# Parallel cranks and coupling rods, two pairs at 90° (no dead centre)

> Why are coupling rods set 90° apart?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 230 (1868)
- Page: https://xemstudio.com/objects/parallel-crank-coupling-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=parallel-crank-coupling-mechanism · Plain text: https://xemstudio.com/objects/parallel-crank-coupling-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 shafts 80 mm apart each carry two cranks of equal radius; equal rods join crank to crank, so the shafts turn together at exactly the same speed (a parallelogram linkage). One pair alone would jam or flip at its two dead centres (rod in line with the cranks); a second pair set at 90° is at full leverage exactly then, so the drive never stops — no flywheel needed.
- Catalog Ø36 crank discs (pin at 12 mm) and 80 mm links: the rod length equals the shaft distance, which makes every rod parallel to the line of shafts.
- Brown's No. 230 (circular into circular motion by two pairs of connecting rods at right angles); the quartered cranks of locomotive coupling rods.
- Parts (each has its own page): crank-disc-36-pin-r12, four-bar-links, dowel-pins-metric.
- Brown's movement No. 230 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| shaft centres | 80 mm |
| crank radius | 12 mm |
| rods | 2 × 80 mm |
| crank phase | 90° |
| ratio | 1 : 1, same direction |

## 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. 230: figure, his text and an animation](https://507movements.com/mm_230.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/parallel-crank-coupling-mechanism/index.json · license: https://xemstudio.com/files/hardware/parallel-crank-coupling-mechanism/LICENSE.md

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

## Questions

**Why are coupling rods set 90° apart?**

Two shafts 80 mm apart each carry two cranks of equal radius; equal rods join crank to crank, so the shafts turn together at exactly the same speed (a parallelogram linkage). One pair alone would jam or flip at its two dead centres (rod in line with the cranks); a second pair set at 90° is at full leverage exactly then, so the drive never stops — no flywheel needed.

## Fits with

- [Crank disc Ø36 × 5, Ø5 bore, Ø3 pin hole at 12 mm](https://xemstudio.com/objects/crank-disc-36-pin-r12/)
- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)