Abstract:
A device for uncoupling a bearing carrier in a turbomachine, the bearing carrier including an upstream part and a downstream part including a plurality of upstream orifices respectively facing a plurality of downstream orifices. The uncoupling device includes rupture screws each passing through an upstream orifice and a downstream orifice, and at least a mechanism for double centering of a rupture screw with respect to the upstream orifice and to the downstream orifice respectively. The mechanism for double centering is independent of the upstream and downstream parts of the bearing carrier and of the rupture screw, and is configured to collaborate with the upstream part of the rupture screw such that when the rupture screw breaks, the upstream part carries with it the mechanism for double centering. A turbomachine can include such an uncoupling device.

Description:
BACKGROUND OF INVENTION 
     Field of Invention 
     The present invention relates to a device for uncoupling a carrier for a bearing of a rotary shaft in a turbomachine. A carrier such as this is able to break its connection with the turbomachine stator upon the onset of imbalance in order to avoid damage to the turbomachine. 
     Description of the Related Art 
     A turbomachine comprises, from upstream to downstream in the direction in which the gases flow, a compressor, a combustion chamber and a turbine. The purpose of the compressor is to raise the pressure of the air supplied to the combustion chamber. The purpose of the turbine is to tap off some of the pressure energy of the hot gases leaving the combustion chamber and convert it into mechanical energy to drive the rotation of the compressor. 
     For that purpose, the compressor and the turbine are made of a first set of fixed components that make up the stator and of a second set of components capable of being rotated relative to the stator and which make up the rotor. 
     The compressor rotor and the turbine rotor form an assembly which is securely connected by a rotary shaft. Rotation of the rotor with respect to the stator is rendered possible by means of bearings, a bearing being a mechanical component that supports and guides a rotor, particularly the shaft of this rotor. This bearing comprises a first part fixed to the rotor shaft and a second part fixed to the stator via a bearing carrier. A rolling bearing assembly is positioned between the two parts of the bearing thus allowing one part of the bearing to rotate relative to the other. The rolling bearing assembly may, for example, be of the ball bearing, cylindrical roller bearing, or taper roller bearing type. 
     A turbomachine may also be of the “twin-spool” type, which means that it has two rotors arranged coaxially, a bearing allowing relative rotation of one of these two rotors with respect to the other. 
     A turbomachine may also comprise a fan, that constitutes the first stage of the compressor. The fan has very large blades known as fan blades, which increase the mass and inertia of the rotor. 
     If a fan blade breaks, imbalance appears on the shaft supporting the fan. Imbalance is a phenomenon that affects the balance of the rotor, the center of gravity of which is no longer precisely on the axis of rotation as it should be. Cyclic loadings and substantial vibrations are therefore imparted to the turbomachine stator, via the bearing carrier, with a great risk of damage that could lead to self-destruction. In order to prevent these undesirable phenomena from being transmitted to the stator, it is necessary to uncouple the bearing carrier, that is to say to interrupt the mechanical transmission of rotation, notably by disconnecting the two parts that form the bearing carrier. 
     Document FR 2877046 describes a solution that consists in using bolted connections that can rupture in order to attach an upstream part and a downstream part that form a bearing carrier. The rupture screw of each bolted connection passes through an upstream hole of an upstream part and a downstream hole of a downstream part of a bearing carrier, the downstream part of the bearing carrier forming an integral part of the casing. The screw head of the rupture screw is adjacent to the hole of the upstream part and is in contact with this upstream part on a plane perpendicular to the axis of the hole. The portion of the rupture screw that passes through the hole is in contact with the inside of the hole via a centering portion and has a portion of reduced cross section liable to rupture when a predetermined tensile force is exceeded, thus uncoupling the two parts that make up the bearing carrier. 
     It will also be noted that with this type of bolted connection that can rupture, the longitudinal positioning of the low-pressure compressor shaft can be achieved via a thrust bearing, in the form for example of a ball bearing, between the drive shaft and the upstream part of the bearing carrier. 
     However, with such a rupture screw, when imbalance appears, the upstream part and the downstream part move relative to one another in a circular relative motion which has the effect of subjecting the rupture screw to shear loadings, because of the tangential contact around this rupture screw, and these may lead to uncontrolled rupturing of the rupture screws. Now, these rupture screws are designed for tensile loadings, and this has a deleterious effect on the uncoupling of the bearing carrier. 
     In order to improve control over the uncoupling function, document EP 2071 138 describes a solution which involves replacing the centering portion described in document FR 2877046 with a means referred to as a “dual-centering” means. This means may in practice consist of the collaboration of a groove and of a rib of complementing shapes, in contact with one another via their two flanks, thus offering two parallel contact surfaces. Such centering, by means of these two parallel surfaces, makes it possible to reduce, if not to eliminate entirely, the ovalizing deformation of the bearing carrier by maintaining permanent contact between the flanks of the groove and of the rib. In order to reduce further, if not to eliminate, the shear forces applied to the rupture screws, document EP 2071 138 also proposes eliminating any contact between each upstream hole and the rupture screw passing through it, allowing said rupture screws to be subjected only to tensile loadings, thus guaranteeing better control over the uncoupling of the bearing carrier. 
     However, the high axial thrust caused by the aerodynamic forces internal to the turbomachine dictates a need for a large-sized thrust bearing. Significant bulkiness generated by this thrust bearing means that said thrust bearing has to be installed on the downstream side of the bearing carrier, while a roller bearing is installed on the upstream side of said bearing carrier. 
     As a result, if blades are lost, the low-pressure compressor shaft is still held longitudinally by the thrust bearing of the downstream part of the bearing carrier. As the upstream bearing secured to the shaft is furthermore a roller bearing, no forward movement drives the upstream part of the bearing carrier. This then results in a risk that the upstream and downstream parts of the bearing carrier might not disengage, and the consequence of this would be that the imbalance generated by the loss of blades would be transmitted in full to the structures. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to remedy these disadvantages and the invention therefore proposes a device for uncoupling a bearing carrier in a turbomachine, this uncoupling device being of the rupture screws type and making it possible, in the event of a loss of blades, not only to reduce the shear forces applied to the rupture screws, if not eliminate these forces, but also to guarantee that the bearing carrier uncouples. 
     To this end, according to the invention, the device for uncoupling a bearing carrier in a turbomachine, this bearing carrier comprising an upstream part and a downstream part comprising a plurality of upstream holes respectively facing a plurality of downstream holes, this uncoupling device comprising rupture screws each passing through an upstream hole and a downstream hole, and at least one dual-centering means for centering a rupture screw with respect to said upstream hole and to said downstream hole, respectively, is notable in that the dual-centering means:
         is independent of the upstream and downstream parts of the bearing carrier and of the rupture screw, and   is able to collaborate with the upstream part of the rupture screw so that when the rupture screw ruptures, the upstream part takes the dual-centering means with it.       

     Thus, by virtue of the invention, when the rupture screw ruptures, the distancing of the upstream part from the downstream part of the rupture screw is accompanied by the removal and distancing of the dual-centering means. The upstream part of the bearing carrier is therefore no longer centered on the downstream part of the bearing carrier, which means that these two parts of the bearing carrier are rapidly separated from one another. The uncoupling of the bearing carrier therefore occurs effectively and the forces generated by any imbalance are not transmitted to the structures. 
     In addition, by maintaining the use of a dual-centering means, it is possible to ensure that, when the rupture screw ruptures, the uncoupling between the upstream part and the downstream part of the bearing carrier occurs only as a result of tension and without a shear component, for even more effective uncoupling. 
     According to one particular embodiment, the dual-centering means is in the form of a tubular component the dimensions of the internal section of which are tailored to the dimensions of the upstream part of the rupture screw. 
     For preference, with the rupture screw comprising an upstream head and a downstream shank, the dual-centering means has a shoulder which is transverse with respect to the head of said rupture screw. Thus, the screw head—and consequently the upstream part of the screw—is surrounded and therefore transversely immobilized by the dual-centering means. 
     For preference also, the dual-centering means has an intermediate portion intended to form a clearance between an upstream hole and the rupture screw that passes through it, so as to prevent any contact between said upstream hole and said rupture screw. Thus, a movement of the upstream part of the bearing carrier does not also take a rupture screw with it as a result of a tangential contact around the rupture screw, and this means that the rupture screw can be made even less subject to shear forces and thereby offers better control over the uncoupling of the bearing carrier. 
     In order for the dual-centering means to act as an intermediary between the bearing carrier and the rupture screw in order to perform the function of centering the two holes, this dual-centering means may have a cylindrical portion able, on the one hand, to pass through an upstream hole and a downstream hole and, on the other hand, to have the rupture screw at least partially passing through it. 
     In one particularly advantageous embodiment, the dual-centering means has a longitudinal securing portion for securing said dual-centering means to the upstream part of the rupture screw. Thus, the dual-centering means is secured longitudinally, namely in the direction in which the gases flow which also corresponds to the direction of relative movement of the two separated parts of the rupture screw (and to the direction of the axis of the upstream and downstream holes of the bearing carrier), to the upstream part of the rupture screw so that this dual-centering means is taken with the upstream part of the rupture screw when said upstream part moves away. 
     According to one particular embodiment of the longitudinal securing portion, this portion is in the form of a flap situated at the upstream end of the dual-centering means. 
     In order to perform the rupture-screw function, the rupture screw may have at least one thinner portion so as to form, upon the rupturing of said rupture screw, on the one hand, an upstream part and, on the other hand, a downstream part of said rupture screw. 
     In that case, given that the rupture screw comprises an upstream head and a downstream shank, the thinner portion is preferably located in the region of the shank. 
     The present invention also relates to a turbomachine comprising a device for uncoupling a bearing carrier according to one of the embodiments described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with the aid of the attached drawing in which: 
         FIG. 1  is a schematic view in axial section of a bearing carrier equipped with an uncoupling device according to a first embodiment of the invention, 
         FIG. 2  is a schematic view in axial section of the uncoupling device of the bearing carrier of  FIG. 1 , before the rupture screw ruptures, 
         FIG. 3  is a schematic view in axial section of the uncoupling device of  FIG. 2 , after the rupture screw has ruptured, and 
         FIG. 4  is a schematic view in axial section of an uncoupling device according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To make the figures easier to understand, identical numerical references will be used to denote technical elements which are similar. 
       FIG. 1  depicts a turbomachine  1  comprising a drive shaft  4 , for example a low-pressure compressor drive shaft, driving a rotor (not depicted) in rotation about the axis of the turbomachine. This shaft is supported here by two bearings, respectively an upstream bearing  2  and a downstream bearing  3  which are connected to one another at a bearing carrier  7 , of essentially frustoconical shape. The bearing  2  is a roller bearing and the bearing  3  is a thrust bearing, of the ball bearing type. 
     The bearings  2  and  3  are connected respectively to a first component  5  and to a second component  11 . The two components  5  and  11  are connected respectively to two ends of a component  6  of the fixed structure of the turbomachine. 
     The components  6  and  11  are joined together by bolts of the non-rupturing type. The connection between the components  5  and  6  forms the bearing carrier  7 , where a plurality of longitudinal rupture screws is located, these together forming a part of the uncoupling device according to the invention. One of these rupture screws carries the reference  8  in  FIG. 1 . 
     The uncoupling device according to the invention is depicted in greater detail in  FIG. 2 . In this figure, the bearing carrier  7  is formed of two ends  5 A and  6 A—referred to hereinafter respectively as the upstream and downstream parts of the bearing carrier—and of the two components  5  and  6 , which are themselves connected to the two bearings  2  and  3  respectively. 
     Arranged in the vicinity of these upstream  5 A and downstream  6 A parts of the bearing carrier are the respectively upstream  5 B and downstream  6 B holes of equal sizes through which a bolted connection of the rupture screw type of the uncoupling device according to the invention is intended to pass. This bolted connection is made up of a rupture screw  8  (described in ample detail in document FR 2 877 046) which passes through the holes  5 B and  6 B, and of a nut  9  intended to collaborate with the screw  8  in order to hold the upstream  5 A and downstream  6 A parts of the bearing carrier  7  firmly together. 
     The rupture screw  8  has a screw head  8 A positioned facing the upstream part  5 A of the bearing carrier, the nut  9  being arranged at the opposite end to this head  8 A, namely facing the downstream part  6 A of the bearing carrier. The screw  8  also comprises a screw shank  8 B, of which a first portion, situated at the opposite end to the head  8 A, is intended to pass through the nut  9 , and a second portion  8 C, situated between the head  8 A and said first part, has a thinner cross section. The dimension of the reduced cross section of this portion  8 C is determined in such a way that said portion  8 C is able to rupture when a predetermined tensile force is exceeded, for example as a result of imbalance occurring upon blade loss, so that the uncoupling of the upstream  5 A and downstream  6 A parts of the bearing carrier occurs at said thinner portion  8 C. 
     In order to center the rupture screw  8  and the upstream  5 B and downstream  6 B holes of the bearing carrier, the uncoupling device according to the invention also comprises a dual-centering means  10  for centering the rupture screw  8  with respect to the upstream hole  5 B and the downstream hole  6 B respectively. This means  10  consists of a component which is independent, firstly, of the rupture screw  8  and, secondly, of the bearing carrier  7  (particularly of the upstream  5 A and downstream  6 A parts of the bearing carrier  7 ). 
     This dual-centering means  10  takes the form of a tubular component of which:
         the internal cross section, which is variable, is tailored to the dimensions of the upstream part of the rupture screw  8 , particularly of the screw head  8 A and of the shank  8 B; and   the external cross section, which is likewise variable, is tailored to the dimensions of the upstream  5 B and downstream  6 B holes.       

     More specifically, this tubular component  10  first of all, upstream, has a first tubular portion  10 A the internal cross section of which is tailored to the screw head  8 A. The component  10  also has a portion  10 B forming a transverse shoulder of said screw head. 
     The component  10  also has, downstream, a second tubular portion  10 C of a dimension smaller than that of the portion  10 A and of which the internal and external cross sections are configured such that this portion  10 C on the one hand passes through the upstream  5 B and downstream  6 B holes and on the other hand has at least part of the rupture screw  8 , and at the very least the shank  8 B, passing through it. 
     Thus, through the intermediary of this portion  10 C, the component  10  performs the dual-centering of the upstream  5 A and downstream  6 A parts of the bearing carrier  7  merely by inserting said portion  10 C in the upstream holes  5 B and downstream  6 B brought into register with one another beforehand. 
     There is thus no longer any need to resort to a dual centering of the upstream and downstream parts of the bearing carrier using a system of grooves and ribs of complementing shapes made directly on said upstream and downstream parts, such a system in fact having the disadvantage of being liable to impede the relative transverse movement of the components  5  and  6  with respect to one another once the rupture screw has broken. 
     On the contrary, by using an independent dual-centering means such as the tubular component  10 , the dual-centering function of centering the components  5  and  6  relative to one another is performed without the structures of these components being altered, as such alteration could have impaired their uncoupling effectiveness. In addition, this independent component  10  can be taken with the screw head  8 A when the screw  8  breaks so that, when said rupture screw  8  ruptures, the dual-centering function of centering the components  5  and  6  is deactivated and said components  5  and  6  are free to move with respect to one another, both longitudinally and transversely. 
     What is more, again through the use of an independent dual-centering means, the dual-centering function is no longer performed by the rupture screws, because if it were, that would have the disadvantage of subjecting the rupture screws to shear forces in addition to the usual tensile forces. 
     The dual-centering means  10  also has an intermediate portion  10 B situated between the first portion  10 A and the second portion  10 C. This intermediate portion  10 B has an external section substantially equal to that of the portion  10 A and an internal cross section substantially equal to that of the portion  10 C. This portion  10 B, the external cross section of which is thus greater than the dimensions of the upstream holes  5 B and downstream  6 B, butts against the upstream part  5 A of the bearing carrier  7  when the bolted connection  8 - 9  is clamping the bearing carrier  7 . 
     The longitudinal thickness of this portion  10 B determines a clearance between the upstream hole  5 B and the rupture screw  8  passing through it (particularly the screw head  8 A), this clearance making it possible to avoid any contact between said upstream hole  5 B and said screw head  8 A. For preference, this longitudinal thickness is determined so that the clearance thus formed prevents any shear forces being applied to the rupture screw  8  in the event of imbalance. 
     According to another embodiment, the dual-centering means may be produced as a plurality of components joined together, provided that these components are independent of the rupture screw and of the bearing carrier  7  and provided that these components are accompanied by the upstream part of the rupture screw  8  when the latter has broken. 
     The rupture of the rupture screw  8  is illustrated in  FIG. 3 . When imbalance appears, the thinner portion  8 C of the rupture screw  8  is broken under the effect of the tensile forces generated by the imbalance, this portion  8 C thus separating into two sub-portions, these respectively being an upstream sub-portion  8 C and a downstream sub-portion  8 C″. Still under the effect of these tensile forces and because the rupture screw  8  is ruptured, the upstream part of said rupture screw—in this instance formed of the screw head  8 A and of the upstream sub-portion  8 C—moves away from bearing carrier  7  in the direction of the arrow F, which occurs along the longitudinal axis X-X′ of the rupture-screw bolted connection  8 - 9 . This longitudinal separation movement of the upstream part of the screw  8  causes longitudinal separation of the dual-centering component  10 , which from then on no longer passes through the holes  5 B and  6 B and frees the upstream  5 A and downstream  5 B parts of the bearing carrier to undergo relative motion both longitudinally (along the axis X-X′) and transversely (perpendicular to the axis X-X′). 
     According to a second embodiment of the dual-centering means  10 , which is illustrated in  FIG. 4 , this means is extended, at the end of the upstream portion  10 A, by an additional portion  10 D to secure the rupture screw  8  longitudinally (in the direction of the axis X-X′) to the dual-centering component  10  when said screw  8  is passing through said component  10  and the screw head  8 A is in abutment against the intermediate portion  10 B. 
     This additional portion  10 D is in the form of a flexible flap able to adopt two successive positions:
         a first position in which the flap  10 D forms an extension of the portion  10 A in the direction of the longitudinal axis X-X′, so that the screw  8  can be inserted into the component  10 ; and   a second position in which the flap  10 D is folded down at least partially in the transverse direction (perpendicular to the axis X-X′) so that said flap  10 D butts against the part of the screw head  8 A situated on the opposite side to that part of said screw head that butts against the intermediate portion  10 B.       

     In that way, when the rupture screw  8  is inserted into the dual-centering means  10 , the intermediate portion  10 B and the additional longitudinal securing portion  10 D butt against the screw head  8 A and thus allow said screw head  8 A and, as a result, the rupture screw  8 , to be immobilized longitudinally. This then ensures that, when the rupture screw  8  has broken, the separating movement of the upstream part of the screw  8  is accompanied, at the same time and at the same speed, by the separation of the dual-centering component  10 , and this has the effect of uncoupling the upstream  5 A and downstream  6 A parts of the bearing carrier  7  more quickly. 
     As depicted in  FIG. 4 , a recess  8 D may be made in the portion of the screw head  8 A onto which the flap  10 D is intended to butt, so that the inclination of said flap  10 D is between 0° and 90° with respect to the longitudinal axis X-X′. In another embodiment, it is possible for no recess to be made, such that the flap  10 D will need to be bent to make an angle of 90° with respect to the axis X-X′. In any event, a person skilled in the art will be able to determine what inclination he wishes to confer upon the flaps bearing in mind, if appropriate, on the one hand, the desirable longitudinal securing force between the screw  8  and the component  10  and, on the other hand, the ease with which the portion  10 D can be bent over.