The gas foil bearing (GFB) technology is a key factor for the transition to oil-free rotating machinery. Among numerous advantages, GFBs offer the unique ability to be lubricated with working fluids such as refrigerants. However, the computational analysis of refrigerant-lubricated GFB–rotor systems represents an interdisciplinary problem of enormous complexity. This work pushes forward existing limits of feasibility and establishes a new strategy that enables stability and bifurcation analyses.

Internal experiments at Liebherr-Aerospace FRANCE on new refrigerant-lubricated compressor designs have shown that under specific operating conditions, a mixture of vapor and liquid appears in the compressor, instead of a single-phase vapor flow. Therefore, refrigerant-lubricated foil bearings behavior is studied, including the likelihood of two-phase flow in the lubricant. We focus on the lubricant behavior only, in the operating conditions of foil bearings. The Thermo-Hydrodynamic approach describes lubricant characteristics such as pressure, density, viscosity, and temperature. It involves the use of a generalized Reynolds equation for turbulent flow, a nonlinear cubic equation of state for two-phase flow and a 3D turbulent thin-film energy equation. Journal bearing global parameters are calculated for steady-state conditions.

Using the foil bearing demonstrator, which is a small, 95=pound thrust turbonect engine developed from an existing jet fuel starter, a series of evaluation tests was conducted to demonstrate that a gas turbie engine, mounted on gas-lubricated, compliant foil bearings, has the potential for qualification in military application.

The performance and durability of advanced, high temperature foil air bearings are evaluated under a wide range (10-50 kPa) of loads at temperatures from 25 to 650 deg C. The bearings are made from uncoated nickel based superalloy foils. The foil surface experiences sliding contact with the shaft during initial start/stop operation. To reduce friction and wear, the solid lubricant coating, PS 304, is applied to the shaft by plasma spraying. PS304 is a NiCr based Cr2O3 coating with silver and barium fluoride/calcium fluoride solid lubricant additions. The results show that the bearings provide lives well in excess of 30,000 cycles under all of the conditions tested. Several bearings exhibited lives in excess of 100,000 cycles. Wear is a linear function of the bearing load. The excellent performance measured in this study suggests that these bearings and the PS 304 coating are well suited for advanced high temperature, oil-free turbomachinery applications.

An investigation of air-lubricated foil bearing technology for application in advanced propulsion and power systems was performed. The work conducted under the program comprised: (a) establishment and documentation of the state-of-the-art of foil bearings, (b) development of analytical tools and design guides to permit calculation of the performance of bending-dominated foil bearings, (c) investigation of potential substrate and coating materials, as well as conduct of start-stop and high speed rub tests with two air-lubricated foil bearings at 1000 deg F, (d) preparation of a layout drawing of a dynamic simulator test vehicle based upon an advanced engine (1980 or later time frame) in the 1000 to 3000 pounds thrust class, as well as a description of the test facility requirements, (e) description of the technology segments of air-lubricated foil bearings that need to be advanced to meet the requirements of the advanced engine application, and definition of the parameters for a long range test effort. (Modified author abstract).

Foil air bearings are self-acting hydrodynamic bearings which rely upon solid lubricants to reduce friction and minimize wear during sliding which occurs at start-up and shut-down when surface speeds are too low to allow the formation of a hydrodynamic air film. This solid lubrication is typically accomplished by coating the non-moving foil surface with a thin, soft polymeric film. The following paper introduces a systems approach in which the solid lubrication is provided by a combination of self lubricating shaft coatings coupled with various wear resistant and lubricating foil coatings. The use of multiple materials, each providing different functions is modeled after oil-lubricated hydrodynamic sleeve bearing technology which utilizes various coatings and surface treatments in conjunction with oil lubricants to achieve optimum performance. In this study, room temperature load capacity tests are performed on journal foil air bearings operating at 14,000 rpm. Different shaft and foil coating technologies such as plasma sprayed composites, ceramic, polymer and inorganic lubricant coatings are evaluated as foil bearing lubricants. The results indicate that bearing performance is improved through the individual use of the lubricants and treatments tested. Further, combining several solid lubricants together yielded synergistically better results than any material alone. DellaCorte, Christopher and Zaldana, Antonio R. and Radil, Kevin C. Glenn Research Center NASA/TM-2002-211482, NAS 1.15:211482, E-13250, ARL-TR-2867