Watt’s way around a patent

The Science Museum Group holds a model of sun and planet gearing made by James Watt, 1782–1784, demonstrating the action designed to produce a rotative motion. Science Museum Group: sun and planet

James Watt was working on the crank and connecting-rod as a way of converting an engine beam’s reciprocating motion into circular motion when he was forestalled by a patent taken out in 1780 by James Pickard. Science Museum Group: sun and planet Watt, instead of contesting the patent, decided to adopt other mechanism, and patented several such mechanisms in 1781. Science Museum Group: sun and planet

Of those mechanisms, only the sun and planet gear came into use. Science Museum Group: sun and planet It is an epicyclic gear, with a sun wheel keyed on the end of the fly-wheel shaft and a planet wheel bolted to the connecting-rod end. Science Museum Group: sun and planet

The planet wheel is held in gear with the sun wheel by a link loose on their centres, or by a pin in the centre of the planet wheel moving in an annular slot. Science Museum Group: sun and planet

When both wheels are of the same diameter, the fly-wheel makes twice the number of revolutions that it would if actuated by a crank and connecting-rod. Science Museum Group: sun and planet The extra revolution is due to the fact that the planet wheel, although going round in a circle, does not revolve on its own axis. Science Museum Group: sun and planet

In some of Watt’s engines the diameter of the planet wheel was less than that of the sun wheel, and shaft speeds were then only about 50 per cent greater than they would have been with a simple crank. Science Museum Group: sun and planet

The drawing is our own, simplified and not to scale. The measurements and the twice-the-crank rule come from the Science Museum Group’s model description.