Patent ID: 7007590

Claim:
A device with at least one extension arm or support arm for multi-linked extension arms, concrete spreader columns, lifting platforms, manipulators and the like, comprising: at least one hydraulic differential cylinder for operating the arm that is articulated on one side and can be raised, lowered and/or telescoped by means of the hydraulic differential cylinder; wherein the hydraulic differential cylinder is provided with a hollow piston rod further comprising a tubular segment and two end pieces arranged at the ends of this segment and designed as a single piece; wherein, with a view to avoiding stress concentrations, the cross section of the wall of the tubular segment over the entire length of the tubular segment, and the transitions from the tubular segment to the end pieces is made to be substantially smooth and plane; wherein, given jointing that establishes material continuity across the joint, at least one of the end pieces is welded to the segment creating a welded joint; wherein the welded joint is made to be substantially flush with the segment and the end piece joined to it by the welded joint by means of finishing in the form of material-removing processing, especially material-removing machining; wherein further the root of the welded joint is eliminated by the finishing, and the region of the welded joint, especially on the internal face of the tubular segment is smoothed, and at least one of the end pieces is provided with an opening through which a cutting tool can be introduced into in order to gain access to a space within the hollow piston rod for performing the said finishing therein.