Common Lubricating Oil Knowledge
1. Classification of lubricants
There are numerous types of lubricants, but they can generally be classified into four major categories based on their physical state: liquid lubricants, semi-solid lubricants, solid lubricants, and gaseous lubricants.
According to the provisions of GBT 498-2014 "Classification Methods and Determination of Categories for Petroleum Products and Lubricants," lubricants and related products are classified as L-category products. Therefore, the general code for lubricants is L, meaning that the first letter of all lubricant codes is L.
(1) Liquid lubricants: including mineral lubricating oils, synthetic lubricating oils, animal and vegetable oils, water-based liquids, etc.
(2) Semi-solid lubricants (grease): Grease appears as a semi-fluid paste at room temperature and pressure, hence also known as solid lubricants. It is formed by thickening base lubricating oil with a thickener in a specific ratio.
(3) Solid lubricants: Solid lubricants are substances that exist in solid form between friction interfaces to provide lubrication, including soft metals, metal compounds, organic and inorganic materials. Commonly used solid lubricating materials in general industry include molybdenum disulfide, graphite, and polytetrafluoroethylene.
(4) Gas Lubricants: Like liquids, gases are fluids and follow the same physical laws of fluids. Therefore, under certain conditions, gases can also serve as lubricants, similar to liquids. Commonly used gas lubricants include air, helium, nitrogen, argon, and others.
(5) Lubricating oil: Lubricating oil is a liquid lubricant, generally referring to mineral oil and synthetic oil, especially mineral lubricating oil.
2. Lubricating oil codes and their meanings
(1) According to the provisions of GBT 7631.1-2008 "Classification of Lubricants and Related Products (Category L) - Part 1: General Grouping," the code for lubricating oil consists of a category, type, and number, written in the form: category + type + number.
(2) Categories refer to the classification of petroleum products, with lubricants being one of them. Lubricant materials are denoted by the letter L.
(3) The category refers to the grouping of lubricating oils, classified according to their application scenarios and represented by corresponding letters: A—Total loss systems; C—Gears; D—Compressors; E—Internal combustion engines; F—Stators, bearings, clutches; G—Guide wheels; H—Hydraulic systems; M—Metalworking; P—Pneumatic tools; T—Steam turbines; Z—Steam cylinders, etc. These are the initial letters in the category column. In practice, the category column may also include one or more additional letters to indicate further subdivisions of the category.
(4) The numbers represent the viscosity grade of the lubricating oil, with their values corresponding to the intermediate kinematic viscosity at 40°C (some may be batch numbers, but this must be specified; otherwise, it refers to 40°C). The unit is mm²/s. According to GB/T3141 "Industrial Liquid Lubricants—ISO Viscosity Classification," there are 20 grades in total: 2, 3, 5, 7, 10, 15, 22, 32, 46, 68, 100, 150, 220, 320, 460, 680, 1000, 1500, 2200, and 3200.
Example: L-AN100 indicates a general-purpose lubricating oil with a viscosity grade of 100 mm²/s, whose kinematic viscosity at 40°C ranges from 90 to 110 mm²/s, with the median kinematic viscosity being 100 mm²/s.
3. Quality indicators of lubricating oil:
The quality indicators of lubricating oil can be divided into two major categories: one is the physical and chemical performance indicators of the oil, and the other is the application performance indicators of the oil. (Mainly introduces several key physical and chemical indicators)
(1) Color: The color of lubricating oil, like all substances, has a corresponding and fixed hue, which is related to the refining degree of the base oil and the additives used. However, during use or storage, it may deteriorate due to oxidation, thereby changing its color, with the degree of discoloration correlating to the extent of deterioration. For instance, a milky white appearance indicates the presence of water or air bubbles; a darker color suggests oxidation deterioration or contamination.
(2) Viscosity: Viscosity is the degree of internal friction resistance in lubricating oil, that is, a measure of internal friction. Viscosity is generally divided into three types: dynamic viscosity, kinematic viscosity, and relative viscosity.
Viscosity is the primary indicator for the classification, grading, quality assessment, selection, and substitution of various lubricating oils.
Relative viscosity: Relative viscosity is the viscosity expressed in conditional units measured using different specific viscometers, generally including three representation methods: Engler viscosity, Saybolt viscosity, and Redwood viscosity.
Kinematic viscosity: Kinematic viscosity is a measure of a liquid's internal friction when it flows under the force of gravity. The unit of measurement is mm²/s. In China, lubricating oils are classified into 20 grades based on their viscosity, known as viscosity grades.
Viscosity is an important quality indicator of lubricating oil. If the viscosity is too low, it can lead to semi-fluid lubrication or boundary lubrication, thereby accelerating the wear of friction pairs and also making oil leakage more likely. Conversely, if the viscosity is too high, the fluidity becomes poor, along with reduced permeability and heat dissipation, increased internal friction resistance, difficulty in starting, and higher power consumption. Therefore, selecting an appropriate viscosity is essential to ensure adequate lubrication of friction pairs.
(3) Viscosity-temperature characteristics: The property of lubricating oil that changes with temperature is called viscosity-temperature characteristics.
Currently, the viscosity index (VI) is commonly used to indicate the quality of viscosity-temperature characteristics. Generally, the higher the VI value of an oil, the greater its viscosity changes with temperature, making it more suitable for applications with variable or wide temperature ranges. The better the viscosity-temperature characteristics of the oil, the higher its VI. Oil with VI=0 is denoted as 0VI, and oil with VI=100 is denoted as 100VI. The viscosity index is an empirical value, derived by comparing the viscosity at 40°C and 100°C against two standard oils: one with good viscosity performance (VI set at 100) and another with poorer viscosity performance (VI set at 0).
(4) Pour point and cloud point
Pour Point: The pour point refers to the highest temperature at which lubricating oil ceases to flow under specified cooling conditions.
Pour point: The pour point is the lowest temperature at which a lubricating oil can still flow under specified cooling conditions.
Both pour point and cloud point indicate the low-temperature performance of lubricating oil, but the pour point better reflects the oil's low-temperature fluidity and is more practical in use than the cloud point. Therefore, the pour point is currently the primary international standard for representing the low-temperature performance of lubricating oil. The pour point is approximately 3°C higher than the cloud point, and the operating temperature of general lubricating oil is 3–4°C higher than the pour point.
(5) Flash Point: The flash point refers to the lowest temperature at which lubricating oil, when heated under specified conditions, produces vapors that mix with air to form a certain concentration and momentarily ignite upon contact with a flame.
(6) Acid value: Acid value refers to the mass of potassium oxide required to neutralize the organic acids contained in 1g of lubricating oil, measured in mg KOH/g.
The acid value has different implications for new and used oil. For new oil, the acid value indicates the refining depth of the oil product, while for used oil, it reflects the degree of oxidation and deterioration of the lubricant during use. An excessively high acid value signifies severe oxidative deterioration.
(7) Moisture: Moisture refers to the mass percentage of water content in lubricating oil. The presence of moisture can deteriorate the viscosity-temperature characteristics when the temperature drops below 0°C, and when the temperature rises, water vaporizes, generating bubbles that disrupt the oil film, causing the oil to emulsify, leading to reduced viscosity and poorer lubrication performance.
(8) Mechanical impurities: All solid impurities suspended and settled in lubricating oil are collectively referred to as mechanical impurities. The presence of mechanical impurities can damage the lubricating oil film and accelerate the wear of friction pairs.
(9) Carbon Residue: Carbon residue refers to the coke-like residue formed after lubricating oil is heated with air introduced, undergoing vaporization and decomposition, expressed as a percentage of the oil's weight.