US2439570A - Electric contact - Google Patents

Description

pr i3, 1948. V|. R. HENSEI. ET Al. 2,439,570

ELECTRIC CONTACT Filed NOV. 10, 1942 /TTORNEYS Patented Apr. 13, 1948 UNITED STATES PATENT- OFFICE ELECTRIC CONTACT Franz R. Hensel, Earl I. Larsen, and Earl F.A

Swazy, Indianapolis. Ind.. assignox-s to P. R. Mallory & Co., Inc., Indianapolis. Ind., a corporation of Delaware i Application November 1o,194z,seriai No. 465,100 sclaims. (ci. 'z5- 136) sistant electric contact.

Other objects of the invention will be apparent l from the description and claims.

In the drawings Figure 1 is a face view of an electric contact embodying features of the invention;

Figure 2 is a side view of the contact and the backing therefor;

Figures 3 and 4 are face and side views, respectively, of a contact suited for resistance welding to a backing;

Figures 5 and 6 show another contact suitable for resistance or projection welding;

Figures 7, 8 and 9 are face view, side view and section of another form of contact suited for resistance welding to a backing; and

Figures 10 and 11 are end and side views of a screw type contact assembly.

Recently some uses for electric contacts have been developed where contact materials of the prior art have failed to perform satisfactorily. One application has arisen in connection with airplane engines where the engine vibrations produce a constant scrubbing" or rotary action of the contacts against each other while they areI The abrasive action accompanying the closed. mechanical rubbing between the contacts when they are closed appears to grind olf some of the contact metal at the surfaces in a powder form. This powder, perhaps aided in some cases by the presence of oil or moisture, produces a high resistance material betweenthe contacts and raises the contact resistance toan excessively high value in a relatively short time. Tungsten contacts have been tried for this use but have proven According to the present invention.- the 'material forming the electric contacts will fall within the following range of contact compositions:

. Per cent Refractory metal carbide selected from the group consisting of tungsten carbide molybdenum carbide, titanium carbide, zirconium carbide, vanadium carbide.

columbium carbide and tantalum carbide '10 to 98 Cobalt 0.25 to 20 Silver 0.25 to 24.9

The term, "silver is intended to include pure silver and high silver alloys, such as coin silver and sterling silver. The principal additive metals in the case of silver base alloys are copper, gold, silicon, nickel, manganese, tin, indium, platinum and palladium.

The preferred refractory carbide is tungsten carbide. Either one or both of the tungsten carbides may be used, the preferred one being WC.

v'ithin the above permissible range of compositions, the following preferred ranges may be mentioned:

The preferred contact composition, particu1arly for applications on engines where vibration is severe, is:

Per cent (1) Tungsten carbide Cobalt g 8 Silver 2 Other specific compositions which may be men.- tioned are:

The preferred method of making the contacts comprises mixing the metal powders together, pressing the powder mixture to form a pressed metal body and then sintering the pressed powder body at an elevated temperature. All the powders used are preferably of -325 mesh iineness and may, for example, have the following average particle size:

, f' Microns Tungsten carbide 1.5 Cobalt 3 to 5 Silver It is preferred to mix the tungsten carbide, or other refractory carbide and cobalt -powders together first and ball mill them for a considerable period of time, such as 48 hours. This promotes the production of cobalt envelopes surrounding the tungsten carbide particles. 'I'he silver powder may then be added and the mixture ball milled again for several more hours, such as 12 hours. It is advisable to add a small amount of a volatile binder such as 1 or 2% glyptal at this stage to promote bonding of the powder mixture. The mixture may then be pressed in a suitable die to form a pressed powder body. A suitable pressed density is 8 grams per c. c. for the preferred composition (l) given above.

The pressed body may then'be presintered in hydrogen at 850 C. for one hour and then cut or dressed to suitable dimensions, allowing for further shrinkage to take place on further sintering. A nal sintering in pure dry hydrogen at a temperature between 1350 C. and 1500 C. is then given to the body in a closed carbon boat or tube. The preferred sintering temperature is 1450 C. The sintered pellets of composition (1) may have a density around 13.9 grams per c. c. and a. hardness around'85-90 Rockwell A.

It will be appreciated that these physical properties are given by way of example only and that the properties of the finished product may be varied depending on the materials and procedures used.

For compositions where the higher silver con- 4 silver to impregnate the remaining pores with silver.

Where operation at extreme temperatures must be provided for. it is of some considerable importance to attach the refractory carbide contact to a .backing having substantially the same low co-etlicient of expansion over a comparatively tents are used, it may be possible in some cases to press and sinter the mixture of refractory then to heat the body in contact with molten wide range of temperatures.

Materials of low co-emcient of expansion which are suitable are Invar, 30 to 45% nickel steels, alloys such as Kovar or Fernico containing 28% nickel, 18% cobalt, balan'ce iron; materials such as Fernichrome containing 37% iron. 30% nickel, 25% cobalt and 8% chromium, also straight ironchromium alloys.

Referring to the drawing, Figures V1 and 2 show a contact assembly comprising a facing disc l0 formed of the sintered refractory carbide composition described herein attached to an Invar or other low expansion alloy backing rivet lI2 by a thin layer Il of brazing solder, such as silver solder. pure copper or a copper-nickel alloy. The backing rivet has a shank portion I3 to enable it to be attached to a support inthe electrical apparatus in which the contact is used.l

Figures 3 and 4 show a button-type contact having a plane facing disc 2l secured by braze 2i to a thin nat disc 22 of backing metal. such as steel, Monel, nickel. bronze or nickel-silver or. where thermal stresses are to be kept low, of Invar or other low expansion alloy. Disc 22 is provided on its back face with a small central rounded dome projection 23 which enables it to be resistance welded to a support such as a leaf spring or other contact arm by clamping the contact and arm between welding electrodes and passing electric current through thejunction to melt the projection and cause welding to the contact arm.

Figures 5 and 6 show another contact button .wherein the facing disc Il of the refractory carbide composition has a rounded face.

Figures '1, 8 and 9 show a contact adapted for resistance welding comprising a refractory facing disc 40 having a plane contact face and a concave under side. Backing l2 is a concave-convex disc, the convex side of-which lits intov the concave side of disc 4l and is joined to it by brazing solder Il. The contact can be secured to a support by resistance welding wherein rim 43 around the concave depression is backing l2 is fused to the support. y

Figures 10 and 11 show a screw type contact and the use of a stress equalizing layer to isolate expansion stresses in the screw backing from the contact face. YThe assembly comprises refractory composition contact face disc 5l secured by a braze layer 5| to a stress equilizing disc 52 which may be of a, non-brittle lowl expansion alloy, such-as Invar or one of the other alloys mentioned. Disc 52 is in turn secured by brazing 'solder layer 53 to the backing screw 5I which may be of relatively high expansion metal such as steel. brass, bronze or the like. Disc I2 prevents the expansion stresses from theibacking screw lfrom being transmitted to the relatively brittle contact facing disc.

Instead of making disc 52 of low expansion alloy, it may be formed of a highly ductile metal or alloy, such as soft silver or copper, which yields suiliciently to absorb thermal stresses.

In comparison tests in an accelerated wear test machine, designed to produce contact abra.-

'sion, it was found that' the preferred material for the present invention formed of tungsten carbide, 8% cobalt and 2% silver, had a contact resistance after 396 hours of operation o! only .12 ohm and showed a material loss for the moving contact of .0030 gm. and tor the stationary contact of .0003 gm., as compared with a contact resistance oi 3.15 ohms and a material loss of .0414 gm. for the moving contact and .0348 gm. for the stationary contact both formed of a high comercial grade of tungsten contact material after only '120 hours of operation. Another high grade tungsten contact material developed a contact resistance of 4.15 ohms and a material loss of .0350 gm. from the moving contact and .0370 gm. from the stationary contact after 120 hours of operation. Of twenty dierent types of contact materials tested none of them exhibited the same advantgeous combination of low contact resistance and low material loss of the preferred material of the present invention.

While specific embodiments of the invention have been described, it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. An electric contact suitable for operation under conditions oi strong vibration formed of a metal composition of 85 to 95% of a refractory metal carbide selected from the group consisting of tungsten carbide, molybdenum carbide, titanium carbide. zirconium carbide, vanadium carbide, oolumbium carbide and tantalum carbide, from 2 to 10% cobalt and 0.5 to 5% silver said contact being characterized by yhigh abrasion resistance and low material loss.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,181,742 Coolidge May 2, 1916 1,979,372 Duhme Nov. 6, 1934 2,049,317 Pinta July 28, 1936 2,281,446 Laise Apr. 28. 1942 2,289,708 Jackson July 14, 1942 2,313,070 Hensel Mar. 9, 1943 FOREIGN PATENTS Number Country Date 345,171 Great Britain Mar. 19, 1931

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