Key components typically include an electric motor, a reducer (including a differential, gears, and bearings), drive half-shafts, rigid axle housings, and wheel hub bearings. Through reasonable design optimization and reliable bench testing, products can be made compact, lightweight, highly efficient, and have a long lifespan.
Types of electric drive axles
The arrangement of electric motors in new energy vehicles can be divided into direct-drive type, parallel shaft type, and coaxial type.
Direct-drive systems replace the engine and transmission of traditional gasoline vehicles with electric motors. The electric drive axle used is derived from the drive axle of traditional gasoline vehicles by increasing the gear ratio and improving gear precision. Because it retains the drive shaft, the space available for the vehicle's battery is limited, and the maximum gear ratio is restricted to 7, it cannot use a high-speed, miniaturized electric motor. Therefore, this electric drive axle is considered a transitional technology in the national new energy vehicle development strategy. This type of drive axle is called the first-generation electric drive axle (see Figure 1). (The classification by generation is not entirely accurate; a more precise term would be type, but let's use this name for now.)
Figure 1 First-generation electric drive dedicated bridge
The parallel shaft structure replaces the engine, transmission, and drive shaft of a gasoline vehicle with an electric motor, integrating the electric motor into a sub-component of the electric drive axle, and arranging it parallel to the output half-shaft of the electric drive axle. This type of drive axle is called the second-generation electric drive axle (see Figure 2).
Figure 2 Second-generation electric drive axle
The coaxial structure is based on the second-generation electric drive axle, in which the motor and the output half-shaft of the electric drive are arranged coaxially. This type of drive axle is called the third-generation electric drive axle (see Figure 3).
Figure 3 Third-generation electric drive bridge
Another type of product, derived from passenger cars, also arranges the motor shaft and output half-shaft in parallel, and independently assigns the load-bearing function of the axle to the rigid axle housing, while the reducer, in conjunction with the CV joint half-shaft, transmits torque. This type of drive axle is called a load-bearing and torsion-resisting separation type electric drive axle (see Figure 4).
Figure 4. Electric drive axle with separate load-bearing and torsion-bearing sections.
Judging from the current research directions of various OEMs and axle manufacturers, second-generation and third-generation electric drive axles can reduce the weight of the electric motor, lower the overall vehicle cost, and increase the vehicle's driving range. Among them, the second-generation electric drive axle is highly favored by the market due to its low initial investment, ease of technical implementation, and high cost-effectiveness. This article will focus on introducing it with examples.
As is well known, fully floating drive axles offer better rigidity and higher load-bearing capacity than semi-floating drive axles, with lower failure rates for bearings and oil seals. Electric drive axles have increased unsprung mass and design load compared to traditional drive axles, placing more stringent demands on their performance. Therefore, this design prioritizes a fully floating second-generation electric drive axle over a semi-floating drive axle.
The design of an electric drive axle mainly includes the following steps:
1) Vehicle dynamics simulation determines the main design parameters of the electric drive axle.
2) Verification of the arrangement of gears and bearings in the reducer.
3) Differential strength design verification.
4) Design verification of half-shaft strength.
5) Design verification of wheel hub bearings.
6) Design verification of rigid bridge housing.
The detailed design of a certain electric drive axle is carried out based on the vehicle parameters and design objectives in Table 1.
Table 1. Parameters and Design Objectives of a Pure Electric Light Commercial Vehicle
Design and development of electric drive axle
1. Vehicle dynamics verification
Based on relevant literature theory and calculation methods [3], a vehicle dynamics calculation table based on EXCEL was compiled. According to the relevant parameters in Table 1, the first generation and the second generation electric drive axles were selected for dynamics calculation, and the calculation results are shown in Table 2 and Table 3 respectively.
Table 2 Dynamic Calculation of First-Generation Electric Drive Axle
Table 3 Dynamic Calculation of Second-Generation Electric Drive Axle
Based on the calculations in Tables 2 and 3, using an electric motor of equal power, the second-generation electric drive axle will provide the vehicle with greater climbing ability, faster acceleration time, and higher transmission efficiency. Furthermore, using a high-speed electric motor eliminates the need for a driveshaft, allowing for the placement of more power batteries, increasing the vehicle's energy density and driving range, while reducing the weight of the electric motor and the overall vehicle cost.
2. Check the arrangement of gears and bearings in the reducer.
Based on the vehicle dynamics simulation results in Table 3, and considering the dimensions of the selected electric motor, the gears and bearings of the second-generation electric drive axle reducer are arranged as shown in Figure 5. Through multiple software analyses and iterative corrections, the final gear and bearing parameters are shown in Tables 4 and 5.
Table 4 Bearing Parameters
Table 5 Parameters of each gear
Figure 5. Gear and bearing arrangement of the second-generation electric drive axle reducer.
The strength and life of the gears and bearings were analyzed using Romax software under the maximum input torque condition. The results are shown in Tables 6 and 7.
Table 6 Bearing Analysis Results
Table 7 Gear Analysis Results
3. Differential strength design verification
Based on the dynamic calculation results of the selected second-generation electric drive axle, the maximum output torque T of the electric drive axle can be obtained. To ensure that the electric drive axle can cope with rapid torque increase and other shock conditions, a reserve coefficient of 1.8 for the output torque is given here; combined with the torque-bearing capacity of the differential used in the traditional drive axle, a certain type of differential assembly is initially selected based on a torque-bearing capacity of 1.8T.
The following sections will conduct design verification of the strength of the half-shaft planetary gear and the differential housing.
According to the formula in Reference 4, the bending strength of the half-shaft planetary gear is calculated to be 742 MPa, which meets the design allowable bending strength of 980 MPa; the extrusion strength between the planetary gear shaft and the gear is 52 MPa, which meets the design allowable extrusion strength of 69 MPa; and the extrusion strength between the planetary gear shaft and the differential shell is 41 MPa, which meets the design allowable extrusion strength of 69 MPa.
The differential housing calculation table based on EXCEL was compiled, and the calculation results are shown in Table 8.
Table 8 Calculation results for differential housing
4. Half-shaft strength design verification
Based on the torsional capacity of 1.8Tmax, the initial selection of the half-shaft rod diameter is φ33.5 mm, and the calculated safety factor of the half-shaft is 1.83, which meets the design requirement of safety factor ≥ 1.8.
A table based on Excel for calculating the spline strength of the half-shaft is compiled. The calculation results are shown in Table 9.
Table 9 Calculation results of half-shaft splines
5. Wheel hub bearing design verification
Based on reference 6, a calculation table for the life of the fully floating rear axle hub bearings based on EXCEL was compiled, and the calculation results are shown in Table 10.
Table 10 Calculation Results of Wheel Hub Bearing Life
6. Rigid bridge housing design verification
Based on reference 6, a calculation table for the strength of a fully floating bridge shell using EXCEL was compiled. The calculation results are shown in Table 11.
Table 11 Calculation results of rigid bridge shell
(Continued)
Conclusion
The second-generation parallel-axis and third-generation coaxial electric drive axles are both evolved from traditional drive axles to varying degrees and are currently the mainstream products in the market.
Parallel-shaft electric drive axles can maximize the use of the rigid axle housing, half-shafts, and other related resources of traditional drive axles. They require lower initial investment, are easier to implement technically, and offer high cost-effectiveness. The technical challenges lie in the design and development of the reducer assembly and related bench testing. Third-generation coaxial electric drive axles, on the other hand, require the electric motor to function as a load-bearing component, thus limiting the use of the rigid axle housing and half-shafts of traditional drive axles. Developing high-speed hollow electric motors and planetary gear sets presents significant technical difficulties and lower cost-effectiveness. However, their high integration and lightweight unsprung mass make them the inevitable future direction for electric drive axles, driving the rapid development of the automotive industry.
