Abstract:
A variable guide vane assembly comprising a variable guide vane having an aerofoil and a spindle at one end of the aerofoil; wherein the vane further comprises a platform positioned between the aerofoil and the spindle, the platform having a surface facing in the direction of the spindle and at least one side face; the assembly also having a washer with an aperture through which the spindle extends, the washer having a portion which overhangs the at least one side face.

Description:
TECHNICAL FIELD OF INVENTION 
       [0001]    The invention relates to turbine engines and variable guide vanes and/or inlet guide vanes for turbine engines in particular. 
       BACKGROUND OF INVENTION 
       [0002]    In a gas turbine engine having a multi-stage axial compressor the rotor is turned at high speed so that air introduced into the compressor is accelerated by the rotating blades and swept rearwards onto an adjacent row of stator vanes. Each rotor stage increases the pressure of the air passing through the stage and at final stage of the compressor the air is many times that of the inlet air pressure. 
         [0003]    In addition to converting the kinetic energy of the air into pressure the stator vanes also serve to correct the deflection given to the air by the rotor blades and to present the air at the correct angle to the next stage of the rotor blades. 
         [0004]    As compressor pressure ratios have increased it has become more difficult to ensure that the compressor will operate efficiently over the operational speed of the engine. This is because the inlet to exit area ratios of the stator vanes required for high pressure operation can result in aerodynamic inefficiently and flow separation at low operational speeds and pressures. 
         [0005]    In applications where high pressure ratios are required from a single compressor spool the above problem can be overcome by using variable stator vanes which permit the angle of one or more rows of stator vanes in a compressor to be adjusted while the engine is running in accordance with the rotational and mass flows of the engine. 
         [0006]    Existing designs for variable inlet guide vanes typically incorporate an integral outer platform which provides for an interface between the aerofoil and also that of the outer spindle. Often known as a “penny”, these platforms are round in geometry and positioned within a counterbore incorporated into a suitable casing to provide for a continuous aerodynamic profile at the casing/penny interface as the vane rotates. 
         [0007]    A non metallic thrust washer may be sandwiched between the penny and the inner base surface of the counterbore to prevent metal to metal contact. 
         [0008]    The counterbore must be sufficiently robust to accommodate the penny and platform and not distort in use. With large vanes the size and depth of the counterbore significantly increases the machining time and cost of the casing. Alternative manufacturing methods, such as forging, to form the casing can exhibit high levels of distortion in manufacture. 
         [0009]    It is an object of the present invention to seek to address these and other problems. 
       STATEMENTS OF INVENTION 
       [0010]    According to a first aspect of the invention there is provided a variable guide vane assembly comprising a variable guide vane having an aerofoil and a spindle at one end of the aerofoil; wherein the vane further comprises a platform having a non-circular plan positioned between the aerofoil and the spindle, the platform having a surface facing in the direction of the spindle and at least one side face; the assembly also having a washer having a circular plan with an aperture through which the spindle extends, the washer having a portion which overhangs the at least one side face. Preferably the washer is supported by the platform surface and portion overhangs one or more of the side faces of the platform. Preferably the plan of the washer is circular. The plan of the platform may be non-circular and may be rectangular, fluted or lobed. 
         [0011]    Advantageously, the washer may be secured to the platform by its positioning between the platform and a further component or by an interference fit between the platform and the washer. Adhesive, or another coating, may be used to further secure the platform with the washer. 
         [0012]    Preferably the washer is formed from plastic. An exemplary plastic is polyimide. 
         [0013]    The washer may be formed from metal having a surface coating. 
         [0014]    The variable guide vane assembly may further comprise a casing having a chamber containing the platform and the washer. Preferably the chamber is stepped from a casing surface wherein a surface of the platform opposing the spindle facing surface is flush with the casing surface. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a schematic of a VIGV and a rotor; 
           [0016]      FIG. 2  is a cross-section of a VIGV mounting arrangement  10 ; 
           [0017]      FIG. 3  is a partially exploded view of a vane platform and thrust washer; 
           [0018]      FIG. 4  is a top view of a vane platform and thrust washer; 
           [0019]      FIGS. 5   a  to  5   d  are plan views ( 5   a - 5   b ) and side views ( 5   c - 5   d ) of alternative washer arrangements. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0020]      FIG. 1  shows a schematic of a variable inlet guide vane (VIGV) or variable stator vane (VSV)  2  in relation to an associated rotor  4 . The VIGV is upstream of the rotor and is rotatable about the axis  6  to change the angle of flow onto the rotor in accordance with the desired operating conditions to improve performance and stability particularly where high pressure ratios are required. 
         [0021]    In the arrangement shown in  FIG. 1 , each individual vane in a VIGV row is supported by bearings  8 ,  10  at the radially inner and outer ends of the vane aerofoil section. The bearings permit the VIGV to rotate or pivot about its spanwise axis  12  which is typically radial, or nearly radial, relative to the compressor or engine axis  14 . The angular position of the vane is controlled by an actuating lever  16 , which is attached to a spigot type extension at the radially outer end of the vane. The actuating lever of each vane in a row is connected to an actuating ring  18 , commonly known as a unison ring which rotates about the axis of the engine  14 . 
         [0022]      FIG. 2  depicts the radially outer bearing arrangement  10  in more detail. The vane spindle  20  is integrally formed with the vane aerofoil  22  and has a platform  24  which has at least one side face which engages a thrust washer  26 . The thrust washer has a circular outer profile, an inner profile that compliments the profile of the platform, and is positioned within an outwardly stepped chamber  28  in the casing  30  carrying the vane. The radially inner surface of the thrust washer provides, with the inner surface of the casing and the radially inner surface of the platform, a substantially flush continuous aerodynamic profile. The radially outer surface of the thrust washer engages the shoulder  32  of the stepped chamber  28  and provides a bearing surface which prevents direct engagement of the aerofoil platform with the casing. 
         [0023]    The actuating lever  16  has a mount which surrounds the vane spindle  20  and has an inner profile which engages the driving teeth  40  on the spindle ( FIG. 3 ). A cylindrical bush  34  separates the outer surface of the actuating lever from the casing whilst permitting the mount to rotate within the casing about the axis  12 . A lever retention feature e.g. a nut, screw or other mechanical fastener secures the actuating lever to the spindle. 
         [0024]    The thrust washer  26  is shown in more detail in  FIG. 3 . The platform  24  in this embodiment has a rectangular shape with an upper surface supporting the underside of the thrust washer and side walls arranged such that the longer sides of the platform engage with the inside surfaces of the washer. Preferably the washer is formed of a suitably formed plastic such as polyimide or a metal provided with a suitable coating which gives an acceptable interface with the platform and/or the casing which offers a combination of low friction and low wear at all contact points with the platform or casing. The upper surface of the thrust washer may be provided with one or more sections  42  which provide relief for the local overhang of the actuating lever. The washer is carried by the upper surface of the platform  24  which faces the spindle. By facing it is meant that it is the surface from which the spindle protrudes. The surface may be sloped towards the spindle, rather than perpendicular as shown in  FIG. 3 , but it will be appreciated that this increases the depth of the platform in the axial direction and may be undesirable. 
         [0025]    A top view of the vane  22 , washer 26  and platform  24  is shown in  FIG. 4  and depicts the profiles of the vane platform and the interior and exterior surface of the thrust washer. 
         [0026]    It will be appreciated that the invention offers a number of advantages. The overhang of the thrust washer, when presented to the platform of the vane, forms a full circular aerodynamic surface at the vane interface with the casing. For large stator vanes in particular the arrangement provides a significant reduction to the time and cost required to machine the vane by virtue of a more efficient utilisation of the aerofoil initial material envelope as machining a platform with a circular cross-section requires significantly more material. By contrast the thrust washer can be made of both cheaper and lighter material which also leads to an overall reduction in the weight of the engine. 
         [0027]    The depth of the washer  26  has a significantly lower profile  48  than the conventional combinations of platform and washer. Accordingly, the manufacturing costs of the casing is reduced. 
         [0028]    The use of a thrust washer can also reduce the profile of the vane which allows the vane, if forged, to be manufactured from smaller stock which results in reduced manufacturing cost of the aerofoil as less material is used with reduced machining time. 
         [0029]      FIGS. 5   a  and  5   b  are top views of alternative washer arrangements. In  5   a  the washer has a star-shaped feature which engages a complementary star shaped platform. In  5   b  the washer has a cross-shaped feature which engages a complementary cross shaped platform.  FIG. 5   c  is a side view of an embodiment of washer which does not have a portion that engages a spindle facing surface of the platform. The washer relies upon an interference fit and/or an adhesive to secure it in position against the platform. A coating can be provided on the top surface of the washer and platform to prevent damage to the casing by the platform when the vane is assembled thereto. In the embodiment of  5   d  the washer has a shoulder  52  which prevents the washer from becoming detached from the platform and entering the air flow through the engine. The arrangement of  FIG. 5   d  has the advantage of being more robust but is thicker than the arrangement of  FIG. 5   c.    
         [0030]    Although aspects of the invention have been described with reference to the embodiments shown in the accompanying drawings it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications may be effected without further inventive skill and effort. For example, the platform  24  is shown as rectangular but may be any other appropriate shape provided the thrush washer may fit around it.