DYNAMIC-RESPONSE AND STABILITY OF PRESSURIZED GAS SQUEEZE-FILM DAMPERS

Compressible squeeze films, an important and interesting area in gas l ubrication, have been relatively neglected in recent times. Aircraft e ngines 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 precl ude the use of oil lubrication. Oil squeeze-film dampers currently bei ng used for rotordynamic control will not be viable at temperatures ab ove 350 degrees F, due to limitations on lubricant oil temperature. A good example of gas SFD application is in conjunction with high temper ature gas lubricated foil bearings, which inherently have low damping. This paper presents an analysis of pressurized air dampers, similar t o a hydrostatic gas bearing. Pressurized air is supplied through a ser ies of orifices in the bearing midplane. Air flows through the orifice s and the resulting pressure forces are calculated using a simple gas- flow model, as in orifice compensated hydrostatic bearings. A small pe rturbation analysis of the shaft center yields the stiffness and dampi ng coefficients, for centered circular orbits. Damping characteristics are studied for a range of parameters such as supply pressure, orific e diameter, pocket volume, orbit size, number of orifices and shaft sp eed. Results show that maximum damping forces are generated for near c hoking flow conditions. The damping coefficient becomes negligible at frequencies above 350 Hz. For damping force to be present, the gas pre ssurization has to exert a force on the rotor opposing the instantaneo us velocity, or, 90 degrees out of phase with displacement. Linear sta bility of unbalanced dampers undergoing centered circular orbits, is a lso investigated, in view of their relevance to rotordynamics. Damper design curves are presented for various parameters.