GB2159895A - Stepped-tooth rotating labyrinth seal - Google Patents

Abstract

A rotating labyrinth seal especially useful for effecting sealing between two plenums in aircraft gas turbine engines comprising a base and a plurality of radially-directed seal teeth rings extending circumferentially around the outer peripheral surface of the base. Each of the seal teeth rings has a body portion 77, and a tip portion 78, with the body portion being substantially thicker than the tip portion. Seal teeth rings of the invention exhibit improved resistance to fatigue crack propagation (Fig.4 not shown) compared to conventional seal teeth rings (Fig. 3B). <IMAGE>

Description

SPECIFICATION Stepped-tooth rotating labyrinth seal Background of the Invention The present invention relates generally to rotating labyrinth seals and particularly to rotating labyrinth seals used in gas turbine engines for the propulsion of aircraft.

Rotating labyrinth seals have a wide variety of uses and one such use is to effect sealing between plenums at different pressures in the gas turbine engines. Such seals generally consist of two principal elements, i.e., a rotating seal and a static seal. The rotating seal, in cross section parallel to the axial length of the engine, frequently has rows of thin tooth-like projections extending from a relatively thicker base radially toward the static seal.

The static seal or stator is normally comprised of a thin honeycomb ribbon configuration. These principal elements are generally situated circumferentially about the axial (lengthwise) dimension of the engine and are positioned with a small radial gap therebetween to permit assembly of the rotating and static components.

When the gas turbine engine is operated, the rotating seal expands radially more than the stator and rubs into the stator seal. The thin honeycomb ribbon construction of the stator reduces the surface area on which the seal teeth rub and thus helps to minimize the heat transferred into the rotating seal.

In addition, the rotating seal teeth tips are made thin in order to thermally isolate them from the surrounding base or shell structure.

The thin tooth configuration is, however, susceptible to handling damage which can result in cracks in the tips of the teeth opposite the base. In some cases, operational, rub-generated cracks may also be formed on the tooth tips. These seal tooth cracks propagate through the teeth radially inward into the supporting shell structure and, left undetected, can eventually spread towards the ends of the support structure resulting in failure of the seal.

The cross-sectional configuration of the seal teeth in the prior art is generally one of a truncated triangular configuration with straight sloping sides meeting at a thin, flat tip. Such prior art configurations of the seal teeth have provided insufficient stalling or arresting of such cracks once they have begun.

Summary of the Invention It is therefore a general object of this invention to provide an improved configuration for the seal teeth of rotating labyrinth seals which provides a greatly increased resistance to crack propagation.

Another important object of this invention is to provide this novel seal tooth configuration which resists crack propagation while still maintaining a thin, lightweight tooth necessary and desirable in gas turbine engines for aircraft propulsion.

These and other objects of the invention are attained by providing a rotating labyrinth seal comprising a plurality of radially-directed seal teeth rings extending circumferentially around the outer periphery of a base, each of the seal teeth rings having a body portion and a tip portion, the body portion being substantially thicker than the tip portion.

More specifically, the body portion has generally parallel body walls which extend radially from the base portion and circumferentially about the base and axial centerline of the engine. The body further has circumferential walls which form the outer circumference of the body portion and lie on either side of the tip portion. The tip portion similarly has generally parallel tip walls extending radially from the circumferential walls of the body and circumferentially about the axial centerline of the engine. The tip walls terminate at a circumferential tip surface which forms the outer circumference of the tip portion.

In order to realize the crack stalling benefits of the invention, the ratio of the thickness of the body to that of the tip should be at least about 5:1.

Additionally, the tip walls should join the body circumferential walls at a fillet whose radius is approximately equal to the thickness of the tip portion. Teeth of the invention have shown at least a 3:1 improvement in tooth and seal life over that of teeth of prior art configurations.

Brief Description ofthe Drawings For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.

FIGURE 1 is a simplified cross-sectional view, in partial cutaway, of an aircraft gas turbofan engine; FIGURE 2 is a partial cross-sectional view of a typical two-stage high pressure turbine of a gas turbine engine; FIGURE 3A is a partial cross-sectional view of a stepped tooth of the present invention for use with a rotating labyrinth seal; FIGURE 3B is a partial cross-sectional view ofa I typical prior art tooth of a rotating labyrinth seal; and FIGURE 4 is a graph of crack length versus tensions-tension fatigue cycles for two rotating labyrinth seal tooth configurations.

Description of the Preferred Embodiments Referring to FIG. 1 of the drawings, there is diagrammatically illustrated a gas turbofan engine, generally designated by the numeral 20. While it is recognized that turbofan engines are well known in the art, a brief description of the operation of engine 20 will enhance appreciation of the interrelationship of the various components by way of background for the invention to be described below. Basically, engine 20 may be considered as comprising core engine 22, fan 24 including a rotatable stage of fan blades 26, and fan turbine 28 downstream of core engine 22 and which is interconnected to fan 24 by shaft 30. Core engine 22 includes axial flow compressor 32 having rotor 34. Air enters inlet 36 from the left of FIG. 1, in the direction of the solid arrow, and is initially compressed by fan blades 26.

A fan cowl or nacelle 38 circumscribes the forward part of engine 20 and is interconnected therewith by a plurality of radially outwardly extending outlet guide vane assemblies 40 (one shown), substantially equiangularly spaced apart around core engine cowl 42. A first portion of the relatively cool, low pressure compressed air exiting fan blades 26 enters fan bypass duct 44 defined between core engine cowl 42 and fan cowl 38, and discharges through fan nozzle 46. A second portion of the compressed air enters core engine inlet 48, is further compressed by axial flow compressor 32, and is discharged to combustor 50 where it is mixed with fuel and burned to provide high energy combustion gases which drive core (or high pressure) engine turbine 52. Turbine 52, in turn, drives rotor 34 by means of shaft 35 in the usual manner of gas turbine engines.The hot gases of combustion then pass through and drive fan (or low pressure) turbine 28 which, in turn, drives fan 24. A propulsive force is thus obtained by the action of fan 24 discharging air from fan bypass duct 44 through fan nozzle 46 and by the discharge of combustion gases from core engine nozzle 54 defined, in part, by plug 56 and cowl 42 of core engine 22. It will be appreciated that the pressure of the various gases within the engine 20 will vary as a function of position along engine axial centerline 58. To isolate the various sections and the pressures therein from each other, rotating labyrinth seals are commonly used.

Referring to FIG. 2 of the drawings, there is shown a partial view of a high pressure turbine section generally designated 60 which is a section of aircraft gas turbine engines which typically utilize rotating labyrinth seals. The high pressure turbine 60 includes a plurality of radially extending stage-one blades suitably mounted in stage-one turbine disks, one set of which is shown and labeled 62 and 74, respectively. High pressure turbine 60 also includes a plurality of radially extending, stage-two blades suitably mounted in stage-two turbine disks, one set of which is shown and labeled 66 and 68, respectively. Stage-one blade 62 and disk 64 lie upstream in relation to downstream stage-two blade 66 and disk 68. The flow of hot gases in high pressure turbine 60 is from upstream to downstream, i.e., from left to right in FIG. 2.

The high pressure turbine 60 further includes a rotating labyrinth seal 70 and a stator or static seal 71. Rotating labyrinth seal 70 is suitably mounted between the stage-one turbine disk 64 and the stage-two-turbine disk 68. Stationary static seal 71 is attached to stage-two nozzle 73. The stage-one nozzle (not shown) lies upstream from the stage-one blades.

Rotating labyrinth seal 70 comprises base 72 and a plurality of seal teeth 74 radially extending from the outer peripheral surface 75 of base 72. The outer circumference of the seal teeth 74 rotate within a small tolerance of the inner circumference of the stator 71, thereby effecting a sealing between stageone plenum 61 and stage-two plenum 63. Base 72 as shown has an annular configuration and a generally arcuate cross section, but other configurations are frequently encountered in gas turbine engines. Seal teeth 74 may be attached to, as by welding, or be integrally machined in seal 70 and extend in ringlike fashion circumferentially about base 72 and axial centerline 58.

Each of the seal teeth 74 according to the present invention, as better shown in FIG. 3A, has body portion 77 and tip portion 78. Each of body portions 77 has generally parallel body walls 79 and 80 which extend radially from the base and circumferentially about the centerline axis of the engine. Body portion 77 also has body circumferential walls 82 and 84 which lie along the outer circumference of body portion 77 on either side of tip portion 78.

Each of tip portions 78 include generally parallel and radially extending tip walls 86 and 88 extending radially from circumferential walls 82 and 84, respectively. Tip portion 78 further has circumferential tip surface 90 which lies along the outer circumference of the tip portion 78, in which tip walls 82, 84 terminate, which surface 90 is coaxial with body circumferential walls 82, 84. In contrast to the seal teeth of the present invention shown in FIG. 3A, there is shown in FIG. 3B a prior art seal tooth 74' which is generally of truncated triangular shape. Generally, 8 is on the order of 15 and the width of tip surface 90' (tut) is on the order of 0.015 inches.

In accordance with the present invention, in order to obtain effective stalling or arresting of propagating cracks the ratio of the distances between body walls 79,80 (body thickness or tb) and tip walls 86,88 (tip thickness or tt) should be at least about 5:1. Preferably, tip walls 86,88 should meet body circumferential walls 82,84 without a transition (fillet) radius, butforfatigue resistance some radius at that juncture is desirable and a juncture or fillet radius, t" approximately equal to the tip thickness, tt, has been found to be the preferred radius.

The effectiveness of seal teeth according to the present invention is shown in FIG. 4. Therein, crack length is plotted as a function of tension-tension fatigue cycles. The tension-tension tests replicate the fatigue-causing hoop stresses to which such seals are commonly exposed during engine operation. A segment of a conventional seal tooth having 6 = 15 tt = 0.015 inches was tested against a segment of a seal tooth of the present invention having tt = 0.015 inches, tb = 0.080 inches, and t5 = 0.015 inches. The material of the teeth was a nickelbase alloy from which such seals are typically manufactured and the initial crack length (X) was 0.020 inches. As can be seen from FIG. 4, the seal tooth of the present invention exhibited a 3 to 1 improvement in cycles to failure over that of the conventional seal tooth.

The invention consists of certain novel features and a combination of parts hereinbefore fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantaqes of the present invention.

Claims (11)

1. A rotating labyrinth seal, comprising a base having an outer peripheral surface and a plurality of radially directed seal teeth integral with said peripheral surface and extending circumferentially about said outer peripheral surface of said base, each of said seal teeth having a body portion and a tip portion, said body portion being substantially thicker than said tip portion.

2. The rotating labyrinth seal of claim 1, wherein each of said body portions has a pair of radially extending body walls which are substantially parallel, the distance between said body walls being the thickness of said body, and wherein each of said tip portions has a pair of radially extending tip walls which are substantially parallel to each other and to said body walls, the distance between said tip walls being the thickness of said tip, said tip walls terminating in a circumferentially extending tip surface.

3. The rotating labyrinth seal of claim 2, wherein the ratio of said body thickness to said tip thickness is at least about five to one.

4. The rotating labyrinth seal of claim 2, further including body circumferential surfaces which lie along the outer circumference of said body portion on either side of the associated tip portion.

5. The rotating labyrinth seal of claim 4wherein said tip walls join said body circumferential surfaces at a fillet having a radius substantially equal to the thickness of said tip walls.

6. A rotating labyrinth seal adapted to effect sealing between two plenums in gas turbine engines, comprising a base having an outer peripheral surface, and a plurality of radially directed seal teeth rings extending circumferentially around said outer peripheral surface of said base, each of said seal teeth rings having a body portion and a tip portion, each of said body portions having a pair of radially extending substantially parallel body walls, the distance between said body walls being the thickness of said body, each of said tip portions having a pair of radially extending tip walls substantially parallel to each other and to said body walls, the distance between said tip walls being the thickness of said tips and wherein the ratio of said body thickness to said tip thickness is at least about five to one.

7. The rotating labyrinth seal of claim 6, wherein each of said body portions has a pair of body circumferential surfaces situated on opposite sides of the associated tip portion.

8. The rotating labyrinth seal of claim 7, wherein each of said tip portions has a tip circumferential surface coaxial to said pair of body circumferential surfaces of the associated body portion.

9. The rotating labyrinth seal of claim 8, wherein said tip walls join said body circumferential surfaces at a fillet having a radius substantially equal to the thickness of said tip walls.

10. The rotating labyrinth seal of claim 6 wherein said base is annular and of an arcuate cross section.

11. A rotary seal substantially as hereinbefore described with reference to Figures 2, 3A and 4 of the drawings.