Abstract:
An apparatus for balancing a shaft of an aircraft engine preferably includes a plate and the use of standard fasteners. The plate provides mounting for nose cone, and the balancing apparatus may be accessed from the front of the assembled engine.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a balancing apparatus and method, and more particularly to an apparatus and method for balancing a shaft of an aircraft turbine engine.  
       BACKGROUND OF THE INVENTION  
       [0002]     The main drive shafts of aircraft gas turbine engines are subject to mass imbalance due to manufacturing variations. The shafts must be balanced prior to assembly in the engine in order to reduce shaft vibration during engine operation.  
         [0003]     Shaft imbalance is characterized by a magnitude of imbalance and an angular direction of imbalance. The magnitude of imbalance caused by an eccentric rotating mass, is a function of the weight of the mass and the radial distance of the mass from the axis of rotation. The angular direction of imbalance is determined by the angular position of the eccentric mass relative to an arbitrary reference direction.  
         [0004]     Due to the subsequent operations of an aircraft turbine engine assembly, there is often a need for further balancing of the rotating parts when the engines are completed. Further balance adjustment however, is normally difficult due to the inaccessibility of the rotating parts and, quite frequently, the unbalance is so severe that it is necessary to strip the turbine engines for rebalancing.  
         [0005]     Therefore, there is a need for an improved balancing apparatus for the shafts of aircraft turbine engines which will enable a shaft to be balanced accurately and simply and which will permit simplified final adjustment after the aircraft engine has become completely assembled.  
       SUMMARY OF THE INVENTION  
       [0006]     One object of the present invention is to provide an apparatus and a method for balancing a shaft of an aircraft engine which is simple and allows for simplified final adjustment after the aircraft engine has been assembled.  
         [0007]     In accordance with one aspect of the present invention, there is an apparatus provided for balancing a shaft of an aircraft engine which comprises a round plate and a plurality of standard mechanical fasteners. The round plate defines a first group of holes axially extending therethrough. The round plate is coaxially attached to the shaft at a forward end thereof. The standard mechanical fasteners are for selectively engaging in at least one of the holes of the first group which is determined in a shaft balancing test, thereby adding a selected balance weight to the plate in the determined hole, in order to balance the shaft.  
         [0008]     The plate preferably also comprises a second group of holes axially extending therethrough for receiving mounting bolts therein for mounting a nose cone to the aircraft engine.  
         [0009]     In accordance with another aspect of the present invention, there is an apparatus provided for an aircraft engine which comprises a nose cone of the aircraft engine, at least one balance weight element, and a member adapted to be centrally mounted to a forward end of a rotatable shaft of the aircraft engine. The member includes a plurality of mounting points by which the nose cone is mounted to the member, and further includes a plurality of attachment points to cooperate with the at least one balance weight element to retain the same to the member. The at least one weight element is retained in a selected one of the attachment points such that the shaft is rotationally balanced.  
         [0010]     In accordance with a further aspect of the present invention, there is a method provided for balancing a shaft of an aircraft engine in which the shaft includes a mounting plate for mounting a nose cone to one side thereof. The method comprises steps of, with the nose cone dismounted, observing a rotational imbalance of the shaft; and accessing the mounting plate through a front opening of a casing of the aircraft engine to install and affix at least one standard fastener in one of a plurality of axial holes of the mounting plate determined during the observation step, thereby rotationally balancing the shaft.  
         [0011]     The method preferably further comprises a step of selecting the at least one standard fastener from a plurality of standard fasteners having identical diameters and different lengths in order to provide a selected balance weight added to the plate.  
         [0012]     An apparatus of the present invention is simply configured and has double functions. The plate is used as a balancing device to selectively carry balancing weights while being used as a mounting plate for supporting the nose cone in its operative position.  
         [0013]     Another advantage of the present invention lies in that standard fasteners rather than specially configured balancing weights are used to balance the shaft of the aircraft engine, resulting in reduced manufacturing expenses of the aircraft engine. This advantage is even more significant when the present invention is applied to small aircraft turbine engines.  
         [0014]     Other features and advantages of the present invention will be better understood with reference to a preferred embodiment described hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Reference will now be made to the accompanying drawings showing by way of illustration, a preferred embodiment in which:  
         [0016]      FIG. 1  is a partial cross-sectional view of an aircraft turbine engine, showing a nose cone mounting plate used as a balancing plate according to one embodiment of the present invention;  
         [0017]      FIG. 2  is a front elevational view of the mounting plate for the nose cone used in the embodiment of  FIG. 1 ;  
         [0018]      FIG. 3  is a partial cross-sectional view of the mounting plate of  FIG. 2 , taken along line A-A, showing the structural details thereof;  
         [0019]      FIG. 4  is a rear elevational view of a clinch nut to be attached to the rear side of the mounting plate  FIG. 3 , showing the cut-away for restraining rotation of the nut;  
         [0020]      FIG. 5  is a front elevational view of the nose cone of  FIG. 1 , showing the mounting recesses thereon; and  
         [0021]      FIG. 6  is a rear elevational view of the mounting plate of  FIG. 1 , showing the inner structure thereof. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Referring to the drawings, particularly  FIGS. 1 and 2 , an aircraft turbine engine generally indicated by numeral  10  includes a main shaft  12  rotatably supported within an engine casing  14  and being driven by a turbine rotor (not shown) of the engine. A fan integrated blade rotor (IBR) assembly  16  is mounted to a forward end of the main shaft  12  to be driven in rotation together therewith. A nose cone  18  is mounted to the forward end of the main shaft  12  by means of a mounting plate  20  such that an annular airflow inlet is defined at the front opening  22  of the engine casing  14 , between the engine casing  14  and the fan IBR assembly  16  together with the nose cone  18 . The nose cone  18  at its rear side defines a central cavity  66  therein to accommodate the forward end of the main shaft  12  and, the fan retaining nut  52  engaged with the forward end of the main shaft  12 .  
         [0023]     Referring to  FIGS. 1, 2  and  3 , the mounting plate  20  is a round plate preferably made of a turned steel, having a central aperture  24 . A front annular rim  26  is disposed coaxially with respect to the round plate  20  and extends axially forwardly from the front side of the plate  20 . The front annular rim  26  divides the plate  20  into a central portion  28  and an annular portion  30  much thicker than the central portion  28 .  
         [0024]     A first group of holes  32  are defined in the annular portion  30  of the plate  20 , axially extending through the plate  20 . The first group of holes  32  are tapped respectively to form inner threads therein for selectively engaging standard specification defined fasteners  33 , such as Military Standard (MS) type screws, which will be further described hereinafter. The first group of the holes  32  are disposed in the circumferential direction of the annular portion  30  of the plate  20 , and are preferably spaced apart substantially equally one from another. The first group of holes.  32  are used as attachment points for adding balance weights to the mounting plate  20 . Therefore, it is preferable to have a great number of the holes  32  disposed in a close relationship one to another in order to allow placement of a selected balance weight element in an accurate angular direction. However, the number of the holes  32  should be limited in order to not jeopardize a predetermined strength and stability of the mounting plate  20  for safely bearing the dynamic airflow loads on the nose cone  18  during engine operation, particularly during flight operation.  
         [0025]     A second group of holes  34  are also defined in the annular portion  30  of the plate  20 , equally and circumferentially spaced apart one from another. In this embodiment, three holes  34  spaced apart by 120 degrees extend axially through the plate  20 , for receiving mounting bolts  36  therethrough. Each of the holes  34  includes a diametrically reduced section  37  at the opening thereof defined in the rear side  38  of the plate  20 . A clinch nut  40  is attached to each hole  34  at the rear side  38  of the plate  20  such that the holes  34  with the attached clinch nuts  40  form a plurality of mounting points for mounting the nose cone  18  of  FIG. 1  to the mounting plate  20 . Each clinch nut  40  has a diametrically reduced front section  42  with thin front edges to be inserted into a hole  34  through the diametrically reduced section  37 . Upon the insertion of the clinch nut  40  into the hole  34 , the thin front edge of the front section  42  of the clinch nut  40  is forced to radially expand, thereby engaging the front section  42  of the clinch nut  40  in the diametrically reduced section  37  of the hole  34 .  
         [0026]     In order to prevent the clinch nut  40  from rotating together with the mounting bolt  36  of  FIG. 1  while the bolt  36  is being tightened, a portion of the clinch nut  40  at one side thereof is cut-away (as illustrated in  FIG. 4 ) such that the cut away side  44  is configured and disposed closely with a section of a rear annular rim  46  of the plate  20 . The annular rim  46  radially and rearwardly extends from the rear side  38  of the plate  20  and has a radius preferably smaller than the radial distance between the hole  34  and the center of the plate  20 . The annular rim  46  is preferably disposed coaxially with the plate  20 .  
         [0027]     The plate  20  further includes a coaxially disposed rear annular rim  48  extending axially and rearwardly from the rear side  38  thereof. The diameter of the annular rim  48  is smaller than the diameter of the annular rim  46 . The inner surface of the annular rim  48  is snuggly fit on a flange  50  of the fan IBR assembly  16  as shown in  FIG. 1 , to center the position of the mounting plate  20  when the mounting plate  20  is mounted to the forward end of the main shaft  12  The central aperture  24  of the mounting plate  20  has a diameter greater than the diameter of the main shaft  12  such that there is a clearance between the mounting plate  20  and the main shaft  12  when the mounting plate is mounted to the forward end of the main shaft  12  and is centered by the annular rim  48  surrounding the annular flange  50  of the fan IBR assembly  16 .  
         [0028]     As illustrated in  FIG. 1 , the mounting plate  20  when mounted on the forward end of the main shaft  12 , is secured thereto by the fan retaining nut  52  which engages threads on the forward end of the main shaft  12  and is tightened to axially press the mounting plate  20  against the radial wall of the annular flange  50  of the fan IBR assembly  16 . Thus, the mounting plate  20  is axially restrained between the fan retaining nut  52  and the IBR assembly  16 . As a further advantage of the present invention, the mounting plate  20  acts as a washer to prevent damaging the fan IBR assembly  16  when torque is applied to the fan retaining nut  52  to tighten same.  
         [0029]     Referring to  FIGS. 1, 5  and  6 , the nose cone  18  generally includes a hollow conical configuration contoured as a smooth convergent extension of the fan IBR assembly  16  when the nose cone  18  is mounted to the mounting plate  20 . The nose cone  18  defines, for example, three recesses  54  on the front and outer side thereof, and forms a small portion of a radial surface (not indicated) as the bottom of each recess  54 . The recesses  54  are disposed in accordance with the respective holes  34  in the mounting plate  20  so that a mounting hole  56  axially extending through the bottom of each recess  54  aligns with a corresponding mounting hole  34  in the mounting plate  20 . The mounting bolt  36  can be inserted through the aligned mounting hole  56  in the nose cone  18  and the mounting hole  34  in the mounting plate  20 , further into the attached clinching nut  40 , and can be tightened to threadedly engage the clinching nut  40 . The recess  54  provides a spaced for placing a tool to tighten the bolt  36  and the radially extending bottom surface of the recess  54  provides a flat base for the head of the mounting bolt  36  to abut.  
         [0030]     The nose cone  18  defines a first outer annular radial surface  58  on the rearward and inner side thereof for contacting or being in a close relationship with the fan IBR assembly  16  such that the nose cone  18  covers the front end of the fan IBR assembly  16 , thereby forming a smooth annular inner wall of the airflow inlet at the opening  22  of the engine casing  14 . The nose cone  18  at the rearward end inner side thereof further includes a second annular radial surface  60  and an annular rim  62 . A radial top surface of the annular rim  62  is preferably disposed in a radial plane determined by the second annular radial surface  60  such that both the radial top surface of the annular rim  62  and the second annular radial surface  60  abut the forward side of the mounting plate  20  at the outer annular portion  30  when the nose cone  18  is mounted to the mounting plate  20 . The annular rim  62  includes an inner diameter corresponding to the outer diameter of the annular rim  26  of the mounting plate  20 , and the annular rim  62  of the nose cone  18  is snuggly fitted around the rim  26  of the mounting plate  20  to center the nose cone  18  with respect to the main shaft  12 .  
         [0031]     An annular recess  64  is defined between the second annular radial surface  60  and the annular rim  62  with the mounting holes  56  extending axially through the bottom of the annular recess  64 . The annular recess  64  is configured to correspond with the position of the holes  32  of the mounting plate  20  (see  FIG. 2 ), and to accommodate one or more of the standard fasteners  33  which are selectively engaged in one or more of the tapped holes  32  for a rotational balance adjustment of the main shaft  12 . The annular recess  64  is further preferably dimensioned to restrain axial movement of the fasteners  33  when the fasteners  33  are engaged in the tapped holes  32  of the mounting plate  20 .  
         [0032]     During a balance adjustment of the main shaft  12 , the mounting bolts  36  are removed and the nose cone  18  is dismounted from the mounting plate  20 . The engine  10  is then started for observation of any rotational imbalance of the main shaft  12  of the engine, which is well known in the art and will not be further described in detail. When the magnitude of the imbalance and the angular direction of imbalance of the main shaft  12  are observed, one or more of the tapped holes  32  are determined as balance weight attachment points and the amount of weight to be added is also determined.  
         [0033]     The next step is to select one or more standard fasteners  33  to act as the balance weights for engaging in the determined one or more tapped holes  32 . The standard fasteners  33  preferably have identical diameters and threads but different lengths. Selection of appropriate lengths of the standard fasteners  33  will provide a match of the amount of balance weights which has been determined.  
         [0034]     A further step of shaft balance adjustment is to access the mounting plate  20  through the front opening  22  of the engine casing  14  for installing and affixing the selected one or more standard fasteners  33  in the determined one or more tapped holes  32 . An annular cavity (not indicated) defined within the fan IBR assembly  16  is disposed behind the mounting plate  20  in order to accommodate a rear section of the selected one or more standard fasteners  33 , which extends out from the rear side of the mounting plate  20 , regardless of the selected length of the standard fasteners  33 . The selected one or more standard fasteners  33  are engaged in the determined one or more tapped holes  32  by means of threads. Nevertheless, it is preferred to apply adhesive to the selected one or more standard fasteners  33  and/or the determined one or more tapped holes  32  in order to provide additional retention of the fasteners  33  in the holes  32 .  
         [0035]     When the main shaft  12  is balanced, the nose cone  18  is placed back in position to cover the mounting plate  20  and the entire front end of the fan IBR assembly  16 , and is then secured to the mounting plate  20  by, for example, the three mounting bolts  36 . Each of the mounting bolts  36  extends axially through the aligned mounting holes  56  in the nose cone  18  and the mounting hole  34  in the mounting plate  20 , and is engaged with the clinch nut  40  by means of threads when the mounting bolt  36  is tightened. The bottom of the annular recess  64  is in direct contact or in a close relationship with the heads of the respective mounting fasteners  33  to provide further additional retention of the fasteners  33 .  
         [0036]     Modifications and improvements to the above-described embodiment of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. For example, the mounting bolts  36  can be engaged within the mounting holes  34  of the mounting plate  20  by threads defined directly in the holes rather than the clinch nut  40  attached thereto. The clinch nut  40  attached to the hole  34  is restrained to prevent rotation by a section of the rim  46 , in this embodiment. However, it can be restrained by any other means well known in the art, such as a pin or other well known stop member affixed to the mounting plate  20  and contacting the clinch nut  40  in a manner such as to prevent rotation thereof. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.