Internal gearings are commonly used in planetary gear boxes, oil pumps and adjustment gear boxes. Due to the smaller addendum diameter compared to pitch diameter tooth root is expanded and accordingly stronger. The concave flanks of the internal gearing and convex of the external lead to a low Hertzian stress on the flank and consequently increased transferable forces. Furthermore this design yields to a narrow osculation of internal and external gear parts and a low constructed space. WELTER zahnrad manufactures internal grindings