Here are some examples of how foil bearings might fit candidate applications. All loads are projected area loads (length x width for radial).
| Item | Lower Risk | More Challenging |
|---|---|---|
| Free-free undamped critical speeds | All well above maximum operating speed | One or more below operating speed |
| Shaft surface speed over normal operating speed range at radial bearing location for “reasonable” shaft configuration | 500 to 740 ft/s (150 to 225 m/s) | Less than 100 ft/s (30 m/s) More than 1000 ft/s (305 m/s) |
| Startup load for “reasonable” sized radial and thrust bearings | Less than 5 psi (34 kPa) | More than 5 psi (34 kPa) |
| Operating static + dynamic load for “reasonable” sized radial bearing (including off design) * | Less than 20 psi (140 kPa) | More than 50 psi (340 kPa) |
| Operating static + dynamic load for “reasonable” sized thrust bearing (including off design) * | Less than 15 psi (100 kPa) | More than 35 psi (240 kPa) |
| Available starting torque | High | Low |
| Damping required for stability and reasonable response* | Low | High |
| Bearing operating temperature | Less than 400 F (200 C) | More than 700 F(370 C) |
| Bearing compartment pressure | Near atmospheric | Sub atmospheric or very high pressure |
| Operating fluid | Reasonably clean, dry air | Very dirty air Occasional liquid present |
