Abstract:
Gear mechanism ( 10 ), in particular for adjusting moveable parts in a motor vehicle, comprising a spur wheel ( 14 ) which is provided with external teeth ( 16 ) and meshes with an internal gear ( 18 ) that is provided with internal teeth ( 20 ), wherein the number of internal teeth ( 20 ) to generate a certain gear step-up ratio is greater by at least one than the number of external teeth ( 16 ) and the spur wheel ( 14 ) and the internal gear ( 18 ) perform an eccentric movement relative to one another, wherein the eccentric movement is directed exclusively by means of the matching tooth geometry of the internal and external teeth ( 20, 16 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a gear mechanism, in particular for adjusting moveable parts in a motor vehicle. 
   An eccentric toothed wheel gear mechanism is known from EP 0 981 696 B1 in which an eccentric wheel embodied as an internal gearwheel is positioned on an eccentric, which is put into rotation by a drive element embodied as an armature. Arranged within the internal gearwheel is a carrier with external teeth, wherein the external teeth cooperate with the internal teeth of the internal gear by meshing in sections so that a reduced output moment can be gripped by the carrier. The lower efficiency of this type of gear design, which is caused by the friction between the gearing and the bearing of the eccentric wheel, has proven to be disadvantageous, particularly in the case of high step-up ratios. In addition, narrow tolerances must be complied with when manufacturing such a gear mechanism, because, on the one hand, jamming of the gearing from the overdetermined bearing and too much play in the gearing, on the other hand, must be avoided. 
   SUMMARY OF THE INVENTION 
   The gear mechanism in accordance with the invention has the advantage that, because of directing the eccentric movement by means of the tooth geometry of the internal and external teeth, an eccentric, on which the spur wheel or the internal gear is positioned in the case of conventional eccentric gears, can be dispensed with. As a result, the bearing of the two toothed wheels that are moved eccentrically towards each other is no longer overdetermined, thereby considerably reducing the friction arising from the bearing in accordance with the invention of the spur wheel or the internal gear. Thus, the efficiency of this type of gear mechanism can be increased considerably in that directing the eccentric movement when mutually rolling off of both gearings is forced exclusively by the mutual gear meshing in accordance with the invention. With such an eccentric-less embodiment of the wobble gear, the very expensive precise manufacturing of the eccentric bearing is eliminated. 
   In order to avoid additional friction from the bearing of the spur wheel, it is connected to the drive element or driven element so that is radially moveable in such a way that the spur wheel can follow the eccentric movement forced by the tooth geometry with minimal friction losses. In doing so, the driving torque or output moment is transmitted in a practically undisturbed manner by the drive element to the driven element. 
   Depending upon the design of the gear mechanism, the internal gear can be driven instead of the spur wheel, wherein the reduced output moment can then be gripped by the spur wheel. Thus, the spur wheel or alternatively the internal gear can be coupled radially flexibly on the corresponding drive element or driven element in order to increase efficiency. 
   An elastic element, which connects the spur wheel or the internal gear to the drive element or the driven element, has proven to be especially advantageous for such a coupling. The elastic element can be embodied as an elastomer for example, whose shape and material properties permit a radial deflection, but is embodied to be relatively rigid against torsion. This type of coupling does not have any mechanical friction surfaces so that efficiency and service life are quite high. 
   In a preferred embodiment, the spur wheel is embodied as an eccentric wheel, which is prevented from rotating around the drive axis via cooperation with housing-mounted guide elements. In this case, the output moment can be gripped directly by the rotatable internal gear. 
   In an alternative design, the spur wheel is arranged so it can freely rotate within a housing-mounted internal gear, thereby achieving a more compact construction. In this case, output takes place advantageously via a carrier, which engages in corresponding receptacles of the spur wheel. 
   If the drive element is embodied as an armature shaft of an electric motor, the internal gear or the spur wheel can be arranged on the motor shaft directly radially free-moving and be coupled with it radially flexibly. In this case, the gear mechanism can be arranged in the motor housing in an especially space-saving manner. 
   In another design, the spur wheel is rotatably mounted on a bridge and arranged within a rotationally secured internal gear. The output takes place in this case via a second internal gear with a different number of teeth and which is positioned on the driven element so that it has free movement radially and executes an eccentric movement vis-à-vis the spur wheel. 
   In order to direct the spur wheel on an eccentric movement vis-à-vis the internal gear and prevent radial displacement of both wheels against each other, it is advantageous to embody the outside diameter of the external teeth of the spur wheel to be greater than the inside diameter of the internal teeth of the internal gear. 
   In order to accomplish directing the eccentric movement without a bearing of the internal gear or the spur wheel on an eccentric, the gear teeth are formed as involute toothing or cycloidal pinion tooth gearing with corresponding tooth geometry in accordance with the invention. 
   If the spur wheel or the internal gear is positioned on the drive shaft or the driven shaft so that it has free movement radially, then jamming of the gearing can be minimized and manufacturing of the gearing can take with broader tolerances. 
   If, when there is meshing in sections of the external teeth in the internal teeth, the spur wheel is no longer displaced radially against the internal gear due to the tooth geometry, then both wheels are directed towards one another on an eccentric movement via the rotational drive, which results in a corresponding stepping down as a function of the difference in the number of teeth. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Different exemplary embodiments of a gear mechanism in accordance with the invention are depicted in the drawings and explained in greater detail in the following description. The drawings show: 
       FIG. 1  A schematic depiction of a spur wheel that is positioned so that it has free movement axially. 
       FIG. 2  Another eccentric gear mechanism with a radial elastic coupling. 
       FIG. 3  Another gear mechanism with a fixed and a rotatable internal gear. 
       FIGS. 4 and 5  An illustration in accordance with the invention of the tooth geometry of involute toothing and cycloidal pinion tooth gearing. 
   

   DETAILED DESCRIPTION 
     FIG. 1  schematically shows a step-down gear mechanism  10  with a drive element  12 , on which a spur wheel  14  with external teeth  16  is positioned so that it has free movement radially. The spur wheel  14  is arranged within an internal gear  18  with internal teeth  20  and has receptacles  22 , which cooperate with housing-mounted locking elements  24  in order to prevent the spur wheel  14  from rotating. The internal gear  18  is connected to a driven element  26 , which is mounted so that it can freely rotate. If driving torque (depicted by arrow  13 ) now acts on the drive element  12 , it is set into rotation around an axis  28 . The spur wheel  14  is connected to the rotating drive element  12  by means of a torsion-proof, but radially free-moving, coupling  30  (depicted by arrow  30 ) and due to the teeth engagement of the external teeth  16  and the internal teeth  20  with a special tooth geometry is forced into an eccentric movement vis-à-vis the internal gear  18 . Since a self-rotation of the spur wheel  14  is prevented by the locking elements  24  arranged on the housing  25 , the internal gear  18  positioned on the axis  28  is set into rotation with driven element  26 , whereby the step-down ratio corresponds to the difference in the number of teeth between the external teeth  16  and the internal teeth  20 . As a result, the output moment  27  depicted by arrow  27  is available on the driven element  26 . It must be specially emphasized that in this case the spur wheel  14  is not positioned on an eccentric, which would force the spur wheel  14  on an eccentric path; rather, the eccentric movement is produced exclusively as a result of the special tooth geometry of the external teeth  14  and the internal teeth  18 , initiated by rotation moment  13 . 
   A concrete execution of a radially free-moving coupling  30  is depicted in another exemplary embodiment of an eccentric gear mechanism in  FIG. 2 . In this case, the spur wheel  14  is connected to a drive element  12  embodied as a drive shaft  32  by means of an elastic element  34 . The rotation moment  13  is transmitted approximately slip-free to the spur wheel  14 , but remains freely moveable radially vis-à-vis the drive shaft  32  within the housing-mounted internal gear  18 . 
   If the drive element  12  is set into rotation, the spur wheel  14  in this design also executes a self rotation in addition to the eccentric movement forced by the tooth geometry because no locking elements  24  are attached. However, the spur wheel  14  features meshing elements  38 , in which the corresponding counter-elements  40  of a carrier  42  positioned on the axis  28  engage. Because of the play between the meshing elements  38  and the corresponding counter-elements  40 , the carrier  42  executes a uniform rotation around the axis  28  and makes an output moment  27  available on the driven element  26 , which is embodied as a driven shaft  33  for example. 
   In another exemplary embodiment in accordance with  FIG. 3 , the drive element  12  has a bridge  44 , on which the spur wheel  14  is rotatably mounted. The drive element  12  in this case is positioned radially fixed on the gear axis  28  so that when driving torque  13  acts, the spur wheel  14  rolls off uniformly in a first housing-mounted internal gear  46  with internal teeth  48 . The second internal gear  18  with the internal teeth  20  is arranged axially offset so it can freely rotate and connected to an output shaft  33  positioned on the axis  28  via a radially elastic coupling  30 . Because of the tooth geometry of the intermeshing external teeth  16  and the internal teeth  20 , the internal gear  18  executes an eccentric movement, which is converted into a uniform rotation of the driven shaft  33  via the radially flexible coupling  30 . In contrast to the exemplary embodiment in  FIG. 2 , in this case the actuation of the spur wheel  14  occurs via a radially rigid bearing and the output via the radially elastically positioned internal gear  18 , which executes an eccentric movement superimposed for rotation. 
     FIG. 4  depicts an enlarged representation of the external teeth  16  of the spur wheel  14  and the internal teeth  20  of the internal gear  18  using the example of involute toothing  49 . If, for example, the internal gear  18  is positioned radially rigidly and the spur wheel  14  that is put into rotation is positioned via an elastic coupling  30  so that it has free movement radially, then the spur wheel  14  executes an eccentric movement just because of the tooth geometry of the external teeth  16  and the internal teeth  20 . The “snapshot” shows the maximum gear meshing with the maximum power transmission at the three o&#39;clock position  50 . If the spur wheel  14  is rotated by the driving torque  13  in a clockwise direction, the teeth  52  of the external gearing  16  are pressed into the tooth spaces  54  of the internal teeth  20 , as depicted by arrow  56  in the six o&#39;clock position  58 . In the process, the tooth tips  60  glide radially along the tooth flanks  62  so that the spur wheel  14  is also forced to rotate on an eccentric path. In  FIG. 4 , the external teeth  16  have a greater outside diameter  64  than the inside diameter  66  of the internal teeth  20 . The tooth geometry of the external teeth  16  and the internal teeth  20  is formed in this case so that the spur wheel  14  cannot be displaced radially vis-à-vis the internal gear  18 ; rather, a radial movement can only occur in connection with a rotation of the spur wheel  14 . This type of eccentric guidance replaces the rotatable bearing of the spur wheel  14  on an eccentric arranged rotationally secured on the drive shaft  32 . Because of the radially free-moving bearing of the spur wheel  14  on the drive shaft  32 , the bearing of the gear mechanism  10  is no longer overdetermined so that bearing friction and jamming due to the eccentric are avoided. In this case, the dipping into one another of the teeth  52  of the internal gear  18  and the spur wheel  14  are used to absorb the reaction forces and specify the path of the spur wheel  14 . In addition, the directing forces applied by the gearing  16 ,  20  for the eccentric movement between the spur wheel  14  and the internal gear  18  act on the same diameter so that the resulting frictional forces are considerably lower than with an eccentric bearing. However, the friction in the case of directing the eccentric movement in accordance with the invention is determined by means of the tooth geometry essentially by a compromise between avoiding jamming and minimizing the play between the two gearings  16 ,  20 . The lower the difference in the number of teeth between the spur wheel  14  and the internal gear  18 , the simpler it is to build up appropriately functioning tooth geometry for directing the eccentric movement. 
   As an example for tooth geometry in accordance with the invention for an involute toothing, the spur wheel  14  has a tooth number of  30 , a real pitch module of 2 mm, a pressure angle of 30°, a tip circle of 62.859 mm, a root circle of 55.13 mm, an addendum modification coefficient of 0.0825 and an axis distance (eccentricity) of −2 mm. The internal gear  18  has a tooth number of −32, a real pitch module of 2 mm, a pressure angle of 30°, a tip circle of −60.83 mm, a root circle of −68.559 mm, an addendum modification coefficient of 0.0825 and an axis distance (eccentricity) of −2 mm. Since both gearings  16 ,  20  cannot be displaced radially against one another, the two gearwheels can only be slid axially into each other in order to produce gear meshing. With such an arrangement, directing the eccentric movement takes place exclusively by means of the tooth geometry. The tooth flanks  62  of one gearing  20  force the tooth tips  60  of the other gearing  16  into the corresponding tooth space  54 . As a result, the gear mechanism  10  is embodied to be eccentric-less. In this connection, neither the spur wheel  14  nor the internal gear  18  are guided by means of an eccentric, but merely arranged to be radially free-moving on the drive shaft or the driven shaft  32 ,  33 . 
     FIG. 5  shows another execution of the gearing  16 ,  20  in accordance with the invention as cycloidal pinion tooth gearing  68 , wherein the internal gear  18  features cylindrical rolls  70  as internal teeth  20 , which are embodied either as freely rotating sleeves  72  or formations  74  fixed to the internal gear. The spur wheel  14  features several circular recesses  76 , which cooperate for example with housing-mounted locking elements  24  or with counter-elements  40  of a carrier  42 . Like the involute toothing  49  in  FIG. 4 , the spur wheel  14  that is set into rotation is forced on an eccentric path because of the tooth geometry. The maximum moment transmission occurs in this case in the 12 o&#39;clock position  53 . The eccentric-less forced directing of the tooth tips  60  along the tooth flanks  62  that are shaped like a segment of a circle is depicted in turn by arrow  56 . 
   It must be noted that, with respect to the exemplary embodiments depicted in all the figures and in description, diverse combination possibilities of the individual features among one another are possible. In particular, the concrete designs of the gear mechanisms (eccentric gear, planetary gear), the formation of the gearing, the designs of the drive element and driven element  12 ,  26  can be varied at will. Essential in this case is just that the eccentric movement of the spur wheel  14  vis-à-vis the internal gear  18  is directed by the tooth geometry of the external and internal teeth  16 ,  20  so that the spur wheel  14  or the internal gear  18  can be arranged so that it has free movement radially vis-à-vis the gear axis  28 . In this case, the radially flexible coupling  30  can be executed as desired. The step-down gear mechanism  10  preferably finds application for adjusting seat parts or for a windshield wiper drive in combination with an electric motor, but can also be used for any other drives.