Kinetic Versus Static Frictional Torque Components in Threaded Fastener Applications

Abstract

This paper investigates the kinetic and static frictional torque components in threaded fasteners during the initial fastener tightening, subsequent torque audit, as well as during the loosening of previously tightened bolts. In less critical applications, the peak kinetic torque value is often used for predicting the bolt preload. The peak value of the tightening torque and its frictional components are mainly determined by the kinetic friction coefficients between the engaged threads and between the rotating nut (or head) and the joint surface. During subsequent quality inspection of the joint after its initial assembly, an audit residual torque value (in the tightening direction) is often used for predicting the residual fastener tension and joint clamp load, as well as for predicting the stability of the clamp load. In contrast with the peak kinetic torque, the audit torque and its thread and under head/nut frictional components are mainly determined by the static friction coefficients. In some cases, the careful application of a breakaway torque in the loosening direction (loosening torque) may be used as a measure for the residual clamp load; similar to the quality audit torque, the loosening torque is determined by the static friction coefficients of the bolted system. An experimental procedure and test set up are proposed to investigate the effect underhead contact radius, thread pitch, surface coating, and fastener head versus nut side tightening on the static and kinetic frictional torque components.

Nonlinear Bonding Analysis of Bolt Connection and Secondary Tightening Strategy

“…The error range of the final pre-tightening force is ±5%~±35%, even up to ±50% [15]. Nassar, Zou et al [16][17][18] analyzed the effect of surface roughness, lubrication method, repeated tightening, and other factors on the friction coefficient, the variation of torque coefficient, and the discrete law of bolt pre-tightening force. The torque-angle method is a combination of torque method and angle method to load bolt together, achieving higher control accuracy than torque method.…”