Welding Positioners Synchronous Lifting. Ball screw jack is used for synchronous lifting welding positioners. Dual ball screw jacks are mounted on both the headstock and tailstock to lifting and lowering the weld frame, a single phase or 3-phases electric motor driven dual ball screw jacks through a worm gear reducer to ensure the whole welding positioner synchronous lifting. Four linear guides mounted on each ball screw jack to ensure the alignment between the headstock and tailstock lift mechanisms, as well as between the pair of ball screw jack on the headstock and tailstock to make sure the weldment is not flexed or bowed.