Abstract:
A turbomachine fan including blades and inter-blade platforms having radial flanges fastened to flanges of a rotor disk by threaded rods inserted into bushes mounted in orifices in the flanges of the platforms is disclosed. Each bush includes at least one deformable part interposed transversely between the threaded rod and an edge of the orifice in which the bush is mounted. The deformable part is intended to allow the platform to pivot in the event of an impact of a blade against the platform and to absorb some of the energy of the impact.

Description:
The present invention relates to a fan of a turbomachine, such as an aircraft turbojet. 
     BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
     In a known manner, the fan provided at the upstream end of a turbojet comprises blades carried by a rotor disk, with platforms arranged between the blades and fastened by radial flanges to flanges of the disk by means of threaded rods inserted through bushes mounted in orifices in the flanges of the platform and through flanges of the disk, these platforms making it possible to guide the air stream entering the turbomachine and to avoid unwanted air flows toward the disk. 
     During the operation of the turbomachine, the breaking of the connection between a blade and the disk can result in the destruction of the adjoining platform. Specifically, the blade then comes up against the lateral edges of the platform and/or against the flanges of the platform, thereby producing a considerable stress on the fastenings used to fasten the platform flanges to the disk flanges and possibly causing these fastenings to break and the platform to be ejected into the air flow path, and resulting in the destruction of parts of the turbomachine that are located downstream. 
     To overcome this problem, it has been proposed to form deformable regions on the lateral edges of the platform. Another solution, which may, if appropriate, be used in combination with the preceding one, is to form abutment regions on the flanges of the disk to prevent contact between the platform and the blade. Hence, the stress is no longer borne by the platform but by the flange of the disk. However, these solutions are not satisfactory since they do not impart any flexibility to the connection between the flanges of the platform and the flanges of the disk and do not allow sufficient absorption of the energy due to the impact between the blade and the platform. 
     SUMMARY OF THE INVENTION 
     The object of the invention is in particular to make available a simple, economic and effective solution to these various problems. 
     Accordingly, the invention provides a turbomachine fan comprising blades carried by a rotor disk and inter-blade platforms having radial flanges fastened to corresponding flanges of the rotor disk by means of threaded rods inserted into positioning bushes mounted in orifices in the flanges of the platforms, wherein each bush comprises at least one deformable part interposed transversely between the threaded rod inserted into the bush and an edge of the orifice in which the bush is mounted, this deformable part being intended to allow the platform to pivot in the event of an impact of a blade against the platform and to absorb some of the energy of the impact. 
     In the event of a blade loss, the deformable region allows a local plastic deformation of the bush at the fastening regions of the threaded rods, thus preventing the fastenings between the flanges of the platform and the flanges of the disk being destroyed and the platform being ejected into the air flow path. The platform is not subject to any deformation and can thus be held in position during the time required to shut down the engine, so preventing major damage to the turbomachine. Maintenance operations are thus facilitated and only the bush deformed by the impact has to be replaced. 
     According to another feature of the invention, the deformable part of the bush is juxtaposed with a rigid part of the bush that is opposed to the deformable part with respect to the threaded rod. 
     The use of a two-part bush makes it possible to replace only that part of the bush which has been subject to deformation following the impact of a blade against the platform. 
     According to another feature of the invention, the deformable part comprises two parallel half-cylinders which are connected by deformable webs and whose concavities are oriented in the same direction, one half-cylinder being applied to the threaded rod and the other to an internal cylindrical surface of the orifice in the flange of the platform. 
     The radial bearing of a half-cylinder on an internal cylindrical surface of the orifice in the flange of the platform makes it possible to optimally transmit the stresses generated by the loss of a blade to the webs, which will deform and thus absorb some of the energy of the impact. 
     In a preferred embodiment of the invention, the ends of the two half-cylinders of the deformable part are aligned with cylindrical ends of the rigid part of the bush and substantially bear on these same ends. 
     Advantageously, the aligned ends of one of the half-cylinders of the deformable part and of the rigid part have interengaged complementary indentations which immobilize the two parts of the bush with respect to one another against translational movement in the orifice in the flange of the platform. 
     According to another feature of the invention, the half-cylinders of the deformable part are connected by two substantially parallel webs joined to intermediate parts of the half-cylinders. 
     In the event of a blade loss, the two webs of material absorb the energy of the impact of the blade against the platform by deforming plastically. The shape and dimensions of these webs can vary quite widely in practice depending on the types of the turbomachines to which the invention is applied. 
     In one variant embodiment, the flange of the platform is fastened to the flange of the disk by means of two parallel threaded rods passing into two adjacent orifices in the flange of the disk and into a common orifice in the flange of the platform in which there is mounted an aforementioned bush comprising a rigid part and a deformable part, the rigid part being formed with an orifice for the passage of one threaded rod and with a half-orifice for the passage of the other threaded rod, the deformable part having the other half-orifice for the passage of the other threaded rod. 
     This variant prevents the platform from being able to turn about the axis of a threaded rod. The integration of a second orifice in the rigid part makes it possible to oppose the pivoting torque of the platform and thus keep it in position. 
     Depending on the various possible configurations, it may be necessary in the event of a blade loss for the bush to perform a rectilinear or curved displacement movement in order to optimally absorb the energy released by the impact of the blade against the platform. 
     The bush can be fastened by crimping in the orifice in the flange of the platform. 
     The invention also relates to a turbomachine, such as an aircraft turbojet, which comprises a fan of the above-described type. 
     The invention additionally relates to a bush intended to be inserted into an orifice in an inter-blade platform flange and comprising a rigid part and a deformable part, the deformable part having two parallel half-cylinders which are connected by webs of material and whose concavities are oriented in the same direction, the two half-cylinders being aligned with and substantially bearing on cylindrical ends of the rigid part. The aligned ends of one of the half-cylinders and of the rigid part advantageously have interengaged complementary indentations. 
     In one variant, the rigid part of the bush comprises an orifice and a half-orifice which are parallel, the half-orifice cooperating with a half-orifice of the deformable part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent on reading the description below given by way of nonlimiting example and with reference to the appended drawings, in which: 
         FIG. 1  is a partial schematic perspective view of a fan according to the invention; 
         FIG. 2  is a perspective view of a deformable bush according to the invention; 
         FIG. 3  is a schematic perspective view representing this bush inserted into an orifice in a flange of a platform; 
         FIG. 4  is a schematic perspective view of a second embodiment of a bush according to the invention, inserted into an orifice in a flange of a platform. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will be made first of all to  FIG. 1 , which represents part of a fan  10  of a turbomachine. A blade  12  of the fan  10 , formed by an airfoil connected at its radially internal end to a blade root, is engaged in a peripheral pocket  14  of a rotor disk  16 . An inter-blade platform  18  comprises radially inwardly extending flanges  20  fastened to radially outwardly extending flanges  22  of the disk. The flanges  20  of the platform  18  are offset axially and radially. Each flange  20  of the platform is fastened by a threaded rod  24  extending along the axis of the turbomachine through a bush  26  inserted into an orifice in the flange of the platform and through a corresponding orifice in the flange  22  of the disk  12 . The threaded rod receives a retaining nut at one of its ends. 
     In a turbomachine, the platforms  18  must be sufficiently close to the blades  12  to prevent any unwanted air flow toward the disk  16 . In the event of a blade loss, the blade  12  is subject to a rotational movement about the axis of the pocket  14  and comes into contact with a lateral edge  28  and/or with the flanges  20  of the platform  18 . The impact of the blade  12  against the platform  18  can break the fastenings of the platform  18  on the disk  16  and eject the platform  18 . 
     In order to absorb some of the energy of the impact in the event of a blade loss, the invention makes provision for a deformable element to be interposed transversely in the means for fastening the platform, thereby making more flexible the connection between the flange  20  of the platform  18  and the flange  22  of the disk and allowing a limited pivoting of the platform  18  in the event of a blade loss, while providing good mechanical strength during normal operation of the turbomachine. 
       FIGS. 2 and 3  show a bush  26  according to the invention, which is intended to be inserted into an orifice in the flange  20  of the platform  18 . The bush  26  is composed of a deformable part  30  and a rigid part  32  which are juxtaposed. The deformable part  30  comprises two parallel half-cylinders  34 ,  36  whose concavities are oriented in the same direction toward the rigid part  32 . The external surface of the external half-cylinder  34  is in contact with an internal cylindrical surface of the orifice in the flange of the platform, the internal surface of the half-cylinder  36  being applied to the threaded rod. The half-cylinders  34 ,  36  are connected in their intermediate parts by two parallel deformable webs  38 . These webs  38  being substantially parallel to a plane tangent to the circumference of the disk  16 . The ends of the two half-cylinders  34 ,  36  of the deformable part  30  are aligned with the ends of the rigid part  32 . The ends  40 ,  42  of the external half-cylinder  34  of the deformable part  30  of the bush  26  have indentations in which there are engaged complementary indentations formed at the ends  44 ,  46  of the rigid part. This method of connecting the deformable  30  and fixed  32  parts makes it possible to prevent any translational movement of one of the deformable  30  or rigid  32  parts with respect to the other. It is also possible to provide a skirt  48  for crimping the bush  26  in the orifice in the flange  20  of the platform  18  so as to ensure that the bush  26  is retained axially in the orifice in the flange  20  of the platform  18 . 
     In the event of a blade loss, the detached blade will strike the edges  28  and the flange  20  of the platform  18 . The energy of the impact is transmitted by the flange of the platform to the deformable part of the bush, which will be compressed to a greater or lesser degree on the threaded rod  24 , the webs  38  deforming plastically and thus allowing the platform  18  to be displaced with respect to the threaded rod  24 . This displacement can occur along a rectilinear or curved path depending on the desired movement of the platform  18  following the impact of the blade  12  against it. The shape and the dimensions of the webs  38  are chosen on the basis of the deformation threshold desired for the deformable part  30 . 
     The two parts of the bush can be made from identical materials or else from different materials. 
     According to one possible variant of the invention (not shown), the bush  26  comprises two deformable parts which are symmetrical with respect to the threaded rod. 
     According to one variant of the invention shown in  FIG. 4 , the rigid part of the bush  26  comprises a second orifice  50  parallel to the first and in which there is inserted a second threaded rod  52  which passes into a second orifice in the flange of the disk. The second orifice makes it possible to oppose a rotation of the platform  18  about a threaded rod and thus keep the platform perfectly in place. 
     The orifice  50  in the bush receiving the second threaded rod  52  is elongated in a transverse direction. Hence, in the event of a blade loss, the impact causes a deformation of the deformable part  30  of the bush  26  and a displacement of the rigid part  32  with respect to this second threaded rod  52 , without butting against this threaded rod. 
     This second orifice can have varied shapes dependent on the desired displacement of the platform  18 . In particular, it can have a curved shape. 
     The platform  18  can comprise bushes  26  with only one orifice and/or with two orifices according to the fastening method appropriate to each flange  20  of the platform  18 . 
     Although bushes  26  as described above are especially suited to their use with flanges  20  of a platform  18 , their applications are not limited to turbomachines and may comprise other types of fastening requiring a certain degree of flexibility in the event of an impact.