Method of producing inner spiral grooved tube and apparatus for producing inner spiral grooved tube
A method produces an inner spiral grooved tube using a first drawing die, a second drawing die, and a revolving flyer. The method includes two twisting-drawing steps. The first twisting-drawing step forms an intermediate twisted tube by reducing the diameter of a linear grooved tube, which has plural straight grooves formed along the longitudinal direction on its inner surface, by passing the linear grooved tube through the first drawing die and then by revolving the liner grooved tube wrapped around the revolving flyer with the revolving flyer, in conjunction with imparting twist to the linear grooved tube. The second twisting-drawing step forms the inner spiral grooved tube by reducing the diameter of the intermediate twisted tube by passing the intermediate twisted tube, which revolves with the revolving flyer, through the second drawing die in conjunction with imparting twist to the intermediate twisted tube.
1. A method of producing an inner spiral grooved tube using a first drawing die, a drawing direction of which is a first direction, a second drawing die, a drawing direction of which is a second direction opposite to the first direction, and a revolving flyer, which is configured to invert a tube route of a tubular material from the first direction to the second direction between the first drawing die and the second drawing die and to revolve around any one of the first drawing die and the second drawing die, the method comprising:
(I) forming an intermediate twisted tube by reducing a diameter of a linear grooved tube, on an inner surface of which a plurality of straight grooves is formed along a longitudinal direction, by passing the linear grooved tube through the first drawing die, and then by conveying the linear grooved tube via the revolving flyer and revolving the linear grooved tube with the revolving flyer, in conjunction with imparting twist to the linear grooved tube; and
(II) forming an inner spiral grooved tube by reducing a diameter of the intermediate twisted tube by passing the intermediate twisted tube, which revolves with the revolving flyer, through the second drawing die, in conjunction with imparting twist to the intermediate twisted tube.
2. The method according to claim 1 , wherein each of diameter reduction ratios of tubular material in in said forming (I) and forming (II) is 2% or more and 40% or less.
3. The method according to claim 1 , wherein
a revolving capstan, which revolves in synchronization with the revolving flyer, is provided on each of a front stage and a rear stage of the revolving flyer for the tubular material to be wound around the revolving capstan.
4. The method according to claim 1 , wherein a guide capstan is provided on each of a front stage of the first drawing die and a rear stage of the second drawing die for the tubular material to be wound around the guide capstan.
5. The method according to claim 1 , wherein a rotary driven capstan in a winding-around direction is provided on each of rear stages of the first drawing die and the second drawing die to impart forward tension to the tubular material.
6. The method according to claim 1 , further comprising:
unwinding the linear grooved tube from an unwinding bobbin as a pre-process to said forming (I) to impart backward tension to the linear grooved tube with a brake configured to restrict rotation in an unwinding direction of the unwinding bobbin.
7. The method according to claim 1 , wherein
a heat treatment is performed on the inner spiral grooved tube formed through said forming (II), thereby obtaining a heat treated inner spiral grooved tube, and
said forming (I) and forming (II) are performed again on the heat treated inner spiral grooved tube to impart an even larger twist angle to the inner spiral grooved tube.