Input: the green puley shaft.
There is a sliding key between the green shaft and the red hollow shaft of two discs.
The blue disc - screw can contact with the two red discs alternately.
Up and down motion of the yellow slider is controlled by the violet lever that causes the pressure at contact places of the three discs.
Be noted that the violet lever represents a multi-bar mechanism used in practice.
The slider reaches max velocity at lower end of its stroke and min velocity at upper end of its stroke.
The pink stopper on the frame (and a not shown brake) sets the highest position of the slider.