# Linear alternator bobbin: Ø37 bore, two 10 mm bays

> What are the dimensions of the coil bobbin for a linear alternator?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Bore = mover Ø35 + 2 × 1.0 gap; flanges at the back-iron sleeve bore Ø52.5; length = sleeve length 30
- Page: https://xemstudio.com/objects/linear-alternator-bobbin/ · JSON: https://xemstudio.com/api/geometry?object=linear-alternator-bobbin · Plain text: https://xemstudio.com/objects/linear-alternator-bobbin/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 coil former of the alternator stator: Ø37 bore (1.0 mm air gap to the Ø35 mover), Ø38.6 core, 30 long, so the coil pair is centred in the 30 mm sleeve.
- Flanges are Ø52.5, the back-iron sleeve's bore (fit: slide): 4 mm end flanges and a 2 mm divider make two 10 mm winding bays.
- Lead-out slots in both end flanges and a crossover notch in the divider; the second bay is wound the opposite way and joined to the first in series.

## 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 |
|---|---|---|---|---|
| mover-to-bobbin gap | 1.0 radial | bobbin bore | designed | alt-bobbin-* |
| bobbin flanges in the sleeve | Ø52.5 | back-iron sleeve bore | slide | alt-bobbin-* |

## Sizes

| parameter | value |
|---|---|
| bore | 37 |
| core OD | 38.6 |
| flange OD | 52.5 |
| length | 30 |
| bays | 2 × 10 |
| divider | 2 |
| end flanges | 4 |

## Where the files are

- NASA (NTRS, public domain): [Linear alternator design for free-piston Stirling convertors (2005)](https://ntrs.nasa.gov/search?q=20050182934) — PDF — US Government work, public domain
- NASA (NTRS, public domain): [Sunpower/NASA 80 We free-piston Stirling convertor (2002)](https://ntrs.nasa.gov/search?q=20020047256) — PDF — US Government work, public domain
- Wikipedia: [Linear alternator](https://en.wikipedia.org/wiki/Linear_alternator) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Free-piston engine](https://en.wikipedia.org/wiki/Free-piston_engine) — web — CC BY-SA 4.0 (text)
- 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/linear-alternator-bobbin/index.json

- alt-bobbin-37-2x10: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-bobbin-37-2x10.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-bobbin-37-2x10.stl) — bobbin: Ø37 bore (1 mm air gap to the mover), two 10 mm bays split by a 2 mm divider with a crossover notch, 30 long, slides into the back-iron sleeve

## How it fits

### Goes in or on
- [Linear alternator back-iron sleeve Ø60.5 × Ø52.5 × 30](https://xemstudio.com/objects/linear-alternator-back-iron-sleeve/): Ø52.5 flanges in the sleeve bore

### Connects to
- [Linear alternator coil Ø50 × 10 (≈ 314 turns)](https://xemstudio.com/objects/linear-alternator-coil/): a coil in each bay
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/): mover inside, 1.0 mm gap

## Questions

**How big is the air gap in the linear alternator?**

1.0 mm radial: Ø37 bobbin bore over the Ø35 mover.

**Why does the bobbin have two bays?**

One coil over each magnet row, wound in opposite directions and joined in series.

## Fits with

- [Linear alternator coil Ø50 × 10 (≈ 314 turns)](https://xemstudio.com/objects/linear-alternator-coil/)
- [Linear alternator back-iron sleeve Ø60.5 × Ø52.5 × 30](https://xemstudio.com/objects/linear-alternator-back-iron-sleeve/)
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/)
- [Build list: Free-piston Stirling, 2 in tri-clamp (flexure-suspended)](https://xemstudio.com/objects/list-free-piston-2in/)