Manufacture of insulated electric cables

In curing an extruded insulation layer of an electric cable by passing the cable through a catenary curing tube, a cable restraining device 8 is disposed at or just prior to the exit end of the catenary tube 11 and serves to apply to the cable 10 a restraint to oppose forces (developed by the hydrostatic pressure of the curing fluid) which tend to extrude the cable through the exit end of the curing tube. In the example shown, the device 8 comprises a set of resiliently flexible cones 9 and a clamping arrangement 12, 13 which is adjustable to adjust the degree of restraints.

This invention concerns improvements in or relating to the manufacture of 
insulated electric cables, and more particularly concerns the manufacture 
of insulated cables by a process wherein an extruded cable insulation 
layer is cured, that is to say vulcanized or chemically cross-linked, by 
the application of heat during passage of the cable through a tubular 
enclosure formed at least in part as a catenary. 
As is well known the use of such a catenary-shaped curing enclosure enables 
the insulation of the cable to be cured from a relatively soft and plastic 
state as it exits from the extruder head at one end of the catenary to a 
relatively firm and rigid state at the opposite end without there being 
any requirement to contact the insulation with supporting or guiding means 
at any intermediate location, the cable simply being suspended between 
spaced apart locations so as to hang in a catenary generally coaxial with 
the catenary-shaped curing enclosure. Means are generally provided for 
controlling the catenary position of the cable within the curing enclosure 
to ensure that the still plastic cable insulation does not come into 
contact with the wall of the curing enclosure and is not damaged thereby, 
such means comprising for example a catenary position sensor within the 
curing enclosure coupled to control the catenary tension in the cable by 
control of the speed operation of a cable haul off means external of the 
curing enclosure. 
In the operation of such an apparatus using a fluid, liquid or gas, within 
the curing enclosure as a medium for applying the required heat to the 
insulation, a difficulty arises on account of the action of the 
hydrostatic pressure of the curing fluid upon the cable which, by virtue 
of the pressure differential between the inside and the outside of the 
curing enclosure at the exit end of the catenary tube, tends to extrude 
the cable through the exit seal customarily provided at the exit end of 
the enclosure. The tension applied to the cable externally of the curing 
enclosure, that is to say the tension applied by the haul off means for 
example, thus is not the same as the tension experienced by the cable 
within the curing enclosure, and under certain operating conditions, for 
example wherein the cable has light-weight conductor(s) and a large body 
of insulation and the curing fluid is relatively dense, the situation can 
arise that the extrusion force developed upon the cable at the exit end of 
the curing enclosure exceeds the normally required external tension 
leading to a loss of control of the cable catenary position within the 
curing enclosure with attendant risk of damage to the cable insulation. 
To avoid or at least substantially reduce the risks attendant upon the 
abovementioned problem, the present invention proposes the provision of a 
cable restraining means at the exit end of the catenary curing tube, or 
adjacent thereto within the tube, for applying to the cable a restraint 
directed oppositely to the forces (developed by the hydrostatic pressure 
of the curing fluid) which tend to extrude the cable through the exit end 
of the catenary curing tube so as to nullify the disadvantageous affects 
of such forces. 
Various means of applying such a restraint to the cable are possible, 
particularly having regard to the fact that in the region of the exit end 
of the catenary tube the cable insulation will be virtually cured and is 
much less susceptible to damage. Thus the restraining means might for 
example comprise braked caterpillars acting on the cable, braking rollers 
or wheels, or other frictional devices. Ideally the restraining means will 
be adjustable for accommodating different or varying cable dimensions, and 
will be operable not only to nullify the effects of the hydrostatic forces 
developed upon the cable but also to provide an excess restraining force 
against which the cable tension control means can operate. 
The presently preferred form of restraining means comprises a plurality of 
generally flexible and resilient conical members which are nested with 
each other and are arranged coaxially of the catenary tube, the conical 
members each being open at its apex for passage therethrough of the cable 
and the nested array of conical members being subject selectively to the 
action of a pressure collar which can be urged axially against the conical 
members so as by a degree of distortion thereof to vary the restraining 
effect of the conical members upon a cable traversing their open apices. 
The action of the pressure collar can advantageously be made dependant 
upon the sensed tension applied to the cable externally of the curing 
enclosure.

Referring to FIG. 1, a cable core 1 is fed by metering caterpillar device 2 
through the cross head 3 of an extruder where a layer of elastomeric 
insulation is extruded continuously onto the cable core 1. The outlet of 
the extruder leads directly into the inlet end of a catenary shaped curing 
tube 4 which, in operation, contains hot fluid under a predetermined 
pressure. As shown, the cable follows a catenary path through the catenary 
tube 4, its position within the catenary tube being controlled through a 
detector 5 which determines the operating speed of a haul-off caterpillar 
6 to determine the tension in the cable. The exit end of the catenary tube 
4 is sealed by means of a sealing arrangement 7. 
FIG. 2 shows a fragmentary sectional view of a restraining means 8, 
embodying the invention, fitted within the bore of a catenary curing tube 
such as that designated 4 in FIG. 1. The restraining means 8 comprises a 
set of frusto-conical members 9 nested or stacked together as shown and 
defining by their aligned open apices a through passage for the cable 10. 
The members 9 are formed of rubber, natural or synthetic depending upon 
the temperatures to which they are to be subjected, and are selected to be 
of a size nominally to suit the dimensions of the cable 10. The members 9 
are securely clamped to the wall 11 of the catenary tube 4, and are 
arranged to be subject to the action of a pair of pressure rings or 
collars 12, 13 the former 12 of which is secured to the catenary tube and 
the latter 13 of which is movable axially of the catenary tube. As will be 
appreciated, movement of pressure collar 13 towards the left (as viewed in 
FIG. 2) will cause a deformation of the members 9 such as to cause them to 
grip more strongly the cable 10, whereas the opposite movement will reduce 
the restraining effect of the members 9. 
FIG. 3 is an exemplary showing of the restraining means 8 of FIG. 2 
installed adjacent to the exit end of a catenary curing tube 4. A pressure 
bypass 14 is provided around the restraining means 8, and there is also 
provided downstream of the restraining means 8 a pneumatic seal 15 and a 
quick release hawser seal 16. FIGS. 4A to 4D show the sequence of 
operations involved in setting up a catenary vulcanization line such as 
that of FIG. 3. In FIG. 4A, the cable start 20 is pulled through the 
catenary tube 4 with a small diameter hawser 21 which gives rise only to a 
very small hydrostatic force acting at the quick release seal 16. Under 
these conditions, the sensed haul off tension is relatively high and this 
causes the pressure collar 13 (FIG. 2) of the restraining means 8 to be 
adjusted towards the right so as to minimize the action of the restraining 
means 8 and permit relatively free passage of the cable start 20. FIG. 4B 
shows the cable start 20 engaged with the restraining means 8, but 
otherwise the conditions are as for FIG. 4A since the restraining means 8 
is bypassed by bypass line 14. When, as shown in FIG. 4C, the cable start 
20 plugs the seal 15, the pressure between seal 15 and hawser seal 16 
reduces towards atmospheric and the hydrostatic force developed upon the 
cable by virtue of the pressure of the curing medium increases 
dramatically. The haul off tension of the cable drops correspondingly, and 
in response to this fall, the restraining means 8 is actuated by 
corresponding leftwards movement of pressure collar 13. In FIG. 4D, the 
hawser seal 16 has opened preparatory to the cable start 20 exiting from 
the catenary tube, and the restraining device 8 adopts a normal operating 
condition where, in dependence upon varying haul off tension, it applies 
to the cable a variable restraining action. 
FIG. 5 shows a control system for adjusting the restraining means 8 of FIG. 
2 according to variations in the sensed haul-off tension. The rubber cones 
9 of the restraining means 8 are compressed in order to provide the 
required braking force by means of a pair of hydraulic cylinders 30, 31. 
The degree of compression, and hence the magnitude of the braking force, 
is determined by the displacement of these cylinders. This displacement is 
determined by a three position hydraulic valve 32 providing a clamping, a 
neutral and a releasing mode. This valve is controlled automatically by an 
electronic logic system 33 fed by a signal from a load cell which monitors 
the haul off tension via the turn round wheel (FIG. 1). Two set points are 
selected, a low and a high. If the haul off tension is below the low level 
the brake 8 is put into clamp until such time as the displacement is 
sufficient to generate enough braking force to increase the haul off 
tension to the low level where it will then change to neutral, i.e. no 
further displacement will take place. Should the cable size increase for a 
given displacement, the braking force will increase and hence so will the 
haul off tension; if the tension increases above the high limit the brake 
will release in a similar manner. This control system is totally 
independent of the catenary position detector 5 which controls the speed 
of the haul off device 6.