jackymechanical

Most of the gears we know are gears with a fixed axis of rotation. For example, mechanical clocks, ordinary mechanical gearboxes, and reducers. Although all the gears above are rotating, their rotation center (which coincides with the center of the circle) is often mounted on the casing through a bearing. Therefore, their rotation axis They are fixed relative to the casing, so they are also called "fixed shaft gear transmission."

        There must be movement. Correspondingly, there is a less well-known type of gears called "planetary gears". Their axis of rotation is not fixed, but is mounted on a rotatable bracket (blue) (Figure The black part is the housing, and the yellow represents the bearing). Planetary gears (green) can rotate around their own axis of rotation (BB) like fixed shaft gears, their axis of rotation also follows the blue carrier (called planet carrier) around the axis of other gears (AA) Rotate. The rotation around its own axis is called "rotation", and the rotation around the axis of other gears is called "revolution", just like the planets in the solar system, hence the name "planetary gear".
  Just like the solar system, the gears with fixed axes that become the center of planetary gear revolution are called "sun gears", as shown in the red gears. There are usually two meshing points on one planetary gear or on two mutually fixed planetary gears, which are in relationship with the two sun gears respectively. As shown in the picture on the right, the gray internal gear axis coincides with the red external gear axis, which is also the sun gear.
        The principle of gear transmission with a fixed axis is very simple. In a pair of gears that mesh with each other, one gear is used as the driving wheel, and power is transmitted from it, and the other gear is used as a driven wheel, and the power is output from it. Some gears are only used as transfer stations, one side meshes with the driving wheel and the other side meshes with the driven wheel, and power passes through it.
In a gear system that includes planetary gears, the situation is different. Because of the planet carrier, that is to say, there can be three rotating shafts to allow power input/output, and a clutch or brake can also be used to limit the rotation of one of the shafts when necessary, and the remaining two shafts are used for transmission. In this way, the relationship between the meshing gears can have many combinations.