August i, 1907.] 



THE INDIA RUBBER WORLD 



333 



The Insulated Wire Manufacture- I. 



By a Practical Man. 



NO department of the rubber industry has made more rapid 

 progress than that devoted to the insulation of wires for 

 electrical uses, though progress in some others may have 

 been more spectacular. The maiuifacture of a valve or a piece 

 of hose is no very exciting matter. Even the specifications of a 

 railway company for air-brake hose, or of a city for its fire hose, 

 while interesting, do not appeal to the imagination. It is within 

 the purlieus of wire insulation that the efficiency of rubber com- 

 pounding comes face to face witli that grim and myster'Ous 

 force found in the electric current. Here is not a question of 

 finish, of color or of cost, so much as resistance and resilience. 

 These matters being well established, users of insulated wire are 

 apt to specify what materials shall be used and how the goods 

 shall be made. That this feature may be the better understood 

 the following specifications are given. 



Here is an easy one : A coating of fine Para is to be put upon 

 the wire, followed by a layer containing 40 per cent, of fine Para, 

 which in turn shall be covered by a layer containing 30 per cent, 

 of fine Para, and finally a rubber-coated tape shall be wound 

 over all. This insulation shall not contain more than 5 per cent, 

 of acetone extract ; a strip with marks upon it 2 inches apart 

 shall stretch to 6 inches between marks, and recover to 2^^ 

 inches in one minute, and then stretch 9 inches without breaking. 



This one is a bit stiflfer : All layers of pure Para must be 98 

 per cent, pure, elastic, tough and free from flaws and holes. The 

 compound to contain 39 to 44 per cent, of fine Para and not 

 more than 3 per cent, of sulphur. This sulphur must be com- 

 bined with Para so that not more than two-tenths of i per cent, 

 shall remain in the compound as free sulphur. A piece one-half 

 inch wide by one-thirty-second thick, shall, on stretching, show a 

 breaking strain of 1. 000 pounds per square inch and stretch 3I/2 

 times its length. Then, subjected to stress of 900 pounds per 

 square inch for one minute, must recover to 50 per cent, of its 

 original length within ten minutes. 



But the higher you go the harder it becomes, and "there's no 

 elevator." This specification is for a cable : 



The insulation must stand stretch of three times a given length 

 without breaking and repeated four times must return within 50 

 per cent, of the original length by ten minutes, and then stretch 

 3J/J times length without breaking. Then, after soaking in water 

 sixty hours, it must withstand an alternating voltage of 2.500. 

 After the cable is made up the insulation of each conductor 

 (wire) shall withstand without rupture for five minutes, a pres- 

 sure of 1,000 volts alternating current, and finally, after seventy- 

 two hours soaking, the insulation resistance of the completed 

 cable shall not be less than 3.000 megohms per mile at 60° F. 

 The compound must contain no reclaimed rubber, rubber sub- 

 stitute, asphaltum, lamp black, paraffine, ozokerite, or oils — 30 

 per cent, pure gum only. What do you think of that, com- 

 placent manufacturers gf "ordinary" rubber goods? 



To insulate a single wire is no great problem, but the assemb- 

 ling and binding into a cable of from 7 to over 2,000 wires is a 

 different matter. Let us examine one form of cable construction. 

 About a single insulated wire as a central conductor are wound 

 six other wires. These are then covered with a layer of fric- 

 tional tape. This bundle is next surrounded by twelve other 

 wires and the whole again taped. Next comes a layer of dry 

 jute covered by a layer of tarred jute. These jute layers are 

 twisted on in opposite directions and bound spirally with cotton 

 thread to keep the fibers snugly in place, while, as a finisher, an 

 armor of galvanized iron wire is wound over all. The outside 

 diameter of such a cable must not exceed a given size, as, for 

 example, l 1-16 inches, nor weigh to exceed 6.500 pounds to the 

 mile. When ready for shipment it will be coiled upon huge 



wooden drums or reels, 4^ feet long by 6'/2 feet high. Each 

 reel will contain from one to two miles of cible. 



In some situations a fireproof cable is desirable. A large num- 

 ber of wires frequently enter into their construction, which are 

 twisted together and covered with a jacket of compounded rub- 

 ber insulation. This jacket is wound with asbestos tape. Then 

 comes another and heavier cover of asbestos fiber, followed by 

 one of cotton, both braided on. A finishing layer of flame-proof 

 compound completes the work. 



Cables are generally made up of bare wires, but not always. 

 What is called an "aerial" cable is sometimes made up of small 

 wires each separately insulated. These are then formed into a 

 cable, which receives a cover of tape, then one of jute and one of 

 braided cotton, covered by a coat of waterproof varnish. 



Some years ago one of the electric lighting companies had a 

 switchboard cable made of unusual size. It contained 61 

 bundles of 37 wires each, or a total of 2257 wires. These wires, 

 twisted into a huge rope, were covered with a heavy coat of 

 rubber insulation, which received a finishing armor of Xo. 4 

 B. W. (i. galvanized iron wire. 



Thus it will be seen that the maker of insulated wire has 

 problems enough and to spare. They have proved stimulating 

 to his inventive faculty, as is evidenced by the large number 

 of patented or specially prepared compounds, chiefly valuable 

 for their insulation properties. Many of them will be found de- 

 scribed in Mr. Pearson's "Crude Rubber and Compounding In- 

 gredients," and it is interesting to observe the various and 

 peculiar substances that have the property of electrical resistance 

 attributed to them. There is "insullac," containing wood or 

 vegetable fiber ; "marloid." employing animal hides ; "viscoid," 

 tar and pitch ; "dermatine," gutta-percha and rubber ; "vul- 

 cabeston," asbestos and rubber ; "kerite," vegetable oils, coal tar 

 and bitumen ; also many patented and safe guarded compositions 

 of which less is known. 



Compounding for wire insulation does not differ essentially 

 from methods employed in the mechanical rubber goods lines. 

 Coniliinations of cheap gums with Para are practicable and val- 

 uable, for, while much insulation is used containing 30 to 80 

 per cent, of fine Para, there is a corresponding demand for cheap 

 stocks always provided they meet insulation requirements. 



One other thing is imperative : Compounds and gums must be 

 clean of grit, sand bark, and the like, for such substances if al- 

 lowed to remain, are almost sure to be found by the electric cur- 

 rent, with disastrous results. So essential is cleanliness that 

 mineral substances are sometimes sifted through silk screens. 

 The average thickness of insulation for small wire being gaged 

 by 64ths of an inch, it will be seen that a grain of sand, pin point 

 in size, is always a ground of anxiety and sometimes the cause 

 of loss. 



Let us review briefly the manufacturing processes necessary 

 to produce an insulated wire. They will be embraced under the 

 following divisions : Mixing of compounds, tubing machine 

 work, single and stranded conductors, cables, chemical analysis, 

 testing and repairing. 



We will assume that the factory has modern appointments, 

 that all rubber used in compounding is clean and dry as bone, 

 that other materials have been carefully dried and sifted. The 

 mill, heated to just the right temperature, is ready for the in- 

 gredients that are to form the compound. The gum is first 

 placed on the rollers and worked a few moments until softened 

 sufficiently to absorb quickly the mineral powders. They are at 

 hand in pail or pan, and are fed into the batch by degrees, or 

 all at once as conditions or practice may require. Possibly there 

 may be over the mill a mixing box, the workman pulling a slide 



