o - s o l i d Even i n l i g h t l y - l o a d e d b e a r i n g s s o l i d - t o - s o l i d
contact.
c o n t a c t can occur a t low
speed when a b e a r i n g i s s t a r t i n g o r s t o p p i n g , and t h i s i s p a r t i c u l a r l y i m p o r t a n t i f a mechanism i s i n t e n d e d t o o p e r a t e i n t e r m i t t e n t l y .
The r e s i d u a l o i l f i l m
w i l l o f t e n g i v e some p r o t e c t i o n under these c o n d i t i o n s , b u t g r e a t e r p r o t e c t i o n
can be o b t a i n e d by t h e use
o f an a n t i - w e a r a d d i t i v e .
T h i s i s u s u a l l y an organic
a c i d such as s t e a r i c a c i d o r a n a t u r a l o i l such as rapeseed o i l , b u t s y n t h e t i c o r g a n i c phosphorus compounds such as t r i - x y l y l phosphate a r e a l s o e f f e c t i v e . I n h e a v i l y - l o a d e d b e a r i n g s o r i n t e r m i t t e n t r o l l i n g b e a r i n g s o r c e r t a i n types o f gear, a more a c t i v e t y p e o f a d d i t i v e known as an Extreme Pressure (EP)
232 a d d i t i v e may be needed.
EP a d d i t i v e s a r e u s u a l l y r e a c t i v e s y n t h e t i c o r g a n i c
compounds c o n t a i n i n g phosphorus, s u l p h u r o r c h l o r i n e , a l t h o u g h lead naphthenate i s s t i l l used i n c e r t a i n gear o i l s , and molybdenum d i s u l p h i d e i s o c c a s i o n a l l y used. EP p r o p e r t i e s a r e assessed i n s e v e r a l d i f f e r e n t ways, b u t t h e most w i d e l y
used t e s t methods a r e p rob ab l y t h e Timken and Four-Ball machines. measured by means o f t h e F ou r-Ba l l Machine,
One c r i t e r i o n ,
i s t h e Mean H e r t z load, and t h i s can
v a r y from 30 f o r a p l a i n m i n e r a l o i l t o 85 f o r a pow erful EP gear o i l . One extreme r equ i reme nt f o r EP p r o p e r t i e s i s i n c e r t a i n types o f metalw orking o p e r a t i o n , and t h e most po we rf u l c h l o r i n e and s u l p h u r - c o n t a i n i n g used i n t h e l u b r i c a n t s f o r such a p p l i c a t i o n s .
additives are
Pow erful EP a d d i t i v e s can be
c o r r o s i v e , and s ho ul d t h e r e f o r e n o t be used where t h e y a r e n o t necessary. A n a t u r a l o i l may be added t o an o i l t o decrease t h e f r i c t i o n i n boundary
l u b r i c a t i o n , and thus reduce t h e power consumption and t h e heat generated.
This
may be p a r t i c u l a r l y i m p o r t a n t i n a mechanism which i s s e n s i t i v e t o f r i c t i o n , The n a t u r a l o i l i s then sometimes c a l l e d a l u b r i c i t y addi-
such as a worm g ear.
t i v e , b u t t h e t e rm i s i l l - d e f i n e d and g e n e r a l l y n o t recommended. We have now c onsi d ere d a l l t h e p r o p e r t i e s r e q u i r e d t o g i v e adequate i n i t i a l
1 u b r i c a t io n , b u t t h e r e a r e two o t h e r c o n s i d e r a t i o n s i n e n s u r i n g continued s a t i s f a c t o r y l u b r c a t i on.
9.3.5
Stab l i t y
An o i l w i 1 decompose c h e m i c a l l y i n s e r v i c e because o f e i t h e r heat (thermal decomposition) o r a co mbi n at i on o f h ea t and oxygen ( o x i d a t i o n ) .
Thermal s t a b i -
l i t y can be improved i n man uf act uri n g i f t h e more u n s t a b l e components o f t h e o i l can be removed, b u t o t h e r w i s e t h e o n l y s o l u t i o n i s t o I n g e n e r a l, however, t u r e s than o x i d a t i o n ,
thermal decomposition takes p l a c e a t much h i g h e r temperaand i t i s t h e o x i d a t i v e s t a b i l i t y w hich determines t h e
maximum t e m p e r a t ure a t wh i ch t h e o i l can be used. i v e l y reduced by t h e use o f a n t i - o x i d a n t s . presence o f oxygen,
keep t h e temperature down.
O x i d a t i o n can be v e r y e f f e c t -
I f t h e temperature i s h i g h i n t h e
i t may t h e r e f o r e be d e s i r a b l e t o use an o i l c o n t a i n i n g an
anti-oxidant. F i g u r e 4 shows t h e r e l a t i o n s h i p between o i l l i f e and temperature f o r t y p i c a l mineral o i l s . Many o f t h e a d d i t i v e s used i n o i l s w i l l a l s o decompose, and t h e u s e f u l l i f e o f t h e o i l may depend on t h e d e p l e t i o n o f such a d d i t i v e s . used up i n p r e v e n t i n g o x i d a t i o n ,
Anti-oxidants are
so t h a t where h i g h l y o x i d a t i v e c o n d i t i o n s occur
th e a n t i - o x i d a n t s may be r a p i d l y exhausted and leave t h e o i l unprotected.
600
1
500
LOO
I
- 300 W [L
200
2W a
5
100
k
L L O W E R TEMPERATLIRE LIMIT IMPOSED BY 'POUR POINT WHICH SOURCE, VISCOSITY, TREATMENT AND ADDITIVES]
- 100 1
I 2
I
l
l
3
4
5
I 10
20
I l l 30 LO 50
100
LIFE (HOURS) Fig.4
200 ,300LOO 500
VARIES WITH OIL,,+ 1,000
2,wO 3,000 L,OOo5.000 10,000
Temperature/l i f e 1 i m i t s f o r m i n e r a l o i l s
N
w
W
234 9.3.6
Contaminants
The q u a l i t y o f a l u b r i c a n t w i l l a l s o d e t e r i o r a t e i n use because o f contamination.
The contaminants may be s o l i d p a r t i c l e s o f wear d e b r i s o r s o l i d decompo-
s i t i o n p r o d u c t s from a f u e l o r t h e l u b r i c a n t i t s e l f , or they may be d i s s o l v e d a c i d s produced by o x i d a t i o n , o r w a t e r from c o n d e n s a t i o n o r f u e l combustion. They may a l s o e n t e r t h e system f r o m o u t s i d e ,
such as when a t m o s p h e r i c d u s t o r r a i n
e n t e r an o i l f i l l e r o p e n i n g . Some o f t h e s e c o n t a m i n a n t s can be removed by f i l t r a t i o n and o t h e r s w i l l s e t t l e o u t i n t h e r e s e r v o i r , b u t i t i s sometimes necessary t o use a d e t e r g e n t o r d i s p e r s a n t a d d i t i v e t o keep t h e contaminants condition.
i n a r e l a t i v e l y harmless d i s p e r s e d
U l t i m a t e l y t h e o i l change p e r i o d i n some systems may be determined
by t h e e x t e n t o f c o n t a m i n a t i o n .
9.4
CHOICE OF
9.4.1
BASE OIL
L i m i t a t i o n s o f Mineral O i l s
M i n e r a l o i l s a r e t h e most w i d e l y used l u b r i c a t i n g o i l s because they a r e t h e cheapest, and f o r many a p p l i c a t i o n s t h e y a r e a l s o t h e b e s t a v a i l a b l e .
The most
i m p o r t a n t l i m i t a t i o n s on t h e i r use a r i s e f r o m h i g h t e m p e r a t u r e s , f l a m m a b i l i t y and c o m p a t i b i l i t y problems, and t h e r e a r e a number o f o t h e r base o i l s w h i c h can t h e n be used as a l t e r n a t i v e s . T a b l e 9.2 shows some o f t h e i m p o r t a n t f a c t o r s i n t h e s e l e c t i o n o f a l t e r n a t i v e base o i l s .
9.4.2
H i g h Temperatures
More and more i n d u s t r i e s a r e r e q u i r i n g l u b r i c a n t s t o o p e r a t e a t temperatures t o o h i g h f o r m i n e r a l o i l s , and a l t e r n a t i v e f l u i d s a r e t h e r e f o r e becoming more and more w i d e l y used.
E s t e r s were f i r s t developed f o r a i r c r a f t j e t e n g i n e s , and
a r e now p r o b a b l y t h e most common l u b r i c a n t i n a p p l i c a t i o n s w h i c h a r e t o o h o t f o r mineral o i l s .
At h i g h e r t e m p e r a t u r e s v a r i o u s t y p e s o f s i l i c o n e can be used, and
f o r even h i g h e r t e m p e r a t u r e s p o l y p h e n y l e t h e r s a r e a v a i l a b l e , b u t b o t h s i l i c o n e s and p o l y p h e n y l e t h e r s have some s e r i o u s d i s a d v a n t a g e s i n comparison w i t h m i n e r a l o i l s and e s t e r s . Figure oils.
5 shows t h e t e m p e r a t u r e / l i f e l i m i t s f o r s e v e r a l s y n t h e t i c l u b r i c a t i n g
These l i m i t s depend on t h e a c c e p t a b l e v i s c o s i t y ,
t h e degree o f oxygen
access and t h e e x t e n t t o which d e t e r i o r a t i o n i s a c c e p t a b l e .
9.4.3
Flammability
Some i n d u s t r i e s , such as a v i a t i o n and c o a l m i n i n g , have l o n g been concerned w i t h the
f l a m m a b i l i t y o f l u b r i c a n t s and h y d r a u l i c f l u i d s .
With increasing
p l a n t i n t e g r a t i o n and l u b r i c a n t system c a p a c i t i e s , t h i s concern i s now e x t e n d i n g
T a b l e 9.2
Fluid
I m p o r t a n t p r o p e r t i e s i n s e l e c t i n g d i f f e r e n t base o i l s .
property
D i - e s t e r Neopentyl polyol (Conplex) Esters
Maximum temperature
250
300
I20
220
320
305
260
450
200
Maximum temperature i n presence of "..,~_..("C)-, oxyygjen
210
240
120
ICO
250
230
200
320
I5O
Minimum temperature due t o increase j n viscositv P C l
-35
-65
-55
-50
-30
-65
-20
0
pC;bsence
Typical Phosphate Ester
Typical Methyl Silicone
Typical Phenyl Hethyl Silicone
Chlorinated Phenyl Hethyl S i 1icone
Polyglycol (inhibited)
Polyphenyl Ether
Nineral
Remarks
Oil
comparison (for 1 .__ For e s t e r s t h i s temperature w i l l be h i g h e r i n the absence o f n e t a l s
o f oxygen
Density ( g l m l )
0.91
1.01
1.12
0.97
1.06
1.04
1.02
1.19
V i s c o s i t y index
145
140
0
200
175
195
160
-60
F l a s h p o i n t (OC)
230
255
200
310
290
270
180
275
to -50
This l i m i t i s a r b i t r a v y . I t w i l l be h i g h e r i f oxygen c o n c e n t r a t i o n i s low and l i f e i s s h o r t T h i s l i n i t depends on the power a v a i l a b l e t o o v e r c m the e f f e c t o f increased viscosity
0.88 to 140
I 5 O to
-____ A h i g h v i s c o s i t y index i s d e s i r a b l e
-
Above t h i s temperature t h e vapour o f the f l u i d m a y b e i g n i t e d by an open _. f l a w ~
Spontaneous i g n i t i o n temperature
Low
Medium
Very h i g h
High
Boundary l u b r i c a t i o n
Good
Good
Very good
High
Very h i g h
Medium
High
LOW
Fair but
F a i r but
Good
Very good
Fair
Good
poor for
poor f o r s t e e l on
This r e f e r s p r i m a r i l y t o anti-wear p r o p e r t i e s when some metal c o n t a c t i s occurring
Non-toxic
Non-tox?c
Non-toxic
Believed t o be low
Believed t o be low
Slicht
~
(None f o r very h i g h temvera-
Nitrile
s t e e l on steel
Toxicity
Slight
Slight
Sane
steel
toxicity S u i t a b l e rubbers
tiitrile. s? 1icone
Cost ( r e l a t i v e t o
mneral
011)
Silicone
Butyl, EPR
Neoprene, v i ton
tieoprene, viton
Viton, fluorosilicone
Nitrile
Corrosive t o some nanferrous metals when h o t
Enhances covosion
lioncorrosive
tioncorrosive
Corrosive in presence of water t o ferrous mta1s
Noncowosive
~
be ~
~
~
~
~~
E f f e c t on metals
Above t h i s temperature the f l u i d may i g n i t e w i t h o u t any f l a r e b e i n g present
Slightly corrosive to nonferrous metals
5
10
n presecce o f water
10
25
5C
60
5
Noncorrosive
250
Noncorrosive when pure
lhese are rough apprOXIPlatlOnS. and vary d i t h q u a l i t y and supp1:r p o s i t i o n .
N
w
u1
N
w
0)
I 1 -100
'
L-POUR
I
I
I
l
l
I
I
1
1
POINT LIMIT FOR SILICONES AND ESTERS-'
Fig.5
Temperature/life limits for some synthetic oils.
-
I
237 t o most i n d u s t r i e s . The b e s t f i r e - r e s i s t a n t
l i q u i d i s w a t e r , b u t i t i s a poor l u b r i c a n t and i n
i t s r e l a t i v e l y u n m o d i f i e d form i s o n l y used as a h y d r a u l i c f l u i d . i t s l u b r i c a t i n g performance f o r m e t a l - c u t t i n g , used.
To
mprove
d e t e r g e n t s and EP a d d i t ves a r e
For more g e n e r a l l u b r i c a n t use, v a r i o u s t e c h n i q u e s a r e employed t o i n -
crease i t s v i s c o s i t y , i n c l u d i n g t h i c k e n i n g w i t h c l a y s and n a t u r a l o r g a n i c substances, m i x i n g w i t h p o l y g l y c o l s , and e m u l s i f i c a t i o n w i t h m i n e r a l o i l . Other f i r e - r e s i s t a n t
l i q u i d s w i t h good l u b r i c a t i n g p r o p e r t i e s i n c l u d e phos-
phate e s t e r s , c h l o r i n a t e d hydrocarbons and c h l o r i n a t e d s i l i c o n e s , b u t these a l l have c o r r e s p o n d i n g disadvantages.
9.4.4
Compatibility
Problems o f i n c o m p a t i b i l i t y can a r i s e w i t h any l i q u i d .
Mineral o i l s are
i n c o m p a t i b l e w i t h n a t u r a l rubber s e a l s and hoses, e s t e r s a r e i n c o m p a t i b l e w i t h n i t r i l e s , and phosphate e s t e r s a r e i n c o m p a t i b l e w i t h many d i f f e r e n t r u b b e r s . M i n e r a l o i l s and most o t h e r c o m b u s t i b l e l i q u i d s a r e i n c o m p a t i b l e w i t h h i g h p r e s s u r e oxygen, and c e r t a i n s p e c i a l f l u o r i n e - c o n t a i n i n g l u b r i c a n t s a r e p r e f e r r e d f o r b r e a t h i n g oxygen systems.
M i n e r a l o i l s a r e u n s a t i s f a c t o r y i n con-
t a c t w i t h r e d - h o t s t e e l s because t h e y produce c a r b u r i s a t i o n , and rapeseed o i l may be used t o a v o i d t h i s problem. Most c o m p a t i b i l i t y problems can be overcome by c a r e f u l s e l e c t i o n o f s u i t a b l e base o i l s and a d d i t i v e s , b u t i n some i n d u s t r i e s such as f o o d s t u f f s ,
pharmaceu-
t i c a l s and chemicals, even t h e s m a l l e s t l e a k o f any c o n v e n t i o n a l l u b r i c a n t may be unacceptable. lubricant.
I n such a case i t may be p o s s i b l e t o use a process f l u i d as a
For example, i n sugar r e f i n i n g t h e h i g h v i s c o s i t y o f syrups and
molasses enables them t o p r o v i d e e f f e c t i v e l u b r i c a t i o n o f b e a r i n g s .
Where t h e
a v a i l a b l e process f l u i d s have i n s u f f i c i e n t v i s c o s i t y f o r e f f e c t i v e hydrodynamic l u b r i c a t i o n , e x t e r n a l p r e s s u r i s a t i o n can be used.
Such use o f process f l u i d s
may sometimes be p r e f e r r e d t o c o n v e n t i o n a l l u b r i c a t i o n , because i t may e l i m i n a t e t h e need f o r s e a l s and g l a n d s .
9.5
GREASES
A l u b r i c a t i n g grease i s a l i q u i d l u b r i c a n t which has been t h ckened t o a semi-solid consistency.
The base o i l may be m i n e r a l o i l , e s t e r
silicone, or
one o f t h e o t h e r s y n t h e t i c o i l s , and many o f t h e a d d i t i v e s used i n l u b r i c a t i n g o i l s a r e e q u a l l y e f f e c t i v e i n greases.
I n a d d i t i o n , s o l i d l u b r c a n t s such as
g r a p h i t e or molybdenum d i s u l p h i d e can c o n f e r i m p o r t a n t advantages i n greases. I t has been s a i d t h a t more i n d u s t r i a l b e a r i n g s a r e l u b r i c a t e d by grease than
by o i l .
To understand t h e reason f o r t h i s we s h o u l d c o n s i d e r a g a i n t h e s i m p l e s t
f o r m o f l u b r i c a t i o n d e s c r i b e d i n S e c t i o n 9.2,
namely t h e use o f a small quan-
t i t y o f p l a i n mineral o i l i n place i n the bearing.
There a r e two disadvantages
600
500
100
300
2 00
100
WITH UNLIMITED OXYGEN I 0
- 100 ~~
1
2
3
1 5
10
20
30 10 50
100
LIFE (HOURS1
Fig.6
Temperature/life
200 300 100500
1.000
l i m i t s f o r greases.
2.000 3.0001.wO
10,000
239 o f t h a t system wh i c h a r e due o n l y t o t h e f l u i d n a t u r e o f t h e o i l .
The f i r s t i s
t h a t i t i s d i f f i c u l t t o r e t a i n t h e o i l i n t h e b e a r i n g f o r any l e n g t h o f tim e . The second i s t h a t i t i s d i f f i c u l t t o s t o p d i r t o r o t h e r c o n ta m in a n ts g e t t i n g i n t o the bearing. The use o f a g r e a s e may overcome b o t h o f t h e s e problems w h i l e r e t a i n i n g t h e s i m p l i c i t y and economy o f t h e system.
The grease i n a b e a r i n g o f t e n d i s t r i b u t e s
i t s e l f q u i t e q u i c k l y i n such a way t h a t a v e r y s m a ll p r o p o r t i o n i s a c t i v e l y l u b r i c a t i n g the bearing, while the bulk i s outside the faces,
c r i t i c a l bearing sur-
f o r m i n g a s e a l a g a i n s t loss o f l u b r i c a n t o r i n g r e s s o f c o n ta m in a n ts .
The c h i e f l i m i t a t i o n on t h e use o f g r e a s e s i s t h e l a c k o f c o o l a n t f l o w t o remove s u r p l u s h e a t when t h e b e a r i n g i s l a r g e , f a s t o r h e a v i l y loaded, b u t t h e r e a r e greases w h i c h w i l l o p e r a t e temperature/life
9.6
a t t e m p e r a t u r e s as h i g h as 250°C.
F i g u r e 6 shows
l i m i t s f o r some t y p i c a l g r e a s e s .
S O L I D LUBRICANTS S t r i c t l y speak i n g , s o l i d l u b r i c a n t s a r e s o l i d m a t e r i a l s w h ic h can be i n t r o -
duced between b e a r i n g s u r f a c e s t o reduce f r i c t i o n or wear, b u t t h e t i t l e i s n o r m a l l y used i n a more r e s t r i c t e d sense t o c o v e r o n l y r e d u c t i o n i n f r i c t i o n o r p r e v e n t i o n o f adh e s i o n o r s e i z u r e .
I t t h e r e f o r e e x c lu d e s h a r d w e a r - r e s i s t a n t
c o a t i n g s and f r i c t i o n s u r f a c e s such as n o n - s k i d c o a t i n g s and b r a k e m a t e r i a l s . Tabl e 9 . 3 l i s t s a l a r g e number o f d i f f e r e n t s o l i d l u b r i c a n t s , b u t t h e m a j o r i t y o f appl i c a t i o n s use e i t h e r molybdenum d i s u l p h i d e , g r a p h i t e or PTFE ( p o l y t e t r a fluoroethylene).
They can a l l be used i n a v a r i e t y o f ways,
powder, rubbed f i l m ,
bonded f i l m o r s p u t t e r e d f i l m .
including free
G r a p h i t e and PTFE may a l s o
be used i n t h e f o r m o f s o l i d b l o c k s , as may many o t h e r p o ly m e r s , and i n t h i s form t hey a r e commonly known as d r y b e a r i n g m a t e r i a l s . The i m p o r t a n t p r o p e r t i e s o f molybdenum d i s u l p h i d e , g r a p h i t e and PTFE a r e l i s t e d i n T a b l e 9.3. The advantages o f t h e common s o l i d l u b r i c a n t s a r e t h e i r w id e te m p e r a tu r e ranges, chem i cal i n e r t n e s s , low v o l a t i l i t y , and t h e f a c t s t h a t t h e y do n o t need s e a l i n g , do n o t need f e e d systems and
g e n e r a l l y do n o t cause c o n t a m i n a t i o n .
T h e i r di sadvant ag e s compared w i t h o i l or grease a r e r e l a t i v e l y h i g h f r i c t i o n , l a c k o f h e a t removal, f a i l u r e t o p r e v e n t c o r r o s i o n , and s te a d y wear i n s e r v i c e . They a r e t h e r e f o r e o n l y used where t h e i r advantages o v e r o i l s and greases o u t weigh t h e i r
di sa d v a n t a g e s ,
such as a t h i g h o r v e r y l o w te m p e r a tu r e s ,
i n vacuum,
where v e r y l o n g l i f e i s r e q u i r e d w i t h o u t r e l u b r i c a t i o n , o r where c o n t a m i n a t i o n is critical.
9.7
GAS LUBRICATION Gases can be used as l u b r i c a n t s i n t h e same way as o i l s , and t h e p h y s i c a l
laws g o v e r n i n q hydrodynamic o i l l u b r i c a t i o n a o o l v a l s n
tn naqe4
The v p r v
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240
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241 v i s c o s i t y o f gases means t h a t t h e f i l m t h i c k n e s s i s a l s o v e r y low, so t h a t hydrodynamic gas b e a r i n g s ("gas-dynamic
bearings")
are 1 imited t o conditions
o f h i g h speed, low load, small c l e a r a n c e s and v e r y c l o s e l y c o n t r o l l e d t o l e r a n c e s . For t h i s reason e x t e r n a l l y p r e s s u r i s e d gas b e a r i n g s a r e more commonly used which can c a r r y h i g h e r loads, a r e l e s s c r i t i c a l on c l e a r a n c e s and t o l e r a n c e s , and can be used a t lower o r even z e r o speeds. Gas l u b r i c a t i o n can be used a t h i g h e r o r lower temperature than o i l o r grease, but t h e i r main advantages a r e i n g i v i n g h i g h s t i f f n e s s i n h i g h speed p r e c i s i o n bearings, such as i n d e n t a l d r i l l s and p r e c i s i o n g r i n d i n g s p i n d l e s , and i n a v o i d i n g s e a l i n g and c o n t a m i n a t i o n problems where a s u i t a b l e process gas i s a v a i l a b l e t o a c t as t h e gas l u b r i c a n t .
242
I() LUBRICANT ADDITIVES Their Application
, Performance and Limitations
D . M . SOUL, L u b r i z o l I n t e r n a t i o n a l L a b o r a t o r i e s , Hazelwood, Derby, U.K.
10.1
I NTRODUCT I ON
The l u b r i c a t i o n o f i n d u s t r i a l p l a n t and equipment covers t h e whole range o f requirements f o r machinery i n c l u d i n g t h e i n t e r n a l combustion engine and t h e gas turbine.
Dis c u ssi o n o f such a wide s u b j e c t i n one chapter would be impossible
b u t a v e r y wide f i e l d o f a d d i t i v e a p p l i c a t i o n s e x i s t s a p a r t from those f o r en g in e and t r a n s m i s s i o n systems.
These i n c l u d e h y d r a u l i c and gear l u b r i c a n t s ,
metal c u t t i n g and f ormi n g f l u i d s ,
t u r b i n e o i l s , compressor and r e f r i g e r a t o r
o i l s , f i r e - r e s i s t a n t f l u i d s , greases, and a m i s c e l l a n y o f s p e c i a l i s e d products. L u b r i c a n t a d d i t i v e s a r e chemicals i n c o r p o r a t e d i n t o e i t h e r a l i q u i d base (m in e r a l o i l , s y n t h e t i c f l u i d or w a t e r ) ,
a semi-fluid,
o r a grease.
I n general,
th e a d d i t i v e s a r e s o l u b l e o r u n i f o r m l y di sp ersed throughout t h e c a r r i e r medium. I n t h i s r e s p e c t t h i s d i s c u s s i o n does n o t cover s o l i d l u b r i c a n t a d d i t i v e s such as g r a p h i t e ,
molybdenum d l s u l p h i d e and polytetrafluoroethylene.
A d d i t i v e s a r e used t o improve t h e performance o f t h e base c a r r i e r m a t e r i a l t o p r o v i d e a d e s i r e d p h y s i c a l , chemical, o r mechanical p r o p e r t y .
They a r e manufac-
t u r e d t o s t r i c t q u a l i t y c o n t r o l standards t o assure c o n s i s t e n t performance i n s i m i l a r a p p l i c a t i o n s , i n many environments t hroughout t h e w o r l d , base m a t e r i a l and t h e a d d i t i v e system.
f o r both the
Many l u b r i c a n t s p e c i f i c a t i o n s c o v e r i n g
a wide range of i n d u s t r i a l processes can o n l y be s a t i s f i e d by t h e use o f these a d d i t i o n a l chemicals and c e r t a i n t e s t s a r e designed s p e c i f i c a l l y t o assess such factors. 10.2
B A S I C PROPERTIES OF LUBRICANTS
All types o f l u b r i c a t i n g f l u i d s d i s p l a y c e r t a i n b a s i c p r o p e r t i e s .
These may
be used t o advantage i n s e l e c t i n g t h e most a p p r o p r i a t e l u b r i c a n t f o r any
243 application.
I n a p p l i c a t i o n s where hydrodynamic l u b r i c a t i o n f i l m s a r e formed,
an u n t r e a t e d base o i l o f t h e c o r r e c t v i s c o m e t r i c p r o p e r t i e s can p r o v i d e a p e r f e c t l y adequate l u b r i c a t i n g medium.
The p r o p e r t i e s w i t h a p p r o p r i a t e t e s t
references f o r s t an da rd e v a l u a t i o n t e s t s a r e g i v e n i n Table 10.1. M in e r a l o i l p r o p e r t i e s a r e dependent on t h e crude o i l source and t h e r e f i n e r y processes o f s e p a r a t i o n and t rea t me nt .
Lubricating o i l fractions o f varying
By s e l e c t i o n o f t h e processes used i n t h e r e f i n e r y ,
v i s c o s i t y a r e produced.
p r o p e r t i e s such as pour p o i n t , c o l o u r and v i s c o s i t y index may be c o n t r o l l e d o r m o d ifie d so as t o produce b e t t e r f i n a l p r o d u c t s .
10.3
LUBRICANT ADDITIVES
I n Ta b le 10.2 i s a l i s t o f t he a d d i t i v e s n o r m a l l y employed i n l u b r i c a n t s . This t a b l e g i v e s t h e g en era l name o f each t ype t o g e t h e r w i t h a summary o f i t s purpose and a suggested mechanism o f a c t i o n . 10.3.1 10.3.1.1
Detergent and D i s p e r s a n t A d d i t i v e s De t e r g en t Types
A v a r i e t y o f me t a l c o n t a i n i n g compounds c a l l e d detergents' have been described T h e i r f u n c t i o n appears t o be one o f e f f e c t i n g a d i s p e r s i o n
i n the l i t e r a t u r e .
o f p a r t i c u l a t e m a t t e r and n e u t r a l i s i n g
d i r t and d e b r i s .
a c i d s r a t h e r than one o f c l e a n i n g up
Some o f t he se m a t e r i a l s a l s o f u n c t i o n as r u s t i n h i b i t o r s and
emu 1 s i f i e r s . The compounds a r e g e n e r a l l y mo l ecu l es h a v i n g a l a r g e hydrocarbon t a i l and a "polar group" head.
The t a i l s e c t i o n serves as a s o l u b i l i z e r i n t h e base o i l ,
w h i l e t h e p o l a r group serves as t h e f u n c t i o n a l p a r t o f t h e molecule, which a t t r a c t s p a r t i c u l a t e contaminants i n t h e l u b r i c a n t . The most w i d e l y used members o f t h e o r g a n o m e t a l l i c c l a s s a r e t h e sulphonates and phenates.
These i n c l u d e b o t h t h e n e u t r a l barium, c a l c i u m and magnesium
s a l t s , and t h e a l k a l i n e o r h i g h l y b a s i c p r o d u c t s prepared from these o r g a n i c s u b s t r a t e s by i n c o r p o r a t i n g metal i n excess o f t h e s t o i c h i o m e t r i c q u a n t i t i e s i n t o those compounds.
Phosphonates and s a l i c y l a t e s a r e a l s o used commercially
as s u b s t r a t e s f o r met al c o n t a i n i n g a d d i t i v e s . Commercial i n t e r e s t i n barium compounds i s d e c l i n i n g due t o c o s t o f product i o n and e n v ir o n me nt al c o n s i d e r a t i o n s . The meaning o f t h e terms " n e u t r a l " Neutral s a l t
-
and "ba sic" metal s a l t used above:
i s a s a l t o f an a c i d which c o n t a i n s t h e s t o i c h i o m e t r i c amount o f metal r e q u i r e d f o r n e u t r a l i z a t i o n o f t h e a c i d i c group present.
Basic s a l t
-
i s a s a l t o f an a c i d which c o n t a i n s more metal than i s required f o r the n e u t r a l i z a t i o n o f the a c i d i c groups p r e s e n t .
Such excess metal may be present
Lu
P P
Table 10.1 Standard L u b r i c a t i n g O i l Tests
Test
Scope o f T e st
P r a c t i c a l use o f
Data
I . P e t . Method Number and Related Methods
S p e c i f i c Gravi t y
Q u a l i t y o f o i l . Naphthenic h i g h , P a r a f f i n i c medium. A ro matic low. B a t ch c o n t r o l .
Volume t o w e i g h t conversions
160/68,
V i scos i t y
R e l a t i o n o f 'b od y' o i l t o t emp era t u re. Norma l l y e v a l u a t e d a t two temperatures, A l s o v a r i o u s methods f o r low tempera t u r e s and v a r y i n g shear r a t e s .
V i s c o s i t y Index. R e f i n i n g processes. 'Flow ' o f o i l i n machinery.
71/79
Flash P o i n t
Volatility, classification o f fluids, inflammability.
Colour
Depth o f c o l o u r r e l a t e d t o r e f i n e r y processes.
Storage and o p e r a t i n g c o n d i t i o n s o f o i l . 34/75 (Pensky-Martin) T a x a t i o n and s h i p p i n g o f Petroleum products. 170/75 (Abel) 36/67 (Cleveland) Contamination. 15/72
Pour/Cloud P o i n t
R e f i n i n g processes dewaxing. E f f e c t o f low temperatures.
Low temperature v i s c o s i t y and o p e r a t i o n .
Acidity
T ra ce resi d ue s from r e f i n e r y processes.
N ormally low f o r new o i l . D e t e r i o r a t i o n o f used o i l .
1 /74
Carbon Residue
Carbon r e s i d u e a f t e r b u r n i n g o f f oil.
Performance o f h i g h q u a l i t y base high. Paraffinic low. Naphthenic
13/78 (Conradson) 14/65 (Ramsbottom)
-
-
-
59/72,
189179
15/67 (Pour) 219/67 (Cloud)
245 due t o a t r u e b a s i c s a l t s t r u c t u r e , b u t a more l i k e l y e x p l a n a t i o n i s t h a t t h e excess metal i s present i n the f orm o f d i s p e r s e d metal compounds.
Sul phonates
10.3.1.1.1
Normal metal sul p ho na t e s d e r i v e d from "mahogany"
a c i d s ( t h e mahogany-coloured
petroleum s u l p h o n i c a c i d s o b t a i n e d as a b y-product d u r i n g w h i t e o i l manufacture) were f i r s t employed as d e t e r g e n t a d d i t i v e s i n commercial crankcase o i l s d u r i n g World War 1 1 .
Almost w i t h o u t e x c e p t i o n ,
was c a l c i u m o r barium.
a d d i t i v e s w i t h r e s p e c t t o detergency, bea r in g metal
t h e metal p r e s e n t i n such sulphonates
P e t ro l eu m sul p ho na t e s were n o t o n l y s u p e r i o r t o e a r l i e r b u t were much l e s s c o r r o s i v e t o s e n s i t i v e
a l l o y s and responded w e l l t o c o r r o s i o n i n h i b i t o r s .
They can be
rep r e s e n te d by t h e g en era l f o rmul a : RxArS03M where R A r r e p r e s e n t s complex a l k y l a r o m a t i c r a d i c a l s d e r i v e d from petroleum and M i s one e q u i v a l e n t o f a p o l y v a l e n t met al .
Supplementing t h e su pp l y o f
n a t u r a l p et roleum sulphonates a r e t h e s y n t h e t i c
sulphonates d e r i v e d f ro m l on g-ch ai n a l k y l s u b s t i t u t e d benzenes (e.g.
polydodecyl
benzene bottoms) o b t a i n e d as b y-pro du ct s i n t h e manufacture o f household d e t e r gents o r wh ic h a r e manufactured s p e c i f i c a l l y f o r t h i s use. Highly basic
su l ph on at es c o n t a i n from 3 t o 10 o r 15 times as much metal as
the c o r r e s p o n d in g normal su l ph on at es. "hyperbasic"
sulphonates,
C a l l e d "overbased",
"superbasic"
or
t he se p r o d u c t s a r e manufactured by h e a t i n g a m i x t u r e
o f c e r t a i n promoters o r s o l v e n t s w i t h a n e u t r a l sulphonate and a l a r g e excess o f metal o x i d e o r h yd roxi d e and c a r b o n a t i n g w i t h carbon d i o x i d e t o c o n v e r t the metal base t o c o l l o i d a l l y - d i s p e r s e d metal carbonate.
Overbased sulphonates possess
t h e a b i l i t y t o n e u t r a l i z e a c i d i c contaminants formed i n l u b r i c a t i n g o i l s and thus reduce c o r r o s i v e wear o f en gi n e components. 10.3.1.1.2
Phosphonates and/or Thiophosphonates
These d e t e r g e n t a d d i t i v e s can be re pre sen t ed by t h e general formula:
R
-
8 P
(XM12
where R i s a l a r g e a l i p h a t i c r a d i c a l o f a t l e a s t 500 m o l e c u l a r w e i g h t . oxygen and/or s u l p h u r , and metal.
500
-
X is
M i s one e q u i v a l e n t o f a monovalent o r p o l y v a l e n t
They a r e pre pa red by f i r s t h e a t i n g a p o l y o l e f i n such a s polybutenes of
2,000 m o l e c u l a r w e i g h t w i t h a phosphorus reagent, g e n e r a l l y phosphorus
p e n t a s u l p h i d e , t o form a complex o r g a n i c phosphorus-sulphur compound which i s the n n e u t r a l i z e d w i t h a metal base.
By steam treatment o r prolonged h y d r o l y s i s
p r i o r t o o r d u r i n g n e u t r a l i z a t i o n , a p o r t i o n o r s u b s t a n t i a l l y a l l o f the sulphur
TABLE 1 0 . 2 Lubricant A d d i t i v e
Additive
Chemicals
Purpose o f A d d i t i v e
How Additive Works
Antioxidant
Hindered phenols hines Organic sulphides Zinc dithiophosphate
Minimizes the formation o f resins, varnish, acids, sludge and polymers
Reduces organic peroxides terminating the oxidation chain. Reduces formation o f acids by decreasing oxygen taken up i n the o i l . Prevents c a t a l y t i c reactions.
Corrosion I n h i b i t o r
Zinc d i thiophosphates Sulphurized terpenes Phosphosulphurized terpenes Sulphurized o l e f i n s
Protects bearing and other metal surfaces from corrosion
Acts as a n t i c a t a l y s t . Coats metal surfaces which p r o t e c t against a c i d and peroxide attack.
Rust I n h i b i t o r
h i n e phosphates Sodi um.ca1 c i urn and Magnesi um sulphonates A l k y l succinic acids F a t t y acids
Protects ferrous metal surfaces against r u s t
Polar molecules are absorbed p r e f e r e n t i a l l y on the metal surface and serves as a b a r r i e r against water. Neutralizes acids.
Detergent
Normal or basic, calcium, b a r i um, magnesium phosphonates, phenates and sul phonates
Reduces o r prevents deposits i n engines operated a t high temperatures
Controls buildup of varnish and sludge by r e a c t i n g w i t h o x i d a t i o n products t o form o i l soluble material which remains suspended i n the o i l .
Dispersant
Polymers such as n i t r o g e n containing polymethacrylates, a l k y l succinimides, and succinate esters high molecular weight amines and amides
Prevents and r e t a r d s sludge formation and deposition under low temperature operating conditions
Dispersants have a strong a f f i n i t y for d i r t p a r t i c l e s and surround each w i t h o i l soluble molecules which keep the sludge frm agglomerating and depositing i n the engine.
F r i c t i o n Modifier
F a t t y acids F a t t y amines Fats
To increase o i l f i l m strength
Highly p o l a r molecules are absorbed on the metal surface and remain i n place t o cushion and keep metal surfaces apart.
Zinc d i a l k y l d i thiophosphate T r i c r e s y l phosphate
Reduce r a p i d wear i n steel-onsteel applications
Antiwear
t o prevent o i l f i l m rupture
A d d i t i v e reacts w i t h the metal t o form a compound which i s deformed by p l a s t i c f l o w t o allow a new d i s t r i b u t i o n of load.
Extreme Pressure
Sulphurized f a t s , o l e f i n s Chlorinated hydrocarbons Lead s a l t s o f organic acids. Amine phosphates
Prevents seizure and welding between metal surfaces under c o n d i t i o n o f extreme pressure and temperature
EP agent reacts w i t h metal surfaces t o form new compounds having lower shear strength than the base metal and i s sheared p r e f e r e n t i a l l y t o the base metal.
V i s c o s i t y Index Improver
Polyisobutylenes Polymethacrylates Polyacryl ates Ethylene propylene copolymers Styrene maleic e s t e r copolymers Hydrcgenated styrene butadiene copolymers
Reduces the r a t e o f change o f v i s c o s i t y w i t h temperature
Polymer molecule assumes a compact curled form i n a poor solvent ( c o l d o i l ) and an uncurled high surface area i n a b e t t e r solvent ( h o t o i l ) . The uncurled form thickens the o i l .
Pour Depressant
Wax a l k y l a t e naphthalene Wax a k l y l a t e d phenols Polymethacrylates
Lowers the pour p o i n t of t h e o i l
Retards the formation o f f u l l - s i z e wax c r y s t a l s by coating o r c o - c r y s t a l l i z a t i o n w i t h the wax.
Anti foam
S i l i c o n e polymers Polymthacrylates
Prevents the fonnation o f s t a b l e foam
Appears t o attack the o i l f i l m surrounding each bubble reducing i n t e r f a c i a l tension. The small bubbles l i b e r a t e d combine t o form large ones which f l o a t t o the surface.
Emu1s i f ie r
Sodium s a l t s of sulphonic acids, sodium s a l t s o f organic acids, f a t t y mine salts
To nake mineral o i l m i s c i b l e w i t h water
Emulsifier i s absorbed a t the o i l - w a t e r i n t e r f a c e t o reduce i n t e r f a c i a l tension r e s u l t i n g i n an i n t i m a t e dispersion o f one l i q u i d i n the other.
Tackiness
Soaps, polyisobutylene and p o l y s c r y l a t e polymers
To provide the o i l w i t h greater cohension
Increases v i s c o s i t y . and s t r i n g y .
Antiseptic
Phenols, c h l o r i n e compounds, Increases emulsion l i f e and formaldehyde bases prevents odour
Prevents and reduces microorganism growth.
Metal Deactivator
T r i a r y l phosphi tes Sulphur compounds Diamines. Dimercapto t h i a d i a z o l e derivatives.
A p r o t e c t i v e f i l m i s absorbed on metal surfaces which prevents contact between corrosive agents and base metal.
Stop the c a t a l y t i c e f f e c t o f metals on o x i d a t i o n and corrosion
Materials themselves are tacky
N
P 4
248 pr e s e n t i n t h e thiophosphonate group, -P(S) (SMl2, can be s u b s t i t u t e d w i t h oxygen t o y i e l d a phosphonate. The h i s t o r y o f these d e t e r g e n t a d d i t i v e s c l o s e l y p a r a l l e l s t h a t o f sulphonate a d d i t i v e s i n t h a t b a s i c and overbased s a l t s have r e p l a c e d normal s a l t s i n a l l b u t a few commercial a p p l i c a t i o n s .
The manufacture o f such b a s i c and overbased
s a l t s i s c a r r i e d o u t by t h e use o f methods l i k e those d e s c r i b e d i n t h e SULPHONATES sect ion.
10.3.1.1.3
Phenates
Phenates and p he na t e -sul p hi d es
have p l a y e d an i m p o r t a n t r o l e as d e t e r g e n t
a d d i t i v e s e v e r s i n c e t h e i r i n t r o d u c t i o n d u r i n g World War 1 1 .
Among t h e e a r l i e s t
a d d i t i v e s o f t h i s t y p e t o g a i n commercial acceptance were: Calcium and ba ri um phenates o f t e r t i a r y - o c t y l p h e n o l s u l p h i d e and t e r t i a r y - a m y l p h e n o l
sulphide having
t h e ge ne ral f ormu l a:
011
R
R
Calcium phenates o f t e r t i a r y - a m y l p h e n o l -formaldehyde condensation p rod uct s.
OH
OH
R
R
Calcium and b ari u m phenates o f p a r a f f i n wax s u b s t i t u t e d phenol h a v i n g t h e g en era l f ormula:
OM
I n a d d i t i o n t o p e r f o r m i n g a d e t e r g e n t r o l e , phenates sulphides
-
-
and e s p e c i a l l y phenate
e x h i b i t s u b s t a n t i a l a n t i - o x i d a n t p r o p e r t i e s and a r e p a r t i c u l a r l y
u s e f u l f o r h i g h temperature f l u i d s . L i k e t h e o t h e r f a m i l i e s o f d e t e r g e n t a d d i t i v e s discussed, phenates have r e p l ace d normal phenates i n most a p p l i c a t i o n s .
b a s i c and overbased Manufacturing
249 te c h n iq u e s f o r such p r o d u c t s a r e s i m i l a r t o those employed f o r b a s i c sulphonates o r phosphonates. 10.3.1.1.4
Alkyl Substituted Salicylates
These a d d i t i v e s can be rep rese nt ed by t h e general formula:
0
II
-
C
OM
where R i s an o i l - s o l u b i l i z i n g o r g a n i c r a d i c a l and M i s one e q u i v a l e n t o f a polyvalent metal.
One o f t h e f i r s t a d d i t i v e s o f t h i s t ype t o see commercial use was t h e z i n c carboxylate o f di-isopropyl
s a l i c y l i c acid.
More r e c e n t l y , c a l c i u m c a r b o x y l -
at e s of lo n g - c h ai n a l k y l s a l i c y l i c a c i d s have been used.
The manufacture of
such a d d i t i v e s i n v o l v e s c a r b o x y l a t i o n o f a metal phenate w i t h carbon d i o x i d e . As w i t h t h e o t h e r d e t e r g e n t a d d i t i v e s discussed, overbasing techniques have been employed t o p rep are improved p r o d u c t s . 10.3.1.2
Mode of A c t i o n o f Det erg en t A d d i t i v e s
A lt h o u g h t h e mechanism o f d et erg en cy i n non-aqueous media such as m i n e r a l o i l s i s n o t f u l l y understood, o f "soap m i c e l l e s "
re sea rche rs have found evidence f o r t h e e x i s t e n c e
i n non-aqueous s o l v e n t s .
There i s reason, then,
t o believe
t h a t d e t e r g e n t a d d i t i v e s i n m i n e r a l o i l s o l u t i o n can a c t i n a manner s i m i l a r t o aqueous soap s o l u t i o n s .
B a s i c and overbased d e t e r g e n t s a l s o possess t h e a b i l i t y
t o n e u t r a l i z e h armf ul i n o r g a n i c and o r g a n i c a c i d s w hich accumulate i n crankcase lubricants during service.
They can a l s o a c t as h i g h temperature s t a b i l i s e r s as
means o f r e d u c in g thermal d eco mpo si t i o n o f o t h e r a d d i t i v e s by n e u t r a l i s i n g small amounts o f a c i d i c p r o d u c t s which c o u l d cause c a t a l y t i c decomposition r e a c t i o n s t o occur.
10.3.1.3
Dispersant A d d i t i v e s
The te r m " d is pe rsan t "
i s p r e s e n t l y used t o designate a d d i t i v e s w hich a r e
capable o f d i s p e r s i n g t h e " c o l d sl ud ge '' formed i n engines operated f o r t h e most p a r t a t r e l a t i v e l y low b u l k crankcase o i l temperatures.
Unless maintained i n
f i n e suspension i n t h e l u b r i c a t i n g o i l , t h i s sludge d e p o s i t s on o i l f i l t e r s , v a lv e t r a i n components, and o i l c o n t r o l r i n g s where i t i n t e r f e r e s w i t h good engine performance. Since known m e t a l - c o n t a i n i n g
d e t e r g e n t s d i d not appear t o o f f e r a s o l u t i o n
t o t h e c o l d s lu d ge problem, re sea rche rs t u r n e d t h e i r a t t e n t i o n t o m e t a l - f r e e
250 o r g a n i c compounds i n t h e hope t h a t an "ash l ess detergent" would p r o v i d e the answer. Such p r o d u c ts can a l s o be used t o e f f e c t d i s p e r s i o n o f i n s o l u b l e m a t e r i a l in oil,
i.e.
c o l l o i d a l dispersions,
and a l s o t o d i s p e r s e w ater i n o i l t o produce
s t a b l e i n v e r t emulsions c o n t a i n i n g up t o f o r t y percent w ater. The compounds which a r e u s e f u l f o r t h i s purpose a r e a g a i n c h a r a c t e r i z e d by a p o l a r group a t t a c h e d t o a r e l a t i v e l y h i g h m o l e c u l a r w e i g h t hydrocarbon c h a i n . The p o l a r group g e n e r a l l y c o n t a i n s one or more o f t h e elements: n i t r o g e n , oxygen and phosphorus.
The s o l u b i l i z i n g c h a i n s a r e based on p o l y i s o b u t y l e n e .
D i s p e r s a n t s may be d i v i d e d i n t o v a r i o u s chemical f a m i l i e s . 10.3.1.3.1
Copolymers
Copolymers which c o n t a i n a c a r b o x y l i c e s t e r f u n c t i o n and one o r more a d d i t i o n a l p o l a r f u n c t i o n s such as amine,
imide, h y d r o x y l , e t h e r , epoxide, phosph-
o r u s e s t e r , c a r b o x y l , an hyd ri de , o r n i t r i l e g e n e r a l l y have d i s p e r s a n t p r o p e r t i e s . Some o f t h e s e polymers e x h i b i t v i s c o s i t y m o d i f y i n g p r o p e r t i e s and thus f i n d a p p l i c a t i o n as m u l t i - f u n c t i o n a l a d d i t i v e s . a r e i n used today:
Three types o f d i s p e r s a n t V I
improver
p o l y m e t h a c r y l a t e s , s t y r e n e - m a l e i c e s t e r copolymers and
ethy le n e - p r o p y le n e copolymers. 10.3.1.3.2
S u b s t i t u t e d Succinimides
Hydrocarbon polymers may be i n t r o d u c e d i n t o molecules by chemical r e a c t i o n . T y p i c a l p r o d u c t s o f t h i s t y p e a r e o b t a i n e d by t r e a t i n g a p o l y o l e f i n , w i t h a molec u l a r w e i g h t i n t h e range 500-2000,
w i t h phosphorus c h l o r i d e and phosphorus sul-
p h i d e and t h e n w i t h rea ge nt s such as ure a, e t h y l e n e o x i d e and b o r i c a c i d . The N - s u b s t i t u t e d l o n g c h a i n a l k e n y l su cci nimides c o n t a i n t h e c h a r a c t e r i s t i c s u c c i n im id e g r o u p i n g
where R c o n t a i n s upwards o f 50 carbon atoms. extensively i n 10.3.1.3.3
T h i s group o f p r o d u c t s i s used
many t yp es o f crankcase l u b r i c a n t .
Amides
H ig h m o le c u la r w e i g h t amides and polyamides a r e u s u a l l y prepared by t h e r e a c t i o n o f h i g h e r f a t t y a c i d s o r e s t e r s w i t h p o l y a l k y l e n e amines.
Products o f
t h i s t y p e a r e g e n e r a l l y used i n t h e l u b r i c a t i o n o f tw o-cycle engines and l i t t l e i n a u t o m o tiv e crankcase l u b r i c a n t s .
251 10.3.1.3.4
Other Chemicals
E x t e n s iv e r e s ea rch i n t o o t h e r cl asse s o f d e t e r g e n t c h e m i s t r y c o n t i n u e s . Products may be produced f rom t h e f o l l o w i n g : P o ly and benzyl amines H ig h m o l e c u l a r w e i g h t e s t e r s whi ch may be used f o r e m u l s i f i c a t i o n purposes Amine s a l t s o f h i g h m o l e c u l a r w e i g h t a c i d s 10.3.1.4
Mode o f A c t i o n o f Di spe rsan t A d d i t i v e s
Based on microscopy and chromatography s t u d i e s , several groups o f researchers have concluded t h a t a shl e ss d i s p e r s a n t s f u n c t i o n by a d s o r p t i o n on contaminant p a r t i c l e s p r e s e n t i n o i l s , t h us keeping them i n suspension.
Although these r e -
searchers have c o n t r i b u t e d much t o an u nd erst anding o f t h e mechanism o f d i s p e r sancy, t h e r e i s wide agreement t h a t i t i s necessary t o e v a l u a t e t h e p r o d u c t s i n p r a c t i c a l t r i a l s s i n c e t h e r e i s l i t t l e i n t h e way o f l a b o r a t o r y t e s t i n g which can compare p r o d u c t s w i t h t h i s f u n c t i o n . 10.3.2
A n t i o x i d a n t s and B e ari n g C o r r o s i o n I n h i b i t o r s
A n t i o x i d a n t s a r e p r o b a b l y employed i n a w i d e r v a r i e t y o f l u b r i c a n t s than any othe r k i n d o f a d d i t i v e .
I n a d d i t i o n t o t h e i r use i n crankcase o i l s and steam
t u r b i n e o i l s , they w i l l be found i n gas t u r b i n e l u b r i c a n t s , automatic t r a n s m is s io n f l u i d s , gear o i l s , c u t t i n g o i l s , greases and h y d r a u l i c f l u i d s . The f u n c t i o n o f an o x i d a t i o n i n h i b i t o r i s t h e p r e v e n t i o n of d e t e r i o r a t i o n ass o c ia t e d w i t h oxygen a t t a c k on t h e l u b r i c a n t base f l u i d .
Inhibitors function
e i t h e r t o d e s t r o y pe roxi d es o r t h e f r e e r a d i c a l s d e r i v e d from p e r o x i d e s .
The
most w i d e l y used a n t i o x i d a n t s i n t h e l u b r i c a n t f i e l d a r e the p h e n o l i c types, s u l p h u r i s e d p o l y o l e f i n s and t h e z i n c d i t h i o p h o s p h a t e s .
The phenols a r e c o n s i -
dered t o be t h e cha i n-b rea ki ng t y p e whereas t h e l a t t e r two a r e b e l i e v e d t o be p e r o x i de d e s t r o y e r s
.
The c o r r o s i o n o f b e a r i n g m e t a l s i n i n t e r n a l combustion engines i s g e n e r a l l y c on s id e r e d t o be due l a r g e l y t o a c i d o r a c t i v e s u l p h u r a t t a c k on t h e b e a r i n g m e t a ls .
The a c i d s i n v o l v e d i n t h e a t t a c k o r i g i n a t e e i t h e r from p r o d u c t s o f
incomplete combustion o f t h e f u e l which f i n d t h e i r way i n t o t h e l u b r i c a n t as blow-by gases o r from t h e o x i d a t i o n o f t h e l u b r i c a n t .
O x i d a t i o n i n h i b i t o r s can
reduce o r e l i m i n a t e t h e l a t t e r m a t e r i a l s and, hence, reduce b e a r i n g c o r r o s i o n . G e n e r a lly ,
i n most i n d u s t r i a l a p p l i c a t i o n s ,
o x i d a t i o n i s t h e major cause o f a c i d
re 1 ease. I n most environments i n which a l u b r i c a t i n g o i l i s employed,
i t comes i n con-
t a c t w i t h a i r o f t e n a t h i g h temperatures and i n t h e presence o f m e t a l s o r chemic a l compounds wh i ch promote o x i d a t i o n o f t h e o i l .
The o i l undergoes a complex
s e r i e s o f o x i d a t i o n r e a c t i o n s and t h e h armf ul r e s u l t s i n c l u d e , p r i n c i p a l l y , an
252 i nc r e a s e i n v i s c o s i t y o f t h e l u b r i c a n t , t h e f o r m a t i o n o f a c i d i c contaminants such as " p e tr o le u m oxya ci ds" and t h e development o f carbonaceous m a t e r i a l . Among t h e more e f f e c t i v e chemicals employed as commercial a n t i o x i d a n t s today are:10.3.2.1
Dith ioph osp ha t e s
Zinc diorganodithiophosphates ( a l s o e f f e c t i v e i n the r o l e o f corrosion i n h i b i t o r s ) . Other me t a l s may a l s o be used. Ashless d i t h i o p h o s p h a t e s , 10.3.2.2
r e p l a c i n g me t a l by amine can be e q u a l l y e f f e c t i v e .
Hin d e re d Phenol
Hin d e r e d phenols ( i . e . phenols i n whi ch t h e h y d r o x y l group i s s t e r i c a l l y bl o c k e d or "hindered") : 2, 6-di-tertiary-butyl-4-methylphenol
4, 4'-methy 1ene b i s (2,6-d i- t e r t ia r y - b u t y 1phenol ) 4, 4'- t h i o b is (2-methy 1-6-t e r t ia r y - b u t y 1 phenol 10.3.2.3
N i t r o g e n Bases
Amines such as:
N-phenyl-alpha-naphthylamine N-phenyl-beta-naphthylamine
Tetramethyldiaminodiphenylmethane Anthranil i c acid P h e n ot h i azi n e and a l k y l a t e d d e r i v a t i v e s 10.3.2.4
S u l p h u r i s e d p o l y o l e f i n s , where t h e sulphur a c t s i n a s i m i l a r manner
t o n a t u r a l l y o c c u r r i n g s u l p h u r chemicals found i n crude o i l b u t removed d u r i n g refining. O f a l l these a n t i o x i d a n t s , phosphates.
t h e most w i d e l y used a r e t h e z i n c d i o r g a n o d i t h i o -
These have a dual f u n c t i o n o f behaving a l s o as a n t i w e a r agents t o
p r o t e c t cam and t a p p e t s c u f f i n g when used i n crankcase o i l s .
These compounds
a r e a l s o used e x t e n s i v e l y as m i l d l o a d - c a r r y i n g a d d i t i v e s i n gear oils and i n hydraulic o i l s . more t h a n
Performance req ui re men t s today g e n e r a l l y d i c t a t e the use o f
one a n t i o x i d a n t .
Hindered phenols a r e f a vou red f o r h y d r a u l i c and t u r b i n e o i l s because they g i v e e x c e l l e n t o x i d a t i o n l i v e s f o r l o n g f i e l d use. antioxidants,
The phenyl naphthylamine
e s p e c i a l l y t h e b e t a form, or t hose c o n t a i n i n g t r a c e s o f i t , a r e
now regarded as c a r c i n o g e n i c ,
They a r e t h e r e f o r e n o t used t o a g r e a t e x t e n t
and a r e more s p e c i f i c t o some types o f grease. 10.3.2.5
Mode o f A c t i o n o f A n t i o x i d a n t s
There i s g e n e ral agreement among independent i n v e s t i g a t o r s t h a t t h e o x i d a t i o n o f a l u b r i c a t i n g o i l i n v o l v e s a c h a i n o x i d a t i o n r e a c t i o n i n w hich i n i t i a l l y -
253 formed o r g a n i c p ero xi de s a t t a c k u n o x i d i z e d o i l and a r e subsequently regenerated by oxygen i n t h e a i r t o c o n t i n u e such a t t a c k . "peroxide theory",
A c c o r d i n g t o t h i s w i d e l y accepted
an e f f e c t i v e a n t i o x i d a n t i s a chemical compound which reduces
org a n ic p e r o x id e s and co nse qu en t l y causes t h e c h a i n r e a c t i o n t o cease. 10.3.3
Co r r o s ion I n h i b i t o r s
I n the a d d i t i v e industry,
t h e t erm " c o r r o s i o n
i n h i b i t o r ' ' i s app1ied;to
a
m a t e r i a l which p r o t e c t s c o r r o s i o n - s u s c e p t i b l e non-ferrous metal components, p r i n c i p a l l y b e a r i n g s , a g a i n s t a t t a c k by a c i d i c contaminants i n t h e l u b r i c a t i n g oil.
A d i f f e r e n t t erm
-
-
rust inhibitor
i s used t o d e s i g n a t e m a t e r i a l s w hich
p r o t e c t f e r r o u s metal s u r f a c e s a g a i n s t r u s t . Among t h e e a r l i e s t t yp es
o f c o r r o s i o n i n h i b i t o r s t o see commercial a p p l i c a Most o f t h ese were n o t p u r e chemicals b u t r a t h e r
t i o n were o r g a n i c p h o s p h i t e s . m ix tu r e s o f mono-,
di-,
and t r i - o r g a n o p ho sphites o b t a i n e d from t h e r e a c t i o n of
alc o h o ls o r h y d r oxyest ers (e.g.
methyl l a c t a t e ,
t r i m e t h y l c i t r a t e ) w i t h phos-
phous t r i c h l o r i de. The m a jo r c l a s s e s o f c o r r o s i o n i n h i b i t o r s i n commercial use a t t h e present time a r e : 10.3.3.1
D ith iop ho sph at es
Metal d i o r g a n o d i t h i o p h o s p h a t e s , e s p e c i a l l y z i n c diorganodithiophosphates. They possess t h e s t r u c t u r e :
RO
RO
\
P
'
QS
MS'
where R i s an a l i p h a t i c o r a roma t i c r a d i c a l and M i s a p o l y v a l e n t metal such as zinc o r n i c k e l .
T h e i r man uf act ure i n v o l v e s f i r s t h e a t i n g an a l c o h o l o r phenol
w i t h phosphorus p e n t a s u l p h i d e t o form d i o r g a n o d i t h i o p h o s p h o r i c a c i d , then neut r a l i z i n g such a c i d w i t h a metal base.
10.3.3.2
Dith io ca rba mat es
Metal diorganodithiocarbamates, e s p e c i a l l y z i n c diorganodithiocarbamates They a r e d e s c r ib e d by t h e g en era l f ormu l a:
R
\ N - C
I R
HS -
SM
254 where R and M a r e as d e f i n e d i n 10.3.3.1.
T h e i r manufacture i n v o l v e s t h e reac-
t i o n o f an o r g a n i c amine, carbon d i s u l p h i d e , and a metal base. 10.3.3.3
Sulphur P ro du ct s
S u l p h u r i z e d terpenes,
f o r example, s u l p h u r i z e d dipentene.
These p r o d u c t s a r e
manufactured by h e a t i n g elemental s u l p h u r w i t h a terpene hydrocarbon and then o p t i o n a l l y washing t h e cru de p r o d u c t w i t h aqueous a l k a l i o r a l k a l i metal s u l p h i d e t o remove d i s s o l v e d o r " c o r r o s i v e " 10.3.3.4
sulphur.
Phosphorus-Sulphur P ro du ct s
Phosphosulphurized t erp en es, f o r example, phosphorus p e n t a s u l p h i d e - t r e a t e d These a d d i t i v e s a r e manufactured by h e a t i n g phosphorus p e n t a s u l -
turpentine.
phid e w i t h a te r p ene hydrocarbon. O f t h e f o u r m aj or cl asse s o f c o r r o s i o n i n h i b i t o r s l i s t e d above, metal d i t h i o -
phosphates
-
p a r t i c u l a r l y zinc dialkyldithiophosphates
commercial acceptance.
-
have achieved t h e w idest
Many o f t h e compounds l i s t e d above may n o t be t r u e c o r r o -
s i o n i n h i b i t o r s b u t f u n c t i o n by i n h i b i t i n g o x i d a t i o n and so reducing t h e format i o n o f c o r r o s i v e c a r b o x y l i c acids. 10.3.3.5
T r i a z o l e s and C h e l a t i n g Agents
B e n z o t r i a z o l e and i t s d e r i v a t i v e s a r e used t o form a s u r f a c e l a y e r on copper and s i l v e r based a l l o y s by c h e l a t i o n .
T h i s process p a s s i v a t e s t h e s u r f a c e and
reduces i t s a b i l i t y t o a c t as a c a t a l y s t towards f l u i d d e g r a d a t i o n by p r e v e n t i n g s o l u b i l i s a t i o n o f sma l l q u a n t i t i e s o f t h e m e t a l . There a r e v e r s i o n s o f t h e chemical whi ch a r e n o t s o l u b l e
and o t h e r which a r e
s o l u b l e i n b o t h s y n t h e t i c base f l u i d s and aqueous systems. 10.3.3.6 Dirnercapto T h i a d i o z o l e D e r i v a t i v e s These a r e p r e pa red by r e a c t i n g h y d r a z i n e and carbon d i s u l p h i d e f o l l o w e d by v a r i o u s r e a c t i o n s t o make t h e m a t e r i a l o i l s o l u b l e .
They a r e e f f e c t i v e i n red-
ucing corrosion. 10.3.3.7
Mode o f A c t i o n o f C o r r o s i o n I n h i b i t o r s
I t can be s a i d t h a t compounds l i k e 10.3.3.5 inhibitors.
and 10.3.3.6
are t r u e corrosion
They f u n c t i o n by r e a c t i n g c h e m i c a l l y w i t h t h e non-ferrous s u r f a c e
o f t h e metal component:
(e . g .
in
an engine, copper-lead o r lead-bronze b e a r i n g s )
t o form t h e r e o n a c o r r o s i o n - r e s i s t a n t , p r o t e c t i v e f i l m .
T h i s f i l m must adhere
t i g h t l y t o t h e b e a r i n g s u r f a c e l e s t i t be removed by d i s p e r s a n t s o r d e t e r g e n t s and expose th e u n d e r l y i n g metal s u r f a c e t o a t t a c k by a c i d i c components
n the
lubricating o i l . 10.3.4
Rust I n h i b i t o r s
The presence o f wat er i n l u b r i c a n t s v a r i e s between v e r y small l e v e l s t o values as h i g h as f o r t y p erce nt (40) i n i n v e r t e mul sion h y d r a u l i c f l u i d s and n n e t y - f i v e
(95) p e r c e n t i n e mul si o n f l u i d s .
I n h i b i t i n g f e r r o u s surfaces a g a i n s t r u s t i n g i s
t h e r e f o r e a r e q u i reme nt i n a l l types o f
oil.
A range o f r u s t i n h i b i t i n g ch emi ca l s i s necessary t o cope w i t h d i f f e r e n t environments,
and t h e i r chemical o r p h y s i c a l a c t i o n i s i m p o r t a n t .
The s t r o n g s u r -
face a d s o r p t i o n e x h i b i t e d by these chemicals t o r e s t r i c t t h e c o n t a c t w i t h w ater means t h a t c a r e f u l c h o i c e i s necessary t o ensure t h a t o t h e r s u r f a c e a c t i v e chemi c a l s , n o t a b l y t h e a n t i w e a r and extreme p ressure members, a r e a b l e t o perform their function. T y p i c a l chemicals used f o r o i l s o l u b l e systems i n c l u d e t h e f o l l o w i n g : A l k e n y l s u c c i n i c a c i d s and d e r i v a t i v e s
Turbine,
hydrau i c and c i r c u -
A l k y l t h i o a c e t i c a c i d s and d e r i v a t i v e s
) Substituted imidazolines
Gear o i l s
Amine phosphates
Preservative o i
Sulphonates, n e u t r a l o r low base
Engine preserva i v e o i l s . Storage, e t c .
S
A d d i t i o n a l f i l m s t r e n g t h i s a chi e ved by t h e a d d i t i o n o f f a t t y m a t e r i a l s such as l a n o l i n i n t h e case o f t h i c k f i l m s t o r a g e co mposition.
10.3.5
V i s c o s i t y Improvers
V i s c o s i t y improvers a r e m a t e r i a l s which improve t h e v i s c o s i t y temperature relationship o f a lubricant.
They a r e g e n e r a l l y o i l s o l u b l e polymers w i t h mole-
c u l a r w e ig h t r a n g i n g from a pp roxi ma t e l y 50,000 t o 1,000,000.
The polymer mole-
c ul e i n t e r a c t s w i t h t h e o i l t o e f f e c t t h e f i n a l o i l v i s c o s i t y .
The h i g h e r t h e
tempature o f t h e system, t h e l a r g e r t h e polymer volume, t h e g r e a t e r t h e t h i c k e n ing e f f e c t , and hence t h e l e s s t h e " t h i n n i n g "
tendendy o f t h e o i l due t o i n c r e a -
sed temperature. I n a d d i t i o n t o v i s c o s i t y c h a r a c t e r i s t i c s , t h e performance o f these polymers i s dependent on t h e shear s t a b i l i t y o r r e s i s t a n c e t o shear and on t h e i r chemical and thermal s t a b i l i t y .
W i t h a g i v e n polymer, t h e shear s t a b i l i t y decreases w i t h
an in c r e a s e i n m o l e c u l a r w e i g h t .
The loss due t o shear i s r e f l e c t e d i n a de-
crease i n v i s c o s i t y o f t h e l u b r i c a n t i n t h e mechanical system. V i s c o s i t y Index o r " V . I . "
i s an a r b i t r a r y number
-
c a l c u l a t e d from t h e obser-
ved v i s c o s i t i e s o f a l u b r i c a n t a t two w i d e l y separated temperatures
-
which
i n d i c a t e s t h e r e s i s t a n c e o f t h e l u b r i c a n t t o v i s c o s i t y change w i t h temperature. The h i g h e r t h e V . I .
va l ue , t h e g r e a t e r t h e r e s i s t a n c e o f t h e l u b r i c a n t t o thicken
a t low temperatures and t h i n o u t a t h i g h temperatures.
The s i g n i f i c a n c e o f V . I .
i s much l e s s w i t h c u r r e n t o i l s p e c i f i c a t i o n s .
Measured low t e m pe rat ure v i s c o s i t y i s now most i m p o r t a n t .
For motor o i l s such
values a r e r e p o r t e d u s i n g t h e Cold Cranking S i m u l a t o r and f o r automatic t r a n s m i ss io n f l u i d s and gear o i l s u s i n g t h e B r o o k f i e l d Viscometer. The use o f hydrocarbon polymers o f low m o l e c u l a r w e i g h t makes i t p o s s i b l e t o form u la t e m u l t i g r a d e d a x l e o i l s .
Reduction i n o i l drag and consequent f u e l
256 s a v in g s may be achieved w i t h lower v i s c o s i t y index values b u t a h i g h degree o f shear s t a b i l i t y .
A l l im p o r ta n t v i s c o s i t y improvers a r e manufactured by processes o f p o l y m e r i The groups o f polymers, s o l u b l e i n o i l , a r e : -
sation.
(i)
P o l yi sob ut en es
(ii)
A l k y l m e t h a c r y l a t e and a c r y l a t e copolymers
( i i i ) Rubber t ype chemicals such as o l e f i n e co-polymers and b ut ad i en e-st yren e copolymers
10.3.5.1
Mode o f A c t i o n o f V . I .
(OCP)
Improvers
improvers e x e r t a g r e a t e r t h i c k e n i n g e f f e c t on o i l a t h i g h temperatures
V.I.
than they do a t r e l a t i v e l y lower temperatures.
The r e s u l t o f such s e l e c t i v e
t h i c k e n i n g i s t h a t t h e o i l s u f f e r s l e s s v i s c o s i t y change w i t h changing temperature.
I t i s b e l i e v e d t h a t t h e s e l e c t i v e t h i c k e n i n g occurs because t h e polymer
assumes a compact,
c u r l e d form i n a poor s o l v e n t such as c o l d o i l , and an un-
c u r l e d h i g h surFace a rea form i n a b e t t e r s o l v e n t such as h o t o i l . Polymers whose s o l u b i l i t y i n o i l changes v e r y l i t t l e w i t h temperature a c t as thickeners,
b u t a r e n o t as e f f e c t i v e V . I .
improvers as a r e those polymers whose
s o l u b i l i t y i s poor a t low temperatures b u t good a t h i g h e r temperatures.
V.I.
improvers a r e more e f f e c t i v e i n i n c r e a s i n g t h e v i s c o s i t y o r d e r o f low v i s c o s i t y o i l s and become p r o g r e s s i v e l y l e s s e f f e c t i v e as t h e v i s c o s i t y o f t h e base o i l i nc r e a s e s .
V.I.
improvers undergo temporary v i s c o s i t y r e d u c t i o n under shear because o f
th e a lig n m e n t o f t h e polymer mo l ecu l es i n t h e d i r e c t i o n o f f l o w .
T h i s temporary
v i s c o s i t y r e d u c t i o n has t h e e f f e c t o f re du ci ng f r i c t i o n i n h i g h shear zones and g i v e s a s i g n i f i c a n t advantage f o r t h e V . I . o f t h e same v i s c o s i t y . s i t y o f the treated
t r e a t e d o i l remains h i g h , thus m i n i m i z i n g o i l consumption.
Polymer-
o i l s a l s o e x h i b i t lower b e a r i n g wear than t h e i r comparable m i n e r a l o i l
counterparts. engine
i m p r o v e r - t r e a t e d o i l o v e r a base o i l
Another advantage i s t h a t i n low shear zones t h e v i s c o -
These f a c t o r s have been i m p o r t a n t i n t h e acceptance o f m u l t i g r a d e
oils.
Polymers used as v i s c o s i t y - i n d e x improvers must be r e l a t i v e l y s t a b l e t o c h a i n s c i s s i o n under h i g h shear r a t e s . shearing, index.
M o l e c u l a r w e i g h t i s d r a s t i c a l l y reduced by
thus c a usi n g a d e t e r i o r a t i o n of t h e p r o p e r t i e s w hich improve v i s c o s i t y
The shear s t a b i l i t y o f a polymer t y p e i s dependent on m o l e c u l a r w e i g h t
and m o le c u la r w e i g h t d i s t r i b u t i o n and i n crea ses w i t h decreasing m o l e c u l a r w eight. I t can be seen t h a t shear s t a b i l i t y and v i s c o s i t y - i n d e x
d i v e r g e n t m o le c ul ar w e i g h t s . based on a compromise between
improvement r e q u i r e
The m o l e c u l a r w eights o f commercial polymers a r e t he se two p r o p e r t i e s .
I n many cases, t h i s has
r e s u l t e d i n t h e use o f l a r g e r amounts o f l ower-molecular-w eight given a p p l i c a t i o n .
polymers f o r a
257 Chemical and thermal index improver.
s t a b i l i t y a r e i m p o r t a n t p r o p e r t i e s o f a good v i s c o s i t y -
For good s t a b i l i t y ,
i t i s i mportant t h a t c a t a l y s t residues be
completely removed d u r i n g t h e man uf act uri n g process.
Some commercial polymers
are f o r m u la te d w i t h i n h i b i t o r s t o enhance t h e i r s t a b i l i t y . commercial v i s c o s i t y - i n d e x improvers a r e m u l t i - f u n c t i o n a l
Several o f the i n t h a t they may be
e f f e c t i v e p o u r - p o i n t d ep ressa nt s and possess dispersency performance.
10.3.6
Pour P o i n t Depressants
Ever s i n c e l u b r i c a t i n g o i l s were p rep are d from crude o i l s , r e f i n e r s have experienced d i f f i c u l t y w i t h c o n g e l a t i o n o f these p r o d u c t s a t low temperatures. P a r t o f t h e d i f f i c u l t y a r i s e s f rom a n a t u r a l t h i c k e n i n g o f t h e hydrocarbons comprising t h e b u l k o f t h e o i l ;
something which can u s u a l l y be c o r r e c t e d by t h e
use o f a s o l v e n t such as kerosene t o reduce v i s c o s i t y . culty
-
t h e more s e r i o u s
part
-
The r e s t o f t h e d i f f i -
a r i s e s from c r y s t a l l i z a t i o n a t low temperatures
o f t h e p a r a f f i n wax p r e s e n t i n a l most a l l l u b r i c a t i n g o i l f r a c t i o n s .
Upon c r y -
s t a l l i z a t i o n , t h i s wax tends t o form i n t e r l o c k i n g networks w hich adsorb o i l and form
a voluminous g e l - l i k e mass which r e s t r i c t s t h e f l o w o r "pour"
o f the o i l .
Pour p o i n t d e pre ssan t s a r e chemicals whi ch modify t h e wax c r y s t a l l i z a t i o n process i n such a manner t h a t t h e o i l w i l l pour a t low temperatures.
Although
some monomeric compounds such as t e t r a ( l o n g c h a i n a l k y l ) s i l i c a t e s , phenyl t r i s t e a r y l o x y s i l a n e , and p e n t a e r y t h r i t o l t e t r a s t e a r a t e have been shown t o be e f f e c t i v e , a l l c o m m e rci a l l y i m p o r t a n t pour p o i n t depressants a r e polymers: (i)
A l k y l m e t h a c r y l a t e polymers and copolymers
( ii )
V i n y l c a r b o x y l a t e - d i a l k y l fumarate copolymers
( i i i ) A l p h a - o l e f i n polymers and copolymers F r i e d e l - C r a f t s co nd en sat i on p r o d u c t s o f c h l o r i n a t e d wax and
(iv)
a r o m a t i c compounds such as n ap ht halene o r phenol (some i n v e s t i g a t o r s do n o t c l a s s t y p e ( i v ) p r o d u c t s as polymers;
others
maintain that
they a r e r e l a t i v e l y low m o l e c u l a r w e i g h t polymers having a p l u r a l i t y o f a r o m a t i c r i n g s and p a r a f f i n wax r a d i c a l s ) . The m o l e c u l a r w e i g h t range o f polymers e f f e c t i v e as pour p o i n i s g e n e r a l l y below t h a t o f polymers used as V . I .
improvers, and
depressants s usually i n
the a r e a o f 500 t o 100,000.
10.3.6.1
Mode o f A c t i o n o f Pour P o i n t Depressants
Pour p o i n t depressants f u n c t i o n by ad sorb i ng on o r c o - c r y s t a l p r e c i p i t a t i n g wax,
thus i n h i b i t i n g l a t e r a l c r y s t a l growth.
i z i n g w i t h the
T h i s promotes growth
o f s m a l l e r c r y s t a l s t h an t h e p l a t e l e t s formed i n t h e absence o f pour p o i n t depressants.
T h i s change d i m i n i s h e s t h e a b i l i t y o f t h e wax c r y s t a l s t o o v e r l a p
and i n t e r l o c k t o f o rm l a r g e conglomerates o f wax w hich would impede t h e f l o w o f the o i 1 .
258 There i s evidence t h a t a l i p h a t i c polymers f u n c t i o n by a c o - c r y s t a l l i z a t i o n mechanism, and t h a t a l k y l a r o m a t i c t y p e s such as c h l o r i n a t e d p a r a f f i n wax-aromatic compound condensation p r o d u c t s f u n c t i o n by a d s o r p t i o n on t h e nascent wax c r y s t a l . 10.3.7
Extreme Pressure A d d i t i v e s
These a d d i t i v e s ,
commonly c a l l e d "E.P."
agents, a r e chemicals which a r e addec
t o l u b r i c a n t s t o prevent d e s t r u c t i v e metal-to-metal P l a i n mineral
contact during l u b r i c a t i o n .
o i l s p r o v i d e good l u b r i c a t i o n when a f i l m o f o i l i s m a i n t a i n e d be.
tween t h e moving s u r f a c e s (hydrodynamic l u b r i c a t i o n ) b u t f a i l t o
p r o v i d e ade-
quate l u b r i c a t i o n when p r e s s u r e and r u b b i n g speeds a r e such t h a t t h e f i l m o f o i l i s squeezed o r wiped o u t . rication",
The l a t t e r k i n d o f l u b r i c a t i o n , c a l l e d "boundary
lub-
i s governed l a r g e l y by parameters o f t h e c o n t a c t i n g s u r f a c e s such as
s u r f a c e f i n i s h , metal shear s t r e n g t h and t h e c o e f f i c i e n t o f f r i c t i o n between t h e c o n t a c t i n g metal s u r f a c e s . Unless such parameters can be chosen t o meet expected pressures and r u b b i n g needs, d e s t r u c t i v e m e t a l - t o - m e t a l 10.3.7.1
Commercial E.P.
contact w i l l take place.
A d d i t i v e s and t h e i r A p p l i c a t i o n
V i r t u a l l y a l l commercial E.P.
a d d i t i v e s a r e o r g a n i c compounds t h a t c o n t a i n onc
o r more elements o r f u n c t i o n s such as s u l p h u r , halogen, phosphorus, c a r b o x y l , o r c a r b o x y l a t e s a l t w h i c h can r e a c t c h e m i c a l l y w i t h t h e metal s u r f a c e under condit i o n s o f boundary l u b r i c a t i o n . The ease w i t h which an E . P . a d d i t i v e t h e metal s u r f a c e , i . e .
i t s "activity",
reacts w i t h
determines t o a l a r g e e x t e n t whether i t
would be used i n a l u b r i c a n t such as a c u t t i n g o i l , a hypoid gear o i l , h y d r a u l i c o i l , o r a steam t u r b i n e o i l . A n assignment o f l i k e l y f i e l d s o f a p p l i c a t i o n o f commercial E . P . a d d i t i v e s T a b l e 10.3
based on t h e i r r e l a t i v e a c t i v i t y i s shown i n T a b l e 10.3
F i e l d s o f A p p l i c a t i o n o f E.P.
Additives
High A c t i v i t y A d d i t i v e s
"Moderate" o r I n t e r m e d i a t e A c t i v i t v Additives
" M i Id" o r Low A c t i v i t v Additives
Straight cutting o i l s
Hypoid gear o i l s
Worm gear o i l s
Drawing compounds
(e.g. m u l t i - p u r p o s e gear 1 ub r i c a n t s )
S p i r a l bevel gear o i l s
Metal-forming l u b r i c a n t s
I n d u s t r i a l gear o i l s
Manual gear box o i l s
Some hypoid gear o i Is
(e.g.
"open gear" o i Is)
I n d u s t r i a l gear o i l s f o r general a p p l i c a t i o n
Motor o i l s Steam t u r b i n e o i l s Jet aircraft turbine oils Gas t u r b i n e o i l s Automatic t r a n s m i s s i o n fluid Hydraulic o i l s ( f i r e r e s i s t a n t emulsion type) I n d u s t r i a l gear o i l s f o r closed o r c i r c u l a t i n g systems
259
10.3.7.2
A u t o mot i ve E . P .
Gear Oils
These o i l s a r e used t o l u b r i c a t e t h e worm, s p i r a l b e v e l , or hypoid gear d r i v e s o f a u t o m ot i ve v e h i c l e s . o f E.P.
S i n ce hyp oi d gears r e q u i r e t h e g r e a t e s t measure
p r o t e c t i o n o f a l l commercial gear d r i v e s , most a d d i t i v e treatments a r e
designed t o g i v e s a t i s f a c t o r y performance i n t h i s environment. H i s t o r i c a l l y , E.P. general c a t e g o r i e s .
a d d i t i v e s f o r use i n gear s e r v i c e f a l l i n t h e f o l l o w i n g T h i s i n f o r m a t i o n sh ou l d n o t be i n t e r p r e t e d as i n d i c a t i n g
t h a t e a r l y treatments a r e obsolete;
i n f a c t , a l l o f t h e l i s t e d treatments a r e
i n commercial use today, many on i n d u s t r i a l a p p l i c a t i o n s . developed a d d i t i v e s , however,
The more r e c e n t l y
en j oy most o f t h e market.
-
E a r l i e s t s u c cessf ul t rea t me nt
Lead soap (e.g.
lead naphthenate p l u s an
a c t i v e o r "corrosive"
organic sulphur
compound.
-
L a t e 1930's
C h l o r i n e and "Moderately a c t i v e ' ' s u l p h u r p r e s e n t i n t h e same o r d i f f e r e n t o r g a n i c mo l ecu l es
-
World War I I p e r i o d
Sulphur, c h l o r i n e and phosphorus i n s u i t a b l e organic c a r r i e r s
Most r e c e n t typ es (1960-
-
)
Sulphur and phosphorus i n s u i t a b l e o r g a n i c carriers
Typical E.P.
a d d i t i v e s used comme rci al l y i n f o r m u l a t i n g gear l u b r i c a n t s have
i nc lu d e d : C h l o r i n a t e d p a r a f f i n wax (40%
-
60% c h l o r i n e )
Chlornaphtha x a n t h a t e ( r e a c t i o n p r o d u c t o f c h l o r i n a t e d naphtha
and a l k a l i
metal x a n t h a t e ) C h l o r i n a t e d p a r a f f i n wax s u l p h i d e s ( r e a c t i o n p r o d u c t o f c h l o r i n a t e d paraf f i n wax and a l k a l i metal s u l p h i d e s ) S u l p h u r i s e d f a t t y o i l s (e.g.
sulphurised l a r d o i l , sulphurised f i s h o i l ,
s u l p h u r i s e d sperm o i I ) S u l p h u r i s e d hydrocarbons such as po l ybu t e nes S u l p h u r i s e d s y n t h e t i c e s t e r s (e.g.
s u l p h u r i s e d methyl o l e a t e o f f a t t y a c i d s )
S u lp h u r c h l o r i d e - t r e a t e d f a t t y o i l s (e. g. S 2 C I 2
A 1 i p h a t i c and a r o m a t i c p o l y s u l p h i d e s (e.g.
-
treated f i s h o i l )
benzyl d i s u l p h i d e , c h l o r o b e n z y l
d i s u l p h i d e , b u t y l d i s u l p hi d e) Phosphosulphurized f a t t y o i l s (e.g.
l a r d o i l heated w i t h s u l p h u r and phos-
phorus pen t a s u 1 ph i d e ) Organic p h o s p h i t e s ( o b t a i n e d by t r e a t i n g a l c o h o l s w i t h P C I )
3
A l k a r y lp h o s p hat es ( o c t y l p h e n o l t r e a t e d w i t h P205)
260 A l k y l phosphates ( a l c o h o l s t e a t e d w i t h P 0 )
2 5
Lead naphthenate Z i n c and l e a d d i - o r g a n o d i t h i o p h o s p h a t e s Z i n c and le a d d i - a l k y l
dithiocarbamates
An e v a l u a t i o n o f t h e p r a c t i c a l e f f e c t i v e n e s s o f a hypoid gear l u b r i c a n t must be c a r r i e d o u t i n f u l l - s c a l e equipment i n t h e l a b o r a t o r y and i n t h e f i e l d . Bench t e s t r i g s have n o t been a b l e t o p e r f o r m t h i s t a s k , a l t h o u g h they a r e useful
i n s c r e e n in g l i k e l y ca nd i da t e s f o r f u l l - s c a l e e v a l u a t i o n .
A l i s t o f these
t y p i c a l bench t e s t s i s g i v e n i n T a b l e 10.4.
10.3.7.3
A n t iwe ar A d d i t i v e s
A lt h o u g h d i s c u s s i n g p r i m a r i l y gear systems,
t h e advent o f h i g h performance
engines f o r passenger c a r s posed new l u b r i c a t i o n problems.
Engine i n s p e c t i o n
began t o r e v e a l un mi st a kab l e evi d en ce o f e xcessive wear and s c u f f i n g o f v a l v e t r a i n components;
h i g h r o t a t i o n a l speeds and i n c r e a s i n g pressures between cams
and l i f t e r f o o t su rf a ces had a p p a r e n t l y combined t o s h i f t l u b r i c a t i o n r e q u i r e ments i n t o t h e boundary r e g i o n .
The f i r s t and perhaps most e f f e c t i v e E.P.
addi-
t i v e f o r c o n t r o l l i n g o r e l i m i n a t i n g wear and s c u f f i n g i n t h e v a l v e t r a i n area was found t o be t h a t v e r s a t i l e a d d i t i v e
-
zinc dialkyldithiophosphate.
Other a d d i t i v e s found u s e f u l f o r t h e c o n t r o l o f v a l v e t r a i n wear i n c l u d e : T r i c r e s y l phosphate D i l a u r y l phosphate Didodecyl p h o s p h i t e S u l p h u r i z e d terpenes S u l p h u r i z e d sperm o i l C h l o r i n a t e d compounds Zinc d i a l k y l dithiocarbamate I t has been mentioned e a r l i e r t h a t t h e z i n c d i o r g a n o d i t h i o p h o s p h a t e p l a y s a v e r y i m p o r t a n t r o l e i n a n t i w e a r h y d r a u l i c o i l s and i n m i l d EP gear o i l s .
The
c h o i c e o f t h e c o r r e c t t ype o f compound depends upon t h e d e s i r e d performance characteristics. alcohols.
A z i n c d i t h i o p h o s p h a t e may be prepared u s i n g a v a r i e t y of
P r im a ry and secondary a l k y l groups a r e used f o r a p p l i c a t i o n s such a s
g a s o l i n e e n g in e and h y d r a u l i c systems and aro matics ( s u b s t i t u t e d phenols) f o r d i e s e l engines where t h e maj o r r o l e i s as a h i g h temperature a n t i o x i d a n t . It is
e s s e n t i a l t h e r e f o r e t h a t t h e performance o f these m a t e r i a l s i s r e l a t e d
t o the application.
G e n e r a l l y , good a n t i w e a r performance means low thermal
s t a b i l i t y , b u t a good a n t i o x i d a n t .
For h y d r a u l i c system use t h e a d d i t i v e must
be r e s i s t a n t t o h y d r o l y s i s a t t emp era t u res t y p i c a l o f h y d r a u l i c c i r c u i t s . An i n d i c a t i o n o f t h e ma j or performance p r o p e r t i e s o f a range o f z i n c diorganod i t h i o p h o s p h a t e s i s g i v e n i n T ab l e 10.5.
T a b l e 10.4 L u b r i c a n t , F r i c t i o n , Wear and Gear T e s t Machines
T e s t Machine
Type o f Contact
Type o f Measurement
Type o f Load ing
Range o f Load ing (kg)
Almen-Weiland
Conforming area, pin/bearing s h e l l s
F r i c t i o n , wear, load-carrying
Mechanical steps
0-2000
F a l e x I P 241/77T
M u l t i p l e line/area
F r i c t i o n , wear, load-carrying
Mechanical continuous
0-2000
Shell 4-Ball I P 239179 T
Multiple point hardened s t e e l b a l Is
F r i c t i o n , wear, 1 oad- ca r r y ing
Dead w e i g h t steps
0-1800
T i mken I P 240/76 T
B l o c k and r i n g . Line
F r i c t i o n , wear, load-carrying
Dead w e i g h t continuous o r step
0-50
Niemann-FZG I P 33h/77 T
Gear. Two t o o t h Load-carrying forms. P i n i o n t e e t h wear r a t e 16. Wheel t e e t h 24. Spur/Case hardened
Dead w e i g h t step, s t a r t under l o a d
1600 'Normal' load
I .A.E. I P 166/77
Gear-one t o o t h Load-carrying form p i n i o n t e e t h 16. Wheel 16. Spur/ Case hardened
Dead w e i g h t step, s t a r t under l o a d
Gear-one t o o t h form pinion/wheel t e e t h 28
Hydraulic a p p l i e d when running
Ryder FTMS 791a-6508
Load-carrying
Up t o a t l e a s t 70 Lever
Speed
Other c h a r a c t e r i s t i c s
m/s
0
-
0.2
0.1-0.25 1500 rprn
0-400 (0-800rpm)
7.3
A l s o used f o r c o r r o s i o n t e s t on specimens a f t e r r u n n i n g Also f o r corrosion t e s t A l s o w i t h d i f f e r e n t loads speed and specimens f o r wear t e s t s . "Roll ing" u n i t f o r b e a r i n g studies A l s o f o r grease s t u d i e s
A l s o runs a t 2175 r.p.m. O i 1 temperature 9OoC
7.9 4000 rpm
A l s o runs a t 2000 and 6000 rpm O i l temperature 200O-6O0C, 400O-7O0C
10000
oil
rpm
temperature 7 4 ' ~ S t a r t up a t no l o a d N CL Q,
262 TABLE 10.5
R e l a t i v e Performance o f Z i n c Di t hiophosphates Substrate
A1 coho1
Performance
Thermal STabi 1 i t y
AntiOxidancy
Hydrolytic Stabi 1 it y
AntiWear
Bearing Protection
Secondary-1
4
2
2
2
Secondary-2
5
1
1
1
1
P r im a r y - 1
2
4
4
4
4
2
P r ima r y - 2
3
3
3
3
3
Aromatic-1
1
5
5
5
Aromatic-2
1
5
5
5 5
5
These a d d i t i v e s a r e t y p i c a l o f those used f o r l u b r i c a n t s Rating
5
T:
worst c o n d i t i o n
1 = best c o n d i t i o n
10.3.7.4
EP a d d i t i v e s f o r T u r b i n e O i l s
Advances i n t h e de si gn o f steam t u r b i n e , gas t u r b i n e ,
and j e t a i r c r a f t t u r b -
i n e engines and t h e i r a s s o c i a t e d g e a r i n g have i n t r o d u c e d problems o f boundary lubrication.
To s o l v e these problems, t h e a d d i t i v e i n d u s t r y has developed E.P.
a d d i t i v e s o f t h e "low a c t i v i t y " v a r i e t y whi ch p r o v i d e p r o t e c t i o n a g a i n s t excessi v e wear a n d
s c u f f i n g o f t u r b i n e en gi n e components.
Such a d d i t i v e s must w i t h -
s ta n d h i g h temperatures -and i n t h e case o f steam t u r b i n e s , w a t e r c o n t a m i n a t i o n w i t h o u t p r o m o tin g c o r r o s i o n o f t u r b i n e e ng i ne components.
-
For use i n gas t u r -
b i n e and j e t a i r c r a f t t u r b i n e engines f o r m u l a t e d w i t h s y n t h e t i c ester-base f l u i d s d i f f e r e n t ch emi ca l s may be r e q u i r e d . For steam t u r b i n e o i l s a d d i t i v e s based on phosphorus and h y d r o l y t i c a l l y s t a b l e c h l o r i n e c o n t a i n i n g chemicals have been used. t i v e s a r e a l s o known t o have been used.
C e r t a i n sulphur-phosphorus a d d i -
I n t h i s case, t h e s u l p h u r i s i n a c t i v e
towards copper a t normal temperatures and becomes r e l e a s e d o n l y a t h i g h operat i ng temperatures
.
For gas t u r b i n e a p p l i c a t i o n s phosphorus compounds, phates, a r e used.
b o t h phosphites and phos-
I n c e r t a i n cases p o l y g l y c o l f l u i d s may a c t i n t h e same way
-
presumably by s t r o n g a d s o r p t i o n o f t h e hydroxy groups t o metal s u r f a c e s . 10.3.7.5
EP A d d i t i v e s f o r C u t t i n g O i l s
Very h i g h p r e ssu res and temperatures a r e developed l o c a l l y between t h e work and t h e c u t t i n g t o o l i n machining o p e r a t i o n s .
The s o - c a l l e d " s t r a i g h t c u t t i n g
o i l s " w i d e l y employed t o c o o l and l u b r i c a t e t h e work and c u t t i n g t o o l a r e m i n e r a l 0 1 1 s which have been blended w i t h s u i t a b l e E.P.
additives.
Since the
l u b r i c a t i o n o f a m e t a l - c u t t i n g o p e r a t i o n i s almost completely w i t h i n t h e boundary
263 region, E . P .
a d d i t i v e s o f high a c t i v i t y are required f o r best r e s u l t s .
I n the
case o f an o r g a n i c s u l p h u r compound, i t s a c t i v i t y s h o u l d be such t h a t a d i l u t e s o l u t i o n (e.g.
1 o r 2 p e r c e n t ) o f t h e compound i n a m i n e r a l o i l w i l l completely
blacken a copper s t r i p w i t h i n one hour a t 100°C.
Sulphur compounds having t h i s
degree o f a c t i v i t y a r e known i n t h e i n d u s t r y as " c o r r o s i v e s u l p h u r " a d d i t i v e s and a r e used e x t e n s i v e l y i n t h e compounding o f commercial c u t t i n g o i l s . Examples o f E.P.
a d d i t i v e s developed f o r use i n c u t t i n g o i l s i n c l u d e s u l p h u r i z e d m i n e r a l
o i l , sulphurized f a t t y o i l s , sulphur chloride-treated f a t t y o i l s , sulphurized o l e f i n s , s u l p h u r c h l o r i d e - t r e a t e d o l e f i n s , benzyl p o l y s u l p h i d e s , c h l o r i n a t e d p a r a f f i n wax, and c h l o r i n a t e d m i n e r a l o i l s . However, metal w o r k i n g and f o r m i n g o p e r a t i o n s r e q u i r e many types o f chemical a d d i t i v e a p a r t f r o m t h e a c t i v e o r c o r r o s i v e s u l p h u r type. sulphur a d d i t i v e s a r e used i n metal removal o p e r a t i o n s .
Generally, the a c t i v e The l i s t below i n d i c a t e s
the i n c r e a s i n g degree o f s e v e r i t y : (i)
T u r n i n g and M i l l i n g
(ii)
D r i l l i n g and Reaming
( i i i ) Tapping and Threading (iv) As v e r y h i g h
Broaching
emperatures a r e reached, t h e c h o i c e o f s u i a b l e a d d i t i v e dep-
ends a l s o on t h e m a t e r i a l
b e i n g machined, so t h a t t h e b e s t c o o l i n g , s u r f a c e
f i n i s h and t o o l l i f e a r e r e t a i n e d . The chemical a c t i v i t y o f s u l p h u r compounds may be chosen t o g i v e p a r t i c u l a r performance i n c u t t i n g o p e r a t i o n s .
When such m a t e r i a l s a r e i n c o r p o r a t e d i n t o
f l u i d s h a v i n g a dual-purpose n a t u r e , such as machine l u b r i c a t i o n , then t h e choice has t o be made w i t h t h e requirements o f t h e h y d r a u l i c o r c i r c u l a t i n g o i l c i r c u i t m e t a l l u r g y matched a g a i n s t t h e metal w o r k i n g o p e r a t i o n . A d d i t i v e s a r e a l s o used i n metal f o r m i n g o p e r a t i o n s such as r o l l i n g , drawing, stamping and f o r g i n g .
For r o l l i n g o i l s t h e g r e a t e s t requirement i s h i g h s u r -
face f i n i s h , which means t h a t t h e a d d i t i v e s must n o t be c h e m i c a l l y a c t i v e .
Gen-
e r a l l y , f a t t y a l c o h o l s and e s t e r s a r e p r e f e r r e d . Drawing and f o r g i n g o p e r a t i o n s r e q u i r e h i g h l y s p e c i a l i s e d p r o d u c t s .
Fatty-
based systems a r e used c o n t a i n i n g a v a r i e t y o f a d d i t i v e s i n c l u d i n g s o l i d l u b r i cants such as g r a p h i t e and molybdenum d i s u l p h i d e .
10.3.7.6
Mode o f A c t i o n o f E.P.
E.P.additives
Additives
f u n c t i o n by r e a c t i n g w i t h r e l a t i v e l y moving s u r f a c e s under
boundary l u b r i c a t i o n c o n d i t i o n s t o f o r m an adherent f i l m which has lower shear s t r e n g t h than t h a t o f t h e metal s u r f a c e s themselves.
T h i s f i l m a c t s as a s o r t
o f s o l i d l u b r i c a n t , and t a k e s o v e r t h e t a s k o f l u b r i c a t i o n when t h e o i l i s no longer a b l e t o p r o v i d e a s e p a r a t i n g and p r o t e c t i v e l u b r i c a t i n g f i l m .
W i t h an
264 a p p r o p r i a t e E.P.
additive,
t h e r e i s l i t t l e o r no f o r m a t i o n o f such " s o l i d l u b r i -
c a n t " under c o n d i t i o n s o f hydrodynamic l u b r i c a t i o n .
I t forms o n l y a t t h e eleva-
t e d temperatures which develop l o c a l l y between metal surfaces under c o n d i t i o n s o f boundary l u b r i c a t i o n .
Because t e mpe rat ure has been shown t o be t h e most
i n f l u e n t i a l parameter i n t h e f u n c t i o n o f E.P.
l u b r i c a n t s , F.P.Bowden and co-
workers have suggested t h a t t h e t erm "Extreme Temperature" l u b r i c a n t s and a d d i t i v e s m ig h t be more a p p r o p r i a t e . Emulsifiers
10.3.8
G e n e r a l l y , an e m u l s i f i e r i s a chemical f o r d i s p e r s i n g e i t h e r w a t e r i n o i l o r o i l i n wa t e r .
The former,
hydraulic fluids,
t h e more d i f f i c u l t system, produces f i r e - r e s i s t a n t
r o c k d r i l l l u b r i c a n t s , and many types o f w ire-draw ing media.
The e m u l s i f i e r may be o f t h e a l k e n y l s u c c i n i m i d e type, f a t t y e s t e r s , o r o t h e r s c o n t a i n i n g f a t t y resi d ue s.
For me t a l wo rki ng and h y d r a u l i c media o f t h e s o l u b l e
o i l ( o i l i n w a t e r ) type, a l a r g e range o f e m u l s i f i e r s i s a v a i l a b l e .
The c h o i c e
depends upon whether i t i s d e s i r a b l e t o have a chemical w i t h i o n i c c h a r a c t e r i s t i c s , a s a l t , o r one w i t h n o n - i o n i c p r o p e r t i e s such as e s t e r s , phenol e t h e r s and o t h e r oxygenated chemicals.
A s h o r t l i s t i s g i v e n below o f some o f t h e
chemicals: Sodium sul p ho na t es T a l l o i l amides E t ha no l amines Qu at ern ary Ammonium s a l t s P o l y a l k y l e n e phenol e t h e r s and a s s o c i a t e d oxygenated p r o d u c t s Ethoxylated f a t t y acids Salts o f f a t t y acids E m u l s i f i e r s a r e c l a s s i f i e d a c c o r d i n g t o a nu merical value, t h e
HLB
number,
o b t a i n e d by e s t i m a t i n g t h e emu l si on s t a b i l i t y when prepared i n a standard way. For o i l - i n - w a t e r
12-15,
s o l u b l e o i l emulsions,
f o r water-in-oil
t h e HLB range f o r e m u l s i f i e r s i s
i n v e r t emulsions i t i s 4-6 when checked i n a naphthenic
base s t o c k .
10.3.9
F r i c t i o n Modifiers
C e r t a i n d e f i c i e n c i e s a r e sometimes their f r i c t i o n a l characteristics.
observed w i t h l u b r i c a n t s w i t h r e s p e c t t o
Where metal s u r f a c e s a r e designed t o s l i d e ,
two opposing r e q ui re men t s may o ccur: (i)
Smooth s l i d i n g w i t h no v i b r a t i o n and minimum c o e f f i c i e n t o f f r i c t i o n .
( i i ) No s l i d i n g maximum c o e f f i c i e n t o f f r i c t i o n f o r engagement o f c l u t c h s u r fa ces o r f r i c t i o n l o c k i n g d evi ces.
265 Additives f o r ( i ) a r e generally fatty-based, such as f a t t y e s t e r s and amides.
n a t u r a l l y - o c c u r r i n g products
They f i n d use i n s l i d i n g motion systems such
as machine t o o l sl i d eways. I n t h e case o f t y p e ( i i ) , t h e c l u t c h o r f r i c t i o n mechanism should engage o r disengage smoothly w i t h o u t v i b r a t i o n . t o control. (i)
Two t ypes o f a p p l i c a t i o n a r e d i f f i c u l t
A c h o i c e o f d i f f e r e n t p r o d u c t s may be made:-
Anti-chatter
i n l i m i t e d s l i p a x l e s , o r o t h e r metal/metal f r i c t i o n
locking units.
I t i s necessary t o reduce t h e s t i c k - s l i p a c t i o n o f
lubricated steel/steel Amide
-
contacts.
metal d i t h i o p h o s p h a t e combinations
Amine d i t h i o p h o s p h a t e s ( i i ) Anti-squawk a d d i t i v e s reduce v i b r a t i o n w hich g i v e s r i s e t o a u d i b l e noise i n clutches o f d i s s i m i l a r material on s t e e l , e t c . ) .
(bronze on s t e e l , asbestos
Chemicals used f o r t h i s purpose i n c l u d e :
N -acyl sa rcosi n es and d e r i v a t i v e s S u l p h u r i s e d f a t s and e s t e r s Organophosphorus a c i d and f a t t y a c i d m i x t u r e s Esters o f dimerised f a t t y acids Fo r m u la t io n s which meet t h e complex performance d e s i r e d i n automatic t r a n s m is s io n f l u i d s , u n i v e r s a l t r a c t o r e n g i n e - t r a n s m i s s i o n o i l s o r machine t o o l systems a r e t h e
r e s u l t o f c a r e f u l ma t ch i ng o f f r i c t i o n m o d i f i e r s i n t h e whole
a d d i t i v e system.
10.4
CONCLUSIONS
The c h a p te r w i l l se rve t o i n d i c a t e t h e complex n a t u r e o f t h e chemicals used i n lubricant formulations.
M a i n l y , t h e d i s c u s s i o n has been about a d d i t i v e s f o r
m i n e r a l l u b r i c a t i n g o i l s a l t h o u g h t h e r e has been r e f e r e n c e made t o s y n t h e t i c f l u i d s and w a t e r c o n t a i n i n g h y d r a u l i c and metal w orking f l u i d s . I n f o r m u l a t i n g l u b r i c a n t s o f any type,
t h e presence o f these a d d i t i v e s re-
q u i r e s t h a t n o t o n l y sh ou l d each i n d i v i d u a l p r o d u c t c a r r y o u t i t s d e s i r e d funct i o n b u t t h a t i n t e r a c t i o n s between more t ha n one chemical must n o t cause d e t e r i o r a t i o n o f performance. s o l u b l e b y - p r o d u ct s.
N e i t h e r must m i x t u r e s r e a c t t o g e t h e r t o form o i l i n -
The f i n a l o i l f o r m u l a t i o n must be s t a b l e o v e r
t h e range
o f o p e r a t i n g and s t o r a g e temperatures. M i x t u r e s o f a d d i t i v e s may be f o r m u l a t e d and used as a package a t a g i v e n tre a tm e n t l e v e l , r e q u i r i n g o n l y
the c a r r i e r f l u i d .
Again, t h e r e must be no
d e t e r i o r a t i o n a f t e r ma nu f a ct u re o r s t o r a g e and the combination must be s e l e c t e d w i t h g r e a t c a r e so t h a t maximum performance i s achieved a t an economic c o s t . The a u t h o r wishes t o acknowledge t h e a s s i s t a n c e o f h i s colleagues i n t h e L u b r i z o l C o r p o r a t i o n f o r r e v i e w i n g t h i s ch ap ter and suggesting a d d i t i o n a l data;
266 also to the Lubrizol Corporation for permission to compile the information REFERENCES 1
2
Smallheer and Kennedy-Smith, Lubricant Additives, The Lezius-Hiles Co., Cleveland, Ohio. C.V.Smallheer, Lectures on Lubricant Additives, Imperial College, London, March 1970.
267
11 CONSUMPTION AND CONSERVATION OF LUBRICANTS
A.R.
LANSDOWN
D i r e c t o r , Swansea T r i b o l o g y Centre, U.K.
11.1
CONSUMPTION
Apart f r o m t h e r e l a t i v e l y small q u a n t i t i e s o f v e g e t a b l e and animal o i l s , almost a l l modern l u b r i c a n t s a r e d e r i v e d f r o m petroleum, e i t h e r by f r a c t i o n a l d i s t i l l a t i o n o r chemical conversion.
I t i s now g e n e r a l l y recognised t h a t t h e
w o r l d ' s remaining s u p p l i e s o f p e t r o l e u m have o n l y a l i m i t e d l i f e . The v a r i o u s e s t i m a t e s o f t h e remaining l i f e o f p e t r o l e u m reserves depend on many assumptions.
The r a t e o f f u t u r e consumption i s i t s e l f dependent on
several assumptions, fuels,
such as t h e a v a i l a b i l i t y and c o m p e t i t i v e n e s s o f a l t e r n a t i v e
improvements i n e f f i c i e n c y o f u t i l i s a t i o n , and f i s c a l
incentives.
The
estimates o f r e m a i n i n g r e s e r v e s a l s o depend on s e v e r a l assumptions, such as t h e r a t e o f d i s c o v e r i n g new r e s e r v e s and t h e p o t e n t i a l f o r u s i n g low-grade sources. For energy purposes t h i s l a s t f a c t o r reaches i t s l i m i t when t h e energy r e q u i r e d t o e x p l o i t a source i s equal t o t h e energy u l t i m a t e l y o b t a i n e d from t h e source. There i s a general tendency t o d i s c u s s t h e f u t u r e o f p e t r o l e u m p u r e l y from the energy s t a n d p o i n t , and t o r e l a t e i t t o t h e phasing i n o f a l t e r n a t i v e energy sources.
There a r e however many p r o d u c t s f r o m p e t r o l e u m which w i l l be l e s s
r e a d i l y r e p l a c e d f r o m non-petroleum sources, and l u b r i c a n t s a r e i n t h i s category
.
A f u r t h e r l i m i t a t i o n on f u t u r e l u b r i c a n t s u p p l i e s i s t h a t n o t a l l crude o i l s can be used t o produce l u b r i c a n t s w i t h o u t expensive, and energy-expensive, chemical p r o c e s s i n g .
The U n i t e d Kingdom's N o r t h Sea o i l i s i n f a c t g e n e r a l l y
u n s u i t a b l e f o r l u b r i c a n t manufacture. We a r e t h u s f a c e d w i t h t h e s i t u a t i o n t h a t w i t h i n t h e w o r k i n g l i f e o f some o f our younger l u b r i c a n t t e c h n o l o g i s t s petroleum-based l u b r i c a n t s w i l l cease t o be p l e n t i f u l and may become e x t r e m e l y expensive.
Conservation o f l u b r i c a t i n g o i l s
should t h e r e f o r e a l r e a d y be a m a t t e r o f s e r i o u s concern, and w i l l i n e v i t a b l y be recognised as such w i t h i n one, o r a t most two, decades.
268 Table 11.1 shows a breakdown o f t h e t o t a l U n i t e d Kingdom consumption o f pe tr o le u m f o r t he ca l en da r year 1977, and i t can be seen t h a t l u b r i c a t i n g o i l s and greases r e p rese nt j u s t o v e r one m i l l i o n m e t r i c tons, o r 1.17% o f t h e t o t a l . A c c u r a t e f i g u r e s f o r t h e breakdown o f l u b r i c a n t types a r e more d i f f i c u l t t o o b t a i n , b u t Table 11.2 g i v e s e s t i m a t e s o f U.K. th e more im p o r t ant c a t e g o r i e s .
l u b r i c a n t consumption i n some o f
The b i g g e s t s i n g l e c a t e g o r y c o n s i s t s o f a u t o -
m o t i v e e n g in e o i l s , c o m p r i s i n g perhaps one t h i r d o f t h e t o t a l l u b r i c a n t consumpt io n . Table 11.1
U n i t e d Kingdom Consumption o f P e troleum Products 1977 ( F i g u r e s from I n s t i t u t e o f Petroleum "Petroleum S t a t i s t i c s " )
Product c a t e g o r y
Consumption (thousand tonnes)
Motor s p i r i t
17,336
Gas, d i e s e l and f u e
oils
47,920
Aviation fuels
4,218
Other f u e l s
9,639
Naphtha/Light d i s t i l a t e
5,179
L u b r i c a t i n g o i l s and greases
1,029
Other p r o d u c t s
2,835
Tota 1
Table 11.2
88,156
E s tima t e d L u b r i c a n t Consumption by Types End Use
TY Pe
Consumption (%)
Av i a t i o n
0.4
Marine
8.5
Tractor
3.0
Motor
Oils
Motor
Greases
35.0 0.1
Industrial
Hydraul i c o i 1 s
16.0
Industrial
B e ari n g o i l s
22.5
5.5 5.5
lndust r i a l
Me t a l wo rki ng
Industrial
Process o i l s
Industrial
I.C.E.
Industrial
Greases
1 .o
Industrial
Fuel as l u b r i c a n t
1 .o
oils
1.5
269 O v e r a l l l u b r i c a n t s account f o r o n l y a l i t t l e o v e r 1 % o f t h e t o t a l p e t r o l e u m consumption, b u t t h e importance o f t h i s 1 % i s increased by t h e two f a c t o r s p r e v i o u s l y mentioned, namely t h e g r e a t e r d i f f i c u l t y o f f i n d i n g s u b s t i t u t e sources f o r l u b r i c a n t s and t h e l i m i t e d range o f crude o i l s which can be used t o produce l u b r i c a n t s . The u l t i m a t e f a t e o f t h e v a r i o u s l u b r i c a n t s i s a l s o d i f f i c u l t t o assess accurately.
Table 11.3 shows some e s t i m a t e s made i n 1973 o f t h e f a t e o f a u t o -
motive engine o i l s .
From t h e c o n s e r v a t i o n p o i n t o f view t h e r e a r e two i n t e r -
e s t i n g aspects o f these e s t i m a t e s .
The f i r s t i s t h a t some 185 000 t o n s were
a p p a r e n t l y p o t e n t i a l p o l l u t e r s o f l a n d and water.
The second
s t h a t some
258,000 tons c o u l d t h e o r e t i c a l l y be reclaimed. An a r t i c l e p u b l i s h e d i n t h e AA "Drive" t h a t the 58,000
magazine i n January,
1974 suggested
t o n s o f engine o i l s changed each year by motor s t s a t home was
p o s s i b l y disposed o f as shown i n Table 11.4.
The l a r g e r quant t y changed i n
garages was, however, g e n e r a l l y disposed o f more l e g a l l y . Since 1974 t h e r e have been more d e t a i l e d surveys, b u t t h e i n c r e a s i n g awareness o f t h e need f o r c o n s e r v a t i o n , has a l s o l e d t o a tendency t o l o n g e r o i l change p e r i o d s and t o a g r e a t e r c o n t r i b u t i o n o f r e - r e f i n i n g t o t h e d i s p o s a l prob 1 em. Both o f these f a c t o r s ,
r e d u c t i o n i n consumption and r e - r e f i n i n g , w i l l
e v i t a b l y assume g r e a t e r importance i n coming years.
Table 11.3
Table 11.4
Estimates o f t h e Fate o f Motor O i l s Burned (exhaust)
25%
95,000 t o n s
Changed (garages)
50%
190,000 t o n s
Changed (home)
15%
58,000
tons
Leaked
5%
Scrapped w i t h v e h i c l e
1%
4,000
Spi 1 l e d
1%
4,000 t o n s
Railway ( ? )
1%
5,000 tons
19,000 t o n s tons
Estimates o f O i l Disposal by " D o - i t - y o u r s e l f " B u r i e d i n garden Poured down d r a i n s
Car Owners 20% 8%
Burned
18%
Taken t o garage e t c . f o r p r o p e r d i s p o s a l
17%
Otherwise disposed o f
37%
in-
11.2
REDUCING CONSUMPTION
There a r e o f cou rse p u r e l y mechanical ways t o reduce consumption, such as e l i m i n a t i n g leakage and i mpro vi ng s e r v i c e a b i l i t y o f engines.
Improvement may
sometimes a l s o be p o s s i b l e by t h e use o f a more v i s c o u s o i l , b u t t h i s must o b v i o u s l y be done w i t h c a u t i o n .
Not o n l y must c a r e be used t o ensure t h a t a
s u i t a b l e v i s c o s i t y i s used, b u t as a ge ne ral r u l e t h e use o f a h i g h e r v i s c o s i t y o i l w i l l mean h i g h e r power consumption, and the r e s u l t i n g energy wastage w i l l pr o b a b ly more t h an o f f s e t any l u b r i c a n t saving. The b e s t p r o s p e c t f o r red uci n g consumption l i e s i n e n s u r i n g t h a t o i l changes a r e n o t c a r r i e d o u t any more f r e q u e n t l y t ha n they need t o be. Even i n i d e n t i c a l systems t h e r a t e o f o i l degradation can v a r y c o n s i d e r a b l y . The f o l l o w i n g a r e some o f t h e f a c t o r s whi ch l e a d t o r a p i d degradation. Dusty o r d i r t y environments High temperatures Very low t e m pera t u res Temperature f l u c t u a t i o n s ,
l e a d i n g t o con densation
High a l t i t u d e Poor f i l t e r maintenance Low o i l l e v e l s Frequent s t o p - s t a r t o p e r a t i o n Short j o u r n e y l e n g t h s i n a v e h i c l e Contamination by ch emi ca l s Contamination by unburned f u e l Contamination by combustion p r o d u c t s Contamination by wear d e b r i s W i t h i n t h e o i l f o r m u l a t i o n t h e f o l l o w i n g f a c t o r s can a l s o lead t o r a p i d de g r a d a t io n . Poor q u a l i t y base o i l , c o n t a i n i n g u n s t a b l e molecules Inadequate a n t i - o x i d a n t c o n t e n t I n s u f f i c i e n t dispersant o r detergent a d d i t i v e s I n s u f f i c i e n t a nt i -we ar o r EP a d d i t i v e s I n s u f f i c i e n t basic additives w i t h sulphur-containing fuel Excessively r e a c t i v e o r unstable a d d i t i v e s Because o f a l l these v a r i a b l e s ,
t h e r e q u i r e d o i l change p e r i o d i n two
i d e n t i c a l e n g in e s may v a r y by a f a c t o r o f t e n i n d i f f e r e n t o p e r a t i n g c o n d i t i o n s .
I n an e r a o f r e l a t i v e l y cheap l u b r i c a n t s , recommended o i l change p e r i o d s w i l l te n d t o be s h o r t enough t o ensure s a t i s f a c t o r y q u a l i t y i n t h e w o r s t s e t o f
c o n d i t i o n s , because t h e economic p e n a l t y f r o m u s i n g an o i l f i l l t o o l o n g w i l l be f a r g r e a t e r t h a n t he v a l u e o f t h e o i l saved.
271 The s o l u t i o n i s o i l q u a l i t y m o n i t o r i n g , which enables t h e o i l r e m a i n i n g i n an i n d i v i d u a l system t o be assessed p e r i o d i c a l l y , and o n l y changed when i t s cond i t i o n i s approaching an u n s a t i s f a c t o r y l e v e l .
As a bonus, o i l m o n i t o r i n g w i l l
a l s o g i v e v a l u a b l e i n f o r m a t i o n about t h e c o n d i t i o n o f t h e engine o r o t h e r l u b r i c a t e d system. O i l m o n i t o r i n g i s an i m p o r t a n t p a r t o f machinery h e a l t h m o n i t o r i n g , which i s described i n d e t a i l i n Chapter 18.
I t i s t h e r e f o r e unnecessary t o d e s c r i b e t h e
various techniques a t l e n g t h here, b u t t h e r e a r e two a s p e c t s which should be mentioned. The f i r s t o f these i s t h e need t o t a i l o r t h e m o n i t o r i n g techniques t o t h e s i z e and importance o f t h e o i l system b e i n g monitored.
A l a r g e c r i t i c a l system
w i l l j u s t i f y t h e e f f o r t and c o s t i n v o l v e d i n f r e q u e n t s p e c t r o g r a p h i c o i l a n a l y s i s or ferrography.
Even a small system, such as a c a r engine, may w e l l j u s t i f y t h e
use o f a magnetic p l u g t o m o n i t o r wear d e b r i s , examination o f a drop o f o i l from the d i p s t i c k by t h e f i l t e r paper technique, o r v i s c o s i t y by t h e F l o s t i c k method. The second f a c t o r i s t h e need t o e s t a b l i s h f o r any system a c r i t e r i o n f o r d e c i d i n g when t o change t h e o i l .
T h i s may be a l e v e l o f a c i d i t y i n t h e T o t a l
Acid Number, a degree o f v i s c o s i t y change, o r t h e v i s i b l e appearance o f contaminants on a f i l t e r - p a p e r , essential
b u t an o b j e c t i v e c r i t e r i o n o r c r i t e r i a w i l l be
i f o i l change i s t o be determined by o i l q u a l i t y .
I t i s a salutary
thought t h a t t h e i n c i d e n c e o f i n f l i g h t engine f a i l u r e s i n c e r t a i n U n i t e d States
A i r Force a i r c r a f t decreased when n o t o n l y r o u t i n e o i l change, b u t r o u t i n e engine overhaul p e r i o d s were d i s c o n t i n u e d i n f a v o u r o f s c h e d u l i n g i n accordance with spectroscopic o i l analysis.
11.3
RECLAMATION AND RE-REFINING
Even when a l u b r i c a n t has d e t e r i o r a t e d so t h a t i t i s no l o n g e r f i t f o r service,
t h e g r e a t e r p a r t o f i t w i l l s t i l l be unchanged.
Much o f t h e degradation
i s by c o n t a m i n a t i o n , w h i l e a f u r t h e r f a c t o r i s d e p l e t i o n o f a d d i t i v e s .
Only a
very small p r o p o r t i o n o f t h e base o i l w i l l u s u a l l y have been degraded, and t h i s w i l l c o n s i s t o f t h e most u n s t a b l e molecules, u s u a l l y o x i d i s e d t o aldehydes, ketones o r c a r b o x y l i c a c i d s . I t i s t h e o r e t i c a l l y p o s s i b l e t o remove a l l t h e contaminants and t h e degraded a d d i t i v e s and base o i l molecules, t o add f r e s h a d d i t i v e s , and t h u s t o produce a l u b r i c a n t which d i f f e r s l i t t l e i f a t a l l f r o m t h e o r i g i n a l .
The processes
used a r e d e s c r i b e d as r e c l a m a t i o n o r r e - r e f i n i n g , depending on t h e e x t e n t o f treatment i n v o l v e d . I n t h e s i m p l e s t case t h e use o f an i n - l i n e f i l t e r t o remove s o l i d contami n a n t s i s a r e c l a m a t i o n technique.
Another example i s t h a t o f a t r a n s f o r m e r
o i l , which degrades i n s e r v i c e t o generate a small b u t unacceptable l e v e l o f
272 o f e l e c t r i c a l c o n d u c t i v i t y and can be c l e a n e d up i n s i t u t o recover t h e r e -
quired insulation l e v e l .
Such techniques a r e commonly known a s " l a u n d e r i n g "
and have been w i d e l y used f o r many y e a r s .
A well-established
system f o r more i n t e n s i v e r e - r e f i n i n g i s t h e a c i d - c l a y
system, which b a s i c a l l y c o n s i s t s o f t h e f i v e components shown i n F i g . 1 .
The
s t r a i n e r a t t h e i n l e t t o t h e waste o i l s t o r a g e tank removes major s o l i d contaminants, w h i l e water and sludge a r e d r a i n e d o f f f r o m t h e bottom.
Contamin-
a t i n g f u e l o r o t h e r v o l a t i l e m a t e r i a l s a r e removed i n t h e steam s t r i p p e r .
The
c l a r i f i e d o i l from t h e i n t e r m e d i a t e s t o r a g e i s t r e a t e d w i t h h o t s u l p h u r i c a c i d , which r e a c t s w i t h most o f t h e r e a c t i v e compounds p r e s e n t and removes them a s a c i d sludge.
Treatment w i t h heated a c t i v e " e a r t h " or " c l a y "
removes t h e
remaining p o l a r compounds, and t h e s o l i d s a r e f i n a l l y removed i n a f i l t e r - p r e s s . The cleaned o i l o b t a i n e d by t h i s process can be made s u i t a b l e f o r use a s l u b r i c a n t base o i l , b u t i s o f t e n blended i n t o heavy f u e l o i l s .
Strainer
-
sludge treatment
Clay and clean oil
Figure 1
A c i d - c l a y r e - r e f i n i n g process
The main disadvantage o f t h i s process i s t h e problem of d i s p o s i n g o f l a r g e amounts o f a c i d and a c i d c l a y .
An a l t e r n a t i v e process developed by t h e
I n s t i t u t e F r a n c a i s du P e t r o l e uses l i q u i d propane p r e c i p i t a t i o n t o remove t h e degraded m a t e r i a l s and r e s i d u a l a d d i t i v e s .
T h i s c o n s i d e r a b l y reduces t h e
273 ge n e r a tio n of waste b y-pro du ct s, economic
b u t a t pre sent t h e process i s probably n o t
.
A Matthys process c o n s i s t s e s s e n t i a l l y o f two d i s t i l l a t i o n stages.
The
f i r s t i s a t atmospheric p ressu re and 180°C and removes w ater and l i g h t hydroThe second t a kes p l a c e a t 3 4 O o - 3 6 O 0 C and i s designed so t h a t the
carbons.
un d e s ir a b le m a t e r i a l s f o rm a coke which i s c a r r i e d o f f i n t h e l i q u i d p r o d u c t s and removed by c e n t r i f u g i n g . Most r e - r e f i n i n g processes r e q u i r e a f i n i s h i n g stage t o produce f r a c t i o n s f o r re-use as l u b r i c a n t s and t h i s may be c l a y treatment o r a conventional hydrofinishing. The r e c o v e r y o f o i l f rom d i l u t e emulsions o r o i l y p l a n t waste-water difficult,
is
b u t i s i mpo rt a nt n o t o n l y f o r o i l c o n s e r v a t i o n b u t f o r e l i m i n a t i o n
of pollution.
The st an da rd procedure i s t o c r a c k emulsions by chemical t r e a t -
ment and a l l o w t h e p rod uct t o s e t t l e i n h o l d i n g tanks u n t i l t h e o i l can be skimmed o f f .
The o i l f r a c t i o n then tends t o c o n t a i n a h i g h p r o p o r t i o n o f d i r t ,
water and t h e t r e at men t chemical.
I t i s l i k e l y t o be uneconomical f o r r e -
r e f i n i n g and i s o f t e n blended i n t o b urn er f u e l . I n r e c e n t y e a rs, t ech ni q ue s have been developed which use p o l y m e r i c membranes t o f i l t e r o u t p a r t i c l e s and d r o p l e t s f rom t h e w ater, e i t h e r by d i r e c t u l t r a f i l t r a t i o n o r by re verse osmosis.
These t ech niques a r e r e p o r t e d t o g i v e a much
cleaner o i l f r a c t i o n as w e l l as a wat er f r a c t i o n s u i t a b l e f o r discharge t o sewers. One problem a s s o c i a t e d w i t h r e - r e f i n i n g o f engine o i l s i s t h a t t h e r e may be a s l i g h t b u i l d - u p o f p o l y n u c l e a r aro mat i cs, which a r e c a r c i n o g e n i c i n higher c o n c e n t r a t io n s .
I t may t h e r e f o r e be d e s i r a b l e t o t r e a t r e - r e f i n e d o i l s by a
f i n i s h i n g process such as s o l v e n t e x t r a c t i o n w hich w i l l remove p o l y n u c l e a r a roma t ic s
11.4
.
ECONOMICS
The economics o f o p t i m i s i n g o i l - c h a n g e p e r i o d s depends m a i n l y on t a i l o r i n g the m o n i t o r i n g tech ni q ue s t o t h e s i z e and importance o f the system.
Systems
which a r e e i t h e r v e r y l a r g e o r o f c r i t i c a l importance a r e a l r e a d y being moni t o r e d by s o p h i s t i c a t e d techniques.
Smaller and l e s s c r i t i c a l systems may
al re a d y j u s t i f y si mp l e m o n i t o r i n g techniques,
and as l u b r i c a n t a v a i l a b i l i t y
decreases, t h e balance w i l l i n e v i t a b l y s h i f t i n f a v o u r o f increased l e v e l s o f m o n it o r in g . The economics of r e - r e f i n i n g a l s o depends on a v a i l a b i l i t y o f l u b r i c a t i n g o i l . During t h e second World War l a r g e q u a n t i t i e s o f l u b r i c a t i n g o i l s were r e - r e f i n e d i n most c o u n t r i e s , b u t t h e i n d u s t r y d e c l i n e d i n t h e f o l l o w i n g t h i r t y years. The e x t e n t o f t h e d e c l i n e v a r i e d i n d i f f e r e n t c o u n t r i e s , b u t nowhere d i d i t
274 c o m p l e t e l y cease.
The reason f o r t h e d e c l i n e may have been p a r t l y p s y c h o l o g i c a l ,
i n t h a t r e - r e f i n e d o i l s were c o n s i d e r e d t o be i n f e r i o r i n some way, b u t t h e main reason was p r o b a b l y f i n a n c i a l .
The economics o f r e - r e f i n i n g d i d n o t p e r m i t more
than a token r e d u c t i o n i n r e t a i l p r i c e s o f r e - r e f i n e d o i l s . I t has been p o s s i b l e f o r many y e a r s t o produce r e - r e f i n e d o i l s o f s i m i l a r q u a l i t y t o new o i l s , b u t i n a p e r i o d o f r e l a t i v e a f f l u e n c e and p l e n t y most people have been h a p p i e r t o pay t h e m a r g i n a l l y h i g h e r p r i c e f o r "new" o i l . The main economic problem has been t h e c o s t o f c o l l e c t i n g and t r a n s p o r t i n g waste o i l , b u t a second f a c t o r has been t h e d i f f i c u l t y o f persuading o p e r a t o r s t o keep wastes o f d i f f e r e n t q u a l i t i e s c l e a n arld separate. I n t h e Federal Republic o f Germany a d e l i b e r a t e move was made t o reduce l u b r i c a n t consumption and encourage r e - r e f i n i n g by t h e i n t r o d u c t i o n i n 1968 o f t h e "Law on Measures t o Ensure t h e Disposal o f Waste O i l " .
T h i s law enabled
o r g a n i s a t i o n s d i s p o s i n g o f waste o i l s by approved methods t o c l a i m an allowance t o cover c o s t s i n v o l v e d i n d i s p o s a l .
The approved methods i n c l u d e d r e - r e f i n i n g .
The funds were r a i s e d by a s p e c i a l d u t y on imported l u b r i c a n t grade o i l , and t h i s had t h e e f f e c t o f i n t r o d u c i n g a p r i c e margin i n f a v o u r o f r e - r e f i n e d o i l s .
As a r e s u l t , i t was e s t i m a t e d i n 1973 t h a t up t o 30% o f t o t a l l u b r i c a t i n g o i l consumption was b e i n g r e - r e f i n e d i n Germany, compared w i t h o n l y 5% i n B r it a i n .
With d e c l i n i n g a v a i l a b i l i t y o f p e t r o l e u m l u b r i c a n t s , t h e p r o p o r t i o n o f r e r e f i n e d l u b r i c a n t s w i l l p r o b a b l y approach more and more t h e t h e o r e t i c a l l i m i t o f 60-70%.
275
12 HEALTH AND SAFETY ASPECTS OF LUBRICANTS A.R.
EYRES,
M.A.,
M.Sc.
Environmental H e a l t h & Product S a f e t y A d v i s o r , Mobil Europe Inc.
12.1
INTRODUCTION
The m a j o r i t y o f m i n e r a l o i l based l u b r i c a n t s and greases a r e r e l a t i v e l y harml es s t o man.
T h e i r use n o r m a l l y i n v o l v e s n o unusual hazards p r o v i d e d t h a t
reasonable c a r e i s taken t o a v o i d e xcessi ve s k i n c o n t a c t o r i n h a l a t i o n o f m i s t s and vapours.
A small number o f p r o d u c t s may, because o f c o m p o s i t i o n a l r e q u i r e -
ments t o meet p a r t i c u l a r t e c h n i c a l performance needs, p r e s e n t a h i g h e r degree of hazard.
Because l u b r i c a n t s a r e m a i n l y composed o f o r g a n i c chemicals which have
some solvency f o r n a t u r a l components o f t h e s k i n , i t i s u n l i k e l y t h a t p r o v i s i o n o f c o m p le te ly s a f e p r o d u c t s c o u l d be p o s s i b l e .
Synthetic lubricants generally
are s i m i l a r i n hazard p o t e n t i a l t o m i n e r a l o i l products. hazards o f l u b r i c a n t s ,
I n order t o review the
i t i s f i r s t o f a l l necessary t o l o o k b r i e f l y a t t h e i r
composition.
12.2
COMPOSITION OF LUBRICANTS
M in e r a l o i l based l u b r i c a n t s a r e p rep are d from base o i l s manufactured from n a t u r a l l y o c c u r r i n g crud e pe t ro l eu m o i l s .
Crude o i l s occur i n many p a r t s o f t h e
wo r ld and t h e i r comp osi t i on v a r i e s a c c o r d i n g t o source. m ix tu r e s o f p a r a f f i n i c ,
I n a d d i t i o n t o complex
i s o p a r a f f i n i c , n ap ht henic ( c y c l o p a r a f f i n i c ) and aromatic
hydrocarbons, some compounds o f s u l p h u r , oxygen and n i t r o g e n w i l l be present p l u s t r a c e s o f a number o f me t a l s.
D i s t i l l a t i o n o f crude o i l , f o l l o w e d by
v a r io u s o t h e r r e f i n i n g processes such as s o l v e n t e x t r a c t i o n , hydrogenation o r a c i d t r e a t m e n t y i e l d s v a r i o u s f r a c t i o n s i n t h e broad c a t e g o r i e s shown i n Table
12.1. A l u b r i c a t i n g o i l f r a c t i o n t y p i c a l l y c o n t a i n s several thousand i n d i v i d u a l hydrocarbon compounds.
A base s t o c k i s u s u a l l y described as p a r a f f i n i c o r
naphthenic depending on t h e predominant t yp e o f hydrocarbon compound p r e s e n t . This i s a f u n c t i o n o f t h e cru de source.
Crude o i l a l s o n o r m a l l y c o n t a i n s p o l y -
c y c l i c a r o m a t i c hydrocarbons, some o f which (4 t o 6 r i n g compounds) a r e known t o be c a r c i n o g e n i c , eg. benz(a) pyrene.
Some r e f i n i n g processes, such as
s o lv e n t o r severe a c i d t r e a t m e n t , remove most o f these p o l y c y c l i c aromatic compounds so t h a t v e r y few a r e p rese nt i n t h e f i n a l l u b r i c a n t base stock.
However,
276 Table 12.1
Mol ecu 1 a r Size Gases L i g h t Naphtha Gasol i n e Heavy Naphtha/Kerosine Gas O i l s
c1
-
'3
-
'8
'4
-
c12
'9
-
'10-
c4
'16 '26
Lubricating Oils
C,7 upwards
Residuum
CZ6 upwards
Boi 1 i n g Range, "C
so 0 20
100 170
-
100
200 270 400
> 300
i t should be n o t e d t h a t t h e p o l y c y c l i c compounds a r e p r e s e n t i n t h e e x t r a c t e d
Because o f t h e i r b o i l i n g p o i n t s , t h e 4 t o 6 r i n g p o l y c y c l i c compounds
material.
a r e n o t n o r m a l l y p r e s e n t i n d i s t i l l a t e f r a c t i o n s b o i l i n g below about 370°C. To o b t a i n l u b r i c a n t performance c h a r a c t e r i s t i c s which c o u l d n o t be p r o v i d e d by t h e base o i l s themselves, v a r i o u s a d d i t i v e s such as a n t i o x i d a n t s , d e t e r g e n t s / d i s p e r s a n t s , e m u l s i f i e r s , b i o c i d e s , a n t i - c o r r o s i v e s and anti-wear/extreme pressure compounds a r e i n c o r p o r a t e d .
P o t e n t i a l h e a l t h hazards o f such a d d i t i v e s
t h e r e f o r e a l s o need t o be considered. For s p e c i a l a p p l i c a t i o n s , a v a r i e t y o f s y n t h e t i c l u b r i c a n t s have been developed.
These a r e based on s y n t h e t i c hydrocarbons and e s t e r s , p o l y g l y c o l s ,
s i l i c o n e s and phosphate e s t e r s .
A d d i t i v e s o f s i m i l a r t y p e s t o those used i n
m i n e r a l o i l based l u b r i c a n t s a r e g e n e r a l l y i n c o r p o r a t e d .
12.3
MINERAL BASE OIL FACTORS
12.3.1
Acute T o x i c i t y
A l l types o f m i n e r a l o i l base s t o c k s have a low o r d e r o f a c u t e ( s h o r t - t e r m )
toxicity.
Acute o r a l LD5O'S ( t h e dose t o t e s t a n i m a l s r e s u l t i n g i n 50 p e r c e n t
m o r t a l i t y ) f o r t h e r a t a r e w e l l above 109 p e r kg of body w e i g h t .
Extrapolated
t o man t h i s equates t o i n g e s t i o n o f m r e than one l i t r e o f o i l t o cause death. Low v i s c o s i t y p r o d u c t s such as k e r o s i n e p r e s e n t a somewhat g r e a t e r hazard because o f t h e danger o f a s p i r a t i o n i n t o t h e lungs, f o l l o w e d by pneumonitis,
if
v o m i t i n g occurs. M i n e r a l o i l s a r e a l s o e s s e n t i a l l y n o n - t o x i c by a b s o r p t i o n t h r o u g h t h e s k i n . Dermal L O 5 0 ' s f o r t h e r a b b i t a r e w e l l above log p e r kg body w e i g h t , a l e v e l g e n e r a l l y c o n s i d e r e d as harmless.
277 12.3.2
Dermatitis
D e r m a t i t i s i s undoubtedly t h e maj o r p o t e n t i a l h e a l t h problem w i t h m i n e r a l o i l p r o d u c ts , r e s u l t i n g f ro m re pe at ed o r p rolonged s k i n c o n t a c t and inadequate skin care.
Primary i r r i t a t i o n and d e f a t t i n g o f t h e s k i n can occur t o v a r y i n g
e x t e n t s , depending on t h e t y p e o f p r o d u c t and t h e degree o f exposure. l i g h t e r p e t r o le u m o i l s w i t h f i n a l b o i l i n g p o i n t s below about 350°C, k e r o s in e , t e n d t o be d i r e c t s k i n i r r i t a n t s .
The
f o r example,
Since the chemical n a t u r e o f
min e r a l o i l ensures t h a t t h e r e w i l l be some solvency e f f e c t on the n a t u r a l f a t s o f t h e s k i n i t i s u n l i k e l y t h a t p o t e n t i a l problems can be t o t a l l y e l i m i n a t e d by product development.
However, a d o p t i o n o f simple measures t o prevent repeated
and prolonged c o n t a c t , t o g e t h e r w i t h good pe rsonal hygiene p r a c t i c e s and c a r e o f the s k i n , can v i r t u a l l y e l i m i n a t e d e r m a t i t i s problems.
D e r m a t i t i s from l u b -
r i c a n t s i s an a v o i d a b l e o c c u p a t i o n a l di se ase i n t h e v a s t m a j o r i t y o f cases.
12.3.3
O i l Mist
A lt h o u g h i t has o f t e n been suggested t h a t i n h a l a t i o n o f o i l m i s t over extended p e r i o d s may l e a d t o an i n crea sed r i s k o f l u n g cancer, evidence from animal t e s t i n g and exposed human p o p u l a t i o n s does n o t support t h i s . animal exposures t o w h i t e o i l m i s t a t increased l u n g cancer r i s k .
For example,
5 and 100 mg/m3 showed no i n d i c a t i o n s o f
I n f u r t h e r work, no i n j u r y o r i n d i s p o s i t i o n from
i n h a l a t i o n o f o i l m i s t was observed amongst animals exposed f o r 18 months t o m i s t f r o m a s u l p h u r i s e d s o l v e n t - e x t r a c t e d n aphthenic base o i l a t 50 mg per c u b ic meter [1,2,3].
Deco uf l e o f t h e U.S.
N a t i o n a l Cancer I n s t i t u t e [4,5]
has
pu b lis h e d two e p i d e m i o l o g i c a l s t u d i e s o f t h e cancer m o r t a l i t y o f workers exposed t o c u t t i n g o i l mists.
The r e s u l t s i n d i c a t e d t h a t exposure t o o i l m i s t s does
n o t pose a hazard i n terms o f r e s p i r a t o r y cancer and f a t a l non-malignant r e s p i r a t o r y disease, b u t may be a s s o c i a t e d w i t h a s l i g h t increase i n cancer o f t h e g a s t r o i n t e s t i n a l system.
A st ud y o f worke rs i n metal machining p l a n t s i n
Germany by Oraschs [ 6 ] showed no adverse r e s p i r a t o r y e f f e c t s among 443 employees i n 17 f a c t o r i e s .
About 63 p e r c e n t o f these employees were r e p o r t e d t o have
been exposed t o h i g h o i l m i s t l e v e l s i n t h e range 40 t o 80 mg/m3.
Comparison
of smokers and non-smokers i n exposed and non-exposed p o p u l a t i o n s suggested i n f a c t t h a t i n h a l a t i o n o f o i l m i s t may p r o v i d e some p r o t e c t i v e e f f e c t a g a i n s t t h e harmful e f f e c t s o f smoking on t h e b r o n c h i a l system. The c u r r e n t T hresh ol d L i m i t Value ( t h e atmospheric c o n c e n t r a t i o n t o which i t i s b e l i e v e d most workers can be exposed f o r 8 hours d a i l y w i t h o u t adverse e f f e c t s on h e a l t h ) p u b l i s h e d by t h e American Conference o f Gernmental Indu s t r i a l Hygienists i s
5
mg/m3.
T h i s has been s e t on t h e b a s i s o f p r e v e n t i n g
nuisance and unpleasantness t o wo rkers r a t h e r than on h e a l t h e f f e c t aspects and i t i s e s s e n t i a l l y an index o f good i n d u s t r i a l hygiene p r a c t i c e s .
I t i s believed
t h a t t h e 5 mg/m3 l i m i t p r o v i d e s a s a f e t y f a c t o r o f a t l e a s t t e n a g a i n s t even r e l a t i v e l y minor changes i n t h e lungs. As w i t h many chemicals,
i n h a l a t i o n o f very h i g h concentrations o f o i l mists
( o r vapours o f more v o l a t i l e p r o d u c t s such as k e r o s i n e ) may cause i r r i t a t i o n o f t h e lungs and may l e a d t o a chemical pneumonia.
12.3.4
O i l Vapours
A t normal temperatures, t y p i c a l l u b r i c a t i n g o i l s do n o t produce any s i g n i f -
i c a n t l e v e l s o f vapour i n t h e working atmosphere.
S a t u r a t e d vapour concent-
r a t i o n s f o r a t y p i c a l l u b r i c a t i n g o i l have been c a l c u l a t e d by Sanderson [7] t o be 0.016 ppm a t 20°C and
1.4
ppm a t 100°C
adverse h e a l t h e f f e c t s w i l l n o t occur.
-
these a r e so low t h a t any
L i g h t e r p r o d u c t s such as d i e s e l o i l and
k e r o s i n e do have t h e p o t e n t i a l f o r e v a p o r a t i o n o f l i g h t ends t o produce s i g n i f i c a n t vapour c o n c e n t r a t i o n s .
For example,
f o r k e r o s i n e a t 20°C i s about 2000 ppm. i r r i t a t i o n o f mucous membranes.
t h e s a t u r a t e d vapour c o n c e n t r a t i o n
These l i g h t e r p r o d u c t s may produce
I n metal machining o p e r a t i o n s , s i g n i f i c a n t
v a p o r i s a t i o n o f l u b r i c a t i n g o i l s may o c c u r a t t h e h i g h t o o l / w o r k p i e c e tempera t u r e s , b u t on c o o l i n g i n t h e s u r r o u n d i n g atmosphere, t h i s vapour w i l l be condensed t o d r o p l e t s o f o i l m i s t .
12.3.5
Skin Cancer
I t has been known f o r many y e a r s t h a t some t y p e s o f m i n e r a l o i l can cause
s k i n cancer w i t h repeated and p r o l o n g e d exposure o v e r l o n g p e r i o d s o f years.
It
should be n o t e d however t h a t s k i n cancer i s n o r m a l l y l e s s s e r i o u s than o t h e r forms o f cancer and i s u s u a l l y c u r a b l e w i t h e a r l y t r e a t m e n t . I n 1922, L e i t c h [ 8 ] r e p o r t e d t h a t S c o t t i s h s h a l e o i l d i s t i l l a t e s caused cancer o f t h e s k i n when p a i n t e d on t o animals.
I t i s not possible t o review a l l
o f t h e e x t e n s i v e s t u d i e s c a r r i e d o u t s i n c e t h a t t i m e , b u t among t h e most i m p o r t a n t were those r e p o r t e d i n 1966 by Bingham and Horton 191, sponsored by t h e American Petroleum I n s t i t u t e .
I t was shown t h a t base s t o c k s prepared by
s o l v e n t r e f i n i n g , which removes p o l y c y c l i c a r o m a t i c hydrocarbons (PCAH), d i d n o t cause tumours i n mice whereas t y p i c a l a c i d r e f i n e d base s t o c k s d i d .
I n 1968,
t h e UK Medical Research Council p u b l i s h e d a r e p o r t "The Carcinogenic A c t i o n o f Mineral O i l s :
A Chemical and B i o l o g i c a l Study" [lo]. The s a l i e n t c o n c l u s i o n s
from t h i s r e p o r t and o t h e r s t u d i e s can be summarised as f o l l o w s :
-
Some f r a c t i o n s o f c e r t a i n crudes f r o m which l u b r i c a t i n g o i l s a r e r e f i n e d have been shown t o produce tumours on s k i n s o f r a b b i t s o r mice.
*
S o l v e n t e x t r a c t i o n methods o f r e f i n i n g which remove a r o m a t i c compounds markedly reduce t h e c a r c i n o g e n i c a c t i v i t y o f t h e r e f i n e d l u b r i c a n t s .
279
.
C a r c i n o g e n i c i t y o f m i n e r a l o i l s appears t o be r e l a t e d t o t h e presence o f PCAH's, some o f which a r e known t o be carcinogens.
*
I t has n o t been p o s s i b l e t o d e f i n e any simple a n a l y t i c a l parameters which c o r r e l a t e w i t h c a r c i n o g e n i c a c t i v i t y . An Ad-hoc Committee o f t h e UK I n s t i t u t e o f Petroleum s t u d i e d a l l a v a i l a b l e
evidence and a d v i s e d member companies i n 1968 t h a t o i l s which have been s o l v e n t r e f i n e d o r t r e a t e d i n o t h e r ways t o a p p r e c i a b l y reduce t h e c o n t e n t o f p o l y c y c l i c aromatic compounds, were l e s s l i k e l y t o promote s k i n cancer
han o i l s which had
n o t been t h u s r e f i n e d . The d i f f i c u l t y o f d e f i n i n g a n a l y t i c a l parameters t o c o r r e a t e w i t h c a r c i n o genic a c t i v i t y i s i l l u s t r a t e d by r e s u l t s r e p o r t e d by Scala [ 11, shown i n Table 12.2.
Table 12.2
A
OIL
B
C
Pyrene, ppm
3.5
18.3
4.4
Benz ( a ) anthracene, ppm
6.6
7.9
2.7
Benz (a) pyrene, ppm
4.5
1.2
0.2
% CA (carbon i n a r o m a t i c r i n g s ) 15.4
15.2
12.4
+
Cancer a c t i v i t y
+
The reason f o r t h i s i s t h e u n p r e d i c t a b l e e f f e c t s o f cocarcinogens, and a c c e l e r a t o r s .
inhibitors
For example, i t has been shown t h a t s u l p h u r and some organo-
sulphur compounds can increase c a r c i n o g e n i c i t y , as a l s o can c e r t a i n t y p e s o f hydrocarbons. So f a r , d e s p i t e e x t e n s i v e research, no e n t i r e l y s a t i s f a c t o r y method f o r
e v a l u a t i n g t h e p o t e n t i a l c a r c i n o g e n i c i t y o f an o i l has been devised. method t o d a t e i s t h e l o n g term p a i n t i n g o f mice s k i n s .
The b e s t
Such t e s t s t a k e two
years t o complete and r e q u i r e h i g h standards o f experimental techniques; are t h e r e f o r e v e r y c o s t l y .
they
There i s a l s o t h e d i f f i c u l t y o f t r a n s l a t i n g r e s u l t s
o f animal t e s t s t o t h e human exposure s i t u a t i o n . t e s t each i n d i v i d u a l f o r m u l a t i o n .
I t i s t h e r e f o r e impossible t o
However, a l a r g e number o f t e s t s have been
done and these f o r m t h e b a s i s f o r c u r r e n t recommendations t o m i n i m i s e hazards. Cases o f o c c u p a t i o n a l s c r o t a 1 cancer have been r e p o r t e d f r o m many c o u n t r i e s i n c l u d i n g France, Sweden, t h e UK and t h e USA.
The i n c i d e n c e i n t h e UK i s
e q u i v a l e n t t o about 5 cases p e r m i l l i o n males p e r year whereas i n Sweden i t i s lower, a t about 1 case p e r m i l l i o n males p e r year.
Wahlberg [ 1 2 ] has r e p o r t e d
t h a t o n l y 21 p e r c e n t o f Swedish cases had had d e f i n i t e exposure t o m i n e r a l o i l .
280 T h i s c o n t r a s t s markedly w i t h a f i g u r e o f 86 per c e n t r e p o r t e d f o r UK cases. No o b v io u s reason f o r t h i s d i f f e r e n c e has been i d e n t i f i e d , b u t f a c t o r s such as p l a n t and p e r s o n al hyg i en e may w e l l be s i g n i f i c a n t .
As these a r e improved,
i nc id e n c e o f s c r o t a 1 cancer can be e xpe ct e d t o decrease e v e n t u a l l y .
The long
l a t e n t p e r i o d f r o m f i r s t exposure t o d i a g n o s i s o f the cancer (10 t o 43 years) means t h a t any changes i n i n c i d e n c e cannot be observed q u i c k l y .
12.3.6
Eye I r r i t a t i o n
I n common w i t h a m u l t i t u d e o f commonly used m a t e r i a l s , eg. soapy water, many l u b r i c a n t s may cause some i r r i t a t i o n i f splashes e n t e r t h e eye.
W i t h the
m a j o r i t y o f p r o d u c t s t h i s w i l l n o t be more t h an v e r y s l i g h t , b u t some such as n e a t s o l u b l e o i l s , may, because t he y c o n t a i n a p p r e c i a b l e amounts o f s u r f a c e a c t i v e m a t e r i a l s such as soaps, be somewhat more i r r i t a t i n g .
12.4
ADDITIVE FACTORS
B e fo r e use i n l u b r i c a n t s , a d d i t i v e s a r e screened f o r t o x i c i t y and s k i n o r eye i r r i t a n c y .
T h i s i s e s s e n t i a l t o assess p o t e n t i a l hazards and determine any
required handling precautions during blending o f the f i n i s h e d l u b r i c a n t .
If i t
appears l i k e l y t h a t an a d d i t i v e may l e a d t o any increased hazard i n t h e blended l u b r i c a n t , f u r t h e r t e s t i n g may be done t o d e f i n e t h i s .
The e x t e n t o f p o s s i b l e
increased h a z a r d i n a f i n i s h e d f o r m u l a t i o n must be assessed t o decide whether o r n o t th e a d d i t i v e sho ul d be r e j e c t e d . I n t h e m a j o r i t y o f p rod uct s, a d d i t i v e s a r e minor i n g r e d i e n t s and t h e p o t e n t i a l hazards a r e e s s e n t i a l l y t ho se a s s o c i a t e d w i t h t h e base m i n e r a l o i l . a d d i t i v e c o n t e n t s a r e h i g h e r , eg.
Where
i n n e a t s o l u b l e o i l s and i n some engine l u b -
r i c a n t s , t h e f i n a l p r o d u c t may be more i r r i t a t i n g than t h e base m i n e r a l o i l .
I f use o f these a d d i t i v e s i s e s s e n t i a l f o r t e c h n i c a l performance reasons, t h e hazards must be c o n t r o l l e d by t h e i mp l eme nt ation o f adequate h a n d l i n g and use precautions.
A wide range of a d d i t i v e s i s used t o a c h i e v e improvements i n l u b r i c a n t p e r formance.
For each a d d i t i v e t ype , v a r i o u s chemical compounds have been found
t o be e f f e c t i v e . p o t e n t i a l hazards.
Very few o f these have been found t o p r e s e n t any s i g n i f i c a n t Among t ho se wh i ch have been t h e cause o f some concern a r e
l ea d compounds, o r t h o isomers o f phosphate e s t e r s , c h l o r i n a t e d naphthalenes, sodium n i t r i t e i n co mbi n at i on w i t h amines, sodium mercaptobenzothiazole and trichloroethylene.
12.4.1
Lead Compounds
One o f t h e e a r l y e f f e c t i v e methods of i mp roving a n t i w e a r and extreme pre s s u r e p r o p e r t i e s o f a l u b r i c a n t was t o i n c o r p o r a t e l e a d soap.
Although t h e r e
281 are no r e p o r t e d cases o f s u f f i c i e n t l e ad a b s o r p t i o n t o cause adverse h e a l t h e f f e c t s , Van Peteghem and Vos [13] r e p o r t e d s l i g h t increases i n b l o o d l e a d l e v e l s i n s t e e l m i l l employees w i t h f r e q u e n t o r prolonged s k i n c o n t a c t w i t h t h i s type o f l u b r i c a n t .
New a d d i t i v e t e chn ol o gy has enabled t h i s t y p e o f f o r m u l a t i o n
t o be l a r g e l y r e p l a c e d o v e r t h e l a s t few ye ars by unleaded l u b r i c a n t s .
12.4.2
Or th o Phosphates
A b s o r p t i o n o f o r t h o t r i c r e s y l phosphate has been shown t o cause c e n t r a l nervous system damage l e a d i n g t o neurornuscular problems and v a r i o u s stages o f paralysis.
The p ara isomer does n o t have t h i s e f f e c t and i s e s s e n t i a l l y i n e r t
p r o v i d i n g t h e c o n t e n t o f o r t h o isomer i s a t a v e r y low l e v e l .
!Suppliers o f
t r i c r e s y l phosphates have f o r many ye ars r e s t r i c t e d the o r t h o c o n t e n t t o l e s s than 1 p e r c e n t i n o r d e r t o a v o i d t h e p o s s i b i l i t y o f c e n t r a l nervous system effects
12.4.3
.
C h l o r i n a t e d Naphthalenes
C h l o r i n a t e d naphthalenes were used f o r a s h o r t p e r i o d many years ago as e f f e c t i v e extreme p r e s s u r e a d d i t i v e s i n c u t t i n g o i l s .
T h e i r use was discon-
t i n u e d when an a s s o c i a t i o n w i t h c h l o r a c n e o f t h e s k i n was found.
The types o f
c h l o r i n a t e d a d d i t i v e now used, eg. c h l o r i n a t e d p a r a f f i n s , do n o t cause t h i s effect.
12.4.4
Sodium N i t r i t e and Amines
Sodium n i t r i t e i n co mbi n at i on w i t h t r i and diethanolamines has been used f o r many y e a r s t o p r o v i d e s a t i s f a c t o r y a n t i c o r r o s i o n p r o p e r t i e s i n aqueous g r i n d i n g fluids,
and, a t lower c o n c e n t r a t i o n s ,
i n some s o l u b l e c u t t i n g o i l s .
R ecently,
small amounts o f n i t r o s a m i n e s , a t y p e o f ch emicalof which some a r e known t o be c a r c i n o g e n i c , have been found i n b o t h c o n c e n t r a t e s and d i l u t e d v e r s i o n s o f such p r o d u c t s [14,15].
Ni t ro sami n es a r e a l s o found i n many foods, d r i n k s and
cosmetics, a r e p r e s e n t i n t h e atmosphere o f c i t y s t r e e t s and a r e a l s o formed w i t h i n t h e body i t s e l f .
Assessment o f any increased hazard from g r i n d i n g f l u i d s
i s d i f f i c u l t because o f t h e problems o f e s t i m a t i n g exposure and e x t e n t o f a b s o r p t i o n i n t o t h e body.
Taking t h e w o r s t p o s s i b l e case,
i t appears t h a t
a b s o r p t i o n c o u l d approach t h a t f ro m foods, b u t t y p i c a l l y i t i s l i k e l y t o be v e r y much l e s s t h a n t h i s .
As a p r e c a u t i o n however, s u p p l i e r s have e l i m i n a t e d the
c o m b in a tio n o f n i t r i t e and amine exce pt i n some c r i t i c a l a p p l i c a t i o n s where i t has n o t y e t been p o s s i b l e t o m e t t h e t e c h n i c a l requirements w i t h a l t e r n a t i v e formulations.
282 12.4.5
Sodium Mercaptobenzothiazole
T h i s compound was used a t one t i m e a s a v e r y e f f e c t i v e c o r r o s i o n i n h i b i t o r i n aqueous l u b r i c a n t s .
I t was l a t e r i d e n t i f i e d as a s t r o n g s k i n s e n s i t i z e r and
i s no l o n g e r used i n a p p l i c a t i o n s where s k i n c o n t a c t i s l i k e l y .
12.4.6
Trichloroethylene
T r i c h l o r o e t h y l e n e was used w i d e l y as a non-flammable d i l u e n t i n open gear l u b r i c a n t s t o e n a b l e a h i g h l y v i s c o u s l u b r i c a n t f i l m t o be e a s i l y a p p l i e d t o gear t e e t h s u r f a c e s .
Exposure t o t r i c h l o r o e t h y l e n e vapour above t h e t i m e
weighted average TLV o f 100 ppm may be hazardous, c a u s i n g depression o f t h e c e n t r a l nervous system w i t h v i s u a l d i s t u r b a n c e s and l a c k o f c o - o r d i n a t i o n , t h e p o s s i b i l i t y o f damage t o t h e l i v e r and kidneys. t r i c h l o r o e t h y l e n e vapours has a l s o o c c u r r e d .
plus
Addiction t o s n i f f i n g
Such exposures on a c o n t i n u i n g
b a s i s a r e u n l i k e l y t o occur e x c e p t perhaps i n c o n f i n e d and p o o r l y v e n t i l a t e d spaces.
However, t o p r o v i d e a g r e a t e r margin o f s a f e t y ,
t r i c h l o r o e t h y l e n e can
be s u b s t i t u t e d w i t h a s l i g h t l y more expensive s a f e r a l t e r n a t i v e , 1.1.1
tri-
c h l o r o e t h a n e , which has a TLV o f 350 ppm. I t has a l s o been i n d i c a t e d t h a t t r i c h l o r o e t h y l e n e i s a c a r c i n o g e n i n animal tests.
However, t h e doses g i v e n by i n g e s t i o n i n t o t h e stomach were s o massive
compared t o p o s s i b l e human exposure t h a t t h e s i g n i f i c a n c e o f these t e s t r e s u l t s can be s e r i o u s l y questioned.
12.5
BACTERIA AND BlOClDES
Water based l u b r i c a n t s and m i n e r a l o i l l u b r i c a n t s contaminated w i t h w a t e r , eg. marine e n g i n e o i l s , can s u p p o r t t h e growth o f b a c t e r i a , y e a s t s and f u n g i . Growth does n o t n o r m a l l y occur i n p r o d u c t s which do n o t c o n t a i n water.
As s u p p l i e d t o
users, p r o d u c t s a r e n o r m a l l y f r e e o f b a c t e r i a , b u t c o n t a m i n a t i o n o c c u r s f r o m a number o f p o s s i b l e sources such as water f r o m engine c o o l i n g systems, t h e water used a s d i l u e n t , r e s i d u a l b a c t e r i a i n p l a n t c i r c u l a t i o n systems,
r e f u s e such a s
c i g a r e t t e s thrown i n t o t h e c o o l a n t , employees s p i t t i n g i n t o t h e p r o d u c t o r even from b a c t e r i a l contamination o f the a i r i n the p l a n t . The b a c t e r i a , y e a s t s o r f u n g i which grow i n aqueous c o o l a n t s o r l u b r i c a n t s contaminated w i t h water a r e n o t n o r m a l l y harmful t o humans.
Although concern
has been expressed t h a t b a c t e r i a l c o n t a m i n a t i o n may l e a d t o increased r e s p i r a t o r y o r s k i n i n f e c t i o n s , i n d u s t r i a l medical a d v i s e r s r e s p o n s i b l e f o r l a r g e metal machining p l a n t s have r e p o r t e d t h a t t h e y can f i n d no evidence f o r t h i s . types o f b a c t e r i a found a r e almost i n v a r i a b l y t h e non-pathogenic
The
t y p e which a r e
harmless t o humans a l t h o u g h v e r y o c c a s i o n a l l y a p o t e n t i a l l y h a r m f u l pathogenic t y p e may be i d e n t i f i e d .
283 For t e c h n i c a l reasons, aqueous c o o l a n t s ,
i t i s d e s i r a b l e t o c o n t r o l b a c t e r i a l growth.
With
t h i s i s ach i eve d e i t h e r by i n c l u s i o n o f small amounts o f b i o -
c i d e s i n t h e o r i g i n a l p r o d u c t o r by a d d i t i o n o f b i o c i d e s d u r i n g use.
By t h e i r
na tu r e , b i o c i d e s a r e mo de rat el y t o h i g h l y t o x i c by i n g e s t i o n and may be s k i n o r eye i r r i t a n t s i n t h e c o n c e n t r a t e d form.
T he r efore,
c o n t r o l l e d t o a v o i d i ncre ase d h e a l t h hazards.
t h e i r use must be c a r e f u l l y
I n the c o n c e n t r a t i o n s n o i m a l l y
used, and p r o v i d e d a p p r o p r i a t e h a n d l i n g p r e c a u t i o n s a r e observed, b i o c i d e s should p r e s e n t no hazard t o h e a l t h . "topping-up"
However, use o f excessive c o n c e n t r a t i o n s
may cause s k i n i r r i t a t i o n .
in
W i t h marine engine o i l s , b a c t e r i a l
problems can be c o n t r o l l e d by c o r r e c t o i l s e l e c t i o n and a p p r o p r i a t e o p e r a t i n g procedures [ 1 6 ] .
12.6
SYNTHETIC LUBRICANTS
A v a r i e t y o f chemical t ype s a r e used as s y n t h e t i c l u b r i c a n t s t o meet opera t i o n a l r e q u ir e m en t s which cannot be s a t i s f i e d adequately w i t h m i n e r a l o i l prod u c t s .
Types i n c l u d e v a r i o u s e s t e r s o f o r g a n i c f a t t y a c i d s , s i l i c o n e s , syn-
t h e t i c hydrocarbons such as p o l y o l e f i n e s , p o l y g l y c o l s and phosphate e s t e r s . There a r e no unusual h e a l t h hazards a s s o c i a t e d w i t h these. skin, s i m i l a r t o t h a t w i t h mineral o i l , pro lo n g e d c o n t a c t o ccurs.
D e f a t t i n g o f the
i s p o s s i b l e i n most cases i f repeated o r
I n t h e case o f phosphate e s t e r s , the use o f t h e
o r t h o isomer s h o u l d be avo i de d as i n d i c a t e d i n 1 2 . 4 . 2 .
12.7
USED AND RECLAIMED OR RE-REFINED OILS
There i s s t r o n g evi d en ce t h a t PCAH c o n t e n t o f m i n e r a l o i l based l u b r i c a n t s i nc r e a s e s d u r i n g use [ 1 7 ] . type o f a p p l i c a t i o n ,
The e x t e n t o f t h e increase appears t o depend on t h e
b e i n g up t o about t e n - f o l d f o r c u t t i n g o i l s and d i e s e l
eng in e o i l s , b u t perhaps one h u n d r e d - f o l d or more f o r g a s o l i n e engine o i l s and quenching o i l s .
Non-engine i n d u s t r i a l l u b r i c a n t s such as h y d r a u l i c , gear and
b e a r i n g o i l s would n o t be expected t o show any s i g n i f i c a n t PCAH increase d u r i n g use because o f t h e l i m i t e d t emp era t u re i n crea ses t o which they a r e subjected.
Much o f t h e in c r e ase i n e ng i ne o i l s appears t o a r i s e from g a s o l i n e combustion products
.
The s i g n i f i c a n c e o f these i n crea ses i n PCAH c o n t e n t i n r e l a t i o n t o any i nc r e a s e d s k i n cancer r i s k i s n o t c l e a r a t present f o r t h e reasons discussed i n 12.3.5.
I n t h e case o f c u t t i n g o i l s , c a l c u l a t i o n s based on o i l m i s t concent-
r a t i o n s a t t h e TLV o f
5
mg/m3 i n d i c a t e t h a t PCAH l e v e l s w i l l be o f the same order
o f magnitude as background atmospheric l e v e l s . m a i n t a i n e d below t h e TLV, risk.
Provided o i l m i s t l e v e l s a r e
i t appears t h e r e should be no s i g n i f i c a n t
increase i n
284 For e n v ir o n m en t a l c o n s e r v a t i o n reasons, t h e r e a r e a t t r a c t i o n s i n r e c l a i m i n g o r r e - r e f i n i n g used l u b r i c a n t s f o r f u r t h e r use.
In specific situations there
may a l s o be economic j u s t i f i c a t i o n f o r re cl amation.
A v a r i e t y o f processes may
be employed f r o m si mp l e c e n t r i f u g i n g and e a r t h f i l t r a t i o n t o a c i d t r e a t m e n t , r e d i s t i l l a t i o n and s o l v e n t e x t r a c t i o n . on t h e p o t e n t i a l hazards,
U n t i l further information i s available
i t i s co nsi d ere d i t would be prudent t o l i m i t use o f
such o i l s t o a p p l i c a t i o n s i n which t h e r e i s l i t t l e s k i n c o n t a c t u n l e s s i t i s c e r t a i n t h a t t h e o i l has o n l y been used where PCAH i n c r e a s e i s u n l i k e l y , o r has been t r e a t e d by a pro cess wh i ch w i l l remove PCAH, eg. s o l v e n t e x t r a c t i o n o f aroma t ic s
.
Used g a s o l i n e e ng i ne o i l s can c o n t a i n up t o about 1 per c e n t of l e a d [ 1 8 ] . T h i s o r i g i n a t e s m a i n l y f r o m l e a d a d d i t i v e s i n g a s o l i n e w i t h perhaps a minor c o n t r i b u t i o n fr o m wear o f e ng i ne p a r t s .
Repeated o r prolonged s k i n c o n t a c t w i t h
these o i l s may r e s u l t i n some i n crea sed a b s o r p t i o n o f l e a d i n t o t h e body. Used c u t t i n g o i l s u s u a l l y c o n t a i n sma l l metal c h i p s o r swarf w hich p r e s e n t an a d d i t i o n a l hazard t o t h e s k i n .
Many o f t he se metal p a r t i c l e s a r e n e e d l e - l i k e
shape and can cause m i c r o - l e s i o n s of t h e s k i n ,
in
l e a d i n g t o a general i r r i t a t i o n .
I t i s a l s o b e l i e v e d t h a t e n t r y o f swa rf i n t o t h e s k i n may d e s t r o y an e l e c t r o n e g a t i v e b a r r i e r beneath t h e s u r f a c e ove r about one square inch around t h e s i t e , thus a l l o w i n g o t h e r m a t e r i a l s such as t h e c u t t i n g f l u i d t o p e n e t r a t e i n t o t h e skin.
12.8 12.8.1
HEALTH AND SAFETY PRECAUTIONS Supplier's Responsibilities
S e c tio n 6 o f t h e H e a l t h and S a f e t y a t Mork A c t e t c .
1974 p l a c e s r e s p o n s i b i l -
i t i e s on s u p p l i e r s " t o ensure so f a r as i s reasonably p r a c t i c a b l e , t h a t t h e substance i s s a f e and w i t h o u t r i s k s t o h e a l t h when p r o p e r l y used" and t o make a v a i l a b l e i n f o r m a t i o n on any r e l e v a n t t e s t s and "about any c o n d i t i o n s necessary t o ensure t h a t i t w i l l be s a f e and w i t h o u t r i s k s t o h e a l t h when p r o p e r l y used." S u p p l i e r s a r e a l s o r e q u i r e d t o e l i m i n a t e o r minimise r i s k s t o h e a l t h and s a f e t y , as f a r as i s r e a son ab l y p r a c t i c a b l e . completely safe products;
S u p p l i e r s cannot be expected t o produce
indeed, i t can be s a i d t h a t t h e r e i s no such t h i n g
t h e r e a r e o n l y s a f e ways o f u s i n g a p rod uct .
-
Even pure w ater can be harmful i f
one d r i n k s t o o much o f i t and many p eo pl e w i l l s u f f e r s k i n problems i f t h e i r hands a r e immersed i n wat er f o r se vera l h ou rs d a i l y . For many y e a r s, r e p u t a b l e l u b r i c a n t s u p p l i e r s have been assessing t h e p o t e n t i a l hazards o f p rod uct s.
O f p a r t i c u l a r importance i s t h e assessment o f
t o x i c i t y o f p o s s i b l e a d d i t i v e s and r e j e c t i o n o f those w hich may l e a d t o s i g n i f i c a n t l y in c r e a s e d r i s k .
As i n d i c a t e d i n S e c t i o n 12.4, a number o f a d d i t i v e s
which may p r e s e n t hazards under some c o n d i t i o n s o f use have been r e p l a c e d i n r e c e n t y e a r s by a l t e r n a t i v e s a f e r m a t e r i a l s .
Care must always be e x e r c i s e d
285 however, t o ensure t h a t a m a t e r i a l w i t h a known r i s k i s n o t r e p l a c e d by a new m a t e r i a l w i t h unknown r i s k which may i n f a c t be much g r e a t e r . The H e a l t h and S a f e t y a t Work Act does n o t r e q u i r e s u p p l i e r s t o d i s c l o s e d e t a i l s o f p r o d u c t comp osi t i on s t o u sers.
W h i l s t most s u p p l i e r s w i l l make
broad c o m p o s i t i o n a l i n f o r m a t i o n g e n e r a l l y a v a i l a b l e , o r even d e t a i l e d informa t i o n on a c o n f i d e n t i a l b a s i s t o a u ser h e a l t h p r o f e s s i o n a l where necessary, f o r m u l a t i o n s a r e p r o p r i e t a r y i n f o r m a t i o n t o t h e i n d i v i d u a l manufacturer i n a c o m p e t i t i v e b u s iness wo rl d.
Co nsi d era bl e re search e x p e n d i t u r e may have been
i n c u r r e d i n d e v e l op i ng new a d d i t i v e s and i n s e l e c t i n g t h e b e s t combination o f a d d i t i v e s t o p r o v i d e a performance b e n e f i t i n a p a r t i c u l a r a p p l i c a t i o n .
Sup-
p l i e r s a r e n a t u r a l l y r e l u c t a n t t o i n crea se t h e p o s s i b i l i t y t h a t t h i s informa t i o n may pass t o t h e i r c o m p e t i t o r s .
I n any case, such i n f o r m a t i o n i s u s u a l l y
o f l i t t l e v a lu e t o t h e use r i n asse ssi ng p o t e n t i a l hazards. v a lu e i s t h e t o x i c o l o g i c a l
O f much g r e a t e r
i n f o r m a t i o n and t h e recommended h a n d l i n g p r e c a u t i o n s
based on t h i s i n f o r m a t i o n . Most s u p p l i e r s a l s o p r o v i d e use rs w i t h b o o k l e t s o r l e a f l e t s recommending safe h a n d l i n g p r e c a u t i o n s and r e v i e w i n g p o t e n t i a l h e a l t h hazards.
T y p i c a l recom-
mendations, t o g e t h e r w i t h p r a c t i c e s o f l a r g e user companies, have been reviewed by t h e I n s t i t u t e o f Petroleum and i n c o r p o r a t e d i n a Code o f P r a c t i c e f o r Metalwor k in g F l u i d s , p u b l i s h e d i n J u l y 1978 1191.
12.8.2
S k in P r o t e c t i o n
The b a s i c r e q ui re men t f o r a v o i d i n g s k i n problems i s t o minimise c o n t a c t . With t h e m a j o r i t y of l u b r i c a n t s , o ccasi o na l s k i n c o n t a c t f o r s h o r t p e r i o d s w i l l cause no problems.
Compliance w i t h t h e normal recommendation t o a v o i d prolonged
o r r e p e a te d s k i n c o n t a c t w i l l be s u f f i c i e n t t o p r e v e n t d e r m a t i t i s and s k i n i r r i t a t i o n problems.
The p r e c a u t i o n s needed t o a v o i d d e r m a t i t i s and s k i n i r r i -
t a t i o n w i l l a l s o p r e v e n t s k i n cancer.
Modern s o l v e n t r e f i n e d types o f m i n e r a l
o i l s ( o r e q u i v a l e n t ) a l s o mi n i mi se t h e r i s k o f s k i n cancer.
With t h e more
i r r i t a n t t y p e s o f p r o d u c t , such as k e r o s i n e and n e a t s o l u b l e o i l s , occasional v e r y s h o r t c o n t a c t i s u n l i k e l y t o cause problems, b u t i f any a p p r e c i a b l e c o n t a c t i s l i k e l y , s u i t a b l e p r o t e c t i v e measures sh ou ld be employed. Contact can be mi ni mi se d by u s i n g s u i t a b l e p r o t e c t i v e gloves and c l o t h i n g , b a r r i e r creams, and t h e p rop er i n s t a l l a t i o n and use o f splash guards on c u t t i n g machines.
P r o t e c t i v e c l o t h i n g which becomes contaminated w i t h o i l should be
changed f r e q u e n t l y and cl e an ed by any l a u n d e r i n g process ( d r y o r wet, o r a c o m b in a tio n o f b o t h ) which produces v i s u a l l y c l e a n garments. comes g r o s s l y contaminated, eg. by spray o r s p i l l a g e ,
I f c l o t h i n g be-
i t should be changed
immediately. S p e c ia l aprons a r e a v a i l a b l e , c o n s i s t i n g o f an impervious back w i t h a de ta c h a b le a b s o rbe nt f r o n t whi ch can be e a s i l y removed f o r c l e a n i n g .
The use o f
286 t h i s t y p e o f p r o t e c t i o n by t o o l s e t t e r s , who a r e p a r t i c u l a r l y l i k e l y t o be exposed t o heavy c o n t a m i n a t i o n i n l e a n i n g o v e r o i l y machines,
i s strongly
Use o f t h i s t y p e o f a pro n removes t h e t e m p t a t i o n t o stow o i l y rags
recommended.
o r t o o l s i n t r o u s e r po cket s, a p r a c t i c e whi ch c o u l d r e s u l t i n t h e s k i n i n t h e g r o i n a r e a b e in g i n p rol o ng ed c o n t a c t w i t h o i l - s o a k e d c l o t h i n g .
Sleeves o f
c l o t h i n g should be s h o r t o r r o l l e d up t o a v o i d c o n t i n u a l f r i c t i o n between o i l soaked c u f f s and t h e s k i n o f forearms and w r i s t s . wear o i l - s o a k e d c l o t h i n g " .
The golden r u l e i s
-
u n d e r c l o t h i n g which may become con t a mi na t e d f rom o i l - s o a k e d o v e r a l l s . c l o t h e s s h o u ld a l s o be changed f r e q u e n t l y . ination,
"Do n o t
I t sho ul d be remembered t h a t t h i s a p p l i e s a l s o * t o Under-
To h e l p minimise c l o t h i n g contam-
s e p a r a te l o c k e r f a c i l i t i e s f o r work and s t r e e t c l o t h e s a r e d e s i r a b l e
i n changing rooms. Employees who come i n t o c o n t a c t w i t h o i l should wash exposed s k i n a t t h e end o f any work p e r i o d , u s i n g warm w a t e r and soap, m i l d d e t e r g e n t o r p r o p r i e t a r y s k i n cleanser.
S t ron g soaps and d e t e r g e n t s and a b r a s i v e type soaps o r c l e a n s e r s
should be avoided.
K e rosi n e, p e t r o l and o t h e r degreasing s o l v e n t s should n o t be
used f o r c l e a n i n g t h e s k i n .
Hands sh ou l d a l s o be washed b e f o r e e a t i n g , d r i n k i n g
or smoking and b e f o r e and a f t e r u s i n g t h e l a v a t o r y .
E a s i l y a c c e s s i b l e washing
and t o i l e t f a c i l i t i e s sh ou l d be a v a i l a b l e and should be w e l l maintained. I d e a l l y , employees sh ou l d shower a t t h e end o f t h e work s h i f t t o remove a l l traces o f o i l from the skin.
I f rag s a r e used t o wipe o i l from t h e s k i n o r
machinery, a p l e n t i f u l su pp l y sho ul d be a v a i l a b l e so t h a t they can be changed frequently.
P r e f e r a b l y t h e y sh ou l d be o f a d i s p o s a b l e t y p e t o a v o i d t h e poss-
i b i l i t y o f a c c u mul a t i o n o f metal c h i p s and sw arf w hich may c u t o r s c r a t c h t h e skin. B a r r i e r creams a r e o f t e n used on t h e assumption t h a t they p r o t e c t t h e s k i n from d i r e c t contact w i t h o i l o r coolant.
Al t hough t h e i r e f f e c t i v e n e s s i s some-
what u n c e r t a i n , the y do have a p a r t t o p l a y i n m i n i m i s i n g d e r m a t i t i s problems by i n c r e a s i n g awareness o f t h e need f o r c a r e o f t h e s k i n .
They can a l s o make
eve n tu a l washing o f t h e s k i n more e f f e c t i v e , p a r t i c u l a r l y where " d i r t y " i n v o lv e d .
work i s
Use o f creams f r o m r e p u t a b l e s u p p l i e r s i s recommended s i n c e l o n g
exp e r ie n c e has enabled them t o a v o i d use o f components w hich may harm t h e s k i n . I t i s im p o r t a n t t o use t h e c o r r e c t t y p e o f cream f o r t h e type o f o i l i n v o l v e d
s i n c e a w a t e r r e s i s t a n t t y p e i nt en de d f o r use w i t h aqueous c o o l a n t s w i l l n o t g i v e e f f e c t i v e p r o t e c t i o n a g a i n s t m i n e r a l o i l based l u b r i c a n t s . Use o f a s k i n r e c o n d i t i o n i n g cream a f t e r work i s a l s o i m p o r t a n t t o h e l p r e p l a c e t h e n a t u r a l f a t s and o i l s removed f rom t h e s k i n by exposure t o l u b r i c a n t s and by washing.
T h i s i s a v e r y i mp ortant p a r t o f a s k i n c a r e programme
t o a v o i d d e r m a t i t i s problems.
I t i s p a r t i c u l a r l y important w i t h o l d e r em-
pl o y e e s who t e n d t o have d r i e r s k i n s , and i n t h e w i n t e r when low temperatures
281 and h u m i d i t y also tend t o cause dryness and c r a c k i n g o f t h e s k i n .
Male
employees a r e o f t e n r e l u c t a n t t o use a s k i n cream because o f t h e i r a s s o c i a t i o n w i t h t h e " s o f t f e m i n i n e touch" o f consumer a d v e r t i s i n g .
There i s no doubt t h a t
the common a s s o c i a t i o n o f m a s c u l i n i t y w i t h tough and rough hands which do n o t need s k i n creams has c o n t r i b u t e d t o numerous cases o f o c c u p a t i o n a l s k i n disease. E f f o r t s t o persuade more men t o use r e c o n d i t i o n i n g creams r e g u l a r l y would undoubtedly be w e l l rewarded i n reduced absence from work, o r need t o t r a n s f e r t o o t h e r j o b s , because o f d e r m a t i t i s .
S u i t a b l e creams a r e a v a i l a b l e f r o m r e p u t a b l e
b a r r i e r cream suppl i e r s . F i n a l l y , an i m p o r t a n t p a r t o f any s k i n p r o t e c t i o n programme i s t o ensure t h a t a l l employees who use o r a r e exposed t o any t y p e o f l u b r i c a n t o r metalworking f l u i d keep a c a r e f u l watch on a l l areas o f t h e i r s k i n and o b t a i n medical a d v i c e a t t h e f i r s t s i g n o f any a b n o r m a l i t y .
Medical a t t e n t i o n should be o b t a i n e d f o r
any c u t s and s c r a t c h e s as w e l l as d i s c o l o r a t i o n , soreness, warty growths.
itching, swelling o r
Awareness o f s k i n d i s o r d e r s and s k i n c a r e can be promoted by
d i s p l a y i n g and d i s t r i b u t i n g p o s t e r s and l e a f l e t s p u b l i s h e d by t h e H e a l t h and Safety E x e c u t i v e and by t h e o i l s u p p l i e r s .
Employees should be aware o f and
observe any s p e c i a l i n s t r u c t i o n s on p r o d u c t package l a b e l s o r i n t h e s u p p l i e r ' s product l i t e r a t u r e .
Under t h e H e a l t h and S a f e t y a t Work A c t ,
i t i s t h e respon-
s i b i l i t y o f t h e employer t o i n f o r m h i s employees o f any known o r p o t e n t i a l hazards t o h e a l t h and t o i n s t r u c t them on t h e a p p r o p r i a t e p r e c a u t i o n s t o be followed.
Constant reminders and proper s u p e r v i s i o n a r e necessary t o ensure
t h a t t h e contempt b r e d o f f a m i l i a r i t y does n o t o v e r r i d e prudence, o r t h e "problems o n l y happen t o o t h e r people" syndrome does n o t become predominant.
12.8.3
O i l M i s t and Vapour
Although exposure i s u n l i k e l y t o c r e a t e a hea t h hazard, c o n c e n t r a t i o n s o f o i l m i s t and vapour i n t h e p l a n t atmosphere shou d be minimised t o a v o i d an unpleasant environment.
The o i l m i s t c o n c e n t r a t on should be m a i n t a i n e d below
t h e Threshold L i m i t Value o f 5mg/m3 and p r e f e r a b y below 2.5mg/m3.
O i l mist i s
determined by use o f sampling pumps t o c o l l e c t m i s t on f i l t e r papers f o r a n a l y s i s by w e i g h i n g o r o t h e r methods.
The Occupational Hygiene Sub-committee o f t h e
I n s t i t u t e o f Petroleum A d v i s o r y Committee on H e a l t h has p u b l i s h e d d e t a i l s o f s u i t a b l e techniques [ 2 0 ] . atmosphere,
As a general guide,
i f m i s t can be seen i n t h e p l a n t
i t i s l i k e l y t o be above 5mg/m3.
I f t h e o i l m i s t l e v e l i s e x c e s s i v e , t h e f i r s t s t e p i s t o t r y t o reduce t h e amount generated. i s produced.
(1)
T h i s w i l l r e q u i r e an assessment o f t h e way i n which t h e m i s t
There a r e two b a s i c mechanisms by which m i s t i s formed:
I n some o p e r a t i o n s o i l may be atomised i n small d r o p l e t form. T h i s may occur
i n some h i g h speed c u t t i n g o p e r a t i o n s o r f r o m m i s t l u b r i c a t i o n systems through
288 over a p p l i c a t i o n o f m i s t o r poor r e c l a s s i f i c a t i o n o f t h e l u b r i c a n t .
In these
cases, m i s t c o n c e n t r a t i o n s may be reduced by adjustments t o t h e method o f appl i c a t i o n o r by m o d i f i c a t i o n t o t he co mpo si t i on o f t h e l u b r i c a n t .
I n metal
machining t h e r a t e , volume f l o w and p o i n t o f a p p l i c a t i o n can a f f e c t t h e degree o f misting.
Proper p o s i t i o n i n g o f sp l ash guards can c o n t r o l t h e escape o f o i l
m i s t i n t o t h e g e n era l p l a n t atmosphere.
S p e c i a l l y formulated a n t i - m i s t c u t t i n g
o i l s have s u c c e s s f u l l y reduced p l a n t m i s t l e v e l s i n some a p p l i c a t i o n s .
Mist
l u b r i c a n t f o r m u l a t i o n s r e q u i r e a c a r e f u l ba l ance between adequate m i s t i n g and reclassification properties.
The r a t e o f a p p l i c a t i o n o f t h e m i s t l u b r i c a n t and
t h e design o f t h e a p p l i c a t i o n system a r e a l s o important. ( i i ) V a p o r i s a t i o n o f l u b r i c a n t may be f o l l o w e d by condensation t o form small droplets o f o i l mist.
I n t h i s case i t may be p o s s i b l e t o p r o v i d e a d d i t i o n a l
c o o l i n g by i n c r e a s i n g t h e volume f l o w r a t e o f o i l a p p l i e d .
I t i s o f t e n thought
t h a t h i g h v e l o c i t y j e t o f o i l i s t h e most e f f e c t i v e means o f c o o l i n g whereas i n f a c t a low v e l o c i t y h i g h volume f l o w w i l l r e s u l t i n less o i l v a p o r i s a t i o n .
With
a h i g h v e l o c i t y j e t t h e r e may a l s o be a g r e a t e r tendency t o f o r m a t i o n o f o i l mi st.
I f f o r m a t i o n o f o i l m i s t and vapour cannot be e f f e c t i v e l y c o n t r o l l e d by app-
l i c a t i o n o r f o r m u l a t i o n changes, l o c a l exhaust v e n t i l a t i o n should be used. Systems i n c o r p o r a t i n g f i l t e r s t o remove o i l so t h a t c l e a n a i r can be r e t u r n e d t o t h e p l a n t atmosphere a r e co mmerci a l l y a v a i l a b l e .
For maximum e f f e c t i v e n e s s ,
th e exhaust hood sho ul d be l o c a t e d as c l o s e t o t h e p o i n t o f m i s t g e n e r a t i o n as poss i b l e
.
O i l m i s t i s sometimes generated by t he use o f a i r j e t s t o remove swarf from
machined p a r t s .
I f t h e r e i s no o t h e r way o f p e r f o r m i n g t h i s o p e r a t i o n , con-
s t r u c t i o n o f an e ncl o sure w i t h l o c a l exhaust v e n t i l a t i o n may be needed.
12.8.4
S k i n Cancer
I f good q u a l i t y s o l v e n t r e f i n e d o i l s ( o r t hose t r e a t e d adequately i n o t h e r ways t o reduce t h e a r o m a t i c c o n t e n t s ) a r e used i n a l l a p p l i c a t i o n s where any s i g n i f i c a n t s k i n c o n t a c t i s l i k e l y , s k i n cancer should n o t be a problem i n f u t u r e , p r o v i d e d t h a t p r e c a u t i o n s di scu ssed under Skin P r o t e c t i o n i n sec.12.8.2 a r e followed.
Because o f t h e l o ng l a t e n t p e r i o d f r o m i n i t i a l exposure t o
occ u r r e n c e ( o f t e n more t h an 20 yea rs) some cases a s s o c i a t e d w i t h prolonged o r rep e a t e d exposure t o p o o r l y r e f i n e d m i n e r a l o i l s many years ago, o r w i t h unsati s f a c t o r y hygiene p r a c t i c e s i n t h e p a s t , can s t i l l be expected t o a r i s e . However, f o l l o w i n g t h e i n t r o d u c t i o n o f s o l v e n t r e f i n e d o i l s i n c u t t i n g f l u i d f o r m u l a t i o n s and t h e i mp l eme nt at i on o f b e t t e r hygiene p r a c t i c e s , fewer cases as t ime passes.
t h e r e should be
289 Because changes o ccur i n co mpo si t i o n o f used o i l s , p a r t i c u l a r l y w i t h gasol i n e engine l u b r i c a n t s and quenching o i l s ,
i t would be prudent t o e x e r c i s e c a r e
w i t h these p r o d u c t s and a v o i d s k i n c o n t a c t as much as p o s s i b l e .
12.8.5
B a c t e r i a and B i o c i d e s
B a c t e r i a i n aqueous c o o l a n t s and m i n e r a l o i l l u b r i c a n t s contaminated w i t h water need t o be c o n t r o l l e d f o r t e c h n i c a l performance reasons. Section 12.5,
t h e r e i s no evidence o f h e a l t h hazards a s s o c i a t e d w i t h t h e o c c u r -
rence o f these b a c t e r i a .
However, a d d i t i o n o f b i o c i d e s t o c o o l a n t systems may
present hazards i n hand1 i n g these m a t e r i a l s . and t o x i c .
As i n d i c a t e d i n
T h e r e f o re,
Biocides a r e normally i r r i t a n t
t h e s u p p l i e r ' s h a n d l i n g recommendations should be c a r e -
f u l l y followed.
12.9
CONCLUSIONS
P r o v id e d t h a t use rs a r e aware o f p o t e n t i a l hazards and f o l l o w recommended h a n d l i n g p r a c t i c e s i n co mbi n at i on w i t h good personal and p l a n t hygiene standards, l u b r i c a n t s s h o u ld p r e s e n t n o undue h e a l t h r i s k s .
The major p o i n t s which need t o
be r e p e a t e d l y s t r e s s e d can be summarised as f o l l o w s : Use good q u a l i t y s o l v e n t r e f i n e d ( o r e q u i v a l e n t ) m i n e r a l o i l s i f t h e r e i s t o be s i g n i f i c a n t s k i n c o n t a c t . Ensure t h a t adequate i n f o r m a t i o n i s a v a i l a b l e t o enable products t o be used s a f e l y . Develop p r o p er awareness o f hazards t hro ugh t r a i n i n g , c a u t i o n a r y notices, supplier publications etc. A v o id r e p e a t ed or pro l on ge d s k i n c o n t a c t . Encourage good p erso na l h ygi e ne w i t h p roper s k i n c l e a n i n g p r a c t i c e s . Provide s u i t a b l e p r o t e c t i v e c l o t h i n g . Encourage p r o p e r use o f b a r r i e r and r e c o n d i t i o n i n g creams. Keep o i l m i s t c o n c e n t r a t i o n s w e l l below 5mg/rn3. E s t a b l i s h good o i l and machine maintenance p r a c t i c e s . Ob t a in e a r l y medical a d v i c e f o r any s k i n problems. Ensure good p r a c t i c e s a r e ma i nt ai n ed w i t h d i l i g e n t s u p e r v i s i o n .
REFERENCES
1 2 3
4 5 6
7
Wagner,W.D., e t . a l . , Am. Ind. Hyg. Assn. J. 1964, 5 , 158. Lushbaugh,C.C., e t . a l . , Arch. Ind. Hyg. 1950. 237. Wagner,W.D., e t . a l . , Unpublished r e s u l t s , USPHS, 1014 Broadway, C i n c i n a t t i , U.S.A. Decoufle,P., Ann. New York Acad. S ci . 1976, 94. Decoufle,P., J. Nat. Cancer I n s t . 1978, 1025. Drasche,H., e t . a l . , Z en t b l . Arb. Med. Arbschutz 1974, lo. Sanderson,J., O i l M i s t - Recent I n t e r e s t s i n Europe. Presented a t Esso Symp. on O i l M i s t and Ni t rosa mi ne s, Stockholm, March 1977.
1,
271, 5,
290 8 9 10
11 12 13
14 15 16
17 18 19 20
Leitch,A., B r i t . Med. J., 1922, 2, 1004. Bingham,E., and Horton,A.W., 'Advances i n B i o l o g y o f S k i n ' , Vol . V I I Carcinogenesis, 1966, Pergamon Press, New York. Medical Research Council, 'The Carcinogenic A c t i o n o f M i n e r a l O i l s : A Chemical and B i o l o g i c a l S t u d y ' . Special Report S e r i e s No. 306, 1968, H.M.S.O. London. J. Occ. Med. 1975, 1_z, 784. Scala,R.A., Wahlberg,J.E., Acta.Derrn. (Stockholm), 1974, 54, 471. Th. Van Peteghem and H. De Vos, B r i t . J. I n d . T e d . 1974, 2, 233. Zingmark,P.A. and Rapp,C. Ambio, 1977, 237. .NIDSH T e c h n i c a l Report ' C o n t r o l o f Exposure t o M e t a l w o r k i n g F l u i d s ' , February 1978, (Pub1 i c a t i o n No. 78-165). Technical B u l l e t i n , ' M i c r o b i o l o g i c a l Degradation o f L u b r i c a t i n g O i l s ' . Mobil O i l Co. L t d . , 1977. Thony,C., e t a l . , Arch. Mal. Prof de Med Trav e t Sec. SOC. ( P a r i s ) , 37. 1975, Clausen,J., and Rastogi,S.C., B r i t . J. Ind. Med. 1977, 208. I n s t i t u t e of Petroleum "Code o f P r a c t i c e f o r M e t a l w o r k i n g F l u i d s " , 1978, Heyden & Son L t d . , London. I n s t i t u t e o f Petroleum Occupational Hygiene Sub-committee, Ann. Occup. 293-297. Hyg. 1975,
6,
36,
2,
18,
291
13
EFFECTIVE CONTAMINATION CONTROL IN FLUID POWER SYSTEMS
J.B.SPENCER,
13.1
Manager, User Support D i v i s i o n , S p erry V i c k e r s .
INTRODUCTION
The s e l e c t i o n o f a f i l t e r and i t s p r o p e r l o c a t i o n i n a h y d r a u l i c system needs as much c a r e and t h e same l e v e l o f e x p e r t i s e as t h e s e l e c t i o n o f o t h e r components such as pumps, v a l v e s and c y l i n d e r s .
Many system designers l o o k no f u r -
t h e r t h a n t h e h y d r a u l i c equipment m a n u f a c t u r e r ' s catalogue f o r guidance, b u t u n f o r t u n a t e l y i t i s s t i l l common f o r h y d r a u l i c equipment manufactuers t o s p e c i f y one g e n e r a l l e v e l o f f i l t r a t i o n such as 25 micrometre, w i t h o u t r e g a r d t o the working p r e s s u r e , environment, o r d u t y c y c l e . a lower s t a n d a r d may be a ccep t a bl e ;
With c e r t a i n types o f equipment
an example o f t h i s being t h e e a r l i e r designs
of pumps, many o f whi ch g i v e l o n g t r o u b l e - f r e e s e r v i c e p r o t e c t e d o n l y by a 0.13 mm s t r a i n e r .
On t h e o t h e r hand, more modern equipment such as t h e m i n i a t u r i z e d
c o n t r o l s w i t h s m a l l e r cl e ara nce s t ha n many servo v a l v e s , w i l l need much h i g h e r standards o f p r o t e c t i o n . U s u a l l y t h e n e x t s t e p i s t o de ci de on t h e l o c a t i o n o f t h e f i l t e r and, again, th e g e n e r a l i z e d recommendations o f t h e f i l t e r manufacturer a r e o f t e n accepted w i t h o u t r e g a r d t o t h e p a r t i c u l a r system requirements.
F i n a l l y , the s i z e o f the
f i l t e r i s f i x e d and sometimes t h e o v e r r i d i n g c o n s i d e r a t i o n i s simply a d e s i r e t o match t h e f i l t e r p o r t s i z e t o t h e di a met er o f t h e a d j o i n i n g pipework.
T h i s may
w e l l ensure t h a t recommended v e l o c i t i e s a r e n o t exceeded b u t much more important and o f t e n o v e r lo oke d i s t h e f i l t e r e f f i c i e n c y and d i r t h o l d i n g c a p a c i t y . I t must be a d m i t t e d t h a t t h e h i t - o r - m i s s
approach described above o f t e n
appears t o a c h ie ve an a c c e p t a b l e r e s u l t , b u t w i t h over 70% o f h y d r a u l i c system f a i l u r e s known t o be due t o poor f l u i d c o n d i t i o n , t h e r e i s a c l e a r need f o r a more s y s t e m a t i c approach t o c o n t a m i n a t i o n c o n t r o l . The need has been a ccen t u at ed by t h e i n c r e a s i n g l y arduous c o n d i t i o n s under whic h systems o p e r a t e .
For example, a p r e s s u r e o f about 70 bar was common i n
i n d u s t r i a l h y d r a u l i c systems f o r many ye ars;
today,
140-210 bar systems a r e
292 commonplace and much o f t h e contaminant f o r m e r l y washed away i s now f o r c e d i n t o t h e c le a r a n c e s where i t does c o n s i d e r a b l e damage.
Smaller o i l r e s e r v o i r s mean
more r a p i d c i r c u l a t i o n and l e s s o p p o r t u n i t y f o r p a r t i c l e s t o s e t t l e o u t .
Higher
o p e r a t i n g temperatures r e s u l t i n t h i n n e r o i l , w hich i n some s i t u a t i o n s may g i v e l e s s p r o t e c t i o n a g a i n s t wear g i v i n g r i s e t o i ncreased contamination. I n t h e f a c e o f t h ese t re nd s,
t h e h y d r a u l i c equipment user wants improved
r e l i a b i l i t y and i t must be e a s i e r and l e s s c o s t l y f o r him t o achieve t h i s i f he works w i t h c l e a n o i l .
I t i s n o t d i f f i c u l t t o keep t h e o i l i n good c o n d i t i o n
p r o v i d e d t h e machine d esi g n i s r i g h t , and t h e rewards f o r doing so a r e b e t t e r r e l i a b i l i t y and l o ng er l i f e f rom b o t h t h e equipment and o i l .
13.1.1
A S y s te ma t i c Approach t o F i l t r a t i o n
To work towards t h e most e f f e c t i v e p r o t e c t i o n c o n s i s t e n t w i t h economy, we must f i r s t d e f i n e o u r aim.
I t i s n o t , as i s w i d e l y assumed, s i m p l y t o separate
o u t p a r t i c l e s l a r g e r t h an a c e r t a i n s i z e chosen a r b i t r a r i l y .
I n s t e a d , we must
ach ie v e s t a b l e l e v e l s o f c o n t a m i n a t i o n a c c e p t a b l e and a p p r o p r i a t e t o v a r i o u s p a r t s o f t h e system. For a s t a b i l i s e d c o n t a m i n a t i o n l e v e l , e c t e d by t h e system f i l t e r s ) .
' d i r t i n ' must equal ' d i r t o u t '
(coll-
' D i r t i n ' i s made up o f i n - b u i l t contaminant,
c o n t a m in a t io n i n t h e i n i t i a l charge o f o i l , and c o n t a m i n a t i o n drawn i n from t h e atmosphere t h r o u gh t h e a i r b r e a t h e r and c y l i n d e r s e a l s
-
a l l o f w hich c o n t r i b u t e
t o t h e g e n e r a t i o n o f p a r t i c l e s by t h e process o f wear. The e f f e c t o f t h e i n - b u i l t c o n t a m i n a t i o n must be c a r e f u l l y considered.
Inev-
i t a b l y i t w i l l be h i g h , even when c a r e i s taken i n t h e p r e p a r a t i o n o f pipework and m a n i f o l d b lo cks.
F l u s h i n g w i l l remove some i n i t i a l contaminant,
but there
a r e many systems where t h i s i s n o t done, and a t t h e f i r s t s t a r t - u p h i g h pressures a r e generated w i t h h i g h c o n t a m i n a t i o n l e v e l s p r e s e n t .
The r e s u l t i s u s u a l l y
r a p i d pump wear and v a l v e m a l f u n c t i o n , t h e f i r s t o f which w i l l almost c e r t a i n l y go u n d e t e c te d a t t h i s time.
A l l h y d r a u l i c systems should be run i n an o f f - l o a d
c o n d i t i o n u n t i l the desired contamination l e v e l i s a t t a i n e d . F i g u r e 1 shows a t y p i c a l r e l a t i o n s h i p between t h e design c o n t a m i n a t i o n l e v e l and t h e a c t u a l l e v e l p r i o r t o s t a r t - u p .
T h i s convenient method o f p r e s e n t a t i o n
f o l l o w s n a t u r a l l y f ro m t h e l o g a r i t h m i c d i s t r i b u t i o n o f p a r t i c l e s i z e t h a t occurs i n practice.
The r e l a t i v e sl o pe s o f t h e i n i t i a l and a c c e p t a b l e Contamination
l i n e s are a c l e a r p o i n t e r t o the type o f f i l t r a t i o n
needed.
The methods used t o d et ermi n e and c o n t r o l c o n t a m i n a t i o n l e v e l s w i l l be d i s cussed i n more d e t a i l l a t e r , b u t a t t h i s st ag e we can summarize t h e p r a c t i c a l and performance req ui re men t s o f t h e f i l t r a t i o n system as f o l l o w s : -
293
Initial contamination level
Fig. 1: Typical relationship between the desired design contamination level and the actual level prior to start up. It is essential that the system is flushed and run at no-load until the acceptable contamination level is achieved.
level
5 10 15 Parliclasire
(i)
25
50
- micrometres(lopa)
100
I t must be ca pa bl e o f red uci n g t h e i n i t i a l c o n t a m i n a t i o n t o t h e d e s i r e d l e v e l w i t h i n an a c c e p t a b l e p e r i o d o f t i me, w i t h o u t causing premature wear o r damage t o t h e h y d r a u l i c components.
(ii)
I t must be cap ab l e o f a c h i e v i n g and m a i n t a i n i n g t h e d e s i r e d l e v e l , i n c l u d i n g a s u i t a b l e f a c t o r o f s a f e t y t o c a t e r f o r a c o n c e n t r a t e d ingress which c o u l d o ccur; f o r example, when a system i s 'topped-up'.
( i i i ) The q u a l i t y o f maintenance a v a i l a b l e t h e end u s e r l o c a t i o n must be acknowledged. (iv)
F i l t e r s must be e a s i l y a c c e s s i b l e f o r maintenance purposes.
(v)
I n d i c a t i o n o f f i l t e r c o n d i t i o n t o s u i t t h e end u s e r ' s requirements must be p r o v id e d.
(vi)
I n c o n tin u ou s p roce ss p l a n t , f a c i l i t i e s must be p r o v i d e d t o a l l o w changing o f elements w i t h o u t i n t e r f e r i n g w i t h p l a n t o p e r a t i o n .
( v i i ) The f i l t e r s must p r o v i d e s u f f i c i e n t d i r t h o l d i n g c a p a c i t y f o r an a c c e p t a b l e i n t e r v a l between element changes. ( v i i i ) T h e i n c l u s i o n o f a f i l t e r i n t h e system must n o t produce u n d e s i r a b l e e f f e c t s on t h e o p e r a t i o n o f components,
e.g.
h i g h back pressures on
seal drains. (ix)
Sampling p o i n t s must be p r o v i d e d t o m o n i t o r i n i t i a l and subsequent l e v e l s
of c o n ta m i na t i o n.
294 13.2 13.2.1
DIRT INPUT
-
TYPES AND SOURCES OF CONTAMINATION
New O i l
A lt h o u g h o i l i s r e f i n e d and blended under r e l a t i v e l y c l e a n c o n d i t i o n s , i t i s u s u a l l y s t o r e d i n drums o r i n a b u l k t a n k a t t h e u s e r ' s f a c t o r y .
A t t h i s point
i t i s no lo n g e r c l e a n , because t h e f i l l i n g l i n e s c o n t r i b u t e metal and rubber
p a r t i c l e s and t h e drum always adds f l a k e s o f metal o r scale.
Storage tanks can
be a r e a l problem because wat er condenses i n them t o cause r u s t i n g and contamin a t i o n fr o m t h e atmosphere f i n d s i t s way i n unless s a t i s f a c t o r y . a i r
breather
f i l t e r s are f i t t e d . I f t h e o i l i s b e i n g s t o r e d under reasonable c o n d i t i o n s ,
t h e p r i n c i p a l contam-
i n a n t s on d e l i v e r y t o a machine w i l l be me t a l, s i l i c a , and f i b r e s .
With o i l s
from r e p u t a b l e s u p p l i e r s , sampling has shown average counts o f 30,000
t o 50,000
p a r t i c l e s above 5 mi crome t re p e r 100 m l , w i t h a r e l a t i v e l y low s i l t l e v e l . Using a p o r t a b l e t r a n s f e r u n i t o r some o t h e r f i l t r a t i o n arrangement,
i t i s possible t o
remove much o f t h e c o n t a m i n a t i o n p r e s e n t i n new o i l b e f o r e i t e n t e r s t h e system and i s ground down i n t o f i n e r p a r t i c l e s . I t must be s a i d i n pa ssi ng t h a t c o n t a m i n a t i o n a r i s i n g f r o m d e l i v e r y and s t o r age v a r i e s w i t h t h e i n d u s t r y .
For example, a i r c r a f t o p e r a t i o n g e n e r a l l y needs
h i g h standards o f c l e a n l i n e s s and f a i r l y q u i c k t u r n o v e r o f s t o r e s , whereas much l on g e r s t o r a g e p e r i o d s a r e t h e r u l e f o r ma ri ne systems and t h e environment may be more d i f f i c u l t t o c o n t r o l .
13.2.2
B u i l t - i n Con t a mi na t i o n
New machinery always has a c e r t a i n amount o f b u i l t - i n contamination.
Care i n
assembly and i n f l u s h i n g t h e system reduces t h i s , b u t never e l i m i n a t e s i t . T y p i c a l b u i l t - i n contaminants a r e b u r r s , c h i p s , m o is t u r e , p i p e dope, wel d s p l a t t e r ,
f l a s h , d i r t , dust, f i b r e , sand,
p a i n t s , and f l u s h i n g s o l u t i o n s .
The amount o f contaminant removed d u r i n g f l u s h i n g depends n o t o n l y on t h e e f f e c t i v e n e s s o f t h e f i l t e r used b u t a l s o t h e v e l o c i t y o f t h e f l u s h i n g f l u i d . Unless h i g h v e l o c i t i e s a r e a t t a i n e d , much o f t h e contaminant w i l l n o t be d i s lodged u n t i l t h e system i s i n o p e r a t i o n , w i t h component f a i l u r e t h e almost certain result.
I r r e s p e c t i v e o f t h e st a nd ard o f f l u s h i n g , an o f f - l o a d running-
i n p e r i o d s h o u ld be regarded as e s s e n t i a l .
Some b u i l t - i n contaminant,
such as
weld s c a l e , o f t e n remains i n t a c t u n t i l h i g h p r e s s u r e o i l i s f o r c e d between i t and t h e p a r e n t m e t a l , thus l o o s e n i n g i t .
13.2.3
Environmental Contamination
Contaminants f r o m t h e immediate surro un di n as can be i n t r o d u c e d i n t o a f l u i d power system.
On l a r g e i n s t a l l a t i o n s such as those w i t h i n steelw orks,
r e l a t i v e l y easy t o a s c e r t a i n t h e e nvi ro nme nt al c o n d i t i o n s , siderably.
it is
though they v a r y con-
For example, a coke oven system operates i n c o n d i t i o n s v e r y d i f f e r e n t
296
from a c o l d m i l l .
Sometimes t h e b e s t s o l u t i o n i s t o p r o t e c t t h e h y d r a u l i c
equipment by p r o v i d i n g a c l e a n room where maintenance can be c a r r i e d o u t under controlled conditions.
Unfortunately,
i t i s n o t uncommon t o see h y d r a u l i c power
sources exposed t o t h e w o r s t p o s s i b l e environment, w h i l e a l o n g s i d e t h e e l e c t r i c s are p r o t e c t e d by p r e s s u r i s e d and temperature c o n t r o l l e d cabins. I n most machine shops t h e r e l a t i v e l y l a r g e contaminant p a r t i c l e s o f 10-15 micrometres do n o t demand a h i g h st an da rd o f a i r f i l t r a t i o n , a l t h o u g h g r i n d i n g machines w i t h o u t e f f e c t i v e e x t r a c t i o n equipment can r e s u l t i n l o c a l i s e d p r o blems.
On t h e o t h e r hand, f o u n d r i e s and st on e q u a r r i e s demand a v e r y h i g h
standard o f f i l t r a t i o n because o f t h e a i r b o r n e a b r a s i v e p a r t i c l e s . The m o b i l e equipment f i e l d p r e s e n t s s p e c i a l problems because t h e o r i g i n a l manufacturer u s u a l l y s e l l s a st an da rd machine t o o p e r a t e i n a w ide v a r i e t y of environmental c o n d i t i o n s .
13.2.4
E n t r y P o i n t s f o r Environmental Con t amination
13.2.4.1
A i r breathers.
Very l i t t l e i n f o r m a t i o n appears t o be a v a i l a b l e on
what t h e f i l t e r w i l l a c t u a l l y ach i eve and p u r e l y nominal r a t i n g s a r e u s u a l l y specified.
There have been i nst an ces where t h e element has shrunk l e a v i n g a
f r e e passage f o r t h e a i r , wh i ch h i g h l i g h t s t h e need f o r more r i g i d e n g i n e e r i n g standards on t h i s t y p e o f p r o d u c t .
The amount o f a i r passing through t h e f i l -
whi ch means f o r example t h a t s i n g l e a c t i n g c y l i n -
t e r w i l l depend on d r a w - o f f ,
ders i n bad environments must r e s u l t i n a g r e a t e r ingress o f contaminant. I t i s encouraging t o see some ma nu f a ct u rers now o f f e r i n g b e t t e r grades of f i l t e r paper and t h a t i n c e r t a i n areas t h e combined f i l l e r / b r e a t h e r unacceptable.
has become
A sep ara t e b r e a t h e r i s more e f f i c i e n t and h e l p s t o r e l e a s e t h e
a i r w h i l e f i l l i n g t a kes p l a c e t hro ug h a s u i t a b l e gauze s t r a i n e r .
13.2.4.2
Power u n i t access p l a t e s .
I n some p l a n t s i t cannot be assumed t h a t
access p l a t e s w i l l always be re pl a ced , though h a p p i l y t h i s s t a t e o f a f f a i r s i s no t as common as i t once was.
I n power u n i t design, good s e a l i n g i s v i t a l , and
i n bad environments such items as s t r a i n e r s should n o t be p o s i t i o n e d i n s i d e t h e r e s e r v o i r i f access r e q u i r e s t h e r e f i t t i n g o f removable p l a t e s .
Other removable
items w i l l a l l o w i n g r e s s d u r i n g maintenance and good design p r a c t i c e should minimize t h i s .
13.2.4.3
Cylinder seals.
Wiper s e a l s cannot be 100% e f f e c t i v e i n removing
very f i n e contaminant f rom t h e c y l i n d e r rod. I f they were, t h e y would remove t h e o i l f i l m from t h e p i s t o n rod, producing a r e s u l t t h a t i s u s u a l l y diagnosed as a l e a k y seal. d r y r o d would q u i c k l y wear o u t t h e se al s.
I n any case, a completely
Where c y l i n d e r s remain extended i n a
296 h e a v i l y contaminated atmosphere c o n s i d e r a b l e q u a n t i t i e s o f f i n e p a r t i c l e s can g e t i n t o t h e system un l ess p r o t e c t i o n such as a b e l l o w s i s p r o v i d e d . I t has been shown t h a t c y l i n d e r p i s t o n r o d s e a l s n a t u r a l l y i n g r e s s about one p a r t i c l e o v er 10 mi cro met re f o r each square c e n t i m e t r e o f swept rod area. Wear o f s e a l s o r wi p ers can i n c r e a s e t h e i n g r e s s i o n r a t e c o n s i d e r a b l y .
Thus
i n bad ambient c o n d i t i o n s a 50mm di a met er r o d i n a 100mm b o r e c y l i n d e r , c y c l i n g a t a speed o f 12 metres p e r minute, c o u l d i n g r e s s about 20,000 p a r t i c l e s ove r 1Omicrometres e very mi nu t e , and t h i s q u a n t i t y c o u l d i n c r e a s e by a f a c t o r o f 100 f o r every 100 hours o f ru nn i ng .
13.2.5
Generated Co nt ami n at i on
Contamination i s c r e a t e d i n t e r n a l l y by t h e o p e r a t i o n o f a h y d r a u l i c system. The generated contaminants a r e p r o d u c t s o f wear, c o r r o s i o n , c a v i t a t i o n , and f l u i d breakdown,
i.e.
decomposition,
oxidation,
etc.
Experience shows t h a t i n
a system which has been c a r e f u l l y f l u s h e d and has f i l t e r e d o i l added t o t h e sealed r e s e r v o i r ( i n c o r p o r a t i n g an e f f e c t i v e b r e a t h e r ) , t h e c o n t a m i n a t i o n w i l l be m a i n l y system generated.
I f t h e i n i t i a l l e v e l i s n o t s a t i s f a c t o r y , t h i s induces wear w hich g r e a t l y a c c e l e r a t e s t h e b u i l d - u p o f generated contaminant.
13.3
EFFECTS OF TYPES AND S I Z E S OF PARTICLES
We know t h a t contaminant p a r t i c l e s a r e o f a l l shapes and s i z e s and t h a t t h e f i n e r t h e y a r e , t h e more d i f f i c u l t i t i s t o count them and t o determine t h e m a t e r i a l o f wh ic h t h e y a r e composed. a b r a s i v e and t h a t ,
However, we can say t h a t t h e m a j o r i t y a r e
i n t e r a c t i n g w i t h s u r f a c e p r o t r u s i o n s , t h e y plough and c u t
fragments f r o m a s u r f a c e .
T h i s wear accounts f o r about 90% o f f a i l u r e s due t o
c o n ta m in a tio n . F a i l u r e s a r i s i n g from contamination f a l l i n t o t h r e e categories:
13.3.1 v a lv e .
C a t a s t r o p h i c F a i l u r e o c c u r s when a l a r g e p a r t i c l e e n t e r s a pump o r For in s tan ce,
i f a - p a r t i c l e causes a vane t o jam i n a r o t o r s l o t , t h e
r e s u l t may w e l l be complete s e i z u r e o f t h e pump or motor.
I n a spool t y p e valve
a l a r g e p a r t i c l e t rap pe d a t t h e r i g h t p l a c e can s t o p a spool c l o s i n g c o m p l e t e l y . Another example of c a t a s t r o p h i c f a i l u r e o c c u r s when t h e p i l o t o r i f i c e of a v a l v e i s b lo c k e d by a l a r g e p a r t i c l e .
F i n e p a r t i c l e s can a l s o cause catastrop.hic
f a i l u r e , for i n s t a n c e i f a v a l v e f a i l s t o o p e r a t e due t o s i l t i n g .
13.3.2
I n t e r m i t t e n t F a i l u r e i s caused by contaminant on t h e s e a t o f a poppet
v a l v e w h ic h p r e v e n t s i t f ro m r e s e a t i n g p r o p e r l y .
I f t h e seat i s t o o hard t o
a l l o w t h e p a r t i c l e t o b e embedded i n t o i t , t h e p a r t i c l e may be washed away when t h e v a l v e i s opened ag ai n .
L a t e r , a n o t h e r p a r t i c l e may p r e v e n t complete c l o s u r e
297 o n l y t o be washed away when t h e v a l v e opens.
Thus a v e r y annoying t y p e o f i n t e r -
m i t t e n t f a i l u r e occurs.
13.3.3
D e g r a d at i o n F a i l u r e f o l l o w s wear, c o r r o s i o n ,
and c a v i t a t i o n e r o s i o n .
They cause increased i n t e r n a l leakage i n t h e system components, b u t t h i s condition i s often d i f f i c u l t t o detect. The e v e n t u a l r e s u l t , p a r t i c u l a r l y w i t h pumps, failure.
i s l i k e l y t o be c a t a s t r o p h i c
The p a r t i c l e s most 1 i k e l y t o cause wear a r e c l e a r a n c e - s i z e p a r t i c l e s
which j u s t pass t h r o u g h t h e cl e ara nce s between moving p a r t s ( F i g . 2 ) .
Particles of similar size to the clearances between moving parts cause the most abrasive wear. Larger particlescannot enter clearance,smaller particlespass through without contact.
a 0 Clearancesize particles interact with both faces simultaneously. Extra contaminationis generated by the disintegrationof the moving parts. Larger particlesare'ground up' by the interactionin the clearances.
8
F i g. 2
I n t e r a c t i o n o f moving p a r t s .
M a n u fa c t u r in g cl e ara nce s w i t h i n h y d r a u l i c components can be d i v i d e d i n t o two p r i n c i p a l zones,
i.e.
around 5 micrometres f o r h i g h pressure u n i t s , and 15-40
micrometres f o r low p r e s s u r e u n i t s .
The a c t u a l clearance may v a r y c o n s i d e r a b l y
depending on t h e t y p e o f u n i t and o p e r a t i n g c o n d i t i o n s i t sees.
Good component
d e s i g n i s im p o r t an t , t hu s m i n i m i s i n g t h e e f f e c t o f small clearances. We s h o u ld t h e r e f o r e l o o k a t t h e f a c t o r s a f f e c t i n g c r i t i c a l clearances and a l s o a t t h e t y p e o f f a i l u r e o c c u r r i n g i n v a r i o u s groups o f components.
13.3.4
Pumps
A l l h y d r a u l i c pumps have component p a r t s which move r e l a t i v e t o one another, s e p a r a t e d by a s mal l o i l - f i l l e d cl e ara nce .
G enerally, these components a r e
loaded toward each o t h e r by f o r c e s r e l a t e d t o pressure, and t h e p r e s s u r e always tend s t o f o r c e f l u i d t h rou gh t h i s cl e ara nce . As t h e f i n i t e l i f e o f most pumps i s determined by a v e r y small q u a n t i t y o f m a t e r i a l b e in g removed f r o m a few su rf a ces,
i t f o l l o w s t h a t i f the f l u i d w i t h i n
298 t h e c l e a r a n c e i s h e a v i l y contaminated, could occur.
r a p i d d e g r a d a t i o n and e v e n t u a l s e i z u r e
W i t h low p r e s s u r e u n i t s t h e d e s i g n p e r m i t s r e l a t i v e l y l a r g e c l e a r -
ances and c o n t a m i n a t i o n has l e s s e f f e c t .
A l s o a t t h e lower p r e s s u r e t h e r e i s
less force a v a i l a b l e t o d r i v e p a r t i c l e s i n t o c r i t i c a l clearances.
Increasing
t h e p r e s s u r e t h e r e f o r e i s o f major s i g n i f i c a n c e i n d e t e r m i n i n g t h e e f f e c t o f c o n t a m i n a t i o n on a pump. Another f a c t o r a f f e c t i n g clearances i s t h e o i l f i l m t h i c k n e s s , which i s a l s o related t o f l u i d viscosity.
An optimum v i s c o s i t y i s used f o r d e s i g n which p r o -
v i d e s good f i l m t h i c k n e s s t o s u p p o r t loads hydrodynamically b u t which i s a l s o low enough t o a l l o w adequate f i l l i n g o f t h e pump w i t h o u t c a v i t a t i o n .
I t i s gen-
e r a l l y found i n p r a c t i c e t h a t f i l t r a t i o n requirements become l e s s c r i t i c a l where h i g h e r v i s c o s i t i e s a r e used, and f o r t h i s reason t h e maximum v i s c o s i t y which i s c o m p a t i b l e w i t h t h e i n l e t c o n d i t i o n s should be chosen.
S i m i l a r l y , good tempera-
t u r e c o n t r o l w i l l be o f b e n e f i t i n t h i s r e s p e c t . The areas i n pumps p a r t i c u l a r l y s u b j e c t t o these c l e a r a n c e problems a r e : -
Gear pump
-
Tooth t o housing, gear t o s i d e p l a t e .
A x i a l p i s t o n pump
-
Shoe t o swashplate, c y l i n d e r b l o c k .
Vane pump
Vane t i p t o cam r i n g , r o t o r t o s i d e p l a t e .
F i g u r e s 3 , 4 and 5 i l l u s t r a t e t h e c r i t i c a l areas d i a g r a m m a t i c a l l y .
Clearance under vane tips depends on speed, pressure and viscosity
ote Some back flow occurs from hlgh to
Fig.3
C r i t i c a l clearances i n a vane pump.
299
In Low pressure
Minimum clearance
Fig.4
&
db Maximum clearance
C r i t i c a l c l e a r a n c e s i n a gear pump.
Where
P
p
= viscosity
=
pressure
Valve plate
Fig.5
C r i t i c a l c l e a r a n c e s i n an a x i a l p i s t o n pump. A l t h o u g h p s t o n c earance i s n o m i n a l l y f i x e d , a c t u a c l e a r a n c e v a r i e s w i t h e c c e n t r c i t y due t o l o a d and v i s c o s i t y .
I n many o f t h e f o r e g o i n g cases t h e cl e ara nces a r e e f f e c t i v e l y s e l f - a d j u s t i n g under o p e r a t i n g c o n d i t i o n s , smaller.
i . e . w i t h i n c r e a s i n g p r e s s u r e clearances become
Under adverse c o n d i t i o n s , and p a r t i c u l a r l y where t h e r e i s shock load-
i ng , t h i s in c r e a ses t h e v u l n e r a b i l i t y t o s m a l l e r contaminant p a r t i c l e s . where c le a r a n c e s a r e n o m i n a l l y f i xed , u nd er
Even
h i g h loads components may t a k e up
e c c e n t r i c p o s i t i o n s whi ch a g a i n makes them v u l n e r a b l e t o s m a l l e r p a r t i c l e s .
It
i s e x tr e m e ly d i f f i c u l t t o be p r e c i s e about e i t h e r t h e magnitude o f these c l e a r ances, p a r t i c u l a r l y under dynamic l o a d i n g , o r t h e e f f e c t o f d i f f e r e n t s i z e p a r t i c l e s i n t h e gaps. However, fr o m t h e d at a we do have and f ro m f i e l d experience gained t o date, we a r e a b l e t o suggest c o n t a m i n a t i o n l e v e l s which, an a c c e p t a b le l i f e f o r most pumps.
i f achieved, w i l l r e s u l t i n
These a r e presented i n t h e n e x t c h a p t e r ,
whic h d e a l s w i t h t h e s p e c i f i c a t i o n o f c o n t a m i n a t i o n l e v e l s . The u s e f u l l i f e o f a pump sho ul d end when i t no longer d e l i v e r s t h e r e q u i r e d o u t p u t a t a g i v e n s h a f t speed, d i s c h a r g e p ressure, and f l u i d temperature. guid e , 15-20% l o s s o f f l o w would i n d i c a t e t h e end o f t h e u s e f u l l i f e .
As a
A l l too
o f t e n d e g r a d a t i o n goes un de t e ct e d until,finally,catastrophic f a i l u r e occurs w i t h v a s t q u a n t i t i e s o f c o n t a m i n a t i o n b e i n g r e l e a s e d i n t o t h e system. owing such a f a i l u r e , t h e system i s n o t t h en p r o p e r l y cleaned,
If, foll-
t h e l i f e o f the
replacement pump w i 1 1 be reduced. I n t h e i n t e r e s t o f t h e end use r, t h e system designer should s p e c i f y t h e m i n i mum a c c e p t a b l e f l o w r a t e f r o m t h e pump t o achieve s a t i s f a c t o r y machine performance.
Means s h o u l d be p r o v i d e d f o r m o n i t o r i n g pump o u t p u t by i n s e r t i n g s u i t a b l e
instrumentation,
e i t h e r t e m p o r a r i l y o r pe rmanently, so t h a t r o u t i n e checks can
be c a r r i e d o u t t o reduce t h e r i s k o f c a t a s t r o p h i c f a i l u r e s . With p i s t o n u n i t s , i t i s u s u a l l y a s i m p l e m a t t e r t o measure case leakage, w hich can be a u s e f u l
gu id e t o pump c o n d i t i o n . Remember, t o t h e end u ser i t i s t o t a l c o s t s t h a t a r e i m p o r t a n t ; o f a low-cost pump may w e l l r e s u l t i n e xpe nsi ve downtime.
the f a i l u r e
I f , by t h e i n c l u s i o n
o f a f l o w meter, such a f a i l u r e can be a voi d ed, t h e i n i t i a l investment i n a flow meter would be f u l l y j u s t i f i e d .
13.3.5
Motors
What has been w r i t t e n about pumps a p p l i e s g e n e r a l l y t o motors o f s i m i l a r des i gn. I t must be remembered t h a t much o f the, contaminant passing through t h e pump
may be r e a c h in g t h e motor a l s o , where t h e r e w i l l be a s i m l l a r performance degradation.
I f , due t o wear, t h e v o l u m e t r i c e f f i c i e n c y o f t h e pump has f a l l e n t o 85%
o f i t s o r i g i n a l v a l u e and t h e v o l u m e t r i c e f f i c i e n c y o f t h e motor has f a l l e n t o , say, 90% o f o r i g i n a l ,
t h en t h e o v e r a l l v o l u m e t r i c e f f i c i e n c y o f t h e pump and
motor w i l l be down t o 0.85 x 0.9
76.5% o f t h e o r i g i n a l value.
For t h i s reason
301 c o n ta m in a tio n c o n t r o l i s p a r t i c u l a r l y i m p o r t a n t i n h y d r o s t a t i c transmissions t o p r o v i d e t h e necessary l e v e l o f f l u i d c l e a n l i n e s s .
13.3.6
D i r e c t i o n a l Valves
The r a d i a l c l e a r a n c e s p e c i f i e d between bore and spool i n most d i r e c t i o n a l v a lv e s i s i n t h e range o f
5
t o 13 mi cro met res.
As i s w e l l know, t h e p r o d u c t i o n
o f p e r f e c t l y round and s t r a i g h t bores i s e x c e p t i o n a l l y d i f f i c u l t , so i t i s unl i k e l y t h a t any spool w i l l l i e e x a c t l y c e n t r a l i n t h e clearance band. inal & i n .
I n a nom-
v a l v e , a good spool i s l i k e l y t o have l e s s than 2.5 micrometres
c 1 ea rance. I n an e l e c t r i c a l l y o pe rat ed v a l v e , shown i n F i g . 6 .
They a r e : Flow f o r c e s
Tank port
Cylinder Return Port
r--
Q MI
Spring force
t h e f o r c e s a c t i n g on t h e s o l e n o i d a r e
+
Pressure port
m
L---
Leakage carrying particles 1r---
r---
Inertia.
Tank port
l---
0 .
AP = Pressure drop across spool
+
1
I
Flow force FaAx AP
Friction
Cylinder pressure Port
Dotted lines show position of spool lands when closed.
Fi g . 6
+
Spring f o r c e
IIRll a
r---
Spring lorce
.
Va l ve spool c r i t i c a l clearances ( w i t h f l o w s and f o r c e s )
Flow, s p r i n g , and i n e r t i a f o r c e s a r e i n h e r e n t f a c t o r s b u t f r i c t i o n f o r c e s are, t o a g r e a t e x t e n t , dependent on f i l t r a t i o n .
I f t h e system i s h e a v i l y contamin-
a t e d w i t h p a r t i c l e s s i m i l a r i n s i z e t o t h e r a d i a l and d i a m e t r a l clearances, h i g h e r f o r c e s w i l l be needed t o move t h e s p o o l . An even worse s i t u a t i o n r e s u l t s f rom s i l t i n g , where contaminant i s forced i n t o t h e c le a r a n ces under p ressu re, e v e n t u a l l y l e a d i n g t o breakdown o f t h e o i l
f i l m and spool hang-up ( F i g . 7 ) . T h i s s i t u a t i o n o ccurs where v a l v e s s u b j e c t e d t o continuous p r e s s u r e a r e operated i n f r e q u e n t l y .
Such v a l v e s sh ou l d p r e f e r a b l y have l o c a l f i l t r a t i o n o f a
v e r y h i g h o r d e r i n t h e a d j a c e n t p r e s s u r e l i n e b u t due account should be taken o f p o s s i b l e p r e s s u r e surges generated d u r i n g component o p e r a t i o n .
The use o f
h i g h e f f i c i e n c y f i l t e r s as a s p e c i a l p r o t e c t i o n f o r s i n g l e u n i t s o r groups o f
302 u n i t s can r e s u l t i n t h e need f o r a v e r y h i g h d i r t c a p a c i t y i f t h e general l e v e l o f f i l t r a t i o n i n t h e system i s much lower.
One large particle can bridge gap
Working pressure
Break in oil film due to dirt concentration
Fig.7
/
1 Silt Build-up
V a l ve spool c r i t i c a l clearance. Eccentricity i s t h e normal c o n d i t i o n i n many cases.
Some idea o f t h e f o r c e s needed t o break t h i s spool hang-up,
compared w i t h
t h e f o r c e s a v a i l a b l e f rom t h e s o l e n o i d , can be gained from t h e example o f a nominal
&
i n . v a l v e o p e r a t i n g a t 210 b ar.
I f a v a l v e o f t h i s t y p e remains s e l -
ecte d i n t h e s p r i n g o f f s e t o r e ne rgi se d p o s i t i o n f o r a l e n g t h y p e r i o d o f time, s i l t i n g t a k e s p l a c e between spool and bo re t o produce t o t a l i m m o b i l i t y .
The
f o r c e needed t o overcome t h i s s t a t e has been found by experiment t o be o f t h e o r d e r o f 135 Newtons, b u t b o t h s p r i n g and s o l e n o i d c o u l d e x e r t o n l y 45 Newtons. Thus t h e e f f e c t o f t h e s i l t i s t o cause t o t a l system f a i l u r e .
13.3.7
P r e s s u re C o n t r o l s
H i g h l y a b r a s i v e p a r t i c l e s i n h i g h v e l o c i t y streams o f o i l erode t h e surfaces w i t h wh ic h they come i n t o c o n t a c t .
T h i s s i t u a t i o n i s common t o p r e s s u r e con-
t r o l l e r s , p a r t i c u l a r l y r e l i e f v a l v e s whi ch a r e s u b j e c t e d t o maximum system pre s s u r e drop and v e l o c i t i e s o f t h e o r d e r o f 30 m/s.
P i l o t c o n t r o l stages gen-
e r a l l y see low volumes a t h i g h v e l o c i t i e s and heavy c o n t a m i n a t i o n a f f e c t s b o t h t h e i r s t a b i l i t y and r e p e a t a b i l i t y . 13.3.8
Flow C o n t r o l s
The c o n t a m i n a t i o n t o l e r a n c e o f f l o w c o n t r o l v a l v e s w i l l depend v e r y much on the o r i f i c e c o n f i g u r a t i o n .
303 F i g u r e 8, f o r example, shows two o r i f i c e s which a r e o f en't r e l y d i f f e r e n t The groove t y p e (a) w i l l t o l e r a t e a h i g h
shape, a l t h o u g h having equal areas.
contamination l e v e l except when used a t low s e t t i n g , whereas
ype (b) i s much
more prone t o s i l t i n g a t a l l s e t t i n g s . With a l l types o f pressure-compensated f l o w c o n t r o l s , t h e performance o f t h e pressure r e d u c i n g element can be c o n s i d e r a b l y a f f e c t e d by c o n t a m i n a t i o n , i r r e s pective o f valve s e t t i n g .
Damage t o t h e m e t e r i n g o r i f i c e can a l s o occur, which
w i l l become p a r t i c u l a r l y apparent a t lower s e t t i n g s .
Note: Throttle profile gives orifice segments of equal area
/
Fig.8
\
Flow c o n t r o l v a l v e t h r o t t l e s e c t i o n s . P r o f i l e (b) i s more prone t o s i l t i n g .
Generally speaking, a l l s p o o l - t y p e c o n t r o l v a l v e s a r e a f f e c t e d by contaminat i o n i n t h e system, e s p e c i a l l y a t h i g h p r e s s u r e s .
The e f f e c t s a r e l i k e l y t o be
magnified i f p r e c i s e a x i a l p o s i t i o n i n g o f t h e spool i s necessary as, f o r example, i n pressure r e d u c i n g v a l v e s where l i m i t e d f o r c e s a r e a v a i l a b l e t o o p e r a t e t h e spool.
On t h e o t h e r hand, poppet v a l v e s , though a f f e c t e d by l a r g e p a r t i c l e s o f
contamination, tend t o be f a r more t o l e r a n t o f s i l t due t o t h e s e l f - c l e a r i n g action o f the seat,
13.3.9
However, e r o s i o n i s s t i l l l i k e l y t o o c c u r .
Summary
I t can be seen from t h e f o r e g o i n g t h a t an i n d i v i d u a l l a r g e p a r t i c l e a r r i v i n g a t the wrong p l a c e a t t h e wrong t i m e can cause c a t a s t r o p h i c f a i l u r e . Surfaces w i t h i n components should be separated by an o i l f i l m , o f which may be c o n t i n u a l l y changing.
the thickness
When t h i s gap i s b r i d g e d by contaminants,
wear w i l l occur thereby g e n e r a t i n g f u r t h e r p a r t i c l e s which may w e l l be ground i n t o many more s m a l l e r p a r t i c l e s .
F i n e p a r t i c l e s i n d i v i d u a l l y o r i n small
304
Fig.9
Suggested a c c e p t a b l e c o n t a m i n a t i o n l e v e l s f o r v a r i o u s h y d r a u l i c systems
Target Contamination Class t o CETOP RP70H
Suggested Maximum Particle Level
5um 15 vm
5vm
Sensitivity
Type o f System
15vm
Suggested Filtration Rating Bx > 75
13
9
4 000
250
Super critical
S i l t sensitive control system w i t h v e r y h i g h r e l i a b i l i t y . Laboratory o r aerospace.
1-2
15
11
16 000
1 000
Critical
High performance servo and h i g h p r e s s u r e l o n g l i f e systems, i.e. a i r c r a f t , machine t o o l s , e t c .
3-5
16
13
32 000
4 000
Very important
High q u a l i t y r e l i a b l e systems. General machine requirements
18
14
130 000
8 000
Important
General machinery and m o b i l e 2-15 systems. Medium p r e s s u r e , medium c a p a c i t y .
19
15
250 000 16 000
Average
Low p r e s s u r e heavy indust r i a l systems, or a p p l i c a t i o n s where l o n g l i f e i s not c r i t i c a l .
21
17
1 000 0 0 0 6 4 0 0 0
Hain
Low p r e s s u r e systems w i t h l a r g e c 1 earances.
10-12
5-25
25-40
Note: This graph assumes viscosity to be within recommended range.
I !I\,.* .
-..-. I ‘=-..I_ -----
Average hydraulic components including most pumps.
0 200
f
‘,Very sensitive t components
UI
i
-. * =..
I
I
I
.--------
7
p,yery tolerant components. I
’.
Figures relate to cumulative particle concentrations above5prnand 15pm respectively (see chapter 8).
Fig.10
Suggested c l e a n l i n e s s l e v e l f o r good l i f e
305 q u a n t i t i e s may n o t cause damage, b u t i f p rese nt i n l a r g e r c o n c e n t r a t i o n s may l ea d t o f a i l u r e t h rou gh s i l t i n g . The o b j e c t i v e must be t o o b t a i n t he most economic combination o f contamina t i o n c o n t r o l and co nt ami n at i on t o l e r a n c e f o r a g i v e n r e l i a b l e system l i f e under known performance and e nvi ro nme nt al c o n d i t i o n s . 13.4
SPECIFYING CONTAMINATION LEVELS
As s t a t e d p r e v i o u s l y , 25 micrometres i s a t y p i c a l general l e v e l o f f i l t r a t i o n , s p e c i f i e d w i t h o u t r e g a r d t o wo rki ng pre ssure , l o c a l environment o r d u t y c y c l e . We know f r o m e x p eri e nce t h a t under s i m i l a r en vironmental and o p e r a t i n a c o n d i t i o n s the e f f e c t i v e n e s s o f a 25 micrometre f i l t e r w i l l v a r y depending on i t s l o c a t i o n i n t h e system.
A l so , under steady f l o w c o n d i t i o n s as t h e pores tend t o c l o g the
f i l t r a t i o n performance may a c t u a l l y improve.
Under v a r y i n g o r i n t e r m i t t e n t
f l o w , however, t h e r e s u l t can be v e r y d i f f e r e n t because contaminant p a r t i c l e s a r e d i s l o d g e d f r o m t h e p ore s a l l o w i n g more f i n e p a r t i c l e s t o pass through. I t i s o b v io u s t h a t t h e c o n d i t i o n s o f use have t h e g r e a t e s t e f f e c t on t h e c ont a m in a t io n l e v e l o b t a i n e d w i t h a s p e c i f i c f i l t e r , which means t h a t i t i s gene r a l l y unwise t o o f f e r w i t h o u t q u a l i f i c a t i o n o r accept w i t h o u t q u e s t i o n a b l a n k e t recommendation o f , say t h e use o f a 25 micrometre f i l t e r .
For t h e user
i t means p u t t i n g h i s investment a t r i s k and f o r t h e manufacturer i t makes t h e a d m i n i s t r a t i o n o f w a r r a n t i e s ext reme l y d i f f i c u l t . From a wide spectrum o f f i e l d da t a , F i g . 9 summarises t h e l e v e l s o f contaminat i o n wh ic h a r e c on si de red a ccep t a bl e f o r most h y d r a u l i c systems. The d e c i s i o n o f whether a system i s c r i t i c a l w i l l depend l a r g e l y on t h e type o f components used and t h e system d esi g n p ressure.
F i g u r e 10 has t h e r e f o r e been
produced t o g i v e t h e recommended co nt ami n at i on l e v e l s w i t h knowledge o f the system p r e s s u r e and h a v i n g an al yse d t h e d i r t s e n s i t i v i t y o f the system componen t s.
13.5
SELECTING THE FILTER
B e f o r e a c h o i c e o f f i l t e r may be made t h e v a r i o u s r a t i n g s used by manufacture r s must be examined. 13.5.1
Nominal R a t i n g
S p e c i f i c a t i o n s MIL-F5504A and MIL-F5504B were e s t a b l i s h e d for d e t e r m i n i n g nominal r a t i n g s .
V e rsi on A d e f i n e s a 10 micrometre f i l t e r as b e i n g a b l e t o
remove 98% by w e i g h t o f a l l p a r t i c l e s o f t h e e l e c t e d contaminant (AC f i n e t e s t d u s t ) l a r g e r t h a n 10 micrometres a t a c e r t a i n h i g h c o n c e n t r a t i o n .
Version B
d e f i n e s a 10 m ic ro met re f i l t e r as b e i n g a b l e t o remove 95% by w eight o f 10-20 m i cr o m e t r e g l a s s beads a t a h i g h c o n c e n t r a t i o n .
Although l i t t l e use has been
made o f t h e s e p a r t i c u l a r s p e c i f i c a t i o n s , many manufacturers use s i m i l a r t e s t s t o p r o v i d e nominal r a t i n g s f o r t h e i r f i l t e r s .
306 Such t e s t s have two major l i m i t a t i o n s .
Firstly,
they do n o t l i m i t t h e maxi-
mum s i z e o f p a r t i c l e a l l o w e d t o pass through t h e element and from t e s t s i t has been found t h a t f i l t e r s meeting these requirements can pass p a r t i c l e s up t o 200 micrometres.
Secondly, t h e h i g h c o n c e n t r a t i o n o f contaminant added i s n o t t y p i -
c a l o f c o n d i t i o n s experienced i n a normal system.
I n practice, the particles
approach t h e f i l t e r i n small c o n c e n t r a t i o n s and those p a r t i c l e s t h a t a r e s m a l l e r than t h e mean p o r e s i z e pass r e a d i l y through t h e f i l t e r as l o n g as t h e f i l t e r medium remains reasonably c l e a n . For t h e reasons g i v e n above t h e r e i s a good case f o r d i s c o n t i n u i n g t h e use o f nominal r a t i n g s .
13.5.2
Absolute Rating
The NFPA F l u i d Power Glossary o f Terms d e f i n e s t h e a b s o l u t e r a t i n g as b e i n g t h e diameter o f t h e l a r g e s t h a r d s p h e r i c a l p a r t i c l e t h a t w i l l pass through a f i l t e r under s p e c i f i e d t e s t c o n d i t i o n s .
T h i s i s an i n d i c a t i o n o f t h e l a r g e s t
opening i n t h e f i 1 t e r - e w n t .
13.5.3
The Bubble T e s t
T h i s i s a t e s t used by manufacturers t o d e t e r m i n e t h e area o f g r e a t e s t porosity.
I t i s achieved by a p p l y i n g a i r p r e s s u r e t o t h e i n s i d e o f t h e f i l t e r e l e -
ment, which i s submerged i n a l i q u i d such as a l c o h o l , which wets t h e f i l t e r media.
The o p e r a t o r r o t a t e s t h e f i l t e r element a t each p r e s s u r e l e v e l and r e c -
o r d s t h e p r e s s u r e a t which t h e f i r s t stream o f bubbles e m i t t e d f r o m t h e f i l t e r element.
The t e s t can be c o n t i n u e d t o measure t h e p r e s s u r e s o f t h e second,
t h i r d , f o u r t h , e t c . l a r g e s t h o l e . By c o n t i n u i n g t o s l o w l y i n c r e a s e t h e p r e s s u r e , a p o i n t i s reached c a l l e d t h e 'open b u b b l e p o i n t ' a t which a i r bubbles appear o v e r t h e e n t i r e s u r f a c e o f t h e f i l t e r element;
t h i s i s a s i m p l i f i e d method o f
measuring t h e mean p o r e s i z e . I t i s claimed t h a t p r e c i s e r e s u l t s a r e n o t o b t a i n a b l e f r o m t h e b u b b l e t e s t ,
which i s u n f o r t u n a t e because such a s i m p l e t e s t would be i n v a l u a b l e .
As i t i s ,
i t s main use i s i n t h e q u a l i t y c o n t r o l o f elements t o ensure t h e r e i s no damage t o t h e media o r a bad s e a l .
13.5.4
Mean F i l t r a t i n g R a t i n g
T h i s i s a measurement o f t h e average s i z e of t h e pores o f t h e f i l t e r media. T h i s i s a v e r y s i g n i f i c a n t r a t i n g , s i n c e i t i s a measure o f t h e p a r t i c l e s i z e above which t h e f i l t e r s t a r t s b e i n g e f f e c t i v e . 'open bubble p o i n t ' method j u s t d e s c r i b e d .
I t can be measured u s i n g t h e
307 13.5.5
Multipass F i l t e r Test
T h i s t e s t , designed t o p r o v i d e a means o f d e s c r i b i n g t h e performance characteristics of a f i l t e r ,
i n v o l v e s t h e co nt i nu ous i n j e c t i o n o f a c o n t r o l l e d conta-
minant i n t o a t e s t system. filter,
As t h e contaminant can be removed o n l y by t h e t e s t
i t w i l l c o n t i n u e t o c i r c u l a t e i n t h e system unless i t i s captured.
The
s e p a r a tio n c a p a b i l i t y o f t h e t e s t f i l t e r i s monitored by a n a l y s i n g upstream and downstream f l u i d samples.
The d i r t h o l d i n g c a p a c i t y i s measured by t h e amount
i n grammes o f t e s t contaminant which can be added t o t h e system b e f o r e a s p e c i f i e d t e r m i n a l p r e s s u r e drop acro ss t h e f i l t e r i s reached. The mathematical r e l a t i o n s h i p whi ch d e s c r i b e s t h e t e s t i s developed from t h e following expression: Number o f p a r t i c l e s Number o f p a r t i c l e s Number o f downstream o f s i z e = o r i g i n a l l y o f s i z e + p a r t i c l e s > xpm > xpm injected o f s i z e >xpm 13.5.6
-
Number o f p a r t i c l e s removed o f s i z e >xpm
Beta R a t i o
The s e p a r a t i o n c h a r a c t e r i s t i c s a r e g i v e n by t h e Beta r a t i o , which i s d e f i n e d as f o l l o w s : o..
-
Number o f upstream p a r t i c l e s l a r g e r than xpm ~~
Number o f downstream p a r t i c l e s l a r g e r than xpm
A Beta r a t i o o f 1 i n d i c a t e s t h a t no p a r t i c l e contamination i s removed.
A
f i g u r e o f l e s s t ha n 1 i s c l e a r l y i m p o s s i b l e unless t h e f i l t e r i s u n l o a d i n g contaminants. For a f i l t e r e x h i b i t i n g a Beta r a t i o g r e a t e r than 1 , t h e downstream concent r a t i o n o f p a r t i c l e s above a g i v e n s i z e w i l l s t a b i l i z e t o g i v e an almost cons t a n t c o n t a m in a t io n l e v e l .
13.5.7
P r a c t i c a l C l a s s i f i c a t i o n o f F i l t e r Performance
Whatever fo r m at i s used by t h e f i l t e r ma nufacturer t o g i v e performance i n f o r mation on h i s p r odu ct ,
t h e degree o f f i l t r a t i o n p r o v i d e d w i l l b a s i c a l l y f a l l
i n t o one o f t h r e e c a t e g o r i e s , depending on t h e degree o f s i l t c o n t r o l .
Typical
data c o r r e s p o n d ing t o these c a t e g o r i e s a r e g i v e n i n Fig.12 though i t should be the m a n u f a c t u r e r ' s r e s p o n s i b i l i t y t o s t a t e i n t o w hich o f t h e t h r e e c l a s s i f i c a tions h i s products f a l l .
A t p r e s e n t t h e r e i s no u n i v e r s a l l y recognised s t idard
c l a s s i f i c a t i o n b u t work be i ng c a r r i e d o u t by v a r i o u s bodies should eventua 1 Y lead t o an i n t e r n a t i o n a l l y agreed d e f i n i t i o n . Two adverse f a c t o r s a f f e c t t h e a c t u a l performance o f f i l t e r s i n s e r v i c e namely p u l s a t i n g f l o w , and t h e sometimes u n c e r t a i n performance o f i n t e r n a l s e a ls and bypass v a l v e s .
308 Fig.12
D e f i n i t i o n o f practical c l a s s i f i c a t i o n categories Category Nomi na 1 Absolute Rating R a t i n g pm
S i l t Control
f to 1
3 to 5
P a r t i a l S i l t Control
3 to 5
10 t o 15
B10-15>75
10 t o 15
25 t o 40
625-40’75
No S i l t C o n t r o l (Chip removal)
13.5.7.1
Beta R a t i o
B3-5
’75
P u l s a t i n g Flow f o r c e s trirough t h e media those f i n e p a r t i c l e s w hich
would o t h e r w i s e lodge among t h e f i b r e s and between l a r g e r p a r t i c l e s a l r e a d y intercepted.
The e f f e c t may be compared w i t h a s i e v e h o l d i n g a m i x t u r e o f
stones, some l a r g e r and some s m a l l e r t ha n t h e openings i n t h e mesh;
when t h e
s i e v e i s s t a t i o n a r y , many o f t h e sma l l stones a r e r e t a i n e d b u t they f a l l through when t h e s i e v e i s shaken.
P u l s a t i n g f l o w t h e r e f o r e increases t h e p r o p o r t i o n o f
s i l t p a r t i c l e s i n t h e system downstream o f t h e f i l t e r , and t h i s i s r e f l e c t e d i n t h e performance curve o f t h e f i l t e r ( F i g . 1 3 ) .
13.5.7.2
Bypassing by i n t e r n a l o r e x t e r n a l v a l v i n g i s a c c e p t a b l e f o r many
systems s i n c e ,
i n a number o f passes, a l l t h e f l u i d e v e n t u a l l y goes through t h e
f i 1t e r . For f i l t e r s o t h e r t ha n t h ose designed f o r permanent bypassing i t would n o r m a l l y be expected t h a t t h e bypass be o p e r a t i v e o n l y when t h e element i s reaching t h e end o f i t s use f u l
life.
T h e r e f o r e t h e e f f e c t s o f premature opening o f t h e
bypass v a l v e o r a f a u l t y i n t e r n a l s e a l need t o be considered.
Since bypassing,
whatever t h e cause, does n o t d i s c r i m i n a t e between f i n e and coarse p a r t i c l e s , t h e e f f e c t i s t o weigh
t h e c o n t a m i n a t i o n p r o f i l e h e a v i l y a t t h e coarse end.
F i g u r e 14 shows t h e e f f e c t o f i n c r e a s i n g percentages o f bypass f l o w s , and i t w i l l be n o t e d t h a t even a t 0.1% bypass t h e maximum p a r t i c l e s i z e has almost doubled.
A t 1% i t has mare t h an t r e b l e d , and a t 10% i t has increased by a f a c -
tor o f five. The p r a c t i c a l s i g n i f i c a n c e o f byp assi n g and t h e need t o make p r o v i s i o n f o r i t depend on t h e l o c a t i o n o f t h e f i l t e r i n t h e system.
For example, on i n l e t
f i l t e r s a bypass v a l v e i s mandatory i n o r d e r t o p r o t e c t t h e pump from c a v i t a t i o n as th e f i l t e r becomes b l ocke d. Pressure f i l t e r s a r e i nt en de d t o p r o t e c t t h e system i f t h e r e i s c a t a s t r o p h i c f a i l u r e o f t h e pump.
I f such a f a i l u r e o ccurs when t h e f i l t e r i s bypassing,
th e n t h e p r o t e c t i o n i s n o n - e x i s t e n t .
There i s o b v i o u s l y a case f o r f i t t i n g non-
bypass p r e s s u r e f i l t e r s where t h e need f o r t h i s t y p e i s e s t a b l i s h e d .
An element
must be f i t t e d t h a t w i l l w i t h s t a n d t h e t o t a l system pressure, w hich increases the c o s t o f t h e f i l t e r .
Cl o gg i ng o f t h e element causes system Performance t o f a l l o f f and a r e l i a b l e i n d i c a t o r i s e s s e n t i a l t o g i v e e a r l y w arning o f t h i s .
309
-
1
106
105
r=-
Q filter
w)'
lnterupted flow performance.
103
102
10
Steady flow
1 102 103
50 70 90 120 m
Particle size
Fig.13
F i l t e r performance d e t e r i o r a t e s under i n t e r m i t t e n t o r p u l s a t i n g flow. The e f f e c t i s more marked w i t h f i n e r p a r t i c l e s which a r e forced through the media.
Fig.14
- micmmetns.
m
Influence o f by-pass leakage on the f i l t e r performance.
Where a bypass i s judged t o be permissible, the system designer must weigh c a r e f u l l y t h e e f f e c t s o f premature opening due t o c o l d s t a r t s and surge flows; the e l i m i n a t i o n o f surges, even those a t low pressure, w i l l reduce the tendency f o r t h e bypass t o open under normal o p e r a t i o n .
A t present t h e r e i s l i t t l e i n f o r m a t i o n a v a i l a b l e on the e x t e n t o f the adverse e f f e c t t h a t p u l s a t i n g f l o w has on f i l t e r performance.
Hopefully, Beta r a t i o s
w i l l e v e n t u a l l y be quoted which do r e l a t e t o c o n d i t i o n s which l i n e f i l t e r s a r e
subjected t o i n a c t u a l p r a c t i c e .
N a t u r a l l y , where stea,dy f l o w e x i s t s r e l a t i n g
more c l o s e l y t o l a b o r a t o r y t e s t c o n d i t i o n s , more accurate p r e d i c t i o n s o f f i l t e r performance can be made.
However, using the l i m i t e d knowledge a t o u r d sposal,
l i k e l y performance l e v e l s f o r each of our categories have been produced i n Fig. 15. On each graph a cumulative count o f both has been shown.
5
and 15 micrometre s i z e pa t i c l e s
The reason f o r using these f i g u r e s i s because o f t h e standard
set by the CETOP RP70H. For ease i n r e l a t i n g t o t h i s document,the range numbers a r e a l s o quoted. a d d i t i o n t o f i x e d f i g u r e s a t t h e two values t h e l i n e s a r e extended t o show expected trends f o r t y p i c a l f i l t e r s .
In
310
PARTIAL SILT REMOVER
SILT REMOVER
CUMIJLATIVECOUNT
CUMULATIVE COUNT SIZE pm
SIZE pm
25,000(18)
64.000(161
16.000(14)
N. B. Range number as per CETOP RP7OH shown in brackets.
N.B. Range number as per CETOP RP7OH shown in brackets.
22
^.
=r o l
105
I
E
h
a
m
i
3- 10‘ e
-a z0
0
i
10’
a 102
10
1
2
5
K)
152025
P*rtlCk
.I20
Fig.15A Assumed mean pore r a t i n g 3 micrometre. Curves show deterioration o f f i l t e r performance w i t h r e s p e c t t o p a r t i c l e s below t h i s s i z e under pulsating flow.
F i g . 1 5 9 Assumed mean p o r e r a t i n g 7 microm e t r e . Curves show d e t e r i o r a t i o n o f f i l t e r performance w i t h r e s p e c t t o p a r t i c l e s below t h i s s i z e under p u l s a t i n g f l o w .
311
CHIP REMOVER CUMULATIVE COUNT SIZE r m
IS
64.000 (16)
N.B. Range number as per CETOP RP70H shown in-brackets
Fig.15C Assumed mean p o r e r a t i n g 15 micrometre. Curves show d e t e r i o r a t i o n o f f i l t e r performance w i t h r e s p e c t t o p a r t i c l e s below t h i s s i z e under p u l s a t i n g f l o w . There i s l i t t l e o r no c o n t r o l o f 5 micrometre p a r t i c l e s .
312 Assuming t h e s e graphs r e l a t e t o performance o f t h e f i l t e r s u p p l i e d by t h e p a r t i c u l a r m a n u fa ct u rer chosen i t i s necessary t o show t h a t t h e actua ment when p l o t t e d s t a y s below t h e l i n e drawn f o r steady o r p u l s a t i n g As an.example,
requirelow.
i f we have a system w i t h a w orking p r e s s u r e o f , say
150 bar
u s i n g components w i t h average d i r t t o l e r a n c e , o u r c l e a n l i n e s s l e v e l graph (Fig.10)
t e l l s us
t h a t t h e d e s i r e d co nt ami n ation l e v e l should be somewhere
between 15/11 and 18/14. R e f e r r i n g now t o Fig.15a and b, we see t h a t under steady f l o w c o n d i t i o n s t h i s c o u l d be achieved by o u r p a r t i a l s i l t c o n t r o l f i l t e r , b u t i f p u l s a t i n g f l o w i s p r e s e n t a s i l t removal f i l t e r i s r e q u i r e d . I t can be assumed t h a t most p ressu res and r e t u r n l i n e f i l t e r s a r e s u b j e c t t o pulsating flow conditions. t i o n on f i l t e r l o c a t i o n .
The reasons a r e g i v e n i n g r e a t e r d e t a i l i n t h e secOn t h e o t h e r hand, o f f - l i n e systems have t h e b e n e f i t
of a steady f l o w r a t e c o n d i t i o n .
I t must be emphasised t h a t t h e onus i s v e r y much on t h e f i l t e r manufacturer t o i d e n t i f y t h e l i k e l y performance o f h i s p a r t i c u l a r media. 13.5.8
F i l t e r Sizing
I t i s c u r r e n t p r a c t i c e f o r man uf act ure rs t o s t a t e a f l o w r a t i n g a t a s p e c i f i c c l e a n p r e s s u r e drop (see F i g . 1 6).
W hi l e t h i s i s a g u i d e t o c a p a c i t y ,
i t may
w e l l be necessary f o r t h e system de si gn er t o p r o v i d e a d d i t i o n a l d i r t c a p a c i t y so
07 07
c-
I
Flow (I/mln) Oil vlscoslty 30 cSt. Element pressuredrop Is directly proportional to fluld viscosity.
Fig.16
T y p i c a l p r e s s u r e d rop s f o r c l e a n s i l t c o n t r o l and c h i p c o n t r o l elements o f s i m i l a r s i z e .
313 as t o ensure t h a t t h e end u s e r o b t a i n s an a cceptable element l i f e .
A l l too
o f t e n f i l t e r s s i z e d p u r e l y on f l o w r a t e have a s h o r t element l i f e .
I n choosing
l a r g e r f i l t e r c a p a c i t i e s g r e a t e r i n i t i a l expense may be i n c u r r e d , b u t t h i s i s almost c e r t a i n t o be re cove red i n reduced r u n n i n g c o s t s ,
i.e.
fewer element
changes, reduced l a b o u r c o s t s and reduced downtime. Co r r e c t f i l t e r s i z i n g n e c e s s i t a t e s r e l a t i n g t h e d i r t e n t e r i n g t h e f i l t e r t o t h e e f f e c t i v e element a rea and t h e maximum a l l o w a b l e p r e s s u r e drop.
The r e l a t -
i o n s h i p o f a r e a t o p r e s s u r e drop i s n o t simple, however, and f i l t e r i n l e t d i r t l e v e l s a r e r a r e l y known. There i s a l a b o r a t o r y t e s t ( t h e 'co mpa rat ive l i f e ' or ' d i r t c a p a c i t y ' t e s t ) which i s designed t o compare t h e d i r t h o l d i n g c a p a c i t i e s o f h y d r a u l i c f i l t e r s . An a r t i f i c i a l contaminant i s added a t a c o n s t a n t r a t e t o a c o n t i n u o u s l y r e c i r c l a t i n g o i l system and t h e r e s u l t a n t i n crea se i n d i f f e r e n t i a l pressure i s p l o t t e d a g a i n s t t h e w e i g h t o f contaminant added, as shown i n Fig.17.
The r e s u l t i n g
c u r v e has a c h a r a c t e r i s t i c f o r m which i s c o n s t a n t f o r a g i v e n f i l t e r media.
"I
[ 6 7
I
0 Total A.C. flne test dust added (grams). Fig.17
Typical d i r t capacity curve f o r hydraul i c f i l t e r element.
Fig.18
25
I
I
50 75 Percent dirt capacity.
1 100
Comparison o f . d i r t c a p a c i t i e s and d i f f e r e n t i a l pressures f o r two d i f f e r e n t f i l t e r s .
314 I n t h e f i r s t stages o f d i r t a d d i t i o n i t can be seen t h a t t h e p r e s s u r e drop increases s l o w l y , whereas l a t e r t h e p r e s s u r e d r o p increases v e r y r a p i d l y .
This
i l l u s t r a t e s why v e r y l i t t l e i s gained i n terms o f element l i f e by a l l o w i n g t h e f i l t e r t o operate a t a h i g h d i f f e r e n t i a l pressure a f t e r the 'knee' o f the curve The c u r v e also i l l u s t r a t e s t h e i r r e l e v a n c y o f a system
has been passed.
designer c o n c e r n i n g h i m s e l f s o l e l y w i t h c l e a n p r e s s u r e drops;
t h e more s i g n i f -
icant f a c t o r i s the d i f f e r e n t i a l pressure across the f i l t e r a f t e r a s p e c i f i e d amount o f d i r t has been added. Except w i t h non-bypass f i l t e r s , t h e maximum p r e s s u r e drop across t h e element i s u s u a l l y determined by t h e bypass v a l v e s e t t i n g .
The system d e s i g n e r must
also check t h a t t h e system performance i s m a i n t a i n e d w i t h t h e p r e s s u r e drop a t i t s maximum value.
I f f i l t e r s a r e o v e r s i z e d , t h e bypass may pass a s u b s t a n t i a l
flow without indicating. I t i s commonly thought t h a t t o o b t a i n l o n g e r s e r v i c e l i f e from a g i v e n f i l t e r
element w i t h o u t s a c r i f i c i n g f i l t e r e f f i c i e n c y , media area i n t o t h e f i l t e r envelope.
i t i s o n l y necessary t o pack more
T h i s i s n o t so s i n c e an optimum area
e x i s t s f o r a g i v e n envelope and i t i s d e t r i m e n t a l t o exceed t h i s a r e a . The comparative l i f e t e s t p r e v i o u s l y d e s c r i b e d i s used t o compare t h e d i r t h o l d i n g c a p a c i t y o f d i f f e r e n t f i l t e r elements, and i n Fig.18 we compare t h e d i r t c a p a c i t y o f two f i l t e r s o f i d e n t i c a l envelope s i z e .
The f i l t e r A has a lower
c l e a n p r e s s u r e drop than f i l t e r B because A has more area.
However, t h e optimum
area has been exceeded and t h e r e f o r e f i l t e r A would have a s h o r t e r l i f e than B f o r a given pressure d i f f e r e n t i a l . How c l o s e p a c k i n g reduces t h e e f f e c t i v e area i s i l l u s t r a t e d i n Fig.19 where t h e p l e a t s c l o s e up under p r e s s u r e and t h e small angles between them c l o g rapidly.
F a t i g u e f a i l u r e s can a l s o o c c u r when p r e s s u r e i s a p p l i e d .
As d a t a on
t h e r e s u l t s o f d i r t c a p a c i t y t e s t s a r e n o t r e a d i l y a v a i l a b l e , we must r e v e r t t o t h e m a n u f a c t u r e r ' s f l o w r a t i n g a t m s p e c i f i c c l e a n p r e s s u r e drop and use t h i s a s
a basis f o r assessing d i r t capacity. Fluid flow +
Fluid flow Effective areas
t
t under high flow
Pleats close up under pressure and small
Wider spacing gives smaller but more
angles between them clog rapidly, reducing the effectlve area
effective filter area
Fig.19A I n c r e a s i n g f i l t e r area w i t h i n a g i v e n package may a c t u a l l y reduce t h e d i r t h o l d i n g capac i t y .
medium
High stress under high flow conditions
Fig.198
t
J
Flow fatigue failure
Changes i n f l o w and p r e s s u r e drop cause t h e s i d e s o f element c o r r u g a t i o n s t o f l e x and t h e root t o stretch, giving rise t o f a t i g u e stresses.
315 Recapping, o u r o b j e c t i v e i s t o a c h i e v e a ' d i r t i n equals d i r t o u t ' c o n d i t i o n . The d e s i r e d d i r t c a p a c i t y o f t h e f i l t r a t i o n system w i l l t h e r e f o r e depend t o a l a r g e e x t e n t on o u r a b i l i t y t o c o n t r o l t h e d i r t i n p u t .
The d i r t i n p u t i s a
product o f i n b u i l t con t a mi na t i o n and i n gre ssed c o n t a m i n a t i o n which, produces system generated con t a mi na t i o n.
i n turn,
L e t us f i r s t examine each source i n
d e t a i l and t h e f a c t o r s c o n t r o l l i n g i t s i n p u t ( F i g . 2 0 ) . Contamination source
Dirt i n p u t
=
I n b u i l t : i n components p ip es, m a n i f o l d s , e t c .
Controller Good f l u s h i n g procedures, system n o t o pe rat ed on load u n t i l acceptable cont a m i n a t i o n l e v e l obtained.
plus P r e sen t i n i n i t i a l charge o f f l u i d
Integrity o f supplier. F l u i d stored under c o r r e c t c o n d i t i o n s ( e x c l u s i o n o f F1 u i d f i 1 t e r e d d i r t , condensation,etc.) during f i l l i n g .
plus lngressed t h rou gh a i r breather
An e f f e c t i v e a i r b r e a t h e r w i t h r a t i n g co mpatible w i t h degree o f f l u i d f i l t r a tion.
plus ln g re ssed d u r i n g f l u i d r e p l e ni sh men t
S u i t a b l e f i l l i n g p o i n t s which ensure some f i l t r a t i o n o f f l u i d b e f o r e e n t e r i n g r e s e r v o i r . T h i s t a s k undertaken by r e s p o n s i b l e personnel. Design should mi ni mi se t h e e f f e c t s .
plus lngressed d u r i n g maintenance plus lngressed t h rou gh c y l i n d e r ro d s e a l s
E f f e c t i v e wiper seals o r , i f airborne con t a mination, rods p r o t e c t e d by s u i t a b l e gaiters
.
plus F u r t h e r generated co nt am i n a t i o n produced as a r e s u l t o f t h e above and the s e v e r i t y o f the duty cycle.
F ig . 2 0
C o r r e c t f l u i d s e l e c t i o n and p r o p e r t i e s ( v i s c o s i t y and a d d i t i v e s ) maintained. Good system design m i n i m i s i n g e f f e c t s o f co nt amination present on system compon en t s.
The p r a c t i c a l st ep s wh i ch c o n t r o l C ontamination i n h y d r a u l i c systems.
Based on Fig.20 we w i l l now grade t h e c l e a n l i n e s s l e v e l o f a system between
1 and 7.
An example o f a grade 1 c l e a n system would be a c l e a n workshop w i t h
e f f e c t i v e c o n t r o l o v e r a l l con t a mi na t i o n i n gress.
A grade
7 d i r t y system would
be, say, a f o u n d ry w i t h l i t t l e o r no c o n t r o l over c o n t a m i n a t i o n i n g r e s s and a system o p e r a t i n g se vera l exposed c y l i n d e r s .
316 Figure 21 w i l l a s s i s t i n making a numerical assessment between these values based on t h e environment and t h e degree o f c o n t r o l over contamination.
Environmental Conditions Degree of Control
Good
Average
Bad
3
6
7
L i t t l e or no c o n t r o l over contamination ingression (many exposed c y l inders)
2
4
5
Some c o n t r o l over contamination ingression (few c y l inders).
1
2
3
Good c o n t r o l over contamination ingression ( g a i t e r e d c y l inders).
Fig.21
Assessment and c l a s s i f i c a t i o n o f system c l e a n l i n e s s l e v e l i n t o 7 grades.
We must now r e l a t e t h i s t o t h e e f f e c t i v e element area and the maximum a l l o w a b l e pressure drop.
The r e l a t i o n s h i p between area and pressure drop i s n o t
simple, but by using very broad approximations o f these values and assuming t h a t the manufacturers f l o w r a t i n g a t a s p e c i f i c c l e a n pressure drop i s a good guide t o d i r c t capacity,
the f o l l o w i n g s e l e c t i o n guides can be used:
To use this graph:
10
1. Decide on maximum acceptable pressure 2. Assess cleanliness grade in SyStem. from drop, this will depend on system requireFig. 22. Draw vertical line through inter. section of pressure drop and cleanliness ments or bypass pressure. Draw horizontal line through this value. grade line.
9
t
30
Bt
3. Read off multiplication factor where
u 7
-
vertical llne crosses horizontal axis. Multiply this factor by the actual flow rate at chosen pressure line location. Now select a filter element to handle this revlsed flow rate at a pressure drop of 1 bar (According to manufacturers clean rating).
2 3
Actual flow x multiplication factor = recommended filter capacity.
ia 6 a 5
?
!I4
i
2 1
1
3
2
4
Multipiiulion faclor.
Fig.22
Pressure l i n e f i l t e r s e l e c t i o n guide.
317
3'0
4\ \ \ \
Tousethisgraph: 1. Decide on maximumacceptable pressure 2. Assess cleanliness grade in system, from Fig. 22. Draw vertical line through interdrop, this will depend on system require ments or bypass pressure. Draw section of pressure drop and cleanliness horizontal line through this value. grade lines.
3. Read off multiplication factorwhere vertical line crosses horizontal axis. Multiply this factor by theactual flow rateat chosen return line location. Now select a filter element to handle this revised flow rate at a pressure drop of 0.3 bar(According to manufacturers clean rating). Actual flow x multiplication factor
= recommended filter capacity.
2
3
4
Mulliplhlion faclor.
Fig.23
Return l i n e f i l t e r s e l e c t i o n guide.
To use this graph: 1. Decideon maximumacceptable pressure 2. Assess cleanlinessgrade in system, from Fig. 22. Draw vertical line through interdrop, this will depend on system require ments or bypass pressure. Draw section of pressure drop and cleanliness horizontal line through this value. grade lines.
3. Read off multiplication lactor where vertical line crosses horizontal axis. Multiply this factor by theactual offline pump flow rate. Now select a filter element to handle this revised flow rate at a pressure drop of 0.3 bar(acc0rding to manufacturers clean rating). Note:Toachieve reasonable lifea minimum multiplication factor of 2 is recommended. Actual flow x multiplication factor
= recommendedfiltercapacity.
2 -
6
4
8
Multlpliutionfactor.
Fig.24
O f f l i n e f i l t e r s e l e c t i o n guide.
318 13.6
LOCATING THE FILTER
13.6.1
Pump I n l e t F i l t r a t i o n
C o r r e c t f i l l i n g o f t h e pump i s v i t a l i f s a t i s f a c t o r y o p e r a t i o n o f t h e hydraul i c system i s t o be achieved.
A l l t o o o f t e n i n s u f f i c i e n t a t t e n t i o n i s p a i d by
t h e d e s ig n e r t o t h e t o t a l pump i n l e t c o n f i g u r a t i o n and, as a r e s u l t , c a v i t a t i o n remains a m a jo r cause o f pump f a i l u r e . For t h i s reason i t i s w o r t h l o o k i n g once more a t t h e b a s i c p r i n c i p l e s o f pump f i l l i n g .
By f a r t h e most common method i s t o use atmospheric pressure
a c t i n g on t h e f l u i d s u r f a c e o f t h e r e s e r v o i r t o f o r c e f l u i d i n t o t h e pump i n l e t chambers.
For convenience, t h e pump i s o f t e n s i t e d above t h e f l u i d l e v e l
(Fig.25). From b a s i c h y d r a u l i c p r i n c i p l e s we know t h a t f o r f l o w t o t a k e p l a c e we must have a p r e s s u r e d i f f e r e n c e .
W i t h t h i s arrangement we r e l y on t h e mechanical
a c t i o n o f t h e pump t o c r e a t e a de pre ssi on a t i t s i n l e t .
I t i s usual f o r manu-
f a c t u r e r s t o q u o t e a maximum a l l o w a b l e d ep ression a t t h e pump i n l e t , w hich i s l i k e l y t o be o f t h e o r d e r o f 0.17 b a r .
Thus w i t h normal p r e s s u r e drops accoun-
t e d f o r , o n l y a v e r y smal l p r e s s u r e d rop can be t o l e r a t e d across t h e f i l t e r . For t h i s reason t h e s i z e and c o s t o f i n l e t f i l t e r s i s o f t e n g r e a t e r than, say, f i l t e r s i n the return l i n e .
Furthermore,
such low p r e s s u r e drops make s i l t
removal v i r t u a l l y i mpo ssi bl e . F i r e - r e s i s t a n t f l u i d s a r e v e r y s e n s i t i v e t o s u c t i o n pressures.
They have
h i g h e r s p e c i f i c g r a v i t i e s t ha n m i n e r a l o i l s , p a r t i c u l a r l y t h e s y n t h e t i c types. T h i s in c r e a s e s t h e p r e s s u r e drop t o t h e pump and a t t h e same t i m e demands a h i g h e r p r e s s u r e t o a c c e l e r a t e t h e f l u i d i n t o t h e pump.
Water g l y c o l s and w ater-
i n - o i l emulsions have a h i g h vapour p r e s s u r e and t h e pump i n l e t depression s h o u ld be l i m i t e d wherever p o s s i b l e t o h a l f t h e v a l u e f o r m i n e r a l o i l s , even when t e m p e r a t u r e i s l i m i t e d t o 5OoC.
With o r w i t h o u t i n l e t l i n e f i l t e r s i t i s
u s u a l l y e s s e n t i a l t o p r o v i d e a p o s i t i v e head a t t h e pump i n l e t when u s i n g these fluids
(Fig.26).
'There must beadepression here to'lift'theoil.
Atmospheric pressure
L l+
Atmospheric pressure
\
Pump is charged at positive pressure.
Fig.25
Ne g a t iv e head t a nk.
F i g.26
P o s i t i v e head tank.
319 I r r e s p e c t i v e o f t h e t y p e o f f l u i d , a p o s i t i v e head w i l l improve i n l e t c o n d i t i o n s because i t increases t h e f o r c e a v a i l a b l e t o c r e a t e t h e r e q u i r e d f l o w . For b o t h n e g a t i v e and p o s i t i v e head i n l e t arrangements, when s i z i n g i n l e t f i l t e r s we must ensure t h a t any f i l t e r (housing and element) and a s s o c i a t e d
1 ) pass t h e f u l l pump volume w i t h i n t h e p e r m i t t e d i n l e t depr-
pipework should:
ession f o r t h a t pump, and 2 ) p e r m i t a bypass f l o w t h a t i s s t i l l w i t h i n t h a t l i m i t when t h e f i l t e r element i s blocked.
( T h i s requirement o f t e n n e c e s s i t a t e s
t h e o p e r a t i o n o f t h e bypass f e a t u r e a t pressures as low as 0.085 b a r , a l e v e l a t which o p e r a t i o n i s seldom c o n s i s t e n t . )
A l l c a l c u l a t i o n s should t a k e i n t o account t h e e f f e c t o f h i g h e r v i s c o s i t y f l u i d , e.g.
a t c o l d s t a r t s , o t h e r w i s e c a v i t a t i o n w i l l occur.
The usual micrometre r a t i n g f o r i n l e t f i l t e r s i s 75 o r 150. a r e a v a i l a b l e i n c o r p o r a t i n g elements down t o 10 micrometre.
However, u n i t s
T h i s means l a r g e
housings and p r o b a b l e o v e r s i z i n g o f t h e i n l e t c o n f i g u r a t i o n . The 75 and 150 micrometre mesh elements w i l l remove most o f t h e p a r t i c l e s above t h e i r r a t i n g b u t a r e r e l a t i v e l y i n e f f e c t i v e i n removing a n y t h i n g s m a l l e r . The a d d i t i o n o f magnets w i l l remove some o f t h e f i n e m e t a l i i c p a r t i c l e s b u t t h e l o c a t i o n o f t h e magnets w i t h i n t h e f i l t e r must be such t h a t under no circumstances can accumulated contaminant break away, thereby passing a conglomerate i n t o t h e pump. An advantage o f t e n claimed f o r i n l e t f i l t e r s i s ease o f s e r v i c i n g . However, i n c o r r e c t re-assembly o f access covers can r e s u l t i n a i r i n g r e s s , which o f t e n goes undetected and can be harmful t o t h e system as d i r t . ,
I n l e t f i l t e r s a r e g e n e r a l l y used i n systems where maintenance procedures do
not prevent q u a n t i t i e s o f large p a r t i c l e s entering the reservoir, f o r instance d u r i n g topping-up.
P r o v i d i n g bypassing i s n o t o c c u r r i n g ,
from t h i s t y p e o f i n g r e s s i o n .
they p r o t e c t t h e pump
However, good r e s e r v o i r design t h a t i n c udes a
mesh screen o r b a f f l e can p r o v i d e an e q u a l l y a c c e p t a b l e c o n t a m i n a t i o n
eve1 a t
t h e pump i n l e t .
heir
Under these circumstances, s t r a i n e r s and f i l t e r s and
a s s o c i a t e d f i t t i n g s can be o m i t t e d f r o m t h e i n l e t l i n e , thus improving pump f i l l i n g conditions. I t i s an encouraging s i g n t h a t more d e s i g n e r s a r e p r o v i d i n g s e p a r a t e f i l l i n g
arrangements, u s u a l l y through some f o r m o f coarse f i l t e r t o reduce t h e need f o r l a s t - c h a n c e pump i n l e t p r o t e c t i o n .
I f t h e f i l l i n g requirements o f t h e pump a r e c r i t i c a l and supercharging i s necessary, as i s t h e case q u i t e o f t e n w i t h l a r g e v a r i a b l e - d i s p l a c e m e n t p i s t o n pumps,
i t i s common t o s i t e a f i l t e r between t h e supercharger and main pump.
For such a p p l i c a t i o n s ,
t h e same g u i d e l i n e s g i v e n f o r t h e s i z i n g o f p r e s s u r e o r
r e t u r n l i n e f i l t e r s can be used.
However, depressions can o c c u r when a v a r i a b l e
pump moves f r o m z e r o t o f u l l displacement, and t h e l i k e l y e f f e c t on f i l t e r p e r formance must be c a r e f u l l y considered.
320
13.6.2
Pressure L i n e F i l t r a t i o n
L e t us s t a r t by d i s c u s s i n g t h e l o c a t i o n o f t h e p r e s s u r e l i n e f i l t e r i n r e l a t i o n t o pump and r e l i e f v a l v e . F i g u r e 27 shows t h e p r e s s u r e f i l t e r l o c a t e d downstream o f t h e r e l i e f v a l v e . For t h e non-bypass t y p e t h e arrangement shown i n Fig.28
i s mandatory.
The
a c t u a l f l o w seen by t h e f i l t e r d u r i n g t h e o p e r a t i n g c y c l e depends on t h e system demand and d u r i n g o f f - l o a d
p e r i o d s t h e r e i s leakage f l o w o n l y i f b l o c k - c e n t r e
d i r e c t i o n a l v a l v e s a r e employed.
Naturally,
i f o f f - l o a d i n g i s achieved through
op e n - c e n t r e d i r e c t i o n a l v a l v e s t h e f i l t e r w i l l see f u l l pump o u t p u t f o r t h i s period.
Pressure filter
Q
Bypass
‘c
Pressure line filler
Rellef
Relief
.-+--A
Tank
Tank
Y ie_l
Strainer
Strainer
Tank
Fig.27
Pump
Tank
P r e s s u re l i n e f i l t r a t i o n w i t h bypass f i l t e r .
Fig.28
Pressure l i n e f i l t r a t i o n w i t h non-bypass f i l t e r .
To in c r e a s e t h e f l o w a cross t h e f i l t e r i t has become common p r a c t i c e t o l o c a t e t h e p r e s s u r e f i 1 t e r between pump and r e 1 i e f v a l v e (Fig.29).
The advo-
c a t e s o f t h i s arrangement p o i n t t o t h e f a c t t h a t t h e r e l i e f v a l v e i s p r o t e c t e d from pump generated d i r t .
A v a l i d p o i n t , however, i s t h a t t h i s generated d i r t
i s caused by pump wear which,
i n turn,
i s d i r e c t l y p r o p o r t i o n a l t o t h e conta-
m i n a t i o n l e v e l o f t h e f l u i d e n t e r i n g t h e pump i n l e t .
The pump i s o f t e n a c o s t l y
component and we should t h e r e f o r e d i r e c t more a t t e n t i o n t o reducing t h e contam i n a t i o n e n t e r i n g t h e pump t o a l e v e l w h i c h w i l l m i n i m i z e wear.
For t h i s
arrangement, a bypass i s mandatory and t h e r e must be an assurance from t h e f i l t e r m a n u fa c t ure r t h a t any f i l t e r m a l f u n c t i o n w i l l n o t r e s u l t i n excessive
321 pressure a t t h e pump o u t l e t .
Relief
Tank
,$!, Strainer
Tank
Fig.29
L o c a t i n g p ressu re f i l t e r b e f o r e r e l i e f v a l v e g i v e s c o n s t a n t f l o w t hro ug h f i l t e r .
Where v a r i a b l e - d i s p l a c e m e n t pumps a r e employed, c a r e f u l a n a l y s i s o f t h e f l o w seen by t h e f i l t e r must be made. pump which i s compensating ( i . e .
Take t h e c l a s s i c case o f a p r e s s u r e compensated low d i spl a cement a t maximum system o p e r a t i n g
pressure), where a low f l o w r a t e e x i s t s a l t h o u g h t h e pump i s s t i l l o p e r a t i n g a t pressure.
With a p r e s s u r e l i n e f i l t e r d i r t removal i s l i m i t e d .
Even when f l o w
demands a r e made t he se a r e 1 i m i t e d t o c y l i n d e r displacement. As i l l u s t r a t e d by Fig.15,
t h e f i l t e r e f f i c i e n c y depends upon t h e t y p e o f f l o w
i t sees, a l t h o u g h c u r r e n t l y no st an da rd t e s t e x i s t s f o r e v a l u a t i n g f i l t e r s when
they a r e s u b j e c t e d t o a b r u p t f l o w and p r e s s u r e changes and mechanical v i b r a t i o n . P r a c t i c e has shown t h e r e i s a r e d u c t i o n i n e f f i c i e n c y b u t i t s e x t e n t depends n o t o n l y on t h e q u a l i t y o f t h e a c t u a l f i l t e r media b u t a l s o on how w e l l t h i s media i s supported.
We must hope t h a t st a nd ard t e s t parameters can be agreed and a
Beta r a t i o quoted wh i ch r e l a t e s more c l o s e l y t o c o n d i t i o n s seen by a f i l t e r i n rea l h y d r a u l i c systems.
I n t h e f u t u r e i t w i l l a l s o be e s s e n t i a l t h a t t h e system
designer has i n f o r m a t i o n on t h e performance o f i n t e g r a l bypass v a l v e s under system o p e r a t i n g c o n d i t i o n s . I n a d d i t i o n t o mechanical v i b r a t i o n and a b r u p t f l o w and p r e s s u r e changes when valves a r e o p e r a te d, a p r e s s u r e l i n e f i l t e r i s a l s o s u b j e c t t o pump p u l s a t i o n s .
322 These e f f e c t s were demonstrated r e c e n t l y on a h y d r a u l i c component t e s t r i g where t h e s o - c a l l e d 10 micrometre nominal f i l t e r proved almost t o t a l l y i n e f f e c t i v e i n removing sub-5 micrometre contaminant. Because p r e s s u r e f i l t e r s have t o w i t h s t a n d t h e f u l l system p r e s s u r e w i t h adequate margin o f s a f e t y , t h e r e i s a tendency t o make these s m a l l , t h u s l i m i t i n t h e i r d i r t holding capacity.
It i s usual f o r manufacturers t o q u o t e a r a t e d
f l o w a t around 1 bar f o r bypass t y p e s and a t a s l i g h t l y h i g h e r p r e s s u r e f o r nonbypass t y p e s which i n c o r p o r a t e h i g h p r e s s u r e d i f f e r e n t i a l c a p a b i l i t y elements. I n summary,
i t can b e s a i d t h a t p r e s s u r e f i l t e r s a r e s u b j e c t e d t o c o n d i t i o n s
f a r removed from t h a t found i n a l a b o r a t o r y f i l t e r t e s t r i g , and because o f t h i s the designer's task i n assessing t h e r e s u l t a n t contamination i s extremely d i f f i cult.
The l e v e l s g i v e n i n F i g . 1 5 a r e based on f i e l d e x p e r i e n c e u s i n g good
q u a l i t y elements. Pressure f i l t e r s may w e l l be used i n c e r t a i n a p p l i c a t i o n s t o p r o t e c t t h e system should c a t a s t r o p h i c f a i l u r e s o f t h e pump occur, o r t o p r o v i d e s p e c i a l p r o t e c t i o n t o a s i n g l e u n i t o r group o f u n i t s .
An example o f t h e l a t t e r would
be a s e r v o v a l v e , where f a i l u r e m i g h t be e x t r e m e l y expensive, though i t should be noted t h a t a p r e s s u r e f i l t e r does n o t p r o t e c t a s e r v o v a l v e f r o m d i r t i n gressed t h r o u g h c y l i n d e r r o d s e a l s .
I n such cases a non-bypass f i l t e r should be
considered w i t h t h e assurance t h a t t o t a l element c o l l a p s e cannot o c c u r .
I f by-
pass types a r e employed, some means o f i n d i c a t i o n should be p r o v i d e d t o g i v e warning o f a p a r t i a l l y b l o c k e d element. From a maintenance p o i n t o f view, changing elements i n v o l v e s s t o p p i n g t h e system u n l e s s e x t e r n a l bypass v a l v i n g i s p r o v i d e d .
This operation o f t e n allows
f r e e a i r i n t o t h e system which must be c l e a r e d b e f o r e s a t i s f a c t o r y machine p e r formance can be o b t a i n e d .
13.6.3
Return L i n e F i l t r a t i o n
The usual r e t u r n l i n e f i l t r a t i o n arrangement shown i n F i g . 3 0 has a l l r e t u r n l i n e s passing through t h e f i l t e r .
D r a i n l i n e s f r o m pumps, motors, and c e r t a i n
v a l v e s should n o t be s u b j e c t e d t o p r e s s u r e surges emanating f r o m t h e system r e t u r n l i n e s and should r e t u r n s e p a r a t e l y t o tank. Where t h e r e a r e h i g h surges (e.g.
due t o u n c o n t r o l l e d decompression o r t h e
r a p i d a c c e l e r a t i o n o f t h e f l u i d column i n t h e r e l i e f v a l v e t a n k l i n e when t h i s v a l v e o p e r a t e s ) i t may be u n d e s i r a b l e t o pass t h e s e t h r o u g h t h e f i l t e r .
To
p r e v e n t c o l l a p s e o f t h e element due t o h i g h - v i s c o s i t y o i l , e.g. a t a c o l d s t a r t , o r when t h e element i s loaded w i t h d i r t , an i n t e r n a l bypass should be provided. When bypassing occurs under m i n i m u m f l o w and s u r g e c o n d i t i o n s t h e c i r c u l a t e d f l u i d should n o t be contaminated by d i r t a l r e a d y r e t a i n e d i n t h e f i l t e r .
The
t y p e o f surge experienced i n most r e t u r n l i n e s w i l l reduce t h e f i l t e r e f f i c i e n c y
323 and i n th e absence o f p r e c i s e data t h e f i l t e r should be s e l e c t e d u s i n g t h e p u l s a t i n g f l o w c o n d i t i o n f rom Fig.15. F u l l f l o w r e t u r n f i l t r a t i o n sho ul d be o f s u f f i c i e n t c a p a c i t y t o handle the maximum r e t u r n f l o w ( i n c l u d i n g any i n excess o f pump f l o w where, f o r example, unbalanced c y l i n d e r s a r e used) w i t h o u t t h e bypass opening.
Linear actuator
I I
I
valve
Directional valve
$0
Return line
Bypass
Strainer
filter
LLI Tank
Tank
Fig.30
B a s ic arrangement o f r e t u r n 1 i n e f i l t r a t i o n .
Careful assessment o f t h e f l o w a cross t h e f i l t e r i s necessary.
The comments
r e l a t i n g t o t h e use o f v a r i a b l e di sp l ace men t pumps w i t h a pressure f i l t e r again ap p ly , s i n c e w i t h p r e s s u r e compensated pumps the f i l t e r sees o n l y t h e volume d i s p l a c e d by t h e c y l i n d e r . For c e r t a i n systems p a r t i a l r e t u r n l i n e f i l t r a t i o n i s acceptable w i t h t h e bypass ( i n t e r n a l o r e x t e r n a l ) always p a s s i n g a percentage o f the flow .
With
t h i s arrangement much depends on t h e con t i n uo us r a t e o f f l o w which we can g e t acr o s s t h e f i l t e r .
S a t i s f a c t o r y performance can sometimes be achieved w i t h as
l i t t l e as 1 0 % p a ssi n g a cross t he a c t u a l element. To summarize, a r e t u r n l i n e f i l t e r does n o t p r o t e c t the system f r o m e n v i r o n -
mental d i r t e n t e r i n g t h e r e s e r v o i r v i a b r e a t h e r s o r d u r i n g topping-up.
However,
i f we s t a r t o f f w i t h a c l e a n r e s e r v o i r and t a k e p r e c a u t i o n s t o p r e v e n t the i ng r e s s o f e n v ir on men t a l c o n t a m i n a t i o n , then experience has shown t h a t e f f e c t i v e f i l t r a t i o n can be a chi e ved e con omi ca l l y w i t h r e t u r n l i n e f i l t e r s .
324 13.6.4
Off-Line F i t r a t i o n
I t has been s t a t e d t h a t t h e e f f e c t i v e n e s s o f f i l t e r s s ted i n b o t h pressure and r e t u r n l i n e s i s reduced by shocks, surges, p u l s a t i o n s
vibrations, etc. t o
an e x t e n t wh ic h depends on t h e t y p e o f media and how w e l l i t i s supported. Steady f l o w r e l a t i v e l y f r e e o f p r e s s u r e f l u c t u a t i o n s p r o v i d e s optimum f i l t e r performance.
The s i m p l e s t way o f a c h i e v i n g t h i s i s t o r e m v e t h e f i l t e r from
t h e main system and p l a c e i t i n an i n de pe nd ently powered c i r c u l a t i n g u n i t where i t s performance i s more p r e d i c t a b l e . Though connected t o t h e system r e s e r v o i r , a t y p i c a l arrangement i s shown i n Fig.31.
The l i k e l y c o n t a m i n a t i o n l e v e l f rom o u r t h r e e f i l t e r r a t i n g s under
steady f l o w c o n d i t i o n s i s g i v e n i n F i g . 1 5 , . a n d a o u r a b i l i t y t o p r o v i d e t h e c o r r e c t f l o w r a t e w i l l d et ermi n e whether o r n o t these l e v e l s can be achieved and ma i n t a ined
.
Fig.31
Layout o f o f f - l i n e f i l t r a t i o n system. Valves (a) and (b) can be p o s i t i o n e d so t h a t t h e u n i t w i l l p r o v i d e a f i l t e r e d f i l l o r di sch arg e.
Assuming reasonable standards o f e n g i n e e r i n g i n t h e design and b u i l d o f t h e system, t h e most r e l e v a n t f a c t o r s i n s e l e c t i n g t h e f l o w r a t e through t h e o f f l i n e f i l t r a t i o n a r e l i k e l y t o be environment and t a n k s i z e .
From f i e l d exper-
i e n c e t o d a t e t h e g u i d e l i n e s g i v e n i n F i g. 32 can be a p p l i e d and t h e f l o w r a t e
325 d e r i v e d can b e used t o d et ermi n e t h e s i z e o f t h e f i l t e r by t h e procedure p r e v iously outlined.
Flow r a t e ( l / m i n ) as percentage o f tank capacity ( I i t r e s )
Environment
5%
Good Average
10%
Bad
20%
Fig.32
Suggested f l o w r a t e s f o r o f f - 1 i n e f i l t r a t i o n .
With o f f - l i n e f i l t r a t i o n t h e d e s i g n e r ' s t a s k i s s i m p l i f i e d because he i s n o t governed by t h e f l o w and p r e s s u r e c h a r a c t e r i s t i c s o f t h e main system.
It
enables h im t o s e l e c t t h e b e s t f i l t e r and t h e b e s t f l o w through i t and then s e l e c t t h e s i z e t o a c h i e v e t h e d e s i r e d f re qu ency o f maintenance. Should t h e d e s i r e d c o n t a m i n a t i o n l e v e l n o t be achieved,
then c o r r e c t i o n s a r e
e a s i l y made t o t h e f l o w r a t e o r t y p e o f f i l t e r w i t h o u t i n any way a f f e c t i n g t h e de s ig n o f t h e main system. Furthermore, t h e o f f - l i n e
i n s t a l l a t i o n can be r u n
p r i o r t o s t a r t i n g t h e main system i n o r d e r t o c l e a n t h e o i l i n t h e t a n k and reduce t h e c o n t a m i n a t i o n l e v e l t h e pump w i l l see a t s t a r t - u p .
By t h e a d d i t i o n
o f s i m p l e v a l v e gear i t can be employed t o f i l t e r t h e i n i t i a l charge o f f l u i d and any used s u b seq ue nt l y i n topping-up.
I d e a l l y , i t should be l e f t running
c o n t i n u o u s l y t o p r o v i d e a complete t a n k o f c l e a n f l u i d ready f o r every s t a r t - u p . Unlike l i n e f i l t e r s ,
the o f f - l i n e
i n s t a l l a t i o n w i l l c o n t i n u e t o c l e a n up t h e
f l u i d when v a r i a b l e d e l i v e r y pumps a r e r u n n i n g a t minimum displacement. Being independent o f t h e main system o f f - l i n e f i l t e r s can be p l a c e d where they a r e most c o nve ni e nt f o r s e r v i c i n g . main system i s n o t a f f e c t e d ;
When element changes a r e necessary t h e
t h e o p e r a t i o n can be c a r r i e d o u t a t any t i m e
w i t h o u t s t o p p i n g o r i n t r o d u c i n g a i r i n t o t h e main system,
thus making t h e v e r y
minimum o f s e r v i c i n g s k i l l s a ccep t a bl e . Whether o r n o t o f f - l i n e f i l t r a t i o n can be t h e s o l e means o f f i l t r a t i o n depends on many f a c t o r s r e l a t i n g t o t h e c h a r a c t e r , q u a n t i t y and o r i g i n o f t h e c o n t a m in a t io n .
As i t i s p a r t i a l f i l t r a t i o n we must decide i f i t i s necessary
t o p r o t e c t i n d i v i d u a l o r groups o f components from s t r a y p a r t i c l e s l i k e l y t o cause a c a t a s t r o p h i c f a i l u r e . The f o r e g o i n g has a t t emp t e d t o show t h a t t h e r e i s a p l a c e f o r i n l e t , pressure, r e t u r n and o f f - l i n e f i l t r a t i o n , wh i ch i s borne o u t by f i l t e r manufacturers who once t o l d us t h a t p r e s s u r e f i l t e r s answered a l l o u r problems and who now o f f e r
a range o f r e t u r n l i n e f i l t e r s , and ma nu f a ct urers o f r e t u r n f i l t e r s who now o f f e r ranges o f p r e s s u r e f i l t e r s . S u r e l y what i s most i m p o r t a n t i s t o r e a l i s e t h a t l i k e a l l branches o f engine e r i n g , system f i l t r a t i o n e n g i n e e r i n g i n v o l v e s t a k i n g a r i s k . A s i n g l e p a r t i c l e
326
Fig. 3 3 A pictorial representation01 the bask liltration W4ualiOn.
327 i n t h e 5 t o 1 0 micrometre s i z e range, c o u l d cause system m a l f u n c t i o n i f i t
say,
a r r i v e d a t t h e wrong p l a c e a t a c r i t i c a l t i m e .
The machine t o o l designer may
w e l l have t h e b e n e f i t o f a p r o t o t y p e on which t o e v a l u a t e t h e f i l t r a t i o n system performance and make changes p r i o r t o t h e f i r s t p r o d u c t i o n b a t c h .
The designer
o f heavy equipment, say, f o r s t e e l w o r k s i s a f f o r d e d no such o p p o r t u n i t y ;
he
must be r i g h t f i r s t t i m e and may w e l l have t o use a l l t h e f i l t e r s e l e c t i o n p r o cedures a v a i l a b l e t o him i n o r d e r t o m i n i m i z e t h e r i s k o f expensive stoppages later.
That r i s k w i l l be w i t h us u n t i l (a) we have more r e a l i s t i c d a t a on f i l -
t e r performance, and (b) maintenance p r a c t i c e s a r e g r e a t l y improved. The message o f t h e p r e c e d i n g c h a p t e r s can b e s t be summarised d i a g r a m m a t i c a l l y (Fig.33).
This high1 ights the f a c t t h a t control over the i n b u i l t contamination
by a p p l y i n g good i n s t a l l a t i o n p r a c t i c e s and u s i n g a l l means a v a i l a b l e t o l i m i t t h e amount o f ingressed c o n t a m i n a t i o n , enables us t o c o n t r o l t h e system generated c o n t a m i n a t i o n t o an a c c e p t a b l e l e v e l . The l e v e l o f c o n t a m i n a t i o n e n t e r i n g t h e pump i s a c r i t i c a l f a c t o r and should be so c o n t r o l l e d t h a t i t p r e v e n t s t h e s o r t o f a c t i o n d e p i c t e d by a mangle. The d i f f i c u l t y i n c o n t r o l l i n g t h i s l e v e l by i n l e t f i l t e r s has been c l e a r l y s t a t e d . I n the main, t h e y should be c o n s i d e r e d o n l y f o r p r e v e n t i n g l a r g e p a r t i c l e s e n t e r i n g t h e pump and causing c a t a s t r o p h i c f a i l u r e . Downstream o f t h e pump t h e a b i l i t y o f a p r e s s u r e f i l t e r t o t r a p p a r t i c l e s w i l l be i n f l u e n c e d t o a l a r g e e x t e n t by any a b r u p t changes o f f l o w and p r e s s u r e , which have t h e e f f e c t o f d r i v i n g p a r t i c l e s t h r o u g h t h e f i l t e r i n g media.
We
know from p r a c t i c e t h a t bypass v a l v e s can m a l f u n c t i o n under c e r t a i n c i r c u m s t a n -
ces, and i t i s e s s e n t i a l t h a t t h e d e s i g n o f t h e f i l t e r p r e v e n t s any m i g r a t i o n o f c o n t a m i n a t i o n i f t h i s should happen. Ingress through c y l i n d e r r o d s e a l s , a l t h o u g h i n t h e s m a l l e r p a r t i c l e range, can n e v e r t h e l e s s add up t o s i g n i f i c a n t q u a n t i t i e s .
The performance o f any
r e t u r n l i n e f i l t e r w i l l , a g a i n , depend upon t h e magnitude o f changes i t sees i n terms o f f l o w and p r e s s u r e . O f f - l i n e f i l t r a t i o n enables optimum f i l t e r performance t o be accomplished, thus e n a b l i n g us t o c o n t r o l c o n t a m i n a t i o n l e v e l s more e f f e c t i v e l y .
Our a b i l i t y
t o remove c o n t a m i n a t i o n depends n o t o n l y on t h e f i l t e r micrometre r a t i n g b u t a l s o on t h e f l o w r a t e across i t , and we must p r o v i d e s u f f i c i e n t d i r t h o l d i n g c a p a c i t y t o ensure an element l i f e a c c e p t a b l e t o t h e end u s e r .
13.7
SUMMARY
Any examination o f t h e s u b j e c t o f c o n t a m i n a t i o n i n v o l v e s f o u r groups o f peop 1 e : (i)
The f l u i d manufacturer o r s u p p l i e r .
(ii)
The h y d r a u l i c equipment and f i l t e r manufacturers.
( i i i ) The manufacturer o f t h e machinery which uses t h e h y d r a u l i c equipment. ( i v ) The end u s e r o f t h e machinery.
328 Each o f these has a commercial r e s p o n s i b i l i t y t o supply equipment which w i l l perform i t s duty s a t i s f a c t o r i l y a t a reasonable p r i c e , and each must have some knowledge o f the c l e a n l i n e s s o f the working f l u i d . The f l u i d s u p p l i e r w i l l supply f l u i d as c l e a n as r e q u i r e d and w i l l charge accordingly. The h y d r a u l i c equipment manufacturer must advise t h e user to whom he i s s e l l i n g h i s equipment on the type o f f l u i d and i t s degree o f c l e a n l i n e s s best s u i t e d t o the h y d r a u l i c equipment he i s recommending f o r a s p e c i f i c a p p l i c a t i o n . The machinery manufacturer, who i s responsible t o the end user f o r supplying equipment, must be aware o f a l l the c o n d i t i o n s t o be met,
i n c l u d i n g supply of
f l u i d s , s e r v i c i n g f a c i l i t i e s , and the type o f r e l i a b i l i t y which has t o be maintained.
I t i s h i s p r e r o g a t i v e t o o f f e r h i s customer the best commercial propo-
s i t i o n and t h i s could mean, f o r example, e i t h e r an inexpensive throwaway u n i t which must be replaced r e l a t i v e l y f r e q u e n t l y , o r a more expensive item f o r which the o n l y service needed i s the occasional exchange o f a r e l a t i v e l y inexpensive component, such as a f i l t e r c a r t r i d g e . The end user has t o make the f i n a l judgement.
He needs t o be a b l e t o compare
the r e a l value o f each i n s t a l l a t i o n o f f e r e d t o him;
each user w i l l place d i f f e r
ent emphasis on the value o f each type o f equipment o f f e r e d .
To the end user
the value o f a pump i s i t s f i t n e s s f o r the purpose, how long i t w i l l perform i t s duty s a t i s f a c t o r i l y , and what the cost o f s e r v i c i n g w i l l be.
He i s n o t i n t e r -
ested i n how many 10 micrometre p a r t i c l e s a r e contained i n 100 m l o f h y d r a u l i c fluid. The u s e r ' s i n t e r e s t i s i n the l e a s t expensive f i l t e r t h a t w i l l p r o v i d e the required degree o f c l e a n l i n e s s .
I n making t h i s assessment, the o r i g i n a l o r
c a p i t a l c o s t o f the equipment has t o be balanced a g a i n s t the c o s t o f service. He may have t o compare, f o r example, the advantages o f f e r e d by a very expensive pump t h a t w i l l operate on ' d i r t y ' o i l w i t h those o f a low-cost pump p l u s f i l t e r .
I f i t i s t o m a i n t a i n i t s present h i g h i n t e g r i t y , the h y d r a u l i c equipment industry needs t o e s t a b l i s h more meaningful s p e c i f i c a t i o n s f o r d i f f e r e n t types of f i l t e r .
The s p e c i f i c a t i o n s must a l l o w the end user t o buy performance w i t h -
out necessarily having t o know how t h i s performance i s achieved.
The people
who need t o w r i t e these s p e c i f i c a t i o n s a r e those who know t h e a c t u a l r e q u i r e ments.
The r e s p o n s i b i l i t y must be d i v i d e d between manufacturers o f f i l t e r
elements, who should know what i s p o s s i b l e , and the manufacturers o f h y d r a u l i c equipment, who should know what i s needed.
These t w o groups should be a b l e t o
communicate i n meaningful terms.
A t present, there a r e no adequate techniques covering a l l aspects o f contami n a t i o n measurement i n h y d r a u l i c f l u i d s . a s t a r t i n the r i g h t d i r e c t i o n . n o t be 100% r i g h t f i r s t time;
This should n o t deter us from making
The c l e a n l i n e s s l e v e l c h a r t , f o r example, w i l l t h e r e w i l l be a c o n t i n u a l need t o review i t i n
329 the l i g h t o f new e xpe ri en ce. The e s c a l a t i n g c o s t o f equipment downtime and maintenance must encourage t h e end u s e r t o c o n s i d e r r u n n i n g c o s t s more c a v e f u l l y , f o r b o t h new and e x i s t i n g plant.
To be a b l e t o do t h i s he must a p p r e c i a t e more f u l l y the p a r t t h a t
c o n t a m i n a t i o n c o n t r o l p l a y s i n d e t e r m i n i n g t hose c o s t s f o r t h e h y d r a u l i c installation.
I t i s hoped t h a t t h i s volume has g i v e n a u s e f u l i n s i g h t t o t h e
r e a l n a t u r e o f t h e problem and has p o i n t e d t h e way t o a systematic approach and more c o s t - e f f e c t i v e s o l u t i o n s .
T h i s c h a p t e r has been e d i t e d by k i n d p ermission o f Sperry V i c k e r s from a more comprehensivb paper " E f f e c t i v e Contamination C o n t r o l i n F l u i d Power Systems",
w r i t t e n by J.B.
Spencer and C.
Balmer, p u b l i s h e d by Sperry Vickers.
330
1L1
SEALS FOR FLUID POWER EQUIPMENT PART ONE
B.D.HALLIGAN,
14.1
C Eng,MIMechE AMPRI, T e c h n i c a l Manager ( P r o d u c t A p p l i c a t i o n s ) James Walker & Co.Ltd.
INTRODUCTION
The f a c t t h a t a s e a l m a n u f a c t u r i n g i n d u s t r y e x i s t s a t a l l o r i g i n a t e s f r o m t he commercial o r p h y s i c a l i m p r a c t i c a b i l i t y o f a c h i e v i n g p e r f e c t and permanent m u t u a l i t y o f adjacent surfaces
-
be t h e y i n dynamic o r s t a t i c r e l a t i o n s h i p .
S e a l i n g d e v i c e s , wh i c h a r e u s u a l l y o f an e s s e n t i a l l y d e f o r m a b l e n a t u r e , p e r m i t t h e f l u i d power d e s i g n e r t o work w i t h i n e c o n o m i c a l l y s e n s i b l e c o n d i t i o n s o f
f i t , surface f i n i s h ,
t o l e r a n c e and f l u i d c l e a n l i n e s s a c r o s s t h e range o f temper-
a t u r e s and p r e s s u r e c o n d i t i o n s w i t h w h i c h he i s l i k e l y t o c o n te n d . The d e s i g n e r o f f l u i d power equipment has,
i n fact,
influenced trends i n the
s e a l i n g i n d u s t r y by g o i n g beyond t h e fundamental r e q u i r e m e n t o f i n t r o d u c i n g an e n v i r o n m e n t a l l y t o l e r a n t component f o r p r e v e n t i n g le a k a g e .
The q u e s t t o m i n i -
mise p r o d u c t i o n c o s t s has a c c e l e r a t e d t h e movement towards s e a l d e s i g n s embodyi n g i n t e g r a l b e a r i n g s and a n t i - e x t r u s i o n d e v i c e s c a p a b le o f s u p p o r t i n g s e a l s a g a i n s t s u b s t a n t i a l e x t r u s i o n gaps a t s i g n i f i c a n t p r e s s u r e s .
Overall i n the
c o n t e x t o f t hose s e a l s s p e c i f i e d f o r l i n e a r dynamic m o t i o n , a much a x i a l l y s h o r t e r s e a l has e v o l v e d wh i c h a l l o w s h x d r a u l i c c y l i n d e r s , f o r example, t o be produced t o more compact p r o p o r t i o n s w i t h t h e consequent s a v i n g i n raw m a t e r i a l c o s t s and i n t h e b u l k o f f l u i d power s u b - a s s e m b lie s . None o f t h i s i s bad.
However i t does mean t h a t a p r o l i f e r a t i o n o f s e a l
desi gns and m a t e r i a l s e x i s t s and t h o s e r e s p o n s i b l e f o r s e l e c t i o n a r e r e q u i r e d t o make t h e i r c h o i c e w i t h d i s c r e t i o n t o e n s u r e t h a t t h e most c o s t - e f f e c t i v e s o l u t i o n i s f oun d .
P i c k i n g a s e a l wh i c h f a c i l i t a t e s t h e achievement o f t h e
cheapest p r o d u c t i o n c o s t s f o r t h e component i n t o w h ic h i t f i t s i s seldom t h e most r easonabl e b a s i s f o r g u a r a n t e e i n g minimum leakage and maximum s e r v i c e a b i l i t y .
331 I t i s an u n f o r t u n a t e f a c t t h a t most f l u i d leakages a r e a t t r i b u t e d t o a f a u l t y seal, whereas i n v e s t i g a t i o n w i l l f r e q u e n t l y show t h a t t h e number o f occasions o f m a l f u n c t i o n due t o a f a u l t y p r o d u c t a r e few.
The r e a l source o f d i f f i c u l t y
u s u a l l y stems from i n c o r r e c t seal s e l e c t i o n , w o r k i n g c o n d i t i o n s which d i f f e r from those considered t o a p p l y , seal containments o u t s i d e s p e c i f i e d t o l e r a n c e s i n terms o f dimension o r f i n i s h , f a u l t y f i t t i n g , be sealed o r ,
importantly,
contaminants i n t h e f l u i d t o
personal i n t e r p r e t a t i o n o f 'leakage'
in quantitative
terms. To understand
t h e l i m i t a t i o n s o f f l u i d seal performance i t i s necessary t o
know a l i t t l e o f t h e m a t e r i a l s most t y p i c a l l y used, t h e o r i g i n s o f fundamental seal designs, how t h e y f u n c t i o n , and t h e i r r e f i n e m e n t t o t h e p r e s e n t s t a t e o f the a r t .
Fortunately,
t h e s e a l i s no l o n g e r an a f t e r - t h o u g h t b u t more o f t e n
receives t h e a t t e n t i o n i t p r o p e r l y deserves as a v i t a l i n t e r a c t i v e element i n any complete f l u i d power system.
14.2
MATERIALS
W i t h i n t h e scope o f t h i s c h a p t e r , a t t e n t i o n w i l l be c e n t r e d on s o l i d e l a s t o meric s e a l s , t y p i f i e d by t h e '0' r i n g f o r s t a t i c connections,
elastomer c o n t a i n -
i n g s e a l s such as c y l i n d e r packings based on p r o o f e d t e x t i l e , and those designs which employ p l a s t i c components f o r v a r i o u s f u n c t i o n s . The compounding o f e l a s t o m e r s i s regarded by many as b e i n g a b l a c k a r t and i t i s t r u e t o say t h a t w i d e l y d i f f e r i n g p r o p e r t i e s can be developed by v a r y i n g
t h e q u a n t i t y and t y p e o f rubber chemicals added t o t h e s t o c k rubber i n e i t h e r i n t e r n a l m i x e r s o r on m i x i n g m i l l s .
The raw m a t e r i a l s u p p l i e d t o t h e seal man-
u f a c t u r e r must u s u a l l y be processed t o t a i l o r t h e p h y s i c a l c h a r a c t e r i s t i c s r e quired f o r the sealing function.
Parameters such as t e n s i l e s t r e n g t h , e l o n g a t i o n
a t break, compression s e t , b r i t t l e p o i n t , and behaviour i n c o n t r o l f l u i d s a r e t y p i c a l o f t h o s e reviewed when assessing performance requirements and l a y i n g down compound s p e c i f i c a t i o n s . i n a d d i t i o n t o a p p l i c a t i o n s u i t a b i l i t y , p r o c e s s a b i l i t y i n mixing, extrusion and moulding sequences must be assured as must s a t i s f a c t o r y post-moulding operat i o n s such as d e - f l a s h i n g . T o m a i n t a i n c o n s i s t e n c y o f seal elastomers r e q u i r e s e x t e n s i v e l a b o r a t o r y
s u p p o r t and c o n t i n u o u s s t a n d a r d t e s t i n g o f t h e compound i t s e l f on a b a t c h - t o batch basis. Tables 14.1
-
1 4 . 3 a r e taken from t h e B r i t i s h Rubber M a n u f a c t u r e r s ' Associa-
t i o n 'Guide t o E l a s t o m e r i c S e a l s ' elastomers,
t o g i v e broad r e f e r e n c e t o most commonly used
t h e i r p r o p e r t i e s , f l u i d c o m p a t i b i l i t y and temperature r e s i s t a n c e .
332 TABLE 14.1 Polymers-types and General P r o p e r t i e s
NATURAL POLY I SOPRENE SYNTHETIC POLYISOPRENE Very good
(Natural Rubber-NR) (Pol y isoprene- I R) General physical p r o p e r t i e s , i n p a r t i c u l a r t e a r resistance, r e s i s t a n c e t o g l y c o l - e t h e r brake f l u i d s and vegetable o i l s .
Poor r e s i s t a n c e t o
Heat, weather, ozone and mineral o i l .
CHLOROPRENE (NEOPRENE-CR) Very good res i stance t o
Ozone, weathering and abrasion
STYRENE-BUTADIENE (SBR) Very good r e s i s t a n c e t o
Abrasion, g l y c o l - e t h e r brake f l u i d s and vegetable o i l s .
Poor r e s i s t a n c e t o
Mineral o i l and ozone.
ACRYLONITRILE-BUTADIENE ( N i t r i le-NBR)
Low, medium, and h i g h n i t r i l e s a r e a v a i l a b l e based on increasing a c r y l o n i t r i l e content which s i g n i f i c a n t l y a f f e c t s low temperature and f l u i d swell properties.
Very good r e s i s t a n c e t o
Mineral o i l , compression set and abrasion.
ISOBUTYLENE-ISOPRENE ( B u t y l - l I R )
CHLORO-ISOBUTYLENE-ISOPRENE (Chloro-Butyl) Very good r e s i s t a n c e t o Poor
POLYBUTADI ENE (Butad iene-BR) Very good r e s i s t a n c e t o
Poor resistance t o POLYSULPHIDE (Thiokol-T) Very good r e s i s t a n c e t o Poor
Tear, weather and gas permeation. T e n s i l e p r o p e r t i e s and mineral o i l resistance.
Tear, abrasion, low temperature, g l y c o l - e t h e r brake f l u i d s and vegetable oils. Offers high resilience. Water, ozone and mineral o i l .
Ozone, mineral o i l s , pet-roleum f u e l s and weather.
A l l other properties.
ETHYLENE PROPYLENE (EPR-EPM) ETHYLENE PROPYLENE TERPOLY MER (EPT-EPDM) Very good resistance t o Weather, ozone, heat, water, steam, g l y c o l - e t h e r brake f l u i d s and vegetable oils. ?oor resistance t o Mineral o i l . CHLOROSULPHONATED POLYETHYLENE (Hypa lon-CSM) Very good resistance t o
Water, ozone, abrasion, a c i d and weather.
333 TABLE 14.1
(contd.)
METHYL-VINYL SILOXANE ( S i l icone-VMQ) PHENYL-METHYL-VINYL SILOXANE ( S i 1 icone-PVMQ) Very good r e s i s t a n c e t o
H i g h and low temperature.
Poor r e s i s t a n c e t o
Abrasion,
TRI FLUOROPROPYL S I LOXANE ( F l u o r o s i 1 icone-FMQ) Very good POLYURETHANE D I - I SOCYANATE (Urethane-AU) Very good r e s i s t a n c e t o
tear
and t e n s i o n .
M i n e r a l o i l and f u e l r e s i s t a n c e . A l l o t h e r p r o p e r t i e s as VMQ. Mineral o i l , abrasion, tear, ozone and weather. O f f e r s h i g h modulus and t e n s i l e p r o p e r t i e s .
Poor
Moist heat resistance.
FLUORINATED HYDROCARBON ( F I uorocarbon-FPM) Very good r e s i s t a n c e t o
High temperature ( i n a i r and most o i l s ) , weather and p e t r o l e u m f u e l .
Poor
Tear s t r e n g t h .
POLYACRYLATE ( A c r y l ic-ACM) Very good
Heat, weather, m i n e r a l o i l and ozone r e s i s t a n c e . Water r e s i s t a n c e .
Po0 r
O f major i n t e r e s t t o t h e f l u i d power i n d u s t r y a r e : -
Acrylonitrile-butadiene ( N i t r i l e Probably upwards o f 80% o f based on n i t r i l e compositions. terms o f good compression s e t
-
NBR)
s e a l s s u p p l i e d t o t h e f l u i d power
ndustry are
The balance o f p r o p e r t i e s a v a i l a b e from NBR i n q u a l i t i e s , a b r a s i o n r e s i s t a n c e , m i era1 o i l
c o m p a t i b i l i t y , ease o f p r o c e s s i n g and low i n i t i a l c o s t favour t h i s c h o i c e . N i t r i l e compositions a r e n o t , however, c o m p a t i b l e w i t h f i r e - r e s i s t a n t
fluids
o f t h e phosphate e s t e r t y p e . Isobutylene-isoprene (Butyl
-
I IR)
S a t i s f a c t o r y i n s e r v i c e w i t h phosphate e s t e r f l u i d s b u t r a t h e r l i f e l e s s from a physical p o i n t o f view.
Not s u i t a b l e f o r m i n e r a l o i l s e r v i c e , n e i t h e r should
b u t y l s e a l s be smeared w i t h m i n e r a l o i l o r grease on f i t t i n g . E t h y l e n e p r o p y l e n e (EPR or
-
EPM)
E t h y l e n e p r o p y l e n e t e r p o l y m e r (EPT
-
EPDM)
Same s e r v i c e f u n c t i o n and l i m i t a t i o n s as b u t y l b u t g e n e r a l l y b e t t e r p h y s i c a l properties
.
F l u o r i n a t e d Hydrocarbon (Fluorocarbon
-
FPM)
The o n l y usual s e l e c t i o n where c o m p a t i b i l i t y w i t h m i n e r a l o i l and phosphate e s t e r i s required.
334 P ol yure t h an e (Urethane
-
Au)
Widely f a v o u r ed as a dynamic se al m a t e r i a l i n t h e US and i n Germany, p o l y ure t h a n e e x h i b i t s e x c e l l e n t a b r a s i o n r e s i s t a n c e and i s v e r y d u r a b l e i n s i t u a t i o n s whic h m ig h t a d v e r s e l y a f f e c t n i t r i l e comp osi tions o r even p r o o f e d f a b r i c s , e.g. passage o f s e a l l i p s o v e r p o r t s i n a c y l i n d e r w a l l . bility,
Poor low temperature f l e x i -
h y d r o l y s i s i n h o t wa t e r and a l o we r o p e r a t i o n a l temperature c e i l i n g a r e
l i m i t i n g factors. Other s e a l component m a t e r i a l s w o r t h y o f mention are:-
-
Textiles
woven c l o t h such as c o t t o n , asbestos,
t e r y l e n e and n y l o n
a r e used as t h e su bst rat um i n elastomer p r o o f e d f a b r i c pa cki ng s f o r medium and h i g h p r e s s u r e d u t i e s . Non-woven m a t e r i a l s such as p o l y e s t e r and polyamide a r e also available. The p r o o f i n g e l ast ome r can be v a r i e d t o s u i t b u t w i l l f r e q u e n t l y be based on NBR and/or c h l o r o p r e n e (Neoprene-CR)
-
PTFE
employed as a back-up r i n g m a t e r i a l f o r ' 0 ' r i n g s . Poor cre ep p r o p e r t i e s r e s t r i c t i t s use as a seal m a t e r i a l i n i t s own r i g h t un l ess e n e r g i s e d by an e l a s t o m e r i c component o r spring.
Nylon and acetal
-
main f u n c t i o n i s h ee l support f o r dynamic s e a l s o r as component b e a r i n g r i n g m a t e r i a l .
-
Hytrel
t h e r e g i s t e r e d t r a d e name o f a Du Pont range o f thermop l a s t i c p o l y e s t e r m a t e r i a l s w hich form a el a st o mers and t h e r m o p l a s t i c s .
b r i d g e between
Has s i m i l a r p r o p e r t i e s t o
p o l y u r e t h a n e b u t e x h i b i t s much b e t t e r low temperature f l e x ibility.
Used c u r r e n t l y as an a n t i - e x t r u s i o n element i n
c e r t a i n s p o o l - t y p e p i s t o n head s e a l s . When r e f e r r i n g t o rubber t e chn ol o gy as a ' b l a c k a r t ' t h e r e i s a t w o - f o l d interpretation.
On t h e one hand i t i s c e r t a i n l y a m a n u f a c t u r i n g area i n f l u e n c e d
by many v a r i a b l e s .
On t h e o t h e r ,
t h e end p r o d u c t used i n a s e a l i n g c o n t e x t i s
i n v a r i a b l y b l a c k i n c o l o u r due t o t h e use o f carbon b l a c k as a f i l l e r necessary f o r toughening the f i n i s h e d a r t i c l e .
Seal i d e n t i f i c a t i o n i s t h e r e f o e a major
problem u n l e s s housekeeping i s o f a h i g h o r d e r w i t h s e a l s p o s i t i v e l y separated and bagged i n s ma l l numbers h a v i n g f u l l s p e c i f i c a t i o n o f m a t e r i a l , s ze, and p a r t number c l e a r l y shown. Colour c o d in g i s one s o l u t i o n b u t t h e r e a l answer l i e s i n t h e use o f s e l f c o l o u r e d e la s to m e rs.
A t t h e moment a good de al o f development work
s being
.
335
TABLE 14.2
R-Recommended A-Acceptable
N-Not recommended
R
A
N
N
N
N
N
N
N
N
N
N
N
R
N
R
TABLE 1 4 . 3 OC
250
200
150
Temperature Range
OC
Any statement of temperature range f o r a given polymer must be qualified by consideration f o r the environment in which i t i s t o work. For example, service temperatures recommended f o r 1iquid applications are l i k e l y t o be s i g n i f i c a n t l y higher than those which would be appropriate t o dry heat s i t u a t i o n s . The values given in Table 3 r e l a t e t o typical f l u i d conditions. N.B. These a r e not continuous r a t i n g s .
W W W W Q, Q,
TTABLE A B i E 14.4 TYPES RECIPROCATING RECIPROCATING APPLICATIONS APPLICATIONS L I P TYPE
OF SEALS FOR RECIPROCATING, ROTARY AND STATIC APPLICATIONS APPLICATIONS APPLICATIONS
U R i n g (Also known as U P a c k i n g )
-
50116 e l a i t m - f o r p r e s ~ u r e up i t o 10 bar--gmd f i l m w r p i t y and l o * pressure sealing characterlst3csideal f o r a i r c y l i n d e r . Ymm t o abrasive *car. e x t r u ~ l ~and n f l u i d t v t t l n g a t hlgher hydraulic p r e s i u ~ e i . Can be f i t t e d in recerrrr r e q u i r i n g no sep*r*te dCLCII. l o l a d e l a i t m i w t h P l i i t i C heel-for OT~IIYTCI UP to 100 b a r d s 10lld rubbcr but hiqher oressure Capability due t o enhanced CItlYIlOn r e l l r t m c e .
350 bbai li I1I1 though though rohs't rohit Fabric-for pressures p r e i i u r e l up up ttoo 350 Fabric-for leis sseennssi itti ivvee Dprrooffiillee *w11 eitend Conitructions ooff leis ? I 1 extend Conitructions range. Frequently Frequently ffi itttteedd dwti thh an an adapt& adaptor rri inngg tthhi iss range. (as shorn shorn iinn the the aise&ly d 5 l m h l y sketcher) sketcher) or 01an an i inntteerrnnaal l (as arc available available s p ~ c a d c r . Spring Spring loaded loaded adaptors l d l p t o r r arc l li ipp spvcadcr. t o aupnent low QrCIIYlt real,ng I b 9 l l t y . Usually rcqulrr separate housing access such I S remyable cover p l a t e but I p c c i a I fabric U-nngr f o r indCcesItble 9movc1 d m available. Other materials include wlyurethdne. P l F t and leather. None o f t W I e designs a r e n o m l l y r e c m n d c d f O T shock loading. rams subject t o l a t e r a l t h r u s t . abrasive condit30nr o r high speed recIpToCation. U n n q 1 seal ~n one d i r e c t i o n only. A double-acting a r m n g m n t M i l l *quire tw ~ e d l it o be r i g ? d l y scpamted and each f a c i n g the p r c s ~ u r cIOUTCC.
AVAILABLE STANDARD STANDARD AVAILABLE MATERIALS MATERIALS
K
0
L
>
0)
L x t r e n r l y wide range of h y d r a d i c rervices-mst types of h y d r a u l i c Dress, r e c i p r o c a t i n g steam punpr, sludge p w s . h y d r a u l i c cylinder; * OilOil and water expanlion glandr, h y d r a u l i c valves.
N o r m l l y pmafed f a b r i c .
fa1 U p - r t m k i m l IlMl omving press rams. r e c i p r o c a t i n g Dmp l a m . l i f t s , cranes. h y d r a u l i c acCuMlators.
Proofed f a b r i c
Others i n c l u d e e i a s t M c r ,
'lFC Or leather
(a) S i n g l e L i p n
Heavy Duty L i p Packi ngs
"T
eU
o w ll-m
V Ring (When i n s e t form, more commonly known as Chevron
( b ) Two-Lips L
e
v
L
la)
Large diameter ram. u p - s t r o k i n g or d o n - s t r o k i n g (*ere f r i c t i o n a l r e s i s t a n c e i s n o t o f paralmunt importance) Wen where subjected t o Shock loads. e.g. i n f o r g i n g p r e i i e s ' h o r i z o n t a l vams of any diameter i n c l u d i n g tho%; o f e x t r w i o h ~ P C I I C I r a m s u b j e c t to I d t e m l t h r u s t : h i g h pressure' r e c i p i o c d t i n g pulps; t b s e heavy duty a p p l i c a t i o n s where packing housing and buIneI are n o t i n F i s t c l a l 5 condition.
337
I
w w -2
338 c a r r i e d o u t t o a ch i eve t h i s end w i t h o u t any s i g n i f i c a n t l o s s i n p h y s i c a l properties.
Coloured f l u o r o c a r b o n rubbers a r e a l r e a d y a v a i l a b l e .
14.3
SEAL DESIGNS
W i t h i n t h e boundaries s e t by f l u i d power systems t h e m a j o r i t y o f s e a l s employed can be c a t e g o r i s e d as l i p o r squeeze t y p e s . F i g u r e 1 i l l u s t r a t e s t h e s i m p l e s t forms u t i l i s e d f o r s t a t i c o r dynamic r e c i procating situations
-
the 'U'
r i n g and t h e '0' seal
-
and i n d i c a t e s how b o t h
r e l y on i n t e r f e r e n c e s t r e s s f o r a t l e a s t a component o f t h e i r f u n c t i o n a l opera t io n .
r--.
-
I.--'
INT
P
P
Fig.1
L i p and Squeeze Seals
Both a r e r e s p onsi ve t o system p r e s s u r e and w i l l generate a r a d i a l s t r e s s g r e a t e r than t h e p r e s s u r e t o be se al e d.
N e i t h e r r e l y on e x t e r n a l l y a p p l i e d
compression t o any r e a l degree as i n t h e case o f t h e s o f t packed gland.
This
i s p a r t i c u l a r l y r e l e v a n t t o squeeze s e a l s whi ch a r e u s u a l l y intended t o f l o a t a x i a l l y i n t h e i r housings. The q u a l i t y o f t h e s e a l i n g c o n t a c t a rea and t h e i n t e r f e r e n c e s t r e s s i n t h e same zone w i l l c o n t r o l low p r e s s u r e s e a l i n g e f f i c i e n c y .
These f a c t o r s p l u s
o v e r a l l s e a l geometry and co mpo si t i o n w i l l s e t t h e p o i n t a t w hich t i m e h y d r a u l i c response t a k e s o v e r f rom t h e manufactured i n t e r f e r e n c e c o n d i t i o n .
The h y d r a u l i c
component may become s i g n i f i c a n t a t p ressu res o f 40 bar f o r an e l a s t o m e r i c ' 0 ' r i n g o r a s h i g h as 200 b a r f o r a r i g i d p r o o f e d f a b r i c packing. S e a l i n g s l a c k o i l and low p ressu res i s g r i t y and i s t h e more d i f f i c u l t c o n d i t i o n . ure s i s a f e a t u r e o f se al containment,
i.e.
g e n e r a l l y a f u n c t i o n o f seal i n t e P r e v e n t i n g leakage a t h i g h pressp r e v e n t i n g e x t r u s i o n o r r a p i d wear
i f i n a dynamic d u t y . Tables 14.4 and 14.5 l i s t t y p i c a l l i p and squeeze types i n common and t r a d i t i o n a l usage.
(Courtesy o f ERMA).
339 Some p a r t i c u l a r p o i n t s t o n o t e on each c a t e g o r y : L i p seals
(i)
-
a r e s i n g l e - a c t i n g t o a p p l i e d pressure and must be separated
by a f i x e d component i n d o u b l e - a c t i n g d u t i e s so t h a t t h e h y d r a u l i c load f r o m t h e element under p r e s s u r e i s n o t passed on t o t h e t r a i l i n g element. For maximum s e r v i c e l i f e m u l t i - r i n g p acking s e t s a r e p r e f e r r e d where t h e s u c c e s s io n o f s e a l i n g edges ensures t h a t breakdown i s n o t sudden.
The use
o f s p l i t r i n g s i s a l s o e n t i r e l y f e a s i b l e w i t h most designs o f t h i s s o r t w i t h o u t s a c r i f i c i n g s e a l i n g performance t o any c r i t i c a l degree.
Savings i n
downtime w i l l be o bvi o us. ( i i ) Squeeze s e a l s
-
have t h e advantage o f b e i n g d o u b l e - a c t i n g and a r e u s u a l l y
housed more e con omi ca l l y t h an t h e i r co rresponding l i p b r e t h r e n .
The l i n e -
age fr o m t h e humble '0' r i n g t o t h e u n i t s e a l s i n contemporary use can be seen f r o m Ta bl e 14.5. Most s e a l s i n t h i s c a t e g o r y use a s i n g l e s e a l i n g zone and damage i n s e r v i c e w i l l be f o l l o w e d by more immediate breakdown than i n t h e case o f m u l t i - l i p p a c k in g s . The squeeze s e a l s a r e r a r e l y used i n s p l i t form a l t h o u g h they can be engineered so t o do. Tables 14.6 and 14.7 d e p i c t t h e e x t e n s i o n o f l i p and squeeze s e a l s t o r o t a r y and s t a t i c f u n c t i o n s . More advanced se al de si gn s a r e d e t a i l e d i n t h e n e x t - s e c t i o n .
14.4
TR IBOLOG I CAL CONS I DERAT I ONS
I t i s h e a r t e n i n g t o f i n d t h e s t u d y o f se al behaviour an e s s e n t i a l p a r t o f t r i b o l o g y seminars,
r e c e i v i n g as much a t t e n t i o n ,
and s u r f a c e topography.
indeed, as bearings,
lubricants
I n r e c e n t ye ars much company research and independent
s tu d y by o r g a n i s a t i o n s such as BHRA has been expended on d e f i n i n g s e a l i n g mechanisms
14.4.1
-
p a r t i c u l a r l y i n r e l a t i o n s h i p t o r e c i p r o c a t i n g motion. F i l m Conditions
A l l dynamic s e a l s r e l y on a co he ren t f l u i d f i l m under t h e i r c o n t a c t area i f t h e y a r e t o f u n c t i o n c o n s i s t e n t l y and p r e d i c t a b l y .
Such f i l m s may stem from
boundary l u b r i c a t i o n i n some modes and be t r u l y hydrodynamic i n o t h e r s .
f i l m w i l l v a r y i n t h i c k n e s s a c c o r d i n g t o se al p r o f i l e , pres s u r e , speed,
surface f i n i s h ,
t y p e o f f l u i d employed
On r e c i p r o c a t i n g d u t i e s i t m i g h t t y p i c a l l y v a r y from 0.25 r o t a r y s h a f t s v a l u es o f 0.6
-
The
interference stress, and i t s temperature.
-
3.0 microns.
On
1.0 mi cro n would u s u a l l y a p p l y .
Leakage f r o m r o t a r y s h a f t l i p s e a l s i s seldom e v i d e n t u n l e s s seal wear down o r u n d e r - l i p c r a c k i n g has t ake n p l a c e
-
a l l o t h e r f e a t u r e s b e i n g equal. However,
340
TABLE
14.5
RECIPROCATING APPLICATIONS
APPLICATIONS
AVAILABLE STANDARD MATERIALS
SQUEEZE TYPE
0-Seal
L i g h t and medium duty, pneumatic and h y d r a u l i c r e c i p r o c a t i n g services, e.g. small c y l i n d e r s , valve spools and stems, rod f i l m wiping.
P r i n c i p a l l y elastomer b u t polyurethane and PTFE a r e a l s o manufactured.
As 0-seal b u t b e t t e r resistance t o s p i r a l t w i s t .
Elastomer.
Pneumatic and h y d r a u l i c c y l i n d e r s p a r t i c u l a r l y where s e n s i t i v i t y i s essential, e.g. weighing machines, t e s t i n g equipment, e t c . P r i m a r i l y f o r p i s t o n heads but also available f o r gland d u t i e s .
Elastomer and PTFE ( p l a i n o r r e i n f o r c e d ) o r elastomer and proofed f a b r i c .
B a s i c a l l y developed f o r hydraulic cylinder rod and p i s t o n head d u t y p a r t i c u l a r l y i n mobile and i n d u s t r i a l h y d r a u l i c environments.
Elastomer/Plastic. Elastomer/Fabric,
Lobed Seal
Energized Sleeve Seal
Supported S i ngle-Acting Sea 1
m
Elastomer/Fabric/Plastic can a l s o be made i n polyurethane.
Supported Double-Acting Seal Developed f o r p i s t o n head duty i n h y d r a u l i c c y l i n d e r s associated w i t h mobile/ indus tr ia 1 hydraul ic industries.
Elastomer/Fabric,
Elastomer/Fabric/Plastic, Elastomer/Plastic.
341
TABLE 14.6 ROTARY APPLICATIONS
APPLICATIONS
AVAILABLE STANDARD MATERIALS
LIP TYPE
Retention o f l u b r i c a n t and exclusion o f f o r e i g n matter from bearings and c r i t i c a l surfaces.
SQUEEZE TYPE
Elastomer/metal, Elastomer/fabric. Spring loading t o l i p normally provided by t o r o i d a l spring o r f i n g e r spring.
The a p p l i c a t i o n o f squeeze-type seals t o r o t a r y d u t i e s requires specialised a t t e n t i o n . Consultation w i t h the seal manufacturer i s recommended.
TABLE 14.7 STATIC SEALS
APPLICATIONS
AVAILABLE STANDARD MATERIALS
GASKETS AND JOINTINGS
The most widely used form o f s t a t i c connection where external mechanical load i s available.
Elastomer, r e i n f o r c e d elastomer, proofed f a b r i c , elastomer bonded cork, compressed asbestos f i b r e and a wide v a r i e t y o f nonelastomer based compositions.
A wide range o f s t a t i c s e a l i n g duties. Cylinder end caps, autoclave doors, pressure vessels, couplings. etc.
Elastomer. proofed f a b r i c , leather, Polyurethane and PTFE.
A l l s t a t i c sealing duties which w i l l permit the use o f the selected 0-seal m a t e r i a l and which a r e o f a design which w i l l contain t h i s m a t e r i a l w i t h i n the confines o f the seal recess a t operating pressure.
Elastomer and PTFE.
LIP TYPE U Ring (Also known as U-Packinal _. . _. .,I
from comnon o r separate source SQUEEZE TYPE
342 i n r e c i p r o c a t i n g a p p l i c a t i o n s passage o f t h e s h a f t through t h e g l a n d w i l l c a r r y t h e o i l f i l m t o t h e atmosphere s i d e o f t h e s e a l .
ROD EMERGINGthick o i l film
Fig.2
ROD RETRACTINGseal energised p r e v e n t i n g t o t a l f i l m iturn
Cond t i o n o f Maximum C o l l e c t e d 0 1 F i l m .
F i g u r e 2 i s a diagrammatic r e p r e s e n t a t i o n o f t h e w o r s t f i l m t r a n s p o r t s i t u a t i o n i n v o l v i n g an emerging c y l i n d e r r o d w i t h s l a c k p r e s s u r e t o t h e g l a n d which r e t r a c t s w i t h t h e s e a l i n g element under l o a d .
I n t h i s mode t h e h e e l o f t h e s e a l
w i l l be e n e r g i s e d and can p r e v e n t t h e r e t u r n o f t h e t o t a l q u a n t i t y o f f i l m c a r r i e d by t h e rod.
The f i t t i n g o f an e f f e c t i v e w i p e r can a g g r a v a t e t h e c o n d i -
tion. To combat t h e emergent f i l m , c a r e f u l a t t e n t i o n i s s e a l geometry.
required t o pressure side
Designs such as t h a t shown i n F i g . 3 w i t h k n i f e - c u t s e a l i n g
edges and a s p e c i f i c r e l a t i o n s h i p between c o n t a c t edge and groove h e i g h t s have proved v e r y s u c c e s s f u l d e s p i t e t h e i r s h o r t a x i a l l e n g t h .
Fig.3
Minimum F i l m
-
Shallow Design
E q u a l l y e f f i c i e n t and h a v i n g t h e advantage o f more t h a n one s e a l i n g edge i s t h e concept shown i n F i g . 4 w h i c h i s a m a r r i a g e o f l i p and squeeze s e a l i n g principles.
343 STANDARD FABRIC CHEVRON GLAND RING
-
F A B R I C CHEVRON I N T E W l E D I A T E RING SPECIAL SYNTHETIC RUBBER HEADER RING
Fig.4
Minimum F i l m
-
M u l t i - L i p Design
F i g u r e 5 i l l u s t r a t e s t y p i c a l improvement i n performance a g a i n s t standard V - r i n g packing under o f f s e t l o a d c o n d i t i o n s and shows i t s a d a p t a b i l i t y even i n s p l i t form. LEAKAGE - cc/hr COMPARISON OF F A B R I C MULTI-LIP PACKING VERSUS S P L I T AN0 ENDLESS L O F I L M CHEVRON WHEN SUBJECTED TO EXAGGERATED ROD KNUCKLING S I Z E : 44.5mm x 60.3mm
(12" x 2:")
4-
GLAND BUSH CLEARANCE 0.5mm(.D20") TEST DURATION 500 HOURS
OIAMETRAL
( 2 ) S P L I T L O F I L M CHEVRON PACKIHG ( 3 1 ENDLESS L O F I L M CHEVRON PACKING
Fig.5
14.4.2
Comparative Seal Performance
Surface F i n i s h
I t i s n o t o n l y t h e numerical v a l u e o f s u r f a c e roughness which i s r e l e v a n t t o seal performance b u t a l s o t h e manner
i n which such a f i n i s h i s achieved.
I n t h e c o n t e x t o f f l u i d power equipment, honed o r r o l l e r b u r n i s h e d b a r r e l f i n i s h e s a r e recommended w i t h an average v a l u e o f between 0 . 4 t o 0.8 p m Ra. L a r g e r c y l i n d e r s should be f i n i s h e d t o 0.8
urn
Ra o r b e t t e r , i f p o s s i b l e , b u t i n
any e v e n t s h o u l d be no worse than 1.6 urn Ra. Rods should i n a l l circumstances be o f a s u r f a c e roughness 0.8 p m Ra o r better.
Many o f t h e s m a l l e r s i z e s w i l l be a v a i l a b l e from p r o p r i e t a r y r o d sup-
p l i e r s t o a s t a n d a r d f i n i s h o f 0.2 pm Ra. i a l , f u n c t i o n , and
All static
Type o f f i n i s h w i l l depend on mater-
t h e d e s i g n e r ' s e x p e r i e n c e o f s i m i l a r equipment.
housing areas may be f i n i s h e d i n t h e range 0.8 t o 1.6 um Ra f o r
p r o o f e d f a b r i c packings.and, p r e f e r a b l y , 0.4 t o 0.8 pm Ra For s m a l l e r h o u s i n g s a p p r o p r i a t e t o '0' r i n g s , e t c .
344 Any l o n g i t u d i n a l marking on rods o r c y l i n d e r s w i l l promote leakage and manuf a c t u r i n g processes posing such r i s k s , e.g. as-drawn tube, r e t r a c t i o n marks on r o l l e r - b u r n i s h i n g heads, should be most c a r e f u l l y monitored. Equally, the achievement o f t o o f i n e a f i n i s h 0.05 I.cm Ra seal.
-
-
perhaps i n t h e area 0.01
-
can prevent the establishment o f a coherent f l u i d f i l m under the
An e r r a t i c performance can f r e q u e n t l y f o l l o w and packing l i f e can be cur-
tailed.
Several p r a c t i c a l
cases have been demonstrated where the d e l i b e r a t e
i n t r o d u c t i o n o f a coarser f i n i s h has restored an adequate q u a l i t y o f sealing. I n any event, on most h y d r a u l i c i n s t a l l a t i o n s the advantages t o be gained by improving f i n i s h e s below 0.2/0.3 pm Ra a r e d i s p r o p o r t i o n a t e l y c o s t l y w i t h o u t o f f e r i n g t a n g i b l e performance improvement. 14.4.3
Seal F r i c t i o n
I n e s t i m a t i n g seal drag loads f o r c r i t i c a l a p p l i c a t i o n s , designers a r e faced w i t h t h e p r a c t i c a l problem o f f i n d i n g even a general idea o f a c t u a l values from seal manufacturers. There a r e r e a l d i f f i c u l t i e s i n e x t r a p o l a t i n g r e s u l t s based on l a b o r a t o r y equipment and applying values t o much l a r g e r p l a n t .
Equally, type o f l u b r i c a -
t i o n , choice o f packing, and degree o f l i p i n t e r f e r e n c e , whether as-moulded o r as a r e s u l t o f compression, a r e a l l q u a n t i t i e s which w i l l i n f l u e n c e the f i n a l result. Work c a r r i e d o u t i n t h i s area w i t h a view t o g i v i n g a n o t i o n a l allowance f o r f r i c t i o n takes due account of seal l e n g t h and expresses seal drag F r i c t i o n load where
=
DL 25.8
x [142
+
(0.8
X
load as :
R)] k g f
R = f l u d pressure (bar)
D z sea L = sea
(2) (2)
contact diameter contact l e n g t h
I t i s o f t e n d e s i r a b l e t o add a contingency allownace o f 15%.
As a r e s u l t , i t
may be seen t h a t a m u l t i - l i p packing o f 1000 mm c o n t a c t diameter and 50 m deep would r e q u i r e 0.2% approx. o f t h e t h r u s t developed on t h i s f u l l area diameter a t a pressure o f 200 bar. Figure 6 makes the p o i n t t h a t any elastomeric and t h e r e f o r e f l e x i b l e seal c o n s t r u c t i o n w i l l s u f f e r an increase i n contact band w i d t h w i t h increasing system pressure.
Even i n the case o f r e l a t i v e l y tough, h i g h modulus m a t e r i a l s
such as proofed f a b r i c o r polyurethane, pressures o f 200 bar o r over may be s u f f i c i e n t t o promote t o t a l a x i a l contact o f t h e seal w i t h the dynamic wear s u r f ace.
345
LOW
HIGH PRESSURE
PRESSURE
I izsi I I I I
CONTACT BAND AREA
Fig.6
I 1
is:!
CONTACT BAND AREA
Seal Contact Band Width
Interposing a loaded PTFE sleeve is an effective means o f limiting seal drag Figure 7 shows
dramatically despite substantial hydraulic pressure increases.
o n e form o f the composite principle utilising a rectangular section elastomeric energiser.
LOADED PTFE SLEEVE
Fig.7
RUBBER ENERGISER
Energised PTFE Sleeve Seal
346 The o r d e r o f f r i c t i o n c o n t r o l a v a i l a b l e by comparison t o comparably t e s t e d
'0' r i n g s and ' U ' r i n g s i s g i v e n i n F i g . 8 .
500
-
COMPARISON OF MOVING F R I C T I O N LOAD AT VARYING F L U I D PRESSURES
u-
.? 400 v
0 < 0
A
z
300
0 I-
$
200
LL -I
5 VI
100 'C' 0
50
100
200
150
250
300
350
O I L PRESSURE (BAR)
Fig. 8
Comparative F r i c t i o n C h a r a c t e r i s t i c s
The p a te n t e d d e s i g n shown i n F i g. 9
combines f r i c t i o n c o n t r o l w i t h maximum
f l l m wiping a b i l i t y f o r the rod s i t u a t i o n .
Fig. 9 E n e r g i s e d PTFE S l e e v e Seal with P r o f i l e d F i l m Wiping Edge
14.4.4
Type o f F l u i d
As f a r as dynamic s e a l s a r e concerned, t h e v a s t m a j o r i t y o f mineral-based o r phosphate e s t e r f l u i d s do n o t p r e s e n t l u b r i c i t y problems. One o f t h e most demanding t ype s o f d u t y f o r any dynamic seal i n v o l v e s p l a i n wat e r a t h i g h p r e ssure whether by d e s i g n o r by t o t a l l o s s o f s o l u b l e o i l content
341 i n a n o m i n a l l y l u b r i c a t e d system.
Hi g h s p e e d s a r e
particularly d i f f i c u l t to
s u s t a i n u n l e s s a minimum s o l u b l e o i l l e v e l o f 2% i s guaranteed. T e s t s conducted on t h r e e - t h r o w m i n i n g pumps o p e r a t i n g a t 250 b a r , 0 . 6 m/s, have shown t h a t f o r even a 2% s o l u b l e o i l c o n t e n t , average seal l i f e w i l l be extended by a f a c t o r o f f o u r by comparison w i t h u n t r e a t e d w ater.
Equally, a
s e p a r a t e l u b r i c a n t feed w i l l produce s i m i l a r l y d r a m a t i c improvements. The t e m p e r a t u re o f h y d r a u l i c f l u i d sho ul d n o t exceed 60°C
i f a t a l l possible,
as s i g n i f i c a n t l y f a s t e r s w e l l and s o f t e n i n g o f p r o o f e d f a b r i c s and s t r a i g h t polymers w i l l o c c u r above t h i s v a l u e .
To i l l u s t r a t e t h e p o i n t , t h e f o l l o w i n g
d a t a i s based on immersion t e s t i n g f o r seven days i n She 1 T e l l u s 27 m i n e r a l o i l . N i t r i l e proofed f a b r i c
Hig h n i t r i l e e l ast ome r
Softening o f
-
room t e mpe rature
+
0 . 7 % volume i n c r e a s e
6OoC 9OoC
2.6%
1 2O0C
3.3%
- 6OoC
0.8%
90°C
2.3%
1 2O0C
3.8%
1.2%
moulded f a b r i c m a t e r i a l u s u a l l y reduces i n t r i n s i c s t r e n g t h and
w i l l normally d i m i n i s h s e r v i c e l i f e .
14.4.5
Filtration
For t h e b u l k o f e l ast ome r p r o o f e d f a b r i c and s o l i d elastomer s e a l s f l u i d f i l t r a t i o n o f 25 mi cro ns sho ul d be p e r f e c t l y adequate from t h e s e a l i n g p e r f o r mance a s p e c t .
S pe ci al s e a l s such as those embodying PTFE wear faces w i l l bene-
f i t f r o m 10 m ic r on f i l t r a t i o n or b e t t e r .
I n e i t h e r event, f i l t r a t i o n equipment
o f f e r i n g a f i n e r c u t - o f f w i l l p r o b a b l y be s p e c i f i e d t o s u i t c o n t r o l v a l v e funct i o n s on a g i v e n pre ss o r h y d r a u l i c component. The m a j o r i t y o f c o n v e n t i o n a l h y d r a u l i c s e a l s w hich a r e t e s t e d in-house work i n c o n j u n c t i o n w i t h no more t ha n a co arse w i r e s t r a i n e r i n any h y d r a u l i c system. However, modern f i l t r a t i o n a i d s must be con si dered an advantage, p a r t i c u l a r l y i f t h e o p e r a t i o n o f t h e p l a n t i n v o l v e s produces a g g r e s s i v e r e s i d u e s .
14.4.6
A i r E n trai n men t
As c y c l e speeds become f a s t e r u r e s must f l u c t u a t e more q u i c k l y .
due t o i n c r e a s i n g work demands, system pressI n many
must be exhausted i n m i l l i s e c o n d s , e.g. moulding presses, e t c .
situations,
f u l l w orking pressure
d i e - c a s t i n g machines, p l a s t i c s i n j e c t i o n
I f a i r i s e n t r a i n e d i n t h e h y d r a u l i c f l u i d such r a p i d
decompressions can be e xcee di n gl y dangerous i f no a u t o m a t i c v e n t i n g i s a v a i l a b l e .
I f one c o n s i d e r s t h a t i n 1O;l o f h y d r a u l i c o i l a t 200 bar and 10°C i t i s p o s s i b l e t o d i s s o l v e n e a r l y 200 1 o f a i r , some i n d i c a t i o n o f t h e magnitude of
348 r i s k w i l l be apparent. The main problem r e l a t e s t o p i s t o n head s i t u a t i o n s where f l u i d c o l l e c t s between two opposed seals.
L i p packings a r e l a r g e l y s e l f - v e n t i n g b u t heavy
i n t e r f e r e n c e polyurethane cup r i n g s a r e o f t e n suspect i n t h i s d i r e c t i o n as they do not r e a c t s u f f i c i e n t l y q u i c k l y . Under no circumstances should two squeeze type seals, e.g.
'0' r i n g s , be
employed on a p i s t o n head as the a i r entrainment contingency can be aggravated by a proven phenomenon known as i n t e r - s e a l pressure whereby t h r e e o r f o u r times system pressure can be b u i l t up i n t h e annular clearance between the seals. Extrusion o f such seals
into t h e
a p p l i e d pressure can be noted i n t y p i c a l cases.
PRESSURE
-
3 TO 4 P
P
P
F i g . 10
I n the gland s i t u a t i o n , t h e r i s k s a r e n o t o f the same degree unless t h e r e i s r e s t r i c t e d access o f working f l u i d t o the packing v i a a s i n g l e p o r t i n the neck bush o r some s i m i l a r feature.
Adequate f l u i d access t o a l l seals should be
assured unless d e l i b e r a t e attempts a r e being made t o reduce t h e a c t u a l pressure a t t h i s p o i n t o r t o dampen pressure v a r i a t i o n s .
14.5
SELECTION
As so many seal designs w i l l apparently meet given c o n d i t i o n s and y e t be s i g n i f i c a n t l y d i f f e r e n t i n m a t e r i a l , s i z e , and p r i c e , the f l u i d power equipment designer can be f o r g i v e n f o r being confused. Table 14.8 i l l u s t r a t e s t h e v a r i a t i o n s t h a t could apply t o a given a c t u a t o r o f f i x e d rod, b a r r e l and s t r o k e dimensions and r e f l e c t s a survey made several years ago when eighteen quotations
were sought f o r a
double cushioning and a s t r o k e l e n g t h o f 18". v a r i e d between €175 and €24.50,
3"
dia. cylinder with
P r i c e s received a t t h a t time
y e t a l l purported t o do the same j o b .
Much w i l l depend on t h e u s e r ' s own experience and preference based on knowledge o f the a p p l i c a t i o n . consequence o f leakage,
his
Schedule maintenance periods, a c c e s s i b i l i t y ,
i n i t i a l cost and a v a i l a b i l i t y w i l l a l l p l a y t h e i r p a r t
i n s t e e r i n g t h e d e c i s i o n towards u l t i m a t e s e c u r i t y o r some o t h e r l e v e l o f costeffectiveness.
349
TABLE 14.8 UPPER
LOWER
UPPER
LOWER
UPPER
LOWER
350 I t i s s i g n i f i c a n t t h a t B r i t i s h S t e e l C o r p o r a t i o n a r e now s e t t i n g t h e i r own standards f o r c r i t i c a l c y l i n d e r d u t i e s and w i l l use m u l t i - l i p packing throughout. Such a s o l u t i o n would be e n t i r e l y u na ccep t a ble t o t h e manufacturer o f e a r t h moving
v e h i c l e s who would seek a more economic s o l u t i o n i n terms o f c y l i n d e r
s i z e and s e a l c o s t . Most s e a l m a nu f a ct u rers w i l l e r r on t h e s i d e o f c a u t i o n i f
t h e y a r e advised
o f in te n d e d s e r v i c e c o n d i t i o n s and i f i n doubt t h i s i s t h e p r e f e r r e d r o u t e t h a t t h e d e s ig n e r s h o u l d t a ke. Equally,
h a v i n g made h i s c h o i c e and b e i n g c o n f r o n t e d w i t h a range o f o v e r a l l
s i z e s f o r a g i v e n di a met er, t h e l a r g e s t se al s e c t i o n a v a i l a b l e should be taken n o t the smallest
-
l i p o r squeeze
-
o t h e r re qu i reme nt s a l l o w i n g .
The s m a l l e r t h e seal
-
-
whether
t h e f i n e r i s t h e wo rki ng t o l e r a n c e band o f i n t e r f e r e n c e and t h e
lower i s t h e c a p a b i l i t y f o r a bso rbi n g mi sal i g nment, v i b r a t i o n , adverse accumulat i v e tolerances,
etc.
This assertion applies equally
t o s t a t i c and dynamic
positions.
14.6
STORAGE BS 3754:1963
' S t o r a g e o f Vu l can i sed Rubber' was prepared under t h e a u t h o r i t y
o f t h e Rubber I n d u s t r y Standards Committee and i n c l u d e s t h e f o l l o w i n g recommend a t ions: "Most v u l c a n i s e d rub be rs change i n p h y s i c a l p r o p e r t i e s d u r i n g s t o r a g e and u l t i m a t e l y may become u n s e r v i c e a b l e , hardening,
f o r example, because o f excessive
softening, cracking, c r a z i n g o r o t h e r surface degradation.
These changes may be t h e r e s u l t of one p a r t i c u l a r f a c t o r o r a combination o f f a c t o r s , namely, t h e a c t i o n o f oxygen, ozone,
l i g h t , heat and humidity."
The d e l e t e r i o u s e f f e c t s o f t he se f a c t o r s may, however, be minimised by carefu
choice o f storage conditions.
14 6.1 14.6.1.1
Recommendations Temperature
The s t o r a g e t empe rat ure should be below 25OC and p r e f e r a b l y below 15OC.
At
temperatures exceeding 25OC c e r t a i n forms o f d e t e r i o r a t i o n may be a c c e l e r a t e d s u f f i c i e n t l y t o a f f e c t the u l t i m a t e service l i f e .
Sources o f heat i n s t o r a g e
rooms should be so a rran ge d t h a t t h e t emp era ture o f no s t o r e d a r t i c l e exceeds 25OC.
The e f f e c t s o f low t emp era t u re a r e n o t permanently
deleterious t o vul-
c a n is e d rubber a r t i c l e s b u t t h ey may become s t i f f e r i f s t o r e d a t low temperat u r e s and c a r e should be t a ken t o a v o i d d i s t o r t i n g them d u r i n g h a n d l i n g a t t h a t temperature.
When a r t i c l e s a r e t ake n f rom low temperature s t o r a g e f o r immediate
use t h e i r t e m p e r a t u re sh ou l d be r a i s e d t o a p p r o x i m a t e l y 3OoC throughout b e f o r e they a r e p u t i n t o s e r v i c e .
351
14.6.1.2
Hu m idi t y
M o i s t c o n d i t i o n s should be avo i de d; condensation does n o t
14.6.1.3
s t o r a g e c o n d i t i o n s should be such t h a t
o ccur.
Light
V u lc a n is e d r u b be r sh ou l d be p r o t e c t e d from l i g h t ,
i n p a r t i c u l a r d i r e c t sun-
l i g h t and s t r o n g a r t i f i c i a l l i g h t w i t h a h i g h u l t r a - v i o l e t c o n t e n t . a r t i c l e s a r e packed i n opaque c o n t a i n e r s , o f storage
14.6.1.4
Unless the
i t i s a d v i s a b l e t o cover any windows
rooms w i t h a r e d o r orange c o a t i n g o r screen. Oxygen and Ozone
Where p o s s i b l e , v u l c a n i s e d rubber sh ou l d be p r o t e c t e d from c i r c u l a t i n g a i r by wrapping, s t o r a g e i n a i r - t i g h t c o n t a i n e r s , o r o t h e r s u i t a b l e means;
this
p a r t i c u l a r l y a p p l i e s t o a r t i c l e s w i t h l a r g e s u r f a c e area t o volume r a t i o s , e.g. pro o fe d f a b r i c , c e l l u l a r ru bb er. As ozone i s p a r t i c u l a r l y d e l e t e r i o u s , s t o r a g e rooms should n o t c o n t a i n any equipment t h a t i s capable o f g e n e r a t i n g ozone,
such as mercury vapour lamps,
h i g h v o l t a g e e l e c t r i c a l equipment, e l e c t r i c motors, o r o t h e r equipment w hich may g i v e r i s e t o e l e c t r i c sparks o r s i l e n t e l e c t r i c a l discharges.
14.6.1.5
De f o r ma t i o n
V u lc a n is e d rubber should, wherever p o s s i b l e , be s t o r e d i n a r e l a x e d c o n d i t i o n f r e e from t e n s i o n , compression o r o t h e r d e f o r m a t i o n . Hig h q u a l i t y re qu i reme nt s f o r s t o r a g e and p e r i o d i c i n s p e c t i o n such as those s p e c i f i e d by th e Aero-Space
I n d u s t r y a r e o b t a i n a b l e from BS
2F.68:1963
"Recom-
mendations f o r t h e s t o r a g e and i n s p e c t i o n i n s t o r e o f v u l c a n i s e d rubber items".
14.7
ASSEMBLY
A lt h o u g h i n d i v i d u a l a p p l i c a t i o n s w i l l i n v o l v e p a r t i c u l a r f i t t i n g problems t h e r e a r e a number o f b a s i c p o i n t s o f good p r a c t i c e which,
i f observed, w i l l
c o n t r i b u t e t o optimum sea l performance: (i)
Check t h a t sea l i s o f c o r r e c t t yp e, p a r t number or s i z e and m a t e r i a l .
(ii)
Ensure t h a t se al i s i n undamaged c o n d i t i o n and clean.
( i i i ) Where p e r m i s s i b l e , smear t h e s e a l i n g edge o f dynamic s e a l s w i t h clean grease.
Con sul t any f i t t i n g i n s t r u c t i o n l a b e l p r o v i d e d by t h e manufac-
t u r e r t o a s c e r t a i n whether f u r t h e r grease a p p l i c a t i o n t o i n t e r - s e a l c a v i t i e s e t c . i s recommended. (iv)
Do n o t t r e a t f l a t g aske t su rf a ces w i t h any form o f j o i n t i n g p a s t e o r lubr i c a n t u n less i n s t r u c t e d so t o do, o t h e r w i s e t h e a b i l i t y o f the gasket t o g r i p t h e a d j a c e n t s e a l i n g faces may be impaired.
352 (v)
Clean a1
se al ho usi n g o r gasket s e a t i n g areas.
Check t h a t o t h e r s u r f a c e s
a d j a c e n t t o t h e passage o f t h e sea l on f i t t i n g a r e a l s o f r e e o f d i r t , s wa r f or o t h e r contaminants. (vi)
Check s e a l h ou si ng dimensions and s u r f a c e f i n i s h t o design recommendations.
( v i i ) I f a s e a l i s l i k e l y t o c o n t a c t t hre ad s, sharp
corners, p o r t s , c i r c l i p s o r
s i m i l a r c o n t i n g e n c i e s d u r i n g t h e assembly o p e r a t i o n then s u i t a b l e f i t t i n g a i d s must be pro vi de d.
The s l i g h t e s t n i c k o r t e a r on a c r i t i c a l edge o f
an e l a s t o m e r i c component w i l l reduce s e a l i n g i n t e g r i t y .
I f f r e q u e n t use
i s envisaged, n o n - m e t a l l i c f i t t i n g sl e eves can be o f advantage s i n c e damage t o a s i m i l a r m e t a l l i c d e v i c e can d u p l i c a t e t h e hazard t o t h e s e a l . ( v i i i ) D o n o t le a ve se al i n p a r t ' a s s e m b l y f o r any l e n g t h o f time i f s e a l i n g edges a r e s u b j e c t t o m i s a l i g n e d loads; f o r example rod seal f i t t e d t o c y l i n d e r w i t h r o d i n p o s i t i o n b u t no g l a n d bush f i t t e d . (ix)
I f a p p r o p r i a t e , a p p l y any post-assembly o p e r a t i o n recommended by s e a l manu f a c t u r e r , f o r example compression o f t h e p r e s c r i b e d amount i n an a d j u s t a b l e g la n d housing; r o t a r y d u t y; and so on.
run ni n g a t h a l f - s p e e d t o a s s i s t seal b e d d i n g - i n on a
f o l l o w i n g up f l a n g e b o l t s a f t e r a p e r i o d a t temperature,.
.. .
353
15
SEALS FOR FLUID POWER EQUIPMENT PART TWO ROTARY SHAFT LIP SEALS
E.D.HALLIGAN,
CEng, MIMechE, A M P R I . , T e c h n i c a l Manager (Product A p p l i c a t i o n s ) James Walker G Co.Ltd.
15.1
INTRODUCTION
For t h e purpose o f s e a l i n g l u b r i c a n t w i t h i n a b e a r i n g o r e x c l u d i n g f o r e i g n m a t t e r from b e a r i n g surfaces, t y p i c a l l y i n Fig.1,
t h e s p r i n g - l o a d e d r o t a r y s h a f t l i p s e a l , as shown
i s w i d e l y accepted.
The n a t u r e o f most designs p r e c l u d e s
t h e use o f t h i s t y p e o f seal f r o m o p e r a t i n g a t s i g n i f i c a n t p r e s s u r e s u n l e s s t h e s e a l i n g l i p i s adequately supported by a shaped p l a t e . much i n excess o f 2'.0
-
For p r e s s u r e c o n d i t i o n s
3.0 b a r , combined w i t h r o t a r y movement i t would be p r e -
f e r a b l e t o c o n s i d e r e i t h e r a compression p a c k i n g o r a r a d i a l f a c e mechanical s e a l as f i r s t c h o i c e .
Fig.1
Standard f a b r i c back s e a l w i t h r e t a i n i n g p l a t e .
3 54 DESIGN
15.2
A wide v a r i e t y o f l i p s e a l designs i s a v a i l a b l e i n terms o f o v e r a l l c o n s t r u c -
t i o n , m a t e r i a l , and l i p p r o f i l e .
Some a r e i n t e n d e d f o r f i t t i n g i n housings
which have no s e p a r a t e c o v e r - p l a t e and may be s u p p l i e d w i t h a r i g i d metal case t o which the s e a l i s bonded, t h e u n i t b e i n g a f o r c e - f i t
Fig.2
i n t h e housing,
(Fig.2).
T y p i c a l metal-encased s e a l .
Others have t h e advantage o f f l e x i b i l i t y by v i r t u e o f h a v i n g a p r o o f e d f a b r i c back ( F i g . 3 a ) w h i l s t r e q u i r i n g t h e p r o v i s i o n o f a r e t a i n i n g p l a ' t e t o n i p a x i a l l y the back o f t h e seal t o p r e v e n t leakage and o b v i a t e r o t a t i o n .
More recent dev-
elopments combine t h e m e r i t s o f f l e x i b i l i t y and s e l f - r e t e n t i o n
i n a metal-
supported a l l - r u b b e r
Fig.3a.
configuration (Fig.3b).
Standard f a b r i c back s e a l .
Fig.3b.
Moulded-in f l e x i b l e s t e e l band s e l f - r e t a i n i n g s e a l .
One o f the s e v e r a l advantages o f t h e f a b r i c back seal i s t h e f a c i l i t y w i t h which s p l i t s e a l s can be f i t t e d w i t h o u t r e d u c i n g performance which, cases,
i s o f t h e same o r d e r o f e f f i c i e n c y as an endless r i n g .
i n many
A rubber i n l a y
i s f r e q u e n t l y moulded i n t o t h e back o f such a seal t h r o u g h which the s p l i t i s effected.
The rubber abutment which r e s u l t s ensures good s e a l i n g across t h e
s p l i t portion. The p r o f i l e o f t h e l i p c o n t a c t area i s s u b j e c t e d t o each m a n u f a c t u r e r ' s design p h i l o s o p h y .
Some h a v e a knife-edge c o n t a c t band and r e l y on heavy as-
355 moulded l i p i n t e r f e r e n c e .
Others depend on t h e s p r i n g t e n s i o n t o u r g e t h e l i p
i n t o intimate shaft contact.
I n p r a c t i c e , a c a r e f u l balance o f i n t e r f e r e n c e ,
s p r i n g c h a r a c t e r i s t i c s and c o n t a c t band w i d t h must be a l l i e d t o knowledge o f the t y p e o f m a t e r i a l b e i n g used, b e a r i n g t ype, and c o n d i t i o n and o t h e r e n v i r o n mental c o n s i d e r a t i o n s such as t emp era t u re, f l u i d and,
i f any, pressure.
A t y p i c a l range o f se al p r o f i l e s i s shown i n F i g . 4 w hich a l s o i l l u s t r a t e s exp e d ie n t used when i n s u f f i c i e n t space i s a v a i l a b l e t o accommodate a p a i r o f seals,
i.e.
t h e s o - c a l l e d d u s t l i p de si gn .
Fig.4
15.3
T y p i c a l se al p r o f i l e s .
MATERIAL
N i t r i l e - b a s e s y n t h e t i c rubber compounds a r e w i d e l y employed as standard mate r i a l s by reason o f t h e i r c o m p a t i b i l i t y w i t h most l u b r i c a t i n g o i l s and greases and t h e i r r e s i s t a n c e t o wear.
There a r e , however, c e r t a i n r o l l i n g o i l s and
c o o l a n t s wh ic h may cause e x c e s s i v e s w e l l o r s h r i n k a g e o f such polymers and a f l u o r o c a r b o n r u b be r,
e.g.
V i t o n , may be a necessary s e l e c t i o n .
This i s p a r t i -
c u l a r l y t r u e o f some palm o i l s o l u t i o n s and o t h e r s o l u b l e types o f o i l . C o n s i d e r a t i o n f o r o p e r a t i o n a l t emp era t u re i s important and, i t i s t h e c o n d i t i o n a t t h e l i p o f t h e s e a l which i s paramount
t h e e n v ir o n m e n t a l t e mpe rat ure .
However,
-
i n t h i s respect, not necessarily
i t may be s a i d t h a t n i t r i l e s a r e f r e -
q u e n t l y used w i t h success f o r i n t e r m i t t e n t s e r v i c e temperatures o f 15OoC and a r e c o n t i n u o u s l y r a t e d f o r 120°C
i n lubricated conditions.
Above these values
e i t h e r a c r y l i c r ubb ers, f l u o r o c a r b o n o r s i l i c o n compounds would r e q u i r e i n v e s t i gation. Many
s i l i c o n e and n i t r i l e rubber s h a f t s e a l s a r e f i t t e d i n automotive a p p l i -
c a t i o n s f o r c r a n k s h a f t and gearbox s e a l i n g ,
i n which areas much use i s b e i n g
made o f grooved s e a l i n g s u r f a c e s which a r e designed t o produce a hydrodynamic
356 e f f e c t i n the
oil film
b e i n g sealed, e f f e c t i v e l y causing t h e o i l t o be pumped
away from t h e s eal l i p (see F i g . 5 ) .
F ig . 5
Metal i n s e r t se al w i t h hydrodynamic a i d .
D e t a i l e d m a t e r i a l recommendations a r e g i v e n i n Appendix 1 .
15.4
SEAL LUBRICATION
I n common w i t h most o t h e r forms o f dynamic s e a l , depend f o r t h e i r e f f i c i e n t and c o n s i s t e n t stable f l u i d f i l m .
r o t a r y s h a f t l i p seals
performance upon t h e presence o f a
The e s t a b l i s h m e n t o f t h i s f i l m , w hich w i l l t y p i c a l l y be i n
t h e o r d e r o f a few mi cro ns t hi ckn ess, may n o t o c c u r immediately, and i t i s n o t uncommon f o r a h i g h pe rcen t a ge o f wear t o occur on r o t a r y s h a f t seal l i p s during t h i s p e r i o d b e f o r e s t e a d y - s t a t e c o n d i t i o n s a r e achieved.
For t h i s reason, i t
i s always good p r a c t i c e t o a p p l y a l u b r i c a n t t o t h e seal l i p and i n t h e seal c a v i t y before f i t t i n g . Where s i n g l e s e a l s a r e housed a t each end o f t h e housing, t h e r e i s n o r m a l l y s u f f i c i e n t b e a r i n g l u b r i c a n t i n c o n t a c t t o p r o v i d e adequate l u b r i c a t i o n . Again, where two s e a l s a r e housed t o g e t h e r ,
i t i s o f t e n found t h a t w i t h b e a r i n g l u b r i -
c a n t on one s e a l and e i t h e r r o l l i n g f l u i d o r r o l l c o o l a n t on t h e o t h e r , f u r t h e r l u b r i c a t i o n i s unnecessary.
I n a l l cases t h e l i b e r a l a p p l i c a t i o n o f grease t o
t h e s e a l s on assembly w i l l ensure l u b r i c a t i o n from t h e b e g i n n i n g and,
i n some
i ns t a n c e s , t h i s w i l l be found s u f f i c i e n t t o l a s t from one f i t t i n g t o t h e n e x t . However, t h e r e a r e many s e a l i n g arrangements, e s p e c i a l l y on r o l l i n g m i l l be a r in g s , and
i n which two o r more s e a l s a r e f i t t e d t o g e t h e r i n t h e same housing
t h e r e i s th e danger t h a t a t l e a s t one w i l l r u n d r y u n l e s s l u b r i c a n t i s
s u p p l i e d from an e x t e r n a l source.
T h i s can b e s t be accomplished by d r i l l i n g a
h o l e t h r o u g h t h e chock c o n n e c t i n g w i t h an a n n u l a r groove i n t h e back o f t h e s e a l housing, as shown i n F i g. 6,
and u s i n g a s p e c i a l t y p e o f seal which has a
s e r i e s o f r a d i a l p o r t s i n t h e base f o r passage o f l u b r i c a n t t o t h e seal l i p . When i t i s d i f f i c u l t o r i m p o s s i b l e t o machine an annular groove i n t h e back o f t h e s e a l housing, shown i n F i g . 7 .
t h i s groove may be i n c o r p o r a t e d i n t h e s e a l s themselves, as
357
Fig.6
Seal l u b r i c a t i o n v i a i n s e a l base.
ports
Fig.7
Seal l u b r i c a t i o n v i a p o r t s and a n n u l a r groove i n seal base.
I n g e n e r a l , a good q u a l i t y m i n e r a l o i l o r grease i s s u i t a b l e f o r seal l u b r i c a t i o n , b u t molybdenised l u b r i c a n t s may be used t o advantage where a p p l i c a t i o n i s infrequent.
Care sh ou l d be taken t o ensure t h a t t h e grease o r l u b r i c a n t w i t h
which t h e s e a l
i s t o come i n t o c o n t a c t i s co mpatible.
are being
used w i t h d i u t e m i n e r a l a c i d s , b u t y l
For example, where s e a l s
compositions may w e l l be used.
T h i s m a t e r i a l i s l i k e l y t o s w e l l i n c o n t a c t w i t h m i n e r a l o i l o r grease and an a l t e r n a t i v e l u b r i c a n t w 1 1 be e s s e n t i a l .
15.5
SHAFT SURFACES
The s e a l i n g a re a o f t h e s h a f t sh ou l d be a f i n e ground f i n i s h o f 0.4 t o 0.8 pm (16
-
32 p i n . ) CLA or Ra f o r most a p p l i c a t i o n s , b u t f o r t h e h i g h e r speed range
i t i s recommended t h a t t h e s u r f a c e f i n i s h be improved t o 0.2
CLA o r Ra.
t o 0 . h pm ( 8 - 1 6 p I n )
I n a l l cases i t i s i m p o r t a n t t h a t t h e s h a f t s e a l i n g area be f r e e
from machining marks, de nt s, b u r r s and scra t ches. Where f l u o r o c a r b o n o r a c r y l i c
rubber s e a l s a r e b e i n g employed,
i t i s also
a d v i s a b l e t o use t h e f i n e r l e v e l o f f i n i s h i n d i c a t e d above i n o r d e r t o e l i m i n a t e p i c k - u p o f t h e s ea l m a t e r i a l .
I f l u b r i c a t i o n i s adequate and f r e e f rom a b r a s i v e c o n t a c t , unhardened m i l d s t e e l s h a f t s w i l l g e n e r a l l y g i v e s a t i s f a c t o r y r e s u l t s under normal o p e r a t i n g conditions.
However, a h a r d e r s h a f t m a t e r i a l i s t o be p r e f e r r e d f o r a p p l i c a t i o n s
where l u b r i c a t i o n i s poor, a b r a s i v e s a r e pre sent, o r speed and p r e s s u r e condit i o n s a r e p a r t i c u l a r l y arduous.
A f i n e machined f i n i s h i s s u i t a b l e f o r t h e housing bore.
358
15.6
FRICTION
The r u b b i n g f r i c t i o n o f t h e seal l i p on t h e s h a f t m a t e r i a l i n e v i t a b l y causes a h i g h e r l o c a l temperature than i s p r e s e n t i n t h e f l u i d b e i n g s e a l e d . e f f e c t w i l l be
This
due t o t h e i n t e r f e r e n c e o f t h e seal l i p on t h e s h a f t m a t e r i a l
and may be aggravated by inadequate l u b r i c a t i o n o r speeds i n excess o f t h a t f o r which a
p a r t i c u l a r seal was designed.
Assuming t h e system temperature t o be
w e l l w i t h i n t h e bounds o f t h e m a t e r i a l c a p a b i l i t y , a h i g h d i f f e r e n t i a l between l i p t i p temperature and f l u i d temperature w i l l be m a n i f e s t e d i n many cases by h a i r - l i n e scores i n t h e seal l i p c o - a x i a l w i t h t h e s h a f t .
Local c a r b o n i s a t i o n
o f t h e rubber compound w i l l a l s o be e v i d e n t . F i g u r e 8 i n d i c a t e s a s e t o f curves d e r i v e d by experiment, p r o j e c t i n g f r i c t i o n a l h o r s e power a g a i n s t s e a l d i a m e t e r f o r a c o n v e n t i o n a l r o t a r y l i p seal design o p e r a t i n g a t 500 r.p.m.
i n m i n e r a l l u b r i c a t i n g o i l a t d i f f e r e n t system
pressures.
300
LOO
500
600
700
800
900
Shaft diameter [ m m l Fig.8
F r i c t i o n a l horse power absorbed by r o t a r y s h a f t l i p s s e a l s , having c o n v e n t i o n a l l i p i n t e r f e r e n c e o p e r a t i n g a t 500 r e v h i n
359
15.7
SPEEDS
Many o f t h e f a b r i c - b a c k e d t y p e o f s e a l s t o g e t h e r w i t h those o f t h e m e t a l supported t y p e a r e w o r k i n g s a t i s f a c t o r i l y on 400 mm r o l l - n e c k d i a m e t e r s a t speeds o f up t o 25 m/s (5000 f t / m i n ) o v e r s u s t a i n e d w o r k i n g p e r i o d s . however,
many f a c t o r s such as s u r f a c e f i n i s h ,
There are,
e c c e n t r i c i t y , and l u b r i c a t i o n
which can l i m i t t h e maximum speed f o r which any seal i s s u i t a b l e , and manufact u r e r s should be c o n s u l t e d i n cases o f doubt.
15.8
ECCENTRICITY
Where p l a i n metal o r s y n t h e t i c c o m p o s i t i o n b e a r i n g s a r e b e i n g employed,
it
i s n o t uncommon t o f i n d t h a t t h e s h a f t i s n o t t r u l y c o n c e n t r i c w i t h t h e seal housing, owing t o b e a r i n g c l e a r a n c e and subsequent wear.
I n t h i s event,
it is
e s s e n t i a l f o r e f f i c i e n t s e a l i n g t h a t t h e seal l i p be capable o f f o l l o w i n g a l l s h a f t movement and,
indeed, on many l a r g e c o m p a r a t i v e l y slow-moving s h a f t s
e c c e n t r i c i t y values o f 2 . 5 mm have been s a t i s f a c t o r i l y accommodated.
Naturally,
t h e s e a l performance i n terms o f e c c e n t r i c i t y c a p a b i l i t y w i l l be speed and s h a f t diameter dependent. Where s p l i t s e a l s a r e f i t t e d , then p a r t i c u l a r c o n s i d e r a t i o n i s necessary t o t h e problem o f s h a f t e c c e n t r i c i t y , s i n c e t h e r e may be a tendency f o r a s p l i t seal t o open a t t h e j o i n .
15.9
PRESSURE
A l t h o u g h few r o t a r y s h a f t l i p s e a l s a r e spec f i c a l l y designed as s t a n d a r d compqnents t o accept s i g n i f i c a n t pressures, t h e use o f m e t a l - s u p p o r t i n g p l a t e s w i l l extend t h e u s e f u l n e s s o f t h i s t y p e o f s e a l
A t y p i c a l p r o f i l e i s shown ' i n
Fig.9.
Fig.9
Seal w i t h shaped s u p p o r t p l a t e f o r p r e s s u r e s up t o 3.0 b a r .
As a r e s u l t o f e x p e r i m e n t a l work on t h e s e a l i n g o f o i l - f i l l e d marine s t e r n glands
and manoeuvring t h r u s t e r s , a seal l i p p r o f i l e has been developed which
s a t i s f a c t o r i l y s u s t a i n s pressures up t o 4 . 0 b a r w i t h o u t t h e use o f a shaped support p l a t e .
The base must be f u l l y supported as i n d i c a t e d i n Fig.10.
360
Q Fig.10
New se al development f o r p ressures up t o 4.0 b a r w i t h o u t shaped sup po rt p l a t e .
Where c o n s t a n t a p p l i e d p r e s s u r e i s n o t a n t i c i p a t e d , ted t h a t g r e a s e - l u b r i c a t e d
i t i s f r e q u e n t l y sugges-
b e a r i n g s a r e equipped w i t h r e l i e f h o l e s and t h a t o i l -
l u b r i c a t e d b e a r i n g s have d r a i n s o f adequate s i z e .
D rains taken f r o m t h e ends
o f t h e b e a r i n g near t h e s e a l s w i l l h e l p t o d i s s i p a t e any l o c a l i s e d p r e s s u r e build - u p .
Where p o s s i b l e , st e ps sho ul d be t aken i n b e a r i n g d e s i g n t o p r e v e n t
escaping h i g h p r e s s u r e o i l i mp i ng i ng d i r e c t l y upon t h e s e a l s . I n some cases where t h e loss o f a sma l l amount o f grease i s unimportant,
a
simple r e l i e f system may be formed by f a c i n g s e a l s away f r o m t h e b e a r i n g and a l l o w i n g t h e s e a l l i p t o be l i f t e d under t h e i n f l u e n c e o f t h e l u b r i c a n t pressure.
15.10
CARE AND HANDLING
F l u i d s e a l s s ho ul d be handled a t a l l t i me s w i t h extreme c a r e s i n c e t h e l i f e o f b e a r in g s o r o t h e r c o s t l y machine p a r t s may depend upon t h e i r e f f i c i e n c y . A t t e n t i o n t o the f o l l o w i n g v i t a l p o i n t s w i l l a s s i s t i n ensuring t r o u b l e - f r e e operation during service.
15.10.1 (i)
Storage The s t o r e
sh ou l d have a c o o l , c l e a n and d r y atmosphere,
f r e e from
d u s t and g r i t . (ii)
Whenever p o s s i b l e , s e a l s should n o t be removed from t h e wrapping i n which they were s u p p l i e d , as t h i s p r o v i d e s p r o t e c t i o n and i d e n t i f i c a t i o n
( i i i ) Avoid u n t i d y s t a c k i n g as t h e w e i g h t may d i s t o r t t h e s e a l s a t t h e bottom o f the stack. (iv)
Seals should never be t h rea de d on w i r e o r s t r i n g as t h i s w i l l damage the l i p s .
361 15.10.2
(i)
H a n d lin g I t must be remembered t h a t seal l i p s a r e extremely v u l n e r a b l e t o damage
and t h e s m a l l e s t n i c k p r o v i d e s a p o t e n t i a l l e a k p a t h . (ii)
Seal r e i n f o r c i n g i n s e r t s , a l t h o u g h adequate f o r t h e i r d u t y , may deform under adverse h a n d l i n g o r s t a c k i n g .
( i i i ) Seals h avi n g met al o u t s i d e su rf a ces may damage o t h e r s e a l s , e s p e c i a l l y
i f the me t a l edges c o n t a c t t he ru bb er p a r t s o f n e i g h b o u r i n g seals. 15.10.3
Fitting
A h i g h p r o p o r t i o n o f f a i l u r e s and leakage o f o i l s e a l s i s due t o i n c o r r e c t f i t t i n g r e s u l t i n g i n damage t o b o t h se al and s e a l i n g s u r f a c e .
S t r i c t attention
t o th e f o l l o w i n g m a t t e r s i s e s s e n t i a l i f b e s t performance i s t o be obtained. (i)
Before f i t t i n g ,
t h e se al should be examined t o ensure t h a t i t i s clean
and undamaged. (ii)
The s e a l i n g l i p sho ul d be smeared w i t h s u i t a b l e c l e a n l u b r i c a n t . Seals used as d u s t e x c l u d e r s should be packed w i t h a compatible grease.
( i i i ) The s e a l i n g l i p , n o r m a l l y , sho ul d f a c e t h e f l u i d t o be sealed. When f i t t i n g ,
i t i s i m p o r t a n t t o ensure t h a t t h e s e a l i n g l i p i s n o t
damaged even by
the s l i g h t e s t n i c k , t h a t the s p r i n g i s c o r r e c t l y
l o c a t e d when i n p o s i t i o n , and t h a t t h e seal i s p r o p e r l y pressed home i n t o t h e ho usi n g rece ss. Examine t h e s h a f t which should be f r e e from a l l roughness and sharp edges and a v o i d pa ssi ng t h e s e a l i n g l i p over keyways, screw threads, o r shoulders.
S h a f t edges o r sho ul d ers should be w e l l rounded o r
chamfered, and where t h i s i s n o t p r a c t i c a b l e a f i t t i n g sleeve s l i g h t l y s h a f t w i t h a l e a d - i n t a p e r should be used.
l a r g e r t h an t h e
A c c o r d ing t o t h e t yp e o f assembly,
i t may be necessary e i t h e r t o f i r s t
p r e s s t h e se al i n t o t h e ho usi n g and subsequently o n t o t h e s h a f t o r , alternatively,
t o pass t he se al ove r t h e s h a f t and then press i t i n t o
t h e h o u si ng . I t i s p r e f e r a b l e f i r s t t o mount t h e seal on t h e s h a f t where circumstances p e r m i t , s i n c e t h i s a l l o w s o b s e r v a t i o n o f t h e l i p d u r i n g assembly. ( v i i ) The assembly sh ou l d n o t be al l o we d t o r e s t f o r any l e n g t h o f t i m e a t an in c o mpl e t e s t a t e o f f i t t i n g , where the w eight o f t h e s h a f t o r housing may be borne by t h e s e a l ,
r e s u l t i n g i n damage o r d i s t o r t i o n t o the
latter. ( v i i i ) W h e n p r e s s i n g t h e sea l i n t o t h e housing, a u n i f o r m p r e s s u r e should be e x e r t e d , p r e f e r a b l y by suitable tool.
means o f an a r b o r press i n combination w i t h a
The di a met er o f t h e t o o l should be s l i g h t l y s m a l l e r
than the diameter o f t h e h ou si ng by 0.1
t o 0.4 mm.
The o u t s i d e surface
362
of the seal can be smeared with a suitable lubricant in order to facilitate fitting. Care must be taken to ensure that the seal does not enter the housing recess in a tilted position, since this will cause damage to the outer surface.
SERVICE PROBLEMS AND THEIR SOLUTIONS
15.11
A number of fault-finding procedures follow which, if taken in sequence, should analyse the reason for a given difficulty with rotary shaft lip seals.
15.11.1
Unacceptable Leakage
This is almost always associated with oil lubricated bearings, since grease is not
a
difficult lubricant to seal. The term "unacceptable" can have wide
interpretation since an occasional drop of oil might be disastrous if it resulted in contamination of the product being handled by the machine concerned in such spheres as the textile, paper, or food industries, whereas it would probably remain unnoticed in a heavy industrial environment. Since the vast increase in
oil prices there is, however, a greater sensiti-
vity to oil losses in any form and leakage rates that hitherto have been ignored are now becoming regarded as unacceptable. When dealing with such complaints it is essential to discover the history of equipment concerned, and this broadly
falls into three categories:
(I)
New equipment recently commissioned, where sealing has been regarded
(11)
as unsatisfactory from the start. Equipment that has been in operation for a period of time and only recently has developed leakage problems.
( I l l ) Equipment that was satisfactory
during its first term of operation but
leakage has occurred after fitting replacement seals during routine maintenance or overhaul. Since trouble tracing is basically a process of eliminating of substantiating faults, the sequence of checks required would vary with each of the above
categories. In order to simp1 ify the procedures and'avoid
irrelevant investigations, the
recommended sequences for each of the above categories is defined by letter symbols to be used in conjunction with the attached fault-finding chart. Category ( ) Full checks in order as A, B, C ,
D and
E until fault is discovered.
Category ( I 1 A, B, C ( 1 ) ,
D ( 2 ) , and D ( 3 ) .
If faults as C ( 1 ) o r D(2) are exhibited, ascer-
tain Deriod o f service with seals. This should be calculated in terms of hours
363 o f r u n n i n g and r e l a t e d t o speed, t e m p e r a t u r e , and o t h e r e n v i r o n m e n t a l c o n d i t i o n s
A moderate speed w i t h good c l e a n l u b r i c a t i o n c o n d i t i o n s and ambient temperat u r e s would n o r m a l l y a n t i c i p a t e a seal
l i f e o f around 10,000 h o u r s .
H i g h speeds, p o o r l u b r i c a t i o n , e l e v a t e d temperatures,
o r p a r t i a l l y abrasive
media c o u l d reduce t h i s t o as l i t t l e as 2,000 h o u r s . The p r o b l e m may t h e r e f o r e be
s i m p l y t h a t o f b e i n g due f o r s e a l replacement.
I f f a u l t s as i n (D3) a r e i n e v i d e n c e , o b t a i n d e t a i l s o f a l l media i n c o n t a c t w i t h seal
( i n c l u d i n g any c l e a n i n g f l u i d s ) as a change o f l i p m a t e r i a l may be
necessary t o o b t a i n c o m p a t a b i l i t y . I f s e a l s d i s p l a y no f a u l t s , check f o r mechanical d e f e c t s as E ( 1 ) and E ( 2 ) . Category A,
B, C(
F a u l t - f i n d i n g Chart
(A)
I s leakage a c t u a l l y o c c u r r i n g f r o m t h e s e a l o r does i t stem f r o m such
sources as b e a r i n g c o v e r f l a n g e s and i s m e r e l y " c o l l e c t e d " housing,
g i v i n g a f a l s e impression?
by t h e s e a l
Check by w i p i n g c l e a n a l l a p p r o p r i a t e
a r e a s and r u n machine t o a s c e r t a i n leakage s o u r c e . (B)
I s l e a k a g e f r o m around s e a l back o r f r o m t h e l i p a l o n g t h e s h a f t ?
wiping (C) I f O / D (1)
b o t h c l e a n and o b s e r v i n g w h i l e machine i s leakage
-
Check by
running.
check t h e f o l l o w i n g :
Is s e a l a good f i t i n h o u s i n g o r i s i t s l a c k ? (On s p l i t s e a l s a s l a c k s e a l w i l 1 d i s p l a y a gap between s e a l e n d s ) .
( 2 ) I f h o u s i n g b o r e i s c o r r e c t s i z e t h e n s e a l O / D dimension i s suspect i f s l a c k i n housing. ( 3 ) I f s e a l i s good f i t i n h o u s i n g , check f o r damage on h o u s i n g b o r e .
(4) Check
h o u s i n g d e p t h t o e n s u r e s e a l i s b e i n g a x i a l l y compressed - i f
applicable. (0 )
If
leakage a l o n g s h a f t , check t h e f o l l o w i n g :
( 1 ) S h a f t s i z e , s u r f a c e f i n i s h , s h a f t damage a t c o n t a c t a r e a . ( 2 ) If ( 1 ) O . K . , ing.
check c o n d i t i o n o f s e a l i n g l i p f o r h a r d e n i n g and/or c r a c k -
I f e i t h e r i n e v i d e n c e , t h e n speed o r t e m p e r a t u r e c o n d i t i o n s a r e
probably incompatible w i t h seal m a t e r i a l .
( 3 ) I f l i p i s s o f t o r s w o l l e n t h i s i s u s u a l l y an i n d i c a t i o n o f chemical i n c o m p a t i b i l i t y w i t h t h e media i n c o n t a c t w i t h t h e s e a l .
(4) I f
( 2 ) and ( 3 ) O.K.,
check s e c t i o n w i d t h o f s e a l w i t h s p r i n g f i t t e d .
364 T h i s s h o ul d be a t l e a s t nominal s e c t i o n +1% immediately on removal and i n c r e a s i n g t o nominal
+3% a f t e r
one hour i n f r e e s t a t e .
S p r i n g may be
shortened by up t o 5% o f i t s o r i g i n a l l e n g t h i f s e c t i o n appears inadequate. (E)
I f checks
S e c t i o n measurement sho ul d be average o f
4
e q u i - d i s t a n t readings.
(C) and (D) do n o t r e v e a l any f a u l t s t h e problem may be due t o
mechanical c o n d i t i o n s and t h e f o l l o w i n g should be checked:
( 1 ) S h a f t t o ho usi n g co nce nt r c i t y s h a f t and h ou si ng b o r e a t
4
-
check by means o f c a l i p e r s between
p o i n t s around p e r i p h e r y .
V a r i a t i o n s o f more
t h a n 0.3 mm r e q u i r e f u r t h e r i n v e s t i g a t i o n . (2) I f s m a l l e s t c a l i p e r measurement o c c u r s between bottom o f s h a f t and housing, t h i s may i n d i c a t e b e a r i n g wear w i t h r e s u l t a n t dynamic e c c e n t r i city.
(Applicable t o horizontal shafts only).
( 3 ) I f b e a r i n g i s O.K. t he n ho usi n g o f f s e t may be r e s p o n s i b l e .
Seal housings
a r e n o r m a l l y c e n t r a l i s e d w i t h t h e s h a f t by means o f a machined r e g i s t e r w i t h th e b e a r i n g h ou si ng .
Where t h i s f e a t u r e
i s n o t i n c o r p o r a t e d then
i t may-be p o s s i b l e t o c e n t r a l i s e t h e housing by s l a c k e n i n g t h e b o l t s and repos i t on i ng .
(4)
I f (11,
( 2 ) and (3) a r e blameless t he n t h e f o l l o w i n g r e q u i r e s i n v e s t i -
gation:
I f bear ngs a r e o i l p r e s s u r e l u b r i c a t e d i s t h e r e adequate drainage t o prevent pressure build-up against the seal?
I f equipment op era t e s on a c o n s t a n t o i l l e v e l p r i n c i p l e , wheels o r b a l l - j o u r n a l
a r e t h e r e gear-
b e a r i n g s i n c l o s e p r o x i m i t y t o t h e seal causing
o i l turbulence or flooding? Where
the l a t t e r s i t u a t i o n exists,
t h e housing lands should be o n l y
m a r g i n a l l y l a r g e r t ha n s h a f t d i ame t e r i n o r d e r t o form a b a f f l e o r , alternatively,
a b a f f l e p l a t e f i t t e d between b e a r i n g and seal housing.
A temporary remedy can be made by u s i n g a 2.5 mm CAF gasket a t t h e bottom o f
the housing, t h e I / D o f which sh ou l d be s h a f t diameter p l u s 0.5 nnn maximum.
A
f u r t h e r gasket o f equal t h i c k n e s s sh ou l d be f i t t e d underneath the r e t a i n i n g p l a t e t o restore the correct s e a l s (see Fig .1 1 and F i g . 1 2 ) .
amount o f a x i a l compression i n t h e case o f r e t a i n e d
365
Fig.11
Showing how t o o l a r g e b o r e d i a m e t e r o f s e a l h o u s i n g l a n d p e r m i t s h i g h v e l o c i t y o i l impingement on s e a l .
B a f f l e p l a t e between b e a r i n g reduce r i s k o f o i l leakage.
and s e a l o r c l o s e - f i t t i n g s e a l would
.A.F gasket .5mm thick
Fig.12
Showing how f i t t i n g o f C.A.F. g a s k e t s can p r o v i d e a temporary remedy f o r s i t u a t i o n i n F i g . 1 1 .
( W i t h acknowledgement t o t h e l a t e Mr.Ray B l a d w i n - S e n i o r A d v i s o r y E n g i n e e r , rotary shaft l i p seals
-
who c o m p i l e d much o f t h e d a t a p r e s e n t e d h e r e ) .
366 APPENDIX 1
ROTARY SHAFT L I P SEAL F(ATERIAL RECOMMENDATION CHARTS
The accompanying c h a r t s show v a r i o u s l u b r i c a n t s and r o l l i n g f l u i d s t h a t a r e commonly used i n t h e metal r o l l i n g i n d u s t r y .
Some o f these a r e n o t c h e m i c a l l y
c o m p a t i b l e w i t h t h e standard n i t r i l e rubbers used i n r o t a r y s h a f t l i p seal p r o d u c t i o n , and i t i s t h e r e f o r e necessary t o employ an a l t e r n a t i v e rubber compound t o achieve s a t i s f a c t o r y r e s u l t s i n terms o f s e a l e f f i c i e n c y and l i f e .
Seal L i p s The rubber compound i n t h e column headed " 1 s t choice"
i s t h e grade l e a s t
a f f e c t e d by t h e media concerned and should be s e l e c t e d wherever p o s s i b l e .
In
most i n s t a n c e s a second c h o i c e i s g i v e n and t h i s rubber can be used w i t h o u t serious detriment t o the seal. Where f l u o r o c a r b o n rubber (denoted by t h e l e t t e r 'A') appears as t h e o n l y c h o i c e , please n o t e t h a t none o f t h e c o n v e n t i o n a l commercial grade rubbers can be used as an a l t e r n a t i v e .
I n i n s t a n c e s where f l u o r o c a r b o n rubber appears as
t h e second c h o i c e i t i s u s u a l l y f o r economic reasons,
i n d i c a t i n g t h a t i t i s only
m a r g i n a l l y s u p e r i o r t o t h e commercial grade compound g i v e n as f i r s t c h o i c e , and t h e small advantage would o f t e n n o t j u s t i f y t h e a d d i t i o n a l c o s t . Seal Backs Although w i t h some media t h e seal back
f a b r i c m a t e r i a l s used f o r c o n s t r u c t i o n o f t h e
s u f f e r l i m i t e d v o l u m e t r i c change,
the effects are not detrimental t o
s e a l i n g e f f i c i e n c y as t h i s p a r t o f t h e s e a l i s f u l l y housed and n o r m a l l y a x i a l l y restrained. Selection o f Materials In r o l l i n g - m i l l applications,
r o t a r y s h a f t l i p seals a r e normally required t o
seal n o t o n l y a g a i n s t b e a r i n g l u b r i c a n t s b u t a l s o r o l l i n g f l u i d s ,
be taken t o s e l e c t a m a t e r i a l which i s c o m p a t i b l e w i t h both.
and c a r e should
T h i s may i n v o l v e
some compromise by s e l e c t i n g a second c h o i c e m a t e r i a l f o r e i t h e r t h e l u b r i c a n t o r the r o l l i n g f l u i d ,
i n o r d e r t o o b t a i n a reasonable degree o f c o m p a t i b i l i t y
w i t h both. I n d u a l s e a l i n g arrangements employing back-to-back
s e a l s , should o n l y one o f
t h e media n e c e s s i t a t e f l u o r o c a r b o n r u b b e r , t h e opposing s e a l c o u l d be made w i t h a l e s s c o s t l y c o m p a t i b l e grade o f rubber p r o v i d i n g t h a t c a r e i s taken i n i d e n t i f i c a t i o n and f i t t i n g . For convenience, rubbers, and
l e t t e r symbols a r e used i n t h e c h a r t s t o i n d i c a t e v a r i o u s
t h e key t o these i s g i v e n :
367 KEY TO MATERIAL CODES Code
Base r u b b e r compound
F I uorocarbon High n i t r i l e Medium h i g h n i t r i l e N i t r i le N i t r i l e with graphite N i t r i l e w i t h p a r a f f i n wax
RECOMMENDED SEAL LIP MATERIALS FOR USE WITH BEARING OILS AND GREASES Seal l i p m a t e r i a l s Brand Name o r Number 1 s t Choice
2nd Choice
B.P.
HCT 80
C
E
B.P.
Sperno 350 HB 'I 450 HB
C
D
B.P.
Energol GR 125-XP
C
0
E
C a s t r o l 98
C
CLP 1 1 4
C
E
C a l y s o l Grease
C
A
C
Duckhams Zero O i l
D
EP 69 O i l
C
D
EP 80 Gear L u b r i c a n t
D
C
Esso E s t i c 65
C
A
Esso Nuray 146
C
A
Esso Pen-0-Led
D
C
EP3
Fuchs MR 40
C
F
Mobi 1 Exu 66/25
C
D
C
F
C
E
C
E
" II
Vacuol i n e ' A A ' 'I 25 x 25R
I,
I'
Heavy
C
E
I'
Mobi l u b e H.D.90
C
E
"
Mobi l e x EP2 Grease
C
F
OM 100 O i l
C
A
Regal Gear O i l
C
B
S h e l l O i l 1624 and 1611
B
C
,I
II
I
EE"
"
Carnea
C
B
"
Faunus ' B '
B
A
I'
Macoma
D
C
"
Nassa 78
C
A
Seal l i p m a t e r i a l s f o r b e a r i n g o i l s and greases (contd.) Seal l i p m a t e r i a
S
Brand Name o r Number 1 s t Choice
2nd C Dice
C
A
Teressa
C
A
"
T i v e l a 75
C
D
"
V i t r e a 75
C
B
"
R e t i n o x Grease
A
C
She1 1 Telona 945 'I
'I
Donax 17
C
B
I'
Aeroshel 1 Grease No.7
C
A
Oil
C
D
Ucon
A l o - J idac
A
Caster O i l ( w i t h w a t e r ) I, I' ( w i t h methyl a t e d
A
B
C
E
C imcoo 1
E
A
Cirncool E 5
B
C
Castrol
A
B
B
A
spirit)
Coolage SL
Croda D4 A Lubrotex
'I
1;
7804
A
Dasco 900
B
A
Esso Somentor N35
C
A
I,
,I
'I
w20
A
6
I'
33 and N60
A
C
Univis ~ 5 8
C
B
Emulsion KF 81
A
-
Germ K i n e t r o l FR3A
A
-
G u l f Mineral Seal
C
D
I'
Cut O i l
C
D
"
93
C
D
B
A
I'
Houghton Permasol
k
B
H u i l e de Laminage 102 P & 982 R
B
C
Hydr i t
B
D
KF 61 R o l l i n g Oil
A
D
Lubricor ' T I
B
E
"
S u l f o n a ED
369 Seal l i p m a t e r i a l s f o r r o l l c o o l a n t s and r o l l i n g o i l s ( c o n t d . )
Seal l i p m a t e r i a l s Brand Name o r Number
1 s t Choice
Mobil Generex 56, 57 and 404D 11 11
"
22 and 24
11
322
A
B
C
B
C
D
P r o s o l 66
A
B
44
B
A
33
A
Solvac 11
A
-
B
A
11
II
'I
2nd Choice
~
~~~
11
11
a00 and "BB"
C
B
M i r o b o 415A
B
A
Ocut G
C
A
Palm O i l
B
A
Quaker T i n n o l 12
E
D
B
D
B
A
B
A
V a c t r a "HH"
I'
11
109
I1
'I
I,
Quakerol 41 ( w i t h S h e l l Carnea 31) 43, 82, a7 M and
88-182 M I'
Qwerl 506
Rol l u p 200 S h e l l Dromus
B
B
D
B
A
B
A
C
B
'I
1270/1307
B
A
I'
1076
D
C
Tayol 316 Emulsion
B
A
Texaco Texatherm 320
A
C
S t e r n o l PL106 and PL107
T r e l l u b 12 A
B
A
Wyrol H40
C
A
370
I(;
SEALS FOR FLUID POWER EQUIPMENT PART THREE COMPRESSION PACKINGS
B.D. HALLIGAN, C.Eng, MIMechE, AMPRI Technical Manager (Product Appl i c a t i o n s ) James Walker and Co. L t d .
16.1
THE PACKED GLAND
Compared t o t h e f i n i t e q u a l i t i e s o f f e r r o u s m e t a l s f o r example, t h e essenti a l l y deformable n a t u r e o f s e a l i n g m a t e r i a l s has i n t r o d u c e d a measure o f v a r i a b i l i t y t h a t causes many commentators t o l o o k on f l u i d s e a l i n g technology as an a r t r a t h e r than a science.
I f t h i s i s t r u e , and manufacturers o f mechan
i c a l f a c e s e a l s would be b u t one area o f v a l i d o b j e c t i o n , then t h e f i e l d o f compression packings i s , a r g u a b l y , t h e b l a c k e s t area o f t h a t a r t . Regarded as an anachronism i n a p e r i o d o f h i g h t e c h n o l o g i c a l achievement, compression packings show no s i g n s o f l o s i n g s i g n i f i c a n t ground i n terms o f p r o d u c t i o n q u a n t i t i e s as new and improved t y p e s p r o l i f e r a t e b o t h i n Europe and elsewhere.
To understand t h i s s i t u a t i o n r e q u i r e s some a p p r e c i a t i o n o f t h e
fundamental mode o f o p e r a t i o n o f t h e a d j u s t a b l e g l a n d o r s t u f f i n g box shown i n Fig. 1.
SYSTEM __c PRESSURE
Fig.1
COMPRESSIVE FORCE &THROUGH GLAND SPIGOT
f
h
Compression Packing
t
Y
371 T h is may be f i l l e d w i t h s p l i t pa cki ng r i n g s chosen from a v a r i e t y o f m a t e r i a l s and c o n s t r u c t i o n s , de scri be d elsewhere, which a r e persuaded to r e a c t a g a i n s t a s h a f t , whether r o t a r y o r r e c i p r o c a t i n g , t o t h e e x t e n t t h a t t h e r a d i a l f o r c e developed exceeds t h e p r e s s u r e t o be sealed. Such a p r i n c i p l e c o u l d n o t be m r e elementary and i t s i n t r i n s i c value c o u l d be f u r t h e r q u e s t i o n e d as pa cki ng s i n t h i s c a t e g o r y used f o r r o t a t i n g o r r e c i p r o c a t i n g equipment r e l y on a c o n t r o l l e d leakage f o r l o n g - t e r m l u b r i c a t i o n purposes,
i f t h e y a r e t o s u r v i v e f o r an adequate p e r i o d .
The c o n t i n u e d j u s t i f i c a t i o n for t h e compression packing might appear obscure a g a i n s t such a background b u t t h e r e can be no doubt t h a t c e r t a i n areas o f appl i c a t i o n e x i s t where no reasonable s u b s t i t u t e i s a v a i l a b l e .
16.1.1
Pumps
Many reasoned and w e l l - r e s e a r c h e d papers have been p u b l i s h e d t o support mechanical s e a l s a g a i n s t s o f t p acki n g and v i c e versa.
There i s no doubt t h a t
th e former have sup pl a nt ed packed g l an ds as o r i g i n a l equipment on the m a j o r i t y o f rotodynamic pumps f o r a v a r i e t y o f process and s e r v i c e f l u i d s b u t they a r e o p e r a t i n g parameters and c o s t c o n s i d e r a t i o n s w hich w i l l f r e q u e n t l y d i c t a t e t h e choice o f s o f t p a cki ng . Table 16.1 compares t h e r e l a t i v e a t t r i b u t e s o f t h e two contenders i n b a s i c terms. I n g e n e r a l i t may be s a i d t h a t , u n l e s s ze ro leakage i s an a b s o l u t e p r i o r i t y , compression p a c k i n gs w i l l r e t a i n an i m p o r t a n t p o s i t i o n wherever r e g u l a r maintenance i s a v a i l a b l e and t h e f o l l o w i n g c o n s i d e r a t i o n s a p p l y :
-
s i m p l i c i t y i n g l a n d d esi g n and a n c i l l a r y equipment ease o f f i t t i n g f l e x i b i l i t y o f su pp l y and spares f o r p l a n t u t i l i z i n g many d i f f e r e n t t y p e s and s i z e s o f pump h a n d l i n g a wide v a r i e t y o f f l u i d s
-
f r e q u e n t a b i l i t y t o c a t e r f o r adverse c o n d i t i o n s w i t h o u t e l a b o r a t e precautions
16.1.2
Valves
I f any doubt e x i s t s r e g a r d i n g s e l e c t i o n on pumps then a much more obvious
c h o i c e o f s o f t p a cki ng a p p l i e s t o t h e v a l v e scene. ment, ease o f f i t t i n g and,
The r e l a t i v e l a c k o f move-
i n t h i s case, l a c k o f leakage requirement f o r l u b -
r i c a t i o n purposes p l u s t h e most d e c i s i v e advantage o f low c o s t , a r e f a c t o r s whic h i d e a l l y r e l a t e t o compression packings. There a r e a r e as where moulded e l a s t o m e r i c s e a l s p r e s e n t a reasonable a l t e r n a t i v e b u t even t h e most e x o t i c compounds would seldom be used above 250°C u n l e s s r e i n f o r c e d by asbestos f a b r i c .
-
w
4 N
TABLE 16.1
Compa r i son I n i t i a l Cost
Re1 i a b i 1 i t y
S o f t Packing
Mechan ica 1 Sea 1
o f t h e o r d e r o f 1 O : l i n f a v o u r o f s o f t packing depending on s i z e and a p p l i c a t i o n APPROXIMATELY EQUAL ample warning o f impending f a i l u r e l i t t l e or no warning o f with p o s s i b i l i t i e s f o r correction end o f u s e f u l l i f e w i t h p o s s i b i l i t y o f sudden complete f a i l u r e
-
Installation
e s s e n t i a l l y simple - r e q u i r i n g no s p e c i a l s k i l l s i f c o r r e c t procedure adopted
s k i l l e d f i t t i n g required p r e c i s e l y defined environment and assembly
Maintenance
r e g u l a r and r e q u i r i n g experience
zero
Spares
f a c i l i t y f o r s t o c k i n g l e n g t h form m a t e r i a l o r complete pre-formed s e t s a t r e l a t i v e l y low c o s t
spare seal components must c o s t can be be a v a i l a b l e substantial
S h af t Wear
can be c o n s i d e r a b l e ; s h a f t sleeves reduce replacement c o s t s
nil
-
f r i c t i o n losses s l i g h t l y h i g h e r w i t h s o f t packing Op era t i n g Costs
leakage losses zero w i t h mechanical s e a l s b u t p o s i t i v e w i t h s o f t packing as l u b r i c a t i o n o f s e a l i n g r i n g s i s e s s e n t i a l
373 16.2
OPERATING PRINCIPLES
By comparison t o t h e se al t yp es d escri b ed i n t h e o t h e r papers e l a s t o m e r i c l i p and squeeze s e a l s
-
-
particularly
compression packings respond t o a p p l i e d
pre s s u r e i n i n v e r s e p r o p o r t i o n to t h e hardness o f t h e i r c o n s t r u c t i o n and r e l y on an e x t e r n a l f o r c e t o produce t h e r a d i a l p ressure r e q u i r e d f o r e f f e c t i v e sealing.
The method o f g e n e r a t i n g t h a t f o r c e can v a r y b u t u s u a l l y (and p r e f e r -
a b l y ) i n v o l v e s a b o l t e d q l a n d s p i g o t as shown i n Fig.1 where c o n t r o l l e d a x i a l movement i s e a s i l y ach i eve d by a dj u st me nt o f t h e r e t a i n i n g n u t s o r studs. Spring l o a d i n g i s sometimes used i n i n a c c e s s i b l e s i t u a t i o n s b u t such a p r o v i s i o n l a c k s t h e f i n e c o n t r o l demanded by some packing types and has a l i m i t e d range o f l o a d c a p a b i l i t y . W h i l s t t h e s e a l i n g f o r c e can be a d j u s t e d t o c a t e r f o r s e r v i c e wear, care must be t a k e n t o a v o i d o v e r compression which w i l l lead t o excessive f r i c t i o n , s h a f t wear and premature p acki n g f a i l u r e . To in c r e a s e d e n s i t y and d i s s i p a t e he at , s o f t packings i n v a r i a b l y contain. l u b r i c a n t s , l o s s o f which, t hro ug h e xcessi ve compression o r o v e r - h e a t i n g i n s e r v i c e , w i l l r e s u l t i n pa cki ng volume l o s s w i t h subsequent r e d u c t i o n i n t h e e f f e c t i v e s e a l i n q r e a c t i o n and c o r r e s p o n d i n g l y i n c r e a s i n g leakage r a t e s .
By
l i m i t i n g compression t o a p o i n t where s l i g h t c o n t r o l l e d leakage i s obtained, adequate l u b r i c a t i o n o f t h e dynamic s u r f a c e s i s ensured and over-compression o f th e p a c k in g avoided.
However, where l u b r i c a t i o n i s a problem
gl a n d c o o l i n g i s r e q u i r e d
-
-
o r a degree o f
a l a n t e r n r i n g can be i n c o r p o r a t e d i n t o t h e gland
area f o r t h e d i s t r i b u t i o n o f a d d i t i o n a l l u b r i c a n t / c o o l a n t (Fig.2a).
The p o s i t i o n
o f a l a n t e r n r i n g w i l l depend on t h e n a t u r e o f t h e a p p l i c a t i o n b u t , s i n c e t h e pa c k in g r i n g s n e a r e s t t h e g l a n d s p i g o t do most o f t h e work, t h e a d d i t i o n a l f l u i d should u s u a l l y be i n t r o d u c e d near t o t h a t area.
ADDITIONAL LUBRICANT/COOLANT
I
RING
DISTRIBUTION PORTS
BARRIER FLUID
374 I f i t i s e s s e n t i a l t h a t t h e f l u i d b e i n g pumped does n o t escape t o atmosphere
(e.g.
a t o x i c medium) t h e l a n t e r n r i n g may serve t o i n t r o d u c e a b a r r i e r f l u i d
a t a p r e s s u r e o f 0.5 t o 1 b a r above t h a t t o be sealed ( F i g . 2 b ) .
Similarly,
where t h e r e i s a r i s k o f severe a b r a s i v e wear t o t h e packing, a f l u s h i n !
fluid
may be i n t r o d u c e d throuoh t h e l a n t e r n r i n g ( F i g . 2 c ) . For a p p l i c a t i o n w i t h n e c a t i v e pump p r e s s u r e s ( i . e .
s u c t i o n ) a supply o f t h e
medium b e i n g sealed can be made through t h e l a n t e r n r i n g t o p r e v e n t a i r - d r a w i n g (Fig.2d).
SUPPLY OF SEALED MEDIUM TO PREVENT AIR-DRAWING
FLUSHING FLU I D
I
I
I f extreme temperatures a r e t o be encountered,
i t i s unlikely that cooling
through t h e l a n t e r n r i n g w i l l be s u f f i c i e n t and recourse must be made t o i n t e r n a l c o o l i n g o f t h e g l a n d housing and s h a f t t o reduce t h e temperature a t t h e gland t o a v a l u e w i t h i n t h e p a c k i n g ' s c a p a b i l i t i e s .
Conversely, when d e a l i n g
w i t h media which c r y s t a l l i z e o r congeal when c o o l (e.g.
sugars, t a r s , e t c . ) ,
t h e p a c k i n g w i l l f a c e r a p i d d e s t r u c t i o n u n l e s s g l a n d h e a t e r s o r a steam j a c k e t e d arrangement a r e employed t o r e s t o r e t h e f l u i d s t a t e b e f o r e s t a r t i n g up. I t should always be remembered t h a t t h e i n c l u s i o n o f a l a n t e r n r i n g i n t o t h e gland area i n v a r i a b l y c o m p l i c a t e s assembly and can p r o v i d e a p o s s i b l e source o f shaft scoring;
t h e y should, t h e r e f o r e , o n l y be c o n s i d e r e d when t h e n a t u r e o f
the a p p l i c a t i o n a b s o l u t e l y demands t h e i r presence.
16.3
GLAND DESIGN
A t t h i s juncture,
few i n t e r n a t i o n a l standards e x i s t t o d e f i n e housing design
f o r s o f t packings b u t t h e dimensions shown i n Table 16.2 should be s a t i s f a c t o r y f o r most a p p l i c a t i o n s .
Housing depths w i l l v a r y w i t h i n d i v i d u a l circumstances,
such as t h e i n c l u s i o n o f a l a n t e r n r i n g , b u t f i v e r i n g s o f square s e c t i o n packings a r e u s u a l l y recommended f o r t h e average,
uncomplicated d u t y .
375 TABLE 16.2
Suggested h ou si ng w i d t h s i n r e l a t i o n t o s h a f t diameters. A l l dimenZions i n mm.
A l l p a c k in g s e xce pt expanded g r a p h i t e Shaft Diameter
Housing Width
Expanded g r a p h i t e Sh aft Diameter
Housing Width
up t o 12
3
up t o 18
3
above 12 t o 18
5
above 18 t o 75
75 t o 150 150 and above
5 7.5
18 t o 25
6.5
25 t o 50
8
50 t o 90
10
90 t o 150
12.5
150
15
10
Other d e s ig n c o n s i d e r a t i o n s wort hy o f n o t e , b u t o f t e n overlooked, can be summarized a s :(i)
The p r o v i s i o n o f an adequate t a pe red ' l e a d i n ' a t t h e mouth o f t h e g l a n d t o f a c i l i t a t e e n t r y o f t h e pa ckina and t o o b v i a t e t h e r i s k o f damage i n t h e assembly o p e r a t i o n .
A minimum o f 15" x 6.5mm u s u a l l y
r e p r e s e nt s good p r a c t i c e . (ii)
The p r o v i s i o n o f a reasonable s u r f a c e f i n i s h on adjacent metal p a r t s p a r t i c u l a r l y t he dynamic su rf a ce. w i l l o c cu r;
The b e t t e r the f i n i s h t h e l e s s wear
0.4um (16 p i n ) CLA o r Ra on the s h a f t and 1.6 pm
(54 u i n ) CLA o r Ra on t h e s t u f f i n g box bore should be i d e a l f o r most applications.
The use o f s h a f t sleeves can g i v e c o n s i d e r a b l e main-
tenance advantage when c o n s i d e r i n g t h e q u e s t i o n o f s u r f a c e f i n i s h . (iii)
The danger o f extreme r u n n i n g cl e ara nces a t t h e g l a n d on t h e s p i g o t si d e.
-
particularly
I n those e x c e p t i o n a l cases where excessive
c le a r a n c e i s u na voi d ab l e, t h e p acki n g should be p r o t e c t e d by an independent r i n g o f s u i t a b l y r o b u s t m a t e r i a l o r c o n s t r u c t i o n which reduces t h e c l e a r a n c e t o a minimum. (iv)
An a llo wa nce for e n t r y o f t h e g l a n d s p i g o t w e l l i n t o the gland area; c e r t a i n l y t o an e x t e n t t h a t exceeds s u b s t a n t i a l l y t h e depth o f the ta p e r e d l e a d i n .
The l e n g t h o f s p i g o t s e l e c t e d must a l s o c a t e r f o r
p a c k in g compression, r e s u l t i n g f rom gland adjustment.
Typical e n t r y
l e n g t h s sho ul d be a t l e a s t two t i mes packing s e c t i o n .
For packings
o f s o f t e r c o n s t r u c t i o n , maximum l e n g t h should be provided.
With
modern p acki n g m a t e r i a l s , b e v e l l e d glands a r e seldom an advantage and can a c t u a l l y promote movement o f t h e s e a l i n g r i n g on t h e s p i g o t s i d e i n t o t h e l i v e cl ea ran ce.
-
376 (v) (vi)
The need t o a v o i d e x c e s s i v e s h a f t misalignment o r whip. The p r o v i s i o n o f adequate s h a f t support.
The p a c k i n g must
n o t be used a s a b e a r i n g .
16.4
PACKING CONSTRUCTION AND MATERIALS
16.4.1
Fibre Material
Mineral
asbestos
Vegetable
cotton
Synthetic
aramid
flax
glass
jute
graphite filament
ramic
graphite f o i l
sisal
nylon polytetrafluoroethylene (PTFE) rayon
16.4.2 Dry
Lubricants Metals
graphite
aluminium f o i l
talc
copper f o i l and w i r e
molybdenum d i s u l p h i t e Wet
l e a d f o i l and w i r e
mica
brass w i r e monel w i r e
tallow
inconel w i r e
castor o i l s t r a i g h t mineral lubricating o i l
stainless steel wire Elastomers
n a t u r a l and s y n t h e t i c
petrolatum
sol i d f r a c t i o n s p a r a f f i n wax soaps s i l i c o n e grease PTFE d i s p e r s i o n s 16.4.3
Construction
The p r i n c i p a l forms o f c o n s t r u c t i o n s f o r f i b r o u s compression packings a r e : (i)
Braided
I n d i v i d u a l yarns a r e b r a i d e d tube o v e r tube and squared o f f .
The d e n s i t y o f t h i s t y p e o f c o n s t r u c t i o n i s h i g h and i d e a l f o r many valve applications. (ii)
Plaited
M u l t i p l e y a r n s a r e interwoven i n p l a i t e d bundles i n such a
fashion that the d i r e c t i o n o f f i b r e f o l l o w s the periphery o f the packing r i n g .
The n a t u r a l c h a r a c t e r i s t i c o f t h i s c o n s t r u c t i o n i s more
s u i t e d t o c e n t r i f u g a l pump a p p l i c a t i o n s than v a l v e s e r v i c e a l t h o u g h t h e i n h e r e n t f l e x i b i l i t y o f t h e f o r m i s p o p u l a r w i t h some users.
Fig.3 (iii)
Basic p a c k i n g c o n s t r u c t i o n s Cross P l a i t
A l l t h e y a r n s a r e i n t e r l o c k i n g and pass d i a g o n a l l y through
t h e p a c k i n g t o p r o v i d e a f i r m c o n s t r u c t i o n o f c o n s i s t e n t d e n s i t y and shape.
Used e x t e n s i v e l y f o r s y n t h e t i c y a r n packings f o r v a l v e s and
pumps. (iv)
Composite asbestos p l a s t i c
This packing category i s o f f a i r l y recent
o r i g i n and i n c l u d e s those t y p e s based upon a b r a i d e d and r e i n f o r c e d asbestos j a c k e t e n c l o s i n g a ' p l a s t i c ' c o r e . t h e s c i e n t i f i c sense,
Although i n a c c u r a t e i n
t h e t e r m ' p l a s t i c ' c o n v e n i e n t l y d e s c r i b e s those
many m i x t u r e s o f asbestos f i b r e and l u b r i c a n t , b o t h m i n e r a l and s o l i d , from which r e a d i l y deformable p a c k i n g m a t e r i a l may be made.
This
p a c k i n g i s w i d e l y accepted f o r d i f f i c u l t v a l v e s e a l i n g d u t i e s .
A l 1 t h e f i b r e - b a s e d c o n s t r u c t i o n s d e s c r i b e d here a r e f r e q u e n t l y r e i n f o r c e d w i t h metal.
T h i s a p p l i e s p a r t i c u l a r l y t o asbestos'based
p r o d u c t s where t h e use
o f metal w i r e i n t h e y a r n can e x t e n d t h e s e r v i c e c a p a b i l i t y o f t h e packing t o
800°C and beyond.
But f o r t h i s f e a t u r e , even t h e b e s t q u a l i t y n o n - m e t a l l i c
asbestos y a r n p a c k i n g would be r e s t r i c t e d t o temperatures o f about 315°C maximum.
A l l o f t h e l u b r i c a n t s shown a r e used i n c o n j u n c t i o n w i t h f i b r e packings o f d i f f e r e n t s o r t s and a r e a p p l i e d by d i p p i n g , c o a t i n g , soaking, nation, dusting, etc.
vacuum impreg-
The prime o b j e c t i s maximum l u b r i c a n t r e t e n t i o n .
Fre-
q u e n t l y , s e v e r a l t r e a t m e n t s and r e p e a t processes a r e employed t o achieve t h i s end.
318
F o i l wrapped deformable asbestos core.
F o i l c r i n k l e d , t w i s t e d and folded upon i t s e l f .
Corrugated f o i l , concertina wound.
Fig.4
T y p i c a l metal f o i l based packing c o n s t r u c t i o n
Fig.5
D o u b l e - b e v e l l e d , e l a s t o m e r p r o o f e d f a b r i c packing f o r a b r a s i v e d u t i e s
379 I n t h e f i e l d o f compression packings, e l a s t o m e r s a r e n o t w i d e l y used a l t h o u g h some b r a i d e d packings do employ y a r n s t h a t a r e t r e a t e d w i t h a rubber p r o o f i n g t o render them more s u i t e d t o d i f f i c u l t wet a p p l i c a t i o n s such as condensate d u t y . Rings o f square o r r e c t a n g u l a r s e c t i o n compression packing, manufactured from folded, r o l l e d o r l a m i n a t e d e l a s t o m e r p r o o f e d c l o t h , a r e s t i l l p o p u l a r f o r r e l a t i v e l y slow moving, lower p r e s s u r e r e c i p r o c a t i n g pumps h a n d l i n g water o f LP steam. One p a r t i c u l a r design, w i t h a moulded, d o u b l e - b e v e l l e d s e c t i o n , made f r o m semi-metallic
rubberised yarn,
i s p a r t i c u l a r l y e f f e c t i v e on r o t a r y a p p l i c a t i o n s
dealing w i t h v i s c o u s media which s o l i d i f y when t h e pump i s i d l e and cause damage t o conventional p l a i t e d packings on r e s t a r t i n ? f r o m c o l d .
T h i s moulded p a c k i n g
i s a l s o s u i t a b l e f o r d u t i e s i n v o l v i n g s o l i d s and a b r a s i v e s (See F i 0 . 5 ) .
16.4.4
Additional Materials
In h i g h performance r o t a r y and r e c i p r o c a t i n g packings, t h r e e m a t e r i a l s which may be regarded as i m p o r t a n t a d d i t i o n s t o t h e seal m a n u f a c t u r e r s ' armoury have become a v a i l a b l e i n t h e l a s t two decades and p l a y a h i q h l y s i g n i f i c a n t p a r t i n extending t h e f r o n t i e r s o f performance o f t h e t r a d i t i o n a l s o f t p a c k i n g : ( i ) PTFE
P o l y t e t r a f l u o r o e t h y l e n e y a r n s p r o v i d e s o f t packings f o r s e r v i c e s
where c o r r o s i v e media a r e b e i n g handled o r freedom f r o m c o n t a m i n a t i o n i s an e s s e n t i a l requirement.
A s e m i - r i g i d f l u o r o c a r b o n p l a s t i c , PTFE i s unique i n possessing almost complete chemical r e s i s t a n c e w i t h i n i t s temperature range which, f i e l d o f a p p l i c a t i o n , spans t h e c r y o g e n i c area t o 250°C.
in this
Another major
advantage r e f e r s t o i t s v e r y low c o e f f i c i e n t o f f r i c t i o n . L u b r i c a t e d p l a i t e d PTFE y a r n packings a r e s u i t a b l e f o r r o t a r y s u r f a c e speeds up t o 8 m/s and a r e a l s o f i n d i n g i n c r e a s i n g acceptance on h i g h speed, h i g h pressure, m u l t i - r a m r e c i p r o c a t i n g pumps. I n s o l i d f o r m t h i s m a t e r i a l i s n o t a c c e p t a b l e as a compression p a c k i n g due t o poor creep p r o p e r t i e s and l a c k o f r e s i l i e n c e .
However, s o l i d
j u n k r i n g s o r spacers i n PTFE a r e o f t e n used t o enhance p a c k i n g p e r formance on arduous pump d u t i e s . ( i i ) Aramid f i b r e
C r o s s p l a i t y a r n packings made f r o m aramid f i b r e , u s u a l l y
o f a d i s t i n c t i v e y e l l o w c o l o u r , a r e becoming i n c r e a s i n g l y p o p u l a r f o r a v a r i e t y o f pump and v a l v e s e r v i c e s h i t h e r t o s a t i s f i e d by PTFE
l u b r i c a t e d asbestos packings.
380 The y a r n has h i g h t e n s i l e s t r e n g t h ,
e x c e l l e n t r e s i l i e n c e , thermal
s t a b i l i t y up t o 250°C and i s r e s l s t a n t t o a wide range o f chemicals. Aramid f i b r e p acki n gs a r e s u i t a b l e f o r speeds up t o 15 m/s and g i v e im p r e s s iv e r e s u l t s w i t h a b r a s i v e media. ( i i i) G r a p h i t e ( a ) Yarn p a cki ng s i n t h i s m a t e r i a l a r e a development f o r r o t a r y pump a p p l i c a t i o n s and p r o v i d e p o s s i b i l i t i e s f o r e x t e n d i n g t h e range o f t h e packed g l a n d beyond bo un da ri es h i t h e r t o e s t a b l i s h e d .
A h i g h c o e f f i c i e n t o f t h erma l c o n d u c t i v i t y ,
low f r i c t i o n and r e s i s t a n c e
t o chemical a t t a c k a r e t h e u s e f u l c h a r a c t e r i s t i c s o f t h i s m a t e r i a l . Temperatures up t o 400°C may be co nsi d ered. I f a good performance i s t o be o b t a i n e d , then c l o s e a t t e n t i o n must be p a i d t o mechanical c o n d i t i o n s such as s h a f t run o u t and f i n i s h .
Care
i n f i t t i n g and r u n n i n g - i n i s a l s o mandatory. ( b ) Expanded g r a p h i t e f o i l i s t h e r e c e n t and dramatic a p p l i c a t i o n o f graphite, p a r t i c u l a r l y i n the context o f valve applications. g r a p h i t e m a t e r i a l s combine t h e w e l l - e s t a b l i s h e d characteristics,
Expanded
thermal and f r i c t i o n
l o n g a s s o c i a t e d w i t h t h e c o r r e c t l y developed use o f
carbon based p r o d u c t s , w i t h a unique f l e x i b i l i t y and r e s i l i e n c e .
The
a t t r i b u t e s o f t h i s e x f o l i a t e d f o rm o f g r a p h i t e bear r e c o r d i n g .
-
e x c e l l e n t r e s i s t a n c e t o compression s e t r e s u l t i n g i n l i t t l e l o s s o f r a d i a l gland f o r c e o r flange seating s t r e s s over long periods (see Table 16.3)
-
no l o s s o f v o l a t i l e s even a t h i g h t emperature thus m i n i m i s i n g frequency o f g l an d ad j ust men t
-
h i g h t e mpe rat ure c a p a b i l i t y p a r t i c u l a r l y i n n o n - o x i d i s i n g environments
-
-
h i g h thermal c o n d u c t i v i t y low f r i c t i o n p r o p e r t i e s
-
self lubricating
e x c e p t i o n a l l y low c h l o r i d e c o n t e n t no adhesion o r c o r r o s i o n problems fire-safe
381 TABLE 16.3 Compress i b i 1 it y / r e c o v e r y ASTM F36-66 Procedure H (Major lo a d 7/mm2)
Expanded graphite lmm t h i c k 1 .o g/cc
(1000 l b f / i n Z )
Expanded graphite 2mm t h i c k 1.0 g/cc
compressibility
39%
33%
recovery
20%
25%
40 N/mm2
40 N/mm2
Stress r e l a x a t i o n BS 1832:1972 Temperature 300°C f o r 16 h r s I n i t i a l s t r e s s o f 40 N/mm2 Residual s t r e s s
N.B.
10 N=l k g f 1 N/mmz=lO kgf/cm2 approx.
Fig.6 shows an i n t e r e s t i n g comparison o f performance on a t e s t g l a n d between h a l f r i n g s o f expanded g r a p h i t e and a l u b r i c a t e d asbestos y a r n packing.
Not
only d i d t h e fo r me r r e q u i r e fewer g l a n d ad j ustments d u r i n g t h e p e r i o d o f t e s t i n g but t h e average leakage r a t e was much l e s s
-
t o the p o i n t o f r u n n i n g v i r t u a l l y
dry f o r p r o t r a c t e d p e r i o d s . Rather t h a n use t h e t a pe f o r m o f expanded g r a p h i t e which i s p r i m a r i l y a usef u l maintenance e x p e d i e n t , moulded r i n g s t o a s e l e c t e d and c o n t r o l l e d d e n s i t y should be t h e f i r s t c h o i c e f o r pump and v a l v e glands. A lt h o u g h more c o s t l y t ha n c o n v e n t i o n a l pa cking m a t e r i a l s , economies o f r a d i a l w i d t h and number o f r i n g s used a r e f e a s i b l e q u i t e a p a r t f r o m t h e performance advantage l i k e l y t o be d e r i v e d f rom t h e use o f expanded g r a p h i t e . One cannot le ave m a t e r i a l s w i t h o u t s p e c i a l r e f e r e n c e t o t h e v i t a l r o l e played by asbestos
-
a much d e n i g r a t e d m i n e r a l f i b r e w i t h o u t w hich economic and prac-
t i c a l s o l u t i o n s t o many s e a l i n g problems would n o t be f e a s i b l e .
Although under-
s ta n d a b le , i n an age o f c o r r e c t awareness o f h e a l t h and s a f e t y m a t t e r s , t h e o v e r - r e a c t i o n a g a i n s t asb est os has r e v e a l e d many i n c o n s i s t e n c i e s .
Motor manuf-
a c t u r e r s may p r o h i b i t i t s use as a p l a n t maintenance m a t e r i a l b u t c o n t i n u e t o use asbestos i n a b r a k e - l i n i n g and c l u t c h - f a c i n g r o l e where r e s i d u a l dust i s evident.
Some u sers may seek t o l i m i t i t s a p p l i c a t i o n i n a safe form as a v a l v e
packing o r gasket b u t p e r p e t u a t e i t s s p e c i f i c a t i o n f o r f i r e - p r o o f p o s i t i o n s and r o o f i n g where t h e m i n e r a l i s c u t i n a d r y form. Hazards e x i s t b u t adherence t o b a s i c a d v i c e on h a n d l i n g asbestos w i l l r e s u l t i n a s e n s i b l e balance between p r o h i b i t i o n and p r a c t i c a l i t y .
In t h i s context,
users o f asbestos based pump and v a l v e pa cki ngs, gaskets o r a l l i e d components might heed, t o advantage, t h e statement i ssu ed by t h e Asbestos I n f o r m a t i o n Committee t o t h e B r i t i s h Valve M a n u f a c t u r e r s ' A s s o c i a t i o n , appended t o these no t e s.
a copy o f which i s
D I S T I L L E D WATER TEMPERATURE OF MEDIUM
TEST CONDITIONS:
- 9OoC - 3.45
PRESSURE OF MEDIUM
bar
U x p a n d e d graphite rings ( i n halves)
-
Lubricated asbestos yarn rings ( s p l i t )
I000 900 800
700 600
‘9L
500 400 300
200 100
0
2.5 m/s
5 m/s
7 . 5 m/s
12.5 m/s
10 m/s P
SHAFT SURFACE SPEED
2.5 2.0
I .5 1 .o
0.5
0
I--
---
I - - - -
.J
& 7.5 m/s
10 m/s
12.5 m/s
SHAFT SURFACE SPEED
I
5
10
1
15 TOTAL TEST TIME-HOURS
20
25
30
383
16.4.5
Selection
The most v e x i n g q u e s t i o n as, f o r many d u t i e s , exist.
so many reasonable a l t e r n a t i v e s
Much w i l l depend on p erso na l e xpe ri en ce, frequency o f maintenance,
o r i g i n a l cost l e v e l , contamination considerations, size, e t c . I n t h e case o f a man uf act ure r pro du ci ng l a r g e q u a n t i t i e s o f v a l v e s o r pumps t o standard dimensions, t h e r e i s much t o be s a i d f o r purchasing sets o r r i n g s r a t h e r t h a n l e n g t h form packing.
W i t h t h e t echniques a v a i l a b l e , packing can be
s u p p lie d ready f o r immediate f i t t i n g w i t h s u b s t a n t i a l r e d u c t i o n i n t h a t o v e r a l l c o s t r e p r e s e n te d by r e c e i v i n g l e n g t h f orm t h a t must be c u t t o s i z e by s k i l l e d personnel.
T h i s economy i s n o t c o n f i n e d t o t h e l a r g e manufacturers b u t i t i s
they who w i l l e n j o y t h e g r e a t e r advantage. On t h e o t h e r hand,
i n many i n st a nce s, t h e problem o f s t o c k i n g r i n g s o r s e t s
ta ilo r - m a d e f o r an assortment o f v a l v e s v a r y i n g i n o r i g i n , can prove i n t o l e r a b l e .
type and dimensions
For t he se cases, t h e r e i s a c l e a r need f o r t h e v e r s a t -
i l i t y o f p a c k in g i n l e n g t h form.
Comparable w i t h t h i s s o l u t i o n i s t h e expedient
provided by those pa cki ng s o f p l a s t i c n a t u r e t h a t a r e a v a i l a b l e i n loose form but t h i s advantage must be weighed a g a i n s t t h e l a b o u r c o s t i n t h e c a r e f u l f i t t i n g r e q u i red. The q u a n t i t y o f p acki n g t o use.and i t s s i z e f o r a g i v e n a p p l i c a t i o n r e l i e s l a r g e l y on t h e e xp eri e nce o f t h e u ser/ man uf acturer i n t h e type o f d u t y being performed, o r on l i a i s o n w i t h a p acki n g s u p p l i e r a t t h e design stage. l a t t e r c o u r s e o f a c t i o n i s always f avo ure d i f any doubt e x i s t s ,
The
s i n c e an exact
knowledge o f t h e c a p a b i l i t i e s and l i m i t a t i o n s o f t h e m a t e r i a l employed can be found o n l y w i t h those s p e c i a l i s t s r e s p o n s i b l e f o r compounding and p r o d u c t i o n . F iv e r i n g s o f square s e c t i o n p acki n g a r e o f t e n accepted as a s u f f i c i e n t number f o r t h e average u nco mpl i ca t e d d u t y b u t t h e r e a r e many pump a p p l i c a t i o n s where t h e presence o f a l a n t e r n r i n g o r s i m i l a r c o n s i d e r a t i o n may d i c t a t e a greater q u a n t i t y . The a p p r o p r i a t e pa cki ng s e c t i o n t o use i n r e l a t i o n t o diameter i s open t o a degree o f i n d i v i d u a l p r e f e r e n c e b u t broad recommendations a r e shown i n Table 1 6 . 2 . To g i v e an idea o f t h e c a p a b i l i t i e s o f t he v a r i o u s m a t e r i a l s and c o n s t r u c t i o n s o f s o f t p a cki ng s which a r e r e a d i l y a v a i l a b l e , r e f e r e n c e may be made t o :
Table 16.5
-
co mpa rat i ve speed performance
Table 16.6
-
co mpa rat i ve temperature performance
Table 1 6 . 7
-
co mpa rat i ve c o s t i n d i c a t i o n .
Table 16.4
s u i t a b i l i t y i n d i f f e r e n t media/speed and temperature l i m i t s
N . B . The statement o f speed and t emp era t u re l i m i t s f o r a g i v e n m a t e r i a l should n o t be c o n s t r u e d as meaning t h a t a pa cki ng w i l l be s u i t a b l e f o r d u t i e s where such maxima a r e j o i n t l y encountered.
384 TABLE 16.4
SYMBOLS
0
-*
Recommended Consult Corrosion inhibitor included
solvents
RECIPROCATING, ROTARY PUMPS & VALVES Lubricated aluminium foil.
1
540
Lubricated braided asbestos
I
350
I
Lubricated plaited asbestos.
315
Plaited, lubricated asbestos impregnated with PTFE dispersion.
290
Plaited. lubricated asbestos impregnated with PTFE disoersion but with no additional lubricant.
290
PTFE impregnated asbestos and glass fibre yarns with suitable lubricant.
I 2 9 0
PTFE yarn impregnated with PTFE dispersion and inen lubricant. Soft lead based foil wrapped round lubricated asbestos COW.
I I 1
7.5
I
CI
lo
20
10
;“t I 8
1 7 5
250
8
260
12
Hydrocarbon resistant lubricated plaited asbestos.
1
I
7
I
Lubricated plaited cotton.
( 9 0 1
7
I
Cross plan aramd hbre yams
I
200
250
I
n
~
Puregraphite foil with no volatile additives
15
10
D
O
RECIPROCATING PUMPS & VALVES Monel wire reinforced asbestoscoverwith plastic core.*
I
Synthetic rubber bonded braided asbestos with brass wire reinforcement.
I
1 0
0
310
Lubricated plaited flax.
VALVES ONLY
480
90
I I T
Constructed from a jacket of asbestos reinforced with inconel wire braided ouer a resilient asbestos core. Lubricated braided asbestos with monel Self-lubricating fibrous asbestos with flake Qraphiteor mica. Lubricated braided asbestosand brasswirereinforced.
540
1
510
PTFE yarn impregnated with PTFE dispersion and inen lubricant but with no additional lubricant.
2 50
Unsintered PTFE cord gland seal for rapid valve packing.
250
I
1 0
0
T A B L E 16.5 MAXIMUM ROTARY SPEEDS FOR PUMP PACKINGS
MATERIAL
I
0
I
ROTARY SPEED I d s )
5
10
15 m/s
20
25
30
T A B L E 16.6 MAXIYUM SERVICE TEMPERATURES OF PUMP PACKINGS MATERIAL
TABLE 16.7 TYPICAL RELATIVE COSTS OF PUMP PACKINGS
MATERIAL
I
RELATIVE COST
I
388 16.4.6
Fitting
I t i s o f t e n assumed t h a t u n s k i l l e d l a b o u r can be used t o repack pump glands b u t t h i s i s t r u e o n l y so l o n g as u n s k i l l e d i s n o t equated w i t h unaware.
Fam-
i l i a r i t y w i t h t h e f o l l o w i n g i d e a l procedure w i l l be more t h a n r e p a i d i n terms o f t r o u b l e f r e e p a c k i n g performance. Where l e n g t h f o r m i s used: ( i ) S p i r a l l y wrap t h e m a t e r i a l around a r o d o f diameter e q u i v a l e n t t o t h e pump s h a f t . ( i i ) Cut t h e r e q u i r e d number o f r i n g s c l e a n l y t o o b t a i n good b u t t - j o i n s . See Fig.7. ( i i i ) Proceed as f o r pre-formed s p l i t p a c k i n g r i n g s .
Fig.7 Where pre-formed s p l i t p a c k i n g r i n g s a r e used: ( i ) C a r e f u l l y remove o l d p a c k i n g ( i n c l u d i n g where a p p r o p r i a t e t h e p a c k i n g on t h e f a r s i d e o f a l a n t e r n r i n g ) . ( i i ) Thoroughly c l e a n a l l s u r f a c e s t h a t w i l l c o n t a c t t h e p a c k i n g and, where p e r m i t t e d , smear w i t h o i l .
Gland and neck bushes, s h a f t s u r f a c e and
b e a r i n g s s h o u l d a l s o be checked f o r s i g n s o f wear and r e c t i f i e d as necessary. ( i i i ) Place f i r s t r i n g o v e r t h e s h a f t by opening t o an ' S ' c o n f i g u r a t i o n t o ensure t h a t bending e f f e c t s a r e spread o v e r t h e whole r i n g .
Fig.8
See Fig.8.
389 ( i v ) I n s e r t f i r s t r i n g i n t o s t u f f i n g box and l i g h t l y bed i n w i t h a s p l i t (wooden) d i s t a n c e p i e c e and g l a n d s p i g o t .
With p l a i t e d packing t h e ' V '
f o r m a t i o n on t h e o u t s i d e di a met er o f t h e r i n g should be p o i n t i n g i n the d i r e c t i o n o f s h a f t r o t a t i o n .
F ig .9 (a)
See Fig.9.
P l a i t e d p acki n g
Eig.g(b)
F o i l wrapped packing
(v) Repeat ( i i ) and ( i v ) w i t h remainder o f r i n g s e n s u r i n g t h a t each r i n g
i s f i r m l y s ea t e d and t h a t t h e b u t t j o i n s a r e staggered by a t l e a s t 90".
N.B.
The
ing s must be f i t t e d i n d i v i d u a l l y and under no circumstances
s h o u ld complete s e t s be f i t t e d as a u n i t . ( v i ) When t h e r e q u i s i t e number o f r i n g s have been f i t t e d , u n t i l t h e s h a f t o r s p i n d l e t o r q u e i n creases. p u l l up t o f i n g e r t i g h t n e s s o n l y .
t i g h t e n gland nuts
Then s l a c k o f f g l a n d and
( I f pump i s t o be s t o r e d b e f o r e use
leave g l a n d s l a c k so t h a t pa cki ng r e s i l i e n c e i s n o t impaired). ( v i i ) Running i n pumps
10 t o 15 minutes.
Prime c a s t i n g and r u n pump up t o o p e r a t i n g speed f o r
I f pump i s n o t f i t t e d w i t h g l a n d c o o l i n g , a c o l d
water spray o v e r t h e g l a n d ho usi n g w i l l a v o i d e x c e s s i v e heat b u i l d - u p d u r i n g t h i s stage.
I f n o leakage occurs,
pressure and s l a c k e n g l a n d f u r t h e r .
s t o p pump, v e n t c a s i n g
Repeat u n t i l leakage s t a r t s .
The c o n t r o l l e d leakage, e s s e n t i a l f o r l u b r i c a t i o n purposes, can then be o b t a i n e d by r u n n i n g t h e pump and e v e n l y t i g h t e n i n g the g l a n d n u t s i n increments o f 2 f l a t s u n t i l ap pro xi mately one drop e v e r y few seconds i s o b ta in e d .
Ap pro xi mat el y 15 mi n ut es should be l e f t between successive
adjustments.
DO NOT OVERTIGHTEN.
( v i i i ) Where lo o s e f o r m m a t e r i a l i s used f o r v a l v e s proceed as ( i ) and ( i i ) and tamp p a c k in g i n t o a dense homogeneous mass p r o g r e s s i v e l y f i l l i n g the housing t o t h e r e q u i r e d degree.
390 ( i x ) Because o f t h e danger o f c o r r o s i o n t h r o u g h e l e c t r o l y t i c a c t i o n , packings c o n t a i n i n g g r a p h i t e should be avoided on v a l v e s o r pumps w i t h s t a i n l e s s o r chrome s t e e l stems.
T h i s r i s k i s most a c u t e when t h e p a c k i n g remains
i n t h e g l a n d d u r i n g s t o r a g e and i s p a r t i c u l a r l y aggravated by t h e presence o f m o i s t u r e .
16.4.7
F a u l t Finding
A m a j o r advantage o f compression packings i s t h a t breakdown i s r a r e l y sudden o r c a t a s t r o p h i c b u t r a t h e r a m a t t e r o f a g r a d u a l b u i l d up o f leakage u n t i l an unacceptable l e v e l i s reached.
Normally, c o n s i d e r a b l e l i f e can be achieved by
c o n t r o l l i n g leakage w i t h f u r t h e r t i g h t e n i n g o f t h e g l a n d n u t s (N.B.
t h e seepage
o f f l u i d which a c t s as a l u b r i c a n t f o r t h e p a c k i n g on r o t a r y a p p l i c a t i o n s should n o t be confused w i t h leakage and t h e r a t e o f one drop e v e r y few seconds should be m a i n t a i n e d ) .
However,
i f o t h e r than r o u t i n e maintenance o r j u s t p l a i n
' f a i r wear and t e a r ' a r e suspected as t h e cause o f leakage and t h e need f o r re-packing,
then t h e f o l l o w i n g h i n t s c o u l d w e l l prove u s e f u l .
( i ) Confirm t h a t t h e p a c k i n g i s r a t e d as s u i t a b l e f o r t h e a p p l i c a t i o n . ( i i ) I f one o r more r i n g s a r e m i s s i n g f r o m t h e s e t , check f o r e x c e s s i v e neck bush c l e a r a n c e a l l o w i n g e x t r u s i o n o f r i n g s i n t o t h e system. I f t h e t o p r i n g has e x t r u d e d between t h e g l a n d f o l l o w e r , a n t i - e x t r u s i o n
r i n g s c o u l d a v o i d replacement o f metal p a r t s . (iii)
I f t h e p a c k i n g ' s r a d i a l t h i c k n e s s appears d i m i n i s h e d i n one o r more
p l a c e s , check f o r an u n d e r s i z e s h a f t o r b a d l y worn b e a r i n g s which c o u l d cause s h a f t whip o r s p i n d l e wobble. ( i v ) I f r a d i a l s e c t i o n o f p a c k i n g d i r e c t l y beneath t h e s h a f t
i s reduced or
premature leakage o c c u r s a l o n g t h e t o p o f t h e s h a f t , check f o r m i s a l i g n m e n t o f s h a f t c e n t r e t o s t u f f i n g box bore. ( v ) I f p a c k i n g i s worn on t h e o u t e r diameter, check f o r loose r i n g s or r i n g s r o t a t i n g w i t h t h e s h a f t due t o i n s u f f i c i e n t g l a n d load. ( v i ) I f p a c k i n g r i n g s have b u l g e s on t h e i r r a d i a l faces,
the adjacent r i n g
was p r o b a b l y c u t t o o s h o r t , c a u s i n g p a c k i n g under p r e s s u r e t o be f o r c e d i n t o t h e gap a t t h e j o i n t . ( v i i ) I f p a c k i n g n e a r e s t g l a n d s p i g o t shows e x c e s s i v e d e f o r m a t i o n w h i l s t o t h e r r i n g s a r e i n f a i r c o n d i t i o n , t h e s e t was p r o b a b l y i n c o r r e c t l y i n s t a l l e d and s u b j e c t e d t o e x c e s s i v e g l a n d tightening.. N.B.
O v e r - t i g h t e n i n g i s u s u a l l y t h e g r e a t e s t s i n g l e cause o f premature p a c k i n g f a i l u r e .
391 ( v i i i ) I f t h e cause o f your p a r t i c u l a r problem i s s t i l l n o t apparent, g i v e equal a t t e n t i o n t o I.D.
and O.D.
leakage and check f o r a rough s t u f f i n g
box bore b e f o r e seeking s p e c i a l i s t a d v i c e .
16.4.8
Standardization
In t h e i n t e r e s t s o f s t o c k c o n t r o l i t i s c l e a r l y an advantage t o r a t i o n a l i s e the v a r i e t y o f packings used i n any p l a n t t o t h a t minimum number which w i l l e f f e c t i v e l y c a t e r f o r a l l t h e c o n d i t i o n s l i k e l y t o be encountered. no o b j e c t ,
If cost i s
then t h e r e a r e s i n g l e , s o p h i s t i c a t e d m a t e r i a l s and c o n s t r u c t i o n s
t h a t w i l l go some way towards s a t i s f y i n g most demands, b u t i t i s d o u b t f u l i f cost e f f e c t i v e n e s s c o u l d be j u s t i f i e d .
Far b e t t e r t o compromise on a small
number o f r e p u t a b l e p r o d u c t s developed f o r t h e areas i n q u e s t i o n , e.g.
pumps,
valves, e t c . There a r e few standards a p p l y i n g t o compression packings on a n a t i o n a l o r i n t e r n a t i o n a l b a s i s , a l t h o u g h many i n d i v i d u a l companies and o r g a n i z a t i o n s have domestic standards which have,
i n many cases, been t h e s u b j e c t o f c o l l a b o r a t i o n
between user and packing manufacturer. BS 4371 : 1968 s p e c i f i e s minimum standards f o r l u b r i c a t e d p l a i t e d c o t t o n , lubricated p l a i t e d f l a x ,
l u b r i c a t e d p l a i t e d o r b r a i d e d asbestos, d r y w h i t e non-
m e t a l l i c p l a i t e d o r b r a i d e d asbestos, p l a i t e d o r b r a i d e d asbestos, m e t a l l i c wire r e i n f o r c e d ,
i n d u r a t e d asbestos,
and l u b r i c a t e d f i b r o u s asbestos and g i v e s
guidance on l i m i t i n g o p e r a t i n g parameters f o r these c o n s t r u c t i o n s . Where packings a r e r e q u i r e d f o r s e r v i c e w i t h p o t a b l e water i n t h e Water A u t h o r i t y d i s t r i b u t i o n system (which covers r e s e r v o i r t o t a p ) , o n l y those mate r i a l s which have gained a N a t i o n a l Water Council Approval may be used.
Such
products have been t e s t e d t o e s t a b l i s h t h a t t h e y produce no c o l o u r , t a s t e o r t u r b i d i t y , a r e n o n - t o x i c and w i l l n o t support m i c r o b i a l growth. S t a t u t o r y Instruments 1978 No. 1927 "The M a t e r i a l s and A r t i c l e s i n Contact w i t h Food R e g u l a t i o n s 1978" r e q u i r e d t h a t compression p a c k i n g m a t e r i a l s , f o r example,
I'
... do
o r l i k e l y t o be,
n o t t r a n s f e r t h e i r c o n s t i t u e n t s t o foods w i t h which they a r e , i n contact,
i n q u a n t i t i e s which c o u l d -
( i ) endanger human h e a l t h o r ( i i ) b r i n g about a d e t e r i o r a t i o n i n t h e o r g a n o l e p t i c (sensory q u a l i t y ) o f such food o r an unacceptable change i n i t s n a t u r e , substance, o r qua1 i t y . " Such r e g u l a t i o n s i n e v i t a b l y r e s t r i c t t h e range o f a v a i l a b l e m a t e r i a l s and lubricants.
C o n s u l t a t i o n w i t h t h e s u p p l i e r i s recommended t o e s t a b l i s h p r e -
f e r r e d grades.
392
16.4.9
The F u t u r e
The research c u r r e n t l y b e i n g undertaken as a d i r e c t r e s u l t o f t h e a n t i asbestos lobby may produce glass/PTFE/graphite match e x i s t i n g m a t e r i a l s .
o r ceramic c o n s t r u c t i o n s which
However, many problems remain t o be s o l v e d i n p r o -
ducing a general purpose p r o d u c t t h a t can compete w i t h asbestos i n terms o f l u b r i c a n t r e t e n t i o n , absorbency o r d u r a b i l i t y a t comparable c o s t . There can be no doubt t h a t g r a p h i t e f o i l p r o d u c t s w i l l , because o f t h i s s i t u a t i o n , see g r e a t e r acceptance d e s p i t e h i g h i n i t i a l c o s t . r e p o r t s a r e e x t r e m e l y encouraging and,
Performance
i r r e s p e c t i v e o f c u r r e n t market con-
s i d e r a t i o n s , t h e m a t e r i a l deserves t o succeed on i t s own m e r i t s .
393 APPENDIX 1 COPY OF STATEMENT ISSUED BY THE ASBESTOS INFORMATION CENTRE,
40 PICCADILLY, LONDON W1V 9PA, TO THE BRITISH VALVE MANUFACTURERS ASSOCIATION, 3 PANNEL COURT, CHERTSEY STREET, GUILDFORD, SURREY,
GU1 4EU
ON 3 0 t h APRIL 1980. SAFETY OF ASBESTOS GLAND PACKINGS AND GASKETS C r y s o t i l e ( w h i t e ) asbestos f i b r e i s a b a s i c c o n s t i t u e n t o f v a l v e packings and gaskets because i t combines i n one m a t e r i a l s o f t n e s s ,
r e s i l i e n c e , absorption
p r o p e r t i e s , s t r e n g t h as a r e i n f o r c e m e n t and, where r e q u i r e d , h e a t r e s i s t a n c e . Asbestos i s o n l y a r i s k t o h e a l t h i f i t s d u s t i s i n h a l e d .
Valve packings and
gaskets based on c h r y s o t i l e ( w h i t e ) asbestos a r e s a f e t o handle by v a l v e users and by maintenance e n g i n e e r s .
A good s t a n d a r d o f i n d u s t r i a l hygiene should be
observed when h a n d l i n g o r u s i n g p r o d u c t s which c o n t a i n asbestos. The A d v i s o r y Committee on Asbestos i n t h e i r i n t e r i m statement and f i n a l r e p o r t p u b l i s h e d i n October 1979 recommend t h a t asbestos d u s t should be k e p t t o t h e lowest p r a c t i c a b l e l e v e l and c a l l f o r a 1 f i b r e / m l c o n t r o l l i m i t .
The r e p o r t
f u r t h e r s t a t e s t h a t t h e presence o f c h r y s o t i l e i s u n l i k e l y t o have produced any m a t e r i a l i n c r e a s e i n t h e r i s k o f l u n g cancer i n t h e general p o p u l a t i o n o r any a p p r e c i a b l e number o f cases o f mesothelioma, and t h e same i s c e r t a i n l y t r u e o f asbestosis.
Gland packings and gaskets w i l l n o t c r e a t e d u s t l e v e l s i n
excess o f t h e c o n t r o l l i m i t , g i v e n normal usage and maintenance. ' L u b r i c a t e d ' packings a r e vacuum impregnated w i t h m i n e r a l o i l s and greases, g r a p h i t e and o t h e r l u b r i c a n t s , and do n o t e m i t d u s t . ' D r y ' p a c k i n g s a r e n o r m a l l y t r e a t e d w i t h a dust-suppressant which s i g n i f i c a n t l y reduces d u s t e m i s s i o n so t h a t i n normal use, n o t be expected t o p r e s e n t a hazard.
i n c l u d i n g maintenance, they would
They a r e n o t commonly used i n v a l v e glands.
'Hard' packings and gaskets (compressed asbestos f i b r e ) a r e made f r o m a comb i n a t i o n o f asbestos, rubber and o t h e r f i l l e r s .
The asbestos f i b r e i s locked
i n t o t h e rubber m a t r i x and w i l l n o t n o r m a l l y c r e a t e d u s t i n use o r maintenance.
I f however, these m a t e r i a l s a r e s u b j e c t e d t o g r i n d i n g r o r o t h e r a b r a s i v e p r o cesses, p r e c a u t i o n s should be taken t o a v o i d i n h a l i n g any dust which may be emitted. ' S o f t ' o r moulded packings a r e n o r m a l l y made f r o m r u b b e r i s e d asbestos c l o t h , and t h e asbestos i s sealed w i t h i n t h e rubber c o a t i n g , so t h a t they a l s o a r e s a f e i n use and maintenance.
394 C r o c i d o l i t e ( b l u e ) asbestos No c r o c i d o l i t e
( b l u e ) a s b e s t o s has been used i n p a c k i n g s and g a s k e t s manuf-
a c t u r e d i n t h e UK f o r s e v e r a l years. however,
incorpo ra te b l u e asbestos.
Some i m p o r t e d p a c k i n g s and g a s k e t s may When l u b r i c a t e d t h e s e p a c k i n g s a r e s a f e
t o handle but r e s p i r a t o r y p r o t e c t i o n w i l l
be needed when t h e y a r e i n a d r y
state. Remova 1 Care s h o u l d be taken when removing o l d p a c k i n g s w h i c h may have l o s t t h e i r lubricants. i.e.
They s h o u l d be damped and t h e n removed w i t h t h e c o r r e c t t o o l s ,
packing e x t r a c t o r s .
Label 1 i n g Any p a c k i n g s o r g a s k e t s w h i c h r e q u i r e s p e c i a l h a n d l i n g p r e c a u t i o n s c a r r y a warning l a b e l .
395
17
CENTRALISED LUBRICATION SYSTEMS DESIGN
J.G.MERRETT,
17.1
Managing D i r e c t o r , E n g i n e e r i n g and General Equipment L t d .
INTRODUCTION
W h i l s t we a r e k e p t c o m p a r a t i v e l y w e l l informed o f t h e l a t e s t developments i n o i l and grease technology and t h e v i t a l r o l e i t p l a y s i n i n d u s t r i a l and commerc i a l applications,
v e r y l i t t l e has been w r i t t e n about t h e e q u a l l y v i t a l "Centra-
I i s e d L u b r i c a t i o n Systems" and some o f t h e methods a v a i l a b l e t o Engineers by which grease and o i l may be t r a n s m i t t e d t o t h e p o i n t o f l u b r i c a t i o n . I n our s o c i e t y where energy, machinery and l a b o u r a r e now (1981) expensive commodities,
i t i s c l e a r t h a t i n t h e p a s t i n s u f f i c i e n t a t t e n t i o n has been p a i d
t o the d i r e c t and i n d i r e c t losses o f energy, occasioned by wear and f r i c t i o n and t o t h e savings o f m a t e r i a l s .
However,
i n 1977 a government f i n a n c e d Ameri-
can Report suggested t h a t $16.25 b i l l i o n p.a.
( a t 1976 v a l u e s ) c o u l d be saved
by a "Strategy f o r Energy Conservation through T r i b o l o g y "
U.K.
[l].
Converted i n t o
(1980) v a l u e s , t h i s i n d i c a t e s an e q u i v a l e n t s a v i n g o f energy through t r i -
bology i n excess o f €1;
b i l l i o n p.a.
I n s h o r t , c o r r e c t l y s e l e c t e d l u b r i c a n t s and t h e i r methods o f a p p l i c a t i o n by C e n t r a l i s e d Systems can e f f e c t s i g n i f i c a n t savings, e.g. machine t o o l s , conveyors, cranes, r o l l i n g m i l l s , b l a s t furnaces, b a l l m i l l s , sugar machinery, paper m i l l s , heavy m o b i l e p l a n t , e t c . t o v a r y i n g degrees, a l l r e q u i r e t h e a p p l i c a t i o n o f lubricants.
A c o r r e c t l y designed and i n s t a l l e d C e n t r a l i s e d L u b r i c a t i o n
System i s t h e e n g i n e e r ' s insurance a g a i n s t some o f t h e severe t r i b o l o g i c a l problems, i . e .
f r i c t i o n and wear, which would o t h e r w i s e occur i f t h e p l a n t and
machinery were i n a d e q u a t e l y l u b r i c a t e d . U n f o r t u n a t e l y , a l l t o o o f t e n i n t h e supply of p l a n t and machinery t h e s e l e c t i o n o f t h e l u b r i c a n t and C e n t r a l i s e d L u b r i c a t i o n equipment a r e considered a t a l a t e stage i n t h e manufacture o f t h e p l a n t which, coupled w i t h t h e c o n f l i c t i n g i n t e r e s t s o f machinery b u i l d e r s , can r e s u l t i n u n s u i t a b l e l u b r i c a t i o n equipment being s e l e c t e d .
L i k e w i s e , t h e l u b r i c a t i o n equipment supply companies have a
r e s p o n s i b i l i t y , n o t o n l y t o know t h e i r own p r o d u c t , b u t a l s o t o a p p r e c i a t e t h e t r i b o l o g i c a l requirements o f t h e p l a n t and machinery which r e q u i r e s t o be
396 lubricated.
F a i l u r e i n t h i s area i n v a r i a b l y r e s u l t s i n the p l a n t user being
p l a c e d i n t h e u n e n v i a b l e p o s i t i o n o f h a v i n g t o a p p l y , a t an e a r l y stage, f o r a d d i t i o n a l c a p i t a l t o r e c t i f y new p l a n t o r , more o f t e n t h a n n o t , t o l i v e w i t h t h e problems and r e c t i f y as and when t h r o u g h a maintenance budget
-
both o f
which the p l a n t u s e r would be t h e f i r s t t o agree i s e n t i r e l y u n s a t i s f a c t o r y . T h i s paper endeavours t o deal w i t h some o f these f a c t o r s a f f e c t i n g t h e c h o i c e o f l u b r i c a n t and l u b r i c a t i n g equipment, t h e b a s i c elements o f t h e machinery t o be l u b r i c a t e d , and t h e c o n d i t i o n s under which i t o p e r a t e s .
17.2
POINTS OF LUBRICATION
Bearings, t h e e s s e n t i a l components o f p l a n t and machinery, may be g e n e r a l l y grouped i n t o j o u r n a l , t h r u s t , conveyor c h a i n p i n and l i n k , a n t i - f r i c t i o n , ways and crane r a i l s .
slide-
Each wear s u r f a c e must be t r e a t e d s e p a r a t e l y w i t h r e g a r d
t o l u b r i c a n t and l u b r i c a t i o n t e c h n i q u e . The l u b r i c a t i o n requirement o f a p l a i n j o u r n a l b e a r i n g i s t h e p r o v i s i o n o f an adequate and c o n s t a n t f l o w o f l u b r i c a n t o f s p e c i f i e d v i s c o s i t y t o g i v e a f l u i d f i l m o f high-load bearing capacity.
The j o u r n a l b e a r i n g has i n h e r e n t l y a
convergence between t h e s h a f t and t h e b e a r i n g .
When r e l a t i v e m o t i o n takes p l a c e
a f i l m o f l u b r i c a n t i s induced between t h e s u r f a c e s , e f f e c t i v e l y s e p a r a t i n g them. Bearings employed t o absorb t h r u s t and p r e v e n t misalignment of s h a f t s v a r y g r e a t l y i n t y p e and l u b r i c a t i o n requirement, w h i l s t a n t i - f r i c t i o n b e a r i n g s r e q u i r e l e s s l u b r i c a n t than p l a i n b e a r i n g s .
Most a v a i l a b l e formulae d e a l i n g
w i t h t h e a p p l i c a t i o n o f grease t o these b e a r i n g s t r e a t speed as an i m p o r t a n t factor.
For small a n t i - f r i c t i o n b e a r i n g s such as those employed i n l i g h t l y
loaded f r a c t i o n a l horse-power motors, t o o much grease can be damaging. I n such cases, recommended l u b r i c a t i o n i n t e r v a l s o f up t o s e v e r a l years have been e s t a blished.
Chain p i n s and l i n k s p r e s e n t m a j o r c r i t i c a l wear p o i n s on f l o o r and
overhead ( i n c l u d i n g Power and Free) conveyors.
I n t h e automobi e i n d u s t r y ,
c h a i n l e n g t h s o f s e v e r a l hundred metres, h a v i n g thousands o f
PO
nts requiring
l u b r i c a t i o n , a r e commonplace ( F i g . 1 ) . Slideway and crane r a i l l u b r i c a t i o n r e q u i r e s t h e r i g h t l u b r i c a n t and t h e r i g h t a p p l i c a t o r (see Fig.10,
S e c t i o n 17.6.1).
Too l i t t l e l u b r i c a n t r e s u l t s i n
r a p i d wear; e x c e s s i v e l u b r i c a n t can be a hazard t o l i f e o r l i m b . Every case i s d i f f e r e n t , y e t i n e v e r y case i t i s c r i t i c a l t h a t t h e l u b r i c a n t i s a p p l i e d i n l i n e w i t h the requirements o f t h a t component,
b o t h w i t h r e g a r d t o mechanical
wear and t o energy conservancy.
17.3
SELECTING THE LUBRICANT
-
O I L OR GREASE?
I n modern machinery l u b r i c a t i o n ,
l u b r i c a n t s and t h e means o f t h e app i c a t i o n
must be considered t o g e t h e r . The b e s t l u b r i c a n t w i l l s e r v e no u s e f u l purpose i f i t i s n o t a p p l i e d a t t h e
397
Fig.1
Excessive p i n and l i n k wear i n a conveyor c h a i n . A 3mm p i n wear p e r p i n on a 732111 conveyor increases t h e c h a i n l e n g t h by 23 metres.
r i g h t t i m e , a t t h e r i g h t p l a c e , and i n t h e c o r r e c t q u a n t i t y .
Conversely, t h e
best l u b r i c a t i o n equipment, a p p l y i n g l u b r i c a n t q u i t e c o r r e c t l y , w i l l be o f l i t t l e use i f t h e l u b r i c a n t i t feeds i s u n s u i t a b l e f o r t h e d u t y i t has t o perform. L u b r i c a n t s g e n e r a l l y , e i t h e r o i l s o r greases, a r e s u p p l i e d by o i l companies supported by s p e c i a l i s t o i l and grease m a n u f a c t u r i n g companies, who w i l l p r o v i d e advice on any p a r t i c u l a r l u b r i c a n t and a p p l i c a t i o n as w e l l as on t h e c h o i c e o f lubricants i n general.
The s u p p l i e r o f t h e l u b r i c a t i o n equipment w i l l a l s o
o f t e n be a b l e t o a d v i s e , e s p e c i a l l y on l u b r i c a t i o n d e s i g n problems, o r a c t as an intermediary
.
The s u b j e c t o f l u b r i c a n t s i s t r e a t e d h e r e o n l y i n i t s v e r y broadest terms under t h r e e headings:"OIL"
o r GREASE";
when t o use one and when t h e o t h e r .
LUBRICATING GREASE;
t h e types and how t o s e l e c t them.
LUBRICATING OILS; types and how t o s e l e c t them.
17.3.1
O i l o r Grease? When t o Use One and When t h e Other
Movement between two d r y s u r f a c e s causes heat and wear.
The purpose o f
i n t r o d u c i n g a l u b r i c a n t between t h e two s u r f a c e s i s t o reduce f r i c t i o n , heat, and wear.
O i l has t h e f o l l o w i n g advantages: and i t scavenges.
A t t h e same t i m e ,
i t flows,
i t penetrates,
i t removes heat,
i t has l i m i t e d s e a l i n g q u a l i t i e s and poor
' s t a y i n g ' power, t h e r e f o r e r e q u i r e s more f r e q u e n t r e p l e n i s h i n g than grease.
398 L u b r i c a t i n g grease, a s e m i - s o l i d p l a s t i c - l i k e m a t e r i a l , has e x c e l l e n t s e a l ing characteristics,
possesses good ' s t a y i n g ' power,
i.e.
i t adheres t o surfaces
more r e a d i l y and l o n g e r t h a n o i l and i t can be an e x c e l l e n t c o r r o s i o n p r e v e n t a tive.
Unlike o i l ,
i t i s a poor conductor o f heat and a poor scavenger;
n o t f l o w o r spread e a s i y where c o n t a m i n a t i o n by
-
i t does
t h e l a t t e r p r o p e r t y can, however, be an advantage
u b r i c a n t s must be avoided such as i n f o o d and b o t t l i n g
and t h e t e x t i l e i n d u s t r es and where t h e l u b r i c a n t i s used as a s e a l i n g medium. Where an a p p l i c a t i o n e n t a i l s HEAT REMOVAL, o i l i s t h e r e f o r e g e n e r a l l y t h e choice.
I t can be appl ed i n t h e f o r m o f a l i q u i d o r as an o i l m i s t ,
m i c r o - f o g comprising an o i l - a i r m i x t u r e .
i.e.
I n severe cases o f h e a t removal, o i l
can be r e c i r c u l a t e d and, d u r i n g t h e c i r c u l a t i o n ,
i t can be c o o l e d and cleaned.
Large t u r b i n e b e a r i n g s and f a s t gear t r a i n s a r e t y p i c a l examples where o i l c i r c u l a t i n g systems a r e used and l a r g e amounts o f o i l a r e c i r c u l a t e d . For a p p l i c a t i o n s where heat removal i s n o t a p r o b l e m , b u t LUBRICATION o r t h e PREVENTION OF INGRESS OF DIRT, WATER, DUST and OTHER EXTRANEOUS MATTER i s l u b r i c a t i n g grease can be used.
-
I t s a p p l i c a t i o n ranges from heavy m i l l b e a r i n g s
and s l i d e s t o t e x t i l e machinery, e t c .
I t i s a l s o t h e most s u i t a b l e where a p p l i -
c a t i o n o f t h e l u b r i c a n t i s r e q u i r e d a t LONG INTERVALS. N e a r l y a l l B a l l and R o l l e r Bearings, except those i n r e s p e c t o f which heat removal i s e s s e n t i a l , a r e g e n e r a l l y grease l u b r i c a t e d .
Where DIRTY/DUSTY and
WET CONDITIONS e x i s t , grease
s generally preferable.
l u b r i c a t i o n makes grease t h e
p r e f e r r e d ' l u b r i c a n t f o r the vast m a j o r i t y o f
The convenience o f grease
r o l l i n g bearings. G e n e r a l l y , o i l l u b r i c a t i o n i s employed i n t h e r e l a t i v e l y few cases where i t i s n o t p o s s i b l e t o t a k e advan age o f t h e m e r i t s o f grease as a r o l l i n g b e a r i n g
1u b r i c a n t . Where motion i s INTERMITTENT o r OSCILLATING, grease i s o f t e n t h e more s u i t able lubricant. Summarising, where heat conveyance away f r o m r u b b i n g s u r f a c e s o r p e n e t r a t i o n o f t h e l u b r i c a n t i s o f importance, o r t h e scavenging f u n c t i o n i s necessary, o i l i s p r e f e r a b l e , w h i l s t l u b r i c a t i n g grease can and,
i n most cases, should be
considered as t h e p r e f e r r e d l u b r i c a n t f o r slow moving machinery,
long l i f e
l u b r i c a t i o n , and where d i r t and d u s t has t o he kept o u t o f t h e b e a r i n g s , s l i d i n g surfaces, etc.
17.3.2
L u b r i c a t i n g Grease
-
The Types and How t o S e l e c t Them
Greases designed f o r l u b r i c a t i o n a r e e s s e n t i a l l y a m i x t u r e o f m i n e r a l o i l and t h i c k e n e r , a c c o r d i n g to t h e a p p l i c a t i o n requirement.
I n t h e most w i d e l y
used modern grease t h e t h i c k e n e r i s a m e t a l l i c soap, u s u a l l y o f l i t h i u m o r calcium, w i t h t h e q u a n t i t y o f o i l a d j u s t e d t o g i v e a s o l i d , s e m i - s o l i d o r semi f 1u i d consistency.
399 The soap f i b r e s form a s t r u c t u r e t h a t r e t a i n s t h e o i l , t h e dimensions and arrangement o f t h e f i b r e s v a r y i n g a c c o r d i n g t o t h e metal and t h e f a t t y a c i d from which t h e soap was made.
The q u a n t i t y , dimensions and d i s t r i b u t i o n o f t h e
f i b r e s a r e t h e main parameters c o n t r o l l i n g t h e s t a b i l i t y and f l o w p r o p e r t i e s o f this lubricating material. One o f t h e most i m p o r t a n t p h y s i c a l f e a t u r e s o f grease i s s t i f f n e s s ( f o r s i s t e n c y ) which i s i n d i c a t e d by a t e s t t h a t measures t h e d e p t h t h a t t h e cone s i n k s i n t o a sample o f grease.
The d e p t h measured i s a t e n t h o f a m i l l i m e t r e ,
and r e f e r r e d t o as t h e degree o f p e n e t r a t i o n .
F i g u r e 2 shows one c l a s s i f i c a -
t i o n system f o r greases. Grade Number.
445
000 OD 0 1 2 3
400
355 310 265 220
4 5 6 Fig.2
Worked p e n e t r a t i o n a t 25'C
175 130
85
-
-
-
-
-
475 430 385 340 295 250 205 160 115
Description Very f l u i d Fluid Semi - f 1 u i d Very s o f t Soft Semi-firm Firm Very f i r m Hard
NLGl c l a s s i f i c a t i o n o f t h e c o n s i s t e n c y o f greases.
Although lithium-based greases today s a t i s f y a v a s t number o f l u b r i c a t i o n requirements f o r cranes, conveyors, f o r g i n g presses, continuous c a s t i n g p l a n t s , r o l l i n g mills, etc.,
t h e r e a r e a number o f s i t u a t i o n s r e q u i r i n g l u b r i c a n t s w i t h
s p e c i a l p r o p e r t i e s , f o r example, t h e a b i l i t y t o w i t h s t a n d h i g h temperatures such as those encountered i n power s t a t i o n t u r b i n e s , bakery oven conveyors, e t c . These l a t t e r greases i n c l u d e t h e c l a y - t h i c k e n e d and o t h e r s o l i d - t h i c k e n e d compounds which w i t h s t a n d c o n s i d e r a b l y h i g h e r temperatures, e s p e c i a l l y when employed w i t h h i g h temperature s y n t h e t i c l u b r i c a n t s , e.g.
polyglycol, s y n t h e t i c esters,
and s i l i c o n e s . The upper temperature l i m i t a t which any grease may be used i s dependent p a r t l y on t h e t y p e o f t h i c k e n e r , p a r t l y on t h e f l u i d and i t s r e q u i r e d s e r v i c e life.
Higher o p e r a t i n g temperatures have t h e e f f e c t o f s h o r t e n i n g t h e l u b r i -
c a n t ' s s e r v i c e l i f e and r e d u c i n g p e r m i s s i b l e o p e r a t i n g speeds.
I t i s therefore
convenient t o express t h e working l i m i t s o f a grease i n terms o f b e a r i n g speeds and temperatures, as shown i n Fig.3.
T h i s diagram shows t h a t l i t h i u m , sodium
and calcium-based greases have upper temperature l i m i t s o f 130, 110 and 7OoC r e s p e c t i v e l y , and t h a t clay-based and calcium-complex greases can be used a t up t o 150°C.
400
Fig.3
17.3.3
Lubricating 011s
Working l i m i t s o f m i n e r a l o i l greases
-
Types and How t o S e l e c t Them
Vegetable and animal o i l s a r e e x c e l l e n t l u b r i c a n t s , b u t have s h o r t l i f e as they o x i d i s e and tend t o go r a n c i d ; as a r e s u l t , t h e i r main use i s f o r 'one s h o t ' a p p l i c a t i o n s such as f o r f o r g i n g o r as a d d i t i v e s f o r m i n e r a l o i l s . G e n e r a l l y , t h e o r i g i n o f t h e o i l determines i t s use ( F i g . 4 )
O i l Origin
ADD1
ication
Mineral O i l
e.g. p e t r o l e u m base f o r general l u b r i c a t i o n o f mechanical p a r t s such as engines, gears and g e n e r a l e n g i n e e r i n g p l a n t .
Vegetable O i l
e.g. c a s t o r , palm, and rape seed o i l s f o r s p e c i a l a p p l i c a t i o n s where h i g h l u b r i c i t y i s d e s i r a b l e such as k i l n s , bakery ovens.
Animal O i l
e.g. sperm o r o t h e r f i s h o i l s from sheep wool f o r a p p l i c a t i o n s such as k i l n s , bakery ovens.
Synthetic O i l
e.g. glycol d e r i v a t i v e s and d i e s t e r f o r extreme h i g h temperature. Fig.4
Origin of Oils
401 F i g u r e 5 i l l u s t r a t e s how v i s c o s i t y o f o i l s change w i t h temperature, becoming t h i n n e r when they a r e heated, b u t they do n o t change v i s c o s i t y a t t h e same r a t e . The r a t e o f v i s c o s i t y change w i t h temperature i s r e f e r r e d t o as the ' v i s c o s i t y index'.
-40-30-20 -10 0 10 20 30 $0 50 60 70 80 90XX)11O1201X)14015MM)710
degrees fahrenheit
Fig.5
17.3.3.1
V i s c o s i t y vs Temperature f o r two o i l s h a v i n g v i s c o s i t y indexes o f 0 and 100 r e s p e c t i v e l y .
The p r o p e r t i e s o f t h e o i l must be c a r e f u l l y considered when d e s i g n i n g
a l u b r i c a t i o n system, e.g.
conveyor chains passing through a p r e - t r e a t m e n t oven
may reach a temperature o f 180/200°C.
O i l s w i t h s p e c i a l i n c l u s i o n s such as
g r a p h i t e o r molybdenum d i s u l p h i d e i n g l y c o l as used i n bakery ovens must have p r o v i s i o n f o r a g i t a t i o n o r r e c i r c u l a t i o n w i t h i n t h e l u b r i c a t o r s t o r a g e tank t o prevent s e t t l i n g o u t o f t h e g r a p h i t e o r molybdenum. Bakery ovens g e n e r a l l y use molybdenum d i s u l p h i d e i n g l y c o l w h i l s t f o r extreme p r e s s u r e l o a d i n g c o n d i t i o n s on Power and Free t r o l l e y s , as i n the c a r i n d u s t r y , chains and t r o l l e y wheels r e q u i r e s p e c i a l l u b r i c a n t s , h a v i n g a h i g h adhesion c h a r a c t e r i s t i c - a s w e l l as i m p a r t i n g r u s t - p r o o f i n g and w a t e r - p r o o f i n g .
402
17.3.3.2
A f u r t h e r s p e c i a l i s t l u b r i c a n t i s rape seed o i l ;
f a t t y vegetable o i l .
t h i s i s a low a c i d
Because o f i t s r e l a t i v e l y h i g h f l a s h - p o i n t
i t i s suitable
f o r h i g h temperature work when r e f i n e d and g i v e n a g r a p h i t e i n c l u s i o n . uses a r e f o r continuous c a s t i n g moulds, f o r g i n g , metals.
Typical
and as a c u t t i n g o i l f o r harder
Here a g a i n , i t s s p e c i a l p r o p e r t i e s must be considered when p r o v i d i n g
t h e l u b r i c a t i n g means.
17.3.4
Summarising L u b r i c a n t S e l e c t i o n
For t h e f i n a l s a t i s f a c t o r y s e l e c t i o n o f l u b r i c a n t t h e f o l l o w i n g f a c t o r s must t h e r e f o r e be determined.
The c o n s t r u c t i o n and m a t e r i a l s employed i n t h e compon-
e n t s t o be l u b r i c a t e d , o p e r a t i n g speeds and l o a d i n g , l i f e requirement o f l u b r i c a n t and machine, o p e r a t i n g environment, e.g.
t h e presence o f w a t e r , steam,
chemicals o r a b r a s i v e m a t e r i a l s i n t h e atmosphere; and l a s t , b u t n o t l e a s t , t h e method o f a p p l i c a t i o n , t h e t y p e o f system t o be employed, t h e diameter of p i p e runs, e t c .
17.4
PIPE DIAMETER vs
FLOW CHARACTERISTICS
To determine t h e l u b r i c a n t f l o w r a t e and volume,
t h e l e n g t h and diameter o f
t h e p i p e l i n e s should be c a r e f u l l y c a l c u l a t e d t o ensure t h a t t h e l u b r i c a n t can s a t i s f a c t o r i l y reach t h e wear p o i n t s a t t h e extreme ends o f t h e p i p e l i n e s .
The
f o l l o w i n g t a b l e summarises some p r a c t i c a l r e s u l t s f r o m t e s t s t o prove optimum measurements and e n d - o f - l i n e p r e s s u r e , u s i n g a l i t h i u m - b a s e d grease o f No.2 consistency a t
15OoC. Nominal bore (mm)
50 38 25 19 17.5
Applied pressure (kPa)
P r e s s u r e drop (kPa/m)
1100
36.1 61.5 144.7 210.3
1 a75
4410 6410
LUBRICATION REQUIREMENTS FOR PLAIN BEARINGS
For normal w o r k i n g c o n d i t i o n s i t has been found t h a t t h e amount o f grease required f o r p l a i n bearings i s equivalent t o a layer o f O . l m m b e a r i n g area (0.1 x d x L) p e r hour o f b e a r i n g o p e r a t i o n .
on t h e developed
Figure
6 illustrates
a c h a r t f o r c a l c u l a t i n g t h e grease requirements f o r p l a i n b e a r i n g s . p l e t o c a l c u l a t e t h e grease r e q u i r e d f o r a b e a r i n g o f long,
i n t e r s e c t diameter and l e n g t h a s a t
*
As an exam-
75mm diameter by 250mm
f o l l o w l i n e o f a r r o w and where i t
i n t e r s e c t s t h e t o p s c a l e , t h i s i n d i c a t e s t h e amount o f grease r e q u i r e d , b e i n g i n t h i s example 6 g m o r 0.21 oz p e r hour.
403
D
LENGTH OF BEARING L
Fig.6
17.6
Grease requirements f o r a p l a i n b e a r i n g .
SELECTING THE RIGHT TYPE OF LUBRICATION SYSTEM
L u b r i c a t i o n systems f o r p l a n t and equipment, whatever t h e a p p l i c a t i o n , i n d i v i d u a l design.
t h e b e a r i n g requirements and t h e d e s i g n e r s ' a p p l i c a t i o n s p e c i f i c a t i o n . regard t o t h e human element, chapter deal w i t h :
17.6.1
require
They must be f u n c t i o n a l and c o r r e c t l y engineered t o s a t i s f y
they must be f o o l - p r o o f .
With
Systems i n c l u d e d i n t h i s
Grease, O i l and Micro-Fog.
Grease L u b r i c a t i o n Systems
A l l C e n t r a l i s e d Grease L u b r i c a t i o n Systems a r e o f t h e n o n - r e c i r c u l a t i n g t y p e and o p e r a t e on t h e t o t a l loss p r i n c i p l e .
They a r e b a s i c a l l y d i v i d e d i n t o D i r e c t
Feeding systems and I n d i r e c t Feeding systems, o t h e r w i s e r e f e r r e d t o as L i n e systems, as i l l u s t r a t e d i n F i g . 7 .
17.6.1.1
D i r e c t Feeding Systems a r e those where t h e volume o u t p u t o f t h e D i r -
e c t Feeding l u b r i c a t i n g pump i s p o s i t i v e ;
t h e pumping p l u n g e r s and means o f
m e t e r i n g t h e o u t p u t t o i n d i v i d u a l wear p o i n t s b e i n g i n c o r p o r a t e d i n t h e l u b r i c a t i n g pump.
T h e r e f o r e D i r e c t Feeding systems o p e r a t e on a P o s i t i v e Volume P r i n -
ciple,
t h e y i n t r o d u c e a metered volume o f l u b r i c a n t i n t o t h e pipes, and
i.e.
s i n c e t h i s volume i s n o t a f f e c t e d by p r e s s u r e i n t h e p i p e s , t h e pumps can work c o n t i n u o u s l y a g a i n s t h i g h back pressures.
- -I
I
GREASE SYSTEMS TOTAL LOSS SYSTEMS
DIRECT FEEDSYSTEMS
INDIRECT FEED M T E M S
I
PR0GRESSI:E MSTEMS
I
I
Y DUAL L I N E
FIG.7
FUNDAMENTAL LUBRICATION
DIVISION OF C E N T R A L I S E D GREASE SYSTEMS.
PARALLEL SVSTEMS
405 D i r e c t Feeding systems can be d i v i d e d i n t o those where each o u t l e t has i t s own m e t e r i n g p l u n g e r (F i g. 8)
and t ho se where a s i n g l e moving plunger d i s t r i b u t e s
p r o g r e s s i v e l y t h e metered amounts o f grease i n t o t h e v a r i o u s o u t l e t s ( F i g . 9 ) . N o r m a lly , d i r e c t f e e d i n g l u b r i c a t o r s o b t a i n t h e i r pumping a c t i o n e i t h e r by means o f moving p l u n g e r s , combined w i t h a system o f p o r t i n g , o r by the use o f p l u n g e r s i n c o n n ect i on w i t h s p r i n g - l o a d e d b a l l v a l v e s .
The l a t t e r type should
be a v o id e d i n t h e case o f d i r t y o r d u s t y c o n d i t i o n s as any i n g r e s s o f e x t r a n eous m a t t e r may l od ge i n t h e s e a t i n g o f t h e b a l l v a l v e and render t h e l u b r i c a t o r inoperative. I t f o l l o w t h a t where a l u b r i c a t o r i s d r i v e n by a moving p a r t o f a machine, an amount o f l u b r i c a n t r e q u i r e d r e l a t i v e t o t h e movement o f t h e s h a f t i n t h e bearing bein g l u b r i c a t e d , a d i r e c t f e e d i n g l u b r i c a t o r should be used (Fig.10) l u b r i c a t o r d r i v e n v i a t h e d i r e c t d r i v i n g elements, e.g.
w i t h the
E c c e n t r i c D r i v e s , Throw
Plates, o r O f f s e t P i n Drives.
17.6.1.2
I n d i r e c t o r L i n e Systems have t h e pumping and m e t e r i n g elements geo-
g r a p h i c a l l y separated;
t he y a r e connected by means o f one o r s e v e r a l p i p e l i n e s .
The a c t i o n i s h y d r a u l i c ,
t h e pump s e r v i n g as a p r e s s u r e c r e a t i n g u n i t f o r pumping
grease i n t o a p i p e l i n e whi ch i s t h e r e b y s e t under pressure.
As t h e pressure
i nc r e a s e s , so t h e v a r i o u s m e t e r i n g elements e j e c t t h e i r charges e i t h e r progressi v e l y , o r a c c o r d i n g t o t h e back pre ssure s a g a i n s t which t h e y operate. D u r in g normal o p e r a t i o n s each element, h aving g i v e n up i t s metered amount,
blocks i t s e l f and w i l l n o t pass any f u r t h e r l u b r i c a n t t o t h e p o i n t s o f a p p l i c a tion.
When a l l elements have g i v e n up t h e i r metered charge, a r a p i d r i s e i n
pre s s u r e o c c u r s i n t h e main l i n e .
U t i l i z i n g e i t h e r t h i s i n c r e a s e i n pressure a t
t h e pump o r a t t h e end o f t h e l i n e , a s i g n a l i s g i v e n i n d i c a t i n g t h a t t h e l u b r i c a t i n g phase i s completed. Thereupon t h e main l i n e has t o be d ep ressurized, w hich i s u s u a l l y e f f e c t e d by opening i t t o t h e grease sup pl y u n i t , e.g.
the reservoir.
Depending on the
t y p e o r make o f system, t h e p l u n g e r s o f t h e m e t e r i n g v a l v e s a r e then r e s e t t o p e r m i t t h e i r f u r t h e r o p e r a t i o n , o r t h e y a r e a l r e a d y s e t f o r another a p p l i c a t i o n phase wh ic h moves them back i n t o t h e i r o r i g i n a l p o s i t i o n , thereby completing a system c y c l e .
The method and mechanics o f r e s e t t i n g depend on t h e p a r t i c u l a r
ty p e o f system; a l s o whether one complete c y c l e o f t h e system i n v o l v e s one o r two a p p l i c a t i o n ( l u b r i c a t i o n ) phases. The fundamental d i v i s i o n o f ' l i n e systems' p a r a l l e l t y p e s o f systems.
i s t h a t between p r o g r e s s i v e and
I n t h e p r o g r e s s i v e system t h e l u b r i c a n t must pass
thro u g h t h e m e t e r i n g elements o r v a l v e s p r o g r e s s i v e l y ,
i.e.
o n l y a f t e r having
a c t u a t e d t h e f i r s t element t o f e ed l u b r i c a n t t o t h e p o i n t o f a p p l i c a t i o n w i l l t h e l u b r i c a n t be passed t o t h e second element, and so f o r t h .
T h i s i s i n con-
t r a s t t o t h e o p e r a t i o n o f t h e p a r a l l e l system, on w hich t h e m e t e r i n g u n i t s a r e
YOl>INNO> OlNW
m
YOU3NN03 a 3 a ~ 33 1 1 ~ 1 s
SNOIWINNOJ L l l A n O
YJMOllOJON 3AVH Sl3aOW (110)V l
*I
:S)ltlOM 1 1 MOH
NOllV3l11!fll 01Slll11lN33 311VWOlflV 11Oj 1102V3111!111 0334 1331110 90P
407
408
Application: Lubiicat/onoluplo I W p ~ ) i n l ~ . ~ l l h g ~ e a l e o r o s . pafl(IcUlar1yon presses. machine lools, packagmg machinery
Introducingcentral lubrication in stages:
Design: A m a n pipe l m m a simple fillinp 01 lubricalion pump loads lo the dislrtbulor; the quantities of lubricant delivered are diitiibuled by the. d w i b u l o r 10the outlels in a parlicular pie. arranged sequence If required. the Iubricanl lrom the distributor can be led lo other distrtbulors lor lurlhet redislribulion
11 . .tg.: L u b r m i c o nof not more lhan 20 points by means 01 a grease nipple screwed into the dislributor andsgreasegun whichisoperaled untiltheflow indjcatoc shows tihat the lubr#calionoperation has been compleletl.
2nd .lago: A hand pump and dlslrlbUIorSin Series. WhlCh distribute the lubr~csnllo the lubrication point6 in a specified manner.
Fig. 11.
Examples o f Progressive Systems.
Fig. 12.
Progressive Lubrication on a plate shear.
409 a c t u a t e d as p r e s s u r e i n c r e a s e s i n t h e main l i n e . Each group i s f u r t h e r d i v i d e d i n t o systems u t i l i z i n g one l i n e o n l y and systems u t i l i z i n g two l i n e s , t h e f o r m e r b e i n g c a l l e d s i n g l e - l i n e systems, t h e l a t t e r d u a l - l i n e systems.
The most p o p u l a r t y p e s o f systems used a r e :
P r o g r e s s i v e Systems, o p e r a t e o n p r e s s u r e / v o lu m e c y c l e s ;
i n t h e i r case t h e
i n c r e a s i n g p r e s s u r e c r e a t e d by t h e pump a c t u a t e s a m e t e r i n g v a l v e w h ic h , h a v i n g g i v e n up i t s s e t amount o f l u b r i c a n t . a l l o w s t h e g r e a s e t o pass i n t o t h e main l i n e leading t o the next metering valve.
When s u f f i c i e n t p r e s s u r e has been
b u i l t up, t h e v a l v e i s a c t u a t e d and l u b r i c a n t a l l o w e d t o f l o w t o t h e n e x t m e te r i n g v a l v e , and so o n p r o g r e s s i v e l y , u n t i l i t r e t u r n s t o t h e l u b r i c a t o r o r where, when s u f f i c i e n t p r e s s u r e has been b u i l t up, a r e v e r s i n g v a l v e i s a c t u a t e d w h ic h reverses t h e f l o w o f t h e grease.
The s e l e c t i o n o f p r o g r e s s i v e systems i s depen-
dent upon t h e number o f p o i n t s t o be l u b r i c a t e d .
F i g u r e 1 1 i l l u s t r a t e s some
o p t i o n s a v a i l a b l e , and F i g . 1 2 shows a t y p i c a l i n s t a l l a t i o n o f a p r o g r e s s i v e l u b r i c a t i o n system on a P l a t e Shear. Dual L i n e Systems ( P a r a l l e l ) , o p e r a t e o n t h e same b a s i c p r i n c i p l e , v i z : t h e m o t o r i z e d l u b r i c a t i n g pump ( F i g . 1 3 )
f o r c e s g r e a s e i n t o one o f two main f e e d
l i n e s i n w hi ch a r e p l a c e d a number o f d u a l - l i n e m e t e r i n g e le m e n ts , each o u t l e t
M U orised BS-A h
p
SC-A Diffaeraial R c Un
MODE L BSA
Fig.13
Dual l i n e system.
410
I_
I
I_
Line 2 pressurising
Line 1 weuurising
2oo
Interval
I
Fig.14
end of line 1 b 2
Dual l i n e system p r e s s u r e .
66
Main Line 2
Fig.15
2
Dual l i n e m e t e r i n g element.
D u a l - l i n e m e t e r i n g elements o p e r a t e h y d r a u l i c a l l y w i t h o u t t h e use o f b a l l s and s p r i n g s o r check v a l v e s .
A c o n t r o l p i s t o n d i r e c t s t h e supply o f grease t o
e i t h e r s i d e of a m e t e r i n g p i s t o n which i s f i t t e d w i t h an i n d i c a t o r p i n , e n a b l i n g easy i n s p e c t i o n o f o p e r a t i o n a t each p o i n t on t h e system.
R e g u l a t i n g screws
f i t t e d t o each i n d i c a t o r housing p e r m i t adjustment down t o 20% o f maximum o u t p u t . The p o s i t i o n o f t h e c o n t r o l p i s t o n ( 2 ) and t h e feed p i s t o n (6) a r e shown i n ' A ' a f t e r the f i r s t p a r t o f the d u a l - l i n e cycle.
P r e s s u r i s e d l u b r i c a n t f r o m main
411 l i n e 1 has moved o v e r c o n t r o l p i s t o n ( 2 ) and then a t t h e upper s i d e o f t h e feed
( 6 ) , d i s p l a c i n g i t and d i s c h a r g i n g a measured q u a n t i t y o f l u b r i c a n t v i a c ro s s p o r t i n g (7) and a cross spool (1) t o o u t l e t ( 8 ) . P o s i t i o n ' B ' i s t h e second
piston
p a r t o f t h e d u a l - l i n e c y c l e and p r e s s u r i s e d l u b r i c a n t from main l i n e 2 has moved c o n t r o l p i s t o n ( 2 ) and d i s p l a c e d t h e m e t e r i n g p i s t o n (6) thus d i s c h a r g i n g a measured s h o t o f l u b r i c a n t v i a cross p o r t (5) and across spool ( 3 ) t o o u t l e t ( 4 ) . For e x tr e m e ly d i r t y and a b r a s i v e environments such as those found i n a B l a s t Furnace, P i g Caster o r Coal P r e p a r a t i o n and Washery P l a n t , i t i s a d v i s a b l e t o have t h e d u a l l i n e m e t e r i n g elements housed i n toughened g l a s s - f r o n t e d p r o t e c t i o n boxes, s i m i l a r t o t h a t shown i n F i g. 16 .
F i g. 16
P r o t e c t i o n box.
412 17.6.1.3
Comparison o f P a r a l l e l Systems - S i n g l e l i n e and D u a l - l i n e .
B o th systems d e p r e s s u r i z e t h e l i n e , b u t i n t h e case o f t h e S i n g l e L i n e System th e r e s e t t i n g o f t h e p l u n g e r s i n t h e m e t e r i n g elements i s u s u a l l y e f f e c t e d by s p r i n g p r e s s u r e , whereas i n t h e Dual L i n e system. when main feed l i n e No.2 i s p r e s s u r i s e d , a second s e r i e s o f dual l i n e m e t e r i n g elements d i s c h a r g e l u b r i c a n t t o the points o f application.
I n d o i n g s o , t hey r e s e t t h e dual l i n e m e t e r i n g
elements i n main l i n e No.1, w i t h which t he y f orm an i n t e g r a l u n i t .
17.6.1.4
S e l e c t i n g Grease L u b r i c a t i o n Systems.
Wherever p o s s i b l e , l u b r i c a t i o n systems sh ould be avoided w hich use equipment i n c o r p o r a t i n g s p r i n g s and v a l v e s , p a r t i c u l a r l y where t h e l u b r i c a t i o n systems a r e r e q u i r e d t o o p e r a t e i n arduous and a b r a s i v e environments. I n g e n e r a l p r a c t i c e , t h e m a j o r i t y o f C e n t r a l i s e d L u b r i c a t i o n Systems used a r e e i t h e r o f t h e ' D i r e c t Feed' o r ' P a r a l l e l - D u a l be in g o p e r a t e d ' m a n u a l l y ' o r ' a u t o m a t i c a l l y ' .
L i n e ' type;
b o t h a r e capable o f
The c h o i c e o f Grease L u b r i c a t i o n
Systems i s u s u a l l y d i c t a t e d by t h e number o f p o i n t s o f a p p l i c a t i o n and t h e i r l o c a t i o n , always b e a r i n g i n mind t h a t f o r utmost r e l i a b i l i t y and minimal maintenance D i r e c t Feed Systems sho ul d be s e l e c t e d wherever i t i s p r a c t i c a l . o f explanation,
By way
a P a r a l l e l - D u a l L i n e system can c y c l e and y e t f a i l t o d e l i v e r
l u b r i c a n t t o some of t h e p o i n t s o f a p p l i c a t i o n w hich i s o n l y e v i d e n t i f t h e o p e r a t o r v i s u a l l y examines t h e p o s i t i o n o f every dual l i n e element i n d i c a t o r pin. Q u i t e o f t e n t h i s i s i m p r a c t i c a l because o f elements b e i n g l o c a t e d i n hazardous areas
o n l y b e in g a c c e s s i b l e when t h e p l a n t and machinery a r e shut down.
T h i s cannot o c c ur w i t h D i r e c t Feed pumps, f o r t h e y w i l l o n l y o p e r a t e e f f e c t i v e l y providing
the points o f application are receiving lubricant;
course, broken f ee d-p i pe s
barring, o f
(which can a p p l y t o b o t h types o f system).
I n prac-
t i c e , a D i r e c t Feed pump can u s u a l l y c a t e r f o r up t o 40 p o i n t s o f a p p l i c a t i o n .
17.6.1.5
L u b r i c a t i o n o f P l a i n o r Sleeve Bearings.
These b e a r i n g s , p a r t i c u l a r l y t h ose o v e r 4" diameter, r e q u i r e as near c o n t i n uous l u b r i c a t i o n as i s p o s s i b l e .
I t i s n o r m a l l y recommended t h a t b e a r i n g s of
t h i s t y p e be f e d by D i r e c t Feeding l u b r i c a t o r s d r i v e n from t h e moving s h a f t . T h i s ensures l u b r i c a n t i s f e d t o t h e b e a r i n g s when t h e s h a f t i s i n o p e r a t i o n and no l u b r i c a n t i s f e d t o t h e b e a r i n g when t h e s h a f t i s i n o p e r a t i v e .
17.6.1.6
Lubrication o f Anti-Friction
( B a l l and R o l l e r ) Bearings.
A n t i - f r i c t i o n b ea ri ng s r e q u i r e c o n s i d e r a b l y s m a l l e r q u a n t i t i e s o f l u b r i c a n t tha n p l a i n b e a r in gs.
Except i n t h e case o f l a r g e a n t i - f r i c t i o n b e a r i n g s , they
w i l l n o t n o r m a l l y be l u b r i c a t e d c o n t i n u o u s l y . Where a n t i - f r i c t i o n be ari n gs a r e c l o s e t o g e t h e r , a hand-operated o r t i m e c l o c k c o n t r o l l e d d i r e c t f e e d i n g pump may be used.
Where they a r e spaced over
413 some d i s t a n c e ,
a l i n e system, e i t h e r hand-operated o r t i m e c l o c k c o n t r o l l e d ,
is
preferred. For l a r g e a n t i - f r i c t i o n b e a r i n g s and t h ose i n s t a l l a t i o n s where t h e g r e a t e s t r e l i a b i l i t y o f D i r e c t Feeding systems i s d e s i r a b l e
b u t t h e number o f p i p e s
should be k e p t low, d i r e c t f e e d i n g pumps w i t h P o s i t i v e D i v i d e r s may be used, s p l i t t i n g v o l u m e t r i c a l l y metered amounts o f l u b r i c a n t independent o f v a r y i n g back p r e s s u r e s . Very f a s t o p e r a t i n g a n t i - f r i c t i o n b e a r i n g s such as those running a t 1400 rpm However, o v e r - g r e a s i n g w i l l do no harm t o l a r g e
should n o t be f e d c o n t i n u o u s l y .
s l o w - r u n n in g a n t i - f r i c t i o n b e a r i n g s where q u i t e o f t e n t h e grease i s used as a s e a l a n t , p r e v e n t i n g d i r t and o t h e r f o r e i g n m a t t e r a n t i - f r i c t i o n bearings
-
t o e n t e r t h e b ea ri ng .
-
the greatest destroyer o f
Therefore i t i s essential t h a t
t h e grease i s k e p t c l e a n a t a l l t i mes, w i t h t h e l u b r i c a t i n g pump c o n t a i n e r o r r e s e r v o i r b o t to m f i l l e d v i a a grease keg o r b u l k grease s t o r a g e system.
17.6.2
O i l L u b r i c a t i o n Systems
O i l L u b r i c a t i o n Systems s e r v e two purposes:
t o l u b r i c a t e and/or c o o l .
On
many a p p l i c a t i o n s . p a r t i c u l a r l y i n t h e absence o f h i g h ambient temperature o r where t h e h e a t g en era t e d i n t h e b e a r i n g s o r t h e gears i s n o t g r e a t . t h e removal o f h e a t by t h e o i l need n o t f e a t u r e as a sep arate c o n s i d e r a t i o n i n t h e s e l e c t i o n o f t h e o i l c i r c u l a t i o n system.
T h i s can be arranged on t h e b a s i s o f l u b r i c a t i o n
c o n s i d e r a t i o n s a l on e.
i n t h e case o f many o t h e r a p p l i c a t i o n s , the
However,
c o o l i n g p r o p e r t i e s o f t h e o i l a r e o f g r e a t importance. O i l systems may t h e r e f o r e be grouped under t h r e e main headings:Group 1
Systems designed f o r l u b r i c a t i o n on a t o t a l loss basis.
Group 2
Systems designed f o r l u b r i c a t i o n and w i t h a small amount o f heat removal.
Group
3
Systems designed f o r l u b r i c a t i o n where an a p p r e c i a b l e degree o f c o o l i n g i s a l s o r e q u i r e d because o f operating conditions.
Group 1 and 2 systems v a r y a c c o r d i n g t o t h e t y p e o f machine and i t s l u b r i c a t i o n requirements.
Systems o f t h e t o t a l l o s s t y p e may be operated e i t h e r manually,
m e c h a n ic a lly , o r mo t o ri sed , whereas systems o f t h e t y p e which c o l l e c t t h e used o i l and r e c i r c u l a t e i t must be a u t o m a t i c .
V arious combinations o f these systems
can be employed, and t h e f o l l o w i n g a r e some t y p i c a l examples: 17.6.2.1
Group 1
- Total
loss systems designed f o r l u b r i c a t i o n purposes o n l y .
I n t h i s t y p e o f system t h e l u b r i c a n t , a f t e r l u b r i c a t i n g t h e b e a r i n g s o r gears, i s n o t used a g a in. pumps.
The group c o n s i s t s o f manual, mechanical o r motor operated
The f o r m e r g e n e r a l l y a p p l i e s on sma l l items o f p l a n t , e.g.
machine t o o l s ,
414 mechanical h a n d l i n g equipment, j i g s and f i x t u r e s , presses, e t c .
These systems
may be f u r t h e r s u b - d i v i d e d i n t o D i r e c t P o s i t i v e Systems or P o s i t i v e S p l i t Systems. D i r e c t P o s i t i v e Systems u s u a l l y comprise one o r more d i f f e r e n t i a l p l u n g e r ty p e o i l l u b r i c a t o r s , e.g.
Se ct i o ns 6.1.1
(F ig.9)
shows t h e o p e r a t i o n o f a man-
u a l l y o p e r a te d 8 - o u t l e t gre ase pump whi ch i s a l s o a d a p t a b l e f o r o i l , w i t h Fig.17 i l l u s t r a t i n g a t y p i c a l a p p l i c a t i o n on a pre ss l u b r i c a t i n g t h e slidew ays and crosshead.
For a p p l i c a t i o n s where a ut oma t i c l u b r i c a t i o n i s r e q u i r e d , mechanical
l u b r i c a t o r s h a v ing u s u a l l y 28 t o 32 pumping u n i t s may be f i t t e d ( F i g . 1 8 ) .
These
l u b r i c a t o r s may be d r i v e n e i t h e r m e c h a n i c a l l y t h r o u g h a r a t c h e t from t h e machine be in g l u b r i c a t e d o r by geared e l e c t r i c motor.
Each pump u n i t can be r e g u l a t e d
fro m z e r o t o maximum, t o f e e d m i n u t e p r e c i s e q u a n t i t i e s o f o i l t o t h e p o i n t s o f lubrication application.
Fig.17
D i r e c t p o s i t i v e system.
415
Fig.18
P o s i t i v e S p l i t System.
D i r e c t p o s i t i v e system.
T h i s t y p e o f system i s used where q u a n t i t i e s o f o i l
per a p p l i c a t i o n p o i n t a r e g r e a t e r t h a n can be s u p p l i e d by t h e p r e v i o u s l y described d i r e c t f e e d i n g p l u n g e r system.
I t comprises o f one o r more small h i g h
p r e s s u r e pumps f i t t e d w i t h i n t e g r a l r e l i e f v a l v e s , and s u p p l i e s l u b r i c a n t t o t h e b e a r i n g s through volume d i v i d e r s .
The d i v i d e r s may be used e i t h e r t o i n -
crease t h e number o f p o i n t s o r t o m o d i f y t h e q u a n t i t i e s f e d t o t h e l u b r i c a t i o n points.
416
Fig.19
17.6.2.2
P o s i t i v e s p l i t system.
Group 2 - Systems designed f o r l u b r i c a t i o n w i t h s m a l l amount o f
heat removal.
T h i s t y p e o f system s u p p l i e s t h e l u b r i c a n t t o t h e b e a r i n g s and
r e t u r n s i t under g r a v i t y through t h e r e t u r n p i p e s and/or drainways t o t h e s u p p l y tank f o r r e c i r c u l a t i o n .
T h i s group can a l s o be s u b - d i v i d e d i n t o D i r e c t P o s i t i v e
Systems o r P o s i t i v e S p l i t Systems.
The former c o n s i s t s o f t h e m e c h a n i c a l l y
operated d i f f e r e n t i a l p l u n g e r t y p e o i l l u b r i c a t o r as d e s c r i b e d i n Group 1.
417 T y p i c a l a p p l i c a t i o n s i n c l u d e paper machines, l a r g e k i l n s , o r g e n e r a l l y where a l a r g e number o f b ea ri ng s a r e t o be f e d p o s i t i v e l y w i t h an a d j u s t a b l e feed. Systems o f t h i s t y p e r e q u i r e a minimum amount o f maintenance and a t t e n t i o n .
The
l u b r i c a t i o n r e s e r v o i r s o f c o n t a i n e r s can be kept f i l l e d e i t h e r by a header t a n k s u p p l y i n g s e v e r a l l u b r i c a t o r s , o r each l u b r i c a t o r can be f i t t e d w i t h s p e c i a l b u i l t - i n s u c t i o n pumps whi ch su pp l y t h e c o n t a i n e r w i t h l u b r i c a n t from t h e main s u p p ly tank.
Each l u b r i c a t o r pump u n i t ( o u t l e t ) can be connected d i r e c t t o t h e
l u b r i c a t i o n p o i n t s o r t o a p o s i t i v e volume d i v i d e r , depending on t h e number o f feeds and t h e l u b r i c a t i o n re qu i reme nt s o f t h e p o i n t s . The l u b r i c a n t i s r e t u r n e d under g r a v i t y t o t h e main supply tank through d r a i n ways o r r e t u r n p i p e s (which can be a rran ge d w i t h b e a r i n g sump l e v e l c o n t r o l dev ic e s ) f o r r e c i r c u l a t i n g t o t h e l u b r i c a t o r c o n t a i n e r s o r header t a n k by means of s u c t i o n pumps, as d escri b ed , o r t o t h e t a n k by a f l o a t - c o n t r o l l e d gear pump (Fig.20).
-
Discharge f i l t e r
I
I b
d
L
&
6
6
b
d
b
b
b
b
1
b b b
b b d
\ l b
/
b
O
b
l
d
b
b b
Reservoir
F ig . 20
System w i t h smal l amount o f heat removal
As i n t h e case o f t h e p r e v i o u s group,
t h e l u b r i c a t o r can be d r i v e n e i t h e r
d i r e c t f r o m t h e machine b e i n g l u b r i c a t e d o r f i t t e d w i t h i t s own motor,
Each
pump can be f e d e i t h e r f ro m t h e machine sump o r from a separate d r a i n and supply tank.
The system i s u s u a l l y i n s t a l l e d on machine t o o l s , sugar machinery, gear
boxes, p r i n t i n g machines, and spe ci al -p urp ose machinery.
17.6.2.3
Group
3
-
Systems designed f o r l u b r i c a t i o n c o o l i n g .
Where t h e r e i s c o n s i d e r a b l e ambient h ea t o r where t h e power t r a n s m i t t e d by t h e p a r t b e in g l u b r i c a t e d c r e a t e s a h i g h degree o f h e a t , t h e c o o l i n g f u n c t i o n o f t h e l u b r i c a n t assumes g r e a t importance.
I t i s necessary i n such cases t o
ap p ly s u f f i c i e n t l u b r i c a n t t o e x t r a c t t h e h eat and t o m a i n t a i n t h e bearings o r gears a t an optimum t emp era t u re.
Arrangements must a l s o be made f o r r e t u r n i n g
t h e l u b r i c a n t t o a sup pl y t a n k f o r c o o l i n g , f i l t r a t i o n , and r e c i r c u l a t i o n , between t h e l u b r i c a t i o n equipment man uf act urer, t h e p l a n t designers, and t h e operators. Such a system u s u a l l y comprises a l a r g e o i l r e s e r v o i r o r s t o r a g e tank; motor d r i v e n pump ( n o r m a l l y a gear t y p e pump a d j a c e n t t o o r on t h e s t o r a g e t a n k ) ; coolers;
f i l t e r s ; p r e s s u r e gauges; a l a r m and f l o w c o n t r o l equipment; t o g e t h e r
w i t h t h e necessary v a l v e s and i n t e r c o n n e c t i n g pipework. and s e l f - c o n t a i n e d w i t h a c a p a c i t y o f 3 8 c c / s e c
The system may be simple
t o 750cc/sec
complex system c a pa bl e o f d e l i v e r i n g s e v e r a l l i t r e s / s e c .
p e r minute, o r a
These systems can be
p r o v i d e d w i t h s i m p l e o r e l a b o r a t e f l o w c o n t r o l , w arning d e v i c e s , and o t h e r i n s t r u m e n t a t i o n a c c o r d i n g t o t h e needs o f t h e i n s t a l l a t i o n (Fig.21).
Fig.21
T y p i c a l example o f l u b r i c a t i n g and c o o l i n g system packaged u n i t f o r t h e l u b r i c a t i o n o f rubber machinery.
419 17.6.3
Micro-Fog L u b r i c a t i o n Systems
Aerosol l u b r i c a t i o n i s t h e g e n e r i c term f o r o i l m i s t o r M i c r o - f o g systems which have been used s u c c e s s f u l l y f o r o v e r t w e n t y y e a r s .
Compared w i t h Centra-
l i s t e d Grease o r O i l l u b r i c a t i o n , a M i c r o - f o g system, t o p e r f o r m t h e same t a s k , r e q u i r e s l e s s l u b r i c a n t and energy and t h e i n i t i a l c o s t i s r e l a t i v e l y low. i s a l s o a h i g h l y f l e x i b l e system,
It
r e a d i l y i n s t a l l e d o n t o e x i s t i n g p l a n t as w e l l
as a t t h e new machine stage.
17.6.3.1
Working P r i n c i p l e s (Fig.22) o u t 1 i n e s t h e e s s e n t i a l and a u x i 1 i a r y
elements o f t h e M i c r o - f o g system and p r o v i d e s a g u i d e t o some o f t h e more common areas o f a p p l i c a t i o n .
During operation,
t h e system produces c o n t i n u o u s l y a dense
c o n c e n t r a t i o n of m i c r o p a r t i c l e s o f o i l which a r e conveyed i n a ' d r y ' f o g i n a low p r e s s u r e d i s t r i b u t i o n system.
On r e a c h i n g t h e p o i n t of a p p l i c a t i o n t h e
' d r y ' f o g i s passed t h r o u g h r e c l a s s i f i e r s , which a r e r e a l l y m e t e r i n g and condens i n g o r i f i c e s , so as t o a c c u r a t e l y f e e d an e x a c t q u a n t i t y o f l u b r i c a n t t o s u i t the operating conditions.
M i cro-foq
1u b r i c a t i o n
lubrication 1u b r i c a t i o n
Fig.22
M i c r o - f o g system.
420 To ensure t h a t t h e f o g reaches t h e r e c l a s s i f i e r d r y ,
the d i s t r i b u t i o n piping
i s sized t o allow o i l p a r t i c l e s t o travel along the piping a t a v e l o c i t y less tha n 7.3 metres/sec,
which i s sl o w enough t o p r e v e n t condensation.
l en c e i n t h e r e c l a s s i f i e r s causes t h e o i l p a r t i c l e s t o 'w et o u t '
The t u r b u -
i n t o the l i n e
l e a d i n g t o o r d i r e c t o n t o t h e b e a r i n g s u r f a c e s where they then form a p r o t e c t i v e f i l m of oil. In o r d e r t o c r e a t e ' d r y '
f og , o i l i s f i r s t drawn i n t o a compressed a i r stream
as i t passes t h r o ug h a V e n t u r i l o c a t e d on t o p o f t h e l u b r i c a t i o n c o n t r o l u n i t (Fig.23).
O i l p a r t i c l e s o f a p p r o x i m a t e l y 0.002mm
i n diameter a r e c o l l e c t e d i n
t h e a i r stream and can be t r a n s p o r t e d l o ng d i s t a n c e s i n t h e d r y c o n d i t i o n .
Lubricator head A i r bypass a d j u s t i n Low pressure w i t c h
lligh pressure s w i t c h Electrical conduit ent Auxiliary requlatcr
Supply t o pressur j e t reclassifier
Waste pipe e x i t
/
Fig.23
Lubrication control unit.
Since M i c r o - f o g systems r e q u i r e no r e t u r n l i n e s they can be designed t o easy i n s t a l l a t i o n and assembly;
hence, l o w c o s t i n s t a l l a t i o n , w i t h o u t t h e problems
a s s o c i a t e d w i t h s i n g l e - l i n e s e r i e s o i l c i r c u l a t i n g systems w hich i n c o r p o r a t e q u i t e s o p h i s t i c a t e d and r e l a t i v e l y expensive v a l v e b l o c k s t h a t have a b u i l t - i n self-reversing operation.
A l t ho ug h such systems may r e q u i r e no separate reverse
and r e c y c l e v a l v e o r v e n t i n g phase d u r i n g t h e l u b e c y c l e , they do have, however, a d i s t i n c t d is a d va nt ag e because t h e y r e q u i r e a m u l t i p l i c i t y o f system t u b i n g (Fig.20). Moreover, Once a M i c r o - f o g system has been i n s t a l e d i t i s much less demandi n g t h a n o t h e r a ut o mat i c l u b e systems.
The system
s e a s i l y assembled,
using a
b u i l d i n g b l o c k approach, whi ch i s designed f o r easy s e r v i c i n g , r e p a i r s and general maintenance.
421 17.6.3.2
O i l Quality.
C o r r e c t o i l s e l e c t i o n i s i m p o r t a n t as some o i l s i n c o r p o r a t e polymers w hich suppress a e r o s o l p r o p e r t i e s , w h i l e h e a v i e r g rade o i l s may r e q u i r e h e a t i n g up t o between 4Oo/45OC
t o a t t a i n t h e v i s c o s i t y f o r maximum o u t p u t . For a l l normal
purposes t h e c o n t r o l u n i t s w i l l p e r f o r m w e l l when w orking w i t h o i l s up t o 700 c e n t i s t o k e s a t 20°C. Summarised l u b r i c a t i n g o i l re qu i reme nt s f o r a s a t i s f a c t o r y M i c r o - f o g system are:(i)
Good a e r o s o l p r o p e r t i e s .
(ii)
Low r a t e o f co nd en sat i on t hro ug h p i p e s .
( i i i ) Low l e v e l o f s t r a y i n g by t h e p a r t i c l e s . (iv)
A h i g h degree o f r u s t i n h i b i t i o n .
(v)
Absence o f c l o g g i n g t en de nci e s a t t h e v e n t u r i n o z z l e o r any polymer precipitation.
17.6.3.3
Compressed A i r
2
A c c e p t in g t h a t most i n d u s t r i a l compressed a i r i s s u p p l i e d a t 7 bar (100 l b / i n ) 2 i n a M i c r o - f o g l u b r i c a t i o n system i t has t o be reduced t o about 2 b a r (30 l b / i n 1. Dur in g i t s passage t h r o u g h t h e v e n t u r i o r i f i c e on t h e c o n t r o l u n i t , a p r e s s u r e
2
dro p o f 0 . 7 bar (10 I b / i n ) t ake s pl a ce.
17.6.3.4
System Design Con si de rat i o ns
To c a l c u l a t e t h e l u b r i c a t i o n re qu i reme nt s o f bearings, an e m p i r i c a l f a c t o r r e f e r r e d t o as a ' l u b r i c a t i o n u n i t '
(L.U.)
has been evolved,
e n a b l i n g a l l moving
s u r f a c e s r e q u i r i n g l u b r i c a n t t o be co nve rt e d t o t h e i r e q u i v a l e n t L.U. The amount o f l u b r i c a n t main
rating.
and bra nch l i n e p i p e bores and r e c l a s s i f i e r nozzles
may t h e n be s i z e d t o p r o v i d e t h e c o r r e c t amount o f l u b r i c a n t a t each l u b r i c a t i n g point.
I n t h i s manner, a n t i - f r i c t i o n b ea ri ngs, j o u r n a l bearings, s l i d e s , gears,
c h a in s , and o t h e r we ari n g s u r f a c e s r e q u i r i n g l u b r i c a t i o n can a l l be converted t o e q u i v a l e n t L.U.
r a t i n g s and se rved by a p p r o p r i a t e l y s i z e d M i c r o - f o g l u b r i c a t i o n
systems.
17.6.3.5
Some T y p i c a l A p p l i c a t i o n s
F i g u r e 24 i l l u s t r a t e s a t h r e e S t ran d Aluminium F o i l M i l l o p e r a t i n g a t 1000 t o 1500 m e t r e s / m in u t e w i t h two 1000 L.U. g e n e r a t i n g heads ( t h i r d a c t s as a standby) s e r v i n g t h e m i l l s t a c k and e x i t a n c i l l a r i e s , w i t h a s e p a r a t e 300 L.U. u n i t serving the entry a n c i l l a r i e s .
2.5 l i t r e s p e r w o r k i n g hour. motor oears.
generating
The t o t a l amount o f o i l used i s l e s s than
F i g u r e 25 i l l u s t r a t e s t h e l u b r i c a t i o n o f v i b r a t o r
422
423
i
I
"i" 'i' 0-
I
Fig.25
17.6.4
Lubrication o f vibrator motor gears.
Check List
Based o n the foregoing, prior to actual selection of the appropr ate lubricant and associated equipment, it is advisable to draw up a check 1 st of known The following list, in simplified form, is for guide purposes only and can naturally be elaborated upon to suit the spec fic needs o f facts and requirements.
the designer o r plant engineer. Specification o f Plant t o be lubricated: Type of plant and machinery to be lubricated. Industrial Application, including operating environmental conditions, e.g. is i t dirty, abrasive, wet, hot, etc.? Surfaces, sizes and speeds to be lubricated, e.g. Bearings (plain), Bearings (roller), Grease (type), Slideways, Chains, etc. Number and Location of lubrication Points (Fixed) and (Moving) and proposed site (if known) for lubricator enabling assessment of pipe and flexes: Frequency plant and machinery operators and whether lubrication needs to be continuous, semi-continuous, or intermittent.
424
-
L u b r i c a n t t ype and s p e c i f c a t i o n S e r v i ces a v a i l a b l e i n t h e P l a n t
-
Grease, O i l , o r M i c r o - f o g ,
etc
e l e c t r i c , pneumatic.
H e a l t h and S a f e t y a spe ct s e.g. normal, flameproof, a c c e s s i b i l i t y any h i s t o r y o f p r e v i o u s l y t r i e d l u b r i c a n t s / e q u i p m e n t . Method proposed f o r f i l l i n g t h e l u b r i c a t i o n t a n k / r e s e r v o i r , e.g. manual, semi-automatic, b u l k storage. Recommended L u b r i c a t i o n Equipment: Why? E s tima t e d performance w i t h any known Case Studies and References. Econom ic s
.
Spares and S e rvi ce. Sometimes equipment s e l e c t i o n i s an i n e v i t a b l e compromise as a r e s u l t o f c o n f l i c t i n g l u b r i c a t i o n re qu i reme nts;
i n such circumstances i t i s
im p o r t a n t t o a p p r e c i a t e a l l t h e f a c t s and t o subsequently gauge p l a n t performance a c c o r d i n g l y . 17 7
SUMMARY
T h i s c h a p t e r has a t t emp t e d t o c l a r i f y t h e more g e n e r a l l y accepted methods o f l u b r i c a t i o n , where necessary i l l u s t r a t i n g a c t u a l examples o f p l a n t and l u b r i c a t i o n equipment.
I t i s n o t i n any way i n t e nded t o i n f e r t h a t t h i s i s t h e o n l y
l u b r i c a t i o n equipment a v a i l a b l e ; from Trade J o u r n a l s , e t c .
o f co urse , t h e r e a r e o t h e r s r e a d i l y determined
L i k e w i s e , t h e r e a r e many more s e l e c t items o f l u b r i -
c a t i o n equipment t a i l o r - m a d e t o meet t h e needs o f s p e c i a l i s t p l a n t . For example, Overhead and F l o o r Conveyors which can t r a v e l up t o 50 metres/ m in u te , u n l e s s e f f e c t i v e l y l u b r i c a t e d (and cl eaned where t h e environment demands) can wear and s e i z e , r e s u l t i n g i n c o s t l y stoppages. l n i t i a l y , c o n v e n t i o n a l s t a t i c l u b r i c a t o r s were used where a m i x t u r e o f a i r and o i l
-
not only
and now more r e c e n t l y j u s t o i l
- was
shot over a gap, w hich r e s u l t e d
n t h e f a i l u r e t o a de qu at el y l u b r i c a t e (Fig.1)
b u t a l s o caused d r i p -
page w i t h c o n s e qu en t i a l p rod uct c o n t a m i n a t i o n and h e a l t h hazards. These problems were c o m p l e t e l y overcome by i n t r o d u c i n g a range o f s p e c i a l purpose l u b r i c a t o r s ;
(F i g. 26 ) shows one such example.
The c o n c l u s i o n i s t o d et ermi n e a l l t h e f a c t s r e g a r d i n g t h e p l a n t t o be l u b r i c a t e d and t o t he n e v a l u a t e t h e l u b r i c a n t s and l u b r i c a t i o n equipment ava i 1a b le .
425
Fig.26
A dog c h a i n assembly engages t h e conveyor c h a i n which moves t h e o i l - d i s p e n s i n g n o z z l e p l a t e s i n and o u t o v e r t h e p i n l i n k s . O i l i s f o r c e d t h r o u g h each n o z z l e .
REFERENCES
1 2
3 4
5
Mechanical L u h r i c a t i o n o f E.O.T. Cranes by D r . H . P e t e r J o s t and P e t e r W. Murray. A F u l l y Automatic B u l k H a n d l i n g L u b r i c a t i o n System f o r a S i n t e r P l a n t by G. W i l l i a m s . An E n g i n e e r i n g Approach t o t h e s e l e c t i o n o f C e n t r a l i s e d Grease L u b r i c a t i o n Systems by D r . H. P e t e r J o s t . Modern B r i t i s h and European Steelworks L u b r i c a t i o n Developments by D r . H. P e t e r J o s t . E!ew M i s t L u b r i c a t i o n Concepts f o r Tapered R o l l e r Bearinos used on High C.H. West and Speed R o l l i n g M i l l Back-up by R o l l s by \!.E.McCoy, P.E. W i l k s .
426
6
7 8 9 10
Aerosol L u b r i c a t i o n Systems - t h e i r c o n t r i b u t i o n t o savings i n o p e r a t i n g and maintenance c o s t s by R.E. K n i g h t and J.G. M e r r e t t . Micro-Fog L u b r i c a t i o n f o r b e a r i n g e f f i c i e n c y by J.G. M e r r e t t . Automatic L u b r i c a t o r s and Cleaners Increase Conveyor L i f e by J.G. M e r r e t t . Automatic L u b r i c a t i o n o f Chain and Conveyor Systems by R.M. Dombroski. The James C l a y t o n L e c t u r e - "Energy Saving through T r i b o l o g y " by D r . H. P e t e r J o s t and D r . J. S c h o f i e l d .
421
18 R.A.
ON CONDITION MAINTENANCE
COLLACOTT, Ph.D.,
Director Head
18.1
-
-
B.Sc(Eng)
,
F. I .Mar. E . ,
F. I .Mech.E.
UK Mechanical H e a l t h M o n i t o r i n g Group
F a u l t D i a g n o s i s Centre, L e i c e s t e r P o l y t e c h n i c .
INTRODUCTION
Maintenance c a r r i e d o u t when r e q u i r e d a f t e r a s i g n i f i c a n t d e t e r i o r a t i o n i n a component as i n d i c a t e d by a sensor o r m o n i t o r e d parameter i s c a l l e d o n - c o n d i t i o n ma i n tenance. I f a person, when v i s i t i n g t h e d o c t o r f o r a h e a l t h - c h e c k was i n v i t e d t o have t h e i r body opened up t o see whether ' e v e r y t h i n g was c o r r e c t ' a most u n s a t i s f a c t o r y s t a t e o f a f f a i r s would e x i s t .
S i m i l a r l y , t o open up a machine i n o r d e r
t o check t h a t i t i s a l r i g h t i s j u s t as u n s a t i s f a c t o r y good can be done t o a machine i n t h i s way.
-
much more harm than
A c c o r d i n g l y i t i s l o g i c a l t o use
d i a g n o s t i c techniques t o assess t h e ' h e a l t h ' o r c o n d i t i o n o f p l a n t and machi n e r y i n j u s t t h e same way as a medical d o c t o r uses symptoms and a i d s t o assess t h e c o n d i t i o n o f human beings
-
and v e r y o f t e n s i m i l a r o r i d e n t i c a l equipment
i s used b o t h by t h e medical d o c t o r and d i a g n o s t i c engineer.
18.2
BACKGROUND
O n - c o n d i t i o n m o n i t o r i n g i s a l r e a d y a c t i v e l y and e f f e c t i v e l y used i n many industries
-
i n a i r c r a f t , nuclear reactors, steel m i l l s , petroleum r e f i n e r i e s ,
s h i p s , power g e n e r a t i o n e t c .
I t i s a technology which has r a p i d l y e v o l v e d
d u r i n g t h e p a s t 5 t o 10 years by u s i n g methods and techniques which have been developed s i n c e 1945. H i s t o r i c a l l y , t h e c h a i n o f e v o l u t i o n i s shown i n Fig.1. 1750
1915 1940 1950 1960 1970
... steam engines ... steam t u r b i n e s ... steam t u r b i n e s ... n u c l e a r r e a c t o r s ... space v e h i c l e s ... ... ... ... ...
Figure 1
-
s i m p l e c o n t e n t s gauges, simple governor p r e s s u r e , tempe rature, c o n t e n t s gauges e t c . automatic c o n t r o l s robotry m i n u t i a r i s a t i o n , remote t e l e m e t r y computer m i n u t i a r i s a t i o n , microprocessors
H i s t o r i c a l chain o f e v o l u t i o n of c o n d i t i o n monitorinq
428
18.3
MANAGEMENT OF CONDITION MONITORING
There a r e t h r e e e f f e c t i v e stages i n t h e management o f an o n - c o n d i t i o n maintenance system which go hand-in-hand w i t h a change i n o r g a n i s a t i o n whereby maintenance i n v o l v e s t h e r u n n i n g o f ( i ) a d i a g n o s t i c c a p a b i l i t y ,
( i i ) a repair
team. The t h r e e stages i n d i a g n o s t i c management a r e :
1)
F a i l u r e Modes and E f f e c t s A n a l y s i s
-
whereby t h e whole/complete p l a n t i s analysed s y s t e m a t i c a l l y t o determine which p a r t s a r e c r i t i c a l and need t o be m o n i t o r e d , a l s o t o a p p r a i s e t h e i r t y p i c a l f a i l u r e cause
2)
M o n i t o r i n g Technique S e l e c t i o n and Sensor A p p r a i s a l
-
whereby t h e most e f f e c t i v e method i s chosen
3)
L i m i t Decision
-
whereby t h e amount o f d e t e r i o r a t i o n which can be p e r m i t t e d i s decided upon
A f u l l account o f a l l t h r e e stages i s g i v e n i n r e f e r e n c e
18.4
[l].
FAILURE MODES AND EFFECTS ANALYSIS
A t y p i c a l example o f t h e way i n which t h i s can be done was e x p l a i n e d by
Venton and Harvey [ 2 ] ,
B r i d g e s [ 3 ] and mentioned by Davies [ 4 ] , i n essence i t
i s t h e use o f i n f o r m a t i o n t o p r e p a r e a numerical assessment o f t h e o r d e r o f e s s e n t i a l i t y o f v a r i o u s components (sub-systems) w i t h i n a p l a n t o r machinery. A t i t s most elementary t h i s may be prepared by a s i m p l e ' D e l p h i ' a n a l y s i s ;
at
i t s more complex i t may i n v o l v e an i n t e g r a t e d d a t a - a p p r a i s a l method u s i n g f a i l u r e r a t e data which can be o b t a i n e d f o r d i f f e r e n t components
-
such f a i l u r e
r a t e data can be o b t a i n e d f r o m a number o f e s t a b l i s h e d d a t a banks [ 5 ] .
A t y p i c a l ' s t a r t i n g ' a p p r a i s a l f o r a marine power p l a n t m i g h t be t h e i n t e r dependency c h a r t f o r a l l sub-systems as shown i n Fig.2.
T h i s a l r e a d y shows t h a t
f o r t h e p a r t i c u l a r m i s s i o n f o r which t h i s a p p r a i s a l was made, h i g h r e l i a b i l i t y o f e l e c t r i c a l g e n e r a t i o n was i m p o r t a n t
-
a s t a t e o f a f f a i r s which was c o n f i r m e d
by a b a s i c PHASE CRITICALITY ANALYSIS and remained unchanged when allowance was made f o r t h e i n h e r e n t s a f e t y t h r o u g h a HAZARDS AND R I S K S ANALYSIS. The causes o f p o t e n t i a l f a i l u r e need t o be e s t a b l i s h e d . be d e r i v e d from h i s t o r i c a l r e c o r d s
-
Again, t h i s must
i t i s i m p o r t a n t t o make, m a i n t a i n and
analyse r e c o r d s i n o r d e r t o p r o v i d e adequate i n f o r m a t i o n upon which t o choose t h e c o r r e c t m o n i t o r i n g methods.
I n a t y p i c a l a n a l y s i s by W i l k i n s o n and
K i l b o u r n [61 t h e f a i l u r e r a t e d a t a o f Table 18.1 was used t o determine t h e best arrangement o f an e l e c t r i c a l g e n e r a t i n g standby p l a n t t o choose t h e most re1 i a b l e system.
429 I nter-dependen t System System
(1)
2) T r a n s m i s s i o n
3) o i l
(2)
(3) ( 4 ) ( 5 ) ( 6 ) (7) ( 8 ) (9)
x
1 ) Main e n g i n e
x
X
(10)
x
(11)
x
(12) (13)
x
X X
fuel
4 ) Compressed
X
X
air
x
5) E l e c t r i c a l
x
x
x
genera t i o n
6) S t e e r i n g
X
gear
7) Deck
X
X
machinery
8) Sea w a t e r
X
9 ) B i I g e and
X
ballast
10) V e n t i l a t i o n
X
11) Exhaust
X
12) C o n t r o l s
13)
Fig.2
x
X
Steam
x
System i n t e r - d e p e n d e n c y A n a l y s i s
As a consequence o f p a s t o p e r a t i o n a l e x p e r i e n c e - e i t h e r by an i n d i v i d u a l user o r f r o m a m a n u f a c t u r e r o r some d a t a c e n t r e such as t h a t p r o v i d e d by L l o y d s R e g i s t e r o f S h i p p i n g o r t h e UK Atomic Energy A u t h o r i t y (Systems Re1 i a b i l i t y S e r v i c e ) - i t i s p o s s i b l e t o p r e p a r e FAULT TREES such as t h a t shown i n
F I ~ [. 7 ~] .
D e t a i l e d e x a m i n a t i o n o f such f a u l t t r e e s w i l l
i d e n t i f y t h e poss-
i b l e p r i m a r y and secondary causes o f f a i l u r e s and t h e l i k e l y symptoms they w i l l produce. F a i l u r e symptoms a r e t h e p h y s i c a l consequences o f a m a l f u n c t i o n s i t u a t i o n .
I f f o r example one c o n s i d e r s b e a r i n g f a i l u r e s t h e r e a r e two q u i t e d i f f e r e n t p h y s i c a l e f f e c t s between r o l l e r element b e a r i n g s and p l a i n b e a r i n g s .
With
r o l l e r e l e m e n t b e a r i n g s e x p e r i e n c e shows t h a t f a i l u r e s a r e a s s o c i a t e d w i t h l o c a l s u r f a c e d e f e c t s , c o n s e q u e n t l y o p e r a t i o n o f d e f e c t i v e b e a r i n g s s e t s up u l t r a h i g h f r e q u e n c y shock waves whi&
can be most e f f e c t i v e l y diagnosed by shock
p u l s e methods p o s s i b l y a i d e d by s t a t i s t i c a l a n a l y s i s by a method c a l l e d 'kurtosis'.
On t h e o t h e r hand, p l a i n b e a r i n g s wear i n a u n i f o r m w h o l e - s u r f a c e
method w i t h t h e r e s u l t t h a t c l e a r a n c e s a r e changed and some i n t e r f e r e n c e ( r u b b i n g ) may ensue;
f o r such c o n d i t i o n s , methods o f c l e a r a n c e measurement by
i n d u c t i v e p i c k - u p s or v i b r a t i o n measurement or even d e b r i s a n a l y s i s may be appropriate.
For t h e two c l a s s e s o f b e a r i n g s d i f f e r e n t sensor systems w i l l be
430
L
aao u
m
FIGURE 3
Typical FAULT TREE for a bearing failure analysis
c
-
PROBLEM IIEFINITION
EVALUATION OF
RMA
EVALUATION
FUhCTlONAL BLOC DIALRAMS ALTERNATIVE SYSTEMS
OBJECTIVES, REOUIREMENTS
I
AN0
I
ANALYSIS REVIEW
t i 1 TRADE-OFF
I I I
OF
ENVIRONMEIITAL
I
OF
FORMULATION Of A N A I Y Y ,
FORMULATION
I
PROGHAM
STUDIES
OUANTITATIVE RMA - ANALYSIS
I
A N A l YSI5
OF RELIABILITY BLOC
DIAGRAMS
CHARACTERISTICS
I
+I
FORMUI.ATION
OF MATHEMATICAL
MODELS
J
f
.1
FAlLlJRE DATA COLLECTION
I
JI CORRECTIVE MAINTENANCE
e
ANALYSIS
f
MAINTENANCE
t
PREDICTION
FAULT TREE ANALYSIS
DATA COLLEC
FIGURE 4
I
I
I
OF
R M A - CHARACTERISTICS
ANALYSIS REVIEW
r TRADE-OFF STUDIES
L
Reliability analysis
1
sequence
I
diagram
-
I
1 1
432 ava i 1 a b I e
.
Thus t h e w h o l e i n i t i a l a n a l y t i c a l s t a g e c a n be r e p r e s e n t e d b y a number o f p r o c e d u r e s w h i c h h a v e been f u l l y s e t o u t i n F i g . 4
TABLE 18.1
[7].
F a i l u r e / r e p a i r data Repair Rate, Repa i r s / h
Mean Time t o Repair, h ( 2 men)
T u r b o - a l t e r n a t o r and c o n t r o l s : a l t e r n a t o r end t u r b i n e end
0.08197 0.122
12.2 8.2
D i e s e l a l t e r n a t o r and c o n t r o l s : a l t e r n a t o r end d i e s e l end
0.08197 0.05051
12.2 19.8
T u r b o - a l t e r n a t o r s y s t e m sea w a t e r c i r c u l a t i n g pump
0.1
10.0
D i e s e l a l t e r n a t o r sea w a t e r c i r c u l a t i n g pump
0.1493
6.7
Condenser
0.1667
6.0
A i r ejector
0.1136
8.8
E x t r a c t i o n pump
0.09434
B o i l e r f e e d pump
0.1429
Waste h e a t e c o n o m i s e r - t y p e b o i l e r
0.03636
27.5
O i l fired boiler
0.05+:
20 . O ”
Composite b o i l e r c i r c u l a t i n g pump
0.09259
10.8
Components
:l:
10.6 7.0
Estimated
To e n s u r e t h a t t h e c h a r a c t e r i s t i c s o f e a c h f a i l u r e / d e f e c t m a l f u n c t i o n i t i s n e c e s s a r y t o be a b l e t o r e c o g n i s e t h e c h a r a c t e r i s t i c s
-
which i s o n l y obtained
through extensive defect r e c o g n i t i o n experience.
18.5
MONITORING TECHNIQUE SELECTION
There a r e b a s i c a l l y f o u r techniques f o r t h e m o n i t o r i n g o f p l a n t and machinery d e t e r i o r a t i o n : 1.
dynamic methods
-
in p a r t i c u l a r v i b r a t i o n monitoring but also i n c l u d i n g t h e u s e o f a i r - b o r n e sounds
433 2.
inspection/integrity
-
s u r v e i l l a n c e methods
which o r i g i n a t e d w i t h n o n - d e s t r u c t i v e t e s t i n g techniques b u t have now extended t o i n c l u d e leak-testing;
odour i d e n t i f i c a t i o n ;
corrosion
m o n i t o r i n g and s t r e s s wave emission
3. contaminant i n s p e c t i o n
-
as a means o f i d e n t i f y i n g wear d e b r i s and r e l a t i n g i t b o t h q u a n t i t a t i v e l y and q u a l i t a t i v e l y t o i t s source
4. trends analysis
-
e f f e c t i v e l y data l o g g i n g e i t h e r as s t r a i g h t sensor o u t p u t s as f o r example temperatures, pressures and speeds o r i n a c o o r d i n a t e d form u s i n g such parameters as s p e c i f i c f u e l consumpt i o n r a t e o r even ' d e l t a s
'
of variations
f r o m the norm
18.5.1
V i b r a t i o n Monitoring
T h i s i s a w e l l e s t a b l i s h e d technique r a n g i n g i n s o p h i s t i c a t i o n from t h e use o f j u d i c i o u s l y p l a c e d d i a l gauges i n c o n j u n c t i o n w i t h stroboscopes t o broadband a n a l y s i s , narrow band a n a l y s i s , a u t o - c o r r e l a t i o n , o t h e r h i g h l y instrumented techniques [8].
s i g n a l a v e r a g i n g and
Most m o n i t o r i n g a p p l i c a t i o n s a r e
s a t i s f a c t o r i l y d e a l t w i t h by means o f : ( i ) p r o x i m i t y probes and pick-ups w i t h p o s s i b l y a cathode ray tube ( c r t ) display
( ii ) seismic v e l o c i t y transducers o r p i e z o - e l e c t r i c accelerometers o u t p u t t i n g to either (a) broad-band ( o v e r a l I ) v i b r a t i o n meters (b) v i b r a t i o n spectrum a n a l y s e r s ( p a r t - o c t a v e o r narrow-band width) ( i i i ) waveform a n a l y s e r s
18.5.1.1
P r o x i m i t y Probes
The o r b i t moved through a s h a f t which i s loose i n i t s b e a r i n g s y e t s u b j e c t e d t o t h e i n f l u e n c e o f v a r i o u s f o r c e s can be observed by u s i n g two p r o x i m i t y probes p l a c e d a t a 90" r e l a t i v e a n g l e and t h e i r o u t p u t s l e d t o an X - Y p l o t t e r o r a CRT.
The r e s u l t i n g d i s p l a y , Fig.5,
can be used t o measure t h e a c t u a l amount
o f o r b i t a l e c c e n t r i c i t y (and t h u s t o determine whether ' b e a r i n g w i p e ' i s 1 i k e l y t o o c c u r ) a l s o t h e shape o f t h e o r b i t i s r e l a t e d t o t h e t y p e o f d e f e c t f o r c e 90
that the source o f troub/e cdn be estab/ished.
434
PICKUP
Fi g .5
S h a ft o r b i t a l a n a l y s i s
18.5.1.2
Seismic V e l o c i t y Transducers/Accelerometers/Vibration Meters/Spectrum A n al yse rs
T h i s i s th e deve l op i ng are a o f i n t e r e s t i n ' s t r a i g h t ' v i b r a t i o n a n a l y s i s . V e l o c i t y t r a n s d u cers a r e most a p p l i c a b l e a t t he lower frequency ranges; erometers a r e e f f e c t i v e a t t h e h i g h e r f req ue ncy ranges;
accel-
a t intermediate
fre q u e n c ie s (around 500 Hz = 500 x 60 = 3000 cpm) e i t h e r transducer i s a p p l i c ab le . With o v e r a l l (wide) bandwidth v i b r a t i o n a n a l y s i s a transducer p i c k s up a s i g n a l d e r i v e d f rom a l l t h e c o n s t i t u e n t f r e q u e n c i e s and t h i s i s measured by the meter
-
which a c c o r d i n g l y t e l l s whether t h e v i b r a t i o n i s i n c r e a s i n g i n
strength,
i e . t h e system i s d e t e r i o r a t i n g .
To t e l l what i s d e t e r i o r a t i n g i n a machine i t i s necessary t o measure t h e v i b r a t i o n s i g n a l produced by each c o n s t i t u e n t component. r e c o r d i n g t h e fr equ en cy 'sp ect rum' f o r t h e machine.
T h i s i s done by
Thus each component w i l l
ge n e r a te v i b r a t i o n s a t a p a r t i c u l a r f req ue ncy ( i t s ' d i s c r e t e ' frequency) and when p l o t t e d as i n Fig.6 produce a ' s p i k e ' on t h e graph a t t h a t frequency. I f the ' s p i k e '
incre ase s w i t h succeeding s p e c t r a i t w i l l mean t h a t a defect
de v e lo p in g i n t h a t p a r t i c u l a r component.
is
To know t h e values f o r t h e d i s c r e t e
fr e q u e n c ie s f o r d i f f e r e n t components i t i s necessary t o make frequency c a l c u l ations o f various kinds
-
i n any event machine designers make these c a l c u l a t i o n s
when machines a r e designed and such i n f o r m a t i o n can be o b t a i n e d when t h e machines a r e b e i ng purchased. Table.
Typical frequencies are given i n the f o l l o w i n g
435
5
FREQUENCY
Fig.6
T y p i c a l frequency spectrum
TABLE 18.2
D i s c r e t e Frequency C a l c u l a t i o n s
V i b r a t i o n Type
Frequency Equation
Simple harmonic
f
Pendu 1 um ( s imp1e )
f = L 9
Pendulum (compound)
f = -1
= -1-
A
2n M
2n L
Bar, u n i f o r m l y loaded, f i x e d b o t h ends Shaft, t o r s i o n a l o s c i l l a t i o n s , s i n g l e f l y w h e e l
f =
w
L2
1 2 271 I
Ball bearing
-
m a l f u n c t i o n o f o u t e r race
nN f = - (1 2
Ball b e a r i n g
-
m a l f u n c t i o n o f i n n e r race
f =
Ball b e a r i n g
-
defective b a l l
D f = N(d
Gear t e e t h
-
d -cos D
B)
nN d ( 1 + - cos 2 D
B)
f = N
tooth defect
ih2)
L!.
- 3.57
f
where, A = s t i f f n e s s o f system M = mass o f system g = acceleration o f g r a v i t y L = l e n g t h o f pendulum h = distance,c.g. t o p i v o t , compound pendulum k = r a d i u s o f g y r a t i o n about c.g., compound pendulum !i= l e n g t h o f b a r E = m o d u l u s o f E l a s t i c i t y (Young’s Modulus) I = second mment of are a o f s e c t i o n about n e u t r a l a x i s w = load p e r u n i t l e n g t h a p p l i e d t o bar T = t o r s i o n a l s t i f f n e s s o f s h a f t CJ .9
gh
2n (k2
1
d -C O S ~8 ) D
t
= shear modulus = p o l a r second moment of area p e r s h a f t = p o l a r moment o f i n e r t i a o f f l y w h e e l = number o f b a l l s i n b e a r i n g N = s h a f t speed (rev/min) d = b a l l diameter D = b a l l p i t c h c i r c l e diameter N1= gearwheel speed (rev/min) t = number o f t e e t h on gearwheel.
G J I n
436 When u s i n g v i b r a t i o n s sensors p a r t i c u l a r c a r e must be taken i n s e l e c t i n g t h e i r mounting p o s i t i o n and i n methods by means o f which they a r e a t t a c h e d . Some v i b r a t i o n d e f e c t s o n l y show up i n t h e r a d i a l d i r e c t i o n , o t h e r s o n l y p r o duce major e f f e c t s i n t h e a x i a l d i r e c t i o n .
I t i s t h e r e f o r e t o be recommended
t h a t 3 sensors be used a t each l o c a t i o n so t h a t t h e v i b r a t i o n s can be e s t a b l i s h e d i n two p e r p e n d i c u l a r r a d i a l p o s i t i o n s as w e l l as t h e a x i a l d i r e c t i o n .
A study o f t h e e f f e c t s o f v a r i o u s d e f e c t s i n d i c a t e s t h e f o l l o w i n g v i b r a t i o n a l characteristics:
TABLE 18.3
V i b r a t i o n C h a r a c t e r i s t i c s o f D e f e c t i v e Components
unbalance
... ..
misalignment
o c c u r s a t s h a f t speed
........
a t s h a f t speed (sometimes 3 o r 4 x )
........
"
p l a i n bearing loose i n housing
...
worn gears
f a u l t y be1 t drive
...
..
........
occurs a t t o o t h mesh frequency = rpm x no. t e e t h
occurs a t b e l t frequency
mechan c a l looseness e l e c t r c a l induced
18.5.1.3
occurs a t 1 / 2 o r 1 / 3 s h a f t speed
... ...
..
.....
in a radial direction i n a r a d i a l o r more dominantly i n an a x i a l d ir e c t ion
in a radial direction in a radial or axial d ir e c t ion in a radial direction
2 x s h a f t speed a t synchronous frequency
........
should disappear o f f power
Waveform A n a l y s i s
The shape o f t h e v i b r a t i o n waves and t h e g e n e r a l i n t e r a c t i o n o f superimposed v b r a t i o n s can be s t u d i e d by p a s s i n g t h e v i b r a t i o n s i g n a l through a time-doma n r e c o r d e r such as an u l t r a - v i o l e t i n genera
(UV) r e c o r d e r .
Such methods a r e
l i m i t e d t o v e r y low frequency ( s t r u c t u r a l ) v i b r a t i o n s and some
transient studies.
18.5.2
I n s p e c t i o n / l n t e g r i t y Surveillance
Methods t o determine t h e presence o f f l a w s which have been adopted f r o m n o n - d e s t r u c t i v e ( n d t ) t e s t i n g methods i n c l u d e t h e f o l l o w i n g : (i)
Dye p e n e t r a t i o n
-
which r e v e a l s c r a c k s a s small as 0.025 pm t o t h e naked eye
431
-
( i i ) Flux t e s t i n g
magnetic m a t e r i a l s magnetised t o r e v e a l t h e presence o f c r a c k s when t h e s u r f a c e i s spread w i t h magnetic p a r t i c l e s / p o w d e r
(iii)
E l e c t r i c a l resistance
-
by which two probes a r e moved o v e r t h e s u r f a c e w i t h an e l e c t r i c a l p.d. them;
between
c r a c k s a l t e r t h e r e s i s t a n c e and
therefore the passing current (iv)
Eddy c u r r e n t t e s t i n g
-
c u r r e n t s induced i n a m a t e r i a l ( n o t n e c e s s a r i l y magnetic) and c r a c k s l o c a t e d by a search c o i l
(v)
Ultrasonic testing
-
whereby c r a c k s a r e l o c a t e d by t h e r e f l e c t i o n o f u l t r a s o n i c waves propagated i n t o the m a t e r i a l ;
t h i s method i s
d e v e l o p i n g i n t o one o f t h e p r i m a r y n d t techniques (vi)
-
Radiographic
by which X-rays p e n e t r a t e t h e s u r f a c e and l o c a t e hidden c r a c k s .
I f one adds t o t h i s c a t a l o g u e o f techniques t h e newer ones which have been d e v e l o p e d e x c l u s i v e l y f o r i n s p e c t i o n / s u r v e i l l a n c e purposes and add
t o t h i s leak
d e t e c t i o n and c o r r o s i o n m o n i t o r i n g , a v a s t a r r a y o f p o s s i b l e methods i s p r e sented.
An i n d i c a t i o n o f t h e range o f these i s g i v e n i n t h e f o l l o w i n g t a b l e :
TABLE 18.4
C l a s s i f i c a t i o n s o f I n t e g r i t y S u r v e i l l a n c e Techniques
Acoustic/ultrasonic
-
Electr ica1
-
Magnetic
-
hysteresis;
Radiography
-
beta-ray backscatter;
Thermal
-
infra-red; compounds
Visual
-
holography;
s t r e s s wave emission;
ultrasonic;
c a p a c i t a n c e ; corona d i s c h a r g e ; c o r r o s i o n probe; eddy c u r r e n t ; microwave; resistance particles;
prints X-ray;
s u r f a c e impedance;
gamma-ray;
neutron
thermographic
borescopes; C.C.T.V.; dye-penetrant; holography p h o t o - e l e c t r o n emission;
438 TABLE 18.5
Some f u r t h e r d e s c r i p t i o n s
Beta-ray B a c k s c a t t e r
t o determine l a y e r t h i c k n e s s ; c o m p o s i t i o n o f aggregates
assess t h e
Capacitance
c r a c k d e t e c t i o n , bond d e f e c t s i n n o n - m e t a l l i c materials
Corrosion-probe
m a t e r i a l loss by c o r r o s i o n measurement
Dye Penetrant
crack penetration using v i s i b l e o r fluorescent dyes
Eddy Current
surface d e f e c t s d e t e c t e d by e l e c t r o m a g n e t i c induction
F l u x Sensors
uses r e s i d u a l o f induced magnetic f l u x p e r t u r ba t i o n s
Magnetic H y s t e r e s i s
measures magnetic changes due t o t h e presence o f faults
Magnetic P a r t i c l e s
s o l i d magnetic p a r t i c l e m i g r a t i o n i n t h e presence o f an a p p l i e d magnetic f i e l d l o c a t e s surface defects
Magnetic P I i n t s
a s t r i p p a b l e p a i n t f i l m under magnetic f i e l d e f f e c t s which i l l u s t r a t e s s u r f a c e d e f e c t s i n f e r romagne t ic mate r ia 1 s
Microwave
infra-red
O p t i c a l Aids
borescopes, f i b r e o p t i c s , CCTV
P h o t o e l e c t r o n Emission
t h e d e t e c t i o n o f spontaneous e l e c t r o n emissions f r o m p l a s t i c a l l y deformed s u r f a c e s
Radiography
p e n e t r a t i o n t o show up d e f e c t i v e and b u r n t - o u t parts
Resistivity
c r a c k o r bond t e s t i n g by e l e c t r i c a l r e s i s t a n c e measurement
S t r e s s Wave Emission
a method f o r l o c a t i n g t o p o s i t i o n and s e v e r i t y o f d e f e c t s i n metal s u r f a c e s and s t r u c t u r e s
U1 t r a son i cs
which uses t r a n s m i s s i o n c h a r a c t e r i s t i c s o f u l t r a - h i g h frequency s o n i c waves t o l o c a t e d e f e c t s .
18.5.3
i n s p e c t i o n o f non-conducting m a t e r i a l s
Contaminant A n a l y s i s
Moving c o n t a c t between t h e m e t a l l i c components o f any mechanical system i s accompanied by wear, which r e s u l t s i n t h e g e n e r a t i o n o f wear d e b r i c p a r t i c l e s . I n a l u b r i c a t e d system these p a r t i c l e s a r e i n suspension i n t h e c i r c u l a t i n g oil.
Under normal c o n d i t i o n s t h e r a t e o f wear i s low and p a r t i c l e s formed a r e
v e r y small,
The s i z e and r a t e o f g e n e r a t i o n o f these p a r t i c l e s increase as t h e
r a t e o f wear increases.
By i d e n t i f y i n g and measuring these m e t a l l i c p a r t i c l e s ,
t h e s u r f a c e f r o m which t h e p a r t i c l e s were worn can be i d e n t i f i e d and t h e r a t e of wear can be determined t o be normal o r abnormal.
439 Techniques f r e q u e n t l y used i n o i l c o n d i t i o n m o n i t o r i n g a r e (i) (ii) (iii) (iv) (v)
Magnetic p l u g i n s p e c t i o n S p e c t r o m e t r i c O i l A n a l y s i s Procedure (SOAP) Ferrography P a r t i c l e Counting ( f o r h y d r a u l i c f l u i d s ) Patch T e s t i n g
A t y p i c a l magnetic p l u g , F i g . 7 ,
i n c o r p o r a t e s a n o n - r e t u r n v a l v e so t h a t i t
can be i n s e r t e d i n t o a p i p e - l i n e and/or withdrawn w i t h o u t loss o f f l u i d . D e b r i s which has been trapped by such a magnetic p l u g can be measured w i t h a magnetometer t o determine t h e amount t h a t has been c o l l e c t e d ; recorded t o determine t h e c o l l e c t i o n t r e n d .
t h i s may then be
A t t h e same time, examination under
a s i m p l e microscope and comparison w i t h Debris R e c o g n i t i o n Drawings/Photographs makes i t p o s s i b l e t o t e l l t h e component from which t h e major amount o f wear m a t e r i a l has a r i s e n , as shown i n Fig.9. When spectroscopy was f i r s t i n t r o d u c e d as a chemical a n a l y t i c a l i n s t r u m e n t
100 years ago,
i t b r o u g h t about a r e v o l u t i o n i n c h e m i s t r y .
I t s advantages were
n o t m e r e l y t h a t spectroscopy was a s e n s i t i v e d e t e c t o r b u t t h a t t h e i n s t r u m e n t c o u l d d e t e c t and measure the q u a n t i t y o f an element p r e s e n t i n t h e sample i n dependently o f how t h e element was i n c o r p o r a t e d i n a compound.
The flame o r
spark o f t h e e m i s s i o n o r a b s o r p t i o n apparatus broke down t h e compounds and each element d i s p l a y e d i t s i n d i v i d u a l s e t o f spectrum l i n e s , Fig.10.
Nothing was
occluded, t h i s was t h e fundamental d i f f e r e n c e f r o m a l l p r e v i o u s l y e x i s t i n g methods of chemical a n a l y s i s . When t h e advantages o f o i l a n a l y s i s were f i r s t r e a l i s e d a t t h e t i m e o f t h e i n t r o d u c t i o n o f d i e s e l locomotives t o t h e r a i l r o a d s , t h e emission spectrograph was adapted t o o i l a n a l y s i s , t h e t h e o r y b e i n g t h a t a r a p i d increase o f a m e t a l l i c element i n t h e l u b r i c a t i n g o i l would imply t h a t a p a r t made o f t h a t element was wearing r a p i d l y .
A i r d i r t , assembly o r r e p a i r d e b r i s , system wear
metals, c o r r o s i o n p r o d u c t s and c o o l a n t water i n h i b i t o r s a r e some of t h e mate r a i l s which may be detected. E l emen t
wavelength (Angstrom u n i t s )
A l umi n ium
3092
Copper
3247
Ch rom i um
3579
Iron
3720
Lead
2833
Sod i um
589 0
Tin
2354
440
Fig.7
Magnetic p l u g ( c h i p d e t e c t o r )
VALVE OPEN-
FLOW
VALVE CLOSED
MAGN
O-RING SEALS BAYONET PINS
Fig.8
Operating p r i n c i p l e o f t h e TEDECO/Muirhead V a c t r i c Magnetic Plug/ Chip D e t e c t o r
B a l l Debris rounded ' r o s e - p e t a l ' r a d i a l l y s p l i t shape
-
-
(b) Track D e b r i s rounded, s u r f a c e break up, c r i s s cross scratches
-
Rol l e r D e b r i s (d) Gear Tooth D e b r i s g e n e r a l l y c u r l e d and i r r e g u l a r shape, g r e y rectangular, p a r a l l e l l i n e s s u r f a c e as s p l a s h o f across width solder Magnetic p l u g d e b r i s source r e c o g n i t i o n
441
40
30
1
D
-
-
20 -
10
IB
I d
0
L9-J-J WAVE- LENGTH
Fip.10
TABLE 18.6
I
C
1 3800
(ANGSTRON UNITS 1
T y p i c a l s p e c t r o s c o p i c spectrum
Element d e t e c t e d and f r e q u e n t source Element
Sources
Aluminium, S i l i c o n Borax, potassium,
Dust and A i r b o r n e d i r t sodium
Coolant i n h i b i t o r r e s i d u e s
Calcium, Sodium Chromium, cooper, tin, zinc
S a l t water r e s i d u e s iron,
lead
Barium, calcium, magnesium, phosphorus, z i n c
Wear, c o r r o s i o n o r r e s i d u a l assembly d e b r i s Engine o i l a d d i t i v e s
Some manufacturers i n c o r p o r a t e s p e c i a l elements i n t o d i f f e r e n t p a r t s o f an engine t o a c t as t r a c e r s so t h a t t h e i r presence i n a sample p r o v i d e s an unambiguous i n d i c a t i o n o f t h e source o f t r o u b l e .
SOAP has developed t o t h e p o i n t where t h e m a j o r i t y o f m i l i t a r y s e r v i c e s , r a i l r o a d s and a i r l i n e s o p e r a t e s p e c t r o g r a p h i c o i l a n a l y s i s as a s t a n d a r d p r o cedure f o r d e t e c t i n g problem areas, schedule o v e r h a u l s and r o u t i n e o i l mon it o r i n g .
44 2 I t m ig h t be tho ug ht t h a t wear p a r t i c l e s c o u l d be examined by v i e w i n g a f i l t e r and measuring t h e number o f p a r t i c l e s .
U n f o r t u n a t e l y f o r most cases
o t h e r t h a n h y d r a u l i c systems t h i s i s n o t p o s s i b l e . c e l l a n e o u s mass o f p a r t i c l e s a r e found.
I f o i l i s f i l t e r e d a mis-
Large and small p a r t i c l e s a r e p i l e d
one on t h e o t h e r so t h a t i t i s n o t p o s s i b l e t o determine t h e i r c h a r a c t e r i s t i c s , size distribution etc.
I n f a c t , f i l t e r e d d e p o s i t s g i v e t h e impression o f being
"just dirt".
01L SAMPLE
COLLECTOR RECEPTACLE
Fig.11
Schematic diagram o f 'F erro gra ph ' ( t h e s t r e n g t h o f the magnetic f i e l d i s g r e a t e s t a t t h e b ot t o m o f t h e s l i d e . )
The m o n i t o r i n g o f wear p a r t i c l e s on t h e F errograph, Fig.11, hand,
i s a t e c h ni qu e i n which most t h i n g s a r e occluded.
on the o t h e r
One o f i t s most v a l -
ua b le c h a r a c t e r i s t i c s stems f ro m t h e f a c t t h a t i t does n o t see every p a r t i c l e and,
i n fact,
ig nore s e v e r y t h i n g exce pt wear metal p a r t i c l e s i n the o i l .
The
s e p a r a t i o n and a n a l y s i s o f wear p a r t i c l e s f o r s i z e d i s t r i b u t i o n , type o f m e t a l , p h y s i c a l shape, c r y s t a l s t r u c t u r e e t c .
i s o f t e n a much more s e n s i t i v e i n d i c a t o r
of t h e wear s i t u a t i o n than a d i r e c t vi e w o f t h e worn surface.
Characteristic
d e b r i s f r o m modes o f f a i l u r e have been d i scussed by S c o t t i n a p r e v i o u s chapter on wear.
443
A l l wear d e b r i s a n a l y s i s t ech ni q ue s a r e p a r t i c l e s i z e dependent and any p a r t i c u l a r t e c h n iq ue i s s e n s i t i v e o n l y t o a s p e c i f i c range o f p a r t i c l e s i z e s . Fig.12 summarises t he e f f i c i e n c y o f these t echniques as a f u n c t i o n o f p a r t i c l e size [g].
ISOA
0
Fig.12
Ferrography
Magnetic plug
10 100 Particle size (microns)
1000
E f f i c i e n c y o f v a r i o u s sensors as a f u n c t i o n o f p a r t i c l e s i z e
The c le a n 1 in e ss re qu i reme nt s o f h y d r a u l i c systems have become more c r i t i c a l i n r e c e n t years.
Systems employing e l e c t r o h y d r a u l i c servo v a l v e s i n n u m e r i c a l l y
c o n t r o l l e d machine t o o l s , h i g h pre ssure systems where pump o r v a l v e clearances may be as s m a ll as 0.5 micrometre a r e p a r t i c u l a r examples.
Analysing p a r t i c -
u l a t e c o n t a m i n a t i o n f o r these cases a r e u s u a l l y c a r r i e d o u t by p a r t i c l e count methods u t i l i s i n g e i t h e r microscopes o r a ut omatic c o u n t e r s . Microscope c o u n t i n g methods (ASTM F312, F 3 1 3 , ARP 598A, I P
2751, a r e p r o -
cedures which a r e con si de red u n s u i t a b l e f o r p a r t i c l e s s m a l l e r than 5 microns. These methods s i z e and co un t s t a t i s t i c a l l y p a r t i c l e s r e t a i n e d on a membrane surface a f t e r t h e f l u i d sample has been f i l t e r e d .
A p a r t f r o m c o u n t i n g being v e r y
time consuming, t h e r e can be v a r i a t i o n when d i f f e r e n t people count t h e same
444 slide.
Automatic c o u n t e r s a r e however now a v a i l a b l e u t i l i s i n g scanning com-
p u t e r s and TV screens. Other in s tr u m ent s u s i n g t he p r i n c i p l e o f l i g h t i n t e r c e p t i o n count p a r t i c l e s suspended i n a l i q u i d f r o m e i t h e r a sample b o t t l e o r d i r e c t f r o m a h y d r a u l i c system.
As these i nst rume nt s o p e r a t e on t h e blockage o f l i g h t p r i n c i p l e , they
measure the p r o j e c t e d a rea o f a p a r t i c l e and then r e c o r d the diameter o f a c i r c l e o f e q u i v a l e n t area.
U n f o r t u n a t e l y t h ey cannot d i f f e r e n t i a t e between
s o l i d p a r t i c l e s and a i r bubbles. The t y p i c a l c o l o u r o f c o n t a m i n a t i o n i n any g i v e n h y d r a u l i c system remains f a i r l y constant.
The darkness o f t h e p a r t i c u l a t e d i s c o l o u r a t i o n o f a f i l t e r
t h e r e f o r e a rough i n d i c a t i o n o f t h e c l e a n l i n e s s o f the t e s t f l u i d .
is
T h i s Patch
Test procedure i s however o n l y g e n e r a l l y a p p l i c a b l e t o gross l e v e l s o f contami n a t io n .
18.5.4
Trends A n a l y s i s
T h i s i s l i t t l e more t h an t h e d at a l og ge r w i t h a memory and t r e n d a n a l y s i s capability.
The most el e men t a ry f o r m i s t h e h a n d w r i t t e n watch l o g w hich i s
inspected and analysed.
As i n p r a c t i c e , r e a l use i s o n l y made o f t h e l o g a f t e r
a f a i l u r e has o c curre d, t h e modern tendency i s t o use sensors t o i n p u t t o a general d a ta system which scans t h e m o n i t o r p o i n t s and produces a r e g u l a r p r i n t out.
I n more advanced form, a computer i s capable o f combining many o f t h e
i n p u t s and e s t a b l i s h i n g t r e n d s which, by mathematical m o d e l l i n g , can be associ a t e d w i t h t h e changes whi ch would occu r f o l l o w i n g s p e c i f i c d e f e c t s and cons e q u e n t ly when t h e system i s i n t e r r o g a t e d ,
i t can s t a t e (through a l o g i c
process) t h e p l a n t and component whi ch i s d e t e r i o r a t i n g ;
w i t h preset-1 i m i t s
such a system w i l l even produce a st at eme nt o f t h e ' u n e x p i r e d l i f e ' remaining. Such advanced t r e n d s m o n i t o r s a r e more commonly known as 'performance' m o n ito r s .
18.6
DETERIORATION LIMITS
Most c l a s s i f i c a t i o n s o c i e t i e s and b od i es o f a s i m i l a r n a t u r e have establ i s h e d s p e c i f i c a t i o n s and codes o f p r a c t i c e which d e f i n e t h e d e t e r i o r a t i o n l i m i t s which s h oul d be a l l o w e d b e f o r e c o r r e c t i v e a c t i o n i s taken.
Typically,
l i m i t s which have been e s t a b l i s h e d f o r d e t e r i o r a t i o n i d e n t i f i e d by v i b r a t i o n methods in c lu d e :
V D I Code o f P r a c t i c e 1056 (October 1964) D I N 45665 (November 1967) B S 4675:
1971
IS0 2372/3 I t does seem t h a t f o r v i b r a t i o n l i m i t s , most people i n general e r r
on
the
445 side o f c o n s i d e r a b l e c a u t i o n a l t h o u g h i n p r a c t i c e , t h e l i m i t must depend upon a large range o f environmental f a c t o r s s p e c i f i c t o each i n s t a l l a t i o n . I t i s o n l y by experience,
b o t h p e r s o n a l and t h a t o f o t h e r t h a t a c t i v e
decision l i m i t s can be reached.
I n t h e whole f i e l d o f c o n d i t i o n m o n i t o r i n g ,
developments a r e o c c u r r i n g a t such a r a p i d speed t h a t o n l y an o r g a n i s a t i o n such as the UK Mechanical H e a l t h M o n i t o r i n g Group through i t s r e g u l a r seminar/ symposia and courses i s i t p o s s i b l e t o a c q u i r e a l l t h e i n f o r m a t i o n consultants
-
-
known t o
needed t o implement and manage an o n - c o n d i t i o n maintenance system.
REFERENCES
1
Collacott,R.A. 'Mechanical Fau t D i a g n o s i s and C o n d i t i o n M o n i t o r i n p ' Chapman & H a l l , London 1977. 2 Venton ,A. D. F. and Harvey ,B. F. R e l i a b i l i t y assessment i n machinery system d e s i g n ' Proc. 1.Mech.E. 973. 3 Bridges,D.C. 'The a p p l i c a t i o n o f r e l i a b i l i t y t o t h e design o f s h i p ' s machinery' Trans. 1.Mar.E. 86, P a r t 6 1974. 4 Davies,A.E. ' P r i n c i p l e s and p r a c t i c e o f a i r c r a f t powerplant maintenance' Trans. 1.Mar.E. 85 P a r t 6 1973. 5 Collacott,R.A. 'Data Sources f o r Re1 i a b i l i t y S t a t i s t i c s ' UKM P u b l i c a t i o n s L t d . , 92 London Road, L e i c e s t e r LE2 OQR 1976. 6 Wilkinson,H.C. and Ki1bourn.D.F. 'The d e s i g n o f s h i p ' s machinery i n s t a l l a t i o n s ' S h i p p i n g World and S h i p b u i l d e r , August 1971. 7 Mathieson,Tor-Chr. ' R e l i a b i l i t y e n g i n e e r i n g i n s h i p machinery p l a n t design' Report IF/R.12 U n i v e r s i t y o f Trondheim,N.I.T. 1973. 8 Col l a c o t t ,R.A. ' V i b r a t i o n M o n i t o r i n g and D i a g n o s i s ' George Godwin L i m i t e d 1978. 9 Pocock,G. ' Introduction t o Ferrography' Symposium on Ferrography, B r i t i s h I n s t . Non D e s t r u c t i v e T e s t i n g , London 1979.
446
I!)
THE TRIBOLOGY OF METAL CUTTING
E. M. TRENT
Department o f I n d u s t r i a l M e t a l l u r g y , U n i v e r s i t y o f Birmingham, Birmingham 815 2TT
19.1
INTRODUCTION
The e f f i c i e n c y o f metal c u t t i n g o p e r a t i o n s i s v e r y l a r g e l y c o n t r o l l e d by t h e behaviour o f t h e work m a t e r i a l and t h e t o o l m a t e r i a l a t t h e i n t e r f a c e between them near t h e c u t t i n g edge o f t h e t o o l .
Metal c u t t i n q i s c a r r i e d o u t on all
the m a jo r c l a s s e s o f m e t a l s and a l l o y s produced commercially.
The machining
o p e r a t i o n s such as b o r i n g , d r i l l i n g , t a p p i n g , t u r n i n q , m i l l i n g , p l a n i n g and sawing a r e v e r y v a r i e d i n c h a r a c t e r .
Ob j ect s as small as instrument p a r t s o r
as l a r g e as i n d u s t r i a l b o i l e r s a r e machined t o remove excess m a t e r i a l and t o ge n e r a t e t h e necessary shapes w i t h t h e r e q u i r e d p r e c i s i o n , b u t , however v a r i e d , a l l machining o p e r a t i o n s have c e r t a i n f e a t u r e s i n common.
19.2
METAL CUTTING PHENOMENA
A l l machining o p e r a t i o n s i n v o l v e t h e use o f one o r more t o o l s o f a wedge shape w i t h a c u t t i n g edge, whi ch remove a t h i n l a y e r f r o m t h e s u r f a c e o f a l a r g e r body as shown i n Fig.1.
The t o o l s a r e always moved a s y m m e t r i c a l l y w i t h
re s p e c t t o t h e wedge a n g l e s o t h a t t h e l a y e r removed
-
t h e " chip"
-
bears
a g a i n s t and moves ove r one s u r f a c e o f t h e wedge, known as t h e "rake face" o f the t o o l .
The t o o l i s so shaped t h a t t h e f r e s h l y c u t metal s u r f a c e does n o t rub
a g a i n s t t h e o t h e r f a c e o f t h e wedge tool.
-
t h e "cl earance face" o r " f l a n k "
o f the
(Fig.1).
The l a y e r o f metal removed i s f i r s t p l a s t i c a l l y deformed by a shearing a c t i o n , r o u g h l y a l o n g a p l a n e A-B
i n Fig.l.,
e x t e n d i n g f r o m t h e t o o l edge A t o
t h e p o s i t i o n B where t h e c h i p sep ara t e s f rom t h e undeformed work m a t e r i a l . i n v o l v e s severe shear s t r a i n
-
This
usually a natural s t r a i n o f a t least 2 but often
v e r y much h i g h e r as can be seen f r o m F i g. 2.
which shows a s e c t i o n t h r o u g h t h e
forming c h i p d u r i n g t h e c u t t i n g o f copper.
The shear plane i s under h i g h com-
p r e s s i v e s t r e s s and t h e t r i - a x i a l s t r e s s c o n d i t i o n i s such t h a t w i t h most d u c t i l e m e t a l s and a l l o y s t h e h i g h shear s t r a i n can be s u s t a i n e d w i t h o u t f r a c t u r e s o t h a t a c ont i nu ou s c h i p i s formed, as w i t h t h e example o f copper.
With
m e t a ls and a l l o y s o f low d u c t i l i t y , however, o r under c u t t i n g c o n d i t i o n s where t h e compressive s t r e s s on t h e shear p l a n e i s low, t h e c h i p may be broken i n t o
447
CLEARANCE FACE
Fig.1
Fig.2
TOOL
Features o f Metal C u t t i n g
S e c t i o n through c h i p and b a r o f h i g h p u r i t y copper a f t e r "quick-stop". Machined a t 122111m i " - ' .
448 small fragments.
A d i s c o n t i n u o u s c h i p i s formed, f o r example, when c u t t i n g
c a s t i r o n o r a f r e e c u t t i n g brass. The c h i p moves across t h e r a k e s u r f a c e o f t h e t o o l away f r o m t h e c u t t i n g edge and breaks c o n t a c t w i t h t h e t o o l s u r f a c e a t some p o s i t i o n C, u s u a l l y r a t h e r i l l defined.
Always t h e c o n t a c t l e n g t h A-C
o f t h e l a y e r b e i n g removed ( t l
10 times t h e feed tl.
i n Fig.1.)
i s g r e a t e r than t h e o r i g i n a l t h i c k n e s s
-
t h e "feed"
-
A-C
i s o f t e n 5 o r even
The work and t o o l m a t e r i a l s a r e i n c o n t a c t a t t h e
c u t t i n g edge and u s u a l l y f o r a s h o r t d i s t a n c e down t h e c l e a r a n c e f a c e o r f l a n k o f the t o o l .
The l e n g t h o f c o n t a c t i n t h i s r e g i o n A-D
than on t h e rake f a c e A-C.
The c l e a r a n c e a n g l e (Fig.1.)
i s u s u a l l y much s h o r t e r which i s u s u a l l y about
6" t o 15" r e s t r i c t s t h e l e n g t h of c o n t a c t on t h i s s u r f a c e , b u t d u r i n g c u t t i n g t h e most common f o r m o f wear i s one i n which a s u r f a c e i s worn on t h e t o o l nearly p a r a l l e l t o the d i r e c t i o n o f c u t t i n g
-
t h e " f l a n k wear land"
T h i s worn s u r f a c e , and hence t h e l e n g t h o f c o n t a c t A-D,
i n Fig.1.
tends t o i n c r e a s e w i t h
c u t t i n g t i m e b u t , t o a v o i d t o t a l t o o l f a i l u r e , t h e f l a n k wear should n o t be a l l o w e d t o become t o o l a r g e b e f o r e t h e t o o l i s reground o r replaced.
19.3 19.3.1
CONDITIONS AT THE TOOL-WORK INTERFACE Tool Forces and Stresses
The f o r c e s a c t i n g on t h e t o o l a r e ( 1 ) t h a t r e q u i r e d t o shear t h e work material over t h e area o f t h e shear p l a n e A-8 (Fig.1.)
and ( 2 ) t h a t r e q u i r e d t o move t h e
c h i p across t h e t o o l rake f a c e c o n t a c t r e g i o n A-C. m a t e r i a l o v e r t h e f l a n k A-D
The f o r c e t o move t h e work
i s small compared w i t h t h e o t h e r f o r c e s and can be
neglected i n a f i r s t approximation.
Tool dynamometers have been developed and
used t o measure t h e f o r c e s a c t i n g on t h e t o o l i n two d i r e c t i o n s d i r e c t i o n o f c u t t i n g Fc and i n t h e d i r e c t i o n o f t h e f e e d F f .
-
i n the
I n mast c u t t i n g
o p e r a t i o n s t h e f o r c e s a c t i n g on t h e t o o l v a r y f r o m a few k i l o g r a m s t o a few hundred k i l o g r a m s . The c u t t i n g f o r c e Fc a c t s n e a r l y normal t o t h e r a k e f a c e o f t h e t o o l and e x e r t s a l a r g e l y compressive s t r e s s on t h i s s u r f a c e .
The f e e d
f o r c e F f i s almost always s m a l l e r than t h e c u t t i n g f o r c e ( t y p i c a l l y 40-60% o f t h e c u t t i n g f o r c e ) and e x e r t s a s h e a r i n g s t r e s s on t h e t o o l s u r f a c e . While t h e f o r c e s a c t i n g on t h e t o o l can be measured w i t h accuracy, even t h e mean v a l u e o f the s t r e s s a c t i n g on t h e c o n t a c t area between work m a t e r i a l and t o o l i s d i f f i c u l t t o d e t e r m i n e because t h e a r e a o f c o n t a c t i s d i f f i c u l t o r impossible t o measure e x a c t l y .
The s t r e s s e s a r e n o t e v e n l y d i s t r i b u t e d on t h e
c o n t a c t area and i t i s n o t easy t o determine t h e s t r e s s d i s t r i b u t i o n on t h i s area.
The general c h a r a c t e r o f t h e s t r e s s d i s t r i b u t i o n on t h e rake f a c e o f a
c u t t i n g t o o l , however,
i s now g e n e r a l l y accepted t o be t h a t suggested by Zorev
111 and shown d i a g r a m m a t i c a l l y i n Fig.3.
The compressive s t r e s s a c t i n g on t h e
rake face i s a t a maximum a t o r c l o s e t o t h e c u t t i n g edge and d i m i n i s h e s t o z e r o a t t h e end o f t h e c o n t a c t area.
The maximum compressive s t r e s s near t h e edge
449
(COMPRESSIVE
STRESS
* ul
CHIP SHEAR STRESS
'b-\ (DISTANCE
, ,
Fig.3
I
'
0
I
'FROM 'CUT
/
S t r e s s d i s t r b u t i o n i n c u t t i n g t o o l ( a f t e r Zorev)
2,000
1,500
Z
-0 4-
5
1,000
CJl
A,
C .c U 0
'
0,
500
I.?
0
50
100
150
200
Cutting speed rn min?
Fig.4
Forces a c t i n g on a t o o l as f u n c t i o n o f c u t t i n g speed. Depth o f c u t 1.25mm. Feed 0.25mm/rev.
450 i s o f t e n t w i c e t h e mean s t r e s s on t h e area o f c o n t a c t .
The shear s t r e s s on t h e
rake f a c e i s more u n i f o r m l y d i s t r i b u t e d as shown i n Fig.3. The v a l u e s o f t h e compressive s t r e s s near t h e edge a r e h i g h r e l a t i v e t o t h e y i e l d stress o f the material being c u t
-
t h e work m a t e r i a l .
For example, i n
c u t t i n g s t e e l t h e compressive s t r e s s near t h e edge may be o f t h e o r d e r o f -2 1500 N mm A major requirement o f a s a t i s f a c t o r y t o o l m a t e r i a l i s t h u s h i g h
.
y i e l d s t r e s s i n compression, and t h e V i c k e r s o r Rockwell hardness v a l u e s a r e u s u a l l y taken as an i n d i c a t i o n o f t h i s p r o p e r t y .
The most commonly used c u t t i n g
t o o l m a t e r i a l s a r e hardened h i g h speed s t e e l and cemented c a r b i d e .
The minimum
hardness o f c u t t i n g t o o l m a t e r i a l s i n common use i s 750 HV (62 Rockwell C). Although t h e r e a r e few r e l i a b l e d a t a f o r t h e s t r e s s e s on t h e c o n t a c t area o f t o o l s i n real c u t t i n g operations,
i t i s c e r t a i n t h a t they are r e l a t e d t o the
y i e l d s t r e s s o f t h e work m a t e r i a l .
Approximate values f o r t h e mean s t r e s s
a c t i n g normal t o t h e r a k e f a c e o f a t u r n i n g t o o l under a standard s e t o f c u t t i n g c o n d i t i o n s a r e shown i n Table 19.1 f o r d i f f e r e n t work m a t e r i a l s . TABLE 19.1
Mean compressive s t r e s s on c o n t a c t area
Work m a t e r i a l
Compressive S t r e s s
-2
N mm
I ron Steel (medium carbon) T i taniurn Copper 70/30 brass Lead
340 770 570
310 420 14
For t h e c u t t i n g o f m a t e r i a l s o f v e r y h i g h y i e l d s t r e n g t h , p a r t i c u l a r l y h e a t t r e a t e d s t e e l s and n i c k e l base a l l o y s , t h e usual s t e e l and cemented c a r b i d e t o o l s may be inadequate because t h e s t r e s s imposed by t h e work m a t e r i a l i s h i g h enough t o deform t h e c u t t i n g t o o l edge even a t v e r y low c u t t i n g speed where t h e t o o l edge temperature i s low.
I t i s g e n e r a l l y c o n s i d e r e d i n a machine shop
t h a t h i g h speed s t e e l t o o l s cannot be used t o c u t s t e e l s w i t h hardness h i g h e r than 350 HV (36 Rc) and t h a t t h e c u t t i n g o f s t e e l w i t h hardness o v e r 550 HV (53 Rc) becomes v e r y d i f f i c u l t even w i t h cemented c a r b i d e t o o l s .
For t h e mach-
i n i n g o f f u l l y hardened s t e e l i t has been more usual t o r e s o r t t o g r i n d i n g u s i n g s i l i c o n c a r b i d e , aluminium o x i d e o r bonded diamond wheels, o r t o shape by e l e c t r o d i s c h a r g e machining (EDM) o r e l e c t r o chemical machining (ECM). Recently t h e i n t r o d u c t i o n o f new c u t t i n g t o o l m a t e r i a l s w i t h s t i l l h i g h e r y i e l d strength tools
-
-
i n c l u d i n g compacted p o l y c r y s t a l l i n e diamond and c u b i c boron n i t r i d e
has made t h e c u t t i n g o f f u l l y hardened s t e e l s , h i g h e r s t r e n g t h n i c k e l -
based a l l o y s , and o t h e r v e r y h a r d m a t e r i a l s , a more f e a s i b l e p r o p o s i t i o n f o r i n d u s t r i a l shaping o p e r a t i o n s .
451 19.3.2
C u t t i n g Speed
One o f t h e most i m p o r t a n t parameters i n metal c u t t i n g i s t h e v e l o c i t y a t which the work m a t e r i a l passes t h e c u t t i n g edge
-
t h e c u t t i n g speed.
This
varies g r e a t l y i n i n d u s t r i a l o p e r a t i o n s f r o m almost zero, f o r example near t h e centre o f a d r i l l , t o 300 m min-'
o r even h i g h e r .
research workers measuring t o o l f o r c e s found t h a t ,
Rather t o t h e i r s u r p r i s e i n g e n e r a l , these f o r c e s do
not i n c r e a s e as t h e c u t t i n g speed i s r a i s e d .
I n many cases t h e f o r c e s decrease, -1 p a r t i c u l a r l y i n t h e speed range up t o 65 m min as shown f o r example i n Fig.4. This has been shown t o be t r u e f o r a wide range o f work m a t e r i a l s and c u t t i n g conditions.
The f o r c e s decrease m a i n l y because t h e a r e a o f c o n t a c t between t o o l
and work decreases as c u t t i n g speed i s r a i s e d .
A l t h o u g h t h e r e have been no v e r y
d e t a i l e d s t u d i e s o f t h e s t r e s s a c t i n g on t h e t o o l s u r f a c e as a f u n c t i o n o f c u t t i n g speed, t h e r e i s no evidence t o i n d i c a t e t h a t t h e s t r e s s a c t i n g on t h e t o o l i s r a i s e d as c u t t i n g speed i s increased. C u t t i n g speed i s o f p a r t i c u l a r importance i n r e l a t i o n t o t h e economics o f machining.
The c o s t o f machining o p e r a t i o n s i s reduced by i n c r e a s i n g t h e r a t e
of metal removal, and t h e main i n c e n t i v e t o t h e development i n machining i n t h e l a s t hundred y e a r s has been t h e r e d u c t i o n o f t h e v e r y h i g h c o s t s by t h e use o f new machines and t o o l s capable o f machining a t increased r a t e s .
In the c u t t i n g
o f h i g h m e l t i n g p o i n t m e t a l s and a l l o y s t h e l i f e o f t h e c u t t i n g t o o l becomes p r o g r e s s i v e l y s h o r t e r as t h e c u t t i n g speed i s r a i s e d u n t i l t h e c o s t o f r e p l a c i n q worn o u t t o o l s more than outweighs t h e advantages o f h i g h e r speed.
I t has been
the a b i l i t y o f t h e c u t t i n g t o o l t o w i t h s t a n d t h e c o n d i t i o n s a t t h e t o o l edge which has l i m i t e d t h e r a t e o f machining o f s t e e l and c a s t i r o n .
The development
and commercial use f i r s t of h i g h speed s t e e l s and then o f cemented c a r b i d e s has enabled c u t t i n g speeds t o be r a i s e d by a f a c t o r o f about 20 t i m e s compared w i t h carbon s t e e l t o o l s and t h e r e a r e s t i l l many o p e r a t i o n s i n which t o o l l i f e i s the f a c t o r l i m i t i n g t h e r a t e o f metal removal. In g e n e r a l as t h e c u t t i n g speed i s r a i s e d n e i t h e r t h e f o r c e s a c t i n g on t h e
t o o l n o r t h e s t r e s s e s on t h e area o f c o n t a c t a r e increased.
The energy expended
i n metal c u t t i n g , however, increases a p p r o x i m a t e l y i n p r o p o r t i o n t o t h e c u t t i n g speed,
i f o t h e r c o n d i t i o n s remain c o n s t a n t .
T h i s energy i s c o n v e r t e d i n t o heat
near t h e c u t t i n g edge, and r a i s e s t h e temperature o f t h e t o o l , r e d u c i n g i t s y i e l d s t r e s s and i n c r e a s i n g t h e r a t e o f t o o l wear.
I t i s t h i s r i s e i n temper-
a t u r e which l i m i t s t h e a b i l i t y o f t h e t o o l s t o w i t h s t a n d i n c r e a s i n g c u t t i n g speed.
19.3.3
The g e n e r a t i o n o f temperatures i n metal c u t t i n g must now be considered. Heat i n Metal C u t t i n g
I n metal c u t t i n g energy i s expended i n t o two main r e g i o n s (1) a l o n g t h e shear p l a n e A-B
(Fig.1.)
where t h e work m a t e r i a l i s sheared t o form t h e c h i p ,
and (2) a t t h e r a k e surface o f t h e t o o l where t h e c h i p i s moved across t h e contact area.
452 The energy expended i n s h e a r i n g t h e work m a t e r i a l t o f o r m t h e ch p m a i n l y r e s u l t s i n r a i s i n g t h e temperature o f t h e c h i p and almost a l l o f t h s h e a t i s c a r r i e d o u t o f t h e system when t h e c h i p breaks c o n t a c t w i t h t h e t o o
.
Since any
one element o f t h e c h i p i s i n c o n t a c t w i t h t h e t o o l f o r o n l y a v e r y s h o r t time
-
-
t y p i c a l l y a few m i l l i - s e c o n d s
o n l y a small p r o p o r t i o n o f t h i s h e a t c o u l d be I t i s probable,
conducted i n t o t h e t o o l under t h e most f a v o u r a b l e c o n d i t i o n s .
as w i l l be shown, t h a t a l l t h e h e a t i n t h e body o f t h e c h i p i s c a r r i e d o u t o f t h e system i n most cases.
A small p r o p o r t i o n o f t h e h e a t generated on t h e
shear p l a n e i s conducted back i n t o t h e body o f t h e workpiece.
The energy ex-
pended on t h e shear p l a n e i s n o r m a l l y t h e lar’gest p a r t o f t h e t o t a l energy o f cutting
-
o f t e n o f t h e o r d e r o f 75 t o 8 0 % o f t h e t o t a l .
The temperature o f t h e
c h i p i s o f t e n r a i s e d t o 200-350°C when c u t t i n g s t e e l o r o t h e r h i g h m e l t i n g point materials. I t i s t h e s m a l l e r p o r t i o n o f t h e t o t a l energy o f c u t t i n g - t h a t expended i n
moving t h e c h i p o v e r t h e t o o l
-
temperatures a t t h e t o o l / w o r k
i n t e r f a c e and t h e c o n d i t i o n s a t t h i s i n t e r f a c e
must t h e r e f o r e be considered.
which i s r e s p o n s i b l e f o r t h e g e n e r a t i o n o f h i g h
The h i g h compressive s t r e s s normal t o t h e t o o l
r a k e s u r f a c e has a l r e a d y been emphasised.
The mean s t r e s s on t h e c o n t a c t area
i s always much h i g h e r than t h e s t r e s s e s n o r m a l l y encountered a t moving i n t e r faces i n e n g i n e e r i n g systems.
The v e r y h i g h s t r e s s e s a l o n e would r e s u l t i n t h e
area o f r e a l c o n t a c t between t h e two s u r f a c e s b e i n g a much h i g h e r p o r p o r t i o n o f t h e apparent c o n t a c t a r e a than is usual f o r s l i d i n g s u r f a c e s , and would t e n d t o promote s e i z u r e .
Other f a c t o r s a r e a l s o f a v o u r a b l e t o s e i z u r e .
The t o o l i s
c o n t i n u a l l y c u t t i n g i n t o clean metal, being brought i n t o contact w i t h surfaces f r e e f r o m o x i d e o r o t h e r l a y e r s which i n h i b i t t h e e x t e n s i o n o f c o n t a c t areas i n many s l i d i n g s i t u a t i o n s .
The c l e a n work m a t e r i a l f l o w s c o n t i n u o u s l y o v e r t h e
t o o l s u r f a c e i n one d i r e c t i o n , sweeping away o x i d e o r o t h e r l a y e r s i n i t i a l l y p r e s e n t on t h e t o o l , which have l i t t l e chance t o r e - f o r m .
Relatively high
temperatures generated a t t h e i n t e r f a c e i n c r e a s e atomic a c t i v i t y and t h i s a l s o tends t o promote s e i z u r e .
19.3.4
Seizure o f t h e Tool/Work
Interface
When these c o n d i t i o n s a r e considered,
i t i s not surprising t o f i n d that
s e i z u r e between the t o o l and work s u r f a c e s i s commonly observed on c u t t i n g tools
[Z]. The process o f f r i c t i o n w e l d i n g i s o f t e n c a r r i e d o u t under con-
d i t i o n s l e s s severe than those encountered i n metal c u t t i n g .
For example,
j o i n t s can be made by f r i c t i o n w e l d i n g a t p r e s s u r e s o f 75 N mm-’ speeds o f 50 m min-’, 750 N mm-’
sound
and p e r i p h e r a l
whereas i n metal c u t t i n g s t r e s s e s on t h e c o n t a c t area o f
o c c u r and h i g h e r speeds a r e o f t e n used.
That s e i z u r e occurs a t t h e
t o o l / w o r k i n t e r f a c e i s confirmed by numerous m e t a l l o g r a p h i c o b s e r v a t i o n s , o f which t h r e e examples w i l l be g i v e n here.
453 Fig.5.
shows a p o l i s h e d and e t c h e d s e c t i o n t h r o u g h t h e rake f a c e o f a
cemented c a r b i d e t o o l and a d h e r i n g work m a t e r i a l a f t e r c u t t i n g s t e e l a t 1 0 0 m -1 min I t shows t h e work m a t e r i a l i n c o n t a c t w i t h t h e t o o l s u r f a c e n o t j u s t a t
.
the tops o f t h e a s p e r i t i e s b u t a t a l l t h e h i l l s and v a l l e y s o f t h e s u r f a c e on a micro scale.
To say t h a t these s u r f a c e s a r e s e i z e d t o g e t h e r means t h a t s l i d i n g
as normally conceived, w i t h t h e two s u r f a c e s separated by a f l u i d f i l m o r i n contact o n l y a t t h e a s p e r i t i e s ,
i s not possible.
The two s u r f a c e s a r e mechan-
i c a l l y i n t e r l o c k e d and/or m e t a l l u r g i c a l l y bonded o v e r t h e whole o r a l a r g e p a r t of the i n t e r f a c e .
That m e t a l l u r g i c a l bonding i s o f t e n i n v o l v e d i s shown by
examples, such as those shown i n Figs.6 and
7, i n which, a f t e r s t o p p i n g c u t t i n g
by p r o p e l l i n g t h e t o o l r a p i d l y f r o m t h e c u t t i n g p o s i t i o n , t h e c h i p remains f i r m l y adherent t o t h e t o o l o r separates f r o m t h e t o o l a t some p o s i t i o n remote from the i n t e r f a c e ,
Fig.5
l e a v i n g a l a y e r o f work m a t e r i a l welded t o t h e t o o l surface.
S e c t i o n t h r o u g h c a r b i d e t o o l w i t h adherent work m a t e r i a l ( w h i t e ) a f t e r cutting steel. Shows s e i z u r e c o n d i t i o n s a t i n t e r f a c e .
Seizure i s n o r m a l l y t h o u g h t o f as a c o n d i t i o n where a mechanism ceases t o f u n c t i o n , as when a b e a r i n g s e i z e s , b u t i n m e t a l c u t t i n g t h e s e i z e d area i s small, t h e r e i s adequate power t o c o n t i n u e c u t t i n g and t h e t o o l i s s rong enough t o r e s i s t t h e s t r e s s e s imposed by s e i z u r e c o n d i t i o n s .
Movement c o n t nues by This gives
shear in t h e work m a t e r i a l i n a r e g i o n a d j a c e n t t o t h e t o o l s u r f a c e . r i s e t o two main s o r t s o f c o n d i t i o n s near t h e i n t e r f a c e .
The f i r s t
ayers o f
work m a t e r i a l s e i z e d t o t h e t o o l a r e s e v e r e l y work hardened and shea transferred t o the next layers.
i s then
I n t h i s way a body o f "dead m e t a l " may be
b u i l t up, adherent t o t h e t o o l m a t e r i a l , which p e r s i s t s f o r l o n g p e r i o d s o f cutting.
Fig.8 shows an example o f t h i s f e a t u r e , known as a " b u i l t - u p
edge".
The b u i l t - u p edge reaches a s t a b l e s t a t e and t h e s i z e and shape depending on the work m a t e r i a l and t h e c u t t i n g c o n d i t i o n s .
I t i s a dynamic s t r u c t u r e w i t h
454
Fig.6
S e c t i o n through h i g h speed s t e e l t o o l and adherent c h i p ( a u s t e n i t i c s t a i n l e s s s t e e l ) a f t e r machining a t 3 0 m min-1.
Fig.7
S e c t i o n throuqh h i g h speed s t e e l t o o l and adherent c h i p fragment a f t e r c u t t i n g low carbon s t e e l a t 107111min-'.
fragments b e i n g c o n t i n u o u s l y added and broken away.
The s h e a r i n g a c t i o n l e a d i n g
t o c h i p f o r m a t i o n may t a k e p l a c e a t a d i s t a n c e o f 300 urn o r more from t h e t o o l surface. A b u i l t - u p edge i s o f t e n formed when c u t t i n g a l l o y s c o n t a i n i n g more than one
phase, such a s s t e e l , c a s t i r o n and a l p h a - b e t a b r a s s
[31.
With such m a t e r i a l s
a b u i l t - u p edge o c c u r s when c u t t i n g a t r e l a t i v e l y low speeds, b u t disappears
455
Fig.8
B u i l t - u p edge formed d u r i n g c u t t i n g low carbon s t e e l a t 15m min
when speed o r f e e d a r e r a i s e d .
-1
.
When c u t t i n g pure m e t a l s and s o l i d s o l u t i o n s a t
almost any speed, and when c u t t i n g two phase a l l o y s i n t h e h i g h e r range o f speeds,
s e i z u r e c o n d i t i o n s a r e observed t o e x i s t a t most o f t h e i n t e r f a c e , b u t
t h e b u i l t - u p edge i s absent.
Movement o f t h e work m a t e r i a l t a k e s p l a c e by
shear c o n c e n t r a t e d i n t o a v e r y t h i n l a y e r a d j a c e n t t o t h e t o o l s u r f a c e u s u a l l y o f t h e o r d e r o f 25-50 um i n t h i c k n e s s . Fig.2.
An example of such a l a y e r i s seen i n
f o r t h e c u t t i n g o f copper and a l a y e r when c u t t i n g a low carbon s t e e l i s
seen a t h i g h m a g n i f i c a t i o n i n Fig.9.
I n t h i s l a y e r t h e work m a t e r i a l behaves
more l i k e a v e r y v i s c o u s l i q u i d than a normal metal and t h e l a y e r i s termed a "flow-zone".
I n t h e flow-zone
l o 4 t o l o 5 p e r second
-
t h e r a t e o f shear s t r a i n i s e x t r e m e l y h i g h
-
and t h e amount o f s t r a i n i s so extreme t h a t o r i g i n a l
s t r u c t u r a l f e a t u r e s (such as p e a r l i t e and f e r r i t e i n s t e e l ) a r e c o m p l e t e l y destroyed.
There i s good evidence t h a t , w i t h i n t h e flow-zone,
dynamic recovery
456 and/or r e c r y s t a l l i s a t i o n a r e t a k i n g p l a c e and t h e behaviour o f t h e m a t e r i a l i s a k i n t o i t s behaviour i n h o t w o r k i n g p r o c e s s e s .
Fig.9
19.3.5
Flow zone a t under s u r f a c e of c h i p , a d j a c e n t t o r a k e f a c e of t o o l , formed d u r i n g c u t t i n g low carbon s t e e l a t 63m min-'.
C u t t i n g Tool Temperatures
The energy expended i n deforming t h e work m a t e r i a l i n t h e flow-zone volume o f metal deformed,
per u n i t
i s much h i g h e r than on t h e shear p l a n e and t h e temp-
e r a t u r e s generated i n t h e flow-zone a r e t h e r e f o r e h i g h e r . t h e flow-zone and t h e t o o l i s v e r y good.
The c o n t a c t between
As has been demonstrated t h e r e i s
continuous m e t a l l i c c o n t a c t i n many cases and heat f l o w s r e a d i l y across t h e boundary t o heat t h e t o o l .
I t i s t h e heat generated i n t h e flow-zone a t t h e
i n t e r f a c e between t o o l and work m a t e r i a l which i s t h e main h e a t source r a i s i n g t h e temperature o f t h e t o o l and c r e a t i n g t h e c o n d i t i o n s under which c u t t i n g t o o l s a r e worn. The r e g i o n s o f t h e t o o l s which a r e heated t o h i g h temperature a r e v e r y l o c a l i s e d and w i t h i n these r e g i o n s temperature g r a d i e n t s a r e v e r y steep, b u t i t i s p o s s i b l e t o study t h e temperature d i s t r i b u t i o n i n some d e t a i l f o r c e r t a i n c o n d i t i o n s o f c u t t i n g by o b s e r v a t i o n o f t h e changes i n s t r u c t u r e o r hardness o f s t e e l t o o l s i n those p a r t s o f t h e t o o l s heated by c u t t i n g a c t i o n above t h e i r tempering temperature.
Fig.10 shows, f o r example, t h e temperature g r a d i e n t s i n
457 a h i g h speed s t e e l t o o l used t o c u t a l o w c a r b o n s t e e l a t a speed o f 76m m i n - l a t a f e e d o f 0.25 mm p e r r e v .
T h i s i s c h a r a c t e r i s t i c o f t h e t y p e o f temperature
d i s t r i b u t i o n f o u n d t o o c c u r i n t o o l s u s e d t o c u t s t e e l u n d e r c o n d i t i o n s where a flow-zone o c c u r s a t t h e i n t e r f a c e . t o o l edge was r e l a t i v e l y l o w
-
Fig.10
shows t h a t t h e t e m p e r a t u r e n e a r t h e
i n t h i s c a s e u n d e r 650°C
-
but there i s a high
t e m p e r a t u r e r e g i o n j u s t o v e r l m m f r o m t h e edge i n t h e d i r e c t i o n o f c h i p f l o w , where,
i n t h i s example,
t h e t e m p e r a t u r e a t t h e i n t e r f a c e was o v e r 800°C.
It i s
f o r t u n a t e t h a t i n c u t t i n g s t e e l and many o t h e r a l l o y s a t h i g h speed t h e r e g i o n o f h i g h e s t t e m p e r a t u r e i s a t a d i s t a n c e f r o m t h e edge where t h e c o m p r e s s i v e s t r e s s o n t h e t o o l i s a maximum ( F i g . 3 ) .
As t h e c u t t i n g speed i s r a i s e d t h e maximum t e m p e r a t u r e o n t h e r a k e f a c e o f t h e tool
i n c r e a s e s r a p i d l y , w h i l e t h e t e m p e r a t u r e n e a r t h e edge i s i n c r e a s e d
more s l o w l y .
The y i e l d s t r e s s o f t h e t o o l m a t e r i a l d e c r e a s e s w i t h r i s i n g tem-
p e r a t u r e and, a s c u t t i n g speed i s r a i s e d , t h e t e m p e r a t u r e a t t h e edge may r e a c h a v a l u e where t h e y i e l d s t r e s s o f t h e t o o l i s r e d u c e d b e l o w t h e c o m p r e s s i v e s t r e s s e x e r t e d by t h e w o r k m a t e r i a l .
The t o o l edge i s t h e n p l a s t i c a l l y deformed
and t h i s l e a d s t o a r a p i d r i s e i n t h e r a t e o f h e a t g e n e r a t i o n a t t h e c u t t i n g edge.
The t o o l t h e n f a i l s c a t a s t r o p h i c a l l y u s u a l l y w i t h i n a f e w seconds.
This
i s t h e m a i n mechanism w h i c h s e t s t h e u p p e r l i m i t t o t h e r a t e o f m e t a l removal w h i c h c a n be a c h i e v e d w i t h h i g h speed s t e e l t o o l s (and, a t a h i g h e r l e v e l o f speeds, w i t h cemented c a r b i d e t o o l s ) when c u t t i n g s t e e l and o t h e r h i g h m e l t i n g point alloys.
Fig.10
T e m p e r a t u r e g r a d i e n t s i n t o o l used t o c u t l o w c a r b o n s t e e l a t 76m m i n - l , 0.25mm/rev f e e d .
458 19.3.6
S1 i d i n g a t t h e TooI/\,dork
Interface
Many o f t h e p e c u l i a r and c h a r a c t e i i s t i c f e a t u r e s o f machining o p e r a t i o n s a r i s e from t h e unusual f e a t u r e o f s e i z u r e a t t h e i n t e r f a c e between t o o l and work m a t e r i a l , b u t these c o n d i t i o n s do n o t e x i s t under a l l c u t t i n g c o n d i t i o n s and on a l l p a r t s o f t h e c o n t a c t area.
The model o f s e i z u r e which has been g i v e n i s t o o
s i m p l i f i e d and must be c o r r e c t e d . A t v e r y low speeds s e i z u r e may n o t o c c u r .
S i m i l a r l y a t t h e p e r i p h e r y o f the
c o n t a c t r e g i o n , even a t h i g h r a t e s o f metal removal, t h e r e i s good evidence t o show t h a t s l i d i n g t a k e s p l a c e a t t h e i n t e r f a c e by a t y p e o f s t i c k - s l i p process. Thus a s e c t i o n through t h e o u t e r edge o f a s t e e l c h i p o f t e n shows a segmented c h i p w i t h a p e r i o d i c s t r u c t u r e a t t h e i n t e r f a c e (Fig.11.) s l i p action.
indicating a stick-
The c e n t r e o f t h e same c h i p shows a flow-zone d e m o n s t r a t i n g
seizure a t t h i s p a r t o f the i n t e r f a c e (Fig.12).
That s l i d i n g o c c u r s i n these
p e r i p h e r a l r e g i o n s may be a t t r i b u t e d t o two main f a c t o r s (1) lower compressive s t r e s s near a f r e e s u r f a c e o f t h e c h i p and ( 2 ) access o f atmospheric oxygen t o t h e i n t e r f a c e a t t h i s p o s i t i o n , r e d u c i n g t h e tendency t o m e t a l l i c bonding. Since t h e mechanisms o f wear may be v e r y d i f f e r e n t under c o n d i t i o n s o f s e i z u r e and s l i d i n g ,
i t i s u s e f u l t o have i n mind a model o f t h e r e a i o n s where
s e i z u r e and s l i d i n g o c c u r most u s u a l l y . o n a c u t t i n g t o o l .
Fig.13 shows such a
map f o r a simple t u r n i n g t o o l .
Fig.11
S e c t i o n through o u t e r edge o f c h i p a f t e r c u t t i n g medium carbon s t e e l a t h i g h speed. Shows s t i c k - s l i p a c t i o n a t i n t e r f a c e .
459
Fig.12
19.4
S e c t i o n t h r o u g h c e n t r e o f same c h i p as Fig.11. Shows flow-zone a t i n t e r f a c e c h a r a c t e r i s t i c o f seizure.
CUTTING TOOL WEAR
While t h e upper l i m i t t o t h e r a t e o f metal removal when c u t t i n g s t e e l o r o t h e r h i g h m e l t i n g p o i n t a l l o y s i s determined by t h e a b i l i t y o f t h e t o o l t o w i t h stand t h e c u t t i n g s t r e s s e s a t e l e v a t e d temperatures, t h e l i f e o f t h e t o o l decreases as t h e c u t t i n g speed i s r a i s e d b e f o r e t h i s l i m i t i s reached.
A t lower
speeds t h e shape o f t h e t o o l i s changed by one o r more o f a number o f d i f f e r e n t wear mechanisms u n t i l i t can no l o n g e r c u t e f f i c i e n t l y .
Fig.14 shows diapram-
m a t i c a l l y on a model t u r n i n g t o o l t h e l o c a t i o n o f t h e c h i e f wear f e a t u r e s observed. "Flank wear" on t h e c l e a r a n c e f a c e o f t h e t o o l o f t e n increases s t e a d i l y w i t h t i m e o f c u t t i n g u n t i l , when a c r i t i c a l amount o f wear i s reached, t h e tempera t u r e on t h i s s u r f a c e s t a r t s t o r i s e r a p i d l y and t o o l f a i l u r e may be sudden. The c r i t i c a l amount o f f l a n k wear v a r i e s under d i f f e r e n t c o n d i t i o n s .but i t may be between 0.4 and 1.5 mm.
To a v o i d complete f a i l u r e , which may be expensive,
t o o l s a r e n o r m a l l y reground o r r e p l a c e d b e f o r e t h e c r i t i c a l wear i s reached. Flank wear may o c c u r a t any c u t t i n g speed b u t t h e wear r a t e increases w i t h
460
X
X
,L I
INCOMPLETE SEIZURE
SECTION X - X
Fig.13
Diagram o f t u r n i n g t o o l showing r e g i o n s o f s e i z u r e and o f s l i d i n g a t the tooI/work i n t e r f a c e
/
/
ACCELERATED WEAR IN REGION OF 1 . DEFORMAT1ON OR SLIDING
RAKE FACE
CRATER
WtAK
$4 ffw
ACCELERATED WEAR IN REGION OF SLIDING
FLANK OR CLEARANCE FACE
Fig.14
Diagram showing wear f e a t u r e s on t u r n i n g t o o l
461 speed as t h e u l t i m a t e l i m i t f o r t h e t o o l m a t e r i a l i s approached.
In the region
o f h i g h speed c u t t i n p t h e r a t e o f f l a n k wear and t h e t o o l l i f e o f t e n f o l l o w t h e r e l a t i o n s h i p g i v e n by T a y l o r [ 4 ] f o r t o o l l i f e i n r e l a t i n g t o c u t t i n g speed Vt"
V t
=
-
c
c u t t i n g speed c u t t i n g t i m e t o f a i l u r e o r t o some s t a n d a r d amount o f wear
n and C
-
c o n s t a n t s f o r a g i v e n t o o l and work m a t e r i a l
" C r a t e r wear"
i s t h e term used f o r a groove o r c r a t e r worn on the rake face
o f t h e t o o l , u s u a l l y a t some d i s t a n c e f r o m t h e c u t t i n g edge ( F i g . 1 4 ) .
Cratering
wear i s c h a r a c t e r i s t i c a l l y observed on t o o l s used a t h i g h c u t t i n g speeds and t h e r a t e o f c r a t e r wear increases as t h e c u t t i n g speed approaches t h e u l t i m a t e l i m i t f o r the t o o l material.
As t h e c r a t e r becomes deeper i t weakens t h e t o o l edge
and may l e a d t o f r a c t u r e o f t h e edge and t o o l f a i l u r e . "Flank wear" and " c r a t e r wear" a r e d e s c r i p t i v e terms and t h e words do n o t imply d i s t i n c t wear mechanisms.
The mechanisms o f wear w i l l now be discussed.
Where s e i z u r e c o n d i t i o n s o c c u r a t t h e t o o l / w o r k i n t e r f a c e a t l e a s t f o u r d i f f e r e n t mechanisms o f wear have been observed and these w i l l be c o n s i d e r e d f i r s t .
Wear
under c o n d i t i o n s o f s l i d i n g a t t h e i n t e r f a c e w i l l be c o n s i d e r e d s e p a r a t e l y .
19.4.1
Abrasion
The a b r a s i v e a c t i o n o f h a r d phases i n t h e work m a t e r i a l , such as o x i d e s o r c a r b i d e s may c o n t r i b u t e t o t h e wear o f c u t t i n g t o o l s .
Abrasion i s , however,
p r o b a b l y n o t a m a j o r cause o f wear under s e i z u r e c o n d i t i o n s u n l e s s t h e p a r t i c l e s o f t h e h a r d phases a r e l a r g e , e.g.,
g r e a t e r than 40 pm, o r p r e s e n t i n v e r y h i g h
c o n c e n t r a t i o n s , as t h e y may be f o r example on t h e s u r f a c e o f c a s t i n g s .
Even
w i t h h i g h speed s t e e l t o o l s t h e a b r a s i v e a c t i o n o f d i s p e r s e d , f i n e h a r d p a r t i c l e s i s p r o b a b l y small because, under s e i z u r e c o n d i t i o n s they r a r e l y impinge on t h e t o o l s u r f a c e i n such a way as t o remove t o o l m a t e r i a l .
W i t h h a r d e r t o o l mat-
e r i a l s such as c a r b i d e s o r diamond, few i f any p a r t i c l e s i n t h e work m a t e r i a l a r e h a r d e r t h a n t h e t o o l s and a b r a s i v e a c t i o n i s l e s s l i k e l y .
The hardness of
c u t t i n g t o o l m a t e r i a l s i s o f more s i g n i f i c a n c e as a measure o f t h e i r a b i l i t y t o w i t h s t a n d h i g h compressive s t r e s s than as a measure o f t h e i r r e s i s t a n c e t o abrasion.
19.4.2
Surface Shearing
When c u t t i n g h i g h e r m e l t i n g p o i n t m e t a l s a t h i g h speeds, t h e i n t e r f a c e temp e r a t u r e , p a r t i c u l a r l y on t h e rake f a c e o f t h e t o o l ( F i g . l O ) ,
may be very h i g h ,
so t h a t t h e y i e l d s t r e s s o f t h e s t e e l t o o l i s reduced t o a v e r y low value i n a
small volume of metal a t t h e i n t e r f a c e .
T h i n l a y e r s o f t h e t o o l m a t e r i a l may
then be sheared away by t h e work m a t e r i a l bonded t o t h e t o o l s u r f a c e .
Fig.15
462 shows an example o f t h i s wearing a c t i o n i n which a c r a t e r i s b e i n g worn on t h e rake face o f a h i g h speed s t e e l t o o l when c u t t i n g carbon s t e e l a t h i g h speed. This wear mechanism i s u s u a l l y observed o n l y where t h e i n t e r f a c e temperature i s above 800°C on s t e e l t o o l s .
It i s one o f t h e mechanisms o f wear r e s p o n s i b l e
f o r c r a t e r wear on s t e e l t o o l s and a l s o f o r t h e f i n a l stages o f f l a n k wear j u s t before complete t o o l f a i l u r e .
T h i s mechanism causes r a p i d t o o l wear.
I t has
not been observed t o o c c u r on cemented c a r b i d e t o o l s .
Fig.15
S e c t i o n through t o o l used t o c u t low carbon s t e e l a t h i g h speed. Showing f o r m a t i o n o f c r a t e r on r a k e f a c e by s h e a r i n g o f h i g h speed t o o l i n r e g i o n of h i g h temperature.
19.4.3
D i f f u s i o n and i n t e r a c t i o n
Under c o n d i t i o n s o f c u t t i n g where t h e t o o l and work s u r f a c e s a r e m e t a l l u r g i c a l l y bonded, t h e t o o l shape can be changed by a process o f d i f f u s i o n and i n t e r a c t i o n between t h e two m a t e r i a l s .
I n t h e s i m p l e s t s i t u a t i o n atoms from
the t o o l m a t e r i a l may d i f f u s e i n t o t h e work m a t e r i a l f l o w i n g o v e r t h e s u r f a c e and be c a r r i e d away by i t
-
i.e.,
t h e t o o l m a t e r i a l i s d i s s o l v e d i n t o t h e work
m a t e r i a l by a process o f t h e same c h a r a c t e r as t h a t of a b l o c k o f s a l t b e i n g d i s s o l v e d by a stream o f water r u n n i n g o v e r i t s s u r f a c e .
Diffusion i s a highly
temperature dependent process and d i f f u s i o n wear o c c u r s a t an a p p r e c i a b l e r a t e o n l y a t r e l a t i v e l y h i g h c u t t i n g speeds where t h e i n t e r f a c e temperature i s h i g h . When c u t t i n g s t e e l s w i t h h i g h speed s t e e l t o o l s wear by a d i f f u s i o n mechanism i s p r o b a b l y s i g n i f i c a n t o n l y where t h e i n t e r f a c e temperature exceeds 650°C. I t has been shown t h a t , even a t m o d e r a t e l y h i g h c u t t i n g speeds o f 25-30 m min
-1
when c u t t i n g s t e e l , temperatures o f 790°C and o v e r o f t e n o c c u r a t p a r t s of t h e
463 i n t e r f a c e , and t h e r e i s a r a p i d r i s e i n t e m p e r a t u r e w i t h f u r t h e r increments i n speed. Simple d i f f u s i o n and a v a r i e t y o f i n t e r a c t i o n s depending on t h e chemical c o m p o s i t i o n s and m e t a l l u r g i c a l s t r u c t u r e s o f t h e t o o l and work m a t e r i a l s , o c c u r across the i n t e r f a c e .
I t i s p r o b a b l e t h a t wear based on a t o m i c i n t e r a c t i o n s i s
t h e most i m p o r t a n t mechanism c h a n g i n g t h e shape o f h i g h speed s t e e l , cemented c a r b i d e o r diamond c u t t i n g tools i n m a c h i n i n g t h e h i g h e r m e l t i n g p o i n t m e t a l s and a l l o y s a t h i g h speeds. Fig.16 shows a s e c t i o n t h r o u g h t h e c r a t e r e d s u r f a c e o f a h i g h speed s t e e l t o o l w i t h a d h e r i n g work m a t e r i a l .
The wear p r o c e s s was one o f d i f f u s i o n ;
t o o l s u r f a c e shows no s i g n s o f p l a s t i c d e f o r m a t i o n .
the
There i s good e v i d e n c e t h a t
wear by d i f f u s i o n i s r e s p o n s i b l e f o r most c r a t e r and f l a n k wear on h i g h speed s t e e l t o o l s where t h e i n t e r f a c e t e m p e r a t u r e i s above
650°C b u t below t h e tem-
perature required f o r the shearing action.
Fig.16
S e c t i o n t h r o u g h h i g h speed s t e e l t o o l i n worn c r a t e r a f t e r c u t t i n g Interface characteristic low a l l o y s t e e l a t 18m m i n - ’ f o r 38 m i n u t e s . o f d i f f u s i o n wear
464 C u t t i n g t o o l m a t e r i a l s h a v e been d e v e l o p e d e m p i r i c a l l y and m o s t h a v e been developed f o r t h e machining o f s t e e l s i n c e t h i s i s t h e main market f o r c u t t i n p tools
[5].
The f i r s t cemented c a r b i d e s p r o d u c e d w e r e a l l o y s o f t u n g s t e n c a r b i d e
and c o b a l t (WC-Co).
These a r e v e r y s u c c e s s f u l f o r t h e c u t t i n g o f n o n - f e r r o u s
m e t a l s and c a s t i r o n a t speeds much h i g h e r t h a n c a n be a c h i e v e d w i t h s t e e l t o o l s because t h e WC-Co a l l o y s have h i g h e r y i e l d s t r e s s and c a n r e s i s t t h e s h e a r i n g a c t i o n a n d d e f o r m a t i o n a t h i g h t e m p e r a t u r e s , a n d a l s o b e c a u s e t h e WC-Co a l l o y s do n o t r e a c t w i t h t h e s e w o r k m a t e r i a l s a t t h e h i g h c u t t i n g speeds. For t h e c u t t i n g o f s t e e l h o w e v e r , t h e WC-Co a l l o y s a r e n o t so s u c c e s s f u l ,
s i n c e r a p i d c r a t e r i n g wear i n p a r t i c u l a r o n t h e r a k e f a c e o f t h e t o o l s g i v e s a v e r y s h o r t l i f e f o r t o o l s used a t speeds n o t much h i g h e r t h a n t h o s e u s e d w i t h h i g h speed s t e e l t o o l s .
Cemented c a r b i d e t o o l s w e r e s u c c e s s f u l f o r c u t t i n g o f
s t e e l a t h i g h e r speeds o n l y when a s m a l l p r o p o r t i o n ( 5 t o 20%) o f T i c o r TaC was added t o t h e WC-Co a l l o y s .
These " s t e e l
c u t t i n g g r a d e s " o f cemented c a r b i d e a r e
a b l e t o m a c h i n e s t e e l a t more t h a n t h r e e t i m e s t h e speed w i t h WC-Co a l l o y s because t h e r a t e o f w e a r , p a r t i c u l a r l y t h e c r a t e r wear o n t h e r a k e f a c e , much l o w e r .
i s so
I t i s now known t h a t t h e i r s u c c e s s must be a t t r i b u t e d t o t h e much
l o w e r r a t e o f s o l u t i o n o f TIC o r TaC i n s t e e l a t e l e v a t e d t e m p e r a t u r e s , compared w i t h t h a t o f WC.
The r a t e o f c r a t e r i n g
in particular
i s a f f e c t e d because i t i s
on t h e r a k e f a c e o f t h e t o o l , away f r o m t h e c u t t i n g edge, temperatures a t t h e tool/work
t h a t the highest
i n t e r f a c e a r e located (Fig.10).
The r a t e o f f l a n k
wear a l s o i s g e n e r a l l y r e d u c e d b y t h e i n c l u s i o n o f T I C a n d TaC i n t h e t o o l m a t e r i a l s when c u t t i n g s t e e l
[5].
I n a more r e c e n t d e v e l o p m e n t t h e r a t e o f wear h a s been d e c r e a s e d s t i l l f u r t h e r b y c o a t i n g cemented c a r b i d e t o o l s w i t h t h i n l a y e r s o f s o l u t i o n r e s i s t a n t The s u b s t a n c e s used f o r t h e c o a t i n a s a r e TIC, T i N o r H f N , w h i c h a r e
materials.
deposited on t h e s u r f a c e o f t h e t o o l s a s l a y e r s known as c h e m i c a l v a p o u r d e p o s i t i o n ("CVD'I).
5
t o 10
jm
t h i c k by a process
These c a r b i d e s a n d n i t r i d e s a r e
t o o b r i t t l e t o be u s e d b y t h e m s e l v e s a s c u t t i n g t o o l s ,
b u t as t h i n l a y e r s w i t h
v e r y f i n e g r a i n s i z e s u p p o r t e d b y t h e t o u g h e r cemented c a r b i d e s u b s t r a t e t h e y w i t h s t a n d w e l l t h e s t r e s s e s o f m a c h i n e shop o p e r a t i o n s , and t h e y a r e l e s s r e a d i l y dissolved i n the steel flowing over the t o o l surfaces. t o o l I i f e may be p r o l o n g e d b y a f a c t o r o f 1.5
to
3
cemeiited c a r b i d e s , o r c u t t i n g speeds 30 t o 60 m m i n
With these coated t o o l s
t i m e s compared w i t h u n c o a t e d
-1
h i g h e r c a n be used f o r t h e
same t o o l l i f e when c u t t i n g s t e e l . Even more r e s i s t a n t t o s o l u t i o n i n s t e e l a t h i g h t e m p e r a t u r e s i s a l u m i n i u m oxide.
A1 0
2 3
( a l u m i n a ) has a h i g h y i e l d s t r e s s a t h i g h t e m p e r a t u r e and t o o l
t i p s a r e s i n t e r e d o r h o t p r e s s e d f r o m a l u m i n a powder. as " c e r a m i c
tools")
A l q 0 3 t o o l t i p s (known
o f h i g h d e n s i t y a n d f i n e g r a i n s i z e c a n be used f o r c u t t i n g
s t e e l and c a s t i r o n a t speeds a s h i g h a s 600-700 m m i n - ' wear.
w i t h v e r y low r a t e s of
These t o o l s , h o w e v e r , a r e l a c k i n g i n t o u g h n e s s and t h e i r u s e i n c u t t i n g
465 s t e e l i s r e s t r i c t e d f o r t h i s reason t o a v e r y small p r o p o r t i o n o f machine shop o p e r a t i o n s on s t e e l . h i g h speed.
They can be more w i d e l y used f o r c u t t i n g c a s t i r o n a t v e r y
Recently cemented c a r b i d e t o o l s have been p u t on the market w i t h
t h i n c o a t i n g s o f A1203 d e p o s i t e d by CVD and these a r e being assessed f o r t h e c u t t i n g o f b o t h s t e e l and c a s t i r o n . Reaction between t o o l and work m a t e r i a l a l s o l i m i t s t h e speeds used when c u t t i n g s t e e l and n i c k e l - b a s e d a l l o y s w i t h diamond t o o l s .
Cubic boron n i t r i d e
i s a s y n t h e t i c m a t e r i a l made by t h e same t y p e o f u l t r a h i g h pressure process used f o r t h e p r o d u c t i o n o f s y n t h e t i c diamond. diamond and, a l t h o u g h i t i s less hard,
I t has a s i m i l a r s t r u c t u r e t o
i t can be used i n c u t t i n g s t e e l s and
n i c k e l - b a s e d a l l o y s a t h i g h e r speeds because i t r e a c t s w i t h them l e s s r e a d i l y than does diamond a t e l e v a t e d temperatures
161.
Thus, under t h e s e i z u r e c o n d i t i o n s which p r e v a i l i n many metal c u t t i n g o p e r a t i o n s , t h e l i f e o f t h e c u t t i n g t o o l s i s o f t e n c o n t r o l l e d by processes o f d i f f u s i o n and i n t e r a c t i o n between t o o l and m a t e r i a l a t h i g h i n t e r f a c e tempera t u r e s when c u t t i n g m a t e r i a l s o f h i g h m e l t i n g p o i n t a t h i g h speeds.
19.4.4
Attrition
I f , under c o n d i t i o n s o f s e i z u r e , t h e temperature-dependent wear processes o f
s h e a r i n g and d i f f u s i o n were t h e o n l y ones r e s p o n s i b l e f o r changing t h e t o o l shape, t o o l l i f e might be expected t o be almost i n f i n i t e a t low c u t t i n g speed. A t low speeds,
however, t o o l s a r e f r e q u e n t l y worn by another mechanism which can
be c a l l e d " a t t r i t i o n " .
Sections through t h e edge o f t o o l s used a t low speeds
o f t e n show t h a t t h e t o o l has been worn by a mechanism i n v o l v i n g b r e a k i n g away from t h e t o o l s u r f a c e o f fragments o f m i c r o s c o p i c s i z e Fig.17.
f o r a h i g h speed s t e e l t o o l .
-
as shown f o r example i n
Such a wear mechanism has been observed
w i t h almost a l l c l a s s e s o f t o o l m a t e r i a l a f t e r c u t t i n g a t low speed, and as t h e speed i s r a i s e d i t becomes l e s s i m p o r t a n t .
I n many cases b o t h d i f f u s i o n and
a t t r i t i o n wear a r e observed on t h e same worn s u r f a c e s . A t t r i t i o n wear seems t o be most severe when t h e machine t o o l l a c k s r i g i d i t y , when v i b r a t i o n o c c u r s o r when t h e r e a r e pronounced i r r e g u l a r i t i e s i n t h e work material.
I t i n v o l v e s an i n t e r m i t t e n t a c t i o n i n which small fragments o f the
t o o l a r e t o r n away t o leave c h a r a c t e r i s t i c a l l y rough worn s u r f a c e s .
High speed
s t e e l t o o l s a r e more r e s i s t a n t t o a t t r i t i o n than a r e cemented c a r b i d e s and the l i f e o f h i g h speed s t e e l t o o l s i s o f t e n l o n g e r a t low c u t t i n g speeds t h a n . t h a t o f cemented c a r b i d e s f o r t h i s reason.
T w i s t d r i l l s f o r example a r e most
commonly made f r o m h i g h speed steels, n o t o n l y because t h e y a r e cheaper but because,
i n many a p p l i c a t i o n s t h e l i f e i s l o n g e r than t h a t o f cemented c a r b i d e
d r i l l s and performance i s more c o n s i s t e n t .
When cemented c a r b i d e t o o l s a r e used
466
Fig.17
S e c t i o n t h r o u g h c u t t i n g edge of h i g h speed s t e e l t o o l a f t e r c u t t i n g s t e e l a t 20m min Worn s u r f a c e c h a r a c t e r i s t i c o f a t t r i t i o n wear
'.
i n t h e low speed range where a t t r i t i o n wear i s dominant, t h e r a t e o f wear i s v e r y dependent on t h e c a r b i d e g r a i n s i z e .
F i n e g r a i n e d cemented c a r b i d e s a r e
much more r e s i s t a n t t o a t t r i t i o n wear than coarse g r a i n e d ones, and WC-Co a l l o y s a r e more r e s i s t a n t t h a n t h e s t e e l c u t t i n g c a r b i d e grades and a r e o f t e n used t o c u t s t e e l a t low speed f o r t h i s reason. Fig.13 shows d i a g r a m m a t i c a l l y those p a r t s o f a t u r n i n g t o o l where s e i z u r e and s l i d i n g a r e most l i k e l y t o o c c u r d u r i n g c u t t i n g .
Rather f r e q u e n t l y more
r a p i d wear i s observed i n t h e s l i d i n g r e g i o n s than a t t h e s e i z e d s u r f a c e s .
For
example, Fig.18 shows a deep tongue o f wear on a t o o l i n t h e s l i d i n g wear r e g i o n where t h e o u t e r edge o f t h e c h i p crossed t h e c u t t i n g edge o f t h e t o o l .
The wear
r a t e was many times h i g h e r a t t h i s p o s i t i o n than i n t h e a d j a c e n t s e i z e d r e g i o n . High r a t e s o f wear a r e o f t e n observed a l s o a t t h e nose o f t h e t o o l i n t u r n i n g o p e r a t i o n s , where t h e o t h e r edge o f t h e c h i p crosses t h e c u t t i n g edge.
467
Fig.18
Clearance f a c e o f t o o l used t o c u t low carbon s t e e l . Shows adherent work m a t e r i a l and deeply worn qroove a t o u t s i d e o f c u t , c h a r a c t e r i s t i c o f wear by s l i d i n g a c t i o n
Such in c r e a s e d wear r a t e i n t h e s l i d i n g r e g i o n s does n o t always occur, and t h e c o n d i t i o n s i n f l u e n c i n g t h e s e v e r i t y o f wear have n o t y e t been s t u d i e d i n sufficient detail.
I t seems most p r o b a b l e t h a t t h e wear i n t h e s l i d i n g r e g i o n s
i n v o l v e s r e a c t i o n between t h e t o o l and work s u r f a c e s and oxyaen o f the atmosphere, which has access t o t h i s p a r t o f t h e i n t e r f a c e .
One p o s s i b l e wear
mechanism i s t h e f o r m a t i o n of o x i d e l a y e r s on t h e tool and the removal o f these p e r i o d i c a l l y by t h e s t i c k - s l i p a c t i o n o f t h e s l i d i n g c h i p (Fig.11).
The worn
s u r fa c e s i n t h e s l i d i n g r e g i o n s a r e n o r m a l l y v e r y smooth and t h i s s o r t o f wear on c a r b i d e t o o l s may be a l most as r a p i d as on s t e e l t o o l s , so t h a t a b r a s i o n by h a r d p a r t i c l e s , a l t h o u g h p o s s i b l e i n t h i s r e g i o n , mechanism o f wear i n most o p e r a t i o n s .
i s p r o b a b l y n o t the main
The r a t e o f s l i d i n g wear may’be i n -
fl ue n c e d by t h e use of c u t t i n g l u b r i c a n t s .
19.5
COOLANTS AND LUBRICANTS
A d i s c u s s i o n o f t r i b o l o g y i n metal c u t t i n g would n o t be complete w i t h o u t c o n s i d e r i n g t h e i n f l u e n c e of c o o l a n t s and l u b r i c a n t s .
The c u t t i n g t o o l i s o f t e n
f l o o d e d w i t h a f l u i d d e s c r i b e d e i t h e r as a c o o l a n t o r as a l u b r i c a n t . types a r e used
-
Two main
water-based f l u i d s c o n t a i n i n g o i l and o t h e r a d d i t i v e s i n sus-
pension o r s o l u t i o n and m i n e r a l o i l s w i t h o r w i t h o u t extreme pressure a d d i t i v e s (ma in ly s u l p h u r and c h l o r i n e c o n t a i n i n g substances).
The f u n c t i o n o f t h e w ater-
468 based f l u i d s i s m a i n l y as a c o o l a n t w i t h t h e o i l p r e s e n t m a i n l y t o p r e v e n t c o r r o s i o n , w h i l e t h e o i l based f l u i d s have r e l a t i v e l y poor c o o l i n g c a p a c i t y and a r e used m a i n l y f o r t h e i r l u b r i c a n t a c t i o n . C o o l a n t s a r e o f t e n necessary t o reduce t h e t e m p e r a t u r e o f machine, t o o l and w o r k p i e c e i n o r d e r t o promote e f f i c i e n t o p e r a t i o n and t o m a i n t a i n d i m e n s i o n a l tolerances.
P r a c t i c a l e x p e r i e n c e shows t h a t a s t r o n g f l o w o f c o o l a n t can a c t
t o i n c r e a s e t o o l l i f e when c u t t i n g a t h i g h speeds, o r t o p e r m i t t h e use o f h i g h e r c u t t i n g speeds.
The d i r e c t i n f l u e n c e o f a c o o l a n t on t h e maximum t e m p e r a t u r e
generated a t t h e t o o l / w o r k
[71.
interface i s usually rather s l i g h t
e r a t u r e i s generated i n t h e f l o w - z o n e a t t h e r a k e f a c e ( F i g . 1 0 ) .
The tempCoolant cannot
p r e v e n t t h e g e n e r a t i o n o f h e a t a t t h i s s u r f a c e and can a c t o n l y t o steepen t h e t e m p e r a t u r e g r a d i e n t s and reduce t h e volume o f t o o l m a t e r i a l h e a t e d t o h i g h t e m p e r a t u r e , b u t i t can have l i t t l e i n f l u e n c e on t h e maximum t e m p e r a t u r e on t h e rake f a c e .
A c t i n g n e a r t h e c u t t i n g edge t h e c o o l a n t can be more e f f e c t i v e and
t h e m a j o r c o o l i n g e f f e c t i s p r o b a b l y t h a t o f r e d u c i n g t h e t e m p e r a t u r e near t h e edge, t h u s i n c r e a s i n g t h e y i e l d s t r e n g t h o f t h e t o o l t o p r e v e n t l o c a l deforma t i o n , and a l s o r e d u c i n g t h e r a t e o f d i f f u s i o n wear on t h e f l a n k o f t h e t o o l . From what has been s a i d about s e i z u r e a t t h e t o o l / w o r k u n l i k e l y t h a t any l u b r i c a n t ,
i n t e r f a c e , i t seems
i n e i t h e r gaseous o r l i q u i d form, can p e n e t r a t e t o
t h a t p a r t o f t h e i n t e r f a c e where s e i z u r e o c c u r s .
L u b r i c a n t s can, however, a c t
e f f e c t i v e l y i n t h e p e r i p h e r a l r e g i o n s where s l i d i n g o c c u r s a t t h e i n t e r f a c e . By p e n e t r a t i n g f r o m t h e p e r i p h e r a l r e g i o n s t h e y may be a b l e t o r e s t r i c t t h e area o f complete s e i z u r e and t h u s t o reduce t h e f o r c e s a c t i n g on t h e t o o l .
Force
measurements have shown r e d u c t i o n s i n b o t h c u t t i n g f o r c e and f e e d f o r c e caused by t h e use o f c o o l a n t s and l u b r i c a n t s a t low c u t t i n g speed.
Many t e s t s have
-1
i n d i c a t e d t h a t t h e i n f l u e n c e o f l u b r i c a n t s i s g r e a t e s t a t speeds below 30 m min w h i l e t h e y have v e r y l i t t l e l u b r i c a t i n g e f f e c t o v e r 60 m min
-1
.
The most
e f f e c t i v e l u b r i c a n t s i n m e t a l c u t t i n g a r e t h o s e w i t h extreme p r e s s u r e a d d i t i v e s , s u g g e s t i n g t h a t s u c c e s s f u l l u b r i c a t i o n i n v o l v e s t h e f o r m a t i o n o f e a s i l y sheared s u r f a c e l a y e r s when t h e l u b r i c a n t s come i n t o c o n t a c t w i t h f r e s h l y exposed m e t a l s u r f a c e s on t h e work m a t e r i a l . When a b u i l t - u p edge i s formed, c o o l a n t s and ' l u b r i c a n t s a r e o f t e n e f f e c t i v e i n g r e a t l y reducing i t s size.
A l a r g e b u i l t - u p edge i s o f t e n r e s p o n s i b l e f o r
v e r y p o o r s u r f a c e f i n i s h and one of t h e most i m p o r t a n t f u n c t i o n s o f a c u t t i n g l u b r i c a n t i s t o improve t h e s u r f a c e where t h i s i s a r e q u i r e m e n t o f t h e s u r f a c e b e i n g machined.
Often,
i n t h i s r e s p e c t , water-based
l u b r i c a n t s , and even w a t e r
i t s e l f , a r e e f f e c t i v e i n r e d u c i n g t h e s i z e o f t h e b u i l t - u p edge. For many c u t t i n g o p e r a t i o n s l u b r i c a n t s a r e e s s e n t i a l .
A t h i g h speeds t o o l
l i f e may be improved by c o o l a n t a c t i o n , b u t t h e use o f l u b r i c a n t s a t l o w e r speeds t o improve s u r f a c e f i n i s h i s n o t always e f f e c t i v e i n r e d u c i n g t h e r a t e o f wear.
I n f a c t tool wear r a t e i s o f t e n i n c r e a s e d by t h e a c t i o n o f l u b r i c a n t s i n
469 t h e s l i d i n g r e g i o n s where i t can p e n e t r a t e .
T h i s a c c e l e r a t i o n o f w e a r may occur
w i t h b o t h s t e e l and cemented c a r b i d e t o o l s .
High r a t e of wear i n r e g i o n s o f I n many e n g i n e e r i n g mech-
s l i d i n g a t t h e i n t e r f a c e has a l r e a d y been discussed. anisms a major f u n c t i o n of
l u b r i c a n t s i s t o p r e v e n t s e i z u r e between moving p a r t s .
I n t h e case o f metal c u t t i n g o p e r a t i o n s , t h e e l i m i n a t i o n o f s e i z u r e i s n o t t h e o b j e c t i v e o f t h e use o f l u b r i c a n t s .
The e l i m i n a t i o n o f s e i z u r e i n many cases
c o u l d r e s u l t i n a d i s a s t r o u s i n c r e a s e i n t h e r a t e o f t o o l wear.
I n most
c u t t i n g o p e r a t i o n s s e i z u r e between t o o l and work m a t e r i a l i s a normal and desirable condition,
r a t h e r than a hazard t o be avoided.
The main u s e f u l fun-
c t i o n s o f c u t t i n g f l u i d s a r e t o reduce temperature and thus increase c u t t i n g e f f i c i e n c y , t o reduce c u t t i n g f o r c e s ,
t o improve s u r f a c e f i n i s h , and t o h e l p
c l e a r c h i p s away f r o m t h e c u t t i n g t o o l . There i s one way i n which s e i z u r e between tool and work m a t e r i a l can be modified,
i f n o t e l i m i n a t e d , t o t h e advantage o f c e r t a i n c u t t i n g o p e r a t i o n s , and
t h i s i s t o i n c l u d e w i t h i n t h e work m a t e r i a l a phase which may i n t e r p o s e i t s e l f between t o o l and work d u r i n g t h e c u t t i n g o p e r a t i o n . f r o m w i t h i n t h e work m a t e r i a l can renew i n t e r f a c i a l
Such a substance a p p l i e d l a y e r s as they a r e swept
away by t h e f l o w o f t h e work m a t e r i a l o v e r t h e t o o l .
Under c e r t a i n c o n d i t i o n s
such substances can be d e s c r i b e d as p e r f o r m i n g t h e f u n c t i o n s o f an i n t e r n a l l u b r i c a n t and a r e g e n e r a l l y known as free-machining a d d i t i v e s . Manganese s u l p h i d e i n s t e e l may a c t i n t h i s way and, when t h e s t e e l c u t t i n g grades o f c a r b i d e a r e used, t h e s u l p h i d e s from t h e s t e e l w i l l o f t e n form an intermediate layer a t the seized tool/work interface.
C e r t a i n calcium-aluminium
s i l i c a t e s i n s t e e l , which l i k e manganese s u l p h i d e a r e p l a s t i c a l l y deformed when t h e s t e e l i s sheared, a l s o f u n c t i o n i n t h e same way as t h e s u l p h i d e s .
Silicate
l a y e r s a r e formed on s t e e l - c u t t i n g c a r b i d e t o o l s a t t h e i n t e r f a c e when c u t t i n g a t h i g h speeds and a r e o f t e n e x t r e m e l y e f f e c t i v e i n r e d u c i n g t o o l wear r a t e and p e r m i t t i n g t h e use o f h i g h e r c u t t i n g speeds. Sulphides i n copper a l s o t e n d t o f o r m a t h i n l a y e r a t t h e t o o l / w o r k
interface
where t h e i r main f u n c t i o n seems t o be t o reduce t h e a r e a o f c o n t a c t on the t o o l rake f a c e and t h u s g r e a t l y t o reduce t h e c u t t i n g f o r c e s which a r e e x t r e m e l y h i g h when c u t t i n g h i g h - c o n d u c t i v i t y copper.
The a d d i t i o n o f l e a d t o brass r e s u l t s i n
an a c c u m u l a t i o n o f l e a d a t t h e b r a s s / t o o l cutting.
i n t e r f a c e under many c o n d i t i o n s o f
The g r e a t e s t b e n e f i t which t h e l e a d c o n f e r s i s t h e b r e a k i n g up o f the
brass c h i p s i n t o small fragments which a r e e a s i l y disposed o f i n h i g h speed a u t o m a t i c machining. O f a l l t h e aspects o f machining t h e f u n c t i o n s o f c u t t i n g f l u i d s as c o o l a n t s
and l u b r i c a n t s have p r o b a b l y r e c e i v e d t h e l e a s t a t t e n t i o n from s c i e n t i f i c research.
There i s much t o be l e a r n e d c o n c e r n i n g t h e ways i n which they a c t t o
achieve t h e r e s u l t s f o r which t h e y a r e used by p r a c t i c a l m a c h i n i s t s .
470 REFERENCES 1
2
3 4
5 6
7
Zorev,N.N., I n t e r n a t i o n a l Research i n P r o d u c t i o n E n g i n e e r i n g (1963), A.S.M.E., P i t t s b u r g h , p.42. Trent,E.M., I . S . I . Report N0.94., 1967, p . 1 1 . W i l l iarn5,J.E. and Rollason,E.C., J., I n s t . , Met., (19701, 144. Boothroyd,G., "Fundamentals of Metal M a c h i n i n g and Machine Tools", McGraw-Hi 1 1 (1975). Trent,E.M., "Metal C u t t i n g " , 1977, B u t t e r w o r t h s . Hibb5,L.E. and Wentorf,R.E. J r . , 8 t h Plansee Seminar ( 1 9 7 4 ) , Paper No.42. Smart,E.F. and Trent,E.M., Proc., 1 5 t h I n t . , Conf., M.T.D.R., (1975) 187.
98,
471
v ( ) ROLE OF LABORATORY TEST MACHINES U
PROFESSOR F.T.
BARWELL, H.H.
JONES
Department o f Mechanical Engineering, U n i v e r s i t y C o l l e g e o f Swansea.
20.1
THE EXPERIMENTAL METHOD
The e s t a b l i s h m e n t o f f a c t s and t h e u n d e r s t a n d i n g o f r e l a t i o n s h i p s between p h y s i c a l phenomena can o n l y be based on experience,
t h e e x p e r i m e n t a l method
should t h e r e f o r e c o n s i s t o f s t r u c t u r i n g e x p e r i e n c e s o as t o produce unambiguous answers t o c e r t a i n q u e s t i o n s . meaning,
For s t r u c t u r e d experience o r experiment t o have
i t must reproduce t h e circumstances s u r r o u n d i n g t h e occurrence o f t h e
phenomena under study.
Otherwise t h e r e s u l t s , though perhaps i n t e r e s t i n g , w i l l
be i r r e l e v a n t t o t h e purpose o f t h e i n v e s t i g a t i o n . T r i b o l o g i c a l i n v e s t i g a t i o n s a r e b e s t c a r r i e d o u t under s e r v i c e c o n d i t i o n s on f u l l s c a l e apparatus b u t t h i s i s seldom p o s s i b l e .
I n f o r m a t i o n may o f t e n be
r e q u i r e d i n advance o f t h e c o n s t r u c t i o n o f t h e machine i n v o l v e d , measurements may n o t be p o s s i b l e i n t h e o p e r a t i o n a l environment ( i n space f o r example) and f u l l s c a l e t e s t i n g may be t o o c o s t l y e s p e c i a l l y when t e s t s t o d e s t r u c t i o n a r e required.
L a b o r a t o r y t e s t methods have t h e r e f o r e been e v o l v e d which serve a
number o f v i t a l f u n c t i o n s i n r e l a t i o n t o t h e p r a c t i c e o f e n g i n e e r i n g . The t e s t i n g o f l u b r i c a n t s by measurement o f t h e i r p h y s i c a l and chemical p r o p e r t i e s i s w e l l developed (See Standard Methods o f T e s t i n g Petroleum and i t s Products,
I n s t i t u t e o f Petroleum and ASTM) b u t t h e c o m p l e x i t y o f t h e r e q u i r e -
ments o f many e n g i n e e r i n g a p p l i c a t i o n s i s such t h a t s a t i s f a c t o r y performance cannot always be p r e d i c t e d f r o m such t e s t s and i t i s necessary t o s i m u l a t e service conditions.
Thus, f o r a p r o d u c t t o be approved f o r supply t o t h e U . S .
Army f o r l u b r i c a t i o n o f I.C.
engines,
i t has t o be s u b m i t t e d t o a s e r i e s o f
engine t e s t s and has t o s a t i s f y c e r t a i n s p e c i f i e d c r i t e r i a .
Once approval has
been g i v e n , b u l k s u p p l i e s may be accepted on t h e b a s i s o f s u f f i c i e n t p h y s i c a l and chemical t e s t i n g t o ensure c o n s i s t e n c y o f c o n s t i t u t i o n . Many i n d u s t r i a l processes i n v o l v e a c o n s i d e r a b l e q u a n t i t y o f raw m a t e r i a l and f u l l s c a l e t e s t i n g may be i n c o n v e n i e n t o r expensive.
I t may n o t always be
p o s s i b l e t o o b t a i n access t o t h e i n t e r a c t i n g s u r f a c e s o f i n t e r e s t and i t may be d e s i r e d t o e x p l o r e t h e e f f e c t o f a v a r i a b l e o v e r a g r e a t e r range than would be p o s s i b l e w i t h e x i s t i n g machinery.
I n these circumstances, t h e c o n s t r u c t i o n o f
s p e c i a l l a b o r a t o r y machines may be e s s e n t i a l .
472
T r i b o l o g i c a l s i t u a t i o n s can g e n e r a l l y be reduced t o t h e c o n s i d e r a t i o n o f i n t e r a c t i n g s u r f a c e s o f g i v e n c o m p o s i t i o n and shape w i t h a p p r o p r i a t e i o a d i n g and r e l a t i v e m o t i o n t o g e t h e r w i t h l u b r i c a n t and e n v i r o n m e n t , and i t i s t h e r e f o r e a t t r a c t i v e t o p r o v i d e s p e c i a l t e s t machines w h i c h p r e s e n t no a p p a r e n t resemblance t o any p r a c t i c a l machine, b u t w h i c h reproduce s u f f i c i e n t l y a c c u r a t e l y the t r i b o l o g i c a l conditions involved.
A number o f machines a r e a v a i l a b l e
c o m m e r c i a l l y which p u r p o r t t o do t h i s and p r o v i d e d c a r e i s taken t o ensure t h e r e l e v a n c e o f t h e t e s t c o n d i t i o n s t o t h e a p p l i c a t i o n , t h e y may produce much useful information. F i n a l l y where an e f f o r t i s b e i n g made t o expand knowledge o f t h e fundamentals u n d e r l y i n g t r i b o l o g i c a l a c t i o n , s p e c i a l equipment i s n e c e s s a r y t o e x t e n d t h e range o f o b s e r v a t i o n and t o i s o l a t e p a r t i c u l a r v a r i a b l e s .
I t i s o f t e n found
t h a t a p p a r a t u s d e v i s e d f o r fundamental r e s e a r c h i s p a r t i c u l a r l y u s e f u l f o r applied investigations.
20.2 20.2.1
LUBRICANT TYPE APPROVAL TESTING MACHINES Engine T e s t s
A great deal o f l u b r i c a n t t e s t i n g i s c a r r i e d o u t i n or der t o s a t i s f y the r e l e v a n t s p e c i f i c a t i o n s f o r e n g i n e o i l , t y p i c a l examples a r e DEF-2101-D U n i t e d Kingdom and MIL-L-46152 TABLE 20.1
Engine T e s t ( s )
P r o p e r t i e s under t e s t
-
P e t t e r W1 gasoline
O i l oxidation, bearing corrosion, lacquer format ion
P e t t e r AV1 diesel
Detergency and h i g h t e m p e r a t u r e stability
O l d s m o b i l e V-8 sequence I I B - gasoline
Low t e m p e r a t u r e r u s t i n g and deposits
F o r d V-8 sequence VC - gasoline
B u i l d up o f d e p o s i t s due t o i n t e r m i t t e n t low temperature operat ion
O l d s m o b i l e V-8 sequence I I I C - gasoline
High temperature o x i d a t i o n
CRC L-38 (CLR) - gasoline
B e a r i n g c o r r o s i o n and shear s t a b i l i t y of multigrade5
C a t e r p i 1 l a r TH - diesel
Ring s t i c k i n g wear and a c c u m u l a t i o n o f deposits
-
MIL-L-46152
i n the United States.
Summary o f T y p i c a l Engine T e s t s
Specification DEF-2101-D
i n the
473
Fig.1
P e t t e r A v l Rig
Fig.2
Fig.3
Caterpillar r i g
Piston examination
474 The main c r i t e r i o n o f acceptance o f a l u b r i c a n t which has been s u b j e c t e d t o t y p e approval t e s t i n g i s t h e c o n d i t i o n o f t h e p i s t o n a f t e r t e s t .
This i s rated
by a panel o f e x p e r t s who assess such f.actors as t h e freedom o f t h e r i n g s , a minimum o f s c u f f i n g o f t h e p i s t o n crown and a minimal amount o f carbon i n t h e t o p r i n g groove.
There should a l s o be no carbon d e p o s i t i n t h e lower r i n g
grooves and t h e p i s t o n s k i r t should be e n t i r e l y f r e e o f l a c q u e r . As w i l l have been g a t h e r e d f r o m p r e v i o u s c h a p t e r s , t h e a c t i o n o f l u b r i c a n t a d d i t i v e s i s complex and t h e r e may be s i g n i f i c a n t i n t e r a c t i o n s o f an u n d e s i r a b l e nature.
For example, some a d d i t i v e s l e a d t o c o r r o s i o n o f c o p p e r - l e a d b e a r i n g s
and i n p e t r o l - e n g i n e s l e a d e d - f u e l s may a f f e c t t h e n a t u r e o f p i s t o n d e p o s i t s ("grey p a i n t " ) .
A d d i t i o n a l t e s t s a r e t h e r e f o r e r e q u i r e d which were o r i g i n a l l y
c a r r i e d o u t f o r 36 hours i n a f o u r c y l i n d e r C h e v r o l e t engine.
The copper-lead
b e a r i n g s o f t h e t e s t e n g i n e were weighed and examined i n o r d e r t o assess c o r r o s i o n and t h e p i s t o n s examined t o assess any d e p o s i t .
A P e t t e r \ / I . spark-
i g n i t i o n engine has been s u b s t i t u t e d f o r t h e C h e v r o l e t e n g i n e i n t h e U.K. engine, t h e CRC L-38,
and an
has been s p e c i a l l y developed f o r l u b r i c a n t a p p r o v a l t e s t s
i n the U.S.A. Although i t may appear a t f i r s t s i g h t t h a t t h e use o f an a c t u a l engine i s a s t r a i g h t f o r w a r d means f o r assessing t h e q u a l i t y o f an o i l , t h e t e s t c o n d i t i o n s are very c r i t i c a l .
The c o m p o s i t i o n o f t h e f u e l , t h e t i m i n g o f t h e v a l v e s , t h e
c o n d i t i o n s o f t h e i n j e c t i o n system a l l may a f f e c t t h e performance o f a l u b ricant.
I n p a r t i c u l a r , t h e exhaust arrangements can markedly a f f e c t r e s u l t s
[la.
20.2.2
Gear T e s t s
The i n t r o d u c t i o n o f h y p o i d gears i n back a x l e s was made p o s s i b l e o n l y by t h e a p p l i c a t i o n o f extreme p r e s s u r e l u b r i c a n t s and t h e maintenance o f adequate l u b r i c a n t q u a l i t y i s essential t o t h e i r successful functioning. qua1 i f y l u b r i c a n t s t o t h e U . K .
(CS 3000) and U.S.A.
(MIL-L-2105C)
Tests t o specifications
a r e c a r r i e d o u t on a c t u a l gears i n a t e s t arrangement o f t h e t y p e shown i n Fig.4.
The l o a d c a r r y i n g and extreme p r e s s u r e c h a r a c t e r i s t i c s o f gear l u b r i c -
a n t s i n a x l e s under c o n d i t i o n s o f high-speed, by low-speed,
low-torque o p e r a t i o n , f o l l o w e d
h i g h - t o r q u e o p e r a t i o n should s a t i s f a c t o r i l y p r e v e n t t h e occ-
u r r e n c e o f gear t o o t h r i d g i n g , r i p p l i n g , p i t t i n g , w e l d i n g , e x c e s s i v e wear, o r o t h e r s u r f a c e d i s t r e s s o r t h e f o r m a t i o n o f o b j e c t i o n a b l e d e p o s i t s when t e s t e d on b o t h u n t r e a t e d and phosphate t r e a t e d gear assemblies i n accordance w i t h t h e s p e c i f i e d procedures. The p r i m a r y a c t i o n o f t h e l u b r i c a n t a d d i t i v e i s t o c o n f e r E.P. on t h e l u b r i c a n t .
properties
T h i s i m p l i e s p r e v e n t i o n o f s c u f f i n g by a c t i v a t i o n o f t h e
c h e m i c a l l y a c t i v e a d d i t i v e s by t h e occurrence o f instantaneous temperature
475
Fig.4
Low speed high torque test r i g hypoid gear and dynamometer installation
Fig.5
Ridging t y p e gear failure
416
Fig.5
Examples o f g e a r damage
477
Fia.6
Rippling type near f a i l u r e
Fig.7
I.A.E.
h i g h speed g e a r r i g
478 " f l a s h e s " between p a i r s o f gears.
The High-speed,
Low-torque t e s t i s used t o
assess t h i s behaviour u s i n g equipment w h i c h embodies an a u t o m o b i l e r e a r - a x l e assembly.
Evidence o f f a i l u r e i s p r o v i d e d by t h e i n i t i a t i o n o f s c u f f i n g .
TABLE 20.2
Summary o f t h e F u l l A x l e Test o f MIL-L-2105C
Test
and CS 3000
Requ i remen t s
Equipment
Moisture Corrosion 7 days
CRC L-33 Federal method 5326
Maximum o f 5% vapour phase c o r r o s i o n o f cover p l a t e , no corrosion o f functional parts.
High Speed Shock Load Test
CRC L-42 Federal method 6507 lP.234/69
Score p r e v e n t i o n equal t o or b e t t e r than RG 1 1 0 o r CRC 10/90 r e f e r e n c e o i l .
Low Speed High Torque Test
CRC L-37 Federal method 6506 I P 232 Proc. 8.
No gear s u r f a c e d i s t r e s s o r deposit formation allowable.
When c h e m i c a l l y a c t i v e a d d i t i v e s a r e used, t h e r e i s always t h e r i s k t h a t they w i l l a c t under c o n d i t i o n s and i n a manner which i s n o t d e s i r e d . t y p e f a i l u r e may o c c u r as i n F i g . 5 and " r i p p l i n g "
example, " r i d g i n g " Fig.6.
The "Low-speed
For as i n
High-torque'' t e s t i s used t o ensure t h a t l u b r i c a n t s a r e
f o r m u l a t e d t o p r e v e n t these t y p e s o f f a i l u r e s f r o m o c c u r r i n g under s e r v i c e conditions. L u b r i c a n t s f o r use i n spur gears a r e u s u a l l y t e s t e d f o r t h e i r a n t i - s c u f f i n g p r o p e r t i e s i n machines which embody means f o r power c i r c u l a t i o n .
One o f t h e
o r i g i n a l machines i n t h i s c l a s s was designed by Mansion and i s known as t h e I.A.E.
machine.
The gears under t e s t a r e geared t o g e t h e r ,
p a r a l l e l s h a f t s , and loaded by t h e " l o c k i n g - u p " s t r a i n i n the shafts.
supported on two
o f a predetermined t o r s i o n a l
The gear s e t s a r e assembled i n a "back-to-back"
relation-
s h i p so t h a t power c i r c u l a t e s c o n t i n u o u s l y and t h e d r i v i n g motor i s o n l y r e q u i r e d t o make up t h e l o s s e s o c c u r r i n g w i t h i n t h e system. Another machine which employs t h e power c i r c u l a t i n g p r i n c i p l e i s t h e "Ryder" machine.
In t h i s d e s i g n one s e t o f gears a c t s as t e s t s e c t i o n and t h e o t h e r i s
o f the single-helical
configuration.
Load i s t h e n a p p l i e d by means o f an a x i a l
f o r c e which i s c o n v e r t e d i n t h e h e l i c a l - g e a r s t o a t o r q u e t e n d i n g t o t w i s t t h e s h a f t thereby l o a d i n g t h e t e s t gears. The F.Z.G.
Machine (Fig.8)
a l s o employs i n v o l u t e gears on t h e p o w e r - c i r c -
ulating principle.
A l l t h e above machines r e q u i r e gears as t e s t p i e c e s and, because each t e s t must be t a k e n t o d e s t r u c t i o n , t h e t e s t procedures a r e v e r y expensive.
There i s
a s t r o n g tendency t h e r e f o r e t o employ disc-machines wherein t h e r e l a t i v e amount
419
Test gears
Lubr i c a n t T
Fig.8
FZG Gear o i l t e s t
o f r o l l i n g and s l i d i n g can be a d j u s t e d t o correspond w i t h t h e events i n t h e meshing c y c l e o f a spur gear.
20.2.3
Simulation o f I n d u s t r i a l Situations
Many i n d u s t r i a l processes r e q u i r i n g l u b r i c a t i o n do n o t correspond t o t h e c o n d i t i o n s o f t h e t y p e approval t e s t o f l u b r i c a n t s , and i t i s necessary t o i n v e s t i g a t e t h e e x i s t i n g t r i b o l o g i c a l s i t u a t i o n t a k i n g i n t o account t h e n a t u r e o f t h e i n t e r a c t i n g m a t e r i a l s , t h e i n d u s t r i a l environment and t h e a p p l i e d f o r c e s and n o t i o n s . I n t h i s circumstance,
i t i s f r e q u e n t l y necessary t o c o n s t r u c t a s p e c i a l
machine t o reproduce w i t h i n t h e l a b o r a t o r y t h e s p e c i f i c c o n d i t i o n s surrounding t h e i n d u s t r i a l problem. An example o f such a machine i s shown i n Fig.9
[2].
This consists o f a
machine designed and c o n s t r u c t e d a t t h e Swansea T r i b o l o g y Centre t o s i m u l a t e the c o n d i t i o n s d e t e r m i n i n g t h e a c t i o n o f t h e s i d e g u i d e s of r o l l i n g m i l l s .
The
s i d e g u i d e s , n o r m a l l y made o f bronze, a r e a c t e d on by t h e edges o f t h e s t e e l s t r i p undergoing c o l d - r o l l i n g which t h e y r e s t r a i n f r o m unwanted sideways movement.
They a r e s u b j e c t e d t o much wear ( F i g . g c ) ,
and moreover, t h e n o n - f e r r o u s
m a t e r i a l t r a n s f e r r e d f r o m t h e guide t o t h e s t e e l was o b j e c t i o n a b l e t o t h e customers o f t h e m a t e r i a l . A c c o r d i n g l y a guide member was f i t t e d w i t h s t r a i n - g a u g e s and t h e r m i s t o r s and s u b j e c t e d t o normal o p e r a t i o n so t h a t t h e f o r c e s a c t i n g on t h e guide c o u l d be determined t o g e t h e r w i t h t h e o p e r a t i o n a l temperature.
The machine i l l u s t r a t e d
480
Fig.g(a)
R o l l i n g M i l l w i t h s i d e guides
i n F i g . g ( b ) was then designed t o embody a c o n t i n u o u s s t e e l b e l t t o r e p r e s e n t t h e c o l d s t r i p beimg r o l l e d i n t h e f u l l s c a l e apparatus and a member which resembled a s i d e guide and which was f o r c e d a g a i n s t t h e edge o f t h e s t r i p w i t h a f o r c e which was determined on t h e b a s i s o f t h e s t r a i n - g a u g e measurements. A f t e r a s e r i e s o f t e s t s had c o n f i r m e d t h a t t h e wear mode o b t a i n e d i n t h e l a b o r a t o r y corresponded e x a c t l y w i t h t h a t observed i n p r a c t i c e , a s e r i e s o f a l t e r n a t i v e m a t e r i a l s was i n v e s t i g a t e d .
I t was concluded t h a t t h e s u b s t i t u t i o n o f
n o d u l a r c a s t - i r o n f o r t h e bronze presented s e v e r a l advantages as f o l l o w s : (a) The r a t e o f wear was reduced (b) The m a t e r i a l was l e s s expensive t o p r o c u r e (c) The c o n t a m i n a t i o n o f t h e p r o d u c t w i t h n o n - f e r r o u s m a t e r i a l was avoided. T h i s example demonstrates t h e p o s s i b i l i t y o f study o f i n d u s t r i a l problems u s i n g
1 a bora t o r y met hods.
20.3
ANALYSIS INTO SYSTEM ELEMENTS AS THE BASIS FOR SELECTION OF LABORATORY TESTS
T r i b o l o g i c a l s i t u a t i o n s encountered i n d i f f e r e n t machines may p r e s e n t c e r t a i n f e a t u r e s i n common i n s o f a r as t h e y a l l embody s u r f a c e s i n r e l a t i v e motion.
481
Fig.9(b)
E x p e r i m e n t a l R i g f o r S i m u l a t i n q S i d e Guide Wear
Fig.g(c)
S i m u l a t e d worn s i d e g u i d e
482 However,
i t w i l l be apparent t h a t t h e n a t u r e and c o n f i g u r a t i o n o f s u r f a c e s may
v a r y w i d e l y as between one machine and another as w i l l t h e modes o f f o r c e and motion.
Therefore t h e r e can be no s i n g l e l a b o r a t o r y r i g which can r e p r e s e n t a l l
tribological situations.
However,
i t i s a l s o t r u e t h a t many machines possess
f e a t u r e s which a r e s u f f i c i e n t l y a l i k e t o j u s t i f y t h e c o n s t r u c t i o n o f t e s t machines f o r t h e purpose o f e v a l u a t i n g l u b r i c a n t s o r m a t e r i a l s o f c o n s t r u c t i o n f o r use t h e r e i n . Great c a r e i s necessary however t o analyse p r e c i s e l y t h e n a t u r e o f t h e t r i b o l o g i c a l s i t u a t i o n s i n v o l v e d i n o r d e r t o s e l e c t t h e c o r r e c t c o n d i t i o n s t o be a p p l i e d i n t h e t e s t machine.
A s c i e n t i f i c a t t i t u d e i s necessary and meaningless
j a r g o n such as " f i l m - s t r e n g t h ' ' o r " l u b r i c i t y "
should be avoided.
(An example o f
a l l o w i n g j a r g o n t o t a k e o v e r i s g i v e n by t h e i n v e s t i g a t o r who s u b j e c t e d t o an
E.P.
t e s t a l u b r i c a n t r e q u i r e d f o r a process imposing h i g h p r e s s u r e a t v e r y low
speed). The f i r s t s t e p i n t h e a n a l y t i c a l process must be t o c h a r a c t e r i z e t h e machine element under study w i t h r e s p e c t t o i t s p o s i t i o n i n t h e k i n e m a t i c c h a i n ; a "Higher P a i r " o r a "Lower
Pair"?
is it
O f t h e lower p a i r s , t h e s l i d i n g p a i r , F i g .
l O ( a ) i s encountered on machine slideways and t h e "Revolute the basis o f the m a j o r i t y o f bearings.
Pair" Fig.lO(b)
is
A p a r t f r o m t h e "screw1' p a i r (which i s a
combination o f t h e s l i d i n g and r e v o l u t e p a i r s ) a l l o t h e r p a i r s a r e h i g h e r p a i r s and a r e so c h a r a c t e r i s e d because t h e y must accommodate r e l a t i v e motion which i s p a r t l y s l i d i n g and p a r t l y t u r n i n g . From t h e p o i n t o f vie!+ o f T r i b o l o g y , t h e i m p o r t a n t d i s t i n c t i o n between lower and h i g h e r p a i r s i s t h a t the former a l l o w c o n t a c t t o be made throughout t h e f u l l e x t e n t o f a s u r f a c e , whereas t h e l a t t e r o n l y a l l o w " p o i n t "
o r "1 ine" c o n t a c t .
Some common examples o f h i g h e r p a i r s a r e shown i n F i g . l O ( d - 9 ) . a r e s a i d t o be "conformal"
and h i g h e r p a i r s a r e "counterformal".
Thus lower p a i r s I t w i l l be
apparent t h a t the p r o f o r m a l d i f f e r e n c e between d i s p e r s e d and c o n c e n t r a t e d cont a c t s w i l l be r e f l e c t e d i n t h e design o f t h e i n t e r a c t i n g components,
in their
m a t e r i a l s o f c o n s t r u c t i o n and i n t h e p r o p e r t i e s r e q u i r e d o f any l u b r i c a n t s applied.
[3].
Fig.11 i l l u s t r a t e s t h e c l a s s i f i c a t i o n o f some common machine
elements i n t o conformal and c o u n t e r f o r m a l c o n f i g u r a t i o n s as w e l l as i n d i c a t i n g t h e mode o f damage c h a r a c t e r i s t i c o f each a p p l i c a t i o n . To be s u c c e s s f u l , l a b o r a t o r y t e s t i n g machines must be s i m p l e and must employ e a s i l y manufactured t e s t p i e c e s .
They must however reproduce t h e c o n d i t i o n s o f
thermal and s t r e s s i n t e n s i t y t o which i t i s a n t i c i p a t e d t h a t t h e l u b r i c a t e d system w i l l be s u b j e c t e d i n s e r v i c e .
The f i r s t broad c l a s s i f i c a t i o n must be
i n t o lower and h i g h e r p a i r s . There a r e r e l a t i v e l y few simple machines a v a i l a b l e commercially f o r t e s t i n g l u b r i c a n t s i n lower p a i r s because, as f a r as l u b r i c a t i o n proper i s concerned,
/
I
v)
.I
Q
U
Q,
L
+ z ,
c 9
(d) Gear tooth contact
lei
&I(
bearmg
IfJ Taper roller
l g J Wheel on roil
bearing
Classification of machine elements into higher and lower pairs
483
FIGURE 10
Nature of Contact
Characteristic Mode of Damage
Lower pair (Conformal 1 Fixtures subject to
Higher pair (Counterformal1
Gears
Hertzian
I
Wheel on rail
Y
~
FIGURE 11
Classification of mode of damage of common machine elements
485 t h i s i s u s u a l l y hydrodynamic and t h e o n l y p r o p e r t y r e q u i r e d o f t h e l u b r i c a n t i s i t s v i s c o s i t y wh i ch can e a s i l y be measured by p h y s i c a l methods.
The i m p o r t a n t
p r o p e r t i e s o f t h e l u b r i c a n t i n t he se a p p l i c a t i o n s a r e r e l a t e d t o such f a c t o r s as detergency o r a n t i - c o r r o s i o n and f u l l s c a l e t e s t s a r e necessary t o safeguard against undesirable i n t e r a c t i o n s . For t e s t i n g m a t e r i a l - l u b r i c a n t co mbi n at i on s under c o n d i t i o n s o f p u r e s l i d i n g , a very c o n v e n ie n t arrangement c o n s i s t s o f t h r e e c y l i n d r i c a l p i n s w hich a r e h e l d i n an upper member w i t h t h e i r axes p a r a l l e l and t h e assembly i s loaded a g a i n s t the s i d e o f an a n n u l a r t e s t p i e c e which i s r o t a t e d about i t s g e n e r a t i n g a x i s . The "Cygnus" (Fig.13)
machine i s o f t h i s t ype .
Both t h e Almen (Fig.13)
and t h e Falex
machines a r e b a s i c a l l y 1 i n e - c o n t a c t machines b u t a c e r t a i n degree o f
c o n f o r m i t y u s u a l l y a r i s e s from.wear o f t h e t e s t pieces.
These two machine types
present t h e common f e a t u r e t h a t two i d e n t i c a l t e s t p i e c e s a r e f o r c e d a g a i n s t a r o t a t i n g member from d i a m e t r i c a l l y opposed d i r e c t i o n s , on t h e s p i n d l e .
i n t e r n a l r a d i u s some 1.5 x 10 which i s 0.35 mm (0.25 "Falex"
a r e "V"
mechanical.
thus balancing the f o r c e
The Almen t e s t p i e c e s f orm segments o f a c y l i n d e r having an
shaped.
-4m
(0.006
i n ) i n d i ame t e r.
i n ) g r e a t e r than t h a t o f t h e s p i n d l e The corresponding t e s t p i e c e s o f t h e
Loading on t h e "Almen"
i s h y d r a u l i c and the "Falex"
R e s u l t s a r e d i f f i c u l t t o e v a l u a t e because o f t h e l o a d i n g methods
u s u a l l y employed [ 4 ] . As r e g a r d s h i g h e r p a i r s ,
t h e r e i s a wi de v a r i e t y o f machines w hich a p p l y
s l i d i n g under c o u n t e r f o r m a l c o n d i t i o n s .
These have g e n e r a l l y been developed t o
assess t h e q u a l i t y o f ge ar l u b r i c a n t s w i t h p a r t i c u l a r r e f e r e n c e t o t h e i n h i b ition o f scuffing. (Stanhope-Seta)
One o f t h e most su ccessf ul o f these has been t h e S h e l l
f o u r b a l l machine (Fig.14).
The problem o f o b t a i n i n g t e s t
pieces wh ic h a r e o f u n i f o r m f i n i s h and m e t a l l u r g i c a l c o n d i t i o n has been s o l v e d by u s i n g b a l l s s e l e c t e d i n batches f r o m commercial p r o d u c t i o n .
Three o f these
b a l l s a r e clamped t o f orm a n e s t i n t o whi ch t h e f o u r t h b a l l ( h e l d i n a chuck) i s forced and r o t a t e d .
The b a l l s a r e 12.7 mm (0.5
i n ) i n diameter and t h e r o t a t -
i on a l speed o f t h e s p i n d l e t o which t h e chuck i s a t t a c h e d i s 1450-1500 r e v min-'. Load may b e a p p l i e d i n increments up t o 800 kg. s e v e r a l procedures,
[5].
The machine may be used i n
t h e most common o f wh i ch i s t h e "Wear-scar
diameter method",
A number o f t e s t s ( u s u a l l y 20) a r e each made on a f r e s h s e t o f b a l l s , load
be in g in c r e a s e d between each t e s t t h e d u r a t i o n o f which i s one minute.
The d i a -
meter o f t h e wear s c a r a pp are nt a f t e r each t e s t i s measured and p l o t t e d a g a i n s t load on l o g a r i t h m i c paper.
Duri n g t h e t e s t s a t t h e lower loads wear i s n e g l i g -
i b l e b u t s u f f i c i e n t r u b b i n g a c t i o n t ake s p l a c e t o leave a measurable mark which when measured and p l o t t e d , g i v e s r i s e t o a s t r a i g h t l i n e w hich c l o s e l y p a r a l l e l s t h a t o b t a i n e d by c a l c u l a t i n g t h e d i ame t e r o f t h e H e r t z i a n c o n t a c t between the loaded b a l l s .
W i t h most l u b r i c a n t s a l o a d i s reached a t w hich a sharp r i s e i n
486
Specimen arrangement
FIG.12
CYGNUS F R I C T I O N AND WEAR TEST MACHINE
Brass lockina Almen blocks
m
at
F I G . 13
w + # j L Falex blocks
Exploded view of pin and
FALEX LUBRICANT TEST MACHINE
V blocks
Cutaway view throughout sample pan
487
Male ball chuck Fourth ball (rotates)
-0
Ball pot lock ring -
(stationary ) Ball Dot insert Ball pot Calibrated arm
Ball pot _ _ mounting disc
Top boll rotates
Ball and flats
Load
Four ball machine
FIGURE 14
Cone and cylinder
488 diameter occurs.
This value i s u s u a l l y s u f f i c i e n t l y d e f i n i t e t o characterise a
l u b r i c a n t and i s c a l l e d t h e i n i t i a l s e i z u r e load.
I n some cases however, r e -
covery may o c c u r a l t h o u g h t h e b a l l s o f t e n become welded t o g e t h e r a t t h e h i g h e r loads.
Another method o f e v a l u a t i o n i s based on t h e t i m e e l a p s i n g under a con-
s t a n t load b e f o r e t h e occurrence o f a sudden i n c r e a s e i n f r i c t i o n . One disadvantage o f t h e f o u r b a l l machine i s t h a t t h e m a t e r i a l o f t h e t e s t p i e c e s , b a l l b e a r i n g s t e e l , i s n o t r e p r e s e n t a t i v e o f m a t e r i a l s w i t h which t h e l u b r i c a n t may be r e q u i r e d t o i n t e r a c t .
A c c o r d i n g l y t h e c e n t r a l b a l l may be
r e p l a c e d by a c o n i c a l l y ended p i e c e and t h e f i x e d b a l l s by small c y l i n d e r s arranged t o form a t r i a n g l e i n a h o r i z o n t a l p l a n e , Fig.14. The Timkin Machine (Fig.15)
embodies a l i n e r a t h e r than a p o i n t c o n t a c t .
The
r o t a t i n g element, formed f r o m t h e r i n g o f a t a p e r r o l l e r b e a r i n g a c t s on a stationary rectangular steel block.
F r i c t i o n can be measured.
Most t e s t i n g machines a r e so arranged t h a t t h e wearing s u r f a c e s a r e i n cont i n u o u s o r repeated c o n t a c t so as t o o b l i t e r a t e t h e i n i t i a l m a n i f e s t a t i o n s o f surface f a i l u r e .
A machine was a c c o r d i n g l y designed a t M.E.R.L.
(now N.E.L.)
which c o n s i s t e d o f two c y l i n d e r s which, i n a d d i t i o n t o r o t a t i o n , c o u l d be t r a v ersed one r e l a t i v e t o t h e o t h e r so t h a t t h e c o n t a c t zone was made t o c o n t i n u a l l y embody f r e s h m a t e r i a l (Fig.16). I n a l l t h e aforementioned machines, o n l y s l i d i n g t a k e s p l a c e whereas i n many mechanisms, place.
i n v o l u t e gears f o r example, r o l l i n g as w e l l as s l i d i n g can t a k e
Disc inachines such as t h e "Amsler"
( a l s o t h e M e r r i t t and S.A.E.
machines) have d i s c s which a r e loaded edgewise t o p r o v i d e v a r i o u s c o m b i n a t i o n s o f r o l l i n g and s l i d i n g ( F i g . 1 7 ) .
-
P i t t i n g t y p e f a i l u r e i s u s u a l l y a s s o c i a t e d w i t h r o l l i n g c o n t a c t and can be simulated i n the laboratory [ 6 ] using a simple m o d i f i c a t i o n o f the four b a l l machine ( F i g . 1 8 ) .
I n s t e a d o f t h e t h r e e lower b a l l s b e i n g clamped i n t o p l a c e
they a r e a l l o w e d t o r o t a t e w i t h i n a s p e c i a l l y designed b a l l r a c e [ 6 ] . i s d e s i r e d t o i n v e s t i g a t e t h e p i t t i n g behaviour o f a s p e c i a l s t e e l ,
Where i t
it i s
p o s s i b l e t o s u b s t i t u t e a c o n i c a l l y ended t e s t p i e c e f o r t h e c e n t r a l b a l l a l t h o u g h t h e t h r e e f r e e b a l l s must be r e t a i n e d .
20.4
A h i g h speed v e r s i o n i s a l s o a v a i l a b l e .
EQUIPMENT FOR B A S I C RESEARCH
The i n v e s t i g a t i o n s pursued i n b a s i c r e s e a r c h l a b o r a t o r i e s may appear t o be u n r e l a t e d t o p r a c t i c e by reason of t h e a r t i f i c i a l c o n d i t i o n s o f t e n imposed i n o r d e r t o e l u c i d a t e some fundamental r e l a t i o n s h i p .
The c l o s i n g o f t h e communic-
a t i o n gap between t h e fundamental i n v e s t i g a t o r and t h e engineer whose p r a c t i c e
lies i n m a n u f a c t u r i n g i n d u s t r y f o r example, has been made e a s i e r by t h e d e v e l opment o f t h e p r a c t i c e o f m o d e l l i n g complex systems u s i n g computers.
Thus i n
Fig.19 t h e r e c t a n g l e marked A r e p r e s e n t s a real. machine-element s u b j e c t e d t o t h e environmental c o n d i t i o n s and l o a d i n g w i t h i n t h e i n d u s t r i a l environment.
Its
489
Lubricant sample
+ Load
Fig.15
Timken t e s t
Fig.16
N.E.L.
c ro ssed c y l i n d e r machine
490
Fig.17
Amsler machine
lL
F
Fig.18
Rolling four ball machine
491
MATHEMATICAL
Fig.19
LABORATORY EXPERIMENTS
E/
The r o l e o f the computer model i n r e l a t i n g b a s i c research t o engineering p r a c t i c e
response t o these c o n d i t i o n s can be determined by a methodical s e r i e s o f t e s t s , r e s u l t s o f which a r e r e p r e s e n t e d by r e c t a n g l e 8.
Assuming t h a t n o t a l l i s w e l l
o r t h a t some development i n p r o d u c t o r p r o d u c t i o n c a p a c i t y i s r e q u i r e d ,
i t may
be necessary t o f o r m u l a t e some p r e d i c t i o n s o f behaviour l y i n g o u t s i d e t h e range o f p r e v i o u s experience.
The system can then be represented by a s e r i e s o f
e q u a t i o n s l i n k e d t o g e t h e r t o form a mathematical model o f t h e machine as indi c a t e d by r e c t a n g l e C.
S p e c i f i c a t i o n s f o r a p p l i e d c o n d i t i o n s and l o a d i n g s would
be f e d i n and t h e computer would produce s o l u t i o n s t o t h e e q u a t i o n s represented by r e c t a n g l e D .
These s o l u t i o n s would p r o v i d e p r e d i c t i o n s o f t h e magnitude o f
o u t p u t q u a n t i t i e s which a r e r e p r e s e n t e d by t h e arrows p o i n t i n g t o the l e f t from
D t o B.
The arrows p o i n t i n g t o t h e r i g h t f r o m €3 t o D r e p r e s e n t t h e r e s u l t s o f
actual t e s t .
I f t h e r e i s agreement between measured o u t p u t s and c a l c u l a t e d pre-
d i c t i o n s , t h e model can be s a i d t o be complete.
I f t h e r e i s divergence, the
model must be r e f i n e d and a d j u s t e d u n t i l a c c e p t a b l e agreement r e s u l t s .
Once
t h i s agreement has been a t t a i n e d , t h e model may be used t o p r e d i c t the behaviour o f an i n f i n i t e v a r i e t y o f machine elements and a p p l i e d c o n d i t i o n s p r o v i d e d they l i e w i t h i n t h e range o f e q u a t i o n s embodied i n t h e model. Attempts t o c o n s t r u c t r e a l i s t i c mathematical models however, f r e q u e n t l y r e v e a l gaps i n understanding o f t h e p h y s i c a l system i n v o l v e d which can o n l y be f i l l e d by c a r e f u l l y c o n t r o l l e d l a b o r a t o r y experiments such as those which were necessary t o e l u c i d a t e t h e n a t u r e and o p e r a t i o n a l r e l a t i o n s h i p s o f e l a s t o hydrodynamic l u b r i c a t i o n (as i n d i c a t e d a t E ) .
Such experiments u s u a l l y form the
b a s i s o f advanced t e s t methods made necessary by developing p r a c t i c e .
492 REFERENCES Cree,J.C. ( 1 9 5 3 ) C a t e r p i l l a r L1 a n d C h e v r o l e t L4 t e s t p r o c e s s , Symposium o n E n g i n e T e s t i n g o f L u b r i c a t i n g O i l . I n s t i t u t e o f Petroleum R o y l a n c e , B . J . (1977) The A p p l i c a t i o n o f E x i s t i n g Knowledge t h e S o l u t i o n o f I n d u s t r i a l T r i b o l o g y Problems. P r o c . I . Mech. E. I n t h e Press. B a r w e l l ,F.T. (1979) ' B e a r i n g Systems - P r i n c i p l e s a n d P r a c t i c e ' Clarendon, Oxford. Meckleburg,K.R. (1975) 'Forces i n the Falex c o n f i g u r a t i o n ' . Trans. ASLE., V01.18, pp. 97-104. Extreme P r e s s u r e P r o p e r t i e s : F r i c t i o n a n d blear T e s t s : Four B a l l I P 239/77. The I n s t i t u t e o f P e t r o l e u m . Standards f o r Machine. Petroleum and i t s p r o d u c t s . 'The e f f e c t o f l u b r i c a n t p i t t i n g B a r w e l 1 , F . T . a n d S c o t t , D . (1956) f a i l u r e o f b a l l b e a r i n g s ' E n g i n e e r i n g , Vo1.182, pp. 9-12.
493
GLOSSARY
Terms and D e f i n i t i o n s
-
Abrasion
Wear by displacement o f m a t e r i a l caused by h a r d p a r t i c l e s o r hard
protuberances.
-
Absolute V i s c o s i t y
-
Additive
see v i s c o s i t y .
A m a t e r i a l added t o a l u b r i c a n t f o r t h e purpose o f i m p a r t i n g new
p r o p e r t i e s o r o f enhancing e x i s t i n g p r o p e r t i e s .
Adhesive Wear
-
Wear by t r a n s f e r e n c e o f m a t e r i a l f r o m one s u r f a c e t o another
d u r i n g r e l a t i v e motion, due t o t h e process o f s o l i d - p h a s e welding.
-
Anti-Wear A d d i t i v e
Area o f Contact
-
An a d d i t i v e used t o reduce wear.
The area o f c o n t a c t between two s o l i d s u r f a c e s i s d e s c r i b e d
i n two ways. ( i ) Apparent Area:
t h e area o f c o n t a c t d e f i n e d by t h e boundaries
o f t h e macroscopic i n t e r f a c e o f t h e bodies ( i i ) Real Area:
t h e sum o f t h e l o c a l areas t r a n s m i t t i n g i n t e r f a c i a l
f o r c e d i r e c t l y between t h e bodies.
Asperities
B,O
Life
-
-
The s m a l l s c a l e i r r e g u l a r i t i e s on a s u r f a c e .
see r a t i n g l i f e .
,
L
B a b b i t t Metal
-
A n o n - f e r r o u s b e a r i n g a l l o y , e i t h e r t i n o r l e a d based con-
s i s t i n g o f v a r i o u s amounts o f copper, antimony, t i n and l e a d .
Base Stock ( o i l )
-
R e f i n e d p e t r o l e u m o i l used i n t h e p r o d u c t i o n o f l u b r i c a n t s
and o t h e r products.
The base s t o c k may be used a l o n e or blended w i t h
o t h e r base s t o c k s and/or a d d i t i v e s .
Bearing
-
A support o r guide by means o f which a moving p a r t i s p o s i t i o n e d
w i t h r e s p e c t t o t h e o t h e r p a r t s o f a mechan’ism.
494 Bearing Area - The p r o j e c t e d b e a r i n g l o a d c a r r y i n g area when viewed i n t h e d i r e c t i o n o f the load.
Beilby layer
-
An amorphous l a y e r o f deformed metal and o x i d e p a r t i c l e s formed
by p o l i s h i n g .
Blending
-
The process o f m i x i n g m i n e r a l o i l s t o o b t a i n d e s i r e d viscous
properties.
Boundary L u b r i c a t i o n
-
A c o n d i t i o n o f l u b r i c a t i o n i n which t h e f r i c t i o n and wear
between two s u r f a c e s i n r e l a t i v e m o t i o n a r e determined by t h e p r o p e r t i e s o f t h e s u r f a c e s , and by t h e p r o p e r t i e s o f t h e l u b r i c a n t o t h e r than b u l k viscosity.
Brinelling
-
I n d e n t a t i o n of t h e s u r f a c e o f a s o l i d body by repeated l o c a l
impact o r impacts, o r by s t a t i c o v e r l o a d .
-
Cavitation Erosion
Wear o f a s o l i d body moving r e l a t i v e l y t o a l i q u i d i n a
r e g i o n o f c o l l a p s i n g vapour bubbles which cause l o c a l h i g h impact pressure
tempe r a t u r e s
0r
Centre L i n e Average (CLA)
-
.
An E n g l i s h measure o f s u r f a c e topography r e p r e s -
e n t i n g t h e average d e p a r t u r e o f a l i n e p r o f i l e o f t h e s u r f a c e f r o m t h e centre line.
Channe i n g
-
The tendency o f a grease t o f o r m a channel by w o r k i n g down a
b e a r i n g o r d i s t r i b u t i o n system, l e a v i n g shoulders t o a c t as a r e s e r v o i r and s e a l .
Clearance R a t i o
-
In a bearing, the r a t i o o f r a d i a l clearance t o shaft radius.
C o e f f i c i e n t of F r i c t i o n
-
The r a t i o o b t a i n e d by d i v i d i n g t h e t a n g e n t i a l f o r c e
r e s i s t i n g m o t i o n between two b o d i e s by t h e normal f o r c e p r e s s i n g these bodies t o g e t h e r .
Composite Bearing M a t e r i a l
-
A s o l i d m a t e r i a l composed o f a c o n t i n u o u s o r
p a r t i c u l a t e s o l i d l u b r i c a n t phase dispensed t h r o u g h o u t a l o a d b e a r i n g m a t r i x t o p r o v i d e c o n t i n u o u s replenishment o f s o l i d l u b r i c a n t f i l m s as wear o c c u r s , and e f f e c t i v e h e a t t r a n s f e r f r o m t h e f r i c t i o n s u r f a c e .
495 Corrosion I n h i b i t o r
-
A d d i t i v e s f o r p r o t e c t i n g l u b r i c a t e d surfaces a g a i n s t
chemical a t t a c k .
They may
be p o l a r compounds w e t t i n g the metal s u r f a c e
p r e f e r e n t i a l l y , or they may a bso rb t h e w ater t o form a w a t e r - i n - o i l e m u ls io n
-
o n l y t h e o i l touches t h e metal.
Some c o r r o s i o n i n h i b i t o r s
combine c h e m i c a l l y w i t h t h e metal t o g i v e a n o n - r e a c t i v e surface.
-
C o r r o s i v e Wear
A process i n which chemical or e l e c t r o c h e m i c a l r e a c t i o n w i t h
t h e environment predominates.
-
Cutting Fluid
A f l u i d applied t o a c u t t i n g tool t o assist i n the c u t t i n g
o p e r a t i o n by c o o l i n g ,
-
Det e r g e n t A d d i t i v e s
l u b r i c a t i n g o r o t h e r means.
compounds which, when blended w i t h l u b r i c a t i n g o i l s ,
d i s p e r s e t he d e t e r i o r a t i o n p r o d u c t s f r o m t h e f u e l and l u b r i c a n t , esp e c i a l l y t ho se formed under h i g h t emp erature c o n d i t i o n s , and, t h u s , m in im is e t h e f o r m a t i o n o f d e p o s i t s l i a b l e t o cause p i s t o n - r i n g s t i c k i n g or o t h e r t r o u b l e .
Di s p e r s a n t A d d i t i v e s
-
compounds which, when blended w i t h l u b r i c a t i n g o i l s ,
m a i n t a i n t h e p r o d u c t s o f combustion f rom t h e fuel i n a f i n e l y dispersed s t a t e , and t h e r e b y mi ni mi se sludge f o r m a t i o n and f i l t e r b l o c k i n g , p a r t i c u l a r l y i n g a s o l i n e engines o p e r a t i n g under c o l d c o n d i t i o n s .
Drop p o i n t
-
temperature a t which a drop o f grease o r o t h e r petroleum product
f i r s t detaches i t s e l f f ro m t h e main b u l k o f m a t e r i a l when a sample i s s t e a d i l y heated under p r e s c r i b e d c o n d i t i o n s .
Duty Parameter
-
A di me nsi o nl e ss number whi ch i s used t o e v a l u a t e t h e p e r -
formance o f b ea ri ng s.
Dynamic V i s c o s i t y
Eccentricity Ratio
-
see: V i s c o s i t y .
-
In a b e a r i n g , t h e r a t i o o f t h e e c c e n t r i c i t y t o t h e r a d i a l
c l earance.
Elasto-hydrodynamic L u b r i c a t i o n
-
A c o n d i t i o n o f l u b r i c a t i o n i n which t h e
f r i c t i o n and f i l m t h i c k n e s s between two bodies i n r e l a t i v e motion a r e determined by t h e e l a s t i c p r o p e r t i e s o f t h e bodies,
i n combination w i t h
t h e v is c o us p r o p e r t i e s o f t h e l u b r i c a n t a t t h e p r e v a i l i n g pressure, temperature and r a t e o f shear.
496 Embeddability
-
The a b i l i t y o f a b e a r i n g m a t e r i a l t o embed harmful f o r e i g n
p a r t i c l e s and reduce t h e i r tendency t o cause s c o r i n g o r a b r a s i o n .
-
Emulsion
A d i s p e r s i o n o f g l o b u l e s o f one l i q u i d i n a n o t h e r i n which i t i s
insoluble.
EP (Extreme p r e s s u r e ) A d d i t i v e
-
A chemical substance c o n t a i n i n g one o r more
elements, e s p e c i a l l y s u l p h u r , c h l o r i n e o r phosphorus, a b l e t o r e a c t w i t h metal s u r f a c e s t o g i v e i n o r g a n i c f i l m s o f h i g h m e l t i n g p o i n t .
The
presence o f these f i l m s h i n d e r s w e l d i n g and s e i z u r e and t h u s p r e v e n t s s c u f f i n g and s c o r i n g , p a r t i c u l a r l y i n gears o p e r a t i n g under h i g h l o a d conditions.
-
Erosion
E r o s i v e wear i s loss o f m a t e r i a l f r o m a s o l i d s u r f a c e due t o
r e l a t i v e m o t i o n i n c o n t a c t w i t h a f l u i d which c o n t a i n s s o l i d p a r t i c l e s .
F a t t y Acids
-
Long c h a i n o r g a n i c a c i d s which o c c u r n a t u r a l l y as t h e i r g l y c e r i d e
e s t e r s i n animal and v e g e t a b l e o i l s and f a t s .
Filler
-
A substance such as l i m e , t a l c , mica and o t h e r powders, added t o
grease t o i n c r e a s e i t s c o n s i s t e n c y or t o an o i l t o i n c r e a s e v i s c o s i t y .
Flash P o i n t
-
The l o w e s t temperature a t which t h e vapour of a l u b r i c a n t can be
i g n i t e d under s p e c i f i e d c o n d i t i o n s .
Flash Temperature
-
The maximum l o c a l temperature generated a t some p o i n t o f
c l o s e approach i n a s l i d i n g c o n t a c t .
Flexure Pivot
-
A t y p e o f b e a r i n g f o r l i m i t e d movement i n which t h e moving
p a r t s a r e guided by f l e x u r e o f e l a s t i c members r a t h e r than by r o l l i n g o r s l i d i n g surfaces.
Fretting
-
The removal o f e x t r e m e l y f i n e p a r t i c l e s f r o m b e a r i n g s u r f a c e s due t o
the i n h e r e n t adhesive f o r c e s between t h e s u r f a c e s p a r t i c u l a r l y under t h e c o n d i t i o n o f small a m p l i t u d e v i b r a t i o n .
F r e t t i n g Corrosion
Friction
-
-
A f o r m o f f r e t t i n g i n which chemical r e a c t i o n predominates.
The r e s i s t i n g f o r c e t a n g e n t i a l t o a common boundary between two
bodies when, under t h e a c t i o n o f an e x t e r n a l f o r c e , one body moves o r tends t o move r e l a t i v e t o t h e s u r f a c e o f t h e o t h e r .
491
-
F r i c t i o n Polymer
An amorphous o r g a n i c d e p o s i t which i s produced when c e r t a i n
m e t a l s a r e rubbed t o g e t h e r i n t h e presence o f o r g a n i c l i q u i d s o r gases.
-
Galling
A severe f o r m o f s c u f f i n g a s s o c i a t e d w i t h gross damage t o t h e surfaces
or failure.
Grease
-
The use o f t h i s f o r m should be avoided.
A l u b r i c a n t composed o f an o i l t h i c k e n e d w i t h a soap o r o t h e r t h i c k e n e r
t o a semi-solid o r s o l i d consistency.
A lime-based grease i s prepared
f r o m a l u b r i c a t i n g o i l and Calcium soap.
Sodium, Barium, L i t h i u m and
Aluminium based greases a r e a l s o used.
Hydraulic F l u i d
-
A f l u i d used f o r t r a n s m i s s i o n o f h y d r a u l i c p r e s s u r e o r a c t i o n ,
not necessarily involving lubricant properties.
May be o i l , water o r
synthetic ( f i r e r e s i s t a n t ) 1 iquids.
-
Hydrodynamic L u b r i c a t i o n
A system o f l u b r i c a t i o n i n which t h e shape and
r e l a t i v e m o t i o n o f t h e s l i d i n g s u r f a c e s causes t h e f o r m a t i o n o f a f l u i d f i l m h a v i n g s u f f i c i e n t p r e s s u r e t o separate t h e s u r f a c e s .
-
Hydrostatic Lubrication
A system o f l u b r i c a t i o n i n which t h e l u b r i c a n t i s
s u p p l i e d under s u f f i c i e n t e x t e r n a l p r e s s u r e t o separate t h e opposing s u r f a c e s by a f l u i d f i l m .
Initial Pitting
-
Surface f a t i g u e o c c u r r i n g d u r i n g t h e e a r l y stages o f o p e r a t i o n
o f gears, a s s o c i a t e d w i t h removal o f h i g h l y s t r e s s e d l o c a l areas and running-in.
-
Journal
That p a r t o f a s h a f t o r a x l e which r o t a t e s o r o s c i l l a t e s r e l a t i v e l y
t o a r a d i a l bearing.
Kinematic V i s c o s i t y
L,o
-
life
Lacquer
-
-
See: V i s c o s i t y .
See: R a t i n g L i f e .
Hard,
lustrous, varnish-like,
o i l i n s o l u b l e d e p o s i t which tends t o
f o r m on t h e p i s t o n s and c y l i n d e r s o f i n t e r n a l combustion engines.
Load C a r r y i n g Capacity
-
The maximum l o a d t h a t a s l i d i n g o r r o l l i n g system can
s u p p o r t w i t h o u t f a i l u r e o r t h e wear exceeding t h e d e s i g n l i m i t s f o r the particular application.
498
-
Lubricant
Any substance i n t e r p o s e d between two s u r f a c e s i n r e l a t i v e motion
f o r t h e purpose o f re du ci ng t h e f r i c t i o n o r wear between them.
-
M i l d Wear
A form o f wear c h a r a c t e r i s e d by removal o f m a t e r i a l i n v e r y small
fragments.
Non-Newtonian V i s c o s i t y
-
The ap pa ren t v i s c o s i t y o f a m a t e r i a l
i n which t h e
shear s t r e s s i s n o t p r o p o r t i o n a l t o t he r a t e o f shear.
Oil
-
A l i q u i d o f ve ge t a bl e , an i mal , m i n e r a l o r s y n t h e t i c o r i g i n f e e l i n g
sl i p p e r y t o t h e touch.
-
Oiliness
That p r o p e r t y o f a l u b r i c a n t t h a t produces low f r i c t i o n under
c o n d i t i o n s o f boundary l u b r i c a t i o n .
The lower t h e f r i c t i o n , t h e g r e a t e r
the o i l i n e s s .
-
O i l M i s t (Fog)
An o i l atomised w i t h t h e a i d o f compressed a i r and then con-
veyed by t h e a i r i n a low-pressure d i s t r i b u t i o n system t o m u l t i p l e points o f lubricant
Pitting
-
application.
Any removal o r displacement o f m a t e r i a l r e s u l t i n g i n the f o r m a t i o n o f
surface c a v i t i e s .
P l a i n B e a r in g
-
Any si mpl e s l i d i n g t yp e o f b e a r i n g as d i s t i n g u i s h e d from f i x e d -
pad, p iv o t ed -pa d o r r o l l i n g - t y p e b ea ri ngs.
Porous B e a r in g powders,
-
A b e a r i n g made f rom porous m a t e r i a l such as compressed metal
t h e po res a c t i n g e i t h e r as r e s e r v o i r s f o r h o l d i n g , o r passages
f o r supplying l u b r i c a n t .
-
Pour P o i n t
The l o we st t emp era t u re a t which a l u b r i c a n t can be observed t o
f l o w under s p e c i f i e d c o n d i t i o n s .
PTFE
-
P o l y t e t r a f l u o r e t h y l e n e , a polymer h a v i n g o u t s t a n d i n g l o w - f r i c t i o n p r o p e r t i e s ove r a wide t emp era t u re range.
PV F a c t o r
-
The p r o d u c t o f b e a r i n g p ressu re and s u r f a c e v e l o c i t y .
499
-
Rating L i f e
The f a t i g u e l i f e i n m i l l i o n s o f r e v o l u t i o n s o r hours a t a g i v e n
o p e r a t i n g speed which 90 p e r c e n t o f a group o f s u b s t a n t i a l y i d e n t i c a l r o l l i n g element b e a r i n g s w i l l s u r v i v e under a g i v e n load.
he 90 per
c e n t r a t i n g 1 i f e i s f r e q u e n t l y r e f e r r e d t o as llL,o-l i f e " or "B,o-l
Redwood V i s c o s i t y
-
ife".
A comrnerical measure o f v i s c o s i t y expressed as t h e t i m e i n
seconds r e q u i r e d f o r 50 c u b i c c e n t i m e t e r s o f a f l u i d t o f l o w through a t u b e o f 10 mm l e n g t h and 1 . 5 mm d i ame ter a t a g i v e n temperature.
Root Mean Square Hei g ht (RMS)
-
An American measure o f s u r f a c e topography
r e p r e s e n t i n g t h e average d e p a r t u r e o f a l i n e p r o f i l e of t h e s u r f a c e f r o m a mean l i n e .
SAE
-
S o c i e t y o f Automotive Engineers.
Sa y b o lt V i s c o s i t y
-
A commerical measure o f v i s c o s i t y expressed as t h e time i n
seconds r e q u i r e d f o r 60 c u b i c c e n t i m e t e r s o f a f l u i d t o f l o w through t h e o r i f i c e o f t h e Standard Sa ybo l t U n i v e r s a l Viscometer a t a g i v e n temp e r a t u r e under s p e c i f i e d c o n d i t i o n s .
-
Sc o r in g
The f o r m a t i o n o f severe s c r a t c h e s i n t h e d i r e c t i o n o f s l i d i n g .
Scratching
Scuffing
-
-
The f o r m a t i o n o f f i n e scrat che s i n t h e d i r e c t i o n o f s l i d i n g
L o c a l i s e d damage caused by t h e o ccurrence o f solid-phase w elding
between s l i d i n g su rf a ces, w i t h o u t l o c a l s u r f a c e m e l t i n g .
-
Severe Wear
A f o r m o f wear c h a r a c t e r i s e d by removal o f m a t e r i a l i n r e l a t i v e l y
l a r g e fragments.
Soap
-
I n l u b r i c a t i o n , a compound formed by t he r e a c t i o n o f a f a t t y ' a c i d w i t h a metal o r metal compound.
Solid Lubricant
-
Any s o l i d used as a powder o r t h i n f i l m on a s u r f a c e t o
p r o v i d e p r o t e c t i o n f r o m damage d u r i n g r e l a t i v e movement, and t o reduce f r i c t i o n and wear.
Spalling
-
Stick-Slip
S e p a ra t i o n o f p a r t i c l e s f ro m a s u r f a c e i n t h e form o f f l a k e s .
-
A r e l a x a t i o n o s c i l l a t i o n u s u a l l y a s s o c i a t e d w i t h decrease i n the
c o e f f i c i e n t o f f r i c t i o n as t h e r e l a t i v e v e l o c i t y increases.
500 Synthetic Lubricant
-
A l u b r i c a n t produced by s y n t h e s i s r a t h e r than by e x t r a c t i o n
o r r e fin e me nt .
-
Th in F i l m L u b r i c a t i o n
A c o n d i t i o n o f l u b r i c a t i o n i n which t h e f i l m t h i c k n e s s o f
t h e l u b r i c a n t i s such t h a t t h e f r i c t i o n between the s u r f a c e s i s d e t e r mined by t h e p r o p e r t i e s o f t h e s u r f a c e s as w e l l as by t h e v i s c o s i t y o f the l u b r i c a n t .
T o t a l A c i d Number (TAN)
-
The q u a n t i t y o f base, expressed i n terms o f t h e
e q u i v a l e n t number o f m i l l i g r a m s o f po t assium h y d r o x i d e t h a t i s r e q u i r e d t o n e u t r a l i s e a l l a c i d i c c o n s t i t u e n t s p r e s e n t i n 1 gram o f sample.
T o t a l Base Number (TBN)
-
The q u a n t i t y o f a c i d , expressed i n terms o f t h e
e q u i v a l e n t number o f m i l l i g r a m s o f p ot assium h y d r o x i d e t h a t i s r e q u i r e d t o n e u t r a l i s e a l l b a s i c c o n s t i t u e n t s p r e s e n t i n 1 gram o f sample.
Va r n is h
-
A d e p o s i t r e s u l t i n g f rom t h e o x i d a t i o n and/or p o l y m e r i s a t i o n o f f u e l s ,
l u b r i c a t i n g o i l s , o r organic c o n s t i t u e n t s o f bearing m a t e r i a l s .
Viscosity
-
That b u l k p r o p e r t y o f a f l u i d , s e m i - f l u i d o r s e m i - s o l i d substance
which causes i t t o r e s i s t f l o w . V i s c o s i t y i s d e f i n e d by t h e e q u a t i o n dv dx
q = -r/-
T i s t h e shear s t r e s s ,
v t h e v e l o c i t y , ds t h e t h i c k n e s s o f an element
measured p e r p e n d i c u l a r t o t h e d i r e c t i o n of f l o w ; t h e r a t e o f shear.
dv/ds i s known a s
V i s c o s i t y i n t h e normal, t h a t i s Newtonian sense
i s o f t e n c a l l e d dynamic o r a b s o l u t e v i s c o s i t y .
Kinematic o r s t a t i c
v i s c o s i t y i s t h e r a t i o o f dynamic v i s c o s i t y t o d e n s i t y a t a s p e c i f i e d temperature and pre ssure .
V i s c o s i t y Index ( V I )
-
A r b i t r a r y s c a l e used t o show t h e magnitude o f v i s c o s i t y
changes w i t h t emp era t u re i n l u b r i c a t i n g o i l s and o t h e r products.
Wear
-
The removal of m a t e r i a l from s u r f a c e s i n r e l a t i v e motion, n o r m a l l y by a b r a s io n , adhesion o r c o r r o s i o n .
-
Wedge E f f e c t
Wettability
-
The e s t a b l i s h m e n t o f a p r e s s u r e wedge i n a l u b r i c a n t .
A term used t o i n d i c a t e t h e ease w i t h which a l u b r i c a n t w i l l
spread o r f l o w o v e r a b e a r i n g su rf a ce.
501 ZDDP
-
I n i t i a l s f o r z i n c dialkyl-dithiophosphate, which i s w i d e l y used a s an extreme p ressu re agent.
I t i s a l s o an e f f e c t i v e o x i d a t i o n i n h i b i t o r
b u t should n o t b e used i n mechanisms w i t h s i l v e r bearings.
Acknowledgement.
Permission t o qu ot e terms and d e f i n i t i o n s r e c e i v e d f r o m
The I n s t i t u t e o f Petroleum and t h e OECD.
502
AUTHOR INDEX
Numbers u n d e r l i n e d g i v e t h e page on which t h e complete r e f e r e n c e i s l i s t e d , o t h e r numbers r e f e r t o t h e page number on which t h e a u t h o r ( o r h i s work) i s mentioned i n t h e t e x t .
221 2
Amateau,M.F.,
210
Archard,J.F.,
13
Attwood,D.G.,
3
Aubert,F.,
lo
220 222
Babichev, 17
2
Bailey,J.M.,
18
Barwell,F.T.,
1
3s
471 482 488 492 13
lo 13 2 Bickerman,J.J., 14 2 Bingham,E., 278 290 4
Beilby,G.,
Block.H.,
12
Booser,E.R., Boothroyd,G., Bosma,R.,
60
Czichos,H.,
4 10
130
Davis,T.A., Oawson,P.H.,
17 29 130
Decoufle,P.,
277
Devine,M.J.,
3
289
290 4 lo
426
Dombroski,R.M.,
1 5 10 11 130
Duckworth,W.E.,
221 fi 14 2
206
Dumbleton,J.H.,
5
12 15 28
Orasche,H.,
289
9
11 13 13
Bridges,D.C.,
428
Brown,T.W.F.,
64
25 2 8 0 Eastham,D.R.,
3 10 130 221
E l w e l l ,R.C.,
445
Engel , P . A . ,
18
Evans,F.C.,
217
Eyres ,A. R.,
275
2
Buckley,D.H.,
4
Bunshah,R.F.,
220
, 130
277
28
79 221
222
428 433
428 445
Davies,A.E.,
Dowson,O.,
2
30 427
492
130
47 49 78 131
Braithwaite,E.R.,
Burke ,A. E.
474
Dobychin,M.N.,
Brainard,W.A.,
Bryce,J.B.,
24
79
Bowden,F.P., Bowen,E.R.,
lo
Crooks,C.S.,
2 23 2 15 2 60 79 461
130
284 290
De Vos,H., 281
7
Blackwell ,J.,
3
Cree,J.C.,
2
14 15 17 25
Bates,T.R.,
Chynoweth,A.,
Collacott,R.A.,
7
Bartenev,G.M.,
fi 52 78 130
5
Clausen,J.,
329
Balmer,C.,
Churchill,J.R.,
Cameron,A.,
130 78
47
3
221
222
fi
Fein,R.S.,
5
Fidler,F.,
17 2 9 130
445
503 Forrester,P.G.,
130 131 208
?21
'0
F o r s y t h , I., 5
Kilbourn,D.F.,
Fowle,T. I., 184 193
196
King,M.,
4 5 58 72 75 78 79 131
Gass,H.,
220 222
Glasser,W.A., Godet,M.,
4
Kruschov,M.M.,
17
9
Lancaster,J.K.,
6
11 217 221
Halling,J.,
330 353 370
lo 221 222 5 1 1
3
Hancock,B.T., Harvey,B.F.,
5
7
11 220 222
2
Hjertzen,D.G.,
132
H o l l igan,P.T.,
9
Hother-Lushington,S.,
Jahanrnir,S.,
13
2
2
2 2
72
Jones,M.H.,
471
3
Justusson,W.M.,
2 lo 1 3
Lund,J.W.,
50 52
to 222 206
28
2
45 47 58 72 75 78 79
Mathieson,T.C.,
429
445
425 7
2 23 2
29
Mear,R.B.,
130
Mear5,D.C..
28
30 485
492
395 426
--
Mills,G.H.,
21 25 27 29 30
Milne,A.A.,
15
3
Ming Feng, I., 13 15 28 Moes,H., 60
2
Moore,M.A.,
221
27
78 277
Merrett,J.G.,
2 3 9 10 11 395 425 426 220
Ludema,K.C.,
Meckleburg,K.R.,
5
Jones,G.J.,
49
130
130 21
McDonald,D.,
132
Johnson,K.L.,
290
McCullagh,P.J.,
a
20
Jarvis,R.A.,
3 8
McCoy,W.E.,
Jarnes,R.D., Jarnieson,D.T.,
Ling,F.F.,
Martin,F.A., 17
130
Hunter,M.S.,
278
Lushbaugh,C.C.,
278 290
Horton,A.W.,
Leitch,A.,
Loy,B.,
Hinterrnan,H.E.,
2 2 lo
75
Lee,C.S.,
Love,P.,
130 14
221
206
Lloyd,K.A.,
2
5
Higginson,G., Hiley,R.W.,
11
7
Leak,D.A.,
Linkinhoker,C.L.,
9
465
223 267
Laventev,V.V.,
Lehrke,W.D.,
445 fl
428
Hastings,G.W., Hibbs,L.E.,
Lansdown,A.R.,
2
20
Halligan,B.D.,
Hirst,W.,
222
220
Grunberg,L.,
Juntz,R.S.,
lo lo 14 9
4
K r a g e l s k y , I .V.,
2
18
Gregory,J.C.,
Jost,H.P.,
426
130
Godfrey,D.,
Jones,M.P.,
3
13
Knight,R.E., Kornbalov,V.S.,
lo 210 221 4
Georges,J.M.,
428
lo
4
Kirk,J.A., Garner,D.A.,
266 445
Kennedy-Srnith,R.,
17
9
Morris,J.A.,
130
Murray,P.W.,
395
425
504 Neale,M.J.,
lo 31 131
2
Newman,A.O.,
64
Nicholson,O.W., Nonnen,F.A.,
80
Smal lheer,C.V.,
210 221
Smart,E.F.,
lo
2
Smi th,A. I . , Soul ,D.M.,
130 P h i l l i p s , R . , 206 221 Pinkus,O., 52 78
242
Spencer,J.B.,
222 291 329
Sridharan,P.,
13
Spalvins,T.,
Perkins,C.A.,
443 445
Pocock,G.,
130 266 468 470 216 221
Shone,E.B.,
79
221
Sternlicht,B.,
130 217 221 Przbyszewski,J.S., 221 222
Pratt,G.C.,
Summers-Smith,O.,
Tabor,D., 2
Rabinowicz,E.,
284
Thony,C.,
-
454 470
Rollason,E.C.,
Ruff,W.,
25
Ryman,F.O.,
479
Tourret,R.,
23
3
221 221 446
216 215
492
-
78
Van Peteghem,T.,
-
278 289 279
Walsh,W.,
1 3 4 5 7 -10 11 13 14 15 17 18 20 21 23 24 25 27
-
3
205 211 212 216 218 221 488 492 -
3
Sharp,W.F.,
219
222 221
222
290
lo
Welsh,N.C.,
131
18 19
15
2
2
465
470
425
Westcott,V.C.,
3
289
Waterhouse,R.B.,
West,C.H.,
290
445
279
Warriner,J.F.,
Wentorf ,R.E.,
-
14 18 20 29
Sherbiney,M.A.,
277
Wahlberg,J.E.,
2 10 426
281 428
Wagner,W.D.,
2
Scott,H.H.,
452 464
468 470
lo
14
2
Venton,A.D.F.,
Schofield,J.,
Shen,C.,
290 lo
283
30
Sanderson,J.,
Scott,O.,
78
Tilley,G.P.,
Trent,E.M.,
49
Scala,R.A.,
lo
52
Tremain,G.R.,
130
Roylance,B. J.,
Saka,N.,
3
130
Roehner,T.G.,
5 1_1 4 5
Thomson ,K.K.,
Richardson,R.C.D.,
Rose,A.,
Thomas,T.R.,
fi
5
Reynolds,O.,
222
221
Tevazwark,J.L.,
290
8
13
8
221 130
216
Teer,D.G.,
2
281
12 15
Taylor,C.M.,
130
Rastogi ,S.C., Reda,A.A.,
Tait,J.,
3 13 a
3
Rafique,S.O., Rapp,C.,
8
221
29
13
Swanson,T.D.,
15
216
130
Quayle,J.P.,
Raask,E.,
78
52
to 13
1 4
Suh,N.P.,
4 9
13 15 24
25 28 29 30
505
-
428 445
Wilkinson,H.C., Wilks,P.E.,
425 425
W i 1 1 iams,G., Williams,J.E.,
454 470 fi 80 130 218 222 205 221 23 9
7
Wilson,R.W., Woldman,N.E., Wright,E.P.,
17 18
Wright,K.H.R.,
Zingmark,P.A.,
Zorev,N.N.,
3
2
221
206
Zakay,V.F.,
Zlatin,L.,
11
5
Williams,D.F.,
281
290
lo
-
448 470
506 f r i c t i o n m o d i f i e r 246 264 hazards 275-290 head 280
SUBJECT INDEX
metal c u t t i n g 262 467 metal d e a c t i v a t o r 247 pour p o i n t depressant 247 257 r u s t i n h i b i t o r 246 254
Abel method 244
t a c k i n e s s 247
Abrasion
v i s c o s i t y index improver 247 250 255
d e f i n i t i o n 17 493 economics 3 Ab r a s iv e Wear 3 12 17 c u t t i n g t o o l s 214 461 467
Adhesion wear t h e o r y 13 493 A i r entrainment 347
A1 ignment, gears 184-187 Almen Weiland t e s t 261
damage 17
Amides 250
e f f e c t o f hardness 17
Analysis
gears 194 202
Auger e l e c t r o n spectroscopy 4
p a r t i c l e s 296
i on spectroscopy 4
p l a i n b e a r in gs 81 97 207
microprobe 4
r e s i s t a n c e 206 213
scanning e l e c t r o n microscopy 4
r o l l i n g b e a r i n g s 173
s p e c t r o g r a p h i c o i l 8 24 439
s e a l s 334 374
x r a y photon
4
Accelerometers 434
A n t i c h a t t e r a d d i t i v e 265
A c i d i t y 244
A n t i c o r r o s i o n a d d i t i v e 276
A c i d tr e a tm e n t 275
A n t i foam a d d i t i v e 247
Add i t ives
A n t i o x i d a n t a d d i t i v e 232 246 251 252
276
a n t i c h a t t e r 265 a n t i c o r r o s i o n 276
A n t i squawk a d d i t i v e 265
a n t i foam 247
A n t i wear a d d i t i v e 231 246 260 276 281
a n t i o x i d a n t s 232 246 251 252 276 a n t i squawk 265 a n t i wear 231 246 260 276 281 493 b i o c i d e s 276 289 c o r r o s i o n i n h i b i t o r 246 251 253 254
495 d e t e r g e n t s 243 245 246 249 276 495 d i s p e r s a n t s 243 249 251 276 495 e m u l s i f i e r s 247 255 264 276 extreme p r e s sure 231 247 258 259
276 467 474 496 e f f e c t on gears 191 193 200 201 e f f e c t on p l a i n b e a r i n g s 1 1 2 e f f e c t on s c u f f i n g 15
493 Area o f c o n t a c t a pparent 452 493 r e a l 452 493 A r t h r i t i s 28 Asbestos packings 377 378 h e a l t h 393 A s p e r i t i e s 493 r o l e i n wear 13 A t t r i t i o n 465 Aus f orming 206
507 B a c t e r i a 282 289 Bad bonding 89 B a r r i e r cream 286 Bearing area 494 materials
Bear ings p l a i n journal heat balance 57 l u b r i c a t i o n requirements 402 m a t e r i a l s e l e c t i o n 43
aluminium a l l o y 83 87
maximum p r e s s u r e 43
aluminium l e a d 210
maximum temperature 71
aluminium t i n 83 210
minimum c l e a r a n c e 65
b a b b i t 82 493
non l a m i n a r 72
copper l e a d a l l o y s 83 86 208 474
performance 39 65
l e a d bronze 83 86 208
plastic
n y l o n 217
porous 32 88
7 217
o v e r l a y 82 84 208
power loss 67 70
phosphor bronze 83 88
p r o f i l e bore 72
p l a s t i c 217
pv f a c t o r 34 498
p o l y a c e t a l 218
s p e c i f i c l o a d r a t i n g 66
porous 88 498
s u r f a c e f i n i s h 47
p t f e 215-218
r o l l i n g element
s i l i c o n bronze 88
a p p l i c a t i o n s 166
s i l v e r 88
c l e a n i n g 171
t h e r m o p l a s t i c 217
c 1 earances 144
thermoset 217
c o r r e c t i o n f a c t o r 146
t r i m e t a l 82 88
damage see f a i l u r e s
w h i t e metal 82 208 Bearings
dimensions 144 d i s m a n t l i n g 168
f l e x u r e 32 36 42
f a t i g u e l i f e 35 139
h y d r o s t a t i c 35 37
f i t s 163
magnetic 32
f r i c t i o n 150
p l a i n journal
grease 152
b e a r i n g l o a d c a p a c i t y 54 61
l o a d c a r r y i n g c a p a c i t y 139
c l e a r a n c e 65
l u b r i c a t i o n 151 151
d e s i g n l i m i t s 46
m a t e r i a l 211
e f f e c t o f bearing temperature 48 60 g r o o v i n g 56 misalignment 67 o i l f i l m s t a b i l i t y 50 64 o i l f i l m t h i c k n e s s 60 o i l o x i d a t i o n 50 s u r f a c e roughness 47 f a i l u r e s see f a i l u r e s f l u i d f i l m 35
mounting 167 n o i s e 159 o i l m i s t 158 o i l s e l e c t i o n 156 performance 39 s e a l s 157 s e l e c t i o n 132 shock p u l s e 163 speed l i m i t 1 4 5 148 149
508 type
a x i a l p i s t o n pump 298
angular contact 134
gear pump 298
b a l l t h r u s t 138
r a d i a l 45 67
c y l i n d r i c a l r o l l e r 135
r a t i o 494
double row a n g u l a r c o n t a c t 134
spool v a l v e 301
d o u b l e row r o l l e r 135
vane pump 298
double row s p h e r i c a l 137
C l e v e l a n d method 244
n e e d l e 136
Cloud p o i n t 235
s e l f a l i g n i n g 133
Cold c r a n k s i m u l a t o r 255
s i n g l e row deep groove 133
Composites
s p h e r i c a l r o l l e r t h r u s t 138
bearing design data 6
t a p e r r o l l e r 136
bearing material 6
r u b b i n g 34
b e a r i n g performance 6
l i f e 140
m a t e r i a l a p p l i c a t i o n 207
m a t e r i a l s e l e c t i o n 210 pv f a c t o r s 41
Compression p a c k i n g see Seals C o n d i t i o n m o n i t o r i n g 427-445
B e i l b y l a y e r 454
Conradson method 244
Beta r a t i o 307
Consumption o i l 267
Bioc i d e s 282 289
C o n t a c t a r e a 184 452 493
Boroscope 437
Contaminant a n a l y s i s 438
Brake m a t e r i a l
7
Contamination
B r i n e l l i n g 178 494
b u i l t i n 292 294
B r o o k f i e l d v i s c o m e t e r 255
c o n t r o l 315
B u i l t up edge 454
e f f e c t on d i r e c t i o n v a l v e s 301
Bypass see f i l t r a t i o n
f l o w c o n t r o l s 302 g e a r s 194
Cams 216
l u b r i c a n t s 234
C a r b u r i s i n g 210 216 219
motors 300
C a r c i n o g e n i c compounds 279
p r e s s u r e c o n t r o l 302
Case h a r d e n i n g 210 219
pumps 297
Cavitation
v a l v e s p o o l s 301
e r o s i o n 101 117-121 397 494
e n v i r o n m e n t a l 294
m a t e r i a l r e s i s t a n t 10 19
g e n e r a t e d 296
s u r f a c e damage 19
l e v e l s 304
Centre l i n e average 494
seals
Ceramics 6 213
s p e c i f i c a t i o n 305
Channeling 494
sources 294 315
C h e l a t i n g agents 254 Chemical vapour d e p o s i t i o n Chromium p l a t i n g 216 Clearance c r i t i c a l 297 299
347
Coolant
7 220 464
467
Copolymers 250
509 Corrosion
Dye p e n e t r a t i o n 436
7
ceramics
Dynamic l o a d r a t i n g 139
copper l e a d a l l o y s 107 i n h i b i t o r 251 253 254
E c c e n t r i c i t y r a t i o 46 51 495
p l a i n b e a r i n g s 102-117
Eddy c u r r e n t t e s t 437
r o l l i n g b e a r i n g s 181
E l a s t o hydrodynamic l u b r i c a t i o n 5 495
r e s i s t a n c e 102-117 see a l s o f a i l u r e s C u t t i n g f l u i d s 6 262 C u t t i n g speeds 449 451 C u t t i n g t o o l s 215 a lu m in iu m o x i d e 450 464 boron n i t r i d e 450 465
polymers 217 traction 5 E l e c t r i c discharge 119 181 r e s i s t a n c e method 437 E l e c t r o chemical machining 450 d i scharge machining 450 Embeddabi 1 it y 81
ceramic 215 464
Emulsions 237 255 282 318
cemented c a r b i d e s 215 450 464
Engine t e s t s 472
c o s t s 451
Eros i o n
h i g h speed t o o l s t e e l 451 464
b e a r i n q damage 117-121
m a t e r i a l for 450 464
ceramics
s i l i c o n c a r b i d e 450
economics 2
wear o f
7 215 459-467
7
p r e s s u r e c o n t r o l 302 steam 38
Debris a n a l y s i s 24 419 c u t t i n g wear 25 26
wear 12 496 see a l s o f a i l u r e s Extreme pressure a d d i t i v e 232 258 259 496
f a t i g u e 27 440
c u t t i n g o i l s 262
la m in a r wear 25 440
e f f e c t on s c u f f i n g 14
r u b b i n g wear 25 26
e f f e c t on p l a i n b e a r i n g s 112
s p h e r i c a l wear 25
e f f e c t on gears 191 193 200 201
s y n o v i a l f l u i d 28 Del ami na t i o n t h e o r y 13 De lp h i a n a l y s i s 428
t u r b i n e o i l s 262 see a l s o a d d i t i v e s Eye i r r i t a t i o n 280
Dermat it i s 277 D e t e r i o r a t i o n l i m i t 444 De t e r g e n t s 243 245 246 249
Fa i 1 u r e s Gears
D i s p e r s a n t s 243 249 251
a b r a s i v e wear 194 202
D i t h i o c a r b o n a t e s 253
alignment 184-187
Di th io p h o s p h a te s 252 253
broken t e e t h 197
Drawing 263
f a t i g u e p i t t i n g 189 197 198 474 498
D r i l l i n g 263 Drop p o i n t 495
foaming 204
Duty parameter 495
f r e t t i n g 202
510 g a l l i n g 191 497
see a l s o f a i l u r e s
g r o o v i n g 188
F a u l t t r e e 429
n o i s e 203
Ferrography 9 25 439 442
o v e r h e a t i n g 203
F i l l e r 496
r i d g i n g 188 474 475
Filtration
r i p p l i n g 201 474 476
e f f e c t o f f i r e r e s i s t a n t f l u i d s 318
s c o r i n g 476
emulsions 318
s c u f f i n g 191 199 474 476
magnets 319
v i b r a t i o n 203 p l a i n bearings a l l o y i n g 125
o f f l i n e 317 324 p r e s s u r e l i n e 316 320 pump i n l e t 318 327
bad bonding 89
r e t u r n l i n e 317 322
bad f i t t i n g 96
s e a l s 347
c a v i t a t i o n 117-121
Fi 1t e r
c o r r o s i o n 102-117
a b s o l u t e r a t i n g 306
e l e c t r i c a l d i s c h a r g e 119
a i r b r e a t h e r 294 295
e r o s i o n 117-121
b e t a r a t i o 307
e x c e s s i v e l e a d 91
bubble t e s t 306
extraneous p a r t i c l e s 97-101
bypass 308 309 320
f a t i g u e 123
c l a s s i f i c a t i o n 307
gas c a v i t i e s 91
d i r t c a p a c i t y 313
o v e r s i z e cuboids 91
e f f i c i e n c y 310 321
thermal c y c l i n g 125
gears 194
v i s c o s i t y 126
l o c a t i o n 318
w i r e wool 122
mean r a t i n g 306
r o l l ing bearings
m u l t i p a s s t e s t 307
a b r a s i v e 173
nominal r a t i n g 305
cage 175
o f f l i n e 317 324
c o r r o s i o n 181
p r e s s u r e l i n e 316 320
e l e c t r i c c u r r e n t 181
p u l s a t i n g f l o w 308
f a l s e b r i n e l l i n g 178
r e t u r n l i n e 317 322 324
i n c o r r e c t mounting 174
s e l e c t i o n 305-317
smearing 177
s i l t c o n t r o l 308 312
v i b r a t i o n 178 wear 172 Falex t e s t 261
s i z i n g 312 F i r e r e s i s t a n t f l u i d 234 318 F l a m m a b i l i t y 234
False b r i n e l l i n g 178
F l a s h p o i n t 235 244 496
Fatigue
F1ash temperature 496
f r e t t i n g 18 p l a i n b e a r i n g s 123 r o l l i n g c o n t a c t 20
F l e x u r e b e a r i n g 32 36 496 performance 42 Flow zone 456
511 F l u i d e r o s i o n 19 Flushing 292 294
Hazards a d d i t i v e s 280
Flux t e s t i n g 437
b a c t e r i a 282 289
Four b a l l machine 232 261 485 487
b i o c i d e s 282 289
Frequency spectrum 435
d e r m a t i t i s 277
Fretting
eye i r r i t a t i o n 280
d e f i n i t i o n 18 496
h e a l t h 545
gears 202
o i l m i s t 277 287
wear d e b r i s 18
o i l vapour 278
see f a i l u r e s
r e c l a i m e d o i l 283
F r i c t i o n m o d i f i e r 264
r e - r e f ined o i 1 283
F r i c t i o n w e l d i n g 452
s c r o t a 1 cancer 279
FZG t e s t machine 261 478
s k i n cancer 278 s y n t h e t i c l u b r i c a n t s 283
G a l l i n g 191 497
H e a l t h and S a f e t y a t Work A c t 284
Gears 184-204
Holography 437
a l ignment 184
H y d r o s t a t i c b e a r i n g 32 34 35
c o n t a c t a r e a 184 f a i l u r e s see f a i l u r e s
IAE t e s t 261
f a t i g u e p a r t i c l e s 25
l n f r a r e d technique 437
f i l t r a t i o n 194
I n s p e c t i o n techniques 436
l u b r i c a t i o n systems 195
I r o n p r i n t i n g 99
m a t e r i a l s e l e c t i o n 210 o i l v i s c o s i t y f o r 193 194
Jost report 1
s c u f f i n g 191 199 t e s t s 261 478
K u r t o s i s 429
Graphite 239 240 242 380 Grease a p p l i c a t i o n 396
Lacquer 472 474 497 Lubricant
c l a s s i f i c a t i o n 399 497
a n t i f r i c t i o n p r o p e r t i e s 231
r o l l i n g b e a r i n g 152
a n t i wear p r o p e r t i e s 231
s e l e c t i o n 224 396
e s t e r s 235
temperature l i m i t s 224 4 0 1
extreme p r e s s u r e p r o p e r t i e s 231
Grooving 188
l i m i t s o f o i l 224
Hardenab i 1 i t y
l i m i t s o f s o l i d 224
l i m i t s of grease 224 e f f e c t o f chromium 211
m i n e r a l o i l 226
e f f e c t o f vanadium 211
polyphenol e t h e r s 236
Hardness
s e l e c t i o n 223-241 400 467
t o o l s t e e l 214
s i l i c o n e s 235
e f f e c t on f a t i g u e s t r e n g t h 207
s o l i d 239
512 v i s c o s i t y 226 229 L u b r i c a t i o n systems
c u t t i n q t o o l t e m p e r a t u r e 456 d i f f u s i o n 462
c o o l i n g 418
e f f e c t o f h e a t 451
d u a l l i n e 409
effect
o f s l i d i n g 458
d i r e c t f e e d 403
e f f e c t o f speed 451
d i r e c t p o s i t i v e 414
e f f i c i e n c y 446
gas 239
f e e d 448
g e a r s 195
f l a n k wear 447
i n d i r e c t l i n e 405
f l o w zone 455
m i c r o f o g 419
h e a t g e n e r a t i o n 451
o i l 413
l u b r i c a n t s 467
p o s i t i v e s p l i t 415
q u i c k s t o p method 447
p r o g r e s s i v e 408
r a k e 447
s e l e c t i o n 403 412
s e i z u r e 452 458
t o t a l loss 413
s u r f a c e s h e a r i n g 461 t e m p e r a t u r e g r a d i e n t 457
Machine e l e m e n t s
tool force
h i g h e r p a i r 482 483 l o w e r p a i r 482 483 M a i n t e n a n c e o n c o n d i t i o n 427-445
448
t o o l wear 459 Molybdenum d i s u l p h i d e 239 240 242 Monitoring techniques
M a g n e t i c b e a r i n g 32
c o n t a m i n a n t a n a l y s i s 8 429 433 438
M a g n e t i c p l u g 24 439 440
s p e c t r u m a n a l y s i s 434
Mater i a 1
t r e n d a n a l y s i s 433
a b r a s i o n r e s i s t a n c e 206
v i b r a t i o n a n a l y s i s 8 429 432 433
c o m p o s i t e 206
w a v e f o r m a n a l y s i s 436
c o r r o s i o n r e s i s t a n c e 206 e l e v a t e d t e m p e r a t u r e 206 207 f i b r e r e i n f o r c e d 206 207 p l a s t i c bearing
7
s e l e c t i o n 205-222 Metal c u t t i n g
Niemann FZG t e s t 261 N i t r i d i n g 210 N o i s e r o l l i n g b e a r i n g s 159 Non d e s t r u c t i v e t e s t i n g 436 dye p e n e t r a t i o n 436
a b r a s i o n 461
e d d y c u r r e n t 436
a t t r i t i o n 465
e l e c t r i c a l r e s i s t a n c e 436
b u i l t up edge 454
f l u x 436
chip
4 4 7 454
r a d i o g r a p h i c 436
c l e a r a n c e a n g l e 447
u l t r a s o n i c 436
c l e a r a n c e f a c e 447 c o o l a n t s 467
O i l d e g r a d a t i o n 270
c r a t e r wear 460 c u t t i n g edge 447 c u t t i n g speed
447 451
f i l m i n s t a b i l i t y 50 64 f i l m w h i r l 72 m i s t 158 277 287 498
513 o x i d a t i o n l i m i t 50
Re-ref i n i n g
s t a n d a r d t e s t s 244
a c i d c l a y 272
vapours 278 287
economics 273
O i l i n e s s 498 Ov e r la y b e a r i n g s 208 O x i d a t i o n r e s i s t a n c e 6 207
Matthys process 273 Ri dg i ng gears 188 474 475 R i p p l i n g gears 201 474 496 R o l l i n g bearings see b e a r i n g s
Packed g l a n d 370 Particles t y p e o f wear 24 440 e f f e c t on f l u i d power systems 296 P a r t i c l e c o u n t i n g 439 Patch t e s t 439 Pensky M a r t i n Method 244 Phenates 248 Phenoles 252 Phosphonates 245 Piston r i n g
Rolling contact fatigue c r a c k p r o p a g a t i o n 23 damage 20 d e f i n i t i o n 20 e f f e c t o f environment 20 e f f e c t o f l u b r i c a n t 23 e f f e c t o f m a t e r i a l 23 p a r t i c l e s 27 Root mean square h e i g h t 499 Rubbing bearings see b e a r i n g s Rust i n h i b i t o r 246 254 Ryder t e s t machine 261
m a t e r i a l s e l e c t i o n 215 wear 216 P i t t i n g 189 197 198 474 498 P l a i n b e a r i n g s see be ari n gs Plastics
7
Porous b e a r i n g s 88 498 Pour p o i n t 235 244 257 498 P r o f i l e b o r e b e a r i n g s 72 PTFE 6 216 239 240 242 Pumps e f f e c t o f contaminant 297 298
S a f e t y l u b r i c a n t 275-290 Sal i c y l a t e s 249 S a ybo lt v i s c o s i t y 499 S co ri ng 476 499 S c u f f i n g 14 cams and tappets 216 e f f e c t o f extreme pressure a d d i t i v e 15 fatigue particles gears 191 199 474 476
PV f a c t o r 34 498
i n c i p i e n t 15
Ra d ia l c l e a r a n c e 45 144
p i s t o n r i n g s 215
Rad io g r a p h ic t e s t 437
s u r f a c e changes 1 5
mechanism o f 14 499
Ramsbottom method 244
Seal l u b r i c a t i o n 356 357
R a t i n g l i f e 499
Sea 1 s
Reaming 263
compression packings 370-394
Reclaimed o i l s a f e t y 283
a b r a s i v e wear 374
Reclamation 9 271
c o m p r e s s i b i l i t y 381
Redwood v i s c o s i t y 499
c o s t 372 387
R e l i a b i l i t y a n a l y s i s 431
f a u l t s 390
514 f i t t i n g 388
lobed 340
f o r pumps 371
0
f o r v a l v e s 371
s i n g l e a c t i n g 340
g la n d d e s ign 374 375 env i ronmen t 384
331 340
Seals r o t a r y l i p 353-369 absorbed power 358
h e a l t h a s pe ct s 391
design 354
i n s t a l l a t i o n 372
e c c e n t r i c i t y 359
leakage 382
f a u l t f i n d i n g 362
ma i ntenance 372
f i t t i n g 361
m a t e r i a l s 376-382
f r i c t i o n 358
packed g la n d 370
hand1 ing 361
r e l i a b i l i t y 372
m a t e r i a l s e l e c t i o n 367-369
r o l l i n g b ea ri ng s 157
maximum pressure 359
s e l e c t i o n 383
maximum speed 359
s h a f t wear 372
s e r v i c e problems 362-365
speed 385
s h a f t s u r f a c e 357
s t a n d a r d i s a t i o n 391 s u r f a c e f i n i s h 375 temperature 386 Seals l i p 338 339 assembly 351
storage 360 Seals s t a t i c compression packing 353 370-394 gasket 341
1 i p 341
c o n t a c t a r e a 338 345
r a d i a l f a c e mechanical 353
e f f e c t o f d e forma t i o n 351
squeeze 341
h u m i d i t y 351
S h e l l f o u r b a l l machine 232 262 485 387
1 i g h t 351
Shock p u l s e measurement 163
oxone 351
S i l t i n g 303
oxygen 351
S i l t removal 308 312
temperature 350
Skin cancer 278 288
energ is e d 345
Skin p r o t e c t i o n 285
f r i c t i o n 344
S o l i d l u b r i c a n t s 6 239 499
leakage 339
S o l vent e x t r a c t i o n 276
m a t e r i a l s e l e c t i o n 331-338
Soap 499
m u l t i l i p 343
S p a l l i n g 213 499
performance 343
Spectroscopy
s e l e c t i o n s 348
Auger e l e c t r o n 4
squeeze 338 339
x r a y photon 4
s to r a g e 350
scanning i o n 4
s u r f a c e roughness 343
ion s c a t t e r i n g 4
Sea 1 s r e c i p r o c a t i ng
Sp ectrographic o i l a n a l y s i s 8 24 439
double a c t i n g 340
Spectrum a n a l y s e r s 434
e n e r g is e d s le eve 340 346
S t i c k s l i p 467 499
515 S t r i b e c k c u r v e 227
S h e l l f o u r b a l l 232 261 485 487
Succinimides 250
Timken 261 488
Sulphonates 245
Thermography 437
Surface f i n i s h
Th iophosphonates 245
e f f e c t on o i l f i l m 48 Surface t r e a t m e n ts
T hre shold l i m i t value 277 T oo l s see c u t t i n g t o o l s
b o r o n i s i n g 219
T o t a l a c i d number 500
c a r b u r i s i n g 210 216 219
T o t a l base number 500
case h a r d e n in g 210 219
T o x i c i t y 235 276 280 282
chemical vapour d e p o s i t i o n 7 220 464 chromosing 219
amines 281 c h l o r i n a t e d napthalenes 281 l e a d compounds 280
chromium p l a t i n g 216 e l e c t r o d e p o s i t i o n 219 flame h a r d e n ing 219 h a r d s u r f a c i n g 219 ion implantation 7 io n n i t r i d i n g 221 i o n p l a t i n g 7 221 molybdenum p l a t i n g 216 n i t r i d i n g 219 220 Noskuff 220
o r t h o phosphates 281 sod ium mercaptobenzothiazole 282 sodium n i t r i t e t r i c h 1 o r o e t h y l ene 282
Trend a n a l y s i s 444 T r ib o l ogy handbook 2 10 module 1 report 1 2 units 1
phosphating 219
T u rbi ne o i l 262
plasma spray 219 s h o t peening 219
U l t r a s o n i c t e s t 437
s i l i c o n i s i n g 219 s u i f BT 219
V i b r a t i o n a n a l y s i s 8 434
Sulphinuz 220
Viscosity
Synovial f l u i d 28
a b s o l u t e 226 600
S y n t h e t i c l u b r i c a n t s 276 500
index 229 500 index improver 229 255
Takiness 247
k i n e m a t i c 226 500
Tappet m a t e r i a l 216
p ressure r e l a t i o n s h i p 231
Tes t machines
temperature r e l a t i o n s h i p 229 401
Almen Weiland 261 485 486 h s l e r 488
Waveform a n a l y s i s 436
c r o s s c y l in d e r 488 Cygnus 485 486
Wea r
Falex 261 486
a b r a s i v e s 194 202
I A E 261 478 Neimann FZG 26
Water based f l u i d s 247 255 264 467
adhesive 12 17 478
chemical 12
516 c u t t i n g t o o l s 215 459 d e b r i s a n a l y s i s 8 24 429 mechanical 12 p i s t o n r i n g 215 r e s i s t a n c e t o a b r a s i o n 213 r e s i s t a n t m a t e r i a l s 213 W e t t a b i l i t y 500 White m e t a ls 82 208 Wire wool f a i l u r e 122
ZDDP 501
