{
 "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": "eccentric-and-strap",
 "name": "Eccentric and strap: Ø30 eccentrics at 3, 5 and 8 mm offset with a split strap",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Drives & mechanisms",
 "year": null,
 "question": "How do I make an eccentric and strap drive with 3D printed parts?",
 "tags": [
  "eccentric",
  "strap",
  "sheave",
  "cam",
  "rotary to linear",
  "mechanism",
  "motion",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "eccentric strap 3d print",
  "eccentric sheave stl",
  "eccentric cam follower",
  "eccentric drive mechanism"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 4,
 "url": "https://xemstudio.com/objects/eccentric-and-strap/",
 "sheet_txt": "https://xemstudio.com/objects/eccentric-and-strap/sheet.txt",
 "sheet": {
  "basis": "Eccentric sheave: a disc on a shaft offset e from its centre gives a stroke of 2e; strap fits the Ø30 disc exactly",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "A disc mounted off-centre on the shaft turns inside a strap; the strap's arm moves back and forth with a stroke of twice the offset.",
   "Three discs: 3, 5 and 8 mm offset for 6, 10 and 16 mm stroke. All take a Ø5 shaft and have a Ø2.5 set-screw hole.",
   "The strap is split so it goes over the disc without taking the shaft apart; its 40 mm arm ends in a Ø3 eye 51 mm from the strap's centre.",
   "The load is spread round the whole strap, so it runs quietly; the usual way to drive a pump or a valve."
  ],
  "table": {
   "columns": [
    "offset e",
    "stroke",
    "disc",
    "shaft"
   ],
   "rows": [
    [
     "3 mm",
     "6 mm",
     "Ø30 × 8",
     "Ø5"
    ],
    [
     "5 mm",
     "10 mm",
     "Ø30 × 8",
     "Ø5"
    ],
    [
     "8 mm",
     "16 mm",
     "Ø30 × 8",
     "Ø5"
    ]
   ]
  }
 },
 "sheet_text": "Eccentric and strap: Ø30 eccentrics at 3, 5 and 8 mm offset with a split strap — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/eccentric-and-strap/  (CC BY 4.0)\nBasis: Eccentric sheave: a disc on a shaft offset e from its centre gives a stroke of 2e; strap fits the Ø30 disc exactly\nCatalog version: 2026-10-20\n\nSizes: see table below\n- A disc mounted off-centre on the shaft turns inside a strap; the strap's arm moves back and forth with a stroke of twice the offset.\n- Three discs: 3, 5 and 8 mm offset for 6, 10 and 16 mm stroke. All take a Ø5 shaft and have a Ø2.5 set-screw hole.\n- The strap is split so it goes over the disc without taking the shaft apart; its 40 mm arm ends in a Ø3 eye 51 mm from the strap's centre.\n- The load is spread round the whole strap, so it runs quietly; the usual way to drive a pump or a valve.\n\noffset e  stroke  disc  shaft\n3 mm  6 mm  Ø30 × 8  Ø5\n5 mm  10 mm  Ø30 × 8  Ø5\n8 mm  16 mm  Ø30 × 8  Ø5\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\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, 4 parts): https://xemstudio.com/files/hardware/eccentric-and-strap/index.json\nAPI: https://xemstudio.com/api/geometry?object=eccentric-and-strap\n",
 "sources": [
  {
   "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
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "stirling-shafts-and-rods",
  "shaft-collars-clips-and-shafts",
  "four-bar-links",
  "scotch-yoke",
  "swash-plate"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "stirling-shafts-and-rods",
     "why": "Ø5 shaft through the disc"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "four-bar-links",
     "why": "Ø3 eye on the strap arm"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "scotch-yoke",
     "why": "pure sine motion"
    },
    {
     "slug": "crank-conrod-flywheel-kit",
     "why": "crank and con-rod"
    }
   ]
  }
 ],
 "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": "eccentric bore",
   "exact": "Ø5",
   "mates": "Ø5 shaft",
   "fit": "slide",
   "files": "eccentric-d30-e"
  },
  {
   "feature": "set-screw hole",
   "exact": "Ø2.5",
   "mates": "M3 tap drill",
   "fit": "designed",
   "files": "eccentric-d30-e"
  },
  {
   "feature": "strap bore",
   "exact": "Ø30",
   "mates": "Ø30 eccentric (turns in it)",
   "fit": "run",
   "files": "eccentric-strap-"
  },
  {
   "feature": "strap split",
   "exact": "1.0",
   "mates": "closes on the eccentric",
   "fit": "clamp",
   "files": "eccentric-strap-"
  },
  {
   "feature": "strap eye",
   "exact": "Ø3",
   "mates": "Ø3 pin (turns in it)",
   "fit": "run",
   "files": "eccentric-strap-"
  }
 ],
 "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 is the stroke of an eccentric?",
   "a": "Twice the offset: a 5 mm eccentric gives a 10 mm stroke."
  },
  {
   "q": "Why use an eccentric instead of a crank?",
   "a": "It sits anywhere along a straight shaft with no crank throw, and the strap carries the load all the way round."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "eccentric-d30-e3",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e3.step",
   "stl": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e3.stl",
   "note": "eccentric Ø30, 3 mm offset (6 mm stroke), Ø5 shaft, Ø2.5 set-screw hole"
  },
  {
   "file": "eccentric-d30-e5",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e5.step",
   "stl": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e5.stl",
   "note": "eccentric Ø30, 5 mm offset (10 mm stroke), Ø5 shaft, Ø2.5 set-screw hole"
  },
  {
   "file": "eccentric-d30-e8",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e8.step",
   "stl": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e8.stl",
   "note": "eccentric Ø30, 8 mm offset (16 mm stroke), Ø5 shaft, Ø2.5 set-screw hole"
  },
  {
   "file": "eccentric-strap-d30",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-strap-d30.step",
   "stl": "https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-strap-d30.stl",
   "note": "split strap for the Ø30 eccentrics, Ø30 bore, 40 mm arm with a Ø3 eye"
  }
 ],
 "markdown": "https://xemstudio.com/objects/eccentric-and-strap/index.md"
}