US2179212A - Resistor unit - Google Patents

Description

Nov, 7, 193% c. J. GANG} 2 179212 RESISTOR UNIT Filed Nov. 25, 1956 Patented Nov. 7, 1939 RESISTOR UNIT Charles J. Ganci, Ridgewood, N. Y., assignor to Ward Leonard Electric Company, a corporation of New York Application November 25, 1936, Serial No. 112,623

3 Claims.

This invention relates to resistor units of the type having an insulating base on which the resistive element is wound or supported and which is covered by an insulating coating such as vitreous enamel.

In making such resistors, it has been the common commercial practice to use a base having a comparatively low coeflicient of expansion, such as porcelain; to use an enamel coating having a higher coeilicient of expansion; and to use a resistive element and terminals having a still higher coefficient of expansion. For example the usual insulating base of ceramic material has a coeflicient of expansion between .000003 and '.000006 per degree C. The usual enamel has a coemcient of expansion from .000009 to .000014 per degree C.- The usual resistive wire has a coeihcient of expansion ranging between .000014 and .000018, while the usual terminals of copper or Monel metal have a coeflicient of expansion of about .0000166 and .000014 per degree C.

In making these resistor units, the resistive conductor and terminals are applied to the base and then coated by the enamel material in powdered or liquid form and then fired at a temperature sumcient to fuse the enamel material and thus form a vitreous insulating coating for protecting the resistive element and terminals. during the firing for forming this enamel coating, the unit as a whole is subjected to the temperature required to fuse the coating and owing to the pronounced difference in expansion between the base and the resistive conductor and terminals, the metal conductor and terminals become loose on the base and the adjacent turns or portions of the resistive element are likely to shift and contact in one or more places and cause short circuits between adjoining portions, besides causing the terminals to move from their proper positions and thus result in many of the units being unsuitable for commercial use.

In order to overcome the above difliculties resort has been taken to form the enamel material of such constituents and in such proportions as to fuse at a lower temperature than that normally required and to bring the firing temperature for the enamel to 500 C., or even less, in an effort to reduce the amount of expansion of the different parts by the use of a lower firing temperature. Such a procedure requires an enamel material very rich in alkalis which is not as durable as thehigher temperature fusing enamels and the presence of these alkalis result in the enamel having a high coeillcient of expansion in the neighborhood of .000014 per degree C.

When an enamel having such a high coemcient of expansion, or even of lower expansion, is used on a base having a still lower coefllcient of expansion, the final product is undesirable by rea son of the innumerable crazes or cracks which form in the enamel during the cooling of the units. This is due to the fact that at thefusing temperatures, the enamel flows and coats the resistive conductor,.terminals and base and when the unit is removed from the furnace, it first cools sufflciently for the enamel to harden, and thereafter, as the enamel and the base shrink during cooling, the enamel contracts more than the base. This imposes tension strains upon the enamel giving rise to the crazes or cracks in the enamel. These crazes not only permit moisture to penetrate the coating and attack the conductor and corrode it but also tend to extend into the material of the base and these cracks in the base weaken it so that the resultant unit often becomes fragile and easily broken.

The Terwilliger Patents No. 1,842,433, granted January 26, 1932, and No. 1,938,674, granted December 12, 1933, were directed to overcoming the above difliculties by providing a ceramic base and an enamel having about the same coefllcient of expansion as that of the resistive conductor. The provision of a base having such a high coeflicient of expansion has the objection that the stresses introduced by heating and cooling tend to set up strains and crack the base and weaken it. Also by making the enamel and base have the same coefiicient of expansion, difflculty arises due to the fact that during the cooling, after fusing the enamel, the outside of the unit cools first and this cooling of the enamel before the cooling of the base places the enamel under tension strains and tends to form crazes in the enamel and weaken the base, giving rise to deterioration of the resistive'element and weakening of the unit. Furthermore, an enamel formed to have the high expansion necessary to be conformable to that of the resistive conductor in accordance with the Terwilliger patents, requires so much alkali and alkaline earths in its composition that its durability is lessened.

Thus the usual prior commercial resistor units have had a base or support of low coeillcient of expansion, an enamel of medium coefficient of expansion and the'resistive conductor and terminals of high coefficient of expansion. The Terwilliger units have a base or support of high 00- eflicient of expansion, an enamel of high coemcient of expansion and the resistive conductor 10 thereby obtain uniformity in the production of such units and avoid the short circuiting of portions of the resistive conductor. Another object is to eliminate the tension strains imposed upon the enamel coating during cooling of the 15 unit and thereby overcome the occurrence of crazes or cracks in the enamel and likewise avoid the weakening of the base resulting from the setting up of crazes or cracks in the enamel coating. Another object is to produce a resultant unit 20 which is more durable than prior products by the making of a crazeless unit which better protects and preserves the resistive conductor from deterioration. Another object is to produce a unit having greater strength and less likely to break by reducing the strains set up in the base and avoiding the presence of cracks in the base resulting from crazes heretofore present in such units. Other objects and advantages will be understood from the following description of the in vention.

1 is a side view of an illustrative resistor unit to which the invention is applicable and showing the unit before the enamel coating is applied; and Fig. 2 is a longitudinal cross-section thereof with the enamel applied. I 7

Referring to Figs. 1 and 2, the insulating body II is shown in the form of a tube having the resistive conductor ll wound thereon. The resistor is shown connected at its end to two terminal bands if, from the projecting ends of which connection may be made to an external circuit. The enamel coating I3 is applied to cover the resistive conductor and terminal hands by dipping the unit in enamel material, by sifting the material thereon, or by spraying or otherwise. The unit is then placed in a furnace and heatedto a sufilciently high temperature to fuse the coating and form the vitreous adhesive insulating enamel coating. Various other forms of resistance devices or units may embody the present invention and the drawing is merely for the purpose of showing typical illustrations to which the invention is applicable.

In the present invention I provide a base or support and an enamel coating and a resistive conductor and terminals of such character and relationship to each other as regards their coefilcients of expansion as to obtainthe objects of the invention. I have found that when firing the unit for forming the enamel coating, using temperatures up to about 800 C., the wire and terminals do not loosen or materially shift in their position when the base has a coefilcient of expansion of at least one-half of that of the resistive conductor and that any increase above three-quarters of that of the resistive conductor is of no advantage in overcoming loosening and shifting of the wire and terminals but introduces the objections of causing crazing of the enamel and weakening of the base and unit as a whole. I have also found that the best results are obtaned when the metal or alloy forming the terminals have a coefficient of expansion of not over three-quarters of that of the resistive conductor. I have also found that a crazeless unit may be produced by use of an enamel coating which has a coefilcient of expansion less than that of the base or support and by the avoidance of crazes the base is not weakened in the procass of manufacture and in fact yields a resultant unit which may be stronger than the base before the application of the enamel coating.

Nickel chromeiron alloy is now usually used for the resistive conductor or wire in the type of resistor units to which this invention'is applicable and has a coefllcient of expansion of about .000017 per degree C. A base adapted to'have proper relationship to such a resistor obtaining certain advantages of this invention may be made by combining the following ceramic materials in the proportions or percentages indicated by the second column:

Kaolin Flint 3o Pyrophyllite 15 Talc 5 This material is finely ground and mixed and then moulded or extruded to the desired form under pressure. It is then fired to mature at a temperature of 1200 C. to 1300 C., according to the usual method of maturing ceramic materials. This insulating base has a coefiicient of expansion of about .0000093 which is more than one-half and less than three-quarters of the coeflicient of expansion of the nickel chrome iron resistor commonly used. The constituents of the base may vary somewhat from the proportions above given and other ingredients used for obtaining advantages of this inventionlprovided the coefilcient of expansion of the base is at least one-half and not more than three-quarters of that of the resistor.

A desirable enamel having a coefilcient of expansion less than that of the base or support and well adapted to cooperate with the particular example of base above given, may be made by fritting together the following ingredients in the .proportions indicated in the righthand column:

The material is ground to a powder and then applied and fused in the usual manner, the fusing of the enamel material above given being at about 625C.

The coemcient of expansion of this enamel is about .0000075 per degree 0. and in cooperation with the particular base above described has the advantage of avoiding crazes and increasing the strength of the unit.

A further feature of the invention is to provide terminals for the unit having a proper relative expansion to that of the resistive conductors. When comparatively massive terminals with reference to the size of the resistive conductor or wire are used and when the terminals have a coemcient of expansion equal to or more than that of the resistive conductor, cracks are likely to form in the enamel over the terminals during the cooling of the unit, even when the base, resistor and enamel are properly related as to their relative expansions. I have found that if terminals are used having a coefilcient of expansion not over three-quarters of that of the resistive conductor, these cracks will not form. An alloy of 52% nickel and 48% iron has an average coefiicient of expansion up to 800 C. of about .0000105 and is well adapted for use as" terminals. This coefilcient is less than seventy-five percent of that of commonly used resistive conductors such as nickel chromium, or nickel chrome iron, or the usual nickel copper alloys. Various other alloys may desirably be used for the terminals provided the coefiicient of expansion is less than seventy-five percent of that of the resistive conductor to which they are connected.

Although particular examples of base, enamel, resistive conductor and terminals have been given for obtaining the advantages of this invention, it will be understood that various other suitable materials may be used without departing from the scope of the invention provided the relative expansions above disclosed are obtained; and by making a unit with a base having a coefficient of expansion from one-half to three-quarters of that of the resistive conductor, an enamel having a coeflicient of expansion less than that of the base, and terminals having a coefficient of expansion less than three-quarters of that of the resistive conductor, all of the advantages of this invention may be obtained, although some of the advantages may be obtained by using less than all of the features thereof.

I claim:

1. In a resistance device, a ceramic base, a

resistive conductor supported thereby, an enamel coating over the resistive conductor having a coefficient of expansion lower than that of the base, said base having a coefiicient of expansion of at least one-half and not more than three-quarters of the coefficient of expansion of the resistive conductor.

2. In a resistance device, a ceramic base, a resistive conductor supported thereby, terminals mounted on the base and connected to the resistive conductor, and an enamel coating covering the resistive conductor and terminals, said terminals having-a coefiicient of expansion not over seventy-five percent of that of the resistive conductor and said base having a coefficient of expansion of at least one-half and not more than three-quarters of the coefficient of expansion of the resistive conductor.

3. In a resistance device, a ceramic base, a resistive conductor supported thereby, terminals mounted on the base and connected to the resistive conductor, and an enamel coating covering the resistive conductor and terminals, said terminals having a coeflicient of expansion not over seventy-five percent of that of the resistive conductor, said enamel having a coeflicient of expansion lower than that of the base and said base having a coeflicient of expansion of at least one-half and not more than three-quarters of the coefficient of expansion of the resistive conductor.

' CHARLES J. GANCI.

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