Xem Studiomeasured geometry

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Variable crank: a spiral-slotted plate moves the pin along a radial slot

How can the throw of a crank be changed while it runs? · Hardware · basis: Xem Studio design following Brown No. 94 (1868)

Also called: variable crank, adjustable stroke crank, spiral crank, manivela variable, verstellbare Kurbel

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

sizes
3 in table

Notes

  • Two plates on the crank shaft: one with a radial slot, one with a spiral slot. The crank pin passes through both. Turning one plate against the other moves the pin in or out along the radial slot, changing the crank radius (and the stroke) without stopping.
  • Here Ø80 plates, slots 6 mm, spiral rising 26 mm per turn (R8 → R34): 0.072 mm of radius per degree. Drawn with the pin at 20 mm (40 mm stroke), the spiral plate turned 166.15°.
  • Brown's No. 94 (variable crank).
  • Parts (each has its own page): variable-crank-disc-radial-slot-80, variable-crank-disc-spiral-slot-80, dowel-pins-metric, linear-shafts-h6.
  • Brown's movement No. 94 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

Variable crank: a spiral-slotted plate moves the pin along a radial… size chart

parametervalue
plates2 × Ø80 × 6
pin radius8 – 34 mm (drawn 20)
adjustment26 mm per turn of the spiral

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. 94: 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

variable-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.
Set it up →

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

Variable crank: a spiral-slotted plate moves the pin along a radial slot, isometric render of a generated part
Render of variable-crank-mechanism-assembly from the free STEP; not to scale.

Fits with

Variable-crank disc Ø80 × 6 with a 6 mm radial slot (R8 → R34), isometric render of a generated part
Variable-crank disc Ø80 × 6 with a 6 mm radial slot (R8 → R34)80.0×80.0×6.0 mm★ 1 file
Variable-crank disc Ø80 × 6 with a 6 mm Archimedean spiral slot (R8 → R34 in one turn), isometric render of a generated part
Variable-crank disc Ø80 × 6 with a 6 mm Archimedean spiral slot (R8 → R34 in one turn)80.0×80.0×6.0 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
Linear shafts Ø3 to Ø25 mm (h6), 100 to 1000 mm long, isometric render of a generated part
Linear shafts Ø3 to Ø25 mm (h6), 100 to 1000 mm long10 sizes★ 70 files

Questions

How can the throw of a crank be changed while it runs?

Two plates on the crank shaft: one with a radial slot, one with a spiral slot. The crank pin passes through both. Turning one plate against the other moves the pin in or out along the radial slot, changing the crank radius (and the stroke) without stopping.