I. lubricating oil function lubricating oil is a liquid lubricant used in various types of machinery to reduce friction and protect machinery and machined parts, and mainly plays the roles of lubrication, cooling, rust prevention, cleaning, sealing and buffering. Lubricating oil accounts for 85% of all lubricating materials, with various types and brands, and now the annual consumption in the world is about 38 million tons. The general requirements for lubricating oil are:
(1) antifriction and wear resistance, reducing friction resistance to save energy, reducing wear to prolong mechanical life and improve economic benefits;
(2) cooling, which requires the friction heat to be discharged from the machine at any time;
(3) sealing, which requires leakage prevention, dust prevention and gas channeling prevention;
(4) Anti-corrosion and anti-rust, which requires protecting the friction surface from oil deterioration or external erosion;
(5) clean washing, which requires cleaning and removing the scale on the friction area;
(6) stress dispersion and buffering, load dispersion, impact mitigation and shock absorption;
(7) Kinetic energy transfer, hydraulic system and remote control motor and friction stepless speed change, etc.
second, lubricating oil composition lubricating oil generally consists of two parts: base oil and additives. Base oil is the main component of lubricating oil, which determines the basic properties of lubricating oil. Additives can make up for and improve the shortcomings of base oil performance and give some new properties, which is an important part of lubricating oil.
1. Lubricating base oil
Lubricating base oil is mainly divided into two categories: mineral base oil and synthetic base oil. Mineral base oil is widely used, with a large dosage (about 95% or more), but in some applications, it is necessary to use products blended with synthetic base oil, thus making the synthetic base oil develop rapidly.
mineral oil base oil is refined from crude oil. The main production processes of lube base oil include: atmospheric and vacuum distillation, solvent deasphalting, solvent refining, solvent dewaxing, clay or hydrogenation supplementary refining. In 1995, the current standard of lube base oil in China was revised, mainly modifying the classification method, and adding two special base oil standards of low pour point and deep refining. The most important thing in the production of mineral lubricating oil is to choose the best crude oil.
the chemical composition of mineral base oil includes high boiling point, high molecular weight hydrocarbon and non-hydrocarbon mixture. Its composition is generally alkanes (linear, branched and multi-branched), cycloalkanes (monocyclic, bicyclic and polycyclic), aromatic hydrocarbons (monocyclic and polycyclic aromatic hydrocarbons), naphthenic aromatic hydrocarbons, organic compounds containing oxygen, nitrogen and sulfur, and non-hydrocarbon compounds such as gum and asphaltene.
2. Additives
Additives are the essence of modern advanced lubricating oil. Proper selection and reasonable addition can improve its physical and chemical properties, endow lubricating oil with new special properties, or strengthen its original properties to meet higher requirements. According to the required quality and performance of lubricating oil, careful selection, careful balance and reasonable blending of additives are the key to ensure the quality of lubricating oil. Commonly used additives include: viscosity index improver, pour point depressant, antioxidant, detergent dispersant, friction moderator, oiliness agent, extreme pressure agent, antifoaming agent, metal passivator, emulsifier, corrosion inhibitor, antirust agent and demulsifier.
introduction to lubricating oil: performance (II)
III. Basic performance of lubricating oil Lubricating oil is a technology-intensive product, a complex mixture of hydrocarbons, and its real performance is a comprehensive effect of complex physical or chemical changes. The basic properties of lubricating oil include general physical and chemical properties, special physical and chemical properties and simulated bench test.
general physical and chemical properties each kind of lubricating grease has its * * * same general physical and chemical properties to show the inherent quality of the product. For lubricating oil, these general physical and chemical properties are as follows:
(1) Appearance (chromaticity)
The color of oil can often reflect its refining degree and stability. For base oil, the higher the refining degree, the cleaner the removal of hydrocarbon oxides and sulfides, and the lighter the color. However, even if the refining conditions are the same, the color and transparency of base oil produced by crude oil from different oil sources and bases may be different.
For the new finished lubricating oil, because of the use of additives, color has lost its original meaning as an index to judge the degree of base oil refining.
(2) Density
Density is the simplest and most commonly used physical performance index of lubricating oil. The density of lubricating oil increases with the increase of the number of carbon, oxygen and sulfur in its composition. Therefore, under the same viscosity or relative molecular weight, the lubricating oil with more aromatic hydrocarbons, more gum and asphaltene has the highest density, the one with more naphthenes and the one with the least alkane.
(3) Viscosity
Viscosity reflects the internal friction of oil products, and it is an indicator of oiliness and fluidity of oil products. Without any functional additives, the higher the viscosity, the higher the oil film strength and the worse the fluidity.
(4) Viscosity index
Viscosity index indicates the degree to which oil viscosity changes with temperature. The higher the viscosity index is, the less the oil viscosity is affected by temperature, and the better its viscosity-temperature performance is, and vice versa.
(5) Flash point
Flash point is an indicator of the evaporation of oil products. The lighter the oil fraction, the greater the evaporation and the lower its flash point. On the contrary, the heavier the oil fraction, the smaller the evaporation and the higher its flash point. At the same time, flash point is an indicator of the fire danger of petroleum products. The hazard grade of oil products is classified according to their flash points. When the flash point is below 45℃, it is flammable, and when it is above 45℃, it is forbidden to heat the oil products to their flash point temperature during storage and transportation. At the same viscosity, the higher the flash point, the better. Therefore, users should choose lubricating oil according to the use temperature and working conditions of lubricating oil. It is generally believed that the flash point is 2 ~ 3℃ higher than the use temperature, so it can be used safely.
(6) Solidification point and pour point
Solidification point refers to the highest temperature at which oil stops flowing under specified cooling conditions. The solidification of oil products is quite different from that of pure compounds. There is no definite solidification temperature for oil products. The so-called "solidification" only loses its fluidity as a whole, and not all components become solid.
the freezing point of lubricating oil is an important quality index indicating the low-temperature fluidity of lubricating oil. It is of great significance for production, transportation and use. Lubricating oil with high freezing point cannot be used at low temperature. On the contrary, it is not necessary to use lubricating oil with low freezing point in areas with high temperature. Because the lower the freezing point of lubricating oil, the higher its production cost, resulting in unnecessary waste. Generally speaking, the freezing point of lubricating oil should be 5~7℃ lower than the lowest temperature in the use environment. However, it is especially important to mention that when selecting low-temperature lubricating oil, the freezing point, low-temperature viscosity and viscosity-temperature characteristics of the oil should be considered comprehensively. Because of the low freezing point oil, its low temperature viscosity and viscosity-temperature characteristics may not meet the requirements.
Both freezing point and pour point are indicators of low-temperature fluidity of oil products. There is no principle difference between them, but the determination method is slightly different. The freezing point and pour point of the same oil product are not completely equal. Generally, the pour point is 2 ~ 3℃ higher than the freezing point, but there are exceptions.
(7) Acid value, base number and neutralization value
Acid value is an index indicating that the lubricating oil contains acidic substances, and the unit is mg KOH/g. Acid value is divided into strong acid value and weak acid value, and the combination of them is the total acid value (TAN for short). What we usually call "acid value" actually means "total acid value (TAN)".
base number is an index indicating the content of alkaline substances in lubricating oil, and the unit is mg KOH/g.
the base number is also divided into two types: strong base number and weak base number, and the combination of them is the total base number (TBN). What we usually call "base number" actually refers to "total base number (TBN)".
Neutralization value actually includes total acid value and total base value. However, unless otherwise specified, the "neutralization value" generally refers only to the "total acid value", and its unit is also mg KOH/g.
(8) moisture
moisture refers to the percentage of water content in lubricating oil, usually by weight. The existence of moisture in lubricating oil will destroy the oil film formed by lubricating oil, make the lubricating effect worse, accelerate the corrosion of organic acids on metals, corrode equipment, and make oil products prone to sediment. In short, the less water in lubricating oil, the better.
(9) Mechanical impurities
Mechanical impurities refer to precipitates or colloidal suspended substances that exist in lubricating oil and are insoluble in solvents such as gasoline, ethanol and benzene. Most of these impurities are sand and iron filings, and some organic metal salts which are insoluble in solvents brought by additives. Generally, the mechanical impurities in lube base oil are controlled below .5% (mechanical impurities below .5% are considered as none).
(1) Ash and sulfuric acid ash
Ash refers to incombustible substances left after burning under specified conditions. The composition of ash is generally considered to be some metal elements and their salts. Ash has different concepts for different oils. For base oil or oil without additives, ash can be used to judge the refining depth of oil. For oil products (new oil) with metal salt additives, ash becomes a means to quantitatively control the amount of additives added. Abroad, sulfuric acid ash is used instead of ash. The method is as follows: a small amount of concentrated sulfuric acid is added before the oil sample is incinerated after combustion, so that the metal elements of the additive are converted into sulfate.
(11) Carbon residue
Under the specified experimental conditions, the burnt black residue formed by heating evaporation and combustion of oil products is called carbon residue. Carbon residue is an important quality index of lubricating oil base oil, and it is a specified item for judging the nature and refining depth of lubricating oil. The amount of carbon residue in lube base oil is not only related to its chemical composition, but also related to the refining depth of the oil. The main substances that form carbon residue in lube oil are gum, asphaltene and polycyclic aromatic hydrocarbons in the oil. Under the condition of insufficient air, these substances are decomposed and condensed by strong heat to form carbon residue. The deeper the refining depth of oil, the smaller the carbon residue value. Generally speaking, the smaller the carbon residue value of blank base oil, the better.
At present, many oil products contain additives of metal, sulfur, phosphorus and nitrogen, and their carbon residue value is very high, so the carbon residue of oil containing additives has lost its original meaning of carbon residue determination. Mechanical impurities, moisture, ash and carbon residue are all quality indexes reflecting the purity of oil products and the degree of refining of lubricating base oil.
special physical and chemical properties in addition to the above general physical and chemical properties, each lubricating oil should also have special physical and chemical properties that characterize its use characteristics. The higher the quality requirements, or the more specific the oil, the more prominent its special physical and chemical properties. The test methods reflecting these special physical and chemical properties are briefly introduced as follows:
(1) Oxidation stability
Oxidation stability indicates the anti-aging performance of lubricating oil, which is required by some industrial lubricants with long service life, and thus becomes a special performance required by these kinds of oils. There are many methods to determine the oxidation stability of oil products. Basically, a certain amount of oil products are oxidized at a certain temperature for a certain period of time in the presence of air (or oxygen) and metal catalysts, and then the acid value, viscosity change and precipitate formation of oil products are determined. All lubricating oils have different automatic oxidation tendencies according to their chemical composition and external conditions. Oxidation occurs with the use process, so some aldehydes, ketones, acids, gums, asphaltenes and other substances are gradually generated. Oxidation stability is the performance of inhibiting the generation of the above substances that are not conducive to the use of oil products.
(2) thermal stability
thermal stability indicates the high temperature resistance of oil, that is, the resistance of lubricating oil to thermal decomposition, that is, the thermal decomposition temperature. Some high-quality antiwear hydraulic oil and compressor oil all require thermal stability. The thermal stability of oil mainly depends on the composition of base oil, and many additives with low decomposition temperature often have adverse effects on the stability of oil. Antioxidants can not obviously improve the thermal stability of oil products.
(3) Oiliness and extreme pressure
Oiliness means that polar substances in lubricating oil form a solid physical and chemical adsorption film on the metal surface of the friction part, thus playing the role of high load resistance and friction and wear resistance, while extreme pressure means that polar substances in lubricating oil undergo tribochemical decomposition on the metal surface of the friction part under high temperature and high load, and react with the surface metal to form a soft (or plastic) extreme pressure film with low melting point, thus.
(4) Corrosion and rust
Steel and other non-ferrous metals are often corroded due to the oxidation of oil products or the action of additives. In general, the corrosion test is to put the red copper strip into oil, leave it at 1℃ for 3 hours, and then observe the change of copper; The corrosion test is that the steel surface will be corroded under the action of water and steam. To determine the rust resistance, 3ml distilled water or artificial seawater is added to 3ml oil test, then the steel bar is placed in it, stirred at 54℃ for 24 hours, and then the steel bar is observed for corrosion. Oil products should have the functions of metal corrosion resistance and rust prevention. In industrial lubricants standard, these two items are usually required.
(5) Anti-foaming property
In the running process of lubricating oil, foam often occurs due to the presence of air, especially when the oil contains additives with surface activity, it is easier to generate foam, and the foam is not easy to disappear. Foam produced in the use of lubricating oil will destroy the oil film, make the friction surface sinter or increase the wear, and promote the oxidation and deterioration of lubricating oil, and also make the lubricating system gas resistant and affect the circulation of lubricating oil. Therefore, foam resistance is an important quality index of lubricating oil.
(6) Hydrolytic stability
Hydrolytic stability indicates the stability of oil products under the action of water and metals (mainly copper). When the acid value of oil products is high or contains additives that can be easily decomposed into acidic substances in water, this index is often unqualified. Its determination method is to add a certain amount of water to the oil test, mix and stir the copper sheet at a certain temperature for a certain time, and then measure the acid value of the water layer and the weight loss of the copper sheet.
(7) Emulsification resistance
industrial lubricants often inevitably mixes some cooling water in use. If the emulsification resistance of lubricating oil is not good, it will form emulsion with the mixed water, making it difficult for water to be discharged from the bottom of the circulating oil tank, which may cause poor lubrication. Therefore, emulsification resistance is a very important physical and chemical property of industrial lubricants. Generally, 4ml of test oil and 4ml of distilled water are stirred violently at a certain temperature for a certain period of time, and then the time for separating the oil layer, water layer and emulsion layer into 4-37-3 ml is observed. Industrial gear oil is prepared by mixing test oil with water, stirring at a certain temperature and 6 rpm for 5 minutes, standing for 5 hours, and then measuring the milliliters of oil, water and emulsified layer.
(8) Air release value
This requirement is stipulated in the hydraulic oil standard, because in the hydraulic system, if the air dissolved in oil cannot be released in time, it will affect the accuracy and sensitivity of hydraulic transmission, and in severe cases, it will not meet the requirements of the hydraulic system. The method for measuring this property is similar to that for antifoaming, except that it is to measure the release time of air (mist) dissolved in the oil.
(9) rubber tightness