{
 "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-bearing",
 "name": "Planar spiral flexure bearing leaves: Ø40 to Ø120, 3-arm and 2-arm, 0.5 and 2 mm",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What is a spiral flexure bearing and what are its dimensions?",
 "tags": [
  "flexure",
  "flexure bearing",
  "planar spring",
  "free piston",
  "nasa",
  "sunpower",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "flexure bearing stirling",
  "spiral flexure spring dxf",
  "planar spring design",
  "free piston stirling flexure",
  "nasa flexure bearing"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 15,
 "url": "https://xemstudio.com/objects/spiral-flexure-bearing/",
 "sheet_txt": "https://xemstudio.com/objects/spiral-flexure-bearing/sheet.txt",
 "sheet": {
  "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°)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "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."
  ],
  "table": {
   "columns": [
    "od_mm",
    "hub_mm",
    "rim_mm",
    "arm_w_mm",
    "bore_mm",
    "arm_radial_span_mm",
    "rim_hole_circle_mm"
   ],
   "rows": [
    [
     40,
     12,
     4.8,
     1.6,
     4,
     9.2,
     35.2
    ],
    [
     60,
     13.2,
     7.2,
     2,
     5,
     16.2,
     52.8
    ],
    [
     80,
     17.6,
     9.6,
     2.5,
     5,
     21.6,
     70.4
    ],
    [
     100,
     22,
     12,
     3,
     8,
     27,
     88
    ],
    [
     120,
     26.4,
     14.4,
     3.5,
     8,
     32.4,
     105.6
    ]
   ]
  }
 },
 "sheet_text": "Planar spiral flexure bearing leaves: Ø40 to Ø120, 3-arm and 2-arm, 0.5 and 2 mm — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/spiral-flexure-bearing/  (CC BY 4.0)\nBasis: 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°)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- 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.\n- 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.\n- 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.\n- 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).\n- Three arms give equal radial stiffness in every direction; two-arm leaves are softer axially but let the hub tilt.\n\nod_mm  hub_mm  rim_mm  arm_w_mm  bore_mm  arm_radial_span_mm  rim_hole_circle_mm\n40  12  4.8  1.6  4  9.2  35.2\n60  13.2  7.2  2.0  5  16.2  52.8\n80  17.6  9.6  2.5  5  21.6  70.4\n100  22.0  12.0  3.0  8  27.0  88.0\n120  26.4  14.4  3.5  8  32.4  105.6\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, 15 parts): https://xemstudio.com/files/hardware/spiral-flexure-bearing/index.json\nAPI: https://xemstudio.com/api/geometry?object=spiral-flexure-bearing\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": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
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  "tolerance_mm": null
 },
 "related": [
  "flexure-stack-spacer",
  "flexure-rim-clamp-plate",
  "flexure-hub-clamp-washer",
  "tri-clamp-flexure-head",
  "flexure-hub-collar",
  "free-piston-displacer"
 ],
 "fits": [
  {
   "role": "Parts that fit it",
   "items": [
    {
     "slug": "flexure-hub-centring-bush",
     "why": "OD = leaf hub bore"
    }
   ]
  },
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "tri-clamp-flexure-head",
     "why": "rim in the flexure-OD seat, 3 × M3"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "flexure-stack-spacer",
     "why": "spacer between leaf rims"
    },
    {
     "slug": "flexure-hub-clamp-washer",
     "why": "washer between leaf hubs"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "compression-springs",
     "why": "coil springs"
    }
   ]
  },
  {
   "role": "Also needed",
   "items": [
    {
     "slug": "flexure-rim-clamp-plate",
     "why": "clamps the stack rims"
    },
    {
     "slug": "flexure-hub-collar",
     "why": "clamps the hubs to the rod"
    }
   ]
  }
 ],
 "dimensions": "exact (nominal, no printing tolerance)",
 "dimension_rule": "All geometry on Xem Studio is exact (nominal, true size) with no printing tolerance. Where a part meets another part, the object lists its mating features (feature, exact size, what it mates with, fit class: press, slide, run, socket, clamp or designed). Apply your process's allowance to those features only; 'designed' gaps are part of the design and stay as drawn.",
 "mating": [
  {
   "feature": "leaf hub bore",
   "exact": "Ø4 / Ø5 / Ø8",
   "mates": "rod, tube or centring bush",
   "fit": "slide",
   "files": "flexure-"
  },
  {
   "feature": "rim holes",
   "exact": "Ø3.2",
   "mates": "M3 screws (clearance)",
   "fit": "designed",
   "files": "flexure-"
  }
 ],
 "fit_classes": {
  "press": "stays put: pin, tube, bearing or magnet held by interference. Typical FDM: hole +0.0 to +0.1 mm on diameter.",
  "slide": "assembles by hand and is held by a grub, pinch or clamp: shaft in a hub, rod in a socket, spigot in a bore. Typical FDM: +0.2 mm.",
  "run": "turns or slides in use: pin in a link, rod through a guide. Typical FDM: +0.3 mm.",
  "socket": "large sliding fit: syringe barrel, sleeve, bobbin in a bore. Typical FDM: +0.3 mm.",
  "clamp": "split part that closes on the mating part with screws: make it at size.",
  "designed": "a gap that is part of the design (piston fit option, displacer gap, air gap): do not change."
 },
 "faq": [
  {
   "q": "What is a flexure bearing in a Stirling engine?",
   "a": "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."
  },
  {
   "q": "How many arms should a spiral flexure have?",
   "a": "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."
  },
  {
   "q": "What are the dimensions of the Ø80 flexure?",
   "a": "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."
  }
 ],
 "files_license": null,
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-40-3arm-w1.6-t0.5.stl",
   "note": "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"
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-40-3arm-w1.6-t2.stl",
   "note": "planar spiral flexure Ø40, 3 arms 1.6 wide, 2.0 thick, hub Ø12 bore Ø4, 3 × Ø3.2 rim holes on R17.6"
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-40-2arm-w1.6-t0.5.stl",
   "note": "2-arm variant Ø40 (lower axial stiffness, less radial)"
  },
  {
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-60-3arm-w2-t0.5.stl",
   "note": "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"
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-60-3arm-w2-t2.stl",
   "note": "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"
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   "note": "2-arm variant Ø60 (lower axial stiffness, less radial)"
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-80-3arm-w2.5-t0.5.stl",
   "note": "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"
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-80-3arm-w2.5-t2.stl",
   "note": "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"
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  {
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-80-2arm-w2.5-t0.5.stl",
   "note": "2-arm variant Ø80 (lower axial stiffness, less radial)"
  },
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-100-3arm-w3-t0.5.stl",
   "note": "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"
  },
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-100-3arm-w3-t2.stl",
   "note": "planar spiral flexure Ø100, 3 arms 3.0 wide, 2.0 thick, hub Ø22 bore Ø8, 3 × Ø3.2 rim holes on R44"
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   "note": "2-arm variant Ø100 (lower axial stiffness, less radial)"
  },
  {
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-3arm-w3.5-t0.5.stl",
   "note": "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"
  },
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   "stl": "https://xemstudio.com/files/hardware/spiral-flexure-bearings/flexure-120-3arm-w3.5-t2.stl",
   "note": "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"
  },
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   "note": "2-arm variant Ø120 (lower axial stiffness, less radial)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/spiral-flexure-bearing/index.md"
}