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Basic knowledge of lubricating oil

Basic knowledge of lubricating oil


1. Specific gravity: the ratio of the mass of a given volume of lubricating oil at 15℃ to that of the same volume of pure water at 4℃.The density of hydrocarbon increases with the increase of its relative molecular weight: with the same relative molecular weight, the density increases according to the order of normal alkane, isomeric alkane, cycloalkane and aromatic hydrocarbon.When expressing specific gravity and density, the measured temperature should be indicated.

2. Viscosity: is the internal friction force of the liquid. When the lubricating oil is in relative motion under the action of external forces, the resistance between oil molecules makes the lubricating oil unable to flow smoothly.The vast majority of lubricating oil according to viscosity classification, it is a variety of mechanical equipment to choose the main index of oil.The lubricating oil with high viscosity has poor flow performance, poor cooling and washing effect, high temperature of friction surface, but good bearing capacity.Oil with less viscosity is the opposite.Therefore in the choice of lubricating oil must be very careful to choose the right viscosity.

The measurement of viscosity is divided into relative viscosity and absolute viscosity.Absolute viscosity is divided into dynamic viscosity and kinematic viscosity: relative viscosity has a variety of methods: engler viscosity, sayer viscosity, redwood viscosity.Kinematic viscosity is equal to the dynamic viscosity divided by the density of the lubricant.The relative and absolute viscosity can be converted to each other.The unit of dynamic viscosity is: per second (Pa. S), also known as milliper second (mPa. S) : the unit of kinematic viscosity is: square meters per second (m/s), but often expressed as cSt (mm/s).

3. Viscosity index: the viscosity of lubricating oil varies with temperature.The higher the viscosity index is, the smaller the tendency of the viscosity of the lubricant to change with the temperature, that is, the smaller the tendency of the viscosity to decrease when the temperature increases, and the smaller the tendency of the viscosity to increase when the temperature decreases.The viscosity index can be obtained by calculating the kinematic viscosity at 40℃ and 100℃ or by looking up the table.

4. Flash point and flash point: it is a safety indicator, which indicates the lowest oil temperature when the oil vapor on the oil surface contacts the flame and flashes under the continuous heating condition.The higher the flash point, the less likely the oil is to ignite.Generally, the flash point of lubricating oil adopts the open cup method, while the light oil with flash point below 150℃ adopts the closed cup method.For the same oil product, the flash point of open cup is 20 ~ 30℃ higher than that of closed cup.Ignition refers to the lowest oil temperature at which the oil is tested after the flash point until the flame of the igniter can cause the oil to ignite and continue to burn for at least 5 seconds.Generally, the flash point of lubricating oil is required to be 20 ~ 30℃ higher than the operating temperature to ensure safety and reduce evaporation losses.

5. Pour point: it is an important low-temperature performance index, indicating the minimum temperature at which the lubricating oil can flow under certain experimental conditions.There are two reasons for the loss of fluidity of oil products at low temperature: one is the increase in the viscosity of oil products at low temperature; the other is that the wax in oil products crystallizes out at low temperature, resulting in the loss of fluidity of oil products.Pour point can be adjusted by adding anticoagulant, but the addition of anticoagulant can not reduce the total content of oil wax, but only change the structure of wax crystal.Long - term storage of oil products, will occur tipping point rebound.

6. Acid value or neutralization value: is an indicator of the total organic acid content in oil products.The value of lubricating oleic acid has a great influence on its performance.Generally, the more serious the oil oxidation, the greater the acid value, the more corrosive the lubricating oil, especially in the presence of water, the more corrosive.In practical use, the change of acid value is commonly used to control the life cycle and storage period of oil.But for some anti-rust oil, as a result of the addition of acid additives, so the acid value is larger is normal.

7. Total alkali value: represents the amount of various alkaline substances in the lubricating oil, especially the oil with alkaline additives, which can be used as an indicator to measure the consumption of alkaline additives in practical use.

8. Oxidation stability: indicates the ability of lubricating oil to resist oxidation and deterioration under the catalysis of heat and metal.Oxidation stability depends on the oil itself chemical composition, and the use conditions, such as temperature, catalyst, humidity, medium, use time, etc.Different oil products have different methods for the determination of oxidation stability.

9. Coning degree: it is an index to measure the consistency of the grease, that is, the degree of soft and hard.At the specified temperature and load, the depth of the standard cone sinking vertically into the grease sample within 5 seconds is called the taper person degree.Therefore, the greater the taper degree, the softer the grease.The grade of grease is based on the size of the taper.

10. Drop point: the grease is heated under specified conditions and becomes soft as the temperature increases. The temperature at which the first drop is dropped from the grease cup is called the drop point.It is a measure of the heat resistance of a grease.Generally, the maximum service temperature of the grease should be 30 ~ 50℃ lower than the drop point. 

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