This invention relates to a buffer device for use with a shock absorption system. It also relates to a shock absorption system comprising one or more of the buffers, and to a rotor assembly for a wind turbine comprising such a shock absorption system.

The type of buffer device described herein is useful in a variety of applications such as in shock absorption or suspension systems for a heavy load. A possible use would be as a vibration damper in rail transport or other heavy transport applications.

Another particularly useful application of such buffers is in wind turbines. Wind turbines typically comprise either two or three turbine blades. A particular issue with two-bladed wind turbines is that, as the blades pass through the vertical, the tower on which the wind turbine is mounted inhibits airflow past the lower blade. This causes an imbalance in the force exerted by each of the two blades, leading to a cyclic torque being applied to the main bearings and shaft of the wind turbine as the turbine blades rotate. This cyclic torque is a cause of fatigue for the main bearings and shaft. Furthermore, the cyclically varying force exerted by the blades is a cause of fatigue for the blades themselves.

The effect is exacerbated by the fact that wind speed often increases at higher altitude. Thus, just as the force exerted by the lower blade reaches a minimum, the force exerted by the upper blade reaches a maximum.

To ameliorate this problem, two-bladed wind turbines are often fitted with a hub that can rotate about a horizontal axis orthogonal to the axis of rotation of the turbine blades. This allows the hub and turbine blades to rock backwards and forwards to absorb the imbalance of forces on the two blades. This rocking motion is known as "teetering".

The teetering motion is normally controlled by solid rubber dampers. These dampers are fitted to a static yoke on the rotor assembly about which the hub teeters. As the hub teeters, the rubber dampers are compressed, which has the effect of limiting the degree of teetering and damping the teetering motion.

However, it has recently been found that the relationship between load and deflection for these buffers is not particularly linear. At high values of deflection the buffers cease to deform to absorb more load as the rubber stiffens. At such high values of deflection, any increase in load is therefore largely absorbed by the turbine blades, main bearings and shaft, which places a higher than acceptable loading on them.

Some experiments have been done to try and improve the linearity by adjusting the shape of the buffer material block. Specifically, we have made the cylinder longer to provide more surface area over which the buffer can bulge in response to the increasing load. In another experiment, we made voids in the cylindrical buffers, again to increase the surface area over which the buffer can bulge in response to the increasing load. However, in both cases the stiffness of the buffers was reduced dramatically so that they cannot be used in wind turbine applications.

In accordance with a first aspect of the invention, there is provided a buffer device for use with a shock absorption system comprising a carriage supported on a chassis, the buffer device comprising first and second members for coupling to the carriage and chassis respectively such that relative motion of the carriage and chassis causes the first and second members to move relative to one another along a deflection axis, wherein the first and second members are coupled together by one or more resiliently flexible members such that their relative motion along the deflection axis results in a shear stress in the or each resiliently flexible member.

We have found that the relationship between load and deflection is very linear over a large range for flexible members operating in a shear mode rather than the normal compression mode of the prior art. Hence, by arranging the buffer device as defined above to cause a shear stress to be developed in the flexible member during deflection, a load-deflection characteristic can be produced, which remains linear over a sufficient range for use with wind turbines and other heavy duty applications as mentioned above. The invention therefore overcomes the above-mentioned problem.

Typically, the first member comprises a buffer pad for receiving a deflection force arising as a result of relative rotation of the carriage and chassis.

The buffer pad may be adapted for coupling to the hub, in use, via a coupling mechanism which bears on the buffer pad as a result of relative rotation of the carriage and chassis.

The or each resiliently flexible member normally comprises one or more resiliently flexible elements.

In essence, the resiliently flexible member provides a flexible coupling between the first and second members that is resiliently deformable. Any material with such characteristics may be used to form the flexible elements. However, the or each flexible element is typically made from an elastomer, preferably a natural rubber. The natural rubber used could include a blend of rubbers.

In a wind turbine application, the rubber composition is typically selected to achieve a value between 40 and 80 on the International Rubber Hardness Degrees (IRHD) scale. When using the IRHD scale, the hardness of an elastomer is measured based on the depth of indentation by an impacting gauge with a standard size and shape. The hardness is obtained by comparing the difference between a small initial force and a much larger final force. The IRHD scale has a range of 0 to 100, corresponding to elastic moduli of zero and infinity respectively. The measurement is made by using a rigid ball to indent the rubber specimen.

In preferred embodiments, the first and second members have coaxial central axes lying on or parallel to the deflection axis.

In these embodiments, each of the first and second members preferably has a coupling surface for coupling to the or each flexible member, the coupling surfaces lying in opposition and tangential to respective planes that are spaced apart and intersect the deflection axis at an acute angle.

By arranging the coupling surfaces in this way, the flexible member adopts a configuration that causes it to be stressed partly in a normal mode and partly in a shear mode. The ratio between normal and shear mode stresses is determined by the angle of intersection of the planes in which the coupling surfaces lie with the deflection axis, steeper angles leading to an increase in shear mode stress. The advantage of this configuration is that it leads to a unit that is very compact along the deflection axis whilst retaining a high degree of linearity. The compactness and linearity can be traded off against each other by adjusting the above-mentioned angle of intersection.

The respective planes to which the coupling surfaces are tangential are typically parallel when the buffer device is in an unloaded configuration.

By "unloaded configuration" we mean a situation in which the carriage and chassis are in rest positions leading to no deflection of the first member relative to the second member.

Preferably, the coupling surfaces are frustoconical surfaces.

The buffer device may comprise a single resiliently flexible member shaped so as to occupy only the volume between the two frustoconical coupling surfaces when the buffer device is in an unloaded configuration.

In this case, the resiliently flexible member is typically formed from an elastomer, preferably a natural rubber.

In one embodiment, the resiliently flexible member comprises a stacked array of resiliently flexible elements spaced apart by separator elements that are less flexible than the resiliently flexible elements.

In this embodiment, the separator elements preferably have a stiffness at least three times as great as the resiliently flexible elements. In a wind turbine application, a suitable value for the stiffness of the resiliently flexible elements is normally in the range of 17 to 22 kN/mm for a turbine capable of generating around 1 MW. Other values may be chosen for other applications, or for different turbine sizes (typically a higher power turbine will require a higher stiffness).

Typically, the resiliently flexible elements are made from an elastomer, preferably a natural rubber, and the separator elements are made from steel.

Each of the first and second members may have an array of coupling surfaces for coupling to the or each resiliently flexible member, each coupling surface in each array being separated from adjacent coupling surfaces in the array, wherein corresponding coupling surfaces in the first and second arrays lie in opposition and tangential to respective planes that are spaced apart and intersect the deflection axis at an acute angle.

The respective planes to which each corresponding pair of coupling surfaces in the first and second arrays are tangential are typically parallel when the buffer device is in an unloaded configuration.

Normally, the coupling surfaces of each array together define portions of respective frustoconical surfaces.

The buffer device may comprise a resiliently flexible member for each corresponding pair of coupling surfaces, each resiliently flexible member being shaped so as to occupy only the volume between the corresponding pair of coupling surfaces when the buffer device is in an unloaded configuration.

Each resiliently flexible member is typically formed from an elastomer, preferably a natural rubber.

Each resiliently flexible member may comprise a stacked array of resiliently flexible elements spaced apart by separator elements that are less flexible than the resiliently flexible elements.

The separator elements may have a stiffness at least three times as great as the resiliently flexible elements. The resiliently flexible elements are usually made from an elastomer, preferably a natural rubber, and the separator elements are made from steel.

The first member and second member preferably have respective opposing surfaces perpendicular to the deflection axis, the opposing surfaces being separated by a resilient member. Typically, this resilient member will be made from natural rubber.

In another embodiment, the first member is surrounded by the second member such that an outer surface of the first member is spaced apart from an inner surface of the second member to define a volume, which is occupied by the resiliently flexible member. In this case, the first and second member are typically cylindrical, in which case the second member is tubular, whereas the first member is usually solid.

In a variant of this embodiment, the first member is surrounded by the second member, each of the first and second member having an array of coupling surfaces for coupling to a resiliently flexible member, each coupling surface in each array being separated from adjacent coupling surfaces in the array, wherein corresponding coupling surfaces in the first and second arrays lie in opposition to define a volume, which is occupied by a respective resiliently flexible member.

In accordance with a second aspect of the invention, there is provided a shock absorption system comprising a chassis; a carriage supported on the chassis and moveable from a rest position relative to the chassis; and one or more buffer devices according to the first aspect of the invention, the first and second members of the or each buffer device being coupled to a respective one of the chassis and carriage, wherein the shock absorption system further comprises a biasing mechanism adapted to displace the first member of the or each buffer device relative to the second member by a predefined offset amount when the carriage is at the rest position.

This aspect of the invention is advantageous because the buffer devices in the shock absorption system can be biased by the predefined offset amount to a particular initial strain to achieve a desired characteristic. For example, they may be biased for maximum energy dissipation or to a point of maximum Young's modulus, depending on the requirements.

In a preferred embodiment, the predefined offset amount is selected to preload the or each buffer device to a desired point in a linear region of a load- deflection characteristic curve of the or each buffer device where the load- deflection characteristic is substantially linear.

This embodiment is particularly beneficial in wind turbine applications, where it is required that the buffers operate linearly. When combined with the shear- mode operation of the first aspect of the invention, a large linear range is achieved. The linearity prevents shock loads from being imparted to the drive train and drive train bearings and ensures gradual energy absorption by the shock absorption system.

By "substantially linear", we do not mean that the characteristic must be precisely linear (although it could be). Instead, the degree of linearity that is acceptable will depend on the application and the skilled person will be able to assess what is acceptable for the application. For example, in a wind turbine application a typical acceptable degree of linearity would be where the load- deflection characteristic has a stiffness value between 17 and 22 kN/mm. Other applications may require different values, which may be below or above this range or overlap with it. Thus, the linear region of the characteristic is that region where all the points on the characteristic have a corresponding stiffness (which is the gradient of the characteristic) within the acceptable degree of linearity.

The desired point may be at or proximal to the point of lowest strain in the linear region.

Alternatively, the desired point may be at or proximal to the midpoint of the linear region. In one embodiment, the carriage is rotatable relative to the chassis and the system further comprises at least one pair of buffer devices according to the first aspect of the invention arranged so that rotation of the carriage relative to the chassis causes an increase in strain in the or each resiliently flexible member of one of each pair of buffer devices and a corresponding decrease in strain in the or each resiliently flexible member of the other of each pair of buffer devices.

In accordance with a third aspect of the invention, there is provided a rotor assembly for a wind turbine comprising a shock absorption system according to the second aspect of the invention, wherein the chassis is a yoke for coupling to a generator, and the carriage is a hub for coupling to at least one turbine blade, the hub and yoke being rotatably coupled via a hinge, wherein the first and second members of the or each buffer device are coupled to the hub and yoke respectively.

In accordance with a fourth aspect of the invention, there is provided a wind turbine comprising a rotor assembly according to the third aspect of the invention, an electric generator rotatably coupled to the yoke and at least one turbine blade coupled to the hub.

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

Figure 1 shows a rotor assembly for a wind turbine;

Figure 2 shows a first embodiment of a buffer device according to the invention;

Figure 3 shows a second embodiment of a buffer device according to the invention;

Figure 4 shows a third embodiment of a buffer device according to the invention;

Figure 4a shows a variant of the third embodiment; Figure 5 shows a wind turbine comprising a rotor assembly with buffer devices according to the invention;

Figure 6 shows a detailed cross-sectional view of the rotor assembly; and

Figure 7 shows a stress-strain characteristic of the buffer devices in the rotor assembly of Figure 6.

Figure 1 shows a wind turbine rotor assembly suitable for use with buffer devices according to the invention. The rotor assembly comprises a hub 1 and a yoke 2. The hub 1 is coupled to the yoke 2 by way of a pair of hinges 3 (only one of which is visible in Figure 1 ). A pair of turbine blades may be mounted on the hub by coupling them to flanges 4a and 4b. The entire rotor assembly is mounted on an input shaft (not visible) of gearbox 5. The gearbox 5 couples the input shaft to an electrical generator, which runs at a higher speed than the input shaft. Thus, as the turbine blades rotate the input shaft of the gearbox is caused to rotate, which in turn causes the generator to rotate at a higher speed and generate electricity.

As the turbine blades rotate, any imbalance in the force exerted by them will cause the hub 1 to teeter relative to the yoke 2 about hinges 3. The teetering motion is controlled by buffers 6. Two buffers 6 are visible in Figure 1 , but it is possible to construct a rotor assembly with more than two buffers 6. Typically, the buffers 6 will be provided in pairs. In a typical hub, the buffers are preloaded as will be described below so that, as the hub teeters in an anticlockwise direction, the strain on one of each pair of buffers increases and the strain on the other of each pair decreases correspondingly. The change in strain on the buffers is reversed during teetering in a clockwise direction.

As will be seen below, each of the buffers 6 has a body, which is clamped to the yoke 2, and a buffer core coupled together by a flexible member made of natural rubber. The buffer core is displaced relative to the body (which remains static relative to the yoke 2 during the teetering motion) as the hub 1 rotates around the hinges 3. This displacement is caused by an actuator mechanism (not shown) coupled to the hub which bears on the buffer core and forces it to deflect as a result of the teetering motion.

Figure 2 shows a cross-section first embodiment of a buffer. In this a buffer core 20 is surrounded by a body 21 . When deployed, the body 21 is clamped to the yoke of a wind turbine rotor assembly as described above and teetering motion will cause the buffer core 20 to deflect relative to the body 21 .

The buffer core 20 and body 21 are typically made from steel. Whilst the body 21 is effectively tubular, the buffer core 20 is normally cylindrical. The volume between the buffer core 20 and body 21 is occupied by a flexible element 22, made from natural rubber, which is firmly bonded to the buffer core 20 and body 21 . It can be clearly seen that as the buffer core is deflected along the deflection axis 23, a shear stress will be induced in the flexible element.

In order to bond the natural rubber flexible element 22 to the buffer core 20 and body 21 , the buffer core 20 and body 21 are both degreased and cleaned, for example by shot blasting. A bonding agent is then applied to the inner surface of body 21 and the outer surface of buffer core 20 where they are required to bond to the rubber. The buffer core 20 and body 21 are then held in the correct relative locations on a jig, and natural rubber from a strip is placed in the void between them. The rubber is compressed to prevent it from flowing away from the buffer core 20 and body 21 and then heat is applied to melt the rubber. The rubber is then allowed to cool to form the flexible element 22 and will be strongly bonded to the buffer core 20 and body 21 .

The axial stiffness required of this kind of buffer in a wind turbine application is in the range of 17 to 22 kN/mm. Figure 2 shows suitable dimensions for the buffer. Of particular importance, are the dimensions labelled "A", "B" and "C". The dimensions "A" and "B" have been determined to provide three different variants of the buffer, which meet the above stiffness requirement. The dimension "C" represents the maximum deflection that the buffer core 20 can undergo relative to the body 21 . A typical value for dimension "C" is 35mm. Increasing the value of dimension "A" helps to prevent fatigue of the natural rubber flexible element 22. In some cases, the value of dimension "A" can exceed that of dimension "C". Tests have shown that this design of buffer provides a good linearity between load and deflection up to a deflection of approximately dimension "A".

Figure 3 shows schematically a cross-section of a second embodiment, in which a buffer pad 30 and a body 31 are coupled together by a flexible element 32 made entirely of natural rubber. The buffer pad 30 and body 31 are typically made from steel and arranged so that their centres are aligned with a deflection axis 33. When deployed, the body 31 is clamped or bolted to the yoke of a wind turbine rotor assembly as described above and teetering motion will cause the buffer pad 30 to deflect relative to the body 31 along the deflection axis 33.

Each of the buffer pad 30 and body 31 has a respective coupling surface 34 and 35. The coupling surfaces 34 and 35 are frustoconical surfaces and lie in planes that intersect the deflection axis 33 at acute angles. When there is no deflection of the buffer pad 30 relative to the body 31 along the deflection axis 33, the planes in which the two coupling surfaces 34 and 35 lie are parallel.

The flexible element 32 is firmly bonded to each of the coupling surfaces 34 and 35 in a process similar to that described with the Figure 1 embodiment. First, the buffer pad 30 and body 31 are both degreased and cleaned, for example by shot blasting. A bonding agent is then applied to the coupling surface 34 of buffer pad 30. The buffer pad 30 is then held in a jig and natural rubber from a strip is placed on the coupling surface 34. The natural rubber strip is then compressed to hold it in place and heat is applied to melt the natural rubber strip. This is allowed to cool when it will be firmly bonded to coupling surface 34. The coupling surface 35 of body 31 is then coated with the bonding agent and then placed in the jig to hold it in the correct location relative to buffer pad 30. The buffer pad 30 and body 31 are then compressed whilst the bonding agent cures. When it has cured, the flexible element 32 will be strongly bonded to the coupling surface 35 of the body 31 . An alternative process requires only one operation by providing a hole in the body 31 through which a mould tool is passed to prevent the molten rubber from flowing into the void between buffer pad 30 and body 31 . The bonding agent can then be applied to the coupling surfaces 34 and 35 of buffer pad 30 and body 31 and the natural rubber strip placed between the two coupling surfaces 34 and 35. The buffer is then compressed with the mould tool in place. Heat is then applied to melt the rubber, which after cooling forms the flexible element 32.

As can be seen from Figure 3, relative displacement of buffer pad 30 towards body 31 will induce a strain in flexible element 32 that is partly shear and partly compressive (i.e. normal). The angle at which the frustoconical coupling surfaces 34 and 35 intersect the deflection axis 33 determines the ratio of shear strain to compressive strain. In a typical example, this angle will be chosen so that the ratio of shear strain to compressive strain is approximately 2:1 . Suitable values for the thickness of the flexible rubber element 32 (i.e. the separation between the coupling surfaces 34 and 35 at zero deflection) are in the region of 60mm to 70mm, which, if the ratio of shear strain to compressive strain is adjusted to 2:1 , results in a shear strain in the order 0.5 at axial deflections in the region of 30mm to 40mm.

The difference between the inner and outer radii of the flexible element 32 (i.e. the length that is bonded to the coupling surfaces 34 and 35) can be adjusted to obtain the desired load-deflection characteristic. This can be determined using finite element analysis. In the case of a 1 MW wind turbine application, the stiffness required is in the range of 17 to 22 kN/mm. The length required is to provide this stiffness can be calculated from the following equation:

, F G - A

K =— =——

X t where: k _s is the stiffness; F is the force applied; x is the corresponding deflection; G is the shear modulus of the rubber; t is the thickness of the flexible rubber element 32 (i.e. the separation between the two coupling surfaces 34 and 35); and A is the contact area between the flexible rubber element 32 and the coupling surfaces 34 and 35.

In Figure 4 a third embodiment is shown in cross-section. In this embodiment, a steel buffer pad 40 is coupled to a steel body by a flexible member made up of an array of five natural rubber elements 42a to 42e. Each adjacent pair of natural rubber elements 42a to 42e is separated by a respective steel separator 43a to 43d.

The buffer pad 40 and body 41 are arranged so that their centres are aligned with a deflection axis. When deployed, the body 41 is clamped or bolted to the yoke of a wind turbine rotor assembly as described above and teetering motion will cause the buffer pad 40 to deflect relative to the body 41 along the deflection axis.

Each of the buffer pad 40 and body 41 has a respective coupling surface 44 and 45. The coupling surfaces 44 and 45 are frustoconical surfaces and lie in planes that intersect the deflection axis at acute angles. When there is no deflection of the buffer pad 40 relative to the body 41 along the deflection axis, the planes in which the two coupling surfaces 44 and 45 lie are parallel.

The buffer device of Figure 4 is made in a similar fashion to that of Figure 3. The coupling surfaces 44 and 45 and the separators 43a to 43d are first cleaned and degreased and a bonding agent is applied. The flexible member is then made up by laying a strip of natural rubber onto coupling surface 44 followed by separator 43d, another strip of natural rubber and so on until coupling surface 45 is placed into contact with the final strip of natural rubber that will form flexible rubber element 42a. The assembled device is then compressed and heat is applied to melt the rubber, which is then allowed to cool. Similar precautions for preventing the molten rubber from flowing away from where it is required are taken as with the other embodiment. Alternatively, the device may be built up in a sequential fashion, one flexible element at a time in a similar manner to that described for the construction of the embodiment of Figure 3. As can be seen from Figure 4, relative displacement of buffer pad 40 towards body 41 will induce a strain in each of the five natural rubber elements 42a to 42e that is partly shear and partly compressive. The angle at which the frustoconical coupling surfaces 44 and 45 intersect the deflection axis determines the ratio of shear strain to compressive strain. In a typical example, this angle will be chosen so that the ratio of shear strain to compressive strain is approximately 2:1 . Suitable values for the thickness of each of the five natural rubber elements 42a to 42e are in the region of 5mm to 20mm, which, if the ratio of shear strain to compressive strain is adjusted to 2:1 , results in a shear strain in the order 0.5 at axial deflections in the region of 30mm to 40mm.

The length of the each of the five natural rubber elements 42a to 42e (i.e. the length that is bonded to the coupling surfaces 34 and 35) can be adjusted to obtain the desired load-deflection characteristic as with the embodiment of Figure 4. This can be determined using finite element analysis. Again, for a wind turbine application the load-deflection characteristic required is in the region of 17 to 22 kN/mm. For a value of 17kN/mm, the length required is 40mm. The rubber used to make the five natural rubber elements 42a to 42e is selected to have a stiffness in this range, as required. The separators 43a to 43d are made from steel or some other material having a stiffness at least three times as high as the rubber used for the five natural rubber elements 42a to 42e.

By using five natural rubber elements 42a to 42e separated by the separators 43a to 43d in this way, the overall shear strain is shared by each of the five natural rubber elements 42a to 42e, which are strained successively as the applied load increases. This is the purpose of the separators 43a to 43d, which ensure that natural rubber element 42e is deflected first, followed by natural rubber element 42d when this has been fully deflected, and so on. Thus, the linearity is improved with respect to the embodiment of Figure 3 because the sum of the strains induced in each of the five natural rubber elements 42a to 42e is reduced for a given force. In a variant of this embodiment, different rubber compositions may be used for each of the five natural rubber elements 42a to 42e to achieve different hardnesses. Each of the five natural rubber elements 42a to 42e could have the same thickness or each may have a different thickness to achieve the desired load-deflection characteristic. In another variant, a first set of the five natural rubber elements 42a to 42e could be made from a first rubber composition with a first hardness, whilst a second set of the five natural rubber elements 42a to 42e could be made from a second rubber composition with a second hardness.

The embodiment of Figure 4 also comprises a central buffer 46. This serves the purpose of increasing the stiffness to the required level. This is desirable in some cases because the five natural rubber elements 42a to 42e are each rather slender and can be significantly strained under heavy loading, leading to fatigue in the five natural rubber elements 42a to 42e. The central buffer 46 effectively adds an offset to the stiffness whilst the five natural rubber elements 42a to 42e control the linearity of the load-deflection characteristic at this offset value of stiffness. As a result, the fatigue in the natural rubber elements 42a to 42e is limited, and the buffer can be used at high loads with a high degree of linearity.

Figure 4a shows a variant of the Figure 4 embodiment, in which the coupling surfaces are arranged in sections rather than as a continuous surface. Together the coupling surfaces forms sections of the same surface. Thus, the buffer pad 40 has coupling surface sections 144a to 144e (not all of the sections are visible in Figure 4a due to its sectional nature), and body 41 has coupling surface sections 145a to 145e.

Each of coupling surface sections 144a to 144e lies opposite and corresponds to a respective one of coupling surface sections 145a to 145e. A flexible member made up of interposed natural rubber strips and separators (as described above with reference to Figure 4) is coupled between each corresponding pair of coupling surface sections (e.g. between 144a and 145a, between 144b and 145b, and so on). In this manner, the body 41 and buffer pad 40 are coupled by several separate flexible members arranged in sections around the central axis of buffer pad 40. The advantage of this is that it simplifies the construction of larger sized buffer device. This is because the manufacture of very large sections is extremely expensive and difficult to keep within tolerance. Also, it is easier to maintain and install the flexible members as this may be done section-by-section. Thus, for larger sized applications there is a particular advantage achieved with this variant.

Figure 5 shows a wind turbine suitable for use with the buffers of the first to third embodiments shown in Figures 2 to 4.The wind turbine comprises a nacelle 50 mounted atop a tower 51 . Within the nacelle 50, a low-speed shaft is coupled at one end to a gearbox, which gears up the rotation of the low- speed shaft to a higher speed for driving an electric generator. A yaw drive is also provided within the tower to rotate the nacelle 50 so that it faces into the wind.

At the front end of the nacelle 50, a rotor assembly 52 is mounted on the low- speed shaft. The rotor assembly is shown in detail in Figure 6. It comprises a yoke 60, which is directly coupled to the low-speed shaft, and a hub 61 , which is coupled to the yoke via a hinge 62 so that it can teeter as the rotor assembly rotates. Turbine blades 53a and 53b are connected to flanges 63a and 63b on the hub. A fairing 54 is placed over the front of the rotor assembly 52 to present a more aerodynamic shape to the wind.

In Figure 6, a pair of buffers 64a and 64b of a similar type to the embodiment of Figure 2 are shown. In these, the outer sleeve (21 in Figure 2) is clamped in the yoke 60 and the inner core (20 in Figure 2) is coupled to hub 61 . The rotor assembly is designed so that when the hub 61 is aligned with the rotational axis of the yoke 60 each of the buffers 64a and 64b is compressed by a predefined amount. Thus, the rotor assembly comprises a biasing mechanism to displace the inner core of the buffer devices relative to the outer sleeves by a predefined offset amount when the hub is at its rest position. This biasing mechanism may simply make use of the design of the hub 61 and yoke 60 so that when the two are assembled, the buffers 64a and 64b are naturally compressed to the required offset. Alternatively, a more elaborate compression system in which the coupling between the hub 61 and the buffers 64a and 64b is effectively movable towards or away from the yoke 60 to adjust the offset to the required amount may be used. This allows a degree of control over the exact bias or offset, which may be useful in some circumstances.

The effect of this biasing mechanism is to pre-load the buffers 64a and 64b to a desired point on the load-deflection (or stiffness) characteristic. An example of a typical load-deflection characteristic is shown in Figure 7. This shows how the stiffness characteristic varies for subsequent cycles of loading as the buffers 64a and 64b absorb energy. In particular, it is clear how the characteristic varies for the first, second, third, and fourth and subsequent cycles as illustrated in Figure 7.

Eventually, the characteristic curve becomes more repeatable over successive cycles, and the energy absorption rate stabilises. It is desirable that the buffers 64a and 64b reach this stabilised condition as quickly as possible when placed in service. This is achieved by pre-loading using the biasing mechanism as a constant initial strain is always placed on the rubber buffers 64a and 64b, thereby effectively artificially accelerating the conditioning process so that repeatable energy absorption results are achieved as soon as the buffers 64a and 64b are placed in service.

Pre-loading the buffers 64a and 64b in this way also improves their fatigue life. This is because the cyclic change in strain is reduced (due to the initial value of strain in each cycle being higher than zero by the offset amount).

By biasing or pre-loading the buffers 64a and 64b to a midpoint of the linear region (point "X"), the buffers 64a and 64b can be caused to operate in a "push-pull" mode of operation over the linear region.

In this "push-pull" mode of operation, as the hub rotates 61 in an anticlockwise direction, the load on buffer 64a increases whilst there is a corresponding decrease in load on buffer 64b. Conversely, when the hub rotates 61 in an clockwise direction, the load on buffer 64b increases whilst there is a corresponding decrease in load on buffer 64a. However, in both cases the increase in loading on the buffer 64a or 64b is linear. In other modes of operation (for example, where there is not the possibility of motion in two opposing senses from the rest position), it may suffice simply to bias the buffers to a point of least strain on the linear region of the stress-strain characteristic, for example point "Y" on Figure 7. This is likely to be of more use in heavy transport and large wind turbine (e.g. 2MW or more) applications because it allows a very long range of deflection before the characteristic becomes significantly non-linear and the kind of deflection that would be expected in this kind of application will typically occur in one sense only.