Light probe installation

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.

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.

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 1/8 (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.RTM. fitting Model Number SS-200-1-0084.
 Turning to FIG. 2, the casing wall 14 is bored to provide a 1/8 inch
 clearance diameter throughhole 30 (i.e., the hole 30 will receive a 1/8
 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 1/8 inch clearance
 hole 30 in the casing wall, the 1/4 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 1/8 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 1/8 inch
 tubing into its final position within the fitting.
 The swage lock fitting 22 and 1/8 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 1/8 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.