Abstract:
One embodiment of the present invention is directed to a wheel hub stress reduction system for retaining a wheel on a vehicle using wheel nuts. The system includes a hub moon having a mounting portion defining a plurality of holes, and a plurality of threaded connectors each received by one of the holes. A maximum tensile stress region is produced in the hub when said connector is tensioned by a wheel nut threadably engaged therewith. The maximum tensile stress region lies beyond a hub radius which bisects said one of the holes. Another embodiment of the present invention is direct it to a method of reducing stress on a wheel hub retaining a wheel on a vehicle using wheel nuts.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a wheel hub stress reduction system which retains wheels on vehicles, such as semis or tractor-trailer trucks, and more particularly to a system employing a contoured connector which mates with a contoured hole defined by a vehicle hub. 
       BACKGROUND 
       [0002]    Conventionally, wheel hubs are formed of cast iron or aluminum, which are machined and assembled to mate with other components of a vehicle. For example,  FIG. 8  is a sectional view of a radial portion of a generally bell-shaped wheel hub H attached in a conventional manner to a vehicle axle (not shown). A connector, such as a stud or bolt B extends through a bored cylindrical hole C defined by a mounting flange D of hub H extending from an interior surface E to an exterior surface F. While only a single bolt B is shown for simplicity, typically a plurality of holes C are equally spaced around the periphery of the hub mounting flange D, each receiving a bolt B, with the number and size of bolts and the bolt pitch circle diameter, depending upon the load rating of the vehicle. 
         [0003]    The bolts B are used to secure together the hub H, sometimes a brake drum G, and a wheel W upon which is mounted a tire T. The bolts B each have a head J at one end, and a threaded portion K at the opposite end. A wheel nut L engages the bolt threaded portion K to secure the wheel W to the hub H. The bolt B has a serrated shoulder portion M which is typically press-fit into cylindrical hole C to affix the bolt to hub H. The bolt head J has undersurface N, which is substantially perpendicular to a longitudinal axis P of bolt B, and is seated substantially flat against the hub interior surface E. 
         [0004]    When mounting wheel W to hub H, wheel nut L is tightened onto the bolt B, which imparts a tensile stress to the hub H in a direction perpendicular to axis P, and a compressive stress perpendicular to undersurface N. The tensile stress commonly occurs in a most critical region of the hub H, along a curved transition between mounting flange D and the barrel portion of hub H at a location radially inward from where material has been removed to form the holes C. The tensile stress may be represented in vector format as a arrow R having a force directed as indicated by the direction of the arrow, and a magnitude represented by the length of the arrow. This tensile stress is imparted to the hub H by the undersurface N of the bolt head J. A compressive stress is imparted by surface N, indicated by arrow S. 
         [0005]    A vehicle hub H is typically subjected to two types of stress which limit service life: (1) the mean tensile stress imparted by tightening the wheel nuts, which has the effect of drawing the hub interior surface E down into hole C; (2) fatigue stress caused by a cyclic load generated when the hub rotates under load such as by cornering on turns. The residual tensile stress, when added to the cyclic stresses, has a negative impact on the service life of the hub H. Additionally, it is quite common for mechanics to over-tighten the wheel nuts L when changing tires, resulting in over-stretching or over-tensioning the bolts B and further increasing the tensile stress, which shortens the service life of the hub H. 
       SUMMARY 
       [0006]    One embodiment of the present invention is directed to a wheel hub stress reduction system for retaining a wheel on a vehicle using wheel nuts. The system includes a hub moon having a mounting portion defining a plurality of holes, and a plurality of threaded connectors each received by one of the holes. A maximum tensile stress region is produced in the hub when said connector is tensioned by a wheel nut threadably engaged therewith. The maximum tensile stress region lies beyond a hub radius which bisects said one of the holes. Another embodiment of the present invention is directed to a method of reducing stress on a wheel hub retaining a wheel on a vehicle using wheel nuts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of a wheel hub stress reduction system according to one embodiment of the invention. 
           [0008]      FIG. 2  is an enlarged sectional view of a radial portion of the stress reduction system of  FIG. 1 . 
           [0009]      FIG. 3  is a side elevational of view of one embodiment of a connector of  FIG. 1 . 
           [0010]      FIG. 4  as a side elevational view of another embodiment of a connector. 
           [0011]      FIG. 5  is an enlarged sectional view of a radial portion of a wheel hub stress reduction system according to another embodiment of the invention using the connector of  FIG. 4 . 
           [0012]      FIG. 6  is a perspective stress diagram showing the tensile stress imparted to the hub when using the wheel hub stress reduction system of  FIG. 1  or  FIG. 5 . 
           [0013]      FIG. 7  is a perspective stress reduction diagram showing the tensile stress imparted to the hub when using a prior art hub and bolting system. 
           [0014]      FIG. 8  is an enlarged radial, sectional view of a prior art hub and bolting system which produces the tensile stress illustrated in  FIG. 7 . 
           [0015]      FIGS. 9A and 9B  are enlarged sectional views each having a vector diagram, with  FIG. 9A  illustrating the prior art system of  FIGS. 7 and 8 , and  FIG. 9B  illustrating the system of  FIGS. 1-3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIGS. 1 through 3  illustrate a wheel hub stress reduction system  10  according to one embodiment of the invention. As best shown in  FIG. 2 , system  10  includes a roughly bell-shaped wheel hub  12  having a barrel portion which attaches to an axle by bearings (not shown). A cylindrical hole  14 , which may be formed by a boring operation, is defined by an outer peripheral mounting flange  15  of the hub  12 . The hole  14  extends from an interior surface  16  to an exterior surface  18  of hub mounting flange  15 . 
         [0017]    A first embodiment of a connector, such as a wheel bolt  20 , is illustrated with the shank  22  having a serrated shoulder  24  at one end, and a threaded portion  25  at an opposing end. The serrated shoulder  24  may be press fit into a cylindrical hole  14  of the hub mounting flange  15 . The bolt shank  22  extends through a hole  26  defined by the brake drum G and a hole  28  defined by wheel W. A wheel nut L threadably engages the bolt threaded portion  25  to mount the tire T on hub  12 . The bolt  20  has a head  30  with an undersurface  32  serving as a contact surface which has a contour centered about a longitudinal axis  34  of the bolt. Typically a plurality of holes  14  are equally spaced around the periphery of the hub mounting flange  15 , each receiving a bolt  20 , with the number of bolts depending upon the load rating of the vehicle. 
         [0018]    The hub mounting flange  15  defines a head seat  35  having a diameter greater than the cylindrical hole  14 . The illustrated seat  35  has a contour which mates the bolt head undersurface  32 , here shown as mating tapered or frusto-conical (also known as a “frustum” or “frustrum”) shapes. As best shown in  FIG. 3 , the bolt head undersurface  32  has an angle φ (“phi”) with respect to the bolt longitudinal axis  34 , as indicated between the dashed lines  34  and  36 , with dashed line  36  indicating a slope angle of the head undersurface  32  and head seat  35 . In the drawings, this slope angle labeled φ (“phi”) is about 45°, although any angle selected in the range of 20° to 80° may be used. The effect on performance of using the illustrated tapered head  32  and tapered seat  35  is discussed below with respect to  FIG. 6 . 
         [0019]      FIGS. 4 and 5  illustrate an alternate embodiment of a connector  40  according to the present invention. As best shown in  FIG. 4 , the connector  40  includes a hub bolt or bolt  41  having a shank  42 . The shank  42  has a non threaded portion  44  at one end which may be optionally serrated to carry a plurality of serrations  45 , and a threaded portion  46  at the opposing end. The bolt  41  has a longitudinal axis  48  upon which is centered a head  50  having an undersurface  52 . In the drawings, the bolt head undersurface  52  has an angle θ (“theta”) with respect to the longitudinal axis  48 , as indicated between dashed lines  48  and  54 , with the dashed line  54  being coplanar with undersurface  52 . In the illustrated embodiment, angle θ is about 90° so the head undersurface  52  is substantially perpendicular to the longitudinal axis  48 , is illustrated for the prior art bolt B of  FIG. 8  discussed in the Background section above. 
         [0020]    The connector  40  includes a spacer member or washer  55  preferably sized to seat against the entire undersurface  52  of bolt head  50 . The washer  55  has a triangular cross-section, illustrated as a right triangle to fit adjacent the mutually perpendicular interface of the head undersurface  52  and the periphery of shoulder  44 . A remaining exposed surface  56  of washer  55  serves as a contact surface for connector  40 . The contact surface  56  is selected to be at angle φ (“phi”) with respect to the longitudinal axis  48 , as indicated in  FIG. 4  between dashed lines  48  and  58 . The angle φ may be selected as described above with respect to bolt  20  of  FIGS. 1-3 , allowing connector  40 , comprising bolt  41  and washer  55 , to be substituted for bolt  20 . 
         [0021]    The connector  40  may be constructed in a variety of different ways. For example, bolt  41  may be formed by cold heading or otherwise forming shoulder  44  and head  50  preferably from a steel material. The spacer member or washer  55  may be formed from a steel material in a stamping operation or other forming operation. Preferably, the bolt  41  is formed by cold heading and washer  55  is formed by stamping. 
         [0022]    Following these initial forming operations, the washer  55  is mounted on the bolt shank  42  and seated against the head undersurface  52 . The washer  55  may be held in place in a variety of different ways, yielding what is known as a captured washer. For example, after washer  55  is installed on shank  41 , the serrations  45  may be formed on shoulder  44 . The ridges of serrations  45  provide shank  41  with an outer diameter which is greater than the outer diameter of shoulder  44 , and greater than the inner diameter of washer  55  to secure the washer to bolt  41 . The threads  46  may be formed on shank  41  either before, after, or during formation of the serrations  45 . As another example, the washer  55  may be compressed or pre-loaded to secure the washer against the head undersurface  52 . In this example, serrations  45  and threads  46  may be formed either before or after washer  55  is installed on bolt  41 . 
         [0023]      FIG. 5  illustrates an alternate embodiment of a wheel hub stress reduction system  60  according to the present invention employing a connector  40 . Here, the connector  40  is substituted for bolt  20  to couple together hub  12 , brake housing G, and wheel W, using wheel nut L to mount a tire T on a vehicle. The interface surface  56  of washer  55  rests against the tapered head seat  35 . Using washer  55  in connector  40  which moves or floats on a shank shoulder  44 , which allows connector  40  to compensate for nonconcentricities of either the bolt head  50  or the cylindrical hub hole  14 . The captured washer  55  promotes full contact of the seating surfaces  5 * 2  and  55  at all times during tightening of the wheel nut L. 
         [0024]    As way of one example,  FIGS. 6 and 7  are stress diagrams comparing the tensile stress imparted to a wheel hub  15  using either wheel hub stress reduction system  10  or  60  ( FIG. 6 ), with the tensile stress imparted to a wheel hub H using the prior art system discussed in the Background Section ( FIG. 7 ) for one specific case.  FIG. 7  represents a typical case, and it has been found that the results are similar for other hub shapes.  FIG. 6  illustrates a stress pattern  70  produced by stress reduction system  10  or  60 . The stress pattern  70  shows different stress levels  72 ,  74 ,  75 ,  76  and  78 , representing increasing levels of stress.  FIG. 7  illustrates a stress pattern  80  on hub H produced by prior art bolts B. The stress pattern  80  shows different stress levels  82 ,  84 ,  85 ,  86  and  88  which represent increasing levels of stress. A comparison of the  FIG. 6  and  FIG. 7  stress levels, in percent (%) of the maximum stress level, is shown in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Stress Levels 
               
             
          
           
               
                 % of Max Stress Level 
                 FIG. 6 
                 FIG. 7 (Prior Art) 
               
               
                   
               
             
          
           
               
                 100 
                   
                 88 
               
               
                 90 
                   
                 86 
               
               
                 50 
                 78 
               
               
                 45 
                   
                 85 
               
               
                 40 
                 76 
               
               
                 20 
                 75 
                 84 
               
               
                 15 
                 74 
               
               
                 0 
                 72 
                 82 
               
               
                   
               
             
          
         
       
     
         [0025]    Of the three types of hub stress described in the Background section above, the stress diagrams of  FIGS. 6 and 7  do not address the fatigue or cyclic stresses, only the mean tensile stress generated by tightening wheel nut L when mounting tire T on a vehicle. The prior art stress pattern of  FIG. 7  shows regions of little or no stress  82  in the barrel portion of the generally bell-shaped hub H, and in pairs of triangular shaped regions extending from opposing sides of each bolt hole C. However, regions of extremely high stress  86  and  88  occurred tangentially along the inboard portion of each of bolt holes C. Transitional regions of stress  84  and  85  lie between the extremely high stress regions  86 ,  88  and the little or no stress regions  82 . 
         [0026]      FIG. 6  also has regions out of little or no stress  72  in the barrel portion of hub  12  and extending circumferentially between each of the bolt holes  14 . The highest areas of stress  78  are pairs of small diamond shaped regions located on opposing sides of each hole  14  and lying in an annular band region encircling hub  12 . Transitional regions of stress  74 ,  75  and  76  lie between stress regions  72  and  78 . The highest levels of tensile stress  78  in stress reduction systems  10 ,  60  are roughly half of the highest stress levels  86 ,  88  experienced using a prior art hub H and bolt B design. 
         [0027]    In addition to the significant reduction in the highest stress levels  78  experienced by the hub  12 , the location of the highest stress levels is vastly improved using stress reduction systems  10 ,  60  over that of the prior art hub H and bolt B assembly of  FIGS. 7 and 8 . As discussed in the Background section above, the extremely high tensile stress  86 ,  88  occurs in a critical region of the hub H. This critical region is located along a curved transition between mounting flange D and the barrel portion of hub H, and at locations inboard from where material has been removed to form the holes C. The curved transition and the material removal each inherently weaken the hub in the critical region. The addition of placing a high tensile stress  86 ,  88  in this critical region, lying along the same radius as each hole C, results in a negative impact on the service life of hub H. The stress reduction systems  10 ,  60  move the highest stress regions  78  out of this critical region and away from any radius intersecting a hole  14  or the contoured seat  35 . 
         [0028]    One possible explanation for this repositioning of the highest stress regions  78  of systems  10 ,  60  from the critical region locations of the highest stress regions  86 ,  88  of the prior art shown in  FIGS. 7 and 8  is illustrated in  FIGS. 9A and 9B .  FIG. 9A  shows the resultant tensile stress as vector R imparted by the flat undersurface N and bolt head J in a radial direction. 
         [0029]      FIG. 9B  illustrates the effect of using a contoured seat  35  with a contoured contact surface  32  or  56 , but for simplicity only system  10  is illustrated. Here, the contoured seat  35  is assumed to be in full contact with the bolt head contact surface  32  or  56 . The total force imparted by bolt head  30  is represented by a vector  90  having a direction which is normal to, or perpendicular to, the contoured seat  32 . Assuming the wheel nuts L in  FIGS. 9A and 9B  are each tightened with the same torque, the magnitude of the forces represented by vector S and vector  92  are equal, and thus, vectors S and  92  have the same length. Each head has an exposed surface which projects beyond said hub interior surface. As seen in  FIG. 9B , each head seat has a first diameter at the interior surface and each head may have a second diameter greater than the first diameter wherein each head has an exposed surface which projects beyond said hub interior surface. Likewise, each of the holes  14  has a first circumference, and each seating surface  35  has a second circumference greater than the first circumference and each head contact surface  32  or  56  has a third circumference sized for a contact fit with said second circumference of said associated hub seating surface  35  when tightened by said wheel nut. In an alternative embodiment, the first diameter may be greater than the second diameter wherein the head has an exposed surface between the hub interior surface and hub exterior surface. In the alternative embodiment, the exposed surface is recessed below the hub interior surface. 
         [0030]    Breaking down vector  90  into an x-y coordinate axis system, vector  90  has a vertical component shown as vector  92  and a horizontal component shown as vector  94 . The terms “horizontal” and “vertical” are relative terms with respect to the view of  FIG. 9B . These results were verified by the test data shown in  FIGS. 6 and 7  for the maximum stress levels  78  and  86 ,  88 , respectively. The horizontal stress vector  94  may impart a residual compressive stress in the critical region of hub  12 . The horizontal stress vector  94  may also be responsible for moving the location of the highest stress levels  86 ,  88  in  FIG. 7  to the location of the highest stress levels  78  in  FIG. 6 , which is out of a critical region. 
         [0031]    Thus, the tensile stress reduction systems  10 ,  60  use a shape where the stud head  30 ,  50  is an angular design or taper that is seated in a countersunk hole  14 ,  35 . This concept produces a lower tensile stress  78  in the critical region of the hub  12  because the forces from the stud mounting torque are directed normal to the connector contact surface  32 ,  56 , instead of perpendicular to the prior art head undersurface N. This normal direction of the force indicated by vector  90  lowers the mean tensile force of the prior art system, indicated by the vector R, and may impart a residual compressive stress indicated by vector  94  in the critical region of hub  12 . The shape of connector  20  has benefit as a monolithic one piece stud design. The two-piece assembled design  40  comprising stud  42  with captured washer  55  promotes full contact of contact surfaces  32 ,  56  with the contoured seat  32  at all times during tightening. 
         [0032]    The present invention has been shown and described with reference to the foregoing exemplary embodiments. It is to be understood, however, that other forms, details, and embodiments may be made without departing from the spirit and scope of the invention which is defined in the following claims.