Abradable Seal Construction

Abstract

An abradable tip seal for a compressor rotor, the seal including closely spaced slots having sleeves of reinforcing material, such as braided fiberglass, embedded therein surrounding each slot to reinforce the thin wall between adjacent slots.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a tip seal for a bladed rotor and more particularly to an abradable tip seal for a compressor rotor.

2. Description of the Prior Art

Compressor case liners made from nonmetallic material and used as shrouds or tip seals for rotating compressor blades to improve compressor performance are well known in the art, such as the abradable coating applied to the compressor casing as shown in Wiles, U.S. Pat. No. 3,346,175. A further example of such an abradable casing liner is shown in Lapac et al, U.S. Pat. No. 3,575,427, of common assignee with the present invention. It is also known to provide slots or openings in these tip seals in the area over which the blade tips pass to improve performance such as shown in Mikolajczak, U.S. Pat. No. 3,580,692, of common assignee with the present invention.

As is often the case, weight and cost are very important factors in the design of these tip seals. In this respect it is desirable to form the tip seals from relatively lightweight and inexpensive compounds such as a thermosetting epoxy resin or rubber. Even these compounds, however are often heavier than desirable. It is possible to reduce the density and thus the weight of compounds such as a thermosetting epoxy resin by the addition of hollow phenolic or glass spheres of small diameter such as is shown in Moorman U.S. Pat. No. 3,271,222 or in the hereinbefore referred to Lapac patent. While these modified compounds are lightweight, they lack strength and may be very brittle. If it is desirable that the tip seal have a plurality of closely spaced openings, the walls between adjacent openings may become quite thin and might break too easily upon contact with a blade tip or even during handling.

It has been found that forming an abradable tip seal having closely spaced slots separated by thin walls using prior art techniques and materials results in either a liner that is strong enough but is too heavy, or a liner that is light enough but is too weak in the area of the thin wall sections and thus does not abrade properly.

SUMMARY OF THE INVENTION

One object of the present invention is an improved tip seal for a compressor rotor. A further object of the present invention is an abradable compressor rotor tip seal which is lightweight yet strong where required.

Accordingly, a compressor rotor tip seal with a plurality of circumferentially spaced slots open at the sealing surface is provided, the tip seal including an individual sleeve of reinforcing material embedded therewithin around each slot to increase the strength of the wall between adjacent slots.

In a preferred embodiment the tip seal is abradable and is made from a molding compound comprising epoxy resin, curing agent, small phenolic hollow spheres and chopped fiberglass particles. The overall specific gravity of the molding compound is approximately 0.8. The sleeves surrounding each slot are made from woven fiberglass material. The tip seal, sleeves included, has a specific gravity of just under 1.0, yet the wall sections between adjacent slots have good strength characteristics in that each wall has a double thickness of fiberglass fabric embedded in it.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of a portion of the fan section of the compressor of a gas turbine engine.

FIG. 2 is a view looking in the direction A of FIG. 1.

FIG. 3 is a sectional view taken along the line 3--3 of FIG. 1.

FIG. 4 is a view of a braided fiberglass sleeve used in the construction of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As an exemplary embodiment of the present invention, consider the portion of the compressor 10 of a gas turbine engine shown in FIG. 1. The compressor 10 comprises an annular case 12, a case liner 14 and a plurality of rotor blades 16, only the tip portion of one blade being shown. In this embodiment the blades 16 are first stage fan blades, and the case liner 14 is an abradable tip seal for these fan blades 16.

The liner 14 in this instance is bonded with an epoxy adhesive 24 along its radially outwardly facing surface 18 to the radially inwardly facing surface 20 of an annular groove 22 formed in the case 12; however, any suitable technique for securing the liner 14 to the case 12, such as with rivets or bolts, is contemplated as being within the scope of this invention.

Referring now to FIGS. 2 and 3, the liner 14 is shown as being comprised of a plurality of arcuate segments 26 circumferentially spaced within the groove 22 and in abutting relationship with each other such as at 28 and 30. The radially inwardly facing circumferentially extending surface 32 of each segment 26 forms a portion of the gas flow path wall 33 of the compressor 10. As best shown in FIG. 2, each segment 26 includes a double row of circumferentially spaced slots 34. (These slots are not shown in the cross section of FIG. 1 for the purpose of clarity.) The slots in this embodiment are incorporated to improve aerodynamic performance of the fan, but they might be incorporated in a tip seal for any number of reasons such as to reduce noise. As seen in FIG. 3, the slots 34 in this embodiment, although extending generally radially outwardly, are skewed with respect to a radial line and they do not extend completely through the segment 26. As stated, their size, shape, and spacing is dictated by aerodynamic and abradability considerations. With regard to slot spacing, the wall portions 36 between circumferentially adjacent slots 34 are quite thin, being on the order of 86 mils at their thinnest section which is at the surface 32. In this embodiment the wall portions 36 are straight and their surfaces 37 are parallel to the engine axis which is in the plane of the drawing and runs from right to left in FIG. 1.

According to he present invention, woven fiberglass sleeves 38 are embedded within the segments 26, one sleeve surrounding each slot 34 in order to reinforce the thin wall portions 36. These sleeves provide each wall section 36 with a double layer of fiberglass and reduce the brittleness and increase the strength of the wall section. It was found that liners 14 made without these fiberglass sleeves 38 were too weak and too brittle at the wall portions 36 to provide proper abradability characteristics. As can be seen in FIG. 3, the sleeves 38 in this embodiment extend from the radially inner face 32 to the radially outerface 18; however, the length of the sleeves 38 and their positions within the segments 26 might vary from design to design according to slot size and spacing and the practical aspects of manufacturing the segments, which in this embodiment are molded.

As stated in the summary of the invention, the preferred material for the liner 14 is a molding compound comprising epoxy resin mixed with small phenolic hollow spheres and chopped fiberglass particles, the overall specific gravity of the molding compound being approximately 0.8. FIG. 4 is a drawing of a sleeve 38 prior to its being molded into the segments 26. The sleeve of FIG. 4 is cut to shape from a continuous tube of woven fiberglass of the appropriate diameter. The sleeves are cured to their desired shape with an epoxy resin to stiffen them so that they will retain their shape during the molding process for the segment 26. As best shown in FIG. 3, the slots 34 are tapered for ease of molding the individual segments 26, the slot circumferential width W decreasing in a radially outward direction. Although, for ease of manufacturing, the preferred embodiment shows a plurality of segments 26, it is contemplated that the liner 14 could be made from a single annular piece.

Although this invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Claims

Having thus described a typical embodiment of my invention that which I claim as new and desire to secure by Letters Patent of the United States is:

1. A compressor case liner for use as an abradable tip seal for a stage of compressor blades in a gas turbine engine, said liner comprised of non-metallic, low density moldable abradable material and having a circumferentially extending radially inwardly facing surface defining a portion of a gas flow path through the engine, said liner including a plurality of closely spaced, radially extending slots circumferentially spaced about said surface, each slot defining an opening in said surface and forming thin wall portions between adjacent slots, said linear including a plurality of separately formed fabric sleeves embedded therein, a sleeve surrounding each one of said slots to strengthen said thin wall portions, said liner having an overall specific gravity of about 1.0 or less.

2. The compressor case liner according to claim 1 wherein said liner comprises a plurality of circumferentially spaced abutting arcuate segments, each segment having a circumferentially extending radially inwardly facing surface, said surfaces of said segments defining, in combination with each other, said inwardly facing surface of said liner, the surfaces of said thin wall portions being parallel to said engine axis, said slots being tapered, decreasing in circumferential width in a radially outward direction.

3. The compressor case liner according to claim 2 wherein said liner material is comprised of phenolic resin hollow spheres in a cured epoxy resin, and said fabric sleeves are comprised of woven fiber glass.