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Offset cranks with slotted arm (variable-speed coupling, Whitworth principle)

How do two offset cranks give a varying speed? · Hardware · basis: Xem Studio design following Brown No. 220 (1868)

Also called: whitworth quick return, offset crank coupling, slotted crank, manivelas descentradas con brazo ranurado, Versetzte Kurbeln mit Schlitzarm

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

sizes
3 in table

Notes

  • Two parallel shafts 5 mm apart: a crank pin on the first slides in a slot on the second. The driven arm runs fast when the pin is nearest its shaft and slow when farthest: speed ratio swings between r/(r − e) and r/(r + e) = 1.71 and 0.71 here.
  • Whitworth's quick return uses exactly this pair (the driven crank then works a slider), with the pin radius larger than the offset so both turn fully round.
  • Brown's No. 220.
  • Parts (each has its own page): crank-disc-36-pin-r12, slotted-crank-arm-30, dowel-pins-metric.
  • Brown's movement No. 220 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

Offset cranks with slotted arm size chart

parametervalue
shaft offset e5 mm
pin radius r12 mm
speed range0.71 – 1.71 × input

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. 220: figure, his text and an animationwebPublisher's page; dimensions are facts

Free files

★ Free files 1 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

offset-slotted-crank-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.
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Plain text: sheet.txt · JSON: /api/geometry?object=offset-slotted-crank-mechanism

Offset cranks with slotted arm (variable-speed coupling, Whitworth principle), isometric render of a generated part
Render of offset-slotted-crank-mechanism-assembly from the free STEP; not to scale.

Fits with

Crank disc Ø36 × 5, Ø5 bore, Ø3 pin hole at 12 mm, isometric render of a generated part
Crank disc Ø36 × 5, Ø5 bore, Ø3 pin hole at 12 mm36.0×36.0×5.0 mm★ 1 file
Slotted crank arm, Ø5 hub, 3 mm slot from 6 to 19 mm, isometric render of a generated part
Slotted crank arm, Ø5 hub, 3 mm slot from 6 to 19 mm37.0×14.0×5.0 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 do two offset cranks give a varying speed?

Two parallel shafts 5 mm apart: a crank pin on the first slides in a slot on the second. The driven arm runs fast when the pin is nearest its shaft and slow when farthest: speed ratio swings between r/(r − e) and r/(r + e) = 1.71 and 0.71 here.