Gear pump, in particular fuel pump

Abstract

A gear pump includes at least two gear wheels, which are disposed in a housing and mesh with one another and pump a fluid from a suction region to a compression region. At least one of the gear wheels includes tooth flanks, which each have at least one generally radially extending recess, which in the region in which the two gear wheels mesh with one another connects a fluid chamber, adjacent to the root region of the gear wheel, to a fluid chamber adjacent to the tip region of that gear wheel.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to improvements in gear pumps, in particular a fuel pump, having at least two gear wheels which are disposed in a housing and mesh with one another and pump a fluid from a suction region to a compression region.

2. Description of the Prior Art

A gear pump of the type with which this invention is concerned, known for instance from German Patent Disclosure DE 101 34 622 A1, employs two gear wheels disposed adjacent to and meshing with one another. The region in which the two gear wheels mesh with one another divides a compression region from a suction region of the gear pump, as a result of the fact that there, in the course of the motion of the gear wheels, a tooth of one gear wheel penetrates a tooth interstice of the other gear wheel. As a result, the fuel located in the tooth interstice is expelled from the tooth interstice.

When the tooth of one gear wheel is in full engagement with the tooth interstice of the other gear wheel, a closed fluid chamber which is filled with fuel forms between the tooth and a root region of the other gear wheel. In the further course of the motion of the gear wheels, the volume of this fluid chamber becomes less and less, which because of the virtually incompressible behavior of fuel, leads to a very pronounced pressure increase. Through system-dictated gaps, some of the enclosed fuel escapes from the closed fluid chamber. In the course of the further motion, the tooth of the one gear wheel leaves the tooth interstice of the other gear wheel again. As a result, an increase in the volume of the closed fluid chamber occurs again, causing a correspondingly pronounced pressure decrease.

The pronounced pressure fluctuations in the closed fluid chamber lead to a major bending stress on the teeth that define it. Moreover, cavitation can occur in the fluid chamber, which additionally stresses the structures defining the fluid chamber. Moreover, the high pressure in the closed fluid chamber presses the gear wheels away from one another, putting additional stress on their bearings. It is therefore known to provide grooves in lateral housing parts; these grooves connect the closed fluid chamber at least intermittently with the compression region or the suction region. As a result, pressure peaks in the closed fluid chamber are dissipated.

OBJECT AND SUMMARY OF THE INVENTION

The present invention has the object of refining a gear pump of the type defined at the outset such that it can be constructed as inexpensively as possible and has a long service life.

This object is attained, in a gear pump of this type, in that at least one of the gear wheels has tooth flanks, which each have at least one recess that in the region in which the two gear wheels mesh with one another connects a fluid chamber, adjacent to the root region, with a fluid chamber adjacent to the tip region.

In an inventive way, it has been found that in conventional gear pumps with gear wheels with typical tooth geometries, in the region where the two gear wheels mesh with one another, there are always two directly adjacent closed fluid chambers, namely one chamber between a tooth of one gear wheel and a root region of the other gear wheel, and one chamber between a tooth of the other gear wheel and a root region of the first gear wheel. The volume of one closed fluid chamber is already increasing again while the volume of the other, immediately adjacent, closed fluid chamber is still compressed. By the provisions of the present invention, these two adjacent fluid chambers are made to communicate with one another.

The rotary angle range in which this total fluid chamber is closed off from the compression region and the suction region is considerably smaller than in the case where the two fluid chambers are separated from one another. This already results in comparatively little compression of the fuel or fluid enclosed in the total fluid chamber. Moreover, the compression in one partial fluid chamber is at least partly compensated for by the already-ensuing expansion in the other partial fluid chamber, which additionally lessens the pressure fluctuation throughout the entire volume.

By the provisions of the present invention, the pressure fluctuations in the closed fluid chambers that are present in the meshing region of the two gear wheels can be reduced by a factor of up to 3 to 4, compared to fuel pumps that do not have the recess of the invention. The bending forces acting on the teeth of the gear wheels are correspondingly less, and hence the gear wheels can be made for instance from a less expensive material. The stress on the bearings of the gear wheels is also reduced, resulting in less friction and making it possible to use less expensive bearings. The reduced pressure fluctuations also mean less risk of cavitation, so that in the gear pump of the invention, a longer service life can be expected. Compared to gear pumps that have slits or grooves on the housing, through which the meshing region can be made to communicate with the compression region and/or the suction region, the gear pump of the invention has improved efficiency, since the pressure in the suction region and compression region is unaffected by the provisions of the invention.

Advantageous refinements of the invention are disclosed. In a first refinement, it is proposed that there is a spacing between one edge of the tooth flank and one edge of the recess. This assures that despite the recess, the corresponding tooth flank of one gear wheel can always be in direct contact with the opposite tooth flank of the other gear wheel, and as a result both alternating stresses on the gear wheels and clacking noises are avoided.

It is also proposed that a spacing be present between the radially outer end of the recess and a tip circle of the gear wheel. This assures good efficiency of the gear pump. The efficiency depends on how well a tooth interstice, present between two teeth of one gear wheel, is sealed off from a circumferential wall of the housing. This sealing remains optimal because provisions made by the invention.

An especially inexpensive embodiment of the gear pump of the invention is possible if the recess includes a lateral oblique flattened face, in particular a polished section.

Alternatively, or in addition to this, it is also possible for the recess to include at least one flute or groove. A generally radial flute or groove of this kind may for instance be disposed centrally in the tooth face, thus avoiding an asymmetrical stress on a tooth.

With a view to the service life, it is favorable if the recess is present only on the non-driving and/or non-driven tooth flanks of a gear wheel. This is in response to the thought that in conventional gear pumps, only one of the two gear wheels is driven. Each tooth of a gear wheel has two tooth flanks facing away from one another. Upon force transmission from one gear wheel to the other, essentially only the leading tooth flanks, in terms of the direction of rotation, are involved for the driving gear wheel, while for the driven gear wheel, essentially only the trailing tooth flanks, in terms of the direction of rotation, are involved in the force transmission. According to the invention, it is therefore proposed that the recesses be disposed only on the respective other, less-stressed tooth flanks.

In a refinement of this embodiment, it is proposed that the recesses be present only on the tooth flanks of the driven gear wheel. As a result, the production costs are lowered.

The advantages of the invention can be attained especially easily and effectively if the gear wheels have a symmetrical tooth geometry, and in particular a simple evolute toothing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which:

FIG. 1 is a schematic plan view on a fuel system with a gear pump having one driving and one driven gear wheel;

FIG. 2 is a perspective view of a region of the driven gear wheel of FIG. 1;

FIG. 3 is an enlarged view of a region of the two gear wheels of FIG. 1 in which these gear wheels mesh with one another;

FIG. 4 is a graph in which the course of the volume of a fluid chamber, enclosed between the two gear wheels, is plotted over the rotary angle for different configurations of the gear pump of FIG. 1; and

FIG. 5 is a view similar to FIG. 2 for an alternative version of a gear pump.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a gear pump is identified overall by reference numeral 10. It functions as a fuel pump in a fuel system 12. To that end, it aspirates fuel from a fuel tank 14 and pumps it to a high-pressure pump 16, from which it passes onward to reach a fuel collection line 18 (or “rail”). A plurality of injectors 20 are connected to this rail and inject the fuel directly into combustion chambers 22 assigned to them.

The fuel pump 10 includes a housing 24, in which there is a recess that forms a pump chamber 26. Two gear wheels 28 and 30 are disposed in the pump chamber 26. The teeth of the gear wheel 28 on the left in FIG. 1 are identified by reference numeral 32, while the tooth interstices between them are identified by reference numeral 34. The teeth of the gear wheel 30 on the right are identified by reference numeral 36 and the corresponding tooth interstices by reference numeral 38. In the fuel pump 10 shown in FIG. 1, only the left gear wheel 28 is driven. A corresponding drive shaft is identified by reference numeral 40. A bearing shaft of the right gear wheel 30, that is, the driven gear wheel, is identified by reference numeral 42.

The geometry of the teeth 32 and 36 of the two gear wheels 28 and 30 is embodied identically, on the order of an evolute toothing. This means that leading and trailing tooth flanks, in terms of the direction of rotation, of the teeth 32 and 36 are embodied symmetrically. The leading tooth flanks, in terms of the direction of rotation, of the left gear wheel 28 are identified by reference numeral 44 and the trailing ones by reference numeral 46; the leading tooth flanks, in terms of the direction of rotation, of the right gear wheel 30 are identified by reference numeral 48 and the corresponding trailing ones by reference numeral 50.

The leading tooth flanks 48, in terms of the direction of rotation, of the right gear wheel 30 have a special feature: They are each provided with a lateral oblique polished section 56. It extends not over the full width of one tooth flank 48, but over only approximately half of it, so that between the polished section 56 and the opposite edge of the tooth 36, there is a spacing A (see FIG. 2). Each gear wheel 28 and 30 is also defined geometrically by a so-called tip circle 58 and a so-called root circle 60. The tip circle 58 is the imaginary line that connects the radially protruding tips of the teeth 36 of a gear wheel 30 to one another. The corresponding line that connects the radially farthest inward regions of the tooth interstices 38 of a gear wheel 30 to one another is called the root circle 60. The polished sections 56 on the teeth 36 of the gear wheel 30 are designed such that they end at a spacing B from the tip circle 58 of the gear wheel 30.

In operation, the gear wheel 28 on the left in FIG. 1 rotates clockwise, while the right gear wheel 30 rotates counterclockwise. In the tooth interstices 34 and 38 that are defined by a respective boundary wall 62 and 64 of the pump chamber 26, fuel is transported from a suction region 66 into a compression region 68. The gear wheels 28 and 30 mesh with one another in a meshing region 70. This means that the teeth 32 of the left gear wheel 28 first enter the tooth interstices 38 of the right gear wheel 30 in the compression region 68 and then emerge from them again in the suction region 66. Analogously, the teeth 36 of the right gear wheel 30 enter the tooth interstices 34 of the left gear wheel 28 in the compression region 68 and leave it again in the suction region 66.

Because of the symmetrical evolute toothing selected, a closed fluid chamber 72 results in the meshing region 70 between the two gear wheels 28 and 30; this fluid chamber is composed of two partial chambers 74 and 76 (FIG. 3). The fluid chamber 72 is separated from the compression region 68 by the contact of the leading tooth flank 44 of the left gear wheel 28 with the trailing tooth flank 50 of the right gear wheel 30 (position 78). The fluid chamber 72 is separated from the suction region 66 by a similar place, identified by reference numeral 80. The trailing tooth flank 46 of the left gear wheel 28 located in the meshing region 70 also contacts the opposed leading tooth flank 48 of the right gear wheel 30 at the point 82. In contrast to the sealing points 78 and 80, however, the contact point 82 is not fluid-tight; instead, the two partial chambers 74 and 76 communicate with one another via a connecting conduit 84 that is created by the polished section 56.

During the rotary motion of the two gear wheels 28 and 30, the tooth 36 of the right gear wheel 30 first moves farther into the tooth interstice 34 on the left gear wheel 28, causing a reduction in the volume of the partial fluid chamber 74. Simultaneously, the tooth 32 of the left gear wheel 28 is already moving out again of the tooth interstice 38 of the right gear wheel 30, and as a result the volume of the partial fluid chamber 76 increases. Some of the progressive volumetric reduction of the left partial chamber 74 is compensated for by the progressive increase in the volume of the right partial chamber 76.

In FIG. 4, the volume V of the fluid chamber 72 is plotted over the rotary angle W of the right gear wheel 30. The corresponding curve is marked 86. Only within an angle range W1 is the fluid chamber 72 separated in fluid-tight fashion from the suction region 66 and the compression region 68. Upon a motion of both gear wheels 28 and 30 within this angle range W1, a maximum compression dV1 of the volume of the fluid chamber 72 results. Corresponding curves that would result if the two partial chambers 74 and 76 were not in communication with one another are also shown in FIG. 4. The curve for the left partial chamber 74 is shown in dashed lines and that for the right partial chamber 76 in dotted lines. It can be seen that an angle range W2 during which the two partial chambers 74 and 76 would be separated in fluid-tight fashion from the suction region 66 and the compression region 68 would be twice as large. It can also clearly be seen that a compression dV2 of each of the two partial chambers 74 and 76 would be several times greater than in the case described above, in which the two partial chambers 74 and 76 communicate with one another.

In the present exemplary embodiment, the compression dV1 of the fluid chamber 72 is smaller, thanks to the connecting conduit 84 that is created by the polished section 56, by more than a factor of 3.5 than the compression dV2 that would exist without the connecting conduit 84. As a result, the pressure stress on the teeth 32 and 36 of the gear wheels 28 and 30 drops. The risk of the occurrence of cavitation whenever a tooth 32 or 36 moves out of the opposed tooth interstice 38 or 34, respectively, is reduced as well, and the loads on the drive shaft 40 and the bearing shaft 42 are decreased. The fuel pump 10 thus has high efficiency, since the radially outer ends of the teeth 32 and 36 can move past the boundary walls 62 and 64 with a slight gap from them, and since in the meshing region 70 as well, a secure separation between the suction region 66 and the compression region 68 is assured.

An alternative embodiment of a gear wheel 30, which may be employed in a fuel pump of the type shown in FIG. 1, is shown in FIG. 5. Those elements and regions that have equivalent functions to elements and regions of the fuel pump 10 as shown in FIGS. 1 through 4 are identified by the same reference numerals. They are not described again in detail.

An essential distinction is that instead of a polished section 56 in the leading-edge tooth flanks 48 of the gear wheel 30, a groove 56 is present, disposed centrally between the two lateral edges and extending overall in the radial direction.

The foregoing relates to 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.

Claims

1. In a gear pump (10), having a housing (24) and at least two gear wheels (28, 30) disposed in the housing (24) and meshing with one another to pump a fluid from a suction region (66) and a compression region (68) in the housing (24), the improvement wherein at least one of the gear wheels has tooth flanks (48), which each have at least one recess (56) that, in a region (70) in which the two gear wheels (28, 30) mesh with one another, connects a fluid chamber (76), adjoining the root region (60) of one gear wheel (30), to a fluid chamber (74) adjacent to the tip region (58) of that gear wheel.

2. The gear pump (10) in accordance with claim 1, further comprising a spacing (A) is provided between one edge of the recess (56) and one edge of the tooth flank (48).

3. The gear pump (10) in accordance with claim 1, further comprising a spacing (B) is provided between the radially outer end of the recess (56) and a tip circle (58) of the gear wheel (30).

4. The gear pump (10) in accordance with claim 2, further comprising a spacing (B) is provided between the radially outer end of the recess (56) and a tip circle (58) of the gear wheel (30).

5. The gear pump (10) in accordance with claim 1, wherein the recesses (56) include a lateral oblique flattened face, in particular a polished section (56).

6. The gear pump (10) in accordance with claim 2, wherein the recesses (56) include a lateral oblique flattened face, in particular a polished section (56).

7. The gear pump (10) in accordance with claim 3, wherein the recesses (56) include a lateral oblique flattened face, in particular a polished section (56).

8. The gear pump (10) in accordance with claim 4, wherein the recesses (56) include a lateral oblique flattened face, in particular a polished section (56).

9. The gear pump (10) in accordance with claim 1, wherein the recess (56) includes at least one flute or groove (56).

10. The gear pump (10) in accordance with claim 2, wherein the recess (56) includes at least one flute or groove (56).

11. The gear pump (10) in accordance with claim 1, wherein the recesses (56) are present only on the non-driving and/or non-driven tooth flanks (48) of a gear wheel (30).

12. The gear pump (10) in accordance with claim 2, wherein the recesses (56) are present only on the non-driving and/or non-driven tooth flanks (48) of a gear wheel (30).

13. The gear pump (10) in accordance with claim 3, wherein the recesses (56) are present only on the non-driving and/or non-driven tooth flanks (48) of a gear wheel (30).

14. The gear pump (10) in accordance with claim 5, wherein the recesses (56) are present only on the non-driving and/or non-driven tooth flanks (48) of a gear wheel (30).

15. The gear pump (10) in accordance with claim 1, wherein the recesses (56) are present only on the tooth flanks (48) of the driven gear wheel (30).

16. The gear pump (10) in accordance with claim 2, wherein the recesses (56) are present only on the tooth flanks (48) of the driven gear wheel (30).

17. The gear pump (10) in accordance with claim 3, wherein the recesses (56) are present only on the tooth flanks (48) of the driven gear wheel (30).

18. The gear pump (10) in accordance with claim 5, wherein the recesses (56) are present only on the tooth flanks (48) of the driven gear wheel (30).

19. The gear pump (10) in accordance with claim 9, wherein the recesses (56) are present only on the tooth flanks (48) of the driven gear wheel (30).

20. The gear pump (10) in accordance with claim 1, wherein the gear wheels (28, 30) have a symmetrical tooth geometry, in particular a simple evolute toothing.