The four-bar linkage is illustrated in model C-1. The model D-5 serves to illustrate the fact that the relative motion between any two links in a 4-bar closed chain can be made equivalent to the rolling of one part on another. The two rolling surfaces upon which the two links are attached are called the centrodes of the pair of links. Viewed from the ‘back’ of model D-5, one can see the motion of the four-bar linkage, similar to C-1, when one of the links is rotated by the geared motion. From the front one can see two non-circular discs roll on one another; one disc is fixed to one of the links of the 4-bar, while the other disc is geared to the motion of the rotating crank linkage on the 4-bar. These two discs represent the so-called reduced centrodes of the linkage.

The idea of centrodes or equivalent rolling surfaces is used today in the design of biomechanical prostheses for joint replacement.

Francis Moon 2004-08-01