US2641670A - Serrated contact - Google Patents

Description

June 9, 1953 H. c. GRAVES, JR 2,641,670

SERRATED CONTACT Filed Aug. 22, 1950 FIG. IA.

l/O FIB )0 FIG. l0.

E ll l2 l3 1| '3 DI? w FIG. IB.

IB I FIG. 28.

FIG. 5.

INVENTOR FIG.4. HERBERT c. GRAVES, JR.

ATTORNEYS Patented June 9, 1953 SERRATED CONTACT Herbert G. Graves, J r., Wilkins'burg, Pa., assignor to Gibson Electric Com corporation pany, Pittsburgh, Pa., a

Application August 22, 1950; Serial No. 180,756

9 Claims.

My invention relates to a novel method of mounting a contact on a contact arm and more particularly relates to novel serrated electrical contacts and a novel method for impregnating such serrated electrical contacts to achieve improved contact mountings.

Electrical contacts have serrations to provide small areas at and around which considerable heat is generated when the contact is urged against the contact carrying arm and the welding current is applied for binding the contact to the arm. The serrations of the contact will pierce the soft copper contact arm to provide a toe-in or dig-in weld. Prior to the welding process, a fusible bonding material, such as silver, is allowed to impregnate the electrical contact. This prevents oxidation of the contact, decreases electrical resistance and provides a material to assist in brazing or welding the contact to the contact arm.

The method of making an electrical contact by mixing silver and a refractory metal powder, such as molybdenum, tungsten, or their carbides, pressing them to form a porous metal with or without a serrated back, sintering and impregnating this contact with silver is'well known in the art. Many variations of this procedure are in present use.

One procedure is to serrate the electrical contact in the pill pressing operation. After the contacts are made by powder presses in pill form with a serrated back, they are sintered and then impregnated with silver. Subsequent to this each contact may go through a punch press operation to press the silver which has been formed in between the serrations intoa form corresponding to that of the serrations.

A second procedure in use is to form a flat contact in a powder pill press, sinter this contact, impregnate it with enough silver so that an excess of silver will manifest itself as a layer of pure silver on one surface of the contact, and then by a coining process form the serrations of the pure silver surface of the contact.

Each procedure is objectionable from at least one of the following standpoints:

1. It requires an extra and comparatively expensive punch press operation.

2. The pyramidal serrations are rather delicate and brittle and a forming or punching press operation tends to crack the serrations.

3. In circular contacts it is almost impossible to make the serrations of the electrical contact line up with the serrations in the punch press coining plunger with the result that there is a definite distortion or breakage at the serrated points.

4. During the impregnation process the molten silver is not fully contained by the truncated pyramidal protrusions and, therefore, runs over the sides of the contact.

In order to avoid these objections, my invention proposes the use of round serrations. This eliminates the necessity of lining up serrated round contacts in the punch press die before striking with the punch, strengthens the serrations and readily holds the molten silver.

Another type of serration' which avoids the 2nd and 4th objectionable features outlined above is the use of truncated pyramidal cavities rather than protrusions. Thus, the serrations are not as delicate as in the protruding type and will not crack in a punch press'operation.

Accordingly, an object of my invention is to provide a novel type of serration for electrical contacts.

Another object of my invention is to provide a serrated round electrical contact whose serrations will easily line up with serrations on-the punch press die. V

A still further object of my invention is to provide two types of serrations for electrical contacts which easily contain and hold the molten silver during the impregnation process.

Another object of my invention is to provide electrical contact serrations which will not crack in a coining operation.

Another object of my invention is to provide a novel serrated electrical contact which is easily mounted on a circuit breaker contact arm.

Still another object of my invention is a method of impregnating electrical contacts to avoid the necessity of a coining operation.

These andother objects of my invention will appear from the following description taken in connection with the following drawings in which:

Figure 1A illustrates a top view of the conventional type of truncated pyramidal formation used on serrated contacts.

Figure 1B is a side view taken along line lb-lb of Figure 1A.

Figure 1C is'a front view taken along line lc -lc of Figure 1A.

Figure 2A illustrates a top view of the novel round serrations on a round contact of my inve'ntion.

Figure 2B isa side view taken along line 217-212 of Figure 2A.

Figure 3A is a top view of the novel serrations which are formed by truncated pyramidal cavities on the novel contactofmy invention.

Figure 3B is a side, view taken along line 3b-3b of Figure 3A and shows the depth of the cavities.

'Figure 3C is a front view taken along line 3c3c of Figure 3A. 7

Figure 4 illustrates the novel serrated contact of my invention pressed against a fiat surface during impregnation. v

Figure 5 is a view in perspective of the final condition of the novel impregnated contact of my invention.

In order to facilitate the description of the invention, the two methods of preparing serrated impregnated contacts will be designated as A and B. Method A is the procedure whereby the serrations are pressed in the electrical contact of silver and a refractory material while it is in the powder pill press, sintered and then impregnated. Following these steps, the serrated electrical contact may or may not 'go through a punch press or restriking operation to press the this material.

silver which hasbeen found in between the serrations into a form corresponding to that of the serrations.

Method B consists of mixing silver and a refractory material, pressing them to form a porous contact similar to that of method A, but without the serrated back. This double fiat surface electrical contact is then sintered and impregnated. Following the impregnating step the contact is sent through a coining or punch pressing operation where the serrations are impressed on the back of the contact.

My invention covers two phases of electrical contacts. The first phase is to provide two novel designs for the serrations used in electrical contacts. This may be used with either the methods of A or B. The second phase of my invention is a procedure which can also be used with either method. The first and second phase of my invention can be used together advantageously.

- The first phase of my invention will nowbe described.

Figures 1A, 1B, and 1C illustrate the conventional type of serrations H on an electrical contact I 0. The surface l3 and space I2 are provided to contain the bonding material such as silver. When the contact is impregnated or covered with a bonding material, this material will be contained in space I2 resting on surface l3.

The truncated pyramidal formations l l or any othe shaped projections, hereinafter referred to as projection type serrations, are formed on the contacts to permit the contact to pierce a circuit breaker copper contact carrying arm to which it is welded or brazed.

A typical application of serrated contacts mounted on a circuit breaker arm is illustrated in my Patent No. 2,030,582 issued February 11, 1936.

When the Welding electrode pressure urges or presses the contact against the contact arm to which it is to be welded, the projections H engage the surface of the contact arm resulting in a high pressure over this small area of engagement' of the combination. Whenthe welding current is passed through the combination, the copper contact arm softens and the projections I I pierce the contact arm to provide a dig-- in weld. The tendency of the projections II to pierce the contact arm is greater when the projections are made in accordance with method A.-

Both of the designs of serrations proposed by this invention have the same function and are used as the projection type of serration. However, these novel serrations also have added advantages and overcome the objectionable feature of the projection type of serrations as will become apparent.

Figures 2A and 2B illustrate the top and side view, respectively, of my novel round or concentric circular ring design serration proposed as one part of the first phase of my invention. Concentric circular hills 2! and valleys 22 circle the center point 28 on the round contact 20. Con tact could be either a round, square or any other desirable shape. The circular hill or projections 2! will serve the same purpose as the projections H of Figures 1A, 1B and 1C and the circular valleys or cavities 22 will serve as a space in'which the bonding material may be contained when the contact is impregnated or flooded with This round serration design will hereinafter be referred to as circular type serrations.

The second design proposed inthe first phase of my invention. is illustrated in Figures 3A, 3B and 3C. The contact 38 is provided with serrations due to the valley or cavities 32 which may be of any desirable size'or shape. It should be noted that the flat surface it of the conventional contact illustrated in Figure 1A is belowthe projections H whereas the flat surface 3! of the novel contact illustrated in Figure 3 is above the cavities or valleys 32. When this contact is impregnated with silver, the portion of silver which acts as a bonding material will be contained in cavities 32 and rest on the surfaces 33... The serrations illustrated in Figure 3 will henceforth be referred to as cavity type serrations.

The many advantages obtained by using either the circular or cavity type serration should be noted at this time. When method A is used, where it is necessary to have a punch press operating on the impregnated serrated contact to obtain the proper serration contours, the contact must be correctly placed in and on the die of the punch press.

If a round contact is used with the projection type of serration as shown in Figure 1A, it is extremely difficult to line up the serrations of the contact with the serrations of the die. However, when a circular type serrated contact is'used as shown'in Figure 2A, this objectionable feature is overcome since it is much easier to line up the contact serrations with the die punch serrations which also have concentric circular grooves.

- Moreover, circular type serrations permit cheaper die cost and maintenance since the orig-. inal machining operation and subsequent polishing steps can be done on a lathe or similar standard rotating machine.

On'the other hand, the die punch associated with the projection type serration involves a rather complicated original grinding operation and subsequent polishing steps. During the performance of this restriking or coining operation, it is almost impossible to make the serrations in the projection type contact line up exactly with the serrations of the die in the punch press.

Inasmuch as the projections are small non-interconnected elements, they are rather delicate and brittle and consequently the punch press operation sometimes results in a distortion, cracking'or breakage of these serrated points.

The difiiculty is overcome in both the circular and cavity type serrated contacts where each of the top surfaces 2| (Figure 2) and 3| (Figure 3) are interconnected to form either a cir-.

cular ring or a cross-hatched series of surfaces. This construction strengthens the serrations so that they are not as delicate or brittle and thus are able to overcome the last-mentioned objection.

When the steps of method A are used in the manufacturing of electrical contacts, the impregnation step requires that a molten material,-

usually silver, be applied to a serrated contact;

In the projection type serrated contact,i'as can aurov be seen in Figure 1, the space l2 which must contain this molten material is opened on all sides so that no supports are available to trap the molten mass and frequently an insufficient quantity of silver orbonding material is attached to the contact in the appropriate areas, thus resulting in an inferior weld.

This objectionable feature is overcome by the design of the circular and cavity type serrated contact where the molten mass or fusiblematerial is easily contained in either the circular grooves 22 or cavities 32 and trapped in place by the circular rings 29 or walls 39 seen in Figures 2 and 3, respectively.

The many advantages of the use of either the circular or cavity type serrated contacts have been outlined only in connection with method A. However, it should be clear that the use of these two types of serrated contacts could be used with equal advantage when method B is used.

The second phase of my invention deals with a means by which serrated contacts can be impregnated to produce a better contact and reduce the number of steps needed in the manufacture of impregnated serrated contacts.

The step in methods A and B of impregnating electrical contacts is old in the art and is done to achieve any of the purposes which will now be described. After the powdered silver and tungsten have been pressed with serrations in the pill form and then sintered, a porous contact results. Impregnating this contact with silver enablesthe entire outside of the contact to be coated with silver, preventing oxidation.

Furthermore, because the contact is porous, the pure silver will fill the cavities of the contact to effectively reduce the electrical resistance of the contact. The third and an important purpose of the impregnation step is to fill a portion of the valleys or cavities with the bonding material of silver to prepare the contact for a good Weld or, if method B is used, to provide a layer of pure silver which can be coined to the serrated shape.

It will be apparent that the amount of im-- pregnating silver must be rather closely controlled or it will fill up and overflow the serrated valleys and hills to obliterate the serrations and defeat the purpose of these serrations. The method of determining the amount of impregnating material which can be added is given by the following formula:

V E W=AB(cV-- where W=Weight (grams) of impregnating silver material to be added to the contact A=Density (gr./cc.) of impregnating material Amt. of silver which can'be impregnated (practically) Amt. of silver which can be impregnated (theoretically) Vol. after impregnation for no excess silver and would give a contact having 100% of its theoretical density. The

curately evaluate this quantity and the only basis which can be used is past experience .on the amount of shrinkage obtained for similar materials. V 1: The quantity B is again a factor whose necessary inclusion in the formula is dictated by past experience. If this quantity were left out of the formula and all other values in the formula were accurately known, the resulting 1 silver weight (W) would, when impregnated into the contact, provide for a compact having,1a..den-- sity equal to of its theoretical density. However, it appears practically impossible" to obtain such a contact, and, therefore, this factor is included so that too large a silver excess will not be obtained.

This is the best formula known to the inventor for calculating the amount of impregnating material to be added to the sintered contact so that no excess of silver isobtained. Unfortunately, it does not always give exactly the calculated result and thus I pick the formula constants (B and C) so that the actual weight of silver added does provide excess which partiallyfills cavities or valleys.

Following the sintering process of method A, the novel impregnation method may be accomplished in the following manner. The sintered serrated contact is placed with the face of its- This is accomplished by placing the sintered ser-- rated contact so that the face of its serrations i's'facing a hard fiat surface'26' of plate 25 as illustrated in Figure 4. The serrations 29 are' pressed as indicated by the arrows 60 by a'ny suitable means against theflat, surface 26 of plate 25.

The end result as seen in Figures 4 and 5 is:

(1) The extreme surface 2| of serrations 29 is not obliterated;

(2) All the surfaces 2| of the contact been pressed into one plane, and;

(3) The extreme outer surfaces 2| and the impregnated silver 23 still form a contour similar to that of the original serrations. 1

(4) The surface of the contact 20 is silver coated.

(5) The porous contact has been impregnated with silver.

The use of this novel'impregnation procedure achieves the purposes previously outlined for the old method of impregnation. That is, the contact 20 has a thin coating 24 of silver on its surface to prevent oxidation. The porous contact 20 also has had its channels filled with the pure 20 have I claim:

sistance of'the contact,rand thirdly, a portion of the space :22 is filled with the impregnating silver 23 to "provide a bonding material through which the contact may be welded or brazed to a contact arm. a Furthermore, this process of impregnation'may be used with contacts containing the serrations of either design outlined in the first phase of this invention, or any design to be used with electrical'contacts having the old projection type serrations.

' The procedure of impregnation outlined in the second phase .of the invention not only accomplishes all the purposes of the old method of impregnationbut also has. additional advantages over" the old method which will now be described.-

Since the final results of this impregnation process result in. a sintered. serrated contact which has all the extreme surfaces 2! in one plane, since only the valleys 22 are partially filled with silver 23, so 'that'the surface still follows a similar contour of the original serration it has been'found that the coining 'or repressing step of method A'is not necessary. Consequently, by using this novel method of impregnation it is possible to eliminate one step. The punch press or restriking operation from method A and B, as previously pointed out, was a very objectionable step due to its many disadvantages, diflicultyof performance and added expense.

- As noted previously in this application, they disclosed herein inasmuch as it will be obviousparticularly to persons skilled in the art thatmany'changes and modifications may be made in these embodiments or methods without departing from the broad spirit and scope of this invention. Accordingly, it is desired that .the invention be interpreted as broadly as possible as defined inthe following claims; v What I claim as new and. desire to-secure by Letters Patent of the United States is:

' 1. In an electrical circuit breaker having acon-- tact carrying arm, an electrical contact mounted;-

on said arm, said contact being made of suitable electrical conducting metals, said contact having any desirable shape and containing serrations on one face, said serrations consisting :of circular concentric rings, said rings consisting of hills forming valleys isolated from each other, said valleys being partially filledgwith another contact material of a lower melting point than the material of said first mentioned contact.

2. In an electrical circuit breaker having a contact carrying arm, an electrical contact mounted on said arm, said contact being made from pressing and. sintering a mixture of silver and refractory metal powders, said pressing op-' eration forming said electrical contact having one face serrated,- said serrations being a series of continuous projecting surfaces effecting cavities'therebetween isolated from each other.

.3. In an electrical circuit breaker having a contact carrying. arm, an electrical contact mounted on said arm, said contact being made from pressing and sintering a mixture of silver and refractory metal powders, said pressing operation formingsaid electrical contact with one face serrated, said serrated face being a series 'ofcontinuous projecting transverse andlongitudinal surfaces effecting cavities therebetween isolated from each other. v

r 4. A round disk'shaped electrical circuit breaker contact; a three dimension design impressed in one face of said contact; said design having circular concentric grooves isolated from each other and being partially filled with contact material so as to leave protruding ridges in said one face of said contact. 7

,5, An electrical contact made from pressing and sintering a mixture of silver and refractory metal powders, said pressing operation forming said electrical contact with one face serrated, said serrated face being a series of continuous projecting transverse and longitudinal surfaces, said transverse and longitudinal surfaces forming a multiple of isolated holes orcavities in said face of said contact. 7

6. A unitary electrical contact made from pressing and-sintering a mixture of silver and refractory powders and impregnating the pressed and sintered contact; said pressing operation forming the first stage of said electrical contact having one face serrated, said serrations being of continuous projecting surfaces forming isolated cavities therebetween; said sintering operation forming the second stage of a porous electrical contact; said impregnation operation depositing and impregnating said porous electrical contact withsilver; a part of said silver being contained in the space between said series of continuous projecting surfaces on one face of;

said electrical contact. v r

'7. An electrical contact comprising a conductive body having one face serrated, said serrations forming a series of recesses in said face isolated from eachother.

, 8. An electrical contact comprising a body of refractory material ofthe class which includes molybdenum, tungsten, tungsten carbide, molybdenum carbide and combinations of said materials; said body having asurfaceiof an electrically conductive metal; said surface being serrated in form, said serrations defining a plurality. of recesses isolated from each other.

9. An electrical contact comprising a body of References Cited in the file of this patent UNITED STATES PATENTS Name Date Allen Oct. 13, 1942 Cox Oct. 24. 1944:

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