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Influence of Surface Roughness on Gear Pitting Behavior

04FTM04

Influence of Surface Roughness on Gear Pitting Behavior T.C. Jao, M.T. Devlin, J.L. Milner and R.N. Iyer, Afton Chemical Corporation and M.R. Hoeprich, The Timken Company In earlier studies, surface roughness had been shown to have a significant influence on gear pitting life. Within a relatively small range of surface roughness (Ra = 0.1 - 0.3 micron), gear pitting life as measured by the FZG pitting test decreases as gear surface roughness increases. This inverse relationship between gear surface roughness and pitting life is well understood in the field. To determine whether this inverse relationship is applicable to a wider range of surface roughness values, we have conducted a pitting study using gears whose surface roughness ranges from 0.1 - 0.6 micron. The results were not completely expected. The study shows that the micropitting area is radically larger when the gear surface roughness is close to the upper limit of the range studied. Plasticity index, which approaches a value of around 3.7 for the rougher gear surface, appears to be responsible for the formation of such large micropitting area. At the same time, the formation of a pit is also greatly delayed. Not only is the pitting life significantly longer, but the initiation of pits can occur near the pitch line. This paper discusses how high surface roughness introduces a wear mechanism that delays the formation of pits.
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