# Diaphragm crosshead hub and tapped backing disc: 2 in, with the displacer rod through the centre

> How do I drive a diaphragm from a crank when the displacer rod passes through its centre?

- Category: Hardware › Stirling & thermoacoustic
- Kind: Hardware
- Basis: Design values from the sealed generator: Ø20 centre clamp as the stock 2 in diaphragm disc, M2 screws (ISO 4762) in ISO 273 medium clearance holes and Ø1.6 tap-drill holes, Ø3 guide bore; the pin height comes from the crank layout
- Page: https://xemstudio.com/objects/tri-clamp-diaphragm-crosshead/ · JSON: https://xemstudio.com/api/geometry?object=tri-clamp-diaphragm-crosshead · Plain text: https://xemstudio.com/objects/tri-clamp-diaphragm-crosshead/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**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 hub clamps the membrane from the crankcase side: a Ø20 × 3 disc with three Ø2.4 holes on Ø13 for M2 × 6 screws, heads up. Under the membrane the backing disc (Ø20 × 2, Ø5 centre hole) has three Ø1.6 holes tapped M2, so nothing stands below it into the working space.
- A Ø8 boss rises to 15 mm with a Ø3 bore straight through disc and boss: the displacer rod runs in it. This 15 mm close running fit is the only way for gas between the working space and the crankcase, the bleed that lets the crankcase follow the working space's mean pressure.
- A bar (z 5.5–9.5, 6 wide) runs out along +X to an arm at x 13.5–18.5 that carries the con-rod pin (Ø3 × 10, pressed) along X, 29.25 mm above the membrane face. The con-rod eye sits at x 8–13, beside the displacer's rod clevis (x ±6), so the two drives pass each other without touching.
- Fits: Ø3 bore run on the Ø3 rod (the bleed); pin hole press on the dowel; disc and backing clamp the membrane.

## 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 |
|---|---|---|---|---|
| guide bore | Ø3 × 15 | Ø3 displacer rod (crankshaft-3x60): the bleed between working space and crankcase | run | crosshead-* |
| con-rod pin hole | Ø3 along X in the arm | dowel-3x10 | press | crosshead-* |
| screw holes | 3 × Ø2.4 on Ø13 | M2 × 6 (ISO 273 medium clearance) | designed | crosshead-* |
| tapped holes | 3 × Ø1.6 (M2 tap drill) on Ø13 | shcs-m2x6 | designed | backing-* |
| centre hole | Ø5 | Ø3 displacer rod (no contact) | designed | backing-* |

## Sizes

| part | feature | value |
|---|---|---|
| hub | disc | Ø20 × 3, 3 × Ø2.4 on Ø13 |
| hub | guide bore | Ø3 × 15 (run) |
| hub | con-rod pin | Ø3 along X at z 29.25, x 8.5–18.5 |
| backing | disc | Ø20 × 2, Ø5 centre hole |
| backing | screw holes | 3 × Ø1.6 (M2 tapped) |

## Where the files are

- ISO: [ISO 273 clearance holes for bolts and screws (fine / medium / coarse)](https://www.iso.org/standard/4196.html) — standard — dimensions are facts (verify at link)
- Wikipedia: [Hagen–Poiseuille equation: plane Poiseuille flow between parallel plates (annular clearance leakage Q = π D δ³ Δp / 12 μ L)](https://en.wikipedia.org/wiki/Hagen%E2%80%93Poiseuille_equation) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (2 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/tri-clamp-diaphragm-crosshead/index.json

- crosshead-2in-p29.25: [STEP](https://xemstudio.com/files/hardware/tri-clamp-diaphragm-crosshead/crosshead-2in-p29.25.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-diaphragm-crosshead/crosshead-2in-p29.25.stl) — crosshead hub: Ø20 × 3 disc with 3 × Ø2.4 on Ø13, Ø8 boss to 15, Ø3 bore 15 long (rod guide and bleed), bar z 5.5–9.5 to an arm x 13.5–18.5, Ø3 pin hole along X at 29.25
- backing-2in-m2-tapped: [STEP](https://xemstudio.com/files/hardware/tri-clamp-diaphragm-crosshead/backing-2in-m2-tapped.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-diaphragm-crosshead/backing-2in-m2-tapped.stl) — tapped backing disc Ø20 × 2: Ø5 centre hole, 3 × Ø1.6 (M2 tap drill) on Ø13

## Questions

**Where does gas pass between the working space and the crankcase?**

Only along the Ø3 rod in the hub's 15 mm bore. With 5 µm radial clearance the mean pressures settle with a time constant of about 8 min; with 10 µm about 64 s. A cycle's leakage is negligible either way.

**Why is the con-rod pin on an arm at the side?**

The displacer rod runs up the axis through the hub, so the diaphragm's con-rod has its own plane beside the displacer's clevis.

## Fits with

- [Tri-clamp membrane with a centre hole: 2 in, for a rod through the diaphragm](https://xemstudio.com/objects/tri-clamp-through-rod-membrane/)
- [Tri-clamp diaphragm centre disc and backing disc: 1.5 and 2 in](https://xemstudio.com/objects/tri-clamp-diaphragm-disc/)
- [Rod clevises for a Ø2 or Ø3 rod or a Ø5 tube, 5 mm gap](https://xemstudio.com/objects/rod-clevises/)
- [Connecting rods 30 to 80 mm centres with 623 pockets](https://xemstudio.com/objects/connecting-rods-623/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Build list: Tri-clamp sealed diaphragm generator, 2 in](https://xemstudio.com/objects/list-tri-clamp-sealed-diaphragm-generator/)