Shaft Key Failure in Hindi || Key Failure || Machine Design

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Shear Failure of Keys

As the term indicates, shear stress is the cause of this type of key failure. Actually, during rotation the shaft and the machine element, say hub, each element exerts equal and opposite force on the key. These opposite forces introduce shear stress along the radius of the shaft. The key is then starts deforming and after some time it fails.

The ultimate stress depends on the material strength of the key and angular velocity of the rotation. You can calculate the shear force applied on the key using the formula given below.

F = π ω l
T = Fr
Where,

F= the shear stress acting on the key,

L= effective length of the key,

ω= angular velocity of the shaft,

The Function of Keys
There are two main functions of keys when they are used to lock the transmission shaft.

The primary function is to transfer torque from the rotating shaft to the rotary element of the machine. The same key works to transmit torque in both direction; from shaft to the machine element, or from machine element to the shaft.
The second function of the key is to restrict relative rotational motion and axial movement between the shaft and the machine element. Some special types of keys are also available, such as feather and splines, keys that allow axial movement between them.

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