Variable stator vane linkage system and method

The linkage system for the variable vane stators includes a pumphandle driven by the linear actuator and a cooperating pumphandle slider bracket where the slider bracket includes a machined hard coated stop for limiting travel of the pumphandle when the vanes are at the full opened position. The pumphandle through the drive link circumferentially positions a synchronizing ring that is connected through a plurality of drive links to each of the vanes circumferentially spaced in the stator for changing the angle of each of the vanes as scheduled by a control and actuator for optimizing the performance of the compressor and gas turbine engine. The steps for assembling the linkage system and actuator for synchronizing the positions of the stops on the actuator and slider bracket relative to a given position of the stator vanes defines a method for providing a rigless variable vane system.

TECHNICAL FIELD 
This invention relates to gas turbine engines and particularly to the 
variable stator vane system and the means for orienting the mechanical 
linkages relative to the vanes and actuator and the method therefor. 
BACKGROUND ART 
As is well known in the gas turbine engine art, it is typical to include 
variable stator vanes in certain stages of compression in the compressor 
section. In order to enhance engine performance, reliability and power 
output, the angle of the vanes are varied to a particular schedule during 
the operating envelope. Compressor efficiency is maximized by orienting 
the angle of attack of the engine working fluid before flowing to the 
compressor blades of the compressor rotor. This requires angular changes 
of each of the vanes in a stator row of vanes. In order to effectuate this 
change, a unison or synchronizing ring (sync ring) by way of linkages is 
attached to each of the vanes and an actuator(s), scheduled by the engine 
control, through a stator linkage system including a pumphandle and slider 
bracket to mechanically position the sync ring(s). 
U.S. Pat. No. 4,755,104 granted to J. H. Castro and R. S. Thompson on Jul. 
5, 1988 entitled "Stator Vane Linkage" and assigned to United Technologies 
Corporation, the assignee common to this patent application, describes a 
typical variable stator vane system of the type which is a concern in this 
invention. As noted in this patent, adjustment of the individual vane is 
carried out by a mount rotatable about a radially oriented axis linking 
each blade of an individual stage together by a plurality of corresponding 
vane arms extending perpendicular to each axis of rotation for each blade. 
Each arm further being joined at the end thereof to the sync ring 
encircling the generally cylindrical compressor case and causing equal 
radial rotation in each linked stator vane in response to relative 
circumferential displacement between the unison ring and the compressor 
case. 
Problems, particularly in maintenance, replacement of and the wear on the 
stator vane system, have occurred resulting in misscheduling of the stator 
vanes. In other words, when the linkages, components or actuators, are 
reassembled under the current rigging procedure (the procedure for setting 
the vane angle relative to the linkage and actuator) mischeduling problems 
have occurred where the angle of the vanes is no longer correlated to the 
input signal of the actuator. This problem is also a result occasioned 
from the wear of certain component parts of the stator linkage system. 
To appreciate the problem, it is best to understand the rigging procedure 
for the heretofore known stator vane system design. A typical system 
consists of an external bellcrank that is actuated by an externally 
mounted hydraulic actuator. Generally an actuator mounted on the wall of 
the fan duct or the compressor case is connected to an externally mounted 
bell crank that, in turn, rotates an internal bellcrank through a torque 
shaft configuration. The internal bellcrank is connected to a pumphhandle 
by a link which rotates about a pivot bolt, and a slider bracket mounted 
to the engine case establishes the plane of rotation. The pumphandle, in 
turn, is connected to a series of sync rings through an equal number of 
links. A single engine will typically employ two of these systems equally 
spaced around the compressor. 
What has been described immediately above is conventional and well known 
technology. 
The procedure for rigging this assembly is as follows: The internal bell 
crank is rotated until a rigging hole in the pumphandle is aligned to a 
rigging hole in the slider bracket. A pin is temporarily inserted into the 
holes to hold the pumphandle in place relative to the slider bracket. An 
adjustable stop screw mounted on the slider bracket is then adjusted to 
contact with the pumphandle and locked down with a jam nut. At this point 
the rig pin is then removed. This now represents the rigged (open) 
position of the pumphandle, snyc rings and vanes. This procedure is 
repeated on the other side of the compressor. In installations where the 
actuator is affixed to a fan duct, the fan duct can now be installed and 
the external bellcrank is inserted through the fan duct and secured to the 
internal bellcrank. In other installations the actuator and external 
bellcrank are connected directly to the compressor case. In either 
embodiment, the final rigging procedure is to then torque the external 
bellcrank until the pumphandle contacts the set screw. The clevis of the 
actuator is then turned until it aligns with the external bellcrank (with 
the actuator fully retracted) and then bolted in place. Ideally, this 
would allow the actuator and pumphandle to contact their stops 
simultaneously. 
As mentioned herein above, this system has evidenced problems occasioned by 
using the wrong size rigging pin, over torquing the stop screw (thus 
yielding the pumphandle), not contacting the pumphandle with the stop 
screw, over-torquing the external bellcrank (also yields the pumphandle), 
and a series of other human error mistakes all of which result in 
mischelduled variable vanes. Since the position of the vanes affects the 
angle of attack of the working fluid medium, the operation of the 
compressor is adversely affected. 
I have found that I can obviate the problems noted above and eliminate the 
complex rigging procedure alluded to in the paragraph immediately above as 
well. In accordance with my invention, a fixed stop is machined on the 
slider bracket to which the pumphandle will contact when its at its 
correct rigging position. This creates a fixed rigging reference point and 
the rigging holes are thusly, eliminated. Since the contact areas on the 
pumphandle and slider bracket can be machined to the same tolerance as the 
rigging holes, there will be no increase in vane misposition due to 
manufacturing and assembling tolerances. The vanes will be set to their 
correct positions when the hardware is bolted to the case, and no further 
internal rigging is required. 
Another problem that is evidenced in the heretofore known variable stator 
vane actuating systems is that as a result of the misrigging the contact 
stresses occasioned by the adjustable stop screw contacting the pumphandle 
prior to the actuator hitting its stop, continuing force of the actuator 
results in an significant over yield of the pumphandle. For the reasons 
enumerated above, the misrigging causes the stops on the actuator and 
pumphandle to become out of sync. Ideally, the stops should hit 
simultaneously if the system is rigged correctly. The problem is even 
further acerbated in a turbofan installation where the actuator is mounted 
on the fan duct. In this type of installation the thermal growth 
differences between the fan duct and the actuator causes the pumphandle to 
contact the stop screw prior to the actuator hitting its stop which causes 
compressive yielding of the pumphandle. Obviously, the problem compounds 
every time the actuator is removed for service without sliding the duct to 
rerig the system. When the actuator is reinstalled, the external bellcrank 
is torqued to where the stop screw contacts the pumphandle which is now 
displaced as a result of the yielding and the actuator clevis is adjusted 
to fit on the external bellcrank. The system is now misrigged and the 
wear/yielding cycle starts again. The heretofore known systems typically 
place a crown configuration on the contact portion of the stop screw which 
causes very high contact stresses when the pumphandle is loaded against 
the stop screw, resulting in pumphandle yielding. Attempts to obviate this 
problem by increasing the contact area of the stop screw/pumphandle so as 
to lower contact stresses, have been unsuccessful to prevent the yield 
problem. 
This invention obviates this yield problem by making the width of the 
machined fixed stop of this invention sufficiently large so that the 
contact stresses are well within acceptable limits for both the pumphandle 
and slider bracket. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an improved adjustable stator 
vane for a gas turbine engine. 
A feature of this invention is to provide for an adjustable stator vane a 
fixed stop mounted on the slider bracket that engages the pumphandle at a 
predetermined position in the operating envelope. 
A feature of this invention is to provide a fixed stop on the slider 
bracket whose contact area with the pumphandle is sufficient so that the 
contact stresses are within acceptable limits for both the pumphandle and 
slider bracket. 
This invention provides a method of rigging the variable stator vane system 
that is characterized as significantly reducing the assembly time in 
comparison to heretofore known systems. 
The fixed stop of this invention is characterized by eliminating human 
error resulting in misscheduling of the vanes and minimizing system wear 
both of which will insure an accurate variable vane schedule for the life 
of the engine. 
The foregoing and other features of the present invention will become more 
apparent from the following description and accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While this invention is being described in its preferred embodiment as 
being incorporated on an axial flow turbofan gas turbine engine where the 
actuator and external bellcrank are mounted on the fan duct, it is to be 
understood, as one skilled in this art will appreciate, the invention can 
be employed with other types of turbofan engines where the actuator and 
external bell crank are mounted elsewhere, or for that matter in other 
types of gas turbine engines. 
The invention includes the method of assembly to obtain a rigless variable 
vane system. The term rigless in the context of this invention means that 
once the mechanism excluding the actuator and external bellcrank of the 
system is adjusted and set (initial rigging), it requires no further 
rigging. This is in contrast to the heretofore known systems that require 
rigging after each maintenance and overhaul of the system. 
Referring now to FIG. 1 which shows a schematic cross sectional arrangement 
of a typical axial flow turbofan gas turbine engine generally indicated by 
reference numeral 10 having an inlet 12 for admitting axially flowing air 
into a forward fan section 14. A portion of the air driven by the fan 
enters the gas generator, or hot core, comprised of a compressor section 
16, diffuser section 18, a combustor section 20 and a turbine section 22. 
The air exiting the fan section 14 bypasses the gas generator and flows 
axially rearward through an annular bypass air passage 24 formed between 
the exterior of the compressor case 26 and a surrounding, coaxial fan duct 
28. The hot core gases exiting the turbine section 22 and the bypass air 
both exit the engine outlet nozzle 30. 
As is typical in the compressor section 16, the air passes through a number 
of axially arranged compressor stages each consisting of a row of stator 
vanes and a compressor rotor. Typically, some of the stator vanes are made 
to vary in order to maximize the angle of attack of the air entering into 
the adjacent blades of the compressor rotor. 
As shown in FIG. 2 which is a cross section of three rows of variable 
stator vanes 32 with the compressor rotors removed that are varied by the 
linkage system generally indicated by reference numeral 36. As shown, a 
radially oriented torque shaft 38 is supported at the radially inward end 
by a spherical bearing 40 secured to the compressor case 26 and universal 
bearing 42 secured to the fan duct 28. Torque shaft 38 is free to rotate 
about its longitudinal axis and rotational motion is imparted thereto by a 
laterally extending drive arm 44 which is connected to the drive shaft 48 
of drive actuator 34 (shown in FIG. 3). Linear actuator 34 supportably 
secured to fan duct 28 in the preferred embodiment is operable by 
hydraulics, but other mediums such as electrical or pneumatic may likewise 
be utilized. The rotational motion of the drive shaft 38 moves sync rings 
54 by a linking means comprising a push rod 56 linking the sync ring 54 
and a pivoted beam or pumphandle 58. Beam 58 is pivoted about an axis 60 
radially oriented with respect to the generally cylindrical compressor 
case 26. The beam 58 is, in turn, linked to the torque shaft 38 by a drive 
link 62 disposed between the beam 58 and a laterally extending internal 
bellcrank 64 secured to the torque shaft 38 intermediate the compressor 
case 26 and fan duct 28. 
Rotational motion of the torque shaft induced by the linear actuator 34 
pivots beam 58 driving the unison ring 54 via the rings links 56. The 
circumferential movement of the sync ring 54 rotates the stator vanes 32 
of an individual stator stage via the linking vane arms 68. 
FIGS. 3 and 4 exemplify a modified stator vane actuation system that is 
used in the rear compressor variable vanes of a turbofan gas turbine 
engine that utilizes this invention. FIG. 3 is an exploded view and FIG. 4 
is a partial perspective view showing the details of the stator vane 
actuation system. As noted the system consists of the external bellcrank 
70 connected to the hydraulic actuator 34 (like reference numerals depict 
like elements in all the Figs.). The external bellcrank 70 is connected to 
the internal bellcrank 72 via the torque shaft configuration 38 (similar 
to that shown in FIG. 2). The internal bellcrank 72 is connected to the 
pumphandle 74 by link 76. Pumphandle 74 rotates about pivot bolt 78 and is 
disposed in pumphandle slider bracket 80. The slider bracket 80 
establishes the plane of rotation. The pumphandle 74 in turn is connected 
to a series of sync rings 82 (only one being described for the sake of 
simplicity and convenience it being understood that the other two sync 
rings are substantially similar to the one being described). Sync ring 82 
is connected to the end of pumphandle 74 by the drive link 84. Drive link 
84 is suitably connected to the pumphandle and the sync ring 82 by 
suitable nut and bolt assemblies as shown in FIG. 3. A single engine will 
typically include two such mechanisms, as described, equally spaced around 
the compressor. 
As is the case with the embodiment in FIG. 2, translation of the linear 
actuator 34 rotates the external bellcrank 70 which, in turn, through the 
torque shaft, rotates the internal bellcrank causing the pumphandle drive 
link 76 to pivot the pumphandle which, in turn, positions the sync ring 82 
for circumferential movement 88. The movement is translated to each of the 
vanes 32 via the connecting links 90. As mentioned in the Background 
portion the linkages must be adjusted to schedule the position of the 
vanes to the input of the actuator 34. This is accomplished by the fixed 
stop mechanism shown in FIG. 5. 
In accordance with this invention and shown in FIGS. 4 and 5 the slider 
bracket 80 is configured with a machined axial hard stop 100. In the 
preferred embodiment the contact area of stop 100 is hard coated with a 
suitable material such as nickle, chrome or their alloys or the like, by a 
well known coating technique such as plasma spray, ion vapor deposition or 
the like. The stop 100 that is machined on the inner face of extension 91 
serves to abut against the front edge 92 of pumphandle at a central 
location. The method is to installing the linkage system and then to 
adjust the vanes to the wide open position and at this point the 
pumphandle engages the hard stop 100. Since the contact areas on the 
pumphandle and slider bracket can be machined to the same tolerances as 
the heretofore used rigging holes no increase vane misposition due to 
tolerances will be realized. The vanes will now be set to their correct 
positions when the hardware is bolted to the case. The actuator 34 is then 
attached by aligning the clevis 96 to fit into the arm of the external 
bellcrank 70 and the stop on the actuator is set by the collar 98. 
From the foregoing it will be appreciated that when the stop of the 
pumphandle is set, the internal hardware is bolted to the case and the 
vanes will be set to their correct position and no further internal 
rigging is required for the life of the engine. This is in contrast to the 
heretofore systems where riggings are required after most maintenance and 
overhaul procedures. 
FIG. 6 which is a partial plan view of the prior art mechanism and is 
included herein to contrast the current method with the heretofore method 
of rigging the linkage system. In the heretofore method the radial holes 
109 and 107 (only the upper hole in the slider bracket is in view) on the 
slider bracket 80 and the pumphandle 74, respectively, are aligned. (The 
rigging holes 107 and 109 are shown in FIGS. 3 and 4 for illustration 
purposes as they no longer serve any useful purpose and hence, can be 
eliminated in accordance with this invention). A rigging pin (not shown) 
is temporarily placed through the holes to hold the pumphandle in place. 
The stop screw 102 which is threadably supported, is adjusted to contact 
with the pumphandle and is locked down with a jam nut. The pin is then 
removed. The external bellcrank is then secured to the internal bellcrank. 
Since the pumphandle is connected to the internal bellcrank and is free to 
move from the stop while being connected, the pumphandle will be in a new 
position away from the stop. Next, the external bellcrank is torqued until 
the pumphandle contacts the set screw 102. Finally, the actuator clevis is 
then turned until it aligns with the external bellcrank (the actuator 
being fully retracted) and then bolted in place. 
As will be appreciated from the foregoing, when the rigging pin is placed 
in the pumphandle rigging hole and the rigging hole of the slider bracket 
which is attached to the compressor case, it positions the pumphandle in a 
fixed position relative to the compressor case. The heretofore 
understanding was that when rigging the variable vanes, adjustments to 
pumphandle running position are made. This is a misconception. The only 
adjustments that are made are to position the stop screw to contact the 
pumphandle in this rigged (open) position. As a result typical errors were 
made during assembly by using the wrong size rigging pin, overtorquing the 
stop screw, not contacting the pumphandle with the stop screw, 
over-torquing the external bellcrank and a series of other mistakes which 
resulted in misscheduling of the variable vanes. 
This invention eliminates this complex assembly procedure and the human 
errors that were incidental thereto. Additionally, the invention 
eliminates the wear/yielding problem by making the width of the machined 
hard stop 100 large enough so that contact stresses are well within 
acceptable limits for both the pumphandle and slider bracket. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be appreciated and understood by 
those skilled in the art that various changes in form and detail thereof 
may be made without departing from the spirit and scope of the claimed 
invention.