Abstract:
A method and an apparatus for strengthening a yarn ( 6 ) is described which is provided with fibers at least in a sheath zone enclosing a core zone. In order to provide advantageous process conditions it is proposed that fibers are needled from the sheath zone through the core zone along the yarn ( 6 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a method for strengthening a yarn which is provided with fibers at least in a sheath zone enclosing the core zone.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    The strength of fibers which are provided with a sheath zone made of fibers which enclose the core zone depends, among other things, on the anchoring of the enveloping fibers in the yarn core. If the yarn is obtained by a twisting of a fiber slubbing, the enveloping fibers are usually well incorporated in the fiber structure as a result of the twisting of the stubbing. If a yarn core is wrapped around with fibers, the connection between the enveloping fibers and the yarn core remains limited to the friction between the enveloping fibers and the surface of the yarn core, thus giving rise to the likelihood that in the case of a respective load, the fiber sheath may be displaced against the yarn core in the longitudinal direction of the yarn. This can lead to a dissolution of the yarn structure, especially whenever the enveloping fibers are provided with a relatively loose connection among one another. The yarn strength also suffers by stresses placed on the yarn which are accompanied by an untwisting of the twist of the yarn.  
         SUMMARY OF THE INVENTION  
         [0003]    The invention is thus based on the object of mechanically strengthening a yarn of the kind mentioned above with the help of a comparably simple method, so that not only higher strength requirements can be met, but also the likelihood of an untwisting of the twist of the yarn can be avoided.  
           [0004]    The invention achieves the above object in such a way that fibers are needled from the sheath zone through the core zone along the yarn.  
           [0005]    Since as a result of this measure the enveloping fibers are anchored additionally in the core zone of the yarn, the mutual connection between the enveloping fibers and the core zone of the yarn is strengthened considerably, which has a direct influence on the yarn strength since the needling of enveloping fibers through the core zone of the yarn produces a fixing of the twist of the yarn. This means that yarns with a predetermined degree of twisting are provided with better strength properties or that a lower twist of the yarn is required for a required yarn strength. This applies to yarns from twisted fiber slubbings, but especially to yarns with a fiber sheath wound about a yarn core, because in this case a mutual connection, which otherwise would not be possible, can be achieved between the enveloping fibers and the yarn core. An additional aspect is that the condensation of the fiber structure which is caused by the needling leads to a certain compensation of thick and thin places, which is disclosed by an even yarn quality.  
           [0006]    In order to perform the needling of yarns, it is possible to assume a conventional apparatus with a drivable needle board reciprocating in the direction of the needle penetration and a stitch base opposite of the needle board. It is merely necessary to ensure that the needles are disposed behind one another in the traveling direction of the yarn and the yarn cannot escape the penetrating needles to the side. For this reason the stitch base is provided with at least one guide groove for the yarn which extends in the direction of yarn passage, with the needles of the needle board penetrating into the guide groove. The side walls of the guide groove, which can be formed by a groove in the stitch base or guide rules provided on the stitch base, prevent a lateral migration of the yarn to be needled, so that the needles need merely be disposed in one row on the needle board in the direction of the guide groove in order to needle the yarn pulled through the guide groove in such a way that the enveloping fibers are pulled through the core zone of the yarn. The enveloping fibers extending through the core zone to the opposite sheath zone substantially prevent any relative movements between the sheath and core zones, so that a yarn needled in this manner is provided not only with favorable strength values, but is also advantageously protected against untwisting.  
           [0007]    Since the desired unity of the fiber structure between the sheath and the core zones of a yarn requires a comparably low stitching density, the passage speed of the yarns to be needled can be kept relatively high in the apparatuses provided for such purposes, which allows a favorable adaptation to the working speed of downstream yarn treatment. In order to enable the simultaneous needling of several yarns, the stitch base can be provided with several parallel guide grooves for one yarn each. The smooth entry and exit of the yarns into and out of the guide grooves can be enforced in a simple way by guide eyes for the yarns.  
           [0008]    In order to obtain a lateral guidance within the guide groove which is advantageous for the needling of a yarn, the clearance of the guide groove can decrease in the direction of needle penetration so that the yarn, depending on its respective thickness, rests on the two side walls of the guide groove when the yarn to be needled is pulled into the guide groove. For this purpose the stitch base can be provided with a convexly arched arrangement at least in the entrance and exit zones, so that in the case of a tensile stress on the yarn to be needled, a force component is obtained which presses the yarn against the stitch base in the guide groove. If the curvature extends over the entire length of the guide groove, this effect is also extended to the needling zone with the advantage that the needle-penetration angle of the needles into the yarn changes along the guide groove, which influences the strength properties accordingly. This effect of the curvature of the stitch base is naturally not dependent on any special arrangement of the cross section of the guide groove which in the case of low requirements placed on the lateral guidance of the yarn can also have a rectangular cross section.  
           [0009]    Although a convex stitch base causes a force component which is perpendicular to the stitch base in the case of a respective tensile load of the yarn which presses the yarn against the stitch base, the provision of a stripper between the stitch base and the needle board is recommended because in this way it is possible to achieve a substantially calmed yarn guidance with a simultaneously lower yarn tension.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The method in accordance with the invention is now explained in closer detail by reference to the enclosed drawing, wherein:  
         [0011]    [0011]FIG. 1 shows an apparatus in accordance with the invention for strengthening a yarn in a simplified, partly sectional side view;  
         [0012]    [0012]FIG. 2 shows a sectional view along line II-II of FIG. 1 on an enlarged scale;  
         [0013]    [0013]FIG. 3 shows an embodiment of an apparatus in accordance with the invention in a representation corresponding to FIG. 1;  
         [0014]    [0014]FIG. 4 shows on an enlarged scale a schematic longitudinal sectional view through a yarn which is strengthened according to FIG. 1, and  
         [0015]    [0015]FIG. 5 shows a schematic longitudinal sectional view through a yarn strengthened according to FIG. 3. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    The apparatus according to FIG. 1 consists substantially of a stitch base  1  and a needle board  2  which is opposite of stitch base  1  and is inserted in a conventional manner in a needle beam  3 . The needle beam  3  is driven reciprocatingly by way of an eccentric drive in the needle-penetration direction of needles  4 . In contrast to conventional stitch bases, the stitch base  1  in accordance with the invention is associated with parallel guide grooves  5  for the yarns  6  to be needled which are held under tensile load between a roller feed  7  and a roller draw-off  8 . Additional guide eyes  9  can be provided for the purpose of improved guidance of the yarns  6 .  
         [0017]    Although the guide grooves  5  are arranged in the form of guide channels in the stitch base  1 , this arrangement is in no way mandatory. The guide grooves  5  could also be formed by guide rules on the stitch base  1 , since the lateral guidance of the yarns  6  is concerned in particular. The channels of the guide grooves  5  are rounded off towards the groove base according to FIG. 2, which leads to an advantageous lateral guidance for the yarns  6 , which are pulled into the guide grooves because the stitch base  1  is provided on the inlet and outlet side with a convex curvature in the direction of passage of yarns  6 , so that force components are obtained as a result of the tensile load of yarns  6 , which force components press against the stitch base  1 . Said force components which occur in the curvature zone are not sufficient, however, in order to allow the omission of a stripper  10  which is disposed between the stitch base  1  and the needle board  2 .  
         [0018]    The embodiment according to FIG. 3 differs from the one according to FIG. 1 merely by the arrangement of the stitch base  1  which is continuously curved in a convex manner from the inlet to the outlet side, so that force components which are perpendicular to the stitch base  1  are obtained over the entire guide length of the guide grooves  5 , which force components press the yarns  6  against the stitch base  1 . A stripper  10  is used nevertheless in order to ensure a calmed yarn guidance which is advantageous for the needling process.  
         [0019]    The embodiments according to FIGS. 1 and 3 differ not only with respect to the forces acting upon the yarns  6 , but particularly by the type of needling as is illustrated by the FIGS. 4 and 5 which each show a yarn  6  with a yarn core  11  and a fiber sheath  12  which can consist of enveloping fibers which are wound about the fiber core  11 .  
         [0020]    The yarn core  11  and the fiber sheath  12  can also be formed by core and sheath zones of a fiber strand twisted into a yarn. Since according to FIG. 1 the needles  4  of the needle board  2  penetrate the yarns  6  perpendicular to the stitch base  1 , enveloping fibers are needled substantially perpendicular to the yarn axis through the yarn core  11  according to FIG. 4, as is illustrated by the indicated fiber bridges  13 . Said fiber bridges  13  connect the fiber sheath  12  with the yarn core  11  and additionally hold the twist of the yarn  6 , which causes the desired yarn strengthening.  
         [0021]    According to FIG. 3, the needles  4  are made to penetrate under different needle-penetration angles the yarns  6  which are guided along the convex stitch base  1 , which leads to fiber bridges  13  which are inclined differently with respect to the longitudinal yarn axis, as is schematically indicated in FIG. 5 in a purely schematic way. The differently inclined fiber bridges  13  lead to a different interfelting between enveloping fibers and yarn core in comparison with FIG. 4, so that an influence on the yarn strengthening can be made through the arrangement of the curvature of the stitch base  1 .  
         [0022]    Although the needles  4  penetrate the yarns  6  substantially in a common axial plane when the needles  4  are not disposed in a mutually slightly offset way, the fiber bridges  13  are usually not disposed in a common axial plane because a twist of the yarn during the passage through the guide grooves cannot be prevented due to the tensile load on the yarns  6 , so that needle penetrations are obtained which are distributed over the circumference of the yarn which is advantageous for an even strengthening of the yarns  6 .