A set of n bolts is to be used to provide a clamping force of F between two components. The load is shared equally among the bolts. Specify suitable bolts, including the grade of the material, if each is to be stressed to K % of its proof strength. The variable K is called the demand factor.
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
sa = K.[s]
The required tensile stress area for the bolt is then
At = P / sa
From a table find the required tensile stress area for the thread. The required tightening torque will be
T = k1 x D x P
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