Abstract:
In an assembly with two concentric shafts, one of which projects beyond the other, a seal support sleeve placed between them has a conical shape and bears on a spacer of the external shaft, so that an assembly nut of the forward end is accessible to be unscrewed from the back 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 behind, without needing to disassemble equipment, such as a fan on the front of the machine, or hindering the installation of a gear.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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 .  
         [0003]     2. Description of Related Art  
         [0004]     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 , and 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  called 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 breaks 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 position in equilibrium in rotation and without necessarily causing any further damage until the machine stops. The shaft end  5  makes this movement tilting around a so-called bearing number  2  behind the previous bearing, that supports its back end and a main shaft in the internal shaft  15 . This bearing is denoted reference  17 . It thus controls the axial position of the internal shaft  15 .  
         [0005]     A movement control 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 a so-called bearing number  3  (reference  18 ) that provides support to the forward end of the external shaft  16 . The previous shaft bearing  17 , being as far backwards as possible to improve the dynamic resistance of the internal shaft  15 , is only separated from it by just sufficient space to contain the gear  20 . An assembly nut is screwed around the forward end of the external shaft  16  to hold it in place against this bearing  18  by stopping in contact on the bearing. This nut, that extends forwards from the bearing  18  and on the movement control 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, overlapping 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 , before maintenance, is usually only possible after disassembling the machine in front of the nut, and particularly the fan  6 , so that it can be reached and unscrewed.  
         [0006]     Refer to French patent 2 783 579 for a description of an assembly by which the nut can be unscrewed from behind using a special tool inserted into the annular space between the shafts, which 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 to be able to remove the high pressure shaft; but the selected assembly consists of screwing the nut in the external shaft whereas it is conventionally screwed around the shaft, which changes the entire design of the forward end of the shaft and it can introduce other disadvantages. In this case, another solution is proposed to provide easy access to the nut holding the external shaft  16  in place from behind without making any major change to the design; the narrow space around the nut makes a few modifications to surrounding parts of the machine necessary, but it will be seen that they are mainly applicable to sealing and movement control devices, for which the design is less restrictive.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     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, characterized in that it comprises a spacer with an adjustment part slid around the external shaft and tightened between the nut and the bearing, and a sealing part arranged around the nut and on which the seal associated with the external shaft rubs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The advantages of this arrangement and other aspects of the invention will be discovered after reading the commentary on the following figures:  
         [0009]      FIG. 1 , already described, is an overview of a gas turbine;  
         [0010]      FIG. 2  shows an_assembly prior to the invention and in which the problems of access to the nut mentioned above were not solved;  
         [0011]      FIG. 3  shows an assembly conform with the invention; and  
         [0012]      FIG. 4  shows a view of another assembly conform with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 2  shows a portion of the internal shaft  15 , the forward end of the external shaft  16 , one end of the number  2  bearing  17 , the number  3  bearing  18 , the assembly nut  19  of bearing  18 , the movement control gear  20 , a crown wheel  21  engaged with the gear  20  and retained between the external shaft  16 , the nut  19  and the bearing  18  to participate in the assembly of the bearing  18  as a thrust bearing, the seal support sleeve  22 , its two seals  23  and  24  rubbing on contiguous portions of shafts  15  and  16 ; and the environment of the gear  20  comprises a transmission shaft  25  connected in rotation to the gear  20  by splines  26  and engaged in a hub  27  of the gear  20 ; a pair of bearings  28  and  29  engaged around the hub  27 ; a housing  30  connected by screwing to an extension  31  of the sleeve  22  and (at right in the figure) to a casing  32  of the stator; and a shell  33  screwed to the housing  30  that retains the bearing  29  on the far end from gear  20 , while the other bearing  28  close to the gear  20  is driven by the hub  27  and slides in the housing  30 ; collars  34  and  35  of the bearing  28  close to the gear  20  and the shell  33  are applied in thrust on the housing  30  and 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. The means of retaining the bearings  28  and  29  include nuts, thrust washers, force fitted sleeves, in a known manner. In this case, the bearing  28  is a ball bearing and the other bearing  29  is a roller bearing, without an internal ring and in which the rollers bear directly on the hub  27 .  
         [0014]     It is worthwhile mentioning the method of fitting the assembly onto the gear  20  after the nut  19  has been screwed into place. The gear  20 , the bearing  28  adjacent to it and the housing  30  are installed by screwing the housing onto the casing  32 , and 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 backwards; 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 is screwed around this shaft  16 , the seal support sleeve  22  completely separates the nut  19  from the annular space  13  between the shafts  15  and  16 .  
         [0015]      FIG. 3  illustrates a first embodiment of the invention and firstly shows a modification to the seal support sleeve, which in this case is assigned reference  38  and which has a conical shape, the seal  39  on the side of the high pressure shaft  16  now having a larger diameter than the unchanged seal  23  on the side of the low pressure shaft  15 ; the seal  39  does not rub on the actual shaft  16 , but rather on a sealing contact surface  41  of a spacer  40  integrated into the high pressure shaft  16  and being provided for this purpose with a thrust contact surface  42  inserted on the said shaft and compressed by the nut  19  between it and the crown wheel  21 .  
         [0016]     The result of this is that the sealing contact surface  41  and the sleeve  38  surround the nut  19  instead of covering it at the front and thus it becomes accessible from behind, using a tool with claws not shown that can be slid in the annular space  43  between the shafts  15  and  16 , and in that the nut  19  is at the forward end of the shaft  16 , the lip  14  that it surrounded having been replaced by the sealing contact surface  41 . The reference  44  is assigned to a locking part of the nut  19  that comprises crab teeth penetrating into the nut  19  and the external shaft  16  to prevent any relative rotation when it is put into place; it is in the form of a split ring and is made of an elastic material so that it can be opened to put it into place.  
         [0017]     Some arrangements may be proposed at the side of the gear  20  to adapt the structure to modify the shape of the seal support sleeve  38 ; there are the two bearings  28  and  29  of the hub support  27  of the gear  20 , the housing  30  fixed to the extension  31  of the seal support sleeve  38  and the casing  32 , but in this case the shell  33  is replaced by a different shell  45  that extends around the two bearings  28  and  29 , and still supports the thrust collar  35  forming the attachment to the housing  30 .  
         [0018]     The assembly is different from the assembly in  FIG. 2 . It is made by fixing the housing  30 , and then consists of inserting the gear  20  fitted with the bearing closest to it (in this case  29 ) into the housing  30 , placing it on a thrust surface  50  at the bottom of the housing  30 , sliding the seal support sleeve  38  above the gear  20  and fixing it to the housing  30 . The shell  45  fitted with the bearing on the remote side of the 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 back and thus lifts the gear  20 . When the collar  35  stops in contact with the housing  30 , the gear  20  engages with the crown wheel  21 . The transmission shaft  25  is finally installed.  
         [0019]      FIG. 4  represents a variant of this idea in which bearings  28  and  29  occupy the same positions as in the previous design and in which the shell  33  is also used; but the thrust collar  34  of the bearing  28  placed on the edge of the housing  30  is replaced by a collar  46  placed under it, and the collar  35  of the shell  33  with which it is in contact. The assembly is made approximately in the same way as before, except that the bearings  28  and  29  must both be put into place after the gear  20  has been put into place in the housing  30 .  
         [0020]     The innovation of the assembly according to the invention can be explained as follows. The only way to access the nut  19  from the front, which was the only possible way before, is to slide the seal support sleeve  22  forwards, separating it from the housing  30 . This movement to lift 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 stop in contact with the teeth of the 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, an assembly was adopted in which the seal support sleeve  38  is pulled clear backwards during assembly of the housing  30  and the gear  20 ; its attachment flange  47  stops in contact with the housing  30  at the back, unlike the situation in the previous assembly. The assembly does not cause any difficulty provided that the gear  20  can be lowered to the bottom of the housing  30  so that it does not collide with the attachment flange  47  or other parts of the seal support sleeve  38  before it is in its final position, which is perfectly possible if the bearing  28 , which is a ball bearing designed to support the gear  20  in the vertical direction (or the radial direction in the gas turbine) is fixed to the housing  30  later after having been inserted below the housing, then lifting the gear  20 .  
         [0021]     The assembly in  FIG. 3  with a single shell  45  for the vertical support bearing  28  of the gear  20  and the other bearing  29 , installed free to slide in the vertical direction, is simpler and probably more rigid, but it makes it necessary to change over the conventional positions of the bearings, and assembly can be more complicated due to the larger size of the shell  45 . This is why the more ordinary assembly in  FIG. 4 , in which the main difference in the method of assembly of bearings  28  and  29  is the position of the collar  46 , has many merits.