# Eccentric and strap driving a slide (10 mm stroke)

> How does an eccentric and strap give reciprocating motion?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 89 (1868)
- Page: https://xemstudio.com/objects/eccentric-strap-slider-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=eccentric-strap-slider-mechanism · Plain text: https://xemstudio.com/objects/eccentric-strap-slider-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

- An eccentric is a crank whose pin is so large that it surrounds the shaft: a disc fixed off-centre on the shaft turns inside a strap, and the strap's rod drives whatever it is pinned to. Here the rod end is pinned to a block sliding in a straight slot, so the block reciprocates exactly like a slider-crank with crank radius = the offset and rod length = strap centre to eye.
- Catalog Ø30 eccentric with 5 mm offset (stroke 10 mm) and its split strap (eye 51 mm from the bore centre), a slide block in the 90 mm slot guide plate; drawn at 60°, block at x = 53.32 mm.
- Brown's No. 89 (eccentric and strap: the steam engine's valve drive).
- Parts (each has its own page): eccentric-and-strap, slide-block-12x6, slot-guide-plate-90, dowel-pins-metric.
- Brown's movement No. 89 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| offset | 5 mm |
| stroke | 10 mm |
| strap | Ø30 bore, eye at 51 mm |
| guide | slot plate 90, 6.2 slot |

## 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. 89: figure, his text and an animation](https://507movements.com/mm_089.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/eccentric-strap-slider-mechanism/index.json · license: https://xemstudio.com/files/hardware/eccentric-strap-slider-mechanism/LICENSE.md

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

## Questions

**How does an eccentric and strap give reciprocating motion?**

An eccentric is a crank whose pin is so large that it surrounds the shaft: a disc fixed off-centre on the shaft turns inside a strap, and the strap's rod drives whatever it is pinned to. Here the rod end is pinned to a block sliding in a straight slot, so the block reciprocates exactly like a slider-crank with crank radius = the offset and rod length = strap centre to eye.

## Fits with

- [Eccentric sheaves Ø30 at 3, 5 and 8 mm offset](https://xemstudio.com/objects/eccentric-sheaves/)
- [Slide block 12 × 6.2 mm for a 6.4 mm slot](https://xemstudio.com/objects/slide-block-12x6/)
- [Slot guide plate 90 × 30 × 5, 6.2 mm slot (74 long)](https://xemstudio.com/objects/slot-guide-plate-90/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)