Abstract:
A turbine engine has a first disk and a second disk, each extending radially from an inner aperture to an outer periphery. A coupling, transmits a torque and a longitudinal compressive force between the first and second disks. The coupling has first means for transmitting a majority of the torque and a majority of the force and second means, radially outboard of the first means, for vibration stabilizing.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to gas turbine engines. More particularly, the invention relates to gas turbine engines having center-tie rotor stacks. 
   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). 
   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. 
   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. 
   Desired improvements in efficiency and output have greatly driven developments in turbine engine configurations. Efficiency may include both performance efficiency and manufacturing efficiency. 
   U.S. patent application Ser. No. 10/825,255, Ser. No. 10/825,256, and Ser. No. 10/985,863 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 OF THE INVENTION 
   One aspect of the invention involves a turbine engine having a first disk and a second disk, each extending radially from an inner aperture to an outer periphery. A coupling, transmits a torque and a longitudinal compressive force between the first and second disks. The coupling has first means for transmitting a majority of the torque and a majority of the force and second means, radially outboard of the first means, for vibration stabilizing of the first and second disks. 
   In various implementations, the second means may include spacers (e.g., as in the Suciu et al. applications or otherwise). The first means may comprise radial splines or interfitting first and second pluralities of teeth on the first and second disks, respectively. The first plurality of teeth may be formed at an aft rim of a first sleeve extending aft from and unitarily-formed with a web of the first disk. The second plurality of teeth may be formed at a forward rim of a second sleeve extending forward from and unitarily-formed with a web of the second disk. The first and second disks may each have an inboard annular protuberance inboard of the respective first and second sleeves. The second means may comprise a spacer having an outwardly longitudinally concave portion having a thickness and a longitudinal extent effective to provide an increase in said force with an increase in rotational speed of the first and second disks. The engine may have a high speed and pressure turbine section and a low speed and pressure turbine section. The first and second disks may be in the low speed and pressure turbine section. The engine may be a geared turbofan engine. A tension shaft may extend within the inner aperture of each of the first and second disks and be substantially nonrotating relative to the first and second disks. The engine may include a vane stage having a number of vane airfoils and having a sealing portion radially inboard of the vane airfoils for sealing with the coupling second means. A third disk may extend radially from an inner aperture to an outer periphery. A second coupling may transmit a torque and a longitudinal compressive force between the third and second disks. The second coupling may include first means for transmitting a majority of the torque and a majority of the force and second means, radially outboard of the first means, for vibration stabilizing. The engine may lack off-center tie members holding the first and second disks under longitudinal compression. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial longitudinal sectional view of a gas turbine engine. 
       FIG. 2  is a partial longitudinal sectional view of a low pressure turbine rotor stack of the engine of  FIG. 1 . 
       FIG. 3  is a radial view of interfitting splines of two disks of the stack of  FIG. 2 . 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a gas turbine engine  20  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 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. 
   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. A low speed shaft  29  carries LPC and LPT rotors and their blades to form a low speed spool. The low speed shaft  29  may be an assembly, either fully or partially integrated (e.g., via welding). The 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 includes the HPC and HPT rotors and their blades. 
     FIG. 2  shows an LPT rotor stack  32  mounted to the low speed shaft  29  across an aft portion  33  thereof. The exemplary rotor stack  32  includes, from fore to aft and upstream to downstream, an exemplary three blade disks  34 A- 34 C each carrying an associated stage of blades  36 A- 36 C (e.g., by engagement of fir tree blade roots  37  to complementary disk slots). A plurality of stages of vanes  38 A- 38 C 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  formed as portions of a core flowpath outer wall  40 . The vane airfoils extend inward to inboard platforms  42  forming portions of a core flowpath inboard wall  43 . The platforms  42  of the second and third vane stages  38 B and  38 C have inwardly-extending flanges to which stepped honeycomb seals  44  are mounted (e.g., by screws or other fasteners). 
   In the exemplary embodiment, each of the disks  34 A- 34 C has a generally annular web  50 A- 50 C extending radially outward from an inboard annular protuberance known as a “bore”  52 A- 52 C to an outboard peripheral portion  54  bearing an array of the fir tree slots  55 . The bores  52 A- 52 C encircle central apertures of the disks through which the portion  33  of the low speed shaft  29  freely passes with clearance. Alternative blades may be unitarily formed with the peripheral portions  54  (e.g., as a single piece with continuous microstructure) or non-unitarily integrally formed (e.g., via welding so as to only be destructively removable). 
   Outboard spacers  62 A and  62 B connect adjacent pairs of the disks  34 A- 34 C. In the exemplary engine, the spacers  62 A and  62 B are formed separately from their adjacent disks. The spacers  62 A and  62 B 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 with (e.g., unitarily formed with or welded to) one of the adjacent disks and extend to a contacting engagement with the other disk. 
   In the exemplary engine, the spacers  62 A and  62 B are 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. The exemplary spacers  62 A and  62 B have outboard surfaces from which one or more annular sealing teeth (e.g., fore and aft teeth  63  and  64 ) extend radially outward into sealing proximity with adjacent portions of the adjacent honeycomb seal  44 . 
   The spacers  62 A and  62 B thus each separate an inboard/interior annular inter-disk cavity  65  from an outboard/exterior annular inter-disk cavity  66  (accommodating the honeycomb seal  44  and its associated mounting hardware). 
   Additional inter-disk coupling is provided between the disks  34 A- 34 C.  FIG. 2  shows couplings  70 A and  70 B radially inboard of the associated spacers  62 A and  62 B. The couplings  70 A and  70 B separate the associated annular inter-disk cavity  65  from an inter-disk cavity  72  between the adjacent bores. Each exemplary coupling  70 A and  70 B includes a first tubular ring-like structure  74  ( FIG. 3 ) extending aft from the disk thereahead and a second such structure  76  extending forward from the disk aft thereof. The exemplary structures  74  and  76  are each unitarily-formed with their associated individual disk, extending respectively aft and forward from near the junction of the disk web and bore. 
   At respective aft and fore rims of the structures  74  and  76 , the structures include interfitting radial splines or teeth  78  in a circumferential array ( FIG. 3 ). The exemplary illustrated teeth  78  have a longitudinal span roughly the same as a radial span and a circumferential span somewhat longer. The exemplary teeth  78  have distally-tapering sides  80  extending to ends or apexes  82 . In the exemplary engine, the sides  80  of each tooth contact the adjacent sides of the adjacent teeth of the other structure  74  or  76 . In the exemplary engine, there is a gap between each tooth end  82  and the base  84  of the inter-tooth trough of the opposite structure. This gap permits longitudinal compressive force to reinforce circumferential engagement and maintain the two structures tightly engaged. Snap couplings or curvic couplings or other spline structures could be used instead of the exemplary spline structure. 
   In the exemplary engine, the couplings  70 A and  70 B transmit the majority of longitudinal compressive force and longitudinal torque along a primary compression path between their adjacent disks. A much smaller longitudinal force may be transmitted via the couplings  62 A and  62 B which may primarily serve to maintain position of and stabilize against vibration of the disks. A particular breakdown of force transmission may be dictated by packaging constraints. In the exemplary engine, the fore and aft ends of the LPT rotor engaging the shaft  29  are formed by fore and aft hubs  90  and  92  extending respectively fore and aft from the associated bores  52 A and  52 C. The relative inboard radial position of these hubs renders impractical a relatively outboard force transmission. An outward shifting of the hubs would increase longitudinal size and, thereby, create packaging and other problems. Thus, the couplings  70 A and  70 B are advantageously radially positioned near the connections of the disk bores  52 A and  52 C to the associated hubs  90  and  92 . 
   The relative inboard position of the main compression and torque carrying couplings may provide design opportunities and advantages relative to alternate configurations. The use of geared turbofans has decoupled the design speed of the low speed spool from the design speed of the fan. This presents opportunities for increasing the speed of the low speed spool. Such increased speeds (e.g., typical operating speeds in the 9-10,000 rpm range) involve increased loading. To withstand increased loading, it may be desired to remove outboard weight such as outboard flanges and bolts that tie the disks together and transmit torque and/or force. A similar opportunity could be presented in the turbine section of the intermediate spool of a three-spool engine (e.g., wherein the fan is directly coupled to the low speed spool). 
   In the exemplary engine, the low speed shaft  29  is used as a center tension tie to hold the disks of the rotor  32  in compression. The disks may be assembled to the shaft  29  from fore-to-aft (e.g., first installing the disk  34 A, then installing the spacer  62 A, then installing the disk  34 B, then installing the spacer  62 B, then installing the disk  34 C, and then compressing the stack and installing a locking nut or other element  96  ( FIG. 2 ) to hold the stack precompressed). 
   Tightness of the rotor stack at the disk outboard peripheries may be achieved in a number of ways. Outward concavity of the spacers  62 A and  62 B may produce a speed-increasing longitudinal compression force along a secondary compression path through the spacers  62 A and  62 B. Additionally, the static conditions of the fore and aft disks  34 A and  34 C may be slightly dished respectively forwardly and aft. With rotation, centrifugal action will tend to straighten/undish the disks  34 A and  34 C and move the peripheral portions  54  of the disks  34 A and  34 C longitudinally inward (i.e., respectively aft and forward). This tendency may counter the effect on and from the spacers  62 A and  62 B so as to at least partially resist their flattening. By at least partially resisting this flattening, good sealing with the honeycomb seals  44  may be achieved across a relatively wide speed range. 
   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 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 spacers  62 A and  62 B such as spacer thickness, spacer curvature or other shape parameters, vane seal shape parameters, and static seal-to-spacer separation (which may include varying specific positions for the seal and the spacer). The iteration may involve varying parameters of the couplings  70 A and  70 B such as the thickness profiles of the structures  74  and  76 , the size and geometry of the teeth  78 , the radial position of the couplings, and the like. 
   One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. 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.