Half-nut link 2 slides along fixed guide a and can be turned about the axis of the guide so that its half-nuts engage screw 1 or screw 3. When the corresponding half-nut of link 2 is engaged to screw 1 having right-hand thread, link 2 slides in one direction along guide a. This direction can be reversed by disengaging link 2 from link 1 and bringing the other half-nut into engagement with screw 3 having left-hand thread and rotating in the same direction as screw 1.
$2228$LS,4L$