Abstract:
A front section for a motor vehicle includes a bumper cladding and a bumper stiffening which extends in the vehicle longitudinal direction between the bumper cladding and a supporting body part. A connecting unit including a blind nut and a threaded fastener is inserted from below into an opening of the body part to fix a rear edge region of the bumper stiffening on an underside of the body part. The blind nut has flexible legs which are reversibly splayed apart above the opening.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to German Patent Application No. 102015004416.8, filed Apr. 2, 2015, which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure pertains to a front section for a motor vehicle which is optimized with respect to different types of collision accidents. 
       BACKGROUND 
       [0003]    DE 10 2009 017 350 A1 discloses a motor vehicle front section with a bumper cladding and a bumper stiffening which extends in the vehicle longitudinal direction between the bumper cladding and cross beam running transversely. Predetermined breaking connections are provided as threaded connections between the cross beam and the bumper stiffening. In the case of a relatively low-energy collision with a pedestrian, the threaded connections should remain intact, in the event of a more energetic collision, possibly with a wall, the shear loading should be sufficient to tear off or tear out the fastener and/or fastener holes. 
         [0004]    The tearing out of the fastener holes necessarily includes damage to the cross beam which involves a costly repair. However, the alternative of tearing off of the fasteners is problematical. The force required for tearing off a fastener generally fluctuates unpredictably from case to case. If it is too low, the stiffening effect of the bumper stiffening in the case of a collision is unsatisfactory; if it is too high, collision forces can be transmitted to the cross beam and damage this, which in this case also significantly increases the costs of a subsequent repair. 
       SUMMARY 
       [0005]    The present disclosure provides a front section for a motor vehicle in which even at higher collision energies than those typical of a pedestrian accident, the repair expenditure can be kept low. The front section for a motor vehicle is typically provided with a bumper cladding and a bumper stiffening which extends in the vehicle longitudinal direction between the bumper cladding and a supporting body part. In a configuration of the present disclosure, a connecting unit which fixes a rear edge region of the bumper stiffening on an underside of the body part is inserted from below into an opening of the body part and has flexible legs which are reversibly splayed apart above the opening. These legs can be compressed again under the action of a sufficient tensile force on the connecting unit so that the connecting unit can pass through the opening again and can release bumper stiffening and body part from one another. Since no material must tear or rupture for compressing the legs, but a continuous bending deformation is sufficient, the force required for withdrawing the connecting unit can be reliably determined by simulation and then set reproducibly. 
         [0006]    The connecting unit includes a blind nut and a threaded fastener. The legs which are part of the blind nut are splayed apart by screwing the fastener into a thread of the blind nut. In order to enable such a splaying, parts of the legs in the unsplayed state can cross the axis of the thread so that the fastener during screwing into the thread must impact against these sections and must push them aside. The thread can be formed in a base plate of the blind nut and the legs extend out from the edges thereof. Such a blind nut can be manufactured cheaply in one piece from flat material such as a steel sheet, in particular by stamping and bending steps. The legs can each include a distal section and a proximal section connecting the distal section to the base plate where the two proximal sections diverge from the base plate. The distal sections can form the aforementioned sections crossing the axis of the thread in the unsplayed state. 
         [0007]    The bumper stiffening and the body part can include sliding surfaces which contact one another at least when the bumper stiffening is pressed back against the body part by a collision. At least one of the sliding surfaces should be inclined towards the vehicle interior in order to convert a collision force directed horizontally towards the vehicle interior into a downward-directed tensile force which acts on the connecting unit. If the bumper stiffening includes a base plate and stiffening ribs projecting upwards from the base plate, one of the sliding surfaces can be formed by an edge of one of these ribs. This can include an upper edge of the relevant rib; As a result of a preferred embodiment, a recess is formed on a rear end of the rib facing the body part and the sliding surface is a part of the edge which delimits this recess. 
         [0008]    Alternatively or additionally, the collision force can be converted into a downward-directed tensile force whereby the bumper stiffening includes a projection in a flank opposite a front side of the body part, which contacts the front side at least when the bumper stiffening is pressed back against the body part by a collision and has a recess underneath the projection. Thus, in the case of a collision, a contact point between the projection and the front side of the body part can form an instantaneous axis of rotation about which the rear edge region can rotate where the connection is released by the rotation. 
         [0009]    If the bumper stiffening includes a base plate and stiffening ribs projecting upwards from the base plate, the flank can be formed by rear edges of the ribs. 
         [0010]    In both the aforementioned embodiments, the ribs can be stiffened by wings projecting in the vehicle transverse direction. Preferably the wings each have a front edge at which they are connected in one piece to the base plate. The wings can extend as far as the rear end of the rib in order to stiffen these directly up to a contact point with the body part. 
         [0011]    The bumper stiffening can be curved in longitudinal section with an upwardly facing concavity so that under suitable collision conditions, the bumper stiffening deviates downwards in the region of the concavity and thereby absorbs collision energy and components arranged above the concavity are safe from any damage as a result of their deviation. The bumper stiffening can cross under at least a front part of an engine compartment and specifically in particular a radiator accommodated there. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements. 
           [0013]      FIG. 1  shows a schematic longitudinal section through a front section of a motor vehicle according to the present disclosure; 
           [0014]      FIG. 2  shows an enlarged detail from  FIG. 1  in longitudinal section; 
           [0015]      FIG. 3  shows the detail of  FIG. 2  in perspective view; 
           [0016]      FIG. 4  shows a perspective view of a fastener blind nut used for fastening a bumper stiffening to a body part of the motor vehicle; and 
           [0017]      FIG. 5  shows a section through the mounted fastener blind nut in the vehicle transverse direction. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. 
         [0019]      FIG. 1  shows a front section of a motor vehicle in a schematic longitudinal section. Located in the engine compartment in the usual manner are an engine  1  and a radiator  2 . The engine compartment is flanked on both sides by longitudinal members  3  which are connected to one another to form a rigid frame. An auxiliary frame  4  carrying the engine  1  is fastened movably in a damped manner to this frame. 
         [0020]    The longitudinal members  3  are provided with flanges  5  at the front ends thereof, to which an upper bumper cross beam  7  is fastened via crash boxes  6 . An intermediate space between the front side of the bumper cross beam  7  and a bumper cladding  8  is filled by a buffer body  9  made of foam. 
         [0021]    A lower bumper cross beam  10  is fastened to the flanges  5  below the upper bumper cross beam  7 . In a central region extending in front of the radiator  2 , the lower bumper cross beam  10  is supported by a bumper stiffening  11  against a collision force acting from the front. A front edge of the bumper stiffening  11  is connected in an arbitrary manner, preferably in a torque-proof manner to the bumper cross beam  10 , here it is inserted in a groove  12  on the rear side of the bumper cross beam  10 . 
         [0022]    A rear edge region  13  of the bumper stiffening  11  is fastened to a front cross beam  14  of the auxiliary frame  4  by fasteners  15  in a manner which will be explained in further detail by reference to  FIGS. 2-4 . The bumper stiffening  11  formed in one piece from plastic has a base plate  16  which extends continuously from the front edge engaging in the groove  12  into the rear edge region  13 . Underneath the radiator  2  the base plate  16  is deflected downwards in the form of a trough extending in the vehicle transverse direction. Stiffening ribs  17  oriented in the vehicle longitudinal direction project from the upper side of the base plate  16 . 
         [0023]      FIGS. 2 and 3  show the rear edge region  13  of the bumper stiffening  11  and the cross beam  14 , once in section in the vehicle longitudinal direction and once in a cutaway perspective view. The ribs  17  which extend over a large part of the length of the base plate  16  with uniform height increase in height towards the rear end  18  thereof as far as an apex point  19  which is located approximately at the same height as an upper side  20  of the cross beam  14  formed here as a rectangular hollow profile. The rear end  18  of the ribs  17  here includes a steeply dropping or vertical edge section  21  as well as, underneath this edge section  21 , a recess  22  which is delimited downwards by an edge  23  which slopes down towards the auxiliary frame  4 . The ribs  17  are stiffened in the region of the rear end  18  by wings  24  projecting in the vehicle transverse direction. The wings  24  are, as is clear from  FIG. 3 , trapezoidal in plan view, where a wide front edge of the trapezium goes over in one piece into the base plate  16  and a rear edge lies opposite a front wall  25  of the cross beam  14 . 
         [0024]    The base plate  16  extends beyond the rear end  18  of the ribs to under the cross beam  14  and is fastened to this by a plurality of fasteners  15  and blind nut  26  which are here each inserted from below into openings  27  in a lower wall  28  of the cross beam  14 .  FIG. 4  shows such a blind nut  26  in perspective view. The blind nut  26  is formed in one piece from a sheet metal blank. It includes an approximately rectangular base plate  29  on which a cylinder shaft  30  is formed around a central opening and provided with an internal thread. At the longitudinal edges of the base plate  29 , legs  31  are angled on both sides, each including a proximal section  32  orthogonal to the base plate  29  (in the unloaded state shown in  FIG. 4 ) and a distal section  33  angled by 90° with respect to the proximal section, which is bent back over the base plate  29 . A U-shaped gap  34  divides the proximal section  32  into two lateral webs  35  and a central finger  36  which is directly connected to the distal section  33  but not to the base plate  29  and projects slightly sideways beyond the webs  35 . The distal sections  33  overlap one another in the extension of the cylinder shaft  30 . 
         [0025]    The openings  27  in the lower wall  28  of the cross beam  14  are rectangular and dimensioned to allow insertion of a blind nut  26  in an orientation in which the longitudinal direction of the base plate  29  coincides with the longitudinal direction of the vehicle. The base plate  29  is longer than the openings  27  so that a stop position of the blind nut  26  is formed by a contact of ends of the base plate  29  with the underside of the lower wall  28 . The length of the fingers  36  is matched to the thickness of the wall  28  so that in the stop position the fingers  36  have completely passed through the opening  27  and their tips lie opposite an upper side of the lower wall  28 . Thus, the blind nut  26  cannot be withdrawn from the opening  27  again without bending at least the fingers  36 . 
         [0026]    In order to fasten the bumper stiffening  11  on the cross beam  14 , the fasteners  15  are passed through holes  37  in the rear edge region  13  of the bumper stiffening  11  as shown in  FIG. 5  and screwed into the thread of the cylinder shaft  30 . The length of the fasteners  15  is determined so that their tips each impact against the distal sections  33  of the blind nut  26  before the head of the fastener  15  presses the bumper stiffening  11  against the base plate  29  of the blind nut  26 . In order to fasten the bumper stiffening  11 , the fasteners  15  are thus screwed so far into the blind nut  26  that they splay the legs  31  apart on the other side. In this way each blind nut  26  is anchored positively on the cross beam  14  but this tight fit can be cancelled if a downwardly directed tensile force acts on the blind nut  26  via the fastener  15 , which is sufficiently strong to bend the upwardly diverging proximal sections  32  towards one another in a parallel orientation. How large this tensile force is can be predefined exactly and reproducibly by selecting the wall thickness of the blind nut  26  and the shape of the legs  31 , 
         [0027]    In the case of a collision in a medium velocity range, preferably between 15 and 40 km/h, the bumper cladding  8  and the bumper stiffening  11  are pushed back towards the vehicle interior and the rear end  18  of each rib  17  comes in contact with the front wall  25  of the cross beam  14 . If the edge section  21  is sufficiently stiff above the recess  22 , a point of the edge section  21  which contacts the front wall  25  can form the axis of a pivoting movement as a result of which, in the view in  FIG. 2  in the anticlockwise direction, the bumper stiffening  11  attempts to escape from the collision force. If instead the rib  17  is resilient above the recess  22  and is compressed, the edge  23  slides obliquely downwards along a rounded corner  38  of the cross beam  14 . 
         [0028]    In both cases, the fasteners  15  and blind nut  26  are subject to a strong, downwardly directed tensile force through which, if this is sufficiently strong, in order to deform the blind nut  26 , this is withdrawn from the openings  27  of the cross beam  14 . The blind nut  26  is thereby deformed in small space so that despite small size they can absorb an appreciable quantity of collision energy. If the blind nut  26  is torn out from the cross beam  14 , the connection between auxiliary frame  4  and bumper stiffening  11  is cancelled. The auxiliary frame  4  is thereby protected from any deformation by forces transmitted by the bumper stiffening  11  and can in any case be deformed if the collision is sufficiently strong in order to compress the entire front section so severely that the longitudinal members  3  are also deformed. Expensive repairs to the auxiliary frame  4  after a collision at moderate speed can therefore be avoided with a high probability which enables a favorable insurance category for the vehicle according to the present disclosure and reduces the operating costs for the holder. 
         [0029]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.