The load on each screw is to be

P = F /n

Specify a bolt made from an SAE grade steel, having a proof strength s psi. Then the allowable stress is

s

_{a}= K.[s]

The required tensile stress area for the bolt is then

A

_{t}= P / s

_{a}

From a table find the required tensile stress area for the thread. The required tightening torque will be

T = k1 x D x P

where

D - nominal outside diameter of threads

P - clamping load

k1- constant dependent on the lubrication present

**Constant dependent on the lubrication present**

For average commercial conditions, use k1 = 0.15 if any lubrication at all is present. Even cutting fluids or other residual deposits on the threads will produce conditions consistent with k1 = 0.15. If the threads are well cleaned and dried, k1 = 0.20 is better. Of course, these values are approximate, and variations among seemingly identical assemblies should be expected. Testing and statistical analysis of the results are recommended.

Related article:

**Bolts, Nuts, Screws, Studs**

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