Cycloidal Reducers, from scratch
A cycloidal reducer takes a fast, gentle spin and turns it into a slow, powerful one.
Start with a single shaft
Here is the input shaft. Drag it.
What we usually want is the opposite: slow but strong.
The usual way: gears
A small gear driving a big one.
But only one or two teeth touch at a time — wear, and backlash.
A different idea: a ring of pins
Fix a ring of smooth round pins into the housing.
Push, don’t spin
Mount a disc on an eccentric and shove its centre around a tiny circle.
On its own the disc would just slide around forever. But the ring of pins is in the way.
Forced to roll
The eccentric can only shove the disc sideways; it can’t spin it. Pressed against the fixed pins, the disc has no choice but to roll along the inside of the ring, like a coin rolling inside a cup.
Several pins share that push at once, and the press-and-react against the fixed ring is what both rolls the disc and lets it carry a load. As the eccentric whips its centre round fast, the disc itself turns slowly, and the other way.
How much it reduces: one fewer lobe
Like the gears earlier, the amount of reduction comes from a count. Here it is the lobes: the disc has exactly one fewer lobe than there are pins, so each input turn slips it back by just one pin.
One pin out of N is 1/N of a turn, so the reduction ratio is simply the lobe count. Drag the slider: more lobes, more reduction.
Collecting the slow turn
We now have our slow turn. But the disc carrying it is also wobbling, and that slow turn is buried under the wobble. Bolt a load straight onto the disc and it would just shake.
Output pins ride in holes wider than themselves by exactly the wobble. The disc wobbles freely in that slack; the hole edge (white arrows) only ever pushes its pin round the slow way. Those pins tie to one plate, the output shaft.
So the wobble is absorbed and only the slow turn reaches the output. Now let’s see how all the parts fit together.
How the output binds to the shaft
Last step: fix that whole ring of output pins onto a single green plate. The plate plus its pins is one rigid piece, and that piece is the output shaft. Drag the slider to slide it together with the rest, or apart.
The green pins stand up from the green plate and plug into the disc’s big holes. The input shaft passes down through a hole in the middle of the green plate without touching it. So the input shaft (up) and the output shaft (down) share the dash-dot axis but turn independently.
The same thing, flat
And here it is straight from above. The thin line from the centre to the disc’s centre is the eccentric crank; the green carrier ties the output pins to the centre axis.
Drag to turn it. Watch the marked lobe creep backward one pin per input turn.
And it’s tiny: a single disc reaches a ratio that would take an ordinary gear train several bulky stages.