Patent Publication Number: US-2019178245-A1

Title: Reduction of cavitation in fuel pumps

Description:
BACKGROUND 
     The subject matter disclosed herein generally relates to the field of fuel pumps, and more particularly to an apparatus and method for reducing cavitation in fuel pumps. 
     Aircraft gas turbine engines receive pressurized fuel from different kinds of fuel pumps including gear-type fuel pumps. The gear pump typically performs over a wide operational speed range while providing needed fuel flows and pressures for various engine performance functions. 
     Gear pumps often comprise two coupled gears of similar configuration and size that mesh with each other inside an enclosed gear housing. A drive gear may be connected rigidly to a drive shaft. As the drive gear rotates, it meshes with a driven gear thus rotating the driven gear. As the gears rotate within the housing, fluid is transferred from an inlet to an outlet of the gear pump. Typically, the drive gear carries the full load of the gear pump drive or input shaft. The two gears may operate at high loads and high pressures, which may stress the gear teeth. 
     For given gear sizes the volume of fluid pumped through the gear pump may partially depend on the geometry of the tooth (e.g., depth, profile, etc.), the tooth count, and the width of the gear. Most gear pumps have gears with about ten to sixteen teeth. As the gears rotate, individual parcels of fluid are released between the teeth to the outlet. A common problem with more traditional gear pumps operating at high rotational speeds is cavitation erosion of the surfaces of the gear teeth and bearings. Cavitation erosion results in pitting of surfaces of the gear teeth that may eventually result in degraded pump volumetric capacity and affect pump operability and durability. 
     BRIEF SUMMARY 
     According to one embodiment, will track claims a fluid gear pump is disclosed. The pump includes a first gear constructed and arranged to rotate about a first axis, the first gear including a concentrically disposed first hub portion and a plurality of first teeth radially projecting and circumferentially spaced about the first hub portion, the first teeth being coated with a vapor-deposited of cavitation resistant coating. The pump also includes a second gear operably coupled to the first gear for rotation about a second axis, the second gear including a concentrically disposed second hub portion and a plurality of second teeth radially projecting and circumferentially spaced about the second hub portion. At a time in operation the plurality of first teeth and the plurality of second teeth contact at first contact point and a second contact point to create a backlash volume interposed between the first contact point and the second contact point. The pump also includes a first bearing abutting and coaxial to the first hub portion; and a second bearing abutting and coaxial to the second hub portion. 
     In any prior embodiment, the coating is deposited by filter arc deposition. 
     In any prior embodiment, the coating is a nickel titanium (NiTi). 
     In any prior embodiment, the coating is deposited by chemical vapor deposition (CVD) or physical vapor deposition (PVD). 
     In any prior embodiment, the coating is at least one of titanium nitride, titanium aluminium nitride (TiAlN), titanium aluminum silicon nitride or a tungsten containing material. 
     In any prior embodiment, the coating is a diamond-like carbon 
     In any prior embodiment, the pump further includes a first shaft on which the first gear is carried, and a second shaft on which the second gear is carried. In this embodiment. The second gear is also coated with the vapor-deposited of cavitation resistant coating. 
     In any prior embodiment, the pump further includes a third gear carried on the second shaft. 
     In any prior embodiment, one or more of the first, second and third gears include a nickel titanium coating thereon. 
     In one embodiment, a method of reducing cavitation during fluid gear pump operation is disclosed. The method includes: rotating a first gear around first axis, the first gear including a concentrically disposed first hub portion and a plurality of first teeth radially projecting and circumferentially spaced about the first hub portion, wherein the first teeth are coated with a vapor-deposited of cavitation resistant coating; rotating a second gear coupled to the first gear about a second axis, the second gear including a concentrically disposed second hub portion and a plurality of second teeth radially projecting and circumferentially spaced about the second hub portion, wherein the plurality of first teeth engage the plurality of second teeth; and transferring fluid from a low pressure side to a high pressure side when the first gear is rotating and the second gear is rotating. 
     In any prior method, wherein the first gear is coated with nickel titanium. 
     In any prior method, the first gear is coated with one of: titanium nitride, titanium aluminium nitride (TiAlN), titanium aluminum silicon nitride, tungsten or tungsten carbide, or a diamond-like carbon. 
     In one embodiment, a fluid gear pump gear arranged to rotate about a first axis is disclosed. The fluid gear pump gear includes a concentrically disposed first hub portion and a plurality of first teeth radially projecting and circumferentially spaced about the first hub portion, the first hub portion and the first teeth being coated with a vapor-deposited of cavitation resistant coating; and a first shaft on which the first hub portion is carried. 
     The fluid gear pump gear of any prior embodiment, wherein the first gear is coated with nickel titanium. 
     The fluid gear pump gear of any prior embodiment, wherein the first gear is coated with one of: titanium nitride, titanium aluminium nitride (TiAlN), titanium aluminum silicon nitride, tungsten or tungsten carbide, or a diamond-like carbon. 
     The fluid gear pump gear of any prior embodiment, further comprising a second gear carried on the first shaft. 
     The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, that the following description and drawings are intended to be illustrative and explanatory in nature and non-limiting. 
    
    
     
       BRIEF DESCRIPTION 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  illustrates a schematic of an aircraft fuel system as one, non-limiting, example of an application of a gear pump of the present disclosure; 
         FIG. 2  illustrates a perspective view of the gear pump with a housing removed to show internal detail; 
         FIG. 3  shows a side view of the drive and driven gears of the gear pump; 
         FIG. 4  is a perspective view of a portion of a gear according to one or more embodiments; and 
         FIG. 5  is a cross-sectional view of a tooth of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
     Various embodiments of the present disclosure are related to the reduction of fluid cavitation within gear pumps. Aircraft engine high pressure fuel pumps typically use a pair of involute gears to generate fuel pressure for the burner injectors. These gears are enclosed in a housing within which they are supported by bearings. In the vicinity of the gear meshing region these bearings form a bridgeland that separates the high and low pressure regions and maintains high pump efficiency. A pump of this description experiences significant pressure oscillations that may lead to the formation and subsequent collapse of cavitation bubbles that may cause material damage. The gears may be especially susceptible to cavitation damage and that results in a deterioration of pump performance and can significantly reduce the useable life of these components. 
     To address these issues a gear shaft that includes gear teeth coated with a cavitation resistant coating. Utilization of such a coating for the gears generally and gear teeth in particular may have the technical effect of reducing cavitation on the gear teeth. Non-limiting examples of such coatings include NiTi (nickel titanium) applied by filtered arc deposition, physical vapor deposition (PVD) or chemical vapor deposition (CVD) applied coatings, such as titanium nitride, titanium aluminium nitride (TiAlN), or titanium aluminum silicon nitride, tungsten or tungsten carbide, or diamond-like carbon, or high velocity oxygen fuel (HVOF)-sprayed carbidethermal spray powders such as WC-10Co-4Cr. In some embodiments, only a portion of a gear tooth may include a coating and the rest formed of another material, such as stainless steel. 
     Referring to  FIG. 1 , one embodiment of a fuel system  20  of the present disclosure is illustrated. The fuel system  20  may be an aircraft fuel system and may include a fuel supply line  22  that may flow liquid fuel from a fuel tank  24  to fuel nozzles  26  of an engine (not shown). A fuel bypass line  28  may be arranged to divert fuel from the supply line  22  and back to the fuel tank  24 . Various fuel system components may interpose the fuel supply line  22  and may include a low pressure fuel pump  30 , a heat exchanger  32 , a fuel filter  34 , a high pressure fuel pump  36 , a metering valve  38 , a high pressure fuel shutoff valve  40 , a screen  42 , a fuel flow sensor  44 , and a fuel tank shutoff valve  45 . The low pressure fuel pump  30  may be located downstream of the fuel tank  24 . The heat exchanger  32  may be located downstream of the low pressure fuel pump  30 . The fuel filter  34  may be located downstream of the heat exchanger  32 . The high pressure fuel pump  36  may be located downstream of the fuel filter  34  and upstream of the fuel bypass line  28 . The metering valve  38  may be located downstream from the bypass line  28 . The high pressure fuel shutoff valve  40  may be located downstream from the bypass line  28 . The screen  42  may be located downstream from the high pressure fuel shutoff valve  40 , and the fuel flow sensor  44  may be located downstream from the screen  42 . It is further contemplated and understood that other component configurations of a fuel system are applicable and may further include additional sensors, valves and other components. 
     The heat exchanger  32  may be adapted to use the flowing fuel as a heat sink to cool other liquids flowing from any variety of auxiliary systems of an aircraft and/or the engine. For example, the heat exchanger  32  may transfer heat from an oil and to the fuel. The oil may be used to lubricate any variety of auxiliary components including, for example, a gear box (not shown) of the engine. Such a transfer of heat may elevate the temperature of the fuel which may make the high pressure fuel pump  36  more prone to cavitation. 
     Referring to  FIGS. 2 and 3 , one non-limiting example of the high pressure fuel pump  36  is illustrated as a gear pump with a housing removed to show internal detail. The housing is shown, generally, by dashed line  61 . 
     The gear pump  36  may be a dual stage pump and may include an fuel centrifugal boost pump housing  46 , an input drive shaft  48  constructed for rotation about a first axis  50 , a coupling shaft  52  constructed for rotation about a second axis  54 , a drive gear  56  with associated bearings  58 , a driven gear  60  with associated bearings  62 , a motive drive gear  64  and a motive driven gear  66  configured for rotation about a third axis  68 . The axis  50 ,  54 ,  68  may be substantially parallel to one-another. The drive shaft  48  may attach to an engine gear box (not shown). The drive gear  56  is engaged and concentrically disposed about the drive shaft  48 . The driven gear  60  and motive drive gear  64  are engaged and concentrically disposed about the coupling shaft  52 . 
     The drive and driven gears  56 ,  60  are rotationally coupled to one another for the pumping (i.e., displacement) of fuel as a first stage, and the motive drive gear  64  and motive driven gear  66  are rotationally coupled to one another for the continued pumping of the fuel as a second stage. 
     In one embodiment, some or all of at least one of the drive gear  56  and the driven gear  60  is coated with a coating as described herein. In one embodiment, all of the one or both of the drive or driven gears  56 ,  60  are formed of a so coated. In another, only the teeth are coated and in yet another, only a portion of one or more of the teeth is coated. As discussed above, examples of suitable coatings include NiTi applied by filtered arc deposition, PVD or CVD applied coatings, such TiN, TiAlN, TiAlSiN, tungsten or tungsten carbide or diamond-like carbon, or high velocity oxygen fuel (HVOF)-sprayed carbide thermal spray powders such as WC-10Co-4C. The same may also apply to the motive drive and motive driven gears  64 ,  66 . 
     It is further contemplated and understood that many other types of gear pumps may be applicable to the present disclosure. For example, the gear pump may be a single stage gear pump, and/or the drive shaft  48  may be attached to any other device capable of rotating the drive shaft  48  (e.g., electric motor). 
     The bearings  58 ,  62  may be inserted into a common carrier  70  that generally resembles a figure eight. A gear bearing face geometry, known in the art as a bridgeland  100  may be sculpted to minimize cavitation and pressure ripple that may deteriorate the integrity of the pump components, discussed further below. The bridgeland  100  separates a low pressure side and a high pressure side of the pump. 
     In operation, the gear pump  36  is capable of providing fuel at a wide range of fuel volume/quantity and pressures for various engine performance functions. The engine gearbox provides rotational power to the drive shaft  48  which, in-turn, rotates the connected drive gear  56 . The drive gear  56  then drives (i.e., rotates) the driven gear  60  that rotates the coupling shaft  52 . Rotation of the coupling shaft  52  rotates the motive drive gear  64  that, in-turn, rotates the motive driven gear  66 . 
       FIG. 4  shows a perspective view of a gear. The gear can be any of the drive gear  56 , the driven gear  60 , the motive drive gear  64  and the motive driven gear  66 . Referring to  FIG. 4 , each of the gears  56 ,  60 ,  64 ,  66  may include a hub portion  72  and a plurality of teeth  74  that may both span axially between two opposite facing sidewalls  76 ,  78 . Each sidewall  76 ,  78  may lay within respective imaginary planes that are substantially parallel to one-another. The hub portion  72  may be disc-like and projects radially outward from the respective shafts  48 ,  52  and/or axis  50 ,  54 ,  68  to a circumferentially continuous face  80  generally carried by the hub portion  72 . The face  80  may generally be cylindrical. The plurality of teeth  74  project radially outward from the face  80  of the hub portion  72  and are circumferentially spaced about the hub portion  72 . The gears  56 ,  60 ,  64 ,  66  may be spur gears, helical gears or other types of gears with meshing teeth, and/or combinations thereof. 
     The hub portion  72  can be coated in one embodiment. In such an embodiment, the spaces between the teeth  74  (shown by reference numeral  75 ) may be coated. The coating can include any of the coatings disclosed herein or other suitable coatings. 
     A suitable coating will have, in one embodiment, a high hardness of over 9 GPa. Such coatings may optionally also have an elasticity of greater than 100 GPa on stainless steels. 
     In another embodiment, the hub portion  72  is formed of metal, such as steel or stainless steel and the teeth  74  are coated while the hub is not. 
       FIG. 5  shows a cross-section taken along lines  5 - 5  of  FIG. 4 . The exemplary tooth  74  includes a base  77  that can formed of steel or stainless steel and, in one embodiment, if formed of the same material as the hub portion  72  ( FIG. 4 ). The base  77  can also be called a tooth or tooth portion herein. The base is coated with a coating  79 . The coating can be any of the coatings discussed herein. In one embodiment, the coating is applied at a thickness (t) of less than 5 micrometers. 
     The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.