15 March 2017

Types of keys in machine design

There are different types of keys are available and there are a number of standards. 
Many ways to classify the keys. so let we check it out types of keys below :

  • Saddle key and Sunk key
  • Square key and Flat key
  • Taper key and Parallel key
  • Key with and without Glib-head
There are some special types of keys are also available. some of the special types of keys are given below :

  • Woodruff key
  • Kennedy key or Feather key 
This special types of keys are used in the following applications depending upon the following factors :

  • Power to be transmitted
  • Tightness of fit
  • Stability of connection
  • Cost 
Saddle key :

A saddle key is a key which fits in the key ways of the hub only there are no keyways on the shaft.
Saddle keys are suitable for light duty only. They tend to work loose rock on the shaft under heavy duty.
There are two types of saddle keys :

  • Hollow saddle key
  • Flat saddle key  
Hollow saddle key :
Hollow saddle keys is a taper key which fits in key ways in the hub and the bottom of the key is a shaped to fit the curved surface of the shaft. 
It is held on by friction, therefore suitable for light loads.

Flat saddle key :
Flat saddle keys is a taper key which fits in key ways in the hub and is flat on the shaft.
It is likely to slip round the shaft under load, therefore suitable for comparatively light loads.  

In both types of saddle keys friction between the shaft, key and hub prevent the relative motion between the shaft and the hub.

In case of the flat key, the resistance of slip is more than that of the hollow key, therefore flat saddle key is slightly superior to hollow shaft key as far as the power transmitting capacity is concerned.

Sunk key :

A sunk key is a key in which half the thickness of the key fits into the keyways on the shaft and the remaining half in the keyways on the hub.

In these keys, keyways are required both on the shaft as well as the hub of the mating element.

This is the standard form of the key.

In sunk key, power is transmitted due to shear resistance of the key.

The sunk key is suitable for the heavy-duty applications because there is no possibility of the key to slip around the shaft.

Square key and flat key :

Sunk keys are used in two ways :

  • Square 
  • Rectangular cross-section
A sunk key with rectangular cross-section is called a flat key.

The flat key is more stability as compared with the square key.

Square keys are used in general industrial machinery while flat keys are more suitable for machine tool applications.

Taper key and Parallel key :

A parallel key which is uniform in width as well as height throughout the length of the key.

A tapper key is also uniform in width but tapper in height.

Standard tapper is 1 in 100.

Tapper is provided due to the following reason :

  • Due to tapper, it is easy to remove the key and dismantle the joint easily.
  • Tapper insures tightness of joint in operating conditions and prevent loosening of the parts.
Glib-head key :

Glib keys are tapered and notched machine keys that are used on power transmission keyed shafts to hold pulleys and gears tightly on the shaft.

Taper keys are often provided with glib-head to facilitate removal.

Glib head key has following advantages as compare with parallel key, taper key :

  • The taper surface results in wedge action and increases frictional force and the tightness of the joint.
  • The taper surface facilitates easy removal of the key, particularly with glib-head keys.
Feather key :

A feather key is a parallel key which is fixed either to the shaft or to the hub and which permits relative axial movement between them. 

The feather key is a particular type of sunk key with uniform width and height.

Feather key used where the parts mounted on the shaft are required to slide along the shaft such as clutches or gear shifting devices. 

Feather key is an alternative to splined connection.

Woodruff key :

A woodruff key is a sunk key in the form of an almost semicircular disk of uniform thickness.

Woodruff keys are used on tapered shaft in machine tools and automobiles.

Advantages of woodruff key :

  • It can be used on tapered shaft because it can align by slight rotation in the seat.
  • The extra depth of key in the shaft prevents its tendency to slip over the shaft.
Disadvantages of woodruff key :

  • The extra depth of key ways in the shaft increases stress concentration and reduces its strength.
  • The key does not permit axial movement between the shaft and the hub.