In addition to reducing wear, mechanical lubrication has other functions

In addition to reducing wear, mechanical lubrication has other functions

There are three types of lubrication: full-film lubrication, boundary lubrication, and mixed lubrication. They are in three different states, but they all rely on lubricating oil to lubricate and resist wear.

　　Lubrication is a frequently encountered problem in equipment maintenance, but what exactly is lubrication? Speaking of lubrication, the scene in our minds may just apply oil or grease to the mechanical friction surface to reduce friction. Although this concept is correct, it is only one aspect of lubrication.

Many substances can be used for lubrication, z* common lubricating oil (liquid oil) and lubricating grease (generally paste at room temperature), grease and lubricating oil are relatively thick, because the grease is used on the basis of lubricating oil The thickener. The oil used for lubrication can be mineral oil derived from petroleum, or artificial chemically synthesized oil, and vegetable oil and animal oil can also be used as lubricating oil.

　　The lubricating oils and greases currently used are mainly derived from petroleum or synthetic oils. The oil before adding additives and thickeners is called base oil and is the main component of lubricating oil. The base oil itself has lubricity, but in order to optimize the overall performance of the base oil, we added some additives. Mineral oil can meet common industrial lubrication requirements, so more than 90% of current oil products are based on mineral oil. The performance of synthetic oil is better, and the use of vegetable oil is mainly for environmental protection.

　　Additives can optimize the performance of base oils, suppress bad properties, enhance beneficial properties, and even give base oils some new properties. However, the more additives are not the better, because additives are chemical substances, and various additives must be properly matched in categories and proportions to achieve the desired effect. In addition, the number of additives should also be used in consideration of occasions. For example, the lubricating oil (motor oil) of a car engine needs to have a better detergency and dispersion effect to remove carbon deposits and soot in the engine, and the lubricating oil used by heavy-duty machinery may be added Extreme pressure additives to increase wear resistance.

　　The effect of lubrication

　　 Reduce friction is one of the main purposes of lubricating oil, but in addition to reducing wear, lubricating oil has other functions, such as lubricating oil film can prevent rust and prevent metal corrosion. Lubricating oil can also play a cleaning role, and the impurities inside the equipment are like toxins in our body, which are removed from the oil by the filter. Cooling effect: friction between metals generates heat, and the lubricating oil flows through the metal surface to take away the heat.

　　 Three states of lubrication

　　 There are three types of lubrication: full-film lubrication, boundary lubrication, and mixed lubrication. They are in three different states, but they all rely on lubricating oil to lubricate and resist wear.

　　1, Full-film lubrication

　　 Full film lubrication is divided into two kinds of hydrodynamic lubrication (hydrodynamic lubrication) and elastohydrodynamic lubrication (elastohydrodynamic lubrication). Hydrodynamic lubrication refers to two sliding friction surfaces that move relative to each other. The two friction surfaces are completely separated by a fluid lubricating film produced by the relative speed to avoid direct contact between them and dry friction and wear. Elastohydrodynamic lubrication is similar to hydrodynamic lubrication in that the contact surfaces are separated by a layer of fluid lubrication film. The difference is that the relative motion between the contact surfaces is rolling, the formed full-film lubrication film bears greater pressure than hydrodynamic lubrication, and the resulting lubrication separation film is thinner than that under hydrodynamic lubrication.

　　 Even if the metal is processed with high precision, the surface is not flat and smooth. Under the microscope, we can see that the metal surface is actually bumpy. Burrs are protrusions that we can already find with the naked eye, and the higher metal protrusions under the microscope are some peaks, which are difficult to see with the naked eye, but they are important for lubrication. In order to achieve full-film lubrication, the thickness of the lubricating film must exceed the height of these peaks and separate them completely to avoid peak-point contact between the two contact surfaces. It can achieve full-film lubrication, which is a good lubrication state of z*, and can effectively protect the contact surface of the machine.

　　2, boundary lubrication (Boundary lubrication)

Although full-film lubrication is ideal, there are some situations where a complete lubrication film may not be formed, such as frequent start and stop of the machine, impact load, speed, and load factors that are not sufficient to form full-film lubrication. These conditions will cause boundary lubrication. . It is good to be able to form a complete lubricating film, but when the conditions are insufficient, a complete lubricating film cannot be formed or the lubricating film becomes thinner (at high temperatures), the transition from liquid friction to dry friction (direct contact between the friction surfaces) occurs before the critical process. State is boundary lubrication. Therefore, in combination with the structure and use of the machine, some equipment requires the use of extreme-pressure lubricants, which contain extreme-pressure additives (EP: extreme-pressure additives), or anti-wear additives (AW: anti-wear additives). Full-film lubrication cannot be achieved. When boundary lubrication occurs, these additives play a role in protecting the mechanical surface. This type of additive can be adsorbed on the metal surface to form a protective layer to avoid metal wear. When boundary lubrication occurs, extreme pressure additives are used to protect the metal surface. Compared with full-film lubrication, boundary lubrication produces more lubricating film friction and generates more heat, but complete dry friction is avoided.

　　The role of the boundary film is actually the physical adsorption and chemical adsorption of the active base of the oily agent on the metal surface to form a firm oil film to ensure lubrication. If the load on the friction pair is large, and at the same time, due to the uneven surface, the pressure at the convex peak is large, and the strength of the film is less than the friction force at the sliding contact, it will cause the rupture of the boundary film and direct metal contact.

　　 Generally speaking, the lubrication state of the mechanical sliding surface not only forms an oil film on a single plane, it becomes an ideal fluid lubrication state, and even a tiny thin oil film can be seen. Moreover, considering the machining accuracy of the mechanical surface and the influence of the unevenness of the surface, there is still some actual state of direct contact, and extremely high pressure occurs at the contact part, and this part of the metal flows plastically. At this time, the lubricating oil is sandwiched between the two metal surfaces, the oil film of the lubricating oil will become thin, and the phenomenon of oil film rupture will occur in the place where the pressure is high, and the result will be changed from fluid lubrication to elastic fluid lubrication to mixed lubrication . Further turn to boundary lubrication.

　　The typical film thickness is 1-50nm, the fluid lubrication effect of the lubricant is reduced, or even does not work at all, the load is almost all borne by the boundary lubrication film formed by the interaction between the asperities and the lubricant and the surface.

　　3, mixed lubrication (Mixed lubrication)

　　The state of mixed lubrication refers to the situation where full film lubrication and boundary lubrication are mixed. We mentioned earlier that if the peak point of the metal surface is higher than the lubricating oil film, it will contact each other. In mixed lubrication, although most areas are in full-film lubrication, when these peaks are in contact, local boundary lubrication will occur.