InputOutputHeld stillA sun gear in the middle, planets around it on a carrier, and a ring with internal teeth around them all: 12, 18 and 48 teeth here. Hold any one of the three, drive another, and take the output from the last. The same gears give 5 : 1 (ring held), 4 : 1 reversed (carrier held) or 1.25 : 1 (sun held). Three planets share the load, so a planetary stage carries far more torque than a single pair of the same size, with the input and output on one axis.