What is Key?
Functions of the key
There are two basic functions of key
- Transmit torque to hub and vice versa
- Prevent relative motion of the shaft and the joined machine element
Types of keys
Keys can be classified on the basis of following factors like.
- Rigidity of connection
- Tightness of fit
- Power to be transmitted
On the basis of above factors, we have following types of keys
- Key with and without Gib head
- Parallel key and taper key
- Flat and square key
- Sunk and saddle key
A saddle is a key which fits in the keyway of the hub only.
Image source: Ques10
Advantage of saddle key
- Since keyway is to be made only in the hub hence its cost is low.
Disadvantage of saddle key
- Power transmission capacity is low
A sunk key is a key in which half the thickness of key fits into the keyway of hub and remaining half remains in the keyway of shaft.
Advantage of sunk key
- Power transmission capacity is high compared to saddle key.
Disadvantage of sunk key
- Since keyway is to be made in both shaft and hub hence its cost is high.
A feather key is a parallel key which allows relative axial movement between shaft and hub. These can be used with gears or clutches.
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Woodruff key is a sunk key in the form of almost semi circular disc of uniform thickness.
Image source: Tandwiel
Advantage of woodruff key
- It can tilt and align itself according to the shaft
- Extra depth of key in the shaft prevents slip of shaft and hub
Disadvantage of woodruff key
- Due to extra depth of keyway, shaft has low strength
- No axial movement is permitted between shaft and hub