Planar spiral flexure bearings (Sunpower / NASA free-piston style): Ø40–120, DXF + STEP — dimension sheet Source: Xem Studio, https://xemstudio.com/objects/spiral-flexure-bearings/ (CC BY 4.0) Basis: Three-arm Archimedean planar springs as used on Sunpower / NASA free-piston Stirling convertors (RE-1000, ASRG, Sunpower EE-35/EG-1000 class) for the displacer and piston suspension; geometry parametrized here, stiffness measured not quoted Catalog version: 2026-10-20 Sizes: see table below - 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. od_mm hub_mm rim_mm arm_w_mm thickness bore_mm arm_radial_span_mm 40 12 4.8 1.6 0.5 steel / 2.0 print 4 9.2 60 13.2 7.2 2.0 0.5 steel / 2.0 print 5 16.2 80 17.6 9.6 2.5 0.5 steel / 2.0 print 5 21.6 100 22.0 12.0 3.0 0.5 steel / 2.0 print 8 27.0 120 26.4 14.4 3.5 0.5 steel / 2.0 print 8 32.4 Files elsewhere (not hosted by Xem Studio; license as stated by the publisher): - NASA (NTRS, public domain): Free-piston Stirling convertor flexure bearing development and life testing (2005) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=20050182933 - NASA (NTRS, public domain): Flexure bearing design for free-piston Stirling engines (1986) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=19860020334 - NASA (NTRS, public domain): Advanced Stirling Radioisotope Generator (ASRG) engineering unit (2011) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=20110016393 - Wikipedia: Flexure bearing — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Flexure_bearing - Sunpower (AMETEK): Free-piston Stirling technology overview — web — Sunpower site [verify license at link] — https://sunpowerinc.com/ - Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/ Free files (CC BY 4.0, Xem Studio; STEP + STL, 30 parts): https://xemstudio.com/files/hardware/spiral-flexure-bearings/index.json API: https://xemstudio.com/api/geometry?object=spiral-flexure-bearings