Input: the small center gear receiving rotation from a motor.
The violet plate with a lever carries 4 gears and two rollers. The rollers alternately contact the yellow disc (its inside wall) and give the screw reciprocating rotation. The lever has three positions corresponding with up, down and dwell of the blue nut-slider motion.
Be noted that the violet lever represents a multi-bar mechanism used in practice.
There is a brake to keep the disc immobile during its dwell (not shown).