Torsional Stresses are dangerous … myth or fact?

If you ask for clarification when you hear people say “ torsional stresses generated during tightening are dangerous to the well being of the fastener” what they usually say is that the combined tensile and shear forces lower the effective yield strength compared to the pure tensile yield, and this drop in strength is what is damaging.

Torsional StressesIn theory, based on the Mohr circle and Von Mises, the combined force yield is lower – the greater the friction means increased torsional/shear effects resulting in a lower strength. However in practical terms this is not an issue for the integrity of the bolt’s load carrying capability.

Research by Newnham, Curley, Wallace and Chapman of SPS Technologies in evaluating effects of high-speed, high-production yield tightening techniques used in the automotive industry, revealed that torsional stresses relieve out of the bolt just after tightening and the reduction accelerates when the working external load is applied.

The result is the tensile capacity of the bolt is the same as its uniaxial yield value. Torque tightening is not dangerous for the bolt.