The cause of gear failure of agricultural tractor

The phenomenon that a part loses its specified function is called failure. The main forms of parts failure can be classified into three categories: strength failure, rigidity failure and wear. Starting from the analysis of failure forms and taking countermeasures, this is a reasonable way to design the working capacity of parts. The reason of failure is mainly in four aspects: design, material, technology, installation and use, so the experimental research in failure analysis should also focus on these aspects. We should make full use of all kinds of macro test and micro observation means, systematically and step by step analyze the changes in the failure parts, so as to find the real root of the failure of mechanical parts. In order to ensure the working ability of parts and avoid failure, the following measures can be taken:

① Control part size. Calculate the size of parts according to the working capacity criteria such as strength and rigidity, and make the size compact as much as possible under the condition of meeting the working requirements.

② Reasonable selection of materials and heat treatment. The material with higher strength limit is selected from the consideration of strength, and the parts with the same size can bear more load without damage; the material with larger elastic modulus is selected from the consideration of rigidity, which can reduce the deformation under the same working conditions; the wear can be greatly reduced by improving the hardness or reasonably selecting the friction pair material.

③ Determine a reasonable structure. Reasonable structure is an important factor to ensure the normal working ability of parts. If the tension device is used to ensure the initial tension, it can prevent slipping and maintain normal operation.

④ Reasonable use and maintenance. The maintenance of gear transmission engineering, such as lubrication, will directly affect its working ability.

Failure analysis is a frontier, cross and comprehensive new discipline, which includes two aspects: logical reasoning and experimental research. In practical application, they should be combined to determine the cause of parts failure, and formulate corresponding remedial and preventive measures to prevent the recurrence of similar accidents. Here we mainly talk about the experimental research. It is necessary to pay enough attention to the important position of part failure and relevant failure analysis in material selection, so as to form the most important failure form combined with common failure forms of parts when analyzing the actual working conditions of parts, so as to lay a correct thinking for correctly determining the most critical performance requirements of parts.

Fracture is the main form of gear failure. The main causes of tooth fracture are as follows:

① Material and hardness. The hardness of the gear is too low, resulting in the use of bending stress is not enough to ensure the bending fatigue strength of the gear.

② Stress concentration caused by too small root radius or scratches.

③ If the surface roughness exceeds the use requirements, the damage caused by machining (such as grinding burn, rolling and cutting scratch).

④ Overload makes the working bending stress of dangerous section of dedendum too large.

Fatigue pitting is another important failure mode. The reason of pitting and spalling on the gear surface is mainly due to the insufficient contact fatigue strength of the gear. The difference between pitting and peeling and wear is that metal is not worn away in the form of particles, but peeled off in the form of blocks, which causes dents on the tooth surface and seriously destroys the correctness of tooth profile. The failure process is as follows: first, a small crack is generated on the tooth surface, the lubricating oil enters into the fatigue crack, and then after repeated engagement, the crack continues to expand and extend, and the lubricating oil continues to fill the deep part of the crack along with the expansion and extension of the crack until a small piece of metal peels off and leaves the tooth surface. This phenomenon destroys the normal meshing performance of gears. The main causes of tooth surface pitting are as follows:

① Material, hardness and defects. The material of the gear does not meet the requirements; the main factor affecting the contact fatigue strength of the gear is the low hardness after heat treatment, which can not guarantee the contact fatigue strength of the gear; in addition, there are defects on the surface or inside of the gear, which is one of the reasons for the insufficient contact fatigue strength.

② Gear accuracy is poor. The accuracy of gear machining and assembly does not meet the requirements, such as poor meshing accuracy and motion accuracy. And the error of the shell center distance of the circular arc gear is too large.

③ The lubricating oil does not meet the requirements. The brand of the used lubricating oil is wrong, the viscosity of the oil is low, and the lubricating performance is poor.

④ The oil level is too high. If the oil level is too high, the oil temperature will rise higher, the viscosity of the lubricating oil will be reduced, the lubricating performance will be destroyed, and the working thickness of the oil film will be reduced.