Information
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Patent Grant
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6491498
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Patent Number
6,491,498
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Date Filed
Thursday, October 4, 200122 years ago
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Date Issued
Tuesday, December 10, 200221 years ago
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Inventors
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Original Assignees
-
Examiners
Agents
- Mack; Brian R.
- Hayes; Christopher T.
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CPC
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US Classifications
Field of Search
US
- 416 189
- 416 190
- 416 191
- 416 192
- 416 195
- 416 92
- 416 97 R
- 415 1734
- 415 1735
- 415 1736
- 415 228
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International Classifications
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Abstract
An improved gas turbine blade is disclosed that utilizes a blade tip shroud with tapered shroud pockets to remove excess weight from the shroud while not compromising shroud bending stresses. Excess shroud weight removed by the tapered pockets reduces overall blade pull while maintaining material necessary to increase shroud stiffness. The resulting rib created between the tapered pockets provides a surface, away from possible areas of stress concentration, for drilling radial cooling holes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to shrouded turbine blades and more specifically to the mass reduction of shrouded turbine blades while not compromising the resistance to shroud bending stresses.
2. Description of Related Art
Gas turbine blades are rotating airfoil shaped components in series of stages designed to convert thermal energy from a combustor into mechanical work of turning a rotor. Performance of a turbine can be enhanced by sealing the outer edge of the blade tip to prevent combustion gases from escaping from the flowpath to the gaps between the blade tip and outer casing. A common manner for sealing the gap between the turbine blade tips and the turbine casing is through blade tip shrouds. Not only do shrouds enhance turbine performance, but they serve as a vibration damper, especially for larger, in radial length, turbine blades. The shroud acts as a mechanism to raise the blade natural frequency and in turn minimizes failures due to extended resonance time of the blade at a natural frequency. A portion of a typical turbine blade with a shroud is shown in FIG.
1
. The figure shows turbine blade
10
with an airfoil section
11
and shroud
12
. The shroud is manufactured integral to the airfoil
11
. The airfoil further contains a leading edge
15
and trailing edge
16
that run generally perpendicular to shroud
12
. Shroud
12
has a thickness and has sidewalls
17
, which are cut to create an interlocking configuration when adjacent turbine blades are present. The interlocking mechanism occurs along two bearing faces
13
. That is, along bearing face
13
is where adjacent turbine blades (not shown) contact shroud
12
. It is the interlocking of the turbine blade shrouds
12
at bearing faces
13
, that creates the means for damping out vibrations as well as for sealing the hot combustion gases within the turbine gas-path. An additional feature of a typical turbine blade shroud is knife edge
14
. Depending upon the size of the blade shroud, one or more knife edges may be utilized. These seals run parallel to each other, typically perpendicular to the engine axis, and extend outward from shroud
12
. The purpose of these seals is to engage the shroud blocks of the turbine casing (not shown) to further minimize any leakage around the blade tip. While the purpose of the shroud is to seal the combustion gases within the flow path as well as to provide a means to dampen vibrations, the shroud has its disadvantages as well.
A drawback to the shroud concept is the weight the shroud adds to the turbine blade. During operation, the turbine blades spin on a disk, about the engine axis. A typical industrial application includes disk speeds up to 3600 revolutions per minute. The blades are held in the disk by an interlocking cut-out between the blade root and the disk. As the turbine blade spins, the centrifugal forces cause the blade to load outward on the turbine disk at this attachment point. The amount of loading on the disk and hence the blade root, which holds the blade in the disk, is a function of the blade weight. That is, the heavier the blade, the more load and stresses are found on the interface between the blade root and disk, for a given revolutions per minute. Excessive loading on the blade root and disk can reduce the overall life of each component.
Another drawback to shrouds is creep curling of the blade shrouds. Depending on the thickness of the shroud, the shroud edges can “curl” up at their ends and introduce severe bending stresses in the fillets between the shroud and blade tip. Shrouds curl due to the bending load on the edges of the shroud from gas pressure loads as well as centrifugal loads. The curling of a shroud is analogous to the bending of a cantilevered beam due to a load at the free end of the beam. An industry known fix to this curling phenomenon is to increase the section thickness of the shroud uniformly which will result in a stiffer shroud and more resistance to curling. The downside to simply increasing the shroud thickness uniformly is the additional weight that is added to the shroud by this additional material.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide an improved turbine blade shroud that reduces the overall blade mass, which in turn reduces the amount of pull on the turbine disk, increasing the life of both the turbine blade root and corresponding disk locations.
It is a further object of the present invention to provide an improved turbine blade shroud that does not compromise shroud bending stresses or mass balance of the shroud.
It is yet another object of the present invention to provide an improved turbine blade shroud that includes a rib section along the shroud for drilling cooling holes such that no high stress concentrations occur from these cooling holes.
In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.
The present invention addresses the issue of eliminating shroud curling while reducing shroud weight, and hence, overall turbine blade weight and not compromising shroud bending stresses. The improved shroud structure removes mass from the shroud while not compromising the structural integrity or performance requirements of the shroud or the overall blade structure. The excess mass is removed by inserting ramped pockets on the outermost surface of the shroud at locations where the mass is not necessary to reduce stress levels at the airfoil to shroud transition.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1
is a perspective view of a typical turbine blade shroud.
FIG. 2
is a perspective view of the turbine blade and shroud of the present invention.
FIG. 3
is a perspective view of the shroud portion of the present invention.
FIG. 4
is a top elevation view of the shroud portion of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment of the improved turbine blade is shown in FIG.
2
. Turbine blade
40
is cast as a single piece construction with multiple details machined into the casting. The turbine blade is comprised of a root section
41
, that may contain a neck area
42
, outward from said root section. Root section
41
contains machined surfaces
43
that match the profile of the turbine disk (not shown) for interlocking said blade root and said turbine disk such that the turbine blade
40
is contained with the turbine disk under centrifugal loads. Outward of said blade root and neck region is a platform section
44
, which is generally planar in shape and connected to root section
41
. Extending outward from platform
44
is an airfoil
45
. The airfoil has an end connected to platform
44
and a tip end, which is connected to a shroud
46
. The shroud
46
is shown in greater detail in FIG.
3
.
FIG. 3
shows the tip of airfoil
45
along with shroud
46
. The shroud extends outward from airfoil
45
and contains a first surface
47
that is fixed to the tip and contoured to the airfoil tip as well as second surface
48
outward of and parallel to surface
47
. Connecting o surfaces
47
and
48
are a plurality of radially extending sidewalls
49
that are generally perpendicular to surfaces
47
and
48
. The surfaces and sidewalls give shroud
46
its thickness that is addressed by the present invention. Shroud
46
also contains knife edges
50
extending outward from surface
48
. Knife edges
50
run across surface
48
and terminate at ends
51
, which are coincident with sidewalls
49
. An additional feature of shroud
46
is recessed pockets
52
in surface
48
and adjacent knife edges
50
. The recessed pockets
52
are separated by a rib
53
and are tapered in depth, with the pockets deepest at regions adjacent the sidewalls
49
and knife edge ends
51
. In the preferred embodiment the improved turbine blade contains two triangular shaped recessed pockets separated by a rib section. Recessed pockets
52
are cast into shroud
46
to remove excess material not required for improved shroud stiffness. This removed material reduces the overall shroud weight and as a result, reduces the amount of blade pull on the turbine disk, as discussed earlier. The material is removed in a tapered fashion such that material necessary to reduce shroud bending stresses, through improved section thickness, which is adjacent rib
53
remains, where as material that is not needed to reduce shroud bending stresses is removed.
FIG. 4
, shows a top elevation view of shroud
46
with recessed pockets
52
, separated from each other by rib
53
. Depending upon engine operating conditions, air or steam cooling of the turbine blade may be required. If such cooling is required, a plurality of cooling holes
54
extending from rib
53
of shroud
46
to cooling passages within the airfoil platform, and blade root section can be machined into the turbine blade. Rib
53
provides a flat plane section for drilling the cooling holes. This rib will ensure that the holes
54
are located far enough away from the recessed shroud pockets so that fillets and corners of the recessed pockets are not cut during drilling of the cooling holes, thereby avoiding the undesirable stress concentrations that can result from sharp corners at the interface of a fillet/corner and the edge of a cooling hole.
While the invention has been described in what is known as presently the 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 within the scope of the following claims.
Claims
- 1. An improved turbine blade comprising:a root section, a platform section of generally planar shape connected to said root, an airfoil extending outward from said platform, said airfoil having a platform end connected to said platform, and a tip end opposite said platform end, a shroud extending outward from said tip end and attached thereto, said shroud comprising: a first surface fixed to said tip end of said airfoil; a second surface in spaced relation to and generally parallel to, said first surface; a plurality of radially extending sidewalls, generally perpendicular to, and connecting said first and second surfaces; a pair of knife edges extending outward from said second surface, each of said knife edges extending across said second surface and having knife ends at said sidewalls; at least one recessed pocket in said second surface, said pocket extending towards said first surface to a predetermined depth wherein each of said at least one recessed pockets varies in depth from the center of said shroud towards said radially extending sidewalls, with the deepest location of each of said at least one recessed pockets immediately adjacent said knife ends.
- 2. The improved turbine blade of claim 1 wherein each of said at least one recessed pockets is generally triangular in shape.
- 3. The improved turbine blade of claim 1 wherein each of said at least one recessed pockets is located adjacent said knife edges.
- 4. The improved turbine blade of claim 1 wherein each of said at least one recessed pockets varies in depth from the center of the shroud towards said radially extending sidewalls, with the deepest location of each of said at least one recessed pockets immediately adjacent one of said sidewalls.
- 5. The improved turbine blade of claim 1 wherein each of said at least one recessed pockets are separated by a rib which extends between said knife edges and is fixed to said second surface, said rib including a plurality of cooling holes, said cooling holes extending from said rib towards said root and through said turbine blade.
- 6. The improved turbine blade of claim 1 wherein said radially extending sidewalls of said shroud are configured in a “Z”-like shape.
- 7. An improved turbine blade comprising:a root section, a platform section of generally planar shape connected to said root, an airfoil extending outward from said platform, said airfoil having a platform end connected to said platform, and a tip end opposite said platform end, a shroud extending outward from said tip end and attached thereto, said shroud comprising: a first surface fixed to said tip end of said airfoil; a second surface in spaced relation to and generally parallel to, said first surface; a plurality of radially extending sidewalls, generally perpendicular to, and connecting said first and second surfaces; at least one knife edge extending outward from said second surface, said at least one knife edge extending across said second surface and having ends at said sidewalls; at least one recessed pocket in said second surface, said at least one pocket extending towards said first surface to a predetermined depth, having a generally triangular shape, and is located adjacent said knife edge.
- 8. The improved turbine blade of claim 7 wherein each of said at least one recessed pockets varies in depth from the center of the shroud towards said radially extending sidewalls, with the deepest location of each of said at least one recessed pockets adjacent said ends of said knife edge.
- 9. The improved turbine blade of claim 8 wherein each of said at least one recessed pockets varies in depth from the center of the shroud towards said radially extending sidewalls, with the deepest location of each of said at least one recessed pockets adjacent one of said sidewalls.
- 10. The improved turbine blade of claim 9 wherein each of said at least one recessed pockets are separated by a rib which extends generally perpendicular across said knife edge and is fixed to said second surface, said rib including a plurality of cooling holes, said cooling holes extending from said rib towards said root and through said turbine blade.
- 11. The improved turbine blade of claim 9 wherein said radially extending sidewalls of said shroud are configured in a “Z”-like shape.
US Referenced Citations (11)