Abstract
The plastic gears, made of the low fractional materials or emploied the grease, break by the bending stress. Since the larger bending deflection with transmitted power the larger real contact ratios of plastic gears, the bending stress calculation formulae of the metalic gears don't be able to apply for the plastic gears. As the studies of the Prof. Yelle's works, the pulsating impact fatigue S-N curves of cantilevers, the impact fatigue lives of quasistatic gears and the power circulation test using PA66, POM, PA66-PE alloy and the pair of PA/POM gears, a practical formula corresponded the ISO standard is proposed by the introducing of the load distribution factor, the thermal effect factor and the molding fault factor.
