In the design of gear and spline teeth, the root fillet area and its maximum tensile stress are of primary concern for the gear designer. In general terms, the tensile stress in the root fillet is based on specific geometries of the design: minor diameter, fillet radius, etc. However, additional concerns regarding the manufacturing method, cutting tool geometry, and process parameters can greatly influence the impact of stress concentration factors in the root fillet area. For a hobbed tooth manufacturing process, the root fillet geometry is controlled by the rack design of the cutter, but also by the number of generating scallops produced by the tool. For a shaping process, the generating scallops are close together and can produce a surface with almost no visible signs of root fillet generating scallops. However, for a hobbing process, the number of threads, number of gashes, and tip radius can create multiple variations of generated scallops. These can create stress concentrations, which can increase the tensile bending stress and potentially impact the service life of the component. For this discussion, stress concentrations caused by root fillet generating scallops will be reviewed. This paper will discuss a specific example regarding parallel-sided splines manufactured with a finish hobbing process and their effects on generating root fillet stress concentrations. To estimate the value of the stress concentrations, Finite Element Analysis was performed on the components for two unique hobbing tool designs. The FE results are compared to actual component field service histories.
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