Study on Mechanism of Bending Ftigue Breakage and Strengthening Method of Gear Teeth

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Bending fatigue tests (pulsator tests) for shot-peened sintered metal gears were carried out, and the effect of shot-peening on the bending fatigue strength of sintered metal gears was determined.The tooth profile changes of gears due to ion-nitriding, tufftriding and case-hardening were examined, and the deformations of gears due to these strengthening methods were determined. The bending fatigue tests for ion-nitrided gears with different case depths under uniform cyclic loading conditions were carried out, and the bending fatigue strength of ion-nitrided gears was determined. Furthermore, the bending fatigue characteristics of the ion-nitrided gears under two-step loading conditions was discussed on the basis of Miner's hypothesis.The main results obtained from this investigation are summarized as follows.1. The surface hardnesses of case-hardened sintered metal gears increase by applying shot-peening, but the case depths of these gears do not change depending on shot-peening. The surface hardnesses do not change with the carbon content (in the range of C0.3-0.5%), but the core hardnesses increase with the carbon content.2. The bending fatigue strengths of case-hardened sintered metal gears become 20-40% larger by applying shot-peening.3. The tooth profile changes of gears due to ion-nitriding and tufftriding are almost equal and fairly small, but those due to case-hardening are fairly large.4. The bending fatigue limits of S45C and SACM645 ion-nitrided gears become maximums for ion- nitriding time 34 and 17 hours, respectively, and are about 40 and 80% larger than the normalized gears. The bending fatigue limits of S45C gears become about 30% larger by tufftriding.5. The bending fatigue breakage of ion-nitrided gears occurs by the propagation of some of small cracks which initiate at the compound layr.