COMMUNICATIONS CABLE

Abstract: A communications cable comprises a plurality of coaxial elements (1) which are stranded around a central structure (2). The central structure (2) is resiliently deformable to allow limited displacement of the coaxial elements (1) when the cable is subjected to bending during installation. The central structure may comprise simply a tube of plastics or rubber material, or a plurality of tubes laid-up together.

Description: COMMUNICATIONS CABLE This invention relates to a communications cable which comprises a plurality of coaxial elements. In cable television systems, use of such cables is envisaged when the multiple star configuration is employed, the multiple-element coaxial cable facilitating cabling from switching points at the centre of each star. For installing these cables in built-up areas, the ducts which are to receive the cables are already crowded with cables for other uses, particularly telephone. As a consequence, the television cables are liable to be subjected to a combination of pulling tensions and bending forces which are higher than those experienced when laying multiple-element coaxial cables for long distance telecommunication trunk links. We have found that the coaxial elements on or around the axis of the cable are the most liable to suffer deformation under the higher forces to which the cable is subjected. Further, we have now devised a multiple-element coaxial cable which overcomes this problem. In accordance with this invention, there is provided a communications cable which comprises a resiliently deformable central structure and a plurality of coaxial elements stranded around that structure. The resiliently deformable central structure may comprise simply a tube of plastics or rubber material, and will deform when the cable is subjected to dynamic bending so as to give the coaxial elements limited freedom to displace and avoid being deformed. For example, whereas a conventional cable might have 30 coaxial elements arranged in 3 layers with 4 elements at. the centre, then a layer of 10 elements and finally a layer of 16 elements, a cable in accordance with this invention might have the 30 coaxial elements arranged in two layers (one of 12 elements and the other of 18 elements) around a central tube. We have found that the provision of a resiliently deformable central structure is sufficient to prevent the coaxial elements being permanently deformed during handling and installation and that no other measures such as embedding the coaxial elements in a cushioning material are required. Instead of a single tube, the central structure may comprise a plurality of resiliently deformable tubes laid-up together, for example say 6 tubes stranded around a central member which may or may not itself be resiliently deformable. Even a single tube could be helically applied around such a central member. The first layer of coaxial elements would be stranded in the opposite direction of lay to the underlying tube or tubes. The or each tube may comprise a semi-air-spaced tube, i.e. with internal longitudinal webs radiating from its axis, with or without a central reinforcing member. The cable, and particularly the tube or tubes. may be filled with a water impermeable medium such as petroleum jelly to prevent migration of water along the cable should it penetrate the sheath at any point. The tube or tubes may serve as carriers for other cable elements, such as optical fibres or control wires. An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-section through a communications cable in accordance with this invention; and Figure 2 is a cross-section, on an enlarged scale, through an alternative form of resiliently deformable tube from which the central structure of the cable may be made. Referring to Figure 1 of the drawings, there is shown a communications cable comprising a plurality of coaxia] elements 1 stranded as two layers (of 12 and 18 elements respectively) around a resiliently deformable central structure 2, which in the example shown comprises a single, hollow plastics tube. The two layers of coaxial element S are st ianded in the sane ol oppo- site directions of lay and are separated by a winding of paper tape 3 around the inner layer. A further winding of paper tape 4 is applied around the outer layer of coaxial elements and the cable is completed by a plastics sheath 6 to the inner surface of which a metal foil moisture barrier 5 is bonded. The tube 2 is selected of optimum characteristics to resist permanent. collapse but to undergo resilient deformation during dynamic bending of the cable during installaiton, so that. the coaxial elements have some freedom to move and avoid being deformed and damaged themselves. The tube 2 may comprise soft PVC, reinforced PVC or polyethylene for example, or a rubber or elastomeric material. As mentioned previously the central structure mag instead comprise a plurality of tubes laid-up together and for example around a central member. Also, the or each tube may comprise a semi-air-spaced tube as shown in Figure 2, wherein the tube is formed internally with a plurality of longitudinal webs 7 radiating from the axis, with or without a reinforcing member along this axis.

Claims: CLAIMS 1. A communications cable comprising a plurality of coaxial elements (1) characterised in that the coaxial elements (1) are stranded around a central structure (2) which is resiliently deformable to allow limited displacement of the coaxial elements (1) when the cable is subjected to bending during installation. 2. A cable as claimed in claim 1, wherein the coaxial elements (1) are arranged in two layers around said central structure (2). 3. A cable as claimed in claim 1 or 2, wherein said central structure comprises a tube (2). 4. A cable as claimed in claim 1 or 2, wherein central structure comprises a plurality of laid-up tubes. 5. A cable as claimed in claim 4, wherein said tubes are laid-up around a central member. 6. A cable as claimed in claim 3, 4 or 5, wherein the or each tube comprises a plastics material. t. A cable as claimed in any one of claims 3 to 6, wherein the or each tube is hollow. 8. A cable as claimed in any one of claims t. to 6, wherein the or each tube is formed internally with a plurality of longitudinal webs (7). 9. A cable as claimed in claim 8, wherein said webs (7) radiate from the axis of the or each tube along which axis a reinforcing member may extend. 10. A cable as claimed in any one of the preceding claims, wherein a plastics sheath (6) extending over said coaxial elements (1) has a moisture barrier (5) bonded to its inner surface.

Inventors: COWLEY, MICHAEL JOHN, OSTERFIELD, JOHN RICHARD
Applicants: PIRELLI GENERAL PLC
Application year: 1985
Main classification: H01B11
IPC codes: H01B7, H01B11