Vacuum pump and permanent magnetic bearing assembly for vacuum pump

Abstract

A passive magnetic bearing assembly for a vacuum pump and a vacuum pump having such a bearing are disclosed. The passive magnetic bearing assembly has a plurality of pairs of ring magnets arranged in a stack, where one pair of ring magnets is located on the stator side of the bearing and the other magnet of the pair is located on the rotor side. The ring magnets are radially magnetized, and each rotor ring magnet contains at least a first harmonic of magnetic inhomogeneity, the magnetic inhomogeneity having a zero peak variation of residual magnetism ΔB i and average residual magnetism B R0 It is measured as a ratio between and , and its value is less than 3%.

Description

[Technical Field]

[0001] The field of this invention relates to vacuum pumps and permanent magnetic bearing assemblies for such pumps. [Background technology]

[0002] Vacuum pumps, such as turbomolecular pumps, may have rotors mounted on magnetic bearings to reduce friction and contamination from lubricating oil.

[0003] Generally, the permanent magnetic bearings (PMBs) in these pumps are made using axially magnetized arrays. Figure 1 schematically shows such a bearing. In some applications, such as scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), and ion traps, stray magnetic fields generated from magnetic bearings can be problematic. There are two types of stray magnetic fields in magnetic bearings: AC stray fields, which are time-varying magnetic fields generated during rotation, mainly due to magnetization errors in the bearing's magnets, and DC stray fields, which are time-invariant intrinsic magnetic fields generated by the magnets. Figure 2 schematically shows the AC and DC magnetic fields of a magnetic bearing. Figure 3 shows two types of magnetization errors, global skew and non-uniformity, in the case of axially magnetized ring magnets.

[0004] Traditionally, AC stray magnetic fields have been reduced by selectively aligning magnetic bearings (see, for example, EP2705263) or by correcting the magnetization error of each magnet. Such techniques are time-consuming and can lead to increased costs. While these techniques address the problem of AC stray magnetic fields, axially magnetized magnets, and by extension, PMBs constructed using them, inherently possess high DC stray magnetic fields, which are more difficult to address.

[0005] It is desirable to provide PMBs that have both low DC and AC stray magnetic fields. Furthermore, a design that does not require selective assembly / alignment or rectification of magnetization errors would be advantageous. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] European Patent EP2705263 Specification [Overview of the project]

[0007] One embodiment provides a passive magnetic bearing assembly for a vacuum pump, comprising a plurality of pairs of ring magnets arranged in a stack, wherein one pair of ring magnets is positioned on the stator side of the bearing and the other ring magnet of the pair is positioned on the rotor side, the ring magnets are radially magnetized, and each ring magnet on the rotor side contains at least a first harmonic of magnetic inhomogeneity, the inhomogeneity being measured as the ratio of the 0 peak variation ΔBi of the residual magnetism to the mean residual magnetism BR0, and the first harmonic of magnetic inhomogeneity is less than 3% in at least the majority of the ring magnets on the rotor side, and preferably all of these ring magnets.

[0008] In some applications of magnetic bearings, a low stray magnetic field is important. The DC stray magnetic field is the intrinsic magnetic field created by a magnet in the surrounding space. The inventors have recognized that radially magnetized ring magnets inevitably have a much lower DC stray magnetic field than axially magnetized ring magnets. Furthermore, it has been recognized that a pair of radially magnetized ring magnets for the stator and rotor, which are the constituent units of a radially magnetized PMB, has an even lower DC stray magnetic field compared to the same pair constructed with axially magnetized ring magnets. However, the AC stray magnetic field is more difficult to control in radially magnetized bearings. Processes such as selective alignment are particularly difficult with such magnets due to the attractive forces between adjacent magnets. The inventors have recognized that radially magnetized magnets not only have a low DC stray magnetic field, but the AC stray magnetic field can also be controlled if a magnet with low non-uniformity is used on the rotor side. Furthermore, the inventors recognized that the sensitivity of the AC stray magnetic field to non-uniformity also depends on the shape of the non-uniformity around the circumference of the magnet, being maximum at the first harmonic and decreasing sharply at higher harmonics. This is particularly pronounced when the magnetic field is measured at a distance from the magnet, such as outside the stator of a vacuum pump, where achieving a low AC stray magnetic field is particularly important.

[0009] The non-uniformity of a magnet is due to the residual magnetism B in its volume. R Caused by fluctuations in the magnetic field, residual magnetism is a property of magnets, and it is the magnetic flux density that remains after the external magnetic field is removed.

[0010] Non-uniformity is due to the average remanent magnetism B R0 The variation of the zero peak in residual magnetism relative to ΔB i It can be defined as the ratio of . Non-uniformity can be determined by measuring the radial magnetic flux density of the magnetic field generated by the magnet in the space surrounding the magnet. The average remanent magnetism is determined by averaging the radial magnetic flux density measured at the same set distance from the magnet to the outer circumference after removing the background magnetic field, such as the Earth's magnetic field.

[0011] By measuring the magnitude of the radial magnetic flux density for any given harmonic, the corresponding non-uniform harmonic can be determined. Therefore, a rotor-side magnet with low magnetic non-uniformity in the first and second harmonics is selected, resulting in low AC and DC stray magnetic fields.

[0012] Highly uniform magnets are becoming increasingly available, and providing a magnetic bearing with radially magnetized magnets, where the rotor magnet is a highly uniformly radially magnetized magnet, is an effective way to reduce both AC and DC stray magnetic fields while providing an effective magnetic bearing.

[0013] In some embodiments, the stator magnets may also be highly uniformly magnetized radially, and in some embodiments, they may have the same or similar non-uniformity requirements as the rotor magnets. While AC stray magnetic field problems occur only with rotating magnets, high uniformity of the bearing's stator magnets can be advantageous, for example, in reducing bearing eccentricity caused by magnetization errors.

[0014] An effective bearing can be provided if the non-uniformity of the first harmonic is less than 3%, but it is preferable that both the first and second harmonics are less than 3% for at least the majority, preferably all, of the ring magnets on the rotor side.

[0015] In some embodiments, it is possible to provide first and possibly second harmonics of less than 2% or preferably less than 1% for at least a large portion, preferably all, of the ring magnets on the rotor side.

[0016] In some embodiments, at least a portion of the ring magnets include NdFeB magnets. In some embodiments, all of the rotor-side ring magnets, and in some embodiments, all of the magnets, include NdFeB magnets.

[0017] NdFeB magnets are high-performance and have high uniformity, so they can be particularly effective magnets for radially magnetized passive magnetic bearing assemblies. NdFeB magnets can contain elements other than Nd, Fe, and B. For example, praseodymium can be added as an alternative to neodymium. Other elements can include one or more of Dy, Ga, Co, Al, and Cu.

[0018] In some embodiments, at least a part of the ring magnet includes a hot-worked NdFeB magnet. In some embodiments, all of the rotor ring magnets, and optionally all of the stator ring magnets, also include hot-worked neodymium magnets.

[0019] Some hot-worked NdFeB magnets can provide particularly uniform magnets and are thus particularly effective in providing a low AC magnetic field.

[0020] In the case of neodymium magnets magnetized radially by hot working, the attractive force is high and assembly, especially selective alignment, is difficult. However, by using a non-magnetic shim between the magnets, the attractive force can be reduced while only slightly impairing the rigidity of the bearing to a negligible extent for the required thickness.

[0021] Magnets magnetized radially can be made of, for example, plastic bonded magnets of Smco, NdFeB, SmFeN, or mixtures thereof.

[0022] In some embodiments, at least a part of the ring magnet includes a heavy rare earth-free ("HREE-free") hot-worked NdFeB having a minimum intrinsic coercive force > 1400 kA / m at 20°C.

[0023] High-temperature emission enrichment (HREE) elements such as dysprosium (and / or other elements like terbium) are often used in magnetic materials to retain magnetism and, consequently, improve high-temperature performance. Hot-worked NdFeB magnets can have high coercivity even without HREE elements like dysprosium, making them a particularly good option for some PMB applications. Because HREE elements are expensive, not readily available, and may require ethical considerations in their procurement, it is advantageous to find magnets that are dysprosium-free or have reduced dysprosium content while still maintaining reasonable coercivity.

[0024] In some embodiments, at least a portion of the ring magnet comprises hot-worked NdFeB having an HREE content of less than 3%, the HREE being non-grain boundary diffused within the magnet, and the ring magnet having a minimum intrinsic coercivity at 20°C greater than 1700 kA / m. In some embodiments, when gain boundary diffusion of HREE is used, the HREE content is less than 1.5%, and the minimum intrinsic coercivity at 20°C greater than 1700 kA / m.

[0025] In other embodiments, magnets with a lower HREE content may be used, which have higher coercivity and may be more effective in certain applications. The grain boundary diffusion properties of HREE have been shown to provide effective thermal protection by reducing the amount of HREE present.

[0026] In other embodiments, the magnet may include a PrFeB magnet.

[0027] In some embodiments, the magnets include plastic bonded magnets. Radially magnetized magnets can be made from plastic bonded magnets, such as Smco, NdFeB, SmFeN, or mixtures thereof.

[0028] Another method for forming magnets with low heterogeneity is to form them using plastic bonding. Plastic-bonded magnets can be manufactured to have desired properties by controlling the manufacturing process, and can provide magnets that are particularly applicable for forming radially magnetized passive magnetic bearings with low heterogeneity.

[0029] In some embodiments, the bearing assembly further includes non-magnetic shims, which are arranged to axially separate the ring magnets in the stack.

[0030] As mentioned above, radially magnetized magnets attract each other, making it difficult to selectively align them to further reduce the AC magnetic field. While the AC magnetic field can be reduced to some extent by using magnets with low heterogeneity, this can be further reduced by selective alignment. Adjacent radially magnetized magnets strongly attract each other, making alignment difficult. This is particularly problematic with hot-worked, radially magnetized NdFeB magnets, where the attractive force is strong, making assembly, especially selective alignment, more difficult. However, by placing non-magnetic shims, defined as materials with a relative permeability of less than 1.1, between the magnets, the attractive force can be reduced with only negligible / relatively low losses of bearing stiffness for the required thickness.

[0031] In some embodiments, shims having a low coefficient of friction on at least a portion of their outer surfaces are preferred to facilitate angular alignment between the magnets.

[0032] In some embodiments, the shim is made of a polymer or fiber-reinforced polymer, such as PEEK or Torlon.

[0033] In some cases, a low coefficient of friction can be provided by the material of the shim, and in other cases, by a low-friction coating.

[0034] A further embodiment provides a vacuum pump including a passive magnetic bearing according to the first embodiment, the vacuum pump including a rotor rotatably mounted in a stator, each pair of ring magnets mounted on the rotor and the other ring magnet of the pair mounted on the stator.

[0035] In some embodiments, the vacuum pump includes a turbomolecular pump.

[0036] In some embodiments, the rotor ring magnets are selectively aligned to reduce the stray AC magnetic field.

[0037] In some embodiments, the ring magnet is selectively oriented vertically to reduce the stray DC magnetic field.

[0038] In addition to reducing the stray AC magnetic field by selectively aligning the rotor ring magnets, the stray DC magnetic field can also be addressed by selectively oriented the stator ring magnets and rotor ring magnets vertically. Magnets with undesirable components of low axial magnetization can be positioned in opposite directions so that their axial magnetic fields point in different directions, thereby reducing the overall magnetically stray DC magnetic field.

[0039] In some embodiments, magnets can be selected such that the undesirable axial magnetization components are at similar levels, so that the overall sum of the DC magnetic field is particularly low due to appropriate orientation.

[0040] Another embodiment provides a method for manufacturing a passive magnetic bearing assembly according to the first embodiment, the method comprising measuring the non-uniformity of at least first harmonics of a plurality of radially magnetized rotor ring magnets, the measurement being performed by determining the radial component of the magnetic flux density harmonics at a set distance from the rotor ring magnets measured at different points around the circumference; calculating the non-uniformity of at least first harmonics by dividing the zero-peak amplitude of the magnetic flux harmonics by the average magnetic flux; selecting rotor ring magnets for at least a majority, preferably all, of the rotor ring magnets such that the calculated non-uniformity of at least first harmonics is below a threshold; and stacking the selected rotor ring magnets with a corresponding set of stator ring magnets to form the passive magnetic bearing assembly.

[0041] In some embodiments, the first and second harmonics are measured, and it is desirable that the heterogeneity of both the first and second harmonics is below a threshold.

[0042] In some embodiments, the multiple radially magnetized rotor ring magnets on which the non-uniformity measurement step is performed are a subset of a batch of ring magnets used to form a PMB, and the resulting statistical analysis is used to determine whether further rotor ring magnets should be tested from the batch, or whether the entire batch of ring magnets can be selected to form a passive magnetic bearing assembly.

[0043] While rotor ring magnets contribute to the AC stray magnetic field, it can be advantageous to adapt stator ring magnets to rotor ring magnets, and therefore, it may be advantageous to test and select them using the same method.

[0044] In some embodiments, the stacking step includes separating the ring magnets in the stack with a non-magnetic shim.

[0045] In some embodiments, the step of determining the magnetic flux includes determining the amplitude and relative phase of different harmonic components of the magnetic flux density, and the step of stacking includes aligning the ring magnets such that the total variation of the magnetic flux around the circumference for the stack of the magnets is reduced for different harmonics, for example, first and second harmonics.

[0046] The total variation of the circumferential magnetic flux described above can include the vector sum of the magnetic flux densities for different harmonics.

[0047] In some embodiments, the magnets forming the array are selected according to the magnitude of their non-uniform harmonics (and / or measured magnetic flux density) so that they compensate for each other when properly aligned and provide an extremely low AC stray magnetic field.

[0048] In some embodiments, the method further includes determining the unintended axial component of the magnetization of each ring magnet by measuring the axial DC magnetic field at a set distance from the ring magnets, and the step of stacking the magnets includes oriented the ring magnets in the stacked state to reduce the overall axial DC magnetic field of the magnetic bearing assembly. In some embodiments, the magnets forming the array are also selected according to the magnitude of the unintended axial component of the magnetization so that they substantially cancel each other out when the axial components are properly oriented to provide an extremely low DC stray magnetic field.

[0049] In some embodiments, instead of determining the unintended axial magnetization component for all magnets, only a portion of the ring magnets are measured for this property. These measurements are used in a statistical quality analysis to confirm the high uniformity and low axial magnetization component of the ring magnets, and more measurements are taken only if the statistical analysis indicates that the magnets are of lower quality than expected.

[0050] In some embodiments, the stray magnetic field of the PMB, particularly the AC stray magnetic field, is tested after assembly or after full assembly of the PMB, rotating half of the pump while the pump or PMB is rotating at a predetermined speed, to determine whether the stray magnetic field is below a predetermined threshold. In some cases, all pumps / PMBs are tested, or only some pumps / PMBs are tested, and the results are statistically analyzed to determine whether the pumps meet the desired requirements or whether further testing or improvements are needed.

[0051] Further specific preferred embodiments are described in the attached independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and may be combined in combinations other than those expressly described in the claims.

[0052] If a device feature is described as being capable of operating to provide a function, it should be understood that this description includes the device features that provide, or are adapted or configured to provide, such function.

[0053] Next, embodiments of the present invention will be described further with reference to the accompanying drawings. [Brief explanation of the drawing]

[0054] [Figure 1] This diagram schematically shows a passive magnetic bearing assembly magnetized in the axial direction. [Figure 2] This diagram schematically shows the AC and DC stray magnetic fields generated from a PMB magnetized in the direction of rotation. [Figure 3] This diagram schematically illustrates the global skew and non-uniformity of a ring magnet magnetized in the axial direction. [Figure 4] This figure schematically shows a magnetized passive magnetic bearing assembly according to one embodiment. [Figure 5] This diagram schematically shows the DC magnetic field radiated by a ring magnet magnetized radially and axially over a radial distance of 100 mm. [Figure 6]This diagram schematically illustrates the global skew and non-uniformity of a radially magnetized ring magnet. [Figure 7] This figure shows examples of first and second heterogeneous harmonics in a ring magnet. [Figure 8] This diagram schematically shows a part of a vacuum pump according to one embodiment. [Figure 9] This figure schematically shows a part of a magnetic bearing assembly according to one embodiment. [Figure 10] This figure shows the axial stiffness of a bearing for different shim thicknesses according to one embodiment. [Figure 11] This is a flowchart illustrating the steps of a method according to one embodiment. [Modes for carrying out the invention]

[0055] Before examining the embodiments in more detail, let's first provide an overview.

[0056] Passive magnetic bearings for vacuum pumps are conventionally made using axially magnetized ring magnets. Advantages of axially magnetized ring magnets include ease of magnetization assembly and the possibility of using multiple magnet formation materials. Disadvantages include the inherently high DC stray magnetic field and the need for selective alignment and correction to reduce the AC stray magnetic field.

[0057] Radially magnetized arrays are more difficult to magnetize and assemble, have fewer materials available for manufacturing, and possess comparable rigidity to arrays magnetized in the corresponding axial direction. However, DC stray magnetic fields are inherently low. Low DC stray magnetic fields are particularly important in some applications, such as ion traps and TEMs.

[0058] Bearings made with axially magnetized magnetic rings are extremely sensitive to skew but less sensitive to non-uniformity, while bearings made with radially magnetized magnetic rings are far more sensitive to a lack of uniformity. The AC magnetic field can be reduced by selectively arranging different rings. However, this is more difficult with radially aligned bearings than with axially aligned bearings because the rings attract each other.

[0059] Passive magnetic bearings using radially magnetized magnet arrays can reduce leakage time-invariant magnetic fields or DC stray magnetic fields. Furthermore, if rotor magnets with at least the required level of uniformity are used, time-varying or AC stray magnetic fields generated during rotation can also be reduced. By using radially magnetized magnets manufactured by a hot-working process from high coercivity HREE or dysprosium (Dy')-free or low HREE or Dy NdFeB magnetic alloys, a material with an extremely fine crystalline structure that is extremely uniformly oriented in the radial direction is obtained, resulting in extremely uniform magnetization and extremely low AC stray magnetic fields.

[0060] As a further method to reduce AC magnetic fields, selective alignment of magnets can be used so that the AC magnetic field from one magnet compensates for the AC magnetic field from another. In radially magnetized arrays, this can be problematic because a strong attractive force acts between adjacent magnets. By using non-magnetic shims interposed between adjacent layers of magnet pairs, the assembly and selective alignment of the magnets can be facilitated, and the attractive force can be reduced. Magnets can be selected with a similar degree of heterogeneity so that compensation between magnets with proper alignment is effectively achieved.

[0061] Figure 4 schematically shows a radially magnetized PMB according to one embodiment. It shows a cross-section passing through one side of the bearing assembly, showing the stator side of the bearing assembly 20 and the rotor side of the bearing assembly 40. The rotor side is composed of a stack of ring magnets 24, and the rotor side is composed of a stack of ring magnets 34. The direction of magnetization is indicated by the arrow.

[0062] Figure 5 schematically shows the DC magnetic fields radiated at a distance of 100 mm by a radially magnetized ring magnet and an axially magnetized ring magnet. It can be seen that the DC magnetic field of the axially magnetized ring magnet is much higher than that of the radially magnetized ring magnet. As mentioned above, the DC magnetic field is the intrinsic magnetic field of the space surrounding the magnet that does not change over time, while the AC magnetic field is due to magnetization errors. Therefore, if a radial rotor magnet with low non-uniformity is supplied, it is possible to create a PMB with low AC magnetic field and DC stray magnetic field.

[0063] Figure 6 schematically shows the global skew and non-uniformity of a radially magnetized ring magnet.

[0064] Figure 7 shows the first and second harmonics of the non-uniformity in the ring magnet. The residual magnetism of both harmonics is plotted against the circumferential angle of the ring. Both harmonics have a peak amplitude ΔB. i While this is 0.1T, the average residual magnetism B R o is 1.35T, and the non-uniformity is 0.1 / 1.35 = 0.074 (7.4%). Therefore, such a magnet is not suitable for the rotor of the PMB according to the embodiment.

[0065] Figure 8 schematically shows a vacuum pump including a passive magnetic bearing 10 according to an embodiment. The PMB 10 includes a stack of rotor magnets 30 and a stack of stator magnets 20 that form both sides of the passive magnetic bearing. The stator bearing magnets 20 are mounted on the stator 4, and the rotor bearing magnets are mounted on the rotor portion with the shaft 2. The pump further includes a motor 1 mounted on the shaft 2 to rotate the shaft 2.

[0066] Figure 9 schematically shows a cross-section through a stack of ring magnets 34 that may form the rotor portion 30 or stator portion of a passive magnetic bearing. In this embodiment, there are non-magnetic shims 32 between each magnet 34. This makes it easier to stack, rotate, and selectively align the magnets, and further reduce the AC magnetic field as needed. One potential problem with shims is that they may reduce the axial stiffness of the bearing. In some embodiments, these shims are made of plastic material, but in other embodiments they may be non-magnetic metal. Plastic material is easy to manufacture, has a low coefficient of friction, and is easy to align, but may not be as stiff or robust as metal shims. A preferred plastic material is fiber-reinforced PEEK.

[0067] Figure 10 shows the axial stiffness for different shim thicknesses, illustrating how the axial stiffness decreases with increasing shim thickness. It can be seen that for smaller shims, there is no significant difference in the axial stiffness of the magnet array.

[0068] Figure 11 shows the steps of a method according to one embodiment. In the first step S10, the magnetic flux at different points on the circumference of multiple ring magnets and at a set distance is determined. In step S20, the average magnetic flux is obtained from these measurements. In step S30, the first and second harmonic non-uniformities of different ring magnets are calculated, and the non-uniformity is evaluated using the respective harmonic 0 peak magnetic flux density amplitude and divided by the average magnetic flux on the circumference. This non-uniformity is ΔB i / B R0 It is defined as follows.

[0069] The background magnetic field can be removed by measuring the background magnetic field and subtracting the measured magnetic field from the measured value. Alternatively, the background magnetic field can be reduced / removed using shielding, or by generating an opposing magnetic field with a set of coils.

[0070] The step S10 of measuring the magnetic flux can include a magnetic probe that measures the components of the magnetic flux density. For example, it can be measured at a predetermined radius R around the magnet for the AC [0-pk] and DC of the magnet, typically at R > ro (outside r of the magnet). The measurement is performed with the axis of the probe located in a plane perpendicular to the axis of the magnet and with the magnet bisected (z = 0). Harmonics of the magnetic flux density can also be calculated, but if the measuring device has a processing circuit that performs, for example, a fast Fourier transform or a spectral analysis, it can also be the direct output of the measuring device.

[0071] ΔB i / B R0 For a given harmonic of ΔB i / B R0 , the ratio Brad[o-pk] / Brad[DC] depends on the magnet size and the position where the magnetic flux density is measured.

[0072] The coefficients can be determined computationally or experimentally, and from the measurement of the magnetic flux density around it, the non-uniformity of the ring magnet for each harmonic can be determined. In practice, since BradDC hardly changes between magnets of different materials and shapes, it is sufficient to measure only the AC magnetic flux component, for example, Brad[0-pk].

[0073] If necessary, the magnitudes of the 0-pk of different 0-pk harmonics can be measured together with their respective phases with respect to a reference position on the magnet. These measurements can be carried out for quality control, but when this step is executed, it can also provide the information necessary to selectively align the magnets in order to achieve a lower AC magnetic field.

[0074] In addition to Brad0-pk, the axial magnetic flux density Bz0-pk (and phase) of the harmonics can also be measured as an option in the same setup. This is to confirm that the magnetic field caused by the global skew error can be ignored for quality control, and also to provide the information necessary to selectively align the magnets in order to achieve a lower AC magnetic field if necessary.

[0075] In step S40, ring magnets are selected whose calculated first and second harmonic inhomogeneities are below a threshold, and in S50, the selected pair of ring magnets is stacked to form a passive magnetic bearing assembly according to the embodiment.

[0076] In some embodiments, instead of performing steps S10 to S30 for each magnet, a subset of the magnets can be measured and used for statistical quality analysis, and further measurements of the ring magnets can be performed only if the number of magnets lacking the required uniformity rises above a threshold.

[0077] In some embodiments, further measurement of the stray magnetic field of the assembled passive magnet is performed while the bearing is rotating at a predetermined speed, and it is confirmed that it is below a desired threshold. Alternatively and / or additionally, the stray magnetic field from an assembled bearing in an operating vacuum pump rotating at full speed can also be tested to determine if it is below a desired threshold. The magnet testing may be performed for each bearing or pump, or, as a statistical analysis, a subset may be tested and further testing performed only if the failure rate reaches a certain level.

[0078] In some embodiments, non-magnetic shims can be placed between magnets when the magnets are stacked, and in some embodiments, the global AC magnetic field can be reduced by selectively aligning ring magnets so that a portion of each magnet is aligned such that the minimum magnetic flux density harmonic (or more) of one magnet matches the maximum magnetic flux of another magnet. In some embodiments, the DC magnetic field is reduced by aligning the magnets such that the axial component of the magnetic field density of the magnet array, including the magnetic field generated by unwanted axial magnetization components, is minimized, canceled out, or at least reduced.

[0079] Unwanted axial magnetization components can be determined using the same setup used to determine non-uniformity. With the same setup (z=0), BzDC can also be measured at a set distance from the center of the magnet. For a magnet that is perfectly radially magnetized, BzDC=0 at z=0, after subtracting the background (i.e., Earth's) magnetic field. If this is not zero, then axial magnetization components exist, potentially increasing the DC magnetic field. Therefore, if extremely low DC magnetic fields are required, the magnets in the stack can be not only oriented (inverted or inverted) but also selectively assembled to create particularly low DC magnetic field arrays.

[0080] The following is a summary and example of the sensitivity of a stray magnetic field to different magnetization errors (global skew and non-uniformity) of different ring magnets magnetized radially and axially.

[0081] Global skew can be quantified by the angle of deviation between the actual magnetization direction and the intended magnetization direction. Magnetization inhomogeneity is represented by the variation ΔB of the remanent magnetization at the 0 peak. i The mean remanent magnetization B of different harmonics R0 It can be quantified by its ratio. For example, in the case of a ring magnet, the following equation describes the first harmonic of the residual magnetism of the surrounding magnet bearing. B R =B R0 +ΔB i cosθ...(1)

[0082] When a magnet is magnetized in the radial and axial directions, the fluctuations in the stray magnetic field around a magnet of a predetermined shape can be evaluated by calculating the fluctuations at a predetermined distance from the magnet's centerline and at a predetermined height z from the plane containing the magnet, and then cutting the magnet in half.

[0083] In the case of heterogeneity, the sensitivity is determined by the different components of the AC stray magnetic field amplitude ΔB (radial, axial, tangential, etc.) and the residual magnetic field 0 peak fluctuation ΔB defined by Equation 1 above. iIt can be calculated as the ratio to. In the case of skew, the sensitivity can be calculated as the ratio of the different components AC of the floating magnetic field amplitude ΔB to the skew angle (degrees).

[0084] Tables 1 and 2 show the results for examples of radially magnetized magnets and axially magnetized magnets of a given geometry for representative distances R from the center line of the magnet. JPEG2026520724000002.jpg38155 Table 1: Sensitivity of Radially Magnetized Ring Magnets

[0085] JPEG2026520724000003.jpg38155 Table 2: Sensitivity of Axially Magnetized Ring Magnets

[0086] From this result, it can be seen that an array using radially magnetized ring magnets is less susceptible to the influence of global skew. Conversely, it is more susceptible to the influence of non-uniformity. Therefore, in order to reduce the AC magnetic field leakage from radially magnetized magnets, it is important to make the magnetization uniformity extremely high.

[0087] In the case of a radially magnetized ring magnet, the sensitivity to the non-uniformity of the alternating current magnetic floating magnetic field changes depending on the shape of the non-uniformity on the circumference within the magnet, especially when the magnetic field is not measured too close to the magnet, for example, outside the stator of a vacuum pump, at positions where it is important to achieve a low AC floating magnetic field, it is maximum at the first harmonic and decreases rapidly at higher harmonics.

[0088] The sensitivity to different harmonics can be demonstrated by calculating the amplitude of the radial magnetic flux density harmonics generated by the harmonics of non-uniformity of the same magnitude.

[0089] Table 3 shows the results calculated for the first four harmonics at z = 0 and three different radii R1 < R2 < R3, similar to the previous example. JPEG2026520724000004.jpg38155 Table 3: Sensitivity of AC stray magnetic fields to harmonics with different non-uniformities

[0090] Therefore, magnet rings with low first and second harmonic inhomogeneity are particularly suitable for these designs.

[0091] As far as plastic-bonded magnets are concerned, the uniformity of magnetization is determined primarily by the uniformity of the magnetic powder within the plastic matrix, especially in the case of magnets made with isotropic powder. Therefore, in order to minimize or at least reduce the AC stray magnetic field, the flow and final density of the powder must be controlled with extreme precision. Compression-molded magnets can generally have higher uniformity than injection-molded magnets. If the powder is anisotropic, the uniformity of magnetization can also be controlled, for example, by the orientation of the powder during the injection molding process.

[0092] Plastic bonded magnets have low strength, defined by their BHmax product, which can result in low bearing rigidity. For this reason, their use as PMBs in high-speed rotary pumps such as turbomolecular pumps is not widespread.

[0093] What is even more interesting in such high-speed rotary pumps is PMB made from sintered magnets. For example, sintered NdFeB with magnets magnetized in the axial direction.

[0094] Sintered NdFeB magnets are extremely magnetic, and at high BHmax products, PMB can achieve high rigidity.

[0095] Radially magnetized neodymium magnet rings are manufactured using a process different from sintering, where the magnetic orientation of the material is imparted by thermomechanical action rather than by an external magnetic field. Therefore, this material is called "hot-worked" NdFeB rather than sintered, but its physical properties (e.g., density) and magnetic properties (e.g., BHmax product) are very similar.

[0096] Radially magnetized neodymium magnet rings can be obtained from different sources in different grades defined by the magnetic strength (BHmax product) and temperature resistance of the material. Temperature resistance is generally improved by adding or increasing the dysprosium content. The higher the dysprosium content, the higher the intrinsic coercivity Hcj and the greater the resistance to demagnetization. Magnets with little or no dysprosium content typically have an Hcj of less than 1000 kA / m.

[0097] When pumps such as turbomolecular pumps operate at temperatures below 120°C or at their maximum temperature, a minimum Hcj of typically 1400 kA / m is desirable to prevent widespread demagnetization during assembly and use. This requires a dysprosium content of approximately 3% (or about half that if grain boundary diffusion technology is used), which corresponds to the standard H grade for such magnets. Depending on the application, higher coercivity may be required, such as 1600 kA / m (equivalent to SH grade), or in some cases, approximately 2000 kA / m (equivalent to UH grade), with a dysprosium content of about 9%.

[0098] Tests have shown that the AC magnetic field generated from hot-worked NdFeB magnets can be higher than the desired value. The results indicate that first-harmonic non-uniformity is the biggest contributor to the AC stray magnetic field, with a value of 4%ΔB at the first harmonic. i / B R It was confirmed that it exceeds 0.

[0099] As a result, the AC stray magnetic fields from each magnet may require selective alignment to keep the sum of the AC stray magnetic fields from the PMB below the limits required for critical applications.

[0100] However, efforts to significantly reduce or eliminate the use of dysprosium, a highly demanded and undersupplied element, while maintaining high intrinsic coercivity and thus the ability to operate at sufficiently high temperatures, motivated detailed research into how process parameters for hot-worked magnet manufacturing can be adjusted to obtain such materials.

[0101] Dy-free materials with high coercivity can be obtained by lowering the hot working temperature in the manufacturing process, as described in the technical document "High performance hot deformed NdFeB magnets" by Dr. K. Hioki. While the typical intrinsic coercivity of Dy-free NdFeB is slightly below 1000 kA / m, Dy-free materials with magnetic force comparable to normal grades tested at intrinsic coercivity of 1500 kA / m or higher can be obtained.

[0102] Furthermore, when ring magnets made from dy-free material are magnetized radially and tested, their high intrinsic coercivity results in lower stray magnetic fields than standard hot-worked NdFeB. Typical magnetization uniformity, judging from measurements available on the websites of such magnet suppliers, is ~2%ΔB for the first harmonic. i / B R o or less.

[0103] By using these magnets in a PMB (Plant Magnetization Block) that uses radially magnetized magnets, low stray AC magnetic fields and extremely low stray DC magnetic fields can be obtained without the need for selective alignment of the magnets. If selective alignment of the magnets is used, even lower AC stray magnetic fields (PMBS) can be obtained.

[0104] Similarly, using similar process parameters, hot-worked Nd-Fe-B magnets can be obtained with a Dy content of less than 3% and a minimum intrinsic coercivity of over 1700 kA / m at 20°C.

[0105] Similar effects can be obtained with these low-dysprosium, high-heat-resistant materials.

[0106] Exemplary embodiments of the present invention have been disclosed in detail herein with reference to the accompanying drawings, but it will be understood that the present invention is not limited to the exact embodiments and that various changes and modifications can be made by those skilled in the art without departing from the scope of the invention as defined by the appended claims and equivalents. [Explanation of symbols]

[0107] 1 motor 2 shafts 4 stators 9 Rotation axis 10 Passive magnetic bearing 20 stator magnet stack 24 Stator ring magnets 30 rotor magnet stacks 32 Non-magnetic shims 34 Rotor ring magnets

Claims

1. A passive magnetic bearing assembly for a vacuum pump, A plurality of pairs of ring magnets arranged in a stack, wherein one pair of ring magnets is positioned on the stator side of the bearing and the other ring magnet of the pair is positioned on the rotor side, comprising a plurality of pairs of ring magnets, The ring magnets are magnetized radially, and each ring magnet on the rotor side contains at least a first harmonic of magnetic inhomogeneity, the inhomogeneity of which has a zero peak variation ΔB of residual magnetism. i and average residual magnetism B R0 A passive magnetic bearing assembly, measured as a ratio, wherein the first harmonic of magnetic non-uniformity is less than 3% in at least the majority of the ring magnets on the rotor side.

2. The passive magnetic bearing according to claim 1, wherein, for at least the majority of the ring magnet on the rotor side, each of the first and second harmonics of magnetic non-uniformity is less than 3%.

3. The passive magnetic bearing assembly according to claim 2, wherein, for at least the majority of the ring magnets on the rotor side, each of the first and second harmonics of the magnetic non-uniformity is less than 2%.

4. The passive magnetic bearing assembly according to claim 2 or 3, wherein, for at least the majority of the ring magnets on the rotor side, each of the first and second harmonics of the magnetic non-uniformity is less than 1%.

5. Each ring magnet on the stator side contains at least a first harmonic of magnetic inhomogeneity, and this inhomogeneity is a zero-peak fluctuation of residual magnetism ΔB i and average residual magnetism B R0 A passive magnetic bearing assembly according to any one of claims 1 to 4, wherein the first harmonic of the magnetic non-uniformity is less than 3% for at least the majority of the ring magnets on the stator side, as measured as a ratio between and .

6. The passive magnetic bearing assembly according to any one of claims 1 to 5, wherein at least a portion of the ring magnet includes an NdFeB magnet.

7. The passive magnetic bearing assembly according to claim 6, wherein at least a portion of the ring magnet includes a hot-worked NdFeB magnet.

8. The passive magnetic bearing assembly according to any one of claims 1 to 5, wherein the magnet includes a plastic bonded magnet.

9. The passive magnetic bearing assembly according to any one of claims 1 to 5, further comprising a non-magnetic shim, the non-magnetic shim being arranged to axially separate the ring magnets in the stack.

10. The passive magnetic bearing assembly according to claim 9, wherein the non-magnetic shim is formed from a material with a low coefficient of friction on the surface that contacts the magnet.

11. The passive magnetic bearing assembly according to claim 9 or 10, wherein the non-magnetic shim is formed from a plastic material.

12. A vacuum pump comprising a passive magnetic bearing assembly according to any one of claims 1 to 11, wherein the vacuum pump includes a rotor rotatably mounted within a stator, each pair of ring magnets being mounted on the rotor, and the other ring magnet of the pair being mounted on the stator.

13. The vacuum pump according to claim 12, wherein the vacuum pump includes a turbomolecular pump.

14. The vacuum pump according to claim 12 or 13, comprising a passive magnetic bearing assembly according to any one of claims 8 to 10, wherein the ring magnets are selectively aligned to reduce the stray AC magnetic field.

15. The vacuum pump according to any one of claims 12 to 14, wherein the ring magnet is selectively oriented vertically to reduce the stray DC magnetic field.

16. A method for manufacturing a passive magnetic bearing assembly according to any one of claims 1 to 11, This includes measuring at least the first harmonic inhomogeneity of a rotor ring magnet magnetized in multiple radial directions, The measurement described above is Determining the radial magnetic flux density at a set distance from the magnetic ring, measured at different points around the circumference, The first harmonic inhomogeneity is calculated by dividing the zero-peak amplitude of each flux harmonic by the average flux around the circumference after removing the radial flux density of the background magnetic field, Selecting a rotor ring magnet for at least the majority of the ring magnets for the rotor such that the calculated first harmonic non-uniformity of the magnetic non-uniformity is below a threshold, The selected rotor ring magnets are stacked with the corresponding set of stator ring magnets to form the passive magnetic bearing assembly. Tested by, method.

17. The method according to claim 16, wherein the stacking step includes the step of separating the ring magnets of the stack with a non-magnetic shim.

18. The method according to claim 16 or 17, wherein the step of determining the magnetic flux includes determining the magnitude and phase of the magnetic flux density harmonics, and the step of stacking includes aligning and selectively selecting the ring magnets such that the total variation of the magnetic flux density harmonics around the circumference is reduced with respect to the stack of magnets.

19. The method according to claim 18, further comprising determining the axial DC magnetic field of each ring magnet by measuring the magnetic field at an axial distance from the ring magnet, wherein the step of stacking the magnets comprises orienting the ring magnets in the stack to reduce the omniaxial DC magnetic field of the magnetic bearing assembly and optionally selecting them.

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