Abstract:
A method for measuring the gap between a multibladed rotatable member and a surrounding housing in a gas turbine engine. In accordance with the method, a blade is removed from the rotatable member and replaced by a distance measuring clearance probe. The distance from the probe to the housing is then measured at a number of axial positions of the rotatable member.

Description:
BACKGROUND 
     This invention concerns a measurement method, and particularly a method for measuring the gap between a multibladed rotatable member and a surrounding housing, and especially a method for measuring such gaps in a gas turbine engine. 
     This invention relates particularly to measuring the clearance between multibladed rotatable members and a surrounding housing in a gas turbine engine. The clearance gaps being measured could be any of the gap between a fan rotor path lining and a fan blade tip; the gap between a compressor seal segment and compressor blade tips; or the gap between a turbine seal segment and turbine blade fins. 
     During assembly and also maintenance of gas turbine engines such as used for example in aircraft, it is important to measure these clearances. Variations in clearance may be encountered due to wear and oxidation of particular segments, for instance in the lining of compressors and turbines. Also, the blades may be of different lengths and/or profiles. 
     Previously such clearances may have been measured by removing the respective rotor or rotors, and using a static or portable coordinate measuring machine, which is a quite time consuming operation. For instance, it may be necessary to remove a number of casings to permit removal of the rotor or rotors, and then to replace the casings to enable the measurement to be carried out. The process must then be repeated to allow the rotor to be replaced. 
     Not all coordinate measuring machines have a sufficient resolution to carry out such work. For instance, in a gas turbine jet engine the compressor requires an axial scan of the lining at sixteen circumferential positions on each stage of the rotor path, with an axial resolution of every 0.1 mm or better. 
     Fan tip clearances have been measured by measuring the gap directly and hand positioning the fan blade forwards and outwards, and measuring the gap between the blade and the casing. 
     SUMMARY 
     According to a first aspect of the invention there is provided a method for measuring the gap between a multibladed rotatable member and a surrounding housing, the method including removing a blade from the rotatable member and locating a distance measuring clearance probe on the rotatable member in place of the blade, and measuring the distance from the probe to the housing at a number of axial positions of the rotatable member. 
     According to a second aspect of the invention there is provided a method for measuring the gap between a rotatable member and a surrounding housing, the method including removing the rotatable member from the housing, and replacing it with an alternative rotatable member bearing a distance measuring clearance probe, and measuring the distance from the probe to the housing at a number of axial positions of the alternative rotatable member. 
     The method may include measuring the distance to the housing during rotation of the rotatable member. 
     The clearance probe may be in the form of a laser measuring device. 
     The rotatable member may be any of a fan rotor, compressor rotor or turbine rotor, with the blade being respectively any of a fan blade, compressor blade or turbine blade. 
     The clearance probe may be arranged to provide radio data transfer during use. 
     The method may also include measuring the blade tip profiles by locating a distance measuring tip probe in the housing or a dummy housing, rotating the rotatable member past the tip probe, and measuring the distance therefrom to the rotatable member. 
     The tip probe may be in the form of a laser measuring device. 
     The tip probe may be arranged to provide radio data transfer during use. 
     A plurality of tip probes may be provided in the housing or a dummy housing, spaced in use across the width of the blade tip, for providing multiple measurements thereacross. 
     The values recorded by the clearance and tip probes may be compared, to provide maximum, minimum and mean gap sizes. 
     The invention also provides a method for measuring the gap between a multibladed rotatable member and a surrounding housing in a gas turbine engine, the method being according to any of the preceding eleven paragraphs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional circumferential sectional view through part of a fan of a gas turbine engine showing a first method according to the invention being carried out; 
         FIG. 2  is a similar view to  FIG. 1  showing a further part of the first method being carried out; 
         FIG. 3  is a diagrammatic circumferential sectional view through part of a fan case of a gas turbine engine showing a second method according to the invention being carried out; 
         FIGS. 4 and 5  are similar views to  FIG. 3  showing different parts of the second method being carried out; 
         FIG. 6  is a diagrammatic circumferential sectional view through part of a turbine of a gas turbine engine showing a third method according to the invention being carried out; and 
         FIGS. 7 and 8  are similar views to  FIG. 6  showing further parts of the third method being carried out. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIGS. 1 and 2  show part of a fan assembly  10  with a fan case  12  which contains a fan rotor  14  which mounts circumferentially therearound a plurality of fan blades  16 . The clearance gap between the case  12  and fan blades  16  is shown at  18 . 
     To measure the gap  18  during construction or maintenance, a one of the fan blades  16  is removed from the rotor  14  and a probe  20  in the form of a laser system is mounted on the rotor  14  by a dummy blade or root fixing  22 . The distance to the fan case  12  from the probe  20  is measured as the rotor  14  is rotated. The probe  20  may be wireless to utilise radio data transfer to transfer data to a data storage unit (not shown). 
       FIG. 2  illustrates a method for determining the profile of the tips of the blades  16 . Three probes  24  again in the form of laser systems are mounted in the case  12  or a similar dummy casing (balancing rig) so as to locate adjacent the tip of the blades  16  as the rotor  14  is rotated. The probes  24  are also connected, and probably by a wireless arrangement, to the data storage unit. 
     The data from the probes  20 ,  24  is combined using software to calculate the gap  18  across the tip of the blades  16 , to provide for instance maximum, minimum and mean values. 
       FIGS. 3 to 5  show part of a compressor assembly  26  of a gas turbine engine. The compressor includes a rotor (not shown) which mounts a plurality of discs  28 . Each disc  28  mounts a plurality of radially outwardly extending blades  30 . The compressor  26  includes a casing  32  with a gap between the rotatable blades  30  and the casing  32 . The rotatable blades  30  are interspersed in adjacent discs  28  by stators  34  mounted on the casing  32 . 
       FIG. 3  shows a compressor assembly  26  where a conventional blade  30  has been replaced by a blade  36  which locates a wireless laser probe  38  for measuring the location of the blade  36  relative to the casing  32 , and thus providing a datum. 
       FIG. 4  shows a one of the discs  28  having been removed and replaced by a wireless laser probe  40  which measures the distance from the rotor to the compressor casing  32 . 
       FIG. 5  shows a further probe  42  which has been mounted to a dummy casing (balancing rig) or casing  32 . The probe  42  measures the distance to the tips of the rotating blades  30  in a similar manner to the probes  24  described above. Again data from the probes  38 ,  40  and  42  will be compared to calculate the gaps between the tips of the rotatable blades  30  and the casing  32 , to provide for instance maximum, minimum and mean values for the different blades  30  during rotation. 
       FIGS. 6 to 8  show a turbine assembly  44  of a gas turbine engine again with a plurality of blades  46  attached by discs to a rotor (both not shown). A gap  50  is provided between the blades  46  and a turbine casing assembly  52 . 
       FIG. 6  shows where a blade  46  has been removed and replaced by a blade  54  which locates a wireless laser probe  56  for measuring the location of the blades  54  relative to the casing assembly  52 , and thus providing a datum 
       FIG. 7  shows a situation where turbine blades  46  have been removed and a probe  58  has been mounted on the respective disc for measuring the distance to the casing  52 . 
       FIG. 8  shows a further probe  60  located in the casing  52  or on a fixture for measuring the distance to the tips  62  of the blades  46  to measure the tip profiles. Again the data received from the probes  56 ,  58  and  60  can be compared to provide an indication of the maximum, minimum and mean values of the tip gaps around the turbine  44 . 
     There are thus described methods for providing repeatable processes for measuring the blade tip gaps in the fans, compressors and turbines of a gas turbine engine. These methods are repeatable and significantly less time consuming than existing arrangements. The methods are therefore more efficient and more reliable for measuring tip clearances. These systems permit 3D topography for seal surfaces and blade clearances to be obtained. 
     Various modifications may be made without departing from the scope of the invention. For instance the method may be usable in other parts of a gas turbine engine. Other types of probes may be usable, and these could be hard wired in particular situations.