What Is A Planetary Gearbox?
An epicyclic gear train (also known as a planetary gear set) 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 slipping. 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, a satellite (ring) gear, and two or more planet gears.
Normally, the sun-gear is driven and thus moves the planet gears locked in the planet carrier and forms 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 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.