Gear oil respect
Gear lubricants are not so much ignored as they are misunderstood. Most of the time they are considered a lower performance lubricant as opposed to hydraulic oil. “That's understandable, because some people think it's not like gear oils are working in high temperatures or high pressures,” says Walt Silveira of Shell Lubricants. “But gear oils actually must be able to provide lubrication in both situations (high heat and pressure) and not fail.”
Complicating these requirements is the fact that the gearboxes on today's machinery are required to transfer more power and be more durable than their predecessors. Teeth and bearing loads in gearboxes are certainly higher, says Tim Copper of Lubrizol Corporation.
Higher loads can result in higher temperatures from friction. This, then, accelerates the oxidation process, which degrades the lubricant and produces sludge, varnishes, gums, and acids.
A common mistake is considering gear lubes no different than engine oil. That is somewhat understandable, as all lubricants are formulated with a base stock oil enhanced with additives. Beyond that simple fact, Silveira says gear lubes differ from other lubricants because they are formulated to provide a film that separates the metal surfaces of gears and bearings.
More specifically, gear oils are designed to provide for elastohydrodynamic lubrication (EHL). This describes a condition in which metal surfaces that contact each other have a low degree of conformity and high contact pressures. In the case of intermeshing gears in a gearbox, the surfaces in motion trap the lubricant film under extreme pressure. When this happens, gear oil's viscosity actually increases to the point where the lubricant forms a pseudosolid film. This almost-solid film separates the surfaces of intermeshing gears. As long as operating conditions (such as speeds, loads, and temperatures) are not excessive, surface contact between gears may never occur due to this remarkable characteristic of gear oil.
But during periods of cold start-up, extremely high operating temperatures, or high shock loading, this full fluid film can be destroyed. Unless a boundary lubricant is present in the gear lubricant when this fluid film is destroyed, wear can take place.
Mark Betner of Citgo explains that many lubricants for manual gearboxes and differentials are hypoid gear oils. These oils contain extreme pressure (EP) additives and antiwear additives.
EP gear oils are capable of performing well in a wide range of temperatures, speeds, and gear sizes to help prevent damage during starting and stopping. The additives used in EP oils are rarely used in motor oils since they can react with water and combustion by-products to form acids that corrode engine parts. For example, the additive sulfur-phosphorus brings antioxidants, demulsifiers, friction modifiers, viscosity modifiers, and metal deactivators to gear oil.