By rolling a lobed disc around inside a ring of pins, with one fewer lobe than there are pins.

The input shaft carries an eccentric bearing. That eccentric makes a lobed disc orbit - the disc does not spin with the input, it wobbles around inside a fixed ring of pins, its lobes engaging the pins as it goes.

Because the disc has one fewer lobe than the ring has pins, a complete orbit advances the disc by exactly one lobe position in the opposite direction. So one full turn of the input produces a very small rotation of the disc, which is the reduction.

That slow rotation is transferred to the output shaft through a set of drive pins passing through oversized holes in the disc, which take out the orbital component and leave only the rotation.

Most units use two discs mounted a half turn apart so the eccentric masses balance each other, which is what keeps a mechanism built around deliberate imbalance running smoothly.

Because many pins share the load at once, instead of one or two gear teeth carrying everything.

In a conventional gear train, torque passes through whichever teeth happen to be in mesh - a small number, and a shock load concentrates on those teeth. Exceed what they can carry and a tooth breaks.

In a cycloidal drive, a large proportion of the pins are in contact with the disc lobes simultaneously. A torque spike is distributed across all of them, so the load per contact stays modest even when the total is several times nominal.

The contacts are also compressive between rounded surfaces rather than bending loads on cantilevered teeth, and compression is a far more forgiving way to load steel.

The result is that cycloidal reducers routinely survive momentary overloads of several times rated torque. That is why they are specified for crushers, mixers, shredders and anything where the load can jam or arrive suddenly.

Both are compact, coaxial and high-ratio, but they earn their place differently.

A planetary shares load between several gear meshes and excels at torque density and efficiency, with low backlash available. It is generally cheaper for a given torque and is the volume choice for servo and mobile drives.

A cycloidal shares load between many pin contacts and excels at shock tolerance and torsional stiffness, with very low backlash and high ratios in a single stage. It handles overload far better and holds accuracy under load better, at higher cost.

Ratio is a practical differentiator: a cycloidal achieves in one stage what a planetary needs two or three stages to reach, which keeps backlash and length down at high ratios.

Choose planetary for torque density and cost; choose cycloidal where shock, stiffness or single-stage high ratio govern. In robotics both appear, often in the same machine at different joints.

Very low backlash and high torsional stiffness - which together are why these units are used in precision positioning.

Backlash can be held extremely low because the disc lobes and pins can be made to run in near-continuous contact, and because the high ratio is achieved in one stage rather than by stacking stages that each add their own free play.

Torsional stiffness matters as much and is often overlooked. A drive can have almost no backlash and still wind up elastically under load, so the output lags the commanded position when torque is applied. Because a cycloidal carries load through many contacts at a large radius, it deflects very little.

For a servo system that combination determines how high the control loop gain can go before instability, and therefore how fast and accurately the axis can move.

Manufacturers publish backlash in arc minutes and stiffness as torque per unit of angular deflection. Both are worth comparing, not just the first.

In two quite different places: precision robotics, and heavy industrial duty with impact loading.

On the precision side they are found in robot joints, positioning tables, indexing mechanisms and machine axes, where the low backlash, high stiffness and single-stage high ratio suit closed-loop control.

On the heavy side they drive mixers, agitators, crushers, shredders, extruders, conveyors and pulp and paper machinery - applications where the load can jam, where material arrives unevenly, and where a conventional gearbox would need to be substantially oversized just to survive the peaks.

What unites them is that both value the ability to take a load the drive did not expect. In robotics that is a collision; in a crusher it is a lump of tramp metal.

They are less commonly chosen for smooth, steady, cost-sensitive duty, where a helical or worm unit does the job for less.

Still a proper one - the shock tolerance is headroom against momentary peaks, not a licence to under-size for the continuous duty.

The published overload capability refers to brief, occasional torque spikes. It does not mean the reducer can run continuously above its nominal rating, and treating it that way overheats the unit and wears the contacts.

So select on the continuous torque with the appropriate service factor for hours run, starts per hour and load character, exactly as for any gearbox. Then check separately that the expected peak or jam torque falls within the published momentary overload rating.

Those are two distinct checks and both must pass. A unit sized only on the peak will be thermally overloaded in normal running; a unit sized only on the continuous duty may be fine day to day and fail the first time the machine jams.

State the jam or stall torque of the driven machine when specifying - it is often the number that decides the selection.

Oil-lubricated with scheduled changes, and wear shows up as increasing backlash rather than as noise.

The rolling contacts between lobes and pins need a lubricant that survives high contact pressure, and the manufacturer's specification should be followed rather than substituted. Oil level depends on mounting orientation as with any gearbox.

The wearing surfaces are the disc lobes, the ring pins and their rollers, and the output drive pins. Wear at any of these increases free play, so backlash growth is the primary condition indicator - which is convenient on a precision drive because it is directly measurable, and less convenient on an industrial one where nobody measures it.

On units in precision service, measuring backlash periodically gives good warning. On heavy-duty units, monitor temperature and vibration, and treat a change in the characteristic sound as worth investigating - the mechanism produces a distinctive note and a change in it usually means something is wearing unevenly.