ROTOR WITH STRESS RELIEF

Abstract: The present invention provides a mechanism for bolting a rotor to a hub without introducing excessive radial stress around the bolt holes when the rotor is operating at high speed. The rotor (1) has a slot cut (35) through its center to elimi­ nate radial stress perpendicular to the slot and thereby avoid the problems of stress cracking of the otherwise weak section. The mounting bolts (501) are necked down and not threaded at the surface of the hub. The bolts are threaded into the hub (501A) at a substantial distance from the top surface of the hub (503) so that they can flex and so that the center of the ro­ tor (1) can move out relative to the hub (32). The hub rotor interface (503) is lubricated to facilitate radial motion of the center of the rotor.

Description: The present invention is related to centrifuges and more particularly to rotors for centrifuges. Centrifuges have been in use for many years. French Patent No. 803,008 to Halleux teaches a centrifuge machine in which a rotating member is secured to a spindle by bolts. Modern centrifuges are rotated at a much greater speed than the early centrifuge machines. If a circular disk is rotated at high speed (for example, 50 thousand RPM) the disk will expand radially. If the disk has a hole in the center to accommodate a drive shaft, high speed rotation will cause the center hole to expand. The expansion of the center hole of the disk results in stresses distributed around the hole in the center of disk. In many centrifuges, the rotor is bolted to a hub which is in turn driven by a spindle or shaft. If a rotor is bolted to a hub, the bolt holes create both radial and tangential stress patterns. The bolts not only hold the rotor down and give it tangential force necessary to cause rotation, but they also constrain the center portion of the rotor from moving outward as the centrifugal forces expand the radius of the rotor. This introduces additional stresses into the rotor, particularly around the bolt holes. In the prior art this problem was addressed by providing a substantially solid hub which was reinforced as required to tolerate the centrifugal stresses generated with negligible strain at the hub, thus allowing for a rigid assembly and concentricity as required for dynamic stability. In many instances the problem was addressed by a combination of making the rotor strong enough to withstand the stress by reducing strain and by removing material from the rotor to reduce the centrifugal forces. For example, in the device shown in US Patent 4,350,283 (Leonian) the material has been removed to the extent that the rotor essentially consists of a bar which has a disk under the bar to impart stability to the structure. Another factor that introduces stress points into a rotor is that rotors generally include tubes that interconnect the separation chamber. Space for these tubes could be provided by having a hole through the disk or by having a slot cut partway through the rotor. Such discontinuities in a rotor introduce further stresses because the portion of the disk with the hole or slot tends to expand at one rate whereas the other portion of the rotor expands at a different rate and hence, in addition to the tangential and radial forces, bending forces are introduced into the rotor resulting in complex, stress distribution patterns. Stress analysis techniques are well known; however, two dimensional stress analysis as applied to a substantially symmetrical design is a much faster operation than is three dimensional stress analysis. Rotors which have asymmetrical features such as holes and openings to facilitate tubes generally require a time consuming three dimensional analysis process followed by complex solutions. An object of the present invention is to provide an improved rotor for a centrifuge. A further object of the invention is to provide a centrifuge where the stress at the center of the rotor have been reduced. Yet another object of the present invention is to provide an improved rotor which is bolted to a hub without introducing excessive radial stress surrounding the bolt holes. Still another object of the present invention is to provide an improved technique for connecting a rotor to a drive hub. A still further object of the invention is to provide a rotor which can easily be designed using simple two dimensional stress analysis techniques. The present invention provides a mechanism for bolting a rotor to a hub without introducing excessive radial stress around the bolt holes when the rotor is operating at high speed. The rotor has a slot cut through its center to eliminate radial stress perpendicular to the slot and thereby eliminate the problem of stress cracking at the otherwise weak section. The mounting bolts are necked down and not threaded at the surface of the hub. The bolts are threaded into the hub at a substantial distance from the top surface of the hub so that they can flex and so that the center of the rotor can move out relative to the hub. The hub rotor interface is lubricated to facilitate radial motion of the center of the rotor. Figure 1 is an overall perspective view of an embodiment of the present invention. Figure 2 is a cut away view showing the bolts that hold the rotor to the hub. Figure 3 is a side view of the mounting bolts. An overall view of a preferred embodiment of the invention is shown in Figure 1. The separation chamber assembly 2 can be removably mounted on disk 1 using retaining mechanism 3. Separation chambers 2A and 2B fit into holes 1A and 1B which are cut into disk 1. Fluid flows to and from chambers 2A and 2B via inlet port 11 and rotating seal 4. Tube 52 interconnects the separation chambers 2A and 2B. The details of the separation chamber assembly 2, the retaining mechanism 3, and the manner of supplying fluid to the separation chambers 2A and 2B are not relevant to the present invention. In order to give an understanding of the size and weights involved, the following typical specifications for disk 1 are given. Disk 1 is constructed from aluminium plate. The rotor diameter is 41.91 cm (16½ inches), the thickness is 5.08 cm (2 inches) and it weighs 14.97 kg (33 pounds). The thickness between the outside edge of holes 1A to 1D and the periphery of the rotor is 3.175 cm (1¼ inches). The holes 1A and 1B have a diameter of 12.7cm (5 inches) and holes 1C and 1C have a diameter of 10.16 cm. Naturally it should be understood that these are merely the dimensions of one particular embodiment of the invention and that other dimensions could be used. As shown in figure 2 disk 1 is attached to the hub 32 by means of bolt 501 and a spring washer 502. Bolt 501 is cut necked down and only threaded at end portion 501A. The interface surface 503 which is located between disk 1 and hub 32 is coated with the conventional low friction non-galling lubricating material. Because of the spring action provided by the necked down portion of bolts 501, disk 1 can move relative to the surface of the hub 32 at the interface 503. In the embodiment shown, bolt 501 is a one quarter inch bolt, 6.35 cm (2½ inches) long, displacements in the order of 0.508 mm (or 20/1000's of an inch) are typical. A typical torque value for the bolts is 6.78 newton meters (60 inch pounds). An example of a low friction interface material 503 which can be used is a plastic sheet or Microseal 100-1 which is graphite based and which is commercially available. The center portion of the surface of disk 1 moves radially more than the surface of hub 32 because the disk 1 has a wider radius than does hub 32. Furthermore, hub 32 is thicker than disk 1 and hence it can withstand more force without expanding. Pins 15A and 15B fit in slot 35 and transfer tangential force from hub 32 to disk 1. Pins 15A and 15B have a sliding fit to the sides of slot 35, hence they do not transmit any radial force to the disk. Slot 35 provides a passageway for tubes 52 (520). Tubes 52 (520) could fit in a passageway or groove that was not cut all the way through disk 1; however, if such a groove were cut in the top of disk 1, there would be shearing or bending stresses in the disk when it rotates. The stress analysis of such a disk would be a three dimensional problem. Slot 35 which is cut all the way through the disk 1 reduces the stress analysis problem from a three dimensional problem to a two dimensional problem. A two dimensional stress analysis of disk 1 requires about one week's effort. A three dimensional stress analysis requires several times that effort, typically followed by a complex solution. Reduction of the time required to do a stress analysis significantly reduces the cost of engineering a rotor and yields a simpler design, hence, significantly reducing the ultimate cost of the rotor. If slot 35 were replaced by a channel which is only cut deep enough to allow the space required by tubes 52 (520), the disk would tend to rupture along the channel after use, thereby damaging the disk and reducing the useful life of the disk. Clamping mechanism 3 holds the separation chamber assembly 2 in position in the disk 1. Holding mechanism 3 does not apply any radial or tangential force to the assembly 2. All radial and tangential forces are applied to assembly 2 by the walls of the openings 1A and 1B. Hub 32 is mounted on the spindle or drive shaft of a conventional centrifuge (not shown). An example of a commercially available centrifuge whereon hub 32 can be mounted is the standard J-6 floor model refrigerated centrifuge commercially available from Beckman Instruments, Corporation, Spinco Division, Palo Alto CA. Slot 35 in disk 1 allows the center of the disk to expand in a direction perpendicular to the center of slot 35. This embodiment shows only one slot 35, however, there could be a second slot perpendicular to slot 35 to relieve the stress along that line also. Figure 3 shows bolt 501 in detail. It should be understood. that there are four identical bolts that hold disk 1 to hub 32. Three of the bolts are designated 501, 501A and 501B. A fourth bolt is located in hole 501C. While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the previously explained and other changes in the form and detail of the embodiment may be made.

Claims: 1. A rotor for a centrifuge, comprising a disk (1), hub means (32) and bolt means (501) for securing a bottom surface of the disk to a top surface of the hub means, the hub means being for attaching the disk to a spindle of a centrifuge, characterised in that the bolt means has a flexible portion and threads at one end (501A) thereof, the hub means having holes which receive the flexible portion to position a fixed end (501A) of the bolt means at a substantial distance from the interface (503) of the disk and the hub means, the bolt means maintaining pressure between the disk and the hub means in the direction of the flexible portion but allowing radial motion of the bottom surface of the disk relative to the top surface of the hub means. 2. A rotor according to claim 1 wherein the flexible portion of the bolt means (501) is a necked down center portion, whereby the bolt means function as springs allowing the disk (1) to move radially relative to the hub means (32). 3. A rotor according to claim 1 or 2 wherein the disk (1) includes first and second openings (1A and 1B) along a diameter of the disk, and further includes a slot (35) connecting the first and second openings, whereby the center of the disk is capable of expanding in a direction perpendicular to the slot. 4. A rotor according to claim 3 wherein the hub means (32) includes pin means (15A and 15B) extending within the slot (35) for transferring tangential forces from the hub means to the disk (1) without transferring radial forces. 5. A rotor according to any one of the preceding claims wherein the interface (503) between the hub means (32) and the disk (1) is lubricated to facilitate radial motion of the disk.

Inventors: PENHASI, HARRY, ARON
Applicants: BECKMAN INSTRUMENTS, INC.
Application year: 1988
Main classification: B04B9
IPC codes: B04B9