Method for manufacturing combined yarn bundle, and method for manufacturing carbon fiber in which resulting combined yarn bundle is used (as amended)

A method for manufacturing a combined yarn bundle by combining two or more carbon fiber precursor yarns using the following rollers (1) to (4), comprising the steps of:

• bringing the two or more carbon fiber precursor yarns which travel in approximately parallel to one another into contact with a first roller at a wrap angle of 20° or more;
• then splitting the two or more carbon fiber precursor yarns into two to be brought into contact with a pair of second rollers, so that the carbon fiber precursor yarns are rotated approximately 90° between the first roller and the pair of second rollers;
• next, sequentially bringing the carbon fiber precursor yarns delivered from one second roller into contact with a third front roller and a third rear roller being substantially offset from one another, and also bringing the carbon fiber precursor yarns delivered from the other second roller into contact with the third rear roller without bringing them into contact with the third front roller, so that these carbon fiber precursor yarns are combined on the third rear roller;
• and thereafter, bringing the carbon fiber precursor yarns delivered from the third rear roller into contact with a fourth roller at a wrap angle of 5° or more to obtain a combined yarn bundle, so that a ratio of a distance L between axes of the first roller and of the pair of second rollers to a yarn width W of the carbon fiber precursor yarn on the first roller, L/W, is 18 or more and a tension of the combined yarn bundle after delivered from the fourth roller is 0.11 cN/dtex or more;
• (1) a first roller;
• (2) a pair of second rollers having an axis approximately orthogonal to both an axis of the first roller and a traveling direction of the carbon fiber precursor yarns immediately after delivered from the first roller, the axes of which are at approximately equal distances L from the axis of the first roller;
• (3) a third front roller and a third rear roller each having an axis parallel to the axes of the pair of second rollers, being arranged in order along a traveling direction of carbon fiber precursor yarns immediately after delivered from the pair of second rollers;
• and
• (4) a fourth roller having an axis approximately orthogonal to the third front rollers and the third rear rollers.