Experimental Verification of Subcritical Whirl Bifurcation of a Rotor Supported on a Fluid Film Bearing

Abstract

The paper presents the results of the experiments that were conducted on a short fluid film bearing with a simple single disk rotor. The behavior of the journal was analyzed as a function of the rotor system parameters such as the load, speed, and imbalance mass. It was verified that the journal bearing can lose stability through either a subcritical or a supercritical bifurcation. In the supercritical bifurcation, the expected gradual increase in the amplitude of the limit cycle was observed, while in the subcritical bifurcation there was a sudden jump to a large limit cycle. In the case of a subcritical bifurcation, it was also observed that the journal bearing became unstable below the rotor threshold speed of instability, following a small perturbation applied to the rotor. Demarcations were made for the stable regions of operation (regions where the rotor is stable below the threshold speed of instability) for the journal bearing based on the obtained experimental results. Experiments were also performed to analyze the effects of imbalance of the rotor on the threshold speed of instability. It was observed that the flexible unbalanced rotor had a lower threshold speed of instability. The limitations of the linear theory of fluid film bearings in predicting these phenomena are discussed.