# Patent

## Title
Trapped spring balance weight and rotor assembly

## Abstract

A balance weight for a turbine rotor includes: a block-like centerbody; a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape; at least one locating structure extending from a radially outer surface of the balance weight; and a limit tab extending radially inward from a distal end of each of the spring arms.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-In-Part of application Ser. No. 12/485,122 filed Jun. 16, 2009, which is currently pending.


BACKGROUND OF THE INVENTION
This invention relates generally to rotating machinery and more particularly to apparatus for balancing rotors.
Gas turbine engines typically include several rotor stages, each having a rotor disk carrying an array of airfoils, i.e., compressor or turbine blades. Turbine rotors must be balanced to prevent damage and excessive loads on bearings and supporting structures, as well as efficiency losses caused by loss of clearance between the airfoils and the surrounding structure (caused by, e.g., shroud rubs).
Despite efforts to first balance their constituent components, turbine rotors still require dynamic balancing following assembly. For this purpose, it is desirable to use balance weights that can be re-positioned to redistribute the mass of the rotor as needed and allow the system unbalance to be fine-tuned to meet precise requirements. Separable balance weights are a common practice in larger gas turbine engines. These include bolts, washers, nuts and other fasteners of varying sizes.
In some gas turbine rotors, notably those in smaller engines, CURVIC couplings and friction joints are assembled using a single bolt or a group of bolts (referred to as a “tie rod” or “tie bolts”) spanning the length of the assembly. A tie bolt configuration weighs less than a conventional bolted joint, but the absence of bolt holes eliminates convenient features on the rotor disk which could otherwise be used to attach separable balance weights. Accordingly, the current state of the art for smaller turbine engines is to balance the assembly by selectively machining a sacrificial surface on the rotating part. Material is removed at the location of peak unbalance to redistribute the mass of the rotor about the axis of rotation. This process is irreversible and risks damaging a component such as an integrally-bladed rotor or “blisk”, which is both safety-critical and expensive.
BRIEF SUMMARY OF THE INVENTION
These and other shortcomings of the prior art are addressed by the present invention, which provides a trapped spring balance weight for a turbine rotor.
According to one aspect of the invention, a balance weight for a turbine rotor includes: a block-like centerbody pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape; at least one locating structure extending from a radially outer surface of the balance weight; and a limit tab extending radially inward from a distal end of each of the spring arms.
According to another aspect of the invention a turbine rotor assembly includes: a rotor element including an annular hub surface and an annular flange surrounding the hub surface, spaced away from the hub surface so as to define a pocket; and at least one balance weight disposed in the pocket, including: a block-like centerbody; a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape; at least one locating feature extending radially outward from the balance weight; and a limit tab extending radially inward from a distal end of each of the spring arms; wherein the spring arms and the centerbody resiliently bear against the flange and the hub surface, respectively, so as to retain the balance weight in the pocket. A radial height of the limit tabs is selected so as to prevent insertion of the balance weight into the pocket if the spring arms are deflected beyond a predetermined limit.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
FIG. 1 is a cross-sectional view of a gas turbine engine constructed in accordance with an aspect of the present invention;
FIG. 2 is an enlarged view of the forward portion of the compressor of the engine shown in FIG. 1;
FIG. 3 is an enlarged view of the aft portion of the compressor of the engine shown in FIG. 1;
FIG. 4 is a perspective view of a balance weight constructed according to an aspect of the present invention;
FIG. 5 is a rear elevational view of the balance weight of FIG. 4;
FIG. 6 is a perspective view of the balance weight of FIG. 4 installed in a rotor disk of the engine of FIG. 1;
FIG. 7 is a front view of a spanner tool for use with a balance weight;
FIG. 8 is a side view of the spanner tool of FIG. 7;
FIG. 9 is a rear view of the spanner tool of FIG. 7;
FIG. 10 is a view of the spanner tool of FIG. 7 in use;
FIG. 11 is a perspective view of a balance weight constructed according to another aspect of the present invention;
FIG. 12 is a rear elevational view of the balance weight of FIG. 11;
FIG. 13 is a perspective view of the balance weight of FIG. 11 installed in the engine of FIG. 1;
FIG. 14 is a cross-sectional view of portion of a compressor of a gas turbine engine, with a balance weight installed therein;
FIG. 15 is a perspective view of the balance weight of FIG. 14;
FIG. 16 is a rear elevational view of the balance weight of FIG. 15; and
FIG. 17 is a perspective view of the balance weight of FIG. 15 in an installed condition.



DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 depicts an exemplary gas turbine engine 10 having a compressor 12, a combustor 14, a high pressure or gas generator turbine 16, and a work turbine 18, all arranged in a serial flow relationship. Collectively the compressor 12, the combustor 14, and the gas generator turbine 16 are referred to as a “core”. The compressor 12 provides compressed air that passes into the combustor 14 where fuel is introduced and burned, generating hot combustion gases. The hot combustion gases are discharged to the gas generator turbine 16 where they are expanded to extract energy therefrom. The gas generator turbine 16 drives the compressor 12 through an impeller shaft 20. Pressurized air exiting from the gas generator turbine 16 is discharged to the work turbine 18 where it is further expanded to extract energy. The work turbine 18 drives an inner shaft 22.
In the illustrated example, the engine is a turboshaft engine, and the inner shaft 22 would be coupled to an external load such as a reduction gearbox or propeller. However, the principles described herein are equally applicable to turboprop, turbojet, and turbofan engines, as well as turbine engines used for other vehicles or in stationary applications. These principles are also applicable to any other type of rotating machinery (e.g. wheels, gears, shafts, etc.) which require balancing.
In the illustrated example, the compressor 12 includes five axial-flow rotor stages and one mixed-flow stage which is positioned immediately upstream of the combustor 14. As best seen in FIG. 2, the first stage rotor 24 of the compressor 12 is an integrally-bladed rotor or “blisk” in which a rotor disk 26 and a plurality of airfoil-shaped compressor blades 28 are formed as one integral component. The aft end of the rotor disk 26 includes an annular hub surface 30 and an annular flange 32 extending over the hub surface 30. Together, the hub surface 30 and the flange 32 define a pocket 34 (best seen in FIG. 6). An inner surface 36 of the flange 32 has an array of grooves 38 formed therein (again, see FIG. 6).
As seen in FIG. 3, the final stage of the compressor 12 includes a rotor disk 40 which carries a plurality of blades 42. The annular impeller shaft 20 extends axially aft from the rotor disk 40. The intermediate section of the impeller shaft 20 includes an annular hub surface 46 and an annular flange 48 extending over the hub surface 46. Together, the hub surface 46 and the flange 48 define a pocket 50 (best seen in FIG. 13). The flange 48 includes an annular array of apertures formed therein. In the illustrated example, as seen in FIG. 13, this array comprises open-ended slots 52 alternating with holes 54.
One or more forward balance weights 60 are installed in the pocket 34 of the first stage rotor 24, and one or more aft balance weights 160 are installed in the pocket 50 of the impeller shaft 20. The exact number, position, and distribution of weights will vary by individual engine. In the particular engine illustrated, only two balance weights are used. Correction of rotor imbalance is accomplished by re-positioning the weights as needed.
FIGS. 4 and 5 illustrate one of the forward balance weights 60 in more detail. It is generally arcuate in shape and comprises a block-like centerbody 62 with resilient spring arms 64 extending laterally outward therefrom. A notch 66 is formed in the radially inner end of the centerbody 62. At the distal end of each spring arm 64, an axially-elongated rail 68 extends radially outward. Opposite each rail 68, a stop block 70 extends radially inward. The forward balance weights 60 may be constructed from any material with an appropriate density and the ability to form the spring arms which can deflect elastically. For example, metal alloys may be used.
With reference to FIG. 6, the forward balance weights 60 are installed into the first stage rotor 24 as follows. The spring arms 64 are deflected radially inward relative to the centerbody 62. They may be held in this position by an appropriate tool or jig. Then the forward balance weight 60 is slid axially into the pocket 34, at the appropriate position. The spring arms 64 are then released. After release, the residual spring force urges the spring arms 64 radially outward against the flange 32 and urges the centerbody 62 against the hub surface 30. The rails 68 engage the grooves 38 in the inner surface of the flange 32 to prevent tangential movement. A mating component (in this case the forward end of an annular shaft 72, seen in FIG. 2) abuts the notch 66 to prevent axial movement of the forward balance weight 60. FIG. 6 shows one of the forward balance weights 60 in an installed condition. During engine operation, centrifugal loading reseats the forward balance weights 60 against the flange 32.
If necessary as indicated by a balancing operation, the forward balance weights 60 can be repositioned circumferentially while the compressor 12 is assembled, for example through use of a spanner-wrench tool. For example, FIGS. 7-9 illustrate a suitable tool 74 which has an elongated handle 76 and a curved head 78 with spanner fingers 80 extending radially inward and laterally outward from its distal ends. As shown in FIG. 10, the tool 74 is inserted into the pocket 34 and used to deflect the spring arms 64 radially inward, disengaging the rails 68 from the grooves 38. The tool 74 may then be moved tangentially in the direction of the arrows, causing the spanner fingers 80 to contact the forward balance weight 60 and push it to a new position. Once the tool 74 is removed, the rails 68 re-engage grooves 38 at the new location. During this operation, the stop blocks 70 contact the annular shaft 72 if an attempt is made to deflect the spring arms 64 too far. This prevents permanent deformation of the spring arms 64.
FIGS. 11 and 12 illustrate one of the aft balance weights 160 in more detail. It is generally arcuate in shape and comprises a block-like centerbody 162 with resilient spring arms 164 extending laterally outward therefrom. An anti-rotation lug 166 extends radially outward from the centerbody 162. At the distal end of each spring arm 164, a shear pin 168 extends radially outward. Opposite each shear pin 168, a stop block 170 extends radially inward. A forward face 172 of the aft balance weight 160 has a convex contour complementary to the cross-sectional profile of the pocket 50 in the impeller shaft 20. The aft balance weights 160 may be constructed from any material with an appropriate density and the ability to form the spring arms which can deflect elastically. For example, metal alloys may be used.
As seen in FIG. 13, the aft balance weights 160 are installed using a method similar to that for the forward balance weights 60, as follows. The spring arms 164 are deflected radially inward relative to the centerbody 162, as shown by the arrows in FIG. 12. They may be held in this position by an appropriate tool or jig. Then the aft balance weight 160 is slid axially into the pocket 50, at the appropriate position. The stop blocks 170 are sized and shaped so as to prevent insertion into the pocket 50 if the spring arms 164 are deflected too far, and thus prevent permanent deformation of the spring arms 164. The spring arms 164 are then released. After release, the residual spring force urges the spring arms 164 radially outward against the flange 48 and urges the centerbody 162 against the hub surface 46. The anti-rotation lug 166 engages one of the slots 52 in the flange 48. The shear pins 168 engage the holes 54 in the flange 48 to prevent axial movement. FIG. 13 shows one of the aft balance weights 160 in an installed condition. During engine operation, centrifugal loading reseats the aft balance weights 160 against the flange 48. If necessary, the aft balance weights 160 can be removed and re-positioned while the compressor rotor is assembled, without any unique jigs or tools.
While the balance weights 60 and 160 have described as “forward” and “aft” weights, it will be understood that these terms are used merely for convenience in description of a particular embodiment. Depending upon the specific engine application and the mating hardware, either design could be used on the forward or aft face of a turbine rotor disk or shaft. Furthermore, the anti-rotation and axial restraint features could be modified or used in different combinations to produce a balance weight suitable for a particular application.
FIG. 14 illustrates a portion of a compressor section of a gas turbine engine, similar in operating principle to the engine 10 described above. A first stage rotor 224 in the compressor section is an integrally-bladed rotor or “blisk” in which a rotor disk 226 and a plurality of airfoil-shaped compressor blades 228 are formed as one integral component. The aft end of the rotor disk 226 includes an annular hub surface 230 and an annular flange 232 extending over the hub surface 230. Together, the hub surface 230 and the flange 232 define a pocket 234 (best seen in FIG. 17). An inner surface 236 of the flange 232 has an array of grooves 238 formed therein (again, see FIG. 17).
One or more balance weights 260 are installed in the pocket 234 of the first stage rotor 224. The exact number, position, and distribution of weights will vary by individual engine. Correction of rotor imbalance is accomplished by re-positioning the weights as needed.
FIGS. 15 and 16 illustrate one of the balance weights 260 in more detail. It is generally arcuate in shape and comprises a block-like centerbody 262 with resilient spring arms 264 extending laterally outward therefrom. A notch 266 is formed in the radially inner end of the centerbody 262. At the distal end of each spring arm 264, an axially-elongated rail 268 extends radially outward. Opposite each rail 268, a stop block 270 extends radially inward. A limit tab 271 extends radially inward from each stop block 270. The balance weights 260 may be constructed from any material with an appropriate density and the ability to form the spring arms which can deflect elastically. For example, metal alloys may be used.
With reference to FIG. 17, the balance weights 260 are installed into the first stage rotor 224 as follows. The spring arms 264 are deflected radially inward relative to the centerbody 262. They may be held in this position by an appropriate tool or jig. Then the balance weight 260 is slid axially into the pocket 234, at the appropriate position. The radial height “H” of each limit tab 271 relative to the stop block 270 (see FIG. 15) is selected to prevent deflection of the spring arms 264 beyond a predetermined limit. More specifically, the height H is set such that the limit tab 271 will interfere with the hub surface 230 before the spring arm 264 can be deflected enough to cause plastic deformation thereof. After insertion, the spring arms 264 are released. After release, the residual spring force urges the spring arms 264 radially outward against the flange 232 and urges the centerbody 262 against the hub surface 230. The rails 268 engage the grooves 238 in the inner surface of the flange 232 to prevent tangential movement. A mating component (in this case the forward end of an annular shaft 272, seen in FIG. 14) abuts the notch 266 to prevent axial movement of the balance weight 260. FIG. 17 shows one of the balance weights 260 in an installed condition. During engine operation, centrifugal loading reseats the forward balance weights 260 against the flange 232. The balance weights 260 may be repositioned as described above for the balance weights 60 and 160. It is also noted that the limit tab feature described with respect to the balance weights 260 may be incorporated in the balance weights 60 or 160.
The balance weight design described herein has several advantages over the current state-of-the-art for small engines. Process control is improved compared to material removal directly from the first stage rotor 24, which introduces local stress concentrations on highly stressed critical rotating parts. Any stress concentration features present on the balance weights 60, 160, or 260 would be generated using precision machining techniques and are therefore more well controlled. Engine cleanliness is also enhanced, as the balance weights do not require any machining at engine assembly and therefore do not create dust or grit that could contaminate the engine system. Finally, cycle time for the balancing process is reduced, because the balance weights can be easily re-positioned while the rotor is loaded in a balance machine, eliminating the re-work loop associated with a material removal balancing process.
The foregoing has described balance weights for a turbine rotor and a balanced rotor assembly. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.

## Claims

1. A balance weight for a turbine rotor, comprising:
a block-like centerbody;
a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape;
at least one locating structure extending from a radially outer surface of the balance weight; and
a stop block extending radially inward from a distal end of each of the spring arms; and a limit tab extending radially inward from and being axially smaller than each of the stop blocks.

2. The balance weight of claim 1 wherein an anti-rotation lug extends radially outward from the centerbody.

3. The balance weight of claim 1 wherein a shear pin extends radially outward from a distal end of each of the spring arms.

4. The balance weight of claim 1 wherein an axially elongated rail extends radially outward from a distal end of each of the spring arms.

5. The balance weight of claim 1 wherein the centerbody includes a notch formed at a radially inner end thereof.

6. A turbine rotor assembly, comprising:
a rotor element including an annular hub surface and an annular flange surrounding the hub surface, spaced radially outwardly away from the hub surface so as to define a pocket between the hub surface and the flange; and
at least one balance weight disposed in the pocket, comprising:
a block-like centerbody;
a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape;
at least one locating feature extending radially outward from the balance weight; and
a limit tab extending radially inward from a distal end of each of the spring arms toward the hub surface within the pocket, the limit tab having a radial height less than the spacing between the hub surface and the flange but sufficient to limit deflection of the spring arms within the pocket to the difference between the radial height of the limit tab and the spacing between the hub surface and the flange;

wherein the spring arms and the centerbody resiliently bear against the flange and the hub surface, respectively, so as to retain the balance weight in the pocket; and
wherein a radial height of the limit tabs is selected so as to prevent insertion of the balance weight into the pocket if the spring arms are deflected beyond a predetermined limit.

7. The turbine rotor assembly of claim 6 wherein an anti-rotation lug extends radially outward from the centerbody and engages an aperture in the flange, so as to prevent axial movement of the balance weight relative to the rotor disk.

8. The turbine rotor assembly of claim 6 wherein each of the spring arms includes a shear pin extending radially outward from a distal end thereof, the shear pins engaging apertures in the flange so as to prevent axial movement of the balance weight relative to the turbine rotor.

9. The turbine rotor assembly of claim 6 wherein each of the spring arms includes an axially elongated rail extending radially outward from a distal end thereof, the rails engaging grooves in the flange.

10. The turbine rotor assembly of claim 6 wherein a stop block extends radially inward from a distal end of each of the spring arms, and a limit tab extends radially inward from each stop block.

11. The turbine rotor assembly of claim 6 further comprising an additional member abutting the pocket so as to retain the balance weight in the pocket in an axial direction.

12. The turbine rotor assembly of claim 11 wherein the centerbody includes a notch formed at a radially inner end thereof which abuts the additional member.

13. A balance weight for a turbine rotor, comprising:
a block-like centerbody;
a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape;
at least one locating structure extending from a radially outer surface of the balance weight; and
a limit tab extending radially inward from a distal end of each of the spring arms;
wherein a shear pin extends radially outward from a distal end of each of the spring arms.

14. A turbine rotor assembly, comprising:
a rotor element including an annular hub surface and an annular flange surrounding the hub surface, spaced away from the hub surface so as to define a pocket; and
at least one balance weight disposed in the pocket, comprising:
a block-like centerbody;
a pair of resilient spring arms extending laterally from opposite sides of the centerbody, the centerbody and the spring arms collectively defining an arcuate shape;
at least one locating feature extending radially outward from the balance weight; and
a limit tab extending radially inward from a distal end of each of the spring arms;

wherein the spring arms and the centerbody resiliently bear against the flange and the hub surface, respectively, so as to retain the balance weight in the pocket; and
wherein a radial height of the limit tabs is selected so as to prevent insertion of the balance weight into the pocket if the spring arms are deflected beyond a predetermined limit; and
wherein each of the spring arms includes a shear pin extending radially outward from a distal end thereof, the shear pins engaging apertures in the flange so as to prevent axial movement of the balance weight relative to the turbine rotor.