Types
There are many different types of bearings.
| Type | Description | Friction | Stiffness†| Speed | Life | Notes |
|---|---|---|---|---|---|---|
| Plain bearing | Rubbing surfaces, usually with lubricant; some bearings use pumped lubrication and behave similarly to fluid bearings. | Depends on materials and construction, PTFE has coefficient of friction ~0.05-0.35, depending upon fillers added | Good, provided wear is low, but some slack is normally present | Low to very high | Low to very high - depends upon application and lubrication | Widely used, relatively high friction, suffers from stiction in some applications. Depending upon the application, lifetime can be higher or lower than rolling element bearings. |
| Rolling element bearing | Ball or rollers are used to prevent or minimise rubbing | Rolling coefficient of friction with steel can be ~0.005 (adding resistance due to seals, packed grease, preload and misalignment can increase friction to as much as 0.125) | Good, but some slack is usually present | Moderate to high (often requires cooling) | Moderate to high (depends on lubrication, often requires maintenance) | Used for higher moment loads than plain bearings with lower friction |
| Jewel bearing | Off-center bearing rolls in seating | Low | Low due to flexing | Low | Adequate (requires maintenance) | Mainly used in low-load, high precision work such as clocks. Jewel bearings may be very small. |
| Fluid bearing | Fluid is forced between two faces and held in by edge seal | Zero friction at zero speed, low | Very high | Very high (usually limited to a few hundred feet per second at/by seal) | Virtually infinite in some applications, may wear at startup/shutdown in some cases. Often negligible maintenance. | Can fail quickly due to grit or dust or other contaminants. Maintenance free in continuous use. Can handle very large loads with low friction. |
| Magnetic bearings | Faces of bearing are kept separate by magnets (electromagnets or eddy currents) | Zero friction at zero speed, but constant power for levitation, eddy currents are often induced when movement occurs, but may be negligible if magnetic field is quasi-static | Low | No practical limit | Indefinite. Maintenance free. (with electromagnets) | Active magnetic bearings (AMB) need considerable power. Electrodynamic bearings (EDB) do not require external power. |
| Flexure bearing | Material flexes to give and constrain movement | Very low | Low | Very high. | Very high or low depending on materials and strain in application. Usually maintenance free. | Limited range of movement, no backlash, extremely smooth motion |
| †Stiffness is the amount that the gap varies when the load on the bearing changes, it is distinct from the friction of the bearing. | ||||||
Read more about this topic: Bearing (mechanical)
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