Abstract:
An apparatus ( 10 ) and method for removing an abradable material from an interior surface ( 16 ) of a fan containment case ( 14 ), without requiring removal of the fan containment case ( 14 ) from its engine ( 12 ). The apparatus ( 10 ) and method generally involve adjusting the elevation of a blast head ( 40 ) and centering a horizontal axis of rotation thereof with the fan containment case ( 14 ) to position the blast head ( 40 ) adjacent the abradable material on the case surface ( 16 ). An erosion media is then sprayed with the blast head ( 40 ) in a substantially radial direction relative to the horizontal axis of rotation of the blast head ( 40 ), such that the erosion media impacts and erodes the abradable material. The blast head ( 40 ) is rotated about the horizontal axis of rotation thereof so that the erosion media is sprayed at the abradable material along the entire circumference of the fan containment case ( 14 ).

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention generally relates to fan containment assemblies for turbomachinery, such as gas turbine engines. More particularly, this invention relates to an automated apparatus and method for removing an abradable material for a fan containment assembly. 
     2. Description of the Related Art 
     Gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. High bypass turbofan engines, widely used for high performance aircraft which operate at subsonic speeds, have a large fan placed at the front of the engine to produce greater thrust and reduce specific fuel consumption. The fan compresses the incoming air, a portion of which is then delivered to the combustion chamber, though a larger portion is bypassed to the rear of the engine to generate additional engine thrust. 
     The fan is circumscribed by a fan containment case such that the case is immediately adjacent the tips of the fan blades. The containment case serves to channel incoming air through the fan so as to ensure that the bulk of the air entering the engine will be compressed by the fan. However, a small portion of the air is able to bypass the fan blades through a radial gap present between the fan blade tips and the containment case. Because the air compressed by the fan blades is used to generate thrust and feed the turbine section of the engine, engine efficiency can be increased by limiting the amount of air which is able to bypass the fan blades through this gap. Accordingly, the fan and containment case are manufactured to close tolerances in order to minimize the gap. However, manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Furthermore, during the normal operation of an aircraft turbofan engine, the fan blades may rub the containment case as a result of a hard landing or a hard maneuver of the aircraft. Any rubbing contact between the fan blade tips and the containment case will abrade the tips of the rotors, tending to further increase the gap between the containment case and blade tips, thereby reducing engine efficiency. The fan is circumscribed by a fan containment case such that the case is immediately adjacent the tips of the fan blades. The containment case serves to channel incoming air through the fan so as to ensure that the bulk of the air entering the engine will be compressed by the fan. However, a small portion of the air is able to bypass the fan blades through a radial gap present between the fan blade tips and the containment case. Because the air compressed by the fan blades is used to generate thrust and feed the turbine section of the engine, engine efficiency can be increased by limiting the amount of air which is able to bypass the fan blades through this gap. Accordingly, the fan and containment case are manufactured to close tolerances in order to minimize the gap. However, manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Furthermore, during the normal operation of an aircraft turbofan engine, the fan blades may rub the containment case as a result of a hard landing or a hard maneuver of the aircraft. Any rubbing contact between the fan blade tips and the containment case will abrade the tips of the rotors, tending to further increase the gap between the containment case and blade tips, thereby reducing engine efficiency. 
     In view of the above, it is well known in the art to cover the portion of the containment case adjacent the blade tips with an abradable material, such that the abradable material will sacrificially abrade away when rubbed by the fan blades. Inherently, as the abradable material is removed, the gap between the blade tips and the surface of the abradable material will increase, necessitating removal and replacement of the abradable material to maintain desirable aerodynamic efficiencies associated with a smooth abradable surface and a small gap between the abradable surface and the fan blade tips. Restoration of the abradable material also becomes necessary if damage has occurred from impacts with foreign objects. 
     A common technique for removing the abradable material is performed with handheld tools, such as an air chisel, after which sandpaper is used to achieve a smooth surface finish. While suitable for use on steel fan cases, air chisels are too aggressive for use on engines with aluminum cases. Aluminum fan cases must be removed from the fan frame, stripped of gearboxes, wire harnesses, controls, etc., and then centered on a turning machine to remove the old abradable material. Any damage that may occur to the base metal must be repaired before applying and bonding the new abradable material. The removal process can be time consuming and expensive, and requires a large maintenance facility to which at least the front of the engine must be transported for disassembly. Due to the special equipment required to perform the machining operation, a limited number of facilities are available for removing fan case abradable material. As a result, additional costs, scheduling and transport problems are common. 
     Accordingly, it would be desirable if an improved technique were available by which the abradable material of a fan containment case could be removed without requiring removal of the fan case from the engine, such that restoration can be performed in the field. 
     SUMMARY OF INVENTION 
     The present invention provides an apparatus and method for removing an abradable material from an interior surface of a fan containment case, without requiring removal of the fan containment case from its engine, e.g., a high-bypass gas turbine engine. The apparatus of this invention generally includes a frame, and means rotatably mounted to the frame for spraying an erosion media in a substantially radial direction relative to a horizontal axis of rotation of the spraying means. The erosion media impacts the abradable material on the interior surface of the fan containment case while the spraying means is positioned within the fan containment case adjacent the abradable material. The apparatus further includes means rotatably mounted to the frame with the spraying means for retrieving the erosion media after impacting the abradable material. Also provided is means mounted to the frame for centering the horizontal axis of rotation of the spraying means relative to the interior surface of the fan containment case, means for adjusting together the elevation of a horizontal central axis of the centering means and the horizontal axis of rotation of the spraying means relative to the frame, and means for positioning the spraying means adjacent the interior surface of the fan containment case. 
     A method made possible with the apparatus of this invention generally comprises the steps of adjusting the elevation of the spraying means and centering the horizontal axis of rotation thereof with the fan containment case to position the spraying means adjacent the abradable material on the interior surface of the fan containment case. The erosion media is then sprayed with the spraying means in a substantially radial direction relative to the horizontal axis of rotation of the spraying means, such that the erosion media impacts and erodes the abradable material on the interior surface of the fan containment case. In addition, the spraying means is rotated about the horizontal axis of rotation thereof so that the erosion media is sprayed at the abradable material along the entire circumference of the fan containment case. At the same time, the erosion media is retrieved after being sprayed and impacted against the abradable material. 
     In view of the above, it can be seen that a significant advantage of this invention is that the apparatus is capable of removing the abradable material from a fan containment case without requiring removal of the case from an engine. Instead, only the fan blades need be removed to gain access to the interior surface of the case. Another advantage of the invention is that the apparatus provides a means by which the spraying means is self-centered with respect to the interior surface of the fan containment case, and the operation of the spraying means can be controlled to follow a specific path along the inner diameter of the fan containment case. As such, the apparatus requires minimal setup, and can be operated in an automated mode without further human supervision. In a preferred embodiment, the apparatus is capable of removing substantially all of the abradable material in roughly a single rotation of the spraying means about its horizontal axis of rotation. With the use of appropriate abradable material and spray pressures, this operation can be completed in about one hour without damaging the substrate beneath the abradable material. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a side view of an apparatus for removing abradable material from a fan containment case in accordance with a preferred embodiment of this invention. 
     FIGS. 2,  3 ,  4  and  5  are a top view, front end view, back end view, and perspective view, respectively, of the apparatus of FIG.  1 . 
     FIG. 6 is a perspective view of a blast head shown mounted to the apparatus of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 through 5 represent an apparatus  10  for removing an abradable material in accordance with a preferred embodiment of the invention. The apparatus  10  is particularly adapted for removing an abradable material from the blade containment case of a turbomachine, such as the fan containment case of a high bypass turbofan engine. As known in the art, high bypass turbofan engines have a fan section within which a fan formed by a number of fan blades is mounted to a hub. A fan section  12  is represented in phantom in FIG. 1, and includes a fan containment case  14  that circumscribes the fan (shown removed), and whose radially inward surface has a channel  16  containing an abradable material. Suitable materials for the abradable material include various lightweight materials known and used in the prior art, including composite materials such as an epoxy filled with hollow glass beads. The apparatus  10  of this invention is adapted to remove the abradable material from the case  14  to permit the application of a new layer of abradable material. While the invention can be used to remove abradable material from containment cases formed of a variety of materials, including steel, the invention is particularly well suited for use on aluminum cases that are more prone to damage during removal of the abradable material by conventional methods. 
     As represented in FIGS. 1 through 5, the apparatus  10  is able to remove the abradable material while the fan containment case  14  remains mounted to the engine. The apparatus  10  is shown as comprising a frame assembly  20  having a base  22 , platform  24  and legs  26  pivotably connected to both the base  22  and platform  24 . A vertical actuator  28  is mounted to the base  22  and connected to the platform  24  to raise and lower the platform  24  relative to the base  22 . The base  22  is shown as being equipped with wheels  29  that enable the apparatus  10  to be moved and positioned in front of the fan section  12 . The wheels  29  are preferably equipped with wheel locks (not shown) of any conventional design to prevent movement of the apparatus  10  once positioned. 
     A shaft assembly  30  is rotatably mounted to the platform  24  in any suitable manner. An electric motor  34  and reduction gear assembly  36  is mounted to the platform  24  for rotating the shaft assembly  30  at an adjustable controlled speed, such as about one rotation per hour, though faster and slower speeds are foreseeable. A radial arm  38  is mounted to the end of the shaft assembly  30  opposite the motor  34 , and extends in a radial direction relative to the axis of rotation of the shaft assembly  30 . At the distal end of the radial arm  38 , a blast head  40  is mounted by a pair of articulating arms  42  whose movement and position, and therefore the radial position of the blast head  40 , are controlled with a radial adjustor  44  mounted to the arm  38 . As will be discussed in greater detail with respect to FIG. 6, the blast head  40  delivers an erosion media (not shown) for the purpose of removing the abradable material from the fan containment case  14 . Finally, the orientation (tilt) of the blast head  40  is adjusted with a tilt adjustor  46 , also mounted to the articulating arms  42 . With the electric motor  34 , the entire assembly—comprising the shaft assembly  30 , radial arm  38 , arms  42  and blast head  40 , etc.—can be caused to rotate about the axis of the shaft assembly  30 , which is oriented horizontally in view of the orientation of the engine fan section  12 . In this manner, the blast head  40  can be caused to follow the inner circumference of the fan case  14  while spraying the erosion media in a generally radial direction, such that the erosion media impacts the abradable material located on the interior surface of the fan containment case  14 , as represented in FIG.  1 . 
     Also shown in FIGS. 1 through 5 is a chuck assembly  48  mounted to a housing  32  that supports the shaft assembly  30 . The chuck assembly  48  is shown as comprising three two-bar arm assemblies  50 , which as shown in FIG. 1 are adapted to engage the fan containment case  14  and center the shaft assembly  30  with the centerline of the fan section  12 . For this purpose, three equiangularly-spaced arm assemblies  50  are preferred, though it is foreseeable that a different number of arms could be used. The arm assemblies  50  are articulated in unison through rods  52  connected to a collar  54  mounted with bearings to the shaft assembly  30 . As such, the chuck assembly  48  has a horizontal central axis that substantially coincides with the axis of rotation of the shaft assembly  30 . An actuator  56  is coupled to the collar  54  to move the collar  54  axially along a given length of the shaft assembly  30 , causing the arm assemblies  50  to pivot in unison toward or away from the axis of rotation of the shaft assembly  30 . The distal end  58  of each arm assembly  50  is adapted to engage the interior surface of the fan containment case  14 , and is equipped with a clamp  60  for gripping the case  14  with the distal end  58 . 
     In FIGS. 1 and 2, an actuator  62  is shown with which the shaft assembly  30 — and therefore simultaneously the blast head  40  and chuck assembly  48 —can be moved axially, i.e., parallel to the axis of rotation of the shaft assembly  30 . Consequently, the blast head  40  can be introduced into the fan section  12 , positioned in the cross-sectional plane of the case  14  containing the abradable material to perform the removal operation, and later retracted from the fan section  12  at the completion of the operation. Simultaneously with the introduction of the blast head  40  in the fan section  12 , the arm assembilies  50  are moved toward the fan section  12 . Once the blast head  40  is properly axially positioned within the fan section  12 , the arm assemblies  50  can be pivoted radially outward with the actuator  56  to engage the rim of the fan containment case  14  and thereby center the shaft assembly  30  within the fan section  12 . Finally, finer adjustments can be made to place the blast head  40  immediately adjacent the abradable material through the radial adjustor  44  and the tilt adjustor  46 . 
     The blast head  40  represented in FIGS. 1 through 5 is shown in greater detail in FIG.  6 . The blast head  40  can be seen to comprise a containment enclosure  64  for containing the erosion media sprayed through a spray head  66  disposed in a wall of the enclosure  64 . The outer surface  68  of the containment enclosure  64  is contoured to correspond to the radius of the fan containment case  14 , and is equipped with a rubber or brush seal (not shown) to provide a sealing action with the interior surface of the fan containment case  14 . The erosion media is delivered to the spray head  66  through a supply hose  70 , and retrieved from the enclosure  64  through a pair of hoses  72  attached to opposite sides of the enclosure  64 . As such, the erosion media is retrieved from the containment enclosure  64  after the erosion media is sprayed from the spray head  66  and has impacted the abradable material on the interior surface of the fan containment case  14 . Thereafter, the spent erosion media containing particles of the abradable material may be discarded, though in a preferred embodiment the erosion media is separated from particles of the abradable material and reused. 
     While spray heads  66  of various designs may be used, a preferred spray head  66  is equipped with multiple nozzles, preferably three as shown, that rotate together about the axis along which the media is sprayed. A blast head  40  equipped with the preferred spray head  66  and media retrieval and reclamation system particularly suitable for use with this invention is the MPC Series of machines available from Abrasive Blast Systems, Inc., of Abilene, Kans. An erosion material found to be suitable for use with the apparatus  10  of this invention is a plastic bead material formed of Type II urea formaldehyde thermoset per Mil-P-85291, and therefore commercially available from a number of sources. In practice, a suitable particle size for the Mil-P-85291 media is about twenty to about thirty mesh, though smaller and larger particle sizes could be used. When using this plastic erosion media within the stated particle size range, suitable operating parameters for the blast head  40  include an adjustable supply pressure of about 50 to about 80 psi (about 3.5 to about 5.5 bar), such as about 65 psi (about 4.5 bar). The volumetric airflow rate from the enclosure  64  is preferably slightly higher than the flowrate to the enclosure  64  to promote a vacuuming effect for reclaiming the media from the enclosure  64 . With these parameters, abradable material has essentially been completely removed from a fan containment casing with a single complete rotation of the blast assembly  40 . Some overlap (e.g., about 370 degrees rotation) is generally desirable to ensure that the entire circumference of the case  14  has been thoroughly treated during the operation. 
     In the preferred embodiment, the apparatus  10  is capable of being operated within minimal supervision by an operator. For this purpose, the apparatus  10  is preferably equipped with controls that cause the shaft assembly  30  to rotate and the blast head  40  to spray the media according to predetermined settings. Automatic safety stops are preferably provided to stop the operation of the blast head  40  if the shaft assembly  30  stops rotating. In FIG. 2, a control unit  74  is seen mounted to the platform  24  by which the rotational speed of the blast head  40  (through the shaft assembly  30 ) is controlled. Connected to this unit  74  is a pendant control  76  with switches, etc., for controlling the operation of the chuck assembly  48  and actuators  28 ,  56  and  62 . 
     In view of the above, the apparatus  10  of this invention can be used to remove abradable material from an interior surface of a fan containment case  14  while the case  14  remains mounted to a high bypass turbofan engine. Prior to the operation, the fan blades are removed from the engine, the platform  24  is raised with the vertical actuator  28  to align the axis of the shaft assembly  30  (and therefore the axis of the chuck assembly  48 ) with the centerline of the fan section  12 . The blast head  40  and arm assemblies  50  can then be introduced into the case  14  by extending the actuator  62 . As a result of actuating the actuator  56  to engage the arm assemblies  50  with the rim of the fan containment case  14 , the axis of rotation of the shaft assembly  30  is accurately centered with the centerline of the fan containment case  14 . The blast head  40  can then be moved in the radial direction with the arms  42  and adjustor  44  until the containment enclosure  64  contacts the interior surface of the case  14  and the spray head  66  is positioned over the abradable material. Thereafter, the spraying operation is commenced, with the blast head  40  closely following the entire inner circumferential surface of the case  14  covered by the abradable material as a result of the operation of the motor  34 , controlled with the control unit  74  to rotate the blast head  40  at a predetermined speed. The control unit  74  can also be employed to terminate the flow of erosion media to and from the blast head  40 . 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the apparatus  10  could differ from that shown. Therefore, the scope of the invention is to be limited only by the following claims.