An Analytic Approach to the Foil Gas Bearing Performance Map

Recent breakthrough improvements in foil gas bearing load capacity, high temperature tribological coatings and computer based modeling has enabled the development of increasingly larger and more advanced Oil-Free turbomachinery systems. Successful integration of foil gas bearings into turbomachinery requires a step wise approach that includes conceptual design and feasibility studies, bearing testing, and rotor testing prior to full scale system level demonstrations. Unfortunately, the current level of understanding of foil gas bearings and especially their tribological behavior is often insufficient to avoid developmental problems hampering commercialization of new applications. A new approach loosely based upon accepted hydrodynamic theory, has been suggested by DellaCorte, et al (1). The proposed “Foil Gas Bearing Performance Map” is intended to guide the integration process. This map, which resembles a Stribeck curve for bearing friction, is useful in describing bearing operating regimes, performance safety margins, the effects of load on performance and limiting factors for foil gas bearings. The previously proposed “Foil Gas Bearing Performance Map” has been based, in large part, on qualitative experience in the authors’ test facilities as well as product development case studies. A limited amount of test data specific to the understanding of the performance map has been experimentally obtained. A project is underway to obtain a larger, more rigorous data set. The objective of the current work is to take an analytic approach toward understanding the behavior of the various regions of this map. These analytic approaches can then be useful in both guiding further experimental investigations as well as providing useful non-dimensionalization schemes which may allow the performance map to be generalized, akin to the Stribeck curve. The foil journal bearing possesses several unique features that distinguish its analysis from those of the traditional hydrodynamic journal bearing. These features include the use of a gas as a lubricant, compliant structures / hydrodynamic surfaces, and interference fits (statically preloaded bearings). The combination of these features is considered in the proposed approach towards explaining the features of the foil gas bearing performance map.