Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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COMPOSITE MATERIAL ROTOR
~ield of the Invention
This invention relates to ultra high speed
centrifuge rotors and in particular to a composite
material rotor of lower density and higher strength of
materials.
Background of the Invention
An ultracentrifuge rotor may experience
60~,000 9 or higher forces which produce stresses on the
rotor body which can eventually lead to rotor wear and
disintegration. All ultracentrifuge rotors have a
limited life before damage and fatigue of the material
comprising the rotor mandates retirement from further
centrifuge use.
Stress generated by the high rotational speed
and centrifugal forces arising during centrifugation is
one source of rotor breakdown. Metal fatigue sets into
conventional rotors following a repeated number of stress
cycles. When a rotor is repeatedly run up to operating
speed and decelerated, the cyclic stretching and relaxing
of the metal changes its microstructure. The small
changes, after a number of cycles, can lead to the
creation of microscopic cracks. As use increases, these
fatigue cracks enlarge and may eventually lead to rotor
failure. The stress on conventional metal body rotors
may also cause the rotor to stretch and change in size.
When the elastic limits of the rotor metal body have been
reached, the rotor will not regain its original shape,
causing rotor failure at some future time.
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Conventional titanium and aluminum alloy rotors
have a respectably high strength to weight
ratio Aluminum rotors are lighter weight than titanium,
leading to less physical stress and a lower kinetic
energy when run at ultracentrifuge speeds; however,
titanium rotors are more corrosive resistant than
aluminum. As the ultracentrifuge performance and speeds
increase, the safe operating limits of centrifugation are
reached by conventional dense and high weight metal
rotors.
One attempt to overcome the design limitations
imposed is indicated in U.S. Patent 3,997,106 issued to
Baram for a centrifuge rotor which is laminated and
consists of two layers of different materials. Wires
(24) are wound around a metal cover 8b which surrounds a
central filler of chemically resistant plastics ~See
Figure 3 of the '106 patent). The Baram '106 patent
envisions greater chemical resistance and lower specific
gravity rotors, which achieve optimum strength, by the
use of a laminate manufacturing process. U.S. Patent
2,974,684 to Ginaven (2,974,684) is directed to a wire
mesh of woven wire cloth 6 for reinforcing a plastic
material liner 7 for use in centrifugal cleaners ~see
Figures 2 and 3).
U.S. Patents to Green (1,827,648), Dietzel
(3,993,243) and Lindgren (4,160,521) have all been
directed to a rotor body made from resin and fibrous
reinforcement materials. In particular, Green '648 is
fibre wound to produce a moment of inertia about the
vertical axis greater than the moment of inertia about
the horizontal axis through the center of gravity of the
bucket so that the rotor bucket is stable at speeds of
7500 to 10,000 RPM (a relatively slow centrifuge speed by
modern standards).
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U.S. Patent 4,468,269, issued August 28, 1984
to the assignee of this application, discloses an
ultracentrifuge rotor comprising a plurality of nested
rings of filarnent windings surrounding the cylindrical
wall of a metal body rotor. The nested rings reinforce
the metal body rotor and provide strengthening and
stiffening of the same. The rings are nested together by
coating a thin epoxy coat between layers. U.S. Patent
3,~13,828 to Roy discloses a design substantially
equivalent to that disclosed by the '269 patent.
None of the conventional designs provide
maximum strength through ultracentrifuge speeds through
the use of a material specifically designed to
accommodate localized stress and resist rotor body
fatigue. Conventional metal bodies, or reinforced metal
body rotors, are subject to metal stress and fatigue
failures during centrifugation.
What is needed is a rotor body of substantial
strength, yet lighter in weight and capable of enduring
increasingly higher loads and speeds. The body should
resist stress and corrosion and be specifically designed
to cope with locali~ed stress.
Summary of the Invention
Disclosed herein is a centrifuge rotor body
made from a plurality of layers of anisotropic
material. (As used in this application, the term
"anisotropic" shall mean a material having properties,
such as bulk modulus, strength, and stiffness, in a
particular direction.) Each layer has a different
modulus of strength, fine tuned to accommodate the
particular stress which said layer would encounter, based
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on the shape, load at the design speed, or size of the
rotor.
In each of the particular layers, selected
portions of the material is oriented in a direction
distinct from the main body of that layer, tc reinforce
and accommodate excessive stress formed at the test tube
receiving cavity of the rotor.
In the preferred embodiment, the anisotropic
material layers are made of a fibrous filament wound
composite material~ where the fiber is graphite and the
resin epoxy. Each of the layers form a composite
material disc and each disc extends radially from the
central axis of the rotor, each disc being secured to
other discs by an epoxy bonding.
Brief Description of the Drawings
Figure 1 is a top plan view of the composite
rotor of this invention.
Figure 2 is an elevated vertical cross-
sectional view of the composite material rotor of this
invention.
Detailed Description of the Preferred Embodiment
With reference to Figures 1 and 2, there is
shown generally a composite material rotor 10 (Figure
2). The rotor 10 is constructed from a plurality of
layered discs, like 26 and 28 (Figure 2).
The composite material selected for the
composition of the rotor of the preferred embodiment
includes (but is not limited to) graphite fiber filament
wound into epoxy resin or a thermoplastic or thermoset
matrix. The fiber volume is in excess of 60%. This
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composition has a density of approximately .065 lb/in3,
which is favorable when compared to conventional rotor
designs including aluminum (.11 lb/in3) and titanium (.16
lb/in3). Alternative fiber filaments include glass,
boron, and graphite. The fibrous material KEVLAR fiber,
an organic fiber made by DuPont, is also a useful
substitute for graphite.
Due to the high stress created by the
ultracentrifuge, material selection has been influenced
by the need for an "anisotropic" material such as
graphite composite filament wound material.
In the preferred embodiment, a vertical tube
rotor 10 is illustrative of the principles of the design
of the subject invention.
Referring to the top plan view of the rotor 10
illustrated in Fig. 1, the varying densities of the
filament design of the rotor 10 is demarcated by circular
boundary lines 24 and 18. The region inward from the
perimeter of circle 18 to the boundary of rotor shaft
cavity 14 is wound to be of similar density to the region
beyond the outer limits of circular line 24. The region
12, between the circular boundary line 18 and 24, is
characterized by a region of more densely wound filament,
as illustrated at region 30 of Figure 2. As the center
of the rotor lO accommodates the insertion from the rotor
underside of the drive shaft 32 (Figure 2) into rotor
drive shaft cavity 14, the top surface of the rotor lO
accommodates the insertion of metal test tube inserts 16
down into the machined cavity 20. A test tube 22 is then
inserted into the insert 16 for a snug fit into the body
of the rotor lO.
In the vertical test tube rotor lO, as
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illustrated in Figures 1 and 2, the stress is maxirnum at
the upper layer, especially region 30 of Figure 2, where
maximum stress is manifested as hoop stress. One test
tube cap (made frotn aluminum, composite material, or
rubber) is loaded into the top of the rotor, for each
test tube. Screwing these caps into the rotor body
causes additional stress to the rotor body at the point
of cap insert.
A critical advantage to the use of composite
material construction is that each layer, such as 26 and
28, forms a disc that is uniquely fine tuned so that the
modulus of elasticity is adjusted to accommodate the
particular stress presented to each of several locations
within and about the rotor 10.
Each of the discs, such as 26 and 28, are
filament-wound around a central core. The fiber filament
is available in at least four types of sizes, one
thousand, three thousand, six thousand, and twelve
thousand fibers per bundle. The preferred embodiment
utilizes a fiber bundle of twelve thousand filaments per
bundle. The filament bundle is wound to provide a range
of two to 10 pounds per bundle of tension depending upon
which of the plurality of discs is being constructed.
The average density of the composite material disc is
.065 lbs/per cubic inch. Those discs experience greater
stresses during operation of the rotor, like disc 2~, are
manufactured with a greater tensile strength than those
discs, like disc 40, which undergoes lesser stresses.
Each disc is individually machined to form the
cavities such as the machined cavity Z0. Once formed,
cured, and machined, the discs are stacked along the
central axis running longitudinally along shaft cavity
1~1, and are secured together by layered application of
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resin epoxy, shown at 41, 34, 36, and 38, sandwiched
between the layered discs 42, 40, 26, and 28. ~fter the
epoxy resin at 41, 34, 36, and 38 is applied between the
disc layers the entire assembly is secondarily cured in
an oven and the composite material rotor 10 is thereby
manufactured.
Each disc is uniquely wound to particularly
respond to the localized stresses which the assembled
rotor will encounter during centrifugation. For example,
disc 26 is formed and manufactured to accommodate
localized stress which differs along the disc radius.
Each disc may be made from a different grade or modulus
strength fiber filament material. Also, the angle of the
fiber windings may be changed from windings parallel to
the horizontal plane. Around the core cavity 14, outward
to circular boundary 18, the fiber is wound at 0 with
respect to the horizontal plane of the rotor 10. As the
filament is wound in the region between 18 and 24, the
filament windings in this vicinity of the machined cavity
20 are deliberately wound at appro~imately a criss-
crossed +45 angle to the horizontal plane, to provide
additional support to surround cavity 20. This criss-
crossed stitching of the filament fiber in the region 12
(Figure 1) between the boundaries 18 and 24 adds
additional support to the cavity 20 to ensure that the
material strength of the rotor will not be diminished by
the presence of machined cavities such as 20. The
optimum strength is obtained when the fiber is wound at
an approximate angle of a criss-crossed +45; however,
use of an angle range, if varied over 10 from a -~45
optimum value in either direction (from +35 to +55
angle from the horizontal), would achieve a superior
strength over the horizontal winding.
Additionally, disc 28 and the disc atop it are
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manufactured from a stiffer, higher modulus, and strength
filament material than the material used to produce
layers 26 and below to accommodate the area of maximum
hoop stress at the top of this vertical tube rotor 10.
Thus, not only would the orientation of the wlnding
differ to accommodate higher stress around the cavlty 20,
but the material comprising the fiber of the filament
wound discs would dlffer, as disc 26 differs from 28, to
fine tune and vary the modules of the discs 26 and 28 to
respond with differing modulus to the differing stresses,
which the discs 26 and 28 would encounter. By having
separate discs, the more expensive, stronger discs would
only be used where needed. A plurality of discs allows a
rotor to be specifically designed to resist greater
localized stress only where it arises.
If a different design than a vertical tube
rotor, such as a fixed angle rotor body, were
contemplated, the maximum stress bearing discs might be
situated about 2/3 of the way down the rotor body, since
the location of maximum stress ln a fixed angle rotor
differs from the location of such maximum stress in a
vertical tube rotor.
It is appreciated that the preferred embodiment
anticipates the use of separate discs comprising the
rotor body, rather than one continual winding defining
the entire rotor. Such a unibody constructlon is
contemplated to be within the scope of this invention,
where the flber ls reorlented to accommodate greater
stress as shown in Figure 2 in the reglon between
boundaries 2~ and 18. However, the preferred embodiment
envisions a plurality of bonded discs rather than a
unitary body fiber wound body due to the apparent
inability of a unibody rotor to overcome residual axially
directed stress that arises when a fiber wound disc
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exceeds an emplrically derived width. Also, a unitary
body filament wound composite material rotor could not
select a plurality of fibrous filaments for various
sections of the rotor body.
While the invention has been described with
respect to a preferred embodiment vertical tube rotor
constructed as described in detail, it will be apparent
to those skilled in the art that various modifications
and improvements may be made without departing from the
scope and spirit of the invention. Accordingly, it will
be understood that the invention is not limited by the
specific illustrative embodiment, but only by the scope
of the appended claims.
