IE46028B1 - Electric cables - Google Patents

Abstract

In the method for producing an electric cable having stress relief, high-tensile stress-relieving members (28, 23) inside the injection head (10) of an extrusion machine are inserted into the still viscous cable sheath (14) in such a way that each stress-relieving member (23) is entirely surrounded by sheath material and is provided with a corrugation after leaving the extruder. The method permits the corrugation of the stress-relieving members (23) to be produced independently of the type of sheath material. The corrugation of the stress-relieving members (23) is brought about by selecting the speed of the sheath material in the injection head (10) of the extruder and the speed of the stress-relieving members embedded in said material to be higher than the speed of the cable core (13). As a result, after leaving the extruder the cable sheath (14) experiences strand widening which causes the corrugation of the stress-relieving members (23).

Description

This invention relates to electric cables, and more particularly to electric cables having undulated tension relief elements embedded in the cable jacket.

U.S. Patent Nos. 3,979,518 and 3,983,313 disclose 5 electric cables having undulated tension relief elements embedded in the cable jacket. In accordance with such patents, the cable jacket is formed of polyethylene and the undulation of the tension relief elements results from the shrinkage of·polyethylene during cooling thereof.

In some cases, it is highly desirable to provide cables with undulated tension relief elements in which the cable jacket is formed from a material other than polyethylene. For example, polyethylene is flammable and in cases where non-flammability is desired the cable jacket should be formed from a non-flammable material, such as polyvinyl chloride; however, polyvinyl chloride does not have the shrinkage properties of polyethylene. As a result, undulation of the tension relief element can not be effected by cooling of the cable jacket.

In accordance with the present invention there - 2 43028 is provided a method for the production of electric cable in a cable extruder head wherein a cable core, a jacketing material and tension relief strands are fed into the extruder head so that the jacketing material produces a tubular jacket for the core having the tension relief strands embedded therein, wherein the tension relief strands and the jacketing material are introduced into the extruder head at a velocity greater than the velocity of the cable core and the velocity of the jacketing material having the tension relief strands therein is thereafter reduced to effect compression of the jacketing material and produce undulations in the tension relief strands.

In this manner, the cable jacket can be produced from materials other than polyethylene, since the shrinkage properties of polyethylene are no longer . required for providing undulations in the tension relief elements.

More particularly, the relative velocities between the cable core and the jacketing material are controlled in a manner to generate an axial compressive force in the cable jacket and tension relief elements to provide the undulations. The greater the difference in the relative speeds, the greater the - 3 46028 undulations and by proper coordination the desired undulation can be achieved. Accordingly, the abrupt axial compression which occurs at the extruder head outlet or beyond the discharge therefrom produces the undula5 tion, whereby undulation of the tension relief elements can be obtained independently of the properties; i.e., shrinkage property, of the jacketing material.

Thus, there is a retarding effect on the jacketing material including the tension relief strands and such retarding effect is provided by the velocity difference between the jacketing material and cable core. In the case of a highly compressible and viscous jacketing material such compressive force is exerted immediately beyond the mouthpiece of the extruder head and there is a swelling of the jacketing material; i.e., the outer diameter of the jacket may be greater than the inner diameter of the mouthpiece.

It is also possible to obtain such a retarding effect with a material of low viscosity and compressibi20 lity within the mouthpiece of the extruder head by providing a mouthpiece having an increasing inner diameter in the direction of flow, whereby the velocity of the jacketing material decreases as it passes through - 4 46028 each larger area. In such a case, the tension relief elements would be fed at a rate corresponding to the velocity of the jacketing material in the smaller diameter area of the mouthpiece in order to provide the desired undulation.

The invention will be further described with respect to the accompanying drawings, wherein: Figure 1 is a schematic axial longitudinal sectional view through the extruder head of a cable extruding machine with a cable being produced therein; and, Figure 2 is a schematic perspective view of a cable segment with partly exposed tension relief elements.

It is to be understood that the general principles for forming the jacketed cable in accordance with the present invention are in accordance with those set forth in the aforementioned United States Patents Nos. 3,879,518 and 3,983,313, with the present invention differing from such patents by the fact that the jacket may be produced from materials other than polyethylene and that the undulations are formed by controlling the parameters within the extruder head, as described, instead of depending on the shrinkage characteristics - 5 4 6 Ο 3 8 of the jacketing material.

Referring to the drawings, a cable extruder head 10 is provided with an opening 11 for coupling the feed screw of an extruder (not shown). Α pre- fabricated cable core 13 made of insulated electric conductors moves in the direction of arrow 12 into head 10 wherein the outer jacket 14 is applied thereon, to provide a finished cable 15, which is cooled off in the commonly known manner.

As particularly shown in Figure 2, the finished cable 15 has a cable core comprised of conductors 15, insulation 17; i.e., five sets of two twisted conductor pairs 19, and a core made of a plastics material.

Around the conductor pairs 19 is a commonly used wrapping 22 made of plastic tape.

The outer jacket 14 is extruded over wrapping which moves with the cable core 13 into the extruder head wherein a variable amount of tension relief elements 23 are provided. These elements may be constructed from groups of yarns or twines having different thread dimensions whereby the number of yarns or twines for each group on the other hand depend on the number of conductor pairs 19, the - 6 4 6 0 2 8 required minimum braking stress of the cable and the thread size. Such numbers also depend on the distance of the groups from each other and the distance of the groups from the inner jacket circumference.

These tolerances are disclosed in the aforementioned United States Patent Specifications. It is essential that the tension relief elements 23 do not run through jacket 14 absolutely parallel to the cable axis (core 21); i.e,, the tension relief elements should be undulated, as shown in Figure 2. This provides an intimate positive locking between the tension relief elements and the jacket material.

Cable core 13 moves through a nozzle 25 in extruder head 10 and discharges into a hollow chamber 30 into which the front ends of channels 26 discharge^, with channels 26 being concentrically arranged with respect to the fed cable core 13 and of which only two channels are shown in Figure 1. The channels 26 are provided with tension relief elements 28 from a supply station, preferably in the form of suspended bobbins 27 which carry the yarn or twines 28.

The front end of extruder head 10 has an outer - 7 460 as matrix or mouthpiece 29 through which the finished cable 15 is with drawn. The tension relief elements 28 are embedded in the jacketing material (thermoplastic or elastomeric) introduced through screw coupling 11, and the funnel-shaped hollow chamber 30 receives the jacketing material with the embedded tension relief elements 28.

The speed or velocity of the jacketing material is dependent upon the exerted pressure and the inner diameter of the mouthpiece 29 and the speed or velocity of· the cable core 13 is equal to the drawingoff speed of the finished cable 15. The relative speed or velocity between the cable core and jacketing material is controlled in a manner such that the jacketing material moves through the head 10 at a velocity greater than the cable core.

The tension relief elements 23 also move at a velocity greater than the velocity of the cable core and such velocity generally corresponds to the flow speed of the jacketing material. As a result, the length of the tension relief elements 23 is longer than the length of cable core 13 or finished cable 15, 6 Ο 2 8 with the additional length being taken up in the undulation of the tension relief elements 23.

As a result, the dimensions of the undulations can be controlled by controlling the relative velocities of the jacketing material and tension relief elements with respect to the cable core. Such velocities can be controlled by controlling and /or adjusting the velocity of the cable core (draw-off speed of finished cable), mouthpiece diameter, pressure on jacketing material, etc, A convenient manner of determining that there is a difference in relative velocities is by measuring the outer diameter of the cable after its discharge from the mouthpiece with respect to the mouthpiece 15 inner diameter. The outer diameter of the cable should preferably be greater than the inner diameter of the mouthpiece with the difference in diameter increasing as the relative speed of the jacketing material and tension relief elements increases with respect to the cable core. It has been found that the degree of undulation increases as the velocity difference increases (and the diameter difference - 9 between the finished cable and mouthpiece inner Λ diameter increases) and such difference should be coordinated to provide the required undulation without the tension relief elements overtaking themselves and forming S-like loops. At the extreme, this could result in retarding the feed movement of the tension relief elements which would result in too great a tension which would reduce cable flexibility. As should be apparent, proper undulation improves, flexibility as a result of the bellows effect which permits bending.

As should be apparent from the above description, as a result of the velocity difference between the jacketing material and cable core, the slower moving cable core has a retarding effect on the faster moving tension relief elements at the time of forming the jacket on the core, whereby the tension relief elements are undulated.

The invention will be further described with respect to the following example; however, the scope of the invention is not to be limited thereby: - 10 4602 EXAMPLE In the following tests, the dimensions of the extruder head, pressure and charging rate of the jacketing material is maintained constant for the several tests with respect to each jacketing material.

The drawing off speed of the cable is changed to thereby change the velocity difference between the jacketing material and cable core. As the velocity of the jacketing material increases with respect to the cable core, the diameter of the . finished cable increases with respect to the mouthpiece inner diameter and the degree of undulation increased. Λ6028 m 9 a) 3 Q> Ό ‘g HOE -rl m $ 0 □ 0 u *H ΜΙ m tn O — 3 3 •Η Ό rd φ !d CM •id 3 e M ω 0 Φ w \ +J Φ a (0 M 34 a (X tjl r4 3 Ή 3 •H O‘H tn H NO 3 +> +> ·— 34 3 3 rfj U « a 3 O •H £ +> rd Φ Φ 1 M +> £ □ Ο -H ω« S Φ φ M •Η Φ (X +J n φ +J Φ g , § g.S H Q ‘ M Φ Φ «Ρ rd φ Μ N S. Φ N (0 fi O'H HjSQ’ •H Φ Φ a a Ό Φ •P ω φ •id rd rd rd rd XJ 34 XJ •rH rd rd Φ a) TO rd r0 tn tn tn rd 3 3 Φ E E •H •H •H Φ b 0 0 E w ω X! 34 XJ E (0 3 3 cn cn ω m in kO in cn m CM rd rd M* cn CM rd rd rd rd rd rH rH rd rd rd rd rd rd CM n CM 0 0 CM n CM kO CM rd CM CM rd rd rd CM CM CM cn in in in m tn in in m m in CM CM CM CM CM CM CM CM CM CM m n m m m co OO CO CO co kD ko kO kD kO r*· Γ r- Γ r- 0 0 0 0 0 0 o 0 0 0 CM CM CM CM CM CM CM CM CM CM rd rd rH rd rH rd rd rd rd rd CM CM CM CM CM CM CM CM CM CM H rd rd rH rd rd rd rd rd rd Ρ» r* r- t*» f*·» Γ*· r* r-- r- r- c* r* r- r* r* U > O > □ > M w w W w ft ft ft ft ft ft ft ft ft ft rd CM cn in kD r* co cn 0 rd PVC = polyvinyl chloride The present invention is particularly advantageous in that it is possible to provide a jacketed cable having undulated tension relief elements without limiting the choice of jacketing material to only those that have certain shrinkage characteristics. Thus, for example, if flammability is a concern;e,g,, for cables used in mines, a jacket material of low flammability such as polyvinyl chloride can be used. Any one of a wide variety of cables can be produced in accordance with the invention and the desirable properties imparted by undulated tension relief elements, and in particular undulated glass fibers, are not limited to self supporting air cables. For example, such properties would also have applicability in cables which have to be drawn in lengths into pipes or shafts or which have to be placed into the ground.

Numerous modifications and variations of the present invention are possible in light of the above teachings and, therefore, within the scope of the appended claims the invention may be practiced otherwise than as particularly described.

Claims (6)

1. A method for the production of electric cable in a cable extruder head wherein a cable core, jacketing material and tension relief strands are fed into the extruder head so that the jacketing 5 material produces a tubular jacket for the core having the tension relief strands embedded therein, wherein the tension relief strands and the jacketing material are introduced into the extruder head at a velocity greater than the velocity of the cable 10 core and the velocity of the jacketing material having the tension relief strands therein is thereafter reduced to effect compression of the jacketing material and produce undulations in the tension relief strands. 15

2. A method as claimed in claim 1, wherein the jacket cable has a diameter greater than the diameter of the outlet of the head,

3. , A method as claimed in claim 1 or 2, wherein the tension relief strands are. glass fibres. 20

4. A method as claimed in any preceding claim, wherein the jacketing material is polyvinyl chloride. - 14 46028

5. A method for the production of electric cable substantially as herein described with reference to the accompanying drawings.

6. An electric cable when produced by the method 5 of any preceding claim .