What Is A Planetary Gearbox?
An epicyclic gear train, often referred to as a planetary gear set, is composed of two gears positioned such that the center of one gear moves in a circular path around the center of another. To facilitate this motion, a component known as the carrier links the centers of these gears.
The carrier itself rotates, guiding the planet gear (sometimes called the planet pinion) as it orbits the stationary gear, which is known as the sun gear or sun wheel.
When the planet and sun gears are in mesh, their pitch circles move together smoothly without slipping. If you imagine tracking a point along the pitch circle of the planet gear, you would see it create a specific path called an epicycloid. In the most straightforward scenario, the sun gear remains fixed while one or more planet gears revolve around it.
This system can also be adapted by adding a fixed outer gear, commonly called a ring gear or annular gear, on the inside of which the planet gear rolls. Under these conditions, the path traced by a point on the planet gear’s pitch circle becomes a hypocycloid.
When an arrangement involves the planet gear simultaneously engaging with both the sun gear and the ring gear, the setup is referred to as a planetary gear train. Typically, in such configurations, the ring gear is held stationary while power is applied to the sun gear.
Why Is It Named A Planetary Gearbox?
The term “planetary gearbox” originates from the way its gears interact and move in concert. Within this mechanism, you’ll find three main components: the sun (or solar) gear, the ring (sometimes referred to as the satellite) gear, and a set of planet gears.
Typically, the sun gear serves as the driving component, setting the planet gears—mounted within the planet carrier—in motion. This assembly, in turn, becomes the output shaft.
The ring gear maintains a fixed position relative to the external structure, which, if you picture it, mirrors the arrangement of planets orbiting the sun—hence the celestial-inspired name.
Historically, ancient engineers utilized similar gear systems in astrology to chart and track the movements of celestial bodies. Given that background, it’s not surprising that this terminology was adopted for the gearbox.
In practical, industrial applications, we often describe these parts in terms of their roles: the sun gear functions as the input shaft, the planet gears and their carrier together form the output, and the ring gear essentially acts as the housing.
Possibilities Of Planetary Gearboxes
It is possible to realize different speeds and turning directions with the same construction. This can be achieved by, for example reversing the gearbox, which offers the following possibilities:
| Driven side | Solid world | To be driven side | Result |
| Input shaft | Housing | Output shaft | Reduction |
| Input shaft | Output shaft | Housing | Reverse movement + Delay |
| Output shaft | Input shaft | Housing | Delay |
| Output shaft | Housing | Input shaft | Acceleration |
| Housing | Output shaft | Input shaft | Reverse movement + Acceleration |
| Housing | Input shaft | Output shaft | Delay |
| In- and output shaft | N.A. | Housing | 1:1 |
Check out our article: What is Shaft?
Where Is The Planetary Gearbox Usually Used (In The Transmission)?
Where is the planetary gearbox usually used (in the transmission):
- In a robot to increase the torque
- In a printing press to reduce the speed of the rollers
- For precise positioning
- In a packaging machine for reproducible products
FAQs
What are the advantages of planetary gearbox?
High precision – up to 1 arcmin. Low noise – the lowest 55 decibels only. High efficiency – the efficiency loss of the planetary gear train is small, so the transmission efficiency can reach up to 97%.
What are the four main components of a planetary gear set?
The most common type of planetary gearset is shown in the figure above. It consists of four major components: the sun, one or more planets, a ring gear and a carrier (often called the arm or the spider).
What is the purpose of using a compound planetary gearset?
Compared to simple planetary gears, compound planetary gears have the advantages of larger reduction ratio, higher torque-to-weight ratio, and more flexible configurations.
How do planetary gears differ from other gears?
Planetary gears are better than spur gears when it comes to lubrication retention. This is due to the planet’s gears being able to retrieve some oil that is lost from the sun’s gear. This also makes the planet’s gears run at higher speeds.
What are the disadvantages of planetary gearbox?
Planetary gearbox has a higher noise level. Load sharing will not be good if the gearbox is not accurate. Planetary gearbox must be grease-lubricated to prevent wear. Cost can be higher for low backlash and longevity.
When should I use planetary gears?
Generally, planetary gears are used as speed reducers. They are used to slow down motors and increase the torque. Torque is the working power of the machine. But we’ve seen our planetary gears turned around to be used as speed increasers, too.