Limitations of Foil Bearings
Foil bearings are hydrodynamic bearings, just like the oil lubricated journal bearings used in most large machinery such as steam turbines, generators or large industrial compressors.
At normal operating speeds, the shaft does not contact the bearing surface. Shaft rotation "drags" fluid (air) into a converging region creating an area of high pressure.
This high-pressure fluid (air) between the rotating shaft and the stationary bearing supports the shaft as it rotates.
As convergence causes the clearance to decrease, the pressure increases and lifts the shaft.
The load that can be supported depends on the relative surface speed, the area of the converging region, the shape of the clearance space between the rotating surface and top foil, the support structure stiffness and the viscosity of the lubricant, which is generally air for foil bearings.
The unique feature of a foil bearing is the compliant operating surface. The surface changes shape depending on load, speed, thermal deformations, etc., to allow the bearing to accommodate levels of misalignment and thermal expansion that would destroy a rigid surface air bearing.
What Foil Bearings Are Not
Foil bearings are not a drop-in replacement for rolling element (ball) bearings. The design constraints and integration requirements are different, it is best if a machine is designed to take advantage of foil bearing capabilities from the start.
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.