Movement No. 91 features a triangular eccentric — a variation of the standard circular eccentric — used to produce intermittent reciprocating rectilinear motion. Instead of a circular disk, a triangular eccentric profile rotates within a surrounding strap or yoke. Due to the flat sides of the triangle, the output rod remains stationary for a portion of each revolution as a flat face passes through the strap, then is driven forward as a corner of the triangle engages and pushes the strap. This dwell-and-advance characteristic makes it particularly suitable for steam engine valve timing, where the valve must remain open or closed for a defined period. This design was notably used in France for controlling steam distribution in engine cylinders.
91. Triangular eccentric, giving an intermittent reciprocating rectilinear motion, used in France for the valve motion of steam engines.