Sun-and-planet gear (Watt's crank substitute), 30 : 30 — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/sun-and-planet-gear-mechanism/  (CC BY 4.0)
Basis: Xem Studio design following Brown No. 39 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- 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.

parameter  value
sun / planet  30 T / 30 T, module 1
arm centres  30 mm
sun turns per rod cycle  2
rod  100 mm

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. 39: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_039.html

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