Failure of the Shaft Key
The force imposed due to the rotation of the shaft causes the failure of the keys. There are two types of key failures.
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= the shear stress acting on the key,
L= effective length of the key,
ω= angular velocity of the shaft,
r= radius of the shaft, and
T= maximum torque transferred by the key.
Crushing Failure of Key
Crushing of a key is the reason of this type of key failure. During rotation the shaft and the hub impose compressive force on the key. This compressive force causes its deformation. The key is then permanently deformed under this force and finally crushing occurs.
This compressive force can be calculated using the parameters of the shaft and the key. The formula for the calculation is given below.
Where all elements are the saem as above and H is the height of the key.