Dynamic Response and Stability of Pressurized Gas Squeeze-Film Dampers

Abstract

Compressible squeeze films, an important and interesting area in gas lubrication, have been relatively neglected in recent times. Aircraft engines are being designed with light weight flexible rotors operating at high speeds and temperatures that may eventually eliminate the use of oil lubrication. A gas or air SFD might be a viable alternative to a conventional oil damper, in high temperature applications that preclude the use of oil lubrication. Oil squeeze-film dampers currently being used for rotordynamic control will not be viable at temperatures above 350°F, due to limitations on lubricant oil temperature. A good example of gas SFD application is in conjunction with high temperature gas lubricated foil bearings, which inherently have low damping. This paper presents an analysis of pressurized air dampers, similar to a hydrostatic gas bearing. Pressurized air is supplied through a series of orifices in the bearing midplane. Airflows through the orifices and the resulting pressure forces are calculated using a simple gas-flow model, as in orifice compensated hydrostatic bearings. A small perturbation analysis of the shaft center yields the stiffness and damping coefficients, for centered circular orbits. Damping characteristics are studied for a range of parameters such as supply pressure, orifice diameter, pocket volume, orbit size, number of orifices and shaft speed. Results show that maximum damping forces are generated for near choking flow conditions. The damping coefficient becomes negligible at frequencies above 350 Hz. For damping force to be present, the gas pressurization has to exert a force on the rotor opposing the instantaneous velocity, or, 90 degrees out of phase with displacement. Linear stability of unbalanced dampers undergoing centered circular orbits, is also investigated, in view of their relevance to rotordynamics. Damper design curves are presented for various parameters.