US3692236A

US3692236A - Self-balancing centrifuge method and apparatus

Info

Publication number: US3692236A
Application number: US85348A
Authority: US
Inventors: Stanley L Livshitz; Robert Alan Mayo
Original assignee: Technicon Instruments Corp
Current assignee: Bayer Corp
Priority date: 1970-10-30
Filing date: 1970-10-30
Publication date: 1972-09-19
Anticipated expiration: 1989-09-19
Also published as: IT938889B; CA936506A; FR2110116A5; SU463254A3; DE2152840A1; CH542658A; NL7114176A; GB1346115A; AU3333171A; BE773364A

Abstract

A centrifuge of the type used for a number of individual test samples in separate sample containers is dynamically balanced automatically by means of a counterbalancing mass contained in an annular chamber within the centrifuge wheel. The centrifuge wheel is mounted as an inverted pendulum about a fixed pivot and spring biased to the vertical position. The counterbalancing mass, which is at the same vertical level as the sample containers, automatically moves angularly to the counterbalancing position at speeds above the speed corresponding to the natural vibration frequency of the pendulum system.

Description

United States Patent v Livshitz et al.

[ 51 Sept. 19, 1972 [54] SELF-BALANCING CENTRIFUGE METHOD AND APPARATUS Inventors: Stanley L. Livshitz, Bronx, N.Y.;

Robert Alan Mayo, Verona, NJ.

Assignee: Technicon Instruments Corporation,

Tarrytown, NY.

Filed: Oct. 30, 1970 Appl. No.: 85,348

US. Cl ..233/26, 74/573 Int. Cl. ..B04b 9/12 Field of Search.....233l23 R, 26, l R, 1 C, 23 A;

References Cited UNITED STATES PATENTS 5/1924 Swancara ..233/26 1/1968 Jarvis ..210/363 9/1959 Gibson ..21'0/363 I 1/1892 Berg ..233/26 1,760,775 5/1930 Peters ..2l0/363 FOREIGN PATENTS OR APPLICATIONS 1,091,525 10/1960 Germany ..2l0/363 Primary Examiner-Jordan Franklin Assistant Examiner-George M. Krizmanich Attorney-S. P. Tedesco and S. E. Rockwell [57] ABSTRACT A centrifuge of the type used for a number of individual test samples in separate sample containers is dynamically balanced automatically by means of a counterbalancing mass contained in an annular chamber within the centrifuge wheel. The centrifuge wheel is mounted as an inverted pendulum about a fixed pivot and spring biased to the vertical position. The counterbalancing mass, which is at the same vertical level as the sample containers, automatically moves angularly to the counter-balancing position at speeds above the speed corresponding to the natural vibration frequency of the pendulum system.

18 Claims, 2 Drawing Figures I0 I II lllll hm H h l t 2O 0; I, I\ it INVENTOR.

ROBERT A. MAYO STANLEY L. LIVSHITZ PATENTEUSEP 1 9 m SHEET 2 OF 2 INYENTOR. ROBERT A. MAYO STANLEY L. LIVSHITZ BY SELF-BALANCING CENTRIFUGE METHOD AND APPARATUS BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to improved centrifuges of the type which are particularly useful for separating small sample quantities of chemical or biological fluids such as blood samples, the samples being contained in a plurality of small tubes which are carried at the periphery of a centrifuge wheel.

2. Description of the Prior Art One of the most serious problems in the operation of centrifuges of the above type is the problem of imbalance of the centrifuge due to unbalanced loading. There are two basic causes for the unbalanced loading conditions. A minor one of these causes is the factor that the individual containers of liquid to be separated by the centrifuge are not necessarily loaded equally with fluid. However, the major cause of unbalance is a failure to fully load the centrifuge, some of the cups or carriers of the centrifuge being allowed to remain empty while the others are filled.

The presently available solutions for these problems of imbalance have not been particularly satisfactory. For instance, it is not always practical to solve the imbalance problem by waiting until the centrifuge is completely filled before running it because there are often urgent tests which must be made immediately without waiting for subsequent samples to be collected. Another obvious solution is to load the centrifuge symmetrically, evenly spacing the full cups and empty cups around the peripheral edge of the centrifuge, or to fill all empty cups with dummy samples of water. This procedure has a number of important disadvantages. For one, it consumes extra operator time. Furthermore, it is extremely important to-maintain absolutely accurate identification of individual samples. One of the means for accomplishing this is to keep the samples in absolutely strict order and this objective is most easily accomplished by loading the samples into the centrifuge cups in an uninterrupted order without empty cups intervening between adjacent samples. Still further, it has been proposed in accordance with the teachings of a prior related co-pending patent application hat a centrifuge of the above described type should be employed in conjunction with an automatic sampling apparatus, and for this purpose the samples must be loaded in a continuous series in adjacent sample cups without interruption. The prior related patentapplication is application Ser. No. 70,199 filed on Sept. 8, 1970 by Stanley L. Livshitz, Alvin Engelhardt, and Stanford L. Adler for a New and Improved Sample Processing and Supply Appraatus and assigned to the same assignee as the present application.

OBJECTS OF THE INVENTION Accordingly, it is an important object of the present invention to provide an improved centrifuge method and apparatus whereby the centrifuge is self-balancing and which will thus automatically compensate for unbalanced conditions.

Other objects and advantages of the invention will be apparent from the following description and the accompanying drawings.

SUMMARY OF THE INVENTION The present invention may be carried out for balancing a centrifuge of the type having a plurality of separate cups arranged around the periphery of the centrifuge wheel for the purpose of holding containers of liquids to be centrifuged by providing an annular chamber within the wheel, pivotally mounting the centrifuge at its axis as a pendulum, spring biasing the centrifuge to a substantially vertical axis position, placing at least two balancing weight bodies loosely within the annular chamber for free movement therein, and then operating the centrifuge at a speed substantially in excess of that corresponding to the natural frequency of the spring biased pendulum system to cause the balancing weight bodies to move within the annular chamber to positions compensating for any unbalanced condition in the loading of the centrifuge cups.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partially in section, illustrating a preferred embodiment of the invention.

FIG. 2 is a partial schematic sectional top view, taken at section 2-2 of FIG. 1, and serving to illustrate features of the dynamic operation of the embodiment of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Referring particularly to FIG. 1, there is shown a centrifuge wheel 10 including a removable upper wheel portion 12, sometimes referred to hereinafter as a tray arranged for holding a number of sample tubes 14 of liquid to be separated by centrifugal action. The wheel 10 also includes a lower, non-removable, portion 16 which is attached for rotation upon a shaft 18. Shaft 18 is an extension of the shaft of a drive motor 20 by means of which the wheel 10 is rotated.

All of the components thus far described are mounted for support on a plate 22 which is pivotally anchored by means of a ball pivot structure 24 to a base supporting member 26. The base member 26 may comprise a part of a larger structure, or it may be securely fastened to a rigid floor. The mounting plate 22 is spring biased to a substantially horizontal position by means of a series of springs 28 spaced around the peripheral edges of the plate. By means of this spring biasing, the shaft 18 of the centrifuge is spring biased to a substantially vertical position. The pivot 24 is concentric with the center line of theshaft 18. The pivotal mounting of the centrifuge, with its spring bias to the vertical position, may be appropriately described as an inverted pendulum mounting and the pendulum possesses a natural oscillation or a vibration frequency about the pivot 24 which is determined by the physical dimensions and characteristics of the system, and particularly by the mass of the wheel 10 and the spring rate of the springs 28.

The lower portion 16 of the wheel 10 defines an annular chamber 30 containing a counterweight mass 32 which automatically shifts within the cavity to counterbalance for any unbalance in the loading of the sample tubes 14. This automatic counterbalancing operation will be described in more detail below.

The tray portion 12 of the wheel 10 includes holders 34 and 34A for the sample tubes 14, and the holders 34 and 34A are pivotally mounted, as at 36, to the peripheral edges of the tray wheel portion 12, so that they may freely swing radially outwardly when the centrifuge wheel is spinning.

As previously mentioned above, the individual sample containers 14 may be filled to various levels, and individual tube holders 34 may remain empty. These two factors may cause the centrifuge to be rather seriously unbalanced. This causes the centrifuge to wobble as it rotates so that the shaft 18 defines a rotating cone of revolution. Thecenter of the pivotal motion is the ball pivot structure 24. The wobbling motion is resisted by the springs 28. The wobbling motion may be referred to hereinafter as rotational vibration, or simply as vibration. The unbalanced condition is illustrated in FIG. 1 by the presence of a sample tube 14 within the holder 34, and the absence of a sample tube within the holder 34A. At speeds below that corresponding to the natural vibration frequency of the system, the counterbalancing mass 32 tends to move within the cavity 30 to the unbalanced side of the centrifugal wheel so that it does not accomplish a compensating function. However, as soon as the speed of rotation of the centrifuge wheel exceeds the natural frequency of the pendulum system, there is a phase reversal of the forces upon the compensating mass 32 and it is caused to shift within the cavity 30 towards the lightly loaded side of the centrifuge wheel to compensate for the unbalanced forces.

The principle of the phase reversal and the automatic positioning of the compensating mass 32 in an inverted pendulum system of this type is explained more fully at pages 6-25, 6-26 of the Shock and Vibration Handbook, Volume I, published by McGraw Hill in 1961. While the compensating mass 32 may consist of a liquid, in the preferred form of the invention it consists of at least two solid bodies which are capable of sliding or rolling within the cavity 30 to accomplish the compensation effect.

Auxiliary buffer springs 42 may be provided, if desired, to prevent the wobbling excursion from becoming too great for speeds at and below that corresponding to the natural vibration frequency in the case of a very serious unbalanced condition. This prevents damage to the apparatus. As another alternative feature, the centrifuge of the present invention may also include a hydraulic energy absorption device 38 having a piston plunger 40. Device 38 provides energy absorption to prevent a drastic build-up of vibration as the natural resonant frequency of the system of the inverted pendulum is approached. The energy absorber 38 may be of the hydraulic fluid type in which fluid is forced through an orifice. The device 38 may be rigidly mounted by means of a threaded boss into a suitable threaded hole within the plate 22. The plunger 40 may simply be provided with a spherical bottom surface which rests against the base 26 and is normally biased onto contact therewith by an internal spring. A number of the energy absorber devices 38 may be provided and may be interspersed between adjacent springs 28 around the periphery of the plate 7. In

one preferred embodiment, three springs 28 and three energy absorbers 28 were evenly spaced around the periphery of plate 22. The energy absorber 38 may be a small hydraulic shock absorber of the type which is commercially available, for instance, under model number designations having the prefix SI-I" from the Special Products Division of Integrated Dynamics Inc. of 2206 Elmwood Avenue, Buffalo, New York 14216. Other similar hydraulic damper structures may be employed such as rotary dampers which may be coupled between plate 22 and base members 26 by means of rotary operating arms and connecting rods. It is also possible to provide for some energy absorption by substituting rubber springs for the springs 28 since the rubber absorbs additional energy as it is deformed. I

FIG. 2 is a partial schematic sctional top view taken at section 2-2 in FIG. 1. As illustrated in FIG. 2, the mass 32 preferably consists of at least two solid bodies 32A and 32B,which are preferably cylinders or spheres having a circular profile when viewed from above. These bodies are rotatably slidable to compensating positions such as illustrated at 32A and 328. The cylindrical form of the bodies 32A and 32B is preferred because the slight frictional restraint caused by the requirement that the cylinders must slide on the bottom Y surface of the chamber 30 provides somewhat greater stability at low speeds and provides quieter operation. At the higher speeds where compensation is obtained, the centrifugal forces upon the bodies 32A and 32B are so much greater than the downward gravity force that the sliding friction is not a serious restraining force. It has been determined that the frictional restraint of the cylinders on the bottom surface of the chamber 30 is often sufficient to keep the cylinders in the dynamic balance position when the centrifuge is turned off and slowed down at the end of an operating cycle. Thus the centrifuge remains balanced as it slows through the speed corresponding to the natural vibration frequency of the system. The frictional resistance of the cylinders on the bottom surface of the chamber is also advantageous in restraining movement of the cylinders to the unbalanced side of the centrifuge during the acceleration phase of the operating cycle. It is within the scope of the invention to enhance this feature of the operation of the device by providing non-skid surfaces on the bottom surface of the changer, or the bottom surface of the cylinders, or both. This non-skid feature may be implemented in many ways, including, for instance, roughened surfaces which provide an interlock effect.

While the counterbalancing mass 32 is referred to generically in FIG. 1, it is represented asa cylinder, such as the cylindrical body 32A, since that is the preferred form for the mass. However, it must be emphasized that when solid bodies are employed, it is much preferred to use at least two such bodies so that symmetrical movements of the bodies around the chamber 30 are possible to maintain symmetrical side to side balance with respect to the point of imbalance of the centrifuge wheel. Thus, as the bodies 32A and 32B move from the position shown in FIG. 2 to the position indicated at 32A and 328', they move in a coordinated manner on both sides of the chamber 30 until a balanced position is achieved. When compensating for a maximum unbalanced condition, the bodies would proceed to the extreme position opposite to the original position until they touched in the bottom most position in FIG. 2. In the event the centrifuge wheel is actually balanced (independent of bodies 32A and 323), then the bodies 32A and 328 end up in positions diametrically opposite to one another. For any intermediate unbalanced condition, the bodies are positioned at some intermediate position such as shown in the drawing at 32A and 32B.

While only two of the counterbalancing bodies 32A and 32B are illustrated in the drawings, it will be understood that three or four or more of these bodies may be employed. However, it is preferably not to fill the cavity 30 more than half full of compensating bodies since the bodies must shift in order to provide a counterbalancing action and if the cavity is more than half full, a maximum compensating action is not achievable through the shifting operation. While large bodies which substantially fill the cross-sectional area of the cavity are illustrated, it is within the scope of this invention to employ smaller bodies which nest together and which may be two or more deep across the toroidal chamber 30.

It is a preferred feature in carrying out the present invention that the upper tray portion 12 of the wheel is easily removable from the remainder of the centrifuge by simply lifting it vertically upward by means of the central handle which is provided on the tray 12. This provides quick removal of the tray and all of the loaded cups 34 and quick transfer of the entire tray and all the samples for a combination with automatic sample analysis apparatus. This particular feature relates to a portion of the subject matter described and claimed in the prior copending patent application Ser. No. 70,199 previously mentioned above.

One important feature of the invention is that the balancing mass 32 is supported within the chamber 30 in a vertical position which is in substantial horizontal alignment with the unbalanced loadsof the cups 34. While this perfect horizontal alignment is not entirely apparent from the static picture given by FIG. 1, when the centrifuge is in operation, the cups 34, whether they are filled or empty, swing radially outwardly about the pivots 36 so that the center of gravity of each cup 34 is effectively raised to be substantially horizontally in line with the mass 32. This feature is important because it provides a more perfect balancing condition when the ultimate balancing is achieved, a condition in which force couples are absent. If the counterbalancing mass 32 had been located at a different elevation, force couples would be generated in the shaft 18 which would necessarily have to be resisted by the pivot 24 and would tend to cause flexing of the shaft 18. These couples can be very detrimental to the apparatus, causing vibration and excessive wear of the motor bearings and the spherical pivot 24, and necessitating heavier construction of the shaft 18.

Because of the perfection of the dynamic balance achievable by means of the present invention, it is possible to provide for satisfactory operation of the apparatus without the use of the fixed pivot 24. In such an arrangement, the entire vertical support of the apparatus is provided through the springs 28 and the energy absorption devices 38. In such a modification, a pivoting or rocking action of the plate 22 is achieved about a pivot point in response to unbalanced conditions. However, the fixed pivot construction employing the spherical ball point pivot 24 is preferred as it provides vertical stability for the apparatus. It will be understood that other universal pivot structures may be employed as alternatives to the spherical bearing shown.

There are a number of important advantages to the pivoted pendulum arrangement of the centrifuge of the present invention. For one, as the centrifuge begins to spin, the initial deflections of the shaft 18 from the vertical position are resisted not only by the springs 28, but also by the gyroscopic forces of the spinning wheel 10. Thus, there is a gyroscopic stabilization effect since the wobbling motion involves a tilting of the axis of rotation.

Furthermore, there is an important advantage to the so-called inverted" pendulum configuration in which the entire centrifuge structure is supported above the pendulum pivot rather than below the pivot. This advantage lies in the fact that the gravity forces acting upon the wheel 10 and the shaft 18 combination, whenever the structure is tilted away from the vertical, tend to increase the apparent mass of the pendulum and thus lower the natural vibration frequency of the pendulum. With the aid of this factor, and by selecting an appropriate ratio between the masses of the centrifuge and the spring rate of the springs 28, the natural frequency of the device is caused to occur at a relatively low rotational speed. Thus, most of the speed range of the device occurs above the speed corresponding to the natural frequency of the system and compensation is accordingly available for most of the acceleration, running, and deceleration cycle of the device.

This is an important advantage of the invention because it means that dynamic balance conditions are approached or achieved before the speed of the device becomes great enough to develop substantial centrifugal forces. In the preferred form of the invention, the speed corresponding to the resonant vibration frequency is less than ten percent of the full operating speed of the device. In an actual physical embodiment, for instance, the speed corresponding to the resonant vibration frequency is about revolutions per minute, and the full operating speed is at least 1,500 revolutions per minute, and more typically about 2,250 revolutions per minute, or higher.

While we have shown and described the preferred embodiment of our invention, it will be understood that the invention may be embodied otherwise than as herein specifically illustrated or described, and that certain changes in the form and arrangement of parts and in the specific manner of practicing the invention may be made without departing from the underlying idea or principles of this invention within the scope of the appended claims.

We claim:

1. A method of balancing a centrifuge of the type having a plurality of separate cups arranged around the periphery of a centrifuge wheel for the purpose of holding containers of liquids to be centrifuged comprising providing an annular chamber within the wheel,

pivotally mounting the centrifuge at its axis as a pendulum,

spring biasing the centrifuge to a substantially vertical axis position,

placing at least two balancing weight bodies loosely within said annular chamber for free movement therein,

7 and then rotating the centrifuge at a speed in excess of that corresponding to the natural frequency of the spring biased pendulum system to cause the balancing weight bodies to move within the annutered at said pivot point.

9. A centrifu e as clai ed in clai 4 wherein the combina on of szii d shaft an said wheel is supported as an inverted pendulum with said pivot lar chamber to positions compensating for any un- 5 point positioned beqeath sfaid f balanced condition in the loading of the centrifuge A cenmfuge as .clalmed m claim 4 when! cups, the balancing weight bodies being positioned sald naimral vlbratlon. frequency corresponds to 8 within the annular chamber at substantially the centrifuge speed which less than ten pelicem of same level as the centrifuge cups during the rotathe fun operaimg sPeed the i tion ofthe centrifuge lo 11. A centrifuge as claimed in claim 8 wherein said supporting means comprises a fixed supporting base mounting and supporting said ball swivel bearing,

a mounting plate axially spaced away from said sup- 2. The method as claimed in claim 1 wherein the centrifuge is mounted as an inverted pendulum.

3. A method as claimed in claim 1 wherein porting base,

the force i the spimgs biasmg i c.enmfuge to a said mounting plate being fixed with respect to the substantially vertical axis position is selected to 15 provide a natural vibration fre uenc of the cenaxialposm-on sa-ld shaft 3 y d h and said spring biasing means comprising compresi correspon mg to a can 1 uge spec w sion springs arranged between said supporting is less than ten percent of the normal full operating base member and said plate and spring biasing said speed ofthe cfmmfugei plate to a position substantially parallel to said 4. A self-balancing centrifuge comprising a rotatable base member shaft 12. A centrifuge as claimed in claim 1 1 wherein mezfns for suppomng sald shaft permfttmg said drive motor is mounted upon said mounting pivotal displacement thereof about a pivot point at plate with its shaft extending vertically upward on F center f of sad fl an axis passing through said pivot point,

Sald Suppomng meanP f Spnng basmg said rotatable shaft being connected to and forming mealls arranged to fi 531d f' an extension of the shaft of said drive motor and stantially vertical position aboutsald pivot point, being supported upon and directly driven thereby a motor connected to mate Said Shaft 13. A centrifuge as claimed in claim 4, wherein a ceflmfuge Wheel mounted P Said shaft at an said centrifuge wheel comprises a light weight ax al position thereon substantially displaced from removable sample; tray including the i h l Said P P portions of said wheel and the cups for receiving Said centrifuge Wheel including a plurality of P sample containers and a portion defining said an- P y connected to the P p y of Said Wheel nular chamber, the sample tray portion of said for mceiving sample containers of liquid t0 be wheel being removable from said annular chamber separated y centrifuge action, portion for the purpose of transferring the sample said wheel defining an annular chamber arranged containers as a group for processing in another apconcentrically about said shaft, paratus,

a counterbalance mass partially filling said annular 14. A centrifuge as claimed in claim 4 wherein chamber, there is provided energy absorption means responsaid counterbalance mass being arranged to shift in sive to displacement movements of said shaft away its angular position within said annular chamber to from the vertical position to absorb the vibration automatically balance said wheel for unequal loadenergy of the combination of said shaft and said ing of said cups at speeds above the natural vibrawheel to thereby limit the vibration amplitudes at tion frequency of the combination of said shaft and low frequencies. said wheel about said pivot point, said annular 15. Acentrifuge as claimed in claim 14 wherein chamber being formed to allow the shift of said said energy absorption means comprises a plurality counterbalance mass to a position which is at subof hydraulic shock absorber devices. stantially the same level as said cups during rota- A Centrifuge as claimed in claim 4 wherein i f id centrifuge h said spring biasing means includes snubber springs 5 A centrifuge as l i d i l i 4 h i operable when the vibration excursion exceeds a said counterbalance mass comprises a plurality of p g t h g if g lf fia spring Solid i rate an o t ere y imit u er v1 ration am- 6. A centrifuge as claimed in claim5 wherein Plitude excursion Prtwent damage {0 the P' each of said solid bodies has a circular profile when P viewed f above. 17. A centrifuge as claimed in claim wherein 7 A t if as claimed in claim 4 wherein the configuration of each of said solid bodies 18 such said supporting means includes means for maintainas to q f slldmg ent Upon the lower suring a substantially fixed vertical position for said of Said chamber for the shlft Posmon to shaft while permitting restrained pivotal defiec achleve tions thereof away from the vertical alignment 25 2238 fi l ag g r'g i gtf slzd z gg d ositiom a e o sai o u c c am ran 3 centrifuge as claimed in claim 7 wherein the surface of each of said bodies in engagement said means for maintaining a substantially fixed vertitherewlth conslst of a rnon'skld Surfacecal position comprises a ball swivel bearing cen-

Claims

1. A method of balancing a centrifuge of the type having a plurality of separate cups arranged around the periphery of a centrifuge wheel for the purpose of holding containers of liquids to be centrifuged comprising providing an annular chamber within the wheel, pivotally mounting the centrifuge at its axis as a pendulum, spring biasing the centrifuge to a substantially vertical axis position, placing at least two balancing weight bodies loosely within said annular chamber for free movement therein, and then rotating the centrifuge at a speed in excess of that corresponding to the natural frequency of the spring biased pendulum system to cause the balancing weight bodies to move within the annular chamber to positions compensating for any unbalanced condition in the loading of the centrifuge cups, the balancing weight bodies being positioned within the annular chamber at substantially the same level as the centrifuge cups during the rotation of the centrifuge.

2. The method as claimed in claim 1 wherein the centrifuge is mounted as an inverted pendulum.

3. A method as claimed in claim 1 wherein the force of the springs biasing the centrifuge to a substantially vertical axis position is selected to provide a natural vibration frequency of the centrifuge corresponding to a centrifuge speed which is less than ten percent of the normal full operating speed of the centrifuge.

4. A self-balancing centrifuge comprising a rotatable shaft, means for supporting said shaft while permitting pivotal displacement thereof about a pivot point at the center line of said shaft, said supporting means including spring biasing means arranged to bias said shaft towards a substantially vertical position about said pivot point, a motor connected to rotate said shaft, a centrifuge wheel mounted upon said shaft at an axial position thereon substantially displaced from said pivot point, said centrifuge wheel including a plurality of cups pivotally connected to the periphery of said wheel for receiving sample containers of liquid to be separated by the centrifuge action, said wheel defining an annular chamber arranged concentrically about said shaft, a counterbalance mass partially filling said annular chamber, said counterbalance mass being arranged to shift in its angular position within said annular chamber to automatically balance said wheel for unequal loading of said cups at speeds above the natural vibration frequency of the combination of said shaft and said wheel about said pivot point, said annular chamber being formed to allow the shift of said counterbalance mass to a position which is at substantially the same level as said cups during rotation of said centrifuge wheel.

5. A centrifuge as claimed in claim 4 wherein said counterbalance mass comprises a plurality of solid bodies.

6. A centrifuge as claimed in claim 5 wherein each of said solid bodies has a circular profile when viewed form above.

7. A centrifuge as claimed in claim 4 wherein said supporting means includes means for maintaining a substantially fixed vertical position for said shaft while permitting restrained pivotal deflections thereof away from the vertical alignment position.

8. A centrifuge as claimed in claim 7 wherein said means for maintaining a substAntially fixed vertical position comprises a ball swivel bearing centered at said pivot point.

9. A centrifuge as claimed in claim 4 wherein the combination of said shaft and said wheel is supported as an inverted pendulum with said pivot point positioned beneath said wheel.

10. A centrifuge as claimed in claim 4 wherein said natural vibration frequency corresponds to a centrifuge speed which is less than ten percent of the normal full operating speed of the centrifuge.

11. A centrifuge as claimed in claim 8 wherein said supporting means comprises a fixed supporting base mounting and supporting said ball swivel bearing, a mounting plate axially spaced away from said supporting base, said mounting plate being fixed with respect to the axial position of said shaft, and said spring biasing means comprising compression springs arranged between said supporting base member and said plate and spring biasing said plate to a position substantially parallel to said base member.

12. A centrifuge as claimed in claim 11 wherein said drive motor is mounted upon said mounting plate with its shaft extending vertically upward on an axis passing through said pivot point, said rotatable shaft being connected to and forming an extension of the shaft of said drive motor and being supported upon and directly driven thereby.

13. A centrifuge as claimed in claim 4, wherein said centrifuge wheel comprises a light weight removable sample tray including the peripheral portions of said wheel and the cups for receiving sample containers and a portion defining said annular chamber, the sample tray portion of said wheel being removable from said annular chamber portion for the purpose of transferring the sample containers as a group for processing in another apparatus.

14. A centrifuge as claimed in claim 4 wherein there is provided energy absorption means responsive to displacement movements of said shaft away from the vertical position to absorb the vibration energy of the combination of said shaft and said wheel to thereby limit the vibration amplitudes at low frequencies.

15. A centrifuge as claimed in claim 14 wherein said energy absorption means comprises a plurality of hydraulic shock absorber devices.

16. A centrifuge as claimed in claim 4 wherein said spring biasing means includes snubber springs operable when the vibration excursion exceeds a predetermined amplitude to increase the spring rate and to thereby limit further vibration amplitude excursion to prevent damage to the apparatus.

17. A centrifuge as claimed in claim 5 wherein the configuration of each of said solid bodies is such as to require sliding movement upon the lower surface of said chamber for the shift in position to achieve balance.

18. A centrifuge as claimed in claim 17 wherein at least one of said lower surface of said chamber and the surface of each of said bodies in engagement therewith consist of a non-skid surface.