<Tr> <Td> Ultrasonic sensing </Td> <Td> ± 5% </Td> </Tr> <P> The preferred bolt preload for structural applications should be at least 75% of the fastener's proof load for the higher strength fasteners and as high as 90% of the proof load for permanent fasteners . To achieve the benefits of the preloading, the clamping force must be higher than the joint separation load . For some joints, multiple fasteners are required to secure the joint; these are all hand tightened before the final torque is applied to ensure an even joint seating . </P> <P> The preload achieved by torquing a bolt is caused by the part of the torque that is effective . Friction in the threads and under the nut or bolt head use up some fraction of the applied torque . Much of the torque applied is lost overcoming friction under the torqued bolt head or nut (50%) and in the threads (40%). The remaining 10% of the applied torque does useful work in stretching the bolt and providing the preload . Initially, as the torque is applied, it must overcome static friction under the head of the bolt or nut (depending on which end is being torqued) and also in the threads . Finally, dynamic friction prevails and the torque is distributed in a 50 / 40 / 10% manner as the bolt is tensioned . The torque value is dependent on the friction produced in the threads and under the torqued bolt head or nut and the fastened material or washer if used . This friction can be affected by the application of a lubricant or any plating (e.g. cadmium or zinc) applied to the threads, and the fastener's standard defines whether the torque value is for dry or lubricated threading, as lubrication can reduce the torque value by 15% to 25%; lubricating a fastener designed to be torqued dry could over-tighten it, which may damage threading or stretch the fastener beyond its elastic limit, thereby reducing its clamping ability . </P> <P> Either the bolt head or the nut can be torqued . If one has a larger bearing area or coefficient of friction it will require more torque to provide the same target preload . Fasteners should only be torqued if they are fitted in clearance holes . </P>

Increasing the angle of a joint is known as