Instability Boundary for Rotor-Hydrodynamic Bearing Systems, Part 1: Jeffcott Rotor With External Damping

Abstract

The instability of rotor-hydrodynamic bearing systems has been a major concern in high-speed rotating machine design. The threshold speed is usually considered to be a barrier that cannot be driven through. However, if some kind of external damping is introduced, the situation may be different. The study is first conducted on the Jeffcott rotor with external damping for both vertical and horizontal configurations. For the vertical system, it has been shown by both analytical and numerical study that there is a threshold speed at which the rotor will get into an upper stable region from the previous unstable region. For the horizontal system, an extended Lund Stability Method is presented which proves that the system may have two threshold speeds. This results in a stability pattern which has one region of instability sandwiched between two regions of stability. Then, the study is expanded to the extended Jeffcott rotor. With the introduction of equivalent journal mass, another threshold speed appears which forms a top and final boundary to the stable operation of the rotor. The results of a numerical study show that with the external damping increased, the first several unstable regions can be reduced or eradicated. The external damping has little effect on the last top threshold which is mainly dependent on the value of journal mass. It has also been shown that a certain value of external damping and small journal mass would be very beneficial to improve the stability characteristics of the system over the entire speed range.