The functions of a transmission are : (1) to change the transmission ratio to meet the traction requirements of different driving conditions, so that the engine can work under favorable conditions and meet the possible driving speed requirements. (2) to realize reverse driving to meet the needs of the car to reverse. (3) to interrupt power transmission. When the engine starts, idles, the car shifts gears or needs to stop to output power, the power transmission to the drive wheels is interrupted. Transmission classification : (1) According to the way the transmission ratio changes, transmissions can be divided into three types: stepped, continuously variable and combined. (a) Stepped transmission: There are several selectable fixed transmission ratios, and gear transmission is used. It can be further divided into: ordinary gear transmission with fixed gear shaft and planetary gear transmission with rotating gear shaft. (b) Continuously variable transmission: The transmission ratio can be continuously changed within a certain range. Common types include hydraulic, mechanical and electric. (c) Combined transmission: It is composed of stepped transmission and continuously variable transmission. Its transmission ratio can be continuously changed within a range of several segments between the maximum and minimum values. (2) According to the operation method, transmissions can be divided into three types: forced operation, automatic operation, and semi-automatic operation. (b) Automatic operation transmission: The selection of gear ratio and shifting are performed automatically. The driver only needs to operate the accelerator pedal, and the transmission can control the actuator according to the engine load signal and vehicle speed signal to realize the gear change. (c) Semi-automatic operation transmission: can be divided into two categories. One type is automatic shifting of some gears and manual (forced) shifting of some gears; the other type is that the gear is selected in advance by a button, and the actuator shifts gears automatically when the clutch pedal is released or the accelerator pedal is released. Transfer case function In multi-axle drive vehicles, a transfer case is provided in order to distribute the output power to each drive axle. The transfer case is generally equipped with high and low gears to further expand the gear ratio and the number of gears when driving in difficult areas. Transfer case operation Each shaft of the transfer case is supported by two tapered roller bearings, and the tightness of the bearings is adjusted by the corresponding adjusting shims. When off-road vehicles are driving on good roads, power to the front axle is usually cut off to reduce power consumption and wear on transmission components and tires. However, when driving off-road, if low-speed power is required, it should be distributed among all drive axles to prevent overloading of the rear and center axles. Therefore, the transfer case operating mechanism must adhere to the following requirements: a low gear should not be engaged without first engaging the front axle, and the front axle should not be disengaged without first disengaging from a low gear.