Key Design

 Key Design

1. Select the material of the keys from any grade of CD or HR steel, having a yield strength lesser than that of the shaft
2. Select square keys for all key locations (at the gear and the pulley) from table (8-15), extend the data for shaft sizes greater than 3¼". Note the key width (a) - the side of the square key
3. Calculate the tangential forces acting at all key locations (at both gears and the 2nd pinion) using the torque values at those locations and the shaft diameters
4. Wt=2T/Dshaft

Where:

Wt is the value of tangential force to be calculated at locations of interest

T is the torque value at the location under consideration

Dshaft is the shaft diameter at the location under consideration

5. Calculate the length required for each key by solving Shear and Bearing stress equations for each key.

Bearing Stress:

Shear Stress:

Where:

Wt is the tangential force

a is the side of the square key

l is the length of the key (to be solved)

Sy is the yield strength of the key material selected

Ssy is the shear yield strength of the key material = 0.5Sy

n is the safety factor

The final value of the length (l) of the key will be the larger of the two values calculated for each location. Generally, key sizes must fall within the following range by length:

If the calculated key length exceeds 1.5Dshaft, and you cannot strengthen the key material further, use two keys placed at 180° at the same section