PURPOSE: To prevent deformation when using a titled tube by giving fine flaws on an outer face of an inner tube formed as a tube, passing this through an outer layer tube and bulge working the laminated tube and forming a bellows.
CONSTITUTION: Fine flaws 13 are formed on an outer face of an inner layer tube 11 formed to a specified size by cutting operation such as hairline etc. This is contained in an outer layer tube 12. Obtained laminated tube 15 is supplied into a bulge forming die and liquid pressure is applied to inner face. The tube 15 is deformed to the outside along the shape of undulation formed in the forming die, and a bellows 16 made up of crest parts 17 and trough parts 18 is obtained. At this time, cavities 19 caused by flaws 13 are formed between tubes 11 and 12, and the cavities communicate with through holes 14 of the tube 12. Thus, the escape passages for air and oil remaining in the tube are formed, and the deformation of inner and outer tubes during using can be prevented.
TAMURA TATSUYA
KOUNO TADAHIRO
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