Investigation into the loosening mechanism of bolt in curvic coupling subjected to transverse loading

It has been widely observed that fasteners turn loose when subjected to dynamic loads in the form of shock, vibration or transverse cyclic loading. This reduces the preload force of bolt and leads to joint failure. Such failures can be catastrophic in safety critical applications. In this paper, the self-loosening of bolts in curvic coupling is analyzed based on the self-rotation of nut in the cases of cyclic torque loads on discs after the preload of bolts. The three-dimensional finite element model for curvic coupling and threads is established in commercial finite element software ANSYS to study the details of the self-loosening process of bolt. Such processes are characterized by microslip at the curvic, the bolt head and the thread contact surfaces. It is found that due to the application of the cyclic transverse load, the nut rotation can occur for only localized slip without complete slip at the bolt head contact surface and the thread surface. However, the curvic surface always shows complete slip under all external loads. The microslip on all contact surfaces are identified to be the major mechanisms responsible for the self-loosening of a curvic coupling. The results obtained agree quantitatively with the experimental observations.