Abstract:
Ends of several electrical wires are joined by a connector which is twisted onto the wire to a predefined torque level by using a unique tool socket. The connector has a body with closed end and an open end for receiving the electrical wires. At least a portion of the hollow body has an equilateral polygonal cross section shape formed by side surfaces which meet at corner sections. The tool socket includes a coupling through which torque is applied and has an aperture for receiving the connector. The aperture has a cross-sectional shape such that the tool socket engages only the connector corner sections and a space exists between the connector side surfaces and the socket. That engagement concentrates torque applied by the tool socket to the connector which causes the corner sections to round upon application of more than the predefined torque level, thus preventing excessive torque from being applied to the connector and the wires.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to connecting electrical wires with twist-on type connectors; and more particularly, to tools for fastening such connectors. 
     The ends of two or more wires of an electrical circuit are often connected together using a twist-on type wire connector. These connectors are available in a variety of sizes and shapes and commonly have a conical shaped body of insulating material, such as plastic, with an opening at the larger end. The opening communicates with a tapered aperture which has helical threads cut in the interior surface of the body. The fastening operation is performed by inserting the stripped ends of two or more wires into the open end and rotating the connector so that the threads screw onto and twist the wires together to form an electrical coupling. An improved connector has a tapered metal spring inserted into the aperture of the insulating body. The spring engages the bare wires and aids in providing a conductive path there between. 
     Twist-on type wire connectors frequently are used by electricians to connect two or more wires in a junction box within a building. In this application, electricians typically twist on the connectors by hand, although manual tools, such as a hexagonal socket wrench or a nut driver, can be used. These connectors also are employed in a variety of electrical appliances. For example, connections between the wires of a ballast in a fluorescent lighting fixture and the electrical supply cord are made in this manner. In a factory, the wire connectors often are attached using a pneumatically or electrically powered nut driver because of the high volume assembly at a fixed location. These power tools have a socket specifically designed to engage the body of the connector. 
     A fastening tool, especially an power-driven one, easily can apply an excessive amount of torque to the connector, thus damaging either the wires or the connector. If cracks in the connector are undetected, a short circuit could occur at the connection. 
     One solution to this problem was to limit the torque with a clutch mechanism between the tool motor and the socket. However, torque limiting devices add additional expense, size and weight to the tool, and require adjustment to the optimum level for each specific wiring application. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a manual or power driven fastening tool for a twist-on wire connector. 
     Another object is to provide a wire connector fastening tool which self-limits the amount of torque that can be applied to the connector during the fastening operation. 
     These and other objectives are fulfilled by a system for joining ends of electrical wires to a predefined torque level, which comprises a twist-on connector and a tool socket specifically designed to cooperate in limiting the amount of torque that the socket is able to apply to the connector. The connector includes a hollow body with an open end in which to receive the wires, a closed end and an outer surface extending between the open and closed ends. At least a portion of the outer surface has elements which form a cross section with a polygonal shape. For example, that portion of the body has side surfaces meeting at outside corners to form a hexagonal cross section. 
     The tool socket includes a coupling by which torque is applied to the tool socket by a driver. An aperture is provided in the tool socket to removably receive the closed end of the connector with side walls of the aperture engaging the portion of the connector&#39;s outer surface. The aperture is significantly larger in cross section than the connector so that a gap exists between the side walls and the outer surface. For example, the aperture may have a polygonal cross section with portions of the side walls between the polygon corners being directed away from the connector to form the gap. The gap results in the transfer of torque between the socket and the connector being concentrated at the outside corners of the connector. This torque concentration causes the elements of the connector, such as the outside corners of the polygon, to deform when the tool socket applies greater than the predefined torque level to the connector. After that deformation, the socket turns freely about the connector inhibiting additional torque from being applied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a twist-on wire connector of a type which can be used with the present invention; 
     FIG. 2 is an axial cross-sectional view through the wire connector with a fastening socket attached thereto; 
     FIG. 3 is a transverse cross-sectional view along line  3 — 3  in FIG.  2  through the wire connector and the fastening socket assembly; 
     FIG. 4 is a transverse cross-sectional view through the wire connector and the fastening socket after an excessive torque has been applied; 
     FIG. 5 is a transverse cross-sectional through the wire connector and a second embodiment of a fastening socket according to the present invention; 
     FIG. 6 is a transverse cross-sectional through the wire connector and a third embodiment of a fastening socket according to the present invention; and 
     FIG. 7 is an axial cross-sectional view through the wire connector with another type of fastening socket attached thereto. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a twist-on wire connector  10  is formed of a hollow body  12  having a general shape of a truncated cone. The body  12  preferably is formed of molded plastic and has an open end  14  which tapers to a smaller diameter closed end  15 . As the outer surface of the body  12  tapers toward the closed end  15 , a transition occurs to six flat surfaces  16 . These flat surfaces  16  define a portion  17  of the body that has an equilateral hexagonal cross-section for engagement by a wrench or socket for fastening the connector  10 . Although the exemplary wire connector  10  has a hexagonal portion  17  various numbers of flat surfaces  16  may be provided to form a body portion with different polygonal shapes for tool engagement. Each flat surface  16  terminates at an edge  18  near the closed end  15  and a conical tip extends from those edges at the closed end. 
     The wire connector  10  also includes a pair of wings  20  which project radially from the body adjacent open end  14 . The radially inner portion of the wings  20  provide exterior longitudinal reinforcement thereby preventing the body  12  from collapsing. The wire connector  10  is fastened onto wires by turning it in the clockwise direction in the orientation illustrated. The curved surface of each wing  20  has grooves which enable the fingers of a user to grip the wire connector during the turning operation. 
     With reference to FIG. 2, the open end  14  of the wire connector has a circular aperture  22  extending axially into the body  12  and terminating a short distance from the closed end  15 . The aperture  22  tapers in a narrowing manner reaching a shoulder  24  approximately one-third the depth of the aperture. The shoulder  24  defines an outer portion  26  of the aperture  22  and a smaller diameter inner portion  28 . A tapered coil spring  30  made of electrically conductive metal is wedged into the smaller inner portion  28 . 
     In use, the stripped ends of two or more wires are inserted into the aperture  22  at the open end  14  of the connector  10 . The closed end  15  of the connector then is placed into a hexagonal socket  32  attached to a square shaft  34  of an electrically or pneumatically powered driver or a manual driver. The power tool then is activated to rotate connector  10  which causes the threaded interior of the aperture  22  to screw onto the stripped ends of the wires twistings the wires together. When the wires have been twisted sufficiently to assure a good electrical connection, the connector  10  is removed from the socket  32 . The wire connector remains on the ends of the wires providing electrical insulation for the connection. 
     In the United States, the Underwriters Laboratory has specified optimum torque levels for attaching different numbers and sizes of electrical wires. Insufficient torque can result in a loose connection which is susceptible to over-heating or disconnection, while application of excessive torque can damage the wires or the connector. 
     As previously noted, electrically or pneumatically powered tools can apply an excessive amount of torque to the connector and break the connector or the wires being fastened. As a consequence, the combination of the wire connector  10  and the tool socket  32  is specifically designed to cooperate and prevent an excessive amount of torque from being applied. That design results in the sharply angled outside corners  38  of the hexagonal connector portion  17  rounding at a predefined torque level allowing the socket  32  to rotate freely about the connector body  12 . Thereafter, torque is not transferred to the connector  10  thus limiting the tool to fastening the wire connector to no greater than the desired torque limit. The yielding of the corners  38  on the connector body  12  not only prevents excessive amount of torque from being applied, but also ensures that the predefined torque level is applied as the corners  38  do not yield until that level has been reached. 
     With reference to FIGS. 2 and 3, the tool socket  32  has a hexagonal cross section aperture  36  within which the closed end  15  of the connector  10  is removably received. The socket aperture  36  is larger than the cross-sectional dimensions of the mating portion of the connector  10  thus producing a loose fit as is particularly evident in FIG.  3 . As is apparent in this figure, the torque exerted on the connector  10  by the socket  32  is concentrated at the outside corners  38  of the hexagonal portion  17  of the connector. In conventional fastening operations, it is desirable to have as tight a fit as possible between the tool socket and the object between fastening, in this case the connector  10 . That tight fit assures the torque will be distributed through a relatively large surface contact area between the components and prevents the tool socket from turning around the object. However, the present concept intentionally provides less than the normally desired tight fit. 
     The relatively loose fit between these components is sufficient to for the tool socket  32  to rotate the connector  10  so as to properly couple wires placed within the connector for fastening. When the predefined torque level for the connection is reached, the angled corners  38  of the hexagonal portion  17  of the plastic connector  10  become rounded as depicted in FIG.  4 . That predefined torque level is too intense for the relatively small amount of plastic material at the connector corners  38  to withstand without deforming. The deformation continues until the socket  32  is able to rotate freely about the connector  10  at which time transfer of torque to the connector ceases. The difference in cross sectional sizes of the connector  10  and the socket aperture  22  and depth D (FIG. 2) that the connector extends onto the socket aperture determine the area of contact between those components and thus the torque magnitude that must be applied before rounding occurs. The strength of the plastic body  12  also is a factor in determining the torque level at which corner rounding occurs. These factors enable the socket-connector combination to be intentionally designed so that the tool socket  32  can not exert more that the predefined torque level on wire connector  10 . 
     FIG. 5 illustrates an alternative design of a tool socket  40  which has an aperture that is formed by six concave curved side walls  42 . The radius of each side wall is more than twice the distance to the center axis  41  of the socket, for example. Adjacent side walls meet at a line that is parallel to the center axis thus defining an inside corner within which a corner  38  of the connector is received. Because of the curving nature of the side walls, the distance from the center axis  41  to the side walls is greatest at each inside corner and decreases going from an inside corner toward a midpoint  44  along each sidewall  42 . Therefore, the hexagonal cross-section portion  17  of the connector  10  is captivated in the aperture so that rotation of the tool socket  40  by the square shaft  34  of the driver will produce rotation of the connector. However, the torque being transferred to from the socket to the connector is concentrated at each outside corner  38  which engages an inside corner of the socket aperture. Thus when the predefined torque limit for this type of connector is exceeded, the corners  38  round allowing the socket to turn freely about the connector. The radius of the side wall curvature defines the area of surface contact between the tool socket  40  and the connector  10 , and thus the torque limit at which rounding occurs. 
     FIG. 6 illustrates a variation of the socket  40  in FIG.  5 . In the third embodiment, socket  50  has an aperture  52  with a dodecagon cross section which by definition has twelve side surfaces and twelve inside corners  54 . The six outside corners  38  of the hexagonal cross sectional portion  17  of the connector  10  nest within six of the inside corners  54  with an open inside corner of socket  50  between each inside corner  54  that is engaged by a connector corner  38 . The twelve side surfaces of the socket aperture  52  angle away from the six exterior flat surfaces  16  of the connector thus concentrating the applied torque to relatively small surface areas of the connector adjacent to corners  38 . This causes the sharply angled connector corners  38  to round when the predefined torque limit is exceeded. 
     Another version of a tool socket  60  according to the present invention is shown in FIG.  7 . This socket  60  has a hexagonal cross section aperture  62  with a relatively large cross section portion  64  within which the closed end  15  of the connector  10  is removably received. The aperture  62  narrows at a shoulder  66  against which abut the edges  18  of the connector flat surfaces  16 . The shoulder  66  defines the depth to which the connector  10  is able to enter the aperture  62  and thus the amount of surface area in which the connector contacts the socket. The torque transferred to the connector  66  and thus the amount of surface area in which the connector contacts the socket. The torque transferred to the connector from the socket during the fastening operation in concentrated in that contact surface area. Therefore by selectively controlling that area with the depth of shoulder  66 , the torque level at which the corners of the hexagonal portion of the connector become rounded can be set to the appropriate magnitude for a given fastening operation. 
     In an variation of the socket  60  in FIG. 7, the portion  64  of aperture  62  is so large in comparison to the cross section of the connector  10  that the socket does not engage the connector flat surfaces  16  or the corners at the meeting point of adjacent flat surfaces. Instead the shoulder  66  has a curved projection which extends into the notches  19  in the edges  18  of the flat surfaces  16 . Thus torque is transferred from the socket to the connector through the surfaces of the notches  19 . The depth of the notches defines the amount of surface area through which the torque is transferred. By defining that surface area, a limit to the amount of torque that may be applied to the connector can be established. Application of a greater magnitude of torque causes the walls of the notches to deform which results in the socket turning on the end of the connector without further torque transfer.