FORM 2
THE PATENTS ACT 1970
(Act 39 of 70)
COMPLETE SPECIFICATION
(See Sectionl0)
TITLE OF INVENTION
BEARING ARRANGEMENT FOR A WIND TURBINE.
APPLICANT(S):
Name: Suzlon Energy Ltd.
Nationality: Indian
Address: One Earth, Opp. Magarpatta City, Hadapsar, Pune-411028,
Maharashtra, India.
The following specification particularly describes the invention and the manner in which it is to be performed

Title:
Bearing arrangement for a wind turbine.
Field of the Invention:
The present invention relates to a new and improved bearing arrangement for a wind turbine and more specifically to the yaw bearing arrangement of a wind turbine used for positioning the nacelle in direction of the wind.
Background of the Invention:
Wind turbines convert kinetic energy from the wind into mechanical energy which can then be used to produce electricity via a generator. The wind turbine comprises a rotor, a generator mounted in a nacelle and a tower. The rotor is formed by a hub or base joining one or several blades which are responsible for transforming the kinetic energy of the wind into rotational movement of the rotor. The hub of the rotor is connected to the generator via a drive train, which transfers the rotational movement of the rotor to the generator. The electric generator then converts the rotational movement to electricity. The generator and the drive train are mounted in the nacelle, which is rotatably mounted on top of the tower. This allows the nacelle to rotate with respect to the tower supporting it such that it allows the rotor to orient in the direction of wind. This rotation of the nacelle and the generator around the length axis of the tower is known as yawing.
The yawing of the nacelle is performed using a yaw bearing arrangement. The yaw bearing arrangement is an important component of the horizontal axis wind turbines. The wind turbine may have a yaw error if the rotor is not properly aligned to the wind. A yaw error implies that a lower share of energy in the wind is passing through the rotor area than desired.
In order to ensure that the wind turbine is producing maximum amount of electric energy at all times, the yaw bearing arrangement is used to keep the rotor facing the wind even when there are slight changes in the wind direction. The yaw bearing arrangement usually comprises a sliding track, sliding pads and at least

one yaw drive. The function of the yaw drive is to rotate the nacelle of the wind turbine in order to align the rotor with the wind direction.
In order to make the movement of the nacelle more controllable, the slide pads can be pre-tensioned towards the sliding track. Due to the pre-tension of the slide pads the friction force between the slide pads and the sliding track is increased, which prevents the nacelle from turning unwantedly due to outer influences, e.g. wind gusts, when the nacelle is in a parked position. The friction force between the slide bearing and the slide pads also decelerates the nacelle when the turning force from the yaw drive is removed. Due to the size of the nacelle, the friction force has to be high enough to overcome the moment of inertia of the nacelle during yawing in order to decelerate the nacelle when the turning force is removed.
To change the direction of the nacelle the yaw drive first has to overcome this friction force, which requires an accordingly powerful yaw drive. This can be solved by one big or several smaller yaw drives; however in both cases the yaw drive will be very heavy and bulky
Document US 2011/0171022 shows a state of the art yaw system. The yaw system comprises hydraulically operated brake callipers. A holding pressure is applied to the brake calliper when the nacelle is in a parked position in order to create a holding force to prevent the nacelle from unwanted yawing. When yawing of the nacelle is desired, the brake callipers are depressurized via a valve. By depressurizing the brake callipers the holding force is removed and the nacelle is able to rotate.
A second yaw system comprising a combination of several yaw modules is shown in US 2010/0109327. Each yaw module can be configured to perform one of a plurality of functions, e.g. driving the rotational movement of the nacelle, active or passive braking of the nacelle and damping of torsional movements of the nacelle. The yaw system can be adapted to different requirements by changing the configuration of the different yaw modules.

In view of the foregoing, not only is there a need to overcome the above mentioned problem of enormous extra weight that is being added to the yaw system of wind turbines but also to overcome hindrance created while generating sufficient amount of electric energy, thereby lowering the efficiency of the wind turbine.
Objects of the Invention:
The object of the present invention is to overcome the disadvantages known from prior art and to provide a yaw system with reduced cost and weight and increased safety and reliability.
Summary of the Invention:
The problem is solved by a bearing arrangement according to claim 1. The bearing arrangement comprises a first bearing member mounted to the tower of the wind turbine and a second bearing member mounted to the nacelle of the wind turbine. The first and the second bearing members are rotatably connected to each other in such that the nacelle is rotatably connected to the top of the tower and can turn around an axis essentially parallel to the length axis of the tower of the wind turbine. The first bearing member can i.e. comprise a sliding track and the second bearing member at least one sliding device. Each sliding device comprises a slide pad configured to be in sliding contact with the sliding track and a first spring set, e.g. a set of spring discs, for applying a load to pre-tension the slide pad towards the sliding track. It is of course also possible to arrange the sliding track and the sliding devices vice versa, so that the first bearing member comprises a sliding device and the second bearing member comprises a sliding track. The bearing arrangement comprises at least one additional spring set, which increases the applied load acting on a slide pad, and an actuating device to disengage, e.g. compress, or engage, e.g. decompress, the additional spring set.
The first spring set pre-tensions the bearing arrangement in order to create a counterforce to the turning force created by the yaw drive. This ensures a

controlled and smooth movement of the nacelle during yawing and also provides a braking force to hold the nacelle in a parked position. By adding an additional active spring set to the bearing arrangement, the friction force of the bearing arrangement can be adapted to different operating states of the wind turbine. When the turbine is in a parked condition the additional spring set is activated, thereby adding an additional load on the slide pad to increase the friction force between sliding track and slide pad. During yawing the additional spring set is deactivated, which reduces the friction force that has to be overcome by the yaw drives and allows the yawing of the nacelle to be performed by less powerful yaw drives.
Since the friction force can be reduced while yawing, the size and the number of yaw drives can be decreased; this reduces cost, weight and required space for the yaw drive.
The present invention aims to reduce the number of components and save costs and assembly time.
In a first embodiment of the invention the first and the additional spring set are mounted in a stack spring configuration, i.e. the additional spring set is mounted on top of the first spring set and shares the same spring axis. The first and the additional spring set are separated by an actuating piston. In an initial state of the bearing arrangement, i.e. during braking or when the nacelle is parked, both springs are equally pre-stressed and compressed. The pre-tension of the two spring sets can be adjusted using a pre-tensioning bolt mounted on top of the spring stack. During yawing the actuating piston moves along the spring axis of the spring stack, which compresses the additional spring set while de-compressing the first spring set. The de-compressing of the first spring set reduces the deflection of the spring set, which also reduces the force acting on the slide pad. At the same time as the deflection of the first spring set is reduced, the deflection of the additional spring set is increased.

In a first embodiment the spring discs are arranged in such a way that the spring force increases linearly with the deflection of the spring. The discs can however also be arranged in such a way that the spring force increases digressive or progressive with the deflection of the spring.
During assembly and/or maintenance of the bearing arrangement the springs are pre-stressed and compressed using the pre-tensioning bolt. The discs in the springs are arranged in such a way that they allow the springs to deflect a certain distance. The springs can be compressed in the range of 0-100 % of the total available deflection distance using the pre-tensioning bolt. In the initial state, both springs are compressed to an equal extent and in such a way that they still allow a further deflection. In a preferred embodiment the springs are pre-tensioned in such a way that they deflect to an extent of 50 % in the initial state and apply 50 % of the maximum spring force on the slide pad. The initial state is reached during braking or parking of the nacelle when the actuator is deactivated and both spring sets are fully engaged. In this state both the first and the additional spring sets are loaded and compressed to an equal extent and are applying a combined force on the slide pad. During yawing of the nacelle, the actuator creates a force compressing the additional spring set and de-compressing the first spring set, so that the deflection of the additional spring set is higher and the deflection of the first spring set is lower than in the initial state. The deflection ratio between the first spring set and the additional spring set can be varied from 50:50 in the initial state to 0:100 during yawing. Preferably the deflection ratio is however limited to 25:75 during yawing, in order to reduce the stress on the springs. The reduced deflection of the first spring set leads to a reduction of the force acting on the slide pad. This leads to a decreased friction force that has to be overcome by the yaw drives during yawing.
In order to allow an engagement and/or disengagement of the additional spring set, the bearing arrangement also comprises an actuating device. During yawing the actuating device retracts the additional spring by adding a retracting force to the spring, so that the spring no longer adds a force on the slide pad. In a preferred

embodiment one actuating device is used to operate two spring stacks mounted next to each other. This further reduces the cost and the weight of the bearing arrangement. The actuating device can be operated hydraulically, electrically, electro-mechanically, mechanically or pneumatically.
In a second embodiment the additional spring set can be mounted separately at a second radial distance from the first spring set. In this embodiment the additional spring acts on a separate slide pad acting on an additional sliding track positioned on a different radial distance from the tower axis than the first sliding track. During yawing of the nacelle the additional spring is fully retracted by the actuating device so that no load is acting on the slide pad. Since no load is applied on the slide pad during yawing the friction of the additional sliding track can be increased, e.g. by non-lubrication of the additional sliding track. By increasing the friction of the additional sliding track the number of slide pads needed to hold the nacelle in a parked position can be reduced which leads to further reduction of weight and costs of the bearing arrangement.
The activation and deactivation of the additional spring pack is controlled by a control logic, which can e.g. be integrated in the turbine controller or in a separate yaw control unit. When a yaw signal is received the additional spring sets are deactivated by the actuator, i.e. the additional spring is retracted from the slide pad in order to reduce the spring force acting on the pad. When a stopping and/or braking signal is received the actuator is deactivated so that the spring force takes over and re-applies the slide pad on the sliding track. Preferably the additional spring set is retracted shortly before yawing is started and reapplied shortly after the yawing is stopped.
The present invention has major benefits compared to the yaw systems known from prior art. Studies have shown that the yaw system known from US 2011/0171022 has major drawbacks when it comes to reliability. In case of a pressure drop in the hydraulic system of the yaw system the braking force is rapidly lost, which can lead to the nacelle spinning uncontrollably due to outer

influences. In worst case this can lead to major damage on the wind turbine due to winding of the power cables connecting the generator in the nacelle with the bottom of the tower. In the present invention on the contrary, a malfunction of the actuator will only lead to the additional spring adding a load on the slide pad which will increase friction and bring the nacelle to a halt.
Since the modules have to be interchangeable with each other, the size of the modules is defined by the largest module, in this case the yaw drive module, which makes the system very heavy and bulky. The additional spring set of the present solution is very compact and light and requires a limited mounting space. In combination with the reduced size and/or number of the yaw drives this gives a significant reduction of weight in comparison to the yaw systems known from prior art.
Especially the first embodiment of the present invention has the benefit that it can be offered as retrofit solution, Since the stacked spring has the same mounting position and diameter as the pre-tensioned springs known from prior art, the pre-tensioned spring only needs to be replaced by the stacked spring without doing any further modification to the turbine. This also makes it easy to adapt the bearing arrangement to the specific requirements of the different wind parks.
The present invention also achieves control over the extent of sliding bearing friction and the same can be increased in certain areas where there is relatively lower sliding friction and finer adjustments can be done in field to increase sliding bearing friction during parked condition if field conditions demand the same.
By reducing the friction of sliding bearing using the application of this concept, it is possible to use existing proven models of yaw drives in new product development thereby, limiting inventory requirements in plants as well as in field across the globe. This also eliminates the need for design of new yaw drives.

Brief description of the drawings:
Fig. 1 illustrates a wind turbine;
Fig. 2 illustrates a pre-tensioned spring set known from prior art;
Fig. 3 illustrates a stacked spring comprising a first pre-tensioned spring set and an additional spring set mounted on top of the first spring set in accordance with an aspect of the present invention;
Fig. 4 illustrates the mounting positions of the spring stack solution; the two spring packs being mounted on top of each other and are mounted at the same radial distance from the rotation axis of the nacelle in accordance with an aspect of the present invention;
Fig. 5 illustrates the actuator used for retracting two spring stacks positioned next to each other on the same track in a two track solution in accordance with an aspect of the present invention;
Fig. 6 illustrates the two track solution; the additional spring pack being mounted at a different radial distance than the first spring pack in accordance with an aspect of the present invention; and
Fig. 7 illustrates the actuator used for retracting two spring sets positioned next to each other in accordance with a second embodiment of the present invention.

Detailed Description of the Invention:
Fig. 1 shows a wind turbine 2 with a tower 3 and a rotatable nacelle 4, which is positioned on the tower 3. Based on the necessary wind tracking, the machine housing 5 is supported on the tower 3, rotatable about a rotation axis 25 via a bearing arrangement 1. The wind direction tracking is performed by the bearing arrangement 1 and a yaw drive. The drive train, comprising a rotor shaft and a gear box, and a generator, connected with the fast shaft of the gear box, are located in the nacelle 4. The drive train is supported on a machine support 5 via a rotor bearing and via the gear box. A rotor flange, on which the hub 6 is arranged, is located on the rotor shaft. The hub 6 accommodates the rotor blades 7 and transmits the forces acting on the rotor blades 7 to the rotor shaft. The bearing arrangement 1 according to the invention is also applicable to other types of wind turbines 2.
Fig. 2 shows a spring set 8 for pre-tensioning a slide pad 9 as known from prior art. The spring set 8 comprises a coned-disc spring 10, a housing 11 for the spring 10 and a tensioning bolt 12 for pre-tensioning the spring set 8. The spring 10 is mounted between two pressure plates 13, one of which is in contact with the tensioning bolt 12 and one which is in contact with the slide pad 9. The slide pad 9 is in sliding contact with a sliding track 14. By turning the tensioning bolt 12 the pre-tensioning force, which presses the slide pad 9 towards the sliding track 14, can be adjusted. However, when the pre-tensioning force is set it remains constant until manually changed by service personnel, e.g. during maintenance of the wind turbine 2.
In order to be able to adapt the pre-tensioning force to different operating states of the wind turbine 2, the present invention adds an additional active spring set 15 to the bearing arrangement 1. The bearing arrangement 1 comprises a first 19 and a second 20 bearing member, one of which is non-rotatably connected to the tower 3 and the other one non-rotatably connected to and/or integrated in the machine support 5.

A first embodiment of the invention is shown in Fig. 3. Here the additional spring set 15 is mounted on top of the first spring set 8, so that a stacked spring 16 is created. The stacked spring 16 comprises the first spring set 8, the additional spring set 15, the pre-tensioning bolt 12 and an actuating piston 17. The actuating piston 17 is mounted between the first spring set 8 and the additional spring set 15 and is powered by an actuator 18 mounted in connection with the stacked spring 16.
It is desired to apply this additional spring force on selected slide pads 9 to increase the friction of sliding bearing during parked condition of the nacelle 4. The use of the additional spring set 15 helps to increase the contribution of sliding bearing friction in the bearing arrangement 1 and is utilized to decrease the size of the bearing arrangement 1 and/or the yaw drives.
During assembly and/or maintenance of the bearing arrangement the springs 8 and 15 are pre-stressed and compressed using the pre-tensioning bolt 12. The discs in the springs 8, 15 are arranged in such a way that they allow the springs 8, 15 to deflect a certain distance. The springs 8, 15 can be compressed in the range of 0-100 % of the total available deflection distance using the pre-tensioning bolt 12. In the initial state, both springs 8, 15 are compressed to an equal extent and in such a way that they still allow a further deflection. In a preferred embodiment the springs 8, 15 are pre-tensioned in such a way that they deflect to an extent of 50 % in the initial state and apply 50 % of the maximum spring force on the slide pad 9. The initial state is reached during braking or parking of the nacelle when the actuator is deactivated and both springs 8, 15 are fully engaged. In this state both the first 8 and the additional 15 spring sets are loaded and compressed to an equal extent and are applying a combined force on the slide pad 9. During yawing of the nacelle 4, the actuator 18 creates a force compressing the additional spring set 15 and de-compressing the first spring set 8, so that the deflection of the additional spring set 15 is higher and the deflection of the first spring set 8 is lower than in the initial state. The deflection ratio between the first spring set and the additional spring set can be varied from 50:50 in the initial state to 0:100 during yawing.

Preferably the deflection ratio is however limited to 25:75 during yawing, in order to reduce the stress on the springs. The reduced deflection of the first spring set 8 leads to a reduction of the force acting on the slide pad 9. This leads to a decreased friction force that has to be overcome by the yaw drives during yawing.
An advantage with this embodiment is that the stacked spring 16 can be mounted in the same socket 21 in the first 19 or the second bearing member 20 as the pre-tensioned spring set 8, shown in Fig. 1. The stacked spring 16 can therefore be offered as a retro-fit module and can easily be installed in existing wind turbines or in new deliveries of already developed wind turbines by exchanging one or more of the state of the art pre-tensioned spring sets 8 known from Fig. 2.
Figure 4 shows the mounting positions 21 of the stacked spring 16. The springs are mounted on the same radial distance R1 from the rotation axis 25 of the nacelle 4 so that all of the slide pads 9 are in contact with the same sliding track 14. The installed springs can be a combination of stacked springs 16 and pre-tensioned springs 8. It is however beneficial to position at least two stacked springs 16 next to each other, since this allows the actuation of two or more stacked springs 16 with one single actuator 18.
The beneficial setup with one single actuator 18 for several stacked springs 16 is shown in Figure 5. The actuator 18 is mounted between two springs 16 situated next to each other and acts simultaneously on both stacked springs 16. When yawing is initiated the actuator 18 creates an axial force on the actuating piston 17 which compresses the additional spring set 15 and releases the pressure from the first spring set 8 until only the pre-tensioning force is applied to the slide pad 9. The actuator 18 can be hydraulically, electrically, electro-mechanically or pneumatically operated,
A second embodiment of the invention, called the "two track solution", is shown in figure 6. In this embodiment the bearing arrangement 1 comprises a second sliding track 22 positioned at a different radial distance than the first sliding track 14. The additional active spring set 15 is positioned in such a way that it acts on a

separate slide pad 23 which is in contact with the second sliding track 22. The first spring set 8 is pre-tensioned with a constant force created by the tensioning bolt 12. The actuator for the two track arrangement of the spring sets 15 is shown in fig. 7 wherein one actuator 18 is used to retract two spring sets 15 positioned next to each other on the same track 22. However it is also possible to use one actuator 18 for each spring set 15. The spring sets 8 on the first track 14 are always pre-tensioned and cannot be retracted. Since the pre-tensioned slide pads 9 are always in contact with the first sliding track 14, it is necessary to lubricate the sliding track 14 in order to reduce the wear of the slide pads 23 during yawing.
The advantage with this embodiment is that since the slide pad 23 of the additional spring set 15 can be retracted during yawing, there is no need for lubrication of the second sliding track 22 in order to reduce the wear of the slide pad 23. The increased friction due to lack of lubrication is on the contrary desired since this increases the friction between the second slide pad 23 and the second sliding track 22 when the nacelle is in a parked position, which reduces the number of slide pads 23 and spring sets 15 needed for providing the necessary holding torque.
Figure 7 shows an actuator 18 used in the two track solution. As already known from the stacked spring 16 solution shown in figure 5 the actuator 18 can be positioned between two spring sets 15 mounted next to each other. In this case the actuating piston 17 protrudes through the coned-disc spring 10 and connects to an actuating bar 24 connected to the actuator 18.
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the scope of the invention are desired to be protected.

List of reference numerals

1 Bearing arrangement 18 Actuator
2 Wind turbine 19 First bearing member
3 Tower 20 Second bearing member
4 Nacelle 21 Mounting socket
5 Machine support '22 Second sliding track
6 Hub 23 Slide pad
7 Rotor blades 24 Actuating bar
8 First spring set 25 Rotation axis
9 Slide pad R1 First radial distance
10 Coned-disc spring R2 Second radial distance
11 Housing
12 Tensioning bolt
13 Pressure plate
14 Sliding track
15 Additional spring set
16 Stacked spring
17 Actuating piston

I/We Claim:
1. Bearing arrangement (1) for a wind turbine (2), the wind turbine (2)
comprising a tower (3) fixed to the ground, a nacelle (4) rotatably mounted
to the tower (3) via the bearing arrangement (1) and at least one yaw drive
for creating a turning torque for turning the nacelle (4) in respect to the
tower (3), the bearing arrangement (1) comprising,
- a first bearing member (19) and a second bearing member (20) mounted rotatably around an axis to each other,
- the first bearing member (19) being non-rotatably connected to the tower (3) or nacelle (4) and the second bearing member (20) being non-rotatably connected to the nacelle (4) or tower (3),
- the first bearing member (19) comprising at least one sliding track (14),
- the second bearing member (20) comprising a plurality of sliding devices
- each sliding device comprising at least one slide pad (9) configured to be in sliding contact with the sliding track (14) of said first bearing member (19) and a first spring set (8) for applying a load to the slide pad (9) towards the slide track (14) of the first bearing member (19),
characterized in, that the bearing arrangement (1) comprises
- at least one additional spring set (15) to increase the applied load acting on a slide pad,
- and an actuating device to disengage or engage the additional spring set (15) in order to adapt the load acting on the slide pad.
2. Bearing arrangement (1) according to claim 1, characterized in, that the
additional spring set (15) is engaged when the nacelle (4) is in a parked
condition and disengaged when the nacelle (4) is turning.

3. Bearing arrangement (1) according to claim 1 or 2, characterized in, that the actuating device (18) is actuated by one or more of, hydraulic actuation or pneumatic actuation or electro-mechanical or mechanical actuation.
4. Bearing arrangement (1) according to one of the preceding claims, characterized in, that the actuating device (18) is connected to a control unit for controlling the position of the actuating device (18).
5. Bearing arrangement (1) according to one of the preceding claims, characterized in that the first spring set (8) and the second spring set (15) consist of coned-disc springs (10).
6. Bearing arrangement (1) according to one of the preceding claims, characterized in that the first spring set (8) and the additional spring set (15) are mounted in series.
7. Bearing arrangement (1) according to claim 6, characterized in that the load acting on the slide pad (9) is adapted by changing a compression ratio between the first spring set (8) and the additional spring set (15).
8. Bearing arrangement (1) according to one of the preceding claims 1 to 4, characterized in that the first spring set (8) and the additional spring set (15) are mounted in parallel.
9. Bearing arrangement (1) according to one of the preceding claims 6 or 7, characterized in that the first spring set (8) and the additional spring set (15) are mounted at the same radial distance from a length axis (25) of the tower
(3).

10. Bearing arrangement (1) according to claim 8, characterized in that the first spring set (8) and the additional spring set (15) are mounted at different radial distances from a length axis (25) of the tower (3).
11. Bearing arrangement (1) according to claim 10, characterized in that a slide pad (23) connected to the additional spring set (15) acts on a second sliding track (22) with a higher friction coefficient than the first sliding track (14).
12. Wind turbine (2) comprising a tower (3) fixed to the ground, a nacelle (4) rotatably mounted to the tower (3) via a bearing arrangement (1) and at least one yaw drive for creating a turning torque for turning the nacelle (4) in respect to the tower (3), characterized in the bearing arrangement (1) being designed according to any one of the preceding claims.
13. Method for operating a wind turbine (2) with a bearing arrangement (1) according to one of the preceding claims, the method comprising the steps of:

- receiving a turning signal;
- sending a signal to the actuating device (18) to disengage the additional spring set (15);
- turning the nacelle (4) to a desired position using yaw drives;
- stopping the nacelle (4) in the desired position; and
- sending a signal to the actuating device (18) to engage the additional spring set (15).