US2207292A - Electric contact and combination thereof - Google Patents

Description

July 9, 1940. F. R. HENSEL. ET A1. 2,207,292

ELECTRIC CONTACT AND COMBINATION THEREOF Filed Api-i1 25, 1939 Dy. z,

J//e/-asc Q//aj 0070/011051 ATTORNEY Patented July 9, 1940 UNITED STATES ELECTRIC CONTACT AND COMBINATION THEREOF Franz R. Hensel, Kenneth L. Emmett, and James W. Wiggs, Indianapolis, Ind., assignors to P. R.

Mallory & Co., Inc.,

Indianapolis, Ind., a corporation of Delaware Application April 25,

2 Claims.

This invention relates to electric make and break contacts and combinations thereof,

The present application is a continuation in part of prior-led copending application S. N. 262,174, filed March 16, 1939 now Patent No.

2,161,575, issued June 6, 1939.

An object of the invention is to improve the characteristics of electric contact elements containing silver.

Another object is to improve' contact combinations in which at least one of the co-operating contacts is formed of a silver alloy.

While a preferred embodiment of the invention is described herein, it is contemplated that a considerable variation may be made in the method of procedure and the combination of elements, without departing from the spirit of the invention.

Figs. 1 and 2 of the drawing illustratetwo preferred embodiments of the invention.

The present invention contemplates an electric contact formed of an alloy composed basically of silver, to which has been added magnesium and cadmium. In the formation of the new contact alloy, it is preferable to provide a composition of the materials specified in the following proportions Per cent Magnesium .05 to Cadmium .5 to 25 Silver Substantially the balance.

The preferred range of magnesium is .1 to 8%, because up to that percentage, the magnesium is taken up in alpha solid solution in silver, and

we have found that silver and magnesium up to 8% can be alloyed very readily and the cast ingots .can be rolled, swaged, or drawn into desirable shapes, such as sheet, bar stock, wire, strip and the like. Silver and cadmium form a solid solution up to 45%. Therefore, alloys of the class described in the present patent, are ternary solid solutions of magnesium, cadmium and silver, except in the cases of high magnesium percentages where a duplex structure will exist.

We have found that the following alloys, which are given by way of example only, have shown very excellent results, when used for electrical contacts' Per cent 1. Magnesium 3.5 Cadmium 7.0 Silver Balance 2. Magnesium 6 Cadmium 15 Silver Balance Magnesium 2 Cadmium Silver Balance 4. Magnesium 15 Cadmium 10 so Silver Balance 1939, Serial N0. 269,976

(Cl. 20o-166) For certain uses contact compositions falling within the range:

, Per cent Magnesium lto 4 Cadmium 5to 20 Balance substantially all silver appear to be a dross, it has been found desirable to melt such alloys, either in a reducing or neutral atmosphere, or to use suitable fluxes, which cover the melt.

The addition of magnesium in the form of a master alloy has also reduced the loss of magnesium, and therefore has made it possible to hold closer tolerances in chemicai composition.

The resultant alloys within the range of ternary solid solutions, have electrical conductivities, ranging from 20 to 40%, International Annealed Copper Standard, according to the respective compositions.'

An alloy, for instance, containing 31/2% magnesium and 7% cadmium, had an electrical conductivity of 22%. The hardness of this material in the as cast condition, was 26 Rockwell F.

After swaging 30%, this hardness increased to vB5 Rockwell F.

We have found that a Brinell hardness of '70 to 130, can be easily obtained with these types of alloys with the proper amount of cold working.

The alloys of the class described have shown very excellent qualities, when used for electrical contact purposes. A comparison test was conducted wherein contacts of similar physical dimensions were tested on a resistive inductive circuit, at 470 cycles per minute, and wherein the current owing in the circuitI was increased periodically, to obtain definite current values of alloys in the nature described above, in comparison with contact materials produced in the prior art. The amount of material transfer of one contact to the other, was used as the prime method of comparison. Y

The most surprising result was found in this test. It was discovered that contacts of the type described, and more particularly those formed of an alloy containing 3.5% magnesium and 7% cadmium, showed no material transfer vwhatsoever. Fine silver under the same conditions gave a material transfer of .010", and coin silver a material transfer of .011.

The test data is tabulated as follows:

exist over a wide range; namely, from 6 to 110 volts D. C. The general superiority of alloys of the present invention is even more apparent at the higher voltages.

'I'he nal contact resistance on these new alloys, the compositions of which are given above, did not substantially exceed'values obtained for fine silver, under identical test conditions. In fact, under these severe operating conditions, the contact resistance actually measured .28 milliohm for the alloy containing 31/2% magnesium, and 7% cadmium.

Alloys of the present invention offer opportunities for new differential contact' combinations with performance values hitherto impossible of attainment. We have found it is possible to use table since tungsten itself oxidizes so badly under the test conditions that it will not pass current. Other materials which give high resistance when operated alone, we have found may be substituted for the tungsten in the above-listed table. The advantageous use of alloys of the present invention is therefore possible in many contact combinations.

For example, we have found that where contact pressures are very small, i. e., less than 50 grams, it is advantageous to employ alloys of the present invention as the positive contact material and materials such as silver-gold, silverpalladium., or silver-platinum as negative contacts.

'Ihe composition of the three alloys, lastnamed above, may be varied to suit the apparatus requirements. If the current, voltage, or operating frequency are high the proportion of noble metal may be increased so as to form up to '75% by Weight of the alloy. If the current and voltage are low, the proportion of noble metal may be decreased, accordingly, to as little as 1% by weight. Pure silver may also be used.

By test, we have found the following combinations give excellent performance:

Low contact pressure combinations Specifically, the results may be tabulated as follows:

Contact material Limiting transfer Contact resist- Dit m l. Positive Negative Current Height ohms .Ag-7 Cd-3.5Mg. W 22amp. Zero 2.0 W W Open circuit at 18 amp. due to contact resistance. Ag Ag 12 amp. .010" 0.8 Ag-7 Cd-3.5 Mg. Ag1`7 Cd-3. 5 12 amp. Zero 0.3

g. Ag W l2an1p. .008 60.0

These tests were made at 12 volts D. C., 160 g.

contact pressure, resistive-inductive load, 470 contact operations per minute.

From the above-listed tabulation, it will be noted that our improved alloys provide a contact combination which has the simultaneous advantages of low contact resistance, high limiting current, and zero transfer.

It is not necessary to limit the contact material opposing our improved alloy to tungsten. 'I'he selection of tungsten was made for the above Under test conditions as illustrated in the above table, we have found it is possible to exchange the platinum and palladium, and to also interchange the gold and palladium.

In general, therefore, we propose to utilize the fullest possibilities of the zero-transfer properties of the alloys of the present invention by changing the opposing contact material to suit the operating load and conditions imposed upon the contact pair.y

We have further found that the wear resistance of our alloys may be increased by adding small amounts of copper, nickel, cobalt, iron, or manganese to them. Such additions raise the softening point of the alloys.

We have also added metals such as nickel, iron, cobalt, and manganese to the opposing differential alloys mentioned above, i. e., to the silver-gold, silver-palladium, and silver-platinum to increase their Wear resistance and hardness.

Such addition metals in small amounts have no appreciably detrimental electrical effects.

Alloys of the present invention are resistant to tarnish by atmospheres containing hydrogen sulfide. It was found that the alloy containing 7% Cd and 3.5% Mg, balance silver, showed no tarnish after AP8 hours in an air-hydrogen sulfide atmosphere which tarnished pure silver and sterling silver in less than 14 hours. Our improved alloy is therefore an improvement for applications which must operate in sulfurous atmospheres, such as railway signal equipment. telegraph apparatus, circuit breakers in smelting plants, etc.

We have found that it is necessary to heat alloys of the present invention above 725 C. in-

order to obtain substantial softening by annealing. Therefore, they are wear-resistant when operated in elevated-temperature atmospheres.

The alloys of the present invention have been found useful for a large number of electrical contact applications, such as for instance, sensitive direct current relays, over-load cut outs, voltage regulators, indicating and recording gauges, thermostatic controls, and the like.

The alloys of the present invention also have a commercial advantage, because the addition of magnesium in the percentages contemplated, provides a material of low specific gravity and therefore, it is possible, with such low density alloys, to produce a large number oiv finished contacts per unit weight of material.

Referring to the drawing Figure l illustrates a contact combination wherein a contact I of a silver base alloy containing both magnesium and cadmium co-operates with a second contact 2 formed of tungsten. Figure 2 shows another contact combination wherein one of the contacts 3 is formed of a silver base alloy containing both magnesium and cadmium and the co-operating contact 4 is formed of an alloy of silver with a precious metal of the group consisting of palladium, platinum and gold.

While the present invention, as to its objects and advantages, has been described herein, as carried out in specific embodiments thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly, within the spirit and scope of the appended claims.

What is claimed is:

l. An electric contact combination for D. C. operation comprising a pair of co-operating contacts, the contact on the positive side. of said pair being formed of a silver 'base' alloy containing both magnesium and cadmium and the contact on the negative side of said pair being formed of an alloy of silver with metal selected from the group consisting of palladium, platinum and gold.

'2. An electric' contact combination for D. C. operation comprising a pair of co-operating contacts, the contact on the positive side of said pair being formed of an alloy of .05 to 15% magnesium, .5 to 25% cadmium and the balance substantially all silver and the contact on the negative side of said pair-being formed of an alloy of silver with metal selected from the group consisting of palladium, platinum and gold.

FRANZ R. HENSEL. KENNETH L. EMMERT. JAMES W. WIGGS.

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