In recent years, several factors have necessitated that relatively large pair size communications cable cores be protected with a cover that exhibits an improved mechanical performance. The cover which is commonly referred to as a sheath system generally includes layers of metal and plastic which are disposed concentrically about the core. Relatively large pair size cores, e.g., 3600 pairs, have increased in popularity, but their size has resulted in cable sheath buckling and/or rupture, particularly in cold weather installations and in the use of high production placing equipment.
Sheath buckling is characterized by distortions, such as ripples, for example, in the sheath that occur when the cable is bent or twisted. These ripples can snag other materials which the cable engages or can become abraded during installation. In some instances, the sheath ruptures and hence no longer protects the core.
These cables usually include a multi-conductor core, an inner metallic tube which is called a shield and which provides protection against external electrical interference, an outer metallic shield and a plastic jacket. Cables of this construction are well known in the industry and have been referred to as Stalpeth cables. See U.S. Pat. No. 2,589,700 which issued on Mar. 18, 1952, in the name of H. G. Johnstone. Each of the shields is usually formed by wrapping a metallic strip about the core to form a longitudinally extending seam. The seam for the outermost shield is usually overlapped with overlapped portions being soldered together. Typically, the shields are corrugated transversely of the longitudinal axis of the cable to facilitate bending of the cable.
It has been determined that an effective method for improving the buckling performance of Stalpeth cable is to increase the cross-sectional stiffness by tightening the cable cross-section. Of course, any tightness in the cable must be accomplished without overly compressing the core, which could affect the electrical performance of the cable. Also, changes to jacket thickness, to flooding compounds, and to jacketing materials have been investigated, but none of these has significantly improved the performance.
In addition to sheath buckling, another area of concern is the diffusion of water vapor through the plastic jacket which may result in an undesirably high moisture level inside the sheath on a cable. See for example E. D. Metcalf "A Bonded Non-Corrugated Aluminum-Polyethylene Sheathing System For Telephone Cable" pp. 235-239 Proceedings 24th International Wire and Cable Symposium December 5-7, 1972. A relatively high moisture level will have a detrimental effect on the transmission characteristics of the cable. The effectiveness of the shield which is made from a single metallic strip formed longitudinally about the cable is enhanced greatly if its resultant seam is sealed. The most effective seal from the moisture barrier point of view is one in which a metal bond exists such as a welded or a soldered seal; however, despite the soldering of the outer shield seen in Stalpeth cable, moisture is able to penetrate the sheath and to enter the core through holes and gaps in the soldered seam.
Besides its inability to prevent the build up of undesirably high moisture levels internally, conventional Stalpeth cable prevents manufacturing difficulties. A continuously soldered seam is difficult to achieve at economical manufacturing speeds because of mismatching of overlapping corrugated portions which comprise the seam. Since the soldering of the seam may require frequent stops and starts of a manufacturing line in order to repair gaps in the seam, the soldering operation must be performed on a separate line from the jacket extrusion which must be continuous. Also, in order to prevent damage to the plastic conductor insulation from the high temperatures of soldering, sufficient core wrap must enclose the conductors. This increases the diameter of the core and results in a core which is less compact than one without the additional protective wrap.
By adhesively bonding the plastic jacket to the outer corrugated shield, it has been found that the resistance of the cable, which is called bonded sheath cable, to moisture diffusion is substantially increased. See, for example, U.S. Pat. No. 3,340,353. Maximum diffusion resistance is obtained by bonding the plaster jacket polyethylene to the coated steel and by bonding overlapping portions of the shield along the longitudinal seam. A study has been made which indicates that a bonded sheath cable should exhibit an improved buckling performance; however, the prior art is seemingly devoid of a cable having a bonded sheath system which simultaneously addresses the problems of moisture diffusion and low temperature buckling.
Bonded Stalpeth cable does offer significant manufacturing advantages over standard Stalpeth. It does not require the soldering of the overlapped portions of the outer shield. Without the necessity of soldering, manufacturing temperatures are reduced from about 300.degree.-400.degree. C. to about 100.degree. C. in the core thereby reducing the probability of damaging the conductor insulation and obviating the need for additional protective wrap for the core. Moreover, the sheath system for bonded sheath cable can be formed in a single line whereas it will be recalled that the standard Stalpeth cable was shielded and then jacketed on another line. Manufacturing difficulties do arise when attempting to nest corrugations of overlapped portions of a corrugated shield to achieve a sealed seam, but this problem has been overcome by flowing adhesive-like material between the overlapping portions as is disclosed in U.S. Pat. No. 4,035,211 which issued on July 12, 1977 in the names of R. G. Bill and E. L. Franke, Jr.
While the use of a bonded sheath which includes a corrugated outer shield overcomes some problems, it may result in an undesirable stressing of the jacket. In fact, G. S. Brockway and G. M. Yanizeski in an article "Elastic State of Stress in a Stalpeth Cable Jacket Subjected to Pure Bending" which was published in Vol. 57 No. 1 January 1978 issue of the Bell System Technical Journal conclude that the probability of spontaneous cracking in a cable jacket is increased by the adherence of the jacket to the soldered steel layer. In an unbonded cable sheath, bending forces cause the jacket to be subjected to uniaxial stresses in a longitudinal direction; however, in a bonded sheath, not only is the jacket stressed in a longitudinal direction, but a significant hoop stress is developed. Unfortunately, this kind of stressing, which is termed biaxial, causes a substantial reduction in the elongation properties of some jacketing materials over those exhibited under uniaxial stress. If the longitudinal seam is left unbonded, the capability of the jacket plastic to resist biaxial stress is especially important since the elongation becomes concentrated in the region where the jacket bridges the seam and because the jacket can be notched by a longitudinal edge of the shield.
The problem of biaxial stressing in bonded sheath cables has not been a problem in the past because bonded sheath cables typically have included an outer jacket bonded to aluminum which is a relatively soft metal. The softness of such a metal allows it to yield to some degree to relieve at least partially any stress concentration. This benefit is not available in bonded Stalpeth cable, for example, in which the outer jacket is bonded to a relatively hard metal such as steel.
Another concern that must be met when using bonded sheath cable is that of delamination. The sheath system must be such that components thereof, i.e., the outer jacket and the outer shield do not delaminate during periods of storage on reels in outside areas when subjected to high temperature. Sheath integrity must also be preserved during installation at relatively low temperatures which may be in the range of -15.degree. C.
Still another concern in bonded sheath cables is the ability of the plastic jacket to contact substantially all the surface area of the corrugated shield. This problem is alluded to by E. D. Metcalf in his priorly-identified paper in which he states that the same uniformity of adhesion could not be produced in bonding a plastic jacket to a corrugated shield as could be provided in bonding a jacket to a flat shield.
It appears that the prior art for bonded sheath cables does not provide a solution to the problem of a relatively large pair size cable which is suitable for underground intallation and which has resistance to moisture infusion as well as the capability of resisting buckling during installation and of resisting delamination. In fact, a review of the prior art seemingly would lead one to conclude that the use of a bonded sheath having a jacket bonded to a corrugated shield to achieve moisture resistance and ease of manufacture engenders other problems.