Foil Bearings


Air and process gas lubricated Foil Bearings are high performance bearings suitable for many high speed applications under 1 MW or so. Under development since the 1960’s, they have been quite successfully applied to certain specialized applications. For example, nearly every commercial aircraft flying today has one or more foil bearing supported machines for cabin pressurization and environmental control. One frequently cited statistic, is a mean time between failure in excess of 100,000 hours in the Boeing 747 machine. Other applications include cryogenic expanders, microturbines and high speed blowers. A variety of other industrial applications are in development. Oil-free turbochargers using foil bearings are also being actively developed.

To help develop new applications, Xdot Engineering and Analysis is uniquely positioned to offer a more collaborative alternative to the high-cost, bearing supplier controlled, black-box development strategy traditionally offered by foil bearing developers. With strong, hands-on experience in the technology, as well as a solid rotordynamics and testing background, Xdot Engineering and Analysis can assist in roles ranging from independent assessments, to subject expertise as a team member, to assembling a team and leading prototype development projects.

How Do They Work?

Foil bearings are very similar in operating principle to the more familiar oil lubricated journal bearings used in numerous applications such as steam turbines or large industrial compressors. As with any hydrodynamic bearing, the shaft load is supported by a self-generated region of high pressure in a thin layer of fluid between the rotating shaft and the stationary bearing. As shown in the figures, this high pressure region is created in the gas filled clearance space that converges in the direction of rotation due to the relative motion between the rotating shaft and the top foil. The load that can be supported is primarily a function of the relative surface speed, the area of the converging region, the shape of the clearance space between the rotor and top foil, the support structure stiffness and the viscosity of the lubricant (generally air for foil bearings). Thermal management also plays an important role for higher loads.

The unique feature of a foil bearing is the compliant operating surface. This compliant operating surface changes shape in response to load, speed, thermal deformations, etc. This compliance allows the bearing to accommodate levels of misalignment and thermal growth that would destroy a rigid surface air bearing. More importantly, it is also a significant source of damping (energy dissipation). This feature allows the bearing designer to accommodate the relatively limited amount of damping available from a pure hydrodynamic air bearing. In this sense, they are conceptually very similar to squeeze-film damper bearings widely used in aircraft and in some industrial applications.

Foil Bearings Versus Other Bearings

The highlighted items in the following table are the usual reasons to use foil bearings:

Gas/Air Foil BearingOil Bearing (Sleeve, Tilting Pad)Rolling Element BearingActive Magnetic Bearing
Maximum Operating Speed at Bearing Surface (Bore) for Radial Bearing Essentially unlimited, 500 to 740 ft/s (150 to 225 m/s) is fairly typicalGenerally 250 to 350 ft/s or less (75 to 105 m/s)1,000,000 DN (bore in mm x RPM) for typical applications, 3,000,000 DN for some aerospace applications.

Equal to a surface speed of 170 to 515 ft/s (52 to 157 m/s)
590 ft/s for typical materials,650 ft/s for special alloys (180 to 200 m/s)
Minimum Required Operating SpeedYes, application dependentYes, application dependentNoneNone
Load Capacity and Typical Projected Area LoadsLow to moderate

50 to 100 psi (0.68 MPa) radial

25 to 35 psi (0.2 to 0.25 MPa) to thrust
Potentially very high

100 to 450 psi (0.68 to 3.1 MPa) radial

250 to 500 psi (1.7 to 3.4 MPa) thrust
Moderate to highLow to moderate

100 psi (0.68 MPa)
Short term overload capabilityLimitedSubstantialGoodLimited
Bearing Operating Temperature RangeCryogenic to 1200+ F (650+ C)Varies with construction and lubricant, but most babbitt surfaced industrial bearings operate in the range of 90 to 180 F (32 to 82 C), and alarm by 250 F (120 C)-20 to 450 F (-30 to 230 C)-300 to 1000 F (-180 to 540 C) claimed by developers
Power LossRadial very low

Thrust moderate
Can be significantGenerally low to moderateGenerally very low
Oil Free?YesNoNoYes (although backup bearings could be grease lubricated)
Misalignment CapabilityLow to moderateModerate, depends on constructionVery low (highly loaded angular contact) to moderate (spherical roller)Moderate
Auxiliary SystemsSource of a limited amount of low pressure "cooling" air (although the actual temperature of this air could be very high in some high temperature applications)Pumps, coolers, filtersNothing for grease lubricated bearings, ranging to pumps, filters, etc. for oil-jet lube at high speeds and loadsControl system electronics, auxiliary bearings, generally some amount of cooling air
Radial Envelope requirementLength in range of 0.5 to 2x shaft diameter, OD in range of 1.25 to 2 times diameterLength in range of 0.5 to 2x shaft diameter, OD in range of 1.25 to 2 times diameter

Plus lube oil system
Length in range of 0.2 to 0.5x shaft diameter, OD in range of 1.5 to 2 times diameter

Plus lube oil system
Length in range of 1 to 2x shaft diameter, OD in range of 1.5 to 4 times diameter

Plus electronics if externaal
WeightGenerally the lightest optionGenerally pretty heavy when pumps, filters, piping etc considered along with the actual bearingsRelatively lightModerate including electronics
StiffnessLowModerate to highHighDepends on tuning, generally are tuned soft
DampingLow to moderateGenerally HighVery LowModerate to high depending on tuning and system dynamics
Shock ToleranceGoodVery GoodModerateCan be poor

Foil Bearing Applications

The following chart gives some indication of how foil bearings might fit candidate applications. Note that all loads are projected area loads (length x width for radial)

ItemLower RiskMore Challenging
Free-free undamped critical speedsAll well above maximum operating speedOne or more below operating speed
Shaft speed over normal operating speed range at radial bearing location for "reasonable" shaft configuration500 to 740 ft/s (150 to 225 m/s)Less than 100 ft/s (30 m/s)

More than 1000 ft/s (305 m/s)
Startup load for "reasonable" sized radial and thrust bearingsLess than 5 psi (34 kPa)More than 5 psi (34 kPa)
Operating static + dynamic load for "reasonable" sized radial bearing (including off design) *Less than 30 psi (200 kPa)More than 50 psi (340 kPa)
Operating static + dynamic load for "reasonable" sized thrust bearing (including off design) *Less than 25 psi (170 kPa)More than 35 psi (240 kPa)
Available starting torqueHigh Low
Damping required for stability and reasonable response*LowHigh
Bearing operating temperatureLess than 400 F (200 C)More than 700 F(370 C)
Bearing compartment pressureNear atmosphericSub atmospheric or very high pressure
Operating fluidReasonably clean, dry airVery dirty air
Occasional liquid present

* Machines that operate at higher pressures and have damper seals may be an exception to these guidelines

Further Reading

We have 300+ papers and 200+ patents related to foil bearings in our database, so it is difficult to point out the "best" ones. However, researchers at NASA Glenn have done a tremendous job over the past decade or so trying to turn proprietary foil bearing art into public domain science. As part of this work, they have developed a number of useful guidelines and rules of thumb. Much of their work is freely available, however, which makes these especially easy to obtain.

It should also be noted that there are some design and analysis codes, including one under development by Xdot. Actual bearing design and development is no longer as much of an empirical "build and bust" effort as some of the older NASA works might lead one to think.

Some of the NASA works that stand-out as being especially relevant for new applications include:

Remaining Technical Challenges and Future Plans for Oil-Free Turbomachinery.

DellaCorte C., and Bruckner R. J., 2010
NASA/TM-2010-216762, presented at the ASME/IGIT Turbo Expo 2010
Nice overview of history and state of the art as of 2010.

The application of Oil-Free technologies (foil gas bearings, solid lubricants and advanced analysis and predictive modeling tools) to advanced turbomachinery has been underway for several decades. During that time, full commercialization has occurred in aircraft air cycle machines, turbocompressors and cryocoolers and ever-larger microturbines. Emerging products in the automotive sector (turbochargers and superchargers) indicate that high volume serial production of foil bearings is imminent. Demonstration of foil bearings in APU’s and select locations in propulsion gas turbines illustrates that such technology also has a place in these future systems. Foil bearing designs, predictive tools and advanced solid lubricants have been reported that can satisfy anticipated requirements but a major question remains regarding the scalability of foil bearings to ever larger sizes to support heavier rotors. In this paper, the technological history, primary physics, engineering practicalities and existing experimental and experiential database for scaling foil bearings are reviewed and the major remaining technical challenges are identified.

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Integration Methodology for Oil-Free Shaft Support Systems: Four Steps to Success.

Howard S. A., DellaCorte C., and Bruckner R. J., 2010
NASA/TM-2010-216827, presented at the 8th IFToMM Conference in 2010
Overview of the process that could be followed to develop a totally new bearing if you have lots of time and money.

Commercial applications for Oil-Free turbomachinery are slowly becoming a reality. Micro-turbine generators, highspeed electric motors, and electrically driven centrifugal blowers are a few examples of products available in today’s commercial marketplace. Gas foil bearing technology makes most of these applications possible.

A significant volume of component level research has led to recent acceptance of gas foil bearings in several specialized applications, including those mentioned above. Component tests identifying such characteristics as load carrying capacity, power loss, thermal behavior, rotordynamic coefficients, etc. all help the engineer design foil bearing machines, but the development process can be just as important. As the technology gains momentum and acceptance in a wider array of machinery, the complexity and variety of applications will grow beyond the current class of machines. Following a robust integration methodology will help improve the probability of successful development of future Oil-Free turbomachinery. This paper describes a previously successful four-step integration methodology used in the development of several Oil-Free turbomachines. Proper application of the methods put forward here enable successful design of Oil-Free turbomachinery. In addition when significant design changes or unique machinery are developed, this four-step process must be considered.

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Load Capacity Estimation of Foil Air Journal Bearings for Oil- Free Turbomachinery Applications.

DellaCorte C., and Valco M. J., 2000
NASA/TM-2000-209782, Presented at the International Joint Tribology Conference in 2000
Pretty good rule of thumb for bearing sizing. Be careful about pushing this rule of thumb too far from the bearing sizes cited.

This paper introduces a simple ""Rule of Thumb"" (ROT) method to estimate the load capacity of foil air journal bearings, which are self-acting compliant-surface hydrodynamic bearings being considered for Oil-Free turbomachinery applications such as gas turbine engines. The ROT is based on first principles and data available in the literature and it relates bearing load capacity to the bearing size and speed through an empirically based load capac- ity coefficient, D . It is shown that load capacity is a linear function of bearing surface velocity and bearing projected area. Furthermore, it was found that the load capacity coefficient, D , is related to the design features of the bearing compliant members and operating conditions (speed and ambient temperature). Early bearing designs with basic or ""first generation"" compliant support elements have relatively low load capacity. More advanced bearings, in which the compliance of the support structure is tailored, have load capacities up to five times those of simpler designs. The ROT enables simplified load capacity estimation for foil air journal bearings and can guide development of new Oil-Free turbomachinery systems.

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A Preliminary Foil Gas Bearing Performance Map.

DellaCorte C., Radil K., Bruckner R. J., and Howard S., 2006
NASA/Tm-2006-214343, Presented at the STLE Annual Meeting in 2006
Has some useful rules of thumb and approaches for matching bearings to applications

Recent breakthrough improvements in foil gas bearing load capacity, high temperature tribological coatings and computer based modeling have 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 thereby hampering commercialization of new applications. In this paper, a new approach loosely based upon accepted hydrodynamic theory, is developed which results in a “Foil Gas Bearing Performance Map” to guide the integration process. This performance 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.

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Stiffness and Damping Coefficient Estimation of Compliant Surface Gas Bearings for Oil-Free Turbomachinery

DellaCorte, C., 2010
NASA/TM-2010-216924, Presented at the International Joint Tribology Conference in 2010
Expands rules of thumb for bearing sizing to include stiffness and damping coefficients. Based on published experimental results.

Foil gas bearings are a key technology in many commercial and emerging Oil-Free turbomachinery systems. These bearings are non-linear 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, a rule-of-thumb, to estimate load capacity. This method has been a valuable tool in system development. The current paper 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.

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