Abstract:
A light probe assembly for mounting in a casing wall comprising a swage lock fitting sized to receive an outer tube; a light probe extending through the fitting and received within the outer tube; wherein the fitting is adapted for attachment to the casing wall, and wherein a remote end of the light probe is seated on an internal shoulder at a forwardmost end of the outer tube.

Description:
TECHNICAL FIELD 
     This invention relates to a light probe assembly for measuring displacement (vibration) of gas turbine compressor blades, and related installation process. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Light probes are currently used to measure vibration in jet engine compressor blades, as a replacement or supplement to, the use of more costly strain gauges. Light probes also have the advantage of measuring vibration in all of the blades on a wheel which realistically cannot be accomplished with strain gauges. On the other hand, the ability to locate the light probes accurately and securely is critical to acquiring reliable data. Typical installations require many probes which make the installation cost a real concern. Various mounting techniques for light probes have been developed, but these have not been satisfactory in all respects. 
     In one prior installation, a 0.94 inch diameter probe was inserted in a 0.105 inch diameter hole in a compressor casing wall. The procedure did not provide for accurate placement, and vibration of the probe itself within the hole created the potential for “white noise” in the results. In addition, air pressure against the probe could result in movement of the probe outside of its measurement range. 
     Other mounting techniques include the use of threaded inserts, but these require additional machining and multi-piece assemblies which create the potential for debris entering the flowpath. 
     The present invention seeks to alleviate the installation problems experienced in the past by providing an inexpensive, pressure tight, and reliable assembly and related installation process for light probes used particularly for measuring displacement of land based gas turbine compressor blades. 
     In accordance with an exemplary embodiment of the invention, a light probe is fitted into a tube, with the forward end of the tube swaged to retain the light probe within a few thousandths of an inch from the tube end. The light probe and surrounding tube are installed in the compressor casing wall by means of a conventional swage lock pressure fitting. During installation, the forward end of the tubing surrounding the light probe is bottomed against the swaged end of the surrounding tube, with the forward edge of the tube flush with the casing inner wall, so that the probe is maintained at a predetermined distance from the inner casing wall, and hence the compressor blades. Tightening of the swage lock cap forms a pressure sealed assembly. 
     Accordingly, in its broader aspects, the present invention relates to a light probe assembly for mounting in a casing wall comprising a swage lock fitting sized to receive an outer tube; a light probe extending through the fitting and received within the outer tube; wherein the fitting is adapted for attachment to the casing wall, and wherein a remote end of the light probe is seated on an internal shoulder at a forwardmost end of the outer tube. 
     In another aspect, the invention relates to a method of installing a light probe in a casing wall comprising a) attaching an elongated tube having a crimped remote end to a fitting body; b) attaching the fitting to the casing wall; c) attaching a separable cap portion of the fitting onto the light probe; and d) inserting the light probe and cap portion into the fitting body with a remote end of the light probe seated on the crimped remote end of the tube. 
     Other features and advantages of the invention will become apparent from the detailed description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a light probe and mounting hardware in accordance with an exemplary embodiment of the invention; 
     FIG. 2 is a side elevation of a compressor casing wall, illustrating the counterbored holes used to facilitate mounting of the light probe assembly shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, the light probe assembly  10  includes a conventional light probe  12  which is to be installed within a compressor casing wall  14  (FIG. 2) in the manner described further hereinbelow. The light probe in the exemplary embodiment has a diameter of 0.094 inch, allowing it to fit snugly within a ⅛ (0.125) inch diameter tube surrounding tube  16 . The forward end of the tube  16  is swaged or crimped inwardly as shown at  18  so that the forward edge  20  of the light probe  12  can be bottomed within the tube  16 , thus retaining the light probe at a predetermined distance (less than 0.010 inch) from the outer forward edge of the tube  16 , and ultimately, from the flowpath when the tube end is arranged flush with the interior surface of the casing wall  14 . 
     A swage lock pressure fitting  22  is used to create a pressure tight mounting of the light probe within the casing wall  14 . The latter may be between about 2 and about 10 inches thick. The swage lock fitting includes a lower part  24  secured to the tube  16  by brazing as shown at  26 . The tube and light probe extend through the fitting  24 , with the proximate end of the tube  16  terminating within an upper part  28  of the fitting  22 . The light probe  12  extends upwardly from the end of the tube  16  and out of the upper fitting part  28  with leads (not shown) connected to an otherwise conventional control set-up. The fitting  22  may be a conventional Swagelok® fitting Model Number SS-200-1-0084. 
     Turning to FIG. 2, the casing wall  14  is bored to provide a ⅛ inch clearance diameter throughhole  30  (i.e., the hole  30  will receive a ⅛ inch diameter tube) counterbored at  32  to receive the fitting lower part  24 . The outermost portion of the casing wall is further counterbored to provide an enlarged diameter portion  34  adapted to receive the enlarged head  36  of the lower fitting part  24 . 
     The installation process for accurately locating the light probe  12  within the casing wall  14  is as follows. After drilling the ⅛ inch clearance hole  30  in the casing wall, the ¼ inch counterbore  32  is formed with an internal thread to receive the exteriorly threaded fitting lower part  24 . Counterbore  34  is formed to receive the enlarged head  37 . The fitting  22  is then installed within the counterbore  32  and torqued to specification. The ⅛ inch tube  16  is then inserted through the fitting lower part  22 , with the swaged end  18  first, until the tube end is flush with the inner surface  15  of the casing wall. The swage lock fitting  22  is tightened in the usual manner to slightly swage and thus temporarily lock the ⅛ inch tubing into its final position within the fitting. 
     The swage lock fitting  22  and ⅛ inch tubing is then removed from the casing wall as an assembly, so that the tube  16  can be brazed to the lower or forward end of the lower part  24  as shown at  26 , thus fixing the tube  16  relative to the lower fitting part  24 . The swage lock cap or upper part  28  and associated ferrules  36 ,  38 , preferably brass, are removed and the upper end of the tube  16  is then machined below the seat of the ferrules  36 ,  38  in the upper part  28 , i.e., the upper end of tube  16  will remain below the ferrules  36 ,  38 . In this way, ferrule  36  may be tightened down and locked in place via ferrule  38  to thereby capture the light probe  12  in the fitting without engaging the upper end of the tube  16 . Before inserting the light probe  12 , however, 0.094 inch diameter trial rod is inserted to verify that the light probe will fit into the ⅛ inch tube and will seat properly at the bottom of the tube. 
     Thereafter, the swage lock fitting  22  and tube  16  is reinstalled into the casing wall  14  and torqued to specification, with an O-ring  40  located below head  36  so as to seal the assembly to the casing wall at the interface of counterbores  34  and  32 . The swage lock upper part  28  and ferrules  36 ,  38  are installed onto the light probe  12  and the light probe is then inserted into the tube  16  until the probe bottoms at the swaged end  18  of the tube  16 . The fitting upper part or cap  28  is then tightened by threading down over an intermediate fitting part  42 , over the ferrules with the lower edge of probe  12  seated at the crimped end of tube  16  which, in turn, is flush with the interior surface  15  of the casing wall. In this way, the location of the lower tip of the light probe can reliably and repeatedly be located at a predetermined distance, e.g. &lt;0.010 inch from the flowpath inside the casing. 
     If necessary, the light probe leads (not shown) are coated with Nichrome to reduce any risk of damage to the leads. 
     while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.