How to design a coaxial 1:2 gear without reversing direction?

Question

I am trying to design a gear that has a 1:2 speed ratio (input:output). This design is very limited in space and for simplicity and space savings requires the input and output shafts to be coaxial.

This leads me to think of using a planetary gearset, but in order to achieve a 1:2 gear ratio, we must use a fixed planet carrier gear, which reverses the direction of the output, which is not desired. Using a fixed ring or sun is not possible when targeting a 1:2 ratio because the ring and sun would have to be the same size. Am I missing something, or is there no simple way to make a 1:2 gear ratio with coaxial shafts, or do I require a reverse gear attached to the output of a planetary gear with fixed planet carrier?

Answer 1

An "orbitless" drive can achieve this in a similar envelope as a planetary:

Orbitless demo

"Nutating" gears can also provide similar results.

A fixed planet carrier planetary will have opposing rotation between the sun and ring, if there is an odd number of planets in line between the sun and ring (most typical planetary applications). If you have an even number of planets on each leg of the carrier and the carrier is fixed, your ring and sun will rotate in the same direction. Crude diagram below.

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Your sun gear will need to have half as many teeth as your ring gear to achieve your 2:1 ratio, that means the planets will need to combine to have half as many teeth as the sun (eg 20 tooth planets, 80 tooth sun, 160 tooth ring). At a minimum you should have 9 teeth on a gear, meaning your smallest possible ring gear would have a 0.6" pitch diameter with 120 diametral pitch (very fine) teeth. In my experience the minimum number of teeth you should have is 24 (regardless of pitch) to avoid undercutting. This would make your smallest ring gear 192 tooth, making your pitch diameter 1.6".

What is your space constraint? Also, your loads and speeds are what should determine your pitch and face widths as well.

Answer 2

EDIT: I misread the original post and suggested a solution that provides 1:2 gearing in one direction only. In other words, a gearing system that works in forward but freewheels in reverse. Reading the question over, this is not what the OP requested.

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I would recommend splitting your two requirements out into separate subassemblies:

1. A 1:2 speed ratio, easily achievable as you suggested via planetary gears
2. One-way driving, which you can use a ratcheting mechanism for.

For ratcheting assemblies, I'd suggest something that looks like DT Swiss's "Star drive" mechanism for bicycle freehub assemblies. See image 4 here; the entire assembly contains just one spring and a set of identical toothed discs to obtain reliable one-way drive. A mechanism like this one can be designed in-line with your shafts, has a low part count, and is proven to work well with outer diameters of about 25 mm (and my guess is that it could go a lot smaller).