The friction pair operates under full film lubrication, which is an ideal situation. However, how to create conditions and take measures to form and meet the state of full film lubrication is a more complicated task. In the long-term production practice, people have continuously explored and researched the principle of lubrication. Some are more mature and some are still studying. A brief introduction to the common lubrication principles of dynamic pressure lubrication, static pressure lubrication, dynamic and static pressure lubrication, boundary lubrication, extreme pressure lubrication, solid lubrication, and self-lubrication is now made.
1.Dynamic pressure lubrication
The lubricant is brought to the friction surface through the rotation of the bearing journal. The oil pressure is generated due to the viscosity of the lubricant and the hydrodynamic effect of the wedge-shaped gap of the oil in the bearing pair. lubricating. The assumption of fluid dynamic lubrication theory is the viscosity of the lubricant, 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 that moves relative to friction is rigid Under load and oil film pressure, its elastic deformation is not considered. Under the above assumptions, for general non-heavy load (contact pressure 15MPa) sliding bearings, this assumption is close to the actual situation. However, when the contact pressure between the rolling bearing and the gear surface increases to 400 to 1500 MPa, the above assumptions are different from the actual situation. At this time, the deformation of the friction surface can be several times the thickness of the oil film, and the elastic deformation of the lubricated metal friction surface and the viscosity of the lubricating oil change with pressure. The pressure distribution in the oil film is more realistic. This kind of lubrication is called elastic fluid dynamic pressure lubrication.
Through a set of high-pressure hydraulic oil supply system, the lubricating oil with a certain pressure is forced into the gap of the friction surface of the moving pair through the throttling damper (such as the gap of the hydrostatic sliding bearing, the flat hydrostatic sliding guide In the gap, the gap of the hydrostatic screw is medium). Before the friction surface starts to move, it is separated by high-pressure oil, forcing the formation of an oil film, thereby ensuring that the motion pair can be fully liquid-lubricated under a certain working load. This lubrication is called liquid hydrostatic lubrication.
3. Dynamic and static pressure lubrication
With the development of science and technology, new types of dynamic and static pressure lubrication bearings have appeared in industrial production in recent years. The combined hydrodynamic and static pressure bearing fully utilizes the advantages of both the hydrodynamic and hydrostatic bearings, and overcomes the shortcomings of both the hydrodynamic and hydrostatic bearings. Main working principle: When the bearing pair is in the process of starting or braking, static pressure liquid lubrication is used to press the high-pressure lubricating oil into the bearing bearing area, and the shaft is floated to ensure the liquid lubrication condition, thereby avoiding the starting condition Or the friction and wear caused by the direct contact between the metal friction surface (the journal surface and the bearing pad surface) due to the failure to form a dynamic pressure oil film during braking. When the bearing pair enters full-speed stable operation, the hydrostatic oil supply system can be stopped, and the dynamic oil film can be formed by dynamic pressure lubrication oil supply, which can still maintain the liquid lubrication condition of the journal in the bearing. This method, in theory, completely avoids semi-liquid lubrication and boundary lubrication during the entire process of starting, running, braking, and reversing of the bearing pair, and becomes liquid lubrication. Therefore, the friction coefficient is very low, as long as the frictional resistance between the molecules inside the liquid which the viscosity of the lubricating oil has is overcome. In addition, the friction surface is completely separated by the static pressure oil film and the dynamic pressure oil film, so if the condition is normal, there will be almost no wear, which greatly prolongs the working life of the bearing and saves kinetic energy consumption.
4. Boundary lubrication (ie boundary friction)
Boundary lubrication is the transition from the lubricant molecules between the friction surfaces and the internal friction between the molecules (ie, liquid lubrication) to the critical state before the friction surfaces directly contact. At this time, there is a layer of adsorbed film on the friction interface. The thickness is usually about 0.1 μm, which has a certain lubricating performance. We call this film a 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 on the metal friction surface by the oily additives and extreme pressure additives in the lubricant, and has little relationship with the viscosity of the lubricant port.
5, extreme pressure lubrication
Extreme pressure lubrication is a special case 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 additive in the lubricant reacts with the metal friction surface to form a layer The chemical reaction film separates the two friction surfaces and reduces the friction coefficient, slows down the wear (or changes the severe wear of the direct contact of the metal surface), and achieves the function of lubrication, which is called extreme pressure lubrication.
Putting a solid powdery lubricant between the friction surfaces can also have a good lubricating effect. There is a solid lubricant between the two friction surfaces, its shear resistance is very small, and a slight external force will cause slippage between the molecules. In this way, the outer friction between the two abrasive surfaces is transformed into the inner friction between the solid lubricant molecules. There are two necessary conditions for solid lubrication. The first is that the solid lubricant should have low shear strength between the molecules, and it is easy to cause slippage. The second is that the solid lubricant must have a strong affinity with the friction surface. Always keep a layer of solid lubricant on the friction surface, and this layer of solid lubricant will not corrode the friction surface. Generally, it is mechanically attached to the metal surface, but it also forms a chemical bond. There are many solid substances with the above properties, such as graphite, molybdenum disulfide, talc and the like. For non-layered solid lubricants or soft metals, it is mainly because of its low shear force, which acts as a lubricant, and then attaches it to the friction surface to form a lubricating film. For the lubricating mechanism of the solid lubricating film that has been formed, its lubrication effect can be explained approximately according to the boundary lubrication mechanism.
In the above-mentioned several kinds of lubrication, during the friction movement, it is necessary to add lubricant to the friction surface. Self-lubricating is mixing solid lubricant powder with lubricating properties with other solid materials and pressing and sintering to form materials, or immersing solid lubricants in porous materials; or directly pressing solid lubricants into materials, as Rub the surface. In this way, in the entire friction process, there is no need to add a lubricant, and it still has a good lubricating effect. The mechanism of self-lubrication includes solid lubrication, boundary lubrication, or both. For example, compressor piston rings, bearing pads, and sleeves made of polytetrafluoroethylene products are self-lubricating. Therefore, in the process of such parts, it can maintain good lubrication without adding any lubricant.