Foil gas bearings are a key technology in many commercial and emerging oilfree turbomachinery systems. These bearings are nonlinear and have been difficult to analytically model in terms of performance characteristics such as load capacity, power loss, stiffness, and damping. Previous investigations led to an empirically derived method to estimate load capacity. This method has been a valuable tool in system development. The current work extends this tool concept to include rules for stiffness and damping coefficient estimation. It is expected that these rules will further accelerate the development and deployment of advanced oil-free machines operating on foil gas bearings.
Foil gas bearings are self-acting hydrodynamic bearings comprised of a series of sheet-metal foil layers from which they derive their name. They are compliant bearings that offer high-speed rotor support while accommodating shaft misalignment and distortion often encountered in turbomachinery. Lightly loaded, low-temperature foil gas bearings are commodities that predominate in the rotor support for aircraft air cycle machines (ACMs). More highly loaded foil bearings operating at high temperatures are an emerging technology making commercial inroads into several markets including aircraft auxiliary power units (APUs), microturbines, gas compressors and blowers, and turbochargers.
The general trend for foil bearings since their initial development over five decades ago is application to larger and more complex rotor systems. As this proliferation occurs, more practitioners will become actively involved with new machine development using foil bearings. Thus, there is a great need for application guidelines to establish the feasibility of proposed rotor systems and to identify existing machines that are good candidates for foil bearing use. Specifically, a method is needed to estimate foil bearing stiffness and damping behavior in order to foster advanced oil-free rotating machine development.
Methods to estimate critical stiffness and damping parameters, however, do not currently exist. The purpose of the methods put forth in this work is to establish simple tools capable of estimating foil bearing stiffness and damping coefficients suitable for oil-free rotor support design work. This has been accomplished by first coalescing all available empirical data on foil bearing performance that has been generated in the author’s own laboratories, and by researchers working in university, government, and industrial laboratories. This information is examined and combined, then used to develop ROT for foil bearing stiffness and damping. These ROTs can then be combined with existing rules for load capacity to obtain credible feasibility assessments for proposed oil-free rotor systems.
The effort described has resulted in algebraic models for foil gas bearings that yield stiffness, damping, and load capacity values as a function of bearing size, design, and operating speed. With these models, one can easily determine the feasibility of building a foil bearing supported machine without incurring the expense of early experimental work. The models presented represent the only known and verified methods to predict conveniently foil bearing performance properties.
This work was done by Christopher DellaCorte of Glenn Research Center.
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