Small planetary gearing with 1:6 ratio

[Jonas]

In my robotic arm, I needed a significant speed reduction for the rotation of the last moving element, i.e. no large torque, but small compact design. I tried several gearing solutions, incl. the well-known planetary gearing with a large 56t turntable. The latter one is compact, but its 1:3 gearing ratio is not sufficiently low and also it suffers from large friction between the two pieces of the turntable.

After some experiments I came with a compact planetary mechanism that uses a small (28t) platform and a 2-stage down-gearing train producing 1:6 ratio, more precisely -1:6. Besides the turntable, 2 24t gears and 4 (or just 2) 8t gears. Here is a couple of pictures:

And a video showing the mechanism on a test bench:

 

Edited May 17, 2021 by Jonas

[HectorMB]

I like it! In particular because it seems to be able to take high torque. I can easily imagine in a rally car or similar. 

 

Nevertheless, you your purpose, did you considered the 42099 planetary gears?

[2GodBDGlory]

Nice! Even more reduction could be achieved by replacing the 12:24 gearing with a 8:28 one, though.

[Jonas]

10 hours ago, HectorMB said:

Nevertheless, you your purpose, did you considered the 42099 planetary gears?

I did not know that special element existed. It seems pretty expensive, though.

2 hours ago, 2GodBDGlory said:

Nice! Even more reduction could be achieved by replacing the 12:24 gearing with a 8:28 one, though.

You are right. In this case, however, bracing would be more difficult.

[Horologist]

Interesting concept, but wouldn't the reduction be greater if the carrier was static and the 28t gear was the output? (12/24)*(8/28) is 1/7.

[Mr Jos]

The 56T large turntable is also 1/7 reduction with a 8T gear, not 1/3?

[Jonas]

5 hours ago, Horologist said:

Interesting concept, but wouldn't the reduction be greater if the carrier was static and the 28t gear was the output? (12/24)*(8/28) is 1/7.

You are right. Unfortunately, in my case, connecting the carrier to the remaining construction would more difficult.

36 minutes ago, Mr Jos said:

The 56T large turntable is also 1/7 reduction with a 8T gear, not 1/3?

The 1:7 ratio holds for the outer gear ring of the turntable. But in my first article I was speaking about the use of the inner 24t ring for a planetary system. Yet, I was wrong with the ratio, sorry, it should be 1:(24/8 +1) = 1:4.

Edited May 18, 2021 by Jonas

[Mr Jos]

1 minute ago, Jonas said:

 

The 1:7 ratio holds for the outer gear ring of the turntable. But in my first article I was speaking about the use of the inner 24t ring for a planetary system. Yet, I was wrong with the ratio, sorry, it should be 1:(24/8 +1) = 1:4.

Aha ok, got it. I hope it will work well with this solution you found.

[Jonas]

Just to explain my motivation for a small and compact design. I am using interchangeable heads in my robotic arm (it is still WIP) and they must use the same interface. Here is an example of two heads: