Patent Publication Number: US-2011062121-A1

Title: Welding method for the manufacture of a removable tow bar for private motor vechiles

Description:
FIELD OF THE INVENTION 
     The present invention relates to a welding method for the manufacture of a removable tow bar for private motor vehicles, which tow bar comprises a coupling member produced as a forging, as well as a ball rod produced as a pivoting member having a free end by which the ball rod is welded to the coupling member by means of a mig welding process. 
     STATE OF THE ART 
     Removable tow bars for private motor vehicles are generally known. They are generally built up of a metal frame or supporting structure fixedly connected to the vehicle, which supporting structure is usually found on the inside of the bumper so that it is generally hidden from view, and a removable ball bar system consisting of the removable ball bar per se and a metal receiver which is fixedly attached to the metal supporting structure by means of bolts or welds and which is intended for receiving the removable ball rod. When fitting the removable ball rod it is inserted into the receiver after which the locking operation in the receiver generally takes place automatically. When the removable ball rod is removed one generally utilizes a control knob which, when turned, causes the ball rod to be released, after which the latter can be removed from the receiver. 
     When ball rods are manufactured, they may be selected to be manufactured in one piece (usually a forging) or to be manufactured in two pieces (a coupling member and the ball rod), which pieces are then fastened to each other after they have been CNC processed. 
     The present invention relates to removable tow bars for which the coupling member and the ball rod are manufactured separately, after which they are fixedly attached to each other. 
     There are various ways to attach the coupling member to the ball rod. One of these ways is the mig welding process. With this welding method the end of the ball rod is connected to the coupling member by an ordinary mig weld. The strength is then the greatest problem. For this reason the end of the ball rod is often turned at a larger diameter than the ball rod per se, after which this end is flattened in order to obtain a larger connecting surface for the weld. Once the end of the ball rod has been flattened, the head end of the ball rod is no longer flat so that an additional CNC milling operation is needed to prepare the surface for the weld. Also a bevel is milled from the head end, which bevel is used as a weld gutter. In this way the desired strength can be obtained, but the method is expensive because a heavy press is needed for flattening the ball rod at the end after which the end is still to be CNC processed before the weld can be made. After this the required welding time is relatively long. Another disadvantage is that the ball rod manufactured in this way has a less fine aspect. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a cost effective welding method which offers sufficient strength, provides a fine aspect, has a relatively short process time and requires a limited investment. To this end the welding method according to the invention is characterised in that the coupling member includes a cylindrical welded lug whose free end is kept at a distance from the free end of the ball rod, so that there is a welding gap between the two ends while the welding process is carried out with at least one stationary or substantially stationary welding gun and the relative movement of the welding gun with respect to the tow bar is caused to occur exclusively or substantially exclusively by the rotating of the tow bar and the coupling member around a horizontal axis, while the welding gap is helically welded full up to the outside surface or just above it. 
     Preferably at the free end of the ball rod or welded lug is provided a spacer stop concentrically situated at the free end and held against the other free end during the welding operation. 
     An embodiment of the welding method according to the invention is characterised in that the other free end not carrying the spacer stop has a central cylindrical hole in which the spacer stop is inserted with a light press fit in such a way that a concentric welding gap having a desired width is developed between the two free ends. This is advantageous in that only one part needs to be connected to the welding manipulator for the welding operation, so that the welding manipulator may be of a simpler construction. 
     A further embodiment of the welding method according to the invention is characterised in that the diameter of the free ends of the ball rod as well as the cylindrical lug on the coupling member are smoothly increased by several millimetres over several millimetres&#39; width. As a result, the tensile stress and compressive stress in the surface of the weld, caused by the work loads, are reduced and thus the load-taking capacity of the product is increased. 
     Preferably two or more stationary or substantially stationary welding guns are used for the welding process. This accelerates the welding process and also improves the distribution of the welding metal in the welding gap. 
     A still further embodiment of the welding method according to the invention is characterised in that through a special blowing apparatus largely surrounding the welding gap protective gas is blown into the welding gap during the welding operation. As a consequence, a better protection of the welding bath is obtained. 
     Again a further embodiment of the welding method according to the invention is characterised in that immediately after the welding operation the weld is machined while the tow bar is being rotated. Prefereably the weld is then machined or milled off. The removal of metal is then carried out on the still very hot (red hot) surface of the tow bar, so that a high degree of smoothness is developed as well as a fine-grained crystal structure. In this way not only a fine aspect is developed, with no longer being visible that it is a welded product, but also the fatigue strength of the product is improved by the fine-grained crystal structure that has developed in combination with the very smooth surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description relating to the appended drawings, the whole given by way of non-limiting example of the welding method according to the invention, will provide better understanding of how the invention can be realised, in which: 
         FIG. 1  shows the coupling member with a cylindrical lug; 
         FIG. 2  shows the bent ball rod; 
         FIG. 3  shows the position of coupling member and ball rod during the welding operation; 
         FIG. 4  shows the helical structure of the weld in the welding gap; 
         FIG. 5  shows a detail drawing of the press fit; 
         FIG. 6  shows a protective gas blowing apparatus partly surrounding the welding gap; 
         FIG. 7  shows a detail cross section of the increase of diameter; 
         FIG. 8   a  shows the tensile stress and compressive stress in a shaft with a bending load plotted against the distance to the centre line; and 
         FIG. 8   b  shows the stress distribution in a shaft in the place of the increased diameter. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The welding method according to the invention is designed for the manufacture of a removable tow bar for private motor vehicles. The tow bar comprises a coupling member  1  made as a forging and a ball rod  3  made as a pivoting member. These two members are welded together by means of a mig welding process in the way elucidated hereinafter. 
     The coupling member  1  is first and foremost provided with a cylindrical lug  5  to which the ball rod is attached. The diameter of this cylindrical lug is then equal to the diameter of the end of the ball rod to be welded thereto; see  FIG. 1 . 
     Subsequently, the end of the ball rod  3  arranged as a pivoting member also has a cylindrical diameter and its head end is provided with a cylindrical and concentric spacer stop  7  which has a diameter that is about half the diameter of the end of the ball rod and has a height of about a quarter of the diameter of the ball rod. Prior to being welded, the ball rod may already have the desired bend. It is simpler for the bending process to be executed prior to the welding operation; see  FIG. 2 . 
     Then, in the welding method according to the invention the two members  1  and  3  described above are placed in a welding manipulator which has a horizontal axis of rotation so that the cylindrical lug  5  of the coupling member  1  abuts the spacer stop  7  at the head end of the ball rod  3 , while the axis of the cylindrical lug on the coupling member coincides with the axis of the end of the ball rod; see  FIG. 3 . In this way a concentric welding gap  9  arises having a depth and a width which are both equal to about a quarter of the diameter of the ball rod. However, the depth and the height of the welding gap  9  need not of necessity be equal to a quarter of the diameter of the ball rod, but it will generally turn out that these values produce a good result in the welding process. The two members  1  and  3  to be united in the welding manipulator are placed such that the axis of rotation of the welding manipulator coincides with the common axis of the end of the ball rod and the lug of the coupling member. The concentric welding gap  9  is then arranged in a vertical plane. 
     During the welding process the product is rotated in the manipulator around a horizontal axis, which coincides with the axis of the end of the ball rod and the axis of the welded lug on the coupling member, while the welding gun  11  is stationary at a specific suitable position with respect to the position of the welding bath and gravity. Generally, in the event of a product rotating counter-clockwise, the welding gun will be positioned in the right-hand top quadrant of the concentric welding gap. This in connection with the advantageous effect of gravity on the welding bath. Welding in this way with a stationary welding gun and a rotating product will provide a helical structure of the weld in the welding gap. The welding parameters are then to be set such that a proper filling of the welding gap is effected in combination with a proper penetration into the side walls of the concentric welding gap.  FIG. 4  shows the helical structure of the weld  13  in the welding gap and the position of the welding gun  15 . 
     For realising an uninterrupted welding process it is necessary for the welding manipulator to have two locking claws whose axes of rotation coincide and which can rotate simultaneously with exactly the same number of revolutions per minute (angle-controlled) and in which the two members to be united can be locked. In this manner it is avoided that a mechanical joint between the two chucks interrupts the welding process during the continuous rotation. A welding manipulator of this type can be realised by using stepping motors for driving the chucks. If such a welding manipulator is not available, in a different manner it may be avoided that the welding process is interrupted, that is, by mechanically connecting, prior to the welding operation, the two members to be welded together, so that the whole product can be lifted from the welding manipulator by only one locking claw. This may be achieved by introducing in the lug  5  of the coupling member  1  a hole  17  having a slight undersize relative to the spacer stop  7  which is situated at the head end of the ball rod  3 , so that the ball rod can be attached to the coupling member with a slight press fit; see  FIG. 5 . In this manner not more than one chuck is required to clamp down the tow bar so as to allow for uninterrupted welding. 
     In order to obtain proper gas protection from the welding bath in the concentric welding gap it is of importance to have the welding manipulator equipped with a special protective gas blowing apparatus  19  which largely surrounds the concentric welding gap and can thus inject the protective gas directly into the welding gap; see  FIG. 6 . In this manner a sufficient quantity of protective gas can stay behind in and around the concentric welding gap for an adequate corrosion protection from the welding bath and the immediate environment. This is of importance because the welding process in this case is a continuous process with much heat power concentrating in a small area. As a result, enormous heat is generated and there is a great chance of oxidation with the rotating shaft. Oxidation inclusions will negatively affect the weld strength and this is avoided in the manner described above. To further increase the strength of the welded joint it is advantageous to apply a slight increase of diameter  21  to the coupling member  1  at the end of the ball rod  3  and at the end of the welded lug  5  over a length of not more than several millimetres, which in a smooth fashion blends with the cylindrical part of the ball rod, or welded lug respectively, see  FIG. 7 , which represents a detail cross section of the increase of diameter  21 . This smooth increase of diameter has two positive effects on the strength. Firstly there is an increase of bending strength as a consequence of the larger diameter (third power of the diameter) but there is yet another effect which especially plays an important part in such a mig weld, which is the fact that the weld stresses in the surface are strongly reduced by the smoothly formed edge.  FIG. 8   a  shows the tensile stress and compressive stress with a bending load in a shaft as a function of the distance to the centre line. It is evident that the tensile stress and compressive stress are largest at the surface, whereas exactly at that location the notch-sensitiveness is greatest at the weld surface and this is disadvantageous. If the shaft at the location of the weld has a slight and smooth increase of diameter, the stress at the surface will be strongly reduced as a result of the elastic effect of the edge. The distribution of stress in the cross section with the edge is shown in  FIG. 8   b . The reduction of tensile stress at the surface can be seen unambiguously. The reduction of stress at the surface has a highly advantageous effect on the load-taking capacity of the weld cut since the notch sensitiveness of the weld is greatest at the surface. 
     To improve the appearance of the weld and also the strength of the weld the welding manipulator can be equipped with a turning or milling installation, so that the weld can be finished immediately after being formed. For this finishing operation a provision can be made for the use of the turning capacity of the manipulator itself, as a result of which this special provision can be realised in a relatively simple manner. Finish-turning the weld does not only produce a fine aspect, but also improves the strength because the notch sensitiveness will be reduced by the smoother surface. 
     Albeit the invention has been described in the foregoing with reference to the drawings, it should be pointed out that the invention is not by any manner or means restricted to the embodiment shown in the drawings. The invention also extends over any embodiment deviating from the embodiment shown in the drawing Figures within the spirit and scope defined by the claims. For example, the head faces at the free ends of the welded lug and the ball rod may also be slightly conically shaped in lieu of flat, so that the welding gap slightly tapers off towards the middle.