Planar spiral flexure bearings (Sunpower / NASA free-piston style): Ø40–120, DXF + STEP

sizes

5 in table

Notes

• A flexure bearing is a stack of flat spiral springs that lets a piston move axially (soft) while holding it on centre (stiff): the free-piston engine's non-contact 'bearing', so the piston runs in a clearance seal with no oil and no wear. NASA's convertors have run for over a decade on them.
• Geometry: 3 arms (radial stiffness symmetric; 2-arm leaves are softer but wobble), each arm sweeping about 330° from the hub to the rim. Axial stiffness per leaf scales with E·t³·w / L³ (L = arm length), so thickness is the big lever: a 0.5 mm steel leaf is ~64× stiffer than a 0.5 mm... halve t and k drops 8×. Stack leaves on spacers to add stiffness in parallel; alternate the spiral hand between leaves to cancel the rotation each spiral induces.
• Materials: spring steel (1095 / C75 / SAE 1074) 0.3–0.8 mm, beryllium copper, or 17-7 PH for the real thing: laser- or water-jet cut from the DXF, deburr, stress-relieve. For demonstrators and motion checks print the 2 mm PETG or nylon versions: they show the motion and survive a few thousand cycles. Stroke limit: keep the leaf's axial deflection under about OD/10 for steel; fatigue life needs stress under ~300 MPa, which means measuring, not guessing: load the stack with known weights, plot deflection, keep the operating stroke in the linear region.
• Natural frequency of the suspended piston: f = (1/2π)·√(k_total/m). Free-piston engines run at this resonance (typically 50–100 Hz for Sunpower-class, 10–20 Hz for hobby builds): tune by adding leaves (k) or mass (m). The gas spring adds to k.
• LES: flexure-suspended displacer on the free-piston test bed; print the stack first, cut steel when the stroke is right.

Sizes

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)	PDF	US Government work, public domain
NASA (NTRS, public domain)	Flexure bearing design for free-piston Stirling engines (1986)	PDF	US Government work, public domain
NASA (NTRS, public domain)	Advanced Stirling Radioisotope Generator (ASRG) engineering unit (2011)	PDF	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 30 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

Plain text: sheet.txt · JSON: /api/geometry?object=spiral-flexure-bearings

Fits with