Abstract:
In an assembly with two concentric shafts, the internal shaft projecting beyond the external shaft, a seal support sleeve placed between the shafts has a conical shape and bears on a spacer mounted on the external shaft between an assembly nut of the forward end of the external shaft and a bearing, so that the nut is accessible to be unscrewed through the space between the two shafts, thus enabling the external shaft to be disassembled. The invention may be applicable to front bearings of gas turbine high pressure shafts and makes it possible to disassemble the high pressure body from the rear, without needing to disassemble equipment, such as a fan on the front of the machine, or hindering the installation of a gear.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This document is a continuation of U.S. application Ser. No. 10/476,372, filed Mar. 24, 2004 now abandoned, which is a National Stage of PCT Application No. PCT/FR02/01506, filed on May 2, 2002. This application is also based upon and claims the benefit of priority from the prior French Patent Application No. 01/05900, filed May 3, 2001. The contents of U.S. application Ser. No. 10/476,372 are incorporated herein by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an assembly for the installation of two coaxial shafts, particularly a so-called low pressure shaft as shown in  FIG. 1 , connecting the low pressure compressor  1  to the low pressure turbine  2  in a gas turbine, surrounded by a high pressure shaft connecting the high pressure compressor  3  to the high pressure turbine  4 . Throughout this specification, the front part of the machine is at the left-hand part of the drawings and the rear part of the machine is at the right-hand part. 
   2. Description of Related Art 
   In this type of machine, the internal shaft  15  (low pressure) projects beyond the front of the external shaft  16  (high pressure) and can finish at a shaft end  5  supporting a large volume fan  6  on which large radius blades are fitted to discharge air into an auxiliary stream  7  surrounding the main gas stream  8 . If there is an accidental breakage in this fan, there will be a severe out-of-balance mass on the internal shaft  15 . Thus, some arrangements need to be made. Firstly, a bearing  9 , also referred to as the number  1  bearing, right at the front of the internal shaft  15 , may be made fusible or breakable; in other words, it may be installed on a low resistance support  10  that is designed to break as soon as an out-of-balance mass develops, so that the resulting large forces that are then generated are not transmitted to the rest of the stator structure. The shaft end  5  is then free to tilt by moving under the effect of the out-of-balance mass until the assembly that it forms with the fan  6  moves into a new equilibrium position in rotation, without necessarily causing any further damage until the machine stops. The shaft end  5  tilts around another bearing  17 , also referred to as bearing number  2 , behind the previous bearing  9 , that supports the back end of the shaft end  5  and a main portion of the internal shaft  15 . This bearing  17  controls the axial position of the internal shaft  15 . 
   A take-off gear  20 , designed particularly to start the machine and activated by a transmission shaft  25  perpendicular to the shafts  15  and  16 , must be placed between this bearing  17  and still another bearing  18 , also referred to as bearing number  3 , that provides support to the forward end of the external shaft  16 . The bearing  17 , being as far backwards as possible to improve the dynamic resistance of the internal shaft  15 , is only separated from the bearing  18  by just sufficient space to contain the take-off gear  20 . An assembly nut is screwed around the forward end of the external shaft  16 , abutting against the bearing  18  to stop axial movement. This nut (shown in  FIG. 2 ), which extends forwards from the bearing  18  and is disposed just above the take-off gear  20 , is difficult to access, particularly because a seal support sleeve must be placed behind the bearing  17  immediately in front of the nut. The sleeve overlaps the forward end of the external shaft  16  and a portion of the internal shaft  15  to create a seal between them. Disassembly of the high pressure shaft  16  for maintenance is usually only possible after disassembling the machine in front of the nut, and particularly removing the fan  6 , so that the nut can be reached and unscrewed. 
   French patent 2 783 579 describes an assembly by which the nut can be unscrewed from behind by using a special tool inserted into the annular space between the shafts. This is more convenient despite the fact that this space is narrow, since there is then no need for any major disassembly of elements of the machine for removing the high pressure shaft, but in that patent the nut is screwed inside the external shaft whereas it is screwed around the shaft in more conventional designs, which changes the entire design of the forward end of the shaft and can introduce other disadvantages. In the present invention, another solution is proposed to provide easy access from the rear part of the machine to the nut retaining the external shaft  16  without making any major change to the design. A few modifications to parts of the machine surrounding the nut remain necessary, but it will be seen that they are mainly concerned with sealing and take-off devices, for which the design may be altered more easily. 
   BRIEF SUMMARY OF THE INVENTION 
   In its most general form, the representative assembly of the invention comprises coaxial internal and external shafts, a forward part of the internal shaft projecting beyond a forward end of the external shaft, a sleeve surrounding the shafts and supporting a seal associated with each of the shafts, a bearing supporting the external shaft close to the forward end and a bearing retaining nut screwed around the forward end, a spacer with an adjustment part slid around the external shaft and tightened between the nut and the bearing, and a sealing part on which the seal associated with the external shaft rubs, the sealing part being arranged around the nut. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages and other aspects of the invention will be discovered after reading the commentary on the following figures: 
       FIG. 1 , already described, is an overview of a gas turbine; 
       FIG. 2  shows a conventional assembly in which the problems of access to the nut mentioned above were not solved; 
       FIG. 3  shows an assembly conforming with the invention; and 
       FIG. 4  shows a view of another assembly conforming with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a portion of the internal shaft  15 , the forward end of the external a shaft  16 , one end of the (number  2 ) bearing  17 , the (number  3 ) bearing  18 , the assembly nut  19  of bearing  18 , the take-off gear  20 , a crown wheel  21  engaged with the take-off gear  20  and retained around the external shaft  16  and between the nut  19  and the bearing  18 , the seal support sleeve  22 , and its two seals  23  and  24  rubbing on contiguous portions of shafts  15  and  16 . The environment of the take-off gear  20  comprises: a transmission shaft  25  connected in rotation to the take-off gear  20  by splines  26  and engaged in a hub  27  of the take-off gear  20 ; a pair of bearings  28  and  29  engaged around the hub  27 ; a housing  30  screwed to an extension  31  of the sleeve  22 , that connects (at the right in the figure) to a casing  32  of the stator; a shell  33  screwed to the housing  30  and in which the bearing  29  remote from take-off gear  20  is retained while the bearing  28  closer to the take-off gear  20  is retained on the hub  27  and slides in the housing  30 ; collars  34  and  35  of the bearing  28  and the shell  33  containing the housing  30  are screwed to it to keep the bearings  28  and  29  in a fixed position along the hub  27 . The attachment screws are not shown to simplify the drawing. In this case, the bearing  28  is a ball bearing and bearing  29  is a roller bearing, without an internal race and in which the rollers bear directly on the hub  27 . 
   It is worthwhile mentioning the method of fitting this assembly onto the take-off gear  20  after the nut  19  has been screwed around the external shaft  16 . The take-off gear  20 , the bearing  28  closer to it, and the housing  30  are installed by fixing the housing  30  onto the casing  32 , the shell  33  and the bearing  29  are then installed by fixing the shell to the housing  30  and the seal support sleeve  22  is fixed to the housing  30  by sliding it backward. Finally the transmission shaft  25  is inserted in the hub  27 . It is seen that the nut  19 , well surrounded by other parts, has become inaccessible unless major disassembly operations are carried out. Since the seal  24  rubs on a lip  14  of the external shaft  16  that extends forwards from the nut  19  and the nut  19  is screwed around the external shaft  16 , the seal support sleeve  22  completely separates the nut  19  from the annular space  13  between the shafts  15  and  16  through which the nut  19  could be unscrewed from the rear of the machine with the dedicated conventional tool comprising claws. 
     FIG. 3  illustrates a first embodiment of the invention. The description will be made by comparison to  FIG. 2 , only the alterations therefrom being indicated.  FIG. 3  firstly shows a modified seal support sleeve is identified as element  38  having a conical shape and the seal, now identified as element  39 , of the external shaft  16  now having a larger diameter than the (unchanged) seal  23  of the internal shaft  15 ; the seal  39  does not actually rub on the external shaft  16 , but rather on a sealing contact surface  41  of a spacer  40  assembled to the external shaft  16  and provided with a thrust contact surface  42  adjusted around the external shaft  16  and compressed between the nut  19  and the crown wheel  21  by the former. 
   The result of this arrangement is that the sealing contact surface  41  and the sleeve  38  surround the nut  19  instead of extending forward therefrom, thus the nut  19  becomes accessible from the rear part of the machine. The dedicated tool can be slid in the annular space  43  between the shafts  15  and  16  to reach the nut  19  which is at the forward end of the shaft  16 , the frontward lip  14  having been replaced by the surrounding sealing contact surface  41 . For the sake of completeness, it has been shown a conventional locking part  44  of the nut  19  comprising crab teeth penetrating into the nut  19  and the external shaft  16  to prevent any relative rotation of the nut  19  and the external shaft  16  when it is installed; the locking part  44  is in the form of a split ring made of an elastic material so that it can be opened during assembly. 
   Some arrangements near the take-off gear  20  are implemented to adapt the design to the modified shape of the seal support sleeve  38 . There are still the two bearings  28  and  29  for supporting the hub support  27  of the take-off gear  20  and the housing  30  fixed to the extension  31  of the seal support sleeve  38  and to the casing  32 , but now the shell  33  is replaced by a different shell  45  that extends around the two bearings  28  and  29  and still comprises the collar  35  for attaching to the housing  30 . 
   The assembly is different from the assembly in  FIG. 2 . After the housing  30  containing the take-off gear  20  fitted with the closer bearing (in this case the roller bearing  29 ) has been fixed, the take-off gear  20  lying on a bottom surface of the housing  30 , the seal support sleeve  38  is slid backwardly above the gear  20  and fixed to the housing  30 . The shell  45  fitted with the bearing remoter to the take-off gear  20  (in this case the ball bearing  28 ) is then inserted into the reaming in the housing  30  and around the hub  27  and the bearing  29 . The bearing  28  supports a spacer ring  51  that finally touches the other bearing  29  during this movement, pushes it up and lifts the take-off gear  20 . When the collar  35  stops in contact with the housing  30 , the take-off gear  20  meshes with the crown wheel  21 . The transmission shaft  25  is then finally installed. 
     FIG. 4  represents a variant in which bearings  28  and  29  occupy the same positions as in the  FIG. 2  design and in which the shell  33  of  FIG. 2  is used, but the collar  34  of the bearing  28  placed on a rim of the housing  30  is replaced by a collar  46  placed under the rim and the collar  35  of the shell  33  contacts the lower face of the collar  46 . The assembly is made approximately in the same way as in  FIG. 3 , except that the bearings  28  and  29  are both installed after the take-off gear  20  has been put in the housing  30 . 
   The innovation of the assembly according to the invention can be explained as follows. The access to the nut  19  from the front, which was the only possible way in conventional designs, required sliding the seal support sleeve  22  in  FIG. 2  forward, separating it from the housing  30 . This movement of the seal support sleeve in the forward direction is no longer useful with the invention, and it is even impossible since the flared rear part of the seal support sleeve  38  would interfere with the teeth of the take-off gear  20 . Thus, it would also be a problem to assemble the housing  30  if the seal support sleeve  38  were moved forwards, since its raised front edge would be stopped in contact with the flared rear part of the housing. Therefore in the embodiment of  FIG. 3  the seal support sleeve  38  is pulled backward during assembly of the housing  30  and the take-off gear  20 ; its attachment flange  48  stops in contact with the housing  30  at the back, unlike in  FIG. 2 . The assembly does not cause any difficulty provided that the take-off gear  20  can be lowered to the bottom of the housing  30  so that it does not collide with the attachment flange  48  or other parts of the seal support sleeve  38  before it is in its final position, which is possible if the shell  45  below the housing  30 , assembled to the housing  30  with an ascending movement, supports both bearings  28  and  29  so that the take-off gear  20  may be lowered below the seal support sleeve  38  as soon as the shell  45  is disassembled. 
   The assembly in  FIG. 3  with a single shell  45  for the bearing  28  providing the take-off gear  20  with thrust support and the bearing  29  sliding in the vertical direction, is simpler and probably more rigid, but it makes it necessary to invert the conventional positions of the bearings  28  and  29 , and assembly can be more complicated due to the larger size of the shell  45 . This is why the assembly in  FIG. 4 , in which the main difference from the prior art is the position of the collar  46 , has merits.