Abstract:
A frangible coupling for interconnecting parts comprising a first ring and a second ring coaxially arranged relative to each other and axially joined via an annular array of fuse ligaments equidistantly spaced apart from each other. The ligaments are configured to fail when an abnormal radial load of a predetermined value causes the first and second ring to move out of their coaxial relationship. When all of the fuse ligaments are severed, the communication between the first and second rings is severed. This allows the first ring to move independently of the second ring, preventing the out of balance load on the first ring being communicated to the second ring.

Description:
BACKGROUND 
   The invention relates to a frangible coupling. In particular it refers to a frangible coupling for turbo machinery. 
   In a conventional gas turbine engine, the fan is used for pressurising ambient air which then passes downstream to a compressor to be further compressed. The air is then mixed with fuel in a combustor, ignited and burned to expand the gas, further increasing the gas pressure before exhausting via a turbine from which energy is extracted. The engine may have a high pressure turbine which powers the compressor, and a low pressure turbine which powers the fan. 
   Other engines utilise power off takes, perhaps directly from one of the turbine stages, to drive independent fans to generate propulsive thrust remote from the propulsion unit. 
   The fan typically comprises an annular array of large fan blade rotors that extend radially outward from a supporting disc. The fan is fixedly joined to a shaft and is rotatable about the axis of the shaft, which is rotatably supported by a number of bearings in communication with a static fan support structure. The concentric alignment of the fan within a surrounding fan casing is maintained by the bearings. The bearings also act as a means to transmit aerodynamic, centrifugal and vibratory loads into the fan support structure. During normal operation the fan is dynamically balanced. 
   In the rare event of the loss of a section of a blade, perhaps because of foreign object damage or failure of the rotor blade material, there may be a substantial rotary imbalance introduced into the fan system. This will be transmitted to the fan support structure. If the engine is fitted to an aircraft, this may lead to undesirable vibrations being transmitted to the airframe body. In extreme cases the aircraft may become difficult to handle or suffer severe structural damage. 
   The engine may be turned off to prevent unnecessary damage to itself and the airframe. However, whilst in flight, there may be no means to stop the aerodynamic windmilling of the damaged engine, which may be enough to cause a substantial imbalanced load and further damage. 
   Likewise, if the fan is driven remotely from the engine it may be desirable to run the damaged fan to generate propulsion. A common requirement is to be able to run the fan up to a predetermined imbalance load, thereby coping with a proportion of blade loss. The fan should only be taken out of use when the imbalance load reaches a certain unacceptable level. It may not be possible for a pilot to make this judgement, requiring some safety feature of the fan to sever the connection between the imbalanced load and the fan structure. 
   In order to accommodate the possibility of such abnormal radial loads the supporting components for the fan may be strengthened. This may have the undesirable effect of increasing the size, weight and expense of the fan structure. Means for the controlled buckling of various parts of engine structure have also been utilised. 
   Another solution is the introduction of a coupling placed between the bearing support structure and the fan support structure that de-couples when the imbalance reaches a predetermined level. Such a device is frequently referred to as a structural fuse or a frangible coupling. When decoupled the connection between the bearing support and support structure is severed, leaving the fan supported by its shaft and at one end by a bearing. 
   Conventional structural fuses are designed to de-couple above relatively low abnormal radial loads. However, an increasingly common requirement is for the fan to carry on rotating and generating useful thrust with a degree of out of balance loading. 
   The fuse has two conflicting requirements. It must withstand any fatigue or normal operational loads but fail reliably under the increased fan blade off load. This presents a load range within which the fuse must be designed. The extra requirement that the fuse not fail under partial fan blade off but fail under full blade off makes the design window prohibitively narrow. 
   Fuse designs exist that utilise shear bolts and spigots that fasten the supporting components of the fan together. However, given the tolerances inherent in the design and materials used, it is not possible to define accurately at what load the fuse will severe the connection. Hence the fuse may sever the connection when the imbalance is below the required lower level, resulting in premature decoupling, or above the higher level, resulting in damage to the fan structure, engine or aircraft. 
   According to the present invention there is provided a frangible coupling for the purpose of supporting a rotatable load having a first ring, a second ring, a plurality of ligaments and a load magnification member, said first ring and second ring interconnected by said plurality of ligaments, with the load magnification member provided on the first ring or rotatable load, there being a small clearance maintained between said member and ligaments adjacent thereto, configured such that, in use, when a load of a predetermined value causes the first and second ring to move relative to one another by a predetermined amount, thereby bringing at least one ligament into contact with said load magnification member, at least one ligament is caused to fail. 
   Preferably the first ring is formed with a flange that is provided with a plurality of semi-circular cross-section cut out portions each of which corresponds closely to the outside diameter of the ligaments part way along the ligaments, thereby defining a small clearance between the ligaments and their corresponding cut out portions in the flange. 
   Preferably at least one ligament is formed with a stress raising feature in the region where it is designed to contact the flange when a load of a predetermined value causes the first and second ring to move relative to one another by a predetermined amount. 
   Preferably the frangible coupling is configured such that at a predetermined out of balance loading induced by the rotatable load at least one ligament is brought into contact with the flange, thereby increasing the stress concentration in the at least one ligament to a level where the at least one ligament fails. 
   The invention provides an internal support structure for rotatable turbo machinery components that will fail when subjected to out of balance forces imparted to the structure caused by a fan blade off or partial fan blade off. The stress raising feature within the fuse accelerates the fracture process, breaking the fuse within a narrower and predictable loading range. This allows the rotor to orbit closer to its new centre of gravity and either transmit a reduced load by a secondary route, or removes the load path altogether. 
   In commonly used aerospace metals there is a linear relationship between load applied and stress induced. This invention employs a means whereby when the load raises to a certain level, the rate of change of stress in the material suddenly is increased, inducing fracture within a much smaller and predictable load range. The stresses acting on the ligaments are magnified, causing them to fail at lower loads than they otherwise would. 
   This load magnification at high loads enables the ligaments to be designed for a long fatigue life at low loads whilst failing positively at higher loads. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and how it may be constructed and operated, will now be described in greater detail with reference, by way of example, to an embodiment illustrated in the accompanying drawings, in which: 
       FIG. 1  is a pictorial representation of a typical gas turbine engine; 
       FIG. 2  is a pictorial representation of a fan powered remotely from an engine; 
       FIG. 3  shows a schematic representation of the relevant section of a fan, illustrating the location of the frangible coupling relative to the rotatable components; 
       FIG. 4  shows a first embodiment of the frangible coupling; 
       FIG. 5  shows a second embodiment of the frangible coupling; 
       FIG. 6  shows the second embodiment distorted by an out of balance force, the distortion is exaggerated for clarity; and 
       FIG. 7  is a diagrammatic representation of the relationship between load applied and stress induced in the frangible coupling. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates the main sections of a gas turbine engine  2 . The overall construction and operation of the engine  2  is of a conventional kind, well known in the field, and will not be described in this specification beyond that necessary to gain an understanding of the invention. For the purposes of this description the engine is divided up into four sections—a fan section  4 , a compressor section  6 , a combustor section  8  and a turbine section  10 . Air, indicated generally by arrow “A”, enters the engine  2  via the fan section  4 . The air is compressed and moves downstream to the compressor  6 . This further pressurises the air, a proportion of which enters the combustion section  8 , the remainder of the air being employed elsewhere. Fuel is injected into the combustor airflow, which mixes with air and ignites before exhausting out of the rear of the engine, indicated generally by arrow “B”, via the turbine section  10 . A cutaway reveals the location of a frangible coupling  12 . 
     FIG. 2  illustrates a fan unit  14  that is driven remotely from an engine. It does not provide compressed air to the engine but is used to generate propulsive thrust remote from the propulsion unit. In  FIG. 2  the fan unit  14  is shown mounted with its central axis vertical. This is only one embodiment, drawn here for illustrative purposes. The fan unit may be mounted in any orientation. 
   For the purposes of this description the fan unit  14  is divided up into 3 sections—a fan rotor section  16 , a compressor section  18  and a drive shaft and gearing arrangement  20 , the latter being shown in a cutaway view. Air, indicated generally by arrow “C”, enters the fan unit  14  via the fan rotor section  16 . The air is compressed and moves downstream to compressor  18 , where it is further pressurised before being exhausted from the fan  14 , indicated generally by arrow “D”. A cutaway reveals the location of the frangible coupling  12 . 
   An enlarged view of fan assembly common to the engine  2  and fan unit  14  is presented in  FIG. 3 . Air, indicated generally by arrow “E”, enters the fan unit  14 , constrained on one side by an outer wall  22  and on the other by a discontinuous inner wall  24 . Support for the inner wall  24  is provided by an array of support members  25  which extend radially towards, and are in communication with, the outer wall  22 . The inner wall  24  comprises several static and rotatable sections, the details of which are not required here to appreciate the invention. The air is pressurised by an annular array of fan rotor blades  26  and then passes downstream, as indicated generally by arrow “F”. 
   The fan blades  26  are fixedly joined to a shaft  28  that is rotatable about the central axis of the fan unit  14 . The shaft  28  is rotabably supported by bearings  30  and  32  at the downstream and upstream ends respectively. The bearing  32  is supported by the nonrotatable frangible coupling  12  via a first static member  36 . The coupling  12  is in communication with a non-rotatable section of the inner wall  24  via a second static member  38 . 
     FIG. 4  shows an enlarged view of the frangible coupling  12 , with the first member  36  and second member  38  removed for clarity. The frangible coupling  12  comprises a first ring  40  axially joined via a row of generally circular cross section fuse ligaments  42  to a second ring  44 . 
   The first ring  40  is formed with a flange  46  that is provided with semi-circular cross-section cut out portions  43  that correspond closely to the outside diameter of the ligaments  42  part way along their length. A small clearance  48  is maintained between the ligaments  42  and their corresponding cut out portions  43  in the flange  46 . The fuse ligaments  42  are equidistantly spaced apart from each other and are formed with a stress raising feature  45 , which, as shown here, may take the form of a narrowed waist. 
   In normal use the primary load path from the fan shaft  28  is through the support bearing  32 , and then through the coupling  12  to the inner wall  24 , transmitted then to the support members  25  which communicate it to the outer wall  22 . 
   Under high out of balance loads the ligaments  42  deflect, the clearance  48  closes and the flange  46  forms part of the secondary load path. The flange  46  acts as a load magnifacation member, and hence a consequence of the out of balance nature of the loading is that the ligament  42  in contact with the flange  46  will carry significantly more load than the other ligaments. This results in the rapid failure of the ligament  42  in contact with the flange  46 . Since the applied load is rotating the adjacent ligament  42  quickly becomes loaded in a similar way and also fails. This process is repeated until all of the ligaments  42  have failed. With the ligaments  42  severed, the first ring  40  is free to move independently of the second ring  44 , allowing the out of balance shaft  28  to oscillate about a new axis, which will result in less damage to the engine support casing than if the out of balance force was transmitted through to the inner wall  24 . 
     FIG. 5  presents an alternative embodiment of the frangible coupling  12 . The coupling  12  comprises a first ring  50  axially joined via a row of fuse ligaments  52  to a second ring  54 . The second ring  54  is formed with a third static member  56  (not shown) that is fixedly joined with a non rotatable section of the inner wall  24  (not shown in this figure). The first ring  50  is fitted with a bearing  58  that rotatably supports the first ring  50  on a shaft  60 . The shaft  60  supports the fan blades  26  (not shown in this figure). The shaft  60  is provided with a disc  62  positioned at about one half of the way between the first ring  50  and the second ring  54 . Extending radially outward from the circumference of the disc  62  is a snub  64  which, in use, acts as a load magnifacation member. 
   In normal in balance operation the blades  26  rotate and cause only small deflections of the shaft  60 . When subjected to abnormally high radial loads the shaft  60  will oscillate, transmitting the oscillation to the bearing  58  and the first ring  50 , causing the ligaments  52  to deflect, as shown in  FIG. 6  (exaggerated). The relative movement of the ligaments  52  and the snub  64  causes them to impact each other as the snub  64  rotates. The impact is sufficient to cause the failure of the ligaments  52 . The first ring  50  will be forced to oscillate with the rotating out of balance load, resulting in the snub  64  impacting on all of the fuse ligaments  52 , breaking them in turn and ultimately severing the connection between the first ring  50  and the second ring  54 . This allows the first ring  50  to move independently of the second ring  54 , allowing the out of balance shaft  60  to oscillate about its new axis, resulting in less damage to the support casing of the fan unit  14  than if the out of balance force was transmitted through to the inner wall  24 . 
   During normal operation in both embodiments the fuse ligaments  52  experience an increase in stress proportional to the load imposed by the rotating load. This is indicated by section “G” of the graph in  FIG. 7 . When an abnormal radial load is applied the stress is increased locally in at least one of the fuse ligaments  52 , increasing the stress per unit force at the critical location on the ligament  52 , indicated by section “H” on the graph. Hence the overall relationship between the load imparted to the first ring  40 , 50  and stress induced in the fuse ligaments  52  is non linear. The sudden increase in fuse ligament stress enables a better control over the loading at which the ligament  52  will fail. 
   The failure of some, but not all, of the ligaments  52  may enable the coupling  12  to accommodate the out of balance load where the more rigid structure provided by the coupling  12 , when all ligaments  52  are intact, would not sufficiently dampen the excessive oscillation. 
   The configurations shown in  FIGS. 1 ,  2 ,  3 ,  4 ,  5  and  6  are diagrammatic. The design and positioning of the frangible coupling, rotor blades, bearings, fan casing and other parts may vary. Likewise the combination and configuration of these components will vary between designs. The relationship presented in  FIG. 7  is an approximation.