There are many different types of bearings.
Type Description Friction Stiffness † Speed Life Notes
||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.
|Ball or Roller Bearing
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
||Off-center bearing rolls in seating
||Low due to flexing
||Adequate (requires maintenance)
||Mainly used in low-load, high precision work such as clocks. Jewel bearings may be very small.
||Fluid is forced between two faces and held in by edge seal
||Zero friction at zero speed, low
||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.
|Fluid or Hydrodynamic Bearing
||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
||No practical limit
||Indefinite. Maintenance free. (with electromagnets )
||Active magnetic bearings (AMB) need considerable power. Electrodynamic bearings (EDB) do not require external power.
||Material flexes to give and constrain movement
||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.
! knife edge bearings
Other bearing types:
- Deep groove ball bearings
- Angular contact ball bearings
- Self-aligning ball bearings
- Cylindrical roller bearings
- Full complement cylindrical roller bearings
- Needle roller bearings
- Tapered roller bearings
- Spherical roller bearings
- Toroidal bearings
- Bearing types
- Thrust ball bearings
- Angular contact thrust ball bearings
- Cylindrical roller thrust bearings
- Needle roller thrust bearings
- Tapered roller thrust bearings
- Spherical roller thrust bearings
- Track runner bearings
- Cam rollers
- Support rollers
- Cam followers
The most common bearing application is the Ball Bearing
Ball Bearing types permit smooth, low-friction movement
between two or more surfaces. This movement is either rotary (shaft
rotating within a mount) or linear (one surface moving along another).
Bearings , are anti-friction devices, each having different anti-friction
characteristics. By far the largest number of bearings are oil-lubricated. The oil film can be maintained through pumping by a pressurization system
(pump). Or it can be maintained by a squeezing or wedging of lubricant produced
by the rolling action of the bearing itself.
General Application Guidelines:
Ball bearings are the less expensive choice in the smaller
sizes and under lighter loads, while roller bearings are less expensive for
larger sizes and heavier loads.
Roller bearings are more satisfactory under shock or impact
loading than ball bearings.
Ball-thrust bearings are for pure thrust loading only. At
high speeds, a deep-groove or angular-contact ball bearing usually will be a
better choice, even for pure thrust loads.
Self-aligning ball bearings and cylindrical roller bearings
have very low friction coefficients.
Deep-groove ball bearings are available with seals built into
the bearing so that the bearing can be pre-lubricated to operate for long
periods reducing maintenance requirements.
Careful consideration of speed requirements are important for
proper bearing application design. Useable speeds are influenced by bearing
size, properties, lubrication and operating temperatures. The permissible speed
varies inversely with mean bearing diameter.
Ball Bearing Useable Life
Ball bearings were formerly rated on the
basis of the compressive stress in the most heavily loaded ball. Except for
static loads, experience has shown that the actual cause of failure is fatigue.
Fatigue characteristics are used for load rating and are dependent on
experimental results. Analytical methods are available to determine a
particular bearings useable life. Consult with manufacturer to determine
typical bearing performance characteristics.
The life of a ball bearing is the
life in hours at some known speed, or the number of revolutions, that the
bearing will attain before the first evidence of fatigue appears on any of the
moving elements. Experience has shown that the life of an individual ball
bearing cannot be precisely predicted. Fatigue characteristics are used for load
Even if bearings are properly
installed, adequately lubricated, protected from foreign matter, and are not
subjected to extreme operating conditions, they can ultimately fatigue. Under
ideal conditions, the repeated stresses developed in the contact areas between
the balls and the raceways eventually can result in fatigue of the material
which results in spalling of the bearing load carrying surfaces. In most
applications, the fatigue life is the maximum useful life of a bearing.
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