Movement No. 117 demonstrates a cam-and-roller yoke mechanism used to drive the valve of a steam engine. A profiled cam rotates continuously on a shaft and is positioned between two friction rollers mounted inside a yoke. As the cam rotates, its varying profile pushes against first one roller then the other, driving the yoke back and forth in a reciprocating rectilinear motion. The two rollers ensure that the cam maintains positive contact in both directions — one roller is pushed as the cam lobe advances, and the other limits the return — eliminating backlash and side play. The yoke, connected to a valve rod, transmits this precise reciprocating motion to the steam engine valve, controlling the timing of steam admission and exhaust. This positive-drive arrangement provides reliable, low-wear valve actuation even at high operating speeds.
117. A cam acting between two friction-rollers in a yoke. Has been used to give the movement to the valve of a steam engine.