Abstract:
A cold forming process for manufacturing ball pivots with a ball area, a cone area and a thread area for installation in ball and socket joints by cold forming is presented, in which a ball pivot blank ( 1 ) with a shaped cone area ( 3 ) and cylindrical areas for the thread ( 2 ) and the ball ( 5 ) is manufactured at first from a bar-shaped semifinished bar stock by extrusion. The extrusion operation is followed by the forming of the ball area as another manufacturing step by means of a rolling processes. At the same time, the thread area can be formed in its final shape. Thus, the ball pivot is manufactured as a whole by cold forming only and it makes it possible to significantly increase the production output per unit of time compared to the processes known from the state of the art. At the same time, it is possible to use less expensive grades of steel, because sufficient fatigue strength of the ball pivot can be guaranteed by the cold forming.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a United States National Phase application of International Application PCT/DE 2004/002614 and claims the benefit of priority under 35 U.S.C. § 119 of German Patent Application DE 103 56 111.0 filed Nov. 27, 2003, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains to a cold forming process for manufacturing ball pivots with a ball area, a cone area and a thread area for installation in ball and socket joints. 
       BACKGROUND OF THE INVENTION 
       [0003]    Machining as well as non-machining manufacturing processes are used, in principle, to manufacture ball pivots that are intended for installation in ball and socket joints. Larger amounts of waste material, which must be disposed of, take place during machining. In addition, there is a disadvantage that the waste material must first be purchased as a semifinished product. In addition, it is disadvantageous that cycle times that would substantially exceed a number of 10 pieces per minute are not possible on the machine tools that are commonly used at present. Moreover, it is necessary in case of machining that at least the thread area must be machined by a rolling operation in order to prepare the necessary threads in this area. The cycle times of such rolling operations are likewise in the above-mentioned range of about 10 pieces per minute. 
         [0004]    As an alternative to machining, it is known in the state of the art that ball pivots can be manufactured according to the cold or hot process by pressing. The pressing operations can be carried out in the direction of the longitudinal axis of the ball pivot, in which case a residual flash is formed at one end of the ball pivot blank, or they are carried out horizontally in a two-part pressing die, in which case a burr is formed circularly around the entire pressed blank. The burrs formed are removed by a finishing machining operation corresponding to the state of the art. Thus, cost-intensive and time-consuming operations are generated until the ball pivot is manufactured in case of non-machining forming of the ball pivot as well. 
         [0005]    In addition, ball pivots that have a multipart design are known from the state of the art, but their manufacture shall not be discussed in greater detail here, because the process according to the present invention pertains to the manufacture of one-part ball pivots. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the present invention is to provide a process for manufacturing ball pivots with a ball area, a cone area and a thread area, in which the number of pieces per unit of time for manufacturing individual ball pivots can be substantially reduced, so that the necessary manufacturing costs can be significantly reduced. 
         [0007]    This object is accomplished according to the present invention by providing a ball pivot blank with a formed cone area and with cylindrical areas for the thread wherein the ball is first manufactured from a bar-shaped semifinished bar stock by means of extrusion, the extrusion flash being arranged at the free end of the area intended for forming the ball. The ball area is subsequently formed in another operation by means of a triggered synchronized rolling process by rolling forming bodies. 
         [0008]    Any machining can be done away with due to the novel combination of the process steps described. During the pressing operation and the subsequent rolling alike, the cycle times are approx. 100 pieces per minute during pressing and in the range of about 50 pieces per minute during rolling. 
         [0009]    The rolling process may be a so-called flat jaw rolling process or a round-jaw rolling process. 
         [0010]    Due to the combination of the manufacturing operations, it is thus possible to manufacture a considerably greater number of ball pivots than has hitherto been known from the state of the art. This becomes possible, among other things, due to the fact that a plurality of functional areas, for example, the ball area, the cone area and the thread area, can be machined simultaneously by a tool during flat jaw rolling or round-jaw rolling. Moreover, the disposal of any waste material, which is absolutely necessary in case of manufacture by machining, is obviously eliminated. 
         [0011]    Since the investment for a rolling machine is in the same cost range as for purchasing a lathe, the new inventive solution also leads to cost advantages in the area of the initial investment for manufacturing ball pivots due to the higher output of workpieces. 
         [0012]    Moreover, it proved to advantageous in the process according to the present invention to form at first a neck area between the cone area and the cylindrical area intended for forming the ball during the flat-jaw or round-jaw rolling. Examples of strength measurements on ball pivots manufactured according to the novel process have revealed that the fatigue strength of the ball pivots can be increased by up to 50% by forming a neck area by rolling. Due to the increase in the fatigue strength by rolling a neck, it also becomes possible to use steels with microalloying elements, whose purchase price is substantially lower than that of the heat-treatable steels necessary for heat treatments. The cold forming of the C—Mn steels alloyed with microalloying elements is sufficient especially in the neck area to reach a sufficient strength. The increase in the strength values can be used to reduce the overall size of corresponding ball pivots at equal external loads. 
         [0013]    Furthermore, it proved to be expedient as another special embodiment of the process according to the present invention that a blind hole-like recess, which is open on the front side, is pressed before the rolling operation into the free end of the ball pivot blank, which free end is intended for forming the ball area. The blind hole-like recess may be formed by the upper die used for pressing. The recess facilitates the forming of the cylindrical partial area of the ball pivot intended for forming the ball area to its final spherical shape. In addition, the recess does, of course, represent saving of material. If the blind hole-like recess, the so-called pole hole, is still present at least partly after the manufacture of the ball area of the ball pivot, the recess may be used, for example, to receive sensors or used as a grease reservoir. Closing the pole hole, for example, by means of a plastic cover is, of course, also conceivable in this connection. 
         [0014]    The operations during both flat-jaw rolling and round-jaw rolling will be explained in greater detail below on the basis of the attached drawings. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the drawings: 
           [0016]      FIG. 1  is the side view of a ball pivot blank before the ball area is formed by means of the triggered synchronized rolling process; 
           [0017]      FIG. 2  is a schematic top view of a flat-jaw rolling device with a ball pivot arranged therein during the rolling operation; 
           [0018]      FIG. 3A  is a sectional view through the flat-jaw rolling device at the beginning of the rolling operation corresponding to section line A-A in  FIG. 2 ; 
           [0019]      FIG. 3B  is a sectional view through the flat-jaw rolling device during the rolling operation corresponding to section line B-B in  FIG. 2 ; 
           [0020]      FIG. 3C  is a sectional view through a flat-jaw rolling device at the end of the forming process for the ball pivot corresponding to section line C-C in  FIG. 2 ; 
           [0021]      FIG. 4  is a schematic sectional view through a round-jaw rolling device with a ball pivot blank arranged therein; 
           [0022]      FIG. 5  is a top view of the round-jaw rolling device according to  FIG. 4 ; 
           [0023]      FIG. 6  is a perspective view of another variant of a round-jaw rolling device with a ball pivot blank arranged therein; 
           [0024]      FIG. 7  is a schematic view through a round-jaw rolling device with a ball pivot blank arranged therein; and 
           [0025]      FIG. 8  is a top view of the round-jaw rolling device according to  FIG. 7 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Referring to the drawings in particular, the ball pivot blank shown in  FIG. 1  is manufactured by means of an extrusion process and comprises essentially a plurality of partial areas of different three-dimensional shapes arranged one after another in the longitudinal direction of the ball pivot. After extrusion, the different areas are to be formed into their final shape by means of the combination of process features that are essential for the present invention, and a ball pivot shape corresponding to the view in  FIG. 3C  is obtained after completion of the cold forming according to the present invention. 
         [0027]    Thus, the ball pivot blank, designated by  1  as a whole, comprises, according to the view in  FIG. 1 , a cylindrical area  2 , which is arranged at a free end of the ball pivot blank  1  and on which a thread for fastening the ball pivot to a machine or body component is located after the completion of the ball pivot. The cylindrical area  2  is joined directly by a cone area  3 . This cone area  3  has, in principle, already its finished dimensions after the extrusion operation, and smoothing of the conical surface can be brought about in a subsequent operation. The cone area  3  is joined, in turn, by another cylindrical area  4 , which comprises the so-called neck section of the finished ball pivot and in which an incision is made as a transition to the area adjoining it corresponding to the view in  FIG. 3C . Another cylindrical area  5 , which may have a semicircular partial area  6  in the transition to the cylindrical area  4 , is arranged at the second free end of the ball pivot blank  1  as the final area of the ball pivot blank  1 . As an alternative to the semicircular shape of the partial area  6 , a conical transition to the cylindrical area  4  is also conceivable here. The cylindrical area  4  mentioned last is to be reduced to a spherical shape within the framework of the process according to the present invention, so that the ball head  18  shown in  FIG. 3C  is obtained at the upper free end of the ball pivot. To facilitate the forming process, an essentially likewise cylindrical recess  7  in the form of a blind hole may be prepared in the cylindrical area  5  at the upper free end. This may happen within the framework of the preceding extrusion operation and is carried out by means of the upper die of the extruder. The two halves of the pressing die are shaped such that the parting line of the two halves is located in the area of the parting line  8 , so that the parting plane of the die and accumulations of burr that may possibly be associated therewith are located outside the cylindrical area  5  that is to be brought to a spherical shape. 
         [0028]    In principle, three possible variants of rolling process are available for the final shaping of the ball pivot blank  1  by means of a triggered synchronized rolling process by rolling forming bodies. These three different processes will be explained below on the basis of the drawings. 
         [0029]    At first, the so-called flat-jaw rolling process is presented as a variant of the rolling process. The corresponding rolling device used in this rolling process comprises two flat jaws  9  and  10 , one of the flat jaws, the one designated by  9  in this view in  FIG. 2 , being designed as a stationary flat jaw, whereas the flat jaw  10  located opposite performs a translatory motion in the direction of arrow P. The roll gap  11  with an inlet area  12  and with an outlet area  13  is located between the flat jaws  9  and  10 . The ball pivot blank  1  is introduced into the roll gap  11  at the beginning of the flat-jaw rolling process in the area of the section line A-A, and the mobile rolling jaw  10  assumes a position corresponding to the dash-dotted view. Due to the displacement of the mobile flat jaw  10  and the resulting rolling motion of the ball pivot blank  1 , the latter is transported through the roll gap  11 . Both the ball pivot and the neck area adjoining same are formed during the motion of the ball pivot blank within the roll gap  11 . For forming, the flat jaws  9  and  10  have, corresponding to  FIG. 3A , a contour that corresponds essentially to that of the ball pivot blank  1  at the beginning of the rolling operation proper. The ball pivot blank  1  is shown for this purpose together with the flat jaw  9  in  FIG. 3A . The contour profile of the other flat jaw  10  shown in  FIGS. 3A ,  3 B and  3 C corresponds, in principle, to that of the flat jaw  9  shown in the figures. The profile of the flat jaws  9  and  10  changes continuously on the lateral surfaces  14  and  15  facing each other to the finished contour profile in the outlet area  13  corresponding to  FIG. 3C  on the finished ball pivot. Due to the translatory motion of the mobile flat jaw  10 , the starting profile of the flat jaw  9  shown in  FIG. 3A  is located, of course, in the lower area in the area of point C on the flat jaw  10  in the view according to  FIG. 2 , whereas the end profile of the flat jaw  9  shown in  FIG. 3C  is located in the area of point A on the flat jaw  10 . Thus, the flat jaws  9  and  10  are symmetrical in the area of the section line B-B in  FIG. 2 . It appears from the change in the contours of the lateral surfaces  14  and  15  of the flat jaws  9  and  10  that the forming of the ball pivot as well as of the neck area takes place in phases one after another. After the ball pivot blank  1  has been introduced into the roll gap  11 , the neck part  16  is formed at first, and a depression is prepared at the same time in the head area  17  of the cylindrical area  5 , which is facilitated by the recess  7  explained on the basis of  FIG. 1  at the free end of the ball pivot blank  1 . The formation of the neck part  16  by means of the rolling process leads to a substantial increase in strength in this area, because, contrary to machining, no material is removed, but the material of the ball pivot blank is rather formed in the area. 
         [0030]    The final forming of the ball head  18  to its final shape is then performed during the further course of the rolling operation between the area of the section lines B-B and C-C, the mobile flat jaw  10  assuming position III indicated by dotted line in  FIG. 2  at the end of the rolling operation. At the same time, smoothing of the surface of the cone area  3  of the ball pivot blank  1  can be performed during the entire rolling operation due to suitable shaping of the contour of the respective lateral surfaces  14  and  15  of the flat jaws  9  and  10 . Moreover, it is, of course, also possible to prepare the necessary thread in the cylindrical area  2  at the lower free end of the ball pivot blank  1 . Thus, a corresponding shaping of the entire ball pivot may thus optionally take place due to the corresponding shape of the flat jaws during a translatory motion of the mobile flat jaw  10 , without removal of material and disposal of material being necessary, unlike in case of machining. In addition, the rolling operation as a whole leads to an increase in the strength of the surface, so that subsequent hardening procedures can be eliminated. 
         [0031]    The described course of the rolling operation for manufacturing the finished ball pivot may also be carried out, as an alternative to the flat jaw rolling process, by means of devices whose rolling jaws have a curved shape. A first shape of round-jaw rolling will be explained below at first on the basis of the views in  FIGS. 4 and 5 . The round-jaw rolling device comprises, corresponding to the views in  FIGS. 4 and 5 , two rolling cylinders  21  and  22 , which are arranged next to one another on parallel axes of rotation  19  and  20 . Analogously to flat-jaw rolling, a roll gap  11 , into which the ball pivot blank  1  is introduced, is formed between the two rolling cylinders. Both rolling cylinders  21  and  22  have on their jacket surfaces the negative form of the contour of both the ball pivot blank  1  and the contour of the finished ball pivot in an another angular area, which may be arranged offset by about 270° on the circumference. The rolling operation is carried out by opposite rotary motions of the rolling cylinders  21  and  22 , the ball pivot itself remaining stationary in the roll gap  11 . 
         [0032]    The views in  FIGS. 4 and 5  show a position of the rolling cylinders immediately after the introduction of the ball pivot blank  1  into the roll gap  11 . At the same time, it can be recognized from the sectional view in  FIG. 4  that a profile, which corresponds essentially to that of  FIG. 3B  of the flat jaw  9  shown there, is formed on the circumference of the rolling cylinders  21  and  22  offset by 180° in the area of the points P 1  and P 2 . A contour corresponding to  FIG. 3C  with the flat jaw  9  shown there is formed on the circumference of the rolling cylinders  21  and  22  in the area of points P 3  and P 4 . The rolling operation to which the ball pivot blank is subjected until it obtains its final shape thus takes place during a rotation of the rolling cylinders  21  and  22  by 270°. Simultaneous smoothing of the cone area  3  of the ball pivot blank  1  as well as the preparation of the necessary thread in the cylindrical area  2  may, of course, likewise be performed during the so-called round-jaw rolling process. 
         [0033]    Devices in which one rolling jaw is formed by a rolling cylinder  23  similar to that of the above-described first variant, whereas the second rolling jaw is formed by a hollow rolling cylinder  24  concentrically surrounding the rolling cylinder, are suitable as another variant of a so-called round-jaw rolling process. Analogously to the above-described alternative rolling processes, a roll gap  11 , into which the ball pivot blank  1  to be formed is introduced, is obtained between the rolling cylinder  23  and the hollow rolling cylinder  24  surrounding this, corresponding to  FIG. 6 . The variable shape of the contour of the ball pivot blank  1  to be rolled is embossed on the inner jacket surface of the hollow rolling cylinder  24  as well as on the outer jacket surface of the rolling cylinder  23 . The rolling operation process is carried out by rotating the rolling cylinder  23  and the hollow rolling cylinder  24  in opposite directions relative to one another, the relative motion of the rolling cylinder  23  and the hollow rolling cylinder  24  being decisive for the shaping. Thus, it may possibly also be sufficient to rotate only one of the cylinders  23  or  24 , while the other is stopped. Both the ball head and the neck part and, moreover, optionally the cone area as well as the thread area of the finished ball pivot can be formed in this process as well. Thus, this other variant of round-jaw rolling also offers the corresponding advantages in terms of high manufacturing output as well as an increase in strength compared to the processes known from the state of the art. 
         [0034]    The forming of the ball pivot may also be carried out with a round-jaw rolling device, which is known per se, as it is shown as an example in  FIGS. 7 and 8 . 
         [0035]    The ball pivot blank  1  is processed in this case by rolling the symmetrical rolling cylinders  25  and  26 , which rotate about their axes of rotation. The ball pivot blank  1  is arranged between the rolling cylinders  25 ,  26  in the roll gap  11 . The ball pivot blank  1  receives its spherical shape due to the correspondingly shaped circumferential geometry of the rolling cylinders  25 ,  26 . A negative contour, which may also form the cylindrically shaped area  5  of the ball pivot blank  1 , is molded in the respective jacket surfaces  27  and  28  of the rolling cylinders  25 ,  26 . The rotating rolling cylinders  25 ,  26  are at first moved toward the ball pivot blank  1 . This feed motion may be brought about, for example, by means of a hydraulic cylinder. 
         [0036]    While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.