Stability and Instability of a Two-Mode Rotor Supported by Two Fluid-Lubricated Bearings

Abstract

This paper is a continuation of the series of papers on application of the improved fluid force model for lightly loaded shafts rotating in a fluid environment. The fluid force model is based on the strength of the circumferential flow. The considered two-mode rotor is supported in two fluid-lubricated bearings, thus it contains two potential sources of instability. The eigenvalue solution predicts thresholds of stability and provide natural frequencies and modes of the system, including the flow-induced mode. The nonlinear model of the rotor/bearing system allows for evaluation of parameters of after instability onset self-excited vibrations (whirl and whip). Experimental data illustrate the dynamic phenomena predicted by the model. In particular, they show an undocumented new phenomenon, the simultaneous existence of two whip vibrations with frequencies corresponding to two modes of the rotor. A radial preload of the rotor results in journal eccentric position inside the bearings, which causes specific changes in the fluid forces (an increase of radial stiffness and reduction of circumferential velocity) providing better stability of the rotor. This effect is illustrated by the experimental data, as well as is predicted by the model.