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 above gears are rotating, their rotation centers (which coincide with the center of the circle) are often installed on the casing through bearings. Therefore, their rotating shafts They are all 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 gear called "planetary gear", whose axis of rotation is not fixed, but is mounted on a rotatable bracket. In addition to the planetary gears can rotate around their own rotation axis like fixed-axis gears, their rotation axis also rotates around the axis of other gears with the carrier (called planet carrier). 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, so it is named "planetary gear".

Just like the solar system, the gears with fixed axes that become the center of revolution of the planetary gears are called "sun gears", as shown in the red gears. There are usually two meshing points on one planetary gear, or on two planetary gears fixed to each other, which are related to two sun gears respectively. As shown in the figure on the right, the axis of the gray internal gear coincides with the axis of the red external gear, which is also the sun gear.

The principle of gear transmission with a fixed axis is very simple. Among a pair of intermeshing gears, one gear acts as the driving wheel, and the power is introduced from it, and the other gear acts as the driven wheel, and the power is output from it. Some gears only serve as a transfer station, with one side meshing with the driving wheel and the other side meshing with the driven wheel, through which power passes.

In a gear system containing planetary gears, the situation is different. Due to the existence of a planetary carrier, that is, there can be three rotating shafts to allow power input/output, and you can also use clutches or brakes to limit the rotation of one of the shafts when needed, and the remaining two shafts are used for transmission. In this way, the relationship between the meshing gears can be combined in many ways