The Z axis is an easier one than the X and Y as the force is one way, when the cutter hits the material....so the spring and 2nd nut only have to hold the weight of the spindle.
The ones I have used were the oldest worn sprained things you could imagine...there is sooo much slop in the thread but as long as the slack is held up no worries...I even ran the thread in and out with coarse grinding paste creating even more slop.... :)