Patent Publication Number: US-8109719-B2

Title: Annular component

Description:
The invention relates to an annular component. 
     In particular it relates to an annular component having a central bore and a sleeve carried on the central bore. 
     Further the invention relates to a stator vane assembly for a compressor, a method of assembly of a stator vane array for a compressor and a method of manufacture of a stator vane array for a compressor. 
     For convenience, the expression “compressor” is used in this specification to embrace fans, which discharge gas (usually air) directly into the surroundings to provide a propulsive force, or discharged into a pipe/duct so as to be pumped along the pipe/duct, and compressors which compress a working fluid (again, usually air) which is subsequently mixed with fuel and ignited either to provide a propulsive jet flow or to drive a turbine, or a combination of the two. 
     Stator vane assemblies for compressors are typically made up of an annular stator vane structure having an annular outer casing joined to an annular inner casing by a plurality of stator vanes to define an annular fluid flow passage. The stator vane structure is supported on the body of the compressor by the attachment of the outer annular casing to an adjacent casing and by a support structure bounded by the inner annular casing. It is known to make such structures entirely from metal. However, while robust, metal structures are heavy. In order to lessen the weight, it is known to manufacture the stator vanes from composite materials, such as that described in U.S. Pat. No. 5,605,440 (Bocoviz et al; Eurocopter). 
     Composite materials (or “composites”) are engineered materials made from two or more constituent materials. The materials generally have significantly different physical or chemical properties and although they bond together to form a finished structure, remain separate and distinct. For example, a composite structure may be made up of reinforcement fibres held together by a matrix, where the matrix is a resin. 
     In one embodiment described in U.S. Pat. No. 5,605,440 the stator vanes surround and are supported by a central support casing made of metal, which is also an inner annular casing that defines the flow path through the fan. The vanes are individually attached to the inner casing. Any expansion and contraction of the inner casing/support structure will be communicated directly to the stator vane structure. Although this may be mitigated to some degree by slotted joints between the vanes and support structure, this requires the vanes to be individually joined to the support casing to build up the array. 
     It is desirable to make composite structures, such as stator vane structures, as one piece and then fit the structure as one unit onto and around the support structure. However, the thermal coefficient of expansion of metal may be significantly greater than that of a non metallic composite structure. Hence a metallic support structure will expand radially outwards at a greater rate than the composite which bounds it. This may put significant stress on the composite structure, causing damage and reducing the operational life of the structure. 
     An object of the present invention is to provide a lightweight composite annular component which can be mounted on and around a support structure, where the thermal expansion of the support structure is reduced to maintain operational stress on the annular component below a predetermined value. 
     According to a first aspect of the present invention there is provided a stator vane assembly for a compressor comprising a support structure which carries and is bounded by an annular stator vane structure comprising a central bore and a sleeve carried on the central bore, wherein the sleeve is disposed between the bearing support structure and bore of the stator vane structure, characterised in that the annular stator vane structure is made from a non-metallic composite material and the sleeve is made from a first material, the coefficient of thermal expansion of the non metallic material being equal to or less than the co-efficient of thermal expansion of the first material. 
     Preferably the first material has a coefficient of thermal expansion which is no greater than five times the co-efficient of thermal expansion of the non metallic composite material. 
     Preferably the sleeve is made from a first material which has a coefficient of thermal expansion which is no greater than twice the co-efficient of thermal expansion of the non metallic composite material. 
     The material of the sleeve is chosen so that the maximum amount it will thermally expand over the expected operational temperature range of the annular component, and thus the amount of force exerted by the sleeve due to thermal expansion of the sleeve, will be below a predetermined value. Additionally the material of the sleeve is chosen so that the sleeve is capable of constraining a predetermined maximum hoop stress. 
     The metallic sleeve on the bore of the annular stator vane structure is configured to limit the thermal expansion of the support structure. The material of the sleeve is chosen such that it can limit thermal expansion forces communicated from the support structure to the annular stator vane structure to below a predetermined level. That is to say, the sleeve limits the maximum hoop stress induced by the support structure on the stator vane structure during an expected operational temperature range. 
     According to a second aspect of the present invention there is provided a method of assembly of a stator vane array for a compressor, characterised in that the array comprises an annular stator vane structure with a central bore made of a non metallic composite material and a sleeve made of a metallic material, the coefficient of thermal expansion of the annular stator vane structure being equal to or less than the coefficient of thermal expansion of the sleeve, the method comprising the steps of inserting the sleeve into the bore, and joining the sleeve to the bore. 
     The sleeve is thus fitted after the annular component (that is to say, the stator vane structure) has been formed. The relative diameters of the sleeve and bore are chosen such that the sleeve can be fitted in place without causing damage to the bore of the composite material. 
     According to a third aspect of the present invention there is provided a method of manufacture of a stator vane array for a compressor, characterised in that the array comprises an annular stator vane structure with a central bore made of a non metallic composite material and a sleeve made of a metallic material, the coefficient of thermal expansion of the annular stator vane structure being equal to or less than the coefficient of thermal expansion of the sleeve, the method comprising the steps of: forming a precursor of the stator vane structure from reinforcement fibres; positioning the sleeve in the bore of the precursor; introducing resin to the fibres and sleeve; and curing the resin such that the sleeve and fibres are bonded to each other. 
     Thus the sleeve can be bonded into place with the resin which bonds the fibres. Thus the sleeve can be fixed in place without causing damage to the composite material of the annular component. 
     Hereinbefore and hereafter a “stator vane structure” is taken to mean the part of the stator vane array formed from a composite material; “stator vane array” is taken to mean the stator vane structure with the protective sleeve fitted; and “stator vane assembly” is taken to mean the stator vane array and support structure assembly. 
    
    
     
       The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a cross-sectional view of a section of a compressor with a stator vane array consisting of a stator vane structure and sleeve, where the stator vane array is mounted on a support structure; 
         FIG. 2  shows a enlarged view and second embodiment of the interface between the stator vane array and support structure as shown in  FIG. 1 ; and 
         FIG. 3  shows the same view as shown in  FIG. 2 , in which a third embodiment of the present invention is presented. 
     
    
    
     A section of a compressor  10  is presented in  FIG. 1 . A stator vane array  12 , consisting of a stator vane structure  11  and sleeve  52 , is mounted on and bounds a bearing support structure  14 , which in turn is disposed around a shaft  16 . Bearings  18 , 20  fitted between the shaft  16  and bearing support structure  14  establish a load path between the shaft  16  and the vane array  12 . Rotatable blades (not shown) attached to the shaft  16  are provided downstream of the stator vane array  12 . An annular inner casing  22  and annular outer casing  24  upstream of the stator vane array  12 , and an annular inner casing  26  and annular outer casing  28  downstream of the stator vane array  12 , define an annular flow path  30 . The stator vane array  12  has annular inner and outer casing walls  32 , 34  which are joined to the inner  22 , 26  and outer  24 , 28  casing walls respectively. In the embodiment shown the outer casing walls  24 , 34 , 28  are provided with flanges  36 , 38 , 40 , 42  for forming a joint between the casings. A static vane  44  extends between the inner casing wall  32  and outer casing wall  34 . 
     A rim  46  towards the downstream end of the casing wall  32  extends radially inwards from the stator vane structure inner wall  32 . The distal end  48  of the rim  46  defines a central bore  50  of the stator vane array  12 . A sleeve  52  is provided on the radially inner surface  54  of the central bore  50 . The stator vane array  12  is thus annular in shape, and defines part of the annular flow path  30 , as well as the annular central bore  50 . As stated above, the vane array  12  is mounted on and bounds the bearing support structure  14 . The bearing support structure  14  located in the central bore  50 , with the sleeve  52  disposed between the support structure  14  and the rim  46 . The sleeve  52  comprises a flat portion  53  which is parallel to the annular bore  50  of the rim  46 . An interference fit is formed between the material of the support structure  14  and the sleeve  52 . A flange  56  extends radially outwardly from the support structure  14  and is located in a recess  58  on the downstream side  60  of the rim  46 . 
     A support arm  62  extends upstream and radially outwards from one side of the support structure  14  towards the upstream end of the radially inner surface of the stator vane inner wall  32 . A seal  64  is disposed between the arm  62  and the inner wall  32 . 
     The walls  32 , 34 , vane  44  and rim  46  of the stator vane structure  11  are formed as one from a non metallic composite material to form continuous ring. The sleeve  52  is made from a first material. The first material may be metallic or a fibre reinforced non metallic material. The support structure  14  is made from a second material, which may be metallic. The stator vane structure  11  has a coefficient of thermal expansion which is less than the co-efficient of thermal expansion of the first material of the sleeve  52 . The thermal co-efficient of expansion of the first material of the sleeve  52  is less than that of the second material of the support structure  14 . Specifically, the sleeve  52  is made from a first material which has a coefficient of thermal expansion which is no greater than ten times the co-efficient of thermal expansion of the non metallic composite material of the stator vane structure  11 , thereby limiting stress due to relative thermal expansion of the sleeve  52  and vane structure  11  during operational use of the component to an acceptable value. 
     The thermal co-efficient of expansion of the first material of the sleeve  52  is no greater than half of that of the second material of the support structure  14 , thereby limiting the radial expansion of the support structure  14  during operational use of the component to an acceptable value. 
     In one embodiment the non metallic composite material is made form an organic matrix composite material where carbon fibres are held in a Bismaleimide (BMI) resin, the first material is a nickel-iron alloy, for example Incoloy 904, and the second material is a titanium alloy. Alternatively Aramid (or “Kevlar®”) fibres can be used instead of carbon fibres. This combination of materials provides for an assembly in which the coefficient of thermal expansion of the sleeve  52  is no greater than 5 times the co-efficient of thermal expansion of the non metallic composite material, and in which the coefficient of thermal expansion of the sleeve  52  is no greater than half that of the support structure  14 . 
     Alternative embodiments of the interface between the rim  46  and the support structure  14  is shown in  FIG. 2  and  FIG. 3 . In  FIG. 2  a bolt  70  ties the flange  56  and rim  46  together. A wedge shaped washer  72  is provided between the bolt  70  and the rim  46  to evenly distribute the clamping force of the bolt  70  on the face of the composite material of the rim  46 . The bolt locates the rim  46  axially on the support structure  14 . 
     The embodiment shown in  FIG. 3  differs only in that instead of the flat sleeve  52  of the previous embodiment, a sleeve  80  is provided which has a substantially “L” shaped cross-section. That is to say, the sleeve  80  has a flat portion  82  which is parallel to the annular bore  50  of the rim  46 , and a second portion  84  which extends substantially at right angles to a flat portion  84 . The second portion  84  sits between the flange  56  and the recess  58 . 
     When the compressor  10  is operating, the shaft  16  is rotated to turn the rotor blades up and downstream of the stator vane  44 . Where there is a heat conduction path to hot components, such as a turbine, the temperature of the shaft  16  and bearing support  14  will rise and consequently they will expand radially outwards. However, the composite material of the annular stator vane structure  11  has a lower coefficient of thermal expansion, and so will expand less than the support structure  14 . The material of the sleeve  52 , 80  has a coefficient of thermal expansion which is less than that of the support structure  14 . Additionally the material of the sleeve  52 , 80  is chosen so that it can constrain the expected maximum hoop stress induced by the support structure  14  during operation of the compressor. That is to say, the radially outward force/stress exerted on the composite material of the vane structure  11  is kept below a predetermined value by the sleeve  52 , 80 . 
     The material of the sleeve  52 , 80  is chosen so that the maximum thermal expansion of the sleeve  52 , 80  over the expected operational temperature range is limited to a predetermined value, thereby limiting the amount of stress communicated to the composite material of the stator vane structure  11  by the expansion of the sleeve  52 , 80 . 
     The predetermined limiting values of force/stress on the composite vane structure are dependent on the material of the composite and the desired life of the vane array  12 . However, it will be appreciated that the sleeve  52 , 80  significantly reduces the peak force/stress induced on the composite structure  11  by the support structure  14 , and therefore will significantly extend its operational life. 
     The choice of first and second materials allows the thermal expansion experienced in operation to be shared by the interface between the support structure  14  and the sleeve  52 , 80 , and between the interface between the sleeve  52 , 80  and the bore  50  of the annular structure  11 . This reduces the maximum expansion that has to be accommodated by either interface. Hence the interference level between the composite bore  50  and the metallic sleeve  52 , 80  can be minimised whilst maintaining an acceptable interference fit over the operational temperature range of the compressor  10 . 
     The stator vane assembly  12  may be manufactured by forming the walls  32 , 34 , vane  44  and rim  46  of the stator vane structure  12  as one and then inserting the sleeve  52 , 80  into the bore  50 , and joining the sleeve  52 , 80  to the bore  50 . An interference fit is provided between the sleeve  52 , 80  and the annulus defined by the bore  50 . It may be required to shrink fit the sleeve  52 , 80  into the bore  50  so as to avoid damage to the surface  54  of the bore  50  during the insertion process. That is to say, the sleeve  52 , 80  can be cooled such that it contracts radially. After insertion, the sleeve  52 , 80  expands and forms an interference fit with the composite material. Hence an interference fit can be achieved without having to force the sleeve  52 , 80  over the radially inner surface  54  of the bore  50 . Forcing the sleeve  52 , 80  over the surface  54  may cause delamination of the composite material, and thus reduce its strength. Additionally or alternatively the sleeve  52 , 80  is bonded into the annulus defined by the bore  50  with a suitable bonding agent. 
     The differing coefficients of thermal expansion allow the level of interference at room temperature between the composite structure  11  and the sleeve  52 , 80  to be less than it would be if the composite structure  11  were fitted directly to the support structure  14 . The lower level of interference means there is less risk of damage to the composite material during installation of the sleeve  52 , 80 . 
     Alternatively the stator vane array  12  may be manufactured by laying up reinforcement fibres to form a precursor of the walls  32 , 34 , vane  44  and rim  46  of the stator vane structure  11  and positioning the sleeve  52 , 80  in the bore  50  of the precursor. In this context “precursor” is taken to mean an array of fibres formed into the shape of the annular stator vane structure defined by the walls  32 , 34 , vane  44  and rim  46 . The matrix, or resin, is then introduced into the precursor, bonding the fibres together in the shape of the annular component structure  11  and bonding the sleeve  52 , 80  into the body of the vane structure  11  to form the stator vane array  12 . Thus the sleeve  52 , 80  can be fixed in place with the resin which bonds the fibres without risking damage to the composite material of the stator vane structure  11 . 
     With the sleeve  52 , 80  in place, the stator vane assembly  12  can be assembled with the support structure  14  with a larger interference level than could be used directly between the support structure  14  and the composite material of the rim  46 , since a close tolerance fit between the sleeve  52 , 80  and the support structure  14  will have no impact on the composite material. 
     Since the sleeve  52 , 80  is fitted to the vane structure  11  during manufacture as a permanent part of the array  12 , and prevents direct contact between composite material of vane structure  11  and support structure  14 , the joint between the stator vane array  12  and support structure  14  can be made and broken as many times as required with no risk of damage to the composite material. 
     In the embodiments shown in  FIGS. 3 , the second portion  84  of the sleeve  80  may be used as a jacking face to assist in disassembly of the stator vane assembly  12  and the support structure  14 . Jacking screws (not shown) acting directly on the face of the recess  58  would cause significant damage, and the second portion  84  acts to protect the composite from this damage.