Bearings can be classified as rolling bearings and sliding bearings. In keeping with the two types of bearings' different forms of motion, rolling bearings give rise to rolling friction, while sliding bearings give rise to sliding friction. Domestic and foreign rolling bearing materials and manufacturing methods have currently reached a high state of maturity, while sliding bearing materials and manufacturing techniques are steadily improving as design and use requirements become increasingly demanding.
Sliding bearings serve to support the rotation of a shaft and protect the shaft and bearing seat by separating the shaft and seat. A crucially important function is reducing the coefficient of friction and wear. Depending on their material and perform, sliding bearings can be classified as self-lubricating sliding bearings and ordinary sliding bearings. Self-lubricating sliding bearings require little or no lubricant during operation, while ordinary sliding bearings do not have an effective lubrication function, and therefore require added lubricant during operation;
Because the shaft and inner surface of sliding bearings form a face-to-face sliding friction couple when in operation, the coefficient of friction and wear must be as small as possible, which imposes high demands on the material of the two friction surfaces. Because the shaft must transmit a certain amount of torque and motion when in operation, shaft materials must typically emphasize hardness and strength. As a result, the material of sliding bearings' working surface must bear most of the responsibility for reducing friction. The materials used for the working surfaces of sliding bearings are usually classified as metallic and non-metallic materials. Metallic materials include bearing alloys, ceramic metal, aluminum alloy-bimetallic, cast iron, bronze, and Babbitt alloy; non-metallic materials include engineering plastic and rubber. With the steady improvement of sliding bearing material technology and manufacturing methods, self-lubricating sliding bearings have become the mainstream type of sliding bearing.
This article chiefly seeks to explain the operating principles of self-lubricating sliding bearings and the current state of this technology.
The lubricating mechanism of sliding bearings functions by forming a lubricating film on the surface of the sliding bearing, and this film separates the motion pair surfaces. Apart from greatly reducing sliding friction, this approach prevents wear by maintaining indirect contact between the motion pair surfaces.
Plain bearings offer a large load-bearing capacity and high rotational precision, and the lubricating film enhances impact-resistance. Because of these advantages, plain bearings are widely used in engineering applications.
The formation of a lubricating film is a necessary condition for sliding bearings to function normally. Factors influencing the formation of a lubricating film include the lubrication method, the relative speed of the motion pair, the physical characteristics of the lubricant, and the roughness of the motion pair surfaces.
In accordance with the bearings' working conditions, the design of sliding bearings generally involves determination of bearing structure, selection of lubricant and lubrication method, and establishment of the bearings' geometric parameters.
Depending on the lubrication method, lubricating films can be classified as fluid lubricating films and solid lubricating films; fluid lubricating films are formed when sliding bearings are lubricated using a fluid medium, and possess a certain degree of compressive strength and cushioning ability. Fig. 1 shows the formation process of a fluid lubricating film. Solid lubricating films are formed when the material forming the working surface of a sliding bearing inherently possesses good self-lubricating performance; during the initial stage of operation, after break-in wear has occurred, self-lubricating material will gradually fill pits in the frictional working surface and form a hard lubricating film. This imposes certain roughness requirements on the frictional working surface, which must ordinarily have a roughness of in the range of Ra0.4 – 1.6; A surface that is too smooth or too rough will impede migration of the lubricant to the working surface, and obstruct the formation of the lubricating film. It is ordinarily required that grinding surface 4 face polished surface 5 in solid lubricant bearings (see Fig. 2). After full formation of a solid lubricating film, movement between the shaft and bearing will consist of frictional movement between the lubricating film and bearing, and the shaft will actually be in only indirect contact with the bearing surface. Figure 3 shows a schematic diagram of the formation of a solid lubricating film. Because it is difficult or impossible to apply lubricant in many situations, solid lubricated sliding bearings are widely used in many types of designs.
Adhesion wear - Occurs when the bearing's safe operating temperature is exceeded or a bare shaft causes adhesion. When a bearing exceeds its acceptable safe operating temperature, the bare shaft will stick to the partially softened bearing friction surface, causing wear.
Burn wear - An excessive operating speed may cause burn wear of the friction surface. If a bearing's operating speed exceeds the acceptable operating speed, this will cause friction and heating of the friction surface, and the abrupt increase in heating may result in burning of the friction surface.
Of course somewhere of plastic sliding bearings is an inevitable phenomenon. When the performance of plastic sliding bearings has been specified, we can only try to comply with the use recommendations of the bearing model as much as possible in order to minimize bearing wear and achieve the best possible performance from plastic plain bearings.