# Rhombic drive lower yoke, 25.4 mm pivot span

> What are the dimensions of the piston 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-lower-yoke/ · JSON: https://xemstudio.com/api/geometry?object=rhombic-lower-yoke · Plain text: https://xemstudio.com/objects/rhombic-lower-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 lower 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 Ø13 × 16 boss on the rod axis (across the pivots) holds the 5 mm tube piston rod (Ø5 bore through; the Ø3 displacer rod runs on up inside it) (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 lower-yoke pivot pins are Ø3 × 16.

## 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-* |
| lower-yoke bore | Ø5 through | 5 mm tube (M3 clamp); displacer rod runs inside the tube | slide | yoke-lower-* |

## Sizes

| parameter | value |
|---|---|
| pivot span | 25.4 |
| bar | 35.4 × 10 × 6 |
| pivots | 2 × Ø3 (run), along the gear-shaft direction |
| boss | Ø13 × 16 |
| rod bore | Ø5 through |
| 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-lower-yoke/index.json

- yoke-lower-piston: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-lower-piston.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-lower-piston.stl) — lower yoke: as the upper, Ø13 × 16 boss with a Ø5 bore through for the 5 mm tube piston rod (M3 grub); the Ø3 displacer rod runs on up inside it

## How it fits

### Connects to
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/): 5 mm tube piston rod in the Ø5 bore
- [Tri-clamp power piston: ring-less full-bore pistons Ø34.8, Ø47.5 and Ø72.9](https://xemstudio.com/objects/tri-clamp-power-piston/): the piston at the other end of the tube rod
- [Tri-clamp flexure head: ISO 2852 cap with Ø60, Ø80 and Ø100 flexure seats](https://xemstudio.com/objects/tri-clamp-flexure-head/): piston plug carries the tube rod
- [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 lower yoke of a rhombic drive hold?**

The 5 mm tube piston rod, clamped by an M3 grub in its Ø13 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 upper yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-upper-yoke/)
- [Rhombic drive link, 30 mm centres](https://xemstudio.com/objects/rhombic-link/)
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/)
- [Tri-clamp power piston: ring-less full-bore pistons Ø34.8, Ø47.5 and Ø72.9](https://xemstudio.com/objects/tri-clamp-power-piston/)
- [Build list: Beta Stirling, rhombic drive, 2 in tri-clamp](https://xemstudio.com/objects/list-beta-rhombic-2in/)