NOMINAL DIAMETER, CLAMP LENGTH AND THREAD PITCH ANALYSIS FOR BOLT PRELOAD AUGMENTATION

Threaded fastening is used mainly for fastening together mechanical parts. Compared to other types of jointing methods such as adhesion, welding, brazing and pressure insertion, threaded fastening has a unique characteristic that elastic energy is built up inside the joint members. Tension in the bolt and compression in the fastened parts are created as a product of action and reaction. These forces can make the joint less susceptible to fatigue and loosening when external load is applied or internal pressure is increased. Since the torque applied to a fastener must overcome all friction before any loading takes place, the amount of friction present is important. It is seen that approximately 50% of the torque applied will be used to overcome bolt head-bearing friction and another 35% to overcome the thread friction and approximately 5% is consumed by prevailing torque. Thus only 10% torque is available to produce clamping force. In this paper, an analysis is presented to study the effect of various parameters such as clamp length, nominal diameter and thread pitch on the preload required for maintaining joint integrity. The suggested design guidelines are useful for proper selection of threaded fasteners used in different assemblies of structures, machine elements etc.