GB2236363A - An arrangement for a magnetic fluid seal - Google Patents

Abstract

Ferro-fluid seals are used to seat rotating parts, for example drive shafts in electric motors. Seals of this type comprise a plurality of parts and necessitate expensive assembly and adjustment operations. However, the seal (20) according to the invention comprises a single part, i.e. the pole ring (25) and the axially shorter of the surfaces delimiting the pole gap expands in cross-section in the direction of the magnetic flux. <IMAGE>

Description

AN ARRANGEMENT FOR A MAGNETIC FLUID SEAL The application relates to an arrangement for a magnetic fluid seal comprising a magnetic circuit, wherein a permanent magnet provides the excitation, in particular for systems with a standing shaft, and comprising at least one active pole gap filled with magnetically conducting liquid, which is vertically permeated by the exciting field and which is bounded by a stationary surface and by a surface rotating about an axis.

The pole gap is the so-called working air-gap.

One delimiting surface rotates, the other is stationary. In the case of a rotating shaft the rotating surface is very large, i.e. axially long (axis of rotation = axis of shaft), and the stationary surface is axially short and relatively small; in case of a stationary shaft the stationary surface is large (because the stationary shaft extends axially) and the rotating surface is small (because it extends axially over a short distance). The surface which in each case extends axially over a short distance can also be described as a pole shoe.

The surface extending axially over a longer distance may, of course, also be a guide surface, e.g.

a seat of a ball bearing. The pole shoe, is thus e.g.

a relatively thin, stamped, soft magnetic disc.

The pole shoe may also be a permanent magnetic, e.g.

a sintered, disc magnetized in the radial direction.

The spatial form of the pole shoe need not be disc-shaped. The pole shoe may also consist of the magnetic pole ring itself and is then a material of high remanence.

In between the rotating and the stationary surfaces the field permeates the working air-gap or pole gap, i.e. the air gap between rotating and stationary surfaces, into which the magnetically conducting and electrically weakly conducting liquid is introduced. Owing to the field forces in this working air-gap or pole gap, the liquid is retained there, even if a substantial pressure differential exists on either side of the same.

Known arrangements of the type mentioned above often consist of one or two flat discs and an axially magnetized permanent magnet located between them. The two discs consist of magnetizable material, and each of them forms a pole shoe for a respective air gap.

At least the gap between one pole shoe and one shaft is filled with a magnetically conducting fluid and thus seals a space located above the seal from a space located below the. seal (Fig. 1).

It is customary for such a magnetic seal to be installed in a motor so that the rotating shaft forms the magnetic return circuit via the two pole shoes which, therefore, do not rotate.

However, there are also systems where it is advantageous for the shaft to be stationary. In this case the seal, i.e. the pole shoes, must rotate (Fig. 2).

It is an object of the invention to simplify the known arrangements of the said seals.

According to a first aspect, the present invention provides an arrangement for a magnetic fluid seal comprising a magnetic circuit, wherein a permanent magnet provides the excitation and comprising at least one active pole gap filled with magnetically conducting fluid, vertically permeated by the exciting field, which is delimited by a stationary surface and by a surface rotating about an axis, characterized in that one (stationary or rotating) surface, which is axially short in relation to the other surface, expands as a cross-section in the direction of the magnetic circuit (flux).

The axially short surface expanding in cross-section can be that of a permanent magnet providing the excitation of the flux or field or that of a soft magnetic pole shoe. In particular, the permanent magnet can be a pole ring magnetized in radial direction. The axially short surface can be the radially outer surface of the pole ring or pole shoe.

In an alternative arrangement, the axially short surface is the inner surface of the pole ring or pole shoe.

In one embodiment, the axially short surface does not rotate and its body is mounted on a stationary shaft. Preferably, said body is axially immediately adjacent a ball bearing jointly forming the magnetic circuit. It also is preferred that the rotating surface is axially at least approximately as long as the maximum axial width of the body.

Alternatively, the axially short surface. does not rotate and its body is made to fit in a bearing bore of a bearing support member.

In another emdodiment, the axially short surface rotates and its body is mounted on a rotating shaft.

Preferably, said body is axially immediately adjacent a radial ball bearing jointly forming the'magnetic circuit. Suitablyt the rotating surface is axially at least approximately as long as the maximum axial width of the body.

The permanent magnetic pole shoe preferably is in the form of a stepped ring, which, in the region of the magnetically acting pole gap, can be axially magnetized.

In a second aspect, the present invention provides an arrangement for a magnetic seal having at least one active pole gap delimited by a stationary surface and a surface rotating about an axis and filled with magnetically conducting fluid, characterised in that one of said surfaces is provided by an axially shortened surface of a one-piece pole ring or shoe.

The seal in accordance with the invention, contrary to the known arrangements of this type, suitably is in one-piece construction, so that it is of a more compact height and is economical to fabricate. Owing to the axially shortened surface of the pole shoe in the region of the magnetically effective air gap the liquid friction is reduced and moreover, a saving in magnetically conducting fluid (sealing fluid) is made.

The one-piece constuction saves additional finishing work, in contrast with the known seals, whose components require mutual adaption.

Further details and advantageous developments of the invention are evident from the embodiments described below with reference to and as illustrated in the drawings.

In the drawings: Fig. 1 is a section through a known arrangement of a seal with a rotating shaft, Fig. 2 is a section through a known arrangement of a seal with a stationary shaft, Fig. 3 is a section through a first embodiment of an arrangement of a seal in accordance with the invention, Fig. 4 is a section through a second embodiment in accordance with the invention, Fig. 5 is a section through the pole ring of the embodiment of Fig. 3, and Fig. 6 is a section through the pole ring of the embodiment of Fig. 4.

Identical parts or parts acting identically are designated in the following Figures by the same reference numerals and described once only. The terms "above" and "below" relate to the representation in the particular Figure.

Fig. 1 shows a shaft 1 (for example of an electric motor) which runs in a bearing 2. The bearing 2 (rolling bearing or plain bearing) is supported in a bearing support member 3. Above the bearing 2, a seal 4 is mounted in the bearing support member so that the space located above it is sealed from the space located below it. The seal 4 consists of a permanent magnet ring 5, pole shoes 6 on both sides and a fluid with magnetic conductivity in the pole gap between the pole shoes 6 and the shaft 1.

In Fig. 2, an embodiment is shown where the shaft 1 is stationary and the bearing support member 3 together with the seal 4 rotates. The remaining parts have already been described in Fig. 1.

The term "standing shaft" means that the same part may be used as a shaft (rotating) and as an axis (stationary), and "... the surface rotating about an axis..." here implies a geometric axis.

Fig. 3 shows an embodiment in accordance with the invention with the shaft 1, onto which is fitted a seal 20. The seal 20 consists of one part only, which is in the form of a pole ring 25. The radial dimension of the seal gap amounts to approx. 0.1 mm.

The sealing fluid is electrically and magnetically conducting. The pole ring 25 preferably consists of a metallic magnet material of high remanence, so that in mounted, filled state a conditionally pressure-proof seal and electrically weakly conducting connection is formed between shaft and rotor. Contact resistances of the order of magnitude from 10 kQ to lMQ (depending on type of construction, degree of filling, material characteristic of the conducting sealing fluid) are sufficient for safe leakage of static charges, such as arise e.g. in plate drives through air friction. The pole ring 25 may also consist of a so-called plastic-magnetic material. In particular plastic-bonded SmCo magnets or others with high remanence are very suitable. To achieve electrical leakage the pole ring has to be metallized on one side with a coat of a thickness of approx. 0.5 to 1 copper or nickel.

The pole shoe or pole ring may also be in the form of a stepped ring 35 (Fig. 4). The magnetization (as also in the embodiment in Fig. 3) may be in the radial or in the axial direction.

In Figs. 5 and 6 the pole rings are represented once again as single components, indicating both directions of magnetization. The pole ring 25 is magnetized in the radial direction and the pole ring 35 in the axial direction, which is made clear by the marking of the magnetic poles (N,S).

The axial magnetization has the additional advantage that no return circuit closing takes place across parts of motor or bearing, and that two adjacent magnetic poles, arranged axially in the air gap, can create high field strengths in the air gap (pole gap 7) even in magnetically weaker material.

In Figs. 3 and 4 embodiments with rotating shaft are shown. Obviously these embodiments are also reversible (i.e. with stationary shaft).

Claims (9)

1. An arrangement for a magnetic fluid seal comprising a magnetic circuit, wherein a permanent magnet provides the excitation and comprising at least one active pole gap filled with magnetically conducting fluid, vertically permeated by the exciting field, which is delimited by a stationary surface and by a surface rotating about an axis, characterized in that one (stationary or rotating) surface, which is axially short in relation to the other surface, expands as a cross-section in the direction of the magnetic circuit (flux).

2. An arrangement in accordance with Claim 1, wherein the axially short surface expanding in cross-section is that of a permanent magnet providing the excitation of the flux or field or that of a soft magnetic pole shoe.

3. An arrangement in accordance with Claim 2, wherein the permanent magnet is a pole ring magnetized in radial direction.

4. An arrangement in accordance with Claim 2 or Claim 3, wherein the axially short surface is the radially outer surface of the pole ring or pole shoe.

5. An arrangement in accordance with any one of the preceding claims wherein the axially short surface does not rotate and its body is mounted on a stationary shaft.

6. An arrangement in accordance with Claim 5, wherein said body is axially immediately adjacent a ball bearing jointly forming the magnetic circuit.

7. An arrangement in accordance with Claim 5 or Claim 6, wherein the rotating surface is axially at least approximately as long as the maximum axial width of the body.

8. An arrangement in accordance with Claim 2 or Claim 3, wherein the axially short surface is the inner surface of the pole ring or pole shoe.

9. Apparatus as claimed in any of Claims 4 to 7, wherein said pole ring is mounted on a rotating shaft.

9. An arrangement in accordance with any one of the preceding claims, wherein the axially short surface does not rotate and its body is made to fit in a bearing bore of a bearing support member.

10. An arrangement in accordance with any one of the preceding claims, wherein the axially short surface rotates and its body is mounted on a rotating shaft.

11. An arrangement in accordance with Claim 10 wherein said body is axially immediately adjacent a radial ball bearing jointly forming the magnetic circuit.

12. An arrangement in accordance with Claim 10 or Claim 11, wherein the rotating surface is axially at least approximately as long as the maximum axial width of the body.

13. An arrangement in accordance with any one of the preceding claims, wherein the permanent magnetic pole shoe is in the form of a stepped ring.

14. An arrangement in accordance with Claim 13, wherein the stepped ring in the region of the magnetically acting pole gap is axially magnetized.

15. An arrangement for a magnetic fluid seal having at least one active pole gap delimited by a stationary surface and a surface rotating about an axis and filled with magnetically conducting fluid, characterised in that one of said surfaces is provided by an axially shortened surface of a one-piece pole ring or above.

16. An arrangement for a magnetic fluid seal substantially as described with reference to and as shown in Figure 3 or Figure 4 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. Apparatus comprising a shaft extending through a guide surface, a working gap being defined between a surface of said shaft and a surface of said guide surface, wherein either the shaft or the guide surface is rotatable about a longitudinal axis, and a magnetic fluid seal for sealing the working gap and comprising a magnetically conducting fluid received in said working gap and a permanent magnet providing magnetic flux within said working gap, and wherein one of said surfaces defining said working gap is shaped to define a working gap whose radial extent decreases along the axial extent of said working gap.

2. Apparatus as claimed in Claim 1, wherein said shaped surface is stepped along its axial extent.

3. Apparatus as claimed in Claim l, wherein said shaped surface is substantially wedge shaped.

4. Apparatus as claimed in any preceding claim, wherein said permanent magnet is in the form of a pole ring.

5. Apparatus as claimed in Claim 4, wherein said pole ring is magnetized in its radial direction.

6. Apparatus as claimed in Claim 4, wherein said pole ring is magnetized in its axial direction.

7. Apparatus as claimed in any of Claims 4 to 6, wherein the one of the surfaces defining the working gap which is not shaped is one of the inner or outer circumferential surfaces of said pole ring.

8. Apparatus as claimed in any of Claims 4 to 7, wherein said pole ring is mounted on a stationary shaft.