How to calculate tightening torque

Written by doug leenhouts
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How to calculate tightening torque
Tightening torque is a function of thread area. (Bolt image by krotex from

Tightening torque is the force applied when a bolt is fastened. Many bolts are given a specific recommendation as to how much torque should be applied when they are fastened. Some popular bolts have tightening torque tables that give the suggested torque based on clamp load, torque-friction coefficients and nominal bolt diameter. Clamp load is also called preload and is usually assumed to be 75 per cent of the bolt proof load multiplied by the tensile stress area, according to

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  1. 1

    Determine the friction coefficient. If the bolt is unplated steel, a very common type, the coefficient is 0.20. If it is plated steel, then the coefficient is 0.15.

  2. 2

    Determine the clamp load. This is done by taking the given bolt proof load for the type of bolt used, multiplying it by 0.75, then multiplying by the stress area of the threads. The stress area for your bolt size can be found in a lookup table. Convert this calculation to pounds.

  3. 3

    Measure the nominal bolt diameter in inches.

  4. 4

    Multiply the friction coefficient by the clamp load by the bolt diameter. This will give you the tightening torque.

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