Title:
Coaxial cable with an undulated drain wire
United States Patent 3896261
Abstract:
A coaxial cable is provided with increased resistance to slippage of the drain wire along a slippery conductive shield surface by undulating the drain wire and imbedding the corrugated drain wire in matching undulated grooves in the outer insulating jacket and inner insulating layer. The grooves require the wire to change shape and to follow a tortuous path in order to slip. Preferably, the coaxial cable is manufactured by a method including the steps of feeding a corrugated drain wire, a web having a conductive shield and a cable having an insulating layer and a central conductor to a jacket extruding means which extrudes a plastic jacket about the drain wire and conductive shield and softens and presses the drain wire into the inner insulating layer thereby forming undulating grooves in the jacket and in the insulating layer with the undulated drain wire imbedded therein.
US Patent References:
Radio antenna
Johnson - December 1939 - 2184997
ELECTRIC POWER CABLE
Plate et al. - October 1969 - 3474189
CABLE SYSTEM
Plate et al. - March 1971 - 3571613
SHIELDED CABLE
Lasley - June 1972 - 3673315
SHIELDED CABLE
Plate - December 1972 - 3707595
Radio antenna
Johnson - December 1939 - 2184997
ELECTRIC POWER CABLE
Plate et al. - October 1969 - 3474189
CABLE SYSTEM
Plate et al. - March 1971 - 3571613
SHIELDED CABLE
Lasley - June 1972 - 3673315
SHIELDED CABLE
Plate - December 1972 - 3707595
Application Number:
05/460718
Publication Date:
07/22/1975
Filing Date:
04/15/1974
View Patent Images:
Export Citation:
Assignee:
Belden Corporation (Geneva, IL)
Primary Class:
Other Classes:
174/107, 174/36
International Classes:
H01B11/18; H01B13/22; H01B7/00
Field of Search:
174/36,12R,107,115,113R
US Patent References:
| 3728474 | SHIELDED POWER CABLE | April 1973 | Arnaudin, Jr. |
Primary Examiner:
Grimley, Arthur T.
Attorney, Agent or Firm:
Fitch, Even, Tabin & Luedeka
Claims:
What is claimed is
1. A coaxial cable having a drain wire with resistance to slipping comprising an elongated central conductor extending longitudinally of the cable, a surrounding layer of insulating material in contact with said central conductor, a conductive shield surrounding said layer of insulating material and in contact therewith, an outer surrounding protective jacket of insulating material disposed about said conductive shield, an undulated drain wire in contact with said conductive shield and having a number of undulations and disposed between said jacket and said insulating layer, an undulating groove formed in said insulating layer and having one side of said undulated drain wire projecting therein, and an undulating groove formed in said jacket receiving the other side of said drain wire therein.
2. A coaxial cable in accordance with claim 1 in which said undulated drain wire extends longitudinally and parallel to said central conductor, said drain wire being disposed inwardly of said conductive shield and in direct engagement with said groove formed in said insulating layer, said conductive shield comprising a longitudinally extending laminate with a metallic foil and an outer plastic layer in engagement with said jacket.
3. A coaxial cable in accordance with claim 1 in which said drain wire extends longitudinally and parallel to said central conductor, said drain wire being disposed radially outwardly of said conductive shield and with said drain wire in direct engagement with said groove in said jacket.
4. A coaxial cable in accordance with claim 1 in which said drain wire is formed with undulations of about 8 per inch and in which said grooves also are formed with eight undulations per inch.
1. A coaxial cable having a drain wire with resistance to slipping comprising an elongated central conductor extending longitudinally of the cable, a surrounding layer of insulating material in contact with said central conductor, a conductive shield surrounding said layer of insulating material and in contact therewith, an outer surrounding protective jacket of insulating material disposed about said conductive shield, an undulated drain wire in contact with said conductive shield and having a number of undulations and disposed between said jacket and said insulating layer, an undulating groove formed in said insulating layer and having one side of said undulated drain wire projecting therein, and an undulating groove formed in said jacket receiving the other side of said drain wire therein.
2. A coaxial cable in accordance with claim 1 in which said undulated drain wire extends longitudinally and parallel to said central conductor, said drain wire being disposed inwardly of said conductive shield and in direct engagement with said groove formed in said insulating layer, said conductive shield comprising a longitudinally extending laminate with a metallic foil and an outer plastic layer in engagement with said jacket.
3. A coaxial cable in accordance with claim 1 in which said drain wire extends longitudinally and parallel to said central conductor, said drain wire being disposed radially outwardly of said conductive shield and with said drain wire in direct engagement with said groove in said jacket.
4. A coaxial cable in accordance with claim 1 in which said drain wire is formed with undulations of about 8 per inch and in which said grooves also are formed with eight undulations per inch.
Description:
This invention relates to a coaxial cable having a drain wire in engagement with a conductive shield.
Coaxial cables with one or more drain wires in engagement with a conductive shield, usually a metallic foil shield, are used in numerous applications in which the drain wire is subjected to a longitudinally directed force causing the drain wire to slip or slide along the slippery metal foil shield. In some instances, the drain wire actually separates from the cable, rendering the cable defective; and, in other instances, the drain wire shifts to a point where it interferes with proper connection of the cable. Automatic insulation stripping machines apply forces causing a shifting of the drain wire in these kinds of cables as a portion of the outer protective jacket of the cable and a portion of the conductive foil shield are severed and removed by the machine. The shifting or pulling of the drain wires from these cables is more likely for very short lengths of cable. In tests of one commercially available coaxial cable having No. 22AWG drain wires laid parallel to the cable, the drain wire has been pulled from a 16 inch length of such cable with longitudinally directed forces in the range of 0.7 to 4.0 pounds. Tests run on cables in which the drain wire has been laid with a spiral wrap about the insulated core, required only slightly more force to pull the drain wire from the cable. However, with the coaxial cable of the present invention, increased resistance to slipping and pulling of a drain wire from a coaxial cable required, in similar tests, at least 12 pounds of force and, in some instances, the drain wire did not pull out with a 20-pound force and, in other instances, the drain wire broke before it separated from the cable.
In order for a coaxial cable having a drain wire, which is more resistant to slippage, to be commercially practical and competitive, it should be capable of being manufactured at high speeds and without expensive equipment. Generally speaking, spirally applied drain wire may provide a few slight increase in the resistance to slippage of the drain wire, but the spiral laying equipment usually slows the speed of manufacture. As will be explained, with the present invention, the coaxial cable may be manufactured at speeds faster than the speeds involved in manufactured cables within spirally laid drain wires.
Accordingly, a general object of the present invention is to provide a coaxial cable of the foregoing kind having a drain wire with increased resistance to slippage and to provide a method of making such a cable.
Other objects and advantages of the present invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:
FIG. 1 is a diagrammatic illustration of an apparatus for practicing a method of manufacture of a coaxial cable constructed in accordance with a preferred embodiment of the invention;
FIG. 2 is a perspective view of a cable with the layers broken away and constructed in accordance with a preferred embodiment of the invention;
FIG. 3 is a cross-sectional view taken substantially along the line 3--3 of FIG. 2; and
FIG. 4 is a cross-sectional view of another embodiment of the invention.
As shown in the drawings for purposes of illustration, the invention is embodied in a coaxial cable 11 having an internal conductor 12 for carrying electrical current. The internal conductor 12 is surrounded by an insulating layer 15 which separates and electrically insulates the conductor 12 from a surrounding conductive shield 17, preferably in the form of a metallic foil. In contact with the metallic surface of the conductive shield 17 is at least one elongated drain wire 19. Surrounding the drain wire 19, conductive shield 17, insulating layer 15 and internal conductor 12 is an outer insulating or protective jacket 21 of insulating material such as a plastic.
Coaxial cables having a conductive shield with a drain wire in contact therewith have been used for numerous applications in which a portion of the outer insulating jacket 15 is stripped away at an end of the cable to expose the drain wire and often the internal conductor for attachment to a given terminal or device. Often the portion of the outer jacket is stripped automatically from the cable by a machine which also severs the cable into a predetermined length, for example, a short piece of cable only several inches in length. Such automatic stripping machines often apply sufficient longitudinally directed forces to the drain wire to cause it to slide along the slippery metallic surface of the conductive outer shield. Usually the conductive shield is a laminate of metallic foil and plastic with the plastic facing outwardly and slightly adhered to the outer extruded plastic jacket. In such cables, it has been found that the drain wire often slips or separates from the cable with small forces, e.g., 0.7 to 4 pounds of force; and there is a demonstrated need for improved resistance to drain wire slippage without a substantial increase in cost of such coaxial cable.
In accordance with the present invention, the drain wire 19 contacting the conductive shield 17 is formed with a series of undulations 23 and it is imbedded in matched grooves 27 and 29 formed in the core layer 15 and jacket 21, respectively, with the peaks or crests 31 and 32 of the respective grooves 27 and 29 resisting a longitudinally directed displacement of the undulated drain wire 19, as heretofore occurred when using an automatic stripping machine for prior art cables. More specifically, the undulated drain wire 19 which hereinafter is termed a corrugated drain wire 19, provides a significantly greater resistance to pulling and slippage than does either a straight drain wire or a spirally laid drain wire of prior art cables of this general kind. For instance, tests indicate that a range of 12 to 20 pounds or more of force are required to remove an undulated drain wire 19 from a 16-inch length of the cable 10 as contrasted to 0.7 to 4.0 pounds for removal of a drain wire of a prior art coaxial cable of 16 inches in length. This greater resistance is due to the fact that the corrugated drain wire 19 must continually change its shape and follow a tortuous path defined by the spaced crests 31 and 32 and valleys therebetween if it is to travel longitudinally relative to the jacket 21 and insulating layer 15. Such change of shape of the drain wire metal and frictional resistance to travel along the tortuous path result in the high resistance despite the fact the drain wire engages a slippery metallic surface on one side thereof.
In accordance with another aspect of the invention, the coaxial cable 11 having increased resistance to drain wire slippage may be manufactured commercially at speeds and costs competitive with commercially available coaxial cables by a method which comprises the steps of, as best seen in FIG. 1, feeding a conductor 12 surrounded by the insulating layer 15 in a forward direction towards an extruder means 35, feeding an undulated drain wire 19 forwardly to the extruder means 35, feeding a web 37 having the conductive shield 17 forwardly for contact with the drain wire 19 and for wrapping about the insulating layer 15, extruding a surrounding jacket 21 of insulating material about the web 37, undulated drain wire 19 and insulating layer 15 at the extruder means 35, and applying sufficient heating and pressure during the extruding step to form an undulating groove 27 in the insulating layer 15 and a matched undulating groove 29 in the jacket 21 thereby providing resistance against pulling of the drain wire 19 from the cable. In the preferred method, a straight drain wire 41 is unreeled from a reel or let-off 43 and travels forwardly to a pair of meshed gears 45 and 47 which turn and corrugate the drain wire with a fixed number and a fixed amplitude of undulations in the drain wire. Also, in accordance with the preferred method, the web 37 having the conductive foil layer 17 thereon is fed with this layer facing inwardly for contact with the drain wire 19 which is sandwiched between the web 37 and the insulating layer 15 in this embodiment of the invention.
Referring now in greater detail to the specific embodiment of the invention and to the coaxial cable shown in FIG. 2, the inner conductor 12 is formed of copper or other suitable conductive material. The central conductor illustrated herein is solid and circular in cross section although it may be stranded or formed in other cross sections. The central conductor may vary considerably in size, and it is generally flexible, as the entire cable 11.
For the purpose of insulating the central conductor 12, the core or layer 15 of insulating material surrounds the conductor 12. The insulating layer 15 is usually extruded about the conductor and is preferably a plastic material such as polyethylene or polyvinyl chloride. Herein, the insulating layer is a solid, single layer although other materials, and composite layers may be used for insulating the internal conductor 12. The insulating layer 15 is of annular cross section and extends longitudinally the length of the cable with its interior surface 46 in direct intimate contact with the outer surface of the conductor 12.
The heat and pressure during the extruding of the jacket 21 within the extruder means 35 softens the insulating layer 15 and causes the formation of the undulated groove 27 in outer surface 47 of the insulating layer 15, the surface 47 being smooth prior to the pressing of the drain wire therein. As it is the drain wire itself which is pushed into the outer surface 47 of the heated insulating layer while in the extruder head, the undulations of the groove 27 match the undulations in drain wire 19 and in the groove 29 which is being formed simultaneously herewith. Herein, it is preferred that the drain wire 19 be disposed directly against the surface 47 of the insulating layer 15 and that conductive shield 17 is between the drain wire and the jacket 21. The web 37 is so thin and pliable that it readily conforms to the undulated drain wire when the extruding plastic for the jacket 21 is pushing the web 37 to conform to the shape of the drain wire. On the other hand, the drain wire 19 may be applied to the exterior side of the conductive shield 17, which is so light and thin in nature, that the drain wire would still form the groove 27 in the heated and softened layer 15 when it is in the extruder means 15. In any event, the drain wire 19 bears direct contact with the crests of only one of the grooves 27 and 29 in either plastic layer 15 or the jacket 21 and is effected by the groove in the other layer although the conductive layer 17 is between the drain wire and the other layer.
In the illustrated embodiment of the invention, the drain wire 19 is disposed internally of the conductive shield 17 and is formed with uniformly spaced undulations, for example, 8 per inch. The number of undulations per inch for the drain wire may be varied from eight, illustrated herein. The amplitude, that is, the height from crest to crest of the undulations may be varied considerably. In one of the examples of a coaxial cable given hereinafter, the crest-to-crest height, i.e., amplitude, is about 0.037 inch whereas in the other example, the height is about one-half of that, viz., 0.019 inch. The undulations are preferably small in amplitude in order to maintain a circular cross section cable when a relatively thin jacket 21 has been applied thereto. Usually, the end users prefer a cable which has a circular cross section. Herein, the drain wire 19 is laid against the insulating layer 15 with the crests being in a plane tangential to the circumferential surface of the insulating layer at the time of entering the extruder means 35 rather than being pointed radially inwardly toward the central conductor. The usual materials may be used for the drain wire 19, for instance, a No. 26 solid copper drain wire or a No. 22 stranded and bunched-tinned drain wire. Of course, other sizes, shapes and conductive materials may be used for the drain wire formation.
The conductive shield 17 is preferably comprised of a flexible foil formed by laminating a thin film of aluminum to one side of a thin insulating tape such as polyethylene or other plastic. When the web 37 is wrapped about the insulating layer 15, the opposite longitudinally extending edges 51 and 53 are overlapped as shown in FIG. 2 with the metal surfaces 53 in contact to provide a complete shield or shorted turn about the interior conductor 12. If desired, another metallic conductive shield in the form of a foil may be applied to the outer side of the insulating tape.
The outer jacket 21 which is formed by extrusion at the extruding means 15 may be formed of a number of materials, but preferably is a plastic material such as polyvinyl chloride or polyethylene. In the specific examples given hereinafter, the jacket 21 has a wall of 0.24 inch thickness of polyvinyl chloride with an outer diameter of 0.251 inch for the first cable and a 0.15 inch wall of polyvinyl chloride for the smaller cable having a 0.077 inch outer diameter. The undulations formed in the outer jacket 21 are in its inner circumferential surface 55 which is in direct contact with the plastic tape side of the conductive shield 17 in this illustrated embodiment of the invention. As the jacket material flows around the drain wire, the extruder tip forces the drain wire 19 radially inwardly in the layer 15 because of the heat and pressure being applied and the softening of the layer 15. The extruder tip and extruding plastic also forms the foil about the drain wire 19. Because both the grooves 27 and 29 are formed simultaneously by the drain wire 19 itself, they are well matched, each having a portion of the undulated drain wire imbedded, i.e., projecting therein in generally the same manner.
To provide a significantly fast method of forming the cable 11, it is preferred that internal conductor 12 and its insulating layer 15 have been previously manufactured to provide an insulated composite conductor being unwound from a reel 57 and it is stripped therefrom and fed forwardly, usually in a continuous manner toward a conventional extruder head within the extruding means 35. The composite conductor 56 merely comprises the internal conductor 12 and the insulating layer 15. Simultaneously, a straight drain wire 41 is fed from the drain wire let-off or reel 43 to the pair of rotatable gear undulators 45 and 47 where, as the drain wire 41 passes between the meshed gears, undulations are generated therein to form the undulated drain wire 19 which is then fed past suitable guides 59 to a position against a side of the insulating layer 15 of the composite wire for travel simultaneously therewith forwardly to a position at which it is joined by the tape 37 which is also fed from a suitable tape let-off or reel means 61. A second drain wire may be corrugated and fed to the opposite side of the composite conductor in this same manner. The web 37 also travels forwardly continuously at the same speed as the composite wire and drain wire 19, and in a well-known manner has one of its longitudinally extending edges 53 bent back to engage the other metallic edge 51 of the shield to form the shorted turn. Thus, the shield 17 has a longitudinally extending seam formed prior to entry into the extruding tip (not shown) which applies simultaneously heat and pressure with the extruding of the jacket material which presses the drain wire 19 into the surface 47 of the insulating layer 15 to form the groove 27. Simultaneously, the matching groove 29 on the other side of the drain wire 19 is being formed in the outer jacket 21. It is to be understood that the term "matching" for the grooves 27 and 29 refers to the general configuration and alignment and that they need not be exactly equal in all respects.
This particular method is found to be quite economical with the use of the usual extrusion temperatures of the jacket 21 being about 340°F. to 370°F. in the extruder means 35. The supply reels 43 of drain wires 41 can be considerably larger than the reels of spiralling machines which supply reels with drain wire and it has been found that without this spiralling equipment that the undulated drain wire may be fed faster through the extruding head. It will be appreciated that additional drain wires may be provided. Also, the undulated drain wire provides a short portion at the stripped area which, when straightened, lengthens slightly for assisting in making connections in an easier manner than when a straight drain wire, which doesn't lengthen, is used. As examples of the invention, coaxial cables have been constructed in accordance with the following dimensions: Cable No. 1 Core diameter = .136" Polyethylene Drain wire size = No. 22 stranded and bunchtinned Shield = Aluminum-Polyester-Vinyl .00085" × 5/8" Jacket = .024" wall -- .215" dia. PVC Undulations Frequency = 8 per inch Crest to Crest = .037" ± .005" amplitude
Cable No. 2 Core diameter = .028" PVC Drain wire size = No. 26 solid copper Shield = Aluminum-Polyester-Vinyl .00085" × 1/4" Jacket = .015" wall -- .077" dia. PVC Undulations Frequency = 8 per inch Crest to Crest = .019" ± .003" amplitude
These examples are exemplary only, as coaxial cables falling within the purview of the invention may be made with other dimensions and materials.
Another embodiment of the invention is illustrated in FIG. 4 which is similar to the embodiment disclosed in FIGS. 1-3 except that the metallic conductive shield 17a faces outwardly and engages the jacket 21a and that the undulated drain wires 19a are disposed outwardly of the conductive shield 17a. The undulated grooves 29a and 27a are formed in the jacket 21a and insulating layer 15a and function, as above described, to limit slipping of the drain wires.
From the foregoing, it will be seen that the present invention provides an improved coaxial cable having drain wires disposed in contact with a slippery conducting shield but interlocked with the outer jacket and the inner insulating layer in a manner that greatly increased forces are required to pull or slip the drain wires. Also, it will be seen from the foregoing that there is provided a quick and economical method of making a cable within a drain wire having increased resistance to slipping in a cable of this kind.
While a preferred embodiment has been shown and described, it will be understood that there is no intent to limit the invention by such disclosure but, rather, it is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention as defined in the appended claims.
Coaxial cables with one or more drain wires in engagement with a conductive shield, usually a metallic foil shield, are used in numerous applications in which the drain wire is subjected to a longitudinally directed force causing the drain wire to slip or slide along the slippery metal foil shield. In some instances, the drain wire actually separates from the cable, rendering the cable defective; and, in other instances, the drain wire shifts to a point where it interferes with proper connection of the cable. Automatic insulation stripping machines apply forces causing a shifting of the drain wire in these kinds of cables as a portion of the outer protective jacket of the cable and a portion of the conductive foil shield are severed and removed by the machine. The shifting or pulling of the drain wires from these cables is more likely for very short lengths of cable. In tests of one commercially available coaxial cable having No. 22AWG drain wires laid parallel to the cable, the drain wire has been pulled from a 16 inch length of such cable with longitudinally directed forces in the range of 0.7 to 4.0 pounds. Tests run on cables in which the drain wire has been laid with a spiral wrap about the insulated core, required only slightly more force to pull the drain wire from the cable. However, with the coaxial cable of the present invention, increased resistance to slipping and pulling of a drain wire from a coaxial cable required, in similar tests, at least 12 pounds of force and, in some instances, the drain wire did not pull out with a 20-pound force and, in other instances, the drain wire broke before it separated from the cable.
In order for a coaxial cable having a drain wire, which is more resistant to slippage, to be commercially practical and competitive, it should be capable of being manufactured at high speeds and without expensive equipment. Generally speaking, spirally applied drain wire may provide a few slight increase in the resistance to slippage of the drain wire, but the spiral laying equipment usually slows the speed of manufacture. As will be explained, with the present invention, the coaxial cable may be manufactured at speeds faster than the speeds involved in manufactured cables within spirally laid drain wires.
Accordingly, a general object of the present invention is to provide a coaxial cable of the foregoing kind having a drain wire with increased resistance to slippage and to provide a method of making such a cable.
Other objects and advantages of the present invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:
FIG. 1 is a diagrammatic illustration of an apparatus for practicing a method of manufacture of a coaxial cable constructed in accordance with a preferred embodiment of the invention;
FIG. 2 is a perspective view of a cable with the layers broken away and constructed in accordance with a preferred embodiment of the invention;
FIG. 3 is a cross-sectional view taken substantially along the line 3--3 of FIG. 2; and
FIG. 4 is a cross-sectional view of another embodiment of the invention.
As shown in the drawings for purposes of illustration, the invention is embodied in a coaxial cable 11 having an internal conductor 12 for carrying electrical current. The internal conductor 12 is surrounded by an insulating layer 15 which separates and electrically insulates the conductor 12 from a surrounding conductive shield 17, preferably in the form of a metallic foil. In contact with the metallic surface of the conductive shield 17 is at least one elongated drain wire 19. Surrounding the drain wire 19, conductive shield 17, insulating layer 15 and internal conductor 12 is an outer insulating or protective jacket 21 of insulating material such as a plastic.
Coaxial cables having a conductive shield with a drain wire in contact therewith have been used for numerous applications in which a portion of the outer insulating jacket 15 is stripped away at an end of the cable to expose the drain wire and often the internal conductor for attachment to a given terminal or device. Often the portion of the outer jacket is stripped automatically from the cable by a machine which also severs the cable into a predetermined length, for example, a short piece of cable only several inches in length. Such automatic stripping machines often apply sufficient longitudinally directed forces to the drain wire to cause it to slide along the slippery metallic surface of the conductive outer shield. Usually the conductive shield is a laminate of metallic foil and plastic with the plastic facing outwardly and slightly adhered to the outer extruded plastic jacket. In such cables, it has been found that the drain wire often slips or separates from the cable with small forces, e.g., 0.7 to 4 pounds of force; and there is a demonstrated need for improved resistance to drain wire slippage without a substantial increase in cost of such coaxial cable.
In accordance with the present invention, the drain wire 19 contacting the conductive shield 17 is formed with a series of undulations 23 and it is imbedded in matched grooves 27 and 29 formed in the core layer 15 and jacket 21, respectively, with the peaks or crests 31 and 32 of the respective grooves 27 and 29 resisting a longitudinally directed displacement of the undulated drain wire 19, as heretofore occurred when using an automatic stripping machine for prior art cables. More specifically, the undulated drain wire 19 which hereinafter is termed a corrugated drain wire 19, provides a significantly greater resistance to pulling and slippage than does either a straight drain wire or a spirally laid drain wire of prior art cables of this general kind. For instance, tests indicate that a range of 12 to 20 pounds or more of force are required to remove an undulated drain wire 19 from a 16-inch length of the cable 10 as contrasted to 0.7 to 4.0 pounds for removal of a drain wire of a prior art coaxial cable of 16 inches in length. This greater resistance is due to the fact that the corrugated drain wire 19 must continually change its shape and follow a tortuous path defined by the spaced crests 31 and 32 and valleys therebetween if it is to travel longitudinally relative to the jacket 21 and insulating layer 15. Such change of shape of the drain wire metal and frictional resistance to travel along the tortuous path result in the high resistance despite the fact the drain wire engages a slippery metallic surface on one side thereof.
In accordance with another aspect of the invention, the coaxial cable 11 having increased resistance to drain wire slippage may be manufactured commercially at speeds and costs competitive with commercially available coaxial cables by a method which comprises the steps of, as best seen in FIG. 1, feeding a conductor 12 surrounded by the insulating layer 15 in a forward direction towards an extruder means 35, feeding an undulated drain wire 19 forwardly to the extruder means 35, feeding a web 37 having the conductive shield 17 forwardly for contact with the drain wire 19 and for wrapping about the insulating layer 15, extruding a surrounding jacket 21 of insulating material about the web 37, undulated drain wire 19 and insulating layer 15 at the extruder means 35, and applying sufficient heating and pressure during the extruding step to form an undulating groove 27 in the insulating layer 15 and a matched undulating groove 29 in the jacket 21 thereby providing resistance against pulling of the drain wire 19 from the cable. In the preferred method, a straight drain wire 41 is unreeled from a reel or let-off 43 and travels forwardly to a pair of meshed gears 45 and 47 which turn and corrugate the drain wire with a fixed number and a fixed amplitude of undulations in the drain wire. Also, in accordance with the preferred method, the web 37 having the conductive foil layer 17 thereon is fed with this layer facing inwardly for contact with the drain wire 19 which is sandwiched between the web 37 and the insulating layer 15 in this embodiment of the invention.
Referring now in greater detail to the specific embodiment of the invention and to the coaxial cable shown in FIG. 2, the inner conductor 12 is formed of copper or other suitable conductive material. The central conductor illustrated herein is solid and circular in cross section although it may be stranded or formed in other cross sections. The central conductor may vary considerably in size, and it is generally flexible, as the entire cable 11.
For the purpose of insulating the central conductor 12, the core or layer 15 of insulating material surrounds the conductor 12. The insulating layer 15 is usually extruded about the conductor and is preferably a plastic material such as polyethylene or polyvinyl chloride. Herein, the insulating layer is a solid, single layer although other materials, and composite layers may be used for insulating the internal conductor 12. The insulating layer 15 is of annular cross section and extends longitudinally the length of the cable with its interior surface 46 in direct intimate contact with the outer surface of the conductor 12.
The heat and pressure during the extruding of the jacket 21 within the extruder means 35 softens the insulating layer 15 and causes the formation of the undulated groove 27 in outer surface 47 of the insulating layer 15, the surface 47 being smooth prior to the pressing of the drain wire therein. As it is the drain wire itself which is pushed into the outer surface 47 of the heated insulating layer while in the extruder head, the undulations of the groove 27 match the undulations in drain wire 19 and in the groove 29 which is being formed simultaneously herewith. Herein, it is preferred that the drain wire 19 be disposed directly against the surface 47 of the insulating layer 15 and that conductive shield 17 is between the drain wire and the jacket 21. The web 37 is so thin and pliable that it readily conforms to the undulated drain wire when the extruding plastic for the jacket 21 is pushing the web 37 to conform to the shape of the drain wire. On the other hand, the drain wire 19 may be applied to the exterior side of the conductive shield 17, which is so light and thin in nature, that the drain wire would still form the groove 27 in the heated and softened layer 15 when it is in the extruder means 15. In any event, the drain wire 19 bears direct contact with the crests of only one of the grooves 27 and 29 in either plastic layer 15 or the jacket 21 and is effected by the groove in the other layer although the conductive layer 17 is between the drain wire and the other layer.
In the illustrated embodiment of the invention, the drain wire 19 is disposed internally of the conductive shield 17 and is formed with uniformly spaced undulations, for example, 8 per inch. The number of undulations per inch for the drain wire may be varied from eight, illustrated herein. The amplitude, that is, the height from crest to crest of the undulations may be varied considerably. In one of the examples of a coaxial cable given hereinafter, the crest-to-crest height, i.e., amplitude, is about 0.037 inch whereas in the other example, the height is about one-half of that, viz., 0.019 inch. The undulations are preferably small in amplitude in order to maintain a circular cross section cable when a relatively thin jacket 21 has been applied thereto. Usually, the end users prefer a cable which has a circular cross section. Herein, the drain wire 19 is laid against the insulating layer 15 with the crests being in a plane tangential to the circumferential surface of the insulating layer at the time of entering the extruder means 35 rather than being pointed radially inwardly toward the central conductor. The usual materials may be used for the drain wire 19, for instance, a No. 26 solid copper drain wire or a No. 22 stranded and bunched-tinned drain wire. Of course, other sizes, shapes and conductive materials may be used for the drain wire formation.
The conductive shield 17 is preferably comprised of a flexible foil formed by laminating a thin film of aluminum to one side of a thin insulating tape such as polyethylene or other plastic. When the web 37 is wrapped about the insulating layer 15, the opposite longitudinally extending edges 51 and 53 are overlapped as shown in FIG. 2 with the metal surfaces 53 in contact to provide a complete shield or shorted turn about the interior conductor 12. If desired, another metallic conductive shield in the form of a foil may be applied to the outer side of the insulating tape.
The outer jacket 21 which is formed by extrusion at the extruding means 15 may be formed of a number of materials, but preferably is a plastic material such as polyvinyl chloride or polyethylene. In the specific examples given hereinafter, the jacket 21 has a wall of 0.24 inch thickness of polyvinyl chloride with an outer diameter of 0.251 inch for the first cable and a 0.15 inch wall of polyvinyl chloride for the smaller cable having a 0.077 inch outer diameter. The undulations formed in the outer jacket 21 are in its inner circumferential surface 55 which is in direct contact with the plastic tape side of the conductive shield 17 in this illustrated embodiment of the invention. As the jacket material flows around the drain wire, the extruder tip forces the drain wire 19 radially inwardly in the layer 15 because of the heat and pressure being applied and the softening of the layer 15. The extruder tip and extruding plastic also forms the foil about the drain wire 19. Because both the grooves 27 and 29 are formed simultaneously by the drain wire 19 itself, they are well matched, each having a portion of the undulated drain wire imbedded, i.e., projecting therein in generally the same manner.
To provide a significantly fast method of forming the cable 11, it is preferred that internal conductor 12 and its insulating layer 15 have been previously manufactured to provide an insulated composite conductor being unwound from a reel 57 and it is stripped therefrom and fed forwardly, usually in a continuous manner toward a conventional extruder head within the extruding means 35. The composite conductor 56 merely comprises the internal conductor 12 and the insulating layer 15. Simultaneously, a straight drain wire 41 is fed from the drain wire let-off or reel 43 to the pair of rotatable gear undulators 45 and 47 where, as the drain wire 41 passes between the meshed gears, undulations are generated therein to form the undulated drain wire 19 which is then fed past suitable guides 59 to a position against a side of the insulating layer 15 of the composite wire for travel simultaneously therewith forwardly to a position at which it is joined by the tape 37 which is also fed from a suitable tape let-off or reel means 61. A second drain wire may be corrugated and fed to the opposite side of the composite conductor in this same manner. The web 37 also travels forwardly continuously at the same speed as the composite wire and drain wire 19, and in a well-known manner has one of its longitudinally extending edges 53 bent back to engage the other metallic edge 51 of the shield to form the shorted turn. Thus, the shield 17 has a longitudinally extending seam formed prior to entry into the extruding tip (not shown) which applies simultaneously heat and pressure with the extruding of the jacket material which presses the drain wire 19 into the surface 47 of the insulating layer 15 to form the groove 27. Simultaneously, the matching groove 29 on the other side of the drain wire 19 is being formed in the outer jacket 21. It is to be understood that the term "matching" for the grooves 27 and 29 refers to the general configuration and alignment and that they need not be exactly equal in all respects.
This particular method is found to be quite economical with the use of the usual extrusion temperatures of the jacket 21 being about 340°F. to 370°F. in the extruder means 35. The supply reels 43 of drain wires 41 can be considerably larger than the reels of spiralling machines which supply reels with drain wire and it has been found that without this spiralling equipment that the undulated drain wire may be fed faster through the extruding head. It will be appreciated that additional drain wires may be provided. Also, the undulated drain wire provides a short portion at the stripped area which, when straightened, lengthens slightly for assisting in making connections in an easier manner than when a straight drain wire, which doesn't lengthen, is used. As examples of the invention, coaxial cables have been constructed in accordance with the following dimensions: Cable No. 1 Core diameter = .136" Polyethylene Drain wire size = No. 22 stranded and bunchtinned Shield = Aluminum-Polyester-Vinyl .00085" × 5/8" Jacket = .024" wall -- .215" dia. PVC Undulations Frequency = 8 per inch Crest to Crest = .037" ± .005" amplitude
Cable No. 2 Core diameter = .028" PVC Drain wire size = No. 26 solid copper Shield = Aluminum-Polyester-Vinyl .00085" × 1/4" Jacket = .015" wall -- .077" dia. PVC Undulations Frequency = 8 per inch Crest to Crest = .019" ± .003" amplitude
These examples are exemplary only, as coaxial cables falling within the purview of the invention may be made with other dimensions and materials.
Another embodiment of the invention is illustrated in FIG. 4 which is similar to the embodiment disclosed in FIGS. 1-3 except that the metallic conductive shield 17a faces outwardly and engages the jacket 21a and that the undulated drain wires 19a are disposed outwardly of the conductive shield 17a. The undulated grooves 29a and 27a are formed in the jacket 21a and insulating layer 15a and function, as above described, to limit slipping of the drain wires.
From the foregoing, it will be seen that the present invention provides an improved coaxial cable having drain wires disposed in contact with a slippery conducting shield but interlocked with the outer jacket and the inner insulating layer in a manner that greatly increased forces are required to pull or slip the drain wires. Also, it will be seen from the foregoing that there is provided a quick and economical method of making a cable within a drain wire having increased resistance to slipping in a cable of this kind.
While a preferred embodiment has been shown and described, it will be understood that there is no intent to limit the invention by such disclosure but, rather, it is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention as defined in the appended claims.