Abstract:
An impeller for use in a containment structure has a hub, a blade attaching to the hub for compressing air as the blade rotates with the hub, and an annulus disposed about the hub whereby the annulus reduces an effect of the hub breaking apart such that a weight of the containment structure is reduced.

Description:
BACKGROUND OF THE INVENTION 
     Auxiliary Power Engine manufacturers are required to demonstrate by test that the auxiliary rotor cases are able to contain damage caused by the failure of high energy rotor and blades. It is known that a “worst-case” rotor failure is defined if the rotor breaks into three equal weight pieces. This is referred to a tri-hub failure. The containment structure/case around a rotor, for instance, must be strong enough to absorb the energy of the three parts when it breaks apart during such a test. 
     To test containment structures, first a rotor, in this case an impeller is deliberately slotted in such a way to fail into three pieces when rotated to specified speed. This impeller is then placed into an engine and the engine is operated at it maximum attainable speed until the impeller fails, breaking into three pieces. 
     SUMMARY OF THE INVENTION 
     According to an exemplar herein, an impeller for use in a containment structure has a hub, a blade attaching to the hub for compressing air as the blade rotates with the hub, and an annulus disposed about the hub whereby the annulus reduces an effect of the hub breaking apart such that a weight of the containment structure is reduced. 
     According to a further exemplar herein a gas turbine engine compressor stage includes a containment structure with a case, a shroud, and a diffuser plate. A hub is in register with the shroud and the diffuser plate. A blade is attached to the hub for compressing air as the blade rotates with the hub. An annulus is disposed about the hub whereby the annulus is configured to absorb energy during break up of said hub into a plurality of parts. 
     According to a further exemplar herein an impeller includes a containment structure, a hub, and a blade in register with the containment vessel that attaches to the hub and compresses air as the blade rotates with the hub. The impeller also includes an annulus disposed about the hub whereby the annulus minimizes an effect of the hub breaking apart such that a weight of the containment vessel is minimized. 
     According to a still further exemplar herein, a method for minimizing weight of a containment structure includes providing a hub having a blade in register with the containment structure; providing an annulus about the hub whereby the annulus minimizes an effect of the hub breaking apart, and reducing a weight of said containment vessel. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of a prior art impeller and its containment structure. 
         FIG. 2  shows a perspective view of an impeller and its containment structure. 
         FIG. 3  shows a method for placing an annulus on a neck. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a prior art gas turbine engine compressor stage  5  with an impeller  10  and its containment structure  15 , prepared for testing, is shown. The impeller  10  has a hub  25  disposed about an axial center line  20  and a compressor blade  30 . The hub  25  attaches to an axle  35  that is supported by bearings  40  and attaches to a turbine (not shown and is known in the art) to rotate the impeller  10  to its maximum attainable speed (typically 110% above its rated speed). Because the hub  25  has several grooves  45  scored or machined into it, the hub  25  is designed to break apart at 110% of rated speed to test the containment structure  15 . 
     The hub  25  has roughly triangular cross-section having a curved hypotenuse  55 . A roughly cylindrical neck  60  attaches the hub  25  conventionally to the axle  35  and axially removed from the blade  30 . The hub  25  may be made of titanium or an Inconel® steel or the like. 
     The containment structure  15  includes a case  90  that acts as an outer band to contain fragments of the impeller  10 . The containment structure  15  also includes a shroud  65  and a diffuser plate  70 , which also function in conjunction with the impeller  10  to channel air  50  to a burner section (not shown) of a gas turbine engine (not shown). The shroud  65  has a curved portion  75  that closely contours a shape of the blade  30 , and the diffuser plate  70  roughly contours to the right side  80  of the hub  25 . The diffuser plate  70  in this example anchors the bearing  40  (in some auxiliary power units, bearing location may be different). 
     The diffuser plate  70  and the shroud  65  merge together to form a passageway  85  which directs air  50  driven by the impeller  10  to a burner section (not shown). The shroud  65 , the diffuser plate  70 , and the passageway  85  are enclosed by the case  90 . 
     For testing purposes, the grooves  45  are machined into the hub  25  so that if the impeller  10  is driven at greater than 110 percent of its rated speed, the impeller  10  breaks into parts that are contained by the containment structure  15 . To contain the failure, the shroud  65 , the diffuser plate  70  and the case  90  must be designed to absorb the energy of the parts of the hub  25  that are hurled into them. However, to absorb this energy the case  90 , the shroud  65  and the diffuser plate  70 , as described herein must be strong and ductile with a sufficient thickness to prevent parts from escaping the case  90 . 
     Referring to  FIG. 2 , an embodiment of a gas turbine engine compressor stage  105  with an impeller  110  and a containment structure  115 , for use with an APU or other gas turbine engine, is shown. The impeller  110  has a hub  125  disposed about an axial center line  120  and a compressor blade  130  attaching to the hub  125 . The hub  125  attaches to an axle  135  that is supported by bearings  140  and attaches to a turbine (not shown and is known in the art) to rotate the impeller  110  and the blade  130  that act as a compressor driving compressed air  150  through passageway  185 . 
     The hub  125  has roughly triangular cross-section having a curved hypotenuse  155 . A roughly cylindrical neck  160  attaches the hub  125  conventionally to the axle  135 . The hub  125  may be made of titanium or an Inconel® steel or the like. 
     The containment structure  115  includes a case  190  that acts as an outer band to contain fragments of the impeller  110 . The containment structure  115  also includes a shroud  165  and a diffuser plate  170 , which also function in conjunction with the impeller  110  to channel compressed air  150  to a burner section (not shown) of a gas turbine engine (not shown). The shroud  165  has a curved portion  175  that closely contours and is in register with a shape of the blade  130  and the diffuser plate  170  roughly contours and is in register with the right side  180  of the hub  125 . The diffuser plate  170  anchors the bearing  140 . 
     The diffuser plate  170  and the shroud  165  merge together to form passageway  185  which directs air  150  driven by the impeller  110  to a burner section (not shown). The shroud  165 , the diffuser plate  170 , and the passageway  185  are enclosed by the case  190 . 
     The grooves  145  machined into the hub  125  so that if the impeller  110  is driven at greater than 110 percent of its rated speed, the impeller breaks into parts that are contained by the containment vessel  190 . 
     An annulus  195  having roughly a rectangular cross section  200  is press or interference fit onto the neck  160  of the impeller  125 . Referring now to  FIG. 3 , after precision machining the diameters (e.g., the outer diameter (“OD”) (step  201 ) of the impeller neck  160  and the internal diameter (“ID”) of the annulus  195 ) that mate between the annulus  195  and the impeller neck  160 , then the annulus  195  may be heated thereby expanding the ID (steps  205 ,  210 ) of the annulus, and the impeller neck  160  may be cooled (steps  215 ,  220 ) thereby shrinking the OD of the neck so the annulus  195  may be slid onto the impeller neck  160 . The annulus may also be heated and the neck cooled simultaneously (steps  205  and  220 ). As the impeller neck  160  and the annulus  195  return to room temperature, an interference fit is formed therebetween. 
     The cross section  200  is rectangular though other shapes are contemplated herein. The annulus  195  is a ring made of a strong material such as Inconel®  625  steel or titanium. By applying the annulus  195  to the neck  160 , as the impeller  110  begins to break apart during testing or during operation due to defect or other reason, enough energy is absorbed by the annulus  195  during the break up that the damage inflicted on the containment structure  115  by the three parts in a worst case impeller failure is less than that inflicted upon the containment structure  15  of  FIG. 1  under similar operating and failure conditions. As such, the case  190 , shroud  165  and diffuser plate  170  may be designed with a reduced thickness relative to the case  90 , shroud  65 , and diffuser plate  70  of  FIG. 1 . For instance, the case  190  and the shroud  165  is two-thirds of the thickness of the corresponding thickness of the case  90  and the shroud  65 . The reduced thickness of case  190 , shroud  165 , and/or diffuser plate  170  collectively have less weight than the weight of the annulus  195 , and therefore the overall weight of the engine is diminished without affecting the ability of the containment structure  115  to perform. As an example, the annulus  195  may weigh about one and one-half pounds (e.g., 0.7 kgs), and the weight shed by the case  190 , shroud  165  and diffuser plate  170  may be three pounds (e.g., 1.4kg) or more. 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.