US 3002173 A
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Description (OCR text may contain errors)
Sept. 26, 1961 R K ALLEN ELECTRICAL CONNECTION Filed June 25, 1957 [i7 V627 0l/- Hobert /C /l//en,
H/ls Attorney States My invention relates to a bus bar assembly and, more particularly, to a conductive metal joint in a bus bar assembly wherein molecular electrical contact is assured.
Within the portion of the electrical industry requiring high current capacity bus bars, a means of connecting bus bars with a reliable low resistance, high capacity, detachable joint has been an economic problem for many years. Except for the noble metals, all conductors have oxides on their surfaces. Because of its low cost and relatively good conductivity, copper is often used for high current capacity bus bars despite the occurrence of oxides and resistance films which inhibit current flow between the contacting surfaces of copper through a supposedly low resistance joint. This is particularly troublesome in corrosive or high temperature applications. With a contact joint of -two inch bus bars having a two inch overlap, it has been the practice of the manufacturing industry to prepare the surfaces to be connected by removing all film before clamping the surfaces together with a high tension bolt. This rather elaborate and expensive procedure results in an apparent contact area of approximately four inches square. However, the actual molecular Contact is substantially less than this. Moreover, at points of no contact, a tarnish will form, particularly at elevated temperatures. When Ithese tarnishes do form, they tend to counteract the bolt tension with a non-conductive layer and thus reduce the support at the conducting surfaces and inhibit current flow to cause an increase in losses and result in additional heating of the joint.
In order to understand completely the scope of my invention, it is necessary to understand the theory of electrical contact joints. In a joint contact, substantially all of the electric current passes from one conductor to the next at points of electrical contact between the conducting metals. Generally speaking, electrical contact is found only at points of metal contact which may be envisioned as direct contact between a conductive atom of one conductor with a conductive atom of another conductor. If one of the contacting atoms is non-conductive, no current will flow except for tunnel effect where an electron will ow through a very short space between the conductive metals when the space is no greater than a few angstroms.
In view of the fact that the competitive cost of making a conductor prevents manufacture of surfaces that are ilat enough and smooth enough to provide molecular contacts or tunnel effect conductance over the entire apparent contacting surface, it becomes obvious that with large contact areas, only high points or peaks (of a few microns in height) actually make electrical contact with an adjacent conductor. Since non-conductive films and oxides are often as thick as the high spots, or hard enough to deform the peaks without being ruptured sufficiently to allow either contact or tunnel effect conductance, it has been the practice to clean carefully the contact surfaces.
Even with carefully cleaned surfaces, the joint resistance is not predictable immediately because of the phenomena of constriction resistance occurring at each contact peak. Constriction resistance may be explained most easily as the effect of contact of the metal at the high points causing a concentration of all the contact current at these very small points. The constriction resistance is a maximum in small circular contact and is inversely proportional to the radius of the contact. However, if the contact point is elliptical, the constriction is reduced,
3,902,173 Patented Sept. 26, 1961 VPice The constriction resistance of a long contact area having a major axis 28 times as great as that of a circle of the same area s approximately 20 percent as great as that of the same area in a circular form. Understanding of the nature of contact resistance will facilitate comprehension of the following discussion.
lIt invariably happens that from time to time such conductive metal joints must be disassembled and reassembled in the field (to replace one of the bus bars or to repair insulation, etc.) where proper surface preparation is not practical. With the tarnish accumulated on the surfaces during use prior to the repair in the field, a large percentage of the stress of the clamping bolt is absorbed by what were formerly contact peaks engaging a nonconductive film or tarnish. Thus, resistance of the reassembled joint is increased and overheating damage is more likely to occur. This has proved particularly expensive within the locomotive equipment history, because a failure of electrical joints in service necessitates the removal of the locomotive to a repair area and often causes the delay of an entire train. One attempt to prevent such Vfailures has been to tinplate the mating surfaces of the joints to provide a soft surface which will deform to provide a greater bearing area and will rupture easily any surface oxides. However, this is very expensive in view of the fact that a very small percent of the joints are subject to such failure and, furthermore, it does not solve directly the problem but merely removes the likelihood of the formation of permanent non-conducting oxides and surface films.
It is therefore a primary object of this invention to provide an economical, simple and reliable electrical connection, suitable for connecting heavy duty bus bars.
Briefly, in carrying out my invention in one of its modiiications, a joint yfor a high current capacity bus bar is formed of two overlapping bus bars which are deformed to reduce the apparent contact area between them and thus increase the actual electrical contact. The contact area is particularly designed to provide a sliding engagement and a sufficient pressure between the contact surfaces under pressure to puncture any non-conducting surface lms. The deformation is arranged to provide a non-circular conducting area to minimize constriction resistance.
The subject matter which I regard as my invention is particularly pointed out and distinctly claimed inV the concluding portion of this specication. My invention, however, as to its organization and method of operation together with Ifurther objects and advantages thereof will bestbe understood by reference to the following description taken in connection with the accompanying drawing, in which:l
FIG. 1 is a top plan View of my invention showing a bus bar provided with male and female deformations;
FIG. I2. is a side plan view of the bus bar shown in FIG. 1;
FIG. 3 is an enlarged detail section view of an assembled joint;
FIG.V4 is a sectional view of a modification of my invention; and
PIG. 5 is a curve of the effect of the shape of a small contact area on constriction resistance.
Referring now -to the drawing, in which like numerals refer to similarparts, in FIG. 1, l have shown a bus bar 10 provided with apertures y11 and 12 suitable for accommodating clamping bolts. Coaxially surrounding the aperture 11, I have provided'an annular groove or recess 14. A coaxial annular ridge 15 surrounds rthe-apenture 12 and is adapted to t into the similar recess 14 of Van adjacent bus bar. The annular ridge 15 is semi-torcidal in shape and rises substantially above Athe flat surface of the bus bar 10.
In order that the annular -ridge 15 and the recess 14 may be coined by a simple stamping operation, as explained below, I prefer to have each end of the bus bar 10. provided with coaxial male and .female members on opposite surfaces as shown in FIG. 2. Thus, when the end of the bus bar 10 is completed, it is adapted to mate with either or both male and female PQrtionsof an adjacent bus bar. Y
Such a mating is illustrated inFIG. 3 wherein a nut 17 is tightened on a bolt 18 to compress the ridge 15 in the recess 14. It should he noted that the engagement of the fridge yin the recess is the only engagement between the overlapping portions of thev bus'bars'ltla and 10b and the height of the ridge 15 is great. enough to provide a space between the bus bars at VtheV overlapping portion of the joint. Thus, Ithe only surface stressedby the bolt 18 vis the outer curved surface y19 and/or the inner curved surface 20. In FIG. 3, theonly contact is 'at 19. Since each integral section of the surface 19 atan angle 6 instead of perpendicular to theV bolt, 18, the only engagement at surface 19 is by a wiping action which will'deform the conducting metals sufficiently to rupture normal voxides and lms. The angle 6, FIGS. 3 and 4, is shown as about 35 which will provideadequate wiping action. For the best wiping action, should be less than 10. However, allowance should be made `for production tolerances to insure engagement of the ridge at the outer sharp corner of the groove 14." Therefore, I prefer to have the angle 0 Ibetween 5 and 45 depending on the practical machining tolerancesy and the metal being machined, as well as the oxides of filmslikely to berencountered.
It should also be observed that a deeperl recess 21 is provided at the bottom of the recess 14. 'I'he recess 21 is machined or punched out to. a depth which will prevent any contact. A contact over the entire surface of the ridge 15, including the region of the vrecess 21, would provide a direct compression resistance to the bolt tension which would prevent suicient strain to cause desirable wiping action at the surfaceV 19 (or 20). Normally, I prefer to have contact at onlyfoneof the surfaces 19 or to insure sufficient deformation ofthefmetal with a reasonable bolt stress. i
By limiting the contact surface, I am able to have the bolt 13 of sufcient tensile `strength to compress the mating surface 19 or 20 and rupture anyoxide of other hlm surface thereon, and to fracture any peaks between the contacting surfaces. Thus, the apparent contact or mating area, according to ymy invention, vis actual electrical Contact area. 'l
In order to facilitate the development of theproper mechanical stresses at the surface `19,'1 prefer .to form a convex annular ridge 15 having a slightly., larger diameter than the concave annular .groove or recess 14. This results in a high pressure contact or contacts formed at the outside edge surface 19 of the `annular groove 14. The use of the outer ridge slightly reduces constriction resistance for a given compression Ibecauselthe perimeter of the outer edge is slightly greater than that of the inner surface 20. Since the area of contact may be assumed to be proportional to the compression of the bolt, the longer perimeter provides, a lowerresistance. The mismatch also allows the use of multiple assemblies without severe permanent deformation of vthe convex ridge 15.
With a 3A; copper bus, bar 10 two incheswide, a load of 2300 pounds in the bolt 18 will rupture oxides, but not cause permanent deformation ofthe surface 19 whereby the joint may be disassembled as required and reassembled repeatedly ,to provide a logw resistanceA connection. Since this vstress is sucicnttoIupture oxides, there need be no special cleaning of the jointeither at the time of original manufacture, orwhen reassembling the joint in the field.
Also, in FIG. 2, I have indicated a method of coining the ridge 15 and recess 14 wherein a punch 25 is forced into the bus bar `10 at a pressure depending on the thickness and composition thereof. For quarter inch coppei and a one inch diameter ridge, the punch pressure should be about 30 tons. A mating female mold (not shown) controls the contour and diameter of the annular ridge 15. A second punch 26 is then used to form the concave surfaces 19.and 20. The critical feature of the molds and punches is that lthe surfaces of contact should not provide a wide area but should instead approach a sharp line Contact as shown at the upper corner of the surface 19. Thus, the sloping edge of the Yridge 15 engages only the peripheral edge of the groove 14 with a wiping action.
Although this is one of the most economical methods of producing my invention on bus bars where mass production techniques may 4be used, my invention may be modified for use on single applications by machining the contours as shown in FIG. 4. In this modification, a female groove 30 is cut from a bus bar 31a coaxially with a clamping bolt aperture 32, and in a bus bar 3117 mating annularridge 34 Vremainsv coaxial with a bolt aperture 35 after cutting away the surrounding copper surface. In this modification, the `annularridge 34 has a triangular cross-section. As shown in FIG. 4, the annular ridge 34 in a bus bar 31h is inserted in the groove 30 of a bus bar 31a. I prefer to have the annular groove 30 designed so that the outer acute lcorners 36 of the groove 30 engage tightly the outer or the innerv surfaces of the ridge 34. Because of the acute angle 0, lthis modification insures a very good wiping action to destroy tarnish and reduce constriction resistance.
According to my invention, the criteria are to provide a small apparent varea of mechanical Contact which is the actual electrical contact area, to provide sufficient mechanical pressure to achieve a good wiping action and substantially complete rupture of high resistance oxides or films covering these areas, and to provide a long (perimeter) area to reduce constriction resistance.
With such a small contact area, Where the apparent area is of the orderof'one percent of the nominal crosssection area of the bus bar, there appears to be a problem of excessive heating with high current capacity bus bar joints. 0bviously, the current within this very small conducting area is much higher density than that normally allowed in conductors. However, I have found that current density as high as 57,000 amperes per square inch does not show appreciable saturation eiIect.
After considerable reilection on this phenomena, I have reached the conclusion that there is little relationship between the apparent area of contact and actual electrical contact in conventional bus bar joints, and that, because of current concentration at the contacting peaks, the current density in a bus bar joint point of contact is actually much greater than Lbefore calculated. This is actually of little consequence, even though it can be assumed that the temperature rise of very small contact points would cause high resistance at that point, because the heat generated is small compared to the amount of thermally conductive metal surrounding it. Thus, although the temperature of the area of contact is high, and its resistance is thereby increased, the depth of the high temperature is so slight that it does not materially aiect the overall resistance of the joint. My invention takes advantage of this phenomena by the use of a relatively small contact area located between relatively large thermally conductive metal masses.
Moreover, since the contact pressure and wiping action of the mating surface 19 or 20 is enough to destroy all films and fracture many of Vthe peaks, I provide an increased metal to metal contact by reducing the apparent mechanical contact, and by eliminating non-conducting support between the joints. In other words, with my invention there is no support between a high point of the .second order and an oxide layer of the adjacent conductor because the stress concentration may be made sucient to rupture all films.
Even taking into account the above factors, the theoretical resistance of the joint of my invention was several times the empirical value until I factored in the constriction resistance advantage obtained by having a line contact for a given area instead of a circular contact area. This reduced the theoretical value by 80 percent to approximate my emperical data. The fact that the line I obtain is curved to form a perimeter of a circle does not materially affect the improvement in constriction resistance.
In order to explain the relation of constriction resistance to the shape of an elliptical contact area, I have shown in FIG. 5 a curve with the constriction resistance plotted on the axis of the ordinates, and the ratio of the major diameter of an elliptical area to the radius of a circle of the same contact area plotted on the axis of the abscissa. The actual maximum value of the constriction resistance depends on the particular metals used and several other factors which have been known for some time. However, with a particular area such as one mil, the maximum constriction resistance occurs when this area is a circle. This point is plotted at the extreme left end of the curve shown. With the area of one mil and similar conditions (same metals, etc.) in the form of an ellipse having a major axis five times the length of the radius of a circle of the same area, this resistance is reduced to 60 percent of the maximum value of the circle. As the ellipse is flattened to have the ratio of :1, the constriction resistance is reduced to 40 percent and at 20:1 to 25 percent. The minimum constriction resistance factor of zero theoretically occurs at infinity when the contact is a straight line.
While I have illustrated and described particular embodiments of my invention, other modifications will occur to those skilled in the art. For instance, the actual contact area may be more than one percent of the overlapping area of the bus bars so long as a controllable pressure may be applied, and my invention can be used with other conductors than copper, with modilications depending on the hardness of the metal and hardness of the films likely to be found thereon. With aluminum, the oxide is particularly troublesome, and I have found that there must be a wiping action between the mating surfaces of more than .015 to obtain the most efcient joint. I intend, therefore, to cover in the appended claims all such modifications as fall within the true spirit and scope of my invention.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. An electrical joint comprising a pair of conductors adapted to be clamped together in an overlapping relationship, the rst of said conductors being providedV with ridge means on a surface thereof, said ridge means presenting at least a pair of oppositely disposed surface portions such that planes tangent to such surface portions respectively will intersect the axis of assembly of the joint at an acute angle, the second of said pair of conductive members being provided with oppositely disposed edge portions located to engage said surface portions of said ridge means, and clamping means for urging and holding said edge and surface portions together under suicient pressure to cause said edge portions to engage said surface portions to produce a wiping action to remove film from the contacting area of said surface and edge portions and deform such contacting area to increase the effective electrically conductive contact area.
2. A bus bar assembly comprising a iirst at conductor having a conductor surface provided with an aperture therethrough for accommodating a clamping bolt and an annular groove coaxial with the aperture intersecting said conductive surface substantially in a sharp edge, a second flat conductor having a conductive surface provided with a similar aperture and an annular ridge coaxially surrounding said similar aperture, said annular ridge presenting surface portions disposed at an acute angle with respect to the axis of said aperture and being disposed at a distance from the axis of said similar aperture equal to the distance of said sharp edge from the axis of the aperture of said first conductor, a bolt adapted to be positioned in said aperture to urge said edge and surface positions into engagement whereby relative movement of the conductors along the axis of the aperture produces a wiping action to remove resistive films from the contacting areas of the edge and ridge portions, and the clamping pressure deforms such edge and ridge portions to increase the percentage of the contact area which is actually electrically conductive.
3. A bus bar assembly as recited in claim 2 wherein the acute angle between the surface portions of said annular ridge and the axis of the aperture is less than 45.
4. A bus bar assembly as recited in claim 3 wherein the length of the actual contact area of the ridgeV and edge portion is at least tentimes its width.
5. A bus bar assembly as recited in claim 2 wherein the acute angle between the surface portions of the ridge and the axis of the aperture is between 10 and 40.
6. An electrical joint comprising a pair of conductors provided with contact surfaces, the contact surface of the rst of said conductors being provided with a ridge thereon, the contact surface of the second of said conductors being providedk with means presenting a relatively sharp edge for engaging a portion of the surface of said ridge and pressure means for urging and holding said contact surfaces together under suicient pressure to cause said edge of said second conductor to produce a wiping action to remove ilm from the contacting area of said surface portion of said ridge and deform such contacting area to increase the eiective electrically conductive area.
References Cited in the le of this patent UNITED STATES PATENTS 1,176,942 Bliss Mar. 28, 1916 1,863,429 Willmore June 14, 1932 2,250,280 Starbird July 22, 1941 2,480,280 Bergan Aug. 30, 1949 2,615,951 Klostermann Oct. 28, 1952 2,820,084 Shaw Jan. 14, 1958 UNITED STATES PATENT FFCE CERTIFICATE OF CORRECTIGN Patent No. 8,002,173 l September 26, 1961 Robert Kilgannon Allen It is hereby certified that error appears in the above numbered patent requiring correction and that the said Lettersl Patent should read as corrected below. A
Column line 10, for "conductor" read conductive line 50, after "conductive" insert Contact Signed and sealed this 20th day of February 1962.
ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents