Worm 1, keyed to shaft 4, rotates about fixed axis A-A and meshes with worm wheel 2 which rotates about fixed axis B. Shaft 4 can also slide axially in fixed guides d-d. Lever 3 turns about fixed axis C and has pin a which engages slot b of shaft 4. The angular displacement of wheel 2 can be the sum of two rotary motions: that obtained from worm 1 as it rotates about axis A-A and that due to the axial travel of the worm. In the latter case, worm 1 is traversed by lever 3 and its operation is that of a gear rack.
$2831$WG,MO$