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Cartwright's parallel motion (twin gears and cranks)

How does Cartwright's parallel motion keep a rod straight? · Hardware · basis: Xem Studio design following Brown No. 328 (1868)

Also called: cartwright parallel motion, cartwright straight line linkage, twin crank straight line, movimiento paralelo de Cartwright, Cartwrightsche Geradführung

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

sizes
4 in table

Notes

  • Two equal gears turn in opposite directions, each with a crank; equal connecting rods join the cranks to the two ends of a crosshead. By symmetry the crosshead's centre can only move on the line midway between the gears: an exact straight line.
  • Here: module 1, 30 T gears at 30 mm, cranks 10 mm, rods 60 mm, crosshead pins 30 mm apart; at 35° the crosshead centre is 65.17 mm above the gear centres.
  • Brown's No. 328 (Dr. Cartwright, 1787).
  • Parts (each has its own page): crank-gear-m1-30t-pin-r10, cartwright-crosshead-30, four-bar-links, dowel-pins-metric.
  • Brown's movement No. 328 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

Cartwright's parallel motion size chart

parametervalue
gears2 × 30 T, module 1, 30 mm centres
crank radius10 mm
rods60 mm (four-bar link)
crosshead pins30 mm apart

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. 328: 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

cartwright-parallel-motion-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=cartwright-parallel-motion-mechanism

Cartwright's parallel motion (twin gears and cranks), isometric render of a generated part
Render of cartwright-parallel-motion-mechanism-assembly from the free STEP; not to scale.

Fits with

Crank gear, module 1, 30 T with a Ø3 crank-pin hole at 10 mm, isometric render of a generated part
Crank gear, module 1, 30 T with a Ø3 crank-pin hole at 10 mm31.99×31.89×8.0 mm★ 1 file
Cartwright crosshead bar, 30 mm pin spacing, centre rod hole, isometric render of a generated part
Cartwright crosshead bar, 30 mm pin spacing, centre rod hole40.0×10.0×5.0 mm★ 1 file
Four-bar links: flat links with 20 to 100 mm hole centres, isometric render of a generated part
Four-bar links: flat links with 20 to 100 mm hole centres7 sizes★ 7 files
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 Cartwright's parallel motion keep a rod straight?

Two equal gears turn in opposite directions, each with a crank; equal connecting rods join the cranks to the two ends of a crosshead. By symmetry the crosshead's centre can only move on the line midway between the gears: an exact straight line.