A key is a piece of mild steel inserted between the shaft and hub or boss of the pulley to connect these together in order to prevent relative motion between them. It is always inserted parallel to the axis of the shaft. Keys are used as temporary fastenings and are subjected to considerable crushing and shearing stresses. A key way is a slot or recess in a shaft and hub of the pulley to accommodate a key.

Types of keys 
The following types of keys are important from the subject point of view 
1. Sunk key 
2. Saddle key 
3. Tangent key 
4. Round key 
5. Splines  key 

1. Sunk key - The sunk key  are provided half in the keyway of the shaft and half in the keyway of the hub or boss of the pulley  . The sunk keys are of the following types :

1. Rectangular sunk key
2. Square sunk key 
3. Parallel sunk key 
4. Gib-head key
5. Feather key 
6. Woodruff key 

2. Saddle key - The saddle are the two following type :

1. Flat saddle key 
2. Hollow saddle key 

3. Tangent key - These are used in large heavy duty shaft .

4. Round key - The round key  are circular in section and fit into holes drilled partly in the shaft and partly in the hub . 

5. Splines


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