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This is one of the plans I sell on my web site. You can take two standard ball nuts which are not to expensive and machine a spacer tube with a ring and apply some heavy duty spring washers to get some preload. When you tighten the ring it pushes appart the nuts to give you about 18 pounds pressure. This takes up the backlash of the screw but allows you to over come any difference in pitch of the screw. I have heard of some people welding the nuts in a jam condition, but this is bad for wear on the screw.