Patent Application
20100266437
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 relight. 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.
A multi-stage pump assembly includes a housing having a constant diameter bore therein. A first gear pump is received in the bore and 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 bore and also has a first gear received in 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 plate is received in the bore and interposed between the first and second gear pumps. The spacer plate is fixed to 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 plate 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 forming a bore in a housing, securing a spacer plate in the housing bore, and assembling first and second gear pumps in the housing bore on opposite faces of the spacer plate.
The method further includes providing first and second journal bearings, and preferably locating the journal bearings at opposite axial ends of the bore 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.
Portions of a fuel supply system 100 are shown in
More specifically, and with continued reference to
Disposed on an opposite axial end or side of the fixed spacer plate 170 is the second gear pump 140b. The second gear pump includes first and second gears 152b, 156b received over and fixedly secured (e.g., pinned) to respective shaft portions 150b, 154b of the first and second shafts. The radially outward extending teeth of each of the first and second gears 152b, 156b 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 184b toward the outlet portion 186b 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 172b of the spacer ring that faces the first and second gears of the second gear pump. The second face 172b is sealed via seal member 174b relative to a fixed bearing member 180b. Again, the fixed bearing member includes portions 184b, 186b, that in conjunction with recesses 194b, 196b on the pressurized bearing 190b, 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 180b is sealingly engaged against the fixed spacer plate with an intermediate seal member 174b 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 190a, 190b 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 180a, 180b 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 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. This is critical as 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
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.
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.
