Full Description JIS B 1759:2019 stipulates the method for estimating the strength exhibited by plastic materials against bending fatigue breakage at the dedendum when they are used for cylindrical gears (hereafter referred to as the gears) that constitute a paralleled gear pair. No corresponding International Standard has been established at this point. Evaluation is made by obtaining the allowable bending stress of the gear materials based on results of an operation test of an external gear pair.