Plastic gears have various merits such as silent running, self-lubrication, and so on, but they also have some demerits such as less load-carrying capacity and unsteady life time. In our sixty-two experiments, most of the failures of plastic gears were fractures of teeth from near pitch points. On basis of these results, a new design equation for bending strength, described by the bending stress at the pitch point, is proposed. In the equation, some coefficients: tooth thickness coefficient, load increasing factor, stress correction factor, etc., are introduced to realize higher load capacity economically by making the best use of the characteristics of plastic materials.
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