Experimental Analysis of the nonlinear Vibrations of a rigid Rotor in Gas Foil Bearings

Abstract

Air bearings and gas foil bearings (GFBs) in particular are characterized by a low-loss operation at high rotational speeds and temperatures, because of their adequate and relatively low lubrication viscosity. Further advantages are the simple design of the bearing and the omission of an oil system. A disadvantage is the low fluid viscosity, which limits the load capacity and damping capacity of the bearing. Even though the bearing wall, which is elastic and sensitive to friction, compensates the mentioned disadvantages by self-regulating the lubrication film and providing external damping. GFBs always show a tendency for nonlinear subharmonic vibrations. In this paper, the nonlinear vibration behavior of a rigid rotor in gas foil bearings is investigated. The rotor is accelerated to approx. 60 000 rpm by means of an impulse turbine. Waterfall charts for a variation of static and dynamic unbalance are recorded using transient coast-downs. The experiments show a variety of nonlinear effects. Their causes are analyzed experimentally. In addition to self-excitation by the fluid film, the rotor is sensitive to high unbalances and the resulting forced vibrations. The nonlinear, progressive system behavior results in excitation orders of 1/2 Ω, 1/3 Ω, and 1/4 Ω that modulate additional frequencies.