Planetary Gear Sets
A simple planetary gear set contains three gear members that remain in mesh with each other. The sun gear is at the center. The planetary carrier circles around the sun like the planets in our solar system. They are surrounded and contained by the ring or annulus gear. The ring gear contains internal teeth.
A single gear set can provide two reduction, two overdrive, and two reverse gear ratios. These modes are achieved by driving and holding these gears with bands and clutches. A clutch drives an input member, while a band or one-way clutch holds another member in reaction. This action causes the third member to output the desired gear mode and ratio.
Notice that when the planet carrier is the driven gear, a reduction gear ratio like first or second gear results. When the carrier is the input, the output is an overdrive ratio. Holding the carrier in reaction causes the pinions to act as idlers, resulting in reverse gear.
Driving two members with no member held in reaction results in a direct 1:1 gear ratio. The internal gears are not rotating; the entire assembly rotates as one. The pinion gears are not walking around the sun or ring gear. They are idle and held stationary. If something is not moving, it is not making noise. That's why no clicking or whining sound is heard when the noisy gear set enters direct drive.