GB2180190A

GB2180190A - Production of optical cable

Info

Publication number: GB2180190A
Application number: GB08614645A
Authority: GB
Inventors: Ronald Yaxley Gill
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1985-09-11
Filing date: 1986-06-16
Publication date: 1987-03-25
Also published as: GB2180190B; GB8522522D0; GB8614645D0

Abstract

A process for producing optical cable having a central strength member (1) of non-conducting material surrounded by a grooved core (4), in which a layer (3) of thermoplastic material softenable by dielectric heating is interposed between the strength member and the core, and the assembly is passed through a dielectric heating unit and twisted while the intermediate layer is in a softened state to cause the grooves to take up a helical or periodically reversing helical path, subsequent cooling leaving the grooves extending in a helical manner about the core axis. <IMAGE>

Description

SPECIFICATION Production of optical cable This invention relates to processes and apparatus for the production of optical cables, that is to say cables incorporating one or more optical fibres within a protective cover, and relates especiallyto the production of such cables which are of the kind incorporating an extruded core ofthermoplastics material having series of longitudinally extending channels, at least one of which accommodates one or more of said op ticalfibres,the core being surrounded in the completed cable, by an outer sheath.

Commonly the channels extend in a helical manneraboutthe core axis, either in theform of a continuous helix or one in which the pitch ofthe helix is periodically reversed, the fibre or fibres being loosely accomodated within the respective channel or channels, which may either be completely closed orbeformed as grooves in the peripheryofthecore.

Hitherto the formation of the helical channels in the core has been effected by twisting the core while it is still in a softened condition after leaving the extruder, but in the case where the channels are provided by grooves in the periphery ofthe core this can give rise to difficulties in introducing the fibres into the grooves at some subsequent stage of manufacture.

In many cases such cables employ a central strength member of electrically conductive material, for example stranded metal wires, and in copending patent application No. 8518691 there is described a processforthe production of optical cable ofthe kind referred to, utilising such a strength member, in which the core is initially formed with straight channels, and after the introduction of the fibres the core is passed through an induction heater arranged to heat the strength member sufficiently to soften at least the adjoining region ofthe core, and the core is then twisted to cause the channels to take up a helical or periodically reversing helical path, the core being subsequently cooled to leave the channels extending in a helical manneraboutthecoreaxis.

Such a process has the advantage that the optical fibres can be introduced into the channels while they are straight, thereby avoiding the difficulty of feeding fibres into helically extending grooves.

However there are certain conditions in which it is not permissible to utilise a metal strength member for an optical fibre cable,forexample adjacent to an electrified railway track. Under these conditions a strength member of non-conducting material for example "Kevlar" (R.T.M.) is commonly employed, and the above technique of forming helically-extending channels in the core ofthe cablecannotthen be used.

According, therefore, to the present invention, in a process for the production of optical cable of the kind referred to having a central strength member of nonconducting material, an intermediate layer of thermoplastic material is formed around the strength member between the latter and the grooved core, and the assembly is passed through a dielectric heater unit arranged to heat the layer sufficientlyto soften it, the core is then twisted to cause the chan- nels to take up a helical or periodically reversing helical path, and is subsequently cooled to leave the channels extending in a helical manneraboutthe core axis.

The core material must, of course, be one which remains relatively unaffected by the dielectric hating process during the softening ofthe intermdeiate layer, and is convenientlyformed of polyethylene or polypropylene with an intermediate layer of polyurethane or PVC.

As in the case ofthe cable described in said copending patent application, the channels may be formed in the core by the extrusion process, although the core may be initially extruded with closed channels andthegroovesformed byslitting the outer wall ofthe core to provide access to the channels. The cable can be completed, following the twisting and cooling processes, by surrounding the core by one or more outer coverings in known manner, eitheras part of a continuous process or as a completely separate process.

One process in accordance with the invention will now be described by way of example with reference to Figures 1 and 2 ofthe accompanying schematic drawings, in which Figure 1 shows, in diagrammatic form, apparatus for carrying outthe process, and Figure 2 illustrates the cross-section of one form of cable core suitable for carrying out the process.

Referring to the drawings, a strength member 1 of Kevlar (R.T.M.) or other non-conducting material is pulled from a drum 1 Othrough an extruder head 2 which deposits around the strength member a layer3 of PVC or polyurethane, and around the layer3 a core 4 of polyethylene or polypropylene, the extruder head 2 being equipped with tooling to produce a multi-grooved core section, for example as shown in Figure 2.

The core assembly, comprising the plastic core section 4, with the embedded strength member 1 and the intermediate layer3, is then passed through a cooling trough Sofa length sufficient to harden the thermoplastics materials ofthe core and intermediate layer, and is subsequently passed through a dryer 6 to removewatertherefrom.

The assembly then passes to a capstan 7 which is arranged to draw the assembly from the extruder, and whose speed is linked to that of the extruder scroll, both being controlled by a control unit 8 in response to signals from a diameter sensing device 9.

A plurality of optical fibres 11 are drawn from reels 12 through multi-head fibre colouring and drying units 13 and are fed by suitable guide means, shown diagrammatically at 14, into the axiallyextending grooves 15 in the core section which is secured against rotation at this position, the fibres being fed into the grooves at a rate sufficient to ensure that they are of greater length than the channels into whichtheyarefed.

The core assembly, now accommodating the optical fibres 1 1,then passes to a cotton whipping head 16which applies a helical open lay binding (not shown) around the assembly for retaining the optical fibres 11 within the respective grooves 15.

The assemblythen passes through a dielectric heating unit 17 which is suitably controlled to heat the intermediate layer 3 sufficiently to soften it. From the dielectric heating unit 17 the assembly passes to a unit 18 which is arranged to grip the core 4, and apply an oscillatory motion to it about the core axis, thus imparting to the core 4, around the softened in- termediate layer3,and hence to the grooves 15 and fibres 11, a periodically reversing helical lay.

At a distance from the dielectric heating unit 17 suf ficient only to allow the softened plastic of the inter- mediate layer3to harden, the cable is wound around a largediametercapstan unit 19 driven at a rate sufficientto apply tension to the core assembly.

The heating ofthe intermediate layer3 asthecore assembly passes through the heating unit is controlled so that the layer is softened sufficientlyto permit the core 4to be twisted, the subsequent cooling and solidification causing the core to be held in the twisted condition.

Instead of the core being extruded with grooves formed by the extrusion process, the core may be extruded with completely closed channels, and the grooves formed subsequently by cutting away strips ofthe outerwall along the central region of each of the channels.

Furthermore the invention could also find use in the manufacture of cables in which the core is formed with closed channels for accommodating one or more optical fibres, the fibre or fibres in such a case being introduced into the respective channel or channels during the extrusion process.

Ajelly-like or powdered substance having waterrepel lent properties, may be introduced into the channel or channels containing one or more optical fibres, the substance being such as to permit a de greeofmovementofthefibre orfibres as the cable is bent.

In one or more of the grooves the optical fibres may be replaced by one or more polyethylene insulated electrical conductors.

The invention includes within its scope optical fibre cable having a central strength member of nonconducting material, an intermediate layer of dielectrically heatable thermoplastic material surrounding the strength member and an outer core of thermoplastic material relatively unaffected by dielectric heating surrounding the intermediate layer, and having a plurality of helically extending grooves containing one or more optical fibres.

Claims

1. A process for the production of optical cable of the kind referred to having a central strength member of non-conducting material, wherein an intermediate layer of thermoplastic material is formed around the strength memberbetweenthelatterand the grooved core, and the assembly is passed through a dielectric heater unit arranged to heatthe layer sufficiently to soften it, the core is then twisted to cause the channels to take up a helical or periodically reversing helical path and is subsequently cooled to leave the channels extending in a helical manneraboutthe core axis.

2. A process forthe production of optical cable according to Claim 1, wherein the core material remains relatively unaffected by the dielectric heating process during the softening of the intermediate layer.

3. A process for the production of optical cable according to Claim 2, wherein the core material is formed of polyethylene or polepropylene and the intermediate layer is formed of polyurethane or PVC.

4. A processforthe production of optical cable according to Claim 1,2 or3,wherein the channels are provided by grooves in the periphery ofthe core, which grooves are formed in the core by the extrusion process.

5. A process for the production of optical cable according to Claim 1,2 or3, in which the channels are provided by grooves in the periphery ofthe core wherein the core is initially extruded with closed channels and the grooves formed by slitting the outerwall of the core to provide access to the channels.

6. A process forthe production of optical cable according to any preceding claim, wherein the cable is completed by surrounding the core by one or more outer coverings.

7. A process for the production of optical cable substantially as herein described with reference to Figures 1 and 2 ofthe accompanying drawing.