Abstract:
This blade attachment device comprises a shell ( 28 ) installed around an outer collar ( 23 ) of the pivot ( 15 ) of the blade ( 27 ) and capable of rotating by an angle sufficient for cleats ( 30, 31 ) to close the ends of the pivot groove and retain the root of the blade ( 27 ) in it, and mobile shims that cooperate to retain the root in the groove. 
     Application to aircraft engines.

Description:
BACKGROUND OF INVENTION 
     The subject of the invention is a device for fastening a variable pitch blade. 
     Many propeller blades have a variable pitch to optimise their operation. The blade is then extended by a pivot rotating in a housing in the propeller hub, or more generally in a support part. The blade and the pivot are often separate parts, the blade being composed of a composite material to save weight, the pivot remaining metallic to resist the forces applied to it. In a number of designs, the pivot is then built into the blade which is moulded over it, and the two parts are therefore inseparable. This is satisfactory in some aspects, but it has the disadvantages that manufacturing is more complicated and that the blade is thicker because it needs to surround the pivot; and continuity between the bottom of the blade and the support part in which the pivot is housed is not good, which creates aerodynamic losses. Other designs consist of constructing the blade and the pivot separately, and holding them together by a dovetail. 
     One known design of this type is shown in  FIGS. 1 and 2  (or in U.S. Pat. No. 5,263,898). A propeller hub  1  comprises a circular housing in which a pivot  2  is housed, to rotate a blade  3 . A groove  4  with a dovetail section and a root  5  of the blade  3  with a corresponding section are shown. Turning is possible due to the roller bearings  6  and the thrust bearings  7  between the hub  1  and the pivot  2 . The root  5  of the blade  3  is held in place in the groove  4  firstly by a shim  8  inserted in the groove  4  and secondly by a shim  11  held in place by bolts  12  extending in the pivot  2  parallel to the groove  4  and on each side of it. The bolts  12  are also used to hold the shim  11  in contact with spring  61  positioned in the groove  4  under the root  5  and the function of which is to hold the blade  3  radially in place when the centrifugal force is not sufficient to keep the root  3  in contact with the groove  4 , particularly when stopped. The spring  61  is slightly curved at rest and the bearing of the shim  11  applies a deformation curvature on it which holds the blade  3  in position. A toothed ring  60  is held in place around the pivot  2  mainly by a locking sector  13  that closes the ring to which it is screwed, and which is also held in place by bolts  12 , supporting the shim  11  from behind. In this design, the pivot  2  may be easily provided with an upper circular platform  14 , that projects in the radial direction beyond the main portions of the pivot and the function of which is to restore continuity of the boundary of the air stream. 
     However, this design has the disadvantage that it requires a large number of parts to hold the blade in the groove. One purpose of the invention is to propose an improvement and thus to facilitate attachment, disassembly or replacement of the blade, and saving weight. 
     BRIEF SUMMARY OF INVENTION 
     In one general definition, the invention relates to a variable pitch blade device comprising a blade, a pivot with an upper groove in which a root of the blade is mounted, and means of retaining the root of the blade in the groove, characterised in that said means comprise shims inserted in the groove and that stop in contact with the blade root, and a shell inserted around the pivot and provided with attachment points to the pivot, and facing cleats closing the groove. 
     The shell is the essential element of the invention; all that is necessary is to slide it around the pivot and fix it to the pivot so that the shell will close the ends of the groove and prevent movements of the blade in both directions, cooperating with the clamping shims which are placed in the groove without needing to be fixed to it. In prior design, the ring is discontinuous and is interrupted in front of the groove and the locking sector must be assembled with it to close the groove; and it does not participate in holding the blade on the other side either, since it is separated from the shim  11 ; it does not have any of the cleats on the shell according to the invention. 
     The shell may be approximately circular in shape provided with a flat on which one of the cleats is installed. 
     The shims may include a retaining shim preventing the root of the blade from sliding in the groove, and a blade root support shim placed on the bottom of the groove, which is a particularly simple arrangement to stop and complement movements of the blade root in the groove. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other aspects of the invention will now be described in more detail with reference to the following figures: 
         FIGS. 1 and 2  already described show a top view and a diametric sectional view of a blade attachment device according to prior art; 
       and the other  FIGS. 3 to 10  illustrate the invention; 
         FIG. 3  shows the isolated pivot, 
         FIG. 4  the assembly of the blade root and the shell on the seal in its condition before assembly, 
         FIG. 5  the assembled state of these parts, 
         FIG. 6  a locking shim, 
         FIG. 7  the assembly of the shims, 
         FIG. 8  the half-platforms covering the joint, 
         FIG. 9  the assembly with the half-platforms and 
         FIG. 10  the isolated shell. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The pivot appears isolated in  FIG. 3 ; it is denoted reference  15  and it comprises a main part  16  (only schematically shown, because the invention does not apply to it), and an upper part  17  through which the blade is attached and which includes the groove  18 . Furthermore, two opposite tabs  19  and  20  in line with each other stand on each side of the groove  18 , on side plates  21  and  22 . The contour of the largest part of the upper part  17  is circular and forms a collar  23 . However, there is a chase  24  formed on one side of the groove  18 , and the upper parts of its lips that delimit it form overhangs  25 ; notches  26  facing each other are formed in the inner edges of the overhangs  25 , thus being arranged beyond the bottom of the groove  18 . 
     Two other elements of the assembly are shown in  FIG. 4 , which we will now describe. The root of a blade  27  is slid into the groove  18 , and a shell  28  is inserted around the collar  23 . The blade  27  (also shown partially) and its root are conventional. The shell  28  comprises an approximately continuous circular hoop  29  in the angular direction of the pivot  15 , two cleats  30  and  31  fixed to the hoop  29  at diametrically opposite positions of it, and two attachment tabs  32  and  33  themselves fixed to the hoop  29  at diametrically opposite positions from each other but perpendicular to the positions of the cleats  30  and  31 . The cleats  30  and  31  and the attachment tabs  32  and  33  rise above the hoop  29 , and the tabs  32  and  33  project radially inwards, while the cleats  30  and  31  do not. 
     The shell  28  can rotate around the pivot  15 , the hoop  29  is sliding on the collar  23 . Since the tabs  19  and  20  of the pivot  15  extend perpendicular to the groove  18 , the state that can be obtained is described below. An appropriate rotation of the shell  28  forces the tabs  32 ,  33  into contact with the bearing faces of the tabs  19  and  20  respectively of the pivot  15 , while the cleats  30  and  31  stand out facing the ends of the groove  18  to close them. This prevents the blade  27  from coming out of the groove  18 . The tabs  19 ,  20 ,  32  and  33  are provided with drillings that are in line with each other when one tab is placed on the other, so that the shell  28  can be fixed to the pivot  15  by bolts  34 , as shown in  FIG. 5 . It can be seen that this assembly system is much simpler than the assembly system shown in  FIGS. 1 and 2 . Other features of the embodiment will now be described. 
     Two shims are added to the device to hold the blade  27  in the groove  18  of the pivot  15  at a determined position. They are shown in  FIGS. 6 and 7 . One is a locking shim  35  designed to hold the root of the blade  27  and prevent it from sliding in the groove  18 . It is usually plane with a polygonal section, and it is inserted into the notches  26 . It comprises a bearing face  36  to support the root of the blade  27  in an upper part, and a drilling  37  in a lower part. This locking shim  35  is composed partly of a rigid material (metallic or composite) and partly of a deformable material (for example at the bearing face  36 ), so that the locking shim  35  can absorb energy during impact of a foreign body on the blade  27  and therefore reduce the forces applied to the root of the blade  27 . The drilling  37  is designed to hold the second shim which is a support shim  38  for the root of the blade  27  placed at the bottom of the groove  18 . It is in the shape of a flat rod. Its end passes through the drilling  37 , and it comprises a hook  39  curved downwards and towards the pivot  15 , which extends under the locking shim  35  thus surrounding a tab  52 . The shims  35  and  38  are free parts, with no assembly means such as bolts to other parts and that are held in place only by blocking, which increases the required simplicity of the design. 
     With reference once again to  FIGS. 4 and 5 , it can be seen that the sizes of the cleats  30  and are unequal, and that the radius of hoop  29  is larger at the location of the smallest cleat  30 , where the hoop is formed into the shape of a flat  40  at the end of straight extension portions  41 . These arrangements make it possible to complete the assembly. The shell  28  is inserted onto the collar  23  in the angular position shown in  FIG. 5  corresponding to assembly, in the absence of the blade  37  and the shims  35  and  38 . The flat  40  extends beyond the projections  25  due to the straight portions  41 , which makes this insertion operation possible. The shell  28  is then turned slightly to put it into the position shown in  FIG. 4 , in which accesses to the groove  18  are clear. The locking shim  35  is slid from below into the notches  26 , the support shim  38  is inserted into the drilling  37 , and then the root of the blade  27  is inserted into the groove  18 , sliding on the locking shim  35  until stopping in contact with the locking shim  35 . An inverse rotation of the shell  28  brings the small cleat  30  immediately in front of the end of the locking shim  38 , and the large cleat  31  in front of the opposite opening on the groove  18 . The root of the blade  27  is then held in groove  18  with little clearance between the cleat  31  and the locking shim  35  that are at fixed positions, and little clearance also in the direction of the pivot axis because the support shim  38  holds it in contact with the top of the groove  18 . The support shim  38  prevents the locking shim  35  from dropping, and the cleat  30  bearing on the support shim  38  prevents its extraction outside the groove  18  even when it only occupies a small part of the section of the groove  18 . 
     The device also comprises two approximately semi-circular half-platforms  42  and  43  and each fitted with a lower tab  44 ,  45 . They are shown in  FIG. 8 . The assembly of the device is completed by installing bearing faces for tabs  44  and  45  on the tabs  19  and  20  respectively, before installing the bolts  34 , the tabs  44  and  45  also being provided with drillings, in line with the previous drillings. The half-platforms  42  and  43  are similar with in particular circular outer edges and  47  forming part of a single circle in the assembled state, but different inner edges  48  and  49  so that they can adjust to the intrados and extrados respectively of the blade  27 . It is thus possible to maintain continuity of the gas flow stream around the blade  27 . This also explains why the half-platforms  42  and  43  are slightly curved inwards and their thickness is not uniform. 
     The completely assembled device is shown in  FIG. 9 . The isolated shell is shown in  FIG. 10 .