From Thingiverse user mattmoses. I cannot resist a good .GIF mechanism animation. [via The Thingiverse Blog]

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## 20 thoughts on “Cycloidal Speed Reducer”

1. Anonymous says:

Someone’s gonna have to explain this one to me… I can’t figure out the relationship between the blue plate (and its purple pins?) and the brown cog (driven by the yellow shaft and cam?).

1. brian maloy says:

It appears that the speed of rotation transferred from the yellow shaft, through the brown plate, to the blue plate is reduced because of the relationship of the purple pegs to the holes they interact with. I guess the speed of the blue plate can be controlled by changing the size of the pegs, holes and cam toothing.

2. Jerry Carter says:

Shaft (yellow) spins cam (yellow) imparting revolution to gear (tan). Gear (tan) impart cyclodial motion to pins (pink) rotating disc (blue) at a speed of reduced ratio. The ratio is, I think, the diameter of the shaft to the diameter of the distance between the pins… but it’s probably a bit more complex than that. The red and tan gear tooth spacing is partly bound by the diameter of these two. Larger diameters of the gears would allow larger diameters of the pins and would reduce the speed of the disc. Think of it as the inverse of your bicycle. The little gear at the back is the shaft above, the big gear you pedal is the blue disc. You pedal slow and the wheel attached to the small gear goes faster, increasing the speed and requiring more work to turn the wheel. Above is the reverse. Less work is required but you get less speed. It’s a good inline assembly to replace offset pullies / gears.

3. Anonymous says:

Ahh, I think got it!

The motive force imparted by each of the 4 hole/pin pairs is 90° out of phase from one pair to the next.

Right?

I kept watching the same hole/pin pair go round and round and wondered how the hole was pulling the pin along for approx. half a rotation. I’m still having a difficult time watching more than one pair at a time but i believe that is the explanation.

2. ejolson says:

@dscotthep:disqus the yellow cam drives the brown cog against the red, causing it to drive the blue plate via the purple pins in the opposite direction.

3. Anonymous says:

Okay, so in order for this to work, the red ring has to be mounted on a stationary platform relative to the drive shaft right? It also reverses rotational direction as well, if I’m looking at it right. (presuming the blue plate is attached to the output shaft on the bottom). Neat stuff, Just the sort of thing I’d fire up a thing-o-matic to see if I could reproduce… um, once I get a thing-o-matic.

4. Alex Young says:

Sooo… does this have a mechanical advantage equivalent to a conventional gearbox with the same ratio?

1. Fibbonachos says:

Nominally, it would have to – conservation of energy means force*distance on both ends has to be the same, unless something inelastic is going on inside.  However, friction is probably a real problem, as well as the wobble created by the inner gear.

5. Sebastian says:

Want to see a real example? Buy a “Thomas the Train” toy engine, one of those that are motorized and running on a AA battery. If you take it apart carefully, you can find this gearing mechanism inside. Be careful, it is a little tricky to re-assemble.

6. Sebastian says:

By the way, before I forget: Thanks, mattmoses, for making this animation. I didn’t quite understand the gearing mechanism when I saw it first, but your animation explains it quite nicely.

### Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and makezine.com. My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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