FIGURE 1: A planetary gear set is made up of a sun gear, planetary gears and a ring gear.  The ring is used as the final drive to the vehicles front or rear drive wheels.  The gears have three modes of operation: Driver, Fixed and Follower. 

 

 

FIGURE 2: Overdrive uses the sun gear in a fixed position, planetary gear in a driver position and the ring gear as the follower.  

 

 

FIGURE 3: Underdrive uses the sun gear in a driver position, the planetary gear set as the follower and the ring gear is in the fixed position.  To drive the ring gear in the proper direction, the planetary gear set uses a second gear to turn the ring gear. 

 

FIGURE 4: Reverse uses the sun gear in a drive position, the planetary gear set in a fixed position with the ring gear in a follower position. One of two types of clutches or bands are used to hold a particular member of the planetary gearset motionless, while allowing another member to rotate, thereby transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body and their sequence controlled by the transmission’s internal programming. Principally, a type of device known as a sprag or roller clutch is used for routine upshifts/downshifts.