1. Field of the Invention
The invention relates to electrically conductive cable. More particularly, the invention relates to cables having low electrical resistance, high tensile strength, good flexibility and which are easy to insulate.
2. State of the Art
Wire is manufactured from ingots using a rolling mill and a drawing bench. The preliminary treatment of the material to be manufactured into wire is done in the rolling mill where white hot billets (square.section ingots) are rolled to round wire rod. The action of atmospheric oxygen causes a coating of mill scale to form on the hot surface of the rod which must be removed. This descaling can be done by various mechanical methods (e.g., shot-blasting) or by pickling, i.e., immersion of the wire rod in a bath of dilute sulphuric or hydrochloric acid. After pickling, the wire rod may additionally undergo a jolting treatment which dislodges the scale loosened by the acid. The remaining acid is removed by immersion of the wire rod in lime water.
The actual process of forming the wire is called drawing and is carried out on the metal in a cold state with a drawing bench. Prior art FIG. 1 shows a simple drawing bench 10. The wire 12 is pulled through a draw plate 14 which is provided with a number of holes, e.g. 16, (dies) of various diameters. These dies have holes which taper from the diameter of the wire 12 that enters the die to the smaller diameter of the wire 12' that emerges from the die. The thick wire rod 12 is coiled on a vertical spool 18 called a swift and is pulled through the die by a rotating drum 20 mounted on a vertical shaft 22 which is driven by bevel gearing 24. The drum can be disconnected from the drive by means of a clutch 26. To pass a wire through a die, the end of the wire is sharpened to a point and threaded through the die. It is seized by a gripping device and rapidly pulled through the die. This is assisted by lubrication of-the wire. Each passage through a die reduces the diameter of the wire by a certain amount. By successively passing the wire through dies of smaller and smaller diameter, thinner and thinner wire is obtained. The dies used in the modern wire industry are precision-made tools, usually made of tungsten carbide for larger sizes or diamond for smaller sizes. The die design and fabrication is relatively complex and dies may be made of a variety of materials including single crystal natural or synthetic diamond, polycrystalline diamond or a mix of tungsten and cobalt powder mixed together and cold pressed into the carbide nib shape.
A cross section of die 16 is shown in prior art FIG. 2. Generally, the dies used for drawing wire have an outer steel casing 30 and an inner nib 32 which, as mentioned above, may be made of carbide or diamond or the like. The die has a large diameter entrance 34, known as the bell, which is shaped so that wire entering the die will draw lubricant with it. The shape of the bell causes the hydrostatic pressure to increase and promotes the flow of lubricant into the die. The region 36 of the die where the actual reduction in diameter occurs is called the approach angle. In the design of dies, the approach angle is an important parameter. The region 38 following the approach angle is called the bearing region. The bearing region does not cause diametric reduction, but does produce a frictional drag on the wire. The chief function of the bearing region 38 is to permit the conical approach surface 36 to be refinished (to remove surface damage due to die wear) without changing the die exit. The last region 40 of the die is called the back relief. The back relief allows the metal wire to expand slightly as the wire leaves the die. It also minimizes the possibility of abrasion taking place if the drawing stops or if the die is out of alignment with the path of the wire.
Although wire drawing appears to be a simple metalworking process, those skilled in the art will appreciate that many different parameters affect the physical quality of the drawn wire. Among these parameters, draw stress and flow stress play an important role. If these parameters are not carefully considered, the drawn wire may have reduced tensile strength. A discussion of the practical aspects of wire drawing can be found in Wright, Roger N., "Mechanical Analysis and Die Design", Wire Journal, October 1979, the complete disclosure of which is hereby incorporated by reference herein.
The wire forming processes described above may be used to form different kinds of wires including wires which are used to conduct electricity and wires which are used as structural supports. Generally, the most important physical characteristic of a wire used to conduct electricity is its electrical resistance and the most important physical characteristic of a wire used for structural support is its tensile strength. In both types of wires, flexibility may be an important characteristic. Generally, a bundle of wire strands which are twisted together to form a cable exhibits much more flexibility than a single wire of comparable diameter. Thus, in both structural and electrical applications, where flexibility is important, stranded cables are used rather than single solid wires. Stranded cables also have the advantage that they do not kink as easily as solid wires and they can be connected to terminals by crimping. Stranded cables have some disadvantages, however. These disadvantages include lower tensile strength and higher electrical resistance than solid wires of comparable diameter. In addition, the rough outer surface presented by stranded cables makes them more difficult to insulate than solid wires. The rough outer surface of bare stranded cables also presents greater wind resistance than a smooth wire.
The electrical resistance of an electrically conductive wire is related to the chemical composition of the wire (which determines its "resistivity"), the length of the wire, and the cross sectional area of the wire. All materials have a measurable "resistivity" which defines the material as a conductor, semi-conductor, or insulator. Resistivity is indicated with the Greek letter .rho. (rho) and is measured in the units ohm-cm or ohm-meter (ohms per unit length). For example, insulators such as polystyrene have a resistivity on the order of 10.sup.20 ohm-cm, semiconductors such as germanium have a resistivity on the order of 10.sup.2 ohm-cm, drawn copper wire conductors have a resistivity of approximately 1.77.times.10.sup.-6 ohm-cm. Resistivity is used to approximate the electrical resistance (R) of a material when it is part of an electrical circuit. The relationship between resistivity and Resistance is given in Equation 1 below, where R is resistance, .rho. is resistivity, L is length, and A is cross sectional area. ##EQU1##
Thus, the resistance of a drawn copper wire which is 100 centimeters long and one centimeter in diameter can be expressed as shown below in Equation 2. ##EQU2##
For convenience, when discussing conductors, resistivity can be expressed in reciprocal-form and called "conductivity". Conductivity is referred to with the Greek letter .sigma. (sigma) and is measured in siemens per meter or S/cm where a siemen is an ohm.sup.-1. Thus, drawn copper wire may be said to have a conductivity of 1.77.times.10.sup.6 S/cm. The conductivity (or resistivity) of a wire is a constant which is independent of cross sectional area. The resistance (or its reciprocal conductance) of a wire is a variable which is based in part on the cross sectional area of the wire.
The twining of several strands of wire to produce a flexible cable results in a cable having an overall diameter D, but which has a smaller cross sectional area than a solid wire with the same diameter. Prior art FIGS. 3 and 4 schematically illustrate an electrical transmission cable 50. The cable 50 is shown consisting of three wire strands 52, 54, 56, each having a diameter "d". In actual practice, an electrical transmission cable may consist of many more conductive strands and one or more steel core strands which serve to enhance the tensile strength of the cable. As shown, the three strands are twined to form the conductive cable 50 having an overall diameter "D" which is approximately 2.15 d. However, the cross sectional area of the conductive cable 50, for purposes of computing the resistance (or conductance) of the cable is not as large as the cross sectional area of a solid wire having a diameter of 2.15 d. Thus, the stranded and twined cable 50 will have a higher resistance than a solid single strand of wire with the same cross sectional diameter. As mentioned above, the twining of the strands presents a rough outer surface on the cable which makes it relatively difficult to cover the cable with extruded insulation and which makes the cable more susceptible to influence by wind. These disadvantages are present in stranded cable, regardless of the number of strands.