Patent ID: 8783085

Claim:
A method for straightening a tube by using a straightener in which three or more pairs of rolls, each pair being formed of two rolls oppositely disposed in a vertical direction, are back-to-back disposed along a traveling direction of a tube to be straightened, wherein a concave shape asymmetric roll, in which a roll diameter of a left roll shoulder as being a relative maximum diameter portion is different from that of a right roll shoulder as being the other relative maximum diameter portion, is used for both upper and lower rolls provided in at least one pair of rolls, except for a pair of rolls located at foremost entry side with respect to the traveling direction of the tube to be straightened among the plural pairs of rolls, and each asymmetric roll is arranged such that the roll shoulder as being a relative maximum diameter portion having a smaller diameter is configured to be located on a tube entry side, while the roll shoulder as being the other relative maximum diameter portion having a larger diameter being located on a tube exit side, where the roll diameter of the shoulder portion of the asymmetric roll refers to a roll diameter of the relevant shoulder portion when the relevant roll is disposed such that the roll central axis is horizontal, and is observed from a direction normal to said axis.