BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates to a centrifuge rotor and, in particular, to a suspension
system for a centrifuge rotor.
Description of the Prior Art
[0002] An important consideration when using a centrifuge apparatus is insuring that the
load carried by the rotor is in balance with respect thereto. This concern is well-founded
since, as rotational speed increases through the critical value, rotor unbalance manifests
itself as vibration. Depending upon the magnitude of the unbalance, such vibration
may be sufficient to damage the centrifuge apparatus.
[0003] For a rotating body, such as a centrifuge rotor. to pass through its critical speed,
two events must occur simultaneously. First, the rotor's principle axis, the axis
through the rotor's center of gravity. must lie parallel to the rotor's spin axis,
the axis passing through the geometric center of the rotor. Second, the rotor's center
of gravity must lie on the spin axis. If these two events were enabled to occur more
quickly after rotor start-up, then the rotor would pass through its critical speed
at an earlier time (i.e., at a lower speed) thus reducing the vibration caused by
a given amount of rotor unbalance.
[0004] In view of the foregoing it would be advantageous to provide a centrifuge that would
maintain vibratory forces at a minimum, thus increasing the amount of unbalance that
the centrifuge would be able to tolerate. One manner of accomplishing this end is
to reduce the critical speed of the rotor. By lowering the rotor's critical speed,
the vibration imposed on the centrifuge by a given rotor unbalance is commensurately
reduced.
SUMMARY OF THE INVENTION
[0005] In accordance with this invention a suspension system for a centrifuge rotor having
a weight W is provided which makes satisfaction of each of the two conditions for
the attainment of critical speed independent of each other. As a result, the critical
speed of the rotor is lowered. The rotor suspension system embodying the teachings
of this invention comprises a coupling member having a first and a second collar each
flexibly connected to a rigid shaft. The rigid shaft has a dimension L. The first
collar is flexibly connected at a first connection point "r" to the centrifuge rotor
while the second collar is flexibly connected to the rotor drive member at a second
connection point "d". One of the flexible joints is thus disposed a predetermined
distance A from the center of gravity of the rotor. The flexible connections of the
coupling exhibit predetermined torsional stiffness K
d and K at their respective points of connection to the drive or the rotor.
[0006] In accordance with this invention the distance A is defined by the relationship:

where
K is the torsional stiffness at the flexible r connection of the coupling to the rotor
at the connection point "r":
kd is the torsional stiffness at the flexible connection of the coupling to the rotor
drive at the connection point "d";
A is the distance that the center of gravity of the rotor lies from the connection
point "r": L is the length of the rigid shaft; and
W is the weight of the rotor.
With the suspension system of the present invention, the satisfaction of the individual
conditions necessary to make attainment of critical speed is made independent of each
other, and the rotor's critical speed is then a function of the torsional stiffnesses
of the coupling, the weight W of the rotor, and the length L of the rigid shaft.
[0007] BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The invention may be more fully understood from the following detailed description
thereof taken in connection with the accompanying drawings, which form a part of this
application, and in which:
Figure 1 is a side elevational view entirely in section of a rotor suspension arrangement
in accordance with the present invention: and
Figure 2 is a force diagram illustrating the forces acting upon the rotor suspension
arrangement in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0009] Throughout the following detailed description similar reference numerals refer to
similar elements in all Figures of the drawings.
[0010] Shown in Figure 1 is a side elevational view entirely in section of a centrifuge
rotor 10 in having a suspension system generally indicated by reference character
12 embodying the teachings of this invention. Since the suspension system 12 may be
used with any well-known rotor configuration, the rotor 10 is illustrated only fragmentally.
The center of gravity of the rotor is diagrammatically indicated by the character
CG. The rotor 10 has a central bore 14 with a suitable top cap 16 secured thereover
as by threaded bolts 18. The central projection of the top cap 16 is sized to receive
a central aperture of a rotor cover knob 20. A generally disc-like rotor cover 22
is secured about the lower periphery of the knob 20 and held in position at that point
by any suitable lock ring 24 as well understood by those skilled in the art. The rotor
10 may be further provided with a windshield 26. The windshield 26 is fragmentally
indicated in the drawings and is conveniently secured to the rotor 10 in a manner
to be discussed herein.
[0011] Motive force for the rotor is derived from a rotor drive arrangement 30 which includes
a drive spindle 32 projecting upwardly into the central bore 14. The upper end of
the drive spindle 32 is provided with a threaded recess 34 for a purpose described
herein.
[0012] The suspension system 12 in accordance with the present invention includes a substantially
annular coupling member generally indicated by the reference character 40. The coupling
member includes first and second collar portions 42A and 42B, respectively. The collars
are flexibly connected through suitable flexible links 44A and 44B to each end of
a substantially rigid shaft portion 46. The shaft 46 has a length L while the center
of gravity CG of the rotor 10 lies a distance A from one of the flexible links 44.
The flexible links 44A and 44B have torsional stiffnesses K
r and K
d respectively associated therewith.
[0013] In practice the collars, flexible links, and rigid shaft are fabricated as a unitary
member such as that provided by Heli-cal
TM rotating shaft flexible couplings such as those sold by Heli-cal Products Company,
Inc. Although such flexible couplings are preferred, any other flexible shaft coupling
may be utilized.
[0014] The collar 42B is secured to a drive adapter 48 by a threaded bolt 50. The drive
adapter 48 is itself secured at the upper end of the drive spindle 32 by a capped
bolt 52 which is received within the threaded recess 34 of the spindle 32. Similarly,
the collar 42A is connected to a ring 56 by a threaded bolt 58. The ring 56 is itself
secured to the rotor by a threaded bolt 60. The bolt 60 extends through the outward
portion of the ring 56 and serves to secure not only the ring 56 but also the central
portion of the windshield 26 to the under surface of the rotor 10.
[0015] As may be better understood by reference to the force diagram shown in Figure 2,
the above-described suspension system serves to suspend the rotor 10 to the drive
spindle 32 at two substantially flexible connection points indicated in the drawings
as connection points "r" and "d". As seen with reference to Figure 1. connection point
"r" is that flexible connection point adjacent the connection of the coupling 40 to
the rotor 10 as defined by the flexible link 44A between the rigid shaft 46 and the
collar 42A. Similarly, the connection point "d" is defined at the flexible connection
point adjacent to the connection of the coupling 40 to the drive spindle 32 defined
by the flexible link 44B which extends between the rigid shaft 46 and the collar 42B.
[0016] As noted earlier, the rotor's critical speed is that rotational speed at which two
events simultaneously occur. These events are, firstly, the parallel alignment of
the principal axis of the rotor (an axis extending through the rotor center of gravity
CG) and the spin axis of the rotor (that is, an axis through the geometric center
of the rotor). Secondly, the rotor's center of gravity CG must lie on the spin axis.
By making the satisfaction of these two conditions independent of each other it is
possible to permit the rotor to attain its critical speed at a lower rotational speed.
As a consequence vibratory forces imposed on the rotor by a given rotor unbalance
are lessened. Thus, the rotor may be enabled to tolerate larger amounts of rotor unbalance.
[0017] The suspension system 12 in accordance with the present invention isolates and makes
independent the satisfaction of the above two conditions imposed on a rotor enabling
it to reach its critical speed. The suspension system 12 acts generally as a kinematic
equivalent of an infinite pendulum. As seen in the force diagram of Figure 2, in order
that the above two conditions be independently achieved, the angle
d must at all times be made to equal the angle
r.
[0018] In accordance with this invention the distance A is defined by the following relationship:

where
K is the torsional stiffness at the flexible r connection of the coupling to the rotor
at the connection point "r":
Kd is the torsional stiffness at the flexible connection of the coupling to the rotor
drive at the connection point "d";
A is the distance that the center of gravity of the rotor lies from the connection
point "r": L is the length of the rigid shaft: and W is the weight of the rotor.
[0019] Utilizing the suspension system of the present invention, as noted earlier, provides
the kinematic equivalent of an infinite pendulum. Thus, the application of a force
to the rotor acting through its center of gravity will result only in the deflection
of the rotor. Simultaneously, a restoring couple imposed upon rotor will result only
in the tilting of the rotor. In this way, the rotor's critical speed becomes a function
only of the torsional stiffnesses of the couplings, the weight of the rotor, and the
length of the shaft.
[0020] Those skilled in the art having the benefit of the present teachings may effect numerous
modifica- - tions thereto. These modifications are, however, to be construed as lying
within the scope of the present invention as defined by the appended claims.