Method Of Making A Communications Cable

Abstract

A communications cable comprises a waterproof core of conductors and a sheath including an unsoldered steel layer for providing mechanical and rodent protection. The steel layer is stretch-formed to attain a tightly registered longitudinal seam which eliminates the necessity of soldering or other means of mechanically joining the seam. Coatings of appropriate thermoplastic materials are placed on the surfaces of the steel layer and drawn into the seam by capillary action to provide a seal against water ingress.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to communications cables. More particularly, it relates to a cable having a waterproof core of conductors and a sheath for mechanical and rodent protection which includes a steel layer that is stretch-formed to attain a tightly registered unsoldered longitudinal seam which draws a suitable flooding compound into the seam to prevent water ingress.

2. Description of the Prior Art

The signal carrying conductors in a communications cable must be protected from many factors in the surrounding environment which would degrade the signal being transmitted. Among these factors are mechanical effects such as abrasion and handling and, in many areas of the country, rodent attacks upon the cables. Other factors which must be considered are lightning strikes and electrical disturbances. Still another factor, and one which is often an effect of one or more of the previously mentioned factors, is the penetration of moisture into the cable core and water into the axial spaces between the sheath layers.

Mechanical and rodent protection is normally provided by metal layers in the sheath surrounding the cable core. Various types of metal layers may be utilized. However, it has been found advantageous to utilize a layer of highly conductive metal in conjunction with a layer of high-strength metal. The conductive layer provides lightning protection and shielding from the electrical disturbances while the high-strength layer provides mechanical strength and protection at low cost and weight and without significantly decreasing cable flexibility because of excessive thickness. Specifically, a structure comprising layers of steel and aluminum as suggested in U.S. Pat. No. 2,589,700, issued to H. G. Johnstone on Mar. 18, 1952, has been utilized. The seam of the steel or high-strength metal layer is soldered, bonded, or otherwise mechanically joined to provide a hermetic seal and prevent moisture penetration and to hold the layers in position about the cable core.

The step of soldering, bonding, or otherwise mechanically joining the high-strength metal layer in a cable sheath is, in most situations, the most expensive step in the entire cable manufacturing operation. For example, if the seam of the high-strength metal layer is to be soldered, it is normally necessary to provide layers or coatings of conductive metal on the surfaces of the high-strength metal layer to permit reasonable soldering rates. Such conductive coatings add considerably to the cost of the metal sheath. Further, because of limitations on solder rates, even with the conductive coatings, the cable manufacturing process normally cannot be carried out as a continuous in-line operation. This necessitates additional handling and adds further to cable costs.

Accordingly, it is an object of this invention to provide a communications cable which has adequate mechanical and moisture protection without the necessity of soldered seams in the metal layers therein.

Another object is to provide a communications cable which may be manufactured in a series of high-speed operations such that the cable may readily be made on a continuous or in-line operation.

Another object is to provide a waterproof, mechanically protected communications cable which may be manufactured at lower cost.

A further object is to provide a simplified method of making a waterproof mechanically protected cable.

SUMMARY OF THE INVENTION

The foregoing objects and others are achieved in accordance with the principles of this invention by a cable comprising a waterproof core of conductors and a sheath including a steel layer which has a tightly registered unsoldered longitudinal seam. The core of conductors is surrounded by a suitable waterproof filler material, such as a mixture of petroleum jelly and a high average molecular weight thermoplastic polymer, which eliminates the necessity of the surrounding sheath forming a hermetic seal. The cable sheath includes a corrugated steel layer which is stretch-formed to attain a tightly registered longitudinal seam which eliminates the necessity of soldering or bonding. The steel layer provides mechanical and rodent protection to the cable. The steel layer is covered with a melted thermoplastic compound that is pulled into the registered seam by capillary action to seal this seam to water ingress. A thermoplastic jacket is extruded over the flooded steel layer for corrosion protection and an aluminum layer is placed under the steel layer to provide lightning protection and shielding.

The unsoldered flooded steel layer and thermoplastic jacket may be used without the underlying aluminum layer over existing cable sheaths to provide mechanical and rodent protection.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be more fully comprehended from the following detailed description and accompanying drawing in which the single FIGURE is a schematic representation, in perspective and partly broken away, showing the cable of this invention.

DETAILED DESCRIPTION

Referring now to the drawing, a cable 101 includes a plurality of conductors or conductor pairs 4 within a cable core 2. The conductors 4 are surrounded by and the interstitial spaces therebetween are filled with a waterproof filler material 6 such as disclosed in the copending application of M. C. Biskeborn et al. Ser. No. 780,314, filed Dec. 2, 1968 and assigned to applicant's assignee; and this disclosure, to the extent pertinent, is hereby incorporated by reference into the instant specification. One example of a suitable waterproof filler material is a 3-to-1 by weight mixture of petroleum jelly and a thermoplastic polymer of high average molecular weight.

About core 2 is a core wrap 8 which may be a suitable plastic or other material. A binder can be placed around core wrap 8 to hold it in position about core 2.

In order to provide lightning protection and shielding for core 2, a layer of conductive metal is placed about the core. A thin aluminum layer 10 having a longitudinal seam 14 therein advantageously can be used for lightning protection and shielding. Longitudinal seam 14 is not required to be soldered or otherwise mechanically joined.

In accordance with this invention, a steel layer 20 having unsoldered overlapping edges 16 and 18 forming a longitudinal seam 17 is longitudinally wrapped about aluminum layer 10 to provide protection from mechanical forces such as abrasion and from rodent attacks. The use of an unsoldered seam 17 for steel layer 20 is possible since the cable core 2 is waterproof. Steel layer 20 and aluminum layer 10 advantageously can be transversely corrugated and meshed with each other to provide a more flexible sheath. Steel layer 20 is stretch-formed and cold-worked as it is wrapped about aluminum layer 10 and edges 16 and 18 are closely meshed to provide a tightly registered overlapping seam 17. The stretch-forming and cold-working insure that edges 16 and 18 retain their respective positions without the necessity for external holding forces after the forming forces have been removed. Thus the tightly registered seam 17 is maintained. Edges 16 and 18 will retain their positions and maintain the tightly registered seam 17 even when cable 101 is would on a reel. The outer or overlying edge 16 of steel layer 20 advantageously can be turned slightly inward toward core 2 to insure that no sharp edges are presented by steel layer 20.

Since steel layer 20 is stretch-formed and cold-worked so that it retains its tightly registered seam 17 after removal of the forming forces therefrom, there is no necessity of soldering edges 16 and 18. Thus, one of the more costly operations, i.e., soldering, previously used in the manufacture of cables having a steel layer is eliminated. Further, the cable of this invention having an unsoldered steel layer can be manufactured in an in-line or continuous operation since the relatively slow soldering step has been removed. Such an in-line operation offers significant economic advantages.

A still further advantage of eliminating the soldered seam is that a much less expensive material can be utilized in steel layer 20. A cold-rolled steel sheath is sufficient for forming unsoldered steel layer 20 of this invention whereas a more expensive tin or copper-coated steel sheath is required where the seam is soldered.

Corrosion protection for steel layer 20 and added protection against water penetration are provided by hot-melt flooding each side of steel layer 20 with respective coatings 12 and 22 of a corrosionproof, waterproof material. This readily can be accomplished by drawing cable 101 through a bath of appropriate material as layer 20 is being applied. Coatings 12 and 22 advantageously might be the same material as is utilized for filler material 6. Protection against water penetration is obtained since coatings 12 and 22, respectively, fill all spaces between steel layer 20 and the adjacent layers 10 and 24 of the cable sheath. Seam 17 is also sealed against water ingress by coatings 12 and 22 being drawn into seam 17 by capillary action of the tightly registered seam. Added mechanical strength is also obtained from the adhesive forces of coatings 12 and 22 which tend to adhere steel layer 20 to adjacent layers 10 and 24.

For added corrosion protection of layer 20 and for additional mechanical and moisture protection, an exterior thermoplastic jacket 24 advantageously is extruded around the exterior surface of layer 20. Thus, the cable sheath comprising an aluminum layer 10, an unsoldered steel layer 20 and a thermoplastic layer or jacket 24 joined by corrosion coatings 12 and 22 provides mechanical, rodent, and waterproof protection at a cost substantially less than the sheaths of prior art cables.

The principles of this invention also can be utilized to provide mechanical, rodent, and waterproof protection to cables which utilize presently known cable sheaths. In such application the stretch-formed steel layers, the layers of corrosionproof, waterproof material, and thermoplastic jacket only are placed over the existing cable sheath. For example, Alpeth is a well-known cable construction that is utilized in very large quantities of cable. However, the Alpeth construction does not have sufficient mechanical, rodent, and waterproof protection for certain applications. Such protection can be provided very economically by placing a stretch-formed layer of cold-rolled steel and an outer thermoplastic jacket, joined by a layer of corrosionproof, waterproof material, according to the principles of this invention, about the existing Alpeth sheath.

It is to be understood that the above-described embodiments are merely illustrative of the principles of this invention. Various modifications thereto might be made by those skilled in the art without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A method of making a communications cable comprising the steps of:

forming a waterproof core of conductors;

longitudinally folding a corrugated layer of conductive metal about said core;

cold-working and stretch-forming a corrugated layer of steel about said layer of conductive metal such that a tightly registered longitudinal seam is formed by the edges of said steel layer without the necessity of mechanically joining said edges; and

extruding a thermoplastic jacket about said steel layer to provide corrosion protection thereto.

2. The method of claim 1 including the step of flooding each side of said steel layer with a corrosion-resistant, waterproof material while said layer is being cold-rolled and stretch-formed.

3. A method of providing mechanical and rodent protection to a communications cable comprising the steps of:

cold-working and stretch-forming a corrugated layer of steel about said cable such that a tightly registered longitudinal seam is formed by the edges of said steel layer without the necessity of mechanically joining said edges; and

extruding a thermoplastic jacket about said steel layer to provide corrosion protection thereto.