Failure analysis and modification of gear shaft

Failure analysis and modification of gear shaft

Main process flow of gear shaft: forging → normalizing → turning → gear hobbing → carburizing → decarburization → quenching → rough grinding → gear grinding → fine grinding.

Among them, the gear shaft is broken after one year of use, and the gear teeth are intact φ Fracture between 90mm outer circle and gear teeth. We analyzed the broken shaft and put forward improvement suggestions from the aspects of gear design, processing and heat treatment to avoid the recurrence of similar situations.

fracture analysis

The fracture of gear shaft has typical shell pattern. From the macroscopic appearance of the crack, the crack is composed of three regions with different characteristics. Area 1 is the crack source, in half moon shape, with black surface and an area of about 6mm2; Area 2 is the crack propagation area, and the surface is fine porcelain; Zone 3 is a transient fault zone with rough shape, large unevenness and radial stripes, accounting for about 50% of the total area. It is a typical fatigue crack fracture.

The fracture of the gear shaft occurs at the undercut. There is a small black area 1 on the section, indicating that cracks appear here earlier. Carefully observe the fracture of the parts. The finish in the undercut groove is very poor, with deep machining tool marks, Here, a small crack is produced during heat treatment. The crack surface is oxidized during heating and cooling, so it is black. The crack is in the undercut and is not easy to find. In the process of use, the gear shaft is subjected to alternating load and the crack expands gradually. At this time, the expansion speed is slow, forming zone 2 (expansion zone). Due to the high hardenability of 18Cr2Ni4W steel, the hardness of the core after quenching reaches 42 ~ 44hrc and the hardness is high, so there is no obvious ring stripe in the expansion zone. When the fatigue crack expands to a certain extent, the effective section of the shaft shrinks, resulting in insufficient strength, instantaneous overload and fracture. This section has the characteristics of rapid fracture. Due to the high hardness of the shaft and large stress during operation, the area formed by post fracture is large, accounting for about 50% of the total area.

Improvement measures

Fatigue damage is very sensitive to defects and generally originates from parts or surface defects with high stress concentration, such as surface cracks, soft spots, inclusions, sharp corner transition and tool marks. The working stress causing fatigue failure is very low, which is often lower than the yield strength of the material. It broke down in less than a year. It can be seen that the defects on the surface of the gear shaft cause high stress concentration, reduce the fatigue strength and greatly shorten the service life of the gear shaft.

The structure of gear shaft parts, processing quality, heat treatment process, material chemical composition and other aspects will lead to shaft failure. Therefore, only by comprehensively considering the quality from the aspects of improving the design structure of parts, strictly controlling the composition of raw materials, improving machining accuracy and improving heat treatment process, can the quality of the shaft be guaranteed and the service life of the shaft be prolonged. Axis. After improvement, the gear shaft is no longer broken.