What Is A Planetary Gearbox?
An epicyclic gear train (also known as a planetary gearset) consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates to carry one gear, called the planet gear or planet pinion, around the other, called the sun gear or sun wheel.
The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary gear(s) roll around the sun gear.
An epicyclic gear train can be assembled so the planet gear rolls on the inside of the pitch circle of a fixed, outer gear ring, or ring gear sometimes called an annular gear. In this case, the curve traced by a point on the pitch circle of the planet is a hypocycloid.
The combination of epicycle gear trains with a planet engaging both a sun gear and a ring gear is called a planetary gear train. In this case, the ring gear is usually fixed and the sun gear is driven.
- Related article: What is Gear?
- Related article: What is Gearbox?
Why Is It Named A Planetary Gearbox?
The planetary gearbox got its name because of how the different gears move together. In a planetary gearbox, we see a sun (solar) gear, satellite (ring) gear, and two or more planet gears. Normally, the sun-gear is driven and thus move the planet gears locked in the planet carrier and form the output shaft.
The satellite gears have a fixed position in relation to the outside world. This looks similar to our planetary solar system and that is where the name comes from. What helped was that ancient gear constructions were used extensively in astrology for mapping and following our celestial bodies. So it was not such a big step to make.
In practice, we often speak from the perspective of the use of planetary gearboxes for industrial automation. That is why we call the sun-gear the input shaft, the planet gears, and carrier the output shaft, and the satellite gear (or ring gear) the housing.
Possibilities Of Planetary Gearboxes
It is possible to realise 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