Classification and selection of lubricants

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(1) Classification of lubricants Lubricants can be classified into four categories based on their physical state: liquid lubricants, semi-solid lubricants, solid lubricants, and gas lubricants. 1. Liquid lubricants: Liquid lubricants are the most widely used and diverse category of lubricant materials, including mineral lubricants, synthetic lubricants, animal and vegetable oils, and water-based liquids. Liquid lubricants are characterized by a wide range of viscosities, providing a broad selection of options for moving parts operating under different load speeds and temperatures. (1) Mineral lubricating oil. It is the most widely used liquid lubricant at present, accounting for about 90% of all lubricants; it is generally formed by adding additives to mineral base oils. (2) Synthetic lubricants. It refers to lubricating oils produced through chemical synthesis methods. (3) Animal and vegetable oils. It refers to lubricants extracted from plants and animals. (4) Water-based liquids. It is a water-based lubricant, available in solution type and emulsion type. 2. Semi-solid lubricants (grease) Semi-solid lubricants, also known as grease, are lubricating materials that are in a semi-fluid state at normal temperature and pressure and possess a colloidal structure. 3. Solid lubricants: The lubricating action of solid lubricants can be of three types. The first type involves the formation of a solid lubrication film on the friction surfaces, and its lubrication mechanism is similar to that of boundary lubrication. The second type is soft metal solid lubricants, which utilize the low shear strength of soft metals to provide lubrication. The third type are substances with layered structures such as graphite, which utilize their structural characteristics to provide lubrication. The most commonly used solid lubricants for equipment lubrication are molybdenum disulfide, graphite, polytetrafluoroethylene, and others. 4. Gas lubricants: Gas is also a fluid, and it obeys the physical laws of fluid lubrication; therefore, under certain conditions, gas can function as a lubricant just like a liquid. The advantages of gas lubricants are a low coefficient of friction, less frictional heat generation at high speeds, low temperature rise, smooth operation, and a wide operating temperature range. The disadvantages of gas lubricants are their low density and low load-carrying capacity; they can only be used in aerodynamic devices operating at 30–70 kPa and in static air devices operating at no more than 100 kPa. (II) Components of lubricants 1. Base oil The base oil is the main component of lubricants, accounting for 80%–95% of their total volume, and it serves as a carrier for additives. Base oils are divided into two categories: mineral oils and synthetic oils. (1) Mineral oil. In our country, mineral oils are generally classified into paraffin-based, intermediate-based, and naphthenic-based categories. (2) Synthetic oil. Synthetic base oils are produced through chemical reactions involving several compounds; they possess high chemical purity and better physical and chemical properties than mineral oils. As a result, they have a wider range of applications and a longer service life, representing the future direction for the development of lubricants. Today, synthetic oils are widely used in aerospace machinery, and their application in industrial machinery is also developing rapidly. Synthetic oils are generally classified into synthetic hydrocarbon oils, ester oils, polyisobutylene oils, polyethers, silicone oils, etc. 2. Additives Additives refer to small amounts of substances added to lubricants, which can significantly improve certain properties of the grease or endow it with new properties. The functions of the additive are as follows: (1) Static dispersant. It is commonly used in internal combustion engine oils to remove varnish and carbon deposits from the cylinder walls and piston rings. It also helps to disperse gums and soot particles throughout the oil, preventing the formation of large particles. (2) Anticorrosives. It delays the oxidation of lubricating oil, extending its service life. (3) Anti-wear agents. Improve the wear resistance and anti-sintering properties of oils, reduce equipment wear, and prevent seizing or sintering. (4) Oil-based agents. Reduce the friction coefficient and improve lubrication performance. (5) Metal passivators. A passivation film is formed on the metal surface to reduce the corrosion of the metal by oils and the catalytic oxidation of oils by the metal. (6) Viscosity index improvers: Increase the viscosity index of oils and improve their viscosity-temperature properties. (7) Rust inhibitor: It acts on the metal surface to prevent rusting or corrosion when the metal comes into contact with water. (8) Antidewaxing agent: It reduces the pour point of oils by slowing down the formation of wax crystals in the oil at low temperatures, thereby improving the oil’s flowability at low temperatures. (9) Antifoam agent: Improves the foaming tendency of oils and causes the foam on the oil surface to dissipate rapidly. (10) Emulsifiers and anti-emulsifiers. Emulsifiers are used in oil emulsions to create a uniform and stable emulsion between oil and water. Anti-emulsifiers are used in general lubricants to help separate water that has mixed into the oil from it more quickly. 3. Thickeners: Thickeners are an important component of grease, and they represent a key feature that distinguishes it from lubricating oils. Grease is composed of a thickener, a base oil, and additives; it is a solid or semi-solid substance formed by the dispersion of the thickener in the base oil. Thickening agents generally can affect the thickness, drip point, and water resistance of fats, and sometimes also their load-bearing capacity. (III) Selection of lubricating oil 1. Factors for selecting lubricating oil The selection of lubricating oil is primarily based on three factors: the actual operating conditions of the equipment, the specifications or recommendations provided by the equipment manufacturer, and the regulations or recommendations given by the lubricating oil manufacturer. In practical applications, the selection of lubricating oil is generally based on the recommendations of the equipment manufacturer. But at the same time, practical operating conditions such as the equipment’s load, speed, and temperature must be taken into account. When selecting lubricating oil, attention should be paid to the following performance parameters of the oil: (1) Viscosity: Viscosity is the parameter used for classifying lubricating oils, and it is decisive for quality assessment and determination. The viscosity of the lubricating oil for equipment is determined by referring to relevant charts based on design or calculation data. (2) Pour point: The pour point is an indicator that indirectly reflects the low-temperature flowability of lubricants during storage, transportation, and use. Experience has shown that the operating temperature must be 5°C to 10°C higher than the pour point. (3) Flash point: It is primarily an indicator of safety during the storage, transportation, and use of lubricants. The principle governing the flash point specification for lubricants is to apply a 1/2 safety factor in accordance with safety regulations, that is, setting it 1/2 higher than the actual operating temperature. Since the oil temperature in the crankcase of internal combustion engine oils should not exceed 120°C, it is specified that the flash point of such oils must be at least 180°C. Given the numerous performance parameters of lubricants and the significant differences between different types, a reasonable decision should be made by taking into account the operating conditions of the equipment, the requirements of the manufacturer, as well as the specifications and descriptions provided for the oil. 2. Substitution of lubricating oil: Each type of lubricating oil has its own performance characteristics; it is necessary to select the appropriate lubricating oil and avoid using substitutes. If substitution is indeed necessary, the following principles should be followed. (1) Try to use oils of the same type or those with similar properties as substitutes. (2) The viscosity must be comparable; the viscosity of the substitute oil shall not exceed ±15% of that of the original oil. Oils with slightly higher viscosity should be given priority for substitution. (3) Quality shall be of a higher standard rather than a lower one. (4) Attention should also be paid to considering the environmental and operating temperatures of the equipment. 3. Mixing of lubricants: It is advisable to avoid mixing lubricants of different types, grades, from different manufacturers, as well as old and new lubricants. The following oils are strictly prohibited from being mixed. (1) Special oils and dedicated lubricants must not be mixed with other types of oils. (2) Oils that require anti-emulsification properties must not be mixed with oils that do not require such properties. (3) Anti-ammonia turbine oil must not be mixed with other turbine oils. (4) Zinc-containing anti-wear hydraulic oil must not be mixed with silver-resistant hydraulic oil. (5) Gear oil must not be mixed with worm gear oil. The oils that can be mixed together are as follows. (1) Products of the same manufacturer with similar quality. (2) Products of the same type but different grades from the same manufacturer. (3) For different types of oils, if both components in the mixture are free of additives. (4) No abnormal phenomena were observed in the blending tests of oils of different types. (5) There are a large variety and quantity of additives added to diesel engine oils, and their properties vary ； Care must be taken when the properties of the oil are not known, to avoid adverse consequences or even lubrication failures in the equipment. (IV) Selection of grease When selecting grease, it is first necessary to clarify the functions it performs, namely its roles in lubrication and friction reduction, protection, sealing, etc. As a lubricating grease for reducing friction, factors such as the temperature range tolerance, load, and speed are primarily considered. As a protective grease, the media and materials in contact are primarily taken into consideration, with emphasis on its protective properties and stability against metals and non-metals. As a sealing grease, the material of the sealing component in contact and the medium involved must be taken into consideration; an appropriate grease should be selected based on the compatibility between the grease and those materials, especially rubber. The selection of grease should be determined by taking into account various factors such as the machine’s operating temperature, speed of rotation, load level, working environment, and method of grease supply. Generally, the following factors should be considered: (1) Temperature. Temperature has a significant impact on lubricants. It is generally believed that once the operating temperature at the point of lubrication exceeds the upper limit for the lubricant’s temperature tolerance, the base oil in the lubricant experiences accelerated evaporation, oxidation, and colloidal degradation. For every 10°C to 15°C increase in temperature, the oxidation rate of the lubricant increases by 1.5 to 2 times, and its lifespan is reduced by half. The operating temperature of the lubrication points also changes depending on the temperature of the surrounding medium. In addition, factors such as load, speed, long-term continuous operation, and excessive grease filling also have a certain impact on the operating temperature of the lubrication points. In situations where the ambient temperature and the temperature during mechanical operation are high, heat-resistant grease should be used; generally, the temperature of the grease should be 20°C to 30°C lower than its dropping point. (2) Rotational speed. The higher the operating speed of the lubricated components, the greater the shear stress exerted on the grease, and the more severe the damage to the fiber framework formed by the thickeners in the grease, which reduces its service life. The operating speed of the equipment doubles, while the lifespan of the lubricant is reduced to 1/10 of its original value. Components that operate at high speeds experience a rapid increase in temperature, which can cause the lubricant to thin out and be lost; therefore, lubricants with a higher viscosity should be used. (3) Load. Choosing the right grease based on the load is one of the key factors in ensuring proper lubrication. For high-load lubrication points, greases with a high viscosity of the base oil, a high content of thickeners, and excellent extreme pressure and anti-wear properties should be used. The cone penetration value of grease is related to the load it can withstand during use. For high loads, grease with a low cone penetration (higher consistency) should be used. If subjected to both heavy loads and shock loads, a grease containing extreme pressure additives, such as one containing molybdenum disulfide, should be used. (4) Environmental conditions. Environmental conditions refer to the working environment of the lubrication point and the surrounding media, such as air humidity, dust, and the presence of corrosive substances. In humid environments or when in contact with water, lubricants with good water resistance can be used. Such as calcium-based, lithium-based, composite calcium, and composite calcium sulfonate greases. Under harsh conditions, grease containing an anti-rust agent should be used, rather than sodium-based greases with poor water resistance. For lubrication points exposed to strong chemical agents, synthetic greases resistant to such chemicals should be used, such as fluorocarbon greases. (5) Others. In addition to the points mentioned above, when selecting grease, economic considerations also need to be taken into account; a comprehensive analysis should be conducted to determine whether the use of this grease results in an extended lubrication period, fewer refillings, less grease consumption, reduced bearing failure, and lower maintenance costs. (6) Relationship between grease viscosity and application.