EP2218904B1

EP2218904B1 - Method for manufacturing a fuel injector servo valve

Info

Publication number: EP2218904B1
Application number: EP09425062A
Authority: EP
Inventors: Mario Ricco; Sergio Stucchi; Raffaele Ricco; Onofrio De Michele; Marcello Gargano; Domenico Lepore; Carlo Mazzarella
Original assignee: Centro Ricerche Fiat SCpA
Current assignee: Centro Ricerche Fiat SCpA
Priority date: 2009-02-16
Filing date: 2009-02-16
Publication date: 2011-09-07
Anticipated expiration: 2029-02-16
Also published as: EP2218904A1; ATE523687T1

Abstract

The manufacturing method regards a fuel injector servo valve (5), wherein a bushing (41) is designed to slide on a fixed stem for opening/closing the servo valve (5) and carries an armature (17), which is free to slide axially on the bushing (41); according to the method, the bushing is made in such a way that it has: a guide portion (61); a shoulder (62), fixed on one side of the guide portion (61) for axial resting of the armature; a flange (24) with an external diameter smaller than the internal diameter of the armature (17), for resting of a spring; an intermediate portion (71) between the flange (24) and the guide portion (61), and with an external diameter smaller than that of the flange (24); and a centring pin (12) for the spring (23). After fitting the armature (17) on the guide portion (61), causing it to pass around the flange (24), a fork (73) is slid in between the armature (17) and the flange (24) for stopping the armature (17) axially; the thickness of the fork (73) is chosen so as to leave a predefined freedom of axial movement for the armature (17) on the guide portion (61).

Description

The present invention relates to a method for manufacturing a fuel injector servo valve, wherein the servo valve comprises a bushing designed to move for a certain axial travel along a fixed stem between an opening position and a closing position. The invention moreover relates to a servo valve produced using the aforesaid method.
Servo valves of the type just described have a discharge duct that exits on a lateral surface of the stem in such a way that, in the closing position, the bushing is subject to a substantially zero axial thrust exerted by the pressure of the fuel. Consequently, the servo valve is of a balanced type and requires relatively small forces for opening and closing it. The bushing is brought into the closing position by a corresponding spring and is controlled so as to be brought into the opening position, against the action of the spring, by a disk-shaped armature, actuated by an electric actuator.
In order to reduce or eliminate the rebounds of the bushing when it is brought into the closing position, the need is felt to separate the armature from the bushing and to displace the armature axially for a travel greater than that of the bushing so as to strike against the latter when it rebounds.
During production of this type of servo valve there is the problem of providing two stop or impact elements for the travel of the armature, which must be fixed with respect to the bushing and must be set on the latter with extreme precision. In addition, during production there is the problem of mounting the armature in a slidable way on the bushing and of fixing thereon one of the stop elements by means of welding, for example laser welding, which presents various difficulties. In particular, the external profile of the armature and the profile of its housing on the bushing must not present interference with the weld material.
DE 10 2006 021741 A1 corresponds to the preamble of claim 1 and discloses a control valve, which opens or closes a connection from a control chamber into a fuel return line by positioning a closing element in a seat or by opening the seat. A bore is embodied in the closing element, and a pin is received in the bore. The diameter of the bore essentially corresponds to the diameter of the seat.
US-A-5299776 discloses a small axial lost motion connection between an armature disk and a needle valve. The armature disk is axially captured on needle valve between a flange and a spring guide retainer. The latter is a ring that has a press fit on the needle valve. The spring guide retainer is pushed onto the needle valve a distance that creates a desired amount of axial lost motion between the armature disk and the needle valve. Once that the desired amount of axial lost motion has been attained, the spring guide retainer is joined to needle valve, such as by welding.
The aim of the invention is to provide a method for manufacturing a servo valve of the type described above that will solve the aforesaid problems referred and presents a high reliability and a limited cost.
According to the invention, the above purpose is achieved by a method for manufacturing a fuel injector servo valve, as defined in Claim 1.
For a better understanding of the invention, described herein is a preferred embodiment, provided by way of example with the aid of the annexed drawings, wherein:

Figure 1 is a partial median section of a servo valve produced according to the method of the present invention;
Figure 2 illustrates, at an enlarged scale, a detail of Figure 1; and
Figures 3 and 4 are similar to Figure 2 and show respective variants of the servo valve of Figure 1.

With reference to Figure 1, designated as a whole by 2 is a hollow body or casing of a fuel injector for an internal-combustion engine, in particular a diesel engine. The casing 2 extends along a longitudinal axis and terminates with a nozzle or nebulizer (not visible in the figure) for injection of the fuel at a high pressure.
The casing 2 has an axial cavity 34, which houses a dosage servo valve 5, comprising a valve body 7 having an axial hole, slidable in which is a rod for control of injection (said rod and said axial hole are not visible in Figure 1). Said rod is controlled by the pressure of the fuel in a control chamber, which is contained in the valve body 7 and is not visible in Figure 1 either. An electric actuator 15 is housed in a portion of the cavity 34 and comprises an electromagnet 16, designed to control an armature 17 having the shape of a notched disk. In particular, the electromagnet 16 comprises a magnetic core 19, which has a polar surface 20 perpendicular to the axis of the casing 2 and is held in position by a support or jacket 21.
The electric actuator 15 has an axial cavity 22 in communication with the discharge of the servo valve 5 for recirculation of the fuel towards the usual tank. Housed in the cavity 22 is a helical compression spring 23, pre-loaded so as to exert an action of thrust on the armature 17 in a direction opposite to the attraction exerted by the electromagnet 16. The spring 23 acts on an intermediate body, designated as a whole by 12a, which comprises a pin 12 defining a centring element for one end of the spring 23. The body 12a moreover comprises an external annular portion defining a flange 24 made of a single piece with the pin 12. A thin lamina 13 made of non-magnetic material is located between a plane top surface 17a of the armature 17 and the polar surface 20 of the core 19 in order to guarantee a certain gap between the armature 17 and the core 19.
The valve body 7 comprises a flange 33 housed in the cavity 34 and kept fixed, in a fluid-tight way, against a shoulder (not visible in the figure) by a threaded ring nut 36, screwed on an internal thread 37 of the cavity 34. The armature 17 is associated to a bushing 41, guided axially by a stem 38, which is made of a single piece with the flange 33 of the valve body 7 and extends in cantilever fashion from the flange 33 itself towards the cavity 22. The stem 38 has a cylindrical lateral surface 39, which guides axial sliding of the bushing 41. In particular, the bushing 41 has a cylindrical internal surface 40, coupled to the lateral surface 39 of the stem 38 substantially in a fluid-tight way, for example with a diametral play of less than 4 µm, or else by means of interposition of annular seal elements (not illustrated).
The fuel comes out of the control chamber of the body 7 through an outlet duct 43, made axially inside the flange 33 and the stem 38. The duct 43 is in communication with at least one substantially radial stretch of duct 44. Advantageously, two or more radial stretches 44 can be provided, set at constant angular distances apart, which give out into an annular chamber 46, formed by a groove of the lateral surface 39 of the stem 38. In Figure 1, two stretches 44 are provided, inclined in the direction of the armature 17.
The annular chamber 46 is obtained in an axial position adjacent to the flange 33 and is opened/closed by a terminal portion of the bushing 41: said terminal portion defines an open/close element 47 for said annular chamber 46 and hence also for the radial stretches of duct 44. Preferably, the open/close element 47 is made of a single piece with the remaining part of the bushing 41 and co-operates with a corresponding stop for closing the servo valve 5. In particular, the open/close element 47 has an internal surface 45 shaped like a truncated cone that is flared towards the end edge and is designed to stop against a joining 49 shaped like a truncated cone, located between the flange 33 and the stem 38.
Advantageously, the joining 49 comprises two surface portions 49a and 49b shaped like a truncated cone, separated by an annular groove 50, which has a cross section shaped substantially like a right angle; i.e., it comprises an internal cylindrical stretch and an external stretch orthogonal to the axis of the casing 2. The surface shaped like a truncated cone 45 of the open/close element 47 engages in a fluid-tight way the portion of surface shaped like a truncated cone 49a, against which it stops in the closing position. On account of the wear between these surfaces 45 and 49a, after a certain time the closing position of the open/close element 47 requires a greater travel of the bushing 41 in the direction of the joining 49, but the diameter of the sealing surface at the most remains defined by the diameter of the cylindrical stretch of the annular groove 50.
The armature 17 is at least in part made of a magnetic material and is formed by a distinct piece, i.e., a piece separate from the bushing 41. It comprises a central portion 56 having a plane bottom surface 57, and a notched external portion 58, with section tapered outwards. The central portion 56 defines an axial hole 59, by means of which the armature 17 engages with a certain radial play along a guide portion 61 defining a terminal collar of the bushing 41. The guide portion 61 projects axially with respect to a flange 60 of the bushing 41 and has an external diameter smaller than the open/close element 47 and than the flange 60.
The bushing 41 has, in a fixed position, a first element for axial stop of the armature 17: said first element is a shoulder 62 that is located at the bottom of the guide portion 61 and, in the particular shown examples, is made of a single piece with the bushing 17, as it is defined by the flange 60.
The body 12a comprises an axial pin 63 for connection with the bushing 41: the pin 63 is made of a single piece with the flange 24, projects axially in a direction opposite to the pin 12, and is inserted in an axial seat 40a of the bushing 41. The seat 40a has a diameter slightly greater than the internal surface 40 of the bushing 41 in order to reduce the portion to be ground for ensuring fluid tightness with the surface 39 of the stem 38.
Notwithstanding the seal between the surface 39 of the stem 38 and the internal surface 40 of the bushing 41, there occurs in general a certain leakage of fuel towards a compartment 48 between the end of the stem 39 and the pin 63. In order to enable discharge of the fuel from the compartment 48 towards the cavity 22, the body 12a is provided with an axial hole 64.
As has been mentioned above, the shoulder 62 defines the first of two elements provided for axial stop of the armature 17 and is located in a position such as to allow the armature 17 to perform a pre-set travel greater than the travel of the open/close element 47, i.e., a relative axial displacement between the armature 17 and the bushing 41.
The body 12a comprises an intermediate portion 71 between the flange 24 and the pin 63. The portion 71 has an external diameter intermediate between that of the flange 24 and that of the pin 63 so that the portion 71 and the pin 63 are joined by an axial shoulder 72, which is set resting against the internal part of an axial end edge 70 of the guide portion 61.
A fork 73 is located axially between the flange 24 and the armature 17, and is fitted onto the portion 71. The fork 73 has an internal lateral surface 78, which is coupled in an axially slidable way to the external cylindrical lateral surface 74 of the portion 71. Alternatively, the fork 73 is located in a fixed axial position against the flange 24, for example by means of a weld.
The fork 73 then has an external lateral surface 80, which at least in part faces, in a radial direction, a lateral surface 81 that defines the axial cavity 22 in so far as the axial thickness of the fork 73 is greater than the lift A by the armature 17, i.e., greater than the empty space present above the surface 17a. In these conditions, the fork 73 is not able to slide in between the polar surface 20 and the armature 17. In particular, the fork 73 has a smaller external dimension D, but close to that of the diameter of the surface 81: in this way, the surface 81 withholds the fork 73 radially against the surface 74 of the portion 71 to prevent it from sliding out of the portion 71 also when the fork 73 comes to rest, by gravity, axially against the surface 17a.
The fork 73 is delimited axially by two opposite surfaces, which are designated by the reference numbers 75 and 76 and are plane. The surface 76 faces the surface 17a of the armature 17 axially and, by the force of gravity acting on the fork 73, tends to come to rest axially against the surface 17a (as illustrated in Figure 1). The surface 75, instead, faces axially a plane surface 65 of the flange 24. When the armature 17 is attracted by the core 19, the fork 73 is raised by the armature 17 towards the flange 24 until its surface 75 comes into axial contact against the surface 65. As shown in Figure 2 for reasons of simplicity, the surfaces 65 and 75 are considered as if they were always in axial contact even though the fork 73 is floating, in so far as the weight of the fork 73 is small as compared to that of the armature 17, and hence its axial movement along the portion 71 does not alter conditions of operation of the armature 17 and of the servo valve 5.
The fork 73 defines the second of the two elements provided for axial stop of the travel of the armature 17 with respect to the bushing 41. In other words, considering, for reasons of simplicity, that the surfaces 65 and 75 will be fixed in contact with respect to one another, the axial distance between the surface 76 and the shoulder 62 is greater than the axial thickness of the portion 56 of the armature 17: the difference between said axial distance and said axial thickness defines the axial play G or maximum relative displacement of the armature 17 with respect to the bushing 41.
When the electromagnet 16 is not energized, the open/close element 47 is kept resting with its surface shaped like a truncated cone 45 against the portion shaped like a truncated cone 49a of the joining 49 by the thrust of the spring 23, which acts through the body 12a so that the servo valve 5 is closed. In the annular chamber 46, there is set up a fuel pressure, the value of which is substantially equal to the supply pressure of the injector. In this condition, normally the armature 17 rests against the shoulder 62 and the lamina 13 rests by gravity on the surface 17a of the armature 17: since the weight of the lamina 13 is negligible with respect to that of the armature 17 and of the bushing 41, for reasons of simplicity it is assumed that the lamina 13 is located adjacent to the surface 20, as represented in Figure 2, in so far as this hypothesis does not jeopardize the described operations.
On these hypotheses, the open/close element 47 has a travel or lift I that is defined by the axial distance between the surface 76 of the fork 73 and the lamina 13. When the electromagnet 16 is energized for opening the servo valve 5, the core 19 attracts the armature 17, which at the start performs a loadless travel, or pre-travel, without affecting the displacement of the bushing 41, until the fork 73 remains axially packed between the surface 17a of the armature 17 and the surface 65 of the flange 24.
At this point, the action of the electromagnet 16 on the armature 17 overcomes the force of the spring 23, via interposition of the fork 73 and of the flange 24, and the armature 17 draws the bushing 41 axially towards the core 19 to enable the open/close element 47 to perform its opening travel; consequently, also with the pressure of the fuel in the chamber 46, the open/close element 47 rises and the servo valve 5 opens.
It is thus evident that the armature 17 performs a lift A greater than the lift I of the bushing 41; i.e., in opening, it performs, along the guide portion 61, a pre-travel equal to the play G between the surface 17a of the armature 17 and the surface 76 of the fork 73.
When energization of the electromagnet 16 ceases, the spring 23, via the body 12a, causes the bushing 41 to perform the travel towards the closing position. During at least one first stretch of this closing travel, the fork 73 remains axially packed between the surfaces 17a and 65, whilst the armature 17 moves substantially together with the bushing 41 and away from the polar surface 20.
At the end of its closing travel, the open/close element 47 strikes, with its conical surface 45, against the portion of surface shaped like a truncated cone 49a of the joining 49 of the valve body 7. On account of the type of stresses involved, the small area of contact, and the hardness of the open/close element 47 and of the valve body 7, after impact, the open/close element 47 rebounds, overcoming the action of the spring 23. Instead, the armature 17 continues its travel towards the valve body 7 along the guide portion 61, i.e., towards the shoulder 62, recovering the play that had formed between the plane surface 57 of the portion 56 of the armature 17 and the shoulder 62 of the flange 60.
After a certain time from impact of the open/close element 47 there is an impact of the plane surface 57 of the portion 56 against the shoulder 62 of the bushing 41, which is rebounding. As a result of this impact between the armature 17 and the bushing 41, the subsequent rebounds of the bushing 41 are markedly reduced or even eliminated as compared to the case where the armature 17 is fixed with respect to the bushing 41.
By appropriately sizing the weights of the armature 17 and of the bushing 41, the travel of the armature 17 and the travel of the open/close element 47, the impact of the armature 17 against the bushing 41 occurs during the first rebound, immediately following upon de-energization of the electromagnet 16 so that both said first rebound and the possible subsequent rebounds are attenuated. The impact between the armature 17 and the shoulder 62 of the bushing 61 can in particular occur upon return of the open/close element 47 into the closing position, i.e., at the end of the first rebound. In this case, the rebounds of the open/close element 47 subsequent to the first are blocked.
In the embodiment shown in Figure 2, the body 12a is a piece distinct from the bushing 41 and is connected to the bushing 41 in a fixed position via welding, after insertion of the pin 63 into the seat 40a. Preferably, the welding operation is performed along a circumference so as to form weld material 77a defined by a continuous bead or else by weld spots in a region corresponding to the sharp edge between the external cylindrical lateral surface 74 of the portion 71 and the axial end edge 70 of the guide portion 61. Preferably, the axial height of the guide portion 61 is less than the axial thickness of the portion 56 of the armature 17, and the weld material 77a remains underneath an ideal plane defined by the surface 17a.
Alternatively, the welding operation is performed along a circumference in a region corresponding to the sharp edge between an axial end edge 83 of the pin 63 and the seat 40a so as to form weld material 77b, defined by a continuous bead or by weld spots, using a welding device appropriately shaped for entering axially into the bushing 41 as far as the compartment 48.
Since welding of the body 12a to the bushing 41 is performed along a circumference it does not require any phasing; i.e., it does not require any particular angular positioning between the bushing 41 and the welding device.
Alternatively, the pin 63 can be sized in such a way as to be connected to the bushing 41 in a fixed position by means of forced interference fit into the seat 40a.
Figures 3 and 4 show two variants of the servo valve 5, the components of which are designated, where possible, by the same reference numbers as the ones used in Figure 2.
In the variant of Figure 3, the external dimension of the fork 73 is decidedly smaller than the diameter of the surface 81 of the axial cavity 22 so that a space is formed between the surfaces 80 and 81. Said space is occupied by the cylindrical terminal portion 86 of a tubular cap 85, which is fitted around the body 12a. The portion 86 surrounds the fork 73, rests axially on the surface 17a of the armature 17, has an external diameter smaller than the diameter of the surface 81, and has an internal diameter that approximates by excess the external dimension D of the fork 73. In this way, the internal surface of the portion 86 is designed to oppose the outer surface 80 of the fork 73 to prevent the fork 73 from sliding radially out of the portion 71 of the body 12a. To enable free axial sliding of the fork 73 on the portion 71 up to the surface 65 of the flange 24, the axial height of the portion 86 must be greater than the axial distance between the surfaces 65 and 17a, considering the armature 17 resting against the shoulder 62. In particular, the portion 86 also performs a function of axial guide for the fork 73.
The cap 85 moreover comprises a cylindrical terminal portion 87 opposite to the portion 86, and an intermediate portion 88 having an internal conical surface that radiuses the internal cylindrical surfaces of the portions 86, 87. The portion 87 surrounds the flange 24 and has an internal diameter that is greater than the flange 24 and is smaller than that of the external diameter of the spring 23. In this way, the spring 23 prevents the cap 85 from sliding axially out of the body 12a, and the flange 24 guides the cap 85 axially during axial displacement of the armature 17.
When the open/close element 47 is in the closing position and the armature 17 is located against the shoulder 62, an axial play B must be envisaged between the spring 23 and the portion 87 of the cap 85. The axial play B must be greater than the axial play G, and is preferably greater than the lift A of the armature 17 in order not to hinder movement of the armature 17. In addition, the axial play B must be smaller than the axial thickness of the fork 73: this condition prevents the fork 73 from possibly moving away radially from the portion 71 and sliding in between the portion 86 and the surface 17a of the armature 17 in the case where the fork 73 is resting axially on the surface 17a and the portion 87 of the cap 85 happens to remain against the spring 23.
In the variant of Figure 4, the body 12a and the bushing 41 are made of a single piece, i.e., they are obtained by machining starting from a single semifinished product, without welds or fits between separate pieces. The single piece that defines the bushing 41 and the body 12a is a bushing designated by the reference number 41a.
According to one variant (not illustrated), the portion 71 is made of a single piece with the bushing 41, whilst the flange 24 and the pin 12 form part of a separate body fixed to the bushing 41.
During production of the servo valve 5, in the case of the solution of Figure 4, the piece 41a is provided in such a way that it has: the guide portion 61 for coupling the armature 17; the shoulder 62 for stop of the armature 17; the pin 12 and the flange 24 with an external diameter less than the axial hole 59 for positioning the spring 23; and the portion 71 between the flange 24 and the guide portion 61 with an external diameter smaller than that of the flange 24.
In the case of the embodiment illustrated in Figure 2, instead, the bushing 41 is provided in such a way that it has: the guide portion 61; and the shoulder 62 located in a fixed position, on one side of the guide portion 61. In addition, the body 12a is provided in such a way that it has: the pin 12; the flange 24; and the portion 71 underneath the flange 24. The body 12a is fixed to the collar 61, preferably by means of a laser welding device (not illustrated). As described above, the welding operation is performed by forming weld material along a circumference, in particular between the external cylindrical lateral surface 74 of the portion 71 and the edge 70. Consequently, welding is carried out at sight, without any need for phasing between the welding device and the bushing 41, i.e., without setting the welding device in a particular predefined angular position with respect to the bushing 41.
After preparation of the piece 41a, or else after fixing of the body 12a to the bushing 41, the armature 17 is fitted on the side of the body 12a, on the guide portion 61, until the armature 17 rests on the shoulder 62, in particular maintaining the axis of the bushing 41 vertical. This is possible since the internal diameter of the armature 17, i.e., of the internal hole 59, is larger than the external diameter of the flange 24.
At this point, the fork 73 is simply slid in towards the portion 71, until the surface 78 rests against the surface 74 so as to stop the armature 17 axially also in the axial direction opposite to the shoulder 62. To calibrate the extent of the axial play G in a fine way, the fork 73 is chosen from among a plurality of forks defining stop spacers that are divided into classes on the basis of their axial thickness. Finally, if so envisaged, the cap 85 may be fitted on the body 12a around the fork 73 and left in a position of axial rest on the armature 17.
The servo valve 5 is then mounted by fixing the valve body 7 in the cavity 34 and fitting the bushing 41 on the stem 38. Finally, the spring 23 is fitted on the pin 12 until the spring 23 itself comes to rest on the flange 24.
From what has been seen above, there clearly emerge the advantages of the manufacturing method according to the invention as compared to the known art.
The solution described is extremely simple in so far as the axial play G of the armature 17 is adjusted at the end of the production cycle with an appropriate choice of the thickness of the fork 73 so as to leave the play G pre-determined in the design stage after the effective distance between the surface 65 and the shoulder 62 has been measured; consequently, the fork 73 comes to define a spacer and an stop element that is mounted after all the operations of production and/or fixing and after the armature 17 has been mounted on the guide portion 61.
Consequently, to obtain the desired axial play G it is not necessary to intervene on the armature 17 but only on the fork 73, which has a calibrated thickness.
Coupling of the fork 73 to the portion 71 does not require any operation of fixing via welding; hence, it does not alter the dimensions of the bushing 41, of the body 12a, or of the armature 17.
Also in the solution of Figure 2, where the body 12a is welded to the bushing 41, the weld material 77a, 77b does not modify the external profile of the bushing 41 or of the body 12a, nor does it alter the surfaces 39, 40 of the stem 38 or of the bushing 41, nor again does it vary the distance between the shoulder 62 and the surface 76. In particular, welding is carried out without any need for phasing between the bushing 41 and the welding device, and the weld material is in view in order to facilitate not only carrying-out thereof, but also quality control. In addition, the weld material 77a, 77b does not create any interference during movement of the armature 17 in the space provided between the shoulder 62 and the surface 76.
It is understood that various modifications and improvements can be made to the manufacturing method and to the servo valve 5 described above, without thereby departing from the scope of protection defined by the annexed claims.
For instance, the armature 17 can be defined by a disk of constant thickness. In addition, the flange 60 can be eliminated so that the shoulder 62 is obtained in the thickness of the bushing 41. The shoulder 62 can also be replaced by an additional stop element, fixed on the remaining part of the bushing 41.
As already mentioned above, the welds described may be made by spot welding rather than by continuous-bead welding. It will be appreciated that the sizing of the weld bead or the weld spots will have to be chosen by taking into account the operation in conditions of fatigue 5 for a sufficient number of cycles.
A spring could be set between the surface 57 of the armature 17 and the flange 33 so as to keep the surface 17a of the armature 17 in contact with the surface 76 of the fork 73 when the electric actuator is not energized. Said possible spring needs to have a stiffness and a pre-loading much lower than those of the spring 23 so as not to affect the dynamics of impact of the armature 17 against the bushing 41 during the phases of rebound described above.
The open/close element 47 could be a separate piece fixed to the remaining part of the bushing 41.

Claims

A method for manufacturing a fuel injector servo valve (5), the servo valve comprising an open/close element (47) fixed with respect to a bushing (41), which is designed to move for a certain axial travel along a fixed stem (38) for opening/closing a discharge duct (43, 44), a spring (23) being provided for keeping said bushing (41) in the closing position, in which said bushing (41) is subject to a substantially zero axial pressure by the fuel; said bushing (41) being movable under the control of an axially perforated armature (17), actuated by an electric actuator (15) against the action of said spring (23);
the method comprising the following steps:
- providing a piece in such a way that it has:
a) a coupling portion (61) for coupling said armature (17);

b) a first element (62) in a fixed position on one side of said coupling portion (61) for axial stop of said armature (17);

c) a flange (24) defining an axial rest for said spring (23), and with an external diameter smaller than the internal diameter of said armature (17);

d) an intermediate portion (71) between said flange (24) and said coupling portion (61), with an external diameter smaller than that of said flange (24);

- fitting said armature (17) on said coupling portion (61), causing said armature (17) to pass around said flange (24) towards said first element (62);

- providing a second element (73) for axial stop of said armature (17); said second element (73) being slid in between said armature (17) and said flange (24) towards said intermediate portion (71);
characterised in that:
- said discharge duct (43, 44) exits from a lateral surface (39) of said stem (38);

- said coupling portion (61) is provided on said piece as a guide portion;

- the thickness of said second element (73) is chosen so as to leave a predefined freedom of axial movement for said armature (17) on said guide portion;

- said piece is provided with a centring pin (12) defining a centring element for one end of said spring (23).
The method according to Claim 1, characterized in that said piece is made by providing a body (12a) comprising said flange (24) and said centring pin (12), providing a bushing (41) comprising said guide portion (61) and said first element (62), and fixing said bushing (41) and said body (12) to one another.
The method according to Claim 2, characterized in that said body (12a) comprises said intermediate portion (71) and a connection pin (63), set coaxial with and opposed to said centring pin (12); and in that said connection pin (63) is inserted in an axial seat (40a) defined by said guide portion (61).
The method according to Claim 3, characterized in that said body (12a) is fixed with weld material along a circumference between said body (12a) and said bushing (41).
The method according to Claim 4, characterized in that said body (12a) is welded by forming weld material (77a) along a circumference between an axial end edge (70) of said guide portion (61) and an external lateral surface (74) of said intermediate portion (71).
The method according to Claim 4, characterized in that said body (12a) is welded by forming weld material (77b) along a circumference between an axial end edge (80) of said connection pin (63) and the internal surface of said axial seat (40a).
The method according to any one of the preceding claims, characterized in that said second element (73) is defined by a fork that is slid in radially.
The method according to Claim 7, characterized in that an internal lateral surface (78) of said fork (73) is set in contact against an external lateral surface (74) of said intermediate portion (71).
The method according to any one of the preceding claims, characterized in that a tubular cap (85) is fitted axially around said flange (24) and said second element (73) after said second element (73) has been slid in.
The method according to Claim 9, characterized in that said tubular cap (85) has an internal diameter smaller than the external diameter of said spring (23).
A fuel injector servo valve, manufactured by applying the method according to any one of the preceding claims.