# Cartwright's parallel motion (twin gears and cranks)

> How does Cartwright's parallel motion keep a rod straight?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 328 (1868)
- Page: https://xemstudio.com/objects/cartwright-parallel-motion-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=cartwright-parallel-motion-mechanism · Plain text: https://xemstudio.com/objects/cartwright-parallel-motion-mechanism/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- Two equal gears turn in opposite directions, each with a crank; equal connecting rods join the cranks to the two ends of a crosshead. By symmetry the crosshead's centre can only move on the line midway between the gears: an exact straight line.
- Here: module 1, 30 T gears at 30 mm, cranks 10 mm, rods 60 mm, crosshead pins 30 mm apart; at 35° the crosshead centre is 65.17 mm above the gear centres.
- Brown's No. 328 (Dr. Cartwright, 1787).
- Parts (each has its own page): crank-gear-m1-30t-pin-r10, cartwright-crosshead-30, four-bar-links, dowel-pins-metric.
- Brown's movement No. 328 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| gears | 2 × 30 T, module 1, 30 mm centres |
| crank radius | 10 mm |
| rods | 60 mm (four-bar link) |
| crosshead pins | 30 mm apart |

## Where the files are

- Henry T. Brown (1868, public domain): [Five Hundred and Seven Mechanical Movements — scan on archive.org](https://archive.org/details/fivehundredseven00browiala) — web — Publisher's page; dimensions are facts
- 507movements.com: [Brown's movement No. 328: figure, his text and an animation](https://507movements.com/mm_328.html) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/cartwright-parallel-motion-mechanism/index.json

- cartwright-parallel-motion-mechanism-assembly: [STEP](https://xemstudio.com/files/hardware/cartwright-parallel-motion-mechanism/cartwright-parallel-motion-mechanism-assembly.step) · [STL](https://xemstudio.com/files/hardware/cartwright-parallel-motion-mechanism/cartwright-parallel-motion-mechanism-assembly.stl) — Assembled mechanism, all parts placed at one pose (STEP keeps each part separate)

## Questions

**How does Cartwright's parallel motion keep a rod straight?**

Two equal gears turn in opposite directions, each with a crank; equal connecting rods join the cranks to the two ends of a crosshead. By symmetry the crosshead's centre can only move on the line midway between the gears: an exact straight line.

## Fits with

- [Crank gear, module 1, 30 T with a Ø3 crank-pin hole at 10 mm](https://xemstudio.com/objects/crank-gear-m1-30t-pin-r10/)
- [Cartwright crosshead bar, 30 mm pin spacing, centre rod hole](https://xemstudio.com/objects/cartwright-crosshead-30/)
- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)