Special hydraulic jacks to make special stretching machines, at the time of making prestressed concrete members of prestressed steel bar tension. Special hydraulic jacks is double-action. Commonly used are both through the heart-and-cone anchor.
Through the heart Jack suitable for tension steel beams or steel wire beams, it mainly consists of tensioning cylinders, cylinder, piston and spring components. Its characteristics are: along the axis of stretch through the heart channel bars (or wire) into the tool at the end of the anchor after anchoring.