EP1686842A1 - Plasma torch - Google Patents

Abstract

The torch has a conical cathode (120) with a point (20) and a half-coupling (200). A cathode screw (300) with a half-coupling (400) fixes the cathode, in operational position, in a cathode-gate (22) by the application of pressure on the cathode. The half-couplings (200, 400) cooperate to form a coupling, between the cathode and the screw, allowing the transmission of a traction force from the screw to the cathode. The point is present on a conical part of the cathode, and penetrates partially a central cavity of an anode to form a space between them to produce an electric arc plasma generator and a passage for a plasmagene gas towards a plasma output opening.

Description

Introduction

The present invention relates to a plasma torch and more particularly to a transferred arc plasma charging torch (P.T.A.).

Many different types of plasma torches are known. Their configuration usually varies depending on the intended application. A known problem in the structure of plasma torches is the alignment of the electrodes for generating the plasma. In general, the anode is annular in shape and has a central orifice in which is disposed the tip of the cathode which is generally conical. In order to ensure optimal plasma generation and to minimize electrode wear, the cathode must be properly centered on the anode port. An improvement to perfect this centering has been proposed in WO 97/20453 by the applicant.

It is also known that service interventions on plasma torches, for example for the replacement of wear parts, are required regularly. When the torch is running, it is in particular the cathode which is subjected to considerable thermal and electrical stresses (temperatures above 15,000 K, arcs of more than 100A, electrical powers exceeding 10kW). It is therefore one of the most rapidly consumed wear parts, even when optimal centering is ensured. As a result, the cathode must be sharpened or replaced frequently. In use, the torches constructed from WO 97/20453 allow, for a given type of torch for a corresponding use, to space the sharpening or exchange of the cathode. In other words, with these torches it is possible to pass operating intervals of about 7 hours at intervals of about 40 hours. Always is it that with the known torches the intervention of replacement or sharpening is complex and laborious. The duration of the intervention reduces the degree of use of the torch. By Moreover, the complexity of the intervention entails a certain risk of damage to the component parts of the torch.

Object of the invention

The object of the present invention is to provide an improved plasma torch which eliminates the aforementioned problems.

General description of the invention

According to the invention, this object is achieved by a plasma torch, in particular a transferred arc plasma charging torch, comprising a torch head having a plasma outlet opening and a protective tube to which the torch head is releasably linked. This torch further comprises an anode disposed in the torch head, the anode having a central cavity communicating with the plasma outlet opening and a cathode comprising a conical portion with a tip on a first end of the cathode, the tip being intended to partially penetrate the central cavity of the anode so as to form between them a space. This space makes it possible on the one hand to produce a plasma-generating electric arc and on the other hand the passage of a plasmagenic gas towards the plasma outlet opening. A cathode holder is disposed in the torch head recessed from the anode. It has a conical cone-shaped cavity that converges towards the anode and is intended to partially receive the cathode leaving its first end free. The cathode holder provides a coaxial orientation and a predetermined spacing between the cathode and the anode and allows a removable mounting of the cathode in the torch head. The torch comprises a fixing means for fixing the cathode in operational position in the cathode holder by applying a pressure on the cathode. According to an important aspect of the invention, the cathode has a first coupling means and the fastening means has a second coupling means. These coupling means cooperate to form a coupling for transmitting a traction force of the fastening means to the cathode. This configuration allows to easily extract the cathode from the torch, for example during its replacement, while ensuring a simple, precise and compact fastening structure.

In an advantageous embodiment, the first coupling means and the second coupling means are designed to allow relative rotation between the cathode and the fixing means. The fact of avoiding the transmission of torques between the fastening means and the conical cathode makes it possible, among other things, to avoid a degradation of both the cathode and the cathode carrier during the assembly resp. disassembly of the cathode.

Preferably, the first coupling means and the second coupling means cooperate to form a coaxial coupling of the cathode and the fixing means. The central axis of the cathode being coaxial with the central axis of the fixing means, the latter can be made in a simple manner, for example in the form of a screw.

Advantageously, provision is made for the first coupling means, the cathode, the second coupling means and the fastening means to be designed to form an assembled extractable assembly by means of an interlocking fit. This interlocking is preferably of the male-female type, the first coupling means forming a male part integral with the cathode and the second coupling means forming a female part integral with the fixing means.

A plasma torch cathode according to the invention comprises a conical portion with a tip on a first end of the cathode. According to an important aspect of the invention, a coupling half is attached to a second end opposite the tip of the cathode. The half-coupling is preferably formed by a coaxial extension with symmetry of revolution. It has a circumferential groove so as to project a reference heel.

Advantageously, the conical cathode comprises a cylindrical portion extending from the conical portion, the length of the cylindrical portion being substantially smaller than the length of the conical portion. Such a configuration is favorable to optimal centering of the cathode in the cathode holder.

In a preferred embodiment, the cathode and the half-coupling are formed in one piece.

A fixing means for the plasma torch cathode according to the invention comprises a contact surface for fixing the cathode in operational position in a cathode holder of a plasma torch by applying a pressure on the plasma torch. cathode. According to an important aspect of the invention, a half-coupling is formed on the fastening means, preferably by at least a first and a second notch of different sections. These cuts are contiguous and arranged in the fixing means in its radial direction so as to release a first radial opening and a second axial opening in the contact surface of the fixing means. This half-coupling allows the insertion of the half-coupling of the cathode described above to form an easily extractable assembly of the plasma torch.

In a simple embodiment, the fastening means is a hexagonal screw, which can be screwed, for example, into the cathode holder of the plasma torch.

Description using the figures

Fig.1:

une vue en coupe longitudinale d'une torche à plasma selon l'état de la technique ;

Fig.2:

une vue en perspective de différentes étapes (A) - (E) d'un remplacement de cathode sur une torche selon la Fig.1 ;

Fig.3:

une vue en coupe longitudinale d'une torche à plasma selon un mode de réalisation préféré de l'invention ;

Fig.4:

une vue de côté agrandie d'une cathode et d'une vis de cathode selon la Fig.3 ;

Fig.5:

une vue de dessous agrandie de la vis de cathode selon la Fig.4 ;

Fig.6:

une vue en perspective de différentes étapes (A) - (C) d'un remplacement de cathode sur une torche selon la Fig.3.

Other features and characteristics of the invention will become apparent from the detailed description of an advantageous embodiment presented below, by way of illustration, with reference to the accompanying drawings. These show:

Fig.1:

a longitudinal sectional view of a plasma torch according to the state of the art;

Fig.2:

a perspective view of different steps (A) - (E) of a cathode replacement on a torch according to Fig.1;

Fig.3:

a longitudinal sectional view of a plasma torch according to a preferred embodiment of the invention;

Fig.4:

an enlarged side view of a cathode and a cathode screw according to Fig.3;

Fig.5:

an enlarged bottom view of the cathode screw according to FIG.

Fig.6:

a perspective view of different steps (A) - (C) of a cathode replacement on a torch according to Fig.3.

Fig.1 shows a longitudinal sectional view of a charging torch P.T.A. according to the state of the art, generally identified by the reference number 10. This plasma torch 10 generally has a symmetry of revolution about a central axis XX '. It comprises a central cathode 12, generally made of tungsten, and an annular anode 14, generally of copper. The anode 14 is provided with a cylindrical central cavity 16. The anode 14 and its central cavity 16 are held coaxially with the axis XX 'by a nozzle 18. The nozzle 18 also constitutes the envelope of the head of the torch 10 and forms with the anode 14 the exit opening torch plasma 10.

The cathode 12 is in the form of a solid of symmetrical revolution around the axis XX '. The cathode 12 is conical on its lower part and cylindrical on its upper part. It should be noted that the cylindrical portion is substantially shorter than the conical portion, which facilitates the centering of the cathode 12 on the anode 14. The conical cathode 12 has on its lower end a tip 20. The cathode 12 is maintained coaxially with the axis XX 'and the central cavity 16 by means of a cathode holder 22.

The cathode holder 22, just like the cathode 12, has the shape of a solid of symmetrical revolution about the axis XX '. A central cavity housing which comprises a conical cavity 24 in the form of a truncated cone is formed in the cathode holder 22. The conical cavity 24 is intended to partially receive the conical portion of the cathode 12 leaving free its lower end. The respective cones of the conical portion of the cathode 12 and the conical frustum of the conical cavity 24 are machined at corresponding angles of 20 ° for example. The cathode holder 22 is configured to overflow the tip 20 and a lower portion of the cone of the cathode 12 in the central cavity 16 of the anode 14. In fact, the cathode holder 22 provides a predefined distance between the cathode 12 and the anode 14. This spacing is used for the passage of plasma gas and the production of an electric arc transforming the gas into plasma. At the top of the conical cavity 24 in FIG. 1, the cathode holder 22 has a central cylindrical cavity 26 whose diameter is slightly greater than the diameter of the cylindrical portion of the cathode 12 in order to receive it. To ensure correct holding of the conical cathode 12, it is mounted in the cathode holder 22 by clamping. As illustrated in FIG. 1, this clamping is effected by a fastening means in the form of a cathode screw 30. The cathode screw 30 bears on an upstanding bearing surface 28 of the conical cathode 12. Thus, the conical portion of the cathode 12 is held in contact and pressure in the conical cavity 24. The cathode holder 22 has, on the wall of its cylindrical cavity 26, an internal screw thread 32 compatible with the external thread 34 of the cathode screw 30. It will be appreciated that the method of attachment described above as well as the short length of the conical cathode 12 allow to obtain a compact fixation while ensuring excellent concentricity between the cathode 12 and the anode 14.

It remains to be noted that the assembly 40, shown in FIG. 1, constitutes the head of the plasma torch 10. The assembly 40 is formed of an assembly of parts, including the cathode holder 22, mechanically held together. by screws. This set of parts 40 has inter alia a connection function for the nozzle 18, a support function for the anode 14 and a housing function for the cathode 12. The assembly 40 is inserted and fixed by its upper part in a protective tube 42 which constitutes the torch 10.

For the sake of brevity, other known aspects of plasma torches such as electrical connections, the supply of gas and powder or the cooling circuit will not be detailed here. All the elements described above are part of a plasma torch according to the state of the art.

As is well known, the cathode 12 is a wear part which must in fact be replaced relatively frequently. Fig.2 shows different steps to be performed when replacing the cathode of a plasma torch 10 according to Fig.1.

As illustrated in (A) in Fig.2, the first step is to remove the torch 10 from a collar 50 which secures it to a machine support (not shown). In step (B), the nozzle 18 is unscrewed and removed from the assembly 40. This releases the anode 14. A high temperature insulator 52, disposed in the assembled state between the cathode 12 and the anode 14 and several seals 54 of the cooling circuit are then released. In step (C), after having unscrewed a screw 56, the protective tube 42 is removed from the assembly 40. Thus a bundle 58 of pipes and supply cables is disengaged. Step (D) then consists in unscrewing the cathode screw 30 and removing it from the cathode holder. In step (E), the assembly 40 is returned with the beam 58. The cathode 12 is then extracted by pushing its tip 20 towards the inside of the assembly 40. The insertion of a cathode of replacement and the winding of the torch 10 is similar, reversing the steps described above.

It should be noted that there are some disadvantages to this relatively complex and laborious procedure. Indeed, during the replacement, one must be careful not to damage the seals 54, the beam 58 and its connections to the assembly 40. The state and the range of the seals 54 must be checked. The anode 14 and the high temperature insulation 52 must be correctly positioned during reassembly. It is generally necessary to check the tightness of the cooling circuit at the level of the anode 14 and the respective connections of the beam 58. Another considerable disadvantage is the need to correctly reposition the torch 10 in the machine. Indeed, the height and orientation of the powder orifices must be correct in order to find the settings obtained before disassembly of the torch 10.

Fig.3 shows a plasma torch 100 of general constitution similar to that of Fig.1. In Fig.3, identical or similar elements are identified by reference numerals identical to those of Fig.2. A novel element is the conical cathode 120. The conical cathode 120 differs from that of the state of the art in that it has on its bearing surface 28, on the side opposite its tip 20, a coupling means in the form of a first half-coupling 200. Another new element is the fixing means, that is to say the cathode screw 300. It has on its side facing the bearing surface 28 of the conical cathode 120, a coupling means in the form of a second half-coupling 400. first and second half-couplings 200, 400 are configured in a conjugate manner so as to cooperate.

Figs. 4 and 5 show in more detail the conical cathode 120 and the cathode screw 300 in coupled configuration. In Fig.4, it will be seen that the first half-coupling 200 essentially comprises two parts, namely a reference bead 202 and a circumferential groove 204. The reference lug 202 and the groove 204, like the conical cathode 120, have a symmetry of revolution about a central axis YY '. The periphery of the reference heel 202 is projecting with respect to the groove 204. The first half-coupling 200 is preferably formed in one piece with the cathode 120. It can be easily obtained by machining, for example on a lathe, a cathode blank. The dimensions of the machining depend inter alia on the dimensions of the cathode.

Fig. 4 also shows the second half-coupling 400 on the cathode screw 300. It is located on the side facing the cathode 120, opposite to a recess hexagon socket 302 for screwing. The second half-coupling 400 consists essentially of two notches 402, 404 contiguous and superimposed along the axis YY 'of the cathode screw 300. The notches 402, 404 form a void space for insertion coupling. The notches 402, 404 are intended to receive the first half-coupling 200 of the cathode 120. The first notch 402 has a height and a diameter slightly greater than the corresponding dimensions of the reference heel 202. It opens on the side wall of the cathode screw 300. The second notch 404 has a height and a diameter slightly greater than the corresponding dimensions of the groove 204. It opens on the side wall and on a contact surface 304 of the cathode screw 300. first half-coupling 200 can thus be inserted laterally in the space formed by the notches 402, 404. The notches 402, 404 are formed radially in the cathode screw 300 in the form of U-shaped notches. For the first and the last second notch 402, 404, the radius of the arcuate segments of the U-shaped ends preferably correspond to the respective radii of the reference heel 202 and the throat 204 increased by a small additional charge. The length of the cuts 402, 404 in the radial direction of the cathode screw 300 (along a line perpendicular to the axis YY ') exceeds the radius of the latter to allow, in coupled configuration, a coaxial introduction of the cathode 120 relative to the cathode screw 300 along the axis YY '.

In Fig.5, it is more clearly seen that the first notch 402 is undercut relative to the second notch 404 in the cathode screw 300. The diameter of the notch 402 is therefore greater than that of the notch 404 so as to project an edge 406 which makes it possible to retain the reference heel 202. The two half-couplings 200, 400 therefore make it possible to form a coupling between the cathode screw 300 and the conical cathode 120. This coupling makes it possible to transmit a pulling force of the cathode screw 300 to the conical cathode 120. In other words, when the cathode screw 300 is unscrewed from the cathode holder 22, it drives the conical cathode 120.

It also appears from Fig.4 and Fig.5 that this coupling allows a relative rotation of the cathode 120 relative to the cathode screw 300 and vice versa. The coupling is therefore non-integral in rotation, which eliminates undesirable frictional wear at the conical portion of the cathode 120 and the conical cavity of the cathode holder 22. In fact, it is important that the carrier cathode 22 is not degraded over time to ensure optimum centering between the cathode 120 and the anode 14. A torque transmission between the cathode screw 300 and the cathode 120 is furthermore undesirable because such torque may interfere with the centering of the cathode 120. It should be noted that in the embodiment described, the attachment of the conical cathode is by a clamping in which the contact surface 304 rests on a bearing surface 306 of the cathode 120. Alternatively, the tightening could be done by pushing the cathode screw 300 on the reference heel 202.

Fig.6 shows different steps when replacing the conical cathode 120 of a torch 100 according to Fig.3. A first step (A) is to remove the protective tube 42 from the head of the torch 100 after removing the screw 56. It will be appreciated that the torch remains fixed in the collar 50 when replacing the cathode. A second step (B) consists in unscrewing the screw of cathode 300. The cathode screw 300 being coupled to the cathode 120, the latter is removed from the cathode holder 22 without further measurement. Indeed, as seen in Fig.3, the cathode screw is accessible through the cylindrical cavity 26 of the cathode holder 22 when the protective tube 42 is removed. Returning to FIG. 6, the first and second half-couplings 200, 400 are uncoupled in a third step (C) in order to release the cathode 120. The insertion of a replacement cathode into the torch 100 is done analogous, by reversing the steps described above. It follows that the replacement of the cathode 120 in a torch 100 according to Fig.3 can be performed without the need to remove the torch 100 of its collar 50 and disassemble the inner parts of the torch 100.

A comparison of the replacement procedures according to FIG. 6 and FIG. 3 shows that all the disadvantages relating thereto as described above have been eliminated thanks to the new configuration of the cathode assembly 120 and the cathode screw 300. It will be appreciated that this improvement can be made without substantial modifications of the other components of the plasma torch.

List of pieces:

plasma torch

10

conical cathode

12

annular anode

14

central cavity (of the anode)

16

buzzard

18

tip (cathode)

20

cathode holder

22

conical cavity (of the cathode holder)

24

cylindrical cavity (of the p.-c.)

26

support surface (of the cathode)

28

cathode screw

30

internal screw thread (from the p.-c.)

32

outer thread (cathode screw)

34

together

40

protection tube

42

necklace

50

H.T. insulation

52

seals

54

screw

56

beam

58

conical cathode

120

cathode screw

300

hexagon socket

302

contact surface

304

bearing surface

306

first half-coupling

200

heel marker

202

throat

204

second half-coupling

400

first notch

402

second cut

404

edge

406

Claims (10)

Plasma torch, particularly transferred arc plasma charging torch, comprising
a torch head which includes a plasma exit aperture;
a protective tube to which the torch head is releasably connected;
an anode disposed in the torch head, the anode having a central cavity communicating with the plasma outlet opening;
a cathode comprising a conical portion with a tip on a first end of the cathode, the tip being intended to partially penetrate the central cavity of the anode so as to form between them a space on the one hand to produce a generator electric arc plasma and secondly the passage of a plasma gas to the plasma outlet opening;
a cathode holder, disposed in the torch head recessed from the anode, the cathode holder having a conical cone-shaped cavity which converges towards the anode and which is intended to partially receive the cathode leaving free its first end, the cathode holder providing a coaxial orientation and a predetermined spacing between the cathode and the anode and allowing a removable assembly of the cathode in the torch head; and
a fixing means for fixing the cathode in operational position in the cathode holder by applying a pressure on the cathode;
characterized in that
the cathode has a first coupling means and the securing means has a second coupling means, which coupling means cooperate to form a coupling for transmitting a tensile force of the fastening means to the cathode. Plasma torch according to claim 1, characterized in that the first coupling means and the second coupling means are designed to allow rotation between the cathode and the fixing means. Plasma torch according to claim 1 or 2, characterized in that the first coupling means and the second coupling means cooperate to form a coaxial coupling of the cathode and the fixing means. Plasma torch according to one of Claims 1 to 3, characterized in that the first coupling means, the cathode, the second coupling means and the securing means are designed to form an extractable assembly assembled by means of a nesting by conjugation of form. Plasma torch according to claim 4, characterized in that the interlock is of the male-female type, the first coupling means forming a male part integral with the cathode and the second coupling means forming a female part secured to the means of fixation. Plasma torch cathode comprising a conical portion with a tip on a first end of the cathode;
characterized by
a coupling half attached to a second end opposite the cathode tip and formed by a rotationally symmetrical coaxial extension having a circumferential groove so as to project a reference bead. Plasma torch cathode according to claim 6, characterized in that the cathode further comprises a cylindrical portion extending from the conical portion, the length of the cylindrical portion being substantially less than the length of the conical portion. Plasma torch cathode according to Claim 6 or 7, characterized in that the cathode and the coupling half are formed in one piece. Fastening means for a plasma torch cathode comprising a contact surface for fixing the cathode in position operating in a cathode holder of a plasma torch by applying a pressure on the cathode;
characterized by
a coupling half formed by at least first and second notches of different sections, which are contiguous and arranged in the fastening means in its radial direction so as to disengage a first radial opening and a second axial opening in the contact surface fastening means. Fixing means for a plasma torch cathode according to claim 9, characterized in that the fixing means is a hexagon socket screw.

Images