GB2551531A

GB2551531A - Rotary seal unit

Info

Publication number: GB2551531A
Application number: GB1610800.3A
Authority: GB
Inventors: Piggott Barry
Original assignee: Wartsila UK Ltd
Current assignee: Wartsila UK Ltd
Priority date: 2016-06-21
Filing date: 2016-06-21
Publication date: 2017-12-27
Also published as: WO2017220962A1; GB201610800D0

Abstract

A rotary seal unit comprises a rotary assembly 8b that is configured to be mounted on a rotary shaft 4 and includes a first sealing element 60, and a static structure 8a that includes a second sealing element 16. The rotary assembly is configured for rotational movement about a rotational axis relative to the static structure. The first and second sealing elements provide a sealing interface 72 between adjacent sealing faces 70 and 30 of the first and second sealing elements. The rotary assembly includes a clamping assembly 41 for clamping the rotary assembly to the rotary shaft. The clamping assembly includes a clamping device 42 and a spacer element 44 that is located between the clamping device and the rotary shaft. The spacer element can be adapted to fit a range of rotary shafts having different diameters.

Description

Rotary seal unit

The present invention relates to a rotary seal unit that provides a fluid resistant seal between a pair of relatively rotating components. The seal unit is suitable for use in numerous applications, for example in marine applications where the seal unit provides a substantially watertight seal between the propeller shaft and the stem tube of a ship or boat, or between an impeller shaft and a housing in a waterjet propulsion system.

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 stem tube into the ship. The seal unit 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 unit of the present invention is designed primarily for use in face seals for marine vessels.

Rotary seals include parts that are subject to wear and must be replaced at regular intervals. Some of these parts are fitted to the propeller shaft and provide a seal against the shaft. Therefore, they must be manufactured to the exact diameter of the shaft.

The propeller shafts of different marine vessel vary enormously in diameter. For example, even for relatively small to medium-sized vessels intended for inland and coastal waters, propeller shafts varying in diameter from about 75mm up to 300mm or larger are common. For larger ocean-going vessels the propeller shafts may be much larger. The situation is made more complicated by the fact that some propeller shafts are made to metric dimensions, whereas others use imperial sizes.

As a result, suppliers of rotary seals have to carry large stocks of replacement parts to ensure that spares are available when required, which is very expensive. Alternatively, seal components can be manufactured to order, but this is again expensive and may introduce a delay in completing the repair or service work.

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

According to one aspect of the present invention there is provided a rotary seal unit comprising a rotary assembly that is configured to be mounted on a rotary shaft and includes a first sealing element, and a static structure that includes a second sealing element, wherein the rotary assembly is configured for rotational movement about a rotational axis relative to the static structure and the first and second scaling elements provide a scaling interface between adjacent sealing faces of the first and second sealing elements, wherein the rotary assembly includes a clamping assembly for clamping the rotary assembly to the rotary shaft, the clamping assembly including a clamping device and a spacer element that is located between the clamping device and the rotary shaft, and wherein the spacer element can be adapted to fit a range of rotary shafts having different diameters.

The provision of a spacer element that can be adapted to fit a range of rotary shafts having different diameters allows the seal unit to be fitted to numerous different shafts. This means that a very wide range of shafts can be catered for using only a limited range of rotary seal units. This substantially reduces the amount of stock that needs to be carried by repairers or suppliers of the rotary seal units, thereby reducing costs and enabling a fast tum-round in servicing and repair operations, which provides cost savings for the end user.

In an embodiment, the spacer element can be adapted to fit a range of rotary shafts having diameters that differ by at least 15mm, preferably at least 20mm, more preferably at least 25mm. This allows the seal unit to be fitted to wide range of different rotary shafts. The seal unit may be part of a set of seal units designed for shafts with different diameter ranges. For example, if each rotary seal unit can be adapted to fit a range of rotary shafts having diameters that differ by 25mm, it is possible with a set of just nine different rotary seal units to accommodate all rotary shafts with diameters in the range 75mm to 300mm.

In an embodiment, the spacer element has a radial thickness of at least 8mm, preferably at least 12mm, more preferably at least 15mm. A large radial thickness makes it possible to adapt the seal unit to fit a wide range of shaft diameters.

In an embodiment, the spacer element can be adapted by machining to fit a range of rotary shafts. This can be done for example by boring or milling to increase the internal diameter of the spacer element to fit the shaft. This is a simple operation that can be carried on location. If it is necessary to cut a radial slot into the spacer element to allow it to be compressed radially, this can be accomplished for example using a hand saw.

In an embodiment, the spacer element is made of metal, for example phosphor bronze or any other suitable metal.

In an embodiment, the spacer element is ring-shaped, optionally including a radial slot.

In an embodiment, the clamping device is ring-shaped, optionally including a radial slot.

In an embodiment, the clamping device is circumferentially adjustable, for example by means of one or more clamping screws.

In an embodiment, the rotary assembly includes a body element that is mounted on the rotary shaft and supports the first sealing element.

In an embodiment, the body element can be adapted to fit a range of rotary shafts having different diameters, for example by machining.

In an embodiment, the body element is fastened to the clamping device.

In an embodiment, the seal unit comprises a face seal in which the sealing interface extends substantially radially relative to the rotational axis.

In an embodiment, the first sealing element comprises a face and the second sealing element comprises a seat.

In an embodiment, the seal unit is water lubricated.

In an embodiment, the seal unit is configured to be mounted on a rotary shaft having a diameter in the range 75mm to 300mm.

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

The first sealing element may comprise a composite material, for example 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 silicon carbide. Alternatively it may be made of a suitable a metal, for example a metal selected from the range comprising copper alloys, phosphor bronzes, gun metals, aluminium bronzes and stainless steels.

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.

The seal unit may be water lubricated.

According to another aspect of the present invention there is provided a propeller shaft assembly including a propeller shaft and a rotary seal unit according to any one of the preceding statements of invention mounted on the propeller 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 an isometric view of a rotary seal unit according to a first embodiment of the invention;

Figure 2 is an end view of the rotary seal;

Figure 3 is a cross-sectional view on line ΠΙ-ΤΠ of figure 2;

Figure 4 is a cross-sectional view on line ΠΙ-ΤΠ of figure 2, showing some optional additional components;

Figure 5 is a cross-sectional view on line V-V of figure 2;

Figure 6 is a cross-sectional view on line VI-VI of figure 2, and Figure 7 is a cross-sectional view on line VII-VII of figure 2.

Figures 1 to 7 illustrate the main components of a rotary seal unit 2 according to a first embodiment of the invention. In this embodiment the rotary seal unit 2 is part of a marine propulsion system and provides a watertight seal between a propeller shaft 4 and a propeller shaft stem tube 6. Alternatively, the rotary seal unit may comprise part of an impeller propulsion system in which it provides a watertight seal between an impeller shaft and an impeller shaft stern tube. Figures 3-7 are partial cross-sectional views, which show only half of the rotary seal: it should be understood that the seal unit 2 extends around the entire circumference of the propeller shaft 4.

The rotary seal unit 2 consists of two separate structures comprising a static structure 8a that is attached to the propeller shaft tube 6 and a rotary assembly 8b that is mounted on the propeller shaft 4 for rotation with the shaft.

The static structure 8a includes a substantially cylindrical tubular seal housing 12, having a first flange 12a at one end that is secured to the end of the propeller shaft stem tube 6 with screws 13a. A seat plate 14 is attached to a second flange 12b at the other end of the housing 12 by bolts 13b and nuts 13c. The seal housing 12 and the seat plate 14 may be made of nylon or any other suitable material (for example, an aluminium bronze). The fasteners 13a, 13b, 13c may be made of stainless steel or any other suitable material. Water vent and drain connections 15 are provided in the upper and lower parts of the seat plate 14.

The scat plate 14 supports a ring-shaped seal scat 16, which is positioned within a circular recess 18 provided on the inner face of the seat plate 14. A sealing strip 20 made of an elastomeric material is positioned between the seat 16 and the seat plate 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 plate 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 silicon carbide. Alternatively, it may be made of any other suitable metal such as a copper alloy, phosphor bronze, gun metal, aluminium bronze or stainless steel.

Depending on the design of the seal, a harder or softer seat 16 may be selected. Silicon carbide typically has a hardness of about 2500Hv. By way of comparison, a suitable copper alloy may have a hardness in the range 125-150Hb, whereas bronzes may have a hardness of about 50-150Hb.

In this embodiment the seal seat 16 has a substantially square cross-section and includes an axial inner face 28, at least part of which forms a seal surface 30. The seat seal surface 30 lies in a plane that is substantially perpendicular to the axis of the propeller 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 rotary assembly 8b is mounted on the propeller shaft 4 for rotation with the propeller shaft. The rotary assembly 8b includes a ring-shaped body 40 (or “spring holder”) that is mounted on the propeller shaft 4 and clamped to the shaft via a clamping assembly 41 comprising a drive clamp ring 42, a clamping screw 43 and a spacer ring 44 (Figs. 2 & 3). In the embodiment shown in the drawings the drive clamp ring 42 comprises a single, ring-shaped component having a radial slot 42a that allows it and the spacer ring 44 to be clamped to the propeller shaft 4 by tightening the clamp screw 43, which connects the two ends of the drive clamp ring 42 on opposite sides of the slot 42a. Alternatively, the drive clamp ring 42 may include two half rings connected by two clamping screws 43.

It should be noted that in the embodiment shown in the drawings, the outer cylindrical surface of the spacer ring 44 and the inner cylindrical surface of the drive clamp ring 42 are stepped to prevent axial movement of the spacer ring 44 relative to the drive clamp ring 42 in the direction away from the body 40 (movement in the opposite direction being prevented by the body itself). This step 44a may however be omitted if not required, or it may be replaced by some other formation that prevents axial movement.

The body 40 is fastened to the drive clamp ring 42 by screws 45 (Fig. 5). The body 40 may for example be made of nylon or any other suitable material. The drive clamp ring 42 and a spacer ring 44 may be made of metal, for example phosphor bronze, or any other suitable material. A sealing element 46, for example an O-cord, is positioned between the body 40 and the surface of the shaft 4, to provide a fluid-tight seal between the body 40 and the shaft 4. A ring-shaped face carrier 48 is slideably mounted on the body 40 to permit axial movement relative to the body, and is urged in an axial direction towards the seat 16 by a number of compression springs 50 (Figures 5 & 6) 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. Rotation of the face carrier 48 relative to the body 40 is prevented by a number of keys 52 provided on the outer circumference of the body 40, which engage corresponding keyways on the inner circumference of the face carrier 48. A seal is provided between the face carrier 48 and the body 40 by an elastomeric bellows 54 that interconnects the body 40 and the face carrier 48, which is secured by a bellows clip 56. The bellows 54 also provides a seal between the drive clamp ring 42 and the body 40, one end of the bellows being clamped between the drive clamp ring 42 and the body 40.

The face carrier 48 carries a ring-shaped face 60, which in this embodiment comprises an axial end part of the face carrier 48. The face 60 includes an annular seal surface 70, which is pressed by the compression springs 50 against the seat seal surface 30. This provides a seal interface 72 between the face seal surface 70 and the seat seal surface 30. The seal interface 72 extends substantially radially from the axis of the rotary shaft 4.

In this embodiment the face carrier 48 is made primarily of a composite material comprising a resin and a fibrous reinforcing material. The composite material may for example consist of 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 carrier 48 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.

In this embodiment the face carrier 48 and the face 60 comprise a single homogeneous component. Alternatively, the face 60 may comprise an insert that is carried by the face carrier 48. The carrier 48 and the face 60 may then be made of different materials to provide optimum mechanical performance. For example, the carrier 48 may be made of a material that is very stiff and mechanically stable, whereas the face 60 may be made of a material that provides low friction, low wear and a stable fluid film within the seal interface 72. For example, the face 60 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 the figures comprises an outside diameter pressurised seal in which the fluid pressure is higher at the outside diameter of the seal 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 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 quickly from the rotary seal as a result of the high thermal conductivity of the seat 16, thus avoiding thermal damage to the components of the rotary seal.

Friction between the face 60 and the seat 16 causes these parts to wear with use. Accordingly, these parts need to be replaced from time to time. Other components such as the body 40, the spacer ring 44, the face carrier 48, the bellows 54, the bellows clip 56, the screws 45 and the springs 50 may also need to be replaced from time to time.

In order to simplify the service process and reduce the number of spare components required by repairers and parts suppliers, the body 40 and the spacer ring 44 are designed to be adaptable to different propeller shaft sizes. The drive clamp ring 42 and the spacer ring 44 are arranged concentrically, with the spacer ring 44 positioned between the drive clamp ring 42 and the propeller shaft 4. The spacer ring 44 is relatively thick, typically having a thickness in the radial direction of about 20mm between its inner and outer diameters. The inner diameter of the spacer ring 44 can be adjusted by machining, allowing it to be matched to the outside diameter of the shaft 4. Similarly, the inner diameter of the body 40 can be adjusted by machining, allowing it to be matched to the outside diameter of the shaft 4.

For a first example, the spacer ring 44 may have an inner diameter of 75mm and an outside diameter of 110mm (i.e. a radial thickness of 17.5mm). In this case, the spacer ring 44 may be machined out to provide an inner diameter in the range 75mm - 102mm. This spacer ring can therefore be adapted to fit a propeller shaft with a diameter in the range 75mm - 102mm.

In a similar manner, larger spacer rings can be adapted to fit larger shafts. Some examples of different sized seals designed to fit a range of different shaft sizes are set out in Table 1 below:

Table 1

As will be apparent from Table 1 above, it is possible to provide rotary seals for propeller shafts with metric diameters ranging from 75mm to 306mm, and imperial diameters ranging from 3 inches to 12 inches, using a set of only twelve different rotary seal units.

In use, the spacer ring 44 is machined to match the diameter of the propeller shaft 4, and a radial slot is then cut in the spacer ring, for example using a hacksaw, allowing it to be compressed against the shaft 4. The drive clamp ring 42 is then fitted over the spacer ring 44. After the rotary assembly 8b has been positioned as required on the propeller shaft 4, the clamping screw 43 is tightened to clamp the drive clamp ring 42 and the spacer ring 44 to the shaft 4.

Figure 4 illustrates two alternative secondary sealing assemblies 110, 120 that may optionally be added to the rotary seal unit described above, in order to provide a secondary seal between the propeller shaft 4 and the stern tube 6, either in an emergency (for example, if the rotary seal fails), or during routine maintenance work. Normally, only one of these secondary sealing assemblies 110, 120 will be provided, depending on the requirements of the vessel. In simple terms, the first secondary sealing assembly 110 is more sophisticated and more expensive, whereas the second secondary sealing assembly 120 is cheaper and simpler. The main components of the rotary seal unit 2 are substantially as described above.

The first secondary sealing assembly 110 is attached to the end of the stern tube 6 and is located between the stern tube 6 and the seal housing 12 of the rotary seal unit 2. The first secondary sealing assembly 110 includes a mounting ring 112 that is fastened to the first flange 12a of the seal housing 12 by screws 13a. The mounting ring provides a radially inwards facing slot 114, which supports an inflatable seal ring 115. The seal ring 115 may be inflated to provide a temporary seal against the propeller shaft 4, either to allow maintenance or in an emergency to prevent water leaking through the rotary seal unit 2.

The second secondary sealing assembly 120 comprises a rubber bung 122, which is mounted on the propeller shaft 4 adjacent the seat plate 14. The bung 122 includes an axial bore 124 through which the shaft 4 passes, the diameter of the bore 124 being slightly less than the diameter of the shaft 4 to provide a relatively tight fit. A jubilee clamp 126 (or similar) is fastened around the rubber bung 122 and can be tightened to prevent axial movement of the bung relative to the shaft 4.

Normally, the bung 122 is located a short distance from the seat plate 14 to avoid rubbing of the parts when the shaft 4 is rotating. In an emergency, for example if the rotary seal fails, the jubilee clip 126 can be loosened and the bung 122 can then be driven axially towards the seat plate 14 to provide a seal. The clip 126 is then tightened to secure the bung in position.

The bung 122 preferably has a frusto-conical part 128 that extends into the seal housing 12 to provide a secure seal against the seat plate 14.

Claims

1. A rotary seal unit comprising a rotary assembly that is configured to be mounted on a rotary shaft and includes a first sealing element, and a static structure that includes a second sealing element, wherein the rotary assembly is configured for rotational movement about a rotational axis relative to the static structure and the first and second sealing elements provide a sealing interface between adjacent sealing faces of the first and second sealing elements, wherein the rotary assembly includes a clamping assembly for clamping the rotary assembly to the rotary shaft, the clamping assembly including a clamping device and a spacer element that is located between the clamping device and the rotary shaft, and wherein the spacer element can be adapted to fit a range of rotary shafts having different diameters.

2. A rotary seal unit according to claim 1, wherein the spacer element can be adapted to fit a range of rotary shafts having diameters that differ by at least 15mm, preferably at least 20mm, more preferably at least 25mm.

3. A rotary seal unit according to claim 1 or claim 2, wherein the spacer element has a radial thickness of at least 8mm, preferably at least 12mm, more preferably at least 15mm.

4. A rotary seal unit according to any one of the preceding claims, wherein the spacer element can be adapted by machining to fit a range of rotary shafts.

5. A rotary seal unit according any one of the preceding claims, wherein the spacer element is made of metal.

6. A rotary seal unit according to any one of the preceding claims, wherein the spacer element is ring-shaped.

7. A rotary seal unit according to any one of the preceding claims, wherein the clamping device is ring-shaped.

8. A rotary seal unit according to any one of the preceding claims, wherein the clamping device is circumferentially adjustable.

9. A rotary seal unit according to any one of the preceding claims, wherein the rotary assembly includes a body element that is mounted on the rotary shaft and supports the first sealing element.

10. A rotary seal unit according to claim 9, wherein the body element can be adapted to fit a range of rotary shafts having different diameters.

11. A rotary seal unit according to claim 9 or claim 10, wherein the body element is fastened to the clamping device.

12. A rotary seal unit according to any one of the preceding claims, wherein the seal unit comprises a face seal in which the sealing interface extends substantially radially relative to the rotational axis.

13. A rotary seal unit according to claim 12, wherein the first sealing element comprises a face and the second sealing element comprises a seat.

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

15. A rotary seal unit according to any one of the preceding claims, wherein the seal unit is configured to be mounted on a rotary shaft having a diameter in the range 75mm to 300mm.

16. A propeller shaft assembly including a propeller shaft and a rotary seal unit according to any one of claims 1-14 mounted on the propeller shaft.