Nonlinear vibrations of a marine propulsion shafting rotor system considering bolt loosening and interface friction coupling

As a critical component of marine propulsion shafting, bolted joints are prone to loosening under the influence of alternating loads and vibration impacts, leading to reduced connection stiffness and potentially inducing thrust bearing misalignment as well as mechanical-magnetic coupling faults. Moreover, under low-speed and heavy-load conditions, bolt loosening exacerbates interface friction in the stern bearing, severely compromising the safe operation of the power system. In response to the issues, a dynamic modeling method for bolt loosening based on the equivalent crack theory is proposed. This method accurately captures the periodic opening and closing behavior of the connection interface during bolt loosening through the breathing effect of the crack. Furthermore, by comprehensively considering the frictional excitation of the stern bearing and the misalignment effect of the permanent magnet thrust bearing, a coupled shafting rotor dynamic model integrating friction and bolt loosening faults is established. Through numerical simulation and experimental validation, the effects of bolt loosening, thrust bearing misalignment, and stern bearing stiffness on the nonlinear vibration response of the shafting are investigated. The vibration response characteristics and evolution laws of bolt loosening faults are characterized, and the dominant excitation sources governing the vibration response at different fault stages are identified, providing support for bolt loosening state recognition based on vibration characteristics.