GB2145169A

GB2145169A - A magnetic fluid seal

Info

Publication number: GB2145169A
Application number: GB08420718A
Authority: GB
Inventors: Dr Kevin Dermot O'grady; Dr John Popplewell; Dr Stuart Winzer Charles
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1983-08-16
Filing date: 1984-08-15
Publication date: 1985-03-20
Also published as: GB8420718D0; GB8322056D0

Abstract

A magnetic fluid seal comprises a stator including poles pieces 16, 18 and permanent magnets 20, a movable magnetic element 12, and bands of a magnetic fluid between the stator and the movable element. A pressurized gas introduced into an annular space 50 between a pair of adjacent bands, is maintained at a pressure equal to or slightly greater than the pressure on that side of the seal to be exposed to a liquid environment 58 held at a higher pressure than a gaseous environment at the other side of the seal. <IMAGE>

Description

SPECIFICATION A Magnetic Fluid Seal This invention relates to magnetic fluid seals, and more particularly to a seal in which magnetic fluid is used to maintain a pressure differential between a liquid environment and a gaseous environment.

Magnetic fluid seals usually comprise a stator element, a movable magnetizable element defining a gap between the stator element, and a permanent magnet for applying a magnetic flux across the gap. The stator or the movable element may be shaped to define a number of mutually adjacent teeth at the gap, and a magnetic fluid introduced into the gap is held by the magnetic flux at the teeth into a discrete number of narrow lines or bands across the gap.Examples of such magnetic fluid seals are described in British Patent Specifications Nos 783881 and 1312698 (United States Patent No. 3620584), and an article describing recent developments of magnetic fluid seals made by Ferrofluids Corporation, Nashua, New Hampshire, USA, appeared in the "Chartered Mechanical Engineer" May 1982 pp 52, 53, published by the Institution of Mechanical Engineers, London SW1, England.

One of the problems associated with magnetic fluid seals occurs when they are used to maintain a pressure differential between a gaseous environment and a liquid environment at a higher pressure than that of the gaseous environment. It has been found that liquid can penetrate between the narrow lines or bands, and the seal in use fails at a pressure differential much lower than it will withstand under static conditions.

According to one aspect of the present invention, in a magnetic fluid seal comprising a stator means, a movable magnetizable element, a plurality of mutually adjacent teeth being defined by the stator means or the movable element so as to define relatively narrow gaps between the other said means or element, and a magnet means for applying a magnetic flux across the gaps so as to retain a magnetic fluid therebetween, there is provided a space between a pair of adjacent said teeth adapted to contain a pressurized gas, and means for pressurizing the gas in the space until it is equal to or greater than the pressure applied on one side of the seal.

In another aspect, the invention provides a method of maintaining a seal formed by bands or lines of a magnetic fluid between a gaseous environment and a liquid environment that is at a higher pressure than that of the gaseous environment, the method comprising maintaining a gas in a space between a pair of adjacent said bands or lines at a pressure equal to or greater than the pressure of the liquid environment.

Preferably, the pressure of the gas in the space is controlled by a gas pressure regulating means controlled by a sensing means arranged to sense the pressure of the liquid environment.

In one application of the invention, the movable element might be rotatable relative to the stator means. Alternatively, the movable element might be slidable relative to the stator means.

The invention will now be further described by way of example only with reference to the accompanying drawing, in which: Figure 1 shows a side representation in medial section of a known form of rotary seal, and Figure 2 shows a part-diagrammatic and side representation in medial section of a rotary seal according to the invention.

Referring now to Figure 1, a rotary seal 10 is shown comprising a magnetizable shaft 12, disposed within a stator 14, the stator 14 comprising two annular magnetizable pole pieces 16, 1 8 respectively separated by an annular, axially magnetized permanent magnet 20. Each pole piece 1 6, 18, defines six inner circumferential teeth 22, and each tooth 22 defines a narrow radial gap 24 between the shaft 12 in order to concentrate the magnetic flux between the stator 14 and the shaft 12 into narrow annular rings of sufficient strength to retain by magnetic attraction bands 26 of a magnetic fluid, each band 26 providing a seal between the pole piece 16, 18 and the shaft 12.

The overall pressure sustaining capability of the rotary seal 10 is the cumulative pressure sustaining ability of the individual bands 26, the pressure of a fluid trapped between adjacent bands 26 increasing progressively through the rotary seal 10 from one side to the other, for example in increments of 31b/in2. The pole piece 18 is joined in sealing engagement to a bulkhead 30 having an aperture 32 through which the shaft 1 2 extends, and the rotary seal 10 separates a gaseous environment 34 at one side of the bulkhead 30, from a liquid environment 36 at a higher pressure.

The rotary seal 10 in operation. has the great advantage that there is no solid/solid contact, the only resistance to rotation being that required to overcome the viscous drag in the magnetic fluid of the bands 26.

Referring now to Figure 2, a rotary seal 40 is shown similar in many respects to the rotary seal 10 of Figure 1 in having a magnetizable shaft 12, but with a stator means provided by two sets 42, 44 of pole pieces 16, 18 separated by annular permanent magnets 20 in a housing 46. An annular space 50 is defined between the shaft 12, the sets 42, 44 and the housing 46, and a gas in the space 50 is pressurized by a source 52 of pressurized gas through a pressure regulator 54.

A remote sensor 56 detects the pressure of a liquid environment 58 at one side of a bulkhead 60, and controls the pressure regulator 54 until an equilibrium state is reached between the gas in the space 50 and the liquid environment 58. At equilibrium, no pressure difference occurs across the set 44 of pole pieces 1 6, 18, a pressure difference occurring only across the set 42 of pole pieces 1 6, 1 8 which has gas on each side.

It has been found with the rotary seal 10 of Figure 1, that the liquid penetrates between some of the bands 26, and when the shaft 12 is rotated the rotary seal 10 fails at a pressure differential much lower than that it will withstand under static conditions. It is thought that this may be due to turbulence in the liquid between the bands 26. In the rotary seal 40 of Figure 2, as there is no pressure difference between the liquid environment 58 and the annular space 50, penetration of the liquid between the bands 26 of the set 44 of pole pieces 16, 18 ought not to occur.

Alternative arrangements can be used for maintaining the gas-filled, annular space 60 at a pressure siightly greater than or equal to the pressure of the liquid environment 58. For example, a bellow (not shown) or a flexible diaphragm (not shown) arranged to be pressurized by the liquid environment 58 and communicating with the space 50.

It will be appreciated, that if desired teeth for retaining bands 26 of magnetic fluid may be defined by the shaft 12 of Figures 1 and 2 instead of by the pole pieces 1 6, 18.

Although the invention has been described in relation to a rotary seal, it will be understood that the invention is also applicable to sliding seals, for example a sliding seal in which lines or bands of a magnetic fluid are held by a stator means to provide a seal between a magnetizable slider member.

The selection of the magnetic fluid for use in the seal does not form part of the invention, such magnetic fluids being known in the art. These magnetic fluids usually comprise a colloidal suspension of magnetic materials such as ferrites, chromium dioxide, or magnetite, in a carrier liquid such as water or a hydrocarbon, with a surfactant, usually a fatty acid. Proprietary magnetic fluids may be obtained from the aforementioned Ferrofluids Corporation, or from Ferrox Limited, Black Horse House, 11 West Way, Botley, Oxford OX20 JB, England.

Claims

1. A magnetic fluid seal comprising a stator means, a movable magnetizable element, a plurality of mutually adjacent teeth being defined by the stator means or the movable element so as to define relatively narrow gaps between the other said means or element, and a magnet means for applying a magnetic flux across the gaps so as to retain a magnetic fluid therebetween, wherein there is provided a space between a pair of adjacent said teeth adapted to contain a pressurized gas, and means for pressurizing the gas in the space until it is equal to or greater than the pressure applied on one side of the seal.

2. A method of maintaining a seal formed by bands or lines of a magnetic fluid between a gaseous environment and a liquid environment that is at a higher pressure than that of the gaseous environment, the method comprising maintaining a gas in a space between a pair of adjacent said bands or lines at a pressure equal to or greater than the pressure of the liquid environment.

3. A magnetic fluid seal substantially as hereinbefore described, with reference to, and as shown in, Figure 2 of the accompanying drawings.