ELECTRICAL CONNECTOR FOR A COAXIAL CABLE AND METHOD OF FITTING SAID CONNECTOR WITH A COAXIAL CABLE

Abstract: Electrical connector for a coaxial cable (10) of the type having a central conductor (12) separated from a braid conductor (14) by an insulation layer (16), and with a jacket (18) around the braid conductor (14), said connector (100) comprising a rear nut (400), a forward body (200), and a medial clamp member (300), the clamp member (300) having a central aperture extending therethrough from front to rear for receiving the insulation layer (16) and central conductor (14) therein, a frusto-conical portion of a smaller diameter at the rear of the clamp member, and a helical groove which is machined into the frusto-conical portion to provide a series of undercuts, and said connector (100) being assembled over the coaxial cable (10) with the nut (400) slipped over the cable (10), the clamp member (300) inserted between the braid conductor (14) and the insulation layer (16) and then rotated in the direction of the helical groove to progressively draw the clamp member (300) within the braid conductor (14) until the entire frusto-conical portion is within the braid conductor (14), such that an edge of each undercut provides a corner surface which engages the braid conductor (14), securing it and providing a resistance to undesirable axial movement of the clamp member (300) away from the cable (10), and with the nut (400) secured to the body (200) by means of interfitting threads (420, 240) to complete the electrical contact and captivate the clamp member (300) therebetween in proper position.

Description: Electrical connector for a coaxial cable This invention relates to an electrical connector for a coaxial cable terminating at a free end and including a central conductor, an insulating layer surrounding the central conductor, a braid conductor surrounding the insulating layer, and a protective sheath surrounding the braid conductor, this electrical conductor comprising a terminal body, a clamp member disposed within the terminal body and a nut assembling the clamp member to the terminal body, the clamp member including a tapered surface, and a bore for receiving the central conductor and the insulating layer and extending between a forward face and a rear face. Many approaches have been suggested for electrically terminating an electrical connector with the end of a coaxialtype electrical cable. One approach involves a significant number of parts which the user must arrange and assemble himself in a proper sequence and position. This is undesirable for many reasons : Many parts may result in possible loss of a part or the failure of the assembler to include the part at all or in its proper location; each part must be separately manufactured and inventoried and detailed assembly instructions must be prepared and supplied; this approach leads to some rather small parts which are easily lost or misplaced and hard to identify and handle. It has been suggested (e.g., U.S. Patent No. 3,373,243) to provide a three-part assembly (a nut, a body, and a clamp member with a frusto-conical shape) for terminating a coaxial cable. The frusto-conical portion of the clamp is pushed axially inward over the central conductor and within the jacket and braid. When the cable jacket is thick, or made of a heavy, non-yielding material, or when the axial movement of the clamp member inward is resisted and the jacket and/or braid layers resist, the clamp member is pushed axially outward. In such cases when the clamp member is not properly positioned substantially fully engaged with the braid, a poor electrical connection is made between the braid and the clamp, creating undesirable resistance in the outer body assembly. If the clamp member is not properly positioned, the central conductor may not be in proper position within the central contact, again creating a poor electrical connection. Although the nut and body are assembled over the clamp and hold the clamp in place, the clamp in the design shown in this patent can move out of position while the nut and body are being assembled. Further, a thick jacket or a tight fitting cable or multi-layer braid resists insertion of the clamp. Other approaches to terminating a coaxial cable require that the cable be prepared by stripping the jacket, braid, insulator layer, and central conductor to progressively longer lengths (in a "stepped" arrangement) prior to insertion of an electrical connector assembly. Such a "stepped" arrangement of the conductor is undesirable in that it requires additional time and effort by the user and provides a chance that the preparation would not be properly done. Such an arrangement is shown in U.S. Patents 3,264,602; 3,107,135; and 3,054,981. Other approaches to obtain a termination of a coaxial connector are shown in U.S. Patents 3,209,287 and 3,634,815. Accordingly, the prior art coaxial terminations lack a design which has few parts, is easy and quick to assemble at low cost, and yet resists outward axial movement of the clamp member, even before the nut and body are assembled. The present invention overcomes the disadvantages and limitations of the prior art arrangements by providing an electrical connector for a coaxial cable terminating at a free end and including a central conductor, an insulating layer surrounding the central conductor, a braid conductor surrounding the insulating layer, and a protective sheath surrounding the braid conductor, said electrical connector comprising a terminal body, a clamp member disposed within the terminal body and a nut assembling the clamp member to the terminal body, the clamp member including a tapered surface, a bore for receiving the central conductor and the insulating layer and extending between a forward face and a rear face, and a spiral groove described by a series of undercuts disposed on the tapered surface such that an edge of each undercut provides a corner surface which engages the braid conductor after the central conductor and the insulating layer are received in the bore and the clamp member is rotated to draw the tapered surface of said clamp member between the insulating layer and the braid conductor. The electrical connector of the present invention is advantageous in that it provides an arrangement which is easy and quick to assemble, and resists outward movement of the clamp member prior to complete assembly of the connector and cable. One way of carrying out the invention is described in detail below with reference to the drawings which illustrate one specific embodiment and a slightly different variant thereof, in which FIGURE 1 is a cross sectional view of an electrical connector and coaxial cable of the present invention. FIGURE 2 is an enlarged side view of a clamp member used in the connector of the present invention, showing the helical groove. FIGURE 3 is a cross sectional view of the clamp mem-' ber of FIGURE 2, looking along the lines 3-3 of FIGURE 2 in the direction of the arrows. FIGURE 4 shows a coaxial cable and a clamp member of the present invention partially inserted over it, with the small rear portion of the clamp member in engagement with the leading edge of the cable braid. FIGURE 5 shows a cross sectional view of the coaxial cable and clamp member of FIGURE 4 when the clamp member is in complete engagement within the coaxial cable and held in place by the helical thread. FIGURE 6 is an enlarged cross sectional view of helical thread structure within a circle identified by an arrow and a numeral VI in FIGURE 3. FIGURES7 and 8 are views of alternate embodiments of the thread structure shown in FIGURE 6. FIGURE 9 is a cross sectional view of an alternate embodiment of the connector of FIGURE 1, in which the nut is a multi-piece member. FIGURE 1 is a cross sectional view of a coaxial cable 10 terminated by a coaxial electrical connector 100 of the present invention. The coaxial cable 10 includes a central electrical conductor 12, an outer braid conductor 14, and a layer of dielectric material 16 between the central conductor 12 and the braid conductor 14. An outer jacket 18 surrounds the conductors. The outer jacket 18 and the dielectric material 16 are both made of electrically insulating materials to electrically isolate the conductors 12,14 from each other and from the environment. The coaxial connector 100, in its preferred embodiment, has three components which are supplied and handled as separate pieces: a forward body 200, an internal clamp member 300 and a rear nut 400. The forward body 200 is of conventional design and includes a mating central contact 210 surrounded by an electrically insulating material 220. An outer electrical contact 230 is disposed outside the insulating material 220, electrically isolated from the central contact 210. When the connector 100 is attached to the cable 10, the central contact 210 is electrically coupled to the central conductor 12 of the cable 10 and the outer contact 230 is electrically coupled to the braid conductor 14 of the cable 10. The body 200 includes an internal thread 240 at the rear end thereof for coupling to the nut 400. The clamp member 300 includes an electrically conductive body 310 and an insulator 320. The insulator 320 fits within an undercut portion 312 extending rearwardly from the front end of the conductive body. The insulator 320 electrically isolates the exposed portion of the central conductor 12 of the cable and the contact 210 from the body 310. The insulator 320 is retained within the undercut in a known manner (e.g., adhesive- ly or frictionally, through pressing in a member in an interference fit). Other structure and features of the body 310 of the clamp 300 will be discussed later in conjunction with FIGURES 2-8. The nut 400 may be of the type described in U.S. Patent 3,373,243. The nut 400 includes a rear portion 410 which is grasped during installation and an external screw thread 420 located forward of the rear portion 410. The screw thread 420 engages the thread 240 of the body 200 to secure the connector assembly 100 together. The nut 400 includes a central aperture 430 through which the cable 10 extends. The nut 400 also includes steps 440 which provide increasingly larger diameters of the central aperture 430 in the. forward region of the nut. Such steps, when used in conjunction with a clamp as described in the present design having a frusto-conical surface of increasing diameter, enables the connector 100, when assembled, to retain the cable 10 therein by captivating the jacket 18 and thereby providing resistance to decoupling forces applied on the cable. The captivating occurs between the steps and the frusto-conical surface. FIGURE 2 is an enlarged view of the external surface of the clamp body 310 of the present invention. The clamp body 310 has a forward portion 313 of a larger diameter and a rear frusto-conical portion 314. The portion 314 tapers from a smaller rear end 314a to a larger forward end 314b, where it meets the larger forward portion 313. A helical groove 316 spirals around the frusto-conical portion 314, making several revolutions. Although a spiral in a form similar to a raised (exposed) screw thread could be used to advantage in the present design, a quicker and less expensive approach is shown in these drawings, that is, to form or cut a groove 316 into the outer surface, which requires the removal of less material. The formation of the helical groove is advantageously formed on an automatic screw machine. Other methods of fabrication such as stamping and rolling, casting, forging and spinning could also be used to advantage to create the clamp 300 of the present invention. Using such other methods of fabrication may make it easier and less expensive to have exposed threads. The pitch and depth of the helical groove are somewhat a matter of design choice. It has been found that a continuous spiral of 40 pitch, 13/100 mm deep, right handed buttress type thread works to advantage. Lefthanded threads could be substituted and used to advantage in the present design although they would be unconventional and more difficult to install manually, as an operator is accustomed to install a screw member with right handed threads. The threads on the nut 400 should be of the same type (i.e., right-handed) as on the clamp so that the application of the nut does not tend to uncouple the clamp from the braid. FIGURE 3 shows a cross sectional view of the clamp body 310 without the insulator. The forward undercut portion 312 for receiving the insulator is shown, as are the enlarged forward portion 313, the helical groove 316 and the. frusto-conical tapering portion 314. FIGURE 4 shows a cross sectional view of the clamp member 300 (with body 310 and insulator 320) being inserted over the "square cut" cable 10. The nut 400 has already been inserted over the cable 10 and is moved to the rear of the forward end of the cable, away from the area of the cable upon which the clamp 300 will be applied. The body 310 of the clamp and the insulator 320 each have a central aperture (311 and 321, respectively). The central aperture 311 of the clamp is large enough to receive the layer of insulation 16 of the cable 10, while the aperture 321 in the insulator is smaller, merely large enough to receive the central conductor 12 therein. The cable 10 has been prepared in a manner which is referred to as a "square cut" in the industry. The square cut preparation has the central conductor 12 extending forwardly of the rest of the cable (outer layers: insulation 16, braid conductor 14, and jacket 18), all of which extend a uniform, but lesser, distance forwardly. The outer rear surface of the clamp 300 has been pressed between the layer of insulation 16 and the braid conductor 14, urging the braid conductor 14 and the jacket radially outward slightly. At the stage shown in FIGURE 4, the helical groove 316 is in contact with the braid conductor 14, and because the braid conductor 14 is being urged radially inward by the surrounding jacket 18, the braid conductor 14 is urged partially into the groove 316. As the body 310 is rotated in the direction of helical groove 316, the groove 316 coacts with the braid conductor 14 to draw the body 310 further in, in the nature of a screw being drawn into wood as it rotates. Similarly, the body 310 is retained by the groove 316 against undesirable axial movement out of the cable 10 because the braid conductor 14 is engaged upon the groove 316. FIGURE 5 shows the clamp 300 when the body 310 fully engages the cable 10. The braid conductor 14 and the jacket 18 extend forwardly on the clamp to the enlarged portion 313 thereof, fully covering the tapering frusto-conical portion 314, with the helical groove 316 being embedded along its length within the braid conductor 14. The insulation layer 16 is well forward within the clamp body in the central aperture thereof. FIGURE 6 is a cross sectional view showing a portion of the clamp body 310 with the thread or helical groove 316 shown. The helical groove 316 shown here is "U" shaped and has corners 316a,316b at the meeting of the groove with the external surface. The rear or trailing corner 316b will engage or "bite into" the braid conductor and prevent it from moving rearwardly, even if the cable is pulled rearwardly with respect to the connector. A portion of the braid conductor will be forced into the groove, providing a more reliable and uniform electrical connection between the braid conductor and the connector. While other types of grooves and threads could be used - either exposed external threads, discontinuous grooves, or varying shapes of grooves - the threads of the type shown in FIGURE 6 are believed the preferred embodiment as they can be formed quite easily on automatic screw machines. Of course, as previously mentioned, the threads could be formed using other techniques. FIGURES 7 and 8 show alternate embodiments of helical threads 316' and 316", where the threads are "V" shaped. These threads are also formable on automatic screw machines with a slightly different tool. In FIGURE 7, a leading corner 316a' is flatter than the leading corner 316a of FIGURE 6 to bite into the braid. In FIGURE 8, the "V" is substantially symmetrical with respect to the surface of the body, giving a front corner 316a" and a rear corner 316b" between which the braid is captivated. The rear corner 316by does most of the engagement of the braid in this instance. FIGURE 9 shows an alternate embodiment of the connector 100 of the present invention. In place of a rear nut of the type shown in FIGURE 1, this embodiment has a rear nut 500 of the type shown in U.S. Patent No. 3,634,815. The nut 500 in this embodiment includes a resilient member 510 having a frusto-conical forward bore 512, with sealing members 520,530 located forward and rearward of the resilient member 510. Since the structure of the nut is of only peripheral concern to the present invention, other nut structures could also be used to advantage. While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made. For example, additional members could be used in the present assembly, and the clamp member of the present invention could also be used for tri-axial cables with appropriate modifications.

Claims: Claims: 1. An electrical connector for a coaxial cable (10) terminating at a free end and including a central conductor (12), an insulating layer (16) surrounding the central conductor (12), a braid conductor (14) surrounding the insulating layer (16), and a protective sheath (18) surrounding the braid conductor (14), said electrical connector (100) comprising a terminal body (200), a clamp member (300) disposed within the terminal body (200) and a nut (400) assembling the clamp member (300) to the terminal body (200), said clamp member (300) including a tapered surface (314), and a bore for receiving the central conductor (12) and the insulating layer (16) and extending between a forward face and a rear face, characterized in that the clamp member (300) further includes a spiral groove (316) described by a series of undercuts disposed on the tapered surface (314) such that an edge of each undercut provides a corner surface which engages the braid conductor (14) after the central conductor (12) and the insulating layer (16) are received in the bore and the clamp member (300) is rotated to draw the tapered surface of said clamp member (300) between the insulating layer (16) and the braid conductor (14). 2. An electrical connector as claimed in claim 1, characterized in that the outer surface of the clamp member (300) is generally frusto-conical in shape with the larger end mounted toward the free end of the cable (10). 3. An electrical connector as claimed in claim 1 having the nut (400) and the terminal body (200) joined by threads (420,240), characterized in that the rotation of the nut (400) to secure it to the terminal body (200) is in a direction to so urge the series of undercuts as to provide greater engagement of the clamp member (300) and the braid conductor (14). 4. The method of assembling the electrical connector according to claim 1 with the coaxial cable, characterized by the steps of: cutting the protective sheath, braid conductor and insulating layer of the cable perpendicular to the axis thereof in a substantially uniform length, shorter than the central conductor; slipping the nut over the cable; inserting the central conductor and the insulating layer in the bore of the clamp member and then a portion of the tapered surface of the clamp member between the insulating layer and the braid conductor; rotating the clamp member in the direction of the spiral groove to progressively draw deeper within the braid the clamp member until the entire tapered surface thereof is within the braid; and assembling the nut and the terminal body over the clamp member to complete the electrical contact and captivate the clamp member therebetween.

Inventors: GOULD, DONALD HOLLENBECK, KAROL, JAMES JOSEPH, KIRBY, ALLAN BLACK
Applicants: THE BENDIX CORPORATION
Application year: 1980
Main classification: H01R9
IPC codes: H01R13, H01R24