Foil Bearings vs Oil, Rolling Element, and Magnetic Bearings

How Foil Bearings Stack Up to Other Types of Bearings

Gas/Air Foil Bearing Oil Bearing (Sleeve, Tilting Pad) Rolling Element Bearing Active Magnetic Bearing
Maximum Operating Speed at Bearing Surface (Bore) for Radial Bearing Generally 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 Speed Yes, application dependent Yes, application dependent None None
Typical Continuous Load Capacity and Typical Projected Area Loads Low to moderate Potentially very high Moderate to high Low to moderate
Short term overload capability Varies, possibly 2 to 3 times static Substantial Good Limited, backup bearings generally required
Bearing Operating Temperature Range 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 Loss Radial very low
Thrust moderate
Can be significant Generally low to moderate Generally very low
Oil Free? No No Yes (although backup bearings could be grease lubricated)
Misalignment Capability Low to moderate Moderate, depends on construction Very low (highly loaded angular contact) to moderate (spherical roller) Moderate
Auxiliary Systems Pumps, coolers, filters Nothing for grease lubricated bearings, ranging to pumps, filters, etc. for oil-jet lube at high speeds and loads Control system electronics, auxiliary bearings, generally some amount of cooling air
Radial Envelope Requirement Length 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 external
Weight Generally heavy when pumps, filters, piping etc considered along with the actual bearings Relatively light Moderate including electronics
Stiffness Low Moderate to high High Depends on tuning, generally are tuned soft
Damping Low to moderate Generally high Very low Moderate to high depending on tuning and system dynamics
Shock Tolerance Good Very good Moderate Can be poor

What Makes Xdot Air and Process Gas Lubricated Foil Bearings a Cut Above the Rest

At Xdot Engineering and Analysis, we take our foil bearings all the way from development, through design to production. Historically, teams who wanted to create machines using foil bearings found themselves up against prohibitive cost, manufacturing, and design challenges. We listened to their concerns and developed our unique, high-performance design to address them.

But we don't stop there. When you choose Xdot for your Air and Process Gas Lubricated Foil Bearings, you get:

Stock sizing.
We are developing a set of stock "frame" sizes that allow us to efficiently customize a bearing to meet the specific needs of new and existing machines.

Custom sizing.
If our stock sizes don't fit your needs, we can produce a custom-sized bearing. Our hardware experience is backed-up with analytical capability so that we get it right the first time, not after a costly build-and-bust development.

Rotordynamic and integration help.
We work with your team to ensure that the overall system is properly designed to work well with foil bearings.

Help developing new applications.
We can assist you with everything from performing independent assessments, to lending subject expertise as a team member, to assembling a team and leading prototype development projects. If you require a more extensive test program, we can leverage our test-rig experience and infrastructure to help ensure a healthy balance of risk versus cost.

If you are planning a new machine and want to consider foil bearings, we encourage you to contact us early in the design process. We can help your team, or subcontract designer, make the most efficient choices and save you time and money down the line.

Get in touch with us to learn more.