Under Graduate Course on
Mach Machin inee Design Design-I -I Design of Machine Elements Shibendu Shekhar Roy Robotics & A.I Lab. (RAIL) Department of Mechanical Engineering
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Key
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- A key is defined as a machine element that is used to connect the
transmission shaft to rotating machine elements like pulley, gear, sprocket, flywheel etc. . Keys are used as temporary fastening of shaft and hub
Functions of Keys
The primary function of the key is to transmit the torque from the shaft to the hub of connecting element or vice-versa
The another function of the key is to prevent relative rotational motion & axial movement (except in case of feather key or splines) between the shaft & the joined m/c elements like gear, pulley etc. Shaft Keyed joint
Consisting of
Hub Key
Keyway is a slot or recess on a shaft and or hub to accommodate a key 6 April 2009
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Key Drawback
The keyway results in stress concentration in the shaft & the part becomes weak
Assembly procedure
For mounting a part at any intermediate location on the shaft,, first the key is firmly placed in the keyway of the shaft & then the hub to be mounted is slide from one end of the shaft till it is fully engaged with the key.
After mounting positioning the hub on the shaft, such that both the keyways are properly aligned, the key is then driven from the end, resulting in a firm joint
Manufacturing process for keyways
Keyway is usually cut by vertical or horizontal milling cutter in case of shaft
Keyway is usually cut by slotting machine in case of hub
Materials
Plain Carbon Steels like 45C8, 50C4 etc.
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Types of Keyways
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Key Rectangular sunk key or Flat key
Sunk Keys
Square sunk key Gib- head key
Saddle keys
Parallel Key
Taper key
Hollow Saddle Key Flat Saddle Key
Types of Keys Woodruff key
Special Keys
Feather or kennedy key Round key
Splines
Factors are considered for selecting of th e type of key for a given applic ation
Power to be transmitted Tightness of fit Stability of connection Cost 6 April 2009
Sunk Key
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- Half the thickness of the key fits into the keyway on the shaft & the remaining half in the keyway on the hub - Power is transmitted due to shear resistance of the key. The relative motion between the shaft & the hub is also prevented by the shear resistance of key
Square 6 April 2009
Machine Design
Rectangular 8
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Rectangular Sunk Key - Sunk key with rectangular cross-section, is also called Flat Key d=diameter of the shaft = diameter of the hole in the hub
Usual proportions of dimensions of key
b= width of key
b=
h=height or thickness of key l=length of key
d 4
h=
2 3
b=
d
l ≥1.5d
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Square Sunk Key - Width & thickness are equal Usual proportions of dimensions of key
d
b=h=
4
l ≥1.5d
Check: Check the dimensions considering mode of failure due to shear & crushing N.B: Flat key has more stability as compared with square key 6 April 2009
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Dimensions of Square & Rectangular Sunk Keys (in mm) [ IS : 2293 ]
Shaft diameter
Key size
Above
Upto & including
6
8
8
Width
Height
Keyway depth In shaft
In hub
2× 2
1.2
1
10
3× 3
1.8
1.4
10
12
4× 4
2.5
1.8
12
17
5× 5
3
2.3
17
22
6× 6
3.5
2.8
22
30
8× 7
4
3.3
30
38
10 × 8
5
3.3
38
44
12 × 8
5
3.3
44
50
14 × 9
5.5
3.8
50
58
16 × 10
6
4.3
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Parallel Sunk Key
IS: 2048
- is a sunk key (with rectangular or square cross-section) which is uniform in width as well as height throughout the length of key
Taper Key
IS: 2292
- is a sunk key which is uniform in width but tapered in height - Bottom surface of the key is straight & the top surface is givena taper - Standard taper is 1 in 100 Designation of Parallel Sunk Keys Width
Height
Length
Example: A parallel key of width 10mm, height 8 mm & a length 50 mm shall be designated as : Parallel key 10×8 ×50 [IS: 2048] 6 April 2009
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Taper is provide for following two reasons
When the key is inserted in the keyways of shaft and the hub & pressed, it becomes tight due to wedge action. This ensures tightness of the joint in operating condition
Due to taper, it is easy to remove the key & dismantle the joint
As compared with parallel key, taper key has f ollowing advantages
The taper surface results in wedge action & increases frictional force & the tightness of the joint
The taper surface facilitates easy removal of the key, particularly with gib head
Gib- head Key
IS: 2293
- It is a rectangular sunk key with a head at one end & taper at top surface to facilitate removal At large end, 6 April 2009
b=d/4; h=(2/3).b=d/6 Machine Design
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Gib Head Key
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Saddle Key
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- Is a key that fits in the keyways of the hub only - There is no such keyway on the shaft Hollow Saddle Key
- Fits in a keyway in the hub & the bottom of the key is concave shaped to match the circular/curve surface of the shaft Flat Saddle Key
Hollow Saddle Key 6 April 2009
Flat Saddle Key
Machine Design
- Fits in a keyway in the hub & the bottom of the key sits on the flat surface machined on the shaft
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Saddle Key
Friction between shaft, key & hub prevents relative motion between the shaft & the hub. Therefore power is transmitted by means of friction
Saddle keys are suitable for light duty & low power transmissionas compared with sunk keys
The resistance to slip in case of flat saddle key is slightly more than that of hollow saddle key. Therefore flat saddle key is slightly superior to hollow saddle key as far as power transmitting capacity is concerned Sunk Key
Saddle Key
-There is no possibility of the key to slip
-Is liable to slip around the shaft when
around the shaft.
subjected to heavy torque
-It can be used in medium & heavy duty
-It can not be used in medium & heavy duty
applications
applications
- It is necessary to cut keyways both on the shaft & the hub.
-Requires keyway only on the hub
Cost is less
Cost is more 6 April 2009
Feather Key
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- Is a parallel sunk key that is fixed either to the shaft or to the hub & that permits relative axial movement between them
- There is a clearance fit between the key & the keyway in the hub. - The hub is free to slide over the key, at the same time, there is no relative rotational movement between the shaft & the hub - It transmits torque & permits some axial movements of hub
N.B: It is an alternative to splined connection 6 April 2009
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Woodruff Key
- is a piece from cylindrical disc having segmental cross-section (in the form of an almost semi-circular disk of uniform thickness)
- Keyway in the shaft is in the form of a semi-circular recess with the same curvature as that of the key. The bottom portion of the key fits into circular keyway in the shaft. - The projecting part fits in the keyway in the hub - Once placed in position, the woodruff key tilts & aligns itself on the shaft
The key is largely used on tapered shafts in Automobile & machine tool construction 6 April 2009
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Woodruff Key
Advantages
Can be used on tapered shaft because it can be aligned by slight rotation in the seat
The extra depth of key in the shaft prevents its tendency to turn over the shaft
Disadvantages
The extra depth of keyway in the shaft increase stress concentration & reduces its strength
The key does not permit axial movement between the shaft & the hub
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Round Key - are circular in section & fit into holes drilled partly in the shaft & partly in the hub - Sometimes the tapered pin is held in tapered holes
It has the advantage that their keyways may be drilled after the mating parts have been assembled
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Splines
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- Splines are keys that are made integral with the shaft. Such shafts are known as Splined Shaft - These types of shafts usually have 4, 6, 10 or 16 splines - They are used when there is a relative axial motion between the shaft & the hub and are also used when the force to be transmitted is large in proportion to the size of the shaft as in Automobile transmission & sliding gear transmission. - These types of shafts usually have 4, 6, 10 or 16 splines - Manufacturing Method: Splines are cut on the shaft by Milling the hub by Broaching
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- Used in gear shifting mechanism in Automobile gear boxes & machine tool gear boxes
Straight Sided Splines
- Stub teeth with pressure angle 30° Types of Splines
- Are specified by module
Involute Splines
- Greater strength - Are used in applications where it is important to keep overall size of assembly as small as possible
Serrations
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Machine Design
- Used as interferance joint 21
Forces acting on Key P’ D P
A P
B C
P’
Key tightly fitted at top & bottom
Shear failure of a tight ly fit ted key 6 April 2009
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