Undercut fillet radius for blade dovetails

Abstract

A turbine or compressor blade assembly includes a blade fixed to a dovetail section attachable to a wheel. The dovetail section has a dovetail shaped to fit in a correspondingly shaped slot in the wheel. A dovetail platform serves as an interface between the blade and the dovetail. An undercut fillet radius is formed at an intersection of the dovetail platform and a dovetail pressure surface, where the undercut radius has a multi-part profile shape configured to attenuate edge of contact stresses. An additional feature is the area where the undercut radius transitions into the P-cut area at the forward end (leading edge) of the dovetail.

Description

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 400 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.

Claims

1. In a turbine or compressor blade assembly including a blade fixed to a dovetail section attachable to a wheel, the dovetail section comprising: 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; andan undercut fillet radius formed at an intersection of the dovetail platform and a dovetail pressure surface, wherein the undercut radius has a multi-part profile shape configured to attenuate edge of contact stresses.

2. A dovetail section according to claim 1, wherein the undercut radius comprises a three-part profile shape.

3. A dovetail section according to claim 2, wherein the three-parts include a large radius part, a small radius part, and a flat part.

4. A dovetail section according to claim 3, wherein an angle between the flat part and the pressure surface is about 40°.

5. A dovetail section according to claim 1, further comprising a P-cut formed adjacent a forward end, wherein an axial location of the undercut fillet radius termination is defined a predetermined distance from the P-cut to accommodate a stress profile resulting from the P-cut.

6. A rotor assembly comprising: a rotor wheel including a plurality of slots; anda plurality of blade assemblies each including a blade and a dovetail section engageable in a respective one of the rotor wheel slots, wherein the dovetail section of each of the blade assemblies comprises: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, andan undercut fillet radius formed at an intersection of the dovetail platform and a dovetail pressure surface, wherein the undercut radius has a multi-part profile shape configured to attenuate edge of contact stresses.

7. A rotor assembly according to claim 6, wherein the undercut radius comprises a three-part profile shape.

8. A rotor assembly according to claim 7, wherein the three-parts include a large radius part, a small radius part, and a flat part.

9. A rotor assembly according to claim 8, wherein an angle between the flat part and the pressure surface is about 40°.

10. A rotor assembly according to claim 6, wherein the dovetail section further comprises a P-cut formed adjacent a forward end, wherein an axial location of the undercut fillet radius termination is defined a predetermined distance from the P-cut to accommodate a stress profile resulting from the P-cut.

11. A method of manufacturing a dovetail section for a compressor or turbine blade assembly engageable with a wheel slot in a rotor wheel, the method comprising: providing a dovetail shaped to fit in the wheel slot; andforming an undercut fillet radius at an intersection of a dovetail platform and a dovetail pressure surface, wherein 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.

12. A method according to claim 11, wherein an angle between the flat part and the pressure surface is about 40°.

13. A method according to claim 11, wherein the dovetail section comprises a P-cut formed adjacent a forward end, and wherein the forming step comprises terminating a forward end of the undercut fillet radius at an axial location a predetermined distance from the P-cut to accommodate a stress profile resulting from the P-cut.