Abstract:
A mounting arrangement for a gas turbine engine blade having a root employs a lightweight, simple and economical leaf spring spacer which biases the blade root in a radially outward direction to minimize unwanted movement of the root within a conforming slot in a blade hub under conditions such as windmilling when centrifugal force alone is inadequate to tightly seal the root within the slot.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    This invention relates generally to gas turbine engines and particularly to a mounting arrangement for gas turbine engine blades such as fan blades. 
         [0003]    2. Background Information 
         [0004]    Turbofan gas turbine engines such as those which power aircraft employ a multiplicity of fan blades attached to a hub mounted on the forward (upstream) end of one of the engine shafts. Typically, such fan blades are provided with a radially outer airfoil shaped portion and a radially inner root portion typically having a dovetail shape. The dovetail shaped root portion is received within a slot which conforms thereto in the fan hub. For ease in attaching the fan blade to the hub by sliding the root portion into the slot and for removal of the blade from the hub by sliding the root portion of the blade out of the slot, the slot is usually slightly larger than the dovetail root portion. This difference in dimensions between the root portion of the fan blade and the slot in the hub results in a clearance between the root portion and the slot. Under normal engine operating conditions when the engine&#39;s rotor is spinning at high speed (several thousand rpm) centrifugal force acting on the fan blade causes the blade to be held tightly in the hub slot. 
         [0005]    However, when the engine is not in use, wind acting on the fan blades can cause the engine&#39;s rotor to slowly turn. This slow turning of the engine rotor in response to winds acting on the fan blades is referred to as windmilling. There is very little centrifugal force acting on the fan blades during such windmilling due to the low rotational speed of the engine rotor in response thereto and thus, the fan blade roots are not tightly held within the conforming slots in the fan hub, resulting in movement between the fan blade root portions and the hub slots in which they are received. This movement of the fan blade root portions within the slots, if unchecked, can result in damage to the fan blade root portions and the slots due to galling and fretting of the surfaces of the root portions and the slots. To minimize such galling and fretting, it has been a practice to employ spacers between the radially innermost end of the root portion and the adjacent portion of the slot which receives the root portion to prevent movement between the blade and the hub during windmilling of the engine&#39;s rotor. In some cases, such spacers actually resiliently bias the root portion and thus the entire blade radially outwardly to tightly secure the blade root portion within the hub slot. Such prior art blade root spacers have taken the form of relatively complex metallic configurations and elastomeric materials secured between rigid clamping members which adjustably compress the elastomeric material to control the elasticity thereof. The complexity and weight of such prior art blade root spacers adds to the cost and weight of the engine, thus detracting from the efficiency thereof. Furthermore, the complexity of such spacers detracts from the ease with which the fan blades are assembled to the hub during engine assembly and removed from the hub for engine maintenance. 
         [0006]    Accordingly, it will be appreciated that a need exists for a simple, lightweight and economical means for minimizing movement of fan blade roots within fan hub slots under conditions such as windmilling and the like. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    In accordance with the present invention, a lightweight, simple and economical leaf spring spacer is disposed between a radially innermost end of a gas turbine engine blade root such as a fan blade root and an adjacent radially innermost surface of a hub slot which accommodates that root. The spacer is held in place by an interference fit between the blade root and the slot such that the spacer exerts a radially outward force on the blade root to secure the blade root within the slot thereby limiting unwanted radial movement and tilting of the blade root within the slot during such conditions as windmilling of the gas turbine engine rotor. In an exemplary embodiment, the blade comprises a fan blade. Also in the exemplary embodiment, the blade root is dovetail shaped, including a pair of radially inner and outer angularly offset longitudinally extending side surfaces, and the slot includes a pair of radially inner and outer angularly offset longitudinally extending side surfaces which are opposed to the radially inner and outer side surfaces of the blade root. The leaf spring spacer is compressed between the radially innermost surfaces of the blade root and the slot which accommodates the root exerting a radially outward force on the root which causes the radially outer side surface of the blade root to bear against the radially outer side surface of the slot, thereby preventing radial movement of the blade root within the slot during the aforementioned windmilling conditions. 
         [0008]    According to an aspect of the present invention, a mounting arrangement for a gas turbine engine blade is provided. The mounting arrangement includes a radially directed axis and a radially inner root received within a slot in an associated blade hub. The mounting arrangement also includes an elongate resilient leaf spring spacer received within said slot between a radially innermost end of said blade root and a radially innermost surface of said slot by interference fit therebetween such that said spacer exerts a radially outward force on said blade root to reduce radial movement of said blade root within said slot. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic view of a turbofan gas turbine engine of the type employing the present invention; 
           [0010]      FIG. 2  is an exploded isometric view of a fan hub and an associated fan blade employed in the turbofan engine of  FIG. 1 ; 
           [0011]      FIG. 3  is a partial front elevation of the fan blade and hub shown in  FIG. 2  taken in a direction of line  3 - 3  of  FIG. 2 ; 
           [0012]      FIG. 4  is a partial front elevation similar to  FIG. 3  but showing the blade mounting arrangement of the present invention in a state of partial assembly; 
           [0013]      FIG. 5  is an isometric view of a blade root spacer employed in the blade mounting arrangement of the present invention; 
           [0014]      FIG. 6  is an isometric view of an alternate embodiment of the spacer shown in  FIG. 5 ; and 
           [0015]      FIG. 7  is a front elevation view similar to  FIG. 3  of the blade mounting arrangement of the present invention employing the alternate embodiment of the spacer shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to  FIG. 1 , a turbofan gas turbine engine  5  has a longitudinal axis  7  about which the rotors  8  of the engine rotate. A fan  10  disposed at the engine inlet draws air into the engine. A low pressure compressor  15  located immediately downstream of fan  10  compresses air exhausted from fan  10  and a high pressure compressor  20  located immediately downstream of low pressure compressor  15 , further compresses air received therefrom and exhausts such air to combustors  25  disposed immediately downstream of high pressure compressor  20 . Combustors  25  receive fuel through fuel injectors  30  and ignite the fuel/air mixture. The burning fuel-air mixture (working medium fluid) flows axially to a high pressure turbine  35  which extracts energy from the working medium fluid and in so doing, rotates hollow shaft  37 , thereby driving the rotor of high pressure compressor  20 . The working medium fluid exiting the high pressure turbine  35  then enters low pressure turbine  40 , which extracts further energy from the working medium fluid. The low pressure turbine rotor provides power to drive the fan  10  and low pressure compressor  15  via low pressure shaft  42 , which is disposed interiorly of the hollow shaft  37 , coaxial thereto. Working medium fluid exiting the low pressure turbine  40  provides axial thrust for powering an associated aircraft (not shown) or a free turbine (also not shown). 
         [0017]    Bearings  43 ,  45 ,  50  and  53  radially support the concentric high pressure and low pressure turbine shafts from separate frame structures  52 ,  54 ,  55  and  56  respectively, attached to engine case  57 , which defines the outer boundary of the engine&#39;s stator which circumscribes rotors  8 . However, it will be appreciated that the present invention is also well suited for mid-turbine frame engine architectures wherein the upstream bearings for the low and high pressure turbines are mounted on a common frame structure disposed longitudinally (axially) between the high and low pressure turbines. 
         [0018]    Referring to  FIGS. 1-4 , the fan  10  comprises a hub  60  mounted on the forward end of low pressure shaft  42  as by bolts  65  and a plurality of blades  70  mounted on hub  60  about the periphery thereof. As best seen in  FIG. 2 , each of the blades  70  has a radial central axis  77  and comprises a radially outer airfoil shaped portion  75  which in a manner well known in the art draws air into the engine and a radially inner root portion  80  which is received within a conforming slot  85  in the periphery of hub  60 . The blades  70  may be formed from a metallic material such as titanium or an alloy thereof, a composite such as a glass-epoxy composite or any combination thereof as is well known in the art. As best seen in  FIG. 4 , root portion  80  is generally dovetail shaped in cross-section, being defined by the radially innermost end of the airfoil portion  75 , a longitudinally extending radially innermost surface  90 , and a pair of longitudinally extending angularly offset side surfaces  95  and  100 . As set forth hereinabove, each slot  85  in hub  60  which receives a dovetail root of one of the fan blades  70  conforms to that root, and includes a longitudinally extending radially innermost surface  105  facing (opposed to) surface  90  of blade root  80  as well as angularly offset radially inner and outer side surfaces  110  and  115  which face (are opposed to) side surfaces  95  and  100  of root portion  80 . 
         [0019]    It will be appreciated that to enable root portion  80  to be inserted into slot  85  for assembly and removed therefrom for disassembly of the fan blades from the hub, slot  85  must be dimensionally larger than dovetail root  80  thereby defining a clearance (e.g., a radial clearance)  120  therebetween. As set forth hereinabove, under windmilling conditions, when the engine is not running and the fan is turned slowly by wind entering the engine, clearance  120  would allow limited radial movement and tilting of blade  70  within slot  85 , thereby causing unwanted wear between the root portion  80  of the blade  70  and the slot side surfaces  95 ,  100  due to frictional galling and/or fretting between the dovetail root portion  80  and the slot  85 . To prevent such wear of the root portion and slot during such windmilling conditions, a leaf spring spacer  125  (see  FIG. 3 ) is disposed within clearance  120  between radially innermost surface  90  of blade root  80  and radially innermost surface  105  of slot  85 . 
         [0020]    Referring to FIGS.  3  and  5 - 7 , the leaf spring spacer  125  includes a longitudinal axis  130  which, when the spacer is assembled with the blade  70  and the hub  60 , is generally parallel to the longitudinal axis  7  of the engine itself. The spacer may be formed from any material having the requisite strength and elasticity such as any of various thermoplastic materials such as Vespel TP-3985 manufactured and sold by E.I. DuPont de Nemours and Company. The spacer  125  is bowed in a radial direction and is retained between root portion  80  and slot  85  by an interference fit therebetween. The spacer  125  includes first and second opposed longitudinally extending major surfaces  135  and  140  and a pair of opposed longitudinally extending lateral side edge portions  145  and  150 . The side edge portions  145 ,  150  may be smooth as shown in  FIG. 5  or notched along the lengths thereof to define tabs  155  as shown in  FIG. 6 . The lateral side edge portions may be generally coplanar to the main body of the spacer or may be radially bent to fill any clearance between the side surfaces  95 ,  100  of the dovetail root  80  and slot  85 . Furthermore, the spacer  125  may be of a uniform thickness or may include thickened portions thereof either along substantial portions of edges  145  and  150  or along the edges of tabs  155  (see  FIG. 6 ). Such thickened portions  145 ,  150  of spacer  125  may conveniently fill any clearances between the root portion  80  of the blade  70  and the disk slot  85  defined by beveled edges of the blade root  80  between adjacent surfaces thereof (see  FIG. 7 ). Also, spacer  125  may include a groove  162  for cooperation with a suitable removal tool (not shown) used in removing the spacer in the disassembly of the fan blade from the hub. As shown in  FIG. 6 , to minimize the weight which the spacer  125  adds to the fan rotor, the spacer  125  may be apertured at one or more locations  160  along length thereof. The apertures  160  are also useful to accommodate balance weights (not shown) which may be necessary to properly balance the fan rotor in the assembly thereof. 
         [0021]    A fan blade  70  is mounted on the hub  60  by first positioning the hub  60  such that the slot  85  in which a fan blade  70  is to be inserted is located at a bottom dead center position and the aft retaining ring is then installed. The blade  70  is then slid into the slot  85  and released so that the radially outer side surface  95 ,  100  of the root  80  rests on the opposed surface of the slot  85 . Spacer  125  is then inserted into the slot. Referring to  FIGS. 2 and 3 , the forward retaining ring  170  is then installed in mating grooves  175  in the periphery of the hub  60  to longitudinally restrain the blade root  80  within the slot  85 . 
         [0022]    Accordingly, it will be appreciated that the blade mounting arrangement of the present invention provides a lightweight, economical and effective arrangement for securing a fan blade  70  to a hub  60  to minimize movement therebetween under windmilling conditions. The simple compact shape of the leaf spring spacer  125  adds minimally to the weight of the fan rotor. No complex and/or heavy mechanisms are necessary to effectively hold the blade root  80  within the hub  60 . 
         [0023]    While the present invention has been described within the context of a gas turbine engine fan blade mounting arrangement, it will be appreciated that the present invention may be employed in the mounting of various other blades to a gas turbine engine rotor such compressor blades or turbine blades. While specific shapes and materials for the blade spacers employed in the present invention have been discussed, it will be appreciated that various modifications thereto may be made without departing from the present invention and it is intended by the appended claims to cover such modifications as may fall within the true spirit and scope of this invention.