{
 "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-cap-ring",
 "name": "Linear alternator cap ring, Ø96 and Ø116",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What holds the linear alternator stator in its housing?",
 "tags": [
  "linear alternator",
  "cap ring",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "linear alternator cap",
  "alternator retaining ring"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 2,
 "url": "https://xemstudio.com/objects/linear-alternator-cap-ring/",
 "sheet_txt": "https://xemstudio.com/objects/linear-alternator-cap-ring/sheet.txt",
 "sheet": {
  "basis": "Same outline and rim circle as the housing's top flange; bore = sleeve bore − 2",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "A flat ring Ø96 (2 in) or Ø116 (3 in) × 4 with a Ø50.5 bore, that holds the back-iron sleeve and bobbin down in the housing.",
   "3 × Ø3.4 holes on the same rim circle as the housing (R35.2 / R44): M3 × 12 screws go up through the top flange and cap into the standoffs that carry the flexure ring and stack B."
  ],
  "table": {
   "columns": [
    "file",
    "outside",
    "bore",
    "thickness",
    "rim circle"
   ],
   "rows": [
    [
     "alt-cap-2in-flex80",
     96,
     50.5,
     4,
     "R35.2"
    ],
    [
     "alt-cap-3in-flex100",
     116,
     50.5,
     4,
     "R44"
    ]
   ]
  }
 },
 "sheet_text": "Linear alternator cap ring, Ø96 and Ø116 — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/linear-alternator-cap-ring/  (CC BY 4.0)\nBasis: Same outline and rim circle as the housing's top flange; bore = sleeve bore − 2\nCatalog version: 2026-10-20\n\nSizes: see table below\n- A flat ring Ø96 (2 in) or Ø116 (3 in) × 4 with a Ø50.5 bore, that holds the back-iron sleeve and bobbin down in the housing.\n- 3 × Ø3.4 holes on the same rim circle as the housing (R35.2 / R44): M3 × 12 screws go up through the top flange and cap into the standoffs that carry the flexure ring and stack B.\n\nfile  outside  bore  thickness  rim circle\nalt-cap-2in-flex80  96  50.5  4  R35.2\nalt-cap-3in-flex100  116  50.5  4  R44\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, 2 parts): https://xemstudio.com/files/hardware/linear-alternator-cap-ring/index.json\nAPI: https://xemstudio.com/api/geometry?object=linear-alternator-cap-ring\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-housing",
  "linear-alternator-back-iron-sleeve",
  "tri-clamp-flexure-head"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "linear-alternator-housing",
     "why": "on the top flange, same 3 × M3"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "linear-alternator-back-iron-sleeve",
     "why": "holds the sleeve down"
    }
   ]
  },
  {
   "role": "Also needed",
   "items": [
    {
     "slug": "threaded-standoffs-metric",
     "why": "standoffs above for stack B"
    }
   ]
  }
 ],
 "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": "cap rim holes",
   "exact": "Ø3.4",
   "mates": "M3 screw clearance",
   "fit": "designed",
   "files": "alt-cap-"
  },
  {
   "feature": "cap ring bore",
   "exact": "Ø50.5",
   "mates": "holds the sleeve and bobbin down",
   "fit": "designed",
   "files": "alt-cap-"
  }
 ],
 "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 does the alternator cap ring do?",
   "a": "It clamps the sleeve and bobbin down onto the housing's ledge; its Ø50.5 bore overlaps the Ø52.5 sleeve bore by 1 mm all round."
  },
  {
   "q": "What screws hold the cap ring?",
   "a": "3 × M3 × 12 through the top flange and cap into the standoffs above."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "alt-cap-2in-flex80",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-2in-flex80.step",
   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-2in-flex80.stl",
   "note": "2in cap ring: holds the sleeve and bobbin down on the housing's top flange; 3 × M3 × 12 through flange and cap into the standoffs for flexure stack B"
  },
  {
   "file": "alt-cap-3in-flex100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-3in-flex100.step",
   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/alt-cap-3in-flex100.stl",
   "note": "3in cap ring: holds the sleeve and bobbin down on the housing's top flange; 3 × M3 × 12 through flange and cap into the standoffs for flexure stack B"
  }
 ],
 "markdown": "https://xemstudio.com/objects/linear-alternator-cap-ring/index.md"
}