Abstract:
A hoop stress relief mechanism is disclosed for use on a rotary body to relieve stress caused by both thermal and centrifugal forces. The mechanism may consist of a J-shaped slot cut from the outer rim of the rotary body a distance inwardly toward the axis of rotation, the slot having a curve in its inward end that curves back towards the outer rim. The J-shaped slot may extend through the rotary body to join its two faces. The J-shaped slot may be fabricated by an electric discharge wire machine. The electric discharge wire machine may make multiple passes in order to smooth the bottom surface of the curved slot portion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to articles of manufacture and methods of fabrication for mechanisms to relieve hoop stress in rotating bodies and, more specifically, to a J-slot modification to a rotating disk having integral cast blades such as those contained in a gas turbine engine.  
         [0002]     Hoop stress is defined as a load measured in the direction of the circumference of a rotating body, the load being created by thermal gradients and centrifugal forces acting in a radial direction outwardly from the axis of rotation of the body. Such stress is particularly acute in the art of gas turbine engine design where the turbine disks may have integrally cast blades. Such turbine disks have been observed to develop fractures along the circumference of the disk during use.  
         [0003]     A number of methods were devised to prevent such fractures. Initially a series of circumferential slots were fabricated into the outer edge of the disk and extending inwardly, the slots being produced using an electric discharge wire machine (EDM). The slots were observed to develop fractures at the inner end nearest the axis of rotation during use, so that a relief hole was drilled at the inner end of the slot to prevent further fracturing. The relief hole was observed to promote increased hot gas ingestion through the disk, so that a rivet or pin had to be inserted through the hole to block such gas ingestion.  
         [0004]     This particular prior art hoop stress relief mechanism is shown in  FIGS. 1 and 2 . According to  FIG. 1 , a prior art hoop stress relief mechanism is shown as fabricated into a section of rotary body  100  with integral blades  110 . A slot  120  may be cut into the rotary body  100  radially from an outer rim  130  to intersect with a hole  140 . The slot  120  and hole  140  extend through the rotary body  100  so that the face  150  and the opposing face (not shown) of the rotary body  100  may be connected. A rivet  160  shown in phantom line may be inserted into the hole  140  and secured, so that hot gasses impacting face  150  may be prevented from flowing through the hole  140  to the opposing face of the rotary body  100 . Fabrication of the prior art hoop stress relief mechanism as shown may comprise the steps of drilling hole  140  through face  150  of the rotary body  100 , using an EDM to fabricate a continuous slot  120  from the outer rim  130  to the hole  140 , and deburring and reaming the hole  140  so that any gouges in the walls of hole  140  may be prevented from serving as sites for fractures in the rotary body  100 . Referring to  FIG. 2 , a plurality of prior art slots  120  and holes  140  are fabricated between blades  110  around the circumference of the rotary body  100  so that hoop stress may be reduced and evenly distributed about the entire circumference of the outer rim  130 .  
         [0005]     The method for fabricating this hoop stress relief mechanism involves a number of manufacturing steps. Referring to  FIG. 1 , the hole  140  must be first drilled through the rotary body  100  and then reamed to remove any objectionable grooves or defects in the hole walls that formed in the drilling process. Next, the slot  120  must be machined from the outer rim  130  of the rotary body  100  to intersect the hole  140 . A rivet  160  must then be installed in the hole  140  to inhibit the flow of hot gasses through the hole  140  thus formed. These steps are used to fabricate the hole-and-slot configuration about the outer rim  130  of the rotary body  100 , and then the rotary body  100  is spun and balanced. The rivets  160  must then be inspected after the spinning operation to ensure that they are still properly seated and not deformed by the centrifugal force generated by the spin operation.  
         [0006]     However, there are a number of problems associated with this mechanism: First, the method of fabricating the hoop stress relief mechanism involves a detailed sequence of operations that must be precisely executed. This sequence consists of drilling a hole of exact proportions through the disk, reaming the hole to eliminate ridges and grooves within the walls of its bore, cutting an EDM slot from the rim of the disk to the hole, inserting a rivet through the hole to prevent hot gas ingestion from an adjacent space, and inspecting the rivet for correct installation and placement. This sequence is labor intensive, time consuming, and exacting, and thus expensive.  
         [0007]     Second, the rivet inserted into the drilled hole is frequently dislodged by vibration, thermal shock, or mechanical means during use. The rivet thus released can cause downstream damage within the turbine. Also, hot gases may subsequently leak through the turbine disk and reduce engine efficiency.  
         [0008]     A third problem is that rivets have varying tolerances, so that when installed, they present a balancing problem. As the turbine rotates more rapidly, rivets that are mismatched as to size, weight, or placement along the circumference of the disk start producing unacceptable vibration. Too much vibration can cause the entire turbine to fail.  
         [0009]     A fourth problem is that there are variations between different tools used to fabricate the holes and slots, which must be accounted for. For example, in a test, 24 holes were drilled with a 0.120″ drill, reamed with a 0.128″ reamer, and finally finished by four 0.1315″ reamers (6 holes each) to determine if tool variation was significant. An analysis of variance of the surface finish as a function of the block (final reamer) yielded a p-value &lt;0.05, that is, the confidence is greater than 95% (p-value is a statement of probability where confidence=1−p-value). This test showed that the reamer/tool is significant and influences the surface finish. Mean surface finish for each tool ranged from 9.8 Ra to 25.9 Ra. Therefore, hole-drilling quality is limited by tool variation and is a problem in production fabrication. Current hole drilling processes impart detrimental flaws to the inner diameter surface of the rivet hole; these flaws can serve as sites at which fractures are initiated.  
         [0010]     A number of similar methods have been found in the prior art to relieve hoop stress in various engine parts. U.S. Pat. No. 3,781,125 teaches the use of a keyhole shaped slotted portion in the outer shroud structure of a nozzle vane structure for a gas turbine engine. The keyhole shaped slot reduces stress due to larger temperature gradients. A threaded sealing member, instead of a rivet, is inserted into the keyhole to restrict gas flow. However, this application is made for a non-rotating shroud, and not for a turbine disk, and therefore does not experience the same problems as would be experienced by a rapidly rotating turbine disk.  
         [0011]     U.S. Pat. No. 4,536,932 teaches a method of forming a turbine disk having integral blades from a plate shaped forging preform. A plurality of slots is formed between the integral blades, and the slots are then closed by the forging process. A rod or wire may be inserted at the base of each slot to increase the radius at the end of the slot. However, this process is amenable only to forging processes and does not address machining issues regarding the slot bases.  
         [0012]     U.S. Pat. No. 5,071,313 teaches the use of T-shaped relief slots of a shroud body of a gas turbine engine. The relief slots are made in the outer portion of a non-rotating shroud for relief from thermal stress and not for centrifugal stress, where balancing and uniformity of the slots is a concern. The teaching is made for a non-rotating shroud, and not for a turbine disk, and therefore does not experience the same problems as would be experienced by a rapidly rotating turbine disk. The teaching does not discuss any considerations in the fabrication of the slots.  
         [0013]     As can be seen, there is a need for a mechanism for relieving hoop stress in a rapidly rotating turbine disk structure, where the mechanism is simple to fabricate, does not allow excessive passage of hot gasses through the turbine disk, does not employ rivets which may become dislodged through use, and does not depend upon uniformity of the machining tools used to fabricate the mechanism.  
       SUMMARY OF THE INVENTION  
       [0014]     In one aspect of the present invention, a hoop stress relief mechanism is provided for a solid rotary body with two faces and an outer rim. The mechanism comprises a slot extending inwardly a distance from the outer rim and providing communication between the first face and the second face, the slot having a first end at the outer rim and a second end, the slot also having a curved slot portion adjoining the second end.  
         [0015]     In another aspect of the invention, a turbine disk with a hoop stress relief mechanism is provided, where the hoop stress relief mechanism comprises a plurality of J-shaped slots fabricated into the rim of the turbine disk.  
         [0016]     In another aspect of the invention, a rotary body with an axis of rotation about which the rotary body rotates is provided, where the rotary body comprises a disk portion with an outer rim and a circular first and second faces; and a slot with a linear slot portion extending inwardly a distance from the outer rim and providing communication between the first face and the second face, the slot having a first end at the outer rim and a second end. The slot also has a curved slot portion adjoining the second end of the linear slot portion, the curved slot portion with a top surface and a bottom surface, the bottom surface being closer than the top surface to the axis of rotation of the rotary body.  
         [0017]     In another aspect of the invention, a method of fabricating a hoop stress relief mechanism in a rotary body with an outer rim, two faces, and an axis of rotation is provided, the method comprising cutting of a plurality of J-shaped slots around the rim of a rotary body, where each slot penetrates the rotary body from face to face.  
         [0018]     In still another aspect of the invention, an electric discharge wire machine may be used to cut the J-slots into the rim of the rotary body.  
         [0019]     In yet another aspect of the invention, a method is provided for fabricating a slot in a rotary body having an outer rim, two faces, and an axis of rotation, where the slot has a linear slot portion extending from a first point at the outer rim to a second point situated inwardly a distance towards the axis of rotation, the slot further having curved slot portion continuing from the second point and curving back towards the outer rim to terminate at a third end, the slot allowing communication between the two faces. The method comprises the steps of cutting the slot in the rotary body with wire of an electric wire discharge machine by making a first pass from the first point to the third point to form the slot; removing a first portion of a recast layer formed along a bottom surface of the curved slot portion by moving the wire inwardly towards the axis of rotation by a first offset and moving the wire in a second pass from the third point to the second point, the second pass being parallel with the path of the first pass; removing a second portion of the recast layer formed along the bottom surface by moving the wire inwardly towards the axis of rotation by a second offset and moving the wire in a third pass from the second point to the third point, the third pass being parallel with the path of the second pass; and removing a third portion of the recast layer formed along the bottom surface by moving the wire inwardly towards the axis of rotation by a third offset and moving the wire in a fourth pass from the third point to the second point, the fourth pass being parallel with the path of the third pass; so that the total portion of the recast layer along the bottom surface that is removed equals the sum of the first, second, and third offsets.  
         [0020]     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  
       [0021]      FIG. 1  shows a front view of a turbine disk illustrating a single, prior art hoop stress relief mechanism fabricated therein;  
         [0022]      FIG. 2  shows a front view of a turbine disk with a plurality of prior art hoop stress relief mechanisms as they are fabricated along the rim of the turbine disk;  
         [0023]      FIG. 3A  shows a front view of a turbine disk illustrating a J-slot hoop stress relief mechanism, according to an embodiment of the invention;  
         [0024]      FIG. 3B  shows a side perspective view of a turbine disk illustrating the placement of the J-slot hoop stress relief mechanism with relationship to the blades of a turbine disk, according to an embodiment of the invention;  
         [0025]      FIG. 4  shows a side perspective view of turbine disk with a plurality of hoop stress relief mechanisms as they are fabricated along the rim of the turbine disk, according to an embodiment of the invention;  
         [0026]      FIG. 5  shows a cross section of a rotary body with a tapered rim angle as defined by blade design, according to an embodiment of the invention;  
         [0027]      FIG. 6  shows a top perspective view of the rim of a rotary body, illustrating the placement of a J-slot between two adjacent blades, according to an embodiment of the invention;  
         [0028]      FIG. 7  shows the geometry of a representative J-slot with the orientation of the slot angle; and  
         [0029]      FIG. 8  shows a representative path taken by an EDM wire in fabricating the J-slot, according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     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.  
         [0031]     The invention provides an innovative mechanism for relieving fractures along the rim of a rotating body that may be caused by hoop stress forces. The innovative mechanism may be a slot extending inwardly from the rim in a generally radial direction and terminating in a curved portion. The slot may extend through the disk of the rotating body. Fabricating such a stress relieving slot in the circumferential rim of a disk may be inexpensive since it does not involve as many steps as the rivet mechanism described previously. There is minimal leakage through the disk and the slot may be essentially free from any hole drilling surface anomalies that may be caused by the drilling process.  
         [0032]     Referring now to the drawings wherein like reference numerals are used throughout the various views to designate like parts and, more particularly, to  FIGS. 3A and 3B , according to these figures, a segment  300  of a rotary body is shown, where the segment  300  has integral blades  110  cast with a disk portion  320 . A hoop stress relief mechanism  330  is shown between the blades  110  of the segment  300  and passing through the disk portion  320  to exit on a front face  350  and a rear face  360  of the disk portion  320 . Referring more particularly to  FIG. 3A  for detail, the relief mechanism  330  may be comprised of a J-slot  370  extending inwardly from a rim  130  of the segment  300 . A linear slot portion  372  of the J-slot  370  may extend a distance before curving back upon itself in a curved slot portion  375  to form the J-slot  370 . A plurality of J-slots  370  may be spaced along the circumference of a turbine disk as shown in  FIG. 4  to provide stress relief for both thermal and centrifugal forces that would tend to cause fractures about the rim  130 . The ends of the curved slot portions  375  may extend in either the direction of rotation of the rotary body  100  or the opposite direction.  
         [0033]     The geometry of a typical J-slot  370  and its relationship to a rotary body  100 , such as a turbine disk, is shown in  FIGS. 5, 6 , and  7 . According to  FIGS. 5 and 6 , the stacking axes  510  of the rotary body  100  may exist in a plane perpendicular to its center of rotation and between its faces  150 , such that the rotary body  100  may be balanced along each side of the stacking plane  510 . Blades  110  may be integrally cast with the rotary body  100  such that they may extend radially from the rim  130  and the center of rotation of rotary body  100 . It should be noted that the rim  130  is not necessarily perpendicular to the stacking axis  510  but may taper towards one face  150  or the other by a disk rim taper angle  520 . The J-slot curved surface  375  may also be tapered with angle  530  from one face  150  to the other face  150  of the rotary body  100 . Each blade  110  may have a fillet  111  at its base where it flares to meet the rim  130  without an abrupt change in the contour between the blade  110  and the rim  130 .  
         [0034]     As shown in  FIG. 6 , a J-slot  370  may be fabricated between two adjacent blades  110 . The J-slot  370  may be fabricated at a slash angle  610 , which is defined as the angle of the J-slot  370  from a plane orthogonal to the plane of the stacking axes  510  drawn through the center of rotation of the rotary body  100 . The J-slot  370  should be constrained to avoid cutting through the fillet  111  of an adjacent blade  110 , which may weaken the blade structure. In order to position the J-slot  370  between two adjacent blades  110  without making contact with the fillet  111  of either blade  110 , it may be necessary to fabricate the J-slot  370  with a non-zero slash angle  610 . Ideally, the leading edge offset  630  and the trailing edge offset  620  from one blade  110  and the blade convex surface offset  640  from the other blade  110  should all be equal; but design and balancing considerations may require empirical adjustment of these values.  
         [0035]     Referring to  FIG. 7 , the geometry of a typical J-slot  370  is shown. Disk rim radius  710  may be defined as the distance from the center of rotation of the rotary disk to the rim  130  taken along the stacking plane  510 . The slot bottom radius  720  may be defined as the distance from the center of rotation of the rotary disk  100  to the bottom  780  of the J-slot  370  taken along the stacking plane  510 . The minor diameter of the J-slot  370  may be defined as two times the radial distance  730  from the bottom  780  of the J-slot  370  to a point  795  which is the point of tangency between the curved slot portion  375  and a radial line extending outward from the center of rotation of the rotary disk  100 . The major diameter  740  of the J-slot curved portion  370  is shown in  FIG. 7 . Point  790  may be defined as the intersection of the slot rim radius  710  drawn through the bottom  780  of the J-slot  370  and circumferential line drawn through point  795 . The linear slot portion  372  may be inclined at a slot angle  770  with respect to the rim  130 , defined as the angle between the linear slot portion  372  and the disk rim radius  710  drawn through point  790 .  
         [0036]     The gap  760  between the slot tip  796  and the linear slot portion  372  may advantageously be of a distance of 0.050 inch or greater to ensure product quality and producibility. The tip angle  765  may be defined by a first line drawn between point  790  and the slot tip  796  and a second line tangent to a circle having its center at the axis of rotation and drawn through point  790 . A tip angle  765  in a range between 20° to 80° may provide acceptable stress relief without failure of the J-slot  370 .  
         [0037]     Using EDM technology, an inventive method for fabricating the J-slot  370  in the rim of a rotary body may use the EDM to remove material in the rotary body according to a predetermined pattern to form the J-slot  370 . Referring to  FIG. 8 , it has been found that an EDM produces a recast layer  875 ,  876 ,  877  along the sides of a slot cut by the EDM. The recast layer  875 ,  876 ,  877  may be defined as the surface that results when an EDM has been used to cut away material. The EDM may generate sufficient heat in cutting away material that the surface along its path may have ridges, waves, and other irregularities. It has been found that in the art of turbine design, a rough surface may have a lower mean time before failure, because the hoop stress produced by rotation is concentrated by such irregularities; a smooth surface may thus have a longer service life. Therefore, it is desirable that the inner surfaces along a path made by an EDM have a smooth, uniform recast layer  875 ,  876 ,  877 . The extent of the recast layer  875 ,  876 ,  877  may be influenced by the speed of the EDM cut, the angle at which the cut is made, and the amount of heat generated thereby. It has been further found that the surface  860  ( FIG. 8 ) of the slot at the bottom of the curved slot portion  375  may be subjected to more hoop stress than the surface  870  along the linear slot portion  372  of the J-slot nearer the rim  130 , and therefore it may be desirable for surface  860  at the bottom portion of the J-slot nearest the axis of rotation of the rotary body  100  to be fairly smooth, i.e. have a smooth or minimal recast layer  876 . The recast layer  877  along top portion of the curved slot portion  375  may not be subjected to the same stress, and its thickness and uniformity may be immaterial.  
         [0038]     The recast layer  876  along the bottom of the curved slot portion  375  may be smoothed by various methods that may be within the scope of the invention. However, a sequence of back-and-forth passes of the EDM as indicated in  FIG. 8  may be advantageously provided by the inventive method to smooth the bottom of the curved slot portion  375 . A first pass  820  may be made as a rough cut through the linear slot portion  372  and the curved slot portion  375 , which may define the general shape of the J-slot. A second pass  830  may be made by reversing the direction of travel of the wire of the EDM as indicated and offsetting the path slightly in a direction normal to surface of the curved slot portion  372 , with the second pass  830  generally following the path of the first pass  820  through the curved slot portion  375 . The second pass  830  may be seen as removing a first portion of the recast layer  876  equal in thickness to the first offset of the path of the second pass  830 . When the wire, while traveling along the path of the second pass  830 , arrives at the junction of the linear slot portion  372  and the curved slot portion  375 , the wire may again be reversed and offset towards the center of the rotary body  100  by a second amount to follow the indicated path of the third pass  840 , thus removing a second portion of the recast layer  876  equal in thickness to the second offset of the path of the third pass  840 . Finally, the wire may again be reversed and offset by a third amount to follow the indicated path of the fourth pass  850 , thus removing a third portion of the recast layer  876  equal in thickness to the third offset of the path of the fourth pass  850 . Thus, the recast layer  876  created by the first pass  820  may be smoothed along the surface  860  of the curved slot portion  375  nearest to the center of the rotary body  100  by removing an amount of material from the recast layer  876  equal to the sum of the first, second, and third offsets.  
         [0039]     For example, it has been found through experimentation that a typical J-slot  370  may be fabricated according to the inventive method by making four passes on a Sodick machine using 0.008″ diameter wire. The second, third, and fourth passes may be offset by amounts of 0.00051″, 0.00063″, and 0.00004″, respectively. Other EDMs having different wire diameters and different offset values for the various passes could be used without departing from the scope of the invention. Current EDM technology can yield a recast thickness of less than 0.0002″ and a 32 Ra finish, resulting in an EDM wire J-slot  370  free from detrimental manufacturing flaws. This process may be repeated for selected locations around the perimeter of the rotary body to form a plurality of J-slots  370  around the rim of the rotary body as shown in  FIG. 4 . However, the number of slots may be equal to or less than the number of blades. Then the rotary body may be balanced to ensure that any manufacturing variations between different J-slots  370  are compensated for. The rotary body may also be spun up to speed to ensure a proper balance. It can be readily seen that use of the inventive method may eliminate three labor intensive manufacturing steps; where these steps are (1) hole drilling and reaming, (2) installation of rivets/pins, and (3) inspection of rivets after spinning (approx. 3.8 hrs/part). This in turn may eliminate the potential damage to the rotor surfaces during the installation and removal of rivets/pins. Furthermore, bin stock providing pins and/or rivets  160  used to reduce leakage through the holes  140  can be eliminated.  
         [0040]     As shown in  FIG. 8 , the geometry of the J-slot  370  may be varied to maximize the stress relieving characteristics of the mechanism. Both the slash angle  610  and the slot angle  770  may be varied to keep hoop stress to which the J-slot  370  is subjected below a design threshold. These angles may be highly dependent upon the targeted service life for the rotary body  100 , the blade geometry at the rim of the rotary body  100  and the number of blades  110 , the material from which the rotary body  100  is manufactured, and the bending stress to which the rotary body  100  is subjected.  
         [0041]     Also, other inventive configurations of the J-slot  370  may be conceived without departing from the scope of the invention. For example, a double J-slot  370  may be fabricated with two curved slot portions  375  each extending in opposite directions from the linear slot portion  372 , in a shape much like an inverted “T” with the tips bent back towards the central shaft.  
         [0042]     An inventive hoop stress relief mechanism and a method for its fabrication have thus been disclosed. The relief mechanism provided by the invention may be a series of J-shaped slots that have been machined about the rim of a rotary body, each slot penetrating the rotary body from face to face. The J-slots may be fabricated into the rim of the rotary body by a electric discharge wire machine, thus reducing the number of time consuming steps required by the prior art method of drilling and reaming holes and installing rivets therein.  
         [0043]     It should be understood, of course, that the foregoing description of the invention 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.