GB1586661A - Electric cables and their manufacture - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRIC CABLES AND THEIR MANUFACTURE (71) We, TELEPHONE CABLES LIMITED, of Chequers Lane, Dagenham, Essex, RM9 6QA a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electric cables and their manufacture, and more particularly is concerned with the provision of an armoured protective covering over an electrically conducting core of such cable.

It is usual to provide a relatively thick protective sheath of insulating material over a conducting core of an electric cable, and the inclusion of metal armouring in the construction of an electric cable is also knawn to be desirable in some cases, to impart additional strength and crush resistance to the cable and to provide protection against mechanical damage to the conductors of the core. One known form of reinforced protective covering consists of a first sheath of plastics material surrounding the cable core, helically wound steel wires laid over this sheath, and a second plastics sheath applied over the wires, these three layers being applied to the core in separate manufacturing operations.

It is an object of the present invention to provide an electric cable having an improved reinforced protective covering over the electrically conducting core, which covering can be applied to the core by a simplified manufacturing procedure.

According to the invention, an electric cable includes, surrounding and electrically conducting core, a tubular sheath of thermoplastic material in which is embedded reinforcement consisting of a plurality of straight elongate tensile members, as hereinafter defined, each consisting of a single strand of metallic or non-metallic material, substantially free from curvature or twisting along its length, said tensile members being disposed longitudinally, parallel to the axis of the core, and individually spaced apart around the core; The electrically conducting core may be of any desired form, - for example of a conventional form comprising a plurality of conductors which may be individually covered with insulating material and may be straight or twisted around one another, in pairs or quads or any other desired multiples, and having tape laid over or wound around the assembly of conductors.

The term " tensile member", as used herein, means a member composed of material having high tensile strength, such that it is capable of withstanding tensile and bending strains to which the cable is subjected in use. The tensile members should be formed of material to which the thermoplastic sheath material is capable of adhering to a sufficient extent to prevent slipping of the tensile members within the sheath. The tensile members may each consist of a single wire of a metal having suitable tensile and frictional properties, for example steel of suitable composition, possibly roughened by surface treatment, or alternatively said members may be formed of non-metallic material, for example glass or plastics material of high tensile strength, and these members may be coated with a material providing adhesion to the thermoplastic sheath material if required. The tensile members may be of circular crosssection, typically of diameter 0 5 to 1 5 millimetres, or may have a non-circular cross-section, for a part or the whole of their length. They are preferably so arranged in the sheath that they are substantially equally spaced apart and substantially equidistant from the cable core: thus if a core is of circular cross-section, the tensile members are preferably arranged in a circle around the core; these members are located at a suitable position within the radial thickness of the sheath, for example somewhat nearer to the outer surface than to the inner surface of the sheath wall.

Suitable thermoplastic materials for use for forming the sheath include, for example, polyethylene and polyvinyl chloride.

In the manufacture of a cable in accordance with the invention, a tubular sheath of thermoplastic material, of the desired total wall thickness and incorporating the reinforcing tensile members within the wall, is formed around the cable core by a single extrusion operation. Thus the reinforced sheath may be applied to the core by-means of an extruder comprising a point and die-head, so designed that the tensile members, in the desired number and arrangement, are fed into and embedded in the thermoplastic material as it is extruded around the cable core.

A specific embodiment of an electric cable in accordance with the invention, and a method of and apparatus for manufacturing the cable, will now be described by way of example, with reference to the diagrammatic drawings accompanying the Provisional Specification, in which Figure 1 shows a portion of the cable, in- elevation, Figure 2 is a cross-section of the cable, drawn on the line II-III in Figure 1, Figure 3 slows, in part-sectional elevation, an extruder die-head tooling designed for use, for forming the reinforced plastics sheath of theicable of Figures 1 and 2, and Figure 4 similarly shows a modified form of die-head tooling which can alternatively be -employed for forming a reinforced cable sheath in accordance with the invention.

Like parts in the different figures of the drawings are indicated by the same reference numerals.

Referring - to - Figures 1 and 2 of the drawings, the cable consists of a core composed of one or more pairs or quads of insulated copper or aluminium wires 1 (a cable having one quad is shown by way of example, but it is to be understood that the specific embodiment is not restricted in respect of the core structure) surrounded by a wrapping of paper or plastics tape 2, and, surrounding the core, an extruded sheath 3 of polyethylene, reinforced by eight steel wires 4 lying parallel to the core and arranged in a circle, equally spaced apart, and located nearer to the outside than the inside of the radial thickness of the sheath.

In Figure, 1, only two of the reinforcing wires 4 are shown, for simplicity, and parts of the tape wrapping 2 and sheath 3 have been, cut away in order to show the conductors 1 of the cable core and their insulation, and the reinforcing wires 4.

In a specific example of a cable of the form shown in Figures 1 and 2, the diameter of the cable core is 4mm, the radial thickness of the polyethylene sheath 3 is 2-9 mm, and the wires 4 are 0 9 mm in diameter, the radial thickness of the polyethylene on the inside and the outside of each of the reinforcing wires being 1 4 mm and 0 6 mm respectively. The number of reinforcing wires employed in a cable of these dimensions may be either larger or smaller than the - number shown, suitable ranging from 4 to 12.

In the manufacture of the cable shown in Figures 1 and 2, the core may be produced in conventinnal manner, and the reinforced sheath can be applied to the core by means of the extruder die-head tooling shown in Figure 3, which comprises a conventional type of extruder die-head,, indicated in outline, at, 5, fitted with a conventional die component 6 and a specially designed point component 7. The point component is formed with the usual central duct 8 through which the cable core can pass, and with a number (for example eight) of additional ducts 9 arranged at regular intervals in a circle around the central duct, and of d suitable diameter for accommodating the reinforcing wires. Holes 11 and 12, corresponding respectively to the ducts 8-and 9 in the point component, are formed in the back plate 10 of the extruder die-head.

The point component is fitted to the extruder head by means of a screwed-in adaptor 13 provided with a locating peg 14, which co-operates with a recess 15 in the point component to position the làttèr correctly so that the ducts 9 are correlated with thé-holes 12 in the back plate; the back plate is atfached and positively located at the back of the extruder head.

In operation of this extruder die-head, for the formation of the cable sheath, the cable core 1, 2- and steel wires 4 (as shown in Figures 1 and 2) are fed respectively through holes 11 and 12 in the back plate and pass through the ducts 8 and 9 respectively, while at the same time polyethylene supplied to the annular channel 16 between the point and die is extruded around the core and reinforcing wires as they emerge from- the point tip 17, so that the wires are embedded in the wall of the polyethylene sheath formed around the cable core.

The shape of the point tip, of the extruder die-head can be modified as desired to control the internal diameter of the extruded sheath. With a tip of the form shown at 17 in Figure 3, the internal diameter of the sheath will be determined by the external diameter of the core so that the sheath will firmly grip the core.

If, however, the sheath is required not to grip the cores but to fit over it fairly loosely, a 'modified form of point tip as shown in Figure 4 may be used. In this case the point tip is formed with an extension constituting an elongated nozzle 18, which prevents the extruded sheath from coming into contact with the cable core immediately on emerging around the tip: in the finished cable the sheath will be in contact with the core but will not grip it. In other respects the diehead tooling shown in Figure 4 is of similar construction to that of Figure 3, apart from the additional optional modification that the central duct 8 is of reduced diameter within the point tip and nozzle, to accommodate a cable core of smaller diameter than that for which the point component of Figure 3 is suitable.

WHAT WE CLAIM IS: 1. An electric cable which includes, surrounding an electrically conducting core, a tubular sheath of thermoplastic material in which is embedded reinforcement consisting of a plurality of straight elongate tensile members, as hereinbefore defined, each consisting of a single strand of metallic or non-metallic material, substantially free from curvature or twisting along its length, said tensile members being disposed longitudinally, parallel to the axis of the core, and individually spaced apart around the core.

2. An electric cable according to Claim 1, wherein each of said tensile members consists of a single metal wire.

3. An electric cable according to Claim 1, wherein the said tensile members are formed of glass or plastics material.

4. An 'electric cable according to any preceding Claim, wherein the said tensile members are of circular cross-section and are of diameter from 0 5 to 1 5 miZi- metres.

5. An electric cable according to any preceding Claim, wherein the said tensile members are substantially equally spaced apart in the sheath and are substantially equidistant from the cable core.

6. An electric cable according to any preceding Claim, wherein the cable sheath is formed of polyethylene or polyvinyl choride.

7. A method of manufacturing an electric cable according to any preceding Claim, wherein a tubular sheath of thermoplastic material, of the desired total wall thickness and incorporating the said tensile members within the wall, is formed around the cable core by a single extrusion operation.

8. A method according to Claim 7, wherein the said sheath is applied to the cable core by means of an extruder comprising a point and die-head, so designed that the said tensile members, in the desired number and arrangement, are fed into and embedded in the thermoplastic material as it is extruded around the core.

9. An electric cable according to Claim 1, substantially as shown in, and as hereinbefore described with reference to, Figures 1 and 2 of the drawings accompanying the Provisional Specification.

10. A method of manufacturing an electric cable according to Claim 9, substantially as hereinbefore described with reference to Figure 3 or Figure 4 of the drawings accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. around the tip: in the finished cable the sheath will be in contact with the core but will not grip it. In other respects the diehead tooling shown in Figure 4 is of similar construction to that of Figure 3, apart from the additional optional modification that the central duct 8 is of reduced diameter within the point tip and nozzle, to accommodate a cable core of smaller diameter than that for which the point component of Figure 3 is suitable. WHAT WE CLAIM IS:

1. An electric cable which includes, surrounding an electrically conducting core, a tubular sheath of thermoplastic material in which is embedded reinforcement consisting of a plurality of straight elongate tensile members, as hereinbefore defined, each consisting of a single strand of metallic or non-metallic material, substantially free from curvature or twisting along its length, said tensile members being disposed longitudinally, parallel to the axis of the core, and individually spaced apart around the core.

2. An electric cable according to Claim 1, wherein each of said tensile members consists of a single metal wire.

3. An electric cable according to Claim 1, wherein the said tensile members are formed of glass or plastics material.

4. An 'electric cable according to any preceding Claim, wherein the said tensile members are of circular cross-section and are of diameter from 0 5 to 1 5 miZi- metres.

5. An electric cable according to any preceding Claim, wherein the said tensile members are substantially equally spaced apart in the sheath and are substantially equidistant from the cable core.

6. An electric cable according to any preceding Claim, wherein the cable sheath is formed of polyethylene or polyvinyl choride.

7. A method of manufacturing an electric cable according to any preceding Claim, wherein a tubular sheath of thermoplastic material, of the desired total wall thickness and incorporating the said tensile members within the wall, is formed around the cable core by a single extrusion operation.

8. A method according to Claim 7, wherein the said sheath is applied to the cable core by means of an extruder comprising a point and die-head, so designed that the said tensile members, in the desired number and arrangement, are fed into and embedded in the thermoplastic material as it is extruded around the core.

9. An electric cable according to Claim 1, substantially as shown in, and as hereinbefore described with reference to, Figures 1 and 2 of the drawings accompanying the Provisional Specification.

10. A method of manufacturing an electric cable according to Claim 9, substantially as hereinbefore described with reference to Figure 3 or Figure 4 of the drawings accompanying the Provisional Specification.