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Planetary gearbox or epicyclic gearbox – what is that?

Simple planetary gearboxes, also called epicyclic gearboxes, have many design advantages. But how are they constructed? This is quickly explained:

  • What is called the sun gear rotates in the center of a planetary gearbox.
  • Spur gears are arranged around this sun gear, evenly distributed around the circumference - hence the word planetary gearbox.
  • These spur gears rotate on a concentric circular path - much like the planets revolve around the sun. For this reason, these gearboxes are also called planetary gearboxes.

In total, a planetary gearbox consists of four components:

1: The case

It is internally toothed and is also called a ring gear. The planets and the sun gear rotate in the ring gear. In most cases, the case is designed to be stationary. The gears are moved by the sun gear, which is coaxial with the output.

2: The sun pinion or sun gear

It is located in the center of the gearbox. The driving forces of the motor shaft usually act directly on the sun gear. The planetary gears arranged around the sun gear then transmit the power from the sun pinion to the ring gear. The number of teeth on the sun gear and the number of teeth on the ring gear determine the gear ratio.

3: The planet gears

These are usually three, in our example four gears, which run between the ring gear and the sun gear. A special feature of planetary gearboxes: in principle, you can use many planet gears here. The more there are, the better the load and torque are distributed in a planetary gearbox. In addition, this allows you to reduce the rolling power. Important: The number of teeth on the planet gears does not affect the gear ratio.

4: The planet carrier

The planet carrier connects the rotating planet wheels to each other. This is done via pins in the planet carrier on which the planet gears are mounted. In most planetary gearboxes, the planet carrier is also the output shaft of the gearbox. Thus, the torque and speed of the planet gears are fed directly to the output shaft of the gear unit by the planet carrier.

What is the key thing about planetary gearboxes?

On the one hand, the simple, but also flexible design. At the same time, planetary gearboxes impress with their high efficiency. This is because only part of the total power has to be transmitted as rolling power. This almost perfect load sharing is also the biggest advantage of a planetary gearbox over a spur gearbox. This is because it is possible to transmit high torques with a high degree of efficiency even in a compact design.


How is the gear ratio determined in a planetary gearbox?

The ratio of a planetary gearbox is established by the interaction between the ring gear and the sun gear. The smaller the sun gear, the larger the ratio. Technically sensible ratios between 3:1 and 10:1 are possible here. Below and above these values, the planet gears or the sun gear usually simply become too small. If higher transmission ratios are nevertheless required, several planetary stages can be connected in series, even in a ring gear. Then, instead of a single-stage planetary gearbox, it is simply necessary to create a multi-stage planetary gearbox.

Are there any other special features of planetary gearboxes?

There are: For example, speeds, directions of rotation and torques can be changed. This is possible when the ring gear is not fixed, but instead the design of the planetary system is changed. However, it is also possible to fix the output shaft and take off the torque via the ring gear.

Where are planetary gearboxes used?

Actually, almost everywhere. This is because epicyclic gearboxes are of great importance in many areas of mechanical engineering. They are found very frequently where high power and high speeds are required with favorable matching of inertia ratios. High ratios can also be achieved easily with planetary gearboxes.

The advantages of epicyclic gearboxes:

  • The input shaft and output shaft are arranged coaxially.
  • The load is distributed over several planet gears.
  • By combining several planetary stages, you have almost unlimited transmission ratios.
  • High efficiency goes hand in hand with low rolling power.
  • They have a favorable performance for their size.
  • They are suitable for a very wide range of applications.


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