11.7  Parallelograms, pantographs and scissors

The parallelogram linkage keeps a part parallel to itself: the drawing lamp, the tool box tray, the bus door, the parallel jaws of a gripper, and its change point, where it can fold into an antiparallelogram. The pantograph scales a motion and the lazy tongs multiply it. The scissor lift, whose actuator force is in Virtual work, Example 2, grows without bound as the lift folds flat: the toggle of Over-centre mechanisms in the opposite role, with the actuator on the weak side, and the reason lifts start their actuators at an angle. Example: the forces in a printed scissor jack.