Patent ID: 7757529

Claim:
A roll forming method for sequentially bending a single long band plate material into a flat tube including flat plate portion pairs opposed in relation to each other, a pair of curved plate portions connecting the ends of the flat plate portion pairs to each other and a folded rib rising upright from one of the opposed flat plate portion pairs and returning to the particular flat plate portion, the flat tube having a substantially θ-shaped section with a substantially elliptic space defined by the flat plate portion pairs, the pair of the curved plate portions and the folded rib, the method comprising: a roll forming step (A) to form a substantially triangular protrusion having a roundish top on a flat plate portion of the band plate material, the substantially triangular protrusion further having two opposed base end portions and a forward end; a roll forming step (B) to closely attach the two opposed base end portions while leaving a hollow portion at the forward end of the substantially triangular protrusion having a roundish top thereby to form a protrusion having the closely-attached base end portions and the forward-end hollow portion; a roll forming step (C) to form the protrusion of step (B) into a folded rib with the entire two sides thereof closely attached to each other; a roll forming step (D) to upset the folded rib from above and thus reduce a depression formed in the flat plate portion of the band plate material by bends between the flat plate portion and the folded rib; a roll forming step (E) to form an L-shaped bend at each transverse end of the band plate material; and a roll forming step (F) to bend an intermediate portion between each L-shaped bend and the folded rib into a semicircle and opposing the flat plate portion pairs to each other while at the same time placing the L-shaped bends into contact with sides of the folded rib.