Model: T01 Parallel Mechanisms: Single and Compound Chains
This model contains two illustrations of linkages that produce motions that keep a link parallel to itself. One example on the right is a simple four-bar linkage forming a rhombus. Since the opposite links are of equal length, any motion of the link opposite the grounded link remains parallel to itself.

In the second linkage on the left, two four-bar kinematic circuits (or chains) are coupled into a compound mechanism in which the outer link remains parallel to both the grounded link as well as the intermediate vertical link. Parallel linkages are often used in conjunction with other mechanisms to translate motion from one position to another in the machine as in James Watt’s parallel motion linkage for the steam engine (1784) that was coupled to his straight-line mechanism (Model S-24).

Francis Moon 2004-07-00