Investigation into the self-loosening behavior of bolted joint subjected to rotational loading

The self-loosening mechanisms of a bolted joint subjected to a rotational load were investigated using three-dimensional FEM. A previous theory regarding the conditions of initiation and progress of loosening was verified. Loosening occurs if the relative rotation angle applied to the bolt reaches a critical value θcr and the thread surface undergoes a complete slip. In addition, loosening progresses if Tsl < Tw < Tst holds, where Tsl, Tst, and Tw are the loosening and the tightening torque of the thread surface, and the slip torque of the bearing surface, respectively. If above conditions hold, bolt tension decreases in proportion to the relative rotation angle of the bolt during complete thread-surface slip while bolt tension does not change during complete bearing-surface slip. If these conditions do not hold, loosening does not progress even if the thread or the bearing surface undergoes complete slip. In order to verify the above loosening mechanism, a loosening test was carried out. It was confirmed that the loosening progressed by the same mechanism as that shown by FEM.