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Planar spiral flexure bearing leaves: Ø40 to Ø120, 3-arm and 2-arm, 0.5 and 2 mm
What is a spiral flexure bearing and what are its dimensions? · Hardware · basis: Archimedean planar springs as on Sunpower / NASA free-piston Stirling convertors; hub Ø = max(bore + 8, 0.22 OD), rim 0.12 OD, arms sweep 330° (2-arm 350°)
Long Earth Society build part. Flexure-suspended displacer on the LES free-piston test bed. The engines
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. Mating features
- sizes
- 5 in table
Notes
- A flexure bearing leaf: a flat spring with a hub, spiral arms and a rim. Stacked leaves let a piston move axially while holding it on centre, so it runs in a clearance seal with no contact.
- Sizes Ø40, 60, 80, 100, 120. Each has a 3-arm leaf 0.5 mm thick (with a DXF), the same 3-arm leaf 2.0 mm thick, and a 2-arm leaf 0.5 mm thick (with a DXF). 3-arm arms sweep 330°, 2-arm 350°, from the hub to the rim.
- Hub Ø12 / 13.2 / 17.6 / 22 / 26.4 with bore Ø4 / 5 / 5 / 8 / 8; rim width 0.12 × OD; three Ø3.2 rim holes on a circle 0.88 × OD.
- Axial stiffness per leaf scales with E·t³·w / L³ (L = arm length), so the 2.0 mm leaf is 64 times the 0.5 mm leaf in the same material. Natural frequency of the suspended mass: f = (1/2π)√(k/m).
- Three arms give equal radial stiffness in every direction; two-arm leaves are softer axially but let the hub tilt.
Sizes
| od_mm | hub_mm | rim_mm | arm_w_mm | bore_mm | arm_radial_span_mm | rim_hole_circle_mm |
|---|---|---|---|---|---|---|
| 40 | 12 | 4.8 | 1.6 | 4 | 9.2 | 35.2 |
| 60 | 13.2 | 7.2 | 2.0 | 5 | 16.2 | 52.8 |
| 80 | 17.6 | 9.6 | 2.5 | 5 | 21.6 | 70.4 |
| 100 | 22.0 | 12.0 | 3.0 | 8 | 27.0 | 88.0 |
| 120 | 26.4 | 14.4 | 3.5 | 8 | 32.4 | 105.6 |
Mating features
Drawn at the exact size. Open press, slide, run and socket features by your process's clearance; leave designed gaps and clamp fits as drawn.
| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| leaf hub bore | Ø4 / Ø5 / Ø8 | rod, tube or centring bush | slide | flexure-* |
| rim holes | Ø3.2 | M3 screws (clearance) | designed | flexure-* |
Where the files are
Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.
| NASA (NTRS, public domain) | Free-piston Stirling convertor flexure bearing development and life testing (2005) | US Government work, public domain | |
| NASA (NTRS, public domain) | Flexure bearing design for free-piston Stirling engines (1986) | US Government work, public domain | |
| NASA (NTRS, public domain) | Advanced Stirling Radioisotope Generator (ASRG) engineering unit (2011) | US Government work, public domain | |
| Wikipedia | Flexure bearing | web | CC BY-SA 4.0 (text) |
| Sunpower (AMETEK) | Free-piston Stirling technology overview | web | Sunpower site verify |
| Long Earth Society | Engine builds | web, video | LES site |
Free files
★ Free files 15 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json
| flexure-40-3arm-w1.6-t0.5 | planar spiral flexure Ø40, 3 arms 1.6 wide, 0.5 thick (DXF outline included), hub Ø12 bore Ø4, 3 × Ø3.2 rim holes on R17.6 | STEP · STL · DXF |
| flexure-40-3arm-w1.6-t2 | planar spiral flexure Ø40, 3 arms 1.6 wide, 2.0 thick, hub Ø12 bore Ø4, 3 × Ø3.2 rim holes on R17.6 | STEP · STL |
| flexure-40-2arm-w1.6-t0.5 | 2-arm variant Ø40 (lower axial stiffness, less radial) | STEP · STL · DXF |
| flexure-60-3arm-w2-t0.5 | planar spiral flexure Ø60, 3 arms 2.0 wide, 0.5 thick (DXF outline included), hub Ø13.2 bore Ø5, 3 × Ø3.2 rim holes on R26.4 | STEP · STL · DXF |
| flexure-60-3arm-w2-t2 | planar spiral flexure Ø60, 3 arms 2.0 wide, 2.0 thick, hub Ø13.2 bore Ø5, 3 × Ø3.2 rim holes on R26.4 | STEP · STL |
| flexure-60-2arm-w2-t0.5 | 2-arm variant Ø60 (lower axial stiffness, less radial) | STEP · STL · DXF |
| flexure-80-3arm-w2.5-t0.5 | planar spiral flexure Ø80, 3 arms 2.5 wide, 0.5 thick (DXF outline included), hub Ø17.6 bore Ø5, 3 × Ø3.2 rim holes on R35.2 | STEP · STL · DXF |
| flexure-80-3arm-w2.5-t2 | planar spiral flexure Ø80, 3 arms 2.5 wide, 2.0 thick, hub Ø17.6 bore Ø5, 3 × Ø3.2 rim holes on R35.2 | STEP · STL |
| flexure-80-2arm-w2.5-t0.5 | 2-arm variant Ø80 (lower axial stiffness, less radial) | STEP · STL · DXF |
| flexure-100-3arm-w3-t0.5 | planar spiral flexure Ø100, 3 arms 3.0 wide, 0.5 thick (DXF outline included), hub Ø22 bore Ø8, 3 × Ø3.2 rim holes on R44 | STEP · STL · DXF |
| flexure-100-3arm-w3-t2 | planar spiral flexure Ø100, 3 arms 3.0 wide, 2.0 thick, hub Ø22 bore Ø8, 3 × Ø3.2 rim holes on R44 | STEP · STL |
| flexure-100-2arm-w3-t0.5 | 2-arm variant Ø100 (lower axial stiffness, less radial) | STEP · STL · DXF |
| flexure-120-3arm-w3.5-t0.5 | planar spiral flexure Ø120, 3 arms 3.5 wide, 0.5 thick (DXF outline included), hub Ø26.4 bore Ø8, 3 × Ø3.2 rim holes on R52.8 | STEP · STL · DXF |
| flexure-120-3arm-w3.5-t2 | planar spiral flexure Ø120, 3 arms 3.5 wide, 2.0 thick, hub Ø26.4 bore Ø8, 3 × Ø3.2 rim holes on R52.8 | STEP · STL |
| flexure-120-2arm-w3.5-t0.5 | 2-arm variant Ø120 (lower axial stiffness, less radial) | STEP · STL · DXF |
Plain text: sheet.txt · JSON: /api/geometry?object=spiral-flexure-bearing
How it fits
Parts that fit it
Flexure hub centring bushOD = leaf hub boreGoes in or on
Tri-clamp flexure headrim in the flexure-OD seat, 3 × M3Connects to
Flexure stack spacer ringspacer between leaf rims
Flexure hub washerwasher between leaf hubsAlternatives
Compression springs, wire Ø0.5–1.6, OD 6–16coil springsAlso needed
Flexure rim clamp plateclamps the stack rims
Flexure hub collarsclamps the hubs to the rodIn these build lists
Free-piston Stirling, 2 in tri-clampNo crank: piston and displacer each hang on a stack of spiral flexures, with a t…You might also need
Questions
What is a flexure bearing in a Stirling engine?
A stack of flat spiral springs that holds the piston on centre while it moves axially, so it runs in a clearance seal with no contact; NASA's free-piston convertors use them.
How many arms should a spiral flexure have?
Three for equal radial stiffness; two-arm leaves are softer axially but let the hub tilt. Alternate the spiral hand between leaves to cancel rotation.
What are the dimensions of the Ø80 flexure?
Hub Ø17.6, bore Ø5, rim 9.6 wide, three arms 2.5 wide, Ø3.2 rim holes on a Ø70.4 circle; 0.5 or 2.0 mm thick.