Abstract:
A method for producing a fork arm ( 18 ) for load-carrying devices, said fork comprising a fork blade ( 5 ) which is substantially horizontal in the operating position, and a substantially vertical fork back ( 20 ) that connects via a fork bend ( 19 ) to said fork blade and is provided with connection elements ( 2, 3 ) for the conveying device, wherein the fork arm consists of a plurality of parts ( 1; 2, 4, 8, 9, 11; 18′, 12  to  17 ) that are connected to one another, at least a number of said parts are welded to one another, and parts ( 1; 2, 4, 8, 9, 11; 18′, 12  to  17 ) of the fork arm are welded to one another by electron beam welding and/or laser welding, wherein the weld penetrates planarly with a depth of at least 15 mm between adjoining surfaces of the parts.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for producing a fork arm for load-carrying devices, said fork arm comprising a fork blade which is substantially horizontal in the operating position, and a substantially vertical fork back that connects via a fork bend to said fork blade and is provided with connection elements for the conveying device, wherein the fork arm consists of a plurality of parts that are connected to one another, and at least a number of said parts are welded together. 
         [0002]    The invention also relates to a fork arm produced according to this method. 
         [0003]    A known fork arm is disclosed in DD 256 050 A3 and is shown and described there in the sole exemplary embodiment as consisting of three steel lamellas welded together at the edge by means of a fillet weld, wherein it is mentioned that the individual lamellas can be adhesively bonded to one another, but without suggesting a manner in which adhesive bonding can be carried out. The outer lamellas can be made here from quality steel, and the inner lamella can be made of conventional construction steel. With the described configuration, the complexity during the production is to be reduced since the individual lamellas can be produced by bending and without forging. 
         [0004]    This known approach could indeed simplify the production; however, the strength of the fork arm cannot be achieved by the steel lamellas that are welded together at the edge by fillet welds, and, in particular, because of the lateral welding using fillet welds, the deflection of the fork arm under load is significantly greater than that of conventional fork arms made of heat treatable steel. Furthermore, the lamella thickness has been selected such that bending the fork bend has to be carried by locally heating the bending area, and energy supply is already very high during bending. A second additional increased energy supply takes place through the selected welding method using fillet welds at the edge of the steel lamellas, wherein a third further energy supply is required for stress relief annealing the welds. 
         [0005]    For simplifying the production, it also became known from DE 195 834 C1 to fabricate the fork blade and/or the fork back or even the whole fork arm from plates that are arranged side by side and are welded together at some places. Through this, forging work can be avoided at least to a certain extent; however, welding at certain areas is complicated and critical work, which overall is detrimental to the strength of the entire fork arm. 
         [0006]    The document EP 0 560 524 A1 likewise shows a fork arm consisting of lamellas, wherein all possibilities of connecting lamellas of a fork arm are left open; however, in particular adhesive bonding is discussed and also welding is discussed in general form, wherein as the sole example, weld beads  52  are described and shown in  FIG. 4  along the lateral edges of a fork arm, which weld beads extend over the entire course of the areas to be welded together. However, this involves the outer surfaces of the lamellas, wherein welding them together effects that the welds behave like two upright flat steel bars, as a result of which these upright flat steel bars bend extensively in the load direction, wherein the individual lamellas follow this movement by axially shifting. The static effect of a fork arm structured in this manner corresponds to an equally dimensioned profiled tube, and overall, the desired strength values cannot be achieved. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the invention to provide a method for producing a high-strength fork arm which is as light as possible and can be produced in an economic manner, and for which the disadvantages of the prior art are eliminated at least to a large extent. In particular, the production shall also proceed very rapidly, wherein, on the other hand, energy supply is very low. 
         [0008]    This object is achieved with a method of the aforementioned kind, in which according to the invention, parts of the fork arm are welded to one another by electron beam welding and/or laser welding, wherein the weld penetrates with a depth of at least 15 mm between adjoining surfaces of the parts. 
         [0009]    Thus, it is essential according to the invention that a planar surface connection is obtained between the individual lamellas, wherein weld depths of ca. 30% of the fork arm width (15% per side surface) or weld depths of at least 15 mm are advantageous in order to obtain an actual planarly extending welded connection that overcomes the above-mentioned problems in terms of strength. However, in many cases it is advantageous to weld over the entire width of the fork arm so that the lamellas or individual parts are welded together over the entire surface. 
         [0010]    Further advantageous configurations of the invention are characterized in the dependent sub-claims. 
         [0011]    The invention offers the advantage that despite low energy supply, high-quality fork arms can be produced, wherein the aforementioned disadvantages of the prior art are eliminated. 
         [0012]    The crystalline structures of the entire fork arm and the individual steel lamellas remain largely undamaged even after welding. Due to the high power density of the electron beam welding or laser welding process, the welds are very narrow, (preferably less than 1 mm), but, on the other hand, they are deep (up to 100 mm) so that a high-strength planarly extending connection between the lamellas is achieved without additional material supply. The strength of the welds is so high that a weld depth of 30% of the fork arm width is sufficient for producing a super high strength fork arm that corresponds to the strength potential of the super high strength sheets of the individual lamellas with a tensile strength of, for example, 1500 N/mm2. However, the weld depth can be reduced or increased as needed. For example, the 30% mentioned above are divided into 15% on each of the side surfaces of the sheet metal lamellas. Taking account of a minimum bending radius specified by the manufacturer of the metal sheets, the metal sheets can be bent cold. 
         [0013]    By using a CNC control, the invention is suitable for producing in large quantities, wherein due to the low energy input, the production costs can be reduced considerably. 
         [0014]    The high welding speed (20 m/min) allows a production speed in intervals of a few minutes because a plurality of lamellas can be welded at the same time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention is explained in more detail below by means of exemplary embodiments which are illustrated in the drawing. In the figures: 
           [0016]      FIG. 1  shows a perspective exploded view of a fork arm according to the invention and the individual parts thereof, 
           [0017]      FIG. 2  shows a perspective partial view from below of gradually shortened lamellas, 
           [0018]      FIG. 3  shows an enlarged side view of the region of the fork bend, 
           [0019]      FIG. 4  shows a side view of a completely assembled fork arm, 
           [0020]      FIG. 5  shows the fork arm of  FIG. 4  in a perspective view from below and from the front, 
           [0021]      FIG. 6  shows another illustration of the fork arm according to  FIG. 4 , 
           [0022]      FIG. 7  shows a detail of  FIG. 6  of straightened sheet metal lamellas of the fork arm, 
           [0023]      FIG. 8  shows a cross-section according to the section A-A of  FIG. 10  with the individual sheet metal lamellas, 
           [0024]      FIG. 9  shows an enlarged detail of  FIG. 8 , 
           [0025]      FIG. 10  shows another illustration of the fork arm according to  FIG. 4 , 
           [0026]      FIG. 11  shows the enlarged region of the fork bend of the fork arm of  FIG. 10 , 
           [0027]      FIG. 12  shows a further embodiment if a fork arm produced according to the invention in a schematic side view, 
           [0028]      FIG. 13  shows an exploded view of the individual parts of the fork arm of  FIG. 12 , 
           [0029]      FIG. 14  shows a further exploded view of a fork arm covered with sheet metal elements, 
           [0030]      FIG. 15  shows a cross-section through the core of the fork arm of  FIG. 14  and the cover elements, and 
           [0031]      FIG. 16  shows in a view as in  FIG. 15  the welded finished state. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]      FIG. 1  shows the perspective exploded view of a first embodiment of a fork arm  18  in steel lamella construction, wherein each individual lamella  1 , for example made of Docol 1500 M (with a tensile strength of 1500 N/mm2), is used with a sheet thickness (for example 2 mm) that allows cold bending without any problems. The fork blade  5  of the fork arm  18 , which fork blade is substantially horizontal in the position of use, transitions via a fork bent  19  into a vertical fork back  20 . As yet to be explained below, the individual lamellas  1  are welded together at the edge by electron beam welding, wherein the weld penetrates with a depth of at least 3 mm between adjoining surfaces of the parts. 
         [0033]    Since the strength of the welds is very high, a weld depth of 30% of the fork arm width is sufficient for producing a high-strength fork arm. However, the weld depth can be reduced or increased as needed, wherein welding through the entire width of the fork arm is also possible. 
         [0034]    The mentioned 30% are divided into 15% on each of the side surfaces of the sheet metal lamellas. In the case of the metal sheets that are used and mentioned as an example, the minimum bending radius r specified by the sheet metal manufacturer is 14 mm. 
         [0035]      FIG. 1  further shows an upper connection element  2  that is welded in a planarly contacting manner to the lamella packet by means of electron beam welding and/or laser beam welding methods. A lower connection element  3  is adapted to the outer radius of the fork bend packet and, if necessary, is also welded thereon in a planarly contacting manner by means of the mentioned welding methods. A wear plate  4  made of, for example, HARDOX 500 is also welded thereon in a planarly contacting manner by means of the mentioned welding method. The connection elements can be made from the steel grade S235JR, for example. 
         [0036]    Advantageously, the individual sheet metal lamellasare increasingly shortened towards the bottom. In  FIG. 2 , the lower surfaces  6  of the shortened lamellas are shown, wherein this shortening from top to bottom has the effect that the fork arm tapers towards the front. Such a tapered fork arm shape is desired in most cases because it facilitates engaging underneath a load. Outer cover lamellas  1  can also be made of another metal, e.g., a non-ferrous metal and can also be welded together with the other ones. 
         [0037]      FIG. 3  shows that the inner radius r of the fork bend is smaller than the outer radius, which outer radius is not illustrated here. Furthermore, the fork bend diagonal d 1  is almost double as thick as the fork arm thickness d. All components to be welded are coherently placed on top of one another so that during welding no additional weld material has to be introduced. Dimensioning that corresponds approximately to the described dimensioning has a particularly advantageous effect on the strength in the region of the fork bend. 
         [0038]      FIG. 4  shows the individual components in the assembled state of the fork arm  18  from the side, and  FIG. 5  shows the finished fork arm  18  in perspective view from the front, from below and from the left side. 
         [0039]      FIG. 6  too shows the finished fork arm  18  with the connection elements  2 ,  3  welded thereto, and the wear plate  4 . It is to be noted here that the lower connection element  3  can also be integrally formed with the wear plate  4 , thereby enclosing the fork bend  19  from the outside. 
         [0040]      FIG. 7  shows the straightened sheet metal lamellas in an enlarged illustration of  FIG. 6 . Said straightening can be carried out, for example, by machining 
         [0041]    From the cross-section of the fork arm blade according to  FIG. 8  and from the enlarged illustration of  FIG. 9  of a detail of  FIG. 8  it is apparent that the welding of the individual sheet metal lamellas is carried out starting from the edge. It is in particular shown how narrow the weld  7  is, for example, only ca. 0.5 mm, and how deep the electron beam welding process penetrates, namely ca. 15 mm in this exemplary embodiment. According to the invention, the weld depth has to be at least 3 mm on both sides, but can also be considerably deeper and can also extend over the entire sheet metal width. 
         [0042]      FIG. 10  shows again a finished fork arm  18 , wherein the enlarged detail illustrated in  FIG. 11  shows that the wear plate  4  and the lower connection element  3  are also welded in planarly contacting manner. 
         [0043]    The described embodiment also has the advantage that due to the lamella construction, the fork arm cannot break abruptly, because due to the interrupted cross-section, a potential crack cannot propagate through the entire cross-sectional area. 
         [0044]      FIG. 12  shows a variant of the fork arm in a further embodiment of the invention in a schematic side view, wherein a heat-treatable steel with a profiled shape is used. The individual components, namely a fork back  9 , a fork blade  11 , an upper connection element  2 , a lower connection element  8 , a wear plate  4  and a fillet piece  10  are also welded together here by electron beam welding and/or laser welding, wherein the weld penetrates with a depth of at least 3 mm between adjoining surfaces of the parts. Here too, the fork bend is designated by  19 . 
         [0045]      FIG. 13  shows the exploded view of the mentioned individual parts of the fork arm from  FIG. 12  prior to welding. 
         [0046]    Only by using the electron beam welding method or laser welding method, there is the possibility that, due to the low energy input, the micro structures of the already hardened and tempered heat-treatable steel (e.g., 36 NiCrMo16 material no. 1.6773 with a tensile strength of 1050 N/mm2) can be largely maintained Annealing and subsequent bending and forging is not required here for the construction of the individual parts shown. Also, stress relief annealing can be eliminated due to the minimal heat input. The assembled combination of the components as shown in  FIG. 12  corresponds optimally to the static requirements. The entire welding process takes less than 10 second per fork arm. 
         [0047]      FIG. 14  shows the exploded view of a further embodiment of a fork arm  18  in which a one-piece base body  18 ′, which has been fabricated beforehand, e.g., according to  FIG. 1 , is covered with cover lamellas  12 ,  13 ,  14 ,  15 ,  16 ,  17  which are made from non-ferrous metal or stainless steel, and, according to the invention, are welded together with the base body  18 ′ from steel by using the electron beam welding and/or laser beam welding method.  FIG. 15  shows in cross-section the base body  18 ′ and the cover elements  12  to  17  in a still unassembled state, and  FIG. 16  shows in cross-section the base body  18 ′ and the cover elements  12  to  17  from stainless steel and non-ferrous metal, respectively, in the assembled state obtained by using the electron beam welding and/or laser welding methods. The use of cover lamellas made from certain other metals is considered for the use of the fork arm  18  in environments with specific requirements such as, e.g., the food industry. In the food sector it is often required to use stainless high-grade steel for the fork arm. By welding stainless steel cover lamellas together with the normal sheet metal lamellas, there is the possibility of significant cost savings. 
         [0048]    It is also to be mentioned, for example, that in the case of explosion-proof fork arms (prevention of spark formation), it is possible to weld, e.g., bronze, or in general a non-ferrous metal, together with steel.