Patent Publication Number: US-10760435-B2

Title: Lock plate for a bladed rotor arrangement

Description:
TECHNICAL FIELD OF INVENTION 
     The present invention relates to a bladed rotor arrangement and an associated lock plate. In particular, the invention relates to a bladed rotor arrangement of a turbine stage of a gas turbine engine or a turbomachine. 
     BACKGROUND OF INVENTION 
     Gas turbine engines comprise a plurality of bladed rotors, each of which comprises a rotor, typically in the form of a disc, and a plurality of rotor blades circumferentially mounted around the radial periphery of the rotor. Each rotor blade has an aerofoil, a platform, a shank and a root. The rotor comprises a plurality of circumferentially spaced axially extending slots defined by radially extending posts. The root of each rotor blade is arranged to locate in a respective one of the axially extending slots. The roots of the rotor blades are generally fir tree or dovetail shaped and the axially extending slots are correspondingly shaped to receive the roots of the rotor blades. The engagement of the roots in the slots is sufficient to radially retain the blades when under centrifugal loading induced during rotation of the rotor. 
     The bladed rotor arrangement also comprises a plurality of lock plates arranged at either or both axial ends of the rotor to prevent the rotor blades moving axially relative to the rotor. The lock plates also act to restrict fluid flowing under the platform of the blades. 
     The radially outer ends of lock plates typically engage grooves provided in the radially inward facing surfaces of the blade platforms, or flanges extending therefrom. The radially inner ends of the lock plates engage circumferentially extending grooves in the rotor or slots defined by additional structural members such as so-called cover plates or and seal plates. 
     In the case of a turbine stage lock plate, the components are exposed to extreme temperatures and pressures, and high levels of centrifugal loading as the rotor spins. And of course, the lock plates are required to maintain the requisite functionality whilst withstanding these extreme forces placed on the components. 
     The lock plates are designed to be undersized in a cold build condition to allow for operational movement resulting from thermal expansion and centrifugal forces, and also to allow for manufacturing tolerances and build clearances. 
     Further, the centrifugal loading is ultimately carried by the blade platform grooves in which the radially outer edges of the lock plates are located. Thus, there is a general requirement to minimise the mass of the lock plates to help reduce the resultant parasitic loading on the blades and any other associated components. 
     The present invention seeks to provide an improved bladed rotor arrangement and associated lock plate. 
     STATEMENTS OF INVENTION 
     The present invention provides a turbine arrangement and a lock plate according to the appended claims. 
     According to the present disclosure there is a turbine arrangement of a gas turbine engine may comprise: a rotor comprising a disc which incorporates a plurality of circumferentially spaced turbine blades, each blade having an aerofoil extending for a platform and a root portion securing the blade to the disc, a lock plate located at an axial end of the root portion of the blade, the lock plate comprising: a plate body having an inward facing surface which is located adjacent the blade root and an outward facing surface, radially inner and outer edge portions, and first and second circumferential edges; a head located at the radially outer edge portion of the plate body; a foot portion located at the radially inner portion of the plate body; a projection extending from the outwardly facing surface away from the turbine blade root in use, the projection being located adjacent to the foot portion, wherein the foot portion and head portion are located within slots provided at least partially by the disc and turbine blade respectively. 
     Providing a projection extending from the outwardly facing surface and local to the foot portion of the lock plate, allows the centre of gravity of the lock plate to be axially spaced from the turbine blade root relative to the geometric centre of the lock plate or a pivot point defined by the head portion. Thus, in use, the lock plate moves away from the turbine blade root when centrifugally loaded. 
     The head portion may include one or more pads. The pads may extend from the radially outer edge of the plate body. The pads may provide a pivot point for pivotally abutting or engaging a corresponding formation on the blade platform. 
     The foot portion slot may be provided by an axially inner wall and an axially outer wall, the foot portion slot being therebetween, and the projection is provided radially proximate to a radially outer periphery of the axially outer wall. 
     The disc slot may be at least partially provided by a seal plate or a cover plate which is attached to the rotor. The seal plate may define a channel between the seal plate and the rotor. The channel may form part of a fluid pathway for delivering cooling air to a turbine component. The fluid pathway may extend between a compressor stage of the gas turbine engine and an internal cooling passage of the rotor blade. 
     The axial extent of the projection may be greater than the outer wall of the disc slot. 
     The radial separation of the projection and the radial periphery of the axially outer wall of the disc slot may be in the range of between 2 mm and 4 mm. The radial separation may be taken when the engine is cold. The nominal cold clearance may be approximately 2.8 mm. 
     The blade slot may be provided in the radially inner face of a blade platform. 
     The blade slot may include a pivot pad which radially opposes a pivot pad on the radial periphery of the lock plate head portion. 
     The projection may be an elongate rib protruding from the outwardly facing surface. The projection may be circumferentially continuous or may be circumferentially segmented. 
     The rib may be circumferentially curved. The rib may extend between the circumferential edges of the lock plate. The radius of curvature of the rib may be concentric with the principal axis of the rotor. 
     The foot portion may include a seal fin located in the disc slot The projection may be located radially outwards of the seal fin. 
     The seal fin may include a chordal inner edge. The chordal edge may be substantially straight. The chord may be a chord of a circle centred on the rotational axis of the turbine. 
     The seal fin may be thinner than the plate body in transverse section. 
     The seal fin may be axially thinner than the plate body. 
     The head portion may include one or more pivot pads on a radially outer edge thereof. There may be a central pivot pad. There may be a pivot pad at each circumferential edge of the lock plate. The pivot pads at the circumferential edge of the lock plates may be half the circumferential length of the central pivot pads. 
     The radially outer edge of the lock plate may include a chamfer on either or both of the outwardly facing surface or the inwardly facing surface, the pivot pad being defined by the chamfer. 
     According to the present disclosure there is a lock plate for covering the root of a turbine blade in a gas turbine engine, may comprise: a plate body having an inward facing surface which is located adjacent the blade root and an outward facing surface, radially inner and outer edge portions, and first and second circumferential edges; a head portion located at the radially outer edge portion of the plate body; a foot portion located at the radially inner portion of the plate body; a projection extending from the outwardly facing surface away from the turbine blade root in use, the projection being located adjacent to the foot portion. 
     Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
     In the description below, unless otherwise stated, the geometric references for axial and circumferential should be taken with reference to the principal axis of the gas turbine engine. The terms upstream and downstream should be taken with reference to the flow stream of the main gas path through the engine. Inward and outward facing surfaces should be taken with reference to the rotor surfaces. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will now be described with the aid of the following drawings of which: 
         FIG. 1  shows a schematic longitudinal section of a conventional gas turbine engine. 
         FIG. 2  shows an isometric view of a high pressure turbine stage of a gas turbine engine, with a cut out to reveal a lock plate assembled in the rotor arrangement. 
         FIG. 3  shows a detailed isometric view of a rotor arrangement including a lock plate. 
         FIG. 4  shows an isometric view from an outward direction of a lock plate according to the invention. 
         FIG. 5  shows the lock plate of  FIG. 4  located in a rotor arrangement. 
         FIG. 6  shows a partial section of  FIG. 5  to illustrate the features of the radially inner portion of the lock plate. 
         FIG. 7  shows a partial section of the radially outer portion of the lock plate and turbine blade platform. 
         FIG. 8  shows an outwardly facing view of the inner disc facing surface of the lock plate. 
         FIG. 9  shows respective outward and inward facing surface end edges of two adjacent lock plates. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     A turbofan gas turbine engine  10 , as shown in  FIG. 1 , comprises in flow series an intake  11 , a fan  12 , an intermediate pressure compressor  13 , a high pressure compressor  14 , a combustion chamber  15 , a high pressure turbine  16 , an intermediate pressure turbine  17 , a low pressure turbine  18  and an exhaust  19 . The high pressure turbine  16  is arranged to drive the high pressure compressor  14  via a first shaft  20 . The intermediate pressure turbine  17  is arranged to drive the intermediate pressure compressor  13  via a second shaft  21  and the low pressure turbine  18  is arranged to drive the fan  12  via a third shaft  22 . In operation air flows into the intake  11  and is compressed by the fan  12 . A first portion of the air flows through, and is compressed by, the intermediate pressure compressor  13  and the high pressure compressor  14  and is supplied to the combustion chamber  15 . Fuel is injected into the combustion chamber  15  and is burnt in the air to produce hot exhaust gases which flow through, and drive, the high pressure turbine  16 , the intermediate pressure turbine  17  and the low pressure turbine  18 . The hot exhaust gases leaving the low pressure turbine  18  flow through the exhaust  19  to provide propulsive thrust. A second portion of the air bypasses the main engine and flows through a bypass duct  23  defined by a fan casing  24 . The second portion of air leaving the bypass duct  23  flows through a bypass, or fan, nozzle  25  to provide propulsive thrust. 
     A portion of the high pressure turbine  16  of the turbofan gas turbine engine  10  is shown more clearly in  FIG. 2 . The high pressure turbine  16  comprises a plurality of nozzle guide vanes  30  which guide hot gases from the combustion chamber  15  onto the turbine rotor blades  36  of a bladed turbine rotor arrangement  32 . The bladed turbine rotor arrangement  32  comprises a turbine rotor  34 , a plurality of turbine rotor blades  36  and a plurality of lock plates  48  and  50 . The turbine rotor blades  36  are mounted on the periphery of the turbine rotor  34  and each turbine rotor blade  36  comprises an aerofoil, a platform  40 , a shank  42  and a root  44 . 
     The turbine rotor  34  comprises a plurality of circumferentially spaced axially extending slots  46  defined by disc posts  52 . The root  44  of each turbine rotor blade  36  locates in a respective one of the axially extending slots  46  in the periphery of the turbine rotor  34 . The turbine rotor  34  in this example comprises a turbine disc. The roots  44  of the turbine rotor blades  36  are generally fir tree shaped and the axially extending slots  46  are correspondingly shaped to receive the roots  44  of the turbine rotor blades  36 . However, the roots  44  of the turbine rotor blades  36  may be dovetail shaped and the axially extending slots  46  shaped accordingly. 
     Lock plates  48 ,  50  are included at the axial extents of the disc posts  52  which define the root receiving axially extending slots. The lock plates  48 ,  50  provide a retaining function to prevent axial movement of the blades relative to the disc, and a sealing function to help reduce fluid flow under the blade platform and around the blade roots. An uncontrolled flow in this area would lead to excessive use of cooling air (which ultimately reduces efficiency) and deleterious levels of poorly distributed cooling air. 
       FIG. 3  shows a more detailed partial section of a turbine section, similar to that shown in  FIG. 2 . The turbine section is a high pressure turbine having a plurality of circumferentially arranged turbine blades  36  in flow series with a vane set. 
     The lock plates  48 ,  50  are plate-like members having a generally flat or planar body defined by a radially outer portion  54 , a radially inner portion  56  and two circumferential edges  58 ,  60  which extend between the radially inner  56  and outer portions  54 . The lock plates  48 ,  50  are arranged in a circumferentially distributed manner and abut one another to provide a full annulus shield around the disc posts and blade roots. The lock plates  48 ,  50  are located adjacent the blade roots  44  and disc posts  52  so as to have an inwardly facing surface  62  and an outwardly facing surface  64 . The radially inner  56  and radially outer  54  portions of the lock plates  48 ,  50  include respective head and foot portions. 
     In relation to the aft or rear lock plate  50  it can be seen that the head portion is located in a groove provided be an inwardly extending flange  66  provided on the radially inner surface of the platform  40 . The foot portion is located in the groove defined between the disc posts and a seal plate  70 . The fore lock plate  48  includes a similar arrangement on the upstream side of the rotor. 
     Under operating conditions the lock plates  48 ,  50  are rotated with the rotor  34  and experience a radial centrifugal force. This force causes the lock plates  48 ,  50  to move radially outboard and into the grooves provided by the blade platform  40 . The mass distribution of the lock plates  48 ,  50  and the centrifugal force is such that the lock plates tend to pivot about the head portion  54 . The amount of rotation, or the rotational force, is dependent on the centre of gravity of the lock plates relative to the pivot point. With existing designs, the centre of gravity is such that the lock plates  48 ,  50  move towards, or inwardly to, the discs for both the forward and rear lock plates. Although this can have advantageous effects, it can also act to open up a gap between the seal plate and the lock plates which can provide an unwanted leakage path for cooling air  68  which is both outside of the lock plates, and being channeled within the lock plates, for the interior passages of the blade and platforms. 
     To help reduce the centrifugally induced moment, it is possible to provide some axial biasing. Such axial biasing may be provided by a spring wire  72  or the like located at an appropriate location between the lock plate  48  and disc  34 . Such a biasing mechanism ultimately adds unnecessary complication and potential points of failure to the rotor arrangement and is not ideal. 
       FIG. 4  shows an isometric view of a lock plate  410  having a plate body  412  with an inward facing surface which is located adjacent the blade root (and obscured from view in  FIG. 4 ) and an outwardly facing surface  464  which faces away from the blade and disc. The lock plate  410  includes radially inner  456  and outer  454  edge portions and first  458  and second  460  circumferential edges which extend between the radially inner  456  and outer  454  edge portions. 
     The radially outer edge portion  454  of the plate body  412  includes a head portion  474 . The radially inner portion  456  includes a foot portion  476 . A projection extends  478  from the outward facing surface  464  of the lock plate  410  away from the turbine blade roots. The projection  478  may take any form which sits proud of the planar surface of the lock plate and substantially moves the centre of gravity axially away from the blade against which it is located. Further, the projection terminates axially beyond the pivot point of the lock plate such that its mass causes the lock plate to move away from the blade under CF loading. Thus, the projection may be one or more outstands or protrusions extending away from the surface of the lock plate. 
     As can be seen in  FIG. 4 , the projection may be in the form of an elongate rib which extends from a fixed end attached to the outwardly facing surface away from the lock plate to a free end. The projection  478  may be perpendicular to the outwardly facing surface  464 . The elongate rib has a longitudinal axis which extends between the circumferential edges of the plate body. In the example shown, the ends of the rib are coterminous with the circumferential edges of the plate, but this need not be the case and the projection may terminate short of the edges. Further, there may be a plurality of discrete projections distributed across the outward face of the lock plate  410 . 
     The rib is longitudinally curved with a radius of curvature which is concentric with the principal axis of the rotor. The radially outer edge of the lock plate is similarly curved in the example shown, having a origin of curvature which is also concentric with the principal axis of the rotor. 
     The purpose of the projection is to move the centre of gravity away from the rotor relative to a pivot point located on a head portion. Thus, the radial position and the radial and axial extent of the projection is determined as a function of the material density and desired centre of gravity and the pivot point provided by the head portion  474 . This is described in more detail below. 
     Although the head portion  474  as shown in  FIG. 4  is circumferentially curved, the radially inner edge of the foot portion has a chordal edge  480 . By chordal edge it will be understood that the radially inner edge foot portion  456  is a straight line which is defined by a chord of a curve centred local to the principal axis of rotor. An advantage of the chordal edge is that it provides an increased sealing engagement with a cover plate against which it abuts when rotated and the centrifugal force. It will be appreciated that the chordal line of the radially inner edge is substantially straight and may include some minor deviation from straight, either locally or along its length. 
     The rib  478  is integrally formed with the lock plate so as to provide a single homogenous structure, for example, by casting or machining from stock. However, there may be instances where the rib is formed as a separate part and attached to the lock plate body by a suitable method of attachment. 
     In the example shown, the lock plate head portion is provided with three separate pivot pads  482   a - c  which are circumferentially distributed along the radially outer edge. There are two circumferentially outer pads  482   a ,  482   c , and one central pad  482   b  located more or less equidistantly between the two outer pads. The two outer pads extend from the respective circumferential edges of the plate body and have approximately half the circumferential length of the central pad such that when two similar lock plates are placed adjacent to each other they form a single pad which is similarly sized to the central pad. It will be appreciated that other configurations of pads may be possible and the circumferential extent and width of the pads will be dependent on the pivoting action which is required. In other examples there may be a greater or fewer number of pivot pads, and they may or may not be evenly distributed around the circumferential edge of the lock plate. 
     The individual pivot pads are provided by rebates in the radially outer edge of the head portion. That is, the peripheral edge of the radially outer portion of the lock plate is indented to provide the pads in the form of upstands. 
       FIG. 5  shows the lock plate of  FIG. 4  in situ. Hence, moving radially outwards there is shown a turbine disc  534  of which a disc post can be seen, a front seal plate  584 , a front cover plate  586 , a lock plate  510 , and a turbine blade platform  540 . The lock plate  510  is located adjacent to and upstream or forward of the rotor  534  and sits in front of the rotor disc posts  552  and blade roots and shank and is held in a plane which is normal to the principal axis of the rotor and engine. 
     The lock plate  510  is held in grooves/slots provided by the turbine blade at the radial outer, and in the rotor disc  534  at the radial inner. In the example, the radially outer groove is defined at least partially by the turbine blade, in particular, the turbine platform. The radially inner groove is defined at least partially by a seal plate  584 . It will be appreciated, that the radially outer and radially inner lock plate retention grooves may be provided by alternative structure. Such structure may include a slotted flange for example. 
     The seal plate  584  is positioned at the upstream axial end of the turbine rotor  534 . It comprises an annular disc having an inner and outer radius, and inwardly and outwardly facing surfaces, the former of which faces inwardly towards the rotor and abuts the upstream disc surface. A separate seal plate may be provided on the downstream side of the rotor  534 . 
     The seal plate  584  is axially retained by a cover plate  586 . The cover plate  586  is fixed to the rotor shaft or an associated member so as to form part of the rotor. The cover plate  586  is an annular structure having an inner bore (not shown) through which a shaft passes. The cover plate  586  also provides a heat shield for the seal plate  584 . 
     The seal plate  584  provides a sealing function and helps guide cooling air from an aperture  90  (see  FIG. 5 ) in the cover plate and into the inlet  592  of the turbine blade cooling passages which is located in the base of the turbine root. The seal plate may define a channel between the seal plate and the rotor. The channel may form part of a fluid pathway for delivering cooling air to a turbine component. The fluid pathway may extend between a compressor stage of the gas turbine engine and an internal cooling passage of the rotor blade. 
     It will be appreciated that other arrangements may be possible and the cover plate and seal plate are both individually optional. In such a case, the lock plate may be retained in a slot provided by an appendage of disc. Such an appendage may be a flange or other mass which is integrally formed with disc. 
       FIG. 6  shows a partial longitudinal section of the foot portion  556  of the lock plate  510  which details the front cover plate  586 , seal plate  584 , lock plate foot portion and turbine disc post  552 . The lock plate is held within a circumferential groove which has a radially outer open end, a radially inner terminal end and axially inner and outer walls 
     The projection  578  provides a mass which acts to move the centre of gravity forwards or away from the disc relative to the geometric centre of the lock plate  510 . The displacement of the centre of gravity is sufficient to cause the forward movement of the lock plate under normal operating centrifugal forces. Hence, in operation, the lock plate  510  is arranged to rock away from the rotor at the radially inner edge when pivoting under CF loads. Moving the lock plate forward at the radially inner portion in this way increases the sealing between the lock plate and the seal plate. 
     The forward movement of the lock plate  510  is provided by a rotation about a fulcrum located towards the radially outer edge of the lock plate  510 . Specifically, in the described example, the lock plate pivots about the radially outer peripheral edge of the lock plate and pivot pads provided thereon. It will be appreciated that the head portion  554  of the lock plate may include the pivot at an edge thereof without the use of discrete pivot pads. 
     The outer peripheries, or outer radii in the described example, of the cover plate and seal plate are substantially similar in extent. Thus, they are approximately radially coterminous. A circumferential groove is provided between the seal plate  584  and the rotor disc  534  to receive a portion of the lock plate  510 . To provide the groove, the seal plate may be spaced from the disc, or have a portion spaced from the disc. In the example shown, the seal plate  584  includes a rebate around the inwardly facing surface of the radially outer portion so as to provide a circumferentially extending groove. The disc  534  may also include some form of rebate in which to accommodate the lock plate  510 . 
     The groove is generally annular and encircles the rotor around the periphery of the seal plate  584 . The groove is substantially axially uniform in radial and circumferential directions and lies in a plane which is generally normal to the axis of rotation. However, it will be appreciated that there may be discontinuities in the groove to allow for anti-rotation or locating features for the lock plates  510 . 
     The foot portion  556  of the lock plate  510  includes a seal fin  588  which extends from the radially innermost edge of the lock plate body. In the example shown, the seal fin  588  extends from the radially inner side of the rib  578 . The seal fin  588  defines the circumferential extent of the lock plate  510  at the foot portion and provides the radially inner edge thereof. 
     The seal fin  588  lies within the circumferential seal groove provided by the disc surface and seal plate  584 . As shown in the example, the seal fin  588  may lie in a plane normal to the rotational axis of the rotor  534  and may be in the same plane or parallel to the lock plate body. The seal fin  588  is spaced from the disc surface and proximate to, if not abutting, the inner surface of the seal groove provided by the seal plate  584 . The seal fin  588  will abut the inner surface of the seal groove under CF loading when the lock plate rocks forward at the radially inner portion. 
     The axial thickness of the seal fin  588  may be less than the lock plate body and less than the seal groove in which it resides to provide the clearance with the disc surface. The axial location of the seal fin may be local to the geometric centre plane of the lock plate body. The radially inner edge of the seal fin  588  provides the chordal edge to the lock plate  584  which is described above in relation to  FIG. 4 . 
     The seal fin  588  may be provided by removing material from either or both of the outwardly facing surface and inwardly facing surface of the lock plate  510 , for example, by grinding or otherwise machining. In the example, the inwardly facing surface of the lock plate  510  is provided with a cut-back or rebated portion from the radially inner edge to provide a step in the inwardly facing surface. The step provides a contact pad for the lock plate to contact the disc and provide axial location thereof and a clearance space to separate the seal fin from the disc surface. 
     In situ, the seal fin  588  is located between the seal plate  584  and the disc  534  surface so as to radially overlap one another in all normally expected operating conditions. Under the CF loading, the seal fin pivots forward and until the seal fin  588  abuts the opposing surface of the seal plate  584  and provides a flow path restriction or seal therebetween. The presence of the chordal edge to the seal fin aids the sealing function. The sealing contact force is increased when the lock plate is subject to CF loading in use. 
     The radial position of the lock plate at rest is provided by an abutment of the seal fin and the terminal end of the seal groove in the seal plate  584 . 
     The rib  578  and the outer peripheral edges of the cover plate  586  and seal plate  584  are radially separated when the engine is at rest (at cold). The separation of the rib  578  and the plates is sufficient to allow for build tolerances and any expected in-service movements to prevent binding or unnecessary stress being induced on the lock plates or blade platforms. The radial separation of the periphery of the rotor outer wall and the projection may be in the range of between 2 mm and 4 mm. The nominal cold clearance may be approximately 2.8 mm. 
     Placing the rib  578  as close to the seal and cover plates as possible allows the separation of the projection and the fulcrum at the head portion to be maximised within the available radial constraint. This increases the effectiveness of the mass in moving the centre of gravity and controlling the movement of the lock plate. Hence, the closer the rib to the plates, or the further from the pivot pads, the smaller the projection can be thereby saving weight. 
     It is also to be noted that the radius of curvature of the outer periphery edge of the cover plate and the inner radius of the projection are concentrically aligned so that the separating gap is uniform along the circumferential length of the rib. 
     The exact size and spacing of the projection relative to the cover plate is determined by the architecture of the engine and the requirements of the CF loading as will be appreciated by the person skilled in the art. Small changes to the size and corresponding mass of the projection can have a significant effect on the axial force exerted on the foot portion. As will be appreciated, the lateral force on the foot portion, and lock plate generally, will determine the dimensions and ultimately the weight of the lock plate. Hence, careful optimisation of the position and size of the rib is preferred. 
     The axial extent of the rib  578 , relative to the principle axis of the engine, is greater than the axial length of the cover plate  586 . That is, the rib extends further upstream than the cover plate  586  and thereby provides an overhanging shield which helps protect the seal plate  584  and disc  534  from radiative heat emitted by the main gas wall. 
     A further advantage of providing a minimal separation between the rib and the outer rim of the cover plate is that it helps provide a tortuous path which further helps restrict any leakage flow path through which cooling air could escape. 
       FIG. 7  shows a detailed section of the head portion  554  of the lock plate  510 . The head portion  554  of the lock plate is located in a groove provided in the radially inner surface of the turbine blade platform  540 . The groove includes a radially inner open end, a radially outer terminal end and axially separated inner and outer walls. In the described example, the groove is formed between a flange  541  which extends radially inwards from the inner surface of the platform and provides outer wall, and a surface of the turbine blade  542  which provides the inner wall. The turbine blade surface may be provided by the shank of the blade or the root. It will be appreciated that the groove may be provided within the end of the flange as is shown in the axially downstream lock plate arrangement of  FIG. 3 . 
     The groove provides a circumferentially extending channel into which the head portion  554  of the lock plate  510  can be fitted and axially restrained in use. The groove is open ended from the radially inner direction and is generally annular being made up from the annular arrangement of similar turbine blades each having corresponding grooves provided in the underside of the platforms. The grooves have a radial component which is normal to the rotational axis of the rotor. 
     The internal surface  543  of the terminal end of the turbine platform groove is shaped to provide a pivot pad  545  which operationally engages with the corresponding pivot pad  582  of the head portion of the lock plate  510 . The blade pivot pad  545  is provided by a step in the radially outer terminal end of the groove. The nose of the step extends circumferentially around the groove and is provided at an approximate axial mid-point thereof. The step is a change in radial extent at the terminal end of the groove and provides a pivot pad which is radially facing, and a riser which is axially facing. In the described example, the riser faces axially towards the disc surface to provide the pivot pad on the axially outer extent of the groove. 
     The pivot pad  545  of the turbine platform  540  extends across approximately half the axial length of the slot at which point there is provided a rebate in a radially outwards direction to provide the step. The pivot pad  545  extends from the inwardly extending flange of the platform axially rearwards towards the turbine shank. Providing a pivot pad  545  which is distinctly formed by a step allows the contact area of the pivot to be more accurately position and predicted in service. It will be appreciated, that the exact dimensions of the pivot pad and its axial extent relative to the slot, will be dependent on the head portion and requirements of the pivot. 
     The head portion  554  is provided by a peripheral edge of the lock plate body. The inside surface of the lock plate includes a scallop or indent  561  (as shown  FIG. 8 ) which provides a circumferential wall  563  on the radially outer edge of the head portion. The wall is provided proximate to the turbine blade groove wall and restricts the amount of bladewards movement of the lock plate. 
     The head portion  554  has at least one pivot pad  582  which corresponds to the equivalent feature on the turbine platforms slot, such that, in unison, the two pads provide a pivot about which the lock plate  510  can turn. 
     The pivot pad  582  of the head portion  554  is provided at the radially extreme edge of the lock plate  510  and may be provided in a perpendicular orientation to the general plane of the lock plate. This outer peripheral edge of the head portion  554  has a pair of chamfers  583   a, b  on the either side of the pivot pad  582  and which define the axial extent of the pivot pad  582 . Thus, the chamfers  583   a,b  are located on the respective outer and inner surfaces of the lock plate body and provide a circumferentially extending surface which extends from the outer or inner surface and terminate at the respective end of the pivot pad  582 . The axial component of the chamfers and resultant location of the pivot pad will be dependent on the turning moment which is required. In the example, the pivot pad  582  is located at an approximate geometric centre of the lock plate body. 
     The outer surface chamfer  583   a  of the head portion  554  lies at an angle to the outward surface of the lock plate in the range of approximately between seventy and eighty five degrees. The purpose of the chamfer  583   a  is to limit the size of pivot pad and provide a shoulder or line contact around which the lock plate can rotate against the pivot pad  543  of the turbine platform groove, and also to provide a clearance space for the lock plate to rotate in to. 
     The inner chamfer  583   b  of the head portion  554  has a steeper incline than the outer chamfer  583   a , relative to the pivot pad. The angle of the chamfer may be in the range of between approximately twenty degrees to forty five degrees. This increased chamfer provides some clearance to allow the lock plate head portion to be inserted more easily in to the turbine platform slot. 
     When assembled the head portion is loosely located in the blade groove. The loose fitting results from the provision of axial clearance in the respective dimensions of the head portion and the blade groove. The clearance allows for ease of assembly and some differential thermal expansion during use. The head portion may have a clearance of between 0.6 mm and 0.2 mm in the axial direction. In some embodiments this will be controlled to approximately +/−0.2 mm in the axial direction. 
     When assembled, the pivot pads of the lock plate and the blade groove axially overlap. The extent of the overlap is determined by the relative sizes of the components but will regardless provide some variability in the position of the pivot pad. This overlap may provide a pivot point variability of between 1 mm and 0.5 mm. 
     The arrangement of the upstream chamfer  583   a  and pivot pad  582  are such that the location of the pivot can be accurately predicted in service when there is substantial movement of the parts relative to one another. This enables a more accurate modelling of the arrangement and more tailored design of the constituent parts. In the particular arrangement shown in  FIG. 7 , the pivot pad is approximately half of the thickness of the lock plate body axial thickness. 
       FIG. 8  shows a rearward projection of the inner facing surface  562  of the lock plate  510  which is located adjacent the disc and turbine blade roots in use. The rearward surface  562  includes a plurality of indents or scallops which are defined by surrounding peripheral walls  563  along the radially inner and outer edges and the circumferential edges of the lock plate body. A mid-span radial wall  565  is provided between the radially inner and outer walls and located at circumferential mid-point. The walls increase the rigidity of the lock plate (or the indents remove weight from the part). The surfaces of the walls also provide abutment portions for the head and foot portions as described above. 
     The radially inner edge of the lock plate  510  as shown in  FIG. 8  includes a chordal edge  567  in the form of a straight edge which extends between the circumferential edges of the lock plate. The chordal edge  567  increases the line contact between the seal plate and the seal fin under various operating conditions where relative movement may be experienced. 
     It will be appreciated that the chordal edge may not be perfectly straight and some minor variation may still perform a similar although perhaps not as optimum function. 
       FIG. 9  shows the edge portions of a lock plate  910  and the corresponding radially extending circumferential edges  948 ,  950  from axially forward and rearward facing directions. The two end surfaces shown are from the ends of a common or circumferentially adjacent lock plates. 
     Each of the lock plate edges  948 ,  950  include a rebate along its radial length to provide a flanged seat into which a corresponding edge from the circumferentially adjacent lock plate can be received. Hence, each lock plate has a rebate in the outwardly facing surface of one lock plate on one circumferential edge, and a rebate in the inwardly facing surface of the other circumferential edge. 
     Each of the radially inner circumferential edge corners of the lock plates include a corner notch  969 . The notch  969  in the example is an approximate quarter round and provides an anti-rotation feature which mates with a corresponding feature on the seal plate or rotor disc. It will be appreciated that the anti-rotation function could be achieved with smaller notches than are presented in this example. However, providing notches with a greater radial extent allows for a larger seal fin and increased sealing functionality. 
     It will be understood that the invention is not limited to the described examples and embodiments and various modifications and improvements can be made without departing from the concepts described herein and the scope of the claims. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more described features.