The sun and planet gear was a method of converting vertical motion to rotary motion and utilised a reciprocating steam engine. It was invented by the Scottish engineer William Murdoch, an employee of Boulton and Watt, but was patented by James Watt in October 1781. It played an important part in the development and growth of the steam locomotive and other devices for rotative motion in the Industrial Revolution.

The sun and planet gear converted the vertical motion of a beam, driven by a small steam engine, into circular motion using a 'planet,' a cogwheel fixed at the end of the pumping rod (connected to the beam) of the engine. With the motion of the beam, this revolved around, and turned, the 'sun,' a larger rotating cog which turned the drive shaft, thus generating rotary motion. An interesting feature of this arrangement,when compared to that of a simple crank,is that when both sun and planet have the same number of teeth,the drive shaft completes two revolutions for each stroke of the beam and not a 1:1 ratio as expected.

See also

Gears | Scottish inventions