Abstract:
This invention relates to a blade assembly comprising: a hub which is rotatable about an axis; at least one blade having an aerofoil portion and a root, wherein the root is coupled to the hub using a primary retention device which prevents radial separation of the root and hub in normal use; and, a secondary retention device which comprises an annular catcher having a restriction through which the root portion cannot radially pass, the restriction being located radially outwardly and radially spaced from the root part during use.

Description:
TECHNICAL FIELD OF INVENTION 
       [0001]    This invention relates to a retention device for a rotating blade. In particular, the invention relates to a secondary retention device which provides a failsafe mechanism to help prevent release of a blade in the event of a failure of a primary retention device. 
       BACKGROUND OF INVENTION 
       [0002]    It is known to use a retention device on rotating blades, for example, on a propeller of an aircraft engine, to help prevent radial separation of the blades from the hub on which they are mounted. It is also known to use a secondary retention device which acts to prevent the radial separation of the blade and hub in the event of a failure of the primary retention device. 
         [0003]    One known propeller assembly which utilises such a secondary retention device is shown in  FIG. 1 . The propeller assembly  10  comprises a hub  12  and a blade  14  having an aerofoil  16  and root  18 . The blade  14  is configured to rotate about axis  19  with hub  12  on bearings (not shown) so as to provide a thrust in the direction indicated by arrow  21 . Bearings  24 ,  25  are provided such that the blade can be rotated about its longitudinal axis so as to provide a pitch control system. During manufacture of the propeller assembly  10 , the root  18  is inserted into the hub  14  via an aperture  20  in the outer surface  22  of the hub  14  and the bearings  24  inserted between respective radially opposing faces  26 ,  28  of the hub  14  and root  18 . The bearings  24  provide the rotational support required of the blade  14  and also act to prevent radial separation of the blade  14  and hub  12  during use. In this way, the bearings  24  act as a primary retention device. 
         [0004]    The secondary retention device in the embodiment shown in  FIG. 1  is a circular wire  32  which is fed into a channel formed from corresponding grooves in the facing surfaces of the hub  12  and the root  18 . In the event of bearing failure, radial separation of the blade  14  and hub  12  is prevented by the wire. 
         [0005]    Although the use of the wire  32  as a secondary retention device can be adequate, it relies on being correctly installed which requires a skilled operator and excessive amounts of time. 
         [0006]    Further, because the wire  32  is neither an integral part of the hub  12  nor the blade  14 , it is subjected to relative movement between these components which can lead to vibration and wear problems. A further difficulty with this system arises due to the concealed location of the wire which makes inspection more troublesome. 
         [0007]    The present invention seeks to overcome some of the problems with known prior art. 
       STATEMENTS OF INVENTION 
       [0008]    In one aspect, the present invention provides a blade assembly comprising: a hub which is rotatable about an axis; at least one blade having an aerofoil portion and a root, wherein the root is coupled to the hub using a primary retention device which prevents radial separation of the root and hub in normal use; and, a secondary retention device which comprises an annular catcher having a restriction through which the root portion cannot radially pass, the restriction being located radially outwardly and radially spaced from the root part during use. 
         [0009]    Providing a restriction in this way allows retention device to be provided which is unloaded in normal use. 
         [0010]    The restriction can be an aperture through which a portion of the blade passes, in use. The aperture may be axi-symmetric. The aperture may be generally circular. 
         [0011]    The annular catcher may comprise at least two segments which abut one another along a parting line, the parting line passing through at least one aperture. The parting line may pass through each aperture. The parting line may lie on the circumferential midline of the annular catcher. 
         [0012]    The two annular segments are attached via a plurality of fasteners. The fasteners may be bolts. 
         [0013]    The annular catcher may be radially located around the hub by three or more locating assemblies. The locating assemblies may include at least one two part connector having male and female portions. At least one part of the locator assembly may be local to the restriction. 
         [0014]    The hub may be nested within at least a portion of the annular catcher. The annular catcher may be fixedly attached to the hub via a flanged portion. 
         [0015]    The annular catcher may be generally polygonal. The apexes of the polygon may each be located at a restriction. 
         [0016]    The annular catcher includes two side rails which are axially spaced relative the axis. 
         [0017]    The blade assembly may further comprise a web portion which extends between side rails. The restriction may be located between the side rails. The side rails may have a substantially constant radial thickness. The axial width of the annular catcher may vary around the circumference of the catcher. The axial width may be at a minimum at the midpoint between each restriction. Either or both of the radially inner and outer surfaces of the catcher may be concave. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0018]    Embodiments of the invention will now be described with the aid of the following drawings of which: 
           [0019]      FIG. 1  shows a known propeller arrangement. 
           [0020]      FIG. 2  shows a propeller gas turbine engine. 
           [0021]      FIG. 3  shows a blade assembly according to the present invention. 
           [0022]      FIG. 4  shows the annular catcher of the assembly shown in  FIG. 3 . 
           [0023]      FIG. 5  shows an alternative blade assembly. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0024]      FIG. 2  shows a twin-spooled, contra-rotating propeller gas turbine engine  210  having a principal and rotational axis  209 . The engine  210  comprises a core engine  211  having, in axial flow series, an air intake  212 , a low pressure compressor  214  (LPC), a high-pressure compressor  215  (HPC), combustion equipment  216 , a high-pressure turbine  217  (HPT), low pressure turbine  218  (LPT), a free power turbine  219  (FPT) and a core exhaust nozzle  220 . A nacelle  221  generally surrounds the core engine  211  and defines the intake  212  and nozzle  220  and a core exhaust duct  222 . The engine  210  also comprises two contra-rotating propellers  223 ,  224  attached to and driven by the free power turbine  219 , which comprises contra-rotating blade arrays  225 ,  226 . 
         [0025]    The gas turbine engine  210  works in a conventional manner so that air entering the intake  212  is accelerated and compressed by the LPC  214  and directed into the HPC  215  where further compression takes place. The compressed air exhausted from the HPC  215  is directed into the combustion equipment  216  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and drive the high, low-pressure and free power turbines  217 ,  218 ,  219  before being exhausted through the nozzle  220  to provide some propulsive thrust. The high, low-pressure and free power turbines  217 ,  218 ,  219  respectively drive the high and low pressure compressors  215 ,  214  and the propellers  223 ,  224  by suitable interconnecting shafts. 
         [0026]      FIG. 3  shows a blade  316  (the radial outer portion of which is not shown) of the propeller assembly  310  of the invention. A blade root  318  of the blade  316  is received in a support cup  320 , which forms part of the hub, where it is supported for rotation about the longitudinal axis  317  of the blade  316  by bearings  322 ,  324 . A typical bearing arrangement is shown but other bearing arrangements are feasible. A pitch control shaft  326  extends across a void within a shroud  328  which is radially inwards of the blade root  318 . The shaft is connected to and transfers rotation from a pitch control mechanism (not shown) to the blade  316 . 
         [0027]    Each support cup  320  comprises a side wall  334  which is circular in cross-section. The interior surface of the side wall  334  is profiled in order to accommodate the bearings  322 ,  324 , and the geometry of the side wall  334  is otherwise configured in order to provide adequate rigidity to support the blade  316  in a stable manner. 
         [0028]    The support cup  320  is one of a plurality of such cups, one for each blade  316 , provided in a support hub  330 . It will be appreciated that the support cups  320  are disposed in a circumferential array about the support hub  330  and are interconnected by suitable webs or bridging structures. The support hub  330  is coaxial with the principal  209  axis of the engine. 
         [0029]    In use, the support hub  330  and support cups  320  are rotationally driven about the principal axis  209  of the engine which results in a centrifugal force on the blade  316  and blade root  318  and hub  330 . A primary retention device in the form of bearings  322  are arranged in corresponding opposing recesses in the blade root  318  and hub and act so as to prevent significant radial movement of the blade  316  relative to the support cup  320 . 
         [0030]    A secondary retention device in the form of an annular catcher  336  is provided radially outwardly of the blade root  318 . The annular catcher  336  is in the form of a collar which includes a restriction  338  through which the blade root  318  cannot radially pass once assembled. Hence, if the primary retention fails during use, for example due to a failure of the bearings  322  or support cup or ring, the blade  316  is retained as part of the engine  210 . 
         [0031]    As shown in  FIGS. 3 and 4 , the annular catcher  336  is in the form of a polygonal ring being generally constructed from two axially separated annular side rails  340  which are joined with an integral intermediate portion or web  342 ; the arrangement providing the annular catcher  336  with an axially extending banded ring like structure which is coaxial with the principal axis  209  of the engine  210 . The intermediate portion  342  connects the side rails  340  so as to provide the necessary strength required to retain the blade root  316  when needed. 
         [0032]    The restriction  338  in the annular catcher  336  which acts to retain the blade root  316  in the event of a failure of the primary retention device is in the form of an axi-symmetric aperture which is coaxially arranged with the longitudinal axis of the support cup  320  and pitch control shaft  326 . In the described embodiment, the axi-symmetric aperture is circular having a circumferential side wall  344  or rail which is located in between the side walls and which provides a radially inner catcher surface  346 . The catcher surface  346  is spaced apart from and opposes the blade root  316  when the primary retention device is operational. The cross-section of the aperture side wall  344  tapers such that the sidewall  344  thickness increases from the inner most circumferential edge, thereby providing the catcher surface  346  with a sloped or slightly conical profile. The aperture side wall  344  or rail is continuous so as to provide a closed circular structure. 
         [0033]    The blade root  318  includes a blade catcher surface  347  which corresponds to the catcher surface  346  of the restriction  338  such that, when placed in contact under a significant centrifugal load, the blade will tend to centralise within the restriction  338  thereby distributing the load across the catcher surface  346  more evenly. 
         [0034]    The annular catcher  336  includes a plurality of such restrictions circumferentially arranged around the support hub to coincide with each support cup and blade. 
         [0035]    As shown in  FIG. 4 , the annular catcher  336  has a generally polygonal construction made up from sections of curved  348  and angled  350  sections. The curved sections  348  form the apexes of the polygon at intervals which coincide with the restrictions  338 . The angled sections  350  are in the form of bridging portions which extend between the curved sections  348  of the annular catcher  326  and have a plurality of facets  352  which give the bridging portions a pinched or waisted appearance which narrows the axial length of the catcher  336  (with respect to the principal axis of the engine) at the midpoint  358  between each restriction  338 . 
         [0036]    The axially separated annular side rails  340  are defined by radially thicker portions or bands located at the axial extremities of catcher  336 . The radial thickness of the side rails is substantially constant around the circumference of the catcher  336 , but the axial length is reduced along the bridging portions to provide the waisted appearance described in the previous paragraph. 
         [0037]    The radially inner  352  and outer  354  surfaces of the intermediate portions are both concave in appearance such that the thickness of the material at the circumferential midline  356  of the catcher  336  is generally less than the side rails  340 . The extent of the concavity changes around the circumference of the catcher so as to provide the thinnest portion at the midpoint  358  between the restrictions. It will be noted that the edges of the catcher are chamfered  368  to help reduce crack propagation. 
         [0038]    The annular catcher  336  is constructed from two annular segments  336   a,    336   b  which are mated along a parting line. The annular segments  336   a,    336   b  are mirror images of each other about a central plane which dissects the annular catcher such that the parting line  360  is coincident with the midline  356 . Thus, each ring segment  336   a,    336   b  includes semi-circular recesses  362   a,    362   b  in the parting line  360  surface which correspond to opposing recesses in the other of the ring segments  336   a,    336   b  which form the restrictions when in place. The ring segments  336   a,    336   b  are fastened together with releasable fasteners in the form of nuts and bolts (not shown). However, it will be appreciated that other mechanisms will be suitable. 
         [0039]    In the unlikely event that the fasteners fail during normal operation, the two annular segments would each retain hoop integrity but may move axially, thereby losing cross key engagement and becoming eccentric. This should give sufficient out-of-balance and allow the engine to be shut down. The bolted interface may also arrest the propagation of an axial crack in half of the annular catcher  336 . 
         [0040]    Although the bridging portions are shown as being continuous, some applications will allow for weight reduction features, such as apertures or voids, to be introduced without detracting from the hoop strength provided by the side rails  340 . 
         [0041]    The annular catcher  336  is positioned relative to the support cup  320  and blade root  316  such that the restriction  338  is separated or spaced from the blade root  318  in normal use. This spacing or separation means that there is no substantial radial load transferred directly from the blade root  318  to the catcher surface  346  during normal use. Thus, in the described embodiment, the only radial load experienced by the catcher  336  is created by its own mass when rotated. A further advantage of the spacing is that, in the event of a failure of the primary retention, the blade  316  experiences some radial movement which leads to an out of balance condition and detectable vibration. Thus the failure can be readily detected. 
         [0042]    The radial spacing is achieved with a locating assembly in the form of a plurality of two part couplers in the form of cross key locators. Thus, as shown in  FIG. 4 , the catcher  336  includes a plurality of notches  364  in the radially inner peripheral surface of the restriction  338  which correspond to a plurality of lugs located on the support hub  330 . There need to be a minimum of three fixings distributed around the circumference of the catcher  338  to provide the necessary centring. As will be noted, the described arrangement includes pairs of locating features, each one of a pair being placed on either side of the restriction  338 . Similar locators may be placed by each restriction around the hub  330 . 
         [0043]    To form the propeller assembly  310 , the blade root  318  is inserted into the support cup  320  and the bearings  322  inserted after any necessary alignment etc. Once the blade root  316  is in place, the annular segments  336   a,    336   b  are positioned around the blade shaft  366  which connects the root  318  and aerofoil portion  314  and bolted together. 
         [0044]    Having a bolted construction of this type is advantageous as it allows the blade  316  to be inserted into the support cup  320  prior to the annular segments  336   a,    336   b  being bolted around the blade shaft  366  of the blade root  316  thereby forming a retention device having a continuous wall which encircles the blade root  316 . 
         [0045]    Other segmented constructions are envisaged as being possible. For example, instead of having two annular segments which abut one another along a single parting line which passes through all of the apertures, there may be multiple segments, each closing one or more apertures. Further, the parting line may not pass along the circumferential midline. 
         [0046]      FIG. 5  shows an alternative arrangement of the propeller assembly  510  in cross section. Thus, there is an annular catcher  536  which is supported by flanges  570  which extend radially inward from the side rails  540  so as to partially envelope the support cup  520  and hub  330 , thereby providing a nested arrangement. The flanges are continuous annular structures and are substantially planar such that the side rails  540  do not have a waisted appearance as per the previous embodiment. The catcher shown in  FIG. 5  is bolted to a portion of the rotor in the form of the aft drive flange  572  which also connects to the support hub  530  so as to rotate the support cups  520  and thus blades  516 . However, the locating assembly of the previous embodiment may be implemented instead of or as well as the flanged arrangement. 
         [0047]    This nested configuration is particularly advantageous in that the catcher  536  provides a more complete containment function in the event of a mechanical breakdown of the blade root  516  or support cup  520  or hub  530 . Further, such an arrangement is significantly more rigid. 
         [0048]    For the avoidance of doubt, the above described embodiments are examples of the broader inventive concept as defined by the scope of the following claims.