The planetary gear system has a very ingenious structure, and its name comes from its movement: a "sun" in the center, with several "planets" revolving around it.

To understand its working principle, we can start with its four core components:

1. "Role Assignment" of the Core Components

A typical planetary gear set consists of the following four parts:
Sun Gear: Located at the center, it rotates on the axis like the sun.
Planet Gears: Usually 3 to 4 of them, they rotate on their own axes and also revolve around the sun gear like planets.
Carrier: A sturdy frame that connects all the planet gears. When the planet gears revolve, they carry the carrier with them.
Ring Gear: A large ring with teeth on the inside, enclosing all the planet gears.

2. How does it work?

The magic of the planetary gear lies in the fact that by locking different parts, completely different rotational effects can be achieved.
Slower but more powerful (reduction): If we fix the outermost ring gear and let the power enter from the central sun gear, the sun gear will push the planet gears to "walk" inside the ring gear. Because it takes a long time for the planet gears to complete one revolution, the output speed of the carrier will be slower, but the force (torque) will be much greater. This is the principle behind the climbing gear in cars.
Faster but less powerful (increase): Conversely, if we let the carrier rotate with the planet gears to drive the central sun gear, the sun gear will rotate rapidly like a spinning top. This is often used in equipment requiring high rotational speed.
Reverse (reverse direction): If we fix the central carrier, preventing the planet gears from moving around, they can only rotate in place. In this case, if the sun gear rotates to the left, the planet gears will act as intermediaries, pushing the ring gear to the right. This achieves reverse power output.

3. Why is it so widely used?

This design is very common in automatic transmissions, power tools, and even bicycle rear hubs, mainly because:
Small size, high power: The power is shared by several planetary gears, rather than relying on a single pair of gears, making it very robust and less prone to failure.
Concentric structure: The input and output are on the same line, saving space and making installation very convenient.
Smooth and quiet operation: Because the gears mesh tightly and the force is evenly distributed, it operates very smoothly and quietly.