A planetary gear refers to a rotating axis that is not fixed and is installed on a bracket (blue) that can rotate (the black part in Figure 1 represents the housing, and the yellow part represents the bearing). In addition to being able to rotate around their own axis of rotation (B-B) like fixed axis gears, planetary gears (green) also rotate around the axis of other gears (A-A) with the blue bracket (called planet carrier). The rotation around one's own axis is called "rotation", while the rotation around the axis of other gears is called "revolution", just like the planets in the solar system, hence its name.
Planetary gear units are divided into single planet row and double planet row according to the number of planetary gear groups installed on the planet carrier.
