Abstract:
The invention relates to a gear pump, especially for a power steering system, having a housing, a first cover and a second cover, the first cover being integrated into the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Patent Application No. PCT/EP2007/004672 filed May 25, 2007, the disclosures of which are incorporated herein by reference in their entirety, and which claimed priority to German Patent Application No. 10 2006 025 182.2 filed May 30, 2006, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a gear pump, especially for a power steering system, having a housing, a first cover and a second cover. 
     As a rule, such pumps consist of many individual parts such as gaskets, gear wheels, bearing shafts, bearing rests, centering pins, etc., so that their assembly is very demanding. The installation of the pump, for example, into a motor-pump aggregate of a power steering system of a motor vehicle is usually done in an awkward manner using screws or similar fasteners. 
     Document DE 203 02 535 U1, and corresponding US Publication No. 2006/0051230, describes a hydraulic pump of the generic type that can be used as a prefabricated assembly in a motor-pump aggregate of a power steering system. The end faces of the housing are each closed by a cover, the two covers being held together by at least one holding part. The hydraulic pump is accommodated in a fluid-filled chamber that is pressurized either by the pump itself or by another means. Therefore, when the individual parts of the pump are designed, it can be assumed that the pressure contributes to holding together the individual parts among each other as well as to affixing the pump as an assembly. In this situation, only a small contact pressure of the individual parts is needed to ensure the start-up of the pump. This contact pressure is necessary in order to seal the intake and pressure areas of the pump. Due to the external pressurization of the housing and of the pump cover, there is only little force differential or no force differential at all between the interior of the pump and the space surrounding the pump, so that the wall thicknesses of the above-mentioned components of the pump can be minimized. The configuration of the pump as a prefabricated assembly has the advantage that each pump can be tested before being delivered or before being installed and that the installation as an assembly is much simpler than the installation of numerous separate individual parts. Moreover, when a pump that has been configured in this manner is assembled, it is not necessary to join the two covers individually to the housing. The holding part ensures that the two covers are pressed against the housing without the need for further affixing means. In the publication cited, either screws or a spring element are used as the holding part, the spring element offering the advantage that the height of the covers can be reduced since there is no need for a thread and no screw head has to be accommodated. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to further simplify the construction of such a gear pump, especially for a power steering system. 
     In order to achieve this object, with a gear pump according to the invention having a housing as well as a first cover and a second cover, the first cover is integrated into the housing, and here, the phrase “a first cover integrated into the housing” means especially that the first cover is formed integrally with the housing. The term “housing” is used to refer to the pump housing or to the integrally formed module comprising the pump housing and other housing sections, for example, a motor housing. The integrally formed housing-cover combination is very simple and can be made with small tolerances, for example, by means of extrusion techniques. As a result of the integration of the first cover into the housing, the number of individual components for a gear pump is considerably reduced; for example, gaskets and centering pins between the housing and the first cover can be dispensed with. Another advantage is that the height of the gear pump according to the invention is less than that of the state of the art. Moreover, in the state of the art, the first cover as well as the housing entail dimensional tolerances. Due to the integral design, only the integrated housing remains as a source of tolerances, so that the requirements in terms of dimensional accuracy of this pump housing can be reduced without increasing the tolerances altogether. 
     The housing of the gear pump can accommodate two intermeshing gear wheels that are each arranged on a bearing shaft, the housing here having two bearing openings for the bearing shafts. Since bearing openings are created in the housing for the bearing shafts of the gear wheels, there is no need for the customarily employed, separate bearing rest. 
     In the illustrated embodiments, the housing has a tub-like cross section in a longitudinal section that runs through the two bearing openings. With such a cross section, the housing can be made in one piece with little effort, especially by means of an extrusion technique. Moreover, the housing can be made by means of extrusion so as to have almost sharp edges (e.g. with a radius of 0.2 mm) at the transition. 
     In one embodiment, the contact surface of the housing with the second cover extends perpendicular to a lengthwise direction of the bearing shafts and runs through the center of the gear wheels. As a result, the housing as well as the cover acquire tub-like cross sections with a geometry that is easy to produce by means of extrusion techniques. 
     Here, a bearing rest can be arranged in the housing on the side facing away from the bearing openings, the two bearing shafts being mounted in this bearing rest. Hence, the gear wheels of the gear pump are securely held via their bearing shafts by the bearing openings of the housing on the one hand and by the bearing rest on the other hand, so that a reliable and proper functioning of the gear pump is ensured. 
     In this embodiment, a sealing element can be arranged in the bearing rest, said sealing element providing axial compensation. In addition to its bearing function for the bearing shafts of the gear wheels, in this case, the bearing rest concurrently secures the sealing element for the axial compensation. This simplifies the assembly of the gear pump. 
     In another embodiment, the second cover is arranged on the housing and two bearing openings are made for the bearing shafts in this second cover. Consequently, on the side of the gear wheels facing the second cover, there is no need for a bearing rest for the bearing shafts so that a smaller number of individual components is needed for the gear pump. 
     The housing of the gear pump may be made of an aluminum alloy. On the one hand, this is easy to process by means of extrusion techniques so that the production of the housing does not pose any problems. On the other hand, the aluminum alloys can be constituted in such a way that the aluminum material forms a low-friction sliding bearing for the bearing shafts of the intermeshing gear wheels. The gear pump thus runs especially smoothly and is correspondingly economical in terms of energy consumption. 
     These advantages of the aluminum alloy as mentioned for the housing of the gear pump, of course, also apply to the second cover, which may likewise be made of an aluminum alloy. 
     As an alternative, the second cover arranged on the housing can also be made of steel. It is especially practical for the second cover to be made of steel for the embodiments of the gear pump in which the second cover does not have bearing openings and is produced as an inexpensive stamped part. 
     In some embodiments, the housing comprises a pump housing and a motor housing. Gaskets and centering pins between the pump housing and the motor housing can be dispensed with, as a result of which the assembly work is simplified and the number of individual components is reduced. 
     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective exploded view of a gear pump according to the state of the art; 
         FIG. 2  shows a perspective exploded view of a gear pump according to the invention, in a first embodiment; 
         FIG. 3  shows a longitudinal section through the assembled gear pump according to the invention, in the first embodiment as depicted in  FIG. 2 ; 
         FIG. 4  shows a perspective exploded view of a gear pump according to the invention, in a second embodiment; 
         FIG. 5  shows a longitudinal section through the assembled gear pump according to the invention, in the second embodiment as depicted in  FIG. 4 ; 
         FIG. 6  shows a detail VI of the longitudinal section as depicted in  FIG. 5 ; 
         FIG. 7  shows a schematic longitudinal section through a gear pump according to the state of the art; 
         FIG. 8  shows a schematic longitudinal section through the second embodiment of the gear pump according to the invention as depicted in  FIG. 5 ; 
         FIG. 9  shows a schematic longitudinal section through the first embodiment of the gear pump according to the invention as depicted in  FIG. 3 ; 
         FIG. 10  shows a schematic longitudinal section through a third embodiment of the gear pump according to the invention; 
         FIG. 11  shows a schematic longitudinal section through a fourth embodiment of the gear pump according to the invention; 
         FIG. 12  shows a schematic longitudinal section through a fifth embodiment of the gear pump according to the invention; 
         FIG. 13  shows a schematic longitudinal section through a sixth embodiment of the gear pump according to the invention; 
         FIG. 14  shows a schematic longitudinal section through a seventh embodiment of the gear pump according to the invention; and 
         FIG. 15  shows a schematic longitudinal section through an eighth embodiment of the gear pump according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a pump according to the state of the art, the pump comprising a hollow-cylindrical housing  10 ′ as well as a first cover  12  and a second cover  14  that can each close the end faces of the housing  10 ′. The pump shown is a so-called external gear pump whose housing  10 ′ accommodates a set of gear wheels with two intermeshing gear wheels  16 ,  18  that are each integrally formed with a bearing shaft  20 ,  22 . The bearing shafts  20 ,  22  are mounted in two opposing bearing rests  24 ,  26 , one of the bearing shafts  20 ,  22  extending through the first cover  12  out of the housing  10 ′ on one end face (drive shaft  22 ). A gasket  28  is positioned between the second cover  14  and the housing  10 ′ or the bearing rest  26 , and another gasket  30  is provided between the first cover  12  and the housing  10 ′ or the bearing rest  24 . In order to correctly position the covers  12 ,  14 , centering pins  32  are attached to the covers  12 ,  14  and/or to the housing  10 ′ and these centering pins  32  engage in corresponding recesses in the covers  12 ,  14  and/or in the housing  10 ′. Additional centering pins  34  are provided on the first cover  12  in order to position the pump in a motor-pump aggregate. Moreover, a pump gasket  35  is provided in order to prevent leakage in the area of openings (intake opening for the hydraulic fluid and opening for the drive shaft  22 ) provided in the first cover  12 . In the embodiment shown, the covers  12 ,  14  are held together with screws  36  in order to hold the pump as a preassembled unit in its assembled state. 
       FIGS. 2 to 6  show a gear pump  38  according to the invention; components whose function corresponds to components of the pump according to  FIG. 1  have the same reference numerals. 
       FIGS. 2 and 3  show a first embodiment of the gear pump  38 , the first cover  12  being integrated into a housing  10 , that is to say, integrally formed with the housing  10 ′ (see  FIG. 1 ). The integrally formed assembly consisting of the housing  10 ′ and the first cover  12  is referred to below as the housing  10 . 
     As can be seen in the longitudinal section in  FIG. 3 , the two intermeshing gear wheels  16 ,  18  with their integrally formed bearing shafts  20 ,  22  are accommodated in the housing  10 , which has two bearing openings  40 ,  42  for the bearing shafts  20 ,  22 . In the longitudinal section in  FIG. 3 , which runs through the two bearing openings  40 ,  42 , the housing  10  has a tub-like cross section. 
     The second cover  14 , like in the state of the art according to  FIG. 1 , is configured as a separate component and is arranged on the housing  10 . However, the second cover  14  has two bearing openings  44 ,  46  to accommodate the bearing shafts  20 ,  22 . Since the bearing openings  40 ,  42 ,  44 ,  46  are formed directly in the housing  10  or in the second cover  14 , no bearing rests are needed in this case. All that is needed in order to compensate for component tolerances in the axial direction is a compressible, elastic tolerance compensating means  48  which, in the present case, has approximately the shape of a bearing rest. 
     The gasket  28  is provided in order to create a seal between the second cover  14  and the housing  10  or the tolerance compensating means  48 . 
     In the embodiment of the gear pump  38  according to the invention shown in  FIGS. 2 and 3 , its components, especially the housing  10  and the second cover  14 , are held together by a specially shaped spring element  50 . The elastically deformable spring element  50  is stirrup-shaped, with a wavy section  50   a  in the middle, whose ends are followed by two side sections  50   b  bent outwards at a right angle, from which, in turn, two short holding sections  50   c  extend approximately at a right angle. The wavy section  50   a , at least in some of its contact points with the second cover  14 , runs in an indentation  52  of the second cover  14 , so as to prevent lateral slippage of the spring element  50 . At the same time, the wavy section  50   a  of the spring element  50  serves as a spacer for components that are adjacent to the gear pump  38 , thus ensuring an adequate cross section at a pressure outlet  53  of the gear pump  38 . In the illustrated embodiment, the two side sections  50   b  lie against the circumferential wall of the housing  10 , in grooves  54  in the circumferential wall, and the two holding sections  50   c  reach behind projections  56  of the circumferential wall of the housing  10 . By means of the indentation  52  on the second cover  14  and the grooves  54  in the housing  10 , the spring element  50  positions the second cover  14  relative to the housing  10 . Moreover, as an alternative or additionally, centering pins  32  can be present. Furthermore, one of the further centering pins  34  which ensure a positioning of the gear pump  38  in a motor-pump aggregate can be seen on the housing  10 . The pump gasket  35  has to be modified as compared to the state of the art in such a way that, in addition to the intake opening and the bearing opening  42  for the drive shaft  22 , it also seals the bearing opening  40  of the bearing shaft  20 . 
     Since the bearing openings  40 ,  42 ,  44 ,  46  in this embodiment constitute sliding bearings for the rotating bearing shafts  20 ,  22 , the second cover  14  and the housing  10  are made of a low-friction aluminum material, for example, an aluminum alloy. Since the cover and the housing do not have a complex shape, in particular, an extruded section can be used as the starting material. 
       FIGS. 4 to 6  show a second embodiment of the gear pump  38 , the housing  10 ′ and the first cover  12  according to  FIG. 1  once again being formed integrally and designated as the housing  10 . 
     The essential difference from the first embodiment according to  FIGS. 2 and 3  is the flat configuration of the second cover  14 . As a result, the more complex gasket  28  (see  FIGS. 2 and 3 ) can be replaced by a simple, flat sealing washer  58 , a surface area of the sealing washer  58  corresponding to a surface area of the second cover  14  and the sealing washer  58  lying over its full surface on the second cover  14 . Since the second cover  14  does not have any bearing openings  44 ,  46  to accommodate the bearing shafts  20 ,  22 , in this embodiment, the bearing rest  26  (see  FIG. 1 ) is used once again to support the bearing shafts  20 ,  22 . A compressible sealing element  60  is provided in the bearing rest  26 , said sealing element  60  providing an axial compensation and thus fulfilling the function of the tolerance compensating means  48  in  FIGS. 2 and 3 . The sealing element  60  may be laid into a receiving groove  62  of the bearing rest  26 , which can be seen especially clearly in  FIG. 6 , where an enlarged section VI of  FIG. 5  is shown. The sealing element  60  is affixed by the receiving groove  62  crosswise to the rotational axes of the bearing shafts  20 ,  22 , whereas, due to its compressibility, it can compensate for component tolerances in the axial direction and it slightly tensions the pump components inside the housing  10  relative to each other and against the housing  10  or the second cover  14  so that the pump components are axially affixed. The axial play is, for example, in the order of magnitude of 0.2 mm. A combination gasket, that is to say, a gasket made up of an elastomer with an integrated support ring, is especially well-suited as the sealing element  60  for a gap of this size. The combination gasket can reliably seal such a gap up to an exerted pressure of approximately 120 bar. 
     The greater complexity of this second embodiment as a result of the required bearing rest  26  is offset by the simpler production of the flat, disk-like second cover  14 . In this case, the second cover  14  can be produced as an inexpensive stamped part, and be made from a material such as steel. 
     Regarding the other features and advantages of the gear pump  38 , which are identical to the first embodiment according to  FIGS. 2 and 3 , reference is made particularly to the description of the figures of this first embodiment. 
       FIGS. 7 to 15  show schematic longitudinal sections through gear pumps  38 ,  FIG. 7  depicting the state of the art and  FIGS. 8 to 15  showing embodiments according to the invention. 
     The known gear pump  38  in  FIG. 1  is depicted as a schematic drawing once again in  FIG. 7 . It comprises the housing  10 ′, the two separate covers  12 ,  14 , the bearing shafts  20 ,  22  for the gear wheels  16 ,  18  as well as the two bearing rests  24 ,  26 . In addition, a separate motor housing  64  is drawn which houses a motor (not shown here) that serves to drive the gear pump  38 . 
     The embodiment according to the invention of the gear pump  38  as shown in  FIG. 8  corresponds to the second embodiment as shown in  FIGS. 4 to 6 . The first cover  12  is integrated into the housing  10  and the housing  10  has bearing openings  40 ,  42  for the bearing shafts  20 ,  22  so that only the bearing rest  26  is still needed. The second bearing rest  24  is eliminated. 
     The gear pump  38  in  FIG. 9  corresponds essentially to the first embodiment as shown in  FIGS. 2 and 3 , the second cover  14  not having any bearing openings  44 ,  46  that directly accommodate the bearing shafts  20 ,  22 . Therefore, instead of the tolerance compensating means  48  as shown in  FIGS. 2 and 3 , the bearing rest  26  is provided to accommodate the bearing shafts  20 ,  22 . 
     A third embodiment of the gear pump  38  is shown in  FIG. 10 , in which the second cover  14 , as seen in the axial direction of the bearing shafts  20 ,  22 , extends partially over the gear wheels  16 ,  18 . In the present case, the contact surface of the housing  10  with the second cover  14  runs perpendicular to a lengthwise direction of the bearing shafts  20 ,  22  and through the center of the gear wheels  16 ,  18 . 
     In a fourth embodiment according to  FIG. 11 , the cover adjacent to the motor housing  64 , unlike the definition in  FIG. 7 , is designated as a second cover  14 . Accordingly, the first cover is located on the upper edge in  FIG. 11  and is integrated into the housing  10 . 
       FIGS. 12 to 15  constitute a fifth to eighth embodiment of the gear pump  38 . In contrast to the embodiments 1 to 4 as shown in  FIGS. 8 to 11 , the motor housing  64  is integrally formed onto the housing  10  so that the housing  10  is the pump housing as well as the motor housing  64 . Due to this integrally formed configuration, the additional centering pins  34  or sealing elements such as the pump gasket  35  are no longer necessary and can be dispensed with (see  FIG. 2 ). As seen in the axial direction of the bearing shafts  20 ,  22 , the second cover  14  in embodiments 5 to 8 only extends to a different distance over the bearing shafts  20 ,  22  and/or the gear wheels  16 ,  18 . In  FIG. 14 , the contact surface of the housing  10  with the second cover  14  runs, for example, precisely through the center of the gear wheels  16 ,  18 . 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.