88 of course some of the hydraulic fluid also flows in the annular space 57 between the proximal shaft 54 and the plug cavity 6 however the pressure at this time is not great enough to create any significant radial expansion in the plug 5 if the plug 5 proceeds through the pre loading step without leaking the operator of the apparatus next inserts the plug 5 into the open end of a tube 3 and intensifies the pressure of the fluid until the shell 11 of the plug 5 is radially expanded in the case of a plug 5 formed from inconel u00ae and having a diameter of about 0 440 inches such pressure amounts to about 28 000 pounds per square inch the application of such pressure to the inlet block 62 creates both a radially expansive force within the plug cavity 6 that is large enough to cause it to expand as well as a tensile force of about 1 700 pounds on the pull rod member 52 the combination of the radially expansive force provided by the hydraulic fluid and the compressive force applied by the pull rod member 52 has four desirable effects on the plug 5 first the radially expansive force expands the relatively thin walled tubular portion 25 of the plug outwardly until the lands 29 forcefully.