The macroscopic fracture morphology of random fracture is a partial map of the macroscopic fracture morphology of the gear test piece teeth when the cycle number N=1.88@106 is broken. This is a typical fatigue fracture. The whole fracture is gray and has obvious shell lines. . According to the shape of the fracture, the fracture can be roughly divided into two regions.
The fatigue source zone and the fatigue expansion zone fatigue source core are located near the midpoint of the pre-fabrication crack along the tooth width direction. The boundary between the fatigue source zone and the fatigue extension zone is not obvious, but it can be seen that in the middle part of the gear, due to the repeated extrusion and friction of the fracture surface, the fracture surface appears very bright and flat, and it can be seen near the two ends of the gear. Obvious beech line.
In the final fracture zone (transient fracture zone), the final fracture zone and the fatigue extension zone have obvious boundary lines, and a large secondary step appears. This breaking zone is a coarse-grained surface, and the trend of change is sharp. This is because the stress exceeds the fracture toughness of the material and the crack occurs due to the instability of the crack. The final fracture zone accounts for about 50% of the total fracture zone. It is also possible to roughly estimate the level of stress. Compared with the general root fracture fracture, the two are very similar and also belong to brittle fracture. It can be seen that this area is a rough plane. Another gear number 215 tooth randomly breaks when the number of cycles N=2.80@106, and the fatigue source is close to the end face of the gear teeth, and the fatigue fringes are not obvious. The final fracture zone accounts for about two-thirds of the total fracture area, indicating a higher stress level. Other features of the fracture are substantially the same as for the 126 tooth fracture. In the test, it was found that the pair of test gears had an eccentric load, which may be the cause of the fatigue crack source on the side of the tooth width end face.
The crack propagation directions were observed. The cracks of the numbers 1-1, 1-31, 1-36, 2-1, 2-10, 2-20, and 2-30 were observed. The results are as follows: When the number of stress cycles N=1.88@106, macro-fatigue cracks of 2mm3mm have been generated at the cracks of the tooth-cutting lines of 1-31 and 1-36.
The initial crack angle of the fatigue crack is observed for cracks that are prefabricated perpendicularly or approximately perpendicular to the tooth profile, such as 1-31, 1-36 and 2-30 gear teeth, etc., the initial crack spread angle is between 70b80b, which is with us. The results of theoretical calculations are consistent. The general trend of crack propagation is from the wire-cut crack to the other tooth root. During the initial length of the fatigue crack (such as 2mm3mm), the change of the expansion direction is small, indicating that the fracture is dominant at this time.
According to the usual practice of fracture mechanics test, after the crack is cut on the test piece, a variable cyclic load is applied to the fatigue test machine to induce fatigue crack along the initial crack direction. This fatigue crack is very close to the true crack tip. However, for the running gear test piece, since the gear geometry and the force are complicated, it is difficult to prefabricate the straight and regular fatigue cracks in the operation, so the wire cutting crack can be directly used without pre-fatigue. crack. Obviously, there is a difference between the corner radius of the crack tip and the real crack.
The random fracture of the actual gear is mostly caused by severe pitting or spalling. The initial crack is non-penetrating, and the prefabricated cracks on the test piece of this test are all penetrating, so it is different from the actual situation. Due to the above reasons, coupled with the limited measurement of crack length and expansion direction, the experimental research in this paper is only preliminary and qualitative. The research on the random fracture test of gear teeth in the stricter sense has yet to be done in the future.

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