A hollow profile ( 1 ) formed of a flat material ( 31 ) having a width (d) has at least one groove ( 6.1, 6.2, 7.1, 7.2 ) provided on an outer side of at least one of the plurality of walls ( 2.1, 2.2, 5.1, 5.2 ) of the profile ( 1 ) and having a bottom ( 8.1, 8.2, 9.1, 9.2 ) spaced form the outer side by depth (t) and two opposite, substantially flat side walls ( 13.1, 13.2, 40.1, 40.2 ) extending substantially perpendicular to the bottom ( 8.1, 8.2, 9.1, 9.2 ), with the groove depth (t) of the at least one groove ( 6.1, 6.2, 7.1, 7.2, 32 ) amounting to from 0.5 to 2 times of the material thickness (d) of the flat material ( 31 ).

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hollow profile formed of a flat material and having a plurality of side walls and at least one groove provided on an outer side of at least one of the plurality of side walls and having a bottom spaced form the outer side by a predetermined depth and two opposite side walls. 
     The present invention also relates to a method of forming the hollow profile. 
     2. Description of the Prior Art 
     Generally, a hollow profile, which is formed of a flat material, is characterized by a small weight while having good static characteristics. For positioning and guiding of connection, parts, e.g., add-on parts, a hollow profile of an assembly system is provided with grooves in which the connection parts are guided. With add-on components being mounted on a hollow profile, often, torsional loads act on the hollow profile. Therefore, from a static point of view, a circumferentially closed hollow profile is preferred. However, the greater is the depth of the groove the smaller is the torsional load a hollow profile can withstand. Therefore, it is desirable to have the groove depth as small as possible. 
     As a rule, the size of the inner bending radius of a sheet, which is bent to 90°, corresponds to the thickness of the sheet. The drawback of this general rule consists in that with a small groove depths, which are accompanied by small bending radii, it is impossible to obtain side walls of a groove which would extend perpendicular to the groove bottom or to the outer side of a profile. The use of a narrow bending radius corresponding to the thickness of a flat material is accompanied by upsetting of the flat material on the inner side of a bent and by stretching of the flat material on the outer side of the bent. This leads to formation of fissures and/or reduction of the material strength. 
     It is known to form notches before bending at the position of the inner radius of a material. The formation of notches permits to use radii as small as the thickness of the flat material. However, this results in reduction of the cross-section of the material and, consequently, in reduction of the material strength. 
     Up to the present, with grooves having a small radius, this problem was bypassed by forming a groove with side walls extending to the groove bottom at angle different from 90°, e.g., at an angle of 45°. If a connection part with rectangular bearing surfaces is loaded sidewise, with such a groove, its guidance in the longitudinal direction of a profile and the prevention of the part from rotation is not insured. When the bearing surface of a connection part is complementary to the shape of a groove, with a side load acting on the connection part, a force component would be produced that would extend at an angle to the direction of force acting on the connection part and that could lift the connection part out of the groove. 
     Accordingly, an object of the present invention is to provide a hollow profile from a flat material with a groove having a small depth and which would insure an adequate longitudinal and lateral guidance for a connection part, whereby the absorption of the torsional stresses would be insured. 
     Another object of the present invention is to provide a hollow profile that can be produced simply and economically. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a hollow profile of the type described above and in which the groove depth amounts to from 0.5 to 2 times of the material thickness of the flat material, insuring that the side walls would extend substantially perpendicular to the groove bottom and would be substantially flat. 
     With a groove according to the invention, the static characteristic of a hollow profile made of a flat material are maintained to a most possible extent, and a reliable longitudinal and lateral guidance of a connection or add-on part along the hollow profile is insured. With a connection part having a shape corresponding to the groove cross-section, the connection part neither rotates nor is displaced sidewise, i.e., in a direction transverse to the longitudinal axis of the profile in its mounted condition on the profile. With the groove bottom being substantially flat, a flat contact between a connection part and a profile is insured, and the opposite side walls, which extend to the bottom at an angle slightly less than 90°, are capable to absorb acting, on the connection part, forces despite a small depth of the groove. Further, the connection part cannot be lifted off the groove despite side forces acting on the connection part. 
     Additionally, e.g., a slide bearing, e.g., of polyethylene can be arranged in the groove. Such a bearing can be provided, e.g., upon an arrangement of a part on the hollow profile. The bearing can be displaceable in the longitudinal direction in order to absorb stresses generated in the part. Such part can represent a support or a conducting part such as a tubular conduit or a ventilation channel. 
     Preferably, the groove depth of the at least one groove amounts to less than 1.5 times of the material thickness of the flat material. With these groove depths, the static characteristics of the hollow profile are maintained to a most possible extent, while a predetermined guidance of a connection part on the hollow profile is insured. 
     Advantageously, at least one groove is provided on each of the walls of a hollow profile. On a wall of a hollow profile, several grooves can be formed next to each other. Preferably, all of the grooves have the same profile, i.e., the same depth and the same width. With an inventive hollow profile designed for use in an assembly system, with grooves having the same profile or shape, a greater flexibility in the use of the hollow profile is obtained. 
     The bottom of the at least one groove extends preferably parallel to a corresponding axis of the hollow profile. With a rectangular hollow profile having four side walls with a groove formed in each of the side walls the grooves bottom extend parallel to respective axes of the hollow profile. The grooves extend, preferably, along the entire length of the profile parallel to respective axes. 
     Advantageously, at least one mounting opening is formed in the bottom of the at least one groove. The mounting opening(s) is (are) designed for receiving fastening means for securing a connection element on the hollow profile. The mounting opening advantageously is adapted to the type of the fastening means, and the opening can have, e.g., a round or rectangular cross-section. 
     Preferably, the flat material for forming the inventive hollow profile is a sheet metal that, preferably, is subjected to a rolling and bending process in order to obtain the profile. Flat material in form of sheet metal is treated easily and economically. By using a rolling/bending process, a plurality of different profiles can be formed. 
     In addition to the rolling/bending process, other processes, e.g., a stamping/bending process, can be used. 
     A method of forming an inventive hollow profile formed of a flat material and including a least one groove provided on an outer side of at least one wall of the hollow profile and having a bottom spaced from the outer side by depth and two opposite side walls, includes the steps of:
         forming a groove with a depth greater than a predetermined end depth of the at least one groove;   pressing a bottom of the groove with the greater depth in a direction opposite a direction of formation of the groove with the greater depth until the predetermined end depth of the at least one groove is substantially reached, with formation, as a result of a groove having a dovetail cross-section;   pressing a projecting material of the dovetail groove backward, and   bending the flat material to a desired shape of the profile.       

     Because the groove depth obtained in the first step is greater than the predetermined end depth of the at least one groove, the inner bending radius can correspond to the thickness of the flat material. Therefore, the material strength of the flat material in the bending region is reduced only slightly. Then, the groove bottom is pressed in a direction opposite the groove formation direction until the predetermined depth is reached. With this step, a dovetail cross-section is formed in the groove. The width of the groove in the plane of a respective side wall of the groove is smaller than the predetermined width of the finished groove. The projecting, in the cross-section, material is then pressed sidewise backward, and after this step, the groove has a desired width. Upon pressing back the projecting material, the material can flow in a free space in the region of the inner radius. As a result of flow of the material, statically favorable material strength and material accumulation are obtained in regions that are subjected, under load, to greatest negative stresses. Finally, the flat material is bent to a predetermined profile. Preferably, the operational steps for forming a groove are parts of a rolling process. 
     The loading of the material takes place not by superimposition of stresses in the main bending region, but is rather displaced to the side walls. There, the stresses can be distributed relatively free, so that the material would not be damaged in this region. Besides, despite the small groove depth, the side groove walls, which have substantially the same width and height and serve as longitudinal and lateral guides for a connection part the shape of which is matched to that of the groove, extend perpendicular to the groove bottom. 
     Preferably after pressing backward the projecting, in the cross-section of the groove, material, in a further step, the groove bottom is straightened. Dependent on the construction of the device for forming the groove, during different operational steps, large deviations of the groove dimensions, which lie outside of predetermined tolerances, takes place. With straightening of the groove bottom, the tolerances are limited to acceptable values. According to an alternative method of forming the groove, the groove is supported, during the backward pressing of the projecting material, sidewise and beneath, e.g., by rolls or other supports, which permits to eliminate the secondary straightening of the groove bottom. 
     Advantageously, the method includes the step of material locking connection of the free longitudinal edges of the bent flat material, which finishes the formation of the inventive hollow profile. Advantageously, the longitudinal edges are connected with each other along their entire length, e.g., are butt-welded, soldered, or glued with each other. 
     The novel features of the present invention, which are considered as characteristics for the invention, are set forth in the appended claims. The invention itself, however both as to its construction and its mode operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing show: 
         FIG. 1  a cross-sectional view of a hollow profile according to the present invention; 
         FIG. 2  a detail view of a groove at an increased, in comparison with  FIG. 1  scale, shown in region II in  FIG. 1 ; 
         FIG. 3  a cross-sectional view of the inventive hollow profile with mounted thereon connection parts; and 
         FIG. 4   a – 4   c  steps illustrating formation of a groove according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A hollow profile  1  according to the present invention, which is shown in  FIG. 1 , has a substantially rectangular cross-section, a first pair of side walls  2 . 1  and  2 . 2  which extend parallel to each other and to one of the axes  3  of the cross-section of the profile  1 , and a second pair of side walls  5 . 1  and  5 . 2  which also extend parallel to each other and to another of the axes  4  of the cross-section of the profile  1 . In the embodiment shown in  FIG. 1 , the hollow profile  1  has a smaller dimension in the direction of the axis  3  than in the direction of the axis  4 . The hollow profile  1  is formed of sheet steel by rolling/bending process, and it has a uniform thickness d. The treated and profiled sheet steel has its opposite end surfaces connected by a butt-joined weld  10 , forming a hollow profile  1 . 
     On the side walls  2 . 1  and  2 . 2 , there are formed, respectively, grooves  6 . 1  and  6 . 2  extending in the longitudinal direction L, and on the side walls  5 . 1  and  5 . 2 , there are formed, respectively, grooves  7 . 1  and  7 . 2  likewise extending in the longitudinal direction L of the hollow profile  1 . The grooves  6 . 1 ,  6 . 2  and  7 . 1 ,  7 . 2  form longitudinal and lateral guide means for shaped and connection parts matching the groove arrangement. The grooves  6 . 1 ,  6 . 2 ,  7 . 1 , and  7 . 2  have all the same width b and depth t. This provides for arrangement in the grooves  6 . 1 ,  6 . 2 ,  7 . 1 , and  7 . 2  of the matching connection parts along the entire longitudinal direction L of the hollow profile and in each of the side walls  2 . 1 ,  2 . 2 ,  5 . 1 , and  5 . 2 . The layout of grooves  6 . 1 ,  6 . 2 ,  7 . 1  and  7 . 2  permits to use a large area of the hollow profile when the hollow profile is used, e.g., as an assembly support in an assembly system. Each of the grooves  6 . 1 ,  6 . 2 ,  7 . 1 , and  7 . 2  has a bottom  8 . 1 ,  8 . 2 ,  9 . 1 , and  9 . 2 , respectively, and a pair of side walls  13 . 1  and  13 . 2 . 
     The bottoms  8 . 1  and  8 . 2  of the grooves  6 . 1  and  6 . 2  extend parallel to planes of respective side wall  2 . 1  and  2 . 2 . The bottoms  9 . 1  and  9 . 2  of the grooves  7 . 1  and  7 . 2  extend parallel to planes of respective side walls  5 . 1  and  5 . 2 . A mounting opening  11 . 1 ,  11 . 2 , through which a fastening element can extend, is formed in a respective groove bottom  8 . 1 ,  8 . 2  in the region of intersection of the axis  4  with the respective bottom  8 . 1 ,  8 . 2 . The mounting openings  11 . 1 ,  11 . 2  extend along the axis  4 . In groove bottom  9 . 1  and  9 . 2  in the region of intersection of the axis  3  with respective bottom  9 . 1 ,  9 . 2 , there are provided, respectively, mounting openings  12 . 1 ,  12 . 2  through which a fastening element can extend. The mounting openings  12 . 1 ,  12 . 2  extend along the axis  3 . 
       FIG. 2  shows, at an increased scale, a detail of groove  13 . 1  in the region II in  FIG. 1 . The deptht of the groove  7 . 2  corresponds approximately to 1.2 times of the thickness d of the sheet steel the hollow profile  1  is made of. The side wall  13 . 1  extends substantially perpendicular to the groove bottom  9 . 2  and has a flat or even section h the height of which amounts to about 70% of the depth t. A connection part, which is arrangeable in the groove  7 . 2 , can be guided in the longitudinal direction L of the hollow profile  1  and in the direction of side walls  13 . 1  and  13 . 2 . 
     A cross-sectional view of the hollow profile  1  shown in  FIG. 1 , together with respective connection parts received in respective grooves  6 . 1 ,  6 . 2 ,  7 . 1 , and  7 . 2 , is shown in  FIG. 3 . In the groove  6 . 1  in the side wall  2 . 1 , there is arranged a first connection part which is formed as an angle  21 . The width W of the angle  21  is minimally smaller than the width b of the groove  6 . 1 . The angle  21  is guided along the side wall  2 . 1  parallel to the plane of the side wall  2 . 1  along the groove side walls  13 . 1  and  13 . 2 . The angle  21  is secured to the hollow profile  1  with a head bolt  22 . 
     A second connection part, an angle  26 , is arranged in the groove  7 . 2  in the side wall  5 . 2 . The angle  26  has a width V which is greater than the width b of the groove  7 . 2 . A leg  27  of the angle  27 , which provides for mounting of the angle  26  on the hollow profile  1 , has a shape complementary to the shape of the groove  7 . 2 . The angle  26  is secured to the hollow profile  1  with a head bolt  28 . Because the groove  7 . 2  has, as other grooves of the hollow profile  1 , its side walls extending substantially perpendicular to the bottom, the leg  27  of the angle  26  contacts the side walls of the groove  7 . 2  along its entire surface. With a sidewise loading of the angle  26 , e.g., in direction of arrow  29 , there are no any essential force components that could lift the angle  26  off the groove  7 . 2 . Moreover, due to the flat contact of the angle  26  with side walls  13 . 1  and  13 . 2  of the groove  7 . 2  or of the angle  21  with the side walls  13 . 1 ,  13 . 2  of the groove  6 . 1 , a respective side wall can absorb a high side force component. 
       FIGS. 4   a – 4   c  show the method of forming a groove in the hollow profile  1 . Below, a rolling process of forming a groove of the hollow profile will be discussed. First, in a flat material  31 , e.g., sheet steel, with a roll (not shown), a groove  32  having a depth T is formed. The depth T is larger than the predetermined end depth t which the finished groove  32  should have. The inner bending radii R 1  and R 2  approximately correspond to the thickness d of the flat material  31 . 
     Then, a groove bottom  33  is pressed in a direction apposite the direction of forming the groove, e.g., in the direction shown with arrow  34 , until the groove  32  has a desired end depth t. With such a process, upon having been formed, the groove  32  has a dovetail cross-section. The inner with U of the groove  32  during this process is smaller than the width d that the groove  32  should have at the end of the manufacturing process. 
     With a first sizing roll  35 , the projecting material  36  is pressed back, which causes flow of material with resulting compacting of the material. Subsequently or during pressing back of the projecting material  36 , the groove bottom  33  is straightened with a dressing roll  37 . In addition, a second sizing roll  38  can be used for pressing back projecting material  39  in a sidewise direction, whereby, in this region also the material is compacted and the material hardness increases. 
     The finished groove  32 , which is formed according to the inventive method, is provided with even or flat sections which occupy a large portion of respective side walls  40 . 1 ,  40 . 2  of the groove  32  and which extend substantially perpendicular to the groove bottom  33 . 
     In summary, there is provided according to the invention, a hollow profile formed of a flat material and having, despite formation of grooves, high static characteristics. Despite a small depth of the grooves, an adequate longitudinal and lateral guidance of connection parts, which are to be mounted on the profile, in the grooves is insured. The profile can be economically produced and has a small weight. 
     Though the present invention was shown and described with references to the preferred embodiment, such is merely illustrative of the present invention and is not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiment or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.