Threaded fasteners or bolted joints are used extensively in automotive assemblies. There are standard procedures to evaluate joint performance under block cycles or road loads. The deciding load case for such joint design is slippage analysis of the joint.
There are studies done to evaluate the theoretical and experimental behavior of these joints. There are different ways of understanding the interaction between the bolt and the nut under different loading scenarios. However, none have provided a satisfactory method of quantifying bolt loosening or loss of clamp load under cyclic loading, where no slippage is observed.
Under varying loads, initial relaxation of the joint is followed by a loss of clamping load. Below a critical value, complete loss of clamping load progresses very rapidly and this results in a loose joint. The loss of clamping load below the critical value happens at a transition stage when both elastic deformations of the bolt as well as external moments contribute to rapid loosening.
This study provides an overall understanding of geometric parameters that contribute to sudden loss of clamping load and increased propensity of loosening. This phenomenon is demonstrated through an example in automotive suspension. The experiment and subsequent analysis aims to identify factors contributing to loosening of a circular joint connected with 4 bolts along a designed bolt circle. The loosening of the joint is caused by subjecting it to a combinatory load which induces both translational as well as rotational load at the center of the pitch circle.
The study developed empirical formulae for assessment with different input parameters and is successfully applied to a number of joints under proving ground or rig loads. The study also proposed minimum bolt pre-loads of the joint to eliminate the propensity of bolt loosening in the joint under a given set of time-varying loads.