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The invention relates to a damped assembly.
In particular it concerns a nozzle guide vane assembly for a compressor or fan stage of a gas turbine engine in which the guide vanes are held in position in an annular ring by means of an intermediate damping medium.
In known nozzle guide vane assemblies individual vanes are held in place between concentric rings by means of inserts of a resilient material such as silicone rubber material. The inserts of resilient material contribute some damping to the assembly as a result of its inherent energy absorbing properties. However, such an arrangement suffers the drawback that the due to the nature of the resilient material the overall assembly can have poor stiffness. This can lead to movement of the vanes relative to their supporting structure allowing vibration and resonant frequencies within the engine running range. This is generally undesirable and in the extreme can lead to structural failure. The invention is intended to overcome this drawback.
According to the broadest aspect of the invention there is provided a damped assembly comprising at least one member carried in a supporting structure by at least one end of the member located in a socket formed in the supporting structure with an intermediate collar of resilient material interposed therebetween.
The invention and how it may be carried into practice will now described by way of example with reference to the accompanying drawing in which:
FIG. 1 shows a nozzle guide vane assembly for a gas turbine engine in which the vanes are located using a resilient collar;
FIG. 2 shows a detailed view of the resilient collar of FIG. 1; and
FIG. 3 shows a section through the collar seated in position over a vane in the assembly of FIG. 1.
Referring firstly to FIG. 1 of the drawings there is shown a segment 2 of an annular nozzle guide vane assembly for a gas turbine engine including at 4 two nozzle guide vanes. The vanes 4 have a hollow interior cavity 6 and are mounted in a supporting structure comprising an annular, radially outer casing, a portion of which is shown at 8, and a concentric inner ring, a portion of which is shown at 10. The overall assembly includes a multiplicity of the vanes 4 spaced apart equidistantly around the rings 8 and 10.
At each of the vane locations an aperture 12 is formed in the outer ring 8 opposite a corresponding aperture 14 in the inner ring 10, both apertures conforming to the cross section of the vanes 4 plus a small gap to receive a collar 16. Into each said aperture there is fitted a collar or boot 16 made of resilient material to the shape of the vane cross-section to form a socket into which one end of the vane 4 is received. Thus, there is a collar or boot 16 of resilient material interposed between adjoining metal parts 8 and 4, or 10 and 4. In engine operation, the rubber collars 16 act to damp relative movement of the metal parts. Each of the collars 16 is formed in an aerofoil shape so that there is an aperture 17 through its middle through which access to the interior 6 of the aerofoil 4 is provided, for example for the passage of cooling air.
Experience in the gas turbine environment has shown that due to the nature of the resilient material the overall system has relatively poor stiffness. This can result in increased axial deflection of the inner ring 10 if the whole assembly is supported by means of cantilevered mounting of the outer casing annulus 8. The extent to which the modal vibration frequencies of the aerofoils can be tuned, to avoid resonances in the engine running range, is limited by the resilient material of which the collars or boots 16 are made.
According to the present invention this drawback is solved by the arrangement illustrated in FIG. 2 in which the collar or boot 16 is modified by the addition of stiffening means. In one embodiment of the invention this stiffening means is in the form of a thin metal plate 18 attached to an end face of the collar. In this example the collar 16 was stiffened by the addition of a metal plate 18 formed of 0.5 mm thick stainless steel bonded to an end surface 20 of the collar 16. The inner and outer peripheries of the plate 18 were formed in the outline shape of an aerofoil cross-section. The dimensions of the aperture 22, defined by the inner periphery of the plate 18 were slightly larger than the corresponding external dimensions of the aerofoil vane 4 and of the end face 20 of the collar 16. Also the external dimensions of the plate 18 were slightly smaller than the corresponding dimensions of the collar face 20. The plate 18 was then bonded to the end surface 20 of the collar 16 in a position to leave a small clearance gap all round the aerofoil 4 after assembly.
The plate 18 was bonded to the collar 16 during a vulcanisation process to cure the silicone rubber material from which it was moulded. The plate 18 was coated with a suitable primer and placed in the mould (not shown) on the uncured silicone rubber. Upon completion of the curing process the stiffening plate 18 and the collar 16 were bonded together well enough to survive intact the mechanical stresses of assembly and use in which the assembly is subject to thermal cycles and simultaneous mechanical stresses.
The stiffness of this collar assembly 16, 18 is influenced by several factors, including thickness of the plate 18, the plate material and the width of overlap with the end face 20 of the rubber collar. These variables may be selected to produce a desired stiffness in the final assembly. The in-plane and bending stiffness of the assembly will be increased by the high in-plane stiffness of the plate 18. Therefore the stiffness of the assembly can be determined by selection of the plate material ie its modulus, thickness and width. The transverse stiffness of the collar assembly is also influenced by all the above factors but is primarily determined by the width of plate overlap, or rather by the clearance between the plate 18 and the vane 4. Lack of clearance acts to constrain local shear deformation of the rubber collar material adjacent to the vane surface ie reducing the width of overlap reduces the transverse stiffness of the collar 16.
In the case of the illustrated example the plate 18 was bonded to one end surface 20 of the collar 16. In another example (not shown) the stiffening means, ie the plate 18 and the collar 16 were formed as a unitary member. The plate 18, was primed on both sides, and was placed in the mould when only partially filled with uncured silicone rubber, so that when filling was complete the plate 18 was fully embedded in the collar 16 with rubber on both sides. The vulcanisation procedure was then carried out as normal.
In further embodiments the inherent properties of the rubber material from which the collar was moulded were modified by inclusions within the body of the silicone rubber. Examples of stiffening materials used are chopped fibres of carbon, glass, and Kevlar (p-phenylene terphthalamide) (Kevlar is a registered trade mark) or glass micro-spheres, ie minute (sub-millimetre) spheres of glass. Such inclusions modify the way and degree to which the rubber deforms when subject to external mechanical stress. Such modified material may be used in addition to a stiffening plate as described above or as an alternative thereto. The thickness and length of the fibres used is dependent upon the design of the rubber boot, the inherent properties of the basic rubber material and the degree of modification of the resilient properties desired.