Research background of fatigue state of heavy load transmission gears

The transmission gear of heavy equipment has low rotation speed in the process of transmission operation, but needs to bear harsh working conditions such as high torque and large impact. The overload and eccentric load caused by improper use and operation will cause stress concentration in the meshing part of the gear during operation, resulting in micro cracks and even tooth root fracture due to fatigue, Cause gear failure and failure of normal operation. In order to improve the reliability of transmission gears of heavy equipment, avoid failure and improve the service life of gears. It is of great significance to study the fatigue crack evolution of transmission gears in heavy equipment.

There are two main methods for gear fatigue crack research: experimental method and numerical simulation method. There are two test methods: bending fatigue strength test and contact fatigue strength test. Bending fatigue strength refers to the number of load cycles when the gear is loaded in the normal direction without any crack. The reason for the crack of the gear is that the contact stress of the gear teeth exceeds the maximum fatigue limit, resulting in the fracture of the gear teeth and the failure of the gear. The content of gear contact fatigue test is to determine the ability of the gear to resist the repeated load in all directions on the gear without damage. The reason is that the contact stress is too large and the speed is too high during meshing operation. Its failure forms are wear, gluing, tooth surface pitting, etc.

Although the test results are reliable from the perspective of human, material and time cycle, some external and accidental factors are easy to affect the test results. For the gear with large size, due to the limitation of its size factors, it increases the difficulty of test-bed matching, and then increases the difficulty of test. Especially in recent years, many studies show that the combination of digital simulation results and test methods is conducive to research, and the results are also persuasive. However, the research on bending fatigue of transmission gears of heavy equipment mostly uses the simulation method, but the test is relatively few.

Most of the faults of the transmission gear system of heavy equipment come from the failure of the gear. The failure forms of the gear are various, such as tooth fracture, tooth surface pitting, tooth surface gluing, tooth surface abrasive wear and plastic deformation. In particular, it should be pointed out that the fracture of gear teeth often causes the meshing dislocation of gears and even jamming due to the fracture of a gear tooth. There are many reasons for tooth fracture. Generally, the fracture mechanism is divided into overload fracture and fatigue fracture. To avoid overload fracture, it is necessary to reasonably design, manufacture and use the gear with reference to the actual working conditions of the gear, and introduce overload protection device into the transmission system when necessary. The fatigue fracture of gear teeth can be improved by controlling the gear stress, reasonably selecting the fillet radius of tooth root, polishing and shot peening the tooth root and reducing harmful residual stress. However, due to the meshing in and out impact of gear teeth, alternating load on tooth surface and some other random factors Fatigue cracks often appear in gear teeth due to material defects and the influence of heat treatment. Under the action of long-term alternating stress, it will cause crack propagation, and finally lead to rapid crack propagation and fracture, resulting in accidents and losses.

Gear bending fatigue is often not easy to find in the process of quantitative change. Once qualitative change occurs, it will lead to the failure of the whole transmission equipment and even serious accidents. Therefore, it is of great significance to study the bending fatigue strength of gears. In the design of gear transmission system in China, the parameters of gear are often determined according to the static strength theory and safety factor criterion, but the bending fatigue failure of gear teeth generally occurs when the static strength of gear teeth is lower than that of gear teeth. This situation is due to the lack of basic data of gear bending fatigue strength in China. Therefore, it is very necessary to study the bending fatigue test of gear.