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Research Papers: Gas Turbines: Structures and Dynamics

Measured Static and Rotordynamic Coefficient Results for a Rocker-Pivot, Tilting-Pad Bearing With 50 and 60% Offsets

[+] Author and Article Information
Chris D. Kulhanek, Dara W. Childs

Southwest Research Institute, San Antonio, TX 78238 e-mail:chris.kulhanek@swri.org The Leland T. Jordan Professor of Mechanical Engineering Turbomachinery Laboratory, Texas A&M University, College Station, TX 77843-3123e-mail:dchilds@tamu.edu

J. Eng. Gas Turbines Power 134(5), 052505 (Feb 29, 2012) (11 pages) doi:10.1115/1.4004723 History: Received May 12, 2011; Revised May 12, 2011; Published February 29, 2012; Online February 29, 2012

Static and rotordynamic coefficients are measured for a rocker-pivot, tilting-pad journal bearing (TPJB) with 50 and 60% offset pads in a load-between-pad (LBP) configuration. The bearing uses leading-edge-groove direct lubrication and has the following characteristics: 5-pads, 101.6 mm (4.0 in) nominal diameter,0.0814 -0.0837 mm (0.0032–0.0033 in) radial bearing clearance, 0.25 to 0.27 preload, and 60.325 mm (2.375 in) axial pad length. Tests were performed on a floating bearing test rig with unit loads from 0 to 3101 kPa (450 psi) and speeds from 7 to 16 krpm. Dynamic tests were conducted over a range of frequencies (20 to 320 Hz) to obtain complex dynamic stiffness coefficients as functions of excitation frequency. For most test conditions, the real dynamic stiffness functions were well fitted with a quadratic function with respect to frequency. This curve fit allowed for the stiffness frequency dependency to be captured by including an added mass matrix [M] to a conventional [K][C] model, yielding a frequency independent [K][C][M] model. The imaginary dynamic stiffness coefficients increased linearly with frequency, producing frequency-independent direct damping coefficients. Direct stiffness coefficients were larger for the 60% offset bearing at light unit loads. At high loads, the 50% offset configuration had a larger stiffness in the loaded direction, while the unloaded direct stiffness was approximately the same for both pivot offsets. Cross-coupled stiffness coefficients were positive and significantly smaller than direct stiffness coefficients. Negative direct added-mass coefficients were obtained for both offsets, especially in the unloaded direction. Cross-coupled added-mass coefficients are generally positive and of the same sign. Direct damping coefficients were mostly independent of load and speed, showing no appreciable difference between pivot offsets. Cross-coupled damping coefficients had the same sign and were much smaller than direct coefficients. Measured static eccentricities suggested cross coupling stiffness exists for both pivot offsets, agreeing with dynamic measurements. Static stiffness measurements showed good agreement with the loaded, direct dynamic stiffness coefficients.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 2

Test rig main section [4]

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Figure 3

Stator and test bearing viewed from the drive end

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Figure 4

Oil exit ports and end seals of test bearing

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Figure 5

50 and 60% offset measured dynamic stiffness at 16 krpm and zero unit load for: (a) direct real, (b) direct imaginary, (c) cross-coupled real, (d) cross-coupled imaginary

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Figure 6

Direct stiffness coefficients at (a) 7 krpm, (b) 10 krpm, (c) 13 krpm, (d) 16 krpm

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Figure 7

Cross-coupled stiffness coefficients at (a) 7 krpm, (b) 10 krpm, (c) 13 krpm, (d) 16 krpm

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Figure 8

Direct damping coefficients at (a) 7 krpm, (b) 10 krpm, (c) 13 krpm, (d) 16 krpm

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Figure 9

Cross-coupled damping coefficients at (a) 7 krpm, (b) 10 krpm, (c) 13 krpm, (d) 16 krpm

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Figure 10

Direct added-mass coefficients at (a) 7 krpm, (b) 10 krpm, (c) 13 krpm, (d) 16 krpm

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Figure 11

Cross-coupled added-mass coefficients at (a) 7 krpm, (b) 10 krpm, (c) 13 krpm, (d) 16 krpm

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Figure 12

Dynamic and static stiffness in the loaded direction for (a) 50% offset, (b) 60% offset

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Figure 13

Dynamic and static stiffness at 16 krpm in the loaded direction for (a) 50% offset, (b) 60% offset

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Figure 14

Spring in series with a fluid film model

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Figure 1

Rocker-pivot tilting-pad journal bearing

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