Note: Descriptions are shown in the official language in which they were submitted.
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, Description
Viscous Damper With Rotor Centering Means
Technical Field
This invention relates to t:win spool engine and
particularly to viscous damped bearings.
Background Art
As is well known, it is common practice to support
the high speed shaft of a twin-spool gas turbine engine
with oil damped radial bearings. It is also generally
known that one can change the natural frequency of the
rotating pressure field as to avoid instability that
comes about due to uncontrollable vibration by spriny
loading the viscous damper. Reissue Patent No. 30,210
given to D. F~ Buono, N. G. Carlson, D. H. Hibner and
15 D- C. Moringiello on February 12, 1980 and assigned to
the same assignee as this patent application, exemplifies
such a system, where a spring is mounted parallel to the
viscous damper. This changes the natural fre~uency of
the pressure field.
~e have, found that use of viscous dampers in a high
speed twin spool engine inherently affects the tip
clearance of the rotor. This is a result of the rotor
sagging within the clearance provided by the oil film.
Obviously, the increased gap allows an excessive amount
of air leakage between the tip of the rotor blade and
its adjacent seal resulting in a gain of thrust specific
fuel consumption ~TSFC). To avoid this gain in TSFC we
provide a centering spring to maintain the concentric
clearance of the rotor which avoids the majority of the
sag. The spring rate of this system is selected so that
it provides maximum support for the rotor weight, for
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given "G" and gyro loads, while maintaining sufficient
flexibility to allow oil film damping under conditions
of high rotor imbalance.
Disclosure of Invention
An object of this invention is to provide a
method of preventing rotor sag for a twin spool engine
that employs viscous damped bearings.
In accordance with a particular embodiment of
the invention, there is provided a rotor enclosed in a
casing having a rotatable shaft supporting a plurality
of compressor blades at one end and a plurality of
turbine blades at an opposite end. At least two bear-
ings are axia]ly spaced along the shaft for rotatably
supporting the shaft. Fluid damper means on one of the
bearings include a cavity adjacent the bearing in which
fluid is fed to support the bearing when in the rotating
mode. The shaft is distortable as a function of the
depth of the cavity and radially displaces the compressor
blades a similar amount~ The tips of the blades are
sufficiently spaced to prevent touching of the inner
diameter of the casing. Means are provided for decreas-
ing -the distance of displacement of the tips of the
blades including a resilient element attached to the
bearing for preventing the bearing from displacing the
depth of the cavity.
A feature of the invention is to utilize a
centering spring for the viscous damped bearing so that
the bearing rotor of the shaft does not bottom against
the bottom wall of the reservoir of the viscous damper
during aircraft maneuvers. The spring rate of the
centering spring is optimized so as to minimize for
rotor to case relative motion while maintaining the
required amount of motion within the viscous damper.
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~ ther features and advantages will be apparent
from the speclfication and clalms and from the accom~
panying drawings which illustrate an embodiment of the
invention.
Brief Description of Drawings
Figure 1 is a partial schematic of the rotor and
support system without the inclusion of this invention.
Figure 2 is a partial schematic identical to
Figure 1 and showing the damped bearing with the present
invention.
Figure 3 is a partial view partly in schematic
and partly in section showing the details of the in-
vention.
Best Mode for Carrying Out The Invention
While this invention is described in its preferred
embodiment with a rotor/shaft for a -twin spool gas turbine
engine being supported by three bearings, it is to be
understood that the scope of the invention is not limited
thereto.
To best understand this invention reference is made
to Figure 1 which shows the rotor generally illustrated
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by reference numeral 10 being supported by three bearings.
Bearings 12 and 1~ are radial load bearings and bearing
16 is the thrust load bearing. The particular bearings
are well known and for further details reference is
hereby made to the models JT9D and JTlOD manufactured by
Pratt & Whitney Aircraft Group of United Technologies
Corporation, the assignee of th:Ls patent application.
As can be seen in Figure 1, the rotor sags because
its weight causes it to seat on the bottom of the viscous
damper wall. The amount of sag is related to the dis-
placement of the bearing relative to the oil damp gap
which occurs when the rotor is sllbject to gravitation
or gyroscopic loacling. In that instance the weight and
inertia of the rotor, that is, shaft 18, stages of
compressor blades 20 and stages of turbine blades 22
force the shaft to distort as shown by the bow of center
line 24.
Figure 2 represents the same rotor system when
employing the present invention. The centering spring 30
suitably supports the bearing so that the gap of the oil
damper 32 remains substantially undisturbed when the
rotor is subject to aircraft maneuvers.
As is apparent from the foregoing, the viscous
damped bearing is located intermediate the compressor and
turbine. This is not to be construed as a necessary
aspect of this invent;on, as one ordinarily skilled in
the art will recognize that the damped bearing can be
located in other locations and more than one bearing may
be damped.
Hence, by employing centering spring 30, the annular
gap defined by the oil reservoir 32 assures that the gap
"A" remains substantially the same when the engine is
in the static condition. In actual test it was found
that the gap allowance could be reduced from .005 inch
to .001 inch which represents a considerable benefit in
TSFC.
Details of a preferred ernbodiment of a viscous
damped bearing centered by a centering spring is shown
in Figure 1. As can be seen the roller bearing generall~
illustrated by reference numeral ~0 comprises inner race
42, outer race 44, sandwiching a plurality of rollers 46
(one being shown) retained in its relative positions by
cage 48. The inner race 42 is supported to shaft 50 by
the end locking members 52 and 54. The outer race 44 is
secured into position and supported to ring element 56
by lock nut and rivet assembly 58. The viscous damper
is obtained by sealing the ends of support ring 56 with
the annular seals 58 and 60 whi~h project beyond the
outer surface 62 of support ring 56 and bear against the
inner diameter of annular ring 64. Note that the bearing
assembly is not supported by the seals 58 but rather are
supported by cantilever spring 70. Hence, the bearing
is centered by the centering spring which comprises a
plurality of cantilevered springs 70 (one being shown).
As noted, one end of spring 70 is secured to the bulk-
head or casing support member 72 by the nut assembly 74and the other end is secured to the extension portion 76
of ring element 56 by nut assembly 78.
As is apparent from the foregoing, the centering
spring 70 serves to maintain the gap "B" in the oil
damper. It is obvious that the centering spring can be
made more effective in terms of supporting the rotor
as being deslgned for increased stiffness. However, this
would result in reduced motion within the ~iscous damper
under conditions of rotor imbalance twhirl) loads. Thus
the support spring rate must be optimized for the two
conflicting requirements.
It should be understood that the invention is not
limited to the particular embodiments shown and described
herein, but that various changes and modifications may
be made without departing from the spirit and scope of
this novel concept as defined by the following claims.
