US2800638A

US2800638A - Electric connector

Info

Publication number: US2800638A
Application number: US361205A
Authority: US
Inventors: Kemper M Hammell
Original assignee: AMP Inc
Current assignee: TE Connectivity Corp
Priority date: 1953-06-12
Filing date: 1953-06-12
Publication date: 1957-07-23
Anticipated expiration: 1974-07-23
Also published as: DE1763068U; DE1057193B; GB754493A; BE529586A; CH335319A; FR1105730A

Description

y 1957 K. M. HAMMELL 2,800,638

ELECTRIC CONNECTOR Filed June 12, 1955 lZ 6a 3 Kemper M flammell United States Patent ELECTRIC CONNECTOR Kemper M. Hammell, Harrisburg, Pa., assignor to AMP Incorporated, a corporation of New Jersey Application June 12, 1953, Serial No. 361,205

Claims. (Cl. 339-276) This invention relates to electrical connectors of the type which are adapted to be crimped onto one or more conductors, and more particularly to such an electrical connector which is especially suitable for crimping onto insulated, coated or dirty conductors.

' Many types of wires which are commonly used for making electrical connections have a layer of tough, elastic, insulating coating such as Formvar or Formex (polyv nyl formal or polyvinyl acetal). In addition, bare electrical wires often become dirty, or develop corrosion layers, such as oxides or sulphides, which effectively increase the contact resistance or even insulate the conductor. When it is desired to make connection to conductors having such external layers or films it is first necessary to pierce or scrape away the insulating coating in some manner before an electrical contact of low ohmic resistancecan be achieved with the current-carrying portion of the wire.

A common procedure has been individually strip or abra'sively clean all such dirty or coated wires before attempting" to make connection thereto, as by the use of wire cutters or similar implements. For certain types of wirecoatings, this approach is feasible, although time-v consuming andexpensive. For other types of wire coatings, such as certain hard and resilient insulation films, this method has proven to be impractical because of the mechanical difficulties of completely stripping off the coating without damaging the conductor; this is particularly true when relatively fine wire, such as the type generally known as magnet wire, is involved.

In the past, attempts have been made to provide solderl'e ss connectors having relatively sharp teeth or ridges which are designed to be forced through a covering of insulation by the application of compressive force. However, tough coatings such as Formvar are nearly impenetrable to connectors of this type.

' Accordingly, it is an object of this invention to provide an electrical connector of the type which is adapted to-be crimped or pressure-forged onto one or more wires, which is capable of partially shearing the wires during the crimping operation, and of jamming the sheared surfacesiri a sticking taper so as to create a permanent and stableelectrical contact with the conducting portion of the wires. Other objects and advantages will be partly obviousfrom and partly pointed out in, the following description of one embodiment of the invention considered together with the accompanying drawings in which: Figure 1 is a cutaway plan view of a connector embodying the present invention and crimped onto a stranded conductor; Figure 2 is an enlarged fragmentary cross-section view of a grooved inner surface of the connector of Figure 1; Figure 3 is a plan of the connector in flat condition before rolling up to ferrule form; and

Figure 4 is an enlarged cross-section view of a modified groove'shown with its coining die. 7 In Figure 1, a terminal connector generally indicated at 2 and having a ferrule portion 4 is shown crimped 2,800,638 Patented July 23, 1957 onto a stranded wire 6 of an insulated cable 8. The ring tongue 10 (shown in Figure 3) of the connector 2 is representative of the various contact portions adapted for attachment to another conductor, a binding post or other terminal device (not shown). Each of the wires 6 may be coated with oxide, sulphide, oil or wax or insulating material from the insulation coating 9.

The cutaway view in Figure 1 shows a number of grooves 12, the edges of which form an abrupt angle adapted to shear through layers on the peripheral strands of the wires 6 and into the metal of the strands to make contact with the current-carrying portions of these strands during a crimping operation wherein the ferrule portion 4 is forced inwardly against the conductor under sufiiciently heavy pressure to deform the metals. The sides of the grooves are sloped with an angle of a sticking taper for the metals concerned, so that when, during such a crimping operation the portions of the wire 6 adjacent the grooves 12 are sheared and driven into the grooves, as indicated in the drawings at 6A, the sheared surfaces are jammed in the taper with such pressure that the surfaces are tightly. held and are tight against corrosion. The crimping pressure will advantageously force the wire to bottom against the deepest surfaces of the grooves and coin itself into every crevice thereof. Because of this bottoming effect, the shearing action is prevented from seriously weakening the wire after piercing the layer of insulation; any additional crimping after the wires reach bottom results only in coining of the conductor material.

It has been found that the dimensions of the grooves may vary Within substantial ranges while still allowing the grooves 12 to be effective in shearing into and permanently gripping the wire 6. Referring now to Figure 2, it has been found, for example, that the width 11 of the groove 12 between the shear edges is particularly important in achieving good shearing action. The depth 13 of the grooves 12 and the breadth of the lands 15 between the grooves are also important, as described hereinafter.

The effectiveness of the connection is a function both of the crimping pressure and the width of the groove.

It is advantageous to provide a groove width which is great enough to afford a sufiicient shearing action at each edge with respect to the insulating layer or coating, and yet sufficiently small that a reasonably large number of grooves may be formed across the inner face of the conductor ferrule, since the effectiveness of such a serrated connector is enhanced by providing additional grooves each adapted to bite into and make effective mechanical and electrical connection to the conductor Wires.

For the commonly used electrical wires, the gap size between the shearing edges of the grooves will ordinarily fall within the range of approximately 0.010 to 0.020 inch. The width dimension of the grooves, for superior results, should approximate the strand diameter of the conductor to about 0.02 inch. With solid wires the width of the grooves may be relatively less than for stranded wire, but in general it is the strand diameter rather than overall wire gauge which controls the dimensions of the groove. Such a dimensional range, additionally, will allow a relatively large number of grooves to be cut into the inner face of the usual types of solderless connector ferrules, which is an advantage not only because of the multiplication of area of fresh metal contact, but because it assures contact with more strands in a multistrand wire, as the spiral lay of a wire brings different strands into position to engage the shear edges in different areas.

There is, however, a limit to the number of grooves which can profitably be crowded into a ferrule, as a substantial land is required between the shear edges of adjacent grooves in order to assure the desired high strength connection of low and stable resistance. Thus with grooves .01-.02 inch in width, their spacing center to center should be at least .025-.030 inch, and in general the width of the supporting lands should not be less than the width of the grooves.

Because of the inward slope of the side walls of the groove 12, the conductor material which is driven into the grooves by the crimping pressure takes on a compressive stress, and thus by its own elasticity maintains a tightly pressed contact against the side walls of the groove. That is, the conductor material flowing into the groove encounters .a continually-decreasing cross sectional area, so that this material is compressed as it is forced into a region of diminishing volume. As is apparent, the amount of compressive stress so built up, and more especially the tendency of the sheared portion ofthe wire to hold tightly to the sides of the groove, is determined partly by the slope given to the groove walls. To minimize the possibility of corrosion it is advantageous to provide for maximum compressive stress on the conductor portion between the sloping sides of the grooves, while keeping the angle small enough to assure a selfholding taper engagement.

A range of optimum taper angles according to this invention, which satisfies these requirements is from to 60. This angle should be smaller for tougher layers surrounding the conductor, and may be larger with soft or frangible coats. For example, when making shear contact through a layer of Formvar, which is extremely tough and resilient, it has been found that a very good taper angle is about 5". Alternatively, when it is desired to shear through a layer of copper sulphide formed on the outer surface of a conductor, a taper angle as large as 50 has been found satisfactory.

The depth of the groove (indicated in the drawing by the numeral 13) should be suflicient to assure that any insulating layer will be sheared through before the sheared portions of the wires 6 bottom in and fill the grooves. This filling, which stops the shearing movement, should occur shortly after completion of shearing through of the insulation layer or coating, and before any of the wires 6 have been sheared to the extent of probable fracture.

In general ,the optimum depth is roughly one half the diameter of the individual strands of the conductors. Thus, with a #14 AN Wire, which has 19 strands each of about .0145 inch diameter, a groove depth of about 0.006 inch has been found to be quite advantageous, whereas in a terminal designed to be crimped onto bared #l2 A. W. G. Wires having strand diameters ranging from 0.0159 to 0.0185 inch, a groove depth of about 0.007 to 0.008 inch has been found advantageous in providing the desired connector characteristics outlined hereinabove. Terminals having these groove depths are not limited to use only with the particular strand diameters indicated, such terminals being adaptable generally for use with a broader range of conductor sizes while still providing superior results to those accomplished without this invention. The range of optimum depth dimensions will, in general, lie between approximately 0.005 and 0.010 inch, with greater depths for enameled wire, and especially poly vinyl methylal enamels.

Within the range of the groove dimensions and slope limitations indicated herein, the grooves will shear cleanly through tough layers of insulation when the connector is crimped. Furthermore, such grooves will produce a stable electrical connection that is permanently maintained by the endwise compression of the sheared portion of the wire, and will create a solid contact without danger of cutting through the conductors. For the predominantly common applications involving relatively smallsized bare wires one may advantageously use a groove width of about 0.010 inch, a depth of about 0.005 inch, and a taper angle of about 50.

Figure 3 shows a solderless connector of the type described hereinabove and in which the ferrule-forming portion 4 is in a flattened condition before crimping onto a conductor. Cut transversely across the conductor-contacting surface of the ferrule 4 are a plurality of substantially parallel grooves 12. The

ear portions

3 and 5 of the ferrule-forming portion 4 are adapted to be folded upwardly (as viewed in the drawing) and wrapped around one or more conductors in telescoping relation thereto, and to be pressed inwardly against such conductors under heavy pressure for forming an intimate, cold-forged connection therewith. This folding and crimping operation may be performed by a variety of tools especially adapted for the purpose, such as are well known in the art of solderless connectors.

To assure good shearing action, it is necessary that the edges of the groove come as near as possible to the very vertex of the dihedral angle. In constructing a connector the grooves may be formed by machining or stamping, but the tools, whether cutting tools or coining dies, should be as sharp as possible to assure efiicient shear edges. In Figure 4, I have shown, somewhat exaggerated and enlarged, a relieved coining die to improve the sharpness of the corner which forms the shear edge. I In this draw ing, the coining die 20 is relieved along the base at 22,

so that when the die is pressed against the fiat ferrule blank 24, the edges 26 of the groove 30 will be initially formed somewhat sharper than the required shear so as to provide a margin of safety.

Although my invention is not limited to any particular metal or combination of metals, it is important for best results that the metal in the shearing edges should be harder than the metal of the wire which is to be engaged. Dead soft copper is therefore too soft for most purposes. I have found that a quarter hard copper or brass (60-70 on Rockwell l5T scale) works well on copper wire of 50 or less Rockwell 1ST hardness. If the grooves are coined into the metal the shearing edges may be worked more severely, and therefore be harder, than other parts of the ferrule. Especially if the shearing edges are first made sharper than the final shear angle, e. g. as shown in Figure 4, and then flattened to give the precise shear angle, the extra working of the metal adjacent the shear edge has a desirable effect.

When a connector constructed in accordance with the present invention is crimped onto a stranded conductor, it is found that the inner strands tend to form outwardly in the region of each groove. The serration pattern is therefore actually apparent through the wire to the center of the stranding. This effect is due to the extrusion of copper in the wire due to pressure exerted by the lands during the crimping, and the temporary absence of such pressure in the areas of the grooves. As a consequence, the outward pressure of the central strands tends permanently to force the outer strands into the serration grooves thereby maintaining a keyed mechanical connection as well as an efficient electrical connection between the conductor and the walls of the groove.

Experience has shown that an extremely good corrosion resistant connection of the type described is formed without necessity of producing as great a reduction in wire cross section as has been required in the past for high quality connections. An important by-product, electrically, of the strongly keyed mechanical connec tion resulting from the present invention is that the wires are prevented from sliding in the crimped connection when under severe stress, as when they are bent sharply near the ferrule. This feature gives better corrosion resistance and much more stable conductivity in the connection.

Comparative tensile tests of standard commercial ferrule-type solderless connectors having the usual V type serrations and connectors differing only in that the grooves were made in accordance with the present invention showed an improvement of approximately 45% in pullout strength.

Connectors embodying the present invention are advantageous not only for use with stranded conductors but also with solid conductors, even those of relatively large size. For example, two or three solid conductors of 0.060 inch diameter each coated with Formvar may be crimped solidly together in the ferrule of a connector of the type hereinabove described, the groove width between the shearing edges being about 0.012 inch wide, the groove depth being about 0.008 inch, and the taper angle of the groove being about 5. With such an arrangement, the groove edges shear cleanly through the Formvar and produce a stable, non-corrosive electrical connection of low ohmic resistance between the conductors. Such a result had not been regarded as possible before the present invention.

Experience has shown that when relatively deep grooves are cut into the ferrule near its mouth the wire 6 leading out of the terminal will be weakened against fatigue; this may result in damage to the conductor strands, as by breaking an individual strand in the sheared region.

Electrical connectors in accordance with the present invention may advantageously be formed with serrations other than the transverse grooves shown in Figure 3. For example, good results may be obtained by serrating the inner face of a ferrule with a number of indentations, the surfaces of each indentation forming a truncated cone or pyramid, the dimensions of which (i. e., width, depth, and slope angle) conform to the pattern set forth hereinabove. The bases of such cones will normally lie in the plane of the inner face of the ferrule, so that the cross-sectional view of each indentation will be similar to that shown in Figure 2; the outer shearing edges of such a frusto-conical indentation may, of course, be raised slightly, analogous to the showing in Figure 4, and for the purposes described with reference to that figure.

When it is desired to crimp onto a stranded wire wherein the conductors are arranged in a substantial spiral with respect to the wire axis, it may be advantageous to cut the grooves of Figure 3 at such an angle with respect to the axis of the ferrule that they make perpendicular contact with the conductor strands. That is, each of the grooves 12 may be arranged so that it intersects the spiral lay of the wire at right angles, or nearly so, to provide a superior grip on the strands in a direction longitudinal of each strand, and thereby improve the pullout strength of the connector.

Alternatively, the grooves may be cut in a direction substantially parallel with the axis of the ferrule, and with dimensions varied gradually from a minimum nearer the mouth of the connector to a maximum, more remote so that the wire will be compressed into a sticking taper in a direction radial of the connector ferrule, and will be taper-wedged by any pull on the wire in a direction longitudinal of the ferrule.

Although certain specific embodiments of this invention and alternatives have been described with particularity, it is desired to emphasize that these are neither limiting nor exhaustive of the invention. They have been included to assist those skilled in the art in applying the principles of this invention, recognizing that the various features of the invention may be modified or adapted to suit the requirements of particular applications.

1 claim:

1. An electrical connector adapted for crimping onto one or more conductors, comprising a ferrule-forming portion having its inner surface formed with a plurality of grooves, the sides of each of said grooves forming surfaces each of which makes a sharp-edged intersection with said inner surface at an included angle more than 90 degrees and less than 120 degrees, said sides as viewed in a longitudinal section through said ferrule-forming portion extending along a straight line to the bottom of the respective groove, the bottom of each of said grooves forming a surface having a substantial width relative to the depth of the groove.

2. An electrical connector adapted for crimping onto one or more conductors, comprising a ferrule-forming portion having its inner surface formed with a plurality of grooves, the sides of each of said grooves forming surfaces each of which makes a sharp-edged intersection with said inner surface at an included angle more than degrees and less than degrees, the bottom of each of said grooves forming a surface that is flat when viewed in a longitudinal section through said ferrule-forming portion, said bottom further having a substantial width relative to the depth of the groove.

3. An electrical connector adapted for crimping onto one or more conductors, comprising a ferrule-forming portion having its inner surface formed with a plurality of grooves, the sides of each of said grooves forming surfaces each of which makes a sharp-edged intersection with said inner surface at an included angle more than 90 degrees and less than 120 degrees, the bottom of each of said grooves forming a well-defined distinct surface parallel to said inner surface of said ferrule-forming portion and having a substantial width relative to the depth of the groove.

4. An electrical connector adapted for crimping into one or more conductors, comprising a ferrule-forming portion having its inner surface formed with a plurality of grooves, the sides of each of said grooves forming surfaces each of which makes a sharp-edged intersect-ion with said inner surface at an included angle more than 90 degrees and less than 120 degrees, said sides as viewed in a longitudinal section through said ferrule-forming portion extending along a straight line to the bottom of the respective groove, the bottom of each of said grooves comprising a surface forming a straight line when viewed in said longitudinal section, said bottom having a substantial Width relative to the depth of the groove.

5. An electrical connect-or adapted for crimping onto one or more conductors, comprising a ferrule-forming portion having its inner surface formed with a plurality of grooves, the sides of each of said grooves forming surfaces each of which makes with said inner surface an included angle more than 90 degrees and less than 120 degrees, the regions of intersection between said side surfaces and said inner surface comprising groove edges which are slightly elevated with respect to said inner surface to form pairs of sharp transverse ridges for shearing insulating coatings on a conductor onto which said connector is crimped, the bottom of each of said grooves forming a surface having a substantial Width relative to the depth of the groove.

6. An electrical connector adapted for crimping onto one or more conductors, comprising a ferrule-forming portion having its inner surface formed with a plurality of grooves, the sides of each of said grooves forming surfaces each of which makes with said inner surface an included angle more than 90 degrees and less than 120 degrees, said sides as viewed in a longitudinal section through said ferrule-forming portion extending along a straight line to the bottom of the respective groove, the regions of intersection between said side surfaces and said inner surface comprising sharp groove edges for shearing insulating coatings on a conductor onto which said connector is crimped, the bottom of each of said grooves forming a surf-ace having a substantial width relative to the depth of the groove, the depth of each of said grooves being approximately one-half the spacing between the edges of the corresponding groove.

7. An electrical connector as set forth in claim 1 wherein said ferrule-forming portion comprises a flat metal blank rolled into tubular shape, said grooves being arranged in side-by-side parallel fashion with each groove extending in the form of a closed ring around the interior of said ferrule-forming portion.

8. An electrical connector as set forth in claim 7 wherein the planes of the rings formed by said grooves are per pendicular to the longitudinal axis of said tubular ferruleforrning portion.

9. An electrical connector as set forth in claim 1 crimped onto a conductor positioned Within the interior of said ferrule-forming portion With said grooves completely filled by portions of said conductor forced into said grooves, the sharp edges of said intersections being sheared into the outer surfaces of said conductor portions to form freshly-exposed conductor surfaces in said grooves and the sloping sides of the grooves pre-ssure-wedging said sheared surfaces throughout a substantial area there-by establishing low-resistance and corrosion-resistant con-tact to said conductor.

10. An electrical connector as set forth in claim 9 crimped onto a conductor having a coating of relatively non-conductive material, the sharp edges of said intersections being sheared into said coating to form freshlyexposed conduct-or surfaces in said grooves and the sloping sides of the grooves pressure-wedging said sheared surfaces throughout a substantial area thereby establishing low-resistance and corrosion-resistant electrical contact to said conductor.

References Cited in the file of this patent UNITED STATES PATENTS;

1,169,642 Heeter J an. 25, 1916 2,259,261 Miller Oct. 14, 1941 2,327,650 Klein Aug. 24, 1943 2,452,932 Johnson Nov. 2, 1948 2,604,508 Bergan July 22, 1952 2,674,725 Buchanan April 6, 1954 2,685,076 Hoffman July 27, 1954