Tribology of discontinuous tightening: An experimental and numerical analysis

Threaded fasteners are widely used across all engineering fields and applications. Conventional tightening methods typically rely on continuous tightening tools, however the weight and size of these tools, together with the need for a reaction bar to counteract the reaction torque, can make them difficult to use in certain situations. An alternative approach is discontinuous tightening, in which impacts are used to deliver the tightening torque. This method allows for smaller, lighter tools and significantly reduces the reaction forces transmitted to the operator. The main challenge with this technique is predicting the joint’s response, both frictional and vibrational, to impulsive loading, which can result in a non-optimal tightening outcome. This research work presents both an experimental and numerical approach to investigate the involved phenomena. A dedicated experimental test bench has been developed to reproduce the most relevant contact conditions under controlled settings, while an explicit dynamic finite element model is employed to investigate the mechanisms involved at the contact scale. The obtained results highlight the role of the vibrational response of the mechanical system in reducing the overall frictional resistance during discontinuous tightening. Increasing the impulsive tangential velocity and decreasing the normal preload leads to a higher vibrational response and a consequently lower equivalent friction coefficient.