{
 "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": "motion-mechanisms",
 "name": "Motion mechanisms: Scotch yoke, swash (wobble) plate, eccentric + strap, Geneva, four-bar links, bell crank, rack",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Drives & mechanisms",
 "year": null,
 "question": "How do I convert rotary motion to linear motion with 3D printed parts?",
 "tags": [
  "mechanism",
  "yoke",
  "swash plate",
  "wobble plate",
  "eccentric",
  "geneva",
  "linkage",
  "rack",
  "motion",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "scotch yoke stl",
  "wobble plate mechanism",
  "swash plate 3d print",
  "eccentric cam follower",
  "geneva mechanism stl",
  "four bar linkage parts",
  "bell crank 3d print",
  "rack and pinion module 1",
  "ross yoke stirling"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 27,
 "url": "https://xemstudio.com/objects/motion-mechanisms/",
 "sheet_txt": "https://xemstudio.com/objects/motion-mechanisms/sheet.txt",
 "sheet": {
  "basis": "Textbook mechanism geometry (Scotch yoke, swash plate stroke = D·sin θ, eccentric stroke = 2e, Geneva with c = R / cos(π/n)); parts sized for Ø5 shafts, Ø3 pins and 623 bearings to match the rest of the shelf",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "Scotch yoke: pure sinusoidal motion, no con-rod; the crank pin (623 bearing on a Ø3 pin) rides in the yoke slot; stroke = 2 × pin offset. Runs the displacer on a beta Stirling without a wrist pin.",
   "Swash / wobble plate: a disc tilted θ on the shaft; a follower touching it at radius r reciprocates r·sin θ each side (Ø40 plate at 15° → 10.4 mm stroke at the rim). Several followers around one plate = multi-cylinder from one shaft; the follower here is an 8 mm square pushrod with a ball end.",
   "Eccentric + strap: a disc offset e on the shaft inside a split ring; stroke 2e, carries load on the whole ring, the quiet way to drive a pump or displacer. Geneva: intermittent motion, n stops per turn; drive-disc radius locks the wheel between indexes.",
   "Four-bar links (20–100 mm centres, Ø3.2) and the bell crank build crank-rockers, parallel motions and the Ross yoke; rack m1 meshes the module 1 gears for linear drives. All pivots are Ø3 dowel pins, Ø3.2 printed holes, or 623 bearings in Ø10.2 pockets."
  ],
  "table": {
   "columns": [
    "mechanism",
    "parts",
    "input → output",
    "key dimension"
   ],
   "rows": [
    [
     "Scotch yoke",
     "yoke + crank",
     "rotary → sinusoidal linear",
     "stroke 20 / 30 mm"
    ],
    [
     "swash plate",
     "plate + follower",
     "rotary → linear, multi-follower",
     "Ø40 at 10 / 15 / 20°"
    ],
    [
     "eccentric",
     "disc + split strap",
     "rotary → linear, high load",
     "e 3 / 5 / 8 mm (stroke 6 / 10 / 16)"
    ],
    [
     "Geneva",
     "wheel + drive",
     "continuous → intermittent",
     "4 / 6 stations, R30"
    ],
    [
     "four-bar links",
     "7 lengths",
     "rockers, parallel motion, Ross yoke",
     "20–100 mm centres"
    ],
    [
     "bell crank",
     "30×30, 20×40",
     "redirect / lever 90°",
     "Ø3.2 pins"
    ],
    [
     "rack",
     "m1 100, m2 150",
     "rotary ↔ linear with the gears",
     "module 1 / 2"
    ]
   ]
  }
 },
 "sheet_text": "Motion mechanisms: Scotch yoke, swash (wobble) plate, eccentric + strap, Geneva, four-bar links, bell crank, rack — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/motion-mechanisms/  (CC BY 4.0)\nBasis: Textbook mechanism geometry (Scotch yoke, swash plate stroke = D·sin θ, eccentric stroke = 2e, Geneva with c = R / cos(π/n)); parts sized for Ø5 shafts, Ø3 pins and 623 bearings to match the rest of the shelf\nCatalog version: 2026-10-20\n\nSizes: see table below\n- Scotch yoke: pure sinusoidal motion, no con-rod; the crank pin (623 bearing on a Ø3 pin) rides in the yoke slot; stroke = 2 × pin offset. Runs the displacer on a beta Stirling without a wrist pin.\n- Swash / wobble plate: a disc tilted θ on the shaft; a follower touching it at radius r reciprocates r·sin θ each side (Ø40 plate at 15° → 10.4 mm stroke at the rim). Several followers around one plate = multi-cylinder from one shaft; the follower here is an 8 mm square pushrod with a ball end.\n- Eccentric + strap: a disc offset e on the shaft inside a split ring; stroke 2e, carries load on the whole ring, the quiet way to drive a pump or displacer. Geneva: intermittent motion, n stops per turn; drive-disc radius locks the wheel between indexes.\n- Four-bar links (20–100 mm centres, Ø3.2) and the bell crank build crank-rockers, parallel motions and the Ross yoke; rack m1 meshes the module 1 gears for linear drives. All pivots are Ø3 dowel pins, Ø3.2 printed holes, or 623 bearings in Ø10.2 pockets.\n\nmechanism  parts  input → output  key dimension\nScotch yoke  yoke + crank  rotary → sinusoidal linear  stroke 20 / 30 mm\nswash plate  plate + follower  rotary → linear, multi-follower  Ø40 at 10 / 15 / 20°\neccentric  disc + split strap  rotary → linear, high load  e 3 / 5 / 8 mm (stroke 6 / 10 / 16)\nGeneva  wheel + drive  continuous → intermittent  4 / 6 stations, R30\nfour-bar links  7 lengths  rockers, parallel motion, Ross yoke  20–100 mm centres\nbell crank  30×30, 20×40  redirect / lever 90°  Ø3.2 pins\nrack  m1 100, m2 150  rotary ↔ linear with the gears  module 1 / 2\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Wikipedia: Scotch yoke — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Scotch_yoke\n- Wikipedia: Swashplate — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Swashplate\n- Wikipedia: Geneva drive — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Geneva_drive\n- Wikipedia: Four-bar linkage — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Four-bar_linkage\n- Wikipedia: Eccentric (mechanism) — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Eccentric_(mechanism)\n- Long Earth Society: Engine and rig builds on extrusion frames — web, video — LES site — https://longearthsociety.com/engineering/\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 27 parts): https://xemstudio.com/files/hardware/motion-mechanisms/index.json\nAPI: https://xemstudio.com/api/geometry?object=motion-mechanisms\n",
 "sources": [
  {
   "publisher": "Wikipedia",
   "title": "Scotch yoke",
   "url": "https://en.wikipedia.org/wiki/Scotch_yoke",
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    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
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  {
   "publisher": "Wikipedia",
   "title": "Swashplate",
   "url": "https://en.wikipedia.org/wiki/Swashplate",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
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  {
   "publisher": "Wikipedia",
   "title": "Geneva drive",
   "url": "https://en.wikipedia.org/wiki/Geneva_drive",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Four-bar linkage",
   "url": "https://en.wikipedia.org/wiki/Four-bar_linkage",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Eccentric (mechanism)",
   "url": "https://en.wikipedia.org/wiki/Eccentric_(mechanism)",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Long Earth Society",
   "title": "Engine and rig builds on extrusion frames",
   "url": "https://longearthsociety.com/engineering/",
   "formats": [
    "web",
    "video"
   ],
   "license": "LES site",
   "license_verified": true,
   "hosted": false
  }
 ],
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 },
 "related": [
  "spur-gears-involute",
  "crank-conrod-flywheel-kit",
  "ratchet-and-pawl",
  "ball-bearings-metric",
  "dowel-pins-metric",
  "shaft-couplers",
  "t-slot-brackets-and-nuts"
 ],
 "faq": [
  {
   "q": "What is a Scotch yoke and when is it better than a con-rod?",
   "a": "A pin in a slot: the slider moves as a pure sine of crank angle with no connecting rod, so it is shorter and has no angularity; the pin bearing takes side load, so put a 623 on it. Good for displacers and pumps."
  },
  {
   "q": "How much stroke does a swash plate give?",
   "a": "Stroke = 2 × r × sin θ for a follower at radius r on a plate tilted θ: Ø40 plate, follower at r = 18, 15° → 9.3 mm. Several followers around the plate run from one shaft."
  },
  {
   "q": "What is the stroke of an eccentric?",
   "a": "Twice the offset: a 5 mm eccentric gives a 10 mm stroke. The split strap goes over the disc and the rod takes the motion."
  },
  {
   "q": "What centre distance does a Geneva mechanism need?",
   "a": "c = R / cos(π/n): for a 6-station wheel of R30, 34.6 mm; the drive pin sits at R·tan(π/n) = 17.3 mm from the drive centre."
  }
 ],
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   "stl": "https://xemstudio.com/files/hardware/motion-mechanisms/rack-m1-100.stl",
   "note": "rack, module 1, 100 mm, 8 high × 8 wide, three M3 mounting holes; meshes with the module 1 gears"
  },
  {
   "file": "rack-m2-150",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/motion-mechanisms/rack-m2-150.step",
   "stl": "https://xemstudio.com/files/hardware/motion-mechanisms/rack-m2-150.stl",
   "note": "rack, module 2, 150 mm"
  }
 ],
 "markdown": "https://xemstudio.com/objects/motion-mechanisms/index.md"
}