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Basic properties of lubricating oils: Lubricating oils are technology-intensive products that are mixtures of complex hydrocarbons, and their actual performance in use is the result of a combination of complex physical or chemical processes. The basic properties of lubricating oils include general physical and chemical properties, special physical and chemical properties, and simulation bench tests. General physical and chemical properties: Each type of lubricant has its common general physical and chemical properties, which indicate the inherent quality of that product. For lubricating oils, these general physical and chemical properties are as follows: Appearance (colority) – the color of the oil often reflects its degree of refinement and stability. For base oils, generally the higher the degree of refining, the more thoroughly the hydrocarbon oxides and sulfides are removed, resulting in a lighter color. However, even under the same refining conditions, the base oils produced from crude oils of different origins and parent categories may vary in color and transparency. For new finished lubricating oils, due to the use of additives, color has lost its original significance as an indicator of the degree of refinement of the base oil. Density is the simplest and most commonly used physical property indicator for lubricants. The density of lubricating oil increases as the amount of carbon, oxygen, and sulfur in its composition rises; therefore, at the same viscosity or the same relative molecular mass, lubricating oils with a high content of aromatics, as well as those with high levels of gums and asphalts, have the highest density, those with a high content of naphthenes have a moderate density, and those with a high content of alkanes have the lowest density. Viscosity: Viscosity reflects the internal friction of an oil and is an indicator of its lubricity and fluidity. Without the addition of any functional additives, the higher the viscosity, the greater the strength of the oil film and the poorer its fluidity. Viscosity index: The viscosity index indicates the degree to which the viscosity of a oil changes with temperature. The higher the viscosity index, the less the viscosity of the oil is affected by temperature, indicating better viscosity-temperature performance; otherwise, the performance is worse. Flash point: The flash point is an indicator of the volatility of oils. The lighter the fraction of the oil, the greater its volatility, and the lower its flash point as well. Conversely, the heavier the distillate of the oil, the lower its volatility, and the higher its flash point. At the same time, the flash point is an indicator of the fire hazard of petroleum products. The hazard level of oils is determined based on their flash point: oils with a flash point below 45°C are considered flammable, while those with a flash point above 45°C are considered combustible. It is strictly prohibited to heat oils to their flash point temperature during storage and transportation. At the same viscosity, the higher the flash point, the better. Therefore, when selecting lubricating oil, users should choose based on the operating temperature and the working conditions of the lubricant. It is generally believed that a flash point 20–30°C higher than the operating temperature ensures safe use. Freezing point and pour point: The freezing point refers to the highest temperature at which the oil stops flowing under specified cooling conditions. The solidification of oils is very different from that of pure compounds. There is no definite solidification temperature for oils; the so-called “solidification” simply means that the oil, as a whole, loses its fluidity. Not all of its components necessarily turn into solids. The freezing point of lubricating oil is an important quality indicator that reflects its flowability at low temperatures. It is of great significance for production, transportation, and use. Lubricants with a high freezing point cannot be used at low temperatures. On the contrary, in areas with higher temperatures, there is no need to use lubricants with a low freezing point. Because the lower the freezing point of the lubricating oil, the higher its production cost, resulting in unnecessary waste. Generally, the freezing point of lubricating oil should be 5~7°C lower than the lowest temperature of the operating environment. It is particularly important to note that when selecting a lubricant for low temperatures, comprehensive consideration should be given to the oil’s freezing point, viscosity at low temperatures, and viscosity-temperature characteristics. Because oils with low freezing points may also have low-temperature viscosity and viscosity-temperature properties that do not meet the requirements. Both the freezing point and the pour point are indicators of the low-temperature flowability of oils; there is no fundamental difference between them, only slight variations in the methods of measurement. The freezing point and pour point of the same oil product are not exactly equal; generally, the pour point is 2–3°C higher than the freezing point, though there are exceptions. Acid value, alkali value, and neutralization value: The acid value is an indicator of the acidic substances present in lubricating oils, with the unit being mgKOH/g. The acid value is divided into strong acid value and weak acid value; the sum of the two gives the total acid value (abbreviated as TAN). What we usually refer to as the “acid value” actually means “Total Acid Number (TAN)”. The alkalinity value is an indicator of the content of basic substances in lubricating oil, with the unit being mgKOH/g. Alkalinity is also divided into strong alkalinity and weak alkalinity; the sum of these two gives the total alkalinity (abbreviated as TBN). What we usually refer to as “alkalinity” actually means “total alkalinity (TBN)”. The neutralization value actually includes the total acid value and the total base value. However, unless otherwise specified, the so-called “neutralization value” generally refers only to the “total acid value,” which is also expressed in mgKOH/g. Moisture: Moisture refers to the percentage of water contained in lubricant, usually expressed as a weight percentage. The presence of water in lubricating oil can destroy the oil film formed by the lubricant, reducing its lubricating effect, accelerating the corrosion of metals by organic acids, causing equipment to rust, and leading to the formation of sediment in the oil. In short, the less moisture in the lubricant, the better. Mechanical impurities refer to precipitates or gelatinous suspensions present in lubricating oils that are insoluble in solvents such as gasoline, ethanol, and benzene. Most of these impurities are sand, gravel, iron shavings, and other such substances, as well as some organometallic salts that are insoluble in solvents and result from additives. Generally, the mechanical impurities in lubricant base oils are kept below 0.005% (levels below 0.005% are considered to be absent). Ash and sulfuric ash: Ash refers to the non-combustible substances that remain after burning under specified conditions. The composition of ash is generally considered to be various metal elements and their salts. Ash content has different meanings for various types of oils; for base oils or oils without additives, ash content can be used to determine the degree of refinement of the oil. For oils with metal salt additives (new oil), ash content becomes a means to quantitatively control the amount of additives added. Abroad, sulfuric acid ash is used in place of ash. The method is: add a small amount of concentrated sulfuric acid after the oil sample is burned but before ash calcination, so that the metal elements of the additive are converted into sulfates. Residue: The charred black residue formed after the heating, evaporation, and combustion of oils under specified experimental conditions is known as residue. Residue is an important quality indicator for lubricant base oils, and it is a parameter specified to determine the properties of the lubricant and the degree of its refining. In lubricant base oils, the amount of residue is determined not only by its chemical composition but also by the degree of refining of the oil. The main substances that cause residue formation in lubricants are gums, asphaltenes, and polycyclic aromatic hydrocarbons present in the oil. Under conditions of insufficient air, these substances undergo decomposition and condensation under high heat to form residue carbon. The deeper the refining of the oil product, the lower its residue value. Generally speaking, the lower the carbon residue value of the base oil, the better. Today, many oils contain additives made of metals, sulfur, phosphorus, and nitrogen; these additives result in high carbon residue values. As a result, the carbon residue measurement for oils containing such additives has lost its original meaning. Mechanical impurities, moisture, ash, and residue are all quality indicators that reflect the purity of oil products, indicating the degree of refinement of the lubricating base oil. Special physical and chemical properties: In addition to the aforementioned general physical and chemical properties, each type of lubricant should also possess special physical and chemical characteristics that reflect its specific usage properties. The higher the quality requirements, or the greater the specificity of the oil, the more pronounced its special physical and chemical properties become. The test methods for assessing these special physical and chemical properties are briefly described as follows: Oxidation stability. Oxidation stability indicates the anti-aging performance of lubricants; many industrial lubricants with a long service life are required to meet this criterion, making it a key property for such types of lubricants. There are many methods for determining the oxidation stability of oils. Basically, a certain amount of oil is oxidized at a specific temperature for a set period of time in the presence of air (or oxygen) and metal catalysts, after which the acid value, changes in viscosity, and the formation of precipitates in the oil are measured. All lubricants have different tendencies toward autoxidation, depending on their chemical composition and the external conditions in which they find themselves. Oxidation occurs over time as it is used, gradually resulting in the formation of aldehydes, ketones, acids, as well as substances such as gums and asphalts. Oxidation stability is the property that prevents the formation of these substances that are detrimental to the use of oil products. Thermal stability refers to the ability of an oil to withstand high temperatures; in other words, it is the lubricant’s resistance to thermal decomposition, which is expressed by the thermal decomposition temperature. Requirements for thermal stability have been established for some high-quality anti-wear hydraulic oils, compressor oils, and similar products. The thermal stability of oils depends primarily on the composition of the base oil; many additives with low decomposition temperatures tend to have an adverse effect on the stability of the oils ； Antioxidants also cannot significantly improve the thermal stability of oils. Oiliness and extreme pressure properties: Oiliness arises from the polar substances in lubricants forming a strong physicochemical adsorption film on the metal surfaces at the points of friction, thereby providing resistance to high loads and frictional wear. Extreme pressure properties, on the other hand, result from these polar substances decomposing due to frictional chemical reactions under high temperatures and heavy loads on the metal surfaces at the points of friction; these substances then undergo frictional chemical reactions with the surface metal to form a soft (or plastic) extreme pressure film with a low melting point, which provides resistance to impacts as well as high loads and high temperatures. Corrosion and rusting: The oxidation of oils or the effect of additives often leads to corrosion of steel and other non-ferrous metals. In corrosion testing, a copper bar is usually placed in oil and left at 100°C for 3 hours, after which the changes in the copper are observed ； The corrosion test involves the formation of rust on the surface of steel under the influence of water and moisture. To determine the rust resistance, 30 ml of distilled water or artificial seawater is added to 300 ml of test oil, a steel rod is placed in this mixture, and it is stirred at 54°C for 24 hours; thereafter, it is checked whether rust has formed on the steel rod. Oils should have resistance to metal corrosion and rust prevention; in industrial lubricant standards, these two properties are typically required to be tested. Antifoam property: During operation, lubricants often form foam due to the presence of air. This is especially true when the oil contains surfactant additives, as foam is more likely to form and it is also difficult for such foam to disappear. Foam formation during the use of lubricant can damage the oil film, cause sintering of the friction surfaces or increase wear, accelerate the oxidation and deterioration of the lubricant, and also cause air blockages in the lubrication system, affecting the circulation of the lubricant. Therefore, antifoaming property is an important quality indicator for lubricants and the like. Hydrolytic stability: Hydrolytic stability characterizes the stability of oil products when exposed to water and metals (primarily copper). When the acid number of an oil product is high, or when it contains additives that readily decompose into acidic substances upon contact with water, this parameter often fails to meet the required standards. Its determination method involves adding the test oil to a certain amount of water, mixing and stirring it with a copper sheet at a specified temperature for a certain period of time, and then measuring the acid value of the water layer and the weight loss of the copper sheet. Emulsification resistance: Industrial lubricants often inevitably come into contact with some cooling water during use. If the lubricant lacks good emulsification resistance, it will form an emulsion with the water that has mixed in, making it difficult to remove the water from the bottom of the circulation tank – which can lead to poor lubrication. Therefore, anti-emulsibility is a very important physicochemical property of industrial lubricants. For ordinary oils, 40 ml of the oil sample is mixed vigorously with 40 ml of distilled water at a certain temperature for a specified period of time, after which the time it takes for the oil layer, water layer, and emulsion layer to separate into 40-37-3 ml is observed ； Industrial gear oil is prepared by mixing the oil with water, stirring at a certain temperature and at 6,000 revolutions per minute for 5 minutes, allowing it to stand for 5 hours, and then measuring the volumes of oil, water, and the emulsion layer. The air release value is a requirement specified in the hydraulic oil standards; in hydraulic systems, if the air dissolved in the oil is not released in a timely manner, it will affect the accuracy and sensitivity of hydraulic transmission, and in severe cases, it will prevent the hydraulic system from meeting its functional requirements. The method for measuring this property is similar to that for antifoaming capacity, except that it measures the time it takes for the air (mist) dissolved within the oil to be released. Rubber sealing performance: In hydraulic systems, rubber is commonly used as a sealing material. In mechanical devices, oils inevitably come into contact with various sealing elements. Oils that have poor sealing properties can cause the rubber to swell, shrink, harden, or crack, thereby affecting its sealing ability; therefore, it is necessary for oils to have good compatibility with rubber. Hydraulic oil standards require a rubber sealing index, which is measured by the change that occurs in a rubber ring of a certain size after being immersed in oil for a specified period of time. Shear stability: In oils to which thickeners have been added, mechanical shear acts during use, causing the high-molecular polymers in the oil to be broken down, which reduces the viscosity of the oil and interferes with proper lubrication. Therefore, shear stability is a specific physical and chemical property that must be measured for such oils. There are many methods for measuring shear stability, including the ultrasonic shear method, the nozzle shear method, the Wicks pump shear method, and the FZG gearbox shear method; all of these methods ultimately aim to determine the rate of viscosity decrease in the oil. Solubility: Solubility is usually expressed in terms of the aniline point. The solubility limit aniline point of composite additives varies with different grades of oils; the limit value for oils with low ash content is higher than that for highly alkaline oils, and the limit value for single-grade oils is higher than that for multi-grade oils. Volatility: The volatility of the base oil is related to fuel consumption, viscosity stability, and oxidation stability. These properties are particularly important for multi-stage oils and energy-saving oils. Rust resistance: This refers specifically to the particular physical and chemical properties that rust-proof greases should possess. The testing methods for this include humidity tests, salt spray tests, laminate tests, and water displacement tests; in addition, there are also greenhouse tests and long-term storage tests. Electrical properties: Electrical properties are unique characteristics of insulating oils, including the dielectric loss angle, dielectric constant, breakdown voltage, pulse voltage, etc. The degree of refinement of the base oil, as well as impurities and moisture, all have a significant impact on the electrical properties of the oil. Special physical and chemical properties of lubricating greases: In addition to their general physical and chemical properties, greases designed for specific applications possess special physical and chemical characteristics as well. Greases with good water resistance are required to undergo a water test ； For low-temperature grease, low-temperature torque needs to be measured ； Multi-effect greases need to be tested for extreme pressure and wear resistance as well as rust prevention properties ； Long-life lubricants require bearing life tests and the like. There are also corresponding test methods for measuring these properties. Other special physical and chemical properties: In addition to their general properties, each type of oil should have its own unique special properties. For example, the cooling rate of quenching oil needs to be determined ； The emulsification stability of the emulsified oil needs to be determined ； The anti-creep coefficient of hydraulic guide rail oil needs to be measured ； The spread of oil mist needs to be measured for spray lubricants ； The freezing point and flocculation point of refrigeration oil need to be measured ； Low-temperature gear oil needs to be tested for pitting, etc. All these properties require a special chemical composition of the base oil, or the addition of certain special additives to ensure them. Instructions for using lubricating oil: Oil storage: 1. It must not be stored upright in an outdoor environment to prevent contamination by moisture and debris. 2. For indoor storage, it can be placed upright with the lid facing up for easy access. 3. Tighten the lid to keep the oil drum sealed. 4. Keep the surface of the barrel clean and the markings clear. 5. Keep the floor clean to facilitate early detection of oil leaks. 6. Keep proper records of inventory entry, with first arrival determining priority. 7. The oil that is drawn frequently is placed on oil drum racks, with the flow of oil being controlled by switches. 8. New oil and used oil should be stored separately; containers that have held used oil must not be used for new oil to prevent contamination. Oil product safety: 1. Oil products should be stored in separate containers, with no flammable materials placed around them. 2. Open flames are strictly prohibited; no kindling is allowed in the oil depot. 3. Be equipped with no fewer than two fire extinguishers. 4. The rags used for wiping the machinery or the oil residues removed therefrom must not be piled up, to prevent fire propagation. 5. Flammable specialty oils or chemical solvents should be stored separately, with flammable material signs placed on them. Usage notes: 1. Consult a lubrication expert to use lubricants of the appropriate specification, and minimize the number of different types of oil used. 2. Each machine is illustrated with simple diagrams showing the areas that require refueling, the type of oil to be used, and the refueling interval; a designated person is responsible for ensuring that the correct type of oil is used. 3. Clean and wipe containers and tools such as oil pumps and oil cans before each refueling. 4. A separate container for each type of oil, with the name of the oil contained therein indicated on the container to prevent contamination. 5. Before changing the oil, the machinery must be thoroughly rinsed with a solvent; water-soluble cleaners must not be used. 6. Keep proper maintenance records for the machinery after each addition or replacement of lubricant. 7. If any abnormalities are detected in the oil or if it has reached the time for oil change, samples should be taken and sent to a professional company for testing. Environment and health: 1. It is strictly prohibited to discharge used oil directly into sewers or soil to prevent environmental pollution. 2. Used oil and waste liquids should be collected in dedicated containers and then handed over to **licensed recyclers for disposal; they must not be dumped carelessly. 3. People with skin allergies or scratches should avoid direct contact with lubricants. 4. Never wear clothes soaked with oil, and do not put oil-contaminated rags into a bag. 5. Do not use dirty cloth scraps to wipe away oil from the skin, as metal shards hidden in those scraps could scratch the skin and cause infections. Glossary of technical terms: Abrasive wear: Mechanical wear that occurs as a result of the relative sliding of two contacting surfaces. Additives: Small amounts of substances added to improve lubrication properties. Adhesion-improving additives: Additives incorporated into oils and greases to enhance adhesion (such as polyisobutylene). Adhesive lubricants: Lubricants to which adhesion-improving additives are added in order to prevent them from being flung off due to centrifugal forces. AF coatings, anti-friction coatings: The most widely used dry film solid lubricants, including those that cure at room temperature and those that require heat curing. The formula contains solid lubricant materials (referred to as “raw materials”) and bonding materials; see “Binders”. Anti-aging: The aging of materials caused by oxidation, overheating, or the presence of certain metals (such as copper, lead, silver, etc.) can be mitigated by adding certain additives (such as antioxidants). ASTM: American Society for Testing and Materials. Base oil: The fundamental component of lubricants and greases. Binder: A non-volatile medium or filler used to enhance the strength of the bond between solid lubricant particles or to improve the adhesion between the solid lubricant film and the friction surface. Loosening torque: The torque required to loosen a bolted connection. Chemical inertness: The ability of a lubricant not to react chemically with certain substances. Coefficient of friction: The ratio of the frictional force between two contacting surfaces to the normal force. Low-temperature performance: For lubricants, this is indicated by cloud point, pour point, and freezing point; for greases, it can be measured using Kesternich flow pressure and low-temperature torque tests. Colloid: Particles (with particle sizes ranging from 10-5 to 10-7 cm) in a stable liquid, existing as a solute without any particle sedimentation. Composite grease: A grease made from metal soaps and thickeners derived from various acids, particularly suitable for high-temperature and long-term use