Experimental Investigation of the Influence of Gas Foil Thrust Bearings on the Stability Behaviour of Rotor-Bearing Systems

Gas foil bearings are a type of aerodynamic bearings which employ compliant foil structures to support rotating machinery, typically at high speeds and low loads. The load bearing lubrication medium is gaseous and, in most cases, ambient air. This avoids an oil system for bearing lubrication and eliminates the associated risks of oil contamination, for example in air cycle machines or turbo compressors for fuel cells. The frictional contact between the foil surfaces increases bearing damping compared to conventional, rigid gas bearings. However, rotor-bearing systems that involve gas foil bearings are susceptible to sub-synchronous vibration which is caused by a self-excitation of the air film beyond the on-set speed of sub-synchronous vibration. This sub-synchronous vibration often features vibration amplitudes that are larger than the synchronous response. Therefore, designers must ensure that a rotor-bearing system operates outside of the speed regime of sub-synchronous vibration. This is typically done by numerical stability assessment, however the effects of gas foil thrust bearings on rotor-bearing system stability are often completely neglected. While measures have been published to influence the onset speed of sub-synchronous vibration by alteration of various parameters of gas foil journal bearings (radial bearings), no experimental studies have demonstrated the influence of gas foil thrust bearings (axial bearings) on sub-synchronous vibrations. In the experimental work presented herein, the axial clearance of gas foil thrust bearings is adjusted on a test rig in which the rotor is fully supported by gas foil bearings (in radial and axial directions). The effect of the clearance variation is discussed in terms of on-set speed of sub-synchronous vibration of the conical rigid body mode, the frequency of vibration and the corresponding vibration amplitudes. The experimental results indicate that gas foil thrust bearing axial clearance can influence the vibrational behavior of the rotor-bearings system, thereby leading to the conclusion that, depending on the mode shape of sub-synchronous vibration, thrust bearings must be considered during the stability assessment of the design phase.