CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/EP2018/050042, having a filing date of Jan. 2, 2018, based off of German Application No. 10 2017 204 340.7 having a filing date of Mar. 15, 2017, the entire contents both of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following describes a method of finishing a bearing ring of a wind turbine bearing; a bearing ring finishing assembly; a wind turbine bearing; and a wind turbine.

BACKGROUND

In the case of a wind turbine generator, the overall stiffness of the combined hub, main bearing, main shaft and generator rotor must be great enough to maintain a satisfactory airgap in the generator even under extreme loading conditions. The wind turbine rotor (the combination of hub and rotor blades) will tend to deflect in various directions during operation of the wind turbine, and the main bearing—which can have a diameter in the order of several meters in the case of a multi-megawatt direct-drive wind turbine—must be sufficiently stiff to be able to withstand these deflections so that the airgap usually in the order of only a few millimetres—remains essentially constant under all operating conditions.

The design of a bearing will be based on information about the surrounding structure, in particular the stiffness of the surrounding structure, and the loading that is to be expected during operation. A bearing such as a wind turbine main bearing must be manufactured to a very high degree of precision in the raceway system (comprising the raceways and the roller elements) in order to keep any angular misalignment between inner ring and outer ring within an acceptable level; to achieve satisfactory load sharing between the rolling elements of the bearing; and to achieve acceptable surface pressure distribution during operation. An estimation of the expected lifetime of a bearing is generally based on detailed information regarding load sharing and surface pressure distribution inside the bearing.

The desired degree of precision for a bearing is achieved during a final machining process, typically by grinding, and is applied over an entire raceway surface (i.e. over 360°) to ensure that the raceway surface is as uniform as possible in the complete circumference with respect to e.g. profile, raceway pitch circle diameter, raceway contact angle etc. It is usual that the raceway is machined to a very high degree of precision, with a tolerance in the order of only a few microns, often as few as 10 μm. While the main bearing of a wind turbine can be manufactured to a very high level of quality, the known machining techniques cannot avoid fatigue damage completely, and this ultimately sets an upper limit on the expected or calculated lifetime of the bearing.

SUMMARY

An aspect relates to provide an improved way of finishing a bearing to overcome the problems mentioned above.

According to the embodiments of the invention, the method of finishing a bearing ring of a wind turbine bearing comprises the steps of identifying a number of local hard zones on a surface of the bearing ring; and removing material from the surface of the bearing such that a bearing ring thickness in a local hard zone is less than a bearing ring thickness outside a local hard zone.

A bearing ring can be finished by using any suitable machining process such as grinding or turning. Generally, grinding is the process of choice. In the following therefore, without restricting the embodiments of the invention in any way, it may be assumed that a final finishing step involves grinding a surface of the bearing ring to achieve a desired profile. The terms “machining” and “grinding” may be regarded as synonyms unless otherwise indicated. The surface that is being machined in this finishing step may be referred to simply as the “machining surface” or “grinding surface”.

The known or established grinding processes have focused on manufacturing unassembled and unloaded bearing rings and raceways with as uniform a surface profile as possibly over the complete circumference of the bearing. The bearing of a wind turbine, when machined to achieve such uniform raceway profiles, may suffer from accelerated fatigue damage on account of so-called “local hard zones” inside the bearing. A local hard zone is a region of a raceway in which significantly higher roller loading occurs compared to the adjacent areas in that same raceway. Roller loading is generally in the order of several tens of kilonewtons (kN) for a large bearing. The surface pressure on a bearing raceway is also higher in a local hard zone than in adjacent areas in that same raceway. Surface pressure is generally in the order of 900-1800 megapascal (MPa). In a wind turbine main bearing that is subject to a conventional uniform grinding step in a finishing stage, relatively large local hard zones over several tens of degrees—arise from the fluctuating character of the loading on the wind turbine rotor (the combination of hub and rotor blades). The existence of local hard zones in a bearing will have a negative influence on the bearing lifetime, and may be understood to be one of the limiting factors in bearing lifetime.

The position of a local hard zone on a surface of the bearing, as well as its shape or contour, can be determined by advanced finite element analysis (FEM) calculations for that specific bearing type and its expected loading. A subsequent step in the inventive method, after identifying a number of local hard zones, is a step of non-uniform grinding (or turning, as appropriate) according to a specific machining profile over the complete circumference of the bearing surface, i.e. over 360°. The resulting non-uniform profile on the machined surface of the bearing will be characterized by transitions from a nominal height to a lower height in each of the local hard zones identified for that bearing type. Since material is removed from the regions identified as the local hard zones, any bearing that is machined using the inventive method will be essentially free of such local hard zones during operation of the wind turbine. The inventive method can therefore extend the lifetime of such a bearing by 10%-30% compared to a bearing that is not finished using the inventive method (i.e. a bearing with a uniform profile over each of its raceways).

A wind turbine bearing manufactured using the conventional finishing techniques is typically expected to have a lifetime of twenty years or so. The inventive method takes into account the complete loading of the bearing, including the negative influence from local hard zones. As a result, the rollers and raceways are not subject to excessive loading in such “local hard zones”, thereby leading to a longer bearing lifetime. The inventive method can effectively reduce the area of a local hard zone to a very significant extent. Instead of extending over an arc spanning several tens of degrees on a bearing raceway, any remaining local hard zones may be completely eliminated, or reduced to within an arc of only a few degrees. The effect of this is to extend the lifetime of the bearing. It is a very significant advantage to be able to extend the lifetime of such a bearing by several years.

According to the embodiments of the invention, the machining assembly comprises a support realized to hold a bearing ring of a wind turbine during a machining procedure; a machining tool such as a grinding wheel arranged to remove material from a machining surface of the bearing ring; and a control unit realized to control at least the machining tool. There are essentially two main ways of achieving the desired machining pattern. In one embodiment, the control unit of the inventive machining assembly is realized to receive information determining the location of one or more local hard zones on the surface to be machined, and the control unit is realized to control the position of the machining tool to reduce the bearing ring thickness in a local hard zone to a greater extent than in a region between local hard zones. In an alternative embodiment, the machining assembly comprises a number of distortion means arranged to force the local hard zones closer to the machining tool. Because of the distorted shape of the bearing ring, the bearing ring thickness is reduced to a greater extent over the local hard zones than in any area between these local hard zones. In this alternative embodiment, there is no need to adjust the machining tool position relative to the local hard zones.

According to the embodiments of the invention, the wind turbine bearing comprises an inner bearing ring and an outer bearing ring, and at least one surface of a bearing ring is ground using the inventive method. According to the embodiments of the invention, the wind turbine comprises a plurality of rotor blades mounted to a hub, a generator with an outer rotor and an inner stator, and a bearing comprising a stationary inner bearing ring and a rotatable outer bearing ring, which outer bearing ring connects the hub to the generator rotor, wherein at least one of the bearing rings is machined in a finishing step using the inventive method. The inventive bearing ring is advantageously able to better withstand the loading arising from combined gravity and wind loads on the rotor, and has a favourably long lifetime as a result. Any wind turbine that incorporates such a bearing can favourably benefit from the extended lifetime of the bearing.

Particularly advantageous embodiments and features of the embodiments of the invention are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein.

The inventive method can be applied to any type of bearing that might benefit from a non-uniform grinding of a surface such as a raceway. For example, the inventive method can be applied to a blade pitch bearing of the roller bearing type. These bearings may also exhibit local hard zones, and may benefit from the inventive method to eliminate or at least significantly reduce the hard zones. However, without restricting the embodiments of the invention in any way, it may be assumed that the bearing is the main bearing of a wind turbine.

An example of a local hard zone on a grinding surface can be a zone or region that corresponds to the future position of a rotor blade relative to the bearing ring when the bearing is installed in a wind turbine. Such a local hard zone or “blade region” can be regarded as the “shadow” that would be cast by a blade mounted to the hub which in turn is mounted to the bearing. These blade regions are geometrically “fixed” in the case of the rotating bearing ring, since this rotates as one unit with the rotor. A blade region can be essentially symmetrical about the “shadow” that would be cast by a blade mounted to the hub. Equally, such a local hard zone may be shifted to one side of the blade axis “shadow”. Any such blade-related hard zone will generally be found on the downwind side of a bearing ring. The loading experienced by an upwind side of a bearing ring is generally of a different nature, and arises from the bending moment through the centre of mass of the wind turbine rotor. The upwind side of a bearing ring may therefore only show a single local hard zone in an upper region of the bearing ring.

Since most wind turbines have three rotor blades, in the following it may be assumed that up to three blade regions are identified for the bearing surface that is to be ground. These three blade regions will be equidistantly arranged about the bearing surface, in the same way that the rotor blades are equidistantly arranged about the axis of rotation of the hub. With three blade regions, there will automatically be three intermediate or non-blade regions.

A bearing for use in a system such as a generator generally has one stationary ring that is mounted to a supporting structure that carries the stationary part of the generator, and one rotatable ring that is mounted to the rotating part of the generator. In a structure such as a wind turbine, there are various ways in which a generator may be realized. The generator might have an outer rotor or field carrying the magnets, and an inner stator or armature carrying the windings, for example. In an alternative embodiment, the generator might have an outer stator and an inner rotor; the rotor may be realized as the armature and the stator might be realised as the field, etc. While any such embodiment is possible, in the following it may be assumed that the generator has a rotating outer field (carrying the magnets) that is connected to the hub by means of a rotatable outer bearing ring, and that the stationary inner bearing ring is mounted to a main shaft. It may be assumed, for the purposes of discussion, that the wind turbine is a large direct-drive wind turbine with such a generator.

The bearing under discussion may be assumed to be a rolling element bearing. Various types of rolling element are also possible, for example a tapered bearing, cylindrical roller bearing, spherical roller bearing etc. The bearing may have one or more rows of rolling elements, whereby the rolling elements of a row are generally guided by means of raceway shoulders and a roller cage. In the following, without restricting the embodiments of the invention in any way, it may be assumed that the bearing is a two-row tapered roller bearing so that each bearing ring inner ring and outer ring has two raceways or seats. A raceway closest to the hub is referred to as an upwind raceway, and a raceway furthest from the hub is referred to as a downwind raceway. For a bearing such as a wind turbine main bearing, the raceways are angled in a “V” formation, as will become clear from the diagrams.

In a preferred embodiment of the invention, the grinding surface is preferably a raceway of the bearing ring, i.e. material is preferably ground from one of the raceways—for example from a raceway on the downwind side of the outer ring.

The inventive method may be performed as part of a final grinding procedure. The bearing ring to be machined or ground will already have been finished to a high level of precision, and only a very small amount of material—e.g. only 100 μm or less—will be removed in the final machining stage. To this end, a grinding pattern is previously determined for the entire bearing ring, whereby the grinding depth for each local hard zone and each intermediate region (a region between local hard zones) is determined in advance. A grinding depth is to be understood as the depth to which material is to be removed from a local hard zone. For the exemplary situation described above for a main bearing for use in a wind turbine with three rotor blades, the desired grinding pattern may comprise an alternating pattern of x and y, where x>y, and whereby x is the grinding depth within each of three blade regions, and y is the grinding depth in each of the intermediate regions. Preferably, there is a smooth transition between the shallow grinding depth (y) and the deeper grinding depth (x) which is described as the so-called grinding depth profile. The optimal grinding depth profile can be calculated by a suitable analysis tool such as advanced FEM analysis.

Parameters and dimensions of the bearing ring will depend to a large extent on the type of machine into which it will be incorporated. For example, the thickness of a wind turbine bearing ring is preferably determined on the basis of loading values that have been estimated or calculated for that type of wind turbine. In conventional wind turbine bearings, the raceways are machined to achieve a profile that is as uniform as possible, i.e. each bearing ring has a constant profile over its entire circumference. This is to be understood to mean that the raceway of a conventional bearing—whether flat or crowned—has a uniform pitch circle diameter and angle over the entire circumference of the raceway. In other words, a radial cross-section through the raceway of a conventional bearing will be the same for all points along the raceway.

In a preferred embodiment of the invention, the grinding depth for a surface is calculated on the basis of a specific predicted loading pattern that will arise from the rotor—i.e. the hub and blades of that type of wind turbine. The specific predicted loading pattern established in the course of the embodiments of the invention takes a “fatigue characteristic load case” into consideration in order to achieve a best possible improvement in lifetime for a specific bearing type. The fatigue characteristic load case is related to a wind speed interval (e.g. a wind speed of 9-11 m/s for a wind turbine that will use that bearing type), and the relevant bearing parameters are generally optimised on the “worst case” assumption that the wind turbine will operate continually in this wind speed interval, at which the power output of the wind turbine is adjusted to its nominal value. It is beneficial to optimize a bearing for the situation to which it will most likely be exposed over much of its lifetime, and usually any such optimization is based on such a “worst case” wear and tear scenario. In addition to considering such a fatigue characteristic load case, the inventive method also effectively eliminates or minimizes any local hard zones in a wind turbine main bearing.

A representative rotor position for obtaining the specific predicted loading pattern in the bearing is a position with one blade pointing vertically downwards, and the other two blades pointing upward and outward. This position is known as the “bunny” position since it is similar to the nose section of a rabbit. In the case of a three-blade rotor, this position is reached three times during each full revolution of the rotor. The loading associated with this position arises from a combination of several loading effects: firstly, the combined mass of the rotor (hub and blades) acts on its centre of gravity (which is at a distance outward from the bearing) resulting in a tilting force on the bearing; secondly, the hub undergoes elastic deformation owing to the flapwise and edgewise bending moments of each of the rotor blades, and this hub deformation is transferred to the bearing. The inventive method takes this fatigue characteristic load case into consideration by the grinding pattern, which comprise three slightly deeper blade regions alternating with three intermediate regions, in a pattern corresponding to the “Y” of the three rotor blades. This grinding pattern favourably allows the bearing to operate, to a large extent, without local hard zones during a high number of operational hours resulting in lower bearing lifetime consumption. The deeper grinding depth (x) within a blade region preferably exceeds the shallower grinding depth (y) by a few microns, for example by 30-100 μm in the case of a main bearing with a diameter of 2-4 m.

The grinding surface can be any surface of a bearing ring that is subject to uneven loading as the rotor rotates. This uneven loading may apply to a flange of the bearing, for example, and the flange surface can be ground using the inventive method after identifying the blade regions on that flange surface. However, it is the raceways of a bearing that are the surfaces most affected by the uneven loading, and in the following, without restricting the embodiments of the invention in any way, it may be assumed that a grinding surface is the raceway of a bearing ring. For example, in the inventive method, material is preferably removed from the raceway in a blade region to achieve an essentially constant bearing ring thickness over the blade region. Preferably, the grinding wheel is controlled to achieve a smooth transition between a local hard zone and a neighbouring region. For example, if a local hard zone extends over 60° of the bearing ring, and the grinding depth in a local hard zone is 80 μm deeper than in an intermediate region, a transition zone can extend over 60°, overlapping the local hard zone and intermediate region, and the grinding depth may gradually increase from the intermediate region into the local hard zone.

Alternatively or in addition, in a preferred embodiment of the invention material may be removed from the raceway in a blade region to achieve an angular correction. To achieve a desired angular correction, the bearing ring thickness may transition gradually from a first thickness at an outer circumference of the grinding surface to a second thickness at an inner circumference of the grinding surface.

The inventive machining assembly may be assumed in the following to be a grinding assembly. The desired grinding pattern can be achieved by appropriate control of the grinding wheel of the grinding assembly. Preferably, the grinding assembly comprise a multi-axis grinding wheel, i.e. a grinding wheel which can at least be controlled to move horizontally as well as vertically. The rate of material removal will depend on how fast the grinding wheel rotates, how quickly it moves relative to the grinding surface, and the position of the active surface of the grinding wheel in relation to the grinding surface. The desired grinding pattern to remove more material from a local hard zone than from an intermediate region can be achieved in a number of ways. For example, the grinding wheel could be controlled to obtain a movement up and down along on axis perpendicular to a raceway of the bearing ring while the bearing ring is being turned as it rests on a horizontal turning table. The grinding wheel position and position of bearing ring on the turning table are fully correlated via a control unit, to ensure a very precise grinding pattern. After obtaining the required circumferential profile for a specific diameter, the grinding wheel is shifted slightly inwards or outwards to a new position corresponding to a different diameter. The grinding process in the circumferential direction is then repeated. This grinding process continues until the complete raceway has been ground to give the desired profile.

A control unit of such a grinding assembly is preferably realised to convert information relating to the grinding profile into appropriate control commands, for example to actuate motors of a turning table and/or the grinding wheel. Control of the grinding assembly components to adjust the grinding wheel position, speed etc. can be done by a suitable computer program that is directly loadable into a memory of a control unit or control module of a grinding assembly and that comprises suitable program elements for performing steps of the inventive method when the computer program is executed by the control unit of the grinding assembly. It is sufficient for a technician or user to identify the so-called “top mark” on the bearing surface (a mark placed by the bearing manufacturer to identify the uppermost point on the bearing when installed) to position the bearing ring in a defined orientation in the grinding assembly, and to input any relevant data to the computer program. The top mark may define “12 o'clock” for a rotor in the “bunny position”. In this case, a top mark is exactly at “12 o'clock” on the outer ring, and is visible on the upwind and downwind sides of the bearing. Similar top marks are generally also present on the upwind and downwind sides of the inner ring, since the inner ring is generally mounted to the main shaft of the wind turbine with its top marks exactly at the “12 o'clock” position.

Alternatively or in addition, the bearing ring itself can be subject to a slight degree of intentional and reversible distortion before grinding, so that the distortion results in the desired grinding pattern. To this end, a suitable distortion means is incorporated in the grinding assembly. In one embodiment, the distortion means comprises a number of shims arranged under the bearing ring at suitable positions. For example, shims behind or under each blade region can effectively “push” the blade region closer to the grinding wheel, resulting in a deeper grinding depth over the blade region. Instead of shims, the turning table can be manufactured with a profiled mounting surface to achieve the desired distortion when the bearing ring is secured to the mounting surface. The bearing ring can be fixed to the turning table in any suitable manner, for example by using magnet arrangement incorporated in the grinding assembly, or by using an arrangement of clamps, bolts, etc. After grinding, any such distortion means are removed or dismounted, so that the bearing ring once again assumes its un-distorted shape, revealing the desired grinding pattern.

