Movement No. 39 presents one of the most historically celebrated gear mechanisms in the entire history of engineering — the Sun-and-Planet gear system, invented and famously employed by James Watt as a substitute for the crank in his steam engines. The story behind this mechanism is as fascinating as the mechanism itself: when Watt sought to convert the reciprocating (back-and-forth) motion of his steam engine piston into continuous rotary motion, the obvious solution was a simple crank. However, the crank had been patented by another inventor, James Pickard, leaving Watt legally barred from using it. Rather than pay royalties, Watt — together with his business partner Matthew Boulton and engineer William Murdoch — devised this elegant alternative. The mechanism consists of two spur gears: the larger sun gear at the center, and the smaller planet gear which is connected to the sun gear’s shaft by a rigid arm that maintains a constant distance between the two gear centers. The planet gear meshes with the outside of the sun gear. As the connecting rod from the piston pushes and pulls the arm carrying the planet gear, the planet gear rolls around the outside of the stationary sun gear — and because the planet is constrained to orbit the sun, this orbital motion is converted into rotation of the output shaft. A notable kinematic property of this arrangement is that if both gears are of equal size, the output shaft rotates twice for every complete orbit of the planet gear — giving twice the rotational speed compared to a simple crank of the same stroke length. Watt’s Sun-and-Planet gear is a landmark example of how engineering constraints can inspire creative and superior solutions.
39. Sun-and-planet motion. The spur-gear to the right, called the planet-gear, is tied to the center of the other, or sun-gear, by an arm which preserves a constant distance between their centers. This was used as a substitute for the crank in a steam engine by James Watt, after the use of the crank had been patented by another party. Each revolution of the planet-gear, which is rigidly attached to the connecting-rod, gives two to the sun-gear, which is keyed to the fly-wheel shaft.