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Research Papers: Gas Turbines: Structures and Dynamics

The Impact of Pad Flexibility on the Rotordynamic Coefficients of Tilting-Pad Journal Bearings

[+] Author and Article Information
Jennifer E. Gaines

Sulzer Turbo Services,
La Porte, TX 77571
e-mail: gainesje@gmail.com

Dara W. Childs

Leland T. Jordan Professor of
Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77840
e-mail: dchilds@turbo-lab.tamu.edu

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 4, 2015; final manuscript received October 23, 2015; published online February 23, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(8), 082501 (Feb 23, 2016) (12 pages) Paper No: GTP-15-1394; doi: 10.1115/1.4032334 History: Received August 04, 2015; Revised October 23, 2015

Static and dynamic load tests were performed on a three-pad, rocker-pivot, tilting-pad journal bearing (TPJB) with three interchangeable pad configurations, each with measurably different pad flexibilities. Measured dynamic-stiffness data for the bearing were readily fitted by a frequency-independent, constant-coefficient [K][C][M] model. The test-bearing had a 101.74 mm diameter with L/D = 0.6. Tests were conducted over the speed range of 6–12 krpm, with unit loads varying from 0.172 to 1.724 MPa. An ISO VG 46 lubricant was used as the test fluid. Pad flexibility was characterized as the change in the pad's bending stiffness or the change in pad thickness. A finite-element model (FEM) was created to predict the structural bending stiffness of each pad configuration, showing a significant pad flexibility increase as pad thickness decreased. To examine the effect of pad flexibility on the rotordynamic coefficients, the measured results were compared across pad configurations and showed that the pad flexibility increase reduced the direct damping coefficients by 12–20%. As pad flexibility increased, the direct-stiffness coefficients could increase or decrease, depending on the unit load. They varied from an increase of 12% at low unit loads to a decrease of 3% at high unit loads. Results show that the pad's structural bending stiffness or flexibility is important when predicting the bearing’s dynamic performance. Damping is consistently overpredicted when neglecting pad flexibility. A nondimensional pad flexibility parameter αflex was developed. It related the average deflection across the pad surface to the pad's arc length and was to relate the pad flexibility of multiple bearings of different sizes. A bearing code was used to predict the percent change in direct damping coefficients for rigid-pad/flexible-pivot and flexible-pad/flexible-pivot models for a surface speed of 54 m/s and a unit load of 783 kPa for the three-pad configuration tested here plus five additional tested bearings from the literature. For the minimum pad thickness configuration tested here, the code predicted a 20% drop in predicted Cxx (off-load axis direct damping) when comparing a model that included pad flexibility with a model that neglected pad flexibility. In terms of αflex, the two thinnest pad configurations tested here are quite flexible compared to both TPJB's pads used in industry and previously tested TPJB pads.

Copyright © 2016 by ASME
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References

Figures

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Fig. 2

Main section of the oil bearing test rig [22]

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Fig. 3

Stator shaker–stinger arrangement as viewed from the NDE

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Fig. 1

Change in pad curvature due to applied moments at the leading and trailing edge of the pad, see Wilkes [16]

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Fig. 4

Bearing configuration, instrumentation attachment, and raw coordinate system viewed from the DE

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Fig. 5

Test-bearing pads with a thickness of 8.5 mm (1), 10 mm (2), and 11.5 mm (3)

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Fig. 11

Measured Kxy (dashed) and Kyx (solid) versus pad flexibility for three shaft speeds and unit loads with uncertainty bars for all shaft speed and unit load conditions

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Fig. 12

Measured Cxx (dashed) and Cyy (solid) versus pad flexibility for three shaft speeds and unit loads with uncertainty bars for all shaft speed and unit load conditions

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Fig. 13

Measured Cxy (dashed) and Cyx (solid) versus pad flexibility for three shaft speeds and unit loads with uncertainty bars for all shaft speed and unit load conditions

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Fig. 14

Measured Mxx (dashed) and Myy (solid) versus pad flexibility for three shaft speeds and unit loads with uncertainty bars for all shaft speed and unit load conditions

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Fig. 15

Measured Mxy (dashed) and Myx (solid) versus pad flexibility for three shaft speeds and unit loads with uncertainty bars for all shaft speed and unit load conditions

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Fig. 6

Measured CC and HC for the pad set with thickness tp = 8.5 mm

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Fig. 7

Measured eccentricity and uncertainty in the loaded directions (y-axis) versus unit load for three-pad thickness

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Fig. 8

Measured attitude angle and uncertainty for the three-pad configuration

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Fig. 9

FEA static deflection predictions for tp = 11.5 mm with a uniform pressure distribution of 689 kPa (100 psi)

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Fig. 16

Schematic of pad flexibility and parameters

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Fig. 17

Percent change in predicted Cxx and Cyy versus the nondimensional pad flexibility parameter, αflex

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Fig. 10

Measured Kxx (dashed) and Kyy (solid) versus pad flexibility for three shaft speeds and unit loads with uncertainty bars for all shaft speed and unit load conditions

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