Abstract:
A suspension component for an automobile, such as a stabilizer bar, is provided and includes at least one localized portion having a strengthened outer surface. The localized portion is positioned at a location of highest stress along the suspension component. A method of forming the suspension component is also disclosed.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention generally relates to a suspension component for an automobile. More specifically, the present invention relates to a stabilizer bar having localized material strengthening, and a method of manufacturing the stabilizer bar.  
           [0003]    2. Description of the Prior Art  
           [0004]    In an automotive vehicle, suspension components, such as a stabilizer bar, helps to keep the vehicle level, particularly when the vehicle is traveling through a curve. The ends of the stabilizer bar are connected to the right and left wheel assemblies of the vehicle. A pair of brackets, positioned between the ends of the stabilizer bar, secure the stabilizer bar to a structural component of the vehicle. Rubber bushings positioned between the stabilizer bar and the brackets provide limited torsional, axial and radial movement of the stabilizer bar relative to the bracket. The rubber bushings also dampen the movement of the stabilizer bar.  
           [0005]    Typically, the goal of the material selection and manufacturing process used to make a stabilizer bar is to form a stabilizer bar with homogenous material properties that meet the highest required yield and fatigue strengths, as determined by calculated finite element analysis procedures. In many cases, cold forming, hot forming, heat treatments, and shot peening operation are performed on the stabilizer bar to achieve the yield and fatigue strength properties. The heat treatments must be applied to the entire stabilizer bar.  
           [0006]    In actual use the highest stresses within the stabilizer bar are realized in specific localized areas along the stabilizer bar, not along the entire length. Therefore, stabilizer bars made by more traditional methods, where the entire stabilizer bar is heat treated, present several disadvantages. One disadvantage is that the stabilizer bar is heavier than it needs to be, because the stabilizer bar is homogenous. Another disadvantage is that the process involves unnecessary cost, as the entire stabilizer bar is subjected to heat treatment, when only a localized area must meet the high stress requirements. Finally, the facilities needed to process the stabilizer bar are larger, and require more energy, than needed, because they must be adapted to treat the entire stabilizer bar.  
           [0007]    Additionally, typical manufacturing methods for suspension components such as stabilizer bars include heating the bar to a high temperature, bending the bar to the desired shape, and then quenching and tempering the bar. These steps are typically required to maintain the strength of the bar after the bar is bent. Quenching and tempering, however, causes de-carbonization of the entire bar and deformation of the bar, which must be later corrected.  
           [0008]    Therefore, there is a need for an improved stabilizer bar, made from a material that can be cold formed to the required shape without requiring heating, quenching and tempering operations, and that includes localized area that are treated to withstand high stresses. Further, there is a need for an improved method of forming a suspension component that allows the suspension component to be cold formed to the required shape and to have localized areas that are treated to withstand high stresses, while the remaining areas of the suspension component are not treated.  
           [0009]    A principle object of this invention is to provide a suspension component that is made from a material that can be cold formed to the needed shape and can be cold work hardening strengthened or heat treated to withstand stress levels that will be experienced by the suspension component.  
           [0010]    Another object of this invention is to provide a suspension component that has localized portions that are specially treated with cold work hardening, shot peening and/or heat treatment to withstand localized stresses that the suspension component will experience, while the remaining portions of the suspension component remain untreated.  
           [0011]    It is also an object of this invention to provide a method of manufacturing a suspension component having localized portions that are work hardening strengthened, shot peened or heat treated to withstand localized stresses that the suspension component will experience, while the remaining portions of the suspension component remain untreated.  
         SUMMARY OF THE INVENTION  
         [0012]    The disadvantages of the prior art are overcome by providing a suspension component, in accordance with the present invention, in which the suspension component includes localized portions that are specially treated using cold work hardening, shot peening and/or heat treatment to withstand localized stresses that the suspension component will experience, while the remaining portions of the suspension component left untreated.  
           [0013]    In a first aspect of the present invention the suspension component is formed from one of micro-alloyed boron steel and vanadium steels containing relatively low levels of carbon, such that the suspension component will have good ductility and fracture toughness, to allow the suspension component to be cold formed, and yet still allow the suspension component to be readily heat treated thereafter if heat treatment localized material strengthening is desired.  
           [0014]    In another aspect of the present invention the suspension component includes localized portions that are shot peened or heat treated after the suspension component has been cold formed into a desired shape.  
           [0015]    In still another aspect of the present invention, the localized portions of the suspension component are induction case hardened. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a plan view of a stabilizer bar of the present invention;  
         [0017]    [0017]FIG. 2 is a plan view similar to FIG. 1;  
         [0018]    [0018]FIG. 3 is a sectional view taken along line  3 - 3  of FIG. 2, wherein the stabilizer bar is solid;  
         [0019]    [0019]FIG. 4 is a sectional view similar to FIG. 3, wherein the stabilizer bar is hollow;  
         [0020]    [0020]FIG. 5 is a flow chart of a method of manufacturing the stabilizer bar of the present invention, including a shot-peeing process; and  
         [0021]    [0021]FIG. 6 is a flow chart of a method of manufacturing the stabilizer bar of the present invention, including a heat treating process.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring to FIG. 1, a suspension component for an automotive vehicle is shown generally at  10 . The suspension component shown is a stabilizer bar, however, it is to be understood, that the teachings of the present invention are applicable to other suspension components. The stabilizer bar assembly  10  includes a stabilizer bar  12  having opposing first and second distal ends  14 , 16 . The stabilizer bar  12  is generally made from steel, and can be solid or hollow between the ends  14 ,  16 . The stabilizer bar  12  further includes at least one bushing assembly  18  mounted thereon. The bushing assemblies  18  are adapted to mount the stabilizer bar  12  to the structure of the automobile.  
         [0023]    Each of the first and second distal ends  14 ,  16  have an attachment point  20 . The attachment points  20  are adapted to connect the distal ends  14 ,  16  to wheel assemblies (not shown) on the automobile. The attachment points  20  can be attached to the distal ends  14 ,  16 , by welding. Alternatively, the attachment points  20  can be formed unitary with the distal ends  14 ,  16 , whereby the distal ends  14 ,  16  are heated, and/or otherwise formed into the shape of the attachment points  20 .  
         [0024]    In operation, the stabilizer bar  12  is adapted to keep the wheels on opposite sides of the automobile level to one another with respect to the automobile. The shape of the stabilizer bar  12  includes a generally straight center section  22  and two arms  24 ,  26  extending generally angularly from opposite ends of the straight section  22 .  
         [0025]    The arms  24 ,  26  are formed by bending the stabilizer bar  12 , such that the arms  24 ,  26  are integral with the center section  22 . The arms  24 ,  26  extend at and angle to the center section  22 . When the distal ends  14  of a first of the two arms  24  is forced upward or downward vertically, the arm  24  acts as a moment arm, thereby transferring torque to the center section  22 . The center section  22  transfers the torque to the opposite arms  26 , to correspondingly force the second distal end  16  upward or downward.  
         [0026]    Referring to FIG. 2, a finite element analysis identifies localized areas  28 ,  30  immediately around the bends between the arms  24 ,  26  and the straight section  22  as the point of highest stress within the stabilizer bar  12  during operation of an automobile. To withstand the higher stresses experienced within the localized areas  28 ,  30 , these localized areas  28 ,  30  include a strengthened outer surface.  
         [0027]    Referring to FIG. 3, a cross section of a solid stabilizer bar  12  shows an outer surface portion  32  and an inner portion  34 . The outer surface portion  32  is strengthened by shot peening the outer surface of the stabilizer bar  12  within the localized areas  28 ,  30 . Alternatively, the surface of the stabilizer bar  12  within the localized areas  28 ,  30  can be heat treated. The heat treatment used to strengthen the outer surface of the stabilizer bar  12  within the localized areas  28 ,  30  can be a traditional quench and temper. However, it is typically difficult to perform a quench and temper operation on a localized portion of an object. To resolve this difficulty, the localized areas  28 ,  30  of the stabilizer bar are preferably induction case hardened, using an eddy current applied to the surface of the stabilizer bar  12  within the localized areas  28 ,  30 . By using an eddy current process to induction case harden the stabilizer bar  12 , the area that is treated, and the depth of the treatment can be closely controlled. Alternatively, the stabilizer bar can be hollow, as shown in FIG. 4.  
         [0028]    The material of the stabilizer bar  12  is preferably steel, however, the particular steel used is important. Preferably the steel selected has good ductility and fracture toughness and high yield strength. This is necessary to allow the stabilizer bar  12  to be bent to the desired shape. The stabilizer bar  12  is cold worked, meaning the stabilizer bar  12  is bent when it is at ambient temperature, rather than at an elevated temperature.  
         [0029]    Further, the steel will be treated to harden the surface within the localized areas  28 ,  30 . Typically, low carbon steels cannot be hardened to the equivalent strength of high carbon steels. With the above in mind, the stabilizer bar  12  of the present invention is preferably formed from a micro-alloyed boron steel, such as 15B21, a vanadium steel, such as 1541V, or other material of similar characteristics. These steels are low carbon steels, but because of the addition of Boron or Vanadium, can be heat treated to hardness and strengths equivalent to high carbon steels. Therefore, the stabilizer bar  12  possesses good ductility and toughness that allows the stabilizer bar  12  to be cold bent to the desired shape, and the localized areas  28 ,  30  can be heat treated to harness and strength levels that meet the requirements of the application.  
         [0030]    Referring to FIGS.  5 , the localized areas  28 ,  30  will be localized work hardened during the cold forming process, due to the work hardening material strengthening mechanism. The shaded areas indicate material plastic flow during cold forming, regardless of sold or hollow bars. Material cold plastic flow will induce work hardening effects further localized strengthening the material.  
         [0031]    Referring to FIGS. 6, a flow chart illustrating method of manufacturing the stabilizer bar  12  according to the present invention is shown generally at  36 . After the steel is received, as shown in block  37  of FIG. 6, the attachment points  20  are formed onto the distal ends  14 ,  16  of the stabilizer bar  12 . Preferably, the distal ends  14 ,  16  are heated, and the attachment points are formed therein by a process known as “Double Eye”, as shown in the blocks indicated by reference numerals  38  and  40 . Then the stabilizer bar  12  is cold formed into the desired shape as shown in block  42 . After the stabilizer bar  12  is cold formed, localized areas of the outer surface are strengthened. In one method, the localized areas of the outer surface of the stabilizer bar  12  are shot peened, to provide compressive forces into the surface of the stabilizer bar  12  which further strengthens the stabilizer bar. This step is illustrated in block  44  of FIG. 6. This step provides the added strength that the localized portions  28 ,  30  need to withstand the high stresses placed on the stabilizer bar  12  within the localized portions  28 ,  30 . After the shot peening, the stabilizer bar  12  can be painted and shipped.  
         [0032]    Referring to FIG. 7, in an alternative process the stabilizer bar  12  is received, as shown in Block  37  of FIG. 6. The attachment points  20  are formed onto the distal ends  14 ,  16  of the stabilizer bar  12 , as shown in blocks  38  and  40 , and, and the stabilizer bar  12  is cold formed into the desired shape, as shown in Block  42 . However, in the alternative process, after the stabilizer bar  12  is cold formed, the outer surface of the stabilizer bar  12  is heat treated within the localized areas  28 ,  30 , which further strengthens the stabilizer bar  12 . This step is illustrated in block  46  of FIG. 7. The heat treatment can consist of a quenching and tempering procedure, an induction case hardening process whereby an eddy current is applied to the surface of the stabilizer bar  12 , or other heat treating method. After the heat treatment, the stabilizer bar  12  can be painted and shipped.  
         [0033]    The foregoing discussion discloses and describes the preferred embodiments of the invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that changes and modifications can be made to the invention without departing from the fair scope of the invention as defined in the following claims. The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation.