Patent Publication Number: US-8125129-B2

Title: Working electrode for an electrodynamic fragmenting installation

Description:
TECHNICAL FIELD 
     The invention concerns a working electrode for an electrodynamic fragmenting installation, changing parts for such a working electrode as well as a use of the working electrode according to the preambles of the independent claims. 
     PRIOR ART 
     In the electrodynamic fragmentation of material, like e.g. concrete, between a working electrode, which is charged with high voltage pulses, and a base electrode, which typically is at zero-potential, high voltage breakdowns are generated through the material that shall be fragmented, whereby a fragmentation of said material is achieved. At each high voltage breakdown, there also occurs a slight material removal at the tip of the working electrode, so that after a certain operating time it is worn-out and needs replacement. Replacement of the electrode might also be necessary upon a change of the material that is to be fragmented by the installation, in order to avoid a contamination of the final product with an undesirable electrode material. In both cases it is, at the electrodynamic fragmenting installations known today, necessary to exchange the entire working electrode including insulator, which is a cost-intensive and time consuming undertaking, last but not least because the working electrodes are, at their connecting side, typically coupled to a system filled with insulating oil. Therefrom results as a further disadvantage that it is uneconomic to use up not completely exhausted electrodes, since the installation work is huge compared to the residual usage time. 
     DISCLOSURE OF THE INVENTION 
     Thus, it is the objective to provide a working electrode which does not have the disadvantages of the prior art or at least partially avoids them. 
     This objective is achieved by the working electrode and the changing parts for such a working electrode according to the independent claims. 
     Accordingly, a first aspect of the invention relates to a working electrode for an electrodynamic fragmenting installation having an exchangeable electrode tip. The working electrode comprises an insulator, e.g. made of plastics or of a ceramic material, with a central conductor made of a material which is electrically well conductive, preferably of a metallic material, e.g. of aluminum, copper or stainless steel, which axially penetrates the insulator. At one end, the central conductor is adapted for connection to a high voltage generator in order to charge the working electrode with high voltage pulses. At its other end, the so called working end, which in operation is immersed into the working area that is filled with process fluid, e.g. water, and the material to be fragmented, the central conductor carries an electrode tip, which in operation forms the starting point for the high voltage breakdowns. The electrode tip is formed by an exchangeable changing part that is of one-piece design or is formed by several pieces. 
     Such working electrodes provide the advantage that upon wear of the electrode or when the material to be fragmented changes, merely the tip of the electrode needs to be exchanged, and e.g. an opening of an oil-filled high voltage system in order to exchange the entire working electrode becomes redundant. By means of this, the maintenance related downtimes and operating costs of electrodynamic fragmenting installations can considerably be reduced. 
     In a preferred embodiment of the working electrode, the changing part comprises a contact area which serves as axial stop of the changing part at the central conductor and at the working end of the central conductor abuts under axial compressive prestress against a stop area of the central conductor. The contact area of the changing part and/or the stop area of the central conductor can e.g. be designed as edgeless stop areas with mere radial extent or also as cone shaped surfaces having a radial and axial extent. Such working electrodes are especially reliable in operation. 
     In a further preferred embodiment of the working electrode, the changing part is, at the end facing away from the electrode tip, connected with the central conductor via a first threaded connection, for fastening the changing part at the central conductor and for generating the compressive prestress between the contact area and the stop area. By means of this, an uncomplicated exchangeability and a safe fastening of the changing part results. 
     In this embodiment it is preferred that the exchangeable changing part between the first threaded connection and the contact area comprises an elongation area, preferably with a length of at least two times, more preferably of at least four times the diameter of the first threaded connection, which according to the principle of an anti-fatigue bolt through elastic elongation is under tensile prestress and thereby generates the compressive prestress between the contact area and the stop area. Preferably, the elongation area of the changing part is designed as anti-fatigue shaft or as anti-fatigue sleeve, wherein in the first mentioned case it preferably forms the outer thread at one end and in the latter case preferably the inner thread of the first threaded connection. 
     Alternatively or additionally it is preferred that the central conductor comprises, between the first threaded connection and the stop area, an elongation area, preferably with a length of at least two times, more preferably of at least four times the diameter of the first threaded connection, which according to the principle of an anti-fatigue bolt through elastic elongation is under a tensile prestress and thereby generates the compressive prestress between the contact area and the stop area. Preferably, the elongation area of the central conductor is designed as anti-fatigue shaft or as anti-fatigue sleeve, wherein in the first mentioned case it preferably forms at one end the outer thread and in the latter case preferably the inner thread of the first threaded connection. 
     Such working electrodes having elongation areas especially provided by their design are robust and can be operated even at strong pressure pulsations in the working area over a long time without any maintenance, since between the central conductor and the exchangeable changing part merely pulsating forces occur, but no alternating forces. 
     In a further preferred embodiment, the changing part is designed as one piece, in another it is formed by several parts, which in the first case provides the advantage of a simple, robust construction and in the latter case provides greater freedom for designing the changing part. 
     In the above case when the changing parts are formed by several parts, it is preferred that the contact area of the changing part is formed by a stop member that is designed preferably as screw nut, preferably as hexagonal nut or screw nut with frontal holes, which stop member together with a further part of the changing part, which further part forms the outer thread or the inner thread of the first threaded connection and is made in one piece together with the electrode tip, forms a second threaded connection. By this it becomes possible to first fasten the electrode tip or the part of the changing part forming said tip, respectively, by means of the first threaded connection to the central conductor and then to generate, by means of the stop member and the second threaded connection, the compressive prestress between the contact area and the stop area. This is particular of advantage in cases where embodiments having long elongation areas between the first and the second threaded connection are used, since in these cases a pre-stressing is possible also without introduction of substantial torsional forces in the elongation area, so that the elongation area can optimally be designed for its function. 
     In still a further preferred embodiment of the working electrode with a first threaded connection according to the claims, the changing part comprises between the electrode tip and the contact area, e.g. shortly before the electrode tip, an area having a not rotationally symmetrical cross section, so that its contour can be engaged in a positive manner with a screwing tool for the screwing and unscrewing of the changing part. In the before mentioned embodiment, it is by this furthermore possible to secure the body of the changing part against turning during tightening of the second threaded connection, by means of which a introduction of torsional forces into the elongation area of this embodiment can completely be obviated. 
     By advantage, the changing part, in order to achieve this, comprises, in the area between the electrode tip and the contact area, at least two parallel surfaces. Such surfaces are easy to produce and permit the turning or securing against turning of the changing part by means of commercially available flat spanners. 
     In another preferred embodiment of the working electrode, the changing part comprises in an area adjacent to its contact area at its outer circumference a circumferential, radial bead. In embodiments in which the stop member is designed e.g. as a screw nut which provides the contact area according to the claims, it is preferred that this stop member at its end facing towards the central conductor comprises at its outer circumference a circumferential, radial bead. 
     In both cases, the radial, circumferential bead serves for the field relief in the area where the central conductor protrudes out of the insulator, by means of which the operating life of the insulator and of the central conductor can considerably be increased. 
     In another preferred embodiment of the working electrode the changing part is in a non-positive manner by means of clamping mounted in a frontal opening in the working end of the central conductor, which preferably is achieved in that the changing part comprises a preferably cylindrical expansion sleeve and a spacing body that at least partially is arranged inside the expansion sleeve, by means of which the expansion sleeve can be expanded in an area in such a manner that it is radially pressed against the wall of the frontal opening and thereby is axially non-displaceable clamped inside the opening. By this, a save fastening of the changing part at the central conductor can be achieved in an easy way. 
     In that case it is of advantage when the spacing body is connected with a driving member for an axial displacing of same relative to the expansion sleeve in order to effect a radial expanding of the expansion sleeve, which driving member protrudes out of the frontal opening at the working end of the central conductor and at it end facing away from the spacing body forms the electrode tip. By means of this it is possible to effect the clamping of the changing part in the central conductor in a simple way by exerting an axial force on the driving member. In case the spacing body and the driving member are formed together as one piece, which is preferred, a particular simple and robust construction results. 
     It also is preferred in this case that the spacing body comprises a preferably conical or pyramidal section for the radial expanding of the expansion sleeve or as a whole is designed as a truncated cone or a frustum of pyramid, since by this huge expanding forces can be generated in a controlled manner. 
     By advantage, the driving member comprises between the electrode tip and the spacing body an outer thread, by means of which an axial force can be exerted onto it for causing a displacement of the spacing body and a resulting radial expanding and clamping of the expansion sleeve in the opening in the central conductor. In this way, relative large displacement forces can be provided in a controlled manner. 
     If in the above case the spacing body is designed in such a manner that an axial displacement of same in direction towards the working end of the central conductor causes a radial expanding of the expansion sleeve, thus the driving member must transmit tensile forces for effecting the clamping of the changing part in the central conductor, it is of advantage when the outer thread of the driving member interacts for generating the axial displacement forces with a respective inner thread of an abutment member, which axially rests on the expansion sleeve. By this a simple construction with only few parts results. 
     Furthermore it is preferred in the before mentioned embodiment when the abutment member is a hexagonal nut or a screw nut with at least two frontal openings, which preferably at its outer circumference comprises a circumferential, radial bead which can serve as field relief. 
     By advantage, the driving member comprises in that case between the spacing body and the outer thread an elongation area preferrably designed as anti-fatigue shaft or as anti-fatigue sleeve, by advantage with a length of at least two times, preferably at least four times of the diameter of the outer thread. 
     Is the spacing body however designed in a manner that an axial displacement of the same in direction away from the working end of the central conductor causes a radial expanding of the expansion sleeve, thus the driving member must transmit compressive forces for effecting the clamping of the changing part in the central conductor, it is preferred that the outer thread of the driving member interacts with a respective inner thread of an abutment member, which axially is connected with the expansion sleeve for transmitting axial tensile forces between the abutment member and the expansion sleeve. If, in the above case, the abutment member is formed as one piece with the expansion sleeve, which is preferred, an as compact as possible construction with a minimum of parts results. 
     By advantage, the expansion sleeve in this case comprises, in the area between the abutment member and the area where it is radially expanded by the spacing body, an elongation area, which preferably has a length of at least two times, more preferably at least four times the diameter of the inner thread of the abutment member. 
     As has already been presented on the basis of some before mentioned preferred embodiments, by providing, on the part of the design, elongation areas for the transmission of the forces that are necessary for generating the compressive forces between parts of the exchangeable changing part and the central conductor, alternating forces between these parts can be avoided even at strong pressure pulsations in the working area, so that especially robust and over a long time maintenance free working electrodes can be made available. 
     Preferably, the driving member comprises, between electrode tip and spacing body, an area of non rotationally symmetrical cross-section, preferably at least two parallel surfaces, which can be engaged in a positive manner with a tool, like e.g. a flat spanner, in order to turn the driving member relative to the expansion sleeve and/or for temporarily securing of same against turning. 
     In still another preferred embodiment of the working electrode, a gasket is arranged between the changing part and the central conductor, preferably an O-ring, for preventing process fluid and dirt from entering into the fastening area between the changing part and the central conductor. In particular in embodiments, in which the changing part is connected with the central conductor via the before mentioned first threaded connection, a fouling and damaging of same can be avoided by means of this. 
     In still a further preferred embodiment of the working electrode, the central conductor comprises, in the area of its working side end where it protrudes out of the insulator, at its outer circumference a circumferential, radial bead. 
     In still a further preferred embodiment of the working electrode, the central conductor comprises, in the area where at its working end it protrudes out of the insulator, an area with a not rotationally symmetrical cross-section, preferably two parallel surfaces, for the positive interaction with a tool, like e.g. a flat spanner. 
     Alternatively or additionally it is preferred that the central conductor, at its working end sided face, comprises at least two frontal holes for the positive interaction with a face spanner. 
     Through these embodiments it becomes possible to secure the central conductor during assembly and/or disassembly of the changing part against a turning within the insulator, which at central conductors that are non-positively, e.g. by force fitting or by shrinking, fastened inside the insulator can lead to a loosening or destruction of the interconnection with the insulator. 
     In preferred embodiments of the working electrode the electrode tip has the shape of a spherical calotte or of a rotation paraboloid. Such shapes provide a locally defined breakdown initiation point, and at the same time a sound service life of the electrode tip. 
     A second aspect of the invention relates to a changing part for a working electrode according to the first aspect of the invention. The changing part comprises an elongate, electrically conductive base body, preferably made of a metal or a metal alloy, which at one end carries a first outer thread for fastening of same at a central conductor of a working electrode and at its other end an electrode tip. Between the electrode tip and the first outer thread there is arranged a second outer thread, which is intended for the screwing on of a stop member having a contact area for axial abutment against a stop area at the central conductor. Between the electrode tip and the second outer thread the base body comprises an area having a non rotationally symmetrical cross-section, so that in a rotational direction around its longitudinal axis it can be positively gripped with a suitable screwing tool, for the screwing and unscrewing, respectively, of the base body in the central conductor of the working electrode and for securing the same against turning during the tightening of a screw nut type stop member arranged on the second outer thread for generating a compressive prestress between the contact area of the stop member and the stop area of the central conductor. Preferably the base body, in order to make this possible, comprises, in the area between the electrode tip and the second thread, at least a pair of parallel surfaces, which can interact with a flat spanner of suitable size. Furthermore, the base body comprises between the first outer thread and the second outer thread an anti-fatigue shaft, preferably with an anti-fatigue shaft length of at least two times, preferably at least four times the diameter of the first outer thread. 
     In a preferred embodiment of the changing part it furthermore comprises, arranged at the second outer thread, a stop member with a contact area for axial abutment against a stop area of the central conductor, which preferably also comprises at least two parallel surfaces for the positive interaction with a flat spanner. 
     A third aspect of the invention relates to another changing part for a working electrode according to the first aspect of the invention. The changing part as well comprises an elongate electrically conductive, preferably metallic base body, which at one end carries an inner thread for fastening the changing part at a central conductor of a working electrode and at its other end ends in an electrode tip. Between the electrode tip and the inner thread there is arranged a stop shoulder for axial abutment against a stop area of the central conductor. Furthermore the base body comprises between the stop shoulder and the electrode tip an area having a non rotationally symmetrical cross section, so that in rotational direction around its longitudinal axis it can positively be driven by means of a suitable screwing tool, for screwing the base body in the receiving opening of the central conductor of the working electrode in order to fasten the changing part at the central conductor and to generate a compressive prestress between the contact area of the stop member and the stop area of the central conductor. For this, the base body preferably comprises, in the area between the electrode tip and the stop member, at least one pair of parallel surfaces, which by means of a flat spanner of a respective size can be engaged. Between the inner thread and the stop shoulder, the base body of the changing part is designed as anti-fatigue sleeve, preferably with an anti-fatigue sleeve length of at least two times, preferably at least four times the diameter of the inner thread. 
     In that case it is preferred that the changing part is of one-piece design. 
     Furthermore it is preferred that the stop shoulder is formed of a circumferential, radial bead of the changing part, which can serve as field relief. 
     A fourth aspect of the invention relates to a changing part for a working electrode according to the first aspect of the invention. The changing part comprises an expansion sleeve and a preferably conical or pyramidal spacing body, which is arranged at least partially inside the expansion sleeve and interacts therewith in such a manner that the expansion sleeve through an axial displacement of the spacing body relative to it can be radially expanded in an area, preferably in an end area of the expansion sleeve. In that case, the spacing body is preferably through material connection, like e.g. through one piece design or by soldering or welding, connected with a driving member for the displacement of the spacing body within the expansion sleeve, which driving member at its end facing away from the spacing body protrudes out of the expansion sleeve and at this end forms an electrode tip with the shape of a spherical calotte or of a rotation paraboloid. Between the electrode tip and the spacing body, the driving member comprises an outer thread, on which a preferably screw nut type abutment member with a respective inner thread is arranged. The abutment member axially rests at the expansion sleeve, so that a rotating of same relative to the driving member can effect an axial movement of the spacing body which is connected with the driving member in direction towards the electrode tip, which in turn causes an increasing expanding of the expansion sleeve. 
     In a preferred embodiment of the changing part, the abutment member is designed as a screw nut with frontal holes, preferably with at least two, more preferably with at least four frontal holes distributed with an equal indexing. In this case it is furthermore preferred that the screw nut with frontal holes at its outer circumference forms a circumferential radial bead, and more preferably, that it has substantially the form of a washer having rounded circumferential edges. By this, the abutment member can also serve as field relief. 
     In a further preferred embodiment of the changing part, the driving member comprises between the spacing body and the outer thread an elongation area which preferably is designed as anti-fatigue shaft or anti-fatigue sleeve, preferably with a length of at least two times, more preferably of at least four times the diameter of the outer thread. 
     A fifth aspect of the invention relates to a changing part for a working electrode according to the first aspect of the invention. The changing part comprises an expansion sleeve and an in particular conical or pyramidal spacing body for a radial expanding of the expansion sleeve upon an axial displacement of said spacing body relative to the expansion sleeve. In that case, the spacing body is, preferably by material connection, like e.g. through one piece design or through welding or soldering, connected with a driving member for a displacement of the spacing body inside the expansion sleeve. The driving member at its end facing away from the spacing body protrudes out of the expansion sleeve and at this end is designed as an electrode tip having the shape of a spherical calotte or of a rotation paraboloid. Between the electrode tip and the spacing body the driving member comprises an outer thread, which interacts with a respective inner thread of an abutment member. The abutment member is connected, preferably through one piece design, with the expansion sleeve, so that a transmission of axial tensile forces between the abutment member and the expansion sleeve is possible and through rotation of the driving member relative to the abutment member an axial movement of the spacing body in a direction pointing away from the electrode tip can be effectuated, which in turn leads to an increasing expanding of the expansion sleeve. 
     In a preferred embodiment of the changing part, the expansion sleeve comprises in the area between the abutment member and the area, where it is radially expanded by the spacing body, an elongation area, preferably with a length of at least two times, more preferably of at least four times the diameter of the inner thread of the abutment member. Typically, such elongation areas are identifiable in that they show a reduced cross-section in order to arrive at an as little rigid as possible elongation characteristic. 
     The changing parts according to the second, third, fourth and fifth aspect of the invention constitute preferred trade goods and allow for the construction of working electrodes in which the electrode tip can in a simple manner be exchanged, without disconnecting the electrode from the voltage supplying system. 
     A sixth aspect of the invention relates to the use of the working electrode according to the first aspect of the invention for the electrodynamic fragmentation of preferably poorly conductive materials like concrete or slag. In such uses, the advantages of the invention become particular clearly apparent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further embodiments, advantages and applications of the invention become apparent from the depending claims and from the following description with reference to the drawings. Therein show: 
         FIG. 1  a longitudinal section through the working end of a first working electrode according to the invention; 
         FIG. 2  a lateral view of the changing part according to the invention of the working electrode of  FIG. 1 ; 
         FIG. 3  a longitudinal section through the working end of a second working electrode according to the invention; 
         FIG. 4  a longitudinal section through the working end of a third working electrode according to the invention; 
         FIG. 5  a longitudinal section through the working end of a fourth working electrode according to the invention; 
         FIG. 6  a longitudinal section through the working end of a fifth working electrode according to the invention; 
         FIG. 7  a longitudinal section through the working end of a sixth working electrode according to the invention; and 
         FIG. 8  a longitudinal section through the working end of a seventh working electrode according to the invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
       FIG. 1  shows the working end of a first working electrode according to the invention in a longitudinal section. As is visible here, the electrode comprises a cylindrical and towards the working end truncated cone shaped insulator  1  made of a thermoplastic synthetic material, in the present case polyethylene, with a central conductor  2  made of stainless steel arranged in its center, which is force fitted into the insulator  1  and thereby is secured therein in a play free manner. The central conductor  2  at its working end sided face comprises in the border area two equally distributed small frontal holes  23  as well as a larger central blind hole bore, which towards the working end, at which the central conductor  2  protrudes out of the insulator  1  thereby forming a circumferential radial bead  14 , is opened and in the area of its closed end forms an inner thread. Arranged in the central bore of the central conductor  2  is a changing part  4  according to the claims, which by means of an end sided outer thread  15  is screwed into the inner thread of the central bore and thus under formation of a first threaded connection  7  according to the claims is affixed to the central conductor  2 . 
     As can be seen when additionally considering  FIG. 2 , which shows the changing part  4  according to the invention in a lateral view, the changing part  4  forms, at its other end, a hemispherical electrode tip  3 , which during operation serves as starting point for the high voltage breakdowns. Between electrode tip  3  and the first threaded connection  7  the changing part  4  comprises a second outer thread  16 , which carries a hexagonal nut  10  that serves as stop member  10  according to the claims, thereby forming a second threaded connection  11  according to the claims. In doing so, the screw nut  10  abuts with its face  5  showing away from the working end, which face forms the contact area  5  according to the claims, axially edgeless and under compressive prestress against the working sided front face  6  of the central conductor  2 , which forms the stop area  6  according to the claims and fluently passes over into the bead  14 . In order to ensure the existence of a compressive prestress between the contact area  5  and the stop area  6  at any time even at heavy pressure pulsations, the changing part  4  comprises in the area between the first thread  7  and the second thread  11  an elongation area designed as anti-fatigue shaft  8 , which has a length of about three times the diameter of the first threaded connection  7 . In order to prevent process liquid and dirt from entering into the central bore in the central conductor  2 , between the anti-fatigue shaft  8  and the second threaded connection  11  there is arranged an O-ring  13  in a circumferential groove on the changing part  4 , which seals the annulus shaped gap formed between the changing part  4  and the wall of the central bore. Furthermore the changing part  4  comprises in the area between the second threaded connection  11  and the electrode tip  3  four surfaces  12  that are arranged relative to each other in each case under an angle of 90°, which can interact with a flat spanner in order to screw and unscrew the changing part  4  into the central conductor  2  and out of same and/or to secure the changing part  4  against turning during the tightening of the second threaded connection  11 . 
     In case it is desired to exchange at the shown working electrode the electrode tip  3 , be it because said tip is worn out or be it because a tip of different material shall be used, first of all the central conductor  2  is secured against turning inside the insulator  1  by means of a face spanner that engages the two frontal holes  23  and, as the case may be, also the changing part  4  is secured against a turning inside the central conductor  2  by means of a flat spanner that engages the surfaces  12  and thereafter the screw nut  10  on the second outer thread  16  is untightened by means of a spanner. Subsequently, the changing part  4  with the aid of a flat spanner is screwed out of the central conductor  2 . Thereafter, a new or different changing part  4  is screwed into the central bore of the central conductor  2  and subsequently the screw nut  10  of this changing part  4  is tightened with a specific torque in order to generate the compressive prestress between the contact area  5  and the stop area  6 , whereby the anti-fatigue shaft  8  is elastically elongated under tensile stress. At the same time, the changing part  4  with a flat spanner at the surfaces  12  and the central conductor  2  by means of a face spanner engaging its two frontal holes  23  are secured against turning in order to avoid a torsion loading of the anti-fatigue shaft  8  and to avoid a turning of the central conductor  2  in the insulator  1 . Preferably, the face spanner for securing the central conductor  2  against turning and the flat spanner for securing the changing part  4  against turning are formed by only one special tool, so that the assembly/disassembly is facilitated and a turning of the changing part  4  relative to the central conductor  2  during a tightening or untightening, respectively, of the screw nut  10  is precluded from the outset. 
       FIG. 3  shows the working end of a second working electrode according to the invention in a longitudinal section. As can be seen, the electrode in this case as well comprises a cylindrical and towards the working end truncated cone shaped insulator  1 , in the center of which there is arranged a central conductor  2  formed by a press fitted cylinder sleeve  19  with a tension anchor  20  fastened inside the sleeve  19  which is having an outer thread. The cylinder sleeve  19  is opened towards the working end and receives inside this opening a changing part  4  according to the claims that is of one piece design, which is within the cylinder sleeve  19  by means of an end sided inner thread  17  formed by it bolted together with the outer thread of the tension anchor  20  and thereby fastened to the central conductor  2 , while forming a first threaded connection  7  according to the claims. At its other end, the changing part  4  forms an electrode tip  3  with the shape of a rotation paraboloid. Between the electrode tip  3  and the inner thread  17  of the first threaded connection  7  the changing part  4  comprises a circumferential radial bead  14 , which serves as field relief and constitutes a stop shoulder  18  according to the claims, which provides the contact area  5  according to the claims, by which the changing part  4  axially abuts under compressive prestress against the face  6  of the cylinder sleeve  19  of the central conductor  2  protruding out of the insulator  1 , which face forms the stop area  6  according to the claims. In order to ensure the existence of a compressive prestress between the contact area  5  and the stop area  6  at any time even at heavy pressure pulsations, the changing part  4  comprises in the area between the first thread  7  and the stop shoulder  18  an elongation area designed as anti-fatigue sleeve  9 , which has a length of about three times the diameter of the first threaded connection  7 . In order to facilitate the screwing and unscrewing of the changing part  4  into the central conductor  2  and to permit a tightening of the first threaded connection  7  for generating the compressive prestress between the contact area  5  and the stop area  6 , the changing part  4  comprises, in the area between the stop shoulder  18  and the electrode tip  3 , two parallel surfaces  12 , which can be engaged with a flat spanner. 
     In case it is desired to exchange at the working electrode shown in  FIG. 3  the electrode tip  3 , the changing part  4  is screwed out of the cylinder sleeve  19  of the central conductor  2  by catching the two surfaces  12  with a suitable flat spanner. Thereafter, a new or different changing part  4  is screwed into the central bore of the central conductor  2  and is tightened with a specific torque, so that via the threaded connection  7  between the changing part  4  and the tension anchor  20  of the central conductor  2  a desired compressive prestress between the contact area  5  and the stop area  6  is generated, in that the anti-fatigue sleeve  9  is elastically elongated under tensile stress. 
       FIG. 4  shows the working end of a third working electrode according to the invention in a longitudinal section. As can be seen, also this electrode comprises a cylindrical and towards the working end truncated cone shaped insulator  1 , in the center of which there is arranged a central conductor  2 . In this case, the central conductor  2  consists of a cylindric metal rod  21  that is press fitted into the insulator  1 , which at the working end of the electrode, at which it protrudes out of the insulator  1 , comprises a central blind hole bore and, arranged therein, an anti-fatigue shaft bolt  22 . The anti-fatigue shaft bolt  22  is fastened with its end facing away from the working end in the central bore by screwing-in in an inner thread at the end thereof and protrudes with its other, working side end, which as well carries an outer thread, out of the central bore of the metal rod  21 , where it forms, together with the inner thread of a cap screw nut shaped changing part  4  a first threaded connection  7  according to the claims. In order to preclude a detrimental torsion loading of the anti-fatigue shaft  8  from the outset, there is arranged between the anti-fatigue shaft bolt  22  and the cylindrical metal rod  21 , in an area direct adjacent to the first threaded connection  7 , a feather key  30 , which after the screwing-in of the anti-fatigue shaft bolt  22  into the cylindrical metal rod  21  has been installed by pushing it in oppositely arranged feather key grooves  31 ,  32  in the thread of the anti-fatigue shaft bolt  22  and in the wall of the central blind hole bore. The changing part  4  in this case is fastened to the central conductor  2  by screwing it onto the threaded end of the anti-fatigue shaft bolt  22 , whereat the front face  5  of the changing part  4  that faces away from the from the working end, which end face forms a contact area  5  according to the claims, under a compressive prestress generated trough an elastic elongation of the anti-fatigue shaft bolt  22  axially abuts against the front face  6  of the cylindrical metal rod  21 , which front face constitutes a stop area  6  according to the claims. As has been indicated earlier, in the present case the changing part  4  is designed similar to a cap screw nut, by comprising a hexagonal area with three pairs of in each case parallel surfaces  12  for the interaction with a screw wrench and a cap  3  with the shape of a rotation paraboloid, protruding from said area, which cap constitutes the electrode tip  3  according to the claims. In case this tip  3  is worn out or a different electrode material is desired, this changing part  4  can be disassembled without difficulty with a screw wrench and be replaced by a new or different one. For ensuring the desired compressive prestress between the contact area  5  and the stop area  6 , the newly assembled changing part  4  is advantageously tightened with a specific torque by means of a torque wrench. 
       FIG. 5  shows the working end of a fourth working electrode according to the invention in a longitudinal section, which substantially differs from the working electrode shown in  FIG. 4  in that the contact area  5  of the changing part  4  ends in a circumferential radial bead  14 , which serves as field relief in the transition area between the insulator  1  and the central conductor  2 . 
       FIG. 6  shows the working end of a fifth working electrode according to the invention in a longitudinal section. As can be seen, the electrode comprises a cylindrical and towards the working end stepwise truncated cone shaped insulator  1  made of synthetic material, with a central conductor  2  of stainless steel arranged in its center, which is press fitted into the insulator  1 . The central conductor  2  comprises at its working end sided face a central cylindrical blind hole bore, which towards the working end, at which the central conductor  2  protrudes out of the insulator  1  under formation of a circumferential, radial bead  14 , is opened. Arranged in the central bore of the central conductor  2  is a changing part  4  according to the claims, comprising a cylindrical, at one end slotted expansion sleeve  24  (not shown in section), which by means of a truncated cone shaped spacing body  25  is radially expanded at its slotted end in such a manner that in the area of this end it is radially pressed against the wall  26  of the blind hole bore and thereby is clamped inside the blind hole bore in an axially non-displaceable manner. The spacing body  25  is formed in one piece design together with a driving member  27  for the axial displacement of same in the expansion sleeve in order to effectuate the radial expanding of the expansion sleeve, which driving member at its end facing away from the spacing body  25  protrudes out of the expansion sleeve  24  and at this end ends in an electrode tip  3  having the shape of a spherical calotte. Between the electrode tip  3  and the spacing body  25  the driving member  27  comprises an outer thread  28 , on which there is arranged an abutment member  29  (not shown in section) with a respective inner thread, which is designed as a hexagonal screw nut. The abutment member  29  axially rests on the expansion sleeve  24  (not on the interior conductor  2 ), so that a rotation of same relative to the driving member  27  can cause an axial movement of the spacing body  25  that is connected with the driving member  27  in direction towards the electrode tip  3 , which in turn leads to an increasing expanding of the expansion sleeve  24  and to an increase of the clamping forces between the wall  26  of the blind hole bore and the expansion sleeve  24 , respectively. In order to achieve an as soft as possible resilience characteristic for the provision of the axial tensile forces of the driving member  27  which ultimately effectuate the clamping forces, the driving member  27  in the area between the spacing body  25  and the outer thread  28  is designed as an anti-fatigue shaft (not visible in the figure). In order to disburden the anti-fatigue shaft of the driving member  27 , during tightening of the screw nut  29  for the purpose of expanding and clamping of the expansion sleeve  24  and during untightening of the screw nut  29  for the purpose of removal of the changing part  4 , from detrimental torsional forces, the driving member  27  comprises in the area between the electrode tip  3  and the outer thread  28  four in each case by 90° at the circumference displaced surfaces  12 , which can be engaged with a flat spanner for securing the driving member  27  against a turning during tightening and untightening, respectively, of the screw nut  29 . 
       FIG. 7  shows the working end of a sixth working electrode according to the invention in a longitudinal section, which from its configuration substantially equals the before discussed working electrode. In contrast to the embodiment shown in  FIG. 6 , in the present case the inner conductor  2  is designed as pure cylindrical sleeve without radial bead and the abutment member  29  as a washer like screw nut with frontal holes having four frontal holes  23  and rounded circumferential edges, which here form the radial bead  14  of the field relief. Furthermore it is eye-catching that the expansion sleeve  24  in this embodiment is shorter and considerably larger in circumference, the spacing body  25  is rather plate shaped in design and the anti-fatigue shaft  8  that is visible here of the driving member  27  is designed shorter than in the example of  FIG. 6 . The insulator  1 , the electrode tip  3 , the surfaces  12  and the outer thread  28  of the driving member  27  are however designed identically. 
       FIG. 8  shows the working end of a seventh working electrode according to the invention in a longitudinal section. As can be seen, also here the working electrode comprises a cylindrical and towards the working end truncated cone shaped insulator  1 , in the center of which there is arranged a central conductor  2 . The central conductor  2 , at its working end sided front face, comprises a central cylindrical bore, which is opened towards the working end, at which end the central conductor  2  under formation of a circumferential radial bead  14  protrudes out of the insulator  1 . The radial bead  14  is equipped with frontal holes  23  for engagement of a face spanner. Arranged in the central bore of the central conductor  2  is a changing part  4  according to the invention, which in the present case comprises an expansion sleeve  24  and a conical spacing body  25  for radially expanding the expansion sleeve  24  through axial displacement relative to same. The spacing body  25  is connected through one piece design with a driving member  27  for displacement of the spacing body  25  in the expansion sleeve  24 , which at its end facing away from the spacing body protrudes out of the expansion sleeve  24  and at this end is designed as electrode tip  3  with the shape of a spherical calotte. Between the electrode tip  3  and the spacing body  25  the driving member  27  furthermore comprises an outer thread  28 , which is screwed into a respective inner thread at the working side end of the expansion sleeve  24 . This area of the expansion sleeve  24  forms an abutment member according to the claims. Spacing body  25 , driving member  27 , outer thread  28  and electrode tip  3  are here formed from a screwing-in part of one piece design, which furthermore possesses surfaces  12  for interaction with a screwing-in tool and screwing-out tool, respectively, and upon a screwing-in into the expansion sleeve  24  automatically effectuates an expanding and a respective clamping of said sleeve in the bore in the central conductor  2 . In order to avoid an introduction of torsional forces into the contact area between the central conductor  2  and the insulator  1 , the central conductor  2  during the screwing-in and screwing-out of this screwing-in part is advantageously secured against turning by means of a face spanner. As can further be seen, the expansion sleeve  24  in the area between the inner thread, which interacts with the outer thread  28  of the driving member  27 , and the area where it is radially expanded by the spacing body  25 , comprises an area  9  which has a significantly reduced cross-section, which area constitutes an anti-fatigue sleeve  9  with a length of about four times the diameter of the inner thread. 
     While in the present application preferred embodiments of the invention are described, it is to be distinctly understood that the invention is not limited thereto and may be otherwise variously embodied within the scope of the following claims.