BREAKAWAY BASE FOR AN ULTRACENTRIFUGE ROTOR

Abstract: A breakaway base for use with an ultracentrifuge rotor wherein the base is designed to fracture when the rotor is above a specified speed so that the rotor will disengage from the drive spindle of the centrifuge. The base (16) has cutout areas (24, 26) to establish a high stress region (44, 46) that is designed to fracture above a specified rotor speed. The cutout areas (24, 26) are designed to receive lugs (52, 54) projecting from the bottom of the rotor. The cutout areas (24, 26) have slotted ledges (32, 34) through which fastening bolts connect to the lugs on the rotor. The fracture of the base in the high stress region in conjunction with the slotted ledges will result in the base fracturing into two parts which will cause rotor disengagement from the drive spindle of the centrifuge and prevent the rotor from spinning at a greater speed.

Description: BREAKAWAY BASE FOR AN ULTRACENTRIFPGE ROTOR Background of the Invention The present invention is directed to ultracen- trifuge rotors and, more particularly, is directed to a mechanical overspeed protection device in the form of a breakaway base on an ultracentrifuge rotor. Ultracentrifuge rotors are designed to withstand stresses within specified limits. The centrifuges into which the rotors are placed in many instances have a capability of imparting rotational speeds greater than the design limits of the rotor. Typically incorporated into centrifuge machines are electrical overspeed protec¬ tion circuits that should cause the centrifuge to cease operation if a preset speed is exceeded; ho evever, these devices are not infallible. Therefore, in certain cases, the speed-limiting device should be an intrinsic part of the rotor itself. If a rotor should exceed its design speed, the G forces on the rotor in the primary stress areas may cause fracturing of the rotor and rapid dissipation of the kinetic energy of the rotor. This not only causes damage to the rotor and the centrifuge, but also presents a possibly hazardous condition to the users of the cen¬ trifuge. Typically, centrifuges are designed to contain any physical fracturing of the rotor. However, damage to the rotor and the centrifuge would be minimized if there are limits on the speed at which the rotor can operate. The Stahl et al. 3,990,063 patent discloses a mechanical overspeed device for an ultracentrifuge ro- tor. Incorporated in this design is a hub member on which the rotor resides for connection to the drive spin- dle. The hub is connected to the rotor by use of bolts that extend through the hub member and into the body of the lower portion of the rotor. Slots are formed in the hub member to establish a stress area. The hub is con- nected by the bolts through apertures in the hub to the rotor. During high speed centrifugation, if the speci¬ fied safe speed is exceeded, stress in the areas adjacent the slots will develop sufficiently to cause a fracture in the hub member. However, the bolts must be sheared in order to allow complete disengagement of the hub mem¬ ber. Further, the insertion of the bolts completely into the body of the rotor creates additional stress regions that may be the source of eventual fracturing in the rotor itself. Another approach to a mechanical overspeed protection device is shown in the Wright 3,961,745 pat¬ ent. This device utilizes a handle on top of the rotor which will rupture if the rotor exceeds a certain speed. The resulting imbalance to the rotor will cause the rotor to disengage from the drive shaft. Another type of mechanical overspeed device is shown in the Stallman et al. 3,101,322 patent wherein a pin is designed to move radially outward in the event of an overspeed condition and engage electrical connector to stop the rotor. In the Pickels 2,666,572 patent a simi¬ lar arrangement is shown having a different shaped pin which is designed to move radially outward and engage a stop switch in the event of a rotor overspeed. The desire to have a mechanical overspeed pro¬ tection device on an ultracentrifuge rotor, in addition to electrical overspeed protection circuits, becomes more pertinent with respect to very high speed ultracentri- fuges which are operating in the range of 30,000 to 100,000 RPM. In many instances these rotors are capable of attaining kinetic energies exceeding one-half million foot-pounds. Therefore, it is desirable to have a mechanical overspeed protection device which will prevent a rotor from reaching a speed at which a possible hazar¬ dous condition could occur. Summary of the Invention The present invention is directed to a mechani- cal overspeed protection device for attachment to an ultracentrifuge rotor. The device utilizes a breakaway base that has cutout areas to establish a fracture path or zone in the base if the rotor should exceed a speci¬ fied speed. The breakaway base is a means for connecting the drive spindle of the centrifuge with the rotor. A pair of projecting lugs on the bottom of the rotor inter¬ connect with the breakaway base. The cutout areas have slotted ledges through which the fastening bolts pass for connection to the rotor lugs. The slots on the ledges are designed to align with the fracture path of the hub so that, if a fracture should occur in the hub, it will be in alignment with the slots on the ledges of the cut-. out area. The bolts do not have to be sheared to permit disengagement of the rotor from the drive spindle. If the rotor should achieve a speed above a specified safe operating speed, the base is designed to fracture and separate from the drive hub as well as from the rotor. Consequently, the drive spindle/rotor inter- face is destroyed and the rotor is prevented from devel¬ oping higher speed and energy levels greater than that which can be contained by the centrifuge. The present invention provides advantages over systems existing in the art. The fastening bolts are designed to enter the projecting lugs in the rotor and OMPI not the main body of the rotor. Therefore, the integrity of the rotor is retained and additional high stress areas are not created in the rotor which may be a source of damage to the rotor in subsequent runs in the centri- fuge. The stress concentrations inside the rotor body are eliminated. The use of the lugs that project from the rotor into the base also provides a means for transmitting the rather considerable torque that is applied when the tube cavity plugs are secured onto and loosened from the rotor body. During this operation the rotor base is normally held in a vise which serves to counteract the applied torque. The present invention is designed so that the peak stress will occur on the innermost surface of each of the cutout areas. This is accomplished by slotting the ledges in the cutout area and leaving only one frac- ture zone on each side of the drive hole. Consequently, the highest stresses are made to occur in a region that ' is relatively simple to analyze. Hence, the intentional fractures can be more consistently predicted to facili¬ tate the design of bases for various rotors. The concept of using an attached base on the rotor allows for easier repair of some rotors which have been damaged as ' a result of drive failure. Since only the rotor bottom is typically damaged when the rotor is disengaged from the drive spindle, the present invention allows for the easy replacement of a damaged base which is a much more simplified and less costly step than re- machining the spindle drive hole in a rotor. Similarly, it must be noted that one of the high stress zones in an ultracentrifuge rotor is the region around the drive hole. Normally, the deeper this recess projects inside the rotor body, the higher the stress will be. By eliminating or reducing the depth of the drive hole, the rotor can be made stronger. Conse- quen ' tly, the use of the attached base will eliminate a drive hole in the rotor body. Brief Description of the Drawings Figure 1 is a perspective view of a vertical tube ultracentrifuge embodying the present invention; Figure 2 is a perspective view of the bottom of the rotor in Figure 1 showing a portion of the present invention; Figure 3 is a perspective view of the breakaway base of the present invention; and Figure 4 is a partial sectional view of the present invention showing the breakaway base connected to the rotor. Detailed Description of the Invention The centrifuge rotor 10 in Figure 1 represents an ultracentrifuge rotor typically known as a vertical tube rotor in that the tube cavities 12 are oriented generally parallel to the spin axis of the rotor in a vertical direction. The rotor 10 is typically made of a very strong metal such as titanium, machined to precise dimensions and configurations, and accurately balanced to withstand tremendous G loads under high speed rotation. Attached to the lower surface 14 of the rotor 10 is a removable or breakaway base 16. Attention is directed to Figure 3 showing the breakaway base 16 which has a general cylindrical config- uration with a lower lip 17. As shown in Figure 4, the base 16 has a central cavity 18 for receipt of a drive spindle hub 20. Located 180° apart and in alignment with each other in the base 16 in Figure 3 are cutout areas 24 and 26. Included in each of these cutout areas 24 and 26 are respective through apertures 28 and 30. Further, located in each of the respective cutout area 24 and 26 are respective ledges 32 and 34 which have respective fastening apertures 36 and 38. Respective slots 40 and 42 are located in the ledges 32 and 34 and are oriented in planar alignment with each other, the center of the base, and the center of each of the fastening apertures 36 and 38. Located between the respective cutout areas 24 and 26 are high stress regions 44 and 46. The existence of the slots 40 and 42 and the apertures 28 and 30 re¬ sults in the regions 44 and 46 being subjected to high stress during high speed centrifugation. During cen- trifugation, the stress experienced by the regions 44 and 46, which are located adjacent the drive hub cavity 18 in the base 16, is significantly greater than the stress on comparison to the larger areas 48 and 50 of the base 16 in Figure 3. In Figure 2, the bottom 14 of the rotor 10 has projecting lugs 52 and 54 which are designed to seat within the cutout areas 24 and 26 of the base 16. When the lugs 52 and 54 are positioned within the cutout areas 24 and 26, any rotational motion imparted to the drive hub 20 by the drive shaft will in turn be imparted to the rotor 14. As shown in Figure 4, the lug 52 (as well as the lug 54 not shown) is designed to be of a thickness sufficient enough to accommodate the length of a fasten- ing bolt 56 which is inserted through the fastening hole 36 of the ledge 32 in the base 16. The aperture 58 of the fastening bolt 56 in the lug 52 is not deep enough to enter into the rotor body portion 10. This is important so that no additional high stress concentration areas are created inside the rotor body. - 5 Located at the interface between the outside faces 51 and 53 of the respective lugs 52 and 54 and the bottom 14 of the rotor are notches 55 to permit turned surface machining of a band approximately the depth of 10 the notch toward the center of the rotor. This area or band will mate with the slight raised rim 57 on the top surface 17 of the breakaway base which is also a machined turned surface. Since the rim 57 is slightly higher than the remainder of the base, only the rim will contact the 15 bottom of the rotor; This will eliminate any slight out of flatness which might otherwise occur if the whole surface of the base contacted the bottom of the rotor. With respect to Figure 3, a locating boss rim 60 20 is positioned on the upper surface 22 and around the central cavity 18 of the base 16. The locating boss is designed to be received in the locating recess 62 in bottom 14 of the rotor 10 in Figure 2. This will assist in the orientation of the rotor 10 in conjunction with 25 the base 16 for the proper orientation of the lugs 52 and 54 in the cutout areas 24 and 26. Turning to the operation of the present inven¬ tion, attention is directed to Figure 4. The rotor 10 has 30 the breakaway base 16 attached with the fastening bolts 56. A centrifuge tube 64 is placed within the tube cavi¬ ty 12 of the rotor 10. Once the tube 64 is in place, a spacer 65 (not shown in section) is placed on top of the tube to support the upper portion of the tube. A plug 66 35 (not shown in section) is threaded into the tube cavity 12 adjacent the top surface 68 of the rotor 10 to secure the spacer and tube. Torqueing of the plug 66 is accom¬ plished by placing of the rotor/base assembly with the rotor base 16 placed in a rotor vise. The small holes 67 in the top of each plug 66 in Figure 1 are for receipt of a tool to torque down the plugs. The flat surface 70 shown in Figure 3 on the base 16 as well as a similar flat surface (not shown) located 180° from the flat space 70 are aligned with conforming flat surfaces in a rotor vise. The rotor will not move as the plug 66 is torqued down tightly in the tube cavity 12. The torque action is resisted by the lugs 52 and 54 being in the cutout areas 24 and 26 of the base 16 which in turn is held in the rotor vise (not shown) by the flat surfaces 70 in the base 16. After all the centrifuge tubes 64, spacers 65, and plugs 66 have been assembled in the rotor 10, the rotor is removed from the rotor vise and placed in the centrifuge machine wherein the drive hub 20 in Figure 4 of the drive spindle (not shown) is received in the cavi¬ ty 18 in the base 16. During normal centrifugation operation the drive hub 20 in Figure 4 will rotate and impart rotational motion into the base 16 as well as the rotor 10. The rotor is designed to operate at a specified maximum speed to ensure safe operation. However, if the speed should exceed the maximum safe speed, fracture of the rotor could result in the rapid dissipation of unacceptably high kinetic energy. The present invention provides a mechanical device to prevent overspeed by automatically disengaging the rotor from the drive hub 20 if the actual speed should happen to exceed the design maximum operational speed. The base 16 has been specifically designed to establish high stress regions 44 and 46 which, during speed above the maximum operational speed, will be sub¬ jected to tremendous stress as a result of the centrifu¬ gal forces generated. These regions are specifically de- signed to fracture at speeds above the maximum opera¬ tional speed, so that the base will separate into two parts 48 and 50. The cutaway areas 24 and 26 with the slots 40 and 42 in the ledges 32 and 34 provide for this separation. Once the stress areas 44 and 46 fracture, the parts 48 and 50 will separate away from the fastening bolts 56 which do not need to be sheared before the base can be disengaged from the drive hub and the rotor. Once the cutaway areas 24 and 26 cease to exist as a result of the base breaking into two parts 48 and 50, the project- ing plugs 52 and 54 will no longer be supported and re¬ ceive rotational motion from the drive hub. The rotor will fall off of the drive hub into the rotor chamber. The only damage which may occur is damage to the rotor itself and to the interior of the centrifuge rotor cham- ber. Because the fastening bolts 56 extend only into the lugs 52 and 54, there is no penetration into the rotor body which would otherwise create an area of high stress concentration during high speed ultracentrifuga- tion. In addition, the fact that the rotor does not have a large cavity for receipt of the drive hub of the drive spindle, no stress concentrations will occur in the lower center of the rotor. The majority of the major stress concentration during centrifugation is by design limited to the lugs 52 and 54 and the base 16. The present invention provides a positive way to prevent the rotor from overspeeding and developing excessive energy. The elimination of the high stress region associated with a drive hole in the center of the OMPI rotor itself reduces the possibility of the rotor failing through its center which is the worst type of failure in a high speed rotor. Because of the removable breakaway base 16, the rotor can be easily repaired by the replacement of the base in the event that the rotor would jump the drive hub as a result of possible excessive imbalance in the rotor or in the case of drive seizure. This is much more effi- cient and less costly than having to remachine a rotor drive hole which would be located in the bottom of the rotor. It is envisioned that embodiments of a breakaway base for an ultracentrifuge rotor other than that specifically shown in this application could be designed within the scope and spirit of the present invention.

Claims: 1. A centrifuge rotor for placement in an ultracentrifuge, characterized by a rotor body (10) for carrying samples to be centrifugated; a breakaway base (16) designed for connection with rotor drive means in ; 5 the centrifuge; engaging means (52 and 54) projecting from the rotor body for connecting the base with the rotor body; securing means (56) to attach the base to the rotor body; an areas (24 and 26) formed in the base for reducing the cross-sectional area of the base .in speci- 10 fied locations (44 and 46) , the areas causing the base to fracture adjacent the locations and disengage the rotor from the drive means and the securing means when the rotor exceeds a specified speed. 2. A centrifuge rotor as defined in claim 1, characterized in that the engaging means comprises at least two lugs (52 and 54) . 3. A centrifuge rotor as defined in claim 2, characterized in that the cross-sectional area reducing means comprises at least a pair of cutout areas (44 and 46) in the base, the lugs (52 and 54) being positioned in " 5 the cutout areas. 4. A centrifuge rotor as defined in claim 3, characterized in that each of the cutout areas has a slotted ledge. 5. A centrifuge rotor as defined in claim 4, characterized in that the securing means are fastening bolts positioned through the slotted ledges and into the lugs, the slotted ledges being oriented 180° from each 5 other on opposite sides of the center of the breakaway base, the base fracturing through its center and in alignment with the slots in the ledges. 6. A centrifuge rotor as defined in claim 1, characterized in that the securing means is a fastener which projects only into the engaging means and not into the rotor body. 7. A centrifuge rotor as defined in claim 9, characterized in that the breakaway base has a uniform flat surface (17) with a raised rim (57) . 8. A centrifuge rotor as defined in claim 4, characterized in that the cutout areas with the slotted ledges cause the base to fracture between the cutout areas when the rotor exceeds the specified speed, the slots in the ledges allowing the fractured base to detach from the securing means and disengage the rotor from the drive means.

Inventors: CHENG DAVID W, CHULAY STEVEN J, CHENG, DAVID, W., CHULAY, STEVEN, J.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1983
Main classification: B04B7
IPC codes: B04B7, B04B9