{
 "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": "linear-alternator-bobbin",
 "name": "Linear alternator bobbin: Ø37 bore, two 10 mm bays",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What are the dimensions of the coil bobbin for a linear alternator?",
 "tags": [
  "linear alternator",
  "bobbin",
  "coil",
  "stator",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "linear alternator bobbin",
  "coil former stl",
  "two bay coil bobbin"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 1,
 "url": "https://xemstudio.com/objects/linear-alternator-bobbin/",
 "sheet_txt": "https://xemstudio.com/objects/linear-alternator-bobbin/sheet.txt",
 "sheet": {
  "basis": "Bore = mover Ø35 + 2 × 1.0 gap; flanges at the back-iron sleeve bore Ø52.5; length = sleeve length 30",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The coil former of the alternator stator: Ø37 bore (1.0 mm air gap to the Ø35 mover), Ø38.6 core, 30 long, so the coil pair is centred in the 30 mm sleeve.",
   "Flanges are Ø52.5, the back-iron sleeve's bore (fit: slide): 4 mm end flanges and a 2 mm divider make two 10 mm winding bays.",
   "Lead-out slots in both end flanges and a crossover notch in the divider; the second bay is wound the opposite way and joined to the first in series."
  ],
  "table": {
   "columns": [
    "parameter",
    "value"
   ],
   "rows": [
    [
     "bore",
     37
    ],
    [
     "core OD",
     38.6
    ],
    [
     "flange OD",
     52.5
    ],
    [
     "length",
     30
    ],
    [
     "bays",
     "2 × 10"
    ],
    [
     "divider",
     2
    ],
    [
     "end flanges",
     4
    ]
   ]
  }
 },
 "sheet_text": "Linear alternator bobbin: Ø37 bore, two 10 mm bays — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/linear-alternator-bobbin/  (CC BY 4.0)\nBasis: Bore = mover Ø35 + 2 × 1.0 gap; flanges at the back-iron sleeve bore Ø52.5; length = sleeve length 30\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The coil former of the alternator stator: Ø37 bore (1.0 mm air gap to the Ø35 mover), Ø38.6 core, 30 long, so the coil pair is centred in the 30 mm sleeve.\n- Flanges are Ø52.5, the back-iron sleeve's bore (fit: slide): 4 mm end flanges and a 2 mm divider make two 10 mm winding bays.\n- Lead-out slots in both end flanges and a crossover notch in the divider; the second bay is wound the opposite way and joined to the first in series.\n\nparameter  value\nbore  37\ncore OD  38.6\nflange OD  52.5\nlength  30\nbays  2 × 10\ndivider  2\nend flanges  4\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- NASA (NTRS, public domain): Linear alternator design for free-piston Stirling convertors (2005) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=20050182934\n- NASA (NTRS, public domain): Sunpower/NASA 80 We free-piston Stirling convertor (2002) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=20020047256\n- Wikipedia: Linear alternator — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Linear_alternator\n- Wikipedia: Free-piston engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Free-piston_engine\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, 1 parts): https://xemstudio.com/files/hardware/linear-alternator-bobbin/index.json\nAPI: https://xemstudio.com/api/geometry?object=linear-alternator-bobbin\n",
 "sources": [
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Linear alternator design for free-piston Stirling convertors (2005)",
   "url": "https://ntrs.nasa.gov/search?q=20050182934",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Sunpower/NASA 80 We free-piston Stirling convertor (2002)",
   "url": "https://ntrs.nasa.gov/search?q=20020047256",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Linear alternator",
   "url": "https://en.wikipedia.org/wiki/Linear_alternator",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Free-piston engine",
   "url": "https://en.wikipedia.org/wiki/Free-piston_engine",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "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,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "linear-alternator-coil",
  "linear-alternator-back-iron-sleeve",
  "linear-alternator-mover",
  "list-free-piston-2in"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "linear-alternator-back-iron-sleeve",
     "why": "Ø52.5 flanges in the sleeve bore"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "linear-alternator-coil",
     "why": "a coil in each bay"
    },
    {
     "slug": "linear-alternator-mover",
     "why": "mover inside, 1.0 mm gap"
    }
   ]
  }
 ],
 "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": "mover-to-bobbin gap",
   "exact": "1.0 radial",
   "mates": "bobbin bore",
   "fit": "designed",
   "files": "alt-bobbin-"
  },
  {
   "feature": "bobbin flanges in the sleeve",
   "exact": "Ø52.5",
   "mates": "back-iron sleeve bore",
   "fit": "slide",
   "files": "alt-bobbin-"
  }
 ],
 "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": "How big is the air gap in the linear alternator?",
   "a": "1.0 mm radial: Ø37 bobbin bore over the Ø35 mover."
  },
  {
   "q": "Why does the bobbin have two bays?",
   "a": "One coil over each magnet row, wound in opposite directions and joined in series."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "alt-bobbin-37-2x10",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/linear-alternator-kit/alt-bobbin-37-2x10.step",
   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/alt-bobbin-37-2x10.stl",
   "note": "bobbin: Ø37 bore (1 mm air gap to the mover), two 10 mm bays split by a 2 mm divider with a crossover notch, 30 long, slides into the back-iron sleeve"
  }
 ],
 "markdown": "https://xemstudio.com/objects/linear-alternator-bobbin/index.md"
}