Abstract:
A pump unit for a hydraulic vehicle brake system with traction control has an electric motor driving a radial piston pump. A rotor shaft of the pump unit has a hollow shaft with two standardized, hardened cylindrical pins that are press-fitted into the ends of the hollow shaft. The rotor shaft can be produced simply, economically, and without metal-cutting machining. The hollow shaft has high bending and torsional strength. The rotor shaft has a small diameter at the bearing points, which makes a small bearing diameter and thus a small installation space for the pump unit possible.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 01/02812 filed on Jul. 25, 2001. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an improved pump unit for a hydraulic vehicle brake system. 
   2. Description of the Prior Art 
   One pump unit, known from German Patent DE 198 09 571 C1, unit has an electric motor as well as a pump that can be driven with the electric motor. The pump of the known pump unit is embodied as a radial piston pump. The known pump unit has a rotor shaft, which is driven to rotate by the electric motor and which drives the pump. The rotor shaft is embodied as a hollow shaft. On an end remote from the pump, the rotor shaft is supported rotatably in a motor housing of the electric motor. An end toward the pump of the rotor shaft embodied as a hollow shaft is press-fitted, in a manner fixed against relative rotation, into an eccentric bush of the pump. The eccentric bush has a bore with which it is pressed, protruding axially, onto the end toward the pump of the rotor shaft. An outer circumferential face of the eccentric bush is cylindrical, and the cylinder is axially parallel and eccentric to the bore of the eccentric bush. On the outer circumference of the eccentric bush, there is a bearing on whose circumference the pump piston or pistons of the piston pump rest. When the motor shaft is driven to rotate, the eccentric bush rotating with it drives the pump pistons into a reciprocating motion for pumping fluid, in the manner known per se for piston pumps. For rotatably supporting the rotor shaft, a bearing is disposed in the bore of the eccentric bush. This bearing is disposed in the axial extension of the hollow shaft. The bearing is braced on a fixed cylindrical rod that penetrates the rotor shaft embodied as a hollow shaft. The cylindrical rod is retained in a manner fixed against relative rotation, outside the ends of the rotor shaft, in a pump housing and in the motor housing. There is a radial gap between the rotationally fixed rod and the rotatable rotor shaft embodied as a hollow shaft. By way of the press-fitted eccentric bush, which is supported rotatably on the pin by the bearing, the hollow shaft is supported rotatably on the end toward the pump. 
   The known pump unit has the disadvantage of being complicated and expensive to produce and manufacture. Another disadvantage is that in the region of the pump pistons, two bearings are disposed one inside the other, namely the bearing in the eccentric bush and the bearing disposed on the outside of the eccentric bush, making a radially large installation space necessary. Moreover, the rotationally fixed rod must protrude axially beyond the rotor shaft embodied as a hollow shaft, so that the rod can be secured. As a result, a great axial length of the pump unit is necessary. 
   International Patent Disclosure WO 94/27045, in FIGS. 1 and 8, discloses at least a pump unit for a hydraulic vehicle brake system with an electric motor, which has a cup-shaped motor housing and a housing cap inserted into it, and having a pump which is drivable by the electric motor and is accommodated in a pump housing; the pump unit has a rotor shaft, which extends from the motor housing through the housing wall and into the pump housing and is supported by means of a ball bearing, whose outer ring is located on one side in the housing cap and on the other in the pump housing. According to FIG. 1, the housing cap is of plastic and is designed such that essentially one-fifth of the length of the outer ring of the ball bearing can be press-fitted in, for the sake of positionally securing the outer ring. The remaining length of the outer ring of the ball bearing, upon the joining of the electric motor to the pump housing, can be inserted into a second stepped bore present in the pump housing. Because of the stepped bores, axial stops for the outer ring of the ball bearing are still available. In terms of its object, this construction has the advantage that the electric motor is in a complete state before it is united with the pump housing and can therefore be subjected to a test operation. Only if this test operation has proceeded satisfactorily is the intrinsically complete electric motor united with the pump housing and thus with the pump. Since the test operation can be performed in such a way that there is less radial stress on the ball bearing than in the later pumping operation, the housing cap is made from plastic. To enable the brush holders, accommodated on the housing cap and thus inside the cup-shaped motor housing, to extend as close as possible to a boundary plane of the pump housing on the cap end and nevertheless enable the outer ring of the ball bearing to be held sufficiently firmly for a test operation of the electric motor, the plastic housing cap is embodied with a greater thickness in one region around the outer ring of the ball bearing, and this thicker region is installed in an indentation that originates on a side of the pump housing toward the motor. The outer ring of the ball bearing, which is press-fitted into the housing cap on one side and on the other is insertable into a stepped bore of the pump housing serves upon insertion as a centering means for orienting the housing cap together with the electric motor relative to the pump housing. In the exemplary embodiment of FIG. 8 of WO 94/27045, an outer ring of the ball bearing protrudes with its part of its length out of the pump housing. Thus an indentation shown in FIG. 1, into which the thickened portion of the housing cap plunges, is omitted. In this respect, less machining expense is necessary for the pump housing of FIG. 8. 
   In a pump unit known from International Patent Disclosure WO 98/17514, an outer ring of a ball bearing is inserted with the majority of its length into a bore step located in the pump housing. The shorter portion of the length of the outer ring extends between attachments, which are located on the face end of a housing wall of the electric motor and are oriented toward a bore step that precedes those into which the outer ring of the ball bearing can be inserted. As a result, as soon as the outer ring of the ball bearing is inserted into the lower-lying bore step of the pump housing, it likewise serves as a means for aligning the electric motor relative to the pump housing. A motor shaft of the pump unit is embodied, adjoining the ball bearing, as an eccentric element located in the pump housing. A needle bearing surrounds the eccentric element, and bearing needles disposed around the eccentric element are encased by a bearing ring, and because of the closeness of the eccentric element to the ball bearing, the bearing ring can run up on an inner ring of the ball bearing. The housing cap is again made from plastic, is embodied as essentially platelike, and therefore dips into a depression that originates at a boundary face, toward the electric motor, of the pump housing and extends in the direction of the bore step associated with the outer ring of the ball bearing. Once again, the outer ring of the ball bearing, during its insertion into the associated bore step, serves as a means for orienting the electric motor relative to the pump housing. Because of the platelike embodiment of the housing cap, brushes of the electric motor are tangent to the boundary plane toward the motor of the pump housing. As a result, the cup-shaped motor housing is advantageously embodied as shorter than the length of the complete electric motor. 
   Other pump units for a hydraulic vehicle brake system are known from German Patent Disclosures DE 198 05 003 A1 and DE 198 49 669 A1, each having an electric motor that has a cup-shaped motor housing and a housing cap inserted into it; each pump unit has a rotor shaft, which extends from the motor housing through the housing cap and into a pump housing and is supported by a ball bearing, whose outer ring is located on one side in the region of the housing cap and on the other in the region of the pump housing. As a result, each housing cap is produced from sheet metal by deep drawing and/or stamping in such a way that pointing outward away from the interior of the motor housing, the housing cap has a tubular stub, which forms a bearing seat for receiving an outer ring of a ball bearing. The outer ring is press-fitted into the bearing seat for the sake of securing it against displacement in axial directions. Beginning at a respective face, toward the housing cap, of the pump housing, there is an opening in the pump housing whose diameter is larger than the outer diameter of the tubular stub that forms the bearing seat. Only in the event that a given rotor shaft is supported at three places, for instance as in FIG. 1 of DE 198 05 003 A1 or FIG. 2 of DE 198 49 669 A1, is the intrinsically complete electric motor orientable relative to the pump housing and thus to the pump in a technologically simple way, thanks to the third bearing. 
   In DE 198 05 003 A1, the housing wall is provided with a support, protruding counter to the pump housing, radially outside the opening in the pump housing that surrounds the bearing seat. For the sake of flush contact by frictional engagement of the protruding support, the housing cap is clamped against the pump housing by elastic deformation. This purpose is served on the one hand indirectly, via one end of the motor housing to be fastened, by fastening means of the kind that also clamp the cup-shaped motor housing against the pump housing. The clamping with elastic deformation of the housing cap is one provision for reducing noise. 
   In the example of the aforementioned DE 198 49 669 A1, the bearing seat, embodied in the form of a tubular stub, has a circular-annular face end, oriented toward the pump, which in this example is flush with a boundary plane of one face end of the outer ring of the ball bearing, which ring is press-fitted into the bearing seat. In this construction, the outer ring of the ball bearing is held by frictional engagement with a press fit in the bearing seat in two axial directions. 
   SUMMARY OF THE INVENTION 
   The pump unit of the invention has a hollow shaft, into the end of which a pin is inserted in a manner fixed against relative rotation. The pin is coaxial with the hollow shaft and protrudes from its end. The hollow shaft together with the pin forms the rotor shaft of the pump unit. The pin protruding from the end of the hollow shaft forms a bearing point, at which the rotor shaft is rotatably supported. The invention has the advantage that in a simple, inexpensive way it makes it possible to embody the rotor shaft as a hollow shaft. The hollow shaft can be made from a pipe or tube which can be ordered by the meter and thus inexpensively and merely has to be cut to the right length. No other machining of the tube is needed. One end face of the pipe forming the hollow shaft can form a shoulder for axial support of a bearing of the rotor shaft. This kind of production of a shoulder for axial support of a bearing requires only a fraction of the effort and expense of metal-cutting production, for instance, of such a shoulder by graduating the diameter of the shaft. Cutting the tube to length to produce the hollow shaft does not require any particular precision, since a total length of the rotor shaft is set by means of an insertion depth of the pin into the end of the hollow shaft. 
   Another advantage of the invention is that a diameter of the bearing point of the rotor shaft, that is, a diameter of the pin inserted into the hollow shaft, is less than an outside diameter of the hollow shaft. The bearing point of small diameter makes a bearing of small diameter possible, and thus a small installation space in the radial direction in each bearing region. Despite the small diameter of the rotor shaft of the pump unit of the invention at the bearing point, the rotor shaft has high bending and torsional strength, because of its embodiment as a hollow shaft. For the same amount of material used, the rigidity of the hollow shaft is greater than that of a solid shaft. 
   Preferably, the pin is press-fitted into the hollow shaft and as a result joined to the hollow shaft in a manner fixed against relative rotation and axial motion. 
   A standardized pin may be used as the pin for the rotor shaft. Such pins can be obtained inexpensively as mass-produced goods. Standardized pins typically have a hard surface, with a reduced diameter tolerance and high surface quality, which makes it suitable as a running face for slidingly bearing the rotor shaft. The hard surface of a standardized pin is equally well suited as a running face on which roller bodies of a roller bearing without an inner ring, such as a needle bearing, can roll. A standardized pin is also suitable for pressing on an inner ring of a roller bearing. As the standardized pin, a hardened cylindrical pin can be considered in particular. 
   One embodiment of the invention provides for producing the hollow shaft as a reformed part, especially by cold forging, instead of cutting it apart from a pipe. The reforming, in particular cold forging, has the advantage that the hollow shaft can be produced quickly and inexpensively, preferably in a single operation. Metal-cutting machining is not needed. By means of the reforming, a densification and/or a hard surface can be achieved. 
   In one embodiment of the invention, the pin inserted into the hollow shaft is integral with an eccentric element for driving the pump pistons of the pump, embodied as a piston pump, of the pump unit of the invention. This embodiment of the invention has the advantage of simple, inexpensive production of the eccentric element; in particular, the eccentric element can be produced integrally with the pin, quickly and simply, preferably in a single operation, as a reformed part and in particular as a cold-forged part. This has the advantage of not requiring a separate eccentric element. Producing the eccentric element integrally with the pin as a reformed part, particularly by means of cold forging, has the advantages that have already been mentioned for producing the hollow shaft by reforming/cold forging, that is, simple, fast production in a single operation, and the compaction of the material because of the reforming. Except for grinding the circumferential surface of the eccentric element, no postmachining or metal-cutting machining of the eccentric element produced by reforming, including of the pin integral with it, is needed. 
   The pump unit of the invention is intended in particular for use in a brake system of a vehicle and is used to control the pressure in wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS (for anti-lock brake system), TCS (traction control system), VDC (vehicle dynamics control) and EHB (electrohydraulic brake system) are used for such brake systems. In the brake system, the pump unit serves for instance to return brake fluid from a wheel brake cylinder or a plurality of wheel/brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into a wheel brake cylinder or a plurality of wheel brake cylinders (TCS or VDC or EHB). The pump unit is needed in a brake system with wheel slip control (ABS or TCS) and/or a brake system serving as a steering aid (VDC) and/or an electrohydraulic brake system (EHB). With the wheel slip control (ABS or TCS), locking of the wheels of the vehicle during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (TCS) can for instance be prevented. In a brake system serving as a steering aid (VDC), a brake pressure is built up in one or more wheel brake cylinders independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from breaking out of the track desired by the driver. The pump unit can also be used in an electrohydraulic brake system (EHB), in which the pump unit pumps the brake fluid into the wheel brake cylinder or wheel brake cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump unit is used to fill a reservoir of the brake system. 
   The pump unit of the invention of one embodiment unites in itself the advantages that the outer ring of the ball bearing, which acts to support the rotor shaft, forms a centering aid for the housing cap and thus for the electric motor, and that noise production is counteracted, and that for that purpose a face end toward the pump of the bearing seat acts as a support, for elastically bracing the housing cap formed from sheet metal. 
   Brushes of the electric motor can be disposed next to the pump housing, and accordingly the motor housing can be made in a shorter length. Compared to the version without a bore step, this version is naturally somewhat more expensive in terms of machining the pump housing, but results in a construction that overall is advantageous. 
   One embodiment provides a short spacing between at least one longitudinal axis of a piston of the pump and the ball bearing, so that the overall combination with other features is distinguished by the advantage, known per se, of the short spacing between at least one pump piston longitudinal axis and the ball bearing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the invention will become apparent from the detailed description contained herein below, taken in conjunction with the drawings, in which: 
       FIG. 1  is a sectional view, in elevation, of a pump unit embodying the invention; and 
       FIGS. 2–6  are views similar to  FIG. 1 , each showing a further embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The pump unit of the invention shown in  FIG. 1  and identified overall by reference numeral  10  has an electric motor  12  and a pump, embodied as a radial piston pump  14 , which can be driven by the electric motor  12 . The radial piston pump  14  has two pump pistons  16 , which are disposed in a boxer arrangement, that is, facing one another. The sectional view shown in the drawing is at an angle of 90° from an imaginary center axis of the pump unit  10 , so that only one of the two pump pistons  16  is visible. 
   The pump unit  10  has a rotor shaft  18 , which at the same time forms a motor shaft of the electric motor  12 . The rotor shaft  18  has a hollow shaft  20 , into both ends of which pins  22 ,  24  are press-fitted. As a result of the press-fitting, the pins  22 ,  24  are joined in a manner fixed against relative rotation and fixed axially to the hollow shaft  20  of the rotor shaft  18 . The hollow shaft  20  is cut from a piece of precision steel pipe and otherwise is not machined. The two pins  22 ,  24  are standardized, hardened cylindrical pins, that is, mass-produced goods. The pins  22 ,  24  are press-fitted over a portion of their length into the hollow shaft  20  and protrude out of the hollow shaft  20 . 
   An armature  26  with armature windings and a commutator  28  of the electric motor  12  are mounted on the hollow shaft  20  in a manner fixed against relative rotation. 
   The two pins  22 ,  24  press-fitted into the hollow shaft  20  form bearing points of the rotor shaft  18 , at which the rotor shaft  18  is rotatably supported: On an end remote from the radial piston pump  14 , the rotor shaft  18  is supported rotatably, by its pin  22  protruding from the hollow shaft  20 , in a bearing bush  30  of sintered metal. The sintered metal of the bearing bush  30  has a porosity and is saturated with a bearing oil, so that the bearing bush  30  has permanent lubrication. The hardened pin  22  has an adequate surface hardness and surface quality for sliding bearing action in the bearing bush  30  with little friction and negligible wear. 
   The bearing bush  30  is retained in a bearing receptacle with an annular spring clamp  32 . The bearing receptacle  34  is embodied as a bulge in a bottom  36  of a cup-shaped motor housing  38 . The spring clamp  32  is riveted to the bottom  36  of the motor housing  38 . The spring clamp  32  retains the bearing bush  30  pivotably in the bearing receptacle  34 , so that an error of angular alignment of the rotor shaft  18  is automatically compensated for; the bearing bush  30  automatically orients itself in alignment with the rotor shaft  18 . 
   The cup-shaped motor housing  38  is closed on an open face end with a housing cap  40  in the form of a perforated disk having a central hole in which a ball bearing  42  is press-fitted, as a further bearing. By crimping, one edge of the hole in the housing cap  40  is shaped into a cylindrical collar, which forms a bearing seat  44  for the ball bearing  42 , into which seat an outer ring  46  of the ball bearing  42  is press-fitted. An inner ring  48  of the ball bearing  42  is pressed onto the pin  24  that is press-fitted, protruding from the hollow shaft  20 , into an end toward the pump of the hollow shaft  20 . The rotor shaft  18 , on its end toward the pump, is rotatably supported by the ball bearing  42 ; the pin  24  forms a bearing point of the rotor shaft  18 . A total length of the rotor shaft  18  is set by means of a press-fitting depth of the two pins  22 ,  24  into the ends of the hollow shaft  20 . 
   Permanent magnets  50  are mounted, surrounding the armature  26 , on an inside circumference of the cup-shaped motor housing. 
   The radial piston pump  14  is accommodated in a hydraulic block that forms a pump housing  52 . The hydraulic block is a component of the hydraulic vehicle brake system that is not otherwise shown. Also accommodated and hydraulically connected to one another in the hydraulic block, besides the radial piston pump  14 , are other hydraulic elements, not shown in the drawing, such as magnet valves, hydraulic reservoirs, and damper chambers. The hydraulic components not shown serve in a manner known per se to provide anti-lock, traction control, and optionally vehicle dynamics control; the radial piston pump  14  is intended for pumping brake fluid in the hydraulic vehicle brake system. Of the hydraulic block that forms the pump housing  52 , only a fraction surrounding the radial piston pump  14  can be seen in the drawing, for the sake of clear illustration. 
   For connecting the electric motor  12  to the pump housing  52  of the pump unit  10 , the motor housing  38  has a radial flange  54  on its open face end; this flange is reformed on the outside, and the motor housing  38  rests on the pump housing  52  with this flange and is screwed by it to the pump housing  52  by means of screws  56 . The screws  56  are screwed into the pump housing  52  between the two pump pistons  16 . Because of the angled section, both a pump piston  16  and a screw  56  are visible in the drawing, but in actuality they are located in imaginary axial planes of the pump unit  10  that are offset at an angle from one another. 
   The pin  24  toward the pump of the rotor shaft  18  protrudes through the ball bearing  42  into a cylindrical eccentric-element chamber  58 , which is mounted in the pump housing  52  coaxially with both the electric motor  12  and the rotor shaft  18 . An eccentric bush  60  is press-fitted, in a manner fixed against relative rotation, onto a free end of the pin  24  that protrudes from the ball bearing  42 . The eccentric bush  60  forms an eccentric element for driving the radial piston pump  14 . The eccentric bush  60  has a cylindrical bore  62 , with which it is press-fitted onto the pin  24 , and a cylindrical outer circumferential face, which is axially parallel and eccentric to the cylindrical bore  62  of the eccentric bush  60  and thus to the rotor shaft  18 . The cylindrical outer circumferential face forms a running face  64  for a needle bearing  66 , which is seated on the eccentric bush  60 . The pump pistons  16  are received axially displaceably in pump bores  68 , which are made radially to the rotor shaft  18  in the pump housing  52  and discharge into the eccentric-element chamber  58 . The pump pistons  16  are pressed by restoring springs, not visible in the drawing, against a bearing ring  70  of the needle bearing  66 . The piston restoring springs are helical compression springs, which are disposed on outer ends, remote from the eccentric bush  60 , of the pump pistons  16 . The eccentric element  60  forms an eccentric element, which when driven to rotate drives the pump pistons  16  to an axial reciprocating motion in the pump bores  68 . The reciprocating motion of the pump pistons  16  brings about pumping of brake fluid in the manner known per se from piston pumps. 
   In the pump unit  10  of the invention, shown in  FIG. 2 , the hollow shaft  20  of the rotor shaft  18  is produced as a cold-forged part and therefore, for production reasons, has a bottom  72  in the region of a longitudinal center of the hollow shaft  20 . Otherwise, the pump unit  10  shown in  FIG. 2  is embodied and functions identically to the pump unit  10  shown in  FIG. 1 . To avoid repetition, see the description of  FIG. 1 . 
   In a pump unit  10  of the invention shown in  FIG. 3 , the eccentric element  60  is integral with the pin  24  that is press-fitted into the end toward the pump of the hollow shaft  20 . The eccentric element  60  is produced jointly with the pin  24  that is integral with it by means of cold forging. The eccentric element  60  is a cylindrical axial portion of the pin  24  that is disposed eccentrically to the pin  24 . A circumferential face of the eccentric element  60  forms the running face  64  for the needle bearing  66  of the radial piston pump  14 . Otherwise, the pump unit  10  shown in  FIG. 3  is constructed like that of  FIG. 1  and functions in the same way. To avoid repetition, see the corresponding description of  FIG. 1 . For identical components, the same reference numerals are used in the drawings. 
   The exemplary embodiment of a pump unit  10  in  FIG. 4  differs from the exemplary embodiment of  FIG. 1  in that a housing cap  40  in the form of a perforated disk is shown with a shallow cone shape, and a gap  74  that keeps a hollow space open is present between the pump housing  52  and the housing cap  40 , and hence radially outside a bearing seat  44  shaped on the order of a cylindrical collar. In the drawing, a width of the gap  74  for the ball bearing  42  is shown larger than at one edge  76 , which defines the outside of the housing cap  40 . This edge  76 , as in the example of  FIG. 1 , is embodied such that it is braced by positive engagement in the open end of the motor housing  38 . As a result, by means of the screws  56 , the housing cap  40  can be clamped indirectly via the radial flange  54  and the motor housing  38  in the direction of the pump housing  52 , so that the bearing seat  44  located on it is pressed in the desired way axially against a stop element  78  located in the pump housing  52 . This stop element  78  is produced for instance by making a bore step  80 , which originates at a boundary face  82  of the pump housing  52  on the side toward the electric motor and is oriented preferably centrally to the eccentric-element chamber  58 . 
   In the state of the electric motor  12  that is not mounted as in  FIG. 4  and accordingly is not yet braced against the pump housing  52 , the perforated-disklike region of the housing cap  40  is preferably flat. The conical illustration of the housing cap  40 , which is shown more qualitatively than quantitatively, shows a deformation on the occasion of assembling the electric motor  12  with the pump housing  52  and accordingly after the screws  56  have been tightened. Because of the elasticity of the material, in this case steel that can be deep drawn, the housing cap  40  has a certain elasticity, with the advantage of a permanent axial contact pressure of the bearing seat  44  against the pump housing  52 . This prevents the housing cap  40  from executing oscillations on the order of an elastic diaphragm, which could cause the emission of noise and possible damage to the ball bearing  42 . 
   As already described for  FIG. 1 , the outer ring  46  of the ball bearing is press-fitted into the bearing seat  44 . In a distinction from that embodiment, for inserting the longitudinal portion of the outer ring  46  of the ball bearing  42  that protrudes from the bearing seat  44 , it is favorable if an associated bore in the pump housing  52  is made in such a way that the outer ring  46  is insertable with little radial play into the bore. In this way, with the aid of the outer ring  46 , the electric motor  12  can easily be aligned with the pump housing  52 , and the outer ring  46  is easily insertable without having to expend effort in terms of press-fitting work. 
   It will be noted that the elastical axial contact pressure of the bearing seat  44  against the stop element  78  of the pump housing  52  described in conjunction with  FIG. 4  can also be made further use of if the rotor shaft  18  described thus far is embodied differently from  FIG. 1  and also differently from  FIGS. 2 and 3 . A rotor shaft that can be used alternatively can for instance be a rotor shaft from the prior art. 
   In the exemplary embodiment of  FIG. 5 , the motor housing  38  has radial indentations  84  extending from the outside inward, and therefore has protrusions  86  oriented radially inward inside the motor housing  38 . These protrusions  86  act as positive-engagement means for additional or sole transmission of some of the clamping force, which can be generated by the screws  56 , and because of this the bearing seat  44  is pressed against the pump housing  52  in a way that is axially secure against vibration. Instead of the radial indentations  84 , naturally a bead, not shown, extending all the way around and aimed at the edge the edge  76  of the housing cap  40  could be provided. 
   It has been noted above that the perforated-disklike region of the housing cap  40  can be flat, for instance, before being put together with the pump housing  52 . However, the possibility also exists of making the perforated-disklike region of this housing cap  40  conically, in such a way that it attains an essentially flat form upon being firmly tightened against the pump housing. 
   The exemplary embodiment of  FIG. 6  differs from the exemplary embodiments of  FIGS. 4 and 5  in that the exemplary embodiment of  FIG. 6  has no bore step  80 , and therefore the bearing seat  44  is pressed against a boundary face  82  of the pump housing  52  that is present on the side toward the electric motor. This exemplary embodiment is preferred for instance if the boundary face  82  is sufficiently flat, for instance as a result of machining. Machining of the boundary face  82  can in fact already be provided so that a radial flange  54  of the motor housing  38  will rest on the pump housing  52  with as little gap as possible. 
   The foregoing relates to the preferred exemplary embodiments 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.