Patent Application
20150115756
The disclosed technology generally relates to bearings, and more particularly to magnetic bearings.
A bearing is a machine element that both reduces friction and constrains motion between moving parts. Many types of bearings exist, but the greatest reduction in friction occurs when a magnetic bearing is employed, which supports a load using magnetic levitation. Magnetic bearings permit relative motion with very low friction and mechanical wear, and thus support the highest speeds of all kinds of bearing.
Some magnetic bearings use permanent magnets and do not require input of power, but do require external stabilization due to the limitations described by Earnshaw's Theorem. Most magnetic bearings use attraction or repulsion to achieve levitation. Review of the prior art, however, indicates that magnetic bearings exploiting magnetic reluctance have not previously been described.
One way to asymmetrically focus magnetic flux employs a horseshoe-shaped electromagnetic coil. Another means to asymmetrically focus magnetic flux employs magnetic bearing designs use variations of the Halbach series. The five magnet linear Halbach series is well known to those skilled in the art as a means to asymmetrically focus magnetic flux. A magnet array with as few as three consecutive magnets, however, can also focus magnetic flux asymmetrically so that north and south poles extend parallel to each other from the same side of the array. These three magnets are configured in linear fashion such that the center magnet is rotated 90 degrees relative to the end magnets, and the end magnets are rotated 180 degrees relative to each other. This type of magnet array will be called a reluctance array. Like the Halbach series, the north and south magnetic poles emanate from one side of the reluctance array.
Magnetic reluctance is defined as the resistance to the flow of magnetic flux through a magnetic circuit as determined by the magnetic permeability and arrangement of the materials of the circuit. Magnetic permeability can be thought of as the ability of a material to allow passage of magnetic flux. It is analogous to the concept of conductivity in electricity. Iron, for instance, has a high magnetic permeability whereas air has low magnetic permeability. Magnetic flux will pass through air, just as an electric spark will cross an air gap, but flux passes much more readily through iron.
The components comprising a magnetic circuit tend to act in such a way as to facilitate the flow of magnetic flux through the circuit, and thus minimize reluctance. This principle is most famously illustrated in Tesla's Switched Reluctance Motor. A ferromagnetic rotor is made to rotate between electromagnets of opposite polarity (stator coils). The rotor is compelled to rotate in order to complete a magnetic circuit through the rotor and stator coils. At the point in the rotation where magnetic flux flows most readily, the magnetic circuit is said to be in a state of minimal reluctance. A series of stator coils are configured in a circle, directing magnetic flux inward towards the ferromagnetic rotor. Successively switching the polarity of the stator coils just ahead of the rotating rotor enables continued rotation. Although the Switched Reluctance Motor employs electromagnets, the reluctance principle also applies to magnetic circuits comprising permanent magnets.
Magnetic reluctance has different and advantageous physical and mathematical properties in comparison to the typical magnetic forces of magnetic attraction and repulsion. Whereas the force between magnets falls off with the inverse of the square of the distance between the magnets, reluctance forces increase in a linear fashion with displacement. For example, when two Halbach series are magnetically coupled across an air gap of distance X, the force between the arrays is only ¼ as strong at a gap distance of 2X. Experimentation has shown that when two arrays are made to slide past each other at a constant gap distance X, like railway cars on parallel tracks moving in opposite directions, reluctance forces will increase in linear fashion over a short displacement, achieve a maximum, then fall to zero in linear fashion. By way of reference, both a rubber band and a steel spring demonstrate linear force-displacement characteristics. Pulling on either is initially easy but becomes harder the more the rubber band or spring is stretched up to the point of failure.
Reluctance is said to be at a minimum when a magnetic circuit employs materials with the greatest permeability and when the path of the magnetic flux completes the magnetic circuit by the most direct route possible. Reducing air gaps between the magnets and/or ferromagnetic components minimizes reluctance; conversely, reluctance increases whenever a magnetic circuit is disrupted by an increased air gap between the magnetic materials comprising the circuit. Air, having relatively low magnetic permeability, resists the flow of magnetic flux. Directing or focusing the path of flux between the magnetic elements by use of magnet arrays such as the Halbach series facilitates completion of a magnetic circuit and minimizes reluctance.
The purpose of the Abstract is to enable the public, and especially scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection, the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the inventive concept(s) of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the inventive concept(s) in any way.
A reluctance magnetic bearing employs magnetic reluctance to achieve magnetic levitation. Once a magnetic circuit has formed between magnets and/or circular magnet s within the bearing, there is a propensity to maintain the magnetic circuit. An outside force that disrupts this magnetic circuit increases reluctance and produces an equal and opposite force within the bearing assembly in an attempt to return to a state of minimal reluctance. A load may be substantially supported without physical contact between bearing surfaces, thus minimizing friction.
The present embodiment relates to a magnetic bearing in which moving parts are held in place and allowed to rotate by employment of magnetic coupling between a ring or disc or circular magnet or magnet (hereafter referred to simply as a circular magnet) and one or more arrays of individual magnets or electromagnets configured to direct flux towards the circular magnet. This magnetic coupling allows for rotation of the circular magnet but otherwise constrains movement of the circular magnet.
A typical bar magnet has north and south poles extending from either end, 180 degrees apart, along the physical axis of the bar magnet. The magnet arrays in one preferred embodiment comprise a group of at least three magnets stacked in linear fashion and configured to focus both the north and south magnetic poles from one side of the array. The magnetic poles extend in the same direction and parallel to each other from one side of the array. Another example of a magnet array is a linear Halbach series of at least five magnets. When Halbach series are noted in the disclosure, it is to be understood that Halbach series are a specific type of magnet array.
Note that in the arrays mentioned above, both north and south poles extend parallel to one another from one side of the array. Another example of an effective array would be the reluctance array. A third example would be a horseshoe-shaped coil or electromagnet. In each example, the intention is that both north and south poles extend substantially from the same side of the array in the direction of a circular magnet. The various means will be hereafter referred to simply as a magnet arrays.
The north and south magnetic poles of each magnet array act as a magnetic pincher extending towards the circular magnet. Several of these magnet arrays may be coupled symmetrically around the outside of a circular magnet with their magnetic poles facing the circular magnet. As a result, the circular magnet is held in place with respect to axial displacement but is still able to rotate on its generally elongate shaft, also called a rotatable shaft, or just shaft.
The magnet array may be attached to a rotatable shaft or cylinder while the circular magnet is fixed, or the circular magnet may be attached to a rotatable shaft or cylinder while the magnet arrays are fixed. Either way, the goal is axial rotation of the ring relative to the magnet arrays. The rotating shaft is generally an elongate shaft that has a linear axis and is configured for rotation around the linear axis, but may also take the form of a cylinder. The linear axis is called the rotational axis. Whether shaft or cylinder, both forms will be referred to as a shaft in this description. The shaft can be a solid rod, or a hollow tube.
Each individual magnet array completes a magnetic circuit by coupling with the circular magnet. The south magnetic pole of the circular magnet is coupled to the north magnetic pole of each magnet array. And the north magnetic pole of the circular magnet is coupled to the south magnetic pole of each magnet array. A zone below a circular magnet is a first zone of increased magnetic flux. A zone above the same circular magnet is a second zone of increased magnetic flux. A state of minimal reluctance exists when the circular magnet resides midway between the north and south poles of each magnet array, such that magnetic coupling occurs between the circular magnet and the magnet arrays. The circular element is restricted between two adjacent zones of magnetic flux created by magnet arrays. The position between zones of increased magnetic flux is a position of least magnetic flux, and therefore least magnetic reluctance, which is a predetermined position to which the shaft is to be directed. An axial load placed on the shaft will displace the circular magnet away from this center position, disrupting the magnetic circuits between the magnet arrays and the circular magnet. Disruption of these magnetic circuits increases reluctance, causing an equal and opposite axial force to be produced by the magnetic bearing. In this way, the axial load is magnetically levitated, and the shaft is restricted in its ability to move in an axial direction.
Alternatively, the magnet arrays may be attached to a shaft located inside a circular magnet with the north/south poles of the magnet arrays extending outwardly towards the ring. Just as above, this embodiment allows for rotation of the shaft but constrains axial displacement of the shaft.
In addition to enabling magnetic reluctance, magnetic coupling between circular magnet and magnet arrays generates an undesirable attractive force that seeks to pull the circular magnet off center and close the gap between circular magnet and magnetic array. Distributing the magnet arrays around the circular magnet would be helpful. But even the most perfect symmetry cannot avoid the limitations implied by Earnshaw's Theorem. Some additional means of centering the circular magnet may be desirable. One such means is a small roller bearing (annular) attached to the shaft, for example, a plain bearing, a fluid bearing, or a magnetic bearing. If a second magnetic bearing, if properly configured, would suffice. The centering bearing serves to maintain the shaft in a central position relative to the magnet arrays but avoids substantially constraining axial displacement. This provision allows for limited axial displacement during the loading of the bearing while maintaining an appropriate gap.
The magnetic reluctance bearing force-displacement curve is initially quite linear up to the capacity of the bearing. This allows the bearing to accommodate a variable load, fluctuating load, or even a vibrating load. This is analogous to the action of an automobile shock absorber as the vehicle travels uneven terrain. Because the force-displacement curve is linear and reproducible, the device can also be graduated and calibrated to function as a weight scale, analogous to a spring scale.
The load-bearing capacity can be increased by an embodiment comprising stacked circular magnets, with appropriate spacing, so as to couple with magnet arrays configured in Halbach series. This embodiment comprises two or more circular magnets, which can be called a circular fly wheel, attached in parallel and coaxially, and alternating magnetic polarity. The magnet arrays are configured as Halbach series, well known to those familiar with the art. Whereas the Halbach series may have a single north pole and a single south pole, the Halbach series comprises additional magnets configured to produce a series of alternating north and south magnetic poles all extending from the same side of each series towards the coaxial circular magnets. The alternating magnetic poles are sufficient in number to magnetically couple with the stacked circular magnets. The number of circular magnet/magnet array couplings may be configured to match the bearing load.
An alternate form of the centering bearing employs a plain bearing. An embodiment is designed in which the circular magnet is attached to the inside of a first cylinder or tube. The magnet arrays are attached on the outside of a second tube of larger diameter such that the first tube fits loosely within the second tube. The inner tube may then rotate within the outer tube. The inner tube functions as a mechanical journal or sleeve rotating in contact with the inner surface of the outer tube which functions as a housing.
An embodiment having a magnet array comprising two magnets and a magnet is also possible, and within the breadth of the claims. It is similar to the reluctance array except that the center magnet has been replaced by a piece of iron or other ferromagnetic material. This magnet serves to conduct magnetic flux between the two magnets so as to facilitate completion of a magnetic circuit within a magnetic bearing embodiment. The magnets in this and other embodiments may be permanent magnets or electromagnets.
While the presently disclosed inventive concept(s) is susceptible of various modifications and alternative constructions, certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the inventive concept(s) to the specific form disclosed, but, on the contrary, the presently disclosed and claimed inventive concept(s) is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the inventive concept(s) as defined in the claims.
In order that the invention may be more fully understood, it will now be described by way of example, with reference to the accompanying drawings. Magnetic field line arrows may be depicted as flowing from the north pole to the south pole. As per convention well known to those familiar with the art, a small circle with a central dot indicates a magnetic north, while an “x” designates magnetic south. The accompanying drawings illustrate variations within three of the multiple elements comprising the magnetic reluctance device: The circular magnet, the magnet arrays, and the centering bearing. Numerous combinations of variations of each element may be employed to create a magnetic circuit that resists perturbation and thereby supports an axial load in accordance with the principles of magnetic reluctance. A few of the variations are illustrated within the accompanying drawings.
In a preferred embodiment shown in
Directing or focusing the path of flux between the magnetic elements by use of magnet arrays such as the Halbach series or magnet array facilitates completion of a magnetic circuit and minimizes reluctance. The magnet arrays 128 in this embodiment focus magnetic flux asymmetrically so that north and south poles extend parallel to each other from the same side of the array. Magnet array 110 comprises three contiguous magnets 110a, 110b, and 110c, such that the center magnet 110b is rotated 90 degrees relative to the end magnets 110a and 110c, and these end magnets 110a and 110c are rotated 180 degrees relative to each other. This magnet array focuses magnetic flux so as to more directly complete a magnetic circuit with circular magnet 120.
Like the Halbach series, the north and south magnetic poles emanate from one side of the magnet array. Each magnet array in this embodiment is configured similarly to direct magnetic flux towards circular magnet 120. The south magnetic pole of magnet array 110 extends from magnet 110a towards the upper surface o120, coupling to its north magnetic pole. The north magnetic pole of magnet array 110 extends from magnet 110c towards the lower surface of circular magnet 120, coupling to its south magnetic pole. In this way a complete magnetic circuit is formed, and forces of magnetic reluctance seek to maintain this circuit.
One might conceive of having other embodiments have a plurality of magnet arrays, with each having at least one magnet, positioned to magnetically couple to magnet 120 so as to complete a magnetic circuit. The circular magnet 120 can be disk shaped, or ring shaped. In one embodiment the circular magnet is disposed circumferentially outside the cylinder of Halbach series such that the north magnetic pole of the circular magnet couples with each individual Halbach series north pole. The circular magnet sits on a plane that essentially bisects the north and south poles of the Halbach series, as shown in
The role of the magnet arrays 128 in the present invention is to bend and focus magnetic field lines 102 towards the circular magnet 120 so as to complete magnetic circuits by the most direct and magnetically permeable route. This implies a minimum of air gaps between magnetic elements within the circuit. Once formed, the complete magnetic circuit allows forces of magnetic reluctance to come into play.
In
The “pincher” effect is illustrated in
Additionally, since a downward force exerted on the shaft results in displacement, the device can also serve as a type of scale for weighing. In fact, the force/displacement curve is linear over most of the displacement, though it does depend on the specific arrangement of magnets.
The friction produced by the rotating flywheel is tunable. Experimentation has shown that if a 5 pound weight is set in motion at 300 rpm's, it takes about six minutes to stop rotating in one embodiment. If the weight of the flywheel is doubled to 10 pounds then set in motion at 300 RPMs, the shaft rotates for about 12 minutes. Friction between the shaft and the centering bearing decreases as the magnetic bearing approaches its load capacity. The source of friction is the centering bearing 124 holding the shaft in place.
A magnet array may take other forms employing various ways of directing and focusing magnetic flux.
One version of the device is a magnetic bearing for a rotating shaft, made up of a generally elongate shaft with a linear axis and configured for rotation around the linear axis with the shaft substantially held within a predetermined position on the linear axis by magnetic forces, wherein the predetermined position results in least magnetic reluctance. It includes a number of stationary magnet arrays surrounding said linear axis of the shaft, with each magnet array made up of at least three magnets, configured to form at least a first and a second zone of increased magnetic flux, wherein the first zone comprises a north magnetic pole and the second zone comprises a south magnetic pole. It also includes a circular magnet element attached to the shaft and held in place on the linear axis in the zone of least magnetic flux with the first and second zones of magnetic repulsion on either side of the circular magnet bearing; and a bearing for centering the shaft so as to maintain a predetermined gap distance between the one or more magnet arrays and the circular ferrous element; wherein the shaft is substantially held in a preselected position on the linear axis and in a predetermined point in a plane of rotation by magnetic forces between the magnet arrays and the circular magnet element.
The device described immediately above also includes a means of centering the shaft in the form of a bearing, such as an annular bearing.
The present invention is intended, among other uses, to serve as a small-scale alternative to a fluid thrust plate for use in impeller-driven applications.
While certain exemplary embodiments are shown in the figures and described in this disclosure, it is to be distinctly understood that the presently disclosed inventive concept(s) is not limited thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various changes may be made without departing from the spirit and scope of the disclosure as defined by the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/895,115, filed Oct. 24, 2013, the disclosure of which is incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61895115 | Oct 2013 | US |
