A journal bearing usually consists of a split cylindrical shell of hard, strong metal held in a rigid support and an inner cylindrical part of soft metal, which holds a rotating shaft, or journal. A self-aligning journal bearing has a spherically shaped support that turns in a socket to adjust to movements of the shaft. Slight misalignment of the shaft can be accommodated in the ordinary journal bearing by wearing of the soft bearing material, often an alloy of tin or lead. Less frequently used are aluminum alloys, steel, cast iron, or a thin layer of silver covered with a thin coating of a soft bearing material. Ideally, a film of lubricant, normally oil, separates journal and bearing so that contact is prevented (see lubrication ). Bearings that are not split are called bushings.
A thrust bearing supports an axial load on a shaft, i.e., a force directed along a shaft's length. It may be a plate at the end of a shaft or a plate against which the collar on the shaft pushes. Large thrust bearings, such as those used to transmit the motive force of a ship's propeller from the shaft to the hull, have blocks that are separated from the collar on the shaft by wedge-shaped spaces filled with oil. Graphite bearings are used in high-temperature situations. Certain plastics make satisfactory self-lubricating bearings for low speeds and light loads and, if additionally lubricated, work at higher speeds and carry greater loads. Rubber and a naturally oily wood, lignum vitae, are used in water-lubricated bearings. Watches and other precision instruments have glass or sapphire pivot bearings. In gas-lubricated bearings a film of gas separates the bearings from the moving machine parts. Magnetic bearings employ magnetic repulsion to separate journal from bearing, reducing friction still further.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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