Sun and Planet Wheels, first devised by James Watt to avoid the use of a crank, which was patented. In his device the epicyclic train arm was replaced by the stationary groove, shown as green, which kept the two wheels in gear. The yellow circle represents the engine shaft, to which the red gear was fastened, purple connecting rod attached to walking beam (here it is attached to the piston). The blue gear was rigidly attached to the end of the connecting rod. While with such an arrangement it is not strictly true that the blue gear does not turn. Since connecting rod rigidly attached to blue gear, the blue gear oscillates while it is hard to see this oscillation. At the same time, the red gear does not rotate smoothly. It's rotation also oscillates around a smooth rotation.

The yellow engine shaft and red gear will make two turns every time the blue gear passes around it.