US2480280A - Electric connector - Google Patents

Description

lg- 30, 1949 M. D. BERGAN 2,480,280

ELECTRIC CONNECTOR Filed Sept. 24, 1945 INVEN TOR MARTIN D. BERAN.

AT'T'DFNEY Patented Aug. 30, 1949 UNITED STATES yPATENT OFFICE ELECTRIC CONNECTOR Msi-un n. nemn, Westfield, N. J., num: to The Thomas Betts Co., Elizabeth, N. J., a corporation of New Jersey Application September 24, 1945, Serial No. 618,218

3 Claims. (Cl. 173-269) The invention relates to an electric connector of the compression sleeve type permanently secured to a conductor.

It has been known in the prior art to intrude a cable end or rod into a socket or recess of a connector formed of some malleable metal, such as copper, and by die pressing, rolling, swedging or otherwise compressing the connector onto the cable end to cause a deformation of -both and a resulting mechanical and electrical bonding action between two electric conductors. Further it has been suggested to roughen the contacting surfaces between a cable and a connector, and also to incorporate abrasive particles in the joint formed therebetween to insure a positive gripping effect in permanently securing the two conductors in their final permanently assembled form. Such assemblies have not proven entirely satisfactory, among other reasons because when used with cables, particularly those formed of fine wire strands, the roughened surfaces or the sharp abrasive particles both have a tendency to cut into and sever the fine cable wires which they engage, and further the resistance which develops across their contacting surfaces often develops thermal and g-alvanie reactions which cause the elements to become heated, often to become hot, especially where high tension electric energy is being transmitted.

Further describing known art it has been the usual practice in forming assemblies of connectors and cables as herein featured, especially where the connectors are made of copper, to exert on the copper connectors a degree of squeeze pressure, just sufficient to cause the necessary bond and herein regarded as moderate when compared with the pressures used in this disclosure. Care had to be exercised, especially where the socket or recess Which received the cable is threaded or otherwise roughened, to regulate the bite of the threads into the fine cable strands in order to obtain the requisite conductivity across the joint and at the same time to avoid damagingv the wire strands. It has been extremely difficult to avoid cutting at least some of the strands and with the pressures necessary to distort a massive aluminum casting, as herein featured, it would be practically impossible with the usual roughened surface to avoid cutting some of the wire strands.

The present disclosure particularly features the use of connectors made of ordinary commercial aluminum. This metal has many advantages when used in the electric art in'addition to its current market cheapness. For instance, it is is highly resistant to most inorganic acids; it alloys readily with copper, nickel, tin and other metals to produce hardness, rigidity and strength in its alloys and it is susceptible to a high degree of finish by polishing or burnishing. Of particular advantage in the instant situation is the fact that it becomes hardened when worked and has high elongation value under radially directed squeeze action.

As is well known freshly formed surfaces of laluminum tamishes readily when exposed to the atmospheric air; an almost invisible coating of its oxide A1203 or alumina forms thereon almost instantly. The presence of this oxide coating is both an advantage and a. disadvantage when used to form connectors of the type herein featured. In its favor it is noted that it permanently adheres to the surface and prevents further 4attack on the metal and thus protects the exposed surface from further deterioration and corrosion. However, the presence of this aluminum oxide film between surfaces intended to be in electric contact is most objectionable. The relatively high coeicient of resistance of aluminum oxide tends to insulate its contacting surfaces from their companion surfaces and thus reduces the conductivity across the joint with resulting heating effects which tend eventually to destroy the emciency of the bond. Particularly in the case of aluminum, and to a less extent in the case of copper and other electrical materials, this oxide film forms so fast that in practice it is impossible even after cleaning it off to make contact with the cleaned surface before a new film is formed. There is no commercially practicable way to avoid the formation of this oxide on aluminum surfaces.

The primary object of the invention is to provide an improved form of assembly formed of two electric conductors, providing a bonding joint therebetween having both high electrical conductivity and mechanical strength.

Broadly this aspect of the invention is obtained by utilizing a sleeve-like connector formed of a ductile, easily distortable, cold-owing metal, protected on its exposed surfaces with a film of insulating layer, and the joint forming surface of which is provided with threads, projections or similar forms of penetrators or scrapers organized and designed to react with the companion surface forming the joint to break through the film, or films as the case may be, in such a way as to scrape away the film or films to form fresh contacting surfaces providing clean metal-to-metal quite ductile and thus can be easily worked; it bonds.

' More specifically `defined the invention features the use of an aluminum connector, the cable receiving bore or recess of which is outlined by a wall, the cable engaging surface of which is'machined to provide an abrasive surface which may be a knurling. In the preferred form of the invention iilustratedthe surface is.

capable of breaking through the tenacious aluminum oxide nlm. It is suggested that this abrasive surface may otherwise be found as by a'metal spray whichleaves a sandpaper rough surface.

Various other objects and advantages of the invention will be in part obvious from an inspection of the accompanying drawings and in part will be more fully set forth in the following particular description of one form of device embodying the invention, and the invent1on also consists in certain new and novel features of construction and combination of parts hereinafter set forth and claimed.

In the accompanying drawings: Fig. 1 is a view in perspective showing a connector and associated cable permanently assem bled and illustrating a preferred embodiment of the completed article aspect of the invention.

Fig, 2 is an enlarged axial cross sectional 1'iew taken-through the device shown in Fig. 1.

Fig. 3 is an enlarged plan view looking down upon a small portion of the serrated or threaded sleeve wall viewing the same from the plane indicated by the line 3-3 of Fig, 2 and showing the impressions made on the aluminum surface of the connector by the strands of the cable.

Fig. 4 is a view partly in section and partly in elevation of the component parts which go to makeup the device shown inv Fig. 1, with the parts located between the jaws of a die press and illustrating the initial method step just before the die press is operated to squeeze the parts into their interlocked and permanently connected position shown in Fig. 2.

Figs. 5 and '6 are each transverse sectional views taken respectively on the lines 5--5 and 6-6 of Fig. 4.

Figs. 7'V to 9 are explanatory views, very much exaggerated, of the joint formed between a portion of the sleeve serrations and one of the surface strands of the cable, the showing in Fig. 7 corresponding to the portion of the surface shown in Fig. 4 before the dies have' squeezed the parts together; Fig. 8 showing the same portion of the joint after the squeeze dies have acted upon the same and with the sleeve distorted into the final form shown in Fig. 2, and Fig. 9 showing in dotted lines the position of ve of the corrugations or threads prior to the dislocations of the same as shown in Fig. 7, superposed on the five corrugavtions when distorted into the distended position shown in full lines corresponding to the nal Fig. 8 form, and

Fig. 10 is an explanatory view of one of the threads highly magnied showing the same at the left relatively narrow and about to bite' through the oxide films on itself and on an outermost strand and at the right shown exaggerated- 1y expanded ln metal-to-metal contact having passed through the two oxide lms.

Referring to the finished structure shown in Figs. 1 and 2, there is disclosed the final commercial assembly II which includes a. metallic connector I2 and a cable I3 intruded into an end of the connector and swedged into permanent connection. Referring to Fig. 2 for a disclosure of the basic parts in detail, the connector I2 includes a sleeve I 4 into the open end of the bore I5 of which is intruded a stripped end portion of the cable I3 shown to be of the braided multiple rope and line aluminum strand type. The cable is of conventional design and includes an inner core I6 of twisted ropes I'I formed of fine wire strands I8.v The cable conductor as thus formed is encircled by a tube of rubber or equivalent insulation I 9.

In the form of the invention herein selected for illustration as a preferred embodiment of the invention, the cable strands are formed of ne aluminum wires and which form of cable has been purposely selected as it is of a type in which with previously known forms of similar connectors the fine strands were most liable to be severed in the act of securing the connectors thereto. Before being inserted in the blank which is to form the finished connector I2, the cable end is previously prepared by stripping back the insulation I9 for a distance to leave exposed nally a proper amount of the bare cable end portion 2| of the cable strands following conventional practices in this respect.

The connector is a. metal casting formed of some easily malleable metal selected from the and connector at least at one end thereof. How- Y ever itis equally within the scope of the disclosure to utilize an open end, or all tubular, type of connector and in this case dependence is placed on the long joint hereinafter described to avoid air infiltration. The bore is twice reduced to form the sleeve I4 and to form an outer cylindrical thin wall cuff 24 having both its internal and external diameters slightly enlarged from those of the sleeve. vThe bore at its inner end adjacent the wall 23 is slightly enlarged from the diameter of the balance of the sleeve bore to form a pocket 25 for receiving the extreme inner distended or rather less compressed end of the cable conductor.

The inner end of the culi bore is dened by an outwardly facing seat 26 of conical form against which the intruded end of the insulation I9 is designed to abut. The outer edge of the cylinder forming the cuff is inwardly beveled to form an end wall 21 against which an inwardly squeezed portion of the insulation is designed to abut in the finished structure `so that the cuff will merge more or less smoothly into the insulation as shown in Fig. 2. A

As by reference to Fig. 4, it will be seen that the insulation I9 is cut back a distance to expose the cable wires for a length slightly less than the length of the sleeve bore and to bring the cut-olf end of the insulation close to but not intruded into the portion of the bore outlined by the conical seat 26.

It is a feature of this disclosure that the conthe scope of the disclosure otherwise to form the connector as by forging or machining it to shape. The sleeve bore is corrugated preferably by cutting the wall with an internal threading machine to form a fine spiral thread 28 of very shallow depth. The depths of the threads or corrugations are dimensioned sufficiently high to pass through the thickness of the aluminum oxide film present, also to pass through whatever may be the thickness of film present on the wire strands, but care must be exercised in dimensionlng the threads or corrugations to be assured that they are not so high that they might sever the line wire strands by biting into them too deeply. In practice the depths of these films vary from flve to ten thousands of an inch. In one embodiment of the invention, the threads are of the single lead type having forty-eight threads per inch. While these threads have the appearance when viewed by the naked eye of being somewhat sharp, however, under the microscope their crowns are definitely and carefully rounded. It is a feature of this disclosure that the crowns be suiiiciently rounded to avoid cutting through the fine wire strands which make up the cable ropes and to provide a smooth surface which will permit longitudinal sliding of each tooth or corrugation on the cable conductor as the corrugations bite inwardly into and through the oxide iilm.

The innermost end of the sleeve bore inl the portion thereof outlining the pocket as herein illustrated is not threaded nor is the bore of the cuff, both being finished with a smooth surface to facilitate longitudinal creeping of the connector 0n the conductor. However, it is within the scope of the disclosure to extend the threads onto the smooth bore portions if necessary.

Considered exteriorly the blank stock from which the connector is formed contains the tongue, tange or contact end 22 and this part of the original blank is not involved in the expansion action herein featured. The tubular portion which is to form the sleeve and cuff is originally of cylinder form with the end which is to form the cuff of slightly greater external diameter than the part which is to form the sleeve as is shown in axial section in Fig. 4.

The parts so prepared with the stripped cable end intruded into the bore of the connector blank are assembled as indicated in Fig. 4 and the assembly located between the compression jaws A and B of a die press of a hydraulic or similar form capable of exerting a squeeze pressure sufiicient as the connector is shrunk onto the conductor to distort the aluminum blank as herein indicated. The work engaging portions of the die surfaces are designed so that the portion of the blank which forms the cuff 24 is retained circular in cross section as indicated in Fig. 5 but the portion which is to form the sleeve is hexagonal in cross section. In general both dies coact to give the connector a more or less standard form such as is shown in Fig. 1.

With the parts in position as shown in Fig 4 the die press is operated either with a single stroke in the case of thin wall castings or with repeated strokes where it is desired to swedge a more rugged form of casting onto the cable end. It is diiiicult to preset any definite pressure required to break through the film of aluminum oxide as the resistance interposed by the cross section of the aluminum casting is a variable factor depending on the mass of material to be deformed. The

6 aluminum oxide film is more tenacious than a copper film and is thus more resistant to break through. 'I'his means that a greater pressure per unit 'surface area is needed in the case of aluminum oxide than would be the case where penetration through other metallic films such as copper oxide is required. In one specific instance a pressure of 50,000 pounds was required to break through the aluminum oxide lm. In vgeneral that pressure is used where a reading of ohmic resistance through the finished joint indicates .that the aluminum film is no longer imposing a resistance across the Joint. The pressures arev controlled by the various types of cables and connectors, also by the hardness of metal alloy employed.

This squeeze action causes the original cylindrical end of the blank to reduce its external diameters changing the length portion which is internally threaded from a cylinder into a hexagon or four-sided, square or diamond form by cold flow of the material worked upon by the dies. The cui 24 elongates to the left from the showing of Fig. 4 as it decreases in both its external and internal diameters. This has the effectl of radially compressing the cut-back end of the insulation I9 and this in turn causes the end of the insulation to become distorted under compression and to creep to the right until it conforms to and is stopped by the bevelled seat 26. This effects a powerful forcing of the end of the insulation into wedging engagement with the conical seat 26. The inward radially directed squeeze action of the cuff acts on the portion of the insulation at the bevelled end 21 at the open end of the bore to wedge the insulation against the bevel. The outer bevel end 21 assists in spreading the axial dimension of the joint between the compressed and incompressed parts of the insulation and provides in effect a soft material gasket at the point where the cable comes out of the connector. Any flexing or bending of the cable is cushioned at this point relieving any flexing or stresses which might otherwise be imposed on the fine wire strands. Of even greater importance is that the long joint formed between the cuil? and the highly compressed rubber-like insulation forms a hermetic seal at the cable end of the connector and this acts to prevent access of atmospheric air to the joint between the bare cable end portion 2| of the conductor core I6 and the sleeve I4 after the bond has been made.

The reduction of cross sectional ar'ea of the threaded sleeve I4 after contact with the end portion 2| of the ca ble core acts to compress the core to an extent to cause its cross sectional area to aoproach closely the aggregate cross sectional area of the metal strands which form the cable core and at the same time to cause some straightening out oi' the twisted strands to approach parallelism. Incidentally there is at least some slight elongation of the portion of vthe core so compressed by reason of this readjustment of the strands as they approach parallelism. It iinally gets to a point where the compacted metal strands tend to resist further reduction in internal diameter of the bore and the continued application thereafter of the squeeze force starts to elongate the sleeve in bot-h axiall directions. In the case of the device illustrated the pressure was continued until the sleeve has increased its length from ten to iifteen percent.

The particular feature stressed in this disclosure is that the threads increase their spacing as an incident of the distortions of the connector.

In the instant case, the pitch at the mid-length of the threaded portion changed from 32 to 28 threads per inch. In general as the threads move relatively away from each other, their pitch is changed and this causes a scraping or abrasive action on the aluminum oxide lm on the connector. This scraping acts to remove the oxide and to break therethrough into the clean metal of high conductivity provided by the freshly cleaned surface of the connector.

Several things happen as the result of this axial movement between cable and connector as they move into their final squeeze position. In passing it may be noted that the coeflicient of linear expansion of the aluminum casting is sub stantially greater than that of the aluminum strands; in fact it is not apparent that there is any material elongation ofthe strands, although there is some elongation which is believed to be more the result of the cable ropes straightening out from their original twisted form rather than to any material expansion of the strands themselves. It is particularly noted that the corruw gations or threads do not bite through the aluminum strands presumably due to the fact that they are rounded. In this way the conductivity through the cable end is not impairedl Of greater importance is the fact that in expanding lengthwise the crowns of the threads become increasingly more and more separated as their opposite ends are approached. Note, for instance that the distance a-b between the rst two crowns at the left of Fig. 7 become slightly longer in Fig. 8, as indicated by the distance a'-b', and these distances progressively increase by increasing increments of length until the distance d-e of Fig. '7, for instance, becomes the very much elongated distance d'-e of Fig. 8.

An effort has been made in Fig. 9 to show the relation of the corrugations by imposing the thread outline of Fig. 7 on that of Fig. 3. In Fig. 9, the original equidistantly spaced apart thread contours are shown in dotted lines and the corresponding contours after radial compression and axial elongation of the connector are shown in full lines.

In an effort to show what are the sequential conditions of the surface which will eventually secure the threaded part of the connector to the exposed end portion 2i of the cable conductor referenceis made to Fig. 7 where the parts at the right end of the threaded portion of Fig. 2 are shown highly magnified and not necessarily in proportion. For convenience of reference the cut is shown through ve crowns of the right hand end of the thread 28 and marked from left to right successively as a, b, c, d and e, and of course equidistantly spaced apart. The threaded portion is shown covered with a film 29 of aluminum oxide shown as a layer of uniform thickness but in actual practice the film is not so uniformly proportioned. There is `also disclosed one of the fine aluminum wire strands I8 at a surface of one of the ropes of the cable. There is also disclosed on the surface of the aluminum wire strand a nlm of aluminum oxide 30.

Apparently what happens is that the shifting serrations lor thread crowns in scraping along the wire strands rub the films on' both of the aluminum surfaces. Consider the movement of any point, say the point e in its engagement with the Wire strand I8. As it moves from e to e' in Fig. 9, the crown, even though rounded, scrapes along the strand for the distance e-e'. Dif ferently considered the length of wire between e' and e may be considered as moving relatively backward from right to left of Fig. 9 and this acts as it scraped along the aluminum surface to break through the aluminum oxide film 29. Likewise, it is seen that all of the Wires forming the surface ofthe end portion 2l brush across the entire surface shown in Fig. 9 with a sweeping action presumably acting to free all or practically all of the surface from the insulating effect of the aluminum oxide. By the same action whatever film 3@ may be on the aluminum strands are likewise swept away or at least broken up.

Referring to the showing at the left of Fig. 10, which may be representative in a highly exaggerated showing of any one of the thread crowns, say crown c, the film 29 Aof aluminumV oxide engages the aluminum oxide nlm 3@ on one of the aluminum strands it just before the start of the squeeze action of the die press and as shown in Fig. 7.

As shown at the right of Fig. 10, the thread crown c has shifted to the right as indicated by the short arrow and in doing so has squashed itself axially,A has broken through both the aluminum oxide lm 29 as well as through the aluminum oxide llm 3@ and 'has also scraped its rounded surface thus bared of aluminum oxide along the bared surface of the strands.

The terrific pressure applied to the outer 'layer of the aluminum strands is transferred therethrough working progressively through all the strands towards the center core strand. The resulting rolling and sliding action of the adjacent strands on each other offers suiicient abrasive friction to break through the oxide nlms on all the strands. This breaking up of the aluminum oxide lms on the strands provides low resistance current paths, starting with Ithe center core strand and passing through the other layers of strands to pass the energy current freely to the connector.

It is a feature of this disclosure that the cuff 2H and the sleeve ld coact to provide a shroud which acts to defeat any tendency of the external atmosphere to reach the fresh contacting surfaces and this tendency to keep the joint liquid, gas and airtight is enhanced first by the closed end wall 23 at one end and also by the sealing effect of the cable insulation i9 as it is clamped by the cuil.' 24 at the opposite end of the joint. The insulation at the end of the cuff also acts to absorb vibration and iiexing and tends to relieve stiins imposed by the compressed portion of the ca e.

If the finished article is sawed lengthwise into separable sections and the` cable end forcefully torn away from the sawed sections, an imprint of cross hatch form will be found on the surface of the sections previously engaged by the wirev strands of the cable core. An attempt has been made to show this imprint in Fig. 3. The strands which had become embedded in the surface formed longitudinally extending, thin and closely positioned parallel grooves or thin lines 3|. The threads or serrations of the sleeve bore formed axially spaced apart transversely extending grooves 32 with the grooves at opposite ends more fully spaced apart than those at the mld-length. It appears from the imprint markings that the wire strands take on a wavy form under the compressive action herein featured and it was from this showing that the wavy configuration of Fig. 8 was taken.

An examination of the surface swedged onto the cable core gives no evidence that any of the 9 extremely ne threadlike strands I8 broke during the swedging operations despite the fact that the aluminum connector stretches for a material percentage of its length as above indicated. It must thus follow that the strands were not stretched beyond their elastic limits but are held distorted under tension.

As the connection maintains its initially good electric bond, it follows that having once provided its close metal-to-metal electric contact, the connection is maintained sealed and it is thus protected from any contact and thus any subsequent oxidizing of the contacting surface is avoided.

The cold working of the aluminum as an incident of securing it to the cable appears to strengthen the metal for it gradually hardens as it approaches its point of nal compression; with this increased strength there is maintained a good sound mechanical as well as an electric joint.

While the invention may be considered as complete as thus described7 it is suggested that a coating of petroleum base lubricant or zinc chromate paint or common red lead be used as a protective measure to seal oi any open pores or crevices in which moisture or oxygen may be pocketed at the time the connector is swedged onto the cable end. These protective coatings are of course applied to the cable end and to the receiving bore of the connector prior to the assembling and compression steps hereinabove described. However, tests have shown that the density of pressures on the cable strands resist penetration of elements causing oxide formation and no protective coatings are necessary. Its use is suggested simply as an added precaution.

By following the practice herein featured, it is possible to secure permanently an aluminum connector to an aluminum conductor and to avoid the ohmic resistance therebetween which heretofore has been present wherever attempts have been made to secure such conductors in electric contact.

I claim:

1. In a device of the class described, the combination o an aluminum sleeve, a fine stranded, aluminum cable core disposed therein. the bore of the sleeve having a mutilated thread with unequally spaced, rounded crowns and with troughs between the crowns, said crowns being in direct metallic-aluminum to metallic aluminum electric and bearing contact with the adjacent core strands, the portions of the adjacent core strands between the crowns bridging across the troughs and the troughs in the portion thereof so bridged containing aluminum oxide.

2. In a device of the class described, the combination of an aluminum sleeve, a line stranded aluminum cable core disposed therein, the bore of the sleeve having a mutilated thread with unequally spaced rounded crowns and with troughs between the crowns, said crowns being in direct metal to metal electric and bearing contact with the adjacent core strands, the portions of the adjacent core strands between the crowns bridging across the troughs.

3. The device defined in claim 2 and in which the rounded crowns are spaced apart progressively greater distances in one direction considered axially of the sleeve.

MARTIN D, BERGAN.

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

UNITED STATES PATENTS Number Name Date 1,711,832 Cooper May 7, 1929 2,008,227 Reilly July 16, 1935 2,151,032 Jensen Mar. 2l, 1939 2,171,280 Tondeur Aug. 29, 1939 2,226,849 Douglas Dec. 3l, 1940 2,251,176 Temple July 29, 1941 2,375,741 Dibner May 8, 1945 2,379,567 Buchanan July 3, 1945 2,405,111 Carlson Aug. 6, 1946

Images