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Research Papers: Gas Turbines: Structures and Dynamics

Measurements Versus Predictions for the Rotordynamic Characteristics of a Five-Pad Rocker-Pivot Tilting-Pad Bearing in Load-Between-Pad Configuration

[+] Author and Article Information
Clint R Carter

Turbomachinery Laboratory, Texas A&M University, College Station, TX 77843-3123crcarter@dow.com

Dara W. Childs

Turbomachinery Laboratory, Texas A&M University, College Station, TX 77843-3123dchilds@tamu.edu

J. Eng. Gas Turbines Power 131(1), 012507 (Oct 13, 2008) (9 pages) doi:10.1115/1.2967728 History: Received April 03, 2008; Revised April 10, 2008; Published October 13, 2008

Rotordynamic data are presented for a rocker-pivot tilting-pad bearing in the load-between-pad configuration for unit loads over the range 3453101kPa and speeds over the range 400013,000rpm. The bearing was directly lubricated through a leading-edge groove with the following specifications: Five pads, 0.282 preload, 60% offset, 57.87deg pad arc angle, 101.587mm(3.9995in.) rotor diameter, 0.1575mm(0.0062in.) diametral clearance, and 60.325mm(2.375in.) pad length. Dynamic tests were performed over a range of frequencies to investigate frequency effects on the dynamic stiffness coefficients. Under most test conditions, the direct real parts of the dynamic stiffnesses could be approximated as quadratic functions of the excitation frequency and accounted for with the addition of an added-mass matrix to the conventional [K][C] matrix model to produce a frequency-independent [K][C][M] model. Measured added-mass terms in the loaded direction approached 60kg. At low speeds, “hardening” direct dynamic stiffness coefficients that increased with increasing frequency were obtained, which produced negative added-mass terms. No frequency dependency was obtained for the direct damping coefficients. The dynamic experimental results were compared to predictions from a bulk-flow computational fluid dynamics analysis. The static load direction in the tests was y. The direct stiffness coefficients Kxx and Kyy were slightly overpredicted. Measured direct damping coefficients Cxx and Cyy were insensitive to changes in either the load or speed in contrast to predictions of marked Cyy sensitivity for changes in the load. Only at the highest test speed of 13,000rpm were the direct damping coefficients adequately predicted. Measurable cross-coupled stiffness coefficients were obtained for the bearings with Kxy and Kyx being approximately equal in magnitude but opposite in sign—clearly destabilizing. However, the whirl frequency ratio was found to be zero at all test conditions indicating infinite stability for the bearing.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 9

(a) Measured and predicted loci plots at 10,000rpm from 345kPa to 2412kPa and (b) comparison of dynamic-stiffness coefficients to the static stiffness at 10,000rpm

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Figure 8

LBP direct mass coefficients versus the load for varying speeds: (a) 4000rpm, (b) 7000rpm, (c) 10,000rpm, and (d) 13,000rpm

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Figure 7

LBP direct damping coefficients versus the load for varying speeds: (a) 4000rpm, (b) 7000rpm, (c) 10,000rpm, and (d) 13,000rpm

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Figure 6

LBP direct and cross-coupled stiffness coefficients versus the load for varying speeds: (a) 4000rpm, (b) 7000rpm, (c) 10,000rpm, and (d) 13,000rpm

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Figure 5

LBP dynamic-stiffness coefficients at 13,000rpm and 345kPa for (a) direct real, (b) cross-coupled real, and (c) direct imaginary

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Figure 4

Photo of the leading edge groove

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Figure 3

Exciter head shown attached to the stator via stingers and load (2)

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Figure 2

Cross-sectional view of the test stand (2)

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Figure 1

Rocker-pivot tilting-pad bearing (1-2)

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