The Influence of Additive Chemistry on Gear Micropitting

Gear micropitting has been a highly visible issue in selected applications in recent years, most notably in large wind turbine transmissions. Various industry groups have addressed the problem from their own area of expertise. This has included evaluation of the gear design characteristics, surface finishing, the use of special coatings, and lubrication. A common approach to improve the lubrication has been first to increase the viscosity and create thicker films, which, in turn, reduce the amount of surface asperity interaction. Another approach from the lubricant side has been to alter the additive chemistry to effect a change in the wear properties of the system. This paper discusses the potential effects observed for different antiwear and EP chemistry on the micropitting of cylindrical gears. Tests were conducted in an FZG test rig which has been used by the industry as a guide to general gear performance. Fluids were examined in a series of experimental designs which served as the iterative process leading toward an optimized additive system. The results show that the EP, or antiscuff agent, was the most effective component at reducing the level of micropitting.