Book excerpt: 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 severesubsynchronousmotions 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.