Abstract:
A bladed rotor disk assembly includes a plurality of circumferentially spaced apart blade root slots extending through the disk at an angle to the disk axial direction. Each slot has a radially inwardly facing load reaction surface along each side thereof extending continuously over less than the full the length of the slot in contact with a corresponding radially outwardly facing load reaction surface of a blade root disposed within the slot. This eliminates highly concentrated reaction loads adjacent the ends of the slot and results in a more uniform load distribution over the remaining smaller reaction surface area, reducing maximum stresses.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to bladed rotor assemblies, and especially to bladed rotor assemblies for gas turbine engines. 
     2. Background Information 
     Bladed rotor assemblies are well known in the art, such as for compressors and turbines of gas turbine engines. In such assemblies, each blade is often attached to the rotor disk by means of a root, integral with the radially innermost end of the blade. The root fits closely within a corresponding blade root slot extending generally axially through the disk rim, but at an angle to the true direction of the disk axis. The disk material disposed circumferentially between a pair of adjacent slots is often referred to as a disk lug. The blade root includes radially outwardly facing reaction surfaces that engage corresponding radially inwardly facing reaction surfaces of a blade root slot. During operation of the rotor, the blade loads are transferred into the disk and disk lugs through these engaged surfaces. Typically, a blade root extends from the front face to the rear face of the disk; and the engaged load reaction surfaces also extend from the front to the rear face of the disk (i.e. the full length of the slot). This is true of bladed disks having conventionally designed dovetail shaped roots and slots, as well as fir tree shaped roots and slots. 
     It is generally desired to keep stresses within the disk and within the blades as low as possible to extend part life. In gas turbine engines designed for flight, it is also desired to minimize the weight of parts, such as disks and blades, consistent with efficient operation, long life and safety. Lighter weight blades also generate lower centrifugal forces and thus may reduce stresses within the disk. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a bladed rotor disk assembly includes a plurality of circumferentially spaced apart blade root slots extending through the disk at an angle to the disk axial direction and having radially inwardly facing load reaction surfaces extending continuously over less than the full the length of the slot in contact with a corresponding radially outwardly facing load reaction surface of a blade root disposed within the slot. 
     By “load reaction surface”, it is meant the surfaces of the blade root and blade root slot that, during operation of the rotor, contact or engage each other to transfer the loads from the blade into the disk. When in contact these surfaces form a “load transfer interface”. 
     More specifically, the present invention eliminates what in the prior art would be portions of the load transfer interface adjacent the ends of the blade root slot, such that the loads over the remaining load transfer interface result in one or more of the following: a more symmetrical load distribution resulting in reduced torque loads on the disk lugs; reduced total loads on the disk lugs and blade roots; and, reduced maximum stress levels in the disk lugs and blade roots. 
     One reason these benefits may occur is because, with conventional root and slot designs, when the blade root load reaction surface along a side of a blade root extends the full length of the slot, the highest and most concentrated reaction loads on that side of the slot occur adjacent one end of the slot, while relatively lower and less concentrated (i.e. more uniform) reaction loads on that same side of the slot occur adjacent the other end of the slot. Therefore, at the low, more uniform reaction load end of the slot, the disk lug material is carrying a relatively small portion of the blade load per square inch of load transfer interface, while at the high reaction load end the disk lug material is carrying a much larger portion of the blade load per square inch of load transfer interface. By eliminating load transfer interface area at the low load end of each side of a slot, the reaction loads over the remaining load transfer interface on each side of the slot becomes more balanced, and results in lower maximum stress. 
     In one embodiment of the present invention a small area of each side of a slot adjacent an end of the slot and which faces what would normally be the low load portion of the root reaction surface is instead spaced from that low load portion such that there is a gap between the blade root and slot over that area. In all other respects, the blade and disk assembly may be considered conventional. The reaction loads over the now smaller load transfer interfaces on each side of the blade root are more balanced than without the gaps, and the maximum stress in the disk lugs is reduced. 
     In another embodiment of the present invention, end portions of the conventional blade root that normally transfer relatively low loads into the disk lugs are removed, providing the benefit of reduced blade weight in addition to more balanced reaction loads over the remaining length of a smaller load transfer interface on each side of the blade root. Total reaction loads, stresses and/or torque on the disk lugs may thereby be reduced. Reduced torque loads means less twisting of the blade lugs, with correspondingly less twisting of the blades. 
     The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric rear view of a portion of a rotor assembly according to one embodiment of the present invention, with some of the blades removed to better show the blade root slots through the rim of the rotor disk. 
     FIG. 2 is a sectional view taken along the line  2 — 2  of FIG.  1  through one of the rotor disk lugs, with the rotor disk axis being in the plane of the figure. 
     FIG. 3 is a diagrammatic sectional view, taken along the line  3 — 3  of FIG. 2, illustrating the differences in reaction loads along the length of the slot as between a disk assembly of the prior art and a disk assembly according to one embodiment of the present invention. 
     FIG. 4 is a view in the direction D of FIG. 3, parallel to the blade root slot length. 
     FIGS. 5 and 6 are schematic views in the directions  5 — 5  and  6 — 6 , respectively, of FIG. 4 showing, for the embodiment of FIG. 1, the disk/blade load transfer interfaces along opposite sides of a blade root slot. 
     FIG. 7 is a schematic view, taken in the direction  7 — 7  of FIG. 2, showing, for the embodiment of FIG. 1, the cross-sectional shape of the blade root and its general orientation relative to the front and rear disk surfaces and the blade platform. 
     FIG. 8 is a sectional view of a rotor assembly, like the sectional view of FIG. 2, but showing a rotor assembly incorporating an alternate embodiment of the present invention. 
     FIG. 9 is a simplified sectional view taken along the line  9 — 9  of FIG.  8 . 
     FIG. 10 is an isometric view in the direction S of FIG. 9 perpendicular to the rear face of the disk, with the blade removed. 
     FIGS. 11 and 12 are schematic views in the directions  11 — 11  and  12 — 12 , respectively, of FIG. 4 showing, for the embodiment of FIG. 8, the disk/blade load transfer interfaces along opposite sides of a blade root slot. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a gas turbine engine rotor assembly  100  incorporating an exemplary embodiment of the present invention includes a rotor disk  102  and a plurality of rotor blades  104 , only one of which is shown. Each blade comprises a root  106 , platform  108 , and airfoil  110 . The disk  102  has a rotational axis  111 , a rear face  112 , a front face  114 , and a rim  116 . A plurality of blade root slots  118  extends through the rim from the rear face to the front face in a direction D (FIG.  3 ). Each pair of adjacent slots defines a disk lug  120  therebetween. The root  106  of each blade is disposed within a respective one of the slots. 
     Referring to FIG. 3, each slot  118  extends in the direction D at an acute angle θ to the direction of the disk axis  111 . Generally, this angle is between about 10° and 30°. In this example θ is 24° and, as best shown in FIG. 4, the blade roots  106  are of the well known “dovetail” shape, although the invention is not limited to use with blades having dovetail roots. The root of each blade has a pair of flat, radially outwardly facing load reaction surfaces  122 A,  122 B, one extending along each side of the root. The surfaces  122 A,  122 B abut corresponding flat, radially inwardly facing slot load reaction surfaces  124 A,  124 B, respectively. The interfaces formed by each of these pairs of contacting surfaces are hereinafter referred to as load transfer interfaces since, during operation of the rotor, the blade loads are transferred into the disk lugs across these interfaces. 
     In the prior art, blade roots and disk slots, as well as the load transfer interfaces, are the same length, which is generally the full length, L (FIG.  3 ), of the slot, as measured in the direction D of the slot. In accordance with the present invention, at least one of the blade root load reaction surfaces  122 A,  122 B, and preferably both, is less than the slot length. This is best seen in FIG. 3, wherein the blade root  106 , although fully within the slot  118 , has oppositely facing end surfaces  126 ,  128  which are perpendicular to the slot direction D. Thus, as best shown in FIGS. 5 and 6, the cross-hatched load transfer interfaces  130 A,  130 B, have respective lengths M and N, corresponding to the respective lengths of the blade root load reaction surfaces  122 A,  122 B. FIG. 7 provides a radially outwardly looking view of the blade  104 , showing the orientation and position of the blade root  106  relative to the blade platform  108  and the disk front and rear faces  114 ,  112 , respectively. 
     Reference is also made to FIGS. 3,  5  and  6  for an understanding of the benefits of the present invention as compared to the prior art. In the present invention, the root load reaction surfaces  122 A,  122 B contact the slot load reaction surfaces  124 A,  124 B between the points Y and Z 1 , and W and X 1 , respectively. Assume, for purposes of discussion, that the blade root load reaction surfaces and slot load reaction surfaces extend the full length of the slot, such that over the radial extent (i.e. from R 1  to R 2  in FIG. 4) of the root load reaction surfaces the root end surfaces  126 ,  128  are substantially in the planes of the disk rear and front faces  112 ,  114 , respectively, as is generally the case with prior art rotor assemblies (i.e. the angle α is 0°, rather than being equal to θ, as shown in FIG.  3 ). In that case, blade loads would be transferred into the disk lugs over the full length L of the slot from X 2  to W on one side of the slot and from Z 2  to Y on the other side. The magnitude of the reaction loads for such a prior art configuration along the lengths L of the respective blade root slot reaction surfaces are represented by the curves  132 ,  134 , which were generated by a computer model of such a configuration. The curves  136 ,  138  of FIG. 3 are generated by a computer model of the same rotor assembly modified according to the present invention (i.e. generally as shown in FIG.  1 ), and represent the magnitude of the reaction loads along the full lengths M (from X 1  to W) and N (from Z 1  to Y) of the blade root load reaction surfaces  122 A,  122 B, respectively. The perpendicular distance from the curves  132 ,  136  to the line X 2 -W, and the perpendicular distance from the curves  134 ,  138  to the line Z 2 -Y represent the magnitude of the reaction load. 
     Compare the “prior art” curves  132 ,  134  to the curves  136 ,  138  for the present invention. Note that, in the prior art rotor assembly configuration, the magnitude of the load along the length of each side of the slot is high at one end of the slot and tapers off to relatively low at the other end. On the other hand, the curves show that, in the rotor assembly configuration of present invention, the loads are more balanced over the blade root length, with high loads near each blade root end, and relatively low loads between the ends. Additionally, the maximum reaction load on each side of the slot is lower in the rotor assembly of the present invention. Computer modeling also indicates that the maximum stress concentration in the disk lugs is lower for the rotor assembly of the present invention, as compared to the prior art. 
     In the foregoing embodiment, the benefits are primarily the result of lowering the weight of the blade by reducing the length of the blade root. That reduces the total load on the disk lugs and corresponding stress levels; and, by having a more balanced load over the length of the root, the stress concentrations are even further reduced. At first glance, it may appear that the reduced reaction load surface areas might negate these benefits; however, the loss of load reaction surface area is not particularly detrimental because the eliminated portions of the prior art reaction surfaces near the ends of the slots (the non-cross-hatched portions of FIGS. 5 and 6) were carrying only a relatively small portion of the total load per unit surface area, as compared to the average load per unit surface area over the full length of the slot. 
     Although in the foregoing embodiment the blade root end surfaces  126 ,  128  are perpendicular to the blade root load reaction surfaces  122 A,  122 B, this is not a requirement. It is preferred, however, that the blade root end surfaces be parallel to each other to maintain symmetry. Thus, a parallelogram cross-sectional shape (in the view of FIG. 3) with the blade root of any length less than the slot length L (in the direction D) may provide a benefit over the prior art by reducing blade weight. Preferably, the angle α is between 0° and θ. Although the blade root end surfaces  126 ,  128  are preferably parallel, they need not be; and, thus, M does not need to equal N, although at least one of them must be less than L. 
     In accordance with another embodiment of the present invention, reference is made to FIGS. 8,  9  and  10 . The rotor assembly  200  includes a disk  202  and blades  204 , only one of which is shown. The disk axis is designated by the reference numeral  211 . The disk has front and rear parallel opposed faces  214 ,  212  adjacent its rim  216 . The disk also has a plurality of circumferentially spaced apart blade root slots  218  defined by and between disk lugs  220 , and extending through the disk rim from the front face  212  to the rear face  214 . As in the previous embodiment, the slots  218  are cut at an angle to the disk axis  211 . Each blade  204  comprises a dovetail-shaped root  206 , platform  208 , and airfoil  210 . In this embodiment, as is also the case in rotor assemblies of the prior art, the blade roots extend the full length of their respective slots, whereby the root end surfaces  226 ,  228  are substantially flush with respective end faces  212 ,  214  of the disk, at least over the radial extent of the root load reaction surfaces. 
     In accordance with this embodiment of the invention, the lugs  220  on each side of a blade root  206  each have pockets  300 ,  302  cut into opposite end faces  212 ,  214  of the disk at the rim to cut back or remove material that would otherwise form a portion of a slot load reaction surface that engages a blade root load reaction surface. Thus, as shown in FIG. 9, wherein the cross-section through the blade root is shown crosshatched, the root load reaction surface  222 A and the slot load reaction surface  224 A both extend from E 1  to F. Similarly, the corresponding reaction surfaces  222 B and  224 B on the other side of the slot extend from G 1  to H. Essentially, the pockets  300 ,  302  create gaps  304 ,  306  between each lug  220  and what are hereinafter referred to as extensions  308  (having a length from E 1  to E 2 ) and  310  (having a length from G 1  to G 2 ) of the blade root load reaction surfaces  222 A,  222 B, respectively. 
     FIGS. 11 and 12 are analogous to FIGS. 5 and 6 of the previously described embodiment, and show the load transfer interfaces  230 A,  230 B, on each side of a slot in the embodiment of FIG. 8. L, M, and N represent the same lengths as in FIGS. 5 and 6. It is readily seen that both embodiments can result in the very same load transfer interfaces. The FIG. 1 embodiment accomplishes this by effectively shortening the length of the blade root; and the FIG. 8 embodiment does this by removing material from the slot surface to create a gap between a portion of the lug and the blade root. In each case the “removed” portion of the prior art load transfer interface was previously located where the prior art reaction loads were relatively low. Moreover, in both embodiments the loads transferred along the length of the load transfer interface are more balanced than those of the prior art, resulting in lower maximum stresses in the lugs. Thus, the general shape of the curves  136 ,  138  in FIG. 3 would be the same for the embodiment of FIG. 8; however, the embodiment of FIG. 1 has the additional advantage of reduced blade weight and correspondingly lower total blade loads to be transferred into the lugs. 
     Although the invention has been described and illustrated with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the invention. For example, gaps similar to the gaps  304 ,  306  between the blade root and slot may be formed by removing a small amount of material from the blade root load reaction surfaces rather than from the disk lugs.