Assembly of Compound Tubes Under Hydraulic Pressure

Abstract

This paper describes a method for inserting a tapered liner into a sleeve while the latter is expanded by hydraulic pressure. The technique avoids many of the limitations associated with traditional shrink fit techniques and autofrettage. The sleeve and liner are manufactured with internal and external tapers, respectively, to give the appropriate interference for the finished compound tube. The liner is mounted on a rod and positioned loosely inside the sleeve. The ends of the sleeve are sealed with plugs, which allow the rod to protrude through each end and which also have hydraulic oil inlets. Once the assembly has been pressurized, the rod is pushed into the vessel to move the liner further into the sleeve generating an interference once the pressure in the sleeve is removed. Insertion of a relatively thin liner can generate high residual compressive stresses at the bore, similar to autofrettage but with a shallower gradient away from the bore. Because the liner is not subjected to plastic strain during manufacture, there is no reduction in compressive strength due to the Bauschinger effect and the maximum compressive stress obtainable is greater than that from traditional autofrettage routes. Such high stresses lead to excess tension in the sleeve, which must be reduced by autofrettaging the sleeve prior to assembly of the compound tube. Such a configuration is suitable for inserting a part-length liner at the chamber for strength and/or wear resistance and tensile stresses can be eliminated to prevent failure of brittle materials, such as ceramics.