Measurement of Drag Torque, Lift Off Speed, and Identification of Frequency Dependent Stiffness and Damping Coefficients of a Shimmed Bump-Type Foil Bearing

This thesis presents measurements characterizing the static and dynamic performance of a BFB configured with shims of two thicknesses (30 ?m and 50 ?m). Parameters of interest include drag torque, rotor lift off speed, and the estimation of force coefficients of a BFB with shims (30 ?m and 50 ?m thick). The thesis also compares those results to those of the original BFB (without shims).

Drag torque measurements during shaft acceleration tests up to 50 krpm show that the lift off speed of both the original bearing and the shimmed bearing increases linearly with applied unit load (W/(LD)). The bearing startup friction factor f=T/(RW) during dry sliding condition for the original bearing (f~0.3) is constant with applied load (W/(LD)), while the bearing in shimmed configurations show a larger friction factor that decreases with load (W/(LD)). Once airborne, a bearing in all three configurations (shimmed and not shimmed) show a similar low (f~0.05 at W/(LD)~ 20 kPa) friction factor that decreases with increasing load.

Bearing dynamic force coefficients are estimated over a frequency range of 200-450 Hz, under a specific load ~14.3 kPa. The shims have an unremarkable effect on the foil bearing direct stiffness coefficients. The shimmed BFB shows increased direct damping coefficients (in particular along the static load direction and at high frequencies) while operating at 50 krpm. The energy dissipated is best characterized with a structural loss factor ?, a function of the bearing elastic (K) properties, and the sliding friction characteristics. Over the narrow arbitrary frequency range from 300-400 Hz and for dynamic motions of amplitude of 20 ?m, the bearing without shims offers a ?? 25% larger than the original bearing, demonstrating that the shimmed bearing dissipates more mechanical energy, albeit the standard deviation in the average loss factor of the shimmed bearings is much larger.

Measurements of the turbocharger (TC) shaft vibration conducted as the shaft accelerates toward 50 krpm (833 Hz) show that a shimmed BFB reduced subsynchronous whirl motions of the TC shaft apparent with the original BFB (WFR~0.30). When supported on a BFB with 50 ?m shims, the TC shaft operates free of subsynchronous whirl motions.

Shimming, therefore is a simple, economical way of increasing energy dissipation in BFBs thereby improving their rotordynamic performance. Alas shimming also offers some undesirable characteristics such as higher startup torque requirements. Note however, that once airborne, the drag friction factor of a shimmed BFB is similar to that of the original bearing.