Characteristics and test properties of rotating electrical machine bearing lubricants
Key considerations with greases and oils include their suitability for an application. Knowledge of lubricant characteristics and test properties helps ensure the correct selection for rotating electrical machine bearings.
- Understand the consistency of grease.
- Understand the meaning of an NLGI number.
- Recognize important characteristics and test properties of greases and oils.
- Lubrication is essential for reducing friction in bearings of rotating electrical machines, affecting temperature control and wear.
- Greases consist of varying compositions of lubricating oil, thickeners/soaps, and additives, with characteristics dependent on these components.
- Key tests for greases and oils include penetration and worked penetration tests, bleeding rate test, and emulsification tests, crucial for determining their suitability for specific applications.
Lubrication is needed to reduce friction between the rolling elements and stationary parts of bearings in rotating electrical machines. By reducing bearing friction, lubricants also help prevent undue temperature rise and dissipate some of the heat that is generated. But some rotating electrical machine applications require different lubricants than others. With that in mind, here’s an overview of some key characteristics and test properties of greases and oils.
Characteristics of greases
Grease technology continues to be a complex topic as new formulations emerge to help solve practical problems. Such solutions typically involve varying the type, hardness and percentages of thickeners/soaps; changing the oil types, viscosity and percentage; and modifying other additives. From a chemical perspective, greases are mixtures consisting of:
Approximately 75% lubricating oil
Approximately 15% thickener/soap
Up to 10% additives
Base oils: The properties of a grease depend primarily on the type of base oil used, as well as on the thickening agent and other additives. Base oils consist of mineral oil or synthetics such as ester oil, synthetic hydrocarbon oil or ether oil. Generally, greases with low-viscosity base oils are best suited for low temperatures and high speeds. Those with high-viscosity base oils are superior at high temperatures and high loads.
Thickening agents are compounded with base oils to keep grease in a semi-solid state. These include metallic soaps (lithium, sodium, calcium and aluminum) and two types of nonsoap thickeners: inorganic (silica gel, bentonite) and organic (polyurea, fluorocarbon). Note that polyurea is a synthetic organic thickener that is widely used for electric motor bearing lubricants because it can withstand temperatures exceeding 250°F (120°C).
Various special characteristics of grease — such as temperature range limits, mechanical stability and water resistance — mostly depend on the type of thickening agent used. For example, sodium-based greases generally provide poor water resistance, while greases with polyurea and other nonmetallic soap thickening agents usually have superior high temperature properties.
Additives: Depending on the purpose, various additives may be used to modify grease properties. These typically include anti-corrosives, rust preventives, fillers, wetting agents, extreme pressure (EP) additives and antioxidants. Antioxidants are used to delay deterioration of greases in most types of rolling bearings. EP additives are best for bearings subject to thrust and/or shock loads but otherwise are not advised because they can shorten grease life.
Grease consistency: Consistency indicates the hardness and fluidity of grease based on penetration units and NLGI (National Lubricating Grease Institute) consistency numbers. As Table 1 shows, each NGLI number covers a range of penetration values. The higher the number, the harder (firmer) the grease and the better it stays in place, which is useful where leakage is a concern. A lower NLGI number indicates a softer grease that flows better. Rolling element bearings use NLGI 1, 2 and 3 greases.
Grease penetration values depend mostly on the base oil viscosity and the percentage of base oil in the lubricant. Penetration is the depth, in tenths of millimeters that a standard weighted cone sinks into grease under prescribed conditions. High-viscosity oils tend to stiffen greases and decrease penetration values, so they work well for heavier loads. They also make greases less susceptible to atomization and thinning at higher temperatures. Low-viscosity oils increase grease penetration values for use with lighter loads and lower temperature applications.
Special-purpose greases are available for low- and high-temperature applications, including synthetic greases for both low and very high temperatures. These special synthetic greases usually are designed to meet specific application issues–primarily very high temperatures. Of these, perhaps the most common are silicone greases, which use silicone oils in place of mineral oils. There are also synthetic greases with low-noise characteristics. Note that while special greases are often well suited for specific applications, they usually are less than optimal in average temperature ranges and applications.
Testing properties of grease
Of the many standardized tests of grease properties, here are a few of the most important ones:
Penetration and worked penetration tests. These tests measure the stiffness or movability of grease as well as its channeling or self-leveling properties.
Accelerated tests: These tests check the oxidation rate or aging properties of grease. Oxidized greases are poor lubricants and tend to accelerate corrosion.
Bleeding rate test: This test determines how quickly the oil in grease tends to separate from the thickener/soap. Judiciously choosing a grease with the appropriate bleeding rate can compensate for the severity of the application.
Emulsification tests: These tests are especially important for applications in humid conditions. A grease that emulsifies easily would normally be flushed out of a bearing very easily in wet applications. At the same time, this type of grease would best dissipate small quantities of moisture.
Characteristics and testing properties of oils
Note: Rotating electric machine bearings use turbine oil. Automotive oils contain detergent additives and should not be used in these applications.
Lubricating oils have simpler compositions than greases, so this discussion covers both the characteristics and test properties of these lubricants.
Lubricant manufacturers or blenders typically use qualification testing to ensure that lubricant blends meet the stated minimum criteria. Table 2 lists some of the more important and useful characteristic properties of turbine oil, including:
Viscosity: Considered the most important property of a lubricant, viscosity provides a lubricating film, cools machine components and seals and controls oil consumption. Liquids that resist flow or flow slowly, such as honey or dish soap, have high viscosity. Liquids that flow easily or quickly, such as water or vegetable oil, have low viscosity. The viscosity index (VI) is the rate at which the viscosity of an oil changes with temperature. The higher the VI, the less the viscosity of an oil changes with temperature.
Oxidation stability: Oxidation stability is the ability of a lubricant to resist chemical combination with oxygen. If that occurs, it can create sludge deposits and increase the viscosity of the oil. Heat, light, metal catalysts, acids formed by water contamination and other contaminants can accelerate the oxidation rate.
Pour point: This is the lowest temperature at which an oil will flow under prescribed test conditions. The number of wax particles that remain after crude oil processing determines the pour point. The more particles there are, the higher the pour point. The fewer there are, the lower the pour point.
Rust resistance test: The rust resistance properties test measures the ability of industrial oils to prevent rust in water contamination situations.
Foaming characteristics test: The foaming characteristics test empirically rates the foaming tendency of lubricating oils and the stability of the foam.
Since some rotating electrical machine bearing applications require different lubricants than others, it helps to understand key characteristics and test properties of greases and oils when making those selections.