The reduction in viscosity of Automatic Transmission Fluids (ATFs) for improving fuel economy results in a reduction in transmission gear fatigue life due to occurring pitting. The aim of this study is to establish a suitable test condition for evaluating differentiation in the gear pitting life between ATFs by using a Forschungsstelle fur Zahnrader und Getriebebau (FZG). In this study FZG gear sets were modified so that gear teeth were misaligned and the contact tooth width was shortened by offset in the direction of the shaft. The fatigue pitting life in response to ATFs under this modified FZG test condition was of the same order as that obtained in the actual transmission test under a severely offset condition. A sulfur-containing additive could play a role on the gear tooth life under severe condition, which was most likely attributed to tribofilm for motion on the gear tooth surface.

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