Offset cranks with slotted arm (variable-speed coupling, Whitworth principle) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/offset-slotted-crank-mechanism/  (CC BY 4.0)
Basis: Xem Studio design following Brown No. 220 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- 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.

parameter  value
shaft offset e  5 mm
pin radius r  12 mm
speed range  0.71 – 1.71 × input

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. 220: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_220.html

Free files (CC BY 4.0, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/offset-slotted-crank-mechanism/index.json
API: https://xemstudio.com/api/geometry?object=offset-slotted-crank-mechanism