# Tri-clamp flexure head: ISO 2852 cap with Ø60, Ø80 and Ø100 flexure seats

> How do I mount flexure bearings on a tri-clamp spool?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: ISO 2852 cap face; pedestal Ø = tube OD; plate Ø = flexure OD + 16 with a flexure-OD × 1 seat and 3 × Ø3.4 on the flexure rim-hole circle (0.88 × OD)
- Page: https://xemstudio.com/objects/tri-clamp-flexure-head/ · JSON: https://xemstudio.com/api/geometry?object=tri-clamp-flexure-head · Plain text: https://xemstudio.com/objects/tri-clamp-flexure-head/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 head of the tri-clamp free-piston engine: it clamps on the spool's open ferrule and carries the piston's flexure stack (stack A).
- Base: 6 mm ISO 2852 cap, pedestal the tube's OD (38.1 / 50.8 / 76.2) to 14 mm so the clamp clears, then a plate 4 mm thick, Ø76 / Ø96 / Ø116. Overall 18 mm.
- On top: a seat Ø60 / Ø80 / Ø100 × 1 mm deep (the flexure OD) and three Ø3.4 holes on the flexure's rim-hole circle (Ø52.8 / Ø70.4 / Ø88). A centre window Ø19.2 / Ø23.6 / Ø28 (hub + 6) lets the rods through.

## 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 |
|---|---|---|---|---|
| flexure pocket in the head / ring | flexure OD | spiral flexure leaf rim | slide | fp-flexure-* |
| head face | ISO 2852 | ferrule face + Type B gasket | clamp | fp-flexure-head-* |
| rim holes (head, ring) | Ø3.4 | M3 screw clearance | designed | fp-flexure-* |
| head / ring window | hub + 12 (Ø29.6 on Ø80) | hub collars and their pinch-screw heads (no contact) | designed | fp-flexure-* |

## Sizes

| spool_in | flange_od_mm | plate_od_mm | flexure_seat_mm | rim_hole_circle_mm | window_mm | height_mm |
|---|---|---|---|---|---|---|
| 1.5 | 50.5 | 76 | Ø60 × 1 | 52.8 | 19.2 | 18 |
| 2 | 64.0 | 96 | Ø80 × 1 | 70.4 | 23.6 | 18 |
| 3 | 91.0 | 116 | Ø100 × 1 | 88.0 | 28.0 | 18 |

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

Index: https://xemstudio.com/files/hardware/tri-clamp-flexure-head/index.json

- fp-flexure-head-1.5in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-1.5in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-1.5in.stl) — 1.5 in flexure head: ISO 2852 cap face, Ø38.1 pedestal clearing the clamp, Ø76 plate with a Ø60 × 1 pocket (= flexure OD), a Ø25.2 window for the hub collars and 3 × M3 (Ø3.4) on the Ø60 flexure's rim holes; the piston's stack A bolts on it
- fp-flexure-head-2in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-2in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-2in.stl) — 2 in flexure head: ISO 2852 cap face, Ø50.8 pedestal clearing the clamp, Ø96 plate with a Ø80 × 1 pocket (= flexure OD), a Ø29.6 window for the hub collars and 3 × M3 (Ø3.4) on the Ø80 flexure's rim holes; the piston's stack A bolts on it
- fp-flexure-head-3in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-3in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-3in.stl) — 3 in flexure head: ISO 2852 cap face, Ø76.2 pedestal clearing the clamp, Ø116 plate with a Ø100 × 1 pocket (= flexure OD), a Ø34 window for the hub collars and 3 × M3 (Ø3.4) on the Ø100 flexure's rim holes; the piston's stack A bolts on it

## How it fits

### Parts that fit it
- [Linear alternator housing for Ø80 and Ø100 flexures (2 in / 3 in)](https://xemstudio.com/objects/linear-alternator-housing/): on standoffs on the flexure head's rim screws
- [Tube piston rods Ø5 × Ø3.4](https://xemstudio.com/objects/tube-piston-rods-5mm/): Ø5 OD in the piston plug socket

### Goes in or on
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/): ISO 2852 face on the open ferrule

### Connects to
- [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/): leaf rim in the flexure-OD seat, 3 × M3
- [Flexure ring for the displacer stack: Ø76, Ø96 and Ø116 with Ø60, Ø80 and Ø100 seats](https://xemstudio.com/objects/tri-clamp-flexure-ring/): 3 × M3 standoffs to the ring above
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/): alternator housing on the same three rim holes

### Also needed
- [Tri-clamp clamp, two-bolt: every flange size ½ to 4 in](https://xemstudio.com/objects/tri-clamp-clamps/): clamp and gasket on the face

## Questions

**What flexure size goes on a 2 inch tri-clamp spool?**

Ø80 leaves on the 2 in flexure head (Ø96 plate); Ø60 on 1.5 in, Ø100 on 3 in.

**How are the flexures held on the head?**

The leaf rim sits in the 1 mm deep seat and three M3 screws pass the Ø3.4 holes on the Ø70.4 circle (2 in) through the stack's spacers and clamp plate.

## 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 ring for the displacer stack: Ø76, Ø96 and Ø116 with Ø60, Ø80 and Ø100 seats](https://xemstudio.com/objects/tri-clamp-flexure-ring/)
- [Flexure hub collars: split collars Ø13.2, Ø17.6 and Ø22 for Ø3 rod and Ø5 tube](https://xemstudio.com/objects/flexure-hub-collar/)
- [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/)
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/)
- [Tri-clamp clamp, two-bolt: every flange size ½ to 4 in](https://xemstudio.com/objects/tri-clamp-clamps/)