Abstract:
Loading platforms ( 16 ) of tail lifts ( 13 ) are usually raised and lowered or pivoted by hydraulic cylinders. Hydraulic cylinders require a hydraulic unit. Furthermore, there is a risk of contamination through unsealed hydraulic lines. To raise, lower and/or pivot the loading platform ( 16 ) of the tail lift ( 13 ), the invention makes provision to use roller screw mechanisms ( 19 ) which are driven by hollow-shaft electric motors ( 23 ). As a result, hydraulic drives for the loading platform ( 16 ) are no longer required. Each roller screw mechanism ( 19 ) is driven by the hollow-shaft electric motor ( 23 ). The unit consisting of the roller screw mechanism ( 19 ) and the hollow-shaft electric motor ( 23 ) operates in principle like a known hydraulic cylinder and also has approximately the same dimensions, with the result that hydraulic cylinders can be replaced by roller screw mechanisms ( 19 ) with hollow-shaft electric motors ( 23 ) without significant modifications to the tail lift ( 13 ).

Description:
STATEMENT OF RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase Under Chapter II of the Patent Cooperation Treaty (PCT) of PCT International Application No. PCT/EP2007/007501 having an International Filing Date of 28 Aug. 2007, which claims priority on German Patent Application No. 10 2006 053 441.7 having a filing date 12 Sep. 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The invention relates to a lifting loading platform, or tail lift, for attaching to a vehicle, with a loading platform and with a lifting assembly which comprises at least one linear drive and which is intended for raising, lowering and/or pivoting the loading platform. 
         [0004]    2. Prior Art 
         [0005]    Tail lifts are used to facilitate the operations of loading and unloading vehicles, particularly when heavy goods are involved. Such a tail lift has a loading platform which is preferably articulated on the rear side of a body of a vehicle such that it can be raised and lowered and usually pivoted as well. The loading platform is arranged on the rear side of the body of the vehicle by means of a lifting assembly. The lifting assembly comprises at least one linear drive which causes the loading platform to be raised, lowered and/or pivoted. It is customary for the lifting assembly to comprise at least one linear drive for raising and lowering the loading platform and at least one further linear drive for pivoting the loading platform. 
         [0006]    The linear drives of known tail lifts are designed as hydraulic cylinders. Although virtually all tail lifts have to date been provided with hydraulic cylinders for moving the loading platform, they have a major disadvantage which particularly concerns the fact that a hydraulic unit is required for operating the hydraulic cylinders and that relatively thick and bulky hydraulic hoses have to be laid from the hydraulic unit to the hydraulic cylinders. If a leak occurs in a hydraulic hose as a result of damage, for example, then oil escapes, a situation which leads to serious contamination and is also critical from an environmental point of view. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Taking this as the starting point, the object on which the invention is based is to provide a tail lift which has simple, cost-effective and nevertheless reliable linear drives for raising, lowering and/or pivoting the loading platform. 
         [0008]    A tail lift for achieving this object comprises a loading platform and a lifting assembly which comprises at least one linear drive and which is intended for raising, lowering and/or pivoting the loading platform, wherein at least one linear drive comprises a roller screw mechanism. The fact that the respective linear drive comprises a roller screw mechanism makes it possible to dispense with a hydraulic unit and with the laying of hydraulic hoses, since the roller screw mechanism makes possible an electric drive, for example. By contrast with conventional linear drives, in particular screw mechanisms, the roller screw mechanism is distinguished by a smooth-running drive with low inherent friction and thus a high degree of efficiency. Consequently, a roller screw mechanism is very particularly suitable for raising, lowering and/or pivoting the loading platform of a tail lift, even when the loading platform bears heavy loads. In particular, screw mechanisms can be subjected to a high degree of loading in spite of their exceptional smooth-running properties in relation to other screw mechanisms. 
         [0009]    Preferably, the respective roller screw mechanism comprises an axially displaceable pushrod which is provided at least partially with a roller thread. The pushrod acts like the piston rod of a hydraulic cylinder, which means that the respective roller screw mechanism can have external dimensions comparable to those of a hydraulic cylinder, with the result that it is possible without significant modifications to provide the tail lift in the manner according to the invention with roller screw mechanisms. 
         [0010]    The roller screw mechanisms are preferably actuated electrically. Provision is made in particular for the respective roller screw mechanism to be assigned an electric motor. Each roller screw mechanism can be driven electrically in a simple manner. The power can be made available from the already present electrical system of a vehicle which, in the case of known tail lifts, serves to drive the hydraulic unit. A hydraulic unit and bulky hydraulic lines are made superfluous by the electrical drive of the roller screw mechanisms. The electrical lines can be laid easily and in the event of damage do not cause any harm as is the case with the oil loss of hydraulic cylinders or hydraulic lines. 
         [0011]    According to one preferred embodiment, provision is made to use a hollow-shaft electric motor as the electric motor. Such a hollow-shaft electric motor is compact and specifically does not have any dimensions which are larger than those of a hydraulic cylinder which can usually be employed. The respective linear drive is then substantially formed from a roller screw mechanism and a hollow-shaft electric motor. 
         [0012]    When the roller screw mechanism is retracted, a part, preferably a large part, of the pushrod can be retracted into the hollow-shaft electric motor, to be precise in a comparable manner to the retraction of a piston rod into a hydraulic cylinder. 
         [0013]    Through the combination of the hollow-shaft electric motor with the roller screw mechanism, the lifting assembly for raising, lowering and/or pivoting the loading platform preferably comprises drives which approximately correspond to hydraulic cylinders in terms of their shape and their dimensions, with the result that, by virtue of the invention, no significant modifications are required to the overall design of the tail lift, in particular to the lifting assembly. 
         [0014]    To ensure that part of the lifting rod can enter the hollow-shaft electric motor when the pushrod is completely or partially retracted, provision is made to arrange the hollow-shaft electric motor concentrically to the longitudinal center axis of the pushrod and around at least part of the latter. For that purpose, the hollow-shaft electric motor has an outer stator which is articulated on the vehicle, the lifting assembly or on the loading platform so as to be non-rotatable with respect to the longitudinal center axis of the pushrod. The hollow-shaft electric motor further has a preferably cylindrical rotor which is surrounded by the stator and which can be driven rotationally about the longitudinal center axis of the pushrod. The rotor of the hollow-shaft electric motor is thereby situated in a space-saving manner in the stator and surrounds that part of the pushrod which is retracted into the hollow-shaft electric motor. In this arrangement, the rotor can also be used in a simple manner to drive a roller screw nut of the roller screw mechanism. A rotation, produced by the rotor, of the roller screw nut about the longitudinal center axis of the pushrod results in the pushrod being displaced axially along the longitudinal center axis such that, as in the case of a hydraulic cylinder, the pushrod protrudes to a greater or lesser extent from the hollow-shaft electric motor which surrounds it. 
         [0015]    In one preferred embodiment of the invention, provision is made for the rotor to be an integral component of the roller screw nut. This makes it possible to form a particularly compact linear drive for the lifting assembly of the tail lift. Alternatively, it is also conceivable to arrange the rotor of the hollow-shaft electric motor concentrically around the roller screw nut or to place the rotor in front of or behind the roller screw nut. In these cases, the rotor and the roller screw nut are connected to one another non-rotatably, but preferably releasably. Here, the pushrod is guided in a freely movable manner through the rotor of the hollow-shaft electric motor. 
         [0016]    Provision is further made for the roller screw mechanism not to have a self-locking facility, that is to say the roller screw nut can be rotated, if appropriate together with the rotor, when the pushrod provided at least partially with the roller thread is subjected to axial loading. Such a roller screw mechanism is distinguished by being particularly smooth-running with low internal friction. It is thus possible for each drive according to the invention to exert relatively high lifting and/or pivoting forces on the tail gate, it also being possible in particular for the tail gate to lift heavy loads. 
         [0017]    In order to be able to reliably lock the tail gate in the desired height or pivoting position even when it is laden with heavy loads, provision is made for each roller screw mechanism or for the hollow-shaft electric motor serving to drive said mechanism to be assigned at least one brake. The brake is preferably designed in such a way that it automatically assumes the braking position and is released, for example electromagnetically, through a supply of power. This ensures that, in the event of a power failure or an interruption in the power supply, the linear drive is locked in the instantaneous position and the loading platform cannot lower or pivot back unintentionally. It is only when the hollow-shaft electric motor is supplied with power and thereby set in operation that the brake is released, thereby ensuring that the roller screw nut is rotated and the pushrod is retracted and extended as a result. The at least one brake is preferably arranged between the roller screw nut and the stator or rotor of the hollow-shaft electric motor. This arrangement is space-saving and allows an effective engagement of the brake in order to lock the roller screw mechanism. 
         [0018]    In a preferred tail lift, provision is made for the lifting assembly to comprise at least one roller screw mechanism with a hollow-shaft electric motor assigned thereto for selectively raising or lowering the loading platform, and at least one further roller screw mechanism with a hollow-shaft electric motor assigned thereto for pivoting the loading platform. It is possible in this way for the loading platform to be raised or lowered and pivoted by means of independent drives. If required, both drives can be moved together to simultaneously raise or lower and pivot the loading platform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0019]    A preferred exemplary embodiment of the tail lift according to the invention is explained in more detail below with reference to the drawing, in which: 
           [0020]      FIG. 1  shows a perspective view of a rear part of a vehicle with a tail lift, 
           [0021]      FIG. 2  shows a perspective view of a linear drive of the tail lift of  FIG. 1 , and 
           [0022]      FIG. 3  shows a side view of the linear drive of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]      FIG. 1  shows a rear part of a vehicle, specifically a truck  10 . The truck  10  has a body  11  which, in the exemplary embodiment shown, is a so-called box body. The body  11  has a rear side  12  which is at least partially open or which is to be opened. The rear side  12  can preferably be opened or closed by means of doors or the like. 
         [0024]    The rear side  12  of the truck  10  is assigned the tail lift  13  according to the invention. The tail lift  13  has a lifting assembly  14  which is directly or indirectly fastened by means of a transversely directed support tube to a vehicle frame  15  of the truck  10 , this frame being represented only by way of an outline in  FIG. 1 . The tail lift  13  also has a loading platform  16 . The loading platform  16  is articulated on the lifting assembly  14  so as to be able to pivot on a lower (in relation to the representation in  FIG. 1 ) transverse edge  17 . The loading platform  16  can additionally be lowered and raised by the lifting assembly  14 .  FIG. 1  shows the closed position of the tail lift  13 , in which position the loading platform  16  is lifted up by the lifting assembly  14  and pivoted into a vertical position behind the body  11  of the truck  10 . When the truck  10  is in a position of movement, the loading platform  16  is thus situated completely behind the body  11 . 
         [0025]    The lifting assembly  14 , which is represented only in part in  FIG. 1 , comprises two identical link arms  18  which can move synchronously in parallel vertical planes. The link arms  18  can be interconnected by means, for example, of a tube or a beam. The lifting assembly  14  comprises linear drives which serve to pivot the loading platform  16  with respect to the lifting assembly  14  and to raise and lower the lifting assembly  14  together with the loading platform  16  fastened thereon. Depending on the particular load-bearing capacity of the tail lift  13  and/or its design, it is possible to provide one or two linear drives for pivoting the loading platform  16  or likewise one or two linear drives for raising and lowering the loading platform  16 . In the case of the tail lift  13  shown ( FIG. 1 ), only a single linear drive for pivoting the loading platform  16  and a single linear drive for raising and lowering the lifting assembly  14  together with the loading platform  16  are provided.  FIG. 1  shows a linear drive. For example, this can be the linear drive for pivoting the loading platform  16 . With respect to the view in  FIG. 1 , and concealed behind the latter, the second linear drive for raising and lowering the loading platform  16  is preferably arranged on another side of the lifting assembly  14 . 
         [0026]    According to the invention, each of the identically designed linear drives comprises a roller screw mechanism  19 . The roller screw mechanism  19  has an elongate pushrod  20 , which is provided at least partially with a roller thread, and a roller screw nut  21  surrounding the pushrod  20  over a region thereof. The pushrod  20  of the roller screw mechanism  19  is non-rotatably connected to the vehicle frame  15 , the lifting assembly  14  or the loading platform  16 . On the pushrod  20 , specifically the roller thread thereof, the roller screw nut  21 , which is arranged concentrically about the pushrod  20 , can be rotated about a longitudinal center axis  22  of the pushrod  20 . As a result, the pushrod  20  is axially displaced in the direction of its longitudinal center axis  22  with respect to the roller screw nut  21 . There are various possible ways of driving the roller screw mechanism  19 , specifically of rotating the roller screw nut  21 . In the preferred exemplary embodiment of the invention that is shown, the roller screw nut  21  is driven by an electric motor, i.e. rotated about the longitudinal center axis  22  of the pushrod  20 . For this purpose, in the preferred exemplary embodiment of the invention that is shown here, the roller screw mechanism  19  is assigned an electric motor, to be precise preferably a hollow-shaft electric motor  23 . In the preferred exemplary embodiment of the invention, each of the identical linear drives thus comprises a roller screw mechanism  19  and a hollow-shaft electric motor  23 . 
         [0027]    The hollow-shaft electric motor  23  and the roller screw mechanism  19  are arranged coaxially one behind the other, with the result that the roller screw mechanism  19  and the hollow-shaft electric motor  23  lie on the same axis, specifically the longitudinal center axis  22  of the pushrod  20 . Because the roller screw mechanism  19 , which serves to form the linear drive according to the invention, and the hollow-shaft electric motor  23  are situated coaxially one behind the other, the linear drive according to the invention has a shape which corresponds to that of the hydraulic cylinders which have been conventionally used to date. In this respect, the hollow-shaft electric motor  23  can be considered to be the cylinder of the hydraulic cylinder, while the pushrod  20  of the roller screw mechanism  19  corresponds to the piston rod of a hydraulic cylinder. 
         [0028]    The hollow-shaft electric motor  23  has an outer stator  24  which is non-rotatably connected to the vehicle frame  15 , the support tube, the lifting assembly  14  or the loading platform  16 . The stator  24  has a cylindrical cavity which is open on one side and in which a cylindrical rotor  25  is situated in a rotatable manner. The hollow-shaft electric motor  23  is assigned a brake  26  which is arranged in front of that end of the hollow-shaft electric motor  23  which faces the pushrod  20 . Connected to the brake  26  in turn is the roller screw nut  21 . In the exemplary embodiment of the invention that is shown, the brake  26  is thus arranged between the hollow-shaft electric motor  23  and the roller screw nut  21 . 
         [0029]    The brake  26  has a fixed outer part  27  which cannot be rotated about the longitudinal center axis  22  and which is connected to the stator  24  of the hollow-shaft electric motor  23 , specifically fastened in front of the stator  24 , by the outer part  27  of the brake  26  and the stator  24  of the hollow-shaft electric motor  23  being screwed together, for example, at their mutually directed end faces. Situated coaxially in the outer part  27  of the brake  26  is an inner part  28  thereof which is non-rotatably connected to the roller screw nut  21  by the inner part  28  of the brake  26  and the roller screw nut  21  of the roller screw mechanism  19  being preferably screwed together at their mutually directed end faces. Moreover, the inner part  28  of the brake  26  is non-rotatably connected to the end face of the rotor  25  of the hollow-shaft electric motor  23 . The rotor  25  of the hollow-shaft electric motor  23 , the inner part  28  of the brake  26  and the roller screw nut  21  are thus connected to form a unit whose parts cannot rotate relative to one another. 
         [0030]    When the brake  26  is released, the roller screw nut  21  can be rotationally driven about the longitudinal center axis  22  of the pushrod  20  by the rotationally driven rotor  25  of the hollow-shaft electric motor  23  via the inner part  28 , which is rotatable therewith, of the brake  26 . As a result of the roller screw nut  21  being rotationally driven about the non-rotatable pushrod  20 , said pushrod is extended or retracted depending on the direction of rotation of the roller screw nut  21 , and consequently the length of the roller screw mechanism  19  is increased or shortened. That part of the pushrod  20  by which the length thereof is shortened with respect to the roller screw nut  21  is accommodated inside the hollow-shaft electric motor  23 . For this purpose, the rotor  25  of the hollow-shaft electric motor  23  is provided with a cylindrical passage  29  whose inside diameter is slightly larger than the maximum outside diameter of the threaded portion of the pushrod  20 . The same applies to the inner part  28  of the brake  26 . By contrast, the roller screw nut  21  is in threaded engagement with the external thread of the pushrod  20 . For this purpose, a plurality of elongate threaded rollers  30  having a relatively small diameter are distributed over the inner circumference of the roller screw nut  21 . The threaded rollers  30  have a continuous external thread which is designed to correspond with the roller thread on the outer circumference of the pushrod  20 , with the result that the thread of the threaded rollers  30  meshes with the thread of the pushrod  20 . By rotating the roller screw nut  21 , the threaded rollers  30  are also rotated about their longitudinal axes which extend parallel to the longitudinal center axis  22  of the pushrod  20 , and in so doing the pushrod  20  is moved axially through the roller screw nut  21 , specifically into or out of the hollow-shaft electric motor  23  depending on the direction of rotation of the roller screw nut  21 . 
         [0031]    The free end of the pushrod  20  that is directed away from the hollow-shaft electric motor  23  is designed as a fastening end  31  which is provided with a transversely directed through-hole  32 . Hence, the pushrod  20  is connected non-rotatably but in an articulated manner to the loading platform  16  or to the lifting assembly  14 , specifically to the free end which faces the loading platform  16 . Likewise, the stator  24  of the hollow-shaft electric motor  23  is provided at its side facing away from the pushrod  20  with a fastening end  33  which also has a through-hole  34 . The fastening end  33  is used to connect the hollow-shaft electric motor  23  non-rotatably but in an articulated manner to a bearing bracket for the lifting assembly  14  on the vehicle frame  15 , in particular the support tube. 
         [0032]    The linear drive shown here comprises the brake  26  because the roller screw mechanism  19  is designed not to be self-locking. Consequently, when loading the loading platform  16 , in particular with a relatively large load, the resulting application of an axial force to the pushrod  20  causes the roller screw nut  21  to rotate. In order to prevent this and thus reliably lock, the loading platform  16  in the set height and inclination, the roller screw nut  21  is prevented from an unwanted rotation by the brake  26 . For this purpose, the brake  26  must only apply a relatively small braking force since only a relatively small force acts in the circumferential direction of the roller screw nut  21 . Consequently, only a small braking force is required in order to prevent an automatic change in the length of the roller screw mechanism  19  when the loading platform  16  is loaded with heavy goods. 
         [0033]    The brake  26  is preferably designed in such a way that it brakes and thus non-rotatably maintains the roller screw nut  21  and/or the rotor  25  of the hollow-shaft drum motor  23  if no power is available at the brake  26  to actuate it. It is only when the brake  26  is disengaged, for example electrically, that it frees the roller screw nut  21  to rotate, with the result that the roller screw nut  21  can then be rotated on the pushrod  20  by the hollow-shaft electric motor  23  in order to change the length of the roller screw mechanism  19 , thus leading to the loading platform  16  of the tail lift  13  being correspondingly pivoted and/or raised or lowered. 
         [0034]    The brake  26  is preferably actuated electrically; however, it can also be actuated in other ways, for example pneumatically or mechanically. 
         [0035]    An electric brake  26  has the advantage that, when the hollow-shaft electric motor  23  is supplied with electric power in order to change the length of the pushrod  20 , the brake  26  is also supplied with electric power, and hence the brake  26  is disengaged and the rotor  25  of the hollow-shaft electric motor  23  can rotationally drive the roller screw nut  21 . As a result, and depending on the particular direction of rotation of the hollow-shaft electric motor  23 , the pushrod  20  is either pushed further along the longitudinal center axis  22  into the cylindrical passage  29  in the rotor  25  of the hollow-shaft electric motor  23  or moved out of the passage in the rotor  25  of the hollow-shaft electric motor  23 . If the rotation of the rotor  25  of the hollow-shaft electric motor  23  is halted due to an interruption in the power supply, this leads to the brake  26  being locked automatically, whereby the roller screw nut  21  is immobilized against rotation, and as a result, even when the loading platform  16  of the tail lift  13  is loaded, the pushrod  20  of the roller screw mechanism  19  cannot be unintentionally retracted into or extended out from the hollow-shaft electric motor, and, consequently, the respective length of the linear drive according to the invention cannot be changed. 
       List of Reference Numbers 
       [0000]    
       
           10  truck 
           11  body 
           12  rear side 
           13  tail lift 
           14  lifting assembly 
           15  vehicle frame 
           16  loading platform 
           17  transverse edge 
           18  link arm 
           19  roller screw mechanism 
           20  pushrod 
           21  roller screw nut 
           22  longitudinal center axis 
           23  hollow-shaft electric motor 
           24  stator 
           25  rotor 
           26  brake 
           27  outer part 
           28  inner part 
           29  passage 
           30  threaded roller 
           31  fastening end 
           32  through-hole 
           33  fastening end 
           34  through-hole