The present invention relates to a thread pull-out mechanism which pulls out needle thread previous to the needle thread tightening motion of the take-up lever and, more specifically, to a thread pull-out mechanism for such a function, having a checking member to check pulling back the needle thread from the take-up lever in pulling out the needle thread from the supply spool.
Generally, the take-up lever, the vertical reciprocatory motion of which being synchronous with the rotation of the arm shaft of the sewing machine, moves downward to supply the needle with an amount of the needle thread necessary for forming a stitch, and then moves upward to tighten the needle thread and to pull out a predetermined amount of the needle thread necessary for forming the subsequent stitch from the supply spool.
Optimum tightening of the needle thread is achieved by enhancing the spring force applied to the tension discs to a certain degree in sewing a comparatively thick fabric, and by reducing the spring force in sewing a comparatively thin fabric. However, when the spring force is reduced to a very low degree in sewing a very thin fabric, and when a large amount of needle thread is wound on the spool or when the spool is supported vertically, resistance to pulling out the needle thread from the spool exceeds the resistance to the movement of the needle thread applied through the tension discs by the spring force, because of the inertia of the spool and a frictional resistance to the rotation of the spool, in pulling out the needle thread from the spool by the upward movement of the take-up lever. Consequently, the take-up lever is unable to pull out a predetermined amount of needle thread from the spool, the tension of the needle thread varies excessively widely and a wavy seam is formed due to an excessive tension applied to the needle thread in tightening the same, and hence it is impossible to form uniform and regular stitches.
To solve such problems, Japanese Utility Model Publication (examined) No. 60-38469 discloses, as shown in FIGS. 6 and 7, a variable pretension device which comprises an annular cam 104 mounted on one end of the arm shaft 102 of a sewing machine beside a balance crank 103, a vertical arm member 105 pivotally supported at the lower end thereof with a pin 106 on the frame 101 of the sewing machine and having a projection engaging the cam surface 104a of the annular cam 104, a thread pull-out arm 108 attached to the top of the arm member 105, and a pretension mechanism 121 which presses the needle thread 120 against a tension block 110 when the take-up lever 115 moves downward in synchronism with the rotation of the arm shaft 102 while the thread pull-out arm 108 pulls out the needle thread 120 from the spool 119. And, a pretension disc 113, an elastic member 116, a tension disc 123, a tension spring 124 and a tension adjusting dial 125 are shown in FIGS. 6 and 7.
The pretension mechanism of this pretension device prevents the reverse movement of the needle thread 120 in pulling out the needle thread 120 from the spool 119 by the needle pull-out arm 108. However, this pretension device has many disadvantages. That is, since the pretension device is driven by a cam mechanism, the parts in sliding contact with each other, such as the cam 104 and the projection 105b of the arm member 105, generate noise during high speed sewing operation, since the cam surface 104a of the cam 104 and the projection 105b of the arm member 105 are in sliding contact, the cam surface 104a and the projection 105b are liable to be abraded during an extended period of use and thereby the amount of the needle thread 120 pulled out in one stitching cycle is variable; since the spring force of the spiral spring 114 of the pretension mechanism for checking the needle thread 120 decreases during an extended period of use, the reverse movement of the needle thread 120 in pulling out the needle thread 120 can not be checked satisfactorily; and since the pretension mechanism comprises many parts, such as the annular cam 104, the arm member 105, the thread pull-out arm 108, the tension finger 109, the tension block 110 and the pretension shaft 112, the pretension device is complex in construction and expensive.
On the other hand, there is provided a thread pull-off mechanism for pulling out the thread of a sewing machine, in U.S. Pat. No. 3,083,661.
Above thread pull-off mechanism is provided with a pull-off finger connected to the take-up crank, having a free end thereof moving reciprocatively up and down, and a predetermined amount of the thread is pulled out from the thread supply source by the pull-off finger. On the front surface of the machine head, there is provided with a pair of guides for guiding the thread in order to pull out the thread by the finger; and these guides are located near the thread tension device. The thread is introduced to the pair of guides via a guide provided on the front top of the machine head.
Between the thread supply source and thread portion to be pulled out by the finger, the thread is bent like L-shape at two points, therefore resistance of the thread passage is comparatively large.
However, in such a case when the thread tension device is adjusted weakly for sewing a thin fabric, and when the thread supply source stores full thread, it is preferable that the resistance of the thread passage is as small as possible, and it is preferable that the resistance is as large as possible in the thread passage between the thread portion to be pulled out by the finger and the thread tension device.