# Flexure stack spacer ring: Ø40 to Ø120 × 2 mm

> What spaces the leaves in a flexure bearing stack?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Ring = the flexure rim: OD × (OD − 2 × 0.12 OD), 2 mm thick
- Page: https://xemstudio.com/objects/flexure-stack-spacer/ · JSON: https://xemstudio.com/api/geometry?object=flexure-stack-spacer · Plain text: https://xemstudio.com/objects/flexure-stack-spacer/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 ring the size of a flexure leaf's rim, 2 mm thick, that sits between leaf rims so the arms can flex without touching.
- Ø40 × 4.8 wide, Ø60 × 7.2, Ø80 × 9.6, Ø100 × 12, Ø120 × 14.4; all 2.0 mm thick, with no rim holes.

## 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 |
|---|---|---|---|---|
| spacer rim holes | Ø3.4 × 3 on R = 0.44·OD (R35.2 on Ø80) | M3 stack screws (clearance) | designed | spacer-* |

## Sizes

| od_mm | id_mm | width_mm | thickness_mm |
|---|---|---|---|
| 40 | 30.4 | 4.8 | 2 |
| 60 | 45.6 | 7.2 | 2 |
| 80 | 60.8 | 9.6 | 2 |
| 100 | 76 | 12 | 2 |
| 120 | 91.2 | 14.4 | 2 |

## Where the files are

- NASA (NTRS, public domain): [Free-piston Stirling convertor flexure bearing development and life testing (2005)](https://ntrs.nasa.gov/search?q=20050182933) — PDF — US Government work, public domain
- NASA (NTRS, public domain): [Flexure bearing design for free-piston Stirling engines (1986)](https://ntrs.nasa.gov/search?q=19860020334) — PDF — US Government work, public domain
- NASA (NTRS, public domain): [Advanced Stirling Radioisotope Generator (ASRG) engineering unit (2011)](https://ntrs.nasa.gov/search?q=20110016393) — PDF — US Government work, public domain
- Wikipedia: [Flexure bearing](https://en.wikipedia.org/wiki/Flexure_bearing) — web — CC BY-SA 4.0 (text)
- Sunpower (AMETEK): [Free-piston Stirling technology overview](https://sunpowerinc.com/) — web — Sunpower site (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (5 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/flexure-stack-spacer/index.json

- spacer-40-t2: [STEP](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-40-t2.step) · [STL](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-40-t2.stl) — stack spacer ring Ø40, 4.8 wide, 2 thick (sets the gap between leaves), 3 × Ø3.4 on R17.6 (the leaves' rim-hole circle) for the stack screws
- spacer-60-t2: [STEP](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-60-t2.step) · [STL](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-60-t2.stl) — stack spacer ring Ø60, 7.2 wide, 2 thick (sets the gap between leaves), 3 × Ø3.4 on R26.4 (the leaves' rim-hole circle) for the stack screws
- spacer-80-t2: [STEP](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-80-t2.step) · [STL](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-80-t2.stl) — stack spacer ring Ø80, 9.6 wide, 2 thick (sets the gap between leaves), 3 × Ø3.4 on R35.2 (the leaves' rim-hole circle) for the stack screws
- spacer-100-t2: [STEP](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-100-t2.step) · [STL](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-100-t2.stl) — stack spacer ring Ø100, 12 wide, 2 thick (sets the gap between leaves), 3 × Ø3.4 on R44 (the leaves' rim-hole circle) for the stack screws
- spacer-120-t2: [STEP](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-120-t2.step) · [STL](https://xemstudio.com/files/hardware/spiral-flexure-bearings/spacer-120-t2.stl) — stack spacer ring Ø120, 14.4 wide, 2 thick (sets the gap between leaves), 3 × Ø3.4 on R52.8 (the leaves' rim-hole circle) for the stack screws

## 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/): between leaf rims, same OD

### Connects to
- [Flexure rim clamp plate: Ø40 to Ø120 × 3 mm, 3 × M3](https://xemstudio.com/objects/flexure-rim-clamp-plate/): stack clamped under the plate

### Also needed
- [Flexure hub washer: Ø12 to Ø26.4 × 2 mm](https://xemstudio.com/objects/flexure-hub-clamp-washer/): matching gap at the hubs

## Questions

**How thick are the flexure stack spacers?**

2.0 mm, the width of the leaf rim (9.6 mm on Ø80).

**What size spacer goes with an Ø80 flexure?**

Spacer-80-t2: Ø80 OD × Ø60.8 ID × 2.0 mm.

## 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 rim clamp plate: Ø40 to Ø120 × 3 mm, 3 × M3](https://xemstudio.com/objects/flexure-rim-clamp-plate/)
- [Flexure hub washer: Ø12 to Ø26.4 × 2 mm](https://xemstudio.com/objects/flexure-hub-clamp-washer/)