United States Patent (19)
`Aloisio, Jr. et al.
`
`(54). BONDEDSHEATH CABLE
`75) Inventors: Charles J. Aloisio, Jr., Atlanta;
`George S. Brockway, II,
`Lawrenceville; Alvin C. Levy,
`Atlanta; Randy G. Schneider,
`Marietta; George M. Yanizeski,
`Roswell, all of Ga.
`
`*
`
`Notice:
`
`73) Assignees: Western Electric Co., Inc., New
`York, N.Y.; Bell Telephone
`Laboratories, Inc., Murray Hill, N.J.
`The portion of the term of this patent
`subsequent to May 4, 1999 has been
`disclaimed.
`(21) Appl. No. 368,183
`Apr. 14, 1982
`22 Filed:
`
`63
`
`Related U.S. Application Data
`Continuation of Ser. No. 225,082, Jan. 14, 1981, Pat.
`No. 4,328,394.
`51) Int. Cl. ............................................... HO1B 7/18
`52 U.S. C. ................................. 174/106 D; 174/107
`58) Field of Search .......... 174/102 D, 106 R, 106 D,
`174/107
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`2,589,700 3/1952 Johnstone ........................... 174/106
`3,340,353 9/1967 Milder ................................. 174/106
`3,376,378 4/1968 Bullock .............
`... 174/107
`3,459,877 8/1969 Bullock et al.
`... 174/07
`3,551,586 12/1970 Dembiak et al. .....
`... 174/107
`3,629,489 12/1971 Jachimowici et al. ............. 174/107
`3,681,515 8/1972 Mildner ............................... 174/107
`3,703,605 11/1972 Dembiak et al. ................... 174/107
`3,711,621 1/1973 Jachimowicj.........
`... 174/23 R
`3,745,232 7/1973 Johnson et al. ..................... 174/107
`3,824,330 7/1974 Lang ..................
`... 174/102 D
`3,826,862 7/1974 Ichiba et al.
`... 174/102 R.
`4,035,211 7/1977 Bill et al........
`... 156/54
`4,109,099 8/1978 Dembiak et al. .
`... 174/107
`4,132,857 1/1979 Scarola et al. ........
`... 174/107
`4,151,365 4/1979 Hacker ..............
`... 174/07
`4,292,463 9/1981 Bow et al. .....
`... 174/107
`4,322,574 3/1982 Bow et al. ........................... 174/107
`4,328,394 5/1982 Aloisio, Jr. et al. ........... 174/106 D
`
`11)
`45
`
`4,439,632
`" Mar. 27, 1984
`
`FOREIGN PATENT DOCUMENTS
`2746929 4/1978 Fed. Rep. of Germany ...... 74/107
`1419843 10/1964 France ................................ 174/107
`6410915 9/1964 Netherlands ........................ 74/106
`OTHER PUBLICATIONS
`Metcalf, E. D., “A Bonded Non-Corrugated Alumi- a
`num/Polyethylene Sheathing System for Telephone
`Cable'; Proceedings of the 21st International Wire and
`Cable Symposium; Dec. 5–7, 1972; pp. 235-239.
`Yanizeski, G. M. et al., "Predicting Fracture Creep; and
`Stiffness Characteristics of Cable Jackets from Material
`Properties”, Proceedings of the 25th International Wire
`and Cable Symposium; Nov. 16-18, 1976, pp. 272-280;
`Bell Laboratories.
`Brockway, G. S. et al., "Elastic State of Stress in a
`Stalpeth Cable Jacket Subjected to Pure Bending', Bell
`System Technical Journal vol. 57, No. 1, Jan. 1978.
`Primary Examiner-J. V. Truhe
`Assistant Examiner-Morris H. Nimmo
`Attorney, Agent, or Firm-F. W. Somers
`57
`ABSTRACT
`A cable which is capable of being made in a large pair
`size and yet which has excellent mechanical properties
`that maintain its integrity notwithstanding extremes in
`temperature during installation and shipping as well as
`during the rigors of installation includes a sheath system
`having a corrugated steel outer shield that is adhesively
`bonded to a plastic jacket. The corrugated steel shield
`which is formed to have a longitudinal overlapped seam
`that is preferably unsealed encloses an aluminum inner
`shield that in turn encloses a multiconductor core. Ad
`vantageously, the sheath system includes a plastic jack:
`eting material which is capable of resisting biaxial
`stresses which are aggravated in a bonded sheath sys
`tem. This results in jacket integrity about the longitudi
`nal seam of the outer shield notwithstanding a notched
`cross-section and an unsupported bridged portion of the
`plastic jacket adjacent to the seam. Of additional benefit
`is a further characterization of the plastic as being one
`which because of its relatively low elastic modulus at
`conventional extrusion times and temperatures is caused
`to fill substantially the corrugations of the outer shield.
`The jacket plastic forms a surface-to-surface bond with
`the shield that is sufficient to prevent delamination of
`the outer shield and the jacket and to prevent buckling
`of the jacket during exposure to temperature extremes.
`9 Claims, 9 Drawing Figures
`
`43 ONGUDNAL SEAN
`45 ACKE
`AOMETALLIC SHIELD
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`U.S. Patent Mar. 27, 1984
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`43 LONGITUDNAL SEAM
`45 ACKET
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`

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`U.S. Patent Mar. 27, 1984
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`U.S. Patent Mar. 27, 1984
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`U.S. Patent = Mar. 27, 1984
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`U.S. Patent Mar. 27, 1984
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`

`1.
`
`BONDEDSHEATH CABLE
`
`This is a continuation of application Ser. No.
`06/225,082 filed Jan. 14, 1981, now U.S. Pat. No.
`4,328,394.
`
`5
`
`10
`
`15
`
`25
`
`4,439,632
`2
`high moisture level will have a detrimental effect on the
`transmission characteristics of the cable. The effective
`ness 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 undesir
`ably high moisture levels internally, conventional
`Stalpeth cable prevents manufacturing difficulties. A
`continuously soldered seam is difficult to achieve at
`economical manufacturing speeds because of mismatch
`ing 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 diame
`ter of the core and results in a core which is less com
`pact 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 plastic jacket 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 manu
`facturing advantages over standard Stalpeth. It does not
`require the soldering of the overlapped portions of the
`outer shield. Without the necessity of soldering, manu
`facturing temperatures are reduced from about
`300-400° C. to about 100° C. in the core thereby reduc
`ing the probability of damaging the conductor insula
`tion 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 corru
`gated 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 Sub
`jected 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
`
`TECHNICAL FIELD
`This invention relates to a bonded sheath cable, and
`more particularly, to a cable having a sheath system
`which includes a corrugated metallic shield which is
`bonded to a plastic outer jacket having sufficiently high
`biaxial stress resistant properties to prevent jacket split
`ting in the vicinity of a longitudinal seam of the shield
`with the bond between the shield and the jacket being
`sufficient to resist buckling during installation at low
`temperatures.
`BACKGROUND OF THE INVENTION
`In recent years, several factors have necessitated that
`20
`relatively large pair size communications cable cores be
`protected with a cover that exhibits an improved me
`chanical 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.
`30
`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
`35
`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 interfer
`ence, an outer metallic shield and a plastic jacket. Ca
`bles 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.
`50
`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 compress
`55
`ing the core, which could affect the electrical perfor
`mance 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 con
`cern 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
`65
`Polyethylene Sheathing System For Telephone Cable'
`pp. 235-239 Proceedings 24th International Wire and
`Cable Symposium December 5–7, 1972. A relatively
`
`45
`
`

`

`O
`
`20
`
`25
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`4,439,632
`4.
`3
`elongation and bond strength characteristics as well as
`cracking in a cable jacket is increased by the adherence
`having a modulus at manufacturing temperatures which
`of the jacket to the soldered steel layer. In an unbonded
`manifests itself in excellent corrugation penetration by
`cable sheath, bending forces cause the jacket to be sub
`the plastic. The elongation properties of the plastic are
`jected to uniaxial stresses in a longitudinal direction;
`such that it resists rupture notwithstanding the biaxial
`however, in a bonded sheath, not only is the jacket
`stressing of the jacket caused by longitudinal bending or
`stressed in a longitudinal direction, but a significant
`hoop stress is developed. Unfortunately, this kind of
`twisting together with circumferential bonding. It has
`been found that the sheath system is essentially notch
`stressing, which is termed biaxial, causes a substantial
`resistant both longitudinally as imprinted by an edge of
`reduction in the elongation properties of some jacketing
`the outer shield and circumferentially by the corruga
`materials over those exhibited under uniaxial stress. If
`the longitudinal seam is left unbonded, the capability of
`tions.
`the jacket plastic to resist biaxial stress is especially
`By using a corrugated shield, it has been found that
`important since the elongation becomes concentrated in
`the peel strength of the bonded plastic which is a mea
`the region where the jacket bridges the seam and be
`sure of the ability to resist delamination of the jacket
`cause the jacket can be notched by a longitudinal edge
`from the shield is substantially greater than that which
`15
`can be explained because of the increased surface area
`of the shield.
`The problem of biaxial stressing in bonded sheath
`over that of a non-corrugated shield at least at relatively
`cables has not been a problem in the past because
`low temperatures. Moreover, a jacket-flat shield geom
`etry having an unacceptable peel strength is converted
`bonded sheath cables typically have included an outer
`to one having more than acceptable peel strength by
`jacket bonded to aluminum which is a relatively soft
`bonding the same plastic to a corrugated shield. This
`metal. The softness of such a metal allows it to yield to
`unexpected result may occur because of the develop
`some degree to relieve at least partially any stress con
`ment of a shear mode between the plastic jacket and the
`centration. This benefit is not available in bonded
`Stalpeth cable, for example, in which the outer jacket is
`corrugated outer shield.
`bonded to a relatively hard metal such as steel.
`BRIEF DESCRIPTION OF THE DRAWINGS
`Another concern that must be met when using
`Other features of the present invention will be more
`bonded sheath cable is that of delamination. The sheath
`system must be such that components thereof, i.e., the
`readily understood from the following detailed descrip
`tion of specific embodiments thereof when read in con
`outer jacket and the outer shield do not delaminate
`junction with the accompanying drawings, in which:
`during periods of storage on reels in outside areas when
`30
`subjected to high temperature. Sheath integrity must
`FIG. 1 is a cross-sectional end view of a cable of this
`also be preserved during installation at relatively low
`invention;
`temperatures which may be in the range of -15° C.
`FIG. 2 is a detail view of a longitudinal seam of a
`shield of one embodiment of this invention;
`Still another concern in bonded sheath cables is the
`ability of the plastic jacket to contact substantially all
`FIGS. 3A and 3B are a series of views in elevation
`35
`the surface area of the corrugated shield. This problem
`showing methods of cable installation;
`is alluded to by E. D. Metcalf in his priorly-identified
`FIG. 4 is an enlarged view in section of a portion of
`paper in which he states that the same uniformity of
`the preferred embodiment of the cable of FIG. 1;
`adhesion could not be produced in bonding a plastic
`FIG. 5 is a graph of elongation versus a characteristic
`jacket to a corrugated shield as could be provided in
`of notch sensitivity;
`bonding a jacket to a flat shield.
`FIG. 6 is an enlarged view to show a portion of a
`shield in which jacket plastic lacks suitable penetration;
`It appears that the prior art for bonded sheath cables
`does not provide a solution to the problem of a rela
`and
`tively large pair size cable which is suitable for under
`FIGS. 7 and 8 are graphs of characteristics of a plas
`tic jacketing material which forms the jacket of the
`ground intallation and which has resistance to moisture
`45
`infusion as well as the capability of resisting buckling
`cable of this invention.
`during installation and of resisting delamination. In fact,
`DETALED DESCRIPTION
`a review of the prior art seemingly would lead one to
`conclude that the use of a bonded sheath having a jacket
`Referring now to FIG. 1, there is shown a cable,
`designated generally by the numeral 20, said cable com
`bonded to a corrugated shield to achieve moisture resis
`50
`prising a core 21 having a plurality of individually insu
`tance and ease of manufacture engenders other prob
`lated conductors 22-22. The core 21 is enclosed by a
`lems.
`core wrap which may be made of a paper tape or of a
`polyethylene terephthalate laminate, for example.
`The core 21 is enclosed in a sheath system which is
`designated generally by the numeral 30. The sheath
`system 30 is designed to protect the cable from the
`ingress of moisture which could degrade the quality of
`the transmission signals, to protect the cable from me
`chanical and electrical damage, and to screen the core
`from electrical interference. The sheath system 30 is
`also capable of resisting buckling during installation.
`Adjacent to the core 21 is a first component of the
`sheath system 30, said first component being a shielding
`layer 31. In a preferred embodiment, the first compo
`nent 31 is wrapped about the core to form a longitudinal
`seam 32 with an inwardly facing surface 33 facing the
`core and with an outwardly facing surface oriented
`
`SUMMARY OF THE INVENTION
`The foregoing problems have been overcome by
`55
`cable of this invention which is referred to as one hav
`ing a bonded sheath and which includes a multipair
`core, a corrugated, inner metallic shield which is en
`closed by a corrugated, outer metallic shield and an
`outer jacket of plastic material. The inner shield which
`preferably is made of aluminum has an open longitudi
`nal seam while the corrugated steel shield has an over
`lapped longitudinal seam. Moreover, the steel which is
`used to form the outer shield has a copolymer adhesive
`coating that causes the jacket to bond to the outer shield
`during extrusion.
`The outer steel shield is not only corrugated but also
`is covered by a plastic material which has particular
`
`40
`
`65
`
`

`

`10
`
`15
`
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`5
`same is available from commercial sources. For exam
`toward other components of the system30. The seam 32
`ple, the combination of a metallic strip which is pre
`is formed so that longitudinal edge portions 36 and 37 of
`coated with an ethylene acrylic acid copolymer adhe
`the layer 31 are either butted together or spaced slightly
`sive-like material is marketed by the Dow Chemical
`apart.
`Company of Midland, Michigan, under designations
`The shielding layer 31 is effective to absorb energy
`X0-5554.21 and X0-5554.28 and is referred to as Zeta
`from stray electrical fields which emanate from sources
`bon(R) plastic clad metal sheathing for electrical wire
`outside the cable 20, Typically, the layer 31 is made
`from a tape of electrical conductor-grade aluminum
`and cable.
`The precoating of the corrugated steel shield 40 may
`alloy approximately 0.020 cm thick.
`be accomplished in several different configurations. In
`Surrounding the shielding layer 31 is an outer second
`one embodiment which is shown in FIG. 2, the surface
`metallic shield which is designated generally by the
`46 of the tape which is to become the outwardly facing
`numeral 40. The outer shield 40 is used to provide me
`surface of the steel shield 40 and an edge portion 52
`chanical protection for the cable 20 such as resistance to
`which is to be a portion of the inwardly facing surface
`animal attack or crushing. Also, the shield 40 imparts to
`are precoated with the material 50 prior to the step of
`the cable suitable strength for resisting buckling during
`forming the tape about the shielded core 21.
`installation of the cable. In a preferred embodiment, the
`By applying a strip 56 of the adhesive material 50
`shield 40 is made of an electric chrome-coated or tin
`along the edge portion 52 of the tape and by coating the
`plated steel tape 41 having a thickness of about 0.015
`entire outwardly facing surface 46 with an adhesive
`C.
`copolymer, the adjacent portions of the overlapped
`It should be apparent that while the preferred em
`20
`portions form a sealed longitudinal seam 57. This cre
`bodiment of this invention is a sheath system 30 which
`ates an effective tubular barrier to moisture penetration
`includes an inner shielding layer 31 and an outer metal
`into the core. The resistance of the cable 20 to moisture
`lic shield 40, the invention is not so limited. For exam
`penetration is also enhanced by the bond created be
`ple, there may be instances where only one shield, the
`tween the precoated outwardly facing surface of the
`shield 40 which is made of a steel-type material having
`25
`shield 40 and the jacket 45. As discussed hereinbefore,
`a relatively high modulus of elasticity, is used. Also, the
`in order to effectively bond the facing surface areas of
`shield 40 because of the material from which it is made
`the overlapped portions of the outer shield 40, it may
`and because of its geometry including its thickness has a
`become necessary to flow additional adhesive-like ma
`stiffness which is substantially greater than than of the
`terial between the overlapped portions. However, if
`30
`inner shield 31.
`there is sufficient contact between the jacket 45 and the
`The shield 40 is manufactured by forming the tape 41
`shield 40, a sealed seam is not necessary to achieve
`about the travelling core 21 with a longitudinal seam 43.
`acceptable resistance to moisture diffusion.
`This may be accomplished for example with methods
`It is important to recognize that while in some cables
`and apparatus shown in application Ser. No. 052,165,
`the outer shield is coated with this adhesive-like mate
`which was filed on June 26, 1979 in the name of W. D.
`35
`rial, such as an acrylic acid copolymer, other arrange
`Bohannon, Jr. and now U.S. Pat. No. 4,308,662. The
`ments come within this invention. For example, it is
`shield 40 includes an inwardly facing surface 44 which
`well known that an improved bond is established be
`faces the shield 31 and an outwardly facing surface 46
`tween a polyethylene jacket and a polyethylene coated
`which faces a next successive component of the sheath
`metallic shield. Consequently, it has been suggested that
`system 30.
`the outer shield be precoated or coextruded with dual
`The next successive component of the sheath system
`layers-one of the acrylic acid copolymer and the
`30 and the outermost component thereof is a plastic
`other, a typical polyethylene. This construction is dis
`jacket 45. It has been found that, by bonding the jacket
`closed in U.S. Pat. No. 4,132,857 which issued on Jan. 2,
`45 to the outer shield 40, buckling during handling and
`1979, in the name of L. S. Scarola.
`installation is resisted by the jacket-shield laminate to a
`45
`The sheath system 30 must provide sufficient strength
`much greater extent than in the standard Stalpeth cable.
`for the cable 20 so that it is capable of resisting buckling
`See. G. M. Yanizeski, E. L. Johnson and R. G. Schnei
`particularly during any of three commonly used tech
`der "Cable Sheath Buckling Studies and the Develop
`niques (see FIG. 3) for installing cables from reels
`ment of a Bonded Stalpeth Sheath' pp. 48-58 Proceed
`58-58 in underground duct 59. In the first (see FIG.
`ings 29th International Wire and Cable Symposium, No
`50
`3A), the so-called C-bend configuration, compressive
`vember 18-20, 1980. In order to provide the cable with
`strains, which cause buckling, are generated as the cable
`a sheath system which is suitable for resisting buckling
`20 is straightened and its reel set is overcome. In an
`and for preventing the infusion of moisture, the shield
`S-bend configuration (see again FIG. 3A), the cable
`40 may include an adhesive-like material 50 which is
`sustains bending beyond that needed to straighten the
`precoated at least along its outwardly facing surface.
`cable and additional compressive strains are generated.
`Then when the jacket 45 of a plastic material, usually
`In a side payout, high production procedure in which
`polyethylene, is applied over the steel shield 40, the heat
`reels are mounted on a flatbed tractor trailer (see FIG.
`of extrusion causes the jacket to become bonded to the
`3B), the cable 20 undergoes bending and torsion as the
`outwardly facing surface of the steel shield.
`cables comes off the top of the reel 58 and then turns to
`The material 50 which is used to precoat the steel
`enter the duct 59. Side payout is the most severe config
`shield 40 is an adhesive material which has the ability to
`uration as far as buckling is concerned while the C-bend
`develop firm adhesion to and prevent corrosion of the
`is the least severe.
`steel. The bonding of the shield 40 to the jacket 45 over
`The plastic material comprising the jacket 45 is char
`a substantial portion of the outwardly facing surface of
`acterized in terms of particular properties which pro
`the shield results in a sheath system 30 which inhibits
`65
`vide excellent resistance to damage to the cable during
`the penetration of moisture into the cable core. In one
`handling and installation and which prevents delamina
`embodiment, the material 50 is comprised of an ethylene
`tion of the jacket 45 from the steel shield 40. The plastic
`acid copolymer and a strip which is precoated with
`
`55
`
`

`

`25
`
`4,439,632
`7
`8
`material which is used to make the jacket 45 must have
`elongation of cable jacket materials may also be plotted
`suitable elongation properties to resist rupture when
`as a function of temperature. See for example FIG. 9 of
`subjected to biaxial stress and notching in a bonded
`C. J. Aloisio and G. S. Brockway "Thermomechanical
`sheath system. This, it will be recalled, becomes impor
`Reliability of Plastics in Transmission Media' pp.
`tant to the integrity of the bonded sheath system of this
`158-163 Plastics and Rubber Materials and Applications
`invention under field conditions.
`November 1979. It has been found that the jacket mate
`These properties also become important to the pre
`rial of the cable of this invention has a low notch sensi
`ferred embodiment of the sheath system of this inven
`tivity which means that it exhibits a relatively high
`tion in which the longitudinal seam is not intentionally
`elongation even when sharply notched.
`bonded (see FIG. 4). In fact, a longitudinal edge portion
`It has been found that the plot of elongation versus
`61 of the tape is directed inwardly toward an underly
`p/d for jacketing grade plastic materials is stepped with
`ing portion as the tape is formed into the shield 40. This
`relatively sharp transitions between steps. While the
`is done in order to prevent the outer overlapping edge
`graph of elongation and p?d which is depicted in FIG.
`portion which forms a step discontinuity in jacket thick
`5 is characteristic of commonly used jacketing materi
`ness along its longitudinal edge from undesirably pro
`15
`als, the transition points from one elongation value to
`truding into the plastic jacket 45. Methods and appara
`another shift for different materials. The significance of
`tus for forming a longitudinal seam with an outer edge
`a transition point is that to one side of it, the plastic
`portion turned inwardly are disclosed in the herein
`behaves in a ductile manner while on the other side, it
`before-identified W. D. Bohannon Jr. application.
`behaves in a brittle manner. These transitions are influ
`The plastic material of the jacket 45 is characterized
`20
`enced by temperature as well as by the bonding of the
`by a biaxial stress resistance which provides the capabil
`jacket 45 to the outer shield 40. A sharp notch creates a
`ity of sustaining stresses across the longitudinal seam of
`biaxial state of stress which is aggravated in a bonded
`the outer shield. In the unsealed seam embodiment
`sheath environment. With an unbonded arrangement,
`which is shown in FIG. 4, the overlapped edge portions
`elongation can occur over a longer distance whereas in
`of the coated metallic shield 40 are free to move relative
`the bonded sheath arrangement it can occur only where
`to each other in the circumferential direction except as
`the jacket 45 bridges the seam. For jacketing plastics,
`confined by the jacket 45 as the cable 20 is handled and
`which are typically used in the communications indus
`installed. This elongation property of the plastic jacket
`try, it is desirable to operate at elongations in the range
`45 prevents jacket splitting in the field during installa
`of 600 to 1000%. Once an operating level of elongation
`tion and is particularly significant in view of the "bridg
`30
`is selected, then the p/d of the plastic material of the
`ing' of the plastic adjacent to the longitudinal edge of
`jacket should occur to the left of that operating range.
`the outer edge portions of the outer shield. Since hoop
`It has been found that plastic materials having a tran
`strength is not provided by the shield 40 across the
`sition point in elongation which occurs to the left (see
`seam, it must be provided by a portion 65 of the plastic
`FIG. 5) of a sharpness ratio p?d value of about 0.7 at
`jacket 45 which bridges between the portion 61a and
`35
`room temperature, i.e. about 23° C., provides suitable
`the inner portion 61b of the overlapped seam.
`The ability of the cable jacket to resistjacket splitting
`notching resistance for the cable of this invention in the
`expected range of installation temperatures, i.e. about
`can be related to a sufficiently low notch sensitivity.
`Notching of the jacket occurs both in a longitudinal and
`-15 C. to 70° C. On the other hand, a material having
`a transition point to the right of a pyd value of 3.5 is not
`in a circumferential direction. First, in the longitudinal
`40
`direction, the overlapping of the longitudinal edge por
`suitable for bonded sheath construction. A material
`which has been found to meet this requirement is one .
`tions of the outer steel shield 40 causes the outer edge
`portion to protrude into the jacket to notch the jacket
`marketed by the Union Carbide Company and desig
`nated DFDA 6059 polyethylene.
`45 in a longitudinal direction. This, of course, can be
`minimized by directing the outer edge portion 61 in
`There are a number of plastic jacketing materials
`45
`such as polyethylene, for example, which exhibit elon
`wardly toward the core, but at the very least there will
`be a notching equal to the thickness of the shield. Se
`gation in the range of 600 to 1000% when uniaxially
`condly, there is a notching in a circumferential direction
`stressed. However, many of these experience a severe
`caused by the peaks of the corrugations of the outer
`decrease in elongation to the range of perhaps 50% or
`shield 40. These corrugations have a range of depth
`less when stressed biaxially, often in the range of tem
`with the maximum peak to valley height of about 0.13
`peratures to which cables are exposed during installa
`cm and are formed at a predetermined number per unit
`tion. This range may extend from a low of about -15
`of length which may be on the order of