Thread tensioning for chain stitch sewing machine

A multi-needle double chain stitch sewing machine for forming a double chain stitch seam without an upper cover thread on a workpiece comprising:

• at least two needles mounted on the right and left of the lower end of a needle bar,
• a looper reciprocating from the right side of the needle bar toward the needles,
• plural thread tension devices for applying thread tension to the needle threads and a looper thread passing through the needles and looper,
• a thread tension changeover device for controlling the thread tension devices to lower the thread tension of the right needle thread and raise the thread tension of the looper thread at the end of a sewing operation in order to form a thread chain,
• said sewing machine further comprising;
• a U-shaped guide fixed near the lower end of the needle bar and having a pair of thread eyes at both opposing end of the U-shaped guide,
• and a thread control plate fixed on the frame of the sewing machine and having a downwardly extending cam,
• wherein an upper half of the cam is formed in a linear form in a direction of the needle reciprocating motion, a lower half of the cam is formed in a sloping curved form to cross between the pair of thread eyes, and when the U-shaped guide descends with the descent of the needle bar, the right needle thread passing through the thread eyes is bended gradually according to the descent of the thread eyes by the lower half of the cam.