The friction pair operates under full film lubrication, which is an ideal condition. However, how to create conditions and take measures to form and comply with the full film lubrication state is a more complicated task. People have continuously explored and researched the lubrication principles in long-term production practice, some are relatively mature, and some are still being studied. Now we will briefly introduce the common lubrication principles of dynamic pressure lubrication, static pressure lubrication, dynamic and static pressure lubrication, boundary lubrication, extreme pressure lubrication, solid lubrication, self-lubrication, etc.

1. Dynamic pressure lubrication

The lubricating oil is brought into the friction surface through the rotation of the journal of the bearing pair. The oil pressure is generated due to the viscosity of the lubricating oil and the hydrodynamic effect formed by the wedge-shaped gap of the oil in the bearing pair, that is, a load-bearing oil film is formed, which is called hydrodynamic pressure. lubricating. The assumption of the hydrodynamic lubrication theory is that the viscosity of the lubricant is equal, that is, the viscosity of the lubricant does not change with changes in pressure at temperature; secondly, it is assumed that the surface undergoing relative frictional motion is rigid, that is, when it is loaded and Under the action of oil film pressure, its elastic deformation is not considered. Under the above assumptions, for general non-heavy load (contact pressure at 15MPa) sliding bearings, this assumption is close to the actual situation. However, when the contact pressure between the rolling bearing and gear surface increases to 400~1500MPa, the above assumed conditions are different from the actual situation. At this time, the deformation of the friction surface can reach several times the thickness of the oil film, and the elastic deformation of the lubricated metal friction surface and the change of lubricating oil viscosity with pressure are two factors to study and calculate the law and thickness of oil film formation, oil film cross-sectional shape and The pressure distribution in the oil film is realistic. This kind of lubrication is called elastohydrodynamic lubrication.

2． Hydrostatic lubrication

Through a high-pressure hydraulic oil supply system, the pressured lubricating oil passes through the throttle damper and is forcibly supplied to the gap between the friction surfaces of the moving pair (such as the gap between the hydrostatic sliding bearing and the gap between the planar hydrostatic sliding guide rail). The gap between the medium and static pressure screws is medium). Before the friction surface starts to move, it is separated by high-pressure oil, forcing the formation of an oil film, which complies with the fact that the moving partner is in a liquid lubrication state under working load conditions. This lubrication is called hydrostatic lubrication.

3. Dynamic and static pressure lubrication

With the development of science and technology, new types of dynamic and static pressure lubricated bearings have appeared in industrial production in recent years. Hydrodynamic and hydrostatic combined bearings take advantage of both hydrodynamic and hydrostatic bearings, overcoming the shortcomings of both hydrodynamic bearings and hydrostatic bearings. Main working principle: When the bearing pair is starting or braking, the hydrostatic liquid lubrication method is used to press the high-pressure lubricating oil into the bearing load-bearing area to float the shaft force, which meets the liquid lubrication conditions, thus avoiding the need for lubrication during startup. Or friction and wear caused by direct contact between the metal friction surface (journal surface and bearing surface) due to the inability to form a dynamic pressure oil film due to speed changes during braking. When the bearing pair enters full-speed stable operation, the static pressure oil supply system can be stopped and dynamic pressure lubrication oil supply can be used to form a dynamic pressure oil film, which can still maintain the liquid lubrication conditions of the journal in the bearing.

This method, theoretically speaking, avoids semi-liquid lubrication and boundary lubrication and becomes liquid lubrication during the entire process of starting, running, braking, and forward and reverse rotation of the bearing pair. Therefore, the coefficient of friction is very low, as long as the friction resistance between the internal molecules of the liquid due to the viscosity of the lubricating oil is overcome. In addition, the friction surface is separated by the static pressure oil film and the dynamic pressure oil film, so if the situation is normal, there will be almost no wear., thus greatly increasing the working life of the bearing and saving kinetic energy consumption.

4. Boundary lubrication (i.e. boundary friction)

Boundary lubrication is the transition from internal friction between lubricant molecules between friction surfaces (i.e., liquid lubrication) to a critical state before direct contact between the friction surfaces. At this time, there is an adsorbed film on the friction interface. The thickness is usually about 0.1μm and has lubrication properties. We call this film the boundary film. The lubrication performance of the boundary film mainly depends on the properties of the friction surface; it depends on the structure of the boundary film formed by the oily additives and extreme pressure additives in the lubricant on the metal friction surface, and has little to do with the viscosity of the lubricating oil port.

5. extreme pressure lubrication

Extreme pressure lubrication is a type of boundary lubrication, that is, the friction pair is under heavy load (or high contact stress), high speed, and high temperature conditions. The extreme pressure additives in the lubricating oil react with the metal friction surface to form a layer of chemical The reaction film separates the two friction surfaces and plays a role in reducing the friction coefficient, slowing down wear (or changing severe wear in direct contact between metal surfaces), and achieving lubrication, which is called extreme pressure lubrication.

6. solid lubrication

Putting solid powdery lubricant between the friction surfaces can also achieve good lubrication effects. There is a solid lubricant between the two friction surfaces. Its shear resistance is very small. Slippage will occur between the molecules with a slight external force. In this way, the external friction between the two grinding surfaces is transformed into the internal friction between the solid lubricant molecules. There are two conditions for solid lubrication. The solid lubricant molecules should have low shear strength and slip easily. Secondly, the solid lubricant should have a strong affinity with the friction surface. During the friction process, the solid lubricant should always use A layer of solid lubricant is maintained on the friction surface, and this layer of solid lubricant does not corrode the friction surface. Generally, it is mechanically attached to the metal surface, but there are also chemical bonds formed. There are many solid substances with the above properties, such as graphite, molybdenum disulfide, talc, etc.

For non-layered solid lubricants or soft metals, they mainly play a lubricating role due to their low shear force, and then allow them to adhere to the friction surface to form a lubricating film. As for the lubrication mechanism of the solid lubricating film that has been formed, its lubrication effect can be approximately explained according to the boundary lubrication mechanism.

7． Self-lubricating

The above mentioned types of lubrication require lubricant to be added between the friction surfaces during the friction movement.. Self-lubricating is to mix solid lubricant powder with lubricating properties with other solid materials and press and sinter them into materials, or immerse solid lubricants in porous materials; or use solid lubricants to directly press them into materials, as Rub the surface. In this way, there is no need to add lubricant during the entire friction process, and it can still have good lubrication. The mechanism of self-lubrication includes solid lubrication, boundary lubrication, or both. For example, compressor piston rings, bearing bushes, bushings, etc. made of polytetrafluoroethylene products are all self-lubricating. Therefore, during the processing of such parts, they can maintain good lubrication without adding lubricant.