Patent Description:
Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.

Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., <NUM> or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.

Wind turbines including direct drive generators eliminate the gearbox, and reliability problems associated with the gearboxes. However, in at least some known wind turbines, rotor bearings, pitch bearings, generator bearings and other bearings may prematurely fail. Because the bearings can be difficult to access and replace, failure of bearings can result in a lengthy and expensive repair process.

To facilitate reducing costs while optimizing turbine availability, bearing replacement and/or repair should be performed rapidly at the wind turbine site with a minimal infrastructure and skill set required. However, known bearings used in wind turbines generally require change-out at the factory or labor intensive and costly on-site repair.

In document <CIT> roller bearing is provided to support a turret of an armoured vehicle. Elastic rollers are provided to support the turret. To protect the roller bearing against overload, e.g. due to exploding grenades, a small gap is provided between the collars.

In document <CIT> a ball bearing supports the nacelle of a wind turbine. To damp vibrations a brake lining is provided between the two races.

The present invention, as defined by the appended claims, is thus provided.

Various aspects and embodiments of the present invention will now be described in connection with the accompanying drawings, in which:.

In some configurations and referring to <FIG>, a wind turbine <NUM> comprises a nacelle <NUM> housing a generator (not shown in <FIG>). Nacelle <NUM> is mounted atop a tall tower <NUM>, only a portion of which is shown in <FIG>. Wind turbine <NUM> also comprises a rotor <NUM> that includes one or more rotor blades <NUM> attached to a rotating hub <NUM>. Although wind turbine <NUM> illustrated in <FIG> includes three rotor blades <NUM>, there are no specific limits on the number of rotor blades <NUM>.

In some configurations and referring to <FIG>, various components are housed in nacelle <NUM> atop tower <NUM> of wind turbine <NUM>. The height of tower <NUM> is selected based upon factors and conditions known in the art. In some configurations, one or more microcontrollers within control panel <NUM> comprise a control system used for overall system monitoring and control. Alternative distributed or centralized control architectures are used in some configurations.

In some configurations, a variable blade pitch drive <NUM> is provided to control the pitch of blades <NUM> (not shown in <FIG>) that drive hub <NUM> as a result of wind. In some configurations, the pitch angles of blades <NUM> are individually controlled by blade pitch drive <NUM>. Hub <NUM> and blades <NUM> together comprise wind turbine rotor <NUM>.

The drive train of the wind turbine includes a main rotor shaft <NUM> (also referred to as a "low speed shaft") connected to hub <NUM> via main bearing <NUM> and (in some configurations), at an opposite end of shaft <NUM> to a gear box <NUM>. Gearbox <NUM> drives a high-speed shaft of generator <NUM>. In other configurations, main rotor shaft <NUM> is coupled directly to generator <NUM>. The high-speed shaft (not identified in <FIG>) is used to drive generator <NUM>, which is mounted on mainframe <NUM>. In some configurations, rotor torque is transmitted via coupling <NUM>. In configurations of the wind turbine, generator <NUM> is a direct drive permanent magnet generator.

Yaw drive <NUM> and yaw deck <NUM> provide a yaw orientation system for wind turbine <NUM>. A meteorological boom <NUM> provides information for a turbine control system, which may include wind direction and/or wind speed. In some configurations, the yaw system is mounted on a flange provided atop tower <NUM>.

<FIG> illustrates one known bearing of the double row type. Bearings of this type can be used in the pitch or yaw system of wind turbines. This configuration is typically chosen to compensate for high loads that occur infrequently. The bearing <NUM> is located at the junction between tower <NUM> and the nacelle's bedplate or mainframe <NUM>. The wind turbine <NUM> can be subject to occasional heavy gusts of wind. These gusts are typically of short duration, however they do exert extreme loads on the wind turbine. The wind can force the nacelle to tilt against one side of bearing <NUM>. This force is transmitted from the main frame <NUM> through the yaw bearing <NUM> into tower <NUM>. The double row bearing <NUM> has the advantage of being able to bear higher loads than known single row bearings, however, the cost of double row bearings is much greater.

Another variation of a known double row bearing is shown in <FIG>. In this bearing a portion of the inner raceway <NUM> extends between the bearings. The outer raceway <NUM> "wraps" around the bearings and the inner raceway extension. However, in both of the bearings of <FIG>, the load experienced between the races is transmitted through the rolling elements (e.g., balls). For bearings subject to high loads, the use of a double or triple row bearing has been required. It would be advantageous if a lower cost, single row bearing could be designed to accommodate high loads as well.

<FIG> illustrates a bearing according to the present invention. The bearing <NUM> can be used as the pitch bearing or yaw bearing in wind turbine <NUM>. The inner race <NUM> includes a shoulder <NUM>, and the outer race <NUM> contains a shoulder <NUM>. In the depicted embodiment the rolling elements are balls <NUM> located between the inner race <NUM> and outer race In other embodiments the rolling elements could be rollers, e.g. tapered, barrel shaped or cylindrical rollers.

The solid arrows <NUM> indicate the load path from the outer race <NUM> to the inner race <NUM> during normal or no load conditions. As can be seen, the load is transmitted from the outer race <NUM> through balls <NUM> to the inner race <NUM>. In such state, the bearing has a low friction torque typical for the bearing type. During extreme loading conditions the load path is changed to pass from outer race <NUM> directly to inner race <NUM>, substantially bypassing bearing <NUM>.

Wind turbine components are generally designed for two major criteria "extreme" and "normal" wind loads. Normal loads can be determined where there is a mean and some fluctuation around it. This produces "useful" power, and components are initially designed (screened) for fatigue and cumulative damage against this repetitive loading. In addition, components are also designed for extreme loads. One definition of extreme loads in the wind turbine field is defined as the <NUM>-Year Gust (e.g., Ve50). Components have to withstand this "one time" each <NUM> years. For example, the <NUM>-Year Gust could be the highest expected velocity of wind expected over a fifty year time period. Fatigue (i.e., repetitive loads) is not necessarily a criterion in this case. Different design rules can be used to address extreme loading scenarios, for example, that the ultimate tensile stress in the material should not be exceeded.

As defined in the IEC <NUM>-<NUM> wind turbine design/safety standard, the largest wind speed to be considered is called "Ve50", which is the maximum gust over a <NUM>-year return period for a <NUM>-second averaging time. Extreme loads can occur during a Ve50 situation. When extreme loads occur the alternate or extreme load path, according to the present invention, is utilized to support them at least partially.

In a wind turbine application, normal loading could take place if the wind is at or below the cut-out speed and is flowing at a substantially constant rate past the blades. An extreme loading condition might occur when a sudden gust of wind appears and places a higher load on the wind turbine components than during a normal loading condition. Extreme loads are often of a shorter duration than normal loads. The term "extreme load" can be defined as any load in excess of normal operating load or any load that puts a large amount of stress on bearing <NUM>. Typically, extreme loads can be caused by gusts of wind and/or wind speeds greater than the rated wind speed or cutout wind speed, or during a Ve50 situation.

The shoulders <NUM> and <NUM> of the inner and outer race are not in contact during normal operating conditions. However, during extreme loading conditions the inner <NUM> and outer <NUM> shoulders make contact with each other and effectively protect bearing <NUM> from excess deformation and/or damage. The dashed arrows <NUM> illustrate the load path during extreme loading. As one example, the blade of a wind turbine could be attached to the outer race <NUM> and the hub could be attached to the inner race <NUM>. In another example the nacelle bedplate could be attached to inner race <NUM> and the tower could be attached to outer race <NUM>.

<FIG> illustrate another embodiment of the present invention, in which the alternate load path for extreme loads makes use of other bearing hardware. The bearing spacing elements <NUM> can be manufactured with an outer diameter slightly smaller than the bearing's <NUM> diameter. During normal loading, the load path <NUM> is from the outer race <NUM> through the balls <NUM> to inner race <NUM>. During extreme loads, the inner and outer races are forced closer to each other, and the spacing elements <NUM> become part of the extreme load path. The extreme load path <NUM> is from the outer race <NUM>, through the bearing spacing elements <NUM> to inner race <NUM>. In this manner, the balls <NUM> can be protected from damage due to extreme loads.

<FIG> illustrate another embodiment of the present invention. In bearings with a cage, the cage can become a load-carrying member during extreme loads.

Effectively, the cage <NUM> functions in similar fashion as the bearing spacing elements of <FIG>. The balls <NUM> are at least partially contained within cage <NUM>, and both balls <NUM> and cage <NUM> are placed between the inner race <NUM> and outer race <NUM>. During normal loads, the load path <NUM> is from the outer race <NUM> through balls <NUM> to inner race <NUM>. During extreme loads, the alternate load path <NUM> is from the outer race <NUM> through cage <NUM> to inner race <NUM>, Typically, the cage <NUM> can be comprised of a plurality of spacing elements connected together via one or more circumferential rings. Shoulder <NUM> can be placed on the inner race <NUM> (as shown) and/or on the outer race <NUM>. The shoulder can be placed above and/or below the cage <NUM> as well.

<FIG> illustrates another embodiment of the present invention, which is an improvement to the type of bearing illustrated in <FIG>. Inner race <NUM> incorporates a pocket <NUM>. Outer race <NUM> includes a projection <NUM>, which extends at least partially into pocket <NUM>. Two series of balls <NUM> are placed above and below projection <NUM>. During normal loads, the load path is from the outer race <NUM> and projection <NUM> through balls <NUM> to inner race <NUM>. During extreme loads, the alternate load path is from the outer race <NUM>, projection <NUM> through pocket <NUM> to inner race <NUM>.

Test data was obtained on a <NUM> MW wind turbine, and the pitch bearing showed a deflection of about <NUM> to about <NUM> at extreme loads. Operating loads were in the range of about <NUM>,<NUM> kNm or less. As one example only, if the shoulders were spaced about <NUM> from each other, then the load on the balls and raceway system (i.e., the pitch bearing) would be reduced by about half. It is to be understood that the shoulders could be spaced from each other by more or less than the amount in the previous example, and the spacing is determined by the requirements of the specific application.

The incorporation of alternate load paths for extreme loads in a bearing has many advantages. The size of the bearing can be reduced, and this translates into lower cost and reduced weight. Towers must support heavy loads, and reducing weight at the top of the tower or in the nacelle is highly beneficial. The bearing can also sustain extreme loads with a reduced rate of failure. Bearings that last longer save in maintenance and replacement costs, as well in avoidance of downtime for the wind turbine.

The shoulders referred to in the description above are preferably enclosed within the lubricated zones of the bearing. In this way a clean contact is provided, preventing dirt to clog the narrow gap. When designed so, a shoulder contact can also be created outside the lubricated zone, accepting possible dirt clogging the gap and interacting during the higher operating loads. Alternatively, a double seal system may be applied where the first and inner set of seals keeps the lubricant in and a second set of seals keeps the dirt out.

<FIG> illustrates another embodiment of the present invention, which incorporates an additional series of rolling elements. A first race <NUM> incorporates an extension <NUM>. Rolling elements <NUM> are placed between the first race <NUM> and the second race <NUM>. An additional series of rolling elements <NUM> are placed between the first race extension <NUM> and the second race <NUM>. During extreme loads, at least a portion of the extreme loads are diverted away from rolling elements <NUM> and directed from extension <NUM>, through rolling elements <NUM> to second race <NUM>. The rolling elements are designed so, that during normal operating load conditions, no load is transferred from extension <NUM> through rolling elements <NUM> to second race <NUM>. The rolling elements <NUM> are slightly undersized with respect to the cooperating races; a cage or spacing element could be incorporated as well. The rolling elements <NUM> are balls. In other embodiments the rolling elements could be rollers, e.g. tapered, barrel shaped or cylindrical rollers.

<FIG> illustrates another embodiment of the present invention, which incorporates a ribbon or spring <NUM> (e.g., a ripple spring). In one embodiment of the present invention spring <NUM> could be a thin metallic material formed into a "ribbon" or stamped into a wavy pattern. For example the thin metallic material could be a metal alloy such as a nickel-steel alloy. The "ribbon" or spring <NUM> could have a non-linear load-deflection curve, which is soft at first, but then would provide a very high reaction after deflecting to a flatter configuration. As one example, the spring <NUM> could be placed on extension <NUM> and, if desired (but not required), a low friction material could be placed on an opposing surface. During extreme loads, at least a portion of the extreme loads are diverted away from rolling elements <NUM> and directed from extension <NUM>, through spring <NUM> to second race <NUM>. The spring <NUM> is designed to not contact second race <NUM> during normal loads, so that no load is normally transferred from extension <NUM> through spring <NUM> to second race <NUM>. It is to be understood that spring <NUM> could be attached to the extension <NUM> and/or the second race <NUM>. The spring <NUM> could also be made from one or more of metallic, sintered metallic, plastics, and reinforced plastic material.

The alternate load path may also comprise layers of lubricating material or a low friction material (e.g., Teflon®, a registered trademark of E. du Pont de Nemours and Company) on one or both of the load bearing surfaces. One or more load-bearing surface (during extreme loads) may also have material formed into specific shapes to help absorb the extreme loads. The sliding or rolling elements of the bearing may also be chosen from one or more of metallic, sintered metallic, plastics, reinforced plastic material.

The first load path has low friction, low load carrying capacity and a first stiffness level, whereas the alternate, second load path could have higher friction, higher load carrying capacity and has a greater stiffness level (as compared to the first load path). The higher stiffness in the second load path could be obtained by having a shape or material difference between the two load paths. As one example, the second load path could include a low friction coating and/or a rippled material.

Claim 1:
A bearing (<NUM>) having a first race (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), a second race (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and rolling elements (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>), wherein said bearing (<NUM>) is a pitch bearing or a yaw bearing for a wind turbine, said bearing (<NUM>) comprising:
a load path (<NUM>, <NUM>, <NUM>) for normal loads, the load path for normal loads (<NUM>, <NUM>, <NUM>) transmitting load from the first race (<NUM>, <NUM>, <NUM>,<NUM>,<NUM>) through the rolling elements (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) to the second race (<NUM>); and
an alternate load path (<NUM>) for extreme loads, said alternate load path being formed between said first race (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and said second race (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and having a higher stiffness, wherein said alternate load path (<NUM>, <NUM>, <NUM>) during normal loading conditions transmits no load and during extreme loading conditions diverts at least a portion of said extreme loads from said rolling elements (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>),