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

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Title: Rotordynamic performance of a rotor supported on bump-type foil bearings: experiments and predictions
Author: Rubio Tabares, Dario
Abstract: 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 bearings ? ? 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 .
URI: http : / /hdl .handle .net /1969 .1 /3854
Date: 2006-08-16

Citation

Rotordynamic performance of a rotor supported on bump-type foil bearings: experiments and predictions. Available electronically from http : / /hdl .handle .net /1969 .1 /3854 .

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