The drive system for a rotating apparatus (such as a centrifuge instrument) typically includes a shaft attached at one end to a motive force (e. g., an electric motor). The shaft supports a load somewhere along its length. One inherent property of such a drive system is load imbalance. Even a very small imbalance in the otherwise symmetrical mass distribution about the drive shaft can cause damaging vibrations. The magnitude of the imbalance places a load on the bearings.
This problem can be addressed by providing a degree of flexibility to the drive shaft. However, as this rotating apparatus accelerates to its operating speed the system passes through several critical speeds. A thin, elongate shaft rotating about its longitudinal axis possesses certain natural frequencies of vibration that become apparent at these critical speeds. Passage through a critical speed thus causes the drive system to vibrate, which in turn magnifies the load imposed on the bearings. This increase in load can significantly reduce the life of the bearing. In the example of a centrifuge instrument the vibration may also have a detrimental effect on the sample that is being processed within the rotor of the instrument.
Several methods have been proposed in the art to solve the problem of vibration generated during passage of a drive system of a rotating apparatus through a critical rotational speed. One solution is to attempt to damp the vibrations imposed on the system. U.S. Pat. No. 5,026,341 (Giebeler) is an example of a rotating system for use in a centrifuge instrument that restricts the deflection of the shaft, thus damping vibration.
Another solution, exemplified by the arrangement disclosed in U.S. Pat. No. 4,236,426 (Meinke et al.), is to alter the physical properties of the drive system in a speed-dependent fashion. As a result, for a first portion of the speed regime of the apparatus, the stiffness of the system is such that the critical speed lies well beyond a predetermined speed threshold. However, once the speed threshold is crossed the stiffness of the system is dynamically altered such that the critical speed of the modified drive lies at a speed that is well below the predetermined threshold. Thus, as the apparatus accelerates through its fill speed regime to its operating speed the critical speeds associated with both the original and the modified drives are avoided. In an arrangement as shown in the Meinke et al. patent, springs, dampers and/or masses are actively coupled and uncoupled in response to the changes in the deflection of the shaft. This coupling and uncoupling has the effect of altering the critical rotational speeds.
Implementation of either of the above-described expedients usually requires a complex mechanism to effect the interaction between rotating and non-rotating parts of the drive system. In either case, technical difficulties can result. Therefore, in view of the foregoing it is believed advantageous to provide an arrangement of the type which dynamically alters the stiffness of the drive system, yet which does so without undue mechanical complexity.