Patent Publication Number: US-6209198-B1

Title: Method of assembling a variable stator vane assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to assembly methods and fixtures therefor. More particularly, this invention relates to a fixture and method for assembling a variable stator vane assembly of a gas turbine engine, by which components of the vane assembly can be selected to compensate for part variances and thereby optimize the operation and service life of the assembly. 
     BACKGROUND OF THE INVENTION 
     Conventional gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. In turbofan engines having multistage compressors, stator vanes are placed at the entrance and exit of the compressor section and between adjacent compressor stages in order to direct the air flow to each successive compressor stage. Variable stator vanes, whose pitch can be adjusted relative to the axis of the compressor, are able to enhance engine performance by altering the air flow through the compressor section in response to the changing requirements of the gas turbine engine. 
     A high pressure compressor variable stator vane assembly  10  is shown in FIGS. 1 and 2. The assembly  10  includes a stator vane  12  mounted within an opening  38  in a casing  22  of a gas turbine engine. As known in the art, in order to alter the pitch of the vane airfoil relative to the axis of the compressor, the stator vane  12  is designed to rotate within the opening  38  of the casing  22 . While various configurations are possible for variable stator vane assemblies, the vane  12  shown in FIGS. 1 and 2 has a radially extending flange  30  from which an annular-shaped portion extends axially to define a pair of seats  28  (unless otherwise noted, radial and axial directions referred to are with reference to the centerline of the vane assembly  10 , and not the radial and axial directions of the engine in which the assembly  10  will be installed). A trunnion  34  also extends axially relative to the flange  30 , and with the seats  28  projects through the opening  38  as seen in FIG.  2 . The vane  12  is secured to the casing  22  with a nut  20  that also secures a spacer  14 , sleeve  16  and lever arm  18  to the trunnion  34 . Rotation of the vane  12  within the opening  38  is caused by actuation hardware (not shown) attached to the lever arm  18 . 
     During engine operation, an overturning moment is created by the gas loads on the vane airfoil, generating reaction forces represented by the arrows “F” in FIG.  2 . As a result, rotation of the vane  12  relative to the casing  22  requires a seal assembly that minimizes wear, friction, and compressor air leakage while subjected to the reaction forces F, as well as withstands the hostile thermal and chemical environment of a gas turbine engine. In FIGS. 1 and 2, a seal assembly is shown as consisting of a bushing  24  and washer  26  between the spacer  14  and flange  30  on opposite sides of the casing  22 . The bushing  24  and washer  26  are preferably molded from composite materials, such as polyimide resin with glass and TEFLON® fibers, in order to be environmentally compatible with the engine environment, as well as provide suitable low-friction bearing surfaces that enable the vane  12  to rotate at acceptable torque levels. 
     The ability to minimize radial air leakage from the compressor through the opening  38  of the casing  22  is an important function of the bushing  24  and washer  26 . As can be appreciated from FIG. 2, the dual functions of the bushing  24  and washer  26  to form an air seal yet enable rotation of the vane  12  are determined by the clearance (radial relative to the axis of the compressor) through the bushing  24  and washer  26  between the flange  30  of the vane  12  and an outer annular surface  36  of the spacer  14 . To minimize compressor air leakage, the vane  12  and spacer  14  must be assembled to the casing  22  so that the minimum possible clearance is achieved. However, an excessively small clearance results in high forces being required to turn the vane  12 , which can overstress the actuation hardware and, in the extreme situation, could completely prevent actuation of the vane  12 , leading to compressor stall. On the other hand, an excessive clearance will not only permit excessive air leakage from the compressor, but will also permit the reaction forces on the vane  12  to cause excessive tilting of the vane assembly  10 . If this occurs, the reaction forces F are more concentrated in the bushing  24  and washer  26  and, in combination with higher leakage through the seal assembly, causes more rapid deterioration of the bushing  24  and washer  26 . 
     From FIG. 2, it can be seen that the clearance through the bushing  24  and washer  26  is determined by the axial offset dimension “D” between the annular surface  36  and a pair of shoulder  32  of the spacer  14 . When the vane  12  and spacer  14  are properly assembled, each of the shoulders  32  abuts one of the seats  28  of the vane  12  as shown in FIG.  2 . Increasing the offset dimension D reduces the clearance through the vane  12  and spacer  14  but increases the actuation torque required to rotate the vane  12 , while decreasing the offset dimensions D increases the clearance but decreases the actuation torque. 
     In the art, variable stator vane assemblies of the type shown in FIGS. 1 and 2 have been assembled to attain a torque level within an acceptable range for the actuation hardware. Because it has been assumed that a close relationship exists between the offset dimension D and the torque required to rotate the vane  12 , spacers  14  with incrementally different offset dimensions D have been purposely manufactured to allow adjustment of both the actuation torque and radial clearance by substituting spacers  14 . After assembly, if the torque required to rotate a vane is outside preestablished torque limits, the nut  20 , lever arm  18 , sleeve  16  and spacer  14  are removed and the spacer  14  replaced with another having a different offset dimension D. For example, if the actuation torque was too high, a spacer  14  with a smaller offset dimension D was installed, while a spacer  14  with a greater offset dimension D is installed if an unacceptably low torque is measured. Once reassembled, torque is again remeasured and the process repeated if the torque remains outside the established limits. 
     Notwithstanding the above, further investigations have shown that the torque required to rotate the stator  12  is surprisingly relatively independent of the spacer  14  installed, and that torque is not a reliable indication of the radial clearance between the vane  12 , spacer  14  and casing  22 . Instead, actuation torque has been found to be primarily determined by irregularities and interferences of the bushing  24  and washer  26  after they have been compressed by the load generated between the flange  30  and spacer  14  by the nut  20 . These irregularities and interferences are not predictable particularly since, while molded to tight tolerances, the composite bushing  24  and washer  26  can distort in the free state due to residual stresses, etc. 
     In view of the above, it can be seen that it would be desirable if a method were available for assembling a variable vane stator assembly to more consistently achieve minimum radial clearances without exceeding acceptable actuation torque levels. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method and fixture assembly for assisting in the matching of components of a variable stator vane assembly of a gas turbine engine. In particular, components of the vane assembly are matched so that part variances are compensated for to minimize radial clearance while also achieving acceptable actuation torque levels, with the result that the operation and service life of the assembly are optimized. 
     According to this invention, the method of this invention generally entails a variable stator vane assembly that includes a stator vane configured to be assembled to a casing with a spacer. The vane has a seat offset from a surface. The spacer to which the vane is to be assembled has first and second surfaces offset relative to each other, the first surface being adapted to engage the seat of the vane, while the second surface is adapted to face the surface of the vane. The vane is installed within an opening in a casing so that a first sealing member is between the casing and the surface of the vane, the casing is between the first sealing member and a second sealing member, and the seat extends through the opening. According to this invention, a fixture is then mounted to the vane so that the casing and the first and second sealing members are clamped between the fixture and the vane under a predetermined load, which can be determined experimentally as the load required to flatten the sealing members and imperfections in their surfaces. The fixture preferably includes a tool body having an annular-shaped surface corresponding to the second surface of the spacer, and is mounted to the vane so that it generates the desired clamping load on the vane and sealing members. Finally, the position of the seat of the vane is detected and a spacer is selected having an offset dimension between its first and second surfaces based on the position of the seat. 
     In view of the above, it can be seen that an appropriate spacer is selected for the vane based on conditions corresponding to what will exist in the final assembly when properly installed. More particularly, the seal assembly composed of the sealing members is compressed under a load that flattens the sealing members and minor surface irregularities that would otherwise create drag torque when the spacer is mounted to the vane. In this condition, the offset dimension required for the spacer to provide the desired radial clearance through the seal assembly can be more accurately determined, with the result that repeated assembly and disassembly of the vane assembly is unnecessary. Accordingly, a significant advantage of this invention is that an improved assembly method is provided that significantly reduces the time to assemble a variable stator vane assembly, and simultaneously more accurately and consistently achieves a vane assembly whose radial clearance is minimized for an acceptable actuation torque level. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a variable stator vane assembly for a gas turbine engine; 
     FIG. 2 is a cross-sectional view of the vane assembly of FIG. 1; and 
     FIG. 3 is a cross-sectional view of a fixtured vane assembly in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method and fixturing for assembling a variable stator vane assembly for use in a gas turbine engine. As represented in FIG. 3, the method entails preassembling a vane assembly of the general type shown in FIGS. 1 and 2 with a fixture  40 , which enables the vane assembly to be more accurately, quickly and repeatably assembled while achieving minimal air leakage and acceptable actuation torque levels. While the invention will be described with reference to the vane assembly  10  of FIGS. 1 and 2, those skilled in the art will appreciate that the invention is applicable to vane assemblies that differ from that shown. 
     As described previously with reference to FIGS. 1 and 2, the variable stator vane assembly  10  includes the stator vane  12  rotatably mounted within the opening  38  in the casing  22  of a gas turbine engine, with the seats  28  and trunnion  34  extending axially relative to the flange  30  and through the opening  38 . The vane  12 , spacer  14 , sleeve  16  and lever arm  18  are all secured to the trunnion  34  with the nut  20 . The seal assembly that reduces leakage through the vane/spacer interface includes the bushing  24  and washer  26 , which may be formed of a variety of materials, preferably composites such as polyimide resin with glass and TEFLON® fibers. While a two-piece seal assembly is shown, different seal assembly configurations and designs can be used with this invention. 
     The radial clearance between the casing  22 , the flange  30  of the vane  12 , and the annular surface  36  of the spacer  14  is determined by the axial offset dimension “D” between the annular surface  36  and the shoulders  32  on the spacer  14 . Therefore, the determination of an optimal offset dimension D is critical to minimizing air leakage through the assembly  10  while maintaining an acceptable torque level required to rotate the vane  12 . However, due to tolerance stacks and by design intent, the bushing  24  and washer  26  can have interferences with the vane  12 , spacer  14  and casing  22 , making a prediction of the radial clearance through the assembly  10  impossible. 
     According to this invention, the fixture  40  serves to determine the optimal offset dimension D under a specified clamping load for the spacer  14  based on the actual dimensions of the vane  12 , casing  22 , bushing  24  and washer  26 , as well as the unpredictable irregularities and interferences between these components that determine the interrelationship between the radial clearance and actuation torque. As represented in FIG. 3, the fixture  40  includes a tool body  42  that is mounted to the vane  12  and casing  22  in lieu of the spacer  14 , sleeve  16  and lever arm  18  shown in FIGS. 1 and 2. An annular-shaped portion  46  of the tool body  42  contacts the bushing  24  and therefore provides an annular-shaped abutment surface  50  that substitutes for the annular-shaped surface  36  of the spacer  14 . The fixture  40  also includes a nut  44  that replaces the nut  20  of FIGS. 1 and 2, and threads onto the trunnion  34  as would the nut  20 . The bushing  24  and washer  26  are assembled with the vane  12  and casing  22  as they would be for the assembly  10  shown in FIGS. 1 and 2. According to the invention, the nut  44  is tightened onto the trunnion  34  to attain a clamping load on the bushing  24  and washer  26  that is sufficient to flatten the bushing  24  and washer  26  and any imperfections in their surfaces, such that a more accurate measurement can be obtained for the offset dimension D required of the spacer  14 . 
     As represented in FIG. 3, the fixture assembly  40  includes a pair of probes  48  that extend through the wall of the tool body  42  and into a cavity within the body  42 . The probes  48 , which can be of any suitable type, such as a linear variable displacement transducer (LVDT), capacitance probe, laser, etc., are used to detect the location of the seats  28  within the cavity. For example, if the locations of the probes  48  relative to the annular-shaped surface  50  of the tool body  42  are known, the location of the seats  28  can be accurately determined relative to the surface  50  or relative to the bushing  24  while subjected to the clamping load. With the location of the seats  28  known, the fixture assembly  40  can be removed and a spacer  14  selected and installed having an offset dimension D that will produce the desired radial clearance for the vane assembly  10 . The load applied to the bushing  24  and washer  26  by the spacer  14  will be less than that applied through the fixture assembly  40 , yet will achieve a desirable minimal radial clearance through the bushing  24  and washer  26  to minimize air leakage through the vane assembly  10 . 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, though a nut  44  is shown as being employed to apply the clamping load through the fixture assembly  40 , it is foreseeable that the clamping load could be generated by other means, such as with hydraulic, pneumatic and other mechanical equipment. Furthermore, the physical configuration of the vane assembly  10  and fixture assembly  40  could vary considerably from that shown in the Figures. Therefore, the scope of the invention is to be limited only by the following claims.