Tightening torque is not the purpose, bolt tension (pre-tightening force) is.

The relationship between the pre-tightening force and the bolt tension is more complicated. It involves bolt and nut material, tooth profile, pitch, material of the connected component, surface roughness, and lubrication method. It is finally simplified into the formula M = kFd, M is torque, and F is bolt tension. , D is the nominal diameter, and k is the coefficient that needs to be measured according to the actual situation.

The accurate method is to use a special instrument for measuring the tensile force of the bolt to fit the k value. The following figure gives the fitting value of the 30CrMnSiA fastener.

If there is no equipment, the bolts can be broken according to the actual connection conditions (including fasteners, connected parts, lubrication conditions, etc.), and the breaking torque is recorded. Take 30 ~ 70% as the tightening torque.