One of the most common parts of an automatic transmission has been around for quite a while. A system of planet gears orbits around a sun gear, set within a ring gear. There are many configurations that these gear trains can be set up, but they all provide great power output, making them ideal for use in automatic transmissions.
The planet gear is ordinarily in place to make the ring gear turn in the same direction as the sun gear. If you've ever watch a set of gears operate, you'll know that when one gear turns, the other does as well, but in the opposite direction. The planet gear set up maximizes this principle, using the planet gears' reverse rotation to provide identical rotation in the outer ring.
Each piece of the configuration, from sun gear to planet gear and ring gear can either be held stationary, used as the input or used to provide output. Each different configuration will give you a different gear ratio. This is the primary reason that they are used within an automatic transmission. By using clutches, brakes and bands, the automatic transmission can hold different pieces of the assembly still and make other pieces move, giving you different gear ratios, which results in different speed outputs. This is what we term "gears" as the vehicle accelerates and shifts from one gear to another. What is actually happening is the transmission is changing the configuration of the planet gears, sun gear and ring gear for more optimal output and greater speed.
Some automatic transmissions have begun utilizing more than one set of sun and planet gears. These compound planet gears systems work like a set of planet gears in some gears and in other gears perform like a single set. The benefits of this are manifold and include increased power output to the drive shaft. This allows the vehicle to access significantly higher speeds and torque, depending on the gear that the vehicle is currently being driven in.
Many different vehicles have taken advantage of this technology, as well as lock up technology, which allows the torque converter to stop spinning at certain speeds and the engine's rotation to power the transmission. All of these configurations hinge on one single technological innovation: the planet gear. Without this amazing gear set, automatic transmissions, as we know them, would not exist. The planet gear allows for an incredible diversity of configurations for an impressive amount of gear ratios. Different manufacturers, obviously, set their gear ratios differently for their vehicles and the applications that those vehicles are designed for. A change in the size of planet gear, sun gear or ring gear will have an immediate impact on the performance of the transmission and the vehicle overall.
There are many different uses of planetary gears, though the automotive industry is possibly the most prevalent. The uses of this gear set have not been lost on other industries, however, and new innovations using these principals are being created as time goes on.