# Rhombic drive link, 30 mm centres

> How long are the links in a rhombic drive?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Rhombic kinematics (tools/rhombic_geom.py): four equal links L = 30 between crank pins at e = 7 on gears 66 apart and yoke pivots at ±12.7
- Page: https://xemstudio.com/objects/rhombic-link/ · JSON: https://xemstudio.com/api/geometry?object=rhombic-link · Plain text: https://xemstudio.com/objects/rhombic-link/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**Exact dimensions.** Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part.

## Notes

- Four identical links join the two crank pins to the two yokes, forming the rhombus. Each is 30 mm between Ø3 holes (fit: run), 38 × 8 × 5 with round ends.
- Two ride each crank pin: the lower link next to the gear, the upper link outside it.
- Link length, gear centres, throw and yoke span together set the phase: here 90.0° with both strokes 20.12 mm.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| link ends | Ø3 | Ø3 pins (turn on them) | run | link-* |

## Sizes

| parameter | value |
|---|---|
| centres | 30.0 |
| outline | 38 × 8 × 5 |
| holes | 2 × Ø3 (run) |
| quantity | 4 |

## Where the files are

- Wikipedia: [Rhombic drive](https://en.wikipedia.org/wiki/Rhombic_drive) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Stirling engine](https://en.wikipedia.org/wiki/Stirling_engine) — web — CC BY-SA 4.0 (text)
- Philips Research / Meijer: [The Philips Stirling engine (rhombic drive papers, 1958–1965)](https://en.wikipedia.org/wiki/Rhombic_drive#History) — web — reference (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (1 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/rhombic-link/index.json

- link-30: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-30.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-30.stl) — rhombic link 30 mm centres, 8 wide × 5, Ø3 pivot holes; four per drive

## How it fits

### Connects to
- [Rhombic drive crank gear, module 1.5, 44 T, 7 mm throw](https://xemstudio.com/objects/rhombic-crank-gear/): Ø3 hole on the gear's crank pin
- [Rhombic drive upper yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-upper-yoke/): Ø3 pin at the yoke's ±12.7 pivots
- [Rhombic drive lower yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-lower-yoke/): Ø3 pin at the yoke's ±12.7 pivots

### Also needed
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 link pins

## Questions

**How many links does a rhombic drive need?**

Four equal links: two on each crank pin, one to the upper yoke and one to the lower.

**What link length goes with this rhombic drive?**

30 mm centres, with a 7 mm throw, gears 66 apart and yoke pivots at ±12.7.

## Fits with

- [Rhombic drive crank gear, module 1.5, 44 T, 7 mm throw](https://xemstudio.com/objects/rhombic-crank-gear/)
- [Rhombic drive upper yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-upper-yoke/)
- [Rhombic drive lower yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-lower-yoke/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Build list: Beta Stirling, rhombic drive, 2 in tri-clamp](https://xemstudio.com/objects/list-beta-rhombic-2in/)