Vane Rings

Abstract

One piece cast metal vane rings of the same shroud diameter and number of vanes but of different throat areas are made using a pattern assembled from several pattern pieces in which the vanes have different angles to the axis of the vane ring shroud. A set of pattern pieces of two or three different angles can thus provide for a number of different throat areas, thus avoiding having to prepare, in a large number of different sizes, expensive molds for making pattern pieces for molds of the same diameter but differing throat areas.

Description

BACKGROUND OF THE INVENTION

This invention relates to one-piece cast metal vane rings and their manufacture.

FIELD OF THE INVENTION

The vane ring is used in the gas turbine engine to direct the exhaust gases coming from the combustion chamber of the engine onto the turbine blades. Such a ring is located, as a stator, between the combustion chamber and the first stage turbine and between each subsequent turbine stage.

The vane rings are made up of inner and outer cylindrical shrouds intervened by an annulus containing a number of vanes inclined to the axis and equally spaced apart circumferentially so that between each pair there is a converging passage whose minimum cross-sectional area is called the throat area substantially equal to the other throat areas. The sum of the throat areas of the vane rings (called the total throat area) determines the amount of air the ring is adapted to pass.

DESCRIPTION OF THE PRIOR ART

Different engines of the same model have different total throat area requirements. So, it is necessary for an engine manufacturer to provide a number of vane rings of the same circumference, having different total throat areas.

One-piece cast metal vane rings, of the type to which the invention applies, are made by casting them in one piece by the lost wax process. A number of different tools are required to make the wax or plastic pattern, and each time a change is made in total throat area, a new set of tools is required. For example, as many as seven tools have been required to obtain the optimum total throat area for a particular engine model while under development and in production.

SUMMARY OF THE INVENTION

The applicatn has now developed a vane ring design which can be produced with a limited number of tools, while providing a large variety of total throat areas in a vane ring of the same circumference. In this design, instead of the respective throat areas between each pair of blades being equal as in the prior art, there are at least two different throat areas, but the total throat area provides the total throat area required of the particular engine.

This is effected, according to the invention, as follows. Some of the vanes are at a different angle from the axis of the engine to other vanes, so that the trailing edges of some of the vanes are offset from a plane perpendicular to the axis of the engine as compared with the trailing edges of adjacent vanes. Through this arrangement throat areas of different dimensions result. The total throat area is thus changed by a small increment for each vane arranged at a different angle. This design, as will be seen, greatly facilitates manufacture of vane rings of the same circumference, having different total throat areas, which can be matched with different throat area requirements of the engine model to which the vane rings are fitted.

In making a vane ring according to prior art, a composite wax assembly is first made by assembling together, in a holder, a plurality of separate, but identical, wax or plastic patterns, in a manner well known. The wax patterns are made by first preparing an expensive steel mold of the desired size and shape. An assembly fixture also has to be made. According to this procedure each time a change in total throat area has to be made, a new set of identical patterns is required. These have to be prepared by first making a new steel mold to be used in the procedure described.

In accordance with the present invention at least two different patterns are made to provide, in turn, at least two different throat areas such that, when the patterns are assembled together to form the ring pattern, a predetermined total throat area is provided. By varying the number of patterns of each angle making up the composite ring pattern, a number of different total throat areas are possible by selecting different numbers of each vane angle pattern. By providing more than two different patterns having respectively different angles from the axis of the engine, a still greater number of different total throat areas can be provided to reduce considerably the number of expensive steel molds needed to vary the total throat area of a ring of a given circumference, for various engine throat area requirements. The different air foil waxes or plastic pieces are arranged so that the same assembly fixture can be used.

BRIEF DESCRIPTION OF THE DRAWING

Having thus generally described the invention, it will be referred to, in more detail, by reference to the accompanying drawings, illustrating preferred embodiments, and in which:

FIG. 1 is a front view of a vane ring showing the leading edges;

FIG. 2 is a rear view of a vane ring showing the trailing edges;

FIG. 3 is a cross-section along lines 3--3 of FIG. 1;

FIG. 4 is an elevation showing two wax or plastic pattern pieces, fitted together in a sub-assembly as they are in a pattern;

FIG. 5 is a front perspective view of the assembly shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring more particularly to the drawings, FIG. 1 shows a special high temperature resistant alloy cast vane ring A made up of an outer shroud 15 and an inner shroud 17 forming therebetween an annulus. Within the annulus, and extending between the shrouds 15 and 17, and integrally connected thereto are a number of spaced apart vanes 19, 21, 23, 25, etc. having respective leading edges 19a, 21a, etc. and trailing edges 19b, 21b, etc. The trailing edge of the inner shroud 17 is shown as 17a. The edge 17a is perpendicular to the axis of the vane ring. Adjacent vanes, for example, as demonstrated by the vanes 19 and 21 (FIG. 3) enclose a throat area as indicated by an arrow 22, and the adjacent vanes 21 and 23 enclose a throat area as indicated by arrow 24, while the throat area between the adjacent vanes 23 and 25 is indicated by the arrow 26.

In accordance with the invention, the throat areas vary between certain pairs of vanes. This is accomplished in accordance with the present invention by varying the inclination or angle of certain of the vanes as compared with other vanes as indicated diagrammatically (exaggerated) in FIG. 3 in connection with the blades 19 and 21 so that the passage between the vanes is of a different minimum cross-section or throat area from that between vanes 21 and 23. The inclination of the individual vane is varied about a pivot point, for example shown at X on a plane parallel to the trailing edge 17a of the shroud 17. In FIG. 3 the vane 19 is shown pivoted counterclockwise about the point X as compared to the vanes 21 and 23 so its trailing edge 19b is slightly out of the plane of the trailing edges 21b and 23b. It will be noted, from the drawing, that the trailing edge 19b of the blade 19 is raised relative to the trailing edge 17a of the inner shroud 17, as compared with the trailing edges 21b and 23b. Likewise, the leading edge 19a is lower than the leading edges 21a and 23a. Consequently, the width of the passage between the blades 19 and 21, as shown by the arrow 22, is less than the width of either of the passages between the pairs of vanes 21, 23 and 23, 25 respectively, as indicated by the arrows 24 and 26. So the throat area between the vanes 19 and 21 is smaller than that between the vanes 21 and 23.

The vane ring A is made by assembling wax or plastic pieces 28, 30, etc. and fitting them together in a fixture to produce the ring pattern which is invested with a skin of a material which becomes the mold, as is well known in the investment casting art. Two pattern pieces are shown fitted together in a sub-assembly B. The fully assembled pattern is of the same shape as the vane ring to be cast. According to the invention, a plurality of different shaped pattern pieces are made, that is to say pieces in which the inclination of certain of the vanes differs from that of other vanes. A preferred number of patterns is three, one having a maximum vane inclination, a second, a minimum inclination and the third an intermediate inclination. What is meant by different shaped composite patterns is patterns which have at least two different vane angles and consequently provide different throat areas between vanes.

A typical example of a vane ring A is as follows. The inner shroud 17 is 9 inches and the outer shroud 15 12 inches in outside diameter. The vanes are approximately 1 and 1/2 inches in radial height. The required total throat area is 13.333 square inches. The ring A has 15 vanes. Vanes 19 and 21 of different angles are used. The angles of vanes 19 and 21 are such that 15 vanes would provide a total throat area of 13.035 square inches using vane 19 alonge and 13.630 square inches for using vane 21 alone. One assembly, using eight of vane 19 and seven of vane 21 generated a total throat area of 13.31 square inches. Another assembly also made up of eight vanes 19 and seven vanes 21 generated a total throat area of 13.35 square inches. This variation is within an acceptable tolerance for total throat areas, since even when making one piece cast metal vane rings by the conventional method, there is always a certain difference in total throat area, resulting from unavoidable slight variations in the assembly technique. This variation in total throat area is normally acceptable when within a .+-. 1 percent tolerance. The percentage throat area difference between a ring made up entirely of vanes having the maximum angle of the vane 19 to the axis of the ring and a ring made up entirely of vanes 21 having a minimum angle to the axis of the ring is between about 4 percent and about 4-1/2 percent total throat area variation. In other words, the mixing of wax or plastic pieces with vanes at different angles permits this relatively wide range of variation in total throat area.

According to the invention, then, the intentional or built-in variation between the two or more different vane angles is considerably greater than any difference which may occur accidentally, even though the variation is quite small. But, the built-in variation is great enough that a plurality of patterns, having different vane angles, can be assembled to provide for a number of different total throat areas, ranging from the total provided by using all patterns of the same angle which would provide the minimum total throat area to the total which provided using all patterns which would provide the maximum throat area. A great number of inbetween variations are possible by different combinations of the respective patterns.

The materials employed in making the vane rings of the invention are well known in the art. For example, the vane ring itself is preferably of a special high temperature resistant alloy. The materials used in the investment casting process are also well known in the investment casting art as are the techniques used. One technique has been used to illustrate the invention and is preferred, but other techniques can equally be used.

The principle of the invention then is to "classify" each individual vane in its pre-investment form (wax or plastic model), select an appropriate vane class, mix and distribution, assemble, invest, etc. and cast. This effectively duplicates the procedure used with individual vane segments prior to the introduction of the integral vane ring. By selecting at least two vane angles, preferably at least three vane angles, a wide selection of total throat areas can be produced. By stocking wax or plastic pieces of different vane angles, the time required to obtain a new vane ring class can be greatly reduced, as it is only necessary to assemble, cast and machine.

Claims

I claim:

1. A one-piece cast metal gas-turbine vane ring, comprising,

spaced apart inner and outer annular shrouds having a common axis extending therebetween,

a plurality of substantially equally spaced apart vanes of the same size, shape and aerodynamic characteristics,

said vanes being inclined at a generally common angle to said axis and having respective trailing edges lying close to a plane perpendicular to said axis with each pair of vanes providing therebetween a passage having a particular throat area,

at least one of said vanes being inclined at a specific minimum angle to said axis and at least one other blade being inclined at a specific maximum angle to said axis such that the difference between the total throat area of the vane ring if all the blades were at said minimum angle and the total throat area of the vane ring if all the blades were at said maximum angle would be not less than 1 percent and not more than 4-1/2 percent in the total throat area variation.

2. A metal vane ring, as defined in claim 1, in which there are vanes having at least three different angles.

3. A metal vane ring, as defined in claim 1, in which said maximum and minimum angles are such that the difference between the total throat area of the vane ring if all the blades were at said minimum angle and the total throat area of the vane ring if all the blades were at said maximum angle would be not less than 2 percent and not more than 4-1/2 percent in the total throat area variation.

4. A metal vane ring, as defined in claim 1, in which there is at least one vane set at an angle intermediate to said maximum and minimum angles.