IHS ESDU Applying, measuring and maintaining pretension in steel bolts. 86014

ESDU 86014 discusses factors affecting the tension in a bolt, and indicates the methods by which a desired tension may be obtained, with a comparison of their relative accuracy and cost. Methods of measuring the bolt loads are also discussed. To determine the bolt pretension achieved by application of a known torque, the results of 1900 tests were analysed to yield curves for both lubricated and unlubricated threads of BSF(BA) and Unified or ISO metric forms. The test data related to nuts of mild and high strength steel seated on mild or high strength steel or aluminium alloy washers, and lubricants ranged from light oil to extreme pressure greases. No significant trend with material or lubricant could be isolated. Techniques for applying a known torque are considered, and their accuracy reviewed. Methods of locking the nut are described, together with their advantages and disadvantages, and include mechanical, frictional, adhesive or deformation techniques.