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Title:
FASTENING MEMBER
Document Type and Number:
WIPO Patent Application WO/2011/073004
Kind Code:
A1
Abstract:
A fastening member (100) for fastening a first component (118) to a second component (110) in an end-to-end configuration along a common axis (A) is disclosed. The fastening member (100) comprises a first portion (124) having a threaded region for engagement with a corresponding threaded region (120) of the first component (118), a second portion (126) for applying a retaining force to the second component (110) to hold the first and second components (118, 110) in sealing engagement, and a deformable portion (130) intermediate the first and second portions (124, 126). The deformable portion (130) is plastically deformable so as to allow relative movement of the first and second portions (124, 126) in use of the fastening member (100), such that a location (129) on the fastening member can be brought into angular alignment with a location on the first or second component (118, 110) upon turning movement of the fastening member. The fastening member is particularly suitable for use as a cap nut in a fuel injector of the type having a cylinder pressure sensor integrated into the cap nut.

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Inventors:
COOKE, Michael (52 Burnt Oak Terrace, Gillingham Kent ME7 1DR, GB)
GUERRATO, Diego (85 Burnt Ash Road, London SE12 8RF, GB)
CALONNE, Jerome (21 Herons Way, Pembury, Tonbridge Kent TN2 4DW, GB)
Application Number:
EP2010/068137
Publication Date:
June 23, 2011
Filing Date:
November 24, 2010
Export Citation:
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Assignee:
DELPHI TECHNOLOGIES HOLDING S.à.r.l. (Avenue de Luxembourg, Bascharage, L-4940, LU)
COOKE, Michael (52 Burnt Oak Terrace, Gillingham Kent ME7 1DR, GB)
GUERRATO, Diego (85 Burnt Ash Road, London SE12 8RF, GB)
CALONNE, Jerome (21 Herons Way, Pembury, Tonbridge Kent TN2 4DW, GB)
International Classes:
F02M61/16; F02M61/14; F16B37/14
Foreign References:
EP1995504A1
RU2140692C1
US4764340A
JP2007205461A
EP1854993A1
EP1854993B1
Attorney, Agent or Firm:
GREGORY, John (Courteney Road, Gillingham Kent ME8 0RU, GB)
Download PDF:
Claims:
Claims

1 . A fastening member (100) for fastening a first component (1 18) to a second component (1 10) in an end-to-end configuration along a common axis (A), the fastening member (100) comprising: a first portion (124) having a threaded region for engagement with a corresponding threaded region (120) of the first component (1 18); a second portion (126) for applying a retaining force to the second component (1 10) to hold the first and second components (1 18, 1 10) in sealing engagement; and a deformable portion (130) intermediate the first and second portions (124, 126); wherein the deformable portion (130) is plastically deformable so as to allow relative movement of the first and second portions (124, 126) in use of the fastening member (100), such that a location (129) on the fastening member can be brought into angular al ignment with a location on the first or second component (1 18, 1 10) upon turning movement of the fastening member.

2. The fastening member of Claim 1 , wherein the location on the fastening member (100) comprises a connection point (129).

3. The fastening member of Claim 1 or Claim 2, wherein the relative movement of the first and second portions (124, 126) is substantially in a direction parallel to the axis (A).

4. The fastening member of Claim 3, arranged so that, in use of the fastening member (100), an initial turning movement of the second portion (126) causes turning movement of the fastening member (100) without deformation of the deformable portion (130), and a subsequent turning movement of the second portion (126) causes turning movement of the fastening member (100) and relative movement of the first and second portions (124, 126) substantially in a direction parallel to the axis (A). The fastening member of any preceding Claim, comprising an internal shoulder (128) where the second portion (124) meets the deformable portion (130), the shoulder (128) being arranged to abut a surface of the second component (1 10) in use of the fastening member (100).

The fastening member of any preceding Claim, wherein the deformable portion (130) comprises a thin-walled region of the fastening member (100).

The fastening member of any preceding Claim, wherein the deformable portion (130) is spaced from the threaded region (120) of the first component (1 18) in use of the fastening member (1 10).

The fastening member of Claim 7, wherein the deformable portion (130) comprises a frusto-conical region (134).

The fastening member of any preceding Claim, wherein at least a part of the deformable portion (130) has a cross-sectional area A/v given by:

A., >

°"y

where Nmm is the minimum retaining force required to form a seal between the first component (1 10) and the second component (1 18); and where ay is the yield stress of the material from which the deformable portion (130) is made.

The fastening member of any preceding Claim, wherein the second portion (126) is shaped to engage with a tool for causing turning movement of the fastening member (100).

The fastening member of Claim 10, wherein the first portion (124) is also shaped to engage with the tool for causing turning movement of the fastening member (100).

A cap nut for a fuel injector, comprising a fastening member (100) accord any preceding Claim. A fuel injector, comprising a cap nut (100) according to Claim 10, a nozzle body holder (1 18) comprising the first component, and a nozzle (1 10) comprising the second component.

A method for assembling an apparatus comprising a first body (1 18) having a first lateral alignment point, a second body (1 10), and a generally tubular fastening member (100) having a second lateral alignment point (129), wherein the first body (1 18), the second body (1 10) and the fastening member (100) are arranged along a common axis (A), and wherein the fastening member (100) is arranged to retain the second body (1 10) against an end (120) of the first body (1 18); the method comprising: applying a predetermined torque to the fastening member (100) sufficient to cause sealing between the second body (1 10) and the first body (1 18); and after applying the predetermined torque, turning the fastening member (100) so as to bring the second alignment point (129) into angular alignment with the first alignment point; wherein turning the fastening member (100) after applying the predetermined torque comprises plastically deforming a portion (130) of the fastening member in an axial direction.

15. The method of Claim 14, wherein turning the fastening member (100) after applying the predetermined torque comprises turning the fastening member (100) through an angle of not more than 360°.

16. The method of Claim 14 or Claim 15, wherein the first and second alignment points comprise electrical connection points, and wherein the method further comprises fitting an axial electrical cable to the apparatus between the electrical connection points.

17. The method of any of Claims 14 to 16, wherein the fastening member comprises: a first portion (124) for engagement with the first body (1 18); a second portion (126) for applying a retaining force to the second body (1 10); and a deformable portion (130) intermediate the first and second portions (124, 126); and wherein applying the predetermined torque to the fastening member (100) and/or turning the fastening member (100) comprises applying a turning force to the second portion (126) of the fastening member.

The method of Claim 1 7, wherein applying the predetermined torque to the fastening member (100) and/or turning the fastening member (100) comprises applying a turning force to the first portion (124) of the fastening member.

Description:
FASTENING MEMBER

Field of the invention The present invention relates to a fastening member. In particular, but not exclusively, the invention relates to a fastening member suitable for use as a cap nut of a fuel injector.

Background to the invention

One type of fuel injector for use in internal combustion engines comprises an elongate, generally tubular or cylindrical nozzle holder body, and a nozzle. The nozzle houses a valve needle of the injector. The valve needle is moveable to control the flow of fuel through outlets in the nozzle. Movement of the valve needle is controlled by an actuator arrangement housed in the nozzle holder body. In use of the injector, the nozzle is positioned in communication with a combustion chamber of an internal combustion engine, and the injection of fuel from the injector into the combustion chamber through the outlets is controlled by operation of the actuator arrangement. As shown in Figure 1 of the accompanying drawings, the nozzle 1 0 comprises an elongate portion 12 of relatively small outside diameter and having an axial bore 14 for receiving the valve needle (not shown), and a collar portion 16 of relatively large outside diameter. The outside diameter of the collar portion 16 approximately matches the outside diameter of an end region 20 of the nozzle holder body 18. The collar portion 16 of the nozzle 10 abuts the end region 20 of the nozzle holder body 18, and is held in position by a fastening member or cap nut 22.

The cap nut 22 comprises a generally tubular element having a first portion 24 with an inside diameter similar to the outside diameter of the end region 20 of the nozzle holder body 18 and the collar portion 16 of the nozzle 10, and a second portion 26 with an inside diameter similar to the outside diameter of the elongate portion 12 of the nozzle 10. The first portion 24 of the cap nut carries internal screw threads that mate with complementary external screw threads provided on the end region 20 of the nozzle holder body 18. The difference in the inside diameter of the first and second portions 24, 26 of the cap nut 22 provides a shoulder 28 of the cap nut 22, which abuts the collar portion 16 of the nozzle 10. In this way, when the cap nut 22 is screwed on to the end region 20 of the nozzle holder body 18, the collar portion 16 of the nozzle 10 is clamped against the nozzle holder body 18 by the cap nut 22.

In some cases, it may be desirable to provide an electrical connection between the nozzle holder body and the nozzle. For example, the present applicant's granted European Patent No. EP-B-1854993 describes an arrangement in which a pressure sensor is integrated into the cap nut of a fuel injector. The pressure sensor is arranged to measure the gas pressure within the combustion chamber associated with the injector, and is located on one side of the cap nut.

In such an arrangement, electrical connections between the injector and a control system of the engine are made at a connection point at an end of the nozzle holder body opposite the nozzle. Consequently, at least one electrical connection, typically a signal cable, must bridge from the connection point on the nozzle holder body to the pressure sensor in the cap nut. Due to space restrictions within the nozzle and the nozzle holder body, it is preferable for the signal cable to be provided on the outside of the body of the fuel injector.

It is desirable to design fuel injector components so that most or all of the assembly of a fuel injector can be automated. For this reason, it is preferred that the signal cable extends in a direction parallel to the common longitudinal axis of the nozzle holder body, the nozzle, and the cap nut.

It will be appreciated that, when the cap nut is screwed on to the nozzle holder body during assembly of an injector of the type described in EP-B-1854993, the angular position of the cap nut pressure sensor may not correspond to the angular position of the connection point on the nozzle holder body. In this event, the signal cable would not extend in the preferred axial direction. Furthermore, due to manufacturing tolerances and other factors, the angular spacing between the pressure sensor and the connection point may vary between injectors, leading to further difficulties in manufacturing the injector using an automated process.

It would therefore be desirable to provide an arrangement which overcomes or alleviates these problems. It is against this background that the present invention has been devised. Summary of the invention

In a first aspect, the present invention provides a fastening member for fastening a first component to a second component in an end-to-end configuration along a common axis. The fasten ing mem ber com prises a fi rst portion having a th readed region for engagement with a corresponding threaded region of the first component, a second portion for applying a retaining force to the second component to hold the first and second components in sealing engagement, and a deformable portion intermediate the first and second portions. The deformable portion is plastically deformable so as to allow relative movement of the first and second portions in use of the fastening member, such that a location on the fastening member can be brought into angular alignment with a location on the first or second component upon turning movement of the fastening member. The location on the fastening member may comprise a connection point. The deformable portion allows relative movement of the first and second portions of the fastening member. Therefore, the fastening member of the invention is useful in applications where it is necessary to bring a location, such as a connection point for an electrical signal cable or a fluid line, or a lug, slot or any other structural feature, on the fastening member into angular alignment with a location on the first or second component. In use, the fastening member can be screwed on to the first component so that a retaining or clamping force is applied to keep the first and second components in contact, and then the fastening member can be turned further to bring a location on the fastening member into angular alignment with a location on the first or second component. The deformable portion deforms when necessary to allow the first portion to move along the threads of the first component, facilitating the further turning movement of the fastening member.

In one embodiment of the invention, the relative movement of the first and second portions is substantially in a direction parallel to the axis. In use of the fastening member, an initial turning movement of the first portion, the second portion or the first and second portions together may cause turning movement of the fastening member without deformation of the deformable portion, and a subsequent turning movement of the first portion, the second portion or the first and second portions together may cause turning movement of the fastening member and relative movement of the first and second portions substantially in a direction parallel to the axis. The fastening member may comprise an internal shoulder where the second portion meets the deformable portion. The shoulder may be arranged to abut a surface of the second component in use of the fastening member. The deformable portion may comprise a thin-walled region of the fastening member. In one arrangement, the deformable portion is spaced from the threaded region of the first component in use of the fastening member. This helps to prevent damage to the deformable portion that could otherwise arise if the deformable portion were to come into contact with the threads of the first component. To this end, the deformable portion may comprise a frusto-conical region.

In one embodiment, at least a part of the deformable portion has a cross-sectional area >Aw given by:

where N mm is the minimum retaining force required to form a seal between the first component and the second component, and where a y is the yield stress of the material from which the deformable portion is made.

The second portion of the fastening member may be shaped to engage with a tool for causing turning movement of the fastening member. For example, the second portion may include flats arranged in a hexagonal configuration for engagement with a hexagonal socket or similar tool. Alternatively, or in addition, the first portion may be shaped to engage with the tool for causing turning movement of the fastening member. When both the first and second portions are shaped to engage with a tool, torsional deformation or twisting of the fastening member during turning movement of the fastening member can be avoided.

The fastening member is particularly suitable for use in a fuel injector, for example of the type having a cap nut including an integrated pressure sensor. Accordingly, from a second aspect, the present invention resides in a cap nut for a fuel injector, comprising a fastening member according to the first aspect of the invention. The cap nut may be arranged to receive a device, for example an electronic device such as a cylinder pressure sensor or other sensor. The device is received in a wall or periphery of the cap nut, laterally spaced from the axis of the cap nut. The device may be received in the second portion of the cap nut. The presence of the deformable portion advantageously allows the angular position of the device to be aligned with a connection point on the first component.

The invention also extends, in a third aspect, to a fuel injector comprising a cap nut according to the second aspect of the invention, a nozzle body holder comprising the first component, and a nozzle comprising the second component. Said another way, the first component of the first aspect of the invention may be a nozzle holder body of a fuel injector, the second component of the first aspect of the invention may be a nozzle of the fuel injector, and the fastening member of the first aspect of the invention may be a cap nut of the fuel injector. The nozzle holder body may comprise a connection point for a signal cable.

A fourth aspect of the invention resides in a method for assembling an apparatus comprising a first body having a first lateral alignment point, a second body, and a generally tubular fastening member having a second lateral alignment point. The first body, the second body and the fastening member are arranged along a common axis, and the fastening member is arranged to retain the second body against an end of the first body. The method comprises applying a predetermined torque to the fastening member sufficient to cause sealing between the second body and the first body and, after applying the predetermined torque, turning the fastening member so as to bring the second alignment point into angular alignment with the first alignment point. Turning the fastening member after applying the predetermined torque comprises plastically deforming a portion of the fastening member in an axial direction. Turning the fastening member after applying the predetermined torque preferably comprises turning the fastening member through an angle of not more than 360°.

The first and second alignment points may comprise electrical connection points, in which case the method may further comprise fitting an axial electrical cable to the apparatus between the electrical connection points.

The fastening member may comprise a first portion for engagement with the first body, a second portion for applying a retaining force to the second body, and a deformable portion intermediate the first and second portions. Applying the predetermined torque to the fastening member and/or turning the fastening member may comprise applying a turning force to the second portion of the fastening member. Alternatively, or in addition, applying the predetermined torque to the fastening member and/or turning the fastening member may comprise applying a turning force to the first portion of the fastening member. In one embodiment of the fourth aspect of the invention, there is provided a method for assembling a fuel injector comprising a nozzle holder body having a first lateral electrical connection point, a nozzle, and a cap nut having a second lateral electrical connection point, wherein the nozzle holder body, the nozzle and the cap nut are arranged along a common axis, and wherein the cap nut is arranged to retain the nozzle against an end of the nozzle holder body. The method comprises applying a predetermined torque to the cap nut sufficient to cause sealing between the nozzle and the nozzle holder body and, after applying the predetermined torque, turning the cap nut so as to bring the second connection point into angular alignment with the first electrical connection point. The method of the fourth aspect of the invention may be carried out using a fastening member according to the first aspect of the invention, or the fastening member may comprise a cap nut according to the second aspect of the invention. The method of the fourth aspect may also be used to assemble a fuel injector according to the third aspect of the invention.

In another aspect, the present invention resides in a fastening member for fastening a first component to a second component in an end-to-end configuration along a common axis. The fastening member comprises a first portion having a threaded region for engagement with a corresponding threaded region of the first component, a second portion for applying a retaining force to the second component to hold the first and second components in sealing engagement, and a deformable portion intermediate the first and second portions.

Preferred or optional features of each aspect of the invention may be used, alone or in appropriate combination, in the other aspects of the invention also.

Brief description of the drawings

Figure 1 of the accompanying drawings, which has been referred to above, is a cross- sectional view of part of a fuel injector having a cap nut of a known type. Embodiments of the present invention will now be described, by way of example only, with reference to the remaining drawings in which like reference numerals are used for like parts and in which: Figure 2 is a cross-sectional view of part of a fuel injector having a cap nut according to one embodiment of the invention;

Figure 3 is a perspective view of the cap nut shown in Figure 2, in isolation; and Figure 4 is a schematic stress-strain curve to illustrate the behaviour of the cap nut of Figures 2 and 3.

Detailed description of preferred embodiments of the invention Figure 2 shows, in cross section, a fastening member or cap nut 100 according to one embodiment of the invention. The cap nut 100 is shown in position in an assembled fuel injector, in which a nozzle holder body 1 18 and a nozzle 1 10 of the fuel injector are connected or fastened to one another by means of the cap nut 100. The nozzle 1 10 comprises an elongate portion 1 12 of relatively small outside diameter and having an axial bore 1 14 for receiving a valve needle (not shown), and a collar portion 1 16 of relatively large outside diameter. The outside diameter of the collar portion 1 16 approximately matches the outside diameter of an end region 120 of the nozzle holder body 1 18. The nozzle holder body 1 18 is a generally cylindrical element that houses an actuator (not shown) for the valve needle. The nozzle 1 10 and the nozzle holder body 1 18 are arranged coaxially along a common axis A, and the collar portion 1 16 of the nozzle 1 10 abuts the end region 120 of the nozzle holder body 1 18.

Referring additionally to Figure 3, the cap nut 100 comprises a generally tubular element having a first portion 124 with an inside diameter similar to the outside diameter of the end region 120 of the nozzle holder body 1 18, and a second portion 126 with an inside diameter slightly larger than the outside diameter of the elongate portion 1 12 of the nozzle 102. The first portion 124 of the cap nut carries internal screw threads that mate with complementary external screw threads provided on the end region 120 of the nozzle holder body 1 18. The cap nut 100 also comprises a deformable portion 130 intermediate the first and second portions 124, 126. The deformable portion 130 is generally tubular, and comprises a cylindrical region 132 connected to a frusto-conical region 134 having a relatively small cone angle. The cylindrical region 132 is adjacent to and connected with the second portion 126 of the cap nut 100, and the frusto-conical region 134 is adjacent to and connected with the first portion 124 of the cap nut. In this way, the deformable portion 130 connects the first portion 124 to the second portion 126. As can be appreciated from Figure 2, the deformable portion 130 is relatively thin in cross-section, compared to the first and second portions 124.

The minimum inside diameter of the deformable portion 130 is located at the cylindrical region 132. This minimum inside diameter is approximately equal to the outside diameter of the collar portion 1 16 of the nozzle 1 10. The inside diameter of the frusto-conical region 134 increases moving from the cylindrical region 132 to the first portion 124 of the cap nut 100. The frusto-conical region 134 meets the first portion 124 approximately mid-way between the inner and outer faces of the first portion 124, so that the end of the frusto-conical region 134 nearest the first portion 124 stands off from the external threads on the lower portion 120 of the nozzle holder body 1 18. In this way, the deformable portion 130 of the cap nut 100 does not foul the threads of the nozzle holder body 1 18, so as to avoid damage to the threads or to the material of the deformable portion 130. Furthermore, if the deformable portion 130 were to be in contact with the threads, the threads may act as local stress-raisers that could cause failure of the deformable portion 130.

The difference in the inside diameter of the deformable portion 130 and the second portion 126 of the cap nut 100 provides a shoulder 128 which abuts the collar portion 1 16 of the nozzle 1 10. In this way, when the cap nut 100 is screwed on to the end region 120 of the nozzle holder body 1 18, the collar portion 1 16 of the nozzle 1 10 is clamped against the nozzle holder body 1 18 by the cap nut 100.

The second portion 126 carries flats (127 in Figure 3) arranged in a hexagonal pattern, to allow the cap nut 100 to be screwed on to the nozzle holder body 1 18 during assembly of the fuel injector. During assembly, a suitable tool is engaged with the flats on the second portion 126, and a torque is applied to the cap nut 100 via the tool so as to cause rotation of the cap nut 100. The second portion 126 also comprises a recess (129 in Figure 3, not shown in Figure 2), for housing a device such as a pressure sensor (not shown). It is necessary to connect the device accommodated in the recess 129 to a corresponding connection point (not shown) provided on the nozzle holder body 1 18 using a signal cable (not shown), once the components of the injector have been assembled. To allow the signal cable to be fitted to the injector using an automated process, it is necessary that the recess 129 and the connection point are in angular alignment once the cap nut 100 is in place. Said another way, the connection point and the recess 129 should lie in a common direction parallel to the axis A of the injector.

The deformable portion 130 of the cap nut 100 is designed so that, during assembly of the injector, the deformable portion 130 can undergo a plastic strain in a direction parallel to the axis A.

During assembly of the injector, the elongate portion 1 12 of the nozzle 1 10 is inserted into the second portion 126 of the cap nut 100, so that the collar portion 1 16 of the nozzle 1 10 is received by the first portion 124 and deformable portion 130 of the cap nut 1 10. The threads of the first portion 124 are engaged with the threads of the end region 120 of the nozzle holder body 1 18, and the cap nut 100 is rotated using a suitable tool. As it rotates, the cap nut 100 moves in an axial direction towards the nozzle holder body.

As noted above, the shoulder 128 of the cap nut 100 abuts the collar portion 1 16 of the nozzle 1 10, and the collar portion 1 16 in turn abuts the end region 120 of the nozzle holder body 1 18. In this way, the cap nut 100 applies an axially-directed clamping force to the nozzle 1 10 and the nozzle holder body 1 18 to retain the nozzle 1 10 in abutment with the nozzle holder body 1 18. The clamping force is sufficient to form a fluid-tight seal between the nozzle 1 10 and the nozzle holder body 1 18. As will be appreciated, when the clamping force is applied, a corresponding axially- directed tensile force is borne by the cap nut 100, and in particular by the deformable portion 130 of the cap nut 100. When the resulting tensile stress in the cap nut 100 exceeds the yield stress of the material of the cap nut 100, plastic deformation of the deformable portion 130 will occur. As will be explained in greater detail below, the cap nut 100 is arranged so that the deformable portion 130 undergoes plastic deformation only when the clamping force reaches or exceeds a minimum level. This minimum clamping force is sufficient to hold the nozzle 1 10 and the nozzle holder body 1 18 in sealing engagement.

Tightening of the cap nut 100 therefore takes place in two consecutive stages. During the first stage, rotation of the cap nut 100 occurs without deformation of the deformable portion 130. The cap nut 100 is turned to increase the clamping force to the minimum level. Since the clamping force achieved is related to the torque applied to the cap nut 100 by the tool, the desired minimum clamping force can be reached by tightening the cap nut 100 to an appropriate minimum torque.

It will be appreciated that, once the minimum torque is reached, the angular orientation of the recess 129 of the cap nut 100 may not correspond to the angular orientation of the connection point on the nozzle holder body 1 18. In other words, the recess 129 and the connection point are angularly misaligned.

In the second stage of tightening, the cap nut 100 is turned further, through an angle corresponding to the angular misalignment of the recess and the connection point. This further turning of the cap nut 100 causes the tensile stress within the deformable portion 130 to increase beyond the yield stress of the material of the deformable portion 130. Consequently, the deformable portion 130 plastically deforms in such a way that the first portion 124 can move relative to the second portion 126 in an axial direction, allowing the first portion 124 to travel along the threaded end region 120 of the nozzle holder body 1 18 without significantly increasing the force that acts to clamp the nozzle 1 10 to the nozzle holder body 1 18 or the corresponding tensile stress in the cap nut 100.

In this way, the recess 129 and the connection point of the nozzle holder body 1 18 can be suitably aligned during the second stage of tightening. A signal cable (not shown) can then be attached to the injector to connect the connection point of the nozzle holder body 1 18 and the device in the recess 129. As shown in Figure 3, a flat 125 is provided on the periphery of the first portion 124 to accommodate the thickness of the signal cable.

By way of comparison, in conventional cap nut such as that shown in Figure 1 , there is no deformable portion. In such a case, once the cap nut is tightened to a suitable torque, any angular misalignment between respective connection points on the cap nut and the nozzle holder body cannot be corrected by further turning of the cap nut. If this were attempted, the torque required to turn the cap nut might exceed the capacity of the tool, the cap nut might fail due to excessive tensile stress, or the nozzle and/or the nozzle holder body might become damaged due to an excessive clamping force.

It is conceivable that, after the first stage of tightening, the respective connection points on the cap nut and the nozzle holder body may already be sufficiently well aligned. In this situation, the cap nut 100 need not be turned further (i.e. the second stage of tightening can be omitted). Alternatively, it may be preferable to turn the cap nut 100 through a further angle in the second stage of tightening, to cause deformation of the deformable portion 130 and to ensure consistency of the alignment.

Referring back to Figure 2, the shape and configuration of the cap nut 100 may have any suitable form, provided that the deformable portion 130 yields only after a sufficient clamping force is established to retain the nozzle 1 10 against the nozzle holder body 1 18, and provided that the cap nut 100 does not fail during assembly or use of the injector.

The material of the cap nut 100 is typically a steel alloy with a relatively high Young's modulus (modulus of elasticity or stiffness). Figure 4 illustrates, schematically, the tensile engineering stress-strain behaviour of the material of the cap nut 100. Initially, as the tensile stress (vertical axis, denoted σ) increases, the material undergoes a linear elastic strain (horizontal axis, denoted ε). Once the tensile stress exceeds the yield stress a y of the material, plastic deformation occurs and the strain or extension of the material increases by a relatively large amount for a small increase in applied stress. Continued plastic deformation results in thinning of the material, until, beyond a certain strain, necking occurs. During necking, the stress required to cause further strain of the material decreases. The maximum tensile stress that can be borne by the material, which is available just before the onset of necking, is known as the ultimate tensile stress OUTS- The strain at this point is denoted SUTS-

Further plastic deformation eventually results in failure of the material, for example by fracture or rupture. This occurs at the failure stress a F and failure strain s F .

In the context of the present invention, it is desirable that necking of the material during deformation of the deformable portion 130 is avoided. If necking were to occur, the tensile stress in the deformable portion 130 may cause deformation to continue to the point of fracture. However, to ensure that a minimum clamping force is applied between the nozzle holder body 1 18 and the nozzle 1 10, some plastic deformation of the deformable portion 130 is always desirable. Consequently, the cap nut 100 is designed such that, at the end of the second stage of tightening, the stress carried by the deformable portion 130 is greater than the yield stress a y but less than the ultimate tensile stress OUTS- This region is highlighted by the box denoted W in Figure 4.

In view of this material behaviour, the dimensions of the deformable portion 130 may be chosen according to the following criteria. If the minimum axial clamping force (i.e. the clamping force sufficient to create a seal between the nozzle 1 10 and the nozzle holder body 1 18 is N min , and σ γ is the yield stress of the material of the cap nut 100, the cross- sectional area A / v of the wall of the deformable portion 130 at its thinnest point (as indicated in Figure 2) is calculated according to the formula:

Accordingly, the deformable portion 130 will plastically deform when the clamping force meets or exceeds the minimum required value.

The minimum cross-sectional area A / v of the wall of the deformable portion 130 depends on the thickness f of the wall and on the diameter D of the deformable portion 130. To ensure that the deformable portion 130 undergoes predominantly axial deformation and to minimise twisting or shear deformation of the deformable portion 130, it is desirable that D is much greater than f. For example, D may be greater than or equal to 20f.

The maximum required axial displacement of the first portion 124 with respect to the second portion 126 is equal to the pitch p of the threads of the first portion 124. An axial displacement equal to p occurs during one full rotation of the cap nut 100 during the second stage of tightening, when the deformable portion 130 is deforming plastically.

To avoid necking, the length L of the deformable portion 130 should be sufficiently long so that the maximum axial strain in the deformable portion 130 does not exceed SUTS- Therefore the length L should satisfy:

P < ae UTS , where a is a safety factor less than 1 to ensure that the material does not neck or rupture. For example, a may be in the range 0.6 to 0.8.

The maximum length L of the deformable portion 130 is, however, constrained by other design constraints of the cap nut 100. Within the overall length /-/ of the cap nut 100, the threaded first portion 124, the deformable portion 130 with length L, and the second portion 126 must be accommodated.

The first portion 124 must be long enough to provide space for a sufficient number of internal threads to engage with the external threads on the end region 120 of the nozzle holder body 1 18. If too few threads are provided, there is a risk of rupture or failure of the threaded joint between the cap nut 100 and the nozzle holder body 1 18.

The second portion 126 must be long enough to allow large enough flats 127 to be provided for engagement of the cap nut fitting tool during assembly of the injector, and to provide sufficient space for the recess 129.

Consequently, the choice of the length L of the deformable portion 130 is a compromise between providing a sufficient length to allow plastic deformation of the deformable portion 130 without necking or failure of the material, and avoiding failure of the threaded joint and difficulties during assembly of the injector.

In some embodiments of the invention, the diameter D of the deformable portion 130 is between approximately 12 mm and approximately 17 mm, the thickness f of the of the wall of the deformable portion 130 is between approximately 0.3 mm and approximately 2 mm, and the length L of the deformable portion 130 is between approximately 10 mm and approximately 40 mm. It will be appreciated, however, that dimensions outside these ranges could also be used if appropriate. Although it is preferable that the deformable portion 130 does not deform by necking, it will be understood that some degree of necking may be acceptable provided that the clamping force applied by the cap nut 100 remains at a sufficient level.

It will be appreciated that the design of the cap nut 100 will be influenced by the choice of material, and that different materials will be appropriate for different applications and cap nut designs. In most cases, a soft engineering steel having good ductility is appropriate. However, the yield stress of the material must be sufficient so that the cap nut 100 is strong enough to withstand the forces that act on it in use. If, in a particular case, the design of the cap nut 100 places constraints on the dimensions of the deformable portion 130, then a particularly ductile alloy could be chosen. To obtain the desired ductility, the cap nut 100 may be heat-treated after manufacture of the cap nut 100.

Typical engineering steels undergo strain-hardening during plastic deformation, as illustrated by the positive gradient of the stress-strain curve of Figure 4 in the working region W. It is desirable, however, that a material with a relatively modest tendency for strain-hardening is chosen, so as to avoid an excessive clamping force being applied when the cap nut 100 is turned to align the connection point and the recess 129.

It is also desirable that the cap nut is arranged so as not to undergo torsional deformation or twisting during assembly, since such deformation could cause misalignment of the recess 129 and the flat 125, misalignment of the cap nut relative to the nozzle holder body 1 18, and/or residual torsional stress in the cap nut.

In a variant of the invention, not shown in the accompanying drawings, the first portion 124 carries additional flats arranged in a hexagonal pattern, similar to the flats 127 on the second portion 126. In use of this variant, during assembly, a suitable tool is engaged with the flats on both the first portion 124 and the second portion 126, and a torque is applied to the cap nut 100 via the tool so as to cause rotation of the cap nut 100. The tool is arranged to permit the first portion 124 to move away from the second portion 126 in an axial direction, in use of the tool.

Because the torque is applied to the first and second portions 124, 126 together in this variant, twisting or torsional deformation of the cap nut 100, and in particular the deformable portion 130, is avoided. Although the present invention is particularly useful in aligning a location on a cap nut with a location on the nozzle holder body of a fuel injector, it will be understood that the fastening member of the invention can be used in other applications. The present invention would find application in any situation in which it is desirable to fasten two components together in an end-to-end manner along a common axis by applying an axially-directed clamping force of an appropriate magnitude. I n such a case, the invention offers the advantage of being able to set the fastening member to a particular rotational or angular orientation with respect to one or both of the components without significantly increasing the clamping force.

Examples of other applications in which the present invention could be used include the alignment of a hydraulic port on a fastening member with an inlet or outlet connection on another component, and the alignment of locating lugs, clearance slots and so on when assembling adjacent components.

As will be appreciated by a person skilled in the art, several modifications and variations of the fastening member could also be made without departing from the scope of the invention as defined in the appended claims.