## 2 Research Object  
The wheel-side electric drive system is a flexible module that is installed on the rear suspension, as shown in Figure 1. Compared with the traditional centralized electric drive system, the wheel-side electric drive system eliminates the differential, the universal joint, the drive half shaft, and other components. Instead, it integrates the functions of the powertrain assembly, suspension assembly, and transmission axle assembly together. As a result, the wheel-side electric drive system has the advantages of compact structure and high transmission efficiency [6]. A three-phase permanent magnet synchronous motor (PMSM) works as the drive power, with the technical parameters shown in Table 1.  
According to the patent [4], the basic structure of the wheel-side electric drive system is shown in Figure 2. In this system, the PMSM drives the input shaft and 1# stage driving gear. The power goes through the 1# stage driving gear, the 1# stage driven gear, the inter-mediate shaft, the 2# stage driving gear, the 2# stage driven gear, and the output shaft. The output shaft is assembled with a wheel sleeve and a hub bearing, ultimately connected to the wheel. All the shafts mentioned above are mounted on the housing. The housing is connected to the main frame through a rubber hinge, and the spring and damper are also fixedly installed on the corresponding position. By defining the relative positional \begin{table}
\begin{tabular}{c c} \hline \hline
**Technical Parameters** & **Data Index** \\ \hline Rated power (kW) & 15 \\ Peak power (kW) & 25 \\ Rated speed (r/min) & 4000 \\ Rated torque (Nm) & 35.8 \\ Peak torque (Nm) & 90 \\ Speed range (r/min) & 0–9500 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the PMSM. Figure 1: Vehicle prototype with the wheel-side electric drive system. relationship of the housing, spring, and damper, the housing acts as a suspension arm with the hinge. The wheel-side electric drive system is driven by two-stage fixed axis helical gears. Among them, the 1\(\#\) stage driving gear is directly fixedly connected to the motor rotor shaft, so there are no bearings in the gearbox, but the positioning is directly achieved by the bearings on the motor rotor shaft. The 2\(\#\) shaft is supported by two deep groove ball bearings, while the axial force can be significantly counteracted by the reasonable design of a gear helix angle. Due to the presence of hub bearings, the 3\(\#\) stage shaft has a strong load-bearing capacity, with a deep groove ball bearing-assisted positioning. The model is shown in Figure 3.  
For the gear parts of the transmission system, they follow the processing flowchart, as Figure 4 shows. After the basic processing of forging, normalization, and turning, the gears  
Figure 3: Modeling of the transmission system. Figure 2: Structure of the wheel-side electric drive system. are ready for hobbing and chamfer. After that, the gears are sent to the heat treatment with carburizing quenching. Then, the thermal deformation must be checked to see whether the parts are ready for form grinding. If they are not ready, then the process parameters in hobbing must be adjusted to improve the thermal deformation. If they are ready, the parts are sent to undergo the process of form grinding. The chassis with awheel-side electric drive system presents a highly modularized design, which can adapt to different types of vehicles, such as city sedans, off-road vehicles, sightseeing vehicles, or golf carts. Correspondingly, the gear tooth profile can be specially modified and tuned according to the specific operating conditions of the vehicle. In gear processing, the free shaping function of form grinding can realize these customization requirements. The final release version can be divided into three categories. The base version is the original state of form grinding, which can cover the basic demand of the vehicle.