Study on bolt loosening mechanism under transverse load considering slip and adhesion status of contact surfaces

The complexity of the thread surface makes it challenging to analyze the mechanism of bolt loosening from a mechanical perspective. To analyze the mechanism of bolt loosening, this paper proposes a mathematical model. Initially, the characteristics of the threaded surface are precisely represented using cylindrical and Cartesian coordinate systems, and the contact relationship of the thread contact surface is derived through normal and tangential vectors. Subsequently, the integral expressions for friction force and torque under transverse load are derived. The results indicate that as the frictional force between the contact surfaces increases, the torque caused by the friction force gradually decreases. Complete slip occurs when the frictional force reaches the critical value, at which point the torque is essentially at its minimum. Furthermore, the comparison between theoretical and finite element results demonstrates that the derived formulas can qualitatively express the loosening mechanism of the bolt under transverse load. Parametric analysis shows that the greater the transverse amplitude, the more likely the contact surfaces will slip. Slip reduces the resistance torque between the contact surfaces, leading to bolt loosening. Increasing the friction coefficient of the thread contact surface and ensuring that the friction coefficient of the bolt head contact surface is sufficiently different from that of the thread contact surface can effectively prevent bolt loosening. This strategy ensures that at least one contact surface maintains adhesion during vibrations, sustaining an adequate resisting torque to counteract loosening.