Christopher DellaCorte and Robert J. Bruckner

Glenn Research Center, Cleveland, Ohio

Remaining Technical Challenges and 

Future Plans for Oil-Free Turbomachinery

NASA/TM—2010-216762

August 2010

GT2010–22086
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Christopher DellaCorte and Robert J. Bruckner

Glenn Research Center, Cleveland, Ohio

Remaining Technical Challenges and 

Future Plans for Oil-Free Turbomachinery

NASA/TM—2010-216762

August 2010

GT2010–22086

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216762 1

Remaining Technical Challenges and 

Future Plans for Oil-Free Turbomachinery

Christopher DellaCorte and Robert J. Bruckner

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

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.

Introduction

Turbomachinery built upon unconventional, Oil-Free, rotor 

support technologies has been maturing since the middle of 

the last century. During this period numerous advances in 

compliant surface gas bearings, electromagnetic bearings, 

materials, solid lubricants and analytical and computer based 

models have been achieved. New Oil-Free rotor system 

designs have been developed and deployed in ever increasingly complex and larger practical machines. The authors’ organization, NASA, has driven much of the pioneering 

development and has made long and deep investments in OilFree technologies because of their uniquely intrinsic and 

enabling value to space power conversion and aeropropulsion 

turbomachinery systems (Refs. 1 and 2).

NASA relevant application examples include air cycle machines (ACM’s) used for aircraft cabin environmental control 

systems, cryogenic turboexpanders and compressors, reusable 

long-life turbopumps, maintenance free auxiliary power units 

(APU’s), aircraft propulsion engines, and closed Brayton cycle 

(CBC) turbine generators for space power. In addition several 

entirely new industries have been built upon the pioneering 

technology demonstrations. These include Oil-Free microturbines, fuel cell blowers, Oil-Free turbochargers and industrial 

air compressors. As Oil-Free products further penetrate 

industrial markets, it seems an appropriate time to explore, 

identify and articulate the remaining major technical challenges and questions in this field. In the following sections of 

this paper, Oil-Free technologies are reviewed and the current 

state of the art is presented. The major remaining technical 

challenges related to bearing scalability are explored and 

advanced approaches to hybridization of bearing technologies 

to overcome limitations are highlighted.

Nomenclature

ACM Air Cycle Machine

APU Auxiliary Power Unit

DN bearing surface velocity parameter

W shaft load

μ fluid viscosity

WLC load capacity at speed

D bearing performance coefficient (lb/in.

3

/krpm)

L bearing axial length (in.)

D shaft diameter (in.)

Krpm shaft speed in thousands of rpm

Technology Background

Foil bearings are self-acting, hydrodynamic fluid film bearings that use the ambient fluid, typically a gas, as their lubricant. In general, foil bearings are capable of supporting lightly 

loaded, high-speed rotating shafts. The lubricating fluid-film is 

generated by the viscous pumping action of the moving shaft 

or runner surface. The fluid film forms between the moving 

surface and a thin, flexible sheet metal foil layer that is supported by a series of spring foils. The foil layer facing the 

moving surface traps the hydrodynamic gas film and the 

supporting spring foils provide compliance, tolerance to 

misalignment and distortion and a host of other attributes such 

as damping (Ref. 3). Figure 1 shows a sketch of a typical 

bump style foil journal bearing.

There are two distinct types of foil gas bearings, journal and 

thrust bearings as depicted in Figure 2. Journal bearings 

support radial loads and thus control rotor orbit. Thrust bearings control axial motion. Though their geometry differs, both 

types of foil bearings operate under the same basic principles, 

namely that the moving surface relies on viscous action to 

drag fluid into the bearing generating hydrodynamic pressure 

that pushes the inner foil surface away from the shaft. 
NASA/TM—2010-216762 2

In turbomachinery systems, thrust loads can be minimized 

through careful sizing and design of aerocomponents and 

judicious selection of operating points. Since no commensurate approach exists for radial loads, scalability of journal 

bearings is critical to the extension of Oil-Free turbomachinery to larger machines. In addition, thrust bearing development and scalability has historically followed the advancement 

of journal bearings since they share similar basic underlying 

physics. While thrust bearings do present major challenges, 

the current paper solely focuses on journal bearings with the 

expectation that advancements in journal bearings will transition to thrust bearings.

In turbomachinery systems, thrust loads can be minimized 

through careful sizing and design of aerocomponents and

judicious selection of operating points. Since no commensurate approach exists for radial loads, scalability of journal 

bearings is critical to the extension of Oil-Free turbomachinery to larger machines. In addition, thrust bearing development and scalability has historically followed the advancement 

of journal bearings since they share similar basic underlying 

physics. While thrust bearings do present major challenges, 

the current paper solely focuses on journal bearings with the 

expectation that advancements in journal bearings will transition to thrust bearings.

Since the ambient bearing cavity gas is used as the working 

fluid no dedicated lubrication system is needed. In essence, the 

gas takes the place of oil used in conventional bearings making foil bearings “Oil-Free.” In addition, foil bearings also 

eliminate the use of rolling elements such as balls or rollers, 

which introduce their own restrictions on performance like 

DN speed limitations. Thus, among the major benefits of using 

foil bearings is their ability to run at high speeds and temperatures without an oil system and this leads to simpler, safer and 

often lower cost engineered systems (Ref. 1).

Technological Development History

The foil gas bearing was originally discovered within the 

tape recording industry. Early technologists seeking to drive 

magnetic tape over recording heads at ever-faster speeds found 

that recording performance degraded at high speeds because the 

tape began to float away and “lift-off” the heads. This floating 

phenomena was deemed a nuisance and Baumeister (Ref. 4) at 

IBM was the first to recognize it as a form of gas hydrodynamics and named it the “foil bearing problem” in reference to the 

flexible oil-lubricated bearings made from metal foils studied in 

Europe in the 1950s. His colleague, W.R. Gross, mathematically modeled the tape motion and envisioned a means to turn such 

a phenomenon into a practical bearing device for high-speed 

spindles. This was essentially a solution seeking a problem and 

Gross published his findings on such flexible gas bearing 

concepts in the literature (Ref. 4). Figure 3 shows a cross 

section of the earliest of foil bearings that resemble a shaft 

braced on three sides with a tape of metal foil held in place by 

spring-loaded rollers.

NASA and the Office of Naval Research (ONR) encouraged 

Gross to apply his bearing concept to support high speed gas 

circulators and compressor-turbines used to cool gas filled 

nuclear reactors in terrestrial and space applications (Ref. 5). In 

both of these cases, the primary need was for a bearing system

capable of operating at high speeds and temperatures for extended periods without maintenance and with no possibility of 

any contamination of the flow stream with oil lubricants.

With significant financial support from the ONR and technical oversight from NASA, Gross and his colleagues demonstrated these recording tape inspired foil gas bearings were 

viable for high-speed rotor support. Among the very first 

applications was the 15 kW Brayton Rotating Unit (BRU) 

shown in Figure 5.
NASA/TM

—2010-2167623

This compressor-turbine machine had a shaft mounted electrical generator and was intended to demonstrate that long life, 

maintenance free power generation could be achieved using 

any heat source, be it nuclear or solar, that could supply 

sufficient hot gas to the turbine inlet. The BRU was originally 

designed to operate on rigid gas bearings but these proved too 

susceptible to particulate contamination, centrifugal and 

thermal shaft-bearing distortions and they lacked adequate 

damping to prevent excessive vibrations and rotordynamic 

instabilities.

The application of the foil bearing to the BRU overcame all 

of its rotor support related technological problems and resulted 

in a milestone game-changing demonstration. Following this 

accomplishment, every CBC power conversion system NASA 

has developed since has relied solely on foil bearings. Detailed 

reports were written and widely disseminated that described 

the design, modeling, manufacturing and testing of the foil gas 

bearings used to support the NASA turbine. Industrial recipients of the information then led foil bearing development for 

ACM’s, turboexpanders, and APU’s (Ref. 5).

Garret-AiResearch deployed foil bearings in commercial 

aircraft cabin pressurization turbomachines in the late 1960s.

These very first fully commercialized Oil-Free machines 

revolutionized environmental control systems (ECS) for 

aircraft (Ref. 6). The foil gas bearings enabled completely 

maintenance free, Oil-Free cabin pressurization and air conditioning to modern aircraft without the possibility of oil fumes 

infiltrating the cabin, a problem common to jets in the 1950s 

and 60s. For several years, Garret-AiResearch enjoyed a 

growing market share owing to their unique and far superior 

technology. In the mid 1970s, Mechanical Technologies 

Incorporated (MTI) and Hamilton Standard developed a 

competing bump style foil bearing which was first deployed 

into ACM’s for fighter aircraft then fully commercialized in 

DC-10 and other aircraft of the day.

In the 1970s, NASA heavily leveraged Department of 

Energy (DOE) funding to demonstrate, in concert with industry, a series of automotive gas turbines utilizing foil bearings.

These engines placed unique demands on the foil bearings in 

that they had to run hotter and faster than the state-of-the-art 

that existed at the time. Hardware contracts were awarded to 

multiple companies and a vigorous in-house test program 

resulted in new high temperature solid lubricant coatings, 

improved bearing designs, bearing test rig facilities and an 

experience base that enabled closely controlled, value added 

contract oversight (Ref. 7). Though the automotive turbines 

did not enter production, they proved that small gas turbines 

could utilize foil bearings and that suitable high temperature 

materials and solid lubricant coatings could be engineered to 

survive the engine environment. Figure 6 shows an example of 

one of these test engines. Numerous reports detailing the 

foundational technologies were published and industrial 

demonstration programs, many funded by the DoD, followed.

Notable examples include Oil-Free auxiliary power units 

(APU)’s for tanks and small aircraft (Ref. 8). 
NASA/TM—2010-216762 4

Interestingly, technologists working on these demonstration 

projects at Honeywell (Garret-AiResearch) in the 1980s took 

early retirements and started a new business to commercialize 

small efficient, foil bearing supported turbomachinery. This 

company, NoMac Industries, was bought by investors and 

eventually became Capstone Turbines, now the world’s 

foremost manufacturer of Oil-Free microturbines (Ref. 9). Foil 

bearings were selected for their long life and low maintenance 

characteristics. In the early 1990s, Capstone relied on the 

information provided in NASA reports on high temperature 

foil bearing coatings and consultations with NASA researchers 

to develop their first successful engines and continues to 

collaborate on advanced engine technology (Ref. 10). Capstone has evolved their product line that includes power 

generation turbines from 30 to over 200 kW and has sold over 

4000 units worldwide. 

The need for robust and reliable low temperature turbomachinery, such as reusable cryogenic rocket engine turbopumps, 

led to NASA funded study, design and demonstration efforts 

for foil bearings and externally pressurized hydrostatic bearings. Test rigs were built and bearings were tested in liquid 

nitrogen, water and liquid hydrogen. The results were widely 

reported and the basic technologies formed the basis of numerous cryogenic turboexpanders and turbocompressors now 

in commercial use (Ref. 11).

Through these development and demonstration projects, the 

basic Oil-Free technology matured. Foil bearing performance 

improved, test capabilities and methods were established and 

suitable bearing materials and tribological coatings were 

proven. One aspect that had been largely lacking was a formalized, widely disseminated development method for designing 

and building a new Oil-Free machine. The Oil-Free turbocharger project served to fill this void.

The Oil-Free turbocharger demonstration project was triggered by the confluence of four factors: 1) the doubling of foil 

bearing load capacity via better structural design; 2) the 

emergence of high temperature solid lubricant coatings tailored for foil bearings; 3) the availability of foil bearing test 

rigs to verify bearing performance; and 4) the establishing of a 

four step hardware development process for risk mitigation. 

With these four factors as a backdrop, work began in earnest in 1995 with NASA’s Oil-Free turbomachinery team 

leading an industry effort that combined the foil bearings of 

Mechanical Technology Incorporated (later the effort was 

transferred to Mohawk Innovative Technologies), the turbocharger technology of Schwitzer Corporation (now BorgWarner Automotive), and the diesel technology of Caterpillar 

Corporation. Under this project, long-life foil bearings were 

demonstrated at temperatures over 650 °C, and to speeds of 

120,000 rpm. Further, a straightforward four-step development 

process was proven to carry new turbomachines supported on 

foil bearings from concept to system level demonstration with 

well-managed risks. The Oil-Free turbocharger, shown in 

Figure 8, was demonstrated in early 1999 and well publicized 

through magazine articles, peer reviewed journal papers and 

conference proceedings (Ref. 12).

The pathfinder turbocharger spurred industrial development 

programs in the US and abroad. To help meet these program 

needs, reports have been written that document foil bearing 

design and manufacturing techniques (Refs. 13 and 14). The 

patent literature captures the current state-of-the-art well by 

detailing the first known production patent for an Oil-Free 

turbocharger (Ref. 15). Further turbocharger development in 

Korea has quantified that Oil-Free turbochargers exhibit a 

significant reduction in friction losses than conventional 

machines, up to 10 percent overall efficiency improvements 

(Ref. 16). 

The development of turbochargers signifies that many important fundamental milestones for Oil-Free turbomachinery 

have been realized. Foil bearing manufacturing methods, and 

solid lubricant coatings with long-life at low and high temperatures have been proven. Though still maturing, commercial 
NASA/TM—2010-216762 5

rotordynamic modeling tools and software packages exist that 

reasonably predict critical speeds, bearing properties and shaft 

dynamic behavior for foil bearing supported systems (Refs. 17

and 18). Lastly, a formalized, four-step method to implement a 

new rotor system has been verified as evidenced by the 

success of the Oil-Free turbocharger demonstration. Using this 

new level of understanding, commercial manufacturers have 

greatly extended to breadth and scope of machines operating 

Oil-Free and a number of intriguing new hybrid technologies 

have been brought forth.

In Asia, several manufacturers produce Oil-Free compressors and blower product lines. Industry markets machines

ranging in size from 50 to 500 hp that compete very well in

the commercial air-handling marketplace. These machines are 

electrically driven, require no maintenance and can be installed at the point where the air is needed, thus eliminating 

the need for elaborate and expensive air piping systems 

prevalent in older factories. Such direct drive machines are far 

more energy efficient than traditional compressors and blowers and eliminate the need for a speed increasing gearbox and 

the potential for oil contamination of products and factories 

(Ref. 19). Like ACM’s, such machines utilize conventional 

foil bearings (35 to 75 mm diameter) and polymer based foil 

coatings to support rotors that weigh up to about 1000 N. For 

larger rotors, either larger bearings or a more hybridized 

approach is required. 

As rotors become heavier and bearings become larger, several characteristics of foil bearings present challenges. Among 

these are high start torque requirements, limited damping and 

limited low speed load capacity (Ref. 20). Recent work with 

large (76 mm diameter) journal bearings operating under high 

static loads (39 kPa) has shown that durable low friction 

coatings and careful minimization of spring preload forces can 

allow bearings to survive even low shaft acceleration during 

prolonged start-up periods. Notwithstanding, long-term 

applications of large foil bearings for heavy rotors could 

benefit from hydrostatic or electromagnetic load sharing, 

especially during low speed operation.

An early demonstration of such a hybrid approach included 

the side-by side demonstration of a large (100 mm diameter) 

foil-magnetic bearing (Ref. 21). Such concepts have been 

patented (Refs. 22 and 23) and recent demonstrations indicate 

that this technology marriage is more capable than either 

bearing technology alone. Hybridization leads to a “smart” 

bearing in which the rotor static weight loads can be relieved 

and active damping can be added via electromagnetics 

(Ref. 24). Figure 9 depicts the hybrid ‘smart” bearing approach.

While externally pressurized hydrodynamic foil and more 

conventional rigid surface gas bearings are an old concept 

(Ref. 25), recent demonstrations show that heavy rotors using 

large foil bearings (100 mm diameter) can be augmented with 

integral, pressurized air supply (Ref. 26). In this work, as 

expected, the augmentation air enabled essentially friction free 

start-up and elimination of foil surface wear. For this particular hybrid bearing which was not optimized for stability, at 

higher speeds, the hydrostatic bearing component needed to be 

curtailed to prevent pneumatic hammering and whirl instabilities. Nonetheless, these examples clearly show a path to the 

Oil-Free support of heavy rotors but highlight the more 

fundamental question concerning the identification of physical 

limits to foil gas bearing size, namely scalability. The following section outlines the dominant physical principles that 

appear to influence the scalability of passive foil gas bearings.

That is, bearings that do not incorporate active clearance and 

thermal management controls, electromagnetic or hydrostatic 

augmentation or other “smart” features.
NASA/TM—2010-216762 6

Bearing Scalability

Foil bearings rely upon a thin (typically less than 10 μm) 

gas film to prevent rubbing contact between the foils and their 

mating surface (the shaft or thrust runner) (Ref. 7). This film 

thickness is determined by the operating speed (surface 

velocity), gas properties (viscosity) and load.

h = f(DN, W, μ)

In general, the functional relationship mirrors the Stribeck or 

Hersey number 

h = f(μDN/W)

in which increases in speed or viscosity lead to increased film 

thickness and increasing load results in decreasing film 

thickness. Practical operating film thicknesses, however, are 

bounded by other factors.

A comprehensive experimental effort conducted in the 

1970s led to measurements of the fluid film thickness for a 

Generation I bump foil bearing operating at varying loads, 

speeds, aspect ratios and preload levels (Ref. 7). In this work, 

capacitance type probes were embedded into the rotating shaft 

operating against a foil bearing enabling the direct measurement of film thickness. The results showed that the minimum 

film thickness (where useful pressure is generated in a bearing) varied only between 5 and 25 μm depending upon load.

Increased speeds led to increased minimum film thickness as 

expected but also had an ancillary effect. Higher speed operation resulted in a larger minimum sustainable film thickness 

than that observed for lower speeds as shown in the following 

figure. At high speeds and loads excessive power loss led to 

thermally limited bearing load capacity.

This result, which has been corroborated through subsequent modeling and experimental work, shows that the film 

thickness is constrained in both the thick and thin film regimes. High shear rates, increased viscous losses and highly 

localized heating characterize thin films. Since typical foil 

bearing materials like nickel based superalloys have high 

thermal expansion coefficients and low thermal conductivities, 

localized heating leads to foil distortion, rupture of the gas 

film and bearing failure. Extensive research has shown that 

poor thermal management is a leading cause of foil bearing 

failure particularly at high speeds and thin films (Refs. 27 and 

28). In this context, one can see that the practical minimum 

sustainable film thickness actually increases at very high 

speeds due to thermal effects thus limiting the practical and 

achievable hydrodynamic pressure unless special care or more 

active cooling systems are employed. The behavior at low 

loads is equally interesting in that films thicker than about 

25 μm cannot be achieved. The reasons for this are many 

faceted. Due to the low gas viscosity, the hydrodynamic effect 

results in useful pressure rise only for thin films. Further, 

should the film thickness rise appreciably above 25 μm, 

pressure loss through side leakage from the bearing effectively 

bleeds the high pressure region, again, unless more actively 

controlled bearing geometry designs are employed. In light of 

these constraints, it is unsurprising that engineering film 

thicknesses are bounded between 5 and 25 μm.

It is important to remind the reader that unlike rigid bearings, foil bearings have no true clearance. At rest, the foil 

surface is spring preloaded against the journal. Above the liftoff speed, the hydrodynamic pressure deflects the foil and its 

elastic supporting structure away from the journal surface 

resulting in a hydrodynamic film thickness one would normally compare to a “clearance” for a rigid bearing thus establishing an eccentricity. Because of these subtle differences 

between foil bearings and conventional gas bearings the 

authors find that considering bearing behavior in terms of 

lubricating film thickness to be a rational approach to understanding foil bearings.

The magnitude of the film thickness has an important impact on bearing scalability. Foil bearings must operate against 

a moving shaft or runner surface, and thus the film thickness 

and the surface velocity dictate the useful pressure and load 

capacity. Since the film thickness is limited, only higher 

speeds provide a potential path to higher pressures and the 

possibility of supporting heavy rotors. When using conventional rigid, fixed clearance bearings at very high speeds, 

centrifugal shaft growth can overwhelm the design clearance, 

but for compliant surface foil bearings, shaft growth is accommodated by the elasticity of the support structure. Nonetheless, maximum surface velocity in a bearing is limited by 

practical engineering concerns.

Bearing surface velocity is constrained by structural material specific strength. As a shaft (or thrust bearing runner) 

rotates, centripetal forces build proportional to the product of 

the radius and the square of the rotational velocity (rω2

). These 

forces result in stresses and strains that act upon the shaft 

causing geometrical growth and distortions. Further, at sufficiently high speeds the stresses can exceed allowable materials 

limits and cause permanent deformation and possibly bursting. 

While careful material selection and structural design can 

minimize these effects, practical purposes using conventional 

engineering materials, accepted practice limits are well 

known.

In turbomachinery, a frequently understood practical limit 

for uncooled high temperature rotating structures is the “maximum exit rim speed” for axial flow compressors, taken to be 

approximately 450 m/s (Ref. 29). For bearing technology, an 

analogous limit, largely driven by the same centripetal loading 

phenomena is the DN limit where D is the shaft diameter in 

mm and N is the rpm. For rolling element bearings the maximum DN while retaining adequate fatigue life is about 

3 million using highly specialized bearings, lubricants and 

cooling methods. For oil lubricated hydrodynamic bearings 

the limit is not quite as well established as unacceptable drag 

losses typically preclude DN values above 2 million 

(100 m/s). However, since such bearings rely upon a rotating 

shaft that has a surface velocity limit of 450 m/s, one can 

translate this value directly into an equivalent DN of 
NASA/TM—2010-216762 7

8.6 million. However, such a simplistic calculation would 

place a foil bearing in the heart of a machine’s fluid flow path 

(turbomachinery) potentially blocking mass flow and limiting 

machine power output. Since turbomachines, especially those 

employing axial flow components, are sized based upon a 

minimum flow path area structural elements like bearings and 

bearing struts must fit within the aforementioned rim diameter.

With this in mind, a more realistic bearing DN (based upon 

the bearing diameter) is half the rim exit speed of 225 m/s or a 

DN of 4.3 million. These values (3 and 4.3 MDN) are reasonably consistent and give rise to the rationale that practical foil 

bearing speeds are limited to below this range. 

Scalability Example

Foil bearing load capacity considerations (based upon maximum speed and minimum sustainable film thickness considerations) allow the estimation of other bearing properties that 

help set reasonable scalability limits for Oil-Free technologies.

For instance, using the widely accepted “Rule-of-Thumb” 

(ROT) for foil bearing load capacity, the maximum load that 

can be supported by a journal foil bearing is (Ref. 3):

WLC=D(LD)DKrpm

A few specific examples follow to illustrate the current understanding of bearing scalability effects.

A 100 mm diameter (D=1, L/D=1) bearing operating at 

20,000 rpm (2 MDN) would be expected to carry about 1, lb

(~6 kN) maximum load. Bearings of this size have been 

produced and tested. Experimental data from the literature 

(Ref. 21) corroborates the load capacity ROT estimates for 

such bearings and is considered within current state-of-the-art.

Extending the ROT approach, a bearing twice the diameter 

and twice the length (200 mm) would support eight times the 

load (10000 lb, 4800 kN) and be operating at approximately 

the previously established DN speed limit (4 million). To 

assess if such a bearing is reasonable, an estimate can be made 

of the machine size and rotor weight that could be supported 

with two (fore and aft) such bearings.

Experiments with bearings ranging from 35 to 75 mm in

diameter, have shown that deadweight static shaft loads must 

be less than 55 kPa (8 psi) to permit manageable start-up 

torque and ensure long cyclic life (Refs. 20 and 30). Typical 

static loads for Oil-Free machines are much lower 14 kPa 

(2 psi). Based upon these static load (14 to 55 kPa) considerations alone, the 100 mm diameter journal bearings used in the 

above example (two per rotor) can support rotors from 300 to 

1200 N (64 to 256 lb). Rotors of this size are typical for air 

compressors in the 75 to 400 kW (100 to 500 hp) range and 

small business class turbofan engines (1,000 lb, 5000 N thrust 

levels).

200 mm diameter bearings, slightly beyond the current 

state-of-the-art, operating with a modest static load (14 kPa) 

could support a rotor weighing from 2400 to 9800 N (496 to 

1988 lb). Rotors of this size class and rotational speed are 

typical for large gas compressors and low-pressure spools for 

medium thrust class (20,000 lb, 100 kN) turbine engines.

The purpose of these bearing sizing calculations is to show 

that based upon load capacity alone, conventional foil bearings 

appear capable of supporting large turbomachinery. These 

load capability estimates, however, are quite simplified and 

ignore other major constraints such as bearing stiffness and 

damping capabilities. The estimates also ignore other issues 

such as manufacturability of physically large and flexible 

structures. Such considerations cannot realistically be assessed 

without validation experiments. Another major concern facing 

scalability of foil bearings is that while load capacity grows 

with the square of the shaft diameter, bearing requirements 

such as shaft dynamic loads often scale with rotating mass (a 

volume characteristic) that grows with the diameter cubed. In 

other words, load capacity is not likely the limiting factor.

Advanced Technology Needs

Compared to oil lubricated bearings, foil gas bearings are 

characterized by relatively low levels of stiffness and damping, typically an order of magnitude lower. This behavior is 

largely due to the low gas viscosity and modest hydrodynamic 

film pressures that result from the operating conditions. Put 

another way, foil gas bearings are an appropriate rotor support 

technology for high-speed, lightly loaded rotors that benefit 

from the elimination of traditional oil systems and their 

complexities. Thus, a rotor design may be adequately supported with respect to load capacity but be unworkable due to 

inadequate bearing dynamic properties. To meet the challenges of heavy aerospace rotors with significant destabilizing 

forces and stringent orbit control requirements, conventional 

foil gas bearing technology alone is likely to be insufficient.

Hybrid approaches in which foil bearing technology is combined with augmentation technology will likely be needed in 

larger Oil-Free rotor support systems.

Hybridization approaches utilizing low speed load sharing 

via hydrostatic gas pressurization is an attractive option. Such 

technology is readily available, scalable to large bearings and 

heavy rotors and adds little complexity. Such pressurization, 

however, must be phased out at higher speeds to avoid pneumatically driven instabilities. Further, such an approach 

requires a compressed air supply and offers little promise for 

improved stiffness and damping capability as well as active 

control.

Combining active electromagnetic technology with foil gas 

bearings offers many advantages. Such a “smart” bearing can 

unload the shaft weight during start-up and provide damping 

capabilities when rotordynamically required. Further, using 

the electromagnetic aspect of the hybrid bearing as the secondary support may reduce these device’s size and electrical 

consumption to a more practicable level. Properly implemented, the bearing could also function as a shaft starter motor 

and integral starter generator. The success of purely magnetic 
NASA/TM—2010-216762 8

bearing supported systems in land based compression machines demonstrate that hybrid smart bearings have real 

potential for more advanced applications. Future systems may 

well be capable of “active in-situ” diagnostics (tracking 

unbalance, shaft crack detection, seal degradation, etc.).

Summary

NASA’s intrinsic need for technologies capable of supporting high speed rotors for power generation, propulsion and 

fluid handling for aeronautics and space missions has been a 

driving force in the development and proliferation of Oil-Free 

technologies. Technological commercialization has been 

achieved in small high-speed, lightly loaded applications like 

aircraft ACM’s, cryogenic turboexpanders, blowers and 

compressors. Demonstration in APU’s, turbochargers and full 

commercialization in microturbines has occurred. For these 

applications, bearing sizes of 100 mm diameter or less have 

been sufficient and the use of hybrid active controlled bearings 

unnecessary.

First principles based scaling suggests that structural limitations preclude bearing size (DN) growth much beyond 4 MDN 

unless hybridization of foil air bearings and active electromagnetic devices is realized. These technologies must also be 

applied to thrust bearings and proven via bearing and system 

demonstrations. With the adoption of such approaches, large 

heavy and high power rotating systems may be developed.

References

1. DellaCorte, C. and Valco, M.J., “Oil-Free Turbomachinery 

Technology for Regional Jet, Rotorcraft and Supersonic 

Business Jet Propulsion Engines,” Proceedings of the 2003 

ISABE Conference, Cleveland, OH, AIAA–2003–1182.

2. Lucero, J.M., and DellaCorte, C., “Oil-Free Rotor Support 

Technologies for Long-life, Closed Cycle Brayton Turbines,”

Proceedings of the AIAA 2nd International Energy Conversion and Engineering Conference (IECEC), pp. 1583–1591, 

Providence, RI, Aug. 2004.

3. DellaCorte, Christopher and Valco, Mark J., “Load Capacity 

Estimation of Foil Air Journal Bearings for Oil-Free Turbomachinery Applications,” Tribology Transactions, Vol. 43, 

No. 4, pp. 795–801, Oct. 2000.

4. Gross, W.A., “Film Lubrication-V. Infinitely Long Incompressible Lubricating Films of Various Shapes,” IBM Research Report RJ 117–5, pp. 69–70, Oct. 1958.

5. Licht, L., and Branger, M., “Design, Fabrication, and Performance of Foil Journal Bearings for the Brayton Rotating 

Unit,” NASA Contractor Report CR–2243, Jul. 1973.

6. Klaass, R.F., and DellaCorte, C., “The Quest for Oil-Free 

Gas Turbine Engines,” Proceedings of the Power Systems 

Conference, New Orleans, LA, SAE Paper 2006–01–3055, 

Nov. 2006.

7. Ruscitto, D., McCormick, J., and Gray, S., “Hydrodynamic 

Air Lubricated Compliant Surface Bearing for an Automotive Gas Turbine Engine: I-Journal Bearing Performance,” 

NASA CR–135368, 1978.

8. Suriano, F.J., 1981, “Gas Foil Bearing Development Program,” U.S. Air Force Report Number AFWAL–TR–81–

2095.

9. Kolanowski, B.F., Guide to Microturbines, Marcel Dekker, 

Ch. 1, pp. 1–15, 2004.

10. Lubell, D., DellaCorte, C., and Stanford, M.K., ”Test Evolution and Oil-Free Engine Experience of a High Temperature 

Foil Air Bearing Coating,” Proceedings of GT2006: ASME 

Turbo Expo 2006, May 8–11, 2006, Barcelona, Spain, 

GT2006–90572.

11. Walton, J.F., and Heshmat, H., “Compliant Foil Bearings for 

Use in Cryogenic Turbopumps,” NASA CP–3282, Vol. 1, 

Sep. 1994, pp. 372–381.

12. Heshmat, Hooshang; Walton, II, James F.; DellaCorte, 

Christopher; Valco, Mark, “Oil-Free Turbocharger Demonstration Paves Way to Gas Turbine Engine Applications,” 

presented at the International Gas Turbine & Aeroengine 

Congress & Exhibition sponsored by the American Society 

of Mechanical Engineers, Munich, Germany, May 8–11, 

2000, 2000–GT–620.

13. DellaCorte, C., Radil, K.C., Bruckner, and Howard, S.A.,

“Design, Fabrication, and Performance of Open Source Generation I and II Compliant Hydrodynamic Gas Foil Bearings,” NASA/TM—2007-214691, ARL–TR–4102, and STLE 

Tribology Transactions, Vol. 51, pp. 254–264, 2008. 

14. Dykas, B., Bruckner, R.J., DellaCorte, C., Edmonds, B.J., 

and Prahl, J.M., “Design, Fabrication and Performance of 

Foil Gas Thrust Bearings for Microturbomachinery Applications,” NASA/TM—2008-215062, ASME Paper GT2008–

50377, and ASME Journal of Engineering for Gas Turbines 

and Power, Vol. 131, pp. 012301–1 to 012301–8, Jan. 2009.

15. LaRue, G.D., Kang, S.G. and Wick, W. (2006), “Turbocharger with Hydrodynamic Foil Bearings,” U.S. Patent 

1,108,488.

16. Lee, Y.B., Park, D.J., and Kim, C.H.; “Stability and Efficiency of oil-Free Turbocharger with Foil Bearings for SUV, 

SAE paper 08SFI–0083, 2008.

17. Howard, S.A., Bruckner, R.J., DellaCorte, C., and Radil, 

K.C.: ”Preliminary Analysis for an Optimized Oil-Free Rotorcraft Engine Concept,” NASA/TM—2008-215064, ARL–

TR–4398, Proceedings of the American Helicopter Society,

AHS Forum 63, Virginia Beach, VA, May 2007

18. Howard, S.A.: ”Rotordynamics and Design Methods of an 

Oil-Free Turbocharger,” NASA/CR—1999-208689, Jan.

1999. 

19. Kim, K.S.; “An Experimental Study on the Performance of 

Air Foil Thrust Bearing for Application to Turbomachinery,”

Ph.D. dissertation, Korean Advanced Institute of Science and 

Technology, Daejon, Korea, Dec. 2006.

20. Radil, K.C., and DellaCorte, C.: “Foil Bearing Starting 

Considerations and Requirements for Rotorcraft Engine Applications,” Army Research Report ARL–TR–4873, Aug.

2009.

21. Swanson, E.E., Heshmat, H., and Walton, J.F. II: “Performance of a Foil-Magnetic Hybrid Bearing,” Journal of Engineering Gas Turbines Power, Vol. 124, Issue 2, pp. 375–383, 

Apr. 2002.

22. Nadjafi, R.H.: “Hybrid Foil/Magnetic Bearing,” U.S. Patent 

6,135,640, Oct. 2000.

23. Heshmat, H., Chen, M., and Walton, J.F., III: “Hybrid FoilMagnetic Bearing,” U.S. Patent 6,353,273, Mar. 2002.
NASA/TM—2010-216762 9

24. Lee, Y.B., Lee S.H., and Kim, S.J.: “Vibration Control and 

Low Power Consumption of the Combined Smart Bearings,”

Proceedings of the 4th World Tribology Congress 2009, Kyoto Japan, Sep. 6–11, 2009, paper F–223. 

25. Gross, W.A., Gas Film Lubrication, John Wiley and Sons, 

New York, NY, pp. 138–114, 1962.

26. Daejong Kim, Donghyun Lee, “Design of three-pads hybrid 

air foil bearing and experimental investigation on static performance,” in print, ASME Journal of Engineering for Gas 

Turbines and Power, 2009.

27. Radil, K.C., and Zeszotek, M., “An Experimental Investigation Into the Temperature Profile of a Compliant Foil Air 

bearing,” NASA/TM—2004-213100 and ARL–TR–3200, 

May 2004.

28. Dykas, B., and Howard, S.A., “Journal Design Considerations for Turbomachine Shafts Supported on Foil Air Bearings,” STLE Tribology Transactions, Vol. 47, pp. 508–516, 

2004.

29. Mattingly, J.D., “Elements of gas Turbine Propulsion,” Ch. 

9, and p. 663, McGraw-Hill, New York, NY, 1996.

30. DellaCorte, C., Valco, M.J., Radil, K.C., Heshmat, H., and 

Lukaszewicz, V.L., “Performance and Durability of High 

Temperature Foil Air Bearings for Oil-Free Turbomachinery,” NASA/TM—1999-209187, Nov. 1999, Tribology 

Transactions, Vol. 43, No. 4, pp. 774–780, Oct. 2000.
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Remaining Technical Challenges and Future Plans for Oil-Free Turbomachinery 

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DellaCorte, Christopher; Bruckner, Robert, J. 

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14. ABSTRACT

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. 

15. SUBJECT TERMS

Turbomachinery; Bearings; Gas turbines; Propulsion 

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