The factors influencing torque-preload relationship of bolted joints in heavy truck

To improve the load consistency in heavy truck assembly, a high-precision fastening instrument is developed to simulate bolted assemblies with various materials and sizes. This instrument enables to control the tightening process with torque, torque-turn, or torque-to-yield strategy. The tightening torque and the preloading force can be measured in real time. Adopting the design of experiment (DOE) method, a tightening tests program, characterized by six variables with two levels each, is designed to investigate the influence of six factors on the torque-preload relationship (nut factor) in bolted joints. These factors include joint materials, fastener class, gasket grade, lubrication, surface roughness, tightening speed and repeated tightening. The effects of these factors on the nut factor and their interactions are evaluated with statistic technique. The results show that the surface roughness and the joint materials are the major factors affecting the nut factor; the interactions between the joint materials and the gasket grade and the interactions between the joint materials and the lubrication also exert considerable influence. The nut factor decreases rapidly with the tightening times, and it remains stable after four tightenings.