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Research Papers: Gas Turbines: Structures and Dynamics

Tilting Pad Journal Bearings—A Discussion on Stability Calculation, Frequency Dependence, and Pad and Pivot

[+] Author and Article Information
Jason C. Wilkes

Research Engineer
Southwest Research Institute,
San Antonio, TX, 78238
e-mail: jason.wilkes@swri.org

Dara W. Childs

Leland T. Jordan Professor of Mechanical Engineering,
Turbomachinery Laboratory,
Texas A&M University,
College Station, TX, 77802
e-mail: dchilds@tamu.edu

Contributed by International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF ENGINEERING for GAS TURBINES AND POWER. Manuscript received July 2, 2012; final manuscript received July 5, 2012; published online October 22, 2012. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 134(12), 122508 (Oct 22, 2012) (17 pages) doi:10.1115/1.4007369 History: Received July 02, 2012; Revised July 05, 2012

For several years, researchers have presented predictions showing that using a full tilting-pad journal bearing (TPJB) model (retaining all of the pad degrees of freedom) is necessary to accurately perform stability calculations for a shaft operating on TPJBs. This paper will discuss this issue, discuss the importance of pad and pivot flexibility in predicting impedance coefficients for the tilting-pad journal bearing, present measured changes in bearing clearance with operating temperature, and summarize the differences between measured and predicted frequency dependence of dynamic impedance coefficients. The current work presents recent test data for a 100 mm (4 in.) five-pad TPJB tested in load on pad (LOP) configuration. Measured results include bearing clearance as a function of operating temperature, pad clearance and radial displacement of the loaded pad (the pad having the static load vector directed through its pivot), and frequency-dependent stiffness and damping. Measured hot-bearing clearances are approximately 30% smaller than measured cold-bearing clearances and are inversely proportional to pad surface temperature; predicting bearing impedances with a rigid pad and pivot model using these reduced clearances results in overpredicted stiffness and damping coefficients that are several times larger than previous comparisons. The effect of employing a full bearing model versus a reduced bearing model (where only journal degrees of freedom are retained) in a stability calculation for a realistic rotor-bearing system is assessed. For the bearing tested, the bearing coefficients reduced at the frequency of the unstable eigenvalue (subsynchronously reduced) predicted a destabilizing cross-coupled stiffness coefficient at the onset of instability within 1% of the full model, while synchronously reduced coefficients for the lightly loaded bearing required 25% more destabilizing cross-coupled stiffness than the full model to cause system instability. The same stability calculation was performed using measured stiffness and damping coefficients at synchronous and subsynchronous frequencies. These predictions showed that both the synchronously measured stiffness and damping and predictions using the full bearing model were more conservative than the model using subsynchronously measured stiffness and damping, an outcome that is completely opposite from conclusions reached by comparing different prediction models. This contrasting outcome results from a predicted increase in damping with increasing excitation frequency at all speeds and loads; however, this increase in damping with increasing excitation frequency was only measured at the most heavily loaded conditions.

Copyright © 2012 by ASME
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Figures

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Fig. 1

Schematic of the journal, bearing, and kth pad in the reference state

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Fig. 2

Rigid body degrees of freedom for a tilting pad

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Fig. 3

Illustration of a pad with a pivot insert

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Fig. 4

Pentagonal clearance measurement at a variety of temperatures (as determined by the mean of pad surface temperatures at the pivot location)

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Fig. 5

Bearing clearance as a function of the average of pad surface temperatures at the pivot location

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Fig. 6

Measured static eccentricity at various speeds with unit loads in the Y-direction from 0–3132 kPa

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Fig. 7

Measured radial displacement of the loaded pad versus applied unit load

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Fig. 8

Loaded-pad clearance versus unit load inferred from strain-gauge measurements at various rotor speeds

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Fig. 9

(a) Real and (b) imaginary components of principal bearing impedances in the loaded direction at 4400 rpm and 3132 kPa unit load

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Fig. 10

(a) Real and (b) imaginary components of principal bearing impedances in the loaded direction at 10,200 rpm and 3132 kPa unit load

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Fig. 11

(a) Real and (b) imaginary components of principal bearing impedances in the loaded direction at 10,200 rpm and 783 kPa unit load

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Fig. 12

Rotor bearing system used to calculate the effect of employing full versus reduced bearing models on system instability

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Fig. 13

Percent relative error in destabilizing cross-coupled stiffness required to cause system instability when employing synchronous and subsynchronous reductions at 4400 rpm and 10,200 rpm at various unit loads

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Fig. 14

Relative error in synchronously reduced principal stiffness and damping coefficients relative to subsynchronously reduced stiffness and damping coefficients

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Fig. 15

Frequency-dependent damping coefficients at 10,200 rpm, 3132 kPa unit load

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Fig. 16

Frequency-dependent damping coefficients at 10,200 rpm, 783 kPa unit load

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Fig. 17

Magnitude of destabilizing cross-coupled stiffness required to cause system instability using subsynchronously and synchronously measured coefficients and predictions using a full bearing model

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