Failure Analysis of Main Helical Gear Pair of Through-drive Axle for a Three-axle Special Vehicle

We can basically conclude that the root cause of tooth breakage and brittle failure of medium drive gear pair is that the actual maximum input torque value of axle main reduction can not reach the maximum allowable value of design and is already less than the actual torque value from engine output to main reduction:

(1) The failure of helical gear pair begins with the main screw. During carburizing, quenching and tempering, problems occur in the gear, and the metallographic structure changes, resulting in low strength of the center part and insufficient depth of the surface hardening layer. When subjected to large loads, the gear breaks brittly and is affected by the secondary helical gear.

(2) Differential case has low hardness, while fastening bolt has high hardness, which easily causes deformation or wear of differential case and stop ring, thus biasing the gear pair of reducer and concentrating the load on the large end of main screw.

In order to improve product reliability, it is decided to make corrections in the following areas:

(1) In order to improve the strength of the gear, the material of the main helical gear pair was changed from 20 CrMnTi to 20 CrMnMoH.Compared with 20 CrMnMoH, 20 CrMnTi contains Ti, which tends to be brittle and has a higher sensitivity to tempering brittleness. Therefore, it has low impact resistance and is easier to meet technical requirements after material changes.

(2) In terms of technical requirements, the requirement for core hardness is relatively low, which is not conducive to the control of core strength.Re-define the material hardness and properly raise the lower limit: after changing the material, the hardness of the core increases to above 37HRC and the surface hardness is controlled to about 60HRC. At this time, the maximum input torque that the axle main helical gear pair can withstand is about 12800N (calculated value).

(3) It is required to strictly control the heat treatment process of gear material. Finished products are submitted to third parties for inspection and provide inspection reports. Our company carries out physical and chemical inspection from time to time.

(4) The length of large screw fastening bolt is too short and the screw-in length is not enough, which may cause thread tooth detachment or loosening and increase bolt length.

(5) The hardness of fastening bolt is too high and does not match with differential case and stop ring, which may easily lead to deformation or wear of parts. Re-select bolts;

(6) In the import inspection, excessive clearance of taper bearing is easy to cause the active helical gear to move, so the inspection and control of clearance of taper bearing are added in the import inspection;

(7) The design input should be reasonable for the selection of axle parameters. It should be considered that special vehicles are prone to impact when driving on bad roads, which causes instantaneous increase of torque and exceeds the bearing range of axle main reduction. According to experience and the recommendation of parameter book, the calculated torque from engine output to axle main reduction should be less than 10% of the maximum allowable input torque value of axle main reduction.Left and right (safety system should be left).

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