Heshmat, Hooshang "Compliant Foil Bearing Technology: An Overview." Handbook of Lubrication and Tribology: Volume II Theory and Design. Second Edition. CRC Press, 2006.

Heshmat, Hooshang “Compliant Foil Bearing Technology: An Overview.”  Handbook of Lubrication and Tribology: Volume II Theory and Design. Second Edition. CRC Press, 2006.

Air-lubricated foil bearings have demonstrated excellent promise for several applications such as air-craft air cycle machines, compressors, blowers, high-speed motors, gas turbines, turbochargers, and other high-speed turbomachinery.  the foil bearing will deform as hydrodynamic pressure is generated.  They can be constructed to deform in an advantageous manner with respect to the application, whether it be for high load capability or for high-speed operation.  It is also possible to introduce frictional damping by causing rubbing as deformation takes place and by selecting material combinations that will optimize the Coulomb damping effect.

Computer codes have been developed and upgraded over the past decade to evaluate the performance of a gas journal bearing using a spring-supported compliant foil as the bearing surface.  The analytical model considers a journal bearing, consisting of a hollow cylindrical sleeve, the inside of which contains springs of corrugated or bump foils lined on top with a very think foil.  The foil is anchored at its leading edge but is free at the trailing end.  The bumps act as a spring, and the foil, when loaded, deflects, producing a film thickness higher than in an equivalent rigid surface.

Since stiffness of the baring will depend upon both the stiffness of the gas film and the stiffness of bump foils, the deflection of the bump foil is particularly important when the gas film is stiff compared with the foil, which is a situation that make  optimum use of the compliance of the bending-dominated bump foil is particularly important when the gas film is stiff compared with the foil, which is a situation that makes optimum use of the compliance of the bending -dominated foil bearing.  Additional computer-based analyses have been developed to calculate the elastic deformation (structural stiffness) and performance of bending-dominated compliant foils (bump and smooth top foils) to the hydrodynamic computer design analysis.  The output of these structural cods provides the appropriate spatial stiffness input.

The computer codes developed by one of the authors, when properly coupled and applied, predict the performance characteristics of gas-lubricated compliant surface foil journal bearings.  Performance characteristics include load capacity, film thickness, pressure and surface deformation profile, power requirements, and dynamic characteristics in the form of stiffness in two degree of freedom.  These performance characteristics are computed as functions of compliant foils’ structural stiffness and bearing geometry, loads, and load direction, running speed and fluid viscosity.  The following assumptions have been made:

1.         The flow is assumed to be laminar, compressible, and isothermal.

2.         The gas is assumed to be ideal.

3.         The bearing operating condition and flow are steady state, and the bearing stiffness coefficients are computed with use of small perturbation on a primary steady-state journal rotating frequency and position.

The aforementioned assumptions still leave the codes applicable to a broad range of applications.  A derivation of the equations governing the performance of gas-lubricated foil journal bearing, along with description of their solution, is given in the next section.  this will be followed by a description of the computer codes including an input description, sample cases, and comparisons with results of experiments.  For more detailed treatment of the subject matter and the design methods employed to integrate compliant foil bearings into function rotor bearing systems, the reader is referred to the upcoming ASME book publication entitled “Foil Bearing Technology – Applications and Integrations.”


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