This model contains two special four-bar linkages connected in parallel. Both four-bar linkages contain parallel pairs of opposite links.[One of the links is the grounded structure.] Parallel four-bar linkages contain a singularitiy when the crank and rocker links are aligned. In this position the connecting link can rotate in two directions. To avoid this singularity engineers often connect another parallel four-bar linkage with a crank that is out of phase with the other linkage. Such dual four-bar linkages were often used in early steam railroad engines to drive the wheels. The singular nature of the parallel four-bar was recognized by Leonardo da Vinci in his Codex Madrid I notebook.

Francis Moon 0000-00-00