This is Flash animation of Automatic transmission. Automatic transmissions use planetery gear system to operate.

Fallowing descriptive images are taken from Garagevitesse web site

	Deceleration Power input: ring gear Power output: planetary carrier Stationary: sun gear When the sun gear is held stationary, only the pinion gear rotates and revolves. Therefore, the output shaft decelerates in proportion to the input shaft only by the rotation of the pinion gear.
	Direct Coupling Power input: sun gear, ring gear Power output: planetary carrier Ring gear rotates with the locked planetary carrier, the input and output shafts rotate at the same rate.
	Reverse Rotation Power input: sun gear Power output: ring gear Stationary: Planetary carrier When the planetary carrier is fixed in position and the sun gear turns, the ring gear turn on its axis and the rotational direction is reversed.
HINT: The reference uses a model to explain the operation using different input and output shafts. In an actual vehicle, the construction is more complex in order to enable the planetary gear unit to appropriately shift gears, as shown by the left diagram.

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