Patent Publication Number: US-9845682-B2

Title: Fan disc

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fan disc for supporting fan blades of a gas turbine engine 
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows a conventional aero engine fan rotor and front bearing housing arrangement. 
     A fan disc  100  supports a circumferential row of fan blades  102 . The disc requires torsional stiffness to resist the 2D vibration mode of the fan rotor. This torsional stiffness is provided by front  104  and rear  106  axially spaced diaphragms which have differential hoop bending stiffnesses. An optional middle diaphragm  108  may be provided to produce a more even radial load distribution across the fan blade root fixings. 
     The disc  100  has a rear drive arm  110  which extends to a bolted joint with the engine&#39;s fan shaft  112 . 
     A front bearing arrangement including a housing  114  and a front set of ball bearings  116  and a rear set of roller bearings (not shown), support and locate the fan shaft. 
     The front set of bearings  116  is axially offset a distance O rearwards from the centre of gravity C of the fan blades  102 . This results in a large overhung fan mass having a relatively low first order vibration frequency with large vibrational amplitudes and high restoring forces. 
     Very large forces are generated during a fan blade off (FBO) event, producing a large bending moment in the fan shaft  112  due to the FBO force and the axial offset O of the front bearing. This bending moment is reacted at the bearings  116  along with the direct forces. 
     The rear drive arm  110  is relatively long, its length being determined by the relative fan disc  100  to bolted joint radial growth in order to provide an amount of isolation to the bolted joint. The length of the arm causes the front set of bearings  116  to be pushed further away from the fan blade centre of gravity C. Moreover, because of the large FBO loads experienced by the bearings  116 , the arm  110  has to be oversized to ensure structural integrity. 
     The already expensive fan disc forging is thus made even more expensive by its accommodation of a long and oversized rear drive arm  110 . 
     An alternative to a bolted joint for connecting the fan disc to the fan shaft is to use a splined joint, as proposed in GB 1556266 and GB 1215300. However, in such spline drive fan discs, the support structure of the foremost bearing and the bearing envelope interferes with the space available for the conventional fan disc rear diaphragm. Consequently, fan disc torsional stiffness is reduced, increasing the risk of coincidence of the 2D fan rotor frequency with an engine order forcing frequency. Also, during FBO, conventional front and rear disc diaphragms provide structural strength to the disc to resist hoop bending distortion and prevent potential disc burst from excessive localised hoop stress in the disc bore. 
     SUMMARY OF THE INVENTION 
     It would be desirable to provide a fan disc which addresses the problems discussed above. 
     Accordingly, in a first aspect, the present invention provides a fan disc for supporting fan blades of a gas turbine engine, the fan disc having:
         a disc body having axially extending slots for receiving, in use, dovetail root fixings of a circumferential row of fan blades; and   an annular drive arm which extends radially inwardly from a forward portion of the disc body to a connector for engaging, in use, with a corresponding connector portion of a drive shaft of the engine;   wherein the fan disc further has a hoop stiffening ring which is removably mounted to a rear portion of the disc body, the stiffening ring being axially spaced from the drive arm to increase the torsional stiffness of the fan disc.       

     Advantageously, by combining the features of a drive arm at the front which extends to a connector and a removably mounted hoop stiffening ring at the rear, the disc can have sufficient torsional stiffness while the fan front bearing can be moved forward, e.g. so that it is coincident with the centre of gravity of the fan. In this way, the fan to bearing overhang can be eliminated, hence the large FBO bending moment can also be eliminated. The improved FBO situation means that the bearing no longer experiences such a large FBO shear force and bending moment, and a smaller fan front bearing can be adopted. In addition, the vibration characteristics of the fan rotor can be improved in that the natural frequency is higher and the amplitude of vibrations is reduced. Furthermore, the arrangement enables a smaller lower cost fan disc forging to be produced to support the fan blades. 
     In a second aspect, the present invention provides a gas turbine engine having a circumferential row of fan blades supported by the fan disc of any one of the first aspect, the axially extending slots of the fan disc receiving dovetail root fixings of the fan blades, and the connector of the drive arm of the fan disc engaging with a corresponding connector portion of a fan drive shaft of the engine. 
     The gas turbine engine may further have a front bearing arrangement radially locating and supporting the fan drive shaft, the front bearing arrangement having a front bearing housing and a set of rotatable bearing members (e.g. a set of ball or roller bearings) which is positioned in the housing axially between the connector portion and the hoop stiffening ring. For example, the set of rotatable bearing members can be approximately axially coincident with the fan centre of gravity. The gas turbine engine may be a geared turbofan engine, the drive shaft being split into a front portion which provides the connector portion and a rear portion which connects to a low-pressure turbine of the engine, the front and rear portions being operatively connected to each other via a power gearbox, wherein the gearbox is located in the front bearing housing. Geared turbofans can provide fuel burn benefits through improved specific thrust, and also a significant noise benefit due to slow speed fan. However, geared fan systems generally require a large space envelope to accommodate the power gearbox. Moving the fan front bearing forwards in conjunction with a front drive arm for the fan disc provides an opportunity to accommodate the power gearbox in the front bearing housing without increasing engine length relative to a direct drive fan system. 
     Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention. 
     The drive arm may be frustoconical in shape, i.e. tapering forwardly as it extends radially inwardly to the connector. 
     The connector may be a splined connector for engaging, in use, with a correspondingly splined connector portion of the drive shaft. Another option, however, is for the connector to be a flange bolted connector for engaging, in use, with a corresponding flanged portion of the drive shaft. According to this option, the flange bolted connector and corresponding flanged portion may have teeth which interlock across the flange joint to enhance torque transfer, e.g. in the form of a curvic or Hirth coupling. Additionally or alternatively, the connector and the drive shaft may have interconnecting locating spigot surfaces to enhance torque transfer between the connector and the shaft. 
     The stiffening ring may be formed of composite material. For example, it may be formed of a metal matrix composite material, such as titanium alloy metal matrix composite or aluminium alloy metal matrix composite. Another option is to form it of a fibre-reinforced polymer composite. Titanium alloy metal matrix composite, in particular, has a relatively high temperature capability. Such composites can combine high stiffness with high specific stiffness. 
     The disc body typically has dovetail root fingers at an outer side thereof which define flanks of axially extending slots. The stiffening ring may then project radially outwardly beyond the outermost surfaces of the dovetail root fingers. By extending radially outwardly further than conventional disc diaphragms, the torsional stiffness of the disc can be improved 
     The disc body may have an aft portion which projects rearwardly of the rear ends of the dovetail root fixings of the fan blades, the stiffening ring being removably mounted to an outer side of the aft portion. According to another option, however, the disc body has dovetail root fingers at an outer side thereof which define flanks of axially extending slots; and the disc body may then have an aft portion formed by a circumferential row of radially outward projections from rear ends of the dovetail root fingers, the stiffening ring being removably mounted to a rear side of the aft portion. Either way, conveniently the stiffening ring may be removably mounted to the aft portion by a row of bolts threaded into axially extending bolt holes formed in the aft portion. 
     The fan disc may further have an annular seal panel attached to or integrally formed with the hoop stiffening ring, the seal panel extending from the stiffening ring to a radially-outward air-washed portion of the seal panel adjacent the trailing edges of the fan blades. 
     Conveniently, the stiffening ring may have a row of forward facing support formations (e.g. pins) which, in use, support corresponding engagement portions of a row of annulus fillers located in the gaps between adjacent fan blades. Typically, such annulus fillers have lids which provide air-washed surfaces extending between the adjacent fan blades. The downstream edge of each lid can cooperate with the air-washed portion of a seal panel of the disc. For example, each lid may have a rear lip which hooks under a forward edge of the air-washed portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  shows a conventional aero engine fan rotor and front bearing housing arrangement; 
         FIG. 2  shows a longitudinal cross-section through a ducted fan gas turbine engine; 
         FIG. 3  shows a side view of the fan disc of the engine of  FIG. 2 ; 
         FIG. 4A  shows a close up view of the rear of the fan disc of  FIG. 3 , with a section A-A indicated,  FIG. 4B  shows a view of the fan disc along section A-A, and  FIG. 4C  shows a further view of the fan disc along section A-A; 
         FIG. 5A  shows a view of the fan disc of  FIG. 3  and the entire front bearing arrangement, and  FIG. 5B  shows an adaptation of the engine in which the fan shaft is split into a front portion and a rear portion operatively connected to each other via a power gearbox; 
         FIG. 6A  shows a close up view of the rear of a variant of the fan disc of  FIG. 3 , with a section B-B indicated, and  FIG. 6B  shows a view of the fan disc along section B-B; 
         FIGS. 7A, 7B and 7C  show respective close up views of the rear of a fan disc to illustrate alternative methods of mounting a stiffening ring to the disc; 
         FIG. 8  shows a close up view of a further variant of the fan disc of  FIG. 3 ; and 
         FIG. 9A  shows a close up view of the connector of another variant of the fan disc of  FIG. 3 , with a section C-C indicated, and  FIG. 9B  shows a view of the joint along section C-C. 
     
    
    
     DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES OF THE INVENTION 
     With reference to  FIG. 2 , a ducted fan gas turbine engine incorporating the invention is generally indicated at  10  and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , an intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . A nacelle  21  generally surrounds the engine  10  and defines the intake  11 , a bypass duct  22  and a bypass exhaust nozzle  23 . 
     During operation, air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow A into the intermediate-pressure compressor  13  and a second air flow B which passes through the bypass duct  22  to provide propulsive thrust. The intermediate-pressure compressor  13  compresses the air flow A directed into it before delivering that air to the high-pressure compressor  14  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors  14 ,  13  and the fan  12  by suitable interconnecting drive shafts. 
       FIG. 3  shows a side view of the fan disc  30  of the engine  10 . The fan disc supports a circumferential row of fan blades  32 . The disc body  34  has axially extending slots which receive the dovetail root fixings of the blades. A frustoconical drive arm  36  tapers forwardly from a forward portion of the disc body to a splined connector  38  which engages with a corresponding splined connector portion  40  of the fan shaft  42  of the engine. 
     The fan shaft  42  has a front bearing arrangement including a bearing housing  50  and a front set of roller bearings  52 . The forward position of the splined connector portion  40 , allows these bearings to be axially located close to fan centre of gravity, eliminating front overhang, and hence eliminating large FBO shear forces and bending moments. Smaller bearings can thus be used. In addition, the vibration characteristics of the fan rotor can be improved, in that the natural frequency is much higher and the amplitude of vibrations is reduced. However, there is now a space conflict at the rear underside of the fan disc  30 , where there is no longer sufficient room to provide a rear diaphragm. 
     Accordingly, the disc body  34  has an aft portion  44  which extends rearwardly of the rear ends of the dovetail root fixings of the blades  32 . A hoop stiffening ring  46  is mounted via a close fitting spigot surface over the outer diameter of the aft portion. A row of bolts  48  threaded into axially extending bolt holes formed in the aft portion secure the stiffening ring to the disc body, and also allow it to be removed therefrom. The stiffening ring in conjunction with the axially spaced drive arm  36  provides sufficient disc torsional stiffness (2D vibration) and structural strength in the event of FBO. Advantageously, the increased axial extension of the disc body enhances the torsional stiffening effect on the fan rotor. 
     As a result of these changes, the fan disc forging envelope can be made significantly smaller, and thus lower cost, than an equivalent conventional disc envelope. 
       FIG. 4A  shows a close up view of the rear of the fan disc  30 , with a section A-A indicated,  FIG. 4B  shows a view of the fan disc along section A-A, and  FIG. 4C  shows a further view of the fan disc along section A-A. The disc body  34  has dovetail root fingers  54  at its outer side which define the flanks of the axially extending slots  56  for receiving the dovetail root fixings of the blades. The stiffening ring  46  provides extra support to the dovetail root fingers during normal engine operation. It also helps to prevent disc polygonising distortions. With reference to  FIG. 4B , the second moment of area for torsional stiffness is provided by a combination of the stiffening ring plus the fan disc rim  58  with radial separation R connected through the dovetail root disc fingers. This arrangement utilises the same principle as I-beam sections to maximise the stiffness improvement within an available space envelope. With reference to  FIG. 4C , the stiffening ring additionally provides extra hoop stress support during FBO events. 
     Referring again to  FIG. 3 , annulus fillers  60  are located in the gaps between adjacent blades  32 . Conventionally such fillers can be fitted to a fan disc by front and rear hooks formed on the outer surface of the disc. Conveniently, however, the stiffening ring can provide a row of formations, such as pins  62 , for supporting corresponding engagement portions of the fillers. This avoids the high cost of providing the rear hook within the fan disc forging envelope. 
     The fan disc  30  may have a an annular seal panel  64  which extends from the stiffening ring  46  to a radially-outward air-washed portion  66  of the seal panel adjacent the trailing edges of the fan blades. Conveniently, the bolts  48  which secure the stiffening ring can also secure the seal panel. The annulus fillers  60  have lids  68  which provide air-washed surfaces extending between the adjacent fan blades. Each lid has a rear lip which hooks under a forward edge of the air-washed portion  66  of the seal panel. 
     A sealing ring  70 , e.g. formed of sheet metal, can also be attached at the bolts  48  to cover the aft ends of the axially extending slots  56 . 
     The stiffening ring  46  can be produced using a composite material such as titanium alloy metal matrix composite (TiMMC). This has a high specific stiffness which therefore increases torsional stiffness while reducing the extra weight of the ring. 
       FIG. 5A  shows a view of the fan disc  30  and the entire front bearing arrangement, including the bearing housing  50 , the front set of roller bearings  52  and a rear set of ball bearings  72 . Advantageously, the disc configuration allows a power gearbox to be incorporated into the engine in the bearing housing without any increase in engine length. In particular,  FIG. 5B  shows an adaptation of the engine of  FIG. 5A  in which the fan shaft is split into a front portion  42   a  and a rear portion  42   b  operatively connected to each other via a power gearbox  74  located in the housing behind the rear set of ball bearings. 
       FIG. 6A  shows a close up view of the rear of a variant of the fan disc  30 , with a section B-B indicated, and  FIG. 6B  shows a view of the fan disc along section B-B. This variant utilises the dovetail profile of the axially extending slots  56  in the aft portion of the disc body to provide a location for an internal clamp  78  to locate the stiffening ring  46 . A re-entrant formation may also be provided to enhance axial retention of the ring. 
       FIGS. 7A, 7B and 7C  show respective close up views of the rear of a fan disc to illustrate alternative methods of mounting a stiffening ring  46  to the disc. None of the alternative methods requires an aft portion  44  extending rearwardly of the rear ends of the dovetail root fixings. Instead they each have an aft portion  44 ′ in the form of a circumferential row of radially outward projections from the rear ends of the dovetail root fingers. 
     In  FIG. 7A , the stiffening ring  46  is in the form of a double ring to provide increased stiffness capability. The ring is removably mounted by bolts  48  to a rear side of the aft portion  44 ′. 
     In  FIG. 7B , the stiffening ring  46  is integrated with the seal panel  64  to provide an enhanced stiffening effect. Again the integral ring and panel is removably mounted by bolts  48  to a rear side of the aft portion  44 ′. In this case, an aluminium alloy metal matrix composite may be more suitable for forming the ring and panel. 
     In  FIG. 7C , the stiffening ring  46  is clamped to the fan disc  30  by an extension  76  from the seal panel  64 . 
       FIG. 8  shows a close up view of a further variant of the fan disc of  FIG. 3 . In this variant, instead of a splined connector, the drive arm  36  has a plain faced, flanged connector  80  which bolts to a corresponding flanged portion  82  of the fan shaft  42 . To provide additional torque transfer capability between the connector and the shaft, locating spigot surfaces  84  are formed on the connector and the shaft. 
       FIG. 9A  shows a close up view of the connector of another variant of the fan disc of  FIG. 3 , with a section C-C indicated, and  FIG. 9B  shows a view of the joint along section C-C. In this variant, the flanged connector  80  and the fan shaft  42  have locating spigot surfaces  84 . However, in addition, to further enhance torque transfer, the flanged connector and corresponding flanged portion  82  of the shaft have interlocking teeth  86  to form a curvic or Hirth coupling. 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. 
     All references referred to above are hereby incorporated by reference.