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Research Papers

Experimental and Analytical Studies on Flexure Pivot Tilting Pad Gas Bearings With Dampers Applied to Radially Compliant Pads

[+] Author and Article Information
Aaron Rimpel

 Southwest Research Institute, San Antonio, TX 78238-5166

Daejong Kim1

 University of Texas at Arlington, Arlington, TX 76019-0018daejongkim@uta.edu

Subtracting the baseline response (i.e., amplitude and phase angle with baseline imbalance level) from the total response with calibrated imbalance masses yields the system response for the calibrated imbalance excitation only. The baseline subtraction method assumes linear superposition is valid.

1

Corresponding author.

J. Tribol 131(4), 041001 (Sep 22, 2009) (10 pages) doi:10.1115/1.3201831 History: Received March 26, 2009; Revised June 13, 2009; Published September 22, 2009

Hydrodynamic flexure pivot tilting pad gas bearings (FPTPGBs) can enable successful operation of oil-free microturbomachinery and FPTPGBs with radially compliant pads permit rotor centrifugal and/or thermal growth to exceed original bearing clearances and achieve higher speeds. This work presents the experimental and analytical study of such bearings and the application of dampers behind the pad radial compliance structure. A time domain orbit simulation method was implemented as the primary analysis tool to predict the rotor-bearing response to imbalance, the presence and location of critical speeds, etc., and to compare with test results. Experiments demonstrate the stable hydrodynamic operation of FPTPGBs with an 28.6mm, 0.8 kg rotor above 120 krpm, for the first time. The rotor-bearing system was intentionally destabilized in tests by increasing bearing clearances and the viscoelastic dampers added behind the FPTPGB pads delayed the onset of subsynchronous vibrations (from 43 krpm without damper to above 50 krpm with damper). Midrange subsynchronous vibrations of the destabilized system initiated at 20krpm were suppressed by 25krpm due to the stabilizing effect of rotor centrifugal growth. The viscoelastic dampers had a negligible effect on suppressing these midrange subsynchronous vibrations in experiments, but this was not demonstrated in simulations, presumed to be due to much lower stiffness contribution of the damper at lower frequencies.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

FPTPGB-C with flexure pivot and compliant beam structures created by wire EDM; photo adopted from Ref. 8

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Figure 2

FPTPGB-C geometry, adopted from Ref. 8

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Figure 3

FPTPGB test rig and instrumentation. Inset: end of rotor with impulse turbine.

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Figure 4

Synchronous response Bode plots for test rig setup No. 1 for (a) front and (b) rear probe stations

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Figure 5

Predicted synchronous response (filtered out subsynchronous for unstable cases) for rotor-bearing system with 700 mg mm imbalance and nominal clearances of 31–34 microns. Results were compared with the major amplitude from rear probe station for test rig setup No. 1.

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Figure 6

Cascade plot of predicted response for 33 micron nominal clearances using orbit simulation. Stability is shown through 140 krpm even though subsynchronous rotor whirl is present from 40 krpm to 80 krpm.

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Figure 7

Predicted synchronous and subsynchronous responses for 33 micron nominal clearances and model comparing effect of centrifugal rotor growth.

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Figure 8

Cascade plot for test rig setup No. 1—Response for rear vertical probe.

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Figure 9

Damaged rotor and bearing after seizure occurred while running at 120+krpm for extended period

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Figure 10

Imbalance response (baseline subtracted) for 288 mg mm imbalance applied in-phase at each end of rotor. Thick lines represent predictions for nominal clearances of 31 microns and 32 microns.

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Figure 11

Response amplitudes normalized by amplitude for 288 mg mm versus added imbalance normalized by 288 mg mm. Solid line represents 1:1 and dashed line represents +5%.

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Figure 12

Adding shims between upper and lower bearing halves to increase vertical clearance

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Figure 13

Cascade plot for test rig setup No. 2 with 76 micron shims and no damper—response for rear vertical probe

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Figure 14

Cascade plot for test rig setup No. 2 with 76 micron shims and with damper—response for rear vertical probe

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Figure 15

Comparison of subsynchronous responses for setup No. 2 with 76 micron shims with and without damper for rear vertical probe

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Figure 16

Coast-down speed versus time for test rig setup No. 2 and 76 micron shims; filled-in symbols represent data points used for curve fit

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Figure 17

Summary of exponential time constant versus shim thickness for coast down tests with test rig setup No. 2

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Figure 18

Predicted vertical responses for rotor-bearing system with split offset (6.3 microns) in bearing, nominal clearance 30 microns

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Figure 19

Predicted vertical responses for rotor-bearing system with split offset (6.3 microns) in bearing without considering damper stiffness, nominal clearance 30 microns

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Figure 20

Predicted vertical responses for rotor-bearing system with split offset (3.2 microns) in bearing, nominal clearance 32 microns

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