Verification of Self-Loosening Mechanisms for Treaded Fasteners (2nd Report, Behaviour and Mechanisms of Bolt Loosening Under Transverse Load due to Vibrations of a Washer Along an Arc)

Abstract

Rotational loosening mechanisms of a bolt were investigated by FEM, under the assumption of a twist at the center of a circular joined structure in which the bolt was set along a certain pitch circle. In this structure, the bolt is loosened by combining the translational and rotational loads. In the case of a large pitch circle in which only loosening rotations occur, the directions of frictional shear forces on the threads were along concentric circles ; however, the instantaneous center was located near the thread surface, and the center was eccentric with the axis of the bolt. If the radius of the pitch circle is set smaller, the instantaneous center moves closer to the center of the bolt, and finally reaches to the same position at the center of the bolt. On the other hand, the directions of frictional shear forces on the threads were calculated theoretically using the inclination and friction on a pressure flank. The results were in good agreement with FE and analysis. By considering these mechanisms, it is estimated that the number of occurrence of loosening in one vibration cycle changes at the border when the diameter value of the pitch circle equals that of the screw threads. If the diameter of the pitch circle becomes smaller than that of the screw threads, the number changes from two to one. With the exception of torsional center-fastened structures, since the pitch circle is very small, loosening of general joined structures will occur twice in one vibration cycle.