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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?
Answer: Viscosity selection for gearboxes is a careful balance between load, speed/reduction ratio, gear geometry and operating environment. Like most applications, a good place to start is with the original equipment manufacturer (OEM) recommendations. This information is commonly posted on the nameplate affixed to the outside of the gear casing.
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?
A: The answer to this question is most definitely “no!” While you certainly shouldn’t be going against the OEM’s recommendations without good reason, extreme ambient operating conditions (e.g. very high or very low temperatures), shock loading or extreme duty cycles, or higher-than-normal load ratings may dictate that a change to the OEM’s recommendations is warranted. It’s always a good idea to consult with a lubrication engineer before selecting a viscosity grade other than the one recommended by the OEM.
Q: When should I consider using a synthetic gear oil?
A: Synthetic gear oils offer some very real advantages in some circumstances. For example, in extremely low temperatures, a synthetic gear oil will have a much lower viscosity than the equivalent grade of mineral oil. This can be an advantage during cold temperature start-up when channeling can cause temporary lubrication starvation, particularly in splash-lubricated gear drives.
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?
A: When switching from mineral to synthetic gear oil, it’s not uncommon to drop down one ISO grade. The reason is due to the fact that synthetic oils typically have higher viscosity indexes than mineral oils. As a result, when you compare the viscosity of, for instance, a ISO VG 680 mineral oil to that of a ISO VG 460 synthetic, they will have very similar viscosities at 160 F. Before applying this rule, it’s important to plot the viscosity-temperature profile of each oil and consider the anticipated operating temperature along with high and low ambient temperatures to insure you select the correct grade for the specific application.
Q: I heard that EP additives are corrosive to yellow metals such as brass and bronze. Is this true?
A: In some circumstances, this can by true. Certain types of EP additives are designed to react with metal surfaces under elevated temperatures to protect them under boundary lubrication conditions. These types of additives are often referred to as “chemically active”, and at elevated temperatures (greater than 140 to 150 F), they can indeed start to react with yellow metals. For this reason, apply caution when selecting gear oils for worm drives; the ring gear is often a yellow metal alloy.
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?
A: For certain gear geometries, particularly worm drives, the dominant frictional force is sliding friction as opposed to rolling friction. Under these circumstances, the lubricant must help reduce the coefficient of sliding friction. To do this, special additives that historically have been comprised of fatty acids are used to help the surfaces slide relative to one another. Another name used to describe this type of oil is “steam cylinder oil” because the same effect is desired in lubricating steam cylinder walls. Compounded oils are not commonly used for steel-on-steel gears.
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?
A: The answer to this question depends on several factors. For example, when applied to a spur gear, which will typically operate with 95 percent or higher efficiency, it’s hard to understand that there is sufficient frictional loss from the lubricant to justify this claim.
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.