How to Size a Key for a Coupling

Connect the gear to the shaft.

A key is a tool that mechanically connects a part such as gear to a shaft. The most common type is a flat key. However, there are different types of keys. They are parallel keys which can be square or rectangular, tapered and gib head keys, pin keys and woodruff keys. A coupling is a device used to connect the end of one shaft to the end of another.
To determine the size of a key for a coupling, you have to do some math. Information needed to work out the equations are the shaft diameter, torque and force.
You can also use a table to determine the size of the key based on the shaft diameter of the coupling.

Instructions

1. Determine the force of the shaft of the key. This equals two times the torque divided by the diameter of the shaft.

2. Divide the force by the width of the key times the length of the key to determine the shear stress.

3. The length of the key should equal two times the torque divide by the width of the key times the shaft diameter times the shear stress.

References

• University of Northern Iowa: Couplings and Keys
• Technology; Clearance and Interference Coupling Fits; Tom Bishop, P.E.; May 2010

Resources

• Falk: Standard Metric Keys and Keyways for Metric Bores with One Key