Nonlinear Dynamic Characteristics Of Rotor-Bearing System Under The Coupled Motions Of Heave And Pitch

Due to the disturbances of wind and wave,the sailing ships can easily encounter complex oscillating motions around the equilibrium position.The vibration with low-frequency and large-displacement generated by ship motions could affect the stability of the ship power plant in the form of transport motion.Therefore,it is necessary to study the dynamic characteristics of marine rotating machinery under oscillating motion.This paper takes the rotor-bearing system,the core component of marine rotating machinery,as the research object,and considers the coupled motions of heave and pitch,as well as the tilting of the rotor in the bearings.The dynamic equation of marine rotor-bearing system is established by the second type Lagrange equation.Besides,the nonlinear dynamic behavior of the system is discussed by using numerical methods.The main work and results are as follows:(1)The dynamic behaviours of the system are analyzed when the rotor speed changes.Based on the short bearing oil film force assumption,the dynamic model of the marine rotor-bearing system under the coupled motion of heave and pitch is established.The numerical calculation results show that complex quasi-periodic bifurcations and inverted bifurcations could occur under the heave and pitch coupled motions after the first instability state of the rotor system;at high speeds,the ship motion could increase the second instability speed of the rotor,the system enters the quasi-periodic motion through the quasi-periodic bifurcation until the rotor finally touches the inner wall of the bearings.(2)The vibration characteristics of the rotor system are discussed when the ship motion parameters change.The transport motion has few effects on the marine rotor system when the amplitude of the heave motion changes within a small range;however,when the amplitude of the heave increases further,the amplitude of the rotor system could increase sharply,and the system turns from a quasi-periodic motion state to a chaotic state.As the pitch amplitude increases,the system transforms from a chaotic state into a quasi-periodic state,and the vibration amplitude of the rotor system undergoes a process of decreasing first and increasing later;in addition,the pitch motion can cause the rotor tilt in the bearing and precession of rotor appears.When the system frequency ratio increases,the rotor system enters the quasi-periodic state through the quasi-period bifurcation from period 2 first and finally transitions to the chaotic state.(3)The dynamic characteristics of the rotor system are studied when parameters of the rotor-bearing system change.When the rotor eccentricity changes,the dynamic bifurcation characteristics of the marine rotor system under the coupled motions of heave and pitch are expressed as:single period?quasi-period? period 2;and when the eccentricity increases to a certain value,the rotor amplitude increases sharply,and the displacement of the rotor in longitude motion is larger than that in lateral motion.When the bearing length-to-diameter ratio increases,the bearing oil film force increases steeply,and the rotor system does not have simple movement forms such as single period and period 2 that the case without considering transport motion;by comparing the amplitude of the rotor without the influence of transport motion,we find that the amplitude of the rotor is smaller than that of the rotor without considering the transport motion when the length-to-diameter ratio is relatively large.