# Linear alternator cap ring, Ø96 and Ø116

> What holds the linear alternator stator in its housing?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Same outline and rim circle as the housing's top flange; bore = sleeve bore − 2
- Page: https://xemstudio.com/objects/linear-alternator-cap-ring/ · JSON: https://xemstudio.com/api/geometry?object=linear-alternator-cap-ring · Plain text: https://xemstudio.com/objects/linear-alternator-cap-ring/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

- A flat ring Ø96 (2 in) or Ø116 (3 in) × 4 with a Ø50.5 bore, that holds the back-iron sleeve and bobbin down in the housing.
- 3 × Ø3.4 holes on the same rim circle as the housing (R35.2 / R44): M3 × 12 screws go up through the top flange and cap into the standoffs that carry the flexure ring and stack B.

## 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 |
|---|---|---|---|---|
| cap rim holes | Ø3.4 | M3 screw clearance | designed | alt-cap-* |
| cap ring bore | Ø50.5 | holds the sleeve and bobbin down | designed | alt-cap-* |

## Sizes

| file | outside | bore | thickness | rim circle |
|---|---|---|---|---|
| alt-cap-2in-flex80 | 96 | 50.5 | 4 | R35.2 |
| alt-cap-3in-flex100 | 116 | 50.5 | 4 | R44 |

## 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 (2 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/linear-alternator-cap-ring/index.json

- alt-cap-2in-flex80: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-2in-flex80.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-2in-flex80.stl) — 2in cap ring: holds the sleeve and bobbin down on the housing's top flange; 3 × M3 × 12 through flange and cap into the standoffs for flexure stack B
- alt-cap-3in-flex100: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-3in-flex100.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-3in-flex100.stl) — 3in cap ring: holds the sleeve and bobbin down on the housing's top flange; 3 × M3 × 12 through flange and cap into the standoffs for flexure stack B

## How it fits

### Goes in or on
- [Linear alternator housing for Ø80 and Ø100 flexures (2 in / 3 in)](https://xemstudio.com/objects/linear-alternator-housing/): on the top flange, same 3 × M3

### Connects to
- [Linear alternator back-iron sleeve Ø60.5 × Ø52.5 × 30](https://xemstudio.com/objects/linear-alternator-back-iron-sleeve/): holds the sleeve down

### Also needed
- [Threaded standoffs, M2.5 and M3 (hex and round)](https://xemstudio.com/objects/threaded-standoffs-metric/): standoffs above for stack B

## Questions

**What does the alternator cap ring do?**

It clamps the sleeve and bobbin down onto the housing's ledge; its Ø50.5 bore overlaps the Ø52.5 sleeve bore by 1 mm all round.

**What screws hold the cap ring?**

3 × M3 × 12 through the top flange and cap into the standoffs above.

## Fits with

- [Linear alternator housing for Ø80 and Ø100 flexures (2 in / 3 in)](https://xemstudio.com/objects/linear-alternator-housing/)
- [Linear alternator back-iron sleeve Ø60.5 × Ø52.5 × 30](https://xemstudio.com/objects/linear-alternator-back-iron-sleeve/)
- [Tri-clamp flexure head: ISO 2852 cap with Ø60, Ø80 and Ø100 flexure seats](https://xemstudio.com/objects/tri-clamp-flexure-head/)