Sockets (14) are provided in each clamping ring (7). They enable form-fit location of the corresponding female threaded element (9) for the threaded section of the respective tensioning screw (8). They are dimensioned so that the head of the screw is rotatable in it. The sockets have predetermined spacing on a circle concentrically encompassing the axis of the clamping ring. At least two sealing rings (6) are provided between the two clamping rings.
