Cycloidal Reducer

It is easy to forget how much of the modern world turns slowly and precisely: the joints of a robot arm, the axis of a tracking telescope, the wrist of a machine that places parts thinner than a hair. Each one needs a fast, gentle motor turned into a slow, powerful, almost play-free motion, and one neat way to do that is the cycloidal reducer. In motion it is hard to follow, so we won’t start with the finished machine. We’ll begin with a single shaft and build it up one piece at a time. Every figure below is live: drag it to turn it, and use the sliders to see what is going on inside.

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.

Gears are simple and cheap, but they have weak spots.

The worst, for precise motion, is that real teeth must mesh with a little clearance. Watch the pair rock back and forth, and drag the slider to change how tightly the teeth mesh.

And that is not the only catch: only one or two teeth ever touch, so a few teeth carry the whole load. To reach a high ratio you stack stage after stage, and the box keeps growing. So we want a drive with no gap to cross, with the load shared across many contacts, small enough to stay a single stage. Let’s build one.

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.

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.

And it’s small

Here is the payoff. To reach a high ratio, an ordinary gear train has to stack stage after stage, and the box keeps growing. A cycloidal drive gets the same ratio from one flat stage. Drag the slider and compare.

Cycloidal vs gears, side by side

We started with the gears’ weak spots; now that the whole drive is built, here is how the two stack up, point for point. Every line is one of those early complaints, answered.

None of this makes gears bad — they are cheaper, simpler, and more efficient per stage, which is why they are everywhere. But where motion has to be slow, strong, and almost play-free inside a small space, the cycloidal drive is hard to beat.