A Keyed Machine Screw Jack has a keyway machined along the length of the lifting screw. A matching key is fastened to the cover of the jack which will eliminate lift shaft rotation. The keyway in the screw causes greater than normal wear on the internal drive sleeve threads, somewhat reducing jack life.
A Inverted translating jack has a lifting shaft that moves through the gear box. The lift shaft extends out from the bottom of the worm screw jack. A nut is integrated with the worm gear such that the worm gear and nut rotate together. When the lift shaft is held to prevent rotation, the lift shaft will move linearly through the gear box to move the load.