Patent ID: 7654122

Claim:
A tube straightening process using a roll straightening machine in which drum type straightening roll pairs are disposed, the rolls of each pair being vertically opposed to each other, in a manner such that directions of rotating axes of rolls in each pair are horizontally oppositely diverted so as to horizontally cross, wherein: in at least three pairs of straightening rolls on an outlet-side of the roll straightening machine, at least an outer layer portion of a roll main body is made of elastic material having spring type hardness Hs (JIS K 6301 A type) of 50 to 100, and an offset amount is imparted to a tube engaged in the three pairs, at least, of straightening opposing rolls on the outlet side such that η defined by the following equation (1) ranges from 1.0×10 −3 to 1.5×10 −3 , the offset amount being set at three positions tube axis where the upper and lower rolls of each pair horizontally cross [ Formula ⁢ ⁢ 1 ] η = 1 R × ( d 2 ) ( 1 ) where a relationship of R=(δ 2 +L 2 )/2δ holds, given that d (mm) is a tube outside diameter, L (mm) is a roll stand span of the roll straightening machine, and δ (mm) is an offset amount.