A New Foil Gas Bearing With Compression Springs as Compliant Structure for Microturbomachinery Applications

Abstract

A new foil gas bearing is introduced in this paper. This foil gas bearing uses series of compression springs as a compliant structure instead of expensive corrugated bump foils. The new foil gas bearing is very simple in structure and easy to manufacture. A theoretical model to estimate stiffness and damping of the spring bump was developed. Measured stiffnesses of individual spring bump agree well with predictions. Load capacity was measured up to 62.5 N at 20,000rpm with both cooled and uncooled bearings. Initial selection of spring geometry rendered rather soft supports compared to other bump foil bearings, and allowed only limited load during the test. Developed cooling method using direct air supply holes machined on the bearing sleeve was very effective to cool the test bearing because the spring bumps are not connected along the circumferential direction, and allow very effective circumferential distribution of cooling air. A series of orbit simulation was performed to estimate critical speed and onset speed of instability. Bump dynamics was directly coupled with the orbit simulation. Critical speed was estimated at around 7500 rpm due to relatively soft support structure. Hydrodynamic instability with WFR 0.5 could be predicted at around 15,000 rpm. The rotor instability is predicted even under the equivalent viscous damping extracted from bump dynamics, implying the viscous damping alone within the bump cannot suppress hydrodynamic instability of foil bearings.