Patent Publication Number: US-7594799-B2

Title: Undercut fillet radius for blade dovetails

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to stress reduction in the interface between a blade dovetail and a wheel slot and, more particularly, to a dovetail section including an undercut fillet radius having a multi-part profile shape formed at an intersection of the dovetail platform and a dovetail pressure surface. 
     FIGS. 1 and 2  show a conventional compressor blade assembly including a blade  12  fixed to a dovetail section  14 , which is attachable to a compressor wheel (not shown). An analysis of a failed blade shows that the failure resulted from fretting on the dovetail pressure surfaces  16  near the small fillet radius  18  that transitions from the blade neck  20  to the dovetail platform  22 . The analysis showed the stress in the small 0.022 fillet radius  18  was substantial enough to grow micro-cracks in the fretted area eventually causing ultimate failure (blade liberation). A subsequent review of several hundred parts showed fretting was prevalent in these areas in nearly all parts observed. 
   An undercut radius concept on compressor blade dovetails has been previously proposed. See, for example, U.S. Pat. No. 6,769,877. A subsequent dovetail section design incorporated a “P-cut” feature  24  as shown in  FIG. 2 . The P-cut feature  24  in the dovetail  14  creates a change in the stress profile unlike that seen on a typical compressor blade dovetail. The prior undercut radius concept did not accommodate this unique stress profile and had a negative affect on the design stress parameters of the P-cut section  24 . 
   BRIEF DESCRIPTION OF THE INVENTION 
   In an exemplary embodiment of the invention, in a turbine or compressor blade assembly including a blade fixed to a dovetail section attachable to a wheel, the dovetail section includes a dovetail shaped to fit in a correspondingly shaped slot in the wheel, a dovetail platform serving as an interface between the blade and the dovetail, and an undercut fillet radius formed at an intersection of the dovetail platform and a dovetail pressure surface. The undercut radius has a multi-part profile shape configured to attenuate edge of contact stresses. 
   In another exemplary embodiment of the invention, a rotor assembly includes a rotor wheel including a plurality of slots, and a plurality of blade assemblies each including a blade and a dovetail section engageable in a respective one of the rotor wheel slots. The dovetail section of each of the blade assemblies includes a dovetail shaped to fit in a correspondingly shaped slot in the wheel, a dovetail platform serving as an interface between the blade and the dovetail, and an undercut fillet radius formed at an intersection of the dovetail platform and a dovetail pressure surface. The undercut radius has a multi-part profile shape configured to attenuate edge of contact stresses. 
   In still another exemplary embodiment of the invention, a method of manufacturing a dovetail section for a compressor or turbine blade assembly engageable with a wheel slot in a rotor wheel includes the steps of providing a dovetail shaped to fit in the wheel slot, and forming an undercut fillet radius at an intersection of dovetail platform and a dovetail pressure surface. The undercut radius is formed with a multi-part profile shape configured to attenuate edge of contact stresses, the multi-part profile shape including at least a large radius part, a small radius part, and a flat part. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a conventional compressor blade assembly; 
       FIG. 2  is a close-up view of the conventional compressor blade assembly dovetail section; 
       FIG. 3  is a perspective view of a dovetail section incorporating features of the invention described herein; 
       FIG. 4  illustrates the interface section of interest between the blade dovetail and a compressor wheel; 
       FIG. 5  is a close-up view of the dovetail/wheel interface incorporating features of the invention described herein; 
       FIG. 6  is a close-up view of a conventional dovetail/wheel interface; and 
       FIG. 7  shows the multi-part undercut radius of the invention and a relative position of the flat part to the dovetail pressure surface. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  is a perspective view of a turbine or compressor blade assembly including a modified dovetail section. The blade assembly includes a blade  12  (airfoil portion), a dovetail platform  22 , and an attachment or root portion (dovetail section)  14  that typically is formed with a dovetail configuration, which enables the blade assembly to be loaded onto a compressor wheel or rotor  30  (see  FIGS. 4-6 ). 
   A P-cut  24  relief slot is formed at the forward end of the dovetail section  14 . This feature reduces the airfoil leading edge stresses making the blade less susceptible to damage on the leading edge. 
   Material is removed from and along the front face of the dovetail pressure surface  16  to form an undercut fillet radius  26  at an intersection of the dovetail platform  22  and the dovetail pressure surface  16 . The undercut radius  26  extends toward a forward end of the dovetail  14 , wherein an axial location of the undercut fillet radius termination is defined a predetermined distance  28  from the P-cut. 
   With reference to  FIGS. 4-6 ,  FIG. 4  illustrates the interface surface of interest between the dovetail section  14  and the compressor wheel  30 .  FIG. 6  is a close-up view of a prior art design 0.022 fillet radius. As noted, it has been discovered that fretting on the dovetail pressure surfaces near the small fillet radii that transitions from the neck to the dovetail platform has caused compressor blade failures.  FIG. 5  illustrates a preferred resolution of the problem including a larger fillet radius at the pressure surface  16  to platform  22  intersection including a multi-part profile shape configured to attenuate edge of contact stresses. 
   A preferred multi-part profile includes at least a three-part profile shape including a large radius part  32 , a small radius part  34 , and a flat part  36 . This three-part design provides an improved stress state in the undercut  26  compared to a single radius design (e.g.,  FIG. 6 ). Finite element analyses were performed on both the prior art and the undercut concept ( FIG. 6  and  FIG. 5 , respectively). The prior art  FIG. 6  results were calibrated to engine-measured stresses thus validating the analysis technique. The undercut concept  FIG. 5  results demonstrated a stress reduction at operating conditions of approximately 40% steady stress and approximately 50% vibratory stress. 
   The flat part  36  and its angular relationship to the dovetail pressure surface  16 , as shown in  FIG. 7 , is important in the area in separation of stresses between the edge of contact  38  and the undercut radii  32 ,  34 . In a preferred arrangement, the angle φ between the flat part  36  and the pressure surface  16  is about 40°. Other undercut angles are possible but must be evaluated carefully. Through design of experiments finite element modeling it was determined that 40° provided the most stress reduction and most stress separation. 
   As noted, the axial location of the undercut fillet radius termination is defined a predetermined distance  28  from the P-cut  24  to accommodate the stress profile resulting from the P-cut  24 . The predetermined distance  28  may be determined using finite element analyses or the like and may vary depending on a size of the blade assembly. Undercut runout/termination must be positioned to accommodate a compromise between manufacturing and desired stress state. An undercut too close to the P-cut relief slot will produce high stresses in the P-cut relief slot. An undercut too far away from the P-cut relief slot will not entirely clean up the prior pressure face 0.022 fillet radius  18  (which is an unacceptable condition). 
   The multi-part profile undercut fillet radius described herein reduces the potential for fretting-related blade failures. The profile shape of the undercut radius serves to attenuate edge of contact stresses to produce a low stress zone between the edge of contact and the larger undercut radius. Moreover, the axial location of the undercut radius termination relative to the P-cut feature serves to meet stress criteria. The design takes into account the unique stress profile of the P-cut feature and provides a solution that enables the P-cut feature to undercut radius transition area to meet its design stress parameters. The three-part profile shape of the undercut radius provides an improved stress state in the undercut compared to a single radius design. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.