Every time a gear tooth comes into mesh, it takes a sharp push from its partner and then lets go a fraction of a second later. Run the gearbox at a few thousand rpm and each tooth absorbs that pulse millions of times an hour. The tooth does not snap from a single overload; it fails slowly, from a fatigue crack growing at the root fillet where the bending stress is highest. Predicting that root stress is the heart of gear strength design. This article explains how to model a gear tooth as a short