TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

Rotordynamic Performance of a Rotor Supported on Bump Type Foil Gas Bearings: Experiments and Predictions

[+] Author and Article Information
Luis San Andrés, Tae Ho Kim

Mechanical Engineering Department,  Texas A&M University, College Station, TX 77843-3123

Dario Rubio

 Bechtel Corporation, 3000 Post Oak Blvd., Houston, TX 77056

J. Eng. Gas Turbines Power 129(3), 850-857 (Dec 14, 2006) (8 pages) doi:10.1115/1.2718233 History: Received June 03, 2006; Revised December 14, 2006

Gas foil bearings (GFBs) satisfy the requirements for oil-free turbomachinery, i.e., simple construction and ensuring low drag friction and reliable high speed operation. However, GFBs have a limited load capacity and minimal damping, as well as frequency and amplitude dependent stiffness and damping characteristics. This paper provides experimental results of the rotordynamic performance of a small rotor supported on two bump-type GFBs of length and diameter equal to 38.10mm. Coast down rotor responses from 25krpm to rest are recorded for various imbalance conditions and increasing air feed pressures. The peak amplitudes of rotor synchronous motion at the system critical speed are not proportional to the imbalance introduced. Furthermore, for the largest imbalance, the test system shows subsynchronous motions from 20.5krpm to 15krpm with a whirl frequency at 50% of shaft speed. Rotor imbalance exacerbates the severity of subsynchronous motions, thus denoting a forced nonlinearity in the GFBs. The rotor dynamic analysis with calculated GFB force coefficients predicts a critical speed at 8.5krpm, as in the experiments; and importantly enough, unstable operation in the same speed range as the test results for the largest imbalance. Predicted imbalance responses do not agree with the rotor measurements while crossing the critical speed, except for the lowest imbalance case. Gas pressurization through the bearings’ side ameliorates rotor subsynchronous motions and reduces the peak amplitudes at the critical speed. Posttest inspection reveal wear spots on the top foils and rotor surface.

Copyright © 2007 by American Society of Mechanical Engineers
Topics: Bearings , Rotors , Motion
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Figure 1

Schematic view of a bump-type GFB. Location of top foil spot-weld relative to vertical plane as in tests.

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Figure 2

Test rig for rotordynamic experiments of rotor supported on GFBs

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Figure 3

Test rotor and foil bearings for rotordynamic tests

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Figure 4

Rotor static displacements at the free end and drive end bearings for increasing applied electromagnetic loads

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Figure 5

Waterfalls of rotor coast down response for two in-phase imbalance displacements. Top graph: u=7.4μm, bottom graph: u=10.5μm. Air pressure at 34.4kPa measurements at rotor free end, vertical plane (YFE).

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Figure 6

(a) Amplitudes of synchronous and subsynchronous motion and (b) whirl frequency ratio (WFR) versus rotor speed for imbalance u=10.5μm (in phase, Test A3). Air pressure at 34.4kPa (5 psig) and measurements at rotor free end, vertical plane (YFE).

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Figure 7

Subsynchronous amplitudes and respective whirl frequencies for tests with two imbalances, (a)u=10.5μm (in phase) and (b)u=7.4μm (out of phase)

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Figure 8

Finite element model of the test rotor (with connecting shaft and flexible coupling included)

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Figure 9

Predicted damped natural frequencies for rotor–foil bearing system (forward modes)

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Figure 10

Predicted damping ratios for rotor–foil bearing system

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Figure 11

Measured and predicted amplitudes of synchronous rotor response for (in phase) imbalance tests. (a)u=7.4μm, (b)u=9.5μm, (c)u=10.5μm. Air pressure at 34.4kPa (5 psig). Measurements at rotor drive end with baseline subtraction, Horizontal (XDE) and vertical (YDE) planes.

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Figure 12

Normalized amplitudes of rotor synchronous response for imbalance tests (in phase). Air pressure at 34.4kPa (5 psig). Measurements at rotor drive end, vertical (YDE) plane, with baseline subtraction.

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Figure 13

Amplitudes of synchronous motions at critical speed (8.4krpm) for increasing air supply pressures and u=3.7μm

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Figure 14

Amplitude frequency spectrum of rotor motion for three supply pressures; (a)40.8kPa, (b)204kPa; and (c)340kPa. Rotor speed 16krpm. Measurements at rotor drive end, horizontal plane (XDE). Imbalance, u=3.7μm.

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Figure 15

Condition of test rotor surface before (top picture) and after (bottom pictures) rotordynamic tests

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Figure 16

Condition of foil bearing surface after rotordynamic tests. Wear near the spot weld line (left) and along the load direction (right) in the drive end bearing.




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