Development of a rotor test rig with a novel controllable preload foil-air bearing

The introduction of a lobed clearance profile into a foil-air bearing can improve rotordynamic performance through the delay of the onset of instability speed and consequent suppression of sub-synchronous vibrations. This can be achieved by shimming (preloading) the foil at three angular locations along its interface with the sleeve. This paper presents the design, manufacture and testing of a rotor test rig with a novel controllable preload foil-air bearing design. The design's motorised micrometer screw assemblies securely prescribe a given preload in real time, eliminating the introduction of potential problems associated with the structural compliance of piezoelectric preloading mechanisms. The shaft of a previous test rig is redesigned to achieve a significantly higher speed with the same motor. The design work is guided by simulations. Experiments confirm that the projected preload is sufficient to virtually eliminate sub-synchronous vibration over the operating speed range. The experiments also confirm that the top speed could be significantly increased by redesigning the shaft for a higher bending critical speed. Although preload increases the friction torque, this is mitigated through the application of diamond-like carbon to the journal, and can be further mitigated by applying preload only in regimes of sub-synchronous vibration activity.