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Eccentric spur gear and pinion on a swinging arm (variable speed)

How does an eccentric gear give a varying speed? · Hardware · basis: Xem Studio design following Brown No. 222 (1868)

Also called: eccentric gear drive, variable speed gear, off centre gear and pinion, engranaje excéntrico y piñón, Exzenterrad mit Ritzel

Exact dimensions. True size, with no printing tolerance added.

sizes
4 in table

Notes

  • A 40-tooth gear turns on a shaft 5 mm off its centre; a 20-tooth pinion on a swinging arm follows it, held at the exact 30 mm centre distance by a link from the gear's centre stud. The pinion's speed varies each turn as its contact point moves nearer and farther from the shaft: between about 0.75 and 1.25 of the mean 2 : 1 ratio.
  • Drawn at 60° of shaft rotation: gear centre at (2.5, 4.33), pinion at (27.9, 20.29), arm pivot at (50, 0). Module 1, teeth in mesh with no backlash drawn.
  • Brown's No. 222 (eccentric wheel with a link in place of the groove of No. 221).
  • Parts (each has its own page): eccentric-spur-gear-m1-40t-e5-b5, spur-gears-module-1, swing-arm-30-holes-3-and-6, dowel-pins-metric.
  • Brown's movement No. 222 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

Eccentric spur gear and pinion on a swinging arm size chart

parametervalue
gear40 T, bore 5 mm off centre
pinion20 T on a 30 mm arm
link30 mm, centre to centre
speed swing0.75 – 1.25 × mean

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

Henry T. Brown (1868, public domain)Five Hundred and Seven Mechanical Movements — scan on archive.orgwebPublisher's page; dimensions are facts
507movements.comBrown's movement No. 222: figure, his text and an animationwebPublisher's page; dimensions are facts

Free files

★ Free files 1 parts. Xem Studio design: free to use, print and sell things from; not to be republished as files. Xem Studio Design License. index.json

eccentric-gear-and-pinion-mechanism-assemblyAssembled mechanism, all parts placed at one pose (STEP keeps each part separate)STEP · STL
Let your AI pull this. Add https://xemstudio.com/mcp to Claude, ChatGPT, Cursor or any MCP client (free, no account, one minute) and it can look up exact sizes, mating fits and free STEP/STL from every chat.
Set it up →

Plain text: sheet.txt · JSON: /api/geometry?object=eccentric-gear-and-pinion-mechanism

Eccentric spur gear and pinion on a swinging arm (variable speed), isometric render of a generated part
Render of eccentric-gear-and-pinion-mechanism-assembly from the free STEP; not to scale.

Fits with

Eccentric spur gear, module 1, 40 T, Ø5 bore 5 mm off centre, Ø3 centre stud hole, isometric render of a generated part
Eccentric spur gear, module 1, 40 T, Ø5 bore 5 mm off centre, Ø3 centre stud hole41.99×41.99×8.0 mm★ 1 file
Spur gears, module 1: 10 to 80 teeth (20° involute), isometric render of a generated part
Spur gears, module 1: 10 to 80 teeth (20° involute)26 sizes★ 26 files
Swing arm / link 30 mm, Ø3 hole one end, Ø6 the other, isometric render of a generated part
Swing arm / link 30 mm, Ø3 hole one end, Ø6 the other40.0×10.0×5.0 mm★ 1 file
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325), isometric render of a generated part
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)5 sizes★ 23 files

Questions

How does an eccentric gear give a varying speed?

A 40-tooth gear turns on a shaft 5 mm off its centre; a 20-tooth pinion on a swinging arm follows it, held at the exact 30 mm centre distance by a link from the gear's centre stud. The pinion's speed varies each turn as its contact point moves nearer and farther from the shaft: between about 0.75 and 1.25 of the mean 2 : 1 ratio.