Note: Descriptions are shown in the official language in which they were submitted.
This invention reLates to electric cables, and more
particularly to electric cables having undulated tension relief
elements embedded in the cable jacket.
U. S. Patents Nos. 3, 879, 518 and 3, 983, 313 disclose electric
cables having undulated tension relief elements embedded in the
cable jacket. In accordance with such patents, the cable jacket
is formed of polyethylene and the undulation of the tension relief
elements results from the shrinkage of polyethylene during cooling
thereof.
In some cases, it is highly desirable to provide cables with
undulated tension relief elements in which the cable jacket is
îormed from a material other than polyethylene. For example,
polyeth~lene is flammable and in cases where non-flammability is
desired the cable jacket should be formed from a non-flammable
mate~ial, such as, polyvinyl chloride; however, polyvinyl chloride
does not have the shrinkage properties of polyethylene. As a
result, undulation of the tension relief element can not be effected
by cooling of the cable jacket~
In accordance with the present invention there is provided a
process for providing a jacketed cable core having undulated
tension relief elements therein by introducing a cable core, a
jQcketing material and tension relief elements into an extruder
head to produce a tubular jacket for the core having the tension
relief elements embedded in the jacketing material, with the radial
undulations of the tension reli~f elements being provided by
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introducing the tension relie~ elements and the jacketing material
at a velocity greater than the velocity of the cable core and
thereafter reducing the velocity of the jacketing material having
the tension relief elements therein to produce the undulations in
the tension relief elements.
In this manner, the cable jacket can be produced from
materials other than polyethylene in that the shrinkage properties
of the polyethylene are no longer required for providing undulations
in the tension relief elements.
~ qore particular ly, the relative velocities between the cable
core and the jacketing material are controlled in a manner to
generate an axial compressive force in the cable jacket and
tension relief elements to pro-vide the undulations. The greater
the difference in the relative speeds, the greater the undulations
and by proper coordination the desired undulation can be achieved~
Accordingly, the abrupt axial compression which occurs at the
extruder head outlet or beyond the discharge therefrom produces
the undulation, whereby undulation of the tension relief elements
can be obtained independently of the properties; i.e,, shrinkage
property, of the jacketing material.
Thus, in accordance with the present invention, there is a
retarding effect on the jacketing material including the tension
relief strands and such retarding effect is provided by the
velocity difference between the jacketing material and cable core.
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In the case of a highly compressible and viscous jacketing
material such compressive force is exerted immediately beyond
the mouthpiece of the extruder head and there is a swelling of
the jacketing material; i. e., the outer diameter of the jacket is
greater than the inner diameter of the mouthpiece.
It is also possible to obtain such a retarding effect with a
material of low viscosity and compressibility within the mouthpiece
of the extruder head by providing a mouthpiece having an increas-
ing inner diameter in the direction of flow, whereby the velocity
of the jacketing material decreases as it passes through each
larger area. In such a case, the tension relief elements would
be fed at a rate corresponding to the velocity of the jacketing
material in the smaller diameter area of the mouthpiece in order
to provide the desired undulation.
The invention will b~ fur~er described with respect to the
accompanying drawings, wherein:
Figure 1 is a schematic axial longitudinal sectional view
through the extruder head of a cable extruding machine with a
cable being made therein; and
Fi gure 2 is a schematic plan v ew of a cable segment with
partly exposed tension relief elements.
It is to be understood that the general principles for forming
the Jacketed cable in accordance with the present invention are
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in accordance with those set ~orth in the aforementioned
United States Patents Nos. 3,879,518 and 3,983,313, with the
present invention differing from such patents by the fact
that the jacket may be produced from materials other than
polyethylene and that the undulations are formed by control-
ling the parameters within the extruder head, as described,
instead of depending on the shrinkage characteristics of
the jacketing material.
Referring to the drawings, a cable extruder head
10 is provided with an opening 11 for coupling the screw of
the extruder (not shown). A premade cable core 13 made of
insulated electric conductors moves in the direction of arrow
12 into head 10 wherein the outer jacket 14 is applied there-
on, to provide finished cable 15, which is cooled off in the
commonly known manner.
As particularly shown in Figure 2, the finished
cable 15 has a cable core comprised of conductors 16,
insulation 17; i.e., five sets of two twisted conductor
pairs 19, and a core 21 made of plastic material. Around
the conductor pairs 19 is a commonly used wrapping 22 made
of plastic bands.
The outer jackek 14 is extruded over wrappings 22
which moves with the cable core 13 into the extruder head
wherein an alternating amount of tension relief elements
23 are provided. These elements may be constructed from
groups of yarns or
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twines having different thread dimensions whereby the number
of groups on the one hand and the number of yarns or twines
for each group on the other hand depend on the number of
conductor pairs l9, the required minimum breaking stress of
the cable and ~he thread size. Such numbers also depend on the
distance of the groups from each other and the distance of the
groups to the inner jacket circumference. These tolerances are
disclosed in the aforementioned United States Patents. It is
essential that the tension relief elements 23 do not run through
jacket l~ absolutely parallel to the cable axis (core 21); i.e., the
tension relief elements should be undulated, as shown in Figure 2.
This provides an intimate positive locking between the tension
relief elements and the jacket material.
Cable core 13 moves through a nozzle 25 in extruder head
lO and discharges into a hollow chamber 30 into which the front
ends of channels 26 discharge, with channels 26 being concentri-
cally arranged with respect to the fed cable core 13 and of which
only two channels are shown in Figure l. The channels 26 are
provided with tension relief elements 28 from a supply station,
preferably in the form of suspended rollers 27 which carry the
yarns or twines 28.
The front end of extruder head lO has an outer matrix or
mouthpiece 29 through which the finished cable 15 is withdrawn.
The tension relief elements 28 are imbedded in the jacketing
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material (thermoplastic or elastomeric) introduced through
screw coupling 11, and the funnel-shaped hollow chamber 30
receives the jacketing material with the embedded tension relief
elements 28.
The speed or velocity of the jacketing material is dependent
upon the e~erted pressure and the inner diameter of the mouth-
piece 29 and the speed or velocity of the cable core 13 is equal
to the drawing-off speed of the finished cable 15. The relative
speed or velocit~r between the cable core and jacketing material
is controlled in a manner such that the jacketing material moves
through the head 10 at a velocity greater than the cable core.
The tension relief elements 23 also ~nove at a velocity
greater than the velocity of the cable core and such velocity
generally corresponds to the flow speed of the jacketing material.
l~s a resultJ the length of the tension relief elements 23 is longer
than the leng~h of cable core 13 or finished cable 15, with the
additional length being taken up in the undulation of the tension
relief elements 2 3 .
As a result, the dimensions of the undulations can be
controlled by controlling the relative velocities of the jacketing
material and tension relief elements with respect to the cable
coreO Such velocities can be controlled by controlling and/or
adjusting the velocity of the cable core (draw-off speed of
finished cable), mouthpiece diameter, pressure on jacketing
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material, etc .
A convenient manner of determining that there is a differencein relative velocities is by measuring the outer diameter of the
cable after its discharge from the mouthpiece with respect to
the mouthpiece inner diameter. In accordance with the invention,
the outer diameter of the cable should be greater than the inner
diameter of the mouthpiece wil;h the difference in diameter increas-
ing as the relative speed of the jacketing material and tension
relief elements increases with respect to the cable core. It has
been found that the degree of undulation increases as the velocity
difference increases (and the diameter difference between the
finished cable and mouthpiece inner diameter increases3 and such
difference should be coordinated to provide the required undulation
without the tension relief elements overtaking themselves and
forming S-like loops. At the e~treme, this could result in
retarding the feed movement of the tension relief elements which
would result in too great a tension which would reduce cable
flexibility. As should be apparent, proper undulation improves
flexibility as a result of the bellows effect which permits bending.
``I As should be apparent from the above description, aæ a
result of the velocity difference between the jacketing material
and cable core, the slower mo-vin~ cable cor e has a retarding
effect on the faster moving tension relief elements at the time of
forming the jacket on the core, whereby the tension relief
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elements are undulated,
The invention will be further described with respect to the
following example; however, the scope of the invention is not to
be limited thereby:
EXAMP:LE
. _
In the following tests, the dimensions of the extruder headJ
pressure and charging rate OI the jacketing material is maintained
constant for the several tests with respect to each jacketing material.
The drawing-off speed of the cable is changed to thereby change
the velocity difference between the jacketing mate:rial and cable
core, As the velocity of the jacketing material increases with
respect to the cable core, the diameter of the finished cable
increases with respect to the mouthpiece inner diameter and the
degree of undulation increases.
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The present invention is particularly advantageous in that it
is possible to provide a jacketed cable having undulated tension
relief element s without limiting the choice of jacketing material
to only those that have certain shrinkage characteristics. Thus,
for example, if flammability is a concern; e. g., for cables
used in mines, a jacket material of low flammability such as
polyvinyl chloride can be used. Any one of a wide variety of
cables can be produced in accordance with the invention and the
desirable properties imparted by undulated tension relief elements,
and in particular undulated glass fibers, are not limited to self
supporting air cables. For e~ampls, such properties would
also have applicability in cables which have to be drawn in
lengths Into pipes or shafts or which have to be placed into the
ground .
Numerous modifications and variations of the present invention
are possible in light of the above teachings and, therefore9 within
the scope of the appended claims the invention may be practiced
otherwise than as particularly described.
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