Influence of gear surface roughness , lubricant viscosity and quality level on ISO 6336 calculation of surface durability Technical Report

The International Organization for Standardization (ISO) provides a method by which different spur and helical gear designs can be compared in terms of the risk for surface fatigue. The calculation of surface durability is based on Hertzian contact theory modified by factors for making adjustments peculiar to gearing. These factors are mainly determined through experimentation, and include surface roughness and lubricant viscosity, as well as the gear quality level. The ISO standard has 13 quality levels, 0 to 12, where 0 describes a more or less theoretically perfect gear tooth. Ten or so tolerance parameters are specified for each gear quality level, for example profile variations and pitch (tooth spacing) variations. In this study, a robustness analysis based on factorial design was performed for a helical gear. The idea of a robust design approach is to be able to find the design least sensitive to variations such as manufacturing variations and product use variations. A number of gear geometries were generated by varying the gear design parameters: pressure angle, helix angle, and face width. Each established gear design was then exposed to different combinations and levels of variations in surface roughness, lubricant viscosity, and quality level. The aim of the robustness procedure was to find out whether the gear designs produced are sensitive to variations in these noise factors; that is, whether the ISO standard can be used to simulate their effect on surface durability. The analysis revealed that the different combinations and levels of variations, separately, influence the load capacity. However, it is not possible to find gear geometry that is more or less sensitive for variations in surface roughness, lubricant viscosity, and quality level. Also, the ISO standard calculation procedure is restricted to only a few tolerance parameters; pitch error, lead deviations (misalignment), and profile form deviation. The factors influenced by these parameters are currently the internal dynamic factor, the face load distribution factor, and the profile load distribution factor.