Tom Bishop, PE
EASA Senior Technical Support Specialist

Lubrication is needed to reduce friction between the rolling elements and stationary parts of a bearing. By reducing bearing friction, lubricants also help prevent undue temperature rise and dissipate some of the heat that is generated. This article describes some of the key characteristics and properties of lubricating greases and oils.

Characteristics of greases
Grease technology continues to be a complex topic as new formulations are developed to help solve practical problems. Such solutions typically involve varying the type, hardness and percentages of soap; changing the oil types, viscosity and percentage; and modifying other additives. Carefully selected lubricating greases are effective in industrial applications at reducing mechanical friction and wear. From a purely chemical perspective, they are mixtures consisting of:

• Approximately 75 percent lubricating oil
• Approximately 15 percent thickener
• Up to 10 percent additives

Grease bases consist of mineral oil or synthetics such as ester oil, synthetic hydrocarbon oil or ether oil. A thickener and other additives are added to the base. The properties of greases are primarily determined by the type of base oil used and by the combination of a thickening agent and various additives. Generally, greases with low viscosity base oils are best suited for low temperatures and high speeds, and greases with high viscosity base oil have superior high temperature and high load characteristics.

Thickening agents are compounded with base oils to maintain the semi-solid state of the grease. The types of bases that are used as thickening agents are metallic soaps and non-soaps. Metallic soap thickeners include lithium, sodium, calcium and aluminum. Non-soap base thickeners are divided into two groups: inorganic (e.g., silica gel, bentonite) and organic (e.g., polyurea, fluorocarbon). Note: Polyurea is a synthetic organic thickener that has been widely used for electric motor bearings 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, a sodium based grease is generally poor in water resistance properties, while greases with polyurea and other non-metallic soaps as the thickening agent are generally superior in high temperature properties. Various additives are added to grease depending on the purpose. Typical additives include antioxidants, extreme pressure additives (EP additives), rust preventives and anti-corrosives. For bearings subject to heavy loads and/or shock loads, grease with extreme pressure additives should be used. Antioxidants are added to grease used in most types of rolling bearings.

Consistency is an index that indicates hardness and fluidity of grease. The higher the NLGI (National Lubricating Grease Institute) number, the harder the grease. For rolling element bearing lubrication, greases with the NLGI consistency numbers of 1, 2 and 3 are used (See Table 1). A lower number indicates that a grease is softer and flows better, while a higher number is firmer (harder) and tends to stay in place, which is useful where leakage is a concern.

While the consistency of oil lubricants is measured in terms of viscosity, the consistency of grease is measured in penetration levels. Penetration is the depth, in tenths of millimeters, to which a standard weighted cone sinks into grease under prescribed conditions. The penetration value of greases is affected by variations in oil viscosity. High viscosity oils tend to make the greases stiffer, so they work well for heavier loads. The higher viscosity oils in greases are also less susceptible to atomization and thinning at higher temperatures. The low viscosity oils are used for reduced loads and lower temperatures. Variations in the percentage of oil present in the greases also affect their penetration value.

Additives are used to modify the grease properties. Common additives include antioxidants, fillers and wetting agents. Antioxidants are used to delay deterioration of greases. Unless a machine is mounted so that there is a thrust load on the bearings, it is gener-ally advisable to use a grease without extreme pressure (EP) additives, which can shorten the life of the grease. Special purpose greases are available for low and high tempera-ture applications, including synthetic greases for both low temperatures and very high temperatures. These special synthetic greases usually are designed to meet specific issues–primarily very high temperatures. Of these, perhaps the most common are silicone greas-es, which use silicone oils in place of mineral oils. There are also synthetic greases with low-noise characteris-tics. Note that while special greases are often well suited for their specific applications, they usually are less than optimal in average temperature ranges and applications.

Testing properties of grease
There are many standardized tests for determining the various properties of grease. Four of the most important tests are described here. The penetration test and the worked penetration tests determine stiffness or movability of grease. They are used as a measure of the channeling or the self-leveling properties of the grease. The oxidation rate or aging properties of the grease are checked by accelerated tests. Oxidized greases form poor lubricants and tend to accelerate corrosion.

The bleeding rate, or rate at which oil tends to separate from the soap, is also an important consideration in greases. Judiciously choosing a grease with the appropriate bleeding rate can compensate for the severity of the application. The emulsification properties of greases are also important, especially for humid applications. A grease that is easily emulsified would normally be flushed out of the bearing very easily in wet applications. At the same time, this type of grease would best dissipate small quantities of moisture.

Characteristics of oils
Lubricating oils are much simpler in composition compared to greases. The testing properties of oils section of this article also provides details about characteristics of lubricating oils.

Testing properties of oils
Typically, lubricant manufacturers or blenders will use qualification testing to make certain that the lubricant blend meets the stated minimum criteria. The oil properties and tests described here are some of the more important and useful.

Table 2 lists typical characteristic properties applicable to a turbine oil that are described here. Considered the most important property of a lubricant, viscosity serves to form a lubricating film, cool machine components, and seal and control oil consumption. Liquids that are resistant to flow or that flow slowly, such as honey or dish soap, have high viscosity. Liquids that are not resistant to flow or that flow fast, such as water or vegetable oil, have low viscosity. The viscosity index (VI) is the rate of change of viscosity of an oil with temperature. The higher the VI, the less the viscosity of an oil changes with temperature. Oxidation stability is the ability of a lubricant to resist the chemical combination with oxygen. If that occurs, it can result in the creation of sludge deposits and increased viscosity. The rate of oxidation is accelerated by heat, light, metal catalysts, acids formed by water contamination and other contaminants.

Pour point is the lowest temperature at which an oil will flow under prescribed test conditions. It is affected by the amount of wax particles removed during the processing of crude. The more wax particles there are, the higher the pour point. The fewer wax particles there are, the lower the pour point. The rust resistance properties test measures the ability of industrial oils to prevent rust in water contamination situations. The foaming characteristics test determines the foaming characteristics of lubricating oils. The test procedure describes methods of empirically rating the foaming tendency and the stability of the foam. Note: Turbine oil is used with rotating electric machine bearings. Automotive oils, which have detergent additives, should not be used in rotating electric machine bearings.