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TECHNICAL FIELD OF THE INVENTION 
     The invention relates to an apparatus for excavating and transporting away materials, in particular soil material, with at least one excavation device which can be rotated about an axis of rotation and with a longitudinal conveyor for transporting away excavated material. 
     PRIOR ART 
     CH-A-331 149 discloses a device for excavating and loading materials that comprises downwardly directed blades which are coupled to a vibrating device and which are intended for cutting out a cross section of material enclosed by the blades. 
     DE-A-32 35 023 discloses an excavation apparatus with a longitudinal conveyor which is intended for transporting away loose waste, said apparatus being intended for gallery support in mining. The front end of the conveyor carries a rotating roller which is arranged transversely to the longitudinal direction of said conveyor. The roller surface is provided with breaker spikes, and a drive motor is arranged inside the roller. The driven roller takes up loose waste and simultaneously levels the floor over which the conveyor is advanced. At the same time, the waste is thrown onto the conveyor. 
     WO-A-2006/074828 discloses an apparatus for cleaning track ballast comprising a transverse strand of a clearing chain, said strand extending transversely with respect to a machine longitudinal direction. As the machine moves slowly forward, ballast underneath the track is dug away by means of the endlessly revolving clearing chain and passed to a cleaning unit. 
     SUMMARY OF THE INVENTION 
     The object on which the invention is based is to provide an apparatus of the type mentioned at the outset which has no exposed movable parts, such as chains or buckets, for example, and can thus have a robust design for comminuting relatively large lumps. Moreover, the excavation apparatus should be suitable for all kinds of materials to be excavated and be able to be used in the smallest spaces. 
     The object is achieved according to the invention in that the excavation device comprises an impeller wheel with a conically extending guide surface arranged about the axis of rotation in a rotationally symmetrical manner and with guide vanes which project radially from the conical guide surface and which form an acute angle with respect to the axis of rotation. 
     In a preferred embodiment, the conically extending guide surface of the impeller wheel has its narrowed end adjoining a conveyor belt and forms a run-off surface for excavated material. 
     Advantageously, the excavation device comprises two impeller wheels with a conveyor belt arranged between them. 
     The guide vanes are preferably bounded by two lateral edges which are parallel to one another, and the width of the guide vanes corresponds substantially to the width of the conically extending guide surface. 
     The guide vanes are preferably twisted with respect to the axis of rotation by an angle such that, as viewed in the running direction, the outer lateral edge leads the inner lateral edge. 
     Advantageously, the outer lateral edge of the guide vanes adjoins a fixed guide plate, at least in a region below the conveyor belt. 
     In one preferred embodiment, the guide plate terminates above the conveyor belt by way of an upper edge situated parallel to the conveying plane of the conveyor belt. 
     Advantageously, a vane covering, which covers the guide vanes, projects inwardly from a peripheral edge of the guide plate. 
     Preferably, the guide vanes are curved with respect to a cross section situated perpendicularly to the lateral edges, and the curved surface of the guide vanes points with its concave curvature in the direction of rotation of the impeller wheels. 
     Preferably, vane teeth project laterally outwardly from the guide vanes. 
     Advantageously, the impeller wheels are each assigned an individually controllable motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Further advantages, features and details of the invention will become apparent from the description given below of preferred exemplary embodiments and with reference to the drawing, in which: 
         FIG. 1  schematically shows an oblique view of an excavation apparatus; 
         FIG. 2  schematically shows an oblique view of a detail of  FIG. 1  with the right impeller wheel removed; 
         FIG. 3  schematically shows a vertical section through the axis of rotation of part of the excavation apparatus of  FIG. 1  as viewed in the conveying direction; 
         FIG. 4  schematically shows the further-simplified vertical section of  FIG. 3  through the axis of rotation of part of the excavation apparatus of  FIG. 1  as viewed counter to the conveying direction; 
         FIG. 5  schematically shows the side view of the arrangement of  FIG. 4 ; 
         FIGS. 6-11  schematically show the layering of material after filling as a function of the direction of rotation; 
         FIGS. 12-14  schematically show various impeller wheel embodiments. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     An excavation machine  10  as represented in  FIG. 1  comprises two impeller wheels  12 ,  14  arranged concentrically and at a distance from one another. Between the impeller wheels  12 ,  14 , which can be rotated about a common axis of rotation y, is arranged a longitudinal conveyor comprising an endlessly revolving conveyor belt  18  having a conveying direction x which is at a right angle with respect to the axis of rotation y and which generally corresponds to the operating direction A of the excavation machine  10 . The impeller wheels  12 ,  14  and the conveyor belt  18  are mounted on a machine frame  16 . 
     Each impeller wheel  12 ,  14  has a central conical guide surface  20  which is arranged in a rotationally symmetrical manner with respect to the axis of rotation y and which encloses an angle of approximately 45° with respect to said axis of rotation. Projecting outwardly from the conical guide surface  20  are guide vanes  22  which are uniformly distributed over the circumference. Here, the projection of the guide vanes  22  into the radial direction intersects the axis of rotation y at an acute angle of approximately 45°, that is to say that the surface of the guide vanes  22  is twisted by an angle of approximately 45° with respect to a radial plane emanating from the axis of rotation y. The guide vanes  22  extend over the full width of the conical guide surface  20  and are bounded by two lateral vane edges  24 ,  26  which are parallel to one another and at a right angle with respect to the axis of rotation y. In relation to a cross section which extends perpendicularly to the lateral edges  24 ,  26 , the guide vanes  22  have a curved design. 
     The conical guide surface  20  tapers from the outer side of each impeller wheel  12 ,  14  toward the inner side situated adjacent to the conveyor belt  18 . 
     The guide vanes  22  are twisted with respect to the axis of rotation y by an angle such that, as viewed in the running direction u, the outer lateral edge  26  leads the inner lateral edge  24 . Here, the curved surface of the guide vanes  22  points with its concave curvature in the direction of rotation u. Vane teeth  28  project outwardly from the outer lateral edge of the guide vanes  22  and substantially tangentially from the curved surface of the guide vanes  22 . 
     Each impeller wheel  12 ,  14  is covered on its inner side by a fixed guide plate  30  with a circular peripheral edge  32  as far as a region situated above the conveyor belt  18 . The guide plate  30  projects by a distance e above the conveyor belt  18  by way of an edge  31  situated parallel to the conveyor belt  18 . The gap between the guide plate  30  and the conveyor belt  18  in the region of the edge  31  is covered by a strip-like cover plate  33  which extends from the edge  31  to the lower-lying conveyor belt  18 . In a further region between approximately the lowest point S of the impeller wheel  12 ,  14  and a point T above the conveyor belt  18 , a vane covering  34  in the form of a band corresponding approximately to the width of the guide vanes  22  projects substantially radially outward from the peripheral edge  32  of the guide plate  30 . 
     As can be seen from  FIG. 3 , the guide vanes  22  guide and press excavated material  41  against the guide plate  30 . The guide channel formed by the guide plate  30  and the vane covering  34  prevents the excavated material flowing out in the rising part of the impeller wheel  12 ,  14 . In the region of this vane covering  34 , between two successive guide vanes  22 , is formed a chamber  36  which is defined by the guide plate and the vane covering  34  and which is open only toward the outer side of the impeller wheel  12 ,  14 , said chamber moving in the direction of rotation of the impeller wheel  12 ,  14  and opening at the end of the guide plate  30  above the conveyor belt  18 . The conical guide surface  20  of the chamber  36  situated above the conveyor belt  18  forms a run-off surface for the excavated material  41  on which said material can run onto the conveyor belt  18 , wherein the inner peripheral edge of the conical guide surface  20  adjoins the lateral edge of the conveyor belt  18 . In addition, a forced displacement of the excavated material  41  in the direction of the conveyor belt  18  is produced by the guide vane  22  situated transversely to the conveying direction x. 
     The two inclined planes, that is to say the orientation of the guide vanes  22  and the taper of the conical guide surface  20 , jointly assist the flow of material, with the result that even bulky or viscous material such as clay can flow off in a favorable manner. 
     As shown in  FIG. 2 , the guide vanes  22  can be provided at their free radial end with a vane part  38  which projects substantially at a right angle with respect to the radial direction of the guide vanes  22  in the running direction u of the impeller wheel  12 ,  14 , said guide vanes thereby being closed. When the guide vanes  22  are designed with a terminally projecting vane part  38 , it is possible to dispense with the vane covering  34 . 
     Each impeller wheel  12 ,  14  is equipped with a motor  40  for producing the rotational movement about the axis of rotation y. The two motors  40  can be controlled independently of one another. 
     In the case of a particular layer structure of a soil material  41  with, for example, a subsoil of clay  42  and an overlying layer of gravel  44 , it is possible, as can be seen from  FIGS. 6-11 , for the filling of the guide vanes  22  to be adapted to the layer structure of the material to be excavated by changing the direction of rotation u of the impeller wheels  12 ,  14  while at the same time exchanging the two impeller wheels  12 ,  14 . 
     With the direction of rotation represented, there is a risk, as shown in  FIGS. 6-8 , that the viscous clay  42  will flow out only in a delayed manner. The emptying time, or the time during which the guide vane  22  moves past the open guide plate  30 , is therefore not sufficient for the clay  42  to flow out completely. The residual clay layer adhering to the guide vanes  22  becomes increasingly thicker, decreases the filling volume and reduces excavation efficiency. 
     In the arrangement shown in  FIGS. 9-11  in which the direction of rotation is reversed, the heavy, loose gravel stones  44  promote the flow of the clay  42  and additionally have a cleaning effect on the impeller wheels  12 ,  14 . The direction of rotation u in the counterclockwise direction is thus advisable in such conditions. 
     The guide channel formed by the vane covering  34  frees the guide vanes  22  above the conveyor belt  18 , which has the following advantages:
         Bulky material, such as wood, ropes, cables, etc., which has been caught in the guide vanes  22  can be removed without problems after stopping the impeller wheel  12 ,  14 .   The guide vanes  22  can be cleaned automatically with the impeller wheel  12 ,  14  running or manually with the impeller wheel  12 ,  14  at a standstill, for example to remove stubbornly adhesive material such as clay, marl, etc.       

     By virtue of its continuous rotational movement, the impeller wheel  12 ,  14  is a highly efficient conveying device for mixed material to be excavated, such as sand, gravel, stones, etc., for example. 
     Since there are no permanent closed cavities, such as, for example, buckets with bases, this excavation system is also highly suited to problematic material to be excavated, such as wet humus, clay, marl, etc. 
     The impeller wheel  12 ,  14  is extremely compact and without exposed, movable parts, such as chains, buckets, etc., for example. Consequently, it can be provided with a robust design such that relatively large lumps which occur, such as masonry, stones, etc., are comminuted without problems. Should an object block the impeller wheel  12 ,  14 , an automatic pressure cut-off protects the device from deformations. If it is found that, for example, a cable is trapped and wound up, the rotational movement can be stopped immediately and the entanglement removed by changing the direction of rotation. The two impeller wheels  12 ,  14  are driven individually via a respective motor  40  and can also be controlled individually. 
     The vane teeth  28  mounted at the outer lateral edge  26  of the guide vanes  22  serve to loosen hard material  41  to be excavated. 
     There are various optimization variants available to the impeller wheel  12 ,  14  depending on the particular area of application:
         When changing the direction of rotation u, the operating direction A can be changed (rearward, pivoting). Adaptation to layer structures of the material to be excavated is likewise possible.   Upwardly closed-off guide vanes  22  having an angled vane part  38  instead of the vane covering  34  are particularly suitable for granular material to be excavated which does not contain bulky material such as wood, for example.   A vane covering  34  which is continuous in the excavation region ( FIG. 12 ) is particularly suitable for problematic material to be excavated, such as large stones and wood. Bulky material is separated out. This arrangement functions for a pivoting operation, but not in the case of forward and rearward travel.   A guide plate  30  which is continuous in the excavation region ( FIG. 13 ) is particularly suitable for bulky material which cannot be comminuted, such as granite blocks and reinforced foundation remains, for example.   A shortened guide plate  30  ( FIG. 14 ) is particularly suitable for bulky material which can be comminuted, such as bricks, for example.       

     LIST OF REFERENCE SIGNS 
     
         
           10  excavation machine 
           12  first impeller wheel 
           14  second impeller wheel 
           16  machine frame 
           18  conveyor belt 
           20  conical guide surface 
           22  guide vane 
           24  inner lateral edge of  22   
           26  outer lateral edge of  22   
           28  vane teeth 
           30  guide plate 
           31  upper edge of  30   
           32  peripheral edge of  30   
           33  cover plate 
           34  vane covering 
           36  chamber 
           38  vane part 
           40  motor 
           41  excavated material/material to be excavated 
           42  clay 
           44  gravel 
         A operating direction of  10   
         x conveying direction of  18   
         y axis of rotation of  12 ,  14   
         u direction of rotation of  12 ,  14   
         e distance between  18  and  31   
         S lower point of  34   
         T upper point of  34

Summary:
A device for excavating and transporting material, especially earth, the device comprising an excavation appliance that can be rotated about a rotational axis (y) in a rotational direction (u), and a longitudinal conveyor for transporting the excavated material. The excavation appliance comprises an impeller having a conically extending guide surface arranged around the rotational axis (y) in a rotationally symmetrical manner, and guide vanes which radially protrude from the conical guide surface and form an acute angle in relation to the rotational axis (y).