Effects of eccentric phase difference between two discs on oil-film instability in a rotor-bearing system

The operating speed of the rotating machinery often exceeds the second order or even higher order critical speeds to pursue higher efficiency. Thus, the second mode whirl/whip can appear when the operating speeds approach or exceed twice the second order critical speed according to the reference A. Muszynska (2005) [1]. In this study, we investigate how the eccentric phase difference between two discs influence the oil-film instability (the first/second mode whirl/whip) in a rotor-bearing system. Firstly, a lumped mass model with 20 degrees of freedom (DOFs) of a rotor system with two discs considering the gyroscopic effect is developed. The graphite bearing and sliding bearing are simulated by a spring-damping model and a nonlinear oil-film force model based on the assumption of short bearings, respectively. The research focuses on the effect of eccentric phase differences of two discs on the onset of instability and nonlinear responses of the rotor-bearing system by using the bifurcation diagrams, spectrum cascades, vibration waveforms, orbits and Poincare maps. The results show that the instability speed increases when the eccentric phase difference becomes larger and it increases almost linearly when the eccentric phase difference is greater than 20 degrees. Moreover, complicated combination frequency components related to the first and second mode whirl/whip frequencies are also excited and the transfer of the self-excited vibration energy between the first and second mode whips can be observed under a larger eccentric phase difference. The study may contribute to a further understanding of the oil-film instability of such a rotor-bearing system. (C) 2013 Elsevier Ltd. All rights reserved.