Study on nonlinear dynamic characteristics of propulsion shafting under friction contact of stern bearings

A nonlinear three degree-of-freedom (3-DOF) mode-coupling model considering perturbations of the stochastic rough surface is proposed. The mechanism model is used to simulate the peak value of the friction vibration line spectrum of the real ship, and the calculation results are compared with the test results of the real ship, which verifies the correctness of the mechanism model. A nonlinear dynamic model of propeller-shaft under friction contact of stern bearing is established, which is based on the proposed mechanism model. The stability and response characteristics of the system under different operating parameters of the model are analyzed. The results show that the larger the friction coefficient between shaft and bearing is, the closer the horizontal and vertical stiffness of stern bearing is, and the smaller the modal damping ratio is, the more unstable the system is. When the system produces unstable friction self-excited vibration due to modal coupling, the friction vibration spectrum presents an obvious line spectrum peak, and the peak frequency corresponds to an unstable frequency obtained from the stability analysis, and a continuous harmonics spectrum will be generated. Under the action of friction, the shaft center makes reverse whirl, and the higher the speed is, the greater the amplitude of whirl is. When the system doesn’t have modal coupling instability, under the coupling effect of perturbations of the stochastic rough surface between the shaft and the bearing and the friction excitation of stern bearing, multiple peaks of friction-induced vibration will be generated within the upper limit of the system analysis frequency of 1000 Hz, and the peak frequency corresponds to the natural frequency of the system. Under the coupling action of the unbalanced propeller rotating eccentric force and its stern bearing friction excitation force, the system produces a vibration peak at the shaft frequency, and the frequency of the friction excitation force is equal to the excitation frequency of the unbalanced propeller rotating eccentric force.