# Variable crank: a spiral-slotted plate moves the pin along a radial slot

> How can the throw of a crank be changed while it runs?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 94 (1868)
- Page: https://xemstudio.com/objects/variable-crank-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=variable-crank-mechanism · Plain text: https://xemstudio.com/objects/variable-crank-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

- Two plates on the crank shaft: one with a radial slot, one with a spiral slot. The crank pin passes through both. Turning one plate against the other moves the pin in or out along the radial slot, changing the crank radius (and the stroke) without stopping.
- Here Ø80 plates, slots 6 mm, spiral rising 26 mm per turn (R8 → R34): 0.072 mm of radius per degree. Drawn with the pin at 20 mm (40 mm stroke), the spiral plate turned 166.15°.
- Brown's No. 94 (variable crank).
- Parts (each has its own page): variable-crank-disc-radial-slot-80, variable-crank-disc-spiral-slot-80, dowel-pins-metric, linear-shafts-h6.
- Brown's movement No. 94 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| plates | 2 × Ø80 × 6 |
| pin radius | 8 – 34 mm (drawn 20) |
| adjustment | 26 mm per turn of the spiral |

## 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. 94: figure, his text and an animation](https://507movements.com/mm_094.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/variable-crank-mechanism/index.json · license: https://xemstudio.com/files/hardware/variable-crank-mechanism/LICENSE.md

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

## Questions

**How can the throw of a crank be changed while it runs?**

Two plates on the crank shaft: one with a radial slot, one with a spiral slot. The crank pin passes through both. Turning one plate against the other moves the pin in or out along the radial slot, changing the crank radius (and the stroke) without stopping.

## Fits with

- [Variable-crank disc Ø80 × 6 with a 6 mm radial slot (R8 → R34)](https://xemstudio.com/objects/variable-crank-disc-radial-slot-80/)
- [Variable-crank disc Ø80 × 6 with a 6 mm Archimedean spiral slot (R8 → R34 in one turn)](https://xemstudio.com/objects/variable-crank-disc-spiral-slot-80/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Linear shafts Ø3 to Ø25 mm (h6), 100 to 1000 mm long](https://xemstudio.com/objects/linear-shafts-h6/)