# Analysis of static strength of transmission gear in heavy equipment

After setting the corresponding analysis conditions, enter the solution analysis stage. After the calculation, you can get the Mises equivalent stress nephogram of the gear. Taking the calculation results under the condition of maximum load fniax = 25kn as an example, when the gear tooth has fatigue failure, there are three Mises equivalent stress nephograms on both sides of the gear tooth and tooth root under tension and pressure under the condition of load Fmax = 25kn, which are shown in Fig. 1 (a), Fig. 1 (b) and Fig. 1 (c).

Because the bending fatigue test is divided into 5 groups, each group is set with different load levels, so the stress value of tooth root in case of fatigue failure of tooth under different load levels is calculated.

From the data, it can be seen that the stress relationship between the compression and tension sides of the gear teeth is: the stress value of the former is about 1.3-1.4 times that of the latter. After carburizing and quenching, the effective carburizing hardening layer of the gear is finally obtained with a thickness of 0.9-1.1mm, and the hardness of the gear surface can reach 58-62hrc.

After gear processing, due to the high hardness and wear resistance in the carburized layer, some residual stress will be left on the surface of the gear. Due to the application of cyclic load, the compressive stress on the pressure side of the gear tooth will gradually accumulate and increase until its stress exceeds the fatigue limit of 42CrMo gear material. At this time, cracks will be initiated. Once the fatigue crack occurs, the stress at the pressure part of the gear tooth will be released in an instant. At the same time, the compressive load perpendicular to the gear tooth will be borne by the fatigue crack, Therefore, the continuous propagation of crack will stop due to the existence of crack. Compared with the tension side of the tooth root, it is just opposite to the compression side of the tooth root. Although the existence of residual stress counteracts the loading of the stress on the tooth surface, that is, when subjected to compressive load, due to the existence of residual compressive stress, part of the compressive load is offset, so that the tensile stress actually acting on the tension side of the tooth root is reduced, Because the previous stress has been concentrated at the crack tip, once the fatigue crack appears, it will accelerate the crack generation rate. Although fatigue cracks occur on both the compression side and the tension side of the tooth root in the bending fatigue test, it is found that the fatigue cracks occur first on the tension side of the tooth root. It can be concluded that the fatigue stress limit of gear bending appears on the tensile side of the gear and determines the bending fatigue strength of the gear. If you want to improve the bending strength of the gear, you should aim at improving the ability of bearing stress on the tensile side of the tooth root.

The Mises equivalent stress nephogram of gear teeth under different loads is shown in Figure 2. It can be seen from the figure that under different loads, the stress gradient on the tensile side of gear teeth is greater than that on the compression side. At the same time, the area of the stress nephogram on the tensile side is small, the stress nephogram on the compression side extends more widely to the shaft hole, and the stress generated by load diffuses to the inside of the gear, It can be found that the stress concentration on the tensile side of the gear tooth is more obvious, while the stress concentration on the compression side is not obvious, and the compression side can bear greater load. Therefore, the most easily cracked part of the gear tooth is the tensile side of the gear tooth.

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