Link 1 of four-bar linkage ADCB turns about fixed axis A. In the position shown by continuous lines in the drawing, points A, D and C lie on a straight line and the mechanism, due to the action of spring 4, is locked. Upon depressing pedal a of link 1 the mechanism is put into its upper position AD'C'B which is shown by dash lines. To return link 1 to its initial locked position, force is applied to its other end. Lug b of lever 3 limits the downward motion of link 1.
$552$LW,4L$