Patent Publication Number: US-2023152185-A1

Title: Compressor system, compressor, and refrigeration cycle apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a compressor system including a compressor having a bearing that supports a rotating shaft, and also relates to a compressor and a refrigeration cycle apparatus. 
     BACKGROUND ART 
     Hitherto, a device that detects wear of a bearing included in a hermetic compressor has been disclosed by Patent Literature 1. According to Patent Literature 1, a plurality of metal pieces are provided around a rotating shaft with a small air gap interposed between the rotating shaft and the metal pieces. The plurality of metal pieces are covered with an insulating mold, such as resin, and are thus formed into a cylindrical part, which is provided around the rotating shaft. The mold forming the cylindrical part and interposed between the rotating shaft and the metal pieces serves as a bearing. If the bearing wears away, the rotating shaft comes into contact with the metal pieces, which changes the electric current flowing through the rotating shaft. In the device disclosed by Patent Literature 1, such wear of the bearing is to be detected based on the change in the electric current. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 3-239901 
     SUMMARY OF INVENTION 
     Technical Problem 
     The device disclosed by Patent Literature 1 has the following problem. To enable the detection of wear of the bearing, the compressor needs to include the cylindrical mold that covers the plurality of metal pieces. Such a design complicates the configuration of the compressor. 
     To solve the above problem, the present disclosure provides a compressor system including a compressor in which wear of a bearing is detectable with a simple configuration, and also provides a compressor and a refrigeration cycle apparatus. 
     Solution to Problem 
     A compressor system according to an embodiment of the present disclosure includes a compressor including a bearing, the bearing supporting a rotating shaft; a sensor configured to measure an index value correlated with a movement of the rotating shaft, the movement being made while the compressor is in operation; and a wear-detecting unit configured to detect a degree of wear of the bearing based on a measured value obtained by the sensor. The bearing is a plain bearing and has a plurality of air gaps arranged at intervals in a circumferential direction such that wear of the bearing changes a positional relationship between an inner circumferential surface of the bearing and the plurality of air gaps and eventually changes a shape of the inner circumferential surface of the bearing. The wear-detecting unit detects the degree of wear of the bearing based on a change in the measured value that is caused by a change in the shape of the inner circumferential surface of the bearing. 
     Advantageous Effects of Invention 
     According to the above embodiment of the present disclosure, the degree of wear of the bearing is detectable by utilizing the fact that the progress of wear changes the positional relationship between the inner circumferential surface of the bearing and the plurality of air gaps and eventually changes the shape of the inner circumferential surface of the bearing. The change in the shape of the inner circumferential surface of the bearing is caused by the plurality of air gaps provided in the bearing. Thus, the degree of wear of the bearing can be detected with a simple configuration realized by providing a plurality of air gaps in the bearing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic sectional view of a compressor system according to Embodiment 1. 
         FIG.  2    is a schematic sectional view of a bearing included in a compressor according to Embodiment 1. 
         FIG.  3    is an enlarged schematic sectional view of a part of  FIG.  2   , including an air gap and peripheral elements. 
         FIG.  4    summarizes a frequency at which an anomalous peak appears in the result of frequency analysis performed by an analyzing unit included in the compressor system according to Embodiment 1. 
         FIG.  5    exemplifies the result of frequency analysis in normal time that is obtained by the analyzing unit of the compressor system according to Embodiment 1. 
         FIG.  6    exemplifies the result of frequency analysis in abnormal time that is obtained by the analyzing unit of the compressor system according to Embodiment 1. 
         FIG.  7    is a schematic sectional view of a bearing included in a compressor according to Embodiment 2. 
         FIG.  8    is an enlarged sectional view of a part of  FIG.  7   , including a second air gap and peripheral elements. 
         FIG.  9    exemplifies the result of frequency analysis at the occurrence of wear by an amount  81  that is obtained by an analyzing unit included in a compressor system according to Embodiment 2. 
         FIG.  10    summarizes a relationship, in Pattern 1, between the bearing having abnormality and the frequency having an anomalous peak to be identified by frequency analysis in a compressor system according to Embodiment 3. 
         FIG.  11    summarizes a relationship, in Pattern 2, between the bearing having abnormality and the frequency having an anomalous peak to be identified by frequency analysis in the compressor system according to Embodiment 3. 
         FIG.  12    summarizes a relationship, in Pattern 3, between the bearing having abnormality and the frequency having an anomalous peak to be identified by frequency analysis in the compressor system according to Embodiment 3. 
         FIG.  13    is a schematic sectional view of a bearing included in a compressor system according to Embodiment 4. 
         FIG.  14    is an enlarged schematic sectional view of a part of  FIG.  13   , including an air gap and peripheral elements. 
         FIG.  15    exemplifies the result of frequency analysis in normal time in the compressor system according to Embodiment 4. 
         FIG.  16    exemplifies the result of frequency analysis in abnormal time in the compressor system according to Embodiment 4. 
         FIG.  17    exemplifies the result of frequency analysis in normal time in a case where air gaps are provided in both a main bearing and an orbital bearing in the compressor system according to Embodiment 4. 
         FIG.  18    illustrates the result of frequency analysis in a case where abnormal wear has occurred only in the main bearing in the compressor system according to Embodiment 4. 
         FIG.  19    illustrates the result of frequency analysis in a case where abnormal wear has occurred only in the orbital bearing in the compressor system according to Embodiment 4. 
         FIG.  20    illustrates a refrigerant circuit of a refrigeration cycle apparatus according to Embodiment 5. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Compressors according to embodiments will now be described with reference to the drawings. In the drawings to be referred to below, the same reference signs denote the same or equivalent elements, which applies throughout the following description of embodiments. The forms of individual elements described throughout the entirety of this specification are only exemplary and do not limit the forms of the elements thereto. Elements illustrated in the drawings may not necessarily be to scale. For easy understanding, terms indicating directions (such as “upper”, “lower”, “right”, “left”, “front”, and “rear”) will be used accordingly. Such terms, however, are only for convenience of description and do not limit the arrangements or orientations of the apparatus and individual elements thereof. 
     Embodiment 1 
     [Configuration of Compressor  100 ] 
       FIG.  1    is a schematic sectional view of a compressor system according to Embodiment 1. The compressor system includes a compressor  100  and a controller  200 . The controller  200  includes a function of detecting wear of bearings included in the compressor  100 . The compressor  100  is, for example, a scroll compressor having a shell filled with low-pressure refrigerant. The compressor  100  is applicable to refrigeration cycle apparatuses, to be described below, for refrigeration uses or air-conditioning uses: such as a refrigerator, a freezer, a vending machine, an air-conditioning apparatus, a freezing apparatus, and a water heater. The compressor  100  suctions refrigerant circulating through a refrigerant circuit included in a refrigeration cycle apparatus, compresses the refrigerant into high-temperature high-pressure refrigerant, and discharges the high-temperature high-pressure refrigerant. 
     First, a configuration of the compressor  100  will be described. As illustrated in  FIG.  1   , the compressor  100  includes a shell  2 , an oil pump  3 , a motor  4 , a compressing mechanism unit  5 , a frame  6 , and a rotating shaft  7 . The compressor  100  further includes a suction pipe  11 , a discharge pipe  12 , a sub frame  20 , an oil-draining pipe  21 , a vibration sensor  60 , a current sensor  61 , and a power-supplying unit  70 . 
     (Shell  2 ) 
     The shell  2  includes a middle shell  2   c , an upper shell  2   a , and a lower shell  2   b . The upper shell  2   a  is provided at the top of the middle shell  2   c . The lower shell  2   b  is provided at the bottom of the middle shell  2   c . The shell  2  forms the outer shell of the compressor  100 . The shell  2  has a bottomed circular cylindrical shape, with an oil sump  3   a  being provided at the bottom thereof. The shell  2  houses the oil pump  3 , the motor  4 , the compressing mechanism unit  5 , the frame  6 , the rotating shaft  7 , the sub frame  20 , the oil-draining pipe  21 , and other relevant elements. The middle shell  2   c  forms a circular cylindrical circumferential wall of the shell  2 . The upper end of the middle shell  2   c  of the shell  2  is closed by the upper shell  2   a , which has a dome shape. The lower end of the middle shell  2   c  of the shell  2  is closed by the lower shell  2   b . The upper shell  2   a  of the shell  2  and the compressing mechanism unit  5  define a discharge chamber  13  therebetween. The discharge chamber  13  provides a high-pressure space. The discharge chamber  13  is positioned above the compressing mechanism unit  5  and receives refrigerant compressed by and discharged from the compressing mechanism unit  5 . 
     (Oil Pump  3 ) 
     The oil pump  3  housed in the shell  2  pumps up oil from the oil sump  3   a . The oil pump  3  is positioned at the bottom inside the shell  2 . The oil pump  3  supplies the oil pumped up from the oil sump  3   a  to lubrication objects, such as bearings included in the compressor  100 , so that the lubrication objects are lubricated. The oil that has been pumped up by the oil pump  3  and has lubricated an orbital bearing  8   c  is, for example, stored in an internal space  6   d , which is provided inside the frame  6 . The oil then flows through oil-feeding grooves  6   c , which extend radially in a thrust bearing  6   b  to be described below. The oil having flowed through the oil-feeding grooves  6   c  flows into an Oldham-ring space, in which an Oldham ring  15  is provided. Thus, the oil lubricates the Oldham ring  15 . The Oldham-ring space has Oldham grooves  15   a  and  15   b  to be described below. The Oldham-ring space is connected to one end of the oil-draining pipe  21 . The oil in the Oldham-ring space flows through the oil-draining pipe  21  and returns into the oil sump  3   a.    
     (Motor  4 ) 
     The motor  4  housed in the shell  2  is positioned between the frame  6  and the sub frame  20  and rotates the rotating shaft  7 . The motor  4  includes a stator  4   b  and a rotor  4   a . The stator  4   b  is fixed to the inner circumferential wall of the middle shell  2   c . The rotor  4   a  is positioned on the inner circumferential side of the stator  4   b . The stator  4   b  rotates the rotor  4   a  with electric power received from the outside of the compressor  100 . The stator  4   b  includes, for example, a stack of iron cores around which coils of different phases are wound. The rotating shaft  7  is fixed to the rotor  4   a . The rotating shaft  7  transmits a rotational driving force generated by the motor  4  to an orbiting scroll  40 . When electric power is supplied to the stator  4   b , the rotor  4   a  rotates together with the rotating shaft  7 . The rotation speed of the rotating shaft  7  is changeable by operating the motor  4  under, for example, inverter control. 
     (Compressing Mechanism Unit  5 ) 
     The compressing mechanism unit  5  housed in the shell  2  compresses fluid that is suctioned into the shell  2  through the suction pipe  11 . The fluid may be, for example, refrigerant. The compressing mechanism unit  5  has a compression chamber  5   a  and an outlet  32 . The refrigerant is compressed in the compression chamber  5   a , and the compressed refrigerant is discharged through the outlet  32 . The compressing mechanism unit  5  includes a fixed scroll  30  and the orbiting scroll  40 . The fixed scroll  30  is fixed to the shell  2 . The orbiting scroll  40  rotates about (i.e., orbits) the fixed scroll  30 . The fixed scroll  30  is, for example, provided at the upper end of the frame  6 , which has a cylindrical shape, in such a manner as to cover an opening of the frame  6 . The fixed scroll  30  is fixed to the frame  6  with fastening parts such as bolts. While the above description relates to a case where the fixed scroll  30  is fixed to the frame  6 , the fixed scroll  30  may alternatively be fixed directly to the middle shell  2   c  of the shell  2 , without being fixed to the frame  6 . 
     The fixed scroll  30  cooperates with the orbiting scroll  40  in such a manner as to compress the refrigerant. The fixed scroll  30  is positioned face to face with the orbiting scroll  40 . The fixed scroll  30  includes an end plate  30   a  and a scroll portion  31 . The scroll portion  31  extends downward from the lower surface of the end plate  30   a . The scroll portion  31  projects toward the orbiting scroll  40  from a surface of the end plate  30   a  that faces toward the orbiting scroll  40 . The scroll portion  31  is a projection having a spiral shape in a cross section taken along a plane parallel to the end plate  30   a.    
     The end plate  30   a  defines the compression chamber  5   a  in combination with the scroll portion  31  of the fixed scroll  30  and a scroll portion  41 , to be described below, of the orbiting scroll  40 . The end plate  30   a  is fixedly provided inside the shell  2 , with the outer circumferential surface thereof facing the inner circumferential surface of the middle shell  2   c , and with the outer circumferential edge of the lower surface thereof being in contact with the upper end face of the frame  6 . The end plate  30   a  has a disc shape. The outlet  32  extends through a central portion of the end plate  30   a . The refrigerant compressed in the compression chamber  5   a  is discharged through the outlet  32 . The outlet  32  is provided at the exit thereof with a discharge valve mechanism  50 . The discharge valve mechanism  50  includes a valve seat  52 , a reed valve  51 , and a reed valve retainer  53 . The valve seat  52  is provided around an open end  32   a  at the exit of the outlet  32 . The reed valve  51  is a leaf spring provided on the valve seat  52  and opens and closes the outlet  32  by the effect of the pressure difference between the inside and the outside. The reed valve retainer  53  is provided on the valve seat  52  and limits the maximum opening degree of the reed valve  51 . The discharge valve mechanism  50  prevents the backflow of the refrigerant discharged from the open end  32   a  at the exit of the outlet  32 . 
     The orbiting scroll  40  is positioned face to face with the fixed scroll  30 . The orbiting scroll  40  is eccentric with respect to the fixed scroll  30 . The orbiting scroll  40  includes an end plate  40   a  and the scroll portion  41 . The scroll portion  41  extends upward from the upper surface of the end plate  40   a . The scroll portion  41  projects toward the fixed scroll  30  from a surface of the end plate  40   a  that faces toward the fixed scroll  30 . The scroll portion  41  is a projection having a spiral shape in a cross section taken along a plane parallel to the end plate  40   a . The end plate  40   a  defines the compression chamber  5   a  in combination with the scroll portion  41  of the orbiting scroll  40  and the scroll portion  31  of the fixed scroll  30 . The end plate  40   a  has a disc shape and undergoes an orbital motion inside the frame  6  with the rotation of the rotating shaft  7 . A thrust load acting in the axial direction on the orbiting scroll  40  is borne by the frame  6 . A surface of the end plate  40   a  that is opposite the surface having the scroll portion  41  serves as the thrust bearing  6   b . The orbiting scroll  40  is prevented from undergoing a spinning motion by the Oldham ring  15  and rotates about the fixed scroll  30 . In other words, the orbiting scroll  40  undergoes an orbital motion with respect to the fixed scroll  30 . 
     The Oldham ring  15  is provided on the thrust bearing  6   b  of the orbiting scroll  40  and prevents the orbiting scroll  40  from undergoing a spinning motion. The Oldham ring  15  that prevents the spinning motion of the orbiting scroll  40  enables the orbital motion of the orbiting scroll  40 . The Oldham ring  15  has catches, which are not illustrated. The catches project from the upper and lower surfaces, respectively, of the Oldham ring  15  in such a manner as to extend orthogonally to each other. The catches of the Oldham ring  15  are fitted in each of the Oldham groove  15   a  provided in the orbiting scroll  40  and the Oldham groove  15   b  provided in the frame  6 . 
     The fixed scroll  30  and the orbiting scroll  40  are housed in the middle shell  2   c , with the scroll portion  31  and the scroll portion  41  thereof that are positioned face to face being in mesh with each other. The space produced between the scroll portion  31  of the fixed scroll  30  and the scroll portion  41  of the orbiting scroll  40  that are in mesh with each other serves as the compression chamber Sa. When the orbiting scroll  40  undergoes the orbital motion with the rotation of the rotating shaft  7 , the refrigerant, which is in a gas state, is compressed in the compression chamber Sa. 
     (Frame  6 ) 
     The frame  6  has a cylindrical shape, with the outer circumference thereof being fixed to the shell  2 . The compressing mechanism unit  5  is positioned on the inner circumferential side of the frame  6 . The frame  6  holds the orbiting scroll  40  of the compressing mechanism unit  5 . The frame  6  bears the thrust-bearing load, which occurs while the compressor  100  is in operation, at the thrust bearing  6   b  of the orbiting scroll  40 . The frame  6  supports the rotating shaft  7  with the aid of a main bearing  8   a  such that the rotating shaft  7  is rotatable. The frame  6  has a suction port  6   a . Refrigerant in a gas state is suctioned through the suction pipe  11  into the shell  2  and flows through the suction port  6   a  into the compressing mechanism unit  5 . 
     A sleeve  17  is provided between the frame  6  and the main bearing  8   a . The sleeve  17  has a cylindrical shape. The sleeve  17  absorbs misalignment that may occur between the frame  6  and the rotating shaft  7 . 
     (Rotating Shaft  7 ) 
     The rotating shaft  7  is connected to the motor  4  and to the orbiting scroll  40  and transmits the rotational force generated by the motor  4  to the orbiting scroll  40 . A portion of the rotating shaft  7  that is above the rotor  4   a  is rotatably supported by the main bearing  8   a , which is attached to the frame  6 . A portion of the rotating shaft  7  that is below the rotor  4   a  is rotatably supported by a counterbearing  8   b , which is provided in the sub frame  20 . The rotating shaft  7  is provided at the lower end thereof with the oil pump  3  that pumps up the oil in the oil sump  3   a . The rotating shaft  7  has an oil passage  7   a . The oil passage  7   a  runs through the inside of the rotating shaft  7 . The oil pumped up by the oil pump  3  flows upward through the oil passage  7   a.    
     The rotating shaft  7  is provided around the outer circumference of the upper portion thereof with a slider  16 . The slider  16  has a cylindrical shape. The slider  16  is positioned on the inner wall of a bottom portion of the orbiting scroll  40 . The orbiting scroll  40  is attached to the rotating shaft  7  with the slider  16  interposed therebetween. Thus, the orbiting scroll  40  rotates with the rotation of the rotating shaft  7 . The orbital bearing  8   c  is interposed between the orbiting scroll  40  and the slider  16 . 
     The rotating shaft  7  is provided with a first balancer  18 . The first balancer  18  is fixed to the upper portion of the rotating shaft  7  by, for example, shrink fitting. The first balancer  18  is positioned between the frame  6  and the rotor  4   a . The first balancer  18  is housed in a balancer cover  18   a . The rotor  4   a  is provided at the lower end thereof with a second balancer  19 . The second balancer  19  is positioned between the rotor  4   a  and the sub frame  20 . The first balancer  18  and the second balancer  19  cancel out any imbalance that may be caused by the orbiting scroll  40  and the slider  16 . 
     (Main Bearing  8   a  and Orbital Bearing  8   c ) 
     The main bearing  8   a  and the orbital bearing  8   c  are each a plain bearing. A plain bearing refers to a bearing including a fixed cylindrical metal or resin part and a rotatable metal part which move relative to each other in such a manner as to form a fluid film of oil between sliding surfaces thereof. 
     (Suction Pipe  11 ) 
     The suction pipe  11  is a pipe through which refrigerant in a gas state is suctioned into the shell  2 . The suction pipe  11  is attached to the circumferential wall of the shell  2 . Specifically, the suction pipe  11  is connected to the middle shell  2   c.    
     (Discharge Pipe  12 ) 
     The discharge pipe  12  is a pipe through which the refrigerant compressed in the compressing mechanism unit  5  is discharged to the outside of the shell  2 . The discharge pipe  12  is attached to the top of the shell  2 . Specifically, the discharge pipe  12  is connected to the upper shell  2   a . The discharge pipe  12  connects the discharge chamber  13 , provided inside the shell  2 , and the refrigerant circuit, provided outside the shell  2 , to each other. 
     (Sub Frame  20 ) 
     The sub frame  20  housed in the shell  2  is positioned below the motor  4  and is fixed to the inner circumferential surface of the middle shell  2   c . The sub frame  20  supports the rotating shaft  7  with the aid of the counterbearing  8   b  such that the rotating shaft  7  is rotatable. The counterbearing  8   b  is a ball bearing but is not limited thereto. The counterbearing  8   b  may be a bearing of any other kind. The counterbearing  8   b  is fitted in a counterbearing-receiving portion, which is fixed to a central portion of the sub frame  20 . 
     (Oil-Draining Pipe  21 ) 
     The oil-draining pipe  21  is connected at one end thereof to the Oldham-ring space as described above, thereby being connected to a space between the frame  6  and the orbiting scroll  40 . The other end of the oil-draining pipe  21  extends downward in the shell  2  and is connected to a space between the frame  6  and the sub frame  20 . The oil-draining pipe  21  allows an excessive portion of the oil flowing into the space between the frame  6  and the orbiting scroll  40  to be discharged into the space between the frame  6  and the sub frame  20 . The oil discharged into the space between the frame  6  and the sub frame  20  flows through the sub frame  20  and returns into the oil sump  3   a.    
     (Vibration Sensor  60 ) 
     The vibration sensor  60  is attached to the shell  2  and measures the vibration of the compressor  100  that is in operation. The vibration sensor  60  is attached to the outer circumferential surface of the shell  2  and at the same height as the main bearing  8   a  or the orbital bearing  8   c . The vibration sensor  60  measures the vibration occurring in the radial direction of the shell  2 . The vibration sensor  60  is, for example, a piezoelectric acceleration pickup. The vibration sensor  60  is connected to the controller  200 . The acceleration measured by the vibration sensor  60  is taken as the vibration value of the compressor  100 , and the vibration value is transmitted to the controller  200 . 
     (Current Sensor  61 ) 
     The current sensor  61  measures the value of an electric current flowing through the compressor  100 : specifically, the value of an electric current flowing through the motor  4 . The current value measured by the current sensor  61  is transmitted to the controller  200 . 
     (Power-Supplying Unit  70  and Power Source  71 ) 
     The power-supplying unit  70  supplies electric power to the motor  4  and includes a power-supplying terminal (not illustrated) and other relevant elements. The power-supplying terminal extends through the shell  2 . Electric power is supplied from a power source  71  to the motor  4  through the power-supplying terminal. 
     [Description of Controller  200 ] 
     The controller  200  includes an analyzing unit  201  and a wear-detecting unit  202 . The analyzing unit  201  performs frequency analysis of the current value measured by a sensor that measures an index value correlated with the movement of the rotating shaft  7  that is made while the compressor  100  is in operation. In Embodiment 1, the sensor is the vibration sensor  60  or the current sensor  61 . The analyzing unit  201  performs frequency analysis of the vibration value measured by the vibration sensor  60  or the current value measured by the current sensor  61 . The wear-detecting unit  202  detects the degree of wear of a bearing of interest based on the result of the analysis performed by the analyzing unit  201 . The main bearing  8   a  and the orbital bearing  8   c  are the objects of wear detection. Details of the processes to be performed by the analyzing unit  201  and the wear-detecting unit  202  will be described separately below. 
     The controller  200  is a piece of dedicated hardware or a CPU (central processing unit) that executes programs stored in a memory. A CPU is also referred to as a processing device, an arithmetic device, a microprocessor, a microcomputer, or a processor. 
     If the controller  200  is a piece of dedicated hardware, the controller  200  is any of the following: for example, a single circuit, a composite circuit, an ASIC (application-specific integrated circuit), an FPGA (field-programmable gate array), and a combination of the foregoing. Individual functions of the controller  200  may be realized by respective pieces of hardware, or such functions may all be realized by a single piece of hardware. 
     If the controller  200  is a CPU, the functions to be executed by the controller  200  are realized by software, firmware, or a combination of software and firmware. The software and the firmware are each written as programs and are stored in a memory. The CPU realizes the functions of the controller  200  by reading the programs stored in the memory and executing the programs. The memory is, for example, a non-volatile or volatile semiconductor memory such as a RAM, a ROM, a flash memory, an EPROM, or an EEPROM. 
     Some of the functions of the controller  200  may be realized by dedicated hardware, while the others may be realized by software or firmware. 
     [Description of Operation of Compressor  100 ] 
     An operation of the compressor  100  will now be described. When electric power is supplied from the power source  71  through the power-supplying unit  70  to the stator  4   b , the stator  4   b  generates a magnetic field. The magnetic field causes the rotor  4   a  to rotate. That is, when electric power is supplied to the stator  4   b , the rotor  4   a  generates a torque, which rotates the rotating shaft  7  supported by the main bearing  8   a  and the counterbearing  8   b . The orbiting scroll  40  connected to the rotating shaft  7  is prevented from spinning by the Oldham ring  15  but undergoes an orbital motion. Through the foregoing process, the compressor  100  changes the capacity of the compression chamber  5   a  provided by the combination of the scroll portion  31  of the fixed scroll  30  and the scroll portion  41  of the orbiting scroll  40 . 
     With the orbital motion of the orbiting scroll  40 , the gas-state refrigerant suctioned into the shell  2  through the suction pipe  11  flows into the compression chamber Sa. As the refrigerant advances toward the center of the compression chamber  5   a , the refrigerant is gradually compressed. The refrigerant thus compressed flows into the outlet  32  provided in the end plate  30   a  of the fixed scroll  30 , opens the discharge valve mechanism  50 , and is discharged through the discharge pipe  12  to the refrigerant circuit provided outside the compressor  100 . 
     In the compressor  100 , the imbalance caused by the motions of the orbiting scroll  40  and the Oldham ring  15  is cancelled out by the first balancer  18  attached to the rotating shaft  7  and the second balancer  19  attached to the rotor  4   a . Furthermore, in the compressor  100 , lubricating oil stored at the bottom of the shell  2  is supplied through the oil passage  7   a  provided in the rotating shaft  7  to relevant sliding parts including the main bearing  8   a , the counterbearing  8   b , and other relevant thrust surfaces. 
     [Operation of Main Bearing  8   a ] 
     When the compressor  100  is activated, the sleeve  17  undergoes a relative motion on the inner circumferential surface of the main bearing  8   a  with oil being present therebetween. Specifically, the sleeve  17  undergoes a relative motion on the inner circumferential surface of the main bearing  8   a  while being pushed against the inner circumferential surface in the direction of the resultant of the centrifugal force exerted by the first balancer  18  and the gas load received from the compressing mechanism unit  5 . In this relative motion, the sleeve  17  rotates while being in contact with the inner circumferential surface of the main bearing  8   a  with oil being present therebetween. In a section orthogonal to the rotating shaft  7 , the sleeve  17  having a cylindrical shape is in contact with the inner circumferential surface of the main bearing  8   a  at a single point, which moves in the circumferential direction in the relative motion with the rotation of the rotating shaft  7 . 
     [Operation of Orbital Bearing  8   c ] 
     When the compressor  100  is activated, the slider  16  undergoes a relative motion on the inner circumferential surface of the orbital bearing  8   c  with oil being present therebetween. Specifically, the slider  16  undergoes a relative motion on the inner circumferential surface of the orbital bearing  8   c  while being pushed against the inner circumferential surface in the direction of the resultant of the centrifugal force exerted by the first balancer  18  and the gas load received from the compressing mechanism unit  5 . In this relative motion, the slider  16  rotates while being in contact with the inner circumferential surface of the orbital bearing  8   c  with oil being present therebetween. In a section orthogonal to the rotating shaft  7 , the slider  16  having a cylindrical shape is in contact with the inner circumferential surface of the orbital bearing  8   c  at a single point, which moves in the circumferential direction in the relative motion with the rotation of the rotating shaft  7 . 
     [Wear Detection] 
     If, for example, the amount of oil that is discharged together with the refrigerant to the outside of the compressor  100  increases, a lack of oil occurs in the compressor  100 . In such a situation, the sleeve  17  and the inner circumferential surface of the main bearing  8   a  directly come into contact with each other, leading to wear over the entire inner circumference of the main bearing  8   a . This also applies to the orbital bearing  8   c . If the rotating shaft  7  and the inner circumferential surface of the orbital bearing  8   c  directly come into contact with each other, wear occurs over the entire inner circumference of the orbital bearing  8   c . That is, the direct contact between the bearing and the cylindrical part provided on the inner side of the bearing causes wear of the bearing. 
     In Embodiment 1, the degree of wear of the bearing is to be detected. Specifically, in Embodiment 1, abnormal abrasion is to be detected, which occurs when the wear of the bearing progresses and the amount of abrasion exceeds a specified value. The specified value may be defined in any way. In the following description, in terms of reliability, the specified value is set at the amount of tolerable wear that represents the limit of the amount of wear. 
     Embodiment 1 relates to a case where only one of the main bearing  8   a  and the orbital bearing  8   c  is the object of detection of abnormal abrasion. The configuration of the bearing that is the object of detection of abnormal abrasion is the same between the case of the main bearing  8   a  and the case of the orbital bearing  8   c . Hence, the bearing to be the object of abnormal abrasion is generally denoted as “bearing  80 ”, and a specific configuration of the bearing  80  will now be described, followed by a method of detecting abnormal abrasion. The case where both the main bearing  8   a  and the orbital bearing  8   c  are the objects of detection of abnormal abrasion will be described in Embodiment 3. 
     [Detailed Configuration of Bearing  80 ] 
     (Outline of Bearing  80 ) 
       FIG.  2    is a schematic sectional view of the bearing included in the compressor according to Embodiment 1.  FIG.  3    is an enlarged schematic sectional view of a part of  FIG.  2   , including an air gap and peripheral elements. 
     As illustrated in  FIG.  2   , the bearing  80  has a cylindrical shape and is formed of, for example, two parts. Specifically, the bearing  80  includes a cylindrical backing metal part  81  and a cylindrical alloy part  82 . The alloy part  82  is provided on the inner circumferential side of the backing metal part  81 . The metal forming the backing metal part  81  has a greater tensile strength than the metal forming the alloy part  82 . The alloy part  82  is made of a highly slidable material, such as a copper alloy or an aluminum alloy. 
     The outer circumferential surface of the alloy part  82  has a plurality of air gaps  83 . The plurality of air gaps  83  are arranged at regular intervals in the circumferential direction. While a case where three air gaps  83  are provided is herein taken as an example, at least two air gaps  83  are to be provided. Providing the plurality of air gaps  83  reduces the strength of the alloy part  82 . Therefore, if the bearing  80  is formed of the alloy part  82  alone, the bearing  80  may be broken under some load. In this respect, the bearing  80  is formed as a combination of the alloy part  82  and the backing metal part  81 . If satisfactory strength is provided, the bearing  80  may be formed of the alloy part  82  alone. 
     The air gaps  83  are each a recess provided in the outer circumferential surface of the alloy part  82 . Referring to  FIG.  3   , the bottom surface,  83   a , of the recess and the inner circumferential surface,  82   a , of the bearing  80  is at a first distance  81  from each other. The first distance  81  is set at the amount of tolerable wear, which is several tens of micrometers, for example. Alternatively, the first distance  81  may be greater than the amount of tolerable wear. The first distance  81  is settable at a value greater than the amount of tolerable wear as follows. 
     As the inner circumferential surface  82   a  of the bearing  80  wears by being in contact with the cylindrical part provided on the inner side of the bearing  80 , the thickness of a portion  82   b  between the bottom surface  83   a  of the recess serving as the air gap  83  and the inner circumferential surface  82   a  of the alloy part  82  (see  FIG.  3   , hereinafter referred to as “wear-tolerable-thickness portion  82   b ”) is reduced. The wear occurs over the entirety of the inner circumferential surface  82   a  of the alloy part  82 . Herein, however, the description focuses on the wear-tolerable-thickness portion  82   b . As the wear progresses, the wear-tolerable-thickness portion  82   b  becomes thinner and more fragile. In such a situation, if the bearing load acting on the bearing  80  is excessive, the wear-tolerable-thickness portion  82   b  collapses into the air gap  83 . Therefore, the sum of: the thickness of the wear-tolerable-thickness portion  82   b  at which the wear-tolerable-thickness portion  82   b  collapses into the air gap  83 ; and the amount of tolerable wear; may be set as the first distance  81 . With such a setting, if the bearing load acting on the bearing  80  is excessive, the collapse of the wear-tolerable-thickness portion  82   b  may occur when the inner circumferential surface  82   a  of the bearing  80  has worn by the tolerable amount. In other words, such a setting prevents the occurrence of a situation where the wear-tolerable-thickness portion  82   b  collapses before the amount of tolerable wear for the inner circumferential surface  82   a  of the bearing  80  is reached. 
     A method of detecting abnormal abrasion according to Embodiment 1 utilizes, as to be described in detail below, a situation where the wear-tolerable-thickness portion  82   b  wears away to make the air gap  83  continuous with the space on the inner side of the bearing  80 . In other words, if the air gap  83  becomes continuous with the space on the inner side of the bearing  80 , the shape of the inner circumferential surface of the bearing  80  changes, which is utilized for the detection of abnormal abrasion. The collapse of the wear-tolerable-thickness portion  82   b  into the air gap  83  also changes the shape of the inner circumferential surface of the bearing  80  and is therefore useful for the detection of abnormal abrasion. 
     [Principle of Abnormal Abrasion Detection] 
     A principle of detection of wear occurring in the bearing  80  will now be described. 
     While the compressor  100  is in operation, the rotating shaft  7  rotates. In a section orthogonal to the rotating shaft  7 , as described above, the rotating cylindrical part provided on the inner side of the bearing  80  is in contact with the inner circumferential surface  82   a  of the bearing  80  at a single point. The contact point moves relative to the inner circumferential surface  82   a  with oil being present therebetween. If any incident occurs such as a backflow of liquid refrigerant into the compressor  100  that may dilute the oil or a reduction in the amount of oil in the compressor  100 , the bearing  80  and the cylindrical part directly come into contact with each other, resulting in wear of the inner circumferential surface  82   a  of the bearing  80 . As the wear of the inner circumferential surface  82   a  of the bearing  80  progresses, the wear-tolerable-thickness portion  82   b  may eventually wears away. 
     Before the wear-tolerable-thickness portion  82   b  wears away, the state of the contact point between the cylindrical part and the inner circumferential surface  82   a  of the bearing  80  is stable. Therefore, a stable oil film is formed between the cylindrical part and the inner circumferential surface  82   a  of the bearing  80 . If the wear-tolerable-thickness portion  82   b  wears away and the air gap  83  becomes continuous with the space on the inner side of the bearing  80 , a sudden change occurs in the shape of the inner circumferential surface  82   a  of the bearing  80 , which deteriorates the formation of the oil film. Consequently, the vibration of the rotating shaft  7  increases. Accordingly, the vibration value and the current value increase. In Embodiment 1, such changes in the vibration value and the current value caused by the change in the shape of the inner circumferential surface  82   a  of the bearing  80  are utilized for the detection of abnormal abrasion. 
     Herein, the first distance  81 , which corresponds to the thickness of the wear-tolerable-thickness portion  82   b , is set at the amount of tolerable wear, as described above. Therefore, the wear-away of the wear-tolerable-thickness portion  82   b  indicates that abrasion of an amount exceeding the amount of tolerable wear has occurred on the inner circumferential surface  82   a  of the bearing  80 . Accordingly, if changes in the vibration value and the current value are detectable, abnormal abrasion is detectable. 
     [Circumferential Width W of Air Gap  83 ] 
     To perform the above detection of abnormal abrasion, the air gap  83  needs to have a circumferential width W that is set as follows. As described above, the bearing  80  and the cylindrical part provided on the inner side thereof are in contact with each other at a single point during operation, and the contact point moves in the circumferential direction in the relative motion with the rotation of the rotating shaft  7 . The sampling interval [s] in the frequency analysis of the vibration value or the current value needs to be shorter than time t1, which is required for the contact point to move by the circumferential width W of the air gap  83 . The reciprocal of the time t1 is regarded as the frequency range for the frequency analysis. It is known that frequency analysis needs to be based on a sampling frequency fs that is 2.56 times the frequency range. Furthermore, the maximum frequency that is settable as the sampling frequency fs is specific to the analyzer to be used for frequency analysis. Herein, a maximum frequency of 51.2 [kHz] is defined as the sampling frequency. Accordingly, the time t1 is expressed as 2.56/51.2×10 3 =0.051×10 −3 . 
     Considering the above, the circumferential width W of the air gap  83  is calculated as follows, on condition that the width W is made longer than a length obtained through the multiplication of the velocity, V, of the rotating shaft  7  by the time t1 required for the contact point to move by the circumferential width W of the air gap  83 : 
         W  [mm]≤ t 1[s]× V  [mm/s]
 
     where 
     velocity V=inside diameter of bearing  80 ×π×rotation speed r, and 
     time t1=0.05×10 −3  [s]. 
     Hence, letting the minimum rotation speed r of the rotating shaft  7  be 30 [rps] and the inside diameter, D, of the bearing  80  be 40 [mm], the following holds: 
         W≥ 0.05×10 −3 ×40×π×30=0.1884.
 
     Thus, in the above example, the circumferential width W of the air gap  83  is set at 0.1884 [mm] or greater. The upper limit of the width W is defined as follows. After the occurrence of wear by δ1, the total circumferential length of portions of the inner circumferential surface of the bearing  80  that are effective as a bearing is reduced by a length corresponding to the number, N, of air gaps  83  times the width W [mm]. Therefore, if the width W is too long, the bearing  80  may lose the function as a bearing after the occurrence of wear by δ1 or greater, leading to a failure of the compressor. Hence, the maximum width that does not cause the failure is regarded as the upper limit of the circumferential width W of the air gap  83 . 
     [Analyzing Unit  201  and Wear-Detecting Unit  202 ] 
       FIG.  4    summarizes a frequency at which an anomalous peak appears in the result of frequency analysis performed by the analyzing unit included in the compressor system according to Embodiment 1. 
     In the bearing  80 , the plurality of air gaps  83  are arranged at regular intervals. Letting the number of air gaps  83  that are arranged at regular intervals be N, as summarized in  FIG.  4   , an anomalous peak appears in abnormal time at a frequency calculated as the product of the number N of air gaps provided in the bearing  80  and the rotation frequency, f, of the motor  4 . Frequency analysis in Embodiment 1 is assumed to be performed by fast Fourier transform (FFT). In FFT, which is based on Fourier transform, it is premised that the signal to be analyzed has periodicity in every time range. Therefore, since the air gaps  83  are arranged at regular intervals, peak values can be obtained through frequency analysis. Herein, the signal to be analyzed refers to the change in temporal data on the current value that is caused by the air gaps  83  or the change in temporal data on the vibration value that is caused by the air gaps  83 . 
       FIG.  5    exemplifies the result of frequency analysis in normal time that is obtained by the analyzing unit of the compressor system according to Embodiment 1.  FIG.  6    exemplifies the result of frequency analysis in abnormal time that is obtained by the analyzing unit of the compressor system according to Embodiment 1. In  FIGS.  5  and  6   , the horizontal axis represents frequency [Hz], and the vertical axis represents vibration intensity [m/s]. The rotation frequency of the motor  4  is denoted by f. Vibration intensity is regarded as acceleration. While  FIGS.  5  and  6    exemplify the results of frequency analysis for the vibration value, similar results are obtained in frequency analysis for the current value. The result of frequency analysis in abnormal time exemplified by  FIG.  6    is for a case where the number of air gaps is 3. 
     In normal time, as illustrated in  FIG.  5   , the first-order component of the rotation frequency f of the motor  4  is to be detected as a major peak. In abnormal time, that is, if abrasion by the amount of tolerable wear occurs, as illustrated in  FIG.  6   , an anomalous peak whose value exceeds a set value appears at a frequency of 3×f, which is the product of “3” denoting the number of air gaps  83  and “f” denoting the rotation frequency. 
     Accordingly, if the result of frequency analysis that is obtained by the analyzing unit  201  contains a peak whose value exceeds a preset value at the frequency of 3×f, the wear-detecting unit  202  determines that abnormality has occurred. 
     The detection of such abnormality by the wear-detecting unit  202  indicates that the life of the compressor  100  may expire soon. Therefore, the controller  200  controls the compressor  100  such that the maximum rotation frequency during operation is reduced. Specifically, the maximum rotation frequency during operation is reduced to, for example, 80% of the maximum rotation frequency in normal time. Furthermore, if abnormality is detected by the wear-detecting unit  202 , the controller  200  notifies the maintenance contractor of the occurrence of abnormality. Since workers for the maintenance contractor are notified of the short remaining life of the compressor  100 , the workers prepare a replacement compressor. 
     The number of air gaps  83  only needs to be plural but is desired to be three or more because of the following reason. In the result of frequency analysis for the vibration value or the current value, as described above, the first-order component of the rotation frequency f of the motor  4  is detected as a major peak. Therefore, if only a single air gap  83  is provided, the change in the vibration value or the current value that is caused by the presence or absence of the air gap  83  may be undetectable. Hence, it is desirable to provide three or more air gaps  83 . Considering the reliability of the bearing  80 , the number of air gaps  83  is desired to be six or less. 
     The object of frequency analysis may be either the vibration value or the current value. If both are subjected to frequency analysis, accuracy of failure detection increases. Specifically, if the results of frequency analysis for the vibration value and for the current value both contain anomalous peaks, respectively, at the frequency expressed as the number N of air gaps  83  times the rotation frequency f, it is determined that abnormality has occurred. Thus, a highly accurate detection result can be obtained, and increased accuracy of failure detection is achieved. 
     The above description relates to a case where only one of the main bearing  8   a  and the orbital bearing  8   c  is taken as the object of detection of abnormal abrasion. If which of the main bearing  8   a  and the orbital bearing  8   c  wears more easily is found in advance from the specifications and usage of the compressor  100 , the plurality of air gaps  83  may be provided only to the one that wears more easily. 
     Advantageous Effects of Embodiment 1 
     The compressor system according to Embodiment 1 includes the compressor  100  including the bearing  80 , the bearing  80  supporting the rotating shaft  7 ; the sensor configured to measure the index value correlated with the movement of the rotating shaft  7 , the movement being made while the compressor  100  is in operation; and the wear-detecting unit  202  configured to detect the degree of wear of the bearing based on the measured value obtained by the sensor. The bearing  80  is a plain bearing and has the plurality of air gaps  83  arranged at intervals in the circumferential direction such that wear of the bearing  80  changes the positional relationship between the inner circumferential surface  82   a  of the bearing  80  and the plurality of air gaps  83  and eventually changes the shape of the inner circumferential surface of the bearing  80 . The wear-detecting unit  202  detects the degree of wear of the bearing  80  based on the change in the measured value that is caused by the change in the shape of the inner circumferential surface of the bearing  80 . 
     Thus, the degree of wear of the bearing  80  can be detected with a configuration realized by simply providing a plurality of air gaps  83  in the bearing  80 . Since the degree of wear of the bearing  80  is detectable, abnormality can be detected in an early stage. Therefore, the compressor  100  can be replaced with a new one before the current compressor  100  completely stops working. 
     The radial distance from the inner circumferential surface of the bearing  80  to at least one of the plurality of air gaps  83  is set at the amount of tolerable wear that represents the limit of the amount of wear. 
     Thus, the occurrence of abnormal wear by an amount exceeding the amount of tolerable wear can be detected. 
     According to Embodiment 1, the bearing  80  includes the cylindrical backing metal part  81 , and the cylindrical alloy part  82  provided on the inner side of the backing metal part  81 . Furthermore, the plurality of air gaps  83  are provided in the outer circumferential surface of the alloy part  82 . 
     With such a configuration, the bearing  80  having the plurality of air gaps  83  can be obtained. 
     The index value is the vibration value representing the vibration of the compressor or the current value representing the electric current flowing through the compressor. 
     That is, the vibration value or the current value can be used as the index value correlated with the movement of the rotating shaft  7  that is made while the compressor  100  is in operation. 
     Embodiment 2 
     Embodiment 2 relates to a configuration in which the bearing  80  has air gaps of a plurality of kinds that are different in the radial distance from the inner circumferential surface  82   a  of the bearing  80 . Now, Embodiment 2 will be described, focusing on the difference from Embodiment 1. 
       FIG.  7    is a schematic sectional view of a bearing included in a compressor according to Embodiment 2.  FIG.  8    is an enlarged sectional view of a part of  FIG.  7   , including a second air gap and peripheral elements. 
     The bearing,  80 A, according to Embodiment 2 has first air gaps  84   a  and second air gaps  84   b , which are of two kinds that are different in the distance from the inner circumferential surface of the bearing  80 A. The first air gaps  84   a  and the second air gaps  84   b  are recesses provided in the outer circumferential surface of the alloy part  82  and having different depths, so that the distance from the inner circumferential surface of the bearing  80 A varies. The distance between the bottom surface,  84   a   1 , of each of the recesses forming the first air gaps  84   a  and the inner circumferential surface  82   a  is δ1, the same as in the case illustrated in  FIG.  3   . The distance between the bottom surface,  84   b   1 , of each of the recesses forming the second air gaps  84   b  and the inner circumferential surface  82   a  is δ2, as illustrated in  FIG.  8   . The relationship between δ1 and δ2 is expressed as δ1&gt;δ2. 
     The first air gaps  84   a  and the second air gaps  84   b  are provided in equal number. three each in this case. The first air gaps  84   a  and the second air gaps  84   b  are arranged alternately and at regular intervals. That is, the air gaps of different kinds are arranged alternately among the different kinds and at regular intervals in the circumferential direction. In such an arrangement, the result of frequency analysis contains an anomalous peak at a frequency expressed as the number of air gaps times f. 
       FIG.  9    exemplifies the result of frequency analysis at the occurrence of wear by δ1 that is obtained by an analyzing unit included in a compressor system according to Embodiment 2. 
     If the amount of wear of the bearing  80 A reaches δ2, as with the case of Embodiment 1, an anomalous peak appears at a frequency of 3×f as illustrated in  FIG.  6   . If the abrasion of the bearing  80 A further progresses and the amount of abrasion reaches δ1, all of the six air gaps of the bearing  80 A become continuous with the space on the inner side of the bearing  80 A. Accordingly, an anomalous peak appears at a frequency of 64 as illustrated in  FIG.  9   . 
     Therefore, if it is known in advance that air gaps of two kinds that are different in the distance from the inner circumferential surface  82   a  of the bearing  80  are provided three each in the bearing  80 A, it is found that a result of frequency analysis containing an anomalous peak at the frequency of 3×f indicates the occurrence of wear by δ2. It is also found that a result of frequency analysis containing an anomalous peak at the frequency of 6×f indicates the occurrence of wear by δ1. 
     While Embodiment 2 relates to a case where air gaps of two kinds are provided, air gaps of three or more kinds may be provided. In the latter case as well, the air gaps are provided in equal number for each of different kinds and are arranged alternately among the different kinds and at regular intervals in the circumferential direction. Thus, the occurrence of wear can be detected in the same manner as above. 
     Advantageous Effects of Embodiment 2 
     According to Embodiment 2, the same advantageous effects as those produced by Embodiment 1 are produced. In addition, the following advantageous effects are produced. In Embodiment 2, since the bearing  80 A has the air gaps of two kinds that are different in depth, the progress of wear is detectable. Therefore, the degree of urgency for the replacement of the compressor can be found. In Embodiment 1 described above, the abnormal wear to be detected is of the amount of tolerable abrasion δ1. In such a situation, there is only a short time left before the compressor  100  completely stops working. Consequently, the compressor needs to be replaced urgently. In contrast, in Embodiment 2, the occurrence of wear by δ2, which is reached before the amount of tolerable abrasion δ1 is reached, is also detectable. Therefore, it is possible to find that there is still some time left before the time for replacement. Accordingly, for example, in a case where a plurality of compressors  100  are to be managed, if there are not enough replacement compressors, replacement can be done in order of priority. 
     Embodiment 3 
     Embodiment 3 relates to a case where both the main bearing  8   a  and the orbital bearing  8   c  are the objects of detection of abnormal abrasion. Now, Embodiment 3 will be described, focusing on the difference from Embodiment 1. 
     If both the main bearing  8   a  and the orbital bearing  8   c  are the objects of detection of abnormal abrasion, both the main bearing  8   a  and the orbital bearing  8   c  are provided with air gaps. It is preferable that the air gaps provided in the main bearing  8   a  be out of phase with the air gaps provided in the orbital bearing  8   c . If the air gaps of the two are in phase with each other, the vibration is amplified, which reduces the reliability of the compressor  100 . 
     Here, three patterns will be given for discussing the case where both the main bearing  8   a  and the orbital bearing  8   c  have the air gaps. 
     Pattern 1: A case where the main bearing  8   a  and the orbital bearing  8   c  have an equal number of air gaps 
     Pattern 2: A case where the main bearing  8   a  and the orbital bearing  8   c  have different numbers of air gaps 
     Pattern 3: A case where Embodiment 2 is applied such that the main bearing  8   a  and the orbital bearing  8   c  each have air gaps of two kinds that are different in depth 
     (Pattern 1) 
       FIG.  10    summarizes a relationship, in Pattern 1, between the bearing having abnormality and the frequency having an anomalous peak to be identified by frequency analysis in a compressor system according to Embodiment 3. 
     Specifically,  FIG.  10    summarizes a relationship between the bearing having abnormality and the frequency having an anomalous peak in a case where the main bearing  8   a  and the orbital bearing  8   c  each have an N number of air gaps that are out of phase with the air gaps of the other bearing. 
     Referring to  FIG.  10   , supposing that N is 3, for example, and that the phase of the air gaps is different between the two bearings, if abnormal abrasion occurs only in the main bearing  8   a , an anomalous peak appears at a frequency of 3×f. Likewise, if abnormal abrasion occurs only in the orbital bearing  8   c , an anomalous peak appears at the frequency of 3×f. If abnormal abrasion occurs in both the main bearing  8   a  and the orbital bearing  8   c , an anomalous peak appears at a frequency of 2×3×f. 
     Accordingly, if the result of frequency analysis contains an anomalous peak at the frequency of 3×f, it is determined that either the main bearing  8   a  or the orbital bearing  8   c  has abnormality. If the result contains an anomalous peak at the frequency of 6f, it is determined that both the main bearing  8   a  and the orbital bearing  8   c  have abnormality. 
     (Pattern 2) 
       FIG.  11    summarizes a relationship, in Pattern 2, between the bearing having abnormality and the frequency having an anomalous peak to be identified by frequency analysis in the compressor system according to Embodiment 3. Specifically,  FIG.  11    summarizes a relationship between the bearing having abnormality and the frequency having an anomalous peak in a case where the main bearing  8   a  has an N number of air gaps and the orbital bearing  8   c  has an M number of air gaps that are out of phase with the air gaps of the main bearing  8   a.    
     The bearing having abnormality and the frequency having an anomalous peak have the relationship summarized in  FIG.  11   . Therefore, if an anomalous peak appears at a frequency of N×f, it is determined that the main bearing  8   a  has abnormality. If an anomalous peak appears at a frequency of M×f, it is determined that the orbital bearing  8   c  has abnormality. If anomalous peaks appear at both the frequency of N×f and the frequency of M×f, it is determined that both the main bearing  8   a  and the orbital bearing  8   c  have abnormality. 
     Thus, if the number of air gaps is made different between the main bearing  8   a  and the orbital bearing  8   c  as in Pattern 3, it is possible to find which of the main bearing  8   a  and the orbital bearing  8   c  has caused abnormal wear, that is, the position of wear. 
     (Pattern 3) 
       FIG.  12    summarizes a relationship, in Pattern 3, between the bearing having abnormality and the frequency having an anomalous peak to be identified by frequency analysis in the compressor system according to Embodiment 3. Specifically,  FIG.  12    summarizes a relationship between the bearing having abnormality and the frequency having an anomalous peak in the following arrangement. The main bearing  8   a  has air gaps of two kinds that are different in depth: an N number of first air gaps  84   a , and an N number of second air gaps  84   b . The orbital bearing  8   c  has air gaps of two kinds that are different in depth: an M number of first air gaps  84   a , and an M number of second air gaps  84   b . In each of the main bearing  8   a  and the orbital bearing  8   c , the air gaps of different kinds are arranged alternately between the different kinds and at regular intervals in the circumferential direction. Furthermore, the phase of the air gaps is different between the main bearing  8   a  and the orbital bearing  8   c.    
     Pattern 3 is based on the same logic as for Patterns 1 and 2. There is a relationship summarized in  FIG.  12   . Since this relationship is known in advance, the position of wear and the progress of wear can be found. For example, if an anomalous peak appears only at a frequency of N×f, it is determined that wear by δ2 has occurred only in the main bearing  8   a . If anomalous peaks appear at two frequencies of N×f and M×f, it is determined that wear by δ2 has occurred in both the main bearing  8   a  and the orbital bearing  8   c . If anomalous peaks appear at frequencies of (2×N)×f and (2×M)×f, it is determined that abnormality with abrasion by δ1 has occurred in both the main bearing  8   a  and the orbital bearing  8   c.    
     While Embodiment 3 relates to a case where air gaps of two kinds are provided, air gaps of three or more kinds may be provided. In the latter case as well, in each of the main bearing  8   a  and the orbital bearing  8   c , the air gaps are provided in equal number for each of different kinds and are arranged alternately among the different kinds and at regular intervals in the circumferential direction. Thus, the occurrence of wear can be detected in the same manner as above. 
     Advantageous Effects of Embodiment 3 
     According to Embodiment 3, since both the main bearing  8   a  and the orbital bearing  8   c  have the air gaps, abnormality can be detected for both of the bearings. 
     Furthermore, since the number of air gaps is made different between the main bearing  8   a  and the orbital bearing  8   c , it is possible to find which of the main bearing  8   a  and the orbital bearing  8   c  has caused abnormal wear, that is, the position of wear. Therefore, if the method of operating the refrigeration cycle apparatus is associated with the position of wear, the method of operating the refrigeration cycle apparatus can be adjusted such that the amount of wear is reduced. Such an operation method contributes to the increase in the reliability of the compressor  100 . An exemplary operation is as follows. If the backflow of liquid refrigerant occurs, the wear of the orbital bearing  8   c  tends to progress faster. Therefore, if abnormal wear of the orbital bearing  8   c  is detected, the operation may be changed in such a manner as to prevent the backflow of liquid refrigerant. 
     In Embodiment 3, the number of air gaps is made different between the main bearing  8   a  and the orbital bearing  8   c . Furthermore, the air gaps of each of the bearings are of a plurality of kinds that are different in depth but are equal in number. Furthermore, in each of the main bearing  8   a  and the orbital bearing  8   c , the air gaps of different kinds are arranged alternately among the different kinds and at regular intervals in the circumferential direction. Thus, both the position of abrasion and the progress of abrasion can be detected. 
     Embodiment 4 
     Embodiment 4 relates to a case where air gaps are provided in the inner circumferential surface of the bearing. Now, Embodiment 4 will be described, focusing on the difference from Embodiment 1. Embodiment 4 relates to a case where one of the main bearing  8   a  and the orbital bearing  8   c  is the object of detection of abnormal abrasion. 
       FIG.  13    is a schematic sectional view of a bearing included in a compressor system according to Embodiment 4.  FIG.  14    is an enlarged schematic sectional view of a part of  FIG.  13   , including an air gap and peripheral elements. 
     The bearing,  80 B, according to Embodiment 4 has a plurality of air gaps  85  in the inner circumferential surface  82   a  thereof. The air gaps  85  are recesses provided in the inner circumferential surface  82   a . Embodiment 4 relates to a case where the first distance  81  (see  FIG.  14   ) between the bottom surface,  85   a , of each of the recesses forming the air gaps  85  and the inner circumferential surface  82   a  of the bearing  80 B is set at the amount of tolerable wear. Alternatively, the first distance may be set at δ2, which is shorter than δ1. The first distance between the bottom surface  85   a  of each of the recesses forming the air gaps  85  and the inner circumferential surface  82   a  of the bearing  80 B may be set based on the degree of wear that is desired to be detected. 
     [Principle of Abnormal Abrasion Detection] 
       FIG.  15    exemplifies the result of frequency analysis in normal time in the compressor system according to Embodiment 4.  FIG.  16    exemplifies the result of frequency analysis in abnormal time in the compressor system according to Embodiment 4. Embodiment 4 relates to a case where the bearing  80 B has three air gaps  85 . 
     In Embodiment 4, the three air gaps  85  are continuous with the space on the inner side of the bearing  80 B in normal time. Therefore, as illustrated in  FIG.  15   , a normal peak exceeding a set value appears at a frequency of 3×f. With the progress of wear of the bearing  80 , if the air gaps provided in the bearing  80 B are reduced to be flash with the inner circumferential surface  82   a  of the bearing  80 B over the entire circumference, the normal peak at the frequency of 3×f disappears as illustrated in  FIG.  16   . Therefore, if it is known in advance that the bearing  80 B has three air gaps  85 , it is found that a result of frequency analysis containing no peak exceeding the set value at the frequency of 3×f indicates the occurrence of abnormal wear. 
     While the above description relates to a case where one of the main bearing  8   a  and the orbital bearing  8   c  is the object of detection of abnormal abrasion, the same detection of abnormality can be achieved in a case where both the main bearing  8   a  and the orbital bearing  8   c  are the objects of abnormal abrasion. In the latter case, the air gaps are provided in the inner circumferential surface of each of the main bearing  8   a  and the orbital bearing  8   c . For example, if three air gaps are provided in the main bearing  8   a  and four air gaps are provided in the orbital bearing  8   c , the results of frequency analysis in normal time and in abnormal time are as illustrated in  FIGS.  17  to  19   . 
       FIG.  17    exemplifies the result of frequency analysis in normal time in a case where the air gaps are provided in both the main bearing and the orbital bearing in the compressor system according to Embodiment 4.  FIG.  18    illustrates the result of frequency analysis in a case where abnormal wear has occurred only in the main bearing in the compressor system according to Embodiment 4.  FIG.  19    illustrates the result of frequency analysis in a case where abnormal wear has occurred only in the orbital bearing in the compressor system according to Embodiment 4. 
     As illustrated in  FIG.  17   , in normal time, normal peaks appear at a frequency of 34 and at a frequency of 4×f. If abnormal wear occurs only in the main bearing  8   a , the result of frequency analysis illustrated in  FIG.  18    is obtained. If abnormal wear occurs only in the orbital bearing  8   c , the result of frequency analysis illustrated in  FIG.  19    is obtained. Therefore, if it is known in advance that the main bearing  8   a  has three air gaps in the inner circumferential surface thereof and the orbital bearing  8   c  has four air gaps in the inner circumferential surface thereof, it is found that a result of frequency analysis containing no normal peak exceeding the set value at the frequency of 3×f as illustrated in  FIG.  18    indicates the occurrence of abnormal wear in the main bearing  8   a . It is also found that a result of frequency analysis containing no normal peak exceeding the set value at the frequency of 4×f indicates the occurrence of abnormal wear in the orbital bearing  8   c.    
     Advantageous Effects of Embodiment 4 
     According to Embodiment 4, the same advantageous effects as those produced by Embodiment 1 are produced. In Embodiments 1 to 3, as the wear progresses, the wear-tolerable-thickness portion  82   b  becomes thinner. Therefore, unless the wear-tolerable-thickness portion  82   b  is designed appropriately, the wear-tolerable-thickness portion  82   b  that has received a bearing load may collapse into the air gaps before the amount of tolerable wear is reached. In such a situation, an anomalous peak may appear before the amount of tolerable wear is reached. Consequently, abnormal abrasion cannot be detected accurately. In view of such a situation, Embodiment 4 does not employ the wear-tolerable-thickness portion  82   b . Therefore, the above collapse can be avoided, and abnormal abrasion can be detected accurately. 
     Embodiment 5 
     Embodiment 5 relates to a refrigeration cycle apparatus including the compressor system according to any of Embodiments 1 to 4. 
       FIG.  20    illustrates a refrigerant circuit of a refrigeration cycle apparatus according to Embodiment 5. 
     The refrigeration cycle apparatus,  300 , includes the compressor system according to any of Embodiments 1 to 4. Specifically, the refrigeration cycle apparatus  300  includes the compressor  100 , a condenser  301 , an expansion valve  302 , serving as a decompressor, and an evaporator  303 . Gas refrigerant discharged from the compressor  100  of the compressor system flows into the condenser  301 , exchanges heat with air flowing through the condenser  301 , thereby turning into high-pressure liquid refrigerant to be discharged. The high-pressure liquid refrigerant discharged from the condenser  301  is decompressed by the expansion valve  302 , thereby turning into low-pressure two-phase gas-liquid refrigerant to flow into the evaporator  303 . The low-pressure two-phase gas-liquid refrigerant having flowed into the evaporator  303  exchanges heat with air flowing through the evaporator  303 , thereby turning into low-pressure gas refrigerant. The low-pressure gas refrigerant is suctioned into the compressor  100  again. 
     Since the refrigeration cycle apparatus  300  configured as above includes the compressor system according to any of Embodiments 1 to 4, abnormality in the compressor  100  can be detected in an early stage. Accordingly, the compressor can be replaced with a new one before the current compressor completely stops working. Such a configuration prevents the occurrence of a situation where the refrigeration cycle stops working to make the customer suffer from disadvantageous effects. 
     The refrigeration cycle apparatus  300  is applicable to apparatuses such as a refrigerator, a freezer, a vending machine, an air-conditioning apparatus, and a water heater. 
     REFERENCE SIGNS LIST 
       2 : shell,  2   a : upper shell,  2   b : lower shell,  2   c : middle shell,  3 : oil pump,  3   a : oil sump,  4 : motor,  4   a : rotor,  4   b : stator,  5 : compressing mechanism unit,  5   a : compression chamber,  6 : frame,  6   a : suction port,  6   b : thrust bearing,  6   c : oil-feeding groove,  6   d : internal space,  6   f : frequency,  7 : rotating shaft,  7   a : oil passage,  8   a : main bearing,  8   b : counterbearing,  8   c : orbital bearing,  11 : suction pipe,  12 : discharge pipe,  13 : discharge chamber,  15 : Oldham ring,  15   a : Oldham groove,  15   b : Oldham groove,  16 : slider,  17 : sleeve,  18 : first balancer,  18   a : balancer cover,  19 : second balancer,  20 : sub frame,  21 : oil-draining pipe,  30 : fixed scroll,  30   a : end plate,  31 : scroll portion,  32 : outlet,  32   a : open end,  40 : orbiting scroll,  40   a : end plate,  41 : scroll portion,  50 : discharge valve mechanism,  51 : reed valve,  52 : valve seat,  53 : reed valve retainer,  60 : vibration sensor,  61 : current sensor,  70 : power-supplying unit,  71 : power source,  80 : bearing,  80 A: bearing,  80 : bearing,  81 : backing metal part,  82 : alloy part,  82   a : inner circumferential surface,  82   b : wear-tolerable-thickness portion,  83 : air gap,  83   a : bottom surface,  84   a : first air gap,  84   a   1 : bottom surface,  84   b : second air gap,  84   b   1 : bottom surface,  85 : air gap,  85   a : bottom surface,  100 : compressor,  200 : controller,  201 : analyzing unit,  202 : wear-detecting unit,  300 : refrigeration cycle apparatus,  301 : condenser,  302 : expansion valve,  303 : evaporator