Abstract:
A platform for a bladed wheel of a turbomachine including a drum and blades of hammer-head attachment type, the foot of which is retained in a circumferential groove of the drum, having bearing and/or retaining faces cooperating with the drum is disclosed. The platform includes at least two openings spaced apart circumferentially and each able to receive the foot of a blade, whereby it constitutes a multiple-blade platform in the form of a piece separate from the blades.

Description:
BACKGROUND OF THE INVENTION 
     1. Field Of The Invention 
     The invention relates to a platform for a bladed wheel of a turbomachine comprising a drum and blades of hammer-head attachment type, the foot of which is retained in a circumferential groove of the drum, having bearing and/or retaining faces cooperating with the drum. 
     The present invention relates in particular but in a non-limiting way to the moving bladed wheels of a low-pressure compressor of a turbomachine. 
     2. Description Of The Related Art 
     Normally, these bladed wheels comprise a series of blades, each forming a separate piece having several parts, these blades being fitted directly onto the drum. 
     Such a blade  20  is shown in  FIG. 1 , on its own, and in  FIGS. 2 to 4  when fitted in a circumferential groove  31  of a drum  30 . 
     In this case, the blade  20  of the prior art that can be seen in  FIG. 1  comprises the following parts:
         a vane  21  extending essentially radially and axially and constituting the part that transmits the kinetic energy to the air stream,   a foot  22  constituting the part that maintains the blade on the rotor by being housed in the drum  30 ,   a platform  23  extending essentially axially and circumferentially, constituting the part that delimits and channels the air stream, and situated between the vane  21  and the foot  22 , and   a radius  24  constituting the linking part between the vane  21  and the platform  23  and having an external progressive transition face with concave profile.       

     After fitting in the circumferential groove  31  of the drum  30  of axis X-X′, as can be seen in  FIGS. 2 to 4 :
         the platforms  23  abut against each other in pairs by their circumferential end face  23   a  (see  FIG. 4 ), hence the blades  20  are immobilized in the circumferential direction relative to the drum  30  thanks to a lock (not represented) fitted between the row of blades  20  and in a locking notch (not represented) of the circumferential groove  31 , and   the foot  22  and the circumferential groove  31  have opposing bearing faces  22   a ,  22   b  and  31   a ,  31   b  (see  FIG. 3 ) which cooperate with each other in pairs, hence the blades  20  are immobilized radially and axially relative to the drum  30  (in rotation and under the effect of the centrifugal force, the inclined bearing faces  22   a  and  31   a  come into contact).       

     In this case, an annular catch  25  is arranged on the internal face of the platform  23 , on the downstream end (see  FIGS. 1 ,  3  and  4 ) or on the upstream end of the platform  23 . As a variant, both the downstream end and the upstream end of the platform  23  are equipped with a catch  25 . 
     This catch  25  forms an abutment beneath the platform  23  which guarantees, when the motor is stopped and the blades  20  are not centrifuged, that the blade  20  bearing this catch remains in the correct position. In practice, the catch  25  forms by its free end a retaining face bearing against the top face (external face) of one of the lateral walls surrounding the circumferential groove  31 : thus, the pivoting of the blade  20  about the foot  22  (about an axis parallel to the perpendicular direction in  FIG. 3 ) is prevented. This catch  25  also serves to ensure the seal-tightness, because it also serves as a bearing surface for the O-ring seal  40  which is housed in an annular ridge  32  of the external face of the drum  30 , close to the circumferential groove  31  (see  FIG. 3 ), in order to prevent the recirculation of air below the blade  20 . 
     In the present text, the terms “internal” and “external” designate radial positions respectively close to the axis X-X′ and distant from the axis X-X′ and the terms “upstream” and “downstream” designate axial positions along the axis X-X′, in the direction of the flow of the air stream, which are arranged respectively one in front of the other. 
     This type of one-piece blade  20  with hammer-head attachment is generally made of a titanium alloy with the application of several technologies: the parts exposed to the air stream, namely the vane  21 , the radius  24  and the top face of the platform  23 , the result of forging, whereas the other parts, namely the bottom face of the platform  23  and the foot, are obtained on completion of a post-forging machining step. 
     Because of the one-piece nature of these blades  20 , it is not possible to make the different parts of the blade  20  using the methods and materials best suited to the geometrical and operating stresses of each of these parts. 
     Also known is how to produce blades that comprise a reduced blade platform (reduced circumferential extent) and which are pieces separate from other pieces consisting of individualized inter-blade platforms. 
     The documents U.S. Pat. No. 6,632,070, US 2007 0020102 and in particular document US 2006 0222502 relate to such a situation. 
     Thus, in the case of the document US 2006 0222502, a blade, an inter-blade platform, a blade, an inter-blade platform and so on are fitted alternately, in the circumferential groove of the drum. This way, on the one hand the blades with reduced platform and on the other hand the individualized inter-blade platforms can be manufactured separately, and therefore using different techniques and/or materials. 
     However, because of the larger number of pieces constituting the bladed wheel (practically twice as many), there are more handling operations when assembling (or disassembling) the bladed wheel. Since the play in the circumferential direction must be as small as possible (play to be taken up), the assembly of a larger number of pieces generates longer times and therefore an excess cost, as well as greater risks of error concerning the positioning of the pieces relative to each other and along the series of blades and inter-blade platforms. 
     Furthermore, with individualized inter-blade platforms, the locking system is more complex because the space generally occupied by the locks is used by the inter-blade platforms, which leads to the use of more complex inter-blade platforms called locking inter-blade platforms. 
     Furthermore, in this case, there is a risk of generating more air leaks, that is recirculations from the duct delimited by the external face of the platforms to the space situated under the platforms, because of the larger possible number of air passages between the reduced platform of a blade and the adjacent individualized inter-blade platform. To optimize the seal-tightness with individualized inter-blade platforms, it has been proposed to modify the geometry of the blade (or of the inter-blade platform), but these attempts have culminated in very complicated geometries, and would be the cause of major excess costs. 
     BRIEF SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide a solution making it possible to overcome the drawbacks of the prior art and in particular offering the possibility of separately manufacturing the platforms and the blades, but without too significantly increasing the number of handling operations when assembling/disassembling and minimizing the risks of recirculation. 
     To this end, according to the present invention, the platform is characterized in that it has bearing and/or retaining faces capable of cooperating with the drum to retain the platform on the drum, in that it covers an angular segment, and in that it comprises at least two openings spaced apart circumferentially and each able to receive the foot of a blade, whereby it constitutes a multiple-blade platform in the form of a piece separate from the blades. 
     In this way, it will be understood that, by no longer using individualized inter-blade platforms, but multiple-blade platforms, the number of pieces stacked in the circumferential direction is reduced, so reducing the handling operations when assembling/disassembling. 
     This solution also offers the additional advantage of making it possible to easily optimize the platform regarding seal-tightness, that is, minimize the risks of recirculation. 
     This solution also offers the advantage of being compatible with a conventional locking system, which is not the case with the individualized inter-blade platform solution. 
     The present invention also relates to a blade designed to be fitted on a drum by means of a platform of the type defined previously, characterized in that it is made in a single piece and comprises a vane and a foot, between which is formed a reduced platform. 
     The present invention also relates to a moving bladed wheel comprising blades of hammer-head attachment type and a drum comprising a circumferential groove able to retain the foot of the blades and having an insertion notch, characterized in that it comprises several platforms of the type defined previously, each covering an angular segment of the bladed wheel and retaining several of its blades in the drum. 
     The present invention also relates to a compressor, in particular a low-pressure compressor, but also, where appropriate, a high-pressure compressor, which comprises a bladed wheel of the type defined previously. 
     Finally, the present invention relates to a turbomachine, in particular a turbo-jet engine, comprising a bladed wheel of the type defined previously. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent from reading the following description, given by way of example, and with reference to the appended drawings in which: 
         FIG. 1 , already described, is a perspective view of a blade according to the state of the art, 
         FIG. 2 , already described, is a perspective view of several blades according to  FIG. 1 , fitted on the drum, 
         FIG. 3 , already described, is a projection view in the direction III of  FIG. 2 , 
         FIG. 4 , already described, is a cross-sectional view in the direction IV-IV of  FIG. 3 , 
         FIG. 5  is a view similar to that of  FIG. 2  for a platform according to the invention, 
         FIG. 6  is a projection view in the direction VI of  FIG. 5 , 
         FIGS. 7 and 8  are cross-sectional views respectively in the directions VII-VII and VIII-VIII of  FIG. 6 , 
         FIGS. 9 and 10  are partial perspective views of the multiple-blade platform according to the invention and of the blade with reduced platform according to the invention, 
         FIGS. 11 to 13  are perspective views illustrating the assembly steps between a multiple-blade platform, blades with reduced platform and a drum, and 
         FIG. 14  is an enlarged cross-sectional partial view in the direction XIV-XIV of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 5 , the drum  130  is partially represented in a portion corresponding to an angular segment spanning four blades with reduced platform  120  (only three of these four blades  120  are represented in  FIG. 5 ) which are each retained by their foot  122  in a circumferential groove  131  of the drum  130  by means of one multiple-blade platform  150 . 
     To this end, the multiple-blade platform  150  comprises, for each blade  120 , an opening  151  opening out into the circumferential groove  131  when the platform  150  is fitted on the drum  130 . 
     The multiple-blade platform  150  also comprises, in the position of each opening  151 , a recess  152  opening out onto the external face of the multiple-blade platform  150  and making it possible to receive a reduced blade platform  123 . This recess  152  is a cavity extending over the entire length, in the axial direction, of the platform  150 , and widthwise, in the circumferential direction, over the same distance as the opening  151  or as the width of the reduced platform  123  of the blade  120 , which is itself at most as wide as the foot  122  of the blade  120 . 
     In order to enable the platform  150  to be fitted on the drum, the platform  150  also comprises two stiffeners  153  comprising lateral bearing faces  153   a  able, on their own, to axially retain the platform  150  on the drum  130  and to reduce air recirculations. 
     To this end, the two stiffeners  153  constitute internal ribs situated at the two upstream and downstream ends of the platform  150  and the two stiffeners  153  comprise (see  FIG. 14 ) lateral bearing faces  153   a  designed to cooperate with the bearing faces  132   a  of the lateral walls  132  of the circumferential groove  131 . Preferably, the lateral bearing faces  132   a  and  153   a  constitute axial support surfaces, namely surfaces that provide full contact between them, for perfect axial retention between the drum  130  and the platform  150 . 
     In this way, furthermore, a labyrinth seal is formed between the drum  130  and the stiffeners  153 , which reduces air recirculations below the platform (internal face), whereby the O-ring seal  40  can be dispensed with. 
     The platform  150  also comprises (see  FIGS. 5 ,  6 ,  10 ,  12  and  13 ) at each of its two circumferential ends, an inter-platform bearing face  154  able to bear, after the platform  150  has been fitted on the drum  130 , against an inter-platform bearing face  154  of the adjacent platform  150 . 
     The platform  150  also comprises, either side of each opening  151 , in the axial direction, two orifices  155  each enabling the passage of a blade catch  125  (see  FIGS. 9 ,  10  and  14 ). 
     Thus, it will be understood that the recess  152  delimits a recessed space, which forms a depression, relative to the external surface of the platform  150 , which passes through the entire thickness of the platform  150  only at the positions of the opening  151  and of the orifices  155 . 
     As can be seen in particular in  FIGS. 9 and 12 , according to the present invention, the blade  120 , which is dedicated to the multiple-blade platform  150  according to the present invention, is made in a single piece and comprises a vane  121  and a foot  122 , between which is formed a reduced platform  123 , the dimensions of which in the axial, circumferential and radial directions precisely correspond respectively to the dimensions in the axial, circumferential and radial directions of the recess  152  in which this reduced platform  123  is able to be inserted. 
     Thus, recirculations between the reduced platforms  123  and the multi-blade platform  150  are avoided. 
     Furthermore, provision is made for the top faces of the reduced platforms  123  and of the multi-blade platform  150  to be flush when each reduced platform  123  is arranged in its recess  152 , so avoiding any aerodynamic disturbance of the air stream. 
     The blade  120  with reduced platform  123  also comprises a radius  124  similar to the radius  24  of the blade  20  described previously in relation to the prior art. 
     Furthermore, the bottom face of the reduced platform  123  has two blade catches  125  arranged axially either side of the foot  122  and oriented towards the foot  122 . 
     These two blade catches  125  pass through the two orifices  155  of the platform  150  to interact with the drum  130 . To this end, the free ends of the catches  125  have internal bearing faces  125   a  (see  FIG. 14 ) which cooperate by resting/bearing against corresponding external bearing faces  132   b  situated on the internal free end of the lateral walls  132  of the circumferential groove  131  of the drum. 
     In this way, the catches  125  of the blade  120  interact with the external bearing faces  132   b  of the drum  130  to radially maintain the platform  150 . The orifice  155  is produced and dimensioned so as to allow this interaction between the blade  120  and the drum  130 . 
     This version with two catches  125  is the one that is preferred and that is represented in the figures (see in particular  FIGS. 7 ,  9  and  14 ), but, according to a variant that is not represented, the reduced platform  123  comprises only a single catch  125 . In this case, the multiple-blade platform  150  is either unchanged and one of the two orifices  155  remains vacant after assembly, or the multiple-blade platform  150  comprises only a single orifice  155 . 
     The interaction between the lateral bearing faces  153   a  of the platform  150  with the lateral bearing faces  132   a  of the drum  130 , guarantees on the one hand the correct axial positioning of the platform  130  in the assembly, and on the other hand that the air recirculations through the bottom of the platform (air leak) will be limited. 
     It should be noted that the relative position between the foot  122  of each blade  120  and the circumferential groove  131  of the drum remains, in a manner similar to the assembly of the prior art presented in relation to  FIGS. 1 to 4 , determined in the axial and radial direction by the contact made, on rotation and under the effect of the centrifugal force, between the inclined internal bearing face  132   c  of the lateral walls  132  of the circumferential groove  131  and the inclined external bearing face  122   a  of the lateral face of the foot  122 . 
     According to the embodiment described previously and illustrated in the Figures, the multiple-blade platform  150  comprises four openings  151  spaced apart circumferentially and each able to receive a blade  120 . In this respect, and according to the present invention, the multiple-blade platform  150  spans an angular segment of several blades  120 , which means that the multiple-blade platform  150  can be produced according to several variants in order to receive two, three or more blades  120  by means of as many openings  151  which are each associated with the recess  152  and with the two orifices  155 . 
     The multiple-blade platform  150  is preferably made of a light metal alloy or of a composite material comprising a thermoplastic matrix and fibres, preferably short fibres, but long fibres can be used. 
     For example, the thermoplastic matrix is of the PEI (Polyetherimide) or PEEK (polyetheretherketone) type and the fibres comprise carbon or glass fibres. 
     Alternatively, provision can also be made for the multiple-blade platform  150  to be made of aluminium alloy. 
     The drum  130  can be the same as the drum  30  of the prior art. However, because, according to the invention, the platform  150  is separate from the blades  120  and can therefore be made, as indicated previously, of a lighter material than the material forming the blades  120  (generally titanium), it is possible to thin the drum  130  which is subject to reduced forces. 
     Thus, it will be understood that, thanks to the invention, the blade  120  with reduced platform is obtained from a “forging” of reduced size compared to that needed to obtain the blade  20  according to the prior art. 
     Furthermore, thanks to the invention, a lightening of the weight of the assembly consisting of the blades  120  and of the multiple-blade platforms  150 , and therefore of the rotor (or bladed wheel) when this assembly is fitted on the drum  130 , is obtained. 
     In relation to  FIGS. 11 to 13 , there now follows an explanation of the fitting between each platform  150 , the drum  130  and the blades  120 . 
     The multiple-blade platform  150  must first be placed on the drum  130  by sliding between the lateral bearing faces  153   a  of the stiffeners  153  and the lateral bearing faces  132   a  of the lateral walls  132  of the circumferential groove  131  of the drum  130  (see  FIG. 13 ). In this step, an opening  151  of the multiple-blade platform  150  is placed (see  FIG. 11 ) opposite an insertion notch  133  forming a single hollow into the bottom of the circumferential groove  131  of the drum  130 . 
     Secondly, the foot  122  of a first blade  120  is inserted into the opening  151  positioned facing the insertion notch  133 , before the entire foot  122  is placed in the circumferential groove  131 . 
     In this step, the catches  125  are positioned automatically in the corresponding orifices  155  (see  FIG. 14 ). 
     Thirdly, the multiple-blade platform  150  is displaced circumferentially on the drum  130  by sliding between the lateral bearing faces  153   a  of the stiffeners  153  and the lateral bearing faces  132   a  of the lateral walls  132  of the circumferential groove  131  of the drum  130 , so as to place the next opening  151  facing the insertion notch  133 , in order to enable the next blade to be fitted into the multiple-blade platform  150 . 
     In this step, the first blade  120  is locked in position, because, with the latter no longer facing the insertion notch  133 , its foot  122  can no longer exit from the circumferential groove  131  and the immobilizing position of  FIG. 14  is obtained. 
     When all the blades  120  of the angular segment corresponding to the circumferential length of the multiple-blade platform  150  are fitted in this way, the next multiple-blade platform  150  and its corresponding blades  120  are fitted, and so on. 
     When all the multiple-blade platforms  150  and all the blades  120  have been fitted, as in the prior art, the whole is locked by the fitting of two locks (not represented) in and either side of the insertion notch  133 . 
     It will be understood that the platform  150  positions the blades  120  circumferentially on the drum  130 . 
     Thanks to the multiple-blade platform  150 , the assembly phases (or, in the reverse direction, disassembling phases) are easy, reliable and rapid. 
     Thus, it will be understood that the presence of a multiple-blade platform  150  improves the delimiting of the artery in which the air stream passing through the moving wheel circulates, while minimizing air recirculations. 
     According to the present invention, the platform  150  is fitted on the drum  130 , solely by the bearing and/or retaining faces, and without additional fixing means such as screws and bolts or welding (no drilling is required on the drum, the platform or the foot of the blades).