US2015476A - Electric cable - Google Patents

Description

Sept. 24, 1935, v HUNTER 2,@l5,476

ELECTRIC CABLE Filed March 24, 1932 INVENTOR ATTORNE )6 Watented Sept. 24, 1935 2,015,476 ELECTRIC CABLE Philip Vassar Hunter, London, England, assignor to Callenders Cable and Construction Company Limited, London, England, a British com- Application March 24, 1932, Serial No. 600,881 In Great Britain April 2, 1931 6 Claims. (61.173-266) This invention relates to the construction of the enclosing sheaths of electric cables and deals particularly with the case where these sheaths are intended to withstand appreciable internal pressure so that it is necessary to use some form of reinforcement. The form of reinforcement hitherto employed in connection with the lead sheaths of cables has consisted of either metal tape or wires usually of steel but sometimes of 10 bronze or other metals. This material is lapped on spirally, necessarily with a comparatively short lay, so as to provide the required circumferential strength.

In some cases paper has been applied to a lead sheath for the purpose of serving as a bedding for such metal reinforcement. The object of this bedding is to prevent the metal reinforcing member or members from cutting or pressing into and thereby damaging the lead sheath. Paper applied under these conditions cannot provide any appreciable reinforcement of the sheath for several reasons, the chief of which is that the modulus of direct elasticity (Young's Modulus) for paper is about three per cent only of that for the metal of the reinforcement. Accordingly,

even if the conditions are adjusted to the best advantage, the paper cannot be made to take a circumferential tensile stress greater than three per cent of the corresponding stress in the metal reinforcement. It is, however, highly probable that even this nominal amount of stress in the paper will not in fact be present since the tension, with which the metal reinforcement is applied, will be greater than that with which the paper was lapped on and accordingly the paper will undergo an initial compression as the metal is wound on. This is in fact essential to the use of the paper as a bedding and further, since the compression is not uniform, local deformation of the bedding will occur. If to these conditions is added the further feature that the layer of paper for bedding will be much thinner than the metal reinforcement, it is seen that the paper can make no real contribution to the reinforcing effect.

I have found that if paper strip is applied to a lead sheath under appropriate conditions, it can act very effectively as a reinforcement and in accordance with the present invention we utilize paper strip for the: purpose of providing the reinforcement. In accordance with the present invention I provide the lead sheath with a covering consisting of strips of paper applied under tension and helically to the sheath, the

- total thickness of the paper being such as to provide a substantial reinforcement of the sheath against the internal pressure. The reinforcing material is protected against mechanical damage by an outer'covering of suitable material. The paper can be lapped on in the manner adopted for the application of paper insulation to conductors in the manufacture of cables, the lay of the strips being short and the adjacent convolutions of a layer abutting or overlapping. It is found that by this means we can obtain from the tensile strength of the paper very considerable reinforcement of the structure. As an example, it is stated that paper having a thickness of about five thousandths of an inch per layer and built up to a. total thickness of .075" provides a satisfactory reinforcement. 15

It is to be understood that in general the paper will beapplied directly to the surface of the lead sheath and in particular it has to be noted that there must not be interposed between the sheath and the paper anything of the nature of a semiliquid material such as impregnating compound I or pitch since such material will under pressure be forced through the paper and thereby prevent the reinforcing effect from being obtained.

One form of construction in accordance with the invention is illustrated by way of example in the accompanying drawing, which shows a perspective view of a cable.

A conductor I has applied to it helically a number of strips of paper to form the insulation 2. 30 The conductor and the insulation are enclosed in a lead sheath 3'. This sheath is reinforced by paper 4 which consists of a number of paper strips applied helically to the inner sheath 3. The strips are applied under tension andbuilt up 35 to the required total thickness. The paper reinforcement is protected from mechanical injury and moisture by an outer lead sheath 5.

The inner sheath has principally the function of establishing a. complete enclosure of the in- 40 sulation of the cable and is therefore made of soft ductile lead. Under internal pressure it may expand slightly until restrained by the paper and the external sheath. The function of the latter is mainly the provision of protection for the 45,

paper reinforcement and may be made of a lead alloy having greater hardness and strength than the inner sheath. Other suitable material which will protect the reinforcement from injury and moisture may, however, be used.

What I claim as my invention and desire to secure by Letters Patent is:

h 1 An electric cable having a conductor, insulation around the conductor, a sheath of soft duc tile lead enclosing the insulation, said insulation 55 being such that an internal pressure will be ciperted on the lead sheath under working conditions, paper reinforcing strips applied helically under tension to the sheath and built up to a suflicient thickness to constitute the principal means of reinforcing the sheath against internal pressure, and an outer sheath of lead of greater hardness and strength than the inner sheath, to protect the paper reinforcing strip against mechanical damage.

2. An electric cable having a conductor, insula tion around the conductor, a lead sheath enclosing the insulation, said insulation being such that an internal pressure will be exerted on the lead sheath under working conditions, paper strips of about five thousandths of an inch in thickness applied helically under tension to the lead sheath and built up to a total thickness of about 0.075 inch to constitute the principal reinforcing means, and an outer covering of suitable material placed over the paper reinforcement to pro tect it against mechanical damage.

3. A cable of a type having concentric lead sheaths, reinforcement therebetween against in ternal pressure, wherein the reinforcement consists substantially entirely of paper.

4. An electric cable comprising a conductor, in

aoisavo sulatlon around the conductor, a lead sheath enclosing the insulation, said insulation being such that under working conditions internal. pressure will be exerted on the lead sheath of sufficient strength to distend the sheath permanently if it 5 is unsupported, reinforcement for the lead sheath supporting it against such pressure, said reinforcement consisting of smooth strips of paper smoothly and tightly applied to the sheath helically under tension in superimposed layers to the necessary thickness, and an outer covering of suitable material placed over the paper reinforcement to protect it against mechanical damage and moisture.

5. A cable comprising concentric lead sheaths,

reinforcement therebetween for resisting internal pressure, said reinforcement consisting substantially entirely of wrapped paper, said cable having a conductor therein.

6. In an electric cable of the type which is subtially entirely of strips of paper Wrapped under tension.

PHILIP VASSAR HUNTER.

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