US2765527A

US2765527A - Sheathing of electric cables

Info

Publication number: US2765527A
Application number: US103722A
Authority: US
Inventors: Fairfield Ronald Mcleod; Aldridge Denis William; Travis William
Original assignee: Connollys Blackley Ltd; BICC PLC
Current assignee: Balfour Beatty PLC; Connollys Blackley Ltd
Priority date: 1948-07-26
Filing date: 1949-07-08
Publication date: 1956-10-09
Anticipated expiration: 1973-10-09

Description

Oct. 9. 1956 R. M. FAIRFIELD ETAL 2,765,527

SHEATHING OF ELECTRIC CABLES 3 Sheets-Sheet 1 Filed July 8 1949 Oct. 9, 1956 R. M. FAIRFIELD ETAL 2,765,527

SHEATHING OF ELECTRIC CABLES 5 Sheets-Sheet 2 Filed July 8, 1949 Inventors PM 8/0 Mel eoo Fa/k'fl'elq Oct. 9, 1956 R. M. FAIRFIELD ET AL 2,765,537

- SHEATHING OF ELECTRIC CABLES Filed July 8, 1949. 3 Sheets-Sheet 3 5o 15 47b 7 r v Patented Oct. 9, 1956 ice STEATG F ELECTRIC CABLES Ronald McLeod Fairtield, Brarnshiii, near Eversley, Denis Wiliiam Aldridge, Rainhili, and William Travis, Southport, England; said Fairiield and said Aldridge assignors to British Insulated Qalienders Cabies Linn ited, London, England, and said Travis assignor to Connollys (Blackley) Limited, Manchester, Engiand, both British companies Application July 8, 1949, Serial No. 103,722 Claims priority, application Great Britain July 26, 1948 12 Claims. (Cl. 29-5 17) The invention relates to the step in the manufacture of electric cables in which a seamless metal sheath is applied over the cable body, which may consist of a single insulated core or a group of cores with or without other members. The invention is concerned with the application of a sheath of metal of a substantially higher extrusion temperature than the lead; particularly it deals with aluminium. The method of application of the sheath employed is that in which there is extruded over the cable body an oversize sheath, that is, a sheath with an internal diameter greater than the external diameter of that body, which is then moved forward from the press, cooled in transit and operated upon to reduce it to a size appropriate for the particular cable body. Where a close fitting sheath is required, the diiierence between the initial internal diameter of the sheath and the external diameter of the cable body is limited to the reduction in internal diameter that can be obtained without producing undesirable workhardening of the sheath, for annealing obviously presents difficulties. Where a small residual clearance is required the initial clearance will be correspondingly greater. However, in both cases the permissible initial clearance Will usually be small and the problem is encountered of avoiding damage to the cable body by heat to which it is subjected, most intensely when in the neighbourhood of the dies of the press.

The risk of damage to the cable body by heat is reduced by maintaining a clearance between the cable body and the sheath until the sheath has reached a safe temperature, by which We mean a temperature at or below which the sheath will not during its further cooling transmit heat to the cable body under the conditions of contact prevailing between the sheath and the cable body at such a rate as to have any substantial adverse eiiect upon the cable body. The risk may be still further reduced by evacuating the clearance.

The type of press at present used for the aluminium sheathing of cables is that in which the metal is inserted in the container in the form of hot solid billets accurately shaped to fit the container. Through the centre of the container passes the cable, separated from the hot metal by a suitably cooled tubular shield and guide. In this form of press extrusion generally takes place in the direction of the stroke of the ram in the container. The extrusion cycle comprises a short charging period during which the billets are inserted in the container round the central tubular shield and an extrusion stage during which a length of sheath is formed which moves forward with the cable body.

Where the operation of extruding a sheath on a length of cable body cannot proceed without interruption, as where, in the case of a billet press, the charge is insutficient to sheath more than a fraction of the length of the cable body, there is, despite the presence of a clearance between the cable body and the sheath, risk of the cable body becoming locally overheated during the charging period, when the sheath is stationary in the press. By the present invention we reduce the risk of such local overheating of the cable body by operating oversize sheath during the interval between two successive periods of extrusion to effect a reduction in its cross-sectional area and reduce its internal diameter and elongate it. This will provide for relative movement at the extrusion die between the cable body and its sheath during the period when the sheath is stationary in the press. Accordingly our invention includes a method of applying a sheath of aluminium or other metal having a substantially higher extrusion temperature than lead, which comprises extruding an oversize seamless sheath of such metal on to the cable body by a discontinuous extrusion operation and after cooling the sheath to a safe temperature, operating upon the sheath to effect a reduction in its cross-sectional area and reduce its internal diameter and elongate it, during the interval between two successive periods of extrusion. This reducing operation may in some cases be confined to the intervals between successive periods of extrusion. It may take place throughout the interval or during some part or parts of the interval.

The invention will now be more fully described, frequent reference being made to the accompanying drawings which are all diagrammatic and not to scale. In these drawings Figure l is an elevation illustrating one form of arrangement for carrying out the aluminium sheathing of insulated electric cables,

Figure 2 is a greatly enlarged cross-section on the line llII of Figure 1, showing an example of a sheathed cable as it leaves the extrusion press shown in Figure 1,

Figure 3 is a greatly enlarged cross-section on the line Ill-III of Figure l of the same cable after leaving the drawing die of Figure 1 by which its sheath has been reduced to a size appropriate for the cable body,

Figure 4 is a plan showing a modified form of the arrangement of Figure 1,

Figure 5 is an elevation illustrating a second form of arrangement for carrying out the aluminium sheathing of insulated electric cable,

Figure 6 is an enlarged view of the drawing die carriage and a support roller carriage of Figure 5,

Figure 7 is a cross-sectional view on the line VIIVII of Figure 6 of the die carriage and its running track shown in Figure 6,

Figure 8 is an elevation of a modified form of arrangement employing a movable drawing die,

Figure 9 is a fragmental elevation on a larger scale of the arrangement shown in Figure 8, and

Figure 10 is a fragmental view, partly in section and partly in elevation, showing an alternative form of cooling device.

In the method illustrated in Figure l, the cable body 1 is drawn oii a supply drum 2 and fed into the press 3, being passed axially through the hydraulic press cylinder 4 and the extrusion ram 5. it is led through the con tainer 6 of the extrusion press by a tubular guide 7. The container is charged with tubular billets made in halves placed together to enclose the cable body and shaped to fit the annular space between the Wall of the container and the guide '7. During the extrusion part of the cycle, the ram 5 moves forward in the container 6 and a length of sheath 8 is formed which surrounds the cable body 1 with a clearance 9, as shown for example in Figure 2. This clearance contains air or other gaseous fluid. The gaseous fiuid may be maintained under a pressure in excess of ambient atmospheric pressure though care must be taken that this is not high enough to distend the hot sheath at the die. Alternatively, save in the case of cables having an impregnated fibrous dielectric, it may be maintained at less than atmospheric pressure. To provide for the application of pressure or vacuum, the guide tube 7 may extend rearwards, through the ram 5 and the hydraulic cylinder 4, and terminate in an appropriate sealing gland. The clearance between the cable body and the extruded sheath can then be maintained under high vacuum by connecting the rear end of the tube 7 to a suitable vacuum plant or under pressure by connecting it to a source of compressed air or gas. By maintaining the gaseous fluid at a very low pressure, in other words, by maintaining a high vacuum in the clearance, the transmission of heat can be reduced to a very low value, for the conditions are then comparable to those which obtain in a vacuum flask because the internal surface of the extruded sheath will have a highly polished silvery surface, and consequently very low rate of heat emission.

On leaving the press, the sheath 8 passes over a number of rollers 10 which maintain it in axial alignment with the dies of the press for a short distance. Whilst so supported it is cooled by water sprays from a line of jets 11 in a header 12, the water being collected in a tank 13, from which it may be passed to a cooler and pumped back to the jets. Alternatively, with the object of cooling the sheath as soon as possible after its formation, it may pass from the extrusion die into and through a liquid-filled cooling tube of the form shown in Figure 10. One end of this tube 60 is coupled to the die or its holder 61 and the other is furnished with a sealing gland 62 through which the sheathed cable 8 leaves the tube. Provision may be made to vary the rate of flow of cooling liquid through the tube or to adjust its temperature or to do both.

After leaving the cooling arrangement the sheath 8 and its contents are guided by rollers 14 into a drawing die 15 through which it is pulled by any suitable form of haul-off device, for example, a power driven capstan 16 round which the sheathed cable makes at least one complete turn before passing to a take-up drum 17. The drawing die 15 reduces the diameter of the tubular sheath 8 to an appropriate extent, for example so that the sheath fits the cable body, as shown in Figure 3.

The reduction in sheath diameter brought about by the die is effected without very materially changing the wall thickness of the sheath. It will be apparent therefore that the drawing down of the sheath 8 by the die 15 is also accompanied by an elongation of the sheath. It follows that within the sheath on the entry side of the die 15, throughout the length between the press 3 and this die 15, the cable body 1 is moving forward relative to the sheath 8 at a fraction of the speed of the cable on the exit side of the drawing die, the proportion being determined by the ratio of reduction of cross-sectional area of sheath in the die. The heating effect on the body of the cable whilst the sheath is stationary in the press during the recharging of the container is reduced by arranging for relative movement between the cable body 1 and the part of the sheath 8 between the press 3 and the drawing die 15 to take place during the charging period. Preferably the normal relative movement, that is the relative speed of cable body to sheath during the extrusion part of the cycle, is maintained, or even increased during the charging period.

This result can be attained by making the speed of extrusion of the sheath 8 from the press 3 greater than the speed of entry of the sheath 8 into the reducing die 15 so that with the aid of guide rollers 19 a dependent loop 18 of slack cable is formed during the extrusion part of the cycle which can be then taken up by continuation of the drawing through the charging period. If the drawing continues at the same speed throughout the cycle the relative speed of the cable body to the sheath is constant. During the charging period the speed of the sheath at the press is zero and hence the speed of the cable body through the press falls to the relative speed of cable body to sheath and may be found to be small. The value of this relative speed of the cable body to the sheath and the minimum length of oversize sheath required, under these conditions, in the loop 18 at the end of the extrusion period can readily be calculated from a knowledge of the increase in length brought about by the drawing die, the duration of the extrusion period, and the linear speed of extrusion of the sheath. For example, if the speed of extrusion is 2.5 feet/sec, the elongation 9%, the extrusion period 33 seconds and the charging period separating two successive extrusion periods 5 seconds:

The length of oversize sheath extruded/cycle =2.5 33=82.5 feet. The length of elongated sheath passing over capstart/cycle =82.5X1.09= feet. Speed of take-up of elongated sheath throughout 90 the cycle =237 feet/sec.

The length of elongated sheath taken up during the charging period =5X2.37l1.85 feet.

If the relative speed of the cable body to sheath is found to be too low to avoid risk of overheating of the body during the charging period, it can be increased by speeding up the rate of drawing during that part of the cycle. This would involve increasing the accumulation of slack in the loop 18 during the extrusion period. In other words, to maintain the same average rate of output of sheathed cable, the speed of drawing would be reduced during extrusion and increased during charging.

Where, to obtain a suitable relative speed of the cable body to sheath, it is necessary to form a large loop, it may be advisable to take precautions to prevent the sheath from being stretched at the die of the press by the weight of sheathed cable in the loop. This may be done in one or more of several ways: the pay-out reel 2 may be appropriately braked during the charging period to place the cable body under tension or the extruded sheath may be passed between a pair of grooved rolls 20 which are braked for the duration of the charging period, or the central part of the loop may be supported on rollers 21 on a vertically slidable frame 22 supported by a counterweight 23 which partially balances the weight of the loop, or the sheathed cable may be guided by rollers 190, as shown in Figure 4, to form a loop 18a in a horizontal or appropriately inclined plane on an appropriate support or supports. The latter may take the form of a smooth table 24 on which the sheath will readily skid. A series of long rollers 25 lying with their axes parallel to the axis of extrusion may be incorporated in the table. In cases where the sheathed body is of large diameter it may be desirable to provide some positively acting means for causing the sheathed cable body to assume a loop of the form required. For example, additional rollers may be provided on the frame 22 on diametrically opposite sides of the sheath to the rollers 21 and a positive drive may be applied to the frame during an initial part or during the whole of its downward movement.

Instead of arranging for the formation of a loop of slack during tthe extrusion part of the cycle, the drawing die can be supported on a carriage and arranged to be moved from the capstan or other haul-off device towards the press during the charging part of the cycle and moved away from the press during the extrusion part of the cycle. This will have the effect of reducing the speed of drawing during the extrusion part of the cycle, whilst during the charging part the speed of the cable body through the press will be a constant fraction of the speed of travel of the drawing die. As the charging period will desirably be short compared with the extrusion period, the speed of approach of the die towards the press may be relatively high, rendering possible a speed of drawing during the charging period sufficiently high to cause the cable body to be moved through the press at a speed that is adequate to prevent overheating of the cable body. Disregarding thermal contraction elfects, this speed may be readily calculated as follows:

Let the speed of extrusion=S, and the speed of travel of the drawing die during the extrusion part of the cycle be Ve and during the charging part of the cycle Vc (negative), then if K is the ratio of elongated length of sheath to initial length of sheath, we have,

Increase in speed o travel of sheath due to elongation during extrusion period=K (SVe)(SVe) Linear speed of sheath at capstan during extrusion period =S+K(SVe)(S-Ve) and Linear speed of sheath at capstan during charging of press =-Vc(K-1) Assuming that the die moves forward during the whole of the extrusion period and back towards the press during the whole of the charging period we have where te and to are the durations of extrusion and charging periods, respectively. Hence effective peripheral speed of capstan during charging =Ve- K 1 2 which is the speed of travel of the core through the press during the charging operation. It will generally be impracticable without arranging for the formation of a loop during the extrusion period, to draw down the sheath at such a rate during the charging period as will enable the capstan speed to be maintained constant throughout the cycle. For constant capstan speed Generally the permissible value of K (which is restricted by the need to avoid excessive work hardening of the sheath) and the value of the ratio of to te+ to will be such as to render Ve greater than S. If during the extrusion period the drawing die travels faster than the extruded sheath it will move away from the step between the drawn down and undrawn parts of the sheath and over a reduced part of the sheath so that during an initial part of its return journey no drawing action will take place. Consequently unless there is a loop of cable for the capstan to draw upon during this initial part of the charging period or a complete stoppage of the oap stan is provided for, rupture of the sheath will occur at the press, where its strength is lowest.

It will be apparent from Equation (2),

capstan speed during charging=Ve- (K 1) that the speed of travel of the core through the press during charging, will be a maximum when Ve has its maximum practical value, namely, when during theextrusion part of the cycle the drawing die moves forward at the speed of extrusion, that is, when the operation of drawing down the sheath is confined to the charging period. If, for example, the length of the extrusion period is 33 seconds and tht of the charging period 5 seconds, and the permissible elongation of the sheath is the speed of travel of the cable body through the press will be found, by substituting in Equation. (2), to be .668. or, where S is, say, 2.5 feet/sec, 1.65 feet/sec. However, to obtain this maximum speed of travel of the cable body through the press during charging, the length of particular example quoted it will be 82.5 feet. Since V0 Va to the rate of travel of the drawing die during the charging operation will also be high, namely 16.5 feet/sec.

The length of travel of the drawing die and its speed may be reduced by further reducing the effective speed of the capstan during the charging period as compared with its speed during the extrusion period, so as to provide for a movement of the cable body through the press during the charging period that will sufiice to avoid risk of local over-heating of the cable body during such period. For instance, we may arrange for the movement of the cable body to be not less than 0.2 feet/sec. during the charging period. The speeds and length of travel of the die to provide such minimum speed of cable body movement can then be found from Equation (2), from which, for the example of press quoted above, Ve=0.30 feet/sec, Vc=2.0 feet/sec. and the length of travel of the die=l0 feet Where it is desired to provide for a movement of the cable body through the press during the charging period equal to some particular proportion, say

n. 7L of the speed of travel of the body during the extrusion period, the speeds and length of travel of the die to effect this can be found from the equation from which (for the example of press quoted above,

a number of rollers 33 each mounted on a separate wheeled carriage 34 which also runs on the track 32. The die carriage 31 and the support roll carriages 34 are coupled together by long flexible couplings 35 to form a train, so that movement of the die carriage towards the press will cause the rollers 33 between it and the capstan 16 to separate and the rollers 33 between it and the press 3 to approach one another, and movement in the opposite direction will have a converse effect. The carriages may be fitted with rubber or spring buffers 36 and the flexible couplings are preferably resiliently extensible to reduce shock at the drawing die, which with the same object in view may be resiliently mounted in its support 37. The number of support roll carriages 34 required will naturally depend upon the size of cable being produced and upon the length of travel of the drawing die. For example, if the length of travel is 10 feet (which quired to ensure a speed of travel of the cable body through the example of press quoted above of not less than 0.2 feet/sec. during the charging operation) it may be found that, in the case of a cable of one inch external diameter a single support roll carriage on each side of the die carriage will sufiice.

it will be understood that with the arrangement shown in Figure 5, the speed of the capstan must be very closely controlled and closely co-ordinated with the speed of travel of the drawing die both during the extrusion period and during the charging period to avoid imposing excessive tension on the hot sheath in the neighbourhood of the press. To this end we may drive the die carriage 31 by a shunt wound D. C. motor 38 driving, through reduction is the length regearing 39, a pinion 40 which engages a rack 41 extending alongside one rail of the track. The motor and gearing are preferably mounted as low as possible to reduce the overall height both of the die carriage and of all the support roll carriages. The capstan may also be driven by a shunt wound D. C. motor. The speed of this motor and that of the carriage motor 38 are accurately adjusted to a given ratio during the extrusion stroke and to another given ratio during the charging stroke, the die carriage motor being reversed for the latter. This changeover of relative speed may be brought about at the beginning of the extrusion period by a limit switch actuated by the press ram at the beginning of its working stroke and again by a second limit switch actuated by the ram at the end of its working stroke. Alternatively, or in addition, a monitoring control may be imposed upon the capstan drive dependent upon the measured tension in the sheathed cable. This may be done by allowing the formation of a slight loop between the capstan and the nearest support roll and varying the speed of the capstan motor in accordance with the depth of the loop. The power supply to the motor may be by way of a third rail and a slipper on the carriage or by cable 42 payed out from a drum 43 at one end of the track. As an additional precaution the sheath may be clamped during the charging period as shown at 20 in Figure 1.

Where the drawing down operation is confined to the charging period, the need to provide for the close co-ordination of the speed of the carriage with that of the capstan both during extrusion and during charging periods to avoid excessive tension in the hot sheath may be eliminated by driving the capstan through a slipping coupling during the extrusion period and during the charging period applying a positive drive that is accurately co-ordinated with the speed of travel of the drawing die towards the press. An example of such an arrangement is shown diagrammatically in Figures 8 and -9. A drawing die is mounted between the adjacent ends of two axially aligned tubular rams 46a, 46b of opposed twin hydraulic cylinders 47a, 47b axially aligned with the extrusion axis of the press 3 and disposed between the press and a draw-off capstan 16. Pressure fluid is admitted to both cylinders to maintain the rams in tension, movement of the rams and the drawing die being effected by creating a pressure difference between the fluid in one cylinder and that in the other. The capstan 16 is driven through a slipping coupling and serves during the extrusion part of the cycle merely to overcome friction between the sheathed cable and the walls of the tubular fixed cable guides 48a, 48b extending through the hydraulic cylinders 47a, 47b and those of the rams 46a, 46b. During the return movement of the drawing die a separate, positive drive is applied to the capstan 16 by means of twin chains 49 attached to the die mounting block 50. These chains pass over twin sprockets 51 which when the chains are being drawn towards the press, ratchet-drive a pinion 52 which engages a spur wheel 53 which through a pinion 54 and spur wheel 55 drives a pinion 56 which engages a ring of teeth 57 on one side of the capstan 16. The diameters of the sprockets, pinions and spurs and toothed ring are chosen to impart an effective peripheral speed to the capstan 16 equal to (K-1) x (the speed of travel of the die), where K is the ratio of elongated length to initial length of sheath, as before. During the return movement of the rams the direction of rotation of the sprockets is reversed but due to the incorporation of the ratchet drive no reverse movement is applied to the capstan.

It will be appreciated that in the arrangement described in the preceding paragraph, the stroke of the reciprocating die equals the length of sheath extruded per cyclein the example of press quoted earlier in this specification, a matter of 82.5 feet. The apparatus is thus very long. This disadvantage may be reduced by using a telescopic ram in place of each of those shown in Figure 8 or by dispensingwith one of the twin cylinders in which case it would be necessary to provide supporting carriages for the remaining ram and probably for the cable also. For example, the die carriage described with reference to Figures 4, 5 and 6, could be driven by a hydraulic ram instead of an electric motor and, if desired, the drive to the capstan could be taken from this ram, provision being made to change the transmission ratio at the end of each stroke.

Naturally we may use the double hydraulic ram arrangement shown in Figure 8 to provide for reciprocation of the drawing die, and employ a separate but co-ordinated two-speed drive to the draw-off capstan or its equivalent. In this case, we can, as will be clear from the description of the arrangement shown in Figures 5, 6 and 7 greatly reduce the length of the apparatus by providing for a drawing down action both during extrusion and charging parts of the cycle. Thus where it is satisfactory to provide for a speed of travel of the cable body through the press of 0.2 feet/sec. during the charging operation, the stroke of the reciprocating die may be reduced to 10 feet, for the aforesaid example of press.

It is to be understood that although we have described and shown examples of apparatus in which the required reduction is effected in a single drawing die, the invention is not limited in this respect and that two or more drawing dies may be used in tandem, each die effecting an appropriate proportion of the required reduction in crosssectional area of the sheath. It is also to be understood that, although the diagrammatic drawings show the drawing die as a fixed wall type of die, the term drawing die used herein is not to be construed as limited to such a die but as including an arrangement of rolls (some or all of which may be power driven) of which the peripheral surfaces co-operate to form a die adapted to reduce and elongate a sheath of appropriate external dimensions as the sheath is passed through the die or the die moved over the sheath.

What we claim as our invention is:

1. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body by means of an extrusion press, comprising extruding a length of oversize seamless sheath of said metallic material on a length of the cable body during two successive time periods of extrusion separated by an interval for recharging the press, cooling the extruded sheath to a safe temperature, drawing down the cooled sheath to elongate it, augmenting the size of a storage loop of sheathed cable during the first extrusion period by extruding at a linear speed of extrusion greater than the linear speed of drawing, recharging the press during said interval and reducing the size of said storage loop during said interval by drawing down and elongating sheath taken from said storage loop during said interval, whereby to advance said cable body through the press during at least part of the said interval.

2. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body by means of an extrusion press, comprising extruding a length of oversize seamless sheath of said metallic material on a length of the cable body during two successive time periods of extrusion separated by an interval for recharging the press, cooling the extruded sheath to a safe temperature, drawing down the cooled sheath to elongate it, augmenting the size of a storage loop of sheathed cable during the first extrusion period by extruding at a linear speed of extrusion greater than the linear speed of drawing, recharging the press during said interval and reducing the size of said storage loop during said interval by drawing down and elongating sheath taken from said storage loop during said interval, the linear speed of travel of the elongated sheath during said interval being the same as during first said extrusion period, whereby to advance said cable 9 body through the press during at least'part of the said interval.

3. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body by means of an extrusion, comprising extruding a length of oversize seamless sheath of said metallic material on a length of the cable body during two successive time periods of extrusion separated by an interval for recharging the extrusion press, cooling the extruded sheath to a safe temperature, drawing down the cooled sheath to elongate it, augmenting the size of a storage loop of sheathed cable during the first extrusion period by extruding at a linear speed of extrusion greater than the linear speed of drawing, recharging the press during said interval and reducing the size of said storage loop during said interval by drawing down and elongating sheath taken from said storage loop during said interval, the linear speed of travel of the elongated sheath during said interval being greater than during first said extrusion period, whereby to advance said cable body through the press during at least part of the said interval.

4. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body by means of an extrusion press, comprising extruding a length of oversize seamless sheath of said metallic material on a length of the cable body during two successive time periods of extrusion separated by an interval for recharging the press, cooling the extruded sheath to a safe temperature, passing the cooled sheath through a drawing die to reduce it and elongate it by causing said die during the first extrusion period to move in the direction of travel of the sheathed cable body at a speed not exceeding the linear speed of extrusion, recharging the press during said interval and during said interval causing said die to move in the reverse direction whereby to draw down and elongate a length of extruded sheath and thereby advance said cable body through the press during said interval.

5. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body by means of an extrusion press, comprising extruding a length of oversize seamless sheath of said metallic material on a length of the cable body during two successive time periods of extrusion separated by an interval for recharging the press, cooling the extruded sheath to a safe temperature, passing the cooled sheath through a drawing die to reduce it and elongate it by causing said die during the first extrusion period to move in the direction of travel of the sheathed cable body at a speed not exceeding the linear speed of extrusion, recharging the press during said interval and during said interval causing said die to move in the reverse direction at a linear speed of travel greater than its linear speed of travel during the first extrusion period, whereby to draw down and elongate a length of extruded sheath and thereby advance said cable body through the press during said interval.

6. A method of forming a sheath of metallic material having a substantially higher extrusion temperature than lead on an electric cable body by means of an extrusion press, comprising extruding an oversize seamless sheath of said metallic material on said cable body during two successive time periods of extrusion separated by an interval for recharging the press, cooling said extruded sheath to a safe temperature, storing a length of said cooled extruded sheath during the first extrusion period, recharging the press during said interval and operating upon at least part of said cooled extruded sheath during said interval to elongate it and advance said cable body through said press.

7. A method of forming a sheath of metallic material having a substantially higher extrusion temperature than lead on an electric cable body by means of an extrusion press, comprising extruding an oversize seamless sheath of said metallic material on said cable body during two successive time periods of extrusion separated by an interval for recharging the press, cooling said sheath to a safe temperature, storing a length of said cooled extruded sheath during the first extrusion period, recharging the press during said interval and causing said cable body to progress through said stored length of sheath during said interval by operating upon at least part of said stored length of sheath to elongate it during at least a greater part of said interval.

8. A method of forming a sheath of metallic material having a substantially higher extrusion temperature than lead on an electric cable body by means of an extrusion press, comprising extruding an oversize seamless sheath of said metallic material on said cable body during two successive time periods of extrusion separated by an interval for recharging the press, artificially cooling said sheath as it is formed, storing a length of said cooled extruded sheath during the first extrusion period, and by operating upon at least part of said cooled extruded sheath during said interval to elongate it, causing an elemental length of said cable body within a hot portion of said sheath to move from said hot portion into a cooled portion of said sheath during said interval.

9. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body by means of an extrusion press, comprising extruding an oversize seamless sheath of said metallic material on said cable body during two successive time periods of extrusion separated by an interval for recharging the press, cooling said sheath to a safe temperature, recharging the press during said interval and operating upon the cooled sheath to elongate it and advance said cable body through said press at least during at least the greater part of the said interval.

10. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body, comprising the successive steps of charging a press with said metallic material, extruding said metallic material in the form of an oversize seamless sheath about a portion of the length of said cable body, stopping said extrusion to recharge the press, recharging the press and thereupon resuming the extrusion of said metallic material in the form of an oversize seamless sheath about a succeeding portion of the length of said cable body, cooling said sheath to a safe temperature, and operating upon the cooled sheath during the recharging of the press to elongate said sheath and advance said cable body through the press during at least part of the time occupied by the recharging operation.

11. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body, comprising the successive steps of charging a press with said metallic material, extruding said metallic material in the form of an oversize seamless sheath about a portion of a length of said cable body, stopping said extrusion to recharge the press, recharging the press and thereupon resuming the extrusion of said metallic material in the form of an oversize seamless sheath about a succeeding portion of the length of said cable body, artificially cooling said sheath to a safe temperature and operating upon the artificially cooled sheath during the recharging of the press to elongate said sheath and advance said cable body through the press during at least part of the time occupied by recharging operation.

12. A method of applying a sheath of metallic material having a substantially higher extrusion temperature than lead to an electric cable body by means of a billet extrusion press wherein the press extrudes an oversize seamless sheath of said material on said cable body during a succession of extrusion operations separated by intervals for recharging the press and wherein the sheath after being cooled to a safe temperature is drawn down to elongate it and reduce the clearance between it and said cable body, characterized by the fact that the cable body is caused to advance through the press during at least the greater part of the interval between the extrusion of two successive billets by storing sheathed cable during an extrusion period preceding said interval and drawing down at least part of the stored cable during at least a greater part of the said interval.

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