Gear stress reduction using internal stress relief features

This paper discusses research into the possibility of reducing gear tooth root stresses by adding internal stress relief features. For many years, gear designs have improved with the incremental addition of design features. Materials have improved, surfaces are selectively hardened with hear treatment and carborization, and shot peening is used to improve surface properties. All of these improvements are related to material attributes. Little has been done to change the gear geometry to improve durability and strength. Although the exterior of the gear is governed by the necessary involute profile of the the teeth, nothing prevents interior changes. In this study holes were drilled along the axis of a test gear segment in an effort to provide stress relief in critical areas. A finite element model was constructed for use in a systematic rest of the effect of hole size and hole placement on tooth root stress. A constant force was applied at the pitch diameter, and all results were normalized with respect to the values obtained for a solid gear. Results show that it is possible to reduce the tooth root tensile stress considerably without producing stresses in the holes greater than on an unmodified gear. These results were verified by photoelastic resting on greatly oversized plastic models. Since gear teeth fail due to fatigue over many cycles, even a slight reduction in the root tensile stress produces a great increase in fatigue life.