Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a filament for use in
optical cables, to optical cables utilizing the filament and to a
method of manufacturing the ~ilament.
It has been previously proposed to manuFacture
optical cable having a central strength member of, for example steel ;
wire, a plastics outer sleeving extruded ar~und the steel wire and
a series of grooves formed in the surface of the plastic sleeving,
each groove containing a dielectric optical waveguide.
In order to ensure that dielectric optical waveguides
are not subject to destructive tensile and compressive stresses
wherever the cable is bent, the grooves are made in helical form.
Thus at a curved part of a cable a dielectric optical waveguide
experiences alternately compression and tension and over the length
of the curve, the stresses at least partially cancel out.
The manuFacturing steps for such cable include
production of a grooved, plastics-coated metal strength member to
provide a central filament for the cable, and the laying of
dielectric optical waveguides into the grooves in the central filament.
In the former, a known practice is to extrude the plastics through
a rotating die, a servo mechanism being utilized to maintain the
correct ratio of die angular velocity to the extrusion rate of
filament in order to maintain the pitch of the helices within a
predetermined range throughout the length of the central filament.
It is necessary to limit the extrusion rate in order to guard against
adverse shear affects resulting when the plastics9 as it is extruded
in one direction, is directed rapidly in a different direction.
Care must also be taken in choosing an extrusion ra~e to avoid
collapse of the grooved structure immediately the malleable~ high
temperature plastics exits from the die.
3U To lay dielectric optical waveguides into an
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appropriately grooved central filament, a planetary stranding
technique has been adopted. In such a technique for laying in, SAy
ten dielectric optical waveguides, ten reels of dielectric optical
waveguide are mounted on a rotatable jig with the central filament
being led through the centre of the jig. The reels revolve around
the longitudinally moving filament with an angular velocity
commensurate with both the pitch of the helical grooves and the
velocity of the central filament. In effect therefore a reel ~ ;
follows a groove around as the central filament is fed through the
jig. A suitable locating device presses payed out dielectric
optical waveguide into the grooves.
Rotation of the reels and their motion around the
central filament does, however, introduce a twist into the laid
dielectric optical waveguide which is unacceptable because of the
internal stresses which result. To compensate for this the reels
are themselves rotated so that the undesirable twist in the
dielectric optical waveguide is pre-empted. The nature of the
movement of the reels somewhat resembles a planet system and accounts
for the name given to this technique.
It will be appreciated that a complex servo mechanism
is required to correctly interrelate the speeds at which:-
1) the central filament is fed through the jig;
2) dielectric optical waveguide is payed out;
3) the jig is rotated, and
4) the reels are rotated.
The optical cable structure of the invention permits the simplification
of operating techniques for manufacture of the cable.
In add;tion, contrasting with known helical - lay
optical cable, the manner in which dielectric optical waveguides are
layed up in the proposed structure inhibits the damage from being
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caused through the cable twisting.
According to a first aspect of the invention there
is provided an elongate filament for an optical cable, the filament
having a surface, which surface deFines a plurality of grooves,
said grooves each having a form of a helix, each said helix changing
hand along the filament.
In preferred embodiments of the filament, at least
one complete pitch of a particular helix exists between each change
of hand thereof and respective helices all change hand in the same
sense at predetermined positions along the filament. It is preferred
that the h lices change hand at regular intervals along the length
of the filament. In a typical optical cable structure, the filament
composition is high density polyethylene surrounding a central
strength member, dielectric optical waveguides being positioned in
the grooves, and an outer plastics sheath surrounding the filament.
In a second aspect of the invention, apparatus For
making the filament comprises an extrusion unit for continuous
extrusion of heated, malleable plastics material through a fixed
die having a plurality of fingers to form said grooves, a twist unit
located downstream of said extrusion unit, said twist unit having a
reciprocally rotatably driveable hollow cylindrical member For
receiving the extruded material with at least some oF the grooves
engaging the member5 the twist unit being driveable to twist the
extruded material thereby to introduce a helical form to the grooves
where extruded material exits the extruded unit. A link mechanism
interadjacent the extrusion unit and the twist unit can relate
reciprocal rotation of the cylindrical member to the rate of
extrusion oF said plastic material. The location of the twist unit
along a feedpath for the extruded -filament is chosen to be where
the filament is comparatively ri~id and can support the engagement of,
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for example, in~ardly pro~ecting fins on the cylindrical member
within the grQoves. Such fins can have blades or wheels of
sufficiently narrow thlckness that they may be inserted within the
grooves.
Alternatively the filament is gripped by a
plurality of resil;ent wheels rotatably mounted on a jig, itself
having a centre of rotation coincident with a central axis of the
filament. The resilient wheels are arranged to bear against the
filament with sufficient pressure to grip the f;lament but
insufficient pressure to permanently d;stort the grooves.
An embod;ment of the invention will be described
by way of example with reference to the accompanying drawings,
in which:-
Figure 1 is a perspective view of a length of filament
according to the invention;
Figure 2 is a schematic representation of apparatus
for making such a filament and laying dielectric optical waveguides
into grooves in the filament,
Fig~res 3 and 4 are respectively a perspective view
and an end view of part of the apparatus for twisting extruded
material to produce the filament;
Figure 5 is a perspective view of apparatus for
laying dielectric optical waveguides into the filamentj and
Figure 6 is a scrap longitudinal sectional view
of part of the laying apparatus of Figure 5.
Referring to the drawings in detail, a filament
for an optical cable has a central steel wire strength member 1
and, extruded over the strength member 1, a sleeve 2 of high
density polyethylene. Formed in the surface of the sleeve and
extending throughout the length of the filament are a number,
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in this case four, circumferentially spaced grooves 3a, 3b~ 3c
and 3d. In use the grooves each accommodate a dielectric
optical ~aveguide in a relatively loose fit, the.whole
being surrounded by an extruded
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plastics sheath (not shown). In order to guard against breakage
of dielectric optical waveguides where the optical cable is bent,
the grooves are made to follow a helical path around the longitudinal
axis of a filament. However, as shown at positions ~ the various
helical paths followed by the grooves change hand (left to right or
right to left) or lay direction. The grooves 3 are advantageously
distributed evenly around the filament so the changes of hand of the
four helical paths take place at the same specific positions along
the length of the filament. The grooves thus have a generally
parallel disposition relative to one another. As is evident from
Figure 1 the changes of hand take place at regular intervals along
the filament.
Turning to Figure 2, there is shown a schematic
representation of apparatus used in the manufacture of an optical
cable utilizing the filament described. Basically the apparatus ~-
comprises three units, an extrusion unit 5, a twist unit 6, and a
laying-in unit 7. To manufacture, steel wire core 1 and a charge
of high density polyethylene 8 are fed into an extrusion unit which
includes a die 9 shown in greater detail in Figure 3. The polyethylene
8 is heated until it is malleable and then extruded around the steel
wire core 1 through the die 9 which is shaped to Form grooves 10 in
the polyethylene as it exits the extrusion unit 5. Some way
downstream of the extrusion unit, the filament, having been cooled
by a trough oF cooling fluid (not shown) becomes relatively rigid
and enters the twist unit which is operable to twist the filament,
therefore introducing the helical form to the grooves where the
polyethylene exits the extrusion unit.
Downstream of the twist unit 6 is the laying-in
unit at which dielectric optical waveguide 11 which is payed out from
reels 12 is set into the grooves 3.
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Referring to the more detailed Figure 3, molten
polyethylene is ex-truded through the die 9 which has four inwardly
projecting straight-walled fingers 13 to form the grooves 3. A twist
unit 6 comprises a mechanism having a central cylindrical bore
through which the extruded filament is pulled, the mechanism having
at one end a gear 14 which is reciprocally rotatable and is driven
by a drive gear 15 which forms part of a drive train from the
extrusion unit, this being shown schematically by arrows B and C
and drive shaft 16, the function of the drive train being to
relate the speed of oscillation of the gear 14 to the extrusion
rate of the extrusion unit 5. Alternatively the rates of extrusion
and drive to the twist unit can be preset to obta;n the required
groove characteristics without the drive train B and C.
Integral with, and adjacent gear 14, is a barrel
member 16 having a series of four evenly circumferentially spaced
slots 17 extending through its wall. Slidably mounted within the
slots for limited radiam movement are four fins 18 having blades ;~
19 of thin cross-section at their inner edges which project into
the barrel 16. Outer edges 20 of the fins 18 are biased radially
inwardly by a spring 21.
In operation of the twist unit 6, the blades 19
interengage in respective ones of the four grooves 3 where the
extruded plastics is relatively cool and rigid and the drive train,
via the gear 14, drives the barrel 16 to kwist the filament 2. Since
the extrusion unit does not rotate, the extruded polyethylene
between the two units 5 and 6 undergoes a shear stress resulting in
the grooves in the most malleable part of the polyethylene, i.e. as
it exits from the extrusion unit 5, being deformed to provide the ;
helical character. The change in hand of the individual helices is
achieved merely by reversing the drive direction of the drive train.
The blades 19 are in an alternative embodiment, (not
shown), replaced by miniature wheels which run in the grooves 3 in
the filament 2 with somewhat less friction than do the blades 19.
In another alternative (not shown) the fi1ament is
gripped at its surface by three wheels of resilient composition. The
arrangement is such that the wheels bear su-Fficiently strongly on the
filament that it can be twisted by the twist unit but insuFficiently
strongly for the grooved surface structure to be permanently distorted.
Figure 4 shows a practical embodiment of the unit
7 for laying dielectric optical waveguides 11 into the grooves 3 of
a f;lament 2. Dielectric optical waveguide is payed out -From Four
reels 12 which ~re evenly circumferentially spaced away from a path
21 along which the grooved filament 2 is drawn. The dielectric
optical waveguides 11 are pulled from the reels by the movement
of the filament itself as will be explained presently. The dielectric
optical waveguides 11 pass through guide means comprising a pair of
rotatable plates 22 and 23. The filament 2 is drawn through the centre
of the two plateswhile the dielectric optical waveguides pass through
the plates at circumferential evenly spaced apertures 24 and 25. The
plate 23 is somewhat thicker than plate 22 and the apertures25 are
lined with tubes 26 which project from the downstream side of the
plate 23. The tubes 26 are inclined towards the axis oF the ~;
filament 2 and their ends 27 are flexible and pressed in-to respective ~-
grooves 3 so that as dielectric optical waveguide is drawn from the
tubes by the filament being drawn past the laying~in unit 7, the
dielectric optical waveguides are automatically located in the bases
of the grooves 3. To aid the drawing out of dielectric optical
waveguide, the outlet ends of the tubes are tapered, the tapered
surface facing radially outward. In addition, the inlet ends of
each of the tubes can be formed with a mouthpiece (not shown) to
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reduce friction effects where dielectric optical wave~uide enters
the tubes.
The circumferential position of the grooves 3, where
they are engaged by tube ends 27, regulate the angular position of
the plate 23~ A geared drive shown schematically as arrow D relates
the rotation of plate 22 to that of plate 23.
In operation the movement of filament 2 past the tube
ends 27 produces rotation of plate 23 determined by the number of
times a helical groove 3 extends around the longitudinal axis of
the filament 2 between adjacent changes of hand or lay direction.
The purpose of the second plate 22 is to prevent the Four fibres
From contacting each other and the central filament. The latter is
undesirable since friction ef-fects would make the pulling of fibre
from the fixed reels 12 much more dif-ficult. The presence of the
plate 22 permits a phased winding of the dielectric optical waveguides
11 around each other and the central filament 2, but withou~ there
being any contact. I-F a number of turns are envisaged between each
change of hand of the helical grooves 3 then a number of intermediate
plates 22 can be sited between the plate 23 and the reels 12 with
20 an appropriate gear drive.
In the embodiment described the drive is such as to
produce angular rotation of ~2 of plate 22 For every angular rotation
of the plate 23.
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