The transfer case is an important part of the automatic transmission which is widely used in multi-axis drive systems because of the multi-axis drive vehicle having high requirements for power, off-road passing, response speed, and reliability [1]. The power output of the transmission is distributed to each drive axle by the transfer case with the driving torque being transmitted to the highest extent, which improves the dynamic, acceleration and passability of the vehicle under various road conditions [2].

The mechanization and electronic development of the transfer case in the Western countries such as the United States and Europe came earlier. After two Gulf Wars, the Exhibition has become more perfect. The structural features of the foreign transfer case are that the front output shaft drive system adopts low-noise multi-row Chain drive. Domestic transfer cases are mainly based on counterfeiting of foreign products. It have certain innovation ability with there being still a certain gap compared with foreign advanced technology [3-5].

There are many forms and types of transfer cases whose core performance is the difference of technical route choices. According to different judgments, the standard can divide the technical route used in the transfer case into different categories. Most transfer cases are slowing down the role of torque increases with higher load than the transmission, thus the transfer gear constant gears are helical gears, bearings are used apered Roller Bearings. According to the different number of shifts, transfer cases can be divided into single-speed transfer cases and double-speed transfer cases;according to the power size, it can be divided into small power transfer case and high power transfer case; according to the number of output shafts, it can be divided into two-axis, three-axis, etc.; according to the different torque distribution it can be divided into symmetrical, asymmetric, twist ;Whether the moment changed it can be divided into fixed torque type, variable torque type [6, 7];

Most of the vehicles initially tend to use single-speed transfer cases. The output power of modern vehicle engines is high, the output torque being still high even under high-speed conditions, and the transmission ratio of various types of automatic and manual transmissions varies widely Having been able to meet the automotive use requirements very well. The use of single-speed transfer case simplified the transfer case structure, making the vehicle simpler to operate. Traditional two-speed transfer cases often require conversions of high and low-level, which not only complicates operations, but also affects vehicle off-road mobility [8], thus the number of vehicles using single-speed transfer boxes is increasing. However, with the introduction of electronic control technology into the vehicle, the conversion of high and low gears is directly determined by the computer according to the road conditions being more and more complicated, thus the double-speed gear transfer case also began to increase.

Symmetrical transfer case distributes the torque evenly to each drive axle. Because the front axle load is less than the rear axle, the travel resistance when the force is higher, the slippage of the front axle wheel is greater than that of the rear axle. Once the front axle suddenly slips, due to the transmission principle of the differential between shafts, the rear axle wheel will not be able to rotate and transmit power, thus wasting the conditions of adhesion between the rear wheel and the ground and limiting the passage of the vehicle [9]; the asymmetrical planetary gear transfer case is rationally allocated to the rear axle. Torque also acts as an inter-axle differential, overcoming the disadvantages of the symmetrical transfer case described above.

The torque that is assigned to each drive shaft in a general gear type transfer case varies with the adhesion of each drive axle. Although the drive axle power with good adhesion is improved, the bridge may be damaged due to overload; Because of the speed of the front wheels and rear wheels can’t be assured the same, the power cycle phenomenon is unavoidably caused during driving, and the driving wheel load is greatly increased, more wear and tear on parts and components, and lower fuel economy. Generally, this type of transfer case will have a sliding sleeve to make it having the ability to switch off the front axle, so fewer and fewer vehicles currently use such transfer cases. The axis asks the differential transfer case has a differential mechanism between the output shafts of the front and rear drive axles, which overcomes the above disadvantages. The transfer box distributes the constant torque to each drive shaft. Each shaft is often in a driving condition and ensures coordinated movement of the bridges. No need to connect the front axle drive equipment. In order to avoid the loss of power when a certain axle wheel slips, the axle shaft asks for a differential lock between the shafts of the differential transfer case so as to lock the output shaft into one when the axle of a certain bridge slips, thereby improving the passivity[10].

The main purpose of this graduation project is to master the basic working principle of the off-road vehicle transfer case through the learning and research of the transfer case, and to achieve the innovative design of the transfer case by creating a three-dimensional model, to school the strength and stiffness of nuclear components by using finite element analysis. Through the verification, kinematics analysis, and structural design research of the existing specific type of transfer case on the market, we can find parts that can be improved to enhance the design, thereby improving its performance and stability.

2. 研究的基本内容与方案

The main content of this design:
1. Master the working principle of the off-road vehicle transfer case;
2. Establish a three-dimensional geometric model of the off-road vehicle transfer case;
3. Intensity analysis of key components of the off-road vehicle transfer case: including verification of the shaft, verification of gears, checking of gear contact strength and checking of gear bending strength;
4. Perform kinematic analysis of the off-road vehicle transfer case.

To determined the dimension parameters through calculation and verification of each component, and to draw parts and general assembly drawings using CAD.