Spring biased drive socket insert for centrifuge rotors

A composite rotor body has a drive hole insert attached. The drive hole insert seats along a conical surface. A disk spring bears against a washer, which in turn bears against the top surface of the composite material rotor body. The preloaded disk spring compresses the conic surface of the rotor body against the mating surface of the drive hole insert. The preload within the disk spring is activate by torquing centering nut until the disk spring is essentially flat. The centering nut captures and centers both the disk spring and washer within a counterbored recess on its bottom face. The contact surface is treated with a form of dry lubricant or release agent, typically particulate Teflon, such that axial translation is facilitated. The disk spring is sized such that the drive hole insert translates axially without rotation along the surface and, effectively, maintains constant in-plane orientation with respect to the rotor body. This occurs as the diameter of the through-the-thickness hole in the rotor body expands and contracts due to operational loads.

This invention relates to centrifuge rotors. More particularly, an 
apparatus and method of attaching a rotor drive hub socket to a composite 
material rotor body is disclosed for resisting dynamic forces imposed by 
centrifugation. 
BACKGROUND OF THE INVENTION 
Centrifuge rotors are fabricated from composite materials such as carbon 
fibers and carbon filaments, as well as with other fiber materials, bound 
into a suitable form with a polymerized synthetic resin. The selection of 
the fiber material, bound with the synthetic resin, is based on an 
exploitation of several unique physical properties of the fibers. For the 
application to centrifuge rotors, the most valuable physical property of 
any rotor construction material is a high strength to weight ratio. A 
second important property is high modulus of elasticity. A third desirable 
property is relatively low density. Composite structures, including 
centrifuge rotors, have been made with a number of different fiber 
materials such as glass filaments, boron filaments, synthetic organic 
fibers such as Dacron, and with various types of carbon fibers and carbon 
monofilaments. In general, the various forms of carbon fibers and 
filaments provide the most desirable combination of the above-mentioned 
properties for use in the construction of centrifuge rotors. 
A rotor properly designed to exploit the above three properties will have 
an advantage over conventional metal rotors (usually aluminum or titanium 
alloys) in that, due to the high strength to weight ratio, there will be 
less centrifugally induced self stress and more of the material of the 
rotor is applied to supporting the centrifugal loads due to the samples 
which are being centrifuged. The high modulus of elasticity allows the 
composite (carbon fibers with the resin binder) to approach or match the 
modulus of metal parts which must be assembled to the rotor and to not 
deform excessively under centrifugal loads. Finally, the low density 
coupled with the high strength to weight ratio allows the design, for 
equivalent sample handling capacity, to be significantly lighter than 
metal rotors. This later advantage is particularly important for large 
volume rotors, which can be too heavy for a small lab technician to lift. 
At the same time, drive bearing life will markedly extended as rotor 
weight is reduced. 
A further property of composite type materials and structures which has 
been exploited in centrifuge rotor and other composite structure design is 
the rather considerable possibility to orient the fibers in the composite 
so that they provide strength where it is needed. In metal construction, 
except for a moderate amount of anisotropic, properties which may be 
established by forging or selective heat treatment, the material has the 
same properties in all directions (isotropic) whether needed or not! Thus, 
as in U.S. Pat. No. 4,781,699 to A. Piramoon, a composite rotor is 
disclosed in which parts of the rotor are in the form of disks made up of 
multiple layers of monodirectional carbon fiber tape stacked and oriented 
so as to vary the angle of successive layers in increments of 45.degree. 
and bound together with a suitable resin. The object being to provide high 
strength at every direction in the plane of the disk but with strength 
normal to the plane only that due to the binding resin. The second 
composite material in this rotor construction consists of an outer ring of 
continuous fibers which are oriented primarily to provide hoop strength to 
give further support to the rotor is resisting the load due to the samples 
and sample holders. 
Unfortunately, the design of composite material rotors is not without some 
special difficulties. One serious difficulty arises from the microphysical 
characteristics of the composite materials. Specifically, where it is 
necessary to provide a machined interface between composite and metal 
part, the surface characteristics of the composite requires quite special 
consideration. An examination of such a machined surface, even when 
finished by precision diamond grinding methods, reveals carbon fibers cut 
through at various angles, very large local variations in fiber versus 
binder areas and a considerable amount of imbedded ground fiber debris. It 
is quite apparent that this surface is not at all well suited to any sort 
of highly localized load bearing. It is, even with careful design measures 
taken to spread the load, very far from an ideal surface for the 
transmission of forces through frictional engagement or for withstanding 
highly oscillating and transient pulse loading. Because of these 
considerations, it has always been recognized that the conventional 
centrifuge drive hub socket, used with metal rotors, was not suitable for 
the composite materials. While it would be entirely feasible to machine 
the composite material, such as that of the lower disk of the above 
mentioned U.S. patent, to the same drive socket dimensions as used in an 
equivalent metal rotor; it is probable that the precision bore of the 
drive socket would be worn to an unacceptable degree after a very few 
cycles. The means for overcoming this problem has been to provide a metal 
drive hole so that the composite material machined interface is not 
subjected to the wear associated with the rotor drive hole. In the 
Piramoon patent the metal insert was designed to have an interference fit 
of approximately 0.0035" on diameter at room temperature and it was 
recognized that the interference would be as little as 0.0005" with the 
loaded rotor at 60,000 rpm. This interference fit was obtained by thermal 
shrinking of the insert in liquid nitrogen, installing it in the precision 
hole provided in the composite plate and allowing it to equilibrate to 
room temperature providing an expansion fit. In practice it has turned out 
that an interference fit of the needed dimensions in a composite material 
is difficult to make to the required reliability. This problem has been 
observed as a tendency for the insert to gradually shift its location by 
turning with respect to the rotor body and, eventually shifting so that 
the rotor becomes unbalanced. Clearly the interference fit between insert 
and rotor has been lost at running speed. Attempts to overcome this 
problem by providing pins against relative rotation have been partially 
successful. Even with the pins against rotation it is clear that there can 
be a certain amount of "working" on the diameter between the composite and 
the insert. It is well known that this sort of working at a frictional 
interface in a rotating system provides additional damping which, in turn, 
lowers the speed margin against the onset of high speed precession. Once 
this precession has started the working at the interface will progress 
rather rapidly and, in the instance of the rather fragile composite 
surface, can be expected to not only generate considerable local damage 
but, due to the changed surface and debris at the interface, rapidly 
increase the damping. 
SUMMARY OF THE INVENTION 
A composite rotor body has a drive hole insert attached. The drive hole 
insert seats along a conical surface. A disk spring bears against a 
washer, which in turn bears against the top surface of the composite 
material rotor body. The preloaded disk spring compresses the conic 
surface of the rotor body against the mating surface of the drive hole 
insert. The preload within the disk spring is activated by torquing a 
centering nut until the disk spring is essentially flat. The centering nut 
captures and centers both the disk spring and washer within a counterbored 
recess on its bottom face. The contact surface is treated with a form of 
dry lubricant or release agent, typically particulate Teflon, such that 
axial translation is facilitated. The disk spring is sized such that the 
drive hole insert translates axially without rotation along surface and, 
effectively, maintains constant in-plane orientation with respect to the 
rotor body. This occurs as the diameter of the through-the-thickness hole 
in the rotor body expands and contracts due to operational loads. 
Basic to the concept of the present invention is the recognition of the 
nature of a machined bore and other machined surfaces in composite 
materials. Such materials are not suited to interference fits under 
dynamic conditions where performance is dependent on the temporal 
stability of the machined surface. In short, under dynamic conditions it 
is expected that there will be more or less continuous changes in both the 
surface and the geometry of any part-to-part interface. 
It will be noted that in the embodiment illustrated in FIG. 2A, the 
frustoconical surface of the insert is forced constantly against the 
mating surface provided in the composite plate. As the rotor is brought up 
to speed it is clear that there will be a local surface motion of the 
conical fit due to the composite disk expanding radially under centrifugal 
load. It is also evident that when the centrifuge is brought to a stop 
there will be a corresponding local motion of the composite disk in the 
centripital direction. Over time it is certain that there will be some 
wear on both parts of this interface. However, the nature of the wear is 
such that it will not defeat the self-centering effect of this feature of 
the design. The disk spring is of a size and force range sufficient to 
compensate for both the inevitable wear at this interface and any similar 
wear at the loading washer composite interface. 
It is also important, in the execution of the designs incorporating the 
concept of this invention, to minimize the possible introduction of 
significant frictional damping which could lead to problems of precession 
and accelerated damage both to rotor and to centrifuge drive. For example, 
if the disk spring is not strong enough or it has not been assembled with 
a high enough clamping force there might be too easy sliding of the 
loading washer or even the disk spring across the washer. 
In short, properly carried out this invention provides a drive hole insert 
for a composite rotor that is self-centering, accommodates an expected 
long term wear at the interface and introduces a minimum of frictional 
damping. 
OTHER OBJECTS, FEATURES AND ADVANTAGES 
An object of this invention is to provide in a composite material rotor a 
drive hole socket formed from a more durable impact resistant material 
than the composite material itself. Accordingly, a drive hole insert is 
disclosed which is formed of metal rather then the easily worn fiber 
reinforced plastics. 
An advantage of the disclosed construction is that it can be adapted to 
composite material rotors of virtually all different configurations. 
A further advantage of the disclosed construction is that the thermal 
expansion interference fits and their accompanying requirements for close 
dimensional tolerances are eliminated. 
A further advantage of the drive hole socket insert is that the design of 
the insert and its attachment to the rotor body do not effectively limit 
the speed of the rotor. 
A further object of this invention is to disclose a Belvelle spring for 
loading of the drive hole insert onto the composite fiber main body of the 
rotor. The combination of the Belvelle spring and the essentially conical 
mating surfaces forms a frictional antitwist interlock between the drive 
hole insert an the rotor body. This interlock resists acceleration and 
deceleration torques. 
An advantage of the design is that it eliminates the need for traditional 
mechanical pinning. Such mechanical pinnings can be a potential source of 
stress intensification both to the rotor body and the drive hole insert. 
A further advantage of the disclosed design is that the assembly occurs in 
simplified steps. Complex steps such as cooling of the insert for a 
thermal expansion interference fit is not required. 
Yet another advantage is that the tolerances of the disclosed dimensions 
used with the joining of the drive hole insert to the rotor body are 
simplified. Exacting, precision measurements are not required. 
A further advantage is that all configurations of the composite material 
rotor body are simple. For example, the configuration of complex surfaces, 
such as screw threads, in the composite material is not required.

Referring to FIG. 1 a typical rotor of the prior art is illustrated. The 
rotor includes a first plate, P1, and a second plate P2. 
The reader will understand that the rotor here illustrated is a so-called 
composite material rotor symmetrically formed about a spin axis 30. 
Typically, plates P1, P2 constitute many separate layers of resin 
impregnated carbon fiber. These layers of resin impregnated carbon fiber 
are pressed together under great force and cured. Thereafter, the plates 
are milled to have the illustrated apertures for receiving the drive hub 
sockets and other apertures necessary for the composite rotor 
construction. 
In the case of plates P1 and P2, the fibers are aligned in parallel 
relation with respect to a radius taken normal to the spin axis 30 of the 
rotor. 
Typically, an outer winding B is wound about the two plates P1 and P2 and 
placed under hoop tension. This hoop tension disposes carbon fibers 
circumferentially around the plates P1 and P2. Typically, the hoop tension 
exerts a radially compressive force on the plates P1 and P2. 
Typically the upper plate P1 and the lower plate P2 are provided with a 
series of apertures T which can contain the canisters 40 for the 
separation of material, such as the classification of sample. As is 
conventional in all such rotors, the rotor bodies and the containing 
apertures are symmetrically constructed with respect to spin axis 30. 
Typically tube inserts T are placed within milled holes H so that sample 
can be classified during rotor rotation. 
It will be understood that where a composite rotor construction is used, 
plates, such as plates P1 and winding B, have anisotropic strength of 
material properties relative to the spin axis 30. The material have high 
stiffness and tensile strength normal to the spin axis 30. These materials 
do not have the same high stiffness and tensile strength properties 
parallel to spin axis 30. In fact, the materials are relatively weak in 
tensile strength along planes parallel to the axis of rotation. 
Such a composite material rotor as illustrated in FIG. 1 has properties 
which impart an improved rotor construction. These properties have been 
previously discussed and will not be further set forth here. 
With reference to FIG. 1 and in the prior art, a carefully drilled 
cylindrical hole 60 was machined within the composite rotor disk P2. 
Thereafter, a mating cylindrical drive hub socket 62 having a surrounding 
lower annular flange 64 was placed in an expansion fit into the milled 
aperture 60. Typically, such an expansion fit includes immersing the 
metallic drive hub, which was preferably of titanium but could alternately 
be of aluminum or stainless steel, into liquid nitrogen. Such immersion 
cooled the drive hub socket and shrunk the drive hub socket. 
Once the drive hub socket was shrunk, it was placed within the rotor body 
and allowed to equilibrate to the temperature of the normal environment. 
This fitting produced a thermal expansion interference fit. The advantages 
and disadvantages of such a thermal expansion interference fit have been 
heretofore carefully set forth. Accordingly, they will not be further 
discussed herein. It will suffice to say that this fit of the drive hub 
socket to the rotor body has severe limitations. 
Referring to FIG. 2A, a drive hub socket H placed within plate P2 of the 
rotor of FIG. 1 is illustrated. This drive hub socket defines a central 
aperture 60 which central aperture is configured for mating with the drive 
hub of a centrifuge, this drive hub being mounted on the end of a spindle 
shaft typically protruding upwardly from a centrifuge (neither being 
shown). Since a particular configuration of such a mating fit is not the 
subject matter of this invention, the aperture for the drive hub is shown 
as an ordinary cylindrical aperture. 
The exterior of the drive hub includes a cylindrical surface 62, a lower 
male conical surface 67 and an extending boss 68. Conical surface 67 
configured at the lower portion of drive hub socket H mates with a female 
conical surface 69 configured at the lower end of the drive hub. 
Boss 68 has attached thereto either by swaging or by preferred threading a 
cap 70. Cap 70 compresses a Belvelle spring 72 onto a compression ring 74 
about threaded boss 68. A Belvelle washer capturing annulus is provided in 
cap 70. Compression rings bears with great force on the upper flat surface 
76 of plate P2. Accordingly, it urges the male conical surface 69 at the 
lower end of the drive hub socket H onto the female conical surface 67 of 
plate P2 with a corresponding great force. 
It can be seen that the slope of the respective conical surfaces is here at 
a preferred angle of 45.degree.. We prefer such slopes to be at least 
30.degree. from the horizontal. Slopes from 30.degree. from the horizontal 
produce two functions. 
First, it will be understood that Belvelle washer 72 exerts a relatively 
great spring force. In a rotor such as that illustrated in FIG. 1, total 
rotor weight is on the order of 11 lbs. Typically, the Belvelle spring 72 
exerts a spring force in the range of 500 to 700 lbs. 
Under such a compressive mating force, the interface between the male 
conical surface 67 on the drive hub socket H and the female conical 
surface 69 of plate P2 has two conforming characteristics. 
First, the taper and interface between the conical surfaces is not 
self-locking. The conical surfaces are free to relatively move one with 
respect to another as the rotor undergoes differential expansion relative 
to the drive hub. 
Second, drive hub socket H is relatively self-centering with respect to the 
cylindrical aperture 78 in plate P2 into which it is placed. This 
self-centering action occurs upon relative movement of the male conical 
surface 69 relative to the female conical surface 67. 
It is preferred that the conical surface 67, 69 be treated with a form of 
dry lubricant or release agent, typically particulate Teflon. Axial 
translation with such lubricant is facilitated. 
In describing the surfaces herein set forth, applicant will use the term 
substantially conical. This term is utilized to cover convex and concave 
contours of the side elevations of the cones between the drive hub H and 
the plate P. While such shapes depart from the precise definition of 
cones, the vagarities of this invention require usage of the term 
"substantially conical." 
Referring to FIG. 2B, an embodiment is illustrated wherein the male conical 
surface 67 is mated to a female conical surface 69, this conical surface 
69 defining a convex section at the female conical surface on plate P2. 
Such a surface assures a conforming contact, which conforming contact 
cannot occur at a corner of the mated surfaces and occurs at the center of 
the mated surfaces. Corner contact can degrade the composite material 
construction and is not stable for the required self-centering fit. 
Referring to FIG. 2C, the linear male concave surface 69 is shown mated to 
a male concave surface 67' having a convex section. 
Referring to FIG. 2D, a convex section 69a is illustrated on drive hub 
socket H and a concave section 69a is illustrated on the rotor body P2. It 
is preferred that the convex section of the rotor body have a slightly 
greater radius of curvature. This slightly greater radius enables a 
central contact away from the corners to occur. 
Likewise, and referring to FIG. 2E, a concave section 69b is illustrated on 
drive hub socket H and a convex section 69b is illustrated on the rotor 
body P2. It is again preferred that the convex section of the drive hub 
socket have a slightly greater radius of curvature. This slightly greater 
radius enables a central contact away from the corners to occur. Further 
variations can be made by the skilled designer. 
Referring to FIG. 3A, a variation of the disclosed invention is set forth. 
A drive hub H with a central aperture 60 is provided with a lower male 
conical surface 80. Lower conical surface 80 mates with a female conical 
surface 82. 
The bearing plate 84 has had its construction changed. Specifically, male 
bearing plate 84 defines a male conical surface 86. Male conical surface 
86 bears down on a female conical surface 88 configured in the upper 
portion of plate P2. 
As before, a central boss 68 having a cap 70 rigidly attached thereto 
compresses a Belvelle washer 72 with great force. 
In the embodiment of FIG. 3A, the male conical surface 80 at the lower 
portion of hub H compresses onto the female conical surface 80. Likewise, 
the male surface 86 defined in compression ring 84 compresses onto the 
female conical surface 88 at the upper portion of plate P2. 
Referring to FIG. 3B, a variation similar to that illustrated in FIG. 2B is 
set forth. Specifically, a convex sectioned female conical surface 82' and 
88' at the respective bottom and top of plate P2 mate with linear male 
conical surface 80, 86. 
Likewise, and with respect to FIG. 3C, the convex section of the conical 
surface is reversed. Specifically, male conical surfaces 80' and 86' at 
the respective bottom and top of the drive hub assembly H have convex 
section and mate with linear conical surfaces 82, 88. 
The reader will understand that the embodiments illustrated with respect to 
FIGS. 3B and 3C will admit of a number of variations. For example, plate 
P2 could contain one convex conical surface and a linear surface. 
Likewise, respective mating surfaces could all be configured in the drive 
hub for matching a convex conical surface to a linear conical surface. 
Other similar variations are possible. 
As has herein been illustrated, the reader will understand this invention 
will admit of a number of embodiments. For example, the attached 
compression of a spring member urging the conical surfaces between the 
drive hub and rotor can be changed to any number of specific embodiments. 
For example, although we have illustrated a threaded cap herein, it can be 
swaged, supplied with keys or provided with other forms of attachment. 
In short, any suitable means for spring compression and means for 
attachment to the hub may be utilized.