Abstract:
An electromotive drive is provided, especially for the pump of a power-assisted steering system of a motor vehicle. The drive includes a housing ( 3 ) with a bearing journal ( 15 ) in which the shaft ( 18 ) of a rotor ( 9 ) is rotationally mounted. The drive also includes a stator ( 7 ) having drive windings. The bearing journal ( 15 ) extends through said stator and supports it. The stator ( 7 ) is supported by the bearing journal ( 15 ), substantially in the transversal direction only. The stator ( 7 ) is coupled with the remaining housing ( 3 ) in a rotationally fixed manner, so as to transmit the engine torque. The arrangement dramatically reduces disturbing noises which occur especially when the motor is operated at full load and which are caused by relatively high-frequency torque variations.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to an electromotive drive system especially suited for use with a pump of a power-assisted steering system of a motor vehicle of the type having a housing, a shaft support containing a shaft on which a rotor is rigidly affixed, and a stator with drive windings which is traversed and retained by the shaft support. 
   As a rule, electromotive pumps are employed with power-assisted steering systems. In these pumps, the motors are typically designed in such a manner that the pumps are operated at full load for only brief periods of time. When the prior art motors with integrated hydraulic power-assisted steering system pumps are operated at full load, is disturbing whistling noises are often generated which are attributable to relatively high frequency torque variations. 
   Known electromotive drives and pumps have an electric motor including a stator and a rotor. The rotor is typically an external rotor that encompasses the stator. The stator is positioned on a shaft support which extends through the stator and is firmly connected with the stator. In addition, for transmission of the torque from the stator to the remaining housing, an axial split pin is installed between the stator and the shaft support. The axial pin is arranged between opposite recesses formed in the interior wall of the stator and the exterior wall of the shaft support. At the underside of the stator, the connecting contacts of the stator windings are joined with a base plate arranged between the stator and the bottom of the housing. The control electronics for the motor or the pump are also typically arranged on the base plate. 
   In prior art motors, the axial split pin is provided for non-positive and positive coupling of the stator and the shaft support. The axial split pin engages with corresponding grooves in the inner wall of the stator and the outer wall of the shaft support. The split pin of the prior art creates a rigid coupling between the rotor masses and the mass of the remaining drive. As a result, the resonant frequency of the combined stator and shaft support system lies within the range of the high frequency torque variations occurring during operation. Thus, the stator/shaft arrangement of the prior art systems are incited to produce disturbing, noise-producing vibrations. 
   Proceeding from said state of the art, it is an object of the invention to create an electromotive drive system, and particularly an electromotive drive for the pump of a power-assisted steering system of a motor vehicle, for which the disturbing noises occurring with known drives are drastically reduced or prevented. 
   SUMMARY OF THE INVENTION 
   The invention proceeds from the recognition that the disturbing noises are typically created by the substantially rigidly coupled stator/shaft support system, which generates a “tuning fork” effect when the resonant frequency of the system falls in the range of the unavoidable high frequency torque vibrations. Such torque variations or vibrations are practically unavoidable with electric motors and which have sufficient amplitude to lead to the disturbing noises, especially when the pump is operated with a full load. 
   According to one aspect of the invention, the rigid coupling between the stator and the shaft support is eliminated with respect to torque transmission. The transmission of torque moment occurs essentially only via the coupling of the stator with the remaining housing and not through the shaft support. The shaft support serves only for positioning the stator in the plane which extends transversely in relation to the shaft support. In the present invention, elimination of the split pin or similar coupling means causing the rigid coupling has resulted in a clear reduction of the disturbing noises. 
   An improved suppression of the disturbing noises is obtained by providing a gap between the interior wall of the stator and the outer wall of the shaft support. The gap is preferably filled at least partially with a viscous medium. Alternatively, or in addition, vibration-absorbing elements are preferably included, for example O-rings, for bridging the gap and coupling the stator and the shaft support. The coupling, however, preferably does not substantially transmit torque. As a result of these measures, further improved noise suppression is achieved. Preferably, the substantially reduced torque transmission at the coupling between the stator and the shaft support alters the resonance properties, e.g. the resonant frequency and damping, of the rotor/shaft support system such that the disturbing noises are essentially eliminated. 
   In a preferred embodiment, the torque transmission from the stator to the remaining housing takes place via a supporting base plate. The base plate preferably includes a punched-out grid. The stator is mounted onto the supporting base plate. This arrangement results in a very simple installation, while at the same time substantially dampens high frequency torque variations or vibrations through the flexibility and damping properties of the base plate, and the overall stator/base plate system. Vibrations of the entire housing, and the detrimental excitation of the rotor/shaft support system potentially excited thereby, are thus reduced or suppressed. 
   In the present invention, in order to avoid a mechanical overload of the base plate, particularly in the case of a base plate having small dimensions, a means is preferably provided at the base plate for non-positive and/or positive torque transmission to the remaining housing. For example, the means include one or more of a surface roughening, denticulation, and fluting on the underside of the base plate. These features produce, in conjunction with appropriate press-on pressure of the plate against the housing, an improved coupling for torque transmission. The roughening, denticulation, or fluting are provided, as desired, at non-plastic-coated conductor tracks of the punched-out grid of the supporting base plate. In this manner, simultaneous electrical contacting of the housing occurs, for example by mass potential. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for the purpose of illustrating preferred embodiment and are not to be construed as limiting the invention. 
       FIG. 1  is a perspective exploded view of the basic components of an electromotively driven pump with a drive in accordance with one embodiment of the invention; and 
       FIG. 2  is a central, vertical longitudinal sectional view of the embodiment of  FIG. 1  in the fully assembled state. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  depicts an electromotively driven pump  1  which includes a housing  3 , an electronic control unit  5 , a stator  7  and a rotor  9 . A protective hood  10  ( FIG. 2 ) joins with the housing  3  and covers the motor. Although the invention will be described with particular reference to the illustrated preferred pump embodiment as shown in  FIGS. 1 and 2 , the drawings are exemplary only and are not to be construed as limiting the invention to any particular application. 
   The housing  3  contains the pumping mechanism and has in an anterior wall a high pressure outlet  11  and in a rear wall an inlet  13  in the area indicated in  FIG. 1 . 
   A cylindrical shaft support  15  extends upwardly from the bottom of the housing  3 . The shaft support  15  contains the driven shaft  18 . The shaft  18  is supported in the shaft support  15  by bearings  17 . The shaft  18  drives the pumping mechanism. The housing  3  preferably consists of aluminum or magnesium cast metal. 
   The electronic control unit  5  is arranged in the housing  3 . The electronic control unit  5  includes a base plate  19  onto which the mechanical, electrical, electro-mechanical and electronic components are attached. The base plate  19  has a recess  21  which engages the shaft support  15  of the housing  3 . The base plate  19  includes an extrusion-coated punched-out grid which supports high current densities, and a printed conductor plate for conducting low current densities. 
   The motor includes a stator  7  having stator windings. The stator  7  also has an axial recess  25  which receives the shaft support  15  of the housing  3 . The stator  7  is rigidly fastened to the base plate  19 , e.g. via a punched-out grid. For example, the stator  7  may be fastened to the base plate  19  by soldering or welding the contact connections of the stator  7  windings to the conductor tracks of the punched-out grid. 
   The rotor  9  is constructed as an external rotor and is rotationally arranged in the housing  3  by a rigid connection to the shaft  18  which in turn is held by the bearings  17 . The rotor  9  joined with the drive shaft  18  by any suitable appropriate fashion. 
   The entire arrangement is preferably covered by the protective hood  10  as shown in  FIG. 2 . The protective hood  10  connects with a collar  27  of a lateral wall of the housing  3 . 
   A pair of receptacles  29  are provided in the housing  3  and are adapted to receive power semiconductors  31  of the electronic control unit  5 . The power semiconductors  31  are preferably power FET&#39;s. The power FET&#39;s normally have small heat sinks  31   a  which cannot guarantee adequate heat transfer. 
   To that end, the small heat sinks  31   a  are disposed on the receptacles  29  in the housing  3  and are so arranged as to have sufficient thermal contact with the housing  3  using any suitable means. 
   Since the heat sinks  31   a  of the power semiconductors  31  also preferably function as electrical contacts, there is preferably provided an electrically insulating but thermally conducting layer between a back side of the small heat sinks  31   a  and the receptacles  29 . Alternatively, a direct electrically conducting contact between the heat sinks  31   a  and the housing  3  is established, conditional upon the heat sinks  31   a  not simultaneously acting as electrical contacts. 
   In the particular represented embodiment, two power semiconductors  31  are included, and are attached to the receptacles  29  by elastic clamps  33 . 
   The regions of the housing  3  below the receptacles  29  preferably have one or more channels through which flows the fluid to be transported by the pump  1 . These regions thus act as heat exchangers for transferring heat out of the power semiconductors  31 . Of course, other measures known to the art can also be incorporated for improving the heat sinking of the power semiconductors  31 . For example, it is preferable to maximize the area through which the fluid is transported in the regions below the receptacles  29 . A plurality of channels can be provided to increase the heat transfer. Additionally, one or more channels preferably includes interior cooling fins. 
   In order to avoid noises which develop with the prior art drives in use heretofore, which noises are caused by relatively high frequency variations of the generated torque moment, the stator  7  is not joined directly to the shaft support  15 . Rather, the shaft support  15  and the stator  7  are arranged such that a gap exists between an inner wall of the stator  7  and an outer wall of the shaft support  15 . 
   In order to attain adequate fixation of the stator  7  while yet providing the gap, one or more O-rings  12  are arranged in grooves  12   a  in the outer wall of the shaft support  15 . The O-rings  12  are preferably sufficiently flexible to minimize undesired rigid coupling between the stator  7  and the shaft support  15 . The O-rings  12  preferably have flexibility and produce a dampening effect, thus acting as vibration-damping coupling elements between the stator  7  and the shaft support  15 . 
   In place of the O-rings  12  or in addition to these O-rings, the gap may also be filled partially with a viscous medium such as a grease, which also avoids or further reduces the rigidity of the coupling. 
   It is to be appreciated that in the preferred embodiment, a substantially rigid coupling between the stator  7  and the shaft support  15  is avoided, as a rigid coupling would support tangential power transmission or the transmission of the torque from the stator  7  to the shaft support  15 . 
   According to the embodiment illustrated in  FIGS. 1 and 2 , the torque moment is not transmitted from the stator  7  through the shaft support  15 . Rather, the torque moment is transmitted directly to the housing  3 , and, more specifically, to the bottom of the housing  3 . The shaft support  15 , aside from providing a mounting for the rotor shaft  18 , serves only to provide axial control or stabilization of the stator  7 . 
   According to the embodiment illustrated in  FIGS. 1 and 2 , the stator  7  is firmly joined with the supporting base plate  19 . As mentioned previously, this joining is preferably realized by soldering or welding of connection contacts to the conductor tracts of the punched-out grid of base plate  19 . 
   Since the base plate  19  is firmly mounted, together with the attached stator  7  in the housing  3 , the torque moment transmission from the stator  7  to the to the housing  3  takes place via the base plate  19 . This arrangement produces an additional benefit in that the base plate  19 , which typically has some flexibility, acts to dampen the high frequency variations of the transmitted torque. This additional benefit is obtained in particular when the base plate  19  is at least partially formed as an extrusion plastic-coated punched-out grid. 
   The vibration dampening properties of the base plate  19  are particularly evident when the base plate  19  is not rigidly connected over its entire area with the housing  3 , but rather is connected only in certain selected areas or spots, e.g. is connected by means of screws. 
   In order to provide reliable safe transmission of torque moment and to avoid a mechanical overload or destruction of the base plate  19 , the base plate  19  is preferably appropriately dimensioned, or additional measures are preferably taken to promote transmission of torque. To that end, in the preferred embodiment means is preferably provided on the base plate  19 , preferably in the attachment region of the stator  7 , for positive and/or non-positive coupling with the housing  3 . Said means are, preferably, a surface roughening or denticulation disposed on the underside of the base plate  19 , which act in conjunction with press-on forces operating in these regions to provide improved torque moment transmission to the housing  3 . This roughening or denticulation is preferably provided on non-plastic coated conductor tracts. These conductor tracts therefore also selectively act as electrical contacts with the housing  3 . 
   Extensions which extend downwardly are also selectively provided at the underside of the plate as desired to promote torque transmission. Such extensions preferably cooperate with stops provided in the housing  3  for the transmission of torque moment. 
   Overall, as a result of the inventive construction, undesirable noise-producing vibrations of prior art stator/shaft support systems are avoided. The shaft support  15  serves only for axial and transverse fixation of the stator  7 , while transmission of the torque moment occurs through a twist-proof fastening of the stator in the housing  3 , preferably by means of a base plate  19 . 
   The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.