![]() ![]() Much of the torque applied is lost overcoming friction under the torqued bolt head or nut (50%) and in the threads (40%). Friction in the threads and under the nut or bolt head use up some fraction of the applied torque. The preload achieved by torquing a bolt is caused by the part of the torque that is effective. Maintaining a sufficient joint preload also prevents relative slippage of the joint components that would produce fretting wear that could result in a fatigue failure of those parts. ![]() This determines the durability of the bolt when subjected to repeated tension loads. The relative stiffness of the bolt and the clamped joint components do, however, determine the fraction of the external tension load that the bolt will carry and that in turn determines preload needed to prevent joint separation and by that means to reduce the range of stress the bolt experiences as the tension load is repeatedly applied. The stiffness of the components clamped by the bolt has no relation to the preload that is developed by the torque. The preload developed in a bolt is due to the applied torque and is a function of the bolt diameter, the geometry of the threads, and the coefficients of friction that exist in the threads and under the torqued bolt head or nut. The applied torque causes the bolt to 'climb' the thread causing a tensioning of the bolt and an equivalent compression in the components being fastened by the bolt. Typically, a bolt is tensioned (preloaded) by the application of a torque to either the bolt head or the nut.
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