Cycloidal gearbox design guide

How a cycloidal drive works

A cycloidal drive turns a fast input into a slow, strong output in a single stage. The motor spins an eccentric cam. The cam makes a lobed disc wobble inside a ring of fixed pins. Because the disc has one lobe fewer than there are pins, each full turn of the motor rolls the disc back by exactly one lobe.

Output pins pass through holes in the disc and pick up its slow rotation. The result: reduction ratio = number of ring pins − 1, and the output turns in the opposite direction to the motor. With 21 pins you get 20:1.

Choosing the ratio, pins and eccentricity

The ratio sets the number of pins. More pins need a bigger pin circle so neighbouring pins keep a gap of at least 1 mm.

The eccentricity shapes the teeth. It must stay below the pin circle radius divided by the number of pins, or the disc profile folds onto itself. Aim for 60–90 % of that limit: too little makes shallow teeth that can skip, too much makes sharp lobes. Thick pins also sharpen the lobes; Torque warns you when a pin gets too close to the undercut limit.

A larger pin circle is the most effective way to carry more torque: the pin forces drop with the radius.

Matching your motor

Output torque ≈ motor torque × ratio × efficiency. Use the torque your motor delivers under load, not its holding torque. Output speed = motor speed ÷ ratio.

Steppers suit low speeds and high ratios. Brushed DC and BLDC motors spin fast, so a ball bearing on the cam becomes essential: a printed plain bearing overheats at high speed. The strength check shows this as a failing cam bearing.

Materials, printing and clearance

PETG is a good default. Nylon and Nylon-CF resist wear and impacts best; PLA is stiff but brittle and softens with heat. Print the discs, cam and output solid (100 % infill or many walls): the strength check assumes solid parts.

The print clearance is the gap between moving parts. Start at 0.2 mm. If the cam is too tight on the motor shaft or the discs bind, increase it by 0.05 mm; if the gearbox has too much play, reduce it. Printing the cam first is a quick fit test.

Bearings and hardware

Steel dowel pins (ISO 8734) are cheap and far stronger than printed pins, so use them for the ring pins and the output pins whenever you can. A deep-groove ball bearing on each cam lobe cuts friction and wear dramatically; an optional bearing in the end cap supports the output.

The bill of materials in the designer lists every pin, bearing and screw with its size and quantity.

Assembly

1. Test-fit the cam on the motor shaft.
2. Bolt the housing to the motor; the screw heads sit in counterbores.
3. Press the ring pins into the housing.
4. Push the cam onto the shaft and fit the cam bearings.
5. Drop in the discs; with two discs, turn the second one until it meshes.
6. Insert the output so its pins pass through the disc holes.
7. Fit the end cap and tighten the assembly screws evenly.
8. Grease lightly and turn by hand: it should run smoothly.

Reading the strength check

For your motor torque, Torque estimates the contact stress between pins and discs, the bending of the ring and output pins, the load on the eccentric bearing, and the torsion of the motor shaft, the shaft joint and the output.

Each check gets a safety factor: how many times the current load the part can carry. Green means it meets your target, yellow means it holds but below target, red means it will likely break. The rated torque is the output torque at which the weakest part reaches your target. These are design estimates from standard cycloidal-drive formulas; always test before relying on a printed gearbox.

Exporting to CAD

STL files are ready to slice. STEP files keep true circles and import into Fusion 360, SolidWorks, Onshape or FreeCAD for further changes. The Fusion 360 script rebuilds the whole assembly as editable components: in Fusion open Utilities › Scripts and Add-Ins, add the folder that contains the script and click Run.

The complete kit bundles everything, plus the bill of materials and a README with these steps.

Compound gear trains

A compound gear train chains spur-gear stages: a small pinion drives a big gear, and that gear is fused to the next pinion (a cluster gear). The ratios multiply: three 3:1 stages give 27:1. Each cluster spins on a fixed steel axle.

Torque designs involute gears with an automatic profile shift for small pinions, so teeth are not undercut. The print clearance becomes the backlash. Bigger modules and wider faces carry more torque; the strength check tells you which stage is weakest.

Planetary gearboxes

A planetary stage has a sun gear in the middle, three to six planets around it on a carrier, and a ring gear with inward teeth. The motor turns the sun, the ring is fixed in the housing, and the carrier turns slowly: the ratio is 1 + ring teeth / sun teeth, typically 3:1 to 10:1. Stacked stages multiply, so two 5:1 stages give 25:1 on one axis.

The load is shared by all planets, which makes planetary gearboxes compact and strong. The planets must be spaced evenly, so sun + ring teeth must divide by the number of planets; Torque checks this and turns each planet to its meshing position in the exported assembly. Each planet spins on a pin held by the carrier plate and a retainer ring, ideally with a small ball bearing.

Split-ring drives for extreme ratios

A split-ring (Wolfrom) drive uses long planets that mesh two rings: a fixed ring and an output ring with a few teeth fewer. Each turn of the carrier moves the output ring by the tooth difference only, so one stage reaches 50:1 to 1000:1: ratio = (1 + fixed ring / sun) / (1 − fixed ring / output ring).

The output ring is profile-shifted so both rings mesh the same planets. The price for the huge ratio is efficiency: much of the power circulates between the two rings, so expect 30–60 % with printed gears. That also makes the drive hard to back-drive, which is often welcome in a robot joint or a winch.

Worm gearboxes

A worm is a screw on the motor shaft. Each turn moves the wheel on by one tooth per thread start, so the ratio is wheel teeth / starts, typically 10:1 to 80:1 in a single stage, with the output at 90° to the motor. Torque designs an involute worm and a helical wheel with the same lead angle: both print flat as twisted extrusions and mesh without interference.

The teeth slide rather than roll, so the efficiency is lower than with spur gears: about 30–45 % with one start, up to 70–80 % with four. With a lead angle below the friction angle the drive is self-locking: the load cannot turn the motor, which is handy for lifts and winches. Sliding makes heat, so Torque checks that the housing can shed it. Grease the worm well.

Which gearbox should I pick?

Pick the cycloidal drive for the most torque in the smallest space, for coaxial output and for shock loads such as robot joints. Pick the compound gear train when you want the simplest parts, any ratio in 1 to 4 stages, and an output beside the motor. Pick a planetary gearbox for coaxial ratios of 3:1 to 1000:1: stacked stages for good efficiency, a split ring for extreme ratios in one compact stage. Pick a worm gearbox for a right-angle output, 10:1 to 80:1 in one quiet stage, and a load that should hold itself when the motor stops.