Abstract:
A mistake proof cable assembly for promoting short time engine assembly and method of assembling same. The mistake proof cable assembly comprises a slotted member defining a slot having an open end, and a cable connected to the slotted member and extending longitudinally through the slot. First and second restrainers are mounted on the cable on opposite sides of the slotted member for limiting free movement of the cable in the longitudinal direction of the slotted member. A lock prevents removal of the cable from the slot through the open end thereof.

Description:
TECHNICAL FIELD 
   The invention relates generally to a cable assembly suited for use in fuel shut-off mechanisms of gas turbine engines, and more particularly to a mistake-proof cable assembly. 
   BACKGROUND OF THE ART 
   Profitability considerations call for short time assembly. As a result, it has been proposed to build gas turbine engines from pre-assembled units to expedite the manufacturing process. One problem that arises from having pre-assembled units is that not all components thereof are always accessible. This is problematic as time is wasted disassembling parts in order to reposition them. For instance, it has been proposed to manufacture a pre-assembled trailing engine case (TEC) comprising a fuel shut-off mechanism, a safety cable and an engine rear cone. In this particular case, the safety cable is not accessible once the rear cone has been installed to complete the pre-assembled TEC. Due to the fact that the TEC is transported and manipulated prior to and during engine assembly, it therefore becomes advantageous to ensure that the safety cable remains in the proper position. 
   As a result, mistake-proof assemblies are important to reduce wasted time in assembly. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide a mistake-proof cable assembly for promoting short time engine assembly. 
   In one aspect, the present invention provides a mistake proof cable assembly comprising a slotted member defining a slot having an open end, a cable connected to the slotted member and extending longitudinally through said slot, first and second restrainers mounted on said cable on opposite sides of said slotted member for preventing longitudinal withdrawal of the cable from the slotted member, and a pin extending transversely through the slot preventing removal of said cable from said slot through said open end thereof. 
   In accordance with a further general aspect of the present invention, there is provided a cable assembly comprising a slotted member defining a slot having an open end, a cable swingable into said slot through said open end about a pivot point on said slotted member, a cable retainer securely mounted to said cable at a distance from said pivot point, said slotted member defining a curved insertion path along which said cable retainer is swung together with the cable to an operation position wherein the cable retainer cooperates with the pivot point to prevent longitudinal withdrawal of the cable from the slot, and a lock to at least limit reverse swinging motion of the cable in the slot. 
   In another aspect, the present invention provides a gas turbine engine safety fuel shut-off mechanism for engaging a fuel control unit to stop a flow of fuel in the event of a turbine shaft rupture, the mechanism comprising a trigger, a lever actuable by said trigger, a slot defined in the lever, the slot having an open end, a cable assembly coupled at a first end thereof to the lever and at a second end thereof to the fuel control unit, the cable assembly having a cable extending longitudinally through the slot, first and second stoppers mounted on the cable on opposite sides of the lever for preventing longitudinal withdrawal of the cable from the lever, and a locking member preventing removal of the cable from the slot through the open end thereof. 
   In yet another aspect, the present invention provides a method of assembling a cable to a slotted member defining a slot having an open end, the method comprising the steps of: a) restraining the longitudinal movement of the cable at opposite sides of the slotted member, and b) independently from step a), restraining the cable from moving out of the slot through said open end thereof. 
   Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 

   
     DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the accompanying figures depicting aspects of the present invention, in which: 
       FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
       FIG. 2  is an axial cross-sectional view of a combustion section of the gas turbine engine of  FIG. 1 , showing a fuel shut-off mechanism incorporating a cable assembly attachment in accordance with a preferred embodiment of the present invention; 
       FIG. 3  is a perspective view showing the fuel shut-off cable installation; 
       FIG. 4  is a side elevational view showing the cable once installed on the lever of the fuel shut-off mechanism; and 
       FIG. 5  is an exploded view of a trailing engine case (TEC) assembly of the gas turbine engine, a rear cone of the assembly being removed to show the fuel shut-off mechanism of  FIGS. 3 and 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
     FIG. 2  illustrates the combustion section showing the combustor  16  and turbine section  18  respectively in further detail. It can be seen that a high (HP) pressure turbine  20  is located axially upstream of a multi-stage low pressure (LP) turbine  22 . The HP turbine  20  includes a tubular HP shaft  24  connected to an HP turbine disc  26 . Similarly, the LP turbine  22  includes an LP shaft  28  connected to an LP turbine disc  30 . The LP shaft  24  can be coaxially mounted internally of the HP shaft  24  and is independently rotatable with respect thereto. Just downstream of the LP turbine  22  is shown a safety fuel shut-off mechanism  32  linked to a fuel control unit  34 . 
   One way in which the gas turbine  10  can fail is that the LP shaft  28  may shear; thereby disconnecting the LP turbine disc  30  from the rest of the shaft  28 . The turbine section  18  can no longer drive rotation once the LP shaft  28  is sheared, thus the latter will slow down. The speed of the LP shaft  28  is measured by a probe (not shown), to monitor the overall speed of the turbine engine  10 , and in part to use this information to control fuel flow. One of the problems associated with a broken LP shaft  28  is that the probe detects the LP shaft  28  slowing down and consequently tries to correct the decrease in speed by signalling the fuel control unit  34  to increasing fuel flow. Since fuel to the combustor  16  drives the HP shaft  24 , the increase in fuel flow causes the HP shaft  24  rotation to speed up and run out of control. As the LP shaft  28  speed continues to decrease, fuel flow continues to increase in the attempt to correct the problem; and thus, the HP shaft  24  speed increases out of control until the HP disc  26  simply flies apart. 
   During an LP shaft  28  shear event, the fuel flow must be shut-off. This is achieved by the fuel shut-off mechanism  32 . Once the LP shaft  28  is sheared, the LP disc  30  tends to move rearwardly due to inertia effects. The fuel shut-off mechanism  32  located behind the LP shaft  28  is engaged by the rearward movement of the LP disc  30 , thereby in turn triggering the fuel control unit  34  to shut-off the fuel. 
   As shown in  FIG. 2 , the fuel shut-off mechanism  32  comprises a trigger  36  adapted to act on a pivotable safety fuel shut-off lever  38 . The lever  38  is connected to the fuel control unit  34  via a cable assembly  40 . The trigger  36  is in close proximity to the LP shaft  28  such that when the latter moves rearwardly the trigger  36  and the LP shaft  28  make contact thereby activating the fuel shut-off mechanism  32 . The trigger  36  is acting on an upstream end  42  of the lever  38  while the cable assembly  40  is connected to an opposite downstream end  44  of the lever  38 . The upstream end  42  of the lever  38  is mounted on a pivot  41 . Upon triggering of the trigger  36  by the LP disc  30 , the lever  38  is caused to pivot upwardly about pivot  41  to shut-off the fuel control unit  34  via the cable assembly  40 . 
   Referring now concurrently to  FIGS. 3 and 4 , a preferred embodiment of the mistake-proof cable assembly comprising the lever  38  and safety cable assembly  40  is illustrated. The downstream end  44  of the lever  38  splits into a pair of parallel fingers  46 , extending in the axial direction, defining a U-shaped slot  48  therebetween. The slot  48  having an open end  50  and a semi-circular closed end  52  through which the parallel fingers  46  unite. The parallel fingers  46  have top and bottom edges  54  and  56 , each defining a straight portion  58  merging into a curved portion  60 . The curved portions  60  of both the top and bottom edges  54  and  56  curve upwards to form rounded extremities  62 . The parallel fingers  46  also include aligned circular apertures  64  for receiving a cotter pin  61  therethrough; such that the pin  61  extends in the transverse direction through the slot  48 . Preferably, the pin  61  extends perpendicular to the axial direction of the slot  48 . 
   Still referring to  FIGS. 3 and 4 , in this exemplary embodiment the cable assembly  40  comprises a cable  66  for extending through the slot  48  between the fingers  46 . A cable retainer or anchoring member, which could for instance be provided in the form of a ball element  68  is securely attached to an upper end of the cable  66 . A second cable retainer, such as a flange  70 , is fixedly mounted to the upper end of the cable  66  at a predetermined distance from the ball element  68 . The cable  66  is axially captively retained in the slot  48  by the ball element  68  and the flange  70  mounted on the cable  66  on opposite sides of the lever  38 . The ball element  68  and the flange  70  limit the longitudinal (or radial) movement of the cable  66  in the slot  48 , such that the cable  66  has a limited freedom of up and down movement. Longitudinal withdrawal of the cable  66  from the slot  48  is thus prevented by the ball element  68  and the flange  70 . 
   Particularly, the ball element  68  includes a ball  72  with a cylindrical sheath (not shown) projecting downwardly therefrom. The ball  72  having a bore  76  inline with the through of the cylindrical sheath such that the cable  66  passes through the cylindrical sheath into the bore  76  and extends out the top of the ball  72 . The ball element  68  is fixedly mounted on the top end  78  of the cable  60  adapted to be seated in a semi-hemispherical recess defined in the top surface of the lever  38  at the semi-circular closed end  52  thereof. Thus, the semi-circular closed end  52  portion of the slot  48  acts as a seat for the ball element  68 . Preferably, the diameter of the ball  72  is slightly greater than the width of the slot  48  between the parallel fingers  46  so as to prevent the ball element  68  from sliding through the slot  48 . Hence, the ball element  68  is seated within the slot  48  partially protruding therefrom above the straight portions  58  of the top edges  54  of the parallel fingers  46 . The curved portions  60  of the top edges  54  help prevent the ball element  68  from moving axially downstream along the slot  48  acting as a stopper. This is due to the fact that the force of gravity helps keep the ball element  68  from sliding up the curved portion  60  in a case where no external forces are being applied to the ball element  68 . 
     FIGS. 3 and 4  show a preferred embodiment of the flange  70  of the cable assembly  40 . Preferably, the flange  70  has first and second cylindrical portions  80  and  82  respectively. The first cylindrical portion  80  having an outside diameter greater than the width of the slot  48  and preferably also greater than the outside diameter of the second cylindrical portion  82 . The first cylindrical portion  80  also having a straight, flat upper surface  84  for abutting the bottom edges  56  of the parallel fingers  46  when the cable  66  is moved upwardly through the slot  48 . The flange  70  is fixedly mounted to the cable  66  below the lever  38  such that when the ball element  68  is seated in the slot  48 , the upper surface  84  of the first cylindrical portion  82  is in spaced relation with the bottom edges  56  of the parallel fingers  46 . The flange  70  prevents reverse movement of the ball element  68  out of the slot  48  in which the cable  66  is inserted. 
   The method of assembling the mistake-proof cable assembly  40  entails restraining the longitudinal movement of the cable assembly  40  about opposite ends of the lever, and restraining the cable assembly  40  from moving transversely in the slot  48 . The cable assembly  40  is generally pre-assembled with the ball element  68  and flange  70  mounted on the cable  66  prior to being coupled to the lever  38 .  FIG. 3  illustrates the possible motion of the cable assembly  40  with respect to the lever  38  when being installed. More specifically, the ball  72  is seated adjacent the semi-circular closed end  52  of the slot  48  and the cable  66  is then swung downwardly within the slot  48 , as shown by arrow A. The ball  72  is free to pivot axially within the slot  48  but is refrained from undergoing axial displacement along the top edges  54  by the curved portions  60  thereof. As the cable  66  is swung axially, the flange  70  also undergoes axial displacement such that the upper surface  84  of the first cylindrical portion  80  thereof contours the curved portions  60  of the bottom edges  56 . 
   According to one embodiment, the cable assembly  40  is mounted on the lever  38  such that the flange  70  is able to slide along the curved portions  60  of the bottom edges  56 , which acts as an insertion ramp. The radius of curvature of the curved portions  60  of the bottom edges  56  is substantially equal to the distance from the center of the ball  72  to the upper surface  84  of the flange  70 . Thus, the curved portion  60  of the bottom edges defines an arcuate insertion path to permit the swinging motion of the flange  70  about the center of the pivoting ball  72 . Furthermore, the force of gravity urges the cable  66  to find a position of equilibrium in the vertical longitudinal direction with the ball  72  resting in the slot  48 . This arrangement provides for easy and reliable assembly as compared to known spring loaded connections. 
   In order to ensure that the cable assembly  40  remains in proper position regardless of the manner in which the fuel shut-off mechanism  32  is manipulated, the cotter pin  61  is then inserted through the circular apertures  64  defined in the parallel fingers  46 . The pin  61  restrains the cable  66  from moving out of the open end  50  of the slot  48 . Other types of locks could be used to prevent reverse motion of the flange  70  about the ball  72 . 
   Thus, in the case where the fuel shut-off mechanism  32  comes pre-assembled as part of a trailing engine case (TEC) assembly  86  as shown in  FIG. 5 , it is advantageous for the longitudinal and transverse movement of the cable assembly  40  to be limited. Since there is no access to the shut-off mechanism  32  when the rear cone  88  is assembled, providing a fuel shut-off mechanism  32  having a cable assembly  40  that will always be sited properly is important to reduce wasted time in engine assembly. 
   Therefore, the preferred embodiment of the fuel shut-off mechanism  32 , which comprises the ball element  68  and the flange  70  mounted to the cable  66  on opposite sides of the lever  38 , advantageously refrains the ball element  68  from moving out of the slot  48  either longitudinally or transversely. Thus, the preferred embodiment of the fuel shut-off mechanism  32  ensures that the cable  66  remains in position prior to and during engine assembly. 
   The above description is meant to be exemplary only, and it should be understood that the mistake-proof cable assembly embodied as part of a fuel shut-off mechanism may also be employed for other applications in different areas of the engine. The present invention does not only apply to a turbine engine with a reverse flow combustor as illustrated in  FIGS. 1 and 2 , but to any engine. Also, one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. For example, the flange may be provided in many shapes and sizes. It may also be provided as a weighted member for pulling the cable and therefore the ball element downwardly. Another example may consist of having a slot with a varying width to further refrain the ball element from moving longitudinally or transversely. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.