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What are planetary gears and how to use them in your designs

What are planetary gears and how to use them in your designs

Planetary gearbox description

Firstly, what are gears? They are circular components with cut teeth, usually used in machines or mechanical systems. They are used for changing direction, speed, and torque.

Now, planetary gears, also called epicyclic gears, are exactly as they sound. They are like planets rotating around the sun. A very similar principle, except they are planets with teeth (spur gears) that fall into each other and rotate around the sun’s gear within another gear, called the ring gear, as you can see in the image below. They were actually invented around 2,000 years ago by the Greeks, who used them to predict planet movements in the solar system. Looking closer at the gears’ design, it makes perfect sense. Nowadays, these types of gears are mostly used for speed reduction and torque increase. And they do so in a compact and reliable way.

There are various types of planetary gearboxes

  • Fixed ring gear
  • Fixed carrier
  • Fixed sun gear
  • Direct drive

What are planetary gears used for?

Well, these gears, are used to change the rotational speed and force of the rotating crank or shaft on the output.

When the rotational speed of the input shaft is 10, a gear with a ratio of 10:1 will result in the rotational speed of the output shaft being 1. That is ten times slower. It works the other way around for the torque, so it will be 10 times stronger on the output shaft.

To give an example, say we have a DC motor with a rotational speed of 10,000 RPM and torque of 1 Nm. Now let’s attach it to a planetary gear with a 10:1 ratio. What this will do is that the speed on the output shaft will now be 1000 RPM but with a force of 10 Nm. See how it works?

What is a gear ratio, and how can I change it?

Well, a gear ratio is basically the rotational speed of the input shaft divided by the rotational speed of the output shaft, like it was shown above. To change these ratios and increase ratios, we need to dig a little deeper into the number of teeth used for the gears. This will determine how much we are able to reduce the speed of our output shaft as well as increase its torque.

The gear ratio is simple when it’s just two gears connected to each other. It’s the number of teeth on the bigger wheel divided by the number of teeth on the smaller wheel. In a planetary gear system, it’s a little more complicated, as it depends on which type of planetary gear. However, say for a fixed ring gear, it’s simply a matter of dividing the number of teeth on the ring gear by the number of teeth on the sun gear.

Multi-stage planetary gearbox

Planetary gears can also be layered, so you can have one, but also two or three-stage gears. It basically means you stack them up to create a bigger transmission ratio. Just have a look at this captivating video. Isn’t it amazing what you can achieve with a little clever mechanics?

Ultimately, it’s a highly effective way to reduce the rotational speed of the output shaft, as every added stage is a multiplication of the ratios. Say our one-stage system has a ratio of 10:1, by adding another stage making it a two-stage planetary gear system will result in a gear ratio of 100:1! It’s done with minimal space, making it a highly compact solution.


By having read this article, you should now know what are planetary gears, how they work and what they are able to achieve in practice.

If you want to dig in deeper, I recommend reading the articles attached below.

Further Resources

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