Abstract:
A type of passive magnetic bearing that provides for both positive radial stiffness and significant levels of passive radial damping. Axial damping and axial forces can also be generated with minor modifications to the basic configuration. The bearing is comprised of a series of magnet, damping, and rigid non-magnetic elements essentially defining a laminated or composite structure. The damping material is directly mounted between the magnets and a rigid material, resulting in constrained layer damping. The range of motion of these floating, or soft mounted, magnets may also be restricted to generate higher stiffness in response to large vibrations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of magnetic bearing or suspension systems. More specifically, the invention is directed towards a passive radial magnetic bearing system that includes damping elements and may also include a mechanism to provide increased stiffness at large rotor amplitudes. 
     2. Description of Related Art 
     Passive magnetic bearings are well known in the art. Many configurations of these types of bearings are possible (e.g. U.S. Pat. Nos. 5,894,181; 5,619,083; 4,072,370; 3,958,842; and 3,614,181 among others). Each of these configurations suffers from a lack of damping. Rotors supported on these types of bearings, therefore, will be poorly damped. This condition results in large vibrational amplitudes when the rotors traverse their critical speeds, increased sensitivity to imbalance forces, and decreased resistance to rotordynamic instabilities. This combination sometimes results in failure of the machines. 
     Many various techniques for introducing damping into passive magnetic systems have been developed (e.g. U.S. Pat. Nos. 5,910,695; 5,679,992; 5,521,448; and 5,386,166). Some of these methods have developed eddy current dampers, but these generally provide very low damping levels. An alternative method, utilized in some of the above patents, is to use a damping material, such as an elastomeric material or a woyen material, to provide the damping. Many different configurations of this approach have also been disclosed. These configurations generally rely on introducing an intermediate housing between the rotor and the machine frame. In general, the stationary portion of the passive magnetic bearing is mounted in the intermediate housing. The damping material is then positioned between the intermediate housing and the machine frame. Undesired rotor vibrational forces are transmitted from the rotor magnets to the stator magnets through the magnetic field. The transmitted vibrational forces cause movement of the stator magnets, and the intermediate housing into which the magnets are mounted. The motion is resisted by the damping material, either in shear or in compression. The resistance of the damping material to the vibrations results in frictional forces, thus dissipating the vibrational energy. 
     This approach has several limitations. First, the intermediate housing represents an additional component that must be manufactured and assembled, adding to system cost and complexity. Secondly, the intermediate housing has a finite, and usually substantial, mass that is added to the bearing mass. This results in a reduction in the resonant frequency of the combined bearing stator and intermediate housing, above which a loss of damping occurs. Finally, in this configuration, all of the forces transmitted through the bearing must pass through the damping element. This limits the designers&#39; ability to independently adjust the stiffness and damping of the bearing system to optimize rotordynamic performance. 
     In addition, several of these configurations rely on a single ring of magnetic material on each of the stator and rotor sections. Variations in the magnetic strength of the rotor and stator magnet materials result in variations of the magnetic forces as one ring rotates relative to the other. This results in “magnetic run-out,” or a mechanical vibration of the rotor due to unbalanced magnetic forces. This sensitivity to variations in the magnetic field strength of the bearing magnets is undesirable. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the current invention to provide a passive magnetic support and damping system without the above listed drawbacks. 
     It is therefore a further object of the present invention to provide a passive magnetic support and damping system that is made of easily manufacturable components in a readily assemblable configuration. 
     It is therefore a still further object of the present invention to provide a passive magnetic support and damping system that provides increased stiffness in response to large amplitude vibrations. 
     It is therefore a still further object of the present invention to provide a passive magnetic support and damping system that is minimally sensitive to variations in the magnetic properties of the permanent magnet materials used. 
     These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a passive magnetic support and damping system in which the rotor portion of the damping system is comprised of a series of disks or annular rings of permanently magnetized material fixedly attached to the rotor of the machine. The stator portion is also comprised of a series of annular rings of permanently magnetized material, which are positioned concentrically with the rotor magnets. The stator and rotor magnets are formed and positioned such that a radial gap is present between said stator magnets and said rotor magnets. At least one, and preferably an even number, of the stator magnets are mounted in a damping material, which, in turn, is fixedly attached to the machine stator. This damping material may be an elastomeric material, a woven material, or any other type of material that exhibits primarily frictional losses in response to shear or compressive strains. The “soft mounted” stator magnet(s) provide damping to the system. The remaining stator magnets are fixedly attached to the machine stator and provide stiffness (“hard mounted”). By varying the number, size, and magnetic strength of the stator magnets mounted in these two ways, the stiffness and damping of the bearing assembly can be varied substantially independently. Further, because only a single stator magnet is interposed between the rotor and each layer of damping material, the resonant frequency of the damping mechanism is very high (i.e. the effective mass of each damper element is minimized). This results in improved damping at higher frequencies than was available in the prior art. 
     An additional feature of the present invention is that the soft mounted stator magnets can be provided with a backing material that limits their displacement. When the soft mounted magnets come into contact with the backing material, they effectively become hard mounted, and contribute additional stiffness to the system. In this manner large excursions of the rotor, which cause large displacements of the soft mounted magnets, will result in increased bearing stiffness, tending to restore the rotor to the nominal position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a passive magnetic support and damping system configured to provide radial stiffness and damping with minimal axial forces. 
     FIG. 1 a  is an enlarged detail view of the inset  1   a  shown in FIG.  1 . 
     FIG. 2 is a cross sectional view of a passive magnetic support and damping system configured to provide radial stiffness and damping and in addition to provide an axial force. 
     FIG. 3 is a cross sectional schematic view of an example showing a small flywheel utilizing passive magnetic support and damping systems to provide radial support and axial pre-load. 
     FIG. 4 is a cross sectional view of a passive magnetic support and damping system configured to provide radial stiffness and damping and in addition to rovide axial damping and an axial force. 
     FIG. 5 is an axial view of the passive magnetic support and damping system displayed in FIG. 2, configured for large diameter and/or high speed systems. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1 one configuration of a radial bearing  10  according to the present invention is shown. The bearing is comprised of a rotor element  20  and a stator element  30 . The rotor element is typically attached to a shaft  21  and is further comprised of at least one rigidly mounted magnet  22 . Said rotor magnet(s) may be directly mounted to the shaft, or may be mounted with a spring element  23  (e.g. a Tolerance Ring; manufactured by USA Tolerance Rings, West Trenton N.J.) with a spring constant significantly higher than the stiffness of the magnetic field between the rotor and stator magnets. Such spring elements allow some differential expansion between the shaft  21  and housing  31  and the magnets  22  and/or  34 , without generating excessive stresses in the magnets. The rigidly mounted rotor magnets may be spaced axially from each other by means of rigid spacers  25 . These spacers may be made of a magnetic material but are preferably non-magnetic. 
     The stator element can be mounted into a housing  31 , which is attached to the non-rotating portion of the machine, not shown. Alternatively, the stator elements can be directly mounted into the machine. The stator element is further comprised of at least one magnet  32  mounted concentrically with the rotor magnet. Said magnet(s) are attached on at least one side, but preferably both sides, to a damping material  33 , such as a viscoelastic material, an elastomer, a woven or felted metal, or another material that exhibits frictional shear losses resulting in damping. The damping material is in turn attached to a structure that is rigidly mounted to the fixed portion of the stator housing. This structure may be another concentrically mounted magnet  34 , a rigd spacer element  35 , the stator housing  31  or any other element that is fixedly attached to the stationary machine. Functionally, the “soft mounted” stator magnets  32  are allowed to follow the radial vibrations of the “hard mounted” rotor magnets  22 . The motion of the soft mounted stator magnets results in shearing of the damper materials  33 . Thus, vibrational motions of the rotor magnets are transmitted through the magnetic coupling, producing vibrational motions of the soft mounted stator magnets. This motion is in turn transmitted to shear or compressive deformations of the damping material, further resulting in frictional dissipation of the vibrational energy. This frictional dissipation provides damping to the rotor through the magnetic coupling. In the illustrated example, additional magnetic coupling between the hard mounted rotor magnets  22  and the hard mounted stator magnets  34  provide bearing stiffness. 
     Also in FIG. 1 it is seen that the soft mounted magnets are surrounded on their outside diametral surface by a radial gap  36  which allows radial displacements to occur. These radial displacements are necessary to generate the damping. The low stiffness of the damping material, however, reduces the overall stiffness of the bearing that would normally be expected of magnets of the given size and material properties. By introducing a rigid backing material  37  which limits the radial displacement of the soft mounted magnets, this stiffness can be partially restored in response to large vibrations. Alternatively, the full radial area shown as gap  36  and element  37  could be filled with additional damping material, increasing the effective damping of the system. 
     In the preferred embodiment, more than one rigidly mounted magnet is present in the rotor portion of the bearing to reduce the effects of variations in the magnetic properties of any one magnet. In addition, in the preferred embodiment, the magnets are magnetized in the axial direction, as shown in the figure. Radially magnetized magnets in both the rotor and stator segments, arranged in opposition, would also work but are more difficult to produce. 
     FIG. 2 illustrates a different embodiment of the present invention in which the rotor element  20  surrounds the stator element  30 . In this illustration, the rotor magnets  22  are positioned about the outside diameter of the stator magnets  32  and  34 . This arrangement results in radial pre-compression of the rotor magnets, which is advantageous for high-speed applications in which the rotational stresses may cause tensile failure of the rotating magnets. Also, an additional magnet  40  has been added to the rotor element to generate an axial force. This is beneficial in situations in which a static load, such as gravity, needs to be countered. Depending on the configuration of the machine, additional magnets generating axial forces could be added to the rotor, the stator, or both. 
     Two possible arrangements of additional magnets  40  exist: 1) the additional magnet can generate a repulsive force between the rotor and stator as shown in the bottom bearing of FIG. 3 or  2 ) the additional magnet can generate an attractive force between the rotor and the stator as shown in the example of FIG.  2 . 
     In the case of FIG. 2, the attraction between the stator magnet  34  and the additional rotor magnet  40  increases the positive radial stiffness and the magnitude of the negative axial stiffness of the bearing. This increased radial stiffness is often advantageous. The attractive force on the additional magnet, however, tends to pull it out of its housing, complicating the task of assembly. 
     In the axial repulsive force configuration (FIG.  3 ), the radial stiffness of the bearing assembly is reduced, but a positive axial stiffness element is introduced. The overall axial stiffness of the bearing remains negative, but the magnitude is reduced. In addition, in this case, the repulsive force tends to keep the additional magnet in the housing, reducing the required complexity of the housing. In the example flywheel shown in FIG. 3, a motor generator, not shown, could be placed internal to the flywheel body  60  inside the cavity  50 . Electrical leads for this device could enter through a hollow shaft  51 . 
     As seen in FIG. 4, this repulsive force between a rotor magnet  22  and the additional magnet  40  can also be utilized to introduce a layer of damping material  41  between the additional magnet and the machine, introducing axial damping. In this case, any undesired axial vibrations of the rotor assembly  20  would be transmitted through the repulsive force between magnets  22  and  40 , resulting in axial motions of the additional stator magnet  40 . This motion would be resisted by the damping material  41  resulting in frictional dissipation of the axial vibration energy, or damping. 
     FIG. 5 represents an axial view of the assembly displayed in FIG. 2, further configured for high-speed applications. In this example the stator magnet(s)  32  (and  34 , not shown) are ring magnets mounted as described earlier. In cases in which the rotational speed is very high, or in which the rotor (outer) magnet  22  diameters are very large, it may be advantageous to manufacture those magnets from sections  22   a . These sections can be bonded to, or otherwise restrained by, the rigid spacers  25 . It is obvious that this same strategy can be used in cases in which the outer magnets are the stator magnets. In this case, the damper material could be positioned between rigid spacers that hold the magnet assemblies in place and fixed portions of the machine. 
     A further embodiment can be illustrated by FIG.  2 . In this illustration, FIG. 2 is an axial cross-section of a translational passive magnetic support and damping system that provides stiffness and damping in the horizontal axis, while allowing translation in the axis that extends perpendicular to the figure such that it extends into and out of the drawing sheet. This embodiment is configured to allow translational movements, such as those used in a linear slide or a positioning table. In this case, Stationary member  30  would consist of a base  31   a  and a series of one or more bar magnets ( 32 ,  34 ). The translational moving portion of the assembly  20  similarly contains bar magnets  22  rigidly mounted into a housing  21   a  or the translational portion of the machine (not shown). The soft mounted magnets  32  are mounted to provide damping in the manner described earlier. The axis of movement in this example is perpendicular to the drawing page, i.e.; it extends into and out of the drawing sheet. 
     The present disclosure should not be construed in any limited sense other than that limited by the scope of the claims having regard to the teachings herein and the prior art being apparent with the preferred form of the invention disclosed herein and which reveals details of structure of a preferred form necessary for a better understanding of the invention and may be subject to change by skilled persons within the scope of the invention without departing from the concept thereof.