US2376815A - Molded lightning-arrester block - Google Patents

Description

Patented May 22, 1945 UNITED STATES PATENT OFFICE MOLDED LIGHTNING-ARRESTER BLOCK Walter G. Roman, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 15, 1942, Serial No. 458,382

My invention relates to hard, molded compositions, and it has particular relation to high-pressure-molded compositions of this nature, having special electrical characteristics which adapt thein for use as the valve-element of a lightning arrester.

Heretofore, lightning-arrester blocks have been made from silicon-carbide crystals, or other particulate semi-conducting material, and a rela-.-

tive non-conducting binder. Formerly, this .binder was commonly clay, but'more recently, a

sodium-silicate binder, or water-glass, has been found to give better results, from the standpoint of desirable lightning-arrester characterisblocks out of the mold resulted in considerable wear on the mold, even though a hardened steel liner was utilized therein, and it also resulted in occasional damage to the arrester-blocks, in getting them out of the mold.

ducting lubricating-material, which was thoroughly mixed in with the ingredients of the block, and which materially. reduced the amount of pressure necessary to press the compressed blocks out of the mold, and to considerably reduce the damage, both to the mold and to the blocks.

It was unexpectedly found, however, that the added lubricating-material not only made it possible to utilize lower pressures, in compressing the material in'the mold, and still obtain the necessary electrical characteristics and life of the arrester, but also resulted in superior, and more uniform electrical properties, less critically. affected by unavoidable variations in the course of the'shop-practice, with respect to all of the electrical properties of the finished product, besides I making the block-length come out more uniformlywith smaller variations dependent upon the precise percentage of water in the mixture during the molding operation, or with respect to the precise molding-pressure. By the addition of the lubricating ingredient, I was enabled to reduce the molding-pressure, in a typical block, from 38 tons to 20 tons, obtaining a block having a lower discharge-voltage, which was practically unaffected by minor shop-variations in moisture-content and molding-pressure, besides also having a lower, and more uniformcut-off voltage, and an increased maximum surge-current discharge-catics. In either case, the mixture had to be 15 pacity. l molded, under considerable pressure, in a hy- An unexpected, but perhaps a most important draulic press; then it had to be forced out of the effect of my invention, is therefore to improve the mold by the application of hydraulic pressure; characteristics of the block and to make these v and then the molded blocks had to be subjected characteristics less critically afiected by unavoidto a heat-treatment for hardening the binder. 2o able minor variations in the shop-practice. When the binder was a clay, the heattreatment While these results and efiects were not at first consisted of a firing-operation at a high temanticipated, I believe, now, that they are a reperature hot enough to vitrify the clay so as, to sult of the lower molding-pressure which was make it a binder. When the binder is sodium needed in order to compress the particles into silicate, the heat-treatment consists in the use of 25 the close juxtaposition necessary for successful only a moderate oven-temperature, of the order binding and desirable electrical properties, and of C., to dry the excess-moisture out of this lower molding-pressure resulted from the rethe mixture and set the sodium-silicate binder. duced friction in causing the particles to slide Since the silicon carbide is a highly abrasive over one another, as the material was being commaterial, the operation of forcing the compressed 30 pressed in the mold. I believe that the high molding-pressures, previously used, resulted in a considerable amount of crushing of the siliconcarbide particles, into a finer powder than the form in which the material was first put into the mold, so that the resulting molded block had, in

from the size of powder-particles originally put into the mold. I believe that this crushing of the silicon-carbide particles, due to the excessive molding-pressures heretofore utilized, was sensitively affected by the stiffness of the mix, or its precise water-content, and by the precise amount of molding-pressure. It is also probable that this crushing was a random and unpredictable phe- 'nomenon, depending upon the precise hardness of the particular batch of silicon carbide which was being used at any moment, this hardness being subject to a certain amount of uncontrolled commercial variation. 2

I believe that the reduced crushing, which I obtain with my reduced molding-pressures, is the principal reason for the superior, and more uniform electricalcharacteristics of the finished block, but whatever may be the cause, I find that although .1 mg

blocks made in accordance with my present invention, have a more uniform and high electrical performance, as compared to blocks without the additional lubricating material,

The use of a reduced molding-pressure also results in considerably less damage to the molds, because of the reduced pressure betweenthe molded block and the side-walls of the mold, during the operation of pressing the block 'out of the mold, after the compressing operation; and the presence of the lubricating material also contributes to a reduction in the friction and abrasion, during the removal of the blocks, as well as the reduction in the frictional-pressure.

With the foregoing and other objects in view, my invention consists in the compositions, parts, combinations and methods hereinafter described and claimed, and illustrated in the accompanying drawing, wherein;

Figure 1 is a vertical sectional view of a lightning arrester embodying my invention in a prefermdnormnnd :Fig. 2.is a diagrammatic sectional view illustrative of the moldingnperations.

In Figure 1, lshow .a;practical .form of vembodiment of my invention, in a lightning-armster .utilizinga, porcelain housing I having a line-terminal! entering at the top, anda groundteuninal lat .thebottom. u'I'he porcelain housing 11 is .hollow,;containing,;at its lower end, a terminaleelectrode 4 which 115 properly sealed to'the bottom of .the housing by..a.sealing-gasketj. Disposedover this bottom terminal-electrode 4 ismy improvedrlightning arrester block 8, constituting the valve-element of the arrester, and consistingpf allarge percent of particles oi an abrasive :semieconducting material, preferablysacarbide, and preferably consisting essentially .of ilicon carbide crystals, held. together withasukfir We 1 elynon-eonducting binder, said binder lbeing preferably a mixture of dium silicateandwater .known as water-g ass.

5 n acco use with mylinvention, e also contains a;relatively,small admixture of a suhstantialiynombinding and relatively nonoonducting :lllbl'lCflDt-rlfifitfil'lfll, preferably cla .a so :uize,'as the lubrican, various other materials ;including s oils e etc. The lubricant which is a e the mixs ould not be ina sumcient quantity to interfcre'seriously-with the bonding properties of the sodiimi-silicatesor other bond which-is used. Experiencehas -shown:that the :productis not very critical as :to the precise amount of lubricant which .is :added, :good .results having been obtained by 'theadditionpf from ,1 t 6% of slay to :the original mixture, before molding. mixture is then molded under high ressure, by which Imean a'pressure of the order ns, more or less, and baked,.as previously described, the silicon-carbide er being a material which ishardened at alow baking-temperature which is considerably lower than :the firing-temperature which is necessary to vitrify the clay, so that the clay whichlhave added to the mix is not vitrified into a binder, but retains its clayey :characteristics, ,as a lubricant.

:Burmounting the-valve-element part of the arr-ester is a series-gap device which may .be of any known ordesired form. LInthe-particular ,form of arrester which I have chosen for illustration, the series-gap .device is shown as comprising a quench-gapportion -14, whichis, in turn, surmounted by a. .spark-gapor. switchinggap structure L5 which is,:in .turn, surmounted is placed.

by a spring it which holds the various parts together under pressure, said spring bearing up against a line-terminal electrode H at the top of the casing.

In Fig. 2, I have diagrammatically indicated the molding process by which a mixture 6' of powdered silicon carbide, sodium silicate, and clay, with about 6% of water, isplaced within a, cylindrical steel mold 2| having a cylindrical hardened-steel liner 22, within which the mix 6' Upper and lower hardened-steel plungers 23 and 24 are then inserted into the mold, with the mix 6' in between, and the plungers 23 and 24 are pressed together in a powerful hydraulic press 25-26, which compresses the material into the desired density, which experience has shown to give the required electrical characteristics for any given batch of silicon carbide, of any given coarseness of grain. Upon the completion of the compressing step, the upper plunger 23 is removed and replaced by a so-called stripper, which consists .of-a: cylindrical tubular member 2 I which presses against theitop of the mold 2|, and permits .the molded materialS to be pressed out of-the mold bythe lower plunger 24, upon the applicationllof rhydraulic pressure, again. After .the compressed cake 6' has been taken out of the mold, it'is dried insan oven, and ready for use.

While I have described ,my invention in a preferred form as to materials, processes, structuresand combinations, and while I haveexplained it in terms of my present theories or beliefs as to its operation, it is to be understood 'that my invention is not limited to anyparticular theory or understanding, and that it is susoeptibleof a fairly wide variety of applications and embodiments, some of which have been outlined hereinabove. I desire, thereforathatmy appended claims shall .be given the broadest construction consistent with their language,

' liclaim as my invention:

:-1., A lightning-arrester valve-element comprising a hard, high-pressure-molded composition. molded .at a pressure .of :the .order of .20 tons. more or less, said composition includinganabrasive; particulate semi-conducting material, air-elatively non-conducting bindertherefor, andarelatimly. small quantit of a substantially nonebinding and relatively non-conducting lubricating material which has lubricating properties in the finished device.

2. -A lightning-arrester.valve element'comprise ing :a hard, high-pressuro-molded composition, molded at a pressure-of the order of .20 --tons. more or less, said composition includinganabrasive particulate semi-conducting-material. :amela- .t-ively non-conducting sodiumesilicate binder therefor, and a relatively small quantity of aznelativelynon-conducting lubricating material which does not become a strong. hard binding-material atoning-temperatures of a: range suitable {or hardening the sodium-silicate binder.

3. A lightning-anester valve-element comprising a hard, high-pressure-molded composition, moldedat a pressure of the order of 20 tons, more unless, said composition anabrasive particulate semi-conducting material, :a binder therefor, and a relatively small :quantityof clay, the binder being of a material which-is capable of .being hardened into a binding-material at a temperature lower than the firingtemperature which is necessary to vitrify theclay and thusv make. it a binder. 1

- '14. Alightning-arrester valve-elementAompria-f ing a hard, high-pressure-molded composition, molded at a pressure of the order of 20 tons, more or less, said composition including an abrasive particulate semi-conducting material, a sodium-silicate binder therefor, and a relatively small quantity of clay which has lubricating properties in the finished device;

5. A molded product having electrical char-' acteristics suitable for lightning-arresters, said product resulting from the process of mixing together silicon-carbide crystals and water-glass, with a small quantity of a non-binding and relatively non-conducting lubricant-material, molding at a molding-pressure of a number of tons, and drying at a moderate oven-temperature of the order of 175 C. until the excess-moisture is driven out and the water-glass binder is set, the lubricant-material remaining lubricating and non-binding after the drying operation.

6. The invention as defined in claim 5, characterized by said lubricant-material being clay.

'7. The method of making a molded product WALTER G. ROMAN.

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