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RESEARCH PAPERS

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

[+] Author and Article Information
Luis San Andrés

Mechanical Engineering Department, Texas A&M University, College Station, TX 77843-3123Lsanandres@mengr.tamu.edu

J. Tribol 128(3), 551-558 (Mar 01, 2006) (8 pages) doi:10.1115/1.2194918 History: Received June 22, 2005; Revised March 01, 2006

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.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Flexure pivot-tilting pad bearing for oil-free test rig

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Figure 2

Geometry of a flexure-pivot pad with orifice for external pressurization

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Figure 3

Schematic cross-sectional view of gas bearing test rig (unit: cm)

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Figure 4

Dimensions of flexure pivot-hydrostatic bearing

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Figure 5

Measured synchronous amplitude of rotor motion supported on flexure pivot gas bearings. Effect of increasing feed hydrostatic pressurization. Test data recorded at left bearing, vertical plane (Ref. 3).

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Figure 6

Flow rate versus supply pressure for test bearings. Measurements (3) and current predictions. Rotor speeds ranged from 10 to 60krpm.

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Figure 7

(a) Predicted gas bearing static journal eccentricity (e∕c) versus rotor speed for increasing supply pressures. Static load W=4.08N (load on pad configuration). (b) Predicted gas bearing attitude angle versus rotor speed for increasing supply pressures. Static load W=4.08N (load on pad configuration).

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Figure 8

Predictions for gas bearing operating at 10krpm and 3.77bar supply pressure. Static load W=4.04N. Left: pressure and film thickness at bearing midplane; Right: three-dimensional pressure field on bearing surface.

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Figure 9

Direct stiffness coefficients (KXX,KYY) for gas bearing versus rotor speed for three magnitudes of gas supply pressure. Comparison of predictions to identified synchronous force coefficients from measurements.

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Figure 10

Direct damping coefficients (CXX,CYY) for gas bearing versus rotor speed for three magnitudes of gas supply pressure. Comparison of predictions to identified synchronous force coefficients from measurements.

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Figure 11

Recorded coast-down rotor speed versus time for three feed pressures (2.36, 3.70, and 5.08bar). Estimation of system time constant and speed coast-down predictions based on drag from bearings only.

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Figure 12

Comparison of predicted and measured imbalance response of test rotor for supply pressure 2.36bar(20psig). Left bearing, vertical plane. Mass imbalances noted.

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