Abstract:
An electrical drive, especially for driving a windshield wiper system of a motor vehicle, comprises a worm gear having a worm shaft and a drive motor having a rotor. The rotor and the worm gear are arranged on axial sections of a shaft. The electrical drive furthermore comprises two shaft bearings on which the shaft is received, only one of said shaft bearings being arranged on the shaft in the vicinity of the rotor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Electrical drives, for example, for use in motor vehicles frequently comprise an electrical drive motor and a reduction gear or a countershaft gearbox. By means of a relevant adaptation of drive motor and gearing to one another, electrical drives in different power ranges, comprising different usable rotational speeds and torques and in different exterior dimensions can be produced. 
         [0002]    In one option, which, for example, is used to drive a windshield wiper unit of the motor vehicle, a rotor of the electrical drive motor and a worm shaft of a worm gear run on a common shaft. The common shaft is normally supported with shaft bearings on both sides of the electrical drive motor. In one modification, a third bearing exists on the end of the common shaft, which faces the worm shaft. The gearing engagement between the worm shaft and the worm wheel is thereby additionally supported and is no longer solely dependent on the torsional stiffness of the worm shaft; however, vibrational and torsional stresses of the rotating shaft can be more easily transmitted via the medial bearing through the use of the three shaft bearings. 
         [0003]    It is the aim of the invention to provide an electrical drive which has an improved bearing support of the shaft thereof. 
       SUMMARY OF THE INVENTION 
       [0004]    According to the invention, an electrical drive comprises a worm gear having a worm shaft and a drive motor having a rotor. The rotor and the worm shaft are arranged on axial sections of a shaft. The electrical drive furthermore comprises two shaft bearings on which the shaft is received, only one of said shaft bearings being arranged on the shaft in the vicinity of the rotor. 
         [0005]    The opportunity advantageously ensues therefrom for installation space to be saved and for a distribution of shaft bearings on the shaft to be implemented such that the bearings are subjected to less stress due to shorter levers and can therefore be expected to have a longer service life. 
         [0006]    The shaft bearing which is not in the vicinity of the rotor can be arranged on a side of the worm shaft facing away from the rotor. Two alternative layout possibilities result therefrom for the shaft bearing arranged on the shaft in the vicinity of the rotor. In a first embodiment, said shaft bearing lies between the rotor and the worm shaft. The end of the worm shaft on which the rotor is arranged is thereby supported only on one side (also: “cantilevered” or “overhung”), and therefore the required installation space for the electrical drive can be reduced. In the second embodiment, the bearing can be arranged on a side of the rotor facing away from the worm shaft. In so doing, the shaft is supported at both ends thereof, whereby an advantageous reduction of leverage forces result when loads are applied to the shaft during the operation of the electrical drive. 
         [0007]    The drive motor can be a brushless DC motor. A motor of this kind requires less installation space along the shaft. This can lead to a further reduction in installation space for the electrical drive. 
         [0008]    The electrical drive can further comprise a housing, in which the shaft bearings and a stator of the drive motor are arranged. A further reduction in installation space can be achieved by integrating the stator into the housing. In addition, an improved protection of the drive motor from contamination and vibration can thereby be achieved. 
         [0009]    A sensor for determining a rotatory position of the rotor in the housing can furthermore be mounted in the housing. A sensor of this kind can particularly be used in connection with a brushless DC motor as drive motor in order to implement an electrical control of said brushless DC motor. The sensor is protected by the housing from harmful environmental influences, such as heat, vibrations and dust. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention is described in detail below with reference to the accompanying figures, in which: 
           [0011]      FIG. 1  shows an electrical drive having a brushless electric motor; 
           [0012]      FIG. 2  shows a modification to the electrical drive from  FIG. 1  and 
           [0013]      FIG. 3  shows an electrical drive having a commutated electric motor. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows an electrical drive  100  having a brushless electric motor. The electrical drive  100  comprises a housing  110  in which the components of the electrical drive  100  are received. A worm shaft  120  and a worm wheel  130  together form a worm gear  140 . A rotor  150  and a stator  160  together form a drive motor  170 . The rotor  150  and the worm shaft  120  are arranged axially one behind the other on a shaft  175 . A first shaft bearing  180  is arranged on the shaft  175  between the rotor  150  and the worm shaft  120 . A second shaft bearing  185  is situated on the right end of the shaft, on a side of the worm shaft  120  which faces away from the rotor  150  of the drive motor  170 . A first position sensor  190  and a second position sensor  195  detect a rotatory position, a rotational speed and/or a rotational direction of the rotor  150 . 
         [0015]    The shaft  175  is normally manufactured from steel. The first shaft bearing  180  and the second shaft bearing  185  can, for example, be roller bearings, in particular ball bearings or even friction (slide) bearings as, for example, self-lubricating bearings. The worm shaft  120  can be integrally embodied with said shaft  175  and the worm thread can be rolled onto said shaft  175  or cut into said shaft  175 . In a further embodiment, the worm thread can be a separate element connected axially or radially to said shaft  175 . The worm shaft  120  can thereby consist of a different material than the shaft  175 , in particular plastic. The material of the worm wheel  130  is selected as a function of the material properties of the worm shaft  120  and the forces to be expected during the operation of the electrical drive  100 . The worm wheel  130  can also be manufactured from plastic. 
         [0016]    The drive motor  170  is a brushless DC motor having an internal rotor  150 . This type of electric motor can require less installation space, particularly in the axial direction, in comparison with a commutated DC motor in a comparable power range. At the same time, the space requirement in the radial direction can be enlarged with respect to the commutated DC motor. The rotor  150  of the drive motor  170  can, for example, be pressed or shrunk onto the shaft  175  or connected in another manner to said shaft. Said rotor  150  carries a number of permanent magnets and if applicable an inference ring, and the stator  160  carries a number of coil windings for generating interacting magnetic fields. Depending on the electrical activation of the coil windings, the permanent magnets of the rotor strive to align themselves into a certain rotatory position with respect to the stator. When the coils of the stator  160  are electrically activated in a suitable manner, the rotor  150  rotates about an axis of rotation of the shaft  175  in a predefined direction at a predefined speed. 
         [0017]    In order to be able to implement the activation of the stator  160  as a function of a rotatory position of the rotor  150 , the rotatory position of said rotor  150  can be determined. The first position sensor  190  and/or the second position sensor  195  can, for example, be used for this purpose. Installation positions, which are different from those depicted, for the position sensors  190  and  195  between said rotor  150  and the housing  110  are likewise possible and not depicted in  FIG. 1 . 
         [0018]    The coils of the stator  160  are activated during operation of the electrical drive  100  on the basis of the determined rotatory position of the rotor  150  such that the rotor  150  rotates and drives the shaft  175 . Radial and axial forces on said shaft  175  are supported by the shaft bearings  180  and  185  on the housing  110 . Said shaft  175  drives the worm shaft  120  which thereupon moves the worm wheel  130  about the axis of rotation thereof. 
         [0019]    Through the use of the worm gear  140 , the electrical drive  100  is designed in a self-locking manner; and therefore when the drive motor  170  is switched off, an external torque acting on the worm wheel  130  is not capable of causing the rotor  150  to rotate. 
         [0020]      FIG. 2  shows a modification to the electrical drive from  FIG. 1 . The essential difference between the electrical drive  100  from  FIG. 2  and the electrical drive from  FIG. 1  is that the first shaft bearing  180  is arranged in  FIG. 2  at a left end of the shaft  175  instead of between the worm shaft  120  and the rotor  150  as in  FIG. 1 . It could thereby be necessary to design the electrical drive according to  FIG. 2  slightly longer along the shaft  175  than the electrical drive  100  from  FIG. 1 . On the other hand, the arrangement of the first shaft bearing  180  shown in  FIG. 2  has the advantage of supporting the shaft  175  in a more precise and resilient manner on account of the extended distance between said first shaft bearing  180  and the second shaft bearing  185 . In addition, flexural vibrations in the shaft  175  are not transferred by said first shaft bearing  180 , and therefore a resonance frequency of said shaft  175  is reduced with respect to the flexural vibrations. 
         [0021]      FIG. 3  shows an electrical drive  100  having a commutated electric motor. The embodiment of the electrical drive  100  depicted in  FIG. 3  is used for comparison with the electrical drives  100  from  FIGS. 1 and 2 . The drive motor  170  is commutated, i.e. brushes  310  are provided, in order to activate the coils in the interior of the drive motor  170  as a function of a rotatory position of the shaft  175 . Position sensors  190  and  195  from  FIGS. 1 and 2  are not required for this purpose. 
         [0022]    The first shaft bearing  180  is situated at a left end of the shaft  175  and is supported at an outer shell  320  of the drive motor  170 . The second shaft bearing  185  is arranged on the shaft  175  between the drive motor  170  and the brushes  310 . 
         [0023]    Because the commutated drive motor is constructed as a matter of the principle involved relatively long along the shaft  175  and due to the additional space requirement for the brushes  310 , a displacement of the electrical drive  100  in the axial direction is greater than that of the electrical drives  100  pursuant to  FIGS. 1 and 2 . Furthermore, the distance between the right end of the worm shaft  120  and the nearest shaft bearing  185  is greater than in the electrical drives  100  pursuant to  FIGS. 1 and 2 , whereby the shaft  175  has to be formed more rigidly to achieve the same load bearing capacity.