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TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

Bump-Type Foil Bearing Structural Stiffness: Experiments and Predictions

[+] Author and Article Information
Dario Rubio

Mechanical Engineering Department,  Texas A&M University, College Station, TX 77843-3123dariorubio10@neo.tamu.edu

Luis San Andrés

Mechanical Engineering Department,  Texas A&M University, College Station, TX 77843-3123lsanandres@mengr.tamu.edu

J. Eng. Gas Turbines Power 128(3), 653-660 (Mar 01, 2004) (8 pages) doi:10.1115/1.2056047 History: Received October 01, 2003; Revised March 01, 2004

Gas foil bearings (FB) satisfy many of the requirements noted for novel oil-free turbomachinery. However, FB design remains largely empirical, in spite of successful commercial applications. The mechanical structural characteristics of foil bearings, namely stiffness and damping, have been largely ignored in the archival literature. Four commercial bump-type foil bearings were acquired to measure their load capacity under conditions of no shaft rotation. The test bearings contain a single Teflon-coated foil supported on 25 bumps. The nominal radial clearance is 0.036mm for a 38mm journal. A simple test setup was assembled to measure the FB deflections resulting from static loads. The tests were conducted with three shafts of increasing diameter to induce a degree of preload into the FB structure. Static measurements show nonlinear FB deflections, varying with the orientation of the load relative to the foil spot weld. Loading and unloading tests evidence hysteresis. The FB structural stiffness increases as the bumps-foil radial deflection increases (hardening effect). The assembly preload results in notable stiffness changes, in particular for small radial loads. A simple analytical model assembles individual bump stiffnesses and renders predictions for the FB structural stiffness as a function of the bump geometry and material, dry-friction coefficient, load orientation, clearance and preload. The model predicts well the test data, including the hardening effect. The uncertainty in the actual clearance (gap) upon assembly of a shaft into a FB affects most of the predictions.

Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Bump-type gas foil bearing

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Figure 2

Schematic view of extended bump strips

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Figure 3

Test setup for static experiments. Side view.

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Figure 4

Load and bearing angular orientations grouped in pairs

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Figure 5

Foil bearing deflection versus static load for all angular positions

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Figure 6

FB1 structural stiffness versus deflection for three shaft diameters. Positions 1–5 (0 deg–180 deg)

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Figure 7

FB1 structural stiffness versus deflection for three shaft diameters. Positions 4–8 (135 deg–315 deg)

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Figure 8

Hysteretic on test foil bearing. Positions 1–5. D2 shaft nominal diameter

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Figure 9

Equivalent foil structural stiffness model

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Figure 10

Predicted and experimental load versus bearing deflection curves for positions 1–5 (0 deg–180 deg) (shaft diameter D2; μf=0.1; lo=2.159mm)

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Figure 11

Predicted load versus bearing deflection curves for increasing dry friction coefficients. (Positions 1–5 (0 deg–180 deg); shaft diameter D2; lo=2.159mm)

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Figure 12

Predicted and experimental structural stiffness versus shaft displacement (shaft diameter D2; μf=0.1; lo=2.159mm; r=0mm)

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Figure 13

Predicted and experimental structural stiffness versus shaft displacement (shaft diameter D1, μf=0.1; lo=2.159mm; r=−0.0127mm)

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Figure 14

Predicted and experimental structural stiffness versus shaft displacement (shaft diameter D3; μf=0.1; lo=2.159mm; r=0.0127mm)

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Figure 15

Bump foil dimensional parameters

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