planetary gears

is also called epicyclic gears is a gear system that consists of one or more outer gears, or planet gears, rotating about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear. Planetary gearing systems may also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.

Epicyclic gearing is used here to increase output speed. The planet gear carrier (green) is driven by an input torque. The sun gear (yellow) provides the output torque, while the ring gear (red) is fixed. Note the red marks both before and after the input drive is rotated 45° clockwise

The carrier (green) is held stationary while the sun gear (yellow) is used as input. The planet gears (blue) turn in a ratio determined by the number of teeth in each gear. Here, the ratio is -24/16, or -3/2; each planet gear turns at 3/2 the rate of the sun gear, in the opposite direction.