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The traditional method for lubricating ball and roller bearings is with grease and oils. These lubricants can be messy, causing significant housekeeping problems and requiring periodic maintenance to replenish the lubricant. Microporous polymeric lubricants (MPL) can significantly reduce or eliminate these problems.
Characteristics of MPLs
MPLs are comprised of two major components, a polymer containing a continuous microporous network and oil contained within these pores. The type of oil incorporated into the polymer can be tailored to the requirements of the application. Examples include FDA/USDA-approved food-grade lubricants to eliminate product contamination and improve housekeeping and safety, or oils with an extreme pressure (EP) additive for high-load applications. Other additives can also be used to alter the lubricant’s properties. Examples include oil property enhancers such as corrosion and oxidation inhibitors and coefficient-of- friction modifiers and lubricating solids such as molybdenum disulfide, graphite and Teflon. The oil content in the polymer can be controlled during processing and the MPL can contain more than 50 percent by weight.
The microporous polymer acts like a sponge releasing and absorbing the oil. The oil is released from the polymer through capillary action to its surface and is transferred to any surface it contacts to provide the necessary lubrication. As the quantity of oil on the surface decreases, the MPL releases more oil. If excess oil becomes present, it is reabsorbed by the porous polymer. For example, as the temperature of the MPL-filled bearing increases, more oil is typically released by the MPL, however, this is reabsorbed by the MPL as the bearing temperature decreases. Because of this, MPLs reduce or eliminate the need for relubrication, therefore minimizing or eliminating maintenance and housekeeping.
A major application of MPLs is the lubrication of ball and roller bearings providing an extended source of lubrication. It is inserted into the space between rolling elements and the race of the bearings (Figure 1).
Figure 1. Bearings Filled with MPL
The MPL provides a continuous source of lubrication. Because it is solid, the MPL can help seal the bearing and reduce foreign contamination of the bearing. Thus it is useful in applications where bearings are exposed to dust or dirt. Reducing the incursion of debris into the bearing can significantly extend the bearing’s life.
MPLs can also be produced in various solid profiles by casting, extruding and injection molding (Figure 2).
Figure 2. Solid Profile Shapes Made from MPL
While they are not designed as load-bearing materials, these solid profiles offer a unique method of delivering lubrication, especially for difficult-to-reach locations. Solid profiles have been used to lubricate railroad and crane wheel flanges, chains, ball screws, linear bearing rails and as lubricating plugs in bushings and sleeves. One special application is the Lubri-sprocket® shown in Figure 3, where it is being used as an idler sprocket to lubricate chains.
Figure 3. Lubri-sprocket Used to Lubricate Chains
MPLs are made by mixing proprietary polymers, oils and special additives. The mixture is packed into the bearing and thermally processed. Several trimming and cleaning operations are required before shipping. Alternately, the mixture can be extruded into cross-sectional shapes or injection molded into specific parts. Because MPLs require thermal processing, the bearings must be processed in the manufacturer’s facility where the MPL is incorporated into the bearing. Therefore, it is not possible to put oil-filled polymer lubricants in a bearing at an existing field operation. This means that MPL-filled bearings must be purchased already filled or sent to the manufacturer to be filled. Nearly any type of bearing can be lubricated with MPLs, including ball, roller, needle, tapered, spherical and cam followers.
Possible Product Advantages
The main advantages of MPLs vs. a grease or oil include:
Possible Product Limitations
While the benefits of MicroPoly® are numerous, there are some limitations. The temperature limitations are listed in Table 2. If these temperatures are exceeded, the polymer softens and can be ejected from the bearing. In addition, MPLs do not dissipate heat rapidly, and as a result, there are rotational speed limitations based on the bearing type and size. Maximum rotational speeds (rpm) at room temperature have been determined for each type of bearing and can be calculated by the following formula using the data in Table 3.
While MPLs generally resist contamination better than greased bearings, this does not make the bearing waterproof and will not prevent corrosion of the bearing. Direct contact with solvents, cleaners and/or acids is not recommended. Repeated exposure will deplete the oil from MPLs, making them less effective.
Because the bearing cavity is filled with the MPLs, the rotational torque is increased compared to grease filled bearings, especially on start-up. This is usually not a problem in most industrial applications.
Examples of Successful Applications