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Primary sourceDocumentOctober 1781

Sun and Planet Gear Patent

James Watt · October 1781

A record of Watt's 1781 patent for the sun and planet gear, which turned the engine's back-and-forth motion into rotary motion.

Watt patented this gear mechanism in his own name, showing his role in protecting and commercialising engineering innovations.

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Close-up photo of a red metal frame with two meshing gears: a blue-grey gear at top and a larger white gear below, with a shaft and bolts visible. Appears to be a Meccano model.
Sun and planet gear, patented by James Watt, 1781.

About this source

What it is

This document records the sun and planet gear, a mechanism for converting the reciprocating motion of a steam engine beam into rotary motion. It was invented by William Murdoch, an employee of Boulton and Watt, but patented by James Watt in October 1781.

Why it matters

Watt patented the gear to get around a crank patent already held by James Pickard. The mechanism let steam engines drive rotating machinery, opening the way for engines to be used in mills and factories rather than just pumping water from mines, a key step in the Industrial Revolution.

Look closer

  • Why would Watt want to avoid using a simple crank, even though it was a well-known solution?
  • What does it tell you about invention that an employee, Murdoch, came up with the idea but Watt took the patent?
  • How did converting motion to rotary power change what steam engines could be used for?

This document is part of the curated material that grounds the AI version of James Watt on Humy.ai — when students ask about related events, the AI can quote it and cite it.

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