###### Abstract  
The braking mode of the battery electric urban bus (BEUB) is different from the friction braking of the traditional fuel bus due to the introduction of a regenerative braking system. The intervention of electromagnetic braking changes the working condition of the main reducer gears, thus affecting their service lives. Based on the Urban Dynamometer Driving Schedule (UDDS) driving cycle condition, the stress-time history of the main reducer gears is calculated. Combined with the static analysis results and the S-N curve of the material, the fatigue lives of the main reducer gears considering electromagnetic braking and traditional friction braking are analyzed. The reverse torque on the driving axle during electromagnetic braking is taken into account to be closer to the real situation. Results show that, under the electromagnetic braking mode, the bending fatigue lives of the tooth root on the convex and concave surfaces of the pinion are 78.5% and 78.9% of that under the traditional friction braking mode, respectively, while the contact fatigue life of the pinion working surface is 78.2% of that under the friction braking mode, indicating that the introduction of the regenerative braking system into the BEUB will cause a significant reduction in the service life of the main reducer gears. This study provides a high-precision fatigue life calculation method for the BEUB main reducer gears and the accurate prediction of their remaining life. b 2022 202215 July 202217 July 202215 July 202215 July 202215 July 202217 July 2022  
system based on electromagnetic braking is introduced into the BEUB. When the regenerative system works, the drive shaft will endure a reverse torque, which changes the stress cycle characteristics of the gear, and then affects their service lives [7]. Therefore, in order to avoid the introduction of the regenerative braking system based on electromagnetic braking, which leads to a sharp decrease in the service life of BEUB main reducer gears, and reduce the waste of maintenance funds and manpower, it is imperative to investigate the influence of electromagnetic braking on the BEUB main reducer gears' fatigue life. Plenty of research with remarkable results has been conducted by scholars in the field of gear fatigue life by using finite element analysis, bench tests, etc. [PERSON] [8] tested the fatigue life S-N curves of four gear steels under low and high cycles, as well as the durability limit, and gave the design curve based on the fatigue test data. [PERSON] [9] and [PERSON] [10] proposed a calculation model to determine the bending fatigue service life of gear tooth roots. The initial crack life was calculated by the finite element method and strain-life method. [PERSON] et al. obtained the unidirectional gear load spectrum consistent with the real working conditions through the ADAMS software. Then, the finite element method is used to calculate the maximum stress of dangerous parts of the gear tooth roots and predict the bending fatigue life of the gear considering the influence of dynamic loads [11]. Based on the actual driving load spectrum, [PERSON] et al. systematically studied the virtual fatigue test method of transmission combined with vehicle and bench test verification methods [12]. [PERSON] used Miner theory and the rain-flow counting method to analyze the influence of different mean stress on uniaxial random loads fatigue tests [13]. [PERSON] summarized and analyzed the basic fatigue formula, load spectrum processing method, and other issues related to fatigue research in practical engineering [14]. Using the cumulative fatigue damage theory, [PERSON] [15] and [PERSON] [16] et al. calculated the unidirectional contact stress spectrum and obtained the contact fatigue life of a transmission gear under the measured speed condition by using a variety of finite element simulation software.