How to build a mechanical 3-way OR gate

For a side project, I need a mechanism with 3 input shafts and 1 output shaft. If any input shaft turns, the output shaft must turn. We can assume only one input shaft turns at a time, if that makes it easier. The gear ratio between each input and the output must be the same. The input shafts cannot be locked relative to each other- if one moves, the others must be able to stay still (which is why 3 connected gears won’t work). The most important factor in the design is compactness (ideally this would be smaller than an inch on each side).

I’ve been thinking about this for a few days and it’s a pretty interesting puzzle so I wanted to share.

So far I’ve only thought of chaining 2 miniature car differentials. A differential has 2 shafts usually used as outputs, but if we use these as the input shafts we get the mechanism we need for 2 inputs. Chaining two differentials with some gears in between would give us what we need, but I don’t like how complex and large that is.

I feel like it might be possible to extend the idea of a differential to more output shafts (which we would use as inputs) but I don’t know how…

Do you all have any ideas?


Some YouTube searching may give you give ideas, sounds like a fun challenge to actually build one so small.

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If the shafts only turn in one direction you could use one way bearings:

Here’s how one of the first programmable binary computers, the Zuse Z1 did it:

If the power for the output is not derived from the inputs and you desire continuous power transmission (which may be desired or not depending on your application):

You could make a n-input OR gate with all of your inputs with a fluid couplings or clutch pack disks activating a cam or linkage to engage the output.

This only works because “excess” motion on any of the inputs is taken up on the fluid coupling or clutch, effectively making a positional control from the continuous rotation inputs.

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I didn’t read carefully enough to see that you wanted the mechanism to work with rotating shafts. Combining differentials will definitely work - they’re just adders, so you can do this (with step-up gearing between differentials as necessary)

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Try a sprag clutch?

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