The present invention relates to a double-twist stranding machine, and in particular to a double-twist stranding machine having an arc structure which rotates around a stationary central structure.

EP 123656 A discloses a double-twist stranding machine comprising a base on which an arc structure can rotate around a stationary central structure. A bobbin is rotated in this stationary structure around its own axis for winding a finished cable pulled into the stationary structure by one or more pulleys, so that it undergoes a double twist due to the rotation of the arc structure, which comprises for this purpose a pair of rotating guides mutually connected by an arc along which the cable runs, thereby simultaneously rotating around the stationary structure.

Both rotating guides of the arc structure of this known stranding machine comprise a pulley which deviates toward the arc, and vice versa, the feed direction of the cable, generally coinciding with the horizontal rotation axis of the arc structure. During the running of the cable in the race of the pulley, the high sliding friction due to the wide contact of the cable with the surface of the race causes a localized torsion of the cable in the rotating guides, so that the resulting cable has a stranding pitch substantially different from the one upstream the machine, with consequent risk of cable imperfections, such as for instance ovalings or swellings, during the passing of the cable in the arc structure.

It is therefore an object of the present invention to provide a stranding machine which is free from said disadvantage. Said object is achieved with a stranding machine, the main features of which are disclosed in the first claim and other features are disclosed in the following claims.

Thanks to the particular rotating guides which is provided with, the stranding machine according to the present invention allows to minimize the friction of the cable in the rotating guides, so that the torsion of the cable tends to "go out" already upstream the machine, i.e. before it goes into the rotating guides, so that the stranding pitch upstream the machine comes close to the final stranding pitch and the obtained cable results free from imperfections.

For this purpose, the rotating guides are provided with a plurality of rollers which can be rotated so that their rotation axis is not perpendicular to the feed direction of the cable, but forms with the latter given angles which allow to minimize the rolling friction and, above all, the sliding friction.

According to a particular aspect of the invention, the axis of the rollers are oriented with the stranding direction of the cable, so that the rotation of the rollers imparts to the cable a force tending to rotate it around its own axis in the stranding direction, so as to facilitate the "exit" of the torsions upstream the machine.

Further advantages and features of the stranding machine according to the present invention will become clear to those skilled in the art from the following detailed and non-limiting description of an embodiment thereof with reference to the attached drawings, wherein:

• figure 1 shows a schematic side view of the stranding machine according to such an embodiment;
• figure 2 shows a first partial and enlarged side view of the stranding machine of figure 1;
• figure 3 shows a second partial and enlarged side view of the stranding machine of figure 1; and
• figure 4 shows a schematic front view of a roller of the stranding machine of figure 1.

Referring to figure 1, it is seen that the stranding machine according to the present invention comprises in a known way a base 1 on which an arc structure 2 can rotate around a horizontal axis 3. The arc structure 2 also rotates around a stationary central structure 4 in which a bobbin 5 is rotated around its own axis for winding a finished cable 6 pulled by one or more pulleys 7 (shown with broken lines) arranged in the same stationary structure 4. Cable 6 comes from the outside of the stranding machine (from the left in figure 1) and undergoes a double twist when it runs toward bobbin 5 in the arc structure 2 rotating around axis 3. The arc structure 2 comprises in turn a pair of rotating guides 2a, 2b mutually connected by an arc 2c along which cable 6 runs.

With reference also to figures 2 and 3, it is seen that, according to the invention, the first rotating guide 2a and/or the second rotating guide 2b of the arc structure 2 are suitably provided with a plurality of rollers 8 along which cable 6 can run pulled by pulleys 7. Each roller 8 is preferably idle, is provided with a concave race and is mounted on a fork support 9 comprising a pin 10 substantially perpendicular to the rotation axis of roller 8, as well as to the portion of cable 6 tangent to the same roller. Pin 10 is inserted into a corresponding hole made in a transversal plate 11 fixed to the rotating guide 2a or 2b. The free end of pin 10 is further preferably threaded, so that the rotation of the fork support 9 can be locked by tightening into such a threaded end a nut 12 which urges against plate 11, in particular through a washer. In the present embodiment of the invention, the rotating guide 2a is provided with three rollers 8 arranged along an arc covering an angle α comprised between 20° and 60°, for example substantially equal to 40°, while the rotating guide 2b is provided with twelve rollers 8 arranged along an arc covering an angle β comprised between 160° and 120°, for example substantially equal to 140°. In particular, angle β is substantially equal to a straight angle minus angle α, i.e. β ≈180° - α.

Finally, figure 4 shows that the revolving supports 9 can be rotated so that the axis of rollers 8 form with the portion of cable 6 tangent thereto not a right angle, as in the known art, but an acute angle comprised between 60° and 90°, for example about 70°. With this arrangement, cable 6 touches the edges of the race of rollers 8 and is not located at the bottom thereof, thus minimizing not only the rolling friction due to the running onto rollers 8, but especially the sliding friction due to the contact with their surface.

Furthermore, the axis of rollers 8 are preferably oriented with the stranding direction of cable 6. For instance, in figure 4 cable 6 has a left stranding direction with respect to the feed direction (indicated with the arrow), so that the axis of roller 8 is oriented so that the stranding direction of cable 6, viewed from the outside, is substantially parallel to the axis of roller 8 or forms a small angle therewith, in particular lower than 20°.