# Watt's linkage (straight-line motion), 60 / 30 / 60

> How straight is Watt's linkage?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 337 (1868)
- Page: https://xemstudio.com/objects/watts-linkage-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=watts-linkage-mechanism · Plain text: https://xemstudio.com/objects/watts-linkage-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 radius arms swinging in opposite directions, joined by a short coupler: the coupler's midpoint moves on a figure-eight whose middle part is very nearly straight — Watt's 1784 parallel motion for a beam-driven rod.
- Here: arms 60 mm, coupler 30 mm, fixed pivots ±60 mm apart horizontally and 30 mm vertically. At 12° the midpoint is at x = 0.01 mm (straight line x = 0); over ±10° of arm swing it stays within about 0.1 mm of the line.
- Brown's No. 337 (radius rod on the lower end of a short link hung from the beam, rod at its centre); the same linkage locates car axles sideways.
- Parts (each has its own page): four-bar-links, watt-linkage-coupler-30, dowel-pins-metric.
- Brown's movement No. 337 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| radius arms | 2 × 60 mm |
| coupler | 30 mm |
| fixed pivots | (−60, 15) and (60, −15) mm |
| tracer | coupler midpoint |

## 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. 337: figure, his text and an animation](https://507movements.com/mm_337.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/watts-linkage-mechanism/index.json

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

## Questions

**How straight is Watt's linkage?**

Two equal radius arms swinging in opposite directions, joined by a short coupler: the coupler's midpoint moves on a figure-eight whose middle part is very nearly straight — Watt's 1784 parallel motion for a beam-driven rod.

**Is Watt's linkage exactly straight?**

No: the midpoint traces a lemniscate-like curve, nearly straight near the middle; Peaucellier's linkage (1864) is the first exact one.

## Fits with

- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/)
- [Watt linkage coupler, 30 mm, centre tracer hole](https://xemstudio.com/objects/watt-linkage-coupler-30/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)