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The effects of different shot peening parameters on the fatigue life of rear axle hypoid pinion gears were evaluated. Shot hardness, shot size, intensity, shot velocity and prior heat treatment were studied and compared to current production. Metallurgical analysis of the carburized SAE 4320 gears along with residual stress gradient analysis at tooth root fillets were performed. The results showed that significant fatigue life enhancement can be achieved by precision shot peening (directed, air nozzle type) and proper determination and tight tolerance of shot peening process parameters and heat treatment.

Shot peening is known to be beneficial for improving fatigue life and stress corrosion resistance of components. Shot peening has historically lacked definition of qualitative process variables and optimum quantitative levels. This problem is compounded by lack of definitive process controls for consistently reproducing the process variables affecting workpiece performance. The authors chose shot peening media as first of a series intended to holistically examine which peening variables affect workpiece performance, how performance is affected, and how these can be metered, controlled, and/or documented. Peening media problems are categorized into seven groups. Each is examined conceptually as to effect on workpiece life. Determination is made as to available equipment, quality assurance, and documentation tools.