Lubrication Oils Greases Lubricant Selection Lubricating Methods

Adequate lubrication of rolling bearings is required for achieving the life calculated for any bearing. In a correctly operating rolling element bearing, a thin film of lubricant separates the rolling elements from the raceways. This film should be of sufficient thickness to actually prevent the rolling element surfaces from touching the inner and outer ring raceways. Contact of the raceway surfaces will result in wear, scoring, and possible seizure. Providing this film is the primary function of the lubricant to four types of internal bearing contact:true rolling contact of the rolling elementhaceways sliding contact between the cage and other bearing components partial slidhgh-olling contact in some bearing types sliding contact between the rollers and guide ribs in roller bearings In addition, the lubricant has several important secondary functions: protection from corrosion exclusion of contaminants flushing away of wear products and debris dissipation of heatThe requirements of a lubricant for rolling element bearings are often more severe than realized. In a rolling element bearing, there are conditions of both rolling and sliding with extremely high contact pressures. The lubricant must withstand high rates of shear and mechanical working not generally prevalent in other mechanical components. For these reasons, proper attention to lubrication is vital for successful bearing operation. Oil is a liquid lubricant which can be pumped, circulated, atomized, filtered, cleaned, heated, and cooled, making it more versatile than grease. It is suitable for many severe applications involving extreme speeds and high temperatures. On the other hand, it is more difficult to seal or retain in bearings and housings and, in general, involves a more complicated system than grease.
Viscosity, the measure of an oil’s thickness, is the most important property of lubricating oil. The selection of proper viscosity is essential and is based primarily on expected operating temperatures of bearings. Excessive oil viscosity may cause skidding of rolling elements and high friction. Insufficient oil viscosity may result in metal-tometal contact of the rolling surfaces.There are two general categories for liquid lubricants: petroleum or mineral oils, and synthetic oils. Mineral oils are lower in cost and have excellent lubricating properties. Synthetic lubricants have been developed to satisfy the need for a wider operating temperature range than is possible with mineral oils. This development has been prompted prompted by the extreme environmental demands of military and aerospace applications. There is a wide range of synthetic types with varying temperature limits. The maximum temperature limits for the common types are given in Table 11. The major disadvantage of synthetics is that they do not have the same load-carrying capacity as do mineral oils at typical industrial equipment operating temperatures. Also, synthetics are rarely compatible with mineral oils, so care must be taken when both types are being used in proximity.Grease is a combination of mineral oil or a synthetic fluid and a suitable thickener (often called soup). The percentage of the oil in grease is usually about 80%, but can range from 70% to 97%. Grease consistency or stiffness is determined primarily by the thickener and base oil viscosity. Greases of a given consistency may be formulated from various combinations of thickener and base oil viscosity so that greases of equal stiffness are not necessarily equal in performance. Greases considered satisfactory lubricants for rolling element bearings are combinations of soap or nonsoap agents, mineral oil, and additives. Soaps such as sodium, calcium, barium, aluminum, lithium, complexes of these soaps, and nonsoaps such as silica and special clays are generally used. Rust and oxidation inhibitors and extreme pressure agents are often added. Lithium and lithium-complex thickeners seem to give the best all-purpose performance, but each type has its advantages.Performance of a gEase depends on several different factors. The lubricating capability of the grease is mainly dependent on the properties of the base oil used. The corrosion protection of the grease is determined by the thickener.the base oil also, it is usually restricted by the thickener. Table 12 shows the maximum temperature limits for the most common thickener types.Be careful to avoid mixing greases of Merent soap bases. The combination will usually be worse than either one by itself, and sometimes worthless. Care should be taken when mixing greases with the same soap bases from different manufacturers, although this is usually not a problem. However, it is not always obvious what the soap base is unless the manufm’s data sheet is consuld. In no case should mineral oil greases be mixed with greases using a synthetic oil. The major criteria for selection of a lubricant is the viscosity. The selection of the proper viscosity oil is especially important for bearings operating in the high load, speed, or temperature ranges. As mentioned in the section OR life adjustment factors, it is necessary to have A between 1.1 and 1.3. The A factor is a mure of the mtio of the oil film thickness to the surface roughness of the raceways in contact. In other words, the oil film needs to be thicker than the race way roughness so that there is never metal-to-metal contact. The viscosity of most oils changes dramatically with a change in temperature. When determining the operating temperature, it is the oil temperature that is important. Generally, the oil temperam is 5" to 20°F greater than that of the bearing housing.The oil film thickness is calculated through the theory of elastohydrodynamic lubrication. This involves the elastic deformations of the raceway contacts, and the pressure-viscosity effects and hydrodynamics of the lubricant. This theory is very complex and best left to computer programs. However, a simplified method for determining if the oil film thickness is sufficient involves using Figures 12 and 13. From Figure 12, find the oil viscosity needed based on the beating size and the operating speed. Then, using Figure 13, combine that viscosity with the operating temperature of the oil to determine what grade of oil is needed. This should give a ballpark estimate of which oil to use: It assumes the use of a mineral oil with a viscosity index of 95. If a much different oil is to be used w e a synthetic, for example), it is best to consult with a bearing manufacturer or oil supplier to make a more detailed calculation. If the A factor is less than 1.1, the bearing life will be reduced. If A is greater than 1.3, the bearing life will be increased. This implies that the highest viscosity possible should be used. However, as the viscosity goes up, so does the operating temperature. As temperature goes up, the oil viscosity goes down and maintenance activity goes up. This all means that there is a practical limit to the life improvement from higher oil viscosity. There are a variety of methods to apply the proper amount The simple oil bath method is satisfactory for low and moderate speeds. The oil level in the housing should not be less than the lip of the outer ring, nor higher than the center of the lowest rolling element. The oil level should only be checked when the bearing is not rotating. Circulating oil is an excellent way to lubricate a bearing, especially on large machines, and can reduce maintenance and prolong the life of the oil in severe operating conditions. of oil to a bearing. The most common are as follows:Oil bath Circulating systems Jet lubrication Mist lubrication Wick feed It can be used with either a wet or dry sump, with the oil usually introduced on one side of the bearing and drained on the other. A system shutoff with loss of pressure is a desired feature. This system is good for all speeds and loads. The main drawback of such a system is the cost. Jet lubrication is a special type of circulating oil system used on very high speed bearings such as in a gas turbine engine. Most of the oil in this method is used for cooling of the bearing. This method can be used at speed levels up to 1.5 million DN with proper design. The jet should be aimed at the largest space between the cage and the ring lands. Mist lubrication is of two types, which are distinguished by the method of generating the mist. In some applications, the mist is generated by a flinger that dips into the oil and throws it into the air in the vicinity of the bearing. Sometimes gears substitute for the flinger. Another way to generate the mist is to spray a jet of oil against the side on the inside of the machinery. This method can be very effective for bearings where cooling is not needed.The second type of oil mist lubrication is when the mist is produced by a special mist generator. The oil mist is formed in an atomizer and supplied to the bearing housing under suitable pressure. This method of lubrication has proven very effective in reducing the operating temperature, not so much by air cooling as by the flow of air, preventing excess oil from accumulating in the bearing. Since the air pressurizes the housing and escapes through the seals, the entrance of moisture and grit is retarded. No drain is needed, as the quantity of oil supplied is very small. The problems with mist oil generators is that the immediate area may be coated with oil and if the oil generator shuts off, the bearings cannot survive long because of the small amount of oil supplied. Also, if there are air pressures created by other parts of the mechanical system, they should be checked to make sure they are not restricting the flow of the air mist under all operating conditions.Wick feed is also suitable for high speeds because, again, a small amount of oil is delivered to the bearing. Careful maintenance is needed to make sure the cup never runs dry and that the wick is always in contact with the source. Grease systems are not as numerous as those for oil. Many bearings come from the manufacturer with a supply ings by hand, filling the internal volume of the bearing one-third to one-half full. A grease gun with a grease fitting on the housing can be used, but care should be exercised not to overfill the housing and cause overheating of the bearing. One method of gauging the amount of grease to add is to add grease slowly while the beating is running until some grease is just visible coming out either seal. The grease fitting should then be removed briefly to allow any grease backpressure to relieve itself, and then be reinstalled.There are automatic grease systems on the market that can relieve a lot of maintenance activity when a number of bearings can be grouped into a system. The bearings and/or their housings need to be packed with grease before the system is operated. The disadvantages of these systems is their initial cost and that all bearings on any one system must be able to use the same grease. Bearings with special needs would need a separate system.