Cam 1 turns about fixed axis A and has two spiral grooves, c and d. Half-nut slides 2 and 3 travel along fixed guides f-f. Pins a and b of slides 3 and 2 engage grooves c and d of cam 1. When cam 1 is turned counterclockwise, slides 2 and 3 move towards each other, closing the half-nuts over the thread of lead screw 4. When cam 1 is turned clockwise, slides 2 and 3 move apart, opening the half-nuts to disengage them from lead screw 4.
$3152$SmC,GC$