Dual durometer twist-on connector

Abstract: A twist-on or wire-nut electrical connector (10) having a rigid, electrically insulative upper body (12), a flexible, elastic, electrically insulative lower skirt (14), and a coil spring (22) within the body for gripping wires (30) which may be inserted therein. The provision of a flexible, elastic skirt (14) allows the insertion of a larger number of wires (or larger sized wires) into the connector; the skirt (14) further deforms to fit more easily within a crowded junction box or other high-density wiring environment. Unlike prior art twist-on connectors, the lower skirt (14) is attached directly to the open end of the polymeric body (12), allowing greater application of torque to the rigid body. In the preferred embodiment, the upper body (12) is formed of polypropylene, the lower skirt (14) is formed of a styrene-butylene compound or an olefinic thermoplastic vulcanizate, and the connector is constructed by multicomponent injection molding.

Description: Background of the Invention 1. Field of the Invention The present invention generally relates to electrical connectors, and more particularly to an improved type of connector known as a twist-on or wire-nut connector. 2. Description of the Prior Art Twist-on connectors (commonly referred to as spring connectors) are well-known in the art. One of the earliest patents disclosing this type of connector is U.S. Patent No. 2,656,204 issued to J. Blomstrand on October 20, 1953. The Blomstrand connector basically comprises a helically coiled spring, into which twisted wire ends are inserted. As the wire ends are inserted, the coil spring expands slightly and resiliently grips the wire ends. Improved versions of the twist-on connector typically include a hard, electrically insulative shell surrounding the coil spring (such as that shown in U.S. Patent No. 3,075,038 issued to W. Schinske on January 22, 1963), and the shell often includes integral wing portions (also referred to as ears, tabs, or fins) which allow the user to exert a greater torque while twisting the connector over the wire ends. Minnesota Mining and Manufacturing Company (3M), assignee of the present invention, markets a line of twist-on connectors under the trademarks Scotchlok, Hyflex and Ranger. One disadvantage inherent in nearly all of the prior art twist-on connectors is the limited range of wire diameters (or absolute number of wires) which the connector can accommodate. As recognized in Underwriters Laboratories' standard 486C for twist-on connectors, this limitation is primarily related to the thickness of the wire insulation. Although a portion of this insulation is removed to allow the wire ends to be twisted together, the insulation must still enter at least slightly into the connector for safety reasons; in other words, no portion of the bare wires should be visible or accessible once the connection is made, to prevent the possibility of a short circuit or other electrical hazard. Figure 7 of the Schinske patent suitably illustrates this problem. In many cases, there is sufficient room within the main body of the connector shell (i.e., within the coil spring) to receive multiple wires, but this room often cannot be fully utilized due to the crowding of the wire insulation at the opening of the rigid skirt of the connector. This construction necessarily results in the wasteful use of additional connectors (sometimes requiring "daisy" chains), and in wasted time on the part of the craftsperson making the electrical connections. One prior art connector which addresses this limitation is shown in U.S. Patent No. 2,890,266 issued to E. Bollmeier on June 9, 1959. That connector utilizes a metal shell surrounding the coil spring, and an elastic sleeve which surrounds the shell and forms a skirt at the opening of the connector. Bollmeier, however, presents additional problems not present in other prior art connectors. Specifically, it is difficult to exert any torque on the inner metal shell since the sleeve tends to slide around the shell as the connector is twisted over the wires. This drawback may, in some instances, be critical since it affects the integrity of the electrical connection and the pullout force required to remove the wires from the connector. It is also unproductive to incorporate the wings or tabs of other twist-on connectors into the Bollmeier device since the wings would be integrally formed with the sleeve, which would just exacerbate the tendency of the sleeve to slide around the inner shell. As a further result of the foregoing drawbacks, a use: of the connector is required to exert a greater gripping force on the connector, which can deleteriously result in deformation of the inner metal shell and the coil spring. Thus, use of this type of twist-on connector is limited to small wire sizes which require relatively low twisting force to secure the wire bundles. It would, therefore, be desirable and advantageous to devise a twist-on connector having a flexible skirt or opening similar to Bollmeier, but which additionally provides enhanced gripping action and greater torque application, to insure a secure wire connection, and which further accommodates a wider range of wire gauges. GB-A-164334 discloses a connector having a body with a closed end, an open end and an internal bore, and located within the bore is a helically wound coil spring. This reference forms the preamble of the subject claim 1. EP-A-0432662 discloses an electrical connector having a body and a skirt but in this reference the skirt is not flexible or elastic. This reference was cited under EPC Article 54(3) and (4). Summary of the Invention According to this invention there is provided an electrical connector as claimed in claim 1 herein. The present connector may be used to connect a wider range of wire gauges (or to connect a larger number of wires) since the skirt can expand to accommodate the bulky insulation surrounding the wires. Unlike the prior art connectors, however, the use of a flexible skirt does not interfere with manual application of the connector and, furthermore, the use of a rigid, insulative upper body allows direct attachment of the torque-application wings. The invention also contemplates various processes used to bond the skirt to the rigid body. Brief Description of the Drawings The novel features and scope of the invention are set forth in the appended claims. The invention itself, however, will best be understood by reference to the accompanying drawings, wherein: Figure 1 is a side elevational view of the dual durometer twist-on connector of the present invention; Figure 2 is a top plan view of the dual durometer twist-on connector of the present invention; Figure 3 is an elevational cross-section taken along lines 3-3 of Figure 2; and Figure 4 is a cross-section similar to Figure 3, but further depicting a plurality of wires inserted into the connector. Description of the Preferred Embodiment With reference now to the figures, and in particular with reference to Figures 1 and 2, there is depicted the dual durometer twist-on connector 10 of the present invention. Connector 10 is generally comprised of an upper shell or body 12 and a lower skirt 14 . Upper body 12 is constructed of any rigid, electrically insulative material, preferably a durable polymer such as polypropylene, polyethylene, or hard vinyl or polyvinyl chloride. A polycarbonite/polybutylene terephthalate alloy may also be used, such as that sold by General Electric Plastics under the brand name XENOY. Body 12 should have a Rockwell hardness of at least R50, allowing high torque application. Lower skirt 14 is constructed of any flexible, electrically insulative material, preferably an elastic polymer such as styrene-butadiene-styrene (SBS), styreneethylene-butylene, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, ethylene-propylene diene terpolmer (EPDM), polychloroprene, copolyester elastomers, modified plastisols, or plasticized vinyl. The preferred materials for skirt 14 are SANTOPRENE (a trademark of Advanced Elastomer Systems of Akron, Ohio), which is an olefinic thermoplastic vulcanizate, and ELEXAR (a trademark of Shell Chemical Co. of Houston, Texas), which is a styrene-ethylene/butylene-styrene compound. This list is not meant to be exhaustive, and other elastomers, as well as natural and synthetic (e.g., urethane or silicone) rubbers, may be used for skirt 14 . As explained further below, skirt 14 is advantageously constructed of a material which bonds well with the material selected for body 12 . The softness of the material selected for skirt 14 depends upon the thickness of skirt 14 (discussed further below), but the acceptable durometer range is about Shore A 18 to Shore D 75. Also visible in Figures 1 and 2 are wings 16 (sometimes referred to as ears, tabs, fins, or extensions) which, as explained further below, are preferably formed integrally with upper body 12 and skirt 14 . Wings 16 extend away from body 12 in a slightly skewed fashion to facilitate clockwise rotation of connector 10 (as viewed in Figure 2) which forces the wires to become further twisted together. Wings 16 may be modified, e.g., by making them retractable as shown in U.S. Patent No. 3,308,229 (Burniston), or by making them frangible or breakaway as shown in copending U.S. Patent Application Serial No. 07/561,699 (filed August 2, 1990). The outer surface of body 12 (as well as the work surface of wings 16 ) is also provided with a plurality of longitudinal grooves 18 which allow stronger gripping for better application of torque. Referring now to Figure 3, connector 10 is depicted in cross-section along lines 3-3 of Figure 2. Figure 3 shows more clearly the two part construction of connector 10 (i.e., upper body 12 and lower skirt 14 ), and the generally tubular construction of both body 12 and skirt 14 . In this regard, it is understood that the term "tubular" is not limited to objects having a circular cross-section, but rather denotes a hollow member of any cross-section. Body 12 has a closed end and an open end, the open end being bonded to or integrally formed with an open end of skirt 14 at a seam or interface 20 (explained further below). Figure 3 further illustrates a coil spring 22 located within an internal bore 24 of body 12 . The inner surface of internal bore 24 preferably has a frustoconical shape, and coil spring 22 is accordingly wound in increasingly smaller diameters to fit snugly within bore 24 . Longitudinal vanes or ribs 26 may be attached to the inner surface of bore 24 to provide some tolerance for minor expansions or deformations of spring 22 and yet still keep spring 22 centered within bore 24 . In the preferred embodiment, there are four such ribs 26 which are molded integrally with body 12 , two of these ribs being shown in cross-section in Figure 3. Coil spring 22 is preferably formed of cold-rolled steel, and has a square cross-section. In this manner, the inwardly extending edge or corner of the spring contacts and work-hardens the wires which are inserted into connector 10 . Coil spring 22 is retained within internal bore 24 of body 12 by an annular flange or rim 28 formed integrally with body 12 . Coil spring 22 may be modified as desired, e.g., by providing dilatable convolutions as taught by Burniston, or by providing an hourglass-shaped coil spring as taught in U.S. Patent No. 3,676,574 (Johansson et al.). Coil spring 22 may also be replaced by other resilient means for gripping the inserted wires, such as a threaded metal retainer as shown in U.S. Patent No. 4,150,251 (Scott). Those skilled in the art will appreciate that the attachment between body 12 and skirt 14 must be sufficiently strong to withstand the stress and shearing forces which are transferred to skirt 14 across interface 20 as body 12 is twisted around the inserted wires. Proper attachment of skirt 14 to body 12 depends on several factors, including the method of assembly, material selection, and the mechanical fit at the interface. The preferred method of joining skirt 14 to body 12 is multicomponent injection molding, also referred to as dual injection molding or two-color molding (not to be confused with co-injection). Of course, other methods are acceptable, including insert molding, ultrasonic welding, solvent welding, or the simple application of an adhesive at interface 20 . Multicomponent injection molding is preferred inasmuch as it requires minimal handling of components. Multicomponent injection molding is also advantageous since there are several materials which can be used in that process and which are suited for use in the dual durometer connector of the present invention. Specifically, the preferred material for body 12 , polypropylene, is easy to form via multicomponent injection molding. The preferred material for skirt 14 is accordingly chosen for its ability to adhere to polypropylene and to be injection molded. Experimentation has revealed that butylene and butadiene compounds bond well to polypropylene and may be used with multicomponent injection molding (see above for the recitation of specific materials for skirt 14 ). It does not matter whether body 12 or skirt 14 is formed in the first mold operation; however, the first component of connector 10 which is molded should preferably still be warm when the second component is molded against it. This will result in an improved bond at interface 20 . In multicomponent injection molding, the two components are molded relatively quickly and thus the first component is still warm when the second material is injected into the mold. The strength of the bond between body 12 and skirt 14 may be maximized by increasing the surface area which forms interface 20 . Therefore, in the multicomponent injection process, the molds are fabricated in such a manner that the connective ends of body 12 and skirt 14 form a tapered hem or margin providing overlapping surfaces at interface 20 . Based on the exemplary dimensions of connector 10 given further below, the width of the overlapping sections is about 5.3 mm. These overlapping surfaces may further be contoured, embossed or otherwise treated to increase the effective area of contact between body 12 and skirt 14 , resulting in stronger joint. Figure 3 also illustrates more clearly the two-segment nature of wings 16 . The upper portion 16a is molded integrally with body 12 while the lower portion 16b is molded integrally with skirt 14 . Upper wing segment 16a also preferably includes a tab portion 17 extending downward. Lower wing segment 16b is molded completely around tab 17 , providing an interlocking fit between the two segments. The actual dimensions of connector 10 may vary considerably depending upon the intended usage. The dual durometer connector could be as long as five centimeters, or as short as one centimeter. Moreover, connector 10 has a wider range of application due to the previously discussed advantages, and a given connector 10 of known dimensions can actually be used in lieu of two or more prior art connectors of different sizes. By way of example, it is useful to note that prior art twist-on connectors come in certain standard sizes which are conventionally color coded. A yellow connector, for instance, typically has an overall length of about 2¾ centimeters and a maximum diameter of just less than one centimeter at its opening; the smallest pair of wires this connector will hold is 18 gauge, while it accommodates a maximum of three 12 gauge wires (this is true of 3M's yellow Scotchlok™ connector). A red connector typically has an overall length of about 3 centimeters and a maximum diameter of about 1.3 centimeters at its opening; the smallest pair of wires that such a red connector will hold is 16 gauge, while it accommodates a maximum of five 12 gauge wires (this range applies to 3M's red Ranger™ connector). Both of these prior art connectors, however, may be effectively replaced by a single connector of the present invention having an approximate overall length of 3.6 centimeters, a maximum diameter of about 1.5 centimeters at the opening of skirt 14 , and an intermediate diameter of about one centimeter at annular rim 28 . Experimentation has shown that a dual durometer connector having these dimensions will still accommodate up to five 12 gauge wires, but will further retain a pair of wires as small as 22 gauge. Thus, a dual durometer connector having these dimensions actually provides a wider range of application than the combined ranges of conventional yellow and red connectors. Such a connector could conveniently be color-coded by coloring one component (e.g., body 12 ) red and coloring the other component (e.g., skirt 14 ) yellow. The thickness of skirt 14 may also vary depending upon its desired flexibility, elasticity and overall strength. As previously mentioned above, a very soft material (shore A 18) may be used, in which case skirt 14 should be about 30/1000" (0.76 mm) thick. Alternatively, a more durable material may be used (up to about shore D 75), in which case skirt 14 could be as thin as about 10/1000" (0.25 mm). In the preferred embodiment, skirt 14 is constructed from a material having a hardness of about shore A 90, and is molded to have a minimum thickness of about 20/1000" (0.51 mm). The advantages of dual durometer connector 10 may best be understood with reference to Figure 4. That figure is similar to Figure 3 except it additionally shows the insertion of several wires 30 . The terminal portions of wires 30 have been stripped, exposing the conductors 32 which are twisted together and inserted within coil spring 22 . As connector 10 is twisted about wires 30 , coil spring 22 expands slightly, and maintains a spring pressure against wires 30 . It would be difficult or impossible to insert this many wires (five are depicted in Figure 4) into a prior art connector, due to the crowding of the bulky insulation surrounding conductors 32 . Dual durometer connector 10 , however, easily accommodates a larger number of wires (or wires of a larger size) because skirt 14 may flex to better conform around wires 30 , and skirt 14 may further expand due to its elastic properties. The friction fit of skirt 14 around wires 30 also provides strain relief, i.e., makes it more difficult to pull wires 30 out of connector 10 . Connector 10 has a further advantage relating to the limited amount of space available in most high density wiring environments. If only two or three wires are connected therein, connector 10 may still be located in a crowded junction box and skirt 14 will not displace extra volume or otherwise interfere with surrounding components since it may deform to optimally fit among other connectors in the junction box.

Claims: An electrical connector (10) comprising: a body (12) having a closed end, an open end, and an internal bore (24), and a helically wound coil spring (22) inserted in said internal bore of said body; characterised in that the internal bore of said body defines a frusto-conical cavity and said body is formed from a rigid, electrically insulative polymer having a Rockwell hardness of at least R50, and a tubular skirt (14) is attached to said open end of said body, said skirt being formed from a flexible, elastic, electrically insulative polymer having a durometer in the range of Shore A 18 to Shore D 75 and said skirt having an axis which is generally aligned with the axis of the internal bore (24). The connector of claim 1 wherein said rigid polymer is selected from the group consisting of polypropylene, polyethylene, polycarbonite/polybutylene terephthalate, hard vinyl, or polyvinyl chloride. The connector of claim 1 or 2 wherein said flexible, elastic polymer is selected from the group consisting of styrene-butadiene-styrene, styrene-ethylene-butylene, styrene-ethylene/butylene-styrene, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, ethylene-propylene diene terpolmer (EPDM), polychloroprene, copolyester elastomers, plasticised vinyl, olefinic thermoplastic vulcanizates, or modified plastisols. The connector of any preceding claim wherein said body (12) has an outer surface, and further comprising a pair of wing extensions (16) attached to said outer surface of said body, said outer surface further having a plurality of longitudinal grooves (18) therein. The electrical connector of any preceding claim wherein said body includes: a plurality of longitudinal ribs (26) extending into said internal bore, in contact with said coil spring (22); and an annular rim (28) extending into said internal bore, proximate said open end of said body, for retaining said coil spring (22) in said internal bore (24).

Inventors: BURTON MARK A, CLIFTON RICHARD B, PAUL GREGG D, BURTON, MARK A., CLIFTON, RICHARD B., PAUL, GREGG D., Burton, Mark A., c/o Minnesota Mining and, Clifton, Richard B., c/o Minnesota Mining and, Paul, Gregg D., c/o Minnesota Mining and
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1992
Main classification: H01R4
IPC codes: H01R4, H02G15