{
 "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": "eccentric-back-gear-mechanism",
 "name": "Gear thrown in and out of mesh by an eccentric bush (lathe back gear)",
 "kind": "hardware",
 "category": "Mechanisms",
 "group": "Gears",
 "year": null,
 "question": "How does an eccentric bush engage a lathe's back gear?",
 "tags": [
  "mechanism",
  "gears",
  "kinematics",
  "507 mechanical movements"
 ],
 "aliases": [
  "back gear",
  "eccentric bush",
  "gear engagement lever",
  "engranaje de retroceso con casquillo excéntrico",
  "Rädervorgelege mit Exzenterbuchse"
 ],
 "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/eccentric-back-gear-mechanism/",
 "sheet_txt": "https://xemstudio.com/objects/eccentric-back-gear-mechanism/sheet.txt",
 "sheet": {
  "basis": "Xem Studio design following Brown No. 56 (1868)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The pinion's axle runs in a bush whose bore is off centre; the bush turns in a fixed housing. With its lever one way the axle sits at the exact centre distance and the gears mesh; half a turn of the lever moves the axle away by twice the eccentricity and the pinion comes clear.",
   "Here module 1, 40 and 20 teeth (catalog gears) at 30 mm; housing bore 32.5 mm from the spindle, bush eccentricity 2.5 mm: engaged at 30 mm, thrown out to 35 mm (2.75 mm clear of the 2.25 mm tooth depth). Drawn engaged.",
   "Brown's No. 56 (lever moving a lathe's back gear in and out of gear by an eccentric).",
   "Parts (each has its own page): spur-gears-module-1, eccentric-bush-16-bore6-e2-5-lever, back-gear-housing-plate-70x40, dowel-pins-metric.",
   "Brown's movement No. 56 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size."
  ],
  "table": {
   "columns": [
    "parameter",
    "value"
   ],
   "rows": [
    [
     "gears",
     "40 / 20 T, module 1"
    ],
    [
     "eccentricity",
     "2.5 mm"
    ],
    [
     "engaged",
     "30 mm centres"
    ],
    [
     "out",
     "35 mm"
    ]
   ]
  }
 },
 "sheet_text": "Gear thrown in and out of mesh by an eccentric bush (lathe back gear) — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/eccentric-back-gear-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)\nBasis: Xem Studio design following Brown No. 56 (1868)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The pinion's axle runs in a bush whose bore is off centre; the bush turns in a fixed housing. With its lever one way the axle sits at the exact centre distance and the gears mesh; half a turn of the lever moves the axle away by twice the eccentricity and the pinion comes clear.\n- Here module 1, 40 and 20 teeth (catalog gears) at 30 mm; housing bore 32.5 mm from the spindle, bush eccentricity 2.5 mm: engaged at 30 mm, thrown out to 35 mm (2.75 mm clear of the 2.25 mm tooth depth). Drawn engaged.\n- Brown's No. 56 (lever moving a lathe's back gear in and out of gear by an eccentric).\n- Parts (each has its own page): spur-gears-module-1, eccentric-bush-16-bore6-e2-5-lever, back-gear-housing-plate-70x40, dowel-pins-metric.\n- Brown's movement No. 56 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.\n\nparameter  value\ngears  40 / 20 T, module 1\neccentricity  2.5 mm\nengaged  30 mm centres\nout  35 mm\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. 56: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_056.html\n\nFree files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/index.json\nAPI: https://xemstudio.com/api/geometry?object=eccentric-back-gear-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. 56: figure, his text and an animation",
   "url": "https://507movements.com/mm_056.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": [
  "spur-gears-module-1",
  "eccentric-bush-16-bore6-e2-5-lever",
  "back-gear-housing-plate-70x40",
  "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 an eccentric bush engage a lathe's back gear?",
   "a": "The pinion's axle runs in a bush whose bore is off centre; the bush turns in a fixed housing. With its lever one way the axle sits at the exact centre distance and the gears mesh; half a turn of the lever moves the axle away by twice the eccentricity and the pinion comes clear."
  }
 ],
 "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": "eccentric-back-gear-mechanism-assembly",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "step": "https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/eccentric-back-gear-mechanism-assembly.step",
   "stl": "https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/eccentric-back-gear-mechanism-assembly.stl",
   "note": "Assembled mechanism, all parts placed at one pose (STEP keeps each part separate)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/eccentric-back-gear-mechanism/index.md"
}