Development of a High Speed Gas Bearing Test Rig to Measure Rotordynamic Force Coefficients

Abstract

The use of gas bearings has increased over the last several decades to include microturbines, air cycle machines, and hermetically sealed compressors and turbines. Gas bearings have many advantages over traditional bearings, such as rolling element or oil lubricated fluid film bearings, including longer life, ability to use the process fluid, no contamination of the process with lubricants, accommodating high shaft speeds, and operation over a wide range of temperatures. Unlike fluid film bearings that utilize oil, gas lubricated bearings generate very little damping from the gas itself. Therefore, successful bearing designs such as foil bearings utilize damping features on the bearing to improve the damping generated. Similar to oil bearings, gas bearing designers strive to develop gas bearings with good rotordynamic stability. Gas bearings are challenging to design requiring a fully coupled thermo-elastic, hydrodynamic analysis including complex non-linear mechanisms such as Coulomb friction. There is a surprisingly low amount of rotordynamic force coefficient measurement in the literature despite the need to verify the model predictions and the stability of the bearing. This paper describes the development and testing of a 60,000 rpm gas bearing test rig and presents measured stiffness and damping coefficients for a 57 mm foil type bearing. The design of the rig overcomes many challenges in making this measurement by developing a patented, high-frequency, high-amplitude shaker system resulting in excitation over most of the subsynchronous range.

Foil Bearing Design Guidelines for Improved Stability

“…Over the past three decades, foil bearings have found a wide range of applications, such as in air cycle refrigeration units [1], microturbine generators [2,3], and turboexpanders for cryogenic application [4]. Foil bearings allow oil-free operation, as lubrication is provided by a gas film.…”

“…The goal of the investigation is to define a set of unified design guidelines for stable foil bearing-supported rotor operation. The objectives are to: (1) conceive a comprehensive model for foil bearing-supported rotors based on existing, state-of-the-art component models; (2) investigate the role of Coulomb friction in subsynchronous vibration; (3) quantify the impact of the axial foil bearing feed line orientation on static load; and (4) characterize the stability of foil bearing-supported rotors and define the conditions under which stable operation is guaranteed.…”

Stiffness and Damping Coefficient Estimation of Compliant Surface Gas Bearings for Oil-Free Turbomachinery

“…Moore and Lubell's test rig is limited to synchronous shaft excitation forcing functions at rather unsteady declining frequencies (Ref. 18). Such experimental complications are not considered criticisms of the research.…”

Rotor dynamic experimental investigation of an ultra-high-speed permanent magnet synchronous motor supported on a three-pad bidirectional gas foil bearing

“…The next generation of propellers and ground power turbines will use oil-free bearings to meet weight and performance requirements. 1 To reduce volumes and weights of machines, some new technologies, such as bearingless permanent magnet machine 2,3 and permanent magnet biased active magnetic bearing, 4 are applied to make the machines run at a high speed, but they all require complicated control techniques. As another type of oil-free bearing, gas foil bearings (GFBs) have many advantages such as uncontrollability, high bearing capacity, low friction power, wide working temperature range, impact resistance, low assembly alignment requirements and superior startstop performance.…”