GB2104165A - Magnetic fluid sealing device - Google Patents

Abstract

A magnetic fluid sealing device has a housing, a shaft (2) accommodated in the housing, a plurality of permanent magnets (5) disposed, in series, coaxially with each other, around the shaft (2) and at least one polepiece (7) disposed between the two adjoining magnets. The polepiece (7) has a concave groove (9) on its inner periphery to form two knife edges (7a, 7b) for concentrating magnetic flux. <IMAGE>

Description

SPECIFICATION Magnetic fluid sealing device This invention relates to a magnetic fluid sealing device for sealing relatively movable surfaces such as rotating shafts, sleeves and the like to maintain a pressure difference across seals orto prevent leak age of a lubricant along the surfaces.

In relatively moving surfaces which may be lubri cated, it is sometimes necessary to interpose seals therebetween to prevent leakage of a lubricant or gas or to maintain a pressure difference across the seals where one of the surfaces such as a rotating shaft passes from one environment at a first pressure into a second environment at another pressure.

As such seals, it has been suggested that magnetic fluid be employed in the gap between relatively movable surfaces. The magnetic fluid comprises a carrier fluid such as water, a hydrocarbon, a fluorocarbon, or a fatty acid and magnet-type particles such as ferrite mixed in the carrier, and is captured in the gap by magnetic flux generated by one of more permanent magnets. In such a magnetic fluid seal, as the relatively movable surfaces do not directly contact each other, they are subject to hardly any wear, whereby the serviceable life of the seal is remarkably extended in comparison with those of mechanical seals. In addition, it provides a positive seal. Therefore, the magnetic fluid seal is often applied to an apparatus used in the environment of a high vacuum such as an X-ray tube apparatus.

Magnetic fluid sealing devices of this kind are disclosed in British Patent No. 783,881 and U.S. Patent No. 3,620,584. In Fig. 5 of the British patent, a shaft of magnetic material is rotatably accommodated in a housing. A plurality of annular permanent magnets are fixed to the inner surface ofthe housing in series in the longitudinal direction ofthe housing with annular polepieces, each being held between two adjoining permanent magnets. Between each of the inner peripheries of the polepieces and the outer surface of the shaft are formed sealing gaps in which the magnetic fluid is entrapped and retained.The two adjoining permanent magnets on opposite sides of each of the polepieces are so arranged that their polarities are symmetrical with respect to that polepiece, that is, the permanent magnets are arranged in the sequence of"N.S-polepiece-S- N-polepiece-N.S". This arrangement ofthe permanent magnets is the same as that of the present invention as discribed hereinafter. In addition, each of the inner peripheries of the polepieces is bevelled so as to form an annular upheaved portion in its central portion in the axial direction ofthe polepieces thereby to provide the desired concentration of the magnetic field. Magnetic flux generated from one of the magnets passes through one of polepieces across one of the gaps and through the shaft and back through another gap and polepiece to the magnet to complete a magnetic flux circuit.

The two magnetic flux flows generated from two adjoining permanent magnets in each polepiece pass across each gap through the upheaved portion of the polepiece in the same direction, and accord in ugly the two magnetic flux flows repu Ise each other to diverge and provide a relatively wide magnetic field for holding the magnetic fluid in the gap. Furth ermore, to avoid magnetic saturation of the polepieces, the thickness of each polepiece is limited, and a very thin plate cannot be used as a polepiece.

Therefore, as the magnetic flux density in the gaps between the inner peripheries of the polepieces and the circumferential surface of the rotating shaft is not very high, the holding force, for holding the magne tic fluid, generated by the magnetic flux passing through each gap is not very great. To increase the holding force, it is necessary to use one or more magnets with strong magnetic field or to form nar row gaps, for example, of less than 20 E.L. Accord ingly, it is difficult to assemble the sealing device so as to maintain narrow gaps between the rotating shaft and the polepieces. In addition, if the number of permanent magnets is increased to increase the number of barriers to confine the magnetic fluid, the sealing device will become large and bulky.

In the U.S. Patent No. 3,620,584, a magnetic fluid sealing unit is positioned between two ball bearings as shown in Fig. 5 of its accompanying drawings.

The fluid sealing unit comprises an annular perma nent magnet and two polepieces on opposite sides of the magnet. The inner periphery of each polepiece is concave in the shape of a triangle in radial section ofthe annular polepiece so as to form two knife edges on opposite ends of its inner periphery in the axial direction of the sealing unit. Magnetic fluid is held within each gap between its concave periphery and a rotating bushing. In this sealing unit, there is no special means to concentrate the magnetic flux passing through the gap, and accordingly the density of the magnetic flux in the gap is not very high. This type ofthe sealing unit cannot provide a complete seal to maintain a great pressure differ ence between two adjoining environments.

In order to increase the effective pressure differ ence, a plurality of polepieces each having a knife edge, defining a sealing gap, opposite to the rotating bushing may be disposed on opposite sides of the permanent magnet as shown in Fig. 6. In this design, a plurality of sealing barriers of magnetic fluid are separately formed in the axial direction of the bush ing. Fig. 7 in the same patent discloses a rotating bushing having a plurality of knife edges to form a plurality of sealing barriers for the same purpose as the sealing device in Fig. 6.

In these prior examples, the magnetic flux is mod erately concentrated at the knife edges ofthe polepieces or the bushing. However, the degree of its concentration is not enough for the sealing devices to maintain a great pressure difference in spite of only a small number of sealing barriers.

It is an object ofthis invention to provide a magne tic fluid sealing device in which a locally strong magnetic field is generated by a repulsive force bet ween two different flux flows passing through the same polepiece thereby to increase the force for holding the magnetic fluid and to maintain a high pressure difference with only a small number of seal ing barriers.

Another object ofthis invention to provide a magnetic fluid sealing device capable of holding the magnetic fluid steadily in spite of wide gaps between a plurality of polepieces and a rotating shaft thereby to facilitate assembly of the device.

According to this invention, there is provided a magnetic fluid sealing device for sealing relatively movable members, comprising: a housing; a shaft of magnetic material relatively rotating with respect to the housing and accommidated therein; a plurality of permanent magnets each having a central hole through which the shaft passes and disposed, in the housing, coaxially with one another in series so that the polarity of the opposite surfaces of two adjoining magnets is the same; and at least one polepiece having a central hole through which the shaft passes and disposed, in general, between the two adjoining permanent magnets, said polepiece having a concave groove on its inner periphery defining the central hole so as to form two knife edges at the flanks or the opposite ends of the inner periphery in the axial direction of the shaft, between each knife edge and the outer surface of the shaft being formed a sealing gap in which magnetic fluid is captured or retained to form a barrierfor maintining a pressure difference.

The nature, utility, and furtherfeatures ofthis invention will be more clearly apparent from the following detailed description with respect to a prefer red embodiment of the invention when read in conjunction with the accompanying drawings.

In the accompanying drawings: Fig. 1 is a side view, in longitudinal section, of an example ofthe device according to this invention; Fig. 2A is a enlarged fragmentary side view, in longitudinal section, of essential parts in Fig. 1; Fig. 2B is a schematic view showing the distribution of magnetic flux in each portion in Fig. 2A; Fig. 2C is a graphical representation showing magnetic flux densities in the portions corresponding to those of Fig. 2B; and Fig. 3 is a graphical representation showing advantageous effects derived according to the device of this invention in comparison with a device of the prior art.

Referring first to Fig. 1, a magnetic sealing device M, according to this invention, has a cylindrical housing 1 and a magnetic rotating shaft 2 inserted through the central portion ofthe housing 1 in its axial direction. One end of the shaft 2 is disposed in a vacuum environment and the other end thereof is disposed in a normal atmospheric environment. The housing 1 is provided with a cylindrical end portion 1a projecting coaxially from an end face of the housing 1. In this end portion 1a, two ball bearings 3 and 4 are disposed at positions spaced apart in its axial direction to rotatably hold the rotating shaft 2.

In the housing 1 are alternately disposed in series in its axial direction a plurality of annular permanent magnets 5, 5,. . and 5 and a plurality of annular polepieces 6, 7,. . and 8. Each of the magnets and the polepieces have central holes 12 and 13 respectively through which the shaft 2 passes. The permanent magnets 5 are so arranged, in general, that the polarity of two permanent magnets 5 and 5 on opposite sides of each polepiece 7 is symmetrical with respect to the polepiece 7. That is, the polarity of the opposite surfaces of two adjoining magnets 5 and 5 is the same each other, and more concretely, the magnets 5 and polepieces 6, 7, 8 are successively disposed in the sequence of "N-S-polepiece-S- N-polepiece-N Bus".

The polepieces 6 and 8 disposed at the two ends of the housing 1 have knife edges 6a and 8a, respectively, at their inner periphery. The inner wall surfaces ofthe end walls ofthe housing 1 are respectively provided with sealing rings 9 and 10 abutting against respective outer flanks ofthe polepieces 6 and 8.

The inner peripheries ofthe polepieces 6,7 and 8 are respectively provided with V-shaped annular concave grooves 9, 9, . . . and 9 so as to form respecitive knife edges 6a, 7a, 7b and 8a at the flanks or the ends of the inner peripheries in the axial direction of the housing 1 or shaft 2 as shown in Figs. 1 and 2A.

Sealing gaps g1, g2,. . and g,, are formed between the knife edges 6a, 7a, 7b and 8a and the cylindrical surface of the shaft 2, in which gaps magnetic fluid is captured at 11, 11,... and 11 to form sealing barriers to maintain the pressure difference between the above mentioned environments on the two sides of the rotating shaft 2.

The distribution of the magnetic flux generated by two adjoining permanent magnets 5 is as shown in Fig. 2B. That is, the magnetic flux from the N pole of a first permanent magnet 5L on the left side as viewed in Fig. 2B passes through the knife edge 7a of a second polepiece 7c and across the gap g4 to flow into the surface portion of the shaft 2. Then, the magnetic flux passes outofthe shaft2 and across the gap g3 adjacent to the gap g4 to flow into the knife edge 7b of a first polepiece 7L.On the other hand, the magnetic flux from the N pole of a second permanent magnet 5C in the center as viewed in Fig. 2B passes through the knife edge 7b of second polepiece 7C and across the gap g5 to flow into the shaft 2, and then across the gap g, to flow into the knife edge 7a of a third polepiece 7R. The two magnetic flux flows generated from the two different magnets 5L and 5C in the second pole piece 7C have the same positive polarities, and accordingly they repulse each other to be separated toward the knife edges 7a and 7b, respectively, and concentrated thereon. The concentration of the magnetic flux flows results in increasing the holding force for holding the magnetic fluid in the gaps g4 and g,.

In the third polepiece 7R, the magnetic flux generated from the second permanent magnet 5C and the magnetic flux generated from the third permanent magnet 5R repulse each other to be separated and concentrated on respective knife edges 7a and 7b of the third polepiece 7R. Accordingly, the density of magnetic flux passing across each gap becomes very high to remarkably increase the force holding for the magnetic fluid so as to completely maintain a vacuum on one side ofthe shaft 2. Furthermore, even if the device is so arranged that the gaps are relatively wide, for example 30 to 50it. it is possible to maintain a relatively great pressure difference whereby the arrangement of the shaft 2 and the polepieces 7 is facilitated.In addition, even if the number of permanent magnets to be used is decreased, it is possible to maintain a relatively great pressure difference, and accordingly useful devices of small size can be manufactured.

Fig. 2C shows the distribution of the magnetic flux density in each portion of the device. From Fig. 2C, it will be appreciated that the density rises steeply in the positions in which the gaps are located and the polarity of magnetic flux passing across the gaps g4 and g5 is opposite to that of magnetic flux passing across the gaps 96 and 97.

The advantageous effect according to this invention will be apparent from Fig. 3. In Fig. 3, the ordinate represents the magnitude of the pressure difference respectively maintained by the device according to this invention and a device of the prior art, and the abscissa represents the number of permanent magnets or sealing barriers. The effectivenesses ofthese devices, in general, are in proportion to the number of the permanent magnets. The capability of maintaining a pressure difference of the device of the present invention is indicated by the line B, while that ofthe device of the prior art (British Patent No.783,881) is indicated by the line A. It will be seen from the lines B and A that the performance, as measured in terms of capability of withstanding and maintaining pressure difference, of the sealing device of this invention is over 1.5 times that of the device ofthe prior art for the same number of sealing barriers.

Claims (3)

1. A magnetic fluid sealing device for sealing relatively movable members, comprising: a hous ing; a shaft of magnetic material relatively rotating with respect to the housing and accommodated therein; a plurality of permanent magnets each having a central hole through which the shaft passes and disposed, in the housing, coaxially with one another in series so that the polarity of the opposite surfaces of two adjoining magnets is the same; and at least one polepiece having a central hole through which the shaft passes and disposed, in general, between the two adjoining permanent magnets, wherein said polepiece has a concave groove on its inner periphery defining the central hole so as to form two knife edges at its flanks or the opposite ends of the inner periphery in the axial direction of the shaft, between the each knife edge and the outer surface of the shaft being formed a sealing gap in which magnetic fluid is captured or retained to form a barrier for maintaining a pressure difference.

2. A magnetic fluid sealing device according to claim 1, in which the inner periphery of each polepiece has a V-shaped groove.

3. A magnetic fluid sealing device substantially as herein described with reference to the accompanying drawings.