The fatigue fracture of the gear reducer rack occurs in the low-speed gear pair.Observing the meshing situation of the entire gear (taking one time in January 1997 as an example), it is found that the meshing gear has a large area of pitting corrosion of the rack due to contact fatigue failure, the spots are dense, and the width of individual chip etching pits is 1 ~ 10mm. 15 shafts are peeled off (numbered No. 2, No. 1), and 2 large gears of the low-speed shaft are peeled off (numbered No. 2, No. 3).Observe the fracture condition of the four racks that have been peeled off: No. 4, 2, and 3 are dark gray ductile fibrous fractures, and there are obvious indentations on the meshing pitch circle. The analysis shows that the No. 4 rack is peeled off first, and extrusion causes 1, No. 2 and No. 3 racks are passively fractured, and the fracture is the normal fracture structure of the fractured tooth caused by fatigue failure caused by overload operation.The surface of the No. 4 rack is light gray, smooth and shiny, and the broken tooth is a cleavage fracture of transgranular fracture, which is an abnormal brittle fracture.
212 Physicochemical analysis of spalled racks.
21211 Hardness Check Rockwell hardness (HRC) is measured at the top surface position of the rack and the position of the middle core of the rack according to the condition of rack spalling.
The shaft gear is moderate (the process specification is HRC56~62), and the difference between the inside and outside is the normal situation after the carburizing and quenching of the tooth surface.
21212 Chemical composition analysis The chemical composition analysis is carried out by taking samples from different positions of the tooth surface and the core of the four racks.
It can be seen from the test results that the composition of the material core is relatively uniform, and the high carbon content at the top of the tooth is due to the carburizing treatment of the tooth surface. The difference is that the carburizing amount of the intermediate gear shaft is higher.
21213 Low magnification histological examination Since the problem occurred on the intermediate gear shaft, the shaft was sectioned transversely, and a low magnification acid pickling macroscopic examination was performed.As shown, the central loose area is offset by the area 23 to the left, and a box-shaped segregation is slightly visible.The segregation on the right side is heavier than the left side, and there are three voids on the edge, which can be analyzed as the traces left by the internal inclusions after cutting and pickling.The surface contact condition of the five teeth on the left is good, with slight pitting corrosion. After running-in, the contact surface expands, the contact stress is correspondingly reduced, and the pitting stops developing, which is the allowable staying pitting.The eighth tooth in question (ie, the No. 1 rack) is just on the right side, which also reflects the material used as the gear shaft. During the manufacturing process of forging and pressing, the center line of the round steel is offset or the raw material is cut into the factory. caused by insufficiency.
For 21214 metallographic microstructure inspection, only sample No. 1 was taken for metallographic structure analysis. It was observed that its oxides were grade 215, sulfides were grade 310, and there were two silicate inclusions. Obviously, non-metallic inclusions were unqualified (standard specification ≤215 grades).
From the above analysis, it can be seen that the material quality of the intermediate gear shaft (20CrMnTi) of the gear reducer has problems, the internal composition is uneven, and the comprehensive factors such as improper processing and manufacturing process and heat treatment process cause microcrystalline cracks.During overload operation, microscopic cracks propagate transgranularly, resulting in macroscopic fission and rack fracture.Its bright fracture can also confirm this.