The gearbox is a gearbox that changes the transmission ratio and direction of movement. It is located between the clutch and the central drive. Its main function is to change the driving force and driving speed (shift) of the vehicle when the engine speed and torque remain unchanged; Enables the vehicle to drive in reverse (reversing); The engine can be stopped without stalling (neutral). According to the transmission type, the gear-type gearbox can be divided into three types: two-shaft type, three-shaft type and composition type. The double-shaft type has two main shafts (excluding the reverse shaft), which are decelerated by only one pair of gears. The three-shaft type has three main shafts (the first shaft, the intermediate shaft and the second shaft), which need to be decelerated by two pairs of gears. The composition type is composed of two transmissions in series or in parallel, so as to obtain more gears with fewer gears.