Hybrid Flexure Pivot-Tilting Pad Gas Bearings: Analysis and Experimental Validation

Abstract

Gas film bearings offer unique advantages enabling successful deployment of high-speed microturbomachinery. Current applications encompass micro power generators, air cycle machines, and turbo expanders. Mechanically complex gas foil bearings are in use; however, their excessive cost and lack of calibrated predictive tools deter their application to mass-produced oil-free turbochargers, for example. The present investigation advances the analysis and experimental validation of hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery. These characteristics are adequate load support, good stiffness and damping coefficients, low friction and wear during rotor startup and shutdown, and most importantly, enhanced rotordynamic stability at the operating speed. Hybrid (hydrostatic/hydrodynamic) flexure pivot-tilting pad bearings demonstrate superior static and dynamic forced performance than other geometries as evidenced in a high-speed rotor-bearing test rig. A computational model including the effects of external pressurization predicts the rotordynamic coefficients of the test bearings and shows good correlation with measured force coefficients, thus lending credence to the predictive model. In general, direct stiffnesses increase with operating speed and external pressurization, whereas damping coefficients show an opposite behavior. Predicted mass flow rates validate the inherent restrictor-type orifice flow model for external pressurization. Measured coast-down rotor speeds demonstrate very low-friction operation with large system time constants. Estimated drag torques from the gas bearings indirectly validate the recorded system time constant.

Hybrid Gas Bearings With Controlled Supply Pressure to Eliminate Rotor Vibrations While Crossing System Critical Speeds

“…The rotordynamic analysis consists of modeling the test rotor with XLTRC 2 ® and prediction of the bearing force coefficients with TILTPADHGB©. Computational code TILTPADHGB©, developed by San Andrés [28], predicts the static and dynamic forced response of fixed or hydrostatic and hydrodynamic tilting pad gas. Table 3 shows the representative radial clearances and preload for the test bearings, as used to obtain predictions.…”

Near-field acoustic levitation and applications to bearings: a critical review

“…Other researchers changed the supporting structures by introducing a tilting pad or compliant metal material. Tilting pad gas bearings have been successfully used in high-speed rotating machinery due to their inherent stability [49][50][51][52]. Foil gas bearings, which have a flexible bearing surface constructed with a metal plate, are widely used in much turbomachinery [53][54][55][56][57].…”

Lumped parameters models for the stability analysis of rotors supported on gas bearings

“…The linearization method was used in [11] to compute the gas film coefficients for a high speed cryogenic turboexpander and in [12] for a heat pump turbocompressor. An interesting analysis of tilting pads is given in [13]. Many other works on the topic can be found in literature, such as [14]- [17].…”