Movement No. 135 demonstrates a refined version of the triangular eccentric (No. 91), used in the steam engine of the Paris Mint. A circular disk carries a triangular tappet in the form of a true Reuleaux Triangle — a curved-sided equilateral shape of constant width. As the disk rotates, the Reuleaux Triangle tappet pushes against the valve-rod, imparting alternating rectilinear motion. The unique geometry of the Reuleaux Triangle creates a characteristic motion profile: at the completion of each stroke, the valve momentarily comes to rest, then is rapidly snapped across to the end of the next stroke. This dwell-and-snap action is ideal for steam distribution valves, where the valve must hold position briefly at each end to allow steam to fully enter or exhaust, then switch quickly to minimize transition time. As a true positive-return cam, the mechanism drives the valve positively in both directions without requiring a return spring.
135. Modification of the triangular eccentric 91, used on the steam engine in the Paris Mint. The circular disk behind carries the triangular tappet, which communicates an alternate rectilinear motion to the valve-rod. The valve is at rest at the completion of each stroke for an instant, and is pushed quickly across the steam-ports to the end of the next.