Further Imbalance Response Measurements of Rotor Supported on Bump-Type Gas Foil Bearings: Operation to 50 krpm

A 2005 TRC report presents imbalance response measurements of a hollow test rotor supported on bump-type gas foil bearings. The top speed of the tests is 25 krpm, the limit of the drive router motor. The 2005 measurements show the gas foil bearings are prone to subsynchronous whirl with amplitudes of motion exacerbated by the imbalance condition of the rotor, i.e. a forced nonlinearity. Presently, a 0.75 kW (1 HP) motor with maximum speed of 50 krpm drives the rotor through a flexible coupling. However, at start up the AC motor does not have enough torque to overcome the dry-friction from the foil bearings contacting the rotor. Hence, the original DC router motor, 1.49 kW (2.0 HP), is installed on the free end of the motor with a centrifugal clutch that engages the test rotor, overcomes the friction in the bearings and accelerates the rotor towards high speeds. The router motor is turned off and the main drive motor spins the test rotor to a top speed of 50 krpm. The rotor is balanced first in a commercial machine and then in-place; the maximum motion amplitudes are less than 12 μm, ~ 27 % of the nominal bearing clearance (c = 45 μm), in the speed range from 2 to 25 krpm. Coast down rotor responses from 50 krpm to rest are recorded only for the baseline imbalance condition and with side pressurization into the bearings set at 0.34 bar (5 psig). The test data show significant subsynchronous motions from 50 krpm to 27 krpm with a main whirl frequency ranging from 20 % to 27% of rotor speed. At lower rotor speeds, subsynchronous motions are negligible. The rotor speed decay curve versus time is linear from 50 krpm to 30 krpm, and from 5 krpm to rest, thus implying rotor rubbing. From 30 krpm towards 5 krpm, the speed decay is exponential showing operation with a gas film in the foil bearings. Measurements of rotor motion are obtained for increasing imbalance masses, in-phase and out-of-phase, in coast down tests from 25 krpm where no subsynchronous vibrations exist. The amplitudes of synchronous motion show passage through a critical speed that decreases from 13 krpm to 9 krpm as the imbalance mass increases. The peak amplitudes of motion appear to increase proportionally with the imbalance, thus giving some assurance into the linearity of the test rotor-gas foil bearing system. A rotordynamic …