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3DAI
Free · runs in your browser · no signup

Planetary Gear Generator (STL)

Pick the sun and planet tooth counts, the number of planets and the module. The tool works out the ring gear, checks that the planets fit and space evenly, shows the ratio for each fixed member, and gives you watertight STL or 3MF files for the sun, planets, ring and carrier.

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How to make a 3D printable planetary gearbox

  1. 1

    Set the teeth

    Sun and planet teeth set the ring (sun + 2 × planet) and the ratio. With the ring held, 18 sun and 18 planet teeth give 4:1.

  2. 2

    Pick the planet count

    Sun teeth + ring teeth must divide by the number of planets so they space evenly. If not, the tool suggests the nearest counts that work.

  3. 3

    Size and fit

    Module sets the tooth size for every gear. Add backlash for printing, a shaft hole in the sun, and pin holes for the planets.

  4. 4

    Download and print

    One STL per part (print the planet as many times as you have planets) or a 3MF with every part laid out flat.

Need more than a gearbox?

Turn a photo or text into a printable 3D model with AI, with an STL download. The AI generators use credits; this gearbox tool does not.

Image to STL →Turn a photo into a printable 3D model with AI.Text to 3D →Describe a part or a figure and get a 3D model.

More free tools: Gear generator · 3D print cost calculator · STL viewer

Planetary gear formulas

These are the rules and ratios the generator uses, with a worked example: an 18-tooth sun, 18-tooth planets, a 54-tooth ring and 3 planets at module 1.25. Every gear, including the ring's inside teeth, has true involute flanks, so the parts roll smoothly together.

Ring teethR = S + 2P (18 + 2 × 18 = 54)
Ring held (most common)ratio = 1 + R / S (1 + 54 / 18 = 4 : 1, sun in, carrier out, same direction)
Sun heldratio = R / (R + S) (54 / 72 = 0.75 : 1, carrier in, ring out, a little faster)
Carrier heldratio = −R / S (-3 : 1, sun in, ring out, reversed: a plain gear train)
Equal spacing(S + R) / N must be a whole number (72 / 3 = 24)
Planets must not touch2a × sin(180° / N) > planet tip diameter (a = sun-planet centre distance)
Centre distance aa = m × (S + P) / 2 (module 1.25: 22.5 mm from the sun to each planet pin)
Pitch diameterssun S × m, planet P × m, ring R × m (22.5, 22.5 and 67.5 mm at module 1.25)

Tips for 3D printing a planetary gearbox

Common starting points for an FDM printer with a 0.4 mm nozzle; your printer may need small changes.

Module vs nozzleModule 1 or more prints cleanly with a 0.4 mm nozzle; module 1.25 to 2 is sturdier. The ring's inside teeth are the hardest part to print, so do not go below 1 on FDM.
BacklashStart at 0.2 mm. A planetary set has two meshes per planet, so it binds sooner than a gear pair. If it is stiff, raise it to 0.25 to 0.3 mm.
OrientationPrint every part flat on the bed. The 3MF is already laid out that way.
Walls and infill4 or more walls so the teeth are solid plastic; 30 to 50% infill for the rest.
PinsUse 3 mm steel pins, bolts or printed pegs through the carrier. The planet hole is the pin size plus the allowance, so the planets spin freely.
Elephant footA flared first layer makes the ring's inside teeth rub. Turn on elephant-foot compensation or chamfer the first layer.
MaterialPLA is fine for demos and light loads. PETG or nylon last longer in a gearbox that runs often.

Planetary gear generator FAQ

It depends on which part is held still. With the ring held, the sun as input and the carrier as output, the ratio is 1 + ring teeth / sun teeth: 18 sun and 54 ring teeth give 4:1. With the sun held it is ring / (ring + sun), and with the carrier held it is ring / sun, reversed. The tool shows all three.
The planets only space evenly around the sun if sun teeth + ring teeth divide by the planet count. For example 18 + 54 = 72 divides by 3 and by 4, so 3 or 4 planets space evenly, but 5 do not. The planets also need room: their tips must not touch each other. If your choice fails, the tool suggests the nearest tooth counts that work.
Ring teeth = sun teeth + 2 × planet teeth, because a planet sits between the sun and the ring on each side. The generator sets it for you. All three gears share the same module and pressure angle so they mesh.
With the ring held, a single stage usually gives between 3:1 and about 10:1. A small sun with large planets gives a higher ratio, but the sun gets weak and below about 17 teeth (at 20°) it is undercut. For more reduction, stack two stages: the ratios multiply.
Start with 0.2 mm, which is the default. Each planet meshes with both the sun and the ring, so small print errors add up; if the set is stiff after a few turns, print again with 0.25 to 0.3 mm. A well-tuned printer can go down to 0.15 mm.
The carrier is the plate that holds the planet pins. In the usual setup (ring held) it is the output and turns slower than the sun with more torque. The generator makes a round carrier with a pin hole for each planet and a centre hole for the output shaft; print a second one for the other side if you want the planets held on both faces.
Yes. It runs in your browser: nothing is uploaded, there is no signup and no watermark, and you can download as many STL and 3MF files as you like. The AI 3D generators linked below use credits; this tool does not.