Sn ap -Fit Design Manual
A s s e m b l y
Table of Conte Contents nts
Topic
Pa r t
Intr Introduction oduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intr Introduction oduction
Snap Snap-F -Fit it Design Applic Applications ations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
Types of Snap-Fits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II
Snap-F Snap-Fit it Beam Beam Design Using Classical Classical Beam Beam Theory . . . . . . . . . . . . . III
Improve Improved d Cantileve Cantileverr Snap-F Snap-Fit it Design D esign . . . . . . . . . . . . . . . . . . . . . . . . . . IV
U & L Shaped Snaps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V
Gener General al Design Design Guideline Guideliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI
English/Metr nglish/Metric ic Conver Conversion sion Char Chart . . . . . . . . . . . . . . . . . . . . . . Inside Inside Back Back Cov Cover
Part I Snap-Fit Design Applications Why use snap -fits? This chapter chap ter w ill give you you a thumbnail sketch of the benefits of snap-fits and the materials materials used to make them.
UNDERCUT
Snapnap -fits fits are the simplest, quickest and mo st costeffectiv effectivee metho d of assembli assembling ng two p arts. When designed designed prop erly, parts with snap-fi snap-fits ts can be assembled assembled and disassembled disassembled n umerous times without any adverse adverse effect on th e assembly. assembly. Snapnap -fits fits are also also the most m ost environmen environ mentall tally y friend friend ly form of assembly because of their ease of disassembly disassembly, making making co mpon ents of different materials easy to recycle.
REQUIRES SLIDE IN M OLD
Although snap-fits can be designed with many materials, the ideal material is thermoplastic because of its high flexibility flexibility and its abil ab ility ity to b e easil e asily y and inex pe nsively nsively molded into complex geometries. geometries. Other advantag advantages es include its relatively relatively high elongation , low coe fficient fficient of friction, friction, and suffici sufficient ent strength and rigidity rigidity to meet th e requirements o f most app lications. lications.
SLOT
NO SLIDE REQUIRED
The designer should be aware that the assembly assembly may may have some “play “play”du ”du e to tolerance stack-up stack-up of the two mating parts. Some snap -fits fits can also increase increase th e cost of an injection injection mo lding lding tool due to th e ne ed for slides slides in the mold. An exp erienced designer can often eliminate eliminate the need for slides by adding a slot in the wall directly below the undercut or by placing the snaps on the edge of the part, so the y face face outw ard (see Figure Figure I-1). I-1).
NO SLIDE REQUIRED, MOLD LESS COMPLEX
Figure I-1 I-1
I-1 I1
S n a p - F i t
D e s i g n
A p p l i c a t i o n s
Part I: Snap-Fit Snap-Fit Design Applica Applica tions Concludin g poin ts: Snapnap -fits fits solve the problem pr oblem o f creating an an inexpe nsive nsive comp onen t that can be q uickly uickly and easily easily joined joined w ith another p iece. Thermop lastics lastics are the ideal material for snap-fits because they have the flexibility and resilience necessary to allow for numerous assembly and disassembly operations.
Door handle bezel
Backside of bezel
Detail of backside of bezel, cantilever design I-2 I2
Partt I I Par Types of Snap-Fits This chapter provides an overview of the different types of cantilever snap-fi snap-fits ts and gives an idea of when wh en the t hey y are used.
When designing de signing a cantilever snap, it is is not unu sual for the designer designer t o go th rough several iterations iterations (changing length, thickness, deflection dimensions, dimensions, etc.) to design design a snap-fit snap-fit with a lower allowable strain for a given mate rial.
Most engineering material applications with snap-fits use the cantilever design (see Figure II-1) and thus this manual will focus on o n th is design. design. The cylindrical cylindrical design design can b e employ emp loyed ed when w hen an unfilled unfilled thermoplasti ther moplasticc material with higher elongation will be used (a typical application is an aspirin bottle/cap bottle/ cap assembly assembly). ).
Other types of snap-fits which can be used are the “U” or “L “L”sh aped cantilever snaps sn aps (see Part V for more detail det ail). ). These are used w hen the strain strain of th e straight straight cantilever cantilever snap can n ot be designed designed b elow the allowabl allowablee strain strain for the given material. Concludin g poin ts: Most applications can employ a cantilever cantilever type snap-fi snap-fitt in the design. design. In applications applications with tight packaging packaging requirements, the “U”or “U”or “L”shape d snap may be required.
Y
CANTILEVER
“U” SHAPED CANTILEVER
Automotive oil filter snaps
“L” SHAPED CANTILEVER
Figure II-1
Cordless screw driver dri ver housing, cantileve cantil ever snap-fi snap-fit t
II-1
Partt II I Par Snap-Fit Beam Design Using Classical Beam Theory A design engineer’s job is to find a balance between integrity of the assembly and strength of the cantilever beam. While While a cantilev cantilever er beam with a deep overhang can make the unit secure,it also puts more strain on the beam dur ing assembly assembly and disassembly. disassembly. This chap ter explains how this balance is achieved.
MATING FORCE
P R
α'
W
α
A typical snapsnap -fit fit assemb ly consists of a cantilever b eam with an overhang at the th e end en d of the th e beam be am (see Figure Figure IIIIII-1). The depth of the overhang defines the amount of deflection during assembly. R
FRICTION CONE
}
P
ENTRANCE ENTRANCE SI DE
α+ β W
β
α
RETRACTION SIDE
Friction riction Coe fficient µ = ta tan β
Mating Force = W W = P tan (α + β)
OVERHANG DEPTH
µ + ta tan α —— —— —— — W =P— 1–µ tan tan α
Figure III-1
Figure III-2
The overhang typically typically has a gentle ramp on the entrance side and and a sharper angle on the retraction side. side. The small small angle angle at the entrance side side ( α) (see Figure III-2) helps to reduce the assembly effort, effort, while the sharp angle angle at the retraction side ( α') m akes disassembly ver very y difficult difficult or impossible impossible depending on the intend ed function. Both the assembly and disassembly force can be optimized by modifyi modifying ng th e angles angles men tioned above.
The main design consideration of a snap-fit is integrity of the assembly and strength of the b eam. The integrity integrity of the assembly is controlled by the stiffness (k) of the beam and th e amount of deflection required for assembl assembly y or disassembly disassembly.. Rigidi igidity ty can be increased either b y using a higher modulus material (E) or by increasing the cross sectional moment o f inertia inertia (I) (I) of the be am. The produc t of these tw o p arameters (EI) (EI) will will determine th e total rigidity of a given given beam length.
III-1
S n a p -F i t B e a m D e s i g n U s i ng C l a s s i c a l B e a m T h e o r y
Part II I: Snap-Fit Snap-Fit Bea Bea m Design Using Classical Classical Beam T heory Cantilever Beam: Deflection-Strain Formulas
The integrity of the assembly can also be improved by increasing increasing the overhang depth. As a result,th e beam has to deflect further and therefore requires a greater effort effort to clear the overhang from the interlocking interlocking hoo k. However, However, as the beam deflection deflection increases,th e beam stress also increases. This will result in a failure failure if the beam stress is above the yield strength of the material.
P
t
L b
Thus the deflection must be op timized timized with respect to the yield strength or strain of the material. This is achieved by optimizing the beam section geometry to ensure that the d esired esired deflection deflection can be reached without exceeding the strength or strain limit of the material.
I) Uniform Uniform Cross Cross Sect ion, Fixed End to Free End
The assembly and disassembly force force will incre incre ase with both stiffness stiffness (k) and and m aximum deflection of the beam (Y). (Y). The force force (P) required to deflect the b eam is proportional to the product of the two factors:
P
t
4
L
k =
Strain:
= 1.50
Y
=
( ) ( )
Eb
Stiffness:
t
3
Y
L2
P
t
2 t
P= kY The stiffness stiffness val value ue (k) de pen ds on beam geometr y as shown in Figure III-3.
L b
Stress or strain induced by the deflection (Y) is also show n in Figure Figure IIIIII-3. 3. The calculated stress or strain value value should be less than the yield yield strength or the yield yield strain of the material in order to prevent failure.
II) Uniform Uniform Width , Height Height Tapers to t/ 2 at Free Free End
When selecting the flexural modulus of elasticity (E) for hygroscopic materials materials,, i.e. i.e. nylon, nylon, care should be taken. In the dry as molded molded state (DA (DAM), the datasheet value value may be used to t o calculate stiffness, stiffness, deflect ion or retention force of snap design. design. Under nor mal 50% relativ relativee humidity conditions h owever, owever, the physical physical prop erties decrease and the refore the stiffness stiffness and and retention force reduce w hile the deflection increases. increases. Both scenarios should be chec ked.
P
Stiffness:
k =
Strain:
= 0 .9 2
Y
=
( )
Eb
t
6.528 t
3
L
( ) L2
Y
b
P t
L
b 4
III) Uniform Height,Width Tapers to b/ 4 at Free Free End End Stiffness: Strain:
k =
P Y
=
= 1 .1 7
Eb 5.136
( ) t
L2
( ) t
L
Y
Where: E = Flexural Modulus P = Force Y= Deflection b = Width of Beam Figure III-3
III-2
3
S n a p -F i t B e a m D e s i g n U s i n g C la s s i c a l B e a m T h e o r y Concludin g poin ts: In a typical typical snap-fi snap-fit, t, the strengt st rength h of a beam beam is depende nt on its geometr geometr y and and maximum deflection dur ing assembly assembly.. The force to assemble assemble and disassemble snap-fit assemblies is highly dependent on the o verhang entrance and ret raction raction angles.
Close-up of automotive fuse box, snap on sides of box
Close-up of automotive fuse box snap
Close-up of automotive fuse box, full view III-3
Partt I V Par Improved Cantilever Snap-Fit Design The cantilever cantilever beam formulas used in conventional snap-fi snap-fitt de sign sign un derestimate the amount of strain strain at the beam/w all all interface interface because they do no t include the deformation deformation in the w all all itself itself.. Instead,th ey assume assume the wall to be comp letely letely rigid rigid with the deflection occur ring only in in th e beam. This This assump assump tion may be vali valid d wh en the ratio ratio of beam length to th ickness is is greater than about 10:1. 10:1. However, owever, to obtain obtain a more accurate accurate prediction of total allowable deflection and strain for short beams, a magnificati magnification on factor should b e app lied lied to th e con ventional formula. This This will enable greater flexibility flexibility in the design w hile taking full advantage advantage of the strain-carrying capability of the material. AlliedS lliedSignal ignal Plastics Plastics has d eveloped evelope d a meth od for estimating these deflection magnification factors for various snap-fit beam/wall configurations as shown in Fig Figure ure IVIV-1. The results of this techn ique, which have been verified both by finite element analysis and actual part testing 1, are show sh own n graphically in in Figure IVIV-1. Figure IV-2 shows similar results for beams of tapered cross-section cross-section (be am thickness de creasing creasing by 1/2 at the tip). Snapnap -Fit Design Examp Examp les 1 & 2 illustrate illustrate this p roce dure for designing snap-fits, snap-fits, including calculating the maximum strain developed during assembly and and predicting the snap-in snap-in force required.
1
Chul S. S. Lee, Alan Dubin and Elmer D. Jones,“Shor ones,“Shor t Cantilever Beam Beam Deflection Analysis Applied to Thermoplastic Snap-Fit Design,”1987 SPE ANTEC, NTEC, held he ld in Los Angele Angeles, s, Californ Californ ia, U.S.A U.S.A..
IV-1
I m p r o v e d
C a n t i l e v e r
S n a p - F i t
D e s i g n
Part IV: Im proved proved Cantilever Cant ilever SnapSnap-Fit Fit Design 8.0
1
ON A BLOCK (SOLID WALL) 7.0
6.0
4
2
ON A PLATE (OR THIN WALL)
Q R O T C A F N O I T A C I F I N G A M N O I T C E L F E D
3
5.0
5
4.0
3.0
2.0
1.0
0 .0 0 .0
1.0
2 .0
3 .0
4 .0
5 .0
6 .0
7.0
ASPECT RATIO, L/t
Uniform Beam, Q Factor Figure IV-1 IV-2
8 .0
9 .0
1 0 .0
1 1 .0
I m p r o v e d
C a n t i l e v e r
S n a p -F i t
D e s i g n
8.0
t/2
7.0
t
6.0
Q R O T C A F N O I T A C I F I N G A M N O I T C E L F E D
2T
5T
5.0
4.0
5T
3.0
2.0 2T
1.0
0 .0 0 .0
1.0
2 .0
3 .0
4 .0
5 .0
6 .0
7 .0
ASPECT RATIO, L/t
Tapere d Beam, Beam , Q Factor Factor Figure IV-2 IV-2 IV-3
8 .0
9 .0
1 0 .0
1 1 .0
I m p r o v e d
C a n t i l e v e r
S n a p - F i t
D e s i g n
Improved Formulas
Allowable Strain Value, MATERIAL PEI PC Acetal Nylon 6(4) PBT PC/ PET ABS PET
b t
P W Y
α L
UNFILLED 9.8%(2) 4%(1) - 9.2%(2) 1.5%(1) 8%(5) 8.8%(2) 5.8%(2) 6% - 7%(3)
o
30% GLASS
2.1%(1)
1.5%(1) Table IV-I
Figure IV-3
NOTES: (1) 70% of tensile yield strain value (2) Plastics astics En En gineerin g. G.G.Trantina. Pl August 1989. (3) V.H.Tru .H.Trumb mbull. ull. 1984 198 4 AS ASME Winter Ann Ann ual Con ference feren ce.. (4) DAM -“Dry As As Molde d” cond co nd ition itio n (5) AlliedSignal test lab
MAXIMUM MAXIMUM STRAIN STRAIN
tY = 1.5 —— —L2 Q
MATING FORCE µ + ta tan α W= P— ——— —— — 1–µ tan tan α bt 2 Eo P= — — — — — — — 6LQ 6LQ
Coe fficient o f Friction Friction (1) MATERIAL
µ 0.20 - 0.25 0.25 - 0.30 0.20 - 0.35 0.17 - 0.26 0.35 - 0.40 0.40 - 0.50 0.50 - 0.60 0.18 - 0.25
PEI PC Acetal Nylo n 6 PBT PC/ PET ABS PET
Where: W = Push-on Force W = Pull-off Force P = Perpendicular Force µ = Coeffici Coefficient ent of Friction Friction α = Lead Angle ng le α = Return Angle b = Beam Width t = Beam Thickne Thickness ss E = Flexural Modulus = Strain at Base L = Beam Length Q = Deflection Magnification agnification Factor (refer to Figure IVIV-2 for p rop er Q values) Y = Deflection ’
’
Table abl e IV-II IV-II NOTES: (1) Material tested teste d against itself
Hub cap with cantilever snaps IV-4
I m p r o v e d
C a n t i l e v e r
S n a p -F i t
D e s i g n
Snapnap -Fit Design Ex ample amp le #1 #1
Snap-Fit nap-Fit Design Ex ample amp le #2 #2
GIVEN:
b t
GIVEN:
P
Y
Material ⇒ Petra 130 (PET)
P W Y
L
α
Material ⇒ Unfilled Nylon Nylon 6
t= L= b= E= µ=
0.10 in 0.50 0.50 in 0.25 in in 1.3 (1 (106) psi 0.2 0.2 (From (From Table able IVIV-II,Coefficien II, Coefficientt of Friction Friction)) α = 30. 30.0° o = 1.5% (From (From Table Table IVIV-I, Allowab le Strain tr ain Value Value))
L b
t
t = Y= L= b=
0.063 in 0.09 0.090 0 in in 0.22 0.225 5 in in 0.242 in
Figure IV-5
DETERMINE: IS THIS TYPE OF O F SNAP-FIT ACCEP ACCEPTA TABL BLE E FOR FOR USE IN IN ® NYLON 6 (CAPRON 8200 82 00 NYL NYLON)
Figure IV-4 IV-4
SOLUTION: DETERMINE:
A) THE MAXIMUM MAXIMUM DEFLECTI DEFLECTIO ON OF OF SNAP SNAP B) THE MA MATING TING FORCE
L — = 3.57 3.57 ⇒ t
SOLUTION:
o =
A) THE THE MAXIMUM MAXIMUM ALLOWA ALLOWABLE DEFLECTIO DEFLECTI ON OF OF SNAP o
2 L Q tYmax = 1. 1.5 —2— —- ⇒ Ymax = —o——— LQ 1.5 t
L — = 5.0 5.0 ⇒ t
tY = 1.5 —— —L2 Q Q = 2.7
(From Q Factor Graph, Figure IV-1)
(0.063)(0.090) 1.5 —————— ——— = 6.2% (0.225)2(2.7)
Therefore, it is accep table for unfilled unfilled Nylon Nylon 6 (See Allow Allowable able Strain tr ain Value Value,T ,Table able IVIV-1).
Q = 2.0 (from Q Factor Graph Graph )
(0.015)(0.5) 2 (2.0) Ymax = —————————— = 0.050 in (1.5)(0.1)
Concludin g poin ts: Unlike con ventional vention al form formulas, ulas, AlliedS lliedSignal ignal includ includes es the t he deflect ion m agnification agnification facto factorr in all all calculati calculations. ons. The examples show h ow t o calculate calculate the m aximum strain strain during assembly assembly and and h ow t o p redict the force needed for assembly.
Therefore, in an actual design, a small smaller er value value for deflection (Y) (Y) would wou ld be ch osen for an added factor factor of safety safety.. B) THE MA MATING TING FORCE bt 2 Eo P=— — — — — — 6LQ 6LQ (0.25)(0.1) 2 (1.3)(10 6) (0.015) P = —————————————————— = 8.1 lb 6(0.5)(2.0) µ + ta tan α W= P— ——— —— — 1–µ tan tan α 0.2 + tan30º W = 8.1 ————————— = 7.1 lb 1 – 0.2 (tan30º) Therefore, it will take 7.1 lb mating force to assemble assemble p arts.
Close-up of automotive hub cap snaps IV-5
Part V U & L Shaped Snaps The cantilever beam snap-fit design isn’t appropriate for all applications. This chap ter defines “L”and ”an d “U “U” shaped snaps and te lls lls when the y are are used. Occasionally a designer will not be able to design a cantilever snap-fit configuration with a strain below the allow allowable able limit limit of the inten ded mate rial. This is is usually usually due to limited limited packaging packaging space w hich can restrict th e length of the snap. This This is the ideal time time to co nsider using either an “L “L”shap ed snap or “U “U”shape d snap.
“L” SHAPED CANTILEVER
Figure V-1
The “L “L”sh aped snap (see Figure VV-1) is formed by designing in slots in the base wall which effectively increases the beam length and flexibili flexibility ty compared to a standard cantilever cantilever beam. This This allows allows the designer designer t o reduce the strain strain d uring assembly assembly below th e allowable allowable limit limit of the selected material. material. It should be noted that adding a slot slot to the base wall may may not b e accep table in some designs for for cosmetic or air air flow con cern s.
The “U”sh “U”sh aped snap (see Figure VV-2) is anoth an oth er way to increase the effectiv effectivee b eam length w ithin a limited limited space envelope. With this design, design, even materials materials with low allowable strain limits (such as highly glass-filled materials) materials) can b e designed to meet assembly assembly requirements. The “U”shap “U”shap ed design usually usually incorp incorp orates the undercut on the outer edge of the part to eliminate the need for slide slide in t he m old, unless a slot slot is acceptable in the w all all from from wh ich the snap p rojects.
“U” SHAPED CANTILEVER
Figure V-2
V-1
U
&
L
S h a p e d
S n a p s
Part V: U & L Shaped Shaped Snaps Snaps “L” SHAPED SNAP–FIT
“L” Shaped Snap-Fit Example A) Calculate Calculate the minimum length (L ( L2) of the slot (see sketch , Figure V-3) in the main wall for for Capron ® 8233 nylon nylon in th e configurat configuration ion below. below. The required required deflection is .38 inches.
P
L1 A
A
b R
B) Calculate Calculate the req uired force (P) to deflect deflec t the snap .38 inches.
t
Section
GIVEN:
A-A
= .03 (with moisture) t = .1 in L1 = .5 in R = .12 .12 in in I = Moment Moment of Inert Inert ia (rectangle) 3 1(.1) bt 3 I = 12 = 12 = 8.333 8.333(10 (10-5) E = 1.3 (10 6) b = 1.0 in Y = .38
8233
L2
Figure V-3 6/ oYt(L1+ R) - 4L13 - 3R(2L12 + R2 + 8L1R) L2 = ——— ——————————— —— – ---––——–————— 12(L1 +R)2
or, Y=
P [4L13+3R(2L12 +R2 + 8L1R) + 12L2(L1 + R) R)2] 12EI
6/ Yt(L1+ R) - 4L13 - 3R(2L12 + R2 + 8L1R) A) L2 = —–––––——— ———————— ——————————— 12(L1 +R)2
Where: L2 = Length of slot as shown in sketch Allowab owable le strain of material m aterial o = All Y = Maximum deflection required in direction of force t = Thickness Thickness L1 = Length Length as shown in sketch R = Radius as shown in sketch (at neutral axis) P = Force b = Beam width E = Flexural Flexural modulus mod ulus I = Moment of inertia
6/.03 6/ .03 (.38)( .38)(..1)(.62) -4( -4 (.5)3 -.36[.5 +.122 + 4(. 4(.12)] = ——————————————— ———————————–– 2 12(.62) L2 = .750 in.
B) Y=
.38 =
P [4L13+3R(2L12 +R2 + 8L1R) + 12L2(L1 + R) R)2] 12EI P [4(.5)3+(.36)[.5 + (12)(1.3)(10 )(8.333)(10 -5) 6
.122 + 8(.5).12]+ 12(.75)(.62) 12(.75)(.62)2]
.38 =
P (4.714) 1.3(103)
P = 104.8 lbs.
V-2
U
&
L
S h a p e d
S n a p s
“U” Shape d Snap –Fit –Fit
“U” Shaped Sna p Example #1 P b
P L1 L1
L2
L2
A
A
R
Section A-A
R
t
Case 1 Case 1 Y=
9(L1 + R)t
[6L1 + 9R {L1(2L1 + 8R) + 3
A) Calculate Calculate the amoun t of deflection at th e tip of the beam for a 1.0 1.0 po und load load
R }+ 2
6L2 (3L12 - 3L1L2 +L22 )]
GIVEN: P= 1.0 lb. I = 0.833 x 10-4 in4 = bt 3 /12 /1 2 (rectan (r ectangular gular cross-section cross-section ) E = 534,000 psi R= 0.15 in L1 = 1.4 in L2 = 0.973 in t = 0.1 in b = 1.0 in
or,
Y=
P [6L13 + 9R {L1(2L1 + 8R) + 18EI
R }+ 2
6L2 (3L12 - 3L1L2 +L22 )]
P [ 6L13 + 9R{L1(2L1 + 8R) + R2} + 6L2(3L12 - 3L1L2 + L22)] 18EI 1 Y= [6(1.4)3 +9(0.15){(1.4) 18(534,000)(0.833 18(534,000)(0.833 x 10-4) (2• 1.4 + 8 • 0.15) 0.15) + (0.15) 2} + 6 (0.973) {3(1.4)2 - 3(1.4)(0.973) + (0.973)2}] = 0.064 in
A) Y = L3 P L2
L1 R
Case 2 Y=
o
3(L1 + R)t
[4L13 + 2L33 +3R {L1(2L1 + 8R) + R2}] or,
Y=
P [4L 3 + 2L 3 +3R {L (2L + 8R) + 1 3 1 1 6EI 6EI
R }] 2
Where: Variables defined on previous pr evious page.
V-3
U
&
L
S h a p e d
S n a p s Concludin g poin ts: Snap -fits can use eith er the th e “L” or “U” “U” shaped design to overcome overcome space limitati limitations. ons. Both the “L”and ”an d “U”sh “U”sh aped snaps snap s effectively effectively reduce redu ce strain du ring assembly, assembly, thu s making it ideal ideal for materials with lower lowe r allowab allowab le strain limits. limits.
“U” Shaped Snap Example #2
L3 P L2
L1 R
Case 2 A) Calculate Calculate the amoun t of deflection at th e tip of the beam for a 1.0 1.0 po und load GIVEN: I = 0.833 x 10 -4 in4 E = 534,000 psi R = 0.15 in L1 = 0.7 in L1 = L2 L3 = 0.273 in t = 0.1 in in
Y = =
P [4L 3 + 2L33 + 3R {L1(2L1 + 8R) + 6EI 6EI 1
Automotive wheel cover
R }] 2
1 [4(0.7) 3 + 2(0.273)3 + 6(534,000)(0.833 6(534,000)(0.833 x 10-4) 3(0.15){0.7(2 • 0.7 + 8(0.15)) + (0.15) 2}]
= 0.012 in
Close-up of above above cover cover backside backside featuri featuring ng the “ L” shaped shaped snap-fi snap-fitt design design (from a top angle)
Inset shot of a “ U” shaped shaped snap-fi snap-fitt design V-4
Part VI General Design Guidelines Three basic issues should be reviewed before finalizing a snap-fi snap-fitt design: design: stress concen tration, creep /relaxation, /relaxation, and fati fatigue. gue. Below are descriptions of these pr oblems and suggestions suggestions to prevent th em. All should be considered as part of good design practice for any ther moplastic moplastic d esign. esign.
betwee n th e parts, relaxation relaxation at the joint joint can result in loss of seal seal pressure, resulting resulting in leakag leakagee o f the contained fluid. fluid. Anoth er p roblem often seen is excessive excessive p lay lay between the parts due to tolerance variations,sometimes resulting in noise and and vibration. Several ways to minimize minimize these p hen omena include: designing designing a low low stress snap beam, designing designing the snap-fi snap-fitt to incorporate a 90° return angle angle so that it relaxes relaxes in tension versus ben ding (see Figure Figure VI-2). I-2). This will will prevent the mating part from slipping slipping past past or b ecoming ecoming loose. loose. Another w ay is to use a large large retu rn angle angle and incre ase the land length in the return angle angle area (see Figure igure VIVI-3). 3). Increasing Increasing th e overhang depth and evaluating evaluating the wor st case scenario in a tolerance study will allow the design to retain given pull-off force even after relaxation occurs.
The single most common cause of failure in snap-fits is stress stress concentration concentration due to a sharp corner between th e snap-fi snap-fitt beam and t he w all all to wh ich it is is attached. Since this location location n ormally ormally coincides with the point of maximum stress,a sharp corner can increase the stress beyond the strength of the material,causing point yielding yielding or bre akage. This is is more cr itical for rigid rigid plastics like glassglass-reinforced reinforced nylon, wh ich h ave relatively relatively low ultimate ultimate elongation. More du ctile ctile materials, materials, like like unreinforced nylon, nylon, tend to yield yield and deform deform be fore the y break, redistributing redistributing the p eak stress over a broader region. One solution is to incorporate a fillet fillet radius radius at the junctu re betw een th e beam and the wall (see Fig Figure ure VI-1), I-1), so th at the ratio of radius to w all thickness thickn ess (R/ (R/ t) is at least 50%. 50%. Going beyond 50 % results in a marginal increase in in stren gth and may cause cause oth er p roblems like like internal voids voids and sink marks. marks. If sink marks marks are an issue, a smaller smaller radius radius can b e used, but it may increase the stress in in this area. Anoth er op tion is to add add the radius radius only on the te nsile nsile side of the beam.
RELAXED POSITION SITI ON (EXAGGERATED) UNDEFORMED POSITION
UNDEFORMED POSITION P
P
RELAXATION IN TENSION
RELAXATION I N BENDING
P = MATING PART FORCE
Figure VI-2
R= .5t MINIMUM
SHARP CORNER
LAND LENGTH
t
POOR DESI GN
GOOD DESIGN
RETURN ANGLE
Figure VI-1 I-1 Creep, or more accurately accurately stress relaxation, relaxation, can result in a reduction of the holding force between the two comp onen ts connecte d by the snap-fi snap-fit. t. Stress relaxati relaxation on will occur graduall gradually y over time. If there is a gasket gasket or seal
OVERHANG DEPTH
Figure VI-3
VI-1
G e n e r a l
D e s i g n
G u i d e l i n e s
Part VI : General Gene ral Design Guidelines Guidelines Fatigue, atigue, or rep etitive etitive loading, loading, is the th ird major major cause of fail failure. ure. Fatigue atigue concer ns p rimarily rimarily apply if hun dreds or th ousands of cycles cycles are anticipated. anticipated. While While the design stress level level might might be well within the strength o f the material, material, the repeated application application of this stress can result in fatigue failure at some point in the future. Some p olymers olymers p erform better t han oth ers in this regard, making them th em ideal cand idates for snap -fits fits or living living hinges that must mu st flex repeatedly repeate dly.. The first first way to avoid avoid a fatigue failure is to choose a material known to perform well in in fatigue. fatigue. This This can be done by comp aring the so-call so-called ed SS-N curves of the m aterials,w hich sh ow t he expected number of cycles to failure at various stress levels levels and and at different different tem peratures of exposure. The secon d way, way, still still using the SS-N curves, is to choo se a design stress level,at the correct temperature,that results in the requ ired number o f load load applications applications pr ior to fail failure. ure. This meth od will usually be con servative since since S-N curves cur ves are typically generated generate d at much m uch higher frequencies than would be anticipated for repeated application of a snap-fit assembly.
Close-up of automotive fuel rail cover, cover, snap-fit design design
For hygroscop ic materials like like nylon, the e ffects ffects of moisture on final final part dimensions and mech anical anical properties also also must be considered. For further information, please con sult the t he All AlliedS iedSignal ignal Plastics Plastics Design Design Solutions Guide.
Concludin g points: There are a numb er of ways to overcome the issues of stress concentration, stress relaxation, relaxation, and fatigue. fatigue. A well thought ou t design and using the right polymer for a given application will minimize minimize these issues. This This allows allows the application application t o ben efit from all the advantages of a snap-fi snap-fitt de sign.
Close-up of truck mirror patch cover
Aerator
Circular ircul ar saw handle handle inset shot fea f eaturi turing ng snap-fit closure and mating mating part VI-2
Notes
English/ English/Me Me tric Conversion Conversion Chart To Co n ve rt En glis h Sy Sys te m
To Me tric Sy Sys te m
Multip ly En glis h Value by by . . .
DISTANCE inch es feet
millimeter s meter s
25.38 0.30478
MASS ounce (avdp ) p ound p ound U.S. ton
gram gram kilogram metr ic ton
28.3495 453.5925 0.4536 0.9072
VOLUME inch 3 inch 3 fluid oun ce quar t (liquid) gallon (U.S.)
centimeter 3 liter cen timeter 3 decimeter 3 (liter) decimeter 3 (liter)
16.3871 0.016387 29.5735 0.9464 3.7854
TEMPERATURE degree F
degree C
(°F –32) / 1.8 = °C
PRESSURE p si p si ksi p si
bar kPa MN/ m 2 MPa
0.0689 6.8948 6.8948 0.00689
ENERGY AND POWER in lb f ft lb f kW U.S. hor sep ower Btu BTU • in / (hr • ft2• ºF)
Joules Joules metr ic h or sep ow er Kw Joules W/ m • °K
0.113 1.3558 1.3596 0.7457 1055.1 0.1442
VISCOSITY p oise
Pa • s
0.1
BENDING MOMENT OR TORQUE ft lb
N• m
1.356
DENSITY lb/in3 lb/ft3
g/cm3 kg/m 3
27.68 16.0185
NOTCHED IZOD ft lb/ in
J/m
53.4
