Stator vane stage in axial flow compressor

A stator vane stage in a split-case, variable geometry axial flow compressor includes two 180-degree spoke-like arrays of stator vanes on upper and lower case halves of the compressor case and a pair of unitary, 180-degree arc shroud ring segments. Each stator vane has a pivot shaft at one end rotatably supported in a radial bore in the corresponding case half by a bushing and a cylindrical vane button at the other end. Each shroud ring segment has a corresponding plurality of cylindrical vane button sockets in an outer surface thereof which rotatably receives a corresponding vane button. The spoke-like array of the stator vanes rigidly positions the shroud ring segments relative to the compressor case. To assemble the shroud ring segments to the stator vanes, the shroud ring segments are first resiliently squeezed radially inward while the vane buttons are sequentially inserted into the corresponding vane button sockets and then released to spring back to their true 180-degree arc shapes in which they are captured on the stator vanes.

FIELD OF THE INVENTION 
This invention relates to stator vane stages in variable geometry axial 
flow compressors in gas turbine engines. 
BACKGROUND OF THE INVENTION 
In typical axial flow compressors in gas turbine engines, an annular 
airflow channel of progressively decreasing area is defined between a 
compressor case and a rotor in the case. Annular rotor blade stages 
motivate flow in the airflow channel and annular stator vane stages 
between the rotor blade stages redirect the airflow. In variable geometry 
axial flow compressors, the stator vanes are rotatable about spoke-like 
radial axes of the case. A hub-like shroud ring on the radially inner ends 
of the stator vanes defines the inner boundary of the airflow channel 
where it traverses the stator vane stage and supports seals which minimize 
leakage. In split-case axial flow compressors of fixed or variable 
geometry, where the case is split in a horizontal center-plane of the 
compressor for assembly purposes, the shroud ring is likewise split into a 
pair of 180-degree arc shroud ring segments. 
Many arrangements have been proposed for attaching shroud ring segments to 
stator vanes in split-case, axial flow compressors. In a fixed geometry 
proposal, a pair unitary or one piece 180-degree arc shroud ring segments 
are attached to the stator vanes through hook-like projections on the 
inner ends of the vanes which seat in individual sockets in the unitary, 
shroud ring segments. In some prior variable geometry proposals, short 
arc-shaped shroud ring segments are assembled with corresponding groups of 
vanes and then unitized into 180-degree arc segments by end plates or like 
connecting devices. In other prior variable geometry proposals, 180-degree 
arc shroud ring segments are formed by bolting together two 180-degree arc 
end pieces. In the latter proposals, inner buttons or projections of the 
vanes are rotatably sandwiched between the bolted-together end pieces. 
While the bolted-together proposals do not require as many individual 
pieces as the multi-segment proposals, they are limited to relatively 
large compressors because the diameters of the shroud ring segments must 
be large enough to accommodate both the inner vane buttons and the bolts 
or other fasteners holding the end pieces together. A stator vane stage 
and method of making the same according to this invention features unitary 
or one-piece 180-degree arc shroud ring segments rotatably connected to 
variable geometry stator vanes. 
SUMMARY OF THE INVENTION 
This invention is a new and improved stator vane stage for a split-case, 
variable geometry axial flow compressor and a method of making the same. 
The stator vane stage according to this invention includes a plurality of 
stator vanes each having a pivot shaft at an outboard end for rotatably 
supporting the vane on an upper or lower half of the case and a 
cylindrical vane button at an inboard end which is rotatably received in a 
complementary cylindrical socket in a corresponding one of an upper or 
lower unitary, 180-degree arc shroud ring segment. Each half of the case 
has a 180-degree array of radial bores which receive bushings and 
respective ones of the vane pivot shafts whereby the stator vanes are 
rotatably supported on the upper and lower halves of the case in 
180-degree spoke-like arrays. The spoke-like mounting of the stator vanes 
prevents radial or lateral bodily shiftable movement of the hub-like 
shroud ring segments so that additional support for the shroud ring 
segments is unnecessary. The method according to this invention of making 
the aforesaid stator vane stage includes the steps of forming a loose 
spoke-like array of stator vanes on the upper and lower halves of the case 
by fitting the vane pivot shafts in the radial bores without the bushings, 
flexing the unitary shroud ring segments by squeezing the ends thereof 
radially inward, sequentially fitting the vane button sockets over the 
vane buttons on the stator vanes, releasing the shroud ring segments to 
permit them to spring-back to their true semi-circular shapes, inserting 
the bushings between the vane pivot shafts and the corresponding radial 
bores in the case, and completing the shroud ring by bolting together the 
upper and lower halves of the case.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2 of the drawings, a schematically illustrated gas 
turbine engine 10 includes a cylindrical case 12 having a longitudinal 
axis 14. The case is split in a horizontal center-plane containing the 
axis 14 and includes a first or upper case half 16 and a second or lower 
case half 18. The upper case half includes a pair of longitudinal edges 
20A,B and a pair of integral flanges 22A,B at the edges 20A,B, 
respectively. The lower case half includes a corresponding pair of 
longitudinal edges 24A,B and a corresponding pair of integral flanges 
26A,B at edges 24A,B. The upper and lower case halves abut at the edges 
20A,24A and 20B,24B and are held together by a plurality of bolts 28 
through appropriate holes in the flanges. 
Within the case 12, the engine 10 includes a split-case, variable geometry 
axial flow compressor 30, an annular combustor 32, and a compressor 
turbine 34. Air enters the compressor at a front end 36 of the case and is 
delivered at a higher pressure to the combustor 32. Combustion of a 
fuel/air mixture in the combustor 32 generates a stream of hot gas motive 
fluid which expands through a nozzle ring 38 and through an annular stage 
of blades 40 of the turbine 34. The motive fluid is exhausted through a 
nozzle, not shown, and a back end 42 of the engine. 
The variable geometry compressor 30 includes a frustoconical rotor 44 
cooperating with the bolted-together upper and lower case halves 16,18 in 
defining an annular airflow channel 46 which progressively decreases in 
cross sectional area toward the combustor 32. The rotor 44 carries a 
plurality of airfoil-shaped blades in the channel 46 arrayed 
circumferentially in a plurality of annular stages 48A-D. A plurality of 
schematically illustrated unison rings 50A-C surround the case 12 and 
operate a plurality of schematically illustrated crankarms 52A-C. Each 
crank arm is connected to a corresponding one of a plurality of stator 
vanes arrayed in a plurality of annular stator vane stages 54A-C according 
to this invention between the rotor blade stages 48A-D. 
The stator vane stage 54A is representative of the stages 54A-C and is 
illustrated in more detail in FIGS. 2-4. The stage 54A includes a 
plurality of stator vanes 56 arrayed annularly in wheel-spoke fashion 
between the bolted-together upper and lower case halves 16,18 and a split, 
hub-like shroud ring 58. Each stator vane 56 includes an airfoil 60, a 
disc-like bearing 62 at the top of the airfoil, a cylindrical pivot shaft 
64 outboard of the bearing 62, and a cylindrical vane button 66 at the 
bottom of the airfoil. The outboard end or stem of each pivot shaft 64 is 
threaded and milled to define a pair of flats 68 for attaching a 
corresponding one of the crankarms 52A thereto for rotation as a unit 
therewith. 
As seen best in FIGS. 2 and 3, each pivot shaft 64 is disposed in a bore 70 
of greater diameter in one of the upper and lower case halves 16-18. Each 
bore 70 is located in a plane perpendicular to the axis 14 and is aligned 
on a corresponding one of a plurality of generally radial or wheel-spoke 
axes 72 of the case 12. A bushing 74 between each bore 70 and the 
corresponding pivot shaft 64 defines a journal bearing between the pivot 
shaft and the corresponding one of the upper and lower case halves. The 
vanes 56 are thus supported on the upper and lower case halves through 
their pivot shafts in 180-degree arrays and in wheel-spoke fashion for 
rotation about the radial axes 72. 
A first washer 76 between the bearing 62 on each vane 56 and a 
corresponding spotface 78 on the upper and lower case halves cooperates 
with a second washer 80 and a nut 82 on the stem of each pivot shaft 
outside the upper and lower case halves in retaining the vanes on the case 
halves. When the unison ring 50A is shifted back and forth in the 
direction of longitudinal axis 14, the crankarms 52A attached to the stems 
of the pivot shafts 64 rotate the vanes 56 about their respective radial 
axes 72. 
The split shroud ring 58 of the vane stage 54A includes a first or upper 
shroud ring segment 84 and a second or lower shroud ring segment 86. Each 
shroud ring segment is a unitary or one-piece 180-degree arc-shaped member 
having no bolts or other fasteners characteristic of earlier sandwich-type 
shroud ring segments. 
The upper shroud ring segment 84 has an outer surface 88 facing the upper 
case half 16, an inner surface 90 opposite the outer surface 88, and a 
pair of planar ends 92A,B. The lower shroud ring segment 86 has an outer 
surface 94 facing the lower case half 18, an inner surface 96 opposite the 
outer surface 94, and a pair of planar ends 98A,B The planar ends 92A,98A 
and 92B,98B abut in the aforesaid horizontal center-plane of the case 12 
when the upper and lower case halves 16,18 are bolted together. 
As seen best in FIGS. 2-3, the cylindrical vane buttons 66 on the stator 
vanes 56 are received in respective ones of a plurality of vane button 
sockets 100 defined by cylindrical bores in the outer surfaces 88,94 of 
the upper and lower shroud ring segments 84,86. The sockets are centered 
on the radial axes 72 of the case and a plurality of bushings 102 
rotatably journal the vane buttons in respective ones of the sockets 100 
so that the stator vanes are rotatable relative to the upper and lower 
shroud ring segments about the radial axes 72. The inner surfaces 90,96 of 
the shroud ring segments carry with a seal material 104 which cooperates 
with a plurality of raised edges 106 on the rotor 44 in preventing airflow 
inside the shroud ring. 
The stator vanes 56 function like the spokes of a wheel to rigidly support 
the shroud ring segments 84,86 on the upper and lower case halves of the 
compressor. When the upper and lower case halves are bolted together at 
the flanges 22A,26A and 22B,26B, the upper and lower shroud ring segments 
84,86 abut at the planar ends 92A,98A and 92B,98B and cooperate to define 
the rigid shroud ring 58. The outer surfaces 88,94 of the shroud ring 
segments cooperate in defining the inside wall of the airflow channel 46 
where the latter traverses the stator vane stage 54A. 
The method of forming the representative stator vane stage 54A according to 
this invention includes the steps of forming the radial bores 70 in the 
upper and lower case halves and forming the cylindrical vane button 
sockets 100 in the upper and lower unitary, 180-degree arc shroud ring 
segments 84,86 as described above. The method further includes the 
following steps. With the upper and lower case halves separated, the pivot 
shafts of each of the corresponding stator vanes 56 are fitted into 
respective ones of the radial bores 70 in the upper and lower case halves 
without the bushings 74, thereby to define on the upper and lower case 
halves loose 180-degree spoke-like arrays of stator vanes as partially 
shown in FIG. 5A. 
Then, each of the upper and lower shroud ring segments 84,86 is pinched or 
squeezed radially to resiliently deflect the planar ends 92A-B toward each 
other and 98A-B toward each other. Turnbuckles, not shown, or similar 
devices may be used to effect and maintain the aforesaid resilient 
deflection of the shroud ring segments. In a stator vane stage having a 
shroud ring of on the order of 12 inches in diameter, the resilient 
deflection of the planar ends of the shroud ring segments toward each 
other may be about 0.4 inches. 
The upper and lower shroud ring segments 84,86 are assembled onto the 
corresponding ones of the stator vanes in the loose arrays by sequentially 
inserting each of the vane buttons 66 into corresponding ones of the vane 
button sockets 100, FIG. 5B. It has been found advantageous to perform 
this step of the method by starting with the vane button on an end or 
outside vane 56 of the 180-degree array of vanes and the socket 100 
adjacent the corresponding one of the planar ends 92A,B and 98A,B of the 
shroud ring segments and to then proceed sequentially to the other of the 
outside vanes and corresponding vane button sockets. Then, the forces 
squeezing the planar ends of the shroud ring segments together are 
released, as by unscrewing a turnbuckle, to permit the segments to spring 
back to their true 180-degree arc shapes, FIG. 5C. In that position, the 
shroud ring segments are captured on the stator vane buttons due to the 
aforesaid spoke-like orientations of the vanes. 
After the shroud ring segments are assembled on the stator vanes, the 
bushings 74 are installed over the pivot shafts of the respective stator 
vanes and seated in the radial bores 70. The bushings are retained on the 
upper and lower case halves by the washers 80 and the nuts 82. In the 
final step, performed after the rotor is positioned between the upper and 
lower case halves, the upper and lower case halves are bolted together at 
the flanges 22A,26A and 2B,26B.