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The kinematic viscosity index of lubricating oils is important for the lubrication of equipment under heavy loads; certain extreme pressure or anti-wear additives are added to greases to enhance their extreme pressure and anti-wear properties. The extreme pressure and anti-wear properties of grease are very important indicators; poor extreme pressure and anti-wear performance can lead to severe wear of equipment, resulting in equipment damage and accidents. What is the significance of the kinematic viscosity index of lubricating oil? (1) Density Density is a simple and commonly used physical property indicator for lubricating oil. The density of lubricating oil increases as the amounts of carbon, oxygen, and sulfur in its composition increase. 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 higher density; those with a high content of naphthenes have a density in between, while those with a high content of alkanes have lower density. (2) Appearance (Chromaticity) The color of the oil often reflects its degree of refinement and stability. For base oils, generally the higher the degree of refinement, 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 sources and parent groups may vary in color and transparency. For new finished lubricants, due to the use of additives, color has lost its original significance as an indicator of the degree of refinement of the base oil. (3) Viscosity index The viscosity index indicates the extent to which the viscosity of an 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. (4) Viscosity Viscosity reflects the internal friction of an oil and is an indicator of its viscosity 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. (5) 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 classification of oils is determined by their flash point: oils with a flash point below 45°C are considered flammable, while those with a flash point above 45°C are deemed combustible. During storage and transportation, it is strictly prohibited to heat oils to their flash point temperature. 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 oil. It is generally believed that a flash point 20–30°C higher than the operating temperature ensures safe use. (6) 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 the two is the total alkalinity (abbreviated as TBN). What we usually refer to as “alkalinity” actually means “Total Basic Number (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. (7) Freezing point and pour point The freezing point is the highest temperature at which the oil stops flowing under specified cooling conditions. The solidification of oils differs greatly from that of pure compounds. Oil does not have a definite freezing temperature; the so-called “freezing” simply means that it loses its fluidity as a whole, and not all of its components turn into solids.