Abstract:
A tow nut for a vehicle bumper assembly having an engagement portion by which a tow eye may be connected thereto and an attachment portion by which the tow nut may be attached to the assembly, characterized in that the tow nut is provided, between the engagement and attachment portions, with a zone of weakness whereby relative movement between the engagement portion and the assembly is permitted, under an impact condition.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates, in broad terms, to the field of vehicular (principally automotive) crash systems and relates, in particular, to vehicular bumper assemblies that are provided with a forwardly-disposed towing facility whereby a vehicle may be towed by another.  
           [0002]    A number of important considerations must be borne in mind during the design and production of such bumper assemblies. First and foremost, the impact-absorbing function of the bumper must be carefully monitored, so as to ensure that as much impact energy as possible is absorbed by the bumper assembly, and thus not transmitted to the main vehicle structure, or to the occupants thereof.  
           [0003]    To this end, bumper assemblies typically comprise a metallic (sometimes rolled) front beam which may be attached, towards a rear part thereof, to a crash box, which is typically designed to deform (“crumple”) under an impact condition. Alternatives to crash box systems are also known, in which the front beam is attached directly to a vehicle chassis rail, with the rail sometimes being provided with a sacrificial element to perform the energy-absorbing function. In both cases, however, the front face of the metallic beam is usually provided, for cosmetic reasons, with a covering, or skin, although such components contribute little to the impact-absorbing characteristics of the overall bumper assembly.  
           [0004]    It has also become common practice, particularly in the domestic and light commercial vehicle industries, to provide, in conjunction with a front bumper assembly, some means whereby the vehicle concerned can be towed by another vehicle, in the case of breakdown, accident damage, insufficient fuel and the like.  
           [0005]    To provide this capability, a so-called “tow nut” is attached to or provided with the forwardly-disposed bumper beam, with the tow nut having an internal thread to receive an externally threaded “tow eye” or “tow hook” having a circular or hooked end, by which a rope or other such line may be used to connect the towed and towing vehicles together. The term “tow nut” should thus be interpreted broadly, by reference to the function that it performs: it need not, of course, be “nut” shaped.  
           [0006]    However, problems have arisen with currently available towing assemblies, principally as the rigid nature of the tow nut and its securement to the forwardly-disposed bumper beam seriously hinders the ability of the beam to crumple under impact conditions. In addition, as it has become the norm to provide a single tow nut towards one end of the forward bumper beam (and thus to one side of the front of the vehicle), the strength and hence impact-absorbing characteristics of the beam vary considerably along its length. As will be understood, this means that a vehicle provided with such a bumper assembly will suffer different types and amounts of damage during a collision, depending on which part of the vehicle is struck.  
           [0007]    It is an object of the present invention to provide an improved tow nut that overcomes or at least reduces these problems. Other objects of the invention are to provide improved towing and vehicle bumper assemblies.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with a first aspect of the present invention, there is provided a tow nut for a vehicle bumper assembly having an engagement portion by which a tow eye may be connected thereto and an attachment portion by which the tow nut may be attached to the assembly, characterized in that the tow nut is provided, between the engagement and attachment portions, with a zone of weakness whereby relative movement between the engagement portion and the assembly is permitted, under an impact condition.  
           [0009]    The zone of weakness may be associated with a reduced thickness portion in the tow nut.  
           [0010]    The tow nut preferably is provided with frangible portion at or near the zone of weakness.  
           [0011]    The attachment portion may comprise part of a flange and the reduced thickness portion may be associated therewith.  
           [0012]    Preferably, the reduced thickness portion is provided by any undercut in the flange.  
           [0013]    The reduced thickness portion may extend about the principal axis of the engagement portion.  
           [0014]    The reduced thickness portion may be generally part-cylindrical.  
           [0015]    Preferably, the engagement portion is generally tubular, with the reduced thickness portion being radially spaced from a wall into the tube.  
           [0016]    Conveniently, the reduced thickness portion is radially spaced from an outer wall of the tube.  
           [0017]    Various depths of undercut maybe employed, and the depth of the undercut may thus be at least half the depth of the flange.  
           [0018]    Preferably, the depth of the undercut is about 80% of the depth of the flange.  
           [0019]    Conveniently, the reduced thickness portion is about 1 mm thick.  
           [0020]    In accordance with a second aspect of the present invention, there is provided a towing assembly for use with a vehicle bumper assembly, the towing assembly comprising an engagement formation by which the assembly may be connected to a towing vehicle and an attachment formation by which the towing assembly is attached to the bumper assembly, the towing assembly being provided, between the engagement formation and the attachment formation, with a zone of weakness whereby relative movement between the engagement formation and the bumper assembly is permitted, under an impact condition.  
           [0021]    The zone of weakness of the towing assembly may be substantially in accordance with the first aspect of the present invention.  
           [0022]    In accordance with a third aspect of the present invention, there is provided a vehicle bumper assembly having a tow nut substantially in accordance with the first aspect of the present invention.  
           [0023]    In accordance with a fourth aspect of the present invention, there is provided a vehicle bumper assembly having a towing assembly substantially in accordance with the second aspect of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The invention, in its various aspects, will now be described, in greater detail, but strictly by way of example only, by reference to the accompanying drawings, of which:  
         [0025]    [0025]FIG. 1 is a perspective view of a vehicle bumper assembly in accordance with the present invention;  
         [0026]    [0026]FIG. 2 is a schematic illustration, from above, of part of the assembly of FIG. 1;  
         [0027]    [0027]FIG. 3 a  is a close-up section of a tow nut in accordance with the present invention;  
         [0028]    [0028]FIG. 3 b  is an axial view, from above, of the tow nut of FIG. 3 a;    
         [0029]    [0029]FIG. 4 is a perspective, schematic, illustration of the outer surfaces of the tow nut of FIGS. 3 a  and  3   b , showing how the nut is welded to a bumper mounting plate;  
         [0030]    [0030]FIG. 5 is an axial view, from above, of the tow nut and welds of FIG. 4;  
         [0031]    [0031]FIGS. 6 a ,  6   b  and  6   c  illustrate, in schematic form, a number of different configurations of recess that may be provided in the tow nut shown in the preceding figures in accordance with the present invention; and  
         [0032]    [0032]FIGS. 7 a  and  7   b  illustrate still further possible configurations of recess in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Referring to the Figures, a vehicle front bumper assembly  10  comprises a forwardly disposed bumper beam  11  which, in this example, is formed by rolling medium to high strength steel sheet, to form the elongate and partially hollow configuration shown. Although not illustrated in the drawings, it will of course be understood that, in practice, the beam  11  will usually be provided with a cosmetic and somewhat protective outer skin which may be formed from a rubber or plastics material, for example. The beam  11  is connected, via mounting plates  12 , to crash boxes  13  which, whilst also formed from a metallic material, are specifically designed to collapse or “crumple”, in the event that the vehicle experiences an impact. The crash boxes  13  are connected, towards their rear ends, to crash box back plates  14  which are fastened, in generally conventional manner, to a rigid part of the vehicle structure.  
         [0034]    A generally circular aperture  15  is provided at one end  16  of the bumper beam  11 , the aperture  15  serving to receive a tow nut  17  which, as shown in FIGS. 3 a ,  3   b ,  4 ,  5 ,  6   a ,  6   b  and  6   c,  has a generally tubular engagement portion  18  which is provided, on its internal surface, with a threaded configuration. The engagement portion  18  is attached to or integral with, at a rear end  19  thereof, a generally circular flange  20  which abuts a rearwardly-facing surface  21  of the mounting plate  12 . The flange may, of course, be of a different configuration. The forwardly-disposed surface of the flange  20  constitutes an attachment portion by which the tow nut  17  may be attached to the mounting plate  12 , and thus secured in relation to the rest of the bumper assembly  10 . As shown in FIGS. 4 and 5, the tow nut  17  is preferably attached to the mounting plate  12  using four evenly circumferentially-spaced welds  22 , although other attachment methods may, of course, also be used. An even, circumferential distribution of weld points is advantageous as this ensures that the tow nut  17  is able to withstand laterally-exerted forces such as may be experienced when the tow nut  17  is used, during transportation of a vehicle, to “tie down” the vehicle to restrict movement thereof during transit. It will however be appreciated that other weld configurations and arrangements could conceivably be employed, and it may be desirable, in cases where extra strength is required, to provide a single circumferentially-extending weld about the attachment portion, to the mounting plate  12 . As shown in FIG. 1, the principal purpose of the tow nut  17  is to receive, on a temporary basis, a tow eye  23  which, in generally conventional manner, comprises an externally-threaded generally elongate shank  24  which, in use, is threadedly engageable with the internally-threaded female tow nut  17 . The shank  24  is attached to or integral with an eye element  25  to which, in appropriate circumstances, a rope or other such towing line may be attached, to enable the vehicle to be towed.  
         [0035]    As shown in FIGS. 3 a ,  3   b ,  6   a ,  6   b  and  6   c , the tow nut  17  is provided, on its underside  26 , with a recessed channel  27  which extends, when the tow nut  17  is viewed axially, in a generally circular manner about the principal axis  28  of the tow nut  17 . The channel  27 , which constitutes a continuous undercut in the flange  20 , creates a similarly generally circular reduced thickness portion  29  that is disposed between the engagement and attachment portions of the tow nut  17 . As shown in FIGS. 6 a ,  6   b  and  6   c , for example, the recess may have a number of differing cross-sections, although, in each case, it will be noted that the apex  30  of the cross-section is offset from the outer wall  31  of the engagement portion  18 . This is to ensure the provision of a shoulder part  32  of generally full-thickness material which, in use, will abut the rearwardly-disposed surface  21  of the mounting plate  12 . The provision of such full-thickness material is important during a towing operation, to ensure that the tow nut  17 , on, being pulled axially away from the mounting plate  12 , is held securely in position. Nonetheless, by selecting an appropriate material, the applicants envisage that it may also be possible to locate the channel directly beneath, or close to, the outer wall  31 .  
         [0036]    As explained above, an important consideration in automotive bumper design is the extent to which the bumper assembly is able to absorb impact forces which are exerted when the vehicle is in collision with another vehicle, or static object, for example. Such energy absorption is important to minimize damage to the main vehicle structure, but principally to reduce the effect of the impact force on the occupants of the vehicle.  
         [0037]    To this end, it is desirable to design vehicle bumper assemblies so that as much crash energy is absorbed by the front end of the vehicle, as possible, with the brunt being taken by the bumper beam  11  and crash box  13  of the assembly  10 . In order for the bumper beam  11  to be able to collapse in the event of an impact, it is important that the components thereof are able to move in a generally rearwardly direction and that the “crumpling” capability of the crash box is not restricted. The provision of the undercut recess  27  in the flange  20  provides this, capability, in that, in the event of a generally axially-directed impact, the tow nut  17  will fail, in shear, at a zone of weakness defined by the reduced thickness portion  29 , disposed just above the apex of the recess  27 . As will be understood, this allows the generally tubular engagement portion to move, generally axially, in relation to the mounting plate  12 , and hence to the rest of the bumper assembly  10 , which in turn allows the bumper beam  11  to collapse, in a somewhat similar direction, towards the crash box  13 . The positioning and configuration of the recess  27  may be adjusted to “tune” the tow nut  17  to fail, in shear, at a variety of different axial forces, with it typically being desired for a shear fail to occur when an axial force of 30 to 40 kN (kilo Newtons) is applied. In practice, the failure force will lie somewhere between, the force at which deformation of the bumper beam  11  commences and the force required to initiate collapse of the rearwardly disposed crash box  13 . What this means is that, in the event of a minor impact, that does not cause the bumper beam  11  to deform, the tow nut  17  will remain intact and firmly secured to the mounting plate  12 . However, where a greater impact is experienced, the tow nut  17  may fail, in shear, with the tubular body of the engagement portion  18  thus being propelled generally rearwardly of the assembly, into the body of the crash box  13 . As the tubular body  18  has now become detached from the mounting plate  12 , it can simply fall towards the rear of the crash box  13 , thus ensuring that the crash box is able to crumple largely unhindered.  
         [0038]    To ensure that the tow nut  17  does not accidentally become detached, in its entirety, from the mounting plate  12 , it will be understood that the weld strength will normally need to exceed the strength of the zone of weakness defined by the undercut recess  27 . This can of course be achieved in a number of ways, although the applicants have found that the total contact area between the welds  22  and the surface  21  of the plate  12  should exceed the upper surface area of the generally disc-shaped zone of weakness.  
         [0039]    As will be understood from the foregoing, the invention, in its various aspects, provides a number of improvements in the field of automotive bumper design, principally as the facility of the tow nut to fail at a specified point, and under a known axial force, enables the forwardly-disposed bumper beam to collapse in the event of an impact, which, in turn, allows the energy absorbing aspect of the assembly to function in an unhindered manner. The offset nature of the undercut recess, however, ensures that the tow nut is sufficiently resilient, when exposed to a towing force, thus ensuring that it does not become separated inadvertently from the mounting plate  12  when a towing operation is underway, or when the associated vehicle is tied down, during transit.  
         [0040]    As shown in FIGS. 7 a  and  7   b , the undercut recess in the flange  20  of the tow nut  17  may alternatively comprise a somewhat disc-shaped cut-out  40 . As shown in the drawings, the provision of a reduced thickness (and hence reduced strength) zone is unaffected by the alternative recess configuration, with it conceivably being desirable to produce the tow nut in this way in an effort to reduce material usage and hence manufacturing costs.  
         [0041]    It should also be understood that, whilst in the examples, the tow nut  17  is shown attached to the mounting plate  12  by abutment of the flange  20  with a rearwardly-facing surface  21  of the plate, it is envisaged by the applicants that the flange  20  could in fact be attached to the plate  12  by abutment with its opposite (forwardly-facing) surface. Thus, by reference to FIG. 3 a , for example, it should be understood that, in use, the mounting plate  12  may in fact be disposed in abutment with the underside  26  of the tow nut  17 .  
         [0042]    The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as, appropriate, may, separately, or in any combination of such, features, be utilized for realizing the invention in diverse forms thereof.