Abstract:
A bushing and washer forming a bearing assembly at the outer radial end of a stator vane, the outer bearing assembly, and the bearing assembly at the inner radial end of the stator vane, the inner bearing assembly, that facilitate durability, effectiveness and reduced cost. Both the inner and outer bearing assemblies are designed to rotate relative to the vane shaft and the respective mating shroud or case to even out the wear around the circumference of the bushing. When a rotating bushing is used, a flange on the bushing is designed to be positioned on the inside of the case, so that a pressure differential across the case applies a force to the vane to move it outwardly against the flange and the flange against the case, thereby minimizing air leakage.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to fan and compressor components of gas turbine engines, and in particular, to bushings and washers used with variable stator vanes in the compressor section of the engine. 
     BACKGROUND OF THE INVENTION 
     In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on a shaft. The flow of hot gases turns the turbine, which turns the shaft and powers the compressor. The hot exhaust gases flow from the back of the engine, providing thrust that propels the aircraft forward. 
     Gas turbine engines generally include a high pressure compressor for supplying combustion air to a combustor, and a turbine. The high pressure compressor, combustor, and turbine are collectively referred to as a core engine. Typically, gas turbine engines also include a low pressure compressor for supplying compressed air, for further compression, to the high pressure compressor, and a fan for supplying air to the low pressure compressor. 
     These compressors typically include a plurality of stages, each stage including in alternating configuration a rotor for moving the air axially and a fixed, radially oriented stator for efficiently directing the flow of air axially. The rotor typically includes an assembly of a plurality of blades radially attached to a rotating disk, the assembly surrounded by a casing. The casing is typically fabricated to be removable, such as by forming the casing into two halves that are then removably joined together. The casing supports the plurality of radially oriented, fixed stator vanes which are attached thereto, while the rotor supports the rotor blades. Each stage of stator vanes are positioned in front of a rotor with the attached blades to efficiently direct air flow to the blades of the rotor. 
     Variable stator vane assemblies are utilized to improve the performance of the engine. For better performance, the rotational speed of the fan and compressor usually need to be different. In general, the high speed compressor rotates about twice as fast as the fan. This is accomplished by attaching the compressor and fan to different spools or shafts which run concentric to each other. In this dual spool configuration, the high pressure compressor is connected to a high pressure turbine by an outer spool. In some configurations, three concentric spools are utilized. Each variable stator vane assembly includes a variable stator vane which extends between adjacent rotor blades. The variable stator vane is rotatable about a substantially radial axis. The orientation of the variable stator vane varies the stagger angle of the vane in a controlled fashion. This allows the vane or vanes to be realigned to change the impingement angle of compressed air on to the rotor blades as the operating condition of the engine changes. The position of the vane is changed by means of a lever arm attached to an actuator ring on the outside of the compressor case. 
     A known variable vane assembly includes a variable vane, a trunion bushing; and a washer. At an outer end, the variable vane assembly is bolted onto a high pressure compressor stator casing and the bushing extends concentrically through an opening in the casing. The washer is positioned above the casing and between the bushing and casing. The variable vane includes a vane stem that extends through the opening in the casing (hereinafter referred to as the “outer end”) and through the bushing and washer. The bushing and washer are referred to herein as a bearing assembly, the bearing assembly positioned radially outboard referred to as the first bearing assembly. The vane also includes a second bearing assembly at its inner radial end. The vane may be shrouded at its inner end to minimize the vibrational effect of flow variations, particularly on the longer vanes. The bearing assembly produces a low friction surface that prevents metal on metal contact. 
     A lever arm is fixedly joined to the vane stem extending outwardly from the vane bushing or first bearing assembly. The distal end of the lever arm is operatively joined to an actuation ring that controls the angle of the vane. All of the vane lever arms in a single stage are joined to a common actuation ring for ensuring that all of the variable vanes are positioned at the same angular orientation relative to the airflow in the compressor stage. 
     Although known variable vane assemblies provide certain advantages as explained above, such vane assemblies have potential gas stream leakage paths which reduce engine efficiency. The primary leakage path is between the outside diameter of the airfoil portion, the stator vane stem, extending through the aperture in the compressor casing and the inside diameter of the bushing. The secondary leakage path is between the outside diameter of an optional metal jacket housing a portion of the bushing or alternatively, the bushing itself and the inside diameter of the aperture opening in the compressor stator casing. Other leakage paths are on either radial end at the airfoil where the airfoil joins the case and the shroud, as well as at the shroud seal, between the shroud and the rotor shaft. Additionally, the high velocity and high temperature of the air can cause oxidation and erosion of the bearing assemblies, which leads to premature failure of the bearing assembly, and eventual inability of the variable vane assembly to function. This will decrease engine efficiency and ability to rapidly respond to power demand changes. 
     Once the bearing assembly fails, an increase in leakage through the opening occurs, which results in a performance loss. In addition, failure of the bearing assembly can allow contact between the stator vane and the casing, which causes wear as a result of vibration and increases overhaul costs of the engine. Accordingly, it would be desirable to provide bearing assemblies fabricated from materials and of a design having performance characteristics that will reduce or eliminate air leakage between the stator vane stem and the compressor casing while providing an increase in the durability of the bushing and washer composition to increase part life. The present invention fulfills this need, and further provides related advantages. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention presents improvements to the bushing and washer forming the bearing assembly at the outer radial end of a stator vane, the outer bearing assembly, and the bearing assembly at the inner radial end of the stator vane, the inner bearing assembly that facilitate durability, effectiveness and reduced cost. 
     Both the inner and outer bearing assemblies are designed to rotate relative to the vane shaft and the respective mating shroud or case to even out the wear around the circumference of the bushing. However, the bushing is designed so that if the relative wear rate between the bushing material and the vane or the shroud/case is high, the bushing will not rotate relative to that material. When a rotating bushing is used, a flange on the bushing is designed to be positioned on the inside of the case, so that a pressure differential across the case applies a force to the vane to move it outwardly against the flange and the flange against the case, thereby minimizing air leakage. 
     The vane is designed to have an axial bearing face adjacent to a vane stem at each end, such that the axial bearing face bears against the bushing flange or a washer to minimize wear and friction. The axial bearing face has a larger diameter for harder, less wearing materials than the adjacent bushing flange and a smaller diameter than the adjacent bushing flange for a softer, more wearing surface to minimize edge effects and “digging-in” abrasion. This configuration can also be applied to a lever arm bearing face which rides against a bushing flange/washer located between it and the casing to reduce wear as the lever arm rotates. The sliding contact forces between the lever arm, vane stems and their respective shroud or case are fully carried by the bushings and washers for the design life of the assembly. 
     The present invention is also a bearing assembly for rotatively positioning a variable vane to an engine casing comprising a bushing wherein a first surface of the bushing contacts a variable vane stem; a second opposed surface of the bushing contacts the engine casing, and the bushing rotates relative to the variable vane stem and engine casing. The bushing may further include a surface positioned such that a pressure differential across the engine casing urges a vane assembly against the bushing, which in turn urges the bushing against an inside wall of the engine casing, thereby closing an air leakage path. 
     Advantages of the present invention are that the bearing assembly and associated, vane and case/shroud design extends system life, increases system temperature capability beyond known values, reduces air leakage and minimizes repair costs when the engine is refurbished. The extended system life of the bearing assembly allowing the variable stator vane to operate over the entire design life of the assembly translates to improved engine performance and lower operating cost of the engine. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a section of a known high pressure compressor for a turbine engine. 
         FIG. 2  is an exploded view of a typical variable stator vane assembly. 
         FIG. 3  is a schematic view of an assembled variable vane assembly. 
         FIG. 4  is a depiction of the leakage path of air across the top portion of the variable stator pivot. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic view of a section of a portion of a typical compressor  100  for a turbine engine (not shown). Six variable stages are shown; however for large engines there typically are more stages, as many as thirteen or fourteen stages. The number of stages is not relevant as each stage operates in the same manner, the overall number of stages being an indication of the volume of air compressed and the degree of compression. A compressor  100  includes a plurality of stages  102 , and each stage  102  includes a rotor disk  108  supporting a plurality of rotor blades  104  axially spaced from a set of radially oriented variable stator vane assemblies  106 . Rotor disks  108  are assembled onto a rotor shaft  110 . For simplicity, only a single spool is shown, however it will be understood by those skilled in the art that multiple spool designs are encompassed by this disclosure. Rotor shaft  110  is also connected at the aft end to a turbine (not shown). Rotor shaft  110  is surrounded by a stator casing  112  that supports variable stator vane assemblies  106  and provides a flow boundary. Vane assemblies may be shrouded at their radially oriented inner ends  124 . These shrouds are not depicted. 
     Each variable stator vane assembly  106  includes a variable vane  114  and a vane stem  116 . Vane stem  116  protrudes through an opening  118  in casing  112 . Variable vane assemblies  106  further include a lever arm  120  extending from variable vane  114  that can be activated by a bellcrank mechanism and actuator (not shown) to rotate variable vanes  114 . The orientation of vanes  114  relative to the flow path through compressor  100  directs air flow therethrough. 
     Variable vane assemblies  106  not only direct air flow through compressor  100 , but also provide a potential leakage pathway to allow air be diverted and to exit compressor  100 , such as through casing openings  118 . This leakage through openings  118  reduces the efficiency of compressor  100 . 
       FIG. 2  is an exploded view of a variable vane assembly  106 . Vane airfoil  210  is shown as a cutaway. Integral vane stem  116  is located at a radially outer end of vane airfoil  210 . Vane stem  116  includes an attachment means  212 , depicted here as a threaded connection, although any other equivalent connection method such as a spline arrangement may be used. Vane stem  116  extends through opening  118  in casing  112 , again shown as a cutaway. Opening  118  includes a counterbore  154  which receives an inner washer  214 . A bushing  216  slides into opening  118  and over upper vane stem  116 , filling the remaining space in opening  118  and preventing contact between casing  112  and upper vane stem  116 . This washer  214  may be replaced by the flange of a flanged rotating bushing, in which case a washer would separate the lever an  120  from the case. A first end  156  of lever arm  120  is assembled over vane stem  116  and is secured to vane stem  116  by a fastening means  224 , here depicted as a locknut, that mates cooperatively with attachment means  212 , depicted as a threaded end of upper vane stem  116 , to secure fastening means  224  to vane stem  116 . Lever arm  120  includes a second end  158  that is integrally attached to the first end  156  by a web  160 . A projection  168  extends from second end  158  and is received by an aperture in actuation ring (not shown). A second bushing  169  fits over projection  168  and into the aperture in actuation ring (not shown) to prevent contact between actuation ring  164  and projection  168 . 
     At the radially inner end of vane assembly  106 , an integral lower vane shaft  226  extends radially inward from vane airfoil  210 . Vane shaft  226  includes a first, large diameter  228  and a second smaller diameter  230 . A bushing  236  is assembled over lower vane shaft  226 , which is received by an optional shroud  231 . A seal  238  is assembled radially inward of the shroud which is contacted by teeth  170  positioned on the rotating apparatus of the engine, the teeth wearing into seal  238  to form a barrier to air leakage. An optional third fastening means  234 , depicted as a locking pin extending through at least one boundary of seal  238 , through shroud  231 , through bushing  236  and through aperture  232  in lower vane shaft  226  secures seal  238 , bushing  236 , and shroud  231  to lower vane shaft  226 . When an optional fastening means  234  is employed, any other mechanical fastening means, such as for example a threaded bolt and locknut may be substituted for the lock pin. Optionally, a washer is place between the axial faced large diameter  228  and shroud  231 . 
       FIG. 3  is a schematic view of a typical prior art variable vane assembly  106  at its upper end in the assembled condition. Variable vane assembly  106  includes a variable vane  302  shown in cutaway. A bushing  304  is positioned on variable vane  302 . A casing  306  supports variable vane  302  and includes a first recessed portion  308 , a second recessed portion  312 , and an inner portion  310  connecting first recessed portion  308  and second recessed portion  312 . An opening, such as opening  314  shown in  FIG. 3 , may be formed adjacent vane stem  302  above the airfoil portion of the vane. 
     Bushing  304  includes a first portion  316  and a second portion  318 . Bushing first portion  316  is in direct contact with casing first recessed portion  308  and separates variable vane  302  from casing  306 . Bushing second portion  318  contacts casing inner portion  310 , being positioned between variable vane  302  and casing  306 . Bushing first portion  316  extends substantially, but just less than, the entire length of casing first recessed portion  308 . To minimize friction torque, this portion  316  may be tapered to provide the most intimate contact at a smaller radius In addition, bushing second portion  318  extends substantially an entire length of casing inner portion  310  and is substantially perpendicular to bushing first portion  316 . Bushing  304  prevents variable vane  302  from directly contacting casing  306 . 
     Variable vane assembly  106  further includes a washer  320 . Washer  320  is substantially flat and includes an outer diameter surface  322  and an inner diameter surface  324 . More specifically, washer  320  includes a first face  326 , a second face  328 , and a thickness  330  that is substantially constant from inner diameter surface  324  to outer diameter surface  322  as shown. This washer  320  may be slightly tapered or provided with a suitable profile to maximize existing torque by ensuring initially, that the most intimate contact with bushing  304  is at a smaller radius, Washer  320  is in direct contact with casing second recessed portion  312  and is coextensive with at least a portion of the length of casing second recessed portion  312 . 
     Variable vane assembly  106  includes a spacer  332  in contact with washer  320 . Washer  320  prevents contact between spacer  332  and casing second recessed portion  312 . Spacer  332  includes a first portion  334  and a second portion  336 . Spacer first portion  334  contacts washer  320  along its second face  328  and has a diameter substantially greater than the diameter of washer  320 . The washer inner diameter is greater than the casing inner portion  310  as well as the inner diameter of spacer  332 . Spacer  332  is separated from bushing  304  by washer  320 . Bushing  304  and washer  320  do not contact each other. Washer  320  prevents spacer  332  from contacting casing  306 . 
     Variable vane  302  also includes a first portion  338 , a vane ledge inner surface  340  and a vane ledge vertical surface  342 , and a vane spacer seating portion  344 . Vane ledge inner surface  340  abuts and transitions to a vane stem fastening surface  346 . Vane stem fastening surface  346  and vane ledge inner surface  340  extend through an opening  118  or aperture in casing  306 . Bushing second portion  318  abuts against inner portion  310  of casing  306 . Bushing second portion  318  prevents vane ledge vertical surface  342  from contacting casing inner portion  310 . 
     Variable vane assembly  106  also includes a lever arm  348 , shown partially in  FIG. 3 , positioned around vane vane  302  and contacting spacer  332  and sleeve  350 . Lever arm  348  is moved by an actuator to adjust the angle of variable vane  302 , and thus alter the direction of air flow through the compressor. 
     In addition, variable vane assembly  106  includes a sleeve  350  contacting lever arm  348 , and a lever arm lock nut  352  contacting sleeve  350 . Lever arm lock nut  352  cooperates with vane stem  346  in holding mating pieces in contact with one another maintaining variable vane assembly  106  securely against casing  306 . 
     Variable vane assembly  106  is assembled by placing bushing  304  on variable vane  302  such that bushing first portion  316  and bushing second portion  318  contact variable vane  302  and are substantially between casing  306  and vane  302 . Variable vane  302  and bushing  304  extend through opening  118  or aperture in casing  306 . 
     Washer  320  is placed on casing  306  adjacent bushing  304 . Spacer  332  is positioned on variable vane  302  and contacts washer  320 . Lever arm  348  is positioned over vane stem  346  and contacts spacer  332 . Sleeve  350  is positioned over vane stem  346  and contacts lever arm  348 . Finally, lever arm lock nut  352  is positioned over vane stem  346  contacting sleeve  350 , locking the assembly in place. 
     Washer  320  and bushing  304  are bearing surfaces in variable vane assembly  106  such as are found in a high pressure compressor. Washer  320  and bushing  304  may be utilized in other environments such as a low pressure compressor variable vane assembly or a turbine rotor vane assembly, their use in the turbine being restricted by their high temperature capability. 
     Solid bushings  304  and washers  320  are fabricated by known techniques, such as by injection molding or by high temperature sintering of ceramics. Ideally, the solid bushing  304  should be durable with effectively good wear characteristics. The bushing  304 , which is readily replaceable, should wear before the casing  306  and vane stem  346 , the casing  306  and vane stem  346  being made of more wear resistant materials. The bushing  304  is made of an inexpensive wear material which is easily replaceable and designed as a consumable item. 
     In a preferred embodiment, bushing  216  rotates relative to vane stem  116 , the benefits of which have heretofore not been appreciated. Such rotation permits even wear around the circumference of the bushing where it contacts vane stem  116  and the casing  112 , thereby improving the service life of bushing  216 . This rotation is most beneficial when the wear rate between bushing  216  and vane stem  116  or casing  112  is relatively low, such as when the wear is less than about 0.0002 inches after sliding effectively about 50,000 feet while experiencing a load of about 25 lbs. However, if the wear rate between bushing  216  and vane stem  116  or casing  112  is relatively high, such as when the wear rate under comparable conditions is about 0.020 inches, bushing  304  is designed to not rotate against vane stem  346  or casing  206 , in order to increase service life. 
     To determine the type of bushing  216  required in a design, physical properties, such as, for example, thermal expansion coefficient, operating temperature range, yield strength and elastic modulus of the mating materials, the forces exerted on the mating materials, wear per cycle and the number of cycles over the expected life are used to determine the relative wear that will be experienced in an application. The wear rate between materials can be determined and the expected wear for an application can be used to determine whether the bushing  216  should be allowed to rotate. For example, even if the wear rate is relatively low, as previously discussed, but galling occurs that causes excessive transfer of one material to the surface of the other, leading to severe roughening of the surfaces, the bushing material would be found to be unacceptable. The bushing  216  is to have an effective thickness sufficient to facilitate manufacturing ease (i.e. reduced cost) and wear life. Thus, if the expected wear during the life of the bushing is expected to be 0.0002, the minimum thickness for manufacturing purposes may be {fraction (1/64)}″, (0.015″), whereas if the expected wear is 0.020″, the minimum thickness may be {fraction (1/32)}″. These values are minimum values based on wear concerns only, and larger bushing thicknesses may be utilized for other reasons such as ease of assembly and handling. For example, a casing material made from M152 and bushing material made from A286 provides a high and unacceptable wear rate as previously discussed. However, a silicon nitride bushing between an A286 vane stem and a M152 case provides an acceptable wear rate, as previously discussed. 
     Bushing clearance should be minimized over the operating temperature regime to minimize air leakage between the bushing  216  and the stator vane assembly  106  or casing  112 , while still permitting bushing  216  to rotate. The air leakage path as shown by arrows  390  in a variable stator vane arrangement is depicted in  FIG. 4 , and a bushing having minimal clearance, that is, that substantially fills a gap between the casing  112  and stator vane is desirable. Optimally, when a rotating bushing is used, the bushing first includes a flange, for example, fabricated to position itself horizontally against casing  112 , so that the differential pressure between the compressor and exterior of the casing applies a force against the stator vane, which is transmitted against bushing and hence against the casing  306 , thereby minimizing air leakage as the gap is closed. Thus, even as the bushing wears due to contact with the casing and stator vane causing a gap, this wear will not result in air leakage as the action of the stator vane against the bushing and against casing  306  will maintain the seal even as the bushing experiences wear. However, the tribological benefits of a rotating bushing are not dependant upon the presence of a bushing flange. 
     It is preferable that a vane have at least one bearing face to ride against a bushing flange, such as bushing first portion  316  or washer  320  to minimize wear and friction. For example, vane first portion  338  which acts as a bearing surface interfaces with bushing first portion  316 . This bearing surface is fabricated with a preselected diameter, the preselected diameter being larger diameter for harder materials which are less likely to wear, such as ceramics and carbides. If however this surface is expected to experience a great deal of wear, such as for example, when the bearing surface is a softer material, such as soft metal as is well known in the art, a composite material, a polymeric material or carbon/graphite material, then the surface will have a smaller preselected diameter This will minimize edge effects, fretting and “digging-in” abrasion. This relationship between diameter size and anticipated wear is also true for the surface of lever arm  348  which bears against, for example, spacer  332 , or washer  320  or bushing flange positioned between the casing  306  and spacer  332 . The preselected diameter size of the vane itself is relative to the engine design requirements and location of the vane within the compressor, but must fall within the acceptable design requirements for the compressor. For example, the range of preselected diameters for vane shafts in the early stages of a GE-90 compressor used to power Boeing 777 aircraft, which is a very large compressor, can be expected to be significantly greater than the vane shafts in the late stages of a T700 or CT7 compressor used to power helicopters. 
     Spacer  332 , bushing  304  and washer  320  are fabricated to ensure that sliding contact forces transmitted, for example, through lever arm  348 , vane stem  346  and shroud  231 ,  FIG. 2 , are fully carried by the spacer  332 , bushing  304  and washer  320  for the expected design life of the compressor. Washer  320  is designed to remain concentric with rotation of the variable vane  302 . In the event that bushings  304  are assemblies constructed of a separate washer  316  and a bushing  318 , it is important to prevent washer  316  from riding up against the mating filet radius, here shown on the variable vane  302 , as this would create undesirable forces. Preferably, the washer is of an effective, preselected thickness to accommodate the filet of portion  316  and is located concentrically of the variable vane  302 . Similarly, washer  320  should be located concentrically and not extend to the edges of spacer  334  nor to the fillet in casing  306  adjacent washer outer diameter surface  322 . 
     Although the bearing assembly of the present invention has been described at the vane stem—casing (outer radial) interface, its principles apply equally well to the vane stem—shroud (inner radial ) interface. Returning to  FIG. 1 , known aircraft gas turbine engines secure the shroud  231  to the vane lower shaft  226  through use of lock pin  234 . Because current designs necessitate use of a shortened bushing at the inner radial interface, vane lower shaft  226  at the inner radial interface should be designed to have sufficient length to facilitate use of standard size bushings. 
     It is advantageous to size the bushing  236  at the inner radial interface and vane such that a standard bushing and vane can provide positioning of the shroud relative to the casing thereby requiring only lock pin  234  and bushing to assure concentricity. The shroud may be split so that two rings pulled together in the axial direction of the engine capture the bushing, or alternatively, multiple segments may be used, which will locate the bushing and seal. Because leakage paths at the shroud do not vent to the bypass channel, leakage losses at this location are of less concern. 
     Although the present invention has been described in connection with specific examples and embodiments, those skilled in the art will recognize that the present invention is capable of other variations and modifications within its scope. These examples and embodiments are intended as typical of, rather than in any way limiting on, the scope of the present invention as presented in the appended claims.