Abstract:
The invention relates to a gear pump in which the amplitudes of the pressure impulses in a pressure chamber can be significantly reduced by a suitable embodiment of a first groove and/or a second groove. The circumferential walls of a pump chamber containing the pressure chamber, in an angular range extending toward the pressure chamber, each have a respective groove communicating with the pressure chamber, by which grooves a radial spacing between the outer circumference of two gear wheels disposed in the pump chamber and the circumferential walls is increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2007/057821 filed on Jul. 30, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is based on a gear pump, in particular for pumping fuel from a fuel tank to a high-pressure fuel pump. 
     2. Description of the Prior Art 
     One such gear pump is known from German Patent Disclosure DE 101 34 622 A1. This gear pump has a housing, in which a pump chamber is formed in which a pair of gear wheels meshing with each other on their outer circumference is disposed. The gear wheels pump the medium to be pumped along pumping conduits, formed between their outer circumference and adjacent circumferential walls of the pump chamber, from a suction chamber into a pressure chamber. In operation of the gear pump, a tooth gap of one gear wheel reaches the pressure chamber, and a tooth gap of the other gear wheel reaches the pressure chamber after it, in chronological succession. The angle of rotation of the gear wheels corresponding to this spacing amounts here to 360°/2z in each case, where z is the number of teeth of the gear wheels. 
     Upon the entry of a tooth gap into the pressure chamber, the volume of the pressure chamber is increased, causing the pressure in the pressure chamber to drop. In operation of the gear pump, pressure pulsations thus occur in the pressure chamber, which have a period corresponding to half the tooth pitch angle of the gear wheels. For a number of teeth z=10, the period is thus 18°, relative to the respective axes of rotation of the gear wheels. In other words, after 18° of an angle of rotation of each of the gear wheels, one tooth gap enters the pressure chamber, and the pressure chamber volume is increased. The pressure pulsations lead to high mechanical stress on the gear pump, which must be taken into account by means of an expensive construction of the gear pump that has the requisite strength. Moreover, the pressure pulsations reduce the pumping capacity of the gear pump. To reduce these pulsations, it has been proposed in DE 101 34 622 A1 that grooves be provided, by which the radial spacing between the outer circumference of the gear wheels and the circumferential walls is increased. By means of this provision, it has already been possible to achieve a significant reduction in the amplitudes of the pressure pulsations. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The invention has the object of furnishing a gear pump in which further reduced amplitudes of the pressure pulsations are achieved. 
     The gear pump according to the invention has the advantage that as a result of the different radial spacings of the grooves in the circumferential walls of the pump chamber, a further reduction of the pressure pulsations is attained. As a result, the mechanical stress on the gear pump is reduced, so that the pump can be manufactured more economically. Moreover, the pumping capacity of the gear pump is improved as a result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One exemplary embodiment of the invention is shown in the drawings and described in further detail in the ensuing description, in which: 
         FIG. 1  shows a gear pump in a cross section; and 
         FIG. 2  shows the course of pressure pulsations in the pressure chamber of the gear pump. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1 , a gear pump is shown that serves in particular for pumping fuel from a fuel tank to a high-pressure pump of a fuel injection system of an internal combustion engine of a motor vehicle. The gear pump has a multi-part housing, with one housing part  10  in which a pump chamber  12  is formed. A pair of gear wheels  14 ,  16 , meshing with one another on their outer circumference and each having a face-end serration, is disposed in the pump chamber  12 . The gear wheel  14  is supported rotatably about an axis  15 , and the gear wheel  16  is supported rotatably about an axis  17 . One of the gear wheels  14 ,  16 , for instance the gear wheel  14 , is driven in a manner not shown in further detail to revolve about its axis of rotation  15  in a direction of rotation  18 , and via a tooth engagement it drives the other gear wheel  16  to rotate about its axis of rotation  17  in a direction of rotation  19 . The teeth of the gear wheels  14 ,  16  define a circular pitch p. 
     The pump chamber  12  has circumferential walls  20 ,  22 , which are oriented toward the outer circumferences of the gear wheels  14 ,  16  and are suitably curved in concave fashion. The gear wheels  14 ,  16  mesh approximately in the middle of the pump chamber  12 ; originating with the tooth engagement of a gear wheel  14 ,  16  on the side pointing in the directions of rotation  18 ,  19 , a suction chamber  24  is formed in the pump chamber  12 , and on the side pointing counter to the directions of rotation  18 ,  19 , a pressure chamber  26  is formed in the pump chamber  12 . Beginning at the suction chamber  24 , one pumping conduit  28 ,  30  each is formed between the outer circumference of the respective gear wheel  14 ,  16  and the adjoining circumferential wall  20 ,  22  of the pump chambers  12  In operation of the gear pump, fuel is pumped out of the suction chamber  24  along the pumping conduits  28 ,  30  into the pressure chamber  26  by the gear wheels  14 ,  16  in their tooth gaps. An inlet from the fuel tank discharges into the suction chamber  24 , and a communication with the high-pressure fuel pump leads away from the pressure chamber  26 . 
     In the circumferential wall  20  of the pump chamber  12  oriented toward the gear wheel  14 , a groove  32  is embodied, by which the radial spacing between the outer circumference of the gear wheel  14  and the circumferential wall  20  is increased. The first groove  32  extends in the direction of rotation  18  of the gear wheel  14  as far as the beginning of the pressure chamber  26 . In a first angular range α 1  of the first groove  32 , there is a radial spacing A r1  between the circumferential wall  20  and the first gear wheel  14 . In a second angular range α 2  of the first groove  32 , there is a radial spacing A r2  between the circumferential wall  20  and the first gear wheel  14 , and A r1  is not equal to A r2 . In the exemplary embodiment shown in  FIG. 1 , A r1  is greater than A r2 . At least one of the first angular range α 1  and the second angular range α 2  is greater than the circular pitch p. In the illustrated embodiment, both the first angular range α 1  and the second angular range α 2  are greater than the circular pitch p. 
     In the circumferential wall  22  of the pump chamber  12  oriented toward the gear wheel  16 , a second groove  34  is embodied, by which the radial spacing between the outer circumference of the gear wheel  16  and the circumferential wall  22  is increased. The second groove  34  extends in the direction of rotation  19  of the gear wheel  16  as far as the beginning of the pressure chamber  26 . In a first angular range β 1  of the second groove  34 , there is a radial spacing A r1  between the circumferential wall  22  and the second gear wheel  16 . In a second angular range β 2  of the second groove  34 , there is a radial spacing A r2  between the circumferential wall  22  and the first gear wheel  14 , and A r1  is not equal to A r2 . In the exemplar embodiment shown in  FIG. 1 , A r1  is greater than A r2 . At least one of the first angular range β 1  and the second angular range β 2  is greater than the circular pitch p. In the illustrated embodiment, the first angular range β 1  is greater than the circular pitch p and the second angular range β 2  is less than the circular pitch p. 
     As a result of this embodiment of the grooves  32 ,  34  a further reduction in the pressure pulsations is attained. Alternatively, the same effect can also be attained by means of a purposeful arrangement of a plurality of grooves, each of constant cross section. However, that variant is more expensive to produce and has a greater tendency to cavitation. 
     To prevent cavitation phenomena: the transition regions  36  between the first angular ranges α 1 , β 1  on the one hand and the second angular ranges α 2 , β 2  on the other are rounded. In a simplified embodiment, the transition regions  36  may also be shortened or even be omitted entirely. 
     In  FIG. 2 , the amplitude of the pressure pulsations in the pressure chamber  26  is plotted over the rpm n of the gear pump. A first line  38  indicates the amplitude of the pressure pulsations of a gear pump according to the prior art. A second line  40  represents the amplitude of the pressure pulsations in a gear pump according to the invention. 
     A comparison of the first line  38  and the second line  40  makes it immediately clear that the amplitudes of the pressure pulsations in the gear pump of the invention can be reduced even more markedly, compared to the gear pump known from DE 101 34 622 A1. As a result, the gear pump of the invention can not only be made lighter in weight and produced more economically, but the pressure regulation in the low-pressure region of the fuel injection system is also improved on the intake side of a downstream high-pressure fuel pump. 
     The foregoing relates to the 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.