# Flexure hub collars: split collars Ø13.2, Ø17.6 and Ø22 for Ø3 rod and Ø5 tube

> How do I clamp a flexure leaf hub onto a piston rod?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Collar Ø = flexure hub Ø (max(bore + 8, 0.22 × OD)); 6 mm thick so the M3 pinch screw's head (Ø5.5) stays within it; 0.8 mm slit; M3 pinch (Ø2.5)
- Page: https://xemstudio.com/objects/flexure-hub-collar/ · JSON: https://xemstudio.com/api/geometry?object=flexure-hub-collar · Plain text: https://xemstudio.com/objects/flexure-hub-collar/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 split clamping collar the size of a flexure leaf's hub: two per stack, one each end, squeeze the leaves, hub washers and spacers onto the rod.
- Ø13.2 (for Ø60 leaves), Ø17.6 (Ø80) and Ø22 (Ø100), all 6 mm thick. b5 collars have a Ø5 bore for the 5 mm tube piston rod; b3 collars a Ø3 bore for the displacer rod.
- A 0.8 mm slit runs from the bore to the rim, closed by an M3 × 10 screw in a Ø2.5 tangential hole at mid-thickness; the screw head stays inside the collar's 6 mm, clear of the leaf beside it.

## 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 |
|---|---|---|---|---|
| hub collar bore (b5 / b3) | Ø5 / Ø3 | 5 mm tube / Ø3 rod | slide | hub-collar-* |
| hub-collar slit | 0.8 | closes on the tube / rod | clamp | hub-collar-* |
| hub-collar pinch hole | Ø2.5 | M3 tap drill | designed | hub-collar-* |

## Sizes

| flexure_od_mm | collar_od_mm | bore_mm | thickness_mm | slit_mm | pinch |
|---|---|---|---|---|---|
| 60 | 13.2 | 5 / 3 | 6 | 0.8 | M3 (Ø2.5) |
| 80 | 17.6 | 5 / 3 | 6 | 0.8 | M3 (Ø2.5) |
| 100 | 22.0 | 5 / 3 | 6 | 0.8 | M3 (Ø2.5) |

## Where the files are

- NASA NTRS: [Free-piston Stirling convertor flexure bearing development and life testing](https://ntrs.nasa.gov/citations/20050182933) — pdf — US Government work
- Long Earth Society: [Tri-clamp Stirling engine builds](https://longearthsociety.com/engineering/) — web, video — LES site
- Wikipedia: [Free-piston engine](https://en.wikipedia.org/wiki/Free-piston_engine) — web — CC BY-SA 4.0 (text)

## Free STEP/STL (6 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/flexure-hub-collar/index.json

- hub-collar-60-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b5.stl) — split hub collar Ø13.2 × 6, Ø5 bore, 0.8 slit, M3 pinch (Ø2.5 tapped) across the slit at mid-thickness: clamps the leaf hubs on the 5 mm tube rod (two per stack)
- hub-collar-60-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b3.stl) — split hub collar Ø13.2 × 6, Ø3 bore, 0.8 slit, M3 pinch (Ø2.5 tapped) across the slit at mid-thickness: clamps the leaf hubs on the Ø3 displacer rod (two per stack)
- hub-collar-80-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b5.stl) — split hub collar Ø17.6 × 6, Ø5 bore, 0.8 slit, M3 pinch (Ø2.5 tapped) across the slit at mid-thickness: clamps the leaf hubs on the 5 mm tube rod (two per stack)
- hub-collar-80-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b3.stl) — split hub collar Ø17.6 × 6, Ø3 bore, 0.8 slit, M3 pinch (Ø2.5 tapped) across the slit at mid-thickness: clamps the leaf hubs on the Ø3 displacer rod (two per stack)
- hub-collar-100-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b5.stl) — split hub collar Ø22 × 6, Ø5 bore, 0.8 slit, M3 pinch (Ø2.5 tapped) across the slit at mid-thickness: clamps the leaf hubs on the 5 mm tube rod (two per stack)
- hub-collar-100-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b3.stl) — split hub collar Ø22 × 6, Ø3 bore, 0.8 slit, M3 pinch (Ø2.5 tapped) across the slit at mid-thickness: clamps the leaf hubs on the Ø3 displacer rod (two per stack)

## How it fits

### Goes in or on
- [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/): one each end of the leaf stack on the hub

### Connects to
- [Tri-clamp piston plug: tube-rod socket for the -rod piston, 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-piston-plug/): b5 collar on the 5 mm tube from the plug
- [Flexure hub centring bush: Ø5 and Ø8 OD × Ø3 / Ø5 bore × 6](https://xemstudio.com/objects/flexure-hub-centring-bush/): collars clamp the stack on the bush

### Also needed
- [Socket head cap screws, M2–M8 (ISO 4762 / DIN 912)](https://xemstudio.com/objects/socket-head-cap-screws-metric/): M3 pinch screw

## Questions

**How is a flexure leaf fixed to the rod?**

Between two split hub collars the size of the leaf's hub, pinched onto the rod with an M3 screw across a 0.8 mm slit.

**Which collar for the piston rod?**

The b5 collar (Ø5 bore) on the 5 mm tube; b3 (Ø3 bore) on the displacer rod.

## Fits with

- [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 hub centring bush: Ø5 and Ø8 OD × Ø3 / Ø5 bore × 6](https://xemstudio.com/objects/flexure-hub-centring-bush/)
- [Flexure hub washer: Ø12 to Ø26.4 × 2 mm](https://xemstudio.com/objects/flexure-hub-clamp-washer/)
- [Tri-clamp piston plug: tube-rod socket for the -rod piston, 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-piston-plug/)