1. Overview and definition of electric drive axle
1. Overview of electric drive axles
With the requirements of various countries for environmental protection, energy conservation and emission reduction, and the country's strategic adjustment of energy structure, the world's energy consumption structure tends to be clean, low-carbon and diversified, which promotes the trend of electrification of the automotive industry. In addition, more and more countries are planning to ban the sale of fuel vehicles at the national level, encourage the development of new energy vehicles, and accelerate the development of industries such as electric drive axles.
2. Definition of electric drive axle
The electric axle is a kind of drive axle, but the power unit is driven by the original internal combustion engine and adjusted to the motor drive, and most of the electric axles integrate the motor into the axle to achieve integration, high efficiency and other functions.
2. Classification of electric drive axles
1. Classification of electric drive axles
According to the different structures, there are mainly integrated electric drive axles for commercial vehicle applications, integrated electric drive systems for passenger car applications, and in-wheel motor distributed drive systems (as shown in the figure).
Integral electric axle
Integrated electric drive system
3. The design of the integral electric drive axle
The development process of the electric drive axle project is generally carried out in the following figure The standard process of auto parts development is carried out in the figure below, and the following takes the parallel shaft integral electric drive axle as an example to mainly illustrate the detailed design process and method of the electric drive axle.
1. Demand input
Before the start of an electric axle project, the R&D department is generally given the basic information of the vehicle, product performance, size boundaries and project nodes by market research or direct needs from customers. For example, the following table is the basic information of a new energy van logistics vehicle.
2. Planning stage
After receiving the design input, the R&D department converts it into quantifiable data, determines the basic performance requirements of the product and conducts a feasibility study, and then provides a first version of the proposal for review.
The project team plans the time node for the project to ensure that it is completed on time.
3. Detailed design
After the scheme is approved, the electric axle assembly is dismantled into three major component systems: reducer assembly, axle housing assembly and braking system. According to the performance requirements of the assembly, the specific performance parameters of each component are decomposed and formulated, and the detailed structural design is carried out.
Taking the new energy vehicle as an example, the following is a preliminary explanation of the basic design methods of each component system.
Fig.1. Typical parallel-shaft integral electric drive axle structure
3.1. Reducer assembly
As the core component of the electric drive axle, the reducer assembly should be focused on in the design stage. Due to the shortcomings of battery life and the high input speed caused by the current trend of high-speed motors, compared with the variable (reduced) gearbox of traditional fuel vehicles, electric vehicles put forward requirements for high efficiency, high torque density, high reliability and high NVH performance, and simple structure for the reducer.
At present, most reducer products adopt single-speed design, which generally adopts two-stage cylindrical helical gear reducer, which is mainly composed of two-stage gear pair, bearing, differential assembly, reducer box and oil seal.
Fig.2. Reducer assembly
3.1.1. Gear pair design
The performance of the gear pair determines the performance of the reducer assembly, and its macro and micro parameters play a key role in the NVH, life and other performance of the electric drive axle.
At present, the basic principles of the design of gear pairs for electric drive systems are as follows:
Tint design:
Because the degree of coincidence has a critical impact on the meshing noise of the gears, almost all cylindrical helical gears for automobile transmissions are designed with thin and high tooth shapes.
Generally, it is required that the coincidence degree of the end face of the first-stage gear pair is 2 or as close as possible, especially to avoid its close to x.5 (x is any integer), and the axial coincidence degree is also as close as possible to the integer, and the total coincidence degree is at least 4;
Due to the sharp tooth deformation of fine and tall teeth, it is necessary to pay attention to the width of the tooth top in the gear design process to avoid the tooth tip hardening during the gear heat treatment.
Gear pair meshing main frequency and frequency doubling avoidance requirements:
When designing the number of gear auxiliary teeth, it is necessary to fully consider the frequency avoidance requirements of the motor. For example, the main order of the 8-pole pair motor and its doubling frequency are mainly multiples of 8, and the number of gear teeth needs to avoid the number of teeth such as 16 and 24.
Low meshing error over the full torque range:
Calculating the meshing error of gear pairs needs to consider the stiffness influence of all components, including the basic properties, clearance and fit relationship of the reducer box, gear shaft and bearing. Generally, the first-level PPTE≤0.4 and the second-level PPTE≤0.7 of the gear pair. For its harmonics, it is also necessary to decrease step by step.
Low slip rate requirements:
Due to the high input speed of the electric drive system, considering the gluing strength of the gear tooth surface, to avoid faults such as tooth surface sintering, it is generally required that the tooth surface sliding rate of the first stage gear pair is ≤ 3, and the second stage can be appropriately relaxed due to the lower speed.
Small backlash design:
The design of the gear tooth thickness tolerance needs to comprehensively consider the machining error of the center distance of the box, and reduce the backlash of the gear pair under the condition of ensuring that the teeth can not be clamped at any temperature, so as to prevent the tooth surface knocking phenomenon at the moment of static start of the transmission system or torque direction switching.
Gear accuracy, chamfering and other requirements:
At present, the accuracy level of the gear pair for electric drive reducer is at least GB 6, the roughness of the tooth surface is ≤ Ra0.8, and the grinding or honing process is generally adopted; the hob design needs to consider the chamfering requirements of the tooth top, and the finished product needs to be controlled at 0.2-0.4mm after considering the heat treatment deformation and machining allowance.
Due to the complexity and many calculation formulas of the gear pair, the design of the gear pair is generally calculated by professional software, and the peak power of the selected motor is Pmax=60kw, the peak torque Tmax=240N.m, and the input speed n=2387 rpm.
KISSsoft software is used as an example to calculate, and the parameters of the gear pair are selected according to the above basic principles of gear pairs, and the gear strength results, coincidence degree and meshing error analysis are shown in Figure 3 and 4.
Fig.3. Strength check of the first-stage gear pair
Fig.4. Meshing error of the first-stage gear pair
After the tooth shape is modified with reference to experience, the meshing analysis is carried out by substituting the load under common working conditions, and the load distribution of the tooth surface is shown in Figure 5.
Fig.5. Load distribution on the tooth surface of the first-stage driving gear
3.1.2. Bearing design
After the parameters of the gear pair are determined, the bearing can be selected according to the stress state of the gear pair, and after the selection is completed, the force decomposition is carried out and the equivalent load of the bearing is calculated, and its life is checked. After determining the bearing life requirements according to the vehicle load spectrum, the damage rate generally cannot exceed 80%, the ball bearing raceway contact stress should not exceed 4000MPa, and the roller bearing should not exceed 4200MPa.
In the case of satisfying the service life, it is necessary to increase the input speed of the reducer assembly and reduce the friction torque of the system, and generally give priority to the use of low rolling resistance bearings such as ball bearings.
3.1.3. Reducer box design
After the gear pair and bearing are determined, the reducer box can be designed, and the influence on NVH and its dynamic stiffness needs to be considered in the box design. The first-order free mode of the box is recommended to be above 1500Hz, and the first-order confinement mode is above 700Hz, and the dynamic stiffness of each position (bearing hole, mounting point, etc.) must be greater than 20000N/mm, so as to minimize the thin-walled large-plane structure. If the follow-up capability permits, the resonance and frequency response simulation and test are carried out for the frequency region with large vibration radiation.
In terms of structure, it is necessary to consider the space layout requirements and assemblyability of the whole vehicle, improve the assembly efficiency and accuracy, and at the same time, lightweight should also be focused.
3.1.4, oil seal
Due to the current trend of high speed of motors, the requirements for the oil seal sealing performance of the input shaft of the reducer are getting higher and higher, with low friction and high temperature resistance. At present, the solution is to improve the performance of oil seal lip materials, often using FPM (fluoroelastomer), and even PTFE (polytetrafluoroethylene) and other rubber materials. The shaft diameter of the fit with the oil seal is reduced as much as possible without affecting the strength and stiffness, and the surface of the mated shaft is finely ground or polished without axial feed, and the surface roughness is at least ≤ Ra0.4 to improve the life of the oil seal.
3.1.5, differential assembly
When the torque and assembly requirements are met, the planetary gears and half-shaft gears adopt a small thrust clearance (0.05-0.15) mm to reduce the transmission system clearance, avoid tooth surface impact and other phenomena, and improve NVH performance.
3.1.6. Lubrication requirements
At present, most of the reducers of the electric drive system adopt splash lubrication mode, mainly using low oil quantity and low viscosity lubricating oil, dry oil pan, forced lubrication and other designs to reduce the power loss of oil stirring and improve the efficiency of the electric drive system.
3.2 Axle Housing Assemblies
The axle housing assembly mainly plays the load, transfer force and torque, which is a key part and has high requirements for the safety factor. At present, most of the integrated electric drive axle housing borrows the axle structure of the original fuel vehicle, and is mature and widely used.
The calculation and verification method of axle housing and half shaft generally refers to Liu Weixin's "Automobile Axle Design".
3.2.1. Axle housing design
The bridge casing mainly plays the role of bearing and supporting, and is a hollow beam structure. During the design, it is necessary to consider the impact coefficient according to different models and road conditions, calculate and check their bending strength, stiffness and fatigue, and carry out bench tests in accordance with GB/T533 and 534 to ensure that the simulation is consistent with the test results (as shown in Figs. 6 and 7).
Fig.6. Schematic diagram of the calculation of static bending stress of axle housing
Fig.7. CAE analysis of static load of axle casing
At present, stamped and welded bridge shells are widely used due to their high material utilization rate and lightweight.
3.2.2. Half-shaft design
It is divided into full floating half shaft, semi-floating half shaft and 3/4 type (less used), and the force and torque transmitted by the two structures are different: the full floating half shaft only needs to check its torsional strength and fatigue, and the semi-floating type needs to add bending moment check on its basis. After the design is completed, the bench test is carried out in accordance with GB/T 293 and 294.
3.2.3, wheel side structure
It is divided into full floating axle and semi-floating axle: the full floating axle needs to check the bearing life of the hub according to the load spectrum and working conditions, and at the same time design the axle hub according to the installation size requirements such as the rim and carry out CAE analysis;
3.3 Braking system
The braking system design is generally based on the vehicle parameters or the requirements of the main engine factory, matching the brake type and calculating its braking torque to meet the braking requirements of the whole vehicle.
Fourth, engineering verification and testing
After the detailed design is completed, the first round of sample trial production and DV test are carried out to verify the feasibility of the design scheme and adjust and optimize the scheme according to the trial production process and test results
5. Production verification and SOP
According to the results of the first round of testing, the second round of small-batch trial production and PV test were carried out after the optimization was completed, and the production feasibility was verified and the pre-batch production preparation was completed.