Patent Publication Number: US-6910866-B2

Title: Controlling the axial position of a fan blade

Description:
FIELD OF THE INVENTION 
     The invention relates to a system for controlling the axial position of removable blades in a turbojet fan. 
     More precisely, the invention relates to a fan rotor comprising a disk having a rim with a plurality of substantially axial grooves that are regularly spaced apart angularly, a plurality of removable blades extending radially outwards from the periphery of said disk, each blade having a blade root received in a respective groove, a downstream flange plate secured to said disk with the downstream faces of the blade roots being in abutment there against, and a removable upstream flange plate secured to said disk for the purpose of retaining the blade roots in the grooves. 
     In the present text, the “upstream face” designates the face facing towards the front of the engine and the “downstream face” designates the face facing towards the rear. 
     BACKGROUND OF THE INVENTION 
     The upstream flange plate of a fan rotor is generally removable so as to enable a blade to be replaced in the event of damage. This can occur in the event of foreign bodies, such as birds, being ingested, in particular during the takeoff and approach stages of an aircraft fitted with turbojets. 
     For assembly reasons, axial clearance is included in the connection between a blade and the disks. The random axial position of a fan blade within this clearance unbalances the rotor and generates vibration in operation. This is particularly true for fan blades that are of large chord. 
     The large centrifugal forces to which blades are subjected in operation can also lead to blades being moved axially in random manner within said assembly clearance. 
     U.S. Pat. No. 4,033,705 discloses a system for retaining blades axially, which system comprises, at each end of a blade root, a tenon which co-operates with two oblique mortises arranged in the rim of the disk on either side of the groove receiving the blade root. The tenons provided on each face of the disk are held radially by a ring secured to the disk. Between each end face of the blade root and the adjacent tenon, that document provides for spring-forming sheet metal acting to prevent the tenon from moving radially while the blades are being assembled, prior to the retaining ring being put into place and fastened. Those springs create assembly clearance at each end of a blade, as can be seen in  FIG. 7  of that document, which clearance is not under control. 
     U.S. Pat. No. 5,282,720 provides for interposing honeycomb elements between the retaining flange plates and the ends of the blade roots, the honeycomb elements serving to absorb a portion of the energy in the event of birds being ingested. During ingestion, the honeycomb deforms progressively, thereby dissipating a portion of the energy. The honeycomb elements must retain their integrity during assembly of the flange plates, and as a result there inevitably exists some axial clearance after assembly, thereby making random axial displacement possible while the fan is in operation. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the invention is to propose a fan rotor as described in the introduction in which the axial position of the blades is controlled by a device that is simple and of low cost, without leading to problems when assembling or removing blades in the event of performing repairs. 
     According to the invention, this object is achieved by the fact that the upstream flange plate is fitted on its downstream face with resilient means for exerting sufficient force on the upstream faces of the blade roots, after assembly, to prevent any axial displacement of the blades during normal operation of the engine. The term “normal” operation of the engine is used to mean operation apart from exceptional events, of the type where foreign bodies are ingested or blades are lost. 
     Thus, once the rotor has been assembled, these resilient means exert sufficient force on the blade roots to ensure that they are held positively pressed against the upstream flange plate, thereby eliminating any axial assembly clearance regardless of the manufacturing tolerances of the blades. 
     Advantageously, that resilient means specific to each blade root are provided. These independent resilient means are preferably constituted by elastomer pegs held in respective orifices formed in the flange plate. 
     When the fan rotor further includes a spacer interposed between each blade root and the bottom of the corresponding groove, said spacer including a radially-extending lug pressing against the upstream face of said blade root, the resilient means advantageously bear against said lugs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and characteristics of the invention appear on reading the following description given by way of example and with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic section of a fan rotor; 
         FIG. 2  is a detailed view of the disposition of the upstream flange plate and a blade, showing the assembly clearance; 
         FIG. 3  is a view similar to  FIG. 2 , showing the clearance taken up by a peg; 
         FIGS. 4A and 4B  are perspective views of a peg; and 
         FIG. 5  is a perspective view of the downstream face of the upstream flange plate fitted with its pegs. 
     
    
    
     MORE DETAILED DESCRIPTION 
       FIG. 1  shows a fan rotor  1  of axis X comprising a disk  2  and a plurality of blades  3  regularly spaced apart angularly around the axis X and extending radially outwards from the periphery of the disk  2 . Each blade  3  has a blade root  4  which is retained in a groove or slot  5  that extends substantially axially in the rim of the disk  2 . The blade roots  4  are of dovetail shape and the grooves  5  are of complementary shape. Under the action of centrifugal forces, when the fan is in operation, said blade roots  4  are held in the grooves  5  by the radially outer walls of the grooves  5 . 
     A spacer  6  is disposed between the base of each blade root  4  and the bottom of the groove  5 . At its end adjacent to the upstream face  7  of the disk  2 , the spacer  6  has a lug  8  which bears against the upstream face  9  of the root  4  of the blade  3 . The function of the spacer  6  is to prevent the blade  3  from dropping into the groove  5  when the fan is stationary, and to absorb a portion of the energy in the event of an impact against the blade, e.g. following ingestion of a foreign body, or breakage of an adjacent blade. 
     On the downstream face  10  of the disk  2  there is secured a downstream flange plate  9  against which the roots  4  of the blades  3  come into abutment. 
     An upstream flange plate  12  is also provided on the upstream face  7  of the disk  2  for the purpose of preventing the roots  4  of the blades  3  from sliding out from the grooves  5  once the rotor  1  has been assembled. The upstream flange plate  12  is fixed to flange pieces of the disk  2  by means of bolts, so that it can be removed, should that be necessary in order to replace a blade  3 . 
     In  FIG. 2 , it can be seen that clearance J exists between the lug  8  and the downstream face  13  of the upstream flange plate  12 . The root  4  of the blade  3  can therefore move through an axial distance J in its groove  5 , and that can lead to the rotor  1  becoming unbalanced, thereby generating vibration in operation. 
     In accordance with the invention, the upstream flange plate  12  has an orifice  15  in front of each blade root  4 , which orifice holds the shank  16  of an elastomer peg  17 , the peg  17  having a head  18  of diameter greater than that of the orifice  15 , and of thickness not less than the clearance J. As shown in  FIG. 3 , the head  18  of the peg  17  is placed on the downstream face  13  of the upstream flange plate  12 , and said head  18  is compressed between the downstream face  13  of the upstream flange plate  12  and the upstream face of the lug  8  of the spacer  6 . 
     The peg  17  thus exerts elastic forces on the lug  8  in an axial direction. Since the lug  8  is pressed against the adjacent blade root  4 , the root is continuously urged towards the downstream flange plate against which it remains in abutment. This simple disposition serves to control the axial clearance of the blades  3  and to prevent the blades  3  from moving axially in random manner. 
       FIGS. 4A and 4B  are perspective views of the peg  17 , and  FIG. 5  is a perspective view of the downstream face  13  of the upstream flange plate  12 . 
     The pegs  17  are naturally engaged in the upstream flange plate  12  prior to the flange plate being assembled to the disk  2 . Reference  20  designates an orifice formed in the upstream flange plate  12  for the purpose of enabling it to be fixed to the flange pieces of the disk  2  by means of bolts. Tightening these bolts causes the heads  18  of the pegs  17  to exert sufficient compression force to prevent any subsequent axial displacement of the blades  3 , as a function of the difference between the thickness of the heads  18  when at rest and the original assembly clearance J. This compression force may vary from one blade to another depending on the manufacturing tolerances of the blades  3  and of the pegs  17 .