Abstract:
A bearing comprises a Bragg grated optical fiber to measure one or more parameters of the bearing. The optical fiber is coupled to the bearing in such a way that at least a part of the optical fiber that comprises Bragg grating is curved. The grating which is curved is adapted to the curvature in such a way that the frequency response is well defined and not spread out. This is achieved by arranging the gratings such that even though the fiber is curved the gratings appear to be substantially parallel instead of being influenced by the curvature.

Description:
TECHNICAL FIELD 
       [0001]    The invention is related to condition monitoring of bearings, in particular in combination with load sensing. 
       BACKGROUND 
       [0002]    Bearings are a very important component in rotating machinery. If a bearing fails, then the complete functionality of the machinery usually also fails. In some applications it might be very difficult or just extremely expensive to replace a failed bearing outside regular scheduled maintenance. Such applications might be deep sea applications, cruise ships or other continuous manufacturing lines. In an attempt to predict when a bearing needs to be replace before it fails and suitable also in an orderly scheduled manner, condition monitoring is done. If the machinery and bearings are physically in a location which is easily accessible, then the condition of a bearing can be assessed by for example vibration measurement. Equipment which is not accessible, such as deep sea applications need other means to assess the condition of a bearing to be able to assess when maintenance is needed. There are many ways of remotely assess the condition of a bearing, however, there seems to still be room for improvement. 
       SUMMARY 
       [0003]    An object of the invention is to define a method and means to monitor the condition and load of a bearing. Load that a bearing is subjected to can influence the bearing life. 
         [0004]    Another object of the invention is to define a unit for monitoring the condition and measure the load of a bearing without any electrical power at the bearing. 
         [0005]    A further object of the invention is to define a Bragg grated optical fiber suitable for monitoring the condition and measuring the load of a bearing by measuring strain on the optical fiber. 
         [0006]    The aforementioned objects are achieved according to the invention by the use of a Bragg grated optical fiber to sense the condition of a bearing and the load of the bearing, the bearing comprising a groove to increase the sensitivity of the fiber to both load variations and vibrations of the bearing. 
         [0007]    The aforementioned objects are further achieved according to the invention by a bearing comprising a Bragg grated optical fiber having a plurality of grated areas, where physically adjacent grated areas are separated in the frequency plane. Suitably adjacent frequency bands correspond to grated areas on opposite sides of a bearing. 
         [0008]    The aforementioned objects are also achieved according to the invention by a Bragg grated optical fiber for attachment on a circular circumference of a bearing, where the gratings are made to be parallel when the optical fiber is attached to the bearing. 
         [0009]    The aforementioned objects are also achieved according to the invention by a bearing comprising a Bragg grated optical fiber for measuring one or more parameters of the bearing. The Bragg grated optical fiber is attached to the bearing such that at least one grating of the optical fiber is curved. According to the invention the grating of the Bragg grated optical fiber is adapted to the curvature in such a way that the influence on the frequency response due to the curvature is minimized. 
         [0010]    Preferably the gratings of the grating are substantially parallel when the optical fiber grating is curved. The gratings can be made before the optical fiber is attached to the bearing. The gratings are then made such that when the expected curvature of the optical fiber is done, the gratings are substantially parallel. The gratings can also be made when the optical fiber is attached to the bearing. The gratings are then being made such that they are substantially parallel when the optical fiber is mounted. In some embodiments the optical fiber has at least one flat surface, that is a non-circular cross section. This is to be able to define the orientation of the optical fiber and thereby attach the fiber to the bearing in a predetermined orientation. 
         [0011]    The aforementioned objects are also achieved according to the invention by a bearing that comprises a Bragg grated optical fiber to measure one or more parameters of the bearing. The optical fiber is coupled to the bearing in such a way that at least a part of the optical fiber that comprises Bragg grating is curved. The grating which is curved is adapted to the curvature in such a way that the frequency response is well defined and not spread out. This is achieved by arranging the gratings such that even though the fiber is curved the gratings appear to be substantially parallel instead of being influenced by the curvature. 
         [0012]    By providing a method and a unit for sensing strain by optical means, condition monitoring and load measurements can be calculated kilometers away from the bearing by the transmission of strain data by optical means. Thus there is no need to provide any electrical power at the bearing. Applications such as monitoring bearings of deep sea pumps, kilometers under the sea surface, is made possible in a reliable manner without any electrical power locally at the bearing. 
         [0013]    Other advantages of the invention will become apparent from the detailed description below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following figures, in which: 
           [0015]      FIG. 1  shows a side view of a bearing in a housing, a typical implementation of the invention, 
           [0016]      FIG. 2  shows a cross section of a part of a bearing according to one embodiment according to the invention, 
           [0017]      FIG. 3  shows a cross section of a part of a bearing according to another embodiment according to the invention, 
           [0018]      FIG. 4  shows a top view of a bearing according to the invention illustrating optical fiber access to measurement groove, 
           [0019]      FIG. 5A  shows a side sectional view of a bearing according to the invention illustrating an example of sensor distribution according to the invention around the bearing, 
           [0020]      FIG. 5B  shows the sensor frequency distribution according to the sensor placement of  FIG. 5A , 
           [0021]      FIG. 6A  shows a conventional optical fiber with Bragg grating, 
           [0022]      FIG. 6B  shows the frequency response of the optical fiber of  FIG. 6A , 
           [0023]      FIG. 7A  shows a conventional optical fiber with Bragg grating that is bent, such as around a circular bearing, 
           [0024]      FIG. 7B  shows the frequency response of the bent optical fiber of  FIG. 7A , 
           [0025]      FIG. 8A  shows a bent optical fiber with Bragg grating according to the invention, 
           [0026]      FIG. 8B  shows the frequency response of the optical fiber according to the invention of  FIG. 8A , 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    In order to clarify the inventions, some examples of its use will now be described in connection with  FIGS. 1 to 8B . 
         [0028]      FIG. 1  illustrates a side view of a bearing  110  mounted in a housing  120 . The bearing  110  comprises an outer  112 , non-rotating, ring, an inner  114 , rotating, ring with rolling elements  116  there between. This is a typical implementation of the inventions. An optical strain gauge, typically a Bragg grated optical fiber strain sensing unit, is according to the invention placed in a groove in the outer  112  non-rotating ring. The groove has to be large enough to accommodate the optical fiber, otherwise the bearing will not fit in the housing  120 . The groove also have to be large enough and placed such that the sensitivity of the strain sensors is increased. This is due to weakening of the outer ring  112 , such that for the same forces on the bearing, the bearing is deformed more thus putting a greater strain on the strain sensors. The groove must at the same time be small enough so that the integrity of the bearing is not jeopardized, that is if the groove is made too large, then the bearing will not be able to sustain its stated capacity. There is thus a compromise between these two conditions, which is also helped by clever placement of the groove. 
         [0029]      FIG. 2  illustrates a cross section of a part of a bearing according to one embodiment of the invention. The bearing is a two row SRB comprising two outer rings  212 ,  213 , each having a corresponding set of rolling elements  216 ,  217 . In this embodiment, each row comprises a corresponding groove  230 ,  231 . Each groove will have a width  235  and a height/depth  234 . Suitable each groove will have a slit  238 ,  239  for placement of the optical fiber. The placement of the groove  230 ,  231  and specifically the corresponding slit  238 ,  239 , is preferably along a load line  237  of each corresponding row. An optical fiber can go completely around a bearing, with strain sensing places, by means of grated section, suitably placed. That is one optical fiber comprising all the required grated sections. Alternatively, several optical fibers can be placed with their corresponding grated sections placed at different places. 
         [0030]      FIG. 3  shows a cross section of a part of a bearing, a CARB, according to another embodiment according to the invention. This embodiment only comprises one outer ring  312  with one row of rolling elements  316 , but multiple grooves  330 ,  331 ,  332 . 
         [0031]      FIG. 4  illustrates a top view of a bearing according to the invention illustrating optical fiber access means  440  to measurement grooves  430 ,  431  in its outer ring  412 . 
         [0032]      FIG. 5A  illustrates a side sectional view of a bearing  500  with a non-rotating outer ring  512 , rotating inner ring  514  with rolling elements  516  there between, according to the invention illustrating an example of sensor distribution  551 ,  552 ,  553 ,  554 ,  555 ,  556 ,  557 ,  558  according to the invention around the bearing. A Bragg grated optical fiber  550 , will comprise one or more strain sensors  551 ,  552 ,  553 ,  554 ,  555 ,  556 ,  557 ,  558 , each sensor defined by a grated section. Each grated section, as seen in detail below in relation to  FIG. 5B  will be represented in the frequency plane by a frequency  571 ,  572 ,  573 ,  574 ,  575 ,  576 ,  577 ,  578 , that will vary in dependence on the strain of the corresponding grated section. 
         [0033]    If a sensor is in the loaded zone  562 , then large variations  580  will result, sensors in the un-loaded zone will show small frequency variations, due to there being only small  584 , if any load and load variations. There are of course sensors in between, with medium variations  582 . According to the invention to use the fiber in an optimal way, to get in as many sensors as possible, without the frequency variations hitting each other, sensors are physically separated such that in the frequency spectrum adjacent sensor frequencies do not vary to a large 580 degree. Since a bearing will always have a loaded zone and an un-loaded zone on its opposite side, then sensor adjacent in frequency, will be placed physically on opposite sides of the bearing. 
         [0034]      FIG. 6A  shows a conventional optical fiber  690  with Bragg grating, arranged flat. It comprises a first strain sensor  651  with a first strain sensor grating separation DA  692 , and a second strain sensor  652  with a second strain sensor grating separation DB  696 . The gratings will be evenly spaced through a cross section of the optical fiber and thus create, as can be seen in  FIG. 6B  nice peak frequency responses  671 ,  672  from each sensor. 
         [0035]      FIG. 7A  illustrates a conventional optical fiber  790  with Bragg grating that is bent, such as around a circular bearing. It also comprises a first strain sensor  751  and a second strain sensor  752 . But as can be seen, when the fiber is bent, the inner side is compressed and the first strain sensor inside grating separation DA- 6   791  and the second strain sensor grating separation DB- 6   795  are both less than before. This in combination with the out side being stretched creating larger separation DA+δ  793 , DB+δ  797 , will create a varying separation across the fiber. This will create, as can be seen in  FIG. 7B , wide frequency responses  779  instead of peaks at the center frequencies  771 ,  772 . 
         [0036]      FIG. 8A  shows a bent optical fiber  890  with Bragg grating arranged in a first  851  and second  852  strain sensor, according to the invention. An optical fiber with Bragg grating according to the invention, will have an equal grating separation DA  892 , DB  896  through the fiber, when the fiber is bent, such as around a circular bearing. As can be seen in  FIG. 8B , we achieve the desired peak frequency responses  871 ,  872 , when the fiber is bent. The grating could be added after mounting of the fiber on the bearing. 
         [0037]    The invention is not restricted to the above-described embodiments, but may be varied within the scope of the following claims. 
         [0038]      FIG. 1  shows a side view of a bearing in a housing, a typical implementation of the invention,
     110  Bearing,     112  Outer, non-rotating, ring of the bearing,     114  Inner, rotating, ring of the bearing,     116  Rolling elements of the bearing, located between the outer and inner ring,     120  Housing.   
 
         [0044]      FIG. 2  shows a cross section of a part of a bearing according to one embodiment according to the invention,
     212  First outer ring belonging to the first row,     213  Second outer ring belonging to the second row,     216  Rolling elements of first row,     217  Rolling elements of second row,     230  First groove for first row,     231  Second groove for second row,     234  Height/Depth of groove,     235  Width of groove,     237  Load line of first row,     238  First optical fiber slit located in first groove,     239  Second optical fiber slit located in second groove,   
 
         [0056]      FIG. 3  shows a cross section of a part of a bearing according to another embodiment according to the invention,
     312  Outer ring,     316  Rolling elements,     330  First groove,     331  Second groove,     332  Third groove,   
 
         [0062]      FIG. 4  shows a top view of a bearing according to the invention illustrating optical fiber access to measurement groove.
     412  Outer ring,     430  First groove,     431  Second groove,     440  Optical fiber passage for entry/exit.   
 
         [0067]      FIG. 5A  shows a side sectional view of a bearing according to the invention illustrating an example of sensor distribution according to the invention around the bearing,
     500  Bearing according to the invention with a optical fiber strain sensor unit,     512  Outer, non-rotating, ring of the bearing,     514  Inner, rotating, ring of the bearing,     516  Rolling elements of the bearing, located between the outer and inner ring,     550  Optical fiber strain sensor unit,     551  SA—strain sensor,     552  SB—strain sensor,     553  SC—strain sensor,     554  SD—strain sensor,     555  SE—strain sensor,     556  SF—strain sensor,     557  SG—strain sensor,     558  SH—strain sensor,     560  Unloaded zone of bearing,     562  Loaded zone of bearing.   
 
         [0083]      FIG. 5B  shows the sensor frequency distribution according to the sensor placement of  FIG. 5A ,
     571  fA center frequency of SA—strain sensor,     572  fB center frequency of SB—strain sensor,     573  fC center frequency of SC—strain sensor,     574  fD center frequency of SD—strain sensor,     575  fE center frequency of SE—strain sensor,     576  fF center frequency of SF—strain sensor,     577  fG center frequency of SG—strain sensor,     578  fH center frequency of SH—strain sensor,     580  Large frequency variation due to large load variations     582  Medium frequency variation due to medium load variations     584  Small frequency variation due to large small variations   
 
         [0095]      FIG. 6A  shows a conventional optical fiber with Bragg grating, arranged flat,
     651  First strain sensor,     652  Second first strain sensor,     690  Optical fiber sensor unit with Bragg grating     692  First strain sensor grating separation DA,     696  Second strain sensor grating separation DB,   
 
         [0101]      FIG. 6B  shows the frequency response of the optical fiber of  FIG. 6A ,
     671  Frequency response of first strain sensor,     672  Frequency response of second strain sensor,   
 
         [0104]      FIG. 7A  shows a conventional optical fiber with Bragg grating that is bent, such as around a circular bearing,
     751  First strain sensor,     752  Second first strain sensor,     790  Optical fiber sensor unit with Bragg grating     791  First strain sensor inside grating separation DA-δ,     793  First strain sensor outside grating separation DA+δ,     795  Second strain sensor grating separation DB−δ,     797  Second strain sensor grating separation DB+δ,   
 
         [0112]      FIG. 7B  shows the frequency response of the bent optical fiber of  FIG. 7A ,
     771  Center frequency response of first strain sensor,     772  Center frequency response of second strain sensor,     779  Frequency width.   
 
         [0116]      FIG. 8A  shows a bent optical fiber with Bragg grating according to the invention,
     851  First strain sensor,     852  Second first strain sensor,     890  Optical fiber sensor unit with Bragg grating     892  First strain sensor outside and inside grating separation DA,     896  Second strain sensor outside and inside grating separation DB,   
 
         [0122]      FIG. 8B  shows the frequency response of the optical fiber according to the invention of  FIG. 8A ,
     871  Frequency response of first strain sensor,     872  Frequency response of second strain sensor,