{
 "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": "fpse-hub-collars",
 "name": "Leaf hub collars: Ø12 and Ø18 bore, M3 grub",
 "kind": "hardware",
 "category": "Hardware",
 "group": "Stirling & thermoacoustic",
 "year": null,
 "question": "How is a flexure leaf's hub held on its rod?",
 "tags": [
  "collar",
  "flexure",
  "hub",
  "tri-clamp",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "flexure hub collar",
  "shaft collar grub"
 ],
 "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/fpse-hub-collars/",
 "sheet_txt": "https://xemstudio.com/objects/fpse-hub-collars/sheet.txt",
 "sheet": {
  "basis": "Design values of the free-piston generator; M3 grub (ISO 4026) in an Ø2.5 tapped hole",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The leaf's hub is clamped axially between a shoulder and a collar (leaf B: the displacer cap's boss and one Ø20 collar) or between two collars (leaves C and D on the Ø18 tube rod). Each collar holds with one M3 × 4 grub.",
   "Fit: the collar bores slide on the rod or tube and are held by the grub."
  ],
  "table": {
   "columns": [
    "collar",
    "od_mm",
    "bore_mm",
    "h_mm"
   ],
   "rows": [
    [
     "displacer",
     20,
     12,
     5
    ],
    [
     "piston",
     28,
     18,
     5
    ]
   ]
  }
 },
 "sheet_text": "Leaf hub collars: Ø12 and Ø18 bore, M3 grub — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/fpse-hub-collars/  (CC BY 4.0)\nBasis: Design values of the free-piston generator; M3 grub (ISO 4026) in an Ø2.5 tapped hole\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The leaf's hub is clamped axially between a shoulder and a collar (leaf B: the displacer cap's boss and one Ø20 collar) or between two collars (leaves C and D on the Ø18 tube rod). Each collar holds with one M3 × 4 grub.\n- Fit: the collar bores slide on the rod or tube and are held by the grub.\n\ncollar  od_mm  bore_mm  h_mm\ndisplacer  20  12  5\npiston  28  18  5\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\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/fpse-hub-collars/index.json\nAPI: https://xemstudio.com/api/geometry?object=fpse-hub-collars\n",
 "sources": [
  {
   "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": [
  "tri-clamp-flexure-leaf",
  "list-fpse-generator-2in"
 ],
 "fits": [],
 "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": "bore",
   "exact": "Ø12 / Ø18",
   "mates": "displacer rod / tube rod (M3 grub)",
   "fit": "slide",
   "files": "all"
  },
  {
   "feature": "grub hole",
   "exact": "Ø2.5",
   "mates": "M3 tap drill",
   "fit": "designed",
   "files": "all"
  }
 ],
 "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 many?",
   "a": "One Ø12 (leaf B) and four Ø18 (leaves C and D)."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "fpse-collar-12x20",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-hub-collars/fpse-collar-12x20.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-hub-collars/fpse-collar-12x20.stl",
   "note": "collar Ø20 × 5, Ø12 bore, M3 tapped radial hole (Ø2.5): clamps leaf B's hub on the displacer rod against the cap's boss"
  },
  {
   "file": "fpse-collar-18x28",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-hub-collars/fpse-collar-18x28.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-hub-collars/fpse-collar-18x28.stl",
   "note": "collar Ø28 × 5, Ø18 bore, M3 tapped radial hole (Ø2.5): two per piston leaf, either side of its hub on the tube rod"
  }
 ],
 "markdown": "https://xemstudio.com/objects/fpse-hub-collars/index.md"
}