A simple planetary gearset can illustrate how to control individual members to produce a particular output or neutral.
This planet carrier is attached to the output shaft. The sun gear is attached to the input shaft from the turbine.
The ring gear is not held. When the input shaft rotates the sun gear, the planet gears idle on their stationary carrier pins, and turn the ring gear in the opposite direction to engine rotation.
This free rotation of the gears provides neutral. No drive is transmitted to the output.
For low gear, a brake band, anchored to the transmission case, is placed around the ring gear. Applying this holds the ring gear stationary.
Now when the sun gear rotates, the planet gears can no longer idle. They must walk around the inside of the stationary ring gear.
The planet carrier must move with them in the same direction as engine rotation, but at a slower speed. And with an increase in torque.
A direct-drive or top-gear condition with a 1 to 1 ratio is obtained by locking together any 2 members of the gearset.
A multi-plate clutch can be used for this purpose.
The outer drum and outer plates are attached to the ring gear. The inner plates and inner drum are attached to the planet carrier.
When fluid under pressure is directed onto the clutch piston, the clutch plates are locked together. This locks the planet carrier to the ring gear.
Now when the sun gear is rotated, the planet gears can neither idle nor walk. The whole gearset turns as one unit to give a direct drive.
For reverse, the ring gear is attached to the output shaft, and the planet carrier is held stationary by a brake band.
Rotating the sun gear causes the planet gears to idle on their stationary pins. This turns the ring gear, and its output shaft, in the opposite direction to engine rotation.
