Abstract:
A low stress seal seals boundaries between rotor sections of a turbofan engine. The low stress seal includes rounded feet on the ends of legs, which rounded feet cooperate with rounded interior corners within a seal groove. The elimination of sharp interior corners reduces stress and prevents the fatigue failures.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to turbofan jet engines and, more particularly, to seals used to separate a high-pressure flow near the center of the engine from a lower pressure flow surrounding the former flow. 
     Modern aircraft use turbofan jet engines to increase efficiency and reduce noise as compared to turbojet engines. While the former engines provide these desirable results, turbofan engines also add some complexity to engine design. The rotating assembly of turbojet engines includes a high pressure compressor comprising a set of rotors that compresses a core flow of air that passes through the high pressure compressor. Some of the compressed air is bled off into a secondary cooling flow that is used to cool hotter parts of the engine. The secondary cooling flow is contained in a cooling flow cavity that is proximal to the spin axis of the rotor, and between the spin axis and the core flow. 
     Known turbofan engines comprise cases and rotors constructed in sections. For example, U.S. Pat. No. 5,338,152 issued Aug. 16, 1994 for “Arrangement for Sealing Structural Members Using a V-Shaped Insert, Particularly in the Case of Turbo-Engines” describes such a seal used at the boundary between the case sections of a turbo-engine. The seal described in the &#39;152 patent resides in a groove in a structural member. The groove includes sharp interior corners and the seal includes legs that angle into the corners. Such sharp corners and legs provide an adequate seal and work well in stationary structures such as engine cases. 
     However, due to the pressure differential between the core flow at the entry to the high pressure compressor and the secondary cooling flow at a high pressure, seals are also required at some of the boundaries between rotor sections. A known seal design, for use at such boundaries, has a “V” cross-section. A seal seat comprises a rectangular groove which straddles the boundary between rotor sections. The groove is on the surface of the cooling flow cavity facing inwardly toward the rotor spin axis, and has sharp (90 degree) interior corners. The ends of the legs of the “V” angle into the sharp interior corners of the rectangular seat. 
     Exemplary of the foregoing, as shown in FIG. 1, includes curvic teeth  18  that join the rotors. A seal  27  is seated in rectangular groove  28  that straddles the curvic teeth  18 . The rectangular groove  28  includes sharp interior corners  29   a  and  29   b . The seal  27  includes a first straight leg  30   a  and a second straight leg  30   b , wherein the first straight leg  30   a  and the second straight leg  30   b  angle into the interior corners  29   a  and  29   b . No feet are provided at the ends of the legs  30   a  and  30   b . The rotor spins at over 25,000 RPM, and the resulting centrifugal force on the interior corners  29   a  and  29   b  by the straight legs  30   a  and  30   b  causes wear leading to fatigue failures at the interior corners  29   a  and  29   b.    
     As can be seen, there is a need for a new seal design that reduces stress on the groove corners, and prevents the resulting failures. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a low stress seal is useable between sections of a rotating assembly. The seal comprises an apex; a first leg and a second leg, wherein the first leg and the second leg extend away from the apex on each side of the apex; a first foot at the end of the first leg; and a second foot at the end of the second leg, wherein the first foot and the second foot are rounded. A first volume and a second volume extend through the sections, wherein the first volume is proximal to the spin axis of the rotating assembly, a boundary between the sections is on a surface of the first volume, the surface faces the spin axis, and the surface faces away from the second volume. A first seal seat and a second seal seat are defined on the surface, wherein the first seal seat and the second seal seat are on opposite sides of the boundary between the sections, and the first foot and the second foot cooperate with the first seal seat and the second seal seat to prevent a flow from the first volume to the second volume. 
     In a second aspect of the present invention, a method for reducing stress in a seal groove at a boundary between rotors in a compressor of a turbofan engine comprises providing seal seats with rounded cross-sections, wherein the seal seats are within the seal grooves and the seal seats straddle the boundary between the rotors; providing a seal with rounded feet, wherein the seal includes an apex, a first leg extends from one side of the apex, a second leg extends from the other side of the apex, and the feet extend from the ends of the legs, wherein the feet are rounded inwardly and the feet cooperate with the seal seats to provide a pressure seal; and inserting the seal into the groove. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art V-seal used in known turbofan engines; 
     FIG. 2 shows a cross-sectional view of a high pressure compressor of a turbofan jet engine, with an arrow indicating the core flow through the high pressure compressor; 
     FIG. 3A shows a seal according to the present invention that is seated in a seal groove at the boundary between rotors in a non-operating compressor; and 
     FIG. 3B depicts a seal according to the present invention seated in the seal groove at the boundary between rotors when the compressor is operating. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Further, while the present invention is described in the context of a compressor for a turbofan engine, the present invention is not so limited. In general, the present invention may be used between any axially stacked members of rotating assemblies that may require pressure to be sealed. As an example, the present invention may be used in a variety of turbine engines. 
     A cross-section of a high pressure compressor  10  of a turbofan engine is shown in FIG.  2 . The cross-section in FIG. 2 is vertically symmetric, and labels are omitted on elements in the bottom half of FIG. 2 to reduce its complexity. The high pressure compressor  10  has a core flow  12  of air that is compressed as the core flow  12  passes through the high pressure compressor  10 . A compressor rotor of the high pressure compressor  10  shown in FIG. 2 may be constructed from rotor sections  15   a ,  15   b ,  15   c ,  15   d ,  15   e , and  15   f  assembled on a tieshaft  14 . The tieshaft  14  includes nuts, or other fittings, at each end and the rotor sections  15   a - 15   f  are forced together by the nuts. Passages in rotor section  15   e  allow some of the core flow  12  to be bled off into a secondary cooling flow  11 , after the core flow  12  has been compressed in the high pressure compressor  10 . The secondary cooling flow  11  is contained in the cooling flow cavity  31  and is used to cool hotter parts of the engine. The cooling flow cavity  31  is proximal to the spin axis of the rotor, and is between the spin axis and the core flow  12 . At the boundary between rotor sections  15   a  and  15   b , a pressure differential exists between the secondary cooling flow  11  (high pressure) and the core flow  12  (low pressure), and a seal  16  is required at this boundary to prevent gasses from escaping from the secondary cooling flow  11  to the core flow  12 . The seal  16  may also be used to seal boundaries between rotor sections  15   d  and  15   e , rotor sections  15   e  and  15   f , and rotor section  15   f  and the following section. 
     A detailed view of a sealing system having the seal  16  residing in a seal groove  17  is shown in FIG.  3 A. Here, the seal  16  is shown as it may appear when the high pressure compressor  10  is not in operation. Curvic teeth  18  join the rotors and provide for both the transmission of torque between rotors, and allow for limited motion of the rotors relative to the adjacent rotor. The seal groove  17  straddles the boundary between rotors and is substantially rectangular in cross section. Although the dimensions can vary, the seal groove  17  is typically about 0.10 to 0.30 inches deep and about 0.30 to 0.60 inches wide. The seal groove  17  may include a first side  19   a  opposite a second side  19   b , a base  20  between the first and second sides  19   a  and  19   b , a first seal seat  21   a  between the first side  19   a  and the base  20 , and a second seal seat  21   b  between the second side  19   b  and the base  20 . The base  20  may be substantially orthogonal to the boundary between the rotors  15   a - 15   f . The seal seats  21   a ,  21   b  are preferably rounded or curved to eliminate the fatigue failures that result from seal seats with sharp corners. Radii of the first and second seal seats are preferably smaller than the radii of a first foot  24   a  and of a second foot  24   b  described below. 
     The seal  16  may preferably comprise an apex  22 , a first leg  23   a  on one side of the apex  22 , and a second leg  23   b  on the other or second side of the apex  22 . A first foot  24   a  is adjacent the first leg  23   a  and a second foot  24   b  is adjacent the second leg  23   b . Each of the foregoing sections or components of the seal  16  smoothly transitions into the adjacent section, i.e., there are no sharp corners. Consequently, the apex  22  may be preferably rounded or curved to both reduce the stress level in the apex  22  and to simplify manufacturing the seal  16 . A typical radius of the apex  22  is about 0.030 to 0.060 inches, although the dimensions can vary for the specific application. Also, even though the apex  22  is described above as having a radius, the present invention contemplates that the apex  22  may have a varying radius and may not comprise an arc of an exact circle. 
     The legs  23   a  and  23   b  preferably extend away from the apex  22  forming a “V”. The first foot  24   a  is at a free end of the first leg  23   a , and the second foot  24   b  is at a free end of the second leg  23   b . The first foot  24   a  may preferably be rounded or curved inwardly towards the second foot  24   b , and the second foot  24   b  may preferably be rounded or curved inwardly towards the first foot  24   a . In order to avoid catching the free end of either the first foot  24   a  or the second foot  24   b  on the curvic teeth  18 , the first foot  24   a  and the second foot  24   b  may continue their free curved ends past the first seal seat  21   a  and the second seal seat  21   b , respectively. Thus, the free ends of the first foot  24   a  and the second foot  24   b  separate or extend away from the seal groove  17  and curl into an inside of the seal  16 . 
     While the specific dimensions can be varied, the cross section of the seal  16  may typically be about 0.30 to 0.50 inches high and about 0.30 to 0.60 inches wide. The length of the legs  23   a  and  23   b  may preferably be of the same length (and can be of different lengths if desired) and typically about 0.10 to 0.60 inches. An angular separation of the legs  23   a  and  23   b  can typically be between about 60 to 135 degrees. The first foot  24   a  and the second foot  24   b  may have the same or a different radius, such as between about 0.030 to 0.060 inches. Yet, even though the first foot  24   a  and the second foot  24   b  can be generally described in terms of a radius, it can be seen from FIG. 3A that they may comprise a part of a polygon with straight sides that generally provide an overall rounded configuration. The free ends of the first foot  24   a  and the second foot  24   b  may preferably separate from a surface of the seal groove  17  adjacent the teeth  18  by about 0.01 to 0.05 inches and/or at an angle of about 5 to 60 degrees. 
     The shape of the seal  16 , when the high pressure compressor  10  is operating, is shown in FIG.  3 B. There are two forces effecting the shape of the seal  16 . The rotor of the compressor  10  spins at a high spin rate and, as a result, the first foot  24   a  may be forced against the first seal seat  21   a  by a first centrifugal force  25   a  and/or the second foot  24   b  may be forced against the second seal seat  21   b  by a second centrifugal force  25   b . Further, due to the pressure differential between the secondary cooling flow  11  and the core flow  12 , there is a uniform pressure  26  on the seal  16 . The result of these forces causes the seal  16  to slightly deform as shown in FIG.  3 B. These forces may combine to push the first foot  24   a  into the first seal seat  21   a  and/or the second foot  24   b  into the second seal seat  21   b , thus providing a sealing force proportional to the pressure differential between the secondary cooling flow  11  and the core flow  12 . 
     As mentioned above, other shapes for low stress seals  16  will be apparent to those skilled in the art and be within the intended scope of the present invention. For example, an arced seal could provide similar behavior as the “V” shaped seal  16  and, thus, come within the scope of the present invention. The seal groove and the seal seats may likewise depart from the above description without departing from the scope of the present invention. Any seal seat including a similar curved sealing surface to reduce stress is intended to come within the scope of the present invention. 
     It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.