Abstract:
A multistage gear pump assembly includes first and second gear pumps that use common shafts and are axially separated by a fixed spacer within the housing. Pressurized bearings are provided at opposite axial ends of the first and second gear pumps. The second gear pump handles cruise and idle operations of the aircraft while the first gear pump stage assists in meeting higher demand modes of engine operation. Otherwise, the first gear pump is maintained at a minimal pressure to reduce energy consumption and still provide desired stability and eliminate issues associated with bearing oil whirl associated with prior known arrangements. When additional assistance is required, such as during takeoff, climb, or windmill relight, the first gear pump advantageously contributes to the increased pressure.

Description:
This application is a continuation-in-part and claims the priority benefit of U.S. application Ser. No. 12/424,745, filed Apr. 16, 2009, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This disclosure relates to a multi-stage pump, and particularly to a multi-stage gear pump assembly used as a fuel pump in an aircraft gas turbine engine. It will be appreciated however, that the disclosure may find application in related environments and applications that encounter the same issues. 
     A large portion of aircraft engine operation involves cruise and idle situations which do not demand large quantities of fuel flow. However, certain circumstances require additional flow, for example during takeoff, climb, or windmill re-light. The fuel pump assembly must be able to satisfy both demands, while adequately addressing associated parameters such as fuel pump size, efficiency, etc. For example, it is known to employ multiple stages of a positive displacement pump assembly to meet the different needs of the aircraft engine to improve efficiency over traditional single stage gear pumps. Typically first and second stages of a multistage gear pump are selectively used. Thus, second gear pump stage is designed to handle the cruise and idle operations of the aircraft while the first gear pump stage is selectively employed in conjunction with the second stage pump to meet the higher demand modes of engine operation. 
     Inclusion of independent gear pumps in the same housing raises a number of issues. For example, when the second pump is functioning at maximum capacity, the first gear pump is operated at a reduced pressure state to reduce energy consumption. In the reduced pressure state, the first pump has a tendency to become unstable. As a result of the teeth of the gears transferring the relatively low load, there is resultant tooth bounce and instability, which could ultimately lead to gear tooth failure. Ideally, a full fluid film without any physical contact between the journal and the bearing surfaces is desired in the bearing assembly. This gear instability can prematurely wear the journal bearing. The bearings that support the arrangement can also become unstable when minimizing pressure to the first pump. A phenomenon known in the industry as bearing oil whirl can occur in journal bearings that are lightly loaded, which could ultimately lead to bearing failure. A conventional arrangement, for example, may drive the second gear stage through the tooth mesh of the first gear stage in order to alleviate the above issues with tooth instability and bearing oil whirl. This results in increased loading on the gear teeth of the first pump, which would require an increase in the gear teeth size or count and increasing weight for example. 
     There are also issues with selectively switching between single and multistage use of the pump. For instance, different forces and stresses result from different modes of operation of the multistage pump. Changing or turning the pressure on and off in connection with one of the gear pump stages has a resultant impact on the stability and efficiency of the pump assembly gears and bearings. 
     There is always a need to reduce the weight and overall envelope size of the pump assembly. Thus, a conventional arrangement where the first and second gear pumps are offset from one another may address a portion of the issues associated with one pump being independent of the other, but it unnecessarily adds additional components, additional wear, additional weight, and increases the overall size of the multistage pump assembly. 
     Accordingly, a need exists for an improved multi-stage pump assembly that addresses these needs and others in a reliable, economic manner. 
     SUMMARY OF THE DISCLOSURE 
     A multi-stage pump assembly includes a housing receiving a first gear pump. The first gear pump has a first drive journal shaft that rotates about a first axis and drives a first gear that operatively engages a second gear rotating about an adjacent second axis. A second gear pump is received in the housing and also has a first gear received on the first drive shaft in spaced relation from the first gear of the first gear pump. The first gear of the second gear pump operatively engages a second gear rotating about a second axis. A spacer is interposed between the first and second gear pumps and fixed to the housing. 
     The spacer extends from the housing and plate is fixed to the housing as a separate plate that is secured to the housing or as an integral part of the housing. 
     The first and second pumps may be differently sized, for example the first gear of the first pump may have a greater axial length than the first gear of the second pump. 
     A pressurized bearing arrangement supports the journal shaft, and preferably includes first and second pressurized bearing portions axially spaced from one another and supporting the common shaft of the first and second gear pumps. 
     In a preferred arrangement, the pump assembly includes first and second fixed bearings disposed on opposite axial sides of the fixed spacer to provide axial thrust load support to the pump assembly, and further includes first and second pressurized, floating bearings disposed on opposite sides of the first and second gear pumps. 
     A method of assembling a multi-stage gear pump assembly includes providing a housing and fixing a spacer in the housing, and assembling first and second gear pumps in the housing bore on opposite faces of the fixed spacer. 
     The method further includes providing first and second journal bearings, and preferably locating the journal bearings adjacent opposite axial ends of the housing to support the shaft. One or more gear stages can be selectively unloaded during pump operation. 
     The shared journal arrangement limits premature wear since the journals are always loaded and provide the needed pre-load to reduce the prospect of bearing oil whirl during periods when the first pump is unloaded, and when the discharge pressure is rapidly turned on or off. 
     Energy consumption is minimized during flight since one or more of the multistage gear stages can be operated at a reduced pressure loading. 
     By locating the gear and stages on the same journal shaft, the load can be transferred through the shaft and not through the teeth, reducing the tooth load and therefore their size. 
     Reduced or limited tooth bounce results from the improved stability. 
     Still other benefits and advantages will become more apparent to one skilled in the art upon reading and understanding the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of a portion of a fuel supply system for an aircraft engine. 
         FIG. 2  is an exploded perspective view of a multi-stage gear pump assembly that includes first and second gear pumps. 
         FIG. 3  is a perspective view of the assembled first and second gear pumps without the surrounding housing. 
         FIG. 4  is a longitudinal cross-sectional view of a second embodiment of a portion of a fuel supply system for an aircraft engine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Portions of a fuel supply system  100  are shown in  FIG. 1  and include a low pressure centrifugal pump  110  and a multistage positive displacement pump assembly or gear pump assembly  120 . In  FIG. 1 , the multistage pump assembly  120  includes a housing  122  having a first end portion  124 , and a second end portion  126  interconnected by a central sleeve  128 . The sleeve  128  is secured at opposite ends to the first and second housing end portions  124 ,  126 , respectively. For example, fasteners secure the end portions to the ends of the sleeve. In addition, the sleeve portion  128  of the housing includes an opening such as a constant diameter bore  130  extending through the housing. In this manner, the housing portion  124  forms a first shoulder at a first end  132  and the second housing portion  126  forms a second shoulder  134  at the opposite end. Between these shoulders and along the extent of the throughbore  130  are received first and second gear pumps  140   a ,  140   b . For purposes of brevity and ease of understanding, since the gear pumps are substantially identical, like reference numerals will refer to like components of the first and second gear pumps. Where appropriate, “a” and “b” suffixes will be used with the reference numerals to identify components associated with the first and second gear pumps, respectively. It will be further appreciated that if additional multiple stages were required, that the additional stages could adopt a structure and function substantially similar to the first and second stage gear pumps as described herein. 
     More specifically, and with continued reference to  FIG. 1  and additional reference to  FIGS. 2 and 3 , the structure of the gear pumps will be described in greater detail. The first gear pump has a first shaft  150   a  that rotates about an associated first axis. The shaft is preferably a hollow shaft or an annular component and received on the shaft is a first gear  152   a  having multiple, circumferentially spaced teeth extending generally radially outward. The first gear  152   a  is a one-piece arrangement with the shaft in this embodiment (i.e., the first gear is integrally formed with the shaft by cutting the gear teeth about a circumferential portion at a desired axial location, or otherwise secured thereto at a predetermined axial location such as being formed as a separate annular first gear that is pinned or bolted to the shaft. A second shaft  154   a  is disposed in parallel relation to the first shaft for rotation about a second axis parallel to the first axis. A second gear  156   a  is likewise preferably a one-piece arrangement with the shaft received on the outer surface of the second shaft and the shafts are spaced a preselected dimension apart so that the gear teeth of the first and second gears  152   a ,  156   a  will mesh with one another. Thus, the second gear is secured to the second shaft in much the same manner and at the same axial location along the second shaft as the first gear is secured to the first shaft. At one end of the first gear pump is a spacer, which in this embodiment is a spacer plate  170 , that is fixed at a predetermined location in the housing bore  130 . The spacer plate is preferably a single piece component that is secured by one or more pins or bolts  171  ( FIG. 1 ), or otherwise secured against axial movement within the bore. A first face  172   a  of the spacer plate faces the first and second gears  152   a ,  156   a  of the first gear pump and includes two openings therethrough that accommodate the first and second shafts, respectively. That is, the spacer plate has the general conformation of a “figure eight” ( FIG. 2 ) with an outer periphery dimensioned for receipt in the bore  130  and the openings dimensioned to receive the shaft  150 ,  154  in parallel relation. Thus, the spacer plate  170  has a bore dimension D 2  that is less than a housing opening dimension D 1 . In addition, a seal member  174   a  is interposed between the spacer plate and a fixed bearing member  180   a . A first surface  182   a  of the fixed bearing that faces the first and second gears has recesses or channel portions  184   a ,  186   a  along mid-portions of the figure eight conformation of the fixed bearing that form one side of or portions of an inlet and outlet for fluid to reach the gear pump. Disposed on the axial opposite side of the first and second gears is a floating journal bearing assembly  190   a . A first axial face  192   a  of the pressurized or floating bearing also includes recesses or cut-out portions  194   a ,  196   a  along mid-portions thereof that cooperate with passage portions  184   a ,  186   a  and together define the inlets and outlets to the gear pump. Inner diameter portions  198   a ,  200   a  of the pressurized bearing are closely received around the external surface of the first and second shafts  150 ,  154 , respectively. As will be appreciated, a hydrodynamic bearing is formed between these adjacent surfaces in order to support the journal shafts during operation. 
     Disposed on an opposite axial end or side of the fixed spacer plate  170  is the second gear pump  140   b . The second gear pump includes first and second gears  152   b ,  156   b  received over and fixedly secured (e.g., pinned) to respective shaft portions  150   b ,  154   b  of the first and second shafts. The radially outward extending teeth of each of the first and second gears  152   b ,  156   b  are designed for interengaging, meshing relation. As the gears rotate, the fluid is advanced or displaced by the individual teeth around the perimeter of the shaft from the inlet portions  184   b  toward the outlet portion  186   b  in the spaces between the individual teeth of the gears. In the same manner as the first gear pump, the second gear pump includes a second face  172   b  of the spacer ring that faces the first and second gears of the second gear pump. The second face  172   b  is sealed via seal member  174   b  relative to a fixed bearing member  180   b . Again, the fixed bearing member includes portions  184   b ,  186   b , that in conjunction with recesses  194   b ,  196   b  on the pressurized bearing  190   b , form a respective inlet and outlet to the second gear pump. Thus, the spacer plate  170  is secured to the housing  128 , and the fixed bearing portion  180   b  is sealingly engaged against the fixed spacer plate with an intermediate seal member  174   b  that also has a figure eight configuration. The spacer plate and the fixed bearings only provide axial thrust load support to the gear pump, and do not function as a journal bearing support to the shafts. The pressurized bearings  190   a ,  190   b  on the other hand, disposed on opposite sides of the first and second gear pumps and at axially outward locations of the gear pumps, are floating bearings that support the journal shafts  150 ,  154  via internal surface  198 ,  200 . 
     The operation of each individual gear pump is generally known in the art. It will be appreciated, however, that the location and placement of the first and second gear pumps within a single diameter bore  130  in end-to-end or back-to-back relation with pressurized bearings at opposite ends is new in the art. This allows both the first and second stages to be pressurized or at least partially loaded during operation. One skilled in the art will also recognize that the spacer plate  170  and fixed bearings  180   a ,  180   b  can be one-piece as long as there is sealing between the first and second gear pump stages. Importantly, however, is a requirement that the spacer plate be axially secured or fixed and able to provide an axial thrust bearing surface. The spacer plate has to be secured axially to resist the potential axial imbalance in thrust loads when the first and second gear stages are run at different discharge pressures. Otherwise, the thrust bearing surfaces could be potentially overloaded from the mismatched pressure if the spacer plate does not adequately resist this loading. 
     A control or valve member is schematically shown by reference numeral  210 . In this manner, and as schematically represented in  FIG. 1 , the second gear pump is typically used for all fuel pump operations such as takeoff, climb, cruise, idle, and windmill relight. The first gear pump, however, is only partially pressurized during the cruise and idle portions of use. That is, when additional fuel flow is demanded by the fuel system, and as required for takeoff, climb, and windmill relight, both the first and second gear pumps can be provided with full pressure. While in the cruise and idle situations, only the second gear pump output is required. The first gear pump flow will be recirculated, and is only pressurized to a partial level. In the minimized pressure state or mode of operation, the first and second bearings  190   a ,  190   b  are always loaded from operation of the pressurized second gear pump so that bearing whirl is not an issue. Moreover, there is no tooth bounce because the bearings are loaded and the load is transferred through the shared shaft  150 ,  154  rather than through the individual gear teeth as in prior known arrangements. Thus, whereas in the past there was an instability issue as a result of extreme pressure loads between on and off situations, such is not the case in the present arrangement. 
     This present arrangement eliminates another shaft and also the associated wear associated with loading the first and second gears of the first and second gear pumps on the first and second shafts, respectively. This reduces the overall weight of the gear pump assembly and reduces the envelope size for the multistage gear pump assembly. Placing the spacer plate between the first and second stages and securing the spacer plate to the housing minimizes the unbraced length of the assemblies. This arrangement increases the strength of the housing by minimizing the deflection and can reduce the weight of the housing if desired. Consequently, securing the spacer plate in the middle between the first and second gear pumps in a straight bore arrangement and sealing between the two stages to minimize cross-flow allows a longer, more flexible shaft that provides for an increased life of the pump since the shaft splines last longer as a result of a more stable arrangement. This structural arrangement also advantageously results in less cavitation and less damage to the gear pump since the loading on the gear teeth can be minimized. The single straight bore arrangement has advantages in manufacturing ease, as well as the preferred method to keep the two gear pumps on the shared shaft running as efficiently as they can with minimal flow loss. 
       FIG. 4  is another embodiment that illustrates portions of a fuel supply system  200 ′. For ease of reference, like components are referenced by like elements with a primed suffix (e.g., fuel supply system  100  of  FIGS. 1-3  is now referred to as fuel supply system  100 ′), while new elements are identified by new reference numerals. It is understood that if a portion of the description of system  2100 ′ is omitted for purposes of brevity, or if a like component is not referenced by a new reference numeral, the prior description of the corresponding component(s) or system of  FIGS. 1-3  still applies to the component(s) or system of  FIG. 4 . 
     More particularly, the primary modification shown in system  100 ′ is the use of an integrated portion  220  of the housing  122 ′ in the multistage pump assembly  120 ′ where the integrated portion is the fixed spacer  170 ′. The integrated portion  220  is formed from the same material as the housing  122 ′ so that the integrated portion serves as the fixed spacer  170 ′. Openings  300 ,  302  are formed in the fixed spacer  170 ′ to accommodate the first and second shafts  150 ′,  154 ′, respectively. The same cross-hatching through the housing  122 ′ and integrated portion  220  demonstrates that the fixed spacer  170 ′ in this embodiment is formed by an integral portion of the housing  122 ′. The opening  130 ′ in the housing  122 ′ includes recesses or counterbores  304 ,  306  that extend axially inwardly from each end of the housing and terminate at the fixed spacer  170 ′. The spacer  170 ′ has a bore dimension D 2  that is less than the housing opening dimension D 1 . 
     The embodiment of  FIG. 4 , and particularly the fixed spacer  170 ′ formed as the integrated portion  220  of the housing  122 ′, is easily manufactured as a cast housing that undergoes separate machining or a housing that is machined to the desired final configuration. The use of the integrated portion  220  also eliminates a potential leakage path that could be present in the embodiment of  FIGS. 1-3 . That is, because the fixed spacer  170  in the embodiment of  FIGS. 1-3  is a separate component (i.e., a plate) that is subsequently secured to the housing  122 , the interface between the spacer plate and the housing could be a potential leak path. Such is not the case with the integrated portion  220  that forms the fixed spacer  170 ′ of the embodiment of  FIG. 4 . There is also an associated structural improvement in the ability of the  FIG. 4  embodiment to handle greater forces or stress in connection with the integrated portion  220  (presuming the same dimensional parameters are used) when compared to a separate plate that is subsequently secured to the housing as described in connection with the fixed spacer  170  of the earlier embodiment described above. 
     The disclosure has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon reading and understanding this specification. For example, one skilled in the art will appreciate that the gears can have different geometries, e.g., different tooth count, different diametrical pitch, different face width, etc., as long as the major diameter is the same. In fact, different geometry may assist in counteracting any potential amplification of a discharge pressure ripple from the first and second gear stages if the two gear stages were identical. It is intended to include all such modifications and alterations in so far as they come within the scope of the appended claims or the equivalents thereof.