1. Field of the Invention
The invention relates to a gripper for a gripper weaving machine and to a weft insertion device with a gripper.
2. Discussion of the Prior Art
To insert a weft thread, either one gripper is used for leading the weft thread through the shed or a feed gripper and pick-up gripper are employed, the weft thread being transferred from the feed gripper to the pick-up gripper inside the shed. The grippers have thread clamps which are controlled positively, that is to say forcibly, outside the shed in the first-mentioned version and inside the shed in the second version. For this purpose mechanical or electromechanical means (GB 2, 059 455 A; EP 0 690 160 A) are employed, the operating element for opening or closing the thread clamp being arranged outside or below the shed. Particularly in the case of thread transfer inside the shed, this results in essentially the following disadvantages.
If mechanical means are used, the operating element must be introduced between the warp threads in order to operate the thread clamp.
If electromagnetic means are used, the trigger magnet must be arranged outside the shed. The airgap between the armature and magnetic core consequently becomes large, and because of this a higher field strength has to be generated, that is to say a winding with a greater number of ampere-turns is required. Since this trigger magnet is capable of slow-action switching, this results in a reduction in the speed of the weaving machine.
In the case of thread transfer inside the shed, the travels of the grippers intersect in the transfer region. That is to say, thread transfer takes place while the feed gripper and the pick-up gripper are still in motion. The activation of the thread clamp takes place at this moment, during which the operating element exerts pressure on the gripper. This results in a greater load and higher wear on the elements involved in operating the thread clamp, such as the gripper band or gripper rod or the batten and band guide or rod guide. Moreover, as a result of pressure being exerted in this way, the band gripper is pressed against the warp threads, thus causing breaks in the warp threads. For this reason, the gripper band and its guides have to be reinforced. As a consequence of this, the moved masses become greater, thus contradicting the requirement for masses which are as low as possible and therefore a reduction in the rotational speed of the weaving machine. It must also be taken into account that inside the shed, because of the high rate of production, the grippers may oscillate in the transfer region and collide with the operating elements. Special measures therefore become necessary in order as far as possible to reduce oscillations with such a frequency and amplitude.
As is known, the clamping force is predetermined by the type of thread. In both the mechanical and the electromechanical control of the thread clamp, therefore, the clamping force has to be set accordingly. For weaving with different types of thread, controls of this type prove at the very least to be restrictive, because, on the one hand, an average clamping force has to be fixed by means of tests and, on the other hand, adaptation of the clamping force while the weaving machine is in operation, and particularly along the path between the operating elements, is not possible and is therefore not provided.
The embodiments described above also have the disadvantage that each point of influence is assigned an arrangement of operating elements for controlling the thread clamp, specifically:
a) feed gripper: activation in the shed center, PA1 b) pick-up gripper: activation in the shed center, PA1 c) feed gripper: activation of the thread read-in on the insertion side, PA1 d) pick-up gripper: thread release on the catching side, and PA1 e) feed gripper: activation of venting on the insertion side. PA1 1) maintenance of up to five arrangements; PA1 2) high failure rate; PA1 3) drop in performance; and PA1 4) high cost outlay. PA1 1. the feed gripper takes over the weft thread from the thread feed; PA1 2. transports the weft thread into the shed; PA1 3. the weft thread is transferred from the feed gripper to the pick-up gripper approximately in the shed center; PA1 4. at the end of the insertion operation, the thread clamp of the pick-up gripper releases the weft thread; PA1 5. the thread clamps are vented for cleaning; and PA1 6. if necessary, the clamping force of the thread clamps is changed.
Up to five arrangements of operating elements are therefore necessary, which are to be activated individually.
This results in the following disadvantages:
The known versions are therefore unsuitable for the individual control of the thread clamps, particularly along the path between the operating elements.
The nearest prior art to the invention may be gathered from the exemplary embodiment of FIG. 17 of EP 0 690 160A. There, a gripper is shown and described, which contains in the housing of the gripper an actuator designed as a piezoelectric element. The expansion of the piezoelectric actuator when a voltage is applied serves for closing the thread clamp, that is to say for bringing the moveable clamping part to bear on the fixed clamping part. The contraction of the piezoelectric actuator in the currentless state serves for opening the thread clamp, that is to say for easing the moveable clamping part from the fixed clamping part by venting. Since the force provided during the contraction of the piezoelectric actuator is low, a prestress of the moveable clamping part against the fixed clamping part is counterproductive and therefore harmful and is to be avoided. In this thread clamp, only two positions of the clamp are possible, that is to say "open" or "closed" but no first holding force as a result of a prestressing of the moveable clamping part against the fixed clamping part. In the event of a power failure, the thread clamp opens automatically and releases the weft thread. An individual control of the magnitude of the force of the piezoelectric actuator cannot be seen. The complexity and size of electromagnetic drives deters a person skilled in the art from installing in the gripper housing.