---
title: "Gears, pulleys, knobs and simple mechanisms"
description: "Spur gears, racks, pulleys, ratchets and knobs from a text description: module, tooth count, bore, keyways. Print-ready STL, 3MF or sliced G-code, watertight."
canonical: https://speakcad.com/use-cases/gears-and-mechanisms
published: 2026-09-03
updated: 2026-09-23
publisher: SpeakCAD
---

# Gears, pulleys, knobs and simple mechanisms

> Spur gears, racks, pulleys, ratchets and knobs from a text description: module, tooth count, bore, keyways. Print-ready STL, 3MF or sliced G-code, watertight.

Mechanical parts have precise definitions, and SpeakCAD speaks that language: module, pressure angle, tooth count, bore, keyway, hub. Describe the pair you need and get meshing gears laid out on the bed.

## Gears from parameters

Give module (or pitch), tooth count and thickness. SpeakCAD generates involute-approximated teeth with the correct pitch diameter so two gears mesh at the sum of their pitch radii, and tells you that centre distance in the reply.

## Bores, hubs and shafts

D-shafts, keyed shafts, hex bores, set-screw hubs and press-fit bearings (608, 625, 623) are all a phrase away. Clearances follow FDM practice: 0.2 mm on a bore that must not spin, 0.4 mm on a free-running one.

## Simple mechanisms

Racks and pinions, ratchets and pawls, cams, lever arms, snap hinges, living hinges for PP and PETG, and geneva drives. For anything with moving parts, print a test pair first; thanks to the free re-download rule, adjusting the backlash spends only the requests that change it (2 credits each), or nothing if you type the new value in the dimensions panel.

## Prompts to try

- A 24-tooth spur gear, module 2, 8 mm thick, 6 mm D-shaft bore with 0.5 mm flat, 4 lightening holes, and a matching 12-tooth pinion. ([open in SpeakCAD](https://app.speakcad.com/signup?next=%2F%3Fprompt%3DA%252024-tooth%2520spur%2520gear%252C%2520module%25202%252C%25208%2520mm%2520thick%252C%25206%2520mm%2520D-shaft%2520bore%2520with%25200.5%2520mm%2520flat%252C%25204%2520lightening%2520holes%252C%2520and%2520a%2520matching%252012-tooth%2520pinion.))
- A GT2 pulley with 20 teeth, 6 mm belt, 5 mm bore, set screw hole M3, flanges 1 mm. ([open in SpeakCAD](https://app.speakcad.com/signup?next=%2F%3Fprompt%3DA%2520GT2%2520pulley%2520with%252020%2520teeth%252C%25206%2520mm%2520belt%252C%25205%2520mm%2520bore%252C%2520set%2520screw%2520hole%2520M3%252C%2520flanges%25201%2520mm.))
- A ratchet wheel 40 mm diameter with 12 teeth and a matching pawl, 5 mm thick, 4 mm pivot holes. ([open in SpeakCAD](https://app.speakcad.com/signup?next=%2F%3Fprompt%3DA%2520ratchet%2520wheel%252040%2520mm%2520diameter%2520with%252012%2520teeth%2520and%2520a%2520matching%2520pawl%252C%25205%2520mm%2520thick%252C%25204%2520mm%2520pivot%2520holes.))

## FAQ

### Are the gear teeth true involutes?

They are close polygonal approximations of involute profiles, which is what most printed gears use. For high-speed or high-load gears, plan a test print and fine-tune the profile shift by chat.

### Can it design both gears at once?

Yes. Ask for the pair and it lays them out side by side with the correct centre distance mentioned in the answer.

## About SpeakCAD

SpeakCAD is the most intuitive CAD for 3D printing, made for people who own a 3D printer and need a part that fits something they have: tap a face and type a size to shape it with simple tools, or describe it to the AI, on your phone or computer, and download a print-ready STL, 3MF or G-code sliced for your printer.

SpeakCAD works inside ChatGPT and Claude as a plugin (an MCP server): ask the assistant for a part with its measurements and SpeakCAD's own CAD AI designs it, shows it in 3D in the chat and hands over a print-ready STL, 3MF or G-code sliced for your printer.

- Live demo, no account: https://speakcad.com/demo
- In ChatGPT, Claude and other MCP clients: https://speakcad.com/chatgpt-plugin, https://speakcad.com/claude-connector, https://speakcad.com/mcp-server
- Pricing: https://speakcad.com/pricing
- FAQ: https://speakcad.com/faq
- Site index for AI assistants: https://speakcad.com/llms.txt