Abstract:
A bumper for a motor vehicle has a cross member disposed transversely in relation to side rails of a vehicle frame and is supported on the side rails by crash boxes. The cross member includes a central longitudinal section having opposite ends, a crash box attachment section with a substantially vertical rear wall and two legs, and an end section having in cross-section a U-shaped configuration defined by two legs and a web which interconnects the legs and forms an outward-projecting bulge. The bulge is joined with the substantially vertical rear wall of the attachment section.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2009 006 609.8, filed Jan. 29, 2009, pursuant to 35 U.S.C. 119(a)-(d), the content(s) of which is/are incorporated herein by reference in its entirety as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a bumper for a motor vehicle. 
     The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
     Bumpers are provided as standard equipment on both the front and the rear of motor vehicles to absorb the impact energy caused by minor impacts and to prevent or minimize damage to the actual structural support of the motor vehicle. A bumper typically includes a cross member which can be attached transversely to the side rails by way of crash boxes. The function of the cross member is to tranfer the energy resulting from an impact into the crash boxes, where the impact energy is converted into deformation energy. The system is here matched such that the crash boxes are located close to the center on the side rail of a motor vehicle, with the impact energy being transferred to the crash boxes and hence to the side rails via the cross member with minimal bending moment. 
     Various bumpers of different designs are known in the art. For example, U.S. Pat. No. 6,349,521 B1 discloses a bumper with a rigid hollow cross member. The cross-section of the cross member varies along its length so as to provide high energy absorption commensurate with the load. 
     EP 1 730 002 B1 discloses a bumper with a cross member having a U-shaped longitudinal profile that is closed off with a cover plate. EP 1 736 369 A2 also discloses a saucer-type cross member. 
     DE 102 06 011 A1 and DE 101 35 903 A1 disclose cross members provided with indentations in the attachment regions with the crash boxes, wherein the indentations engage in the open ends of the crash boxes. The intent is here to provide transverse stability to the cross member. 
     Bumpers must satisfy regulatory requirements as well as demands from consumer protection agencies aimed at protecting pedestrians. In addition, requirements from the classification tests of insurers (RCAR, GDV) must be satisfied. The automobile manufacturers typically also require optimal utilization of the installation space, ease of installation, high energy absorption capability and lowest possible weight as well as acceptable costs. In addition, the goal of reducing CO 2 -emission requires a consequent implementation of lightweight construction in vehicle design. However, the required weight reduction and the insurance classification tests tend to contradict one another. 
     It would therefore be desirable and advantageous to address this problem and to obviate other prior art shortcomings by providing a fully functional bumper which is lightweight and has an improved loading capacity. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a bumper for a motor vehicle has a cross member which is attached to longitudinal side rails of the vehicle frame by way of crash boxes. The cross member has a center longitudinal section and ends, which are connected to a corresponding crash box attachment section and an end section. Each crash box attachment section has a U-shaped configuration and a substantially vertical real wall and an upper and a lower leg. The end sections also have a U-shaped cross-section defined by a web and two legs. According to the invention, each web of an end section connects into the rear wall of a crash box attachment section by forming a bulge, which improves the loading characteristic and, in particular, increases the stiffness of the cross member in the region where the cross member is attached to the crash boxes. The impact energy is transmitted to the crash box by way of the cross member with a small bending moment. Due to the increased stiffness, the profile depth of the cross member can be reduced in the outer region, i.e., in the end sections. In addition, the wall thickness of the cross member can be reduced. Altogether, a bumper with an improved loading capacity can be implemented in lightweight construction with optimal weight. 
     Advantageously, the bulge projects outward in the direction to the crash box. The web of the end sections is altogether rounded, particularly over the vertical height. This measure also increases the stiffness characteristic and the energy absorption capacity of the bumper. 
     The bulge is constructed so as to have a greater width than the web of the end section. Preferably, the bulge has a rounded or drop-shaped contour. 
     According to one advantageous embodiment of the invention preferred in practical applications, the center longitudinal section of the cross member has also a U-shaped cross-sectional configuration with a web and two legs. The crash box attachment sections have greater widths than the center longitudinal section and the end sections and are also deeper. The vertical height of a crash box attachment section is greater than the vertical height of the web in the center longitudinal section. 
     According to yet another embodiment of the invention, outwardly oriented front webs connect from the free ends of the legs of the end sections and/or the crash box attachment sections and/or the center longitudinal section. 
     The cross member is constructed as a single saucer-shaped or U-shaped piece and is preferably made of sheet steel. The web(s) of the cross member is/are arranged on the side of the bumper facing the motor vehicle. The front side of the cross member is closed off with a cover plate, which is joined to the front webs. The joints can be implemented as spot joints and prepared by, for example, spot welding, punch-riveting or similar joining techniques. Preferably, the cover plate is connected to the front webs with an adhesive. In this way, a further reduction in weight and increase in stiffness can be attained. This measure provides additional benefits in that the adhesive layer has an insulating function and prevents contact corrosion between the cross member and the cover plate. The use of non-galvanized sheet metal, in particular hot-formed and hardened steel profile, now becomes possible or is at least made easier. Advantageously, use of an adhesive is favored for connecting a cross member and a cover plate made from a lightweight metal, for example sheet aluminum, or from a combination of different materials, for example, when the cross member is made of sheet steel and the cover plate is made of a lightweight metal. 
     In another advantageous embodiment, the rear wall of a crash box attachment section may have an opening which may be used, for example, to insert a towing lug into a lug attachment unit arranged inside the crash box. 
     In addition, the vertical height of the web of the center longitudinal section can vary. In particular, the web can be higher and wider at the center of the center longitudinal section than in the regions adjacent to the attachment sections. Moreover, regions of the front sections in the center longitudinal section may have chamfers. The chamfers are also provided in the center of the center longitudinal section, thereby increasing the stiffness and the loading characteristic in this region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
         FIG. 1  is a front view on the cross member of a bumper without a cover plate according to the present invention; 
         FIG. 2  shows in a perspective view a detail from the right end of the cross member according to  FIG. 1  with a cover plate; 
         FIG. 3  shows the detail of  FIG. 2  in a different perspective view; 
         FIG. 4  represents a cross-section of the diagram taken along the line IV-IV of  FIG. 2  with a schematically indicated crash box that is attached to a schematically indicated frame member of a motor vehicle; 
         FIGS. 5 to 7  show three different details of an end section in different perspective views; 
         FIG. 8  shows in a perspective view a detail of the left end of the cross member according to the  FIG. 1  with a cover plate; 
         FIG. 9  shows the detail of  FIG. 8  in a different perspective view; and 
         FIG. 10  represents the diagram of  FIG. 8  in a side view with a schematically indicated crash box. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Throughout all the Figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the Figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
     Turning now to the drawing, and in particular to  FIG. 1 , there is shown a bumper  1  for a motor vehicle according to the invention. The bumper  1  includes a cross member  2  which can be attached to a side rail of a motor vehicle (schematically shown as element  32  in  FIG. 4 ). The Cartesian coordinate system shown in  FIG. 2  illustrates the x-axis pointing in the direction of the motor vehicle, the y-axis pointing to the longitudinal direction of the cross member  2 , and the z-axis aligned with the vertical width of the cross member  2 . The cross member  2  is attached to the side rails by way of crash boxes  3  which are schematically indicated in  FIGS. 4 and 10 . The crash boxes  3  are provided to absorb, in addition to cross member  2 , the energy resulting from an impact by converting the energy into deformation energy. 
     The cross member  2  is formed in one piece from sheet metal, in particular sheet steel, and has an altogether saucer-shaped or U-shaped configuration. The cross member  2  has a center longitudinal section  4  with two ends  5 ,  6 , with a corresponding crash box attachment section  7 ,  8  and an end section  9 ,  10  being attached to the two ends  5 ,  6 , in that order. As illustrated in  FIGS. 2 and 3 , the crash box attachment sections  7 ,  8  are U-shaped and have a substantially vertical rear wall  11  which is connected to an upper leg  12  and a lower leg  13  (see also, for example,  FIGS. 4 ,  7  and  9 ). The rear wall  11  faces the motor vehicle. Accordingly, the legs  12 ,  13  face away from the motor vehicle. The upper leg  12  and the lower leg  13  may each have openings  34  for attachment of crash boxes  3 , for example with screws, as illustrated for example in  FIG. 4 . The end sections  9 ,  10  also have a U-shaped cross-section and in addition a web  14 , as well as two forward facing legs  15 ,  16  (see also  FIGS. 5 and 8 ). 
     The center longitudinal section  4  also has a U-shaped cross-section with a web  17 , an upper leg  18  and a lower leg  19 . The center longitudinal section  4  transitions via widening transition sections  20 ,  21  to corresponding crash box attachment sections  7 ,  8 . The vertical height H R  of the rear wall  11  of a crash box attachment section  7 ,  8  is greater than the vertical height H Sm  of the web  17  in the center longitudinal section  4 . 
     The webs  14  of the end sections  9 ,  10  are rounded along their vertical height and curved towards the motor vehicle. The end sections  9 ,  10  are also curved along the length of the cross member  2 , with the width of each leg  15 ,  16  increasing from the respective end  5 ,  6  to the corresponding crash box attachment sections  7 ,  8  (see  FIGS. 5 to 7 ). 
     The crash box attachment sections  7 ,  8  are configured in form of consoles. Each end section  9 ,  10  transitions by way of a transition section  22 ,  23  into a widened crash box attachment section  7 ,  8 . The webs  14  of the end sections  9 ,  10  are connected with the rear wall  11  of a crash box attachment section  7 ,  8  through the formation of a bulge  24 . The bulge  24  is rounded and projects outwardly towards the motor vehicle or towards the crash box  3 . The width b A  of the bulge  24  is greater or wider than the height H SE  of the web  14  of an end section  9 ,  10 . 
     The structure of the web  14  and its transition into the bulge  24  results in a corrugation-like reinforcement in the region of the end section  9 ,  10  and of the crash box attachment sections  7 ,  8 , which can improve the loading characteristic of the cross member  2 . With the increased stiffness, the profile depth of the cross member  2  in the end sections  9 ,  10  can be reduced. In addition, the wall thickness of the cross member  2  can be reduced in comparison to conventional designs. 
     Outwardly oriented front webs  25 ,  26  are extend from the front or free ends of the legs  15 ,  16  of the end sections  9 ,  10  and the legs  12 ,  13  of the crash box attachment sections  7 ,  8 , as well as the legs  18 ,  19  of the center longitudinal segment  4 . The front webs  25 ,  26  extend over the entire length of the cross member  2 . 
     As shown in  FIGS. 3 to 7  and  FIG. 10 , the front side of the cross member  2  is closed off by a cover plate  27  which is joined with the front webs  25 ,  26 , in particular with an adhesive  35 . The cover plate  27  further increases the stiffness of the bumper  1 . 
     Connecting the cross member  2  and cover plate  27  with an adhesive further reduces the weight and increases the stiffness. In addition, the adhesive layer insulates the front webs  25 ,  26  and the cover plate which can reduce contact corrosion. This simplifies or makes possible the use of non-galvanized sheet metal and/or hot-formed profiles. 
     As illustrated, for example in  FIGS. 2 to 7 , the cover plate  27  has a depression  28  facing the interior of the cross member  2 . This corrugation-like shape of the material additionally increases the stiffness of the cover plate  27  and of the bumper  1  in general. 
       FIG. 1  and  FIGS. 8 to 10  further illustrate openings  29 ,  30  disposed in the rear wall  14  of the crash box attachment section  8  and, perpendicular thereto, also in the cover plate  27 . These openings are provided for insertion of a towing lug in a lug attachment unit (not shown) disposed inside the crash box  3 . 
       FIG. 1  also shows that the web  17  of the center longitudinal section  4  has a varying height. The vertical height H Sm  of the web  17  is greater at the center of the cross member  2  than near the crash box attachment sections  7 ,  8 . Moreover, chamfers  31 ,  32  are provided on the front webs  25 ,  26  in the center of the center longitudinal section  4 , wherein the chamfers  31 ,  32  face away from the front webs  25 ,  26  towards the web  17  of the center longitudinal section  4 . 
     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.