Abstract:
A support for a bearing that guides a shaft in a turbomachine is disclosed. The support includes an annular part made of a shape memory material which retains an initial shape when the load applied to it remains below a threshold value and which deforms, absorbing energy when the applied load becomes at least equal to the threshold value. The annular part is capable of reverting at least approximately to its initial shape when the applied load drops back below the threshold value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to supports for bearings that guide rotor shafts in turbomachines such as aircraft bypass turbojet engines. 
     2. Description of the Related Art 
     Turbojet engine fan blades may be subjected to damage, for example following ingestion of foreign objects such as birds or blocks of ice. 
     In general, the fan is robust enough to withstand the effects of such an ingestion and to continue to operate, possibly at a lower speed. 
     However, ingestion of a foreign body can sometimes cause one or more fan blades to break, this being liable to have the effect of causing significant imbalance requiring the engine to stop, and capable of subjecting the structure of the turbojet engine to considerable cyclic forces, at least while the engine is slowing down to its windmilling or autorotation speed, that is to say to its speed of free rotation as a result of its movement through the atmosphere. 
     In order to avoid imbalance forces being transferred to the structure of the turbojet engine, it has been proposed for the low pressure compressor shaft that bears the fan to be decoupled from the stator. 
     This shaft is generally radially guided by two bearings supported by the stator. A first bearing, sometimes known as the upstream bearing or thrust bearing, comprises an antifriction ball bearing positioned at the upstream end of the shaft and is supported by an annular support connected to an intermediate casing of the turbojet engine, while a second bearing, commonly known as the downstream bearing, comprises an antifriction roller bearing positioned downstream of the first bearing. 
     Devices for decoupling the upstream bearing and comprising “rupture” screws for connecting the support of this bearing to the intermediate casing have been proposed, as have other devices comprising a bearing support that has a local striction engineered to allow the support to deform when the applied load exceeds a predetermined load, and other devices proposed have included a corrugated bearing support intended to buckle under a certain load. Devices comprising “rupture” pins in the region of the upstream bearing have also been proposed. 
     However, when the upstream bearing has been decoupled from the stator it no longer centres the low-pressure compressor shaft and even though the loads resulting from the imbalance are not of such a great magnitude once the autorotation speed has been reached, these loads are then essentially borne by the downstream bearing, with a risk therefore of breaking the latter. 
     To avoid this disadvantage, devices have been proposed in which the downstream bearing is, for example, mounted on a support that is articulated to allow the shaft to become off-centred without destroying this downstream bearing, but these devices lead to additional mass penalizing turbojet engine performance. 
     Furthermore, with the known devices and should the low pressure compressor shaft break, the upstream part thereof is no longer axially retained by the upstream bearing, thus potentially endangering the turbojet engine. 
     BRIEF SUMMARY OF THE INVENTION 
     It is a particular object of the invention to provide a simple, economical and effective solution to these problems while at the same time avoiding the aforementioned disadvantages. 
     A particular object of the invention is to allow the low-pressure compressor shaft to be recentred once one of its guide bearings has been uncoupled, as soon as the turbojet engine has reached its autorotation speed so as to avoid the other guide bearing becoming destroyed and preserve the structure of the turbojet engine. 
     To this end, the invention proposes a support for a bearing that guides a shaft in a turbomachine, such as a turbojet engine, characterized in that it comprises an annular part made of a shape memory material which retains an initial shape when the load applied to it remains below a threshold value and which deforms, absorbing energy when the applied load becomes at least equal to the threshold value, this annular part being capable of reverting at least approximately to its initial shape when the applied load drops back below the threshold value. 
     The shape memory annular part, by deforming, decouples the bearing it supports from the casing when the shaft is transmitting to the bearing loads that exceed a predetermined threshold value, so as to absorb the mechanical energy generated by these loads and prevent these loads from being transmitted to the casing and thence to the entire turbojet engine structure. 
     These loads are typically imbalance loads resulting from the ingestion of a foreign body, such as a bird or a block of ice, which has damaged the turbomachine fan. 
     Once the engine has stopped, and as soon as the turbojet engine has reached its autorotation speed at which the loads transmitted by the shaft are of lower magnitude, the annular part reverts more or less to its initial shape and thus allows the bearing once more to centre the shaft. 
     Further, the bearings are capable of ensuring axial retention of the upstream end of the shaft, even during uncoupling. 
     According to another feature of the invention, the bearing support comprises controlled means of heating the shape memory annular part to cause this annular part to revert to its initial shape when the applied load is below the threshold value. 
     The heating means are particularly suited to the scenario in which the shape memory material is of the “single-acting” type and is initially in the martensite state capable of deforming substantially under the effect of load, thereby absorbing mechanical energy in order to permit the uncoupling of the bearing support, the heating means thereafter allowing the shape memory material to be made on command to switch to the austenitic phase in order to cause it to revert to a learned shape more or less identical to its initial shape so that the bearing support reverts to its initial shape and is once more able to centre the shaft. 
     Advantageously, the heating means comprise means of conveying hot air onto the shape memory annular part. 
     The hot air may, for example, be tapped off the turbojet engine high pressure compressor. 
     The bearing support according to the invention advantageously comprises means of guiding hot air onto the shape memory annular part, these means for example comprising a sheet metal deflector mounted around the shape memory annular part. 
     In the case of a material of the “single-acting” type, the sheet metal deflector allows better guidance of the hot air intended to trigger the transition of the shape memory material into the austenitic phase. 
     As an alternative, the shape memory material is a material of the superelastic type which is in the austenitic phase when the applied load is below the threshold value and which changes into the martensitic phase with elastic deformation when the applied load becomes at least equal to the threshold value. 
     This means that the substantial ability of superelastic shape memory materials to deform can be put to good use in avoiding the need to resort to means of heating the annular part of the bearing support. 
     Which of the two alternative forms is chosen may be governed by the respective abilities of the various shape memory materials to absorb mechanical energy. 
     According to another feature of the invention, the shape memory annular part connects the upstream and downstream ends of the support together, and preferably comprises thin longitudinal blades uniformly distributed about the axis of the turbomachine. 
     Thin blades such as this are relatively commonplace in the shape memory component marketplace and have a good ability to return to a pre-learned shape. 
     According to another feature of the invention, the ends of the thin blades are brazed or welded to the end parts of the support. 
     The bearing support according to the invention advantageously comprises an annular sealing envelope positioned radially on the inside of the shape memory annular part. 
     This annular sealing envelope is particularly useful in instances where the shape memory annular part comprises thin longitudinal blades of the type mentioned hereinabove, in order to seal the space between the bearings and thus prevent oil from leaking from that space. 
     The envelope also constitutes a means of guiding hot air onto the shape memory annular part, this being particularly beneficial in instances where the shape memory material is of the “single-acting” type. 
     The invention also relates to a turbomachine comprising a shaft guided in upstream and downstream bearings which are supported by supports fixed to a casing, characterized in that at least one of the supports is a bearing support of the type described hereinabove. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be better understood, and further details, advantages and features thereof will become more clearly apparent, from reading the following description given by way of nonlimiting example with reference to the attached drawings in which: 
         FIG. 1  is a partial schematic view in axial section of a turbomachine of a known type; 
         FIG. 2  is a partial schematic view in axial section on a larger scale of a turbomachine according to the invention; 
         FIG. 3  is a perspective partial schematic view of the turbomachine of  FIG. 2 ; 
         FIG. 4  is a thermomechanical diagram illustrating how the invention works. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an aircraft bypass turbojet engine  10  comprising a fan  12  formed of a disk  14  bearing blades  16  and mounted upstream of a low-pressure compressor  18 , the disk  14  being connected by an annular flange  20  to an upstream end flange  22  of a shaft  24  of the low-pressure compressor intended to drive the rotation of the fan  12  about the axis  26  of the turbojet engine, in a way that is well known. 
     The shaft  24  is guided radially and retained axially by two bearings  28  and  30  supported by an intermediate casing  32 . 
     A first bearing  28  positioned near the upstream end of the shaft  24 , commonly termed “thrust bearing” or “upstream bearing”, comprises an antifriction ball bearing formed essentially of a radially internal annular cage  34  and of a radially external annular cage  36  delimiting a substantially toric cavity in which balls  38  run freely. 
     The radially external annular cage  36  is connected to the upstream end of a substantially annular or slightly frustoconical envelope that forms a bearing support  40 , the downstream end of which has an annular flange  42  fixed by screws  44  to a component  46  connected to the intermediate casing  32 . 
     A second bearing  30  is positioned downstream of the first bearing  28  and comprises an antifriction roller bearing formed essentially of a radially internal annular ring  48  bearing rollers  50  on which a radially external annular ring  52  rests. 
     The radially external annular ring  52  has a flange  54  for attachment to the intermediate casing  32 . 
     In operation, the bearings  28  and  30  radially centre and axially retain the low-pressure compressor shaft  24 , the bearing  28  in particular providing axial retention of the upstream end of the shaft should this shaft break. 
     In the event of an incident that damages the fan and introduces an imbalance, such as the loss of a fan blade following ingestion of a foreign body into the turbojet engine for example, the bearings  28  and  30  are subjected to considerable cyclic forces and transmit a significant proportion of these forces to the intermediate casing  32 . 
     Although the engine is generally stopped as soon as such an incident occurs, it nonetheless takes the turbojet engine a certain length of time to reach its autorotation speed, that is to say the speed at which it rotates freely as a result of its movement through the atmosphere. 
     During this space of time, the upstream bearing  28 , which is closest to the fan  12  and thus experiences the majority of the imbalance forces, runs the risk of being damaged and of damaging the intermediate casing  32  to which it is connected, and thereby the entire structure of the turbojet engine. 
     It is therefore beneficial, in order to protect the upstream bearing  28  and to spare the structure of the turbojet engine from the imbalance forces, to allow uncoupling between this upstream bearing  28  and the intermediate casing  32 , that is to say to minimize the forces transmitted to the intermediate casing by the shaft  24  of the low pressure rotor. 
     When the turbojet engine reaches the autorotation speed, it is, however, desirable for means of centring the shaft to be available once again, so as to prevent the downstream bearing  30  from withstanding all the imbalance loads alone as these loads, although of lesser magnitude under these conditions of free rotation, are nonetheless liable, in the long term, to damage this downstream bearing  30 . 
     In order to meet this need, the invention proposes to replace the upstream bearing support  40  of the turbomachine  10  with a bearing support capable of deforming in order to allow the upstream bearing  28  to become uncoupled when the loads applied to the upstream bearing are too great, and capable of reverting to a shape close to its initial shape when the applied loads have reduced sufficiently, so that the upstream bearing  28  can once again serve to centre and to axially retain the shaft  24 . 
     As shown by  FIGS. 2 and 3 , this new upstream bearing support denoted by the reference  56  comprises an annular or slightly frustoconical part formed of thin blades  58 , for example ten of these, uniformly distributed about the axis of the turbojet engine and connecting the upstream end  36  of the bearing support  56 , which bears the radially external cage of the upstream bearing  28 , to its downstream end  60  that bears the flange  42  for attachment to the component  46  connected to the intermediate casing  32 , these thin blades being, for example, welded or brazed to the ends  36  and  60  of the bearing support  56 . 
     In a first embodiment, the thin blades  58  are made of a shape memory material of the “single-acting” type, such as an alloy of nickel and of titanium, commonly known as Nitinol. 
     The shape memory material, which is initially in the martensitic phase, gives the thin blades  58  a good ability to deform and the possibility, when heated, of changing into the austenitic phase, reverting to a learned shape. 
     In order for the bearing support  56  to revert to its initial shape after having been deformed, the thin blades are prepared beforehand so that, in the austenitic phase, they adopt a shape similar to their initial shape prior to deformation, as will be explained in greater detail in a later paragraph dealing with the operation of the invention and more specifically with the use of “one-way” memory effect of the shape memory material, with reference to  FIG. 4 . 
     To heat the thin blades  58 , the turbojet engine  10  comprises means for directing onto the thin blades air at a temperature higher than the austenitic phase transition temperature of the shape memory material of which the thin blades are made, it being possible for this hot air to have been, for example, tapped from a high-pressure compressor located downstream of the low-pressure compressor  18 . 
     These means essentially comprise a deflector  62  formed of a substantially cylindrical sheet metal envelope surrounding the thin blades  58  and which at its downstream end has an annular flange  64  for attachment to the flange  42  of the bearing support  56 . 
     Ducts  66  for guiding the hot air are formed in a downstream frustoconical part  68  of the deflector  62  and are positioned facing openings formed in the flange  42  to allow the hot air to flow in the upstream direction. 
     In order to seal the space between the bearings, that is to say the volume between the upstream  28  and downstream  30  bearings and the bearing support  56 , given the bladed structure of the latter, a cylindrical envelope made of a flexible material such as an elastomer joins the upstream  36  and downstream  60  ends of the bearing support  56  together in such a way as to form an apron  70 . 
     This apron  70  is positioned radially on the inside of the bearing support and, as appropriate, keeps the hot air near the thin blades  58 . 
       FIG. 4  is a diagram representing the deformation ε of the thin blades of shape memory material as a function of the mechanical stress σ applied to them and of temperature T, during the operation of the turbojet engine described previously. 
     During the course of the preparation of the thin blades  58 , they will have been given a shape corresponding to the normal geometry of the bearing support  56 , when the thin blades were in the austenitic phase at a high temperature, this state being represented by point A 0  in  FIG. 4 , then the thin blades will have been cooled without applying any stress to them in order to cause them to change without deformation into the martensitic phase, in a transition  72  culminating in the state M 0  of  FIG. 4 . 
     During normal operation, the thin blades  58  are more or less in this state M 0 , so that the upstream bearing  28  performs its functions of centring and of axially retaining the low pressure compressor shaft  24 . As long as the mechanical stress σ applied to the thin blades is below a threshold value σ 0 , their rigidity remains relatively high, their deformation ε not exceeding a relatively low value ε 0 , thus allowing them precisely to guide the upstream part of the shaft  24 . 
     When the fan suffers damage, for example following the ingestion of a foreign body and, in particular, in the event of the loss of a blade, considerable imbalance forces are transmitted to the bearings that guide the shaft  24 , and more especially to the upstream bearing  28  which is closest to the fan. 
     The mechanical stress applied to the thin blades  58  then exceeds the threshold value σ 0  which means that the rigidity of the thin blades drops sharply, as indicated by the curve  74  in  FIG. 4 , and at the same time their ability to absorb mechanical energy increases. 
     The forces transmitted to the upstream bearing  28  are then dissipated by the deformation of the thin blades  58 , and are therefore not transmitted to the intermediate casing  32  to which this bearing  28  is connected. 
     Under the effect of the loads applied to them, the thin blades thus reach an uncoupled state represented by the Point M d . 
     At the same time, the engine is stopped by an automatic control system or on the command of the aircraft pilot, and its speed therefore gradually decreases down to the autorotation speed caused by the movement of the aircraft through the atmosphere, which means that the imbalance load is also decreased. 
     This change in stress is represented by the curve  76  in  FIG. 4 , which corresponds to the special case in which the applied load returns to substantially zero. 
     As soon as the loads have decreased appreciably and no longer present any danger to the structure of the aircraft, the automatic control system or the pilot commands that hot air tapped from the turbojet engine high pressure compressor be sent to the thin blades. 
     Subjected to a temperature higher than their transition temperature T t , the thin blades change into the austenitic phase along the curve  78  and revert to their initial shape through a memory effect, thus recentring the low pressure compressor shaft  24 . 
     The bearing support  56  proposed by the invention therefore allows uncoupling between the upstream bearing  28  and the intermediate casing  32  when the loads transmitted by the low pressure compressor shaft  24  are too great, while at the same time offering the possibility of recentring this shaft as soon as the loads have dropped back to an acceptable level. 
     The shape memory material of the thin blades is chosen such that they remain in the elastic domain of the martensitic phase, in which the rigidity of the thin blades remains substantially constant irrespective of the applied stress, under normal operating conditions, and so that they depart from this elastic domain as soon as the applied loads exceed a threshold value σ 0  liable to endanger the turbojet engine. The material is also chosen such that its rigidity, in the aforementioned elastic domain, allows it to meet the overall flexibility requirement for the bearing support  56 . 
     The device comprising the bearing support  56 , the deflector  62  and the apron  70  also has the advantage of not significantly increasing the mass of the turbojet engine and of comprising a limited number of parts. 
     In a second embodiment, instead of using a one-way shape memory effect, the bearing support  56  according to the invention may work by virtue of a superelastic effect of the shape memory material of which the thin blades  58  are made. 
     In this embodiment, the shape memory material is chosen such that it is in the austenitic phase at turbojet engine normal operating temperatures. 
     The imbalance forces applied to the bearing support  56  cause the shape memory material to pass under stress into the martensitic phase in which this material has an enormous capacity for elastic deformation. 
     As soon as the applied loads disappear or reduce sufficiently, the material returns to the elastic domain of the austenite, reverting to its initial shape. 
     This embodiment of the invention has the advantage of not requiring any means of heating the shape memory material, and therefore proves to be simpler to implement than the previously described first embodiment. 
     However, which variant is chosen will essentially depend on the cost and availability of the various types of shape memory materials.