Abstract:
A locking device ( 36 ) for use in retaining an assembly of rotor blades against movement around a rotor disc on which they are mounted comprises a body member having first and second portions interconnected by a weakened region ( 66 ) whereby a force applied to turn the first portion ( 38 ) relative to the second portion ( 40 ) can cause them to shear apart at said weakened region ( 66 ) thereby facilitating release of seized devices to enable blade removal.

Description:
FIELD OF THE INVENTION 
     This invention relates to locking devices. More particularly, but not exclusively, the invention relates to locking devices for locking rotary compressor blades to the rotary discs upon which the blades are mounted. 
     BACKGROUND OF THE INVENTION 
     In some high pressure axial flow compressors, the blades are retained in circumferential grooves in the rim of the compressor disc. In order to prevent the blades moving around the disc, one or more locking devices are provided in the groove or grooves. 
     A problem with such locking devices is that the temperatures and stresses experienced in use of a gas turbine engine can result in the locking devices seizing in the groove. During servicing of the engine, it is often necessary to dismantle the compressor, which means that the seized locking devices need to be drilled out which can result in damage to the disc. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this invention there is provided a locking device for locking a first article in a recess of a second article, the device being securable in a secured condition in said recess to lock the first article to the second article, the device comprising a weakness, whereby a force can be applied to said locking device to break the device at said weakness to release the device from said secured condition thereby allowing the first article to be removed from the second article. 
     The preferred embodiment of this invention is particularly suitable for use in preventing circumferential movement of first article, in the form of compressor or turbine blades of a gas turbine engine, around a second article comprising a support member in the form of a disc on which the blades are mounted. 
     The locking device may include a body member and securing means to secure the locking device to the second article wherein the weakness extends across a region of at least one of the body member and the securing means. 
     Preferably at least one of the securing means and the body member comprises first and second portions, wherein the weakness extends between the first and second portions of one of the body member and the securing means. The force may be applied to effect relative turning movement of said first portion relative to said second portion to cause the first portion to shear relative to the second portion at said weakness and to separate therefrom. 
     The body member may define a bore extending therethrough and the securing means may include an elongate member to extend through the bore to engage the second article, thereby securing the locking device against the second article. 
     In a first embodiment, the body member includes said first and second portions and said weakened region. The securing means may be securable to said body member at said first portion. The elongate member may extend through the region of the bore through the second portion to engage the second article. 
     The weakened region may extend at least partially around the body member, preferably substantially wholly therearound. The weakened region may define a boundary between the first and second portions and may comprise a groove or concavity. 
     Alternatively, or in addition, at least a part of the bore through the region of the second portion adjacent the first portion may be wider than the bore extending through the first portion to create the weakened region at the junction of said first and second portions. Preferably, the wider portion of the bore extends from the first portion to the adjacent end of the bore in the second portion. 
     Conveniently, the securing member is generally cylindrical in configuration and may be in the form of a bolt or a screw, suitably a grub screw. 
     In a second embodiment, the securing means includes said first and second portions, and said weakness. The first portion may include said elongate member which may extend through the second portion. The second portion is preferably engageable with the body member. The second portion is preferably engageable with the body member. The second portion and the body member may be provided with threads to co-operate with each other. Preferably, the threads on the body member are internal threads within the bore. 
     The region of weakness may extend at least partially around the securing means, and preferably substantially wholly therearound. 
     The first portion may be receivable in an indentation in the first article, for example, in a root shroud of an adjacent rotor blade. The first portion is preferably configured and/or sized to enable the first portion to be turned relative to the first article. Conveniently the first portion is of a substantially circular cross-section, or may be any other suitable configuration, for example, triangular or hexagonal, to allow a torque applying device, e.g. a spanner, to be applied thereto to turn the first portion relative to the second portion. 
     The body member may be shaped to be received in the recess which may be a groove extending circumferentially around said support member. The second article may include flange means extending partially over and spaced from the base of the recess, the second portion of said body member being adapted to engage the flange means when the device is located in the recess and the securing means actuated to lock the device to the second article. 
     Preferably complementary threads are formed on the part of said bore extending through the body member, and on a corresponding part of the securing means. In the first embodiment, the threaded parts of the bore and of the securing member are on regions thereof which are radially outwardly located in use. 
     According to another aspect of this invention there is provided a rotor assembly for a gas turbine engine, the rotor assembly including a plurality of rotor blades assembled on a rotor disc, and at least one locking device as described above engaged with a groove in the disc, wherein each rotor blade located adjacent the, or each, locking device defines an indentation to receive a part of the first portion of said body member therein, the indentation being configured and/or sized to allow the first portion to turn relative to the second portion. The indentations of adjacent blades may together define an access aperture for the locking device. Conveniently, the aperture defined by a pair of adjacent blades is substantially circular. In the preferred embodiment, the invention defined in each of said adjacent blades is substantially semi-circular. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, in which 
     FIG. 1 is a sectional side view of the upper half of a gas turbine engine; 
     FIG. 2 is a diagrammatic perspective sectional view showing the rotor of a high pressure compressor; 
     FIG. 3 is a vertical cross-section through one embodiment of a locking device; 
     FIG. 4 is a diagrammatic perspective view of a variation of the embodiment shown in FIG. 3; and 
     FIG. 5 is a vertical cross-section through a further embodiment of a locking device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a gas turbine engine is generally indicated at  10  and comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high pressure compressor  14 , a combustor  15  a turbine arrangement comprising a high pressure turbine  16 , an intermediate pressure turbine  17  and a low pressure turbine  18 , and an exhaust nozzle  19 . 
     The gas turbine engine  10  operates in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  which produces two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression take place. 
     The compressed air exhausted from the high pressure compressor  14  is directed into the combustor  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through and thereby drive the high, intermediate and low pressure turbines  16 ,  17  and  18 , before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbines  16 ,  17  and  18  respectively drive the high and intermediate pressure compressors  14  and  13  and the fan  12  by suitable interconnecting shafts. 
     Referring to FIG. 2, there is shown a part of the high pressure compressor  14 , which comprises a rotary disc  20  on which are mounted a plurality of compressor blades  22 . The disc  20  defines a circumferentially extending groove  24  having opposed flanges  26 ,  28  to hold the blades  22  on the disc  20 . 
     Each of the blades  22  comprises an aerofoil section  30 , a blade root  32  and a root shroud  34  provided between the blade root  32  and the aerofoil section  30 . 
     In order to prevent the compressor blades  22  moving circumferentially around the groove  24 , locking devices  36  are provided. Typically, two or more locking devices  36  are provided at spaced intervals around the groove. A first embodiment of a locking device  36  is shown in more detail in FIG.  3  and comprises a body member  37  having first and second portions  38 ,  40  which define therethrough a bore  42 . 
     The first portion  38  is of a cylindrical configuration, and the second portion  40  is shaped to enable it to be received in the groove  24  and to engage the flanges  26 , 28 . The second portion  40  includes opposite outwardly extending shoulders  44  terminating at a bevelled surface  46 . The shoulders  44  are disposed within the generally circular aperture defined by aligned indentations of the root shrouds  34  of selected adjacent blades  22 , as explained below. The second portion  40  has a cylindrical surface defining a circular section  47  extending from the each bevelled surface  46 . A further bevelled surface  49  extends outwardly from each circular section  47  and corresponds to the shape of the inner surface of the groove  24 . The second portion  40  locates the locking device  36  in the groove  24  by engagement with the bevelled inner surface of the groove  24 . 
     The bore  42  has a radially outer end region  48  extending through the first portion  38 , which is provided with internal screw threads  50 . A securing member, in the form of a bolt or a grub screw  52 , has complementary external screw threads  54  formed at its radially outer end region  56  such that the grub screw  52  can be screwed into the bore  42 . The radially outer end of the grub screw  52  is provided with a shaped blind recess  53  to receive a suitable driver, for example a hexagonal ended key, or screwdriver (not shown). 
     The radially inner end of the bore  42  is counterbored to form an increased diameter region  58  extending through the second portion  38 . The radially inner shank region  60  of the grub screw  52  which is devoid of screw threads is of lesser diameter than the region  58  such that an annular gap  62  is formed between the shank  60  and the wall of the counterbored region  58  of the bore  42 . The gap  62  is large enough to allow the second portion  40  of the body member to fall away from the first portion  38  without hindrance when breakage occurs between the first and second portions  38  and  40  as described hereafter. 
     An annular concavity  64  is formed around the radially inner end of the first portion  38  adjacent the second portion  40 . The concavity  64  and the counterbored region  58  together provide an annular weakened or breakable region  66  extending around the bore  42  between the first and second portions  38 ,  40 . 
     Referring again to FIG. 2, the compressor  14  is assembled in the normal manner, such that two locking devices  36  are arranged at a predetermined spacing which may be generally opposite each other around the disc  20 , conveniently, but not essentially at 180° from each other. The blades  22  are located in the groove  24  in known manner. The blades  22  adjacent, and on opposite sides of, each of the locking devices  36  are provided with root shrouds  32  having indentations  68  defined therein. The two indentations  68  together form a circular aperture in which the first portion  38  of the locking device  36  is received, thereby allowing access thereto by the aforementioned driver. 
     In FIG. 3, the locking device  36  is shown within the groove  24  of the disc  20 . The groove  24  has shaped side walls  25  which correspond in part to the outwardly extending surface  49  of the second portion  40  of the locking device  36 , and a radially inner wall  27  against which the radially inner end of the grub screw  52  can abut. 
     When all the blades  22  have been fitted in the groove  24 , the grub screws  52  are tightened into the threaded region  56  of the respective locking devices  36  and the inner end  55  of each abuts against the radially inner wall  27  of the groove  24 . Further tightening of grub screws  52  then pushes the body member  37  radially outwardly until each bevelled wall  46  of the shoulders  44  on the second portion  40  engages against the respective bevelled inner wall  29  of the flanges  26 ,  28 . In this position, the outwardly extending surfaces  49  engage the inner walls  25  of the groove  24 . The circular section  47  locates in a circular opening in the groove  24  to prevent circumferential movement of the blades  22  around the disc  20 . 
     When it is desired to dismantle the compressor  14  it is necessary first to remove the locking devices  36 . The conditions during use of the compressor  14  frequently cause the grub screws  52  to seize to the inner wall of the bore  42  at threads  50 ,  54  so that they cannot be removed. However, when the screw  52  is turned the first portion  38  shears relative to the second portion  40  at the weakened region  66 . This causes the second portion  40  to break away from the first portion  38  and fall into the groove  24 . The first portion  38  can then be removed from the groove  24  of the disc  20  together with the grub screw  52 . When this has been repeated for all the locking devices  36  in the groove  24 , the blades  22  can be slid around the groove  24  and removed therefrom. 
     FIG. 4 shows a variation of the embodiment shown in FIGS. 2 and 3, which differs therefrom in that the first portion  38  is of a hexagonal configuration. This allows a spanner, for example a socket spanner to be fitted over the first portion and a turning force applied thereto to shear the first portion  38  from the second portion  40 . It will be appreciated that the first portion  38  can be any suitable configuration to enable a corresponding socket to fit over it to apply the turning force thereto. Further, with this embodiment, the apertures defined by the indentations  68  of adjacent blades  22  is of sufficient size to receive therein a socket of a socket spanner to fit over the first portion  38 . 
     Referring to FIG. 5, there is shown a second embodiment of a locking device  136  which comprises a main body member  140  defining therethrough a bore  142  having internal threads  150 . 
     The body member  140  is of a similar shape to the second portion  40  of the first embodiment, and performs the same function, i.e. it is received in the groove  24  on the rotary disc  20  to secure the compressor blades  22  in place. Accordingly, the body member  140  has many of the same features at the second portion  40  of the first embodiment, and these have been designated with the same reference numeral. 
     Securing means in the form of a bolt  152  includes a first portion comprising a head  151  having a blind recess  153  shaped to receive a suitable driver, for example a hexagonal ended key or a screwdriver. The bolt  152  also includes a second portion in the form of a shank  155  having external threads  154  which can pivotally engage the threads  150  of the bore  142 . The shank  155  defines therein a hollow  156 , and an elongate projecting member  160  extends from the head  151  through the hollow  156  in the shank  155  to project from the open end of the shank  155 . A V-shaped groove  159  extends around the bolt  152  between the head  151  and the shank  155 . The radius of the bolt at the groove  159  is reduced and an annular weakened or breakable region  161  is defined between the groove  159  and the hollow  156 . 
     In use, the elongate member  160  abuts against the radially inner wall  27  of the groove  24  in the disc  20  to push the body member  140  radially outwardly to engage against the flanges  26 ,  28  of the groove  24  of the compressor disc  20 , in the same way as the above screw  52  of the first embodiment. 
     As can be seen, the elongate member  160  is connected to the head  151  of the bolt  152 , but not connected to the shark  155 . Thus, in the situation where the bolt  152  is seized to the main body member  140 , a turning force applied to the head  151  will cause the head  151  to sheer from the shank  155  at the weakened region  161 . Since the elongate member  160  connected to the head  151 , the elongate member is removed with the head  151 , thereby releasing the body member  140 . When the body member  140  has been released, the blades  22  can be removed. 
     Various modifications may be made without departing from the scope of the invention. For example, alternative means of locking the locking members in place may be employed and the region of weakness in the body member may be formed in different ways and in different locations. 
     Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.