Patent Publication Number: US-RE39630-E

Title: Turbine blisk rim friction finger damper

Description:
TECHNICAL FIELD 
     The present invention relates generally to turbines and more particularly to a damper for dampening vibration in a turbine disk. 
     BACKGROUND OF THE INVENTION 
     Discussion 
     Turbine disks are commonly subject to high cycle fatigue failure due to resonant vibration and fluid-structure instabilities. Disks have several critical speeds wherein operation of the disk at any one of these speeds creates an amplified traveling wave within the disk, inducing potentially excessive dynamic stresses. At each of these critical speeds the wave is fixed with respect to the housing and can be excited by any asymmetries in the flow field. The resulting resonant vibration prevents the operation of conventional turbine disks at critical speeds. Fluid-structure instabilities arise due to coupling between the surrounding fluid and the disk, which can also induce excessive stresses and prevent operation at speeds above a threshold stability boundary. 
     In conventional turbine disks with separate blades assembled onto a disk, blade damping techniques are typically employed to reduce resonant response as well as to prevent the fluid-structure instability that results from the coupling of aerodynamic forces and structural deflections. Accordingly, it is common practice to control blade vibration in the gas turbine and rocket engine industry by placing dampers between the platforms or shrouds of individual blades attached to the disk with a dovetail or fir tree. Such blade dampers are designed to control vibration through an energy dissipating friction force during relative motion of adjacent blades in tangential, axial or torsional vibration modes. Blade dampers, in addition to the blade attachments, provide friction dampening for both disk and blade vibration. 
     This damping mechanism, however, is not feasible for integrally bladed turbine disks (blisks) unless radial slots are machined between each blade to introduce blade shank flexibility. The added complexity of the slots increases the rim load on the turbine disk and defeats some of the cost, speed and weight benefits of the blisk. Consequently, the lack of a blade attachment interface results in a significant reduction in damping and can result in fluid-structure instability at speeds other than the disk standing wave critical speeds. 
     Rim dampers have been utilized by the gear industry to reduce vibration in thinly webbed large diameter gears. In such applications a split ring or series of spiral rings are preloaded in one or more retainer grooves on the underside of the gear rim. At relatively low rim speeds the centrifugal force on the damper ring provides damping due to relative motion when the gear rim experiences vibration in a diametral mode. This method of friction damping, however, is not feasible at high rim speeds because the centrifugal force on the damper ring is of sufficient magnitude to cause the damper to lock-up against the rim. Lock-up occurs when the frictional forces become large enough to restrain relative motion at the interface, causing the damper ring to flex as an integral part of the rim. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a damper for an integrally bladed turbine disk which employs a plurality of fingers to reduce the vibration of an integrally bladed turbine disk. The damper is primarily intended to reduce vibration when the integrally bladed turbine disk vibrates in a diametral mode shape. However, the damper is also effective in reducing the vibration of turbine blades mounted on the disk rim. 
     It is another object of the present invention to provide a damper having a profile which applies a frictional contact force continuously over a disk profile to direct the contact force normal to the disk surface. 
     In one preferred form, the present invention provides a damper for reducing vibrations in an integrally bladed turbine disk. The damper includes an annular member and a plurality of fingers. The annular member is configured so that it is retained by a radial step on the inside face of the integrally bladed turbine disk rim. Alternatively, conventional fasteners may be employed to couple the annular member to the integrally bladed turbine disk rim. The plurality of fingers are coupled to and concentrically spaced around the annular member. Each of the fingers is adapted to provide relative circumferential motion with respect to the inside face of the integrally bladed turbine disk when the integrally bladed turbine disk vibrates in a diametral mode shape. The annular member is configured to provide structural support to the fingers so that they apply a contact force to the integrally bladed turbine disk that is directed normal the disk surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional view of an integrally bladed turbine disk assembly constructed in accordance with the teachings of the present invention; 
         FIG. 2  is a longitudinal cross-sectional view of a portion of the integrally bladed turbine disk assembly of  FIG. 1  illustrating the integrally bladed turbine disk; 
         FIG. 3  is an enlarged portion of the integrally bladed turbine disk illustrated in  FIG. 2 ; 
         FIG. 4  is a front elevational view of a portion of the integrally bladed turbine disk assembly of  FIG. 1  illustrating the damper; 
         FIG. 5  is an enlarged portion of the damper illustrated in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of the damper taken along the line  6 — 6  of  FIG. 4 ; 
         FIG. 7  is a cross-sectional view of the integrally bladed turbine disk assembly of  FIG. 1 ; 
         FIG. 8  is a cross-sectional view of an integrally bladed turbine disk assembly constructed in accordance with an alternate embodiment of the present invention; 
         FIG. 9  is a longitudinal cross-sectional view of the integrally bladed turbine disk assembly of  FIG. 8 ; 
         FIG. 10  is a front elevational view of a portion of the integrally bladed turbine disk assembly of  FIG. 8  illustrating the damper in greater detail; 
         FIG. 11  is an enlarged view of a portion of the damper illustrated in  FIG. 10 ; and 
         FIG. 12  is a cross-sectional view of a portion of the damper taken along the line  12 — 12  of FIG.  10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to  FIG. 1  of the drawings, a turbopump  10  wherein various embodiments of the present invention may be effectively utilized is shown in a cross-sectional view. The turbopump  10  is shown to include an integrally bladed turbine disk assembly  12  having an integrally bladed turbine disk  14  and a damper  16 . 
     In  FIGS. 2 and 3  a portion of the integrally bladed turbine disk  14  is shown in cross-sectional view. The integrally bladed turbine disk  14  is symmetrical about a longitudinal axis  20  and includes a unitarily formed rotor portion  22  having a plurality of radially extending blades  24  and an axial face  26 . In the particular embodiment illustrated, a damper cavity  28  having a first cavity portion  30  and a second cavity portion  32  is formed into the axial face  26 . The first cavity portion  30  is formed into the axial face  26  in a direction perpendicular to the longitudinal axis  20 . The first cavity portion  30  includes an annular face  34  and a radial lip portion  36 . The second cavity portion  32  includes an arcuate inner surface  38  which intersects the annular face  34 . 
     The damper  16  is shown in  FIGS. 4 through 6  to include an annular member  40  and a plurality of T-shaped fingers  42  that are coupled to and spaced circumferentially around the annular member  40 . In the particular embodiment illustrated, the annular member  40  is a continuous hoop that is sized to engage the annular face  34  of the first cavity portion  30 . Each of the plurality of T-shaped fingers  42  includes a base portion  44  and a leg portion  46 . The base portion  44  is coupled to the annular member  40  and extends radially inward therefrom. The leg portion  46  is coupled to a distal end of the base portion  44  and extends tangentially therefrom. The T-shaped fingers  42  include an arcuate outer surface  48  which is configured to cooperate with the arcuate inner surface  38  in the second cavity portion  32  in a manner that will be discussed in detail below. 
     Preferably, the annular member  40  and the plurality of T-shaped fingers  42  are integrally formed. Construction in this manner permits each of the T-shaped fingers  42  to be formed by a pair of circumferentially-spaced, tangentially-oriented slots  50  and a pair of circumferentially-spaced, radially-extending slots  52 . As shown, each of the radially-extending slots  52  intersects one of the tangentially-oriented slots  50 . 
     In  FIG. 7  the damper  16  is shown in operative association with the integrally bladed turbine disk  14 . The damper  16  is preferably cooled in a liquid gas, such as liquid nitrogen, and shrunk-fit to the damper cavity  28  during the assembly of the integrally bladed turbine disk assembly  12 . The annular member  40  provides the damper  16  with continuity to permit it to be retained in position relative to the integrally bladed turbine disk  14 . The annular member  40  also provides a mechanism for preloading the plurality of T-shaped fingers  42  against the arcuate inner surface  38 . 
     In operation, the radially-extending slots  52  and tangentially-oriented slots  50  effectively decouple the tangential motion of the annular member  40  from the T-shaped fingers  42 . Due to high centrifugal forces present in the integrally bladed turbine disk assembly  12 , the annular member  40  is forced against the annular face  34  with sufficient force to cause lock-up. During lock-up, relative movement between the annular member  40  and the annular face  34  is inhibited. Due to the presence of the radially-extending slots  52  and tangentially-oriented slots  50 , the T-shaped fingers  42  are permitted to move tangentially at the frictional interface  54  between the integrally bladed turbine disk  14  and the damper  16  when the integrally bladed turbine disk assembly  12  vibrates in a diametral mode shape. The friction interface  54  includes an area where the annular member  40  and the T-shaped fingers  42  contact the annular face  34  and the arcuate inner surface  38 , respectively. Vibration of the integrally bladed turbine disk  14  in a diametral mode causes tangential motion between the T-shaped fingers  42  and the arcuate inner surface  38 . The circumferential length and thickness of the radially-extending slots  52  and tangentially-oriented slots  50  are selected to optimize the damping, centrifugal force, and relative tangential motion for a particular application. 
     Another unique feature of the damper  16  is the configuration of its contact surface  60  (shown in FIG.  6 ). The contact surface  60  includes the arcuate outer surface  48  of the T-shaped fingers  42  and the annular outer surface  62  of the annular member  40 . The contact surface  60  is configured in a manner wherein the annular member  40  provides a first contact force and the T-shaped fingers  42  provide a second contact force. The first contact force provided by the annular member  40  is applied to the integrally bladed turbine disk  14  in a radial direction through the annular outer surface  62 . The arcuate outer surface  48  causes the second contact force applied by the T-shaped fingers  42  to vary constantly from a radial direction to an axial orientation (i.e., against a radially extending portion of the axial face  26  of the integrally bladed turbine disk  14 ). Consequently, the majority of the damper centrifugal load is transferred to the integrally bladed turbine disk  14  through the annular member  40  while the T-shaped fingers  42  provide a much smaller contact force. Configuration in this manner prevents lock-up between the T-shaped fingers  42  and the integrally bladed turbine disk  14 . 
     The frictional characteristics of the contact surface  60  may be controlled through the finishing of contact surface  60  to a desired surface finish or through the application of a coating, such as silver plating or molydisulfide. Silver plating is highly desirable as it is resistant to fretting which can result from micro-motion between the damper  16  and the integrally bladed turbine disk  14 . 
     While the integrally bladed turbine disk assembly  12  has been described thus far as including a damper  16  with T-shaped fingers  42  which is shrunk-fit to a damper cavity  28  during the assembly of the integrally bladed turbine disk assembly  12 , those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. For example, the damper  16 ′ may be coupled to a face of the integrally bladed turbine disk  14 ′ as illustrated in  FIGS. 8 and 9 . In this arrangement, integrally bladed turbine disk assembly  12 ′ is shown to include a pair of dampers  16 ′ which are coupled to the integrally bladed turbine disk  14 ′ via a plurality of fasteners  100 . Integrally bladed turbine disk  14 ′ is symmetrical about its longitudinal axis  20 ′ and includes a unitarily formed rotor portion  22 ′ having a plurality of radially extending blades  24  and an pair of axial faces  26 ′. 
     In the particular embodiment illustrated, a damper cavity  28 ′ having a first cavity portion  30 ′ and a second cavity portion  32 ′ is formed into each of the axial faces  26 ′. The first cavity portion  30 ′ is formed into the axial face  26 ′ in a direction parallel the longitudinal axis  20 ′. The first cavity portion  30 ′ includes an plurality of fastener apertures  102 . The second cavity portion  32 ′ is illustrated to include a circumferentially extending wall member  104  which is skewed to the first cavity portion  30 ′, thereby providing the second cavity portion  32 ′ with a shape corresponding to a truncated inverse cone. Those skilled in the art will understand that the shape of second cavity portion  32 ′ may be tailored in a desired manner to achieve specific design goals and as such, the second cavity portion  32 ′ may alternatively be arcuately shaped. 
     In  FIGS. 9 through 12 , the damper  16 ′ is shown to include an annular member  40 ′ and a plurality of fingers  42 ′ that are coupled to and spaced circumferentially around the annular member  40 ′. In the particular embodiment illustrated, the annular member  40 ′ is a flange that abuts the first cavity portion  30 ′. Each of the plurality of fingers  42 ′ includes a base portion  44 ′ and an end portion  46 ′. The base portion  44 ′ is coupled to the annular member  40 ′ and extends radially inward therefrom. The end portion  46 ′ is coupled to a distal end of the base portion  44 ′ and extends therefrom to contact the second cavity portion  32 ′. The fingers  42 ′ include an outer surface  48 ′ which is configured to cooperate with the wall member  104  of the second cavity portion  32 ′ in a manner that will be discussed in detail below. Preferably, the annular member  40 ′ and the plurality of fingers  42 ′ are integrally formed. Construction in this manner permits each of the fingers  42 ′ to be formed by a pair of circumferentially spaced, radially extending slots  52 ′. As shown, each of the radially extending slots  52 ′ terminates at a slot aperture  110  which is employed to reduce the concentration of stress at the intersections between annular member  40 ′ and each of the plurality of fingers  42 ′ when damper  16 ′ is in operation. 
     In  FIGS. 8 and 9 , the plurality of fasteners  100  are illustrated to include a plurality of externally threaded fasteners  114 , a plurality of internally threaded nuts  116  and a plurality of dog-bone washers  118 . Each of the dog-bone washers  118  is positioned over a pair of circumferentially adjacent fastener apertures  120  and  102  formed into the annular member  40 ′ and the first cavity portion  30 ′ of the integrally bladed turbine disk  14 ′, respectively. Externally threaded fasteners  114  are placed through fastener apertures  120  and  102  and internally threaded nuts  116  are threadably engaged to the externally threaded fasteners  114  such that a clamping force is generated by fasteners  100  to retain annular member  40 ′ such that annular member  40 ′ will not rotate about the longitudinal axis  20 ′. 
     In operation, the radially extending slots  52 ′ effectively decouple the tangential motion of the annular member  40 ′ from the fingers  42 ′. The radially extending slots  52 ′ permit the fingers  42 ′ to move tangentially at a frictional interface  54 ′ between the integrally bladed turbine disk  14 ′ and the damper  16 ′ when the integrally bladed turbine disk assembly  12 ′ vibrates in a diametral mode shape. The friction interface  54 ′ includes an area where the fingers  42 ′ contact the wall member  104  of the second cavity portion  32 ′. Vibration of the integrally bladed turbine disk  14 ′ in a diametral mode is transmitted to and absorbed by damper  16 ′. In this regard, the vibrations cause tangential motion in the plurality of fingers  42 ′ relative to wall member  104  so that the energy of the vibrations is absorbed in the friction interface  54 ′ by frictional contact between the plurality of fingers  42 ′ and the wall member  104 . 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.