Abstract:
A method for measuring blade tip clearance for a gas turbine engine having a fan case assembly and a rotor having a plurality of blades may include the steps of removably attaching a measurement tool to the fan case assembly, the measurement tool having a sensor, rotating the rotor, and measuring blade tip clearance for the plurality of blades during the rotation of the rotor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The exemplary embodiments relate generally to gas turbine engines and more specifically to apparatus for measuring the clearance of blade tips. 
         [0002]    Gas turbine engines, steam turbines, aircraft engines, jet engines and other axial flow turbomachinery are typically designed to minimize the radial gaps between the blade tips and the blade housings or cases. Gaps between the blade tips and the cases can reduce efficiency by allowing gas or air to leak into the downstream stages of engine operation. The gaps between the blade tips and the cases are a function of engine speed and temperature, and the gaps changes during engine operation. High operating rotational speeds can cause radial elastic growth in rotating hardware (i.e. blades), resulting in radial blade tip growth. Additionally, high temperatures cause thermal expansion in the case and in the rotating hardware. Currently several inspection methods for determining the gap between the blade tips and the fan cases at operating speed are being used. 
         [0003]    One method for determining the gap between the blade tips and the case utilizes a thin metal rod inserted and fastened into an axially drilled bolt, the resulting assembly being inserted into a mount plate attached to the fan case. The end of the rod is located where the blade tips should be. The method requires that the engine be operated for a specified time period after which the amount of wear on the rod is measured to determine the change in the gap between the blade tips and the case. The method is insufficient in that the thin metal rods often bend or break which renders measurement thereof moot. In addition, metal liberated from the thin metal rod, either as pieces or as powder can cause damage to the engine. Further, making these thin metal rods can be both difficult and time consuming because each rod must be custom made using a measurement of distance from the fan case to the blade tip. Further, such a method suffers from errors such as measurement, data recording, and machining. It is often the case that the thin metal rods are made either too short or too long. Short rods do not rub the blade tip, while long rods bend or break. Further still, this assembly requires addition of holes in the fan case, which may weaken the case and possible cause structural damage after an extended period of use. 
         [0004]    Another method utilizes a taper gage and gage block to determine the tip clearance for each individual blade. The gage block is placed on the interior of the fan case and the taper gage is placed on top of the block. The taper gage slides along the gage block until it contacts the blade. The technician may then read the taper gage to determine the gap between the blade and the case. This process is repeated for each blade. This is very time consuming and leads to longer manufacturing and overhaul times. This technique may also be prone to errors. These errors may include, bridging of the fan case by the gage block, measurement reading errors and parallax error when reading the taper gage. Furthermore, with the rake angle that may be applied to the blade tip, it may not be possible to read the gage at the point of contact. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    In one exemplary embodiment, a method for measuring blade tip clearance for a gas turbine engine having a fan case assembly and a rotor having a plurality of blades may include the steps of removably attaching a measurement tool to the fan case assembly, the measurement tool having a sensor, rotating the rotor, and measuring blade tip clearance for the plurality of blades during the rotation of the rotor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0006]      FIG. 1  is a cross-sectional schematic view of an exemplary gas turbine engine. 
           [0007]      FIG. 2  is a cross-sectional view of an exemplary fan assembly. 
           [0008]      FIG. 3  is a perspective view of an exemplary embodiment of a measurement tool. 
           [0009]      FIG. 4  is a cross-sectional view of an exemplary fan assembly shown having an exemplary embodiment of a measurement tool installed thereupon. 
           [0010]      FIG. 5  is a close-up cross-sectional view of the area  5  circled in  FIG. 4 . 
           [0011]      FIG. 6  is a top view of an exemplary fan assembly taken along line  6 - 6  in  FIG. 4 , shown having an exemplary embodiment of a measurement tool installed thereupon. 
           [0012]      FIG. 7  is a bottom view of an exemplary fan assembly shown having an exemplary embodiment of a measurement tool installed thereupon. 
           [0013]      FIG. 8  is a perspective view of an exemplary embodiment of a bushing shown an installed condition. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  illustrates a cross-sectional schematic view of an exemplary gas turbine engine  100 . The gas turbine engine  100  may include a fan assembly  102 , low-pressure compressor  104 , a high-pressure compressor  106 , a combustor  108 , a high-pressure turbine  110 , and a low-pressure turbine  112 . The fan assembly  102  and low-pressure compressor  104  may be coupled to the low-pressure turbine  112  through a shaft  114 . The high-pressure compressor  106  may be coupled to the high-pressure turbine  110  through a shaft  116 . In operation, air flows through the fan assembly  102 , low-pressure compressor  104  and high-pressure compressor  106 . The highly compressed air is delivered to the combustor  108 , where it is mixed with a fuel and ignited to generate combustion gases. The combustion gases are channeled from the combustor  108  to drive the turbines  110  and  112 . The turbine  112  drives the fan assembly  102  and low-pressure compressor  104  by way of shaft  114 . The turbine  110  drives the high-pressure compressor  106  by way of shaft  116 . 
         [0015]      FIG. 2  illustrates a cross-sectional view of an exemplary fan assembly  102 .  FIG. 2  shows the bottom portion of the fan assembly. Tip clearance is typically measured at the bottom center of the fan case. It should be noted that the measurement can occur at any position around the circumference of the fan casing and the exemplary embodiments should not be limited to just the bottom portion. The fan assembly  102  may include a rotor  118 , which may receive a plurality of fan blades  120 . Alternatively, the rotor  118  may be a blisk where a plurality of airfoils integral with the rotor  118  extend outwardly therefrom. The fan blades  120  extend radially from a platform  122  to a tip  124 . A fan case assembly  126  radially bounds the tip  124 . The fan case assembly  126  may include a fan case  128  and a fan shroud  130 . The fan shroud  130  has a radially inner surface  132 . Together, the blade tip  124  and inner surface  132  of the fan shroud  130  define a gap  134 . 
         [0016]      FIGS. 3-7  illustrate an exemplary embodiment of a measurement tool  136  for measuring the height of the gap  134  or the distance between the blade tip  124  and the inner surface  132  of the fan shroud  130  at various points along the axial length of the blade tip  124 . The measurement tool  136  has a frame  138 . The frame  138  may include a backing portion  140  and an extended portion  142 . It should be noted that any configuration of the frame  138  might be used so long as the measurement tool  136  may be attached securely to the fan case assembly  126 . The backing portion  140  may include an attachment system  144 . Any attachment system known in the art may be used so long as the measurement tool  136  may be attached securely to the fan case assembly  126 . In one exemplary embodiment, the attachment system may include a plurality of clamps  146 . The clamps  146  may be any clamping mechanism known in the art, such as but not limited to, a cam clamp, over center clamp, vice clamp, or any other similar clamp. The clamps  146  may include a lever  148  and a cam  150 , which may cooperate with a screw  152  and a bushing  154  to securely attach the measurement tool  136  to the fan case assembly  126 . The clamps  146  may have a locked and unlocked position. The unlocked position is shown in  FIG. 3 . The locked position is shown in  FIG. 4  and will be described in more detail below. The clamps  146  may be spaced apart such that the screw  152 , bushing  154  and post  156  may be placed into existing holes in the fan case assembly  126 . This allows the measurement to occur without modification to the fan case assembly  126 . 
         [0017]    An arm  158  may be attached to the frame  128 . In one exemplary embodiment, the arm  158  may be attached to the extended portion  142  with a screw  160  or any other attachment mechanism. In another exemplary embodiment, the arm  158  may be attached using a hinge and spring mechanism that may bias the arm  158  into contact with the fan case  128 . Any attachment mechanism known in the art may be used. The arm  158  may be attached at one end and free at the other. The free end may include a sensor  162 . The sensor  162  may be any sensor known in the art that can measure the distance between two points. In one exemplary embodiment, the sensor  162  may be a non-contact displacement sensor, such as, but not limited to, a capacitive position sensor or an optical sensor. The sensor  162  may have a lead  164  that may pass from the sensor  162  along a channel  166  to electronic components located remotely from the measurement tool  136 . As shown in  FIG. 5 , the arm  158  may have a protrusion  168  for contacting the surface  132  and stabilizing the sensor  162 . In one exemplary embodiment, the protrusion  168  may be spherical. Any number of arms  158  may be used and any number of sensors  162  may be used on each arm  158 . The arm  158  may be any length or width so long as when the measurement tool  136  is attached to the fan case assembly  126 , the sensor or sensor  162  are placed in appropriate measurement locations such as, but not limited to, the center of the blade  170 , the leading edge  172 , and/or the trailing edge  174 . The arm  158  may be attached such that it is easily removed or replaced, for example, should a blade  120  contact the arm  158  during measurement and break or to take measurements in different locations by changing to a different arm. 
         [0018]    The measurement tool  136  may be installed onto the fan case assembly  126  through fan case forward flange  176 . The forward flange  176  may have a plurality of holes  178  for receiving screws  152 , bushings  154  and posts  156 . The lever  148  may then be actuated into the locked position. The screw  152  may be pulled towards the forward flange and cause the bushing  154  to expand along slots  180 , as shown in  FIG. 8 . The bushing  154  may apply axial force to the forward flange  176  and pull the measurement tool  136  against the forward flange  176  to substantially eliminate any gaps therebetween. The location and configuration of the measurement tool  136  may be predetermined so as to align the sensor  162  with the area of the blades  120  to measure. Once the measurement tool  136  is locked into position, the sensor  162  may begin to take measurements as the fan blades  120  are rotated. The sensor  162  can obtain data for each of the fan blades  120  in a single rotation; however, it should be understood that data may be obtained for more than one rotation. The measurement tool  136  may be attached and take measurements in a relatively short period of time, while ensuring accurate measurements. Furthermore, the tool does not require any additional holes or structural transformation of the fan case assembly since it uses established holes for assembly. This leads to a reduced production cycle and produces accurate and reliable tip clearance measurements. 
         [0019]    This written description discloses exemplary embodiments, including the best mode, to enable any person skilled in the art to make and use the exemplary embodiments. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.