Abstract:
A preload measuring apparatus to measure the preload of a rolling bearing that includes a bearing support, a pressurization device, an excitation device, a vibration sensor, and a control section. The bearing support supports a rolling bearing by supporting both ends of a shaft inserted into an inner ring of the rolling bearing. The pressurization device loads a predetermined weight in the axial direction onto the rolling bearing. The excitation device applies vibration onto the rolling bearing by the drive of the piezoelectric elements. The vibration sensor detects the vibration generated on the rolling bearing by the vibration application of the excitation device. The control section operates the excitation device in the situation that the pressurization device presses the rolling bearing, and obtains the resonance frequency by the output signal from the vibration sensor and calculates the preload of the rolling bearing according to the obtained resonance frequency before the predetermined weight as loaded.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a preload measuring apparatus of a rolling bearing which is assembled into each type of precision rotational portions such as a spindle motor, rotary actuator, rotary encoder, or the like, of a video tape recorder (VTR), hard disk drive (HDD), laser beam printer (LBP), or similar device. 
     A rolling bearing which is assembled into a precision rotational portion such as a spindle motor, rotary actuator, rotary encoder, or the like, used in a VTR, HDD, LBP, or similar device, is required to be very accurately produced to prevent a whirling motion and a deflection in the axial direction. Accordingly, a rolling bearing to support the spindle is used under the condition that a preload in the axial direction is applied. By applying the preload, the rigidity of the bearing is kept high, the deflection accuracy of the shaft is increased, and the slippage of a ball at the high speed rotation is prevented. 
     As such the rolling bearing to support the spindle of the VTR, HDD, LBP, or the like, there are following rolling bearings: for example, as shown in FIG.  11 ( a ), a pair of respectively independent rolling bearings assembled onto a rotation shaft by press fitting or adhesion; as shown in FIG. 11 ( b ), a rolling bearing which has double-row raceways in the outer ring, and to which the preload is applied by press fitting or adhering the divided inner ring to be moveable in the axial direction; or as shown in FIG. 11 ( c ), a rolling bearing which has double-row raceways in the inner ring and the outer ring respectively, and to which the preload is applied at assembling by changing the dimension of the pitch of the raceway. 
     In the other words, in the rolling bearing shown in FIG. 11 ( a ), an inner race member thereof comprises a shaft and two inner rings attached to the shaft, and an outer race member thereof comprises two outer rings which are respectively mated with the inner rings. In the rolling bearing shown in FIG.  11 ( b ), an inner race member comprises a shaft and two inner rings attached to the shaft, and an outer race member thereof comprises a common outer ring having two raceways which are respectively mated with the two inner rings. In the rolling bearing shown in FIG.  11 ( c ), an inner race member thereof comprises a shaft having two raceways, and an outer race member thereof comprises an outer ring having two raceways which are respectively mated with the two raceways of the shaft. 
     However, in any method of the above description, it is difficult to measure the preload after the assembling of the rolling bearing, because the preload is different according to the positional dimension of each member. 
     Accordingly, for example, Japanese Patent Examined Publication No. Hei. 2-61100 disclose a preload measuring method wherein the resonance frequency of the bearing apparatus itself is obtained by applying a minute vibration onto the rolling bearing, and this resonance frequency is converted into the preload by using a constant relationship between the resonance frequency of the rolling bearing and the preload. 
     However, in such the preload measuring method, there is still a problem which will be described below. That is, noise vibration is generated due to a disturbance of the mechanical system when a minute vibration is applied onto the rolling bearing, and therefore the preload can not be accurately measured due to this noise vibration. 
     Further, as the rolling bearing becomes small and then the mass of the outer ring becomes small, the resonance frequency of the rolling bearing appears on the high frequency side. As shown in FIG. 7, the vibration peak appears, for example, near 20 kHz at which the noise is relatively strong. At that time the resonance frequency is covered with the noise, so that it becomes difficult to detect the resonance frequency. Accordingly, the accuracy of the measurement is lowered, and the rigidity or the preload of the rolling bearing which are calculated, becomes incorrect. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve such the conventional problems, and an object of the present invention is to provide a preload measuring apparatus of a rolling bearing which can accurately measure the preload working on the rolling bearing by preventing the influence due to vibration of the disturbance. 
     The above-mentioned object can be achieved by a preload measuring apparatus for measuring a preload of a rolling bearing which includes an inner race member, an outer race member, and a plurality of rolling elements rotatably interposed between the inner race member and the outer race member, the preload measuring apparatus , according to the present invention comprising a bearing support member, a pressurization member, an excitation member, a vibration sensor and a controller. The bearing support member supports both ends of the inner race member in the axial direction of the rolling bearing. The pressurization member loads a predetermined weight to one of the both ends of the inner race member in the axial direction through the bearing support member. The excitation member applies vibration to the inner race member in the axial direction while the predetermined weight is loaded on the inner race member the pressurization member. The vibration sensor detects a vibration which is generated on the outer race member and is caused by the vibration of the inner race member. The controller is capable of calculating a resonance frequency of the rolling bearing through an output signal of the vibration sensor and also calculating a true preload of the rolling bearing in accordance with the thus calculated resonance frequency, the true preload defining a preload of the rolling bearing before the predetermined weight is loaded on the inner race member. 
     In the above-mentioned preload measuring apparatus according to the present invention, it is preferable that the inner race member comprises an inner ring and a shaft rigidly secured to the inner ring, and the bearing support supports both ends of the shaft in the axial direction. 
     In the above-mentioned preload measuring apparatus according to the present invention, it is preferable that the inner race member comprises an inner ring and a shaft rigidly secured to the inner ring, and the bearing support member supports one end of the shaft in the axial direction and one end surface of the inner ring in the axial direction. 
     In the above-mentioned preload measuring apparatus according to the present invention, it is preferable that the inner race member comprises a shaft having a raceway on its peripheral surface. 
     In the above-mentioned preload measuring apparatus according to the present invention, it is preferable to further comprises a pressurization detect member detecting a pressure of the inner race member which is loaded by the pressurization means, wherein when an output of the pressurization detect member reaches a predetermined value, the excitation means begins to apply the vibration to the inner race member. 
     In the above-mentioned preload measuring apparatus according to the present invention, it is preferable that the excitation member comprises at least two piezoelectric elements applying vibration to the inner race member while loading the predetermined weight on the inner race member in the axial direction, the piezoelectric elements having reverse phases and the same amplitudes each other. 
     In the above-mentioned preload measuring apparatus according to the present invention, it is preferable to further comprises a weight member attached to the outer race member, wherein the vibration sensor detects the vibration of outer race member through the weight member. 
     In the above-mentioned preload measuring apparatus according to the present invention, it is preferable that the loading operation of the pressurization member is stopped when a resonance frequency output from the vibration sensor coincides with a predetermined frequency. 
     In addition, the above object can be attained by a preload measuring apparatus, according to the present invention, to measure a preload of the rolling bearing comprises 
     a bearing support member for supporting a rolling bearing by supporting both ends of a shaft inserted into an inner ring of the rolling bearing to be measured, 
     a pressurization member for loading a predetermined weight in the axial direction onto the rolling bearing, 
     an excitation member for applying vibration onto the rolling bearing by the drive of a piezoelectric element, 
     a vibration sensor to detect a vibration generated on the rolling bearing by the vibration application of the excitation member, and 
     a control section wherein the excitation member is operated under the condition that the pressurization member presses the rolling bearing, the resonance frequency is obtained from an output signal from the vibration sensor, and the preload of the rolling bearing before the predetermined weight is loaded, is calculated according to the obtained resonance frequency. 
     In the preload measuring apparatus of the rolling bearing, the vibration can be detected while noise components are being removed because a predetermined weight is loaded onto the bearing by the pressurization member, therefore, the resonance frequency of the rolling bearing can be accurately obtained. As a result, a true preload of the rolling bearing can be accurately obtained by conversion processing. Further, the measurement of the preload can be quickly carried out by the continuous automatic control, and high productivity measurement can be carried out. 
     It is preferable that, in the calculation of the preload, the true resonance frequency f a  is obtained from a predetermined weight F2 value, the resonance frequency f M  when the weight F2 is loaded, and a correction factor K. Then a preload F a  is obtained from the true resonance frequency f a  and an contact angle α of the bearing. 
     Further, it may be allowed that a weight member is fixed to the outer ring of the rolling bearing, and a probe of the vibration sensor is contacted with a portion of the weight member to detect the vibration. 
     According to this, even when the size of the bearing becomes small and then the rigidity and the mass of the bearing becomes small, the vibration peak of the resonance frequency can be detected without being covered with noises. Accordingly, the preload of the rolling bearing can be accurately obtained. 
     Further, as a production method of the rolling bearing using the preload measuring apparatus of the rolling bearing, in the case where a rolling bearing applied preload is produced, the rolling bearing comprising the first member having the first peripheral surface, the second member arranged concentrically with the first member and having the second peripheral surface opposite to the first peripheral surface, the first raceway formed on the first peripheral surface, the second raceway formed on the portion opposite to the first raceway of a portion of the second peripheral surface, the third raceway formed on a portion shifted from the second raceway in the axial direction of the second peripheral surface, the third member which is supported by the first member with sufficient engagement strength and supported concentrically with the first and second member, and has the third peripheral surface opposite to the second peripheral surface, the fourth raceway which is formed on the portion opposite to the third raceway of a portion of the third peripheral surface, and each plurality of balls which are respectively provided between the first raceway and the second raceway, and between the third raceway and the fourth raceway, wherein an appropriate preload is applied onto the each plurality of balls by adjusting the engagement depth of the third member to the first member, each plurality of balls are respectively inserted between the first raceway and the second raceway, and the third raceway and the fourth raceway, then the third member is forced to move in the axial direction to the first member, in the situation that the engagement depth is shifted from the length necessary for appropriate preload application and thus preload application can not be conducted on the ball, and the preload is applied to make the engagement depth to be the length necessary for appropriate preload. The production method of the rolling bearing applied preload comprises that an piezoelectric element having the sufficient rigidity to the strength in axial direction is arranged in series with the third member and the member which forces to move the third member in the axial direction, each component of the rolling bearing is vibrated according to the current flowing into the piezoelectric element while the resonance frequency of the rolling bearing is being measured, the third member is press fitted to the first member, and the press fitting operation onto the third member is completed in the situation that the resonance frequency almost coincides with the predetermined frequency. The production method of the rolling bearing can be allowed that the mass is increased by attaching the weight member to the second member. 
     According to the production method of the rolling bearing, the third member and the first member into which the third member is press fitted, can be vibrated with the small vibration energy, and the vibration level of the resonance frequency of the rolling bearing apparatus can be raised. As the result, the engagement depth of the third member to the first member can be exactly regulated with the small energy consumption. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration showing a first embodiment of an overall structure of a preload measuring apparatus according to the present invention. 
     FIG. 2 is a block diagram showing a structure of a control. section of the preload measuring apparatus. 
     FIG. 3 is a flow chart showing a measuring process of the preload according to the preload measuring apparatus. 
     FIG. 4 is a graph showing a relationship of an axial weight and a movement amount of a feed shaft. 
     FIG. 5 is a graph showing a relationship of the preload and a resonance frequency according to the difference of an contact angle of the bearing. 
     FIG. 6 is a view of the basic dynamic model explaining a relationship of the vibration of a piezoelectric element and the vibration in axial direction of the rolling bearing. 
     FIG. 7 is an illustration showing a power spectrum of the bearing vibration in a second embodiment of the present invention. 
     FIG. 8 is a view showing the dynamic model when a weight member is added to an outer ring in the second embodiment of the present invention. 
     FIG. 9 is a view showing a second embodiment of a concrete structure according to the present invention. 
     FIG. 10 is a view explaining a production method of the rolling bearing in the third embodiment of the present invention. 
     FIGS.  11 ( a ) to  11 ( c ) sectional views of the rolling bearings explaining conventional various preload loading methods. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, preferable embodiments of a preload measuring apparatus of a rolling bearing according to the present invention will be detailed, referring to the drawings. 
     FIG. 1 shows an overall structure in a first embodiment of the preload measuring apparatus of the rolling bearing according to the present invention. FIG. 2 is a block diagram showing a concrete structure of a control section of the preload measuring apparatus. 
     Initially, as shown in FIG. 1, a preload measuring apparatus  100  of the present embodiment comprises, mainly, a measuring apparatus main body  20  and a control section  30 . In the main body  20 , one end of the shaft  12  is inserted and fixed in a recessed portion of a receiving side tool  14   a  and the other end is set to be able to contact with a pressing side tool  14   b  under the condition that a shaft  12  is inserted into an inner ring  10   a  side of the rolling bearing  10  which is a measuring object. The control section  30  is connected to the measuring apparatus main body  20  and controls the input/output data to the measuring apparatus main body  20 . The receiving side tool  14   a  and the pressing side tool  14   b  correspond to a shaft supporting means. 
     Herein, the rolling bearing which is a measuring object, is one of a type shown in FIG.  11 ( b ), which is divided into  2  portions on the axial line. The rolling bearing has inner ring  10   a ,  10   a  having raceways on the outer periphery side. The rolling bearing also has an outer ring  10   b  which is provided on the outer peripheral side of the inner ring, and has double-row raceways on the inner peripheral surface of the outer ring opposite to the raceways of the inner ring. However, any one of other types may be applied. 
     The measuring apparatus main body  20  comprises, from its lower portion in order, a load cell  21  fixed on a base board, a lower piezoelectric element  22  fixed on the load cell  21 , a receiving side tool  14   a  which is fixed on the lower piezoelectric element  22  and supports the lower end of the shaft  12 , a pressing side tool  14   b  pressing the upper end of the shaft  12 , an upper piezoelectric element  23  fixed on the pressing side tool  14   b , a feed shaft  25 , and a coupling section  28 . One end of the feed shaf  25  is connected to the upper piezoelectric element  23  and the feed shaft  25  is supported so that it can move in the arrowed direction (vertical direction) shown in FIG. 1 by a guide holder  24 . The coupling section  28  is connected to the other end of the feed shaft  25  and is driven through a ball screw  27   a  and a nut  27   b  by the rotation of a pulse motor  26  for feeding and pressurizing. Further, the measuring apparatus main body  20  comprises a vibration sensor  29  which detects a vibration by contacting a probe  29   a , for example, with the end surface of the outer ring  10   b  of the rolling bearing  10 . 
     Then, the load cell  21 , the lower piezoelectric element  22 , the upper piezoelectric element  23 , the pulse motor  26 , and the vibration sensor  29  are connected to the control section  30 , and input/output of each data of them is conducted. 
     Incidentally, the pulse motor  26  corresponds to the pressurization means, and piezoelectric elements  22  and  23  correspond to the excitation means. 
     The load cell  21  detects the weight loaded in the axial direction of the rolling bearing  10  in real time. As shown in FIG. 2, the output signal from the load cell  21  is amplified by a load cell amplifier  31 , and successively transferred to a central processing unit (CPU)  33  through an A/D converter  32 . 
     The lower and upper piezoelectric elements  22  and  23  excite the rolling bearing  10  held between the receiving side tool  14   a  and the pressing side tool  14   b  through the shaft  12 . After a oscillation signal of a swept sin oscillator  34  is amplified by a power amplifier  35  by a command from the CPU  33 , the amplified signal is inputted into the piezoelectric elements  22  and  23 , and then the shaft  12  is excited. Incidentally, a swept sin wave (for example, its frequency is about 0-10 kHz) which is swept is appropriately used as the inputted oscillation signal. 
     These piezoelectric elements  22  and  23  have a sufficient rigidity in the axial direction of the rolling bearing. The pair of piezoelectric elements  22  and  23  are driven in opposite phases and at the same amplitudes each other. When one piezoelectric element is elongated, the other piezoelectric element is contracted by the same amount. That is why compression of the shaft  12  of the rolling bearing  10  by the both piezoelectric elements is prevented, and the rolling bearing  10  is made to assuredly vibrate in the axial direction. 
     When the pulse motor  26  for the feeding and pressurization is rotationally driven, the rotation is converted into the straight movement by the ball screw  27   a  and the nut  27   b , and then the feed shaft  25  is moved. After the shaft  12  is moved to the contact position of the pressing side tool  14   b , the shaft  12  is further pressed. By pressing the shaft  12 , the shaft  12  is compressed, as the result, the preload is loaded onto the inner and outer rings of the rolling bearing  10 . The pulse motor  26  is rotationally driven through a pulse motor control driver  36  which is operated by a command from the CPU  33 . 
     The vibration sensor  29  , for example, a moving magnet type vibration detection sensor, detects the resonance frequency of the rolling bearing  10 . The vibration sensor  29  comprises the probe  29   a  to detect the vibration, and a matching transformer  29   b  which generates amplification effect by the pressure-up of voltage due to an increase of the impedance. The detection signal from the matching transformer  29   b  is inputted into low pass filter  37 , and a high frequency component (for example, more than 15 kHz) is removed. This low pass filter  37  has an effect that the reflected frequency in the high frequency area is removed and the discrete frequency in the low frequency area is selected. Thus the low pass filter  37  functions as a kind of a low-pass filter. Further, the signal from the low pass filter  37  is transferred to the CPU  33  through an amplifier  38  and an A/D converter  39 . 
     The CPU  33  reads a program based on a calculation method which will be described later, from an auxiliary memory apparatus (which is not shown) and controls each units. Then the CPU  33  reads the data obtained from each unit and calculates the preload of the rolling bearing  10 . Concretely, each setting is conducted by an input operation from the attached key-board  40 . While the CPU  33  drives the pulse motor  26 , and the piezoelectric elements  22  and  23  through the interface, it successively reads the weight change data and the vibration data, conducts each processing and calculation, and displays the result on the display  41  attached to the control section  30 . 
     Next, according to a flow chart in FIG. 3, a process to measure the preload of the rolling bearing by the preload measuring apparatus will be described. 
     Initially, as shown in FIG. 1, after the bearing apparatus  10  is set between the receiving side tool  14   a  and the pressing side tool  14   b , the control section  30  and each unit are started up by turning on the power supply to them, and the program is started by the operation of the key board  40 . Then, the CPU  33  drives the pulse motor  28  so that the feed shaft  25  is lowered at a predetermined speed from standby position (step  1 , hereinafter, abbreviated to S 1 ). At that time, the weight loaded to the rolling bearing  10  is successively detected by the load cell  21 . 
     When the output of the load cell  21  reaches a predetermined first setting weight F1 (for example, 5-10 kg), that is, the feed shaft is lowered and the pressing side tool  14   b  comes into contact with the shaft  12 , the rotation speed of the pulse motor  26  is switched from the rapid feeding mode to the slow feeding mode. Further, as the pulse motor  26  is rotated in such the situation, the shaft  12  is pressed, and the weight is loaded onto the rolling bearing  10 . This weight acts on the rolling bearing  10  as a preload (S 2 ). 
     Then, when a second setting weight F2, that is, the output of the load cell  21  reaches a weight at which the preload is measured, the pulse motor  26  is stopped (S 3 ). 
     Thus, the feeding speed of the pulse motor  26  is set to two stages as shown in FIG.  4 . The pulse motor feeds at the rapid feeding mode until the first setting weight F1 is detected. As the weight F1 is detected, the pulse motor is switched to the slow feeding mode until the weight F2 is detected. 
     After the second setting weight F2 is reached and the pulse motor  26  is stopped by a stop signal, the CPU  33  applies the swept sin wave onto the piezoelectric elements  22  and  23  using this stop signal as the trigger signal. Accordingly, the piezoelectric elements  22  and  23  generate vibration. This vibration is transmitted to the shaft  12 , and the rolling bearing  10  is vibrated (S 4 ). Thereby, after the stop of the pulse motor  26 , the vibration can be loaded onto the rolling bearing at once, and simple and smooth automatic processing can be realized. 
     Next, a process to obtain the preloaded amount of the rolling bearing and its calculation principle will be described. 
     Generally, the resonance frequency f which acts on the rolling bearing while receiving the vibration, is expressed by the following equation (1).              f   =       1     2      π              K   m                 (1)                                
     Herein, m is the mass of the bearing, and K is a rigidity of the bearing. 
     When the resonance frequency measured when the second setting weight F2 is loaded onto the bearing, is f m , a true (at the initial condition) resonance frequency f a  in which the weight is not loaded, is obtained by subtracting the frequency component for the weight F2 from the resonance frequency f M . Accordingly, the resonance frequency f a  can be expressed in an equation (2) as follows: 
     
       
         f Δ =f M −F2×kc  (2) 
       
     
     Herein, kc is a previously obtained correction coefficient [Hz/kgf]. The resonance frequency f a  obtained by the above equation (2) is substituted into the equation (1) and the rigidity K is obtained. Then the true preload F a  is obtained from the equation (3) using the obtained K. 
     
       
         F a =α·f(K)  (3) 
       
     
     Herein, α is a contact angle of the bearing and a known value. As shown in FIG. 5, a relationship between the resonance frequency and the preload changes corresponding to the magnitude of the contact angle α. That is, when the contact angle α is large (in other words, the radial gap is large), the resonance frequency is higher than the case where the contact angle α is small (in other words, the radial gap is small), even when the preload is the same. 
     To be obtained the above described true preload F a  by the control section  30 , initially, the resonance frequency f M  is obtained (S 5 ), when the vibration is applied by the piezoelectric elements  22  and  23 . 
     Concretely, after the high frequency component is removed from the signal from the vibration sensor  29  by low pass filter  37 , the signal is amplified by the amplifier  38 , and further, sampled by an A/D converter  39 . Thereby, for example,  1024  digital data is generated. Then FFT processing is conducted to this digital data, therefore, the power spectrum is obtained. The maximum frequency component of this power spectrum is defined as the resonance frequency f M . Next, the true resonance frequency f a  is obtained from the obtained resonance frequency f M  and the weight F2 detected by the load cell  21 , by using the equation (2) (S 6 ). 
     Then, the true preload F a  is obtained from the true resonance frequency f a , the rigidity K of the bearing, and the contact angle α of the bearing, by using the equation (3). (S 7 ) 
     Such the calculation processing is automatically conducted by previously storing the program in the control section  30 , and then conducting the calculation according to the measuring data read from each unit by a command from the CPU  33 . In this case, the calculation result is successively displayed on the display  41 , and the judgement of good or bad or the information serving for each condition setting in the production process, can be clearly and rapidly presented to the operator of the preload measuring apparatus  100 . 
     After that, one cycle of the measurement is completed by a series of returning operation (S 9 ) such as the data storing, stop of the drive of piezoelectric elements  22 ,  23 , and the weight removing by the reversal rotation of the pulse motor  26 . Then the apparatus stands by for the next measurement. 
     As described above, according to the present embodiment, the generation of the noise vibration due to the disturbance such as the back rush or the like can be prevented by obtaining the resonance frequency under the condition that the weight is applied on the bearing, and converting the preload from the obtained resonance frequency. Accordingly, the highly accurate preload measurement can be conducted. 
     Further, the measurement can be carried out at once after the weight reaches the weight F2 by using the second setting weight F2 to measure the resonance frequency as the trigger signal. Accordingly, the measurement can be continuously conducted, the automatic measurement becomes easy, and the measurement with the high productivity can be conducted. 
     Next, referring to FIGS. 6-9, a second embodiment of the present embodiment will be described. Incidentally, in each drawing, the same portion as the first embodiment is denoted by the same code and the explanation will be omitted, and only different portions will be described by using different codes. 
     A preload measuring apparatus of the second embodiment can carry out the high accurate preload measurement to the small rolling bearing. When the mass of the bearing is small, the resonance frequency appears in the high frequency area side in which noise level is high, and S/N ratio of the resonance frequency to be detected is worse. Therefore, in the present embodiment, a weight member having the tapered inner diameter which fits into the outer peripheral portion of the outer ring, is added to the rolling bearing, thereby, the resonance point is shifted to the low frequency side, and then the lowering of the detection accuracy of the resonance frequency is prevented. The vibration sensor brings the probe in contact with this weight member, not with the outer ring of the rolling bearing, to carry out the measurement. However the other structure of the preload measuring apparatus is almost the same as in the first embodiment. 
     Initially, a reason why the weight member is added to the rolling bearing at the time of the preload measurement, will be described. 
     As shown in FIG. 6, When the amplitude on the input side is x and the amplitude on the output side is X, an amplitude ratio is expressed by the following equation (4).                X   x     =     1     2      ζ          1   -     ζ                 2                     (   4   )                                
     Herein, ζ is expressed in an equation (5) as follows.              ζ   =     C     2        mK                 (5)                                
     When ζ is small, the following equation (6) is obtained.                  X   x     ≅     1     2      ζ         =       mK     C             (6)                                
     Accordingly, when the spring constant K, and the attenuation coefficient C are constant, the following equation (7) is obtained.                X   x     ∝     m             (7)                                
     That is, if the mass becomes “a” times, the amplitude becomes {square root over (α)} times. Further, the following equations (8) and (9) exist between the resonance frequency f and the mass m.              f   =       1     2      π              k   m                 (8)               f   ∝       1   m               (9)                                
     That is, when the mass becomes “a” times, the resonance frequency becomes        1     a                            
     times. 
     As can clearly be seen from the equation, the resonance frequency of the rolling bearing having the small mass appears on the high frequency side. As shown in a relationship between the frequency and the amplitude in FIG. 7, for. example, when the resonance frequency generates in the vicinity of 20 kHz in which the noise level is large, it is becomes insufficient to separate of the noise from the frequency component of the bearing vibration. Therefore, the resonance frequency can not be accurately obtained. 
     On the one hand, when the mass is increased, and thereby, the resonance frequency is shifted to the low frequency side in the vicinity of, for example, 4 kHz in which the noise level is relatively small, the influence due to the noise is decreased, and the resonance frequency can be obtained under the condition that the S/N ratio is large. Therefore, the measuring accuracy is increased. 
     Accordingly, in order to increase the mass of the rolling bearing, as shown in a dynamic model in FIG. 8, the weight member is added to, for example, the outer ring. Therefore, the mass m of the overall bearing can be increased. Thereby, the resonance frequency f is shifted to the low frequency area side. 
     As the concrete structure of the dynamic model, as shown by the structure of the main portion of the preload measuring apparatus  200  of the present embodiment in FIG. 9, the rolling bearing  60  is held between piezoelectric elements  22  and  23  through the pressing side tool  51 , a bearing fixing tool  52 , and the receiving side tool  53 . That is, the bearing fixing tool  52  is inserted into the inner ring of the rolling bearing  60 , one end side of the rolling bearing is contacted with a collar portion of the bearing fixing tool  52 , and the other end side of the rolling bearing  60  is contacted with the upper end portion of the receiving side tool  53 . Thereby the rolling bearing  60  is held on the shaft line of the feed shaft  25 . 
     Further, a ring-shaped weight member  55  is attached to the outer ring of the rolling bearing  60 . The weight member  55  has a taper (for example 5° relative to the axial direction thereof) so that the outer peripheral portion of the outer ring just fits in the inner peripheral surface of the weight member. The probe  29   a  of the vibration sensor  29  is contacted with for example, the upper surface of the weight member  55  for the detection of the vibration. Incidentally, as the direction of the taper, as shown in the drawing, the taper may expand to the upward direction, or reversely, expand to the downward direction. In any direction, it maybe allowable if the trouble is not generated for the pick-up of the vibration sensor  29 . 
     It can also be considered that the vibration sensor  29  larger than that shown in the drawing is actually used, however, the degree of freedom of setting is enhanced by the down sizing of the shaft of the pressing side tool  51 , and the degree of freedom of the vibration measuring position is also increased. 
     The other stricture of preload measuring apparatus  200  is the same as the preload measuring apparatus  100  of the first embodiment, and the process of the preload measurement is also the same as described above. Incidentally, the pressing pressure by the feed shaft  25  may be neglected, because the weight in the axial direction is loaded onto the bearing by the weight member. 
     In the preload measuring apparatus of the present embodiment, even when the size of the rolling bearing is decreased, thereby, the mass of the bearing is decreased and the resonance frequency of the bearing is increased, the mass of overall bearing is increased by providing weight member on the outer ring, and thereby the resonance frequency is lowered. Accordingly, while the lowering of the measuring accuracy due to noises is being prevented, the vibration peak of the resonance frequency can be stably, highly and accurately detected. 
     Next, referring to FIG. 10, the third embodiment of the present invention will be described. In the present embodiment, the preload measuring apparatus is shown, which is applied to the production apparatus of the rolling bearing. Incidentally, in also the present embodiment, the common portion to the first embodiment is denoted by the common code, and its explanation is omitted. 
     FIG. 10 shows main structure of the production apparatus of the rolling bearing comprising the preload measuring apparatus  300  of the present embodiment. 
     In a shaft  80  which is the first member, its small peripheral portion  80   a  and a large peripheral portion  80   b  are connected by a stage portion  80   c . On the outer peripheral surface of the large peripheral portion  80   b  which is the first peripheral surface, the first deep-slot type inner ring raceway  81 , which is the first raceway, is formed. Further, an inner ring  70   a , which is the third member, has a little smaller diameter than the outer diameter of the small peripheral portion  80   a  in the free condition. This inner ring  70   a  forms the second deep-slot type inner raceway  70   b , which is the fourth raceway, on the outer peripheral surface which is the third peripheral surface. 
     Further, in the inner peripheral surface, which is the second peripheral surface, of the outer ring  70   c  which is the second member, deep-slot type outer ring raceways  70   d ,  70   d , which are the second and the third raceways, are formed. The weight member  55  is fixed on the outer peripheral surface of the outer ring  70   c , and the tapered inner peripheral surface of the weight member  55  is engaged with the outer peripheral surface of the outer ring  70   c.    
     Such the shaft  80 , inner ring  70   a  and the outer ring  70   c  are assembled such that the inner ring  70   a  is engaged with the outside of the shaft  80 , and the balls  82 ,  82  are arranged among the first and second inner ring raceways  81 ,  70   b , and both outer ring raceways  70   d ,  70   d . The balls  82 ,  82  are held by the holder  83 . In this assembling time, the preload is not yet applied onto the balls  82  and  82 . 
     After each member of the structure has been assembled, the shaft  80  is engaged with the receiving side tool  84 , and the leading edge of the pressing side tool  65  is struck against the end surface of the inner ring  70   a . By narrowing the gap between the receiving side tool  84  and the pressing side tool  85  by a push-in apparatus  86 , the inner ring  70   a  is pushed in outside the small peripheral portion  80   a  of the shaft  80 , and thereby, the preload is applied onto balls  82  and  82 . 
     The piezoelectric elements  22  and  23  are respectively held between the receiving side tool  84  and the base board  87 , and between the pushing side tool  85  and a push-in arm  88 . These piezoelectric elements  22  and  23  have the sufficient rigidity along in the push-in direction of the inner ring  70   a  (the vertical direction in FIG.  10 ). These piezoelectric elements  22  and  23  are driven corresponding to the signal amplified by the power amplifier  35 , which is amplified the oscillated signal from the swept sin oscillator  34  of the control section  30  shown in FIG.  2 . 
     In the embodiment shown in the drawing, the above described control. section  30  outputs the signal for the resonance frequency detection of the rolling bearing, and the signal for the force necessary for pushing in the inner ring  70   a  onto the small peripheral portion  80   a , that is, the signal for decreasing the stick slip. The pair of piezoelectric elements  22  and  23  are driven in the opposite phases, and with the same amplitudes. That is why it is prevented that the inner ring  70   a  is pushed in onto the small peripheral portion  80   a  accompanied by the vibration of the rolling bearing by both piezoelectric elements  22  and  23  (both piezoelectric elements  22  and  23  are simultaneously elongated, thereby, the pushing-in operation is conducted), and thereby, the shaft  80  and the inner ring  70   a  are sufficiently vibrated in the axial direction. 
     On the one hand, the probe  29   a  of the vibration sensor  29  is contacted with the end surface of the weight member  55 , and the output from the sensor  29  is inputted into the control section  30 . The control section  30  regulates the displacement amount of the push-in arm  88  by the push-in apparatus  86 . 
     At the time of production of the rolling bearing, when the inner ring  70   a  is pushed in onto the small peripheral portion  80   a , and an appropriate preload is applied onto each of balls  82  and  82 , as disclosed in Japanese Patent Examined Publication No. Hei. 6-344233. It discloses that while the resonance frequency of the rolling bearing is being measured by the vibration sensor  29  in the same manner as in the first embodiment, the pressure oil is fed in the push-in apparatus  86 , and the inner ring  70   a  is fitted in and engaged with the small peripheral portion  80   a  of the shaft  80  by pressing the inner ring  70   a  by the push-in arm  88 . Then, when the above described resonance frequency almost coincides with the predetermined frequency, the feeding of the pressure oil in the push-in apparatus  86  is stopped, and the fitting-in operation is completed. Thus, the rolling bearing onto which an appropriate preload is applied, is completed. 
     Incidentally, in each of embodiments, the preload is measured by loading the weight onto the inner ring side, and measuring the resonance frequency of the outer ring side, however, the reverse structure in which the weight is loaded onto the outer ring side and the resonance frequency of the inner ring side is measured, may also be allowable. 
     As described above, in the preload measuring apparatus of the rolling bearing of the present invention, the vibration is provided under the condition that a predetermined weight is loaded onto the bearing, thereby, the vibration generated in the rolling bearing can be detected while the noise components are being removed. Accordingly, the resonance frequency of the rolling bearing can be accurately obtained. As the result, the true preload of the rolling bearing can be accurately obtained by the conversion processing. Further, the measurement of the preload can be rapidly conducted by the continuous automatic control, and the measurement with the high productivity can be conducted. 
     Further, when the weight member is fixedly provided onto the rolling bearing, it is prevented that the detection accuracy of the resonance frequency is lowered accompanied by the down sizing of the bearing, and the measurement of the preload can be highly accurately conducted. 
     The foregoing detailed description of the present invention is provided for the purpose of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.