WO2006133703A1

WO2006133703A1 - Magnetic device for transfer of forces

Info

Publication number: WO2006133703A1
Application number: PCT/DK2006/000334
Authority: WO
Inventors: Torben Ole Andersen; Frank Thorleif JØRGENSEN; Peter Omand Rasmussen
Original assignee: Aalborg Universitet
Priority date: 2005-06-13
Filing date: 2006-06-13
Publication date: 2006-12-21

Abstract

The invention provides a magnetic device arranged to transfer a force between two mechanical elements upon relative movement of these elements, e.g. the device may be used in a gear box etc. The device includes two mechanical elements (209, 210) with respective set of magnets connected thereto in order to magnetically transfer a force between the two mechanical elements (209, 210). A third mechanical element (204) is arranged to functionally interact with the two mechanical elements (209, 210) to cause a cycloid movement between the two set of magnets, during operation of the magnetic device. In preferred gear box embodiment, the two set of magnets are arranged in respective circular patterns are operate in a cyclo gear configuration, i.e. with the first mechanical element (209) with magnets is connected to an input shaft (201), and it (209) is arranged eccentrically inside the second mechanical element (210) with magnets, wherein the second mechanical element (210) forms an output shaft. The third mechanical element (204) serves to rotationally fix the first mechanical element (209) so that it performs a combined orbital and rotational relative to the second mechanical element (210). Due to the magnetic transfer of forces this embodiment is suited for applications where media separation is crucial. In another embodiment the first mechanical element has two sets of magnets arranged on each side, and these set of magnets are arranged to perform magnetic interaction with respective set of magnets on an output shaft and a fixed part, e.g. a housing, and wherein the first mechanical element is connected to an input shaft such that it performs a combined rotational and axial movement.

Description

MAGNETIC DEVICE FOR TRANSFER OF FORCES

Field of the invention

The invention relates to the field of magnetic devices for transfer of forces. More specifically, the invention provides magnetic devices to be used in gear boxes to transfer a torque between two shafts at a gear ratio different from one.

Background of the invention

Transfer of forces by means of magnets is well-known, also in configurations for use in gear boxes to transfer a torque from one shaft to another at a certain gear ratio. By means of magnetic interaction between two magnets, it is possible to transfer a force without mechanical friction.

US 2,243,555 and US 5,569,967 describe simple magnetic gear configurations based on traditional mechanical two gear wheels. In general, such magnetic gears with a simple configuration have a rather limited capability to transfer large torques due to the limited force that can be transferred between the single magnets. This limitation highly depends on the size of the air gap between the magnets. The torque limitation of a magnetic gear also depends on the configuration, i.e. the number of single magnets that contribute to transferring force at the same time. In simple configurations, e.g. spur gear configurations, only one set of magnets contribute to force transfer at the same time, and thus these configurations have a poor efficiency in terms of torque versus space required for the gear box. This means that such magnetic gear boxes need to be 30-40 times the size of a corresponding mechanical gear in order to transfer the same torque.

US 5,633,555 and US 3,301,091 describe more complicated gear configurations. E.g. a permanent magnet version of a mechanical planetary gear is described in US 5,633,555. The planetary gear configuration provides the advantage that more than one magnets contribute to transfer forces at the same time, and thus it can be expected that such magnetic gear is capable of transferring a relatively high torque in relation the volume required. This is described in "A novel high-performance magnetic gear" by K. Atallah, and D. Howe, (2001) IEEE Transactions on Magnetics, Vol. 37, Issue 4 Part 1, pp. 2844-2846. A disadvantage with the planetary gear configuration, however, is the fact that it provides a rather low gear ratio. Therefore, several gear stages are required to produce a high gear ratio, e.g. gear ratios larger than the order of 10:1.

Summary of the invention Thus, following the above, it may be seen as an object of the present invention to provide a magnetic device capable of providing a high efficiency in terms of maximum force for a given size and still with the capability to transfer force at a high gear ratio in a single gear stage, if required. The invention provides a magnetic device arranged to transfer a force between two mechanical elements upon relative movement of these elements, the device comprising:

- a first mechanical element connected to a first set of magnets,

- a second mechanical element connected to a second set of magnets arranged to magnetically interact with the first set of magnets so as to magnetically transfer a force between the first and second mechanical elements, and

- a third mechanical element arranged to functionally interact with the first and second mechanical elements to cause a cycloid movement between the first and second set of magnets, during operation of the magnetic device.

By cycloid movement between first and second sets of magnets is understood that following a selected magnet from the first set of magnets a cycloid movement relative to a selected magnet from the second set of magnets can be observed. To provide such cycloid relative movement, it is understood that at least two of the first, second and third mechanical element are arranged to rotate in relation to each other.

A device according to the invention is efficient since it is suitable to implement in embodiments where a large number of magnets of the first and second sets of magnets are in magnetic interaction at the same time. In addition, the configuration with three mechanical elements movably arranged in relation to each other allow implementations having a high gearing ratio between the two mechanical elements, e.g. gearing ratios in the range 10:1 to 250:1 which is possible with a reasonable number of magnets of the first and second set of magnets.

The principles of the device are applicable for embodiments suited a large variety of applications, i.e. low power as well as high power applications. In general, the magnetic transfer of force is frictionless, while it is possible to produce e.g. gear box embodiments where input and output shafts are not in physical contact. This means that the input and output shafts can be separately sealed. E.g. within clean industries such as the food industry, this is advantageous since this enables easy dismantling and reassembling for cleaning purposes, and no lubricating between the moving elements is required.

The high efficiency is advantageous in high power applications such as gear boxes in ships connecting engine and propeller shaft, or connecting blade shaft and generator in wind turbines, since the design allows a reduced size and weight for a given torque compared with known magnetic gears.

For use e.g. as a yaw gear for wind turbines or other break down critical elements, the magnetic device according to the invention is advantageous since the magnetic force transfer causes the device to have a built-in force limiting effect that prevent serious damage in case of overload. I.e. upon a maximum force or torque being applied, the device will not be able to function properly, however no permanent damage will occur in contrast to mechanical gears which will suffer from broken teeth of gear wheels and thus permanent damage in case of an overload. Such permanent damages often involve high costs due to function stop and due to repair costs, e.g. in case of break down of off-shore wind turbines.

In some embodiments the second mechanical element is fixed while the force is transferred between the first and third mechanical elements. In other embodiments the third mechanical element is fixed while the force is transferred between the first and second mechanical elements.

In different embodiments the first mechanical element and any one of the second and third mechanical elements are arranged to perform different movements in relation to each other: a radial movement, an axial movement, a linear movement, or any combination of such movements.

As mentioned, the device may be a gear (or form part of a gear box) arranged to provide a gear ratio between the two mechanical elements. In preferred embodiments, the gear is arranged to transfer a torque between first and second shafts at a gear ratio. The third mechanical element may be fixed while the first and second shafts are connected to the respective first and second mechanical elements. Alternatively, the second mechanical element is fixed while the first and second shafts are connected to the respective first and third mechanical elements.

In a preferred embodiment the first set of magnets are arranged on an outer periphery of the first mechanical element in a first circular pattern, and wherein the second set of magnets are arranged at an inner periphery of the second mechanical element in a second circular pattern with a diameter larger than the first circular pattern, and wherein the first set of magnets are positioned within the second set of magnets. In other words, in this embodiment the device has a cyclo gear configuration.

Preferably, an axis of rotation of the first shaft is in the centre of the second set of magnets, and wherein the first shaft is connected to the first mechanical element via an eccentric mechanism so as to move the first set of magnets eccentrically in relation to the second set of magnets. Preferably, the eccentric mechanism includes a bearing with a centre axis different from the axis of rotation of the first shaft.

In one embodiment, the first mechanical element is arranged to engage with the third mechanical element so as to force the first mechanical element to perform a combined orbit and rotational motion within the second mechanical element.

The engagement between the first and third mechanical elements may include two or more pins connected to the third mechanical element, these two or more pins being arranged to fit into respective holes of the first mechanical element. However, it is appreciated that a mechanical coupling between the first and third mechanical elements may be formed in many alternative ways such as know by the skilled person.

Preferably a number of magnets of the first set of magnets is smaller than a number of magnets of the second set of magnets. Thus, a gear ratio different from one can be obtained, depending on the actual configuration.

Preferably, the magnetic device includes a housing. The second or the third mechanical element may be mounted fixed to the housing.

In another embodiment, the first mechanical element is arranged to move in a combined rotational and axial movement. Preferably, a third set of magnets is connected to the first mechanical element and located on an opposite side of the first mechanical element relative to the first set of magnets, and at the same time a fourth set of magnets is attached to a third mechanical element. Preferably, the first mechanical element is arranged to move, upon rotation of an input shaft so as to provide a magnetic interact between the first and second set of magnets, as well as a magnetic interaction between the third and fourth set of magnets.

In order to reduce magnetic unbalance, it may be preferred to include into the magnetic device an additional set of first, second and third mechanical elements similar to the first set of first, second and third mechanical elements, wherein the two sets of mechanical elements are mounted together with their rotating elements in opposite angular positions.

The magnets of the first and second sets of magnets are preferably permanent magnets, however it is appreciated that for some applications it may be preferred to use electromagnets, e.g. electromagnets based on superconductors.

The magnets of the respective first and second sets of magnets are preferably arranged with their north and south poles in an alternating pattern.

Brief description of the drawings

In the following the invention is described in more details with reference to the accompanying figures of which

Fig. 1 shows front and sectional views of a preferred gear embodiment,

Fig. 2 shows side and sectional views of the embodiment of Fig. 1,

Fig. 3a-3h illustrates for the embodiment of Figs. 1-2 different positions of the rotating elements during parts of one full rotation of the input shaft,

Fig. 4 shows front and sectional views of another preferred gear embodiment, and Fig. 5a, 5b illustrate a preferred gear embodiment with the same physical configuration but with different parts fixed.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. Description of preferred embodiments

Figs. 1 and 2 illustrate different views of a preferred radial gear embodiment. The principle function of gear is similar to a mechanical cyclo gear, and even though the first and second shafts 1, 10 are denoted input and output shafts in the following, it is appreciated that the gear can be used in reverse mode as well.

In the illustrated embodiment a first mechanical element 9 is a circular metal plate with a first set of permanent magnets 8, 13 attached to its outer periphery. Thus, the first set of permanent magnets are arranged in a circular pattern in alternating north pole 8 (black color) and south pole 13 (white color) order, the magnet directions pointing away from the centre of the first mechanical element 9. The second mechanical element 10 is a circular rotor with a second set of permanent magnets 7, 12 attached to its inner periphery, these magnets 7, 12 following the same color code as the first set of magnets. For illustration purposes, a total of 8 magnets are shown on the first set of magnets 8, 13, while there are 10 magnets in the second set of magnets 7, 12.

In this embodiment, the second mechanical element 10 is integrally formed with the output shaft 10, and thus the same reference sign is used. As seen, the diameter of the circle formed by the first set of magnets 8, 13 is smaller than the circle formed by the second set of magnets 7, 12, and the first mechanical element 9 is positioned within the second set of mechanical elements 10 such that the first set of magnets 8, 13 and the second sets of magnets 7, 12 face each other and they are thus able to magnetically interact and thereby transfer a force.

The air gap between the two sets of magnets 8, 13, 7, 12 is large enough to allow the first mechanical element 9 to be eccentrically positioned relative to the output shaft 10. A number of magnets of the first and second sets of magnets are different. Thus, where the magnetic resistance is least, the north and south magnets face each other and create a torque between the first mechanical element 9 and the second mechanical element, i.e. the output shaft 10. As the input shaft 1 is rotated, the position where north and south magnets have the lowest distance will move, and a torque will influence the output shaft 10 and force it to rotate. An eccentric mechanism 5 is connected the input shaft 1. The input shaft 1 is mounted in a bearing 2, wherein the bearing 2 is mounted in a housing 4 about which the gear is formed. In this embodiment, the housing 4 constitutes the third mechanical element. The housing 4 also has two bearings 11 mounted therein, these bearings 11 serving to support the output shaft 10.

The first mechanical element 9 engages with two pins 3 mounted to the housing 4. As seen, the two pins 3 are positioned in two holes in the first mechanical element 9 formed eccentrically to a centre of the first mechanical element 9. The first mechanical element 9 is driven by a bearing 6 connected with the eccentric mechanism 5. The bearing 6 and thus the input shaft 1 is centrally positioned in relation to the first mechanical element 9.

Figs. 3a-3h serve to illustrate the relative movement of the input shaft 1, the first mechanical element 9 and the output shaft 10. Since the two pins 3 are also seen in Figs. 3a-3h, the position of first mechanical element 9 relative to the stationary housing 4, i.e. the third mechanical element, is also seen. In the altogether 8 illustrations, the effect of the input shaft 1 being rotated clockwise in steps of 45° can be observed, and thus the effect of the eccentric mechanism 5 is sketched. In Fig. 3a the eccentric mechanism 5 is in the position with the least possible distance between opposite magnets 7 and 8. Thus, in this situation there is large magnetic force between magnets 7 and 8 due to the small distance. For the angular opposite side of the first mechanical element 9 the distance between the first and second set of magnets is relatively small, and thus the magnetic interaction between magnets on this side is small. The total result is that output shaft 10 is forced to rotate to a certain angular position.

In Fig. 3a the synchronous angle of the gear is 0° and this corresponds to the angular position of the output shaft 10 for the case with no torque load of the output shaft 10. In Fig. 3b the input shaft 1 is rotated 45° thus causing the eccentric mechanism 5 to rotate and perform a parallel displacement. The parallel displacement of the eccentric mechanism causes the minimum distance between the two sets of magnets to move such that the largest magnetic force is now between the south pole magnet 12 of the output shaft 10 and the south pole magnet 13 of the first mechanical element 9. Hereby, the output shaft 10 will rotate 9°, and thus with a 45° rotation of the input shaft, a gear ratio of 1:5 is achieved. The remaining Figs. 3c-3h merely serve to illustrate an entire orbital rotation of the first mechanical element 9. It is also easily seen from Figs. 3a-3h that the interaction between the first 9, second 10 and third mechanical elements 4 is such that e.g. magnets 7, 8 perform a cycloid movement relative to each other.

From Fig. 3a-3h it is seen that even though the set of opposing magnets with the minimal distance provides the highest contribution to transfer of forces and thus torque, the nearest neighbouring magnets will also provide substantial contribution to the total torque. This is especially the case in configuration with many closely spaced magnets where a large number of neighbouring magnets will contribute to the total torque transfer. Thereby, a high torque-volume efficiency is obtained compared to traditional magnetic gear configurations. Calculations indicate that a torque-density of 183 kNm/m³ can be reached with this configuration, compared to 3-400 kNm/m³ that can be obtained for mechanical gear. However, compared to known magnetic gear a significant efficiency improvement is achieved. 5

Fig. 4 illustrates a gear embodiment with axial interaction. An input shaft 104 is supported in two main bearings 105, 108. A third bearing 105' is mounted on the middle of the input shaft 104. This third bearing 105' has its axis displaced relative to an axis of rotation of the input shaft 104. A first mechanical element 109, mounted on the third bearing 105', has a

10 first set of permanent magnets 103 attached on its first side. The first set of magnets 103 on the first mechanical element 109 are arranged for magnetic contact to a second set of permanent magnets 102 on a stationary or fixed part 101 of the gear. If the number of magnets in the first (103) and second set of magnets (102) is the same, a magnetic coupling is provided. However, if this number is different, a gear ratio different from one is

15 obtained between the input shaft 104 and the fixed part 101 of the gear.

The first mechanical element 109 has a third set of magnets 103' attached on its second side, i.e. the side opposite the first side. This third set of magnets 103' serving to magnetically interact with a fourth set of magnets 102' connected to the output shaft 115, 20 the output shaft 115 being supported by a bearing 113 mounted to the fixed part 101. If the number of magnets of the third 103' and fourth set of magnets 102' is different, a gear ratio different from one is obtained between the first mechanical element 109 and the output shaft 115.

25 The function of the gear is as follows: upon rotation of the input shaft 104, the first mechanical element 109 is driven, while still the first mechanical element 9 is kept rotationally fixed in relation to the fixed part 101 of the gear. With a different number of magnets of the third and fourth set of magnets, and as there will only be one location where a magnetic resistance is minimal between the third 103' and fourth set of magnets

30 102', a gear ratio between the first mechanical element 109 and the output shaft 115 is provided upon rotation of the input shaft 104.

The first mechanical element has the first 103 and third set of magnets 103' attached on respective sides, both of the first 103 and third set of magnets 103' with magnet directions 35 in axial configuration. The magnets are placed in a circular pattern as numbers on a clock. The magnet direction should be alternating in upward and downward directions. Also the second 102 and fourth set of magnets 102' should be configured in circular patterns.

Upon rotation of the input shaft 104, the first mechanical element 109 will perform a 40 turning/twisting movement due to the configuration of the input shaft 104 - the type of movement also known to be performed by a circular plate dropped from a slightly tilted position onto a flat table. In some embodiments the number of magnets of the first (103) and second set of magnets (102) is the same, while a gear ratio is achieved between input shaft 104 and output shaft 115 by a different number of magnets on the third (103') and fourth set of magnets (102'). However, it is appreciated that the gear ratio can as well obtained by different number of magnets of the third (103') and fourth set of magnets (102'), while the first (103) and second set of magnets (102) have the same number of magnets.

Fig. 5a shows a perspective view of a gear embodiment with a principle corresponding to the embodiments described in Figs. 1, 2 and 3. Arrows indicate direction of rotation, i.e. ' input shaft 201 and output shaft 210 rotate oppositely. An input shaft 201 is connected to a first mechanical element 209 via an eccentric mechanism 205. In the specific embodiment 42 magnets are attached to the first mechanical element 209, while 44 magnets are attached to the second mechanical element 210, i.e. this second mechanical element 210 serving as output shaft. A third mechanical element 204, e.g. part of a housing, engages with the first mechanical element 209 via three pins 203.

If in general a number of magnets on the first mechanical elements is denoted A, and the number of magnets on the second mechanical elements is denoted B, a gear ratio for the configuration in Fig. 5a is: G = B/(B-A), and thus it is appreciated that high gear ratios can be obtained with reasonable number of magnets.

Fig. 5b shows the same configuration as in Fig. 5a, however as illustrated the second mechanical element 210 is now fixed, e.g. to be rigidly connected to a housing, while the third mechanical element 204 serves as output shaft. The arrows indicate that input shaft 201 and output shaft 204 in this case rotate in opposite directions. For the configuration of Fig. 5b, the gear ratio is give by: G = A/(A-B), and as in Fig. 5a high gear ratios can easily be obtained.

For the configurations of Figs. 5a and 5b it is found that a minimum distance of 0.5 mm and a maximum distance between two sets of magnets is suitable. However, it is appreciated that this depends on the actual application, e.g. on the bearing tolerances etc.

The embodiment of Fig. 5a has the advantage that input shaft 201 and output shaft 210 are physically separated, i.e. there is also an electrical separation between the two. Thus, in applications where separation of media is important, the embodiment of Fig. 5a is advantageous. Also the Figs. 1 and 2 are suited for media separation in this manner.

It is to be understood that reference signs in the claims should not be construed as limiting with respect to the scope of the claims.

Claims

1. A magnetic device arranged to transfer a force between two mechanical elements upon relative movement of these elements, the device comprising:

- a first mechanical element (9, 109, 209) connected to a first set of magnets (8,

13, 103),

- a second mechanical element (10, 101, 210) connected to a second set of magnets (7, 12, 102) arranged to magnetically interact with the first set of magnets (8, 13, 103) so as to magnetically transfer a force between the first (9,

109, 209) and second mechanical elements (10, 101, 210), and

- a third mechanical element (4, 115, 204) arranged to functionally interact with the first (9, 109, 209) and second mechanical elements (10, 101, 210) to cause a cycloid movement between the first (8,13, 103) and second set of magnets (7, 12,

102), during operation of the magnetic device.

2. Magnetic device according to claim 1, wherein the second mechanical element (10, 101, 210) is fixed, and wherein the force is transferred between the first (9, 109, 209) and third mechanical elements (4, 115, 204).

3. Magnetic device according to claim 1, wherein the third mechanical element (4, 204) is fixed, and wherein the force is transferred between the first (9, 209) and second mechanical elements (10, 210).

4. Magnetic device according to any of the preceding claims, wherein the first mechanical element (9, 209) and any one of the second (10, 210) and third mechanical elements (4, 204) are arranged to perform a radial movement in relation to each other.

5. Magnetic device according to any of claims 1-3, wherein the first mechanical element (109) and any one of the second (101) and third mechanical elements (115) are arranged to perform an axial movement in relation to each other.

6. Magnetic device according to any of claims 1-3, wherein the first mechanical element and any one of the second and third mechanical elements are arranged to perform a linear movement in relation to each other.

7. Magnetic device according to any of the preceding claims, wherein the device is a gear arranged to provide a gear ratio between the two mechanical elements.

8. Magnetic device according to claim 7, wherein the gear is arranged to transfer a torque between first (1, 104, 201) and second shafts (10, 115, 204) at a gear ratio.

9. Magnetic device according to claim 8, wherein the third mechanical element (3, 4) is fixed, and wherein the first (1) and second shafts (10) are connected to the respective first (9) and second mechanical elements (10).

5 10. Magnetic device according to claim 8, wherein the second mechanical element (210) is fixed, and wherein the first (201) and second shafts (204) are connected to the respective first (209) and third mechanical elements (204).

11. Magnetic device according to claim 8, wherein the first set of magnets (8, 13) are 10 arranged on an outer periphery of the first mechanical element (9) in a first circular pattern, and wherein the second set of magnets (7, 12) are arranged at an inner periphery of the second mechanical element (10) in a second circular pattern with a diameter larger than the first circular pattern, and wherein the first set of magnets (8, 13) are positioned within the second set of magnets (7, 12). 15

12. Magnetic device according to claim 11, wherein an axis of rotation of the first shaft (1, 201) is in the centre of the second set of magnets (7, 12), and wherein the first shaft (1, 201) is connected to the first mechanical element (9, 209) via an eccentric mechanism (5, 205) so as to move the first set of magnets (8, 13) eccentrically in relation to the second

20 set of magnets (7, 12).

13. Magnetic device according to claim 12, wherein the eccentric mechanism (5, 205) includes a bearing with a centre axis different from the axis of rotation of the first shaft (1, 10).

25

14. Magnetic device according to claim 12 or 13, wherein the first mechanical element (9, 209) is arranged to engage with the third mechanical element (4, 204) so as to force the first mechanical element (9, 209) to perform a combined orbit and rotational motion within the second mechanical element (10, 210).

30

15. Magnetic device according to claim 14, wherein the engagement between the first (9, 209) and third mechanical elements (4, 204) include two or more pins (3, 203) connected to the third mechanical element (4, 204), these two or more pins (3, 203) being arranged to fit into respective holes of the first mechanical element (9, 209).

35

16. Magnetic device according to any of claims 11-15, wherein a number of magnets of the first set of magnets (8, 13) is smaller than a number of magnets of the second set of magnets (7, 12).

40 17. Magnetic device according to any of claims 8-16, the device further comprising a housing (4).

18. Magnetic device according to claim 17, wherein the second (10, 210) or the third mechanical element (4, 204) is mounted fixed to the housing (4).

19. Magnetic device according to claim 10, wherein the first mechanical element (109) is arranged to move in a combined rotational and axial movement.

5 20. Magnetic device according to claim 19, wherein a third set of magnets (103') is connected to the first mechanical element (109) and located on an opposite side of the first mechanical element (109) relative to the first set of magnets (103), and wherein a fourth set of magnets (102') is attached to a third mechanical element (115).

10 21. Magnetic device according to claim 20, wherein the first mechanical element (109) is arranged to move so as to provide a magnetic interaction between the first (103) and second set of magnets (102) as well as a magnetic interaction between the third and fourth set of magnets.

15 22. Magnetic device according to claim 21, wherein all of the first, second, third and fourth set of magnets (103, 102, 103', 102') are arranged in respective circular patterns.

23. Magnetic device according to any of claims 19-22, wherein the first mechancial element (109) is connected to the first shaft (104) via a bearing (105') eccentrically

20 positioned relative to the first shaft (104) in order to cause the combined rotational and axial movement of the first mechanical element (109).

24. Magnetic device according to any of claims 19-23, wherein the first mechanical element (109) is rotationally fixed in relation to the second mechanical element (101).

25

25. Magnetic device according to any of claims 9-24, including an additional set of first (9), second (10) and third mechanical elements (4) similar to the first set of first (9), second (10) and third mechanical elements (4), wherein the two sets of mechanical elements are mounted together with their rotating elements in opposite angular positions so as to

30 reduce magnetic unbalance.

26. Magnetic device according to any of the preceding claims, wherein the magnets of the first (8, 13) and second sets of magnets (7, 12) are permanent magnets.

35 27. Magnetic device according to any of the preceding claims, wherein the magnets of the respective first (8, 13) and second sets of magnets (7, 12) are arranged with their north and south poles in an alternating pattern.