Abstract:
The invention is directed to a friction wheel drive with a driving roller capable of being driven in a rotary manner, which is mounted on a bearing unit so as to be rotatable about an axis of rotation. The bearing unit is displaceably guided transversely to the axis of rotation, and a circumferential surface of the driving roller can be brought into driving engagement with a friction surface. The bearing unit is coupled to a first mechanical forced guidance system, by which the driving roller, responding to a driving force acting in a first direction, can be automatically pressed against the friction surface with a contact pressing force that increases as the driving force increases. The bearing unit is also coupled to a second mechanical forced guidance system, by which the driving roller, responding to a driving force acting in an opposite second direction, can be automatically pressed against the friction surface with a contact pressing force that increases as the driving force increases.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a notional stage application of PCT/DE2010/001408 filed Dec. 3, 2010, claiming priority from Application DE 102010004858.5-12 filed on Jan. 18, 2010 in Germany. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a friction wheel drive for transmitting driving forces in changing directions, such as in handling and conveying equipment. Drives of this kind are known and have the disadvantage, among other things, that the contact pressing force of the driving rollers has to be set relatively high in order to make it possible to transmit the maximum drive forces to be expected, without excessive slip. This results in unnecessary wear and poor efficiency. 
       BACKGROUND OF THE INVENTION 
       [0003]    The problem of the invention consists in improving a friction wheel drive, which is known per se, in which a circumferential surface of a driving roller co-operates with a friction surface, in such a way that the transmission of driving and braking forces of varying magnitudes in both directions is in each case possible by automatically increasing the contact pressing force of the driving roller to a degree appropriate to the coeffi-cient of friction. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    A friction wheel drive of this kind is advantageous in that the contact pressing force with which the driving roller is pressed against the friction surface is not laid down permanently, such as in line with the maximum drive force to be expected, but is instead adapted, automatically and independently of the direction of drive, to the driving force to be transmitted, or the torque to be transmitted, so that on the one hand, acceptable slip is not exceeded and on the other hand, unnecessarily great pressing forces are avoided, which would otherwise reduce the efficiency of the drive and cause unnecessary wear. 
         [0005]    The first and/or the second forced guidance may take the form of a first and/or second mechanical coupling member linking the bearing unit to a mount, relative to which the bearing unit is guided so as to be displaceable transversely to the axis of rotation. 
         [0006]    In particular, it is contemplated that displacement of the bearing unit parallel to the friction surface simultaneously and automatically produces a displacement of the bearing unit in the direction of the friction surface. The contact pressing force is preferably proportional to the driving force. It is preferably contemplated that the displacement in the direction of the friction surface is proportional to the displacement parallel to the friction surface. 
         [0007]    In a first embodiment, it is contemplated that two two-armed pivot levers, which are arranged, in a rest position of the bearing unit with no drive force, on both sides of, in particular symmetrically to, a plane including the axis of rotation and perpendicular to the friction surface, with first lever arms on a pressing surface of the bearing unit parallel to the friction surface which can in each case be supported with a second lever arm on a transmission surface of the bearing unit running in each case perpendicularly to the friction surface (or substantially perpendicularly, where the difference compared to the vertical direction can correspond approximately to the friction angle of the friction pair pressure roller/friction surface). The pivot levers may be hinged to the mount. 
         [0008]    In this context, it may be contemplated that the bearing unit is mounted to be pivotable about a swivelling axis arranged on a carriage which is displaceable perpendicularly to the friction surface and parallel to the axis of rotation of the driving roller. 
         [0009]    In the process, the carriage may by guided to be displaceable either along a linear guide or about a swivelling axis perpendicular to the friction surface arranged parallel to the axis of rotation. 
         [0010]    In a further embodiment, it may be contemplated that a pressure member can be pressed against a counter-surface opposite the friction surface, which is a further outer surface of a friction-drive body, parallel to the friction surface, where two two-armed pivot levers, which are arranged, in a rest position of the bearing unit with no drive force, on both sides of, in particular symmetrically to, a plane including the axis of rotation of the driving roller and perpendicular to the friction surface, with first lever arms on a transmission surface of the bearing unit arranged perpendicularly to the friction surface in each case and with second lever arms which can be supported on a pressing surface, running parallel to the friction surface, of a pressure body supporting the pressure member displaceably mounted perpendicularly to the counter-surface. The pivot levers may be hinged to the mount. 
         [0011]    In this context, it may be contemplated that the bearing unit is guided to be displaceable along a linear guide aligned parallel to the friction surface or, perpendicularly to the friction surface, about a swivelling axis arranged parallel to the axis of rotation of the driving roller especially on a mount. 
         [0012]    In this context, it may be contemplated that the bearing unit is guided to be displaceable along a linear guide aligned parallel to the friction surface or, perpendicularly to the friction surface, about a swivelling axis arranged parallel to the axis of rotation of the driving roller especially on a mount. 
         [0013]    The pressure member may be spring-biased relative to the bearing unit. 
         [0014]    In a further embodiment, it may be contemplated that the bearing unit is force-guided on a control surface, which is especially V-shaped in design, and which is arranged, in a rest position of the bearing unit with no drive force, on both sides of, in particular symmetrically to, a plane including the axis of rotation of the driving roller and running perpendicular to the friction surface, thus causing displacement of the bearing unit parallel to the friction surface simultaneously to displacement in the direction of the friction surface. 
         [0015]    In this context, it may be contemplated that the bearing unit is displaceably guided along a first linear guide aligned parallel or perpendicular to the friction surface on a carriage which for its part is displaceably guided along a second guide aligned perpendicularly to the first linear guide, especially on the mount. 
         [0016]    In a manner analogous to the embodiments described above, each of the linear guides may be replaced by a swivel bearing with a swivelling axis kept parallel to the axis of rotation of the driving roller, retained on the carriage or on the mount. 
         [0017]    In one embodiment, it is contemplated that the bearing unit is mounted on a carriage that is displaceable perpendicularly to the friction surface so as to be pivotable about a swivelling axis arranged on the carriage and parallel to the axis of rotation of the driving roller, the carriage being force-guided along a control surface which is in particular arcuate in design, and which is arranged, in a rest position of the bearing unit with no drive force, on both sides of, in particular symmetrically to, a plane including the axis of rotation of the driving roller and running perpendicular to the friction surface, thus causing displacement of the bearing unit parallel to the friction surface simultaneously to displacement in the direction of the friction surface. 
         [0018]    In this context, it may be contemplated that the carriage is guided to be displaceable perpendicularly to the friction surface along a linear guide aligned perpendicularly to the friction surface or about a swivelling axis arranged parallel to the axis of rotation of the driving roller, especially on a mount. 
         [0019]    In the above-mentioned embodiments with two angle levers, it may be contemplated that the transmission surfaces are arranged in a V shape at an angle to the above-mentioned plane which, or whose arc tan value, corresponds to a length ratio of the first and second lever arms. The angle may be chosen such that a distance between transmission rollers of the first lever arms and the pressing surface and respective distances between transmission rollers of the second lever arms and the transmission surfaces remain substantially constant despite changes in the distance between the swivelling axis of the bearing unit and the swivelling axis of the pivot levers. 
         [0020]    In the embodiments without a displaceable pressure body, it may be contemplated that a pressure member which is retained on the mount and which can be pressed against a counter-surface opposite the friction surface, which is a further outer surface of a friction-drive body, parallel to the friction surface, is spring-biased relative to the bearing unit. 
         [0021]    In addition, it may be contemplated that the bearing unit and/or the pressure body is/are displaceably guided about a swivelling axis perpendicular to the friction surface, and it may be contemplated that the swivelling axis coincides with a swivelling axis on which a mount is hinged to a frame so as to be swivellably movable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Further advantages and features of the invention will become clear from the following description of several worked embodiments, reference being made to a drawing in which: 
           [0023]      FIG. 1  shows a view of a first embodiment of the invention, seen from above, 
           [0024]      FIG. 2  shows a view of the first embodiment, seen from the front, 
           [0025]      FIG. 3  shows a side view of the first embodiment, 
           [0026]      FIG. 4  shows a perspective view of the first embodiment from one direction, 
           [0027]      FIG. 5  shows a perspective view of the first embodiment from a different direction, 
           [0028]      FIG. 6  shows a view of a second embodiment of the invention, seen from above, 
           [0029]      FIG. 7  shows a perspective view of the second embodiment from one direction, 
           [0030]      FIG. 8  shows a perspective view of the second embodiment from a different direction, 
           [0031]      FIG. 9  shows a view of a third embodiment of the invention, seen from above, 
           [0032]      FIG. 10  shows a perspective view of the third embodiment, 
           [0033]      FIG. 11  shows a view of a fourth embodiment of the invention, seen from above, 
           [0034]      FIG. 12  shows a perspective view of the fourth embodiment, 
           [0035]      FIG. 13  shows a view of a fifth embodiment of the invention, seen from above, 
           [0036]      FIG. 14  shows a perspective view of the fifth embodiment, 
           [0037]      FIG. 15  shows a view of a sixth embodiment of the invention, seen from above, 
           [0038]      FIG. 16  shows a perspective view of the sixth embodiment from one direction, 
           [0039]      FIG. 17  shows a perspective view of the sixth embodiment from a different direction, 
           [0040]      FIG. 18  shows a view of a seventh embodiment of the invention, seen from above, 
           [0041]      FIG. 19  shows a perspective view of the seventh embodiment, 
           [0042]      FIG. 20  shows a view of an eighth embodiment of the invention, seen from above, 
           [0043]      FIG. 21  shows a view of the eighth embodiment, seen from above from the opposite direction, 
           [0044]      FIG. 22  shows a perspective view of the eighth embodiment, and 
           [0045]      FIG. 23  illustrates the link between the frictional connection and slip. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0046]      FIGS. 1 to 5  show different views of a first embodiment of a friction wheel drive of the invention, which, in the application illustrated, is used to drive individual carriages of a conveyor means. 
         [0047]      FIG. 2  shows a schematic view, from the front, of an individual carriage  2  of a conveyor means, which has a chassis  4  on which are mounted support rollers, or idlers,  6 , which are rotatable about horizontal axes, and guide rollers  8 , which are rotatable about vertical axes. The support and guide rollers  6 ,  8  run substantially without play in a pair of guide rails  10 , which are fixed parallel and spaced apart on a supporting frame  12 , so that the carriages can be moved in a longitudinal direction or conveying direction  13 . 
         [0048]    Fixed to each carriage  2  is a plate-like drive blade  14 , which points vertically downwards and is aligned in the longitudinal direction  13 , with the help of which a driving force acting in the longitudinal direction can be transmitted to the carriage  2  by means of a friction wheel drive  16 . 
         [0049]    Since a chain of carriages  2  is as a rule joint together by couplings and, in addition, each carriage  2  possesses a certain inertia, it is sufficient if at least one friction wheel drive  16  is disposed on the frame  12 , or if a plurality of friction wheel drives  16  are disposed over the length of the conveyor means, spaced apart from one another to some extent. 
         [0050]      FIG. 1  shows a view of a friction wheel drive of this kind, seen from above, where part of the frame  12  and the drive blade  14  are illustrated, which extends in the longitudinal direction  13 . 
         [0051]    A mount  22  is mounted on the frame  12  so as to be pivotable about a swivelling axis  24 , which is vertical in this case, and which provides an alignment compensation between the drive blade  14  and the friction wheel drive. The mount  22  has a flat base plate  26 , to which a further flat plate  28  is fixed, spaced apart from it and parallel to it. Extending between the plates  26 ,  28  are cylindrical journals  30 ,  32  and mounting brackets  34 ,  36 . 
         [0052]    On the lower plate  26 , a carriage  38  is guided on two linear guides  40  arranged spaced apart and parallel to each other along axes of travel  42  relative to the plate  26 . The two linear guides  40  form a first linear guide in this connection. 
         [0053]    A bearing unit  44 , on which a rotatably driven driving roller  46  is mounted so as to be rotatable about an axis of rotation  48 , is guided on the carriage  38  along two linear guides  50 , which form a second linear guide, so as to be longitudinally displaceable along axes of travel  52 , the second linear guide  50  being aligned perpendicular to the first linear guide  40 . 
         [0054]    A drive motor  20  is disposed beneath the mount  22  and is in driving connection with the driving roller  46 ; it may be firmly joined to the bearing unit or arranged separately and may be connected to the driving roller via a rigid or flexible driving connection, such as a chain, belt or shaft drive. 
         [0055]    A circumferential surface  54  of the driving roller  46  can be brought into engagement with a friction surface  56  forming an outer surface of the drive blade  14 , the friction surface  56  being substantially parallel to the second linear guide  50  in operation. 
         [0056]    The bearing unit  44  has two transmission surfaces  60 ,  62 , which run parallel to one another and to the first linear guide  40 , and a pressing surface  64 , which runs perpendicular thereto and parallel to the second longitudinal guide  50 . 
         [0057]    Two two-armed pivot levers  70 ,  72  are mounted on the mount  22  on the journals  30 ,  32 , which are fixed rigidly to the mount, so as to be pivotable about swivelling axes  74 ,  76  arranged perpendicular to the linear guides  40 ,  50  and parallel to the axis of rotation  48  of the driving roller  46 . On the end of each first lever arm  78 , there is a transmission roller  82 , which is rotatable about the axis of rotation  80 , a distance between the axis of rotation  80  and the swivelling axis  74  or  76  of the respective pivot lever being a The transmission rollers  82  can roll on the pressing surface  64 . 
         [0058]    On the end of each second lever arm  84  of the pivot levers  70 ,  72 , there is likewise a transmission roller  86 , which is mounted so as to be rotatable about an axis of rotation  88  with a distance b between the respective swivelling axis  74 ,  76  of the respective pivot lever  70 ,  72  and which can roll on the transmission surface  60  or  62  facing the pivot lever concerned. 
         [0059]    The lever arrangement  70 ,  72  is preferably symmetrical to a plane  89  including the axis of rotation  48  of the driving roller  46  and perpendicular to the friction surface  56 , so that the pressing effect of the friction wheel drive is the same in both directions of drive. 
         [0060]    As is shown by  FIG. 5  in particular, a pressure roller  90  forming a pressure member is provided, which is mounted on a journal  94  and runs freely about an axis of rotation  92  arranged parallel to the axis of rotation  48  of the driving roller  46 . The journal  94  is firmly joined to the mount or the plates  26 ,  28 , so that the axis of rotation  92  does not change position relative to the mount  22 . Biased tension springs  96  are disposed between mounting brackets  34 ,  36 , which are mounted on the mount  22  so that they do not change position, and the bearing unit  44  and in this way define a biasing force of the driving roller  46  towards the pressure roller  90 . 
         [0061]    In operation, when a carriage  2  moves in the longitudinal direction, the drive blade  14  enters the area between the driving roller  46  and the pressure roller  90  of a friction wheel drive  16 , the rollers being biased against each other with a predetermined contact pressing force by means of a suitable choice of tension springs  96 , so that friction forces in the linear guides etc. are overcome. Irrespective of any driving force, there is therefore already a force of the driving roller  46  which is directed orthogonally to the friction surface. 
         [0062]    When the driving roller  46  is driven with a drive torque in a chosen direction of rotation (forwards or backwards), a reaction force which acts on the driving roller in the opposite direction to the driving force becomes effective parallel to the friction surface  56 , i.e. parallel to the axes of travel  52  of the second linear guide  50 , along which the bearing unit  44  is displaceable relative to the carriage  38 . Depending on the remaining play, the elasticity of the driving roller  46 , and the elasticity of the pressure roller  90  etc., the bearing unit  44  is displaced parallel to the friction surface under the effect of the reaction force described. In the process, one of the pivot levers  70 ,  72 , which is located in the direction of the displacement movement, is swivelled about the swivelling axis  74  or  76  because of the contact between a second lever arm  84 , or its transmission roller  86 , and the transmission surface  60  or  62 , so that the transmission roller  82  located at the end of the first lever arm  78  begins to press on the pressing surface  64  and moves the bearing unit  44  in a direction perpendicular to the friction surface  56  by a distance that results from the ratio a/b of the lengths of the lever arms between the first and second lever arms, or the effective lever arm length ratio, if the lever arms include an angle other than 90°. 
         [0063]    The driving roller  46  is thus pressed against the friction surface with a contact pressing force that varies in strength depending on the driving force acting at any particular time and is roughly proportional to the driving force, as a result of which a considerable reduction in wear and an improvement in efficiency can be achieved compared to a contact pressing force that is permanently fixed. In particular, the automatic boosting of the contact pressing force acts in both directions, i.e. irrespective of the direction of drive. 
         [0064]      FIGS. 6 to 8  illustrate a second embodiment of the invention, in which the bearing unit  44  is mounted on a first linear guide  40 , which is fixed to the mount  22  and is displaceable parallel to the friction surface  56 , while a pressure body  100  carrying the pressure roller  90  is guided on a second linear guide  50 , which is fixed to the mount  22  and is displaceable perpendicularly to the friction surface  56  and perpendicularly to the first linear guide  40 . Between the bearing unit  44  and the pressure body  100 , there are provided tension springs  96 , which bias the two members against each other. 
         [0065]    The transmission surfaces  60 ,  62  running perpendicularly to the friction surface are arranged to the side of the bearing unit  44  here, as with the first embodiment, whereas the pressing surface  64  running perpendicularly thereto is formed on the pressure body  100 . Any displacement of the bearing unit  44  parallel to the friction surface caused by the reaction force generated when driving the driving roller  46  thus acts via the pivot levers  70 ,  72  to cause a relative movement of the pressure body  100  and thus of the pressure roller  90  perpendicularly to the friction surface towards the bearing unit  44 , likewise irrespective of the direction of drive of the driving roller  46 . 
         [0066]      FIGS. 9 and 10  illustrate a third embodiment of the invention, in which the arrangement of the pressure roller  90 , bearing unit  44 , carriage  38  and first and second linear guide  40 ,  50  corresponds to the first embodiment. Only the pivot levers  70 ,  72  are re-placed by a V-shaped control surface  102  and a control roller  104  co-operating with it and rotatably mounted on the bearing unit  44 . Proceeding from an apex  106  which is furthest away from the friction surface  56 , the control surface  102  approaches the friction surface in both directions at a shallow angle, so that in the case of displacement parallel to the friction surface, the bearing unit  44  undergoes a corresponding, lesser movement towards the friction surface. The two regions of the control surface  102  on either side of the apex  106  are preferably symmetrical to the plane  89  and run at identical angles thereto, or to the friction surface, which corresponds to the identical, mirror-image design of the pivot levers  70 ,  72  from the embodiments described above and ensures that the pressing effect is the same in both directions. The angle at which the control surface, or its two regions on either side of the plane  89  or the apex  106 , runs relative to the friction surface determines the relationship between the displacement parallel to the friction surface and the displacement perpendicular to the friction surface and thus also corresponds to the relationship between the contact pressing force and the reaction force. 
         [0067]      FIGS. 11 and 12  show a fourth embodiment of the invention, the structure of which is similar to the third embodiment with regard to the control surface  102 , though the bearing unit  44  is mounted not along a linear guide parallel to the friction surface, but rather on the carriage  38  about a swivelling axis  110  parallel to the axis of rotation  48  of the driving roller  46 , which in turn is displaceably guided along a linear guide  50  arranged perpendicular to the friction surface. Tension springs  96  bias the bearing unit  44  on the carriage  38  towards a pressure roller  90  which is disposed in a fixed position relative to the mount  22 . 
         [0068]    Much the same as with the third embodiment, a control roller  104  is mounted on an end of the bearing unit  44  opposite the driving roller  46 ; it is rotatable about an axis of rotation parallel to the axis of rotation  48  of the driving roller  46  and can roll on a control surface  112  which is disposed in a fixed position relative to the mount  22 . Unlike the control surface  102  of the third embodiment, the control surface  112  is arcuate in design, so that the local distance from the swivelling axis  110  of the bearing unit  44  is reduced constantly as the swivelling angle of the bearing unit increases, based on the initial position shown, where there is no driving force. The control surface  112  is shaped in such a way here that the axis of rotation  48  of the driving roller  46  is increasingly displaced towards the friction surface as the bearing unit  44  is increasingly deflected or swivelled about the swivelling axis  110 . 
         [0069]      FIGS. 13 and 14  show a fifth embodiment, in which the bearing unit  44 , much the same as in the fourth embodiment, is hinged to the mount  22  about a swivelling axis  110  running parallel to the axis of rotation  48  of the driving roller  46 . In other respects, the structure corresponds to the second embodiment according to  FIGS. 6 to 8 . It can be seen in this example that in the present connection, in principle any linear guide can be replaced by a swivel bearing, provided that the distance between the axis of rotation of the driving roller, or the axis of rotation of the pressure roller, and the respective swivelling axis must not be made too small, so that the movements which occur remain small relative to the swivelling radius. For this reason, it was, for example, possible in the fifth embodiment also to replace the linear guide  50 , with which the pressure roller  90  is guided, by a swivel bearing, it being necessary for the swivel arm bearing the pressure body to extend by about 90° to the direction of movement of the linear guide. With small deflections, the displacement of the pressure roller is then approximately parallel to the linear guide of the fifth embodiment. 
         [0070]      FIGS. 15 to 17  show a sixth embodiment of the invention, which is similar in structure to the first embodiment. Unlike the first embodiment, the second linear guide  50 , with which the bearing unit  44  there is guided on the carriage  38 , is replaced in the sixth embodiment by a swivel bearing with a swivelling axis  110  running parallel to the axis of rotation  48  of the driving roller, so that the bearing unit  44  is pivotable relative to the carriage  38 , which can move along the axes of travel  42  of the first linear guide  40  perpendicularly to the friction surface  56 . As in the first embodiment, the axis of rotation  92  of the pressure roller  90  and the swivel axes  74 ,  76  of the pivot lever  70 ,  72  are attached in a fixed position relative to the mount  22 . 
         [0071]    The transmission surfaces  60 ,  62  are not parallel to one another and perpendicular to the friction surface  56 , as is the case in the first, second and fifth embodiments, but are instead arranged in a V shape, in each case at an angle to a plane  91  which includes the axis of rotation  48  of the driving roller  46  and the swivelling axis  110 . The angle referred to, indicated by O in  FIG. 15 , corresponds to the arc tan value of the length ratio a/b of the lever arms of the pivot levers  70 ,  72 , which results in an automatic adjustment when the driving roller  46  suffers wear. Wear corresponds to a reduction in the diameter of the driving roller  46 , so that the tension springs  96  cause the bearing unit  44  to move correspondingly further towards the pressure roller  90  than is shown in  FIG. 15 . In the process, the angle of inclination of the transmission surfaces  60 ,  62 , which is adapted to the lever arm length ratio, ensures that no additional play occurs between the transmission surfaces  60 ,  62  and the transmission rollers  86  or between the pressing surface  64  and the transmission rollers  82 . 
         [0072]    This kind of automatic adjustment effect thanks to transmission surfaces arranged in a V or wedge shape can also be implemented in the second and fifth embodiments. 
         [0073]      FIGS. 18 and 19  illustrate a seventh embodiment of the invention, which shows an example of how it is possible to dispense with linear guides completely. This embodiment is similar to the fifth embodiment according to  FIG. 13 , with the difference that the pressure body  100  bearing the pressure roller  90  is not mounted on a linear guide which is displaceable perpendicularly to the friction surface, but is instead mounted so as to be pivotable about a further swivelling axis  114 , which runs parallel to the swivelling axis  110  about which the bearing unit  44  is pivotally mounted. In this embodiment, the further swivelling axis  114  advantageously coincides with the swivelling axis  24 , about which the mount  22  is pivotally mounted on the frame  12  for the purpose of alignment compensation. 
         [0074]      FIGS. 20 to 22  show an eighth embodiment of the invention, which differs from the embodiments described above in that the bearing unit  44  is mounted on a mounting arm  122 , together with the driving roller  46  and drive motor  20 , so as to be pivotable about a swivelling axis  120 , which in turn is mounted on the frame  12  so as to be pivotable about a swivelling axis  124 . No mount is required here. The swivelling axis  124  corresponds to the swivelling axis  24  and provides for an alignment compensation if the position of the friction-drive body  14  deviates to one side. A pressure roller  90  is mounted on a pressure body  100 , which takes the form of a swivel arm, which is likewise mounted on the frame  12  so as to be pivotable about the swivelling axis  124 . The pressure body  100  has a lateral pressure surface  126 , against which a first or second pressure roller  128 ,  130  can rest. The pressure rollers  128 ,  130  are rotatably mounted on the bearing unit  44 . A first axis of rotation  132  of the first pressure roller  128  is a first distance c from a central plane  134  including the axis of rotation  48  of the driving roller  46  and the swivelling axis  120  of the bearing unit  44 , while a second axis of rotation  134  of the second pressure roller  130  is a distance d from the central plane  134 . The distances c, d are not the same in the embodiment illustrated, but are selected such that when there is a (slight) swivelling movement of the bearing unit  44  about the swivelling axis  120 , which is caused by a driving force introduced by the driving roller in one direction or the other, the pressure body  100  is pressed by the first or second pressure roller  128 ,  130 —with the same force as far as possible—against the counter-surface  58  of the friction-drive body  14 , so that the different distance between the pressure points at which the first and second pressure rollers some into contact with the pressure surface  126  is compensated by the swivelling axis  124 . 
         [0075]    In operation, because of the drive of the driving roller  46  and its contact with the drive blade or friction-drive body  14 , the bearing unit  44  is subject to a swivelling moment about the swivelling axis  120  and a more or less slight deflection, so that, depending on the drive direction, the first or second pressure roller  128 ,  130  comes to rest against the pressure surface  126 , and moves the mounting arm  122  about the swivelling axis  124  against the counter-surface  58 , so that both the driving roller  46  and the pressure roller  90  are each pressed with the same orthogonal force against the friction surface  56  or the counter-surface  58  respectively. 
         [0076]    It is expedient for the distance c to be made smaller than the distance d, because the first pressure roller  128  associated with the distance c acts on a shorter lever length of the mounting arm  122  than the second pressure roller  130  associated with the distance d, so that this ultimately ensures that the pressing force of the pressure roller  90  and driving roller  46  is the same for both drive directions of the driving roller  46 , based on the same driving moment of the driving roller in each case. 
         [0077]    The relationships in the transmission of the frictional force in the form of a driving force of the driving roller acting tangentially to the friction surface cannot be equated to normal static or sliding friction, so that Coulomb&#39;s law of friction (friction force as the product of the coefficient of friction and perpendicular force) does not apply unchanged. The tangential transmission of force between a driving roller and a friction surface presupposes that slip occurs between them. The contact area can be divided up into an adhesion region and a sliding region, with the share of the sliding region growing as the driving force increases and the perpendicular force remains constant. 
         [0078]    The behaviour of a friction wheel drive is characterised by the frictional connection/slip curve f t =f(sAR) shown in  FIG. 23 , where the frictional connection describes the ratio of transmitted driving force to perpendicular force, f t =F A /F N , and the slip is defined as the difference in speed between the circumferential speed of the driving roller and the speed of the friction-drive body, based on the circumferential speed of the driving roller: VAR 
         [0000]        S   AR =( V   AR   −V   k )/( V   AR ). 
         [0079]    If the transmitted driving force increases while the perpendicular force remains constant, the slip also increases. When the slip is great, the tangential frictional connection and coefficient of sliding friction are the same, and from this it is possible to establish the theoretically maximum transmissible driving force as the product of the perpendicular force and the coefficient of sliding friction. 
         [0080]    If the frictional connection-slip curve is known for a given pair of materials, then for the permissible boundary slip S AR, zul , the frictional connection f t, zul  corresponding to that slip can be found. This makes it possible to determine the transmissible driving force for a given perpendicular force, which may be limited for design reasons for ex-ample, and that transmissible driving force may perhaps be exceeded for a short time. 
         [0081]    Proportional pressing between the driving roller and the friction surface, which can be achieved with the invention, means a linear increase in the perpendicular force together with the driving force with a constant frictional connection, e.g. f t =0.5, and hence also constant slip. 
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of reference numerals 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  2 
                 Carriage 
               
               
                  4 
                 Chassis 
               
               
                  6 
                 Support roller 
               
               
                  8 
                 Guide roller 
               
               
                 10 
                 Guide rail 
               
               
                 12 
                 Frame 
               
               
                 13 
                 Longitudinal/conveying direction 
               
               
                 14 
                 Drive blade (friction drive body) 
               
               
                 16 
                 Friction wheel drive 
               
               
                 20 
                 Drive motor 
               
               
                 22 
                 Mount 
               
               
                 24 
                 Swivelling axis 
               
               
                 26, 28 
                 Plate 
               
               
                 30, 32 
                 Journal 
               
               
                 34, 36 
                 Mounting brackets 
               
               
                 38 
                 Carriage 
               
               
                 40 
                 First linear guide 
               
               
                 42 
                 Axis of travel 
               
               
                 44 
                 Bearing unit 
               
               
                 46 
                 Briving roller 
               
               
                 48 
                 Axis of rotation 
               
               
                 50 
                 Second linear guide 
               
               
                 52 
                 Axis of travel 
               
               
                 54 
                 Circumferential surface 
               
               
                 56 
                 Friction surface 
               
               
                 58 
                 Counter-surface 
               
               
                 60, 62 
                 Transmission surfaces 
               
               
                 64 
                 Pressing surface 
               
               
                 70, 72 
                 Pivot lever 
               
               
                 74, 76 
                 Swivelling axis 
               
               
                 78 
                 First lever arm 
               
               
                 80 
                 Axis of rotation 
               
               
                 82 
                 Transmission roller 
               
               
                 84 
                 Second lever arm 
               
               
                 86 
                 Transmission roller 
               
               
                 88 
                 Axis of rotation 
               
               
                 89 
                 Plane 
               
               
                 90 
                 Pressure roller (pressure member) 
               
               
                 91 
                 Plane 
               
               
                 92 
                 Axis of rotation 
               
               
                 94 
                 Journal 
               
               
                 96 
                 Tensioning screw 
               
               
                 100  
                 Pressure body 
               
               
                 102  
                 Control surface 
               
               
                 104  
                 Control roller 
               
               
                 106  
                 Apex 
               
               
                 110  
                 Swivelling axis 
               
               
                 112  
                 Control surface 
               
               
                 114  
                 Further swivelling axis 
               
               
                 120  
                 Swivelling axis 
               
               
                 122  
                 Mounting arm 
               
               
                 124  
                 Swivelling axis 
               
               
                 126  
                 Pressure surface 
               
               
                 128  
                 First pressure roller 
               
               
                 130  
                 Second pressure roller 
               
               
                 132  
                 First axis of rotation 
               
               
                 134  
                 Central plane 
               
               
                 136  
                 Second axis of rotation 
               
               
                 a 
                 Distance 80-74, 76 
               
               
                 b 
                 Distance 88-74, 76 
               
               
                 c 
                 Distance 132, 134 
               
               
                 d 
                 Distance 136, 134 
               
               
                 Φ 
                 Angle between 60, 62 and 91