Abstract:
A gear pump comprises a casing having an inlet, an interior, and an outlet. An inlet gear is positioned at the inlet and pressurizes fluid received at the inlet. A drive gear is positioned at the outlet of the casing, the drive gear receiving fluid pressurized by the inlet gear to output pressurized fluid at the outlet. A speed-reduction gear is meshed to the drive gear and connected to the at least one inlet gear, the speed-reduction gear having a greater number of teeth than the drive gear to reduce a rotational speed from the drive gear to the inlet gear, such that the inlet gear has a lower speed that the drive gear. An input shaft is coupled to the drive gear and receives a rotational input to actuate the drive gear.

Description:
TECHNICAL FIELD 
     The technical field relates to positive displacement pumps, and more particularly to gear pumps used with high-speed power sources. 
     BACKGROUND 
     The speed of gear pumps is limited by cavitation. By increasing an inlet pressure of pumps, pumps may operate greater speeds without causing cavitation. As the speed of gear pumps is limited by the inlet pressure, gear pumps are commonly used with cumbersome boost pumps, pressurized tanks and the like to feed pressurized fluid to the inlet. Such techniques, however, present problems were space or weight may be an issue, and may also present unwanted costs and complexity. There is therefore a need for improvement. 
     SUMMARY 
     According to one aspect, there is provided a gear pump comprising a casing having an inlet adapted to receive a fluid, an interior to receive gears to pressurize the fluid, and an outlet to output pressurized fluid; at least one inlet gear positioned at the inlet and adapted to pressurize fluid received at the inlet; a drive gear positioned at the outlet of the casing, the drive gear adapted to received fluid pressurized by the at least one inlet gear to output pressurized fluid at the outlet; a speed-reduction gear meshed to the drive gear and connected to the at least one inlet gear, the speed-reduction gear having a greater number of teeth than the drive gear to reduce a rotational speed from the drive gear to the at least one inlet gear, such that the at least one inlet gear has a lower speed that the drive gear; and an input shaft coupled to the drive gear and adapted to receive a rotational input to actuate the drive gear. 
     In accordance with another aspect, there is provided a method for operating a gear pump comprising: actuating a drive gear with a rotational input; driving an inlet gear through a gear assembly meshed with the drive gear such that inlet gear rotates slower than the drive gear; inletting a fluid supply to the inlet gear whereby the inlet gear pressurizes the fluid supply, and feeds the fluid supply to the drive gear; and outletting the fluid supply further pressurized by the drive gear. 
     Further details of these and other aspects of the improvements presented herein will be apparent from the detailed description and appended figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is an isometric view, partly sectioned, of a high-speed gear pump in accordance with an embodiment of the present application; 
         FIG. 2  is an isometric sectioned view of the high-speed gear pump of  FIG. 1 , with gear rotational directions and fluid flow paths illustrated; 
         FIG. 3  is an isometric view of a gear assembly of a two-stage high-speed gear pump in accordance with another embodiment of the present application; and 
         FIG. 4  is an isometric view of the gear assembly of the two-stage high-speed gear pump of  FIG. 3 , with gear rotational directions and a fluid flow illustrated. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a high-speed gear pump is shown at  10 , for pumping fluids such as oil and fuel. The gear pump  10  has a casing  12  accommodating a gear assembly. To illustrate an interior of the gear pump  10 , a portion of the casing  12  is removed from  FIGS. 1 and 2 . The casing  12  has a fluid inlet  14  and a fluid outlet  16 . 
     The gear pump  10  has an input shaft  18  connected to a power source (not shown), such as a high-speed electric motor or the like. A drive gear  20  is directly coupled to the input shaft  18 . The drive gear  20  is positioned adjacent to the outlet  16  in the casing  12 . 
     A first driven gear  21  and a second driven gear  22  are positioned adjacent to the inlet  14  in the casing  12 . The first driven gear  21  is meshed with the drive gear  20 , and is thereby driven by the input shaft  18 . The second driven gear  22  is meshed with the first driven gear  21 , whereby the first driven gear  21  transmits actuation from the drive gear  20  to the second driven gear  22 . 
     The drive gear  20  has a smaller number of teeth than the first driven gear  21  and the second driven gear  22 . The driven gear  21  and  22  may or may not have the same number of teeth. In  FIGS. 1 and 2 , the drive gear  20  has eight teeth, while the driven gears  21  and  22  both have twelve teeth. Therefore, the gear ratio of the gear assembly results in a smaller rotational speed for the driven gears  21  and  22  than for the drive gear  20 , as the driven gear  21  acts as a speed-reduction gear in the gear assembly. 
     Referring to  FIG. 2 , a direction of rotation of the gears  20 ,  21  and  22  is depicted, as are paths of the fluid flow within the casing  12  from the inlet  14  to the outlet  16 . As illustrated by path A, a first portion of the fluid received by the inlet  14  is pressurized by passing between the first driven gear  21  and an interior of the casing  12 , to reach the outlet  16 . As illustrated by path B, a second portion of the fluid received by the inlet  14  is pressurized by passing between the second driven gear  22  and an interior of the casing  12 . The pressurized fluid illustrated by path B is then at least partially pressurized by passing between the drive gear  20  and the casing  12 , as illustrated by path C. The pressure of the fluid at the outlet  16  is therefore a mix of the pressures of the fluids coming from paths A and C. 
     As the pressure of the fluid is higher at the exit of path B than at the inlet  14 , the drive gear  20  may rotate faster than if it were at the inlet  14 , without causing cavitation. The arrangement by which the smaller and faster gear is at the outlet  16  while the larger and slower gear is at the inlet  14  allows the use of a rotational input of higher speed without causing cavitation. 
     Moreover, in order to increase the pressure at the inlet to gear  20 , the leakage of fluid to path C may be controlled, to return some pressurized fluid to the outlet to gear  22 . The leakage is controlled by a direct path from outlet  16  by a cored line or by increasing the clearance between the gear  20  and the housing. By this leakage, the speed of the drive gear  20  may be increased. 
     The second driven gear  22  may be the only inlet gear, namely the only gear receiving fluid from the inlet  14 . Other configurations are considered as well. 
     Referring to  FIG. 3 , a two-stage high-speed gear pump is illustrated at  30 . For clarity purposes, the gear pump  30  is shown without a casing. The gear-pump  30  has an input shaft  31  connected to a power source, such as a high-speed electric motor or the like. Drive gear  32  is coupled to the input shaft  31 . Outlet-stage driven gear  34  is meshed with the drive gear  32 , whereby rotational actuation of input shaft  31  is transmitted to the driven gear  34  through the drive gear  32 . The drive gear  32  has a smaller number of teeth than the driven gear  34 , and therefore rotates faster than the driven gear  34 . The driven gear  34  acts as a speed-reduction gear in the gear assembly. In  FIG. 3 , as an example, the drive gear  32  has eight teeth, whereas the driven gear  34  has twelve teeth. 
     The outlet-stage driven gear  34  has a coupling shaft  36  by which it is directly coupled to an inlet-stage first gear  38 . The inlet-stage first gear  38  therefore rotates with the outlet-stage driven gear  34 . 
     The inlet-stage first gear  38  is meshed with an inlet-stage second gear  40 . In  FIG. 3 , the first gear  38 , and the second gear  40  of the inlet stage have the same number of teeth, namely eight teeth. The first gear  38  and the second gear  40  may have a different number of teeth. 
     Referring to  FIG. 4 , a direction of rotation of the gears  32 ,  34 ,  38  and  40  is depicted, as are paths of fluid flow from the inlet to the outlet of the gear pump  30 . 
     The inlet stage of the gear pump  30  comprises the first gear  38  and the second gear  40 . Accordingly, inlet fluid D is pressurized by passing through paths E 1  or E 2 , respectively between the tips of the first gear  38  and an interior of the casing (not shown) and between the tips of the second gear  40  and an interior of the casing (not shown). The pressurized fluid from paths E 1  and E 2  them reaches the second stage, as illustrated by path F. 
     The outlet stage of the gear pump  30  comprises the drive gear  32  and the driven gear  34 . The pressurized fluid from the path F is partly directly about the drive gear  32  in path G 1  and about the driven gear  34  in path G 2 , to respectively be pressurized between the drive gear  32  and the casing (not shown), and the driven gear  34  and the casing (not shown). The outlet fluid H is therefore a mix of the pressurized fluid from paths G 1  and G 2 . 
     In the gear pump  30 , the gear with the higher speed is the drive gear  32 . As it is at the outlet of the gear pump  30 , the drive gear  32  is fed pressurized fluid from the inlet stage, whereby it may rotate at higher speed without causing cavitation. By the gear reduction resulting from the gear arrangement of the gear pump  30 , the first and second gears  38  and  40  at the inlet rotate at lower speeds as a function of the inlet pressure. 
     Leakage may be controlled across the inlet stage and outlet stage. By limiting the leakage, the inlet pressure is increased, thereby enabling the gears of the gear pump  30  to rotate faster. 
     As is shown in  FIG. 3 , the second gear  40  has a shaft. Other pump stages may be stacked to the two stages of the gear pump  30 , in a multi-stage configuration. The first gear  38  may be the only inlet gear, namely the only gear receiving fluid from the inlet, or the only gear in the first stage. Other configurations are considered as well. 
     In operating the gear pump  10  ( FIGS. 1-2 ) and the gear pump  30  ( FIGS. 3-4 ), the tip velocity of the gears  21 / 22  and gears  38 / 40 , respectively, may be controlled as a function of the measurement of the fluid inlet pressure, so as not to cause failure due to cavitation. By maintaining a higher inlet pressure, the gears may rotate faster. 
     The gear pump  10  ( FIGS. 1-2 ) and the gear pump  30  ( FIGS. 3-4 ) may be used as fuel pumps. In such a use, the gear pumps  10  and  30  have a compact and simple design. Moreover, the gear pumps  10  and  30  are self-lubricating and may therefore be used in environments where auxiliary lubrication systems are not available. In turbine engine applications, the drive gears may be smaller when receiving a rotational input from the accessory gear box, thereby resulting in a compact gear pump. 
     Still other modifications will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the scope of the appended claims.