Abstract:
A method for improving the wear characteristics of ID bushings comprising the steps of providing an ID bushing comprising electro-graphitic carbon.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     The present invention relates to a bushing constructed of electro-graphitic carbon for reducing wear in gas turbine trunnions.  
         [0003]     (2) Description of the Related Art  
         [0004]     In gas turbine engines, the variable vanes of the high compressor are held at the inner diameter by an inner diameter, or ID, trunnion bushing. With reference to  FIG. 1 , there is illustrated the construction of a portion of an engine including the ID bushing  23 . The bushing  23  is situated between trunnion  15  and two ID shroud halves  22 ,  22 ′. A variable vane  17  is attached to trunnion  15  via platform  21 .  
         [0005]     The bushing  23  is assembled/clamped between the ID shroud halves  22  and is typically constructed of a wear resistant and low friction material. Typically, bushing  23  is fabricated from graphite filled polyimide materials capable of continuous operation up to 650° F. Unfortunately, a bushing  23  constructed of such polymides is not capable of withstanding the high temperatures and loads of advanced high performance compressors. At present, bushings  23  are limited to 650° F. to 700° F. peak excursions as extended periods of exposure tend to rapidly degrade the bushing  23  resulting in metal to metal contact between the trunnion  15  and the ID shrouds  22 ,  22 ′. The negative aspects arising from such metal to metal contact are two-fold. Firstly, the metal to metal contact serves to degrade, and destroy, the physical trunnion  15  and ID shrouds  22 ,  22 ′ resulting in a potentially catastrophic engine failure mode by wearing through the trunnion  15 , liberating, and entering the engine core. Secondly, such metal to metal contact serves to wear away the trunnion  15  and the ID shrouds  22 ,  22 ′ so as to alter the physical geometry of both. As the geometry of the parts change, the tightness of the fit between the trunnion and the ID shrouds  22 ,  22 ′ is similarly altered. Such an alteration in the geometry ultimately results in an angular displacement of a variable vane  17 . Such displacement of variable vane  17  can be catastrophic. Specifically, if a variable vane  17  is displaced with respect to adjacent vanes by more than 6°, a catastrophic surge may be induced. It is therefore of the utmost importance that the trunnion  15  and ID shrouds  22 ,  22 ′ operate in such a manner as to maintain their shapes and, thus, maintain a constant variable vane  17  angle.  
         [0006]     What is therefore needed is a self lubricating bushing  23  which does not suffer material breakdown at high temperatures and which serves to maintain the fit and orientation of ID shrouds  22 ,  22 ′ and trunnion  15  during and after thermal exposure.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, it is an object of the present invention to provide a bushing constructed of electro-graphitic carbon for reducing wear and improving thermal stability in gas turbine trunnions.  
         [0008]     In accordance with the present invention, a method for improving the wear characteristics of ID bushings comprises the steps of providing an ID bushing comprising electro-graphitic carbon.  
         [0009]     In accordance with the present invention, a wear resistant ID bushing comprises a bushing comprising electro-graphitic carbon.  
         [0010]     In accordance with the present invention, a bushing assembly comprises an ID bushing comprising electro-graphitic carbon, a trunnion, and an ID shroud wherein the ID bushing is located in contact with the trunnion and the ID shroud. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1 A  diagram of the gas turbine engine ID bushing assembly of the present invention.  
         [0012]      FIG. 2 A  cross-section diagram of an electro-graphitic carbon bushing  23  of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     It is a central teaching of the present invention to disclose an inner diameter, or ID, bushing  23  composed of a graphite based substance, preferably electro-graphitic carbon. The ID bushing  23  of the present invention does not suffer significant breakdown even at temperatures approximating 1050° F. In addition, the ID bushing  23  of the present invention both self lubricates as well as maintains the appropriate distance between ID shrouds  22 ,  22 ′ and the trunnion  15 . As a result, the ID bushing  23  of the present invention may operate for extended periods of time at high temperatures while maintaining its geometry so as to avoid unwanted deflection of the variable vane.  
         [0014]     With reference to  FIG. 1 , there is illustrated the bushing assembly  3  of the present invention. ID bushing  23  is generally cylindrical and is situated between ID shrouds  22 ,  22 ′ and trunnion  15 . With reference to  FIG. 2 , there is illustrated in detail a cross-section of an ID bushing  23  of the present invention. As noted, ID bushing  23  is fabricated from a carbon based substance, preferably electro-graphitic carbon. With ID bushing  23  thusly formed, it is preferable to chamfer or otherwise machine the ID bushing  23 . Were one to allow ID bushing  23  to extend at either end parallel to normal line  35 , the result would be an increase in the probability of damage to the ID bushing  23 . In operation, the stresses transmitted to the trunnion  15  and bushing  23  from the variable vane  17  can be substantial. These forces serve to encourage the trunnion  15  and bushing  23  to rock in a pendulum-like motion away from being parallel to bushing centerline  31 . As a result of this rocking motion, severe stress is applied to the ends of the ID bushing  23  nearest to normal line  35 . If the interior edge of ID bushing  23  were to remain a right angle, these forces could cause ID bushing  23  to chip or otherwise fragment. It is therefore preferable to machine a chamfer  37  into the interior edges of ID bushing  23 . In a preferred embodiment, the angle θ between the chamfered surface  55  of ID bushing  23  extending at an angle θ from normal line  35  is between 5 and 85°. Most preferably, the chamfer angle θ is approximately 45°. While illustrated with respect to a chamfer surface  55  extending in a linear fashion at a chamfer angle θ, the present invention is not so limited. Rather, the present invention is drawn broadly to encompass any and all shapes to which chamfer surface  55  might be machined including but not limited to curves.  
         [0015]     In operation, the bushing  23  of the present invention has been seen to experience no wear of the trunnion after durations of operation in excess of forty hours. It was observed that, as the trunnion rotated and moved with respect to the ID bushing  23 , and subsequently wore upon ID bushing  23 , the electro-graphitic carbon of the ID bushing  23  adhered to and filled voids created in the outer surface of the trunnion  15 . In this manner, the electro-graphitic carbon of the ID bushing  23  was self lubricating and acted to provide a very stable lubricious graphite-to-graphite contact surface. In addition, as the graphite distributed itself about the trunnion  15 , the total volume of the graphite remained unchanged. As a result, there was maintained a constant spacing between the trunnion  15  and the ID shrouds  22 ,  22 ′ equal to the original thickness of the ID shroud  23 . The geometry of the trunnion  15  with respect to the ID shrouds  22 ,  22 ′ remained constant and therefore avoided any unwanted deflection of the variable vane  17 .  
         [0016]     Tests conducted at 850° F. confirm that the ID bushing  23  of the present invention exhibits a 3.5× wear resistance over the bushings known in the art over a sixty-five hour period and continued to run up to 207 hours with the same amount of wear as polymide designed bushing experienced at sixty-five hours.  
         [0017]     It is apparent that there has been provided in accordance with the present invention a bushing constructed of electro-graphitic carbon for reducing wear in gas turbine trunnions. which fully satisfies the objects, means, and advantages set forth previously herein. While the present invention has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.