That's a vise, right? Not a jack? Do you mean the screw? Guess I've never heard of it called a jack, though I guess it Jack's the jaws together? Next question, htf does that get broken moving? Fall out of an airplane? Several order pieces I would have expected to break before the screw. Last question, is it not repairable?
Huh? Do you have the pieces of the screw out? Or does it not move the movable jaw? I would expect the nut to break off long before the screw breaks. That's happened to many a vise that has fallen wrong.