US 3240867 A
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Description (OCR text may contain errors)
March 15, 1966 R, D. MADDOX 3,240,667
SHIELDED CONDUCTOR IN AN EXTENSIBLE CABLE Filed Oct. 9, 1962 United States Patent O 3,240,867 SHIELDED CONDUCTOR IN AN EXTENSIBLE CABLE Richard D. Maddox, deceased, late of Richmond, Ind., by Clara A. Maddox, executrix, Richmond, Ind., assiguor to Belden Manufacturing Company, Chicago, Ill., a corporation of Illinois Filed Oct. 9, 1962, Ser. No. 229,464 4 Claims. (Cl. 174-69) This application is a continuation-in-part of application Serial No. 183,573, tiled March 29, 1962, now abandoned.
This invention relates to coiled electrical cords and more particularly to an extensible and retractable electric cord of the type which includes at least one insulated wire having suitable shielding thereabout.
The use of shielded type coiled cords has become increasingly more prevalent, and such cords are incorporated in numerous electrical measuring and communication systems. For example, coiled cords are commonly utilized on telephone receivers, as microphone cables for radio transceivers, as probe cables for Geiger counter equipment, etc. These coiled cords generally employ one or more insulated signal carrying wires which require suitable electromagnetic and/ or electrostatic shielding. The function of the shielding is to preclude extraneous voltages or signals from being picked up and adversely affecting the signal carried by one or more of the wires contained in the coiled conductor and/or inhibit signals from being transmitted by the shielded signal carrying wires.
When shielding is employed, it is necessary that the the coiled cord or conductor retain the extensible and retractable properties which characterize such cords. Initially attempts were made to utilize a braided form of shielding similar to that which proved satisfactory as shielding for insulated conductors or wires included in various cable structures. However, attempts to incorporate this type of braided shielding in coiled cords did not meet with great success.
More particularly, attempts to use the round strands conventionally incorporated in braided shields led to a diameter buildup larger than is desirable in retractile or coiled cords. In addition, the nature of the conventional braided shields is such that the degree of flexibility and pliability required in coiled cords cannot be realized. Moreover, previous shielding braided from conductive textiles, plastics and elastomers have not proven to possess the most effective electrical shielding properties.
To overcome the problems incident to the use of the previously available braided shields, recourse has been made to producing retractile cords by providing shielding about selected insulated wires contained therein in the form of spirally wound metallic wire or in the form of a plurality of helically wrapped metallic ribbons. These latter and other non-braided forms of shielding, while proving satisfactory for certain applications, do not possess the advantageous qualities of braided shielding as required in coiled cord constructions.
It is a prime object of the present invention to provide an improved retractile cord construction including at least one insulated wire having a flexible braided metallic shield thereabout.
VA further object of the invention resides in the provision of a retractile cord including at least one wire shielded with flexible metallic braid that possesses the highly desirable properties of extensibility and retractability.
Still another object of the invention is to provide a retractile cord including at least one insulated wire 3,240,867 Patented Mar. 15, 1966 ice shielded with a flexible metalized braid which can withstand numerous extension and retraction cycles and which is relatively economical to manufacture.
Other objects and advantages of the present invention will become apparent from the following description thereof when considered in conjunction with the accompanying drawings wherein:
FIGURE l is a perspective view, partially broken away, of one type of extensible and retractable coiled cord embodying the principal features of the present invention;
FIGURE 2 is an enlarged perspective view of one of the insulated wires shown in FIGURE l having an improved braided metallic shield of the type contemplated by the present invention;
FIGURE 3 is an enlarged cross-sectional View of a iilamentary strand of a flexible metalized lm utilized in the braided shield illustrated in FIGURE 2; and
FIGURE 4 is a cross-sectional View taken along the line 4-4 of FIGURE 2.
As depicted in the drawings, a preferred embodiment of the present invention includes a retractile cord having an outer insulating sheath set in helical form and `having at least one insulated wire contained therein which is provided with a braided metallic shield. In accordance with the provisions of the invention, the braided metallic shield is fabricated of a plurality of thin metalized ribbon-like strands or filaments which exhibit good flexible properties and are capable of withstanding the normal tensile stress imparted thereto. These characteristics of the braided shielding contemplated by the invention insure that the coiled cord containing the shielded wire retains the desired elastic properties and is capable of withstanding numerous expansion and retraction cycles.
Referring in particular to FIGURE 1, there is disclosed one form of retractile cord 10 which includes at least one insulated wire 11 having a braided metallic shield 12 of the type contemplated by the present invention. The coiled cord 10 is formed with a plurality of helical turns or convolutions 10a having a substantially uniform diameter. Oppositely disposed substantially straight end portions 10b and 10c of the cord 10 extend tangentially from the coiled central portion and act as terminals for the cord. The cord includes an outer insulating sheath or jacket 13 which is fabricated of a. material such as an extruded synthetic rubber. A thin coating 14 of a protective tissue-like material, preferably composed of cellulosic fibers, is provided along the inner peripheral surface 13a of the sheath.
In a preferred embodiment of the invention, the jacket or sheath 13 is extruded about a plurality of individual wires 11, each of which preferably includes a stranded central conductor 15 and an insulating sleeve 16 fabricated of a material such as rubber, plastic, etc. The individual wires 11 are wound in lspiral fashion about each other to provide a compact cord and the coating 14 serves to protect the Wires during the extrusion of the sheath 13.
At least one of the insulated wires 11 is provided with the braided metallic shield 12 which extends along the outer periphery of the sleeve 16. As previously described, the shield serves to isolate a selected wire 11 from stray electrical signals that may have an adverse effect on the operation of the system wherein the cord is utilized and/ or precludes signals from being transmitted by the shielded Wires. The cord 10, which is illustrative of only one of the many types of cords wherein the braided metallic shield 12 can be utilized, also includes a drain or ground wire 17 that is wound in spiral fashion with and around the shielded and unshielded wires 1v1. The drain wire may also be woven in the braid itself or laid parallel thereto.
Referring to FIGURES 2-4, the braided shield 12 which extends along the outer peripheral surface of the insulating sleeve 16 of the signal carrying wire 11, is fabricated in accordance with conventional braiding methods from a plurality of metalized flamenta-ry strands or ribbon-like members 12a. Each of the filamentary strands or members 12a is extremely thin and is characterized by a high degree of exibility. These characteristics of the strands which comprise the braid insure that the elastic properties of extensibility and ret-ractability are maintained in the composite coiled cord including the braided shielding.
As shown in FIGURE 3, each metalized lilamentary strand 12a includes a thin central or base layer 12b formed of a suitable flexible material, which in one embodiment of the invention has a `thickness of approximately .002 and a width of approximately .02". The preferred thickness Iand width of the lamentary strands will of course vary when braided shielding fabricated therefrom is utilized with insulated wires of different diameter.
Although the flexible base layer must be sufliciently thin to preclude substantial diameter build up in the coiled cord incorporating the braided shielding, the material must be sufficiently strong to withstand the tensile stress impa-rted thereto during normal use of the cord. Materials which satisfy these requirements and which can be satisfactorily utilized as the thin flexible base material are polyethylene terephthal-ate, polytetratiuoroethylene and polychloroltrifluoroethylene. These materials are commercially available under the trade ma-rks Mylar, Teflon and Kel-F, respectively. Mylar is the preferred base material since it is relatively inexpensive and has high tensile strength in small cross section. In addition, extremely thin coatings or layers 12a (eg. .00035) of conductive materials such as aluminum, silver, copper, etc. can be readily bonded to each surface of a Mylar strand so that during subsequent expansion and retraction of the cord incorporating the shield, the continuity of the metallic coatings is not disturbed.
The. braid 12 provided about the insulating sleeve 16 of the conductor 11 differs from the usual ribbon-like conductive shield that is helically or spirally wound about a wire disposed within a coiled cord. In this connection, the thin metalized strands forming the braid 12 possess flexible properties which particularly lend the shielding braided therefrom to advantageous use in coiled cords. Moreover, when problems of R-F pickup are encountered, the individual iilamentary strands forming the braid and crossing each other repeatedly do not develop or sustain any inductive eifect as would be the case with conventional spirally served shielding.
It has been found that the flex life of shielding braided from the metalized filamentary str-ands has been substantially improved and the noise level in a coiled cord incorporating such a braid is minimized when the individual filamentary strands 12a are braided so that the various strands merge to define a braid angle of less than about 45 and greater than about 17 (where the braid angle is equal to the angle dened between a longitudinally extending line that bisects the angle at which two strands merge and one of the strands). For braid angles less than this, the strand of the braid on the outside of a bend is subject to undesirable stretching. It has further been found that such advantageous results are optimized when the aforedescribed braid angle .is approximately 26.
To provide adequate shielding when utilizing filamentary strands having 4the dimensions described above in connection with one preferred embodiment, sixteen such strands were braided to provide an overall coverage of approximately 95%. Since the braid is relatively thin, this coverage is achieved without any substantial buildup in the diameter of the cord incorporating the conductors provided with the braided shielding. Preferably, the overall coverage should not exceed approximately An extensible `and retractable coiled cord including at least one insulated wire having a thin metalized shield braided thereabout can be produced in any desired length in accordance with conventional coiled cord producing methods. That is, the several interwound conductive members or wires 11 are enclosed by the extruded outer sheath or jacket 13, and the entire cord is helically wound about a mandrel with the ends thereof remaining substantially straight to provide the terminal portions 10b and 10c. Thereafter, the mandrel together with the cord wrapped in helical fashion thereabout is subjected to successive heating and cooling steps which effect a setting of the sheath in coiled form. Preferably, the coiled cord is worked after cooling to reverse the direction of the convolutions 10a and thereby increase the tendency of the cord to remain in a contracted state.
It should be understood that the foregoing description embodied in the present application is merely illustrative of the application of the invention. Various modifications in the structural features of the described coiled cord incorporating a shield braided of thin strands of metalized pexible material incorporated therein could be devised by those skilled in the art Without departing from the invention.
Various features of the invention are set forth in the accompanying claims.
What is claimed is:
1. In an extensible and retractable coiled cord that includes an outer insulating sheath formed with a plurality of helical convolutions with the insulating sheath set in such helical formation, a plurality of insulated wires disposed Within the sheath and extending along the length thereof, a exible braided shield provided about at least one of said insulated wires, which shield comprises a plurality of ribbon-like strands of a thin, strong, flexible non-conductive base material having a thin laye-r of conductive material bonded to each surface thereof, said conductive layers being substantially thinner than the thickness of said ribbon-like strands of said non-conductive material, the braid on said one insulated wire having a braid angle of between approximately 17 and 45, whereby a minimum of stretching is imparted thereto.
2. In an extensible and retractable coiled cord that includes an outer insulating sheath formed with a plurality of helical convolutions with the insulating sheath set in such helical formation, a plurality of insulated wires disposed within the sheath and extending along the length thereof, a flexible braided shield provided about at least one of said insulated wires, which shield comprises a plurality of ribbon-like strands of a thin, strong, flexible non-conductive base material having a thin layer of conductive material bonded to each surface thereof, the braid on said one insulated wire having a braid angle of approximately 26 whereby a minimum of stretching is imparted thereto.
3. The product of claim 1 in which the non-conductive base material is polyethylene terephthalate.
4. The product of claim 2 in which the non-conductive base material is polyethylene terephthalate.
References Cited by the Examiner UNITED STATES PATENTS 2,293,677 8/ 1942 Slayter 174-102 2,367,944 1/1945 Ingalls 138-124 X 2,759,990 8/ 1956 Bean 87--9 2,764,625 9/ 1956 Ingmanson 174-69 2,939,905 6/ 1960 Canfield 174-69 X JOHN F. BURNS, Primary Examiner. JOHN P. WILDMAN, Examiner,