Horizontal Bending Basics
Long screws mounted horizontally may sag due to the weight of the screw. If the sag is significant, the screw threads will be compressed on the top section of the screw and extended on the bottom section of the screw. In severe cases, the nut may actually seize on the screw as the running clearance between the screw and nut threads disappears.
The calculations involved are beyond the scope of this section, but the designer needs to be aware of the possibility of problems due to screw sag. If a long span is being used, increasing the screw diameter will decrease the sag and help prevent nut seizure. For more detailed information and the governing equations on horizontal bending of screw shafts, see Roton Engineering Bulletin No. 974.