Foil Bearings - Technology Whose Time Has Come - Part 2

By Peter Chapman

foil bearing-33

This second part of a two-part post reviews foil bearing development history, the technical sweet spot for use, and why this is indeed a technology whose time has come. Part 1 provided an excellent introduction to what foil bearings are, and why they are an excellent choice in certain situations.

The History of Foil Bearings

Development of foil bearings began in the 1950’s by Garrett AiResearch (now Allied Signal) for use in high-speed air cycle machines (ACM), which are used on aircraft to manage the cooling, heating, and pressurization of the cabin. During the 1960’s, Mechanical Technology, Inc. (MTI, Latham, NY) began developing a number of gas-lubricated bearing designs and by the late 1960’s introduced its Hydresil™ foil bearing design. MTI investigated using foil bearings in a number of different technologies, including Chrysler’s Automotive Gas Turbine Engine. Since then, foil bearing designs have been adapted to operate in machines ranging from compressors and blowers, gas turbines and turbochargers, to high-speed motors and generators. Most recently there has been significant interest in their use in blowers for automotive fuel cells, refrigerant-based systems, as well as power cycle machinery using supercritical carbon dioxide as the working fluid. In these applications, process fluid purity is of utmost importance.

In 2009, MSI acquired MTI’s foil bearing technology, and continues to build on that legacy today.

Technical Sweet Spot

Unfortunately, as good as foil bearings sound, they are not suitable for all applications. Certainly all applications could benefit from a simple, compact, clean, maintenance-free, highly efficient technology like foil bearings. However, they are not necessarily suitable for all applications.


It is easy to postulate that supporting something on a film of air would be more difficult than on a film of oil, or on an array of rolling metal balls. In fact, foil bearings offer much less load-carrying capacity than oil-lubricated or rolling element bearings. Therefore, they are suitable for applications where the loads are light to moderate, on the order of 15-30psi/100-100kPa. Similarly, a much higher surface speed is required to develop the hydrodynamic film with a gas versus a liquid. Therefore, high rotational speeds are desired. As such, machines that are lightly loaded running at high speeds are well suited for foil bearings.

The chart below shows the wide range of applications that foil bearings have been designed for over the years, plotting size (in this case journal diameter) vs. shaft speed. The “sweet spot” for radial foil bearings tends to be in the 1 to 2 inch/2.5 to 5 cm diameter size and 40,000 to 120,000 rpm speed range. Lower speeds with larger diameters are feasible, but become more challenging due to increased rotor weight and reduced surface speed. The practical lower speed limit of foil bearings is generally considered to be in the 5,000 to 10,000 rpm range.

foil bearing experience chart

Market Dynamics – A Technology to Watch

Foil bearings have often been referred to as an enabling technology; that is, they allow for the creation of designs that otherwise would not be possible without them. The efficiencies of turbines, compressors, and electric motors can often benefit by running at higher speeds. As advancements in these technologies permit them to operate at higher speeds, foil bearings may be the supporting technology that will allow it to happen. Higher operating speeds often result in smaller machine size to achieve a given power level. There is a push in industry toward high speed, direct-drive systems, allowing for higher power density through more compact designs and higher overall systems efficiencies. Reduced machine maintenance also promotes more favorable life-cycle costs. Though they have been around for nearly 70 years now, foil bearings may soon be common in many everyday machines and devices. If you’d like to speak with an experienced foil bearing design engineer about your system requirements and if this technology makes sense for your application, contact us.


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