Grease lubrication is generally suitable for low to medium speed applications where the operating temperature of the bearing is below the grease’s limit temperature. No anti-friction bearing grease is suitable for all applications. Each grease has only limited performance and characteristics. Grease consists of base oil, thickener and additives. Bearing grease usually contains petroleum base oil thickened with a certain 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. Ingredients of Grease Base Oil Thickener Additive Grease Mineral Oil Synthetic Hydrocarbon Ester Substance Perfluorinated Oil Silicone Lithium, Aluminum, Barium, Calcium and Compound Soap Unscented (inorganic) particles Glue (clay), carbon black, silica gel, PTFE soap-free (organic) polyurea compound rust inhibitor dye tackifier metal passivator antioxidant anti-wear extreme pressure additive calcium-based and aluminum-based greases have excellent water resistance, Suitable for industrial applications that need to prevent moisture intrusion. Lithium-based greases have multiple uses and are suitable for industrial applications and wheel-end bearings.
Synthetic base oils, such as esters, organic esters and silicones, when they are used with commonly used thickeners and additives, the maximum operating temperature is usually higher than the maximum operating temperature of petroleum-based oils. The operating temperature range of synthetic grease can be from -73°C to 288°C. The following are the 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 Using the thickeners in Table 27 with synthetic hydrocarbon or ester-based oils, the maximum operating temperature can be achieved Increase by about 10°C.
°C °F °C °F
Lithium 193 380 121 250 good
Lithium complex 260+ 500+ 149 300 good
Composite aluminum base 249 480 149 300 excellent
Calcium sulfonate 299 570 177 350 excellent
Polyurea 260 500 149 300 Good
The use of polyurea as a thickener is one of the most significant developments in the lubricating field for more than 30 years. Polyurea grease shows excellent performance in a variety of bearing applications, and in a short time, has become recognized as a ball bearing pre-lubricant. Low temperature Under low temperature conditions, the starting torque of grease lubricated bearings is very important. Some grease can only function normally when the bearing is running, but it will cause excessive resistance to the start of the bearing. In some small machines, it may not start when the temperature is extremely low. In such a working environment, it is required that the grease has 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 very small at a low temperature of -73°C. In some cases, these greases perform better than lubricants in this regard. The important point about grease is that starting torque is not necessarily a function of grease consistency or overall performance. Starting torque is more like a function of the individual performance of a specific grease, and it is 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 grease under various base oil conditions. After years of experiments with grease-lubricated bearings, its empirical methods show that the lubricating grease life will be halved for every 10°C increase in temperature. For example, if the service life of a grease at a temperature of 90°C is 2000 hours, when the temperature rises to 100°C, the service life is reduced to approximately 1000 hours. Conversely, after lowering the temperature to 80°C, the service life is expected to reach 4000 hours.