MULTI-CONDUCTOR CABLE

Abstract: A multi-conductor cable such as an electric power cable, a signal-transmission cable or optical fiber cable, including a core made of a plurality of insulated conductors, a sheath surrounding the core, and a plurality of foamed plastic string fillers which fill the void space between the core and the sheath and between the insulated conductors.

Description: This invention relates generally to cables and, more specifically, to multi-conductor cables such as electric power cables, signal-transmission cables and optical fiber cables. A multi-conductor cable is generally composed of a core of a plurality of insulated conductors, a sheath surrounding the core, and a filler occupying the interstices within the core and between the sheath and the core. The filler serves to provide circular cross section of the cable, to prevent the deformation of the cable and to improve the tensile strength of the cable. Paper tapes, jute and split yarns of plastic films have been hitherto used as the filler. Since the filler is employed in relatively a large amount, it is desired to provide a filler which is inexpensive, light in weight and high in tensile strength. US-A-3288916 discloses a cable which comprises a core made of a plurality of insulated conductors and a sheath surrounding the core. The void space between the core and the sheath contains space filler in the form of elongated tubes of a foamed plastics material. There is, however, no space filler in the void space between adjacent insulated conductors within the core. US-A-3843831 discloses a cable in which at least three elongated fillers, which extend the length of the cable, define with a surrounding sheath elongated recesses each of which receives an insulated conductor. The void space between the surrounding sheath and the resulting core of conductors and fillers contains so-called "minor fillers". There is provided in accordance with the present invention a cable which, as in US-A-3843831, comprises: a core made of a plurality of insulated conductors, and a sheath surrounding the core, the void space between the insulated conductors of the core and between the core and the sheath containing space filler which comprises a plurality of strings. The cable in accordance with the present invention is characterized in that each of the strings is a composite formed of a foamed plastics layer integrally bonded to a reinforcing strip which forms an external surface of the string. The present invention will now be further described with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of the cable according to the present invention; Figs. 2 and 3 are enlarged, cross-sectional views diagrammatically showing filler strings used according to the present invention; Fig. 4 is a partial, enlarged perspective view diagrammatically showing a further embodiment of a filler string according to the present invention; and Fig. 5 is an enlarged, cross-sectional view diagrammatically showing the use of filler strings of this invention in conjunction with conventional space fillers. Referring first to Fig. 1, the cable according to the present invention includes a core made of a plurality (three in the particular illustrated case) of insulated conductors 20 each made of an electric wire 1, an insulator 2 such as of a plastics material and a covering 3. The core is surrounded by a sheath 4 formed, for example, of a plastics material such as polyethylene. The plastics sheath 4 may be extruded over the core and, if desired, bonded to the core. If necessary, a shielding tape (not shown) may be provided between the core and the sheath 4 for enclosing the bundled conductor assembly. The void space or interstices between the insulator conductors 20 and between the sheath 4 and the conductors 20 are filled with space filler including a plurality of composite strings 5 formed of a layer of a foamed plastics material integrally bonded to a reinforcing strip of a plastics material. Illustrative of suitable foamed plastics materials are foamed polyolefins such as polypropylenes, polyethylenes and polybutenes, polystyrenes and polyurethanes having an expansion ratio of about 3-100, preferably about 5-80. Above all, the use of a foamed polypropylene is particularly preferable for reasons of its excellent electrical and mechanical properties, high resistance to heat and inexpensiveness. Examples of suitable strings 5 before being filled in the void space of the cable are schematically shown in Figs. 2 and 3 by way of a cross section on a line perpendicular to the longitudinal axis of each string 5. It is preferred that the string 5 be in the form of a tape. Preferably, the tape has a thickness of about 0.5-5 mm and a width of about 5-150 mm. String 5 in the form of a tape can deform into any shape by application of mechanical stress. Thus, when the tapes are packed in the void space of the cable in a compressed state, the void space can be substantially entirely filled with the tapes without leaving any voids because of the elastic deformation of the tape fillers. Consequently, the resulting cable becomes uniform in size and in mechanical strength throughout its length. Fig. 2 depicts a preferred example of the string 5. The string 5 is a composite string formed of a thin strip tape 6 and a foamed plastics layer 7 integrally provided over the surface of the tape 6. The tape 6 serves as a reinforcing member and is preferably formed of a plastics film having a thickness of about 5-50 µm. The plastics film is preferably uniaxially stretched in a direction parallel to the lengthwise direction. Examples of suitable plastics films include polypropylene films, polyethylene filns, polybutene films, polyester films and polyacetal films. The composite string 5 shown in Fig. 2 may be prepared, for example, by a method including the steps of providing a stretched plastics film, superimposing a foamed plastic resin layer on the film, using, if necessary, an adhesive, and severing the resulting laminate into strings in a direction parallel to the stretching direction of the film. Fig. 3 depicts another example of the composite string according to the present invention. The composite string 5 is composed of a foamed plastics layer 8 with both its sides being bonded to two plastics tapes 9. With the composite strings of this embodiment, the production of a cable may be performed more easily since the strings have an improved slippage. That is, the friction between the strings, between the insulated conductors and the strings, between the guides and the strings and between the flared assembling tube and the strings which is caused during the assembling and bundling step may be reduced because each string has an outer surface covered with the plastics tapes 9. Fig. 4 illustrates a further embodiment of the composite string 5. The string 5 is longitudinally folded to have a U-shaped or C-shaped cross section with its plastics tape 10 forming the outer surface of the string and with its foamed plastics layer 11 forming the inside surface. This structure can exhibit the same friction-reducing property as that of Fig. 3 but is more preferred because it is easier to prepare, and because of its lightness in weight. By using a combination of the plastics tape 10 and the foamed plastics layer 11 which has a more tendency to shrink than the tape 10, the composite string can be spontaneously curved. Alternatively, the desired curved structure may be obtained by compositing the plastics tape 10 with the foamed plastics layer 11 at such a temperature as to cause the shrinkage of the foamed plastics layer 11. The filler strings 5 according to the present invention may be used in conjunction with conventional fillers such as slit yarns, paper tapes and the like. Fig. 5 schematically illustrates such an embodiment. The fillers include composite foamed plastic strings 5 here shown schematically and slit yarns 12. It is preferred that at least 50 vol % of the space filler filled in the cable be occupied by the foamed plastics strings 5 to satisfactorily accomplish the objects of the present invention. The cable according to the present invention may be prepared in any known manner. For example, a desired number of insulated conductors and a suitable number of the strings are assembled by a flared forming tube to form a core continuously travelling in the lengthwise direction, with its upstream portion being covered with a plastic sheath by extrusion and cooled, thereby to continuously produce a cable. The cable thus obtained is light in weight because of the space fillers formed largely of a foamed plastic. Further, since the strings are elastically deformable, they can be easily assembled together with the insulated conductors to form a core and they can completely and evenly fill the void space within the cable sheath, so that the cable not only shows an improved elasticity but also has a substantially uniform cross section and uniform physical properties throughout the length.

Claims: A cable comprising: a core made of a plurality of insulated conductors (20), and a sheath (4) surrounding the core, the void space between the insulated conductors (20) of the core and between the core and the sheath (4) containing spacer filler comprising a plurality of strings (5), characterized in that each of said strings (5) is a composite formed of a foamed plastics layer (7;8;11) integrally bonded to a reinforcing strip (6;9;10) which forms an external surface of said string (5). A cable as claimed in Claim 1, wherein each of said strings (5) has a thickness of 0.5-5 mm and a width of 5-150 mm. A cable as claimed in Claim 1, wherein said reinforcing strip (6;9;10) is formed of a plastic material which has been stretched in the lengthwise direction. A cable as claimed in any preceding claim, wherein each of said strings is longitudinally folded or curved to have a U-shaped or C-shaped latitudinal cross section with its reinforcing strip (10) forming the external surface of the string. A cable as claimed in any preceding claim, wherein each of said strings includes two plastics reinforcing strips (9) and a foamed plastics layer (8) sandwiched between the two plastics strips (9). A cable as claimed in any preceding claim, wherein said space filler comprises at least 50% by volume of said strings (5).

Inventors: FUJIMURA, SHIGEKI, SAKAMOTO, ITARU, YOSHIMURA, SHOHEI
Applicants: JAPAN STYRENE PAPER CORPORATION
Application year: 1986
Main classification: H01B7
IPC codes: H01B7