Plain Bearing

(or simple bearing), a support or guide of a mechanism or machine in which friction occurs during slippage of the mating surfaces. A distinction is made between radial and axial (thrust) plain bearings according to the direction of loading. A distinction is also made, according to the method of lubrication, among hydrodynamic, hydrostatic, gas-dynamic, and gas-static bearings—in which the lubricant is air or a neutral gas—and solid-lubricant bearings. There are many types of bearing designs, including self-aligning, segmented, and self-lubricating.

Radial plain bearings usually consist of a sleeve and two or more bushings, which completely or partially encircle the shaft. Such bearings operate mostly under conditions of fluid or semifluid friction. They are lubricated through holes in the bushings, by oil rings, or by local spiral grooves and pockets. Radial bearings are used in the journal boxes of railroad cars and in the mounts for internal-combustion engines, turbogenerators, and other machines. Heavily loaded bearings, such as those for the rolls in rolling mills, have diameters of 140-1,200 mm. The relative gap of such bearings, which is the ratio of the difference between the diameters of the bushing hole and journal on the shaft to the diameter of the bushing hole, (D – d)/D, is 0.0003–0.002, and the length-diameter ratio is 0.6–0.9. Such bearings can operate at relative sliding velocities of 0.2-60.0 m/sec and unit pressures of 5-25 meganewtons per sq m (MN/m²), or 50-250 kilograms-force per sq cm (kgf/cm²). In highly stressed internal-combustion engines, the unit pressures on plain bearings may be as high as 30-35 MN/m², or 300-350 kgf/cm². High-speed fluid-friction bearings of this type are made with rigid bushings or with self-aligning bushings in the form of rocking, free, or ring-shaped “floating” liners.

Axial plain bearings are simple or segmented thrust bearings; they operate in a similar manner to end gaskets, slide blocks, and crossheads. A segmented thrust bearing consists of fixed or rocking pillow blocks formed by a set of sectors, and also a thrust plate or ring on the rotating shaft. The pillow blocks have a slight tilt relative to the plane of the thrust plate. Self-aligning capability is provided by springs, rocking supports, a hydraulic system, or elastic deformation. Thrust bearings are widely used in turbogenerator and hydrogenerator mounts. The diameter of the thrust plate in the bearings of large hydrogenerators may be up to 4-5 m, and the disk may carry a load up to 4,000 tons-force.

Hydrodynamic and gas-dynamic bearings operate under conditions such that the friction surfaces are separated by a layer of fluid or gas formed by the pressure that arises in the viscous lubricating layer as a result of relative motion of the surfaces. In hydrostatic and gas-static bearings the frictional surfaces of the parts, whether moving relative to one another or at rest, are completely separated by the lubricating material, which is supplied under pressure to the gap between the surfaces. There are also bearings called hydrostatodynamic bearings, which operate sometimes as hydrostatic bearings (for example, during start-up) but mainly as hydrodynamic bearings.

Plain bearings that operate with separation of the friction surfaces by a lubricating layer are designed using the hydrodynamic theory of lubrication. The minimum thickness of the lubricating layer (usually measured in microns), the pressure in the layer, and the temperature and flow rate of the lubricating materials are determined in the design computations. Bearings of this type are fabricated from metallic and nonmetallic antifriction bearing materials.