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TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

A New Bump-Type Foil Bearing Structure Analytical Model

[+] Author and Article Information
Sébastien Le Lez, Mihaï Arghir, Jean Frene

Laboratoire de Mécanique des Solides, Université de Poitiers, Téléport 2-SP2MI, Boulevard Marie et Pierre Curie, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France

J. Eng. Gas Turbines Power 129(4), 1047-1057 (Apr 27, 2007) (11 pages) doi:10.1115/1.2747638 History: Received April 26, 2007; Revised April 27, 2007

A gas bearing of bump foil type comprises an underlying structure made of one or several strips of corrugated sheet metal covered by a top foil surface. The fluid film pressure needs to be coupled with the behavior of the structure for obtaining the whole bearing characteristics. Unlike in classical elasto-aerodynamic models, a foil bearing (FB) structure has a very particular behavior due to friction interfaces, bump interactions, and nonisotropic stiffness. Some authors have studied this complex behavior with the help of three-dimensional finite element simulations. These simulations evidenced a lack of reliable analytical models that can be easily implemented in a FB prediction code. The models found in the literature tend to overestimate the foil flexibility because most of them do not consider the interactions between bumps that are highly important. The present work then develops a model that describes the FB structure as a multidegree of freedom system of interacting bumps. Each bump includes three degrees of freedom linked with elementary springs. The stiffnesses of these springs are analytically expressed so that the model can be adjusted for any dimensions and material properties. Once the stiffness matrix of the whole FB structure is obtained, the entire static system is solved taking friction into account. Despite its relative simplicity, comparisons with finite elements simulations for various static load distributions and friction coefficients show a good correlation. This analytical model has been integrated into a foil bearing prediction code. The load capacity of a first generation foil bearing was then calculated using this structure model as well as other simplified theoretical approaches. Significant differences were observed, revealing the paramount influence of the structure on the static and dynamic characteristics of the foil bearing. Some experimental investigations of the static stiffness of the structure were also realized for complete foil bearings. The structure reaction force was calculated for a shaft displacement with zero rotation speed, using either the multidegree of freedom model or the usual stiffness formulas. The comparisons between theoretical and experimental results also tend to confirm the importance of taking into account the bump interactions in determining the response of the structure.

Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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The six DOF model

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Figure 3

Boundary conditions for calculating the elementary stiffness

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Figure 4

Calculation of the elementary stiffness k1

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Figure 5

Calculation of the elementary stiffness k2

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Calculation of the elementary stiffness k3

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Figure 7

Calculation of the elementary stiffness k4

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Figure 8

Calculation of the elementary stiffness k1bis

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Figure 9

Calculation of the elementary stiffness k3bis

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Figure 10

Calculation of the transmission force angle θd

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Figure 11

Calculation of the transmission force angle θdbis

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Figure 12

Stiffness matrices

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Figure 13

Bumps deflection for various load distributions

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Figure 14

The influence of the structure on the foil bearing load capacity

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Figure 15

Experimental static load tests

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Figure 16

Coordinate system of the foil bearing

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Figure 17

Load distribution on the bumps

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Figure 18

Bearing structure static loading—spot weld location: 180deg

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Figure 19

Bearing structure static loading—spot weld location: 270deg

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