During the installation test of a large helical gear of a loader drive axle during its development, it was found that the gear failed when it worked for 200~1 300h.The inner diameter and outer diameter of the large helical gear are 210 mm and 380 mm respectively.The macro-morphology of the failed large helical gear is shown in Figure 1. The gear material is 20CrMnTi steel.Refer to JB/T 6041-2013 Technical Requirements for Gear Pairs of Wheel Construction Machinery Drive Axle Main Reducer: After hardening, the surface hardness of the parts is 58-64 HRC and the core hardness is 33-45 HRC.Gear heat treatment is carried out according to JB/T 5944-1991 General Technical Specifications for Heat-treated Parts of Construction Machinery.In order to find out the failure cause of broken teeth of this large helical gear, the author checks and analyses it and gives improvement measures.
(1) The early failure of large helical gear of loader drive axle is mainly due to the small radius of curvature at the fillet corner of the tooth root, the dimension of engagement spot and the low hardness of the center of the gear, which causes the stress concentration at the tooth root to be strengthened. Under the bending stress during the loading test, fatigue cracks are initiated at the tooth root and propagate, resulting in the failure of the broken teeth of the gear.
(2) The stress concentration at the root of the tooth can be reduced and the bending fatigue strength of the teeth can be improved by replacing the gear cutting tool with bigger fillet on the top of the tooth, changing to 20CrMnTiH steel with good hardenability, adjusting the tooth shape properly and increasing the strong shot peening process.
(3) The service life of the modified large helical gear has exceeded the specified service life (2 000 h) and no early failure of broken teeth has occurred.