Metal working – Method of mechanical manufacture – Impeller making
Reexamination Certificate
1998-12-16
2001-04-03
Cuda, I (Department: 3726)
Metal working
Method of mechanical manufacture
Impeller making
C029S889230
Reexamination Certificate
active
06209198
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to assembly methods and fixtures therefor. More particularly, this invention relates to a fixture and method for assembling a variable stator vane assembly of a gas turbine engine, by which components of the vane assembly can be selected to compensate for part variances and thereby optimize the operation and service life of the assembly.
BACKGROUND OF THE INVENTION
Conventional gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. In turbofan engines having multistage compressors, stator vanes are placed at the entrance and exit of the compressor section and between adjacent compressor stages in order to direct the air flow to each successive compressor stage. Variable stator vanes, whose pitch can be adjusted relative to the axis of the compressor, are able to enhance engine performance by altering the air flow through the compressor section in response to the changing requirements of the gas turbine engine.
A high pressure compressor variable stator vane assembly
10
is shown in
FIGS. 1 and 2
. The assembly
10
includes a stator vane
12
mounted within an opening
38
in a casing
22
of a gas turbine engine. As known in the art, in order to alter the pitch of the vane airfoil relative to the axis of the compressor, the stator vane
12
is designed to rotate within the opening
38
of the casing
22
. While various configurations are possible for variable stator vane assemblies, the vane
12
shown in
FIGS. 1 and 2
has a radially extending flange
30
from which an annular-shaped portion extends axially to define a pair of seats
28
(unless otherwise noted, radial and axial directions referred to are with reference to the centerline of the vane assembly
10
, and not the radial and axial directions of the engine in which the assembly
10
will be installed). A trunnion
34
also extends axially relative to the flange
30
, and with the seats
28
projects through the opening
38
as seen in FIG.
2
. The vane
12
is secured to the casing
22
with a nut
20
that also secures a spacer
14
, sleeve
16
and lever arm
18
to the trunnion
34
. Rotation of the vane
12
within the opening
38
is caused by actuation hardware (not shown) attached to the lever arm
18
.
During engine operation, an overturning moment is created by the gas loads on the vane airfoil, generating reaction forces represented by the arrows “F” in FIG.
2
. As a result, rotation of the vane
12
relative to the casing
22
requires a seal assembly that minimizes wear, friction, and compressor air leakage while subjected to the reaction forces F, as well as withstands the hostile thermal and chemical environment of a gas turbine engine. In
FIGS. 1 and 2
, a seal assembly is shown as consisting of a bushing
24
and washer
26
between the spacer
14
and flange
30
on opposite sides of the casing
22
. The bushing
24
and washer
26
are preferably molded from composite materials, such as polyimide resin with glass and TEFLON® fibers, in order to be environmentally compatible with the engine environment, as well as provide suitable low-friction bearing surfaces that enable the vane
12
to rotate at acceptable torque levels.
The ability to minimize radial air leakage from the compressor through the opening
38
of the casing
22
is an important function of the bushing
24
and washer
26
. As can be appreciated from
FIG. 2
, the dual functions of the bushing
24
and washer
26
to form an air seal yet enable rotation of the vane
12
are determined by the clearance (radial relative to the axis of the compressor) through the bushing
24
and washer
26
between the flange
30
of the vane
12
and an outer annular surface
36
of the spacer
14
. To minimize compressor air leakage, the vane
12
and spacer
14
must be assembled to the casing
22
so that the minimum possible clearance is achieved. However, an excessively small clearance results in high forces being required to turn the vane
12
, which can overstress the actuation hardware and, in the extreme situation, could completely prevent actuation of the vane
12
, leading to compressor stall. On the other hand, an excessive clearance will not only permit excessive air leakage from the compressor, but will also permit the reaction forces on the vane
12
to cause excessive tilting of the vane assembly
10
. If this occurs, the reaction forces F are more concentrated in the bushing
24
and washer
26
and, in combination with higher leakage through the seal assembly, causes more rapid deterioration of the bushing
24
and washer
26
.
From
FIG. 2
, it can be seen that the clearance through the bushing
24
and washer
26
is determined by the axial offset dimension “D” between the annular surface
36
and a pair of shoulder
32
of the spacer
14
. When the vane
12
and spacer
14
are properly assembled, each of the shoulders
32
abuts one of the seats
28
of the vane
12
as shown in FIG.
2
. Increasing the offset dimension D reduces the clearance through the vane
12
and spacer
14
but increases the actuation torque required to rotate the vane
12
, while decreasing the offset dimensions D increases the clearance but decreases the actuation torque.
In the art, variable stator vane assemblies of the type shown in
FIGS. 1 and 2
have been assembled to attain a torque level within an acceptable range for the actuation hardware. Because it has been assumed that a close relationship exists between the offset dimension D and the torque required to rotate the vane
12
, spacers
14
with incrementally different offset dimensions D have been purposely manufactured to allow adjustment of both the actuation torque and radial clearance by substituting spacers
14
. After assembly, if the torque required to rotate a vane is outside preestablished torque limits, the nut
20
, lever arm
18
, sleeve
16
and spacer
14
are removed and the spacer
14
replaced with another having a different offset dimension D. For example, if the actuation torque was too high, a spacer
14
with a smaller offset dimension D was installed, while a spacer
14
with a greater offset dimension D is installed if an unacceptably low torque is measured. Once reassembled, torque is again remeasured and the process repeated if the torque remains outside the established limits.
Notwithstanding the above, further investigations have shown that the torque required to rotate the stator
12
is surprisingly relatively independent of the spacer
14
installed, and that torque is not a reliable indication of the radial clearance between the vane
12
, spacer
14
and casing
22
. Instead, actuation torque has been found to be primarily determined by irregularities and interferences of the bushing
24
and washer
26
after they have been compressed by the load generated between the flange
30
and spacer
14
by the nut
20
. These irregularities and interferences are not predictable particularly since, while molded to tight tolerances, the composite bushing
24
and washer
26
can distort in the free state due to residual stresses, etc.
In view of the above, it can be seen that it would be desirable if a method were available for assembling a variable vane stator assembly to more consistently achieve minimum radial clearances without exceeding acceptable actuation torque levels.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, there is provided a method and fixture assembly for assisting in the matching of components of a variable stator vane assembly of a gas turbine engine. In particular, components of the vane assembly are matched so that part variances are compensated for to minimize radial cl
Bowen Wayne R.
Lammas Andrew J.
Cuda I
General Electric Company
Herkamp Nathan D.
Hess Andrew C.
LandOfFree
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