Patent Publication Number: US-8528181-B2

Title: Alignment of machine components within casings

Description:
FIELD OF INVENTION 
     The present disclosure relates to a technique and apparatus for accurately aligning heavy machine components of generally circular cross-section within surrounding casings, and has particular relevance to alignment of annular combustors within the casings of large, heavy-duty gas turbine engines. 
     BACKGROUND 
     Correct positioning of an annular combustor within the casings of a gas turbine engine is very important, because precise alignment with respect to the injection of fuel, inflow of air and the turbine is required to avoid excessive stresses on combustor components and to aid proper combustion. Incorrect alignment of the combustor increases stresses on combustor components that interface with the turbine nozzle guide vanes, resulting in decreased component life. 
     A known method of combustor alignment utilizes the principle of cross-key location, shims being used between confronting location faces of the cross-keyed components to enable the making of fine adjustments to combustor alignment. However, to obtain satisfactory alignment of the combustor in this way can be very time-consuming, particularly when the assembled combustor is large and heavy. Several iterations of the alignment procedure may be required, involving the use of several different thicknesses of shims between each set of confronting location faces. Moreover, a completely correct alignment cannot be guaranteed. 
     Therefore, to save time and reduce costs during manufacturing assembly of an engine and during rebuild of an engine after maintenance actions, it will be advantageous to have a faster and more precise way of obtaining correct combustor alignment. 
     SUMMARY 
     The disclosure is directed to a method to accurately align a machine component of generally circular cross-section within a surrounding machine casing that includes a bottom half of the casing and a top half of the casing. The bottom half and top half, in use, are bolted together at a split line occupying a horizontal plane. The component and the bottom half of the casing are provided with complementary interdigitating members at three circumferentially spaced-apart locations, which include first and second locations at the split line on respective first and second horizontally opposed sides of the component, and a third location at bottom dead center. The method includes the steps of:
         (a) lowering the machine component into the bottom half of the casing to engage the interdigitating members at the three locations;   (b) engaging jacking apparatus at each of the three locations, the jacking apparatus being independently operative at each location to reposition the component within the bottom half of the casing, thereby to attain a jacked position of the component;   (c) inserting shims between the interdigitating members at the three locations to maintain the jacked position of the component; and
 
repeating steps (b) and (c) as often as necessary to attain a desired position of the component within the bottom half of the casing.
       

     The disclosure is also directed to an apparatus to accurately align a machine component of generally circular cross-section within a surrounding machine casing. The casing includes a bottom half of the casing and a top half of the casing bolted together at a split line occupying a horizontal plane, the component and the casing each have a longitudinal axis. The component and the bottom half of the casing are provided with complementary interdigitating members that engage each other at three circumferentially spaced-apart locations. The locations include first and second locations at the split line on respective first and second horizontally opposed sides of the component, and a third location at bottom dead center. The apparatus includes:
         (a) mutually confronting location faces provided on the interdigitating members at each of the first and second locations, the location faces being positioned and oriented such that shims are insertable therebetween for vertical positional adjustment and axial positional adjustment of the component within the bottom half of the casing;   (b) mutually confronting location faces provided on the interdigitating members at the third location, the location faces being positioned and oriented such that shims are insertable therebetween for altering an attitude of the component within the casing and aligning the longitudinal axis of the component with a vertical plane containing the longitudinal axis of the casing;   (c) jacking apparatus at each of the three locations, the jacking apparatus being independently operative at each location to incrementally reposition the component to attain a desired jacked position of the component within the bottom half of the casing and to facilitate insertion of shims between the interdigitating members at the three locations to maintain the desired jacked position of the component after the jacking apparatus has been removed.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, which are not to scale: 
         FIG. 1  is a diagrammatic cross-sectional plan view of a gas turbine engine to which the invention can be applied, the cross-section excluding the core of the engine and being taken on a horizontal, diametric split plane of the engine casing; 
         FIG. 2  is a diagrammatic cross-section of the combustor and adjacent parts on the underside of the engine of  FIG. 1 ; the cross-section is taken in a vertical plane including the longitudinal axis of the engine; 
         FIG. 3A  is an enlarged view of the part of  FIG. 2  within the rectangular outline  3 A, comprising combustor location features; 
         FIG. 3B  is a view on horizontal section line  3 B- 3 B in  FIG. 3A ; 
         FIG. 3C  is a partial view on arrow  3 C in  FIG. 3A , showing hidden detail of the combustor location features; 
         FIG. 4A  is partial view on arrow  4 A in  FIG. 1 , showing a side elevation of combustor location features located at the horizontal split plane of the engine casing; 
         FIG. 4B  is a plan view on arrow  4 B of the combustor location features in  FIG. 4A ; 
         FIG. 5  diagrammatically illustrates a device to aid accurate adjustment of the location of the combustor within the casing using the combustor location features of  FIGS. 3A to 3C ; and 
         FIG. 6  diagrammatically illustrates a device to aid accurate adjustment of the location of the combustor within the casing using the combustor location features of  FIGS. 4A and 4B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Introduction to the Embodiments 
     One aspect deals with a method to accurately align a machine component of generally circular cross-section within a surrounding machine casing that comprises a bottom half of the casing and a top half of the casing that in use are bolted together at a split line occupying a horizontal plane. The component and the bottom half of the casing are provided with complementary interdigitating members at three circumferentially spaced-apart locations comprising first and second locations at the split line on respective first and second horizontally opposed sides of the component, and a third location as near as possible to bottom dead centre. The method comprises the steps of:
         (a) lowering the machine component into the bottom half of the casing to engage the interdigitating members at the three locations;   (b) engaging jacking apparatus at each of the three locations, the jacking apparatus being independently operative at each location to incrementally reposition the component within the bottom half of the casing, thereby to attain a jacked position of the component;   (c) inserting shims between the interdigitating members at the three locations to maintain the jacked position of the component; and   (d) repeating steps (b) and (c) as often as necessary to attain a desired position of the component within the bottom half of the casing.       

     A preferred arrangement of the jacking apparatus is such that jacking at the first and second locations raises (or lowers) the component within the bottom half of the casing, whereas jacking at the third location alters the component&#39;s attitude within the bottom half of the casing. While the component is raised on the jacking apparatus, it is possible not only to adjust the component&#39;s axial position within the bottom half of the casing, but also to align the component&#39;s longitudinal axis with a vertical plane containing the casing&#39;s longitudinal axis. 
     The method is facilitated by apparatus that in a preferred embodiment includes:
         (a) mutually confronting location faces provided on the interdigitating members at each of the first and second locations, the location faces being positioned and oriented such that shims are insertable therebetween for vertical positional adjustment and axial positional adjustment of the component within the bottom half of the casing;   (b) mutually confronting location faces provided on the interdigitating members at the third location, the location faces being positioned and oriented such that shims are insertable therebetween for altering an attitude of the component within the casing and aligning the longitudinal axis of the component with a vertical plane containing the longitudinal axis of the casing;   (c) jacking apparatus at each of the three locations, the jacking apparatus being independently operative at each location to incrementally reposition the component to attain a desired jacked position of the component within the bottom half of the casing and to facilitate insertion of shims between the interdigitating members at the three locations to maintain the desired jacked position of the component after the jacking apparatus has been removed.       

     Preferably, the jacking apparatus at each of the first and second locations includes:
         (a) a base plate fixed to the bottom half of the casing at the split line;   (b) a lifting plate, a first end thereof making line contact with the base plate;   (c) an incrementally adjustable jack acting between the base plate and a second end of the lifting plate, such that when the jack raises or lowers the second end of the lifting plate, the lifting plate pivots about the first end thereof; and   (d) a connection between the lifting plate and an interdigitating member that is fixed to the component, whereby raising or lowering of the lifting plate correspondingly raises or lowers the component.       

     The jacking apparatus at each of the first and second locations may further include a screw jack arrangement acting between the base plate and opposed sides of the interdigitating member that is fixed to the component, thereby to adjust the axial position of the component within the bottom half of the casing while the lifting plates at the first and second locations are raised on their jacks. 
     It is preferred that the jacking apparatus at the third location includes:
         (a) a base plate fixed to an interdigitating member that is fixed to the bottom half of the casing;   (b) a head plate fixed to the component;   (c) a connecting member fixed to the head plate and connecting the head plate to the base plate such that the head plate and the component fixed thereto is moveable with respect to the base plate and the casing, thereby to alter the attitude of the component within the casing;   (d) an incrementally adjustable jack acting between the base plate and the connecting member and operative to move the component as aforesaid.       

     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a gas turbine engine  10  has an engine core  11  including an annular air intake duct  12 , compressor inlet guide vanes  14 , multiple stages of compressor rotor blades  16  separated by compressor stator blades  17 , a combustor entry duct  18 , an annular combustor  20 , turbine inlet nozzle guide vanes  22 , multiple stages of turbine rotor blades  24  separated by turbine stator blades  25 , and an exhaust duct  26 . The compressor rotor blades  16  and the turbine rotor blades  24  are mounted on respective compressor and rotor drums  28 ,  30 , these in turn being mounted on a rotor shaft  32 , which defines the engine&#39;s longitudinal and rotational axis. Front and rear ends of the rotor shaft  32  are supported for rotation in respective bearing arrangements  34 ,  36 , the front bearing races  38 ,  40  being held in a housing  42  supported by aerodynamically shaped struts  44  that extend across the intake duct  12 , and the rear bearing race  46  being held in a housing  48  supported by aerodynamically shaped struts  50  that extend across the exhaust duct  26 . 
     The engine  10  has robust exterior and interior casings, constructed from several axially consecutive casing sections, to support the various components of the engine core  11  (for simplicity of illustration in  FIG. 1 , divisions between axially consecutive casing parts are not shown). Hence, compressor and turbine stator blades  17 ,  25  are mounted in the surrounding inner casing sections  51 ,  53 . To support the rotating parts of the engine core  11  within the exterior casing, the front and rear bearing housing support struts  44 ,  50 , are fixed to respective front and rear casing sections  52 ,  54 , which define the intake and exhaust ducts  12 ,  26 . The combustor entry duct  18  is supported from a smaller diameter mid-casing section  56 , while the outer shell of the combustor  20  is supported within the large diameter casing section  58  at three locations. One of the combustor support location is at the 6 o&#39;clock position and is therefore hidden underneath the combustor in the view of  FIG. 1 , but is indicated by reference  60  in  FIG. 2 . The other two combustor support locations  62 ,  64 , are diagrammatically indicated in  FIG. 1 , at the 3 o&#39;clock and 9 o&#39;clock positions on the outer circumference of the combustor  20 . As will be explained later, support locations  62 ,  64  are different from support location  60 . 
     Looking now at the more detailed view of  FIG. 2 , a major portion of the compressed air  66  at the rear of the combustor entry duct  18  is turned through an angle approaching 180 degrees by deflector vanes  68  and flows into a plenum chamber  70 , which is defined between the outer wall of the combustor entry duct  18  and the large diameter casing section  58 . Most of the air  76  that flows into the plenum chamber  70  enters the front end of the combustor  20  as combustion air  76   a  and is mixed with fuel that enters the combustor through an annular array of equi-angularly spaced-apart pairs of fuel lances  72 . However, a proportion  77  of the air  76  flows through the gap between the combustor  20  and the casing section  58  into the chamber  78  and is used to cool the outside of the combustor. After use for this purpose, a proportion  77   a  of air  77  is used to cool the radially outer combustion liner  84 , as shown by the arrows, the combustion liner  84  being double-walled as shown, so that the cooling air can flow between the walls. To obtain similar cooling of the radially inner combustion liner  82 , a minor proportion  66   a  of the compressed air  66  at the rear of the combustor entry duct  18  is not turned into the plenum chamber  70 , but as shown by the arrows, is allowed to continue rearward through duct  80  for a short distance before being turned through nearly 180 degrees and channeled between the double walls of the radially inner combustion liner  82 . Hence, for both inner and outer combustion liners, cooling occurs due to cooling air flowing between their double walls as well as over the liner&#39;s external surfaces. 
     In the combustion chamber, combustion is initiated in the swirling flow  74  in Zone  1  and completed in Zone  2 , from where the combustion gases are channeled into the turbine through the annular array of nozzle guide vanes  22  at the combustor exit. It should be noted that the nozzle guide vanes  22  are hollow so that a proportion  77   b  of air  77  can pass through them for cooling. 
     It will be understood that the combustor components are subject to high heat stresses from the combustion gases and that combustor misalignment within the exterior casings could allow leakage of hot combustion gases from the combustor and/or result in excessive mechanical stress, perhaps causing damage to some components. In  FIG. 2 , the components most likely to be affected by misalignment include:
         The so-called “tipping segments”  86 , which connect the radially inner combustion liner  82  to the nozzle guide vanes  22 , control leakage of compressed air into the hot gas path at the exit of the combustor, and prevent backflow of hot gases from the combustor into the combustor entry duct  18 .   The so-called “bone segments”  88 , which connect the radially outer combustion liner  84  to the nozzle guide vanes  22 , control leakage of compressed air into the hot gas path at the exit of the combustor, and prevent leakage of hot gases from the combustor into the chamber  78 .       

     As previously mentioned, the combustor  20  is supported within the exterior casing at the three locations  60 ,  62  and  64 , which will now be explained in more detail. 
     As shown in  FIG. 2  and  FIGS. 3A to 3C , location  60  comprises five location features, namely three blocks  90 ,  92 ,  94  that protrude inwardly from the casing section  58  and two blocks  96 ,  98  that protrude outwardly from a bolting flange  100  on the combustor. Blocks  90 ,  92  and  94  are preferably cast integrally with the casing section  58 , though they could alternatively be welded or bolted onto it. Blocks  96  and  98  may be cast integrally with the bolting flange  100 , or welded onto it, but are preferably bolted onto it. Blocks  90  and  92  comprise flanges with a substantially rectangular cross-section whose longitudinal dimension extends at right angles to the rotational axis of the engine  10 ; block  94  is a robust cylindrical tine or prong located mid-way between the flanges  90 ,  92 ; and blocks  96  and  98  comprise a pair of robust projections with a rectangular or square cross-section, which in the assembled engine fit in the gap  95  between the flanges  90  and  92 , one on each side of the cylindrical tine  94 . 
     As shown in  FIGS. 3A and 3B , flanges  90  and  92  are axially spaced-apart by a gap  95  and are each provided with a pair of flat, substantially rectangular location faces  90   a ,  90   b  and  92   a ,  92   b , each location face being in a vertical plane oriented normally to the engine&#39;s rotational axis. Location faces  90   a  and  92   a  face each other across the gap  95 , as do location faces  90   b ,  92   b.    
     Tine  94  is provided with a pair of flat circular location faces  94   a ,  94   b  on opposing sides of the tine, the plane of each location face being oriented parallel to a vertical plane coincident with the engine&#39;s rotational axis. 
     Projections  96  and  98  are each provided with three flat location faces  96   a  to  96   c  and  98   a  to  98   c  that confront corresponding location faces on flanges  90  and  92  and tine  94 . Projection  96  has a pair of circular location faces  96   a ,  96   b  on its axially opposed sides, so that in the assembled engine, location face  96   a  confronts location face  90   a  on flange  90  and location face  96   b  confronts location face  92   a  on flange  92 . Similarly, projection  98  has a pair of circular location faces  98   a ,  98   b  on its axially opposing sides, so that in the assembled engine, location face  98   a  confronts location face  90   b  on flange  90  and location face  98   b  confronts location face  92   b  on flange  92 . A rectangular or square location face  96   c  and  98   c , respectively, provide the third location face on each projection  96 ,  98  and are arranged so that in the assembled engine, location faces  96   c  and  94   a  confront each other, as do location faces  98   c  and  94   b.    
     As shown in  FIGS. 1 ,  4 A and  4 B, location  62  comprises three location features  102 ,  104  and  106 . In  FIG. 4A , the wall of the exterior casing section  58  is shown partly broken away to reveal them. Location features  102 ,  104  are axially spaced-apart so that there is a gap  103  between them and comprise blocks of rectangular section that project inwardly from the inner side of the exterior casing section  58 . Blocks  102 ,  104  are preferably integrally cast with casing section  58 , though they could alternatively be welded or bolted on. Location feature  106  is a T-shaped block that is preferably bolted onto the bolting flange  100  of the combustor  20 , though it could alternatively be cast integrally with the flange  100 , or welded on. When the combustor  20  is correctly assembled in the engine, the stem of the T-shaped block is positioned between the two rectangular section blocks  102 ,  105 , and the top surface  106   a  of the T-shaped block  106  is substantially in-line with the engine casing&#39;s horizontal split plane  108 , which is aligned with the engine&#39;s rotational axis. 
     Location  64  is on the diametrically opposite side of the engine and except for being a mirror image of location  62 , is structurally identical thereto. 
     Each block  102 ,  104  has two flat location faces  102   a ,  102   b  and  104   a ,  104   b , with each block&#39;s location faces being set at right angles to each other. Location faces  102   a  and  104   a  are in mutually parallel vertical planes which are oriented normally to the engine&#39;s rotational axis, while location faces  102   b  and  104   b  share a common horizontal plane. T-shaped block  106  has four flat circular location faces  106   b  to  106   e . Location faces  106   b  and  106   e  confront location faces  102   b  and  104   b , respectively, and therefore lie in a common horizontal plane, whereas location faces  106   c  and  106   d  confront location faces  102   a  and  104   a , respectively, and therefore lie in parallel vertical planes oriented normally to the engine rotational axis. 
     It has been the practice to install the assembled combustor  20  by using overhead lifting equipment to lower it into the bottom half of the engine casing so that outwardly pointing projections  96  and  98  on bolting flange  100  are inserted in the gap  95  between inwardly pointing flanges  90  and  92  on exterior casing section  58 , with one projection  96 ,  98  located on each side of the central cylindrical tine  94 . Simultaneously, the downwardly pointing stem of the T-shaped block  106  on bolting flange  100  is inserted in the gap  103  between the inwardly pointing blocks  102 ,  104 . When located correctly within the engine, the combustor  20  can be bolted securely to other engine static structure. To achieve the correct location, the combustor remains attached to the lifting equipment while it is adjusted to its correct position and orientation, relative to the previously installed ring of nozzle guide vanes  22  and other engine internals, by insertion of shims between the confronting location faces described above. 
     Adjustment by insertion of shims is achieved as follows. 
     When the combustor  20  is suspended at locations  62  and  64 , shimming at the 6 o&#39;clock position, location  60 , enables adjustment of combustor position by:
         centering, so that the combustor&#39;s centre is in a vertical plane that coincides with the engines&#39; rotational axis, and   tilting, comprising adjustment of its attitude within the casing, specifically the pitch angle of the combustor&#39;s longitudinal axis relative to the longitudinal axis of the casing, so that the combustor&#39;s exit annulus is at the correct attitude for attachment to the nozzle guide vane annulus  22 .
 
Centering is achieved by inserting shims between the central inwardly pointing tine  94  and the outwardly pointing projections  96 ,  98 , i.e., between location faces  94   a / 96   c , and/or between location faces  94   b / 98   c . Changes of tilt angle are achieved by inserting shims between the inwardly pointing flanges  90 ,  92  and the outwardly pointing projections  96 ,  98 , i.e., between location faces  90   a / 96   a  and/or  90   b / 98   a , and between location faces  92   a / 96   b  and/or  92   b / 98   b.  
       

     Shimming at the 3 o&#39;clock and 9 o&#39;clock positions, locations  62  and  64 , enables adjustment of combustor position by:
         aligning the combustor vertically, so that the combustor&#39;s centre is in a horizontal plane that coincides with the engines&#39; rotational axis, and   aligning the combustor axially, so that the combustor&#39;s exit annulus can dock correctly with the nozzle guide vane annulus  22 .       

     Vertical alignment is achieved by inserting shims between the blocks  102 ,  104  and the cross-bar of the T-shaped block  106 , i.e., between location faces  102   b / 106   b , and/or between location faces  104   b / 106   e . Axial alignment is achieved by inserting shims between the blocks  102 ,  104  and the stem of the T-shaped block  106 , i.e., between location faces  102   a / 106   c , and/or between location faces  104   a / 106   d.    
     The position of the combustor relative to the nozzle guide vanes  22  is critical for combustor integrity and service life. Precise alignment is required for proper combustion and to avoid interference fits between the combustor exit annulus and the nozzle guide vane annulus, which could result in excessive stresses on the “tipping segments”  86  and the “bone segments”  88  ( FIG. 2 ). However, because inserting shims between location faces at one of the locations  60 ,  62 ,  64  affects spacing between location faces at the other two locations, the above-described shimming procedure has to be an iterative process of successive approximations to the ideal position of the combustor, involving the insertion and removal at each location of shims having different thicknesses. As such, it is very time-consuming. Moreover, the overhead lifting equipment used to suspend the combustor while the shim thicknesses are adjusted is difficult to control to the required degree of accuracy for exact positioning of the combustor. Consequently, we have developed the following apparatus and method to reduce the severity of these problems and increase the speed and accuracy of positioning.  FIGS. 5 and 6  show the apparatus, which includes incrementally adjustable jacks  121 ,  138  to enable a speedier and more accurate positioning process. “Incrementally adjustable”, means that the jacks are controllable to give small discrete jacking movements of, say, the order of one millimeter. 
     Referring first to  FIG. 5 , this shows a simplified, part-sectional, enlarged side-view of location  60  comprising the location features  90 ,  92  and  96 , but with the features  94  and  98  omitted for clarity. To assist correct positioning of the combustor  20  with respect to its tilt relative to the nozzle guide vanes, a fixture  110  is sized to fit through an access hole (not shown) in the side of the casing  58 . Fixture  110  has a head plate  112  that bolts on to the combustor&#39;s bolting ring  100  (or is otherwise detachably fixed thereto), a cranked arm  118 , and a pedestal  113 , comprising a horizontal base portion  113   a  and a vertical portion  113   b , portion  113   b  being rigidly fixed to base  113   a  by, e.g., welding. Head plate  112  spans at least two circumferentially spaced bolt holes  114  on the bolting ring  100  and is fixed thereto by corresponding circumferentially spaced bolts  115 , which are screwed into the bolt holes  114 . 
     To secure the fixture  110  to the casing  58 , pedestal base  113   a  is hooked around the location flange  90 , whereby the flange projects through an aperture  116  in the base plate, the aperture being a close fit to the flange. Pedestal base  113   a  is thereby able to firmly support a lower horizontal portion  118   c  of the cranked arm  118 , which is captured in a channel  113   c  of the pedestal&#39;s base portion  113   a . Together, channel  113   c  and the base  113   a  comprise linear bearing surfaces for the horizontal portion  118   c  of the cranked arm  118 . The bearing surfaces may be lined as required by a low-friction coating, such as PTFE, or the like. This allows forward and backward movements of the arm  118  generally parallel to the rotational axis of the turbine, as will now be explained. 
     The lower horizontal portion  118   c  of the cranked arm  118  is joined to the upper horizontal portion  118   a  by a vertical portion  118   b , and a hydraulic cylinder jack  121  acts between the arm&#39;s vertical portion  118   b  and the pedestal&#39;s vertical portion  113   b , whereby the arm can be moved incrementally backwards or forwards relative to the pedestal  113  and casing  58  by the action of the hydraulic jack&#39;s plunger  120 . The hydraulic cylinder  121  is pressurised through a flexible armored hydraulic tube  122 , which is connected to a hand-operated hydraulic pump (not shown). A suitable hydraulic pump and cylinder combination is, for example, an Enerpac® P142 pump and an Enerpac® RSM 100 cylinder, see http://www.enerpac.com. Because the pedestal  113  is immovably engaged with the flange  90 , incremental fore-and-aft movements of the arm  118  can be used to incrementally change the combustor&#39;s tilt angle while the combustor  20  is suspended at locations  62  and  64 , shims being inserted as appropriate to maintain the position against the pivot weight of the combustor after removal of pressure from the hydraulic cylinder  121 . Between hydraulically assisted adjustments of pitch angle, centering of the combustor can be accomplished by insertion of shims between tine  94  and projections  96 ,  98 , as noted previously. All shims at location  60  are initially installed undersized to allow for insertion of additional shims after final positioning of the combustor using apparatus installed at locations  62  and  64 , as described below. 
     Turning now to  FIG. 6 , a fixture  130  is provided to assist correct positioning of the combustor with respect to its vertical and axial alignment. It should be understood that the apparatus now to be described in connection with location  62  is duplicated at location  64  on the opposite side of the engine  10  as a “mirror image” (laterally inverted) version, thereby enabling the same types of adjustments to be made on both sides of the engine. Therefore, the following description of the apparatus associated with location  62  will also suffice for a description of the apparatus associated with location  64 . 
       FIG. 6  is a diagrammatic side elevation of location  62  looking outwards from the combustor, the bolting flange  100  of the combustor  20  thereby being excluded from the view. It comprises a base plate  132 ; a lifting plate  134  overlying the base plate; a screw-threaded tie rod  136  that connects the lifting plate to the T-block  106  through a large hole or slot  132   a  in the base plate, for adjustment of the T-block&#39;s vertical position relative to the base plate  132 ; and twin threaded bolts  137   a ,  137   b , which pass through axially opposed end-pieces  132   b ,  132   c  of the base plate to enable adjustment of the T-block&#39;s axial position relative to the base plate. The base plate  132  and the lifting plate  134  may be machined from two pieces of steel bar or plate stock. 
     The base plate  132  has a horizontally extending skirt or platform portion  132   d , which is hidden in  FIG. 6  but whose thickness is indicated by the dashed line. The platform portion  132   d  extends over, and is seated on, the engine casing&#39;s horizontal split plane  108  and is fixed thereto by bolts or setscrews (not shown). 
     With regard to the tie rod  136 , its bottom end is secured in a threaded hole  106   f  in the top of the T-block  106  and its top end  136   a  is constituted by a ball swivel  136   b  that is held in a PTFE lined steel bearing race within the tie rod end  136   a . A suitable tie-rod for use in this embodiment is a McMaster-Carr® tie-rod with a right-hand thread and a ball joint rod end, part number 607451K281, see http://www.mcmaster.com. The top side of the lifting plate  134  is provided with a support groove  134   c  for the tie rod end  136   a.    
     With regard to the lifting plate  134 , it may be described as having a pivot end  134   a  and a jacking end  134   b . The underside of the pivot end  134   a  is provided with a part-cylindrical portion  134   d , through which the lifting plate makes line contact with the top side  132   e  of the base plate. To facilitate incremental raising and lowering of the jacking end  134   b  of the lifting plate, the underside of the jacking end  134   b  is seated on a hydraulic cylinder  138 . This is pressurised through a flexible armored hydraulic tube  139 , which is connected to a hand-operated hydraulic pump (not shown). The Enerpac® hydraulic pump and cylinder combination noted previously can be used here. The hydraulic cylinder&#39;s plunger  140  contacts the top side  132   e  of the base plate  132 . Hence, when the hydraulic cylinder  138  is pressurised or depressurized, the lifting plate  134  pivots about its pivot end  134   a  as its jacking end  134   b  is raised or lowered by small increments in and out of the hydraulic plunger  139 , thereby raising or lowering the T-shaped block  106  and the attached combustor  20  through the tie rod  136 . As the jacking end  134   b  of the lifting plate is raised or lowered, the ball swivel  136   b  enables the top end  136   a  of the tie rod to move by small increments as required within the support groove  134   c . Ball swivel  136   b  also enables the tie rod to remain vertically oriented as the vertical position of the combustor is adjusted and maintained by inserting shims between the location faces  102   b / 106   b  and  104   b / 106   e.    
     Regarding axial positioning of the combustor  20 ,  FIG. 6  shows that the part of the base-plate  132  which projects inwardly from the casing  58  over the T-shaped block  106 , is shaped like a horizontally aligned square bracket   with two downward-pointing arms  132   b ,  132   c . Threaded bolts  137   a ,  137   b  pass through corresponding axially extending threaded holes  132   f ,  132   g  in the downward-pointing arms  132   b ,  132   c  and flat ends of the bolts bear against axially opposed flat ends  106   g ,  106   h  of the cross-bar of the T-shaped block  106 . The bolts  137   a ,  137   b  run parallel to the engine&#39;s rotational axis and when rotated in a complementary manner (e.g., bolt  137   a  clockwise and bolt  137   b  the same amount counterclockwise), they cause the T-block  106 , and hence the combustor  20 , to move to-and-fro axially relative to the base plate  132  and the fixed structure of the engine, in particular the nozzle guide vane annulus  22 . In effect, the bolts act like a screw jack arrangement to move the combustor axially with respect to the engine casing. This enables the axial position of the combustor to be adjusted and then maintained by inserting shims between the location faces  102   a / 106   c  and  104   a / 106   d.    
     The fixture  130  and hydraulic jack  138  at locations  62  and  64  also facilitates minor side-to-side adjustment of the combustor (i.e., horizontal movements normal to the engine&#39;s rotational axis) while it is raised on the hydraulic jack, the correct positioning being maintained by inserting (or removing) shims between the location faces  94   a / 96   c  and  94   b / 98   c  at location  60 . 
     Once the position of the outlet of the combustor  20  (as defined by the tipping segments  86  and the bone segments  88 ,  FIG. 2 ) has been satisfactorily adjusted relative to the inlet side of the nozzle guide vane annulus  22  as described above, the combustor can be secured in its final position within the engine and the fixtures  110  and  130 , with their associated hydraulic jacks, can be removed. Final assembly of the engine can then continue. Furthermore, during maintenance of the engine, the fixtures  110  and  130  allow adjustment of the shims without disassembly or removal of the combustor from the engine, so reducing engine outage time and enabling more exact alignment of the combustor. Proper combustor alignment reduces stresses on Zone  2  of the combustor, resulting in increased component life. 
     Whereas the above description has focused mainly on the use of hydraulic jacks to incrementally adjust the position of a machine component within a machine casing, other types of jacking apparatus, such as screw jacks, may be substituted for hydraulic jacks, provided such apparatus is controllable to move the component by small amounts. 
     The present invention has been described above purely by way of example, and modifications can be made within the scope of the invention as claimed. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. Each feature disclosed in the specification, including the claims and drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and its cognates, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.