Abstract:
A pivot ring arrangement for a stage of variable stator vanes (VSVs) ( 38 ) in a gas turbine engine comprises a plurality of segments ( 12 ) secured in a segment carrier ( 22 ). The segments are injection molded from self-lubricating material, obviating the need for separate bushes for the VSV spindles ( 36 ).

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to gas turbine engines, and more particularly to mounting arrangements for variable stator vanes in gas turbine engines. 
   It is known to provide variable stator vanes (VSVs) in gas turbine engines, to improve their performance and stability, especially where high pressure ratios are required. Such vanes are able to pivot about a substantially radial axis, so as to change the angle they present to the incoming air. VSVs that are situated before the first rotor stage of a compressor are commonly referred to as variable inlet guide vanes (VIGVs). 
   VSVs have a spindle at each end; the spindles are located in bushes fitted into a pivot ring. For ease of assembly and disassembly, the pivot ring is usually made in two semicircular sections. Known pivot rings are commonly made from aluminium, and require complex machining during manufacture, both to form the functional shape of the ring and for weight reduction. Bushes, typically of Vespel, are fitted into holes in the ring and provide bearing surfaces for the VSVs. The fitting of the bushes adds further cost and complexity to the manufacture of the pivot ring. 
   It is an objective of this invention to provide a mounting arrangement for VSVs that overcomes the disadvantages of cost and complexity associated with known pivot rings. 
   BRIEF SUMMARY OF THE INVENTION 
   According to one aspect of this invention, a pivot ring segment for a gas turbine engine is made from a self-lubricating material. 
   The material may be an advanced engineering polymer (AEP) material. 
   Preferably, the segment is injection moulded. 
   The segment may have at least one flattened portion to accommodate the movement of a variable vane. 
   According to a second aspect of the invention, a pivot ring arrangement for a gas turbine engine comprises a plurality of pivot ring segments according to any of the preceding four paragraphs secured in a segment carrier. 
   Preferably, the thermal expansion coefficient of the segment carrier is substantially the same as that of the segments. 
   The segment carrier may be made of corrosion-resistant steel or of titanium alloy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which 
       FIG. 1  is an isometric view of a segment for a pivot ring assembly according to the invention; 
       FIG. 2  is a section (on the line II-II of  FIG. 1 ) through the segment of  FIG. 1  and its adjacent components in the engine; and 
       FIG. 3  is an isometric view of the segment of  FIG. 1 , showing two variable stator vanes in place. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , a pivot ring segment  12  is injection moulded from an advanced engineering polymer (AEP) material, such as Torlon®. (AEP materials are distinguished from other engineering polymers by their high temperature capability (Torlon® is usable up to 250° C.) and they retain their mechanical properties (UTS, stiffness, etc.) close to that maximum operating temperature.) 
   Holes  14  accommodate the spindles of two variable inlet guide vanes (VIGVs) and allow them to rotate. Further features  16  provide a clearance for the aerodynamic “penny” of the VIGV. Twenty-nine of these segments  12  are arranged in an annular array around the circumference of the engine, with their end faces  18  abutting. 
     FIG. 2  is a section through a single pivot ring segment  12  and its adjacent components in the engine, on the line II-II of  FIG. 1 . The pivot ring segments  12  are located in a segment carrier  22 , by two circular bosses  34  on each segment  12 . The segment carrier  22  is made from corrosion-resistant steel and forms a continuous ring around the engine. The segment carrier  22  is secured to a seal carrier  24  by a plurality of bolts  26  spaced around its circumference, and to the front bearing housing  28  by a plurality of bolts  30 , likewise spaced around its circumference. The VIGV spindle  36  fits through the hole  14 , allowing the VSV  38  to rotate about an axis of rotation  32 . 
   Because the pivot ring segments are formed from Torlon®, there is no need for separate low-friction bushes to support the VIGVs. 
     FIG. 3  shows the pivot ring segment of  FIG. 1 , with two VIGVs  38  in place. As the VIGVs are rotated in use, the corners  42 ,  44  of the aerofoils move essentially in a linear fashion. However, because the VIGVs  38  are arranged in a circle, the radially inner face  46  of the pivot ring presents a circular profile. In conventional pivot ring assemblies, it has been necessary to cut away the corners  42 ,  44  of the aerofoils so that their essentially linear movement does not cause them to foul against the circular profile of the pivot ring inner face  46  at the extremes of the vane&#39;s movement. In the present invention, the injection moulding technique permits flattened portions  20  to be incorporated into the segments  12 , so that the linear movement of the aerofoil corners  42  can be accommodated without compromising the form of the VIGV  38 . The gap between the vane and the pivot ring is therefore smaller than in known pivot ring assemblies, which reduces losses. 
   The invention therefore provides a mounting arrangement for VIGVs which has lower cost, weight and complexity than conventional arrangements. 
   It will be appreciated that various modifications may be made to the embodiment described without departing from the scope of the invention. 
   For example, the pivot ring segments may be made from a different material, provided that its frictional properties are suitable (dynamic coefficient of friction below about 0.5). Two other materials with similar wear characteristics to Torlon® are Celazole® and Vespel®. 
   The pivot ring segments may be made by other means than injection moulding (Celazole® and Vespel®, for example, cannot be formed by injection moulding). 
   The number of VIGVs accommodated by each pivot ring segment may be different. 
   The segment carrier may be made from an alternative material, for example from titanium alloy. While corrosion-resistant steel offers the greatest cost saving over the prior art design, a titanium alloy component would offer a greater weight reduction, and this may be of greater benefit in some circumstances. The segment carrier may be made in one piece, or it may be fabricated from two or more parts, for example by welding.