Comparison of Symmetric and Asymmetric Gears for EV Gearbox

Table 1 presents a comparative analysis between the 2nd symmetric tooth gear stage of a two-stage EV gearbox and its optimized asymmetric tooth version. The objective is to assess the potential advantages associated with the implementation of asymmetric tooth gears.

The symmetric gear stage was designed using conventional methods in accordance with established standards, whereas the proposed asymmetric gear stage was developed employing the Direct Gear Design methodology. Due to the differences in design approaches, the normal moduli and helix angles of the symmetric and asymmetric gears exhibit slight variations. The asymmetric gear stage has an increased drive pressure angle, which is intended to increase the load-carrying capacity of the drive tooth flanks. The coast pressure angle is reduced with the aim of increasing the contact ratio on the drive flank. To achieve the same objective, tooth tip thicknesses are minimized to the allowable lower limit for carburized gears, ranging from 0.3 to 0.4 times of the module value.

Table 2 presents the results of the stress analysis, demonstrating a significant reduction in tooth root stress for asymmetric gears. This improvement is achieved through an increase in tooth root thickness and the optimization of the root profile. Moreover, an increased drive flank pressure angle, along with an increased contact ratio, contributes to lower drive flank contact stress. Additionally, the increased coast flank contact ratio leads to a modest decrease in coast flank contact stress in asymmetric gear designs.

The maximum contact stress is a primary factor in determining the size of a gear stage. Compared to symmetric tooth gears, optimized asymmetric tooth gears enable a reduction in gear stage dimensions while maintaining equivalent drive flank contact stress levels under identical drive flank loading conditions. Tables 3 and 4 provide a comparison between the symmetric gear stage and the reduced-size asymmetric tooth gear stage. The modified dimensions and parameters are highlighted in blue.

The dimensional reduction of the asymmetric tooth gear stage, as demonstrated in Table 3 and Figure 8, facilitates an assessment of the potential weight savings achievable through the application of asymmetric gears. The size reduction of asymmetric gears is characterized by their transverse cross-sectional profile, while maintaining identical face widths compared to symmetric gears. Consequently, gear weight can be approximated as proportional to the transverse cross-sectional areas of both symmetric and asymmetric gears, as illustrated in Figure 8.

Gear weight is primarily determined by its structural design, which typically includes a central bore and machined relief areas aimed at weight reduction, as well as specific features that support integration with other gearbox components. Figure 9 presents common configurations for gear body design.