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Bearing (mechanical)

A bearing is a device to permit constrained relative motion between two 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.

Bearing friction

Low friction bearings are often important for efficiency, to reduce wear and to facilitate high speeds. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces.

• By shape, gains advantage usually by using spheres or rollers.

• By material, exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)

• By fluid, exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching.

• By fields, exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.

Combinations of these can even be employed with the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.

An example of a four-point contact ball bearing

Principles of operation

Animation of ball bearing

There are at least six common principles of operation:

• sliding bearings, usually called "bushes", "bushings", "journal bearings", "sleeve bearings", "rifle bearings", or "plain 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.

Motions

Common motions permitted by bearings are:

• Axial rotation e.g. shaft rotation

• Linear motion e.g. drawer

• spherical rotation e.g. ball and socket joint

• hinge motion e.g. door

Loads

Bearings vary greatly over the size and directions of forces that they can support.

Forces can be predominately radial, axial (thrust bearings) or moments perpendicular to the main axis.

Speeds

Bearings vary typically involving some degree of relative movement between surfaces, and different types have limits as to the maximum relative surface speeds they can handle, and this can be specified as a speed in ft/s or m/s.

For rotational bearings generally performance is defined in terms of the product 'DN' where D is the diameter (often in mm) of the bearing and N is the rotation rate in revolutions per minute.

Generally in terms of relative speed of the moving parts there is considerable overlap between capabilities, but plain bearings can generally handle the lowest speeds while rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which have no known upper speed limit.

Life

Fluid and magnetic bearings can potentially give indefinite life.

Rolling element bearing life is statistical, but is determined by load, temperature, maintenance, vibration, lubrication and other factors.

For plain bearings some materials give much longer life than others. Some of the John Harrison clocks still operate after hundreds of years because of the lignum vitae wood employed in their construction, whereas his metal clocks are seldom run due to potential wear.