Limitations of bench testing for gear lubricants

Base oil type, viscosity, and additive packages of gear lubricants influence gear failures as wear, scuffing, micropitting and pitting as well as power loss and efficiency. Numerous test methods have been developed to quantify these influences. Different test equipment like a gear test rig, a twin disk machine and the Shell four ball tester are shown and discussed. Results of lubricants in these bench tests on scuffing, pitting and friction behaviour are shown and compared to each other and to gear performance. It can be summarized that poor correlation has to be stated for the bench tests with 100% sliding rate. For the twin disk simulation poor correlation was found for scuffing results, good relative but poor absolute correlation for pitting results and both good relative and absolute correlation for the frictional behavior. From these investigations it has to be concluded that bench test results applied to gears have to be regarded very critically and can only be interpreted correctly with excellent experimental background and experience.