mandrel and the internal diameter of the tube 50 are matched in such a way that the mandrel can be introduced into the tube moreover the mandrel is provided with a row of orifices 40 which lead to a cavity in the interior of the mandrel which cavity is connected to a line 32 for a pressure medium a pressure medium is fed via the lines 30a b c e the pressure line 30d is not shown in the figure to the individual drive mechanisms 24 25 26 33 and 39 in fig3 the two sets of clamping devices 20a b 21a b are shown in the position for the smallest distance between the sets in addition the figure shows the said cavity 42 in the mandrel 27 a gasket 41 between one end of the tube 50 and the set of clamping devices 21a b and a gasket 34 between the other end of the tube 50 and the set of clamping devices 20a b a gasket 35 between the mandrel 27 and the set of clamping devices 20a b can also be seen an outlet valve 36 is located in the lower clamping device 21b in this way a closed space which can be subjected to pressure through the orifices 40 is formed in the mandrel 27 fig4 shows the two sets of clamping devices shifted back to their starting positions while the surrounded parts of.