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, 20213 yr by Jonas

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?

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

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.

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.

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

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, 20213 yr by Jonas

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.

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: