ABUTMENT CONFIGURATION

TECHNICAL FIELD

The invention relates to a main rotor shaft for a wind turbine, and particular an arrangement for a main bearing for use with that shaft.

BACKGROUND

Although many different types of wind energy generators exist today, the most common type is the horizontal axis wind turbine or "HAWT". HAWTs, hereinafter simply 'wind turbines', are in widespread use in on-shore and off-shore settings.

In order to capitalise on economies of scale, there has been a general trend for wind turbines to be designed with ever larger rotor disc diameters in an effort to increase the energy capture potential, thereby lowering the average cost of energy production. This principle has contributed to year-on-year increases in global installed capacity in an effort to re-balance the energy generation mix away from non-renewables such as oil and gas, towards renewables such as wind and solar.

However, the upward trend of wind turbine size comes with its challenges since the wind turbine towers must be taller, the blades must be longer and stronger, and the nacelles must be larger and heavier. The centrepiece of the wind turbine can be considered to be the main rotor shaft, since it carries the hub and rotor blades and harnesses the rotational energy generated by the blades so that it can be converted to electrical energy by the generator. The main rotor shaft and, thus, the housing within which it supported, therefore must be incredibly robust to withstand the huge forces generated during energy production. What is more, the main rotor shaft is typically designed with a life comparable to the rated lifetime of the wind turbine itself, which is usually in the region of 20 years.

In one arrangement, the main rotor shaft extends through a main shaft or 'bearing' housing and is rotatably supported within that housing by two main shaft bearings: a forward bearing supports the end of the shaft near to the hub, that is the 'front' or 'forward' end, and a rear bearing supports the end of the shaft distal from the hub, that is the 'back' or 'rear' end. The bearings function to ensure that the main rotor shaft can rotate smoothly and also transfer axial loads and bending moments to a bed-plate or base-frame via the main bearing housing. This arrangement is generally effective at decoupling the gearbox of the wind turbine from the axial thrust and bending forces of the main rotor shaft, so that only torque is transferred to the gearbox.

The trend towards heavier blades and hubs means that the main rotor shaft and, therefore, the supporting bearings, are required to deal with higher loads, and so there is constant pressure to design the rotor shaft assemblies to handle the loads more effectively.

It is against this background that the invention has been devised. SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a main rotor arrangement for a wind turbine, comprising a main rotor shaft that extends in an axial direction and includes a hub connection flange at its hub-connection end, wherein the shaft flange extends radially inwardly so as to transition into a generally cylindrical first shaft portion having a first diameter. The first shaft portion transitions to a second shaft portion at a shoulder, wherein the second shaft portion has a reduced diameter compared to the first shaft portion. The main rotor arrangement further comprises a forward bearing carried on shaft and which includes an inner ring, and outer ring and a roller set, wherein the inner ring has a radially inner surface in contact with the second shaft portion and which defines an inner ring shoulder which abuts the shaft shoulder defined between the first shaft portion and the second shaft portion.

The main rotor arrangement of the invention beneficially provides a simple and effective means of locating and retaining a main bearing at the correct position along the length of the main rotor shaft. The shoulder of the arrangement, defined between two portions of the rotor shaft having different diameters, advantageously negates the need for time consuming and wasteful processing required to form an upstanding bearing backing rib that is required in prior art systems. Furthermore, the shoulder of the invention can be positioned closer to the hub-end of the rotor shaft than is possible in prior art arrangements, which reduces the load on the bearing generated by the hub and rotor of the wind turbine. In this way, the likelihood of damage or failure of the main bearing is reduced, potentially avoiding the need for expensive repairs which may take the wind turbine out of action for a prolonged period of time.

The main rotor arrangement of the invention is to be considered in the context of a utility- scale wind turbine, which would typically have a power rating of at least 1 MW. The invention therefore extends to such a wind turbine comprising a main rotor arrangement in accordance with the invention.

The size and relative dimensions of the main rotor shaft may be such that the diameter of the second shaft portion may fall in the range of 80% and 98% of the diameter of the first shaft portion; and the diameter of the first shaft portion may fall in the range of 25% and 200% of the length of the main rotor shaft. Relative to the size of the main rotor shaft, it is envisaged that the depth of the shoulder should be relatively shallow so as to reduce machining waste. For example, for a shaft diameter of approximately 1.6m, it is envisaged that the shoulder depth should not exceed 10cm.

The shoulder may be defined in principle at any point along the inner ring, wherein the position of the shoulder defines a shoulder length. A longer shoulder length means that the bearing may be positioned closer to the hub-connection end of the shaft, which may provide a benefit in terms of withstanding shaft loads more effectively. It is envisaged that the shoulder length should be greater than 25% the total length of the inner ring, although that length may be between 60% and 95%.

In some embodiments, the main rotor arrangement may be provided with a creep prevention element for preventing circumferential creep of the forward bearing. The creep prevention element may take the form of a pin that engages between the bearing and the shoulder.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a front view of a wind turbine, comprising a main rotor arrangement according to the invention; Figure 2 is a schematic view of a drivetrain of the wind turbine of Figure 1 , including a main rotor arrangement;

Figure 3 is a perspective view of a main rotor arrangement;

Figure 4 is a longitudinal section view of the main rotor arrangement comprising known arrangements of main shaft and main bearings;

Figure 5 is an illustrative view of a main shaft arrangement in accordance with an embodiment of the invention; and

Figure 6 is an illustrative view of a main shaft arrangement in accordance with an embodiment of the invention, including a creep prevention pin. DETAILED DESCRIPTION

The present invention relates to an improved arrangement for a main bearing and main rotor shaft of a wind turbine, in which a shoulder of the main rotor shaft engages with a shoulder of the bearing to locate the bearing in the correct position on the main rotor shaft and to prevent the bearing from shifting out of position toward the hub-end of the wind turbine during operation. As will be described in more detail later, the invention beneficially allows the main bearing to be supported at a position on the main rotor shaft which is closer to the hub-end of the wind turbine, thus reducing the load transmitted to the bearing by the hub and rotor. Furthermore, the present invention reduces the time-consuming machining process that is required to provide the main rotor shaft with an integral bearing backing rib which is typical in current systems.

With reference to Figure 1 , a wind turbine 2 includes a nacelle 4 that is supported on a generally vertical tower 6, which is itself mounted to a foundation 8. The foundation 8 may be on the land, or wholly or partially underwater. The nacelle 4 houses a number of functional components, some of which are shown schematically in Figure 2, by way of example. Such a configuration would be well known to a skilled person.

Here, the nacelle 4 is shown as housing at least in part, the main rotor arrangement 10, a gearbox 12 and a generator 14. For brevity, some typical components have been omitted from Figure 2 as they are not central to this discussion, for example a power converter and yaw drive. However, the presence of such components is implicit and such component would be well understood by the skilled reader. The main rotor arrangement 10 includes a hub 16 coupled to a main rotor shaft 18, which is rotatably supported in a main shaft housing 20 by a bearing arrangement 22. Note that the main shaft housing 20 is sometimes referred to in the art as a main bearing housing, and will be referred to as such from now on. In this embodiment, the bearing arrangement 22 comprises a forward bearing 24 and a rear bearing 26. The hub 16 is connected to a plurality of rotor blades 27, although three blades are typical in a HAWT. The blades 27 are acted on by the wind and therefore torque is applied by the hub 16 to the main rotor shaft 18 which causes it to rotate within a main bearing housing 20. An input or 'forward' portion of the main rotor shaft 18 comprises a flange 18a, by which means the main rotor shaft 18 is connected to, and driven by, the hub 16. Here the flange 18a is shown as being connected to a further flange 29 that is associated with the hub 16, such that the two flanges form a coupling between the hub 16 and the main rotor shaft 18. The flange 18a can therefore be considered to be at the hub-connection end of the main rotor shaft 18.

An output portion 18b of the shaft 18 provides input drive to the gearbox 12. The gearbox 12 steps up the rotational speed of the main rotor shaft 18 via internal gears (not shown) and drives a high-speed gearbox output shaft 28. The high-speed output shaft 28 in turn drives the generator 14, which converts the rotation of the high-speed output shaft 28 into electricity. The electrical energy generated by the generator 14 may then be converted by other components (not shown here) as required before being supplied to the grid, for example, or indeed any electrical consumer. So-called "direct drive" wind turbines that do not use gearboxes are also known. The gearbox 12 may therefore be considered optional.

At this point it should be noted that although in this embodiment two support bearings 24, 26 are shown that provide support to the main rotor shaft 18 at forward and rearward positions, arrangements are also known in which the rearward bearing is omitted and, instead, rear support for the main rotor shaft 18 may be provided by the generator 14.

The main bearing housing 20 is supported on a base frame 30, which can also be known as a bed plate. Although not shown here, the base frame 30 may be coupled to a yaw drive at the upper part of the wind turbine tower 6 to enable the base frame 30 and, thus, the entire nacelle 4 to yaw with respect to the tower 6 so as to enable the direction of the hub 16 to be adjusted with respect to the wind direction. The base frame 30 is typically a cast component, for example of steel or iron, and has the function to transfer the main shaft loads from the shaft 18, through the bearings 24, 26, the main bearing housing 20, and the base frame 30, and into the wind turbine tower 6. Whereas Figure 2 was schematic in nature to show a general arrangement of components, Figures 3 and 4 illustrate a more practical realisation of a main bearing housing 20 and main rotor shaft 18 for a better understanding of the configuration of the relevant components. It should be noted that whilst the general layout of the main bearing housing 20 and main rotor shaft 18 shown in Figures 3 and 4 is applicable to the present invention, the main inventive concept is not illustrated in these drawings, but will be described later with particular reference to Figures 5 and 6. However, Figures 3 and 4 should assist in providing useful context for the present invention. Note that the same reference numerals are used to denote components corresponding to those in Figure 2. Referring to Figures 3 and 4, the main rotor shaft 18 is tapered along its length to provide a relatively larger circumference at the forward end 32 of the shaft 18 and a relatively smaller circumference at the rearward end 34 of the shaft 18. It should be noted that it is not essential that the main rotor shaft 18 is tapered. However, this configuration may provide certain advantages as it allows the shaft 18 to support a larger forward bearing 24, capable of more effectively managing the substantial loads to which it is subjected in use.

The forward and rear bearings 24, 26 are situated between the main rotor shaft 18 and main bearing housing 20, at forward and rearward positions, respectively, along the length of the shaft 18. The forward and rear bearings 24, 26 together enable the main rotor shaft 18 to freely rotate with respect to the main bearing housing 20 during wind turbine operation, about a rotor axis R that extends through the centre of the main rotor shaft 18.

The forward and rear bearings 24, 26 each include an inner ring 36, an outer ring 38 and a plurality of generally cylindrical rolling elements 40, more simply referred to as rollers, supported therebetween. Note that the inner ring 36 may sometimes in the art be referred to as a cone, whereas the outer ring 38 may sometimes be referred to as a cup. A typical wind turbine bearing for use in utility-scale applications, typically exceeding 1 MW in power output, must withstand high loads and operate reliably over an extended lifetime. In this embodiment, the forward and rear bearings 24, 26 are tapered roller bearings having tapered inner and outer races 42, 44 and tapered rolling elements 40 designed to accommodate combined axial and radial loads. In other embodiments, different types of bearings may be used, for example cylindrical roller bearings or spherical roller bearings (not shown). Cylindrical roller bearings utilise rows of cylindrical rolling elements that are in linear contact with races of the inner and outer rings. Spherical roller bearings include two rows of barrel-shaped rolling elements, which are supported between an outer race and two inclined inner ring races. Because the centre of the sphere of the outer ring race is at the bearing axis, these bearings are self aligning and can withstand misalignment of the shaft relative to the housing.

The main bearing housing 20 comprises a front flared portion 46 that defines a forward bearing seat 48 and a rear flared portion 50 that defines a rear bearing seat 52. To secure the bearings 24, 26 in position, the main rotor shaft 18 includes a forward bearing retainer 54 or 'rib' for retaining the forward bearing 24 in the forward bearing seat 48 and a rear bearing retainer groove 56 for holding a backing element such as a rear bearing clip 58, circlip, lock nut or similar structure that retains the rear bearing 26 in the rear bearing seat 52.

Focusing now on the forward bearing retainer 54, and with particular reference to Figure 4, a standard forward bearing retainer 54 of the current state of the art may comprise a rib 60 in the form of a protrusion that extends radially outwards from an outer surface 62 of the main rotor shaft 18, and extends about the entire circumference of the main rotor shaft 18. This retaining rib 60 includes an abutment surface (not shown) facing away from the forward (hub) end 32 of the shaft 18, against which a corresponding abutment surface (not shown) of the inner ring 36 of the forward bearing 24 contacts in use.

In this way, the forward bearing 24 is located at the correct position along the length of the shaft 18, and is prevented from drifting towards the hub-end 32 of the shaft 18 during assembly and operation. Nevertheless, there are problems associated with the above- described configuration of the retaining rib 60 and forward bearing 24. For example, the processing time and material cost associated with machining the retaining rib 60 into the main rotor shaft 18 is significant. Also, the retaining rib 60 must be precisely dimensioned and positioned on the shaft 18, resulting in strict tolerance requirements that further increase the cost of producing this retaining rib60. A further consideration is that the retaining rib 60 may be located on the shaft at a point where there are high stress concentrations during use. If this is the case, then stress relieving features may also need to be integrated into the structure of the shaft to mitigate stress issues relating to the position of the rib. Thus, any enhancement to this process or to the configuration of the forward bearing 24 and retaining feature 54 would be beneficial. The present invention provides an arrangement for a main bearing 24 and main rotor shaft 18 of a wind turbine 2 which addresses at least one of the problems associated with the current state of the art. Referring to Figure 5, a forward roller bearing 24 of a wind turbine 2 in accordance with a first embodiment of the invention is shown. It should be noted that although the bearing in the illustrated embodiment is a tapered roller bearing, the present invention is not limited to use with tapered roller bearings, but may equally be applied to a system having another suitable bearing type.

When the forward bearing 24 is assembled between the main rotor shaft 18 and the main bearing housing 20, the inner ring 36 abuts with the outer surface 62 of the main rotor shaft 18 and the outer ring 38 of the forward bearing 24 is received in the forward bearing seat 48 of the main bearing housing 20.

Still referring to Figure 5, a portion of the main rotor shaft 18 in the vicinity of the forward bearing 24 is shown, in which a portion of the hub connection flange 18a can be seen. At its outermost point, the flange 18a is in a plane that is perpendicular to the axis of the rotor shaft 18. The hub connection flange 18a extends radially inwardly towards the rotor axis R (not shown in Figure 5) such that the flange 18a transitions into a generally cylindrical first shaft portion 68 having a first diameter. The first shaft portion 68 is generally parallel to the shaft axis.

The first shaft portion 68 transitions to a second shaft portion 70 at a shoulder 72, the first and second shaft portions 68, 70 being spaced axially along the rotor shaft 18. The shoulder 72 defines a step extending about the circumference of the rotor shaft 18 between the first and second shaft portions 68, 70, such that the second shaft portion 70 has a second diameter smaller than the first diameter of the first shaft portion 68. In this embodiment of the invention, the diameter of the first shaft portion 68 is approximately 1600mm, whereas the depth of the shoulder 72 is approximately 80mm, although it is envisaged that the depth should be no greater than 100mm. The diameter of the second shaft portion is approximately 95% of the diameter of the first shaft portion 68. The diameter of the first shaft portion of the main rotor shaft is approximately 50% of the length of the main rotor shaft 18. It should be noted that the above dimensions are specific to this embodiment, and may vary in other embodiments on the invention. Specifically, in other embodiments the diameter of the second shaft portion may fall anywhere in the range of 80% and 98% of the diameter of the first shaft portion, and the diameter of the first shaft portion may fall anywhere in the range of 25% and 200% of the length of the main rotor shaft.

A shoulder surface 74 provides the transition region between the outer surfaces of the first and second shaft portions 68, 70. Here the shoulder surface 74 is substantially perpendicular to an outer surface 75 of the main rotor shaft 18 in the region of the second shaft portion 70. In this context, although it is currently envisaged that the shoulder surface 74 is vertical, some variation may be tolerated, and expected. For example, the shoulder surface 74 may be inclined with respect to the outer surface 75 of the second shaft portion such that, in some embodiments, the shoulder surface 74 forms an angle with the second shaft portion 70 that is up to 10° off-vertical. Furthermore, the shoulder surface 74 may be inclined to provide either a slope or an overhanging ledge between the first and second shaft portions 68, 70. Thus, it will be clear to the skilled reader that the shoulder surface 74 may have many different forms provided it is able to perform its required function as a robust stop for the inner ring 36 of the forward bearing 24 during assembly and operation.

Referring now to Figures 4 and 5, the inner ring 36 of the forward bearing 24 comprises a slanting radially outer surface 76 defining an inner ring raceway or track in which the rolling elements 40 of the inner ring 36 are received and guided between front and rear guiding flanges 76a, 76b. The inner ring 36 also defines a stepped radially inner surface 78 that abuts with the outer surface 62 of the main rotor shaft 18. In use, the stepped inner surface 78 of the inner ring 36 defines an inner ring shoulder 79 that abuts the shoulder 72 of the main rotor shaft 18, such that the stepped inner surface 78 is in contact with at least part of the outer surface 62 in the region of the first and second shaft portions 68, 70, and the entire shoulder surface 74. In this way, the forward bearing 24 is correctly located on the rotor shaft 18 and prevented from drifting towards the hub-end 32 of the shaft 18 during assembly and operation.

The inner ring shoulder 79 is provided at a position A along the axial length L of the inner ring 36 of the forward bearing 24. The inner ring shoulder position A defines a shoulder length, I , of the inner ring 36, which in this embodiment forms 25% of the total axial length L of the inner ring. Here, the total length L is taken between front and rear faces 36a, 36b of the cone/inner ring. This percentage may vary in other embodiments. For example, the length I may define a larger percentage of the total axial length L of the inner ring 36 to allow the inner ring 36 to sit closer to the hub 16 when the rotor arrangement 10 is assembled, for example between 60 and 95%. In this way, loads acting on the forward bearing 24 from the rotor and hub may be reduced (as is described in more detail below). The shoulder 72 of the main rotor shaft 18 and stepped inner ring 36 of the forward bearing 24 provide a simple and effective means of locating and retaining the forward bearing 24 at the correct position along the length of the rotor shaft 18. The configuration of the shoulder 72, as a step defined between two portions of the rotor shaft 18 having different diameters, advantageously negates the need for the time consuming and wasteful process required to form the rib 60 in the current state of the art. Furthermore, providing the retaining feature 54 as a shoulder 72 transitioning from the hub connection flange 18a allows the retaining feature 54 to be positioned closer to the hub-end 32 of the rotor shaft 18 than would otherwise be possible. The loads on the forward bearing 24 are mainly due to forces generated by the rotating rotor and the weight of the hub 16. Reducing the distance between the bearing 24 and the hub 16 acts to reduce the lever arm length (i.e. the length between the forces on the hub and the bearing on which said forces act), which in turn reduces the load (turning moment) on the bearing 24 that is generated by said forces. This reduces the likelihood of damage and/or failure of the bearing 24 which may necessitate expensive and time consuming repairs to be undertaken.

It should be noted at this point that the bearing 24 in practice would also include a roller cage that keeps the rollers is a spaced configuration about the circumference of the cup and cone of the bearing, although this feature is not illustrated here for clarity.

Referring now to Figure 6, a second embodiment of the invention is shown, in which a pin 80 is provided to prevent circumferential movement of the bearing 24 on the rotor shaft 18. This may be referred to as the prevention of "creep".

The creep prevention pin 80 is a generally cylindrical shaped member 82 which is received in corresponding pin-receiving openings 84, 86 provided in the main rotor shaft shoulder 72 and the stepped inner surface 78 of the inner ring 36, respectively. Thus, in embodiments including a creep prevention pin 80, the main rotor shaft shoulder 72 must have sufficient depth to accommodate the pin 80. When the bearing 24 is assembled on the main rotor shaft 18 for use, the creep prevention pin 80 is received in the corresponding openings 84, 86 of the main rotor shaft 18 and inner ring 36 of the forward bearing 24. In this way, the creep prevention pin 80 prevents circumferential movement of the inner ring 36, and thus the forward bearing 24, about the rotor shaft 18. In this embodiment, a single pin 80 is used to prevent creep prevention. In other embodiments, multiple creep prevention pins 80 could be included in the set-up, spaced at intervals about the circumference of the rotor shaft/bearing. Moreover, it is envisaged that instead of pins, disc-like elements could be engaged between cooperating recesses defined by the main rotor shaft shoulder 72 and the stepped inner surface 78 of the inner ring 36. Such a structure would have the effect of 'keying' the inner ring 36 to the shoulder 72 thereby preventing circumferential creep. A serrated structure could also be provided. It should also be noted that the creep prevention pin 80 may have various different shapes and dimensions in other embodiments of the invention, provided it has sufficient strength to retain the bearing 24 in position during wind turbine operation without shearing or breaking. The creep prevention pin 80 of this embodiment of the invention provides a way of preventing circumferential movement of the bearing 24 on the rotor shaft 18, which can be effected using a cheap and simple component.

It should be noted that the specific embodiments described above represent examples of how the inventive concept may be implemented. Thus, the word 'pin' in creep prevention pin should not be considered limiting but could cover similar objects and elements which serve the same purpose of creep prevention. The skilled person would appreciate that the embodiments could be modified without departing from the inventive concept as defined by the claims.