Hart's Straight-Line Linkage

Hart's straight-line linkage

In 1874, Hart discovered some linkages that turn rotational motion into straight line motion. The bow-tie shape in this linkage is a contra-parallelogram which is itself an interesting linkage.


  1. Cut out the pieces from the pattern in card stock or boxboard. You can use a manila folder.
  2. Punch out the holes (or use a knife to cut them out if the hole-punch doesn't reach). And use a knife to cut the tabs.
  3. Bend the tabs marked "A" up and curl them in so they will fit through the hole in another piece marked "A." Once the tabs are through the hole, flatten them down again.
  4. Do the same for tab B. Note that these tabs will go down to the underside of the model.
  5. Assemble the rest of the pieces as shown in the diagram.
  6. Hold down the rectangular piece onto the table and move the linkage from point F. As the linkage is moved, the hole in the long bar describes a straight line.


  • The topmost long bar in the model sometimes catches on the bar below it at point B. You can tape a slip of paper over the bar at point B so the long bar can slide over it easier.