Abstract:
There is provided a ball joint provided with a ball stud having a spherical head portion in an end portion, a ball seat to which the spherical head portion is slidably fitted, thereby pivoting the ball stud universally, and a housing receiving the ball seat, the housing being structured such that a closed-end cylindrical main body portion and a flange portion extending outward in a radial direction from an edge portion of the main body portion are formed in accordance with sheet metal processing. The flange portion is formed thinner than a thickness of the main body portion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a ball joint which is, for example, used in a connection portion of a stabilizer for a vehicle, and to a method of manufacturing a housing therefor.  
           [0003]    2. Related Art  
           [0004]    [0004]FIG. 4 shows a conventional embodiment of this kind of ball joint. The ball joint is constructed such that a spherical head portion  2   a  formed in an end portion of a ball stud  2  is slidably fitted to a resin ball seat  3  received within a cylindrical housing  4 . The ball stud  2  is pivoted relating to the ball seat  3  in a universal manner around the spherical head portion  2   a , that is, in such a manner as to freely tilt and rotate around an axis. A rivet-like engagement portion  3   b  formed in a bottom portion of the ball seat  3  extends through a bottom portion of the housing  4  so as to be engaged therewith, whereby the ball seat  3  is restricted to the housing  4 , thereby preventing it from rotating around the axis and coming off from the housing  4 . In this case, reference numeral  6  denotes a bevel-like dust cover, and reference numeral  7  denotes a support bar integrally adhered to the housing  4 .  
           [0005]    In this case, as shown in enlarged FIG. 5, the housing  4  is constituted by a main body portion  4   a  formed in a closed-end cylindrical shape, and a ring-like flange portion  4   b  formed in an edge portion of the main body portion  4   a , and is integrally formed in accordance with a sheet metal process from a steel plate. Further, the support bar  7  is electric resistance welded at a position a distance  c  apart from a lower surface of the flange portion  4   b . This is because the welding is applied so as to avoid a circular arc surface formed in a cross portion between the lower surface of the flange portion  4   b  and the outer peripheral surface of the main body portion  4   a , and a space for burrs generated in welding is secured. Further, the spherical head portion  2   a  of the ball stud  2  is pressure inserted by elastically deforming the ball seat  3 . In order to prevent a crack from being generated in the bottom portion of the ball seat  3  in pressure insertion thereof, it is necessary to provide a certain degree of thickness in the bottom portion. Accordingly, it is impossible to move the ball stud  2  very close to the bottom portion side of the ball seat  3 . For the reason mentioned above, a center O of the spherical head portion  2   a  of the ball stud  2  is arranged at a position a distance e shifted from a center line  7   a  of the support bar  7 .  
           [0006]    Since the difference of distance e exists between the center of the spherical head portion  2   a  of the ball stud  2  and the center line  7   a  of the support bar  7 , a bending moment of P×e with respect to a test load P generated by a tensile test or a buckling test in an axial direction of the support bar  7  is applied to the support bar  7 . Accordingly, since a strength of the support bar  7  cannot be obtained while a strength obtained by multiplying a cross sectional area of the support bar  7  by a material strength is generally secured, there have been cases in which it is necessary to make the support bar  7  thick for the purpose of satisfying a required specification.  
           [0007]    Further, a drawing load F of the ball stud is generally expressed by a smaller value among values calculated by the following formula (1) or (2).  
             F= 4× N×d   3 ×π/4+μ× F′×η× ( l   1   ×π×D )   (1)  
             F=N×π×D×l   2    (2)  
           [0008]    In this case, parameters in the respective formulas mentioned above are as follows.  
           [0009]    d: diameter of the shaft in engagement portion  3   b    
           [0010]    N: shear resistance of the ball seat  3   
           [0011]    μ: coefficient of friction between the housing  4  and the ball seat  3   
           [0012]    η: component force conversion efficiency in the outer peripheral direction due to contact angle between the ball stud  2  and the ball seat  3  when a drawing load is applied to the ball stud  2   
           [0013]    l 1 : distance of the linear portion extending from the crossing point between a normal line and the inner peripheral surface toward the flange portion  4   b  (distance of the flange portion  4   b  to the end point of the circular arc surface) when the normal line is formed passing through the center O of the spherical head portion  2   a  of the ball stud  2  on the inner peripheral surface of the housing  4   
           [0014]    F′: component serving to provide friction force between an inner peripheral surface of the housing  4  and the ball seat  3  when a drawing load is applied to the ball stud  2   
           [0015]    D: diameter of spherical head portion  
           [0016]    l 2 : length of thick portion extending from crossing point between normal line and inner peripheral surface toward axial direction of ball stud  2  when a normal line is formed passing through center O of spherical head portion  2   a  of the ball stud  2  on inner peripheral surface of ball seat  3 .  
           [0017]    In general, the value calculated by the formula (1) is smaller in the drawing load F. In accordance with the formula (1), in order to increase the drawing load F, the spherical head portion may be made large, or the housing may be made longer so as to set the distance l 1  of the linear portion of the inner peripheral surface to be large. However, in such a countermeasure, weight and size are increased, and material costs are unnecessarily increased.  
         SUMMARY OF THE INVENTION  
         [0018]    Accordingly, objects of the present invention are to provide a ball joint which can increase a tensile strength and a buckling strength of a support bar and can increase a drawing strength of a ball stud, and to provide a method of manufacturing a housing therefor.  
           [0019]    The present invention provides a ball joint comprising: a ball stud having a spherical head portion in an end portion; a ball seat to which the spherical head portion is slidably fitted, thereby pivoting the ball stud universally; and a housing receiving the ball seat. The housing is structured such that a closed-end cylindrical main body portion and a flange portion extending outward in a radial direction from an edge portion of the main body portion are formed in accordance with a sheet metal process. The flange portion is formed so as to be thinner than a thickness of the main body portion.  
           [0020]    In accordance with the ball joint having the structure mentioned above, since the flange portion is formed to be thinner than the thickness of the main body portion, it is possible to move the center line  7   a  of the support bar  7  in FIG. 5 closer to the center O of the spherical head portion  2   a  of the ball stud  2 . Accordingly, the position difference  e   between both can be small so as to increase a tensile strength and a buckling strength of the support bar  7 .  
           [0021]    It is desirable that an outer radius of curvature in a cross section of the crossing point between the flange portion and the main body portion be made smaller than a thickness. Accordingly, the distance l 1  of the linear portion of the housing inner peripheral surface becomes long, so that it is possible to increase the drawing strength of the ball stud.  
           [0022]    In this case, in order to make the position difference  e   between the support bar and the spherical head portion, it is desirable that the ball stud be arranged as close as possible to the bottom portion of the ball seat.  
           [0023]    Accordingly, it is preferable to form a flat portion in a bottom portion side of the spherical head portion and to set a distance between a center of the spherical head portion and the flat portion to 0.25 to 0.4 times the diameter of the spherical head portion. Further, it is effective to make a thickness of the bottom portion of the ball seat as small as possible. In this case, by providing with a convex portion having a thickness larger than a periphery in a center of the bottom portion, it is possible to prevent a crack from being generated in the bottom portion in pressure inserting the spherical head portion into the ball seat.  
           [0024]    When arranging the ball stud close to the bottom portion side, an angle of swing of the ball stud becomes narrow. In order to secure a wide angle of swing, it is possible to form a shaft portion connected to the spherical head portion in a tapered shape which is tapered toward the spherical head portion side.  
           [0025]    Next, the present invention also provides a method of manufacturing a housing for a ball joint provided with a closed-end cylindrical main body portion and a flange portion extending outward in a radial direction from an edge portion of the main body portion. The method comprising: forming a closed-end cylindrical body by sheet metal forming; working so as to reduce a thickness of a portion near an opening portion of the cylindrical body; and expanding and opening a portion at which the thickness is reduced, or a forward portion from a portion close to the opening portion, so as to form a flange portion.  
           [0026]    In accordance with the manufacturing method mentioned above, since the flange portion is expanded and formed in the portion thinner than the thickness of the raw material, a crossing portion between the flange portion and the main body portion is constructed in the portion thinner than the thickness of the raw material. Accordingly, an outer radius of curvature in a cross section of the crossing portion becomes smaller than the thickness of the raw material. In this case, a well-known method can be employed for the process of reducing the thickness close to the opening portion of the formed body. For example, it is possible to employ a method of clamping an outer periphery of the formed body by a metal mold and inserting a punch having an outer diameter larger than an inner diameter of the formed body to the opening portion. In this case, it is desirable that the thickness after being thinned be between 55 and 65% of the thickness of the raw material. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIGS. 1A, 1B,  1 C and  1 D are views showing a ball joint in accordance with an embodiment of the present invention, in which FIG. 1A is a cross sectional view along a line A-A in FIG. 1C, FIG. 1B is a view as seen from an arrow B in FIG. 1A, FIG. 1C is a view as seen from an arrow C in FIG. 1A and FIG. 1D is a view as seen from an arrow D in FIG. 1A;  
         [0028]    [0028]FIG. 2A is side an elevational view showing the ball joint shown in FIG. 1A in an enlarged manner, and FIG. 2B is a side cross sectional view showing a bottom portion of a ball seat enlarged;  
         [0029]    [0029]FIGS. 3A, 3B and  3 C are side cross sectional views showing a method of forming a housing in accordance with an embodiment in the order of FIGS. 3A to  3 C;  
         [0030]    FIGS.  4 A,  4 B,and  4 C are views showing a ball seat constituting a conventional ball joint, in which FIG. 4A is a cross sectional view along a line A-A in FIG. 4B, FIG. 4B is a view as seen from an arrow B in FIG. 4A, and FIG. 4C is a view as seen from an arrow C in FIG. 4A; and  
         [0031]    [0031]FIG. 5 is a view showing the ball joint shown in FIG. 4A enlarged. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]    A description will be given below of an embodiment in accordance with the present invention with reference to the accompanying drawings.  
         [0033]    [0033]FIG. 1 shows a ball joint  1  used in a connection portion of a stabilizer for a vehicle. The ball joint  1  in accordance with the embodiment is provided with a ball stud  20  having a spherical head portion  20   a  in an end portion, a ball seat  30  to which the spherical head portion  20   a  is slidably fitted, and a housing  40  to which the ball seat  30  is pressure inserted so as to receive the ball seat  30 . A rivet-like engagement portion  30   b  formed in a bottom portion of the ball seat  30  extends through a bottom portion of the housing  40  so as to engage therewith, whereby the ball seat  30  is clamped to the housing  40 , so that it is possible to prevent the ball seat  30  from rotating around an axis and coming off from the housing  40 . Further, one end of a support bar  70  is firmly fixed to the housing  40 .  
         [0034]    The ball stud  20  pivots in the ball seat  30  in a universal manner around the spherical head portion  20   a , that is, in such a manner as to freely tilt and rotate around the axis. FIG. 2 is a view showing the ball joint  1 , in accordance with the embodiment, in an enlarged manner. As shown in FIG. 2A, a shaft portion  20   b  connected to the spherical head portion  20   a  of the ball stud  20  is formed in a tapered shape tapered toward the spherical head portion  20   a . Further, a front end portion of the spherical head portion  20   a  has a large portion cut off, and a flat portion  20   c  is formed thereat. A distance from a center O of the spherical head portion  20   a  to the flat portion  20   c  is set to be 0.25 to 0.4 times the diameter of the spherical head portion  20   a.    
         [0035]    The ball seat  30  is formed in a closed-end cylindrical shape having a flange portion  31  in an upper end edge, and a spherical seat  32  to which the spherical head portion  20   a  is fitted is formed in an inner portion thereof. As shown in FIG. 2B, a convex portion  33  which is thicker than a peripheral portion is formed in a center of a bottom portion of the ball seat  30 . The ball seat  30  is formed, for example, by a resin such as a polyacetal, a polybutylene terephthalate or the like. A bevel-like dust cover  60  is provided between the flange portion  31  of the ball seat  30  and a flange portion  21  formed in the ball stud  20 .  
         [0036]    The housing  40  is structured such that a flange portion  42  is integrally formed in an upper end edge of a closed-end cylindrical main body portion  41 , and an inner portion thereof is formed as a receiving portion for the ball seat  30 . A thickness of the flange portion  42  is made thinner than a thickness of the main body portion  41 . Further, an outer radius of curvature of a cross section in a crossing portion between the main body portion  41  and the flange portion  42  is made smaller than the thickness of the main body portion  41 .  
         [0037]    Next, a description will be given of a method of manufacturing the housing  40  with reference to FIG. 3. For manufacturing the housing  40 , a transfer metal mold in which a plurality of working stages are provided in one metal mold is preferably employed. First, as shown in FIG. 3A, a formed body  45  having the shape of the housing is draw molded from a steel plate. Next, a hole  40   a  to which the engagement portion  30   b  of the ball seat  30  is inserted is formed in the next working stage. In this state, the formed body  45  has a uniform thickness t 1 .  
         [0038]    Next, the formed body  45  is set in a lower mold (not shown) of the next stage so as to clamp an outer periphery, and a punch is inserted to an opening portion of the formed body  45 . Accordingly, a thickness near the opening portion moves to a side of the opening portion, whereby the thickness is reduced. Therefore, as shown in FIG. 3B, a portion  45   a  having the thickness t 1  of the raw material and a portion  45   b  having a thickness t 2  thinner than the thickness t 1  are formed in a tubular portion of the formed body  45 , and these two portions  45   a  and  45   b  are smoothly connected by curved surfaces having radii R 1  and R 2  of curvature. In this embodiment, the thickness t 2  is set to be about 60% of the thickness t 1 , whereby it is possible to make the radius R 2  of curvature smaller than the thickness t 1 .  
         [0039]    Next, the formed product  45  is transferred to the next stage, and as shown in FIG. 3C, is expanded by the portion having the radius R 2  of curvature, whereby the housing  40  constituted by the main body portion  41  and the flange portion  42  is formed. In this case, in the crossing portion between the main body portion  41  and the flange portion  42 , the radius R 2  of curvature formed by the thickness reduction mentioned above is left as it is. Accordingly, an outer radius of curvature of the crossing portion is smaller than the thickness t 1  of the raw material.  
         [0040]    In the ball joint  1  having the structure mentioned above, since the flange portion  42  is compression molded so as to be thinner than the thickness of the main body portion  41 , it is possible to reduce a size h from a center line  70   a  of the support bar  70  to an end surface of the flange portion  42 . Accordingly, it is possible to increase a tensile strength and a buckling strength of the support bar  70 . In particular, in the embodiment mentioned above, since the outer radius R 2  of curvature in the cross section of the crossing portion between the flange portion  42  and the main body portion  41  is made smaller than the thickness t 1 , the distance l 1  of the linear portion of the housing inner peripheral surface is long, whereby it is possible to increase the drawing strength of the ball stud  20 .  
         [0041]    Further, in the embodiment mentioned above, since it is possible to form the structure to be as thin as possible while preventing the bottom portion from being cracked, by cutting off a large part of the front end portion of the spherical head portion  20   a  so as to form the flat portion, and forming the convex portion  33  in the center of the bottom portion in the ball seat  30 , it is possible to arrange the ball stud  20  significantly closer to the bottom portion side. As a result, as shown in FIG. 2A, the center O of the spherical head portion  20   a  is arranged on the center line  70   a  of the support bar  70 , and it is possible to substantially completely prevent the bending moment from being generated in the support bar  70 . Further, in the embodiment mentioned above, since the shaft portion  20   b  of the ball stud  20  is formed in the tapered shape, there is an advantage that a wide angle of swing can be secured.