A bearing is a component used to reduce friction in a machine. Bearings may be classified broadly according to the motions they allow and according to their principle of operation.

Major Types

Common motions include linear and rotary. A linear bearing allows motion along a straight line, for example a drawer being pulled out and pushed in. A rotary bearing allows motion about a center, such as a wheel on a shaft or a shaft through a housing. Common kinds of rotary motion include both one-direction rotation and oscillation where the motion only goes through part of a revolution.

Principles of Operation

There are at least six common principles of operation: sliding bearings, usually called "bushings", "journal bearings", or "plain bearings"; rolling_element bearings such as ball 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; and flexure bearings, in which the motion is provided by a load element which bends.

History and Development

An early type of linear bearing was an arrangement of tree trunks laid down under sleds. This technology is known to date at least as far back as the construction of the Pyramids of Giza. Modern linear bearings use a similar principle, sometimes with balls in place of rollers.

The first plain and rolling-element bearings were wood, but ceramic or glass can be used, and steel, other metals, and plastic are all common today.

Rotary bearings are required for many applications, from heavy_duty use in vehicle axles and machine shafts, to precision clock parts. The simplest rotary bearing is the sleeve bearing, which is just a cylinder inserted between the wheel and its axle. This was followed by the roller bearing, in which the sleeve was replaced by a number of cylindrical rollers. Each roller behaves as an individual wheel. The first practical caged-roller bearing was invented by horologist John Harrison in his H3 chronometer of 1760.

An early example of a wooden ball bearing (see rolling_element bearing), supporting a rotating table, was retrieved from the remains of a Roman ship in Lake Nemi, Italy. The wreck was dated to 40 BC. Leonardo da Vinci is said to have described a type of ball bearing around the year 1500. One of the issues with ball bearings is that they can rub against each other, causing additional friction, but this can be prevented by enclosing the balls in a cage. The captured, or caged, ball bearing was originally described by Galileo in the 1600s. The mounting of bearings into a set was not accomplished for many years after that. The first patent for a ball race was by Philip Vaughan of Carmarthen in 1794. The modern, self-aligning design of ball bearing is attributed to Sven Wingquist of the SKF ball-bearing manufacturer in 1907.