# Rhombic drive upper yoke, 25.4 mm pivot span

> What are the dimensions of the displacer yoke in a rhombic drive?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Yoke pivots at ±12.7 (rhombic kinematics, tools/rhombic_geom.py), pivot axes parallel to the gear shafts, rod axis across them; bore at the rod, M3 tap drill Ø2.5
- Page: https://xemstudio.com/objects/rhombic-upper-yoke/ · JSON: https://xemstudio.com/api/geometry?object=rhombic-upper-yoke · Plain text: https://xemstudio.com/objects/rhombic-upper-yoke/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

- The upper yoke of the rhombus: a 35.4 × 10 bar, 6 thick along the gear-shaft direction, with Ø3 pivot holes (fit: run) at ±12.7 running parallel to the gear shafts.
- A Ø11 × 16 boss on the rod axis (across the pivots) holds the Ø3 displacer rod (fit: slide), with an M3 grub (Ø2.5 tapped) entering at mid-height; an M3 × 4 grub sits flush.
- Both yokes sit in one plane on the rod axis, in front of the two link planes; the upper-yoke pivot pins are Ø3 × 12.

## 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 |
|---|---|---|---|---|
| yoke pivots | Ø3 | Ø3 pins (turn on them) | run | yoke-* |
| yoke rod bore | Ø3 / Ø5 | displacer rod / 5 mm tube (M3 clamp) | slide | yoke-* |
| yoke clamp screw hole | Ø2.5 | M3 tap drill | designed | yoke-* |

## Sizes

| parameter | value |
|---|---|
| pivot span | 25.4 |
| bar | 35.4 × 10 × 6 |
| pivots | 2 × Ø3 (run), along the gear-shaft direction |
| boss | Ø11 × 16 |
| rod bore | Ø3 |
| grub hole | Ø2.5 (M3) |

## 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-upper-yoke/index.json

- yoke-upper-displacer: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-upper-displacer.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-upper-displacer.stl) — upper yoke: bar 35.4 × 10 × 6, Ø3 pivot holes 25.4 apart along the gear-shaft direction, Ø11 × 16 boss on the axis with the Ø3 displacer-rod bore and an M3 grub

## How it fits

### Connects to
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/): Ø3 displacer rod clamped in the boss
- [Rhombic drive lower yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-lower-yoke/): the displacer rod runs on down through the lower yoke's tube
- [Rhombic drive link, 30 mm centres](https://xemstudio.com/objects/rhombic-link/): two links on the ±12.7 Ø3 pivots

### Also needed
- [Set screws (grub screws), M3–M8 (ISO 4026)](https://xemstudio.com/objects/set-screws-metric/): M3 clamp screw

## Questions

**What does the upper yoke of a rhombic drive hold?**

The Ø3 displacer rod, clamped by an M3 grub in its Ø11 boss; two links pivot on it at ±12.7.

**Which way do the yoke pivot holes run?**

Parallel to the gear shafts, across the rod: the links swing in planes square to the shafts.

## Fits with

- [Rhombic drive lower yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-lower-yoke/)
- [Rhombic drive link, 30 mm centres](https://xemstudio.com/objects/rhombic-link/)
- [Rhombic drive crank gear, module 1.5, 44 T, 7 mm throw](https://xemstudio.com/objects/rhombic-crank-gear/)
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/)
- [Build list: Beta Stirling, rhombic drive, 2 in tri-clamp](https://xemstudio.com/objects/list-beta-rhombic-2in/)