Applying torque to increase the pre-load minimises the effect of the load forces and so increases fatigue life. Source: Parmley, R.O.,ed., 2000. Illustrated sourcebook of Mechanical Components. New York: McGraw-Hill. pp19 -12.
Standards relating to bolt design Standard grade specifications give the minimum mechanical properties for the bolt. Source: Oberg, E., et al., eds. Machinery's handbook 26th ed. New York : Industrial Press, 2000. pp. 1488
British Standards Institution, 1964. BS. 3580: 1964. Guide to design considerations on the strength of screw threads.
Gives formulae and details the relationships between material strength and features of bolt design and manufacture
Lists variables such as lubrication used, types of washers, coarse or fine threads
States “it is recommended that torque-tension relations be established experimentally for the conditions of the particular application.”
British Standards Institution, 2005. BS EN 14399-2:2005. High-strength structural bolting assemblies for preloading. Suitability test for preloading.
The principle of the test is to tighten the assembly and to measure, during tightening, the following parameters: the bolt force; the relative rotation between the nut and the bolt; the torque, if required; the bolt elongation, if required.
Cobb, B.J. Preloading of bolts. In: Parmley, R.O.,ed., 2000. Illustrated sourcebook of Mechanical Components. New York: McGraw-Hill. pp 19-12 to 19-16
But practical variations are listed: applied torque, thread design, friction forces between touching surfaces.
So safety margins and tests are required.
Design curves are produced based on the stress-torque equation and the preload-torque equation: For a given bolt diameter read up to the desired load for the tightening torque and read down for the material strength requirement.
ESDU 86014: Applying, measuring and maintaining pretension in steel bolts
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.