The heavy duty screw jacks are mechanical linear actuators able to lift up to 800 tons (4 units). They are generally used to move the work piece to a comfortable height for assembly or machine operators, but also for loading and offloading it to and from transportation equipment. 
The jacking units can be connected by spreader beams or simply to the jacking wings of the vessel to lift or lower.
Each unit is composed by an electric motor, a gear speed reducer, the screw (or worm shaft) and a worm gear all contained in a solid and stable steel frame. The screw jacks enable to move heavy loads slowly, accurately and safely. The advantage of such system is the simplicity.
The screws are regularly inspected for wearing, but as additional safety a carbon steel nut is added to the mechanism. This nut is at the bottom of the worm gear and is lifted by this; in normal conditions the safety nut does not support any load. 
The failure of the threads of the worm gear will cause the load to be transferred to the safety nut or safety disc preventing the load from falling. The safety disc can replace the worm gear to complete the lift and allow replacement of the worm gear after its completion