Method of manufacturing electrical cable, and resulting product, with reduced required installation pulling force

Abstract

Disclosed are cable types, including a type THHN cable, the cable types having a reduced surface coefficient of friction, and the method of manufacture thereof, in which the central conductor core and insulating layer are surrounded by a material containing nylon or thermosetting resin. A silicone based pulling lubricant for said cable, or alternatively, erucamide or stearyl erucamide for small cable gauge wire, is incorporated, by alternate methods, with the resin material from which the outer sheath is extruded, and is effective to reduce the required pulling force between the formed cable and a conduit during installation.

Claims

1. A method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable having a reduced installation pulling force through a conduit, the method comprising: introducing pellets comprising polymeric nylon material to an extrusion head; advancing at least one conductor wire through the extrusion head; introducing polyvinyl chloride (PVC) into the extrusion head; melting the PVC into a molten state in the extrusion head; melting the nylon pellets into a molten state in the extrusion head; extruding the molten PVC around the conductor wire to form a first layer of a sheath; extruding the molten nylon around the first layer of the sheath to form a second layer of the sheath; and, introducing a pulling lubricant into the THHN cable manufacturing process at at least one stage selected from; compounding the pulling lubricant with the polymeric nylon material to form lubricated pellets from which the sheath is to be formed, mixing the pulling lubricant with the polymeric nylon pellets before introduction of the mixture into the extrusion process, or introducing the pulling lubricant at a point after the polymeric nylon material has been introduced into the extrusion head.

2. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 1, wherein the pulling lubricant is introduced at a point after the polymeric nylon material has been introduced into the extrusion head, and wherein the step of introducing the pulling lubricant at a point after the polymeric nylon material has been introduced into the extrusion head further comprises introducing the pulling lubricant directly into the extrusion head.

3. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 1, wherein the pulling lubricant comprises silicone.

4. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 3, wherein the pulling lubricant comprises a high molecular weight silicone based pulling lubricant.

5. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 1, wherein introducing the pulling lubricant at least in part causes the resulting THHN electrical cable to have an average coefficient of friction against an interior surface of a PVC conduit setup of less than about 0.2.

6. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 3, wherein the pulling lubricant comprises a low molecular weight silicone based pulling lubricant.

7. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 1, wherein the at least one conductor comprises a plurality of grouped conductors.

8. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 1, wherein the at least one conductor is a large gauge conductor.

9. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 8, wherein the at least one conductor comprises an AWG 4/0 conductor.

10. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 1, wherein the THHN electrical cable has a round profile.

11. The method of manufacturing a thermoplastic high heat-resistant nylon-coated (THHN) electrical cable according to claim 1, wherein melting the nylon pellets in the extrusion head comprises causing the nylon polymer to reach at least about 500° F.

12. A THHN electrical cable manufactured according to the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other details and aspects of the invention, as well as the advantages thereof, will be more readily understood and appreciated by those skilled in the art from the following detailed description, taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a diagrammatic representation of typical equipment used in the manufacture of cable in accordance with the present invention, when mixing the lubricant with the nylon material prior to extrusion;

(3) FIG. 2 is a graphical representation of test data comparing the effect of different pulling lubricants in small size THHN cable in which the outer sheath material is nylon;

(4) FIG. 3 is a graphical representation of test data comparing both the effects of different pulling lubricants and different percentages of pulling lubricant in large size THHN cable in which the outer sheath material is nylon;

(5) FIGS. 4 and 5 are representations of test devices which may be used to create the aforementioned test data;

(6) FIG. 6 is a section view of a THHN cable produced in accordance with the process of the present invention;

(7) FIG. 7 is a diagram illustrating a first type of joist-pull test apparatus used to characterize the present invention; and

(8) FIG. 8 is a diagram illustrating a modified type of joist-pull test apparatus used to characterize the present invention.

DESCRIPTION

(9) Referring initially to FIG. 1, there is depicted typical equipment 11 for manufacturing electrical cable 12 in accordance with one process of the present invention. The outer sheath of the cable is of an extruded polymer material, specifically nylon. The equipment 11 may include a reel 13 which supplies conductor wire 14 to an extruding head 15. In flow communication with the extrusion head is a tank 16 of the nylon pellets 17. A tank 18 with the desired pulling lubricant 19 is adapted to be in flow communication with the tank 16 by way of conduit 22, thus enabling the mixing of the pulling lubricant with the nylon pellets 17, the mixture thereafter introduced into the extruder. Alternatively, the tank may be adapted to be in fluid communication with the extruder or extrusion head 15, by way of conduit 23, downstream from the point of entry of the nylon material, thus allowing the pulling lubricant to mix with the nylon material while in its molten state in the extruder or extruder head. A cooling box 20 for cooling the extruded product is provided, and a reel 21 is positioned for taking up the resulting cable assembly 12. When the final cable construction is such that there are multiple layers of sheath material, the pulling lubricant should desirably be incorporated into the outermost layer.

(10) As is therefore evident, the pulling lubricant can be mixed with the material from which the outer sheath is to be extruded prior to extrusion or, alternatively, introduced into the extruding head for subsequent mixing with the molten extrusion material as the sheath is being formed. As a further alternative, the pulling lubricant can be initially compounded with the polymeric material of the pellets themselves in a process upstream of that depicted in FIG. 1, thereby forming lubricated polymeric pellets, thus eliminating the need for tank 18 and conduits 22 and 23.

(11) Polymeric materials that can be used for an insulating layer or outer sheath of different type of cable include polyethylene, polypropylene, polyvinylchloride, organic polymeric thermosetting and thermoplastic resins and elastomers, polyolefins, copolymers, vinyls, olefin-vinyl copolymers, polyamides, acrylics, polyesters, fluorocarbons, and the like. As previously described, for the THHN cable of the present invention, the conductor core of a single solid or stranded conductor is surrounded by an insulating layer of PVC covered by an outer sheath of a polyamide (e.g., nylon).

(12) In accordance with the testing subsequently described, it has been determined that, for THHN cable, silicone oil is the preferred pulling lubricant. For small gauge THHN wire, erucamide is an alternative preferred pulling lubricant, to be incorporated in the nylon sheath.

(13) The efficacy of these pulling lubricants for the nylon sheath, and specifically an optimum range for the quantity of such lubricants, in accordance with the invention, has been proven by the use of various tests. Prior to discussing the results of such tests, these test methods and their equipment are described as follows:

(14) Testing Methods

(15) Coefficient of Friction Test

(16) Referring now to FIG. 4, diagrammatically illustrated is the apparatus for a coefficient of friction test. The coefficient of friction test apparatus was developed to give a consistent way to determine the input values necessary to use the industry-standard program published by PolyWater Corporation to calculate a real-world coefficient of friction for a given cable being pulled in conduit. Given the inputs for the conduit setup, the back tension on the wire, and the pulling tension on the pulling rope, this program back-calculated a coefficient of friction for the cable by subtracting the back tension from the pulling tension and attributing the remaining tension on the rope to frictional forces between the cable and the conduit.

(17) The overall setup used a pulling rope threaded through ˜40′ of PVC conduit (appropriately sized for the cable being pulled) with two 90° bends. Three 100′ pieces of THHN cable were cut and laid out parallel to one another in line with the first straight section of conduit, and the rope connected to them using industry-standard practice. The other end of the THHN cable was attached to a metal cable which was wrapped around a cylinder with an air brake to allow the application of constant back tension on the setup.

(18) The metal cable was threaded through a load cell so that it could be monitored in real-time, and continuously logged. The pulling rope was similarly threaded through a load cell and constantly monitored and logged. Values for both back tension and pulling tension were logged for the time it took to pull cable through the conduit run. These values were then averaged and used in the PolyWater program to calculate coefficient of friction.

(19) Specific Type THHN Tests

(20) Initial tests of small gauge Type THHN wire were performed using the small-scale tension tester shown in FIG. 5. In this test, multiple individual American Wire Gauge (AWG) size 12 THHN wires were provided on the payoff and attached to a metal pulling tape that was threaded through an arrangement of ½″ conduit that included about 50 feet of straight conduit and four 90° bends. A pulling rope was attached to the other end of the pulling tape and a tugger was used to pull the cable arrangement through the conduit. The rope was threaded through a pulley arrangement that used a load cell to monitor rope tension while the wire was pulled through the conduit. This tension was continuously logged and averaged to give an average pulling force for the pull. This force correlated directly to the coefficient of friction for the cable.

(21) Using the data obtained from the small scale pull test of FIG. 5, FIG. 2 illustrates a comparison of the different required pulling forces for a small gauge cable consisting of multiple (AWG) size 12 THHN conductors. The test subjects had 0.25-0.85% of two different potential pulling lubricants, erucamide and stearyl erucamide, mixed into the outer nylon sheath. Results of the test are shown in FIG. 2 and compared to the results for the standard THHN product without any pulling lubricant and with the externally applied industry-standard Y77. This test shows that erucamide is one preferred lubricant for small gauge THHN cable, in an optimum percentage of approximately 0.85%, by weight.

(22) Next, large gauge Type THHN cable was tested. Using the coefficient of friction test of FIG. 4, FIG. 3 illustrates the different values of surface coefficient of friction of the exterior surface of the sheath, for cables consisting of three individual large gauge AWG 4/0 THHN conductors, for varying percentages of the pulling lubricant, silicone oil, of varying molecular weights. The two lubricants compared in FIG. 3 are a high-molecular weight silicone oil (HMW Si) and a lower molecular weight silicone oil (LMW Si). Comparison results are shown for this same THHN cable arrangement lubricated with industry-standard Y77, as well as with respect to three other trial pulling lubricants, fluorinated oil, molydisulfide, and stearyl erucamide. The results in FIG. 3 illustrate that, while other pulling lubricants can reduce the coefficient of friction of the exterior surface of the cable, the preferred pulling lubricant for THHN cable, and particularly large gauge THHN cable, is a high molecular weight silicone oil added at a level of approximately 9%, by weight, or higher.

(23) In accordance with an advantage of the present invention, the pulling lubricant that is incorporated in the sheath is present at the outer surface of the sheath when the cable engages, or in response to the cable's engagement with, the duct or other structure through which the cable is to be pulled. For the THHN cable of the present invention, where the outer sheath is of nylon and the preferred pulling lubricant is high molecular weight silicone oil, this silicon-based lubricant permeates the entire nylon sheath portion and is, in effect, continuously squeezed to the sheath surface in what is referred to as the “sponge effect,” when the cable is pulled through the duct.

(24) Compounding with Pulling Lubricant

(25) As previously described, the pulling lubricant may be incorporated into the extruded sheath (or the outer layer of the cable sheath if the sheath is of multiple layers) by initially compounding the lubricant with the (outer) sheath material to be extruded. To prepare the lubricated blend of the present invention, the resin and additional components, including the pulling lubricant, are fed into any one of a number of commonly used compounding machines, such as a twin-screw compounding extruder, Buss kneader, Banbury mixer, two-roll mill, or other heated shear-type mixer. The melted, homogeneous blend is then extruded into strands or cut into strips that may be subsequently chopped into easily handled pellets. The so-prepared lubricated pellets are then fed into the extruder for forming the outer sheath.

(26) THHN Cable

(27) THHN and THWN-2 are types of insulated electrical conductors that cover a broad range of wire sizes and applications. THHN or THWN-2 conductors are typically 600 volt copper conductors with a sheath comprising an outer layer of nylon surrounding a layer of thermoplastic insulation and are heat, moisture, oil, and gasoline resistant. THHN cable is primarily used in conduit and cable trays for services, feeders, and branch circuits in commercial or industrial applications as specified in the National Electrical Code and is suitable for use in dry locations at temperatures not to exceed 90° C. Type THWN-2 cable is suitable for use in wet or dry locations at temperatures not to exceed 90° C. or not to exceed 75° C. when exposed to oil or coolant. Type THHN or THWN-2 conductors are usually annealed (soft) copper, insulated with a tough, heat and moisture resistant polyvinylchloride (PVC), over which a polyamide layer, specifically nylon, is applied. Many cables, including those addressed by the present invention, can be “multi-rated,” simultaneously qualifying for rating as THHN or THWN-2.

(28) Referring now to FIG. 6, there is illustrated a THHN cable 24 constructed in accordance with the process of the invention. The cable is characterized by a sheath comprising an extruded layer 25 of PVC insulation material and an overlying extruded thin layer 26 of nylon, the sheath surrounding a central electric conductor 27 which is usually, though not exclusively, of copper. The only limitation on the type of pulling lubricant to be incorporated into the extruded outer nylon sheath is that it be sufficiently compatible with nylon to be co-processed with it, and particularly when compounded with nylon, that it be robust enough to withstand the high processing temperature for nylon, which is typically about 500° F. However, it has been found that for THHN cable, this lubricant is preferably a high molecular weight, high viscosity silicone fluid; for small gauge THHN wire, as an alternative, erucamide or stearyl erucamide.

(29) Two industry-standard processes can be used to produce this product, the so called co-extrusion method and the tandem extrusion method. In both processes, the conductor, either solid or stranded, is first introduced into the extrusion head where the heated, melted PVC insulation compound is introduced and applied to the circumference of the conductor. In the co-extrusion process, the melted nylon compound is introduced into the same extrusion head and applied together with the PVC to the conductor, in a two-layer orientation. In the tandem process the PVC-coated conductor leaves the first extrusion head and is introduced into a second, separate extrusion head where the melted nylon is applied to the surface. In both cases, the final product is then introduced into a cooling water bath and ultimately the cooled product is wound onto reels. In either case, the nylon material is preferably initially compounded with the pulling lubricant to provide the so-lubricated extrusion pellets.

(30) As shown in FIG. 2, small gauge THHN cable prepared, as described, with nylon as the outer layer of the sheath, and containing 0.25%, 0.50% and 0.85%, by weight, of stearyl erucamide, had an average pulling force of 18.1 lbs, 16 lbs and 18.5 lbs, respectively. Even better, small gauge THHN cable containing 0.25%, 0.50% and 0.85%, by weight, of erucamide had an average pulling force of 13.2 lbs, 10.3 lbs and 9.6 lbs, respectively. Comparably, the pulling force for a THHN cable with no pulling lubricant was measured at 38.5 lbs, and THHN cable with only Y 77 applied to the exterior surface was measured at 15.3 lbs. FIG. 3, on the other hand, illustrates the results when silicone oil is used in THHN cable, compared to other potential lubricants, illustrating silicone oil as a much preferred pulling lubricant for this type cable.

(31) To understand the effects of the jacket lubricant system on the ease of pull, variations of the UL (Underwriters Laboratories, Inc.) joist pull test were utilized.

(32) The joist pull test outlined in UL719 Section 23 establishes the integrity of the outer PVC jacket of Type NM-B constructions when subjected to pulling through angled holes drilled through wood blocks.

(33) The first variation of the test apparatus (see FIG. 7) consists of an arrangement of 2″×4″ wood blocks having holes drilled at 15° drilled through the broad face. Four of these blocks are then secured into a frame so that the centerlines of the holes are offset 10″ to create tension in the specimen through the blocks. A coil of NM-B is placed into a cold-box and is conditioned at −20° C. for 24 hours. A section of the cable is fed through corresponding holes in the blocks where the end protruding out of the last block is pulled through at 45° to the horizontal. The cable is then cut off and two other specimens are pulled through from the coil in the cold-box. Specimens that do not exhibit torn or broken jackets and maintain conductor spacing as set forth in the Standard are said to comply.

(34) Pulling wire through the wood blocks provides a more direct correlation of the amount of force required to pull NM-B in during installation. Because of this relationship, the joist-pull test is initially the basis for which ease of pulling is measured, but a test for quantifying this “ease” into quantifiable data had to be established.

(35) Accordingly, and as shown in FIG. 8, a variable-speed device was introduced to pull the cable specimen through the blocks. An electro-mechanical scale was installed between the specimen and the pulling device to provide a readout of the amount of force in the specimen. To create back tension a mass of known weight (5-lbs) was tied to the end of the specimen.

(36) Data recorded proved that NM-B constructions having surface lubricates reduced pulling forces.

(37) A 12-V constant speed winch having a steel cable and turning sheave was employed; the turning sheave maintains a 45 degree pulling angle and provides a half-speed to slow the rate of the pulling so that more data points could be obtained. Holes were drilled in rafters whereby specimens could be pulled by the winch.

(38) It was found using this method that lubricated specimens yielded approximately a 50% reduction in pulling force when compared to standard, non-lubricated NM-B specimens. The results are shown in Tables 1 and 2 wherein the data was recorded at five second intervals.

(39) TABLE-US-00001 TABLE 1 Specimen Description Test Pt. Manufac- Manufac- Manufac- Manufac- Manufac- Manufac- Control Control Present Descr. turer A1 turer A2 turer A3 turer B1 turer B2 turer B3 1 2 Invention 1.sup.st Point 26.8 48.3 37.8 37.4 16.5 41.9 24 2.sup.nd Point 34.6 51.1 35.2 38.1 41.6 42 20.5 3.sup.rd Point 33.7 46.8 32 33 40.2 38.7 20 4.sup.th Point 38.6 49.8 34.7 34.6 41.3 29.5 17.4 5.sup.th Point 33.1 44.8 34.2 32.5 41.3 34.3 20.2 6.sup.th Point 28.6 44.7 32.2 33.2 42.5 35.9 15.8 7.sup.th Point 5.5 51 32.2 33.9 41.1 37 17.2 8.sup.th Point 26.8 49.2 33.9 33 40.9 38.4 17.3 9.sup.th Point 21.9 52.5 32.6 30.6 42.7 37.3 21.9 Average 30.51 48.69 33.87 34.03 41.45 37.22 19.37 AAA— Denotes Outliers Test in Table 1 performed at a constant speed with winch using ½ speed pulley Test in Table 2 performed on cable with 5 #weight suspended at building entry Std. Prod. Average Present Invention 37.6289 19.37

(40) TABLE-US-00002 TABLE 2 Specimen Description Test Pt. Manufacturer A Manufacturer B Control 1 Control 2 Control 3 Invention A Invention B Descr. 14-2 14-2 14-2/12-2 14-2/12-2 14-2/12-2 14-2/12-2 14-2/12-2 1.sup.st Point 34 32.6 50 47.5 40.2 21.5 12.3 2.sup.nd Point 35 35.7 50.6 38.3 37.5 22.9 12.8 3.sup.rd Point 35.5 31.2 46.7 43.2 27.5 29 12.1 4.sup.th Point 37.7 35 44.5 46 36.8 22.4 14.9 5.sup.th Point 40.5 30.6 46.2 39.5 36 23.3 11.9 6.sup.th Point 32.9 28.8 40.9 35.7 41.2 21.1 12.5 7.sup.th Point 44.2 32.4 52.8 37.5 37 21.6 11.7 8.sup.th Point 43 32.4 40.7 T1.1 31.7 22.5 11.7 9.sup.th Point 43.4 30.5 40 31.1 19.2 11 10.sup.th Point 40 11.6 Average 38.62 32.13 45.82 38.50 35.99 22.61 12.25 14-2/12-2 14-2/12-2 14-2/12-2 Control Avg. Invention A Invention B 40.103241 22.61 12.25

(41) Although the aforementioned description references specific embodiments and processing techniques of the invention, it is to be understood that these are only illustrative. For example, although the description has been with respect to electrical cable, it is also applicable to other types of non-electrical cable such as, for example, fiber optic cable. Additional modifications may be made to the described embodiments and techniques without departing from the spirit and the scope of the invention as defined solely by the appended claims.