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Analysis of low temperature and high temperature environment of grease lubricated bearings

by:JNSN     2021-12-25
Grease lubrication is generally suitable for medium and low speed applications where the bearing operating temperature is lower than the limit temperature of the grease. No anti-friction bearing grease can be suitable for all applications. Each type of grease has only limited performance and characteristics. Grease consists of base oil, thickener and additives. Bearing grease usually contains petroleum-based oil thickened with a metal soap. In recent years, organic and inorganic thickeners have been added to synthetic base oils. Table 26 summarizes the composition of typical greases. Table 26. Grease composition Base oil thickener additives Grease mineral oil Synthetic hydrocarbon esters Perfluorinated oil Silicone lithium-based, aluminum-based, barium-based, calcium-based and complex soap soap-free (inorganic) particles Glue (clay), carbon black, silica gel, PTFE soap-free (organic) polyurea compound rust inhibitor, dye, tackifier, metal passivator, antioxidant, anti-wear and extreme pressure additives, calcium-based and aluminum-based greases have excellent water resistance, It is suitable for industrial applications that need to prevent the intrusion of moisture. Lithium-based grease has many uses and is suitable for industrial applications and wheel end bearings. Synthetic base oils, such as esters, organic esters and silicones, when they are used together with commonly used thickeners and additives, the maximum operating temperature is usually higher than that of petroleum-based base oils. The operating temperature of synthetic grease can range from -73°C to 288°C.   The following are general characteristics of thickeners commonly used with petroleum-based oils. Table 27. General characteristics of thickeners used with petroleum-based oils. Thickeners typical dropping point maximum temperature. Water resistance. The thickeners in Table 27 can be used in conjunction with synthetic hydrocarbon or ester base oils to set the maximum operating temperature. Improve about 1C. °C °F °C °F Lithium base 193 380 121 250 Good lithium complex base 260+ 500+ 149 300 Good complex aluminum base 249 480 149 300 Excellent calcium sulfonate 299 570 177 350 Excellent polyurea 260 500 149 300 Good polyurea The use of urea as a thickener is the most significant development in the lubrication field in more than 30 years. Polyurea grease has shown excellent performance in a variety of bearing applications, and in a short period of time, it has become a recognized pre-lubricant for ball bearings. Low temperature Under low temperature conditions, the size of the starting torque of grease lubricated bearings is very important. Some greases can only function normally when the bearing is running, but cause excessive resistance to the starting of the bearing. In some small machines, it may not start when the temperature is extremely low. In such a working environment, the grease is required to have the characteristics of low temperature starting. If the operating temperature range is wide, synthetic grease has obvious advantages. The grease can still make the starting and running torque at a low temperature of -73°C. In some cases, these greases will perform better than lubricants in this regard. The important thing about grease is that starting torque is not necessarily a function of grease consistency or overall performance. The starting torque is more like a function of the individual performance of a specific grease, which must be determined by experience. High temperature: The high temperature limit of modern greases is usually a comprehensive function of the thermal stability and oxidation resistance of the base oil and the effectiveness of oxidation inhibitors. The temperature range of the grease is determined by the dropping point of the grease thickener and the composition of the base oil. Table 28 shows the temperature range of the grease under various base oil conditions. After years of experiments on grease-lubricated bearings, the empirical method shows that: every 1C increase in temperature, the service life of the grease will be halved. For example, if a certain grease has a service life of 2000 hours at a temperature of 9C, when the temperature rises to 10C, the service life will be reduced to approximately 1000 hours. Conversely, after lowering the temperature to 8C, the service life is expected to reach 4000 hours.
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