Measurement of Drag Torque, Lift off Speed and Rotordynamic Force Coefficients in a Shimmed BFB

Abstract

Oil free microturbomachinery relies on gas bearings, in particular bump type foil bearings (BFBs), to make nearly frictionless systems with improved efficiency, long life and extended maintenance intervals. Rotors supported on generation I BFBs often show large amplitude sub synchronous whirl motions that limit their application into high speed conditions. Mechanically preloading a BFB through shimming is a common practice that improves rotordynamic performance. This paper quantifies the effectiveness of shimming on the forced performance of a BFB (L = 38.1 mm, D = 36.5 mm) that comprises of a single top foil and bump foil strip. The dry-friction torque (T) during startup is proportional to the applied load and increases with shim thickness. The bearing lift-off shaft speed, establishing operation with a gas film, also increases with load. The friction factor f = T/(RW) during dry friction operation at start up increases with shim thickness albeit decreasing with applied load. Once the bearing is airborne, the bearing–shimmed or not–shows approximately the same low friction factor, f ~ 0.05 under a specific load W/(LD) ~ 20 kPa. Dynamic loads spanning 200–450 Hz excite the BFB in a rotordynamics rig operating at 50 krpm (833 Hz). A static vertical load, W/(LD) acts on the bearing. The bearing direct stiffnesses increase with increasing excitation frequency while the damping coefficients decrease slightly. The stiffnesses for the various BFB configurations offer unremarkable differences. The direct damping coefficients of the shimmed BFB are up to 30 % larger than the coefficients of the original bearing. The frequency averaged material loss factor for BFB with 50 µm shims \((\bar{\gamma }\sim 0. 6 2)\) is 25 % larger than that for the original bearing \((\bar{\gamma }\sim 0. 4 7 )\). As expected, a shimmed BFB dissipates more mechanical energy than a BFB without shims.