According to the different lubricating materials between the friction pairs, lubrication can be divided into fluid (liquid, gas) lubrication and solid lubrication (see lubricant). According to the different friction states between the friction pairs, lubrication can be divided into fluid lubrication and boundary lubrication. The lubrication state between fluid lubrication and boundary lubrication is called mixed lubrication, or partial elastic fluid dynamic lubrication. Fluid lubrication Under appropriate conditions, the two mutually friction surfaces can be separated by a layer of viscous fluid with a certain thickness (more than 1.5 to 2 microns), and the external load is balanced by the fluid pressure. Most of the molecules in the fluid layer are not affected by the ionic electric field of the friction surface and can move freely, that is, the friction only exists in the lubrication state between the fluid molecules. The friction coefficient of fluid lubrication is very low (less than 0.01). According to the way the lubricating film pressure is generated, fluid lubrication can be divided into dynamic lubrication and static lubrication. In traditional lubrication mechanics research, the friction body and the lubricating fluid are regarded as rigid bodies and viscous fluids (Newtonian fluids) respectively. In fact, the friction body is an elastic body, but sometimes it can be simplified as a rigid body. Fluid dynamic lubrication that needs to consider the effects of elastic deformation and pressure on viscosity is called elastic fluid dynamic lubrication. When the friction body is in a plastic state, the fluid dynamic lubrication that needs to consider the plastic effect is called plastic fluid dynamic lubrication. The traditional research method of fluid lubrication began in 1886, and the founder was O. Reynolds of the United Kingdom. Later generations collectively referred to the research results of traditional lubrication mechanics as classical lubrication mechanics. In fluid lubrication, the viscosity of the fluid is generally evaluated by viscosity. Figure 1 shows a model that assumes that the fluid is incompressible and flows in a lamellar manner. The relationship between the viscous shear resistance of the fluid to tangential motion, that is, the shear stress τ and the velocity gradient (the rate of change of the fluid velocity u along the direction perpendicular to the laminar direction y) is where η is the proportional constant, that is, the viscosity, also known as dynamic viscosity. The above relationship is called the internal friction law of fluid laminar flow (Figure 2), also known as Newton's internal friction law. Fluids whose flow behavior conforms to this law are called Newtonian fluids. For lipid plastic bodies (called non-Newtonian fluids), the corresponding internal friction law is where τ0 is the initial shear resistance of the lipid. Sometimes the dependence of fluid flow on time should also be considered. The Reynolds equation is the basic equation that describes the pressure distribution of the fluid dynamic lubrication film. The traditional Reynolds equation is based on the motion equation of viscous fluid, also known as the Navier-Stokes equation. It is simplified based on certain assumptions after being combined with the mass continuity equation. The universal Reynolds equation that describes the pressure distribution of the fluid lubrication film is: where v1 and v2 are the velocities of the boundary surfaces 1 and 2 along the x direction respectively; t is the time; η is the dynamic viscosity of the fluid; p is the pressure of the fluid film; h is the density of the fluid; and h is the film thickness. The two terms on the left side of this equation characterize the film pressure distribution, and the three terms on the right side indicate the causes of the fluid dynamic lubrication film pressure, namely the wedging effect, the surface stretching effect, and the squeezing effect. Usually the surface stretching effect is very small and can be ignored. When the film thickness h does not change, the squeezing effect can also be ignored. Therefore, under most working conditions, the wedging effect of the lubricating fluid is the main term that generates the film pressure. For gas hydrodynamic lubrication, a state equation must be added to the general Reynolds equation. If the lubricating gas is considered to be a real gas and satisfies the polytropic relationship, the additional equation is: Where T is the absolute temperature; R is the gas constant of the specific gas; n is the polytropic expansion index, n=cp/cv, cp and cv are the specific heat at constant pressure and specific heat at constant volume, respectively. When n=1, it is isothermal flow; when n=1.401 (air), it is adiabatic flow. In addition, when the temperature in the lubricating film changes greatly, causing a significant change in viscosity, an energy equation must be added to the general Reynolds equation to solve it simultaneously. Boundary lubrication The lubrication state when there is a thin film (boundary film) between two mutually rubbing surfaces. This phenomenon usually occurs when the machine is started or stopped. Boundary film can be divided into adsorption film and reaction film (Figure 3). The film formed by the polar molecules in the lubricant adsorbed on the friction surface is called adsorption film. Adsorption film is further divided into physical adsorption film and chemical adsorption film. ① Physical adsorption film: The attraction of molecules firmly adsorbs polar molecules on the solid surface, and they are arranged in a directional manner to form a surface film with a thickness of one to several molecular layers. ② Chemical adsorption film: The surface film formed by the degradation or polymerization reaction of certain organic compounds in lubricating oil (such as dialkyl dithiophosphates, dibasic acid diol esters, etc.), or the chemical binding force generated by the exchange of valence electrons of polar molecules in lubricating oil with electrons on the metal surface, which makes the polar molecules of metal soap arranged in a directional manner and adsorbed on the surface to form a surface film. Additives in lubricating oil, such as extreme pressure agents containing organic compounds such as sulfur, phosphorus, and chlorine, react chemically with the metal surface to form a surface film that can withstand a large load, which is called a reaction film. Under the action of frictional heat generated when the convex peaks on the two friction surfaces are in direct contact and relative motion, the reaction film is continuously formed and destroyed. When the adsorption film reaches saturation, the polar molecules are closely arranged, and the cohesive force between molecules gives the film a certain load-bearing capacity, preventing the two friction surfaces from directly contacting each other. Figure 4 is a model of the lubrication effect of the adsorption film. When the friction pair slides relative to each other, the adsorption film slides relative to each other like two brushes, which can play a lubricating role and reduce the friction coefficient. The reaction film has a high melting point, is not easy to adhere, has low shear strength, has low friction resistance, and can be continuously destroyed and formed, so it can prevent direct contact between metal surfaces and play a lubricating role. Factors affecting the lubrication performance of the adsorption film include the structure and adsorption amount of polar molecules, temperature, speed and load. When the number of carbon atoms in the polar molecules increases, the friction coefficient decreases. When the adsorption amount of polar molecules reaches saturation, the lubrication performance of the film is good and stable. When the operating temperature exceeds a certain range, the adsorption film will be scattered or desorbed, and the lubrication will fail. Usually, the friction coefficient of the adsorption film decreases with the increase of speed until a certain value. Under normal working conditions, the friction coefficient of the adsorption film is the same as that of dry friction and is not affected by the load. The reaction film has a strong anti-adhesion ability under extremely high pressure, and its lubrication performance is more stable than any adsorption film. Its friction coefficient increases with the increase of speed until a certain value. Reaction films are often used under heavy load, high speed and high temperature conditions. Under certain working conditions, the ability of the boundary film to resist rupture is called the strength of the boundary film. It can be expressed by the critical pv value, critical temperature value or critical friction coefficient. ① Critical pv value: In normal boundary lubrication, when the load p or speed v increases to a certain value, the temperature of the friction pair suddenly rises, and the friction coefficient and wear increase sharply. The corresponding pv value when the boundary film strength reaches the limit value is called the critical pv value. ② Critical temperature value: When the temperature of the friction surface reaches the degree of disorder, softening or melting of the boundary film, the adsorption film desorbs, the friction coefficient increases rapidly but still has some lubrication effect. The temperature at this time is called the first critical temperature. When the temperature continues to rise to the point where the lubricating oil (grease) polymerizes or decomposes, the boundary film completely breaks, the friction pair becomes sticky, and the wear increases sharply, the temperature is called the second critical temperature. The critical temperature is the main parameter for measuring the strength of the boundary film. ③ Critical friction times: The number of repeated frictions when the boundary film reaches lubrication failure is called the critical friction times
Adding lubricant between two surfaces in relative friction to form a wear-reducing layer of lubricating oil film can reduce the friction coefficient, maintain friction resistance, and reduce power consumption. For example, under good liquid friction conditions, its friction coefficient can be as low as 0.001 or even lower. At this time, the friction resistance is mainly the low shear resistance of the mutual sliding between molecules in the liquid lubricating film. Lubricants between friction surfaces can maintain wear caused by hard particle wear, surface rust, and bite welding and tearing between metal surfaces. Therefore, if enough lubricants are supplied between friction surfaces, good lubrication conditions can be formed, the oil film can be prevented from being damaged, and the matching accuracy of parts can be maintained, thereby greatly maintaining wear. Lubricants can reduce the friction coefficient and maintain the generation of friction heat. We know that the work done by the running machinery to overcome friction is all converted into heat, part of which diffuses outward from the body, and part of which continuously increases the temperature of the machinery. The centralized circulation lubrication system using liquid lubricants can take away the heat generated by friction, play a role in cooling, and control the operation of the machinery within the required temperature range. The surface of the machinery will inevitably come into contact with the surrounding medium (such as air, water, water vapor, corrosive gases and liquids, etc.), causing the metal surface of the machinery to rust, corrode and be damaged. Especially the high-temperature workshops of metallurgical plants and chemical plants are more serious in corrosion and wear. For the cylinders and pistons of steam engines, compressors, internal combustion engines, etc., lubricating oil can not only lubricate and reduce friction, but also enhance the sealing effect, so that there is no air leakage during operation and improve work efficiency. Grease has a special effect on forming a seal, which can prevent water or other dust and impurities from penetrating into the friction pair. For example, the use of oil-immersed packing coated with grease has a good lubricating effect on the seal of the water pump shaft head, and can prevent leakage and dust impurities from penetrating into the pump body and play a good sealing role. In addition, lubricating oil also has the effect of reducing vibration and noise.
