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Sun-and-planet gear (Watt's crank substitute), 30 : 30

How does a sun-and-planet gear replace a crank? · Hardware · basis: Xem Studio design following Brown No. 39 (1868)

Also called: sun and planet gear, watt sun and planet, planetary crank, engranaje sol y planeta, Sonnen-und-Planetengetriebe

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

sizes
4 in table

Notes

  • A planet gear fixed to the end of a rocking rod is held in mesh with a sun gear on the output shaft by a free arm: each to-and-fro cycle of the rod carries the planet once round the sun.
  • With equal gears the sun makes two turns per cycle (1 + N_planet/N_sun), which is why it was used: twice the shaft speed of a plain crank.
  • Brown's No. 39 is the arrangement used by Watt as a substitute for the crank. Module 1, 30 + 30 teeth, centre distance 30 mm.
  • Parts (each has its own page): spur-gears-module-1, sun-and-planet-arm-30, sun-and-planet-rod-100, linear-shafts-h6.
  • Brown's movement No. 39 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

Sun-and-planet gear (Watt's crank substitute), 30 : 30 size chart

parametervalue
sun / planet30 T / 30 T, module 1
arm centres30 mm
sun turns per rod cycle2
rod100 mm

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. 39: 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

sun-and-planet-gear-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=sun-and-planet-gear-mechanism

Sun-and-planet gear (Watt's crank substitute), 30 : 30, isometric render of a generated part
Render of sun-and-planet-gear-mechanism-assembly from the free STEP; not to scale.

Fits with

Spur gears, module 1: 10 to 80 teeth (20° involute), isometric render of a generated part
Spur gears, module 1: 10 to 80 teeth (20° involute)26 sizes★ 26 files
Sun-and-planet arm, 30 mm centres, Ø10 holes, isometric render of a generated part
Sun-and-planet arm, 30 mm centres, Ø10 holes48.0×18.0×5.0 mm★ 1 file
Sun-and-planet rod, 100 mm, with Ø10 × 13 planet boss, isometric render of a generated part
Sun-and-planet rod, 100 mm, with Ø10 × 13 planet boss118.0×18.0×18.0 mm★ 1 file
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 does a sun-and-planet gear replace a crank?

A planet gear fixed to the end of a rocking rod is held in mesh with a sun gear on the output shaft by a free arm: each to-and-fro cycle of the rod carries the planet once round the sun.