Nihon Kikai Gakkai ronbunshu (Jul 2020)
Influences of tooth flank finishing after fine particle shot peening on tooth surface texture and fatigue life of vacuum carbonitrided gears
Abstract
Vacuum carbonitriding has recently been attracting attention as a heat treatment to reduce the size and weight of power transmissions. In this study, all of the chromium molybdenum steel test gears were vacuum carbonitrided after hobbing. The roughness of test gears was finished by a gear grinding machine to about 3 μmRz on tooth profile. These gears were called Gear G. Additionally, fine particle shot peening was applied on some of Gear G to improve the bending strength by providing work hardening and compressive residual stress on tooth flank. These gears were named Gear GS. Their load carrying capacity was investigated through running tests using a gear testing machine. The endurance limit of Gear G was about 2150 MPa in maximum Hertzian stress at the pitch point. This is an improvement of about 25 % over that of the carburized gear. However, the endurance limit of Gear GS was lower than that of Gear G. In order to investigate this factor, each of the surface topology were observed using a laser microscope. Comparatively, the rate of contact area was decreased after shot peening. This means that the surface topology of Gear GS must be improved, since failure occurs in shorter cycles on Gear GS. In order to simulate the surface roughness finished by the gear lapping technology which has been developed by the authors, some of Gear GS were polished to about 1 μmRz on tooth profile. These gears were called Gear GSP. According to running tests of Gear GSP, the fatigue life was longer than those of Gear G and GS, because the rate of contact area was improved by polishing. It is concluded that if the vacuum carbonitrided gear was treated by grinding, fine particle shot peening and the gear lapping in turn, the appropriate load carrying capacity may be obtained.
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