METHOD FOR JOINING TWO PERIPHERALLY-GROOVED CABLE CORES AND THE JOINT THUS OBTAINED

Abstract: The ends of two grooved cores (1, 2), of the type apt for housing the optical fibres in telecommunication cable, are joined by mechanically connecting the inner, resistent elements (11, 12) and by reconstructing the thermoplastic covering provided with grooves that was previously removed, for carrying out the jointing of the resistant elements. The reconstruction takes place by melting a piece (4) of core, devoid of the resistant element, in the space delimited by a heat-shrinkable sleeve (5), by a plurality of metallic shaped-sections (6) inserted into the grooves (15, 16) and by the jointed resistant elements (11, 12) of the two core pieces.

Description: The present invention concerns a method for joining the ends of two extended elements provided with peripherical grooves, commonly known as grooved cores, in particular, of the type which are intended for housing one or more transmissive optical elements in telecommunication cables, as well as the joint thus obtained. By the term 'grooved core' is meant a structure comprising at least one central element, resistant to tractions and compressions, and a covering of a thermoplastic material, provided with one of more grooves upon its outer surface which are apt for housing the transmissive elements of the optical, and eventually of the electrical type. Preferably, the inner resistant element is a metalic wire disposed in a central position, around which a thermoplastic material covering is formed through extrusion means. The grooves can be either rectilineal, a closed, or an open-helix (i.e. with periodic directional inversions), and they can be extended for the entire outer surface, or else, on only one strip of it. The transmissive element of the optical type is preferably an optical fibre of the type that is provided with at least one primary covering. The invention is directed to realizing the jointing of the ends of two grooved cores of the above-described type. The grooved cores have to be connected to each other in diverse circumstances. For example, the joint of an optical fibres cable, requires the jointing of the grooved core as an operative phase. Moreover, the same grooved core, considered as a product apart that is destined for being used in optical fibre cables and, in general, for connections realized with optical elements, could require the said jointing for diverse motives. In fact, although the construction of grooved cores having any whatsoever length is theoretically possible, the construction of lengths that are not excessively long is preferable both from a qualitative viewpoint of the product, as well as for the successive transportation of the same. The jointing of the ends of the grooved cores, is presently realized by mechanically connecting together the internal resistant elements, after the removal of an appropriate length of thermoplastic covering, and thereafter, reconstructing the thermoplastic covering, with the grooves correctly fitted together without any discontinuities that could negatively influence the transmission characteristics of the optical fibres that are housed therein. According to the known technique, for reconstructing the portion of thermoplastic material, recourse is had to special moulds inside which the two extremities of the cores are positioned preferably after the connection (by welding, ferrules etc.) of the central element, and wherein there is injected the thermoplastic material which has to form the reconstructed layer. This method presents diverse drawbacks. Firstly, it requires a special mould and the necessary equipment for the fusing and moulding processes. Secondly, the mould of this type can be used only for a given grooved core, and a new mould must be prepared should a grooved core having diverse dimensions be used, or if the grooves are diverse in number, form, or lay-out etc. Such a type of mould is costly and cannot be got ready in a short period of time and moreover, it must be substituted if the grooved core has to undergoes even a minimum modification - for example, in the pitch of the groove and/or in their depth. Moreover, this operation requires a certain amount of ability - seeing that it necessitates an appropriate dosage of the material introduced, its correct temperature etc. To end with, since the mould is formed by two or more parts connected together, the formation of burrs - in correspondence of the jointing line of the mould is unavoidable. Hence, said burrs have to be removed by means of a successive mechanical machining for eliminating the risk of introduc-ing attenuations into the optical fibres. Another known joint, substantially having the drawbacks above reported, is the one described in US-A-4 172 746. In fact in the joint according to the above patent a termination, comprising a machine made cylindrical ceramic grooved element is to be adhesively secured to each cable end, the optical fibers of the two cables are adhesively secured inside the grooves of said ceramic grooved elements and the so terminated cable ends are secured one to the other by means of a complex mechanical structure. Therefore, the aim of the present invention is to realize a method that is particularly simple, for joining the extremities of two grooved cores, which does not require the use of any costly equipment and that results as being easy to effectuate on any whatsoever type of grooved core. A further aim of the invention is to realize the jointing in an optimal manner without requiring the need of further operations, and with using the minimum equipment that is hence, cheap and also extremely simple to use and which can be got ready in a brief period of time for any whatsoever type of grooved core. This invention consists of a method of joining the ends of two cable cores (1, 2), each comprising at least one central elongated strenght member (11, 12) surrounded by a covering of thermoplastic material having on its peripheral surface one or more grooves (15, 16), which are apt to house optical fibres, said method being characterized by the following steps: a) applying a tube (5) over one of the ends of the cores to be joined, said tube (5) consisting of heat-shrinkable material and having an inner diameter which is slightly greater than the outer diameters of the core; b) removing the thermoplastic covering from an end portion of each core c) mechanically connecting the strenght members of each core, thereby forming a continuous strenght member having a section bare of thermoplastic covering; d) inserting onto the bare section a sleeve made of the same thermoplastic material as the covering and having grooves in its peripheral surface corresponding to the grooves of the cores to be joined, a diameter equal to the diameter of the cores, a lenght shorter than the heat-shrinkable tube and a longitudinal slot, said sleeve being obtained by removing the strenght member from a predetermined lenght of a similar core and longitudinally slotting said lenght of remaining covering; e) aligning the grooves (15, 16) of the cores and the grooves of the sleeve by inserting into each groove of the sleeve a metallic structural element (6) having a greater length than the sleeve and a cross sectional shape which conforms with the shape of the grooves, so that the end portions of said elements extend into the respective grooves of the termoplastic covering of the cores; f) positioning the heat-shrinkable tube (5) at least over the sleeve (4) and over the elements (6); g) applying heat to the entire assembly of sleeve (4), tube (5) and elements (6) so as to cause the the tube to shrink and the underlaying thermoplastic sleeve to melt locally, whereby the sleeve is fused to the thermoplastic covering of the cores, respectively, and the groove configuration is maintained; h) removing the heat-shrinkable tube (5) and the metallic structural elements (6) from the joint thus obtained. Moreover, this invention leads to a joint, made between two grooved cores, comprising at least one resistant element and a plastic covering having, on its outer surface, one or more grooves that are apt for housing transmissive elements of the optical type, which are obtained through said method. The invention will now be described - with reference to a non-limiting example of an embodiment, illustrated in the attached figures from 1 to 4 which show the main phases of this method. The two grooved cores 1 and 2, shown in the FIG. 1, are of the type having a central resistant element (11, 12), around which there is foreseen a thermoplastic covering (13, 14) of nylon, for example, upon whose outer surface, six, equidistributed, helicoidal open-helix grooves (15, 16) i.e. grooves, wherein the direction of the helix is periodically inverted, are provided. The resistant elements for each of the cores, could also be more than one, for example, all twisted together. Upon one of the two cores, is inserted a tube 5 of a heat-shrinkable material, with an inner diameter that is slightly greater than the diameter of the grooved core. By 'diameter of the grooved core' is intended here the diameter of the tube circumference, where the core itself is inscribed. The softening temperature for the tube 5 is higher than that of the thermoplastic material of the core. In FIG. 1 the tube 5 (just shown partially) is positioned on the core 2. The end portions of said two grooved cores (1, 2), are bared of their thermoplastic covering, for a tract of a suitable length, that can be of 5- 20 mm, for uncovering the end portions of the resistant elements 11, 12, that are thereafter joined together mechanically, for example, by means of a ferrule or else, through other known means. Generally speaking, the connection is such as to determine a slight thickening in corresponding of the jointing between the resistant elements. In FIG. 1, there is shown the configuration assumed by the two grooved cores 1 and 2, after the connection of the resistant elements 11 and 12. There is then prepared a sleeve 4, having a length h which is equal to the tract, bared of its covering, of the so-formed resistant element. This sleeve is obtained from a length of a grooved core (which is identical to the grooved cores to be joined) with making a longitudinal cut 8 through which the resistant element is removed. Said longitudinal cut can be rectilineal, or else, curvilinear with extending, for example, between two grooves, in such a manner that none of the sleeve grooves become nicked, or altered in any way, by the longitudinal cut. Sleeve 4 is then positioned, again thanks to the longitudinal cut 8, on the joined resistant element (as is visible in FIG. 2) with causing, as far as possible, the grooves 15 and 16 of the core ends to be joined so as to correspond with the grooves present on the sleeve. Successively, into each groove there is inserted a metallic structural element 6 having a section equal to that of the groove, a greater length than the sleeve length h , and a configuration equal to that of the core grooves. More particularly, the element 6 (shown in FIG. 2) presents an open-helix configuration, for coupling with the open-helix configuration of both core grooves. In the instance of closed-helix, or rectilineal grooves, the elements 6 would have a corresponding configuration. The elements 6 are obtained by simply drawing a soft pliable material, for example, copper or aluminium, which is then shaped manually. Their elasticity can eventually compensate for any imperfect reproduction of the configuration of the grooves. Moreover, their cost results as being rather negligible, and they can be prepared as a function of the dimensions of the grooved cores, of the shape of the transversal section of the grooves, and of the configuration of the same, in a simple and rapid manner. Once the structural elements 6 have been inserted into the grooves of the cores and of the sleeve in such a manner as to connect them together, then the heat-shrinkable tube 5 is caused to slide and to be positioned on the sleeve. The tube length is greater than the sleeve length, but it is preferably lesser than the length of the structural elements. Thereafter, the heating is carried out in a uniform way, either with hot air, or with a flame 7, that is made to slide longitudinally (as shown in FIG. 3), with simultaneously rotating it around the core. The heat causes a uniform thermo-shrinkage of the tube 5, around the underlying thermoplastic material, and also the localized melting of the underlying thermoplastic material, which becomes shaped, or moulded by the structural elements 6 and by the inner surface of the tube 5 in such a way as to form helical grooves that are absolutely devoid of any discontinuity and/or burrs. As already indicated, the tube 5 is made out of a thermo-shrinkable material, having a higher melting temperature when compared to the melting temperature of the thermo-plastic material of the cores, said latter being equal to about the temperature at which said thermo-shrinkage occurs. The material of the said tube can, for example, be a fluoropolymer. The inner diameter of tube 5 is slightly greater than the diameter of the grooved cores, so that a same tube is practically suited for being used with grooved cores with diverse diameters, apart from, of course, absorbing eventual constructive tolerances that could be encountered between grooved cores that are produced, for example, at different times. Once the joint is cooled, tube 5 is removed, by making a longitudinal cut, from the joined grooved core in which the said thermoplastic material does not present any discontinuities, and where even the swelling, due to the ferrule 20, has been absorbed. The method, according to the invention, results as being particularly advantageous and economical. In fact, it allows for completely eliminating the costs involved with using a mould, and this latter is substituted by a low priced component such as the structural elements 6 and thermoshrinkable sleeve 5. Moreover, the method results as being extremely flexible, since, contrary to what could be had with moulds, it is made independent of any dimensions, and of any particular configuration of the grooved core. Moreover, the necessary components 5 and 6, if required, can be got ready in a very short period of time. Moreover, the elimination of the mould itself, allows for dosing, with precision, the quantity of thermoplastic material that is necessary for reconstructing. In fact, except for the slight excess due to the thickening, in correspondence of the joint between the resistant elements, the sleeve 4 contains the exact quantity of material that is necessary for reconstructing the h length tract. The joining operations do not require any particular ability, since no critical operations exist, not even for the heating necessary for melting, and to end with, no supplementary working operations are necessary for the elimination of burrs. The invention is also intended to include different types of grooved cores and in particular, cores having rectilineal, closed-helix grooves, or yet other configurations for said grooves.

Claims: Method of joining the ends of two cable cores (1, 2), each comprising at least one central elongated strenght member (11, 12) surrounded by a covering of thermoplastic material having on its peripheral surface one or more grooves (15, 16), which are apt to house optical fibres, said method being characterized by the following steps: a) applying a tube (5) over one of the ends of the cores to be joined, said tube (5) consisting of heat-shrinkable material and having an inner diameter which is slightly greater than the outer diameters of the core; b) removing the thermoplastic covering from an end portion of each core c) mechanically connecting the strenght members of each core, thereby forming a continuous strenght member having a section bare of thermoplastic covering; d) inserting onto the bare section a sleeve made of the same thermoplastic material as the covering and having grooves in its peripheral surface corresponding to the grooves of the cores to be joined, a diameter equal to the diameter of the cores, a lenght shorter than the heat-shrinkable tube and a longitudinal slot, said sleeve being obtained by removing the strength member from a predetermined lenght of a similar core and longitudinally slotting said length of remaining covering; e) aligning the grooves (15, 16) of the cores and the grooves of the sleeve by inserting into each groove of the sleeve a metallic structural element (6) having a greater length than the sleeve and a cross sectional shape which conforms with the shape of the grooves, so that the end portions of said elements extend into the respective grooves of the termoplastic covering of the cores; f) positioning the heat-shrinkable tube (5) at least over the sleeve (4) and over the elements (6); g) applying heat to the entire assembly of sleeve (4), tube (5) and elements (6) so as to cause the tube to shrink and the underlaying thermoplastic sleeve to melt locally, whereby the sleeve is fused to the thermoplastic covering of the cores, respectively, and the groove configuration is maintained; h) removing the heat-shrinkable tube (5) and the metallic structural elements (6) from the joint thus obtained. Method, according to CLAIM 1, characterized by the fact that the said grooves (15, 16) have a helicoïdal configuration and are equidistributed on the peripheral surface of the core. Method, according to CLAIM 1 or 2, characterized by the fact that the ends of the strength members (1, 2) are caused to be joined together by means of a ferrule (20).

Inventors: BORTOLIN, BRUNO, DOTTI, ENRICO
Applicants: PIRELLI CAVI S.P.A.
Application year: 1987
Main classification: G02B6
IPC codes: B29C63, H02G1, F16B4, G02B6, B29C65