Enter the total torque transmitted by the shaft and the diameter of the shaft into the calculator to determine the key force.

Key Force Formula

The following equation is used to calculate the Key Force.

KF = T*2/D
  • Where KF is the key force (lbf)
  • T is the torque applied to the key (lbf-in)
  • D is the diameter of the shaft (in)

To calculate the key force, multiply the torque by 2, then divide by the diameter of the shaft.

What is a Key Force?


A key force is a total force, including direct and shear force, acting on a key during an operation.

How to Calculate Key Force?

Example Problem:

The following example outlines the steps and information needed to calculate the Key Force.

First, determine the torque applied to the key. In this example, the torque applied to the key is found to be 50 lbf-in.

Next, determine the shaft diameter. For this problem, the shaft diameter is found to be 4 in..

Finally, calculate the Key Force using the formula above:

KF = T*2/D

KF = 50*2/4

KF = 25 lbf


What is the significance of the diameter of the shaft in calculating key force?

The diameter of the shaft is crucial in calculating the key force because it directly influences the distribution of force across the key. A larger diameter means the force is distributed over a larger area, potentially reducing the stress on the key. Conversely, a smaller diameter concentrates the force, which could increase the risk of key or shaft failure.

How does the torque applied to the key affect the key force?

The torque applied to the key is directly proportional to the key force. As the torque increases, the key force increases accordingly. This relationship is critical in designing mechanical systems, as it helps ensure that the key and shaft can withstand the operational forces without failing.

Can the formula for calculating key force be used for any shaft and key material?

While the formula KF = T*2/D provides a basic calculation for key force, the material properties of the shaft and key, such as strength, ductility, and wear resistance, also play a significant role in their performance under load. Therefore, while the formula offers a starting point, engineers must also consider material properties and safety factors in their designs to prevent mechanical failure.