# Jockey (tightening) pulley engaging a slack belt

> How does a jockey pulley start and stop a belt drive?

- Category: Mechanisms › Belt and friction drives
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 5 (1868)
- Page: https://xemstudio.com/objects/jockey-pulley-belt-tightener-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=jockey-pulley-belt-tightener-mechanism · Plain text: https://xemstudio.com/objects/jockey-pulley-belt-tightener-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

- The belt is made a little long, so it slips idle on the pulleys; a pulley on a lever (the jockey or tightener) pressed on the back of the slack strand takes up the slack and the drive starts. Lift the lever and it stops — a clutch without moving the belt.
- Here Ø40 and Ø80 pulleys at 140 mm, a 16 × 1.5 belt of 472.33 mm (0.98 mm longer than an open belt) and a Ø30 jockey pressing the upper strand in 8 mm. The jockey also adds wrap on the small pulley: 169.91° instead of 196.43°.
- Brown's No. 5. Put the jockey on the slack side (here the upper strand when the small pulley drives clockwise).
- Parts (each has its own page): flanged-flat-pulley-40, flanged-flat-pulley-80, flanged-flat-pulley-30-bore6, flat-belt-16x1-5-jockey-drive-40-80-c140, jockey-lever-bore6, dowel-pins-metric.
- Brown's movement No. 5 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| pulleys | Ø40 and Ø80 at 140 mm |
| jockey | Ø30, 8 mm deflection |
| belt | 16 × 1.5, 472.33 mm |
| wrap on Ø40 | 169.91° |
| lever | 51.48 mm + 40 mm handle |

## 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. 5: figure, his text and an animation](https://507movements.com/mm_005.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/jockey-pulley-belt-tightener-mechanism/index.json · license: https://xemstudio.com/files/hardware/jockey-pulley-belt-tightener-mechanism/LICENSE.md

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

## Questions

**How does a jockey pulley start and stop a belt drive?**

The belt is made a little long, so it slips idle on the pulleys; a pulley on a lever (the jockey or tightener) pressed on the back of the slack strand takes up the slack and the drive starts. Lift the lever and it stops — a clutch without moving the belt.

## Fits with

- [Flanged flat-belt pulley Ø40, 16 mm face, Ø8 bore](https://xemstudio.com/objects/flanged-flat-pulley-40/)
- [Flanged flat-belt pulley Ø80, 16 mm face, Ø8 bore](https://xemstudio.com/objects/flanged-flat-pulley-80/)
- [Flanged flat-belt pulley Ø30, 16 mm face, Ø6 bore](https://xemstudio.com/objects/flanged-flat-pulley-30-bore6/)
- [Flat belt 16 × 1.5 for a Ø40 / Ø80 drive at 140 mm with a Ø30 jockey pulley (loop drawn)](https://xemstudio.com/objects/flat-belt-16x1-5-jockey-drive-40-80-c140/)
- [Jockey (tightener) lever, 51.48 mm centres, Ø6 holes](https://xemstudio.com/objects/jockey-lever-bore6/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)