A magnetic levitation motor

Abstract

The utility model relates to a kind of magnetic suspension motor, comprising: stator assembly, stator assembly includes magnetically conducting shaft and the fixed part fixed coaxially to one end of magnetically conducting shaft;Rotor assembly, rotor assembly includes shell and the multiple magnets of the uniform arrangement along shell inner wall, the inner surface of multiple magnets is collectively enclosed with axial cavity, axial cavity at least one end has opening;The magnetically conducting shaft of stator assembly is at least partially located in axial cavity, and radial interval is formed between the outer circumferential surface of magnetically conducting shaft and the inner surface of multiple magnets;Fixed part constitutes the axial location of rotor assembly;By coaxially sleeving magnetically conducting shaft in the axial cavity surrounded by circumferentially uniformly arranged magnet, and magnetic suspension effect is formed using the radial interval therebetween, the non-contact motion of rotor assembly along magnetically conducting shaft axial is realized.In the entire motion stroke, more balanced magnetic suspension thrust can be output, and simultaneously, the motor completely relies on the magnetic field interaction between permanent magnet and magnetically conducting shaft to realize weight compensation.

Description

Technical Field

[0001] This utility model relates to the field of drive mechanism technology, specifically to a magnetic levitation motor. Background Technology

[0002] Weight-compensated motors, as functional components commonly used in mechanical equipment, automated devices, and precision motion systems, are primarily used to counteract or balance the weight of supported parts, thereby achieving lightweight, low-energy consumption, and high-precision motion control. Currently, common weight compensation methods include mechanical spring-type compensated motors, pneumatic compensated motors, and magnetic spring-type compensated motors. Among these, mechanical spring-type compensated motors are characterized by their simple structure and low cost; however, their output thrust is linearly related to the extension length, making it impossible to achieve a constant compensation force throughout the entire stroke range. This can easily lead to uneven force output, limiting their application in precision motion systems. Pneumatic compensated motors can provide relatively constant thrust compensation within a certain stroke range, and their performance is more stable than mechanical springs. However, these devices typically rely on external air sources and air circuit systems, resulting in complex structures and high maintenance requirements.

[0003] Furthermore, the size of the cylinder and piping limits its application in compact spaces or portable devices, making it particularly unsuitable for applications with strict requirements on installation space and weight. Magnetic spring-type compensated motors achieve contactless compensation through the magnetic repulsion or attraction between the magnet and the magnetically conductive element. Theoretically, they can achieve a relatively balanced thrust output without external energy input, offering advantages such as fast response and long lifespan. However, existing magnetic springs typically consist of magnets, stainless steel tubes, end caps, and magnetic rings. To achieve a certain thrust and stroke, their overall length is relatively long, resulting in a less compact structure and limiting their application in short-stroke or miniaturized devices. In addition, traditional magnetic springs still have room for improvement in terms of magnetic field distribution uniformity, force curve smoothness, and structural stability.

[0004] Existing weight-compensated motors have varying degrees of shortcomings in terms of force output balance, system compactness, and applicable environment. Therefore, it is necessary to study and improve the structure of such weight-compensated motors; to solve the above problems, the concept of this application is proposed. Utility Model Content

[0005] The purpose of this utility model is to overcome the shortcomings of the prior art. This application provides a magnetic levitation motor to solve the problem of achieving a compact structure and outputting relatively balanced thrust throughout the entire stroke range without the need for an air source.

[0006] To achieve the above objectives, this utility model provides the following technical solution: a magnetic levitation motor, comprising: a stator assembly, the stator assembly including a magnetic guide shaft and a fixing member coaxially fixed to one end of the magnetic guide shaft; a mover assembly, the mover assembly including a housing and a plurality of magnets uniformly arranged along the inner wall of the housing, the inner surfaces of the plurality of magnets together forming an axial cavity, the axial cavity having an opening at at least one end; the magnetic guide shaft of the stator assembly is at least partially located within the axial cavity, and a radial gap is formed between the outer peripheral surface of the magnetic guide shaft and the inner surfaces of the plurality of magnets; wherein, the fixing member constitutes an axial limit for the mover assembly.

[0007] By coaxially fitting the magnetically conductive shaft within an axial cavity formed by circumferentially uniformly arranged magnets, and utilizing the radial spacing between them to create magnetic levitation, non-contact movement of the mover assembly along the magnetically conductive shaft is achieved. This structural design allows the device to output a more balanced magnetic levitation thrust throughout its entire stroke, effectively overcoming the force imbalance problem caused by the linear relationship between the output force and stroke of traditional mechanical springs. Simultaneously, the device relies entirely on the magnetic field interaction between the permanent magnets and the magnetically conductive shaft for weight compensation, eliminating the need for an external air source or complex piping system, significantly simplifying the system structure and reducing usage and maintenance costs.

[0008] Preferably, the lateral surfaces of adjacent magnets are spaced apart to form at least two axially penetrating gaps, and the two gaps are arranged opposite each other. By setting the gaps, magnetic interference between adjacent magnets can be avoided, the linearity of the magnetic flux density distribution can be improved, and thus a more stable compensating force output can be achieved throughout the entire motion stroke.

[0009] Preferably, at least two strip-shaped positioning grooves are formed on the end wall of one axial end of the outer casing; each positioning groove extends radially along the outer casing and is symmetrically distributed; the position of each positioning groove corresponds one-to-one with the position of each gap and is connected. The function of the positioning grooves is to divide the axial cavity of the outer casing into two independent spaces by engaging positioning workpieces, such as positioning plates, with the two corresponding positioning grooves when installing multiple magnets, so as to accommodate the magnets separately. In this way, after the two magnets are assembled, two gaps are formed between them, and each gap corresponds to the position of the corresponding positioning groove, thereby facilitating the installation of the magnets.

[0010] Preferably, the axial length of the magnetic guide shaft within the axial cavity is less than the axial length of the axial cavity itself. This design ensures that the mover has sufficient compensation stroke under magnetic levitation, while avoiding mechanical contact between the magnetic guide shaft and the end of the housing, thus improving service life and flexibility.

[0011] Preferably, the fixing member has the same outer diameter as the outer shell and is coaxially arranged, with their outer peripheral surfaces forming a continuous and smooth outer peripheral surface. This structural design makes the overall appearance of the device neat, reduces airflow interference and frictional resistance, and facilitates the device's embedding or installation in compact spaces.

[0012] Preferably, the axial length of the outer shell is greater than the axial length of the fixing member to enhance its structural rigidity and support strength. Furthermore, this design improves the structural stability of the mover assembly during operation, reduces the risk of deformation, and thus further enhances motion accuracy and suspension balance.

[0013] Preferably, the outer peripheral wall of the magnetic guide shaft has at least two symmetrically arranged axial planes, the extension direction of which is parallel to the axis of the magnetic guide shaft. This design prevents the magnetic guide shaft from rotating during operation, thereby maintaining the stability of the magnetic field distribution and improving the symmetry and reliability of the device.

[0014] Preferably, each magnet faces the magnetic axis with its inner magnetic pole, and the polarity of the inner magnetic poles of each magnet is the same. This arrangement causes the magnetic field to generate a repulsive force in the radial direction, achieving magnetic levitation support and thus outputting a stable compensation force curve.

[0015] Preferably, the magnet is a permanent magnet, and its two surfaces facing and away from the magnetic guide shaft have magnetic poles of opposite polarity to form a magnetic field direction perpendicular to the axis of the magnetic guide shaft. This magnetization method causes the magnetic field lines to be concentrated in the radial direction, enhancing the magnetic levitation effect and improving the linearity and controllability of the compensation force output.

[0016] Preferably, the inner diameter of the axial cavity is larger than the outer diameter of the magnetic shaft, thereby achieving zero mechanical friction movement. This makes the movement of the mover assembly extremely smooth and sensitive, and without wear, greatly improving the service life and reliability of the device.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects: This magnetic levitation motor includes: a stator assembly comprising a magnetic shaft and a fixing member coaxially fixed to one end of the magnetic shaft; and a mover assembly comprising a housing and a plurality of magnets uniformly arranged along the inner wall of the housing, the inner surfaces of the plurality of magnets collectively forming an axial cavity, the axial cavity having an opening at at least one end; the magnetic shaft of the stator assembly is at least partially located within the axial cavity, and a radial gap is formed between the outer circumferential surface of the magnetic shaft and the inner surfaces of the plurality of magnets; wherein the fixing member constitutes an axial limit for the mover assembly. By coaxially fitting the magnetic shaft within the axial cavity formed by the circumferentially uniformly arranged magnets, and utilizing the radial gap between them to form a magnetic levitation effect, non-contact movement of the mover assembly along the axial direction of the magnetic shaft is achieved. This structural design enables the device to output a more balanced magnetic levitation thrust throughout the entire movement stroke, effectively overcoming the problem of unbalanced force output caused by the linear relationship between the output force and stroke of traditional mechanical springs. Meanwhile, the device relies entirely on the magnetic field interaction between the permanent magnet and the magnetic shaft to achieve weight compensation, without relying on an external air source or complex pipeline system, which significantly simplifies the system structure and reduces the cost of use and maintenance.

[0018] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall structure of this utility model; Figure 2 This is a cross-sectional structural diagram of the present invention; Figure 3 This utility model Figure 1 A magnified structural diagram of part A.

[0020] Reference numerals: 1. Stator assembly; 11. Magnetic shaft; 12. Fixing component; 2. Mover assembly; 21. Housing; 22. Magnet; 23. Axial cavity; 24. Gap; 25. Positioning groove; 26. Interval. Detailed Implementation

[0021] In this utility model, unless otherwise explicitly specified and limited, the terms "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Detachable installation methods are varied, such as through plug-in and snap-fit ​​connections, or through bolt connections, etc.

[0022] The present invention will now be described in more detail with reference to specific embodiments. However, the implementation of the present invention is not limited thereto. The embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. For process parameters or conditions not specifically specified, conventional techniques can be referred to.

[0023] Please see Figures 1-3 As shown, the technical solution adopted in this specific embodiment is: a magnetic levitation motor, comprising: a stator assembly 1, the stator assembly 1 including a magnetic shaft 11 and a fixing member 12 coaxially fixed to one end of the magnetic shaft 11; a mover assembly 2, the mover assembly 2 including a housing 21 and a plurality of magnets 22 uniformly arranged along the inner wall of the housing 21, the inner surfaces of the plurality of magnets 22 together forming an axial cavity 23, the axial cavity 23 having an opening at at least one end; the magnetic shaft 11 of the stator assembly 1 is at least partially located within the axial cavity 23, and a radial spacing 26 is formed between the outer peripheral surface of the magnetic shaft 11 and the inner surfaces of the plurality of magnets 22; wherein, the fixing member 12 constitutes an axial limit for the mover assembly 2, and enables the mover assembly 2 to move axially along the magnetic shaft 11.

[0024] In this embodiment, the magnetic levitation motor mainly comprises two parts: a stator assembly 1 and a mover assembly 2, both coaxially arranged along the same central axis. The stator assembly 1 includes a magnetic shaft 11 and a fixing member 12 coaxially fixed to one end of the magnetic shaft 11. Preferably, the magnetic shaft 11 is made of a high-permeability material, such as industrial pure iron or silicon steel, to form an efficient magnetic circuit channel. The fixing member 12 can be connected to the magnetic shaft 11 by interference fit, threaded connection, or welding, and its function is to serve as the mounting reference for the entire stator assembly 1. In addition, the fixing member 12 may be provided with mounting holes to fix it to an external support structure by bolts. The mover assembly 2 includes a housing 21 and a plurality of magnets 22 evenly arranged along the inner wall of the housing 21. Preferably, the housing 21 is a housing with an opening at the end, and the housing 21 may be made of a non-magnetic material, such as aluminum alloy or austenitic stainless steel, to avoid forming additional magnetic short circuits. The inner surfaces of the plurality of magnets 22 together form an axial cavity 23, at least one end of which has an opening designed to allow the magnetic shaft 11 of the stator assembly 1 to extend into it. The magnetic shaft 11 of the stator assembly 1 is at least partially located within the axial cavity 23, and a radial gap 26 is formed between the outer circumferential surface of the magnetic shaft 11 and the inner surfaces of the plurality of magnets 22. When the plurality of magnets 22 of the mover assembly 2 generate a magnetic field, this magnetic field interacts with the magnetic shaft 11 made of highly permeable material, generating a radial magnetic levitation force within the radial gap 26 region. This magnetic levitation force allows the mover assembly 2 to levitate on the magnetic shaft 11 of the stator assembly 1, thereby achieving mechanical contactless support. The fixing member 12 constitutes an axial limit for the mover assembly 2; the fixing member 12 is located at the end of the axial movement path of the mover assembly 2. When the moving part 2 moves along the axial direction of the magnetic shaft 11 and approaches the fixing member 12, the end face of the housing 21 of the moving part 2 will make mechanical contact with the fixing member 12, thereby preventing the moving part 2 from moving further in that direction.

[0025] During assembly, the axial cavity 23 of the mover assembly 2 is sleeved onto the magnetic shaft 11, maintaining a predetermined radial distance 26 between them to ensure uniform magnetic flux density distribution and form magnetic levitation force. Through the magnetic field interaction between the magnet 22 and the magnetic shaft 11, a stable compensating force can be output without any air source or mechanical spring. Furthermore, through this design, the magnetic levitation force between the magnet 22 and the magnetic shaft 11 achieves weight compensation, enabling a more balanced thrust output throughout the entire stroke range, eliminating the need for an air source or air tubing, thus simplifying the structure. Simultaneously, compared to traditional magnetic springs, its overall length is shorter, resulting in a more compact structure under the same stroke and thrust conditions. It should be noted that in an alternative solution, the magnetic shaft 11 can be a hollow structure to reduce weight or accommodate sensor cables. Preferably, the housing 21 can be made of non-magnetic materials, such as aluminum alloy or stainless steel, to avoid magnetic field interference. Additionally, a guide slip ring can be provided on the outer periphery of the housing 21 of the mover assembly 2 to further improve guiding accuracy and motion smoothness.

[0026] The lateral surfaces of adjacent magnets 22 are spaced 26 apart to form at least two axially penetrating gaps 24, and the two gaps 24 are arranged opposite each other. In this embodiment, a plurality of magnets 22 are arranged circumferentially along the inner side of the housing 21, and a small gap 24 is formed between adjacent magnets 22. The gap 24 is used to prevent direct magnetic flux coupling or magnetic saturation between magnets 22, ensuring that each magnet 22 forms an independent magnetic field distribution, making the overall magnetic field distribution along the magnetic guide axis 11 more uniform. However, by setting the gap 24, magnetic interference between adjacent magnets 22 can be avoided, and the linearity of the magnetic flux density distribution can be improved, thereby achieving a more stable compensation force output throughout the entire movement stroke. It should be noted that, preferably, the width of the gap 24 can be determined according to the size of the magnet 22, and can be 0.2mm to 2mm; in an alternative solution, a non-magnetic spacer can be set between adjacent magnets 22 to maintain equidistant gaps 24 during assembly and enhance structural stability.

[0027] At least two strip-shaped positioning grooves 25 are formed on the end wall of one axial end of the outer casing 21. Each positioning groove 25 extends radially along the outer casing 21 and is symmetrically distributed. The position of each positioning groove 25 corresponds one-to-one with the position of each gap 24 and is connected to it. In this embodiment, the function of the positioning grooves 25 is to divide the axial cavity 23 of the outer casing 21 into two independent spaces to accommodate the magnets 22 when installing multiple magnets 22, by engaging a positioning workpiece, such as a positioning plate, with the two corresponding positioning grooves 25. In this way, after the two magnets 22 are assembled, two gaps 24 are formed between them, and each gap 24 corresponds to the position of the corresponding positioning groove 25, thereby facilitating the installation of the magnets 22. In addition, this structural design is not only applicable to the case of two magnets 22, but can also be extended to the configuration of multiple positioning grooves 25 and multiple magnets 22, for example, arranging three or more magnets 22 in the axial cavity 23 of the outer casing 21.

[0028] The axial length of the magnetic guide shaft 11 within the axial cavity 23 is less than the axial length of the axial cavity 23. In this embodiment, the insertion length of the magnetic guide shaft 11 is less than the total length of the axial cavity 23, allowing the mover assembly 2 to have a certain travel space on the magnetic guide shaft 11, enabling it to move freely up and down. However, this design ensures that the mover has sufficient compensation travel under magnetic levitation, while avoiding mechanical contact between the magnetic guide shaft 11 and the end of the housing 21, thus improving service life and flexibility. It should be noted that different compensation force curves can be achieved by adjusting the length of the magnetic guide shaft 11 or the distribution of the magnets 22.

[0029] The fastener 12 has the same outer diameter as the outer shell 21 and is coaxially arranged, with their outer peripheral surfaces forming a continuous and smooth outer peripheral surface. In this embodiment, the fastener 12 and the outer shell 21 have the same outer diameter and are installed along the same central axis. After assembly, their outer peripheral surfaces form a smooth transition surface without any steps. This design makes the overall appearance of the device neat, reduces airflow interference and frictional resistance, and facilitates embedding or installation in compact spaces. It should be noted that, preferably, a chamfer or rounded transition can be used at the connection between the fastener 12 and the outer shell 21 to further improve assembly accuracy and appearance continuity.

[0030] The axial length of the outer shell 21 is greater than the axial length of the fixing member 12. In this embodiment, the thickness of the outer shell 21 along the axial direction is designed to be greater than the thickness of the fixing member 12 to enhance its structural rigidity and support strength. However, this design can improve the structural stability of the mover assembly 2 during operation, reduce the risk of deformation, and thus further improve motion accuracy and suspension balance.

[0031] The outer peripheral wall of the magnetic guide shaft 11 is provided with at least two symmetrically arranged axial planes, the extension direction of which is parallel to the axis of the magnetic guide shaft 11. In this embodiment, the outer contour of the magnetic guide shaft 11 is designed as a structure with symmetrical axial planes, such as a double-plane cylinder. The axial plane portion is used to cooperate with the axial cavity 23 to restrict the rotational degree of freedom of the magnetic guide shaft 11. However, this design can prevent the magnetic guide shaft 11 from rotating during operation, thereby maintaining the stability of the magnetic field distribution and improving the symmetry and reliability of the device. It should be noted that, preferably, four symmetrical planes can be provided to achieve higher guiding accuracy.

[0032] Each magnet 22 faces the magnetic axis 11 with its inner magnetic poles, and the polarity of the inner magnetic poles of each magnet 22 is the same. In this embodiment, the inner magnetic poles of multiple magnets 22 all face the magnetic axis 11, and their polarities are consistent. This forms a ring-shaped magnetic field distribution with the same polarity around the magnetic axis 11. However, this arrangement causes the magnetic field to generate a repulsive force in the radial direction, achieving magnetic levitation support and thus outputting a stable compensation force curve. Preferably, the magnets 22 can be neodymium iron boron permanent magnets 22 to obtain a higher magnetic energy product; an alternative is that the outer magnetic poles can be fitted with magnetic sleeves to adjust the magnetic field strength distribution.

[0033] The magnet 22 is a permanent magnet, and its two surfaces facing and away from the magnetic guide shaft 11 have opposite magnetic poles to form a magnetic field direction perpendicular to the axis of the magnetic guide shaft 11. In this embodiment, the magnet 22 is a permanent magnet, such as N-grade neodymium iron boron, and its magnetization direction is perpendicular to the axis of the magnetic guide shaft 11. The inner side of the magnet 22 has an N pole, and the outer side has an S pole, forming a stable magnetic field in the radial direction. However, this magnetization method causes the magnetic field lines to be concentrated in the radial direction, enhancing the magnetic levitation effect and improving the linearity and controllability of the compensation force output. It should be noted that, in an alternative solution, the magnetization direction of the magnet 22 can be changed to an oblique radial direction to achieve a special force curve output; preferably, a magnetic guide ring can be added to the outside of the magnet 22 to further concentrate the magnetic flux and improve the magnetic efficiency.

[0034] The inner diameter of the axial cavity 23 is larger than the outer diameter of the magnetic shaft 11, thereby achieving zero-friction movement. This makes the movement of the mover assembly 2 extremely smooth and sensitive, without wear, greatly improving the service life and reliability of the device. Preferably, the dimensional difference between the inner diameter and the outer diameter, i.e., the width of the radial interval 26, remains consistent along the entire axial length of the magnetic shaft 11 to ensure the uniformity of the magnetic field distribution and magnetic levitation force. An alternative solution is that the width of the radial interval 26 can be optimized based on the simulation results of the magnetic field line distribution, using a non-uniform width at different axial positions to achieve a specific force curve output.

[0035] During operation, the stator assembly 1 remains stationary, while the mover assembly 2 can slide freely along the magnetic shaft 11. When the mover is subjected to external gravity, the interaction between the magnet 22 and the magnetic shaft 11 generates a magnetic levitation force to offset part or all of the gravity, achieving constant force weight compensation. By rationally designing the distribution of the magnets 22 and the magnetic field strength, the output force can be kept balanced within a certain stroke range. Compared with traditional magnetic springs, this device significantly shortens the overall length under the same stroke and thrust conditions, achieving a compact structure, convenient installation, and simplified maintenance.

[0036] The foregoing, in conjunction with the embodiments and accompanying drawings, has clearly and completely described the concept, specific structure, and technical effects of this utility model, so as to fully understand the purpose, features, and effects of this utility model. Obviously, the described embodiments are only a part of the embodiments of this utility model, not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of this utility model without creative effort are all within the scope of protection of this utility model. Furthermore, all connections / linkages mentioned herein do not simply refer to direct contact between components, but rather to the ability to form a better connection structure by adding or reducing connecting accessories according to specific implementation conditions.

[0037] The above description of the specific embodiments of this utility model is only used to further illustrate this utility model and should not be construed as limiting the scope of protection of this utility model. Any non-essential improvements and adjustments made to this utility model by technical engineers based on the above description of the utility model shall fall within the scope of protection of this utility model.

Claims

1. A magnetic levitation motor, characterized in that, include: A stator assembly, the stator assembly including a magnetic shaft and a fastener coaxially fixed to one end of the magnetic shaft; A mover assembly, the mover assembly including a housing and a plurality of magnets uniformly arranged along the inner wall of the housing, the inner surfaces of the plurality of magnets together forming an axial cavity, the axial cavity having an opening at at least one end; The magnetic shaft of the stator assembly is at least partially located within the axial cavity, and a radial gap is formed between the outer peripheral surface of the magnetic shaft and the inner surfaces of the plurality of magnets; wherein the fixing member constitutes an axial limit for the mover assembly.

2. The magnetic levitation motor according to claim 1, characterized in that, The lateral surfaces of adjacent magnets are spaced apart to form at least two axially penetrating gaps, and the two gaps are arranged opposite each other.

3. The magnetic levitation motor according to claim 2, characterized in that, At least two strip-shaped positioning grooves are provided on the end wall of one axial end of the outer shell; each positioning groove extends radially along the outer shell and is symmetrically distributed; the position of each positioning groove corresponds one-to-one with the position of each gap and is connected.

4. The magnetic levitation motor according to claim 1, characterized in that, The axial length of the magnetic guide shaft within the axial cavity is less than the axial length of the axial cavity itself.

5. The magnetic levitation motor according to claim 1, characterized in that, The fastener has the same outer diameter as the outer shell and is coaxially arranged, and the outer peripheral surfaces of the two together form a continuous and smooth outer peripheral surface.

6. The magnetic levitation motor according to claim 1, characterized in that, The axial length of the outer casing is greater than the axial length of the fastener.

7. The magnetic levitation motor according to claim 1, characterized in that, The outer peripheral wall of the magnetic shaft is provided with at least two symmetrically arranged axial planes, and the extension direction of the axial planes is parallel to the axis of the magnetic shaft.

8. The magnetic levitation motor according to claim 1, characterized in that, Each of the magnets faces the magnetic axis with its inner magnetic pole facing it, and the polarity of the inner magnetic poles of each magnet is the same.

9. The magnetic levitation motor according to claim 1, characterized in that, The magnet is a permanent magnet, and its two surfaces facing and away from the magnetic guide shaft are magnetic poles with opposite polarities to form a magnetic field direction perpendicular to the axis of the magnetic guide shaft.

10. The magnetic levitation motor according to claim 1, characterized in that, The inner diameter of the axial cavity is larger than the outer diameter of the magnetic shaft.

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