{
 "version": "2026-10-20",
 "license": "Dimension sheets: CC BY 4.0. Standard and published-size files: CC BY 4.0. Xem Studio's own designs (files_license = Xem Studio Design License): free to use and sell things from, not to redistribute as files — https://xemstudio.com/license/. Other publishers' files: their own license, linked beside them.",
 "slug": "third-system-pulleys-7-to-1-mechanism",
 "name": "Pulleys of the third system, three pulleys (7 : 1)",
 "kind": "hardware",
 "category": "Mechanisms",
 "group": "Pulleys and tackle",
 "year": null,
 "question": "How does the third system of pulleys work?",
 "tags": [
  "mechanism",
  "pulleys and tackle",
  "kinematics",
  "507 mechanical movements"
 ],
 "aliases": [
  "third system of pulleys",
  "spanish burton",
  "7 to 1 pulley",
  "compound pulley",
  "sistema de poleas de tercera clase (7:1)",
  "Rollensystem dritter Art (7:1)"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": false,
 "free_files": 1,
 "url": "https://xemstudio.com/objects/third-system-pulleys-7-to-1-mechanism/",
 "sheet_txt": "https://xemstudio.com/objects/third-system-pulleys-7-to-1-mechanism/sheet.txt",
 "sheet": {
  "basis": "Xem Studio design following Brown No. 21 (1868)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "Every rope ends on the load: rope 1 runs from the load over the fixed pulley to the first runner, rope 2 from the load over that runner to the second, rope 3 from the load over the second runner to the hand. Tensions double up the chain — P, 2P, 4P — and the load gets all three: W = 7P (2ⁿ − 1 with n = 3).",
   "Pitch-Ø80 fixed block, pitch-Ø40 and pitch-Ø20 runners; rope ends on a load bar at x = −40, 20 and 50, whose weight stem sits on the resultant (x = −10).",
   "Brown's No. 21 (a combination following the doubling rule; the single Spanish burton, No. 16, is the same system with two pulleys).",
   "Parts (each has its own page): tackle-beam-240-sockets, tackle-block-40-top-stem, tackle-block-20-top-socket, tackle-block-10-top-socket, rope-sheave-3mm-pitch-80, rope-sheave-3mm-pitch-40, rope-sheave-3mm-pitch-20, ropes-3mm-third-system-pulleys-7-to-1, tackle-weight-d40x40-socket, load-bar-100-sockets-m40-20-50, dowel-pins-metric.",
   "Brown's movement No. 21 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size."
  ],
  "table": {
   "columns": [
    "parameter",
    "value"
   ],
   "rows": [
    [
     "fixed block",
     "pitch Ø80"
    ],
    [
     "runners",
     "pitch Ø40 and Ø20"
    ],
    [
     "rope tensions",
     "4P, 2P, P"
    ],
    [
     "mechanical advantage",
     "7"
    ]
   ]
  }
 },
 "sheet_text": "Pulleys of the third system, three pulleys (7 : 1) — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/third-system-pulleys-7-to-1-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)\nBasis: Xem Studio design following Brown No. 21 (1868)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- Every rope ends on the load: rope 1 runs from the load over the fixed pulley to the first runner, rope 2 from the load over that runner to the second, rope 3 from the load over the second runner to the hand. Tensions double up the chain — P, 2P, 4P — and the load gets all three: W = 7P (2ⁿ − 1 with n = 3).\n- Pitch-Ø80 fixed block, pitch-Ø40 and pitch-Ø20 runners; rope ends on a load bar at x = −40, 20 and 50, whose weight stem sits on the resultant (x = −10).\n- Brown's No. 21 (a combination following the doubling rule; the single Spanish burton, No. 16, is the same system with two pulleys).\n- Parts (each has its own page): tackle-beam-240-sockets, tackle-block-40-top-stem, tackle-block-20-top-socket, tackle-block-10-top-socket, rope-sheave-3mm-pitch-80, rope-sheave-3mm-pitch-40, rope-sheave-3mm-pitch-20, ropes-3mm-third-system-pulleys-7-to-1, tackle-weight-d40x40-socket, load-bar-100-sockets-m40-20-50, dowel-pins-metric.\n- Brown's movement No. 21 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.\n\nparameter  value\nfixed block  pitch Ø80\nrunners  pitch Ø40 and Ø20\nrope tensions  4P, 2P, P\nmechanical advantage  7\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Henry T. Brown (1868, public domain): Five Hundred and Seven Mechanical Movements — scan on archive.org — web — Publisher's page; dimensions are facts — https://archive.org/details/fivehundredseven00browiala\n- 507movements.com: Brown's movement No. 21: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_021.html\n\nFree files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/third-system-pulleys-7-to-1-mechanism/index.json\nAPI: https://xemstudio.com/api/geometry?object=third-system-pulleys-7-to-1-mechanism\n",
 "sources": [
  {
   "publisher": "Henry T. Brown (1868, public domain)",
   "title": "Five Hundred and Seven Mechanical Movements — scan on archive.org",
   "url": "https://archive.org/details/fivehundredseven00browiala",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "507movements.com",
   "title": "Brown's movement No. 21: figure, his text and an animation",
   "url": "https://507movements.com/mm_021.html",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "tackle-beam-240-sockets",
  "tackle-block-40-top-stem",
  "tackle-block-20-top-socket",
  "tackle-block-10-top-socket",
  "rope-sheave-3mm-pitch-80",
  "rope-sheave-3mm-pitch-40",
  "rope-sheave-3mm-pitch-20",
  "ropes-3mm-third-system-pulleys-7-to-1",
  "tackle-weight-d40x40-socket",
  "load-bar-100-sockets-m40-20-50",
  "dowel-pins-metric"
 ],
 "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": [],
 "faq": [
  {
   "q": "How does the third system of pulleys work?",
   "a": "Every rope ends on the load: rope 1 runs from the load over the fixed pulley to the first runner, rope 2 from the load over that runner to the second, rope 3 from the load over the second runner to the hand. Tensions double up the chain — P, 2P, 4P — and the load gets all three: W = 7P (2ⁿ − 1 with n = 3)."
  }
 ],
 "files_license": {
  "name": "Xem Studio Design License",
  "holder": "Xem Studio",
  "url": "https://xemstudio.com/license/",
  "note": "Xem Studio's own design: free to use, print, modify and sell things made from it; not to be republished or redistributed as files."
 },
 "offers": [
  {
   "file": "third-system-pulleys-7-to-1-mechanism-assembly",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "step": "https://xemstudio.com/files/hardware/third-system-pulleys-7-to-1-mechanism/third-system-pulleys-7-to-1-mechanism-assembly.step",
   "stl": "https://xemstudio.com/files/hardware/third-system-pulleys-7-to-1-mechanism/third-system-pulleys-7-to-1-mechanism-assembly.stl",
   "note": "Assembled mechanism, all parts placed at one pose (STEP keeps each part separate)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/third-system-pulleys-7-to-1-mechanism/index.md"
}