Abstract:
A rotor has a central shaft having a central longitudinal axis. The rotor has a longitudinal stack of a plurality of disks surrounding the shaft. An aft hub couples the stack to the shaft. The aft hub has a proximal portion and a distal portion. The distal portion tapers at a lower characteristic half angle than does the proximal portion.

Description:
BACKGROUND 
       [0001]    The disclosure relates to gas turbine engines. More particularly, the disclosure relates to gas turbine engine rotor stacks. 
         [0002]    A gas turbine engine typically includes one or more rotor stacks associated with one or more sections of the engine. A rotor stack may include several longitudinally spaced apart blade-carrying disks of successive stages of the section. A stator structure may include circumferential stages of vanes longitudinally interspersed with the rotor disks. The rotor disks are secured to each other against relative rotation and the rotor stack is secured against rotation relative to other components on its common spool (e.g., the low and high speed/pressure spools of the engine). 
         [0003]    Numerous systems have been used to tie rotor disks together. In an exemplary center-tie system, the disks are held longitudinally spaced from each other by sleeve-like spacers. The spacers may be unitarily-formed with one or both adjacent disks. However, some spacers are often separate from at least one of the adjacent pair of disks and may engage that disk via an interference fit and/or a keying arrangement. The interference fit or keying arrangement may require the maintenance of a longitudinal compressive force across the disk stack so as to maintain the engagement. The compressive force may be obtained by securing opposite ends of the stack to a central shaft passing within the stack. The stack may be mounted to the shaft with a longitudinal precompression force so that a tensile force of equal magnitude is transmitted through the portion of the shaft within the stack. 
         [0004]    Alternate configurations involve the use of an array of circumferentially-spaced tie rods extending through web portions of the rotor disks to tie the disks together. In such systems, the associated spool may lack a shaft portion passing within the rotor. Rather, separate shaft segments may extend longitudinally outward from one or both ends of the rotor stack. 
         [0005]    Desired improvements in efficiency and output have greatly driven developments in turbine engine configurations. Efficiency may include both performance efficiency and manufacturing efficiency. 
         [0006]    U.S. patent publications 20050232773A1, 20050232774A1, 20060099070A1, 20060130456A1, and 20060130488A1 of Suciu and Norris (hereafter collectively the Suciu et al. applications, the disclosures of which are incorporated by reference herein as if set forth at length) disclose engines having one or more outwardly concave inter-disk spacers. With the rotor rotating, a centrifugal action may maintain longitudinal rotor compression and engagement between a spacer and at least one of the adjacent disks. This engagement may transmit longitudinal torque between the disks in addition to the compression. 
       SUMMARY 
       [0007]    One aspect of the disclosure involves a gas turbine engine rotor. The rotor has a central shaft having a central longitudinal axis. The rotor has a longitudinal stack of a plurality of disks surrounding the shaft. An aft hub couples the stack to the shaft. The aft hub has a proximal portion and a distal portion. The distal portion tapers at a lower characteristic half angle than does the proximal portion. 
         [0008]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a partial longitudinal sectional view of a gas turbine engine. 
           [0010]      FIG. 2  is a partial longitudinal sectional view of a high pressure compressor rotor stack of the engine of  FIG. 1 . 
           [0011]      FIG. 3  is an enlarged view of an aft hub of the stack of  FIG. 2 . 
           [0012]      FIG. 4  is a partial longitudinal sectional view of a prior art gas turbine engine. 
           [0013]      FIG. 5  is a static force diagram for the aft hub of the compressor rotor stack of the engine of  FIG. 4 . 
           [0014]      FIG. 6  is an at-speed force diagram for the aft hub of the compressor rotor stack of the engine of  FIG. 4 . 
           [0015]      FIG. 7  is a static force diagram for the aft hub of the compressor rotor stack of the engine of  FIG. 1 . 
           [0016]      FIG. 8  is an at-speed force diagram for the aft hub of the compressor rotor stack of the engine of  FIG. 1 . 
           [0017]      FIG. 9  is a partial longitudinal sectional view of an alternate high pressure compressor rotor stack. 
           [0018]      FIG. 10  is a partial longitudinal sectional view of a second alternate high pressure compressor rotor stack. 
           [0019]      FIG. 11  is a partial longitudinal sectional view of a third alternate high pressure compressor rotor stack. 
       
    
    
       [0020]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0021]      FIG. 1  shows a gas turbine engine  20 . The exemplary engine  20  is a two-spool engine having a high speed/pressure compressor (HPC) section  22  receiving air moving along a core flowpath  500  from a low speed/pressure compressor (LPC) section  23  and delivering the air to a combustor section  24 . High and low speed/pressure turbine (HPT, LPT) sections  25  and  26  are downstream of the combustor along the core flowpath  500 . The exemplary engine further includes a fan  28  driving air along a bypass flowpath  501 . Alternative engines might include an augmentor (not shown) among other systems or features. 
         [0022]    The exemplary engine  20  includes low and high speed spools mounted for rotation about an engine central longitudinal axis or centerline  502  relative to an engine stationary structure via several bearing systems. The low speed shaft  29  carries LPC and LPT rotors and their blades to form the low speed spool. Alternative fans may be directly driven by one of the spools. The low speed shaft  29  may be an assembly, either fully or partially integrated (e.g., via welding). The exemplary low speed shaft is coupled to the fan  28  by an epicyclic transmission  30  to drive the fan at a lower speed than the low speed spool. The high speed spool similarly includes the HPC and HPT rotors and their blades and a high speed shaft  31 . 
         [0023]      FIG. 1  shows an HPC rotor stack  32  mounted to the high speed shaft  31  across a forward portion  33  thereof. The exemplary rotor stack  32  includes, from fore to aft and upstream to downstream, a plurality of blade disks  34  each carrying an associated stage of blades  36  (e.g., by engagement of dovetail blade roots (not shown) to complementary disk slots). A plurality of stages of vanes  38  are located along the core flowpath  500  sequentially interspersed with the blade stages. The vanes have airfoils extending radially inward from roots at outboard shrouds/platforms  39  ( FIG. 2 ) formed as portions of a core flowpath outer wall  40 . The vane airfoils extend inward to inboard tips  42 . The tips face stack spacers  43  forming portions of a core flowpath inboard wall  44 . 
         [0024]    In the exemplary embodiment, each of the disks  34  has a generally annular web  50  extending radially outward from an inboard annular protuberance known as a “bore”  52  to an outboard peripheral portion  54  (e.g., bearing an array of blade attachment slots). The bores  52  encircle central apertures of the disks through which the portion  33  of the high speed shaft  31  freely passes with clearance. Alternative blades may be unitarily formed with the peripheral portions  54  (e.g., as a single piece with continuous microstructure (an integrally bladed rotor (IBR) or “blisk” machined from a single piece of raw material)) or non-unitarily integrally formed (e.g., via welding so as to only be destructively removable). 
         [0025]    The outboard spacers  43  connect adjacent pairs of the disks  34 . In the exemplary engine, some of the spacers  43  are formed separately from their adjacent disks. The spacers  43  may each have end portions in contacting engagement with adjacent portions (e.g., to peripheral portions  54 ) of the adjacent disks. Alternative spacers may be integrally formed with (e.g., unitarily formed with or welded to) one of the adjacent disks and extend to a contacting engagement with the other disk. For example, the spacer between the exemplary last two disks is shown unitarily formed with the last (aft/rear) disk. 
         [0026]    The spacers may be outwardly concave (e.g., as disclosed in the Suciu et al. applications). The contacting engagement with the peripheral portions of the adjacent disks produces a longitudinal engagement force increasing with speed due to centrifugal action tending to straighten/flatten the spacers&#39; sections. 
         [0027]    In the exemplary engine, the high speed shaft  31  is used as a center tension tie to hold the rotor stack  32  in compression. The disks may be assembled to the shaft  31  from fore-to-aft (or aft-to-fore, depending upon configuration) and then compressing the stack and installing a locking nut or other element to hold the stack precompressed). 
         [0028]    Tightness of the rotor stack at the disk outboard peripheries may be achieved in a number of ways. Outward concavity of the spacers may produce a speed-increasing longitudinal compression force along a secondary compression path through the spacers. Additionally, the static conditions of the fore and aft disks may be slightly dished respectively forwardly and aft. With rotation, centrifugal action will tend to straighten/undish the fore and aft disks and move their peripheral portions longitudinally inward (i.e., respectively aft and forward). This tendency may counter the effect on and from the spacers so as to at least partially resist their flattening. The engine operational condition affects the distribution of forces and torques along the length of the rotor stack. For example, in a compressor stack driven by a downstream turbine, the operationally-induced longitudinal torque increases from upstream to downstream. Similarly, the compression provides a downstream-increasing longitudinal tension partially counteracting the precompression and any speed-increasing longitudinal compression associated with the spacers or other rotor geometry. Similarly, any rub between the blade tips and the engine case will provide a downstream-increasing torque and tension component. Thus, the components of rotor torque do both to compression and rub are maximum at the last/downstreammost/rear/aft stage and at any adjacent rear hub structure coupling the rotor stacks to the driving turbine section. The precompression force is, therefore, selected to provide sufficient at-speed compression to counter the operational tensions at the last stage and rear hub. Sufficient force must be maintained across a variety of speeds and operating conditions. For example, at given speeds, acceleration and deceleration may have largely opposite effects on loading relative to steady-state operation. 
         [0029]      FIG. 1  shows a rear hub  70  coupling the HPC disks to the high speed shaft  31  and to the disks  72  of the HPT. Generally, the hub  70  includes a portion  74  extending forward and outward to be coupled to/engaged an associated/coupled one of the HPC disks (e.g., the last/rear disk). 
         [0030]      FIG. 2  shows the portion  74  as extending forward and outward from a junction  76  with a portion  78  for connecting to the shaft and a portion  80  for connecting to the HPT. The exemplary portion  78  extends to an inner/ID region  82  which may engage the shaft radially and longitudinally. The exemplary region  82  is longitudinally retained to the shaft by a threaded nut  84  restricting relative rearward movement of the region  82 . The engagement between the region  82  and the nut  84  allows transmission of compression through the stack and corresponding tension through the shaft forward portion  33 . The exemplary portion  80  extends as a tube/shaft rearward to a junction  90  with a corresponding forward portion of a front/forward hub  92  of the HPT. The exemplary junction  90  is a flanged bolt circle. 
         [0031]      FIG. 2  shows the portion  74  as including a proximal/aft/inboard portion (subportion)  100  and a distal/outboard/forward portion  102 . The exemplary portion  74  carries a bore  104  via a web  106  extending inward from the junction  108  of the portions  100  and  102 . The exemplary web  106  is unitarily formed with the distal portion  102 . As is discussed further below, the proximal portion  100  has a greater half angle than the distal portion  102  (i.e., the portion  100  is more radial and the portion  102  is more longitudinal). 
         [0032]      FIG. 3  shows an exemplary junction  118  between the portion  74  and the rearmost disk  34 . The outboard peripheral portion  54  of the rearmost disk  34  includes an inward and aft facing shoulder formed by an aft-facing surface  120  and an inward facing surface  122 . A rim  123  of the hub distal portion  102  is accommodated within the shoulder. An exemplary front surface  124  of the rim engages the surface  120 ; an outer diameter (OD) surface  126  engages the surface  122 . The exemplary junction  118  may similarly include a shoulder having surfaces  130  and  132  (on distal portion  102 ) and a rim  133  of the proximal portion  100  having a forward surface  134  and an OD surface  136 . 
         [0033]      FIG. 4  shows a prior art center-tie rotor stack which may serve as a baseline for reengineering to a configuration such as  FIG. 1 . The hub portion  140  extends forward and outward from a proximal root at a junction  142  to a distal rim  144 . The rim  144  engages the aft-most disk. The engagement may be by one or more of a radial and/or axial interlocking or frictional interference fit. The hub portion  140  is outwardly concave along essentially its entire length so as to increase in slope or half angle from the junction  142  to the rim  144 . Thus, a proximal portion  150  will be characterized by a smaller half angle than a distal portion  152 . A boundary between the portions  150  and  152  may be somewhat arbitrarily defined. However, one convenient location would be a junction between separate pieces. Another convenient location would be a bore. Alternative prior art hubs are frustoconical as opposed to arcuate in section. 
         [0034]    In a static condition (i.e., with the engine at zero speed) the hub may impart an axial compression force to the HPC stack. The hub may also impart an outward radial force creating a hoop tension in the aft-most disk. These engagement forces may be normalized such as in units of force per circumferential linear dimension, or units of force per angle about the engine centerline  502 .  FIG. 5  shows an exemplary diagram of the net normalized static force wherein the net force  510  has an axial component  512  and a radial component  514 . The exemplary forced vector  510  is off longitudinal/axial by an angle θ 1 . The vector  510  may be near parallel to a terminal slope of the distal section  152 . 
         [0035]    Operational factors may tend to alter the net force with rotational speed. For example, the hub may tend to bow outward with increased speed. With a simple frustoconical hub, the art has known this bowing may have deleterious effects. Accordingly, the baseline hub includes an effective inward static bow provided by its outward concavity. Specifically, with a simple frustoconical hub, the induced outward bowing may tend to draw the forward rim of the hub rearward and decrease the engagement force with speed. With the  FIG. 4  hub having a static inward bow, the straightening effect of the speed-imposed outward bow tends to shift the rim forward and increases the engagement force with speed. This helps maintain integrity of the stack during operation. For example,  FIG. 6  shows an at-speed situation wherein the axial force has increased to  512 ′ and the radial force has increased to  514 ′ for an overall force of  510 ′. 
         [0036]    Contrary to conventional wisdom, the rotor of  FIG. 1  has a configuration resembling an overall outward bow. Specifically, the slope or half angle of the distal portion  102  ( FIG. 2 ) is lower/smaller than that of the proximal portion  100 . Although the individual portions  100  and  102  are shown concave outward, other variations are possible and are discussed below. For example,  FIG. 2  shows the hub  74  as having a total radial span R S  that includes the portions  78  and  82 . Exemplary hub longitudinal span L S  is defined only for the portion  74  and may extend from the base  160  of a channel formed by the forward surface of the junction  76 . An exemplary longitudinal span L S1  of the portion  100  may be measured from the base  160 /forward surface of the junction  76  to the rim surface  134 . The longitudinal span L S2  of the portion  102  may be measured from the front surface of the web  106  to the rim surface  124 . The radial span R S1  of the portion  100  may be measured from a center of the section of the portion  100  at the same longitudinal position as the base  160  to the OD surface  136 . Similarly, the radial span R S2  of the portion  102  may be measured from a center of the section of the portion  102  at the front face of the web  106 . Exemplary L S1  and L S2  are at least each 25% of L S , more narrowly, 30%. Exemplary half angle θ may be measured relative to a median  540  of the section of the respective portions  100  or  102 . The overall half angle of the portions may be measured as a mean or a median (e.g., averaged over length). Exemplary mean or median half angles of the distal portion  102  are at least 10% less than of the proximal portion  100 . Exemplary mean or median half angles of the distal portion  102  are 0-40°, more narrowly, 20-40°. Exemplary terminal portions of the half angles (e.g., along terminal regions adjacent the rim  123 ) may be in a similar angle range. In the  FIG. 3  embodiment, exemplary portions  100  and  102  are, both, over majorities of their respective lengths or longitudinal spans, concave outward. In alternative examples discussed below, one of the two (e.g., the distal portion  102 ) may alternatively be concave inward. 
         [0037]      FIG. 7  is a static force diagram for the engine of  FIG. 1 .  FIG. 8  is an at-speed force diagram. Exemplary operational speeds are 10,000-24,000 revolutions per minute (RPM), more narrowly, 17,500-21,500 RPM. A reengineering to such a configuration may provide greater control over the static relationship and speed-dependent relationship between axial and radial loads. For example, the configuration of the distal portion  102  may be selected to reduce at-speed radial loading. This may be achieved by reducing local slope or half angle at the junction  118 . It also may be achieved by reduced outward concavity, increased thickness, or other engineering factors. The proximal portion  100  may, however, be configured to be primarily responsible for the speed-increasing axial load. Whereas the axial load will be transmitted through both portions  100  and  102 , the radial load may be interrupted. For example, the provision of the bore  104  and web  106  can resist transmission of high radial loads at the junction  108  from being passed to the junction  118 . 
         [0038]    In the exemplary reengineering, one possible attribute is a reduction in the axial precompression force  522  ( FIG. 7 ) relative to the prior art axial precompression  512 . This may be accomplished along with a reduction in the static radial force  524  and net force  520 . The reengineering may provide a reduction in the at-speed radial force  524 ′ relative to the baseline force  514 ′. This reduction may advantageously be accompanied at least by a proportionately smaller reduction in the axial force  522 ′ relative to the at-speed axial force  512 ′. However, the axial force may advantageously be either essentially maintained or even increased (e.g., as shown in  FIG. 8 ). A reduction in the at-speed radial force ( 524 ′ being reduced relative to  514 ′) may allow for reduced strength and mass of the last disk (e.g., reducing its web thickness, bore size, etc.). The exemplary reengineering essentially maintains a speed-induced component  528  of the at-speed radial force relative to the baseline speed-induced component  518 . In the exemplary reengineering, the baseline hub has both static and at-speed radial forces (e.g., force per linear circumferential dimension) greater than the associated longitudinal forces. In distinction, the reengineered hub has both static and at-speed longitudinal forces greater than the associated radial forces. More narrowly, the longitudinal forces may be at least 120% or 150% of the radial forces, yet more narrowly 150-500%. For the at-speed forces, these relationships may be present across the entireties of the operational speed range (e.g., the ranges identified above) or may be present at least at a single operational speed in such ranges. 
         [0039]    The foregoing principles may be applied in the reengineering of an existing engine configuration or in an original engineering process. Various engineering techniques may be utilized. These may include computer simulations and actual hardware testing. The simulations/testing may be performed at static conditions and one or more non-zero speed conditions. The non-zero speed conditions may include one or both of steady-state operation and transient conditions (e.g., accelerations, decelerations, and combinations thereof). The simulation/tests may be performed iteratively. The iteration may involve varying parameters of the location of the junction  108 , shape and thicknesses of the portions  100  and  102 , attributes of the bore and web  104  and  106  and attributes of the last disk. Such a reengineering may change one or more additional attributes of the engine (beyond the preload and at-speed load values and relationships). For example, reduction in preload may allow reduction in weight or use of lighter or lower cost/performance materials elsewhere in the stack (e.g., relatively forward). This may be the case even where hub mass and/or the cost/performance of hub materials are increased. Additional changes may occur relatively downstream/aft in the stack. For example, reduction in the parasitic radial load on the last disk may reduce the needed strength of the last disk and thus reduce the massiveness of its bore, web, and rim. Such reductions may improve rotor thermal response and reduce stress-causing thermal gradients, yet further increasing performance envelope. Bore size reduction may permit a slight further reduction in engine length. 
         [0040]      FIG. 9  shows an alternate reengineered hub  200  wherein the forward and outward extending portion  202  is divided into a generally outwardly (relative to the centerline) concave proximal portion  204  and a generally outwardly convex distal portion  206 . A webless bore  208  is formed proximate a junction between the proximal and distal portions. The outward convexity allows the exemplary distal portion  206  to be nearly longitudinal in the vicinity of a junction  210  of its rim  212  and the last disk. Relative to the concave distal portion  102 , the convex distal portion  106  may reduce the relative radial load to axial load for the junction  210  versus the junction  118 . This may reduce the needed strength/size/mass of the bore and web of the mating downstreammost/aftmost disk  34 . This may simultaneously or alternatively increase the available operating speed. In such an embodiment, an overall (e.g., mean or median) half angle of the convex distal portion may be relatively high compared with a relatively low terminal angle in a region near the junction  210 . For example, the overall angle may be in a range of 30-60° whereas the terminal angle may be in a range of 0-20°. Similarly, an average angle over a forward half of the distal portion  206  may be in a range of 5-30°. 
         [0041]      FIG. 10  shows yet an alternative hub  300  having a portion  302  connecting to the stack but lacking a portion connecting directly to the shaft. Rather, the hub extends rearward to a junction  304  with the HPT hub. Accordingly, a combined compression is applied across the HPC and HPT stacks and associated with a continuous tension along the high speed shaft (e.g., as opposed to a tension interrupted by the missing junction between the hub  302  and shaft. The shaft portion  302  has a proximal portion  310  and a distal portion  312  which may be otherwise similar to those of the hub  200 . However, the absence of a portion connecting with the shaft allows the bore  314  to be relatively radially inward with a web  316  extending to the portion  302 . 
         [0042]      FIG. 11  shows a hub  400  otherwise similar to the hub  300  but with the proximal portion  410  and distal portion  412  formed as separate pieces with a similar rim-and-shoulder junction  413  to that of the  FIG. 2  embodiment. 
         [0043]      FIG. 12  shows an alternative high speed spool which, except, as described below, may be similar to that of  FIG. 2 . The high speed shaft  620  extends further aft than the shaft  33  of  FIG. 2  to pass within the bores of disks  622  and  624  of the high pressure compressor section. A nut  626  replaces the nut  84  and is positioned aft of the HPC disks. In the illustrated embodiment, forward of the HPC the shaft  620  includes a stop  628  which has a forward face abutting a rear face of an HPC hub ID region  630  (replacing the region  82 ). The exemplary region  630  is at the terminus of a rearwardly inwardly converging portion  632  replacing the portion  78  of  FIG. 2 . 
         [0044]    Other single- and multi-spool configurations are possible. The hub features may be implemented in various such configurations and on various such spools. For example, implementation on an LPC hub (e.g., in a two- or three-spool configuration) may involve exemplary operating speeds in the range of 2,500-11,000 RPM. 
         [0045]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied as a reengineering of an existing engine configuration, details of the existing configuration may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.