{
 "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-drive-kit",
 "name": "Rhombic drive kit (Philips beta Stirling): twin crank gears, links, yokes, frame",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Drives & mechanisms",
 "year": null,
 "question": "How does a rhombic drive Stirling engine work and what parts does it need?",
 "tags": [
  "rhombic drive",
  "beta",
  "philips",
  "mechanism",
  "gears",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "rhombic drive stirling engine",
  "rhombic drive mechanism",
  "beta stirling drive",
  "philips stirling rhombic",
  "rhombic drive 3d print",
  "stirling engine wheel pushes displacer and piston"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 10,
 "url": "https://xemstudio.com/objects/rhombic-drive-kit/",
 "sheet_txt": "https://xemstudio.com/objects/rhombic-drive-kit/sheet.txt",
 "sheet": {
  "basis": "Philips/Meijer rhombic drive: two counter-rotating crank gears, four equal links, two yokes; displacer rod through the piston; geometry here e = 8, link 40, gear centres 32 (module 1, 30 T), scaled for a 2 in tri-clamp beta",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The rhombic drive is the classic beta mechanism (Philips 1950s): two identical gears mesh and turn opposite ways, each carrying a crank pin; four equal links join the pins to an upper yoke (displacer rod) and a lower yoke (piston rod, a tube the displacer rod runs through). Because the two cranks are mirror images every side load cancels: the piston and displacer move on the axis with no piston slap, perfectly balanced, and the phase angle is fixed by the geometry (about 90° for these proportions).",
   "Geometry rules: gear centre distance c, crank throw e, link length L. Stroke ≈ 2e for both yokes; the displacer leads the piston by an angle set by c/L and e/L (Meijer's charts); with c = 32, e = 8, L = 40 you get ~85–95° and strokes of 16 mm. Keep L ≥ 4e. Gears must be meshed with the pins 180° apart (one at top dead centre when the other is at bottom): mark a tooth.",
   "Parts: 2 crank gears (module 1 30 T, Ø5 shafts in 625 bearings in the frame plates), 4 links (40 mm, Ø3 pins), upper yoke (Ø3 displacer rod clamp), lower yoke (Ø5 tube piston rod, Ø3.6 clearance for the displacer rod), 2 frame plates, 4 Ø3 crank pins. Rod tube: 5 mm K&S brass (3.1 ID) is the piston rod with the Ø3 displacer rod inside.",
   "LES: the 2 in beta on the rhombic drive is the balanced, quiet one; matches the Ø46 × 90 tri-clamp displacer at 16 mm stroke, with the syringe or a lapped Ø20 power piston."
  ],
  "table": {
   "columns": [
    "parameter",
    "value_mm",
    "note"
   ],
   "rows": [
    [
     "gear centres c",
     32,
     "30 T module 1 gears"
    ],
    [
     "crank throw e",
     8,
     "stroke 16"
    ],
    [
     "link length L",
     40,
     "≥ 4e"
    ],
    [
     "yoke span",
     32,
     "= c at mid-stroke"
    ],
    [
     "displacer rod",
     "Ø3",
     "through Ø5 tube"
    ],
    [
     "piston rod",
     "Ø5 tube (3.1 ID)",
     "K&S brass"
    ],
    [
     "bearings",
     "4 × 625",
     "frame plates"
    ]
   ]
  }
 },
 "sheet_text": "Rhombic drive kit (Philips beta Stirling): twin crank gears, links, yokes, frame — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/rhombic-drive-kit/  (CC BY 4.0)\nBasis: Philips/Meijer rhombic drive: two counter-rotating crank gears, four equal links, two yokes; displacer rod through the piston; geometry here e = 8, link 40, gear centres 32 (module 1, 30 T), scaled for a 2 in tri-clamp beta\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The rhombic drive is the classic beta mechanism (Philips 1950s): two identical gears mesh and turn opposite ways, each carrying a crank pin; four equal links join the pins to an upper yoke (displacer rod) and a lower yoke (piston rod, a tube the displacer rod runs through). Because the two cranks are mirror images every side load cancels: the piston and displacer move on the axis with no piston slap, perfectly balanced, and the phase angle is fixed by the geometry (about 90° for these proportions).\n- Geometry rules: gear centre distance c, crank throw e, link length L. Stroke ≈ 2e for both yokes; the displacer leads the piston by an angle set by c/L and e/L (Meijer's charts); with c = 32, e = 8, L = 40 you get ~85–95° and strokes of 16 mm. Keep L ≥ 4e. Gears must be meshed with the pins 180° apart (one at top dead centre when the other is at bottom): mark a tooth.\n- Parts: 2 crank gears (module 1 30 T, Ø5 shafts in 625 bearings in the frame plates), 4 links (40 mm, Ø3 pins), upper yoke (Ø3 displacer rod clamp), lower yoke (Ø5 tube piston rod, Ø3.6 clearance for the displacer rod), 2 frame plates, 4 Ø3 crank pins. Rod tube: 5 mm K&S brass (3.1 ID) is the piston rod with the Ø3 displacer rod inside.\n- LES: the 2 in beta on the rhombic drive is the balanced, quiet one; matches the Ø46 × 90 tri-clamp displacer at 16 mm stroke, with the syringe or a lapped Ø20 power piston.\n\nparameter  value_mm  note\ngear centres c  32  30 T module 1 gears\ncrank throw e  8  stroke 16\nlink length L  40  ≥ 4e\nyoke span  32  = c at mid-stroke\ndisplacer rod  Ø3  through Ø5 tube\npiston rod  Ø5 tube (3.1 ID)  K&S brass\nbearings  4 × 625  frame plates\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, 10 parts): https://xemstudio.com/files/hardware/rhombic-drive-kit/index.json\nAPI: https://xemstudio.com/api/geometry?object=rhombic-drive-kit\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": [
  "spur-gears-involute",
  "tri-clamp-displacers-and-heads",
  "brass-tube-ks",
  "ball-bearings-metric",
  "dowel-pins-metric",
  "crank-conrod-flywheel-kit",
  "stirling-configurations-guide"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "stirling-linkage-fittings",
     "why": "gear plates on the rhombic frame head"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "tri-clamp-power-pistons",
     "why": "-rod piston and its tube rod to the lower yoke"
    },
    {
     "slug": "tri-clamp-displacers-and-heads",
     "why": "displacer rod to the upper yoke"
    },
    {
     "slug": "tri-clamp-free-piston-kit",
     "why": "piston plug carries the tube rod"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "ross-yoke-kit",
     "why": "alpha with parallel cylinders"
    }
   ]
  },
  {
   "role": "Also needed",
   "items": [
    {
     "slug": "ball-bearings-metric",
     "why": "4 × 625 in the plates"
    },
    {
     "slug": "dowel-pins-metric",
     "why": "Ø3 link pins"
    },
    {
     "slug": "brass-tube-ks",
     "why": "5 mm tube piston rod over the Ø3 rod"
    },
    {
     "slug": "crank-conrod-flywheel-kit",
     "why": "flywheel"
    }
   ]
  }
 ],
 "faq": [
  {
   "q": "What is a rhombic drive?",
   "a": "Two counter-rotating crank gears and four links forming a rhombus between two yokes; the top yoke drives the displacer, the bottom yoke the piston, both on one axis. Fully balanced, no side load, fixed phase angle."
  },
  {
   "q": "How do I set the phase angle on a rhombic drive?",
   "a": "It is built into the geometry (gear spacing, throw and link length) and the 180° meshing of the gears; with c 32 / e 8 / L 40 it is about 90°. Change e/L to adjust."
  },
  {
   "q": "What stroke does this rhombic kit give?",
   "a": "16 mm for both displacer and piston (2 × 8 mm throw); scale e and L together for other strokes, keeping L ≥ 4e and c ≈ 4e."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "crank-gear-30t-e8-left",
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   ],
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   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-30t-e8-left.step",
   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-30t-e8-left.stl",
   "note": "crank gear 30 T module 1, Ø5 bore hub, Ø3 crank pin at 8.0 mm (stroke 16.0); left (identical; mesh them 180° apart)"
  },
  {
   "file": "crank-gear-30t-e8-right",
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   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-30t-e8-right.stl",
   "note": "crank gear 30 T module 1, Ø5 bore hub, Ø3 crank pin at 8.0 mm (stroke 16.0); right (identical; mesh them 180° apart)"
  },
  {
   "file": "link-40-1",
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   "price_usd": 0,
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   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-1.stl",
   "note": "rhombic link 40.0 mm centres, Ø3.2 pivots (4 needed)"
  },
  {
   "file": "link-40-2",
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   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
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   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-2.stl",
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  {
   "file": "link-40-3",
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   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
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   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-3.stl",
   "note": "rhombic link 40.0 mm centres, Ø3.2 pivots (4 needed)"
  },
  {
   "file": "link-40-4",
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   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-4.step",
   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-4.stl",
   "note": "rhombic link 40.0 mm centres, Ø3.2 pivots (4 needed)"
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  {
   "file": "yoke-upper-displacer",
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   "license": "CC BY 4.0 (Xem Studio)",
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   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-upper-displacer.stl",
   "note": "upper yoke (displacer rod Ø3) span 32 mm pivots"
  },
  {
   "file": "yoke-lower-piston",
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   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-lower-piston.stl",
   "note": "lower yoke (piston on a Ø5 tube rod, displacer Ø3 rod passes through) span 32"
  },
  {
   "file": "gear-frame-plate-c32",
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   "price_usd": 0,
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   "step": "https://xemstudio.com/files/hardware/rhombic-drive-kit/gear-frame-plate-c32.step",
   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/gear-frame-plate-c32.stl",
   "note": "frame plate with Ø16.2 pockets for 625 bearings (Ø5 gear shafts) on 32 mm centres, a Ø16 rod clearance and 2 × M3 mounting holes at (±20, −15) for the rhombic frame head (print two)"
  },
  {
   "file": "crank-pin-3x14",
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   "stl": "https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-pin-3x14.stl",
   "note": "Ø3 × 14 crank pin (dowel) for the gears"
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 ],
 "markdown": "https://xemstudio.com/objects/rhombic-drive-kit/index.md"
}