Crack propagation path and life of helical gear root of Metro gearbox

In recent years, China’s urbanization process is accelerating, and people travel more and more. Urban rail transit system is of great significance to relieve the traffic pressure. With the continuous development of urban rail transit system, the accidents of fatigue failure of the gearbox bevel gear of metro train also increase. The main failure modes of gear are tooth surface contact fatigue and tooth root bending fatigue, but with the continuous development of gear surface treatment technology, the contact fatigue strength of gear has been greatly improved, so tooth root bending fatigue has become the main failure mode of gear.

Many scholars at home and abroad have studied the bending fatigue and crack growth of gears. ]A method for predicting the bending fatigue life of gears based on the dynamic load spectrum is proposed and designed. The calculation model of the bending fatigue life from root crack initiation to propagation is derived. A formula for calculating the bending fatigue strength of helical gear tooth root based on the friction between teeth is put forward by analytic method. The influence of the friction between teeth on the stress and bending fatigue of helical gear tooth root is studied. In this paper, the law of root crack propagation is studied by using the extended finite element method, and the simulation results obtained by the traditional finite element method are compared, which proves the correctness of the simulation results. The process of crack initiation and propagation of heavy-duty surface gear was studied, and the fatigue life of gear with different surface roughness and surface treatment process was predicted. In this paper, the magnitude of gear engagement force under variable speed transmission is simulated, and a method to calculate the magnitude of gear engagement force under different speeds is provided.

At home and abroad, there have been some researches on the crack propagation of helical gear root. However, due to the different application environment of helical gear, it bears completely different loads in the service process, which affects the prediction of residual life. Therefore, in this paper, the load-bearing situation of the Metro gearbox helical gear is studied in detail, and the propagation mechanism of the helical gear root under the initial crack state is further analyzed, the growth law of the crack propagation path is mastered, and the crack life of the helical gear root is predicted.

(1) By comparing the tooth root bending stress calculated by GB / T 3480.5-2008 standard with the bending stress obtained by transient dynamic simulation, it is found that the stress obtained by the two methods is basically consistent, which verifies the accuracy of the finite element calculation model and provides data support for the subsequent calculation of tooth root crack growth.

(2) The failure mode of the root bending fatigue of the helical gear of Metro gearbox is studied and analyzed, and the change rule and distribution of the first main stress of the root under different operation conditions are given.

(3) The propagation mechanism of the root of the helical gear of the Metro gearbox under the initial crack state is obtained through in-depth analysis. The crack propagation path of the pinion is simulated and analyzed, and the growth law of the crack propagation path is mastered. It can be seen from the research results that although the stress in the middle position along the tooth width tangential direction is large, due to the different geometry and stress state, when the tooth root of helical gear expands, it starts from the edge along the tooth width tangential direction, which is different from the way of static strength failure.

(4) In this paper, the crack propagation life of a gear with an initial crack length of 4 mm and a width of 2 mm and a crack location at the end face of the gear tooth is analyzed. The cycle life of the pinion with the initial crack is 16 × 105 times. This provides a new method to predict the service life of the Metro gearbox helical gear from crack initiation to gear fracture failure, which can effectively avoid major accidents and economic losses caused by gear failure.