Of all the lubricated components out there, there are few more pervasive than gearboxes. From high-speed gearing in turbomachinery to slow-turning gear reducers, lubricant selection, application and condition have perhaps the single-largest effect on the reliability and longevity of gearboxes. Here are some of the more common questions and responses I’ve received over the years on the topic of gearbox lubrication and gear oils in general. Question: What is the correct viscosity to use in a typical gear reducer? In surveying this data, we’ll often see viscosity referenced not in terms of ISO grades, but rather the American Gear Manufacturers Association (AGMA) grade or occasionally in Saybolt Universal Seconds (SUS). The AGMA grade is a numeric rating that correlates directly with gear oil viscosity grades. For example, an AGMA 5 oil is equivalent to an ISO VG 220 oil, AGMA 6 to ISO VG 320, AGMA 7 to ISO VG 460, and so on. You also may see the letters S or EP appended to the AGMA grade. This refers to the need for a synthetic (S) oil or an oil formulated with extreme-pressure (EP) additives. For older gearboxes, lubricant specifications are sometimes given in the older and now-defunct unit of SUS. For example, a gearbox manufacturer may state the need to use a “700-second oil”. To convert to the appropriate ISO viscosity grade, a good rule of thumb is to divide the SUS value by 4.6. Therefore, a 700-second oil becomes an ISO VG 150. Q: Should I always stick with the OEM’s recommendation? Q:
When should I consider using a synthetic gear oil? Likewise, at higher operating temperatures caused by high ambient temperatures or the process itself, synthetic gear oils will have a higher viscosity than the equivalent grade of mineral oil and will typically resist oxidative and thermal breakdown better than mineral oil. A rule of thumb is to use a mineral oil if the operating temperature is below 160 degrees Fahrenheit, but consider synthetics or premium mineral-based oils (such as Group III gear oils) if the operating temperature is likely to exceed 180 F. Of course, there are other reasons why a synthetic oil might be advisable, such as for extended oil drain or other operational reasons. When using synthetic gear oils, pay close attention to the type of synthetic in use. Many synthetic gear oils are made from polyalphaolefin (PAO) basestocks, which are compatible with conventional mineral oils. However, we increasingly see the use of polyglycol gear oils, which have excellent lubricity while helping to keep the gearbox clean of deposits due to their natural detergency and “clean-burning” tendency. In fact, some gear manufacturers are factory-filling their boxes with polyglycol-based oils. Polyglycols are incompatible with hydrocarbon basestocks (mineral or PAO synthetic), thus requiring extreme caution in helping to prevent accidental mixing and cross-contamination. When switching from a hydrocarbon oil to a polyglcol, perform a thorough cleaning and flushing to help prevent hydrocarbon residues from reacting with the polyglycol gear oil. Q: We recently switched to a synthetic oil because of
higher operating temperatures. Our lubrication supplier is recommending an ISO VG 460 synthetic in place of the ISO VG 680 mineral oil we’ve always used. Why the change in viscosity grade? Q: I heard that EP additives are
corrosive to yellow metals such as brass and bronze. Is this true? A good way to check to see if an oil is “chemically active” is to look at the specification sheet provided by the lubricant manufacturer. On the sheet, you will typically see a test referred to as “copper strip corrosion” (ASTM D130). This tests how chemically reactive an oil is to copper and copper-containing alloys. For gear oils, a 1a rating is typically a good indicator that the oil is chemically inert, while higher ratings (such as 1b or 2a) might indicate possible problems when used in certain gearing at elevated operating temperatures. Often, a lubricant manufacturer will specifically state “this oil should not be used in gearboxes containing yellow metals”; other times, the manufacturer will simply state that the oil is appropriate for “steel-on-steel” applications – the implication being that they should not be used in situations where the ring gear is brass or bronze.
Q: For some of our gear drives, the OEM is calling for a “compounded oil”. What does that mean? Q: My lubricant supplier says that if we switch to a synthetic, we can expect to see a 10 percent reduction in energy consumption. Is this true? Likewise, if the dominant frictional loss is not due to fluid friction but rather the process or some other mechanical factor, again it’s unlikely that you’ll see any difference. However, in some situations where there’s plenty of sliding friction and the lubricant is responsible for most of the energy loss, there are circumstances where a 5 to 10 percent drop in energy has been seen. As with everything, consider all factors rather than simply accepting anecdotal evidence, no matter what the source.
What is EP gear oil?Extreme pressure (EP) gear oils are used to minimize wear and scuffing in automotive and industrial applications under high load conditions, especially where high peak or shock loads are encountered.
What is gl2 gear oil?Venom® Synthetic GL-2 Gear Oils are non-corrosive to gear and bearing materials such as steel, copper, bronze, Babbitt, and cadmium-nickel; these oils provide superior foam resistance and water separation.
What is non EP gear oil?Non-EP Synthetic Gear Lubricants (SL-SGLX) are designed for use in a wide range of gear applications. They are formulated to provide a long oil life with excellent oxidative stability, film strength, lubricity and wear protection. They offers outstanding protection — even in the most demanding applications.
What is 140 grade gear oil?Gear Oil 140 is a premium, heavy duty, mineral automotive gear oil manufactured with modern extreme pressure additives and friction modifiers meeting the requirements of API GL-5/6 and MT-1 as well as many other manufacturer specifications.
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