PURPOSE: To reduce a cost by making a hollow shaft of a double-pipe which consists of an outer pipe, in which a spiral groove is provided along the outer peripheral surface thereof by forming processing, and an inner pipe, which is inserted to this outer pipe integrally and has a strength equal to the outer pipe or more than it.
CONSTITUTION: A hollow shaft 11 is made of a double-pipe which consists of an outer pipe 13 made of thin aluminium pipe, in which one spiral groove 12 having a U-shape section is provided along the outer peripheral surface thereof by swaging as forming processing, and an inner pipe 14 made of thick carbon steel pipe for mechanical structural use and is inserted to this outer pipe 13 integrally. The inner pipe 14 made of thick carbon steel pipe is inserted into the outer pipe 13, in which the spiral groove 12 is provided, to fix both the pipes 13, 14 integrally. Necessity of mechanical finish is thereby eliminated to reduce a cost.
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ANDO YASUO