Abstract:
A tool for crimping a tubular crimp sleeve onto the end of a cable. The tool includes a frame structure, having two mutually opposing frame legs, which receives a crimping insert for a selected sleeve size. The insert includes two blocks that are brought together by means of a screw which, with the aid of a lever, is screwed through one of the frame legs so as to press the insert blocks together. A spring element strives to separate the two insert blocks, and at least one of the frame legs includes a recess for receiving the end of a corresponding one of the insert blocks.

Description:
This application is a divisional application of U.S. Ser. No. 09/269,206 filed Mar. 31, 1999 now U.S. Pat. No. 6,326,546 which is the national phase under 35 USC §371 of PCT International Application No. PCT/SE97/01617 which has an International Filing Date of Sep. 25, 1997, which designated the United States of America and was published in English and claims priority from 9603614-0 filed Oct. 3, 1996 in Sweden which is claimed herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cable strain relief of the kind having a plurality of conductors which are embraced by a cable casing and including a sleeve. 
     The invention also relates to a tool for fitting such a strain relief to a cable of corresponding diameter. 
     2. Description of the Related Art 
     The present invention relates to a cable strain relief of the kind defined in the preamble of claim 1. 
     The invention also relates to a tool for fitting such a strain relief to a cable in accordance with the preamble of the independent claim directed to the tool. 
     A strain relief is a connection element that is coupled to the circumference of a cable and enables forces that act in the length direction of the cable, i.e. axially acting forces, to be transferred to an external construction (e.g. a cable cabinet). Such a cable will normally include a bundle of individual, insulated conductors covered with screen braiding. An insulating sleeve is provided on top of the braiding. 
     Such strain reliefs will preferably be electromagnetically impervious, i.e. provide electromagnetic insulation (EMI), and must also be capable of transferring forces effectively, so as to prevent individual fibers from being subjected to tensile forces in junction boxes or the like. 
     A known type of strain relief intended for this purpose is comprised of two tubular parts that have co-acting, conical surfaces. One part (the inner part) has slits that extend axially from one end thereof, such as to form axially extending tongues. When the two parts are fitted together axially, the free ends of the tongues will be bent inwardly against the outside of the cable. The outer insulation is removed at the end of the cable and the braiding is folded back around the end of the remaining insulating sleeve. Consequently, when the two tubular parts are fitted together, parts of the braiding threads will be clamped between adjacent tongues. This often results in unsatisfactory anchoring of the strain relief to the cable. 
     Other known strain reliefs incorporate a U-shaped element whose legs are bent in towards the cable so as to overlap each other. This solution can result in damage to the individual conductors in the cable, and may also cause EMI-leakages to occur through the overlap. 
     Earlier known strain reliefs are expensive and require the use of complex devices for fitting the reliefs to cables, and also result in joints of greatly differing qualities. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the invention is to provide a strain relief that can be fitted to cables of different standard diameters with the aid of a simple tool, such as to obtain an EMI-tight connection and a uniform result on each occasion. 
     Further objects of the invention will be evident from the following text, either directly or indirectly. 
     The invention is basically concerned with establishing a strain relief for a screened cable that includes a plurality of conductors, by crimping a crimp sleeve on that part of the cable at which the braiding has been folded back over said cable, said crimp sleeve incorporating means which function to achieve an EMI-tight coupling of the sleeve to the wall of an apparatus housing at the cable leadthrough or transit. Crimp sleeves of this kind are available in various diameters, lengths and wall thicknesses adapted to different cable diameters, so as to ensure that the crimp sleeve will be effectively anchored to the cable with the aid of a crimping tool and therewith provide an EMI-impervious cable connection. 
     In order to enable a generally uniform crimping force to be applied with sleeves of different diameters and different wall thicknesses, the larger crimp sleeves include a circumferential groove that reduces the crimping force required to achieve the requisite strain relief anchorage of the crimp sleeve to the cable. 
     The inventive crimping tool for crimping such strain relief sleeves may include a tool frame structure that has two generally parallel and mutually opposing frame members. Two opposing crimp inserts are placed between the frame members. The inserts include on their mutually facing sides recesses for crimping a sleeve of corresponding diameter on a corresponding cable. A screw meshes with a threaded hole through one frame member, so that the screw can be screwed in a direction towards the second frame member such as to bring the two inserts together. The inserts include means for guiding relative movement in said direction. The two inserts also include spring means which strive to move the inserts apart in said direction. 
     The two frame members have shallow recesses for receiving and localizing respective inserts. The tools can be used in conjunction with a set of insert pairs which each include crimping recesses that are adapted to crimp sleeves of correspondingly different sizes. In other respects, the insert pairs have generally equal outer dimensions in order to enable the insert pairs to be readily swapped in the frame structure. Owing to the relative guiding of the inserts and the spring means, the pair of inserts will be held automatically in the frame structure as soon as the inserts are inserted thereinto, and can readily be replaced manually with another pair of inserts, by first pressing the fitted pair of inserts together against the action of the spring means and then tipping the inserts out of one of the recesses and out of the frame structure. The recesses in the frame members are shallow recesses. The frame structure may conveniently have an elongated support arm and the screw may include a lever for facilitating rotation of the screw. 
     The invention will now be described in more detail with reference to an exemplifying embodiment thereof and also with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an inventive strain relief fitted to the end of a cable and connected to the wall of an apparatus housing. 
     FIG. 2 is a cross-sectional view taken on the line A—A in FIG.  1  and images the strain relief prior to being crimped on the end of the cable. 
     FIG. 3 illustrates the configuration of the strain relief subsequent to being crimped on the end of the cable. 
     FIG. 4 shows a tool for crimping the strain relief sleeve. 
     FIG. 5 is a sectional view of a crimping tool insert. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modification is within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     FIGS. 1 and 2 illustrate a cable  1  comprising a core  2  that includes a plurality of insulated conductors  24 , said core  2  being provided with screening braiding  3  which, in turn, is embraced by a cable casing  4 . A length of casing  4  has been removed from the outermost end of the cable  1  and the braiding folded back over the remaining end part of the casing  4 . A strain relief sleeve  10  is shown fitted over the backwardly folded screening part  3 ′ of said end section. The front part of the sleeve  10  includes an externally threaded section  12  and also an outwardly open circumferential groove  11  between its ends. An opening  13  through the casing wall, for instance in the groove  11 , enables it to be ascertained whether or not the screen  3 ′ is located immediately beneath the sleeve  10 . FIG. 1 also shows the wall  20  of an apparatus housing. The wall  20  includes an opening  21  having a thread that will mesh with the outer thread  12  on the sleeve. The sleeve  11  is anchored to the wall  20  by means of the screw joint  12 ,  22 . The sleeve  10  lies in intimate contact with the screen  3 ,  3 ′ around the whole of its circumference and tightly engages the wall  20  via the screw joint, so that the sleeve  10  will provide an EMI-tight connection to the apparatus housing (provided that the sleeve  10  and the wall  20  are made of a suitable material in this respect). 
     The sleeve  10  is crimped onto the cable  1  so as to obtain an EMI-tight connection therewith. By crimping is meant that the sleeve  10  is subjected to plastic deformation from an essentially circular, round state into a polygonal shape, as illustrated in FIG.  3 . Such crimping of the strain relief sleeve  10  provides a highly durable anchorage of the end of the cable  1  to the sleeve  10  with respect to tensile forces or strain acting in the length direction of the cable, while providing, at the same time, an EMI-tight connection between the sleeve  10  and the cable  1  and subjecting the insulated conductors  24  to stresses that are generally safe with respect to the integrity of the conductors  24 . 
     The crimping technique requires a larger material thickness of the sleeve  10  in the case of large diameter cables  1  (with maintained crimping deformation pattern), which normally means that the crimping tool must exert crimping forces that increase markedly with increasing diameters of the cable  1  and the sleeve  10 , by providing larger sleeves  10  with a circumferential groove  11  between its ends, and can limit the increase in requisite crimping forces that must be exerted by the crimping tool. This enables the use of a simple, and therewith cost favorable tool that includes exchangeable inserts that cover a whole series of differently sized sleeves  10  adapted to cables  1  of standard sizes. 
     FIGS. 4 and 5 illustrate one such simple tool  30  with associated inserts  40 . 
     The crimping insert  40  is comprised of two mutually co-acting blocks  41 ,  42 , which may be mutually identical. Each of the blocks  41 ,  42  has an orthogonal parallelepipedic shape and each of the mutually opposing sides of the blocks includes a recess  43 , said recesses together defining an hexagonal opening when the blocks  41 ,  42  are in mutual abutment. The hexagonal shape corresponds to the final external shape  10 ′ (FIG. 3) of the crimped sleeve  10 . Each block  41 ,  42  has a guide pin  44  on one side of the recess  43  and a corresponding aperture or hole  45  on the other side thereof. A helical spring  48  is placed on the bottom of each aperture  45 . Each block  41 ,  42  is therewith designed so that the pin  44  of one block  41 will fit into the hole  45  in the other block, and vice versa. 
     Although the blocks  41 ,  42  are mutually identical as a result of the illustrated construction of the insert  40 , it will be obvious to the person skilled in this art that the blocks  41 ,  42  can be constructed differently with respect to the pins  44 , the holes  45  and the springs  48 , while retaining the function of the blocks. FIG. 4 illustrates a tool handle  31  which has at one end a frame structure  50  formed by the end-part  32  of the handle, a so-called yoke  33  which is carried by two bolts  34  that extend perpendicularly through the yoke  33  and through the handle part  32  and take up forces that strive to move the yoke  33  away from the handle part  32 . The yoke has a recessed part  36  which receives an adjacent end of the block  42 . A guide plate  35  is carried on the inside of the frame structure  50 , parallel with the yoke  33 . The guide plate has a recessed part  36  with a bottom plate  39  which is movable along the bolts  34  and which lies normal to the handle part  32 . 
     When the blocks  41 ,  42  (FIG. 5) are pressed together so as to bring their adjacent surfaces  46  essentially into contact with one another, the insert  40  can be inserted laterally into the frame structure  50  in alignment with the recess  36 , whereafter the insert  40  is allowed to expand under the action of the springs  48  to the state shown in FIG. 4, where the insert  40  is thus held by the expansion forces of the springs  48 . The insert  40  can, nevertheless, be easily removed from the frame structure, by first compressing the insert and then tilting it out of the frame structure  50 . 
     A series of inserts  40  that have essentially identical external dimensions but recesses  43  of mutually different sizes can be used in conjunction with the tool  30 . These inserts  40  can be readily exchanged in the tool  30 , which has an extremely simple construction as evident from the a foregoing. 
     When the crimp sleeve has been crimped firmly to the cable with a radial load that is distributed generally uniformly in the length direction of the sleeve, the sleeve will be deformed radially more pronouncedly at its ends (i.e. obtain a smaller diameter) than in its central region. This effect is apparently due to weakening of the sleeve wall by the centre groove  11 . This results in the section of cable located in the sleeve between its ends being stretched axially to some extent, which could be detrimental to the conductors in the cable. Instead, the cable section is compressed in the crimp sleeve, wherewith inclination of the end-edges of the sleeve provides a particularly effective transfer of axial forces between the crimped sleeve and the cable. 
     It will be noted in particular that the crimped sleeve and the crimping force engage all conductors, conductor insulation, screens and the like in the cable, so that all cable parts will obtain an axial force-coupling to one another and to the crimped sleeve, such that said cable components will not experience any relative axial movement when axial forces are applied. Furthermore, the crimping affords radial compression of the cable, so that the cable will be sealed against axial fluid throughflows between the cable components. It will also be noted that the crimp sleeve is a single ring-shaped element, which facilitates work in fitting the sleeve. 
     Although the crimp sleeve is shown in FIG. 1 to be provided with an axial tubular extension having an outer thread, it will be understood that this extension can be omitted and a separate sleeve nut, or the like, that axially couples the sleeve to an externally threaded leadthrough sleeve on an apparatus housing, or the like, may be used instead. 
     The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not. to be regarded as a departure -from the spirit and scope of the invention, and all such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims.