In the case of a thread feed device which is known from Belgium Pat. No. 900 041, an air jet located close to the thread supply device serves as an assistant device, which, either alone or in cooperation with an arm which is adapted to be resiliently pivoted transversely to the thread path, tries to keep small thread tension variations for the thread storage and feed device during each thread draw-off cycle. Thread tension variations also result from the cyclically changing unwinding paths of the thread from the supply device which has the form of a bobbin. More specifically, the thread is stored on the supply bobbin with a criss-cross winding formation, and the unwinding of the thread in an axial direction periodically changes the direction of the path along which the thread leaves the bobbin thus creating oscillating variations of the resistance of the yarn. The assistant device takes over part of the drawing-off work of the thread storage and feed device. In particular, in the case of the high thread speeds in air-jet weaving machines, a reduction of the number of thread breakages is thus aimed at. However, the air jet and the pivotable arm represent a source of malfunction in the thread path, since it is difficult to exactly adapt the influence of the pneumatic assistant device on the thread to the speed of the thread in the thread storage and feed device.
The present invention is based on the task of providing a thread feed device and a method by which the number of thread breakages in general, and the number of thread breakages in the thread storage and feed device in particular, can be reduced even further.
A friction drive acting on the thread mechanically and with a certain amount of slip (slip-feed) is able to adjust, with the aid of an assist feed, the thread speed upstream of the thread storage and feed device sensitively enough to avoid undesirably strong strains on the thread in the thread storage and feed device. This results in a considerable decrease in thread breakages in general and in thread breakages in the thread storage and feed device in particular. The friction drive can be adapted to the drawing-off speed of the thread storage and feed device in a simple manner. It also damps tension peaks resulting from the changing unwinding paths of the thread (short-time thread drag) from the bobbin. The friction drive has a simple structural design, it requires little space and it can easily be adapted to various thread qualities. The surface, which is in contact with the thread, must not be sticky, but it should be comparatively smooth, it should guarantee gentle treatment of the thread and, moreover, it should be resistant to wear. A rotating surface made of hard chromium-plated aluminum or a surface made of plasma-coated ceramics, for example, will serve this purpose well.
The modulation of the influence on the thread permits a precise adaptation to the respective thread speed, e.g. by selecting the rotating surface of the friction drive.
The drive provides energy for the thread; depending on the thread tension, the thread takes up the amount of energy required for obtaining the necessary thread speed. The major part of this task no longer has to be fulfilled by the thread storage and feed device. A modulation of the influence on the thread can also be effected by influencing the driving force transmission in the friction drive.
The influence on the thread can be modulated by varying the looping angle of the thread in the friction drive, a larger looping angle causing a stronger assist feed, whereas a smaller looping angle causes a weaker assist feed. In addition to the tension of the thread, the looping angle is used as a control variable for the amount of slip.
When the looping angle is approx. 90.degree., there will be the additional advantage that the thread deflection from the bobbin to the storage device, which has hitherto been necessary in the case of a conventional arrangement of a vertical bobbin and of a horizontal thread storage and feed device, will be carried out by the friction drive itself. The friction roll can be light and its inertia can be small and it can rapidly be accelerated to the speed required and then be decelerated again. Short-time thread drags originating from the bobbin are compensated for by the friction roll.
A clutch member has the effect that the thread can run with the friction roll also without direct drive of said friction roll, if this is advantageous under certain operating conditions. In particular in the slow-down phase, the thread is accurately decelerated via the engaged clutch and the decreasing driving speed, and this prevents, at least to a large extent, after-running of the thread due to its mass and, consequently, an undesirable excessive supply of thread.
A brake member can be used for rapidly decelerating the friction roll or for decelerating said friction roll in a purposeful and modulated manner during operation, a desired thread tension being thus produced. If desired, a free wheel blocking in one direction can be provided as well.
A specially selected surface coating of the friction roll is one of the factors determining the amount of slip. In adaptation to the thread quality, different surface coatings can be provided in an axially juxtaposed mode of arrangement, the respective adequate surface coating being then used in each case.
A speed control unit effects an accurate control of the friction drive in response to the thread speed determined by the thread storage and feed device. It is, however, also possible to drive the friction drive independently and to control the actual speed of the thread only via the modulation of the influence, e.g. by means of the slip. Also particularly sensitive or weak threads can thus be processed at high speeds and without any risk of breakage.
It is possible to achieve a uniform adaptation of the thread speed for the thread storage and feed device. Even in the case of high speeds and a comparatively sharp deflection, the thread will no longer break in the thread storage and feed device. The intentionally effected speed difference causes the slip, which is required for the assist feed (deceleration or acceleration).
From the point of view of control technology, the feed is effected with an increasing amount of slip from the very beginning.
The operating range is subdivided into a plurality of successive ranges. In the first range, the thread is--e.g. in the slow-down phase of the thread storage and feed device--decelerated for the purpose of preventing after-running because the friction drive runs less fast than the thread or stands still. In the next range, the friction drive can be disconnected from its drive, the thread is then subjected to the mechanical motion resistance of the friction drive, and this suppresses variations in the tension. In the subsequent range, a surplus speed is available due to the fact that the friction drive is driven faster than the thread, and, if necessary, the thread will take up said surplus speed at least partially with a certain amount of slip. Also in this speed range variations of the tension in the thread are suppressed by means of the exactly controllable assist feed.
The assist feed decelerates in the range of low thread speeds, whereas it pushes from behind in the case of higher thread speeds.
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