Investigation of Effects of Additive for ATF and Test Condition on Pitting Life of Gear Tooth in the FZG Pitting Test

Abstract

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.

Evaluation of Mechanical Characteristics of Tribofilm Formed on the Surface of Metal Material Due to Friction under Lubrication with Automatic Transmission Fluid

“…The tribological properties can possibly change depending on the type of lubricant, and a wide selection of lubricants, additives, and mating materials is present [20,21]. The thickness, hardness, and surface roughness [22][23][24][25] of the sulfur-or phosphorous-based tribofilm produced on the surface were reported to be key factors affecting the pitching life of the friction surface. When the thickness of the tribofilm (referred to as the boundary lubrication film in the references) formed on the surface after friction is 40-50 nm, it is classified as a thin film; and when the thickness is >130 nm or more, it is classified as a thick film.…”