GB2576066A

GB2576066A - Improved wind turbine shaft and drive assembly

Info

Publication number: GB2576066A
Application number: GB1904431.2A
Authority: GB
Inventors: John Mellor Andrew; Wrigth Chy
Original assignee: Turbine Eng Developments Ltd
Current assignee: Turbine Eng Developments Ltd
Priority date: 2018-03-29
Filing date: 2019-03-29
Publication date: 2020-02-05
Also published as: GB201904431D0; GB201805208D0

Abstract

Wind turbine comprising a bed 11, rotor, gearbox 15, generator 20, shaft 12 and a hub 14 with a plurality of blade mounts, wherein the shaft is mounted on the bed by at least two spaced apart bearings 13, the gearbox and generator are mounted on the bed. The shaft inputs to the gearbox, wherein the output is connected via a coupling to the generator input shaft. The shaft includes a hub mounting portion with a hub seat and adjacent flange. The hub is in an interference fit with the shaft/hub seat. The hub and flange have corresponding pluralities of holes and studs with nuts, or aligned holes wherein at least the hub holes are threaded and threaded bolts pass through the flange holes and engage with the threads of the hub. The blade mounts may be welded or fastened, the bearing may be lip sealed, the shaft may be electroless nickel. Also disclosed is a turbine further comprising a high or low speed shaft brake assembly which may be hydraulic or pneumatic. Various flexible couplings are disclosed. There may be sensors and/or controllers for the brake assembly, and a tower yaw motor.

Description

Improved Wind Turbine Shaft and Drive Assembly

Field of the Invention

The present invention relates to wind turbines and in particular to the assembly through which rotational energy developed by the blades of the wind turbine is delivered to a generator which converts rotational energy to electrical energy.

Background of the Invention

Wind turbines have operating parameters, one such parameter being wind speed. If the wind speed is too low the blades will not turn, and if the wind speed is too great the blades will rotate faster than is safe for the turbine. In the case of wind speeds that are too great it is desirable to stop the blades from rotating. If the blades are not prevented from rotating they may rotate at such great speed that damage is caused. In order to achieve this wind turbines are provided with brakes which are used to slow the blades down to a standstill and then to hold the blades in a stationary condition.

The braking systems of wind turbines that are currently available do not always succeed in bringing the blades to a halt. The adoption of fail to safe braking systems in wind turbines is almost universal. It has been noticed that such braking systems (which include a brake disc and a calliper mounting pair of brake pads which are held off the disc by air pressure in a disengaged configuration and forced against the disc into an engaged configuration by springs when air pressure is release) can fail to halt the blades. It is understood that such failure occurs due to heating of the brake pads during a time period of a few seconds following application of the brake pads to the brake disc. If the brake pads heat to too great a temperature then the braking system will experience brake fade. The usual solution to braking problems is to increase the brake pad and/or disc size and to use a spring capable of exerting a greater force. The problem associated with increasing the spring force is that the resulting increased braking effort is reacted through the structure of the wind turbine, hence subjecting the structure to greater forces.

Wind turbines generally include a gearbox to which the shaft mounting the rotor forms an input, the gearbox having an output shaft connected to a generator and gears having a different number of teeth so that the output shaft rotates faster than the input shaft. Typically, wind turbines having a power output of up to 750 kw have the braking system mounted on the output shaft. This is considered advantageous since the braking torque is lower on the shaft that is rotating faster.

A wind turbine is described in W02004088131. This document describes a self-regulating wind turbine. A mechanism is provided so that if the turbine enters an overspeed condition, the resulting increase in centrifugal force causes the blades to change their orientation to the wind direction so that less energy is transferred from the wind to the turbine, thereby limiting the speed of rotation of the turbine.

Whilst the provision to vary the pitch of the turbine blades is useful, in essence this is only used where there has been a failure of a braking mechanism. It is known to use pneumatic braking system on wind turbines. The function of the braking system is to stop the blades from rotating when the wind speed is outwit the operating parameters of the wind turbine.

The blades of a wind turbine are attached to a hub, the hub being mounted on a shaft. In one type of wind turbine the hub carrying the blades is secured on a shaft by means of taper locks situated between the shaft and the hub, one to the front of the hub and one to the rear. In order to receive the taper locks the inner surface of the hub is machined to provide a sloping surface that corresponds to the taper of the taper lock. The hub provides blade mounts to which individual blades are attached. Where the blade mounts are welded to the hub the hub can become distorted due to the heat generated during the welding process. This distortion can cause the slope of the sloping surface to be disturbed. If this occurs, when the taper lock is inserted between the hub and the shaft, instead of the taper lock and the hub engaging with each other about the whole of the internal surface of the hub, the taper lock engages at points on the internal surface of the hub. The point loading of the hub and taper locks can cause stress build ups which may reduce the service life of the hub and/or taper lock. Further, there may be some relative movement between the taper locks and the hub as the turbine rotates. Such relative movement between the taper locks and the hub indicates that the hub is not held tightly on the shaft. In this scenario the hub may therefore rock relative to the shaft causing unwanted stresses within the shaft, hub and supporting structures.

The rotor shaft is mounted on a bed by means of at least two spaced apart bearings. It is known to adopt laminar seals for the bearings. Laminar seals require a sleeve to be mounted on the shaft to each side of the bearing races. The bearing is packed with grease, which moves out through the seal slowly, thereby creating a barrier of grease immediately adjacent the bearing seal. The problem with bearings having such seal arrangements is that the lubricating grease seeps out through the seal constantly and therefore the bearing requires the lubricant grease to be topped up constantly. In the case of a wind turbine having long service intervals, even where an autolubrication system is provided the correct delivery of lubricant grease to the bearing cannot be guaranteed.

It would be desirable to provide an improved arrangement for mounting the rotor hub on the shaft.

It would also be desirable to provide an improved arrangement for mounting the shaft on to the turbine bed.

It would also be desirable to provide an improved braking mechanism for a wind turbine. Summary of the Invention

According to the invention there is provided a wind turbine comprising a bed, a rotor, a gearbox, a generator and a shaft, wherein: the rotor includes a hub having a plurality of blade mounts and the hub is mounted on the shaft, the shaft is mounted on the bed by means of at least two spaced apart bearings, the gearbox and the generator are mounted on the bed and the shaft forms an input to the gearbox, the output of the gearbox being connected to an input shaft of the generator by means of a coupling, and wherein the shaft includes a hub mounting portion which comprises a hub seat and a flange adjacent the hub seat, and wherein the hub is an interference fit with the hub seat, the hub and flange each including a plurality of aligned holes, the holes in at least the hub being threaded and the hub mount including a plurality of bolts, which threaded bolts pass through the holes in the flange and the threads of the bolt engage with the threads of the holes in the hub.

The blade mounts may be welded to the hub. Alternatively, the blades may be attached to the hub by an arrangement including a pin or a bolt.

The spaced apart bearings are lip sealed bearings.

Preferably, the shaft is coated with electroless nickel.

The wind turbine may further comprise a high speed shaft brake assembly, wherein a first part of the high speed shaft brake assembly is mounted on the gearbox output to rotate therewith and a second part of the high speed shaft brake assembly is in fixed relationship with respect to the bed.

The wind turbine may further comprise a low speed shaft brake assembly, wherein a first part of the low speed shaft brake assembly is mounted on the shaft or a component of the gearbox rotating with and at the same speed as the low speed shaft to rotate therewith and a second part of the low speed shaft brake assembly is in fixed relationship with respect to the bed.

The low speed shaft brake assembly and/or the high speed shaft brake assembly may include at least one hydraulically actuated brake calliper.

The low speed shaft brake assembly and/or the high speed shaft brake assembly may include at least one pneumatically actuated brake calliper.

The lows speed shaft brake assembly and/or the high speed shaft brake assembly may include a brake disc, which may be ventilated.

The coupling connecting the output of the gearbox to the input shaft of the generator may be a flexible coupling.

The flexile coupling may include two hubs, one attached to the gearbox output and the other to the generator input shaft, each hub being provided with teeth and the flexible coupling comprising an elongate element having teeth on one side thereof, the teeth of the elongate element being shaped and dimensioned to mesh with the teeth of the hubs, the flexible coupling further comprising a retaining band surrounding the elongate element and the toothed parts of the hubs.

The retaining band may be elastomeric.

The flexible coupling may comprise a flexible ring attached to a hub mounted of the output shaft of the gearbox and to a split coupling mounted on the input shaft of the generator.

The flexible ring may be an elastomeric ring.

The flexible ring may be attached to the hub by a first plurality of fasteners and to the split coupling by a second plurality of fasteners.

The second plurality of fasteners may extend in a direction substantially perpendicular to the direction in which the first plurality of fasteners extend.

The split coupling is situated within the flexible ring.

An end face of the hub may abut an end face of the flexible ring.

The wind turbine may further comprise a controller and at least one sensor, the at least one sensor including a hub speed monitor, the output of the at least one sensor comprising an input to the controller, and the controller having at least one output, the at least one output being a brake actuator control signal.

The wind turbine may comprise a plurality of sensors and a plurality of outputs, the plurality of outputs including a hydraulic brake actuator control signal and a pneumatic brake actuator control signal.

Preferably, the at least one sensor includes a hydraulic pressure sensor and a pneumatic pressure sensor.

The at least one sensor may include a clock.

According to another aspect of the invention there is provided a wind turbine comprising a bed, a rotor, a gearbox, a generator, a shaft and a brake, wherein: the rotor includes a hub having a plurality of blade mounts and the hub is mounted on the shaft and the shaft is mounted on the bed by means of at least two spaced apart bearings: the gearbox and the generator are mounted on the bed and the shaft forms an input to the gearbox, an output of the gearbox being connected to an input shaft of the generator by means of a coupling; and wherein the brake includes a low speed shaft brake assembly associated with the shaft and/or a high speed shaft brake assembly associated with the output of the gearbox.

Preferably, a first part of the high speed shaft brake assembly is mounted on the gearbox output to rotate therewith and a second part of the high speed shaft brake assembly is in fixed relationship with respect to the bed.

Advantageously, a first part of the low speed shaft brake assembly is mounted on the shaft or a component of the gearbox rotating with and at the same speed as the shaft to rotate therewith and a second part of the low speed shaft brake assembly is in fixed relationship with respect to the bed.

The wind turbine assembly may comprise at least one yaw motor configured for rotating the wind turbine assembly relative to a tower upon which the wind turbine assembly is mounted. The wind turbine assembly may comprise two or three yaw motors. The or each yaw motor may be provided with a pinion gear wheel on its output shaft, the pinion gear wheel configured to engage with a ring gear mounted on the tower.

The bed and a tower on which the wind turbine assembly is mounted may be provided with corresponding parts of a slew ring connection which provides for the wind turbine assembly to rotate relative to the tower by means of the at least one yaw motor.

The bed may be provided with mounts for receiving yaw motors. It is preferred that the mounts are located such that pinion gear wheel of the or each yaw motor engages with the ring gear of the tower.

Brief Description of the Drawings

In the Drawings, which illustrate a wind turbine of the prior art and a wind turbine according to the invention, and which are by way of example:

Figure 1 is a cross-sectional elevation of a wind turbine shaft and hub assembly of the prior art;

Figure 2a is plan view of the wind turbine according to the invention;

Figure 2b is a side view of the wind turbine shown in Figure 2a;

Figure 2c is a front view of the wind turbine shown in Figure 2a;

Figure 2d is a rear view of the wind turbine shown in Figure 2a;

Figure 3 is a schematic representation of wind turbine shaft and hub assembly of the invention;

Figure 4 is a side view of the turbine shaft illustrated in Figure 2;

Figure 5 is a cross-sectional elevation of a coupling between a high speed output of a gearbox and a generator input shaft;

Figure 6 is a schematic representation of a brake arrangement;

Figure 6a is a side view of the brake arrangement illustrated in Figure 6;

Figure 7 is a schematic representation of the control system for operation of the brakes of the wind turbine;

Figure 8 is a plan view of a wind turbine assembly according to an embodiment fo the invention;

Figure 9 is a side view of the wind turbine illustrated in Figure 8;

Figure 10 is en end view of the output of the high speed gearbox of the wind turbine assembly illustrated in Figures 8 and 9;

Figure 11 is a cross-section on the axis A-A shown in Figure 10;

Figure 12 is a top plan view of the bed plate of the wind turbine assembly illustrated in Figures 8 and 9; and

Figure 13 is a side view of the bed plate illustrated in figure 12.

Detailed Description of the Drawings and Preferred Embodiments

Figure 1 illustrates an example of the prior art. A shaft 1 is supported in bearings 2 (one of the bearings being shown) and provides a mounting region for a hub 3. The inner surface of the hub includes two tapered parts 3a, 3b. The angle of taper of the tapered parts 3a, 3b corresponds to the angle of taper of taper locks 7 which secure the hub 3 on to the shaft 2. The shaft 2 includes a key way 2a. A key 2b placed in the key way engages with a corresponding key way provided by the taper locks 7 to transfer rotational power generated by blades 5 from the hub 3 to the shaft 1. The hub 3 has blade mounts 4 attached thereto. The blade mounts are welded to the hub 3.

Referring now to Figures 2a to 2d, a wind turbine assembly 10 comprises a bed 11 on which other components of the assembly are mounted. Those components include a shaft 12 mounted in two spaced apart bearings 13, the bearing housings thereof being attached to the bed 11 by means of bolts 13a.

A hub 14 is mounted on one end of the shaft 12, with the other end of the shaft being connected to a gearbox 15. The gearbox 15 has an output shaft 15a which is connected to the input shaft of a generator 20, the gearbox being configured so that the output shaft thereof rotates at a faster speed than the shaft 12.

Referring also to Figures 3 and 4, the shaft 12 includes a flange 12a having a plurality of holes12b spaced apart radially relative to one another. The shaft also provides hub seats 12c, 12d, the hub 14 being an interference fit on these hub seats. The hub 14 includes a plurality of studs 14a, which are threaded externally. The studs are spaced radially relative to one another so that they may align with the holes 12b of the flange 12a. Studs 14a pass through the holes 12b of the flange and mate with the threads of nuts 14b to secure the hub 14 to the shaft 12.

As can be seen from Figure 3, the shaft 12 is equipped with a brake assembly comprising a brake disc 18 and three radially spaced apart callipers 19, each having a pair of brake pads for engagement with opposing sides of the disc 18.

The output shaft 15a of the gearbox 15 is connected to the input shaft 21 of the generator by means of a flexible coupling 22, as illustrated in Figure 5. The input shaft of the generator 20 amounts a hub 26, with the output shaft 15a of gearbox 15 mounting a corresponding hub 27. The illustration shows the teeth 27a formed in the end of the hub 27. The hub 26 is provided with similar teeth. These teeth do not intermesh. Instead the teeth and spaces between of each hub are aligned and an internally toothed band 28 is provided around the two hubs 26, 27, the internal teeth thereof intermeshing with the teeth of both hubs. A securing band 29 is positioned around the internally toothed band 28 to hold it in place. Both bands 28, 29 are made of a material that exhibits a certain amount of flexibility, and is formed of an elastomer in the illustrated embodiment.

The internally toothed band 28 is elongate with two ends and is wrapped around the aligned hubs 26, 27. The retaining band 29 is a continuous band with surrounds the internally toothed band 28 holding it in place.

In the illustrated embodiment brake sets are shown on both the shaft 12, the low speed shaft, and the output shaft 15a of the gearbox 15, the high speed shaft. The brake set associated with the high speed shaft comprises a disc 16 (mounted on hub 27) and brake callipers 17 and the brake set associated with the low speed shaft comprises a disc 18 and brake callipers 19. In the drawing only two brake callipers are shown for each brake disc. However, more than two callipers may be provided as shown in Figure 6 below.

Referring now to Figure 6, a ventilated brake disc 40, which may be mounted on either or both of the low speed shaft 12 and the high speed shaft 15a, is provided with two sets of callipers 41,42. The first set of callipers 41 is connected to a hydraulically powered actuation system. The brake callipers 21 may be fail to safe or not. With either type of brake calliper, the actuation thereof is controlled by a control system which measures the rotational speed of the hub, and adjusts the force applied by the brake pads to the disc in accordance with the speed of rotation of the hub.

The control algorithm adjusts the braking force in proportion to the speed of rotation, so as the hub rotates more slowly the braking force is reduced. Parameters associated with the hub may be sensed and the outputs of such sensors used in the control algorithm. For example, the temperature at the hub may be sensed.

The callipers 42 are actuated pneumatically and are fail to safe type brake callipers, that is the callipers comprise springs to press the brake pads against the brake disc and pressurised air is introduced into the calliper to compress the springs, thereby releasing the brake pads from the disc.

In this illustrated embodiment the function of the callipers 42 is to hold the hub stationary once its speed of rotation has been reduced to zero revolutions per minute. The controller issues a signal to the pneumatic system which cuts the supply of pressurised air to the callipers 42, resulting in the springs thereof pressing the brake pads against the disc 40.

In another embodiment, the pneumatically operated callipers and associated brake pads may be used to assist the hydraulically operated callipers in reducing the rotational speed of the hub 14.

Referring now to Figure 7, a control system for operation of the brakes of the wind turbine comprises a controller in the form of a programmable logic controller 30 which receives inputs from a hub speed sensor 31, a clock 32 a hydraulic pressure sensor 33 and a pneumatic pressure sensor 34. The hydraulic pressure sensor 33 senses the hydraulic pressure acting on the brake calliper 41, whist the pneumatic pressure sensor senses the pneumatic pressure acting on the brake calliper 42. The PLC 30 is programmed to generate outputs which control the hydraulic pressure experienced by the brake calliper 41 and the pneumatic pressure experienced by the brake calliper 42.

The pic 30 may be programmed to adjust the hydraulic pressure experienced by the calliper 21 according to the sensed hub speed. The clock 32 allows the duration of application of hydraulic fluid pressure to the calliper 21 to be measured and controlled in relation to time. For example, the hydraulic brake calliper 21 may be actuated for a first period of time at a first pressure and subsequently for a second period of time at a second pressure.

The pic 30 may be programmed such that the pneumatic brake actuator is applied when the hub speed detected by sensor 31 has fallen below a certain threshold, for example when the hub is at a standstill. Further, the pneumatic pressure sensed provides the PLC 30 with information about the wind turbine. If the pressure is below the threshold pressure required to open the calliper 22, then the hub is stationary because the pneumatic brake calliper 22 is fail to safe.

Figures 8 to 13 illustrate another embodiment of a wide turbine assembly according to the invention. The embodiment is similar to that shown in Figures 2a to 2d. As such like numerals are used to indicate like parts.

The bed 11’ differs from the bed illustrated in Figures 2a to 2d in that a mounting is provided with a mounting point for a yaw motor 50. The yaw motor 50 mounts a pinion wheel 51 on its output shaft. The pinion wheel 50 engages with a ring gear (not shown) which is mounted on the tower on which the whole wind turbine assembly 10 is mounted.

The wind turbine assembly includes a pneumatic brake control cabinet 55 and a hydraulic brake control cabinet 56.

Figure 12 illustrates a bed 11’ which is provided with three mounting points 49 for yaw motors 50. The provision of a plurality of yaw motors 50 provides for redundancy, allowing the position to the wind turbine assembly 10 to move relative to the tower if one of the yaw motors should fail. The provision of multiple yaw motors 50 allows greater force to be generated or the same force to be generated with smaller motors.

The flexible coupling 22 used in the embodiment shown in Figures 8 and 9 is illustrated in Figures 10 and 11 and comprises a hub 27 attached to the high speed output shaft 15a and a split coupling comprising two parts 60a, 60b attached together by bolts 60d and to the generator input shaft 25 by a key 60e which sits in a key way 60c. The split coupling is attach to the hub 27 by means of a flexible ring 65 which is fastened to the hub 27, by bolts 66 in the illustrated example, and to the split coupling parts 60a, 60b by bolts 67 (the two bolts 67 are for the purpose of illustration, the number and type of bolt used being selected according to the requirement of the coupling).

Figure 10 illustrates the pneumatic brake callipers 42. In this embodiment hydraulically powered brake callipers 19 are provided for the low speed output shaft as illustrated in Figure 9. The brake calliper arrangement showing Figure 6 may be used in the embodiment illustrated in Figures 8 and 9.

The wind turbine of the invention is particularly advantageous with respect to the prior art. The arrangement for mounting the hub on the shaft reduces the possibility of the hub rocking on the shaft. The bearing arrangement is simpler, less costly in assembly and less likely to fail for want of lubricant. The braking arrangement ensures both sufficient braking force to bring the hub to a stand still whilst also controlling the braking force so as not to overload the whole wind turbine structure.

Claims

1. A wind turbine comprising a bed, a rotor, a gearbox, a generator and a shaft, wherein: the rotor includes a hub having a plurality of blade mounts and the hub is mounted on the shaft and the shaft is mounted on the bed by means of at least two spaced apart bearings: the gearbox and the generator are mounted on the bed and the shaft forms an input to the gearbox, an output of the gearbox being connected to an input shaft of the generator by means of a coupling; and wherein the shaft includes a hub mounting portion which comprises a hub seat and a flange adjacent the hub seat, and wherein the hub is an interference fit with the hub seat, a respective one of the hub and flange including a plurality of studs and the other a plurality of corresponding holes each for receiving one of the studs therein and a nut associated with each stud; or each of the hub and flange including one of: a plurality of aligned holes, the holes in at least the hub being threaded and the hub mount including a plurality of bolts, which threaded bolts pass through the holes in the flange and the threads of the bolt engage with the threads of the holes in the hub.

2. A wind turbine according to Claim 1, wherein the blade mounts are welded to the hub or attached to the hub by fasteners.

3. A wind turbine according to Claim 1 or 2, wherein the spaced apart bearings are lip sealed bearings.

4. A wind turbine according to any preceding claim, wherein the shaft is coated with electroless nickel.

5. A wind turbine according to any preceding claim, further comprising a high speed shaft brake assembly, wherein a first part of the high speed shaft brake assembly is mounted on the gearbox output to rotate therewith and a second part of the high speed shaft brake assembly is in fixed relationship with respect to the bed.

6. A wind turbine according to any preceding claim, further comprising a low speed shaft brake assembly, wherein a first part of the low speed shaft brake assembly is mounted on the shaft or a component of the gearbox rotating with and at the same speed as the shaft to rotate therewith and a second part of the low speed shaft brake assembly is in fixed relationship with respect to the bed.

7. A wind turbine according to Claim 5 or Claim 6, wherein the low speed shaft brake assembly and/or the high speed shaft brake assembly includes at least one hydraulically actuated brake calliper and/or at least one pneumatically actuated brake calliper.

8. A wind turbine according to any preceding claim, wherein the coupling connecting the output of the gearbox to the input shaft of the generator is a flexible coupling.

9. A wind turbine according to Claim 8, wherein the flexile coupling includes two hubs, one attached to the gearbox output and the other to the generator input shaft, each hub being provided with teeth and the flexible coupling comprising an elongate element having teeth on one side thereof, the teeth shaped and dimensioned to mesh with the teeth of the hubs, the flexible coupling further comprising a retaining band surrounding the elongate element and the toothed parts of the hubs.

10. A wind turbine according to Claim 9, wherein the retaining band is elastomeric.

11. A wind turbine according to Claim 9, wherein the flexible coupling comprises a flexible ring attached to a hub mounted of the output shaft of the gearbox and to a split coupling mounted on the input shaft of the generator.

12. A wind turbine according to Claim 10, wherein the flexible ring is an elastomeric ring.

13. A wind turbine according to Claim 11 or 12, wherein the flexible ring is attached to the hub by a first plurality of fasteners and to the split coupling by a second plurality of fasteners.

14. A wind turbine according to Claim 13, wherein the second plurality of fasteners extend in a direction substantially perpendicular to the direction in which the first plurality of fasteners extend.

15 . A wind turbine according to any of Claims 11 to 14, wherein the split coupling is situated within the flexible ring.

16. A wind turbine according to any of Claims 12 to 15, wherein an end face of the hub abuts an end face of the flexible ring.

17. A wind turbine according to Claim 5 to 6 or any claim dependent thereon, further comprising a controller and at least one sensor, the at least one sensor including a hub speed monitor, the output of the at least one sensor comprising an input to the controller, and the controller having at least one output, the at least one output being a brake actuator control signal.

18. A wind turbine according to Claim 17, wherein the wind turbine is a wind turbine according to Claim 6 and comprises a plurality of sensors and a plurality of outputs, the plurality of outputs including a hydraulic brake actuator control signal and a pneumatic brake actuator control signal.

19. A wind turbine according to Claim 18, the at least one sensor includes a hydraulic pressure sensor and a pneumatic pressure sensor.

20. A wind turbine according to any of Claims 17 to 19, the at least one sensor includes a clock.

21. A wind turbine comprising a bed, a rotor, a gearbox, a generator, a shaft and a brake, wherein: the rotor includes a hub having a plurality of blade mounts and the hub is mounted on the shaft and the shaft is mounted on the bed by means of at least two spaced apart bearings: the gearbox and the generator are mounted on the bed and the shaft forms an input to the gearbox, an output of the gearbox being connected to an input shaft of the generator by means of a coupling; and wherein the brake includes a low speed shaft brake assembly associated with the shaft and/or a high speed shaft brake assembly associated with the output of the gearbox.

22. A wind turbine according to Claim 21, wherein a first part of the high speed shaft brake assembly is mounted on the gearbox output to rotate therewith and a second part of the high speed shaft brake assembly is in fixed relationship with respect to the bed.

23. A wind turbine according to Claim 21 or 22, wherein: a first part of the low speed shaft brake assembly is mounted on the shaft or a component of the gearbox rotating with and at the same speed as the shaft to rotate therewith and a second part of the low speed shaft brake assembly is in fixed relationship with respect to the bed; and/or the low speed shaft brake assembly and/or the high speed shaft brake assembly includes at least one hydraulically actuated brake calliper and/or at least one pneumatically actuated brake calliper

24. A wind turbine according to any preceding claim, and comprising at least one yaw motor.

25. A wind turbine according to any preceding claim, wherein:

the wind turbine further comprises a tower, the bed attached to the tower; or the wind turbine further comprises a tower, the bed attached to the tower, the bed and the tower comprising co-operating parts of a slew ring connection; or the wind turbine further comprises a tower, the bed attached to the tower, the bed and the tower comprising co-operating parts of a slew ring connection, the at least one yaw motor providing with a pinion gear wheel and the tower with a ring gear, the at least one yaw motor operable to rotate the bed relative to the tower.