Patent Publication Number: US-6986637-B2

Title: Stub axle

Description:
The present invention relates to stub axles and more particularly to stub axles used with regard to intermediate pressure compressors of a turbine engine. 
   Referring to  FIG. 1 , a gas turbine engine is generally indicated at  10  and comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high pressure compressor  14 , combustion equipment  15 , a high pressure turbine  16 , an intermediate pressure turbine  17 , a low pressure turbine  18  and an exhaust nozzle  19 . 
   The gas turbine engine  10  works in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  which produces two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
   The compressed air exhausted from the high pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines  16 ,  17  and  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbine  16 ,  17  and  18  respectively drive the high and intermediate pressure compressors  14  and  13 , and the fan  12  by suitable interconnecting shafts. 
   It will be noted that the intermediate pressure compressor  13  is located upon a stub shaft  1 . The stub shaft  1  is generally, or at least possibly, upstream of the intermediate pressure compressor location bearings. If this stub shaft fails in use, it will be understood that the torque load of the intermediate compressor on the turbine is lost and therefore the turbine may over speed. Failure of the intermediate shaft  1  rearwards of any axial supports such as a location bearing allows the turbine  17  to move axially rearwards towards the stator  2 . Such action would cause interference braking between the turbine and the stator and therefore limit over speed. However, failure of sections of the shaft  1  forwards of any axial support may not allow the turbine to move rearwardly into interference engagement with the stators and therefore prevent a brake limiter to over speed. In such circumstances, failure of the intermediate compressor stub shaft  1  in a section forward of any location bearing can present and cause a hazardous condition for the turbine  17 . 
   In accordance with the present invention there is provided a stub shaft for a turbine engine, the shaft comprising a shaft end associated with a compressor, the shaft characterised in that a failure mass is secured upon the shaft end and an intermediate mass secured at an intermediate location on the shaft, and a mass providing rotational balance for the shaft in normal use whereby upon failure of the shaft the failure mass is displaced and the shaft vibrates to cause compressor surge. 
   Typically, the shaft end is forwards of an axial mounting for the stub shaft. 
   Generally, vibration causes radial displacement of the stub shaft whereby there is interference engagement with stable structures such as stators to drag brake the shaft and limit rotational speed. 
   Generally, the shaft end is bell shaped. Furthermore, the failure mass is secured at a rim of that bell shape. Further advantageously, the intermediate mass is secured at a crown part of the bell shaped end. 
   Normally, the balance mass is placed along the stub axle to balance couple between the failure mass and the intermediate mass for rotational balance of the stub shaft in normal use. 
   Possibly, one or more of the masses is moveable to achieve tuning of rotational balance for the stub shaft. 
   Also in accordance with the present invention there is provided a turbine engine incorporating a stub shaft as described above. 

   
     An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic cross section of a typical turbine engine; and 
       FIG. 2  is a schematic cross-section of a stub shaft in accordance with the present invention. 
   

   In the event of an intermediate pressure shaft failure of a gas turbine the torque load applied to the intermediate pressure compressor is lost. Thus, for a short period of time the turbine has a significant pressure reduction across it which in turn leads to rotational acceleration. This acceleration will lead to an over speed condition where the forces applied to the turbine are too high and the turbine will therefore itself fail. More importantly, with such high levels of kinetic energy containment of the failing fragments of the turbine is unlikely and so there may be hazardous consequences. 
   Generally, as indicated above, if the intermediate pressure shaft fails rearwards of its axial supports (such as locational bearings) then the shaft will tend to move axially rearwards towards stationary structures such as stators whereupon interference friction will occur which although causing damage to the turbine blades will prevent occurrence of over speed. However, failure of sections of the shaft forwards of such axial support does not allow or cause turbine movement axially with resultant interference with stable structures in order to prevent over speed. Generally, the intermediate compressor stub shaft is a section of the intermediate pressure shaft located forwards of the location bearings. Generally, interference slowing of the stub shaft is only really beneficial when the interference is “downstream” of the failure or break. 
   In accordance with the present invention upon intermediate pressure compressor stub shaft failure the compressor is arranged to surge without recovery. This surge reduces the pressure gradient across the turbine and so inhibits the accelerating force causing over speed rotation of the turbine. Such surging occurs due to the detrimental effect of vibration on the blade aerodynamics especially tip clearance. 
   As indicated previously,  FIG. 1  is a schematic cross section of a gas turbine engine. As can be seen, the intermediate pressure compressor  13  is secured upon a stub shaft  1 . The stub shaft  1  generally takes a bell shaped drum cross-section at a shaft end  3  associated with the intermediate pressure compressor  13 . The shaft  1  connects the intermediate pressure compressor  13  with the intermediate pressure turbine  17  with an intermediate pressure flow passing in the direction of arrowhead A to the high pressure compressor  14 . 
     FIG. 2  is a schematic cross-section of a stub shaft  101  in accordance with the present invention. As can be seen, the stub shaft  101  incorporates a shaft end in the form of a bell shaped drum  102 . This bell shaped drum  102  is associated with the intermediate pressure compressor or turbine ( 13  or  17  in  FIG. 1 ) with a stub axle  103  extending away from the bell shaped drum  102  in order to provide the stub shaft. This stub axle  103  rotates about a centre line X—X and typically as indicated previously is coupled to the intermediate pressure turbine ( 17  in  FIG. 1 ). 
   In accordance with the present invention a number of mass elements  104 ,  105 ,  106  are located about the stub shaft  101 . A failure mass  104  is secured about a peripheral edge or rim of the bell shaped drum shaft end  102 . An intermediate mass  105  is located at an intermediate position within the stub shaft  101 . A balance mass  106  is located at a position along the length of the stub axle  103 . All the masses  104 ,  105 ,  106  are held in a rotational balance. Thus, the balance mass  106  essentially equalises the mass couple created between the failure mass  104  and intermediate mass  105 . 
   It is necessary that the masses  104 ,  105 ,  106  are balanced. Typically, a normal balanced relationship is when the failure mass  104  is placed at a top dead centre position near a rim  107  of the shaft end  102  whilst the intermediate mass  105  is placed at a bottom dead centre position near to a crown edge  108  of the shaft  101  whilst as a result of their relatively different displaced positions both longitudinally and radially relative to the centre of rotation (line X—X) it is necessary to provide the balance mass  106  at an appropriate position along the axle  103 . Typically, the intermediate mass  105  is positioned at a position at which location bearings for the stub shaft  101  are located in use. Thus, with all the masses  104 ,  105 ,  106  in position there is a substantially neutral couple effect to enable smooth and balanced rotation of the shaft  101  in use. The balance mass  106  is generally placed at a top dead centre position as indicated above at a position along the axle  103  in order to create this balanced couple effect between the masses  104 ,  105 ,  106 . Failure of the shaft between these masses  104 ,  105 ,  106  will cause vibration. 
   As indicated above, failure of the stub shaft  101  will result in displacement of the failure mass  104 . This displacement will result in the intermediate pressure compressor and the masses  104  or  105  becoming detached from the stub shaft  101 . In such circumstances, the masses  104 ,  105 ,  106  are now unbalanced and rotation of the shaft  102  causes vibration. In short, the bell shape drum  102  is out of balance with the masses  104 ,  105  on one side. This vibration will cause compressor surging. 
   In order to ensure sufficient severity of vibration the out of balance couple between the masses  104 ,  105  should be large. As indicated previously, it is by the vibrations that the compressor surges and an intermediate pressure turbine over speed is prevented. 
   In order to reduce the bending effects presented to the stub shaft  101  by the relative locations of the masses  104 ,  105 ,  106  it will be understood that the angles between the respective masses  104 ,  105 ,  106  may be changed in particular embodiments provided that rotational and longitudinal balance is maintained in normal operation of the shaft  101 . Alternatively, bending effects may be avoided by increasing shaft  101 ,  102  strength. 
   The masses  104 ,  105 ,  106  will typically be formed from a relatively high density material which has sufficient physical characteristics to withstand heat and centrifugal forces presented during operation of a turbine engine. The masses  104 ,  105 ,  106  will be secured at their relative locations by appropriate means including adhesive, brazing and other finishing techniques including use of bolts and rivets. Generally, as shown, the masses  104 ,  105 ,  106  will be secured upon outer surfaces of the stub shaft  101 . Although shown as single elements, the masses  104 ,  105 ,  106  may be formed by a plurality of elements appropriately secured and located about the stub shaft  101  in order to achieve their functional necessities in terms of a displaceable failure mass, a intermediate mass and a balance mass for normal operation. 
   It will be appreciated that the particular mass of each mass  104 ,  105 ,  106  will be dependent upon stub shaft  101  and end  102  mass and balanced mass locations as well as the necessity to achieve rotational balance in normal operation and sufficient vibration for over speed control in accordance with the present invention upon displacement of the balance masses,  104 ,  105 ,  106  relative to each other. 
   The balance mass  106  as well as the intermediate mass  105  may be arranged when there is failure of the stub shaft  101  resulting in displacement of the failure mass  104 , to provide an interference friction contact in order to slow rotation of the shaft  101 . Similarly, the balance mass  106  may become slightly displaceable upon vibration as a result of the unbalanced state when the failure mass  104  is displaced whereby that balance mass  106  comes into interference contact with a stable structure in order to provide braking of IP turbine ( 17  in  FIG. 1 ) rotation. Generally, as indicated above the most hazardous situation with regard to stub shaft  101  failure are those where the failure is forward of the location bearings for the shaft  101 . In  FIG. 2  broken line  109  shows such a position. In this situation as described previously, the failure mass  104  is detached from the intermediate mass  105  and balance mass  106 . Thus, the portion of the bell shape drum  102  detached with the mass  104  causes that portion to be out of balance. If this failure mass  104  is of sufficient magnitude there are enough vibrations in the compressor associated with that portion of the drum  102  whereby the compressor surges which in turn reduces the pressure across the turbine associated with the stub shaft with the result that a turbine over speed conditions is prevented. 
   It is imbalance between the masses  104 ,  105 ,  106  which creates vibration so that these functions within the present stub shaft arrangement may be interchangeable. 
   Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.