Patent Publication Number: US-2022234665-A1

Title: Free-moving transport cart, conveying system and treatment plant

Description:
The invention relates to a free-moving transport cart for conveying workpieces, in particular vehicle bodies, on a conveying line, having
     a) a chassis which defines a main axis and main orientation of the transport cart, and comprises a chassis frame which supports a plurality of, in particular at least four, wheel arrangements each having one or more wheels;   b) a fastening device for at least one workpiece, which is coupled to the chassis by means of a connecting device;   c) a wheel guiding system for the wheels of the wheel arrangements, which defines a basic chassis configuration in which the wheel arrangements define a horizontal travel plane.   

     In addition, the invention relates to a conveying system for conveying workpieces, in particular vehicle bodies, on a conveying line, and to a treatment plant for treating workpieces. 
     In the case of such treatment plants, one or more treatment devices are arranged along a conveying line which is present, the treatment devices being able to be in particular a coating cabin, a dryer or a workstation. The workpieces here can be treated with materials or media in a treatment device, for example painted, or assembled or else mechanically processed, for example ground or polished, in a workstation. In the case of vehicle bodies, a workstation can be formed in particular by an assembly station in which the body shell is provided with the vehicle components. 
     The travel plane defined by the transport cart in practice coincides with a horizontally flat travel floor on which the transport cart moves. One or more of the roller arrangements which are present support one or more driven and/or steerable wheels, whereas other wheel arrangements support wheels which rotate only passively and are not driven, but may also be steerable. The main axis of the transport cart is generally the longitudinal axis thereof. The direction of said longitudinal axis generally coincides with the main conveying direction in which the transport cart moves. Even in the case of omnidirectional transport carts which do not always permit an unambiguous assignment of front and rear, a corresponding reference axis can, however, be defined. 
     In the above-discussed operations, but also generally on the conveying line, for a satisfactory process sequence the workpieces have to follow the conveying line with a high degree of accuracy. In particular in the case of the automated coating of the workpieces with the aid of painting robots, the movement of the workpieces and the movement of the robots are coordinated with one another. If the workpieces deviate too far or too suddenly from a basic path, this can have negative effects on the coating result. 
     However, also in the case of the manual assembly steps mentioned, the working sequences of the workers are matched to the movement sequence and to the corresponding position and orientation of the workpieces. 
     During their travel along the conveying line, the workpieces are intended to follow a stable path, and therefore the transport carts are preferably provided with an unsprung wheel guiding system. 
     An unsprung wheel guiding system is intended to be understood as meaning a system in which no resilient elements with restoring forces can act indirectly or directly on the wheel arrangements or wheels. 
     However, in the case of such a wheel guiding system, there is the risk, in the event of a floor unevenness, of the transport cart tilting to the side or at least partially, i.e. at least with one of the wheel arrangements, losing the necessary floor contact. In particular if this involves a driven and/or steerable wheel arrangement, the control of the transport cart can then be restricted. This will be explained once again further below in conjunction with  FIG. 4 . 
     For a controlled movement of the transport cart and of the workpiece, it is therefore desirable for the continuous floor contact of the wheel arrangements to be ensured and for the workpiece at the same time to be able to be conveyed stably in terms of movement. 
     It is therefore an object of the invention to provide a transport cart, a conveying system and a treatment plant of the type mentioned at the beginning which take this concept into account. 
     This object is achieved in the case of a transport cart of the type mentioned at the beginning in that
     d) the wheel guiding system guides the wheels of the wheel arrangements in such a manner that at least one wheel of at least one wheel arrangement is movable out of the travel plane without the remaining wheel arrangements completely leaving the travel plane.   

     In this way, a wheel can evade or, better, can follow a floor unevenness, with the remaining wheels being able to remain in floor contact. The evading wheel likewise remains in floor contact over the floor unevenness. As becomes clear further below, tilting of the workpiece can be at least reduced by such a wheel guiding system when the transport cart travels over a floor unevenness. 
     It is particularly advantageous if the wheel guiding system comprises at least one swing-axle structure which is mounted on the chassis frame so as to swing about a swing axis and which extends in the direction of the main axis, wherein the swing-axle structure carries a first wheel arrangement on the one side of the swing axis and carries a second wheel arrangement on the other side of the swing axis. By this means, a mechanically relatively simple construction is formed which meets all the desired requirements. 
     In a preferred variant, the swing axis is arranged centrally between the first and the second wheel arrangement. 
     Alternatively, it can be favorable if the swing axis is arranged eccentrically between the first and the second wheel arrangement. 
     Since, if there is only one swing axis, tilting of the transport cart may still occur, the wheel guiding system preferably comprises a first swing-axle structure and a second swing-axle structure which are spaced apart from one another, i.e. are arranged one after the other, in the direction of the main axis. 
     In this case, it is preferred that the swing axis of the first swing-axle structure and the swing axis of the second swing-axle structure run at least in a common vertical plane, in particular coaxially, in the basic chassis configuration. 
     Alternatively, it can be favorable if the swing axis of the first swing-axle structure and the swing axis of the second swing-axle structure are offset in a horizontal direction perpendicular to the main axis in the basic chassis configuration. For this purpose, swing-axle structures in which the swing axis is arranged eccentrically between the wheel arrangements are preferably considered here. 
     It can be of advantage if the wheel guiding system for at least one swing-axle structure which is present comprises a blocking device by means of which a swinging movement of the swing-axle structure about the swing axis is blockable or releasable. This is particularly favorable in the event of two swing-axle structures, the swing axes of which run in a common vertical plane and in particular coaxially. 
     The blocking device advantageously then comprises a movable blocking element which is movable, in particular in a motorized manner, between a blocking position, in which it blocks a swinging movement of the swing-axle structure, and a release position, in which it releases a swinging movement of the swing-axle structure. 
     In the case of two swing-axle structures, it is favorable if there is a respective blocking device with a blocking element for the first and the second swing-axle structure, and that the wheel guiding system comprises a safety device which ensures that at least always one blocking element of the two blocking devices takes up its blocking position and blocks the associated swing-axle structure. 
     In order to even further reduce tilting of the transport cart to the side, it is favorable if, in the case of at least one swing-axle structure, the first wheel arrangement and/or the second wheel arrangement is mounted movably on the swing-axle structure. 
     Preferably, the movably mounted wheel arrangement comprises a carrying structure which carries or supports one or more wheels and is mounted on the swing-axle structure so as to be pivotable about a pivot axis, wherein the pivot axis runs in particular parallel to the swing axis of the swing-axle structure. 
     It is particularly advantageous if, in the basic chassis configuration, the pivot axis of the carrying structure is located lower in the vertical direction than the wheel axes of the one or more wheels of the movably mounted wheel arrangement. 
     It is also favorable if the movably mounted wheel arrangement comprises at least one first and one second wheel which are each mounted separately by means of a bearing structure which is in each case fastened pivotably about a pivot axis to the carrying structure, the bearing structure running in particular parallel to the swing axis of the swing-axle structure and to the pivot axes of the carrying structures of the wheel arrangements. 
     As discussed above, the wheel guiding system is preferably an unsprung wheel guiding system. 
     The wheel guiding system is distinguished in particular in that it is configured in such a manner that, within the scope of the possible movement of the at least one wheel or of one of the wheel arrangements out of the travel plane, the transport cart always remains statically determined. 
     With regard to the conveying system, the abovementioned object is achieved in that the conveying system comprises a multiplicity of transport carts having some or all of the above-explained features. 
     In the case of the treatment plant, the abovementioned object is achieved in that such a conveying system is present. 
     Said conveying system is particularly advantageous if along the conveying line there is a travel space for the chassis, which is connected in an upward direction to a conveying space by a connecting passage in such a manner that the chassis is movable in the travel space and the fastening device is carried along in the conveying space, with the connecting device extending through the connecting passage. 
     Preferably, along the conveying line at least one treatment device is arranged, in particular a dryer, a workstation or a coating device, which comprises a floor and/or a partition with the connecting passage. 
    
    
     
       Exemplary embodiments of the invention will now be explained in more detail below with reference to the drawings, in which 
         FIG. 1  shows schematically a cross section of a treatment plant with a conveying space which is connected by a connecting passage to a travel space, arranged below said conveying space, for a conveying system by means of which workpieces are conveyed on a conveying line, wherein the conveying system comprises a multiplicity of free-moving transport carts; 
         FIG. 2  shows a partial longitudinal section of the treatment plant according to  FIG. 1  with a transport cart; 
         FIG. 3  shows a view from below of a chassis of a transport cart, wherein four wheel arrangements each having a pair of wheels are shown, and a wheel guiding system according to the invention is illustrated; 
         FIG. 4  shows a front view of a transport cart according to the prior art, in which the workpiece is tilted in relation to a vertical longitudinal plane when the transport cart travels over a floor unevenness; 
         FIG. 5  shows a front view and a larger detailed view of a transport cart with a wheel guiding system according to a first exemplary embodiment in the region of a floor unevenness; 
         FIG. 6  shows a transport cart with a wheel guiding system according to a second exemplary embodiment, wherein a front view of the transport cart and four movement phases A, B, C, D during the overcoming of a floor unevenness are shown in a perspective view in each case; 
         FIG. 7  shows a transport cart with a wheel guiding system according to a third exemplary embodiment, wherein a front view of the transport cart and four movement phases A, B, C, D during the overcoming of a floor unevenness are shown in a perspective view in each case; 
         FIG. 8  shows a front view and a larger detailed view of a transport cart with a wheel guiding system according to a fourth exemplary embodiment in the region of a floor unevenness; 
         FIG. 9  shows a front view and a larger detailed view of a transport cart with a wheel guiding system according to a fifth exemplary embodiment in the region of a floor unevenness. 
     
    
    
     DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS 
       FIGS. 1 and 2  schematically illustrate a treatment plant, denoted overall by  10 , for treating workpieces  12  which are illustrated by way of example as vehicle bodies  14 . 
     The treatment plant  10  comprises a treatment device  16  with a housing  18  which defines a treatment space  20 . The treatment device  16  can also be a workstation  22  in which assembly operations, quality control operations or the like are undertaken and in which the treatment space  20  can be open upward and optionally open on all sides. 
     However, the invention will be explained below using the example of a treatment device  16  in which the treatment space  20  is designed as a treatment tunnel  24  and comprises two tunnel walls in the form of sidewalls  26  and two further tunnel walls in the form of a ceiling  28  and a floor  30 . Irrespective of its specific design, i.e. whether open or closed, the treatment space  20  in all cases has a floor  30 . Even in the case of a treatment space  20  closed in this way, the treatment device  16  can be a workstation  22 . 
     Such a treatment device  16  with tunnel walls  26 ,  28  and  30  is in particular a dryer  32 , in which the treatment tunnel  24  predefines a dry tunnel. However, such a treatment device  16  may also be a treatment device with a corresponding treatment tunnel in which the workpieces  12  are painted in particular automatically with the aid of painting robots which guide an application device, or manually. 
     The workpieces  12  are conveyed with a conveying system  34  on a conveying line  36  through the treatment space  20 , i.e. here the treatment tunnel  24  of the treatment device  16 , and also outside the treatment device  16 . In the latter case, for example, between two treatment devices  16  which are present along the conveying line  36 , or on the path toward the treatment plant  10  or on the path away from the treatment plant  10 . 
     Along the conveying line  36 , the workpieces  12  thus move in a conveying space  38  above the floor  30  which extends at least in sections along the conveying line  36 . The floor  30  can also be present in front of and/or behind treatment devices  16  which are present in each case. In the region of the treatment device  16 , the conveying space  38  coincides with the treatment space  20  thereof. Accordingly, the conveying space  38  can likewise be opened or closed. 
     The conveying line  36  can comprise one or more curved sections, wherein a curved section can be present in the region of and/or outside the treatment device  16 . 
     The treatment device  16  is operated continuously and accordingly has an input at one end side and an output at the opposite end side, of which only the input is denoted by  40  in  FIG. 2 . The input  40  and the output can be designed as a lock, as is known per se and in itself. However, the treatment space  20  can also be configured as a batch system and can optionally only have a single entrance via which the workpieces  12  are conveyed into the treatment space and are also conveyed again out of the latter after the treatment. This single entrance can also optionally be designed as a lock. 
     The conveying system  34  comprises a multiplicity of free-moving transport carts  42  on which the workpieces  12  are transported and which move on a travel floor  44 . The transport carts  42  are floor-based and are designed as free-moving transport vehicles in the sense of driverless transport systems that are familiar to a person skilled in the art as what are referred to as DTSs. The transport carts  42  are drivable and steerable independently of one another. 
     The terms horizontally and vertically that are used here always relate to the transport vehicle  42  when the latter is completely ready for operation and travel on a horizontal and flat section of the travel floor  44 . 
     Each transport cart  42  comprises a chassis  46  with a chassis frame  48  which defines a main axis, shown by an arrow  50 , and main orientation of the transport cart  42 . In the exemplary embodiment here, the main axis  50  is the longitudinal axis and is also referred to below as longitudinal axis  50 . In the main orientation following therefrom, the chassis  46  has a front  46   a  and a corresponding rear  46   b . The main orientation is oriented, in general terms, on the basis of the direction in which the transport vehicle  42  moves forward. To this end, a corresponding front  42   a  can be assigned even to omnidirectional transport carts  42 . 
     The chassis frame  48  supports at least four wheel arrangements  52  which can be seen in  FIG. 3  and are denoted there by  52 . 1 ,  52 . 2 ,  52 . 3  and  52 . 4 . 
     Each wheel arrangement  52  comprises one or more wheels  54  and can consequently have one individual wheel  54  or can comprise a pair of wheels  56  consisting of two wheels  54  or else more than two wheels  54 . Each wheel  54  which is present is rotatable about a wheel axis  58  assigned to the respective wheel  54 . If two or more wheels  54  are present, they are arranged, in a wheel arrangement  52 , at least axially parallel with respect to the wheel axes  58 . The wheels  54 , the pairs of wheels  56  and the wheel axes  58  are not provided in all the figures and are not always all provided with reference signs. 
     The term “wheel” should be understood as meaning also rollers or rolls or the like and, in particular in the case of omnidirectional transport carts  42 , also omni wheels or Mecanum wheels or the like. 
     A wheel arrangement  52  can be driven and can therefore propel the transport cart  42 . If a driven wheel arrangement  52  is intended to be denoted in the figures, said wheel arrangement bears the additional index “-d” from the word “drive”. 
     For this purpose, the driven wheel arrangement  52 - d , depending on the configuration, correspondingly comprises one or more driven wheels  54 . If a driven wheel arrangement  52 - d  comprises more than one wheel  54 , it can suffice if only one of said wheels  54  of the wheel arrangement  52 - d  is driven. The driven wheel or wheels  54  of a driven wheel arrangement  52 - d  is or are coupled to a drive device  60 , which is shown schematically only in  FIG. 3 . 
     In addition, a wheel arrangement  52  can be steerable and can therefore change the direction of movement of the transport cart  42 . If a steerable wheel arrangement  52  is intended to be referred to in the figures, it bears the additional index “-s” from the word “steer”. A driven and steerable wheel arrangement is thus denoted by  52 - d - s.    
     For this purpose, in the case of the present exemplary embodiment, the steerable wheel arrangement  52 - s  is rotatable with the aid of a steering device  62 , likewise shown only in  FIG. 3 , about a steering axis of rotation  64  which runs vertically when the transport cart  42  rests with its wheel arrangements  52  on a horizontally flat section of the travel floor  44 . 
     In practice, individual wheels  54  are also drivable separately and are each coupled to a dedicated drive device  60 . If a plurality of separately driven wheels  54  are present in a wheel arrangement  52 , the steering movement can be achieved in a manner known per se and in itself in that said wheels  54  are rotated with different directions of rotation and/or rotational speeds about their wheel axis  58 . In this case, there are therefore no driving and steering devices  60  and  62  that are separate from one another. 
     Both the driving device  60  and the steering device  62  operate in practice with electric motors which are coupled directly or via transmission elements to the wheel arrangement  52  or to an associated wheel  54 . 
     In the present exemplary embodiment, the transport cart  42  is formed omnidirectionally, and therefore all of the wheel arrangements  52 - s  are steerable in this manner. If, as discussed above, omni wheels or Mecanum wheels are used, the rotation about the steering axis of rotation  64  can be omitted. The steering device  62  then coordinates, for example, the wheel activations necessary for the change in direction. 
     In the present exemplary embodiments, four wheel arrangements  52  are provided, in each case one at the front on the left and right and at the rear on the left and right in the direction of the longitudinal axis  50 . Two drivable and steerable wheel arrangements  52 - d - s  are provided here diagonally at the front on the right and at the rear on the left; alternatively, wheel arrangements  52 - d - s  can also be provided at the front on the right and at the rear on the left. The two other wheel arrangements  52 - s  are merely steerable. However, four driven and steerable wheel arrangements  52 - d - s  can also be provided. 
     Specifically, in  FIG. 3 , according to the above-explained terminology, the wheel arrangements  52  are denoted by  52 . 1 - d - s ,  52 . 2 - s ,  52 . 3 - s  and  52 . 4 - d - s , with the view from below being taken into consideration there. 
     In the case of modifications which are not shown by themselves, non-steerable wheel arrangements  52  can also be present, for example two front wheel arrangements  52 - d - s  in the direction of the longitudinal axis  50  can be driven and steerable and two rear wheel arrangements  52  in the direction of the longitudinal axis  50  can be non-driven and non-steerable. 
     In the case of modifications which are likewise not shown separately, more than four wheel arrangements  52 , in particular six, eight or ten wheel arrangements  52 , can also be provided. The number and arrangement of driven and/or steerable and neither driven nor steerable wheel arrangements  52 - d ,  52 - s ,  52 - d - s  and  52  is coordinated here with the local conditions and requirements imposed on the conveying system  34 . 
     The transport cart  42  comprises a fastening device  66  to which a workpiece  12  or a corresponding workpiece carrier for workpieces  12  can be fastened. For the fastening of vehicle bodies  14 , the fastening device  62  in the present exemplary embodiment comprises a carrying profile  68  with bearing bolts  70  which cooperate in a manner known per se and in themselves with counter elements on the vehicle body  14  such that the vehicle body  14  can be fixed on the fastening device  66 . 
     The fastening device  66  can also have a plurality of sets of such bearing bolts  70  which are adapted to different vehicle bodies  14  of different dimensions and configurations such that the fastening device  66  can be used flexibly for different types of vehicle body. The fastening device  66  therefore directly receives a vehicle body without the vehicle body  14  being fastened on a workpiece carrier, such as, for example, a skid which is known per se and in itself. 
     The chassis  46  of the transport cart  42  is coupled to the fastening device  68  by means of a connecting device  72 . The connecting device  72  comprises at least one upwardly pointing strut  74 .  FIG. 2  illustrates on the one hand a connecting device  72  with a single such strut  74  and on the other hand a connecting device  72  with two struts  74 . 1  and  74 . 2  which are illustrated by dashed lines in  FIG. 2 . In the case of two such struts  74 . 1 ,  74 . 2 , the stability is possibly increased in comparison to only one strut  70 . 
     Each strut  70  which is present couples the chassis  46  of the transport cart  42  to the fastening device  68 . In a further modification, more than two struts  70  can also be present. The transport cart will be described below with respect to the exemplary embodiment with the single strut  70 ; what is stated with regard thereto applies analogously to transport carts  42  having a plurality of struts  70  or else differently designed connecting devices  72 . 
     In the exemplary embodiment shown here, the strut  70  is at any rate shown as a rectilinear vertical strut, but may also have geometries differing therefrom. In particular, geometries of the strut  70  with a C-shaped section or with a section which is inclined from below upward in relation to a vertical plane which is parallel to the longitudinal axis  50  and to the transport direction are considered here. 
     The treatment plant  10  and the conveying system  34  are coordinated with one another in such a manner that only part of the conveying system  34  is moved in the conveying space  38  while the other part of the conveying system  34  is moved outside the conveying space  38 . 
     For this purpose, outside the conveying space  38 , a guiding region  76  with a travel space  78  likewise arranged outside the conveying space  38  is provided, in which the chassis  46  of a respective transport cart  42  is moved, wherein the conveying space  38  and the guide region  76  and the travel space  78  are upwardly separated by a partition  80 . In the present exemplary embodiment, this partition  80  is a section  82  of the floor  30 , wherein the guiding region  76  with the travel space  78  is arranged below the floor  30 . 
     An arrangement of the guiding region  76  and of the travel space  78  “outside” the conveying space  38  should be understood in such a manner that there is a structural separation between the conveying space  38  and the guiding region  76  and the travel space  78  by means of the partition  80  which has been discussed. However, this does not mean that the guiding region  76  and optionally the travel space  78  cannot protrude at least in regions into the conveying space  38  and can overlap in cross section with the conveying space  38  and optionally with the treatment space  20 . 
     The travel space  78  can be open toward the surroundings of the treatment device  16  or of the remaining conveying line  36 ; at any rate, there does not have to be a dedicated housing for the travel space  78 . In the present exemplary embodiment, the travel space  78  is, however, bounded, at least in the region of the treatment device  16 , by a dedicated travel space housing  84  which comprises the partition  80 . In other words, in the present exemplary embodiment, the section  82  of the floor  80  is therefore part of the travel space housing  84 . Alternatively, in the case of the treatment device  16 , the sidewalls  26  of the housing  18  can also extend downward beyond the floor  30  such that they laterally bound the travel space  78  there; in this case, said travel space  78  is then upwardly separated from the treatment space  20  by the entire floor  30 . 
     The travel space  78  is now connected to the conveying space  38  via a connecting passage  86  in the partition  80 . The connecting passage  86  is complementary with respect to the connecting device  72  of the transport carts  42 . 
     The connecting device  72  extends through the connecting passage  86  such that the fastening device  66  together with the workpiece  12  is located in the conveying space  38 —and therefore in the treatment space  20  in the treatment device  16 —and the chassis  46  of a transport cart  42  is located in the travel space  78 . 
     In the present exemplary embodiment, the connecting passage  86  is consequently rectilinear in cross section. In the case of the above-discussed modifications of the struts  70 , the connecting passage  86  is correspondingly angled and designed in the manner of a labyrinth in cross section. In the case of differently designed struts  70  or a connecting device  72  conceived differently once again, the connecting passage  86  is correspondingly adapted thereto. The flow path between the conveying space  38  and the travel space  78  can be shielded by shielding elements, such as, for example, laminae or the like arranged in imbricated form. 
     There is a preferably unsprung wheel guiding system  88  for the wheels  54  of the wheel arrangements  52 . The wheel guiding system  88  defines a basic chassis configuration in which the wheel arrangements  52  define a horizontal travel plane  90 . In said basic chassis configuration, the wheel axes  58  of the wheels  54  of the wheel arrangements  52  run horizontally. If the travel floor  44  is horizontally flat, the travel plane  90  coincides with the travel floor  44 , as  FIGS. 1 and 2  show. In  FIG. 3 , the travel plane  90  is the plane of the paper. 
       FIG. 4  first of all illustrates a situation in which an unsprung transport cart  42 , without a further compensating device, travels with a wheel arrangement  52 , here the non-driven and steerable wheel arrangement  52 . 2 - s , with a pair of rollers  56  over a floor unevenness, denoted by  92 , of the travel floor  44 . In such a situation, two undesirable effects are in focus: 
     Firstly, the chassis  46  of the transport cart  42  is with respect to a vertical reference plane  94  which tilts laterally in the direction of the longitudinal axis  50  of the transport cart  42 . This is illustrated by a comparison plane  96  which runs through the longitudinal axis  50  of the transport cart  42 . 
     As can be seen, the floor unevenness  92  also causes the workpiece  12 , in the present exemplary embodiment the vehicle body  14 , to tilt in relation to the vertical reference plane  94 . 
     Secondly, it may occur that a driven and steerable wheel arrangement  52 - d - s  loses contact with the travel floor  44  since the chassis  46  is raised by the floor unevenness  92 . In  FIG. 4 , for example, the wheel arrangement  52 . 4 - d - s , which lies behind the plane of the paper in  FIG. 4  and is therefore indicated with a dashed reference line and which can be seen at the bottom on the left in the case of the transport cart  42  in  FIG. 3 , is lifted off the travel floor  44 , and therefore no wheel  54  of the wheel arrangement  52 . 4 - d - s  has floor contact any more. The full driving and steering control for the transport cart  42  is lost at this moment. 
     This should already be avoided for a satisfactory process sequence since, in particular during a painting operation, the movement of the workpiece  12 , i.e. here the vehicle body  14 , and the movement of the application devices by the painting robots are coordinated with one another. A change in the planned movement of the workpiece through the treatment device  16  may therefore reduce the quality of the coating obtained. 
     Furthermore, it may occur, however, because of the two undesirable effects, firstly the tilting of the transport cart  42  and secondly the loss of the control of the movement of the transport cart  42 , that the connecting device  72  of the transport cart  42  collides with the sidewall of the connecting passage  86 . This may result in an extreme case in jamming of the system and in damage of the transport cart  42 . 
     In order to counteract this, the wheel guiding system  88  guides the wheels  54  of the wheel arrangements  52  in such a manner that at least one wheel  54  of at least one wheel arrangement  52  is movable out of the travel plane  90 , in particular because of a floor unevenness  92 , without the remaining wheel arrangements  52  completely leaving the travel plane  90 . This wheel  54  can therefore carry out a maximum movement out of the travel plane  90 . The end of this movement is predefined by the construction if a further movement of said wheel  54  out of the travel plane  90  is mechanically stopped as it were. 
     By this means, it is possible that the wheels  54  which are not affected by the floor unevenness  92  do not lose contact with the travel floor  44 . The control of the movement, i.e. the propulsion by the driving device  60  and the predetermined travel direction by the steering device  62 , is thereby maintained for the transport cart  42 , even in the event of floor unevennesses  92 . 
       FIGS. 5 to 9  show preferred exemplary embodiments in which the wheel guiding system  88  comprises at least one swing-axle structure  98  which is mounted on the chassis frame  48  so as to swing about a swing axis  100  and which extends in the direction of the longitudinal axis  50  of the chassis  46  and in particular parallel thereto. The swing-axle structure  98  carries a first wheel arrangement  52  on the one side of the swing axis  100  and a second wheel arrangement  52  on the other side of the swing axis  100 . In the present exemplary embodiment, these are the wheel arrangements  52 . 1 - d - s  and  52 . 2 - s  in the case of the swing-axle structures  98  in each case illustrated at the front  46   a  of the chassis  46 , the corresponding reference signs not always being indicated again. 
     The swing axis  100  runs horizontally in practice in the basic chassis configuration, but may also be inclined upward or downward up to a certain degree in relation to a horizontal plane. In the basic chassis configuration, the swing axis  100  is arranged higher in the vertical direction than the wheel axes  58  of the wheels  54  that belong to the swing-axle structure  98 . 
     In the case of the wheel guiding system  88  shown as the first exemplary embodiment in  FIG. 5 , there is a single swing-axle structure  98  which rigidly supports the two wheel arrangements  52 . The swing axis  100  is arranged there on the swing-axle structure  98  centrally between the two wheel arrangements  52 . The swing-axle structure  98  is shown by way of example at the front  46   a  of the chassis  46 . By contrast, the other two wheel arrangements  52 , i.e. at the rear  46   b  here, are coupled rigidly to the chassis frame  48  in said wheel guiding system  88 . 
     Alternatively, the swing-axle structure  98  can be provided at the rear  46   b  and the wheel arrangements  52  coupled rigidly to the chassis frame  48  at the front  46   a  of the chassis frame  46 . 
     A swing-axle structure  98  with two wheel arrangements  52  fastened rigidly thereto defines a system with a point of articulation, namely the swing axis  100 . 
     If one of the two wheel arrangement  52  supported by the swing-axle structure  98 , by way of example the wheel arrangement  52 . 2 - s  in  FIG. 5 , travels over a floor unevenness  92 , the swing-axle structure  98  is deflected about the swing axis  100  and the wheels  54  of the wheel arrangement  52 . 2 - s  move upward out of the travel plane  90  with a vertical direction component. 
     The remaining wheel arrangements  52 , i.e. the opposite wheel arrangement  52 . 1 - d - s  on the swing-axle structure  98  and the two wheel arrangements  52 . 3 - s  and  52 . 4 - d - s  at the rear  46   b  of the chassis  46 , which wheel arrangements are located behind the plane of the paper in  FIG. 5 , remain in contact with the travel floor  44  and do not leave the travel plane  90 . Only the opposite wheel arrangement  52 . 1 - d - s  on the swing-axle structure  98  tilts. 
     Of the pair of wheels  56  shown here in the case of the wheel arrangement  52 . 1 - d - s , although the inner wheel  54  moves out of the travel plane  90 , the outer wheel  54  remains in floor contact with the travel floor  44  and consequently the wheel arrangement  52 . 1 - d - s  does not leave the travel plane  90  completely, but rather only partially. The driving and steering control of the transport cart  42  via the wheel arrangement  52 . 1 - d - s  is maintained via the outer wheel  54 . 
     Furthermore, even a lateral tilting of the workpiece  12  is at least reduced; there is substantially only a slight raising of the chassis  46  of the transport cart  42  in the front region and thereby also merely a slight raising of the workpiece  12 . 
     If, however, during the further movement of the transport cart  42 , the wheel arrangement  52 . 4 - d - s  at the rear  46   b  of the chassis  46  arrives at the floor unevenness  92 , the chassis  46  is tilted since the wheel arrangements  52  at the rear  46   b  are coupled rigidly to the chassis frame  48 . 
       FIG. 6  therefore shows, as a second exemplary embodiment, a wheel guiding system  88  which a first swing-axle structure  98 . 1 , which is mounted on the chassis frame  48  so as to swing about a first swing axis  100 . 1 , and a second swing-axle structure  98 . 2 , which is mounted on the chassis frame  48  so as to swing about a second swing axis  100 . 2 . The first and the second swing-axle structure  98 . 1 ,  98 . 2  are spaced apart from one another in the direction of the longitudinal axis  50  of the chassis  46 . The chassis frame  48  is shown by dashed lines in  FIG. 6 . In addition,  FIG. 6  illustrates wheel arrangements  52  having only a single wheel  54 . 
     The wheel arrangements  52 . 1 - d - s  and  52 . 2 - d  are mounted rigidly on the swing-axle structure  98 . 1  at the front  46   a  of the chassis  46 . The wheel arrangements  52 . 3 - s  and  52 . 4 - d - s  are mounted rigidly on the swing-axle structure  98 . 2  at the rear  46   b  of the chassis  46 . 
     In both swing-axle structures  98 . 1 ,  98 . 2 , the associated swing axis  100 . 1  or  100 . 2  is arranged on the swing-axle structure  98 . 1  or  98 . 2  centrally between the two wheel arrangements  52 . 
     In the basis chassis configuration, the two swing axes  100 . 1  and  100 . 2  each extend parallel to the longitudinal axis  50  of the chassis  46  and lie at least in a common vertical plane, i.e. in the vertical reference plane  94  that is not shown in  FIG. 6 , but may be offset in the vertical direction. In the present exemplary embodiment, the first swing axis  100 . 1  and the second swing axis  100 . 2  run coaxially. 
     For each swing-axle structure  98 , the wheel guiding system  88  comprises a blocking device  102  by means of which a swinging movement of the respective swing-axle structure  98  about the swing axis  100  can be blocked or released. For the two swing-axle structures  98 . 1  and  98 . 2 , in the present exemplary embodiment there are consequently two blocking devices  102 , of which, for the sake of clarity, only the blocking device  102  of the swing-axle structure  98 . 1  has been provided with reference signs and is explained. 
     In the present exemplary embodiment, the blocking device  102  comprises a movable blocking element  104  which is movable between a blocking position, in which it blocks a swinging movement of the swing-axle structure  98 . 1  or  98 . 2 , and a release position, in which it releases a swinging movement of the swing-axle structure  98 . 1  or  98 . 2 . The blocking element  104  is moved by a motor  106  which is shown only in the front view of the chassis  46 . 
     In  FIG. 6 , the blocking element  104  is shown schematically as a fork which, in the blocking position, engages around a horizontal section of the swing-axle structure  98 . 1  or  98 . 2 . However, configurations which are different therefrom are also conceivable, for example a bolt which, in its blocking position, can engage in an opening in the swing-axle structure  98 , or a latch which, in its blocking position, can latch with a counter element on the swing-axle structure  98 , or the like. 
     In addition, the wheel guiding system  88  comprises a safety device  108  which ensures that at least always one blocking element  104  of the two blocking devices  102  takes up its blocking position and blocks the associated swing-axle structure  98 . 1  or  98 . 2 . Otherwise, i.e. if both blocking elements  104  of the two blocking devices  102  were to take up their release position at the same time, the chassis  46  would tilt unstably and about the swing axes  100 . 1  and  100 . 2  which are coaxial here. 
     For this purpose, the safety device  108  comprises a position sensor device  110  which is likewise shown only in the front view and which detects the position of the blocking elements  104  and interacts with a control device  112  which moves a blocking element  104  into the release position only when the respective other blocking element  104  takes up its blocking position. 
     In addition, the wheel guiding system  88  comprises a swing-sensor device  114  by means of which a swinging movement of each swing-axle structure  98 . 1  or  98 . 2  can be detected. The swing-sensor device  114  likewise interacts with the control device  112 . 
     In the basic chassis configuration of the wheel guiding system  88 , the swing-axle structure  98 . 1  at the front  46   a  of the chassis  46  is released and the swing-axle structure  98 . 2  at the rear  46   b  is blocked, as the movement phase A illustrates. During this movement phase A, the wheel arrangement  52 . 2 - s , for example, now arrives at a floor unevenness  92  and, in the movement phase B, travels onto the floor unevenness  92 . 
     In the process, the swing-axle structure  98 . 1  swings about its swing axis  100 . 1 . This is basically the same situation that has been explained with regard to  FIG. 5 . During the further movement of the transport cart  42 , the floor unevenness  92  is traveled over by the wheel arrangement  52 . 2 - s  and, in the movement phase C, the swing-axle structure  98 . 1  pivots back again; the wheel guiding system  88  is again in its basic chassis configuration. 
     The swinging movement of the swing-axle structure  98 . 1  during the travel over the floor unevenness is detected by the swing-sensor device  114  and transmitted to the control device  112 . The control device  112  correlates the travel movement of the transport cart  42  and the swinging movement of the swing-axle structure  98 . 1 ; it can be deduced from the data that the floor unevenness  92  is now located between the wheel arrangement  52 . 2 - s  on the front swing-axle structure  98 . 1  and the wheel arrangement  52 . 4 - d - s  on the rear swing-axle structure  98 . 2 . 
     The control device  112  now activates the motor  106  for the blocking element  104  of the front swing-axle structure  98 . 1  such that said blocking element is moved into its blocking position and blocks the front swing-axle structure  98 . 1 , as the movement phase C in  FIG. 6  illustrates. The blocking element  104  for the rear swing-axle structure  98 . 2  is then moved into its release position by the control device  112  correspondingly activating the motor  106 ; the movement phase C in  FIG. 6  also shows this. 
     During the further movement of the transport cart  42 , the rear swing-axle structure  98 . 2  can pivot about the swing axis  100 . 2  when the wheel arrangement  52 . 4 - d - s  travels onto the floor unevenness  92 , as is shown in the movement phase C according to  FIG. 6 . 
       FIG. 7  shows, as a third exemplary embodiment, a modified wheel guiding system  88  with a first and a second swing-axle structure  98 . 1  and  98 . 2 . 
     In a modification with respect to the exemplary embodiment according to  FIGS. 5 and 6 , the swing axes  100 . 1 ,  100 . 2  in the swing-axle structures  98 . 1 ,  98 . 2  are arranged eccentrically there on the swing-axle structure  98  between the two wheel arrangements  52 . The distance between the swing axis  100  and one of the two wheel arrangements  52  is therefore greater than the distance between the swing axis  100  and the other of the two wheel arrangements  52 . 
     The first and the second swing-axle structure  98 . 1  and  98 . 2  are mounted on the chassis frame  48  in such a manner that the swing axes  100 . 1  and  100 . 2  thereof in the basic chassis configuration are offset in a horizontal direction perpendicularly to the longitudinal axis  50 . In the present exemplary embodiment, the two swing axes  100 . 1  and  100 . 2  therefore run parallel, but not coaxially. However, a non-parallel profile of the swing axes  100 . 1  and  100 . 2  is also possible. 
     This laterally offset mounting of the two swing-axle structures  98 . 1  and  98 . 2  means that the chassis  46  is stable for travel without further measures; the risk of tilting to the side, as would be the case in the exemplary embodiment according to  FIG. 6  without the blocking elements  104 , does not exist. 
     Accordingly, both on blocking elements  104  and on associated motors  106 , the safety device  108  with sensor device  110 , the control device  112  and the swing-sensor device  114  can be omitted. 
     In order nevertheless to detect floor unevennesses, it is possible to retain the swing-sensor device  114  and an associated control device  112  which, for example, can store the data and/or can forward same to a master central controller. 
     In the case of the exemplary embodiments according to  FIGS. 5 to 7 , the roller arrangements  52  are mounted rigidly on the swing-axle structures  98  and  98 . 1  and  98 . 2 . 
       FIGS. 8 and 9  each show, as a fourth and fifth exemplary embodiment, a wheel guiding system  88  in which the wheel arrangements  52  are mounted movably on an associated swing-axle structure  98 . 
     For this purpose, each wheel arrangement  52  comprises a carrying structure  116  which carries or supports the wheel  54  or the wheels  54  and which is in each case mounted on the swing-axle structure  98  so as to be pivotable about a pivot axis  118  which runs parallel to the swing axis  100  of the swing-axle structure  98 . 
     Accordingly, the wheel arrangements  52 , which can be seen in  FIG. 8 , of the wheel guiding system  88  are each mounted on the swing-axle structure  98  so as to be pivotable about a pivot axis  118 . In the present exemplary embodiment, the pivot axes  118  are arranged lower in the vertical direction, in the basic configuration of the chassis of the wheel guiding system  88 , than the swing axis  100  of the swing-axle structure  98 . In the present exemplary embodiment, the pivot axes  118  of the wheel arrangements  52 , in the basic chassis configuration of the wheel guiding system  88 , are also arranged lower in the vertical direction than the wheel axes  58  of the one or more associated wheels  54  of the respective movably mounted wheel arrangement  52 . 
     A swing-axle structure  98  with two wheel arrangements  52  fastened thereto about a respective pivot axis  118  defines a system with three joints or points of articulation, namely the swing axis  100  and the two pivot axes  118 . 
     As  FIG. 8  illustrates, a wheel arrangement  52 , here the front wheel arrangement  52 . 2 - s , can deviate with two degrees of freedom when it travels onto a floor unevenness  92 . This is advantageous in particular when one pair of wheels  56  is used, if only one of the two wheels  54  of the pair of wheels  56  travels onto a floor unevenness  92 . 
     Each wheel arrangement  52  can again have only one single wheel  54 . If the wheel arrangement  52  comprises a plurality of wheels  54 , as is shown in  FIG. 8  using the example of two wheels  54  in each case, the spatial arrangement and spatial relationship of the wheels  54  of a wheel arrangement  52  with respect to one another does not change and therefore moves conjointly if the wheel arrangement  52  is tilted as a whole about the pivot axis  118 . 
     The wheel arrangement  52 , and thus the pair of wheels  56 , can firstly pivot about the pivot axis  118  in such a manner that one of the wheels  54  of the pair of wheels  56  still remains in the travel plane  90  and only the other wheel  54  of the pair of wheels  56  travels over the floor unevenness  92 . In the process, the swing-axle structure  98  undergoes a slight swinging movement about the swing axis  100 . 
     The chassis  46  is raised less here than in the case of a swing-axle structure  98  with a rigid mounting of the wheel arrangements  52 . The workpiece  12  can thereby be guided even more quietly and with a more stable movement over floor unevennesses  92 . 
     In the case of the wheel guiding system  88  according to  FIG. 9 , a wheel arrangement  52  comprises a plurality of wheels  54 , with again two wheels  54  as a pair of wheels  56  for each wheel arrangement  52  being shown by way of example. 
     In this wheel guiding system  88 , the spatial arrangement and spatial relationship of the wheels  54  of a wheel arrangement  52  can change with respect to one another. For this purpose, each wheel  54  is once again mounted separately by means of a bearing structure  120  which, for its part, is fastened to the carrying structure  116  so as to be pivotable about a pivot axis  122 , said pivot axes running parallel to the swing axis  100  of the swing-axle structure  98  and to the pivot axes  118  of the carrying structures  116  of the wheel arrangements  52 . 
     In the case of the exemplary embodiment shown in  FIG. 9 , the pivot axes  122  of the bearing structures  120  in the case of a wheel arrangement  52  in the basic chassis configuration of the wheel guiding system  88  are arranged lower in the vertical direction than the pivot axis  118  of the carrying structure  116  and are located in a common horizontal plane. The pivot axes  112  of the bearing structures  120  in the case of a wheel arrangement  52  in the basic chassis configuration are arranged here in the vertical direction between the pivot axis  118  of the supporting structure  116  and the wheel axes  58  of the wheels  54  of the wheel arrangement  52 . In a horizontal direction, the two pivot axes  122  of the bearing structures  120  are arranged on either side of a vertical plane which runs through the pivot axis  118  of the carrying structure  116 . The pivot axis  118  of the carrying structures  116  of the wheel arrangement  52  is arranged at approximately the same height as the swing axis  100  in the basic chassis configuration. 
     If then, in the case of one pair of wheels  56  with two wheels  54  in a wheel arrangement  52 , one of the wheels  54  travels onto a floor unevenness  92 , the carrying structure  116  of the wheel arrangement  52  and the two bearing structures  120  of the wheels  54  can pivot in opposite directions about the respective pivot axes  118  and  122 . It is thereby possible for the wheel axes  58  of the wheels  54  to run horizontally despite the floor unevenness  92  and for the wheels  54  not to tilt with respect to a horizontal plane. 
     In the variants according to  FIGS. 8 and 9 , in each case there can be again only one swing-axle structure  98 , as has been described in conjunction with  FIG. 5 , or there can be in each case a front and a rear swing-axle structure  98 . 1 ,  98 . 2 , as has been explained in conjunction with  FIGS. 6 and 7 ; in the case of two swing-axle structures  98 . 1 ,  98 . 2 , the respective swing axes  100 . 1 ,  100 . 2  can therefore run alternatively in one and the same vertical plane and also coaxially with respect to one another, as shown in  FIG. 6 , or can be offset in a direction perpendicular to such a plane, as  FIG. 7  illustrates. 
     In all the above-explained exemplary embodiments, the respective load which is removed to the travel floor  44  by a certain wheel arrangement  52  remains substantially unchanged when a floor unevenness  92  is traveled over. The only small influences thereon by the reduced tilting of the chassis  46  or of the workpiece  12  to the side or in the transport direction are not taken into consideration here. 
     In all the above-explained exemplary embodiments, the wheel guiding system  88  leads thereto and is configured in such a manner that the transport cart  42  always remains statically determined within the scope of the possible movement of the wheel  54  or of one of the wheel arrangements  52  out of the travel plane  90 . By this means, it is consequently prevented that the transport cart  42  can wobble in relation to the travel floor  44  within the meaning of a static underdeterminedness. Therefore, only if a floor unevenness  92  leads to a wheel arrangement  52  having to be moved further out of the travel plane  90  than the construction permits is the transport cart  42  raised in such a manner that the transport cart  42  is statically underdetermined and wobbles in relation to the travel floor  44 .