Brakes – Inertia of damping mass dissipates motion – Resiliently supported damping mass
Reexamination Certificate
1999-12-03
2001-11-20
Oberleitner, Robert J. (Department: 3613)
Brakes
Inertia of damping mass dissipates motion
Resiliently supported damping mass
C188S378000, C267S285000
Reexamination Certificate
active
06318527
ABSTRACT:
TECHNICAL FIELD
This invention is directed to vibration isolator springs in a vibration isolator assembly, and more particularly, to an improved apparatus and method of mounting said springs in said assembly for use with a helicopter main rotor system.
BACKGROUND OF THE INVENTION
Mast-mounted vibration isolators are well-known in the art for canceling or substantially reducing vibratory forces active on a helicopter rotor. While most such devices are referred to as “vibration absorbers”, this may be viewed as a misnomer inasmuch as these devices typically isolate the energy produced by cyclic in-plane and out-of-plane loads rather than absorb the energy as the name implies. Such devices typically comprise: a hub attachment fitting for mounting to the main rotor hub such that the isolator is rotated in a plane parallel to the main rotor disc, and a spring-mass system mounted to and rotating with the hub member. The troublesome in-plane forces comprise (n−1) frequency vibrations and (n+1) frequency vibrations. By (n−1) vibrations we mean the vibrations which oscillate at a frequency equal to the number of blades (n) minus 1 times rotor rpm, i.e. (n−1) * rotor rpm, and by (n+1) vibrations we mean the vibrations which oscillate at a frequency equal to the number of blades (n) plus 1 times rotor rpm, i.e. (n+1) * rotor rpm. Taking a four-bladed rotor as an example, these vibrations are also sometimes referred to as 3P and 5P vibrations. The spring arm-mass system is tuned in the non-rotating condition to a frequency equal to n * rotor rpm (e.g., 4P for a four-bladed rotor) at normal operating speed, so that in the rotating condition it will respond to both N+1 and N−1 frequency vibrations (i.e., 3P and 5P).
FIGS. 1
a,
1
b
and
1
c,
depict a prior art vibration isolator similar to that described and illustrated in U.S. Pat. No. 5,901,616 to Miner et al. (hereafter ‘Miner’ and assigned to the assignee of the present invention) which is hereby incorporated by reference. As shown a vibration isolator
100
comprising a circular inertial mass
102
is supported from an inner hub
104
by resilient spring arms
106
so as to be capable of oscillation in any direction within its plane of rotation. The isolator
100
of
FIG. 1
a
is mounted to the rotor mast
108
of
FIG. 1
b
to render the configuration of isolator/mast
110
of
FIG. 1
c.
This prior art isolator
100
is capable of canceling both (n−1) and (n+1) frequency vibrations of a helicopter rotor in a single installation. As shown in
FIG. 1
a
the spiral shaped spring arms
106
are fastened to both the inner hub
104
and the circular inertial mass
102
with bolts at each of the spring arm ends
112
and
114
. These spring arms
106
are typically made from graphite composites consisting of multiple parallel layers of graphite held together by epoxy as is well known in the art.
The root end retention of these composite spring arms
106
which are incorporated in main rotor helicopter vibration isolators are most highly loaded and structurally the most critical in the isolator design. The operation of the isolator creates a high concentrated bending moment that must be reacted at the retention areas
116
and
118
by a bolted connection consisting of a metal hub (inner metal hub
120
and outer metal hub
122
) and respective metal retention plates
124
,
126
(
FIG. 1
a
). This retention configuration creates a very abrupt load transfer from the composite spring arm
106
to the bolted metal retention at both inner and outer retention areas
116
,
118
. A severe prying of the spring arm under the bending load generates very high transverse shears and corresponding interlaminar shear stresses which are the most critical loading for a composite isolator spring arm.
Previous main rotor isolator designs, such as Miner and Vincent et al. (U.S. Pat. Nos. 4,145,936 and 4,225,287 ) relied upon a bolted rigid spring-to-hub connection. This type of design configuration generates a very sudden load transfer of the isolator spring arm bending moment into the rigid metal retention, creating very high interlaminar shear stresses in the composite spring arm. Since a given composite material selected for the spring arm design has a specific interlaminar shear stress strength, a higher spring arm stress must be reduced by either thickening or widening the spring arm geometry which compromises the design to other than optimum.
While the teachings disclosed in the Miner and the Vincent patents provide a baseline for design and development of the vibration isolator described therein, they do not address the issue of reducing interlaminar shear stress at the root retention area of the spiral shaped springs.
Without reduction of this shear load, the springs must be over built throughout to compensate for this factor, increasing cost and weight; both important factors in helicopter design.
A need, therefore, exists for an apparatus and method of attaching spring arm ends to the inner and outer hubs of a vibration isolator which reduces interlaminar shear loads, and, inter alia, facilitates optimum design requirements for a composite spring arm, provides improved structural efficiency, reduces fabrication costs, and reduces the weight of a helicopter.
SUMMARY OF THE INVENTION
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the present invention.
A novel apparatus and method are provided to reduce the interlaminar shear loading in the retention area of composite springs used in a vibration isolator for use with a helicopter main rotor. Uniquely configured nylon pads are attached to the spring arm retention area to provide a stress dissipation area between the composite spring arm and the metal retention components. This allows optimization of the isolator spring arm design. The thickness of the nylon pads and the planform geometry are calculated based on the isolator stiffness, frequency and composite material generic properties to create the highest strength composite spring arm for a given stiffness, and frequency design requirements. In addition the uniquely configured pads allow a simple constant width and thickness isolator spring arm design. The location of bolt through-holes in the retention area is also calculated to avoid piercing high concentrations of interlaminar stress loading.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.
REFERENCES:
patent: 3317166 (1967-05-01), Janssen
patent: 4145936 (1979-03-01), Vincent et al.
patent: 4225287 (1980-09-01), Vincent et al.
patent: 4323332 (1982-04-01), Fradenburgh
patent: 4619349 (1986-10-01), Braun
patent: 4645423 (1987-02-01), Ferris et al.
patent: 4779483 (1988-10-01), Andra et al.
patent: 4874292 (1989-10-01), Matuska et al.
patent: 5901616 (1999-05-01), Miner et al.
patent: 5954480 (1999-09-01), Schmaling et al.
Byrnes Francis E.
Schmaling David N.
Oberleitner Robert J.
Sikorsky Aircraft Corporation
Williams Thomas J.
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