CN207584030U - A kind of multilevel sleeve type device for sealing magnetic fluid - Google Patents

Abstract

The utility model relates to a multi-stage sleeve type magnetic fluid sealing device, comprising a shaft, a casing, a left pole shoe ring, a middle pole shoe ring, a right pole shoe ring, an inner permanent magnet ring, an outer permanent magnet ring and a sleeve; The left pole shoe ring and the right pole shoe ring are set between the shell and the shaft; the shaft between the left pole shoe ring and the right pole shoe ring is fitted with more than two sleeves at intervals along the axial direction; The inner permanent magnet ring is installed in the annular groove I, and the inner permanent magnet ring is provided with an outer permanent magnet ring on the inner wall of the corresponding shell; the left pole shoe ring, the right pole shoe ring and the middle pole shoe The inner circle of the ring is provided with axial pole teeth; the end face of the sleeve corresponds to the end faces of the left pole shoe ring, the right pole shoe ring and the middle pole shoe ring; the purpose of this utility model is to solve the existing single The problem of low pressure resistance exists in the multi-stage sleeve type ferrofluid seal device and the multi-magnetic source ferrofluid seal device.

Description

A multi-stage sleeve type magnetic fluid sealing device

technical field

The utility model belongs to the field of mechanical engineering sealing, in particular to a multi-stage sleeve type magnetic fluid sealing device.

Background technique

Ferrofluid is a liquid magnetic material that is sensitive to magnetic fields and can flow. It is a stable colloid system formed by nano-scale (1~100nm) magnetic particles coated with surfactants, which are highly dispersed and suspended in the carrier liquid. Even under the long-term action of gravity, centrifugal force or strong magnetic field, the suspended nano High-grade magnetic particles will not agglomerate and can maintain stable magnetic properties. Magnetic fluid sealing can solve the above problems well. Compared with traditional sealing technology, magnetic fluid sealing technology has the advantages of zero leakage rate, no solid friction, long life and high reliability, and has been widely used in various industries. This is of great significance to engineering sealing. Ferrofluid seals are mainly used in low-pressure gas seals such as vacuum and dustproof. With the further development of research and application, the application of ferrofluid in high-pressure gas seals, water seals, and oil seals is imminent.

One of the methods to improve the pressure resistance performance of magnetic fluid seals under large gaps is to increase the number of magnetic sources in the magnetic circuit of magnetic fluid seals and improve the shape of pole pieces, such as the seal described in reference 1 (the patent publication number is CN202332310U). Device and the sealing device described in reference 2 (publication number is the patent of CN205877200U). Although the sealing performance of the two sealing devices described in the above documents has been greatly improved compared with ordinary magnetic fluid, there is still room for further improvement in the sealing performance of the existing sealing structure.

Utility model content

The purpose of this utility model is to provide a multi-stage sleeve type magnetic fluid sealing device, thereby solving the problem of low pressure resistance existing in the existing single-stage sleeve type magnetic fluid sealing device and multi-magnetic source magnetic fluid sealing device, and at the same time, the assembly is simple , easy to process, making this sealing technology successfully applied in high-speed and heavy-duty fields.

The technical scheme of the utility model is as follows:

The multi-stage sleeve-type magnetic fluid seal device includes a shaft, a casing, a left pole shoe ring, a middle pole shoe ring, a right pole shoe ring, an inner permanent magnet ring, an outer permanent magnet ring, and a sleeve;

The left pole shoe ring and the right pole shoe ring are set between the casing and the shaft; more than two sleeves are set on the shaft between the left pole shoe ring and the right pole shoe ring along the axial interval; The various sleeves are separated by the middle pole shoe ring, and the middle pole shoe ring is set between the shell and the shaft; an annular groove I is provided on the outer surface of each sleeve, and the said The inner permanent magnet ring is installed in the annular groove I, and the inner wall of the corresponding shell of the inner permanent magnet ring is provided with an outer permanent magnet ring; the axial width of the inner permanent magnet ring is less than the axis of the outer permanent magnet ring. To the width, and there is a gap between the outer circular surface of the inner permanent magnet ring and the inner circular surface of the outer permanent magnet ring;

The outer circular surfaces of the left pole shoe ring, the right pole shoe ring and the middle pole shoe ring are in contact with the shell, and the inner circle surfaces of the left pole shoe ring, the right pole shoe ring and the middle pole shoe ring face the outer circle of the shaft The surface extends and corresponds to the outer circular surface of the shaft; the inner circular surfaces of the left pole shoe ring, the right pole shoe ring and the middle pole shoe ring are provided with axial pole teeth, and the axial pole teeth are along the diameter Extending toward the outer circular surface of the shaft, there is a gap between the outer circular surface of the shaft, and the magnetic fluid is arranged in the gap;

The end face of the sleeve corresponds to the end faces of the left pole shoe ring, the right pole shoe ring and the middle pole shoe ring. The end face of the sleeve is provided with pole teeth I, and the pole teeth I are perpendicular to the left pole shoe ring. , the end faces of the right pole shoe ring and the middle pole shoe ring extend, and there is a gap between the end faces, and the magnetic fluid is arranged in the gap.

The sleeve includes an inner circular surface I, an outer circular surface I, an inclined end surface I, and a vertical end surface I, and the axial length of the outer circular surface I is longer than the length of the inner circular surface I; the outer circular surface The two ends of I are respectively connected with the upper end of the vertical end surface I, the two ends of the inner circular surface I are respectively connected with the lower end of the inclined end surface I, and the upper end of the described inclined end surface I is connected with the lower end of the vertical end surface I; The above-mentioned inclined end face I and vertical end face I together form the end face of the sleeve, and pole teeth I are respectively provided on the inclined end face I and the vertical end face I.

The left pole shoe ring includes an inner circular surface II, an outer circular surface II, an inclined end surface II, and a vertical end surface II; the axial length of the outer circular surface II is longer than the length of the inner circular surface II; The right end of the circular surface II is connected to the upper end of the vertical end surface II, the right end of the inner circular surface II is connected to the lower end of the inclined end surface II, and the upper end of the inclined end surface II is connected to the lower end of the vertical end surface II;

The inclined end face II and the vertical end face II together form the right end face of the left pole shoe ring, corresponding to the inclined end face I and the vertical end face I on the left end face of the adjacent sleeve respectively;

An annular groove II is provided on the outer circular surface II, and a sealing ring I is arranged in the annular groove II.

The right pole shoe ring includes an inner circular surface III, an outer circular surface III, an oblique end surface III, and a vertical end surface III; the axial length of the outer circular surface III is longer than the length of the inner circular surface III; The left end of the circular surface III is connected to the upper end of the vertical end surface III, the left end of the inner circular surface III is connected to the lower end of the inclined end surface III, and the upper end of the inclined end surface III is connected to the lower end of the vertical end surface III;

The inclined end face III and the vertical end face III together form the left end face of the right pole shoe ring III, corresponding to the inclined end face I and the vertical end face I on the right end face of the adjacent sleeve respectively;

An annular groove III is provided on the outer circular surface III, and a sealing ring II is arranged in the annular groove III.

The middle pole shoe ring includes an inner circular surface IV, an outer circular surface IV, an inclined end surface IV, and a vertical end surface IV, and the axial length of the outer circular surface IV is longer than the length of the inner circular surface IV; The two ends of the circular surface IV are respectively connected to the upper end of the vertical end surface IV, the two ends of the inner circular surface IV are respectively connected to the lower end of the inclined end surface IV, and the upper end of the said inclined end surface IV is connected to the lower end of the vertical end surface IV ;

The inclined end face IV and the vertical end face IV together form the end face of the middle pole shoe ring, corresponding to the inclined end face I and the vertical end face I on the end face of the adjacent sleeve respectively;

An annular groove IV is provided on the outer circular surface IV, and a sealing ring IV is arranged in the annular groove IV.

The multi-stage sleeve-type ferrofluid seal device also includes a left magnetic isolation ring and a right magnetic isolation ring; the left magnetic isolation ring is arranged on the left side of the left pole shoe, and is arranged close to the inner wall of the shell; the The right magnetic isolation ring is arranged on the right side of the right pole shoe, and is set close to the inner wall of the shell.

The multi-stage sleeve-type magnetic fluid seal device also includes a left bearing and a right bearing, the left bearing and the right bearing are respectively sleeved on the shaft, and the left bearing is arranged on the left side of the left magnetic ring, The right bearing is arranged on the right side of the right magnetic isolation ring.

The inner permanent magnet ring and the outer permanent magnet ring are respectively arranged in 2-10 groups at intervals.

The axial pole teeth and the pole teeth I are respectively composed of 2 to 10 annular pole teeth arranged at intervals.

Both the inner permanent magnet ring and the outer permanent magnet ring are axially magnetized permanent magnets, and the directions of the magnetic force lines of the inner permanent magnet ring and the outer permanent magnet ring are opposite.

The right side of the right bearing is provided with an end cover, and the end cover is sleeved on the shaft to seal the right end of the housing, and a sealing medium is filled between the right bearing and the end cover.

The utility model aims to solve the existing technical defects, and provides a multi-stage sleeve type magnetic fluid sealing device, which can solve the problem of insufficient pressure resistance of the sealing device under the condition of a large gap.

The utility model designs a special structure that embeds the permanent magnet in the polygonal sleeve. The sleeve has corresponding pole teeth axially and laterally, and the permanent magnet is embedded between the pole shoes. Magnetic fluid is injected into the radial, lateral and axial sealing gaps formed by the shaft to realize multi-stage sleeve-type magnetic fluid seals.

The utility model greatly improves the magnetic field strength in the sealing gap through a unique structural design, thereby greatly enhancing the pressure resistance of the magnetic fluid seal; the two-stage sleeve-type sealing structure can reduce the loss of the magnetic fluid when the seal fails, and increases the sealing capacity. Multiple pressure bearing capacity and self-healing capacity of the device. The pressure resistance and sealing reliability of the magnetic fluid seal under the condition of a large gap are further improved, and its safe working range is expanded.

Description of drawings

Fig. 1 is the structural representation of sealing device described in the utility model;

The serial numbers and corresponding names in the figure are as follows:

1-shaft, 2-housing, 3-left pole shoe ring, 4-middle pole shoe ring, 5-right pole shoe ring, 6-inner permanent magnet ring, 7-outer permanent magnet ring, 8-sleeve, 9- Annular groove 1, 10-axial pole teeth, 11-left separated magnetic ring, 12-right separated magnetic ring, 13-left bearing, 14-right bearing, 15-end cover, 16-sealing medium.

31-inner circular surface II, 32-outer circular surface II, 33-oblique end surface II, 34-vertical end surface II, 35-annular groove II, 36-sealing ring I;

41-inner circular surface IV, 42-outer circular surface IV, 43-oblique end face IV, 44-vertical end face IV, 45-annular groove IV, 46-sealing ring IV;

51-inner circular surface III, 52-outer circular surface III, 53-oblique end face III, 54-vertical end face III; 55-annular groove III, 56-sealing ring II;

81-inner circular surface I, 82-outer circular surface I, 83-oblique end surface I, 84-vertical end surface I, 85-polar tooth I.

Detailed ways

Below in conjunction with accompanying drawing, the utility model is further described.

As shown in Figure 1, the multi-stage sleeve-type magnetic fluid seal device includes a shaft 1, a housing 2, a left pole shoe ring 3, a middle pole shoe ring 4, a right pole shoe ring 5, an inner permanent magnet ring 6, Outer permanent magnet ring 7, sleeve 8;

The left pole shoe ring 3 and the right pole shoe ring 5 are sleeved between the housing 2 and the shaft 1; the shaft 1 between the left pole shoe ring 3 and the right pole shoe ring 5 is sleeved axially at intervals More than two sleeves 8; each of the sleeves 8 is separated by the middle pole shoe ring 4, and the middle pole shoe ring 4 is set between the shell 2 and the shaft 1; each sleeve 8 An annular groove 19 is provided on the outer circular surface, and the inner permanent magnet ring 6 is installed in the annular groove 19, and an outer permanent magnet ring 7 is arranged on the inner wall of the shell 2 corresponding to the inner permanent magnet ring 6; The axial width of the inner permanent magnet ring 6 is less than the axial width of the outer permanent magnet ring 7, and there is a gap between the outer circular surface of the inner permanent magnet ring 6 and the inner circular surface of the outer permanent magnet ring 7;

The outer circular surfaces of the left pole shoe ring 3, the right pole shoe ring 5 and the middle pole shoe ring 4 are in contact with the shell 2, and the inner circles of the left pole shoe ring 3, the right pole shoe ring 5 and the middle pole shoe ring 4 The surface extends toward the outer surface of the shaft 1 and corresponds to the outer surface of the shaft 1; the inner surface of the left pole shoe ring 3, the right pole shoe ring 5 and the middle pole shoe ring 4 Pole teeth 10, the axial pole teeth 10 extend radially toward the outer circular surface of the shaft 1, leaving a gap with the outer circular surface of the shaft 1, and the magnetic fluid is arranged in the gap;

The end face of the sleeve 8 corresponds to the end faces of the left pole shoe ring 3, the right pole shoe ring 5 and the middle pole shoe ring 4, and the end face of the sleeve 8 is provided with pole teeth I85, and the pole teeth I85 are vertical Extending to the end faces of the left pole shoe ring 3 , the right pole shoe ring 5 and the middle pole shoe ring 4 , there is a gap between them and the end faces, and the magnetic fluid is arranged in the gap.

The sleeve 8 includes an inner circular surface I81, an outer circular surface I82, an inclined end surface I83, and a vertical end surface I84. The axial length of the outer circular surface I82 is longer than the length of the inner circular surface I81; The two ends of the surface I82 are respectively connected to the upper end of the vertical end surface I84, the two ends of the inner circular surface I81 are respectively connected to the lower end of the inclined end surface I83, and the upper end of the described inclined end surface I83 is connected to the lower end of the vertical end surface I84; The inclined end surface I83 and the vertical end surface I84 together form the end surface of the sleeve 8, and the inclined end surface I83 and the vertical end surface I84 are respectively provided with pole teeth I85.

The left pole shoe ring 3 includes an inner circle surface II31, an outer circle surface II32, an inclined end surface II33, and a vertical end surface II34; the axial length of the outer circle surface II32 is longer than the length of the inner circle surface II31; The right end of the outer circular surface II32 is connected to the upper end of the vertical end surface II34, the right end of the inner circular surface II31 is connected to the lower end of the inclined end surface II33, and the upper end of the described inclined end surface II33 is connected to the lower end of the vertical end surface II34;

The inclined end face II33 and the vertical end face II34 together form the right end face of the left pole shoe ring 3, corresponding to the inclined end face I83 and the vertical end face I84 on the left end face of the adjacent sleeve 8 respectively;

An annular groove II35 is provided on the outer circular surface II32, and a sealing ring I36 is arranged in the annular groove II35.

The right pole shoe ring 5 includes an inner circular surface III51, an outer circular surface III52, an oblique end surface III53, and a vertical end surface III54; the axial length of the outer circular surface III52 is longer than the length of the inner circular surface III51; The left end of the outer circular surface III52 is connected to the upper end of the vertical end surface III54, the left end of the inner circular surface III51 is connected to the lower end of the inclined end surface III53, and the upper end of the inclined end surface III53 is connected to the lower end of the vertical end surface III54;

The inclined end face III53 and the vertical end face III54 together form the left end face of the right pole shoe ring III5, corresponding to the inclined end face I83 and the vertical end face I84 on the right end face of the adjacent sleeve 8 respectively;

An annular groove III55 is provided on the outer circular surface III52, and a sealing ring II56 is arranged in the annular groove III55.

The middle pole shoe ring 4 includes an inner circular surface IV41, an outer circular surface IV42, an inclined end surface IV43, and a vertical end surface IV44, and the axial length of the outer circular surface IV42 is longer than the length of the inner circular surface IV41; The two ends of the outer circular surface IV42 are respectively connected to the upper end of the vertical end surface IV44, the two ends of the inner circular surface IV41 are respectively connected to the lower end of the inclined end surface IV43, and the upper end of the said inclined end surface IV43 is connected to the lower end of the vertical end surface IV44 connect;

The inclined end face IV43 and the vertical end face IV44 together form the end face of the middle pole shoe ring 4, corresponding to the inclined end face I83 and the vertical end face I84 on the end face of the adjacent sleeve 8 respectively;

An annular groove IV45 is provided on the outer circular surface IV42, and a sealing ring IV46 is arranged inside the annular groove IV45.

The multi-stage sleeve type ferrofluid seal device also includes a left magnetic isolation ring 11 and a right magnetic isolation ring 12; The inner wall is set; the right magnetic isolation ring 12 is set on the right side of the right pole shoe 5 and is set close to the inner wall of the shell 2 .

The multi-stage sleeve type magnetic fluid seal device also includes a left bearing 13 and a right bearing 14, the left bearing 13 and the right bearing 14 are respectively sleeved on the shaft 1, and the left bearing 13 is arranged on the left septum On the left side of the magnetic ring 11 , the right bearing 14 is arranged on the right side of the right magnetic isolation ring 12 .

The inner permanent magnet ring 6 and the outer permanent magnet ring 7 are respectively arranged in 2-10 groups at intervals.

The axial pole teeth 10 and the pole teeth 185 are respectively composed of 2-10 annular pole teeth arranged at intervals.

The inner permanent magnet ring 6 and the outer permanent magnet ring 7 are axially magnetized permanent magnets, and the directions of the magnetic force lines of the inner permanent magnet ring 6 and the outer permanent magnet ring 7 are opposite.

The right side of the right bearing 14 is provided with an end cover 15, and the end cover 15 is sleeved on the shaft 1 to encapsulate the right end of the housing 2, and the space between the right bearing 14 and the end cover 15 is filled with a sealing medium 16.

Claims (10)

1. A multi-stage sleeve-type magnetic fluid seal device, including a shaft (1), a housing (2), a left pole shoe ring (3), a middle pole shoe ring (4), a right pole shoe ring (5), an inner The permanent magnet ring (6), the outer permanent magnet ring (7), and the sleeve (8), are characterized in that: The left pole shoe ring (3) and the right pole shoe ring (5) are set between the casing (2) and the shaft (1); the left pole shoe ring (3) and the right pole shoe ring (5) Two or more sleeves (8) are installed on the shaft (1) in the axial direction; the sleeves (8) are separated by the middle pole shoe ring (4), and the middle The pole shoe ring (4) is set between the casing (2) and the shaft (1); an annular groove I (9) is provided on the outer surface of each sleeve (8), and the inner permanent magnet ring ( 6) Installed in the annular groove I (9), the inner permanent magnet ring (6) is provided with an outer permanent magnet ring (7) on the inner wall of the shell (2) corresponding to the inner permanent magnet ring (6); the inner permanent magnet ring ( 6) The axial width is smaller than the axial width of the outer permanent magnet ring (7), and there is a gap between the outer circular surface of the inner permanent magnet ring (6) and the inner circular surface of the outer permanent magnet ring (7); The outer circular surfaces of the left pole shoe ring (3), right pole shoe ring (5) and middle pole shoe ring (4) are in contact with the shell (2), and the left pole shoe ring (3), right pole shoe ring (5) and the inner circle of the middle pole shoe ring (4) extend toward the outer circle of the shaft (1) and correspond to the outer circle of the shaft (1); the left pole shoe ring (3), Axial pole teeth (10) are arranged on the inner circles of the right pole shoe ring (5) and the middle pole shoe ring (4), and the axial pole teeth (10) are arranged radially outward of the shaft (1). The circular surface extends, and there is a gap between the outer circular surface of the shaft (1), and the magnetic fluid is arranged in the gap; The end face of the sleeve (8) corresponds to the end faces of the left pole shoe ring (3), the right pole shoe ring (5) and the middle pole shoe ring (4). Teeth I (85), the pole teeth I (85) extend vertically to the end faces of the left pole shoe ring (3), the right pole shoe ring (5) and the middle pole shoe ring (4), leaving space between the end faces There is a gap in which the magnetic fluid is disposed. 2. The multi-stage sleeve-type ferrofluid seal device according to claim 1, characterized in that: the sleeve (8) includes an inner circular surface I (81), an outer circular surface I (82), an inclined end surface I (83), vertical end surface I (84), the axial length of the outer circular surface I (82) is longer than the length of the inner circular surface I (81); the two sides of the outer circular surface I (82) The ends are respectively connected to the upper end of the vertical end surface I (84), the two ends of the inner circular surface I (81) are respectively connected to the lower end of the inclined end surface I (83), and the upper end of the inclined end surface I (83) is connected to the The lower end of the vertical end face I (84) is connected; the inclined end face I (83) and the vertical end face I (84) together form the end face of the sleeve (8), the inclined end face I (83), the vertical end face I ( 84) are respectively provided with pole teeth I (85). 3. The multi-stage sleeve type ferrofluid seal device according to claim 2, characterized in that: the left pole shoe ring (3) includes an inner circular surface II (31), an outer circular surface II (32), Inclined end surface II (33), vertical end surface II (34); the axial length of the outer circular surface II (32) is longer than the length of the inner circular surface II (31); the outer circular surface II (32) The right end of the inner circular surface II (31) is connected to the upper end of the vertical end surface II (34), the right end of the inner circular surface II (31) is connected to the lower end of the inclined end surface II (33), and the upper end of the inclined end surface II (33) is connected to the vertical The lower end of the end face II (34) is connected; The oblique end surface II (33) and the vertical end surface II (34) together form the right end surface of the left pole shoe ring (3), and the oblique end surface I (83) on the left end surface of the adjacent sleeve (8) , the vertical end face I (84) correspond respectively; An annular groove II (35) is provided on the outer circular surface II (32), and a sealing ring I (36) is arranged inside the annular groove II (35). 4. The multi-stage sleeve type ferrofluid seal device according to claim 2, characterized in that: said right pole shoe ring (5) includes an inner circular surface III (51), an outer circular surface III (52), Inclined end surface III (53), vertical end surface III (54); the axial length of the outer circular surface III (52) is longer than the length of the inner circular surface III (51); the outer circular surface III (52) The left end of the inner circular surface III (51) is connected to the upper end of the vertical end surface III (54), the left end of the inner circular surface III (51) is connected to the lower end of the oblique end surface III (53), and the upper end of the oblique end surface III (53) is connected to the vertical The lower end of the end face III (54) is connected; The oblique end surface III (53) and the vertical end surface III (54) together form the left end surface of the right pole shoe ring III (5), and the oblique end surface I (83) on the right end surface of the adjacent sleeve (8) ), the vertical end face I (84) correspond respectively; An annular groove III (55) is provided on the outer circular surface III (52), and a sealing ring II (56) is arranged inside the annular groove III (55). 5. The multi-stage sleeve type ferrofluid seal device according to claim 2, characterized in that: the middle pole shoe ring (4) includes an inner circular surface IV (41), an outer circular surface IV (42), Inclined end face IV (43), vertical end face IV (44), the axial length of the outer circular surface IV (42) is longer than the length of the inner circular surface IV (41); the outer circular surface IV (42) The two ends of the inner circular surface IV (41) are respectively connected with the upper end of the vertical end surface IV (44), and the two ends of the inner circular surface IV (41) are respectively connected with the lower end of the inclined end surface IV (43). The upper end is connected with the lower end of the vertical end face IV (44); The inclined end face IV (43) and the vertical end face IV (44) together form the end face of the middle pole shoe ring (4), and the inclined end face I (83) and vertical end face of the adjacent sleeve (8) The straight end faces I (84) correspond respectively; An annular groove IV (45) is provided on the outer circular surface IV (42), and a sealing ring IV (46) is arranged inside the annular groove IV (45). 6. The multi-stage sleeve type ferrofluid seal device according to claim 1, characterized in that: it also includes a left magnetic spacer ring (11) and a right magnetic spacer ring (12); said left magnetic spacer ring (11 ) is set on the left side of the left pole piece (3), and is set close to the inner wall of the shell (2); the right magnetic isolation ring (12) is set on the right side of the right pole piece (5), close to the shell (2) ) of the inner wall setting. 7. The multi-stage sleeve type magnetic fluid seal device according to claim 1, characterized in that it further comprises a left bearing (13) and a right bearing (14), and the left bearing (13) and the right bearing (14) ) are respectively set on the shaft (1), the left bearing (13) is set on the left side of the left magnetic spacer ring (11), and the right bearing (14) is set on the right side of the right magnetic spacer ring (12). side. 8. The multi-stage sleeve-type ferrofluid seal device according to claim 1, characterized in that 2-10 groups of the inner permanent magnet ring (6) and the outer permanent magnet ring (7) are respectively arranged at intervals. 9. The multi-stage sleeve-type ferrofluid seal device according to claim 1, characterized in that: said axial pole teeth (10) and pole teeth I (85) are respectively arranged at intervals of 2 to 10 Composed of ring pole teeth. 10. The multi-stage sleeve-type ferrofluid seal device according to claim 1, characterized in that: the inner permanent magnet ring (6) and the outer permanent magnet ring (7) are both axially magnetized permanent magnets , the directions of the magnetic force lines of the inner permanent magnet ring (6) and the outer permanent magnet ring (7) are opposite.