Lever 1 oscillates about fixed axis A through a constant angle. Pawl 2 is connected by turning pair B to lever 1 and engages the teeth of ratchet wheel 3, turning it intermittently counterclockwise about axis A. Mask, or shield, 4 turns about axis A and shields a number of teeth of ratchet wheel 3, lifting pawl 2 out of engagement for a part of its stroke. The angle of rotation of ratchet wheel 3 to each stroke of lever 1 can be varied by turning shield 4 to cover more or less teeth in the path of pawl 2. Shield 4 is held in the adjusted position by pin a which enters the required hole b in a fixed plate.
$2700$RG,4L$