U.S. patent number 3,567,846 [Application Number 04/733,528] was granted by the patent office on 1971-03-02 for metallic sheathed cables with roam cellular polyolefin insulation and method of making.
This patent grant is currently assigned to General Cable Corporation. Invention is credited to William J. Brorein, Fred F. Polizzano.
United States Patent |
3,567,846 |
Brorein , et al. |
March 2, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
METALLIC SHEATHED CABLES WITH ROAM CELLULAR POLYOLEFIN INSULATION
AND METHOD OF MAKING
Abstract
The metallic sheathed electrical cables of this invention have
foamed cellular polyolefin dielectric insulation which is
fusion-bonded to the inside of an annealed sheath to obtain better
electrical and mechanical characteristics. The sheath is applied to
a foam-insulated core and then sunk down by drawing through a die
or reducing rolls to make the tube fit the insulated core snugly.
Controlled heating of the sheathing melts the part of the
insulation, or adhesive material, when used, which is in contact
with the sheath to produce the fusion bond. The heating period is
short and is followed by a quench. This controlled heating and
cooling is also used to anneal the metallic sheath.
Inventors: |
Brorein; William J. (Whippany,
NJ), Polizzano; Fred F. (Allendale, NJ) |
Assignee: |
General Cable Corporation (New
York, NY)
|
Family
ID: |
24948001 |
Appl.
No.: |
04/733,528 |
Filed: |
May 31, 1968 |
Current U.S.
Class: |
174/102R; 29/828;
156/48; 174/110F; 74/502.5; 174/36 |
Current CPC
Class: |
H01B
11/1839 (20130101); Y10T 29/49123 (20150115); Y10T
74/20456 (20150115) |
Current International
Class: |
H01B
11/18 (20060101); H01b 007/18 () |
Field of
Search: |
;174/102--110.8,36
;156/48 ;29/429 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,419,843 |
|
Oct 1964 |
|
FR |
|
6,612,759 |
|
Apr 1967 |
|
NL |
|
Primary Examiner: Myers; Lewis H.
Assistant Examiner: Grimley; A. T.
Claims
We claim:
1. An electrical cable including in combination a core having at
least one conductor, a sunk down tubular metal sheath surrounding
the core, and foamed dielectric insulation filling the space
between the core and the sheath, the insulation being bonded to the
core and being under some radial pressure between the core and the
sheath, said insulation filling irregularities in the inside
surface of the sheath and being fusion-bonded to the sheath.
2. The electrical cable described in claim 1 characterized by the
insulation being a cellular-foamed polyolefin and a hermetic seal
is formed between the cellular-foamed polyolefin and the metallic
sheath.
3. The electrical cable described in claim 1 characterized by the
insulation being from the group consisting of foamed polyethylene
and foamed polypropylene, and the sheath being from the group
consisting of aluminum and copper.
4. The electrical cable described in claim 1 characterized by the
fusion bond of the insulation to the sheath including a thin layer
of insulation denser than the underlying foam and filling in
irregularities in the inside surface of the sheath the fused
insulation being less than 20 percent of the radial thickness of
all of the foam insulation.
5. The electrical cable described in claim 1 characterized by the
foam being in two layers including an inner layer and an overlying
outer layer that has a softer foam than the inner layer to minimize
the effects of sheath compression and to achieve more uniform
bonding and hermetic sealing to the sheathing.
6. The electrical cable described in claim 1 characterized by the
bond of the foam to the sheath including an outer layer of
adhesion-promoting material that bonds the foam insulation to the
inside surface of the sheath at a temperature lower than the fusion
temperature of the foam.
7. An electrical cable including a conductor core, insulation
surrounding the core, including a foamed dielectric, a metal sheath
surrounding the insulation and holding the foamed dielectric under
some compression, and a thin layer of adhesion-promoting material
on the outside of the foamed insulation between the foam and the
metal sheath and bonded to both the foam and the metal sheath.
8. The electrical cable described in claim 7 characterized by the
adhesion-promoting material being from the group consisting of
amorphous polypropylene and polypropylene copolymers, and
copolymers of polyolefin and acrylic acid.
9. The electrical cable described in claim 1 characterized by the
metal sheath being annealed and being from the group consisting of
aluminum and copper.
Description
RELATED PATENTS, APPLICATIONS AND PUBLICATIONS
A metal sheathed electrical cable and method of making it are
disclosed in U.S. Pat. No. 3,356,790 issued Dec. 7, 1967 to
Polizzano and Robinson and in the U.S. Pat. application of Oscar G.
Garner, Ser. No. 517,706, filed Dec. 30, 1965, now U.S. Pat. No.
3,430,330 for Aluminum-Sheathed Coaxial Cable. Other
aluminum-sheathed cables and methods of manufacture are described
by Hollingsworth and Raine in the Institution of Electrical
Engineers Proceedings, Dec. 1954.
BACKGROUND AND SUMMARY OF THE INVENTION
Metal-sheathed electrical cable with foam insulation may have one
or more conductors with polyethylene or polypropylene insulation;
and the sheath is typically aluminum or copper with a relatively
thin wall. The conductors may be individually insulated and then
covered with an extruded foam polyolefin belting as is typical of
video-pair or cables of more than one conductor. One or more layers
of insulation may be used in the sheath. The single or plural
conductors, with or without individual insulation, that are
surrounded by the foam insulation described herein, will be
referred to as the "core" in this specification.
Any of three basic methods may be used for making this cable. The
first and preferred method is to longitudinally fold a strip into
an oversized cylindrical tube over the insulated core, on a
continuous basis, and to weld this tube together along the butted
edges of the strip; then to sink or draw down this tube to provide
a snug fit over the insulated core. When superior adhesion is
desired, or lower bonding temperature preferred, the strip or the
core may be precoated with an adhesion-promoting material.
The second method is to pull the insulated core into an oversized
tube, and then draw the metal tube through a die or sinking rolls
in order to sink down this tube to provide a snug fit over the
insulated core. The insulated core may be precoated with an
adhesion-promoting material if desired, for this method of
operation.
The third method is to extrude an oversize tube, usually aluminum,
over the insulated core, and to sink this tube down to provide a
snug fit over the core. The insulated core may be precoated with an
adhesion-promoting material if desired, for this method.
This invention provides for heating the metallic sheath quickly
above the melting or softening point of the foamed polyolefin, or
to activate the adhesion-promoting material when used, and then
rapidly cooling to control the depth of melt and solidify the
materials. This is done after the tube has been sunk down by
drawing through the die or reducing rolls and fitted snugly over
the insulated core. The terms "reducing rolls," "sinking roller,"
or "forming rolls" may be used interchangeably.
In addition, this invention provides for extruding or fabricating
the foam-insulated core to make full benefits of this heat-treating
process, although normally extruded cores can be used
successfully.
By melting the outer layer of foam polyolefin which is compressed
against the metallic sheath, the radial compression which extends
inward to the center conductor is reduced. The thin shell on the
outer surface of the insulation, which is formed during this
melting operation, has less effect on the effective dielectric
constant than the radial compression, which causes an increase in
density and dielectric constant, so a net reduction in effective
dielectric constant is obtained at points of compression near the
critical area closest to the central conductors where the
dielectric constant has the greatest effect. In addition, when the
foam insulation is heat-bonded to the metallic sheath, lower
initial compression may be used when the sheath is applied over the
foam core, thus minimizing the dielectric compression problem.
The outer layer of the foam-insulated core, when melted by the
application of heat to the metallic sheath, flows on to the inner
side of the sheath and fills any irregularities in the sheath, and
when cooled it provides a bond of high strength to the sheath as
well as providing a hermetic seal. With typical, commercially
available foamed cellular polyethylene materials, this bond is of
such high strength that the foamed material itself must be torn
apart when it is pulled away from the metallic sheath.
These heat-treated cables, by virtue of the intimate bond between
sheath and insulated core, are hermetically sealed against
longitudinal leakage, whereas untreated cables will form water and
air channels at very low pressures. Heat-treated cables have been
tested up to 30 p.s.i. (gauge) air pressure with no leakage,
whereas untreated cables leak at less than 1 p.s.i. (gauge).
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagrammatic view of apparatus for making the
electrical cable of this invention;
FIG. 2 is a fragmentary diagrammatic view showing a modification of
part of the apparatus shown in FIG. 1;
FIG. 3 is a diagram illustrating the heating and cooling of the
cable when using the apparatus shown in FIG. 1;
FIG. 4 is a diagram showing the heating and cooling of the cable
when using the apparatus shown in FIG. 2;
FIG. 5 is a greatly enlarged sectional view through the cable on
the section line 5-5 of FIG. 1;
FIG. 6 is a view corresponding to FIG. 5 but taken on the section
line 6-6 of FIG. 1;
FIG. 7 is a view similar to FIG. 6 but showing a modified form of
cable;
FIG. 8 is a view similar to FIG. 6 but showing another modification
in which more than one layer of foam insulation is used;
FIG. 9 is a sectional view illustrating the way in which void areas
may exist within the sheath after sinking of the sheath; and
FIG. 10 is a view similar to FIG. 9 after the voids have been
filled in accordance with the method of this invention.
DESCRIPTION OF PREFERRED EMBODIMENT
In making the cable of this invention, an insulated core 10 is
first placed in a metal sheath 12. This can be done by a number of
methods, as explained in the description of the background of this
invention, and the methods and conventional. Whether the sheath is
formed around the insulated core by folding a strip or tape
longitudinally and welding it, or whether the insulated core is
drawn into a seamless tube, the insulated core is always of smaller
diameter than the inside diameter of the sheath 12.
The sheath 12 is lubricated by lubricant 14 discharged over the
outside of the sheath 12 from a supply nozzle 16. The sheath is
pulled through a sinking die 20 by a caterpillar capstan 22. In
place of the sinking die 20, reducing rolls can be used, if
desired. Beyond the sinking die 20 the cable sheath passes through
a cleaning chamber 26 in which cleaning fluid 28 is discharged
against the surface of the sheath from a nozzle 30.
FIG. 5 shows the cable before passage through the sinking die 20.
The cable illustrated consists of a center conductor or core 32
surrounded by foamed plastic insulation 34, and the core 32
insulated by the foamed plastic 34 is loose in the sheath 12. As
previously explained, the core 32 can have a plurality of
conductors and these conductors can be covered with their own
individual insulation.
After passage through the sinking die 20 the diameter of the sheath
12 is reduced so that it fits snugly around the foamed plastic
insulation 34. It is desirable to have the die 20 of a size to put
the foamed plastic insulation under some compression, the purpose
of which will be explained in connection with other figures.
Beyond the caterpullar capstan 22 the sheath 12 passes through an
induction heater 40 which raises the temperature of the sheath high
enough to bond the foamed plastic to the inside surface of the
sheath. This can be done by melting the surface of the foamed
plastic which is in contact with the sheath. The heat should not be
excessive because melting of the foam to an excessive depth will
reduce the volume of the foam so that it no longer fills the inside
of the sheath. The permissible amount of melting depends upon how
much the foam is compressed by the sheath. The reason that some
compression is desirable is that it causes the foam, when heated to
a softening temperature and flowable condition, to flow as
necessary to touch all portions of the inside surface of the sheath
12.
If the sheath is not completely round, then the softened foam will
accommodate itself to any lack of circularity. Where the inside
surface of the sheath is not completely smooth, the softened foam,
when under some compression, flows into the irregularities so as to
have contact with the entire inside surface of the sheath. This
results in a better bond and is also useful in obtaining hermetic
sealing between the foamed plastic insulation and the inside
surface of the sheath 12.
Another type of irregularity results from variations in the inside
diameter of the sheath. These may be periodic and caused by minor
eccentricity of the rolls by which the sheath is made. Such
irregularities cause variations in the compression of the
insulation in the sheath at axially spaced locations and this can
result in the setting up of standing waves when the cable is used
to conduct electricity. This invention eliminates this cable
problem because the foamed plastic, when softened, adjusts to any
irregularities in inside diameter of the sheath and produces a
substantially uniform pressure on the insulation with resulting
improvements in the electrical characteristics of the
insulation.
Although the maximum permissible melting of the foamed insulation
depends upon the compression, it is preferable to have the depth of
melting less than 10 percent of the radial thickness of the foam,
and in any event, less than 20 percent.
The plastic used for the foamed insulation of this invention is
preferably a polyolefin such as polyethylene having a percentage of
air of about 45 percent to 55 percent. These values are given by
way of illustration. Polypropylene can also be used.
Commercially available foamed polyethylene material, such as Union
Carbide's DFA 4860, DFD 4960 and others, will bond, with no extra
adhesive materials, to clean copper, aluminum or steel, if the
temperature of the sheath is raised quickly to about 300.degree. F.
to 850.degree. F. for up to 10 seconds and is then quickly quenched
or cooled to control the depth of melt. These are not limiting
conditions but are typical of usual processing speeds. For example,
small cables with aluminum sheaths can be successfully bonded when
heated to 600.degree. F. for only 2 or 3 seconds while larger
cables require longer periods of time to insure the desired depth
of melt which is usually held to about 0.001 inch up to 0.020 inch,
depending on the size of cable, but these limits do not cover all
sizes and types to which the invention can be applied.
The foamed insulation can be bonded to the inside of the sheath at
lower temperature if an adhesion-promoting material is used. When
the sheath is formed around the insulated core, such
adhesion-promoting material can be applied to the surface of the
sheath which will constitute the inside of the sheath after
forming, or it can be applied to the outside surface of the foamed
insulation. When the insulated core is pulled into an already
formed seamless sheath, it is impractical to coat the inside of the
sheath and the adhesion-promoting material is applied to the
outside of the insulated core before pulling the core into the
seamless tubing.
The advantage of using adhesion-promoting material is that it melts
at a lower temperature than that of the foamed insulation. Examples
of suitable adhesion-promoting materials are the amorphous
polypropylene family as made by Avisun Corporation, such as Oletac
TD-133 and these may be used for special designs where it is not
desirable to heat the sheath over about 300.degree. F. to
400.degree. F. Polyolefin and acrylic acid copolymers may also be
used to promote adhesion. This type is known also as polyolefin
copolymers containing carboxyl groups and is useful in maintaining
a bond under severe environmental conditions.
Close beyond the heater 40 the sheath 12 passes through a quenching
chamber 44 in which water or other cooling fluid 46 is discharged
against the sheath 12 from one or more nozzles 48. This quenching
provides a control on the depth of melting. The period of time
between the heating and quenching depends upon the axial spacing of
the quenching chamber 44 from the heater 40 and upon the speed of
travel of the cable. The depth of melt can be controlled by
changing the amount of heating or the speed of travel of the cable
or the spacing of the quenching chamber from the heater. Means for
changing the speed of travel of the cable are shown
diagrammatically in FIG. 1 as a motor 50 which drives the capstan
22, the motor being supplied with power from a power line 52
through a speed controller 54.
Beyond the quenching chamber 44, the cable sheath is advanced by
another capstan 22' having driving means similar to the capstan 22
and indicated by the same reference characters with a prime
appended.
The reason for the use of two capstans 22 and 22' is to avoid
excessive pull on the cable sheath while it is heated by the heater
40. Considerable pull is necessary to advance the cable sheath
through the sinking die 20, or reducing rollers if rollers are
used, and the tension imparted to the sheath by this pull is more
than the tube can withstand without stretching when highly
heated.
Although is is more economical to assemble the insulated core and
sheath in a continuous operation with the sinking of the sheath and
the heating and quenching operations of this invention, it is not
essential that these operations be combined. Foam-insulated cable
with the sheath fitting snugly around the insulated core can be
supplied from reels on which it has been stored and can be treated
by the heating and quenching of this invention; and in such cases
it is not necessary to use two caterpullar capstans, since the
cable is subject to very little tension when merely unwound from a
reel.
In addition to the bonding of the foamed insulation to the sheath
and the equalizing of the pressures in the insulation, the heating
of the sheath by the heater 40 serves another important purpose. In
the use of aluminum and copper sheaths, the metal is work-hardened
by the sinking operation which reduces the diameter of the sheath
to fit snugly around the insulated core. This hardening makes the
cable stiff. The heating of the sheath in accordance with this
invention anneals the sheath and substantially increases the
flexibility of the cable.
Although the single heating and quenching step illustrated in FIG.
1 can be used to effect both the fusion bond and a degree of
annealing of the sheath, better results are obtained with a
two-stage heating and quenching, such as illustrated in FIG. 2.
The apparatus shown in FIG. 2 includes the heater 40 and the
quenching chamber 44; and also includes a second heater 60 with a
quenching chamber 64 located beyond the quenching chamber 44 in the
direction in which the cable sheath 12 travels. In the quenching
chamber 64 the sheath is quenched by water 66 or other cooling
fluid discharged against it from a nozzle 68 in the same manner as
already described for the quenching chamber 44. Elements 40 and 44
are closer than in FIG. 1.
FIGS. 3 and 4 illustrate the difference in the operation of the
invention when using the single heating and the two-state heating
of FIGS. 1 and 2, respectively. FIG. 3 shows the sheath heated
rapidly to a temperature of approximately 600.degree. F. to
750.degree. F. in a period of time T-1. As the sheath passes beyond
the heater, it cools during a dwell time T-2 as it passes from the
heater to the quenching chamber. The sheath is then cooled quickly
during a period of time T-3 to ambient temperature. This heating is
not ideal for either annealing the sheath or fusing the foamed
insulation, but is a practical and effective compromise if both the
annealing and fusing are to be performed in the same operation.
FIG. 4 shows the two-stage heating and quenching of FIG. 2. The
sheath is heated rapidly to a higher temperature than in FIG. 3,
for example, approximately 800.degree. F. and is immediately cooled
so as to prevent excessive melting of the foamed insulation. As the
cable travels to the next heating step, any plastic which softened
or melted during the annealing heating has an opportunity to cool.
The sheath is then reheated to a temperature sufficient to cause
the foamed insulation to bond to the sheath and the heating period
T-P is long enough to produce the desired depth of softening
necessary for equalizing pressures and producing flow into any
irregularities. The heating during this period T-P is kept at a low
enough temperature so that the foam obtains the desired temperature
gradients. A temperature of 400.degree. F. is shown in FIG. 4 as
illustrative. The temperature used for this second heating period
may be higher or lower, depending upon whether adhesion-promoting
material is used, as previously described and depending upon the
softening point of the particular foam used.
FIG. 6 shows the final cable with the outer sheath 12 of annealed
metal fused to a bonded outer layer of the foamed insulation 34,
this outer layer being designated by the reference character
34'.
When an adhesion-promoting material is used on the inside of the
sheath 12 or on the outside of the insulation 34, it forms a layer
35 as shown in FIG. 7. FIG. 7 shows a cable which is similar to
FIG. 6 except that the core contains two conductors 70 each of
which is covered with a layer of insulations 72 which may be of any
desired type. These conductors 70, with their insulation 72, are a
twisted pair and for purposes of this invention are considered a
core surrounded by foamed insulation 34 which serves the purpose of
the conventional belting layer.
FIG. 8 shows a modified form of the cable of this invention. In
this modification the core consists of a conductor 76 and the
foamed insulation between this conductor core 76 and the sheath 12
is applied in two layers instead of the single layer shown in FIG.
6. These two layers include an inner layer 78 of foamed insulation
and an outer layer 79 which is also foamed insulation. In practice,
these two layers 78 and 79 may be extruded successively or
simultaneously and the outer layer 79 is a softer or less dense
foam than the inner layer 78. The softer layer 79 has the advantage
of accommodating itself more easily to irregularities in the sheath
12.
FIGS. 9 and 10 show an example of one type of irregularity with
which this invention is particularly useful. In FIG. 9 a sheath 90
has a seam 92 which is welded with a flash or bead 94 which holds
the insulation 34 spaced from the inside surface of the sheath 90
on both sides of the flash or bead 94 so as to leave void areas 96
on each side of the flash or bead 94. This would leave a
substantial area of the insulation unbonded to the sheath if the
insulation did not flow to fill up the voids 96.
FIG. 10 shows the way in which the insulation 34 flows in and fills
up the voids on both sides of the flash or bead 94 when the
insulation 34, which is under some compression, is heated to its
softening point.
The preferred embodiments of the invention have been illustrated
and described, but changes and modifications can be made and some
features can be used in different combinations without departing
from the invention as defined in the claims.
* * * * *