Experimental Measurement of the Shank Torque As a Function of the Stiffness and Frictional Characteristics of the Bolted Joint

Abstract

During tightening, the amount of torque given by the difference between the tightening torque, which is directly applied by the torque wrench, and the underhead torque, flows through the screw shank towards the threaded portion. This torque combines with the axial preload to bring about the overall stress state of the screw at tightening. Upon release of the torque wrench, a certain amount of the shank torque is released due to the elastic springback of the screw-plates system. In the literature [1], this phenomenon is just briefly treated by a few authors: they generally agree that approximately a half of the initial shank torque is released just a few seconds after torque wrench removal. This indication is given regardless of the frictional [2] and stiffness [3] parameters, which govern the joint. The present contribution aims at assessing, if there is any effect of the following parameters on the amount of shank torque being released after wrench removal: (i) the ratio between the torsional stiffness of the screw and of the plates, (ii) the friction coefficients in the underhead and in the thread. The experimentation has been run on a M20 8.8 class socket head screw, which has been instrumented by a double array of strain gauges, to simultaneously measure both the axial preload and the torque acting on its shank. Two different types of joined members have been examined: a cylindrical sleeve whose diameter is twice the screw diameter (compliant joint) and a rectangular plate whose transverse dimensions are more than ten times larger than the screw diameter (stiff joint). The underhead and thread friction coefficients have been controlled by properly selecting lubrication conditions. The main outcome of the work is that the torsional stiffness of the joined members does have an impact on the residual shank torque. A simple mathematical model has also been implemented, in order to predict the residual shank torque during the design phase.

Research on the relationship between the tightening torque and the preload of composite bolted structures

“…Croccolo et al 30 studied the influence of design and manufacturing process parameters on the friction coefficient of the bolt bearing surface, including material composition, surface roughness grade, surface treatment process, and repeated tightening times. Croccolo et al 31 evaluated the influencing factors of key physical quantities on the elastic release of torque. Croccolo et al 32,33 tested the friction properties of hexagonal head steel bolts, and studied the effects of coating and lubrication on the friction coefficient through experiments.…”

Effect of tightening process parameters on the T-F curve of bolts in composite structures

“…The literature 12 investigated the influence of design and manufacturing process parameters on the friction coefficient of the bolt bearing surface, including material composition, surface roughness grade, surface treatment process and number of repetitions of tightening. The literature 13 assessed the influence of key physical quantities on the elastic release of torque, including the ratio of the bolt to the rotational stiffness of the part being connected. In the literature, 14,15 the frictional properties of hexagonal-head steel bolts were tested, and the effect of coating and lubrication on the coefficient of friction was experimentally investigated.…”