Classification and selection of lubricating materials

Thread Content

I. Classification of lubricant materials: The lubricants used in equipment are classified by form into liquid lubricants, gas lubricants, semi-solid lubricants, and solid lubricants. In 1986, the International Organization for Standardization (ISO) issued \"Methods of classification and determination of categories for petroleum products and lubricants\" along with its series of standards, providing a common language used worldwide for lubricant materials. GB/T7631.1 \"Classification of Lubricants and Related Products (Class L)\\" divides lubricants into 19 groups based on their application areas. Classification criteria for various groups established based on application areas: A – Fully lost-systems; B – Demolding; C – Gears; D – Compressors (including refrigerators and vacuum pumps); E – Internal combustion engines; F – Spindles, bearings, and clutches; G – Guides; H – Hydraulic systems; N – Electrical insulation; P – Pneumatic tools; Q – Heat conduction; R – Temporary protection/anti-corrosion; T – Turbines; U – Heat treatment; X – Applications requiring grease; Y – Other applications; Z – Steam cylinders; S – Special lubrication applications. II. Types and grades of lubricating oils Lubricating oils are the most widely used lubricants in equipment lubrication applications. Primarily petroleum product refineries. There are distillate oils, residue oils, and blended oils, as well as synthetic hydrocarbon lubricants produced by synthetic methods. 1. Types of lubricating oils: The classification of lubricating oils under the old standards was based on the type and structural characteristics of the lubrication equipment. There are a total of fifteen groups. According to GB/T7631.1, there are 19 groups in total for lubricants and related products based on their application areas. The types mainly used for equipment lubrication include groups A, C, D, E, F, G, P, T, X, Z, S, etc. For more detailed classification criteria for group lubricants, refer to the various standards in GB/T7631. 2. The grade classification for lubricating oils, such as GB3141, is equivalent to the ISO3448 standard, which establishes the «Viscosity Classification for Industrial Lubricating Oils». This classification is based on the dynamic viscosity of the lubricating oil at 40°C. III. Types and grades of grease Grease is also known as butter or dry oil. It is a semi-solid product made from a liquid lubricant thinned with a thickener. The grease has good adhesion, strong rust resistance, excellent sealing properties, and is suitable for harsh operating conditions; it is particularly appropriate for lubricating rolling bearings. 1. Types of grease: The old standard for classifying greases was based on the type of thickener used. The classification of the GB7631.8X group (greases) is equivalently based on the classification standards of the international ISO6743/9X group. This standard is determined based on the operating conditions of grease applications. 2. Grease grade: The grease grades are divided into 9 consistency levels based on the penetration value. 3. Grease symbol markings The new standard grease symbol markings consist of the following components: (1) “L” is the code for lubricants ； (2) The first letter X indicates the grease group ； (3) The second letter refers to the lowest operating temperature ； (4) The third letter indicates the maximum operating temperature ； (5) The fourth letter refers to the water resistance and rust prevention performance of the grease under water-contaminated conditions ； (6) The fifth letter refers to the lubricating performance of the grease under high-load or low-load conditions. Example: A type of grease, under the following conditions: Minimum operating temperature: -20℃ ； Maximum operating temperature: 160℃ ； Environmental conditions: Subjected to washing ； Corrosion resistance: No corrosion protection required ； Load condition: High load ； Consistency grade: 00. The code for this grease is XBEGB00. Under this standard, a grease has only one code, which corresponds to the most stringent operating conditions under which that grease is used (such as temperature, water contamination, and load). 4. Main quality indicators of lubricating oil 4.1 Viscosity Viscosity is an indicator that shows the degree of thickness of lubricating oil. The viscosity level is determined by the cohesive force between oil molecules; it is the main factor that determines the thickness of the oil film and serves as the key criterion for selecting lubricants. The methods for expressing viscosity include dynamic viscosity, kinematic viscosity, Engler viscosity, Leidenfrost viscosity, and Say viscosities. Both our country and the International Organization for Standardization use kinematic viscosity. GB/T 265 “Determination of kinematic viscosity of petroleum products and calculation of dynamic viscosity” specifies the method for determining kinematic viscosity and the method for calculating dynamic viscosity. Viscosity is an important quality indicator of lubricating oils; it plays a significant role in their classification, quality assessment, and selection. Viscosity is also the main parameter used to measure blended alternative oils. After blending two similar types of oils, their viscosity must be measured; they can only be used if the requirements are met. 4.2 Viscosity Index (VI) The property of a fluid’s viscosity changing with temperature is known as its visco-temperature behavior. It is expressed by the viscosity index (V1). The calculation of the viscosity index is carried out in accordance with the method specified in GB/T2541 \"Tables for Calculating the Viscosity Index of Petroleum Products\". The higher the viscosity index of a lubricant, the smaller the change in its viscosity when the temperature changes. Lubricants such as hydraulic oil, diesel engine oil, and gear oil all require a high viscosity index. 4.3 Flash Point The flash point is an indicator of the safety of lubricants. The flash point of a oil is the lowest temperature at which, under specified conditions, the oil is heated; when the oil vapor that evaporates mixes with air to reach a certain concentration and comes into contact with an open flame, a flash fire occurs. The methods for determining flash point include the open cup method and the closed cup method. GB/T261 \"Method for determining the flash point of petroleum products (closed-cup method)\", GB/T267 \"Method for determining the flash point and ignition point of petroleum products (open-cup method)\", or GB/T3536 \"Method for determining the flash point and ignition point of petroleum products (Cleveland open-cup method)\") specify the methods for determining the flash point. The flash point indicates the amount of light fractions present in the oil. It is one of the important indicators for the production, storage, transportation, and use of oil products. Mechanical equipment that operates at high temperatures, such as air compressors, internal combustion engines, and air hammers, require lubricants with a high flash point. The flash point should generally be 20~30°C higher than the operating temperature. Using lubricants with a flash point lower than the specified requirement is unsafe; it can lead to fires or even explosions. 4.4 Freezing point and pour point The freezing point and pour point are indicators of the low-temperature fluidity of lubricants. Under specified test conditions, the highest temperature at which an oil loses its fluidity is called the freezing point of the oil. In other words, the lowest temperature at which the cooled oil can still flow is referred to as the oil’s pour point. GB/T3535 \"Method for Determining the Pour Point of Petroleum Products\" specifies the method for determining the pour point. The pour point indicates the oil’s performance at low temperatures. Lubricants for machinery operating at low temperatures, construction equipment used for outdoor work in cold regions during winter, and vehicles require a low pour point. The pour point of general lubricating oils should be 15~30°C lower than the lowest operating temperature. The pour point of lubricating oil also affects the low-temperature starting performance, wear, and fuel consumption of internal combustion engines. 4.5 Moisture Moisture refers to the amount of water contained in the oil. It is expressed as the percentage of water content relative to the oil. GB/T260 \"Method for Determining Water Content in Petroleum Products\" specifies the method for determining water content. Moisture accelerates the oxidation of lubricating oil and promotes the corrosion of metals by low-molecular-weight organic acids, thereby damaging the integrity of the lubricating oil film and altering its properties. The new oil should be free of moisture. Rain exposure during storage and transportation of oils, or the mixing of water-based liquids during use, can increase the moisture content in those oils. The moisture absorption of lubricating oil and low-temperature dew formation in the oil sump are also causes of increased moisture. 4.6 Neutralization value, acid value, and alkali value: The acid value is the weight of KOH required to neutralize the organic acids contained in 1 g of the oil. Expressed in mgKOH/g. The alkali value is expressed as the weight of KOH in equivalent amounts to the acid required to neutralize the basic components in the oil, measured in terms of lg. The neutralization value is a *common term for the acid number and alkali number of petroleum products. GB/TF264, GB/T4945, and GB/T7304 specify the methods for determining acid value, alkali value, and neutralization value. The acid value is an indicator of the degree of refining of new oil. An increase in the acid value of lubricating oil during storage and use indicates its oxidative degradation. Lubricants with too high an acid value can corrode machine components, and the oil should be replaced immediately. 4.7 Water-soluble acids or bases Water-soluble inorganic acids, low-molecular-weight organic acids, or basic substances present in petroleum products and lubricants are referred to as water-soluble acids or bases. GB/T259 specifies the method for determining water-soluble acids or bases. Water-soluble acids or bases indicate the degree of refinement of the lubricating oil. Pollution or oxidation that occurs during storage and transportation can also cause changes in the levels of water-soluble acids or bases. 4.8 Mechanical Impurities and Insolubles Solid substances contained in lubricating oil are collectively referred to as mechanical impurities or insolubles. Dust, dirt, and iron filings that contaminate lubricating oil during processing, storage, and use, along with wear-generated particles, all contribute to an increase in the impurity content of the oil. Generally, the oil sample is diluted with a solvent and then filtered; the weight of the insoluble substances is determined and expressed as a percentage. The content value varies slightly depending on the solvent used. GB/T511 \"Method for determining mechanical impurities in petroleum products and additives (gravimetric method)\") and GB/T8926 \"Method for determining insoluble matter in used lubricating oils\" specify the methods for determining the content of mechanical impurities and insoluble matter in lubricating oils. Impurities are harmful substances in lubricating oil that can accelerate component wear and clog the oil passages. Lubrication systems and hydraulic systems are generally equipped with filters to remove mechanical impurities from the oil. If the lubricating oil or hydraulic oil only has an excessive level of mechanical impurities, it can be reused after being filtered using a precision oil filter. 4.9 Oxidation Stability Oxidation stability is an indicator of a lubricant’s ability to resist oxidation. Under specified conditions, the oil sample is heated and catalyzed to determine the neutralization value or insoluble matter of the oil. The method for determining the oxidative stability of oils is carried out in accordance with the provisions of GB/T12581 \"Method for determining the oxidative properties of mineral oils with inhibitors\". The degree of oxidation stability determines the service life of the oil. Adding antioxidant additives to lubricating oil can improve its oxidation stability. Lubricants for precision machine tools, oils for hydraulic equipment, and oils for devices such as turbines and internal combustion engines require good oxidation stability. 4.10 Residue and Aging Characteristics Residue indicates the degree of refining of the oil. Under specified conditions, the oil sample is heated and air is introduced into it to cause the oil to age and form carbonaceous residues. The carbon residue value of the samples before and after aging is determined, with this value being used to indicate the aging characteristics of the lubricant. GB/T268 \"Method for determining residue in petroleum products (Kensky method)\\" and GB/T12709 \"Method for determining aging characteristics of lubricating oils (Kensky residue method)\\" specify the methods for determining residue. A high residue carbon content accelerates component wear and clogs the oil passages. The carbon residue level of lubricating oils used in equipment such as air compressors and internal combustion engines should be low. 4.11 Anti-emulsification property The anti-emulsification property is an indicator that reflects the ability of lubricating oil to separate from water. It involves stirring oil and water together under specified conditions to create an emulsion. The time required to allow the oil and water to separate again when left at a certain temperature represents the anti-emulsification property, expressed in minutes. GB/T7305 \"Method for Determining the Emulsion Resistance of Petroleum and Synthetic Fluids\" specifies the method for measuring the emulsion resistance of lubricants. Lubricating oils for steam turbines and hydroturbs must have good emulsion resistance. 4.12 Other quality indicators of lubricating oil There are many other indicators that reflect the quality of lubricating oil, and relevant standards can be selected for testing based on the requirements for using that oil. (1) Foaminess (foaming property), in ml per unit. Comply with GB/T12579 standard. (2) Corrosion test (copper sheet, 100°C, 3h), grade. Comply with GB/T5096 standard. (3) Wear resistance. Implement SH/T0306 ； SH/T0307 ； SH/T0189 standard. (4) Hydrolytic stability. Comply with standard SH/T0301. (5) Thermal stability. Comply with standard SH/T0209. (6) Filterability. Comply with standard SH/T0210. (7) Shear stability. Comply with standard SH/T0103. (8) Scratch resistance test. Comply with standard SH/T0519. (9) Temperature at groove formation °C. Comply with standard SH/T0030. IV. Main quality indicators of grease 1. Penetration The penetration is an indicator of the thickness or hardness of the grease. It is expressed by the depth to which a standard cone of a specified weight sinks into the grease within 5 seconds, while keeping the grease at a specified temperature. Unit: 0.1mm. GB/T269 \"Method for determining the penetration of greases and petroleum greases\" specifies the method for measuring the penetration of greases. Penetration indicates the fluidity and plastic strength of grease. To a certain extent, it reflects the load-carrying capacity of the grease as well as the degree of its loss. For high-load friction pairs, grease with a low cone penetration should be selected. Penetration is the parameter used to classify grease grades. 2. Dropping point: The dropping point is an important indicator of the heat resistance of grease; it represents the lowest temperature at which the grease begins to melt and drip. That is, under specified conditions, the grease is heated until it melts, and the temperature at which the first drop begins to fall is measured, expressed in °C. GB/T4929 \"Method for Determining the Dropping Point of Greases\" specifies the method for determining the dropping point of greases. The dropping point is the basis for selecting the operating conditions for grease. The dripping point of ordinary grease should be 20~30°C higher than the operating temperature. 3. Other quality indicators of grease There are also some other quality indicators for grease. Relevant standards can be selected for testing according to the requirements of the usage conditions. (1) Oil separation by steel mesh (100°C, 24h) %. Comply with standard SH/T0324. (2) Evaporation rate (% at 99°C, 22h). Comply with GB/T7325 standard. (3) Corrosion grade (T2 copper sheet, 100°C, 24h). Comply with GB/T7326 standard. (4) Water loss by dripping (38°C, lh) % . Comply with standard SH/T0109. (5) Wear resistance (75°C, 1200 r/min, 392 N, 60 min): wear mark diameter d, mm. Comply with standard SH/T0204. V. Additives for Lubricants 1. Functions of Additives With the development of industrial technology, modern equipment demands high speed, high performance, high automation, high efficiency, and a long service life; as a result, simple mineral oil-based lubricants are no longer sufficient for meeting these requirements. Adding a small amount of other substances to lubricants can improve their performance and endow them with new properties. These substances are called additives for lubricants. The functions of additives include the following: (1) improving the properties of lubricating materials and reducing the freezing point of oils. Quickly eliminate foam from the oil. Improve viscosity-temperature properties. Improve the tackiness and slipperiness. Increases oil film strength. (2) Protect oils and fats from oxidation and deterioration, thereby extending their shelf life. Improves antioxidant capacity. Improve resistance to corrosion. Improve emulsification resistance. (3) Protect metals from corrosion and improve oil adhesion and oiliness. Improve the oil’s resistance to corrosion. Passivated metals enhance rust resistance. (4) Improve the performance of lubricating greases under harsh working conditions and enhance their extreme pressure and anti-wear properties. Improve the resistance of components to scratching. Improve the self-repair capability against wear of mechanical components. 2. Classification and Properties of Lubricant Additives Lubricant additives can be classified by function into detergents and dispersants, anti-oxidant and anti-corrosion agents, extreme pressure and anti-wear agents, oiliness agents and friction modifiers, anti-oxidants and metal deactivators, viscosity index improvers, rust inhibitors, pour point depressants, antifoam agents, etc. Within each group, names and codes were established based on the chemical name of the additive. VI. Selection of Lubricant Materials 1. General Requirements for Lubricant Materials The following properties are required of lubricant materials: (1) Lubricant materials must have good anti-friction properties, capable of altering the coefficient of friction, reducing frictional resistance, and minimizing wear on mechanical components. (2) It must have appropriate fluidity to form a lubricating film between the moving parts of the friction pair. (3) It has a certain degree of oiliness and a certain strength of oil film. (4) It has good chemical stability and is not prone to oxidation or deterioration during use. (5) It has good defoaming and anti-emulsification properties. (6) It has excellent rust and corrosion resistance, does not corrode metal components, and does not cause aging or deformation of rubber seals. (7) It has a certain sealing capacity, capable of preventing impurities from entering the friction area. (8) It has low volatility and can maintain a stable viscosity for an extended period of time. (9) Newly purchased lubricating materials must meet quality standards. Pure and free of impurities. 2. Selection of lubricating materials: Choosing the right lubricating materials is key to proper equipment lubrication. The selection of lubricating materials should be made based on factors such as the motion characteristics of the friction pair, the materials comprising the friction pair, the operating load, operating temperature, clearance between the components, the lubrication method, and the lubrication equipment. The classification of lubricant materials is based on the application scenario. It is necessary to thoroughly understand the specifications, grades, properties, and application ranges of various lubricating materials in order to select them appropriately. 3. General principles for selection: (1) Based on the load on the friction pair; the greater the load, the higher the viscosity of the lubricating oil should be ； The grease penetration should be low. For low-speed, high-load friction pairs in a boundary lubrication condition, the adhesiveness and extreme pressure properties of the lubricant must be taken into account. (2) Depending on the operating speed of the friction pair, for high-speed, light-load friction pairs, lubricants with lower viscosity or greases with higher penetration should be used. Taking the effect of centrifugal force into account, rolling bearings should use lubricating oil with a higher viscosity or lubricating grease with a lower penetration within the allowable temperature rise range. For high-speed and high-load friction pairs, lubricants with higher viscosity or greases with lower penetration should also be used, and lubricating materials containing oil additives and extreme pressure additives should be employed whenever possible. (3) Based on the manufacturing precision of the friction pair, for friction pairs with high manufacturing precision and small clearances, lubricants with lower viscosity or greases with higher penetration should be selected. For gear drives with low precision as well as wire ropes, lubricants with higher viscosity or greases with lower penetration should be used. (4) Depending on the material of the friction pair, if the hardness of the friction pair material is low, a lubricant with high viscosity and good oiliness or a grease with a low penetration should be used; if the hardness of the friction pair material is high, a lubricant with low viscosity or a grease with a high penetration should be used. (5) Depending on the position, direction, and operating conditions of the friction pair, for friction pairs operating in vertical or non-horizontal directions, lubricants with higher viscosity should be used. Where it is necessary, grease should be employed, especially in areas where refueling is not feasible due to long-term operation; grease lubrication is preferred in such cases. (6) Depending on the working environment and temperature, for machinery that operates in wet or humid conditions, lubricants or greases with good oiliness, rust resistance, and anti-emulsification properties should be selected. For high operating temperatures, lubricants with a high flash point, high viscosity, good oxidation stability, and heat resistance should be used, or greases with a low penetration value, high drip point, and heat resistance. For low operating temperatures, water-free lubricants with a low pour point and low viscosity, or greases with a high penetration value and heat resistance, should be used. In cases of large temperature fluctuations, lubricants with a high viscosity index and low pour point, or greases with a wide operating temperature range, should be used. For machinery operating in environments with corrosive media, lubricants with excellent corrosion and rust resistance should be used. The fertilizer industry, as well as machinery operating in environments with ammonia present, should use lubricants with good resistance to ammonia. Food processing machinery should use lubricants and greases that are harmless to the human body and suitable for use in food machinery. (7) Depending on the lubrication system selected, oil holes, nozzles, and oil cups that require manual oiling due to wear should use lubricants with an appropriate viscosity ； For lubrication areas that absorb oil using oil wicks or oil felt, lubricants with a lower viscosity should be used. For thin oil circulation lubrication systems, lubricating oils with low viscosity and good oxidation stability should be selected. A grease with a high penetration index should be used in centralized dry oil lubrication systems. 4. Selection of lubricating materials for common friction pairs and equipment – The operation and maintenance manuals for various mechanical devices recommend suitable lubricating materials. In principle, it should be used according to the required variety and grade. When domesticating the oils used for imported equipment, addressing temporary shortages of substitutes, and improving lubrication performance, it is necessary to select materials with comparable or superior properties, taking into account the characteristics of the originally used lubricants and the lubrication requirements of the equipment. (1) Lubrication of machine tool spindle bearings Machine tool spindle bearings include sliding bearings, rolling bearings, and hydrostatic bearings. ① Sliding bearings: there are single oil wedge and multiple oil wedge types. Multi-oil wedges can be further divided into multi-wafer adjustable types and integral types. The operating clearance of sliding bearings is generally between 0.004 and 0.06 mm. An oil film is formed at the friction area between the oil wedge and the components, reducing the friction between the spindle and the bearings. When selecting lubricating oil, factors such as the bearing clearance, operating load, sliding speed, and operating temperature need to be taken into account. ② Rolling bearings: The bearings used in machine tool spindles are generally high-precision bearings. The breaks between tasks are quite short. When sharing a lubrication system with gear drives, lubricating oil should be used for lubrication. Lubricating grease is used to lubricate the compact and lightweight moving spindle components. ③ Hydrostatic bearing: Relies on pressurized oil to keep the spindle in a state of complete fluid friction within the bearing. Hydrostatic bearings have a high load-carrying capacity, high operating speeds, and good precision retention. The choice of lubricating oil depends on factors such as the throttling type and load level. (2) Lubrication of guides: Guides are classified by spatial position into horizontal guides and vertical guides, and by mode of motion into sliding guides and rolling guides. Horizontal sliding guides are further divided into hydrodynamic guides and hydrostatic guides. The contact area of the guide rails is relatively large; under the action of the sliding seat and the load, the lubricating oil is easily squeezed away, resulting in uneven distribution of the oil and easy disruption of the oil film. The lubrication of hydrodynamic guide rails is a mixed lubrication of boundary lubrication and fluid lubrication. During low-speed, high-load movement, the guide rails experience significant static frictional resistance; the use of ordinary lubricants often leads to a \"creep\" phenomenon. Therefore, for the lubrication of horizontal guides on heavy-duty machine tools, L-G guide oils with good anti-creep properties are generally chosen. When using a single system for hydraulic and guide rail lubrication, L-HG hydraulic oil should be selected. For the guides of forging equipment and heavy machinery, high-viscosity L-AN full-loss oil is generally used. Cylinder oil or grease should be used when working in environments with high temperatures. (3) Lubrication of gear drives Gear drives are the most widely used type of mechanical transmission. In gear transmission, the specific load on the tooth contact line is very high, and sliding occurs. To prevent tooth surface scuffing and adhesion, the selection of lubricating oil should take into account factors such as transmission speed, load, and operating temperature. ① Light load: For gear transmissions where the surface load P is less than 500 N/mm2 or the maximum sliding speed of the gear surfaces is less than 1/3 of the tooth pitch, oils such as L-HI hydraulic oil for full-loss systems, or L-CKB industrial gear oil, should be used. ② Medium load: For gears that operate under a tooth surface load of 500–1100 N/mm2, it is advisable to use L-CKB gear oil with a higher viscosity, or L-CKC medium-load extreme pressure industrial gear oil with extreme pressure properties. ③ Heavy load: Gears that are subjected to a tooth surface stress greater than 1100 N/mm2 and operate at high temperatures should use L-CKD heavy-duty extreme pressure industrial gear oil. ④ For heavy-duty, high-friction worm gear transmissions, L-CKE or L-CKE/P worm gear oil should be used. ⑤ Vehicle gear oil requires good performance at both high and low temperatures. GL-3, GL-4, or GL-5 vehicle gear oil should be used. ⑥ For gear lubrication in gearboxes where no leakage is permitted, L-CKG gear lubricating oil or a mixture of L-X series industrial grease and lubricating oil can be used. ⑦ For open gear drives, L-CKH and L-CKM types of open gear oils are used. (4) Lubrication of air compressors: The cylinders and pistons of air compressors operate under high temperature and pressure. Lubricants are required to have a high flash point, good oxidation stability, and a low tendency to form carbon deposits. For oil-lubricated pistons or air compressors with an exhaust pressure of ≤1 MPa and an exhaust temperature of ≤160°C, and a stage pressure ratio of <3, or those with an exhaust pressure of >1 MPa and an exhaust temperature of ≤140°C and a stage pressure ratio of ≤3, L-DAA compressor oil should be used. Exhaust pressure ≤ 1 MPa, exhaust temperature: > 160°C, or exhaust pressure > 1 MPa and exhaust temperature 140–160°C℃ ； For oil-lubricated piston air compressors with a pressure ratio >3, L-DAB medium-load compressor oil should be used. Air compressors that have a strong tendency to develop carbon deposits in their exhaust systems should use L-DAC heavy-duty compressor oil. This is done to reduce carbon buildup in the exhaust system and prevent fires or explosions caused by such buildup. (5) Lubrication of machine tools: The selection of lubricants for machine tools is specified in the GB/T7632 standard. VII. Selection and Management of Hydraulic Oil 1. Selection of Hydraulic Oil (1) General requirements for the working fluid in hydraulic systems The working fluid used in hydraulic systems serves both as a medium for transmitting power and as a lubricant for hydraulic components. Hydraulic oils are usually formulated from refined mineral oils and synthetic oils; other petroleum products, synthetic fluids, or water-based emulsions can also be used. The following requirements apply to hydraulic fluids: ① It must have appropriate viscosity and good viscosity-temperature characteristics. The viscosity of the hydraulic fluid should be compatible with the system. Severe leakage occurs when the viscosity is too low, resulting in high power consumption ； Conversely, if the viscosity is too high, frictional resistance increases, wear is greater, and the system’s sensitivity is low. Hydraulic systems that are used intermittently in general, as well as those equipped with oil temperature control, should use hydraulic oil of lower viscosity. Hydraulic systems with high power output and long operating times should use hydraulic oil with higher viscosity. Hydraulic oil is also required to be less affected by temperature changes, that is, to have a high viscosity index. ② It has good lubricating and anti-friction properties. Hydraulic oil is used to lubricate components such as pumps, cylinders, and hydraulic valves. These components have high precision and small clearance, requiring hydraulic oil to possess good lubricity and anti-friction properties. Hydraulic systems and CNC machines operating under high pressure require good wear resistance and a high viscosity index to enhance system reliability. ③ It has good oxidation stability. Hydraulic systems require large amounts of oil, and it is necessary that the oil can be stored and used for an extended period of time. Gums, asphalts, carbon residues, and other substances formed as a result of the oxidation and deterioration of oil are highly harmful to hydraulic systems. An increase in oil temperature accelerates the oxidation process of the oil; therefore, antioxidant and anti-corrosion additives should be added to hydraulic oil to enhance its resistance to oxidation and corrosion. ④ It has excellent anti-foaming properties. Hydraulic systems can develop foam due to poor sealing of the pipes and the absorption of air at the oil inlet, which leads to unstable pressure, slow operation, as well as shocks and vibrations. The antifoaming additives added to hydraulic oil can effectively accelerate foam breakdown and improve the antifoaming performance of the hydraulic oil. ⑤ Free from moisture and mechanical impurities. Moisture can cause corrosion of hydraulic components; generally, the water content in hydraulic oil should be ≤0.025%. Mechanical impurities can block oil passages and valve holes, affecting the proper operation of the system. Generally, new oil is not allowed to contain mechanical impurities. The hydraulic oil in use can become contaminated with impurities and wear debris, resulting in an excessive level of impurities. Oil should be filtered at this time. ⑥ It has good rust resistance. Water and water-soluble acids or bases in hydraulic oil can cause corrosion and rusting of metals. Hydraulic oil should have good rust resistance to protect hydraulic components from corrosion. ⑦ Depending on the application area of hydraulic oil, some equipment requires it to have good performance at high and low temperatures ； Some require high-temperature resistance and fireproof properties. It must be selected correctly based on the operating conditions of the hydraulic oil. (2) Selection of hydraulic oil: The oil used in hydraulic systems depends mainly on factors such as the structure of the oil pump, operating temperature, operating pressure, and the environment. ① Hydraulic pump: The hydraulic pump is the heart of a hydraulic system. A hydraulic pump of a certain design has an allowable range for the viscosity of the oil; the minimum allowable viscosity of the oil depends on the size of the lubrication and sealing clearances of the bearings. The maximum allowable viscosity of the fluid is limited by the pump’s suction capacity, which is determined by the pump’s design and manufacturing precision. The viscosity of oil used in general hydraulic systems is 10~150 mm2/s (at 40°C). ② Operating temperature: The operating temperature of the hydraulic system should generally be below 60°C, with temporary temperatures reaching up to 90°C. For light loads and low oil temperatures, hydraulic oil with a lower viscosity should be used ； Hydraulic oils with higher viscosity should be used in heavy-duty hydraulic systems operating at high temperatures. ③ Working pressure: When the system operates under high pressure, the viscosity of the oil should be high ； The system operates at low pressure, so the viscosity of the oil should be low. ④ Working environment: The type of hydraulic oil should be selected based on the working environment. For hydraulic machinery used outdoors in cold regions during winter, low-viscosity hydraulic oils such as L-HR, L-HV, and L-HS should be selected. ⑤ Operating conditions and leakage factors: Hydraulic cylinders that undergo reciprocating motion should use hydraulic oil with a low viscosity. Due to high system leakage, hydraulic oil with a higher viscosity should be used. ⑥ Requirements for equipment performance: The hydraulic system of CNC machine tools should use anti-wear hydraulic oil with good viscosity-temperature properties, or hydraulic oil specifically designed for NC equipment. Equipment that shares a single system for hydraulic and guide rail lubrication should use L-HG hydraulic oil with good anti-scuffing properties. (3) Common oils for hydraulic systems ① L-H group hydraulic oils are classified according to GB/T 11388.1. There are 5 types and 28 grades of L-H group hydraulic oils, with their quality specifications further divided into first-class and top-quality categories. ②L—TSA turbine oil boasts excellent antioxidant and rust-resistant properties, as well as a long service life. It can be used as oil for general hydraulic systems. ③ The spindle oil for the L—F group consists of two varieties, L—FC and L—FD, with 18 grades in total, and can be used as oil for general hydraulic systems. ④ Specialized hydraulic oils include aviation hydraulic oil, drilling rig hydraulic oil, marine hydraulic oil, etc. It is characterized by a wide temperature adaptation range. Certain hydraulic machinery for outdoor work can also be used. 2. Management of oils for hydraulic equipment (1) General requirements for the management of oils used in hydraulic equipment. The oils used in hydraulic equipment serve as a medium for transmitting power, as well as a lubricant for precision components such as pumps and valves. High cleanliness standards are required for the oil, and strict management must be implemented during its use. The requirements for the management of hydraulic oil are as follows: ① When topping up the hydraulic oil tank, only the specified types and grades of oil must be used; mixing oils of different types is not permitted. ② The oil filtering system must be strictly implemented to prevent oil contamination and ensure that the oil remains clean. ③ Maintain the required oil level to prevent air from being drawn in, which could affect the quality of the oil and the performance of the hydraulic system. ④ Regularly clean or replace the oil filter element. ⑤ Oil replacement should be based on quality. Replace it immediately if the oil quality exceeds the specified standards. ⑥ Prevent other liquids (cooling water, cutting oil, or coolant) from mixing into the hydraulic oil reservoir. Once the issue is identified, the cause must be determined; after thorough resolution, the oil tank should be completely cleaned and the oil replaced. ⑦ The oil temperature should be kept below 70°C to prevent the oil from oxidizing and deteriorating. Cooling should be used when the temperature is too high. (2) Purification of oil for hydraulic systems: To control the temperature rise of the oil, hydraulic systems typically have large oil sumps (tanks) and use large amounts of oil; frequent oil changes lead to waste of resources and increased costs. For general hydraulic applications, oils with good antioxidant and anti-corrosion properties should be chosen to extend the oil’s service life. The cleanliness of hydraulic oil has a significant impact on the performance of hydraulic systems. Mechanical impurities in the oil can block valve holes and oil passages, causing the system to malfunction. It can also cause wear to the oil pump bearings, plungers, vanes, and gears. In hydraulic systems, failures caused by substandard oil (wear-related failures and performance-related failures) account for over 80% of the total number of failures. Mechanical impurities in hydraulic oil originate from contamination during refueling, maintenance work, poor sealing, as well as wear debris. According to analysis, grinding debris accounts for over 75% of all mechanical impurities. Therefore, well-performing hydraulic systems are equipped with various types of oil filters to purify the oil and ensure the proper operation of the hydraulic system. ① Oil filter: The oil filter is an auxiliary component of the hydraulic system. There are various types such as mesh type, slit type, sintered type, and paper type. Based on filtration precision, they can be classified as phase-separated (100pm), standard (10μm), fine (5μm), and ultra-fine (1μm). The coarse filter is generally installed at the oil inlet, with a filtration particle size of 40–300 μm; its purpose is to prevent large particles of impurities from entering the oil pump and causing damage to it. Hydraulic systems are equipped with fine filters at the pump’s output pipe to remove small particle impurities, protect valve components, and reduce system malfunctions. Some hydraulic systems are equipped with return oil filters or separate circulation filtering devices that continuously remove impurities generated in the oil during operation, thereby purifying the oil. Various oil filters should be cleaned and inspected regularly; any damage detected should be repaired or replaced immediately. When a paper oil filter has poor oil filtration capacity, its filter element should be replaced right away. ② Hydraulic oil purification system: If the viscosity, moisture content, and acid value of the hydraulic oil meet the quality requirements, it can still be used after filtration. The purification protocol for hydraulic oil requires the following: a. Filtering every time it is serviced: Maintenance of the hydraulic system can lead to contamination of the oil. It is necessary to carry out maintenance work, clean the oil filter, as well as clean the oil tank and filter and purify the oil ； b. Regular filtration: To remove wear debris and organic oxidation products generated in the oil, a filtration and purification system is established based on the operating conditions of the equipment. Generally, fine filtration should be carried out every 1500 hours, in conjunction with filtering and purification, as well as the cleaning of the oil tank and filters. VIII. Substitution of Lubricants and Selection for Higher Performance 1. Substitution of Lubricants (1) Domestic substitution for lubricants used in imported equipment. The lubricants recommended for use in foreign-imported equipment vary depending on the country of production, the manufacturer, and the age of the equipment; as a result, there is a wide variety of lubricant types and grades. Since the International ISO Organization issued a series of standards for lubricants, the standards for petroleum products and lubricants in various countries have also begun to align with international standards. Develop and adopt new standards that correspond to ISO standards. At this time of transition between old and new standards, the substitution of imported equipment’s lubricating materials with domestic alternatives must be approached with great caution. You can follow these steps to make the selection: ① Determine the category and key quality specifications of the lubricant by referring to the grade and manufacturer name of the lubricant recommended in the equipment’s operating manual. ② If possible, the remaining lubricating oil in the oil pool should be sampled for the determination of physical and chemical properties. ③ By taking into account the friction properties of the lubricated parts of the equipment, the magnitude of the load, the lubrication method, and the requirements of the lubrication systems, and by comparing them with the performance and key quality indicators of domestically produced lubricants, an alternative grade is determined. ④ The general viscosity index should be consistent with the viscosity of the recommended oil, or slightly higher by 5% to 15% of the recommended oil viscosity (at 40°C dynamic viscosity). ⑤ The additives contained in oils generally do not have testing capabilities available to the users; therefore, it is advisable to choose oils with good performance and high quality. For hydraulic oils, it is preferable to use L-HM or L-HV hydraulic oil, rather than L-HL hydraulic oil. For circulating lubrication oils, it is advisable to avoid using L-AN fully consumed oils and instead opt for L—HL hydraulic oil. ⑥ Pay attention to changes in the device’s performance during use, and thoroughly investigate the causes of any issues that arise. (2) Substitution for temporary oil shortages: In equipment lubrication management, temporary shortages of certain oils can sometimes occur. To ensure the proper operation of the equipment, substitute oils must be selected. As a general rule, when replacing lubricating oil, it is advisable to use an oil of the same type or one with similar properties, with viscosity levels that are comparable or slightly higher. When replacing the oil, the oil reservoir must be cleaned thoroughly of any residual oil to avoid adverse reactions. Generally, L-HL hydraulic oil can be used as a substitute for L-AN fully lost foam oil ； Replace L-HI with L-FC, L-FD spindle oil, and L-TSA turbine oil. Hydraulic oil ； Replace L-HM hydraulic oil with L-HV and L-HS hydraulic oil ； Replace L-G guide rail oil with L-HG hydraulic oil, etc. However, the replacement of spindle oil in precision machine tools must be approached with caution. 2. High-performance alternatives for lubricant materials (1) The necessity for high-performance lubricant materials: The pursuit of higher performance, greater reliability, automation, faultlessness, and maintenance-free operation in modern equipment imposes higher demands on the performance and quality of lubricant materials. Achieving higher performance in lubricant materials is of great significance for improving both economic and social benefits for enterprises. Products such as mechanical oils and water-containing calcium-based greases, which were widely used in equipment in the past, have poor performance and a short service life; their use should be gradually reduced. According to analysis, using high-performance lubricants can reduce the waste of labor and resources resulting from frequent oil changes. The overall benefits can result in savings of over 20%. Therefore, it is essential to improve the performance of lubricant materials. (2) Ways to improve the performance of lubricating materials: ① For large lubrication oil tanks, use oils with good antioxidant and anti-corrosion properties to reduce the use of L-AN oil. The oil used in L-AN fully loss systems has poor antioxidant and anti-corrosion properties, and is mainly used for loss-type lubrication. Replacing the oil reservoir in existing equipment with L-HL hydraulic oil can extend the service life of the lubricating oil. ② Increase the use of multi-effect lithium-based greases, and reduce the application scope of calcium-based and sodium-based greases. Extends the service life of lubricant and reduces the workload associated with maintenance and replacement. ③ For lubricants used in special applications, high-performance synthetic lubricants should be selected based on the operating conditions, in order to reduce the use of ordinary lubricants and improve the reliability of lubrication. ④ Use energy-saving anti-wear additives to improve the lubrication performance of equipment. Currently, the market offers a variety of imported and domestic energy-saving anti-wear agents. There are polymer-based anti-friction materials as well as ultra-fine alloy anti-friction materials, both of which are said to possess the ability to repair and heal themselves from wear. When using it, start with a small trial amount; once the effects and experience are gained, then expand its application scope. IX. Properties and Applications of Common Lubricant Materials The production of lubricant materials is at a transition point between old and new standards, and new standards for some lubricant materials have not yet been established. The products also use old standards and corporate standards.