Imperforate bowl: centrifugal separators – Including vibration damping means
Reexamination Certificate
1999-05-03
2001-02-06
Cooley, Charles E. (Department: 1723)
Imperforate bowl: centrifugal separators
Including vibration damping means
C494S083000, C494S084000, C464S180000
Reexamination Certificate
active
06183408
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to centrifuge systems and more specifically to a rotor shaft assembly used in a centrifuge system.
BACKGROUND OF THE INVENTION
Large centrifugation systems typically use a removable rotor for holding sample containers which contain the sample to be separated. The rotor is covered by a rotor lid and then placed into an instrument chamber wherein the rotor is spun during centrifugation.
A typical centrifuge assembly is shown with reference to FIG.
6
. The centrifuge assembly
10
consists of a rotor assembly
12
which is connected by a rotor shaft assembly
14
to a drive motor
16
. With reference to
FIG. 7
, the rotor assembly
12
includes a rotor body
110
which has a rotor chamber consisting of a plurality of chambers
112
for receiving configuration sample containers, not shown, which hold the sample being centrifuged and an interior upper chamber
114
. The separation between the canister chambers
112
and the interior upper chamber
114
is shown by a dashed line
113
. Interior chamber
114
is the volume which remains within the rotor chamber after the insertion of the centrifugation containers. At the upper end of the rotor body
110
is an annular opening defined by an edge
150
. The opening in the top of the rotor body
110
is intended to be covered by a lid assembly
15
. Rotor body
110
includes an axial bore
120
formed through the spin axis of the rotor body, extending from an open end within interior chamber
114
to an open end
125
at the bottom of the rotor body. Axial bore
120
includes one or more locking pins
130
which project into the interior volume of the axial bore.
Setting up the rotor assembly for a centrifugation run includes placing the rotor into a instrument chamber, not shown. The instrument chamber includes a spindle hub
20
which is part of the rotor shaft assembly
14
and is received in the axial bore
120
of the rotor body
110
. The inserting end of the spindle hub
20
is slotted to engage locking pins
130
, thus locking the spindle hub into position relative to the rotor body. The spindle hub
20
is coupled to the quill shaft
22
and the bottom of the shaft is coupled to a drive motor, or other type of rotation source, which provides the torque to spin the rotor. A housing
26
encloses a large portion of the shaft and forms the outer enclosure of the rotor shaft assembly
14
.
A common problem that exists in centrifugation systems is that the centrifuge rotor becomes unbalanced and vibrates when the rotational speed in the system reaches a critical speed. Normally, the rotor rotates about its geometric center of gravity. As the speed of the rotor increases, it reaches a critical speed which is defined as “the angular speed at which a rotating shaft becomes dynamically unstable with large lateral amplitudes due to resonance with the natural frequencies of the lateral vibration of the shaft.” (McGraw-Hill Dictionary of Scientific and Technical Terms, 5th Edition, 1994.) At the critical speed, the rotor experiences a radial displacement that is synchronous with rotation, the maximum displacement being at the critical speed. As the speed continues to increase beyond the critical speed, the radial displacement decreases until the only radial displacement remaining is that associated with rotor imbalance. Additionally, since the centrifuge operator must load the samples into the chambers, this can create a further imbalance to the rotating system, which can magnify the radial displacement and vibrating effect. During normal use, the rotor generally passes through its critical speed when accelerating from a stopped position to its normal operating speed and, after centrifugation is completed, when decelerating back to a stopped position.
When designing a shaft for a rotor assembly used in a centrifuge system, the design objectives are usually in conflict. One objective is to have a very flexible shaft in order to minimize bearing loads at super-critical speeds. A second objective is to have a stiff shaft in order to minimize rotor displacement at the critical speed. In conventional designs, the stiffness of the shaft is usually chosen as a compromise between these two competing objectives. Generally in the prior art, the stiffness of the shaft has been designed to have a linear characteristic in that the shaft would have the same stiffness at any rotor speed.
With reference to
FIG. 8
, rotor shaft assemblies in the prior art generally have a linear stiffness characteristic. As shown in
FIG. 8
, as the amount of radial displacement, shown on the x-axis, increases, the amount of force resisting the displacement or stiffness, shown on the y-axis, also increases proportionally, resulting in a graph
71
that is a straight line.
In U.S. Pat. No. 5,683,341 to Giebeler, there is a discussion of a prior art shaft design which has a high amount of stiffness for enabling the rotor to be maintained vertically aligned even though some imbalance forces are present. This design is then compared to the design of Giebeler which relies on the rotor being supported above its center of gravity to keep the rotor upright, rather than relying on the stiffness of the shaft. U.S. Pat. No. 5,827,168 to Howell attempts to minimize the vibrations of a centrifuge by using sliding and damping bearings to restrain vertical movement of a disk rotatably attached to the centrifuge's drive shaft.
It is the object of the present invention to provide a rotor shaft assembly having an improved characteristic of stiffness, such that the shaft can maintain the required stiffness to compensate for rotor displacement at the critical speed, but can be flexible at super-critical speeds in order to minimize bearing loads.
SUMMARY OF THE INVENTION
The above object has been achieved by a rotor shaft assembly having a non-linear stiffness characteristic. The shaft assembly provides stiffness when the rotor reaches the critical speed, but allows for the shaft to be flexible at super-critical speeds.
In a first embodiment of the invention, the rotor shaft assembly has a bearing sleeve mounted either on the inside of the spindle hub or on the outside of the housing in order to decrease the clearance between the housing and the spindle hub. This allows the shaft to be flexible within the clearance, but causes the housing to limit the radial displacement of the shaft when the rotor and spindle hub experience radial movement at the critical speed.
In a second embodiment of the invention, a support tube surrounds the lower portion of the shaft with a small clearance existing between the support tube and the shaft. When the radial displacement of the rotor shaft exceeds the clearance, the support tube provides higher resistance to any additional radial movement.
The rotor shaft assembly of the present invention allows the shaft to be flexible at super-critical speeds to minimize bearing loads, but also provides for a stiff shaft at the critical speed by limiting the amount of radial movement of the shaft. In this way, both of the desired design objectives discussed above may be achieved.
REFERENCES:
patent: 2346432 (1944-04-01), Heintz
patent: 2827229 (1958-03-01), Blum
patent: 3233825 (1966-02-01), Lomb
patent: 3604617 (1971-09-01), Patterson
patent: 3676723 (1972-07-01), Drucker
patent: 3770191 (1973-11-01), Blum
patent: 3779451 (1973-12-01), Lehman
patent: 3902659 (1975-09-01), Brinkmann et al.
patent: 3938354 (1976-02-01), Lehman
patent: 4201066 (1980-05-01), Nolan, Jr.
patent: 4205779 (1980-06-01), Jacobson
patent: 4214179 (1980-07-01), Jacobson et al.
patent: 4226359 (1980-10-01), Jacobson
patent: 4236426 (1980-12-01), Meinke et al.
patent: 4322030 (1982-03-01), Jacobson et al.
patent: 4324440 (1982-04-01), Steigenberger et al.
patent: 4334718 (1982-06-01), Hirt et al.
patent: 4511350 (1985-04-01), Romanauskas
patent: 4513566 (1985-04-01), Rajsigi et al.
patent: 4568324 (1986-02-01), Williams
patent: 4846773 (1989-07-01), Giebeler et al.
patent: 4890947 (1990-01-01), Williams et al.
patent: 4897075 (1
Godin Charles N.
Petch Derek G.
Wright Herschel E.
Beckman Coulter Inc.
Cooley Charles E.
Kivinski Margaret A.
May William H.
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