This paper aims to investigate experimentally the disengagement of needle thread from rotating hook in an industrial single-needle lockstitch sewing machine with respect to tightening tension.For this purpose, the disengagement of the thread from rotating hook was photographically observed, and the resultant changes in needle thread tension were recorded simultaneously. A rotating hook, manufactured by Hirose Manufacturing Co., Ltd., model HSL-B was used, which was provided with a rotating hook section jib. Sewing speed was 2,250 spm, and the sewing threads used were cotton, and filament threads of polyester and nylon.The experimental investigation above revealed that the timing of the disengagement of needle thread from rotating hook is affected by the elongation properties of the sewing threads.If the cotton thread low in elongation was used, the needle thread first slips out of the jib and thereafter slips out of the `rotating hook bobbin case holder position bracket'. This is because when the needle thread becomes tight the rotating hook is beyond the correct angular position. The needle thread behavior therefore is abnormal. In this case, it is estimated that if the shortage of needle thread slack can not be compensated by the thread take-up spring or by thread elongation, the thread is frequently drawn out of the tension disc to result in an insufficient tightening tension.When polyester or nylon threads higher in elongation than cotton are used disengagement of needle thread from the rotating hook is normal since the needle thread first slips out of the bracket and thereafter slips out of the jib. When the disengagement occurs in this order the timing of disengagement has no effect on the tightening tension.

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