How Gear Teeth Fail: Predicting Root Bending Stress with the Lewis Equation

Tooth bending at the root fillet is the failure mode that destroys gearboxes, and predicting it is essential for cost-effective design. The Lewis equation, unchanged since 1892, remains the foundational tool for sizing gears—making this calculation critical before committing to production. Understanding how module, face width, and tooth count control the stress equation empowers engineers to make the right trade-offs early.