Abstract:
A turbomachine rotor disk including inter-blade platforms fixed to ribs delimited by cavities in which blade roots are retained, and protective liners mounted between the flanks of the cavities of the disk and the blade roots is disclosed. The protective liners have a C-shaped cross section so that they can be fitted translationally and retained radially on the ribs of the disk and constitute a locking device which locks the platforms on the ribs of the disk.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
     The present invention relates to a fan rotor disk of a turbomachine such as an airplane turbojet engine. 
     In the known art, a rotor disk is formed at its periphery with an alternation of cavities and of ribs and bears a plurality of blades each formed of an airfoil section connected to a root engaged axially and retained radially in a cavity belonging to the disk. Platforms are fixed between the blades by radial flanges connected to corresponding radial flanges formed on the ribs of the disk. 
     The dynamics involved in mounting and removing the platforms onto and off the ribs of the disk entails that the platform perform a translational movement along the rib so that orifices in the flanges of the platform engage with pegs or rods provided on the flanges of the disk. 
     In a turbomachine, the lateral edges of the platforms have to be close enough to the blade airfoil sections to prevent parasitic flow of air toward the disk. The airfoil sections of a fan have a curved profile which means that the lateral edges of the platforms have also to be curved. However, because of the fitting and removal dynamics, there needs to be some clearance between the edges of the platforms and the airfoil sections and this gives rise to air leaks toward the disk. This clearance is particularly large at the axial ends of the platform. Thus, the buildup of blade/platform clearances over the entire disk leads to a reduction in the overall efficiency of the turbomachine. 
     In the prior art, solutions were focused on restoring the seal between the platform and the blade. However, techniques such as this entail the incorporation of additional components, making the turbomachines heavier. 
     SUMMARY OF THE INVENTION 
     It is a particular objective of the invention to provide a simple, economical and effective solution to these various problems. 
     To this end, the invention proposes a turbomachine fan rotor disk comprising, at its periphery, blades the roots of which are retained in cavities of the disk, and inter-blade platforms fixed to ribs delimited by the cavities in which the blade roots are mounted, protective liners being mounted between the flanks of the cavities of the disk and the blade roots, wherein the protective liners have a C-shaped cross section so that they can be fitted translationally and retained radially on the ribs of the disk and form means of locking the platforms onto the ribs of the disk. 
     In the known art, the liners are used to protect the blade roots against friction on the internal walls of the cavity. The liner is usually made of a material that has greater resistance to wear than the blade root and than the disk. 
     According to the invention, the liners perform an additional function by radially retaining the intern-blade platforms in position on the ribs of the rotor disk using locking means. The invention therefore makes it possible to reduce the clearance between the blade and the platform because the platform no longer needs to be inserted axially, locking being afforded by the liners. The lateral edges of the platform can thus perfectly correspond to the curvature of the blade airfoil section. 
     According to another feature of the invention, the platforms on the disk comprise roots pressed against the ribs of the disk, these roots being engaged and retained in cutouts or openings in the liners. 
     In a preferred embodiment, the liners can be moved translationally on the ribs between a position in which they free and a position in which they retain the roots of the platforms and are immobilized in their retaining position by an annular component mounted on the upstream face of the disk for axially retaining the blade roots in the cavities of the disk. 
     This system of locking using a translational movement of the protective liner allows the platform to be mounted on a rib of the disk with a minimum of clearance. 
     According to another feature of the invention, the roots of the platforms comprise radial uprights and axial rims extending in the upstream direction from the radial uprights. When the protective liner is in the locked position, the axial rims lie inside the protective liner and can thus be used to hold the platform in position on the rib of the disk. 
     Advantageously, the cavities and ribs of the disk and the protective liners have a helicoidal profile. 
     In certain turbomachine configurations, the rotor blade roots are mounted in cavities of the disk that make an angle with the axis of the disk. Thus, when the blades are mounted in the cavities of the disk, there are unequal thicknesses of rib radially retaining the blades on each side of the blade root and this can give rise to premature blade root wear. The use of helicoidal profiles for the cavities and ribs of the disk makes it possible to keep constant thicknesses on each side of the blade roots over the entire length of the disk, the liners having a helicoidal profile so that they can be fitted onto the ribs. 
     The platforms are advantageously mounted on the disk by radial translation. 
     The invention also relates to a turbomachine such as an airplane turbojet engine and which comprises a disk of the type described hereinabove. 
     The invention further relates to a protective liner for a blade root in a turbomachine and which has a C-shaped cross section and comprises cutouts or openings formed in its wall connecting the legs of the C. The protective liner may have a helicoidal profile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent from reading the following description which is given by way of nonlimiting example and with reference to the attached drawings in which: 
         FIG. 1  is a partial perspective view of a disk bearing a blade and a platform according to the prior art; 
         FIG. 2  is a schematic perspective view of a protective liner fitted onto a rib of the disk according to the invention; 
         FIG. 3  is a schematic side view of an inter-blade platform according to the invention locked onto a rib of the disk; 
         FIG. 4  is a schematic view from the left of the disk of  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is made first of all to  FIG. 1  which depicts part of a turbomachine disk  10  bearing a blade  12  according to the prior art. The disk  10  at its periphery comprises an alternation of cavities  14  and of ribs  16  extending longitudinally over the entire length of the disk  10 . The rotor blade  12  formed of an airfoil section  18  connected to a blade root  20  is engaged and retained radially in a cavity  14  of the disk  10 . A platform  22  is positioned on a rib  16  of the disk  10 , the edges  24  of the platform  22  being positioned close enough to the airfoil section  18  of the contiguous blade  12  that flows of air toward the disk  10  are prevented. The platform  22  is fixed by inwardly-extending radial flanges onto outwardly extending radial flanges  26  of a rib  16  of the disk  10 . Rods  28  inserted in the flanges  26  of the disk  10  and the flanges of the platform  22  radially retain the platform  22  on the rib  16 . The dynamics of fitting and removing the platform  22  dictate that the flanges being gauged onto the rods  28  through a platform  22  movement along the central axis of the rib  16 . Now, because the airfoil section  18  is curved, it is necessary to have some clearance between the edges  24  of the platform  22  and the airfoil section  18  so as to allow the platform  22  to move about the central axis of the rib  16 . This clearance is of the order of 3 mm and is greatest at the axial ends of the platform  22 . Air can thus circulate between the platform  22  and the blade  12 , reducing the performance of the turbomachine. 
     The invention makes it possible to reduce the clearance between the edges  24  of the platform and the airfoil section  18  of the blade  12  by altering the dynamics involved in mounting the platforms on the ribs  16  of the disk  10  and by using protective liners to lock the platforms onto the disk. 
     In the prior art, liners were used to protect the blade roots  20  engaged in the cavities  14  from friction between these blade roots and the flanks of the cavities  14  of the disk. The interposition of an element such as a protective liner between the rib  16  and the blade root  20 , the resistance to wear of which is lower than the wear resistance of the blade root  20  and of the disk  10 , makes it possible to spare the blade root  20 . 
       FIG. 2  schematically depicts a protective liner  30  according to the invention, translationally engaged onto a rib  16  of a disk  32  according to the invention, just part of which is visible. The protective liner  30  has a C-shaped cross section allowing it to be engaged axially and retained radially on the rib  16 . The central part  34  of the protective liner  30  extends over part of the rib  16  of the disk and is connected by substantially radial uprights  36  to the lateral edges  38  of the liner  30  and extends over the entire length of the disk  32 . The lateral edges  38  positioned inside the cavity  14  bear against part of the rib and radially retain the protective liner  30 . The central part  34  is substantially parallel to the peripheral external surface of the rib  16  of the disk  32  and cutouts  40  at its axial ends are substantially parallel to a plane perpendicular to the axis of the disk  32 . 
     The protective liner  30  according to the invention not only prevents damage to the blade roots  20  but also holds the platform on the rib  16  of the disk  32  by forming means of locking the platform in position. To do that, the central part  34  of the liner  30  comprises an opening  42  and cutouts  40  at its axial ends. 
       FIG. 3  depicts a platform  44  according to the invention comprising roots  46  extending radially inwards, which roots are formed of radial uprights  48  and axial rims  50 . 
     A platform  44  positioned on a rib  16  of the disk  32  is locked in place by translationally inserting the protective liner  30  onto a rib  16  of the disk  32 . The liner  30  is moved along the rib  16  of the disk  32  as far as a position such that the roots  46  of the platform  44  can be inserted by a radial translational movement in the direction of the arrow A, into a cutout  40  and the opening  42  in the protective liner  30 . The platform  44  depicted in  FIG. 3  comprises two roots  46  which are axially offset and which after insertion through the protective liner  30  rest on the rib  16  of the disk  32 . The protective liner  30  is then given a translational movement in the direction of the arrow B on the rib  16  so that the axial rims  50  are radially retained by the central part  34  of the protective liner  30 , thus radially locking the platform  44  onto the disk  32 . Indeed, in operation, the platform  44  is subjected to the effect of centrifugal force and suffers a radial displacement halted by the axial rims  50  which butt against the central part  34  of the protective liner. 
     Inserting the platform  44  radially means that the curvature of the edges of the platform  44  can coincide perfectly with the curvature of the airfoil section  18  and that the clearance between the platform  44  and the blade  12  is thus smaller. 
     In the embodiment of the protective liner  30  depicted in the drawings, the lateral upstream ends of the liner  30  form projections  52  with respect to the upstream face of the disk  32  when the liner  30  is in the position that locks the platform  44 . These projections  52  are intended to collaborate with an annular component, not depicted, mounted on the upstream face of the disk  32  for axial retention of the blade roots  20  and of the protective liners  30 . 
     In a preferred embodiment of the invention depicted in  FIG. 4 , the cavities  14  and the ribs  16  together with the protective liners  30  have a helicoidal profile. This profile makes it possible, when the cavities  14  are at an angle to the axis of the disk  32 , to keep a constant thickness e 1  over the entire length of the disk  32 , this thickness also being substantially equal to the thickness e 2  because the cavities  14  and ribs  16  follow the cylindrical profile of the disk  32 . This type of profile thus makes it possible to reduce the wear on the blade roots  20 . 
     Should a blade be lost, the protective liner  30  may also act as a limit stop or alternatively may deform to prevent contact between the platform  44  and the loose blade  12  as such contact may lead to ejection of the platform  44 . 
     The protective liner  30  may have a variable number of cutouts  42  and openings according to the number of roots  46  that the platform  44  requires. 
     The protective liners are made of metal and have a thickness ranging between 0.1 and a few millimeters.