WHAT IS AUTOMATIC TRANSMISSION SYSTEM
Automatic Transmission can automatically change gear ratios as the vehicles moves, free from having to shift gear manually. like other transmission systems on vehicle, its allow to run at a relatively high rotation speed without human interference. In this type of vehicle, driver not need to change the gear a smart automatic system change the speed ratio according to vehicle running condition.
Hydraulic transmission system consist major three components:
1. Torque Converter
2. Planetary Gear Train
3. Hydraulic Controls
Torque Converter:
The torque converter takes the place of clutch on a conventional manual transmission. The goal of the torque converter is to provide a means by which to connect and disconnect the engines power to the driven load.
To know more about Torque Converter click here
Planetary Gear Train:
It consisting of planetary gear sets as well as clutches and bands. It is mechanical system that provide different gear ratios.
A planetary gearset consists of a sun gear in the center, planet gears that rotate around the sun gear,a planet carrier that connects the planet gears and ring gear on the outside that meshes with the planet gears. by using clutches or bands, we can prevent certain components from moving and we can alter the input and output of the system and thus change of overall gear ratio.clutches or bands are used to just change which components rotate and which stay stationary. these clutches are actuated by the valve body, their are sequence controlled by the transmission's internal programming.
Principally, a type of device known as a sparg or roller clutch is used for routine upshifts or downshifts. operating much as a ratchet, it transmits torque only in one direction, free-wheeling in the other. the advantage of this type of clutch is that its eliminates the sensitivity of timing a simultaneous clutch release & apply on two planetaries. simply taking up the drivetrain load when actuated , and releasing automatically when the next gear's sparg clutch assumes the torque transfer. These band semi manually controlled for low range or reverse gear.manually gear operate on the planetary drum's circumference. these bands are not working when driver select drive/overdrive range. The torque being transimitting by the sparg clutches instead bands are used for braking.
Hydraulic Control:
An oil pump uses for controlling various clutches and bands modifying the speed of the output depending on the speed of car. A pump is typically gear pump mounted between the torque converter and planetary gearset.
1. It draws transmission fluid from a sump and pressurize it which is needed for transmission component to operate.
2.The input for the pump is connected to torque converter housing, which is bolted with engine flywheel so pump provide the pressure whenever the engine is running.
3. pump works only when the engine in running condition. so this is impossible to push start a vehicle equipped with automatic transmission.
A GOVERNOR is connected to the output shaft which regulates the hydraulic pressure depending on the vehicle speed. engine load is monitor either by a throttle cable or a vacuum modulator.
The valve body is the hydraulic control center the receives pressurized oil from main oil pump. This pressure is regulated and used to run a network of spring-loaded valves, check balls and servo pistons The valves use the pump pressure and the pressure from a centrifugal governor (as well as hydraulic signal from the range selector valves and the throttle valve or modulator) to control which ratio is selected on the gearset as the vihicle and engine change speed, the difference between the pressure changes, causing different sets of valves to open and close. these clutch and brake band actuators controlling the operation of the planetary gearset to select the optimum gear ratio for current operating conditions.
Now a days, the valves are controlled by electro-mechanical servos which are controlled by the electronic engine control unit (ECU) or separate transmission control unit (TCU).
