Machinery Lubrication

Machinery Lubrication March April 2018

Machinery Lubrication magazine published by Noria Corporation

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www . | March - April 2018 | 51 "Although polishing wear can have both good and bad consequences, it usually is best to avoid any wear mode." Chemical Polishing Chemical polishing happens when a chemica l is corrosive enough to remove the surface layer of metal, exposing the underlying metal, which typically is reflective and very polished. This type of polishing is most like the silver polish mentioned earlier. In this mechanism, a mildly corrosive compound is applied to a surface. e compound then reacts with the surface material, forming a soft film on the metal. When the polish is removed, it takes this soft film with it, leaving behind an unoxidized and often brightly polished surface. The Pros of Polishing Wear Polishing wear can be used to your advantage. W hen machine parts are highly polished, there tends to be less drag or friction between the parts. is will help with the equipment's eff iciency by reduci ng a mp d raw, f uel consumption and possibly even the operating temperatures. Machine surfaces are often rough with microscopic projections known as asperities. ese asperities greatly contribute to the friction between moving parts. e larger the asperities, the more friction will be generated, which must be overcome during operation. A good example of this would be rubbing two pieces of sandpaper against each other. Each piece of grit on the paper represents an asperity. If rubbed together, the individual pieces of grit come in contact and require more force to pass over each other. When machine parts are finely polished, it is like reducing or removing the grit from the paper. Less or smaller grit means less force and less surface damage when these parts move relative to one another. In terms of lubrication, more polished machine surfaces require a smaller lubricating f ilm for protection. e goal of a well-oiled machine is to have a lubricating film that is larger than the surface asper- ities. When the surfaces are very rough, the film must be larger. is is typically achieved by using higher viscosity lubricants. e higher the viscosity, the more viscous drag in the machine and the more fuel or energy that must be consumed to churn through the lubricant. If the machine parts are finely polished and have smaller asperities, the lubricant film can be smaller, and subsequently a lower viscosity lubricant can be used. is means less viscous drag and better energy efficiency. The Cons of Polishing Wear Although polishing can be bene- ficial in many ways, it can also be a negative process if it occurs uninten- tionally. Unintended polishing may take place when a concentration of solid contaminants reaches a certain point in both the size and number of hard particles. While the machine is operating, this mixture of oil and particles moves throughout the system. ese particles begin cutting into the machine surfaces, leaving behind small, linear scrapes that ultimately lead to a polished surface. A not her ex a mple of how polishing can happen unintention- ally is when the wrong additive package is selected for a piece of equipment. Many gear oils rely on the addition of extreme-pressure (EP) additives to aid in lubricating moving parts in boundary lubri- cation regimes. Some of these EP additives are chemically aggressive and actually eat or pit into machine parts, causing chemical polishing. When the additive package is matched appropriately with the machine, this chemical reaction will be mild, and the soft metal f ilm formed by the polishing process can be beneficial in reducing machine wear during boundary lubrication conditions. When the additive package is not matched correctly, the resulting chemical wear becomes far too extreme, removing more surface metal and ultimately reducing the machine's surface profile. As the surface profile is reshaped, the parts do not mate together as well and in some cases can lead to catastrophic failure. Machines most susceptible to aggressive chemical polishing are those that employ the use of a soft metal or a softer metal alloy. A good example would be a worm-drive gearbox. Frequently, the worm is made of steel, while the worm wheel is made of brass or some copper alloy. Chemically aggressive EP additives will attack the soft copper. is polishing process typically will go beyond the mild surface polish and turn into severe chemical corro- sion, leading to the eventual failure of the machine. Prevention To ensure the oil you use will not cause severe chemical polishing, check the lubricant's technical data sheet for the results of the ASTM D130 test, which is known as the copper strip corrosion test. How the lubricant performs on this test will tell you how corrosive the fluid and additives are to softer metals. e test takes a strip of freshly polished copper and subjects it to the candidate fluid. After it is heated, the strip is examined for corrosion. e results are reported on a scale ranging from 1A (virtu- ML

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