BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a measurement bearing, in particular for a wheel set of a rail vehicle, having a rotational axis and at least one first bearing ring. The first bearing ring defines a main supporting direction and a circumferential direction extending transversally with respect to the main supporting direction. The first bearing ring is also designed to take up bearing forces which have at least one force component in the main supporting direction and, during operation, said bearing forces are applied to the first bearing ring via a plurality of rolling bodies. The present invention also relates to a bogie for a rail vehicle with such a measurement bearing. Finally, the invention relates to a method for determining the mechanical loads on a measurement bearing. 
     2. Description of the Related Art 
     Roller bearings are used in a large number of different forms to transfer radial and/or axial loads between components rotating in relation to one another. Here, the components of the roller bearing may be subject to various load cases (e.g. static and/or dynamic load, rotating and/or stationary load, etc.) according to the application, for which the bearing must be designed accordingly. It is indeed possible to extensively model the bearing and the expected load cases and to use simulation calculations for the design of such a bearing. The problem remains, however, that for the expected load cases a certain simplification must necessarily be applied. Difficulties also arise in taking account of production and assembly tolerances in such model calculations. 
     It is also known for the actual load cases affecting a bearing during operation to be estimated by analysis of the contact pattern or the deterioration pattern of a worn bearing. Here again, however, it is difficult to draw any conclusions about the actual load situation during operation. 
     The abovementioned models of the roller bearing are also used, inter alia, in methods for identification of the (deterioration) state of a roller bearing of this type. For example, such a method is known from EP 1 197 415 B1. Here, dynamic measurements (via acceleration sensors) in the area of a bearing are used to draw conclusions about its level of deterioration by means of modelling the bearing. 
     Through a later analysis of the measured bearing a further refinement of the model can indeed take place. But the problem remains that only secondary information on the actual loads on the bearing can be obtained. 
     Wheel set bearings of rail vehicles are as a rule subject to comparatively high loads, whilst at the same time for such wheel set bearings a long service life is demanded. This applies in particular to the wheel set bearings of high speed vehicles. Despite comparatively great efforts in the design of wheel set bearings and high quality of manufacture, from time to time damage is observed, the causes of which are unclear. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to provide a measurement bearing or a method of the type described at the beginning, which overcomes the above disadvantages and, in particular, with a simple design allows a reliable measurement of the actual loads on the bearing. 
     The present invention is based on the technical teaching that with a simple design a reliable measurement of the actual loads on the bearing is possible if a recess is made in a bearing ring of the bearing, in which a corresponding sensor device (with one or more sensors or the like) is arranged, in order to sense, during operation of the bearing, preferably, in real time, one or more sensing variables which is or are representative of the loads acting on the bearing in this area. 
     It has turned out that such an arrangement, in particular such a recess, can be created in such a way that in this way no appreciable weakening of the bearing, in particular, no appreciable reduction of the rigidity of the bearing, results. Therefore, the measurement bearing can at the same time be designed in such a way that its mechanical characteristics at least substantially correspond to those of a conventional bearing (as intended for the application concerned). Accordingly, from the measurements on the measurement bearing, highly accurate conclusions can be drawn about the actual load situation of the conventional bearing during operation. The measurements on the measurement bearing can thus be used in an advantageous manner for the design of such a conventional bearing in order to optimise its design. 
     According to one aspect of the invention for this purpose a measurement bearing is provided, in particular for a wheel set of a rail vehicle, having a rotating axis and at least one first bearing ring. The first bearing ring defines a main supporting direction and a circumferential direction extending transversally with respect to the main supporting direction. The first bearing ring is also designed to take up bearing forces which have at least one force component in the main supporting direction and during operation said bearing forces are applied to the first bearing ring via a plurality of rolling bodies. The first bearing ring has at least one first recess in which a sensor device is arranged, wherein the sensor device is designed to sense a sensing variable which represents the bearing forces which are applied to the bearing ring. 
     Here, the main supporting direction of the bearing designates the direction in which, according to its purpose, the bearing predominantly has to take up forces. For a purely radial bearing the main supporting direction is the radial direction of the bearing, while for a purely axial bearing the axial direction of the bearing represents the main supporting direction. In mixed types (for example tapered roller bearings, barrel-shaped roller bearings, etc.) the main supporting direction can, if necessary, have an appropriate inclination towards the radial direction or the axial direction. In rolling bodies with shell surfaces, which run linearly in a section (containing the rolling body axis), the main supporting direction is generally defined by a surface normal on the running surface of the rolling bodies (assigned to the bearing ring concerned). 
     The first recess can basically be arranged at any suitable point in the first bearing ring. Preferably the first recess is arranged in the contact area of the first bearing ring, i.e. the area of the first bearing ring which mainly takes up the loads during operation of the bearing. As a rule, this contact area lies in the projection of the rolling bodies in the main supporting direction. Preferably, the first recess is therefore arranged in the area of a projection of a movement area of the rolling bodies along the main supporting direction, wherein the movement area of the rolling bodies corresponds to the toroid defined by the rolling bodies rotating around the axis of rotation during operation. 
     In order to guarantee ease of access to the first recess, in particular to allow ease of fitting of the sensor device, the recess is preferably open to one side of the bearing ring. Here it is preferably a case of the side of the first bearing ring not turned towards the running surface of the rolling bodies. Preferably the first recess is open to the side of the first bearing ring turned away from the rolling bodies, since in this way easy positioning of the sensor device at any point in the bearing ring is possible, without a large recess in the first bearing ring being necessary with the associated weakening of the first bearing ring. 
     Depending on the application it may basically be sufficient to provide the first recess with the first sensor device as a single recess in the first bearing ring. Preferably, however, at least a second sensor device is arranged in the first bearing ring, in order to achieve a greater spatial resolution when considering the actual loads in the bearing. Preferred variants of the measurement bearing according to the invention are accordingly characterised in that the first bearing ring has at least one second recess, in which a further sensor device is arranged, wherein the further sensor device is designed to sense a sensing variable which represents the bearing forces which are applied to the bearing ring. 
     It shall be understood here that in addition to these two sensor devices, further sensor devices may also be provided in order to increase the spatial resolution of the measurement. It must be ensured here, however, that the mechanical weakening of the bearing caused by the additional recesses, in particular the associated reduction in rigidity, still remains within a tolerable limit in order to allow reliable statements to be made on the actual load conditions during operation of a conventional bearing. 
     In order to keep the localised weakening of the first bearing ring in the area of the respective recess as low as possible, the first recess and the second recess are an appropriate distance apart. Preferably, it is then provided that the first recess and the second recess are arranged a distance apart in the circumferential direction and/or transversally to the circumferential direction. 
     Preferably, the first recess and the second recess are arranged a distance apart in the circumferential direction by a first circumferential angle α 1 , wherein the rolling bodies during operation define a division which corresponds to a second circumferential angle α 2 , and the first circumferential angle α 1  is greater than the second circumferential angle α 2 . Here, with N being the number of rolling bodies, the second circumferential angle α 2  is calculated according to: 
     
       
         
           
             
               
                 
                   
                     
                       α 
                       2 
                     
                     = 
                     
                       
                         360 
                         ⁢ 
                         ° 
                       
                       N 
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Preferably, the first circumferential angle α 1  is at least 110% of the second circumferential angle α 2 , preferably at least 130% of the circumferential angle, more preferably at least 150% of the second circumferential angle, since, in this way, a particularly favourable arrangement of the sensor devices with a simultaneous lower weakening of the first bearing ring is possible. In this connection it should be noted that a small distance between two recesses with regard to transmission of the sensor signals over a short path (for example over appropriate signal lines) to the outside of the bearing ring is an advantage. 
     In further preferred variants of the measurement bearing according to the invention, the first recess and the second recess are arranged in the area of a projection of a movement area of the rolling bodies along the main supporting direction, wherein the first recess and the second recess are arranged a transversal distance a apart from one another in a spacing direction running transversally to the circumferential direction. The movement area of the rolling bodies again corresponds to the toroid that is defined by the rolling bodies rotating around the axis of rotation during operation. This movement area has a transversal dimension b in the spacing direction. The transversal spacing a is 30% to 70% of the transversal dimension b, preferably 40% to 60% of the transversal dimension b, more preferably 45% to 55% of the transversal dimension b. 
     In this connection it should be noted that a small distance of the two recesses (in the transversal direction and in the circumferential direction) with regard to transmission of the sensor signals over a short path (for example over appropriate signal lines) to the outside of the bearing ring is an advantage. 
     In order to achieve the lowest possible weakening of the measurement bearing compared to a corresponding conventional bearing, the depth of the respective recess is also preferably limited appropriately. Preferably, it is provided that the first recess has a depth dimension t in the main supporting direction and that the first bearing ring has a thickness dimension d in the main supporting direction, wherein the depth dimension t is 30% to 70% of the thickness dimension d, preferably 40% to 60% of the thickness dimension d, more preferably 45% to 55% of the thickness dimension d. 
     The forwarding or the transmission, respectively, of the measurement signals of the sensor device from the bearing ring to an appropriate processing unit can basically take place in any suitable manner. In particular, the transmission of the measurement signals can take place completely or partially wirelessly. In preferred variants, because these are of a particularly simple design, at least one duct opening into the first recess is provided, designed to lead signal lines connected with the first sensor device to the outside of the first bearing ring. 
     The duct can be created in any suitable manner. In particular, in the first bearing ring a suitable drill hole can be provided which opens into the first recess. A particularly simple to manufacture variant of the measurement bearing according to the invention is characterised in that in the first bearing ring at least one first groove running in the circumferential direction and opening into the first recess is provided. Preferably then in addition at least one second groove running transversally to the circumferential direction is provided which opens into the first groove, in order, for example, via this, to lead signal lines connected with the sensor device to the outside of the first bearing ring. 
     Here again, in order to achieve the lowest possible localised weakening of the first bearing ring, it is preferably provided that the first recess and the second groove are arranged a distance apart in the circumferential direction by a third circumferential angle α 3 , the rolling bodies (in the manner already described above) during operation define a division which corresponds to a second circumferential angle α 2 , and the third circumferential angle α 3  is greater than the second circumferential angle α 2 . Here again it is preferably provided that the third circumferential angle α 3  is at least 110% of the second circumferential angle α 2 , preferably at least 130% of the second circumferential angle α 2 , more preferably at least 150% of the second circumferential angle α 2 . 
     In this connection it should again be noted that a small distance between the second groove and the two recesses (in the circumferential direction) with regard to transmission of the sensor signals over a short path (for example over appropriate signal lines) to the outside of the bearing ring is an advantage. 
     As mentioned, the invention can be used for any kind of bearings. It can be particularly easily and advantageously used in connection with radial bearings. Preferably the first bearing ring therefore defines a radial direction and the main supporting direction corresponds to this radial direction. The invention can be used particularly advantageously also in configurations with a plurality of bearing rings arranged next to one another. Preferably the first bearing ring defines an axial direction and, in the axial direction, a second bearing ring is provided alongside. Here, the two bearing rings must not be directly adjoining one another. Rather, between the first bearing ring and the second bearing ring an intermediate ring can be arranged. 
     A particularly advantageous configuration with regard to the measurements to be performed results if the second bearing ring with reference to a plane of symmetry running perpendicular to the axial direction is designed to be symmetrical with the first bearing ring. 
     The present invention can be used for any types of roller bearings, for example, for ball bearings, barrel-shaped roller bearings or needle roller bearings. Particular ease of use is achieved with cylindrical roller bearings or tapered roller bearings. Furthermore, the measurement can be carried out at any point in the bearing. Here, the location of the measurement is typically determined by the type of load on the bearing (stationary load or rotating load). Here, it is of particular interest to perform the measurements on the bearing ring that is subject to a rotating load. Here, in the case of a radial bearing, the latter can be an external ring or also an internal ring of the radial bearing. In the case of a wheel set bearing the first bearing ring therefore typically forms an inner ring of the measurement bearing. Furthermore, the first bearing ring in this case preferably forms a bearing ring of the measurement bearing that rotates during operation. 
     The sensor device can basically have any suitable design, in order to sense the sensing variables representing the load on the bearing ring. Because of its particularly simple construction and its ease of handling it is preferably provided that the first sensor device comprises at least one strain gauge connected with the first bearing ring. Preferably, at least two strain gauges are provided, which are preferably arranged diametrically opposed to each other in the first recess, so that two measuring planes can be created in a simple manner with a single recess. 
     Here, essentially any alignment of the strain gauges is possible. Because of the rotational movement, however, it is preferably provided that at least one strain gauge is essentially aligned in the circumferential direction. 
     The transmission of the measurement signals from the sensor device to a processing unit for further processing can basically take place in any suitable manner. Preferably, at least in sections, wireless transmission is provided. It is therefore preferably provided that the first sensor device is connected with a transmitter of a telemetry device. 
     The present invention further relates to a rail vehicle with a measurement bearing according to one of the preceding claims. This allows the variants and advantages previously described to be achieved to the same degree so that in this connection reference is made to the above statements. 
     Furthermore, the present invention relates to a method for determining the mechanical loads on a measurement bearing, in particular for a wheel set of a rail vehicle, in which a measurement bearing is used having a rotational axis and at least one first bearing ring. The first bearing ring defines a main supporting direction and a circumferential direction extending transversally with respect to the main supporting direction. Furthermore, the first bearing ring is designed to take up bearing forces which have at least one force component in the main supporting direction, and, during operation, said bearing forces are applied to the first bearing ring via a plurality of rolling bodies. By means of a first sensor device, during operation, a sensing variable which represents the bearing forces which are applied to the bearing ring is sensed and output in the form of first measurement signals, wherein the sensor device is arranged in a first recess of the first bearing ring. The first measurement signals from the first sensor device are then used to determine the mechanical loads. This likewise allows the variants and advantages previously described to be achieved to the same degree so that in this connection reference is made to the above statements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further preferred embodiments of the invention can be inferred from the dependent claims or from the description below of preferred embodiments, which makes reference to the attached drawings. It is shown in: 
         FIG. 1  a schematic sectional view of part of a preferred embodiment of the measurement bearing (along line I-I from  FIG. 2 ); 
         FIG. 2  a top view of a development of the measurement bearing from  FIG. 1 ; 
         FIG. 3  a schematic side view of part of the rail vehicle according to the invention with the measurement bearing from  FIGS. 1 and 2 ; 
         FIG. 4  a schematic sectional view of part of a further preferred embodiment of the measurement bearing according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     In the following, by reference to  FIGS. 1 to 3 , initially a first preferred embodiment of the measurement bearing  101  according to the invention is described which is used in a preferred embodiment of the rail vehicle  102  according to the invention. 
     As can be inferred from  FIG. 3 , the rail vehicle  102  has a body  102 . 1 , supported by a bogie  102 . 2 . The bogie  102 . 2  comprises two wheel sets  102 . 3 , on which the bogie frame  102 . 4  is supported by wheel set bearing devices  102 . 5 . Each wheel set bearing device  102 . 5  comprises a measurement bearing  101  with an inner part  101 . 1 , which rests on the axle shaft (not shown) of the wheel set  102 . 3 , while its outer part (not shown) rests in the wheel set bearing housing (not shown) of the wheel set bearing device  102 . 5 . 
     The measurement bearing  101  is in the form of a cylindrical roller bearing and has three parts, its form being mirror-symmetrical to a plane of symmetry  101 . 2 . Here the measurement bearing comprises a first part  101 . 3 , a second part  101 . 4  and a third part  101 . 5 . 
     The first part  101 . 3  of the measurement bearing  101  comprises a first bearing ring in the form of an inner ring  103 . The first part  101 . 3  of the measurement bearing  101  also comprises a plurality of N roller bodies in the form of cylindrical rollers  104 , which run on a bearing surface  103 . 1  of the inner ring  103 . Via the cylindrical rollers  104  inter alia the corresponding proportion of the weight of the body  102 . 1  is supported in a main supporting direction, here, the radial direction R of the measurement bearing  101  (z direction in  FIG. 1 ). 
     The inner ring  103 , on its inside turned towards the axle shaft, has a first recess pointing in the radial direction in the form of a first blind hole  105  and second recess pointing in the radial direction in the form of a second blind hole  106 . 
     Furthermore, the inner ring  103 , on its inside, has a first groove  107  running in its circumferential direction, which opens into the first recess  105 . It similarly has a second groove  108  running in its circumferential direction U, which opens into the second recess  106 . The two grooves  107  and  108  are connected together via a third groove  109  running obliquely to the circumferential direction U. The third groove  109 , in the axial direction of the measurement bearing  101  (y direction in  FIG. 1 ), extends completely through the inner ring  103 . 
     In the first recess  105  a first sensor device  110  is arranged while in the second recess  106  a second sensor device  111  is arranged. The first sensor device  110  and the second sensor device  111  each comprise two strain gauges, arranged diametrically opposed to one another, so that in each case they are aligned essentially in the circumferential direction U of the inner ring  103 . 
     The strain gauges  110 . 1 ,  110 . 2  of the first sensor device  110  are connected via first signal lines (for example, enamelled copper wires) with a transmitter  112 . 1  of a telemetry device  112 , which rests on the axle shaft (not shown). The first signal lines here are led in the first groove  107  as far as up to the third groove  109  and then via the third groove  109  out of the inner ring  103  to the transmitter  112 . 1  (as shown in  FIG. 2  by way of example but for clarity only in full for the strain gauge  110 . 1 ). 
     The strain gauges of the second sensor device  111  are connected via second signal lines (for example, enamelled copper wires) with the transmitter  112 . 1 . Here, the second signal lines are run in the second groove  108  as far as up to the third groove  109  and then via the third groove  109  likewise out of the inner ring  103  to the transmitter  112 . 1  (as shown in  FIG. 2  by way of example but for clarity only for one of the two wire strain gauges). 
     The sensor devices  110  and  111  deliver signals in a sufficiently known manner through the voltage changes resulting from the deformations (serving as sensing variables) of the strain gauges, which represent the loads acting on the inner ring  103  at the respective measuring point. Here, the sensor devices  110  and  111 , depending on the available sampling rate, provide real time data on the current loads on the measurement bearing. In the present example very high time resolutions are achieved so that in relation to the angle of rotation of the measurement bearing  101  an angular resolution of up to 1° can be achieved. 
     The transmitter  112 . 1  sends the measurement signals from the sensor devices  110  and  111  to a processing unit  112 . 2  of the telemetry device  112 , which provides appropriate further processing of the signals from the sensor devices  110  and  111  and from this delivers the actual loads on the measurement bearing  101 . 
     As mentioned, the third bearing part  101 . 5  has a design that is symmetrical with the first bearing part  101 . 3  and has the same number of sensor devices or strain gauges, which are likewise connected via a groove  101 . 6  (connecting the two third grooves  109  of the first bearing part  101 . 3  and the third bearing part  101 . 5 ) running in the axial direction (y direction in  FIG. 1 ) with the transmitter device  112 . 1  (as shown in  FIG. 2 , but in an incomplete manner for clarity). 
     The spatial resolution of the measurement is therefore defined by the four measuring planes in each case, aligned perpendicularly to the axial direction (y-direction in  FIG. 1 ) in the first and third bearing parts  101 . 3 ,  101 . 5 , which are defined by the four strain gauges of the two sensor devices  110  and  111  in the first bearing part  101 . 3  and the four strain gauges of the two sensor devices in the third bearing part  101 . 5 . This means, therefore, that there are eight such measuring planes defined in the measurement bearing  101 . 
     It will be understood here that, with other variants of the invention, other numbers of measuring planes can also be defined, for example by providing further recesses with further sensor devices (each with one or more strain gauges). It will also be understood that a different number of strain gauges may be provided in the each of the first and second recesses. 
     The recesses  105 ,  106  and the grooves  107 ,  108  are designed in such a way that the annular cross-section of the inner ring  103  is weakened only very minimally so that there is only an insignificant change to the high rigidity of the bearing  101 . Thus, there is total comparability with unmodified serial production bearings. 
     In order to keep the localised weakening of the first bearing ring in the area of the respective recess as low as possible, and to maintain sufficient coverage of the loaded area of the measurement bearing  101  by the measurement, the first recess  105  and the second recess  106  are spaced an appropriate distance apart in the circumferential direction and transversally to the circumferential direction. 
     Thus the first recess  105  and the second recess  106  are arranged a distance apart in the circumferential direction by a first circumferential angle α 1  (as shown in the case from  FIG. 2  by the length r·α 1 , where r is the internal radius of the inner ring  103 ), corresponding to 150% of the circumferential angle α 2 . The second circumferential angle α 2  is defined by the division of the cylindrical rollers  104  and again is calculated using the above equation ( 1 ), that is: 
     
       
         
           
             
               α 
               2 
             
             = 
             
               
                 
                   360 
                   ⁢ 
                   ° 
                 
                 N 
               
               . 
             
           
         
       
     
     This allows a particularly advantageous arrangement of the sensor devices  110 ,  111  and, at the same time, a low weakening of the first bearing ring  103 . In this connection it should be noted that a small distance between two recesses  105 ,  106  with regard to transmission of the sensor signals over a short path (for example over appropriate signal lines) to the outside of the bearing ring  103  is an advantage. 
     As can be inferred from  FIGS. 1 and 2 , the first recess  105  and the second recess  106  are arranged in the area of a projection of a movement area of the cylindrical rollers  104  in the main supporting direction, i.e. in the radial direction R. Here, the first recess  105  and the second recess  106  are spaced apart in a spacing direction running transversally to the circumferential direction U (y direction in  FIG. 1 ) by transversal spacing a. 
     The movement area of the rolling bodies again corresponds to the toroid that is defined by the cylindrical rollers  104  rotating around the axis of rotation of the measurement bearing  101  during operation. This movement area has a transversal dimension b in the spacing direction. The transversal spacing a of the first and second recesses  105 ,  106  is approximately 50% of the transversal dimension b. 
     In this connection it should again be noted that a small distance between the two recesses  105 ,  106  (in the transversal direction and in the circumferential direction) with regard to transmission of the sensor signals over a short path (for example over appropriate signal lines) to the outside of the bearing ring  103  is an advantage. 
     In order to achieve the lowest possible mechanical weakening of the measurement bearing compared with a conventional bearing, the depth of the respective recess  105 ,  106  is also limited. To this end, the first recess  105  and the second recess  106 , in the main supporting direction (z direction in  FIG. 1 ), have a depth dimension t, wherein the depth dimension t is approximately 50% of the thickness dimension d of the bearing ring  103 . 
     In order to also achieve the lowest possible localised weakening of the first bearing ring  103 , the first recess  105  and the third groove  109  are arranged spaced apart in the circumferential direction U by a third circumferential angle α 3  (as shown in the case of  FIG. 2  by the length r·α 3 ), which is 150% of the second circumferential angle α 2 . 
     Thanks to the total number of measuring planes, from the strain gauges of which the signals are simultaneously recorded, by means of bearing force measurements over the respective circumference in each case a measurement of the load zones is obtained. For the purposes of evaluation the measurement data from each measured area can be “rotated back” to a reference angle by the processing unit  112 . 2 . 
     In a further embodiment, grooves with strain gauges can be arranged on the rear wall of the bearing  101  in order to (also) sense the longitudinal forces on the bearing  101 . 
     Second Embodiment 
     In the following, with reference to  FIG. 4 , a further preferred embodiment of the measurement bearing  201  according to the invention is described. In terms of its structure and functionality the measurement bearing  201  essentially corresponds to measurement bearing  101 , so that here only the differences will be looked into. In particular, like components will be given reference numbers that have been increased by a value of 100. Where in the following no differing description is provided, reference is made to the statements above concerning the first embodiment with respect to these components. 
     The only difference from measurement bearing  101  from  FIG. 1  is that the measurement bearing  201  is not designed as a cylindrical roller bearing but as a tapered roller bearing. Here, the first recess  205  and the second recess  206  (with the respective annular channel  207  or  208  and the respective sensor devices  210  or  211 ) are arranged in such a way that their longitudinal axes run parallel to the main supporting direction H, which is aligned perpendicularly to the running surface  203 . 1  of the cylindrical roller  204  on the inner ring  203 . 
     The present invention has been described above using examples in which the sensor devices are arranged in the inner ring of the measurement bearing. It shall be understood, however, that, in other variants of the invention, additionally or alternatively sensor devices can also be arranged in the outer ring of the measurement bearing. 
     Furthermore, the present invention has been described solely using examples in which the measurement bearing is designed as a radial bearing. It will be understood, however, that the invention can also be used with axial bearings. 
     Finally, it will be understood that the invention can be used not only in connection with the wheel set bearings of rail vehicles, but also for any other applications in which the most accurate possible determination of the actual loads on a bearing is desired and advantageous. In particular, the invention can be used for any other types of vehicle (such as land, air and water vehicles), any mobile and stationary systems, for example wind powered systems, and so on.
A measurement bearing includes a rotational axis and at least one first bearing ring. The first bearing ring defines a main supporting direction and a circumferential direction extending transversally with respect to the main supporting direction. The first bearing ring also has at least one first recess in which a sensor device is arranged. The sensor device is adapted to sense a sensing variable which represents the bearing forces which are applied to the first bearing ring. The first bearing ring further has at least one second recess, in which a further sensor device is arranged. The first recess and the second recess are arranged a distance apart in the circumferential direction and transversally to the circumferential direction.