Abstract:
A device for controlling variable-pitch blades in a turbomachine compressor, including at least one control ring surrounding an external casing of the compressor, and a control shaft connected to the ring and to the rod of an actuator, the body of which is mounted such that it can pivot about an axle borne by a support mechanism of the control shaft so as to guarantee a precise relative positioning of the axis of pivoting of the actuator with respect to the control shaft.

Description:
BACKGROUND 
     The present invention relates to a device for controlling variable-pitch blades in a turbomachine such as an aircraft turbojet or turboprop. 
     A turbomachine of this type comprises one or a plurality of variable-pitch guide blade stages which are mounted between the wheels of a compressor. These variable-pitch blades are carried by the turbomachine stator and are adjustable in position about the axes thereof to optimise gas flow in the turbomachine engine. 
     Each variable-pitch blade comprises a cylindrical pivot at each of the ends thereof, said pivots defining the axis of rotation of the blade. The radially external pivot is engaged in a cylindrical duct of an external annular casing of the turbomachine and is connected by a connecting rod to a ring which is mounted about the casing and is in turn connected by a lever to a control shaft actuated by a cylinder. The control shaft runs parallel to the casing axis and may be connected to one or a plurality of the abovementioned rings to control one or a plurality of guide blade stages. 
     The cylinder rod is connected to the control shaft by a radial arm rigidly connected to the control shaft. The linear movement of the cylinder rod causes the control shaft to rotate about the axis thereof and the or each ring to be driven in rotation about the casing. The rotation of a drive ring is transferred by the corresponding connecting rods to the external pivots of the blades of one stage and rotates same about the axes thereof. 
     DESRCRIPTION OF THE RELATED ART 
     In the prior art, the cylinder is pivotably mounted about an axis parallel with the casing axis, said axis being carried by means, such as a cap, fixed on the casing independently from the support and rotational guidance means of the control shaft. The cylinder pivoting axis is generally in the vicinity of the end of the cylinder, next to the side opposite the movable rod thereof (see for example document U.S. Pat. No. 3,779,665). 
     This type of control device is the subject of recurrent maintenance operations during which the various parts of this device are disassembled and removed from the casing, and refitted on the casing. The production and assembly tolerances of these parts mean that the relative positioning of the cylinder and the control shaft is subject to variation, conveyed by a loss of precision in the variable-pitch blade control. For example, differences in the relative position of the cylinder and the control shaft in the region of approximately several tenths of a millimetre have been observed, which may give rise to imprecision in the angular pitch of the guide blades in the region of several tenths of a degree. 
     BRIEF SUMMARY 
     The aim of the invention is specifically that of avoiding this drawback simply, effectively and economically. 
     For this purpose, it relates to a device for controlling variable-pitch blades in a turbomachine compressor, comprising a control ring rotatably mounted about an external casing of the compressor and to be connected by means such as connecting rods to variable-pitch blades, and a cylinder wherein the body can pivot about a fixed axis and wherein the movable rod is connected to the control shaft by connecting rod means carried by a control shaft guided in rotation by supporting members attached to the casing, characterised in that the pivoting axis of the cylinder body is carried by the control shaft supporting members and by at least one tab attached on the casing. 
     The invention makes it possible to define a precise mounting position of the cylinder pivoting axis in relation to the control shaft and locate this relative position after maintenance disassembly and reassembly operations, while retaining the parallelism between the pivoting axis of the cylinder and the control shaft, regardless of the manufacturing and assembly tolerances of the parts of the control device. 
     In one embodiment of the invention, the cylinder body comprises a first cylindrical radial blade foot guided in rotation in an orifice of a tab rigidly connected to one of the supporting members of the control shaft, and a second cylindrical radial blade foot, diametrically opposite said first blade foot, and guided in rotation in an orifice of a second tab attached on the casing. Said second tab is independent from the supporting members of the control shaft. 
     The second blade foot is advantageously engaged in an eccentric guide ring housed in the orifice of the second tab to compensate for any misalignment between the orifices of the two attachment tabs. Two radially grooved washers are preferably engaged on the eccentric ring and inserted between the second tab and an external annular rim of the ring, one of the washers being rigidly connected in rotation with the tab and comprising radial grooves engaging with complementary radial grooves of the other washer which is locked in rotation on the ring. The washers may be clamped axially between the tab and the annular rim of the ring by an insert attached to the tab and pressing axially on the annular rim of the ring, on the side opposite the tab. 
     In one alternative embodiment, the cylinder body comprises two diametrically opposed radial blade feet defining the pivoting axis of the cylinder body and received in orifices or notches of two tabs of a supporting clamp attached on the casing, each blade foot being further connected by a connecting rod to the control shaft. The clamp and the tabs thereof are independent from the supporting members of the control shaft. Each connecting rod connecting a blade foot to the control shaft may comprise at one end an orifice wherein the blade foot is guided in rotation and at the opposite end thereof an orifice whereby the connecting rod is rotatably mounted on the control shaft. 
     The or each blade foot is preferably integral with the cylinder body, for example by means of foundry. 
     The control shaft is for example supported and guided in rotation by V-shaped supporting members attached to annular flanges of the casing. 
     The control shaft is preferentially connected to the cylinder rod and to the control ring by radial arms with which it is rotatably rigidly connected. It may be connected by two radial arms, with which it is rotatably rigidly connected, to two parallel control rings, for the control of two annular rows of variable-pitch blades. The device may control a greater number of annular rows of variable-pitch blades, for example four. The control shaft is then connected by four radial arms to four control rings of the abovementioned type. 
     The pivoting axis of the cylinder may be situated substantially mid-way from the ends of the cylinder body and in the vicinity of the end of the cylinder body, situated on the side of the movable rod of said cylinder. 
     The invention also relates to a turbomachine compressor, comprising at least one device for controlling variable-pitch blades as described above, as a turbomachine, such as an aircraft turbojet or turboprop, comprising at least one such device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be understood more clearly and other features, details and advantages of the invention will emerge more clearly on reading the following description provided as a non-limitative example and with reference to the appended figures, wherein: 
         FIGS. 1 and 2  are partial perspective schematic views of devices for controlling variable-pitch blades according to the prior art relative to the present invention, 
         FIGS. 3 and 4  and partial perspective schematic views of a control device according to the invention, 
         FIGS. 5 and 6  are partial perspective schematic views of a further control device, 
         FIG. 7  is a partial perspective schematic view of an alternative embodiment of the device according to the invention, 
         FIG. 8  is a view corresponding to  FIG. 7  and represents a further control device, 
         FIG. 9  is a partial perspective schematic view of a further alternative embodiment of the device according to the invention, 
         FIG. 10  is a perspective schematic view of the centring and guiding system in  FIG. 9 , 
         FIGS. 11 and 12  are perspective and side schematic views of radially grooved washers of the system in  FIG. 10 , respectively. 
     
    
    
     DETAILED DESCRIPTION 
     The devices for controlling variable-pitch blades represented in  FIGS. 1 and 2  are each mounted on a substantially cylindrical external casing  10  of a turbomachine compressor such as an aircraft turbojet or turboprop. 
     The casing  10  bears one or a plurality of variable-pitch blade stages, one of which is partially represented in  FIG. 2 . The blades  11  of a stage are evenly distributed about the axis of revolution of the casing  10 , and each comprise a vane connected at the radially external end thereof to a radial cylindrical pivot  12  running along the axis of rotation of the blade and which is engaged in a cylindrical duct  13  of the casing. The radially external end of said pivot is connected to one end of a connecting rod  15  wherein the other end is connected to a control ring  14 ,  14 ′. 
     The control device may comprise one or a plurality of rings  14 ,  14 ′. It comprises two thereof in the example in  FIG. 1  and only one in the case of  FIG. 2 . Each ring  14 ,  14 ′ encompasses the casing  10  and is connected by a lever  16  to a control shaft  18  running substantially parallel with the axis of revolution of the casing. The control shaft  18  is further connected to the movable shaft  22  of a hydraulic, pneumatic or electric cylinder  20 . 
     The control shaft  18  is carried at the ends thereof by two supporting members  24  attached on the casing  10 . Each supporting member  24  in  FIG. 1  comprises a cylindrical housing wherein a bearing  26  for centring and guiding one end of the shaft  18  is received. The members in this case are substantially V-shaped and each comprise two jambs applied and attached, for example using screw-nut means, at the radially internal free ends thereof onto the casing  10 . The housings for receiving the bearings  26  are situated at the junction between the two jambs of the members  24 . The control shaft  18  runs, in the case of  FIG. 1 , along the entire axial dimension of the casing  10 , and the supporting members  24  are attached on annular flanges  27  provided at the axial ends of the casing. 
     Each lever  16  is hinged at a first end on an axis  28  carried by a cap  30  of the corresponding ring  14 ,  14 ′. The second end of the lever  16  is hinged on an axis  32  carried by a radial arm  34  rotatably rigidly connected to the control shaft  18 . The movable rod  22  is further hinged on an axis  35  carried by a further radial arm  36  rotatably rigidly connected to the control shaft  18 . The axes  28 ,  32  and  35  are parallel with each other and the casing axis. 
     The radial arms  34  and  36  are angularly offset in relation to the others about the axis of the control shaft  18 . The radial arms  34 ,  36  and the levers  16  have given lengths. These various parameters (angular offset between the arms and lengths of the arms and levers) are predetermined such that a linear movement of the movable rod  22  of the cylinder  20  causes, by rotating the control shaft  18 , a given angular movement of each ring  14 ,  14 ′ about the casing axis, and is conveyed by the rotation of the corresponding variable-pitch blades  11  about the axes thereof. 
     In the prior art, the body  38  of the cylinder  20  is pivotably mounted on supporting means  39  attached on the casing  10 , about an axis  40  parallel with the casing axis. These supporting means  39  are independent from the control shaft  18  and the supporting members  24  of said shaft. The pivoting axis  40  is situated in the vicinity of the end of the cylinder  20 , situated on the side opposite the movable rod  22  thereof. 
     The manufacturing and assembly tolerances of the various parts of the control device, and particularly those of the cylinder  20  and the means  39  for attaching the cylinder  20  on the casing  10 , mean that the relative positions of the various parts are not sufficiently precise, and are not reproducible after each disassembly/reassembly cycle. This is conveyed by imprecision in the angular offset of the blades  11  which may impair the performances of the compressor and the turbomachine. 
     The present invention makes it possible to solve this problem using the means for supporting the control shaft which are connected to the pivoting axis of the cylinder so as to define a precise mounting position of said pivoting axis in relation to the control shaft. 
     Reference will now be made to  FIGS. 3 and 4  wherein the members already described with reference to  FIGS. 1 and 2  are designated with the same figures increased by one hundred. 
     In the embodiment of the invention represented in  FIGS. 3 and 4 , the pivoting axis  140  of the cylinder  120  is connected to the supporting members  124  via the control shaft  118  and two connecting rods  150  running along either side of the cylinder  120 , between the pivoting axis  140  and the control shaft  118 . The axis  140  and the shaft  118  are parallel with each other. 
     The pivoting axis  140  is situated substantially mid-way from the ends of the cylinder body. In this instance, it is defined by two diametrically opposed cylindrical radial blade feet  141  in relation to the cylinder axis and running along either side of the cylinder body  138 . Each blade foot  141  is integral with the cylinder body  138 , and comprises one free end which is centred and guided in rotation in an orifice of a first end of a connecting rod  150 . The second end of each connecting rod  150  comprises an orifice traversed by the control shaft  118 . 
     Bearings  152  are mounted in the orifices of the connecting rods  150  for centring and guiding the shaft  18  and the axis  140  in rotation. The connecting rods  150  are mounted rotating freely on the control shaft  118  and on the pivoting axis  140 , and are not rotated about the axis of the shaft  118  during the rotation of said shaft. Each connecting rod  150  extends in this instance between the radial arm  136  connected to the cylinder  120  rod and a radial arm  134  connected to the lever  116  of a ring  114 ,  114 ′. 
     The pivoting axis  140  is further supported by a supporting clamp  160  attached on the casing  110 . This clamp  160  is substantially U-shaped and comprises two parallel tabs  162  connected to each other by a median portion  164  attached to the casing  110  by screw-nut type means. The tabs  162  of the clamp extend on either side of the cylinder  120  body and comprise orifices or notches  166  for receiving the blade feet  141  defining the pivoting axis  140 . The notch  166  of each tab is, in this instance, substantially U- or C-shaped and opens onto one side of the tab  162 , situated on the side of the cylinder rod  122 . 
     The control device functions as follows. The cylinder  120  is powered to move the rod  122  thereof in a linear fashion over a predetermined axial stroke. The movement of the cylinder rod is converted into a rotation of the control shaft  118  about the axis thereof and a rotation of the rings  114 ,  114 ′ about the casing axis. The movement of the rod  122  also causes slight pivoting of the cylinder  120  about the axis  140 , said axis being rotationally guided in the orifices of the rotatably mounted connecting rods  150  on the control shaft. 
     During a cylinder  120  maintenance operation, requiring the disassembly of the cylinder and the refitting thereof on the casing  110 , the relative position of the pivoting axis  140  of the cylinder in relation to the control shaft  118  is preserved. 
     Reference is now made to  FIGS. 5 to 8  wherein the items already described with reference to  FIGS. 1 and 2  are designated using the same figures increased by two hundreds. 
     In the control device in  FIGS. 5 and 6 , the pivoting axis  240  is carried directly by one of the supporting members  224  of the control shaft  218 , the axis  240  and the shaft  218  being parallel with each other. 
     The cylinder  220  has, in this instance, been brought closer to a supporting member  224  wherein one of the jambs is rigidly connected to a cylinder support tab  270 . The pivoting axis  240  is situated in the vicinity of the end of the cylinder, on the side of the movable rod  222  thereof, and only runs on one side of the cylinder. This axis  240  is defined by a cylindrical radial blade foot  241  wherein one end is engaged into an orifice of a radial cylindrical boss of the cylinder body, and wherein the other end is received in a bearing  272  mounted in a corresponding cylindrical housing of the tab  270 . 
       FIG. 7  represents an alternative embodiment of the control device according to the invention.  FIG. 8  represents a further control device. In both cases, the devices differ from the embodiment in  FIGS. 5 and 6 , particularly in that a second cylindrical radial blade foot  241  is diametrically opposite the first blade foot  241 , in relation to the cylinder axis, such that the cylinder pivoting axis  240  runs on either side of the cylinder body  238 . Said second blade foot  241  is also integral with the cylinder body, and comprises a free end received in a bearing  282  carried by a further tab  280  running parallel with the first tab  270 . 
     In the example in  FIG. 7 , the second tab  280  is attached onto the casing  210  by screw-nut type means. In the case of  FIG. 8 , this tab  280  is connected to the tab  270  of the member  224  and forms a U-shaped clamp therewith. 
     In both cases mentioned above, the bearings for centring and guiding the blade feet  141 ,  241  of the cylinder are locked axially in the orifices of the support tabs  270 ,  280  or the ends of the connecting rods  150  by nuts screwed onto the threaded portions of the blade feet. 
     The alternative embodiment represented in  FIG. 9  differs from that in  FIG. 7  essentially by the system for centring and guiding the blade foot  241  in the orifice of the second tab  280  and by the means for locking said system axially in said orifice. 
     The manufacturing and assembly tolerances may give rise, after the attachment of the second tab  280  by screws  300  or similar on the casing  210 , to a misalignment between the orifices of the tabs  270 ,  280  for housing the blade feet  241 . To remedy this drawback, the system for centring and guiding the blade foot  241  engaged in the orifice of the second tab  280  particularly comprises an eccentric cylindrical ring  302  to compensate for said misalignment, said system being represented in  FIG. 10 . The eccentric ring  302  comprises an external cylindrical surface engaging with the internal cylindrical surface of the orifice of the tab  280 , and an axial cylindrical bore wherein the internal surface engages with the external cylindrical surface of the blade foot  241 . The internal and external surfaces of the ring are, for example, offset in relation to each other by approximately 0.5 millimetres. The ring  302  is intended to be rotated about the second blade foot  241 , in the orifice of the second tab  280 , until the axial bore thereof is aligned with the orifice of the first tab  270 . 
     Two radially grooved washers  304 ,  306  are engaged on the ring  302  and inserted axially between one face of the second tab  280 , situated on the side opposite the cylinder  220 , and one external annular rim  308  of the ring  302  ( FIGS. 9 to 12 ). These washers  304 ,  306  lock the ring  302  in rotation in the orifice of the second tab  280  once said ring is in the abovementioned position wherein the axial bore thereof is aligned with the orifice of the first tab  270 . 
     A first washer  304  comprises a lateral face pressing on the abovementioned face of the second tab  280  and comprising protruding tappets  310  engaging with grooves (not shown) having a corresponding shape provided on the tab to rigidly connect the first washer  304  of the tab  280  in rotation. The other face of said first washer  304  comprises evenly distributed radial grooves  312  about the axis thereof, for example forty in number. These grooves  312  engage with radial grooves  312  having a complementary shape formed on one of the faces of the second washer  306  also comprising a tappet  314  or similar engaging with a notch  316  of the ring to rigidly connect the second washer  306  with the ring  302  in rotation. The notch  316  may be formed on the external cylindrical surface of the ring  302  and on the face of the external rim  308  of the ring situated on the side of the tab  280 , or merely on said face of the rim  308  to avoid weakening the eccentric ring. In this case, the tappet  314  is replaced on the washer  304  by a similar tappet system to that of the washer  306 . 
     Each groove  312  of the first washer  304  (or of the second washer) engages with the grooves of the second washer  306  (or the first washer) to define a given angular position of the ring  302  about the axis thereof. The greater the number of grooves of each washer, the greater the number of different angular positions of the ring in the orifice of the second tab, and the more precise the adjustment of the abovementioned alignment. The spacing between two angular positions of the ring is determined by the spacing between two successive grooves of each washer. 
     The centring and guiding ring  302  is, in this instance, locked axially in the orifice of the second tab  280  via an insert  318  mounted and attached on the tab, and not by a nut screwed onto the blade foot, as was the case in the previous embodiments. 
     The insert  318  comprises an end comprising an orifice for inserting a screw  320  fastened in a corresponding orifice of the second tab  280 . The opposite end of the insert  318  pressing axially on the external rim  308  of the ring  302 , on the side opposite the tab  280 , to lock said ring axially and clamp the radially grooved washers  304 ,  306  against each other. 
     The second tab  280  of this embodiment is mounted and attached as follows after mounting and attaching the cylinder body on the first cylinder support tab  270 . The second tab  280  is presented on the cylinder body, the second blade foot  241  of the cylinder  220  is engaged in the orifice of said tab and the system represented in  FIG. 10  is engaged about the blade foot  241 , in the orifice of the tab. The ring  302  is then rotated about the axis thereof, incrementally, until the end of the tab  280  rests on the casing  210 . In this position, the tab  280  is positioned correctly on the casing  210  (the orifices or the slots  322  provided at the radially internal end of the tab  280  being aligned with corresponding orifices of the casing  210 ) and may be attached on the casing by the screws  300 , and the orifice of said tab is centred on the pivoting axis of the cylinder defined by the first attachment tab  270  and by the two cylinder body blade feet  241 . 
     The control devices according to the invention described and represented in  FIGS. 3 ,  4  and  7  offer the advantage, in relation to the prior art, that the relative position of the pivoting axis of the cylinder and the control shaft axis, remains invariable after maintenance operation disassemblies and reassemblies, making it possible to retain variable-pitch blade control precision. 
     The blades controlled by the devices according to the invention may be of the type represented in  FIG. 2 , or any other type used in aircraft turbojets or turboprops.