This is based on the relationship of the lifting screw diameter and the length. Lifting screws in tension are rated for the full capacity of the screw jack, regardless of length, providing they do not rotate faster than the critical speed. For the lifting screw in compression, capacity is limited by the screw jack's permissible buckling load. The permissible buckling load of a lifting screw is reduced as the lifting screw gets longer. Use the maximum extended lifting screw length when using the permissible buckling load charts to determine screw jack load capacity.