The automobile industry is a very important part of the world today, and the traditional automobile industry has been on the road of new intelligence and new energy after years of development. Now, with its oil shortage, environmental pollution, climate warming and other problems, each country began to launch a new development strategy to maintain the sustainable development of the automobile industry and real-time response to the international situation, improve their competitiveness, therefore, the new energy automobile becomes the 21st century automobile industry development main hot spot.
Foreign Development, the first electric vehicle produced in the United States, and the American Electric Vehicles Alliance also proposed by 2040 a clear new energy vehicle development goals and action plans. Japan exempted new energy vehicles from a variety of taxes a decade ago. Germany says renewables will account for 47% of its energy by 2020. [1]
In February this year, the Ministry of Industry and Information Technology organized the launch of the compilation of the new energy automobile industry development plan (2021-2035) , which showed that The new top-level planning route is to vigorously develop fuel-cell vehicles and subsidize charging stubs.
Meanwhile, Wan Gangye, vice-chairman of the National Committee of the Chinese People's Political Consultative Conference, said, "the future of new energy vehicles is fuel cell vehicles. "
According to the China charging alliance, 480,000 private charging posts are expected to be added in 2019, and 120,000 public charging posts are expected. By the end of 2019, the total number of charging infrastructure will reach about 1.4 million units, and the charging industry will enjoy explosive growth.
Thus, as a new generation of automotive engineers, the study of new energy vehicles is a major trend.

And the study of new energy vehicles, speed reducer can be said to be one of the very worthy research direction.
Because in the traditional automobile, the gearbox and the engine may say is the automobile most important two parts, is also closely related, is indispensable [2] . However, for new energy vehicles, because the motor itself has a very high speed and torque output range, so the role of the gearbox began to be replaced by the reducer.
If the electric vehicle is driven by the motor directly instead of the reducer, then the motor is required to have the characteristics of low speed and high torque, and the motor speed is proportional to the power when the Torque is certain, and the speed of the drive motor with the same power is lower The bigger the weight and volume of the motor, the bigger the size and weight of the motor, and the output power curve may not meet the requirements of the vehicle. At the same time, the motor reducer solution can also reduce the overall weight and manufacturing cost of the drive system. For example, the scheme of "motor reducer rear axle" and "motor rear axle" , the overall weight of the scheme with reducer than the scheme without reducer not only the overall weight of at least 100 kg less, the cost of more than 10,000 yuan.
But if the electric vehicle does not have the reducer, the output to the wheel Torque is too small, it is difficult to meet the climbing and other needs of large torque. And daily use, most of the time is in flat road driving, if the selection of large torque motor, the price will be extremely high, the car cost-effective down.
At present, there are many directions in the research of reducer in China. For example, Zhou Jianxing, Sun Wenlei, Liu Geng and other researchers study the vibration and noise of reducer [3] , Li Yixuan, Wang Wei, Zhang Ming focus on the study of vibration characteristics of reducer planetary gear transmission system under impact load [4] , Liu Shihua, Li Peijun, gong Weilun and others mainly study the meshing phase relation in the planetary gear transmission [5] , Zhu Zhengping studies the efficiency and self-locking analysis of the planetary gear reducer [6] , Yan Yifei, jingjianping prefers to study the fault dynamic simulation and feature extraction of planetary gear reducer [7] , in addition, there are also the dynamic simulation of NGW planetary gear reducer and the reduction design of Planetary Frame [8] , the dynamic simulation research of planetary gear reducer based on Pro / E and Adams [9] and the multi-objective optimization design of planetary gear reducer [10] .
Ulgheru, V and Malcoci, I have studied the anti-vibration dynamic model of planet-semiconductor gear reducer [11] , the research group Concli, Franco; Conrado, Edoardo; Gorla, carlo et Al. calculated the power loss of planetary gear reducers: Measurement and Computational Fluid Dynamics [12] , Hsieh, long-chang; Tang, Hsiu-Chen focused on the study of meshing efficiency of 2K-2H planetary gear reducers [13] , in addition there are planetary gear reducer dynamic analysis [14] and four, five, six link planetary gear train transmission ratio and efficiency range analysis [15] .
Thus it can be seen that the research on all directions of planetary gear has been involved at home and abroad. However, at present, most of the research is on single-stage planetary gear reducers, while the research on double-stage planetary gear reducers is still rare This graduation project carries on the research of the two-stage planetary gear reducer, studies its structure design method, proposes the strength check calculation method, improves the transmission performance of the two-stage planetary gear reducer, the application of two-stage planetary gear reduction in new energy vehicle has certain value.
Because of the single-stage reducer caused by motor torque output in one go, perhaps uninterrupted power output to accelerate the start, but not conducive to the vehicle's economy and comfort. Especially in order to pursue the performance of the Model s with high-speed motor, the power consumption of the high-speed motor is large, and the first gear ratio of the single-stage reducer is large, which makes the cruising state of the vehicle in the high-speed critical point, and the economy is not high.
At present, the reason why most single-stage deceleration schemes are adopted is mainly because the characteristics of motor are different from those of internal combustion engine. The drive motor generally has the characteristics of low speed constant torque and high speed constant power, and can produce large torque at very low speed Unlike the diesel locomotive, you need to slow down, increase the Torque, get going.
With the single-stage reducer, the power performance of the pure electric passenger vehicle depends entirely on the driving motor, and the performance of the driving motor is required to be higher, that is, the driving motor is required to provide higher driving torque in the constant torque region At the same time, it can provide a higher speed in the constant power region to meet the requirements of accelerating, climbing and high-speed driving.
When electric vehicles reach the speed limit after there is no room for improvement, so the speed is restricted, high-speed economy is not high. At the same time, the adoption of a single-stage reducer is not conducive to the efficiency of the high-power drive assembly system, because the single transmission ratio can not usually take into account both the power and the economy of a pure electric passenger vehicle The driving motor can not work at the high efficiency point in most cases, especially at the highest or lowest speed and under the condition of low load, the efficiency of the driving motor will drop below 60-70% A serious waste of on-board Electric Energy and reduce the driving range.
However, due to the weak supply chain of the reducer, the weak integrated control ability of the whole vehicle factory, the development technology of the two-stage reducer and the low cost-performance ratio, which limit the economy and power of the new energy vehicle at present, the research of the two-stage reducer is more important.
And in the reducer, because the planet gear transmission mechanism uses many planet gears to carry on the power division, thus effectively enhances its bearing capacity [16] , simultaneously has the good coaxiality [17] , there are also the advantages of large transmission ratio, small size, light weight, less material consumption [18] , so the planetary gear has high research value.
In conclusion, the new energy double-stage planetary gear reducer is chosen as the research direction.
At the same time, for the individual, the research can also promote students to consult more literature, using the engineering graphics, theoretical mechanics, materials, mechanics and computer related knowledge. Master from the principle of the study and plan, to specific structural design ideas and design methods.
Graduation design is also a basic training for students to use the knowledge, theory and skills they have learned to solve the problems of engineering and technology within the scope of their current specialty The improvement of students'individual ability is also irreplaceable.

2. 研究的基本内容与方案

2.1Research (design) the basics:

Learn about different types of planetary gear reducers (NGW type planetary gear reducers, double row straight planetary gear reducers [18] , Z-X-K type planetary gear reducers [19] , ZZ, PF, ZK, ZJ type planetary gear reducers [20] , NLQ and NGW-s type planetary gear reducers [21] , etc.) , according to the demand of the new energy vehicle, this paper analyzes and selects the suitable type of reducer and its structure type and scheme, studies its structure design method, proposes the strength checking calculation method, carries on the calculation analysis and the check to the reducer and the parts, to improve the transmission performance of the two-stage planetary gear reducer, the assembly drawing and the main parts drawing of the two-stage planetary gear reducer are drawn.

2.2Technical options and measures:

1. Determine the original conditions and data of the two-stage planetary gear reducer
2. The structural types of double-stage planetary gear reducer and its components are analyzed
3. The Structure Design Scheme of double-stage planetary gear reducer and its components is determined (tooth fitting calculation-calculation of main parameters of gear-calculation of meshing parameters of high-speed / low-speed stage-calculation of Geometric dimensions-checking Calculation of Assembly conditions-calculation of transmission efficiency -)
Design, calculation and analysis of two-stage planetary gear reducers and their components (gear shafts, shafts, planetary frames and related components)
4. Drawing the Assembly drawing and the drawing of the main parts of the double-stage planetary gear reducer (the design drawing must include the assembly drawing and the drawing of the parts, and the total drawing amount should be reduced to not less than 2.5 sheets of 0 # drawing, of which the machine drawing amount should be reduced to not less than 1.5 sheets of 0 # drawing, and the 3D drawing in the machine drawing (to be converted into 2D drawing) should not be less than 1 sheet of 0 # drawing, and the contents of all drawings should not be repeated;
5. Kinematics analysis of two-stage planetary gear reducer (initial idea using solidworks for simulation analysis)

2.3 goals
2.3.1 purpose:
(1) the design drawing shall include the assembly drawing and the parts drawing. The total drawing quantity shall be reduced to not less than 2.5 sheets of 0 # drawing, of which the machine drawing quantity shall be reduced to not less than 1.5 sheets of 0 # drawing And Machine Drawing three-dimensional drawing (to be converted to two-dimensional drawing) paper shall not be less than 1 Sheet 0 # drawings, all drawings shall not be repeated;
(2) graduation Product requirements document not less than 10,000 words;
(3) not less than 15 papers of graduation project, 5 of which are in foreign languages, each of which is not less than 200 words;
(4) no less than 1,400 words in the opening report;
(5) to translate materials in foreign languages relevant to the field of study into Chinese of not less than 20,000 printed characters and not less than 5,000 words;
(6) complete a certain amount of programming tasks, there should be the source program and debugging calculation results (arranged by the instructor) .
(7) the copy ratio of the full text of the graduation Product requirements document shall not be higher than 30% and that of the single chapter shall not be higher than 35% .

2.3.2 two-stage planetary gear reducer objectives:
Taking into account the motor power, speed, vehicle load and full load quality, maximum speed and conventional speed and other requirements, design a two-stage planetary gear reducer which meets the design requirements, has practical significance and is really feasible On this basis, the mass and volume of the reducer are reduced, the transmission ratio and load-carrying capacity are increased, the transmission is stable and the transmission efficiency is high.