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Effect of Uncontrolled Heat Treat Distortion on the Pitting Life of Ground, Carburized and Hardened Gears

98FTM04

Heat treatment causes distortion of gear tooth geometry. To improve tooth quality, particularly after carburizing and hardening, gears are generally ground. For low distortion, stock removal is small up to a maximum of 0.005 inch from each tooth flank. This allows minimum surface hardness reduction with no possible detrimental effect on gear pitting life. Uncontrolled distortion, on the other hand, results in an undue amount of stock removal with a significant loss of tooth surface hardness. Lower surface hardness accelerates contact-stress-induced tooth pitting that reduces gear life. For heavily distorted gears, the pitting life may be reduced as much as 30 percent after grinding. For a realistic evaluation of the pitting life of ground, carburized and hardened gears with uncontrolled distortion, derating factors are necessary and established for a number of gear materials.
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