# Triangular eccentric: curved triangle turning in a yoke (motion with dwells)

> How does a triangular eccentric give a pause at each end of the stroke?

- Category: Mechanisms › Cams
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 91 (1868)
- Page: https://xemstudio.com/objects/triangular-eccentric-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=triangular-eccentric-mechanism · Plain text: https://xemstudio.com/objects/triangular-eccentric-mechanism/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- 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

- A curved triangle of constant width turns about one of its corners inside a frame whose faces are exactly that width apart, so it always touches both. The frame moves to and fro, but stands still while a corner-centred arc rolls on a face: an intermittent reciprocation from continuous rotation.
- Here width 30 mm: stroke 30 mm, with a dwell at each end of the stroke; drawn at 25°.
- Brown's No. 91 (triangular eccentric giving an intermittent reciprocating motion, used in France for slide valves).
- Parts (each has its own page): triangular-eccentric-reuleaux-w30, triangular-eccentric-yoke-w30, dowel-pins-metric.
- Brown's movement No. 91 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| cam | Reuleaux triangle, width 30 |
| pivot | one corner |
| stroke | 30 mm |
| yoke opening | 30 mm |

## 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. 91: figure, his text and an animation](https://507movements.com/mm_091.html) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/triangular-eccentric-mechanism/index.json · license: https://xemstudio.com/files/hardware/triangular-eccentric-mechanism/LICENSE.md

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

## Questions

**How does a triangular eccentric give a pause at each end of the stroke?**

A curved triangle of constant width turns about one of its corners inside a frame whose faces are exactly that width apart, so it always touches both. The frame moves to and fro, but stands still while a corner-centred arc rolls on a face: an intermittent reciprocation from continuous rotation.

## Fits with

- [Triangular eccentric: curved (Reuleaux) triangle of width 30, pivoted at a corner](https://xemstudio.com/objects/triangular-eccentric-reuleaux-w30/)
- [Yoke 42 × 72 with a 30 mm wide opening and two 8 × 8 rods](https://xemstudio.com/objects/triangular-eccentric-yoke-w30/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)