Abstract:
In a centrifuge comprising a rotor shaft assembly, a diaphragm disposed about the rotor shaft assembly reduces noise and vibration. The diaphragm permits the rotor shaft assembly to pivot off a vertical axis while substantially limiting horizontal displacement thereof. Also, where a centrifuge includes a rotor shaft and a drive shaft, a member situated between the rotor shaft and the drive shaft substantially limits vertical displacement of the rotor shaft while allowing angular deflection of the rotor shaft with respect to the drive shaft.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a centrifuge rotor shaft assembly, and more particularly to a centrifuge assembly where a diaphragm is disposed about the rotor shaft assembly to permit the rotor shaft assembly to pivot while substantially limiting horizontal displacement thereof. Also, a member situated between a rotor shaft and a rotor shaft substantially limits vertical displacement of the rotor shaft, while allowing angular deflection of the rotor shaft with respect to the drive shaft. 
     2. Description of the Prior Art 
     A centrifuge instrument is a device by which liquid samples may be subjected to a centrifugal force. The samples are typically carried in tubes situated within a member known as a centrifuge rotor. The rotor is mounted at the top of a rotor shaft, which is connected to a drive shaft that provides a source of motive energy. Centrifuge drive systems must be designed to accommodate unbalanced rotating loads. The imbalance may exist initially when loading samples into the centrifuge rotor, or it may result from a tube failure during operation of the centrifuge. The imbalance represents a non-uniform distribution of matter throughout the mass of the rotor. 
     Any given mass, or centrifuge rotor, has a geometric center based on the dimensions of the mass, and a mass center based on the distribution of matter within the mass. The mass center is also referred to as the center of gravity. In an actual mass or centrifuge rotor, the mass center is offset from the geometric center due to machining errors and density variations. A rotating mass mounted on a drive and suspension system, has a critical speed at which the mass laterally shifts its axis of rotation from rotating about its geometric center to rotating about its mass center. 
     Centrifuge drive systems operate below and above a critical speed. Below the critical speed, the centrifuge rotor rotates about its geometric center. Above the critical speed, the centrifuge rotor attempts to rotate about its mass center. Because centrifuge drive and suspension systems need to have some type of spring in the system to allow the transition through critical speed, the centrifuge rotor approaches rotation about its mass center. A vibration is induced because centrifuge rotor mass center and the centerline of the drive system do not fully align. The amount of vibration that the rotor produces at a given speed is dependent on the distance between the rotor&#39;s mass center and drive geometric center. If the components of the drive system for the centrifuge are rigidly interconnected, then the vibration would subject the drive system to damaging stresses that could possibly destroy the centrifuge. Accordingly, centrifuge drive systems are typically designed to enjoy a certain degree of flexibility. 
     For a centrifuge rotor to approximate rotation about its mass center, the rotor shaft must be allowed to horizontally shift its axis of rotation. Accordingly, two flexible joints are required between the drive shaft and the rotor shaft. Flexible shafts and gyros, which are well known in the prior art, both allow the required horizontal shift. 
     A flexible shaft must bend or deflect in order to allow a rotor to spin about its mass center. The greater the flexibility of the shaft, the further it can be deflected to accommodate the horizontal shift and thus reduce the load on the centrifuge motor bearings, motor suspension and instrument frame. However, there is a tradeoff. Greater flexibility is generally achieved by reducing the diameter of the flexible shaft. Smaller diameter shafts have a greater difficulty in making the critical speed transition, and they can be more easily damaged by an unbalanced rotor or by a rotor that has been dropped on the shaft. Smaller diameter shafts also limit the amount of torque that can be transmitted, thus limiting the acceleration rate. 
     Gyro systems are more robust and less expensive to replace than flexible shaft systems. A gyro system is basically comprised of a rotor shaft pivotally connected to a drive shaft or motor shaft through an intermediate coupling. The intermediate coupling serves as a universal joint that allows the axis of the rotor shaft to assume a position different from that of the drive shaft. The centrifuge rotor is connected to the rotor shaft with a flexible coupling. 
     The problem associated with centrifuge operation above critical speed is well recognized in the prior art. The following patents illustrate several mechanisms that have been developed to reduce vibrations. 
     U.S. Pat. No. 3,770,191 (Blum) discloses a centrifuge drive system that automatically causes the center of gravity of a rotor to become aligned with the axial center of the drive system. An articulated rotor shafts permits lateral movement of the rotor whereby the geometric center of the rotor can be displaced so that its center of gravity becomes aligned with the axis of the drive system. A sliding block element is disposed about the articulated rotor shaft to reduce undue vibration of the shaft. 
     U.S. Pat. No. 4,568,324 (Williams) discloses a drive shaft assembly including a damper disposed between a flexible shaft and a bearing shaft. The damper accommodates the flexure of the flexible shaft while damping vibrations that are imposed on the flexible shaft by a rotor. 
     U.S. Pat. No. 5,827,168 (Howell) discloses a disk, rotatably attached to a centrifuge drive shaft, for reducing vertical vibrations of the drive shaft. Damping bearings are positioned against a surface of the disk to reduce vibrations thereof. 
     FIG. 1 shows a cross section of a typical centrifuge gyro drive shaft assembly of the prior art. A gyro housing  10  generally encloses one end of a rotor shaft  15  and one end of a drive shaft  25 , which are interconnected through a coupling  20 . The other end of drive shaft  25  is housed within a motor  40 . Rotor shaft  15  is supported within gyro housing  10  by bearings  30   a  and  30   b , and flexible mounting  35 . The flexible mounting  35  is composed of a bearing housing  36  and two elastomeric rings  37   a  and  37   b . A rotor (not shown) is positioned on top of rotor shaft  15 . 
     At rest, and at speeds below the critical speed, rotor shaft  15  and drive shaft  25  share a common vertical axis  45 . During centrifuge operation, motor  40  provides a rotational motive force that rotates drive shaft  25 , coupling  20  and rotor shaft  15 . Motor  40  accelerates, thus increasing the angular velocity of rotor shaft  15 . At the critical speed, the rotational axis of rotor shaft  15  shifts both horizontally and at an angle away from vertical axis  45 . This shift is permitted by flexible mounting  35 . 
     Bearings  30   a  and  30   b  are horizontally displaced by the horizontal displacement or shift of rotor shaft  15 . Flexible mounting  35  compresses and expands to accommodate the displacement of bearings  30   a  and  30   b . As with any spring mass system, the elastic stiffness of flexible mounting  35  results in a resonant frequency that is within the normal operating range of most centrifuge systems. 
     A drive assembly configured as shown in FIG. 1 suffers from several inherent deficiencies. First, the horizontal shift of rotor shaft  15  and bearings  30   a  and  30   b  is itself a source of resonant vibration. A resonance is undesirable in a system where an objective is to minimize vibration. Second, to accommodate the shift and provide an adequate degree of torsional flexibility, flexible mounting  35  is typically composed of an elastomer. As rotational velocity increases, the elastomer becomes less flexible, and less responsive to the horizontal shift. Third, the elastomer is not a very good thermal conductor. Consequenty; heat generated by bearings  30   a  and  30   b  is not efficiently dissipated, and they are therefore stressed and susceptible to premature fatigue. 
     Another undesirable degree of freedom can be found in the vertical movement of rotor shaft  15 . Because bearings  30   a  and  30   b  are mounted by elastomeric rings  37   a  and  37   b , rotor shaft  15  can move vertically. This vertical movement introduces another mode of vibration at a resonant frequency within the normal operating range of most centrifuge systems. 
     There is a need for a centrifuge drive assembly that can accommodate the tendency of a rotor to shift its axis of rotation from its geometric center to its mass center while minimizing vibration introduced by horizontal displacement of the drive shaft assembly. 
     There is also a need for a centrifuge drive assembly that minimizes vibration caused by a vertical displacement of a rotor shaft while allowing angular deflection of the rotor shaft with respect to a drive shaft. SUMMARY OF THE INVENTION 
     The present invention provides a centrifuge assembly that comprises a rotor shaft assembly and a diaphragm disposed about the rotor shaft assembly. The diaphragm permits the rotor shaft assembly to pivot off a vertical axis while horizontal displacement of the drive shaft assembly is substantially limited. 
     This unique centrifuge assembly typically comprises a rotor, a rotor shaft assembly and a diaphragm flexibly secured about the rotor shaft assembly. The rotor shaft assembly may include a rotor shaft coupled to the drive shaft via an intermediate coupling, and, optionally, a gyro housing enclosing one end of the rotor shaft and one end of the coupling. 
     In one embodiment, the diaphragm is comprised of a plurality of radially directed bars. 
     In a second embodiment, the diaphragm is comprised of an inner flange and an outer flange having a common center point. The flanges are connected by radially directed bars. 
     In a third embodiment, the diaphragm is a disk with a centrally located hole. The disk provides flexible security throughout a 360° arc. 
     The centrifuge may additionally comprise one or more springs to vertically support the rotor shaft assembly. The springs can be situated beneath the base of the rotor shaft assembly, or formed from an elastomeric ring and disposed about a load bearing perimeter of the rotor shaft assembly, or can be incorporated into a drive coupling. 
     The present invention allows nutation of the rotor about the rotor shaft assembly and limits horizontal displacement of the axis of rotation of the coupling. Accordingly, the vibration associated with the horizontal displacement is substantially reduced due to the avoidance of any resonant frequencies within the operating range of the centrifuge rotor. That is, the greater the horizontal stiffness, the higher the resonant frequency is pushed above the operating range of the centrifuge. 
     Additionally, a member situated between a rotor shaft and a drive shaft limits vertical movement of the rotor shaft while allowing angular deflection of the rotor shaft with respect to the drive shaft. The member takes up a gap between the rotor shaft and the drive shaft caused by manufacturing tolerances. In one embodiment, the member is comprised of a cylindrical spacer and two disk-shaped pads. In a second embodiment, the member is comprised of a first sleeve disposed substantially around an end of the rotor shaft, a second sleeve disposed substantially around an end of the drive shaft, and a column disposed between the two sleeves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a centrifuge gyro drive shaft assembly of the prior art; 
     FIG. 2 is a cross section of a centrifuge drive shaft assembly; 
     FIG. 3 is a top planar view of a diaphragm according to one embodiment of the present invention; 
     FIG. 4 is a top planar view of another embodiment of a diaphragm according to the present invention; 
     FIG. 5 is a top planar view of still another embodiment of a diaphragm according to the present invention; 
     FIG. 6 is a cross-sectional of a centrifuge assembly according to the present invention, including springs for vertical support of a rotor shaft assembly; 
     FIG. 7 is a top planar view depicting the relationship between the springs and diaphragm bars; 
     FIG. 8 is a cross-sectional view of a centrifuge drive shaft assembly with another embodiment of a spring; 
     FIG. 9A is a graph depicting the vibratory force produced by a conventional gyro of the prior art; 
     FIG. 9B is a graph depicting the vibratory force produced by a horizontal spring gyro of the present invention; 
     FIG. 10 is a cross-sectional view of one embodiment of a member situated between a rotor shaft and a drive shaft according to the present invention; 
     FIG. 11A is a cross-sectional view of a second embodiment of a member situated between a rotor shaft and a drive shaft according to the present invention; 
     FIG. 11B is a top planar view of a sleeve with a slit as seen along line  11 B— 11 B of FIG. 11A; 
     FIG. 12A is a side elevation of a flexible coupling; and 
     FIG. 12B is an end view of a flexible coupling as seen along line  12 B— 12 B of FIG.  12 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a cross section of a centrifuge assembly  100  according to the present invention. Centrifuge assembly  100  has a motor  150 , a motor housing  140 , a diaphragm  130 , a rotor shaft assembly  105 , a drive spud  187  and a rotor (not shown). 
     A drive shaft  145  is coupled to a rotor shaft  115  via a coupling  120 . It also includes a gyro housing  110 , which encloses one end of rotor shaft  115  and one end of coupling  120 . Rotor shaft  115  is supported within gyro housing  110  by bearings  163 . Drive spud  187  is pivotally connected to rotor shaft  115 , and the rotor is positioned on top of drive spud  187 . 
     Diaphragm  130  is disposed about coupling  120  and flexibly couples rotor shaft assembly  105  to motor housing  140 . Diaphragm  130  is, optionally, connected to gyro housing  110  by bolts  125   a  and  125   n , and connected to motor housing  140  by bolts  135  and  135   n . As will be described below, diaphragm  13  to pivot on a rotor shaft assembly pivot point  155 . 
     During centrifuge operation, motor  150  provides a rotational motive force that rotates drive shaft  145 , coupling  120 , rotor shaft  115 , drive spud  187 , and ultimately the rotor. At speeds below a critical speed the rotor rotates about its geometric center. The rotor&#39;s geometric axis is located at an axis  175   a , which coincides with a vertical axis  165 . Gyro housing  110 , rotor shaft  115  and drive shaft  145  are also centered along vertical axis  165 . Diaphragm  130  lies in a plane substantially perpendicular to drive shaft  145 . 
     At and above the critical speed, the rotor rotates about its mass center. The mass center is offset from the geometric center by a distance  180 . The rotor&#39;s mass center aligns with axis  175   a , and consequently, the rotor&#39;s geometric axis is forced to shift horizontally to axis  175   b . The relationship between axis  175   a  and  175   b  as shown in FIG. 2 represents an instant in time. As the rotor rotates about its mass center at axis  175   a , the rotor&#39;s geometric axis revolves around axis  175   a . That is, the geometric axis travels in a circle with a centerpoint at axis  175   a  and a radius of distance  180 . Since axis  175   a  coincides with vertical axis  165 , which is also the axis of drive shaft  145 , the rotation of the rotor shaft about its mass center is concentric with the rotation of drive shaft  145 . 
     Since the rotor is pivotally connected to drive spud  187  at drive spud pivot point  185 , the rotor and its geometric axis are allowed to pivot along an arc  170  and remain vertical. However, the axis of rotor shaft  115  is deflected from vertical axis  165  to an axis  190 . Axis  190  is defined by endpoints at drive spud pivot point  185  and rotor shaft assembly pivot point  155 . As the rotor rotates about its mass center at axis  175   a , axis  190  revolves, and defines a cone of precession, around vertical axis  165 . As seen in FIG. 2, the rotor shaft assembly  105  is permitted to pivot with respect to drive shaft  145  and vertical axis  165  when the rotor is rotating. 
     As the axis of rotor shaft  115  is deflected to axis  190 , diaphragm  130  permits gyro housing  110  to pivot along an arc  160  so that the centerline of gyro housing  110  likewise coincides with axis  190 . In this illustration, which shows an instant in time, gyro housing  110  pivots on rotor shaft assembly pivot point  155  in a counter-clockwise direction as shown by arc  160 . The side of gyro housing  110  that is connected to diaphragm  130  by bolt  125   a  moves down, and the other side of gyro housing  100 , which is connected to diaphragm  130  by bolt  125   n , moves up. During centrifuge operation, gyro housing  110  oscillates about vertical axis  165 . This oscillatory movement on the part of gyro housing  110  is referred to as“mutation”. Gyro housing  110  is thus permitted to pivot off vertical axis  165  but its horizontal displacement is substantially limited. 
     In an actual centrifuge system, the difference between a rotor&#39;s mass center and geometric center, i.e., distance  180 , is typically about 0.05 (50 thousandths) inches, and arc  160  represents about 1° of angular displacement off the vertical axis  165 . The nutation of a gyro housing  110  is barely discernible to the naked eye, but a tremendous amount of force must be constrained. For example, a 57 pound rotor rotating at 9,000 cycles per minute (CPM) is subjected to approximately 6,000 pounds of centrifugal force. 
     Gyro housing  110  nutates, and diaphragm  130  flexes, at the same rate that the rotor rotates. Diaphragm  130  must be flexible enough to accommodate the nutation of gyro housing  110 , yet strong enough to endure the stress imposed during centrifuge operation. Ideally, diaphragm  130  would have a zero spring rate and freely allow the rotor to shift its axis of rotation from its geometric center to its mass center. However, all objects oscillate at a natural frequency that is a function of their spring rate and mass. In practical application, diaphragm  130  is designed with a spring rate greater than the operating frequency of the centrifuge system. That is, a lower spring rate can be used in a centrifuge system with a heavy rotor and a low operating frequency, than in a system with a light rotor or high operating frequency. Several alternative embodiments of diaphragms are presented below. 
     FIG. 3 is a top planar view of one embodiment of a diaphragm  192  according to the present invention. Diaphragm  192  is comprised of a plurality of radially directed bars  193  disposed about the circumference of a coupling  199  at regular angular intervals  198 . Bars  193  are connected to a motor housing  194  by bolts placed through holes  195 , and connected to a gyro housing  196  by bolts placed through holes  197 . Bars  193  are approximately 0.180 inches wide and 0.060 inches thick, and manufactured of stainless steel. 
     FIG. 4 shows another embodiment of a diaphragm  200  according to the present invention. An outer flange  210  and inner flange  215  share a common center point  220 . Inner flange  215  and outer flange  210  are connected by radially directed bars  225 . Bars  225  are spaced at regular angular intervals  240  to partition diaphragm  200  into substantially equal arcs. Diaphragm  200  is connected to a gyro housing by bolts placed through holes  230 , and connected to a motor housing by bolts placed through holes  235 . Bars  225  are approximately 0.180 inches wide and 0.060 inches thick. Diaphragm  200  is manufactured of stainless steel. 
     FIG. 5 depicts still another embodiment of a diaphragm  300 , comprising a disk  310  with a centrally located hole  315 . Diaphragm  300  is connected to a gyro housing by bolts placed through holes  320 , and connected to a motor housing by bolts placed through holes  325 . Diaphragm  300  is manufactured of 16 gauge stainless steel. 
     FIG. 6 is a cross-sectional view of a centrifuge assembly m which vertical springs provide support for a rotor shaft assembly. A drive shaft  445  is coupled to a rotor shaft  415  via a coupling  420 . It also includes a gyro housing  410 , which encloses one end of rotor shaft  415  and one end of coupling  420 . A flexible drive spud  487  is pivotally connected to rotor shaft  415 , and a rotor (not shown) is positioned on top of drive spud  487 . A diaphragm with radially directed bars  430   a  and  430   b  is disposed about coupling  420 . Springs  450   a  and  450   b  are positioned to support rotor shaft assembly  405 . 
     Springs  450   a  and  450   b  are intended to relieve some of the vertical force imposed upon diaphragm bars  430   a  and  430   b  by the combined weight of rotor shaft assembly  405  and the centrifuge rotor. Springs  450   a  and  450   b  serve to extend the useful life of diaphragm bars  430   a  and  430   b.    
     Springs  450   a  and  450   b  can be a manufactured of a metallic or elastomeric material. Practical examples include helical springs, wound springs, machined springs and elastomeric springs such as a Lord FlexBolt™, manufactured by Lord Corporation of Erie, Pa. However, elastomeric springs, as compared to metallic springs, provide better damping of vertical and oscillatory ringing of rotor shaft assembly  405   
     FIG. 7 is a top planar view showing the relationship of springs to diaphragm bars. Springs  450   a  and  450   b , and bars  430   a  and  430   b , are subsets of a plurality of springs  450   a - 450   n , and bars  430   a - 430   n , respectively. Springs  450   a - 450   n  and bars  430   a - 430   n  are disposed about the perimeter of coupling  420 . Any given spring  450   a - 450   n  is located in an arc  460  formed between two adjacent bars  430   a - 430   n.    
     FIG. 8 is a cross-sectional view of a centrifuge assembly with another embodiment of a spring for vertical support of a rotor shaft assembly. A rotor shaft assembly  505  includes a gyro housing  520  generally enclosing one end of a rotor shaft  525  and one end of a drive shaft  535 , which are interconnected through a coupling  515 . A flexible drive spud (not shown) and a rotor shaft (not shown) are positioned on top of rotor shaft  525 . A diaphragm  530  is disposed about coupling  515 . Spring  510  is disposed about a load-bearing perimeter of gyro housing  520 . 
     Spring  510  is a solid elastomer ring. It absorbs some of the vertical force imposed upon diaphragm  530  by the combined weight of rotor shaft assembly  505  and the centrifuge rotor. Spring  510  serves to extend the useful life of diaphragm  530 . 
     FIGS. 9A and 9B are graphs comparing the performance of a conventional gyro (FIG. 9A) to a horizontal spring gyro of the present invention (FIG.  9 B). The horizontal axes of these graphs represent rotor cycles per minute (CPM) and the vertical axes represent units of acceleration (G). 
     A conventional gyro, represented in FIG. 9A, produces significant vibrations of approximately 7G at 6k CPM (ref.  610 ), and increases to approximately 14.3G at 18.8k CPM (ref.  620 ). 
     In contrast, a horizontal spring gyro of the present invention, represented in FIG. 9B, produces vibrations of approximately  4 G at 6k CPM (ref.  630 ) and 2G at 18.8k CPM (ref.  640 ). The vibrations of the horizontal spring gyro are significantly lower than those of the conventional gyro in the range of 6k CPM to 18.8k CPM. Vibratory acceleration peaked at approximately 32.3G at 20.5k CPM (ref.  650 ). 20.5k CPM is therefore the resonant frequency of the system. The frequency at which the peak occurs is adjustable by altering the thickness and width of the bars in the various embodiments of the diaphragm of the present invention. As the bars are made thicker and wider, the spring rate and the resonant frequency of the system increases. The spring rate can be increased to set the resonant frequency above the operating frequency range of the system. 
     FIG. 10 shows one embodiment of a member situated between a rotor shaft and a drive shaft for limiting vertical displacement of the rotor shaft. A member  725  is situated between a rotor shaft  705  and a drive shaft  710 . Member  725  is accommodated within an axially directed center hole through a coupling  730 , and is held in place by coupling  730 . 
     Member  725  is comprised of a metal cylindrical spacer  720  and two rubber disk-shaped pads  715   a  and  715   b . However, a spacer  720  or pad  715   a  alone may be adequate in some applications. Spacer  720  and pads  715   a  and  715   b  can be made of metal, rubber, nylon, polymeric material or any stiff elastomeric material. 
     Downward movement of rotor shaft  705  is limited by member  725 . Pads  715   a  and  715   b  will compress to allow an angular deflection of rotor shaft  705  in relation to drive shaft  710 . 
     FIG. 11A shows a second embodiment of a member situated between a rotor shaft and a drive shaft for limiting vertical displacement of the rotor shaft. A member  750  is situated between a rotor shaft  705  and a drive shaft  710 . Member  750  is accommodated within an axially directed center hole through a coupling  730 , and is held in place by coupling  730 . 
     Member  750  is comprised of a column  760  disposed between a first sleeve  755  and second sleeve  765 . Sleeve  755  slides over and substantially around an end of rotor shaft  705 . Sleeve  765  slides over and substantially around an end of drive shaft  710 . Member  750  can be made of metal, rubber, nylon, polymeric or any stiff elastomeric material. 
     The diameter of column  760  is small enough, and flexible enough, to allow an angular deflection of rotor shaft  705  in relation to drive shaft  710 . Vertical movement of rotor shaft  705  will be limited by the firmness of column  760 . 
     Referring to FIG. 11B, sleeve  765  includes axial slits  770 . Sleeve  755 , in FIG. 11A, also includes slits. The slits  770  allow sleeves  755  and  765  to more easily slide over the ends of their respective shafts  705  and  710 . 
     As shown in FIGS. 12A and 12B, coupling  730  includes a clamping mechanism  775  to compress slits  770  and secure sleeves  755  and  765  to shafts  705  and  710 , respectively. A single piece flexible shaft coupling such as that shown in FIGS. 12A and 12B is available from Helical Products Co. of Santa Maria, Calif. Generally, coupling  730  can be any type of shaft coupling with a center hole. 
     Alternatively, instead of including and compressing slits  770 , sleeves  755  and  765  can be secured to shafts  705  and  710  using set screws (not shown). 
     Those skilled in the art, having the benefit of the teachings of the present invention may impart numerous modifications thereto. Such modifications are to be construed as lying within the scope of the present invention, as defined by the appended claims.