Bearings lubed-for-life

Traditionally, greases and oils have been used to lubricate ball and roller bearings. These lubricants can be messy, causing significant housekeeping problems and requiring periodic maintenance to replenish the lubricant. Solid lubricants, such as microporous polymeric lubricants (MPLs), can significantly reduce or eliminate these problems.

MPLs are made up of two major components, a polymer containing a continuous microporous network and oil that is contained within these pores. The type of oil incorporated into the polymer can be tailored to the requirements of the application. For example, the oil can be an FDA/USDA-approved food-grade lubricant to eliminate product contamination and improve housekeeping and safety, or an oil with an extreme pressure (EP) additive for high load applications.

Other additives can be used to alter the lubricant's properties and include oil property enhancers such as corrosion and oxidation inhibitors, 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 over 50% oil by weight.

The microporous polymer acts much like a sponge, releasing and absorbing oil. Oil is released from the polymer to its surface through capillary action. It is then 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 re-absorbed by the porous polymer.

As the temperature of the MPL-filled bearingincreases, more oil is typically released by the MPL. This oil is reabsorbed by the MPL as the bearing temperature decreases. Because of this, MPLs reduce or eliminate the need for relubrication, 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 molded into the space between rolling elements and the races of the bearing (Fig. 1). The MPL provides a continuous source of lubrication.

Because it is solid, the MPL helps shield the bearing and reduce contamination. This is useful in applications where bearings are exposed to dust or dirt. Reducing the incursion of debris into a bearing can significantly extend its life.

MPLs can also be produced in various solid profiles by casting, extruding, and injection molding (Fig. 2). 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 crane wheel flanges, ball screws, and linear bearing rails, and as lubricating plugs in bushings and sleeves. One application is an MPL sprocket (Fig. 3) being used as an idler to lubricate chains.

MPLs are made by mixing proprietary polymers, oils, and additives. The mixture is packed into the bearing and thermally processed. Because MPLs require this treatment, the bearings must be processed in the manufacturer's facility, where the MPL is incorporated into the bearing.

It is not possible to put oil-filled polymer lubricants into a bearing in the field. Bearings must be purchased MPL-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.

Various formulations of MPLs provide operating temperature ranges from -40 F to 350 F. If the high-temperature limit is exceeded, the polymer softens and can be ejected from the bearing. MPLs do not dissipate heat rapidly, and as a result, there are rotational speed limitations based on bearing type and size. Maximum rotational speeds (rpm) at room temperature have been determined for each type of bearing and can be calculated by using the Ndm Value from the table in the following formula. The Ndm value is a factor relating maximum rotational speed to bearing size. Ndm values for various bearing designs are shown in the table.

Ndm Value

_____________

Maximum rpm = ½(Bore + O.D.) in mm.

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, or acids is not recommended. Repeated exposure will deplete the oil from MPLs, making them less effective.

Because the bearing cavity is filled with MPL, rotational torque is increased compared to grease-filled bearings, especially on startup. This is usually not a problem in most industrial applications.

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If you have any questions about microporous polymeric lubricants contact the author at 937-743-8061. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com.