NZ204647A - Optical fibre cable:inserting excess length fibres into extruded package during extrusion - Google Patents

Description

204647 Priority Date(s): .^V?..

Complote Specification Filed: ,4.'S^ Class: fp.i. M/m... . Af.n&v/M..

M.q././3(3/W p'Jwic^n 'J^-V!!!"" Hwiwfli! P.O. Journal. No: • """ * NEW ZEALAND <20 JUN1983^| 4»otn€= PATENTS ACT 1953 COMPLETE SPECIFICATION Improvements in or relating to Optical Fibre Cable.

We, TELEPHONE CABLES LIMITED, of Chequers Lane, Dagenham, Essex. RM9 6QA, England, a Company incorporated under the Laws of England, hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described and ascertained in and by the following statement:- 204647 This invention relates to optical fibre cables, that is to say cables of the kind incorporating one or more optical fibres accommodated in a protective cover, which is also known as a package and will 5 hereinafter be referred to as such, and relates also to a method of manufacturing such cables.

According to the invention in an optical fibre cable the protective package comprises an extrudate formed with a plurality of longitudinally 10 extending closed channels, and one or more optical fibres are loosely accommodated in at least one of the channels, the fibre or fibres having been introduced into the channel or channels during the extrusion process and each having a natural length which is 15 greater than the corresponding length of the channel in which it is accommodated.

By the natural length of a fibre is meant the length of the fibre if laid along a straight line.

The incorporation of the fibre or fibres 20 within the channels during the extrusion of the , package in the manufacture of the cable is simplified, and by the use of a fibre or fibres having a natural length which is greater than that of the respective channel or channels the risk of the fibre or fibres 25 being subjected to tensile stress and therefore becoming damaged during manufacture or subsequent use of the cable is significantly reduced.

The fibre or fibres may, for example, be introduced into the extrudate at a rate greater than 30 the rate at which the extrudate is formed.

The extrudate, which is preferably; but 2 04 6 not necessarily, of circular form, may incorporate one or more strength or strain bearing members, hereinafter referred to simply as strength members, extending along it to assist in preventing the optical 5 fibre or fibres being subjected to an excessive tensile force during manufacture or subsequent use of the cable. The strength member or members may be of a metal such as steel or alumium, conveniently of helically wound stranded form, or alternatively of a 10 plastics material such as the aromatic polyamide known as 'Kevlar or any other material having suitable tensile strength.

However by employing a suitable high strength material for the extrudate, the use of 15 separate strength members may not be necessary, at least for some applications.

If one or more strength members having an appropriate coefficient of thermal expansion is/are employed the member or members may be arranged to be 20 heated prior to the extrusion process, such that on subsequent cooling and contraction it reduces the overall length of the package, compared with the natural length (as above defined) of any fibre or fibres within it.

The extrudate may comprise a central core with a plurality of longitudinally extending fins radiating from it to join the outer wall of the extrudate, the channels being formed between adjacent fins. The fins may have any suitable shape of 30 cross-section and any convenient number of fins and channels may be provided depending upon the diameter of the cable and the number of optical fibres required to be accommodated.

Where the extrudate has a central core 35 the strength member or members, where provided, may be embedded within the core, the member or members then conveniently acting as a carrier on to which the 204647 _I4_ package is extruded during manufacture. However the core may alternatively be hollow and provide an additional channel for accommodating one or more optical fibres. One or more strength members may then 5 be incorporated if required in another part or other parts of the extrudate, for example in the outer wall or one or more of the fins.

The extrudate can, however, take alternative extrudable forms with the closed 10 longitudinally-extending channels shaped and disposed in.any other convenient manner. Sharp angles within the channels into which a fibre could become wedged are preferably avoided.

The extrudate is conveniently formed of a 15 readily extrudable insulating material such as polyethylene, polypropylene or pvc, but any other suitable insulating material capable of being extruded could alternatively be employed.

It is sometimes desirable to incorporate one 20 or more electrical conductors in an optical fibre cable, for example to provide electrical power to repeaters or to provide a monitoring facility.

Accordingly one or more electrical conductors may be accommodated in at least one of the cable channels.

If two or more such conductors are disposed within the same channel they should, of course, be mutually - . insulated, for example by being individually provided with insulating coatings or sheaths. However in some cases strength members themselves could be formed of, 30 or incorporate, a metal of suitably high electrically conductive properties and be utilised for this purpose. In the case of a steel wire employed as a strength member the wire may, for example, be copper clad to enhance the conductivity of the member. 35 A cable may comprise a plurality of packages as above described each accommodating one or more optical fibres, and formed into a composite cable, for example by being wound in helical fashion. 204 <=> -a* In some cases the units may be wound around a common central strength member and in such a case the individual packaging units need not themselves incorporate a strength member within the central core, 5 although it may, in some instances, be desirable to provide a central carrier for the packaging unit to extrude on to. Such a carrier, as it is not required to provide any substantial degree of tensile strength, may be in the form of one or more glass 10 fibres, not necessarily of the optical type, or a thin metal wire, although other materials could alternatively be employed for this purpose.

According to another aspect of the invention in a method of forming an optical fibre 15 cable a package of extrudable insulating material is formed, by extrusion, with a plurality of longitudinally-extending channels, and one or more optical fibres are introduced into at least one of the channels during the extrusion process, the rate 20 of introduction of the fibre or fibres being such that in the finished cable the fibre or fibres is/are prevented from being subjected to any excessive tensile stress .

During manufacture the package may be 25 twisted following the extrusion process such that any channels spaced from the package axis, and hence the optical fibre or fibres accommodated therein, are disposed in a helical fashion about said axis. Such cable has the advantage that when it is bent 30 virtually no compression or extension of the optical fibre(s) occurs, since the or each optical fibre is approximately evenly distributed about the neutral axis of the cable so that tension on a section of optical fibre on the outside of the bend is relieved 35 by slack in an adjacent section of the fibre on the inside of the bend. The shorter the pitch of the helically wound fibrc(s), the smaller the bend radius that can be accoinmodated in this manner. However the 20464 pitch must not be so short that the minimum allowable radius of curvature of the fibre(s) is approached, or that friction between the fibre(s) and core prevents the core from sliding over the fibre(s) when the cable 5 is bent. In practice a relatively shallow pitch has been found to represent the best compromise (typically, for a cable of 10mm diameter, 0.5m). In a commonly employed refinement.of this cable construction, the fibre pitch is periodically reversed at intervals as 10 short as half a turn, so that the or each fibre has approximately a sinusoidal configuration projected onto a cylindrical surface coaxial with the package.

This construction allows the cable to be fed to a take-up spool without having to be untwisted. A 15 further advantage is that it allows torsional strains to be accommodated, since a twist in a given direction will respectively wind up and unwind oppositely pitched adjacent channel sections or vice versa.

It will be understood that the terra "helical" 20 when applied to optical fibres and cables in accordance with the invention includes not only helices wound with a continuous sense, but also helices wound with a periodically reversing sense with a period as short as half a turn (i.e. a cylindrical sinusoidal 25 configuration) and other configurations sufficiently similar to these to confer strain relieving properties onto an optical fibre within an appropriate optical cable.

In the manufacture of such a cable, hot 30 thermoplastic material is continuously extruded from a die member and conveyed to a cooling zone from which the developing package emerges in a set but flexible condition, one or more interior channels parallel to but displaced from the package axis are 35 continuously formed within the package and at least one optical fibre is continuously conveyed into a said channel by a delivery tube which extends within the channel downline through the cooling zone at 204647 least to a point at which the package is sufficiently cool to prevent any appreciable contamination of the optical fibre surface or adhesion of the optical fibre to the channel wall, and the hot 5 section of the developing package is twisted by first torsional gripping means rotating about its axis to form said channel or channels into said helical configuration.

The or each channel is preferably formed by 10 an extrusion over a second tube forming part of the die-member. The delivery tube is preferably flexible, allowing the direction of rotation of the torsional gripping means to be periodically reversed, and of sufficiently small outside diameter to avoid contact 15 with the hot (and therefore sticky) sections of the channel walls. The delivery tube may be made of stainless steel and may be of the type used in hypodermic syringes for example. Alternatively the delivery tube may be in the form of a small diameter 20 closely wound helical spring, to give increased flexibility.

The cooling zone is preferably located intermediate the die member and torsional gripping means .

The torsional gripping means may comprise a pair of opposed spring loaded grip-rollers and may either oscillate torsionally to produce a cylindrical sinusoidal optical fibre configuration or may rotate continuously about the cable axis. Second torsional 30 gripping means may grip the package downline of the first torsional gripping means but upline of pulling means arranged to pull the completed package assembly. In the former case the second torsional gripping means may torsionally oscillate in phase with, but 35 with a larger amplitude of oscillation than, the first torsional gripping means and in the latter case may rotate slightly faster than the first gripping means. 2. ,-i \J -41 O In each case the channels are temporarily twisted (and therefore extended) to a slightly greater extent than the fibres within them, and since the core is elastic at this stage the core and channels untwist 5 slightly after passing through the second gripping means and allow the optical fibres to slacken off within the channels. As a result the fibres are not held tautly against the radially innermost surface of their associated channels, but with suitable control 10 of the degree of overtwist introduced by the second gripping means may be arranged to lie on the neutral axes of their associated channels. As a result the fibres in the completed cable are virtually unstrained (and therefore free of microbends) and may slide freely to accommodate bends in the cable. Alternatively, as mentioned above, the optical fibres may be fed at a fractionally greater rate than the speed of travel of the package, thereby achieving a similar effect to that produced by overtwist. In 20 order to further facilitate movement of the fibres within the channels, a solid lubricant such as talc may be coated onto the optical fibre surfaces.

Several different optical cables in accordance with the invention will now be described 25 by way of example with reference to Figures 1 to 9 of the accompanying schematic drawings, in which Figure 1 represents a transverse cross-section of one form of cable in accordance with the invention, Figure 2 represents, diagrammatically, one form of apparatus for manufacturing such a cable, Figures 3 and 4 illustrate, also diagrammatically, parts of two different forms of apparatus, and Figures 5 to 9 represents transverse cross- sections of a number of different cables.

Referring first to Figure 1 the cable comprises a package 1 formed as an extrusion of, for 2046 example polyethylene, and comprising a central core 2 having a plurality, in this case four, longitudinally extending, uniformly spaced fins 3 relating therefrom and joined at their outer ends to 5 a circular-sectioned outer wall Each of the channels 5 formed between adjacent fins 3, with the exception of one, contains two optical fibres 6, and the remaining channel contains a pair of electrical conductors 7, for 10 example of copper, mutually insulated from each other.

Both the optical fibres 6 and the conductors 7 are loosely accommodated within the respective channels 5, and the core 2 contains, embedded therein, a strength member 8 of helically 15 wound steel strands.

The fibres 6, the conductors 7 and the strength member 8 are introduced into the cable during the extrusion process, which will now be described with reference to Figure 2.

The optical fibres 6, the conductors 7 and the strength member 8 carried by respective drums 6A, 7A, 8A are led through guides (not shown) into an extruder head 11 from which a polyethylene package 1 of the shape shown in Figure 1 is extruded on to the 25 strength member 8, the fibres 6 and the conductors 7 being guided into the respective channels that are formed.

From the extruder head 11 the cable is drawn by a belt capstan 13 into a water trough 12 to 30 harden the polyethylene and is finally wound on to a take-up drum 14.

Means (not shown) are employed to ensure that the fibres are slightly longer than the surrounding package, so as to avoid them being 35 subjected to any excessive tensile stress.

The cable may be twisted periodically in opposite directions to give the channels 5, and hence 204647 the optical fibres 6 accommodated therein a reversing helical configuration, and one form of apparatus for achieving this is illustrated in Figure 3.

In use of this apparatus polypropylene at a temperature of approximately 200°C is extruded from an extruder 21 to form a package 22 of approximately 10mm diameter which is drawn through a water bath 23 by a conventional tractor 24.

The extruder comprises a conventional die component 25 and a pierced point component 26.

Molten polypropylene is forced through the annular gap between components 25 and 26 as shown by the arrows ) and simultaneously a central reinforcing cable 27 is drawn through a central hole in the point component 26. The point component is provided with a plurality of tubes 28 of approximately 2mm external diameter (only two of which are shown) symmetrically disposed about the central hole, through which thin flexible steel 20 delivery tubes 28a extend into the developing core to a position 29 in the water bath 23 at which the temperature of the core is approximately 80°C.

Coated optical fibres 30 of 250/ira diameter are positively fed through the flexible tubes 28a from a 25 reel 31 and emerge at position 29 into channels 32 formed in the package by the tubes 28. The channels 32 and all points downline of position 29 are at temperatures below 80°C, which has been found to be sufficiently cool to prevent contamination or 30 adhesion to the channel walls of the optical fibre surfaces. The tubes 28a can be slid in and out so as to terminate at the most suitable position in the ^ water bath in response to the particular extrusion rate, extrusion temperature and ambient conditions 35 obtaining.

Downline of the water bath 23 torsional gripping means 39 comprising a pair of opposed spring-loaded grip rollers 33 mounted on an * ■*-; 2046 * annular gear 34 grip the cooled cable. The torsional gripping member 39 is torsionally oscillated by a pinion drive from a reversible motor 35 and continuously periodically twists the entire 5 section of cable between the die component 25 and rollers 33 as shown by the curved arrow A. Annular seals 36 in the walls of the water bath provide the necessary torsional freedom of movement for the core assembly. The section of the package lying 10 within the water bath is too cool for significant plastic deformation to occur, but a significant > permanent periodically reversing twist is imparted to the hot section 37 of the package emerging from the J die component 25. The projecting ends of the flexible tubes 28a follow the oscillations of the package but make contact only with the cool sections of the channels 32 (which in this Figure are shown straight for the sake of clarity but which are in fact formed in a cylindrical sinusoidal configuration). 20 The twist imparted to the channels 32 implies that the optical fibres 27 need to be fed at a rate slightly greater than the rate of feed of the package, particularly if the fibres 30 are to lie on the neutral axes of the channels 32. This is achieved 25 by providing a stepped portion 47 on the reel 31 of slightly reduced (with respect to the neutral axis of the reinforcing cable) diameter (shown greatly "— exaggerated for the sake of clarity) over which the cable 27 runs and drives the reel. The speed of 30 rotation of the reel 31 is measured by a tachometer 38 and used to control the rate of reversal of a motor 35 and hence the pitch of the sinusoidally disposed channels 32.

Figure 4 illustrates part of apparatus for 35 achieving an alternative method of "overfeeding" the optical fibres into the cable. The arrangement is generally similar to that shown in Figure 3, except » / > / 204^4/ that second torsional gripping means 39a is provided and the optical fibres 30 are not positively fed into the channels but are drawn in with the package. This is achieved by initially temporarily 5 gripping the walls of the channels 32 (indicated diagrammatically) against the fibres at a point (not shown) downline of the gripping assembly 39a. The torsional gripping means 39a torsionally oscillates in phase with, but with a greater amplitude than the 10 gripping means 39 in response to the rate of feed of the reinforcing cable 27 as determined by the tachometer 38. This control is indicated dia-grammatically by the arrowed lines connected to tachometer 38. The section of package material 22 15 between gripping means 39 and 39a is elastic and therefore partially relaxes its twist after passing downline of the latter. The optical fibres 30 are not of course appreciably elastic and are drawn into the package at a slightly enhanced rate by the 20 oscillations of gripping means 39a. The net result is that the fibres are somewhat slacker downline of the gripping means 39a and the degree of extra slack can be controlled by controlling the amplitude of this oscillation to cause them to lie approximately 25 on the neutral axes of the channels 32.

The sinusoidal configuration of the channels 32 and flexure of the tubes 28 is indicated by dotted lines in Figure 4 and a similar configuration would be achieved by the apparatus of 30 Figure 3- In each case the fibres 30 may be passed through a vibrating tray of solid lubricant such as talc before being fed into the core. It will be apparent that in each case the or each torsional gripping means 39, 39a may rotate continuously in one 35 direction to achieve a unidirectional helical configuration of the channels 32 (so that the tubes 28a need not be particularly flexible), the "overfeed" 2 04 6 effect in Figure 4 then being achieved by rotating the gripping means 39a slightly faster than the gripping means 39.

Figure 5 shows an alternative form of 5 cable comprising a number, for example six as shown, of cable units 10 each similar in construction to that illustrated in Figure 1 but of smaller diameter, and having in place of the central strength member 8, a single wire or fibre 15 which provides a carrier 10 on to which the respective package is extruded, optical fibres being similarly introduced into the channels 5 formed between adjacent fins 6 during the extrusion process.

The individual cable units are formed 15 substantially as described with reference to Figures 2, 3 or 4, and are then wound helically around a common insulated strength member 16 utilising a known cable winding technique. The central strength member can again be formed from helically wound 20 steel strands, conveniently coated with polyethylene or polypropylene.

The whole assembly may be surrounded by an outer protective sheath.

In some cases the individual carriers 15 25 may be dispensed with.

In a modification each of the individual packages may incorporate a strength member as in the case of the cable described with reference to Figure 1; the individual units may be wound round 30 a common central strength member, as in the Figure 5 embodiment, or may simply be wound together in helical fashion with the common central strength member being omitted.

Typically the diameter of the cable 35 illustrated in Figure 1 would be of the order of 10mm, although each of the units forming the cable illustrated in Figure 5 could have a diameter of only 204647 3mm. However the size will, of course depend on the construction and the number of optical fibres which need to be accommodated.

Whilst the cable units of Figures 1 and 5 5 have been shown with four fins any other convenient number, for example five or six, may be employed, and instead of polyethylene other extrudable plastics material could be used to form the package.

One such cable having six fins 3 radiating 10 from a central core 2 is illustrated in Figure 6, this cable being formed in a similar manner to that of Figure 1. In addition the core 2 of this cable has embedded within it a plurality of strength members 8, for example two as shown, of helically 15 wound copper clad steel strands which provide conductors for the transference of electrical power.

A single strength member may take the place of the plurality of strength members in the core 2 if desired, and one or more electrical 20 conductors may be accommodated in at least one of the cable channels 3.

In a modification of the cable illustrated in Figure 6 the core 2 is made hollow, as shown in Figure 7, and it may then accommodate one or 25 more optical fibres as at 6.1. One or more strength members, which may or may not be formed of or incorporate a metal having good electrically conductive properties, so that they can be used for the transference of electrical power, are then 30 embedded in another part of the extrudate, for example the outer wall as shown at 17. A further modification is illustrated in Figure 8.

An alternative form of cable is illustrated in Figure 9. This again comprises an 35 extrudate 1 of polyethylene or polypropylene having a central core 2, in which is embedded a strength member 8, and provided with fins 3 radiating from it 204.4 m as in the first embodiment. This cable, however, incorporates an inner wall 18 spaced radially from the core 2 and the outer wall 19 to provide an inner and an outer series of channels 5.1, 5.2 5 respectively. Some or all of the channels accommodate one or more optical fibres as at 6.

Electrical conductors may alternatively be provided in one or more of the channels, or may replace the single strength member 8. In a 10 further modification the core 2 is hollow and incorporates one or more loosely fitting optical fibres or conductors, one or more strength members, if needed, then being incorporated in the part or parts of the extrudate other than the core. 15 The invention is not restricted to cables having the forms of extrudate illustrated and other shapes of extrudate formed with longitudinally extending channels of any convenient shape of cross section into which optical fibres, and possibly 20 electrical conductors, are introduced during the extrusion process, could alternatively be employed.

For example in a further modification in which a cable has a plurality of channels spaced 25 around a central core with the outer wall of the package forming the walls of the channels, at least one of the channels contains an elongate member capable of being drawn radially outwards from an end of a length of cable to split the respective section 30 of the wall and provide access to any fibre or fibres within the respective channel for jointing purposes. Alternatively at least one section of wall bounding a channel may be formed with a region extending along the cable which is of thinner cross-35 section than the' remainder of the wall, enabling it to be more easily cut to provide access to any fibre or fibres within the channel, and in some cases 2 u4o it may be formed with at least one flap closing the respective channel but openable to provide access to any fibre or fibres within it.

Claims (27)

204647 -17- WHM

1. An optical fibre cable comprising an insulating ■X 10 15 20 25 30 i package formed as an extrusion with a plurality of longitudinally extending closed channels, and one or more optical fibres loosely accommodated in at least one of the channels, the fibre or fibres having been introduced into the channel or channels during the extrusion process and each having a natural length which is greater than the corresponding length of the channel in which it is accommodated.

2. An optical fibre cable according to Claim 1 wherein the package incorporates one or more strength members.

3. An optical fibre cable according to Claims 1 or 2 wherein the package comprises a central core with a plurality of longitudinally extending fins radiating from it to join the outer walls of the package, the channels being formed between adjacent f ins.

4. An optical fibre cable according to Claim 3 having one or more strength members embedded within the core.

5. An optical fibre cable according to Claim 3 wherein the core is hollow and provides a channel accommodating one or more optical fibres or electrical conductors.

6. An optical fibre cable according to any preceding Claim wherein the package consists of polyethylene, polypropylene or pvc.

7. An optical fibre cable according to any preceding Claim having one or more electrical conductors accommodated in at least one of the channels.

8. An optical fibre cable according to Claim 2 having at least one strength member formed of copper clad steel wire. V 204647 -18-

9. An optical fibre cable comprising a plurality of cables according to any preceding Claim wound helically around a central strength member.

10. An optical fibre cable according to any pre-5 ceding Claim in which at least one channel bounded by the outer wall of the package contains an elongate member capable of being drawn radially outwards from an end of a length of cable so as to split the respective section of wall to provide access to any fibre or fibres 10 within it.

11. An optical fibre cable according to any one of Claims 1 to 9 wherein the part of the outer wall of the package which bounds each said channel is formed with a region extending along it of thinner cross-section 15 than the remainder.

12. An optical fibre cable according to any one of Claims 1 to 9 wherein at least one section of the outer wall of the package which bounds a channel incorporates a flap which closes the channel but is openable to 20 provide access to any fibre or fibres within it.

13- A method of manufacturing an optical fibre cable wherein a package of extrudable insulating material is extruded, with a plurality of longitudinally extending closed channels, and one or more optical fibres are intro-25 duced into at least one of the channels during the extrusion process so as to be loosely accommodated therein, the rate of introduction of the fibre or fibres being such that in the finished cable each fibre has a natural length greater than the corresponding length of the channel in 30 which it is accommodated.

14. A method of manufacturing an optical fibre cable according to Claim 13 wherein the fibre or fibres v is/are introduced into the channels at a rate greater than the rate of extrusion. 35 15. A method according to Claim 13 in which ■ . v

~ SAUGI986' 204647 -19- the package is twisted following the extrusion process such that any channels spaced from the package axis and the optical fibre or fibres accommodated therein are disposed in a helical 5 fashion about the axis.

16. A method according to Claim 15 comprising continuously extruding from a die member a package of hot plastics material having a plurality of interior channels parallel to but spaced from the 10 package axis, conveying the package to a cooling zone from which it emerges in a set but flexible condition, conveying at least one optical fibre into a said channel by a delivery tube which extends along the channel from the die member through the 15 cooling zone at least to a point at which the plastics material is sufficiently cool to prevent any appreciable contamination of the optical fibre surface or adhesion of the optical fibre to the channel wall, and twisting the hot section of the developing 20 package by a torsional gripping means rotating about the axis of the package to form the channels into said helical configuration.

17- Apparatus for carrying out the method of Claim 16 comprising a die member from which a 25 package of hot plastics material, having a plurality of interior channels parallel to but spaced from the package axis can be continuously extruded, means for conveying the package through a cooling zone, at least one delivery tube which is arranged to lie 30 within an associated channel for conveying an optical fibre into the channel and which extends from the die member through the cooling zone, and torsional gripping means and means for engaging the package and capable of rotation about the axis of the 35 package so as to twist the hot section of the package and form the channels into said helica"1 configuration. 204647 -20-

18. A method according to Claims 15 or 16 wherein the direction of twist is periodically reversed at intervals of not more than half a turn so that the or each fibre has a substantially 5 sinusoidal configuration projected on to a cylindrical surface coaxial with the package.

19. Apparatus according to Claim 17 for carrying out the method of Claim 18 wherein the or each said delivery tube is in the form of a flexible 10 metal tube.

20. Apparatus according to Claim 17 for carrying out the method of Claim 18 wherein the or each said delivery tube comprises a closely wound helical spring. 15 21. Apparatus according to any one of

Claims 17, 19 or 20 wherein the cooling zone is located intermediate the die member and the gripping means .

22. Apparatus according to any one of 20 Claims 17, 19, 20 or 21 wherein the torsional gripping means comprises a pair of opposed spring loaded rollers.

23- Apparatus according to any one of Claims 17, 19, 20, 21 or 22 including means for 25 oscillating the gripping means about the axis of the package.

24. A method according to Claim 18 wherein the package is given a degree of overtwist, and is subsequently allowed to relax slightly before the 30 plastics material has set.

25. Apparatus according to Claim 23 for carrying out the method of Claim 24 including second torsional gripping means arranged to grip the package downline of the first torsional gripping 35 means but upline of pulling means arranged to pull the completed package assembly, and means for torsionally oscillating the second gripping means in phase with but with a larger amplitude than the 204647 -21- first gripping means.

26. A method according to any one of Claims 13 to 16, 18 or 24 wherein a solid lubricant is applied to the fibre surfaces prior 5 to the extrusion process. to 16, 18, 24 or 26 of manufacturing an optical fibre cable according to Claim 4 wherein the or each strength member is heated prior to the extrusion 10 process such that on cooling it results in a reduction in the overall length of the package. 28. A method of manufacturing an optical fibre cable carried out substantially as hereinbefore described with reference to Figures 2, 3 or 4 of 15 the accompanying drawings. 29. An optical fibre cable substantially as shown in and as hereinbefore described with reference to Figure 1 or any one of Figures 5 to 9 of the accompanying drawings .

27. A method according to any one of Claims 13 Kr h^tholr authorised Ajcstt*, 5 AUG 1986^