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How does the temperature affect the selection of grease?

Time：

2022-01-10

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When buying grease, the temperature range in use is a factor to be considered. Grease has a certain temperature range, and there are differences in the applicable temperature range of different greases. The components of grease are base oil, thickener and additives, and the temperature of grease is also related to these three components. For a certain grease, the manufacturer will give a maximum use temperature and minimum use temperature, the user should remember that when choosing, the user needs to consider the highest and lowest temperature of the lubrication part (usually the lowest temperature when starting).

The minimum operating temperature of grease

The hardness of grease will increase when the temperature drops, which will affect the normal delivery and cause the grease can't reach the lubrication point adequately, and the machine lacks lubrication when it starts cold. The minimum use temperature of grease must be higher than the minimum ambient temperature at cold start. At lower temperature (e.g. close to -30℃), synthetic grease should be used, such as grease using PAO as base oil.

The maximum use temperature of grease: grease has an indicator is dropping point (dropping point), but dropping point is not equal to the highest use temperature, the maximum use temperature of grease allowed is generally lower than dropping point 30 ℃ to 50 ℃, do not use over temperature, especially the high temperature for a period of time. More than the dropping point of the grease even if cooled down, the performance has been destroyed, should not be used.

Grease performance at high temperature

The immediate danger of using lubricating oil over temperature is to accelerate oxidation and deterioration, and to generate acidic substances. It is worth noting that if there is local high temperature at the lubrication point, it will also cause oxidation and deterioration of grease, and gradually spread out. Local high temperature may be caused by poor bearing installation, but also may be due to too much or not enough grease, the wrong oil, by vibration, excessive load and other problems caused.

Grease oxidation will become darker and the acid produced will cause the thickener to decompose, causing the base oil to be lost from the thickener. Grease is actually thickened lubricating oil, thickener like a sponge to absorb the base oil to avoid excessive loss, so lubrication is mainly by oil lubrication. If the base oil is lost, although there is still grease on the surface, but the lubricating ability is almost gone. During the oxidation process, we can observe the change of grease consistency, such as hardening or thinning, flowing oil. The common grease is generally made of lithium thickener and mineral base oil, which can meet the lubrication requirements of most common working conditions. If there is high temperature (above 60 degrees Celsius), the grease will accelerate the oxidation, when close to 90 degrees Celsius, the oxidation of common grease seriously restricts the performance of grease, these cases need to use better grease, generally use synthetic oil and high-grade thickener, such as compound lithium base, polyurea, compound calcium sulfonate and other types of thickener. This type of grease has better performance than ordinary grease in general temperature and high temperature.

In the range of grease use, the temperature characteristics of grease also make a difference. As mentioned before, grease is mainly lubricated by base oil, if the base oil has better viscosity-temperature characteristics (high viscosity index), then the viscosity is relatively stable during the temperature change. The classification of base oil is stipulated by API, and there are five classifications at present. It is necessary for users to know the base oil composition of grease when purchasing.

Although the main components of grease are base oil and thickener, the base oil and thickener are not affected by temperature to the same extent. Generally speaking, the base oil is relatively more affected by temperature change, while for the consistency of thickener, the consistency changes relatively slowly before reaching the dropping point. Therefore, it is necessary to understand the performance of the base oil when purchasing grease.

Low temperature performance of grease

At low temperature, grease will become thicker, i.e. hardness will increase. The low temperature performance of grease is also directly related to the base oil. Just like edible oil, liquid at room temperature, low temperature will be liquidity reduction or even stop flowing. When it drops to a certain temperature, the lubricant stops flowing, and this temperature is called the pour point (pour point), and lubricants with low pour points can be used at lower temperatures. Mineral oils generally have a pour point of -20 to -30°C, but the pour point is generally required to be lower than the operating temperature.

Low temperature torque (low temperature torque) is one of the important indicators of grease low temperature performance, low temperature, grease thickening, hardness increase, will have a blocking effect on the bearing. Low temperature torque is in the low temperature (-20 ℃ below), grease blocking the degree of rolling bearing rotation, grease low temperature torque by the starting torque and the average value of torque after a period of operation. The smaller the starting torque value of grease measured in low temperature, the smaller the starting power consumption; on the contrary, if a grease has a large low temperature starting torque. Then the phenomenon of stuck bearing may occur, which is not suitable for use in low temperature.

Therefore, in the purchase of grease, the use of lubrication point temperature (maximum and minimum temperature) must be taken into account, the allowable use of grease temperature range should include the use of grease temperature range. The indicators related to temperature include dropping point, maximum use temperature, minimum use temperature, pour point of base oil, low temperature torque, viscosity index of base oil and other indicators.

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