The method and assumptions used for the application of EHD theory to the calculation of gear tooth oil film thicknesses for the design and analysis of industrial gear drives is presented. A nomograph, utilizing readily available gear geometry, operational, and lubricant parameters, is illustrated which allows rapid determination of calculated gear tooth oil film thicknesses for a wide range of gear drive conditions. Gear tooth surface distress is related to the specific film thickness, λ, the ratio of calculated oil film thickness to the magnitude of the composite surface texture. The term “surface texture” is introduced for gear contacts to indicate that surface attributes coarser than roughness importantly relate to tooth surface distress, but a sophisticated method for its quantitative assessment has not been developed. Data from several hundred petroleum lubricated laboratory tests and closely followed field applications which include through hardened gears of 1 in. to 15 ft in diameter are used to correlate specific film thickness and gear tooth surface distress. Curves are given to predict the probability of occurrence of such distress over the range of pitch line velocities of 4–35,000 ft/min.

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