The integrity of a threaded fastener joint, either shear or tension, is maintained by the preload in the fastener. It has been shown that the optimum joint has as high a preload as possible. However, equally important is the need to accurately control the preload to a specified value. In many instances the reason for failure of bolted joints is incorrect initial preload, loss of the preload due to loosening, or relaxation of the preload due to a variety of mechanisms.
The fundamental problem, however, is to assemble the joint and accurately achieve the required initial preload in the fastener. A number of methods are available for controlling or achieving the preload during assembly. These are discussed in this section.
The methods described are generally amenable to theoretical analysis, and where relevant this is included. It is important to note that, although apparently simple, the bolted joint is extremely complex. Simple theoretically "correct" methods suffer from inaccuracies, some quite subtle. The accuracy of the analysis depends on the specification of materials and geometrical parameters, which in practice are impossible to specify without resorting to empirical data derived from tests on actual or representative joints. Friction data for some commons joint materials are given in Annex C.
This section relates to the method of torque tightening, which is the most common method of tightening threaded fasteners. Other methods intended for obtaining more precise preload during tightening are presented in Section 12.
The torque tightening method utilises the relationship between applied torque and preload induced in the fastener. The relationship is dependent on both geometry and effective coefficients of friction at the "under head" bearing region and in the threads.