Gearbox mainly refers to the automobile gearbox, which has two types: manual gearbox and automated gearbox. Manual gearbox is made up of gear and axle, changing speed and torque through different gear combination. While automated gearbox AT is composed of hydraulic torque converter, planetary gear and hydraulic control system, changing speed and torque by hydraulic transmission and gear combination. It has three functions. Firstly, change transmission ratio. Secondly, under the premise of invariable rotation, enable the automobile to drive back. Thirdly, using neutral, severance the power transmission to make the engine start, idle, and convenient for engine to shift gears or to carry on dynamic output.