Patent Publication Number: US-6699168-B2

Title: Rotary centrifuge having pivoting buckets for holding samples

Description:
BACKGROUND 
     Embodiments of the present invention relate to a rotary centrifuge for centrifuging samples. 
     A rotary centrifuge rotates sample containers containing samples to apply centrifugal forces to the samples. The sample may be, for example, a fluid to which centrifugal forces are applied to separate, for example, components of the fluid that have different densities. Typically, the rotary centrifuge has a rotatable hub to receive pivoting buckets and a drive mechanism to rotate the hub. The pivoting buckets each comprise a receptacle to receive a sample container and a closing cap. A trunnion attached to the bucket has pivot pins that seat in corresponding holes in the hub of the centrifuge to allow the bucket to pivot as the hub is rotated. Trunnion springs may also be used to allow the buckets in their pivoted position to be displaced radially outwardly at high rotational velocities until the buckets are supported by a circumferential surface of the hub to reduce the centrifugal load on the bucket itself while still allowing the centrifugal forces to still operate on the sample in the bucket. 
     However, such conventional trunnion and bucket systems have several problems. One problem is that the interfaces and joints of conventional trunnion and bucket systems are often not as strong as desirable. For example, the joint between the trunnion and pivot pins can weaken at high rotational speeds. In addition, the trunnion spring mechanism that allows the bucket to slide radially outwardly at high speeds is also difficult to manufacture with sufficient strength and resilience. Also, when multiple components are assembled to make a trunnion and bucket system, such systems are more susceptible to failure from mis-assembly or misalignment of the different components. Another problem arises when the cap is not properly attached to the receptacle of the bucket. During operation of the centrifuge, vibrations may cause the cap to rotate and loosen off the receptacle, causing the sample held inside to be damaged. 
     Thus, it is desirable to have a bucket, trunnion, and trunnion spring, that is strong, resilient and provides improved ease of assembly and manufacture. It is also desirable to have a receptacle cap that remains securely attached to the receptacle during operation of the centrifuge. It is further desirable for the cap to be easily attached to and removed from the receptacle. 
     SUMMARY 
     A bucket is capable of holding a sample container in a rotary centrifuge. The bucket comprises (a) a receptacle to receive the sample container; and (b) a trunnion joined to the receptacle, the trunnion comprising: (i) a plurality of cutouts that each define a flexible span; and (ii) pivot pins to allow the bucket to pivot under the application of a centrifugal force generated by the rotary centrifuge. 
     A bucket capable of holding a sample container in a rotary centrifuge, the rotary centrifuge comprising an external seat, and the bucket comprising: 
     (a) a receptacle to receive the sample container, the receptacle comprising a seating surface; and 
     (b) a trunnion joined to the receptacle, the trunnion comprising: 
     (i) a plurality of cutouts that each define a flexible span that is sufficiently flexible to flex under application of a centrifugal force generated by the rotary centrifuge to allow the seating surface of the receptacle to seat against the external seat of the rotary centrifuge whereby the centrifugal force applied on the pivot pins may be reduced; and 
     (ii) pivot pins to allow the bucket to pivot under the application of the centrifugal force. 
     A bucket capable of holding a sample container in a rotary centrifuge, the bucket comprising: 
     (a) a receptacle to receive the sample container, the receptacle comprising an open end having an internal surface with a groove, the groove having an opening, an end, and a width that decreases in size from the opening to the end; 
     (b) a cap capable of closing the open end of the receptacle, the cap comprising pegs that are sized to fit in the groove; and 
     (c) a trunnion comprising a pair of pivot pins to allow the bucket to pivot under the application of a centrifugal force generated by the rotary centrifuge. 
     A bucket capable of holding a sample container in a rotary centrifuge, the bucket comprising: 
     (a) a receptacle to receive the sample container, the receptacle comprising an open end having an internal surface with a groove therein, the groove having an opening, an end, and a width that decreases from the opening to the end; 
     (b) a cap capable of closing the open end of the receptacle, the cap comprising pegs that are sized to fit in the groove; and 
     (c) a trunnion comprising: 
     (i) a plurality of cutouts that each define a flexible span that is sufficiently thin to flex under application of a centrifugal force generated by the rotary centrifuge; and 
     (ii) a pair of pivot pins to allow the bucket to pivot under the application of the centrifugal force. 
    
    
     DRAWINGS 
     These features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings which illustrate examples of the invention. However, it is to be understood that each of the features can be used in the invention in general, not merely in the context of the particular drawings, and the invention includes any combination of these features, where: 
     FIG. 1 is a schematic perspective view of a rotary centrifuge according to an embodiment of the present invention; 
     FIG. 2 is a perspective view of a bucket, cap and trunnion according to an embodiment of the present invention; 
     FIG. 3 is a cross-sectional side view of the bucket of FIG. 2 showing a sample container in the bucket; 
     FIG. 4 is a schematic cross-sectional side view of a portion of a hub of the rotary centrifuge of FIG. 1; 
     FIG. 5 is a cross-sectional side view of the bucket of FIG. 2 showing a tapering groove in an internal surface of the bucket for receiving pegs of a self-seating cap; 
     FIG. 6 is a top view of the bucket of FIG. 2; 
     FIG. 7 a  is a cross-sectional side view of a bucket and an external seat in a stationary state of the rotary centrifuge; 
     FIG. 7 b  is a cross-sectional side view of the bucket of FIG. 7 a  as it begins to seat on the seating surface as the rotary centrifuge accelerates; 
     FIG. 7 c  is a cross-sectional side view of the bucket of FIG. 7 b  continuing to seat on the external seat as the rotary centrifuge continues to accelerate; 
     FIG. 7 d  is a cross-sectional side view of the bucket of FIG. 7 c  completely seated on the external seat; 
     FIG. 7 e  is a cross-sectional side view of the bucket and seating surface of FIG. 7 d  after the seating surface is partially deformed by the centrifugal force generated in the rotary centrifuge; 
     FIG. 7 f  is a cross-sectional side view of the bucket being displaced in the partially deformed seating surface of FIG. 7 e;    
     FIG. 8 a  is an angled perspective view of the cap of the bucket of FIG. 2 showing the pegs of the self-seating cap; 
     FIG. 8 b  is an side view of the self-seating cap of FIG. 8 a;    
     FIG. 8 c  is an top view of the self-seating cap of FIG. 8 a ; and 
     FIG. 9 is a schematic diagram of the pegs of the cap of FIG. 8 a  engaging the tapering groove in the internal surface of the bucket of FIG.  5 . 
    
    
     DESCRIPTION 
     An exemplary version of a rotary centrifuge  100  according to an embodiment of the present invention as schematically illustrated in FIG. 1, is suitable for rotating a sample in a sample container to generate a centrifugal force in the sample. The sample container is exposed to the centrifugal force to separate components of the sample. For example, the rotary centrifuge  100  may separate fluid components having different densities. The illustrative version of the rotary centrifuge  100  provided herein should not be used to limit the scope of the invention, and the invention encompasses equivalent or alternative versions, as would be apparent to one of ordinary skill in the art. 
     Generally, the rotary centrifuge  100  comprises a rotatable hub  110  having a plurality of circumferentially spaced apart bucket carriers  115  comprising sockets  120  which receive the pivoting buckets  130 , for example, the hub  110  may have at least about four bucket carriers  115  that are angularly spaced apart and distributed. In the version shown, the rotary centrifuge has six bucket carriers  115  that are located about 60° apart. The hub  110  comprises a peripheral carrier ring  272  that has seating surfaces  270  to support the buckets  130  in operation. The hub  110  may also have indentations  111  along its outer periphery to reduce the mass of the hub  110  which would otherwise would cause undesirable stresses in the regions between the sockets  120  of the hub  110  during rotation of the hub. In one embodiment, the hub  110  is made from a metal, such as titanium or aluminum. 
     The rotary centrifuge  100  further comprises a motor  112  to rotate the hub  110  about a rotation axis  113  to generate a centrifugal force in samples that are in the buckets  130 . For example, the motor  112  may be a rotary electric motor. The motor  112  typically comprises an axle  114  that is engaged in a slot (not shown) of the hub  110  to allow the motor  112  to rotate the hub  110 . In one embodiment, the motor  112  rotates the hub  110  at an angular velocity of from about 1,000 to about 40,000 rpm. 
     The buckets  130 , as shown in FIGS. 2 and 3, are supported by the bucket carriers  115  of the hub  110  that allow the buckets  130  to pivot and swing radially outwardly as the hub  110  rotates and angularly accelerates. In one version, as shown in FIG. 1, the bucket carriers  115  are integral with the hub  110  (as shown) and comprise sockets  120  having pin slots  271  that have an apex  280  as shown in FIG.  4 . The pivot pins  140  of the bucket  130  are supported in the apex  280  of the pin slots  271  of the bucket carriers  115 , such that the hub  110  is stationary, the buckets  130  remain vertically oriented and when the hub is rotating the buckets pivot about the pins  140  to a radially horizontal position. The apex  280  typically has a curvature that is complementary to the shape of the pin  140 . In another version (not shown), the bucket carriers  115  are secured to the hub  110  (or to arms extending from the hub) by suitably matched bolts or rivets and mounting holes. 
     The buckets  130  are capable of holding sample containers  150  in the rotary centrifuge  100 , as illustrated in FIGS. 2 and 3. Each bucket  130  comprises a receptacle  160  capable of receiving a sample container  150 . For example, the receptacle  160  may be shaped to match the external shape of the sample container  150  and sized slightly larger than the sample container  150  to snugly receive the sample container  150 . Each receptacle  160  has an open end  163  at its top through which a sample container  150  is inserted and a closed end  165  at its bottom to support the sample container  150 . 
     The bucket  130  further comprises an seating surface  190 , as shown in FIG. 2, that in operation, contacts an external seat  270  of rotary centrifuge  100  to stabilize the position of the bucket  130  and reduce the load applied to the bucket components. For example, the external seat  270  may be formed by a surface of the ring  272  of the hub  110 , as shown in FIG.  4 . In this version, the seating surface  190  comprises a convex surface of the receptacle  160  that mates with a corresponding concave external surface  270  of the ring  272  of the hub  110 . As the bucket  130  swings upwardly into a horizontal plane, centrifugal forces pull the bucket  130  radially outwardly. At particular rotational velocities, the bucket  130  is pulled out sufficiently far to allow the bucket seating surface  190  to contact and rest on the external seat  270  of the ring  272 . This allows the external seat  270  to relieve the load of the centrifugal forces that is being applied to the pivot pins  140 . For example, the bucket  130  may seat on the ring  272  at rotational speeds of from about 2000 to about 4000 rpm. In the seated position, the centrifugal forces applied to the samples in the buckets  130  continue to be along radial axes  274  normal to the centrifuge rotation axis  113 , as shown in FIG.  4 . 
     The bucket  130  also comprises a trunnion  170  that is joined to the receptacle to allow attachment of the bucket  130  to the carrier assembly  115 , as illustrated in FIGS. 5 and 6. In the version shown, the trunnion  170  extends upwardly from the open end  163  of the receptacle  160 . The trunnion  170  may comprise a metal, such as for example titanium. Each trunnion  170  comprises one or more pivot pins  140  that allow the bucket  130  to pivot in engagement with the bucket carriers  115  under an applied centrifugal force. The trunnion  170  typically comprises a pair of pivot pins  140  that oppose one another and are positioned symmetrically along a pivoting axis  182  about which the bucket  130  can rotate. The pivot pins  140  may be shaped as, for example, cylindrical protrusions, concave stumps, or tapered rods. The pivoting allows the centrifugal forces to be applied along the length of the sample containers thereby increasing the effect of the centrifugal forces on the volume of the samples. 
     Returning to FIG. 5, the trunnion  170  also comprises a trunnion spring  180  that allows a radially outward displacement of the portion of the receptacle  160  of the bucket  130  below the pivot pins  140 . In one version, the trunnion spring  180  comprises a plurality of cutouts  220  that each define a flexible span  200  that is sufficiently thin to flex under application of the centrifugal force. The cutouts  220  further define side supports  210  between adjacent of cutouts  220  that serve to support the spans  200  thereby allowing the spans to flex within the gap between the supports  210 . At least one of the cutouts  220 , may be, for example, substantially oval in shape. In one version, the flexible spans  200  are arcuate members having a tapering thickness that tapers to a minimum at about the center of the span  200 . For example, the minimum thickness of each span may be, for example, less than about 100 mils, or even less than about 50 mils. Preferably, the spans  200  comprise two sets of opposing spans  200  with the pivot pins  140  mounted on a shoulder  201  between the spans. In operation, as the trunnion spring  180  flexes under an applied centrifugal force, the opposing spans  200  flex in a similar shape to thereby allow the pivot pins  140  to remain aligned to each other. In one version, the trunnion spring  180  is capable of flexing a sufficient distance to allow the receptacle  160  to be displaced by at least about 20 mils relative to the pivot pins  140 , and may additionally be sufficiently inflexible to limit displacement of the receptacle  160  to less than about 50 mils relative to the pivot pins  140 . As shown in FIG. 6, the trunnion spring  180  may be attached to the receptacle  160  along a second axis  184  that is substantially orthogonal to the pivoting axis  182  of the pivot pins  140 . This structure and attachment allow the trunnion spring  180  to suitably flex as force is applied between the receptacle  160  and the pivot pins  140 . 
     In one version, the trunnion  160  and receptacle  160  form an integral unitary member, as shown in FIG.  5 . This integral bucket  130  is substantially absent a material interface between the receptacle  160  and the integral trunnion  170 . For example, the receptacle  160  and the trunnion  170  may be machined from a unitary piece of a material, such as single bar stock of metal, such as titanium. This integral bucket  130  is typically stronger and more durable than a bucket that is formed from assembling separate parts. Furthermore, the integral bucket  130  may be more easily manufactured than an assembled bucket. However, the trunnion  160  and receptacle  170  may also be separate pieces (not shown) that are joined together, for example, by conventional joining systems, such as for example, a screw joint, welding or bolts. 
     During operation of a conventional rotary centrifuges, the centrifugal force generates a side-loading force on the pivot pins  140  at high rotational speeds when the seating surface  190  of the bucket  130  is seated on the external surface  270  of the hub  110 . The side-loading force is generated parallel to the axis of rotation  113  of the hub  110  and can degrade the structural integrity of the pivot pins  140  or even break the pins  140 . The side-loading force can also damage the trunnion spring  180  by the application of a sideways shearing force on the spring  180 . For example, if the bucket  130  seats in a position that is not fully horizontal, or if the bucket  130  is not fully seated, the pivot pins  140  and trunnion spring  180  are subjected to the side-loading force. 
     In one version of the present invention, the pivot pins  140  and seating surface  190  are adapted to allow the bucket  130  to seat on the ring  272  substantially without generating a side-loading force on the pivot pins  140 . In this version, the receptacle  160  comprises a longitudinal axis  167  passing centrally therethrough, and the pivoting axis  182  of the pivot pins  140  are horizontally offset by a predefined distance from the longitudinal axis  167 , as shown in FIG.  6 . In one embodiment, the pivot pins  140  are offset from the longitudinal axis  167  by from about 10 to about 30 mils, such as by about 20 mils. 
     In the initial stationary position of the rotary centrifuge  100 , as shown in FIG. 7 a , the pivot pins  140  rest at the apex  280  of pin slots  271  (see FIG. 4) and gravity causes the buckets  130  to remain in a substantially vertical orientation. When the hub  110  rotates, the bucket  130  swings upwardly, as shown in FIG. 7 b , and the seating surface  190  of the bucket  130  approaches and eventually contacts the external seat  270  of the ring  272  at the contact point  281 . For example, the longitudinal axis  167  of the bucket  130  may form an angle with the radial axis  274  of from about 0.5 to about 3 degrees. At the same time, the centrifugal force that acts on the bucket  130  as a result of the rotation of the hub  110  flexes the trunnion spring  180  and allows the bucket  130  to be displaced radially outwardly. 
     As the rotational velocity of the hub  110  increases, the centrifugal force on the bucket  130  increases causing the bucket  130  to further pivot about the contact point  281 , as shown progressively in FIGS. 7 c  and  7   d , to become fully seated on the seat  270  of the ring  272 . The pivot pins  140  become displace upwardly along the pin slots  271  from their resting surfaces  280  by a vertical distance  141 . As the hub  110  is further rotated to higher angular acceleration, the bucket  130  pivots on the resting surfaces  280  as its seat  270  moves outwardly and upwardly toward the inner seat  270  of the ring  272 . For example, the pivot pins  140  may displace upwardly by a distance of from about 10 to about 35 mils in the pin slots  271 . As this movement continues, the bucket  130  becomes approximately horizontal, until its seating surface  190  eventually comes to rest completely against the seating surface of the ring  272 , as shown in FIG. 7 d.    
     With increased rotational velocities, the centrifugal force temporarily deforms the seat  270  of the ring  272 , including retracting a lower portion of the seat  270 , as shown in FIG. 7 e . For example, the seat  270  of the ring  272  may be deformed such that a portion of the seat is horizontally displaced by a distance  142 . As a result, the pivot pins  140  and the bucket  130  are displaced downward along the pin slots  271 , as shown in FIG. 7 f . For example, the pivot pins  140  may be displaced downwardly by from about 10 to about 35 mils. In one embodiment, the pivot pins  140  are returned to their seated positions on the resting surfaces  280  of the pin slots  271 . Thus, the side-loading force that would otherwise damage or destroy the pivot pins  140  is at least reduced, and may even be eliminated. By decreasing the side-loading force, the offset pivot pins  140  increase the durability of the bucket  130 . The firm seating of the bucket  130  on the ring  272  allows the ring  272  rather than the pivot pins  140  to support the centrifugal force on the bucket  130 . 
     The bucket  130  also comprises a cap  230  to close the open end  163  of the receptacle  160 , as illustrated in FIGS. 8 a  to  8   c . The cap  230  may comprise a first o-ring  295  to seal the cap  230  against the bucket  130 . The o-ring  295  may comprise, for example, a fluoroelastomer. The cap  230  has a handle  240  adapted to be grasped to remove the cap  230  from the bucket  130 . For example, the handle  240  may comprise a loop-shaped protrusion with a finger hole  242  to facilitate a tight grip. The handle  240  may also be adapted to be grasped by a robot arm. The geometry of the finger hole  242  is adapted to withstand the centrifugal force without deforming or breaking, while having a low overall mass to minimize the weight of the bucket  130  on the carrier assembly  115 . The cap  230  may be made from aluminum. 
     In another version, the open end  163  of the receptacle  160  has an internal surface that comprises a groove  250 ,  255  therein, and the bucket cap  230  comprises a peg  260  that fits in the groove  250 ,  255 , to allow the cap  230  to self-seat and close the bucket  130 , as illustrated in FIG.  9 . The groove  250 ,  255  is sized to receive the peg  260 , and has a first portion  250  that is substantially vertical. The groove  250  also has a second portion  255  having a tapering width that decreases from a first larger width to a second smaller width. In one embodiment, the first portion  250  is in the trunnion  170  and the second portion  255  is in the receptacle  160 . Typically, the second portion of the groove  255  comprises a first internal wall that is substantially parallel to a plane that is normal to the longitudinal axis  167 , and a second internal wall that is at an angle relative to the normal plane. For example, the second wall  252  may slope down toward the first wall  251 . In one embodiment, the groove  255  is shaped as a right-triangle. 
     To close the bucket  130 , an operator aligns the cap  230  with the receptacle  160  and pushes the cap  230  into the receptacle  160  such that the peg  260  slides down the first portion of the groove  250 , as in positions (a) and (b), until the cap  230  contacts the first o-ring  295 . Then, the operator rotates the cap  230  with respect to the receptacle  160  to slide the peg  260  along the top of the second portion of the groove  255 , as in positions (c), (d), and (e), sliding the cap  230  beside the o-ring  295 . For example, the operator may rotate the cap  230  clockwise, looking down onto the bucket  130  from the side of the cap  230 , by turning the handle  240 . In one embodiment, the pegs  260  and groove  255  are adapted to allow a rotation of the cap  230  in the bucket  130  of from about ⅙ to about ½ of a whole revolution, such as from about ¼ to about ½ of a turn. This turning angle may be preferable because it can be easily executed by a human operator with one twist of the hand that minimizes disturbance of the sample  105 . When the bucket  130  is being centrifuged, the peg  260  slides in the second portion of the groove  255 , such as into position (f). The groove  255  is shaped such that under the application of the centrifugal force the cap  230  slides toward the first internal wall  251  of the groove  255  until the cap  230  closes the bucket  130 . 
     The groove  250 ,  255  maintains a suitable seal between the cap  230  and the receptacle  160 . If the cap  230  is not entirely securely attached to the receptacle  160 , the centrifugal force produced by the motor  112  causes the cap  230  to self-seat into the receptacle  160 . For example, if the cap  230  is only partially placed into the bucket  130  such that the cap peg  260  is at position (e), the radially outward centrifugal force that is generated when the bucket  130  is being rotated and is in a substantially horizontal orientation, causes the cap  230  to slide radially outwardly such that the cap peg  260  becomes securely locked by the centrifugal force at position (f). In another example, if the cap peg  260  is at position (d), the centrifugal force causes the cap  230  to slide out such that the cap peg  260  is at position (d′). The groove  255  may additionally be advantageous because, if the cap  230  is initially not fully screwed in the receptacle  160 , the width of the groove  255  allows a surface of the cap  230  to support the cap  230  on the receptacle  160  rather than having the pegs  260  support the weight of the cap  230 . 
     Sample containers  150  are provided for placement in the buckets  130  of the rotary centrifuge  100 , as shown in FIG.  3 . The sample container  150  comprises a tube having open and closed ends  282 ,  285 , respectively, the open end  282  having an outer surface  294 . For example, the sample container  150  may be an elastomer test tube, such as comprising a polyallomer or polycarbonate. In one version, the bucket cap  230  (as shown) or a second cap (not shown) is adapted to close the sample container  150 . After centrifugal operation, the motor  112  decreases the angular velocity of the hub  110  to decrease the magnitude of the centrifugal force and smoothly return the buckets  130  to their original upright positions. When the hub  110  has come to a stop, the caps  230  may be removed from the buckets  130  to by pulling their handles  240  to access the sample containers  150 . 
     Although the present invention has been described in considerable detail with regard to certain preferred versions thereof, other versions are possible. For example, the present invention could be used with other rotary centrifuges, such as a rotary centrifuge that allows the sample to be placed directly into the bucket. Thus, the appended claims should not be limited to the description of the preferred versions contained herein.