Patent Publication Number: US-6665924-B2

Title: Centrifuge having a spring-loaded nut for securing a rotor to a drive cone

Description:
The present invention relates to a centrifuge instrument, and more particularly, to a centrifuge in which a rotor hold-down screw, a nut and a spring are arranged to secure a rotor to a drive cone. 
     BACKGROUND OF THE INVENTION 
     A centrifuge instrument is a device by which a liquid sample may be subjected to a centrifugal force that separates the liquid sample into its constituent parts. The sample is typically carried in a tube situated within a member known as a centrifuge rotor. The rotor is mounted on a drive cone, which is connected to a drive shaft that provides a source of motive energy to rotate the rotor. 
     The centrifugal force that advantageously acts upon the sample also acts upon the rotor that holds the sample. If the rotor separates from the drive cone during centrifuge operation, it could damage the centrifuge instrument. If the rotor is thrown from the centrifuge instrument, it could damage external equipment or injure a person in the vicinity of the instrument. Accordingly, the rotor must be adequately secured to the drive cone. 
     The mounting of the rotor to the drive cone is typically accomplished by way of a rotor hold-down screw. A central region of the rotor rests upon the drive cone. The rotor hold-down screw is routed through a cover hold-down screw knob and the central region of the rotor, and tightened into a threaded channel in the drive cone. That is, the rotor hold-down screw is threaded directly into the drive cone. To better secure this arrangement, a lock washer is sometimes placed between a lower surface of the rotor hold-down screw and an upper surface of the cover hold-down screw knob. 
     As compared with an older centrifuge instrument, a newer centrifuge instrument typically has a stronger motor, better drive design, and a more powerful refrigeration system. These features of the newer instrument provide the operator with the advantages of faster acceleration and deceleration, an ability to run the rotor with a greater degree of imbalance, and a possibility of having a colder rotor chamber temperature. 
     The centrifuge instrument may accept any one of a plurality of different centrifuge rotors depending upon the separation protocol being performed, and it is not uncommon for an operator to use an old rotor on a new centrifuge instrument. However, the capability of the new instrument to more rapidly change its speed and temperature, and to operate with a greater degree of imbalance, also increases the chance for the rotor hold-down screw to become loosened, and thus increase the opportunity for the rotor to become separated from the drive cone. 
     The present invention provides an improved arrangement for securing a rotor to a drive cone in a centrifuge instrument, and is suitable for an environment in which the centrifuge speed or temperature are rapidly changed. 
     SUMMARY OF THE INVENTION 
     An apparatus for securing a rotor to a drive cone in a centrifuge includes (a) a nut, movable in an axial direction in the drive cone, (b) a screw, for passing through a region of the rotor and for engagement with the nut, and (c) a spring located between a surface of the nut and a surface of the drive cone. The screw, when tightened into the nut, moves the nut in the axial direction to compresses the spring between the surface of the nut and the surface of the drive cone. The nut may include a protrusion that fits into a slot in the drive cone to limit rotation of the nut with respect to the drive cone. The spring can be any of a double-spring washer, a compression spring, a disc spring or a belleville spring washer. 
     The apparatus can also include a retaining ring for holding the nut and the spring in the drive cone. Such a retaining ring is located adjacent to a perimeter region of the nut, within an annular groove of the drive cone. 
     A preferred embodiment of the present invention is an apparatus for securing a rotor to a drive cone in a centrifuge, where the apparatus includes (a) a nut, movable in an axial direction in the drive cone, (b) a screw for passing through the rotor and for engagement with the nut, (c) a spring located between a surface of the nut and a surface of the drive cone, and (d) a retaining ring for holding the nut and the spring in the drive cone. The retaining ring is located adjacent to a perimeter region of the nut, within an annular groove of the drive cone. The screw, when tightened into the nut, moves the nut in the axial direction to compress the spring between the surface of the nut and the surface of the drive cone. The nut has a protrusion that fits into a slot in the drive cone to limit rotation of the nut with respect to the drive cone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a centrifuge instrument having a rotor secured to a drive cone in accordance with the present invention. 
     FIG. 2 is a cross section of the centrifuge of FIG. 1 showing a detailed view of an arrangement of a rotor hold-down screw and a nut. 
     FIG. 2A is a view of a nut and a drive cone of a centrifuge as seen along line  2 A— 2 A of FIG.  2 . 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross section of a centrifuge  100  having a rotor  135  adapted for holding at least one sample secured to a drive cone  130  in accordance with the present invention. Centrifuge  100  includes a rotor hold-down screw  105 , a cover hold-down screw  110 , a cover  115 , a nut  120 , and a spring  125 . 
     Rotor  135  sits upon drive cone  130 . Cover  115  is placed over rotor  135  and held in place by cover hold-down screw  110 . Rotor hold-down screw  105  is routed or passed through cover hold-down screw  110 , cover  115 , and a central region of rotor  135  and tightened into nut  120 . An o-ring  155  holds cover hold-down screw  110  and rotor hold-down screw  105  in place on cover  115 . Thus, o-ring  155  holds cover  115 , cover hold-down screw  110 , and rotor hold-down screw  105  together as an assembly. 
     Spring  125  is positioned between a surface  140  of drive cone  130 , and a surface  145  of nut  120 . Rotor  135  is secured to drive cone  130  through a cooperative arrangement of rotor hold-down screw  105 , nut  120  and spring  125 . During operation of centrifuge  100 , a rotational force is provided by a motor (not shown) that causes drive cone  130  to rotate. The rotational force is transferred from drive cone  130  through drive pins  150  to rotor  135 . 
     Spring  125  can be implemented as one or more spring elements, or a series of springs, for example, as two spring elements arranged back-to-back. Suitable spring elements include, but are not limited to, a double-spring washer, a compression spring, a disc spring or a belleville spring washer. The belleville spring washer is preferred because it provides very high loads in a confined space, e.g., the space between surface  140  and surface  145 . 
     FIG. 2 is a cross section of centrifuge  100  showing in detail the arrangement of rotor hold-down screw  105  and nut  120 . During assembly of centrifuge  100 , spring  125  is pre-loaded into drive cone  130 , nut  120  is inserted and a retaining ring  225  is installed adjacent to a perimeter region of nut  120  in an annular groove  230  in drive cone  130  to hold spring  125  and nut  120  in place. 
     Rotor hold-down screw  105  has threads  205  that engage corresponding threads  210  in nut  120 . Assume that rotor hold-down screw  105  is left-hand threaded, so that when it is rotated counter-clockwise threads  205  more fully engage threads  210  and rotor hold-down screw  105  is tightened into nut  120 . Nut  120  is movable in an axial direction, e.g., up and down in FIG.  2 . Tightening of rotor hold-down screw  105  draws nut  120  upward and compresses spring  125  between surfaces  140  and  145 . Spring  125  opposes the compression and attempts to expand, thus exerting a force that has a tendency to lock rotor hold-down screw  105 , nut  120 , and drive cone  130  in positions relative to one another. Nut  120  has two protrusions  220  that fit into slots  215  of drive cone  130 . 
     FIG. 2A is a view of nut  120  and drive cone  130  as seen along line  2 A— 2 A of FIG.  2 . Protrusions  220  are confined to slots  215 , and thus, rotation of nut  120  is limited with respect to drive cone  130 . This arrangement of protrusions  220  and slots  215  provides for a transfer of torque from nut  120  to drive cone  130  when rotor hold-down screw  105  is being either threaded or unthreaded into nut  120 . 
     The arrangement of rotor hold-down screw  105 , nut  120  and spring  125  prevents a loosening of rotor hold-down screw  105  from nut  120  during centrifuge operation. This arrangement is particularly advantageous when the temperature or operating speed of centrifuge  100  is rapidly changed, or where rotor  135  is not balanced. 
     It should be understood that various alternatives and modifications of the present invention could be devised by those skilled in the art. Nevertheless, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.