AU4268100A - Mechanical shaft to housing seal - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

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Name of Applicant/s: Actual Inventor/s: Address for Service: Invention Title: M.I.M. Holdings Limited Roland Thomas Dixon and David Gerard Hinds BALDWIN SHELSTON WATERS 60 MARGARET STREET SYDNEY NSW 2000 'MECHANICAL SHAFT TO HOUSING SEAL' Details of Associated Provisional Application No. PQ1357 dated 01 Jul 1999 The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 28455AUP00 Field of Invention The present invention relates to a mechanical shaft seal for fluid sealing an interface between a shaft and a through hole in a housing.

The invention has been developed primarily for use in high rotational speed or high wear applications such as grinding mills, and will be described herein after with reference to that application. However, it will be appreciated by those skilled in the art that the invention is not limited to use in this field.

Background to the Invention In the context of the present application, mechanical seals are used to allow a shaft to rotate within a through hole formed in a wall or partition, such as that formed by a ooooo machinery housing. This enables torque to be transferred from an external power source :to, for example, gears or milling components within the housing. Since many grinding, :i milling and other such applications require the existence of a lubricating fluid within the housing, the seal is often required to prevent undesirable egress of fluid and other materials from within the housing. Resilient interference fit seals can sometimes be used, but harder mechanical seals are preferential in situations where high rotational speeds or the potential for high wear rates exist.

Mechanical seals typically include a shaft sleeve which rotates with the shaft, and an inlet flange affixed to the housing through which the shaft passes. The required fluid seal is created between the shaft sleeve and the inlet flange at a seal surface interface.

The interface is normally orientated perpendicular to the axis of rotation of the shaft, as with clutch plate assemblies in an automobile clutch assembly. Alternatively, the contact interface can be circumferential, as in a drum brake assembly.

-3- Pressure is applied to the contact surfaces by means of mechanical or hydraulic force, or some combination of the two. Mechanical force is imposed by either the rigidity of the assembly or by means of a resilient spring. Hydraulic force is imposed through fluid pressure applied behind one or both of the contact surfaces by means of a fluid supply gland. Lubrication for the contact surfaces can be supplied either by egress of fluid from the gland via the seal surface interface, or from direct egress of a fluid from the interior of the housing.

A substantial disadvantage of such mechanical seals is the requirement for a relatively strongly constrained positional relationship between the shaft and the through hole in the housing. Typically, and particularly in high wear applications such as ooooo S. .grinding mills, mechanical seals are formed from relatively hard metals such as "Tungsten Carbide which are machined to a close interference tolerance. Unfortunately, o• this results in a seal which offers little tolerance for shaft movement relative to the housing in either axial or radial directions. Where relative movement of the shaft is forcibly imposed due to, for example, warping or eccentricities in the shaft or even low frequency vibration, the result can be increased fluid leakage, increased wear or seal failure where relative movement between the shaft and housing is too great.

The problem is exacerbated when the housing contains material which itself can increase the failure rate of such mechanical seals. For example, in a grinding mill, abrasive particles of various sizes are immersed in a lubricating fluid, and can cause virtually instantaneous seal failure if they become lodged between the seal contact surfaces.

It is an object of the invention to provide a mechanical seal arrangement that overcomes or at least substantially ameliorates, one or more of the disadvantages of the prior art, or at least provides the public with a useful alternative to known mechanical seal arrangements.

Summary of Invention According to a first aspect of the invention there is provided a seal for providing fluid sealing at an interface between a housing and an axially extending shaft entering a through hole in the housing for rotation with respect thereto, the seal allowing degrees of axial and radial movement of the shaft with respect to the housing whilst maintaining the fluid sealing, the seal including: shaft seal disposed on the shaft for rotation therewith, the shaft seal having a first sealing surface extending substantially axially and a second sealing surface having at o• •:least a radially extending component; a housing seal associated with the housing and constrained against substantial rotation with the shaft, the housing seal having a third sealing surface extending o oe substantially axially and a fourth sealing surface having at least a radially extending *component; the seal being configured such that, in use, the first and third sealing surfaces sealingly engage each other at a first sealing interface whilst the second and fourth sealing surfaces sealingly engage each other at a second sealing interface, to allow degrees of axial and radial movement of the shaft with respect to the housing whilst maintaining the fluid sealing.

Preferably, the seal includes bias means acting on the housing seal or the shaft seal, wherein the first and third sealing surfaces sealingly engage each other in an axially slidable manner at the first sealing interface whilst the second and fourth sealing surfaces are urged by the bias means into sealing engagement with each other at the second sealing interface, thereby allowing degrees of axial and radial movement of the shaft with respect to the housing whilst maintaining the fluid sealing. More preferably, the bias means acts on the housing seal to urge it, and thereby the fourth sealing surface, towards the second sealing surface.

Preferably also, the shaft seal includes a first seal portion and a second seal portion, the first and second seal portions being separate components and the first and ***second sealing surfaces being disposed on the first and second seal portions respectively.

In a preferred form, the housing seal includes a third seal portion and a fourth seal :i portion, the third and fourth seal portions being separate components and the third and fourth sealing surfaces being disposed on the third and fourth seal portions respectively.

Preferably, the first sealing interface is axially spaced from the second sealing "interface to define an annular fluid cavity therebetween. More preferably, the seal further includes a fluid gland for introducing a fluid into the annular fluid cavity under S"pressure. Even more preferably, the sealing interfaces are configured to allow a predetermined amount of fluid weepage to lubricate the first, second, third and fourth sealing surfaces.

Preferably, the seal is configured such that the first interface allows greater weepage of the fluid than the second interface, thereby preferentially weeping fluid into the housing. More preferably, the fluid is introduced to the annular cavity at a pressure -6higher than a fluid pressure within the housing, thereby preventing egress of material from the housing to the annular cavity via the first sealing interface.

Preferably also, the housing seal is disposed at least partially within an annular recess associated with the housing, the annular recess being sized and configured to allow radial movement of the housing seal therewithin. More preferably, the seal further includes means for constraining rotational movement of the housing seal within the annular recess.

Preferably also, the third seal portion is disposed at least partially within an annular recess associated with the housing, the annular recess being sized and configured to allow radial movement of the third seal portion therewithin. More preferably, the seal further includes means for constraining rotational movement of the third seal portion "within the annular recess.

•According to a second aspect of the invention there is provided a seal for providing fluid sealing between a housing and an axially extending shaft entering a through hole in the housing for rotation with respect thereto, the seal including: a shaft seal disposed on the shaft for rotation therewith, the shaft seal having a first sealing surface and a second sealing surface; oo00 S•a housing seal associated with the housing and constrained against substantial rotation with the shaft, the housing seal having a third sealing surface and a fourth sealing surface, the seal being configured such that, in use, the first and third sealing surfaces engage each other at a first sealing interface adjacent an interior of the housing whilst the second and fourth sealing surfaces engage each other at a second sealing interface remote from the interior of the housing; and -7a fluid delivery means for introducing a lubricating fluid to an area intermediate the first sealing interface and the second sealing interface; wherein each of the first and second sealing interfaces has an associated leakage tolerance, the leakage tolerances being selected such that the first sealing interface allows greater lubricating fluid leakage than the second sealing interface for a given lubricating fluid delivery pressure.

Preferably, the first sealing interface is axially spaced from the second sealing interface, and an annular fluid cavity is defined therebetween.

Preferably, the lubricating fluid delivery means includes a fluid gland for introducing a fluid into the annular fluid cavity under pressure.

Other exemplary aspects of the invention are set out in the numbered paragraphs at the end of this specification.

S. ""Brief Description of Drawings S-Preferred embodiments of the invention will now be described, by way of example oo o only, with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a mechanical seal providing fluid sealing between a housing and a shaft entering the housing via a through hole, the mechanical seal being in 0accordance with the invention; Figure 2 is a sectional view of components from Figure 1 associated with the housing; Figure 3 is a sectional view of components from Figure 1 associated with the shaft; -8- Figure 4 is a sectional view of an alternative mechanical seal for providing fluid sealing between a housing and a rotating shaft passing therethrough, in accordance with the invention; Figure 5 is a sectional view of yet another mechanical seal in accordance with the invention; and Figure 6 is a sectional view of yet another mechanical seal according to the invention.

Detailed Description Referring to the drawings, a mechanical seal 1 provides fluid sealing at an interface between a housing 2 and an axially extending shaft 4 where the shaft 4 enters a through *!*hole 6 in the housing 2. In the example illustrated, the housing 2 forms part of a :grinding mill (not shown), and the shaft 4 provides torque from an external power source such as an electric or diesel motor (not shown).

The mechanical seal 1 includes a shaft seal 8 disposed on the shaft 4 for rotation therewith. In the embodiment shown in Figures 1 to 3 the shaft seal includes a first seal portion 10 and a second seal portion 12, upon which are formed first and second sealing surfaces 14 and 16 respectively. The first sealing surface 14 extends substantially S"axially, whilst the second sealing surface 16 extends radially. The first and second seal portions 10 and 12 are affixed to the shaft 4 in such a way that they rotate when the shaft 4 rotates. This can be achieved by means of a tight interference fit between the shaft 4 and each of the first and second seal portions 10 and 12 respectively. Alternatively, in other embodiments, rotation can be prevented by threaded fasteners, interlocking teeth on the seal portion and shaft, or by any other suitable mechanism.

-9- The second seal portion is sealed against the shaft 4 by means of an O-ring 29.

The mechanical seal 1 also includes a housing seal 18 mounted to the housing 2, the housing seal 18 including a third seal portion 20 and a fourth seal portion 22. The third seal portion 20 is mounted within an annular seat 24, configured such that the third seal portion 20 can move in a radial plane relative to the annular seat 24 but is constrained against rotation with the shaft 4. In the embodiment shown, this is accomplished by means of two diagonally opposite splines 26 formed in the radially outer periphery of the annular seat 24, which mesh with corresponding recesses 28 in the outer periphery of the third seal portion 22. As with the first and second seal portions 10 and 12, rotation can be constrained by any known mechanism, although in this case there must be provision for ***movement of the third seal portion 20 in a radial plane. In this case, the recesses 28 are relatively larger than the splines 26, which allows a degree of movement in the radial plane whilst limiting the amount by which the third seal portion 20 can rotate with respect to the housing 2. A third sealing surface 30 is formed on an inner periphery of the third seal portion The fourth seal portion 22 is mounted in an annular recess 40 formed at one axial end of a cylindrical carrier 42. The cylindrical carrier 42 is in turn slidably mounted to the housing 2 by means of guide shafts 44 (only one of which is shown). The guide shafts 44 pass through axial holes 46 formed in a main flange 48 on the carrier 42. This enables axial sliding motion of the carrier 42, and hence the fourth seal portion 22, with respect to the housing 2, whilst preventing relative rotation with respect thereto.

The guide shafts 44 include end flanges 50 and a bias spring 52 extends between the end flanges 50 and the main flange 48 formed on the carrier 42. The bias spring 52 is compressed, and axially biases the carrier 42 to the right of Figures 1 and 2.

As with the third seal portion, the fourth seal portion 22 is constrained from rotation with respect to the cylindrical carrier 42, and thereby the housing 2, by means of an interlocking recess and key arrangement 34. However, in this case there is no requirement for radial movement and so the fourth seal portion 22 can be constrained from rotation in any of the ways described in relation to the first and second seal portions and 12. A fourth sealing surface 32 is formed on an axial face of the fourth seal portion 22. An axial surface of the third seal portion 20 is sealed by means of an O-ring oooeo 27 to the housing 2.

An O-ring 54 is used to seal the housing 2 and carrier 42 together. It will be appreciated that as the carrier 42 moves axially with respect to the housing 2, the O-ring 54 ensures that they remain sealed with respect to each other.

As best shown in Figure 1, in use, the first sealing surface 14 abuts the third :sealing surface 30 at a first sealing interface 56, whilst the second sealing surface 16 abuts the fourth sealing surface 32 at a second sealing interface 58. Between the first sealing interface 56 and the second sealing interface 58 there is formed an annular cavity for containing a fluid supplied under pressure from a fluid gland (not shown) via the housing 2. By careful selection of sealing tolerances at the first and second sealing interfaces 56 and 58, and the selection of the applied pressure of the fluid into the gland, a controlled amount of fluid weepage from the annular cavity 60 is allowed, thereby lubricating the seal 1. This also acts to ensure that the abrasive materials from within the -11 grinding mill do not enter the first or second sealing interfaces 56 and 58, which would cause relatively rapid failure of the mechanical seal 1. Fluid weepage is also allowed from the cavity through the second sealing interface, again, to provide lubrication. It is desirable to select sealing tolerances of the first and second sealing interfaces such that s fluid/preferentially weeps through the first interface. This reduces spillage of lubricating fluid from the second sealing interface into the area surrounding the housing, as well as providing a greater flow of the lubricating fluid where it is most needed to induce preferential weeping via the first interface 56.

It will be appreciated that many other forms and configurations of mechanism seal can be implemented, depending upon such factors as the desired sealing characteristics 0..

*0and the environment in which the seal is to operate.

One example of an alternative embodiment is shown in Figure 4, in which features o0 similar to those in previous Figures are designated with like numerals. As with the previous embodiment, the mechanical seal 1 includes first, second, third and fourth sealing surfaces 14, 16, 30 and 32. However, the first and second sealing surfaces 14 and 16 are both formed on a single piece shaft seal 8. This embodiment reduces the number of individual components required to implement a seal according to the invention. However, there are some commercial disadvantages associated with the practical implementation of this embodiment. In particular, the axially extending sealing interfaces tend to wear out of tolerance relatively faster, necessitating replacement, or at least complex re-machining.

By contrast, the earlier embodiments' use of a spring loaded radial seal ensures that a relatively consistent tolerance can be maintained as the seal interface wears. As -12the seal interface wears to the point where the bias force of the springs is below a minimum standard, spacers (not shown) can be positioned co-axially with the springs to recompress them, thereby increasing the force and bringing the tolerance back within an acceptable range. In the embodiments shown, it is still necessary to replace or remachine the seal portions having an axially extending interface relatively more often.

However, this is preferable to having to replace both seals as often.

The use of a seal having two radially extending sealing interfaces is also possible.

However the various springs and other components required for the interface adjacent the interior of the housing can be exposed, depending upon the particular application, to corrosive, abrasive or otherwise undesirable substances within the housing. For this 5 0 reason, the use of a seal with a generally axially extending seal interface adjacent the housing is preferred.

e¢ Yet another embodiment of the invention is shown in Figure 5, in which the shaft seal 8 and the housing seal 18 each take the form of single components. In the 6500 embodiment shown, the housing seal 18 is again retained within an annular seat 24.

Opposite axial faces 62 and 64 of the housing seal 18 are sealed to the housing 2 by 6eo oo* means of corresponding O-rings 66 and 68 formed within their own annular seats 69 and •go• 70. This results in the formation of an annular cavity 71 adjacent an outer periphery 74 of the housing seal 18. This cavity 71 provides the radial space required for the housing seal 18, and thereby the shaft seal 8 and shaft 4, to move within the radial plane.

The cavity 71 can also accept a fluid introduced under pressure into the peripheral cavity 71 via a fluid supply gland 72. A lubrication hole 73 is formed radially in the housing seal 18 to enable fluid under pressure to pass from the cavity 71 to the interface -13between the housing seal 18 and the shaft seal 8. This ensures adequate lubrication, which enables easier axial movement of the shaft with respect to the housing 2 whilst maintaining an adequate seal.

It will be appreciated that in other embodiments (not shown) the lubricating fluid can be supplied under pressure directly to the area intermediate the first and second seal interfaces. In that case, no annular cavity is required.

It will also be understood that the seal need not allow for radial or axial movement, although one or both of these aspects are preferred. In the case where no such movement is allowed, the preferential weepage of lubricating fluid into the housing becomes 10 particularly important to the invention. For example, one can modify the embodiment of S0 Figure 5 such that radial movement is not provided. In that case, the seal can be 0* considered to have a first sealing interface 90 on one side of the fluid port 71, and a 00o0 second sealing interface 92 on the other side. The leakage tolerances of the first and second sealing interfaces 90 and 92 are then selected to allow preferential weeping of 00 fluid into the housing via the first interface 90. The fluid pressure is selected such that 000.

0enough fluid passes through the second sealing interface to provide sufficient lubrication 0000 and cooling thereof.

00 Figure 6 shows yet another embodiment of the invention, in which the first and second sealing interfaces 56 and 58 extend axially. In other respects, the embodiment shown in Figure 6 is relatively similar to that shown in Figures 1 to 3, and like features are indicated with corresponding reference numerals. As with other embodiments, the Figure 6 embodiment can be altered such that either or both axial and radial movement is constrained. In such modified embodiments, the tolerances of the first and second -14sealing interfaces 56 and 58 are selected to reduce preferential weeping of lubricating fluid from annular cavity 60. Alternatively, where the seal is configured to allow axial and radial movement of the shaft 4 with respect to the housing 2, the preferential weeping aspect of the invention is optional.

In other embodiments not illustrated, the shaft seal 8 (the first portion 10, where provided) is disposed in an axial seat on the shaft 4 such that the shaft seal 8 can move radially with respect to the shaft, but is constrained to rotate with the shaft. This would similarly allow relative radial movement between the shaft 4 and housing 2 whilst maintaining fluid sealing therebetween.

10 In yet other embodiments, also not shown, either or both of the sealing interfaces can include sections that are curved in cross-section or inclined at an angle between the •axial and a plane normal to the axis.

At least in the preferred embodiment, the mechanical seal according to the invention provides a novel way of allowing both radial and axial movement of a shaft with respect to a housing, whilst maintaining a seal therebetween, not withstanding the potential existence of abrasive materials which preclude the use of resilient seals. As such, the present invention presents a commercially significant improvement over prior art seals.

Those skilled in the art will appreciate that the various embodiments described are exemplary only, and that many different and varied embodiments can be constructed without departing from the spirit and scope of the present invention as envisaged within this specification.

Claims (29)

1. A seal for providing fluid sealing at an interface between a housing and an axially extending shaft entering a through hole in the housing for rotation with respect thereto, the seal allowing degrees of axial and radial movement of the shaft with respect to the housing whilst maintaining the fluid sealing, the seal including: a shaft seal disposed on the shaft for rotation therewith, the shaft seal having a first sealing surface extending substantially axially and a second sealing surface having at least a radially extending component; a housing seal associated with the housing and constrained against substantial 10 rotation with the shaft, the housing seal having a third sealing surface extending *.substantially axially and a fourth sealing surface having at least a radially extending component; the seal being configured such that, in use, the first and third sealing surfaces sealingly engage each other at a first sealing interface whilst the second and fourth sealing surfaces sealingly engage each other at a second sealing interface, to allow *m adegrees of axial and radial movement of the shaft with respect to the housing whilst maintaining the fluid sealing.

2. A shaft seal according to claim 1 including bias means acting on the housing seal or the shaft seal, wherein the first and third sealing surfaces sealingly engage each other in an axially slidable manner at the first sealing interface whilst the second and fourth sealing surfaces are urged by the bias means into sealing engagement with each other at the second sealing interface, thereby allowing degrees of axial and radial movement of the shaft with respect to the housing whilst maintaining the fluid sealing. -16-

3. A seal according to claim 2 wherein the bias means acts on the housing seal to urge it, and thereby the fourth sealing surface, towards the second sealing surface.

4. A seal according to claim 1 wherein the shaft seal includes a first seal portion and a second seal portion, the first and second seal portions being separate components and the first and second sealing surfaces being disposed on the first and second seal portions respectively.

A seal according to claim 1 wherein the housing seal includes a third seal portion and a fourth seal portion, the third and fourth seal portions being separate components and the third and fourth sealing surfaces being disposed on the third and fourth seal portions respectively.

6. A seal according to claim 1 wherein the first sealing interface is axially spaced o from the second sealing interface to define an annular fluid cavity therebetween.

7. A seal according to claim 6 including a fluid gland for introducing a fluid into the annular fluid cavity under pressure.

8. A seal according to claim 7 wherein the sealing interfaces are configured to allow a *predetermined amount of fluid weepage to lubricate the first, second, third and fourth :i sealing surfaces.

9. A seal according to claim 8 and being configured such that the first interface allows greater weepage of the fluid than the second interface, thereby preferentially weeping fluid into the housing.

A seal according to claim 7 wherein the fluid is introduced to the annular cavity at a pressure higher than a fluid pressure within the housing, thereby preventing egress of material from the housing to the annular cavity via the first sealing interface. -17-

11. A seal according to claim 1 wherein the housing seal is disposed at least partially within an annular recess associated with the housing, the annular recess being sized and configured to allow radial movement of the housing seal therewithin.

12. A seal according to claim 11 including means for constraining rotational movement of the housing seal within the annular recess.

13. A seal according to claim 3 wherein the third seal portion is disposed at least partially within an annular recess associated with the housing, the annular recess being sized and configured to allow radial movement of the third seal portion therewithin.

14. A seal according to claim 13 including means for constraining rotational 10 movement of the third seal portion within the annular recess.

.15. A seal for providing fluid sealing between a housing and an axially extending shaft entering a through hole in the housing for rotation with respect thereto, the seal including: ee a shaft seal disposed on the shaft for rotation therewith, the shaft seal having a first sealing surface and a second sealing surface; a housing seal associated with the housing and constrained against substantial rotation with the shaft, the housing seal having a third sealing surface and a fourth sealing surface, the seal being configured such that, in use, the first and third sealing surfaces engage each other at a first sealing interface adjacent an interior of the housing whilst the second and fourth sealing surfaces engage each other at a second sealing interface remote from the interior of the housing; a fluid delivery means for introducing a lubricating fluid to an area intermediate the first sealing interface and the second sealing interface; -18- wherein each of the first and second sealing interfaces has an associated leakage tolerance, the leakage tolerances being selected such that the first sealing interface allows greater lubricating fluid leakage than the second sealing interface for a given lubricating fluid delivery pressure.

16. A seal according to claim 15 wherein the first sealing interface is axially spaced from the second sealing interface, and an annular fluid cavity is defined therebetween.

17. A seal according to claim 15 wherein the shaft seal includes a first seal portion and a second seal portion, the first and second seal portions being separate components and the first and second sealing surfaces being disposed on the first and second seal portions respectively.

18. A seal according to claim 15 wherein the housing seal includes a third seal portion o and a fourth seal portion, the third and fourth seal portions being separate components and the third and fourth sealing surfaces being disposed on the third and fourth seal oooo portions respectively.

19. A seal according to claim 15 wherein the lubricating fluid delivery means includes a fluid gland for introducing a fluid into the annular fluid cavity under pressure.

A seal according to claim 15 wherein the lubricating fluid is introduced at a pressure higher than a fluid pressure within the housing, thereby preventing ingress of fluids or materials from the housing into the first sealing interface.

21. A seal according to claim 15 wherein the housing seal is disposed at least partially within an annular recess associated with the housing, the annular recess being sized and configured to allow radial movement of the housing seal therewithin. -19-

22. A seal according to claim 21 further including means for constraining rotational movement of the housing seal within the annular recess.

23. A seal according to claim 18 wherein the third seal portion is disposed at least partially within an annular recess associated with the housing, the annular recess being sized and configured to allow radial movement of the third seal portion therewithin.

24. A seal according to claim 23 including means for constraining rotational movement of the third seal portion within the annular recess.

A seal according to claim 1 or claim 15 wherein one or more of the first, second, third and fourth sealing surfaces is formed from a wear-resistant metal..

26. A seal according to claim 25 wherein the metal is tungsten carbide.

.27. A shaft seal according to claim 1 including a spring acting on the housing seal or S•the shaft seal, wherein the first and third sealing surfaces sealingly engage each other in an axially slidable manner at the first sealing interface whilst the second and fourth sealing surfaces are urged by the spring into sealing engagement with each other at the second sealing interface, thereby allowing degrees of axial and radial movement of the :::shaft with respect to the housing whilst maintaining the fluid sealing.

28. A seal for providing fluid sealing between a housing and an axially extending shaft entering a through hole in the housing for rotation with respect thereto, the seal including: a shaft seal disposed on the shaft for rotation therewith, the shaft seal having a first sealing surface and a second sealing surface; a housing seal associated with the housing and constrained against substantial rotation with the shaft, the housing seal having a third sealing surface and a fourth sealing surface, the seal being configured such that, in use, the first and third sealing surfaces engage each other at a first sealing interface adjacent an interior of the housing whilst the second and fourth sealing surfaces engage each other at a second sealing interface remote from the interior of the housing; a fluid cavity for allowing the introduction a pressurised lubricating fluid to an area intermediate the first sealing interface and the second sealing interface; wherein each of the first and second sealing interfaces has an associated leakage tolerance, the leakage tolerances being selected such that the first sealing interface allows greater lubricating fluid leakage than the second sealing interface for a given lubricating fluid delivery pressure.

.29. A seal for providing fluid sealing at an interface between a housing and an axially extending shaft entering a through hole in the housing for rotation with respect thereto substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings. DATED this 26th Day of June, 2000 M.I.M. HOLDINGS LIMITED Attorney: JOHN B. REDFERN Fellow Institute of Patent Attorneys of Australia of BALDWIN SHELSTON WATERS