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.

The influence which several factors relating to the

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To determine the most suitable porosity of a fiber assembly used as a sound absorbent, we measured the sound absorption coefficients of fiber assemblies having air spaces behind them by the standing wave method and investigated their sound absorption characteristics.The results :(1) Sound absorption characteristics change from a viscosity-resistance type to, successively, a mixed type and a resonance type as they decrease in porosity.There are two types of absorption characteristics of the resonance type.One is a fibrous resonance type, which is for a thick sample.The other is a board resonance, for a thin sample.(2) The relation between the most suitable porosity Pe (%) at which a fiber assembly has the maximum sound absorption, i.e., 1.00, and the thickness T (cm) is shown as follows :where a' is a constant which is decided by fiber fineness and b' is another constant. If T is constant, the relation between Pe and fiber fineness d (denier) is shown as follows :where C is a constant decided by T.(3) The relation between the total surface area S (cm2) of fibers constituting a fiber assembly of porosity Pe (%) and T (cm) is shown as follows: S = a Tb x 104 where a and b are constants.A fiber assembly which meets this equation has the maximum sound absorption coefficient at a certain frequency, if it has no back air space, or at an optional frequency if it has a back air space suited to the frequency.

This article presents the results of an investigation into the relation between the normal incident sound absorption coefficient and the apparent density of glass wool boards used as a sound absorbent.(1) Only one kind of glass wool boards 64 kg/m3 in apparent density and 2.5 cm in thickness out of 20 kinds made by 3 manufacturers has given a sound absorption characteristic belonging to the mixed type. The sound absorption characteristics of the other kinds of glass wool boards have been shown by experiment to belong to the viscosity-resistance type.(2) The relation between effective apparent density Dea (kg/m3) at which a glass wool board gives 1.00 in sound absorption coefficient and the thickness T (cm) is :Dea=aT -b where a and b are constants fixed by the manufacturer.(3) The relation among apparent density Da (kg/m3), thickness T (cm), distance d (cm) between the center of a sample and the rigid wall, and frequency f o at which the sample gives the maximum sound absorption coefficient is expressed by the following empirical formula : f0= (c/4-aD3a'T)d-' where a and t are constants and c is the speed of sound.