Abstract:
A middle support member is used to provide axial support and control to the tie shaft. The middle support member includes a high pressure compressor coupling nut that applies a preload that allows the high pressure compressor stack to be installed separately from the high pressure turbine rotor through a kickstand.

Description:
BACKGROUND 
     This application relates to a method of assembling a gas turbine engine, wherein both a compressor rotors and the turbine rotors are assembled using a tie shaft connection. 
     Gas turbine engines are known, and typically include a compressor, which compresses air and delivers it downstream into a combustion section. The air is mixed with fuel in the combustion section and combusted. Products of this combustion pass downstream over turbine rotors, driving the turbine rotors to rotate. 
     Typically, the compressor section is provided with a plurality of rotor serial stages, or rotor sections. Traditionally, these stages were joined sequentially one to another into an inseparable assembly by welding or separable assembly by bolting using bolt flanges, or other structure to receive the attachment bolts. 
     More recently, it has been proposed to eliminate the welded or bolted joints with a single coupling which applies an axial force through the compressor rotors stack to hold them together and create the friction necessary to transmit torque. 
     SUMMARY 
     A gas turbine engine has a compressor section carrying a plurality of compressor rotors and a turbine section carrying a plurality of turbine rotors. The compressor rotors and the turbine rotors are constrained to rotate together with a tie shaft. An upstream hub provides an upstream abutment face for the compressor rotors stack. A downstream hub bounds the upstream end of the compressor rotor and abuts the compressor rotor stack against the upstream hub. 
     The downstream hub creates a middle support used to provide radial support for a high pressure rotor and control to the tie shaft preload. The middle support also includes a high pressure compressor coupling nut that applies a preload that allows the high pressure compressor stack to be installed separately from the high pressure turbine rotor. The middle support is essential to control the dynamic stability of the long high pressure rotor spanning the distance between its forward and aft supports. The aft support includes a multiple layer interference fit between the shaft and the most downstream turbine rotor. The multi-layer fit accomplishes simultaneously radial support for the rotors stack and dynamic stability for a high pressure spool. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional perspective view of a turbine engine according to the claims. 
         FIG. 2  is an enlarged view of the turbine engine with a middle support member according to the claims. 
         FIG. 3  is an enlarged view of a high pressure turbine rotor aft end support member according to the claims. 
     
    
    
     DESCRIPTION 
       FIG. 1  illustrates a turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally including a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. In the illustrated arrangement, by-pass air flows longitudinally around the engine core through a by-pass duct  20  provided within the nacelle. The compressor  14  and turbine  18  may be connected in a variety of ways, such as through a shaft, through one or more tie shafts, through a transmission, etc. 
     Referring to  FIG. 2 , a long span between supporting bearings  350  and  330  creates rotor dynamic problems for bearing preload and rotor stability. Bearings apart from being mounted on the shafts and housings have to be preloaded properly for their proper functioning. Preloading is the methodology by which the internal clearance in the bearing is removed by applying a permanent thrust load to it. In other terms, the bearing is pushed to such an extent that it has to move only in the groove (raceway) and cannot move axially in either direction. Preloading may be needed for several reasons such as to eliminate the radial and axial play in the bearing which would be inherently present even after a bearing is mounted radially on a shaft, eliminate all the unnecessary clearances, which may induce a rigidity to the bearings and thus to the system the bearing supports and by reducing the clearances, the rotational accuracy of the bearing may be controlled. Thus, it helps to reduce the non-repetitive run out that could occur because of the clearances. 
     To address these requirements, a support  340  may be provided between bearings  330 ,  350  and a compressor rotor stack  313  and a turbine rotor stack  324  may be configured to retain a tight radial fit with a tie shaft  322 . Axial preload in the compressor rotor stack  313  and the turbine rotor stack  324  may generate the friction between adjoining rotor faces required for torque transmission. A downstream hub  341  may act as a middle support member to address these requirements. The downstream hub  341  may allow the compressor rotor stack  313  to be assembled separately with a temporary preload applied by a high pressure compressor (HPC) coupling nut  332 . The HPC coupling nut  332  may be axially preloaded to satisfy dynamic stability requirements and to prevent the HPC coupling nut  332  from whirling. 
       FIG. 2  schematically illustrates a gas turbine engine  10  incorporating a combustion section  311 , shown schematically, a compressor rotor stack  313  having a plurality of compressor rotors  338 , and a turbine rotor stack  324  having a plurality of turbine rotors  325 . As shown, an upstream hub  334  may be threadably secured to the tie shaft  322  at the upstream side of the compressor rotor stack  313 . The downstream hub  341  may be positioned downstream of the compressor rotor stack  313 , and may contact the downstream-most of the compressor rotors  338 . The compressor rotor stack  313  may be sandwiched between the downstream hub  341  and the upstream hub  334 , and secured by the HPC coupling nut  332 . The downstream hub  341  may abut the turbine rotor stack  324  which in turn may be secured with a high pressure turbine (HPT) lock nut  327  as shown in  FIG. 3 . A downstream lock nut  401  may bias a plurality of seals and bearings against the turbine rotors  325 . The HPT lock nut  327  and the downstream lock nut  401  may be threadably engaged to the same tie shaft  322 . The HPT lock nut  327  applies primary preload to the compressor rotor stack  313  and the turbine rotor stack  324 . As shown in  FIG. 3 , the HPT lock nut  327  may be threadably received on threads  458  of the tie shaft  322 .  FIG. 3  illustrates the downstream lock nut  401  and the HPT lock nut  327  threadably engaged to the tie shaft  322 . Initially, the upstream hub  334  ( FIG. 2 ) may be threadably assembled to the tie shaft  322  and the compressor rotors  338  and the downstream hub  341  may be stacked together and secured by the HPC coupling nut  332  which may apply an axial preload force to hold the compressor rotors  338  against a kickstand  343  of the downstream hub  341 . An internal compression load may be created in the compressor rotor stack  313  to react tension load in the tie shaft  322 . 
     The kickstand  343  of the downstream hub  341  is designed as a soft spring to enable a secondary load path from the HPC coupling nut  332  through the kickstand  343 , downstream hub  341 , and compressor rotor stack  313 . The secondary load path may prevent rolling and may ensure self alignment with the mating face of the HPC coupling nut  332 . The kickstand  343  of the arrangement may also generate radial and axial reactions at the downstream hub  341  interface with the most downstream of the compressor rotors  338 . The secondary load path applies a preload that is mostly temporary as it decreases significantly after the HPT lock nut  327  is tightened—the residual secondary preload may also create loaded contact between the kickstand  343  of the downstream hub  341  and the HPC coupling nut  332  even for conditions when the HPC coupling nut  332  tends to separate. 
     As shown in  FIG. 3 , radial preload may be applied to the turbine rotor stack  324  through a first fit  420  between bearing  330  and an intermediary sleeve  465 , a second fit  430  between intermediary sleeve  465  and a high pressure turbine (HPT) rotor arm  467 , and a third fit  440  between HPT rotor arm  467  and the tie shaft  322 . 
     The turbine rotors  325  may be axially preloaded and secured by the HPT lock nut  327  which may apply an axial preload force to hold the compressor rotor stack  313  and turbine rotor stack  324  together and produce the necessary friction to transmit torque. When the HPT lock nut  327  is tightened, the primary load path is transferred from the kickstand  343  to the cylindrical portion of the downstream hub  341  and through the turbine rotor stack  324 , producing internal compression load in the compressor rotor stack  313  and turbine rotor stack  324  and tension load in the tie shaft  322 . 
     The arrangement of the three fits  420 ,  430 , and  440  may ensure that the compressor rotor stack  313  and turbine rotor stack  324  are reliably held together, will be capable to resist the forces to be encountered during use, and will transmit the necessary torque and satisfy dynamic stability requirements. All these functions may be accomplished within a minimal radial envelope and with a low-profile locking ring  485 . 
     As a result of the arrangement described above, axial preload may be achieved with a single fastener, the tie shaft  322 . The preload may be distributed between the primary path and the secondary path, via the kickstand  343 , in a balanced manner such that there is a minimum loss in clamping capability while the dynamic stability is maintained for a long-span, high speed rotor, for example, a rotor that turns at a rate greater than 20,000 RPM. As illustrated in  FIG. 3 , the three fits  420 ,  430 ,  440  accomplish simultaneous radial support for the rotors stack, dynamic stability for the high pressure spool, and a leak-proof joint. 
     Although embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention. In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.