Movement No. 157 is a modification of Movement No. 156, replacing the slotted bell-crank arrangement with a connecting-rod linkage. Instead of a crank-pin sliding directly in a slot on the bell-crank arm, the crank-pin on the rotating disk is connected to one arm of the bell-crank via a separate connecting-rod. As the disk rotates, the crank-pin traces a circular path and drives the connecting-rod, which pushes and pulls one arm of the bell-crank, causing it to oscillate about its fixed pivot. The other arm of the bell-crank translates this oscillation into alternating rectilinear motion of a connected rod or slide. Substituting a connecting-rod for the slot eliminates the sliding friction between the pin and slot, producing smoother operation and reducing wear — especially at higher speeds. The connecting-rod also introduces a slightly different motion profile compared to the slotted version, offering additional design flexibility for tuning the output motion characteristics.
157. A modification of the movement last described; a connecting-rod being substituted for the slot in the bell-crank.