# Flexure hub centring bush: Ø5 and Ø8 OD × Ø3 / Ø5 bore × 6

> How do I centre a flexure with a 5 mm hub hole on a 3 mm rod?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: OD = flexure leaf hub bore (Ø5 for Ø60/Ø80 leaves, Ø8 for Ø100), ID = rod or tube, 6 long = two 2 mm leaf hubs and the 2 mm hub washer between them
- Page: https://xemstudio.com/objects/flexure-hub-centring-bush/ · JSON: https://xemstudio.com/api/geometry?object=flexure-hub-centring-bush · Plain text: https://xemstudio.com/objects/flexure-hub-centring-bush/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 plain sleeve that centres flexure leaves whose hub hole is bigger than the rod: the two leaf hubs and the hub washer between them sit on its OD, the rod runs through its bore, and the two hub collars (bore = rod) clamp its ends.
- Hub-bush-60-b3 and hub-bush-80-b3: Ø5 OD × Ø3 bore × 6 for Ø60 / Ø80 leaves on the Ø3 displacer rod. hub-bush-100-b3: Ø8 × Ø3 × 6. hub-bush-100-b5: Ø8 × Ø5 × 6 for Ø100 leaves on the 5 mm tube piston rod.

## 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 |
|---|---|---|---|---|
| centring bush OD | leaf hub bore (Ø5 / Ø8) | flexure leaf hub | slide | hub-bush-* |
| centring bush bore | Ø3 / Ø5 | Ø3 rod / 5 mm tube | slide | hub-bush-* |

## Sizes

| file | od_mm | bore_mm | length_mm | for |
|---|---|---|---|---|
| hub-bush-60-b3 | 5 | 3 | 6 | Ø60 leaves, Ø3 rod |
| hub-bush-80-b3 | 5 | 3 | 6 | Ø80 leaves, Ø3 rod |
| hub-bush-100-b3 | 8 | 3 | 6 | Ø100 leaves, Ø3 rod |
| hub-bush-100-b5 | 8 | 5 | 6 | Ø100 leaves, 5 mm tube |

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

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

- hub-bush-60-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-60-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-60-b3.stl) — centring bush Ø5 OD × Ø3 ID × 6: fills the two leaf hubs and the hub washer (Ø5 bores) and centres them on the Ø3 displacer rod; the b3 collars clamp its ends
- hub-bush-80-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-80-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-80-b3.stl) — centring bush Ø5 OD × Ø3 ID × 6: fills the two leaf hubs and the hub washer (Ø5 bores) and centres them on the Ø3 displacer rod; the b3 collars clamp its ends
- hub-bush-100-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b3.stl) — centring bush Ø8 OD × Ø3 ID × 6: fills the two leaf hubs and the hub washer (Ø8 bores) and centres them on the Ø3 displacer rod; the b3 collars clamp its ends
- hub-bush-100-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b5.stl) — centring bush Ø8 × Ø5 × 6: centres the Ø100 leaves and hub washer on the 5 mm tube piston rod

## 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/): OD = leaf hub bore

### Connects to
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/): Ø3 displacer rod or 5 mm tube in the bore

### Also needed
- [Flexure hub collars: split collars Ø13.2, Ø17.6 and Ø22 for Ø3 rod and Ø5 tube](https://xemstudio.com/objects/flexure-hub-collar/): collars clamp the stack ends

## Questions

**How do I fit a 5 mm flexure hub on a 3 mm rod?**

On the Ø5 × Ø3 × 6 centring bush: the two leaf hubs and the hub washer sit on its OD, the rod passes its bore, and hub collars clamp the ends.

**Why is there a b5 bush only for Ø100?**

The Ø60 and Ø80 leaves already have a Ø5 hub bore for the 5 mm tube; only the Ø100 leaves (Ø8 hub bore) need a Ø8 × Ø5 bush.

## 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 collars: split collars Ø13.2, Ø17.6 and Ø22 for Ø3 rod and Ø5 tube](https://xemstudio.com/objects/flexure-hub-collar/)
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/)