Abstract: Hydraulic apparatus for the generation of large tensile forces by means of a number of stacked power units, each having a hydraulically energized power cell, the stack of units being disposed in alignment with the member to be stressed. The power cells are connected and parallel, in their surfaces rest against supporting plates which provide a clearance therebetween so as to permit an expansion of said cells when they are subjected to hydraulic pressure. A similarly supported return cell causes the power cells to return to their initial positions when hydraulic pressure is removed from them.