scope:

There is a wealth of literature and test results regarding the subject on single directional gear tooth bending stress and life relationships (S-N curves), they have been published on various journals and handbooks over the past decades, and several of them had been widely accepted and adapted as industry standards by different gear societies around the world. However, very limited information regarding the bi-directional tooth bending life has been revealed. To fill in the above mentioned gap for practical usages, the authors first intended to apply the traditional fatigue theories such as modified Goodman, Gerber, Morrow or similar theories with minor modifications to derive a series of S-N equations for different loading conditions, but the correlation with the actual test results was not satisfied. Nevertheless, from the observation on these test results, the slopes and endurance limits on the fitted S-N curves from all the test points were reasonable closer to each other, as long as the test gears were produced by the same material and similar manufacturing process. Based on the above observation, the author proposes a new approach that uses the common (or averaged) slope and endurance limit, and a series of S-N curve equations on any loading conditions can be derived, once the single directional S-N curve has been obtained.