WO2017149263A1

WO2017149263A1 - Rotary seal assembly

Info

Publication number: WO2017149263A1
Application number: PCT/GB2016/050564
Authority: WO
Inventors: James Robb; Dean Smith; Clive Wilson; Giles CLUFF
Original assignee: Wärtsilä Uk Limited
Priority date: 2016-03-03
Filing date: 2016-03-03
Publication date: 2017-09-08

Abstract

A rotary seal assembly (2) comprises first and second sealing elements configured for rotational relative movement about a rotational axis and providing a sealing interface between adjacent first and second sealing faces of the first and second sealing elements. The first sealing element comprises a composite material that includes fibrous reinforcing elements within a resin matrix, and the second sealing element comprises a metallic body having a coating on the second sealing face, said coating having thermal conductivity exceeding that of the metallic body.

Description

Rotary Seal Assembly

The present invention relates to a rotary seal assembly that provides a fluid resistant seal between a pair of relatively rotating components. The seal assembly is suitable for use in numerous applications, for example in marine applications where the seal assembly provides a substantially watertight seal between the propeller shaft and the hull of a ship or submarine, or between an impeller shaft and a housing in a waterjet propulsion system. The seal assembly may also be used in other applications, for example in the generator turbine of a hydroelectric generator or a tidal turbine.

The primary purpose of a rotary seal is to prevent fluid flowing between a pair of rotating components. For example, in the case of a propeller shaft of a ship, the seal prevents seawater flowing through the stern tube into the ship. As well as preventing the flow of fluid, the seal assembly must be robust, reliable, resistant to wear, and have low friction characteristics. Typically, rotary seals are lubricated by a thin film of fluid that forms in the sealing interface between the opposed components of the seal (the face and the seat). The lubricating fluid is typically oil, water or a mixture of fluids. The present invention is concerned primarily, but not exclusively, with water lubricated seals.

Many rotary seal configurations are known including face seals where the sealing interface extends substantially radially from the axis of the rotary component, and lip seals where the sealing interface is cylindrical and coaxial with the axis of the rotary component. The seal assembly of the present invention is designed primarily for use in face seals (including both outside diameter and inside diameter pressurised face seals), but it is also applicable to other kind of rotary seal including lip seals. An important characteristic of a rotary seal is its pressure- velocity (PV) limit, which is calculated as a function of the mean rotational interface velocity and the net closing pressure of the seal interface. A typical value for low to medium duty rotary seals in marine applications is in the range 5-20 Bar metres per second (Bar.m/s). The present invention is concerned primarily, but not exclusively, with medium to high duty applications having a PV limit of about 50 Bar.m/s or higher. A high PV factor places an increased demand on the rotary seal assembly, which must be carefully designed to provide the required performance.

The performance of a rotary seal at high PV values depends largely on the materials selected for the components that form the sealing interface, as well as the overall design of the rotary seal. Typically, seawater lubricated bronze/composite interfaces can achieve an upper PV limit of about 21 Bar.m/s. For higher PV values silicon carbide (SiC) materials can be used for the sealing components, which can provide an operating PV value in the range 15-70 Bar.m/s, with an upper limit of about 120 Bar.m/s. However, one problem with the use of SiC is that the material is brittle and can fracture when subjected to a shock load if it cannot be suitably isolated from the load. This is a major problem with naval vessels, which may experience severe shock loads during combat or, for example, as a result of an underwater explosion. Failure of the seal in such a situation can compromise the safety of the vessel and ultimately could cause the vessel to sink. The brittle nature of SiC also limits the usefulness of this material in other marine and non-marine applications.

It is an object of the present invention to provide a rotary seal assembly that mitigates one or more of the aforesaid problems.

According to one aspect of the present invention there is provided a rotary seal assembly comprising first and second sealing elements configured for rotational relative movement about a rotational axis and providing a sealing interface between adjacent first and second sealing faces of the first and second sealing elements, wherein said first sealing element comprises a composite material that includes fibrous reinforcing elements within a resin matrix, and wherein said second sealing element comprises a metallic body having a coating on the second sealing face, said coating having thermal conductivity exceeding that of the metallic body. In other words, the metallic body has a first thermal conductivity and the coating has a second thermal conductivity that is higher than the first thermal conductivity.

The combination of a first sealing element comprising a composite material and a second sealing element comprising a metallic body with a high thermal conductivity coating produces a rotary seal assembly that is capable of supporting a high PV duty, but which also has a very high level of shock resistance. It is therefore highly suitable for use in naval vessels, as well as numerous other applications.

The composite material of the first sealing element may comprise a resin and a fibrous reinforcing material. In an embodiment the resin is an epoxy resin. Alternatively, the resin may for example be a phenolic resin or a polyethylene resin. The fibrous reinforcing material may for example include fibres selected from glass, aramid, nylon, carbon, PTFE or natural fibres, for example cotton. The fibrous reinforcing material may be provided in the form of individual filaments or as a woven or non-woven cloth.

In an embodiment, the first sealing element is a filament-wound product or component. Alternatively it may be, for example, a cloth-wound component, a flat laminate component (formed either by mechanical or vacuum induced moulding), a low pressure moulded component or an injection moulded component.

The composite material may include a friction modifier, for example graphite or PTFE, to reduce friction between the first and second sealing elements. The second sealing element may be made of a metal selected from the range comprising copper alloys, phosphor bronzes, gun metals, aluminium bronzes and stainless steels. In one embodiment the second sealing element is made from a chromium zirconium copper alloy, which provides a high thermal conductivity, good thermal stability, and a high hardness.

The metallic body of the second sealing element may have a thermal conductivity of at least 20W/mK, more preferably at least 50W/mK, yet more preferably at least 200W/mK. The high thermal conductivity ensures that heat generated by friction is conducted away from the sealing elements, so reducing the risk of thermal degradation. The coating on the second sealing element may have a thermal conductivity of at least 300W/mK, preferably at least 600W/mK, more preferably at least lOOOW/mK. The high thermal conductivity of the coating also helps to ensure that heat generated by friction is conducted away from the sealing elements, to reduce the risk of thermal degradation. The coating may have a hardness of at least 1000 Hv, preferably at least 2000 Hv, more preferably at least 3000 Hv. The high hardness helps to ensure low wear, thus increasing the working life of the seal assembly.

The coating may comprise a diamond coating, which preferably has a thickness in the range 8-70 microns. A diamond coating has been found to be very suitable, having a very high thermal conductivity and hardness. Other coating materials with suitable properties are also available.

In one embodiment the sealing assembly comprises a face seal in which the sealing interface extends substantially radially relative to an axis of rotation. In this embodiment the first sealing element may comprise a face and the second sealing element may comprise a seat. The first sealing element may include a carrier component and a sealing component that is carried by the carrier component, wherein the carrier component and the sealing component are made of different materials and/or have different physical properties. For example, the sealing component may be made from a material having a lower coefficient of friction.

In another embodiment the seal assembly comprises a lip seal in which the sealing interface is substantially cylindrical and coaxial with the rotational axis.

In an embodiment the first sealing element is configured to rotate and the second sealing element is stationary.

The seal assembly may be water lubricated.

The rotary seal may have a PV limit of at least 10 Bar.m/s, preferably at least 30 Bar.m/s, more preferably at least 50 Bar.m/s. The first and second sealing faces of the first and second sealing elements may have a static coefficient of friction of less than 0.6. In an embodiment the first and second sealing elements have a static coefficient of friction in the range 0.3 - 0.5.

According to another aspect of the present invention there is provided a waterjet propulsion system including an impeller shaft and a rotary seal assembly according to any one of the preceding statements of invention mounted on the impeller shaft.

According to another aspect of the present invention there is provided a propeller shaft assembly including a propeller shaft and a rotary seal assembly according to any one of the preceding statements of invention mounted on the propeller shaft. According to another aspect of the present invention there is provided a turbine including a turbine shaft and a rotary seal assembly according to any one of the preceding statements of invention mounted on the turbine shaft.

Various embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view showing part of an outside diameter pressurised face seal assembly comprising a first embodiment of the invention;

Figure 2 is a simplified cross-sectional representation of a face seal assembly comprising a second embodiment of the invention, being a variant of the seal assembly shown in Figure l ; Figure 3 is a cross-sectional view showing part of an inside diameter pressure face seal assembly comprising a third embodiment of the invention;

Figure 4 is a cross-sectional view showing part of a lip seal assembly comprising a fourth embodiment of the invention;

Figure 5 is a diagrammatic representation of a waterjet propulsion system that includes a seal assembly according to a fifth embodiment of the invention;

Figure 6 is a sectional side view of an open water lubricated propeller system; Figure 7 is a sectional side view of a closed water lubricated propeller system, and Figure 8 is a sectional side view of a submarine propulsor shaft.

Figure 1 illustrates the main components of a rotary seal assembly 2 according to a first embodiment of the invention. In this embodiment the rotary seal assembly comprises part of a waterjet propulsion system, which provides a watertight seal between an impeller shaft 4 and an impeller shaft stern tube 6. It should be understood that the seal assembly 2 extends around the circumference of the impeller shaft 4, only one half of the seal assembly being shown in the drawing.

The rotary seal assembly 2 consists of two separate structures comprising a stationary structure 8a that is attached to the impeller shaft tube 6 and a rotary assembly 8b that is mounted on the impeller shaft 4 for rotation with the shaft. The stationary structure 8a includes a mounting ring 10 that is bolted to the end of the impeller shaft stern tube 6, a substantially cylindrical tubular housing 12, one end of which is attached to the mounting ring 10, and a seat housing 14 that is attached to the other end of the housing 12. These components may all be made for example of an aluminium bronze material or any other suitable material. The mounting ring 10 supports an inflatable seal 15, which may be inflated to provide a temporary seal against the impeller shaft 4, either to allow maintenance or in an emergency to prevent water leaking through the rotary seal assembly 2.

The seat housing 14 supports a ring-shaped seal seat 16, which is positioned within a circular recess 18 provided on the inner face of the seat housing 14. A sealing strip 20 made of an elastomeric material is positioned between the seat 16 and the seat housing 14, providing a fluid-tight connection between the two parts and also permitting a small amount of relative movement between the seat 16 and the seat housing 14.

The seat 16 is preferably made of a material that has a high thermal conductivity, good thermal stability, and a high hardness.

For example, the seat 16 may be made of a metal such as chromium zirconium copper alloy, for example ASTM18150, which has a thermal conductivity at 20°C of about 330W/mK. Alternative suitable materials include phosphor bronzes and gun metal such as PD102, PD104, PB 1, LG1, LG2, LG4, LB 4 and SAE660, or aluminium bronzes such as CA104, NES833, NES834, C63000 or AB2, or stainless steels such as duplex, super duplex and other chrome steels. Some of these materials may have a lower thermal conductivity than the copper alloy mentioned above. For example, bronzes typically have a thermal conductivity of about 30-80W/mK and some stainless steels may have a thermal conductivity of only about 20W/mK. Such materials are unlikely to be suitable for high PV applications, but may be suitable for lower duty applications. Preferably, the seat 16 is made of a metal that has a thermal conductivity of at least 20W/mK, more preferably at least 50W/mK, yet more preferably at least 200W/mK.

Depending on the design of the seal, a harder or softer seat may be selected. The copper alloy described above has a hardness in the range 125-150Hb, whereas bronzes may have a hardness of about 50-150Hb. By comparison, silicon carbides typically have a hardness of about 2500Hv.

In this embodiment the seal seat 16 has a substantially square cross-section comprising a radial outer face 22 that engages the sealing strip 20, a radial inner face 24, an axial outer face 26 located within the recess 18, and an axial inner face 28, at least part of which forms a seal surface 30 of the rotary seal. The seal surface 30 of the seat 16 lies in a plane that is substantially perpendicular to the axis of the impeller shaft 4 and provides a flat surface against which a face 60 of the rotary seal rotates, as will be described below. In this embodiment the face 60 comprises a first sealing element and the seat 16 comprises a second sealing element.

The seal surface 30 of the seat 16 is provided with a coating 32 that has a high thermal conductivity, a high hardness and a low coefficient of friction. In this embodiment the coating 32 is based on diamond, although alternative coating materials having similar properties may also be used. The diamond coating 32 used in certain embodiments of the invention described herein has a hardness of about 3500-4000Hv and a thermal conductivity of at least lOOOW/mK. More generally, the coating may have a thermal conductivity of at least 300W/mK, preferably at least 600W/mK, more preferably at least lOOOW/mK, and a hardness of at least lOOOHv, preferably at least 2000Hv, more preferably at least 3000Hv. In the embodiment of the invention described herein where the seat 16 has a diamond coating and the face 60 comprises a composite material, the static coefficient of friction between the two seal surfaces is typically about 0.3 (wet) to 0.5 (dry). By comparison, the diamond coated surface typically has a static coefficient of friction against itself of about 0.025. The coating 32 may for example consist of a diamond coating created by a chemical vapour deposition process (CVD), having a thickness in the range 8-70 microns. Alternatively, the coating may be formed by other processes, for example by physical vapour deposition (PVD), by high velocity oxygen fuel spraying (HVOF) or a plasma arc process.

The rotary assembly 8b is mounted on the impeller shaft 4 for rotation with the impeller shaft. The rotary assembly 8b includes a ring-shaped body 40 that is mounted on the impeller shaft 4 and clamped to the shaft via a drive clamp ring 42 and a spacer ring 44. An O-ring seal 46 is positioned between the body 40 and the surface of the shaft 4. A ring-shaped face carrier 48 is slideably mounted on the body 40 to allow for axial movement relative to the body, and is urged in an axial direction towards the seat 16 by a number of compression springs 50 (see Figure 2) that act between the body 40 and a flange 51 that extends radially inwards from the main ring-shaped part of the face carrier 48. An elastomeric seal 52 is positioned between the face carrier 48 and the body 40. A seal is also provided by an elastomeric bellows 54 that interconnects the body 40 and a bellows clamp 56 that is attached to the face carrier 48. The face carrier 48 supports a ring-shaped face 60, which is located within a recess 62 in the face carrier 48. A sealing strip 64 made of an elastomeric material is positioned between the face 60 and the face carrier 48, providing a fluid-type connection between the two parts and also permitting a small amount of relative movement between the face 60 and the face carrier 48. The face 60 includes an axial outer part 60b that faces towards the seat 16, which is pressed by the compression springs 50 against the seat seal surface 30. This provides a seal interface 72 between the face seal 70 and the seat seal surface 30. The seal interface 72 extends substantially radially from the axis of the rotary shaft 4.

The face 60 is made primarily of a composite material comprising a resin and a fibrous reinforcing material. In an embodiment the composite material comprises an epoxy resin and a fibrous reinforcing material that may include fibres of glass, nylon, aramid, carbon, PTFE or natural fibres, for example cotton. Alternatively, the composite material may include a phenolic resin or a polyethylene resin. The composite material may also include fillers and/or friction modifiers, for example graphite or PTFE. The face 60 may be formed as a filament-wound component, a cloth-wound component, a flat laminate component (formed either by mechanical or vacuum induced moulding), a low pressure moulded component or an injection moulded component.

The face 60 may comprise a single homogeneous component or, as illustrated by the embodiment shown in Figure 1, the face 60 may comprise a holder 60a and an insert 60b that is carried by the holder 60a and forms the ring-shaped face seal 70. The holder 60a and the insert 60b may be made of different composite materials to provide optimum mechanical performance. For example, the holder 60a may be made of a material that is very stiff and mechanically stable, whereas the insert 60b may be made of a material chosen to provide low friction, low wear and a stable fluid film within the seal interface 72. For example, the insert 60b may include a fibrous reinforcing material that is selected to provide low friction and/or low wear, and it may include friction modifiers such as graphite or PTFE.

The rotary seal shown in Figure 1 comprises an outside diameter pressurised seal in which the fluid pressure is higher at the outside diameter of the sealing interface 72 than at the inside diameter of the interface. A relatively high pressure liquid, for example seawater, fills an outer chamber 80 between the housing 12 and the seal interface 72. An inner chamber 82 between the shaft 4 and the seal interface 72 is normally filled with relatively low pressure air. Accordingly, water in the outer chamber 80 tries to flow radially inwards through the seal interface 72 into the inner chamber 82. The sealing surfaces of the face 60 and the seat 16 are normally separated at least partially from one another during relative rotation of the sealing elements by a thin fluid film within the seal interface 72. The face 60 is pressed against the seat 16 by the springs 50 and also by the differential fluid pressure acting on the face carrier 48. The flow of fluid through the seal interface 72 is therefore minimal. During rotation of the shaft 4 the fluid film that forms between the face 60 and the seat 16 lubricates the seal ensuring a very low level of friction between the face 60 and the seat 16. Usually the liquid within the outer chamber 80 will be water and the rotary seal may thus be referred to as a water lubricated seal. Although the level of friction between the face 60 and the seat 16 is very low, aided by the relatively low coefficient of friction between the composite material forming the face 60 and the diamond coating 32 on the seal surface 30 of the seat 16, rotation of the shaft 4 will still cause heat to be generated by friction within the seal as a result of the high PV factor for which the seal is designed. This heat is removed very quickly from the rotary seal as a result of the very high thermal conductivities of the seat 16 and diamond coating 32, thus avoiding thermal damage to the components of the rotary seal.

The rotary seal also has a very high level of shock resistance, owing the strength and resilience of the materials selected for the face 60 and the seat 16. The seal is therefore very efficient and reliable and is suitable for use in naval vessels and other challenging applications.

Figure 2 illustrates the main components of an outside diameter pressurised rotary seal assembly 2 according to a second embodiment of the invention, being a variant of the rotary seal shown in Fig. 1. The previous description of the rotary seal shown in Fig. 1 therefore applies equally to this embodiment, except as indicated below.

In this embodiment, the tubular housing 12 and the seat housing 14, which form part of the stationary structure 8a, comprise a unitary structure which is attached at one end to the mounting ring (not shown). The ring-shaped rotary seal seat 16, which is positioned within a circular recess 18 provided on the inner face of the seat housing 14, is substantially as described in the first embodiment. A sealing strip 20 made of an elastomeric material is positioned between the seat 16 and the seat housing 14. The seat 16 is preferably made of a material that has a high thermal conductivity, good thermal stability and a high hardness. For example, the seat may be made of a metal such as chromium zirconium copper alloy, or any of the other materials described previously. The seal surface 30 of the seat 16 is provided with a coating 32, preferably based on diamond, which has a high thermal conductivity, a high hardness and a low coefficient of friction. The diamond coating 32 preferably has a thickness in the range 8-70 microns and may be created by chemical vapour deposition (CVD), physical vapour deposition (PVD), high velocity oxygen fuel spraying (HVOF) or a plasma arc process. The rotary assembly 8b is mounted on the impeller shaft 4 for rotation with the impeller shaft. The rotary assembly 8b includes a ring-shaped body 40 that is mounted on the impeller shaft 4 and secured by a drive pin 86 for rotation with the impeller shaft 4. A ring-shaped face carrier 48 is mounted on the body 40 to allow for axial movement relative to the body, and is urged in an axial direction towards the seat 16 by a number of compression springs 50 that act between the body 40 and a flange 51 that extends radially inwards from the main ring-shaped part of the face carrier 48. An elastomeric seal 52 is positioned between the face carrier 48 and the body 40. Bellows are not used in this embodiment.

The face carrier 48 supports a ring-shaped face component 60, which is located within a recess 62 in the face carrier 48. A sealing strip 64 made of an elastomeric material is positioned between the face 60 and the face carrier 48, providing a fluid-type connection between the two parts and permitting a small amount of relative movement between the face 60 and the face carrier 48. The face 60 includes an axial inner surface 66 that is accommodated within the recess 62 and an axial outer surface 68 that faces towards the seat 16. The axial outer surface 68 carries a ring-shaped face seal 70 that is pressed by the compression springs 50 against the seat seal surface 30, providing a seal interface 72 between the face seal 70 and the seat seal surface 30.

The face 60 is made primarily of a composite material comprising a resin and a fibrous reinforcing material, as described previously. In this embodiment the face 60 has a unitary structure.

In use, the rotary seal operates substantially as described above in relation to the first embodiment. Any heat generated by friction within the seal is removed very quickly from the rotary seal as a result of the very high thermal conductivities of the seat 16 and diamond coating 32, thus avoiding thermal damage to the components of the rotary seal. Figure 3 illustrates the main components of an inside diameter pressurised rotary seal assembly 102, comprising a third embodiment of the invention. In this embodiment the rotary seal assembly comprises part of a propeller system and provides a watertight seal between a propeller shaft 104 and a stern tube (not shown). As in the previous embodiments, it should be understood that the seal assembly 102 extends around the circumference of the propeller shaft 104, only one half of the seal assembly being shown in the drawing. The rotary seal assembly 102 includes a stationary structure 108a that is attached to the stern tube and a rotary assembly 108b that is mounted on the propeller shaft 104 for rotation with the shaft. The stationary structure 108a includes a seat housing 114 that is bolted to the end of the stern tube. The seat housing 114 may be made for example of an aluminium bronze material or any other suitable material.

The seat housing 114 supports a ring-shaped seat 116, which is positioned within a circular recess 118 provided on the inner face of the seat housing 114. A sealing strip 120 is positioned between the seat 116 and the seat housing 114, providing a fluid-tight connection between the two parts and permitting a small amount of relative movement between the seat 116 and the seat housing 114.

The seat 116 is preferably made of a material that has a high thermal conductivity, good thermal stability, and a high hardness. For example, the seat may be made of a metal such as chromium zirconium copper alloy, or from any other suitable material as described above.

The seal seat 116 has an axial inner face 128, at least part of which forms a seal surface 130 of the rotary seal. The seal surface 130 lies in a plane that is substantially perpendicular to the axis of the propeller shaft 104 and provides a flat surface against which the face part of the rotary seal rotates, as will be described below.

The seal surface 130 of the seat 116 is provided with a coating, preferably based on diamond, which has a high thermal conductivity, a high hardness and a low coefficient of friction, as described above. The coating preferable has a thickness in the range 8-70 microns and may for example be created by chemical vapour deposition (CVD), physical vapour deposition (PVD), high velocity oxygen fuel spraying (HVOF) or a plasma arc process.

The rotary assembly 108b includes a ring-shaped body 140 that is mounted on the propeller shaft 104 for rotation with the shaft. A ring-shaped face carrier 148 is slideably mounted on the body 40 to allow for axial movement relative to the body, and is urged in an axial direction towards the seat 116 by a number of compression springs 150 that act between the body 140 and a flange 151 that extends radially inwards from the main ring-shaped part of the face carrier 148. An elastomeric seal 152 is positioned between the face carrier 148 and the body 140. Relative rotation between the face carrier 148 and the body 140 is prevented, for example by means of drive pins (not shown).

The face carrier 148 supports a ring-shaped face component 160, which is located within a recess 162 in the face carrier 148. A sealing strip 164 made of an elastomeric material is positioned between the face 160 and the face carrier 148, providing a fluid-type connection between the two parts and permitting a small amount of relative movement between the face 160 and the face carrier 148. The face 160 includes an axial outer part 164 that faces towards the seat 116, which is pressed by the compression springs 150 against the seat seal surface 130. This provides a seal interface 172 between the face seal 170 and the seat seal surface 130.

The face 160 is made primarily of a composite material comprising a resin and a fibrous reinforcing material. In a preferred embodiment the composite material comprises an epoxy resin and a fibrous reinforcing material that may include fibres of glass, nylon, aramid, carbon, PTFE or natural fibres, for example cotton. Alternative composite materials may also be used as described above. In this embodiment the face 160 comprises a single homogeneous component. However, it may alternatively consist of a holder and an insert of a different material, as described in relation to the embodiment shown in Figure 1.

The rotary seal shown in Figure 3 comprises an inside diameter pressurised seal in which the fluid pressure is higher at the inside diameter of the sealing interface 172 than at the outside diameter. The relatively high pressure liquid, for example seawater, fills an inner chamber 180 between the propeller shaft 104 and the seal interface 172. An outer chamber 182 on the outside of the seal interface 72 contains relatively low pressure air. Accordingly, water in the inner chamber 180 tries to flow radially outwards through the seal interface 172 into the outer chamber 182. The sealing surfaces of the face 160 and the seat 116 are normally separated at least partially from one another during relative rotation of the sealing elements by a thin fluid film within the seal interface 172.

In use, the face 160 is pressed against the seat 116 by the springs 150 and also by the differential fluid pressure acting on the face carrier 148. The flow of fluid through the seal interface 172 is therefore minimal. During rotation of the shaft 104 the fluid film between the face 160 and the seat 116 lubricates the seal, ensuring a very low level of friction between the face 160 and the seat 116. Usually the liquid within the inner chamber 180 will be water and the rotary seal may thus be referred to as a water lubricated seal.

As with the previous embodiments, rotation of the shaft 104 will cause heat to be generated by friction. This heat is removed very quickly from the rotary seal as a result of the very high thermal conductivities of the seat 116 and diamond coating 132, thus avoiding thermal damage to the components of the rotary seal.

The rotary seals illustrated in Figs. 1-3 are all face seals in which the sealing interface extends substantially radially from the axis of the rotary component, the face 60 comprises a first sealing element and the seat 16 comprises a second sealing element. The invention is also applicable to rotary lip seals where the sealing interface is cylindrical and coaxial with the axis of the rotary component. An embodiment of a rotary lip seal is illustrated in Fig. 4.

The rotary lip seal 202 illustrated in Fig. 4 comprises a cylindrical rotary shaft 204, and a static assembly 206 comprising a cylindrical housing 208 that surrounds a part of the shaft 204 (only part of the housing being shown) and a sealing ring 210 that is carried by the housing 208. The sealing ring 210 is pressed against the cylindrical surface of the shaft 204 by a circular garter spring 212, and also normally by differential fluid pressure. The sealing ring 210 divides the annular gap between the housing 208 and the shaft 204 into a high pressure chamber 214 on one side of the sealing ring 210 and a low high pressure chamber 216 on the other side of the sealing ring. The radially innermost surface of the sealing ring 210 is pressed against the cylindrical surface of the shaft 204 to provide a seal at a sealing interface 218. The sealing interface 218 is substantially cylindrical and coaxial with the rotational axis of the shaft 204. In this embodiment the sealing ring 210 comprises a first sealing element.

The sealing ring 210 is made primarily of a composite material comprising a resin and a fibrous reinforcing material. In a preferred embodiment the composite material comprises an epoxy resin and a fibrous reinforcing material that may include fibres of glass, nylon, aramid, carbon, PTFE or natural fibres, for example cotton. Alternative composite materials may also be used as described above. In this embodiment the sealing ring 210 comprises a single unitary component. However, it may alternatively consist of a holder and an insert of a different material, as described in relation to the embodiment shown in Figure 1. The periphery 220 of the shaft 204 comprises a second sealing element and is contacted by the sealing ring 210. The periphery 220 of the shaft 204 is preferably made of a material that has a high thermal conductivity, good thermal stability, and a high hardness. For example, the periphery 220 may be made of a metal such as chromium zirconium copper alloy, or from any other suitable material as described above. The periphery 220 may consist of a cylindrical sleeve or liner that is mounted on the shaft 204, or alternatively the entire shaft 204 including the periphery 220 may be made of a suitable seat material.

The periphery 220 of the shaft 204 provides a cylindrical seal surface that is substantially coaxial with the axis of the propeller shaft 204 and has a cylindrical surface that rotates against the sealing ring 210. The surface of the periphery 220 is provided with a coating 222 that has a high thermal conductivity, a high hardness and a low coefficient of friction, as described above. In this embodiment the coating 222 comprises a diamond coating, which preferably has a thickness in the range 8-70 microns and may for example be created by chemical vapour deposition (CVD), physical vapour deposition (PVD), high velocity oxygen fuel spraying (HVOF) or a plasma arc process. Alternative coating materials having similar properties may also be used.

In use, the sealing ring 210 is pressed against the periphery 220 of the shaft 204 by the garter spring 212 and also by the differential fluid pressure acting on the sealing ring 210. The flow of fluid through the seal interface 218 is therefore minimal. During rotation of the shaft 204 the fluid forms a film within the seal interface 218, thus lubricating the seal and ensuring a very low level of friction between the sealing ring 210 and the shaft 204. Usually the liquid within the high pressure chamber 214 will be water and the rotary seal may thus be referred to as a water lubricated seal. Alternatively, the liquid within the high pressure chamber 214 may be oil or an environmentally acceptable lubricant (EAL). As with the previous embodiments, rotation of the shaft 204 will cause heat to be generated by friction. This heat is removed very quickly from the rotary seal as a result of the very high thermal conductivities of the periphery 220 of the shaft 204 and the diamond coating 222 provided on the periphery 220, thus avoiding thermal damage to the components of the rotary seal. Figure 5 illustrates a marine waterjet propulsion system 300 that includes a seal assembly 302 according to an embodiment of the invention. The seal assembly 302 may for example be similar to that shown in any one of Figures 1 to 4 as described above. The waterjet propulsion system 300 is conventional apart from the seal assembly 302 and includes an inlet duct 304 that leads from an inlet opening 306 in the bottom part 308 of a vessel's hull to an impeller housing 310 in the vessel's transom 312. An outlet duct 314 leads from the impeller housing 310 to a nozzle 316. The propulsion system also includes a conventional hydraulic steering mechanism 318 and a reversing bucket assembly 320.

The impeller housing 310 contains an impeller (not shown), which is driven through an impeller shaft 322. The shaft 322 passes through an impeller stern tube 324 that extends through a wall of the inlet duct 304. The seal assembly 302 is mounted on a forward end of the stern tube 324 and provides a water-tight seal between the shaft 322 and the stern tube 324, preventing water from entering the hull of the vessel.

Figure 6 illustrates a marine propeller shaft mechanism 400 that includes a seal assembly 402 according to an embodiment of the invention. The seal assembly 402 may for example be similar to that shown in any one of Figures 1 to 4 as described above. The propeller shaft mechanism 400 is conventional apart from the seal assembly 402 and includes a propeller 404 and a propeller shaft 406 that passes through a stern tube 408 in the stern of the vessel's hull 410. The shaft 406 is supported within the stern tube 408 by plain bearings 412. The seal assembly 402 is mounted on a forward end of the stern tube 408 and provides a water-tight seal between the shaft 406 and the stern tube 408, preventing water from entering the hull of the vessel. In this embodiment, the propeller shaft mechanism 400 is an open water lubricated system, in which clean filtered water is fed from a reservoir 414 into the annular space between the propeller shaft 406 and the stern tube 408 via an inlet duct in the seal assembly 402.

Figure 7 illustrates an alternative marine propeller shaft mechanism 500 that is similar in many ways to the propeller shaft mechanism shown in Fig. 6 and includes two seal assemblies 502 according to an embodiment of the invention. Each seal assembly 502 may for example be similar to one of the seal assemblies shown in any one of Figures 1 to 4 as described above. The propeller shaft mechanism 500 is conventional apart from the seal assemblies 502 and includes a propeller 504 and a propeller shaft 506 that passes through a stern tube 508 in the stern of the vessel's hull 510. The shaft 506 is supported within the stern tube 508 by plain bearings 512. A seal assembly 502 is mounted at each end of the stern tube 508 and provides a water-tight seal between the shaft 506 and the stern tube 508. In this embodiment, the propeller shaft mechanism 500 is a closed lubricated system, in which a lubricating fluid, for example a mixture of glycol and water, is fed from a reservoir 514 into the annular space between the propeller shaft 506 and the stern tube 508 via an inlet duct in the stern tube 508.

Figure 8 illustrates a submarine propulsor shaft mechanism 600 that includes a seal assembly 602 according to an embodiment of the invention. The seal assembly 602 may for example be similar to that shown in any one of Figures 1 to 4 as described above. The propeller shaft mechanism 600 is conventional apart from the seal assembly 602 and includes a propeller 604 and a propeller shaft 606 that passes through a stern tube 608 in the stern of the submarine's hull 610. The seal assembly 602 is mounted on a forward end of the stern tube 608 and provides a water-tight seal between the shaft 606 and the stern tube 608, preventing water from entering the hull of the submarine.

A seal assembly similar to that shown in any one of Figures 1 to 4 as described above may also be used in numerous other applications, including for example to seal the turbine shaft of a generator turbine in a hydroelectric generator or a tidal turbine.

Claims

1. A rotary seal assembly comprising first and second sealing elements configured for rotational relative movement about a rotational axis and providing a sealing interface between adjacent first and second sealing faces of the first and second sealing elements, wherein said first sealing element comprises a composite material that includes fibrous reinforcing elements within a resin matrix, and wherein said second sealing element comprises a metallic body having a coating on the second sealing face, said coating having thermal conductivity exceeding that of the metallic body.

2. A rotary seal assembly according to claim 1, wherein the composite material of the first sealing element comprises a resin and a fibrous reinforcing material.

3. A rotary seal assembly according to claim 1 or claim 2, wherein the first sealing element is a filament-wound product.

4. A rotary seal assembly according to any one of the preceding claims, wherein the composite material includes a friction modifier.

5. A rotary seal assembly according any one of the preceding claims, wherein the second sealing element is made of a metal selected from the range comprising copper alloys, phosphor bronzes, gun metals, aluminium bronzes and stainless steels.

6. A rotary seal assembly according to any one of the preceding claims, wherein the metallic body of the second sealing element has a thermal conductivity of at least 20W/mK, more preferably at least 50W/mK, yet more preferably at least 200W/mK.

7. A rotary seal assembly according to any one of the preceding claims, wherein the coating has a thermal conductivity of at least 300W/mK, preferably at least 600W/mK, more preferably at least lOOOW/mK.

8. A rotary seal assembly according to any one of the preceding claims, wherein the coating has a hardness of at least 1000 Hv, preferably at least 2000 Hv, more preferably at least 3000 Hv.

9. A rotary seal assembly according to any one of the preceding claims, wherein the coating comprises a diamond coating, which preferably has a thickness in the range 8-70 microns.

10. A rotary seal assembly according to any one of the preceding claims, wherein the sealing assembly comprises a face seal in which the sealing interface extends substantially radially relative to an axis of rotation.

11. A rotary seal assembly according to claim 10, wherein the first sealing element comprises a face and the second sealing element comprises a seat.

12. A rotary seal assembly according to claim 10 or claim 11, wherein the first sealing element includes a carrier component and a sealing component that is carried by the carrier component, and wherein the carrier component and the sealing component are made of different materials and/or have different physical properties.

13. A rotary seal assembly according to any one of claims 1-9 in which the seal assembly comprises a lip seal in which the sealing interface is substantially cylindrical and coaxial with the rotational axis.

14. A rotary seal assembly according to any one of the preceding claims, wherein the first sealing element is configured to rotate and the second sealing element is stationary.

15. A rotary seal assembly according to any one of the preceding claims, wherein the seal assembly is water lubricated.

16. A rotary seal assembly according to any one of the preceding claims, having a PV limit of at least 10 Bar.m/s, preferably at least 30 Bar.m/s, more preferably at least 50 Bar.m/s.

17. A waterjet propulsion system including an impeller shaft and a rotary seal assembly according to any one of the preceding claims mounted on the impeller shaft.

18. A propeller shaft assembly including a propeller shaft and a rotary seal assembly according to any one of claims 1-16 mounted on the propeller shaft.

19. A turbine including a turbine shaft and a rotary seal assembly according to any one of claims 1-16 mounted on the turbine shaft.