Abstract:
A lubrication system is disclosed for a bearing assembly, the bearing assembly having at least one bearing. The lubrication system may comprise a pump circuit, an injection circuit, an extraction circuit and at least one three-way valve. The pump circuit may comprise at least a pump. The injection circuit may provide lubricant to the bearing during an injection mode of operation. The extraction circuit may extract lubricant from the bearing during an extraction mode of operation. A flow combination topology of the first three-way valve may allow operation of the lubrication system in the injection mode or in the extraction mode or in both modes simultaneously.

Description:
[0001]    This application claims the benefit of European Patent Application no. 13382004.3 filed on Jan. 10, 2013 and U.S. Provisional Patent Application Ser. No 61/776,552 filed on Mar. 11, 2013, the disclosures of which are hereby incorporated by reference in their entirety for all purposes. 
         [0002]    The present invention relates to bearings and more specifically to lubrication systems for bearing assemblies. 
       BACKGROUND ART 
       [0003]    Large bearings, and particularly large rolling bearings, are typically submitted to heavy loads. Such heavy loads may be imposed to these bearings either in stress conditions or at rest. Although each bearing may be appropriately selected and dimensioned for a particular heavy load application, proper lubrication may generally be regarded as an important factor for its overall performance during the bearing&#39;s estimated service life. 
         [0004]    Although rolling bearings often work well in non-ideal conditions, sometimes minor problems may cause bearings to fail quickly and unpredictably. For example, under a stationary (non-rotating) load, small vibrations can gradually press out the lubricant (or grease) between the races and rollers or balls of the bearing. This situation is known as false brinelling. To avoid false brinelling, accurate and local lubrication is required, particularly in the area where the load transmission between rolling elements and raceways takes place. The quality of the lubricant is also of utmost importance in order to prevent failure modes. As a consequence, old lubricant needs to be removed at a regular basis in order to maintain sufficient lubrication properties. 
         [0005]    Another factor that affects performance of a bearing is the volume of lubricant in the bearing. Typically, bearings are delivered filled with their corresponding lubricant at an ideal percentage, which in some applications may be about 60% of their free volume. Regular lubrication cycles intend to maintain the volume of lubricant inside the bearing close to that ideal percentage. However, after a significant operating time, the volume of lubricant inside the bearing may divert from the ideal percentage. 
         [0006]    If the volume of lubricant falls short of the ideal percentage, then the hydro dynamic lubricant layer may become thin or disappear in certain points or areas. Consequently, the friction at the load transmission areas may increase and so does the risk of wear initiation. Furthermore, the increased friction increases in turn the temperature of the remaining lubricant. Thus, the lubricant may suffer accelerated aging damage which may affect its performance. In extreme cases, damaged additives, soaps and oil may even accelerate the initiation of fatigue signs inside the bearing. 
         [0007]    On the other hand, bearings working with a level of lubricant above the ideal level may be exposed to local excess pressure inside the component due to the combined effects of accumulation of lubricant close to the bearing inlets and rolling motion of the balls or rollers in the same area. An overpressure inside a bearing may lead to failure of the sealing means. For example, in case of 4 points contact slew bearings, rubber seals may pop out of their housing and leave the component exposed to lubricant leakage and external agent contamination. This last consequence may be critical for the service life of bearings since it may generally lead to indentation effects or corrosion of the raceways in case water or solid contaminants enter inside the bearing. The final effect may be the requirement for substitution of the component well before its expected service life. 
         [0008]    The above problems are of particular importance in large bearings in the wind energy sector. Wind turbine generators employ large bearing assemblies, such as the main, pitch and yaw bearing assemblies that are subject to particularly heavy loads. Furthermore, the orientation of the bearings, with respect to, for example, the wind turbine rotor blade, may lead to high concentrations of lubricant in certain areas during periods of low or no wind. For example, in a pitch bearing assembly of a wind turbine generator, certain zones of the bearings may suffer from lubricant concentration and the bearings may potentially be affected by an internal overpressure at these zones. One such zone may be the bearing zones in the rotor plane at the trailing edge of the blades of the wind turbine. A combination of gravity and centrifugal forces may cause concentration of lubricant and overpressure in the affected zones. 
         [0009]    To mitigate these problems, manufacturers of lubricant systems have developed specific products with the aim of bringing fresh lubricant to the bearing at reduced intervals. Control systems of lubrication systems range from local fixed intervals lubrication control solutions to external lubrication control solutions. In the latter case, controllers may also manage when and how much lubricant is applied to the bearing. 
         [0010]    With respect to the lubricant recovery, some bearings&#39; designers suggest closed outlets for keeping the lubricant inside the bearing and a passive extraction of the lubricant only during maintenance operations. In some alternative solutions, individual deposits may be fixed to the bearing outlets for a continuous passive recovery of the lubricant. These deposits may be emptied or replaced during maintenance operations. 
         [0011]    Finally, active systems have been developed for the used lubricant recovery. In some cases, lubricant may be extracted from the bearing outlets, while in others a small intermediate deposit may be connected to the bearing outlet with a suction element which extracts the lubricant from these deposits. 
         [0012]    In wind turbine generators, the most advanced existing solutions offer control of the lubricant injection only, depending on the working conditions of the bearing. In these cases, the lubricant extraction from the bearing is carried out by active systems which depend on the lubricant injection. 
         [0013]    However, although the existing lubrication control systems may be able to monitor precisely the amount of injected lubricant, the amount of remaining lubricant inside the bearing remains out of their scope. Furthermore, the distribution of lubricant inside the bearing remains unknown. 
       SUMMARY OF THE INVENTION 
       [0014]    There is a need for a new lubrication system that at least partially resolves some of the above mentioned problems. It is an object of the present disclosure to fulfil such a need. 
         [0015]    In a first aspect, a lubrication system is disclosed for a bearing assembly, the bearing assembly having at least one bearing. The lubrication system may comprise a pump circuit, an injection circuit, an extraction circuit and at least a first three-way valve. The pump circuit may comprise at least a pump. The injection circuit may provide lubricant to the bearing during an injection mode of operation. The extraction circuit may extract lubricant from the bearing during an extraction mode of operation. The first three-way valve may have a first port coupled to the pump circuit, a second port to the lubricant injection circuit and a third port to the lubricant extraction circuit. A flow combination topology of the first three-way valve may allow operation of the lubrication system in the injection mode or in the extraction mode or in both modes simultaneously. The three-way valves may be T-shaped 3-way valves to allow operation of the lubrication system in either or both modes. 
         [0016]    One aspect of the proposed solution is that the lubricant injection circuit may be dissociated or decoupled from the lubricant extraction circuit. Therefore any discrepancy between injection and extraction of lubricant may be compensated after each lubrication cycle. 
         [0017]    In some embodiments, the lubrication system may comprise a first monitoring element and/or a second monitoring element, coupled to the injection circuit and/or the extraction circuit, respectively, for measuring the volume of lubricant injected and/or extracted in the bearing during a lubrication cycle. The first three-way valve may be operable in response to a measurement of the monitoring elements. The first three-way valve may define a passageway between the pump and the extraction circuit when a monitoring element indicates a lubricant volume above a desired value and a passageway between the pump and the injection circuit when a monitoring element indicates a volume below a desired value. 
         [0018]    The purpose of the monitoring elements is to provide information about the quantity of lubricant injected to, and about the quantity of lubricant extracted from, the bearing during an injection, extraction or combined injection-extraction mode. A central control system may receive this information and act accordingly on the lubrication system. Thanks to the monitoring elements, the central control system may be able to keep a balance between the injected and extracted volumes of lubricant. Moreover, since the central control system may be aware of the initial volume of lubricant inside the bearing (as this parameter is part of the quality control plan of the component at the manufacturing warehouse), it may also be able to inform about possible deviations in regards to under- or over-filling. As a result, failure modes of operation of the respective bearing may be prevented. 
         [0019]    In some embodiments, the lubrication system may comprise a local pressure measuring instrument, coupled to the bearing, for measuring the pressure of lubricant at a proximal point of the bearing. The extraction circuit may then include a first extraction sub-circuit and a second extraction sub-circuit. The first extraction sub-circuit may be coupled to the bearing substantially at or near the proximal point. The first extraction sub-circuit may be selectively operated in response to a lubricant pressure measurement by the local pressure measuring instrument at the proximal point that is above a desired value. Accordingly, the injection circuit may include a first injection sub-circuit and a second injection sub-circuit. The first injection sub-circuit may be coupled to the bearing substantially at or near the proximal point. The second injection sub-circuit may be connected to the bearing further away from the proximal point. The first injection sub-circuit may be selectively operated in response to a lubricant pressure measurement by the local pressure measuring instrument at the proximal point that is below a desired value. 
         [0020]    The main advantage of the local pressure measuring instrument is that, apart from controlling the overall volume of lubricant inside the bearing, the lubrication system may also control the distribution of lubricant within the bearing. This may help avoid local lubricant concentrations and, consequently, any sealing failures at the affected points. This is also particularly advantageous in pitch bearing assemblies of wind turbine generators, where specific local lubricant accumulation points may appear due to the topology of the pitch bearing assembly and due to the centrifugal forces subject to the rotation of the wind turbine rotor. These points may be the points at the cross-section of the pitch bearings plane with the rotor plane. 
         [0021]    In some embodiments the monitoring elements may be flow meters and/or manometers. A flow-meter may be connected to a recovery line in order to inform the central control system about possible failures in the recovery line, such as leakage or line breakage. The flow-meter may also deliver information about the used lubricant volume extracted from the bearing. This information may be more reliable than the calculation of the volume of extracted lubricant from the bearing depending on the number of lubrication cycles and the metering of the extraction devices. Accordingly, the central control system may be informed about the volume of lubricant injected thanks to the accurate metering of the injection devices which deliver always the same volume of lubricant at each injection cycle. 
         [0022]    By accurately measuring the volume of lubricant injected and extracted it is possible to predict with great accuracy the amount of lubricant present inside the bearing at any given moment. Therefore, should there be any discrepancy between the expected volume and the measured volume, it is possible to take early corrective measures, such as provide extra injection cycles or extra extraction cycles. 
         [0023]    In some embodiments, the injection circuit and the extraction circuit may be coupled to a plurality of bearings. The lubricant injection circuit may include a plurality of injection devices. Each injection device may be coupled to one bearing of the plurality of bearings, respectively. Each injection device may comprise a plurality of injectors. Each injector may be coupled to the respective bearing at a different point. Accordingly, the extraction circuit may include a plurality of extraction devices. Each extraction device may be coupled to one bearing of the plurality of bearings, respectively. Each extraction device may comprise a plurality of extractors. Each extractor may be coupled to the respective bearing at a different point. 
         [0024]    In some embodiments, the injection devices may be arranged in series in the injection circuit. Accordingly, the extraction devices may be arranged in series in the extraction circuit, respectively. In these embodiments, the lubrication system may further comprise a second three-way valve coupled to the pump. The first three-way valve may be coupled to the first injection device and to the first extraction device in the series, respectively, and the second three-way valve may be coupled to the last injection device and to the last extraction device in the series, respectively. The advantage of having two 3-way valves in a circuit is that a more homogenous pressure may be achieved throughout the lubrication circuit during a lubrication cycle. 
         [0025]    In some embodiments the 3-way valve may be manual and in other automatic, such as an electro-valve. In case of manual valves, the central control system may deliver to the maintenance staff information regarding the volume of lubricant inside the bearing, as well as a recommendation of used lubricant to be extracted or fresh lubricant to be injected. In case of electro valves, the disassociation of the injection and extraction circuits may be performed automatically on demand of the central control system. The corresponding operations of lubricant injections or extractions required to keep the volumes balanced may also be performed by the central control system. In embodiments where a plurality of valves is present, some of the valves may be automatic while others may be manual. 
         [0026]    In both cases, manual or automatic, the lubrication system may be able to operate extra injection or extraction cycles in order to keep the volume of lubricant balanced inside the bearing. This may be possible due to the respective volumes registered by the central control system. 
         [0027]    In another aspect, a pitch bearing assembly for a wind turbine generator is disclosed. The pitch bearing assembly may comprise three bearings and a lubrication system according to any of the embodiments described above. The advantage of having a dissociated lubrication system in a pitch bearing assembly is that the volume and distribution of lubricant in the respective bearings may be better controlled, thus increasing the durability of the bearings and reducing the maintenance and repair costs. As a result, the service life of the pitch bearings may be extended which may significantly reduce the maintenance costs of the overall wind turbine generator. 
         [0028]    In yet another aspect, a wind turbine generator is disclosed comprising a bearing assembly according to any of the embodiments described above. 
         [0029]    Additional objects, advantages and features of embodiments of the invention will become apparent to those skilled in the art upon examination of the description, or may be learned by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Particular embodiments of the present invention will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which: 
           [0031]      FIG. 1  is a block diagram of a bearing assembly according to an embodiment; 
           [0032]      FIG. 2  is a block diagram of a bearing assembly according to another embodiment; 
           [0033]      FIG. 3  is a block diagram of a bearing assembly according to another embodiment; 
           [0034]      FIG. 4  is a block diagram of a bearing assembly according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0035]      FIG. 1  is a block diagram of a bearing assembly according to an embodiment. Bearing assembly  200  may include three bearings  205 ,  205 ′ and  205 ″ and lubrication system  210 . Lubrication system  210  may include pump circuit  220 , injection circuit  230 , extraction circuit  250  and  3 -way T-shaped valves  270 ,  275 . Each of the  3 -way valves  270 ,  275  may have a first port A, A′ coupled to the pump circuit  220 , a second port B, B′ coupled to the injection circuit  230  and a third port C, C′ coupled to the extraction circuit  250 . 
         [0036]    Pump circuit  220  may include pump  222  and main line  225 . Main line  225  may have two branches,  225 A and  225 A′, one coupled to first port A of valve  270  and the other coupled to first port A′ of valve  275 . Each of the two branches  225 A and  225 A′ may be split in an injection branch  235  and an extraction branch  255 . The main line  225 , and consequently, the injection branch  235  and the extraction branch  255 , are used to supply pressure to the injection circuit  230  and to the extraction circuit  250 , respectively. 
         [0037]    Injection circuit  230  may include injection branch  235  and three injection devices  240 ,  240 ′,  240 ″. The injection branch  235  supplies pressure to the injection devices  240 ,  240 ′,  240 ″. The injection branch  235  may be connected at one end to port B of valve  270  and at the other end to port B′ of valve  275 . The injection devices  240 ,  240 ′ and  240 ″ may be distributed along the injection branch  235  at or near their respective bearings  205 ,  205 ′ and  205 ″. Each injection device  240 ,  240 ′ and  240 ″ may comprise a lubricant deposit (not shown), a plurality of injection lines  245 - 1  to  245 -n,  245 ′- 1  to  245 ′-n and  245 ″- 1  to  245 ″-n, and a pumping element, respectively, for pumping lubricant to the bearing. Injection circuit  230  may further include a pressure measuring device  247 , such as a manometer, for measuring the pressure of the lubricant at a point along the injection branch  235 . 
         [0038]    Extraction circuit  250  may include extraction branch  255  and three extraction devices  260 ,  260 ′,  260 ″. Extraction branch  255  may be coupled at one end to port C of valve  270  and at the other end to port C′ of valve  275 . The extraction devices  260 ,  260 ′ and  260 ″ may be distributed along the extraction branch  255  at or near their respective bearings  205 ,  205 ′ and  205 ″. Each extraction device  260 ,  260 ′ and  260 ″ may comprise a plurality of extraction lines  265 - 1  to  265 -n,  265 ′- 1  to  265 ′-n and  265 ″- 1  to  265 ″-n, respectively, for extracting lubricant from their respective bearing  205 ,  205 ′,  205 ″. Each extraction device  260 ,  260 ′ and  260 ″ may also be connected to a flow-meter  262 ,  262 ′ and  262 ″, respectively, for measuring the volume of lubricant extracted from the respective bearing. Extraction circuit  250  may further include a recovery tank  268  for collecting all extracted lubricant via a recovery line  267 . Extraction circuit  250  may further include a pressure measuring device  257 , such as a manometer, for measuring the pressure of the lubricant at a point along the extraction branch  255 . 
         [0039]    The 3-way valves  270 ,  270 ′ may assume 4 distinct positions. In a first position, the ports B, B′ and C, C′ may be closed and no injection or extraction of lubricant takes place. In a second position, the ports B, B′ and C, C′ may all be open and the lubrication system may simultaneously inject and extract lubricant. In a third position, ports B, B′ are open and C, C′ are closed, and in this situation, the lubrication system may only inject lubricant to the bearings. In a fourth position, ports B, B′ are closed and ports C, C′ are open, and thus the lubrication system may only extract lubricant from the bearings. 
         [0040]    In a typical scenario, the 3-way valve moves from the previously mentioned first position to the previously mentioned second position initiating a lubrication cycle. The pump circuit may be equipped with a deposit of fresh lubricant and an electrical motor which actuates pumping elements and may provide lubricant at a given pressure to the injection circuit. An integrated control system may deliver to the wind turbine generator control information about the amount of fresh lubricant inside the deposit and the pressure at the pump inlet during a lubrication cycle. The WTG control may interact with the pump and start a lubrication cycle on demand. The lubrication line may be connected to the pump and feed the injection devices of the injection circuit with fresh lubricant. It may also simultaneously actuate the extraction devices. The injection devices may deliver a metered quantity of fresh lubricant to the bearing through the injection lines after finalizing each pressuring cycle in the main line where they are connected. The extraction devices may extract the same metered quantity of used lubricant from the bearing and bring it through the extraction circuit to the used lubricant deposit. Once the lubrication cycle is completed, the flow meters and the pressure meters communicate their values to a central lubrication monitoring centre. In case all communicated values are within limits, no action takes place and the 3-way valve assumes the first position until the next lubrication cycle. In case at least a value is off limits then a selective injection or extraction takes place. 
         [0041]    In one example scenario, the value that is off limits is the value of a monitoring element indicating that the volume of lubricant extracted during the lubrication cycle is more than the foreseen volume. In that case, more lubricant needs to be injected. The extra amount to be injected may be calculated as the absolute difference between the foreseen volume and the actual volume as counted by the flow meter. Once the extra amount has been calculated, the valve assumes the third position and an injection cycle takes place until the extra amount has been injected. Once the monitoring element confirms injection of the extra amount, the valve assumes the first position. 
         [0042]    In another example scenario, the value that is off limits is the value of a flow meter indicating that the volume of lubricant extracted during the lubrication cycle is less than the foreseen volume. In that case, more lubricant needs to be extracted. The extra amount to be extracted may be calculated as the difference between the foreseen volume and the actual volume as counted by the flow meter. Once the extra amount has been calculated, the valve assumes the fourth position and an extraction cycle takes place until the extra amount has been extracted. Once the flow meter confirms extraction of the extra amount, the valve assumes the first position. 
         [0043]      FIG. 2  is a block diagram of a bearing assembly according to another embodiment. Bearing assembly  300  may include one bearing  305  and lubrication system  310 . Lubrication system  310  may include a pump circuit and an injection circuit similar to the pump circuit  220  and the injection circuit  230  described with reference to  FIG. 1 . Furthermore, lubrication system  310  may include primary extraction sub-circuit  350 , secondary extraction sub-circuit  350 SE and 3-way T-shaped valves  370 ,  375 ,  370 SE and  375 SE. Each of the 3-way valves  370 ,  375  may have a first port A, A′ coupled to the pump circuit, a second port B, B′ coupled to the injection branch  335  and a third port C, C′ connected to a lubrication line that leads to a first port A, A′ of the 3-way pumps  370 SE,  375 SE, respectively. Each of the 3-way valves  370 SE,  375 SE may have a second port B, B′ coupled to the extraction branch  355 SE of the secondary extraction sub-circuit  350 SE and a third port C, C′ connected to the primary extraction sub-circuit  350 . 
         [0044]    Primary extraction sub-circuit  350  may include extraction branch  355  and an extraction device  360 . Extraction branch  355  may be connected at one end to a lubrication line that leads to port C of valve  370 SE and at the other end to port C′ of valve  375 SE. The extraction device  360  may comprise a plurality of extraction lines for removing lubricant from bearing  305 . The extraction device  360  may also be connected to a flow-meter, as discussed with reference to  FIG. 1 , for measuring the volume of lubricant extracted from the respective bearing. Primary extraction sub-circuit  350  may further include a pressure measuring device  357 , such as a manometer, for measuring the pressure of the lubricant at a point along the extraction branch  355 . 
         [0045]    Now, secondary extraction sub-circuit  350 SE may have an extraction branch  355 SE and an extraction device  360 SE. Extraction branch  355 SE may be coupled at one end to port B of valve  370 SE and at the other end to port B′ of valve  375 SE. The extraction device  360 SE may comprise a plurality of extraction lines for extracting lubricant from bearing  305 . The extraction device  360  may also be connected to a flow-meter, as discussed with reference to  FIG. 1 , for measuring the volume of lubricant extracted from the respective bearing. Secondary extraction sub-circuit  350 SE may further include a pressure measuring device  357 SE, such as a manometer, for measuring the pressure of the lubricant at a point along the extraction branch  355 SE. 
         [0046]    The secondary extraction sub-circuit may be connected to the bearing substantially at or near a point on the bearing where concentration of lubricant is anticipated. In that way, should the device  357 SE measure an increased pressure, it may be possible to selectively operate only the secondary lubricant extraction sub-circuit by appropriately orientating the 3-way valves. For example, if there is a need to only remove lubricant from a pressure point, the 3-way valves  370  and  375  shall have the vertical axis of the T-shape facing outwards, while the 3-way valves  370 SE and  375 SE shall have the vertical axis of the T-shape facing towards the A, A′ points. The term “vertical axis” is used to denote the perpendicular bisector line in a T-shape. Therefore the pump circuit may be connected only with the secondary extraction sub-circuit  355 SE and only extraction device  360 SE may be operable. 
         [0047]      FIG. 3  is a block diagram of a bearing assembly according to another embodiment. Bearing assembly  400  may include three bearings  405 ,  405 ′,  405 ″ and lubrication system  410 . Lubrication system  410  may include a pump circuit and an injection circuit similar to the pump circuit  220  and the injection circuit  230  described with reference to  FIG. 1 . Furthermore, lubrication system  410  may include primary extraction sub-circuit  450 , secondary extraction sub-circuit  450 SE and 3-way T-shaped valves  470 ,  475 ,  470 SE and  475 SE. Each of the 3-way valves  470 ,  475  may have a first port A, A′ connected to the pump circuit, a second port B, B′ coupled to the injection branch  435  and a third port C, C′ connected to a lubrication line that leads to a first port A, A′ of the 3-way pumps  470 SE,  475 SE, respectively. Each of the 3-way valves  470 SE,  475 SE may have a second port B, B′ coupled to the extraction branch  455 SE of the secondary extraction sub-circuit  450 SE and a third port C, C′ connected to the primary extraction sub-circuit  450 . 
         [0048]    Primary extraction sub-circuit  450  may have an extraction branch  455  and three extraction devices  460 ,  460 ′,  460 ″ distributed along the extraction branch  455 , each device for extracting lubricant from a bearing, respectively. Extraction branch  455  may be connected to a lubrication line that leads at one end to port C of valve  470 SE and at the other end to port C′ of valve  475 SE. A pressure measuring device  457  may monitor the lubricant pressure along the extraction branch  455 . 
         [0049]    Now, secondary extraction sub-circuit  450 SE may have an extraction branch  455 SE and three extraction devices  460 SE,  460 SE′,  460 SE″ distributed along the extraction branch  455 SE. Each of the extraction devices  460 SE,  460 SE′,  460 SE″ shall be connected at or near their corresponding bearing  405 ,  405 ′,  405 ″, respectively, at a point where lubricant concentration may take place or where lubricant pressure tends to be higher. Extraction branch  455 SE may be connected at one end to a lubrication line that leads to port B of valve  470 SE and at the other end to port B′ of valve  475 SE. The extraction devices  460 SE,  460 SE′,  460 SE″ may also be connected to a flow-meter, as discussed with reference to  FIG. 1 , for measuring the volume of lubricant extracted from the respective bearing. Secondary extraction sub-circuit  450 SE may further include a pressure measuring device  457 SE, such as a manometer, for measuring the pressure of the lubricant at a point along the extraction branch  455 SE. 
         [0050]    The secondary extraction sub-circuit may be connected to the bearing substantially at or near a point on the bearing where concentration of lubricant is anticipated. In that way, should the device  457 SE measure an increased pressure, it may be possible to selectively operate only the secondary lubricant extraction sub-circuit by appropriately orientating the 3-way T-valves valves. For example, if there is a need to only remove lubricant from pressure points of the bearings, the 3-way valves  470  and  475  shall have the vertical axis of the 3-way valves&#39; T-shape facing outwards, while the 3-way valves  470 SE and  475 SE shall have the vertical axis of their T-shape facing towards the A, A′ points. Therefore the pump circuit may be connected only with the secondary extraction sub-circuit  455 SE and, thus, only extraction devices  460 SE,  460 SE′,  460 SE″ may be operable. It should be noted that it may also be possible to provide extra manometers and extra 3-way valves along the secondary lubricant extraction sub-circuit  455 SE, so that pressure may be measured at each bearing and the lubricant extraction may selectively be performed at one, two or all the bearings at any given moment. 
         [0051]      FIG. 4  is a block diagram of a bearing assembly according to yet another embodiment. Bearing assembly  500  may include three bearings  505 ,  505 ′,  505 ″ and lubrication system  510 . Lubrication system  510  may include a pump circuit similar to the pump circuit  220  described with reference to  FIG. 1 , and an extraction circuit similar to the extraction circuit discussed with reference to 
         [0052]      FIG. 3 . Furthermore, lubrication system  510  may include a primary injection sub-circuit  530 , a secondary injection sub-circuit  530 SI and 3-way T-shaped valves  570 ,  575 ,  570 SI,  575 SI,  570 SE and  575 SE. Each of the 3-way valves  570 SI,  575 SI may have a first port A, A′ coupled to the pump circuit, a second port B, B′ connected to the secondary injection branch  535 SI and a third port C, C′ connected to a lubrication line that leads to a first port A, A′ of the 3-way valves  570 ,  575 , respectively. Each of the 3-way valves  570 ,  575  may have a second port B, B′ coupled to the primary injection branch  535  of the primary injection sub-circuit  530  and a third port C, C′ connected to a lubrication line that leads to the first port A, A′ of the 3-way valves  570 SE,  575 SE′, respectively. The second port B, B′ of the 3-way valves  570 SE,  575 SE′, may be coupled to a secondary extraction sub-circuit and the third port C, C′ may be coupled to a primary extraction sub-circuit. The primary extraction sub-circuit may include extraction devices  560 ,  560 ′ and  560 ″. Accordingly, the secondary extraction sub-circuit may include extraction devices  560 SE,  560 SE′ and  560 SE″. 
         [0053]    Primary injection sub-circuit  530  may have an injection branch  535  and three injection devices  540 ,  540 ′,  540 ″ distributed along the injection branch  535 , each device for injecting lubricant to a bearing, respectively. Injection branch  535  may be connected at one end to a lubrication line that leads to port B of valve  570  and at the other end to port B′ of valve  575 . A pressure measuring device  547  may monitor the lubricant pressure along the injection branch  535 . 
         [0054]    Now, secondary injection sub-circuit  540 SI may have an injection branch  535 SI and three injection devices  560 SI,  560 SI′,  560 SI″ distributed along the injection branch  535 SI. Each of the injection devices  560 SI,  560 SI′,  560 SI″ shall be connected at or near their corresponding bearing  505 ,  505 ′,  505 ″, respectively, at a point where lubricant shortage may take place or where lubricant pressure tends to be low. Injection branch  535 SI may be connected at one end to a lubrication line that leads to port B of valve  570 SI and at the other end to port B′ of valve  575 SI. Secondary injection sub-circuit  450 SE may further include a pressure measuring device  547 SI, such as a manometer, for measuring the pressure of the lubricant at a point along the injection branch  535 SI. 
         [0055]    The secondary injection sub-circuit may be connected to the bearings substantially at or near a point on the bearings where shortage of lubricant is anticipated. In that way, should the device  547 SI measure a reduced pressure, it may be possible to selectively operate only the secondary lubricant injection sub-circuit by appropriately orientating the 3-way T-shape valves. For example, if there is a need to only inject lubricant to low pressure points of the bearings, the 3-way valves  570 SI and  575 SI shall have the vertical axis of the 3-way valves&#39; T-shape facing towards the A, A′ points. Therefore the pump circuit may be connected only with the secondary injection sub-circuit  530 SI and, thus, only injection devices  560 SI,  560 SI′,  560 SI″ may be operable. It should be noted that it may also be possible to provide extra manometers and extra 3-way valves along the secondary lubricant injection sub-circuit  530 SI, so that pressure may be measured at each bearing and the lubricant injection may selectively be performed at one, two or all the bearings at any given moment. 
         [0056]    Although only a number of particular embodiments and examples of the invention have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof are possible. Furthermore, the present invention covers all possible combinations of the particular embodiments described. Thus, the scope of the present invention should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.