Sun and planet gear

[2] It played an important part in the development of devices for rotation in the Industrial Revolution.

[citation needed] The sun and planet gear converted the vertical motion of a beam, driven by a steam engine, into circular motion using a 'planet', a cogwheel fixed at the end of the connecting rod (connected to the beam) of the engine.

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 double stroke of the beam instead of one.

The planet gear is fixed to the connecting rod and thus does not rotate around its own axis.

This link appears, at first sight, to be similar to a crank but the drive is not transmitted through it.

The Whitbread Engine of 1785
Animation of Sun & Planet gear using Meccano . Notice that the sun and flywheel rotate twice for every circuit of the planet when they have a 1:1 ratio of teeth.