Air foil bearings (AFB’s) are widely used in small to midsized turbomachinery. They are simple in construction, offer very low drag friction, and have very high reliability at high speed operations. This paper presents experimental imbalance response of a 4.84 kg rigid rotor (operating below bending critical speed) supported by two hybrid air foil bearings with 50 mm in diameter. The concept of “hybrid” in this paper utilizes the hydrostatic augmentation of the load capacity during the start up and shut down. The hybrid air foil bearings were designed with three top foils for enhanced stability. Imbalance responses in cylindrical mode are presented up to 44,000rpm with different supply pressures. As the supply pressure is increased from 2.67 to 4 bar, the bearing stiffness increases slightly, resulting in slightly larger vibration (and reduced damping ratio) during the trans-critical speed operation. Hydrodynamic instability was observed with whirl frequency ratios of about 0.17∼0.2 depending on the supply pressures. Tests were also conducted to investigate the effect of supply pressure on the rotordynamic stability. The test results show that the hybrid operation is very effective to suppress the subsynchronous vibrations at high speeds.

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