Design Guidelines for Torque Controlled Tightening of Bolted Joints

This paper proposes a set of design guidelines to assist the Engineer when torque tightening of threaded fasteners is to be used for product assembly. Discussed are the factors which affect the bolt preload (tension) such as frictional scatter, the accuracy of the torque tightening method, and the effect of fasteners having a prevailing torque characteristic. The consequence which the use of special bolts, such as those having flange heads or reduced bolt shanks, have on the tightening torque and preload is also considered.

It is shown that as the variation in the coefficient of friction increases, the correct tightening torque to specify to avoid bolt failure, and the maximum preload which can be ensured based upon this torque, both reduce in magnitude. An improvement in the reliability of products using bolted joints assembled by the torque tightening method can be assisted by use of the proposed guidelines. This is accomplished by ensuring that the maximum preload value which can be ensured, after allowing for frictional scatter and other effects, is used as the basis of the product's design.