direction of pressing of the compressing ring fig2 illustrates the increases of the taper u03b1 of the supporting surface 3 from e g 20 degrees at the outer circumference to e g 75 degrees at the inner circumference while the pressing surface 6 of the pressing ring 5 has a constant taper u03b4 e g 60 degrees also the pressing surface 6 is formed on an axially projecting 34 nose portion 34 5a due to which the compression ring may be moved into the space surround by the concave conical supporting surface 3 in the initial position of the compression ring 5 indicated by the full lines in fig2 the undeformed sealing ring 4 having a cross sectional diameter d9 just touches the supporting surface 3 and the pressing surface 6 at the dotted line position 5 39 the compression ring is indicated at its innermost position this position is determined by the circumferentially compressed core cross sectional circle d8 just contacts the supporting surface 3 as well as the pressing surface 6 and corresponds to the smallest pipe diameter dmin the effect of the concave conical shape of the supporting surface 3 is that when pressing the sealing ring 4 the latter will be initially subjected to a relatively large force component directed to the axis and as a consequence of this may be easily compressed in the circumferential direction this provides for a better balance between the sealing pressures acting on.