US3812445A - Be-cu contact for nema outlet - Google Patents

Abstract

A fingered electrical contact for a wiring device is provided to exert a high yieldable gripping force on an inserted power prong. The contact metal is a copper-beryllium alloy hardened to give the properties of spring steel.

Description

I United States Patent 1 1 3,812,445 Stefani May 21, 1974 BE-CU CONTACT FOR NEMA OUTLET 2,745.080 5/1956 Offerman 339/258 P 2.865,0l0 12 1958 T l 339 I64 [751 lnvemo" Joseph Swim", Warwlck, 3,153,840 10/1964 V2622: 72/364 [73] Assignee: General Electric Company,

7 Providence, R.l. [22] Filed: Sept. l6, 1968 Primary Examiner-Joseph H. McGlynn Attorne A em, or FirmPaul E. Rochford; P. L. A 21 Appl. N6; 871,415 schlamiv, g

a Related Application Data [63] Continuation-impart of Ser. No. 546,273, April 29,

1966 abandoned 52 us. Cl 339/32 R, 29/630, 72/364, [57] ABSTRACT 113/119, 339/164 R [51] Int. Cl H0lr 29/00 A ngere ele trical contact for a wiringdevice is [58] Field of Search 339/32, 33, 164, 165, 258; provided to exert a high yieldable gripping force on an 1 13/ 1 19; 29/630; 72/324, 364, 379, 700 inserted power prong. The contact metal is a copperberyllium alloy hardened to give the properties of [561 References Cited pring steel.

7 UNITED STATES PATENTS 2.743.428 4/l956 Martines 339/258 P 1 Claim, 15 Drawing Figures PATEMEUHAYZI 1m 3,812,445

' sum 1 (If 2 FIG.3

INVENTOR JOSEPH P STEFAN] BY Me /Z M ATTO NEY PATENEB F5621 GM SHiET 2 BF 2 FIG. 10

FIG-.14

M OF TE NT ES R N H P E S O J BE-CU CONTACT FOR NEMA OUTLET This application is a continuation-in-part of the application Ser. No. 546,273 of the same inventor filed Apr.

29, 1966 now abandoned.

The present invention relates to an electrical wiring device and more particularly to a wiring device having electrical contacts of unusually strong prong retaining capability.

The effectiveness of a wiring device for its intended purpose of conveniencing the temporary making and subsequent breaking of electrical connections has been developed to a degree where electrical outlets are provided in most buildings having electrical service. Heavier and heavier currents are available from such outlets for normal household, factory and institutional use as equipment requiring heavier temporary supply of electrical current and greater reliability of supply of current have come into more common use.

Reliability of connection is ensured by use of the locking device type of electrical connector but special electrical caps and receptacles must be provided and these must have specially shaped power blades and receiving connectors.

Greater reliability of maintained connection can be ensured in part by increasing the retentive pressure exerted on a prong by the prong receiving contacts of an electrical connector. However it is known that materials which have greater capability to conduct electricity will generally have poorer physical and work resistance properties.

One difficulty resulting from the poorer mechanical properties of highly conductive metals used in electrical receptacles is the tendency of the contacts to be sprung by the repeated insertion of the power blades as required to make electrical contact. This is-familiar to most persons who have used convenience outlets in older homes. As the contact fingers of conventional convenience outlets are sprung there is a tendency to employ power blades of greater thickness to ensure contact. This has a tendency to cause further spreading of contact fingers and can cause the separation of the contact fingers of new convenience outlets beyond that satisfactory for retaining the thinner power blades under acceptable blade retaining pressure. Poor contact results in heating of the connection. As heating results in oxidation, resistance at the contact is increased and the heating effect is further increased. Hazardous heating under high loads can result.

While it is highly desirable that the spacing between contact fingers of a convenience outlet remain essentially constant and that the contact pressure exerted on blades inserted between the fingers not diminish due to prolonged usage or due to use of blades of first greater and subsequently of lesser thickness, it will be appreciated that too great a contact pressure can also be detrimental. This is particularly so where the metal of the contact fingers is sufficiently hard as to cause a scoring or cutting of the surface of the blades inserted between the contact fingers. Also too great a pressure between the contact fingers can require use of an excessive pressure to insert the power blades. This, in turn, can cause too great a pressure of the contact on the housing and can cause deforming or even a breaking of the housing itself.

Similarly where the inserted power blade is held between contact fingers with excessive pressure the with- It is accordingly one object of the present invention to provide a blade contact for use in convenience outlets and the like which is not subject to requiring excessive force to establish or to break contact and which is also not subject to loss of adequate power, blade contact pressure.

Another object is to provide a contact which may be conditioned by repeated insertion of smooth hardsurfaced, over-sized power blade contacts without undue loss of the ability of the contact to exert retentive pressure on smooth-hard surfaced power blade contacts of conventionally smaller thickness.

Other objects will be in part apparent an in part pointed out in the description which follows.

In one of its broader aspects the object of the present invention are achieved by providing a fingered contact formed of a beryllium copper alloy in the semihardened condition and heated to a final spring hardness, the fingers of said contact retaining their spring pressure within prescribed pressure limits following re peated cycling with over-sized power blades.

The prescribed limits for a specific embodiment of the present invention as illustrated in the accompanying figures is the capability of a fingered contact to develop sufficient spring pressure on a smooth steel blade having a thickness of 0.055 inches under a pull of 1.5 pounds for seconds after said contact has been subjected to 20 conditioning cycles comprising the insertion and withdrawal of a smooth steel blade without holes having a thickness between 0.73 to 0.75 inches.

This standard is that specified by the National Electrical Manufacturers Association NEMA Standards, Publication No. WD2-l963. According to Federal Specification W-C 596 b (GSA-FSS) of Oct. 22, 1967 the finish on power blades used in such tests must be an 8 microinch finish grind in a direction normal to the direction of insertion of the blades.

The invention will be understood with greater clarity through the reading of the description which follows with reference to the accompanying drawings in which:

FIG. 1 is a plan view showing the contact of the present invention in duplex form in place in a duplex housing shown in phantom;

FIG. 2 is a plan view of the cover, shown also in phantom of the outlet housing of FIG. 1;

FIG. 3 is an elevational view of one form in which the duplex form of contact of the present invention may be made for use with the duplex outlet housing of FIGS. 1 and 2;

FIG. 4 is a perspective view of a contact similar to that shown in FIG. 3;

FIG. 5 is a perspective view of a contact similar to that shown in FIG. 3 but which may be formed by the joining of the separate contact end portions to a con ductor strip portion;

FIG. 6 is a perspective view of a single contact adapted for use in a single outlet;

FIG. 7 is a perspective view of a duplex contact similar to that shown in FIG. 5, one of the contact ends being shown in exploded view;

FIG. 8 is a horizontal section showing the array of fingered contacts as used in a modification of the separated fingered contact of FIG. 7; 2

FIG. 9 is a similar section of an array of fingered contacts illustrating a further modification of the separated fingered contact of FIG. 7;

FIG. 10 is a plan view of a duplex housing for an outlet of the present invention;

FIG. 11 is a sectional view taken along the lines ll11 of FIG. 10;

FIG. 12 is a split view similar to that of FIG. 11 but showing one contact member entering and another contact member in place in a duplex housing;

FIG. 13 is a plan view of a housing for a single outlet;

.FIG. 14 is a sectional view of the housing of FIG. 13 taken along the line l414 of FIG. 13;

FIG. 15 is a perspective view of a contact member similar to those suitablefor use in the housing of FIGS. 13 and 14.

Referring now to the drawings and particularly to FIGS. 1 and 2, there is shown a pair of duplex receptacle contacts 10 and 12 disposed in mirror image relation each to the other, and also disposed in the respective insulating cavities l4 and 16 of the lower section 18 of an insulating housing shown in phantom. The upper section 20 of the insulating housing, shown in FIG. 2 also in phantom, when placed in position over the lower section, provides two pairs of power blade openings 22 and 24, in registry with the blade receiving fingered ends of the duplex contacts 10 andl2.

Openings 24 will admit a blade aligned either parallel or normal to the length of the conductor strip 10. The alignment of the two blades of a power connector to be plugged into the convenience outlet will determine the rating or power level to be supplied from the convenience outlet and to be received through the connector according to established conventions.

The power blade will normally be either parallel or normal to the connector strip, but the problem or need for establishing a high contact pressure between the blade and the contact fingers is substantially the same for the blade in either its parallel or normal orientation. Similarly the problem of avoiding having the blade receiving fingers 26 and 28 sprung by insertion normal to the connector strip of blades of excessive thickness is similar to that of having the finger 30 sprung by insertion parallel to the connector strip of power blades of excessive thickness.

To acquire this ability to resist being sprung the metal part is formed of a copper beryllium alloy which is an annealed and partially work hardened state at the time of part formation, and which is later heat treated to impartthe spring properties of spring steel. In general pursuant to the present invention the metal from which the part is formed is in as hardened a state as willpermit the part to be formed with reliability and facility.

It will be realized that not all metalparts are formed with equal reliability and facility but that this depends on the form of the part, the tolerances to be achieved,

the angles and the tolerances of the angles to be formed, the tooling used and a number of other factors familiar to those knowledgeable in the mechanical arts.

An important feature of the present invention is'the formation of the copper beryllium fingered contact through a heat treatment to provide fingers in any array adapted to receive and grip a power blade with the appropriate contact pressure in either parallel or normal orientation to the conductive connector strip, but wihtout the high cost fixturing or jigging and multiple step heat treating normally necessary to achieve this result with the copper-beryllium alloy employed using techniques known in the art.

It will be evident from a consideration of the shapes and spacings of the fingers of FIGS. 1 and 3 that when the fingers do not make contact with each other and when the spring pressure depends primarily on the flexing of the contact metal the amount of spring pressure developed on a power blade inserted between opposed fingers will depend on the spring properties of the contact metal, including the fingers, and also on the initial spacing of the area of closest proximity of the individual'fingers, i.e., the portion of the finger surface which will deliver the-spring pressure to the inserted power blade.

The difficulty of forming parts, such as the fingered contacts of the Figures, from a metal stock, such as from a beryllium-copper alloy strip, due to dimensional changes, including shrinkage, and due to differential stress relief of the metal are well known.

In describing the formation of a U-shaped spring, for example, and of a 360 spring, elements of which will be seen to be represented in the contacts of the Figures, an authoritative article in the July 16, 1964 issue of Machine Design provides the following recommended procedure for forming the springs through fixturing:

Recommended procedure for both shapes is:

l. The formed part is given a partial free-body heat treatment 1 hr at 650 F. Since the inside of the radius was compressively deformed during bending, the part will close somewhat during the partial heat-treatment.

2. The part is cooled to room temperature, then fitted into an accurately profiled steel fixture, putting the outside of the bend under an elastic compressive stress.

3. The part is again heated to 650 F for 1 hr, and the bend sets on the fixture, producing the desired shape. I

It might appear logical to eliminate some of the three step procedure by overforming the piece, putting it on the fixture and heating for 2 hr without interruption. This simplified procedure doesnt produce satisfactory results however; the shrinking cannot be accurately predicted, and the bend usually moves farther than is desired. Thus, the partial heat-treatment is an essential step in using simple fixtures and achieving shape control.

Surprisingly, and contrary to the indications provided by this source, it has now been found that the parts shown in the Figures can be produced to have very small and highly accurate clearances between the confronting positions of the fingers with only a single heat treatment.

Furthermore, and even more surprisingly, this novel contact forming process is now accomplished without the use of fixturing.

The surprising nature of the distinction of the method taught herein is further evident from the teaching of the US. Pat. No. 3,153,840 of A.W. Vincent issued Oct. 27, 1964. In this patent the inventor teaches that where parts having free ends are given a single heat treatment, that the'only result of this treatment is the warping or distortion of the parts. In other words, the Vincent teaching is that a single heat treatment step cannot be used without ruining the parts unless the tongues are held in place by jigs or fixtures.

For the sake of clarity, and to distinguish from jigged or held parts, parts which are not held or jigged are referred to in this application by the terminology movable parts. Specifically, where fingers are designated as movable, it is to be understood that they are free from any jigging or holding or bridging with a fixed metal part attaching them to other metal parts.

Considering now the first of these two improvements no initial heat treatment is given to the part after it is formed. The strip material from which the part is formed should preferably be about three eighths hard and may be between one quarter and one half hard and also one which, on physical testing, undergoes an elongation of between 14 and 35 percent, and preferably between and percent. An important aspect of the present invention is the discovery that the dominant material variable which must be determined in order to determine the suitability of the strip stock for formation of the fingered electrical contacts as illustrated in the Figures is the degree to which a partially hard stock may be elongated.

In order to obtain the optimum results employing the teaching of the present invention, it is important to understand certain relationships between the characteristics of the material used in preparing the fingered contact from strip stock, the form and dimensions of the fingered contact formed, and the heat treatment and post heat treatment afforded, to the fingered tion within a relatively narrow band of percentages normally of the order of IO percentage points.

For example, starting with a strip stock which is extensible to between 25 and percent, once the form of the fingered contact and the dimensions of the clearances between the contact fingers have been set for this copper beryllium strip stock it is highly preferred thatas the dominant material variable is indicated to be the extensibility of this stock, and inasmuch as the extensibility may be selected from among those in the broad range of 14 to 35 percent, it will be understood that as the extensibility of .the material is reduced from a higher .to a lower value within the range by furtherrolling of the stock, the "hardness of the stock is increased and its formability as well as its extensibility is reduced.

To produce parts, particularly fingered contactparts with uniform and reproducible clearance from stock within this broad range, it will be understood that other factors being constanttthe percentages of elongation of the stock should be maintained pursuant to this inventhe extensibility of the material used in forming additional parts to these same dimensions and form should be within the 25 to 35 percent range. On the other hand, where the parts are formed with material of perhaps 20 percent extensibility, it is highly preferred that material used for forming additional parts be extensible within the 14 to 25 percent range.

The choice of an upper or lower or other range indicated above will be in part dependent on the complexity of the part to be formed and the angles which are used in the parts. For example, to form a part to tighter angles or to bends of smaller radius, the stock selected should be less roll hardened and accordingly more extensible. For such parts a choice of extensibility within the range of 25 to 35 percent will be preferred.

On the other hand, where because of the form of the parts there is less need for complex forms and less need for tight or small radius angles, folds, or bends, the preferred choice would be for the material which has been more work hardened and which, accordingly, is' less subject to change during a heat hardening of the formed part.

It must also be appreciated then in selecting the form or angles or bends to beincluded in apart that in general the larger the radius of a bend, the lower will be the degree of control in holding dimensions or changing dimensions to certain tolerances during the heat hardening. It will be noted, for example, that in ribbing the fin- .gered contact of the present invention the ribs are applied to the portions which have the shallower angles of the structure.

By roll hardness, or hardness due to rolling, as used in this application, is meant any hardness imparted to the material from whatever source prior to the formation of the part from the stock and accordingly prior to the heat treatment of the formed part. Accordingly, al-

though the hardness is stated herein to be a roll hardness it may very well be a hardness imparted as the result of rolling and/or as a result of any other treatment of the stock prior to part formation and is distinct from the hardness imparted to the formed part by a heat treatment to give the formed part the hardness and other properties of spring steel.

To illustrate the present invention a part such as that illustrated in FIGS. 1 and 3 was formed from a strip of 31 2 mils thickness of a three eighths hard copper beryllium alloy designated as alloy 165. This is an alloy normally'having a'beryllium content between 1:60 and 1.85 percent and for the purpose of forming parts pursuant to this invention, preferably having a beryllium content of at least 1.7 percent.

The strip was formed into the part as shown in FIG. 3, to provide a blade receiving clearance 32 of 20 to 30 mils and a blade receiving clearance34 of 2to 10 mils.

The part was vapor degreased and then heated in a reducing atmosphere for 4 hours without interruption at a temperature of within a range of 605 to 625 and preferably 615?. The parts are preferably kept in the reducing atmosphere as their temperature is brought up to heat treating temperature and as they are cooled.

The yield strength of the metal of the strip before the heat treatment is preferably 70,000 to 90,000 psi and is also preferably increased to at least 140,000 psi by the heat treatment.

Using an alloy 165 strip stock as described above particularly one having the indicated combination of starting properties, it was found that parts having fingers of desired spring properties and having the required power blade receiving clearances can be formed without the need for the interrupted heat treatment heretofore required.

It is important in selecting the treatment steps that a distinction be maintained between the treatment used when the ends or fingers are freely movable and those used as taught in the Vincent U.S. Pat. No. 3,l53,840 when the fingers are necessarily jigged or fixtured or bridged by a connecting metal element. The formation of the article as is described above resembling that in FIG. 3 was carried out as is indicated with the fingers free to move under the influence of the heat treatment.

Referring now to FIG. 4 a form of the device of the present invention isshown which embodies the second improvement referred to above, and accordingly the additional improvement to aid in overcoming the limitations of the prior art as set out above.

Specifically in FIG. 4, and in FIG. 6 wherein a like improvement is shown, a rib 40 is pressed centrally along the contact finger during the finger forming operation and prior to heat treatment. It has been found that such lengthwise rib can function effectively as an integral jig in further limiting the change of the spacing of the fingers during the heat treatment operation. Through the use of this rib more complex forms of fingers can be employed or greater control of dimensional change of fingers of simpler form can be achieved.

The effect of the integral jig or ribbing is that-of an improvement over the plain or unribbed finger for certain applications as explained above. However it will be understood that the unribbed fingered contact prepared pursuant to the teaching of this invention by an uninterrupted heat treatment yields uniquely uniform products of unusual blade retaining capabilities.

The ribbed structure offers certain other advantages. For example, the rib adds the advantage of increasing the force transmitted along the finger without deflection or bending of the beam itself. Such deflection is, of course, of the material of the length of the finger acting as a beam and a greater portion of the spring action of the contact is thus generated in the base portion 46 as a torsion of this portion. This improvement can accordingly be used to effectively gain greater distribution of the spring action and in this way limit the tendency of the contact finger to be sprung by an excessive deflection in the finger itself.

The rib extends outwardly from the confronting surfaces of the fingers and does not quite extend up the length of the finger far enough to reach the area to be contacted by the power prong. As is most evident from FIG. 3, the upper ends of the fingers are bent out in V formation to provide a guide and wedge surface for the introduction by wedging of the power prong. To concentrate the spring developed pressure at the upper end of the fingers and thus allow the finger to serve as a spring along essentially its entire length, the portions of the fingers below the contact area also diverge toward the base of the fingered contact.

Referring now more particularly to FIG. 5, the device represents a departure from those shown in the other Figures principally in two respects.

The first is the inclusions of a fulcrum point 52 in the form of a protuberance formed'by dimpling the inside surface of the contact base 56. This protuberance corresponds to the lowermost end of the rib 40 of FIG. 4 and serves essentially the same function as that served by the lowermost end of the rib. This protuberance serves as a fulcrum point in bearing against the side wall of the cavity into which the contact strip is placed in a convenience outlet insulating housing as shown in FIG. 1. The bearing of the ends of the duplex strip against the end walls of the cavities provides an interference fit which, because it is made through this fulcrum point, permits the base 56 to undergo greater de flection with the deflection of finger 58. This action accordingly serves to increase the extend of torsional force developed in the base 56 as a result of deflection of the finger 58. The lowermost end of rib 40 performs an essentially equivalent function. 1 v

An alternative means of achieving the effect of the interference fit is through use of a protuberance or wedge on the casing or housing of the outlet as is described below with reference to FIGS. 10 through 14.

As will also be pointed out more fully below, the degree of pressure exerted longitudinally on the contact strip from the housing may be increased over that indicated immediately above in describing the interference fit of the contact strip lengthwise in the housing. This control of the longitudinal compression of thestrip is useful in control of the spacing between the contact fingers particularly fingers such as 33 and 35 illustrated in FIG. 1. Such control of the spacing also has the effect of modifying the degree of compression exerted by these fingers on each other where they are in contact or the pressure exerted on a power blade inserted therebetween.

The second respect in which the duplex contact of FIG. 5 is distinct is in the combination of a distinct strip of conductor metal 54 between the fingered ends of the contact. This separate strip is mechanically and electrically joined to the bases of the fingered ends by the bolts 51 and 53. It may be conveniently made of brass, for example, as the principaladvantages of the copper beryllium alloy is in the fingered ends of the duplex contact structure.

One further advantage of the use of a metal such as brass as the conductor strip is that it facilitates the breaking off of the mechanical and electrical central link so as to separate the duplex contact for separate circuit wiring. A still further advantage of the use of brass is the higher conductivity and also, of course, still another is the lower cost of the metal.

An alternative way in which an increased spring pressure may be developed by the fingers is by closing" the base from which the fingers extend, or from which they are, in this construction, essentially cantilevered. This closing of the base is accomplished by securing the free end such as the free end 42 of FIG. 4 to mechanically link it to the other end" 43 of the base of the fingered end of the contact. Such a closed base of a fingered contact end is illustrated at the right hand end of FIG. -5, the left hand end being shown with the base open to illustrate the advantages of use of the protuberancc fulcrum point 52. The two ends of the figured contact at the right of FIG. are overlapped and joined to conductor strip 54 by rivet 51.

The linking of two ends of the fingered contact end has additional advantages which will be explained more fully below where it is desired to alter the dimensions between the contacts such as contacts 33 and 35 of FIG. 1 or the equivalent contacts of FIG. 5 by the attachment of a rivet such as 51 to hold the ends together. The part, that is the separable fingered contact part, in this illustration is made with a separation of the fingered contacts and accordingly a separation of the holes in the base which are penetrated by the rivet 51 so that in the assembly when the rivet is inserted the holes are brought into alignment and accordingly the fingers of the contact are brought closer together or into greater compression if they are abutting. This provides, in other words, an added method for adjusting the pressure which can be exerted by fingered contacts such as 33 and 35 on a power blade inserted therebetween.

As shown in the right hand end of FIG. 5, the base has two actual ends inasmuch as it, is not formed integrally with the elongated conductive connection portion of the duplex contact structure.

Considering now more specifically the structure of FIG. 6, it will be evident that this is a single contact structure which resembles in its essentials-that of the left hand portion of FIG. 4.

The rib 60 corresponds to that 40 of FIG. 4 and a second rib, only partly visible in FIG. 6, also corresponding to like ribs on the innermost fingers of FIG. 4, is located at the bend 62 in finger 64. This shorter rib e'xtends longtudinally of'the finger approximately half the distance to the folds above and below the fold 64 which it intersects.

The length of strip 68 provides the portion of the contact which is electrically connected by a connector not shown with the wire carrying current to the convenience outlet. Like parts of the other contacts have like functions.

Referring now particularly to FIG. 7, a structure is provided which includes a number of advantages in addition to those described above.

One such advantage is the closed base of the fingered contact end. Closing in this case is accomplished through a second material by bolting one end 71 of the base shown at the left end in exploded view, with a suitable bolt or rivet extending through the aligned holes 73 and 74 of the end of the conductor strip 75, and the end 71 of the fingered contact base 76.

The other end 77 of the contact base 76 is provided with a stud 78 adapted to engage the conforming opening 79 of conductor strip 75 and to be mechanically locked therein at the time the rivet 72 is secured in place.

It is evident that as the base of the fingered contact of FIG. 7 is closed a spring bias can be established in the base, which bias will serve to bias, in turn, the spring action of the fingers of the contact. In other words, it is evident that in the first place the simple mechanical closure of the base of a fingered contact such as that shown in FIG. 4 without developing any spring bias in the base will by itself reduce the freedom of the base to yield to torsional forces developed in the base as a result of deflection of the fingers. By closure is meant, of course, the mechanical linking of the metal band forming the base so that a closed band is formed as shown in FIG. 7 as contrasted with the open base as shown in FIG. 4. Where in effecting the closure the base elements are not simply mechanically linked as referred to above, but the base is itself subjected to com-. pression so as to develop a spring bias before the me chanical linkage is completed it is this biasing of the base which in turn biases the spring action of the fingered contacts extending up from the base. Accordingly from such biasing closure the base is put and remains under a spring bias and the fingered contacts similarly placed and remain under a spring bias.

An additional feature of the structure of FIG. 7 is explained with reference again to FIG. 1 and comparison to the fingers'as arrayed in FIG. 1. When a power blade is inserted into space 32 it is evident that the finger 31 will deflect to the greatest degree because'the thickness of this finger is presented parallel to that of the power blade and is parallel to the blade. The fingers 33 and 35 are disposed with their width normal to that of an entering power blade so that the latter fingers are disposed to resist deflection to a greater extent than finger 31. Accordingly, since the fingers33 and 35 resist deflection, most of the actual deflection which does occur with insertion of a blade into space 32 is that of the fin ger 31. The finger 31 thus becomes the more yielding finger and blades 33 and 35 are in this sense the unyielding fingers or, more properly, the less yielding fingers, as some deflection does occur.

When a power blade is inserted in space 34 each of the two blades 33 and 35 may be thought of as yielding to a roughly equal degree and accordingly both may be I characterized as more yielding in the above frame of reference.

Also from FIG. 1 it is evident that the spaces between the fingers 32 and 34 adapted to receive the power blade are aligned ina T form with the cross member of the T, i.e., clearance 32, aligned parallel to the conductor strip portion between the two end fingered contacts of the duplex contact structure.

Finger 33 will normally yield more than finger 35 because in the version shown in FIGS. 1 and 3 the innermost blade 35 is anchored to and in fact formed as an integral extension of the conductor strip joining the two end fingered contacts. Finger 33 is subject to greater deflection because its base is not anchored or held as immobile as that of finger 35. It is withinthe scope of the present invention to increase the spring pressure on a blade acted on by a finger such as finger 33 by anchoring or biasing the base of this finger as described heretofore.

Returning now to the structure of FIG. 7, there is shown a fingered contact providing a T formation of spaces between power blade receiving fingers. This formation is found between two yieldable fingers set at right angles and a third less yieldable finger.

The less yieldable finger is formed by abutting two fingers, one forming the end of the conducting strip joining the fingered contacts, and the other formed as a parallel and matching end finger of the fingered contact. The two matching fingers are mechanically joined at their respective bases by the rivet 72.

The finger 81 is the more yieldable of those which contact a power blade inserted with the width of the blade and finger arranged in parallel because the width of the finger 82 is disposed normal to that of the power blade.

The double finger is less yieldable than finger 82 when a power blade is inserted with its width parallel to that of finger 82 because the width of both of the double fingers aredisposed normal to that of the power blade.

A further modification of the fingered contact is shown in cross-section in FIG. 8 and includes an angled finger (a finger having a lengthwise fold and an angular cross-section) in place of the double finger of FIG. 7. This angled finger provides the additional advantage of presenting a finger of low yieldability to each of two directions at right angles. This is used most conveniently as the less yielding of a three finger contact in receiving power prongs in either of two alignments forming an L formation. A horizontal sectional view through the three fingers of the contact taken slightly above the base of the fingered contact is shown in FIG. 8.

A further modification shown in FIG.- 9 as a horizontal sectional view through the array of fingers is particularly suitable for use with blades in a T formation and employs edge ribbing to reinforce the parallel pair of fingers such as 33 and 35 of FIG. 1. The edge ribbing is at the edge of the fingers corresponding to 33 and 35 of FIG. 1 and proximate the perpendicularly extending finger corresponding to 31 of FIG. 1. The edge ribs extend in'opposite direction and are aligned parallel to the third finger of the T. The edge ribbing accordingly forms additional surface to bear against a power prong inserted parallel to the contact strip.

Through the use of ribbing as taught herein, the dimensional changes of the parts such as those shown during a heat hardening has been virtually eliminated.

Clearances of 3 to 6 thousandths between fingers can be set in the part before heat treatment and are found to be maintained within that range following heat treatment without any fixturing and with a single and uninterrupted heat treatment.

An important aspect of the present invention is the discovery that through ribbing as taught above, it is possible to limit and even eliminate the closing movement of the fingers either together or apart on heat treatment without eliminating the ability of the fingers, and of the fingered contact, to apply effective spring pressure to power blades within predetermined limits inserted into contact with the heat hardened fingered contact.

While the foregoing specification for establishing and holding certain clearances and tolerances between parts of the fingered contact of this invention is valuable in permitting parts to be formed and then heat treated without appreciable dimensional changes resulting from the heat hardening, it will be reazlied that the maintenance of precise controls in the dimensions as well as in the extensibility of the stock from which the parts are formed, and in other specifics of the mate rial and parts, is often difficult and expensive. Accordingly while it is feasible pursuant to this invention to I be formed which fulfills the performance requirements or standards, such as the NEMA Standard specified above, but which allows a wider tolerance of material, dimensional, processing, and other variables in attaining such performance standards.

Referring again to FIG. 1 it will be noted for example that where the clearance 32 is formed from a part folded without ribbing to the configuration shown, after the part is punched from strip stock with the length of the strip 12 oriented normal to the length of the strip stock where the stock has been selected to have the essential properties discussed above, it is found that the clearance 32 will enlarge by a generally predictable amount averaging 8 to 10 mils for a 20 mil starting clearance but extending over a range of 6 to 12 mils. This can be substantially reduced by ribbing of finger 31 in accordance with the present invention.

By contrast the dimension of the clearance 34 may not change appreciably where the starting strip has been selected based on its having the combination of properties as discussed above.

However, the variation of dimensions of clearance 34 from initial, as formed, to final, as heat treated is far greater than that of clearance 32 because of. the far greater complexity of the forms of the portions of the contact which determine the clearance. For an unribbed part, formed from material having the narrow band of extensibility as indicated above, initial clearance 34 having dimensions of 2 to 10 mils will have final dimensions in the range of 0 to [2 mils. Ribbing of fingers 33 and 35 as taught above will reduce this variability by approximately 50 percent.

Moreover this ribbing is useful not only in improving the predictability and/or control of the final dimensions of the clearances, even of those resulting from complex parts, but also in improving the as formed clearances of the parts.

The foregoing treats generally of the fingered contact of this invention and of how to form it and how to close it or mount it for most effective use.

The specification of performance test which a contact of this invention should desirably meet is that established by the National Electrical Manufacturers Association as indicated above. It will be appreciated that the performance of thecontact will usually be in the conventional insulating housing for convenience outlets and as indicated above there may be appreciable influence on the performance, particularly where the contact fingers do not extend up from a closed base, as a result of the interference fit of the contact base into the cavities of the insulating housing. Some forms of this interaction are more fully described in the section which follows.

Referring now to FIG. 10 the base of a duplex outlet housing is seen in top plan view. The base is essentially the same in outline and internal structure as that shown in phantom in FIG. 1.

With specific reference to the Figure a pair of contact receiving cavities 84 and 86 are provided at one end of the base and a similar pair of contct receiving cavities 88 and 90 are provided near the other end of the base. A contact strip such as 12 of FIG. 1 is introduced into the base to locate the fingered end portions thereof in the end portions 84 and 88 of insulating cavity 16.

The wire attachment portions of the contact strip are located as the contact strip is inserted into the cavities 14 and 16 of base 18 of FIG. in registry with the pairs of wire entry ports 92 and 94 of cavity 16 and 96 and 98 of cavity 14.

Similarly the central break off portion of the contact strip is positioned where its severance from the strip will leave one fingered contact and associated pair of wire attachment means secured in each end of the contact strip receiving cavity 14 or 16.

A cavity 100 at one end of the base receives grounding contact and wire attachment means for the mounting strap to which the grounding contacts are mounted in conventional fashion.

What is provided pursuant to the invention in the receptacle base, in addition to the cavity to receive the contact strips, is means to act on and to longitudinally compress the contact strips as they are inserted into the respective cavities of the base. These meansare as indicated above alternative means for developing the interference fit developed as a result of providing protuberances on the ends of the contact strips. The manner of. their use and operation in relation to the contact strips is best described with reference now to FIGS. 11 and 12.

Referring first to FIG. 11, a vertical sectional view of the base of FIG. 10 is shown taken along the lines 11-11 of FIG. 10.

Within the cavity 16, at the fingered contact receiving end portions 84 and 88 thereof, are located the beveled wall portions l02 and 104. These portions 'are formed integrally with the insulating base 18 and are designed to act on the contact strip to generate in the strip a longitudinal compression as a result of an interference fit similar to the longitudinal compression developed as a result of the action of the protuberances 52 on the end walls of the cavity bases 14 and 16.

The action of the beveled end wall 102 may be explained best with reference to FIG. 12. In the split illustrationthere is shown in the upper portion the relation of an entering fingered contact to the end section of the base 18. As is seen the conact once it enters the end portion '84 of the cavity can contact the end wall surface 106 to engage this wall with a lowerpressure interference fit or under essentially no pressure at all. However it is also evident that as the fingered contact is urged further into the cavity 16 an interference fit or longitudinal compression of the strip will be developed or increased as the entering edge of the base of the fingered contact rides up the beveled surface 102.

One result of this wedging of the contact strip into the cavity 16 is the partial closure of the base of the fingered contact and accordingly of the distance between the fingers of the contact where they are initially separated or an increase in the pressure between the ends of the fingered contacts where they are initially in contact.

Referring again to FIG. 12, it is seen that fingers 33 and 35 are shown to have an appreciable clearance 34 therebetween as the base of the contact first enters the end portion 84 of cavity 16. At the lower portion of the FIG. 12 there is shown a pair of fingered contacts 133 and 135 in a closed or abutting position after the contact strip is fully inserted into cavity 16 and accordingly after the fingered end portion 110 of the strip has entered the lower portion 88 of cavity 16. The end surface of the contact strip is accordingly seen to be pressed in an interference fit against the shelf surface 118 similar to the shelf surface 108 of end portion 84 of the cavity 16.

Turning now to the FIGS. 13 and 14 an outlet base is shown adapted for inclusion of single fingered contacts with associated wire attachment means to serve as a single outlet as contrasted with the duplex outlet structure of FIGS. 10 through 12.

The parts provided and their association is however quite similar to that provided in each half or the duplex outlet particularly when the connecting break off tab is removed from the duplex contact strip as when the duplex outlet is to be wired with the conventional split circuit wiring arrangement.

Referring specifically to FIG. 13 it will be seen that fingered contact receiving cavities 86 and 87 are provided similar to the receiving cavities 84 and 86 of FIG. 10. Also pairs of wire entry ports 93 and 97 are associated with the single outlet contact strip placed therein similar to the pairs of ports 92 and 96 of FIG. 10.

End compartment 128 accommodates grounding wire connectors of a conventional grounding strap not shown.

What is made clearer from the single outlet base of FIGS. 13 and 14 is that the longitudinal compression or interference fit provided for the duplex outlet and par ticularly the longitudinal compression of the strip thereof is provided separately for each half of the duplex strip. It will be noted that there is a beveled surface at each end portion 84 and 86 of the duplex base 18. Similarly for the single outlet base of FIG. 13 a beveled surface 103 is providedbetween an end wall entry surface l07and an end wall interference shelf surface 109 of FIG. 14.

The longitudinal compressive force is accordingly de veloped between the end wall surfaces 103 and 109 and the opposite end wall surface 121 best seen in FIG. 13.

For the duplex outlet structures the compressive forces are developed between the beveled and shelf surfaces such as 102 and 108 of FIG. 12 and a separator'surface such as 120. Partial retention. may also be provided by appropriate configuration of the underside of the cover such as 20 shown in FIG. 2.

The end wall surface 121 of FIG. 13 may be provided with an adjacent rib 123 to hold the end of the wire attachment portion such as 68 of FIG. 6 in place in a single receptacle base.

A further alternative form of wedge surface to induce an interference fit in a receptacle base is illustrated in FIG. 15. In this illustration the parts of the device are similar to those shown in FIG. 5 but the wedge surface takes the place of the protuberance such as 52 of FIG. 5. It will be apparent that the insertion of the fingered contact of FIG. 15 by inserting the base surface into a cavity such as is illustrated in phantom in FIG.

1 and which cavity is not provided at its end surfaces with compression inducing wedges will produce a similar effect in inducing longitudinal compression of the contact strip and partial closure or increased pressure between the opposed fingers 134 and 136.

It will further be understood that whereas the foregoing is described in terms of increasing the interference fit and accordingly the longitudinalcompression of fingered contact members as they are inserted into a receiving cavity of an insulating housing that the increased compression can be accomplished pursuant to this invention prior to insertion into a cavity. In such cases the compression can be accomplished manually or by means of a tool prior to insertion and the part may then even undergo some relaxation or extension once in place in the cavity as the compressive force of a tool or the like is removed.

The importance of the longitudinal compression in practice of the present invention is as indicated above in adjusting clearance or spring pressure between fingers, particularly where such fingers are opened or separated in the heat treatment.

It should be understood that fingers can have excessive separation after heat treatment either because they were too closely positioned or too widely separated prior to heat treatment. When too widely separated prior to heat treatment they may remain too widely separated. If too closely positioned they may abut during heat treatment and be bent into too wide a separation.

Usually as indicated above the increase of a separation which is initially too wide will occur in a clearance such as 32 of FIG. 1 whereas decrease of a separation which is too close will occur in a clearance such as 34 of FIG. I.

Because of the change of dimensions resulting from heat treatment, it is usually preferred to mechanically close the base of a lingered contact after the heat treatment as this closure can serve the same function as compressing the contact base after heat treatment through insertion into a suitably sized base as explained above.

What is claimed is:

l. A fingered contact of a copper beryllium alloy of spring hardness having multiple spring fingers rising from a common base, the free ends of said fingers being freely movable and being aligned to receive power blades in either of two positions at right angles, a first of said fingers being stiffened by ribbing to be the least yieldable, a second and third being less stiffened by ribbing to be less yieldable, and said ribbing serving as integral jigging for maintaining predetermined clearances of said fingers as said contact is heat treated to spring

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