Enter the torque applied to the bolt, the diameter of the bolt, and the coefficient of friction for the bolt contact to determine the clamping force.

Clamping Force Formula

The following formula is used to calculate the axial clamping force of a bolt. 

F = T / K * D
  • Where F is the clamping force (N, lbs)
  • T is the torque applied to the bolt (N-mm, in-lb)
  • K is the friction of the bolt contact
    • Steel = .2
    • Cadmium = .161
    • Lubricated = .17
  • D is the diameter of the bolt ( mm, in )

To calculate clamping force, divide the bolt torque by the bolt friction constant, then multiply by the diameter of the bolt.

Clamping Force Definition

What is a clamping force? A clamping force is a measure of the force applied to an object that is used to oppose separating forces that are trying to move that object from its current location.

Example Problem

How to calculate clamping force?

  1. First, determine the torque applied to the bolt.

    Using a torque wrench, the torque being applied to the bolt in this example is found to be 300 in-lbs.

  2. Next, determine the diameter of the bolt.

    The bolt being used in this application is a measure to be 2 inches.

  3. Next, determine the friction of the bolt.

    In this example, the bolt is a standard steel bolt. The friction is assumed to be .2. In reality, this could vary slightly depending on several factors, but .2 is a good estimate.

  4. Finally, calculate the clamping force.

    Using the formula, the clamping force is found to be 300/(2*.2) = 750 lbs of force.

About Clamping Force

What is clamping force measure in? A clamping force is typically measured using units of lbs or newtons. Clamping forces use the same units as any other force metric.

Does clamping force increase with the number of bolts? Increase the number of bolts on a fixture will increase the clamping force. Typically the total clamping force would be approximately the sum of each of the clamping forces provided by the individual bolts.

How much clamping force do I need? One way to determine how much clamping force you need is to first determine a factor of safety. That is how much more than the exact required amount you want to design for. In general, a factor of safety of 2 or more is recommended. Next, determine the exact force that the object may see during operation and then multiply by the factor of safety.