Abstract:
A brush device for an electric machine having at least one brush and at least one element such as a helical, scroll, or plunger spring for pressing each brush against a rotating contact element, for example a commutator. In at least one further element for example a spring tab, counteracts a lifting of each brush away from the contact element. To this end, the spring tab is inclined at an angle, as a result of which it exerts a weaker action when the brush moves in the direction toward the contact element than it does counter to this direction. As a result, the brush can be repositioned, but is prevented from lifting away from the contact element. This also reduces the danger of vibrations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application Ser. No. of PCT/DE 2004/001007 filed on May 14, 2004. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is directed to an improved brush device for an electrical machine. 
   2. Description of the Prior Art 
   A brush device of the type with which this invention is concerned has at least one brush and at least one element for pressing the brush against a preferably rotating contact element. As a rule, the pressing element is a helical spring or a scroll spring. The contact element can be a commutator or a slip ring. The pressing element should reposition the brush as it wears down and prevent it from lifting off, for example due the contact element being out of round. The more powerful the pressing element is designed to be, the less danger there is of a liftoff. But this also increases the wear. Finally, the necessary play in the guidance of the brush also permits the occurrence of undesirable vibrations due to fluctuations in the coefficient of friction. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   The brush device for an electrical machine according to present invention has the advantage that it reduces the danger of the brush lifting away from the contact element. It permits a spring for pressing the brush to be embodied in a weaker form, which has a positive effect on the wear. It is also possible to reduce vibrations due to the required play in the guide. A more exact brush positioning is thus possible, which results in a more precise commutation and reduced noise. To this end, the brush device for an electric machine, which has a brush and at least one element for pressing the brush against a preferably rotating contact element, has an additional element that counteracts a lifting of the brush away from the contact element. In this case, the at least one element is preferably clamped at an end oriented away from the brush and rests against the brush with its other end. It is advantageous if, between the ends of the at least one element, at least one first section is provided, which exerts a more rigid action counter to the pressing direction than it does in the pressing direction, i.e. the at least one element provides more friction against the brush when it attempts to move away from the commutator than when the brush moves toward the commutator. 
   If noncircularities of the contact element require a certain residual flexibility, then a second section is provided between the at least one first section and the clamped end; this second section, at least in the direction counter to the pressing direction, exerts a weaker action than the at least one first section and preferably exerts the same elastic action both in and counter to the pressing direction. 
   If the brush-pressing element engages the brush obliquely in relation to the pressing direction, then the brush is less likely to vibrate in its guide. 
   In an inexpensive embodiment, the at least one pressing element is embodied as integrally connected to the at least one element. 
   If the contact element has only one rotation direction, then it is advantageous to situate the at least one element on the side of the brush oriented away from the rotation direction of the contact element. If the contact element has two different rotation directions, then it is advantageous for the at least one element to rest against a side of the brush situated in a plane with the rotation direction of the contact element. However, the element can also rest against both of the sides mentioned above. 
   In a preferred embodiment form, the at least one element is a spring tab; the spring tab rests with one edge against the brush and a section adjoining this is inclined in the direction of the contact element. In this way, the at least one element lends more friction against the brush moving away from the commutator than for the brush to move towards the commutator. 
   In an advantageous modification, the at least one element is a spring tab and the at least one pressing device is a scroll spring. In this connection, it is cost-effective for an element embodied in the form of a spring tab and a scroll spring for pressing the brush against the contact element to be connected to each other by means of a bracket. 
   An electric machine equipped with a brush device according to the present invention has the advantage that it can last longer because the brushes wear down less quickly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will become apparent from the detailed description contained herein below, taken in conjunction with the drawings, in which: 
       FIG. 1  shows a cross section through a brush device according to the invention in an electric machine, 
       FIG. 2  shows a second, modified brush device, 
       FIG. 3  shows a third, modified brush device, 
       FIG. 4  shows a fourth, modified brush device, 
       FIG. 5  shows a longitudinal section through a fifth, modified brush device, and 
       FIG. 6  shows a longitudinal section through a sixth brush device, 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a simplified, cross-sectional depiction of a part of an electric machine  10  more particularly a brush-equipped DC motor that can be used, for example, in a power window, a wiper drive unit, etc. of a motor vehicle. It can, however, also be a generator. 
   For the case in which it is a brush-equipped DC motor, the shaft  12  of the electric machine  10  supports a commutator  14 . In the case of a generator, this would be a slip ring. In both cases, however, it is a contact element that preferably rotates. 
   A brush device  16 . 1  is also shown in the drawing. The brush device  16 . 1  has the required number of brushes  18 , but has at least one brush  18  and at least one sliding contact. The brush  18  is made of carbon or another suitable material; it is contained in a guide  20  in a sliding fashion with play; the guide  20  is oriented toward the shaft  12  of the electric machine  10  and its two side walls  21 . 1  and  21 . 2  are visible in the drawing. The guide  20  is preferably part of a brush holder not shown in further detail. An arrow that points toward the shaft  12  indicates the pressing direction  22  in  FIG. 1 . The brush device also has a helical spring  24 , which constitutes an element that presses the brush  18  against the commutator  14 . The helical spring  24  can rest against a limit of the guide  20  at its end oriented away from the commutator  14 . 
   In addition to the helical spring  24 , a spring tab  26 . 1  is also provided, which rests against the brush  18  laterally and counteracts a lifting of the brush  18  off of the commutator  14 . To this end, the spring tab  26 . 1  is clamped at an end  28  oriented away from the brush  18 . The end  28  can, for example, be integrally connected to the brush holder, not shown in further detail, as part of a plastic injection molding process. The spring tab  26 . 1  can, however, also be a metal part that has been extrusion coated with plastic or can simply be pressed-fitted into place. With the other end  30  and its edge, the spring tab  26 . 1  rests against the brush  18 . The section  32  of the spring tab  26 . 1  adjoining the end  30  and its edge is inclined in the direction of the commutator  14  and the pressing direction  22 . This means that the section  32  has a higher coefficient of friction and exerts a more rigid action counter to the pressing direction  22  than it does in the pressing direction  22 . Thus, the spring tab  26 . 1  provides greater resistance against the brush  18  moving away from the commutator  12  than it does for movement of the brush  18  towards the commutator  12 . The inclination angle α between the pressing direction  22  and the section  32  is less than 90° and greater than 0°. Preferably, the angle lies in a range from 60° to 80°; a particularly favorable angle has turned out to be 70°. However, standard production tolerances of a few degrees must be taken into count. 
   Finally, the spring tab  26 . 1  shown in  FIG. 1  is situated on the side  36  of the brush  18  oriented away from the rotation direction of the commutator  14  indicated by the arrow  34 . As a result, during rotation, the spring tab  26 . 1  and the movement of the commutator  14  both act to press the brush  18  against the side wall  21 . 2 . In  FIG. 1 , the spring tab  26 . 1  is embodied so that it is virtually impossible for the brush  18  to move counter to the pressing direction  22  since the inclination of the spring tab  26 . 1  provides a much greater coefficient of friction in this direction and thus prevents the brush from being deflected counter to the pressing direction  22 . Preferably, the spring tab  26 . 1  inhibits or prevents the brush  18  from moving counter to the pressing direction. This inhibiting action, however, can also be accompanied by a certain residual elasticity. As the helical spring  24  repositions the brush  18  as it wears down, the spring tab  26 . 1  can pivot slightly away from the brush  18 , although it is still able to rest against it. Care must be taken here to dimension the helical spring  24  so that it overcomes the spring force generated by the spring tab  26 . 1  and the friction in the guide  20 . 
     FIG. 2  shows a modified spring tab  26 . 2 . The remaining components are the same and have been provided with the same reference numerals as in  FIG. 1 , and reference is made to  FIG. 1  with regard to these parts. Between the first section  32  of the spring tab  26 . 2  and the clamped end  28 , there is a second section  38  that extends essentially perpendicular to the pressing direction  22 . To this end, the spring tab  26  is bent between the two sections  32  and  38 . As a result, the second section  38  exerts a weaker action than the first section  32 , at least in the direction counter to the pressing direction  22 , and consequently exerts an elastic action that is preferably equal both in and counter to the pressing direction  22 . The second section  38  can also be inclined in relation to the position shown. It is also possible to use the length  40  of the second section  38  to influence the rigidity or elastic behavior of the spring tab  26 . 2 . The elastic behavior of the spring tab  26 . 2  lends it a residual flexibility that is advantageous if the commutator  14  is very out of round. 
     FIG. 3  shows a brush device  16 . 3 , which has a modified brush  18 ′. The remaining components are the same and have been provided with the same reference numerals as in  FIG. 1 , and reference is made to  FIG. 1  with regard to these parts. The end  42  of the brush  18 ′ oriented away from the commutator  14  has a bevel  44 . A scroll spring  46 , for example, engages this bevel. The spring force thus acts obliquely to the pressing direction  22  and obliquely to the brush  18 ′, which further stabilizes the brush  18 ′ in its guide  20 . The spring force acts in the same direction as the rotation direction  34  of the commutator  14 . 
     FIG. 4  shows a brush device  16 . 4 . The only difference from the device in  FIG. 3  is that the brush  18 ′ is rotated by 180° around its longitudinal axis. As a result, the spring force of the scroll spring  46  acts counter to the rotation direction  34  of the commutator  14 , which also stabilizes the brush  18 ′ in the guide  20 . 
     FIG. 5  shows a particularly inexpensive brush device  16 . 5 . In this instance, the spring tab  26 . 3  and the scroll spring  46 ′ are embodied of one piece. The spring tab  26 . 3  is connected to the scroll spring  46 ′ by means of a bracket portion  48 . The bracket portion  48  has a first section  48 . 1  that adjoins the scroll spring  46 ′ and extends parallel to the side wall  21 . 3  of the guide  20 . The first section  48 . 1  is adjoined by a second section  48 . 2  that protrudes out from the plane of the first section  48 . 1  and is situated in a lateral recess  50  in the side wall  21 . The second section  48 . 2  is adjoined via an arc-shaped third section  48 . 3  to the spring tab  26 . 3 , which is oriented toward the brush  18 ′. The second section  48 . 2  and the spring tab  26 . 3  thus enclose a V-shape that is situated in the likewise V-shaped recess  50  of the side wall  21 . 3 . The section  52  of the recess  50  that adjoins the end of the guide  20  oriented toward the commutator  14  is farther away from the brush  18 ′ than the remaining region of the side wall  21 . 3 . This facilitates installation of the brush device  16 . 5 . It is naturally also possible to embody the recess in the form of a simple lateral through opening, etc. 
   Another aspect of the brush device according to  FIG. 5  is that the spring tab  26 . 3  rests against the side  54  of the brush  18 ′ that lies in a plane with the rotation direction of the commutator  14 . This is advantageous in reversible electric machines  10 . It is naturally also possible to embody or situate the spring tab  26  so that it rests against both this side  54  and the side  36  ( FIG. 1 ). 
     FIG. 6  shows a brush device  16 . 6  equipped with a plunger spring  56  and a brush  18 ″ embodied in the form of a hammer brush. As elements that counteract a lifting of the brush  18 ″, bristles  60  are provided on a surface  58  oriented parallel to the pressing direction  22  of the plunger spring  56 ; these bristles  60  are inclined in relation to the pressing direction  22  and rest against the plunger spring  56 . As a result, the plunger spring  56  can move in the pressing direction  22 , but is prevented or at least hindered from moving counter to the pressing direction  22 . The bristles  60  thus indirectly counteract a lifting of the brush  18 ″ via the plunger spring  56 . The bristles  60  can also be provided on both sides of the plunger spring  56 ; they can be comprised of metal or plastic. In lieu of the bristles  60 , it is also possible, for example, to provide a saw tooth geometry or a number of spring tabs. 
   In all of the above-mentioned elements  26 . 1 ,  26 . 2 ,  26 . 3 ,  60  that counteract a lifting of the brush  18 ,  18 ′,  18 ″, it is important that they exert a less rigid action in the pressing direction  22  than they do counter to the pressing direction  22 . Preferably, they demonstrate an even more rigid behavior counter to the pressing direction than the springs  24 ,  46 ,  46 ′, and  56  themselves. 
   The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.