With the increasingly serious energy and environmental problems, new energy vehicles have attracted much attention. With its advantages of zero emissions, low noise, and extensive power sources, electric vehicles have become the focus of research in various countries. There are many similarities in the body structure between electric vehicles and fuel vehicles, but there are significant differences in the structural form and working environment of power transmission system. The transmission system of electric vehicle omits torsional damping components such as torque converter and clutch, and the system is shown as an under-damping system; At the same time, the transmission system adopts the structural form of multi-stage deceleration and low-gear transmission, which makes the power transmission path shorter and the number of cycles significantly increased. These new features of the electric vehicle transmission system bring new theoretical and technical problems, in which the transmission system life prediction and system reliability are the bottlenecks that restrict the further improvement of performance.
Compared with the traditional internal combustion engine, the driving motor, as the power source of electric vehicles, has a torque dynamic response of 100 times faster, a speed of 2-3 times higher, and has high-frequency fluctuations and strong impact. During the driving process of electric vehicles, the helical gear transmission system is affected by the torque from the motor and the driving conditions, so the actual load of the helical gear has strong time-varying and randomness; At the same time, a certain impact load is often generated during the meshing process, so the stress situation of the helical gear is complex, and the effect of these forces significantly affects the dynamic meshing process of the helical gear, leading to pitting, fracture and other fatigue failures. At present, there have been many studies on the fatigue life prediction of related parts, but there are few studies on the fatigue life prediction based on the fatigue damage theory to calculate the dynamic load of the electric vehicle helical gear transmission system under the cyclic condition by using the motor dynamic model. Therefore, considering the high-speed and high-frequency dynamic characteristics of the motor, obtaining the fatigue load spectrum of the helical gear pair of the electric vehicle transmission based on the transient road conditions and predicting its dynamic fatigue life are of great significance to the design innovation and technical upgrading of the electric vehicle transmission system in China.
1) Based on the dynamic control model of permanent magnet synchronous motor (PMSM) and the cycle driving conditions, a method for calculating the load history of electric vehicle transmission under cycle driving conditions is proposed by using computer simulation.
2) For the load cycle counting of helical gears, the data of the contact stress spectrum of helical gears are first extracted and reorganized to make the load of a single gear tooth become a continuous load history, and then the rain flow counting method is used to count the load cycle to obtain the mean value of the contact stress amplitude of helical gear teeth under the cycle working condition – frequency relationship.
3) Based on the modified P-S-N curve, the contact fatigue life of high-speed helical gears in transmission is calculated by Miner linear cumulative fatigue damage theory, which provides theoretical basis and method for dynamic fatigue life prediction of helical gears in electric vehicle transmission system.
For the acquisition of the load history of the helical gear of the electric vehicle transmission, the most ideal method is the actual vehicle test. The test of the actual working condition load of the transmission system can not be replaced by any numerical simulation results, and can reflect the actual operation of the transmission more comprehensively. However, this method requires a high test system, and has a long cycle and high cost. Secondly, the random load data of the electric vehicle transmission system obtained by the real vehicle test method can reproduce the actual working conditions of the transmission system, but the amount of data is too large and the practicability is not strong. At present, the load history of transmission helical gears is mainly obtained by computer simulation. Taking the high-speed helical gear of a certain electric vehicle with constant transmission ratio as the research object, the control model of permanent magnet synchronous motor for vehicle is established, and the model is simulated and calculated based on the instantaneous road conditions, and the simulation results are verified on the real vehicle; The dynamic torque of the motor obtained from the simulation is taken as the driving torque of the high-speed gear pair, and the helical gear is converted into the equivalent spur gear to calculate the contact stress spectrum of the tooth surface under the cyclic condition; The rain-flow counting method is used to count the load cycles and obtain the relationship between the amplitude and frequency of the contact stress of the helical gear. Finally, the modified P-S-N curve and the fatigue cumulative damage theory are used to predict the contact fatigue life of the transmission helical gear.