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

摘要

Gas foil bearings (GFB) appear to satisfy most requirements for oil-freeturbomachinery, i.e. relatively simple in construction, ensuring low drag friction andreliable high speed operation. However, GFBs have a limited load capacity and minimalamounts of damping. A test rig for the rotordynamic evaluation of gas foil bearings wasconstructed. A DC router motor, 25 krpm max speed, drives a 1.02 kg hollow rotorsupported on two bump-type foil gas bearings (L = D = 38.10 mm). Measurements ofthe test rotor dynamic response were conducted for increasing mass imbalanceconditions. Typical waterfalls of rotor coast down response from 25 krpm to restevidence the onset and disappearance of severe subsynchronous motions with whirlfrequencies at ~ 50% of rotor speed, roughly coinciding with the (rigid mode) naturalfrequencies of the rotor-bearing system. The amplitudes of motion, synchronous andsubsynchronous, increase (non) linearly with respect to the imbalance displacements.The rotor motions are rather large; yet, the foil bearings, by virtue of their inherentflexibility, prevented the catastrophic failure of the test rotor. Tests at the top shaft speed,25 krpm, did not excite subsynchronous motions. In the experiments, thesubsynchronous motion speed range is well confined to shaft speeds ranging from 22krpm to 12 krpm. The experimental results show the severity of subsynchronous motionsis related to the amount of imbalance in the rotor. Surprisingly enough, external airpressurization on one side of the foil bearings acted to reduce the amplitudes of motionwhile the rotor crossed its critical speeds. An air-film hovering effect may haveenhanced the sliding of the bumps thus increasing the bearings?? damping action. Thetests also demonstrate that increasing the gas feed pressure ameliorates the amplitudes of subsynchronous motions due to the axial flow retarding the circumferential flow velocitydevelopment. A finite element rotordynamic analysis models the test rotor and usespredicted bearing force coefficients from the static equilibrium GFB load analysis. Therotordynamic analysis predicts critical speeds at ~8 krpm and ~9 krpm, which correlatewell with test critical speeds. Predictions of rotordynamic stability are calculated for thetest speed range (0 to 25 krpm), showing unstable operation for the rotor/bearing systemstarting at 12 krpm and higher. Predictions and experimental results show goodagreement in terms of critical speed correlation, and moderate displacement amplitudediscrepancies for some imbalance conditions. Post-test inspection of the rotor evidencedsustained wear at the locations in contact with the bearings' axial edges. However, thefoil bearings are almost in pristine condition; except for top foil coating wear at thebearing edges and along the direction of applied static load.