Purchasing gear lubricants: be careful when playing the numbers game
Written by John Sander
Background
An assumption has been made that anyone reading this has a basic knowledge of most gear types and their geometric makeup. Gears are a unique application for lubrication in that they provide a different challenge than other lubricant applications. In non-gear applications, the moving surfaces are in sliding motion or rolling motion. Gear teeth, however, experience both rolling and sliding motion at the same time. With these interesting conditions in mind, the formulator has numerous ingredients available to build a gear lubricant that provides proper protection. See Table 1 for a list of common ingredients used in various combinations for gear oil formulas. A formula does not have to contain every one of these additives, and there are multiple additives available to provide specific functions. It is the job of the lubricant formulator to choose the proper ingredients to provide synergistic performance in a given application.
So, how does a formulator choose the proper ingredients for gear oil for a specific application? To answer this question, it might help to define how gear oils are categorized. As you can see in Figure 1, gear oils are categorized first based upon the application as either an open gear lubricant or an enclosed gear lubricant. As these names would suggest, an open gear lubricant is not enclosed in a gearbox or oil sump. As such, open gear lubricants are generally formulated as either high viscosity fluids or greases. This paper will focus on enclosed gears only.
Enclosed gears are generally contained within a gearbox or some other device in which the gears can be bathed or showered with a coating of fluid lubricant. Enclosed gear oils are categorized next as either automotive or industrial gear oils. Both types can be subcategorized as synthetic or mineral oil, which describes the base fluid. Automotive gear oils are then broken down into a final set of subcategories that describe the application in which it will be used, while industrial oils are subcategorized based upon the type of additives used in the formula.
