Note: Descriptions are shown in the official language in which they were submitted.
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GROOVED SUPPORT MEMBERS FOR OPTIC FIBERS
This invention relates to apparatus for providing a
grooved support member for optical fibers.
Optical cables comprise a plurality of optical
fibers and some protective support for the fibers. To
provide support, some cable structures have a central support
member or core provided with a plurality of circumferentially
spaced ribs which extend along the support member and project
outwardly from it. Grooves defined between the ribs house
one or more fibers. To enable the cable to be flexed during
storage and installation and also in use while preventing
tensions being applied to the fibers, it is common practice
for the grooves to extend longitudinally of the cable while
lying at an angle to the axis of the support member. In one
accepted manner of doing this, the grooves extend in true
helical fashion along the support member. In another
accepted manner of causing the grooves to lie at an angle to
the core axis, the grooves extend along the core in
sinusoidal manner, i.e. in which the grooves lie at angles to
the axis, alternately, in one direction around the support
member and then in the other. With the sinusoidal
arrangement, the grooves normally do not complete a
revolution around the support member in either direction.
The sinusoidal arrangement for the lay of the optical fibers
is one which has been found to be most desirable in that it
is the most convenient path for the optical fibers to follow
for field repair and branch splicing purposes performed upon
the cable. An optical cable structure in which the grooves
may extend either helically or sinusoidally along the support
member is described in U.S. Patent 4,361,381, granted
November 30, 1982 and entitled "Optical Cable" in the name of
R.J. Williams.
In U.S. Patent 4,205,899, entitled "Optical Cables"
and granted June 3, 1980 to F.D. King and T.S. Swieciki,
there is described a commercially used method for producing a
grooved support member for supporting fibers in which the
grooves extend in sinusoidal fashion. As described in U.S.
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Patent 4,205,899, the support member comprises a central
metal strength member and a surrounding plastics sheath or
sleeve which is formed upon the strength member by extrusion.
The extrusion die is formed with a plurality of inwardly
extending fingers or projections which are in a fixed
position and which produce the grooves in the sheath during
the extrusion process. To cause the grooves to follow their
sinuous paths in spite of the fact that the projections are
stationary, a twist unit is provided downstream from the
extruder and this twist unit operates by gripping upon the
plastics material of the sheath in a position where the
driven material is relatively rigid, and then the twist unit
is reciprocally rotated about the axis of the support member.
This motion of the twist unit is imparted to the support
member and causes the member to twist reciprocally along its
length immediately downstream from the position at which it
leaves the extruder orifice so that the grooves are deformed
to extend around the support member alternatively in one
direction and then in the other dependent upon the direction
of movement of the twist unit. A problem with the commercial
method of forming the sinusoidal grooves is that to achieve a
desired degree of movement of the grooves around the axis of
the support member in each direction, then the twist unit
needs to be rotated in each direction for a distance
substantially in excess of the desired degree of movement.
This is because of the lag in the twist between the position
at which the twist is applied at the twisting unit and the
die orifice at which the twist first becomes effective. For
instance if, in the finished support member, each groove is
desired to move angularly around the axis of the support
member by approximately 320, then the distance between the
twist unit and the die orifice dictates that the twist unit
must be rotated for many revolutions to produce this degree
of angular movement. The number of revolutions required by
the twist unit is proportional to the distance between twist
unit and die orifice. However, in practice, there is a
minimum distance between the twist unit and die orifice and
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this is governed by the location of a cooling unit for
solidifying the plastics material for the sheath to be
capable of being gripped by the twist unit without causing
damage. Unfortunately, this minimum distance still requires
a substantial number of revolutions of the twist means to
achieve the angular movement of the grooves around the axis
of the support member. In addition, some of the twist
already applied to the plastics sheath between the die
orifice and the twist unit in one direction is immediately
cancelled by twisting movement of the sheath in the opposite
direction. As can be seen therefore, there are many
variables that make it extremely difficult to predict the
number of rotations of the twist unit required for a desired
angular movement of the grooves. These complications are
aggravated by the fact that the amount of resultant twist
also depends upon the plasticity of the particular plastics
material and also upon the structure and strength of the
particular strength member being used. For instance, it has
been common practice to use a strength member which is a
steel cable formed from steel filaments twisted together.
With this structure, the resistance to torsional twisting is
greater in one direction than in the other and although the
twist unit may be rotated for an equal number of rotations in
each direction, the strength member will tend to return to a
position of stability after the twisting operation takes
place thereby resulting in the groves being angularly
displaced in one direction around the support member to a
greater extent than in the other direction. Such a situation
is not considered to be desirable in the final construction
of cable.
The present invention seeks to provide apparatus
for forming a grooved support filament in which the degree of
angular movement of sinusoidal grooves around the axis of the
support member may be more controlled.
According to the present invention is provided
apparatus for making a grooved support member for carrying
optical fibers comprising an extrusion die defining a die
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aperture havin~ circumferentially spaced and fixed pro-
jections definlng grooves in plastics material to be extruded
therethrough, the extrusion die disposed upon a passline for
passage of a strength member for the grooved support member
through the extrusion die and die aperture to extrude a
plastic sheath onto the strength member, a twisting means
located upstream from the die along the passline for gripping
the strength member as said member is fed through the twist-
ing means and along the passline, the twisting means compris-
ing a plurality of gripping wheels rotatably mounted onopposite sides of the passline with their peripheral surfaces
facing towards the passline, the wheels on each side of the
passline spaced apart along the passline with the wheels
staggered in position along the passline from one side of the
passline to the other and a means for rotating the gripping
wheels alternately in one direction and then in the other
direction around the passline.
With the apparatus according to the invention, a
better degree of control is possible so that the applied
twist upstream from the extrusion die orifice can be more
accurately determined to produce a desirable angular movement
of the sinuous grooves around the support member. With the
apparatus of the invention, there is direct control of the
twist movement of the strength member itself, because the
strength member is twisted directly and twist is not imparted
to it through the plastics sheath as in the prior method of
operation. Due to this, because the rigidified plastic6
sheath is not being relied upon for the purpose of producing
the twist in the grooves, then the position for application
of the twist to the strength member may be relatively closer
to the die orifice than has previously been possible. As a
result, there may be little difference between the amount of
rotational twist applied to the strength member and the
desired angular movement of the grooves which are produced.
For instance, with the strength member being twisted from a
position close to and upstream of the extrusion die, then for
a finished angular movement of the grooves of about 320
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around the axis of the support member in each direction, any
twist unit need only be rotated between 360 and 500 around
the axis of the strength member.
One embodiment of the invention will now be
described, by way of example, with reference to the accom-
panying drawings, in which:-
Figure 1 is an isometric view of part of a groovedsupport member for optical fibers of a cable;
Figure 2 is a schematic side elevational view of
apparatus for making the support member shown in Figure 1 and
for laying optical fibers into grooves of the support member;
Figure 3 is a side elevational view, partly in
cross-section and on a larger scale than Figure 2 of a twist-
ing means incorporated in the apparatus;
Figure 4 is a cross-sectional view of the twisting
means taken along line IV-IV in Figure 3;
Figure 5 is a side elevational view of parts of the
twisting means showing them in use; and
Figure 6 is an enlarged cross-sectional view
through part of the twisting means.
As shown in Figure 1, a support member 10 for
optical fibers comprises a central strength member 12 formed
from fiberglass rod, or alternatively from cabled steel
wires, and an extruded plastics sheath 14 surrounding the
strength member 12. The plastic sheath 14 is formed with a
plurality of circumferentially spaced ribs 16 which extend
longitudinally of the support member and follow paths such
that they define between them grooves 18 which follow
sinusoidal paths along the core as shown by Figure 1. Each
groove extends along its path in each direction around the
axis of the support member for a subtended angle of approxi-
mately 320 so that it does not form a complete revolution
around the support member in either direction. The support
member 10 may be used to form part of a cable structure as
described in U.S. Patent 4,361,381, entitled "Optical Cable"
and granted on November 30, 1982 to R.J. Williams. In that
patent, the support member is referred to as a crush
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resistant core and a steel strength member which the core
surrounds.
The support member 10 is made and optical fibers 20
(Yigure 2) are laid into the grooves 18 by the apparatus
shown in Figure 2. As shown generally by Figure 2, the
apparatus comprises, at an upstream end of the passline for
the strength member 12, a twisting means 22 followed further
downstream by an extruder 24 and laying device 26 for laying
the optical fibers 20 into the grooves 18. The strength
member 12 is fed through the twisting means 22 along its
passline and through the extrusion die orifice (not shown) of
extruder 24, the extruder operating to extrude the plastics
sheath 1~ onto the strength member and at the same time form
the grooves 18 in the sheath. For the purpose of forming the
grooves, the die is provided with a plurality of
circumferentially spaced and fixed projections which assist
in defining a die orifice of complementary shape to the
grooved sheath. After the groove support filament has been
cooled, for instance by passage through a water cooling
trough 28, it then proceeds through the laying head 26. The
optical fibers are provided from separate spools and proceed
through the head 26 for laying into the grooves. The laying
head may be of any suitable construction and may for instance
be of the construction described in U.S. Patent 4,483,134
entitled "Laying of Optical Waveguides onto a Support
Filament", dated November 20, 1984 and granted to G. McKay
and R.J. Williams. To complete the cable (not shown), the
support filament is then provided with a core wrap, metal
shield and jacket in a conventional manner.
The strength member is rotationally twisted from a
twisting position upstream from the die 24. The twisting
means 22 is disposed closely adjacent to the die as no part
of the apparatus is located between them. The distance
between the twisting station and the die orifice is
exceedingly small thereby minimizing any unaccountable and
unforeseen reasons for producing variation in path shape of
the grooves for a predetermined degree of twist of the
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strength member. Thus, a greater degree of control of the
path shape is producible for a predetermined degree of rota-
tion of the twisting means. It has been found for instance
that for a sinusoidal shape of the grooves in which each
groove extends around the core for approximately 320, then
the twisting means is merely required to rotate around the
passline for between 360 and 500, dependent upon the
strength and torsional characteristics of the strength member
and the exact distance between the twisting station and the
die orifice. Exact rotational movement of the twisting means
may be easily set to achieve the desired sinusoidal path of
the grooves. Further to this, because the rotation at the
twisting head is minimized, then this effectively minimizes
the work required from the twisting means itself and from any
power unit involved so that energy used upon and wear on the
apparatus are also reduced to a minimum. The twisting means
22 is illustrated in detail in Figures 3 and 4.
~ As shown in Figure 3, a housing 32 has two main and
parallel walls 34, which lie on opposite sides of a chamber
of the housing and short end walls 36 which extend inwardly
towards the passline to connect the walls 34 to two bearing
sleeves 38 coaxially aligned at opposite ends of the housing.
The bearing sleeves 38 are rotatably mounted in bearings 40
carried by a machine frame 42 for rotatably mounting the
housing around the passline which extends through the sleeves
and through the housing.
The twisting means is provided with gripping means
for gripping the strength member as it iB fed along its
passline. The gripping means comprises a plurality of grip-
ping wheels 44 rotatably mounted on opposite sides of thepassline with peripheral surfaces facing towards the pass-
line. Wheels are spaced apart in series at each side of the
passline in staggered positions from side-to-side of the
passline. The wheels are rotatably mounted upon mounting
means in the form of blocks 46 which are slidable between two
opposing chamber closing plates 48, secured by screws 50 to
the walls 32 of the housing. The plates 48 effectively close
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the chamber except for a narrow slot 52 defined by each plate
48 in alignment with the passline. A wheel adjusting means
is provided for adjustably positioning the blocks 46 and thus
the wheels towards and away from the passline. The wheel
adjusting means comprises adjustable screw threaded elements
in the form of screws 54 disposed one at each end of the
walls 34 of the housing and extending through these walls in
screw threaded engagement therewith. Outer ends of the
screws 54 are provided with screwdriver slots 56 and the
inner ends have larger diameter pads 58 which have end sur-
faces lying in engagement with the blocks 46 to act as thrust
surfaces for moving the blocks inwardly as the screws are
moved into the walls 34. Thus, dependent upon screw posi-
tion, the outer positional limits of the blocks are adjusted.
A resilient means is provided to urge each of the
blocks away from the passline if the screws 54 are moved in
an outward direction, the resilient means thus operating in
the opposite direction from the thrust provided by the screws
54. The resilient means comprises compression springs 60
which, as shown in Figure 4, are received within blind bores
62 of one of the mounting blocks 46 and lie in abutting
engagement against an opposing surface 64 of the other block.
Means is also provided for rotating the twisting
means and this comprises a driving pulley 66 secured to one
of the sleeves 38, the driving pulley being drivable by a
belt or chain means 6~ from a prime mover which conveniently
is a reversible electrical motor tnot shown). In use of the
apparatus, the strength member 12 is fed through the housing
along the passline and lies with the wheels 44 in staggered
relationship at each side of it. Wheels are positioned
relative to the passline so that each wheel provides a side-
ways thrust upon the strength member to cause it to be
slightly distorted from its true path as shown in Figure 5
during its movement through the twisting means. The applica-
tion of the force in this manner to the strength member issufficient to impart a rotational twisting movement to the
strength member from the twisting means as the twisting means
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is rotationally reciprocated around the passline. Applica-
tion of a lateral load to the strength member is assisted by
the structure of each of the wheels 4~ which, as shown in
Figure 4, is provided with a peripheral V-shaped groove 70
which maintains the strength member on its path and at the
junction 72 of the groove with the peripheral surface, the
wheel makes a point contact with the strength member in the
section shown in Figure 6. Thus any lateral load applied by
the wheels to the strength member is applied in each case
along a short circumferential distance of each wheel which is
merely a line contact thereby maximizing the pressure applied
to the strength member.
With the above apparatus and with the twisting
means described in the embodiment, a positive non-slipping
rotational twisting action is applied from the twisting means
to the strength member although the strength member is devoid
of any surface feature which could be mechanically engaged
and which could assist in the twisting operation.
As the strength member is moved along its path, the
twisting means rotates to cause twisting rotation of the
strength member alternately in one direction and then in the
other. This twisting movement of the strength member extends
through the die orifice and downstream from the die with the
degree of twist diminishing progressively from the point of
application. As the projections at the die orifice for
forming the grooves are fixed, then the strength member
twists relative to these projections and causes the unsolidi-
fied strength member immediately downstream from the die
orifice to twist relative to the die projections. Thus, the
grooves in the sheath are caused to extend around the axis of
the support member after issuing from the die first in one
direction and then in the other. The twisting influence upon
the sheath is effective downstream from the die orifice to a
position at which the sheath has cooled and solidified to
become sufficiently ~igid to resist twisting.
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