# Gas-spring piston Ø14.9 × 15 and Ø19.9 × 20

> What size is the gas-spring piston in a free-piston Stirling engine?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Gas spring k = γ·p·A² / V; piston drawn at bounce-space bore − 0.1 (0.05 mm radial gap as drawn)
- Page: https://xemstudio.com/objects/gas-spring-piston/ · JSON: https://xemstudio.com/api/geometry?object=gas-spring-piston · Plain text: https://xemstudio.com/objects/gas-spring-piston/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 small piston on the rod that compresses a sealed bounce space and makes a gas spring: Ø14.9 × 15 for a Ø15 bore, Ø19.9 × 20 for a Ø20 bore.
- Solid, with a Ø5 bore (fit: slide) for the rod and 0.5 mm chamfers. As drawn it is 0.1 mm under its bore, a 0.05 mm radial gap.
- Stiffness k = γ·p·A²/V: the Ø20 piston at 1 bar with 20 cm³ of bounce space gives about 690 N/m; a smaller volume is stiffer.

## 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 |
| gas-spring piston in its bore | Ø15 / Ø20 (= bore) | Ø15 / Ø20 bounce-space bore | run | gas-spring-piston-* |

## Sizes

| file | diameter | length | bore it runs in | rod bore | k at 1 bar, 20 cm³ |
|---|---|---|---|---|---|
| gas-spring-piston-15x15 | 14.9 | 15 | 15 | 5 | ≈ 220 N/m |
| gas-spring-piston-20x20 | 19.9 | 20 | 20 | 5 | ≈ 690 N/m |

## 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/gas-spring-piston/index.json

- gas-spring-piston-15x15: [STEP](https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-15x15.step) · [STL](https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-15x15.stl) — gas-spring piston Ø15 (= the Ø15 bounce-space bore; clearance-seal running fit) × 15, Ø5 rod bore
- gas-spring-piston-20x20: [STEP](https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-20x20.step) · [STL](https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-20x20.stl) — gas-spring piston Ø20 (= the Ø20 bounce-space bore; clearance-seal running fit) × 20, Ø5 rod bore

## How it fits

### Connects to
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/): on the Ø5 rod
- [Free-piston Stirling displacer Ø29 × 75 and Ø39 × 100](https://xemstudio.com/objects/free-piston-displacer/): spring for the displacer rod

### Alternatives
- [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/): flexure stiffness instead of gas

## Questions

**How stiff is a gas spring?**

K = γ·p·A²/V; Ø20 at 1 bar with 20 cm³ gives about 690 N/m.

**What keeps a gas spring's mean position from drifting?**

Centring ports: two 0.5 mm holes the piston uncovers at mid-stroke.

## Fits with

- [Free-piston Stirling displacer Ø29 × 75 and Ø39 × 100](https://xemstudio.com/objects/free-piston-displacer/)
- [Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40](https://xemstudio.com/objects/free-piston-power-piston/)
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/)
- [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/)