TECHNICAL FIELD 
       [0001]    The present invention relates to a vehicle seat, in particular to an assembly group of an adjustment mechanism of a vehicle seat. 
       BACKGROUND 
       [0002]      FIG. 1  shows a perspective view of a section of a height adjustable seat part  1  of a vehicle seat according to the prior art comprising the seat part  1  and a backrest (not shown) connected to the seat part  1 . The seat part  1  is arranged on a vehicle floor of a vehicle (not shown) longitudinally adjustable via a guide rail arrangement  7 . The seat part  1  comprises a first side part  21  and a second side part (not shown) forming a seat frame, wherein a rear cross tube  4  and a front cross tube (not shown) extend there between. The rear cross tube  4  is mounted pivotably to the side parts  21  and carries a first rocker  3  and a second rocker (not shown) which form a rear rocker pair. An arc weld  40  axially secures an end  41  of the cross tube  4 . 
         [0003]    A front rocker pair is formed by a third and fourth rocker which form together with the first and second rocker two parallel four joints for height adjustable connection of the side parts to the guide rail arrangement  7 . 
         [0004]    The guide rail arrangement  7  is formed by two parallel guide rail pairs extending along a vehicle longitudinal direction from which a section of a first guide rail pair is shown in  FIG. 1 . The upper guide rail  71   a  is pivotably connected to the rocker  3 , while the lower guide rail  71   b  is connected to the vehicle floor. 
         [0005]    The rockers are part of a height adjustment mechanism together with a manual drive unit or an electromotive drive unit  8  which allows for raising or lowering the seat part  1  relative to the guide rail arrangement  7  or vehicle floor, respectively, by pivoting the rockers. The drive unit  8  comprises a manually driven lever or hand wheel or an electrical motor and a gear with an output pinion, which are connected to the side part  21 . The output pinion engages in a toothing of a tooth segment connected to the cross tube  4  and can be rotated in respect to the tooth segment for adjusting the seat height. 
         [0006]    When operating the height adjustment mechanism the first rocker  3  arranged on the first end  41  of the cross tube  4  is driven and moved by means of the drive unit  8 , whereby the adjustment movement is transferred synchronically to the second rocker connected to a second end of the cross tube  4  via the cross tube  4 . In this manner the first and second rockers of the rear rocker pair are pivoted, the seat part  1  is adjusted thereby and the third and fourth rockers of the front rocker pair are moved passively. Thus, the rear and front rocker pairs can be pivoted for this purpose along a pivot direction parallel to each other in order to raise or lower the seat part  1  relative to the guide rail arrangement  7  by pivoting the rear and front rocker pairs. 
         [0007]    To leave sufficient space between the side part  21  and the first rocker  3  for mounting the drive unit  8  to the side part  21  and to ensure a play-free connection of the first rocker  3  and the side part  21 , a plastic spacer  9  is arranged on the first end  41  of the cross tube  4  between the side part  21  and the first rocker  3 . For optical and design reasons it is required that the visible elements of the seat part  1 , in particular the guide rail arrangement  7  and the rockers have to be coated. However, since the coating of the visible parts has to be done after the assembly of the height adjustment mechanism by a supplier of the seat part  1  the complete seat part  1  including a side part and the cross tube  4  are also coated which causes additional costs. 
         [0008]    Moreover, coating of the above identified part requires a need for a non-conductive bushing whereas no coating allows for the potential to just use grease without bushings or a conductive bushing. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the present invention to reduce the complexity of the vehicle seat, in particular the part complexity of the height adjustment mechanism of the vehicle seat and thus to facilitate the assembly of the vehicle seat and the height adjustment mechanism, respectively. 
         [0010]    It is an additional object of the present invention to provide a cost effective concept by making use of standardized parts for different sizes and configurations of vehicle seats and for manufacturing vehicle seats of varying size and structure. 
         [0011]    It is a further object of the present invention to reduce the number and size and thus the surface area to be coated. 
         [0012]    In a first aspect, a vehicle seat comprises a seat part for providing a seat surface for a seat occupant, which seat part comprises two side parts between which a cross tube extends along a pivot axis and an adjustment mechanism comprising a plurality of rockers which are pivotably coupled to the seat part. The cross tube is divided into a long tube element and a short tube element, the long tube element extending between the side parts of the seat part and the ends of the cross tube are inserted into openings of the side parts whereas the short tube element is connected in a torsionally locked manner both to a first rocker and to the long tube element. 
         [0013]    The cross tube hence combines a long tube element extending between the side parts of the seat part whereby the ends of the long tube element are inserted into openings of said side parts and a short tube element which is connected in a torsionally locked manner both to a first rocker and to the long tube element. The separation of the cross tube into a long tube element and a short tube element reduces the complexity of the height adjustment mechanism and thus of the vehicle seat and facilitates the assembly of the vehicle seat and the height adjustment mechanism, respectively, and provides a cost effective concept by using a standard long tube element for different sizes and configurations of vehicle seats and different short tube elements for adapting the cross tube to the individual size and structure of different vehicle seats for manufacturing vehicle seats of varying size and structure. 
         [0014]    Moreover, a separation of the cross tube into a long tube element and a short tube element allows for a coating of those parts only which have to be coated and to leave the remaining parts such as the side parts and the cross tube uncoated, thus reducing the number and size and thus the surface area of the height adjustment mechanism to be coated. 
         [0015]    In one aspect, the outer diameter of at least the end of the long tube element is smaller than the inner diameter of the short tube element such that the long tube element may be inserted into the short tube element. This facilitates a connection of the long tube element and the short tube element in a torsionally locked manner by securing the interfacing parts of said long tube element and said short tube element together by laser welding, screwing, riveting or press-fitting. 
         [0016]    In another aspect the interfacing parts of the long tube element and the short tube element have a shape other than circular such that a rotation of the long tube element effected by the drive unit is transmitted to the short tube element with or without laser welding, screwing, riveting or press-fitting those two elements. 
         [0017]    In a further aspect, the end of the short tube element facing one of the side parts abuts the respective side part forming a spacer between the respective side part and the first rocker such that no additional spacer is necessary for leaving sufficient space between the side part and the first rocker for arranging the drive unit at the side part and for ensuring a play-free connection of the first rocker and the side part, thus reducing the number of parts of the adjustment mechanism. 
         [0018]    Furthermore, in order to improve the necessary torsional lock between the short tube element and the first rocker the short tube element has a tube section extending along a longitudinal direction and bead sections formed on the short tube element and radially protruding with respect to the tube section, whereas the first rocker has a planar section and a first opening formed in the planar section, wherein the short tube element extends through the first opening such that the tube element with the bead sections abuts the planar section of the first rocker. 
         [0019]    In an embodiment of the present invention the adjustment mechanism comprises a guide rail arrangement for the longitudinally adjustable arrangement of the vehicle seat in a vehicle and a height adjustment mechanism comprising a plurality of rockers which are pivotably coupled to said seat part and said guide rail arrangement, which connect said seat part to said guide rail arrangement so as to be adjustable in height, said rockers being pivotably mounted on the guide rail arrangement via a holding element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The various features and advantages of the present invention may be more readily understood with reference to the following detailed description and the embodiments shown in the drawings. Herein, 
           [0021]      FIG. 1  shows a perspective view of a section of a height adjustable seat part of a vehicle seat according to the prior art; 
           [0022]      FIG. 2  shows a perspective partial view of an adjustable seat part of a vehicle seat according to the present invention; 
           [0023]      FIG. 3  shows a perspective partial view of an adjustable seat part of a vehicle seat according to  FIG. 2  prior to connecting the short tube element and the long tube element; 
           [0024]      FIG. 4  shows a separate sectional view of the interfacing parts of the short tube element and the long tube element after assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Subsequently, embodiments of the invention shall be described in detail with reference to the drawings. In the drawings, like reference numerals designate like structural elements. 
         [0026]    It is to be noted that the embodiments are not limiting for the invention, but merely represent illustrative examples. 
         [0027]      FIG. 2  shows in a perspective partial view of an adjustment mechanism, in particular a height adjustment mechanism, for a vehicle seat a side part  21  which in connection with a second side part and rear and front cross tubes extending there between forms a seat frame as described herein above. The side part  21  comprises a bore  210  through which a long tube element  5  extends into a short tube element  4  which is connected in a torsionally locked manner to a first rocker  3 . The first rocker  3  is pivotably mounted to a holding element  10  via a joint  11 . The holding element  10  is firmly connected to an upper guide rail  71   a  which is slidable connected to a lower guide rail  71   b  which is connected to a (not shown) vehicle floor. The upper guide rail  71   a  and the lower guide rail  71   b  form one of two parallel guide rail pairs of a guide rail arrangement  7  extending along a vehicle longitudinal direction. 
         [0028]    The first rocker  3  is part of the adjustment mechanism together with a manual or electromotive drive unit  8 , which upon rotation in either direction raises or lowers the seat part  1  relative to the guide rail arrangement. The drive unit  8  comprises either a manually driven lever or handwheel or an electrical motor and a gear with an output pinion, which are connected to the side part  21 . The output pinion engages in a toothing of a tooth segment  80  connected to the long tube element  5  according to  FIG. 3  and can be rotated in respect to the tooth such that, by pivoting the first rocker  3 , the vehicle seat may be adjusted in its height position, for instance. 
         [0029]    The first rocker  3 , as visible in  FIGS. 2 and 3 , comprises a planar section  30 , a first opening  31  and a second opening  32  formed therein at opposite ends of the planar section  30  of the first rocker  3 . The planar section  30  herein is substantially planar, which shall be understood to mean that the section in a substantially planar fashion extends along a plane, but may include a curvature for an increased stability as visible in  FIGS. 2 and 3 , such that the planar section  30  not necessarily is entirely flat. 
         [0030]    The first opening  31  serves to receive the short tube element  6 , whereas the second opening  32  formed in the opposite end of the planar section  30  of the first rocker  3  receives the joint  11  for pivotably connecting the first rocker  3  and the holding element  10  which is connected to the upper rail  71   a  of the guide rail arrangement  7  such that, by pivoting the first rocker  3 , the vehicle seat may be adjusted in its height position. 
         [0031]    The planar section  30  is surrounded by a flange  33  protruding from the planar section  30  along a longitudinal direction along which the short tube element  6  extends when it is connected to the first rocker  3 . The flange  33  may for example serve to increase the structural stability of the first rocker  3 . 
         [0032]      FIG. 3  shows in a perspective partial view the height adjustable seat part of a vehicle seat according to  FIG. 2  prior to connecting the long tube element  5  and the short tube element  6 . 
         [0033]    As visible from  FIGS. 3 and 4  the short tube element  6  comprises a tube section  60  extending along a longitudinal direction and two bead sections  61 ,  62  formed on the tube section  60  for axially fixing the short tube element  6  to the first rocker  3 . The bead sections  61 ,  62  may for example be formed by cold forming or by hot forming the short tube element  6  such that the part of the planar section  30  of the first rocker  3  surrounding the first opening  31  of the first rocker  3  is received in between the bead sections  61 ,  62  such that the short tube element  6  is axially fixed to the first rocker  3 , as shown in  FIG. 4 . 
         [0034]    For connecting the first rocker  3  to the short tube element  6 , an axially inner bead section  62  is formed on the tube section  60  of the short tube element  6  in a certain distance from an end of the tube section  60  such that the inner bead section  62  radially protrudes from the tube section  60 . 
         [0035]    The short tube element  6  is then inserted with its free end adjoining the inner bead section  62  into the first opening  31  of the first rocker  3  such that the inner bead section  62  abuts, with an inner side opposite an outer side of the planar section  30  of the first rocker  3  around the first opening  31  of the first rocker  3 . Subsequently, an axially outer bead section  61  is formed on the short tube element  6 , thus axially fixing the short tube element  6  to the first rocker  3 . 
         [0036]    For torsionally locking the short tube element  6  to the first rocker  3  such that torsional forces may be transferred in between the short tube element  6  and the first rocker  3  different techniques may be used. 
         [0037]    In a first embodiment the first opening  31  in the planar section  30  of the first rocker  3  has a cross-section other than round, i.e. for example a hexagonal cross-section, a beveled cylindrical cross-section and the like in combination with a corresponding portion of the tube section  60  between the inner bead section  62  and the end of the tube section  60  of the short tube element  6 . While forming the inner bead section  62 , or after forming the inner bead section  62 , the cylindrical portion of the tube section  60  between the inner bead section  62  and the end of the tube section  60  of the short tube element  6  is suitably deformed using a stamping technique to obtain a profile at least of the part of the tube section  60  between the inner bead section  62  and the axially outer bead section  61  other than round, i.e. by forming for example a bevel at the cylindrical portion of the tube section  60  or by forming a hexagonal portion. 
         [0038]    In a second embodiment protrusion elements may be formed to the inner bead section  62  and/or axially outer bead section  61  of the short tube element  6  such that the protrusion elements engage with openings radially extending from the first opening  31  of the first rocker  3 . Via the engagement the first rocker  3  is torsionally locked to the short tube element  6  such that torsional forces may be transferred in between the short tube element  6  and the first rocker  3 . 
         [0039]    In a third embodiment the inner bead section  62  and/or axially outer bead section  61  of the short tube element  6  may be laser welded or riveted to the planar section  30  of the first rocker  3  around the first opening  31  of the first rocker  3  such that the short tube element  6  is torsionally locked to the first rocker  3  to transfer torsional forces between the short tube element  6  and the first rocker  3 . 
         [0040]    As mentioned above a tooth segment  80  of the drive unit  8  is fastened to the long tube element  5  in particular by forming two foldings  51 ,  52  at both sides of a bore of the tooth segment  80  according to  FIG. 4  which press-fit the tooth segment  80  to the long tube element  5 . 
         [0041]    Alternatively, corresponding to the torsional locking between the short tube element  6  and the first rocker  3  as described herein above a cross-section other than round may be formed both to the bore of the tooth segment  80  and the portion of the long tube element  5  between the two foldings or beads  51 ,  52  for torsionally locking the tooth segment tooth segment  80  to the long tube element  5  to transfer torsional forces between the tooth segment  80  and the long tube element  5 . 
         [0042]    The outer diameter of the long tube element  5  is smaller than the inner diameter of the short tube element  6  by a degree that at least an interfacing part  50  of the long tube element  5  is insertable in the tube section  60  of the short tube element  6 . After insertion of the interfacing part  50  of the long tube element  5  in the tube section  60  of the short tube element  6  the interfacing part  50  and an interfacing part of the tube section  60  are secured together by laser welding as indicated by a weld  12  in  FIG. 2 , screwing or riveting. 
         [0043]    Alternatively, the outer diameter of the long tube element  5  is smaller than the inner diameter of the tube section  60  of the short tube element  6  by a degree that the interfacing part  50  of the long tube element  5  may be press-fitted to the interfacing part of the tube section  60 . 
         [0044]    In a further alternative the interfacing part  50  of the long tube element  5  and the tube section  60  of the short tube element  6  have a shape other than circular to assure a torsionally locked connection between the long tube element  5  and the short tube element  6  by means of positive locking. 
         [0045]    As shown in  FIG. 4  after connecting the interfacing part  50  of the long tube element  5  and the tube section  60  of the short tube element  6  the front end of the short tube element  6  facing the side part  21  abuts the surface of the side part  21  around the bore  210  forming a spacer between the side part  21  and the first rocker  3 . 
         [0046]    For coating the visible parts of the height adjustment mechanism it is not necessary to coat the complete height adjustment mechanism but merely the guide rail arrangement  7  and the rockers but not the large surface of the side parts and the long tube element  5  and short tube element  6  forming the cross tube. 
         [0047]    The embodiment described herein is not limiting for the invention as set forth in the claims, but merely provide illustrative examples. The invention can be carried out in an entirely different manner in entirely different embodiments. 
       REFERENCE NUMERALS 
       [0000]    
       
           1  adjustable seat part 
           3  (first) rocker 
           4  (rear) cross tube 
           5  long tube element 
           6  short tube element 
           7  guide rail arrangement 
           8  drive unit 
           9  plastic spacer 
           10  holding element 
           11  Joint 
           12  weld 
           21  side part 
           40  Arc weld 
           41  first end of the cross tube 
           50  interfacing part of the long tube element 
           51 ,  52  foldings 
           60  tube section of the short tube element 
           61  outer bead section of the short tube element 
           62  inner bead section of the short tube element 
           71   a  upper guide rail 
           71   b  lower guide rail 
           80  tooth segment 
           210  bore 
           30  planar section of the first rocker 
           31  first opening of the first rocker 
           32  second opening of the first rocker 
           33  flange
A vehicle seat comprises a seat part for providing a seat surface for a seat occupant, said seat part comprising two side parts 21 between which a cross tube 5, 6 extends along a pivot axis, and an adjustment mechanism comprising a plurality of rockers 31 which are pivotably coupled to said seat part. The cross tube 5, 6 is divided into a long tube element 5 and a short tube element 6. The long tube element 5 extends between the side parts 21 of said seat part and is inserted into openings 210 of said side parts 21. The short tube element 6 is connected in a torsionally locked manner both to a first rocker 31 and to said long tube element 5.