Failure Analysis of Eccentric Gear Spindle of Failure Analysis Press Chu Shujun (Jinan Second Machine Tool Group Co., Ltd., Jinan 250022) The shaft fracture is a fatigue fracture and has the characteristics of low cycle fatigue fracture. The main reason for the fracture of the gear main shaft is found. At the stress concentration and overload operation, the working stress of the shaft is too large, causing the fracture subject matter, the low-cycle fatigue fracture stress of the gear spindle, the chemical composition of the gear spindle (w% 1 situation, the D31-400t press produced by our factory, user After two months, the eccentric gear main shaft (see) of the press was found to be broken. The broken part was broken along the cross section of the oil hole on the right side of the shaft as indicated by the arrow. The shaft material is 45 steel, and the process is forging-normalizing- Roughing and quenching and tempering-machined intermediate frequency quenching (45~52HRC) tempering-grinding. In order to find out the cause of the main shaft fracture, the fracture was examined and analyzed by scanning electron microscopy and optical microscope.
2 Raw material analysis 2.1 Chemical composition analysis The chemical composition of the gear spindle was measured by a Swiss ARL 3460 direct reading spectrometer. The results are shown in the attached table.
The measured value of the element 45 steel standard value 2.2 hardness measurement measured the surface hardness of the gear spindle is 48 ~ 49HRC, in line with the part design requirements (45 ~ 52HRC); the core hardness (30mm from the surface) is 199HB2.3 metallographic examination in the oil hole section The metallographic sample was cut with a wire cutter.
The test specimens of non-metallic inclusions were observed after polishing (not eroded) under a microscope of 100 times, and evaluated according to GB/T 10561-1989. Grades of brittle inclusions, grade 2 of plastic inclusions; grade 1 of loose inclusions.
The microstructure of the hardened layer was tempered fine martensite + cryptocrystalline martensite, and the martensite morphology was grade 6. The core structure was blocky, reticular ferrite + sorbite. The grain size is 6 grades. See (3) Determination of the hardness of the hardened layer. The depth of the hardened layer on the surface of the spindle is 1. 1.8 mm, the depth of the effective hardened layer is 1.07 mm, and the microhardness distribution of the hardened layer. The curve shows that the above test results show that the quality of raw materials and heat treatment are in line with the technical requirements of the main shaft. 3 Fracture analysis 3.1 Macroscopic fracture of the macro-fracture fracture main axis, its section is basically gray, accompanied by a silver-white bright area, see a in the picture The whole section is basically flush, but it is higher at the bottom of the section, and its slope is 45 with the axial direction. In the figure, b is found near the oil hole of the section and 6 radial steps are found. The height is about 1mm, which is the fatigue source before fatigue. It is clearly visible along the line. Most of the entire section is advancing into the fatigue crack propagation zone. Only a small part of the instantaneous fracture zone 3.2 crack observation is taken at the section to take a 30mm thick specimen for thermal acid etching test. It is found that the surface of the fracture is 3mm and There are more than 10 discontinuous circumferential directions near the oil hole The cracks are about 35~60mm long. See the circumferential crack along the section. Take a number of axial metallographic samples at the near oil hole crack. Under the microscope, the crack is fine and straight, and the depth is about 1.1mm. The structure around the crack crack is normal, and there is no oxidative decarburization. See the C step at the observation. The grain is deformed and the grain is elongated toward the step. See 3.3 Micro-fracture with SM-25S Scanning Electron Microscope for Gear Spindle The oil hole section was observed, and it was found that there was a secondary crack on the surface of the oil hole. See the tire pattern at 1 mm from the surface. See 01 for the welcome of the table. Welcome to the welcome advertisement. The crack of the crack is 4 mm. The chemical composition and hardness of the shaft are analyzed. And the results of microstructure inspection show that the quality of the raw materials and heat treatment are in line with the technical requirements of the shaft. The six radial steps near the oil hole observed in the macroscopic fracture and the circumferential welcome to the crack, and the grains deformed at the step indicate the oil. The stress concentration at the hole is serious, and it becomes the tire pattern observed under the electron microscope of the fatigue source. It can only appear under the condition of low-cycle fatigue fracture. Its appearance indicates that the stress on the shaft is close to or exceeds the yield of the material. Therefore, the design requirements and the user's production site were inspected. It was found that the safety factor of the shaft design was small, and the equipment was in high load operation during service. The result increased the local stress concentration at the oil hole of the shaft and made the stress peak high. The fatigue strength of the material gap is reduced, and the working stress of the shaft is too large to cause the fracture. 5 Conclusion The fracture of the eccentric gear of the press belongs to the fatigue fracture. The observation of the fracture shows that the fracture has a tire pattern and the characteristics of the low-cycle fatigue fracture. Stress concentration at the oil hole, and overloading causes the working stress of the shaft to be excessively broken.
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