Investigations on the nonlinear dynamic characteristics of a rotor supported by porous tilting pad bearings

Abstract

Porous tilting pad bearings (PTPBs) combine the advantages of tilting pad and porous gas bearings and can be used in rotary motion and precision machine tools. This research mathematically investigates the dynamic characteristics of a rotor supported by PTPBs. A nonlinear rotor dynamics model that considers the unsteady gas film as well as translational and angular gyroscopic motions is established in this paper. The predicted results agree well with the experimental data when the rotor accelerates to ~ 30 krpm. Subsynchronous responses with large vibration amplitude are observed at high rotational speeds. The subsynchronous vibrations are related to both the fluid circumferential average velocity and eccentricity ratio. Meanwhile, the nonlinear dynamic responses of the system are analyzed by using waterfall plots, fast Fourier transforms, rotor orbits, Poincare maps, and bifurcation diagrams. The results suggest that the adaptive motions of pads can enhance the stability of the system and increase the onset speed of the instability. Given that the eccentricity ratio increases along with the rotor mass (within a certain scale) and the pressure ratio of unload pads, the system stability is enhanced at the same time. Both the preload of PTPBs and pad installation can influence the system stability. Those PTPBs without preload and those pads with loads in between can also benefit the stability of the system. The findings of this work can help designers avoid subsynchronous vibrations at high rotational speeds.