Effect of Hardened Depth on Fatigue Strength of Carbonitrided Gears

To elucidate the effect of hardened depth on the fatigue strength of carbonitrided gears, gears with three kinds of hardened depths were fatigue-tested using a power circulating gear testing machine. In the case of gears with the shallowest and intermediate hardened depths, the failure mode was spalling. In the case of the gear with the greatest hardened depth, the failure modes were spalling and tooth breakage due to bending at the tooth fillet. The surface durability was higher with greater hardened depth. The bending strength of the gear with the greatest hardened depth was lower than that of the gear with intermediate hardened depth under the same loading condition. Therefore, when carbonitrided gears are designed, it is important to determine the optimum hardened depth of those gears. The change in dynamic performance of every gear was slight during the fatigue processes, since the change of tooth profile of every gear was slight.