1. Field of the invention
This invention relates to a rotary thread take-up lever in a sewing machine.
2. Description of the Related Art
In a sewing machine, its rotary thread take-up lever performs a series of operations as follows: That is, the rotary thread take-up lever supplies the upper thread to the needle which is supplied from the bobbin, and then loosens the thread as much as required for the shuttle race body to scoop up the needle and for the thread to pass through the rotary hook. Furthermore, the rotary thread take-up lever pulls up the upper threaded immediately when the thread passes through the rotary hook, and supplies the upper thread which is used for formation of a seam in synchronization with the feeding of the work, and pulls up the upper thread, and tightens the thread. In the description, the term "shuttle" includes the shuttle race body and the rotary hook rotatable inside the shuttle race body.
The rotary thread take-up lever, which is a kind of thread take-up lever, performs the above-described series of upper thread supplying and lifting operations with the aid of the continuous rotation of a thread take-up lever element (described later).
A conventional ordinary rotary thread take-up lever is as shown in FIGS. 6(a) and 6(b). That is, the conventional rotary thread take-up lever 20 includes a rotary thread take-up lever element 24 which has a base portion 22 and an arm 23 extending from the base portion 22. More specifically, the base portion 22 is fixedly secured to a rotating member 21 which is turned in one direction around a horizontal axis in association with a spindle of the sewing machine such as a needle bar driving crank shaft. The arm 23 extends outwardly from the base portion 22 with respect to the rotating axis.
The rotary thread take-up lever 20 is turned clockwise. When the upper thread forms a single thread loop around the arm 23 of the rotary thread take-up lever (cf. FIGS. 8 and 9), the thread loop is slidably moved back and forth along the front and rear edges 26 and 25 of the arm 23 during the rotation of the thread take-up lever element 24, thus achieving the above-described series of operations of the rotary thread take-up lever. In order to prevent the thread loop from coming off the arm during the rotation of the thread take-up lever element 24, the arm 23 has locking fingers 27 and 28 at its free end while the base end 22 has protrusions 29 and 30 at both sides.
FIG. 7 is a graphical representation indicating a thread take-up lever's motion curve A and a shuttle's motion curve B of an industrial zigzag sewing machine having the above-described conventional rotary thread take-up lever 20 and a full-turn shuttle. In FIG. 7, the horizontal axis indicates the angles of rotation of the arm shaft of the sewing machine, and the vertical axis indicates the lengths of the upper thread supplied by the rotary thread take-up lever. In the vertical axis, the mark "0" corresponds to the position of the throat plate's upper surface. The rotary thread take-up lever's motion curve A is to indicate the lengths of the upper thread supplied by the rotary thread take-up lever, while the shuttle's motion curve B is to indicate the lengths of the upper thread taken up by the shuttle. The lengths of the upper thread thus taken up are based on a thread take-up lever motion scale. The needle bar is located at the highest position (the top dead point) in its range of movement when the angle of rotation of the spindle is 0.degree. and it is located at the lowest position (the bottom dead point) when 180.degree..
For a period of time between the time instant f and the time instant a in FIG. 7, the upper thread is supplied while sliding on the gently curved portion (indicated by the dotted line 31 in FIG. 6) of the rear edge 25 of the arm 23. FIG. 8 shows the state of the rotary thread take-up lever 20 and the upper thread 16 at the time instant a. The upper thread 16 is held by one point on the rear edge 25 until the time instant a. The upper thread 16 is suitably tensioned by a pair of thread guides (not shown) which are provided respectively on the seam side and on the thread supplying source side.
When the rotary thread take-up lever 20 is further turned, at the time instant a' in FIG. 7, the upper thread 16 is locked by the above-described locking fingers 27 and 28, and it is supplied until the time instant b (in FIG. 7) while being supported by two thread holding points 32 and 33 on the rear edge 25. FIG. 9 shows the state of the thread take-up lever 20 and the upper thread 16 at the time instant b (in FIG. 7). As the thread take-up lever 20 is further turned, the upper thread 16 is pulled upwardly.
The conventional rotary thread take-up lever 20 thus designed suffers from the following problems:
The length M of the thread supplied by the thread take-up lever is much larger than the maximum length m of the thread which is taken up by the shuttle, and therefore the maximum length D of the waste thread is also excessively large. In addition, while the shuttle pulls in the thread, the latter is abruptly released (F in FIG. 7). Hence, in the case where the sewing machine is operated at high speed, the thread is liable to move unstably, as a result of which the resultant seams are irregular, and the thread may be broken.
The maximum length D of the waste thread may be decreased by reducing the distance between the center of rotation O of the rotary thread take-up lever 20 and the thread holding points 32 and 33; i.e., the radius of rotation R1 of the thread holding points 32 and 33. However, in this case, the thread take-up lever's motion curve A corresponding as shown in FIG. 7 is shifted, as a whole, to the left with respect to the shuttle's motion curve B, and the length E of the waste thread which is required for the upper thread 16 to come out of the shuttle is also decreased. Hence, it is not practical to reduce the radius of rotation R1 of the thread holding points 32.