Effect of Forging Grain Flow on Performance of Hypoid Gear Set - Case Study

Gear sets are one of the prime components in rear axle assembly that drives the wheel by taking the power from gear box. Typically forged gears are used for automotive applications in order to perform to its maximum level while in their service. In this case study heavy truck axle hypoid gear sets with deeper gear ratio (> 6) are considered. The batch-A & batch-B samples are produced through two separate forging routes. Batch A sample is processed through press forge route & Batch B samples are through ring rolling route. Both the batches (4 samples of each) are tested for gear set fatigue bench test validation with particular torque. Test suspended upon failure of the parts. The failure mode of both the batch A & batch B are fatigue in nature & fractured at the tooth root due to root bending phenomenon. But the life of batch A is 58892 ring rear cycles where that of Batch B are is 46496 ring gear cycles. There is drop of 21% in life of gearset performance. Fracture analysis conducted & complete metallurgical results tabulated shows no significant difference in raw material, heat treatment, dimensional & any other parameters. Ishikawa diagram plotted to understand the root cause. The only difference is forging process. Both the samples are analyzed for the grain flow pattern. The batch A gears being made through press forging route are having the well defined grain flow pattern runs in part in radial direction. The batch B gears being made through ring rolling route are having the grain flow in the circumferential direction. In gear tooth cutting method, the cutter moves from outer diameter to inner diameter in both the batches. While doing so the batch A gears having radial grain flow, the cutting action will be along the grain flow. The grain flow in the tooth is not cut & they will have good bonding with the gear tooth root. This grain flow bonding enhances the resistances to tooth root bending fatigue characteristics. But in case of batch B gears having the circumferential grain flow, the cutter cuts the tooth in transverse direction of grain flow. This makes the individual tooth will not have grain flow bonding at the tooth root. Such grain flow pattern hinders the resistance to gear tooth root bending fatigue characteristic. This leads to the early failure of batch B gear than batch A gears. This case study shows that grain flow has great effect on fatigue performance. The gear tooth of press forge can take more deflection or bending fatigue loading condition than the ring rolling route, resulting in higher part performance.