Robert J. Bruckner 

Glenn Research Center, Cleveland, Ohio

Performance of Simple Gas 

Foil Thrust Bearings in Air

NASA/TM—2012-217262

February 2012
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Robert J. Bruckner 

Glenn Research Center, Cleveland, Ohio

Performance of Simple Gas 

Foil Thrust Bearings in Air

NASA/TM—2012-217262

February 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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NASA/TM—2012-217262 1

Performance of Simple Gas Foil Thrust Bearings in Air

Robert J. Bruckner 

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Foil bearings are self-acting hydrodynamics devices used to support high speed rotating machinery.

The advantages that they offer to process fluid lubricated machines include: high rotational speed 

capability, no auxiliary lubrication system, non-contacting high speed operation, and improved damping 

as compared to rigid hydrodynamic bearings. NASA has had a sporadic research program in this 

technology for almost 6 decades. Advances in the technology and understanding of foil journal bearings 

have enabled several new commercial products in recent years. These products include oil-free 

turbochargers for both heavy trucks and automobiles, high speed electric motors, microturbines for 

distributed power generation, and turbojet engines. However, the foil thrust bearing has not received a 

complimentary level of research and therefore has become the weak link of oil-free turbomachinery. In an 

effort to both provide machine designers with basic performance parameters and to elucidate the 

underlying physics of foil thrust bearings, NASA Glenn Research Center has completed an effort to 

experimentally measure the performance of simple gas foil thrust bearing in air. The database includes 

simple bump foil supported thrust bearings with full geometry and manufacturing techniques available to 

the user. Test conditions consist of air at ambient pressure and temperatures up to 500 °C and rotational 

speeds to 55,000 rpm. A complete set of axial load, frictional torque, and rotational speed is presented for 

two different compliant sub-structures and inter-pad gaps. Data obtained from commercially available foil 

thrust bearings both with and without active cooling is presented for comparison. A significant 

observation made possible by this data set is the speed—load capacity characteristic of foil thrust 

bearings. Whereas for the foil journal bearing the load capacity increases linearly with rotational speed, 

the foil thrust bearing operates in the hydrodynamic high speed limit. In this case, the load capacity is 

constant and in fact often decreases with speed if other factors such as thermal conditions and runner 

distortions are permitted to dominate the bearing performance.

1.0 Foil Thrust Bearing State of the Art

One of the earliest published works on the idea of a foil bearing, in journal bearing form and using oil 

as the lubricant, was published in 1953 by H. Blok of the Technical University at Delft in Holland 

(Ref. 1). In this work Blok describes an experimental apparatus in which a foil is wrapped around 180° of 

a journal and loaded via a system of weights and pulleys. The results of friction and load tests are 

presented with theoretical equations representing the accommodation of the foil membrane to the 

hydrodynamic pressure. Over the past several decades the use of foil bearings has enabled the 

development of many unique, high speed machines.

The benefits foil bearings offer to high speed turbomachinery can be significant. Among these 

benefits are reduced machine weight due to the elimination of the oil systems, removal of the traditional 

diameter—rotational speed limit imposed by rolling element bearings, and a synergistic use of working 

fluid as a lubricant enabling a contaminant free working fluid without the need for seals, filters, and 

ancillary subsystems. However in order to realize these benefits, foil bearing must be able to support the 

required machine load, provide the requisite rotordynamic stiffness and damping, and operate reliably and 

without burden to the machine. Excess burden to a turbomachine usually manifests itself in the 

detrimental use of working fluid as bleed flow to cool or pressurize specific components.
NASA/TM—2012-217262 2

Figure 1.—Drawing and photograph of a simple foil thrust bearing.

A typical foil thrust bearing is shown in Figure 1. The bearing consists of a solid backing plate with 

an annulus of discrete bearing pads or petals. Each of these pads is constructed from a smooth and 

continuous top foil that is fixed on the leading edge and free on the remaining three edges. The backside 

of the top foil is supported by a compliant foundation. The hydrodynamic pressure rise is initially created 

by the physical contraction of the film thickness between the top foil and the thrust runner. The compliant 

foundation can take any number of styles and forms. 

The foil thrust bearing operates in the same manner as all other hydrodynamic bearings. As viscous 

fluid is dragged through a converging channel the momentum given the fluid through the moving shaft is 

transformed into elevated pressures. Thus a significant load can be supported with a very small drag 

force. Theoretically the supported load to drag force ratio can reach 1000, however in practice this ratio is 

in the low hundreds. According to the governing Reynolds equation, all hydrodynamic lubrication follows 

certain behaviors such that supported load increases with absolute viscosity and decreases with the square 

of film thickness. Hydrodynamic behavior with respect to speed is variable. Figure 2 (Ref. 2) highlights 

some of the pertinent features. At low relative surface speeds the load support increases with speed and 

the bearing analysis can proceed in a simplified fashion with the assumptions of incompressible lubricant 

and zero side leakage. This proportional increase of load capacity with respect to shaft speed has given 

rise to the load capacity rule of thumb for foil journal bearings, DellaCorte (Ref. 3). However, as shaft 

speed continued to increase the load capacity reaches a limit or high-speed asymptote. This limit
NASA/TM—2012-217262 3

Figure 2.—Generalized behavior of hydrodynamic 

lubrication (Ref. 2). Figure 3.—Load capacity of foil thrust bearing cross 

plotted from data Heshmat (Ref. 4) showing high 

load capacity speed drop off.

Figure 4.—Load capacity versus shaft speed graph 

showing high speed drop off. Data from 

References 5 and 6.

is experienced in foil thrust bearings. As shaft speed is further increased a region of decreasing load 

capacity is reached that is attributed to high rates of viscous heat generation and surface distortions of the 

bearing and runner. Figures 3 and 4 demonstrate this phenomenon. In Figure 3, data from Heshmat 

(Ref. 4) is cross plotted to generate a load capacity versus speed chart. The data shows a drop in load 

capacity from 60,000 to 70,000 rpm. Figure 4, obtained by plotting data published in References 5 and 6, 

contains load capacity data from 25,000 to 55000 rpm. The uncooled thrust bearing shows a decrease in 

load capacity over this speed range, while cooling is shown to enable a modest increase in load capacity.

The asymptotic behavior is also shown analytically in Figure 4 as is calculated by the methods of 

Bruckner (Ref. 7).
NASA/TM—2012-217262 4

The analysis of foil thrust bearings has been hampered by the lack of a self-consistent data set that 

includes load, torque, and speed data along with a detailed description of the bearing design. Few such 

data sets exist in the open literature. It is the objective of this work to provide such a database so that the 

understanding and analysis of foil thrust bearings can be advanced by a broad spectrum of potential users 

of the technology.

2.0 Experimental Method 

NASA Glenn has developed two test rigs specifically designed to test the performance of single sided 

foil thrust bearings. The vertical thrust bearing rig is driven by a 5 kW electric motor and is capable of 

speeds up to 21,000 rpm. The vertical rig also has a high temperature capability where the bearing cavity 

temperatures can be controlled up to 800 °F. The horizontal thrust bearing rig is driven by an air turbine, 

has a practical speed limit of 60,000 rpm, and an ambient temperature limit. Both thrust bearing rigs 

utilize ambient pressure air as the lubricant and are not capable of high pressure environments. In addition 

to the two test rigs NASA has also developed a method to produce simple bump foil thrust bearings 

(Ref. 8). The design of these thrust bearings had the objective of creating research grade bearings. The 

intent of research grade bearings is to possess repeatable, albeit modest performances. In order to 

accomplish this objective the inter-pad spacing was increased, the inlet contraction was lengthened, and 

the compliant foundation was softened.

Two different bump foil designs have been tested to date. These two configurations are shown in 

Figure 5. All performance testing of these two bump foil configurations was performed without forced 

cooling. The tests of the first bump foil configuration are described in detail in Dickman (Ref. 9). The 

primary objectives of this test were threefold: to develop final manufacturing and test techniques, to 

quantify bearing-to-bearing variability, and to generate an open source performance database. A summary 

of the results is shown in Figure 6. Load and torque data is presented for shaft speeds of 14, 19, 21, and 

40 krpm. At modest loads the load torque characteristic is repeatable for the three bearings that were 

tested. However, the load capacity showed large variability. At 21 krpm the load capacity varied from 40 

to 60 lb. This variability in load capacity is typical of all foil bearings and is due to the hand-made nature 

of the current state of the technology.

Figure 5.—Rendering of the bumpfoil configurations used for the initial open source thrust bearing test program. 

Bump foil tested by Dickman on left. Bump foil used in the TEE experiments on right.
NASA/TM—2012-217262 5

Figure 6.—Simple foil thrust bearing data from Dickman (Ref. 8).

The second bump foil configuration was tested with two different top foils in order to investigate the 

inter-pad separation. The bearing designated TEE01 was comprised of the original top foils having a 15°

separation between leading and trailing edge. The bearing designated TEE02 used a top foil that extended 

to the next pad such that there was no trailing edge to leading edge separation. Data for this test is 

presented in Figure 7 for speeds of 19, 21, and 40 krpm. Data comparison between Figures 6 and 7 is not 

possible because two different bump foil configurations were used. In the later the stiffness distribution 

was concentrated near the inner radius of the bearing causing the bearings to underperform when 

compared to the Dickman data. Qualitative comparison indicates that the bearing with the larger top foil, 

TEE02, has higher torque at a given load and speed than that of the original configuration. Additionally, 

the extra pad area did not have a significant effect on the load capacity of the bearing at higher rotational 

speeds. Although at low speeds (14 and 19 krpm) the load capacity for TEE02 showed small increases 

with large increases in frictional torque.

An important and consistent observation was made from the commercial bearings tested by Dykas, 

the open source bearings tested by Dickman, and the open source TEE bearings were the asymptotic load 

capacity limit of 20 psi. This crude estimation of ultimate load capacity was obtained by calculating true 

loaded area from observations of witness marks following high speed load capacity tests. The total load 

divided by the true load area for uncooled thrust bearings appears to be consistent across different bump 

foil designs and tribological wear couples.
NASA/TM—2012-217262 6

Figure 7.—Simple foil thrust bearing data from the inter-pad separation effect.

3.0 Summary and Conclusions

Analysis of existing historic foil thrust bearing data shows that unlike the journal bearing counterpart, 

foil thrust bearing load capacity does not increase linearly with speed. For the thrust bearing the 

theoretical high speed asymptote is often reached and a high speed load capacity drop-off may be 

encountered. The oil-free turbomachinery designer must take this into account when designing new 

articles of high speed rotating machinery that utilize this technology.

A preliminary database has been generated that shows robust and repeatable performance of foil 

thrust bearings up to ~80 percent of their load capacity. However, variability in load capacity from 

seemingly identical bearings is on the order of ± 25 percent. Based on witness mark observations from 

high speed load capacity tests the asymptotic high speed limit of foil thrust bearings operating in air 

appears to be 20 psi when true pad area is used. The foil thrust bearing performance database can be used 

by analytic model developers to validate and verify foil bearing predictive codes.
NASA/TM—2012-217262 7

References 

1. Blok, H. and vanRossum, J., “The Foil Bearing – A New Departure in Hydrodynamic Lubrication,”

Lubrication Engineering, 1953. 

2. Hamrock, Bernard, J., Fundamentals of Fluid Film Lubrication, McGraw-Hill, Inc., New York, 1994.

3. DellaCorte, C., Valco, M., “Load Capacity Estimation of Foil air Journal Bearings for Oil-Free 

Turbomachinery Applications,” NASA/TM—2000-209782, 2000.

4. Heshmat, H., “Major Breakthrough in Load Capacity, Speed and Operating Temperature of Foil 

Thrust Bearings,” Proceedings of the WTC 2005 World Tribology Congress III, WTC2005-63712, 

Washington, DC, 2005.

5. Dykas, B., “Factors Influencing the Performance of Foil Gas Thrust Bearings for Oil-Free 

Turbomachinery Applications,” Ph.D Dissertation, Case Western Reserve University, May 2006.

6. Dykas, B., Prahl, J., DellaCort, C., and Bruckner, R., “Thermal Management Phenomena in Foil Gas 

Thrust Bearings,” Proceedings of GT2006, ASME Turbo Expo 2006: Power for Land, Sea, and Air, 

May 8–11, 2006, Barcelona, Spain, GT2006–91268.

7. Bruckner, R. J., “Simulation and Modeling of the Hydrodynamic, Thermal, and Structural Behavior 

of Foil Thrust Bearings”. PhD dissertation, Case Western Reserve University, Cleveland, OH, 2004.

8. Dykas, B., Bruckner, R., Dellacorte, C., Edmonds, B., Prahl, J., “Design, Fabrication and

Performance of Foil Gas Thrust Bearings for Microturbomachinery Applications,” Journal of 

Engineering for Gas Turbines and Power, Vol. 131/012301-1, January 2009.

9. Dickman, J., “An Investigation of Gas Foil Thrust Bearing Performance and its Influencing Factors,” 

Master’s Thesis, Case Western Reserve University, May 2010.
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14. ABSTRACT

Foil bearings are self-acting hydrodynamics devices used to support high speed rotating machinery. The advantages that they offer to process fluid lubricated machines include: 

high rotational speed capability, no auxiliary lubrication system, non-contacting high speed operation, and improved damping as compared to rigid hydrodynamic bearings. 

NASA has had a sporadic research program in this technology for almost 6 decades. Advances in the technology and understanding of foil journal bearings have enabled 

several new commercial products in recent years. These products include oil-free turbochargers for both heavy trucks and automobiles, high speed electric motors, 

microturbines for distributed power generation, and turbojet engines. However, the foil thrust bearing has not received a complimentary level of research and therefore has 

become the weak link of oil-free turbomachinery. In an effort to both provide machine designers with basic performance parameters and to elucidate the underlying physics of 

foil thrust bearings, NASA Glenn Research Center has completed an effort to experimentally measure the performance of simple gas foil thrust bearing in air. The database 

includes simple bump foil supported thrust bearings with full geometry and manufacturing techniques available to the user. Test conditions consist of air at ambient pressure 

and temperatures up to 500 °C and rotational speeds to 55,000 rpm. A complete set of axial load, frictional torque, and rotational speed is presented for two different compliant 

sub-structures and inter-pad gaps. Data obtained from commercially available foil thrust bearings both with and without active cooling is presented for comparison. A 

significant observation made possible by this data set is the speed-load capacity characteristic of foil thrust bearings. Whereas for the foil journal bearing the load capacity 

increases linearly with rotational speed, the foil thrust bearing operates in the hydrodynamic high speed limit. In this case, the load capacity is constant and in fact often 

decreases with speed if other factors such as thermal conditions and runner distortions are permitted to dominate the bearing performance.

15. SUBJECT TERMS

Foil bearings; Thrust bearings; Journal bearings; Hydrodynamics; Turbomachinery 

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