Parallel Motion Linkage

The Parallel Motion Linkage is a special case of Four Bar Chain in which the equal in lengths of opposite sides of the parallelogram ensure that input and output motions remain parallel.

There are several good examples of these in action. The Pantograph is a parallel motion linkage used by electric trains to pick up power from overhead cables. Another use is the Pantograph used in drawing to copy sketches. Other uses are in containers such as tool boxes and sewing boxes which open up to reveal multiple shelves and containers which need to be held horizontal.

James Watt used a Parallel Motion Linkage to ensure the movements of both the piston and the pump rod in his Beam Engines remained parallel.