US1842433A - Resistance device and insulator - Google Patents

Description

Jan. 26, 1932. c. o. TERWILLIGER 1,842,433

RESISTANCE DEVICE AND INSULATOR Filed Dec. 28, 1928 Patented Jan. 26, 1932 UNITED STATES,

PATENT OFFICE ,cnanms o. rEawILLIeEa; or MOUNT VERNON, new 2031:, assrenon re WARD LEONARD mncrarc comranv, A conroaa'rron or nnwxonx RESISTANCE DEVICE AND INSULATOR Application filed December 28, 1828. Serial No. 328,938.

This invention relates to improved resistance devices or resistance units and improved insulators or insulating material. It is particularly applicable to those forms of resistance devices in which the resistive conductor is Wound or applied to a support of insulating material and covered with an insulating coating, such as vitreous enamel. The invention is -also applicable where the apparatus or device including the insulating material is s'ubjected during manufacture, or during use, to comparatively high temperatures. For example, the improved insulatin material is ada ted to be moulded aroun metal portions 0 the device and then fired to form the insulator and support for the metal parts. 7

It has been customary in the manufacture of resistance units, in the form of tubular units, to use porcelain as the refractory tubular'insulating support. On this tube the-resistive conductor. of the proper size and length'is wound and attached to suitable terminals, which latter may be affixed to the tube prior to the winding of the resistive conductor, or after the winding of the conductor. The next operation is to apply an adhesive vitreous enamel coating to the tube over the conductor and terminals. This is accomplished by dipping the unit in the enamel material, or spraying the enamel material over the unit and conductor, after which the unit-is heated or fired in a furnace to the necessary temperature to melt or mature theenamel to form a glazed surface over the unit, thus forming an insulating protective coating impenetrable to moisture. In

order to mature the enamel, temperatures of from 1.000 to 1,200 degrees F, or more are required.

The usual units of this type are composed of materials having relativeexpanslon and coeflicients of expanslon as follows: v

Porcelain 2.54.6 or .0000025 to .0000046 Resistive conduct0l 1 4. or .0000140 Terminals 16. or .0000160 Enamel 9.41. or .0000090 to .000011 Thefirst column of numerals in the above table indicates approximately the relative expansion of the difierent parts over the range of the usual enameling temperatures; and the case of fine conductors, they ma shift to such an extent that they touch each other at diflerent points and form short-circuits. This causes a lack of uniformit in the resistance of the units when finis ed. When the variations are of sufficient importanc'e, they cannot be used for the pur ose intended, requiring their rejection. Xnother difiiculty arises on account of the unequal expansion of the parts due tothe enamel flowing into the space between the resistive conductor and the refractory tube when the enamel has become softened during the maturing operation. The greater expansion of the conductor with reference to the porcelain body causes the turns of the conductor to sag and allow the softened enamel to pass between the turns of the conductor and the tube.

Then when the units are cooled, after the maturin operation, the resistive conductor is place under considerable strain, and in the actually break in one or more places an cause an open circuit in the resulting unit.

Another trouble difficult to overcome arises I in the adhesive vitreous enamel coating itself due to the difi'erent expansions of the parts composing-such resistance units. This difiiculty is the development, unless extreme care be taken in the manufacture, of innumerable cracks called crazin This crazin sometimes permits molsture to reach t e coated conductor and in time deteriorates the same and sometimes is sufliciently present to cause the unit to have an unsi h ance, instead of a smooth glazed surface.

These and other difliculties have required tly appeaiw very great care in the manufacture of tubular. resistance units, and also units of other forms, such as plate or disk forms, causing increase in time and cost of manufacture and constant inspection of the work as it progresses, as well as careful testing of the final product.

Recently the demand for high resistance units occupying comparatively small space has made it necessary to use an extremely small size of resistive conductor and to also wind the conductor on the tube to a very close )itch. Thus the difficulties due to the diferent expansion of the parts become particularly serious in the manufacture of such small high resistance units.

The main object of this invention is to produce an insulating body which will not only have high insulating qualities comparable to that of porcelain, but will have a coefficient of expansion within the range of temperatures to which the body is to be subjected, that will substantially correspond or more nearly correspond to the expansion of the metal portions and other parts with which the insulating body is united, and thereby avoid the difficulties above referred to.

Another object is to produce an improved insulating refractory body, which requires less time of firing than that of the usual porcelains.

Another object is to produce an insulating body which will have insulating properties equal to, or superior to, that of porcelain and have equal or greater strength and toughness than that of porcelain. Another object is to produce an insulating material not only having substantially the sameexpansion as the resistive conductor, or other metal parts, but one which will also withstand the sudden expansion and contraction without cracking or breaking during the manufacture thereof and during the firing to form the enamel coating. Other objects and advantages of this invention will be understood from the following description and accompanying drawings.

Fig. 1 is a side view of an illustrative resistance unit to which the invention is applicable and showing the unit before the enamel coating is applied; Fig. 2 is a longitudinal cross-section thereof with the enamel applied; Fig. 3 is a plan view of another form of illustrative resistance device to which the invention is applicable; and Fig. 4 is a crossscction showing the improved insulating .material moulded about metal parts.

Referring to Figs. 1 and 2, the improved insulating body 10 is shown in the form of a tube and has wound around the same the resistive conductor 11. The latter has connected at its endstwo terminal bands 12. The enamel coating 13 is applied to cover the resistive conductor and terminal bands, the enamel material first being applied by dip- Fig. 3 shows another form of resistance .unit or device wherein the improved insulating body is in the form of a disk with an upturned circular edge 14. The resistive conductor 11' is applied thereto by positioning the same on the face thereof, usually in zigzag form and has its ends connected to terminals 15 mounted in the upturned rimof the insulating body. An enamel coating 13' is applied to cover and adhesively hold the resistive conductor in position upon the insulating body. Various other forms of resistance devices or resistance units may embody my invention and the illustrations are of known types to which the'present invention is applied.

Fig. 4 illustrates an example of the use of this improved material where it is moulded about parts before firing, and where the parts may be metals of a character capable of withstanding the heat of the maturing temperatures. The improved insulating material 16 is shown moulded about metal terminal posts 17, and in the material, holes 18 are formed before firing adapted for securing the terminal block to a support.

It is evident that if the parts forming a resistance unit, or other devices, have substantially the same coefficient of expansion over the range of temperatures to which they are subjected when combined, during manufacture, the difliculties already explained are overcome. In accordance with the present invention, the insulating body is one which, in addition to having high insulating properties, toughness and strength, also has sub stantially the same coefficient of expansion as that of the part or parts with which it is combined. The improved material will also withstand the sudden expansion and contraction during the enameling without breakage. It also has a much greater maturing range than conventional porcelains, permittin the temperature during the firing of the insulating body to extend over a wider range without danger of making the finished body either o-veror under-matured.

If the expansion of the insulating body be made substantially the same as that of the resistive conductor, it is evidently desirable that the enamel be made of material having the same coefficient of expansion. This may be readily accomplished by selecting the materials forming the enamel of such ingredients and in such proportions as to give the desired degree of expansion, as is understood by those skilled in the art. For example, if a nickel chromium conductor be used having an expansion of say 14 during the maturing of the enamel, then a suitable enamel having substantially the same relative expansion ma be formed from the following materials: elspar 60 lbs., borax 42 lbs, carbonate of soda lbs, saltpeter 2 lbs. and cobalt oxide lb.

With reference to the refractory insulate ing body which will have the required in sulating properties, strength and toughness, the withstanding of manufacturing temperatures and operations, and also have substantially the same coefficient of expansion as the resistive conductor, or other parts with which it is to be combined, it has heretofore been impossible to produce an insulating body of the requiredcharacter, particularly one well adapted for combination with the resistive conductor and adhesive enamel coating.

1 have found that an insulating body fulfilling the above requirements and overcoming the above explained difiiculties may be made of a number of different materials. The natural mineral or rutile l have found to be particularly advantageous as a principal ingredient of the improved insulating material. This mineral ore, even when not combined with other ingredients when it has been properly ground, moulded and tired, fulfills the above requirements, being superior to porcelain in all of the desired qualities. But rutile when used alone, and ilmenite after treatment with acid to remove most of the iron, although having high insulating properties and being strong and tough and adapt ed to withstand the effects of manufacture and although having a coefficient of expansion over the enameling range of about 9.67 compared with 2.5 to to for porcelain, does not have a coetficient of expansion as high as that of the usual resistive conductor. 'lhus such material when used alone, although superior to porcelain in all respects, for the purposes of this invention, and although advantageous in overcoming the diiiiculties previously described, would be-even more advantageous if its coefiicient of expansion were that of the usual resistive conductor.

The mineral sore rutile has titanium dioxide as its main, or principal, ingredient and has of using the mineral ore rutile, other materials may be used having titanium dioxide as its principal ingredient, and equivalent to rutile. For example, ilmenite, or any other titaniferous ore may be used after treating the same to remove most of the iron and other impurities.

When titanium dioxide is heated to sintering and thencooled, it produces a mass of needle-like formation; and it is this property which makes the same valuable as a binder for the present improvement in forming insulating refractory bodies.

I have found that various ceramic materials can be added to rutile, or titanium dioxide material and thereby increase the coetficient of expansion of the resulting material to be the same as that of the resistive conductor and other parts, or even higher, if desired, for other special purposes; and at the same time maintain its toughness and strength, ability to withstand temperature changes and other manufacturing effects and likewise have high insulating properties.

it have found that flint, or apatite, and other similar materials when separately combined with rutile, or material sufficiently high in titanium dioxide, may be used to fulfill all of the above-described requirements includinga coefficient of expansion substantially equal to that of the resistive conductor or other metal parts. Also flint and apatite, or other similar materials, may be, in some cases, combined and used with the rutile or titanium dioxide material. if apatite or flint, for example, be used by themselves, however, the results are unsatisfactory. For example, apatite when finely ground and formed into a suitable refractory unit and subjected to a temperature of 2400 F, will not produce a suthciently strong body and is extremely brittle. lf flint be utilized in the same way, it will not produce a sulliciently strong body, being quite crumbly. A flint body has a very high coeilicient of expansion equal to about 20.4 over the enameling range between 26 and 626 centigrade, but will not withstand the enameling operation. An apatite body has a coe'liicient of expansion of about 18.3 between the enameling range of temperatures, but does not withstand thermal shock. l have found, however, that if titanium dioxide be added to either flint or apatite so as to form as little as 10% of the combined material, the resultant material is greatly strengthened l have also found that various amounts of titanium dioxide may be combined with either flint or apatite, or a combination thereof, or with other similar materials, to produce an insulating body having the desired properties and giving, according to the proportions used, a coetlicient of expansion of the resultant insulating refractory material substantially the same as that of the resistive conductor and other parts with which it may be desired to use the improved insulating material. In fact, I have found that titanium dioxide material may have combined therewith in proper proportions other materials having high coeihcients of expansion and secure a coefficient of expansion very much higher than that required for use in combination with a resistive conductor. For example, a mixture of 25% rutile, 25% flint and 50% apatite by-weight gave a comparative expansion over the range of enameling temperatures as highas 21.34 compared with an expansion of 9.67 when rutile is used by itself. Furthermore, this mixture and others of similar character all give a strong and tough material and well adapted to withstand the temperature changes during formation of the refractory material, and also during the maturing of the enamel.

.tube should likewise have an expansion of 14 for best results. That is, the enamel, resistive conductor and tube should have substantially the same expansion over the enameling range. A refractory tube for this particular urpose may be formed of a mixture by welght of rutile two parts and fiint one part approximately, or it may be formed by a combina- I tion of rutile, flint and apatite. If it be as-- sumed that the expansion of the resistive conductor be 12 approximately, such as is the case with iron wire, then the refractory tube should have an expansion of substantially the same amount. In such a case, the mixture may be formed by weight of one part rutile and two parts apatite approximately.

In forming this improved insulating material, the rutile, flint or apatite is finely ground to pass through a screen of 200 mesh, or even finer. iVhere necessary,"the mineral ore is chemically treated by acid to remove undesired oxides according to known methods, such as in the case of'ilmenite, and then finely ground. The finely ground material is then moulded or extruded, according to the usual practise followed with porcelain material, to the desired form under pressure. It is then fired to mature it, as in the case of forming porcelain. Where the material is to be matured while united with the parts it is to support or insulate, it is moulded about the parts, such as the metal contacts, terminals, or conducting portions and then matured. If the material be formed by extrusion, and metal parts are to be united with the insulating material before being fired, this is brought about by machining the formed material and inserting or attaching the metal parts.

To mature or fire the improved material, it is subjected to a temperature of 2000 F. to 2800 F., depending on its particular composition. The maturing or firing does not cause a fusing of the material, but a sintering takes place resulting in a strong refractory body of high insulating properties free from blisters or cracks. During the maturing, the material is merely brought to incipient vitrification, that is, partially vitrified and a homogeneous body is obtained more conveniently than in the making of porcelain and superior thereto in characteristics already described.

The usual process for maturing porcelain takes from 24 to 36 hours, but with my improved material, one to three hours at the maturing temperature is usually sufiicient.

I have further found that it is desirable, in some cases, to fire my improved insulating material when maturing it, in a reducing atmosphere, thereby securing a tougher and stronger refractory body. This may be due to the fact that otherwise the slight amount of iron in the rutile, or similar titanium dioxide material, is oxidized or caused to form iron oxides of higher valency; but this does not occur in a reducing atmosphere. When the iron is oxidized to higher valencies, it may be that this does not permit the titanium dioxide to have the same binding power as when this oxidization is not permitted to take place, or it may be due to the titanium dioxide being'reduced to a lower valency.

It will be understood that my improved insulating refractory material ma be formed of various materials and in various proportions to suit the particular requirements of the particular arts with which it is combined, as hereinbefore explained, without departing from the scope of this invention. It will also be understood that this invention capable of many uses and adapted for use in various forms of devices and apparatus Without departing from the scope thereof.

I claim:

1. A resistance device comprising an insulating refractory support and reslstor, and an insulating fused coating covering said support and resistor, said support containing a titanium dioxide ore as a principal ingredient.

2. A'resistance device comprising an in sulating refractory support and resistor, and an insulating fused coating covering said support and resistor, said sup ort containing rutile as a principal ingredlent.

3. A resistance device comprising an insulating refractory support and resistor and an insulating fused coating covering said. support and resistor, said support containing titanium dioxide as a principal ingredient and another material as a principal ingredient having a higher coefiicient of expansion than titanium dioxide.

4. A resistance device comprising an in sulat-ing refractory support and resistor, and an insulating fused coating covering said support and resistor, said support containing titanium dioxide as a principal ingredient and apatite as a principal ingredient.

5. A resistance device comprising an insulating refractory support and resistor, and

an insulating fused coating covering said support and resistor, said support containing titanium dioxide as a principal ingredient and flint as a principal ingredient.

6. A resistance device comprising an insulating refractory support and resistor, and an insulating fused coating covering said support and resistor, said support containing titanium dioxide as a principal ingredient and apatite as a princlpal ingredient and flint as a principal in edient.

' CHARLES TERWILLIGER.

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