{
 "version": "2026-10-20",
 "license": "Dimension sheets are written by Xem Studio from the cited sources and are free to use (CC BY 4.0, attribute 'Xem Studio'). Linked files belong to their publishers under the license shown; Xem Studio does not host them. 'license_verified: false' means the license shown is from memory or the publisher's page as last read and should be confirmed at the link before commercial use.",
 "slug": "spiral-flexure-bearings",
 "name": "Planar spiral flexure bearings (Sunpower / NASA free-piston style): Ø40–120, DXF + STEP",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What is a spiral flexure bearing and how do I make one for a free-piston Stirling engine?",
 "tags": [
  "flexure",
  "flexure bearing",
  "planar spring",
  "spiral spring",
  "free piston",
  "nasa",
  "sunpower",
  "stirling",
  "free piston",
  "long earth society"
 ],
 "aliases": [
  "flexure bearing stirling",
  "spiral flexure spring dxf",
  "planar spring design",
  "free piston stirling flexure",
  "nasa stirling flexure bearing",
  "sunpower flexure",
  "flexure stack"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 30,
 "url": "https://xemstudio.com/objects/spiral-flexure-bearings/",
 "sheet_txt": "https://xemstudio.com/objects/spiral-flexure-bearings/sheet.txt",
 "sheet": {
  "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",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "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."
  ],
  "table": {
   "columns": [
    "od_mm",
    "hub_mm",
    "rim_mm",
    "arm_w_mm",
    "thickness",
    "bore_mm",
    "arm_radial_span_mm"
   ],
   "rows": [
    [
     40,
     12,
     4.8,
     1.6,
     "0.5 steel / 2.0 print",
     4,
     9.2
    ],
    [
     60,
     13.2,
     7.2,
     2,
     "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,
     12,
     3,
     "0.5 steel / 2.0 print",
     8,
     27
    ],
    [
     120,
     26.4,
     14.4,
     3.5,
     "0.5 steel / 2.0 print",
     8,
     32.4
    ]
   ]
  }
 },
 "sheet_text": "Planar spiral flexure bearings (Sunpower / NASA free-piston style): Ø40–120, DXF + STEP — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/spiral-flexure-bearings/  (CC BY 4.0)\nBasis: 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\nCatalog version: 2026-10-20\n\nSizes: see table below\n- 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.\n- 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.\n- 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.\n- 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.\n- LES: flexure-suspended displacer on the free-piston test bed; print the stack first, cut steel when the stroke is right.\n\nod_mm  hub_mm  rim_mm  arm_w_mm  thickness  bore_mm  arm_radial_span_mm\n40  12  4.8  1.6  0.5 steel / 2.0 print  4  9.2\n60  13.2  7.2  2.0  0.5 steel / 2.0 print  5  16.2\n80  17.6  9.6  2.5  0.5 steel / 2.0 print  5  21.6\n100  22.0  12.0  3.0  0.5 steel / 2.0 print  8  27.0\n120  26.4  14.4  3.5  0.5 steel / 2.0 print  8  32.4\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- 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\n- 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\n- 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\n- Wikipedia: Flexure bearing — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Flexure_bearing\n- Sunpower (AMETEK): Free-piston Stirling technology overview — web — Sunpower site [verify license at link] — https://sunpowerinc.com/\n- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 30 parts): https://xemstudio.com/files/hardware/spiral-flexure-bearings/index.json\nAPI: https://xemstudio.com/api/geometry?object=spiral-flexure-bearings\n",
 "sources": [
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Free-piston Stirling convertor flexure bearing development and life testing (2005)",
   "url": "https://ntrs.nasa.gov/search?q=20050182933",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Flexure bearing design for free-piston Stirling engines (1986)",
   "url": "https://ntrs.nasa.gov/search?q=19860020334",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Advanced Stirling Radioisotope Generator (ASRG) engineering unit (2011)",
   "url": "https://ntrs.nasa.gov/search?q=20110016393",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Flexure bearing",
   "url": "https://en.wikipedia.org/wiki/Flexure_bearing",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Sunpower (AMETEK)",
   "title": "Free-piston Stirling technology overview",
   "url": "https://sunpowerinc.com/",
   "formats": [
    "web"
   ],
   "license": "Sunpower site",
   "license_verified": false,
   "hosted": false
  },
  {
   "publisher": "Long Earth Society",
   "title": "Engine builds",
   "url": "https://longearthsociety.com/engineering/",
   "formats": [
    "web",
    "video"
   ],
   "license": "LES site",
   "license_verified": true,
   "hosted": false
  }
 ],
 "xem": {
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  "sheet_free": true,
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 },
 "related": [
  "free-piston-core",
  "linear-alternator-kit",
  "springs",
  "crank-conrod-flywheel-kit",
  "nasa-fpse-references",
  "printer-calibration-coupons"
 ],
 "faq": [
  {
   "q": "What is a flexure bearing in a Stirling engine?",
   "a": "A stack of flat spiral springs that holds the piston centred while letting it move axially, so it can run in a clearance seal with no contact, no oil and no wear; the suspension used in NASA's free-piston convertors."
  },
  {
   "q": "How many arms should a spiral flexure have?",
   "a": "Three: radial stiffness is then the same in every direction. Two-arm leaves are softer axially but let the hub tilt; stack leaves with alternating spiral hand to cancel rotation."
  },
  {
   "q": "What material for flexure springs?",
   "a": "Spring steel (1074/1095, C75) 0.3–0.8 mm, beryllium copper, or 17-7 PH stainless, laser or water-jet cut from the DXF; printed PETG/nylon 2 mm leaves for motion checks only."
  },
  {
   "q": "How do I set the frequency of a free-piston engine?",
   "a": "f = (1/2π)√(k/m): measure the stack stiffness with weights, weigh the moving assembly, and add leaves or mass until the resonance sits where the thermodynamics want it (10–20 Hz for a hobby engine)."
  }
 ],
 "files_license": null,
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   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-100-2arm-w3-t0.5-steel.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-100-2arm-w3-t0.5-steel.stl",
   "note": "2-arm variant Ø100 (lower axial stiffness, less radial)"
  },
  {
   "file": "spacer-100-t2",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "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",
   "note": "stack spacer ring Ø100, 12 wide, 2 thick (sets the gap between leaves)"
  },
  {
   "file": "clamp-plate-100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/clamp-plate-100.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/clamp-plate-100.stl",
   "note": "rim clamp plate Ø100, 3 × M3"
  },
  {
   "file": "hub-clamp-100-b8",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/hub-clamp-100-b8.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/hub-clamp-100-b8.stl",
   "note": "hub clamp washer Ø22, Ø8 bore"
  },
  {
   "file": "flexure-120-3arm-w3.5-t0.5-steel",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-3arm-w3.5-t0.5-steel.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-3arm-w3.5-t0.5-steel.stl",
   "note": "planar spiral flexure Ø120, 3 arms 3.5 wide, 0.5 thick (laser-cut spring steel / BeCu, DXF included), hub Ø26.4 bore Ø8, 3 rim holes"
  },
  {
   "file": "flexure-120-3arm-w3.5-t2-print",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-3arm-w3.5-t2-print.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-3arm-w3.5-t2-print.stl",
   "note": "planar spiral flexure Ø120, 3 arms 3.5 wide, 2.0 thick (printable PETG/nylon demonstrator), hub Ø26.4 bore Ø8, 3 rim holes"
  },
  {
   "file": "flexure-120-2arm-w3.5-t0.5-steel",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-2arm-w3.5-t0.5-steel.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-2arm-w3.5-t0.5-steel.stl",
   "note": "2-arm variant Ø120 (lower axial stiffness, less radial)"
  },
  {
   "file": "spacer-120-t2",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "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",
   "note": "stack spacer ring Ø120, 14.4 wide, 2 thick (sets the gap between leaves)"
  },
  {
   "file": "clamp-plate-120",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/clamp-plate-120.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/clamp-plate-120.stl",
   "note": "rim clamp plate Ø120, 3 × M3"
  },
  {
   "file": "hub-clamp-120-b8",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/hub-clamp-120-b8.step",
   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/hub-clamp-120-b8.stl",
   "note": "hub clamp washer Ø26.4, Ø8 bore"
  }
 ],
 "markdown": "https://xemstudio.com/objects/spiral-flexure-bearings/index.md"
}