Planetary Gearbox Set: Explaining the Principle of Work

The primary function of a planetary gearbox set is to perform various operations, including deceleration, reversing, acceleration and direct connection in applications such as gearhead motors, turbine engines, car mirrors, automatic transaxle vehicles and many others. The set itself consists from planetary gears, brakes and clutches. The gears are connected to the clutches and brakes which are designed to control the amount of power delivered to the gear sets. Planetary units are particularly useful in cases when space and weight as opposed to reduction and torque are the primary concerns for the proper operation of one application. To get a better insight into the uses and features of the planetary gearbox set it’s essential to first examine its construction and mechanisms.


In terms of construction the most basic model of planetary gearbox consists of three types of gears: a sun gear, planet gears (along with the gears’ carriers) and a ring gear. Arranged around the central axis of rotation, the planet gears are designed to connect with the sun and fixed ring gear. They are mounted on a rotating component, known as cage or carrier, whose function is to produce high-torque output at a low speed. The set of connected gears rotating around the sun gear is very similar to the planets orbiting around the sun, hence the name planetary gearbox. The three main types of gears can function as a power input or output, while in some cases are maintained static. The function that each of these has in the overall operation of the planetary gearbox set defines the gear ratio. By changing the input, output and static elements, the gear unit is able to perform a variety of operations. Let’s take a look at how each operation is performed .


The planetary gearbox is able to decelerate when the sun gear is fixed, while the ring gears provide the power input and the carrier delivers the output. With the sun gear being static, only the planet gears can rotate, thus slowing down the output shaft in proportion to the input.


For the gearbox to perform reverse operations, the planetary carrier should be held stationary, while the sun gear delivers the input and the ring gear provides the power output. When the carrier is in a static position while at the same time the sun gear turns, the planet gears rotate around its axis and as a result their direction of rotation is reversed.


The gearbox can accelerate when the sun gear is in a fixed position with the planetary carrier providing the power input and the ring gears the output. In this case the carrier turns clockwise causing the planet gears to rotate around the sun gear in the same direction. How much the ring gear accelerates will depend on the number of teeth on the ring and sun gears.

Direct connection

The gearbox can provide direct connection when both the sun and ring gear provide the power input, while the carrier has the function to deliver the output. Rotating at equal speeds, the ring and sun gear connect together and as a result the planetary carrier is able to turn at the same speed as well.