Abstract:
A blade retention system for retaining blade roots in slots of a rotor hub disk, the rotor blade having a radially outward blade tip requiring surface material removal by grinding to acquire a finished ground surface, having a blade root retention tab with an elongated web of thickness less than the gap between the slot and blade root and with at least two deformable protrusions extending radially to a height exceeding said gap, the protrusions adapted to deform under compression radially inwardly on sliding engagement with the blade root in the slot and to exert a reaction force radially outwardly on the blade root of a magnitude sufficient to secure the blade tip during surface removal grinding of the blade tip against radial displacement and against rotational displacement.

Description:
TECHNICAL FIELD 
   The invention relates to a blade retention system with retention tab for securing blades to a rotor in a gas turbine engine. 
   BACKGROUND OF THE ART 
   Turbine rotors and compressor rotors include individually removable blades that are conventionally mounted in a peripheral array of individually manufactured blades in interlocking slots in the rotor disc that match the blade roots. High rotational speeds require that the blades be securely mounted and blades are exposed to high temperature variations during operations as well as axial loading from flow of gas over the air foiled and platform surfaces. Individual blades are periodically removed during repairs and inspection. Preferably any blade locking mechanism is installed and rapidly removed with no damage to the rotor hub and blade root. 
   Many different types of locking devices are provided in the prior art. One low cost method of retaining small blades is with counter sunken rivets, which extend through the slot. The riveting operation on the bladed disk assembly is unreliable and frequently requires rework. Riveting machines are relatively expensive and limit the location where the work can be performed. Inconsistent load is applied by the crushed rivet and therefore the radial load on the blade varies depending on the installation. 
   Even when complex blade root locking systems are used, at times the blades loosen during grinding of the blade tips thereby creating variations in the outer rotor assembly diameter. Since blade tip clearance is of critical importance in maintaining the efficiency of the engine, variation in the outer rotor assembly diameter is undesirable. 
   As well, rivets have been known to develop insufficient axial resistance to the axial loads imparted on the blades and allow the blades to slide within the slots in the rotor and thereafter rub against adjacent components causing contact damage. 
   Many of the blade retention systems of the prior art involve relatively complex and expensive interlocking components that are not readily removed during repair operations. Some systems provide a resilient radial outward force that is variable and do not adequately support the blade roots radially during grinding of the blade tips. 
   It is an object of the present invention to provide a simple inexpensive blade retention scheme that adequately supports the blade during grinding operations of the blade tip. 
   It is a further object of the invention to provide a retention scheme that supports the blade with sufficient radial load to grind the rotor assembly without use of complex tooling or the need for high speed grinding. 
   Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below. 
   DISCLOSURE OF THE INVENTION 
   The invention provides a blade retention system, for use with a rotor hub disk with peripheral circumferential array of spaced apart blade retention slots extending between the forward and rearward hub faces, each slot having a radially inward floor and side walls adapted for sliding engagement with a blade root of a rotor blade. The blade root has a bottom surface which when engaged in the slot, is radially spaced from and adjacent the slot floor thus defining a gap extending between the forward and rearward hub faces at said selected broach angle. The blade root retention tab has an elongated web with a thickness less than said gap and including a preformed transverse flange extending from a first end of the web and a second end that extends from the slot and is bent on installation into an installation flange. The web is adapted to be disposed in the gap when the web rests on the slot floor to permit sliding engagement of the blade root in the slot while the preformed flange engages one of the forward and rearward faces of the rotor hub disk. The web of the tab includes at least two deformable protrusions extending radially to a height exceeding the gap, so that the protrusions deform on sliding engagement with the blade root in the slot and secure the blade radially outward during grinding of the tip for example. 

   
     DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be readily understood, one embodiment of the invention is illustrated by way of example in the accompanying drawings. 
       FIG. 1  is an axial cross-sectional view through a turbo fan gas turbine engine illustrating the basic components of the engine and specifically the location of rotors and blades to which the invention applies. 
       FIG. 2  is an axial cross-sectional view through the blade root mounted in the slot of a rotor in accordance with the invention using the retention tab with two deformable protrusions as shown in dashed outline. 
       FIG. 3  is a perspective view of a segment of the forward face of the hub included to installed blades. 
       FIG. 4  is a perspective view of a segment of the aft face of the hub. 
       FIG. 5  is a plan view of an individual retention tab. 
       FIG. 6  is an elevation view of the retention tab. 
       FIG. 7  is a cross-sectional view through the retention tab to show the details of the protrusions. 
     Further details of the invention and its advantages will be apparent from the detailed description included below. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows an axial cross-section through a turbo-fan gas turbine engine. It will be understood however that the invention is equally applicable to any type of engine with a compressor and turbine section such as a turbo-shaft, a turbo-prop, or auxiliary power units. Air intake into the engine passes over fan blades  1  in a fan case  2  and is then split into an outer annular flow through the bypass duct  3  and an inner flow through the low-pressure axial compressor  4  and high-pressure centrifugal compressor  5 . Compressed air exits the compressor  5  through a diffuser  6  and is contained within a plenum  7  that surrounds the combustor  8 . Fuel is supplied to the combustor  8  through fuel tubes  9  which is mixed with air from the plenum  7  when sprayed through nozzles into the combustor  8  as a fuel air mixture that is ignited. A portion of the compressed air within the plenum  7  is admitted into the combustor  8  through orifices in the side walls to create a cooling air curtain along the combustor walls or is used for cooling to eventually mix with the hot gases from the combustor and pass over the nozzle guide vane  10  and turbines  11  before exiting the tail of the engine as exhaust. 
     FIG. 2  shows details of the blade retention system for use in a rotor assembly such as a compressor or turbine rotor for example. As shown in  FIGS. 2 ,  3  and  4 , the rotor hub disk  12  has a thickness between the forward face  13  and rearward face  14  with a peripheral circumferential array of spaced apart blade retention slots  15  extending between the forward and rearward hub faces  13  and  14  at a selected broach angle β as is conventional. Each slot  15  had a radially inward wall and side walls adapted for sliding engagement with the blade root  16  of a blade  17 . The root  16  also has lateral sides disposed at the broach angle β and includes a bottom surface  18  which when engaged in the slot  15  is radially spaced from and adjacent the slot  4  floor thus defining a gap “g” as shown in  FIG. 2  extending between the forward and rearward faces  13  and  14  at the selected broach angle β. 
   In order to retain the blade root  16  and exert a radially outward force sufficient to maintain the position of the blade  17  during grinding of its tip  19 , the invention provides a novel blade root retention tab  20 . The details of the tab are illustrated best in  FIGS. 5 ,  6  and  7 . The tab  20  has an elongated web  21  having a thickness “t” that is less than the gap “g”. The tab includes a preformed transverse flange  22  that extends from a first end of the web  21 . As best shown in  FIG. 2 , the web  21  is adapted to be disposed in the gap “g” when the web  21  rests on the slot floor to permit sliding engagement of the blade root  16  into the slot  15 . In the embodiment shown, the preformed flange  22  engages the rearward face  14  of the rotor hub disk  12 . However, it will be understood that the arrangement can be easily reversed such that the flange  22  engages the forward face  13 . 
   As shown in  FIGS. 5 ,  6  and  7 , the web  21  includes at least two deformable protrusions  23  that extend radially to a height “h” that exceeds the dimension of the gap “g”. As shown in  FIG. 2  in dashed outline, the protrusions  23  before installation of the blade root  16  extend upwardly, however once the blade root  16  slides over the tab  20 , the protrusions  23  are deformed and pressed radially inwardly resulting in an interference fit. As shown in  FIG. 4 , the blade root  16  preferably includes a chamfer  24  on the leading edge which is oriented transverse to the broach angle β to apply an evenly distributed force to guide the deformation of the protrusions  23 . 
   Referring to  FIGS. 5 ,  6  and  7 , in the embodiment shown the tab  20  includes two protrusions  23  that are symmetrically spaced apart from a mid point of the web  21 . However it will be understood that any number of protrusions  23  can be provided preferably in a symmetrical pattern in order to enhance the even distribution of force to hold the blade roots  16  during grinding of the blade tip  19 . The protrusions  23  as illustrated in the embodiment of  FIGS. 6 and 7  are simple undulations of the web  21  formed by a press for example with a central portion of the protrusion  23  having a constant radius of curvature “r” as illustrated in FIG.  7 . As shown in  FIG. 6 , the preformed flange  22  is resiliently biased by over bending through an angle α such that the rotor surface engagement pad  25  remains in contact with the rearward face  14 . As illustrated in dashed outline in  FIGS. 6-7  and in solid outline in  FIG. 2 , the initially straight second end of the web  21  is bent into an installation flange  26  once the blade root  16  is in place. To prevent relative axial movement between the blade roots  16  and the rotor hub disk  12  on installation, the blade root  16  includes a trailing edge with rotor engagement abutments in the form of two fingers  27  that straddle the end of the web  21  in the installed position as best seen in  FIGS. 3 and 2 . In the installed position, the installation flange  26  is bent parallel to the two fingers  27  in an opposite direction. As a result, forward or rearward movement of the blade root  16  relative to the rotor hub disk  12  is prevented by interference with flanges  22  and  26  or fingers  27 . 
   Therefore, on installation the radially extending protrusions  23  incorporated into the retention tab  20  provide sufficient radial load to permit grinding of the blade tips  19  of the rotor assembly without the use of complex tooling or the need for high speed grinding. The interference induced by the protrusions  23  and the bent flanges  22  and  26  provide sufficient load to prevent the blade roots  16  from releasing from the slots  15  and in forward direction. The abutment fingers  27  prevent rearward axial motion of the blade root  16  within the slots  15 . 
   The retention tab  20  is designed with a width and thickness “t” to fit within the slots  15  in the rotor hub  12 . The preformed flange  22  engages either the forward or rearward face  13 ,  14  of the rotor hub disk  12 . Preferably, the slot  15  has a flat bottom matching the shape of the retention tab  20 . An elliptical shape for the bottom floor of the slot  15  would minimize stress in the rotor hub  12 , but on contact with a flat retention tab  20  would promote damage due to the line contact with the edges of the tab  20 . 
   The assembly procedure requires simple tooling as follows. The rotor hub  12  is installed into its backing plate (not shown). The retention tab  20  is inserted into position in the slot  15  and held in place as the retention tab  20  is compressed forward with the backing plate. The backing plate also provides support for the retention tab  20  against buckling while inserting the blade root  16  in a sliding motion within the slot  15 . The blade roots  16  are installed by sliding them axially over the retention tabs  20  and deformable protrusions  23 . The two fingers  27  that are cast in the blade root  16  ensure that blades  17  are installed in the proper orientation in a mistake proof manner. The blade root  16  slides within the slot  15  and scratching of the rotor hub disk  12  is eliminated since movement is between the blade root  16  and the retention tab  20 . 
   Preferably, the leading edge of the blade root  16  includes a chamfer  24  normal to the broach angle β to ease transition over the deformable protrusions  23 , thereby facilitating assembly of the blade roots  16  into the slots  15 . Once the two fingers  27  of the blade root  16  abut the rotor hub  12 , the other end of the tab  20  may be bent upwardly to form an installation flange  26  and complete the assembly. 
   Spring back of the flange  26  is minimized through use of low ductility material for the tabs  20 . Bending of the tab  20  during installation can be performed with a soft mallet or a roller with hydraulic tooling. Assembly time is reduced significantly in comparison to riveting for example. 
   Although the above description relates to a specific preferred embodiment as presently contemplated by the inventors, it will be understood that the invention in its broad aspect includes mechanical and functional equivalents of the elements described herein.