The friction pair operates under full film lubrication, which is an ideal condition. However, how to create conditions and take measures to form and meet the full film lubrication state is a relatively complex task. People have continuously explored and studied the lubrication principle in long-term production practice, some of which are relatively mature, while others are still under research. Here is a brief introduction to the lubrication principles of commonly used dynamic pressure lubrication, static pressure lubrication, dynamic static pressure lubrication, boundary lubrication, extreme pressure lubrication, solid lubrication, self-lubricating, etc.  
1. Dynamic pressure lubrication
By rotating the journal of the bearing shaft, lubricating oil is brought into the friction surface. Due to the viscosity of the lubricating oil and the fluid dynamic effect formed by the wedge-shaped clearance of the oil in the bearing pair, oil pressure is generated, forming a load-bearing oil film, which is called fluid dynamic lubrication. The assumption of fluid dynamic pressure lubrication theory is that the viscosity of the lubricant is equal, that is, the viscosity of the lubricating oil does not change with the change of pressure at a certain temperature; Secondly, it is assumed that the surface undergoing relative frictional motion is rigid, meaning that its elastic deformation is not considered under load and oil film pressure. Under the above assumptions, for sliding bearings that are generally not overloaded (with a contact pressure of 15MPa), this assumption is close to the actual situation. However, when the contact pressure between the rolling bearing and the gear surface increases to 400~1500MPa, the above assumed conditions are different from the actual situation. At this point, 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 in lubricating oil viscosity with pressure are two factors that are more suitable for studying and calculating the formation law and thickness of the oil film, the cross-sectional shape of the oil film, and the pressure distribution inside the oil film. This type of lubrication is called elastohydrodynamic lubrication.  
2. Static pressure lubrication
Through a high-pressure hydraulic oil supply system, lubricating oil with a certain pressure is forcibly supplied to the clearance of the friction surface of the moving pair (such as in the clearance of the static pressure sliding bearing, the clearance of the flat static pressure sliding guide rail, the clearance of the static pressure screw, etc.) through the throttle damper. Before the friction surface begins to move, it is separated by high-pressure oil, forcing the formation of an oil film, thereby ensuring that the moving pair can be completely in a liquid lubrication state under certain 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 bearings with dynamic and static pressure lubrication have emerged in industrial production in recent years. The combination of hydrodynamic and hydrostatic bearings fully utilizes the advantages of both hydrodynamic and hydrostatic bearings
Overcoming the shortcomings of both liquid dynamic pressure bearings and liquid static pressure bearings. Main working principle: When the bearing pair is in the starting or braking process, static pressure liquid lubrication is used to inject high-pressure lubricating oil into the bearing bearing bearing area, lift the shaft force, and ensure liquid lubrication conditions, thereby avoiding 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 caused by speed changes during the starting or braking process. 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 condition of the journal in the bearing.  
In theory, this method completely avoids semi liquid lubrication and boundary lubrication throughout the entire process of starting, running, braking, and forward and reverse rotation of the bearing pair, becoming liquid lubrication. Therefore, the coefficient of friction is very low
As long as the frictional resistance between molecules inside the liquid due to the viscosity of lubricating oil is overcome, it is sufficient. In addition, the friction surface is completely separated by static pressure oil film and dynamic pressure oil film, so if the situation is normal, there is almost no wear generated
This greatly extends the working life of the bearing and saves kinetic energy consumption.  
4. Boundary lubrication (i.e. boundary friction)
Boundary lubrication is the transition from internal friction between lubricant molecules on the friction surface (i.e. liquid lubrication) to a critical state before direct contact with the friction surface. At this point, there is a thin film of adsorption on the friction interface, usually around 0.1 µ m thick, which has certain lubricating properties. We call this thin film the boundary film. The lubrication performance of boundary films mainly depends on the properties of the friction surface; It depends on the structure of the boundary film formed on the metal friction surface by the oil-based additives and extreme pressure additives in the lubricant, and has little to do with the viscosity of the lubricating oil port.  
5. Extreme pressure lubrication
Extreme pressure lubrication is a special case of boundary lubrication, where the friction pair is under heavy load (or high contact stress), high speed, and high temperature conditions. The extreme pressure additive in the lubricating oil reacts with the metal friction surface to form a chemical reaction film, separating the two friction surfaces and reducing the friction coefficient, slowing down wear (or changing severe wear caused by direct contact of the metal surface), and achieving lubrication. This is called extreme pressure lubrication.  
6. Solid lubrication
Placing a solid powder lubricant between the friction surfaces can also achieve a good lubrication effect. There is a solid lubricant between two friction surfaces, which has very low shear resistance. With a slight external force, molecules will slip between them. This transforms the external friction between the two grinding surfaces into internal friction between solid lubricant molecules. There are two necessary conditions for solid lubrication. Firstly, the solid lubricant molecules should have low shear strength and be prone to slip; Secondly, solid lubricants should have a strong affinity with the friction surface. During the friction process, a layer of solid lubricant should always be maintained on the friction surface, and this layer of solid lubricant should not corrode the friction surface. Generally, it is mechanically attached to metal surfaces, but there are also chemical bonds formed. There are many solid substances with the above properties, such as graphite, molybdenum disulfide, talc powder, etc.  
For non layered solid lubricants or soft metals, the main reason is their low shear force, which provides lubrication and allows them to adhere to the friction surface to form a lubricating film. For the lubrication mechanism of the already formed solid lubricating film, its lubrication effect can be approximately explained by the boundary lubrication mechanism.  
7. Self lubrication
The various types of lubrication mentioned above require the addition of lubricant between the friction surfaces during frictional motion
Self lubrication is the process of mixing solid lubricant powder with other solid materials that have lubricating properties, pressing and sintering them into materials, or immersing them in solid lubricants in porous materials; Alternatively, solid lubricants can be directly pressed into materials as friction surfaces. In this way, no lubricant needs to be added throughout the entire friction process, and it can still have a good lubricating effect. The mechanism of self-lubricating includes solid lubrication, boundary lubrication, or both. For example, compressor piston rings, bearing shells, shaft sleeves, etc. made of polytetrafluoroethylene products are self-lubricating, so in the process of such parts, they can maintain good lubrication without adding any lubricants.