Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates generally to fiber
optic cables, and more particularly to a novel fiber optic
cable and method of making the same wherein one or more fiber
optic elements are disposed within a nonmetallic tubular
shield over which a nonmetallic braided strength member is
coaxially formed in tight fitting relation so that an axial load
applied to the cable is taken by the braided strength member
so as to protect the fiber optic element from undesirable elon-
gation and damage.
The use of low-loss fiber optic cables has found
wide acceptance in many industrial and commercial applications,
including the fields of computer technology and communications.
Low-loss fiber optic cables offer many desirable advantages
over metallic conductors including use for long distance
transmission without repeaters r immunity from cross talk, greater
bandwidth capabilities, lighter weight, and potential for lower
cost signal communication systems.
As a practical matter, it has been found that signi-
ficant problems are encountered which inhibit wide utilization
of the desirable features of fiber optic cables over metallic
conductors. One basic problem is the inherent fragility of
glass fibers which makes more difficult the production of fiber
optic cables which are flexible and can withstand bending,
twisting, impact, vibration, etc. The basic approach to solving
this problem has been to provide means for strengthening and
buffering the individual fiber optic elements so that subsequent
bundling, cabling and field usage will not damage or adversely
affect the optical properties of the fiber optic elements.
As a result of the fragility of the glass fibers
employed in fiber optic cables, the glass fibers are capable of
withstanding only relatively low elongation per unit length
when subjected to tensile loading. Some fiber optic elements
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will fail by breaking when subjected to approximately 1%
elongation while other types of fiber optic elements can with-
stand approximately 6% elongation without failure. Because
the fiber optic elements are capable of withstanding only
relatively low elongation, it is known to provide a nonmetallic
axial strength member within a fiber optic cable to carry
the fiber optic elements so that the axial strength member
takes the major portion of any tensile load applied to the
cable.
In addition to employing axial strength members within
fiber optic cables to prevent the fiber optic elements from
being subjected to potentially damaging tensile loads, it is also
known to employ nonmetallic strands of suitable strength material
externally along the length of a bundle of one or more fiber
optic elements in a fiber optic cable to improve the tensile
strength characteristics of the cable without damage to the fiber
optic elements. In general, however, the nonmetallic strands
are positioned in parallel relation to the longitudinal axis
of the cable along the length of the cable. One problem with
this technique for preventing damaging tensile loading of the
fiber optic elements is that during the manufacturing process,
some lengths of the reinforcing strands may be longer than other
lengths. This results when the fiber optic cable is curved
around one or more pulleys or sheaves during manufacture with
the result that the high strength strands adjacent the shorter
radius of curvature have slightly shorter length than the
strands adjacent the outer or larger radius of curvature. When
the resulting fiber optic cable is subjected to a tensile
load, the shorter reinforcing strands are subjected to greater
tensile loading than the longer strands and may fail, thereby
causing failure of the fiber optic cable.
One of the primary objects of the present invention
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is to provide a novel fiber optic cable and method of making the
same wherein the cahle includes a nonmetallic braided strength
member disposed coaxially along the length of a tube bundle in
the form of one or more fiber optic elements carried within a
tubular shield, the nonmetallic braided strength member being
operative to take any tensile loads applled to the cable so as to
prevent damaging elongation and failure of the fiber optic
elements.
A more particular object of the present invention is to
provide a novel fiber optic cable and method of making the same
wherein one or more fiber optic elements are disposed within a
tubular nonmetallic shield and a nonmetallic braided strength
member is disposed coaxially along the length of the tubular
shield in tiqht fitting relation thereon, the braided strength
member having greater tensile strength and lower elongation per
unit length than the fiber optic elements so that any axial load
applied to the cable is substantially taken by the braided
strength member rather than the fiber optic elements.
The use of a textile braid layer in connection with
fiber optic cables is not, per se, novel. For example,
applicant's copending Canadian application, Serial No. 284,368,
filed August 9, 1977, now Canadian Patent No. 1,091,071,
discloses a fiber optic cable wherein a textile braid is applied
coaxially along a plurality of fiber optic bundles to provide
improved abrasion resistance for the fiber optic bundles. U.S.
Patent No. 3,691,001, dated September 12, 1972, discloses a fiber
optic bundle having an outer protective sheath comprising a
cylindrical braid member snugly fitted on the cylindrical surface
of an optical fiber bundle and impregnated with a synthetic resin
material to form the protective sheath. In somewhat similar
fashion, U.S. Patent No. 3,855,897, dated December 24, 1974,
discloses a method of protecting a bundle
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of flexible optical Eibers wherein the fibers are inserted
axially into a tubular braid of intertwined filaments which are
thereafter axially stretched to dec:rease the diameter of the
braid until it compresses the inserted bundle whereafter the
stretched tubular braid is impregnated with a synthetic resin
composition and solidified. Neither the aforenoted copending
application nor the aforenoted two U.S. patents disclose the
use of a nonmetallic braided strength member in a fiber optic
cable wherein the nonmetallic braided strength member has
greater tensile strength and lower elongation per unit length
than the fiber optic elements so that the braided strength
member takes any tensile load applied to the cable and prevents
damaging tensile loading of the fiber optic elements as
intended by the present invention.
In accordance with one feature of the present
invention, a fiber optic cable has at least one fiber optic
element disposed coaxially within a nonmetallic tubular
flexible and substantially radially noncompressible shield
layer which extends along the length of the fiber optic element
in loose fitting relation thereon. A nonmetallic braided
strength member is disposed coaxially along the length of the
shield layer in tight fitting relation thereon, the braided
strength member including a plurality of discrete strands of
nonmetallic material braided together along the length of the
tubular shield layer so as to cover substantially the full
outer peripheral surface of the tubular shield layer. The
discrete strands are braided so that the discrete strands are
wound in intertwined opposite helical relation along the length
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of the shield layer in a manner to prevent the cable from
undergoinq a rotational twisting action about its longitudinal
axis when subjected to a tensile loading, the braided strength
member having greater tensile strength and lower elongation per
unit length than the fiber optic element so that the major
portion of any tensile load acting on the cable is taken
substantially by the braided strength member.
In accordance with another feature of the invention, a
method of making a flexible fiber optic cable includes the step
of forming a tubular shield layer along the length of at least
one fiber optic element in loose fitting relation thereon, the
tubular shield layer being flexible and substantially radially
noncompressible. Next, a nonmetallic tubular braided strength
member is formed coaxially along the length of the tubular
shield layer in tight fitting relation thereon, the braided
strength member including a pluality of discrete strands of
nonmetallic material braided together along the length of the
tubular shield layer so as to cover substantially the full
outer peripheral surface of the shield layer. The discrete
strands are braided so that the discrete strands are wound in
intertwined opposite helical relation along the length of the
shield layer in a manner to prevent the cable from undergoing a
rotational twisting action about its longitudinal axis when
subjected to a tensile loading, the braided strength member
having greater tensile strength and lower elongation per unit
length than the fiber optic element so that the major portion
of a tensile load applied to the cable is taken substantially
by the braided strenqth member. Lastly, an outer protective
jacket is formed coaxially along the length of the braided
strength member.
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Further objects and advantages of the present
invention, toqether with the orqanization and manner of
operation thereof, will become apparent from the following
detailed description of the invention when taken in conjunction
with the accompanying drawing wherein like reference numerals
designate like elements throughout the several views, and
wherein:
FIGURE 1 is a fragmentary view of a length of fiber
optic cable constructed in accordance with the present
invention, various layers of the cable being exposed for
purposes of illustration;
FIGURE 2 is a transverse sectional view tken
substantially along line 2-2 of FIGURE l;
FIGURE 3 is a fragmentary longitudinal view similar to
FIGURE 1 but illustrating an alternative embodiment of a fiber
optic cable constructed in accordance with the present
invention;
FIGURE 4 is a transverse sectional view taken
substantially along line 4-4 of FIGURE 3;
FIGURE 5 is a fragmentary longitudinal view similar to
FIGURE 1 but illustrating still another embodiment of a fiber
optic cable in accordance with the present invention; and
FIGURE 6 is a transverse sectional view taken
substantially along line 6-6 of FIGURE 5, looking in the
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direction of the arrows.
Referring now to the drawing, and in particular to
FIGURES 1 and 2, a fiber optic cable constructed in accordance
with one embodiment of the present invention is indicated
generally at 10. The fiber optic cable 10 is made from
nonmetallic components and includes an elongate fiber optic
element 12 which is of known design and conventionally comprises
a fiber optic glass core having a glass or plastic cladding layer
formed coaxially over the core to effect the desired refraction.
For example, fiber optic elements are commercially available
which range in size and elongation from an outer diameter of
approximately .005 inch and an elongation of approximately 1%
for a fiber optic element having a glass core and glass cladding,
to a slightly larger size fiber optic element having approximately
5-6% elongation and having a glass core and a plastic cladding
layer.
A protective nonmetallic tubular shield 14 is formed
by conventional extruding techniques coaxially along the length
of the fiber optic element 12 so as to be relatively loose
fitting thereon. The nonmetallic tubular shield 14 may, for
example, be formed of a suitable polycarbonate material which '
may be extruded in tubular form over the fiber optic element and,
in combination with the fiber optic element, forms a tube bundle.
The tubular shield 14 forms a flexible but relatively radially
noncompressible protective shield layer for the fiber optic
element.
In accordance with an important feature of the
invention, a nonmetallic braided tubular strength member,
indicated generally at 18, is disposed coaxially along the
length of the tubular shield 14 in tight fitting relation thereon.
The braided strength member 18 comprises a plurality of tightly
braided strands of nonmetallic material such as suitable high
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strength yarn which has greater tensile strength and lower
elongation per unit lenyth than the associated fiber optic
element 12. In the illustrated embodiment, the braided strength
member 18 comprises an eight strand braid, six strands of which
are indicated at 18a-f, wherein the strands are braided so as
to establish three picks per inch on a tubular shield 14 having
an outer diameter of approximately .079 inch. As used herein,
the term "pick" refers to the crossing of one braid strand with
another. Thus, three picks per inch is defined as three strand
crossings per inch, as considered along a longitudinal line or
surface element along the braid layer 18. The braided strands
18a-h comprising the eight strand braided strength member 18
are disposed in closely braided relation so as to cover substan-
tially all of the outer circumferential surface of the underlying
tubular shield 14, as well as being braided in tight fitting
relation on the tubular shield 14.
AlternativeIy, the braided strength member 18 may
comprise a sixteen strand braid formed to establish six picks
per inch.
- 20 The strands 18a-h comprising the braided strength
member 18 may each comprise a high strength yarn having the
desired strength characteristics. For example, the individual
strands of the braid may be made from a suitable aramid material,
*
~ an example of which is KE~LAR, a product of Du Pont Company.
Making the braided strength member 18 from individual KEVLAR
strands having 1420 denier, identified by Du Pont as KEVLAR 49,
has been found to be particularly satisfactory for the intended
purpose of the braided high strength member 18.
Depending on the selected materials from which the
braided strength member 18 and tubular shield 14 are made,
it may be desirable to interpose an abrasion resistant layer
between the tubular shield and the braided strength member.
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In the illustrated embodiment, a protective abrasion resistant
layer 20 is formed along and coaxially over the tubular shield
14 in closely fitting relation thereon. The abrasion resistant
B layer 20 may comprise a polyester film material, such as MYLAR~
a product of Du Pont Company, and is formed so that the longi-
tudinal axis of a strip of polyester film extends longitudinally
along the tubular shield 14 with the opposite lateral edges 20a
and 20b of the wrapped film strip preferably being in slight
overlapping relation and extending longitudinally along the
underlying tubular shield.
With the abrasion resistant layer 20 formed coaxially
over the tubular shield 14, the braided strength member 18 is
then formed coaxially over the abrasion resistant layer, as
by a continuous braiding operation, so as to be formed tight
against the abrasion resistant layer and underlying shield
layer. By forming the braided strength member tightly against
the underlying substantially radially noncompressible shield 14,
the braided strength member cannot undergo a reduction in
diameter which would allow undesirable longitudinal lengthening
of the strength member when the fiber optic cable 10 is
subjected to tensile loading.
Preferably, an outer protective jacket 24 is formed
by conventional extrusion techniques coaxially along the outer
peripheral surface of the braided strength member 18. The
jacket 24 provides protection for the braided strength member
18 in preventing abrasion or cutting thereof, and is of
conventional design and material. For example, the exterior
jacket 24 may be formed from a suitable PVC or polyurethane
material.
In making the fiber optic cable 10 as thus described,
with an eight strand braided strength member 18 covering
substantially all of the outer circumferential surface of the
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underlying abrasion resistant layer 20 and protective tubular
shield 14, and with the individual strands of the braided strength
member being made from No. 49 KEVLAR, and with the braided
strength member formed over an outer diameter of approximately
5 .079 inch, it has been found that the elongation and tensile
strength characteristics of the cable may vary in relation
to the number of picks per inch in accordance with the following
table:
Picks Per Percent Elongation at Breaking Strength
Inch 150 lb. tensile load (lbs. force)
2.28 1.1 484
3 1.1 480
3.75 1.2 400
6 2.1 385
10.8 230
From the aforedescribed table, it is seen that an eight
strand braided strength member 18 having approximately 2.28 - 3
picks per inch results in a desirable elongation factor of
approximately 1.1 percent at 150 lb. tensile load and provides
a breaking strength for the corresponding fiber optic cable of
approximately 480 lbs. This is particularly suitable for
preventing excessive and damaging elongation of the associated
fiber optic elements in the cable. Preferably, the eight
strand braided strength member 18 is formed with approximately
2.28-6 picks per inch when formed on a .079 diameter.
Ideally, a relatively low value braid angle, i.e.
few picks per inch, is desirable. Eowever, the braid angle
should not be selected at such a low value that gaps are
created between the braided strands through which the tubular
-~ 30 shield 14 and associated fiber optic element could project when
the fiber optic cable is bent around curves. Should this occur,
the benefit of the braided strength member would be substantially
reduced.
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FIGURF,S 3 and 4 illustrate cm alternative embodiment
of a fiber optic cable, indicated generally at 30, constructed
in accordance with the present inventlon. The fiber optic cable
30 is generally similar to the aforedescribed fiber optic cable
10 except that a second braided strength member 32 is formed
along the outer peripheral surface of the previously formed
jacket 24 in tight fitting relation thereagainst. The braided
strength member 32 is substantially the same as the braided
strength member 18 and is formed from a plurality of braided
strands of nonmetallic material such as yarn made from a suitable
aramid, for example, KEVLAR N~. 49 wherein the individual strands
are of 1420 denier. The braided strength member 32 may also be
formed of eight or sixteen individual strands and formed in
similar fashion to the high strength braid member 18. An
outer jacket 34 is preferably formed coaxially along the length
of the outer braided strength member 32 and may be formed of a
suitable material, such as PVC or polyurethane.
FIGURES 5 and 6 illustrate still another embodiment
of a fiber optie cable, indicated generally at 40, constructed
in accordance with the present invention. The fiber optic cable
40 includes a plurality of substantially identical fiber optic
bundles 42a, 42b and 42c each of which comprises a conventional
fiber optic element 44 over which a protective tubular shield
or sheath 46 is formed in coaxial loosely fitting relation.
The coaxial sheaths 46 are preferably formed from a suitable
polycarbonate material which is flexible yet sufficiently rigid
to provide crush resistance. Such sheaths can be extruded with
little shrink back and facilitate stripping to simplify term-
ination of the fiber optic cables.
In the embodiment of FIGURES 5 and 6, the fiber optic
bundles 42a-c are carried by an axial carrier member 48 which
may comprise a relatively high strength yarn having suitable
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strength characteristics. For this purpose, the carrier member
48 may also be made of a suitable strength yarn made from an
aramid such as KEVLAR. The fiber optic bundles 42a-c are
retained in assembled relation along the length of the carrier
member 48 by sui~able means such as a spiral tape wrap layer 50
which extends the full length of the coaxial cable about the
tube bundles so as to coaxially cover the tube bundles and axial
carrier member and maintain them in assembled relation. The
tape forming the cover layer or retaining layer 50 may comprise
a suitable PVC tape or other suitable material such as a polyester
film material. The tube bundles 42a-c are preferably helically
wound about the axial carrier member 48 as illustrated in FIGURE 5.
A braided strength member 18' having a substantially
identical construction and physical properties as the afore-
described braided strength member 18 is formed coaxially alongthe length of the wrap layer 50, underlying tubular bundles
42a-c and axial strength member 48 and has greater tensile
strength and lower elongation per unit length than the fiber
optic elements within the fiber optic bundles 42a-c so that a
major portion of any tensile load applied to the fiber optic
cable 40 is substantially taken by the braided strength member.
In this manner, the braided strength member will absorb tensile
loads applied to the fiber optic cable and will prevent undesirable
elongation of the fiber optic elements 42a-c to a point where
they could fail or otherwise be harmfully damaged so as to
lose their transmitting properties. The braided strength member
18' is formed so as to fit tightly against the underlying tape
layer or wrap 50 and fiber optic bundles so that tensile loading
of the braided strength member does not cause a reduction in
diameter thereof and a corresponding axial elongation which
could result in excessive and detrimental elongation of the
fiber optic elements 42a-c.
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In similar fashion to the aforedescribed fiber optic
cable 10, the fiber optic cable 40 of FIGURES 5 and 6 preferably
has an outer protective jacket 52 formed thereon in coaxial
relation along the length of the braided strength member 18'.
Thus, in accordance with the present invention,
various embodiments of a novel fiber optic cable construction
are provided wherein one or more fiber optic bundles in the
form of one or more fiber optic elements are carried within
one or more protective tubular shields over which a tubular
braided strength member is tightly formed so as to maintain
the fiber optic bundles in substantially fixed relative orien-
tation. In all instances, the braided strength member has
greater tensile strength and lower elongation per unit length
than the associated fiber optic elements so that the braided
strength member is adapted to take the major portion of any
tensile load applied to the corresponding fiber optic cable and
prevent the associated fiber optic elements from being subjected
to tensile loading which might effect undesirable elongation
thereof to a point of failure.
While preferred embodiments of the present invention
have been illustrated and described, it will be understood to
those skilled in the art that changes and modifications may
be made therein without departing from the invention in its
broader aspects~ Various features of the invention are defined
in the following claims.
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