Dynamic behavior analysis of cracked rotor based on harmonic motion

Abstract

In the present study the additional slope and bending moment at crack position are used in analyzing the dynamic behavior of a general cracked rotor. The nonlinear motion of the cracked rotor, which results in the harmonic vibration, is simulated using the response including bending moment and the additional slope recursively. Even though the change of the orbit at the subcritical speed occurs, the magnitude of additional slope does not change if the crack-induced dynamic bending moment is smaller than the gravity-induced static bending moment at the corresponding critical speed range; the cause of the orbit change is the high value of the displacement influence coefficient at the corresponding critical speed. Only at the speed range where the dynamic bending moment is enough large to affect the total bending moment, the change of additional slope occurs with the speed change and it becomes one of the causes of the drastic orbit change. In the present research model, the orbit change due to the large dynamic bending moment as well as the high influence coefficient occurs at around subcritical speeds of the second critical speed. The continuous operation of the cracked rotor at such speed range having large dynamic bending moment may produce the fast crack propagation. And also it is analyzed that the second vibration mode happens when the speed closely approaches half of the second critical speed.