Air Foil Bearings Research Articles

Parametric Studies on Static and Dynamic Performance of Air Foil Bearings with Different Top Foil Geometries and Bump Stiffness Distributions

Experimental and analytical studies on air foil bearings have been performed extensively over the past decades, and significant improvement in load capacity and rotor dynamics stability have been reported. Often, advanced air foil bearings are believed to have complicated bump foil structure to provide unique underlying support mechanism, which in turn make the bearing very stable and have high load capacity. However, all the analytical studies on air foil bearings so far assume a circular profile of top foil with uniform bump stiffness distribution because detailed information on bump stiffness distribution and overall bearing shape is not known to the public. This paper investigates load capacities and rotordynamic performances of two different types of air foil bearings, e.g., circular cylindrical bearings with single continuous top foil and noncircular preloaded bearings with three top foil pads. Within the two subcategories, stiffness variations along axial and circumferential directions were given to have a total of four types of air foil bearings with different overall bearing shapes and stiffness distributions. Overall static and dynamic performance of the four different types of air foil bearings are presented and compared via calculations of load capacities and dynamic force coefficients, modal stability analyses, and time domain orbit simulations. The major difference of load capacities comes from the overall bearing shape (circular continuous foil or preloaded three pad) rather than spatial variation of bump stiffness within the bump foils. Preloaded three-pad bearings have significantly reduced load capacity compared to the circular bearings because of small pad arc length. Rotordynamic performance is also much more sensitive to the overall bearing shape than spatial variation of bump stiffness and damping within the bump foils. The linear stability analyses predict modal natural frequencies very close to those from the orbit simulations. However, onset speeds of instability from these two approaches are quite different, manifesting the limitation of the linear stability analyses. The orbit simulations show the three-pad bearings have higher onset speeds of instability than circular bearing (47,000rpm versus 24,000rpm).

Vibration Characteristics of a 75kW Turbo Machine With Air Foil Bearings

Air foil bearings are very attractive bearing systems for turbomachinery because they have several advantages over conventional bearings in terms of oil-free environment, low power loss, long life, and no maintenance. However, most of the developed machines using air foil bearings are limited to small and high-speed rotors of 60,000–120,000 rpm, since the increase in power of turbomachinery requires lower rotor speed and greater loading in bearings, which makes it difficult to use air foil bearings for large machines. In this paper, a 75 kW turboblower using air foil bearings is introduced, and the vibration characteristics of the machine have been investigated experimentally under a wide range of operating conditions, including compressor surge in the performance test. The machine is designed to be fully air lubricated and air cooled, and its operating speed is 20,000–26,000 rpm with maximum pressure ratio of 1.8. The results show that the air foil bearings offer adequate damping to ensure dynamically stable operation in the whole range.

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.

A Systems Approach to the Solid Lubrication of Foil Air Bearings for Oil-Free Turbomachinery

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 nonmoving 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.

小型高速マイクロガスタービン用ラジアルフォイル軸受特性の計算法(FR1 マイクロエネルギー変換)

Generators with small gas turbines that generate less than 100kW are drawing attention as a power source of cogeneration systems. Dynamic pressure type air foil bearings are expected to serve as shaft bearings. The design that these bearing are simply designed, easy to produce and maintenance free without requiring lubrication, holds the key to miniaturization of micro-gas turbines. However, the details of such design guidelines have not been clarified though this technology is used in APC for aircraft units. In this study, in order to obtain design guidelines of foil bearing, in making the foil to be the discretion beam, the modeling of the foil was carried out. Taking the frictional forces of attachment part of the foil and the protrusion, etc. into consideration, static characteristic and dynamic characteristic were analyzed to examine performance and stability of the radial foil bearing.

공기 포일 베어링으로 지지되는 무급유 터보 과급기 회전체 설계에 대한 연구

The feasibility study on the oil-free turbocharger supported by air foil bearings is investigated. Using the perturbation method, dynamic characteristics of air foil bearings are calculated based on the stati c equilibrium position of a turbocharger rotor is predicted. With collaboration of calculated stiffness and damping of foil bearing, rotordynamic analysis is performed using the finite element method. The effects of bump compliance and bearing clearance on rotordynamic characteristics of the oil-free turbocharger such as the critical speeds, eccentricity ratio, vibration amplitude and stability are investigated.

Suppression of Subsynchronous Vibrations Due to Aerodynamic Response to Surge in a Two-Stage Centrifugal Compressor with Air Foil Bearings

An investigation was conducted on the suppression of subsynchronous vibrations due to aerodynamic response to surge in a two-stage centrifugal compressor with air foil bearings. Unsteady aerodynamic response to surge caused excessive subsynchronous shaft vibration which might result in reduced bearing life. Notably, subsynchronous vibrations associated with rigid mode frequencies were more severe than any other subsynchronous vibrations. The objective was to develop rotordynamic stability near the surge line in a two-stage compressor by the modification of bearings into high damping foil bearings. Viscoelastic foil bearings, which consist of a viscoelastic top foil supported by a series of bump foils, were adapted to replace bump type foil bearings in a two-stage compressor. Investigations were made in the form of a compressor operating test near the surge line with the conventional bump foil bearing and the viscoelastic foil bearing respectively. Testy results showed the effectiveness of the viscoelastic foil bearing for suppressing the subsynchronous vibrations, including subsynchronous vibrations associated with rigid mode frequencies of the shaft. Thus, rotordynamic stability was improved with a viscoelastic foil bearing, which has enhanced damping characteristics near the surge line. Scheduled for Presentation at the 58th Annual Meeting in New York City April 28–May 1, 2003

Development of Advanced Turbo Blowers Using High-Speed BLDC Motors and Foil Air Bearings

Development of Advanced Turbo Blowers Using High-Speed BLDC Motors and Foil Air Bearings

The Effect of Journal Roughness and Foil Coatings on the Performance of Heavily Loaded Foil Air Bearings

Foil air bearing load capacity tests were conducted to investigate if a solid lubricant coating applied to the surface of the bearing's top foil can function as a break-in coating. Two foil coating materials, a conventional soft polymer film (polyimide) and a hard ceramic (alumina), were independently evaluated against as-ground and worn (run-in) journals coated with NASA PS304, a high-temperature solid lubricant composite coating. The foil coatings were evaluated at journal rotational speeds of 30,000 rpm and at 25 °C. Tests were also performed on a foil bearing with a bare (uncoated) nickel-based superalloy top foil to establish a baseline for comparison. The test results indicate that the presence of a top foil solid lubricant coating is effective at increasing the load capacity performance of the foil bearing. Compared to the uncoated baseline, the addition of the soft polymer coating on the top foil increased the bearing load coefficient by 120 percent when operating against an as-ground journal surface and 85% against a run-in journal surface. The alumina coating increased the load coefficient by 40 percent against the as-ground journal but did not have any effect when the bearing was operated with the run-in journal. The results suggest that the addition of solid lubricant films provide added lubrication when the air film is marginal, indicating that as the load capacity is approached foil air bearings transition from hydrodynamic to mixed and boundary lubrication. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

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 journal's 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

Thrust-Washer Evaluation of Self-Lubricating PS304 Composite Coatings in High Temperature Sliding Contact

PS304 self-lubricating composite coatings were successfully deposited on steel substrates at various plasma spray facilities using mixtures blended from commercially obtained constituent particles. Coatings were evaluated in thrust-washer tests against Inconel X-750 at low contact pressures to 40kPa, sliding speed of 5Amis, and either ambient temperature or 500 °C chosen to simulate conditions in airfoil bearings during startup and shutdown contact. Wear factors for all PS304 coatings tested, regardless of contact pressure and temperature, ranged from 1–3*10−4 mm3/Nm while coefficients of friction of approximately μ =0.5 were measured in all cases. While wear and friction behavior of PS304 in air foil bearings appear to have been simulated, surface roughening was observed in these thrust-washer tests which used continuous sliding contact, as opposed to the evolution of smoother surfaces observed in high-temperature foil bearings experiencing cyclic startup/shutdown. Wear-induced surface smoothening of PS304 was additionally simulated in thrust-washer tests with sliding contact instead imposed intermittently.

Performance and Durability of High Temperature Foil Air Bearings for Oil-Free Turbomachinery

The performance and durability of advanced, high temperature foil air bearings are evaluated under a wide range (10 to 50 kPa) of loads at temperatures from 25° to 650 °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, PS304, 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 PS304 coating are well suited for advanced high temperature, oil-free turbomachinery applications. Presented at the 55th Annual Meeting Nashville, Tennessee May 7–11, 2000

Rotordynamics and Design Methods of an Oil-Free Turbocharger©

The feasibility of supporting a turbocharger rotor on air foil bearings is investigated based upon predicted rotordynamic stability load accommodations, and stress considerations. It is demonstrated that foil bearings offer a plausible replacement for oil-lubricated bearings in diesel truck turbochargers. Also, two different rotor configurations are analyzed and the design is chosen which best optimizes the desired performance characteristics. The method of designing machinery for foil bearing use and the assumptions made are discussed. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

Evaluation of Advanced Solid Lubricant Coatings for Foil Air Bearings Operating at 25° and 500°C

The tribological properties of one chrome oxide and one chrome carbide based solid lubricant coating were evaluated in a partial-arc foil bearing at 25° and 500°C. Start/stop bearing operation up to 20,000 cycles were run under 10 kPa (1.5 psi) static deadweight load. Bearing friction (torque) was measured during the test. Specimen wear and SEM/EDS surface analyses were conducted after testing to understand and elucidate the tribological characteristics observed. The chrome oxide coating which contains both Ag and BaF2/CaF2for low and high temperature lubrication, exhibited low friction in sliding against Al2O3 coated foils at 25° and 500°C. The chrome carbide coating, which lacked a low temperature lubricant but contained BaF2/CaF2 as a high temperature lubricant, exhibited high friction at 25°C and low friction at 500°C against both bare and Al2O3 coated superalloy foil surfaces. Post test surface analyses suggest that improved tribological performance is exhibited when a lubricant film from the coating transfers to the foil surface. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

Foil Air Bearings Cleared to Land

This article explores use of foil air bearings in land-based turbomachinery. A machine with foil air bearings is more reliable than one with rolling element bearings because it requires fewer parts to support the rotative assembly and needs no lubrication. Foil air bearings can handle severe environmental conditions such as the ingestion of sand and dust. A reversed pilot design at the cooling flow inlet prevents large particles from entering the bearing's flow path, and smaller particles are continually flushed out of the bearing by the cooling flow. Many applications of foil air/gas bearings other than air cycle machines have been built and successfully tested, but nothing appears to be currently in production. Foil bearings have strong potential in several applications. Among these are small general aviation gas turbine engines; oil-free cryogenic turboexpanders for gas separation plants; auxiliary power units for various aerospace and ground vehicles; and, taking advantage of automated manufacturing methods, automotive gas turbine engines, vapor-cycle centrifugal compressors, and commercial air/gas compressors.

A New Foil Air Bearing Test Rig for Use to 700°C and 70,000 rpm

A new test rig has been developed for evaluating foil air bearings at high temperatures and speeds. These bearings are self-acting hydrodynamic air bearings which have been successfully applied to a variety of turbomachinery operating up to 650°C. This unique test rig is capable of measuring bearing torque during start-up, shutdown and high speed operation. Load capacity and general performance characteristics, such as durability, can be measured at temperatures to 700°C and speeds to 70,000 rpm. This paper describes the new test rig and demonstrates its capabilities through the preliminary characterization of several bearings. The bearing performance data from this facility can be used to develop advanced turbomachinery incorporating high temperature oil-free air bearing technology. Presented as a Society of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, United Kingdom, September 9–12, 1997

Development of CdO-graphite-Ag coatings for gas bearings to 427 °C

A CdO-graphite coating was developed and successfully tested in foil air bearings for long periods up to a temperature of 427 °C (800 °F) and at a normal contacting load (during starting and stopping) of 14 kPa (2 lbf in −2) based on the bearing projected area. The addition of ultrafine silver to the CdO-graphite improved the coating endurance. At 427 °C, the CdO-graphite-Ag coating performed better than CdO-graphite without silver, both for extended periods at 14 kPa (2 Ibf in −2) and for limited periods at 35 kPa (5 lbf in −2). At 288 °C (550 °F), the coating was tested for an extended period up to 28 kPa (4 Ibf in −2) and has also successfully completed high speed shock tests to an acceleration level of 100g.

Friction and Wear Results From Sputter-Deposited Chrome Oxide With and Without Nichrome Metallic Binders and Interlayers

Friction and wear tests were conducted on optimized sputtered Cr2O3 and Cr2O3 with metallic binder coatings. The coatings were applied on the bearing surface of journal foil air bearings and were tested against chrome-carbide-coated journal surfaces. The objective of the study was to develop a coating system which would withstand 9000 start-stops and high-speed rubs (maximum acceleration, 100 gs) in temperatures ranging from room temperature to 650° C. The Cr2O3 coating completed the test sequence and the coating consisting of Cr2O3 with metallic binders completed 3000 start-stops. The coefficient of friction of the coatings at 650° C was found to be about half that at room temperature. It was concluded, therefore, that the coatings should perform much better in a high temperature environment alone. The decrease in friction at high temperature is attributed to oxidation and interactions of the coatings and substrates at the interface temperature.