Patent Description:
In a typical `horizontal axis' wind turbine, a nacelle mounted on a tower supports a rotor that includes a plurality of blades. The nacelle houses a main bearing arrangement which supports the rotor so that it is able to rotate as the blades are driven by the wind. The rotor is supported on the input end of a powertrain of the wind turbine which typically includes a gearbox and a generator, although direct drive wind turbines are known that do not include a gearbox. In a known configuration, the main rotor of the wind turbine is coupled to a so-called 'low-speed' shaft which is in turn coupled to the input end of the gearbox. The gearbox also includes a high-speed output shaft which is coupled to the generator.

In some wind turbine powertrains, the powertrain includes a conduit that extends along its rotational axis and which serves to carry electrical and/or hydraulic services to the rotating hub. This conduit is often referred to as a `pitch tube' by those skilled in the art. One of the challenges with this approach is that the pitch tube must be located precisely in order to pass through the generator and the gearbox safely. What is more, the pitch tube passes through different environments such as air filled and oil filled cavities, so the pitch tube must be sealed against its surrounding components to guard against contamination and leakage.

Known approaches to seal the running surfaces of rotating shafts include annular lip seals and labyrinth seals. These are two examples, but others are known. In both approaches, it is important that the rotating shaft is supported in an accurate concentric orientation with respect to the annular seal. Excessive tolerance results in increased wear and less effective sealing functionality. In the case of a pitch tube, it can be the case that the sealing position on the shaft is displaced along the shaft axis a significant distance from where the shaft is rotationally supported and this may lead to concentricity issues.

It is against this background that the invention has been devised <CIT> is a relevant example of prior art.

According to a first aspect of the invention there is provided apparatus comprising a first shaft and a second shaft supported in a substantially concentric relationship so that they are able to rotate relative to one another about a rotational axis; wherein one of the first and second shafts passes through a bore defined in the other of the first and second shafts. A seal arrangement is located between the first and second shafts, said seal arrangement comprising a seal member and a bearing, the seal member being locked from rotational movement relative to the first shaft, and comprises a first portion that is rotatably mounted on the second shaft by the bearing and a second portion that is configured to seal against a running surface defined by the second shaft.

Advantageously, since the seal arrangement is rotatably mounted on the same shaft against which it forms a seal, the sealing arrangement accommodates for alignment errors between the first and second shafts. This may particularly be the case where both shafts are rotatable. But it may also be the case where the outer shaft is stationary and in effect forms a housing for the inner shaft. In either situation, lateral movement of the inner shaft transverse to the rotational axis may be significant, particularly where that shaft is supported by bearings which are remote from the sealing point.

The invention has particular use in a wind turbine application, where the first shaft is a gearbox output shaft and the second shaft is a pitch tube that passes through the gearbox output shaft.

In one embodiment, the seal arrangement comprises a radial surface that opposes the first shaft thereby defining an annular gap. The annular gap provides in effect a cushion which accommodates alignment errors between the first and second shafts. In order to close this gap, the seal arrangement may include a flexible annular seal. The annular seal may take various forms suitable to close the gap and prevent leakage of fluids therethrough. However, in one embodiment the annular seal take the form of a sealing ring which is flattened in form to resemble an annular membrane.

The flexible annular seal may extend between a surface of the first shaft and a surface of the seal arrangement to close the annular gap. In this way, the flexible annular seal in effect locks the rotational movement of the seal arrangement to the rotational movement of the first shaft. This can be particular advantageous in circumstances where the first shaft has a relatively high rotational speed to the second shaft as the high rotational speed imparted to the seal arrangement can improve the operation of the seal.

In order not to affect the rotation of either of the two shafts, the bearing on which the first portion of the seal arrangement is mounted to the first second shaft preferably provides a very low friction interface. In one embodiment the bearing may be a roller bearing, although friction bearings and ball bearings are other options.

Notably, in some embodiments the bearing that supports the sealing arrangement on the respective shaft is different to, that is to say, it is not the same as the bearing arrangement that rotatably supports that shaft with respect to the other shaft.

The seal arrangement may be selected to provide an effective seal for the particular application that the apparatus is intended for use. In some circumstances it may be preferable to use a type of labyrinth seal. This may be the case where the speed differential between the two shafts is high, for example between <NUM> and <NUM> rpm, and so a low friction seal is particularly desirable. In other circumstances, other rotational seals may be appropriate, for example lip seals.

In the above apparatus, the first portion and the second portion of the seal arrangement may be defined by a seal member. Suitably, the sealing member may be defined by a single part and may be shaped to accommodate the bearing within it, therefore providing a particular compact and elegant configuration.

Notably the seal arrangement is compact and so does not take up excessive space between shafts. For example: in one embodiment, the second portion of the sealing arrangement is separated from the bearing by an axial distance along the rotational axis that is less than the radial distance of an annular volume defined between the first shaft and the second shaft; in one embodiment the axial length of the sealing arrangement taken along the rotational axis is substantially the same as a radial distance of an annular volume defined between the first shaft and the second shaft.

In another aspect, the invention provides an apparatus comprising a housing defining an internal bore, a shaft that extends through the internal bore and which is rotatably mounted with respect to the housing so as to rotate around a rotational axis (R); a seal arrangement between the internal bore of the housing and the shaft, locked from rotational movement relative to said housing; wherein the seal arrangement comprises a seal member that is spaced from the internal bore of the housing and includes a first portion that is rotatably supported on the shaft by a bearing and a second portion that is configured to seal against a running surface defined by the shaft.

Notably, in this embodiment the bearing that supports the sealing arrangement on the respective shaft may be different to, that is to say, it is not the same as the bearing arrangement that rotatably supports that shaft with respect to the housing.

It will be appreciated that preferred and/or optional features of the first aspect of the invention may be combined with the other aspects of the invention. The invention in its various aspects is defined in the independent claims below and advantageous features are defined in the dependent claims below.

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:.

Note that features that are the same or similar in different drawings are denoted by like reference signs.

The invention relates to an arrangement of a shaft rotating in a housing which is provided with a dynamic seal. The housing may be stationary, but the invention has particular use in embodiments where the housing is itself a rotating shaft so that the arrangement comprises a pair of coaxial and nested shafts rotating at different speeds with a seal between them. In such an arrangement, providing an adequate seal between the shafts can be challenging, particularly if one or other of the shafts has tolerance errors which compromise the mutual concentricity of the shafts.

Such sealing arrangements may be applied in many different industries, but in order to give the invention a context in which it will be better understood, the technology will now be described in more detail with reference to a wind turbine where a so-called pitch tube extends axially through a hollow rotating shaft. As is known by the skilled person, a pitch tube is a hollow shaft that passes through one or more sections of the wind turbine powertrain in order to provide a protected conduit for hydraulic and electrical services between the stationary reference frame of the nacelle and the rotating reference frame of the hub. However, sealing the pitch tube reliably and where needed within the interior of the powertrain components is challenging.

In order to place the embodiments of the invention in a suitable context, reference will firstly be made to <FIG> which illustrates a typical Horizontal Axis Wind Turbine (HAWT) <NUM> that includes an electrical power generating arrangement, as is shown in <FIG>. Although this particular image depicts an on-shore wind turbine, it will be understood that equivalent features will also be found on off-shore wind turbines. In addition, although such wind turbines are referred to as `horizontal axis', it will be appreciated by the skilled person that for practical purposes, the axis is usually slightly inclined to prevent contact between the rotor blades and the wind turbine tower in the event of strong winds.

The wind turbine <NUM> comprises a tower <NUM>, a nacelle <NUM> rotatably coupled to the top of the tower <NUM> by a yaw system (not shown), a rotating hub or 'rotor' <NUM> mounted to the nacelle <NUM> and a plurality of wind turbine rotor blades <NUM> coupled to the hub <NUM>. The nacelle <NUM> and rotor blades <NUM> are turned and directed into the wind direction by the yaw system.

As mentioned above, the nacelle <NUM> houses an electrical power generating arrangement <NUM>, which includes a powertrain or drivertrain. Both terms are used in the technical art, and 'powertrain' will be used from now on for consistency. As will be understood by those skilled in the art, the wind turbine powertrain comprises a gearbox <NUM>, and a generator <NUM> which are driven by a main shaft <NUM>.

The main shaft <NUM> is supported by a main bearing housing <NUM> and is connected to, and driven by, the rotor <NUM> thereby providing input drive to the gearbox <NUM>. The gearbox <NUM> steps up the rotational speed of the low speed main shaft <NUM> via internal gears (not shown) and drives a gearbox output shaft (not shown in <FIG>). The gearbox output shaft in turn drives the generator <NUM>, which converts the rotation of the gearbox output shaft into electricity. The electricity generated by the generator <NUM> may then be converted by other components (not shown) as required before being supplied to an appropriate consumer, for example an electrical grid distribution system.

With reference generally to the gearbox <NUM>, its housing <NUM> is generally cylindrical in form, which is due to the specific type of gearbox that is used in this particular powertrain arrangement. As the skilled person would know, an epicyclic gearbox comprises a series of planet gears that are arranged about a central sun gear, and which collectively are arranged within an encircling ring gear. Such a gearbox may include more than one stage of planet gears. The ratio of the number of teeth between the ring gear, the planet gear and the sun gears may be used to determine the gear ratio of the gearbox. For clarity, fine detail of the gearbox will not be described in further detail here as the gearbox is not the principal subject of the invention. Suffice to say that other gearbox configurations could also be used.

Further details of the powertrain will now be described with reference also to <FIG> which shows some components more clearly. It should be noted here <FIG> is schematic in form and so for brevity and clarity some components of the generator are not shown or described so as not to detract from the focus of this discussion.

As can be seen, the main shaft <NUM> is supported in the main bearing housing <NUM> by a front bearing <NUM> and a rear bearing <NUM>. The axially-spaced bearings <NUM>,<NUM> ensure that the main shaft <NUM> is supported securely to rotate about the main rotational axis, shown here as R, despite the high loads that are imposed upon it. Since the gearbox <NUM> is an epicyclic gearbox, the main shaft <NUM>, the gearbox <NUM> and the generator <NUM> are all centred on and share the same rotational axis R.

The main shaft <NUM> is coupled to the gearbox <NUM> at a gearbox input shaft <NUM>. Similarly, the gearbox <NUM> is coupled to the generator <NUM> by a gearbox output shaft <NUM>. More specifically, the gearbox output shaft <NUM> is coupled to a rotor <NUM> of the generator <NUM>, hereinafter `generator rotor'.

Turning now to the generator <NUM>, in the arrangement shown it is an IPM (interior permanent magnet) electric machine having an external stator <NUM> which surrounds the generator rotor <NUM>. By 'external', it is meant that the generator stator <NUM> is in a radially outer position relative to the generator rotor <NUM> and surrounds it as compared to generator designs in which the rotor is external to the stator. The magnetically active components of the generator <NUM> are contained within a generator housing <NUM> which in this arrangement is cuboidal in form, as can be seen in <FIG>.

A cylindrical magnet package <NUM> is supported on the generator rotor <NUM> and is arranged to rotate around the rotational axis R. The magnet package <NUM> is supported by a generator rotor hub <NUM> and a support flange <NUM>. The exact structural configuration of the generator rotor hub <NUM> and the support flange <NUM> is not critical. However, functionally these components serve to transmit drive from the generator rotor hub <NUM>, as transmitted from the gearbox, through the support flange <NUM> to the magnet package <NUM>, thereby ensuring that the magnetically active parts of the generator rotor <NUM> are coupled to the gearbox output shaft <NUM>.

As is common in many designs of wind turbine powertrain, an elongate hollow conduit or shaft in the form of a "pitch tube" <NUM> is provided in order to transfer hydraulic and electrical services from the stationary frame of the nacelle to the rotating frame of the main rotor hub. As is shown in the figures, the pitch tube <NUM> extends along the rotational axis R and passes through the generator <NUM>, the gearbox <NUM> and extends into the main shaft <NUM>. The precise form of the pitch tube is not crucial to the invention and so further explanation is omitted.

Although the pitch tube <NUM> passes through powertrain components that rotate at a relatively high rotational frequency, such as the gearbox output shaft <NUM> and the generator rotor <NUM>, the pitch tube <NUM> rotates at the same frequency as the main shaft <NUM>. Therefore, the pitch tube <NUM> must be supported rotationally with respect to the generator rotor <NUM> and the gearbox output shaft <NUM>. For this purpose the pitch tube <NUM> is rotationally supported by suitable bearings at various points along its axial length. The precise configuration of the bearings is not critical but, as shown in <FIG>, in this arrangement the pitch tube <NUM> is supported at a first (rear) bearing <NUM> and a second (front) bearing <NUM>. The rear bearing <NUM> is located at a central aperture provided in a rear face <NUM> of the generator housing <NUM>, and the front bearing <NUM> is defined within the gearbox <NUM>, and is shown schematically in <FIG>. Together the two bearings <NUM>, <NUM> function to support the pitch tube <NUM> in a concentric relationship relative to the gearbox <NUM> and the generator <NUM> about rotational axis R. The skilled person would appreciate that other configurations are acceptable.

Since the pitch tube <NUM> extends through different components of the powertrain, suitable seals are necessary to ensure that ambient environments of the powertrain are not contaminated by oil-filled environments. For instance, the internal environment of the generator needs to be protected from oil and grease inside the gearbox. However, the sealing demands can be challenging to satisfy because it can be difficult to ensure that the pitch tube is truly concentric with the gearbox output shaft. In circumstances where the pitch tube is slightly misaligned, the effectiveness of conventional rotational seals may be compromised which can lead to admittance of oil mist into the generator and increased wear on the running surface of the pitch tube.

To this end, the embodiments of the invention provide a seal arrangement <NUM> which serves to seal the pitch tube <NUM> against the high speed output shaft <NUM> of the gearbox. An embodiment of the seal arrangement <NUM> is indicated generally in <FIG>. However, due to the scale of the drawing, a more detailed view of the circled region A of <FIG> is shown schematically in <FIG>.

At this point it should be noted that <FIG> can be considered to be a simplified view so as to focus on the main features of the inventive concept without obscuring them with unnecessary detail. Here, the gearbox output shaft <NUM> can be seen in a radially outer position relative to the pitch tube <NUM>. It should be noted that details regarding how the gearbox output shaft <NUM> is rotationally supported are not shown for clarity in this view. However, the skilled person would understand that the gearbox <NUM> would be provided with suitable bearing arrangements to ensure that the output shaft is adequately supported.

The gearbox output shaft <NUM> is hollow and as such defines an axial internal bore <NUM>. The pitch tube <NUM> passes through the internal bore <NUM> and, in this way, extends through the gearbox <NUM> from the generator side to the main shaft side. As has been mentioned, the pitch tube <NUM> is supported on respective bearings <NUM>, <NUM> independent from the gearbox output shaft <NUM> so that the pitch tube <NUM> is able to rotate relative to the gearbox output shaft <NUM> about the rotational axis R. The gearbox output shaft <NUM> and the pitch tube <NUM> are therefore supported in mutually concentric orientations such that the pitch tube <NUM> is nested within the gearbox output shaft <NUM>.

The pitch tube <NUM> has an outside diameter that is smaller than the internal diameter of the gearbox output shaft <NUM> so that an annular volume <NUM> is defined between these two components. It is this annular volume <NUM> that must be sealed in order to prevent contamination between the oil-laden environment inside the gearbox <NUM> and the ambient environment of the generator <NUM>. To this end, the seal arrangement <NUM> is located in the annular volume <NUM> between the pitch tube <NUM> and the gearbox output shaft <NUM>. More specifically, in this embodiment the seal arrangement <NUM> is located in the annular volume <NUM> in a position proximate to the opening <NUM> of the internal bore <NUM> of the gearbox output shaft <NUM>.

In a typical rotating shaft seal, a seal member such as an annular lip seal would be mounted on the gearbox output shaft <NUM> and seal against a running surface of the pitch tube <NUM>. However, such a conventional arrangement would be vulnerable to concentricity errors between the two shafts which could lead to excessive wear on the running surface of the pitch tube and/or increased leakage from the seal.

In contrast to the conventional arrangement as discussed above, notably the seal arrangement <NUM> of the invention is mounted, supported or otherwise affixed to the pitch tube <NUM>. More specifically, the sealing arrangement <NUM> is rotatably mounted to the pitch tube <NUM> so that it is able to rotate relative to it. As can be seen in the Figures, the sealing arrangement <NUM> is mounted directly to the pitch tube <NUM> in that there are no other shafts or housing components between them. The sealing arrangement <NUM> is therefore fixed to the pitch tube <NUM> in such a way that it is able to rotate whilst fixed to the pitch tube. Expressed another way, the pitch tube <NUM> carries the sealing arrangement <NUM>. Moreover, and as will be appreciated from the discussion that follows, it should be noted that the bearing that supports the sealing arrangement <NUM> on the pitch tube <NUM> is different to the bearing arrangement that rotatably supports the pitch tube and the gearbox output shaft <NUM>. For the avoidance of doubt, it should be noted that this distinction applies to each embodiment of the invention discussed below.

In this embodiment the seal arrangement <NUM> includes two components; a bearing <NUM> and a seal member <NUM>. The bearing <NUM> is shown here as a roller bearing as this is a convenient way to provide a low friction interface between the pitch tube <NUM> and the seal member <NUM>. However, it is envisaged that other types of bearings would be acceptable, for example different types of ball bearings, roller bearings and friction bearings.

Whereas <FIG> shows a side view of the seal member <NUM>, <FIG> shows a perspective view in which detail may be better appreciated.

The seal member <NUM> in this embodiment is an annular component which is generally cylindrical in outer form. The interior form of the seal member <NUM> is shaped to define two sections: a first section <NUM> serves a connecting function to connect or fix the seal member <NUM> to the bearing <NUM>; whilst a second section <NUM> serves the sealing function. The two sections <NUM>,<NUM> each define approximately half of the axial length of the seal member <NUM>.

In the illustrated embodiment, the first section <NUM> of the seal member <NUM> includes a circular socket <NUM> defined by a cylindrical inner wall <NUM>, the inner diameter of which is comparable to that of the outer diameter of the bearing <NUM> on which it is mounted. In the arrangement shown, the socket <NUM> has a frustoconical base or floor <NUM> at which point the first section <NUM> merges into the second section <NUM>. The second section <NUM> therefore has thicker wall section compared to the first section <NUM>.

The seal member <NUM> may be mounted on the bearing <NUM> by way of a press fit since the internal diameter of the socket <NUM> is comparable to the outer diameter of the bearing <NUM>. Additional or alternative measures may be taken to ensure a secure fixing. For example, a suitable bonding agent may be applied to the contact surfaces of one or both of these components. Furthermore, a suitable circlip, set screw or pin arrangement may be integrated into the seal arrangement <NUM> to ensure that the seal member <NUM> is connected securely to the bearing <NUM>. It is also envisaged that heat shrinking may be a suitable option for securing the seal member onto the bearing.

The second section <NUM> of the seal member <NUM> provides the sealing function. In this embodiment the second section <NUM> defines a labyrinth seal <NUM>. It should be appreciated at this point that the precise form of seal is not crucial. As illustrated, the labyrinth seal <NUM> is depicted as a simple non-isolating radial labyrinth seal featuring a plurality of axially-spaced annular sealing fins <NUM> that are defined by the second section <NUM> of the seal member. In the usual way, the seal member <NUM> is configured such that the sealing fins <NUM> define a very small gap with the underlying running surface of the pitch tube <NUM> to provide a non-contact seal. Drain channels <NUM> are provided in the seal member <NUM> so that the fluid captured between the fins <NUM> can be expelled radially outwards. Other types of labyrinth seals would also be acceptable in this application, such as axial labyrinth seals and isolating labyrinth seals. For instance, the pitch tube could be provided with counter fins that extend radially in a direction away from the pitch tube and interdigitated with the fins of the sealing member. Moreover, other seal types such as lip seals could be used. Combinations of seal types may also be used. The skilled person would understand, however, that different types of seal may increase the complexity of the arrangement.

The seal member <NUM> may be made from any suitable material. Different materials may be more suited to different applications. For example, types of engineering plastics and rubbers (e.g. nylon, PEEK, FKM or EPDM) may be suitable for some applications, as would metal seals (e.g. steel).

Advantageously, since the seal arrangement <NUM> is rotatably mounted on the pitch tube <NUM>, the sealing fins <NUM> of the seal member <NUM> are accurately positioned with respect to the opposed running surface irrespective of how accurately the pitch tube <NUM> is mounted with respect to the gearbox output shaft <NUM>. This ensures an accurate seal is established.

Referring specifically to <FIG> it will be noted that the outer diameter of the seal arrangement <NUM> is less than the internal diameter of the gearbox output shaft <NUM>. This ensures that the seal arrangement <NUM> is able to run freely within the annular volume <NUM> and also provides space to accommodate eccentric mounting between the pitch tube <NUM> and the gearbox output shaft <NUM>. Due to the difference in diameters, an annular gap <NUM> is defined between the radially outer surface <NUM> of the pitch tube <NUM> and the internal bore <NUM>. Expressed another way, the seal member <NUM> is spaced from the internal bore <NUM>. It will be noted that any eccentricity in how the pitch tube <NUM> is mounted with respect to the gearbox output shaft <NUM> will cause the annular gap <NUM> to be non-uniform around its circumference. In effect, therefore, it may be considered that any tolerance between the pitch tube <NUM> and the gearbox output shaft <NUM> is transferred radially outwards to the annular gap <NUM> between the seal arrangement <NUM> and the gearbox output shaft <NUM>.

In order to seal the annular gap <NUM>, the seal arrangement <NUM> includes a cover in the form of a flexible annular seal <NUM> that closes the annular gap <NUM>. The flexible annular seal <NUM> is also shown in <FIG> in perspective view.

In the illustrated embodiment, the flexible annular seal <NUM> is in the form of a ring or membrane seal. The ring seal <NUM> is shaped so as to be generally planar in cross section. When installed in the annular gap <NUM>, the plane of the ring seal <NUM> is transverse to the rotational axis R. The planar shape of the ring seal <NUM>, together with the material from which it is made, provides it with radial resilience so that it is able to stretch in the radial direction in order to accommodate non-uniformity in the annular gap <NUM>. Although it is envisaged that a flat ring would be acceptable for the ring seal <NUM>, the illustrated embodiment is shaped to provide a greater degree of flexibility.

To this end the ring seal <NUM> comprises radially inner and outer flanges or lips <NUM>, <NUM> which are connected by an intermediate ridge, rib or bridge <NUM>. The curved shape of the bridge <NUM> links the inner and outer flanges <NUM>,<NUM> out of the general plane of the ring seal <NUM> and therefore provides a greater degree of flexibility to allow the inner and outer flanges <NUM>,<NUM> to move towards and away from each other. This is illustrated diagrammatically in <FIG>, which depicts the ring seal <NUM> adapting to a non-uniform annular gap <NUM> in which the lower part of the ring seal <NUM>, in the orientation shown in the diagram, is stretched relative to the upper part, wherein the majority of the stretch is accommodated by the bridge <NUM>.

The two radial flanges <NUM>,<NUM> of the ring seal may be secured to their respective mounting surfaces in any suitable way. Although not shown here, it is envisaged that the flanges <NUM>, <NUM> could be adhesively bonded or mechanically fastened to the surfaces to which they are required to be attached. With reference to <FIG>, it can be seen that the ring seal <NUM> is positioned so that it extends between a surface of the gearbox output shaft <NUM> and a surface of the sealing arrangement <NUM> and, as such, closes the annular gap <NUM>. More specifically, in the illustrated embodiment, the ring seal <NUM> extends between respective adjacent axial-facing surfaces <NUM>, <NUM> of the gearbox output shaft <NUM> and the sealing arrangement <NUM>. The adjacent axial-facing surfaces <NUM>, <NUM> are aligned in the axial direction along the rotational axis R.

The ring seal <NUM> may be attached to the associated mounting surfaces by any suitable means, including clamping rings and bolts, as is appropriate and as would be apparent to the skilled person.

In addition to closing the annular gap <NUM>, the ring seal <NUM> serves as a radial linkage between the seal member <NUM> and the gearbox output shaft <NUM>. Since the seal member <NUM> is mounted on the bearing <NUM>, the ring seal <NUM> acts as a torque rest and as such makes seal member <NUM> rotate in synchronisation with the gearbox output shaft <NUM>. Since the gearbox output shaft <NUM> rotates at a higher speed than the pitch tube <NUM>, this helps to ensure the labyrinth seal operates optimally as such seals tend to be more effective at capturing leaking fluids due to the higher radial forces.

From the above discussion, it will be noted that the seal arrangement <NUM> is relatively compact in the axial direction along the rotational axis R. This means that the sealing function of the seal member <NUM> is predictable and reliable and does not take up excessive space in the annular volume <NUM> between the two shafts <NUM>. Notably, the second portion <NUM> of the seal member <NUM> is positioned close to the bearing, as shown by the dimension D1 on <FIG>. More specifically, dimension D1 is less than the radial distance of the annular volume <NUM> between the gearbox output shaft <NUM> and the pitch tube <NUM>, which is indicated on <FIG> as dimension D2. The dimension D1 is also less than the diameter of the pitch tube <NUM>, which is indicated as dimension D3. It will also be noted that the depth of the seal member <NUM>, that is the axial length of the seal member <NUM> when oriented as shown in <FIG>, as indicated by the dimension D4 is substantially equal to that of the dimension D2. What is more, due to the provision of the socket <NUM>, the bearing (<NUM>) is accommodated within the axial length of the seal member <NUM> and, in this embodiment, does not protrude from the socket <NUM>. This contributes to the space-efficient package of the seal arrangement <NUM>.

The skilled person would appreciate that the illustrated embodiments are provided as an indication of one way in which the inventive concept may be implemented. Some variants have been described above, but others will be apparent. Therefore, the scope of the invention should be determined by the terms of the appended claims rather than the features of the illustrated embodiments.

In the embodiments discussed above, the seal arrangement <NUM> is configured to be mounted directly to the pitch tube <NUM> and also to seal against a running surface on the pitch tube <NUM>. Expressed in another way, the seal arrangement <NUM> provides a radially inward facing seal. However, it is envisaged that the principles of the invention apply also to a seal arrangement that may be configured to provide a radially outer seal between two rotating shafts. Such a configuration is depicted in <FIG>, in which it will be noted the components are common with the previous Figures, and so the same reference numerals will be used for clarity. A detailed description of this arrangement will not be provided and only the differences will be explained.

In this embodiment, the seal arrangement <NUM> is located in the same position as in <FIG>, namely in the annular volume <NUM> between the pitch tube <NUM> and the generator output shaft <NUM> at an axial position adjacent to the open end of the gearbox output shaft <NUM>. However, it will be apparent that the orientation of the seal arrangement <NUM> has been inverted so that the bearing <NUM> is fixed to the internal bore <NUM> of the gearbox output shaft <NUM> and the seal member <NUM> is opposed to and so seals against a running surface on the gearbox output shaft <NUM>. It follows, therefore, that the annular gap <NUM> that is closed by the ring seal <NUM> is now defined by the radially inward surface of the seal arrangement <NUM> and the outer surface of the pitch tube <NUM>. Notably in this embodiment there is provided an optional set of counter fins <NUM> mounted on a carrier <NUM>. The counter fins <NUM> oppose the fins of the labyrinth sealing member <NUM> and thus provide additional isolation for the seal arrangement <NUM>. More specifically, the counter fines <NUM> are interdigitated with the fins of the sealing member <NUM> as would be understood by the skilled person. The counter fins <NUM> and associated carrier <NUM> may form an annular component that could be received over the sealing member <NUM> and the bearing <NUM>. The combined assembly of the counter fins <NUM>/carrier <NUM>, the sealing member <NUM> and the bearing <NUM> could therefore be installed as a single component.

A suitable drain channel <NUM> could be provided in the outer shaft <NUM> in order to drain leakage fluid from the seal arrangement <NUM> through to a suitable drain or sump.

Claim 1:
Apparatus comprising:
a first shaft (<NUM>) and a second shaft (<NUM>) supported in a substantially concentric relationship so that they are able to rotate relative to one another about a rotational axis (R); wherein one of the first and second shafts (<NUM>,<NUM>) passes through a (<NUM>) bore defined in the other of the first and second shafts (<NUM>,<NUM>);
a seal arrangement (<NUM>) between the first and second shafts (<NUM>,<NUM>), said seal arrangement (<NUM>) comprising a seal member (<NUM>) and a bearing (<NUM>), the seal member (<NUM>) being locked from rotational movement relative to said first shaft (<NUM>);
wherein the seal arrangement (<NUM>) comprises a first portion (<NUM>) that is rotatably mounted on the second shaft (<NUM>) by the bearing (<NUM>) and a second portion (<NUM>) that is configured to seal against a running surface defined by the second shaft (<NUM>).