Manufacture of a flexible core for an optical cable

A helically grooved flexible optical cable core comprising a plurality of helically laid flexible bodies 1 of sector-shaped cross-section is manufactured by drawing a multiplicity of flexible non-metallic reinforcing elements 2 under tension through electrically insulating plastics material 3 in a semi-liquid state; causing the plastics encapsulated reinforcing elements to pass through an elongate die 23 having an outlet orifice 25 of sector-shape, which die, between its ends, is of such a shape that reinforcing elements adjacent the arcuate surface 4 of the body are travelling at a greater linear speed than reinforcing elements remote therefrom, and which has a blade 26 protruding inwardly of the die and forming in the arcuate surface of the body a longitudinally extending groove 5; causing the body as it emerges from the die to follow a helical path with its grooved arcuate surface outermost and so cooling the helically advancing body that the plastics material sets; partially filling the groove in the arcuate surface with a grease and continuously feeding an optical fibre into the grease in the groove; laying up the helically advancing body with a plurality of helically advancing bodies of similar cross-section and construction to form a flexible core of circular cross-section which is rotating about its longitudinal axis; and taking up the rotating core in such a way that any residual torsion in the core arising from said rotation is removed.

This invention relates to optical cables of the kind comprising a flexible 
core having in its outer circumferential surface a plurality of 
circumferentially spaced, helically extending separate grooves in at least 
one of which is loosely housed at least one optical fiber suitable for the 
transmission of light. 
The generic term "light" includes the ultra-violet, visible and infra-red 
regions of the electromagnetic spectrum and the invention is especially, 
but not exclusively, concerned with optical cables of the aforesaid kind 
for use in the communications field in which the optical fibers are 
adapted for transmission of light having a wavelength within the range 0.8 
to 2.1 .mu.m. 
The invention is particularly concerned with an optical cable of the 
aforesaid kind in which the flexible core is of approximately circular 
transverse cross-section and comprises a plurality of flexible elongate 
bodies of approximately sector-shaped transverse cross-section helically 
laid-up together, each of which bodies throughout its length is made of a 
composition comprising a multiplicity of longitudinally stressed flexible 
elongate non-metallic reinforcing elements substantially encapsulated in 
extruded plastics material and each of at least some of which elongate 
bodies has in its outer arcuate surface a longitudinally extending groove. 
The flexible core of the optical cable of the aforesaid kind with which 
the invention is particularly concerned will, for convenience, hereinafter 
be referred to as "a flexible core as hereinbefore described". 
It is an object of the present invention to provide an improved method of 
manufacturing a flexible core as hereinbefore described. 
According to the invention the improved method comprises the steps of 
drawing a multiplicity of flexible elongate non-metallic reinforcing 
elements under tension in the directions of their lengths through 
electrically insulating plastics material which is in a liquid or 
semi-liquid state; causing the multiplicity of non-metallic reinforcing 
elements encapsulated in electrically insulating plastics material in a 
liquid or semi-liquid state to pass through an elongate die having an 
outlet orifice having a transverse cross-section approximating to a sector 
of a circle to form a flexible elongate body of approximately 
sector-shaped transverse cross-section, which die, between its ends, is of 
such a shape that non-metallic reinforcing elements constituting or 
adjacent to the arcuate surface of the flexible elongate body so formed 
are travelling at a greater linear speed than non-metallic reinforcing 
elements remote therefrom, and which has means protruding inwardly of the 
die and forming in the arcuate surface of the body a longitudinally 
extending groove; causing the flexible elongate body as it emerges from 
the die to follow helical path within an imaginary cylinder of 
substantially circular transverse cross-section in such a way that the 
arcuate surface of the body is maintained outermost and so treating the 
helically advancing body that the plastics material in which the 
reinforcing elements are encapsulated sets; at least partially filling the 
groove in the arcuate surface of the helically advancing flexible elongate 
body with a water-impermeable medium of a grease-like nature and 
continuously feeding into the water-impermeable medium in the groove at 
least one optical fibre; laying up the helically advancing flexible 
elongate body housing in its groove at least one optical fiber with a 
plurality of helically advancing flexible elongate bodies of similar 
transverse cross-section and construction to form a flexible core of 
approximately circular transverse cross-section which has in its outer 
surface a plurality of circumferentially spaced, helically extending 
grooves in which optical fibres are loosely housed and which is rotating 
about its longitudinal axis; and taking up the flexible core in such a way 
that any residual torsion in the core arising from said rotation is 
removed. 
Preferably, the flexible elongate bodies of approximately sector-shaped 
transverse cross-section having optical fibers loosely housed in 
longitudinally extending grooves in their arcuate surfaces are 
manufactured concurrently and the helically advancing bodies are laid up 
together downstream of a station at which optical fibers are continuously 
fed into the grooves of the bodies. A single flexible elongate body or a 
sub-assembly of at least two flexible elongate bodies at each of a 
plurality of longitudinally spaced laying up stations, but, preferably, 
all the helically advancing flexible elongate bodies are laid up together 
at a single station. 
Between its ends, the elongate die preferably extends-in a smoothly curved 
arcuate path, the internal surface of the die defining the arcuate surface 
of the flexible elongate body having the greater radius of curvature. By 
virtue of the fact that the elongate die between its ends follows a 
smoothly curved path, the non-metallic reinforcing elements immediately 
adjacent that part of the inner surface of the die having the greater 
radius of curvature will travel under tension at a linear speed greater 
than that of the non-metallic reinforcing elements remote therefrom. 
Preferably, over at least a major part of the length of the smoothly 
curved elongate die, the internal surface of the die defining the arcuate 
surface of the sector-shaped flexible elongate body and the internal 
surface of the die defining the apex of the sector-shaped flexible 
elongate body are so curved that the transverse cross-section of the 
smoothly curved elongate die is substantially constant in shape and size 
to ensure that the non-metallic reinforcing elements are drawn through the 
die at the desired linear speeds relative to one another. 
At its entry end, the elongate die preferably opens into and forms part of 
an open-topped chamber into which the electrically insulating plastics 
material in a liquid or semi-liquid state is fed, preferably through an 
inlet port intermediate of the ends of the die. The multiplicity of 
flexible elongate non-metallic reinforcing elements preferably are drawn 
under tension approximately vertically downwardly into the open topped 
chamber of electrically insulating plastics material in a liquid or 
semi-liquid state. 
The means by which a longitudinally extending groove is formed in the 
arcuate surface of the flexible elongate body preferably takes the form of 
a blade or plurality of fingers which protrudes inwardly of and at least 
partly along the die at or near its outlet end. Preferably, the blade or 
each finger has a transverse cross-section approximating in shape to the 
frustum of a cone and the blade of plurality of fingers extends along at 
least a major part of the length of the die. 
In order to cause the flexible elongate body as it emerges from the die to 
follow a helical path within an imaginary cylinder of substantially 
circular transverse cross-section in such a way that the arcuate surface 
of the body is maintained outermost, preferably immediately downstream of 
the smoothly curved die the flexible elongate body passes into a helically 
extending channel of substantially V-shaped cross-section in the outer 
surface of a substantially rigid elongate member and, as the flexible 
elongate body travels along the helically extending channel, the 
electrically insulating plastics material in which the non-metallic 
reinforcing elements are encapsulated in so treated that it is caused to 
set. To maintain the shape of the helical groove formed in the arcuate 
surface of the flexible elongate body, preferably as the body travels 
along the helically extending channel in the outer surface of the rigid 
elongate member a helical blade engages in and maintains the shape of the 
groove in the arcuate surface of the flexible body until the electrically 
insulating plastics material has set. 
Preferably, the smoothly curved arcuate path of the elongate die and the 
shape of the helically extending channel of the rigid elongate member are 
so related that the relative positions of the non-metallic reinforcing 
elements within the flexible elongate body being formed are maintained 
substantially constant and the transition from the single curve of the 
elongate die to the compound curve of the helically extending channel is 
accomplished without any undesirable strain being imposed on the elements. 
There may be some relative movement between adjacent or neighbouring 
elements which the semi-liquid state of the electrically insulating 
plastics material allows. 
To facilitate feeding of water-impermeable medium of a grease-like nature 
into the groove in the arcuate surface of the flexible elongate body, 
preferably, at a position at which the groove in the arcuate surface of 
the helically advancing flexible elongate body is uppermost, 
water-impermeable medium of a grease-like nature is continuously fed 
downwardly into the groove, e.g. by causing the grease-like 
water-impermeable medium to flow downwardly on the surface of and to drip 
from a downwardly extending substantially rigid elongate guide. 
The flexible elongate body is drawn through the smoothly curved die and 
helically channelled elongate member by any convenient means. Having 
regard to the relatively slow speed at which the flexible elongate body is 
formed, the drawing means preferably takes the form of a pair of 
longitudinally spaced and reciprocating clamps each of which 
intermittently and out of sequence with the other grips the advancing 
body, advances a predetermined distance, releases the body and returns to 
its original position so that the body is drawn continuously from the 
smoothly curved die; alternatively, the drawing means may be a pair of 
driven endless belts which are rotatably driven bodily around the axis of 
the helically advancing body. 
After the helically advancing flexible elongate bodies have been laid up 
together to form the flexible core rotating about its longitudinal axis, 
preferably the flexible core is drawn by a pair of longitudinally spaced 
and reciprocating clamps each of which intermittently and out of sequence 
with the other grips the advancing flexible core, advances a predetermined 
distance, releases the core and returns to its original position so that 
the advancing flexible core is drawn continuously but, in this case, to 
accommodate for the fact that the advancing flexible core is rotating 
about its longitudinal axis, whilst each clamp is gripping the core and 
advancing said predetermined distance, the clamp is constrained to rotate 
with the core. 
The take up means by which any residual torsion is removed from the 
rotating flexible core may be a drum twister or it may take the form of a 
rotatably driven turntable on to which the rotating flexible core is so 
coiled that any residual torsion in the core is removed. 
Preferably, the optical fiber is so fed into the water-impermeable medium 
in the groove that it lies immediately below the surface of the medium so 
that, when a tensile force is applied to the flexible core of which the 
flexible elongate body forms a part causing the core to stretch, the 
optical fiber can move radially inwardly through the medium towards the 
axis of the core, thereby substantially reducing risk of unacceptable 
tensile force being applied to the optical fiber. To this end, preferably 
the optical fiber is drawn from a source of supply towards the station at 
which it is fed into the water-impermeable medium in the groove in a path 
which lies at an acute angle to the path of the helically advancing body 
so that, over a predetermined axial length of the body, an excess length 
of optical fiber is provided. Preferably, also, the optical fiber is drawn 
from the source of supply by longitudinally spaced and reciprocating 
clamps operating in a manner similar to that of, and preferably 
mechanically coupled to, the clamps of the other drawing means. The source 
of supply of optical fiber preferably is so mounted that it is bodily 
rotatable in space and is operatively coupled to the take-up means so 
that, when the take-up means rotates to remove any residual torsion from 
the rotating flexible core, the source of supply of optical fiber rotates 
to remove any twist in the optical fiber that would otherwise by imparted 
by rotation of the advancing flexible core about its longitudinal axis. 
Downstream of the means drawing the laid up flexible elongate bodies 
forming the rotating flexible core, at least one tape of electrically 
insulating material or other flexible binder may be helically lapped round 
the laid-up bodies, the direction of lay of the helically wound tape being 
opposite to that of the laid-up bodies. 
The flexible elongate non-metallic reinforcing elements of the flexible 
elongate body preferably are made of an aromatic polyamide such as that 
sold under the trade name `KEVLAR`; an alternative non-metallic material 
of which they may be made in non-optical glass. 
The electrically insulating plastics material in which the non-metallic 
reinforcing elements are encapsulated preferably is a theromoplastics 
material, such as polypropylene or nylon but, in some circumstances, it 
may be a thermosetting plastics material, such as a polyester or epoxy 
resin or polyurethane. 
Other electrically insulating plastics materials in which the non-metallic 
reinforcing elements may be encapsulated are thermotropic liquid 
crystalline polymers such as wholly aromatic polyesters, 
aromatic-aliphatic polyesters, aromatic polyazomethines, aromatic 
polyester-carbonates and wholly or non-wholly aromatic polyester amides. 
Where the plastics material in which the non-metallic reinforcing elements 
are encapsulated is a thermoplastics material, the plastics material will 
be introduced into the chamber at such a temperature that the material is 
in a liquid or semi-liquid state and the chamber and/or elongate die will 
be heated to maintain the material at that temperature. Cooling air will 
be directed on to the helically channelled elongate member in order to 
cause the plastics material to set. 
Where the plastics material is a thermosetting material, the helically 
channelled elongate member will be heated in order to cause the plastics 
material to set. 
The invention also includes apparatus for carrying out the improved method 
as hereinbefore described. 
The invention further includes an elongate flexible core as hereinbefore 
described when manufactured by the improved method hereinbefore described. 
The flexible core as hereinbefore described manufactured by the improved 
method of the invention has the important advantage that an optical cable 
of which it forms a part is so flexible that it can be readily handled, 
and wound on and unwound from a cable drum, without substantially any risk 
of danger to an operative arising from an inherent torsion or stiffness in 
the cable. Moreover, the flexibility of the flexible core is such that an 
optical cable having a flexible core manufactured by the improved method 
of the invention can be readily strung in long lengths between poles, 
towers or other upstanding supports and used as an aerial cable. A 
flexible core manufactured by the improved method of the invention has the 
further important advantage that the multiplicity of non-metallic 
reinforcing elements are distributed substantially evenly throughout each 
flexible elongate body of sector-shaped transverse cross-section and that, 
as a consequence, when a cable incorporating the core is stressed, strain 
is applied substantially evenly across the transverse cross-sections of 
all the flexible elongate bodies.

A preferred flexible core as hereinbefore described comprises six flexible 
elongate bodies 1, each of sector-shaped transverse cross-section, 
helically laid up together. As will be seen on referring to FIG. 1, each 
body throughout its length is made of a composition comprising a 
multiplicity of longitudinally stressed flexible elongate non-metallic 
reinforcing elements 2 encapsulated in extruded plastics material 3 and 
has in its outer arcuate surface 4 a longitudinally extending groove 5. 
As will be seen on referring to FIG. 2, the preferred optical cable 
comprises a flexible core 8 of circular transverse cross-section 
comprising six flexible elongate bodies 1 of sector-shaped transverse 
cross-section helically laid up together, a plastics tape 9 helically 
wound around the laid up bodies and an overall protective sheath 10 of 
plastics material. The groove 5 in each of the flexible elongate bodies 1 
is substantially filled with a water-impermeable medium 6 of a grease-like 
nature and an optical fiber 7 is immersed in the water-impermeable medium 
in each groove immediately below the surface of the medium. 
Referring to FIGS. 3 and 4, the apparatus employed in the manufacture of 
the preferred flexible core of the optical cable shown in FIG. 2 comprised 
six chambers and associated dies generally referred to at 20 and six 
helically channelled rigid rods 30, one of which is associated with each 
die. Downstream of the six chambers and associated dies 20 and helically 
channelled rods 30 is a station 40 at which water-impermeable medium 6 of 
a grease-like nature is continuously fed into the groove 5 in the arcuate 
surface 4 of each of six flexible elongate bodies 1 emerging from the 
helically grooved rods. Downstream of the station 40 are six pairs of 
reciprocating two-part clamps generally referred to at 50 by means of 
which the flexible elongate bodies 1 are continuously drawn from the dies 
20 and helically channelled rods 30. Each two-part clamp has in its 
clamping surface a helical slot in which a flexible body 1 can be so 
gripped that helical advance of the body is temporarily restrained. Beyond 
the clamps is a station 60 at which optical fibers 7 are continuously fed 
into the water-impermeable medium 6 in the grooves 5 of the flexible 
elongate bodies. The optical fibers 7 are supplied from sources 61 from 
which they are drawn by six pairs of reciprocating two-part clamps 
generally referred to at 62 mechanically coupled to the clamps 50. As will 
be seen in FIG. 4, the optical fibers 7 in travelling from the sources 61 
to the station 60 follow a path which is inclined at an acute angle to the 
path followed by the flexible elongate bodies 1. Downstream of the station 
60 are two guide stations 70 and 71 which serve to orientate the advancing 
flexible elongate bodies 1 so that they can be laid up together as they 
pass through a forming die 80 which will press the laid up bodies radially 
inwardly together to form the flexible core 8 shown in FIG. 2. Beyond the 
forming die 80 is a pair of reciprocating two-part clamps 90 for drawing 
the laid-up flexible elongate bodies 1 now forming a flexible core 8 
through the forming die, which reciprocating clamps are mechanically 
coupled to the clamps 50 and 62. Downstream of the clamps 90 is a taping 
head 100 and a turntable 110 that can be rotatably driven to remove any 
residual torsion from the flexible core. 
As will be seen on referring to FIG. 5, each of the six chambers and dies 
20 comprises an open topped chamber 21 which, at the bottom of the 
chamber, blends by means of a passage 22 of approximately triangular shape 
into a smoothly curved elongate die 23 which, between its ends, extends in 
a smoothly curved arcuate path and has a substantially constant transverse 
cross-section approximating to a sector of a circle. Immediate of the ends 
of the die 23 is an inlet port 24 by means of which electrically 
insulating plastics material 3 in a semi-liquid state can be fed into the 
die and hence the open topped chamber 21. The internal surface of the die 
23 defining the arcuate surface of the flexible elongate body 1 has the 
greater radius of curvature and, over a major part of the length of the 
smoothly curved elongate die, the internal surface of the die defining the 
arcuate surface of the sector-shaped flexible elongate body and the 
internal surface of the die defining the apex of the sector-shaped 
elongate body are so curved that the transverse cross-section of the die 
is substantially constant in shape and size to ensure that the 
non-metallic reinforcing elements 2 are drawn through the die at the 
desired linear speeds relative to one another. Extending along a major 
part of the length of the internal surface of the die defining the arcuate 
surface of the sector-shaped flexible elongate body 1 near the outlet end 
25 is a blade 26 which protrudes inwardly of the die and which has a 
transverse cross-section approximating in shape to the frustum of a cone. 
At its downstream outlet end 25, the elongate die 23 opens into one end of 
a helically extending channel 32 of V-shaped cross-section in the outer 
surface of a rigid rod 30 of circular cross-section which extends 
substantially horizontally from the body of the chamber 20. A helically 
extending blade 33 protrudes into the helically extending channel 32 
throughout its length and serves to maintain the shape of the groove 5 
formed by the blade 26 in the arcuate surface 4 of the flexible elongate 
body 1. 
In use, a multiplicity of flexible reinforcing elements 2 of aromatic 
polyamide are drawn under tension in the direction of their lengths and 
downwardly through electrically insulating thermoplastics material 3 which 
is continuously fed into the open topped chamber 21 through the inlet port 
24 and which is at such a temperature that the thermoplastics material is 
in a semi-liquid state. The multiplicity of reinforcing elements 2 
encapsulated in electrically insulating thermoplastics material 3 in a 
semi-liquid state pass through the elongate die 23 to form a flexible 
elongate body 1 which has a transverse cross-section approximating to a 
sector of a circle having in its arcuate surface 4 a groove 5. The 
multiplicity of reinforcing elements 2 are substantially evenly 
distributed throughout the cross-sectional area of the body 1. Immediately 
downstream of the smoothly curved die 23, the flexible elongate body 1 
passes into the helically extending channel 32 in the outer surface of the 
rigid rod 30 with the helical blade 33 engaging in the groove 5, and, as 
the flexible elongate body travels along the helically extending channel, 
cooling air is directed from a source (not shown) onto the rigid rod to 
cause the electrically insulating thermoplastics material 3 of the 
advancing body set. The smoothly curved die 23 is of such a shape that 
reinforcing elements 2 constituting and adjacent to the arcuate surface of 
the flexible elongate body 1 are travelling at a linear speed greater than 
that of reinforcing elements remote therefrom. Since the flexible elongate 
body 1 is travelling along the helical channel 32 in the outer surface of 
the rigid rod 30, the body is rotating about the rigid rod and the arcuate 
surface of the body is maintained outermost. 
From the rigid rods 30, the six helically advancing flexible elongate 
bodies 1 pass through the station 40, so positioned that at the station 
the grooves 5 in the arcuate surfaces 4 of the bodies are uppermost, and 
at this station water-impermeable medium 6 of a grease-like nature is 
continuously fed downwardly into the grooves by causing the medium to flow 
downwardly on the surface of and to drip from a downwardly extending rigid 
elongate guide (not shown). 
The six helically advancing flexible elongate bodies 1 being manufactured 
concurrently are withdrawn continuously from the smoothly curved die, 
around the helically channelled rod 30 and through the station 40 by the 
six pairs of longitudinally spaced and reciprocating two-part clamps 50, 
each of which clamps intermittently and out of sequence with the other so 
grips an advancing body that helical movement of the body is restrained 
temporarily, advances a predetermined distance, releases the body which 
recovers the helical orientation temporarily inhibited by the clamp, and 
returns to its original position. 
Beyond the reciprocating clamps 50, the helically advancing flexible 
elongate bodies pass through a station 60, which is so positioned that the 
grooves 5 in the arcuate surfaces 4 of the bodies are uppermost, and 
optical fibers 7 from the sources 61 are fed continuously into the 
water-impermeable medium in the grooves. The optical fibers 7 are 
continuously withdrawn from the sources 61 by the longitudinally spaced 
and reciprocating clamps 62 each of which is mechanically coupled to the 
clamps 50. Each clamp 62 intermittently and out of sequence with the other 
grips the advancing optical fibers, advances a predetermined distance, 
releases the optical fibers and returns to its original position so that 
the optical fibers are drawn continuously from the sources 61. The optical 
fibers 7 are continuously advancing in a path which lies at an acute angle 
to the path followed by the helically advancing flexible elongate bodies 1 
so that, over a predetermined axial length of each body, an excess length 
of optical fiber is provided and, when the optical fiber is fed into the 
water-impermeable medium 6 in a groove 5 it will lie immediately below the 
surface of the medium. The sources 61 of optical fiber are so mounted that 
they are capable of rotating bodily in space and are so mechanically 
coupled to the rotatably driven turntable 110 that, as the turntable is 
rotatably driven to remove any residual torsion in the flexible core 8, 
each of the sources 61 is bodily rotated to remove any twist from the 
optical fiber 7 being continuously withdrawn therefrom which would 
otherwise be imparted by the rotation of the flexible core about its axis. 
Downstream of the station 60, the helically advancing flexible elongate 
bodies 1 pass through guides 70 which appropriately orientate the 
helically advancing bodies so that they are helically laid up together to 
form a flexible core of circular cross-section as they pass through and 
are pressed radially inwardly together by the forming die 80 to form the 
flexible core 8. 
The flexible core 8 emerging from the forming die 80 and rotating about its 
axis is continuously drawn by the longitudinally spaced and reciprocating 
clamps 90 which are mechanically coupled to the clamps 50 and 62. Each of 
the clamps 90 intermittently and out of sequence with the other grips the 
advancing flexible core, advances a predetermined distance, releases the 
core and returns to its original position but, as the advancing flexible 
core is rotating about its axis, whilst each clamp 90 is gripping the core 
and advancing said predetermined distance, the clamp is constrained to 
rotate with the core. 
Immediately downstream of the reciprocating clamps 90 at a taping head 100, 
a plastics tape 9 is helically wound around the flexible core 8, the 
direction of lay of the tape being opposite to that of the flexible 
elongate bodies 1. The flexible core 8 is fed downwardly on to the 
turntable 110 which is rotatably driven at such a speed that, in addition 
to facilitating coiling down of the core, the rotating turntable also 
removes any residual torsion in the core by completing approximately one 
revolution for each pitch of the laid up helically extending bodies 1. 
To complete manufacture of the optical cable shown in FIG. 2, in a separate 
operation the sheath 10 of plastics insulating material is extruded over 
the plastics tape-covered flexible core 8.