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TECHNICAL PAPERS

Design of Flexure Pivot Tilting Pads Gas Bearings for High-speed Oil-Free Microturbomachinery

[+] Author and Article Information
Kyuho Sim

Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123

Daejong Kim

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

J. Tribol 129(1), 112-119 (Jul 27, 2006) (8 pages) doi:10.1115/1.2372763 History: Received May 31, 2005; Revised July 27, 2006

Abstract

This paper introduces flexure pivot tilting pad gas bearings with pad radial compliance for high-speed oil-free microturbomachinery. The pad radial compliance was for accommodation of rotor centrifugal growth at high speeds. Analytical equation for the rotor centrifugal growth based on plane stress model agreed very well with finite element method results. Parametric studies on pivot offset, preload, and tilting stiffness were performed using nonlinear orbit simulations and coast-down simulations. Higher preload and pivot offset increased both critical speeds of the rotor-bearing system and onset speeds of instability due to the increased wedge effect. Pad radial stiffness and nominal bearing clearance were very important design parameters for high-speed applications due to the physically existing rotor centrifugal growth. From the series of parametric studies, the maximum achievable rotor speed was limited by the minimum clearance at the pad pivot calculated from the rotor growth and radial deflection of pads due to hydrodynamic pressure. Pad radial stiffness also affects the rotor instability significantly. Small radial stiffness could accommodate rotor growth more effectively but deteriorated rotor instability. From parametric studies on a bearing with $28.5mm$ in diameter and $33.2mm$ in length, optimum pad radial stiffness and bearing clearance are $1–2×107N∕m$ and $35μm$, respectively, and the maximum achievable speed appears $180krpm$. The final design with suggested optimum design variables could be also stable under relatively large destabilizing forces.

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Figures

Figure 1

Figure 2

Figure 3

Bounded subsynchronous rotor orbit for base line design

Figure 4

Amplitudes of vibration components from 87 to 115krpm (extracted from fast Fourier transform of the simulated vibration signal along the X direction)

Figure 5

Centrifugal growth of the steel hollow shaft (3mm thickness and 28.52mm outer diameter)

Figure 6

Simulated net clearance at pivot versus radial stiffness with speed as a curve parameter

Figure 7

Coast-down simulations of peak-to-peak amplitudes of εX with respect to pad radial stiffness (C=40μm)

Figure 8

Coast-down simulations of peak-to-peak amplitudes of εX with respect to pad radial stiffness (C=35μm)

Figure 9

Coast-down simulations of peak-to-peak amplitudes of eX with pad radial stiffness of 1.0×107N∕m for various nominal clearances

Figure 10

Effect of external destabilizing forces on bearing forces

Figure 11

Effect of external destabilizing forces on rotor orbits; cross-coupled stiffness above 8×105N∕m initiates sub synchronous whirl

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