Variable crank: a spiral-slotted plate moves the pin along a radial slot — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/variable-crank-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 94 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- 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.

parameter  value
plates  2 × Ø80 × 6
pin radius  8 – 34 mm (drawn 20)
adjustment  26 mm per turn of the spiral

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- 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
- 507movements.com: Brown's movement No. 94: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_094.html

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/variable-crank-mechanism/index.json
API: https://xemstudio.com/api/geometry?object=variable-crank-mechanism