# Ratchet wheels and pawl (anti-reverse / one-way drive)

> How do I design a 3D printed ratchet and pawl?

- Category: Hardware › Stirling & thermoacoustic
- Kind: Hardware
- Basis: Sawtooth ratchet: tooth depth 3 mm, 85/15 flank split; pawl pivot 3 mm; dimensions chosen for printing at 0.4 mm nozzle
- Page: https://xemstudio.com/objects/ratchet-and-pawl/ · JSON: https://xemstudio.com/api/geometry?object=ratchet-and-pawl · Plain text: https://xemstudio.com/objects/ratchet-and-pawl/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- Ratchet tooth: the drive face is radial, the back face slopes; the pawl must contact the drive face at or below the tooth tip so load pushes it deeper, not out. Pivot the pawl so its contact point trails the pivot by 10–15°.
- 12 teeth = 30° steps (coarse, strong); 24 = 15°; 36 = 10°. Spring: a printed cantilever or a 0.5 mm music-wire torsion spring on the pivot.
- Where it goes: fishing-reel style anti-reverse, hand-wound starters, a one-way drive between a Stirling flywheel and a generator so the generator never back-drives the engine.

## Sizes

| part | teeth | step_deg | od_mm | bore_mm |
|---|---|---|---|---|
| ratchet-12t | 12 | 30 | 40 | 5 |
| ratchet-24t | 24 | 15 | 40 | 5 |
| ratchet-36t | 36 | 10 | 40 | 5 |
| pawl-24 | — | — | 24 long | 3.2 |

## Where the files are

- Wikipedia: [Ratchet (device)](https://en.wikipedia.org/wiki/Ratchet_(device)) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Pawl](https://en.wikipedia.org/wiki/Pawl) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds that use these parts (videos and write-ups)](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (4 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/ratchet-and-pawl/index.json

- ratchet-12t-d40: [STEP](https://xemstudio.com/files/hardware/ratchet-and-pawl/ratchet-12t-d40.step) · [STL](https://xemstudio.com/files/hardware/ratchet-and-pawl/ratchet-12t-d40.stl) — ratchet wheel Ø40 × 6, 12 teeth 3 mm deep, Ø5 bore
- ratchet-24t-d40: [STEP](https://xemstudio.com/files/hardware/ratchet-and-pawl/ratchet-24t-d40.step) · [STL](https://xemstudio.com/files/hardware/ratchet-and-pawl/ratchet-24t-d40.stl) — ratchet wheel Ø40 × 6, 24 teeth 3 mm deep, Ø5 bore
- ratchet-36t-d40: [STEP](https://xemstudio.com/files/hardware/ratchet-and-pawl/ratchet-36t-d40.step) · [STL](https://xemstudio.com/files/hardware/ratchet-and-pawl/ratchet-36t-d40.stl) — ratchet wheel Ø40 × 6, 36 teeth 3 mm deep, Ø5 bore
- pawl-24: [STEP](https://xemstudio.com/files/hardware/ratchet-and-pawl/pawl-24.step) · [STL](https://xemstudio.com/files/hardware/ratchet-and-pawl/pawl-24.stl) — pawl 24 mm, Ø3.2 pivot, for the Ø40 wheels; spring not modeled

## Questions

**How many teeth should a ratchet have?**

Fewer teeth are stronger: 12 teeth (30° steps) for a hand-wound starter, 24–36 for fine positioning. Tooth depth 3 mm prints cleanly.

**Which way does the pawl point?**

Its contact point trails the pivot in the locking direction so load drives it into the tooth; the back of the tooth lifts it on the free stroke.

**Can a printed ratchet hold an engine flywheel?**

Yes at model scale: a 12-tooth PETG wheel with a 6 mm face holds several N·m; keep the pawl thick (6 mm) and pivot it on a 3 mm steel pin.

## Fits with

- [Spur gears, involute 20°: module 0.5, 1 and 2, 12–60 teeth](https://xemstudio.com/objects/spur-gears-involute/)
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/)