In the operation of the gear loader of the gear, frequent advances and retreats, or the user often uses the loader as a bulldozer, which often causes the loader to be blocked and instantaneously impacted. The bevel gear is often broken due to the power factor. When the gear transmits torque, in addition to bearing a large contact stress, the root is also subjected to asymmetric bending alternating stress, and its ability to resist bending stress mainly depends on the strength of the gear. The maximum dangerous stress is generated near the root of the tooth, and stress concentration is likely to occur at the sharp corner of the root, which is prone to microcracks. Changes in the working conditions of the gear have a great influence on the fatigue strength of the material, first of all, whether the loader is in good running condition. Generally speaking, under the given conditions of design, the parts have sufficient safety factor, but in actual work, due to the overload of the loader or the occasional occurrence of variable speed operation, the instantaneous overload greatly increases the bending stress. When overloaded, the bending stress tends to exceed the allowable bending stress by 2 to 3 times. Generally, the force of the gear determined according to the power and the rotational speed is only a theoretical value, and the actual value may greatly exceed this value. This makes it possible to force the part to the maximum or to cause microcracks in the weak part, so that it breaks.
The failure of the bevel gear is a premature fatigue fracture, indicating that the gear is overloaded during operation. Further comprehensive analysis of the fracture shows that compared with the normal fracture, the instantaneous static fracture fracture area is larger, the fatigue arc is sparse and clear, the distance between the arc belts is large, and the distribution is uneven. These characteristics indicate that the bevel gear pair is Early fatigue fracture due to overload operation.
In addition, the main tooth surface of the large bevel gear is severely crushed, and the auxiliary tooth surface is subjected to a large impact load, which causes the auxiliary tooth surface crack. It also shows that the gear pair has been subjected to a large overload, and the tooth shape is greatly deformed. As a result, the normal meshing condition of the gear pair is destroyed, so the bending stress is greatly increased, which is also one of the factors of premature breakage of the gear, and is often the cause of the first failure of the large bevel gear and the most damage.
Analysis of Gear Overload Capability At present, China's construction machinery industry mostly adopts automobile standards when designing bevel gear transmission. The main parameters of the bevel gears are not well reflected in the characteristics of off-highway use and road use. For example, road vehicles have certain requirements for stability and noise, not for highway use. For example, loaders, scrapers, bulldozers, etc. have very low speeds and are less sensitive to noise. In fact, the noise generated by the main drive and the engine work. The comparison of places is very small. At the same time, in terms of strength design, we mainly refer to the limited chart, and the important factors affecting the strength of the corner radius and the tooth thickness ratio are not precisely considered. Sometimes the manufacturing department is limited to existing processing conditions and does not guarantee design requirements, which will affect the performance of the gear.
The results of the conventional design (original design) are as follows: the pressure angle is 20°, the helix angle is 35°, and the other coefficients are: working tooth height: 1.560 full tooth height: 1.733 large gear tooth top height: 0.270 height correction factor: 0.510 radial Gap coefficient: 0.173 tangential correction factor: 25 calculated geometric parameters of the tooth blank.
From the design point of view, the strength of the gear pair is lower. The allowable contact stress of the 20CrMnTi material in the original design specification is 3430, and the allowable bending stress is 808. The unit is MPa. The safety factor of the design static stress of the gear has been Nearly 1, according to the information of Carter 966E, the allowable contact stress of the corresponding material is 2800 MPa, the allowable bending stress is 700 MPa, and the criterion of the static strength of 35 according to the fatigue strength theory is not sufficient. The tool number used for machining should be larger, 15.5, so that the pressure angle of the gear surface can be consistent with the design requirements. The working tooth surface has a large pressure angle, which improves the bending strength of the gear during operation.
Conclusion According to the above data and analysis, the overload capacity of the gear pair is relatively low. Due to the influence of the position of the contact zone during processing or installation, the defects of the original purchased product materials and the quality of the heat treatment are extremely likely to cause the above four forms. The gear is broken.
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