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Rotordynamic performance of a rotor supported on bump-type foil bearings: experiments and predictions

by Rubio Tabares, Dario

Abstract (Summary)
Gas foil bearings (GFB) appear to satisfy most requirements for oil-free

turbomachinery, i.e. relatively simple in construction, ensuring low drag friction and

reliable high speed operation. However, GFBs have a limited load capacity and minimal

amounts of damping. A test rig for the rotordynamic evaluation of gas foil bearings was

constructed. A DC router motor, 25 krpm max speed, drives a 1.02 kg hollow rotor

supported on two bump-type foil gas bearings (L = D = 38.10 mm). Measurements of

the test rotor dynamic response were conducted for increasing mass imbalance

conditions. Typical waterfalls of rotor coast down response from 25 krpm to rest

evidence the onset and disappearance of severe subsynchronous motions with whirl

frequencies at ~ 50% of rotor speed, roughly coinciding with the (rigid mode) natural

frequencies of the rotor-bearing system. The amplitudes of motion, synchronous and

subsynchronous, increase (non) linearly with respect to the imbalance displacements.

The rotor motions are rather large; yet, the foil bearings, by virtue of their inherent

flexibility, prevented the catastrophic failure of the test rotor. Tests at the top shaft speed,

25 krpm, did not excite subsynchronous motions. In the experiments, the

subsynchronous motion speed range is well confined to shaft speeds ranging from 22

krpm to 12 krpm. The experimental results show the severity of subsynchronous motions

is related to the amount of imbalance in the rotor. Surprisingly enough, external air

pressurization on one side of the foil bearings acted to reduce the amplitudes of motion

while the rotor crossed its critical speeds. An air-film hovering effect may have

enhanced the sliding of the bumps thus increasing the bearingsÂ? damping action. The

tests also demonstrate that increasing the gas feed pressure ameliorates the amplitudes of subsynchronous motions due to the axial flow retarding the circumferential flow velocity

development. A finite element rotordynamic analysis models the test rotor and uses

predicted bearing force coefficients from the static equilibrium GFB load analysis. The

rotordynamic analysis predicts critical speeds at ~8 krpm and ~9 krpm, which correlate

well with test critical speeds. Predictions of rotordynamic stability are calculated for the

test speed range (0 to 25 krpm), showing unstable operation for the rotor/bearing system

starting at 12 krpm and higher. Predictions and experimental results show good

agreement in terms of critical speed correlation, and moderate displacement amplitude

discrepancies for some imbalance conditions. Post-test inspection of the rotor evidenced

sustained wear at the locations in contact with the bearings' axial edges. However, the

foil bearings are almost in pristine condition; except for top foil coating wear at the

bearing edges and along the direction of applied static load.

Bibliographical Information:

Advisor:San Andres, Luis; Childs, Dara; Rediniotis, Othon

School:Texas A&M University

School Location:USA - Texas

Source Type:Master's Thesis

Keywords:foil bearings subsynchronous vibrations structural stiffness

ISBN:

Date of Publication:05/01/2003

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