The present invention relates to the field of turbomachines such as an axial
compressor of a gas turbine engine, and is particularly intended for the
variable-pitch stator blades of the machine.
An articulated system, such as the variable-pitch stator blades of a gas
turbine engine compressor, comprises parts that move relative to one
another. Figures 1 and 2 show schematically a variable-pitch stator blade
mounted in the casing 3 of the machine. The stator blade comprises an
aerofoil 12, a plate or platform 13 and a rod forming a pivot 14 at one end.
The pivot 14 is housed in a bore or radial orifice made in the wall of the
casing 3 via various bearings. The blade is held by this end only. The other
end holds an annular floating element 16 in which it is mounted so as to
pivot via a second pivot 17. The ring is provided with sealing means for the
portion of the rotor 18 that is adjacent to it. The pivot 14 swivels in the
corresponding bore of the casing by means of bearings, for example a
bottom bearing 4. The platform 13 is housed in a cavity in the form of a
counterbore machined in the wall of this casing. The wall of the casing is in
radial contact with the platform 13 either directly or by means of a bush or
shim. The top portion of the pivot 14 is held in a top bearing 5. The
opposite face of the platform 13 relative to the bearing 4 forms the base of
the aerofoil and is swept by the gases set in motion by the compressor. This
face of the plate is shaped so as to ensure the continuity of the stream
formed by the casing. A nut holds the blade in its housing and a lever
actuated by appropriate control members controls the rotation of the blade
about the axis XX of the rod in order to place the latter in the required
position relative to the line of the gaseous flow. The relative movements
result from the sliding of the surfaces in contact with one another.
In the case of an axial compressor of a gas turbine engine or else an axial
compressor only of air or another gas, such as a blast furnace or natural
gas, the aerofoil 12 is subjected over the whole of its length to the
aerodynamic and pressure forces generated by the gaseous flow. The
component of these forces oriented perpendicularly to the chord in the
pressure side to suction side direction, usually passing via the axis of the
pivot, is the greatest. It is noted however that, in the case of major
deflections, the.component may move away from this axis. The aerofoil is
also subjected to axial forces of static pressure directed upstream because
of the pressure difference between downstream and upstream. The resultant
force is illustrated by the arrow F in the figures. The result of this is the
application of a moment that, associated with the rotation of pitch about the
axis XX over an amplitude that may reach and exceed 40 degrees, creates
an intense zone of friction. This friction leads secondarily to wear of the
plate and/or the bushes. This first zone 20 of intense friction is located on a
portion of the surface of the plate. It is indicated by crosses in Figure 2.
Therefore, in normal operation of the machine, because of these tilting
forces applied to the aerofoil 12, the plate presses via this first zone 20
against the surface of the housing made in the wall of the casing, while on
the portion diametrically opposed to the pivot, the pressing forces are zero
or very slight.
In the aeronautical field, any excess weight should be avoided and
independently of the excess pressure of any excess load, there is also an
attempt to eliminate any weight that fulfils no function whether it be
mechanical or aerodynamic.
The applicant also has the constant objective of finding solutions that make
it possible to lighten the machine without, for all that, compromising its
performance and its reliability. Any weight saving improves the efficiency
of the machine and makes it possible to reduce operating costs.
In pursuit of this objective, the applicant has therefore arrived at the present
invention that relates to a variable-pitch stator blade.
According to the invention, the turbomachine variable-pitch stator blade
comprising an aerofoil, extended on one side by a pivot by which it is
mounted so as to rotate in a bore of the casing of the turbomachine, and a
plate, between the aerofoil and the pivot, perpendicular to the line formed
by the aerofoil and the pivot, is characterized in that, since the face of the
plate opposite to the aerofoil comprises a first zone and a second zone, the
first zone being subjected to an intense friction with the wall of the casing
because of the transverse forces applied to the aerofoil and the second zone
being subjected in normal operation to a less intense friction than the first
zone, the thickness of the plate on the second zone is reduced relative to the
thickness of the plate on the first zone.
Variable-pitch blades, particularly of an axial compressor, of the prior art
have a plate of uniform thickness if no account is taken of the curvature
and/or non-linearity of the gas stream. So, thanks to the invention, it is
possible to reduce the weight of this portion of the blade without
compromising its functionality, namely ensuring the continuity of the
stream and reducing leakages along the pivot.
The second thinner zone extends practically over an arc of 60 to
120 degrees about the axis of the pivot.
For an axial compressor, the first zone is situated on the suction side and
the second zone extends from the pressure side of the aerofoil.
Preferably, the second thinner zone is delimited by a border - in particular
the upper face of this border is in the extension of the flat top surface of the
plate - that is thicker than the first zone so as to form a decompression
chamber between the periphery of the plate and the pivot which makes it
possible to improve the seal. In addition, this border makes it possible to
form a contact in the case where the forces are reversed, particularly when
compressor pumping phenomena occur. In addition, this arrangement is
advantageous when it comes to assembling the machine as it prevents the
parts from tilting exaggeratedly.
A simple and economic means of producing blades with a second zone
arranged in this way is to machine the plate. Depending on the tool chosen,
the cavity has a bottom that is flat, curved or else any other shape.
Other features and advantages will emerge from the following description
of a non-limiting embodiment of the invention with reference to the
drawings in which:
Figure 1 shows, in section along the axis of the machine, an example
of a conventional variable-pitch stator blade mounted in a compressor
casing,
Figure 2 shows the same blade seen from above,
Figure 3 is a view in perspective of a blade portion having the
features of the invention.
Figure 3 shows a stator blade on its own in its portion close to the pivot 14.
The plate 13 is seen from above in perspective. According to one
embodiment of the invention, machining has been carried out on the face of
the plate turned towards the casing 3 to make a cavity 22A in the second
zone 22 that is not subjected to the compression forces resulting from the
application of the force F on the aerofoil 12. This cavity 22A here has been
machined by means known per se. The bottom of the cavity is flat; it could
be curved if the machining head were ball-shaped. Shapes other than that
shown are possible. In addition, instead of machining, the cavity may
originate from casting, forging or powder metallurgy. The cavity preferably
extends over an arc, of a circle for example, of 60 to 120 degrees,
advantageously corresponding to the arc subtending the zone of intense
friction. The function of this cavity is to reduce the weight of the blade but
not reduce its mechanical characteristics. The resulting thickness of the
plate is therefore sufficient to ensure the mechanical strength of the plate. It
can be seen that a border 23 has been retained on the periphery of the plate.
This border has a dual function. The first is to form a decompression
chamber reducing the air leakages between the stream of the turbomachine
and the pivot 14 through the bore of the casing in which the pivot 14 is
housed. The second function is to form a bearing surface in the case of
reversal of the forces resulting from an operating anomaly of the
turbomachine, such as the pumping of the compressor, or else to simplify
the assembling operations. The width of this border need not be constant.
For example, it could be wider in a zone to be reinforced. Advantageously,
its upper plane is in the plane of the plate facing the casing.
A solution has been described in which the thickness is reduced on the
upper face of the plate. However, it is also a part of the invention to carry
out this thickness reduction by forming a cavity in the face of the plate
situated on the side of the gas stream or by thinning the plate via this face.

Claims
1. Turbomachine variable-pitch stator blade comprising an aerofoil
(12), extended on one side by a pivot (14) by which it is mounted so
as to rotate in a bore of the casing (3) of the turbomachine, and a
plate (13), between the aerofoil and the pivot, perpendicular to the
line formed by the aerofoil and the pivot, characterized in that, since
the face of the plate opposite to the aerofoil comprises a first zone
(20) and a second zone (22), the first zone (20) being subjected to an
intense friction with the wall of the casing (3) because of the
transverse forces applied to the aerofoil (12), the thickness of the
plate on the second zone (22) is reduced relative to the thickness of
the plate of the first zone (20).
2. Blade according to Claim 1, whereof the thinner portion of the
second zone (22) extends over an arc of 60 to 120 degrees about the
pivot (14).
3. Blade according to Claim 1, whereof the first zone is situated on the
suction side and the second zone extends from the pressure side of
the aerofoil.
4. Blade according to Claim 1, 2 or 3, whereof the second thinner zone
is delimited by a border (23) so as to form a decompression chamber
between the periphery of the plate (13) and the pivot (14).
5. Blade according to one of the preceding claims, whereof the second
thinner zone (22) forms a cavity obtained by machining the plate
(13).
6. Blade according to the preceding claim, whereof the cavity has a flat
or curved bottom.
7. Blade according to one of Claims 5 or 6, whereof the cavity is
elongate in an arc of a circle.
8. Turbomachine comprising at least one stator blade according to one
of Claims 1 to 7.