US20080232972A1

US20080232972A1 - Blade fixing for a blade in a gas turbine engine

Info

Publication number: US20080232972A1
Application number: US11/690,452
Authority: US
Inventors: Richard Bouchard; Patrice Ekedi
Original assignee: Pratt and Whitney Canada Corp
Current assignee: Pratt and Whitney Canada Corp
Priority date: 2007-03-23
Filing date: 2007-03-23
Publication date: 2008-09-25
Also published as: CA2622945A1

Abstract

The blade fixing comprises a firtree having a width between two opposite surfaces, each defining a plurality of successive necks separated from one another by arcuate lobes. Each of the opposite surfaces is substantially flat and substantially parallel to the other at the narrowest point of a first one of the necks that is closest to the platform.

Description

TECHNICAL FIELD

The invention relates to an improved fixing for a blade used in a gas turbine engine.

BACKGROUND

Gas turbine engines are often provided with rotors in the compressor and/or the turbine sections thereof where blades are removably connected to the periphery of the corresponding rotor. While many arrangements have been suggested in the past to connect the blades to the rotors, room for further improvements still exists.

SUMMARY

In one aspect, the present concept provides a blade fixing for a rotor blade in a gas turbine engine, the blade having a platform with the blade fixing projecting therefrom, the blade fixing comprising a firtree having a width between two opposite surfaces, each surface defining a plurality of successive necks separated from one another by arcuate lobes, the firtree having a local minimum width at a narrowest portion of each of said necks and a local maximum width at an apex of each of said lobes, each of the opposite surfaces being substantially flat and substantially parallel to the other at the narrowest portion of a first one of the necks that is closest to the platform.
In another aspect, the present concept provides a firtree for a blade in a gas turbine engine, the firtree generally projecting from a platform of the blade, the firtree comprising at least two pairs of opposite arcuate lobes, each lobe of a pair being on a corresponding side, the lobes generally decreasing in size between successive lobes starting from a first of the lobes with reference to the platform, the platform and the first lobe being separated on each of the opposite sides by a neck having a substantially flat section extending parallel to a medial plane of the firtree, the necks uninterruptedly blending to adjacent lobes.
In a further aspect, the present concept provides a fixing for an airfoil blade for a gas turbine engine, the fixing comprising a firtree projecting radially from a platform of the airfoil blade and having a plurality of pairs of opposed lobes, the platform and lobes being areas of maximum fixing width separated by corresponding necks, the necks being areas of minimum fixing width, one of the necks closest to the platform having a radial height which is larger than a radial height of the other necks.
Further details of these and other aspects will be apparent from the detailed description and figures included below.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying figures, in which:

FIG. 1 schematically shows a generic turbofan gas turbine engine to illustrate an example of a general environment in which the improved blade fixing can be used;

FIG. 2 is a perspective view of an example of a blade with an example of the improved blade fixing; and

FIG. 3 is a schematic side view of the bottom part of the blade shown in FIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a gas turbine engine 10 of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan 12 through which ambient air is propelled, a multistage compressor 14 for pressurizing the air, a combustor 16 in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases and a turbine section 18 for extracting energy from the combustion gases. The engine 10 is an example of an environment in which the improved blade fixing can be used. Blades with the improved blade fixing can be used in the fan 12, the compressor section 14 and/or the turbine section 18.
FIG. 2 illustrates an example of a rotor blade 20 with an example of the improved blade fixing, which blade fixing comprises a firtree 22. The blade 20 also comprises a platform 26 under which projects the blade fixing. An airfoil 24 projects over the platform 26 and the firtree 22 projects below the platform 26 on the opposite side. The exact shape of the platform 26 and of the airfoil 24 illustrated in FIG. 2 is not relevant in the present specification.
The firtree 22 includes two opposite profiled and continuous side surfaces 22 a, 22 b. These surfaces 22 a, 22 b are shaped so as to define a plurality of opposite pairs of necks 30, 32, 34 separated from one another by opposite pairs of arcuate lobes 40, 42. They are provided for connecting the blade 20 to a corresponding profiled slot, called disk broach, made in the periphery of a rotor (not shown). A connector element (not shown) is also generally provided to prevent to the blade 20 from axially moving out of its slot. The firtree 22 prevents the blade 20 from moving radially outward under the intense centrifugal force when the rotor is rotated at very high speeds. It also transfers other forces between the rotor and the blade 20.
One of the goals in the design of the blade fixing is to minimize the blade weight while still being capable of withstanding the harsh conditions in which it is subjected. While providing a blade fixing with a firtree design can effectively reduce weight, the design of the blade fixing must take into account the stress concentrations at some of the locations, particularly where the width of the firtree 22 is narrower compared to other locations. These locations correspond to necks of the firtree 22.
In the improved firtree 22, the two necks 30 closer to the platform 26 on the two opposite side surfaces 22 a, 22 b are mutually separated by a width that remains locally minimal over a given radial height (d) instead of being minimal only at a specific point, in which case the radial height is virtually zero. The radial height is measured in a direction parallel to a medial axis M of the firtree 22. The corresponding neck 30 on each side has a portion that is substantially flat and extends parallel to the medial plane M of the firtree 22. These flat neck portions have an infinite or almost infinite radius of curvature and an angle being nil with reference to the medial plane M of the firtree 22. The radius of curvature of the narrow portion of the first necks is larger than the radius of curvature on either side of these portions. There is a local maximum width at an apex of each lobe 40, 42.
Also, each of the three necks 30, 32, 34 has a surface uninterruptedly blending to adjacent surface areas so as to prevent any abrupt junction anywhere on the side surfaces 22 a, 22 b. The junctions are thus made using fillets.
In the illustrated embodiment, the opposite side surfaces 22 a, 22 b are substantially symmetrical with reference to the medial plane M of the firtree 22. Also, as illustrated, the pairs of lobes 40, 42 are two in number and the lobes 40, 42 decrease in size as a function of the distance from the platform 26, the maximum width of the lobes 40, 42 decreasing towards the extremity of the firtree 22. Necks 32, 34 away from the platform 26 have an arcuate profile at a narrowest portion of such necks 32, 34. Alternative designs are possible as well, especially for the tip 44 of the firtree 22, which illustrated tip 44 is only optional.
The above description is meant to be exemplary only, and one skilled in the art will recognize that other changes may also be made to the embodiments described without departing from the scope of the invention disclosed as defined by the appended claims. For instance, the present invention is not limited to a blade as exactly illustrated herein. The blades can have shapes and proportions that vary in accordance with the needs. The flat neck portions can be provided anywhere on the firtree. Using a plurality of flat neck portions simultaneously is also possible. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.

Claims

1. A blade fixing for a rotor blade in a gas turbine engine, the blade having a platform with the blade fixing projecting therefrom, the blade fixing comprising a firtree having a width between two opposite surfaces, each surface defining a plurality of successive necks separated from one another by arcuate lobes, the firtree having a local minimum width at a narrowest portion of each of said necks and a local maximum width at an apex of each of said lobes, each of the opposite surfaces being substantially flat and substantially parallel to the other at the narrowest portion of a first one of the necks that is closest to the platform.

2. The blade fixing as defined in claim 1, wherein the remainder of the plurality of successive necks have an arcuate profile at the narrowest portion of the such necks.

3. The blade fixing as defined in claim 1, wherein each of the surfaces have a radius of curvature at the narrowest portion of the first neck which is larger than a radius of curvature on either side of the narrowest portion of the first neck.

4. The blade fixing as defined in claim 1, wherein the narrowest portion of each of the first necks has a radius of curvature which is larger than a radius of curvature at the narrowest portion of any other neck of the plurality.

5. The blade fixing as defined in claim 1, wherein the opposite side surfaces are substantially symmetrical with reference to a medial plane of the blade fixing.

6. The blade fixing as defined in claim 1, wherein the lobes decrease in width as a function of a distance between the lobes and the platform.

7. The blade fixing as defined in claim 1, wherein opposite portions on the surfaces immediately adjacent to the platform are part of the first necks.

8. A firtree for a blade in a gas turbine engine, the firtree generally projecting from a platform of the blade, the firtree comprising at least two pairs of opposite arcuate lobes, each lobe of a pair being on a corresponding side, the lobes generally decreasing in size between successive lobes starting from a first of the lobes with reference to the platform, the platform and the first lobe being separated on each of the opposite sides by a neck having a substantially flat section extending parallel to a medial plane of the firtree, the necks uninterruptedly blending to adjacent lobes.

9. The firtree as defined in claim 8, wherein the firtree is substantially symmetrical with reference to the medial plane.

10. A fixing for an airfoil blade for a gas turbine engine, the fixing comprising a firtree projecting radially from a platform of the airfoil blade and having a plurality of pairs of opposed lobes, the platform and lobes being areas of maximum fixing width separated by corresponding necks, the necks being areas of minimum fixing width, one of the necks closest to the platform having a radial height which is larger than a radial height of the other necks.

11. The fixing as defined in claim 10, wherein the sides of the firtree have profiles that are substantially symmetrical with reference to the medial plane.

12. The fixing as defined in claim 11, wherein the lobes decrease in size as a function of the distance of the lobes from the platform.