U.S. patent number 3,816,644 [Application Number 05/346,600] was granted by the patent office on 1974-06-11 for low noise cord with non-metallic shield.
This patent grant is currently assigned to Belden Corporation. Invention is credited to Robert B. Cole, Byron B. Giffel.
United States Patent |
3,816,644 |
Giffel , et al. |
June 11, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
LOW NOISE CORD WITH NON-METALLIC SHIELD
Abstract
A low noise flexible multiconductor electric cord having a
number of insulated electrical conductors, at least one of which
has a non-metallic electrically conductive material enveloping the
same to define an electrical shield, a grouping of relatively small
exposed electrical drain or ground strands positioned generally
parallel to and in intimate contact with the shield of each of the
shielded primary conductors to provide electrical continuity
between the grounding strands and the shielding material, and an
outer insulating jacket enveloping all of the conductors and ground
strands.
Inventors: |
Giffel; Byron B. (Richmond,
IN), Cole; Robert B. (New Westville, OH) |
Assignee: |
Belden Corporation (Chicago,
IL)
|
Family
ID: |
23360167 |
Appl.
No.: |
05/346,600 |
Filed: |
March 30, 1973 |
Current U.S.
Class: |
174/115; 174/36;
174/113R; 174/116; 174/120SC |
Current CPC
Class: |
H01B
7/04 (20130101); H01B 9/028 (20130101); H01B
11/1091 (20130101) |
Current International
Class: |
H01B
7/04 (20060101); H01B 9/02 (20060101); H01B
9/00 (20060101); H01b 009/02 () |
Field of
Search: |
;170/113R,113C,115,116,12SC,12SR,69,36,130,131A,126,16SC,15SC,11V |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Grimley; A. T.
Attorney, Agent or Firm: Anderson; William E.
Claims
What is claimed is:
1. A flexible, low noise miniature type cord, comprising, in
combination:
a plurality of flexible electrical conductors, each of which
includes an electrically insulating sheath enveloping the same,
each conductor comprising one or more strands, each strand
comprising a non-metallic thread and one or more electrically
conductive metallic ribbons;
at least one of said conductors having a non-metallic, flexible
electrically conductive shielding material enveloping said
insulating sheath to define an electrical shield;
a plurality of at least eight flexible electrically conductive
grounding strands positioned generally parallel to and in contact
with each of said shielded conductors to provide electrical
continuity between said grounding strands and said shielding
material, each of said strands comprising a non-metallic thread and
one or more electrically conductive metallic ribbons twisted around
one another; and
an outer electrically insulating flexible jacket enveloping said
conductors and grounding strands, said jacket having sufficient
radial compression to maintain said grounding strands and shielding
material in intimate contact with one another.
2. A cord as defined in claim 1 which includes three of said
electrical conductors arranged in a generally triangular
configuration, one of said conductors being said shielded conductor
and said plurality of grounding strands being positioned in at
least one of the valleys between said shielded insulated conductor
and an adjacent insulated conductor, and textile filler strands
extending along the remaining valleys.
3. A cord as defined in claim 1 including a central core of
non-conductive textile material and six of said insulated
conductors, two of which have said enveloping shielding material,
said conductors being symmetrically arranged so that each of said
conductors are immediately adjacent to other conductors and said
core, said two shielded conductors being spaced from one another by
one or more of said unshielded insulated conductors.
4. A cord as defined in claim 1 wherein said electrically
conductive non-metallic material comprises electrically conductive
polyvinyl chloride having a volume resistivity of up to about 12.5
ohms centimeters.
5. A cord as defined in claim 1 wherein said outer adjacent jacket
comprises a flexible elastic insulating material and is set in the
form of a series of self-retracting coiled convolutions, said
coiled convolutions normally being adjacent one another in
retracted relation and being spreadable relative to one another
responsive to force being applied to extend the cord.
6. A cord as defined in claim 1 wherein each of said conductors
comprises four of said strands, each of said strands including two
of said ribbons, each ribbon having cross sectional dimensions of
about 1 mil by 9 mils.
7. A cord as defined in claim 1 wherein said outer insulating
jacket is extruded plastic material of uneven thickness, said
jacket having a generally circular outer periphery and being of
increased thickness in those locations where valleys occur between
adjacent conductors and/or grounding strands and/or textile filler
strands.
8. A cord as defined in claim 1 wherein said conductors comprise a
plurality of electrically conductive strands, each strand being
equal or smaller than the equivalent of about 22 gauge AWG.
9. A flexible multi-conductor miniature electric cord having
superior low noise and extended flexibility life characteristics,
comprising in combination:
a plurality of flexible insulated primary electric conductors;
at least one of said primary conductors having an electrically
conductive non-metallic extruded material enveloping the same and
defining an electric shield;
a plurality of uninsulated electrical metallic grounding strands,
each of said strands comprising a non-metallic thread and one or
more electrically conductive metallic ribbons twisted around one
another, said plurality of grounding strands being grouped together
and positioned generally parallel to and in intimate contact with
said shielded primary conductor to provide electrical continuity
between said grounding strains and said shielding material; and
an outer insulating jacket surrounding said conductors and sized to
provide sufficient tightness to maintain said grounding strands in
intimate contact with said shielded primary conductors.
Description
This invention generally relates to flexible electric cords or
cables and, more specifically, to miniature multiconductor shielded
electric cords that may be either retractile cords or straight bulk
cordage.
Although retractile or coiled cords as well as the uncoiled cords
or bulk cordage have long been manufactured incorporating one or
more shielded conductors, i.e., such shielded conductors have a
suitable electromagnetic and/or electrostatic shielding material
that envelopes the electrical insulating material covering the
metallic conductors. The function of the shielding material is to
preclude extraneous voltages or signals being picked up and carried
by one or more of the conductors which may detrimentally affect the
signal being transmitted by the shielded conductors.
Previous constructions of cable shields have consisted of low
coverage copper or tinsel or copper alloy braids, serves,
aluminum-polyester tapes as well as semiconducting fabrics. All
these shields exhibit one or more of the disadvantages of high
self-generated noise, large diameters, high cost, poor shielding,
or reduced flex life in addition to reduced initial
flexibility.
For relatively small diameter multi-conductor cords, commonly
referred to as miniature or sub-miniature cords which are used in
the telephone headset industry, and for microphone cables of tape
recorders, medical instrumentation, etc., undesirable hum and
self-induced or triboelectric noise are important factors that
contribute to the operational performance of a cord during use.
Additionally, since such cords are typically subjected to
considerable flexing during normal usage, it is also important that
the cord be capable of being flexed over an extended period of time
without breaking the conductors or grounding strands therein or
without detrimentally affecting the operational characteristics of
the cord.
Accordingly, it is a primary object of the present invention to
provide a miniature multi-conductor cord that includes at least one
shielded conductor and exhibits excellent flexibility and extended
flex life, as well as improved operational characteristics
including substantially lower self-induced noise and hum.
It is another object of the present invention to provide a cord
having the aforementioned desirable attributes which is suited for
fabrication as either straight, bulk cordage or in a coiled
retractile configuration.
Still another object of the present invention lies in the provision
of having at least one shielded conductor in the cord wherein the
shield comprises a layer of semiconductive plastic material
overlying the insulating sheath that envelops the conductors and
also includes a group of extremely small exposed or uninsulated
grounding strands positioned generally parallel to and in intimate
contact with the conductive plastic shielding material so that
electrical continuity between the shielding material and grounding
strands is maintained, thereby reducing the probability of the
shield becoming open circuited.
A related object lies in the provision of extruding the conductive
shielding material around an insulated conductor which results in
significantly increased production speeds compared to serving and
other comparable procedures and thereby favorably affects
production costs.
A more specific object of the present invention lies in the
provision of having a group of small diameter uninsulated strands
in intimate contact with the shielding material to provide drain or
ground strands that may be easily processed at a point of
termination, with the combination of the grounding strands and
shielding material also providing improved electrical operational
characteristics.
Yet another specific object of the present invention is to provide
a cord having the above desirable attributes wherein the shielding
material for the shielded conductor is easily applied and may have
a thin wall thickness which is helpful in producing a small overall
cord diameter.
Other objects and advantages of the present invention will become
apparent upon reading the following detailed description, while
referring to the attached drawings, in which:
FIG. 1 is a cross sectional view, partially in perspective, and
illustrating a cord construction embodying the present invention,
and particularly illustrating three primary conductors; and
FIG. 2 is a sectional view of a cord construction also embodying
the present invention, and particularly illustrating a six primary
conductor configuration.
While the present invention is susceptible of various modifications
and alternative constructions, certain preferred embodiments are
shown and described herein. It should be understood, however, that
it is not intended to limit the invention to the specific forms
disclosed. On the contrary, it is intended that all substitutions,
equivalents and modifications be covered as may be included within
the spirit and scope of the present invention as expressed in the
appended claims.
Turning now to the drawings and specifically FIG. 1, a short length
of electric cord 10 embodying the present invention is shown.
Broadly stated, the cord 10 is shown to include three primary
electrical conductors indicated generally at 12, 14 and 16, with
each of these conductors having one or more electrically conductive
central strands 18 and an insulating sheath 20. The conductor 12
also has a layer 22 of non-metallic, electrically conductive
shielding material applied to and enveloping the insulating
material 20. Positioned generally parallel to and in intimate
contact with the shielding material 22 is a grouping of uninsulated
or exposed strands 24 which are in electrical contact with the
material and provide a ground or drain wire which may easily be
processed or terminated at terminating points. A pair of
non-conductive textile filler strands 26 are also provided in the
construction illustrated to provide for more even overall symmetry
and balance in the resulting cord so that during normal use, the
cord will not be prone to flex repeatedly in a specific direction
which would tend to reduce the overall flex life of the cord. An
outer jacket 28 is shown to surround all of the interior components
of the cord. While a cross sectional view of the cord taken any
where along its length would result in the same relative positions
of the interior components relative to one another, it should be
understood that the conductors and strands are twisted along their
length and the jacket 28 is applied to maintain the twist of the
interior components as is conventional practice.
The foregoing broad description of the electric cord embodying the
present invention, while appearing to be similar to existing
configurations, nontheless belies the significant advance in the
art, particularly in terms of its operational characteristics and
flex life, which will be clearly set forth hereinafter. More
specifically, the various parameters that are used to measure the
operational effectiveness of an electric cord will be set forth and
the significant improvements that result from the present invention
will become readily apparent.
In keeping with the present invention, the size of the miniature
cords to which the present invention is primarily directed and
particularly the three conductor construction shown in FIG. 1 may
of course vary, but may be on the order of 0.130 inches (130 mils)
diameter for straight or bulk cordage, the diameter being slightly
increased to about 0.150 inches (150 mils) diameter in the event a
retractile cord is produced. The slightly increased diameter for
the retractile cord is largely a function of increased jacket
thickness that is desirable to provide the requisite elasticity or
resiliency for the coiled configuration. With such outside
diameters, the individual conductors 14 and 16 are typically about
37 mils in diameter including the insulating sheath 20 and the
shielded conductor 12 is of slightly increased diameter due to the
additional thickness of the shield and is preferably about 47 .+-.
2 mils diameter. The grouping of drain or ground strands 24 may of
course vary depending upon the number of individual strands that
are incorporated into the cord, but with a grouping of 10 drain
strands 24, the effective diameter approximates 29-30 mils, it
being realized that the grouping is not perfectly symmetrical. With
the conductors 12, 14 and 16 being arranged in a generally
triangular configuration, the grouping of ground strands 24 is
positioned immediately adjacent the shielded conductor 12
preferably in the valley between either the conductor 14 or 16 and
the shielded conductor 12. In the shown configuration, it is then
preferable that filler material such as a textile thread or cotton
strand or the like 26 be placed in the two remaining valleys to
gain the desirable symmetry and fill out the configuration to
approach circularity in the overall outer configuration.
While each of the primary conductors 12, 14 and 16 are shown to
have a total of four strands 18, it should be understood that a
greater or small number may be utilized depending upon the size and
kind of strands that are used. For example, each of the strands 18
may comprise fine metal wires or tinsel which is a small textile
strand or cord such as polyester or the like, around which one or
more ribbons of a copper cadmium alloy or other low resistance
metal are wrapped, with each of the ribbons typically having a
cross sectional area of 1 mil by 9 mils. Thus, with a total of four
of each strands 18, with each strand having 2 ribbons therein, it
is seen that 8 electrically conductive ribbons would be present.
The insulating material 20 is preferably extruded to the strands 18
and is preferably comprised of polypropylene or other flexible high
resistance electrical insulating material.
In accordance with an important aspect of the present invention,
and referring to the shielded conductor 12, it is noted that the
construction of this conductor is substantially similar to the
other conductors 14 and 16, except for the application of an
electrically conductive shielding material to the outer surface of
the electrical insulation 20. The electrically conductive shielding
layer 22 is preferably extruded over the insulation so as to
provide a homogeneous layer of electrically conductive material,
such as semi-conductive polyvinyl chloride, for example. It has
been found that semi-conductive polyvinyl chloride having the
volume resistivity of up to 12.5 ohm centimeters is sufficiently
conductive to adequately shield the conductor, although it is
preferred that volume resistivities be within the lower end of the
range, such as 4.8 ohm centimeters, for example. In this
connection, a semi-conductive polyvinyl chloride having a volume
resistivity of 4.8 ohm centimeters is sold under the trade name of
"ABBEY-100" by the Abbey Plastic Corporation of Hudson, Mass. Using
a conductive polyvinyl chloride shielding material with a wall
thickness of about 5 mils, only increases the diameter of the
conductor 12 by about 10 mils over the diameter of the other
primary conductors 14 and 16 and therefore does not appreciably
alter the symmetry of the overall cord configuration. Extrusion of
the plastic shielding material 22 over the insulation 20 of the
conductor 12 permits the use of high speed extruders and thereby
eliminates the use of relatively slow serving machines, and may
result in considerable production cost reductions.
While the conductive polyvinyl chloride is preferred for the
shield, it should be understood that other semi-conductive plastic
materials may be used, such as a semi-conducting polyethylene, for
example. However, it is believed that polyethylene is principally
somewhat inferior because it does not have favorable aging
characteristics, and its electrical resistivity often increases
with time. It is also preferred that the conductive shielding
material 22 be dissimilar to the insulating material 20 so that the
two materials will not bond together at the interface which would
inhibit easy stripping of the shield by a user at a termination
point. Thus, a semi-conductive polyvinyl chloride for the shielding
material has been found to be readily strippable relative to a
polypropylene insulating material 20.
In accordance with yet another aspect of the present invention, the
group of uninsulated drain or grounding strands 24 are shown to be
generally parallel to and immediately adjacent the conductive
shielding material 22 so as to be in electrical contact with it.
The individual drain or ground strands 24 are preferably made from
a ribbon of conventional size, such as 1 mil by 9 mil cross
sectional area wrapped around a polyester thread or the like to
form a single strand. The construction of FIG. 1 illustrates a
total of 10 of such strands 24 in the grouping and, while this
number is not particularly critical, it is preferred that at least
8 to 10 and preferably at least 10 of such strands be used. If more
than 10 strands 24 are used, the positive effect that may be gained
would probably not justify the additional cost. However, if fewer
than 10 of such conductors are used, it has been found that the
flex life of the cords as a whole may be appreciably reduced. This
is partially due to the manner in which the flex life is measured.
It is preferred that the flex life is determined by flexing the
cable until there is an increase in the measured resistance of 25
percent within the grouping of the drain strands, when they and the
strands of the primary conductors are connected in series with one
another, due to breakage of the individual strands. It is seen that
if a total of eight wires are in the group, only two need to break
before the critical limit of 25 percent increased in resistivity is
achieved. On the other hand, if 10 of such strands are used, it is
seen that three wires need to break before the 25 percent increase
is reached and, in actual use, it has been found that the use of 10
of such grounding strands is preferred from the standpoint of
overall cord flex life. The group of grounding strands 24 are shown
to be positioned in the valley between the conductors 12 and 14,
with the inside of the outer extruded jacket 28 effectively
maintaining the grounding strands 24 into intimate contact with the
shielding material 22. In this connection, the outer insulating
jacket 28 is preferably of a flexible plastic material such as
polyvinyl chloride, or other flexible insulating material. The
outer jacket is extruded under sufficient radially inward pressure
so as to urge the group of grounding strands 24 into intimate
contact with the shielding material 22 so as to provide electrical
continuity between them. Under such conditions, the probability of
an open shield along the length of the cord is greatly reduced due
to the combination of the conductive drain or grounding strands and
the extruded semi-conductive polyvinyl chloride shield material.
Additionally, the operational characteristics have been
significantly improved, in terms of the factors such as hum and
self-induced noise.
Turning now to an important aspect of the present invention, and
referring to the following Tables I and II which vividly illustrate
the comparison of the operational characteristics of the cord
embodying the present invention as compared to a cord having
similarly sized primary conductors, but which includes a served
conductive shield comprising uninsulated tinsel strands. In other
words, the cord having a served tinsel shielded conductor was
tested along with a cord substantially identical to the structure
shown in FIG. 1 and the test results are evident from the following
Tables.
It should be noted that the cord designated as "A" in the Tables is
a three-conductor cord having metallic strands served around the
insulation for the shielded conductor to define the shield, and
cord "B" is a three-conductor cord embodying the present invention
and having the extruded non-metallic electrically conductive
polyvinyl chloride shield and a total of ten grounding strands have
been positioned generally parallel to and in intimate contact with
the shielding material. Cord "B" is substantially the construction
illustrated in FIG. 1.
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TABLE I
Test Cord A Cord B
__________________________________________________________________________
Hum Freq. (Hz) 60 26 mv 3.5 mv 400 58 mv 32 mv 1 k 1.1 v 38 mv 5 k
1.5 v 42 mv 7 k 1.6 v 46 mv 9 k 1.7 v 48 mv 11 k 1.7 v 49 mv 13 k
2.2 v 48 mv 15 k 1.6 v 49 mv
__________________________________________________________________________
As is evident from the measurements shown in Table I, cord "B"
embodying the present invention has greatly improved
characteristics and exhibits on the order of 7.5 times better hum
suppression than cord "A" at 60 Hz, almost 2 times better at 400
Hz. and on the order of 30 times better for frequencies measured
between 1 k Hz and 15 k Hz. It is noted that the hum test was made
by placing the two cords within a brass tube approximately 18
inches long and having an inside diameter of about 3/8 to 5/8 inch,
wherein an oscilloscope was connected to a cord and a variable
frequency generator connected to the brass tube with a 700 VAC
signal being applied. The test essentially measures the amount of
signal that penetrates the shielding material and appears on the
shielded conductor itself.
With respect to self-induced noise, it was found that the cord "B"
had an average of 9.5 peak to peak millivolts as compared with a
measurement of 114 millivolts average for cable "A," which shows an
improvement on the order of 12 times better for cord "B" embodying
the present invention. The self-induced noise test was made
following the method described in MIL-C-17 which is also a National
Bureau of Standards test procedure and is not set forth in detail
for that reason.
With respect to the flex life of cord "B" embodying the present
invention, it was tested by bending the cord 60.degree. each side
of vertical around a 1/8 inch radius at the rate of 85 cycles per
minute. The test was terminated when the measured electrical
resistance of the primary conductor strands and group of grounding
strands connected in series, increased 25 percent, due to breakage
of individual strands within the group. It has been found that the
average flex life of cord "B" is at least 200,000 cycles and
measurements approaching 800,000 cycles have been experienced. It
is noted that if less than 10 grounding strands are incorporated
into the cord, that the flex life decreases. In this connection,
when a total of only 8 wires were incorporated into the design, the
flex life was found to drop to an average of about 160,000 to
170,000 cycles on the average.
Although it is shown from the foregoing that the flex life and
self-induced noise characteristics are indeed superior for the cord
embodying the present invention, Table II is set forth to
illustrate other operational characteristics such as capacitance,
D-C resistance of the shielded drain or grounding strands as well
as cross talk between various primary conductors within the two
cords and is included to complete the data for the comparison.
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TABLE II
Test Cord A Cord B
__________________________________________________________________________
Cross Talk Freq. (Hz) dB Isolation dB Isolation 40 K 64 69 70 K 58
60 160 K 48 46 400 K 34 27 1 M 15 10 1.6 M 14 13 4.0 M 13 14 10 M
21 31 16 M 26 43 40 M 28 35 CAPACITANCE (pfd 1 ft. at 1 KH.sub.2)
Shielded conductor to drain wire 50.1 58.3 Unshielded conductor to
drain wire 37.4 49.4 DC RESISTANCE (Ohms per 100 ft.) Shield 233.2
Shield and drain wires 228.2
__________________________________________________________________________
As is evident from Table II, the cross talk, capacitance and d-c
resistance characteristics are comparable for the cords "A" and
"B." It should be noted that the cross talk was measured according
to the provisions of military specification MIL-C-23437 and
MIL-C-23553.
In accordance with another aspect of the present invention and
turning to FIG. 2 which illustrates another cord also embodying the
present invention wherein six (6) primary conductors are
incorporated into the configuration, it should be understood that
the construction is quite similar to that shown in FIG. 1, but
includes four unshielded insulated primary conductors and two
shielded insulation conductors as well as two groups of grounding
strands. It is preferred that the shielded conductors be separated
from one another and, in the illustrated embodiment, they are
diametrically opposed relative to one another. Due to the symmetry
of the configuration shown, it is preferable that a central textile
core filler 30 having a diameter comparable to the diameters of the
primary conductors be incorporated and it has been found that a
cotton filler core is adequate for this purpose. Because of the
inclusion of substantially similar components within the
construction, the designator numbers of FIG. 1 are used in FIG. 2
since the individual primary conductors as well as the group of
grounding strands are substantially identical to those shown in the
configuration of FIG. 1. It should also be understood that while
only three and six primary conductor cords are specifically
illustrated in the drawings, cord configurations having 2, 4, or 5
primary conductors as well as even greater numbers of primary
conductors may be produced in accordance with the present
invention.
Thus, miniature electrical cords embodying the present invention
have been described herein in detail which have many desirable
attributes and have been shown to demonstrate superior operational
characteristics in terms of hum and self-induced noise as well as
in the flex life of the cord. One desirable aspect of the present
invention is the improved processability that is achieved, since
the shield is ready for termination merely by stripping off the
conductive shielding material and terminating the drain wire. This
and the other previously mentioned desirable attributes are
believed to satisfy all of the objects and advantages that have
been set forth herein.
* * * * *