Abstract:
A gas turbine engine having an oil cavity architecture and bearing placement which reduce heat rejection and oil system complexity by enclosing the reduction gearbox bearings and at least the shaft bearings supporting the high pressure shaft in the same oil cavity.

Description:
TECHNICAL FIELD 
   The present invention relates generally gas turbine engines, and more particularly to an improved architecture for a gas turbine engine. 
   BACKGROUND OF THE INVENTION 
   Traditional gas turbine engine layouts necessitate a plurality of oil lubricated bearings spaced apart throughout the engine, and hence require a plurality of oil cavities which contain and feed oil to these bearings. The greater the number of oil cavities required, the greater the cost, weight and complexity of the engine design. Additionally, more oil cavities increases the possibility of potential oil leakages associated with external pipes used to transfer oil to and between the various spaced apart bearing cavities.  FIG. 1  illustrates such a typical gas turbine engine layout having six bearing cavities, generally identified as A through F, each of which may comprise several individual bearings therewithin. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a gas turbine engine having an improved architecture. 
   Therefore, in accordance with the present invention, there is provided a gas turbine engine comprising: a primary reduction gearbox enclosing a plurality of reduction gearbox bearings therewithin; at least a high pressure shaft rotatably supported by a plurality of high pressure shaft bearings and having a compressor and a turbine mounted thereto; a gas generator portion defining a shaft bearing cavity therewithin, the shaft bearing cavity containing at least the high pressure shaft bearings; and wherein the primary reduction gearbox and shaft bearing cavity are engaged in unrestricted oil flow communication and define a single oil cavity within which the reduction gearbox bearings and the high pressure shaft bearings are enclosed. 
   There is also provided, in accordance with the present invention, a gas turbine engine comprising: a casing; a primary reduction gearbox having a plurality of reduction gearbox bearings and an accessory gearbox having a plurality of accessory gearbox bearings; at least a first shaft having a compressor and turbine mounted thereto, the first shaft being rotatably supported by at least two shaft bearings; and wherein the reduction gearbox, the accessory gearbox and the shaft bearings are disposed in a single oil cavity within the casing, and wherein unrestricted oil flow is possible between the reduction gearbox bearings, the accessory gearbox bearings and said shaft bearings. 
   There is further provided, in accordance with the present invention, a gas turbine engine comprising at least outer and inner concentric shafts, at least said outer shaft having a compressor and turbine mounted thereto, each of said shafts being respectively rotatably supported by at least two outer shaft bearings and two inner shaft bearings, the outer shaft bearings being spaced apart from a turbine end of the outer concentric shaft such that the turbine is disposed between said bearing and said end, such that the outer concentric shaft is cantilevered from the outer shaft bearings. 
   There is also provided, in accordance with the present invention, a free turbine gas engine comprising at least outer and inner concentric shafts rotatably supported by a plurality of shaft bearings, the outer shaft being a free turbine shaft and having a turbine and compressor mounted thereto, the outer shaft bearings disposed forward of the compressor such that the bearings are isolated from the turbine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
       FIG. 1  is a schematic cross-section of a prior art gas turbine engine having a plurality of bearing cavities spaced throughout the engine; and 
       FIG. 2  is a somewhat schematic cross-section view of a gas turbine engine in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 2 , the gas turbine engine  10  of the present invention is of a type preferably provided for use in subsonic flight and generally described in United States Patent Application Publication No. US2003/0115885 (incorporated herein by reference). The gas turbine engine  10  generally comprises a gas generator module  12  including, in serial flow communication, a multistage compressor portion  18  for pressurizing the air, a combustor portion  22  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine portion  20  for extracting energy from the combustion gases. 
   The compressor portion  18  includes an air inlet  24 , a booster stage or boosted rotor type low pressure (LP) compressor  26  (which may be of the type described in U.S. Pat. No. 6,488,469, incorporated herein by reference), and a centrifugal impeller type high pressure (HP) compressor  28  at the outlet end of a compressor air flow duct  30 . 
   The turbine portion  20  generally includes a high pressure (HP) turbine rotor  31  and a low pressure (LP) turbine rotor  33 , mounted to concentric outer HP shaft  38  and inner LP shaft  36  respectively. These main shafts or “spools” are concentrically arranged, preferably with the HP shaft  38  outside the LP shaft  36 . This is typically done simply due to geometry restrictions, as the HP shaft generally connects rotating elements which are disposed closer together in the engine. The outer HP shaft  38  drives the HP compressor impeller  28 , while the inner LP shaft  36  drives the LP compressor rotor  26 . The LP shaft  36  also preferably drives the reduction gearbox module  14  and the accessory gearbox module  16 . One will appreciate, however, that these components may be driven by different shafts. 
   The outer HP shaft  38 , concentric with and disposed surrounding the inner LP shaft  36 , is cantilevered at the rear of the engine from HP shaft bearings disposed forward of both the HP turbine  31  and the HP compressor  28 , within a shaft bearing cavity  62  (indicated with a dashed line in  FIG. 2 ). The shaft bearing cavity  62  preferably includes almost all of the LP and HP shaft bearings, with the exception of the rear LP shaft bearing  52 . The shaft bearing cavity  62  opens into, and thus is in free (i.e. unrestricted) fluid flow communication with, both the reduction gearbox  14  and the accessory gearbox  16 , thereby defining a large single oil cavity  60  (outlined in  FIG. 2  by the large solid line  59 ) which will be described in greater detail below. The fluid communication between various portions of the single cavity is “unrestricted” in the sense that there is an absence of conduits or other restrictions requiring pressure to force the oil between such portions of the single cavity. Rather, engine attitude permitting, oil may simply flow under force of gravity alone within the various portions of the cavity. Any narrowed portions of the cavity are preferably sized relative to the oil viscosity, etc., so that “spent” oil may pass to the oil-collection portion of the cavity independent of a separate pumping means, as will be described in more detail below. While the shaft bearings used may be of many different types, in the embodiment depicted in  FIG. 2  a roller bearing  42  and a ball bearing  44  support the outer HP shaft  38  and engage the shaft forward of the impeller of the HP compressor  28 . Atypically, there are thus preferably no bearings located on the HP spool aft of the HP compressor  28 . The inner LP shaft  36  is supported at opposed ends, particularly at a forward end by a roller bearing  50  and a main ball bearing  48  and at a rear end by a rear bearing  52 . This rear LP shaft bearing  52  can be an oil-less bearing or a bearing having its own cavity. 
   In use, the operation of the gas generator  12  causes output rotational power to be delivered by the LP turbine shaft  36 . As the LP shaft rotates, which can be at speeds upward of 25,000 to 30,000 revolutions per minute (RPM) (the HP shaft rotating at speeds of up to 50,000 RPM or higher), torque is transferred to RGB input shaft  51  via the RGB tower shaft  41 , and then through the reduction gear train  55  to the RGB output shaft  53 . The reduced speed of the RGB output shaft  53  is typically around 2000 RPM, but will largely depend on the particular application of the engine. 
   The reduction gearbox module  14  receives input power from the LP shaft  36  via an RGB tower shaft  41 , which is in meshed engagement with the LP shaft  36  via bevel gears  45  and  47 , respectively disposed on the LP shaft  36  and the tower shaft  41 . The tower shaft extends through the inlet gas path  30  within a fairing  43 . A bevel gear set  49  transfers rotational power to an RGB input shaft  51  which, in turn, drives an RGB output shaft  53  through a reduction gear train  55 . The output shaft  53  terminates (in this example) in a propeller flange (not shown) for connection with a suitable propeller (not shown). A plurality of RGB bearings  54  suitably journal all RGB rotating shafts. 
   The accessory gearbox module  16  is generally driven by the LP shaft  36  via an AGB tower shaft  61 . The AGB output shaft  72  is used to drive accessory devices such as fuel pumps, starter generators, mechanical fuel controls, air/oil separators, and oil pumps, etc. A plurality of AGB bearings  56  suitably journal all AGB rotating shafts. 
   The term “forward” as used herein is defined as meaning a position within the engine towards the “cool” end of the engine, namely the upstream end of the engine in typical aircraft installations. 
   By cantilevering the HP shaft  38 , the so called “hot end” HP shaft bearing (roller bearing  42 ) is located forward of both the HP turbine  31  and the HP compressor  28  and within the transmission area of the engine. As this rearmost HP shaft bearing  42  is isolated from the hottest part of the engine, namely the high pressure turbine region, the heat rejection to the oil is therefore greatly reduced. Although cantilevered low-speed shafts are known, cantilevered high-speed shafts are not known nor is the use of this mechanism to isolate the HP bearings from the hottest section of the engine. 
   Having a cantilevered HP shaft  38  also allows for a simplified assembly/disassembly process. Particularly, the HP turbine rotor  31  and/or the LP compressor rotor  26  can be removed if necessary without disturbing the oil system contained within the monolithic single oil cavity  60 . Such simplified assembly/disassembly of elements previously inaccessible without affecting the oil system significantly reduces the possibility of contamination thereof. 
   Preferably, the large single oil cavity  60  contains all of the bearings for the reduction gearbox (RGB)  14 , the accessory gearbox (AGB)  16  and the outermost main shaft bearings. This engine architecture or layout significantly differs from typical turboprop or turbofan engines, which generally comprise a plurality of bearing oil cavities as depicted in  FIG. 1 . The novel structure and layout of the present gas turbine engine  10 , particularly the offset nature of both the RGB  14  and the AGB  16  and the cantilevered HP shaft  38 , permits almost all of the major oil-lubricated bearings to be disposed in the one large bearing oil cavity  60 . The single main oil cavity  60  of the present invention permits the deletion of such external pipes and the associated hardware, saving cost and weight. 
   Further, enclosing all major oil-lubricated bearings, within a single cavity permits the overall heat rejection to the oil to be reduced, thereby lowering oil cooling requirements. As mentioned, this is due in part to the isolation of the bearings relative to the hottest section of the engine, but also in part because the amount of hot secondary air flow blown into the cavity, to create a pressure-differential to seal the oil within the cavity, is reduced due to the reduction in overall number of oil-air interfaces. Particularly, the main bearing cavity  60  preferably comprises only two main shaft seals about the low and high pressure shafts  36 , 38 , a forward shaft seal  64  located just rearward (or downstream) of the boosted LP compressor rotor  26  and a rear shaft seal  66  located just forward (or upstream) of the HP compressor impeller  28 . As the amount of secondary air flow into the oil cavity is reduced, the amount of heat which is thus introduced the oil is accordingly reduced. Therefore, the total amount of heat which much be removed from the oil is lower, thereby reducing the size, cost and weight of the oil cooling system required. The main bearing cavity  60  also provides a single large volume of oil having a significant thermal mass, thereby providing a more beneficial mass-to-surface area ratio than possible with several individual smaller cavities further decreasing the engine heat rejection to the oil. 
   Another advantage of the large main oil cavity  60  is the elimination of the need for a separate AGB oil scavenge system. Particularly, with the present apparatus only gravity is required in normal operating attitudes to drain the oil within the cavity  60  down to the bottom, wherein the oil tank (not shown) is preferably located. Thus, no external or internal pipes or scavenge pumps are necessary. As all main bearings are located within the main cavity  60  and the oil tank is disposed at the bottom thereof, the possibility of an oil flow mismatch between the main oil pressure and that of the scavenge system is therefore eliminated. This significantly reduces the potential for bearing cavity flooding. 
   The only engine bearing not disposed within the main bearing cavity  60  is the rear LP shaft bearing  52 . As noted above, this rear LP shaft bearing  52  is not a major load bearer, and therefore can be a relatively smaller roller or air bearing. If an air bearing is used, no other bearing cavity is required other than the main oil cavity  60 . However, if a rolling element bearing (such as a ball or roller bearing) which requires oil lubrication is to be used, a small secondary bearing cavity  58  is provided to contain the oil therein. 
   Although the present invention has been described and depicted with respect to a turboprop engine, it is to be understood that the present invention may be employed in any gas turbine engine having at least a primary reduction gearbox. Accordingly, turboprop, turboshaft, or geared turbofans, depending on the suitability of their particular layouts, may be configured having the principles of the present invention described above. 
   The embodiments of the invention described above are intended to be exemplary. Those skilled in the art will therefore appreciate that the forgoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.