Oil is more common as an industrial lubricant because it flows so easily into critical areas. However, oil reservoirs aren’t always reliable — or even possible. Picture an overhead conveyor system with spray-lubricated wheels, where the oil drips to the floor. Even if it’s not that obvious, oil can create untidy hazards — often inescapable, since lubes are essential to protect moving parts from wear and corrosion. Or are messes avoidable? Grease is particularly effective at improving containment while reducing contamination and replenishment schedules — and friction too. What makes a grease different from plain oil is its added gellant, or thickener, that keeps the oil where it’s needed most — between moving surfaces subject to friction or vibration.
All greases are colloids, a permanent suspension where microscopic particles of gellant are dispersed through the oil. Too large to dissolve and too small to settle out, the gellant particles create a weblike matrix that keeps oil in place. Many materials are used for this purpose and offer unique benefits. For example, aluminum complex soap has closely packed fibers that make for smooth grease with very high water resistance. Kevin D. Akin, director of Product Services and Support at Nye Lubricants Inc. based in Fairhaven, Mass. adds, “Filmforming anti-wear additives can prevent electrical continuity on a switch contact, while anti-oxidants are beneficial in protecting hydrocarbon oils.” And except for polyureas, lubricant thickeners are generally compatible with thickeners in their same family. However, as with any design, greases all have their particular limitations; damping greases are one example.
Some damping greases are so tacky, objects must be forced through them; this shear resistance minimizes free-motion problems like backlash, coasting, and wear. Though it’s more the base oil’s viscosity (often highly viscous PAOs) that gives damping greases their tackiness, they are thickened with silica and PTFE. Silica thickener has a high separation point and works with all base oils. But because PTFE is a very slippery and malleable molecule, PTFEthickened damping greases offer better shear resistance. (In contrast, silica molecules under high shear tend to break down.)
In other greases designed for very high temperatures, PTFE also improves formulas — to survive prolonged exposure to temperatures to 260°C. Another gellant that adds thermal and lubricity stability is clay; it can raise the dropping point, the temperature at which grease becomes liquid, or begins to separate oil. Other times, when temperature resistance (to 120°C) and water resistance is a priority, lithium soap is used. (If the thickener resists water like lithium or calcium soaps do, then rust and corrosion inhibitors are added to protect surfaces from rusting.)
Akin points out the caveats here: “Some additives can be detrimental. Soaps with otherwise good water resistance — like lithium and calcium — do not have good salt-water resistance. However, complex versions of these are generally better in both fresh and salt-water performance. Another example is molybdenum disulfide, which is an EP additive for metal-on-metal applications. It can accelerate wear on plastics.”
When sticky’s too sticky
Chain is a perfect candidate for grease. Its long contacting surfaces can’t often be enclosed economically, so grease offers a perfect solution. In fact, newer formulas address some of the more specific problems of these chain/grease systems. In cold environments, conventional chain oil tackifying agents — which ensure adherence of oil to chain surfaces — tend to thicken excessively and increase oil viscosity. If extreme enough, this thickening can block and prevent protective oil films from reaching internal chain components. Unprotected chain parts then seize or rust, leading to sticky linkages, erratic conveyor motion, and increased energy consumption.
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To improve chain lubrication and simplify maintenance, some synthetic oils are made to stay fluid in the cold. Polyalphaolefin (PAO) synthetic oils in particular give excellent lubrication at high and low temperatures. Unlike oils made in conventional fractionation processes, they are built from smaller molecular building blocks, and synthesized on a molecular level to provide specific performance parameters. The synthetic oil adheres to chains through frequent washdowns, but without emulsifying and blocking lubricant from internal chain surfaces. Even at -18°C its tackifying agent doesn’t solidify, so the lower viscosity is maintained, allowing oil to penetrate internal roller surfaces. With periodic reapplication, it can protect chain for years. Dave Como of the Molykote Lubricants Expertise Group at Dow Corning Corp., Midland Mich., points out one caveat: “The length of time that a product protects depends on the specifics of the given application. for example, desired re-lube intervals. Another thing to keep in mind is that PAO-based lubes aren’t always the most suitable lube. The specifics of the application will determine the best base oil to use.”
Other formulations include polyolester (POE) or polyisobutylene (PIB) synthetics, and hydroprocessed mineral oils. Though PIBs typically act as a tackifier, in some open gear and chain greases PIB may comprise 100% of the base oil. There is a catch: though the formulations do fine when very cold, hot temperatures can harm them. Because oilbased products can volatilize above 200°C, solid lubricants are sometimes more suitable. They’re either dispersed in carrier fluids that promote their penetration into surfaces, or included in resin-bound coatings that act much like lubricating paints.
Esters are formed from the reaction between an acid and an alcohol (or other compound) with an hydroxyl radical. Ester from alkyl or aryl alcohol and phosphoric acid is called alkyl or aryl phosphate. Often, aryl-phosphate esters are used as additives to help lubrication films stay put for augmented anti-wear protection from mineral oil and synthetic lubricant bases. This is particularly important in hydraulic, turbine, and general circulatory oil applications. But when oils are exposed to water, these additives can break down to release small amounts of phenolics — carbolic acids, from the benzene used in the original chemical reaction.
Another problem is the heavy metals that esters sometimes contain. For example, zinc dialkyl dithiophosphate helps lubrication films stay on metal to keep rusting air and moisture from contacting surfaces. However, zinc is one of the heavy metals being proposed for regulation by the U.S. Environmental Protection Agency. If put in effect, pending regulations could limit the allowable release of zinc and other heavy metals into the environment by metal processing and manufacturing industries, and dramatically reduce the amounts allowed in effluent.
Newly developed trialkyl phosphate oil additives are a safer alternative to aryl and alkyl phosphates. The low-viscosity additives don’t release phenolic materials and are ashless — in other words, metal-free. Even so, trialkyl phosphates stabilize lubricants and effectively fight wear in hydraulic, gear, and circulatory oils. According to Indianapolisbased Great Lakes Chemical Corp., the liquid additive is also compatible with many mineral and synthetic oils, so it can be used alone or in conjunction with other metal passivators, dispersants, demulsifiers, and anti-foam additives.
In particular, extreme-pressure applications benefit from the additive. They traditionally use alkyl-phosphate additives, which are difficult to handle and readily hydrolyze. (Alkyl phosphates can also interact with other basic metal salt additives, not to mention insoluble salts in hard-water applications.) On the other hand, neutral trialkyl phosphates are free from such disadvantages and work effectively with anti-oxidant sulfur carriers to provide enhanced extreme-pressure performance in gear and metalworking oils, to name two examples. In these applications, trialkyl phosphates also eliminate the need for additional lubricity additives. Finally, trialkyl phosphates do not contain chlorine or sulfur, which are sometimes undesirable. “It provides an excellent ashless alternative to zinc dialkyl dithiophosphate without sacrificing performance,” says David Phillips, Great Lakes technical manager, Performance Additives and Fluids.
Another cleaving mechanism
Grease gellants and thickeners are indispensable in motion components, but in certain situations no tackifier is completely appropriate. For these applications, solid microporous polymeric lubricants (MPLs) offer an alternative. MPLs are made up of two major components: a polymer (with a continuous microporous network) and oil in its pores. The microporous polymer acts much like a sponge, releasing and absorbing lubricant through capillary action to its surface, and transferring lubricant to surfaces it contacts. As the quantity of oil on contacting surfaces decreases, the MPL releases more oil. Conversely, if there’s too much oil, the porous polymer “wipes” it up and reabsorbs it. Temperature increases also release more oil from the MPL until temperature decreases and oil is reabsorbed. Because of this, MPLs reduce or eliminate the need for relubrication — helpful when location prevents maintenance.
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For example, tapered roller thrust bearings in truck axle kingpins are difficult to maintain and usually fail because of dirt, water, road salt, and other severe road conditions. MPLs inserted into the space between rolling elements and races can provide continuous lubrication and reduce contamination to extend bearing life. This helps these kingpin bearings meet warranty life requirements. Another use is on tapered roller bearings in coal mines, which can seize if contaminated with too much coal dust. MPLs prevent this — and the potential for coal dust fires. And the system isn’t just useful in bearings: solid profiles have been used to lubricate railroad and crane wheel flanges, chains, and ball screws. One special MPL idler sprocket lubricates chains. While they are not designed as load-bearing materials, solid MPL profiles do offer unique lube-delivery mechanisms, especially for difficult- to-reach locations.
However, as Alan J. Heckler, Ph.D. of PhyMet Inc., Springboro, Ohio, explains, “While MPL-coated components generally resist contamination better than greased components, this does not make treated parts waterproof; nor does it prevent corrosion. Direct contact with solvents, cleaners, and acids is not recommended because repeated exposure depletes oil from MPLs.”
Other limitations exist as well. Because MPLs require thermal processing, treated parts must be processed in manufacturer facilities; it’s not possible to put MPLs on parts already operating in the field. Also, MPLs do not dissipate heat rapidly. To prevent the polymer from softening (and ejecting from bearings) application temperature limits should be observed.
Where Ndm values depend on the bearing type and the bore and OD are in measured in mm.
Dave Pierman, product manager of lubrication and delivery systems at The Timken Co., Canton, Ohio, points out a final design consideration: “Because MPLs are placed in bearing cavities, rotational torque is slightly higher than that of grease-filled bearings, especially on startup. That’s because the force imposed by the MPL slightly increases friction.” Even so, this is usually not a problem in industrial applications.