Abstract:
A wear sensor comprising:
       an insulating substrate having a top surface and a bottom surface;   a conductive electrode formed on said top surface of said insulating substrate;   an insulating wear lining material having a first side secured to said top surface of said insulating substrate and conductive electrode, an opposite second side that will be worn down by relative motion between the wear sensor and a moving component;   one or more contact points where the electrical properties between the electrode and the moving component can be measured; and   one or more perforations through the thickness of the substrate and electrode, through which an adhesive may flow, thereby increasing the peel strength between the wear sensor and race or between the wear sensor and the wear liner.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
       [0001]    This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 61/713,735, filed Oct. 15, 2012 by Iosif Izrailit et al. for SENSOR FOR WEAR MEASUREMENT, METHOD OF MAKING, AND METHOD OF OPERATING SAME (Attorney&#39;s Docket No. NANO-19 PROV), which patent application is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to wear sensing devices in general, and more particularly to a sensor suited to the measurement of wear in bearings that employ a low friction wear lining material, instead of balls or rollers to support the load. More particularly, the invention relates to the electrical measurement of capacitance or other electrical impedance parameters between a movable surface and an electrode, which may be positioned within or on the back side of the wear liner, and its correlation to wear. 
       BACKGROUND OF THE INVENTION 
       [0003]    Condition based maintenance programs rely upon inspection to identify those parts that are nearing their end of life. Bearings are no exception to this rule. The replacement of a bearing before it is fully worn out may be wasteful, but waiting too long to replace a bearing can be catastrophic in some applications, particularly with rotorcraft and aircraft. It is known in the art to place sensors inside a bearing to measure wear. Discenzo (U.S. Pat. No. 7,551,288) disclosed a system for monitoring bearing wear that employed an optical fiber embedded in the bearing and operatively coupled to an interferometric system. Such a system will measure wear at only one point, and that point may not coincide with the area of maximum wear. Bearings with multiple optical fibers were disclosed to try to remedy this defect, but the overall complexity required for this measurement rendered the solution cost prohibitive. 
         [0004]    It is the goal of this invention to provide a sensor that will detect wear in any location within the bearing, and enable timely replacement, using a cost effective method. 
       SUMMARY OF THE INVENTION 
       [0005]    These and other objects are addressed by the provision and use of a novel insulating wear liner with a sensor for detecting wear on a bearing. More particularly, a sensor having an electrode can be inserted in, or on the back of, a wear liner in a bearing for monitoring the wear of the wear liner by monitoring the impedance parameters such as capacitance, inductance, and resistance, or a combination thereof, between the electrode and a movable component, such as a shaft. 
         [0006]    In one form of the present invention, there is provided a wear sensor comprising: 
         [0007]    an insulating substrate having a top surface and a bottom surface; 
         [0008]    a conductive electrode formed on said top surface of said insulating substrate; 
         [0009]    an insulating wear lining material having a first side secured to said top surface of said insulating substrate and conductive electrode, an opposite second side that will be worn down by relative motion between the wear sensor and a moving component; 
         [0010]    one or more contact points where the electrical properties between the electrode and the moving component can be measured; and 
         [0011]    one or more perforations through the thickness of the substrate and electrode, through which an adhesive may flow, thereby increasing the peel strength between the wear sensor and race or between the wear sensor and the wear liner. 
         [0012]    In another form of the present invention, there is provided a wear sensor comprising: 
         [0013]    a conductive filament that is deposited on or woven into an insulating, wear resistant material, such that the capacitance between the conductive filament and a moving component increases as the wear resistant material is worn away. 
         [0014]    In another form of the present invention, there is provided a method for sensing wear in a sleeve bearing for a moving component, the method comprising: 
         [0015]    providing a wear sensor comprising:
       an insulating substrate having a top surface and a bottom surface;   a conductive electrode formed on said top surface of said insulating substrate;   an insulating wear lining material having a first side secured to said top surface of said insulating substrate and conductive electrode, an opposite second side that will be worn down by relative motion between the wear sensor and a moving component;   one or more contact points where the electrical properties between the electrode and the moving component can be measured; and   one or more perforations through the thickness of the substrate and electrode, through which an adhesive may flow, thereby increasing the peel strength between the wear sensor and race or between the wear sensor and the wear liner;       
 
         [0021]    positioning the wear sensor inside of a wear liner of a sleeve bearing; and 
         [0022]    measuring at least one electrical property between the electrode of the wear sensor and the moving component so as to determine the wear in the sleeve bearing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These and other objects, features and advantages of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein: 
           [0024]      FIG. 1A  illustrates a new sleeve bearing with a sensor inserted into a wear liner in accordance with the present invention; 
           [0025]      FIG. 1B  illustrates a symmetrically worn sleeve bearing with a sensor inserted into a wear liner in accordance with the present invention; 
           [0026]      FIG. 1C  illustrates a non-concentrically worn sleeve bearing with a sensor inserted into a wear liner in accordance with the present invention; 
           [0027]      FIGS. 2A-2D  illustrate a sleeve bearing with a capacitive sensor inserted into a wear liner in accordance with the present invention, wherein the capacitive sensor is to be measured with a probe contact; 
           [0028]      FIGS. 3A-3D  illustrate a sleeve bearing with a capacitive sensor inserted into a wear liner in accordance with the present invention, wherein the sleeve bearing comprises an antenna with significant inductance for creating a resonant LC circuit; 
           [0029]      FIGS. 4A-4D  illustrate a spherical bearing with a capacitive sensor inserted into a wear liner in accordance with the present invention, wherein the capacitive sensor is to be measured with a probe contact; 
           [0030]      FIGS. 5A and 5B  illustrate a method of interrogating a capacitive sensor inserted into a wear liner of a spherical bearing in accordance with the present invention, using a capacitance meter; 
           [0031]      FIGS. 6A-6D  illustrate a spherical bearing with a capacitive sensor inserted into a wear liner in accordance with the present invention, and wherein the spherical bearing comprises an antenna with significant inductance for creating a resonant LC circuit; 
           [0032]      FIGS. 7A and 7B  illustrate a method of interrogating the capacitive sensor described in  FIGS. 6A-6D  with a tracking generator, matching network, interrogating antenna, and a spectrum analyzer; 
           [0033]      FIG. 8A  illustrates a new square telescoping bearing with a sensor inserted into a wear liner in accordance with the present invention; 
           [0034]      FIG. 8B  illustrates a worn square telescoping bearing with a sensor inserted into a wear liner in accordance with the present invention; and 
           [0035]      FIGS. 9A-9C  depict a process flow for producing the wear liner of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]    The present invention comprises an insulating wear liner with a sensor that is positioned either within the liner or placed on the non-wearing surface of the liner. The sensor is comprised of a conductive electrode and one or more pads for interrogating the electrical properties of the sensor. The liner is situated between the race and the moving part. 
         [0037]    By way of example but not limitation, a sensor may be positioned inside of the wear liner of a sleeve bearing, and the capacitance between the wear liner and the shaft can be calculated in the new condition of the shaft and wear liner, and after wear by a shaft. 
         [0038]    Looking now at  FIG. 1A ,  FIG. 1  illustrates a new sleeve bearing with a sensor inserted into the wear liner according to this invention. 
         [0039]    The new, unused sleeve bearing is assembled with a shaft which has radius R shaft . The shaft is centered in the bearing, concentric with the race, which has a radius R race . The sensor conductive electrode is positioned inside the liner, having radius R sensor , such that all three are concentric and R race &gt;R sensor &gt;R shaft . 
         [0040]    We assume the liner has a uniform dielectric constant of ∈. The new bearing, with no wear, will have a capacitance C new  between the sensor and the shaft, which is given by: 
         [0000]    
       
         
           
             
               C 
               new 
             
             = 
             
               
                 2 
                  
                 π 
                  
                 
                     
                 
                  
                 
                   ɛ 
                   0 
                 
                  
                 ɛ 
               
               
                 ln 
                  
                 
                   ( 
                   
                     
                       R 
                       sensor 
                     
                     
                       R 
                       shaft 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0041]    Table 1 shows a calculation of capacitance for a new shaft bearing. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Calculation of capacitance for new liner in a sleeve bearing 
               
             
          
           
               
                 NEW LINER 
                   
                 inch 
                 Value 
                 Metric Unit 
               
               
                   
               
             
          
           
               
                 Wear liner thickness 
                 T 
                 0.012 
                 0.00030 
                 m 
               
               
                 sensor position 
                 Sp 
                 0.006 
                 0.00015 
                 m 
               
               
                 Diameter of Shaft 
                 Dsh 
                 0.500 
                 0.01270 
                 m 
               
               
                 Diameter of Race 
                 Dr = Dsh + 2T 
                 0.524 
                 0.01331 
                 m 
               
               
                 Diameter of Sensor 
                 Ds = Dsh + 2Sp 
                 0.512 
                 0.01300 
                 m 
               
               
                 Bearing Length 
                 L 
                 0.500 
                 0.01270 
                 m 
               
               
                 Dielectric constant of liner 
                 e 
                 2 
                 2 
               
               
                 Permittivity of vacuum 
                 e0 
                   
                 8.85E−12 
                 F/m 
               
               
                 Radius of shaft 
                 Rsh = Dsh/2 
                   
                 0.00635 
                 m 
               
               
                 Radius of race 
                 Rr = Dr/2 
                   
                 0.00665 
                 m 
               
               
                 Radius of sensor 
                 Rs = Ds/2 
                   
                 0.00650 
                 m 
               
               
                 Capacitance sensor-shaft 
                 C = 2 * pi * e * e0/(ln(Rs/Rsh) 
                   
                 4689.2 
                 pF/m 
               
               
                 Capacitance Bearing, pF 
                 Cb = C * L 
                   
                 59.6 
                 pF 
               
               
                   
               
             
          
         
       
     
         [0042]    There will also be capacitance between the sensor electrode and the outer race, but this value should be constant over the life of the bearing. Between the sensor electrode and the moving shaft, there will be wear. Accordingly, the thickness of the wear liner will decrease, and the shaft will exhibit more play. One aspect of this invention is the effect of concentricity on the measured capacitance of a sensor embedded in a wear lining. We recognize two wear modes that could occur, concentric uniform or non-concentric non-uniform. 
         [0043]    To illustrate uniform wear, we consider a bearing that is worn with perfect symmetry so that some of the wear liner is removed from its entire circumference. Next, we position the shaft in perfect concentricity with the race and sensor electrode.  FIG. 1B  illustrates a symmetrically worn sleeve bearing with a sensor inserted into the wear liner according to the present invention. 
         [0044]    In this arrangement, there are two capacitors in series, one made of air C air , and another made from the remaining liner C liner  The air gap, having thickness W, will have a capacitance based on the radial gap, R liner =R shaft +W. The capacitance of that gap will follow: 
         [0000]    
       
         
           
             
               C 
               air 
             
             = 
             
               2 
                
               
                 
                   πɛ 
                   0 
                 
                 ( 
                 
                   
                     ɛ 
                     air 
                   
                   
                     ln 
                      
                     
                       ( 
                       
                         
                           ( 
                           
                             
                               R 
                               shaft 
                             
                             + 
                             W 
                           
                           ) 
                         
                         
                           R 
                           shaft 
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0045]    Likewise, the wear liner will have a capacitance based on its thickness, equal to R sensor −R liner , or R sensor −(R shaft +W) 
         [0000]    
       
         
           
             
               C 
               liner 
             
             = 
             
               2 
                
               
                 
                   πɛ 
                   0 
                 
                 ( 
                 
                   
                     ɛ 
                     liner 
                   
                   
                     ln 
                      
                     
                       ( 
                       
                         
                           R 
                           sensor 
                         
                         
                           
                             R 
                             shaft 
                           
                           + 
                           W 
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0046]    The total capacitance, C T , will follow that of two capacitors in series; C T =(C air ×C liner ) (C air +C liner ). Table 2 shows the result of this calculation. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Concentrically worn sleeve bearing 
               
             
          
           
               
                 CONCENTRIC WEAR 
                   
                 inch 
                 Value 
                 Metric Unit 
               
               
                   
               
             
          
           
               
                 Wear liner thickness 
                 T 
                 0.012 
                 0.000305 
                 m 
               
               
                 sensor position 
                 Sp 
                 0.006 
                 0.000152 
                 m 
               
               
                 Diameter of Shaft 
                 Rsh = Dsh/2 
                 0.500 
                 0.012700 
                 m 
               
               
                 Diameter of Race 
                 Rr = Dr/2 
                 0.524 
                 0.013310 
                 m 
               
               
                 Diameter of Sensor 
                 Rs = Ds/2 
                 0.512 
                 0.013005 
                 m 
               
               
                 Bearing Length 
                 L 
                 0.500 
                 0.012700 
                 m 
               
               
                 Dielectric constant of liner 
                 e 
                 2 
                 2 
               
               
                 Permittivity of vacuum 
                 e0 
                   
                 8.85E−12 
                 F/m 
               
               
                 Radius of shaft 
                 Rshaft 
                   
                 0.00635 
                 m 
               
               
                 Radius of race 
                 Rr 
                   
                 0.00665 
                 m 
               
               
                 Radius of sensor 
                 Rsensor 
                   
                 0.00650 
                 m 
               
               
                 Wear 
                 W 
                 0.004 
                 0.00010 
                 m 
               
               
                 Radius of liner 
                 Rliner = Rshaft + Wear 
                   
                 0.00645 
                 m 
               
               
                 Capacitance shaft to liner 
                 Cair = 2 * pi * e0(1/ln(Rliner/Rshaft) 
                   
                 3503 
                 pF/m 
               
               
                 Capacitance liner to electrode 
                 Cliner = 2 * pi * e0(e/ln(Rsensor/Rliner) 
                   
                 14180 
                 pF/m 
               
               
                 Total Capacitance/m 
                 CT = (Cair * Cliner)/(Cair + Cliner) 
                   
                 2809 
                 pF/m 
               
               
                 Capacitance 
                 C = CT * L 
                   
                 35.7 
                 pF 
               
               
                   
               
             
          
         
       
     
         [0047]    The resulting capacitance is lower than the value calculated in Table 1 for the new bearing. We note that this is the case only if the shaft is held at the center. If loaded, the shaft will be non-concentric and the following example will apply. 
         [0048]    Next, to illustrate the non-concentric, non-uniform case, we consider a bearing that has been loaded and worn preferentially on one side. The result is that the shaft is no longer concentric with the sensor.  FIG. 1C  illustrates a non-concentrically worn sleeve bearing with a sensor inserted into the wear liner according to the present invention. 
         [0049]    The capacitance of two cylinders eccentrically located one inside the other with radii (R shaft ) and (R sensor ), respectively, but with the centers of the two cylinders having a distance (W) apart, will be larger than in the concentric case. Ignoring the replacement of the worn-away dielectric with air, the capacitance would be: 
         [0000]    
       
         
           
             C 
             = 
             
               2 
                
               π 
                
               
                   
               
                
               
                 ɛ 
                 0 
               
                
               
                 ɛ 
                  
                 
                   ( 
                   
                     1 
                     
                       a 
                        
                       
                           
                       
                        
                       
                         cosh 
                          
                         
                           ( 
                           
                             
                               
                                 - 
                                 
                                   ( 
                                   
                                     
                                       W 
                                       2 
                                     
                                     - 
                                     
                                       R 
                                       shaft 
                                       2 
                                     
                                     - 
                                     
                                       R 
                                       sensor 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                               / 
                               2 
                             
                              
                             
                               R 
                               shaft 
                             
                              
                             
                               R 
                               sensor 
                             
                           
                           ) 
                         
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0050]    The capacitance is calculated for an eccentrically worn sleeve bearing in Table 3. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Non-concentric wear of a sleeve bearing 
               
             
          
           
               
                 WORN LINER 
                   
                 inch 
                 Value 
                 Metric Unit 
               
               
                   
               
             
          
           
               
                 Wear liner thickness 
                 T 
                 0.012 
                 0.000305 
                 m 
               
               
                 sensor position 
                 Sp 
                 0.006 
                 0.000152 
                 m 
               
               
                 Diameter of Shaft 
                 Dsh 
                 0.500 
                 0.012700 
                 m 
               
               
                 Diameter of Race 
                 Dr = Dsh + 2T 
                 0.524 
                 0.013310 
                 m 
               
               
                 Diameter of Sensor 
                 Ds = Dsh + 2Sp 
                 0.512 
                 0.013005 
                 m 
               
               
                 Bearing Length 
                 L 
                 0.500 
                 0.012700 
                 m 
               
               
                 Dielectric constant of liner 
                 e 
                 2 
                 2 
               
               
                 e0 
                 e0 
                   
                 8.85E−12 
                 F/m 
               
               
                 Radius of shaft 
                 Rsh = Dsh/2 
                   
                 0.006350 
                 m 
               
               
                 Radius of race 
                 Rr = Dr/2 
                   
                 0.006655 
                 m 
               
               
                 Radius of sensor 
                 Rs = Ds/2 
                   
                 0.006502 
                 m 
               
               
                 Eccentric Wear 
                 W 
                 0.004 
                 0.000102 
                 m 
               
               
                 Capacitance/m shaft to sensor 
                 C = 2 * pi * e * e0 * (1/(acosh(−(W{circumflex over ( )}2 − Rsh{circumflex over ( )}2 − Rs{circumflex over ( )}2)/2Rsh * Rs) 
                   
                 6440.4 
                 pF/m 
               
               
                 Capacitance of Bearing 
                 Cb = C * L 
                   
                 81.8 
                 pF 
               
               
                   
               
             
          
         
       
     
         [0051]    In Table 3, we see that the capacitance is significantly higher for the non-concentric worn bearing than for the new bearing. A notable aspect of this invention is that the capacitance between a metallic shaft and a sensor placed inside or behind the wear liner will increase with concentric or non-concentric wear, as long as the shaft is loaded. The capacitance is an inverse function of the liner thickness. Accordingly, the capacitance increases rapidly as the liner thickness approaches zero. 
         [0052]    Between the two previous examples, we expect to find the non-uniform, non-concentric case to be prevalent, as the loading and wear of bearings is rarely uniform. As such, we can relate the wear of a bearing to a measurable increase in capacitance between the shaft and the sensor. 
         [0053]    The capacitance measurement can be made at different frequencies. A standard frequency for capacitance measurement is 10 kHz. Measurements taken at a higher frequency improve the sensitivity of the measurement, but also increase the error due to interference. The optimal frequency for accuracy will depend on the electromagnetic interference in the environment surrounding the bearing. The measurement of Q factor, which can be calculated from the active and inductive current components in the sensor, provides information about the status of the liner. If at any point the gap between the sensor and the ball approaches zero, Q will drop rapidly toward zero. It will also be electrically shorted at this point. A Q under 5 indicates that the bearing needs immediate replacement, and a Q above 20 indicates a bearing with good health. The electrical shorting of the sensor and ball can also be used as an indicator that the wear liner has failed in at least one spot, and therefore needs replacement. 
         [0054]    Turning again to  FIG. 1 ,  FIG. 1A  illustrates a new sleeve bearing (without wear)  200 , comprising an outer race  201 , a movable shaft  203 , a wear liner  206  and a sensor  205  inserted into wear liner  206 . 
         [0055]      FIG. 1B  illustrates the sleeve bearing  200  of  FIG. 1A  after symmetric wear of wear liner  206 . The symmetric wear of wear liner  206  results in a worn sleeve bearing  200  having an equal air gap  222  between wear liner  206  and movable shaft  203 , with erosion of all wear on sleeve bearing  200  lining up to sensor  205 . 
         [0056]      FIG. 1C  illustrates an asymmetrically, a non-concentrically worn sleeve bearing  200  with a sensor  205 , where shaft  203  is closer to sensor  205  in one location than in another location. An air gap  222  is created by the removed material. 
         [0057]    Looking now at  FIG. 2A ,  FIG. 2A  illustrates a sensor  205  for a sleeve bearing  200 , comprising a race  201 , a shaft  203 , a sensor  205  and a wear liner  206 . Sensor  205  comprises a conductive trace  208  sandwiched between a lower and upper layer of insulating substrate  209  which may be of differing thicknesses ( FIG. 2B ). When sensor  205  is laid flat ( FIG. 2C ), conductive trace  208  can be seen in detail, along with tabs  210  that extend from sleeve bearing  200 . Slots  215  formed on sensor  205  assist in the flow of adhesive between layers. Electrode pads  213  are positioned on the surface of tabs  210  which can be probed with a capacitance meter to measure the capacitance between one electrode pad and shaft  203  ( FIG. 2D ). 
         [0058]    Looking now at  FIG. 3A ,  FIG. 3A  illustrates a sensor  205  for a sleeve bearing  200 , comprising a race  201 , a shaft  203 , a sensor  205  and a wear liner  206 . Sensor  205  comprises a conductive trace  208  sandwiched between a lower layer of insulating substrate  209  and an upper layer of insulating substrate  214  ( FIG. 3B ). When sensor  205  is laid flat ( FIG. 3C ), conductive trace  208  can be seen in detail, along with tabs  210  that extend from sleeve bearing  200 . Slots  215  formed on sensor  205  assist in the conformation of the sensor to surface variations, and to flow of adhesive between layers. Electrode pads  213  are positioned on the surface of tabs  210  which can be taken together as a connection point  220  for an antenna  221 . 
         [0059]      FIG. 4A  illustrates a spherical bearing  200  comprising a race  201 , a ball  202 , a shaft  203 , a sensor  205 , a wear liner  206  and an insulator  207 . Sensor  205  comprises a conductive trace  208  sandwiched between two layers of insulating substrate  209  ( FIG. 4B ). When sensor  205  is laid flat ( FIG. 4C ), conductive trace  208  can be seen in detail, along with tabs  210  that extend from sleeve bearing  200 . Holes  211  formed on sensor  205  assist in the flow of adhesive between layers. Strain relief cuts  212  formed on sensor  205  enable sensor  205  to deform into a more conformal shape. Electrode pads  213  are positioned on the surface of tabs  210  for the interrogation of sensor  205  ( FIG. 4D ). Viewed end on, after installation, electrode pads  213  may be touched with one probe of a capacitance meter. 
         [0060]    Looking now at  FIGS. 5A and 5B ,  FIGS. 5A and 5B , illustrate a method of interrogating sensor  205 . As shown in  FIG. 5A , a probe  225  of a precision capacitance meter  230  makes contact with an electrode pad  213  on the circumference of spherical bearing  200 . Assuming that race  201  and shaft  203  are both conductive and electrically connected elsewhere, the capacitance measured by the precision capacitance meter shall be comprised of the capacitance between ball  202  and sensor  205 , which is electrically in series with the capacitance between sensor  205  and race  201 . 
         [0061]      FIG. 6A  illustrates a spherical bearing  200  comprising a race  201 , a ball  202 , a shaft  203 , a sensor  205 , a wear liner  206  and an insulator  207 . Sensor  205  comprises a conductive trace  208  sandwiched between two layers of insulating substrate  209  ( FIG. 6B ). When sensor  205  is laid flat ( FIG. 6C ), conductive trace  208  can be seen in detail, along with tabs  210  that extend from sleeve bearing  200 . Holes  211  formed on sensor  205  assist in the flow of adhesive between layers. Strain relief cuts  212  formed on sensor  205  enable sensor  205  to deform into a more conformal shape. Electrode pads  213  are positioned on the surface of tabs  210  for the interrogation of the sensor ( FIG. 4D ). Viewed end on, after installation, tabs  210  and electrode pads  213  are connected at a point  220  to an antenna  221 , which may be mounted on the face of race  201 . 
         [0062]    Looking now at  FIGS. 7A and 7B ,  FIGS. 7A and 7B  illustrate a method of interrogating sensor  205  wirelessly. A signal produced by a tracking generator  235  is coupled through a matching network  240  to a loop antenna  245 , which interacts with sensor antenna  221 , for measuring bearing wear remotely. The output frequency of tracking generator  235  is varied over time, and at one moment will match the frequency of the LC circuit created by the sensor&#39;s capacitance and the antenna&#39;s inductance. At that moment, a spectrum analyzer  250  will detect the resonance frequency. The shift in resonant frequency shift from the change in sensor capacitance will correspond to the reduction in the wear liner thickness. Preferably, sensor antenna  221  may be placed in a detent, which is a circumferential groove in bearing race  201 . 
         [0063]      FIG. 8A  illustrates a new square telescoping bearing with a sensor inserted into the wear liner in accordance with the present invention. 
         [0064]      FIG. 8B  illustrates a worn square telescoping bearing with a sensor inserted into the wear liner in accordance with the present invention. 
         [0065]    One illustrative procedure for producing a device according to the present invention is shown in  FIGS. 9A-9C . In  FIG. 9A , there is shown an insulating substrate  209  with a metallic coating  208 . In  FIG. 9B , a second layer of insulator  214  is applied to sandwich the electrode, which may be patterned. In  FIG. 9C , at least one hole or a pattern is cut out, producing a sensor that can be inserted into a bearing. Holes in the sheet are expected to improve the bonding with the substrate. 
         [0066]    Turning back to  FIG. 8A ,  FIG. 8A  is an end-view of a new, un-worn telescoping structure comprising an outer sleeve  201 , an inner shaft  203  and a wear lining  206 , which has been instrumented with a sensor  205  part-way through wear lining  206 . In  FIG. 8B , wear liner  206  has been worn, leaving an air gap  222  and a reduced lining thickness on one side. The capacitance of this system can be modeled as the sum of the four parallel plate capacitors. Capacitance in this system is equal to the product of the permittivity of free space ∈ 0 , the dielectric constant ∈ and the area A divided by the distance d: C=∈∈ 0 A/d. 
         [0067]    Comparing  FIG. 8A  to  FIG. 8B , the lining thickness on the sides is unchanged, but in  FIG. 8B  the upper and lower distances are changed. At the bottom, the thickness of wear lining  206  has been reduced by wear, and a corresponding air gap  222  has opened up above shaft  203  at the top. The upper capacitor will have a lower value than before as the distance between shaft  203  and sensor  205  is increased by air gap  222 . The lower capacitor will have a much higher value than before, as it has a distance between shaft  203  and sensor  205  that is reduced by the same distance as air gap  222 . The increase in capacitance for the lower capacitor will more than make up for the decrease in capacitance for the upper capacitor. This is clear because the function 1/d is nonlinear. It approaches infinity as the quantity ‘d’ gets small, and it approaches zero as ‘d’ gets large. 
         [0068]    We note that a similar type of measurement could be made if the wear liner material was conductive, and the resistance was measured as a function of wear. 
         [0069]    There are two methods to measure the capacitance of the sensor. The first is to measure the value directly with a probe and a capacitance meter. The other alternative is to measure the resonant frequency of the combination of the sensor&#39;s capacitance and the attached antenna&#39;s inductance. A similar measurement could be implemented using an inductive sensor and a distributed capacitor to create the resonant circuit. 
         [0070]    The preceding examples should be construed as non-limiting, as other methods of implementing the sensor are possible. Also, other methods can be used to measure the wear in addition to capacitance, including inductance and resistance. 
       MODIFICATIONS OF THE PREFERRED EMBODIMENTS 
       [0071]    It should be understood that many additional changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the present invention, may be made by those skilled in the art while still remaining within the principles and scope of the invention.