Planetary Gear Sets

A simple planetary gear set contains three gear members that remain in mesh at all times. The sun gear is at the center. It is surrounded by pinion gears that are contained in the planetary carrier. They are surrounded and contained by the ring or annulus gear. The ring gear contains internal teeth.

Planetary gear set gear ratios.

A single gear set is capable of providing two reduction, two overdrive, and two reverse gear ratios. These different 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 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. When the carrier is held in reaction, the pinions act as idlers, and the output is reversed.

Whenever two members are driven with no member held in reaction, a direct 1:1 gear ratio results, the internal gears are not rotating; the entire assembly rotates as one. This is why there is no clicking or whining sound heard when the faulty gear set enters direct drive. The entire gear set is turning, not the individual gear set members; the pinion gears are not walking around the sun or ring gear, but are idle and held stationary.