The inventors of the present invention have determined that there are numerous shortcomings with the methods and apparatus of the background art relating to gear pumps, specifically external gear pumps.
FIGS. 1(a)–(c) are partial sectional views showing the operation of an external gear pump of the background art. FIGS. 2(a)–(d) are partial sectional views showing the operation of an internal gear pump of the background art. FIG. 3 is an isometric view of an internal gear shaft of an internal gear pump of the background art having an internal and an external gear component. FIG. 4 is a perspective view of an external gear pump according to the background art. As seen in FIGS. 2(a)–(d) and FIG. 3, internal gear pumps generally include a rotating shaft 1 providing a rotational force to a rotor 2, e.g., a larger external gear assembly. A smaller, idler gear 3 is positioned within the external gear assembly, e.g., internal to the rotor 2. Internal gear pumps are generally well-suited for high-viscosity fluids that require medium to low pressure applications. Internal gear pumps are self-priming, relatively simple in design, e.g., only two moving parts and are relatively easy to maintain.
External gear pumps also rely upon the intermeshing of two gears to pump a fluid. However, as seen in FIGS. 1(a)–(c), two identical gears 5, 6 are positioned in a side-by-side arrangement within a generally oval-shaped housing 7 of an external gear pump 12. Typically, only one of the shafts 8, 9 are powered by a motor or other motive force, thereby operating both gears 5,6, as the drive gear 5 in turn drives the driven gear 6 through the meshing arrangement of the two gears 5,6. One of skill in the art will appreciate that only two gears 5,6 are shown in the accompanying figures to simplify the following discussion. However, the present inventors submit that it is common to add additional gears onto a pump shaft, e.g. in a double geared design where each shaft includes concentric gears and adjacent pumping chambers.
As seen in FIG. 1(a), a fluid such as fuel oil is supplied at a pump inlet 10 side of the pump 12. As the gears 5,6 rotate, the fluid is drawn into the teeth 11 of the gears 5,6. As seen in FIG. 1(b), the fluid travels around the circumference of the gears 5,6, e.g., between the sides of the housing 7 and gears 5,6, with relatively little or no fluid passing between the gears themselves. As seen in FIG. 1(c), the pressurized fluid is forced through a pump outlet side 13 of the pump 12. As the individual teeth 11 of the gears release from their meshing arrangement on the inlet side of the pump 12, a low-pressure region is formed that draws additional fuel into the pump inlet 10 to continue the pumping process.
External gear pumps are particularly advantageous for medium to high-pressure applications involving a wide range of fluids and materials, e.g., external gear pumps may be utilized for corrosive and non-corrosive fluids with optimization of construction materials depending on the application. Further, external gear pumps are typically favored in the background art due to the fact that the gears 5,6 and shafts 8,9 of the pump 12 are supported on both sides by bearings, e.g., in contrast to the overhanging design of the internal gear shaft 1 shown in FIG. 3, thus providing relatively “quiet” pump operation in wide ranges of applications, including high pressure hydraulic applications.
Since external gear pumps are supported on both sides of the gears 5,6 of the pump 12 by bearings, it is generally thought by those skilled in the art that premature wear of the gears 5,6, shafts 8,9 and housing 7 is mitigated by the balancing of loads and prevention of deflection of the shaft(s). FIGS. 5 and 6 of the present application show a common external gear pump assembly of the background art. FIG. 5 is a cross-sectional view of an external gear pump having a pair of shafts and a pair of bearing supports according to the background art. FIG. 6 is a cross-sectional view of an external gear pump according to the background art taken along a line extending through the pumping chamber.
A more detailed description of the operation and construction of the external gear pump shown in FIGS. 5 and 6 is provided in U.S. Pat. No. 6,035,718 to Schmidt et al., the entirety of which is hereby incorporated by reference. The external gear pump 12 includes shafts 8, 9 (drive shaft 8), gears 5, 6, (drive gear 5) a pump inlet 10, a pump outlet 13, and gear teeth 11 contacting a wear layer 16 of an interior 15 of the pump chamber at contact points 11′. Spaces 14 between the gear teeth 11 and the interior 15 of the pump chamber allow entrapped fluid to be pumped by the pump 12 during operation. The housing 7 includes journal bearing blocks 20, 20 on either side of, e.g., along lateral surfaces 17, 17, the gears 5,6. The journal bearing blocks 20,20 include journal bearing bores 21, 21 for accommodating and supporting the shafts 8,9 of the gear pump 12.
The present inventors have determined that gear pumps of the background art suffer from the following disadvantages. Forces acting on the gear area of a gear pump are a result of the pressure differential from pump inlet to pump outlet and the pump torque. As described hereinabove, a high pressure region is created at the outlet 13 side of the pump 12 and a relatively low pressure region is created at the inlet 10 side of the pump 12 during any pumping operation. Accordingly, the shafts 8, 9, gears 5,6 and teeth 11 experience a reactive force F that will act toward the pump inlet's 10 low pressure region during operation. The present inventors have determined that this reactive force F ultimately results in premature wearing of the housing, gears, teeth and shafts as tooth contact 11′ and wear.
Further, as bearings and related equipment wear, the shafts 8,9 are more likely to experience deflection that will contribute to additional wear and acceleration of the degradation of pump components. As seen in FIG. 6, the reaction force F is generally opposite to the direction of fluid flow shown leaving the pump outlet 13 and is concentrated along the inlet 10 side of the external gear pump 12 where the gear teeth 11 contact the inlet side of the interior 15,16 of the pump housing 7.
With respect to pump torque, the pump torque acts along a line action in a direction equal to the tooth pressure angle, e.g., typically 25 degrees, through the pitch circle at the point of tooth contact. The forces on the drive and driven gear teeth are equal and opposite. The reaction force for the drive gear is 25 degrees from normal at a direction toward the high-pressure side, e.g., opposite the differential pressure force. The reaction force of the driven gear is 25 degrees from normal toward the low-pressure side, e.g., acting in the same direction as the differential pressure force. However, both of these forces are considerably less than the differential pressure force described hereinabove. The net result is a force on each gear toward the low-pressure side with the drive gear net force being somewhat less than the driven gear net force.