# Linear-alternator stator that clamps round a 2 in tri-clamp tube: four coils outside the pressure wall

> How do I take electricity out of a pressurised free-piston engine without a wire through the wall?

- Category: Stirling engines › Tri-clamp engine parts
- Kind: Hardware
- Basis: Coil window and pitch from the alternator design in tools/gen_ta_power.py; gap field from a magnetic-circuit estimate
- Page: https://xemstudio.com/objects/canned-linear-alternator-stator-2in/ · JSON: https://xemstudio.com/api/geometry?object=canned-linear-alternator-stator-2in · Plain text: https://xemstudio.com/objects/canned-linear-alternator-stator-2in/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 coils sit outside the honed spool: a split bobbin clamps on the Ø50.8 tube with four 14 mm bays at the magnet rows' 32 mm pitch; the coils are wound in place, alternate hands, and joined in series; a split return-path shell (Ø66 / Ø74) clamps over them with three M4 × 16 and nuts each side. The shell has to carry the magnets' flux back; what it is made of is the builder's choice within that.
- The spool wall (1.40 after honing) and a 0.5 bobbin wall sit in the magnetic gap: the wall must not carry magnetic flux, or it short-circuits the magnets. In the 25 Hz field the eddy-current loss in a 1.4 mm wall is a fraction of a watt.
- Estimate: gap field 0.29 T (Br 1.3 T magnets, magnetic-circuit estimate with a 0.8 leakage factor), 369 turns of 0.45 mm wire per coil, Bl ≈ 79 T·m, coil resistance ≈ 31 Ω, inductance ≈ 132 mH; damping available Bl²/R ≈ 203 N·s/m. Measure Bl (EMF ÷ velocity) on the built engine; the model's power scales with it.
- No electrical pass-through, no wire in the gas: the generator's pressure boundary is only tri-clamp joints.
- Fits: bobbin bore clamps the tube (the 1 mm split is the take-up); shell bore clamps the bobbin flanges; ear holes are M4 clearance.

## 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 |
|---|---|---|---|---|
| bobbin bore | Ø50.8 | 2 in tube outside (honed spool) | clamp | stator-bobbin-* |
| bobbin flanges and dividers | Ø66.0 | return-shell bore Ø66.0 | clamp | stator-bobbin-* |
| split | 1.0 total (0.5 per half) | the other half | clamp | all |
| coil bays | 4 × 14 wide, Ø51.8–Ø66 | coils wound in place (Ø51.8 / Ø65.8 winding) | designed | all |
| coil to shell | 0.1 radial | return-shell bore | designed | stator-coil-* |
| ear holes | 6 × Ø4.5 | M4 clearance (ISO 273 medium) | designed | stator-iron-* |

## Sizes

| part | value |
|---|---|
| bays | 4 × 14 at 32 |
| coil | Ø65.8 / Ø51.8 |
| shell | Ø74 / Ø66 × 112 |
| turns per coil | 369 |
| Bl | ≈ 79 T·m |
| coil resistance | ≈ 31 Ω (all four) |

## Where the files are

- Xem Studio: [tools/gen_ta_power.py: the network model and every dimension on this page](https://xemstudio.com/) — code — CC BY 4.0
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (3 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/canned-linear-alternator-stator-2in/index.json

- stator-bobbin-half-2in-112: [STEP](https://xemstudio.com/files/hardware/canned-linear-alternator-stator-2in/stator-bobbin-half-2in-112.step) · [STL](https://xemstudio.com/files/hardware/canned-linear-alternator-stator-2in/stator-bobbin-half-2in-112.stl) — bobbin half (y ≥ 0.5): wall Ø50.8 / Ø51.8, end flanges and dividers to Ø66, four 14 mm coil bays at 32 pitch, 112 long
- stator-coil-2in-14x7: [STEP](https://xemstudio.com/files/hardware/canned-linear-alternator-stator-2in/stator-coil-2in-14x7.step) · [STL](https://xemstudio.com/files/hardware/canned-linear-alternator-stator-2in/stator-coil-2in-14x7.stl) — coil winding volume Ø65.8 / Ø51.8 × 14: about 369 turns of 0.45 mm wire
- stator-iron-half-2in-112: [STEP](https://xemstudio.com/files/hardware/canned-linear-alternator-stator-2in/stator-iron-half-2in-112.step) · [STL](https://xemstudio.com/files/hardware/canned-linear-alternator-stator-2in/stator-iron-half-2in-112.stl) — return-path shell half (y ≥ 0.5) Ø74 / Ø66 × 112, ears x ±36..46 with Ø4.5 at x ±41.5, z 14, 56, 98

## Questions

**Why outside the tube?**

Then the only things in the pressurised gas are the moving magnets: no feedthrough to seal, and the coils are cooled by the room.

**How are the coils joined?**

In series, alternate hands, because the rows under them alternate N and S: all four voltages then add.

## Fits with

- [Free-piston mover for a 2 in honed spool: piston and alternator magnets in one, Ø48.00 × 130](https://xemstudio.com/objects/free-piston-mover-2in/)
- [Tri-clamp honed cylinder spool: 2 in × 6 in, bore finished Ø48.00](https://xemstudio.com/objects/tri-clamp-honed-cylinder-spool/)
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/)
- [Build list: Thermoacoustic-Stirling ring generator, three 2 in units, pressurised](https://xemstudio.com/objects/list-ta-ring-3x2in/)