Method of adjusting a triggering clearance and a trigger

A method of adjusting a triggering clearance and a trigger wherein the triggering clearance can be readily adjusted directly on the assembly line by selectively fixing an axial adjustive position between first and second triggering members.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a trigger and method for adjusting a triggering clearance suited for use in fuel shut-off mechanisms of gas turbine engines.

2. Description of the Prior Art

In gas turbine engine assembly, it is not always possible to premachine every part to the exact right dimension prior to assembly because, in some cases, the accuracy required is within the range of the tolerance stack-up on the engine. Thus, the exact dimension can only be ascertained once a group of engine parts come together during assembly. This is problematic as time is wasted in assembly having to disassemble parts in order to accurately machine a particular part to the exact required dimension.

In one particular case, the trigger of the fuel shut-off mechanism needs to be positioned in spaced relation with other engine components to define a triggering clearance that must be very accurately controlled. The accuracy required is often within the range of the tolerance stack-up on the engine, and therefore the trigger must undergo a custom grinding operation during assembly to obtain the required triggering clearance. As the trigger must be very accurately machined, it is not uncommon for grinding error to occur thus further delaying engine assembly. Customization and rework add unwanted cost and time to assembly

Accordingly, there is a desire to provide a method of adjusting a triggering clearance between engine components to reduce wasted time and effort in assembly.

SUMMARY OF THE INVENTION

It is therefore an aim of the present invention to provide a method of adjusting a triggering clearance for promoting short time engine assembly.

It is also an aim of the present invention to provide a trigger for adjusting a triggering clearance on the assembly line.

Therefore, in accordance an aspect of the present invention provided is a method of adjusting a triggering clearance comprising the steps of mounting a trigger having first and second engaged members for axial displacement when triggered, and adjusting the triggering clearance by selectively fixing an axial adjustive position between the first and second members.

In accordance with another aspect of the present invention is provided a method of adjusting a triggering clearance in a turbine engine between a fuel shut-off mechanism and an engine part comprising the steps of mounting a trigger of the fuel shut-off mechanism for axial displacement by a ruptured engine part, and adjusting the triggering clearance by selectively fixing a first member of the trigger in an axial adjustive position relative to a second member of the trigger.

In accordance with a further aspect of the present invention is provided a triggering assembly allowing for the adjustment of a triggering clearance directly on an assembly line, the triggering assembly comprising a trigger axially movable under a predetermined pressure, said trigger having a first member securely engageable in a relative adjustive axial position to a second member, an adjustment of the relative adjustive axial position varying the triggering clearance.

In accordance with yet another aspect of the present invention is provided a gas turbine engine safety fuel shut-off mechanism for actuating a fuel control unit to stop a flow of fuel in the event of a rotor shaft rupture, the mechanism comprising a trigger spaced from a rotor by a triggering clearance, the trigger having first and second members engaged in a relative axial position, the relative axial position between the first and second members being adjustable to adjust the triggering clearance, the trigger being axially displaceable when triggered by the rotor to actuate the fuel control unit.

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 PREFERRED EMBODIMENTS

FIG. 1illustrates a twin-spool turbofan engine10of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan12(or low pressure compressor) through which ambient air is propelled, a high pressure compressor14for further pressurizing the air, a combustor16in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section18for extracting energy from the combustion gases. Preferably, engine10includes a turbine exhaust case (TEC)23according to the applicants copending application Ser. No. 10/892,297, filed Jul. 16, 2004, herewith incorporated by reference. The TEC23is bolted by a flange25integrated at the front end thereof to the engine case27.

The turbine section18comprises a low pressure turbine17having at least one last downstream rotor stage including a turbine rotor securely mounted on a turbine shaft21drivingly connected to the fan12to form the low pressure spool of the engine10. The turbine section18further includes a high pressure turbine19drivingly connected to the high pressure compressor14via a tubular shaft13concentrically mounted about the shaft21. The high pressure compressor14, the high pressure turbine19and its shaft13form the high pressure spool of the engine10. The low spool and the high spool are independently rotatable with respect to one another.

One way in which the gas turbine10can fail is that the low pressure (LP) shaft21may shear; thereby disconnecting the LP turbine disc from the rest of the shaft. The low pressure turbine17can no longer drive rotation of the low pressure compressor12once the LP shaft21is sheared, thus the latter will slow down and the former will speed up out-of-control. The speed of the LP shaft21is measured by a probe, to monitor the overall speed of the turbine engine10, and in part to use this information to control fuel flow. One of the problems associated with a broken LP shaft21is that the probe detects the LP compressor12slowing down and consequently tries to correct the decrease in speed by signalling the fuel control unit to increasing fuel flow. Since fuel to the combustor16drives the high pressure (HP) shaft13, the increase in fuel flow causes the HP shaft13rotation to speed up to non-typical speeds. As the LP shaft speed continues to decrease, fuel flow continues to increase in the attempt to correct the problem; and thus, the HP shaft speed increases until the HP disc simply flies apart. In addition, with the LP shaft21broken, the LP turbine17will also speed up out-of-control until the LP disc simply flies apart.

During an LP shaft shear event, the fuel flow must be shut-off. This is achieved by a fuel shut-off mechanism20, partly shown inFIG. 2, located axially downstream of the LP shaft21. More specifically the fuel shut-off mechanism20is assembled to the LP shaft rear bearing housing22such that it is positioned behind a LP shaft bolt24locking bearing26in place.

Once the LP shaft21is sheared, the LP disc tends to move rearwardly due to pressure effects. The fuel shut-off mechanism20located behind the LP shaft bolt24is engaged by the rearward movement of the LP disc, thereby in turn triggering the fuel control unit (not shown) to shut-off the fuel. Since HP and LP disc run away occurs quickly, the clearance between the shut-off mechanism20and the bolt24must be as small as possible, but not too small. A too small clearance is problematic as the fuel shut-off mechanism20may be accidentally tripped during a dynamic event such as bird or ice ingestion in which the LP shaft may bend and move around a bit, but not break.

As shown inFIG. 2, the fuel shut-off mechanism20comprises a trigger28adapted to act on a safety fuel shut-off lever30pivotally mounted on a support32. The lever30is connected to the fuel control unit via a cable assembly (not shown). More specifically, the trigger28is loosely housed by the support32for axial movement with respect thereto.

When in the assembled position, the trigger28is in close proximity to the LP shaft bearing26and bolt24, such that when the LP shaft21moves rearwardly, the bolt24makes contact with the trigger28, thereby activating the fuel shut-off mechanism20. The trigger28acts on an upstream end34of the lever30while the cable assembly (not shown) is connected to an opposite downstream end of the lever30(not shown). The upstream end34of the lever30is mounted on a pivot36. Upon triggering of the trigger28by the LP shaft bolt24, the latter moves axially causing the lever30to pivot upwardly about pivot36to shut-off the fuel control unit via the cable assembly (not shown).

FIG. 2depicts a preferred embodiment of the trigger28suited for use in the fuel shut-off mechanism20to manually adjust a triggering clearance38. The trigger28for adjusting the triggering clearance38has an adjustable length and comprises a first member that can be provided as a pin40, fixed in a relative axial position to a second member that can be provided as a sleeve42, such that the relative axial position there between is adjustable. When the trigger28is mounted in position proximal to the bolt24, adjusting the relative axial position between the first and second members adjusts the triggering clearance38. The trigger28is adapted to be axially displaced when triggered by the ruptured LP shaft21.

Preferably, the pin40and sleeve42are threadedly engaged and the axial position is adjustable. The pin40has a preferably hexagonal head44at an upstream end46thereof, a threaded portion48at a downstream end50thereof and a central cylindrical portion51inbetween. Preferably the threaded portion48has a slightly smaller diameter than the central portion72. The threaded portion48includes external threads52disposed on the exterior surface of the pin40. The sleeve42can be provided in the form of a cap having an axial opening56defined at an upstream end58thereof and an abutting or capping surface60defined at a closed downstream end62thereof. The capping configuration is preferred since it provides thread protection. The abutting surface60is adapted to contact the lever30for actuation thereof when the trigger28is triggered. The opening56extends axially a majority of the length of the sleeve42and includes internal threads64for mating with the external threads52of the pin40. Thus, the pin40is threaded into the internally threaded sleeve42such that the threaded portion48of the pin40is received in the opening56of the sleeve42. As the pin40can be screwed and unscrewed relative to the sleeve42, the relative axial position between the two members is adjustive. Screwing the pin40causes an increase in the number of external threads52mating with internal threads64thereby shortening the distance from the head44of the pin40to the abutting surface60of the sleeve42. Unscrewing the pin40causes the opposite effect. Therefore, the overall length of the trigger28is determined by the relative axial position between the pin40and the sleeve42.

Still referring toFIG. 2, it can be seen that the sleeve42has a flange66extending radially outwards for engagement with the support32when in the assembled position. The support32is configured to mate with the flange66to prevent rotational movement of the sleeve42. Hence, the sleeve42is prevented from rotating when the pin40is screwed or unscrewed. The flange66is preferably rectangular with a pair of straight sides68that abut corresponding straight surfaces70of the support32for preventing rotation of the sleeve42. Notably, the support32does not prevent the sleeve42from sliding axially as the sleeve42is only loosely fitted in a corresponding recess defined in the support32. However, the support does limit the freedom of motion of the sleeve42in the forward axial direction by virtue of the engagement of flange66with the corresponding inner shoulder of the support32(seeFIG. 2).

FIG. 2shows the trigger28mounted to the bearing housing22. More specifically, the bearing housing22defines an axial bore72with first and second diameter portion74and76respectively for receiving the pin40of the trigger28. The first diameter portion74has substantially the same diameter as the central portion51of the pin40for ensuring a tight fit while the second diameter portion76, aft of the first, is larger in diameter. The trigger28, supported by the support32, is mounted to the bearing housing22by inserting the pin40through the axial bore72from within the bearing housing such that the head44protrudes forwardly from an upstream opening78thereof. The central portion51of the pin40extends through both the first and second diameter portion74and76, fitting snugly through the first diameter portion74.

As the head44of the pin40extends into a cavity80of the bearing housing22filled with lubricating oil, it is advantageous to create a seal. Firstly, by the loose fit between the first diameter portion74and the central portion51and secondly by the incorporation of a packer seal82about the central portion51of the pin40compressed in the second diameter portion76.

The bearing housing22and support32also engage when assembled together to better seal the axial bore72as well as to provide greater structural rigidity. The support32has a forwardly extending cylindrical piece84for axial insertion into the second diameter portion76of the bearing housing22. The cylindrical portion80is adapted to fit between the central portion51and the second diameter portion76adjacent the packer seal82. Depending on the relative axial position between the pin40and the sleeve42, the cylindrical piece84of the support32may apply an axial pressure against the packer seal82bettering the seal.

Furthermore, the trigger28comprises locking means86for securing the relative adjustive axial position between the pin40and the sleeve42once the desired gap38. Thus, once the triggering clearance38has been regulated through the adjustment of the trigger28, it is necessary to fix the relative axial position between the pin40and the sleeve42. It should be understood that securely fixing the pin40and the sleeve42together can be achieve in a variety of ways. For instance, the locking means86can comprise a locking plate88mounted at the upstream opening78of the axial bore72as depicted inFIGS. 2 and 3. The locking plate88is preferably circular with an L-shaped tongue90extending both radially and axially. The locking plate88defines a central hexagonal aperture92for receiving the hexagonal head44of the pin40therethrough.

Notably, the pin head44and the shape of the aperture92can be any appropriate shape so long as both parts remain functional, that is, the pin can be screwed at the head44and the locking plate88can prevent the pin40from rotating once in place. The head44of the pin40is preferably hexagonal to facilitate screwing thereof by a tool passing axially through the bearing26into the cavity80of the bearing housing22. Also, the axially extending portion of the tongue90facilitates manipulation of the locking plate88by way of a tool positioned through the bearing housing22as aforementioned.

The bearing housing22includes a corresponding locking base94for engaging with the locking plate88. Preferably, the locking base94is integral to the bearing housing22, including five outcroppings96equally spaced in a circular array for abutting the periphery of the locking plate88as illustrated inFIG. 3. Each pair of adjacent outcroppings96defining a slot98there between for receiving the tongue90. Therefore, the locking base94includes five slots98for positioning the locking plate88in five different orientations. The locking plate88and corresponding locking base94are designed to accommodate any angular position of the hexagonal head44thereby maintaining the pin40in a relative axial position with the sleeve42.

Moreover, the locking means86can further comprise a spring-loaded retaining clip100adapted to be received in a corresponding circumferential groove defined in the bearing housing22to axially retain the locking plate88as depicted in this exemplary embodiment. The retaining clip100prevents the pin40from freely moving axially in the forward direction by locking the locking plate88to the locking base94. For instance, in a case where the pin40is completely unscrewed from the sleeve42, the retaining clip100prevents the pin40and locking plate88from falling forward into the cavity80of the bearing housing22.

FIGS. 2 and 3show the retaining clip100engaged with the locking base94adjacent the locking plate88. More specifically, the outcroppings96have lips102that extend radially inward against which the retaining clip100abuts when positioned. The retaining clip100is preferably annular with an open end104providing flexibility. To install the retaining clip100it is first compressed, then positioned adjacent the locking plate88and released so as to engage behind the lips102of the outcroppings96.

Therefore, the locking means86of the trigger28exemplified in this embodiment limit the freedom of motion of the pin40rotationally by way of a locking plate88and base94and a retaining clip100respectively.

Now referring back toFIG. 2, the method of adjusting the triggering clearance38entails first mounting the trigger28having first and second “telescopically” related portions for axial displacement when triggered. In this exemplified embodiment the first and second portions are the pin40and sleeve42, however it should be clear that alternative embodiments exist without departing from the scope or depth of the invention. The sleeve42and the pin40are mounted to the support32for axial movement relative thereto in response to a pushing action of the low pressure shaft bolt24. Second, the method entails adjusting the triggering clearance38by selectively fixing a relative axial adjustive position between the first telescopically related portions, namely the sleeve42and the pin40. The sleeve remains in its set position relative to lever30and the pin40is screwed further into the sleeve42or partly unscrewed therefrom to vary the effective length of the trigger28and, thus, adjust the gap38. The pin40is rotated counter clockwise or clockwise by engaging a tool with the hexagonal head44while the sleeve42is locked against rotation.

To adjust the relative axial position between the pin40and the sleeve42, the pin40can be screwed or unscrewed until a position is selected and fixed. The relative axial position can be fixed by a number of locking means86as aforementioned. For example, the locking plate88can be installed along with the retaining clip100.

The method of adjusting the triggering clearance38above-described is advantageous over prior art methods as the triggering clearance38can be adjusted on the assembly line when the turbine engine10is being assembled. Due to the fact that the triggering clearance38varies depending on the range of the tolerance stack-up on the engine, it is preferable to be able to quickly and easily adjust the triggering clearance38on the assembly line. The elimination of the time required to grind the pin to the required length represents a major cost saving.

FIG. 4illustrates another embodiment of the present invention. The reference numerals used for various elements in this embodiment correspond to the reference numerals utilized in the preferred embodiment but have been raised by 100.

The trigger128illustrated inFIG. 4comprises locking means186differing from the preferred embodiment above-described. A helical coil106is mounted between the external threads152of the pin140and the internal threads164of the sleeve142to provide substantial resistance when screwing or unscrewing the pin140; therefore, limiting the rotational freedom of motion of the pin140. For example, up to 14 pounds of torque can be required to turn the pin140with the helical coil106insert installed.

Notably, a similar outcome can be attained without the use of the helical coil106but with a deformed sleeve242that is slightly flattened on opposing sides rather than perfectly annular. Thus, the shape of the deformed sleeve242renders the task of screwing and unscrewing the pin240arduous.

Also, the pin140comprises a flange108extending radially from the central portion151adjacent the threaded portion148thereof. The flange108is adapted to abut surface110of the support132for limiting forward axial freedom of motion of the pin140.

FIG. 5illustrates yet another embodiment of the present invention. The reference numerals used for various elements in this embodiment correspond to the reference numerals utilized in the preferred embodiment but have been raised by 200.

The trigger228illustrated inFIG. 5comprises a threaded portion248that is larger in diameter than the central portion251. Such a configuration limits the axial freedom of motion of the pin240as the threaded portion248is adapted to abut surface210of the support232. In this exemplary embodiment the rotational freedom of motion of the pin240relative to the sleeve242is restricted by a helical coil insert206as in the embodiment depicted inFIG. 4.

Furthermore, it can be seen that the sleeve242comprises a first axial opening256at an upstream end258thereof and a second axial opening261at a downstream end262thereof. The abutting surface260is provided by a pin cap212that is preferably integral to the pin240rather than by the sleeve242. The pin cap212abuts the lever230of the fuel shut-off mechanism220for actuation thereof when the trigger228is triggered. Thus, in this particular embodiment the central portion251of the pin240requires a diameter less than or equal to the diameter of the threaded portion248to ensure that the sleeve242can be mounted from the upstream end246of the pin240. The pin cap212is larger than opening261to prevent pin240from falling into the bearing cavity.

Notably, in still another embodiment of the present invention, the pin cap212can be provided as a separate part to the pin240to allow the sleeve242to be mounted from the downstream end250of the pin in the case where it was necessary to have a central portion251with a diameter larger than the diameter of the threaded portion248. The pin cap212can be welded or brazed in place once the sleeve242is threadedly engaged with the pin240.

The trigger28embodied herein enables the pre-assembled turbine exhaust case (TEC) comprising the fuel shut-off mechanism, the safety cable and the engine rear cone to be quickly assembled to the rest of the turbine engine. The trigger28is designed to facilitate manipulation and adjustment thereof.