Patent ID: 12252049

DETAILED DESCRIPTION

In the various figures, the same references designate identical or similar elements. For conciseness, only those elements which are useful to understanding the described embodiment are shown in the figures and are described in detail below.

In the following description, when referring to absolute position qualifiers, such as the terms “front”, “rear”, “top”, “bottom”, “left”, “right”, etc., or relative qualifiers, such as the terms “above”, “below”, “upper”, “lower”, etc., or to orientation qualifiers, such as “horizontal”, “vertical”, etc., unless otherwise specified these are in reference to the orientation of the figures or of a vehicle seat in its normal position of use.

In particular, the longitudinal direction X means the longitudinal direction of the seat. The longitudinal direction X of the seat is considered to be the same as the longitudinal direction of the motor vehicle in which the seat is mounted. This longitudinal direction X corresponds to the normal direction of travel of the vehicle. The longitudinal direction X is horizontal. The transverse direction Y of the seat thus corresponds to the transverse or lateral direction of the motor vehicle. This transverse direction corresponds to a direction perpendicular to the normal direction of travel of the vehicle. The transverse direction Y is horizontal. Finally, the vertical direction Z is a vertical direction of the seat, perpendicular to the longitudinal and transverse directions.

FIG.1schematically represents a vehicle seat10, in particular a motor vehicle seat, mounted on a slide mechanism12.

The seat10illustrated inFIG.1comprises a seating portion14, with a seating portion frame16, a backrest18, with a backrest frame20and a base22, and a headrest24. The backrest frame20here is pivoting about a first transverse axis A1, relative to the base22. To do so, a hinge mechanism26is provided on each side of the seat10, between the backrest frame20and the one of the two hinge gusset plates28forming the base22in the example shown.

The seating portion frame16is mounted so as to pivot relative to the backrest frame20about a second transverse axis A2. In addition, near its front end, the seating portion frame16is mounted so as to pivot about a third transverse axis A3, relative to a connecting rod30arranged on each side of the seat10. Each connecting rod30is mounted so as to pivot about a fourth transverse axis A4relative to one of the slides32of the slide mechanism12. More specifically, each connecting rod30is mounted so as to pivot about the fourth transverse axis A4relative to the movable profile34of a respective slide32.

Each movable profile34is part of a respective slide32and is associated with a fixed profile36. The fixed profile36is also called a rail or female profile. The fixed profile36is fixed to the floor38of the motor vehicle.

In this example, the seat10comprises a manual control element40, called a release bar, for controlling the sliding of the slides32. This manual control element40allows in particular locking and unlocking the systems for stopping the sliding of the movable profiles34relative to the respective fixed profiles36. Once these stop systems have been unlocked, the manual control element40can also be used as a gripping element, to cause the movable profiles34to slide relative to their respective fixed profiles36of the slide32, in the longitudinal direction X. The fixed36and movable34profiles of the slides32are generally made of metal.

Alternatively, the movement of the movable profiles34relative to the fixed profiles36is controlled by means of one or more actuators.

In the following, we will focus more particularly on the backrest18of the seat10, as illustrated byFIGS.2to5.

As indicated above, the backrest18comprises a backrest frame20and a base22. The base22makes it possible to fix the backrest frame20to the movable profiles34of the slides32. More specifically, each hinge gusset plate28is fixed to the movable profile34of a respective slide32. A lock may be provided to selectively lock/unlock the base22and therefore the backrest18with respect to the movable profiles34of the slides32.

The backrest frame20comprises in particular, according to the example illustrated, two uprights42, an upper crosspiece44, and a lower crosspiece46. The two uprights42, the upper crosspiece44, and the lower crosspiece46are fixed together here to form a framework48. The framework48is substantially rectangular here. The backrest frame20further comprises, in the example ofFIG.2, a metal plate50fixed to the framework48.

The two hinge mechanisms26are connected together by a rod52enabling the two hinge mechanisms26to be controlled simultaneously.

Furthermore, the two hinge gusset plates28are connected together by a crosspiece54. Here the crosspiece is formed by a tube54. The tube54extends mainly along an axis A5, transverse to the seat10. Thus, the longitudinal direction of the tube54(or direction of the longitudinal axis A5of the tube54) mounted on the seat10corresponds substantially to the transverse direction Y of the seat10.

Here the tube54has a polygonal section, more precisely rectangular, even more precisely square. The tube54thus has four sides541,542,543,544, with parallel pairs. Side is understood to mean a side of the tube54, the wall forming this side of the tube54. Each side541,542,543,544is thus in three dimensions, even if one of its dimensions is in principle much smaller than the other two, in particular at least ten times smaller, preferably at least one hundred times smaller than the other two.

The tube54is received, at each of these two longitudinal ends, in a hole56in a respective hinge gusset plate28. As is more particularly visible inFIG.3, the hole56here has a “U” section to facilitate the formation of the folded tongue66, described below, by stamping.

As is more particularly visible inFIG.4, a first end58, here a free end, of a first side541of the tube54is offset along the direction of the longitudinal axis A5of the tube54, relative to a second end60, here a free end, of a second side542of the tube54, the first and second sides541,542being parallel. The first58and second60ends of sides541,542are in the vicinity of a same longitudinal end of the tube54. Thus, here the second side542is shorter than the first side541, in the direction of the longitudinal axis A5of the tube54, in the vicinity of the longitudinal end of the tube54considered.

As is particularly visible inFIG.5, near the first end58, the tube54and the hinge gusset plate28define a first area of contact62of the first side541with a first surface64of the hinge gusset plate28. Here, the first surface64is the surface facing the tube54, of a tongue66extending from an edge of the hole56. The tongue66extends substantially perpendicularly to the remainder of the hinge gusset plate28, near the hole56.

Near the second end60, the tube54and the hinge gusset plate28define a second area of contact68of the second side542with a second surface70of the hinge gusset plate28. Here, the second surface70of the hinge gusset plate28is the surface of a ramp72extending from the vicinity of the hole56. The ramp72and the tongue66extend here one on either side of the hole56, along the direction of the longitudinal axis A5of the tube54. In other words, the ramp72and the tongue66extend in two opposite directional orientations, from the vicinity of the hole56in the hinge gusset plate28. Viewed in the cross-section ofFIG.5, the ramp72is inclined in the direction of the first side541of the tube54, near its end74furthest from the hole56, here a free end.

As is particularly visible inFIG.5, the first and second areas of contact62,68are offset along the direction of the longitudinal axis A5of the tube54. In addition, the gusset plate28and the tube54are shaped here so that the first side541of the tube54is not covered on its face opposite the face in contact with the tongue66. Thus, as illustrated inFIG.5in particular, it is possible to create a first laser weld76between the first side541and the tongue66by means of a laser78, then a second laser weld80between the second side542of the tube54and the ramp72of the hinge gusset plate28with the same laser78, kept within a same half-space delimited by the extension plane P1of the first side541or the extension plane P2of the second side542of the tube54. In other words, the tube54and the gusset plate28are shaped so that the first and second areas of contact62,68, can be reached by a laser beam emitted from a same half-space delimited by an extension plane of the second side542.

Thus, for example, the first weld76is created by placing the laser78in a first position E1located perpendicularly to the first side541of the tube54, in the half-plane delimited by the extension plane P2of the second side542of the tube54. Once the first weld76has been created, the laser78is moved to a second position E2located perpendicularly to the second side542of the tube54, at the second area of contact68, in order to create the second weld80there. Advantageously, the movement of the laser78from the first position E1to the second position E2is carried out while always keeping the laser78in an orientation allowing the emission of a laser beam normal to the first and/or the second sides541,542of the tube54. In other words, the laser78is moved from the first position E1to the second position E2, by translational movement(s), preferably along one or more straight lines. The corresponding movement is advantageously devoid of any relative rotation of the assembly of the tube54with the hinge gusset plate28.

It should be noted that according to the example illustrated inFIG.5, the face of the first side541in contact with the tongue66and the face of the second side542in contact with the ramp72have the same orientation. We define here the orientation of a surface, by the direction and the directional orientation of a normal to this surface. Thus, here, these two faces are oriented in a direction perpendicular to the axis A5of the tube54, facing the same way (here towards the top ofFIG.5). With such an orientation of the faces of the first and second sides541,542in contact with the hinge gusset plate28, it is possible to create the first and second welds78,80with an incident laser beam, each time striking a side541,542of the tube54. This makes it possible to obtain a better quality of laser welding because the laser beam strikes the thinnest part. Here, in fact, the sides541,542of the tube54are thinner than the sheet metal from which the hinge gusset plate28is formed.

Alternatively, the first position E1′ of the laser78for the creation of the first weld76is located between the extension plane P1of the first side541of the tube54and the extension plane P2of the second side542of the tube54. In this case, the two positions E1′, E2of the laser78for the creation of the two welds76,80are within a same half-space delimited by the extension plane P1of the first side541of the tube54.

In both cases, the two welds76,80can be made while keeping unchanged the orientation of the laser78relative to the tube54. This is particularly practical for implementing the assembly of the tube54with the hinge gusset plate28. Indeed, it is then possible to create the two welds76,80at the same station of a production line, without having to pivot the laser78relative to the tube54.

The first and second welds76,80are for example parallel.

In the following, an example100of a method of producing a backrest18as described above is described in more detail, with reference toFIGS.5to8.

The method100of producing a backrest18comprises, according to the example ofFIG.6, a first step102consisting of providing on the one hand at least two uprights42, an upper crosspiece44and/or a lower crosspiece46which are intended to form the backrest frame20, and, on the other hand, two hinge gusset plates28and a tube54which are intended to form the base22. A metal plate50intended to be integrated into the backrest frame20may also be provided in this first step102.

During a second step104, the tube54is inserted into the hole56of at least one of the two hinge gusset plates28, as shown inFIG.7. Preferably, during this second step104, the tube54is preferably inserted into the two hinge gusset plates28, a respective longitudinal end of the tube54being inserted into a hole56of each hinge gusset plate28.

Once the tube54is inserted into a hole56of a hinge gusset plate28, the tube54and the hinge gusset plate28define a first area of contact62of a first side541with a first surface64of the hinge gusset plate28. Here, the first surface64is the surface, facing the tube54, of a tongue66extending from an edge of the hole56. The tongue66extends substantially perpendicularly to the remainder of the hinge gusset plate28, near the hole56.

Similarly, the tube54and the hinge gusset plate28then also define a second area of contact68of a second side542with a second surface70of the hinge gusset plate28, near a second end60of the second side542. Here, the second surface70of the hinge gusset plate28is the surface of a ramp72extending from the vicinity of the hole56. Here the ramp72and the tongue66extend one on either side of the hole56, along the direction of the longitudinal axis A5of the tube54. The ramp72is inclined in the direction of the first side541of the tube54, near its end74furthest from the hole56, here a free end.

The ramp72ensures contact between the second surface70of the gusset plate28and the second side542of the tube54, despite the manufacturing tolerances of the tube54and the hinge gusset plate28.

The first and second areas of contact62,68are offset along the direction of the longitudinal axis A5of the tube54. In addition, the gusset plate28and the tube54are shaped here so that the first side541of the tube54is not covered on its face opposite the face in contact with the tongue66. Thus, the first and second areas of contact62,68can be reached by a laser beam emitted by a laser located in a same half-space, delimited by the extension plane of the first side541or the second side542of the tube54.

Advantageously, during this second step104, the two uprights42, the upper crosspiece44and/or the lower crosspiece46, and the metal plate50where applicable, are also assembled together, so as to form a preliminary assembly of the backrest frame20. In this preliminary assembly of the backrest frame20, the two uprights42, the upper crosspiece44and/or the lower crosspiece46, and the metal plate50if applicable, are assembled together but are not fixed together, at least not permanently fixed together.

During a third step106, a first laser weld76is created by emitting a first laser beam L1with a laser78in a first position E1, in the direction of the first area of contact62. Advantageously, the first laser beam L1has a direction of incidence that is substantially normal to the first area of contact62. One will note here that the first position E1of the laser78is within the half-space delimited by the extension plane P2of the second side542of the tube54, not comprising the first side541.

During a fourth step108, the laser78is moved relative to the assembly formed by the tube54received in each hinge gusset plate28, to a second position E2of the laser78. It is noteworthy that the first and second positions E1, E2of the laser78are within the same half-space delimited by the extension plane P2of the second side542of the tube54, not comprising the first side541. Thus, advantageously, it is possible during all of the fourth step108to keep the laser78within the same half-space delimited by the extension plane P2of the second side542of the tube54, not comprising the first side541.

During this fourth step108, it is also advantageous to move the laser78relative to the tube54and each hinge gusset plate28only by translational movement(s), without any relative rotation of the laser78with respect to the tube54. Such a displacement only by translational movement(s) is more easily achievable than a displacement including at least one relative rotation. Such displacement only by translational movement(s) may in particular be easily carried out on the same station of an assembly line. Furthermore, such a displacement only by translational movement(s) limits the adjustments necessary between the creation of the two welds with the laser78. In particular, it is unnecessary in principle to readjust the focal length of the laser78after such a movement.

During a fifth step110, a second laser weld80is created by emitting a second laser beam L2with the laser78from the second position E2, in the direction of the second area of contact68. Advantageously, the second laser beam L2has a direction of incidence substantially normal to the second area of contact62.

The first and second laser welds76,80are for example parallel.

Finally, during a sixth step112, the two uprights42, the upper crosspiece44and/or the lower crosspiece46, and the metal plate50if applicable are fixed together. This sixth step112may in particular include or even consist of the creation of laser welds82. Advantageously, here, these laser welds82are created with the same laser78as for the first and second welds76,80, the laser78being kept within the same half-space delimited by the extension plane P2of the second side542of the tube54, not comprising the first side541. During this sixth step112, it may be particularly advantageous to move the laser78, relative to the backrest81ready for welding, by translational movement(s) only. Here, each time, the laser beam emitted by the laser78is directed onto the metal plate50whose thickness is less than the thickness of the walls of the two uprights42, of the upper crosspiece44and/or of the lower crosspiece46to which the metal plate50is fixed by laser welding. Welds82of better quality are thus created.

FIG.9illustrates a second example of the assembly of a tube54in a frame part28of a vehicle seat10. This second exemplary assembly differs from the first assembly described above in that the tube54is arranged, in a direction perpendicular to the axis A5of extension of the tube54, between two tongues66,84extending, in the section view ofFIG.9, substantially parallel to the axis A5of extension of the tube54, from the edge of the hole56in the hinge gusset plate28.

Near the first end58, the tube54and the hinge gusset plate28define a first area of contact62of the first side541with a first surface64of the hinge gusset plate28. Here, the first surface64is the surface, facing the tube54, of a tongue66extending from an edge of the hole56. The tongue66extends substantially perpendicularly to the rest of the hinge gusset plate28, near the hole56.

Near the second end60, the tube54and the hinge gusset plate28define a second area of contact68of the second side542with a second surface70of the hinge gusset plate28. Here, the second surface70of the gusset plate28is the surface of a second tongue84, extending from the vicinity of the hole56. The first and second tongues66,84extend here one on either side of the hole56, along the direction of the longitudinal axis A5of the tube54. In other words, the first and second tongues66,84extend in two opposite directional orientations, from the vicinity of the hole56in the hinge gusset plate28. Here, the first and second tongues66,84are substantially parallel.

As in the first example, the first and second areas of contact62,68are offset along the direction of the longitudinal axis A5of the tube54. In addition, here the gusset plate28and the tube54are shaped so that the first side541of the tube54is not covered on its face opposite the face in contact with the tongue66. Thus, it is possible to create a first laser weld76between the first side541and the tongue66by means of a laser78, then a second laser weld80between the second side542of the tube54and the second tongue84of the hinge gusset plate28with the same laser78, kept within a same half-space delimited by the extension plane P1of the first side541or the extension plane P2of the second side542of the tube54.

Thus, also in this example, the tube54and the gusset plate28are shaped so that the first and second areas of contact62,68are reachable by a laser beam emitted from a same half-space delimited by an extension plane of the second side542.

Thus, for example, the first weld76is created by placing the laser78in a first position E1located perpendicularly to the first side541of the tube54, within the half-plane delimited by the extension plane P2of the second side542of the tube54. Once the first weld76has been created, the laser78is moved to a second position E2located perpendicularly to the second side542of the tube54, at the second area of contact68, in order to create the second weld80there. Advantageously, the movement of the laser78from the first position E1to the second position E2is carried out while always keeping the laser78in an orientation allowing the emission of a laser beam normal to the first and/or second sides541,542of the tube54. In other words, the laser78is moved from the first position E1to the second position E2, by translational movement(s). The corresponding movement is advantageously devoid of any relative rotation of the assembly of the tube54with the hinge gusset plate28.

Alternatively, the first position E1′ of the laser78for the creation of the first weld76is located between the extension plane P1of the first side541of the tube54and the extension plane P2of the second side542of the tube54. In this case, the two positions E1′, E2of the laser78for the creation of the two welds76,80are within a same half-space delimited by the extension plane P1of the first side541of the tube54.

Here, however, the face of the first side541in contact with the tongue66and the face of the second side542in contact with the second tongue84have opposite orientations. Here, these two faces are oriented in a direction perpendicular to the axis A5of the tube54, but facing in opposite directions.

As a result, the second laser weld80is created by means of an incident laser beam, impacting the surface of the second tongue84formed by the hinge gusset plate28and not the second side542of the tube54as in the first exemplary assembly. By doing so, the second weld80may be significantly less resistant and/or more difficult to produce than in the context of the first exemplary assembly. On the other hand, however, the geometry of the tube54and of the hinge gusset plate28are significantly simpler.

The two welds76,80may, however, be created while keeping the orientation of the laser78unchanged relative to the tube54. This is particularly practical for implementing the assembly of the tube54with the hinge gusset plate28. Indeed, it is then possible to create the two welds76,80on a same station of a production line, without having to pivot the laser78relative to the tube54.

The second exemplary assembly illustrated inFIG.9may be produced by implementing a method substantially identical to the one described above for producing the first exemplary assembly.

The present disclosure is not limited to the examples indicated above but is capable of numerous variants accessible to those skilled in the art.

In particular, in the examples described, the assembly described is that of a tube with a hinge gusset plate. Alternatively, the tube may be assembled to a flange. Such a flange rigidly connects the backrest frame to the seat support. The seat support may in particular be a movable profile of a slide or the floor of the passenger compartment. A similar assembly may be provided for a tube into any frame part. Such a frame part is advantageously made from a metal sheet, shaped in particular by stamping, bending, cutting, and/or any other process for shaping a metal sheet, accessible to those skilled in the art.

This description relates to a method for assembling a tube in a frame part of a vehicle seat, in particular of a motor vehicle seat. Also described is a method of manufacturing a vehicle seat backrest making use of this method for assembling a tube in a frame part of a vehicle seat. Also described is an assembly of a tube in a frame part of a vehicle seat, in particular of a motor vehicle seat, a backrest comprising such an assembly, and a vehicle seat, in particular a motor vehicle seat, comprising such a backrest.

A comparative vehicle seat backrest may be fix one or more crosspieces to elements forming all or part of the uprights of the seat frame. The comparative elements in question, hereinafter referred to as frame parts, may in particular be a flange or a gusset plate of a hinge mechanism. The crosspiece is for example formed by a tube of circular section. A square section tube is preferred, however, because such a tube is more rigid.

In one example, the comparative crosspiece is fixed to the frame part by MAG welding (for “Metal Active Gas”). To do this, the crosspiece may be inserted into a hole through the frame part. Then, near the end of the crosspiece, a MAG weld is made between the crosspiece and the frame part.

When the tube has a square section, two welds on two opposite faces of the tube may be used in order to provide good mechanical strength.

Although laser welding technology is generally preferred, it is difficult to implement as a replacement for MAG welding of the tube to the frame part. In this case, indeed, it is necessary to rotate the laser relative to the assembly formed by the tube and the frame part, in order to create the two welds. Such movement of the laser relative to the assembly of tube and frame part generally uses a change of station on the assembly line, which lengthens the manufacturing time of the seat and increases its cost.

This disclosure seeks to improve the situation.

To this end, a method is described for assembling a tube into a frame part of a vehicle seat, the method comprising the steps of:

i) providing a tube extending mainly along the direction of a longitudinal axis of the tube, an end portion of the tube being such that a first free end of a first side of the end portion is offset along the direction of the longitudinal axis of the tube, relative to a second free end of a second side of the end portion;

ii) providing a frame part in the form of a shaped metal plate;

iii) arranging the tube in contact with the frame part, so that:the first side of the end portion of the tube and a first surface of the frame part define a first area of contact of the tube with the frame part, near the first free end, andthe second side of the end portion of the tube and a second surface of the frame part define a second area of contact of the tube with the frame part, near the second free end,

the first and second areas of contact of the tube with the frame part being offset relative to each other, along the direction of the longitudinal axis of the tube;

iv) successively creating, by means of a same laser, a first weld at the first area of contact then a second weld at the second area of contact.

Thus, advantageously, the first and second contact surface, where the welds are made, can be more easily accessible by a laser beam.

According to preferred embodiments, the method comprises one or more of the following features, taken alone or in combination:at least the end portion of the tube is of polygonal section, in particular rectangular, more particularly square;the laser is moved between the first and the second weld within a half-space delimited by an extension plane of the first or second side;between the first weld and the second weld, the laser is moved relative to the tube and to the frame part, by translational movement(s) only;the first and second sides are parallel;the face of the first side in contact with the first surface and the face of the second side in contact with the second surface have the same orientation;the second surface is on a ramp of the frame part, the ramp being inclined relative to the direction of the longitudinal axis of the tube;the frame part has a hole in a face of the frame part, substantially normal to the direction of the longitudinal axis of the tube, the hole being shaped to receive at least one among the first free end of the first side of the tube and the second free end of the second side of the tube, the hole preferably being shaped to receive only one among the first free end of the first side and the second free end of the second side of the tube;

the first and second surfaces extend one on either side of the hole, along the direction of the longitudinal axis of the tube;the ramp is divergent in the direction of insertion of the tube into the hole of the frame part;in step iii), a laser beam is emitted on a side of the tube each time, the thickness of the first and second sides of the tube being less than the thickness of the frame part at the first and second surfaces respectively; andthe frame part is a flange or a hinge gusset plate, of a vehicle seat backrest.

According to another aspect, a method of manufacturing a vehicle seat backrest is described, comprising the steps of:

a) providing at least two uprights, an upper crosspiece and/or a lower crosspiece, two frame parts, and a tube,

b) assembling the tube to at least one among the two frame parts, by implementing a method as described above in all its combinations;

c) assembling together the uprights, the upper crosspiece and/or the lower crosspiece, and the two frame parts.

Preferably, in step c), a plate is also fixed to the uprights and/or to the upper crosspiece and/or to the lower crosspiece, preferably by laser welding.

According to another aspect, also described is an assembly of a tube into a frame part of a vehicle seat, comprising:a tube extending mainly along a direction of a longitudinal axis of the tube, an end portion of the tube being such that a first end of a first side of the end portion is offset along the direction of the longitudinal axis of the tube, relative to a second end of a second side of the end portion,a frame part of a vehicle seat, in the form of a shaped metal plate, the frame part defining a first surface and a second surface which are distinct,

the tube and the frame part being shaped to define a first area of contact of the first side with the first surface near the first end and a second area of contact of the second side with the second surface near the second end,

the tube and the frame part being fixed together by laser welding at least at the first area of contact and at a second weld at the second area of contact.

According to preferred embodiments, the assembly comprises one or more of the following features, taken alone or in combination:the tube and the frame part are shaped so that the first area of contact of the first side with the first surface and the second area of contact of the second side with the second surface are reachable by a laser beam emitted from a same half-space delimited by an extension plane of the first or second side;at least the end portion of the tube is of polygonal section, in particular rectangular, more particularly square;the first and second sides are parallel;the first and second welds are parallel;the face of the first side in contact with the first surface and the face of the second side in contact with the second surface have the same orientation;the second surface is on a portion of the frame part forming a ramp, inclined relative to the direction of the longitudinal axis of the tube;the frame part has a hole in a face of the frame part, substantially normal to the direction of the longitudinal axis of the tube, the hole receiving at least one among the first end of the first side of the tube and the second end of the second side of the tube, the hole preferably receiving only one among the first end of the first side and the second end of the second side of the tube;the first and second surfaces extend one on either side of the hole, along the direction of the longitudinal axis of the tube; andthe frame part is a flange of a backrest or a gusset plate of a hinge of a vehicle seat backrest.

According to another aspect, a vehicle seat backrest is described, comprising at least two uprights, and an upper crosspiece and/or a lower crosspiece, two frame parts, and a tube assembled together, at least one frame part and the tube forming an assembly as described above, in all its combinations.

According to yet another aspect, a vehicle seat is described comprising a seating portion and a backrest as described above, in all its combinations, the backrest being fixed to the seating portion.

A method for assembling a tube (54) into a frame part (28) of a vehicle seat (10) comprises the steps of:

i) providing a tube (54) such that a first free end (58) of a first side (54i) of an end portion of the tube (54) is offset longitudinally relative to a second free end (60) of a second side (542) of the end portion;

ii) providing a frame part (28) in the form of a metal plate;

iii) arranging the first and second free ends (58;60) of the tube (54) in contact with the frame part (28), in two longitudinally offset areas of contact; and

iv) successively creating, by means of a same laser (78), a weld (76;80) at the first and second areas of contact (62;68).