Center Strength Member Cable

Abstract

A high strength, lightweight electrical cable having a center strength wire rope member and a plurality of shielded and insulated conducting wires arranged around the center strength member is disclosed, the conducting wires being shielded through the use of a spirally wound layer of conducting material and being cushioned through the use of a soft insulation layer and a plastic filler interposed between the center strength member and the conductors. The conducting wires are formed in bundles and are spirally wound around the center insulator and strength member. A sheath consisting of an open braid of nylon strands secures the conductor bundle, and the entire assembly is jacketed by means of an extruded layer of polyurethane which penetrates into the nylon braid, thereby providing a strong waterproof shield for the conductors. The center strength member is torque-balanced to avoid twisting and imparting rotational movement to a suspended load member.

Description

BACKGROUND OF THE INVENTION

Electrical cables having both electrical conductors and separate strength members have been in common use for many years, and the configuration of such cables varies with the use to which the cable is put. In an application for airborne sonar use, there are some special requirements since the cable must support a sonar transducer which is rapidly lowered a substantial distance into the ocean and then reeled back to a hovering helicopter, and must carry power to the transducer and signal information from the transducer. It is desirable to deploy the transducer and recover it as quickly as possible, and special stresses occur at certain points in the operating cycle, such as when the transducer is initially started up on recovery and when the transducer breaks from the surface. Thus the cable must be sufficiently strong to withstand these stresses and yet should be as light as possible because of the need to support the structure from a helicopter. The cable is subject to wear and abrasion because it is reeled in and out from a drum over a sheave. The transducer on the end of the cable tends to rotate on its vertical axis if there are forces available to cause this rotation. The cable should preferably not contain an inherent force tending to cause this rotation, since the means correcting the sonar signal for orientation of the transducer is somewhat limited in its ability to track and compensate for such rotation. The above requirements favoring small size and lightness also run counter to the need for including a substantial number of signal wires, each of which carries received signals of small magnitude which must not be subjected to radiation from the power cable or cables connected to the transducer. Despite the above requirements it is highly desirable, if not essential, that the load carrying member carry the load under all conditions and not stretch or elongate such as to place part of the load on the conducting wires.

With cables now in use, a number of the above shortcomings have been experienced. To avoid twisting and rotation of the transducer and to permit the shield to do double duty, one design is used in which a heavy external shield of basket-weave configuration also serves as the load-bearing strength member. This member, which is just under the outside jacket is very stiff in bending and in twisting modes, but the crossed shielding wires rub against each other under load as the cable passes over the sheave, causing the shielding and load-carrying wires to break, and frequently the broken ends punch through the outside insulation layer. This, of course, permits salt water to enter the cable. Another failure mode experienced which was at least partly caused by elongation of the shielding structure due to stress, involved rucking or puckering and separation of the outside insulation jacket which was fastened to the shielding layer, but which could not anchor very tightly because the shielding layer was woven so tightly that the molded jacket could not penetrate it to any degree to bond to a lower layer.

Because of the shortcomings of the cable having an outside shield and strength member, designs have been tried using a center strength member. This moves toward solving some of the above problems but may introduce new ones. Such a cable has little resistance to twisting, and if the center wire rope is not carefully torque-balanced, a twisting action can be imparted to the transducer which is too rapid for the compensating means to handle. The cable still may deteriorate rapidly from abrasion, and so it remains a requirement that the center strength member carry the entire load at all times to avoid loading the conductors and that the resulting cable be capable of withstanding the abrasion forces and transverse loading forces resulting from rapid reeling and passing over a sheave of fairly small diameter.

THE DRAWING

The single FIGURE is a perspective view of a section of a cable according to our invention with various layers cut away to reveal the internal construction.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, a section of a cable made according to our invention is shown with portions of the respective layers removed to show its construction. The center strength member consisting of a stranded wire rope 10 is surrounded by means of a thick layer of soft insulating material 12. The wire rope 10 is actually formed of three cables of seven strands each, and this has proven to be quite efficient for the particular loading required. Other arrangements can be used, of course, to meet the strength requirements for each particular application. The wire rope 10 is carefully torque-balanced to avoid introducing twisting forces since it is undesirable that the load carried on the end of this cable be subjected to substantial rotational forces. Spirally wound around the outside of the soft plastic layer 12 are a plurality of conducting members 14, 16, 18, 20, 21, 22, 24, 26 and 28. The spiral angle chosen is somewhat higher than normal, about 20.degree. with respect to the cable center line. A small layer of soft plastic filler material (not shown) which becomes somewhat fluid under pressure is inserted between the conducting members to hydraulically distribute transverse loads to minimize fatigue. Since the wire rope has a very high modulus of elasticity, it elongates only slightly under loads, and the spirally wound conductor members remain virtually unloaded and therefore relatively free of fatigue stresses as the cable is repeatedly tensioned in service. An open braided sheath 30 of fabric such as nylon is used to secure the bundle of conducting members in place, and the entire assembly then has an extruded jacket 32 of polyurethane applied under pressure so that the polyurethane material is driven into and through the braid. In this manner the polyurethane is forced into the spaces around the individual conductor members and through the braided layer 30 in such manner as to insure that the jacket 32 is firmly secured and will not separate from the internal structure.

The individual conductor members 14-28 vary as to the numbers of individual conducting wires they contain, but they are of substantially the same overall diameter and quite small relative to the diameter of the entire cable. This factor plus the use of the soft plastic filler and the resilient insulation layer 12 tends to minimize the concentration of forces on any particular conducting member, even during reeling over a sheave or onto the storage reel. Conductor 14 has a single large stranded conducting wire 34, and it is supplied with an insulating jacket 36. This jacket is, in turn, surrounded by means of a spirally wound electrical shield 38 of conducting material such as fine copper wire, and it, in turn, has a plastic insulating layer 40.

Conductor 16 has a plurality of individual conducting wires 42, 44, 46 and 48, and each of these has its separate insulating jacket. These are, in turn, contained within a sleeve 50 contained within a spiral shield 52 which is essentially identical to shield 38, and it, in turn, is covered with an insulating layer 54. Conductors 20, 22 and 26 are identical to conductor 16. Conductors 18, 21 and 24 are the same except that they contain only three individual conducting wires. By alternating these three- and four-wire conductors, the strands may be packed very tightly around the center insulation layers 12. The individual conductor members are, as described above, of essentially the same diameter which minimizes stresses on any individual conductor member as the cable is reeled or passes over a sheave under load. Each of the conductor members has a spirally wound shielding layer to minimize electrical interference problems. This is particularly useful in the sonar application described since a substantial amount of power may be carried by the conductor 14.

It is apparent that the number and arrangement of conductor members may be varied to suit requirements of any given application. In some applications it may not be necessary to provide shielding for all conductor members, but this depends upon the nature and type of signals carried, as is well known in the art.

Claims

We claim:

1. An electrical cable comprising

a torque-balanced wire rope member capable of carrying substantially the entire load on said cable,

a thick layer of soft insulating material surrounding said wire rope member,

a plurality of conductor members of small diameter relative to said cable spirally wound around said layer, each of said members including at least one conducting wire, an insulating layer around said wire, a spirally wound shield of conducting wires in at least some of said conductor members and an insulating layer around said shield,

an open braid of fabric material wrapped around said conductor members, and

an insulating jacket surrounding said braid.

2. An electrical cable as set forth in claim 1 wherein most of said conductor members include a plurality of conducting wires, each of said wires is insulated and said shield surrounds all of the said plurality of wires in each conductor member.

3. An electrical cable as set forth in claim 1 wherein said conductor members are wound around said soft insulating layer in a spiral angle of approximately twenty degrees.

4. An electrical cable as set forth in claim 2 wherein said insulating jacket is an extruded layer of polyurethane forced through said braided fabric material.

5. An electrical cable as set forth in claim 2 wherein a layer of soft plastic filler material is inserted around said soft insulating material and between said conductor members.