Carbonitrided gears having three kinds of standard pressure angles were fatigue-tested using a power circulating gear testing machine, in order to elucidate the failure mode and changes in tooth profile and dynamic performance during the fatigue process. Two kinds of failure modes were observed. One was tooth breakage due to bending at tooth fillet, the other was spalling near the working pitch point. Surface durability of gears for spalling depended on the peak amplitude of the ratio of orthogonal shear stress to Vickers hardness. The surface durability was higher with smaller pressure angle. Under the same Hertzian stress, the bending fatigue life of a gear with the smallest pressure angle was the longest. The fatigue strength evaluated under normal load increased with increasing presure angle. The changes of the dynamic performance during the fatigue process were nominal for all three kinds of standard pressure angle.
