Low temperature and high temperature environment analysis of grease lubricated bearings

Grease lubrication is generally suitable for low to medium speed applications where the bearing operating temperature is below the limit temperature of the grease. No single anti-friction bearing grease is suitable for all applications. Each grease has only limited properties and characteristics. Greases consist of base oils, thickeners and additives. Bearing greases typically contain petroleum-based oils thickened with some kind of metallic 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 Ester Perfluorinated Oil Silicone Lithium, Aluminum, Barium, Calcium and Complex Soap Free Soap (Inorganic) Particulate Condensation Glue (clay), carbon black, silica gel, PTFE soap-free (organic) polyurea compounds, rust inhibitors, dyes, adhesives, metal passivators, antioxidants, anti-wear, extreme pressure additives, calcium-based and aluminum-based greases with excellent water resistance, Ideal for industrial applications where protection against moisture intrusion is required. Lithium-based greases are versatile and 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, typically have higher maximum operating temperatures than petroleum-based oils. The operating temperature range of synthetic greases can be from -73C to 288C. The following are general characteristics of thickeners commonly used with petroleum-based oils. Table 27. General Characteristics of Thickeners Used with Petroleum Base Oils Typical Dropping Points of Thickeners, Maximum Temperature Water Resistance Raise about 10C. CFCF 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 Polyurea as thickener is One of the most significant advances in lubrication in over 30 years. Polyurea grease has shown excellent performance in a variety of bearing applications, and has become a recognized pre-lubricant for ball bearings in a short period of time. Low temperature In low temperature conditions, the magnitude of the starting torque of grease lubricated bearings is very important. Some greases can only function properly when the bearing is running, but will cause excessive resistance to the starting of the bearing. In some small machines, it may fail to start when the temperature is extremely low. In such a working environment, the grease is required to have low temperature starting characteristics. Synthetic greases have clear advantages if the operating temperature range is wide. The grease can still provide low starting and running torque at low temperatures of -73C. In some cases, these greases will 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. Breaking torque is more of a function of the individual properties of a particular grease and is determined empirically. High temperature: The high temperature limit of modern greases is usually a combined function of the thermal stability and oxidation resistance of the base oil and the effectiveness of the oxidation inhibitor. 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 ranges of the greases for various base oil conditions. After years of testing of grease lubricated bearings, the empirical method shows that for every 10C increase in temperature, the service life of the grease will be halved. For example, if a grease has a service life of 2000 hours at 90C, when the temperature rises to 100C, the service life drops to approximately 1000 hours. Conversely, after dropping the temperature to 80C, the service life is expected to reach 4000 hours.