{
 "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": "rhombic-crank-gear",
 "name": "Rhombic drive crank gear, module 1.5, 44 T, 7 mm throw",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Drives & mechanisms",
 "year": null,
 "question": "What are the dimensions of the crank gears in a rhombic drive?",
 "tags": [
  "rhombic drive",
  "gear",
  "crank",
  "beta",
  "philips",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "rhombic drive gear",
  "rhombic drive crank gear stl",
  "30 tooth module 1 crank gear",
  "rhombic drive dimensions"
 ],
 "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/rhombic-crank-gear/",
 "sheet_txt": "https://xemstudio.com/objects/rhombic-crank-gear/sheet.txt",
 "sheet": {
  "basis": "Involute spur gear module 1.5, 44 teeth, 20° pressure angle (pitch Ø66); two of them on 66 mm centres; crank pin at e = 7, a quarter tooth pitch off a tooth centre; rhombic kinematics in tools/rhombic_geom.py",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The two counter-rotating gears of the rhombic drive are this one part used twice: module 1.5, 44 teeth, pitch Ø66, tip Ø69, 6 face, Ø12 × 5 hub with an M3 grub (Ø2.5 tapped), Ø5 bore (fit: slide).",
   "Crank-pin hole Ø3 (fit: press) at 7 mm from the axis and 2.045° (a quarter tooth pitch) from a tooth centre. With the two gears mirror images of each other (both pins at the same height), a tooth of one then sits in a gap of the other on the 66 mm centre line: the pin layout and the mesh agree.",
   "With the 30 mm links and yoke pivots at ±12.7 the displacer leads the piston by 90.0° and both strokes are 20.12 mm."
  ],
  "table": {
   "columns": [
    "parameter",
    "value"
   ],
   "rows": [
    [
     "module",
     1.5
    ],
    [
     "teeth",
     44
    ],
    [
     "pitch Ø",
     66
    ],
    [
     "tip Ø",
     69
    ],
    [
     "face width",
     6
    ],
    [
     "hub",
     "Ø12 × 5"
    ],
    [
     "bore",
     "Ø5"
    ],
    [
     "crank pin",
     "Ø3 at 7, 2.045° off a tooth"
    ],
    [
     "gear centres",
     66
    ]
   ]
  }
 },
 "sheet_text": "Rhombic drive crank gear, module 1.5, 44 T, 7 mm throw — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/rhombic-crank-gear/  (CC BY 4.0)\nBasis: Involute spur gear module 1.5, 44 teeth, 20° pressure angle (pitch Ø66); two of them on 66 mm centres; crank pin at e = 7, a quarter tooth pitch off a tooth centre; rhombic kinematics in tools/rhombic_geom.py\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The two counter-rotating gears of the rhombic drive are this one part used twice: module 1.5, 44 teeth, pitch Ø66, tip Ø69, 6 face, Ø12 × 5 hub with an M3 grub (Ø2.5 tapped), Ø5 bore (fit: slide).\n- Crank-pin hole Ø3 (fit: press) at 7 mm from the axis and 2.045° (a quarter tooth pitch) from a tooth centre. With the two gears mirror images of each other (both pins at the same height), a tooth of one then sits in a gap of the other on the 66 mm centre line: the pin layout and the mesh agree.\n- With the 30 mm links and yoke pivots at ±12.7 the displacer leads the piston by 90.0° and both strokes are 20.12 mm.\n\nparameter  value\nmodule  1.5\nteeth  44\npitch Ø  66.0\ntip Ø  69.0\nface width  6.0\nhub  Ø12 × 5\nbore  Ø5\ncrank pin  Ø3 at 7, 2.045° off a tooth\ngear centres  66.0\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Wikipedia: Rhombic drive — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Rhombic_drive\n- Wikipedia: Stirling engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Stirling_engine\n- Philips Research / Meijer: The Philips Stirling engine (rhombic drive papers, 1958–1965) — web — reference [verify license at link] — https://en.wikipedia.org/wiki/Rhombic_drive#History\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/rhombic-crank-gear/index.json\nAPI: https://xemstudio.com/api/geometry?object=rhombic-crank-gear\n",
 "sources": [
  {
   "publisher": "Wikipedia",
   "title": "Rhombic drive",
   "url": "https://en.wikipedia.org/wiki/Rhombic_drive",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Stirling engine",
   "url": "https://en.wikipedia.org/wiki/Stirling_engine",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Philips Research / Meijer",
   "title": "The Philips Stirling engine (rhombic drive papers, 1958–1965)",
   "url": "https://en.wikipedia.org/wiki/Rhombic_drive#History",
   "formats": [
    "web"
   ],
   "license": "reference",
   "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,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "rhombic-link",
  "rhombic-frame-head",
  "rhombic-crank-pin",
  "rhombic-upper-yoke",
  "rhombic-lower-yoke",
  "list-beta-rhombic-2in"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "rhombic-frame-head",
     "why": "Ø5 gear shafts in the head's 625s, 66 mm centres"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "rhombic-crank-pin",
     "why": "Ø3 pin pressed in the gear at 7 mm"
    },
    {
     "slug": "rhombic-link",
     "why": "two links ride each crank pin"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "spur-gears-involute",
     "why": "other involute gears"
    }
   ]
  },
  {
   "role": "Also needed",
   "items": [
    {
     "slug": "stirling-crankshafts",
     "why": "Ø5 gear shafts"
    }
   ]
  }
 ],
 "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": "gear bore",
   "exact": "Ø5",
   "mates": "Ø5 gear shaft",
   "fit": "slide",
   "files": "crank-gear-"
  },
  {
   "feature": "gear crank-pin hole",
   "exact": "Ø3",
   "mates": "Ø3 crank pin",
   "fit": "press",
   "files": "crank-gear-"
  },
  {
   "feature": "gear hub grub hole",
   "exact": "Ø2.5",
   "mates": "M3 tap drill",
   "fit": "designed",
   "files": "crank-gear-"
  },
  {
   "feature": "gear teeth",
   "exact": "module 1.5, 44 T, 20°, on 66 mm centres",
   "mates": "the other crank gear (no backlash drawn)",
   "fit": "run",
   "files": "crank-gear-"
  }
 ],
 "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 gears does a rhombic drive use?",
   "a": "Two identical spur gears turning opposite ways; here module 1.5, 44 T on 66 mm centres, each with a crank pin at 7 mm."
  },
  {
   "q": "How are the rhombic gears timed?",
   "a": "Mesh them with the two crank pins level and mirror images of each other; the pin hole sits a quarter tooth pitch off a tooth so that position meshes."
  },
  {
   "q": "What stroke do these gears give?",
   "a": "20.12 mm on both yokes with the 30 mm links (about 2√2 × the 7 mm throw at a 90° phase)."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "crank-gear-m1.5-44t-e7",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-m1.5-44t-e7.step",
   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-m1.5-44t-e7.stl",
   "note": "crank gear module 1.5, 44 teeth (Ø66 pitch, Ø69 tip), 6 face, Ø12 × 5 hub with an M3 grub (Ø2.5 tapped), Ø5 bore, Ø3 crank-pin hole at 7 mm, 2.045° (a quarter tooth pitch) off a tooth centre; two per drive, meshed on 66 mm centres"
  }
 ],
 "markdown": "https://xemstudio.com/objects/rhombic-crank-gear/index.md"
}