Abstract: Electromagnetic vibration in rotating machinery is endowed with nonlinear characteristic due to the mechanic and electromagnetic coupling. According to the nonlinear relation of unbalance magnetic pull(UMP) with respect to rotor eccentricity, an analytical expression of UMP is deduced and a kinetic equation is set up to express electromagnetic vibration characteristics of a cracked rotor-bearing system with nonlinear oil film. Numerical method is adopted to analyze bifurcation and chaos characteristics of the system. The influence of UMP and crack depth on rotor movement is analyzed through Poincaré maps, amplitude spectrum and bifurcation diagrams. Simulation results reveal that dynamic responses of this system advance to occur due to UMP and the amplitude increases. The spectrum is composed of continuous and broad band frequencies. As crack depth increases, transitory chaotic motion of the rotor system appears in the sub-critical speed region. But the chaotic motion region decreases and its characteristics change around the critical speed, where period-doubling bifurcation almost disappears. And in the supercritical speed domain there is a long region for period-seven. It is indicated that this research may contribute to further understanding of failure mechanism of such a rotor-bearing system with the mechanic and electromagnetic coupling.

Key words: Bifurcation, Chaos, Crack model, Rotor-bearing system, Unbalanced magnetic pull