WO2015021994A1

WO2015021994A1 - Mounting assembly of a gear drive system

Info

Publication number: WO2015021994A1
Application number: PCT/EP2013/002421
Authority: WO
Inventors: Peter Klein Meuleman; Jascha Van Pommeren
Original assignee: Aktiebolaget Skf
Priority date: 2013-08-13
Filing date: 2013-08-13
Publication date: 2015-02-19

Abstract

The invention resides in a gear drive system (100) for enabling driven rotation of a first component (110) relative to a second component (120), comprising a drive gear (135), a driven gear (140) and a mounting assembly (130) for supporting the drive gear. The mounting assembly and drive gear are mounted to a reference surface (125) on one of the first and second components. The driven gear is mounted on an annular section (112) on the other of the first and second components, whereby teeth of the driven gear are in meshing engagement with teeth of the drive gear The mounting assembly comprises a mobile part (132) to which the drive gear (135) is mounted, whereby the mobile part is connected to the reference surface (125) via a linkage arrangement. The mounting assembly further comprises retaining means (137a, 137b), in fixed connection with the mobile part, which radially retain the annular section in a manner which permits rotation of the driven gear (140) and maintains a constant spacing between the meshing gear teeth. The linkage arrangement (133, 134) is adapted to constrain the mobile part (132) to one degree of freedom, such that the drive gear (135) attached to the mobile part is obliged to follow a positional variation of a centre axis (140A) of the driven gear relative to the reference surface (125).

Description

Mounting assembly of a gear drive system

Field of the invention

The present invention relates to a gear drive system comprising a drive gear and a driven gear and is more particularly directed to a mounting assembly that supports the drive gear. The gear drive system of the invention is particularly suitable in applications where a centre axis of the driven gear may be subject to positional variations relative to a centre axis of the drive gear, such as in wind turbine applications. The invention further relates to a wind turbine that is provided with a gear drive system for enabling a pitch angle of a turbine blade to be adjusted relative to a turbine hub.

Background of the invention

Wind turbines are designed to convert wind into electricity, by turning a shaft connected to the generator positioned in a wind turbine housing, also known as a nacelle. The rotation of the generator shaft is achieved by wind turbine blades, normally three, that are connected to a hub, whereby the hub is rotationally coupled to the generator shaft. To enable optimization of the output power of the wind turbine, the blades are rotational about their longitudinal axis, for adjusting a pitch angle of the blade. In this way, the blades can be used to control the amount of wind power transferred from the wind to the generator.

In conventional wind turbines, a slewing bearing is generally used for rotational support of each of the turbine blades relative to the turbine hub. One ring of the slewing bearing is mounted to the hub and the blade is mounted to the other ring of the slewing bearing. Such a slewing bearing may be a ball or roller bearing having a dimension similar to the diameter of the root of the blade (in modern turbines up to 3 meters). For rotating a blade about its longitudinal axis, the bearing ring to which the blade is attached may be provided with a ring gear or a segment of a ring gear having teeth that engage with e.g. a motorized pinion gear that is fixedly attached to the hub. An example of such a pitch drive system is disclosed in WO 2013/071936 In use, pitch bearings are subjected to high loads and consequently can experience deformations that cause sub-optimal engagement and additional wear of the pinion gear. One solution for addressing this problem is to use relatively large gear teeth in the pitch drive system, enabling greater radial variation in gear-to-gear distance. Misalignment can be addressed by crowning the gear teeth, which has the disadvantage of reducing the line contact between gear teeth, resulting in increased stress levels.

Consequently, there is room for improvement.

Summary of the invention

It is an object of the invention to provide a gear drive system in which optimal gear teeth alignment is ensured in combination with optimal gear teeth geometry. According to the invention, this object is achieved by means of a mounting assembly for a drive gear of the gear drive system that enables said drive gear to follow positional variations of a driven gear of the gear drive system.

Specifically, the invention resides in a gear drive system for enabling driven rotation of a first component relative to a second component, comprising a drive gear, a driven gear and a mounting assembly for supporting the drive gear. The mounting assembly and drive gear are mounted to a reference surface on one of the first and second components. The driven gear is mounted on an annular section on the other of the first and second components, whereby teeth of the driven gear are in meshing engagement with teeth of the drive gear. In accordance with the invention, the mounting assembly comprises a mobile part to which the drive gear is mounted, whereby the mobile part is connected to the reference surface via a linkage arrangement. The mounting assembly further comprises retaining means, in fixed connection with the mobile part, which radially clamp the annular section in a manner which permits rotation of the driven gear and maintains a constant spacing between the meshing gear teeth. The linkage arrangement is adapted to constrain the mobile part to one degree of freedom, such that the drive gear attached to the mobile part is obliged to follow a positional variation of a centre axis of the driven gear relative to the reference surface. As a result, teeth of the drive gear remain in alignment with teeth of the driven gear, making it possible to employ gears with a fine gear module in the gear drive system.

Gear module defines tooth size in a gear and is the ratio of pitch diameter to number of teeth. When comparing gears of the same size and same gear ratio, the use of smaller teeth has a number of benefits. Due to the relatively greater number of gear teeth, the contact ratio is higher, leading to reduced stiffness variations, smoother running and no undercutting of the drive gear. The relative sliding speed between the teeth is lower, resulting in less friction and less wear. Furthermore, in combination with optimal tooth shape, the higher contact ratio increases gear strength and enables higher drive torque to be achieved. A drawback of gears with a fine gear module is the reduced radial tolerances and reduced ability to accommodate positional variations between the gear teeth. In a gear drive system according to the invention, the positional variations are accommodated by the linkage arrangement. The positional variations are generally attributable to deformations in a bearing construction that supports the driven component. The deformations experienced in use by the supporting bearing construction depend on the design of the construction and on the applied loads. In some constructions, a radial deformation is experienced, leading to a linear displacement of the driven gear axis relative to the reference surface in a radial direction.

In a first embodiment of the gear drive system of the invention, the linkage arrangement between the mobile part of the mounting assembly and the reference surface is adapted to permit only a radial displacement of the mobile part and drive gear relative to the reference surface.

Suitably, the mounting assembly comprises a fixed part that is mounted to the reference surface, whereby the fixed part and mobile part are connected via the linkage arrangement. In one example of the first embodiment, the linkage arrangement is formed by a parallel arrangement of first and second leaf springs or first and second elastic hinges that enable flexure in radial direction. The first and second leaf springs or elastic hinges may be arranged above and below the drive gear, or may be arranged at one side of the drive gear.

In a further example, of the first embodiment, the linkage arrangement comprises three links between the mobile part and the reference surface, whereby each end of each link has a pivoting head. The links are arranged to enable only a radial displacement of the mobile part relative to the reference surface. Other options are possible, such as elastic joints, or linear guides, and the linkage arrangement may be formed by any connection that enables radial displacement of the mobile part. When the driven gear axis undergoes a displacement relative to the reference surface, the drive gear axis is obliged to follow this displacement because of the retaining means. The retaining means may comprise rollers or elements with a sliding surface which are in fixed connection with the mobile part and which bear against radially opposite surfaces of the annular section. In a preferred example, the retaining means comprises a roller or sliding element that is attached to the pinion gear and bears against a radial surface adjacent to the driven gear. Advantageously, this radial surface has a diameter that is equal to the pitch circle of the driven gear. The retaining means further comprises a roller or sliding element that is mounted to the mobile part and bears against an oppositely oriented radial surface of the annular section. The retaining means thus ensure an optimal position of the drive gear teeth relative to the driven gear teeth and ensure that this position is maintained.

In some applications, the bearing construction that supports the driven component experiences deformations that cause a displacement of the driven gear axis relative to the reference surface, which displacement has a radial component and a tangential component. In a second embodiment of the invention, the linkage arrangement is adapted to enable the mobile part and drive gear to move relative to the reference surface in a direction with a radial and tangential component.

In one example of the second embodiment, the linkage arrangement comprises a parallel arrangement of links that enable flexure in a radial direction, such as first and second leaf springs, which are arranged at one side of the drive gear. The linkage arrangement further comprises a pivot element that interconnects the mobile part of the mounting assembly and the parallel leaf springs. To ensure that the drive gear axis follows the displacement of the driven gear axis, the retaining means comprises three retaining elements in the form of rollers or sliding elements. Suitably, first and second retaining elements bear against one radial surface of the annular section and the third bears against an oppositely oriented radial surface of the annular section. The first and second retaining elements are arranged at either tangential side of the third roller, meaning that optimal tooth alignment is ensured also in tangential direction.

In other applications, the bearing construction that supports the driven component experiences deformations that cause a tilting displacement of the driven gear axis relative to the reference surface, about a virtual rotation point. In a third embodiment of the invention, the linkage arrangement is adapted to enable the mobile part and drive gear to tilt relative to the reference surface about the same virtual rotation point. In one example of the third embodiment, the linkage arrangement comprises five links with a pivoting head at each end. The links may also be executed as elastic elements. Suitably, the mobile part of the mounting assembly comprises a top part parallel to the reference surface. Three links are arrangement between the top part and the reference surface, such that a virtual extension of each link converges at the rotation point. The mounting assembly also comprises a wall part mounted that extends from the reference surface. The linkage arrangement further comprises two links between the top element of the mobile part and the wall element which constrain the mobile part in the tangential direction, and transfer reaction forces from the drive gear to the reference surface.

As will be understood, the linkage arrangement employed in a gear drive system according to the invention may be configured in a variety of different ways, depending on the positional variations of the driven gear axis relative to the reference surface.

In an advantageous execution, the system of the invention is a pitch drive system, whereby the first component is a wind turbine hub and the second component is a wind turbine blade. The high loads on the blade which are transmitted to the hub through a supporting bearing arrangement are likely to cause deformations which in turn lead to positional variations between the axis of the driven gear and the reference surface on which the drive gear is mounted. The drive gear typically comprises a pinion gear which is preferably mounted to the hub, whereby a ring gear is provided on a radial surface that is in connection with the blade. It is also possible to mount the pinion drive to the blade and to provide the ring gear on the hub. In pitch drive applications, a limited amount of relative rotation between the hub and blade takes place. Typically, no more than approximately 90 degrees of relative angular displacement is needed. In one example of a pitch drive system according the invention, the driven gear is an arcuate gear segment that spans an arc of between 90 and 120 degrees.

The invention will now be further described with reference to the accompanying drawings.

Brief description of the drawings

In the drawings:

Figure 1A: illustrates a perspective view of a first embodiment of a gear drive system according to the invention, executed as a pitch drive system in a wind turbine for enabling driven rotation of a turbine blade relative to a turbine hub;

Figure 1 B illustrates a cross-section of a detail of the gear drive system of

Figure 1A;

Figure 2: illustrates a perspective view of a further example of a gear drive system according to the first embodiment of the invention;

Figure 3: illustrates a schematic front view of a second embodiment of a gear drive system according to the invention;

Figure 4a: illustrates a cross-section view of a third embodiment of a gear drive system according to the invention, executed as a pitch drive system;

Figure 4b: illustrates a perspective view of the pitch drive system of Figure 4a.

Detailed description of the invention

Figure 1A illustrates a perspective view of an example of a drive gear system 100 according to a first embodiment of the invention, a cross-section of which is shown in greater detail in Figure B. The system comprises a drive unit having a drive gear (pinion gear) 135 coupled to a motor 136. The drive unit is mounted to a first component via a mounting assembly 130. The system further comprises a driven gear 140 that is mounted to a second component. In the depicted example, the first component is a hub 120 of a wind turbine and the second component is a dynamic frame 110 that is rotationally supported relative to the hub via a bearing arrangement. The bearing arrangement may comprise a slewing bearing or two axially spaced bearing units of smaller diameter. The dynamic frame 110 is adapted for connection of a wind turbine blade or forms part of the blade, and the system 100 is a pitch drive system for adjusting a pitch angle of the blade. When supporting the turbine blade, the bearing arrangement experiences high loads that cause a certain amount of deformation. In the depicted example, a radial deformation is expected and according to the invention, the drive unit of the pitch drive system is coupled to the driven component 110 in a manner that enables a centre axis 135A of the pinion gear to remain at a constant distance from a centre axis 140A of the driven gear 140. The mounting assembly 130 comprises a fixed part 131 that is fixedly attached to a reference surface 125 on the hub 120. The assembly further comprises a mobile part 132 that is connected to the fixed part 131 via a first leaf spring 133 and a second leaf spring 134, which are arranged in parallel and enable flexure in a radial direction R. In this example, the fixed and mobile parts 131 , 132 of the mounting assembly are spaced from each other along the pinion gear axis 135A, whereby the first and second leaf springs 133, 134 are respectively arranged above and below this axis. The drive unit 135, 136 is fixedly attached to the mobile part 132 of the mounting assembly by means of e.g. bolts 150. The fixed part 131 of the mounting assembly suitably comprises an opening through which the motor 136 extends, and which has a sufficient clearance to allow for a radial displacement of the drive unit relative to the fixed part 131.

Teeth of the pinion gear 135 are in meshing engagement with teeth of a ring gear 140 that is mounted to an annular section 112 of the dynamic frame 110. The pinion gear and ring gear have a fine gear module, and thus have smaller gear teeth than the gears commonly used in pitch drive systems. As mentioned earlier, the use of smaller gear teeth has many benefits, such as improved strength and better wear behaviour. On the other hand, the relatively smaller radial tolerances mean that smaller radial deflections between the gears can be accommodated.

In the system according to the embodiment of Figs 1 a and 1 b, the pinion gear axis 135A follows the radial deflections of the ring gear axis 140A. This is enabled by the leaf springs 133, 134 and in that the pinion gear is radially retained relative to the annular section. In the depicted example, the retaining means comprises a first roller 137a that is attached to the mobile part 132 of the mounting assembly and which bears against a radially inner surface 113 of the annular section. The retaining means further comprises a second roller 137b, attached to the pinion gear 135, which bears against a radially outer surface 115 of the annular section. Preferably, the radially outer surface 115 has a diameter equal to the pitch circle diameter of the ring gear 140. The first and second rollers 137a, 137b permit the ring gear 140 to rotate relative to the mobile part 132 and maintain a constant and optimal spacing between the meshing gear teeth. Thus, a linear displacement of the ring gear centre axis 140A relative to the reference surface 125 in either radial direction causes a corresponding displacement of the pinion gear centre axis 135A. The displacement of the pinion gear axis relative to the reference surface 125 is taken up by the leaf springs 133, 134, ensuring that the teeth of the pinion gear and of the ring gear remain in alignment.

A further example of a pitch drive system 200 according to the first embodiment of the invention is shown in Figure 2. Again, the system is configured to maintain a constant centre-to-centre distance between a centre axis of the pinion gear 235 and a centre axis of the ring gear 240, due to radial deformations of a bearing arrangement that supports the driven component to which the ring gear is mounted. The pinion gear 235 and drive motor 236 are mounted to a reference surface (not shown) on the hub via a mounting assembly. In this example, the mounting assembly comprises a fixed part 231 and a mobile part 232 which are arranged in a direction transverse to the centre axis 235A of the pinion gear. The fixed part 231 is attached to the reference surface; the drive unit comprising the pinion gear 235 and motor 236 is attached to the mobile part 232 and the fixed part and mobile part are connected by a parallel arrangement of first and second leaf springs 233, 235 that flex in radial direction. The pinion gear 235 is again retained in both radial directions relative to the ring gear 240 by means of a first roller 237a attached to the mobile part 232, which bears on a radially inner surface of the annular section, and by means of a second roller 237b attached to the pinion gear 235, that engages a radially outer surface of the 215 of the annular section. The drive gear axis 235A is thus obliged to follow radial displacements of the ring gear axis relative to the reference surface, whereby the corresponding radial displacement of the drive gear axis 235A relative to the reference surface is taken up by the leaf spring arrangement 233, 234.

In a second embodiment of the invention, the drive gear system is configured to enable the centre axis of the drive gear to follow the centre axis of the driven gear in a combination of a radial direction and in a tangential direction. A schematic front view of an example of such a system 300 is shown in Figure 3. The system depicted in Figure 3 is similar to the system described with reference to Figure 2 and comprises a mounting assembly having a fixed part 331 that is arranged to one side of a mobile part 332, whereby the two parts 331 , 332 are connected via a parallel arrangement of first and second leaf springs 333, 334 that flex in radial direction R. The drive motor 336 and associated pinion gear 335 are attached to the mobile part 332 of the mounting assembly and the fixed part 331 is attached to a reference surface. To enable the mobile part and pinion gear 335 to move relative to the reference surface in a tangential direction T, as well as the radial direction R, the mounting assembly further comprises a hinge element 338 connected between the parallel leaf spring arrangement 333, 334 and the mobile part 332. A pivoting connection between the hinge element 338 and mobile part 332 enables the mobile part to pivot about a pivot axis 338A about which the mobile part 332 can pivot, which, in turn, enables motion in tangential direction T. To ensure that the pinion gear 335 also follows a displacement of the ring gear in radial and tangential direction, the mounting assembly comprises first, second and third retaining elements in the form of rollers 337a, 337b and 337c attached to the mobile part.

In this example, the ring gear 340 has a C-shaped cross-section and has an inner flange 341 and an outer flange. The gear teeth are provided on a radially outer surface of the outer flange. The first roller 337a is arranged with its centre axis directly beneath the pinion gear axis, and bears against a radially inner surface of the inner flange 341. The second and third rollers 337b, 337c bear against a radially outer surface of the inner flange 341 and are arranged at either side of the first roller 337a in tangential direction. The three retention points constrain the motion of the mobile part, relative to the ring gear, in both radial directions and in both tangential directions. As a result, the radial spacing and tangential alignment of the drive gear teeth relative to the ring gear teeth remain constant. In some applications, the loads on a driven component may cause the supporting bearing construction to experience deformations that result in tilting of the driven gear axis about a rotation point. According to a third embodiment of the invention, the drive unit is supported on the reference surface and is coupled to the driven component in a manner that enables the drive gear axis to tilt about the same rotation point.

Fig. 4a shows a cross-section of a bearing arrangement which rotationally supports a wind turbine blade (not shown) relative to a wind turbine hub 420. For adjusting a pitch angle of the blade, a pitch drive system according the invention is provided. The system comprises a driven gear 440 mounted to a dynamic frame 410 to which the blade is attachable, and comprises a drive unit having a pinion gear 435 that is mounted to a reference surface 425 on the hub 420 via a mounting assembly 430. The hub comprises a static frame having a first bearing seat 421 and a second bearing seat 422, axially spaced from the first seat. The first and second bearing seats are joined by a static conical section 427 which has three open regions defined between three static legs 428 arranged circumferentially on the static frame at a largest diameter thereof. Similarly, the dynamic frame has a dynamic conical section 417 which has three dynamic legs 418 arranged circumferentially at a largest diameter of the dynamic frame, with openings between these legs. The legs 418 of the dynamic frame pass through the openings between the legs of the dynamic frame and the legs of the static frame pass through the opening between the legs of the static frame. Thus, a restricted amount of relative rotation can take place between the static and dynamic frames. In blade pitch systems, no more than approximately 90 degrees of relative rotation is required, which the depicted assembly permits.

The dynamic frame is rotationally supported on the static frame by means of first and second bearing units 460, 470, which are mounted to the first and second bearing seats 421 , 422 respectively. In this example, the second bearing unit 470 consists of a radial spherical plain bearing, whereby an outer ring 471 of the bearing is mounted to the second bearing seat 422 of the static frame and an inner ring 472 of the bearing is mounted to the dynamic frame. It is also possible for the inner ring to be mounted to the static frame and the outer ring to be mounted to the dynamic frame. The first bearing unit 460 also comprises a radial spherical plain bearing, but has inner ring connected to the first bearing seat 421 and has an outer ring connected to the dynamic frame 410. It is also possible for the inner ring to be connected to the dynamic frame and for the outer ring to be mounted to the static frame. The first bearing unit 460 further comprises first and second spherical thrust plain bearings for transmitting axial loads. For the depicted bearing construction, finite element analysis of the construction predicts that the dynamic frame 410 and ring gear 440 attached thereto will tilt about a rotation point 480, under the expected applications loads. The pitch drive system is configured such that the pinion gear 435 is able to tilt about the same rotation point 480.

The gear drive system again has a mounting assembly 430 with a fixed part 431 mounted to a reference surface 425 of the hub 420. The mounting assembly further has a mobile part 432 to which the pinion gear is attached. As described with reference to the embodiment of Figs 1 a and 1 b, a constant spacing between the teeth of the pinion gear and ring gear 440 is maintained by first and second rollers 437a, 437b, in connection with the pinion gear, that bear on opposite radial sides of the annular section to which the ring gear is mounted.

To enable the centre axis of the pinion gear 435 to tilt relative to the reference surface 425, in response to a corresponding tilt of the ring gear axis, the mobile part 432 of the mounting assembly is connected to the fixed part via an arrangement of five linkages. The arrangement is more clearly depicted in the perspective view of the mounting assembly in Fig 4b. The fixed part 431 of the assembly has a base element 431 a arranged on the reference surface 425 and has a wall element 431 b perpendicular to the base element. The mobile part 432 of the assembly has a top element 432a, parallel to the base element 431 a, and has a front element 432b to which the pinion gear 434 and first retaining roller 437a are attached. The base element of the fixed part and the top element of the mobile part are connected by three vertical links 333a, 333b, 333c. In this example, the links are rods with a pivot joint at each end. Two rods 333a, 333b are arranged at one side of the drive motor 436 and a third rod 333c at the other side. The ends of each rod are arranged on the top element 432a and on the base element 431 a such that each an extension of each rod points converges at the rotation point 480, as best seen in Figure 4a.

The three vertical links 333a, 333b, 333c enable the mobile part of the mounting assembly and the pinion gear axis to tilt about the rotation point 480 when a deformation of the bearing construction causes the ring gear axis to tilt. However, they do not constrain the mobile part 331 in tangential direction. The linkage arrangement thus comprises a further two horizontal links 434a, 434b between the wall element 431 b of the fixed part and the top element 432a of the mobile part. As a result, the mobile part is constrained to one degree of freedom and the pinion gear axis is obliged to follow the ring gear axis. The two horizontal links 434a, 434b also serve to transmit gear reaction forces from the mobile part 432 to the fixed part 431 of the mounting assembly and to the hub 420. A drive gear system according to the invention may be executed in a variety of different ways, depending on the positional variation of the driven gear axis relative to the reference surface on which the drive gear is mounted. The embodiments which have been described are suitable for use in wind turbine application, but the invention is not restricted to pitch drive systems and may be employed in any application where the centre axis of the driven gear experiences positional variations.

Furthermore, a number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.

Claims

A gear drive system (100, 200, 300) comprising a drive gear (135, 235, 335, 435) in meshing engagement with a driven gear (140, 240, 340, 440) for enabling driven rotation of a first component (1 0, 410) relative to a second component (120,420), wherein the drive gear is mounted to a reference surface (125, 425) on one of the first and second components via a mounting assembly (130, 430), and wherein the driven gear is mounted to an annular section (112) on the other of the first and second components, characterized in that:

• the mounting assembly comprises a mobile part (132), which is moveably connected to the reference surface (125) via a linkage arrangement (133, 134, 233, 234, 333, 334, 338), whereby the drive gear is mounted to the mobile part;

• the mobile part (132, 232, 332, 432) comprises retaining means (137a, 137b, 237a, 237b, 337a, 337,b, 337c, 437a, 437b) that engage with the annular section (112) in a manner which permits rotation of the driven gear (140) and maintains a constant relative position between meshing teeth of the drive gear and of driven gear; and in that

• the linkage arrangement constrains the mobile part (132, 232, 332, 432) to one degree of freedom, such that the drive gear mounted to the mobile part is obliged to follow a positional variation of a centre axis of the driven gear (140A) relative to the reference surface (125, 425).

The gear drive system according to claim 1 , wherein the retaining means comprise a first retaining element (137a, 237a, 337a, 437a) that bears against a radially inner surface (113) of the annular section (112) and comprise a second retaining element (137b, 237b, 337b, 337c, 437c) that bears against a radially outer surface (115) of the annular section.

The gear drive system according to claim 2, wherein one of the retaining elements (137b, 237b, 437b) is mounted to the drive gear (135, 235) and bears against a radial surface (115) adjacent to the driven gear (140, 440), whereby a diameter of the radial surface is equal to the pitch circle of the driven gear.

4. The gear drive system according to any preceding claim, wherein the positional variation comprises a radial displacement and the linkage arrangement (133, 134, 233, 234) is configured to enable only a radial displacement of the mobile part relative to the reference surface.

5. The gear drive system according to claim 4, wherein the linkage arrangement comprises a parallel arrangement of first and second leaf springs (133, 134, 233, 234) that enable flexure in a radial direction (R).

6. The gear drive system according to claim 5 wherein the first and second leaf springs (133, 134) are arranged parallel to a centre axis (135A) of the drive gear (135) in radial direction.

7. The gear drive system according to claim 5, wherein the first and second leaf springs (233, 234) are arranged at one side of the drive gear axis (135A) in tangential direction.

8. The gear drive system according to claim 1 or 2, wherein the positional variation comprises a displacement in a combination of a radial direction (R) and a tangential direction (T), and wherein the linkage arrangement is configured to enable only a corresponding displacement of the mobile part relative to the reference surface.

9. The gear drive system (300) according to claim 8, wherein the linkage arrangement comprises a parallel arrangement of first and second leaf springs (333, 334) that enable flexure in radial direction (R) and further comprises a hinge element (338) that interconnects the mobile part (332) and the leaf springs, whereby the mobile part is pivotably connected to the hinge element.

10. The gear drive system (300) according to claim 9, wherein:

• the annular section of the second component comprises an annular flange (341);

• the retaining means comprises first, second and third retaining elements (337a, 337b, 337c), mounted to the mobile part (332);

• the first and second retaining elements bear on one radial surface of the annular flange (341) and the third retaining element bears on an opposite radial surface of the flange; and

• the first and second retaining elements are arranged at either side of the third retaining element in tangential direction (T).

1 1. The gear drive system according to claim 1 or 2, wherein the positional variation comprises a tilting displacement about a rotation point (480) and wherein the linkage arrangement is configured to permit only a tilting of the mobile part relative to the reference surface (425) about the same rotation point (480).

12. The gear drive system according to claim 11 , wherein the mobile part (432) of the mounting assembly (430) comprises a top element (432a) parallel to the reference surface (425) and wherein the mounting assembly comprises a fixed part (431 ) with a wall element (431 b) that extends from the reference surface, whereby:

• the top element and the reference surface are connected by three links (433a, 433b; 433c) with pivotable connections at each end, arranged such that a virtual extension of each link intersects at the rotation point (480); and

• the wall element (431 b) and the top element (432a) are connected by two links (434a, 434a) with pivotable connections at each end, arranged to constrain the mobile part (431 ) in tangential direction.

13. The gear drive system according to any preceding claim wherein the retaining means comprises a roller or a retaining element with a sliding surface.

14. The gear drive system according to any preceding claim, wherein the first component is a wind turbine blade and the second component is a wind turbine hub (120, 420).

15. The gear drive system according to claim 14, wherein the driven gear (140) is provided on a slewing bearing that supports the blade relative to the hub (120) or is provided on a dynamic frame (110, 410) to which the blade is attached, the dynamic frame being supported by two axially spaced bearing units (460, 470).