US2414463A - Electrical contact - Google Patents

Description

Jan. 2l, 1947. T. M. GUNN ETAL ELECTRICAL CONTACT Filed Sept. l0, 1943 'FIG-2.

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aiaadi Townsend lid. Gunn, Attiehoro, and Walter J. Wilheire, Piainvilie, assignors to Metals it Controls Corporation, .ittllelboroy Mass., a corporation oi Massachatsettsv application Septemher ld, lllserslai. No. Stillld either; and the provision oi an attachment of the class described which is very accurate permanent and reliable. @ther objects wiil he in part obvious and in. |part pointed out l'iereinafter.

The invention accordingly comprises the elements and combinations of eiements, steps and sequence ci steps, features of construction, and arrangements of parts which will he erremplied in the structures hereinafter described, and the` scope of the application of which will he indicated in the following claims.

In the accompanying drawing, in which is illus-s trate@ one of various possible embodiments of the invention,

l is a top plan view of a contact emhodying the invention;

Fia. 2 is a bottom plan view of Fis. l;

Fis. 3 is a crosssection oi a composite strip prior to a forming operation;

Figfe is a cross-section illustrating a forming operation;

Fis. 5 :is a vertical'section taken on line of Fia. l;

Fis. 6 is a cross-section at location t--t o Fis. 2 but showing the nished contact attached to a bimetal sheet;

Fig. 7 is a side elevation or the iinished contact;

Fig. E is a side elevation showing hcw the assembly of Figs. 6 and is made on a spot Welder;

Fig'. 9 is a. section showing an alternative form of contact button before attachment; and

Fig. lo shows the button of Fig. 9 in attached position.

Similar reference characters indicate corresponding parts throughout-the several views of the drawing.

It is commonly necessary` to attach electrical contacts to thin sheets such as of bimetallic thermostatic material which, due to its thinness of section, is easily damaged by the attaching process. Electrical spot-welding has been found to (Gl. gilt-R66) be one of the best methods of making such an attachment, but even with it certain dilculties are encountered in practice. This is because lthe bimetal is usually a high-resistance material, such as for example a nickel-steel and invar combina.- tion, the total thickness of which is, for example, of the order of .006 to .016 inch. An example of the contact to he attached is a button composed of a silver facing on a steel or Monel backing, the total thickness of which button is, for example, of the order of 1% to of an inch. Other metals and thicknesses are sometimes used, but the above are exemplary.

No attempt is ordinarily made to weld silver directly to bimetal, because silver has such a low resistance. that it would reduire an excessive amount ci current to bring about its fusion to the himetal; and then the high-resistance bimetal would he so overheated by this amount of current that'it would burn away. This overheating is due not only to the high-resistance characteris tics oi the bimetal but also to its thin cross section. Therefore, heretofore it has been the practice to employ the steel or Monel backing mentioned in order to more closely match the relative resistance of the actual surfaces to-be joined, and

thus he able to use a lower value of` current. Nevertheless, some diiilculties are met with in the case of very thin blmetal having rather high resistance qualities. The present invention over-` comes these diculties.

Referring now more particularly to the drawing, numeral l indicates a. contact button generally, and numeral t the thin bimetal strip to which the contact l is to be attached. The contact button l is composed of a steel or Monel backing 5 having welded thereto in the usualmanner a silver face l. The buttons are made from a sheet constituted by welded steel (or Monel) and silver components (see Fig. 3).

The present invention comprises applying a. layer I3 of solder about .002 inch thick on the steel or Monel sheet opposite the silver sheet, preferably .prior .to punching, as indicated in Fig. 3. Then :the organized silversteel-solder sheet is placed in a coming press to form the individual buttons as shown in Fig. 4. A matrix die 5 provides the convex surface for the silver sheet 1. This convex surface is shown at the bottom in Fie. 4. The opposite matrix' die 8 has a face which impresses upon the solder-covered steel or Monel face a. grid-like impression shown at the top o1' Fig. 4. The face of this contour is shown in Fig. 2. The grid-like contour. is squeezed into the steel or Monel backing .through the solder aardgas.

'.3 without removing the4 solder, and the result is a grid-like back facing in the steel or Monel which is entirely covered with solder of about the same forming operation has been that not enough ows out around the edge of the "Dutton to make the usual undesirable uneven colg lar of solder which is often seen on ordinary sol dered contacts. Any excessive uneven lateral coll lar has various electrical disadvantages which it is important to avoid if possible. For example, it

often prevents the most desirable current dieB tribution around a contact.- A typical nished re sult of the invention is shown in Figs. d and "Z,

The attachment is by means ci welding can at the then practically solderless raised faces s Attachment of the button i to the strip ti is made in a spot-welding machine l5 (Fig. d) hav= ing high-conductivity, cold-operating electrodes il. A hot-electrode Welder is not used, such as for example one employing carbon electrodes. This is an important point, since it is desired to cause the heating for welding and soldering to occur only near the faces of the raised grid portions S which faces are on the interior of the joint, and not by heat conduction from an electrode through the silver l, steel 5 and strip The reason for this is that in the latter case the temperature gradient would be from the electrodes toward the grids 9, thus subjecting the entire thickness of the strip 3 to a higher temperature than necessary for a given temperature at the point of welding` and soldering, With the high conductivity cold 'electrodes in the welding machine, the parts 9 are rst heated to a higher temperature than the surrounding parts `simply v because they have a'smaller cross section than the adjacent parts and thus heat up faster and y accomplish the interior welding and soldering before overheating or burningof the strip Vcan occur.

We have found that the use of carbon electrodes caused eirtensive annealing of the bimetal,

' due to the length of time required in the total attaching cycle. With the carbon electrodes using v solder alone, it is necessary to hold the' electrodes clamped together to position the contact on the bimetal until the solder solidies. Also, the heating part of the cycle is slow because of the l fact that if suiicient current is used to heat the button and bimetal rapidly, the silver surface of the button will melt rst because the heat engenderedtherein comes, for a large part, from the carbon electrodes.

In the present method, the grid projections act as rapid heaters for the solder at the one particular .place where it is necessary to have this heat.'4

This heat may reach 2700 F. due to the Welding that takes place. However, it occurs so rapidly that little or no annealing of the bimetal around the contact takes place, yet, due .to the high temperature, the solder immediately flows during the cycle. Also, the weld that is made serves to clamp the contact on the bimetal in position while the solder solidiiies.

It is to be understood, as to Fig. 8, that the usual pressure of the welding machine electrodes is applied and current traverses the button l and strip 3, the solder on the faces of the raised grid por-tions S is melted and is squeezed out laterally into the grooves H therebetween. In this position the solder takes up its holding functions. As the solder disappears from under the parts 9, they come into contact with the strip 3 and are joined thereto by direct spot-welds.

The grooves Il accommodate enough solder so and by the solder i3 `now niled in the grooves Eli. The result is a relatively heavy contact attached both by welding and .soldering (but in dide'rent audition the attachment is relatively-accurate and clean-cut around the edges of the button i. The welded .part of the contact `is constituted by a eld of separated areaaand the soldered parte are constituted by a network of solder lines. i.the relative areas oi the neld of squares and oi the network can be varied somewhat, and also the shape oi the grids can he varied. The values above given are :for a 1/4 inch button about .06 inch thick having .a silver facing on a steel or Monel backing attached to a bimetal strip of chrome-steel alloy and Invar totaling about .00@ inch thick, the attachment being made ,on the chrome-steel side of the strip.

Reduction of attachment areas of buttons has heretofore been attempted by providing a single small area in order to gain the resistance needed for temperature increase at' the welding point. In practice this involved a current density oi about 33,000 ampereslper square inch or higher Ain the case of a it inch contact necked down to I/ inch to provide a single area of weld oi about .003 inch area. Ii the whole V4 inch area were to be welded down the current density would be the same, but the total current required would be enormously greater.

It has been suggested heretofore to use muitiple projections on a contact instead of a single projection to increase resistance and localize heat without overheating, but the current-carrying capacity of such a joint will nowhere equal,

plished. The weld serves to give mechanical strength to the joint and alsohigh conductivity, due to the autogenous joint where it occurs. It also acts as a clamp while the solder solidies.

The solder gives the additional electrical con-- tact between the contact button and the thin bimetal which the welded zones do not supply. Furthermore, the projections serve as heaters for the solder to enable an vextremely short soldering-welding time to be used to avoid unwanted annealing. Therefore, there results an attachment which covers the entire area oi the contact button, is mechanically strong, and is done with lower power and less deleterious annealing than would be the case if a. joint of the same area were made by other known methods. Y

As an example above, we have used a thin sheet of high-resistance bimetal. However, the eiiect can be advantageously used where a heavy cona certain area,

tact is to be soldered to a thin sheet of copper. In that case the grid form can be used ii the contact has a relatively thick portion of steel, as shown above. However, if it is possible to use a thin section of the high-resistance heater metal. next to the solder, the grid form may not have to be used, as indicated in Fig. 9 wherein numeral 2| is a silver contact, 2l is a ilat disc oi highresistance heater metal and the solder. In this case when attachment isgnede, the heater metal 23 melts the solder 25 and forms a soldered connection to the sheet 2l as shown in Fig. 10.

In this case only a soldered connection is desired to the thin low-conductivity copper sheet 2l. Thus, the layer 23 of high-resistance metal acts as an eficient adjacent heater for the solder while preventing destruction of the thin sheet to which the contact is being attached, just as it prevents destruction in theA bimetal sheet in the case of a contact which is welded, as above described.

It is to be understood that .in th fo ci the invention any inaccuracies ci gagement :between the contact areas `will automatically be lled in with solder'which will perform the desired holding functions.

In view oi the above.7 it will be seen that the several objects ci the invention are achieved and other advantageous results atted.

As many changes could be made in the above v constructions and processes without departing from the scope oi the invention, it is yintended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a ting sense.

We claim:

1. An electrical contact comprising a member having a relatively 'low-resistance face and a relatively high-resistance bach to which said face is attached, said back being provided with a eld of spaced, raised welding areas with depressed spaces therebetween, solder in said spaces, and a support attached to said back directly at said raised areas and through the solder in said spaces.

2. An electrical contact assembly comprising i relatively thin blmetallic support, a contact button comprising a relatively high-resistance element adjacent said support and a relatively low-resistance element on its other side, said button being directly attached to the support over and solder connecting the button to the support over another area.

3. An electrical contactassembly comprising a relatively thin bimetaliic support, a contact button comprising a relatively high-resistance element adjacent said support and a relatively lowresistance element on its other side, said button being ydirectly attached to the support over a certain area, and solder connecting the button to the support over another area, said directly attached and soldered areas being both substantially within the periphery of the button.

4. An electrical contact comprising a button having a low-resistance face and a high-resistance back attached to said face, said back having its rearward face formed as a grid of raised and low portions, a support contacted by said raised portions and ydirectly attached thereto, and solder ln said low portions attaching said button and support.

5. A contactu comprising a relatively low-resistance conducting face member, a relatively high-resistance heater member forming a. backing for said face, solder on said backing member for attachment to a support, said backing member having a high enough relative resistance with respect to the resistance of the face member so that upon application ci current to the contact seid solder is transformed into a fastener between the backing and the support without substantially damaging either the face member, the

support, or the backing member.

d. A contact comprising a relatively low-resistance conducting face member, a relatively highresistance heater member forming a backing for said face, solder on said backing member for attachment to a support, said backing member having a high enough relative resistance with respect to the resistance of the face member so that upon application of current to the contact said solder is transformed into a' fastener between the backing and the support without substantially damaging either the face member, the support or the backing member, said current being suicient also to weld the backing member to the support Without substantial damage.

7. A contact for attachment to supports comprising a face made of material of relatively high conductivity, a solder fastening part, and a layer of relatively high-resistance metal between the face and the solder adapted upon application of suilicient current to melt the solder without substantial damage to the supports or the face.

8. A contact for welding and soldered attachment to supports comprising a face made of material of relatively high conductivity, a solder fastening part, and a layer of relatively high-resistance metal between the face and the solder, and having a. reticulated face for positioning against the support and adapted upon application of suicient current to melt the solder and to weld the reticulated face of the high-resistance metal to the bimetallic sheet without substantial damage to the latter or the face.

'rowNsnNn M. GUNN. WALTER J.

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