# Piston stop bumper Ø30 and Ø40 × 6

> What stops a free-piston Stirling piston from hitting the end of its cylinder?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: OD = liner bore (fit: press); recess 0.6 × bore
- Page: https://xemstudio.com/objects/piston-stop-bumper/ · JSON: https://xemstudio.com/api/geometry?object=piston-stop-bumper · Plain text: https://xemstudio.com/objects/piston-stop-bumper/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

- An end stop for the free-piston core: Ø30 or Ø40 (the liner bore, fit: press) × 6 tall, a 2 mm closed face over a 4 mm deep recess of 0.6 × the bore (Ø18 / Ø24), with a 1 mm rounded edge.
- One at each end of the stroke; the stroke is kept to 85 % of the free travel between them. Over-stroke, when the alternator load drops, is the usual way a free-piston engine fails.

## 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 |
|---|---|---|---|---|
| rod bores | Ø5 | Ø5 rod | slide | all |
| stop bumper OD | Ø30 / Ø40 | liner bore | press | stop-bumper-* |

## Sizes

| file | OD | height | recess | face |
|---|---|---|---|---|
| stop-bumper-30 | 30 | 6 | Ø18 × 4 | 2 |
| stop-bumper-40 | 40 | 6 | Ø24 × 4 | 2 |

## Where the files are

- NASA (NTRS, public domain): [Free-piston Stirling engine design and analysis (RE-1000 test results) (1984)](https://ntrs.nasa.gov/search?q=19840017543) — PDF — US Government work, public domain
- NASA (NTRS, public domain): [Dynamics of free-piston Stirling engines (1987)](https://ntrs.nasa.gov/search?q=19870011457) — PDF — US Government work, public domain
- Wikipedia: [Free-piston engine](https://en.wikipedia.org/wiki/Free-piston_engine) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Beale number](https://en.wikipedia.org/wiki/Beale_number) — web — CC BY-SA 4.0 (text)
- Ohio University: [Stirling cycle machine analysis (Urieli), free online](https://www.ohio.edu/mechanical/stirling/) — web — Author's site
- 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/piston-stop-bumper/index.json

- stop-bumper-30: [STEP](https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-30.step) · [STL](https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-30.stl) — end-stop bumper Ø30 OD (= the Ø30 bore) × 6
- stop-bumper-40: [STEP](https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-40.step) · [STL](https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-40.stl) — end-stop bumper Ø40 OD (= the Ø40 bore) × 6

## How it fits

### Goes in or on
- [Free-piston cylinder liner Ø30 × 120 and Ø40 × 160](https://xemstudio.com/objects/free-piston-cylinder-liner/): in the Ø30 / Ø40 bore

### Connects to
- [Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40](https://xemstudio.com/objects/free-piston-power-piston/): at each end of its travel

## Questions

**What stops a free-piston engine from over-stroking?**

The alternator load first; bumpers at both ends of the stroke for when the load drops, with the stroke kept to 85 % of the free travel.

**How big is the stop bumper for a 40 mm bore?**

Ø40 × 6 with a Ø24 × 4 recess.

## Fits with

- [Free-piston cylinder liner Ø30 × 120 and Ø40 × 160](https://xemstudio.com/objects/free-piston-cylinder-liner/)
- [Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40](https://xemstudio.com/objects/free-piston-power-piston/)
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/)