Stress correction factor in load capacity estimation standard, JIS B 1759, for plastic gears

The present study is part of a series aimed at enhancing the reliability of JIS B 1759, ”Estimation of tooth bending strength of cylindrical plastic gears.” This study examined the stress correction factor converting the nominal tooth bending stress into the local one. The current version has a problem with underestimating the load capacity of internal gears, but considering the contact ratio under load has alleviated the problem. Nevertheless, some issues remain, which led the present paper to determine an alternative stress correction factor through finite element analyses of internal virtual spur gears. Before discussing the alternative factor, examining the fillet profile of injection-molded internal spur gears led to proposing a single circular arc as the standard fillet. Based on that, FEAs of internal virtual spur gears with a single circular fillet determined the modified stress correction factor, which can relieve JIS B 1759 of the underestimations. The modified factor enables the standard to estimate load capacity regardless of external or internal gears, which improves the reliability of the standard.