Abstract:
Provided is an accessory gearbox for a gas turbine engine that contributes minimally to engine weight and complexity. An accessory gearbox comprises a stationary frame that is circumferentially disposed about a central longitudinal axis of the engine. The frame includes a forward-directed axial face and a rearward-directed axial face for fastening the frame to the engine. A central bore accepts a rotational driving means and an outermost rim circumscribes the outer diameter of the frame. A compartment is located between the bore and the rim. At least one primary accessory is mounted to a face and proximate the compartment, remotely engaging the driving means and able to be driven concurrently therewith.

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The invention relates to an accessory gearbox and more specifically to an accessory gearbox integrated with a stationary frame of a gas turbine engine. 
   (2) Description of the Related Art 
   In an axial-flow gas turbine engine, incoming air is directed rearward by a large diameter fan, where the air is typically split into two discrete streams: a core air stream and a bypass air stream. The core air stream enters the engine and is pressurized in series by one or more rotating compressors and is then mixed with fuel in a central combustor. The fuel and air mixture is burned in and expelled from the combustor as hot combustion gases. The hot combustion gases are directed rearward to one or more turbines disposed downstream of the combustor. The turbines extract power from the combustion gases and use the power to rotate the compressors via a common rotor shaft or rotor spool. Conversely, the bypass air stream is directed outside the engine through one or more bypass ducts. The exhausting combustion gases in combination with the bypass air stream generate a propulsive thrust for powering a vehicle such as an aircraft. 
   Besides producing thrust, a gas turbine engine also supplies power for various systems and accessories essential to the engine and aircraft operation as well as passenger comfort. These systems and accessories are typically powered pneumatically by bleeding core air from the compressors or powered mechanically by engaging the driving means. Since the driving means are located proximate a central longitudinal axis of the engine, a transfer shaft is necessary to transmit the power to an accessory gearbox mounted in an external location. The accessory gearbox allows each accessory to run at an optimum speed and in an accessible location for ease of maintenance, reduced weight and minimum complexity. Accessory gearboxes are conventionally mounted beneath the engine, proximate the combustor, or beneath a cylindrical case surrounding the fan. 
   Accessory gearboxes mounted beneath the engine require an angle gearbox and two separate transfer shafts to transmit power from the driving means. The first transfer shaft, sometimes referred to as a tower shaft, transmits the power from the driving means, radially downward to the angle gearbox. The second transfer shaft, sometimes referred to as a lay shaft, transmits the power axially rearward from the angle gearbox to the accessory gearbox. The requirement for an angle gearbox housing, lay shaft and accessory gearbox housing adds substantial weight and complexity to the engine. Accessory gearboxes mounted beneath the engine also contribute to engine flexure, which is known to negatively affect compressor and turbine clearances and thus reduces engine fuel efficiency. Also, the area beneath the engine houses many pipes, wires and brackets, which must be tightly packaged around the accessory gearbox. The inaccessibility of the gearbox causes difficulties for maintenance personnel during diagnostic and repair procedures. 
   Accessory gearboxes mounted beneath the fan case require an extended tower shaft because the fan is very large in diameter. A mid-span bearing and an angle gearbox are used to transmit power from the driving means. The tower shaft transmits the power from the driving means, substantially radially downward and through the fan case to the angle gearbox. The power is transmitted axially forward from the angle gearbox to the accessory gearbox. The requirement for an angle gearbox housing and extended tower shaft with a mid-span bearing adds substantial weight and complexity to the engine. Also, accessory gearboxes mounted beneath the fan case increase the frontal area of the engine, increasing the aircraft drag and thus increasing the engine fuel burn. 
   What is therefore needed is an accessory gearbox that contributes minimally to engine weight and complexity, while not negatively impacting engine fuel efficiency. 
   BRIEF SUMMARY OF THE INVENTION 
   Provided is an accessory gearbox for a gas turbine engine that contributes minimally to engine weight and complexity while improving engine fuel burn. An accessory gearbox, in accordance with the current invention, comprises a stationary frame that is circumferentially disposed about a central longitudinal axis of the engine. The frame includes a forward-directed axial face and a rearward-directed axial face for fastening the frame to the engine. A central bore accepts a rotational driving means and an outermost rim circumscribes the outer diameter of the frame. A compartment is located between the bore and the rim. At least one primary accessory is mounted to a face and proximate the compartment, remotely engaging the driving means and able to be driven concurrently therewith. 
   Since the inventive accessory gearbox is integrated into an engine frame, there is no need for an extended tower shaft with mid-span bearing, angle gearbox housing, lay shaft or accessory gearbox housing. The elimination of these components from beneath the engine or fan case contributes minimally to engine weight and provides additional area for housing pipes, wires and brackets in order to simplify engine maintenance. A reduction in both engine flexure and frontal area also improves engine fuel burn. 
   Other features and advantages will be apparent from the following more detailed descriptions, taken in conjunction with the accompanying drawings, which illustrate by way of examples accessory gearboxes in accordance with several embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a simplified cross sectional view of a gas turbine engine showing a prior art accessory gearbox configured for external mounting under an engine. 
       FIG. 2  is a simplified cross sectional view of a gas turbine engine showing a prior art accessory gearbox configured for external mounting on a fan case. 
       FIG. 3  is a simplified cross sectional view of a gas turbine engine showing an accessory gearbox configured for integral mounting in accordance with an embodiment of the current invention. 
       FIG. 4  is an isometric rear view of an intermediate frame showing an accessory gearbox configured for integral mounting in accordance with an embodiment of the current invention. 
       FIG. 5  is a partial top schematic sectional view showing an accessory gearbox configured for integral mounting in accordance with an embodiment of the current invention. 
       FIG. 6  is a partial top schematic sectional view showing an accessory gearbox configured for integral mounting in accordance with another embodiment of the current invention. 
       FIG. 7  is a partial top schematic sectional view showing an accessory gearbox configured for integral mounting in accordance with yet another embodiment of the current invention. 
   

   When referring to the above listed drawings, like reference numerals designate identical or corresponding parts throughout the various views. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIGS. 1 ,  2  and  3 , air  10  enters a gas turbine engine  12  via a forward mounted fan  14 , where the air  10  is directed into two discrete streams: a core air stream  16  and a bypass air stream  18 . The core air  16  is pressurized in series by a low-pressure compressor  20  and a high-pressure compressor  22 , before being mixed with fuel and burned in a centrally mounted combustor  24 . The core air  16  exits the combustor  24  as combustion gases  26 , which expand in series through a high-pressure turbine  28  and then a low-pressure turbine  30  before being exhausted from the engine  12 . The bypass air  18  is directed-outside the engine  12  through one or more bypass ducts  32  (not shown). The exhausting combustion gases  26  in combination with the bypass air  18  generate a forward propulsive thrust. The low-pressure turbine  28  drives the low-pressure compressor  20  via a low rotor shaft  34  revolving about a central, longitudinal axis  36  of the engine  12 . Similarly, the high-pressure turbine  30  drives the high-pressure compressor  22  via a high rotor spool  38 . Although two compressors  20 ,  22  and two turbines  28 ,  30  are shown in each of the engines  12  depicted in the illustrations, more or less may be used. 
   In each of the prior art engines  12  illustrated in  FIGS. 1 and 2 , the high rotor spool  38  remotely drives an externally mounted accessory gearbox  40 . With specific reference to  FIG. 1 , a tower shaft  42  engages the high rotor spool  38  proximate the central axis  36  through a bevel gear set  44  and transfers the power radially outward through another bevel gear set  44  within an angle gearbox  46 . The power is then transferred approximately rearward via a lay shaft  48  to an accessory gearbox  40  mounted beneath the engine  12 . Now referring specifically to  FIG. 2 , the high rotor spool  38  drives a tower shaft  42  proximate the axis  36  through a bevel gear set  44 , which then transfers the power radially outward through another bevel gear set  44  within an angle gearbox  46 . The power is finally transferred forward from the angle gearbox  46  to an accessory gearbox  40  mounted to a case  50  circumscribing the fan  14 . Deficiencies with each of the prior art accessory gearbox mounting configurations have previously been discussed in the aforementioned sections. 
   Referring now to  FIG. 3 , those skilled in the art will appreciate an accessory gearbox  40  integrated into an engine frame  52  for a minimal contribution to engine weight and reduced complexity. Although an intermediate frame  52  is described and illustrated in each of the inventive embodiments below, it is to be understood that any compartmentalized frame  52  may be used. 
   The intermediate frame  52  is a stationary, nearly cylindrical structure disposed between the low-pressure compressor  20  and the high-pressure compressor  22 . A forward axial face  54  is fastened to the low-pressure compressor  20  and a rearward axial face  56  is fastened to both the high-pressure compressor  22  and to a pylon  58  used for attaching the engine  12  to the aircraft. As best illustrated in  FIG. 4 , the frame  52  further includes a central bore  60  for allowing the low rotor shaft  34  and the high rotor spool  38  to pass there through. An inner hub  62  circumscribes the bore  60 , and provides support for the low rotor shaft  34  and the high rotor spool  38  by way of roller bearings  64  (see  FIG. 3 ). A compartment  66  is spaced radially apart from the inner hub  62  by a number of circumferentially spaced inner struts  68 , forming an inner diameter annular duct  70  there between. The inner duct  70  directs the core air  16  from the low-pressure compressor  20  to the high-pressure compressor  22 . An outermost rim  72  is spaced radially apart from the compartment  66  by a number of circumferentially spaced outer struts  74 , forming an outer diameter annular duct  76  therebetween. The outer duct  76  directs the bypass air  18  from the fan  14  to one or more bypass ducts  32  (not shown). 
   A tower shaft  42  extends substantially radially outward from the bore  60  through the inner hub  62  and an inner strut  68  to the compartment  66 . The circumferentially lowermost inner strut  168  is used to facilitate the return of lubricating fluid by gravity from the roller bearings  64  to the compartment  66 . The tower shaft  42  is supported by bearings  78 , and a first end  80  is driven by the high rotor spool  38  through a first bevel gear set  44 . The tower shaft  42  may be driven at the same speed or at a different speed as the high rotor spool  38 . A second end  84  directly drives at least one primary accessory  86  mounted to an axial face  54 ,  56  and proximate the compartment  66 . Each primary accessory  86  is driven at an optimum speed by an appropriately sized second bevel gear set  44  and accessory shaft  90 . Although the high rotor spool  38  drives the tower shaft  42  in each of the illustrated examples, the low rotor shaft  34  may also be used as a tower shaft  42  driving means. 
   As further illustrated in  FIGS. 5 and 6 , at least one secondary accessory  92  may also be mounted to an axial face  54 ,  56  and proximate the compartment  66 . Each secondary accessory  92  may be indirectly driven by the tower shaft  42  through a straight gear set  94  engaged with an adjacent accessory  86 ,  92 . By altering the straight gear  94  ratios between accessories  86 ,  92 , the accessories  86 ,  92  may be driven at their optimum speed. The accessories  86 ,  92  are distributed circumferentially about the axial faces  54 ,  56  based upon available area and system requirements. An accessory  86 ,  92  may include at least one of a starter, an integrated drive generator (IDG), a fuel pump, a hydraulic pump, a lubrication/scavenge pump, a de-oiler, and a permanent magnet alternator (PMA). In the illustrated embodiments, the starter functions as a primary accessory  86  and is directly driven with the tower shaft  42  to enable engine  12  starting. 
   As the example of  FIG. 7  illustrates, a secondary accessory  92  may be mounted in series with a primary accessory  86  as well. The secondary accessory  92  may be driven at the same speed as the primary accessory  86  by sharing a common accessory shaft  90 , or may be driven at a different speed be means of a gear set (not shown). 
   At least one tertiary accessory  96  may be mounted to a face  54 ,  56  and proximate the compartment  66  as well. The tertiary accessories  96  are typically used for maintenance or to monitor the health of the accessory gearbox  40 . The tertiary accessories  96  are not driven and may include an oil filter, a metal chip detector, and an oil pressure-regulating valve. 
   While the present invention has been described in the context of specific embodiments, applications and vehicles thereof, other alternatives, modifications and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended that the present invention embrace those alternatives, modifications and variations as fall within the broad scope of the appended claims.