Manufacture Of Multiconductor Cables

Abstract

A multiconductor cable core is provided with a barrier of sealing material by pumping the sealing material from a thermostatically controlled storage vessel to a cable feeding station while at a temperature just above that at which crystallization begins and transferring sealing material from the cable feeding station to the cable core while still at such a temperature. The material is cooled to effect crystallization by abstraction of heat by the insulated conductors constituting the core to cause sufficient sealing material to become solidified to form an effective moisture barrier. The sealing material while at such a temperature is preferably continuously circulated around a ring main in which is located a plurality of cable feeding stations for transferring the material to a core curing a conventional stranding process.

Description

This invention relates to telecommunication cables of the kind comprising a multiplicity of plastics insulated conductors enclosed within a waterproof sheath. If such cables, whether buried in the ground or drawn into ducts, become locally damaged to an extent to allow water to enter through the damaged sheath, the water will travel along the cable through the interstices between the insulated conductors and between the conductors and the sheath and so will have an adverse effect upon the electrical characteristics of the whole cable length, for instance by shorting of circuits via pinholes in the wire insulation.

With the object of preventing water that has entered through a defective sheath or joint casing from travelling even a short distance along the cable from the point of entry it has been proposed to fill the interstices between the insulated cable conductors and between them and the cable sheath throughout the entire length of the cable with water impermeable filling material.

For forming a continuous barrier throughout the length of a cable it is desirable to use a waterproof impermeable material which will not drain under the influence of gravity or such hydrostatic pressure as may arise in the event of damage to the cable sheath or joint casing but which will permit relative sliding movement of the plastics insulated conductors or group of conductors over one another during such bending of the cable as occurs during manufacture and installation of the cable. Examples of such materials are:

A. Microcrystalline petroleum waxes

B. Mixtures of microcrystalline petroleum waxes and oils, for instance petroleum jelly

C. Low molecular weight, high Malt Flow Index polyethylene or polypropylene of a semisolid or greaselike nature

D. Mixtures of petroleum jelly, microcrystalline petroleum waxes, polyethylene, polysioutylene and aluminum stearate.

E. A BLEND OF TWO OR MORE OF FILLING MATERIALS (A.) TO (D.).

The choice of filling material is to some extent limited by the method of introducing the filling material into the cable core and by the nature of the dielectric of the conductors forming the core.

In the manufacture of plastics insulated multiconductor cables that are filled throughout with water impermeable filling material, it has been considered necessary to transfer such material from a storage vessel to the cable and to apply it to the cable under superatmospheric pressure by some form of pump. It is difficult however to pump such material in its solid condition because almost inevitable cavitation will occur within the storage vessel and cause a break in the feed to the pump. If pumping in such condition can be achieved, at the required rates of flow, for example 100 ft./min. (30.7 m./min.) at 4 gallons/min. (18.2 litres/min.) in a pipe having a cross-sectional area of 1 sq. in. (6.5 sq. cms. ), some breakdown of the crystal bonds in the material will result thus degrading the material by lowering its viscosity, so rendering it less capable of forming a barrier permanently resistant to water under pressure.

Degradation of the material occasioned by pumping is reduced if pumping takes place while the material is at a temperature just above the temperature at which crystallization begins and it has been proposed to provide a cable core with a barrier of sealing material by pumping the sealing material from a thermostatically controlled storage vessel to a cable feeding station while at a temperature just above that at which crystallization begins and cooling the material to effect crystallization while transferring it from said cable feeding station to the cable core. We have now found that it is not necessary to cool the sealing material while it is transferred from the cable feeding station to the cable core and that a core can be provided with a satisfactory barrier of sealing material if the sealing material is transferred to the cable core while the material is still at a temperature just above that at which crystallization begins and is cooled to effect crystallization by abstraction of heat by the insulated conductors constituting the core.

Accordingly the present invention consists of a method of providing a multiconductor cable core with a barrier of sealing material which comprises pumping the sealing material from a thermostatically controlled storage vessel to a cable feeding station while at a temperature just above that at which crystallization begins, transferring the sealing material from the cable feeding station to the cable core while still at such a temperature, and cooling the material to effect crystallization by abstraction of heat by the insulated conductors constituting the core to cause sufficient sealing material to become solidified to form an effective barrier to the passage of moisture along the core.

The rate of delivery of sealing material to the core may be adjusted in relation to the linear speed of travel of the core but preferably the rate of delivery of sealing material is such that, for any linear speed of travel at which the core is required to travel, sufficient material will be applied to the core to form an effective barrier when it becomes solidified.

By a temperature just above that at which crystallization begins is meant a temperature at which the sealing material is in a sufficiently liquid state to permit pumping of the material to be effected with substantially no degradation of the material occurring. Where, as is preferred, the sealing material is a mixture of microcrystalline petroleum waxes and oils, such as petroleum jelly, the temperature at which crystallization begins is approximately 70.degree. C. and the temperature just above the crystallization temperature at which the material can be pumped without causing substantial degradation is approximately 76.degree. C.

The invention also includes apparatus for carrying out the aforesaid method comprising a thermostatically controlled storage vessel for storing the sealing material while at a temperature just above that at which crystallization begins, the vessel being connected to one or more than one cable feeding station having means for applying sealing material to the cable core and, connected between the storage vessel and the applicator means of the or each cable feeding station, means for withdrawing sealing material from the storage vessel and for controlling the rate of delivery of the material to the applicator means.

It is preferred to have a feeding station at each stranding point and to transfer sealing material in a liquid state to the core at, or shortly in advance of, the closing die of the stranding head, in which case the number of feeding stations in operation will be the same as the number of layers of conductors, pairs or quads laid up around the center conductor, pair or quad, but where the first layer is laid up around a group of two or more conductors, pairs or quads it may be advantageous to have a feeding station at the point where these two or more conductors, pairs or quads are brought together to form a core center. A further feeding station may be located at the outlet end of the final closing die but we have found that this is not generally necessary.

Preferably the sealing material is applied to the core through an applicator die constituting or forming a separable part of the closing die associated with a stranding head and in this case the sealing material is applied to the surface of the last-formed layer of conductors, pairs or quads, to the surface of each conductor of the layer of conductors, pairs or quads being brought down by the closing die onto said last-formed layer, and to the surface of the layer of conductors, pairs or quads formed at said closing die, thereby ensuring that the cable interstices are completed filled with sealing material. Each feeding station may have its own individual source of supply and surplus material (i.e. material that is not carried forward through the stranding head) in a liquid or semisolid state may either be returned to the storage vessel to be brought back to the requisite temperature in or on its way to such vessel or it may be collected for reprocessing by the supplier.

Alternatively there may be a common source of supply for some or all of the feeding stations at a machine or for some or all of the feeding stations at a number of machines. In such case sealing material is maintained in a thermostatically controlled storage vessel at a temperature just above that at which crystallization begins and is continuously circulated through a ring main which is thermally insulated and may be trace heated to maintain the circulating material at the appropriate temperature. From this ring main sealing material while still at such a temperature is bled off at intervals and fed to the feeding stations. Transfer of the sealing material to the cable core is preferable controlled by a valve at each feeding station but it may be controlled by a metering pump. Surplus material from the feeding stations cannot in this case be fed back into the ring main unless it is first brought back to the temperature of the circulating material. It will generally be preferable either to pump it back into the common storage vessel or to collect it for reprocessing. Preferably the sealing material is circulated in the ring main at a much higher rate than the aggregate rate at which it is withdrawn for transfer to the cable core, for example at a rate 5 to 10 times as great. Such circulation is preferably effected by a pump having an intake in a part of the storage vessel remote from the part to which the circulating material is returned.

The invention will now be described in more detail, and by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of apparatus suitable for use in providing a multiconductor cable core with a barrier of sealing material which extends throughout the length of the cable core, while the cable core is being manufactured by a conventional stranding machine,

FIG. 2 is an end view of an applicator die of the apparatus diagrammatically illustrated in FIG. 1,

FIG. 3 is a section on the line III--III in FIG. 2, and

FIG. 4 is a diagrammatic illustration of an alternative form of apparatus for providing a continuous barrier of sealing material in a multiconductor cable core.

Referring to FIG. 1, the apparatus comprises a storage vessel 1 for the sealing material thermostatically controlled in any convenient manner and, connected to the lower part of the vessel, a ring main 2 comprising an outward pipe 3, in which is connected a pump 5, and a return pipe 4. At each end of a plurality of locations spaced at intervals along the length of the return pipe 4, of which three only are shown, is a cable feeding station 11 at which sealing material at a temperature just above that at which crystallization begins and hence in a liquid state is continuously bled off from the ring main 2 and transferred to the cable core. Each cable feeding station 11 has a valve 12 for controlling the rated of flow of sealing material being bled off from the system 2 and a die 15 for applying the material to the cable core. The die 15 is detachably secured to the entry end of the closing die of a stranding head.

The storage vessel 1 and all piping of the ring main 2 are lagged and the piping is provided with steam trace heating. In the lower part of the storage vessel 1 there are provided stainless steel steam heating coils 7 for use when the sealing material in the vessel has been allowed to go solid, as for instance when commencing circulation of the sealing material in preparation for applying sealing material to a cable core. During circulation of the sealing material and application of the material to a cable core the sealing material in the thermostatically controlled storage vessel 1 is maintained in a liquid state by any suitable method.

At each feeding station 11 sealing material, which is being pumped around the ring main 2 in a liquid state, is continuously bled off and transferred in a liquid state into the applicator die 15. The rate of delivery of the sealing material in a liquid state to the cable core via each applicator die 15 is controlled by its associated valve 12. The sealing material is cooled to effect crystallization by abstraction of heat by the insulated conductors constituting the core to cause sufficient sealing material to become solidified to form an effective barrier to the passage of moisture along the core. Circulation of the sealing material through the ring main 2 will generally be maintained at a rate 5 to 10 times as great as the aggregate rate at which it is withdrawn from the ring main at the cable feeding stations 11.

Surplus material spewing from each applicator die 15 is returned to a collecting tank 16 where any loss of temperature may be compensated by trace heating the tank. From the collecting tank 16 the surplus material is pumped by a scavenge pump 18 back through a filter 17 into the upper part of the storage tank 1 from where, after it has been brought back to the temperature of the circulating material, it can be recirculated through the ring main 2. At each feeding station 11, a pipe 19 is provided for feeding sealing material directly from the ring main 2 into the collecting tank 16 when required, as for instance preparatory to starting up the sealing operation.

As will be seen on referring to FIGS. 2 and 3, each of the dies 15 by which the sealing material in a liquid state is applied to the cable core during its manufacture comprises a body 21 having a smoothly tapering bore 22 which is bellmouthed at its entry end and which, at its exit end, corresponds to or is somewhat greater than the circumscribing circle of the cable core passing through the die. The outer surface of the wall of the body 21 is stepped at three locations along its length to form three cylindrical portions, 23, 24 and 25. Portion 23 at the exit end of the die is externally screw threaded. In the portion 24 of the die wall are three ports 26 for entry of sealing material which are distributed uniformly around the axis of the die and are inclined to it so as to impart to the liquefied material a component of movement in the same axial direction as the direction of travel of the cable core passing through the die. A sleeve 27, which has an internal diameter corresponding to the external diameter of the cylindrical portion 25 and which has at one end an inwardly directed internally screw-threaded flange 28, is screwed on to the end portion 23 of the body 21 until the sleeve fits tightly over the cylindrical portion 25 and abuts the step thereof. A hole 29 in the wall of the sleeve 27 is bounded by an upstanding continuous wall 31 and provides access for flow of sealing material in a liquid state to an annular chamber 30 lying between the internal surface of the sleeve 27 and the cylindrical portion 24. The wall 31 bounding the hole 29 is internally screw threaded for connection to a pipe carrying sealing material from the feeding station 11. Sealing material in a liquid state is fed from the annular chamber 30 to the cable core passing through the die via the ports 26. As previously stated the die 15 is detachably secured to the entry end of the closing die of a stranding head.

The rate of flow of sealing material into each die 15 is at least sufficient to maintain a "pool" of material in a liquid state in the annular chamber 30 and in the bell-mouth of the die. Surplus material from each die 15, if any, is allowed to fall into its associated collecting tank 16 to be reheated and pumped back into the storage tank as previously described.

By way of example it is mentioned that in providing a multiconductor cable core comprising 100 pairs laid in six stranded layers with a continuous barrier of petroleum jelly having a drop melting point within the approximate range 73.degree. to 78.degree. C. using the method and apparatus described with reference to FIGS. 1 to 3; the petroleum jelly in a liquid state and at a temperature of approximately 76.degree. C. is delivered to the core at each of six applicator dies at a rate of approximately 2 gal./min. (9.1 litres/min.), the linear speed of travel of the core being approximately 225 ft./min. (69 m./min.).

Although in the foregoing description reference is made to the continuous filling of a single multiconductor cable core during its manufacture, it will be appreciated that the ring main may serve two or more adjacently sited cable-making machines.

In the alternative form of apparatus shown in FIG. 4, each stranding head of a conventional stranding machine has at its inlet end an applicator die 45 which is of a form similar to that shown in FIGS. 2 and 3 and at which sealing material in a liquid state is fed to a cable core from an individual thermostatically controlled tank 41 in which the sealing material is maintained in a liquid state in any suitable manner, for instance by heating coils 47. The sealing material in a liquid state is fed to the applicator die 45 through a pipe 42 constituting the cable feeding station, and filter 43 by a metering pump 44. Surplus material spewing from the applicator die 45 falls back into the tank 41 where it is reheated and pumped back to the applicator die. In advance of the applicator die 45 a pipe 46 is provided for feeding sealing material being pumped through the pipe 42 directly back into the tank 41 when required, as for instance preparatory to starting up the sealing operation.

As in the method using the first form of apparatus sufficient sealing material is caused to become solidified by abstraction of heat by the insulated conductors constituting the core to form in effective barrier to the passage of moisture along the core.

The process of fully filling a multiconductor cable in accordance with the present invention has the advantage that it eliminates substantially all mechanical working of the sealing medium--generally petroleum jelly--and so eliminates the separating out of oil from the wax which occurs under the action of shear brought about by pumping such material when in its solid state.

Claims

What I claim as my invention is:

1. A method providing a cable core comprising a multiplicity of insulated conductors with a barrier of sealing material which will not drain under the influence of gravity or such hydrostatic pressure as may arise in the event of damage to the sheath of the cable but which will permit relative sliding movement of the insulated conductors over one another during such bending of the cable as occurs during manufacture and installation of the cable, which method comprises the steps of:

a. pumping the sealing material from a thermostatically controlled storage vessel to a cable feeding station while at a temperature just above that at which crystallization begins,

b. transferring the sealing material from the cable feeding station to the cable core while still at such temperature, and

c. cooling the material to effect crystallization by abstraction of heat by the insulated conductors constituting the core to cause sufficient sealing material to become solidified to form an effective barrier to the passage of moisture along the core.

2. A method as claimed in claim 1, wherein the sealing material in the storage vessel is maintained at a temperature just above that at which crystallization begins and is continuously pumped from the storage vessel and circulated around a ring main to one or more cable feeding stations located in the ring main:

3. A method as claimed in claim 2, in which the cable core is being manufactured by the conventional stranding process employing a stranding machine having at least one stranding head and associated closing die, wherein the sealing material is applied to the core at the closing die of each stranding head of the stranding machine.

4. A method as claimed in claim 3, in which the first layer of conductors of the cable core is laid up around a group of at least two conductors forming a core center, wherein sealing material at a temperature just above that at which crystallization begins is delivered through an applicator die at the point where the two conductors are brought together to form the core center.

5. A method as claimed in claim 2, wherein surplus material spewing in a liquid or semisolid state from an applicator die at each cable feeding station is returned to the storage vessel and is brought back to the requisite temperature.

6. A method as claimed in claim 2, wherein the sealing material is pumped around the ring main at a rate substantially exceeding the aggregate rate of flow of material from the cable feeding stations to the cable core.

7. A method as claimed in claim 1, wherein each of a plurality of cable feeding stations is fed with sealing material from a separate source of supply.

8. A method as claimed in claim 7, wherein surplus sealing material spewing in a liquid or semisolid state from an applicator die at each cable feeding station is returned to the storage vessel associated with the die and is brought back to the requisite temperature.

9. Apparatus for use in providing a cable core comprising a multiplicity of insulated conductors with a barrier of sealing material which will not drain under the influence of gravity or such hydrostatic pressure as may arise in the event of damage to the sheath of the cable but which will permit relative sliding movement of the insulated conductors over one another during such bending of the cable as occurs during manufacture and installation of the cable, which apparatus comprises:

a. a thermostatically controlled storage vessel for storing the sealing material,

b. means for maintaining the sealing material at a temperature just above that at which crystallization begins,

c. at least one cable feeding station connected to the storage vessel,

d. means at the feeding station for applying sealing material to the cable core, and

e. connected between the storage vessel and the applicator means of the cable feeding station, means for withdrawing sealing material from the storage vessel and for controlling the rate of delivery of the material to the applicator means.

10. Apparatus as claimed in claim 9, wherein the thermostatically controlled storage vessel is connected to a ring main for the sealing material in which is connected means for continuously withdrawing sealing material from the storage vessel, driving it around the ring main and back into the vessel, and a cable feeding station is provided at at least one location in the ring main.

11. Apparatus as claimed in claim 10, wherein the means for controlling the rate of delivery of the material to the applicator means is a valve.

12. Apparatus as claimed in claim 10, wherein each of a plurality of applicator means has associated with it a heated tank for collecting surplus material spewing from the applicator means, a pipe extends from the collecting tank to the storage vessel, and a scavenge pump is provided for pumping the surplus material from the collecting tank back into the storage vessel.

13. Apparatus as claimed in claim 9, wherein each of a plurality of thermostatically controlled storage vessels is connected to a separate cable feeding station.

14. Apparatus as claimed in claim 13, wherein each means for withdrawing sealing material from the storage vessel and controlling its rate of delivery to the applicator means is a metering pump.

15. Apparatus as claimed in claim 13, wherein each storage vessel is so located with respect to the applicator means of its associated cable feeding station as to collect surplus material spewing from the applicator means.

16. Apparatus as claimed in claim 9, wherein the applicator means comprises a die which forms at least a part of the closing die of a stranding head of a stranding machine and which has a number of ports for entry of sealing material, which ports are distributed uniformly around the axis of the die and are inclined to it so as to impart to sealing material entering the die a component of movement in the same axial direction as the direction of travel of the cable core.