Analysis of low-temperature and high-temperature environments of grease lubricated bearings

Grease lubrication is generally suitable for low to medium speed applications where the bearing operating temperature is below the grease limit temperature. No single anti-friction bearing grease is suitable for all applications. Each grease has limited properties and characteristics. Greases consist of a base oil, thickeners, and additives. Bearing greases usually contain a petroleum base oil thickened with some kind of 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. Composition of Greases Base Oil Thickener Additive Greases Mineral Oil Synthetic Hydrocarbon Esters Perfluorinated Oils Silicones Lithium, Aluminum, Barium, Calcium, and Complex Soaps Soap-free (Inorganic) Microgels (Clay), Carbon Black, Silica Gel, PTFE Soap-free (Organic) Polyurea Compounds Rust Inhibitors Dyes Tackifiers Metal Deactivators Antioxidants Antiwear Extreme Pressure Additives Calcium and aluminum greases have excellent water resistance and are suitable for industrial applications where protection from water intrusion is required. Lithium greases are versatile and are suitable for industrial applications and wheel end bearings.
Synthetic base oils, such as esters, organic esters, and silicones, when used with common thickeners and additives, generally have higher maximum operating temperatures than petroleum-based oils. The operating temperature range for synthetic greases can be from -73°C to 288°C. Below are the general characteristics of common thickeners used with petroleum-based oils. Table 27. General Characteristics of Thickeners Used with Petroleum-Based Oils Thickener Typical Dropping Point Maximum Temperature Water Resistance Using the thickeners in Table 27 with synthetic hydrocarbon or ester base oils can increase the maximum operating temperature by approximately 10°C.
°C °F °C °F
Lithium 193 380 121 250 Good
Lithium Complex 260+ 500+ 149 300 Good
Aluminum Complex 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 the most significant development in lubrication in more than 30 years. Polyurea greases have shown excellent performance in a variety of bearing applications and have become an accepted pre-lubricant for ball bearings in a short period of time. Low Temperatures At low temperatures, the starting torque of grease-lubricated bearings is very important. Some greases only work when the bearing is running, but they cause too much resistance to the starting of the bearing. In some small machines, it may not be possible to start when the temperature is very low. In such working environments, the grease is required to have the characteristics of low temperature starting. Synthetic greases have a distinct advantage when it comes to operating over a wide temperature range. They can provide very low starting and running torques at temperatures as low as -73°C. In some cases, these greases can outperform oils in this regard. An important point about greases is that starting torque is not necessarily a function of grease consistency or overall performance. Starting torque is more a function of the individual properties of a particular grease and is determined by experience.
High Temperatures: The high temperature limits of modern greases are usually a function of the thermal stability and oxidation resistance of the base oil and the effectiveness of the oxidation inhibitors. The temperature range of a 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 greases for various base oils. An empirical approach based on many years of testing grease-lubricated bearings has shown that grease service life is halved for every 10°C increase in temperature. For example, if a grease has a service life of 2000 hours at 90°C, the service life drops to approximately 1000 hours at 100°C. On the other hand, by lowering the temperature to 80°C, the service life can be expected to reach 4000 hours.