# Patent

## Title
Variable dual spring blade root support for gas turbines

## Abstract

An adjustable blade root spring device for turbine blade fixation in turbomachinery. The device is designed to be placed in a space in a rotor disk cavity adjacent to a tip of a blade root, where the device applies a radial outward force on the turbine blade to fix the blade position in the rotor disk. The device includes a wave spring with integral end blocks which is compressed by a bolt and a coil spring. When the wave spring is compressed in length, it increases in height and makes contact with the rotor disk and the turbine blade. The force of the wave spring on the turbine blade can be adjusted via the bolt, and the coil spring provides an increased compliance range. The body of the device has an oblong cross-sectional shape, thereby preventing rotation of the device in the space between the blade and the disk.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part application of U.S. patent application Ser. No. 14/457,504, titled ADJUSTABLE BLADE ROOT SPRING FOR TURBINE BLADE FIXATION IN TURBOMACHINERY, filed Aug. 12, 2014, which claimed the benefit of the priority date of U.S. Provisional Patent Application Ser. No. 61/892,824, titled ADJUSTABLE BLADE ROOT SPRING FOR TURBINE BLADE FIXATION IN TURBOMACHINERY, filed Oct. 18, 2013.
This application also claims the benefit of the priority date of U.S. Provisional Patent Application Ser. No. 62/111,785, titled ADJUSTABLE BLADE ROOT SPRING FOR TURBINE BLADE FIXATION IN TURBOMACHINERY, filed Feb. 4, 2015.


BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates generally to a device for fixing a turbine blade's position relative to a rotor disk in a combustion gas turbine and, more particularly, to an adjustable blade root spring device which can be freely inserted in a space beneath the blade root, then compressed via an axial bolt so that a wave spring increases in height and presses the blade radially outward relative to the rotor disk, thus positively engaging the blade root with its mating surfaces in the disk even when no centrifugal load is present, where the wave spring includes integral machined end blocks and the device has an oblong body cross-sectional shape to prevent rotation in the space.
Description of the Related Art
Combustion gas turbines are clean-burning, efficient devices for generating power for a variety of applications. One common application of combustion gas turbines is in power plants, where the turbine drives a generator which produces electricity. Such stationary gas turbines have been developed over the years to improve reliability and efficiency, but the continuous improvement quest never ends.
Turbine blades are airfoils which are arranged circumferentially around a rotor disk inside the turbine, where rows of rotating blades are alternately positioned between rows of stationary turbine vanes. Because turbine blades are directly exposed to combustion gases, they get extremely hot. Blades are also subject to combustion gas pressure, centrifugal force and vibration. Thus, turbine blades may become damaged or worn over time, and they therefore need to be easily replaceable.
A common and reliable design for the attachment of turbine blades to the rotor disk is where the blade root has an inverted “fir tree” shape, and the disk has a complementary fir tree shaped cavity. With this design, a blade can be installed in a disk by simply sliding the blade in a longitudinal direction (parallel to the rotational axis of the turbine) so that the blade root fir tree engages with the mating cavity in the rotor disk. In this design, there is necessarily some looseness between the blade root and the disk cavity, both to allow for easy installation and removal, and to allow for differing radial growths due to thermal expansion and/or centrifugal forces. When the turbine is running at operational speed, centrifugal force pulls the blades radially outward so that the looseness is all taken up, and contact points on the branches of the fir tree are pressed tightly against each other. However, turbines are sometimes operated in a low-speed “stand by” mode, where the centrifugal force of rotation is not enough to overcome the force of gravity, and as a result, each blade experiences radial inward/outward and rocking movements on each rotation of the turbine. Over long durations, these repeated movements of the blade relative to the disk cause excessive wear on contact points of the blade fir trees and disk cavities, as well as on blade tip shrouds.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, an adjustable blade root spring device for turbine blade fixation in turbomachinery is disclosed. The blade root spring device is designed to be placed in a space in a rotor disk cavity adjacent to a tip of a blade root, where the device applies a radial outward force on the turbine blade to fix the blade position in the rotor disk. The device includes a wave spring with integral end blocks which is compressed by a bolt and a coil spring. When the wave spring is compressed in length, it increases in height and makes contact with the rotor disk and the turbine blade. The force of the wave spring on the turbine blade can be adjusted via the bolt, and the coil spring provides an increased compliance range. The body of the device has an oblong cross-sectional shape, thereby preventing rotation of the device in the space between the blade and the disk.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional diagram of a combustion gas turbine, showing an arrangement of blades and vanes;
FIG. 2 is an illustration of a turbine blade attachment to a rotor disk with a fir tree cavity shape;
FIG. 3 is an exploded-view illustration of an adjustable blade root spring device for fixing the turbine blade of FIG. 2 in position in the rotor disk;
FIG. 4 is another exploded-view illustration, from a different point of view, of the adjustable blade root spring device for fixing the turbine blade in position in the rotor disk;
FIG. 5 is an illustration of the adjustable blade root spring device of FIG. 3 as fully assembled;
FIG. 6 is a side view illustration of the adjustable blade root spring device in the space between the turbine blade and the rotor disk;
FIG. 7 is an illustration of a bent tab washer which can be installed between a washer and a head of a bolt in the adjustable blade root spring device;
FIG. 8 is an exploded-view illustration of the bent tab washer in position between the washer and the head of the bolt;
FIG. 9 is an illustration of two turbine blades attached to the rotor disk, with one of the adjustable blade root spring devices inserted into each of the two spaces between the blade and the disk;
FIG. 10 is an exploded-view illustration of a second embodiment of an adjustable blade root spring device for fixing the turbine blade in position in the rotor disk;
FIG. 11 is an exploded-view illustration, from a different point of view, of the adjustable blade root spring device of FIG. 10;
FIG. 12 is a cross-sectional view of the ribbon portion of a wave spring included in the adjustable blade root spring device of FIGS. 10 and 11;
FIG. 13 is an end cross-sectional view illustration of side walls of the adjustable blade root spring device showing how the side walls are beveled to correspond to the shape of the blade root of the turbine blade;
FIG. 14 is an illustration of the adjustable blade root spring device of FIGS. 10 and 11 as fully assembled;
FIG. 15 is a side cross-sectional view illustration of the adjustable blade root spring device of FIGS. 10-12 in the space between the turbine blade and the rotor disk; and
FIG. 16 is an end cross-sectional view illustration of the blade root of the turbine blade attached to the rotor disk, with the adjustable blade root spring device of FIGS. 10-12 inserted into the space between the blade root and the disk.



DETAILED DESCRIPTION OF THE EMBODIMENTS
The following discussion of the embodiments of the invention directed to an adjustable blade root spring device for turbine blade fixation in turbomachinery is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the blade root spring device is discussed below in the context of a combustion gas turbine, but the device may also be applicable in fixing blades of a steam turbine, or blades in other rotating machinery.
FIG. 1 is a cross-sectional diagram of a combustion gas turbine 10, such as the type which is used to drive an electrical generator in a power plant. As is understood by anyone familiar with turbomachinery, the turbine 10 includes a series of blades (22, 24, 26, 28) and vanes (30, 32, 34, 36). As can be seen in FIG. 1, the blades 22-28 and the vanes 30-36 are arranged in alternating rows along the length of the turbine 10. Each row of blades is attached to a rotor disk which rotates with a power shaft, where the power shaft drives downstream machinery such as a generator. For example, the fourth row blades 28 are attached to a rotor disk 40. The vanes 30-36 are fixed in place, being attached to inner and outer casings. The vanes 30-36 are airfoils which serve to direct and accelerate the flow of combustion gas as it expands, turns the blades 22-28 and passes through the turbine 10.
Modern combustion gas turbines such as the turbine 10 operate at very high temperatures and pressures for both efficiency and power density reasons. Even with advances in material technology, the turbine blades 22-28 can eventually become worn or damaged due to the temperatures, pressures and forces they experience. Therefore, it is necessary to be able to replace individual turbine blades in a straightforward manner, while ensuring that the blade-to-disk attachment mechanism is strong enough to withstand the applied loads and vibration.
FIG. 2 is an illustration of a turbine blade 100, which could be any of the blades 22-28 of FIG. 1, and its attachment to a rotor disk 110. In his design, which has proven to be reliable and effective, the blade 100 has a blade root 102 with an inverted “fir tree” shape, and the rotor disk 110 has a complementary fir tree shaped cavity 112. With this design, the turbine blade 100 can be installed in the rotor disk 110 by simply sliding the blade 100 in a longitudinal direction (parallel to the rotational axis of the turbine) so that the fir tree shape of the blade root 102 engages with the mating cavity 112 in the rotor disk 110. In this design, there is necessarily some looseness between the blade root 102 and the disk cavity 112, both to allow for easy installation and removal of the blade 100, and to allow for differing radial growths due to thermal expansion and/or centrifugal forces.
When the turbine 10 is running at operational speed, centrifugal force pulls the blade 100 radially outward so that the looseness is all taken up, and contact points on the branches of the fir trees are pressed tightly against each other. However, turbines are sometimes operated in a low-speed “stand by” mode, also known as “turning gear” operation, intended to maintain the turbine 10 in a state of operational readiness. Turning gear operation typically occurs at speeds less than 100 rpm. In turning gear operation, the centrifugal force of rotation is not enough to overcome the force of gravity. As a result, each blade experiences radial inward/outward and rocking/tilting movements on each rotation of the turbine shaft due to the looseness of the blade root 102 in the fir tree shaped cavity 112. Over long durations, these repeated movements of the turbine blade 100 relative to the rotor disk 110 cause excessive wear on contact points of the blade fir trees and disk cavities, as well as on blade tip shrouds. In order to prevent this motion of the blade 100 relative to the disk 110 during low speed turbine operation, it is desirable to install a device in a space 120 below the tip of the blade root 102, in the bottom of the cavity 112, where the device can apply a radial outward force on the bottom of the blade 100. The device can be designed such that the force is sufficient to keep the blade 100 pressed radially outward (“upward” in FIG. 2) so that the blade root 102 is fixed in position in the cavity 112.
In addition to helping to avoid wear between turbine blade root fir trees and disk slots during turning gear operation, installation of blade root fixation devices that keeping the turbine blades 100 forced in their running position at all times may help avoid balance variations from run to run that can potentially contribute to rotor vibration. Noticeable improvements in vibration behavior coincided with the installation of such devices on the blades 100 of multiple turbine units.
Other devices which have been developed to provide the radial outward force on the blade 100 have several drawbacks. Some such devices cannot be expanded after being placed into the space 120; thus, these devices are difficult to insert into the space 120, and they scrape along the tip of the blade root 102 and the bottom of the cavity 112 and damage these surfaces when being inserted. Other such devices cannot be adjusted to provide a desired amount of radial force on the blade 100 in spite of part-to-part dimensional variations, or allow for an initial radial force adjustment but the radial force changes dramatically with thermal expansion during turbine operation.
FIGS. 3 and 4 are exploded-view illustrations of a new adjustable blade root spring device 200 for fixing the turbine blade 100 in position in the rotor disk 110. The adjustable blade root spring device 200 overcomes the drawbacks of other turbine blade fixation devices. As will be discussed in detail below, the device 200 can be easily inserted into the space 120 in an uncompressed state, thereby not damaging the blade root 102 or the cavity 112. The device 200 has a design which distributes contact forces along the length of the blade root 102 and the bottom of the cavity 112, and is robust to the tolerances of the parts involved. The device 200 is also designed to self-adjust to changes in the height of the space 120 due to thermal expansion.
The device 200 includes an outer body 202 including two flat side walls 204 spaced apart by a first end block 206 at one end and a second end block 208 at the other end. The outer body 202 can be fabricated of separate pieces, with the side walls 204 being attached to the end blocks 206 and 208, or the outer body 202 can be machined from a single piece of material. At one end of the outer body 202—the end including the second end block 208—a fixed washer 210 is attached by welding or brazing. Thus, the outer body 202 defines a shape which is bounded on the two sides by the side walls 204, on the two ends by the end blocks 206 and 208, and open on the top and bottom. A wiggle spring assembly 212 comprises a wiggle spring 214, a spring bracket 216 and an L-bracket 218. The spring bracket 216 is attached, preferably by welding, to one end of the wiggle spring 214. The L-bracket 218 is welded to the other end of the wiggle spring 214 as shown.
The wiggle spring 214 is made of a flat piece of a nickel-based alloy—selected for its corrosion and oxidation resistance and high strength at elevated temperatures—loosely folded into an accordion shape, as shown in FIGS. 3 and 4. In one embodiment, the wiggle spring 214 may be fabricated from a strip of INCONEL® alloy X-750 having a width in a range of 0.15-0.25 inches and a nominal thickness of 0.025 inches. The design of the wiggle spring 214 is significant in several regards. First, the wiggle spring 214 must be folded so that the pitch is not too fine and not too coarse. If the wiggle spring 214 were to be made with a much finer pitch (say, 10 or 20 folds instead of 3 or 4), then the wiggle spring 214 would not exhibit the desired increase in height when compressed, as discussed below. If the wiggle spring 214 were to be made with a more coarse pitch (say, 1 or 2 folds), then the wiggle spring 214 would likely buckle in an uncontrolled manner when compressed, instead of providing the desired jacking motion. It has been found that, for the turbine blade 100, where the space 120 has a depth of approximately 6-8 inches, the wiggle spring 214 should have 3-4 full folds along its length.
The design of the wiggle spring 214 shown in FIGS. 3 and 4, with three upper apex points 220 and four lower apex points 222, has been shown to provide good results for the intended application. The bend radius at the apex points 220 and 222 is also important, as too small of a bend radius can create excessive stress concentration at the apex points 220/222, and too large of a bend radius affects the compliance properties of the wiggle spring 214. It has been found that a bend radius at the apex points 220/222 in a range of 0.08-0.12 inches is optimal.
The adjustable blade root spring device 200 also includes a coil spring 224, a washer 226 and a bolt 228. To begin assembly of the device 200, the wiggle spring assembly 212 is placed down into the outer body 202 such that the wiggle spring 214 is between the end blocks 206 and 208 and between the side walls 204, a block portion 230 of the L-bracket 218 is inside the outer body 202 and abutted against an inner face of the end block 208, and a block portion 232 of the spring bracket 216 is outside the outer body 202. Next, the bolt 228 is placed through a hole 234 in the washer 226, through the coil spring 224 and through a hole 236 in the spring bracket 216. The bolt 228 is then threaded into a threaded hole 238 in the end block 206. The bolt 228 is threaded into the threaded hole 238 until the head of the bolt 228 is in contact with the washer 226, the washer 226 is in contact with the coil spring 224, and the coil spring 224 is in contact with the end of the block portion 232 of the spring bracket 216. At this point, the wiggle spring 214 is not compressed from its as-manufactured shape, the assembly of the device 200 is complete, and the device 200 is ready to be inserted into the space 120.
FIG. 5 is an illustration of the device 200 as fully assembled. In this view, it can be seen how the device 200, when assembled, is long and slender and can be slid into the space 120 between the blade root 102 and the bottom of the cavity 112. Before inserting into the space 120, the device 200 is assembled as shown in FIG. 5, but the coil spring 224 and the wiggle spring 214 are not compressed. Therefore, the wiggle spring 214 is at its free, or as-manufactured, length and height. After inserting into the space 120, the bolt 228 is tightened such that the coil spring 224 is compressed and bears against the spring bracket 216, thus compressing the wiggle spring 214.
When compressed, the wiggle spring 214 becomes shorter in length and taller in height, thereby providing a jacking effect between the blade root 102 and the bottom of the cavity 112. FIG. 6 is a side view illustration of the device 200 in the space 120 between the blade 100 and the disk 110. In this view, with the nearer of the side walls 204 not shown, it can be clearly seen how the upper apex points 220 press “up” (radially outward) on the bottom of the blade 100, and the lower apex points 222 press “down” (radially inward) on the rotor disk 110. It can also be seen in FIG. 6 how threading the bolt 228 into the end block 206 (not shown in FIG. 6) compresses the coil spring 224 against the spring bracket 216, thus squeezing the wiggle spring 214 into a shorter length and a taller height. The bolt 228 can be tightened to any suitable degree, which could be a prescribed number of turns beyond first loading of the coil spring 224, or a prescribed torque, or a prescribed amount of bolt rotation after reaching a torque threshold, where the torque threshold could be established to correspond to the wiggle spring 214 providing the desired compressive force against the blade root 102 and the bottom of the cavity 112.
It is important that the bolt 228 does not back out after it is tightened to provide the desired compression of the wiggle spring 214, as discussed above. Standard lock washers or other friction devices may be used to prevent undesired turning of the bolt 228 after installation of the device 200. However, it may be desirable to include a feature which provides positive fixation of the head of the bolt 228 to prevent rotation after installation.
FIG. 7 is an illustration of a bent tab washer 240 which can be installed between the washer 226 and the head of the bolt 228. As shown in FIG. 7, the bent tab washer 240 is in its as-manufactured state, with tabs 242 bent almost perpendicular to the plane of the sheet metal from which the bent tab washer 240 was stamped, but still leaving space to allow for turning the head of the bolt 228. The bolt 228, while shown in the figures as having a standard hex head, can have any head design suitable for the application. For example, the bolt 228 could include a recess for an allen wrench or star wrench, or straight or Phillips screwdriver slots. The bolt 228 could also have a square or rectangular four-sided head. It is simply required that the bolt 228 can accept a wrench or screwdriver for turning, and that its head has flats which can be held in position by the tabs 242 of the bent tab washer 240.
FIG. 8 is an exploded-view illustration of the bent tab washer 240 in position between the washer 226 and the head of the bolt 228. As can be seen in FIGS. 7 and 8, the bent tab washer 240 also includes an engagement tab 244 which fits into a notch 246 in the washer 226. The engagement tab 244 prevents rotation of the bent tab washer 240 relative to the washer 226. After the bolt 228 has been tightened as discussed above during installation of the device 200 in the space 120, the tabs 242 are bent or squeezed such that they press against the flats on the head of the bolt 228. In this final configuration, rotation of the bolt 228 relative to the bent tab washer 240 is prevented. The bent tab washer 240 is further prevented from rotating relative to the washer 226 as discussed above, thus fixing the bolt 228 and the entire device 200 in the desired, compressed position.
It is also noted that the device 200 can easily be removed from the space 120, by reversing the installation steps described above. This is important because turbine blade removal and replacement is occasionally necessary, and it is desirable to reuse the device 200.
FIG. 9 is an illustration of two of the turbine blades 100 attached to the rotor disk 110, with one of the adjustable blade root spring devices 200 inserted into each of the two spaces 120. On the left side of FIG. 9, the device 200 is partially inserted into the space 120. As discussed above, the device 200 can be freely inserted into the space 120, without scraping or scratching the surfaces of the blade 100 or the disk 110, because the device 200 is not vertically expanded until after insertion into the space 120. On the right side of FIG. 9, the device 200 is fully inserted into the space 120. It can be seen how the shape of the fixed washer 210 and the washer 226 generally match the shape of the space 120, thus precluding any washer rotation. This prevents the outer body 202 from rotating when the bolt 228 is tightened during installation, and prevents either the outer body 202 or the washer 226 from rotating after the device 200 has been installed.
Several features of the adjustable blade root spring device 200 warrant further discussion. First, as mentioned above, the device 200 can be installed and removed without damaging the turbine blade 100 or the rotor disk 110. This is important both for ease of installation and because any scraping or scratching of the blade 100 and the disk 110 could not only damage these components, but also create a potential foreign object damage problem in the turbine 10.
Another valuable feature of the device 200 is that the radial load applied to the blade 100 can be adjusted as desired. This is accomplished by simply specifying a torque or angular rotation to apply to the bolt 228 which results in a compressive force on the wiggle spring 214 which provides the desired radial blade force. This adjustability of radial force allows the device 200 to be used in different turbine applications and operating conditions. Furthermore, the bolt 228 can be further adjusted if necessary, after installation of the device 200 and reassembly of the turbine 10. This further adjustment of the bolt 228 can be accomplished without significant disassembly of the turbine 10 by simply providing an access port/hole through a lock plate which covers the end of the blade root 102 and the cavity 112.
It is also noteworthy that the device 200 does not require any special features to exist on either the turbine blade 100 or the rotor disk 110. This is important because it is undesirable to make design changes to parts—such as the blade 100 or the disk 110—which have been validated for production, and which have been proven in field operation. The device 200 can be used with existing fir tree designs of the blade root 102 and the disk cavity 112.
The device 200 is also designed to evenly distribute the radial force along the bottom of the blade 100. Even in the presence of manufacturing tolerances and surface irregularities, where the height of the space 120 may not be perfectly uniform along its depth, the apex points 220 and 222 of the wiggle spring 214 will each make contact with the blade 100 or the disk 110, and the radial force at each of the apex points 220/222 will tend to balance out. That is, for example, one of the upper apex points 220 will not tend to take all of the radial force, to the exclusion of the other upper apex points 220, because the adjacent sections of the wiggle spring 214 will naturally compress further to prevent this from happening.
Furthermore, the radial load applied by the device 200 self-compensates when the height of the space 120 changes due to thermal expansion. This is made possible by the presence of the coil spring 224. Consider a design where the coil spring 224 is not included in the device 200, and the wiggle spring 214 is directly compressed by the bolt 228 pressing against the spring bracket 216. In such a design, a desired radial force could be applied to the blade 100 by the wiggle spring 214 when the bolt 228 is tightened. However, if the height of the space 120 increases slightly due to thermal expansion, the amount of radial force applied to the blade 100 would drop dramatically, or disappear completely, because the apex points 220 and 222 of the wiggle spring 214 would quickly lose contact with the blade 100 and the disk 110. This is because, in this fictional design with no coil spring, the spring bracket 216 would be experiencing a positional constraint associated with the installed position of the bolt 228.
Returning to the actual design of the device 200 shown in the preceding figures, including the coil spring 224, it can be seen that a much more robust load compensation is inherent. Again, consider that the height of the space 120 increases slightly due to thermal expansion. Because the coil spring 224 applies a force boundary condition to the spring bracket 216—not a positional boundary condition as in the no-coil-spring sign discussed above—the device 200 will maintain most of the radial force on the bottom of the blade 100. Specifically, the wiggle spring 214 will further compress as necessary to maintain contact with the blade 100 and the disk 110, and the coil spring 224 will uncompress by the same amount. However, the amount that the coil spring 224 uncompresses will be small in comparison to its preload compression, thereby maintaining nearly the same amount of preload.
In order to achieve the load-compensation effect described in the preceding two paragraphs, the coil spring 224 may be specified with a spring rate in a range of 150-200 pounds/inch. The amount of coil spring preload on the spring bracket 216 may be in a range of 25-50 pounds, resulting in a radial force of the wiggle spring 214 on the bottom of the blade 100 of 150-250 pounds. These design specifications dictate that the coil spring 224 is compressed by a non-trivial amount, on the order of 1⁄4 inch, when the bolt 228 is tightened during installation of the device 200. Thus, if the height of the space 120 increases due to thermal expansion, the coil spring 224 will uncompress only slightly, the axial load on the wiggle spring 214 will also change only slightly, and the radial force of the wiggle spring 214 on the bottom of the blade 100 will also change only slightly. The coil spring 224 thereby provides the desired blade force self-compensation in the device 200.
Using the device 200 described above, the turbine blades in a gas turbine engine can be securely held in position relative to the rotor disk, even during low speed turbine operation where centrifugal forces are low. The positive turbine blade fixation achieved with the adjustable blade root spring device 200 prevents excessive blade wear during turning gear operation of the turbine, resulting in both improved turbine reliability and lower maintenance cost.
The device described above has been found to perform well in a particular turbine design. For a different turbine however, with different blade size and different size of cavity between blade root and disk, a different design of blade root spring device is beneficial.
FIGS. 10 and 11 are exploded-view illustrations of an adjustable blade root spring device 300 for fixing the turbine blade 100 in position in the rotor disk 110. The adjustable blade root spring device 300 has been optimized for turbines with a smaller space 120 between the blade 100 and the disk 110. As was the case with the device 200 discussed above, the device 300 has many features which make it more desirable than other prior art turbine blade fixation devices. However, the device 300 has some design differences as compared to the device 200.
The device 300 includes an outer body 302 including two side walls 304, a first end 306 and a second end 308. In a preferred embodiment, the outer body 302 is machined from a single piece of material, such as INCONEL® alloy X-750. As can be seen in FIGS. 10 and 11 (and later in FIG. 15), the side walls 304 of the outer body 302 define an oblong cross-sectional shape (approximating an ellipse with straight clipped sides parallel to the major axis) which is designed to fit into, and be unable to rotate within, the cavity 120. Thus, no component like the end washer 210 is needed with the device 300. The outer body 302 is open on the top and bottom.
In the device 300, a wave spring 312 replaces the wiggle spring assembly 212 of the device 200. The terms “wave spring” and “wiggle spring” are synonymous in the context of the disclosed devices. The name “wave spring” is being used with the adjustable blade root spring device 300 because the wave spring 312 has been substantially changed from the design of the wiggle spring assembly 212. The wave spring 312 includes a floating end block 316 and a fixed end block 318. In a preferred embodiment, the wave spring 312—including the end blocks 316/318—is machined from a solid block of material, and then formed into the desired wave shape. In this embodiment, the wave spring 312 is made of INCONEL® alloy 718 having a width in a range of 0.15-0.30 inches and a nominal thickness (in the wave area) of 0.025 inches. The number of waves in the wave spring 312 has also been changed from the wiggle spring 214, as discussed below.
As discussed above regarding the wiggle spring 214, the wave spring 312 must be folded so that the pitch is not too fine and not too coarse. Because the device 300 is intended for applications where the height of the space 120 is smaller than for the device 200, the number of bends or folds in the wave spring 312 must be greater in order to maintain the same pitch angle. It has been found that, for a turbine blade/disk arrangement where the space 120 has a height of approximately 0.28-0.3 inches and a depth of approximately 6-7 inches, the wave spring 312 should have 5 folds along its length.
The design of the wave spring 312 shown in FIGS. 10 and 11, with four upper apex points 320 and five lower apex points 322, has been shown to provide good results for the intended application. The bend radius at the apex points 320 and 322 is also important, as too small of a bend radius can create excessive stress concentration at the apex points 320/322, and too large of a bend radius would affect the compliance properties of the wave spring 312. It has been found that a bend radius at the apex points 320/322 in a range of 0.105-0.115 inches is optimal.
In order to eliminate a source of part-to-part variability and eliminate brazing of small components, the wave spring 312 is manufactured from a single piece of material. Starting with a block of material (such as INCONEL® alloy 718) which is long enough to accommodate the wave spring 312 in its fully-straightened shape, the block is cut to form the end blocks 316 and 318 at the two ends, connected by a flat ribbon of material. The flat ribbon of material is the spring section of the wave spring 312. The cutting operations on the wave spring 312 can include any combination of conventional machining, wire EDM, laser cutting, etc. Features can then be machined into the end blocks 316/318 as required. These features include a through-hole 336 in the floating end block 316, the end block 318 shaped to fit into a slot 330 in the outer body 302, and detailed features such as fillets and rounds.
FIG. 12 is a cross-sectional view of the spring section of the wave spring 312. It has been found to be helpful to bevel the edges of the spring section of the wave spring 312, at a taper angle 314 of about 10° (+/−1°), in order to provide better clamping or holding of the wave spring 312 during subsequent machining. The bevel may not extend all the way across the thickness of the material in the spring section. This detail is shown in FIG. 12.
After the wave spring 312 has been machined, it is formed into the final (“design position” or “as-manufactured”) wave shape. The wave spring 312 is designed and manufactured such that, after forming of the wave shape, all of the top surfaces are coplanar. That is, the four upper apex points 320 and the upper surfaces of the end blocks 316/318 are all in the same “horizontal” plane after manufacturing. This is important in order for the vertical loads applied by the wave spring 312 to the bottom of the turbine blade root 102 to be distributed evenly. The five lower apex points 322 are of course also coplanar, and inherently distribute the loads on the disk 110 via compression of the wave spring 312.
The adjustable blade root spring device 300 also includes a coil spring 324, a washer 326, a bolt 328 and a bent tab washer 340. The washer 326 is flat, and has a shape which substantially matches the cross-sectional shape of the outer body 302. Similar to the device 200, the bent tab washer 340 has an engagement tab which fits into a mating notch in the washer 326 to prevent rotation of the bent tab washer 340 relative to the washer 326.
To begin assembly of the device 300, the wave spring 312 is placed down into the outer body 302, between the side walls 304, such that the fixed end block 318 of the wave spring 312 is fitted into the slot 330 in the outer body 302, and the floating end block 316 of the wave spring 312 is outside the outer body 302. Next, the bolt 328 is placed through the bent tab washer 340, through the washer 326, through the coil spring 324 and through the hole 336 in the floating end block 316. The bolt 328 is then threaded into a threaded hole 338 in the first end 306 of the outer body 302. The bolt 328 is threaded into the threaded hole 338 until the head of the bolt 328 causes the bent tab washer 340 to engage with the washer 326, the washer 326 is in contact with the coil spring 324, and the coil spring 324 is in contact with the floating end block 316. At this point, the engagement of the bolt 328 very slightly compresses the coil spring 324, holding the assembled device 300 in an un-loaded, stable condition. The wave spring 312 is not compressed from its as-manufactured shape, the assembly of the device 300 is complete, and the device 300 is ready to be inserted into the space 120.
FIG. 13 is an end cross-sectional view illustration of the side walls 304 of the adjustable blade root spring device 300 showing how the side walls 304 are beveled to correspond to the shape of the blade root 102 of the turbine blade 100. As shown at arrows 400, the upper surfaces of the side walls 304 are beveled at an angle parallel to the bottom surface of the blade root 102. This is important because at steady state and over speed conditions of the turbine 10, the outer body 302 may deflect outward and contact the bottom of the blade root 102. Should this occur, it is desirable to have surface contact and not have sharp edges of the side walls 304 hit the blade root 102.
FIG. 14 is an illustration of the device 300 as fully assembled. In this view, it can be seen how the device 300, when assembled, is long and slender and can be inserted into the space 120 between the blade root 102 and the bottom of the cavity 112. Before inserting into the space 120, the device 300 is assembled as shown in FIG. 14, and the coil spring 324 and the wave spring 312 are not compressed. Therefore, the wave spring 312 is at its free, or as-manufactured, length and height. After inserting into the space 120, the bolt 328 is tightened such that the coil spring 324 is compressed and bears against the floating end block 316, thus compressing the wave spring 312. The bolt 328 is further tightened to compress the wave spring 312 sufficiently to create the desired radial preloads on the blade root 102 and the bottom of the cavity 112. After the bolt 328 is tightened to the desired degree, the tabs of the bent tab washer 340 are bent to bear against flats on the head of the bolt 328 to prevent further turning of the bolt 328, thus completing installation of the device 300. The device 300, assembled as shown in FIG. 14 and before installation in the space 120, has a length of approximately 6 inches, a width of approximately 0.6 inches, and a height of approximately 0.27 inches. Of course, depending on the available space 120 between the blade root 102 and the bottom of the cavity 112, larger and smaller sizes of the device 300 can also be made, which may drive changes in the thickness and the number of waves in the wave spring 312.
FIG. 15 is a side cross-sectional view illustration of the device 300 in the space 120 between the blade 100 and the disk 110. In this view, with the nearer of the side walls 304 not shown, it can be clearly seen how the upper apex points 320 press “up” (radially outward) on the bottom of the blade 100, and the lower apex points 322 press “down” (radially inward) on the rotor disk 110.
FIG. 16 is an end cross-sectional view illustration of the blade root 102 of the turbine blade 100 attached to the rotor disk 110, with one of the adjustable blade root spring devices 300 inserted into the space 120. It can be seen how the end-view shape of the device 300 (the body 302 and the washer 326) generally match the shape of the space 120, particularly along its lower portion. This prevents rotation of the device 300 in the space 120 when the bolt 328 is tightened during installation or after the device 300 has been installed.
The adjustable blade root spring device 300 provides many desirable features, as discussed previously for the adjustable blade root spring device 200. One feature of the device 300 is its ability to be installed and removed in an unexpanded state, without scraping against and damaging the turbine blade 100 or the rotor disk 110, and then expanded once in position. The device 300 is designed so that the radial load applied to the blade 100 can be adjusted as desired via tightening of the bolt 328, and to evenly distribute the radial force along the bottom of the blade 100. The radial load applied by the device 300 also self-compensates when the height of the space 120 changes due to thermal expansion, by virtue of the properties of the coil spring 324.
Like the device 200, the device 300 does not require any special features to exist on either the turbine blade 100 or the rotor disk 110. The device 300 can be used with existing fir tree designs of the blade root 102 and the disk cavity 112, being particularly suited for smaller sizes of the space 120. The device 300 can also be easily removed from the space 120, by reversing the installation steps described above.
The positive turbine blade fixation achieved with the adjustable blade root spring device 300 prevents excessive blade wear during turning gear operation of the turbine, resulting in both improved turbine reliability and lower maintenance cost. The device 300 has been optimized for ease of assembly and effectiveness in the smaller blade-disk cavities found in some gas turbine engines.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.

## Claims

1. An adjustable blade root spring device for fixing a turbine blade in position in a rotor disk in a gas turbine, said device comprising: an outer body including opposing side walls and first and second ends, where the outer body has a cross-sectional shape approximating an ellipse with flat sides parallel to its major axis, and the outer body has a central opening bounded by the side walls and open on top and bottom; a wave spring comprising a plurality of straight segments folded in an alternating fashion into a wave shape, with an apex point between each of the plurality of straight segments, where the wave spring includes a floating end block at a first end and a fixed end block at a second end, and where the wave spring is placed substantially within the central opening of the outer body such that the floating end block is outside of the first end of the outer body and the fixed end block is fitted into a slot in the second end of the outer body; a coil spring; a flat washer; a bent tab washer; and a bolt, said bolt arranged to pass through the bent tab washer, the flat washer, and through the coil spring and then through a hole in the floating end of the wave spring, said bolt being threaded into a threaded hole in the first end of the outer body, where threading the bolt into the threaded hole compresses the coil spring against the floating end and causes the wave spring to decrease in length and increase in height such that the wave spring presses the turbine blade radially outward relative to the rotor disk.

2. The device of claim 1 wherein the turbine blade includes a root portion having an inverted fir tree shape, the rotor disk includes a cavity shaped to receive the root portion of the turbine blade, and the cavity further includes a space adjacent to the root portion of the turbine blade where the device can exert a force radially outward on the turbine blade and radially inward on the rotor disk.

3. The device of claim 2 wherein the flat washer has a shape which matches the cross-sectional shape of the outer body, and where the shape fits within the space adjacent to the root portion of the turbine blade but does not permit rotation of the flat washer or the outer body within the space.

4. The device of claim 2 wherein the wave spring presses the turbine blade radially outward relative to the rotor disk with a force of at least 150 pounds, and the force fixes the turbine blade in position in the rotor disk in any static or dynamic configuration of the gas turbine.

5. The device of claim 4 wherein the coil spring has a stiffness which causes the coil spring to compress by a compression amount when the bolt is tightened during installation of the device, and the compression amount is sufficient to absorb any relaxation of the wave spring due to increased height of the space adjacent to the root portion of the turbine blade during turbine operation, while still maintaining the force of at least 150 pounds.

6. The device of claim 5 wherein the stiffness of the coil spring is in a range of 150-200 pounds/inch.

7. The device of claim 1 wherein the bent tab washer includes an engagement tab which engages a notch in the flat washer and prevents rotation of the bent tab washer relative to the flat washer, where tabs on the bent tab washer can be bent into a position which prevents rotation of the head of the bolt after the device has been installed in the gas turbine.

8. The device of claim 1 wherein the wave spring, including the floating end block, the fixed end block and a spring section therebetween, is machined from a single piece of a nickel-based alloy.

9. The device of claim 8 wherein the spring section has a width in a range of 0.15-0.30 inches and a nominal thickness of 0.025 inches.

10. The device of claim 1 wherein the wave spring is shaped so that four upper apex points contact the turbine blade and five lower apex points contact the rotor disk, and the four upper apex points are coplanar with upper surfaces of the floating end block and the fixed end block.

11. The device of claim 10 wherein the four upper apex points each provide a substantially equal contact force on the turbine blade, and the five lower apex points each provide a substantially equal contact force on the rotor disk.

12. A gas turbine engine rotor assembly, said assembly comprising: a plurality of turbine blades, each of the plurality of turbine blades including a blade root portion having an inverted fir tree shape; a rotor disk designed to hold the plurality of turbine blades in a circumferential arrangement around an outer periphery of the rotor disk, said rotor disk including a plurality of cavities, with one of the plurality of cavities for each of the plurality of turbine blades, where each of the plurality of cavities has a fir tree shape designed to receive the blade root portion of one of the plurality of turbine blades, and where each of the plurality of cavities also includes a space adjacent to and radially inward from the blade root portion; and an adjustable blade root spring device inserted into the space in each of the plurality of cavities, where the adjustable blade root spring device includes: an outer body including opposing side walls and first and second ends, where the outer body has a cross-sectional shape approximating an ellipse with flat sides parallel to its major axis, and the outer body has a central opening bounded by the side walls and open on top and bottom; a wave spring comprising a plurality of straight segments folded in an alternating fashion into a wave shape, with an apex point between each of the plurality of straight segments, where the wave spring includes a floating end block at a first end and a fixed end block at a second end, and where the wave spring is placed substantially within the central opening of the outer body such that the floating end block is outside of the first end of the outer body and the fixed end block is fitted into a slot in the second end of the outer body; a coil spring; a flat washer; a bent tab washer; and a bolt, said bolt arranged to pass through the bent tab washer, the flat washer, and through the coil spring and then through a hole in the floating end of the wave spring, said bolt being threaded into a threaded hole in the first end of the outer body, where threading the bolt into the threaded hole compresses the coil spring against the floating end and causes the wave spring to decrease in length and increase in height such that the wave spring presses the turbine blade radially outward relative to the rotor disk.

13. The rotor assembly of claim 12 wherein the flat washer has a shape which matches the cross-sectional shape of the outer body, and where the shape fits within the space adjacent to the root portion of the turbine blade but does not permit rotation of the flat washer or the outer body within the space.

14. The rotor assembly of claim 12 wherein the bent tab washer includes an engagement tab which engages a notch in the flat washer and prevents rotation of the bent tab washer relative to the flat washer, where tabs on the bent tab washer can be bent into a position which prevents rotation of the head of the bolt after the device has been installed in the gas turbine.

15. The rotor assembly of claim 12 wherein the wave spring, including the floating end block, the fixed end block and a spring section therebetween, is machined from a single piece of a nickel-based alloy.

16. The rotor assembly of claim 15 wherein both edges of the spring section of the wave spring are machined with a taper angle of between 9 and 11 degrees in order to provide improved clamp holding of the wave spring during subsequent machining operations.

17. The rotor assembly of claim 12 wherein the wave spring is shaped so that four upper apex points contact the turbine blade and five lower apex points contact the rotor disk, and the four upper apex points are coplanar with upper surfaces of the floating end block and the fixed end block.

18. A gas turbine vibration reduction system; comprising:
an outer body configured to reside in a space between a rotor disk and a blade root of a gas turbine rotor, said outer body having a cross-sectional shape which prevents rotation of the body within the space; a wave spring comprising a plurality of straight segments folding in an alternating fashion into a wave shape, with an apex point between each of the plurality of straight segments, said wave spring substantially contained within the outer body such that said wave spring includes a first end block fixed within the outer body and a second end block positioned outside the outer body; a compression screw arranged on an exposed end of the outer body such that the screw operatively adjusts a compression of the wave spring; and a coil spring arranged coaxially surrounding the compression screw and passing through the second end block of the wave spring to maintain a residual contact pressure between the compression screw and the wave spring.

19. The vibration reduction system of claim 18 wherein the wave spring, including the first and second end blocks and a spring section therebetween, is machined from a single piece of a nickel-based alloy.

20. The vibration reduction system of claim 18 wherein the wave spring is shaped so that four upper apex points contact the blade root and five lower apex points contact the rotor disk, and the four upper apex points are coplanar with upper surfaces of the first and second end blocks.