CA2265855C

CA2265855C - A strain relief and a tool for its application

Info

Publication number: CA2265855C
Application number: CA002265855A
Authority: CA
Inventors: Per Karlsson
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-10-03
Filing date: 1997-09-25
Publication date: 2006-01-10
Anticipated expiration: 2017-09-25
Also published as: EP1215787B1; SE9603614D0; HK1049071B; DE69735432D1; KR100468289B1; US20020056565A1; AU721552B2; CA2265855A1; WO1998015044A1; ATE300110T1; ATE320102T1; HK1049071A1; DE69733780D1; EP1215787A2; AU4578497A; DE69733780T2; KR20000048777A; SE507262C2; SE9603614L; JP2001501434A

Abstract

An EMI-tight strain relief for a screened cable includes a tubular crimp sleeve which is crimped onto the end of the cable and which includes means (12) for securing the strain relief in an opening in the wall of an apparatus housing. In a series of crimp sleeves (10) at least one large sleeve (10) will have a circumferential groove (11) for equalizing the necessary crimping force. A tool for crimping the sleeves (10) on the cable includes a frame structure (50) which receives a crimping insert for the sleeve size concerned in a manner which enables the insert to be replaced. The insert includes two blocks (41, 42) that are brought together by means of a screw which, with the aid of a lever (37), is screwed through a frame leg so as to press the insert blocks (41, 42) together.

Description

ï»¿1015202530CA 02265855 2004-12-23wo 98/15044 PCTISE97/016171A strain relief and a tool for its applicationThe present invention relates to a strain relief for a screen cable, particularly foruse in an opening through the wall and to an interior of a housing for apparatus.The invention also relates to a tool for fitting such a strainrelief to a cable in accordance with the preamble of theindependent claim directed to the tool.A strain relief is a connection element that is coupled to thecircumference of a cable and enables forces that act in thelength direction of the cable, i.e. axially acting forces, tobe transferred to an external construction (e.g. a cable âcabinet). Such a cable will normally include a bundle ofindividual, insulated conductors covered with screen braiding.An insulating sleeve is provided on top of the braiding.such strain reliefs will preferably be electromagneticallyimpervious, i.e. provide electromagnetic insulation (EMI), andmust also be capable of transferring forces effectively, so asto prevent individual fibers from being subjected to tensileforces in junction boxes or the like.A known type of strain relief intended for this purpose iscomprised.of two tubular parts that have co-acting conicalsurfaces. one part (the inner part) has slits that extendaxially from one end thereof, such as to form axially extendingtongues. when the two parts are fitted together axially, thefree ends of the tongues will be bent inwardly against theoutside of the cable. The outer insulation is removed at theend of the cable and the braiding is folded back around the endof the remaining insulating sleeve. Consequently, when the twoï»¿1015202530CA 02265855 l999-03- llwo 93/15044 PCT/SE97I016172tubular parts are fitted together, parts of the braidingthreads will be clamped between adjacent tongues. This oftenresults in unsatisfactory anchoring of the strain relief to thecable.Other known strain reliefs incorporate a U-shaped element whoselegs are bent in towards the cable so as to overlap each other.This solution can result in damage to the individual conductorsin the cable, and may also cause EMIâleakages to occur throughthe overlap.Earlier known strain reliefs are expensive and require the useof complex devices for fitting the reliefs to cables, and alsoresult in joints of greatly differing qualities.Accordingly, the object of the invention is to provide a strainrelief that can be fitted to cables of different standarddiameters with the aid of a simple tool, such as to obtain anEMI-tight connection and a uniform result on each occasion.Further objects of the invention will be evident from thefollowing text, either directly or indirectly.This object is achieved fully or partially with the strainrelief according to Claim 1. The object is also achieved eitherfully or partially with the inventive tool defined in theindependent claim directed to the tool.Further embodiments of the invention are defined in theaccompanying dependent claims.ï»¿1015202530CA 02265855 l999-03- 11W0 93,150.â PCTISE97/016173The invention is basically concerned with establishing a strainrelief for a screened cable that includes a plurality ofconductors, by crimping a crimp sleeve on that part of thecable at which the braiding has been folded back over saidcable, said crimp sleeve incorporating means which function toachieve an EMIâtight coupling of the sleeve to the wall of anapparatus housing at the cable leadthrough or transit. Crimpsleeves of this kind are available in various diameters,lengths and wall thicknesses adapted to different cablediameters, so as to ensure that the crimp sleeve will beeffectively anchored to the cable with the aid of a crimpingtool and therewith provide an EMIâimpervious cable connection.In order to enable a generally uniform crimping force to beapplied with sleeves of different diameters and different wallthicknesses, the larger crimp sleeves include a circumferentialgroove that reduces the crimping force required to achieve therequisite strain relief anchorage of the crimp sleeve to thecable.The inventive crimping tool for crimping such strain reliefsleeves may include a tool frame structure that has twogenerally parallel and mutually opposing frame members. Twoopposing crimp inserts are placed between the frame members.The inserts include on their mutually facing sides recesses forcrimping a sleeve of corresponding diameter on a correspondingcable. A screw meshes with a threaded hole through one framemember, so that the screw can be screwed in a direction towardsthe second frame member such as to bring the two insertstogether. The inserts include means for guiding relativemovement in said direction. The two inserts also include springmeans which strive to move the inserts apart in said direction.ï»¿1015202530CA 02265855 l999-03- llwo 93/15044 PCT/SE97l016l74The two frame members have shallow recesses for receiving andlocalizing respective inserts. The tool can be used inconjunction with a set of insert pairs which each includecrimping recesses that are adapted to crimp sleeves ofcorrespondingly different sizes. In other respects, the insertpairs have generally equal outer dimensions in order to enablesaid insert pairs to be readily swapped in the frame structure.Owing to the relative guiding of the inserts and the springmeans, the pair of inserts will be held automatically in theframe structure immediately the inserts are inserted thereinto,and can readily be replaced manually with another pair ofinserts, by first pressing the fitted pair of inserts togetheragainst the action of the spring means and then tipping saidinserts out one of the recesses and out of the frame structure.The recesses in the frame members are shallow recesses. Theframe structure may conveniently have an elongated support armand the screw may include a lever for facilitating rotation ofthe screw. ~The invention will now be described in more detail withreference to an exemplifying embodiment thereof and also withreference to the accompanying drawings.Figure 1 shows an inventive strain relief fitted to the end ofa cable and connected to the wall of an apparatus housing.Figure 2 is a crossâsectional view taken on the line A-A inFigure 1 and images the strain relief prior to being crimped onthe end of the cable.Figure 3 illustrates the configuration of the strain reliefsubsequent to being crimped on the end of the cable.Figure 4 shows a tool for crimping the strain relief sleeve.Figure 5 is a sectional view of a crimping tool insert.ï»¿1015202530CA 02265855 2004-12-23W0 98,150.â PC'IâISE9â7I0l6l'IFigures 1 and 2 illustrate a cable 1 comprising a core 2 thatincludes a plurality of insulated conductors 24, said core 2being provided with screening braiding 3 which, in turn, isembraced by a cable casing 4. A length of casing 4 has beenremoved from the outermost end of the cable 1 and the braidingfolded back over the remaining end part of the casing 4. Astrain relief sleeve 10 is shown fitted over the backwardlyfolded screening part 3â of said end section. The front part ofthe sleeve 10 includes an externally threaded section 12 andalso an outwardly open circumferential groove 11 between itsends. An opening 13 through the casing wall, for instance inthe groove 11, enables it to be ascertained whether or not thescreen 3â is located immediately beneath the sleeve 10. Figure1 also shows the wall 20 of an apparatus housing. The wall 20includes an opening 21 having a thread that will mesh with theouter thread 12 on the sleeve. The sleeve 10 is anchored to thewall 20 by means of the screw joint 12, 22. The sleeve 10 liesin intimate contact with the screen 3, 3â around the whole ofits circumference and tightly engages the wall 20 via the screwjoint, so that the sleeve 10 will provide an EMI-tightconnection to the apparatus housing (provided that the sleeve10 and the wall 20 are made of a suitable material in thisrespect).The sleeve 10 is crimped onto the cable 1 so as to obtain anEMI-tight connection therewith. By crimping is meant that thesleeve 10 is subjected to plastic deformation from anessentially circular, round state into a polygonal shape, asillustrated in Figure 3. Such crimping of the strain reliefsleeve 10 provides a highly durable anchorage of the end of thecable 1 to the sleeve 10 with respect to tensile forces orï»¿1015202530CA 02265855 l999-03- 11W0 9915944 PCT/SE97/016176strain acting in the length direction of the cable, whileproviding, at the same time, an EMI-tight connection betweenthe sleeve 10 and the cable 1 and subjecting the insulatedconductors 2% to stresses that are generally safe with respectto the integrity of the conductors 24.The crimping technique requires a larger material thickness ofthe sleeve 10 in the case of large diameter cables 1 (withmaintained crimping deformation pattern), which normally meansthat the crimping tool must exert crimping forces that increasemarkedly with increasing diameters of the cable 1 and thesleeve 10, by providing larger sleeves 10 with acircumferential groove 11 between its ends, and can limit theincrease in requisite crimping forces that must be exerted bythe crimping tool. This enables the use of a simple, andtherewith cost favorable tool that includes exchangeable_ inserts that cover a whole series of differently sized sleeves10 adapted to cables 1 of standard sizes.Figure 4 and 5 illustrate one such simple tool 30 withassociated inserts 40.The crimping insert 40 is comprised of two mutually coâactingblocks 41, 42, which may be mutually identical. Each of theblocks 41, 42 has an orthogonal parallelepipedic shape and eachof the mutually opposing sides of the blocks includes a recess43, said recesses together defining an hexagonal opening whenthe blocks 41, 42 are in mutual abutment. The hexagonal shapecorresponds to the final external shape 10â (Figure 3) of thecrimped sleeve 10. Each block 41, 42 has a guide pin 44 on oneside of the recess 43 and a corresponding aperture or hole 45on the other side thereof. A helical spring 48 is placed on theï»¿1015202530CA 02265855 l999-03- 11W0 98,150â PCT/SE97/016177bottom of each aperture 45. Each block 41, 42 is therewithdesigned so that the pin 44 of one block 41 will fit into thehole 45 in the other block, and vice versa.Although the blocks 41, 42 are mutually identical as a resultof the illustrated construction of the insert 40, it will beobvious 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 functionof said blocks. Figure 4 illustrates a tool handle 31 which hasat one end a frame structure 50 formed by the end-part 32 ofsaid handle, a so-called yoke 33 which is carried by two bolts34 that extends perpendicularly through the yoke 33 and throughthe handle part 32 and take up forces that strive to move theyoke 33 away from the handle part 32. The yoke has a recessedpart 36 which receives an adjacent end of the block 42. A guideplate 35 is carried on the inside of the frame structure 50,parallel with the yoke 33. The guide plate has a recess part 36with a bottom plate 39 which is movable along the bolts 34 andwhich lies normal to the handle part 32.When the blocks 41, 42 (Figure 5) are pressed together such asto bring their adjacent surfaces 46 essentially into contactwith one another, the insert 40 can be inserted laterally intothe frame structure 50 in alignment with the recess 36,whereafter the insert 40 is allowed to expand under the actionof the spring 48 to the state shown in Figure 4, where theinsert 40 is thus held by the expansion forces of the springs48. The insert 40 can, nevertheless, be easily removed from theframe structure, by first compressing the insert and thentilting it out of the frame structure 50.ï»¿1015202530CA 02265855 l999-03- 11W0 98,150â PCT/SE97/016178Shown in Figure 4 is a screw 37 which is threaded through athrough-penetrating opening in the handle part 32 and actsagainst the bottom plate 39 in the recess 36 in the plate 35,such as to push the plate 35 towards the yoke 33. The screw 37is provided with a long, lateral lever 38 by means of whichheavy torque can be exerted manually on the screw 37, whilerotational forces can be counteracted at the same time, byholding against the handle end 31. The screw 37 and its co-acting thread in the handle 31 form a transmission mechanismfor linear movement of the insert blocks 41, 42 towards oneanother when crimping a sleeve 10 on the endâsection of a cable1.A series of inserts 40 that have essentially identical externaldimensions but recesses 43 of mutually different sizes can beused in conjunction with the tool 30. These inserts 40 can bereadily exchanged in the tool 30, which has an extremely simpleconstruction as evident from the aforegoing.When the crimp sleeve has been crimped firmly to the cable witha radial load that is distributed generally uniformly in thelength direction of the sleeve, the sleeve will be deformedradially more pronouncedly at its ends (i.e. obtain a smallerdiameter) than in its central region. This effect is apparentlydue to weakening of the sleeve wall by the centre groove 11.This results in the section of cable located in the sleevebetween its ends being stretched axially to some extent, whichcould be detrimental to the conductors in the cable. Instead,the cable section is compressed in the crimp sleeve, wherewithinclination of the end-edges of the sleeve provides aparticularly effective transfer of axial forces between thecrimped sleeve and the cable.ï»¿1015CA 02265855 l999-03- llwo 93/15044 PCT/SE97/01617It will be noted in particular that the crimped sleeve and thecrimping force engage all conductors, conductor insulation,screens and the like in the cable, so that all cable parts willobtain an axial force-coupling to one another and to thecrimped sleeve, such that said cable components will notexperience any relative axial movement when axial forces areapplied. Furthermore, the crimping affords radial compressionof the cable, so that the cable will be sealed against axialfluid throughflows between the cable components. It will alsobe 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 Figure 1 to be providedwith an axial tubular extension having an outer thread, it willbe understood that this extension can be omitted and a separatesleeve nut or the like that axially couples the sleeve to anexternally thread leadthrough sleeve on an apparatus housing orthe like instead.

Claims

CLAIMS:

1. A strain relief for a screen cable (1), for use in an opening through a wall (20) of an apparatus housing, comprising a plurality of conductors (24) which are surrounded by a common screen, and a cable casing embracing said screen, wherein the strain relief includes a sleeve (10), characterized in that the strain relief sleeve (10) is a crimp sleeve having a tubular wall with an inspection opening (13) therethrough, said tubular crimp sleeve being crimped on the cable (1) on the screen (3') folded back over the cable casing;
and further characterized in that the crimp sleeve (10) embodies means (12) for coupling the strain relief sleeve (1) to said wall (20) of the apparatus housing via a cable leadthrough or transit (21).

2. A strain relief according to claim 1, characterized in that in a series of crimp sleeves (10) of mutually different diameters, at least one sleeve of larger diameter has a circumferential groove (11) which functions to reduce the force required to crimp said sleeve.

3. A strain relief according to claim 1 or claim 2, characterized in that the crimp sleeve (10) and the apparatus housing wall (20) are adapted to provide an EMI-tight connection of the cable (1) to the interior of the housing.