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Research Papers: Gas Turbines: Structures and Dynamics

Experimental Analyses of a First Generation Foil Bearing: Startup Torque and Dynamic Coefficients

[+] Author and Article Information
Laurent Rudloff, Mihai Arghir, Olivier Bonneau, Pierre Matta

Institut PPRIME, CNRS UPR3346, Université de Poitiers, Futuroscope Chasseneuil 86962, France

The torque of a needle bearing might seem too high for an accurate measurement of the foil bearing. A ball bearing would have had a lower torque. The needle bearing was selected due to accessibility requirements that were not evident in the early stages of development. The torque of the bearing can be lowered by eliminating every two needles.

J. Eng. Gas Turbines Power 133(9), 092501 (Apr 14, 2011) (9 pages) doi:10.1115/1.4002909 History: Received May 08, 2010; Revised May 25, 2010; Published April 14, 2011; Online April 14, 2011

This paper presents the results of the experimental analysis of static and dynamic characteristics of a generation 1 foil bearing of 38.1 mm diameter and L/D=1. The test rig is of floating bearing type, the rigid shaft being mounted on ceramic ball bearings and driven up to 40 krpm. Two different casings are used for startup and for measurement of dynamic coefficients. In its first configuration, the test rig is designed to measure the startup torque. The foil bearing casing is made of two rings separated by a needle bearing to enable an almost torque free rotation between the foil bearing and the static load. The basic results are the startup torque and the lift-off speed. In its second configuration, a different casing is used to measure the impedances of the foil bearing. Misalignment is a problem that is minimized by using three flexible stingers connecting the foil bearing casing to the base plate of the test rig. The test rig enables the application of a static load and of the dynamic excitation on the journal bearing casing and can measure displacements, forces, and accelerations. Working conditions consisted of static loads comprised between 10 N and 50 N and rotation frequencies ranging from 260 Hz to 590 Hz. Excitation frequencies comprised between 100 Hz and 600 Hz are applied by two orthogonally mounted shakers for each working condition. Stiffness and damping coefficients are identified from the complex impedances and enable the calculation of natural frequencies. The experimental results show that the dynamic characteristics of the tested bearing have a weak dependence on the rotation speed but vary with the excitation frequency.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

The tested foil bearing

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Figure 2

The test rig for startup measurements

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Figure 3

Characteristic startup measurements

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Figure 4

Schematic cut of the torque measurement casing

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Figure 7

The test rig with the casing for dynamic measurements

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Figure 8

Proper modes of the stingers and the casing

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Figure 9

Displacement of the bearing center for increasing static load

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Figure 10

Waterfall plot of the unloaded bearing

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Figure 11

Graphical solution of the nonlinear equation (13)

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Figure 12

Dynamic coefficients

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Figure 13

Natural frequencies of the bearing: (a) static loads between 10 N and 40 N and (b) static load of 50 N

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Figure 14

Waterfall plots for 570 Hz rotation frequency and 7 μm excitation amplitude

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Figure 15

Wrapped circularity measurements

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Figure 16

Upper and lower limit of the circularity measurement

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