Brian Dykas

The role of radial clearance on the performance of foil air bearings

Load capacity tests were conducted to determine how radial clearance variations affect the load capacity coefficient of foil air bearings. Two Generation III foil air bearings with the same design but possessing different initial radial clearances were tested at room temperature against an as-ground PS304 coated journal operating at 30000 rpm. Increases in radial clearance were accomplished by reducing the journals outside diameter via an in-place grinding system. From each load capacity test the bearing load capacity coefficient was calculated from the rule-of-thumb (ROT) model developed for foil air bearings. The test results indicate that, in terms of the load capacity coefficient, radial clearance has a direct impact on the performance of the foil air bearing. Each test bearing exhibited an optimum radial clearance that resulted in a maximum load capacity coefficient. Relative to this optimum value are two separate operating regimes that are governed by different modes of failure. Bearings operating with radial clearances less than the optimum exhibit load capacity coefficients that are a strong function of radial clearance and are prone to a thermal runaway failure mechanism and bearing seizure. Conversely, a bearing operating with a radial clearance twice the optimum suffered only a 20% decline in its maximum load capacity coefficient and did not experience any thermal management problems. However, it is unknown to what degree these changes in radial clearance had on other performance parameters, such as the stiffness and damping properties of the bearings. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

Journal Design Considerations for Turbomachine Shafts Supported on Foil Air Bearings

Foil air bearings can offer substantial improvements over traditional rolling element bearings in many applications and are attractive as a replacement to enable the development of advanced oil-free turbomachinery. In the course of rigorous testing of foil journal bearings at NASA Glenn Research Center, shaft failure was repeatedly encountered at high ambient temperature and rotational speed, with moderate radial load. The cause of failure is determined to be excessive non-uniform shaft growth, which increases localized viscous heating in the gas film and eventually leads to a high-speed rub and destruction of the bearing and journal. Centrifugal loading of imbalance correction weights and axial temperature gradients within the journal due to the hydrodynamic nature of the foil bearings, determined by experiment and finite element analysis, are shown to be responsible for the non-uniform growth. Qualitative journal design guidance is given to aid in failure prevention.

Design, Fabrication, and Performance of Foil Gas Thrust Bearings for Microturbomachinery Applications

ABSTRACT Amethodologyforthedesignandconstructionofsimplefoilthrust bearingsintendedfor parametricperformancetestingandlow marginal costs is presented. Features drawn from a reviewof the openliterature are discussed as they relate to bearingper-formance. The design of fixtures and tooling required to fab-ricate foil thrust bearings is presented, using conventional ma-chining processes where possible. A prototype bearing with di-mensionsdrawnfrom theliteratureis constructed,with allfabri-cationsteps described. Aload-deflectioncurvefor thebearingispresented to illustrate structural stiffness characteristics. Start-stop cycles are performed on the bearing at a temperature of425 ◦ C to demonstrate early-life wear patterns. A test of bearingload capacity demonstrates useful performance when comparedwith data obtained from the open literature. Introduction Foil gas bearings represent an enabling technology foradvanced oil-free turbomachinery systems. Operating athigh speeds and temperatures, these next-generation turboma-chines will present tribological challenges that conventional oil-lubricated rolling element bearings may be unable to meet, in-cluding shaft speeds well above three million DN and bearingtemperatures in excess of 400

Thermal Management Phenomena in Foil Gas Thrust Bearings

Thrust foil gas bearings operate at high speeds on a very thin fluid film which experiences high shear. Shear induced viscous losses result in localized heating which must be managed to prevent thermal distortions and failure. The current work examines the need for thermal management in thrust foil bearings as evidenced by reduced performance in uncooled bearings. Measured bearing power loss is a few hundred watts from twenty-five to fifty-five krpm (89–196 m/s runner surface velocity based on mean bearing diameter). Modeling of the thrust runner demonstrates a potential for stress-induced running surface deformations greater than the gas film thickness (>10 μm), and radial temperature gradients in the bearing foils can exceed a few degrees Celcius per millimeter. Air flow forced through the foil structure achieves improvements in bearing load capacity and demonstrates a need for increased understanding of the thermal environment in these bearings.© 2006 ASME

Preliminary Experimental Characterization of the Gas Film in Foil Thrust Bearings

Foil gas thrust bearings are a critical component enabling oil-free turbomachinery, but because of the strong effect bearing compliance has on the hydrodynamic gas film, current numerical simulation efforts require experimental measurement of gas film quantities for verification. In this study, experimentally measured bearing torque over a range of speeds and loads is used to draw conclusions about the gas film characteristics. Assuming an isothermal gas film of constant thickness, the torque measurements give typical reference values for bearing compressibility number and film thickness under normal operation. These data are presented for use in the development of more accurate foil thrust bearing numerical models.Copyright

Experimental Measurement of Thrust Foil Bearing Temperature Profiles

Gas foil thrust bearings provide axial support for modern Oil-Free turbomachine rotors, reducing maintenance costs and overall system mass. Large temperature gradients can arise in these bearings due to the viscous dissipation in the gas film and the high thermal conduction resistance of the support structure, which consists of thin superalloy foils. Relatively little experimental temperature data exists for this type of bearing, due in large part to the difficulty in installation of instrumentation. In the current work, temperature gradients in the uncooled bearing are measured across a speed range from 25,000 rpm to 65,000 rpm at low to moderate loads. Bearing torque is measured to correlate temperature gradient magnitude with bearing power loss.Copyright