Sliding bearings, also known as plain bearings or sliding bearings, comprise a range of mechanical components that go by different names but are usually classified as fluid film bearings whose purpose is to reduce friction between a rotating, reciprocating or sliding surface (such as a shaft) and a stationary surface (such as a housing). The lubricating film is usually oil, although water is sometimes used in special cases. “Dry running” bearings obtain their slip through PTFE or other low-friction materials. Sliding bearings are different from ball and roller bearings, which use rolling elements to reduce friction, but in both cases the goal is the same. Sliding bearings can be grouped in any number of ways; for the purposes of this article, they will be discussed by material.

• Bronze and bimetallic bearings
• Carbon insert bearings
• Polymer bearings

These machine elements go by many names, including sleeve bearings, bushings, journal bearings, plain bearings, friction bearings, and more.

Many large rotating machines, such as steam turbines, support their shafts on fluid film bearings commonly known as journal bearings. Automotive engines, and virtually all engines, are made this way. These bearings are typically manufactured using compliant materials such as bimetallic bearings or various copper alloy bearings, including leaded bronze bushings, aluminum bronze bushings, tin bronze bushings, etc. As the shaft rotates in the bearing, its motion creates an area of high oil pressure that lifts the shaft out of the bearing, creating what is called hydrodynamic lubrication. A shaft running on this oil film can theoretically run forever, as there is no wear or fatigue associated with rolling bearings. Of course, the machine must start and stop, and this is where the adaptability of the bearing material comes into play. As the oil film thins, the bearing material must resist welding and galling to the hardened surface of the journal. Generally, copper is used for higher loads than bimetallic alloy bearings, but it is less compliant, so shafts running in copper alloy bearings must be rigid and in close alignment with the bearings.

Bearings that carry shafts that are not parallel to the bearing surface may have difficulty developing hydrodynamic lubrication. Similarly, bearings with oil grooves may have difficulty establishing full film lubrication if the grooves are located in the load area. There is a sacrificial aspect to bearing materials. If the shaft “grazes” the bearing surface, the softer bearing material should be able to withstand the damage while leaving the hardened, expensive-to-repair journal undamaged. Because the position of the shaft in the bearing gap varies with the load, fluid film bearings have a greater ability to withstand shock loads than rolling element bearings. Although journal bearings are associated with radial loads, fluid film thrust bearings are also available, including the tilting pad variety for very large machines. Journal bearing designs are tailored to specific applications, as design pressures can vary widely.

Sintered bronze bearings are made by compressing a powder mixture of copper/tin or iron/copper into the shape of the bearing and then heating the shape to a temperature between the melting points of the two metals. This produces a porous material that absorbs oil and releases it under heat and pressure. In theory, such bearings are self-lubricating, although provisions for adding grease or oil to the inside of the bearing are usually included. Unlike journal bearings, many porous bronze bearings operate in the boundary or mixed film regions of the hydrodynamic curve where constant or occasional contact between the bearing and the shaft occurs.

Bronze bearings are available in cylindrical and flanged shapes to handle radial and axial loads. This material is also available in ribbon form for linear applications, such as machine tracks.

Graphite is used as a shaft lubricant in many instances where the environment is too tough for rolling-element bearings. In the typical design, cast bronze is machined to the dimension of the bearing and holes are bored into the outer surface and through to the interior bearing surface. Graphite plugs are then installed in these holes. Variations include bronze with graphite impregnation bushing and steel backings PTFE sliding bushing, which provide for both dry and self-lubricated operation. Graphite bearings are often used in high-temperature settings.

Polymer bearings are typically injection molded with a solid lubricant or wound with a lubricating wire and represent the latest approach to sliding bearings. Known for its high slip characteristics, PTFE is often bonded to a thin metal housing to produce a plain bearing that can run dry without cold flow. Heat can reduce the effective range of these bearings; metal housings are sometimes used to dissipate the heat generated during operation. Molded polymer bearings usually have the lubricant dispersed throughout the bearing body. Humidity can affect any of these bearings, causing expansion. Many manufacturers can offer bearings made from low moisture-absorbing materials for use in humid environments.

The smoothness of the shaft plays a role in the success of a polymer bearing: it should be neither too smooth nor too rough so that the bearing can maintain its rated life. In fact, compatibility of the shaft material with the bearing material is a consideration for any fluid film bearing.

Three basic influences govern fluid film bearings: viscosity, rotational speed and bearing unit load. Hydrodynamic lubrication itself depends on three basic assumptions: the presence of relative motion between the shaft and the bearing; wedging action as the shaft moves eccentrically against the bearing wall; and the presence of a suitable fluid in the bearing. While the steps required to properly design a conventional oil film plain bearing are beyond the scope of this paper, there are several important requirements for their successful deployment

• The bearing needs to achieve thick film lubrication.
• Sufficient lubricant must be available.
• The temperature range needs to be maintained.
• The oil needs to flow completely through the bearing.
• Deflection of the shaft should be considered.
• Starting and stopping needs to be done at low bearing pressures, otherwise film lubrication should be considered.

Selection of self-lubricating sliding bearings is a somewhat different task, but considerations of bearing load, operating speed, operating temperature, etc. are of similar importance. Manufacturers will provide design assistance in the selection of their products. Sintered and self-lubricating bearings are available in standard lengths, bore diameters and outside diameters to meet almost any requirement, and custom products are often available. These bearings are usually pressed into the housing bore, and the inside diameter is often reduced once the bearing is installed.