Stability Analyses on Flexure Pivot Tilting Pad Gas Bearings for Microturbomachinery

Presented are stability analyses of a flexure pivot tilting pad gas bearing with radial compliance using linearized perturbation method. The radial compliance was intended to accommodate large rotor centrifugal growth at high speeds. The rotor centrifugal growth was considered in the perturbation analyses as a physical mechanism that reduces the gas film thickness. The rotor growth reduces effective bearing clearance and could be a stabilizing mechanism at intermediate speed ranges as well as a limiting factor to the maximum achievable rotor speed for given bearing clearance and pad radial stiffness. The bearing stability was very sensitive to the bearing clearance. The smaller clearance is desirable to achieve higher stability margin at low speed ranges below 100 krpm. However, at higher speeds above 100 krpm, the larger clearances provide more stability margins. The cross-coupled stiffnesses and damping coefficients become near zero at resonance frequency of pad tilting motion, and direct stiffnesses become near zero as excitation frequency approaches the resonance frequency of pad radial motion.