Patent Publication Number: US-8979502-B2

Title: Turbine rotor retaining system

Description:
TECHNICAL FIELD 
     The described subject matter relates generally to gas turbine engines, and more particularly to a turbine rotor retaining system of a gas turbine engine. 
     BACKGROUND OF THE ART 
     A gas turbine engine rotor assembly conventionally comprises a plurality of circumferentially spaced airfoils which extend radially outwardly from a rotor disc. During engine operation the rotor assembly is rotated at a high speed, thereby creating a centrifugal force acting on engine components. Axial forces are imparted to the airfoils as fluid passes through the rotor assembly. The rotor assemblies often include a dovetail or firtree attachment mechanism for coupling the engine components together and for resisting the centrifugal force acting on the components. Axial retaining mechanisms such as rabbets, bolts, tangs, pins or split rings may also be provided to counteract axial loads on the airfoil. The high-speed rotation of the rotor assembly causes a centrifugal load associated with the retaining mechanisms, which may result in vibration. In order to reduce vibration, the retaining mechanisms often have to be balanced, which incurs additional time and costs. 
     Accordingly, there is a need to provide an improved rotor retaining system for a rotor assembly of a gas turbine engine. 
     SUMMARY 
     In one aspect, the described subject matter provides a rotor retaining system for a rotor assembly of a gas turbine engine, the rotor assembly including a shaft, a rotor disc mounted on the shaft, a plurality of blades extending radially from the rotor disc and a cover plate attached to a side of the rotor disc, the rotor retaining system comprising: a first retaining nut threadingly engaging the shaft and retaining the rotor disc in position on the shaft; a second retaining nut disposed around the first retaining nut, threadingly engaging the rotor disc and retaining the cover plate in position relative to the rotor disc; a washer engaging both the first and second retaining nuts to interlock one retaining nut relative to another to impede relative rotation between first and second retaining nuts; and a wire removably engaging one of the first and second retaining nuts and disposed axially adjacent to the washer to retain the washer in position. 
     In another aspect, the described subject matter provides a rotor assembly of a gas turbine engine comprising: a shaft; a rotor disc mounted on the shaft and retained in position by a first retaining nut which threadingly engages the shaft in a first rotational direction; a plurality of blades received in circumferentially spaced slots in the rotor disc and axially extending across the rotor disc; a cover plate attached to the rotor disc and retaining the blades in the respective slots of the rotor disc, the cover plate being retained in position relative to the rotor disc, by a second retaining nut which threadingly engages the rotor disc in a second rotational direction opposite to the first rotational direction; a washer positioned adjacent the first and second retaining nuts to impede relative rotation between the first and second retaining nuts; and a wire removably engaging one of the first and second retaining nuts and disposed axially adjacent to the washer to retain the washer in position. 
     In a further aspect, the described subject matter provides a method for retaining components of a rotor assembly of a gas turbine engine, the method comprising: a) providing a first retaining nut threadingly engaging the rotor assembly in a first rotational direction for retaining a first one of the components in position; b) providing a second retaining nut threadingly engaging the rotor assembly in a second rotational direction opposite the first rotational direction, for retaining a second one of the components in position; c) using a washer to interlock the first and second retaining nuts one relative to another to impede relative rotation between the first and second retaining nuts; and d) engaging a wire to one of the first and second retaining nuts for retaining the washer in position. 
     Further details of these and other aspects of the described subject matter will be apparent from the detailed description and drawings included below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying drawings depicting aspects of the described subject matter, in which: 
         FIG. 1  is schematic cross-sectional view of a turbofan gas turbine engine illustrating an exemplary application of the described subject matter; 
         FIG. 2  is a partial perspective view of a compressor assembly of the turbofan gas turbine engine of  FIG. 1 , with a front portion thereof cut away to show internal structures of the assembly; 
         FIG. 3  is a partial enlarged view of the compressor assembly illustrated in in  FIG. 2 , showing the details of a rotor retaining system for the rotor assembly; 
         FIG. 4  is an front elevational view of a washer used in the rotor retaining system of  FIG. 3 ; 
         FIG. 5  is an front elevational view of a damper wire used in the rotor retaining system of  FIG. 3 ; and 
         FIG. 6  is a partial rear elevational view of a first retaining nut, showing a recess in a slot configuration defined in an end section of the retaining nut. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a turbofan gas turbine aircraft engine presented as an example of the application of the described subject matter, including a housing or nacelle annular outer case  10 , a annular core casing  13 , a low pressure spool assembly (not numbered) which includes a fan assembly  14 , a low pressure compressor assembly  16  and a low pressure turbine assembly  18 , connected together by a shaft  12 , and a high pressure spool assembly (not numbered) which includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24 , connected together by a hollow shaft  20  which is positioned coaxially around the shaft  12 . The annular core casing  13  surrounds the low and high pressure spool assemblies in order to define a main fluid path (not numbered) therethrough. In the main fluid path there is provided a combustor to constitute a gas generator section  26 . 
     It should be noted that the terms “radial”, “axial” and “circumferential” used in the description below refer to orientation about an engine central axis (not numbered) as shown in  FIG. 1 . The terms “upstream” and “downstream” used in the description below generally refer to the direction of a gas flow from an engine inlet  11  to an engine outlet  28  as shown in  FIG. 1 . The terms “forward” and “rearward” used in the description below also generally refer to the direction toward the engine inlet  11  and the engine outlet  28 , respectively. 
     Referring to  FIGS. 1-6 , a rotor assembly  30  in for example the high pressure compressor assembly  22  includes a rotor disc  32  mounted on the hollow shaft  20  to rotate together therewith. The rotor disc  32  is axially located immediately downstream of an impeller  34  of a centrifugal compressor which may also be part of the high pressure compressor assembly  22  and is mounted to the hollow shaft  20  to rotate together therewith. The rotor disc  32  is locked in an axial location on the shaft  20  by a retaining nut  36 . The retaining nut  36  has an annular body (not numbered) with inner threads (not shown) which engage outer threads (not shown) on the shaft  20 . Therefore, the retaining nut  36  can be rotated about the shaft  20  to axially move forward against a radial contact surface  38  of the rotor disc  32  in order to prevent the rotor disc  32  from rearward axial movement relative to the shaft  20 . 
     The rotor assembly  30  includes a plurality of circumferentially spaced airfoil blades  40  which extend radially outwardly from the rotor disc  32 , each having a root section (not numbered) received in respective slots  42  of the rotor disc  32 . The slots  42  are circumferentially spaced one from another on an outer periphery (not numbered) of the rotor disc  32  and axially extend across the outer periphery. A cover plate  44  having an annular body (not numbered) which may be in a dish-like profile, is attached to a rear side of the rotor disc  32 . The cover plate  44  has an annular large-diameter axial end  46  defined at one side of the cover plate  44  and an annular small-diameter axial end  48  at the other side of the cover plate  44 . The annular large-diameter axial end  46  of the cover plate  44  is disposed against a radial surface (not numbered) of the outer periphery of the rotor disc  32  and partially covers each of the slots  42 , to thereby prevent the blades  40  from rearward axial withdrawal from the respective slots  42 . The cover plate  44  is in turn, retained in position relative to the rotor disc  32  by a retaining nut  50  which threadingly engages the rotor disc  32  at a rearward end portion which is formed by a cylindrical wall (not numbered) of the rotor disc  32 . The cylindrical wall is positioned around and radially spaced apart from the retaining nut  36 . 
     The retaining nut  50  includes an annular body (not numbered) having inner threads (not shown) for engagement with outer threads (not shown) on the rearward end portion of the rotor disc  32  such that the retaining nut  50  can be rotated to move forwardly on the rearward end portion of the rotor disc  32  against the annular small-diameter axial end  48  of the cover plate  44 , thereby pushing the annular large-diameter axial end  46  of the cover plate  44  forward to be in tight contact with the radial surface of the outer periphery portion of the rotor disc  32 . 
     A washer  52  is provided adjacent to and engaging both the retaining nuts  36  and  50 , to interlock the retaining nuts  36  and  50  one to another, thereby preventing the respective retaining nuts  36  and  50  from rotation, which will be further described hereinafter. A damper wire  53  which is a single metal wire forming a split ring as shown in  FIG. 5 , removably engages for example with the retaining nut  50  for retaining the washer  52  in position and for damping vibration of the rotor assembly  30  during engine operation. Therefore, the damper wire may be selected to provide proper mass and resiliency such that the damper wire  53  is enabled to damp vibration when the damper wire  53  is attached to the rotor assembly  30 . 
     In one embodiment, the washer  52  may be formed with a metal ring having for example, opposed radial surfaces  54  (only one shown). The washer  52  may have a first lock member  56  integrated with the metal ring of the washer  52  and extending radially inwardly from the metal ring. The washer  52  may further include a second lock member  58  integrated with the metal ring and extending radially outwardly from the metal ring of the washer  52 . The first and second lock members  56 ,  58  may be circumferentially aligned one with another according to this embodiment, as shown in  FIGS. 3 and 4 . Nevertheless, the first and second lock members  56 ,  58  may be circumferentially spaced apart, according to another embodiment as shown in  FIG. 2 . Alternatively, the first lock member  56  may be disposed in a circumferential location as illustrated by the broken line in  FIG. 4 , to be diametrically opposite the second lock member  58 , in consideration of weight balances of the washer  52 . 
     According to one embodiment, the retaining nut  36  may have a threaded section  60  containing inner threads for threaded engagement with the shaft  20  and an annular end section  62  provided with at least one recess  64  having an opening on the radial surface of the annular end section  62 . The recess  64  may be formed as a slot extending radially through the annular end section  62  for receiving the first lock member  56  of the washer  52 . A second recess (not shown) similar to the recess  64 , may be provided in the annular end section  62  of the retaining nut  36 , diametrically opposite the recess  64  in consideration of weight balance of the retaining nut  36 . 
     According to one embodiment, the retaining nut  50  may also include an annular end section  68  and a threaded section  66  containing inner threads for threaded engagement with the annular rearward end portion of the rotor disc  32 . The annular end section  68  may be provided with a plurality of recesses  70 , for example, circumferentially spaced apart one from another. The recesses  70  have an opening on the radial annular surface of the annular end section  68  of the retaining nut  50 . The recesses  70  each may extend radially outwardly from an annular inner surface (not numbered) of the annular end section  68  of the retaining nut  50  such that the second lock member  58  may be selectively received in one of the recesses  70 . The retaining nut  50  may further include a circumferential annular groove  72  extending radially outwardly from the annular inner surface of the annular end section  68  of the retaining nut  50 . The circumferential annular groove  72  may circumferentially extend through the respective recesses  70 , and may also be more shallow than the recesses  70 . Therefore, the circumferential groove  72  may be segregated by the respective recesses  70  into separate circumferential groove sections, each section being positioned between adjacent recesses  70 . The damper wire  53  may be received in the circumferential groove  72  or in the circumferential groove sections thereof, as shown in  FIG. 3 . 
     When both retaining nuts  36  and  50  are tightened in position to retain the rotor disc  32  and the cover plate  44  in their respective positions, the washer  52  is positioned radially between the annular end section  62  of the retaining nut  36  and the annular end section  68  of the retaining nut  50 . The retaining nuts  36  and  50  are tightened such that one of the recesses  70  of the retaining nut  50  circumferentially aligns with the recess  64  of the retaining nut  36 . Therefore, the first and second lock members  56 ,  58  can be axially moved into and received in the respective aligned recesses  64 ,  70  in the respective retaining nuts  36  and  50 . The threaded engagement between the retaining nut  36  and the shaft  20  is in a rotational direction opposite to the rotational direction of the threaded engagement between the retaining nut  50  and the annular end portion of the rotor disc  32 . When one of the retaining nuts  36  and  50  is rotated in a direction to loosen the threaded engagement, the other of the retaining nuts  36  and  50  is to rotated in the same direction, which further tightens the threaded engagement of said other one of the retaining nuts  36 ,  50 , because the two retaining nuts  36  and  50  are interlocked together by the washer  52 . Therefore, the retaining nuts  36  and  50  prevent each other from rotation while the washer  52  is in position. The washer  52  in turn is axially restrained between a radial surface (not numbered) of the selected radial recess  70  of the retaining nut  50  and the damper wire  53  which is fittingly received in the annular groove  72  in the retaining nut  50 . 
     In the embodiment wherein the first and second lock members  56 ,  58  are not circumferentially aligned with each other as shown in  FIG. 2 , the retaining nuts  36  and  50  are tightened such that a selected one of the recesses  70  aligns with the second lock member  58  of the washer  52  when the first lock member  56  aligns with the recess  64  in the retaining nut  36 . Therefore, in all embodiments the washer  52  can be axially moved into position and interlocks the retaining nuts  36  and  50  one to another, to prevent relative rotation therebetween. 
     According to one embodiment, the retaining nuts  36  and  50  of the rotor assembly  30  are prevented from rotation by only one washer  52  with the damper wire  53 . The lock members  56  and  58  of the washer  52  can be conveniently placed in the recess  64  of the retaining nut  36  and a selective one of the recesses  70  of the retaining nut  50  respectively, when the washer  52  is axially forwardly moved into an annulus (not numbered) between the end sections  62  and  68  of the respective retaining nuts  36 ,  50 . There is no need for a crimp action which is conventionally required to crimp tabs of a washer for locking a nut. In contrast to a conventional retaining ring, the mass/weight of the damper wire  53  is relatively small and an imbalance effect which could be caused by the damper wire  53  may be small enough to be ignored, and therefore no balancing action is required. The damper wire  53  is spring loaded within the annular groove  72  in the retaining nut  50  and therefore does not require a catcher feature. The spring loaded damper wire  53  performs not only as a lock ring but also as a damper to absorb vibration energy of the rotor assembly  30 . 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departure from the scope of the described subject matter. For example, a turbofan gas turbine engine has been described as an exemplary application of the subject matter, however, it should be understood that gas turbine engines of various types may also be applicable for the described subject matter. The rotor retaining system as described in the above-noted embodiment is for a high pressure rotor assembly of a gas turbine engine, however the above-described rotor retaining system may also be applicable to other rotor assemblies of gas turbine engines, such as low pressure rotor assemblies, low or high pressure turbine assemblies, etc . . . The damper wire may be attached to either one of the retaining nuts. The plurality of recesses for selective engagement with a lock member of the washer may also be defined in either one of the retaining nuts. Still other modifications which fall within the scope of the described subject matter will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.