bearing

A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.
Contents
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* 1 Overview
* 2 Principles of operation
* 3 Motions
* 4 Friction
* 5 Loads
* 6 Speeds
* 7 Play
* 8 Stiffness
* 9 Service life
o 9.1 Fluid and magnetic bearings
o 9.2 Rolling element bearings
o 9.3 Plain bearings
o 9.4 Flexure bearings
o 9.5 Short-life bearings
o 9.6 L10 life
o 9.7 External factors
o 9.8 Bearing current
o 9.9 Summary
* 10 Maintenance
* 11 History
* 12 Types
* 13 See also
* 14 References
* 15 External links

[edit] Overview

Plain bearings use surfaces in rubbing contact, often with a lubricant such as oil or graphite. A plain bearing may or may not be a discrete device. It may be nothing more than the bearing surface of a hole with a shaft passing through it, or of a planar surface that bears another (in these cases, not a discrete device); or it may be a layer of bearing metal either fused to the substrate (semi-discrete) or in the form of a separable sleeve (discrete). With suitable lubrication, plain bearings often give entirely acceptable accuracy, life, and friction at minimal cost. Therefore, they are very widely used.

However, there are many applications where a more suitable bearing can improve efficiency, accuracy, service intervals, reliability, speed of operation, size, weight, and costs of purchasing and operating machinery.

Thus, there are many types of bearings, with varying shape, material, lubrication, principle of operation, and so on. For example, rolling-element bearings use spheres or drums rolling between the parts to reduce friction; reduced friction allows tighter tolerances and thus higher precision than a plain bearing, and reduced wear extends the time over which the machine stays accurate. Plain bearings are commonly made of varying types of metal or plastic depending on the load, how corrosive or dirty the environment is, and so on. In addition, bearing friction and life may be altered dramatically by the type and application of lubricants. For example, a lubricant may improve bearing friction and life, but for food processing a bearing may be lubricated by an inferior food-safe lubricant to avoid food contamination; in other situations a bearing may be run without lubricant because continuous lubrication is not feasible, and lubricants attract dirt that damages the bearings.
[edit] Principles of operation
Animation of ball bearing

There are at least six common principles of operation:

* plain bearing, also known by the specific styles: bushings, journal bearings, sleeve bearings, rifle bearings
* rolling-element bearings such as ball bearings and roller bearings
* jewel bearings, in which the load is carried by rolling the axle slightly off-center
* fluid bearings, in which the load is carried by a gas or liquid
* magnetic bearings, in which the load is carried by a magnetic field
* flexure bearings, in which the motion is supported by a load element which bends.

[edit] Motions