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Plain bearings and sleeve bearings are designed for extreme environments and can withstand high or low temperatures. Others are suitable for hazardous environments with high levels of shock and vibration. Applications for plain bearings and [sleeve bearings] include food processing machinery, home appliances, packaging equipment, office machines, and automotive equipment.

There are two broad types of bearings used in machinery today: plain and rolling element bearings. This article targets the special lubrication requirements of plain bearings, also known as sleeve bearings and journal bearings.The plain bearing consists of a shaft, also called a journal, and a supporting component, which may be a shell around the shaft called a sleeve, a half shell that the shaft fits into, two half shells (top and bottom parts) or a multipart shell.

Plain bearings are used for high radial loads (perpendicular to the axis of the shaft) and low to high speeds. Typical applications include turbines, large milling systems, engine cranks, compressors, gearboxes, shaft bearing supports, etc.The components that are separated by the oil film in a plain bearing are the bearing liner and the shaft. The shaft is composed of high-quality, wear-resistant, structurally strong steel. The bearing liner may be made of a single layer or multiple layers, depending on the design features of the equipment 

Under normal operating conditions, the lubrication regime will be a hydrodynamic full-fluid film. A hydrodynamic film occurs when there is sufficient lubricant between the lubricated surfaces at the point of loading to form a fluid wedge that separates the sliding surfaces. In this state, the lubricated components do not touch each another, reducing friction and wear.Another condition that can occur is boundary film lubrication. This is when the film that separates the fag bearings surfaces undergoes significant loss resulting in a high load of metal-to-metal contact. This happens any time the relative motion of component surfaces are slow and no oil film is formed.

Operating under proper speed, surface area, viscosity and oil volume, a plain bearing can support very heavy loads. The balance between these conditions is important. If the load or the speed changes, the lubricant viscosity must be adjusted to compensate for the change. There is no simple formula that is used to calculate the viscosity requirements for oil lubricated plain bearings, but the ZN/P formula demonstrates the results of complex calculations used to arrive at the proper clearance. 

There are several factors that can wipe or damage a plain nachi bearing surface. Abrasive wear is one of the most common. If the wear is caused by a hard particle rubbing between the lubricated surfaces, it is called three-body wear. Wear caused by an asperity on one surface cutting the other surface is called two-body abrasion

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