{
 "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-displacer",
 "name": "Free-piston displacer: Ø39.4 sealed shell and cap for a 2 in tri-clamp engine",
 "kind": "hardware",
 "category": "Hardware",
 "group": "Stirling & thermoacoustic",
 "year": null,
 "question": "What does the displacer of a pressurised free-piston Stirling look like?",
 "tags": [
  "displacer",
  "free piston",
  "stirling",
  "tri-clamp",
  "long earth society"
 ],
 "aliases": [
  "free piston displacer",
  "sealed displacer stirling",
  "pressurised stirling displacer"
 ],
 "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-displacer/",
 "sheet_txt": "https://xemstudio.com/objects/fpse-displacer/sheet.txt",
 "sheet": {
  "basis": "Design values of the free-piston generator; collapse margin from the thin-tube buckling relation p = 2E/(1 − ν²)·(t/D)³ at E 200 GPa (assumed)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "Ø39.4 shell, 0.8 wall, 209.9 long with its cap: it reaches from the bottom of the hot spool (the expansion space under it) through joint A into the 3 in cooler spool, where its cold face meets the compression space under leaf B.",
   "It runs with a designed 0.30 radial gap in the liner and the cooler insert, never touching: leaf B at joint B holds its cold end on the axis and the rod's gland holds the other end of the rod.",
   "Sealed with the gas it was assembled in. Thin-tube collapse pressure of the shell: about 37 bar for a wall of modulus 200 GPa, against 16 bar absolute at the relief setting; the cap's flange takes the end load on the shell's rim.",
   "Moving mass with its rod, plug, stinger and gas-spring piston: 365 g (at 7.9 g/cm³)."
  ],
  "table": {
   "columns": [
    "part",
    "size"
   ],
   "rows": [
    [
     "shell",
     "Ø39.4 × 206.9, wall 0.8"
    ],
    [
     "cap",
     "spigot Ø37.8, flange Ø39.4 × 3, boss Ø16 × 8"
    ],
    [
     "gap",
     "0.30 radial (designed)"
    ],
    [
     "travel",
     "±6 room, ±4.47 at the design point"
    ]
   ]
  }
 },
 "sheet_text": "Free-piston displacer: Ø39.4 sealed shell and cap for a 2 in tri-clamp engine — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/fpse-displacer/  (CC BY 4.0)\nBasis: Design values of the free-piston generator; collapse margin from the thin-tube buckling relation p = 2E/(1 − ν²)·(t/D)³ at E 200 GPa (assumed)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- Ø39.4 shell, 0.8 wall, 209.9 long with its cap: it reaches from the bottom of the hot spool (the expansion space under it) through joint A into the 3 in cooler spool, where its cold face meets the compression space under leaf B.\n- It runs with a designed 0.30 radial gap in the liner and the cooler insert, never touching: leaf B at joint B holds its cold end on the axis and the rod's gland holds the other end of the rod.\n- Sealed with the gas it was assembled in. Thin-tube collapse pressure of the shell: about 37 bar for a wall of modulus 200 GPa, against 16 bar absolute at the relief setting; the cap's flange takes the end load on the shell's rim.\n- Moving mass with its rod, plug, stinger and gas-spring piston: 365 g (at 7.9 g/cm³).\n\npart  size\nshell  Ø39.4 × 206.9, wall 0.8\ncap  spigot Ø37.8, flange Ø39.4 × 3, boss Ø16 × 8\ngap  0.30 radial (designed)\ntravel  ±6 room, ±4.47 at the design point\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Ohio University (Urieli): Stirling cycle machine analysis: ideal adiabatic and simple analysis (after Urieli & Berchowitz, Stirling Cycle Engine Analysis, 1984) — web — author's site — https://www.ohio.edu/mechanical/stirling/\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-displacer/index.json\nAPI: https://xemstudio.com/api/geometry?object=fpse-displacer\n",
 "sources": [
  {
   "publisher": "Ohio University (Urieli)",
   "title": "Stirling cycle machine analysis: ideal adiabatic and simple analysis (after Urieli & Berchowitz, Stirling Cycle Engine Analysis, 1984)",
   "url": "https://www.ohio.edu/mechanical/stirling/",
   "formats": [
    "web"
   ],
   "license": "author's site",
   "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": [
  "fpse-displacer-rod",
  "fpse-displacer-gas-spring",
  "fpse-displacer-cylinder",
  "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": "shell OD",
   "exact": "Ø39.4",
   "mates": "liner and cooler-insert bores Ø40.00 (0.30 radial gap)",
   "fit": "designed",
   "files": "fpse-displacer-shell"
  },
  {
   "feature": "shell bore",
   "exact": "Ø37.8",
   "mates": "cap spigot (sealed joint)",
   "fit": "press",
   "files": "fpse-displacer-shell"
  },
  {
   "feature": "spigot",
   "exact": "Ø37.8 × 6",
   "mates": "shell bore (sealed joint)",
   "fit": "press",
   "files": "fpse-displacer-cap"
  },
  {
   "feature": "rod socket",
   "exact": "Ø12 × 14",
   "mates": "displacer rod Ø12",
   "fit": "press",
   "files": "fpse-displacer-cap"
  },
  {
   "feature": "boss top face",
   "exact": "Ø16",
   "mates": "leaf B's hub, clamped by the collar",
   "fit": "clamp",
   "files": "fpse-displacer-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": "Why is the displacer sealed?",
   "a": "Vented, its whole inside would be dead volume in the working space. Sealed, it only has to carry the charge pressure; its cap's flange rests on the shell's rim so the load goes straight into the shell."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "fpse-displacer-shell-39.4",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-displacer/fpse-displacer-shell-39.4.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-displacer/fpse-displacer-shell-39.4.stl",
   "note": "displacer shell Ø39.4 × 206.9, wall 0.8, closed hot end 2 (1.0 outer round)"
  },
  {
   "file": "fpse-displacer-cap-39.4",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-displacer/fpse-displacer-cap-39.4.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-displacer/fpse-displacer-cap-39.4.stl",
   "note": "displacer cap: spigot Ø37.8 × 6 (2 wall) into the shell, flange Ø39.4 × 3 on its rim, boss Ø16 × 8 with the Ø12 × 14 rod socket"
  }
 ],
 "markdown": "https://xemstudio.com/objects/fpse-displacer/index.md"
}