Composite material centrifuge rotor

Abstract: The rotor in­ cludes a top disk (T), a bottom disk (B) and a spacer belt (S) spac­ ing the two disks apart to form a rotor assembly (A). Both disks are fabricated by overlying multi­ ple layers of tape (14, 16, 18, 20) the tape being made from fibers held together with resin. The tapes are stacked in multiple layers normal to the spin axis of the rotor (30). When the tape is compiled in multiple layers to a thickness of the top and bottom disk, the tape is compressed between plates (P₁, P₂) and cured at a tem­ perature well above the autoclave temperature of sterilization of the rotor. Thereafter, a spacer belt (S) consisting of a spirally wound resin impregnated fiber is placed to space the top and bottom disks apart. Thereafter, the assembly is wound by a peripheral belt (B) with resin impregnated high stiff­ ness fiber, which is cured and shrinks during curing to exert hoop tension around the periphery of the rotor assembly.

Description: This invention relates to centrifuges and more particularly to a composite material centrifuge rotor applicable for use in modern high speed or so-called "ultracentrifuges". An ultracentrifuge rotor may experience 600,000 g or higher forces which produce stresses on the rotor body which can eventually lead to rotor wear and disintegration. All ultracentrifuge rotors have a limited life before damage and fatigue of the material comprising the rotor mandates retirement from further centrifuge use. Stress generated by the high rotational speed and centrifugal forces arising during centrifugation is one source of rotor breakdown. Metal fatigue sets in with conventional rotors following a repeated number of stress cycles. When a rotor is repeatedly run up to operation speed and is decelerated, the cyclic stretching and relaxing of the metal changes its microstructure. The small changes, after a number of cycles, can lead to the creation of microscopic cracks. As use increases, these fatigue cracks enlarge and may eventually lead to rotor failure. The stress on conventional metal body rotors may also cause the rotor to stretch and change in size. When the elastic limits of the rotor metal body have been reached, the rotor will not regain its original shape, causing rotor failure at some future time. Metal rotors are usually constructed of titanium and aluminum alloys. Titanium and aluminum alloy rotors have a respectably high strength to weight ratio. Aluminum rotors are lighter weight than titanium, leading to less physical stress and a lower kinetic energy when run at ultracentrifuge speeds; however, titanium rotors are more corrosion resistant than aluminium ones. As the ultracentrifuge performance and speeds have increased, the safe operating limits of centrifugation have been reached by conventional metal rotors. One attempt to overcome the design limitations imposed is indicated in U.S. -A- 3,997,106 issued to Baram for a centrifuge rotor which is laminated and consists of a first disk (Figure 3 top disk) and a second disk 8b, a filler material being provided between the disks wherein a radial fibre hoop (24) is wound under tension and cured over the periphery of said assembly and over the outside surfaces of said first and second disks and said spacer ring whereby said rotor assembly at said first disk, second disk and spacer ring are radially compressed with respect to the spin axis of said rotor equal and opposite to the forces of radial tension of said rotor. The Baram '106 patent envisions greater chemical resistance and lower specific gravity rotors, which achieve optimum strength, by the use of a laminate manufacturing process. U.S. -A- 2,974,684 to Ginaven is directed to a wire mesh of woven wire cloth 6 for reinforcing a plastic material liner 7 for use in centrifugal cleaners (see Figures 2 and 3). U.S. -A- 1,827,648, 3,993,243, 4,160,521 have all been directed to a rotor body made from resin and fibrous reinforcement materials. In particular, '648 is fiber wound to produce a moment of inertia about the vertical axis greater than the moment of inertia about the horizontal axis through the center of gravity of the bucket so that the rotor bucket is stable at speeds of 125 to 167 rps( 7500 to 10,000 RPM) (a relatively slow centrifuge speed by modern standards). U.S. -A- 4,468,269 discloses an ultracentrifuge rotor comprising a plurality of nested rings of filament windings surrounding the cylindrical wall of a metal body rotor. The nested rings reinforce the metal body rotor and provide strengthening and stiffening of the same. The rings are nested together by coating a thin epoxy between layers. US A 3913828 discloses a design substantially equivalent to that disclosed by the '269 patent. None of the conventional designs provide maximum strength through ultracentrifuge speeds through the use of a material specifically designed to accommodate localised stress and resist rotor body fatigue. Conventional metal bodies, or reinforced metal body rotors, are subject to metal stress and fatigue failure during centrifugation. What is needed is a rotor body of substantial strength, yet lighter in weight and capable of enduring increasingly higher loads and speeds. The body should resist stress and corrosion and be specifically designed to copy with localised stress. Metal rotors have the convenience of being adaptable to well known metal working techniques. Providing that reasonable care is utilized in metal working and abrupt metallic interfaces are avoided, the isotropic strength and materials property of metal remains unchanged. That is to say all the metallurgical properties of metal are substantially unchanged in any given direction of the metal with respect to the spin axis of the rotor. In the following material, it will become apparent that the disclosed composite rotor does not include isotropic properties. In fact, the rotor is specifically designed to have maximum resistance against centrifuged induced radial tension along planes parallel to the spin axis of the rotor. Applicant will utilize the term "anisotropic" to describe this property. An object of this invention is to set forth a unitary disc construction and process of disk construction from which rotor plates can be manufactured. According to one aspect of the present invention there is provided a composite laminated rotor of disk construction having a radial tensioning hoop formed from filaments wound around the periphery of the rotor characterised by: at least one disk, said disk fabricated from laminated layers with longitudinally extending fibres for reinforcing, said fibres bound together with resin; said laminated layers having sequentially repeating patterns of fibre directions, each layer having longitudinally extending fibres arranged in parallel and adjacent layers having fibres arranged in different angular orientations with respect to a selected radius vector extending normal to the spin axis of said rotor, each said laminated layer being normal to the spin axis of said rotor; and the radial tensioning hoop is cured to shrink under tension over the periphery of said disk whereby said disk is radially compressed with respect to the spin axis of said composite rotor construction equal and opposite to the dynamic forces of radial tension on said disks. An advantage of the disk construction is that it has an anisotropic radial reinforcement with fibres of great stiffness. The fibres oriented at one radial direction in one layer can transmit their stiffness through the cure resin to fibres oriented at a different radial direction at a different adjacent layer. Yet another advantage of the disk construction is that the disk can be fabricated up to a practical limit of about 4.064cm (1.6 inches). From the fabricated disk, a composite rotor construction can be made. According to a further aspect of the present invention there is provided a process for constructing a composite laminated rotor of resin and fibre comprising the steps of: fabricating first and second laminated disks using thin layers of fibre reinforced material having longitudinally extending fibres bound together with resin; cutting and placing said thin layers with said longitudinally extending fibres into pieces with said pieces centred about the spin axis of said rotor; orienting said respective thin layers to repeating patterns of fibre directions, each layer having longitudinally extending fibres arranged in parallel and adjacent layers having fibres arranged in differing orientations with respect to a selected radius vector from said spin axis of said rotor; pressing and curing said thin layers together to form the top and bottom disks for said rotor; placing a spacer hoop between said first and second disks to form a hollow rotor assembly (A); winding resin soaked fibre under tension around the exterior of said hollow rotor assembly; and curing said resin soaked fibre windings to shrink said windings onto said hollow rotor assembly whereby said rotor assembly is prestressed by hoop tension against radial loading of said rotor while centrifuging. Advantageously as the layers are stacked, they are oriented so as to vary the longitudinal and radial direction of the high stiffness fibres. When the layers are cured, the rough edges are removed by machining, typically by the use of an end mill. The end mill is operated to trim the edges while the cured disk still remains securely clamped. For this clamping the original pressing plates may be used; alternatively, other plates may be substituted. Preferably, the layers for each of the disks are compressed between two metal pressing plates, the pressing plates being typically of aluminium or steel. The metal pressing plates are then further utilised to maintain the integrity of the upper and lower disks while necessary holes or cavities for receiving the drive hub and centrifuge sample buckets are configured. These holes are typically made by drilling a pilot hole and thereafter introducing an end mill to gradually relieve the hole from the pilot hole diameter to its full diameter in successive increments without disturbing the lamination. The disks are preferably notched at their confronting surfaces by the end mill to receive a radially wound fibre ring. When the disks are confronted over the ends of the fibre ring, a rotor assembly is formed. The hoop tension is chosen so that the belt radially pretensions the rotor assembly with a force that exceeds any dynamic centrifugal force on the belt at designed centrifuge speed. There results a rotor having about 25% the weight of an all metal titanium rotor which enables faster acceleration and declaration to centrifuging speed and reduces both containment requirements and drive train wear. When the rotor disclosed herein is fully loaded with buckets, sample tubes and mounted on a centrifuge, the resultant assembly is 60% lighter than an all metal (titanium) prior art counterpart. A further advantage of the disk is that it may be worked to essentially the same tolerance as an all metal rotor without delamination or damage. A further advantage of this aspect of the invention is that the fibre winding, when cured and shrunk, radially prestresses the rotor disk equal and opposite to the dynamic radial loading of centrifuging. An interference fit between the radial band and the rotor disk is created which does not become separated upon centrifuging. An advantage of this aspect of the invention is that the density of the rotor is vastly reduced. Not only is the fibre laminate constructed 1/3 the density of titanium but the hollow construction of the rotor further reduces the weight of the rotor in the order of about 25% over the weight of a titanium rotor of equal size. Yet another advantage is that the rotor has imparted anisotropic radial stiffness. That is to say the stiffness is normal to the rotor spin axis and in the direction of radial stress imparted by centrifugal force from the spin axis. Fibres having a stiffness of up to 4.82 x 10¹¹Pa (70 million lbs/inch²) may be used (as distinguished from titanium having a stiffness of 1.034 x 10¹¹Pa (15 million lbs/inch²). Further, the stiffness of the fibre can be varied for the particular rotor construction utilized. An advantage of the rotor is that acceleration and declaration times are vastly reduced. Thus, long dwell times, while the samples reaches centrifuging speed or declarations from centrifuging from centrifuging speed, are no longer required. Consequently, the classifying cycle time for specimens is reduced. Both operator savings and apportioned centrifuge costs per sample are likewise reduced. A further advantage of the requirement is that since the rotor is lighter, the kinetic energy contained in a high speed rotor spinning at centrifuging speed is reduced. Consequently, the containment requirements of the centrifuge can be reduced. Containment belts can either tolerate a higher speed rotor of the disclosed construction of alternatively can be lightened when used exclusively with composite rotors. Yet a further advantage of the light composite rotor is that it produces less bearing and drive train wear on the centrifuge. Moreover, since the rotor is of lighter construction, the effects of rotor imbalance are lessened. Yet an additional advantage of the disclosed rotor is that it is hollow. Consequently the hollow portions of the rotor do not contribute to centrifugal dynamic loading. Only the disks and radial belts contribute to centrifugal dynamic loading. Other objects, features, and advantages of this invention will become more apparent after referring to the following specification and attached drawings in which: Fig. 1 is a perspective view of the completed rotor of this invention broken away to identify the component parts; Fig. 2 is a perspective schematic illustrating the fabrication of either the top or bottom disk of the rotor of Fig. 1 showing specifically the placement of the longitudinally reinforced fiber laminated layers in an alternately radial pattern between upper and lower clamping and curing plates; Fig. 3 is a perspective schematic illustrating the disk of Fig. 2 with the working tool of an end mill forming sample bucket holes and a spacer ring notch, the particular disk here illustrated being a bottom disk; Fig. 4 is a perspective schematic illustrating top and bottom disks trapping a radially wound spacer ring there between to form a rotor assembly of this invention; Fig. 5 illustrates the rotor assembly of Fig. 4 clamped between two spool plates with the assembly being radially wound with a resin impregnated tow of carbon fibers immediately prior to curing and shrinking of the fibers to complete the rotor illustrated in Fig. 1; and Fig. 6 illustrates the section of an alternative embodiment of the composite rotor of this invention. Referring to Fig 1, a completed rotor R is illustrated having top disk T and bottom disk B confronted across spirally wound spacer ring S to form a rotor assembly A. Rotor assembly A is wound by a tensioned belt C. As will hereinafter become apparent, belt C exerts a radial tension inwardly on the rotor assembly A. This inward radial compression is opposite to the dynamic radial tension that occurs during centrifuging. The rotor construction is hollow. Consequently, a sample bucket is placed within the rotor as shown in Fig 1. The sample bucket has sufficient side wall thickness to resist all centrifugal forces on the sample and is constructed from conventionally forged and machined titanium. In describing the invention, the fabrication of the top and bottom disks will first be set forth with respect to Figs 2 and 3. Secondly, the construction of the rotor assembly will be explained with respect to Fig 4. Finally, the fabrication and shrinking of the belt C over the rotor assembly A will be set forth with respect to Fig 5. Referring to Fig 2, the process for the formation of disks T and B is illustrated. Typically, two aluminum pressing plates P1 and P2 are used as part of a press mechanism (otherwise not shown) for the curing of the individual laminated layers. The disks are each formed from approximately 240 to 250 layers of tape trapped between the pressing plates. The tape consists of parallel strands of longitudinally extending fibre having high stiffness impregnated and held together with partially cured resin. The tape is cut in typically square sections to exceed the full diameter of the rotor. Such tape can be purchased from the Celanese Plastic & Specialities Company of Chatam, New Jersey. The tape is sold under the Trade Mark CELION 12K. The tape comprises fibre tows having 12,000 fibres in each tow. The curing temperature of the resin utilised for holding the fibre together in the commercial tape is in the range of 160°C (350°F). This 160°C (350°F) temperature is selected to be well above the 114°C (260°F) autoclave temperature of sterilisation of the completed rotor assembly. Orientation of the tape is specifically illustrated. Referring to Fig 2 it will be seen that spin axis 30 of the rotor has an arbitrary radius taken normal to spin axis 30. It will be appreciated that all of the individual laminate layers 14, 16, 18 and 20 of the rotor are normal to spin axis 30. It is in this plane that the disk is required to have the improved anisotropic properties set forth herein. The fibres of layer are all aligned parallel to radius vector 32. The fibre of layers 16 are oriented clockwise 45° (+45°) using the convention herein from the fibres of layer 14. The fibres of layer 18 are counter-clockwise from the fibres of layer 14 at 45° (45° using the convention herein). Finally, the fibres of layer 20 are placed at right angles to those fibres of layer 14. The disk is fabricated by many successive layers. Utilizing the apparatus herein shown, as many as 250 layers of such tape can be utilised by build a disk 4.064 cm (1.6 inches) in height. As the layers are stacked, the variation of the fibres angulation is repeated as described. Once the tape is compiled to the full height of the disk, both curing and pressing occur. The reader will appreciate that woven fibres constituting fabric having straight woven fibres could as well be used. Typically the press plates with the trapped tape layers are placed within an oven elevated to the required curing temperature of 170°C (370°F). Thereafter the plates are urged towards one another under gradually increasing compression. Initially, the compression begins at 17.23 x 10⁴Pa (25 lbs/inch²). Over a period of 4 hours the pressure between the plates P1 and P2 is increased. The pressure increases in increments until, at the end of 4 hours, a total pressure of 1.37 x 10⁹Pa (200 lbs/inch²). Total curing time is 6 hours at the 160°C (350°F) temperature. The reader will appreciate that the final two hours of curing occurs at the full 1.37 x 10⁴Pa (200 lbs/inch²) pressure between plates P1 and P2. Once pressing and curing is completed, the successive tape layers between the plates will extend outwardly from the plates in a "dog eared" fashion. Trimming is required. Referring to Fig 3, trimming can be understood. Typically, plates P1 and P2 will be clamped towards one another by clamps (not shown). Thereafter, an end mill M1 typically rotating and trimming around the periphery of plates P1, P2 and disk B therebetween will trim the side edge debris from bottom disk B. Preferably end mill M1 is provided with ultrasonic vibration and the trimming cuts of the end mill M1 are made incrementally so as not to interfere with the lamination of the individual layers of bottom disk B. It will be remembered from Fig 1 that the finished rotor includes hole H having sample bucket 40 therein. It is sample bucket 40 which contains the sample which is classified during centrifugation. The method of forming the precise aperture for the sample bucket 40 is illustrated with respect to Fig 3. Typically, a drill D1 penetrates plate P1, disk B and plate P2 therebelow. Thereafter, an end mill M2 gradually machines out the pilot hole of drill D. Referring to end mill M3, the mill is shown completing hole H for the sample bucket 40 within the rotor. It has been found that if the sample holes are 2.54 cm to 3.81 cm (1 to 1.5 inches) in diameter, a pilot hole in the order of 1.27 cm (1/2 inches) can be drilled. Thereafter, an end mill is utilised to cut away no more than 0.0125 cm (20/1000ths of an inch) at a time. It is preferred if the end mill is sonically vibrated. Referring back again to Fig 1, it will be understood that rotor assembly A includes notch 50 in upper disk T and notch 52 in lower disk B. Machining of notch 52 in lower disk B is illustrated in Fig 3. In Fig 3, an end mill M4 is shown creating notch 52. This notch 52 is sized to receive a fibre wound spacer belt S. Construction of the fibre wound spacer belt can be understood by referring to US-A-4738656. Simply stated, spacer belts S is formed by winding individual fibres over an aluminium mandrel and curing those fibres to form the spacer belt. Thereafter, the mandrel is cryogenically cooled and removed. Once the mandrel is removed, the resultant cylinder of fibre wound resin impregnated particles can be cut to form spacer belts S of the desired length. Referring to Fig 4, the rotor assembly A can be understood. The rotor assembly A includes a top disk T, a bottom disk B and spacer belt S therebetween. Notch 52 on bottom disk B receives the bottom portion of spacer belt S. Notch 50 on top disk T receives the upper position of spacer belt S. The entire rotor assembly is fitted together, ready for final winding as illustrated in Fig 5. It will be appreciated that top disk T includes 8 holes H1-H8. These holes are aligned to register with holes H1-H8 in bottom disk B. Additionally, bottom disk B includes a hub hole 60 which hole provides for an attachment of the spin apparatus of the rotor. There now remains to be described the placement of belt C on the exterior of the rotor assembly A. This can be understood with respect to the schematic of Fig. 5. Referring to Fig. 5, rotor assembly A has been trapped between two spool plates P3 and P4. These respective spool plates are bolted across rotor assembly A by bolts 70 extending through the respective holes H1-H8 and corresponding holes H1, H8. Plates P3 and P4 extend radially beyond the side edges of the spin assembly A. This enables winding of belt C. Belt C is formed from a multifiber tow. The fiber is lead from a spool 80 and dipped through a resin bath 90. The fiber then passes through a brake 92 and a bobbin mechanism 94. Bobbin 94 tends to feed the multiple stranded fiber tow onto and over the periphery of the rotor assembly A. The rotor assembly A is typically spun on a shaft 100. It is spun under a torque so that the fiber tow extending between brake 92 and rotor assembly A is approximately under 88.96N (20 lbs.) of tension. Bobbin mechanism 94 enables the fiber to be placed over the surface of the rotor mechanism A in an even distribution. The particular fiber herein used is manufactured by the Union Carbide Corporation of Concord, California in the Specialty Polymers and Composites Division. It is sold under the trademark THORNEL T-40. In this particular case it has a stiffness of 2.7579x10¹¹Pa (40 million lbs./inch²). The fiber has a group of individual strands bound together in a tow which tow contains approximately 3,000 or more strands. Once the fiber is wound to a thickness of belt C, the two plates P3 and P4 and the wound fiber are cured, again utilizing a resin having a curing temperature in the range of 170°C (370°F). This construction causes the fibres to shrink over the periphery of rotor assembly A. It will be understood that the belt C exerts a hoop tension around the periphery of the rotor assembly A. This hoop tension actually causes the diameter of belt A to shrink by as much as 8.40 x 10⁻⁶M (3,000ths of an inch) for a rotor having a diameter of the order of approximately 20.32 cm (8 inches). More importantly, belt C exerts upon the rotor assembly A a radial tension. This radial tension is inward and to and towards the spin axis 30 of the rotor. It is opposite to and always in excess of the forces dynamically produced in the rotor during centrifuging. Having set forth the embodiment of Fig 1, short reference will be made to an alternate embodiment of the rotor utilising the section of Fig 6. Referring to Fig 6, a rotor R2 is disclosed. Rotor R2 includes an enlarged spacer belt S. Further, the respective holes Hx and Hy are shown canted as is common in some centrifuge rotors. The reader will understand that this alternate rotor construction is manufactured by precisely the same techniques. Consequently, the rotor construction of Fig 6 will not be further discussed herein. The reader will understand that whereas layers of tape having their fibres extending in one direction for the fabrication of the disks of the rotor assembly are disclosed, fabric woven with fibres extending substantially straight in more than one direction could as well be used provided that the sequential layers of fabric impart the alternating directions of radial reinforcement here shown.

Claims: A composite laminated rotor of disk construction having a radial tensioning hoop formed from filaments wound around the periphery of the rotor characterised by: at least one disk, said disk fabricated from laminated layers (14, 16, 18, 20) with longitudinally extending fibres for reinforcing, said fibres bound together with resin; said laminated layers (14, 16, 18, 20) having sequentially repeating patterns of fibre directions, each layer having longitudinally extending fibres arranged in parallel and adjacent layers having fibres arranged in different angular orientations with respect to a selected radius vector (32) extending normal to the spin axis (30) of said rotor, each said laminated layer (14, 16, 18, 20) being normal to the spin axis (30) of said rotor; and the radial tensioning hoop (8) is cured to shrink under tension over the periphery of said disk whereby said disk is radially compressed with respect to the spin axis (30) of said composite rotor construction equal and opposite to the dynamic forces of radial tension on said disks. The composite rotor of claim 1 and wherein said composite rotor includes first and second disks (T, B). The composite rotor of claim 2 and wherein said first and second disks (T, B) include a spacer ring (S) therebetween. The composite rotor of any of claims 1 and 3 wherein said laminated layers (14, 16, 18, 20) include tape having said fibres extending in one direction. The composite rotor of any of the above claims wherein said top disk (T) and said bottom disk (B) include registered holes (H) for receiving a sample holder (40). The composite rotor of claim 5 wherein said registered holes (Hx, Hy) are angularly inclined with respect to the spin axis (30) of said rotor. A process for constructing a composite laminated rotor of resin and fibre comprising the steps of: fabricating first and second laminated disks (T, B) using thin layers of fibre reinforced material (14, 16, 18, 20) having longitudinally extending fibres bound together with resin; cutting and placing said thin layers (14, 16, 18, 20) with said longitudinally extending fibres into pieces with said pieces centered about the spin axis (30) of said rotor; orienting said respective thin layers (14, 16, 18, 20) to repeating patterns of fibre directions, each layer having longitudinally extending fibres arranged in parallel and adjacent layers having fibres arranged in differing orientations with respect to a selected radius vector (32) from said spin axis (30) of said rotor; pressing and curing said thin layers (14, 16, 18, 20) together to form the top and bottom disks (T, B) for said rotor; placing a spacer hoop (S) between said first and second disks (T, B) to form a hollow rotor assembly (A); winding resin soaked fibre under tension around the exterior of said hollow rotor assembly (A); and curing said resin soaked fibre windings to shrink said windings onto said hollow rotor assembly (A) whereby said rotor assembly (A) is prestressed by hoop tension against radial loadings of said rotor while centrifuging.

Inventors: PIRAMOON ALIREZ M, PIRAMOON, ALIREZ M.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1988
Main classification: B04B7
IPC codes: B29C53, B04B5, B04B7