US2394591A - Electrical resistor - Google Patents

Description

Feb. 12, 1946. B. c. BRICKER 2,394,591

ELECTRICAL RESISTOR Filed Sept. 9, 1944 j A l wlw/W);

www@ MW i -/igT-' 10614 10 ATTORNEY Patented Feb. 12, 194s n ELECTRICAL BESISTOB Burton c. ancrer, mw", N. J., signor to E. I. du Pont de Nemours Oompany, Wilmington, Del., a corporation of Delaware Application September 9, 1944, Serial No. 553,367

8 Claims.

This invention relates to electrical resistors. More particularly, it relates to electrical resistors comprising a. cylindrical, generally tubular core,

- an electrical resistance wire spirally wound on said said wire and core and the base of said metallic connecting members.

Electrical resistors of this genral type have been produced for many years, however, prior to the present invention the art has found it impossible to consistently produce such electrical resistors which will meet the highest specifications demanded by users of these resistors. The U. S. Navy has long demanded, in accordance with their grade 1, class 1, specification for wire-wound electrical resistors, that such resistors withstand the following test:

Operating the resistor at a hot-spotftemperature of 275 C.; immediately thereafter quenching the resistor in a bath of boiling saturated salt water; then quenching in ice cold saturated salt water; immediately thereafter again operating the resistor at a temperature of 275 C.; and repeating this cycle nine times, the resistor not to change in resistance by more than 10% of its initial value.

Prior to this invention, the art had been unable to produce, consistently, wire-wound electrical resistors having a vitreous enamel protective coating which would meet this specification of the Navy. A great many different combinations of materials used in the several elements of such resistors have been tried heretofore without successfully meeting the higher specifications demanded in the art. The dimcultyariseg from the fact that the insulating core, the resistance wire, the vitreous enamel covering, and usually the metal fastening tabs are all composed of different materials which have different coemcients of expansion, and the fact that the vitreous enamel must be fired on the assembled core, wire and tabs at an elevated temperature.

In order to obtain the necessary high resistance in a relatively small u nlt, the resistance wiremust be very fine in size and must be wound in very closely spaced helices onthe core. k(ireat care must therefore be taken to avoid contact of the wire windings with each other, and to avoid undue strain on the wire as a result of differences in expansion and contraction of the parts during the firing and cooling of the vitreous enamel coating. Furthermore, the particular combination of materials used in constructing the various parts of the resistor together with the particular procedural steps employed in making the resistor are of utmost importance.

It is an object of this invention to produce an improved wire-wound, vitreous enamel coated electrical resistor.

It is another object of this invention to provide a wire-wound, vitreous enamel coated electrical resistor which will consistently meet the highest specifications demanded in the arts.

It is a further object of this invention to provide a process for the production of wire-wound, vitreous enamel coated electrical resistors which will consistently produce such resistors capable of withstanding the most rigid test specifications.

Other objects of the invention will appear hereinafter.

The objects of this invention are accomplished by first applying to a, wire-wound electrical resistor core, a thin slip of clay, preferably kaolin or ball clay containing 2% to 50% bentonite, based on the total solids content of said slip, drying the clay slip, coating the wire-wound core containing the dry clay with a vitreous enamel slip containing 28% to 55% lead oxide, 5% to 20% zinc oxide, 10% to 30% boric oxide, 4% to 10% alumina and 11% to 32% silica, and firing said enamel coating. Such vitreous enamels are described in the copending application of Alden J. Deyrup, Serial No. 551,041, filed August 24, 1944.

In order that electrical resistors which will withstand the most rigid tests demanded of them be produced more readily and consistently, it is preferred that the wire-wound core, after application of the clay slip and drying the Same, be first coated with an enamel slip containing between 35% and 55% lead oxide, 5% and 20% zinc oxide, 10% and 30% boric oxide, 4% and 10% alumina.

and 11% and`32% silica, firing said coating, then providing the resistor with a second coating comprising .an enamel slip containing between 28% and 35% lead oxide, 5% and 20% zinc oxide, 10% and 30% boric oxide, 4% and 10% alumina, and 11% and 32% silica, and ring the second enamel coating. f

The present invention will be more clearly understood by reference to the following'detailed disclosure when taken in connection with the accompanying illustrations, in which:

Fig. 1 is a fragmentary sectional view of a wire-wound core containing a coating of a clay slip and an over-coating of an enamel slip.

Fig. 2 is a similar sectional view of a wirewound core containing a red enamel coating anda second unilr'ed coating of an enamel slip.

Fig. 3 is another fragmentary sectional view of s wire-wound core containing two fired enamel coatings.

Referring to the several views shown in the drawing, reference numeral 6 designates a cylindrical hollow insulating core upon which is wound a resistance wire 8. One end of the resistance wire is shown at l I in contact with metal fastening member I0. The fastening members I are clamped about the ends of core 0 in any desired manner and the wire is fastened thereto by brazing, welding or the like.

The assembled core 6, wire 8 and the base of fastening members l0 are covered by means of a thin clay slip l2 and allowed to dry thoroughly after which a slip of enamel Il is applied over the dry clay.

The assembly, including core, wire fastening members, clay, and enamel slip, is then nred in a conventional manner to produce a resistor having a smooth, homogeneous enamel surface with the clay merged with the enamel adjacent the core, wire and members.

A second coating I6 of an enamel slip may be applied over the fired enamel coating as shown in Fig. 2. This second enamel slip may then be fired to produce a preferred resistor structure comprising two enamel coatings as shown in Fig. 3.

The electrically insulating resistor core 6 is generally composed of a ceramic material, for example, steatite porcelain, magnesium silicate, or magnesium aluminum silicate. A preferred core is one composed of a ceramic material having a linear expansion coefcient between 6 l0 and 7 10-6 per degree centigrade.

The resistor core is wound in a conventional manner with an electrical resistance wire, for example, a nickel-chromium alloy or nickel-copper alloy wire, and the wire securely held in place by means of metal fastening tabs or connecting elements to which the ends of wire are fastened to form an electricity conducting unit from one tab, through the wire to another tab.

The clay slip may comprise an aqueous suspension of kaolin or ball clay, preferably, however, it comprises an aqueous suspension of 98% to 50% kaolin or ball clay and 2% to 50% bentonite, percentages, by weight, based on the total solids content of the slip.

The wire-Wound core, including the fastening tabs, is coated with the clay slip by first making the clay slip comprising water and clay, Sulliciently fluid so that it will flow between wire coils and drain from the spaces between wire coils to leave a substantially uniform coating on the exposed surfaces of the core, wire and tabs. It will be readily apparent that if the slip is thick it will not flow between Wire coils but will bridgethe space between coils, and a coating of uniform thickness on the wires and core cannot be obtained. The thinness or fluidity of the slip will depend in large measure upon the size of the wire, the spacing between wires, and the method of application. In general, the clay slip should contain between 3 and 30 parts water for 1 part solids, and preferably between and 10 parts water for 1 part solids.

The coating of clay slip is then applied to the wire-wound core by brushing, spraying or dipping, after which the core is allowed to drain, for example, by suspending the same from one end thereof. The clay slip'is then dried by contact with the atmosphere at room temperature, by forced air drafts, or by mild baking.

The wire-wound core containing the dry ingofclayisthencoatedwithsvitreous slip, after which the enamel slip is dried then fired in the usual manner, generally at peratures from 650 to 750 C. A second enamel may be applied and nred over the first enamel coating.

As above stated, the enamel used in providing the vitreous coating on the resistor of the present invention must contain 28% to 55% lead oxide (PbO), 5% w 20% zinc oxide (ZnO), 10% to 30% boric oxide (B203). 4% to 10% alumlnl (A1201), and 11% to 32% silica. 'I'he above in-l gredients must be within the ranges set forth whether one or more enamel coatings are applied to the electrical resistor. The enamel must not contain to exceed 1% NaaO or its chemically equivalent amount of other alkali oxides, and not to exceed 0.5% nuorine in the form of nuorlne compounds.

The production of electrical resistors comprising a wire-wound core and a vitreous enamel protective coating is largely dependent upon highly empirical arts, and highly desirable results are dimcult of achievement. For this reason such resistors which will consistently pass the abovesaid Navy specifications were unobtainable for a period of years, although many attempts were made to produce them. The result achieved through the use oi vitreous enamels of compositions herein described, in conjunction with the step of first applying a clay slip to the wound resistor tube, represents a surprising technical advance in the resistor art. I am unable to explain the surprising nature of the result, because it is a generally accepted view that failure of enameled resistors depends upon crazing, cracking, or separation of the enamel, and in many other arts, clay coatings are used as nonadherent layers, to which glassy coatings will not stick. Nevertheless, I have discovered the remarkable fact that when used as herein described, in conjunction with the improved enamel compositions described and claimed in the copending application of Deyrup, Serial No. 551.041, the clay coating makes it possible to obtain excellent quality resistors, passing the extremely severe Navy test, in a rapid, efllcient, and dependable commercial manner. Without the useof the invention herein described, the full advantageous properties of the improved enamels referred to can only be consistently realized by resorting to other difficult and economically undesirable procedures, for example, winding on a grooved tube, a procedure technically satisfactory but commercially dlsadvantageous.

As above set forth, electrical resistors, which will consistently pass the most rigid tests involving mechanical shock and vibration, and rapid expansion and contraction due to heating and cooling, can be more readily made by first applying and firing to the wire-wound resistor an enamel containing between 35% and 55% lead oxide, between 5% and 20% zinc oxide, between 10% and 30% boric oxide, between 4% and 10% alumina, and between 11% and 32% silica, and then applying and firing over the first enamel a second enamel containing between 28% and 35% lead oxide, between 5% and 20% zinc oxide, between 10% and 30% boric oxide, between 4% and 10% alumina and between 11% and 32% silica. These compositions also must contain not to exceed 1% NazO or the chemical equivalent amount of an alkali oxide, or exceeding 0.5% uorine.

Of the above two enamels the one containing between 35% and 55% lead oxide is more iluid than the one containing the smaller amount of lead oxide. The high lead oxide content enamel will therefore, more readily flow between the wire coils to merge with the coating of clay and form a smooth uniform outer surface. The high lead oxide content enamel is superior to the low lead oxide content enamel as a first coat. If the low lead oxide enamel is used as a first, or only, coat, great care must be taken to avoid bridging of the wire coils and to avoid pin holes in the coating due to low degree of fluidity.

The high lead oxide content enamel does not have the excellent resistance to crazing of the low lead oxide content enamel, and if the former is used as the only coat great care must be taken in the application and firing of the enamel to avoid producing of a coating which will craze.

If the high lead oxide content enamel is rst applied and fired and the low lead oxide content enamel is then applied and fired, electrical resistors having an excellent appearance and an excellent resistance to sudden changes in temperature, and to mechanical shock, can be readily and consistently produced without diillculty.

Since it is obvious that many changes and modincatlons can be made in the particular details above set forth without departing from the nature and spirit of the invention, it is to bel understood that the invention is not to be limited to the above-said details except as set forth in the appended claims.

I claim:

l. The method of making an electrical resistor which comprises applying to a wire-wound electrically insulating core a thin clay slip, drying said clay slip, applying to said clay-coated wirewound core an enamel slip containing between 28% and 55% PbO, between 5% and 20% ZnO, between and 30% B202, between 4% and 10% alumina, and between 11% and 32% silica, and drying and firing said enamel slip.

2. The method of making an electrical resistor which comprises applying to a wire-wound electrically insulating core a thin clay slip, drying said clay slip, applying to said clay-coated wirewound core an enamel slip containing between 35% and 55% PbO, between 5% and 20% ZnO, between 10% and 30% B202, between 4% and 10% alumina, and between 11% and 32% silica, drying and nring said enamel, applying over said nred enamel an enamel slip containing between 28% and 35% PbO, between 5% and 20% Z110. between 10% and 30% B202, between 4% and 10% alumina, and between 11% and 32% silica, :und drying and ilring the last-applied enamel 3. The method of making an electrical resistor which comprises applying to a wire-wound electrically insulating core a slip comprising an queous suspension of a clay and bentonite in Proportion ot 98% to 50% clay and 2% to 50% bentonite, and containing 3 to 30 parts by weight of water for each part solids, drying said clay IUD. pplying to said clay-coated wire-wound core an enamel slip containing between 28% and 55% PbO, between 5% and 20% ZnO, between 10% and 30% B204, between 4% and 10% alumina, and between 11% and 32% silica, and drying and ring said enamel slip.

4. The method ci making an electrical resistor which comprises applying to a wire-wound electrically insulating core a slip comprising an aqueous suspension of clay and bentonite in a proportion of 08% to 50% clay and 2% to 50% bentonite, and containing 3 to 30 parts by weight of water for each part solids, applying to said clay-coated wire-wound core an enamel slip containing between 35% and 50% PbO, between 5% to 20% ZnO, between 10% to 30% B203, between 4% to 10% alumina, and between 11% to 32% silica, drying and ring said enamel, applying over said fired enamel an enamel slip containing between 28% and 35% PbO, between 5% to 20% ZnO, between 10% to 30% B203, between 4% to 10% alumina and between 11% and 32% silica, and drying and ilring the last applied enamel slip.

5. An electrical resistor which comprises a core of ceramic material, an electrical resistance wire wound on said core, metal fastening members on said core and connected to said wire, a vitreous insulating enamel coating vpositioned about said core, wire and base of said members. and a clay merged with the enamel adjacent said core, wire and base, said enamel containing between 28% and55% PbO, between 5% and 20% ZnO, between l0% and 30% B203, between 4% and 10% alumina, and between 11% and 32% silica.

6. An electrical resistor which comprises a core of ceramic material, an electrical resistance wire wound on said core, metal fastening members on said core and connected to said wire, a vitreous insulating enamel coating positioned about said core, wire and base of said members, and a clay merged with the enamel adjacent said core. wire and base, said enamel containing between 35% and 55% PbO, between 5% and 20% ZnO, between 10% and 30% B202, between 4% and 10% alumina, and between 11% and 32% silica, and a second vitreous insulating enamel coating positioned about the above-said enamel coating, said second enamel coating containing between 28% and 35% PbO, between 5% and 20% ZnO, be-

tween l0% and 30% B203, between 4% and 10%` alumina, and between 11% and 32% silica.

7. An electrical resistor which comprises a core of ceramic material, an electrical resistance wire wound on said core, metal fastening members on said core and connected to said wire, a vitreous insulating enamel coating positioned about said core, wire and base of said members, and a claybentonite mixture comprising 98% to 50% clay and 2% to 50% bentonite merged with the enamel adJacent the core, wire and base, said enamel containing between 28% and 55% PbO, between 5% .and 20% ZnO, between 10% and 30% 320x, between 4% and 10% alumina, and between 11% and 32% silica.

8. An electrical resistor which comprises a core of ceramic material, an electrical resistance wire wound on said core, metal fastening members on said core and connected to said wire. a vitreous insulating enamel coating positioned about said core, wire and base of said members, and a claybentonite mixture comprising 98% to 50% clay and 2% to 50% bentonite merged with the enamel adjacent said core, wire and base, said enamel containing between 35% and 55% PbO, between 5% and 20% ZnO, between 10% land 30% BsOs, between 4% and 10% alumina and between 11% and 32% silica, and a second vitreous insulating coating positioned about the above said enamel coating, said second enamel coating containing between 28% and 35% PbO, between 5% and 20% ZnO, between 10% and 30% B202, between 4% 'and 10% alumina and between 11% and 32% silica.

BURTON C, BRICKER.

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