Abstract:
The invention concerns a motor vehicle air duct also serving as crosspiece forming part of the structure of the vehicle. The invention is characterised in that the duct comprises two half-shells ( 2, 12 ) including each a tubular portion ( 6, 16 ) and two lateral flanks ( 4   a,    4   b;    14   a,    14   b ); it further comprises: two stop elements ( 22   a,    22   b ) defining each a rib projecting relative to one of the contact surfaces of one of the half-shells and having a stop surface arranged immediately proximate to an abutting surface ( 40   a,    40   b ) provided on the other half-shell ( 12 ), a continuous weld bead ( 13   a,    13   b ) extending between the contact surfaces of the half-shells which are in contact with each other.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to an air duct of a motor vehicle. 
   The invention relates more precisely to an air duct of the structuring type, which supports some of the forces to which the vehicle is subjected, the air duct acting in particular also as a crosspiece forming part of the structure of the vehicle and supporting various accessories. 
   The invention aims to provide, in a simple, reliable and inexpensive manner, an air duct having a high degree of mechanical strength and satisfactory sealing with respect to the air that has to circulate inside it. 
   As indicated in the introductory part of FR-A-2 796 577, it is known to produce a motor vehicle air duct extending in a direction of elongation and comprising two half-shells arranged opposite one another, such that each half-shell has:
         a tubular portion extending in the direction of elongation of the duct, the tubular portions defining between them an air cavity extending in said direction of elongation,   two lateral flanks extending one on each side of the tubular portion and each having a contact surface in contact with one of the contact surfaces of the other half-shell, and   a continuous weld seam extends substantially in the direction of elongation of the duct between the contact surfaces of the half-shells which are in contact with one another.       

   Although such a solution provides sealing between the two half-shells, it is not entirely satisfactory as regards mechanical strength in the case of impact. The applicant has noticed that the continuous weld seam has the advantage over a discontinuous weld (for example an ultrasonic weld, which is constituted by a succession of spots) of providing sealing between the half-shells but that it also has a weakness in terms of mechanical strength. 
   When an accident occurs, the air duct is subjected to multi-axial mechanical forces in different regions located in particular at the steering column and the airbag module. Those forces have a component which shears the weld seam and when one of the weld seams is broken in one region, the breakage is propagated rapidly in the direction of elongation of the duct, so that the duct opens laterally. The mechanical strength of the duct is then considerably reduced. 
   This is particularly true in the case of front impact where the front supports of the vehicle have a tendency to move towards one another and to cause the structuring air duct to be bent. 
   consequently, the solution indicated in the introductory part of FR-A-2 796 577 has not proved to be mechanically satisfactory. 
   In addition, JP-A-61 160316 discloses an air duct comprising two half-shells which are in contact with one another by way of lateral flanks and which are held together by foaming. In order to prevent the foam from entering the inside of the duct, baffles formed by ribs projecting relative to the contact surfaces are provided. 
   However, the various embodiments illustrated in JP-A-61 160316 are not entirely satisfactory, in particular as regards the assembly of the two half-shells by screwing, which is very time-consuming. 
   Moreover, the technique of foaming is relatively complex to implement, can be used economically only in certain special cases (duct near the outer surface of the dashboard) and does not, as such, provide a high degree of cohesion between the half-shells and consequently a strong resistance on the part of the duct towards the stresses to which the vehicle is subjected. 
   SUMMARY OF THE INVENTION 
   In order to overcome the above-mentioned disadvantages, the invention proposes that said half-shells also have two stop elements having the following features:
         each stop element defines a rib extending in the direction of elongation of the duct and projecting relative to one of the contact surfaces,   each stop element has, in a direction perpendicular to the contact surface relative to which it projects, an end region,   each stop element has a stop surface extending between the end region and one of the contact surfaces, said stop surface belonging to one of the half-shells and being in the immediate vicinity of an abutment surface provided on the other half-shell.       

   The continuous weld seam and the stop elements are two entirely complementary means. The stop elements not only enable the positioning of the half-shells relative to one another to be facilitated but above all enable the mechanical strength of the duct with regard to the weld seam to be considerably increased. When the duct is subjected to said strong forces, the stop elements abut the stop surfaces, absorb some of the forces to which the duct is subjected and thus relieve the weld, in particular by efficiently countering the torsional and shearing forces to which the weld is subjected. For its part, said continuous weld seam, in a single operation, provides sealing between the half-shells and connects them to one another. 
   Such a continuous weld seam can be produced by providing a strip on two of the contact surfaces and by causing the two half-shells in contact with one another to vibrate or by using a heating mirror. 
   According to a first feature, the invention proposes that:
         each stop element is located between the tubular portion of one of the half-shells and one of the flanks of said half-shell,   the duct also has two recessed regions complementing the stop elements,   said recessed regions incorporate said abutment surfaces.       

   This solution gives a relatively simple, strong and compact duct. 
   According to an additional feature, the invention proposes that:
         each tubular portion has an internal surface partially delimiting the air cavity,   each stop element has an internal surface partially delimiting the air cavity,   the internal surface of the stop elements forms an extension of the internal surface of each tubular portion without forming a step relative thereto.       

   Thus, although the stop elements are located in the immediate vicinity of the air cavity, they do not project into said air cavity and do not disturb the flow inside it. Problems of noise or loss of head are thus avoided. 
   In order to simplify the production of the duct and to simplify the positioning of the two half-shells, the invention proposes that:
         the two stop elements are both located on one of the half-shells,   the two recessed regions are provided on the other half-shell.       

   In order further to improve the strength of the duct, the invention proposes that the duct also comprises, in said end region of the stop elements, at least one weld seam connecting the two half-shells. 
   This weld seam contributes all the more to improving the mechanical strength of the duct because it is not located in the same plane as the weld seam between the contact surfaces. 
   Advantageously, the stop surfaces form an angle of from 45 degrees to 90 degrees with the flat contact surfaces in order to create an efficient abutment for opposing the torsional and shearing forces to which the duct is subjected. 
   According to another advantageous feature of the invention, the weld seam is produced by vibration, so that the duct has the following features:
         the weld seam is formed by a strip extending in the direction of elongation of the duct,   it comprises at least one rectilinear portion and one curved portion,   the stop elements are present in the rectilinear portion and absent from the curved portion.       

   Bearing in mind the relative displacement (translation) between the two half-shells which is necessary in order to produce this type of weld, it is not possible to arrange the stop elements in the curved portion. 
   The invention will emerge even more clearly from the following description which is given with reference to the appended drawings in which: 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a view of a duct according to the invention in cross-section according to the plane marked I in  FIG. 3 , 
       FIG. 2  is a view on an enlarged scale of the detail marked II in  FIG. 1 , 
       FIG. 3  is a perspective exploded view of the duct, 
       FIG. 4  is a view in conformity with  FIG. 2  of a variant of the duct, 
       FIG. 5  is a view in conformity with  FIG. 2  of another variant of the duct. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1  to  3  illustrate a duct  1  comprising an upper half-shell  2  and a lower half-shell  12  extending in a direction of elongation  10 . Each half-shell  2 ,  12  comprises a front flank, upper  4   a  and lower  14   a  respectively, a rear flank, upper  4   b  and lower  14   b  respectively, and a tubular portion, upper  6  and lower  16  respectively, extending between the respective front and rear flanks. The tubular portions  6 ,  16  have an internal surface  26 ,  36  delimiting an air cavity  30 , in this case in two parts. 
   The upper front flank  4   a  has a front upper surface  5   a  in contact with a front lower surface  15   a  belonging to the front lower flank  14   a , while the upper rear flank  4   b  has a rear upper surface  5   b  in contact with a rear lower surface  15   b  belonging to the rear lower flank  14   b . All these contact surfaces  5   a ,  5   b ,  15   a ,  15   b  are substantially flat and extend in a substantially horizontal plane P containing the direction of elongation  10 . 
   The fact that the plane P is substantially horizontal promotes resistance to the forces generated by a front impact where the front supports of the vehicle have a tendency to cause bending of the duct  1  in a vertical plane containing the direction of elongation  10 . 
   The upper contact surfaces  5   a ,  5   b  and lower contact surfaces  15   a ,  15   b  are connected to one another by means of two strips  13   a ,  13   b  (only one has been shown in  FIG. 3 ) which project from each of the lower contact surfaces  15   a ,  15   b  and which merge with the respective upper contact surface  5   a ,  5   b , in this case by setting in relative vibration the surfaces in contact with one another. 
   The upper half-shell  2  also comprises a rectilinear front rib  22   a  projecting from the front upper contact surface  5   a  and a likewise rectilinear rear rib  22   b  projecting from the rear upper contact surface  5   b . The ribs  22   a ,  22   b  are located between the upper tubular portion  6  and each of the upper flanks  4   a ,  4   b . They tend to extend the upper tubular portion beyond the upper flanks  4   a ,  4   b  and each have an end region  32   a ,  32   b  forming a crest, an internal surface  34   a ,  34   b  and an external surface  46   a ,  46   b.    
   The internal surfaces  34   a ,  34   b  form an extension of the internal surfaces  26 ,  36  of the upper  6  and lower  16  tubular portions. In particular, the internal surface of the duct  1 , including the internal surfaces  34   a ,  26 ,  34   b  and  36 , has neither a step nor an interruption in curvature. 
   The external surfaces  46   a ,  46   b  of the ribs  22   a ,  22   b , respectively, are substantially flat and extend substantially perpendicularly to the upper contact surfaces  5   a ,  5   b  from which they project. 
   The lower half-shell  12  has, opposite each of the ribs  22   a ,  22   b , a recessed region  24   a ,  24   b  comprising a base surface  38   a ,  38   b  and an abutment surface  40   a ,  40   b.    
   The abutment surfaces  40   a ,  40   b  are located opposite and in the immediate vicinity of (the spacing has been increased in the Figures for reasons of clarity) the external surfaces  46   a ,  46   b  in order to act as an abutment for the ribs  22   a ,  22   b  when the duct  1  is subjected to torsional or shearing forces. 
   In addition, the ribs  22   a ,  22   b  are welded to the lower half-shell  12  in their end region  32   a ,  32   b , by way of a strip  42   a ,  42   b  projecting from the base surface  38   a  of the recessed regions  24   a ,  24   b.    
   As illustrated in  FIG. 3 , the duct has rectilinear portions and curved portions. The ribs  22   a ,  22   b  are present only in the rectilinear portions and extend in the direction of elongation  10 . On the other hand, the ribs  22   a ,  22   b  are absent from the curved portions owing to the constraints imposed by the vibration welding process. By using another welding process, for example, by heating mirror, it would be possible to arrange the ribs along the entire length of the duct  1 . 
   The duct  1 , in this case its lower half-shell  12 , incorporates supports  48   a ,  48   b  for securing to the structure of the vehicle. 
   Depending on the level of force that the duct has to withstand, the duct is advantageously produced from acrylonitrile butadiene styrene (ABS) copolymer, or from polypropylene charged with talc at from 10% to 40% (by weight) and reinforced with glass fibres. Other materials would also be suitable. In particular, it is generally advantageous for the duct to be produced from a hybrid material comprising a metal core overmolded with plastics material. 
     FIG. 4  illustrates a variant of the duct which is basically distinguished from the embodiment illustrated in  FIGS. 1  to  3  in that the ribs have been located slightly away from the tubular portions  106 ,  116  of the half-shells  102 ,  112 . Elements corresponding to those in  FIGS. 1  to  3  have a reference increased by 100. 
   Bach recessed region  124   a  comprises a flange  150   a  having a first surface  152   a  defining a portion of the internal surface  136  delimiting the air cavity and a second surface  154   a  forming an abutment surface opposite the internal surface  134   a  of the corresponding rib  122   a . Each of the recessed regions  124   a  in which one of the ribs  122   a  is inserted thus defines near the air cavity  130  a groove delimited by the base surface  138   a  and the abutment surfaces  140   a ,  154   a  extending one on each side of said rib  122   a . The flanges  150   a  prevent the strips  142   a  from extending, after welding, into the air cavity  130  and generating air flow noise. 
     FIG. 5  illustrates a variant of the duct which is basically distinguished from the embodiment illustrated in  FIGS. 1  to  3  in that the ribs have been located well away from the tubular portions of the half-shells, so that each upper flank  204   a  is located between one of the ribs  222   a  and the upper tubular portion  206  of the upper half-shell  202 . Elements corresponding to those in  FIGS. 1  to  3  have a reference increased by 200. 
   Each internal surface  234   a  is to act as an abutment for one of the ribs  222   a  and, more precisely, for the abutment surface  240   a  of the corresponding recessed region  224   a . Thus there is no risk that, after welding, the strips  242   a  will extend into the air cavity  230  and generate air flow noise.