In the world of mechanics, the sun gear doesn't refer to a specific part number, but rather to a position and function. Just like the star at the center of the solar system, any gear located at the core of a planetary gear set, surrounded by other gears, is called a "sun gear."
We can understand it from the following common aspects:

1. The core "commander"

Sun gears can be found in almost all automatic transmission cars, cranes, and even electric screwdrivers. It's usually a small circular gear with teeth on its outer edge, mounted on a central shaft. It's the source or convergence point of power; all the force is distributed to the surrounding gears (planetary gears) through it.

2. Two common "appearances"

Although the name is consistent, in practical applications, sun gears mainly have two common tooth profiles:
Straight-toothed sun gear: The teeth are straight, simple to manufacture, and powerful, but may produce a "humming" sound at high speeds.
Helical-toothed sun gear: The teeth are cut at an angle. This design is most common in modern passenger cars because it rotates very quietly and smoothly, and the meshing is tighter.

3. Versatile role-playing

The sun gear performs different tasks in different machines:
In a reducer: It rotates quickly, transmitting power to the surrounding gears, making the output force greater, acting like a "power amplifier."
In a differential: It connects the axles of the left and right wheels, flexibly adjusting the speed of both sides during turns to prevent the car from skidding.

4. Why use this structure?

The sun gear is widely considered "useful" mainly because it allows machines to be compact and powerful.
Because it's in the middle, several planetary gears rotate around it simultaneously, sharing the load. Therefore, the sun gear doesn't need to be very large to withstand significant force.
This compact "one-to-many" design allows our car transmissions to occupy less space.