Experimental Analysis on the Tightening Torque - Preloading Force Relationship in Threaded Fasteners

The aim of this study is to provide an experimental methodology useful to determine the friction coefficients in bolted joints and, therefore, to relate precisely the tightening torque to the preloading force. The components under investigation are some clamped joints made of aluminium alloy and used in front motorbike suspensions to connect the steering plate (fork) to the legs and the legs to the wheel pin. The aluminium alloy is realised by a casting or a forging process afterwards anodized or spry-painted in surface. Some specific specimens have been appropriately designed and realised with the same process of the actual components. The bolt torque is given by a torque wrench whereas the preloading force has been evaluated by means of a strain gauge. Thread and underhead friction coefficients have been studied separately, by applying an axial bearing located between the bolt head and the flange of the specimen. The overall friction coefficient and the torque coefficient (nut factor) have been calculated. Experimental tests have been carried out by applying the Design of Experiment (DOE) method in order to obtain an accurate mathematical model that involves the significant friction variables and their interactions. The results of this preliminary study have been, then, applied to those connections used in front motorbike suspensions to lock the steering plates with the legs and the legs with the wheel pin, by means of one or two bolts. The preloading force, produced during the tightening process, should be evaluated accurately, since it must lock safely the shaft, without overcoming the yielding point of the hub. Firstly, the applied tightening torque has been precisely related to the imposed preloading force by means of the friction coefficients definition. The tensile state of clamps, have been evaluated both via FEM and by leveraging some design formulae proposed by the Authors as functions of the preloading force and of the clamp geometry. Finally the results have been compared to those given by some strain gauges applied on the tested clamps: the discrepancies between numerical analyses, design formulae and experimental results remains under a threshold of 10%.