# Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets

> What does the moving magnet part of a linear alternator look like and what size is it?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Moving-magnet layout as in Sunpower / NASA convertors; pockets at the Ø10 × 3 disc magnet; checked in the generator (alt_check)
- Page: https://xemstudio.com/objects/linear-alternator-mover/ · JSON: https://xemstudio.com/api/geometry?object=linear-alternator-mover · Plain text: https://xemstudio.com/objects/linear-alternator-mover/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 moving part of the linear alternator: a Ø35 × 26 carrier on a Ø12 × 6 hub, 32 mm overall, with a Ø5 bore (fit: slide) and a Ø2.5 radial hole (M3 tap size) in the hub for a grub screw.
- Two axial rows of 8 pockets (fit: press) for Ø10 × 3 disc magnets, faces out, 12 mm apart along the axis, the second row turned half a pocket. Row 1 all north out, row 2 all south out: the flux through each coil reverses as the mover passes.
- With magnets it is about 58 g, which adds to the piston's moving mass. It clamps the 5 mm piston tube or a Ø5 rod and sits centred in the coils at mid-stroke, 1.0 mm from the bobbin bore.

## 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 bore | Ø5 | 5 mm tube or Ø5 rod (M3 grub) | slide | alt-mover-* |
| magnet pockets | Ø10 × 3 | Ø10 × 3 disc magnet | press | alt-mover-* |
| mover grub hole | Ø2.5 | M3 tap drill | designed | alt-mover-* |

## Sizes

| parameter | value |
|---|---|
| carrier | Ø35 × 26 |
| hub | Ø12 × 6 |
| bore | Ø5 |
| magnet pockets | 2 rows × 8, Ø10 × 3 |
| row pitch | 12 |
| air gap to bobbin | 1.0 |
| mass with magnets | ≈ 58 g |

## 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-mover/index.json

- alt-mover-35-2x8-10x3: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-mover-35-2x8-10x3.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-mover-35-2x8-10x3.stl) — mover: Ø35 × 26 carrier with 2 axial rows of 8 Ø10 × 3 magnet pockets (faces out; row 1 all N out, row 2 all S out, rows offset half a pocket), 12 mm pitch, Ø5 bore, Ø12 hub with an M3 grub: clamps the 5 mm piston tube or a Ø5 rod

## How it fits

### Goes in or on
- [Linear alternator bobbin: Ø37 bore, two 10 mm bays](https://xemstudio.com/objects/linear-alternator-bobbin/): runs inside the Ø37 bore, 1.0 mm gap

### Connects to
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/): clamps the Ø5 rod
- [Tri-clamp flexure head: ISO 2852 cap with Ø60, Ø80 and Ø100 flexure seats](https://xemstudio.com/objects/tri-clamp-flexure-head/): clamps the 5 mm piston tube
- [Planar spiral flexure bearing leaves: Ø40 to Ø120, 3-arm and 2-arm, 0.5 and 2 mm](https://xemstudio.com/objects/spiral-flexure-bearing/): flexures keep it centred in the gap

### Also needed
- [Neodymium disc magnets, common sizes](https://xemstudio.com/objects/neodymium-disc-magnets/): 16 × Ø10 × 3 in the pockets
- [Set screws (grub screws), M3–M8 (ISO 4026)](https://xemstudio.com/objects/set-screws-metric/): M3 grub in the hub

## Questions

**Why two rows of magnets on a linear alternator mover?**

Each coil must see the flux reverse as the mover passes; two rows of opposite polarity under two oppositely wound coils do that.

**What magnets go in the mover?**

Sixteen Ø10 × 3 disc magnets, eight per row, faces out.

## Fits with

- [Linear alternator bobbin: Ø37 bore, two 10 mm bays](https://xemstudio.com/objects/linear-alternator-bobbin/)
- [Linear alternator housing for Ø80 and Ø100 flexures (2 in / 3 in)](https://xemstudio.com/objects/linear-alternator-housing/)
- [Neodymium disc magnets, common sizes](https://xemstudio.com/objects/neodymium-disc-magnets/)
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/)
- [Tri-clamp flexure head: ISO 2852 cap with Ø60, Ø80 and Ø100 flexure seats](https://xemstudio.com/objects/tri-clamp-flexure-head/)
- [Build list: Free-piston Stirling, 2 in tri-clamp (flexure-suspended)](https://xemstudio.com/objects/list-free-piston-2in/)