each of the paired camming projections 7 has a greater axial length than the guide projections 6 and has an inside face 70 inclined in an axial direction to closely conform to the inclined outside face 60 of the adjacent one of guide projections 6 it is to be observed that with axial movement of the resilient tubular casing 3 relative to the tubular frame 1 the camming projections 7 in each pair act upon the corresponding pair of guide projections 6 through the interengagement of inclined side faces 60 and 70 of the respective projections 6 and 7 to force the ribs 5 formed on the tubular casing 3 circumferentially toward each other against the resiliency thereof until the ribs 5 are placed in abutting engagement with each other to close the gap 4 in the wall of the tubular casing 3 obviously the gap 4 closed in this manner will become open again under the resiliency of the casing 3 when the latter is moved axially in the opposite direction incidentally the other tubular casing or frame 1 should be designed with an internal diameter which affords a radial distance between the walls of inner and outer tubular casings 3 and 1 sufficient to ensure smooth interengagement and relative sliding movement between the paired guide and camming projections 6 and 7 respectively provided on the inner and outer casings 3 and 1 though each pair of camming projections 7 has been shown and described above as integrally formed on a block welded to the tubular frame 1 such projections may alternatively be formed separate and individually secured directly.