Working Principle and Technical Status of Self-lubricating Sliding Bearings

Bearings can be divided into rolling bearings and sliding bearings. Rolling friction occurs when the rolling bearing works, and sliding friction between the surfaces occurs when the sliding bearing works. These are two different ways of movement. Currently, the materials and manufacturing technologies of rolling bearings both domestic and abroad are already very mature. However, the materials and manufacturing technologies of sliding bearings are gradually being reformed and developed with the continuous improvement of design and service requirements.

The function of the sliding bearing is to support the operation of the shaft, to separate the shaft from the housing bore and protect the shaft and the housing bore. One of its important functions is to reduce the friction coefficient and reduce the degree of wear. Sliding bearings can be divided into self-lubricating sliding bearings and general sliding bearings. Self-lubricating sliding bearings can achieve with no or less oil during operation, while general sliding bearings do not have such good lubrication performance themselves, so they must be lubricated during operation by adding oil.

The sliding friction pair is a surface-to-surface type of friction between the shaft and the inner surface of the sliding bearings. The friction coefficient and the abrasion are required to be as small as possible during operation. This puts higher requirements on the materials of the two friction surfaces. The shaft material should be hard and strong since the shaft must transmit a certain force during operation. So we concentrate the requirements for reducing friction on the material of the sliding bearing working surface. The materials of sliding bearing working surface are usually divided into two categories: metal and non-metal. Metals include alloys, ceramic metals, aluminum alloys--bimetals, cast iron, bronze, Babbitt Metal, etc….; non-metals are mainly divided into engineering plastics and rubber. With the continuous innovation and progress of sliding bearing material technology and manufacturing technology, self-lubricating sliding bearings have become the mainstream product. This article mainly states the working principle and technical status of self-lubricating sliding bearings.

About the mechanism sliding bearing lubrication and its limit PV value

When the sliding bearing works, a lubricant film can be formed on the surface of the sliding bearing to separate the surfaces of the moving pair. It reduces the sliding friction greatly. Because there is no direct contact between the surfaces of the moving pairs avoids abrasion of the sliding bearings. The large bearing capacity and the high rotation precision of the sliding bearing, along with the anti-impact effect of the lubricating film formed during the movement lead to a widely used of sliding bearings in engineering projects.

The formation of the lubricating film is the basic condition for the normal operation of the sliding bearing. The factors that affect the formation of the lubricating film are the lubrication method, the relative speed of the motion pair, the physical properties of the lubricant and the roughness of the surface of the motion pair. The design of the sliding bearing should determine the structure type and geometric parameters of the bearing, select the lubricant and the lubrication method according to the working conditions of the bearing.

There are two lubrication film which is fluid lubrication film and solid lubrication film according to the different lubrication methods. The fluid lubricating film is a lubricating film with certain pressure resistance and buffering performance formed by the sliding bearing under the lubrication of the fluid medium. Graph 1 shows the process how the fluid lubrication formed. The solid lubricating film has a good self-lubricating performance due to the material of the sliding bearing working surface. A solid lubrication film formed after the self-lubricating material is gradually transferred to the pits on the working surface of the grinding shaft during initial operation. Therefore, the lubricating method of the solid lubricating film has certain requirements on the roughness of the shaft working surface, and it is generally required to be between Ra 0.4 and 1.6. Too smooth or too rough surfaces go against to the transfer of lubricant to the working surface and hinder the establishment of a lubricating film. The general requirement for solid lubrication bearings is from Grinding Surface 4 to Polishing Surface 5 (refer to figure 2). When the solid lubricating film is completely formed, the motion between the shaft and the bearing has actually been converted into a frictional motion between the lubricating film and the bearing, and the shaft is no longer in direct contact with the bearing surface. Figure 3 is a schematic diagram of the formation of a solid lubricating film. Solid lubricating sliding bearings are adopted by most designers as a result of that many applications are not suitable or too difficult to add or re-add oil to the bearings.