A planetary gear is a type of gear system comprised of spur gears. In planetary gearing (also known as epicyclic gearing), a center gear, called a sun gear, serves as the input and driver of the set. Three or more “driven” gears (referred to as planets) rotate around the sun gear.
Finally, the planets engage with a ring gear from the inside, which makes an internal spur gear design. Because the planet gears are evenly distributed around the sun, planetary gear trains are known to be extremely rugged designs. Another benefit of a planetary gear set is that it is easy to convert to a different ratio by simply changing out the carrier and sun gears.
A planetary gear set uses spur gears that move opposite of each other within the same plane. While spur gears are a more basic type of gear in terms of engineering since they do not utilize specialty angles or cuts like bevel or herringbone gearing, they are complex in the tooth shape design. Depending on the application, this tooth design will determine where the teeth make contact, which then determines the available power, torque, and speed potential of the gears.