Abstract:
According to the present invention, there is provided a bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, an inner member including said hub wheel and formed therewith double row inner raceway surfaces each arranged opposite to each of said double row outer surfaces, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: there are arranged a separate outer or inner ring on at least one of said outer and inner members, and a preload varying means arranged at an abutting portion between said outer and inner members for varying the preload applied to the bearing, said preload varying means can be extended or contracted based on an output signal from a detecting sensor for detecting the running condition of vehicle.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a bearing apparatus for a wheel of vehicle for rotatably supporting the wheel of vehicle such as an automobile, and more particularly to a bearing apparatus for a wheel of vehicle having a preload varying means built in the bearing apparatus for variably setting an optimum preload suitable for the running condition of vehicle.  
         [0003]     2. Description of Background Art  
         [0004]     The power transmitting apparatus for transmitting an engine power to wheels is required not only to allow power transmission from an engine to wheels, but to allow radial and axial displacement from wheels as well as momentum displacement caused by bounds of body of vehicle during running on a rough road and cornering of vehicle. Accordingly, as shown in  FIG. 12 , one end of a driving shaft  100  arranged between an engine and a driving wheel is connected to a differential apparatus  102  via a sliding type constant velocity universal joint  101  and the other end thereof is connected to a wheel  105  via a bearing apparatus for a wheel  104  including a stationary type constant velocity universal joint  103 .  
         [0005]     As shown in  FIG. 11 , this bearing apparatus for a wheel of vehicle  104  comprises a hub wheel  16  on which a wheel  105  is mounted at one end thereof, a double row rolling bearing  107  rotatably supporting the hub wheel  106 , and an outer joint member  108  of the stationary type constant velocity universal joint  103  for transmitting the power of the driving shaft  100  to the hub wheel  106 . The double row rolling bearing  107  comprises an outer member  110 , a separate inner ring  109  press fitted on the outer periphery of the hub wheel  106 , and a double row rolling elements  111  rollably contained between the hub wheel  106 /the inner ring  109  and the outer member  110 .  
         [0006]     In such a bearing apparatus for a wheel of vehicle  104 , it has been carried out to apply a predetermined preload to the bearing apparatus in order to assure a desired bearing rigidity. The control of the bearing preload has been carried out by precisely finishing the abutting surfaces between the hub wheel  106  and the inner ring  109  as well as by tightly connecting the hub wheel  106  and the outer joint member  108  by fastening a securing nut  112  with a predetermined torque (axial force). Of course the bearing preload influences not only the bearing life, but the environmental problems such as safety driving and an improvement of fuel consumption etc. That is, as shown in  FIG. 9 , since the rotational torque is proportional to the bearing preload, it is possible to contribute to the improvement of fuel consumption by reducing the preload to reduce the rotational torque. On the contrary, since the inclination angle of bearing essential factor for determining the rigidity of bearing is inverse proportion to the bearing preload, as shown in  FIG. 10 , the bearing rigidity can be improved by increasing the preload to reduce the inclination angle of bearing and thus the inclination of brake rotor (not shown) caused during the cornering run of vehicle can be also suppressed.  
         [0007]     If it is possible to control the bearing preload in an optimum manner in accordance with the running condition of vehicle, it will be able to provide a bearing apparatus for a wheel of vehicle ideal in the safety drive and improvement in fuel consumption. However no bearing apparatus has been developed yet which can variably set the optimum bearing preload in accordance with the running condition of vehicle.  
         [0008]      FIG. 8  shows a bearing unit  50  which can vary the bearing preload. This bearing unit comprises two rolling bearings  51  and  51 , inner and outer cylindrical portions  52  and  53  for holding the rolling bearings  51  and  51 , an electrostrictive element  54  arranged on one end of one of the cylindrical portions  52  and  53 , and a voltage controller  55  for controlling voltage applied to electrostrivtive element  54 , and the cylindrical portion  52  together with the electrostructive element  54  is secured to stationary portions  56  and  57  of the bearing unit  50 . Stabilization of preloading can be obtained by automatically setting the distance between inner rings of the bearings at a predetermined value with using a temperature sensor (not shown) for detecting temperature variation of the inner cylindrical portion  52  and the stationary portion  56  and then by extending or contracting the electrostrictive element  54  with using the voltage controller  55  for controlling the voltage applied to the electrostrictive element  54  in accordance with the temperature variation (cf Japanese Laid-open Patent Publication No. 224216/1999).  
       SUMMARY OF THE INVENTION  
       [0009]     However this bearing unit  50  is directed to that which can only suppress variation of the preload set initially of two rolling bearings  51  and  51  which would be caused by temperature variation with using the electrostrictive element  54  and thus it is different from that which intends to variably set the optimum preload in accordance with the running condition of vehicle. Accordingly it is impossible to variably set the optimum preload in accordance with the running condition of vehicle by using the bearing unit  50  of the prior art which can only suppress the preload of the bearings  51  and  51  caused by temperature variation. This is because the load applied to the bearing for a wheel of vehicle always randomly changes in accordance with the running condition of vehicle e.g. whether it runs on straight or curved roads.  
         [0010]     It is, therefore, an object of the present invention to provide a bearing apparatus for a wheel of vehicle which can variably set the optimum preload in accordance with the running condition of vehicle.  
         [0011]     For achieving the object of the present invention, there is provided a bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, an inner member including said hub wheel and formed therewith double row inner raceway surfaces each arranged opposite to each of said double row outer surfaces, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: there are arranged a separate outer or inner ring on at least one of said outer and inner members, and a preload varying means arranged at an abutting portion between said outer and inner members for varying a preload applied to the bearing, said preload varying means can be extended or contracted based on an output signal from a detecting sensor for detecting the running condition of vehicle.  
         [0012]     According to the bearing apparatus of the present invention, since there are arranged a separate outer or inner ring on at least one of said outer and inner members, and a preload varying means arranged at an abutting portion between said outer and inner members for varying a preload applied to the bearing, said preload varying means can be extended or contracted based on an output signal from a detecting sensor for detecting the running condition of vehicle, it is possible to suppress the inclination of the brake rotor by increasing the bearing preload when the bearing rigidity is required during cornering of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the vehicle is running on a straight road, it is possible to reduce the rotational torque and the friction loss of the bearing by reducing the bearing preload and thus to improve the fuel consumption.  
         [0013]     The preload varying means may be any one of an electrostrictive element, a magnetostrictive element, a shape memory element, or a member having a coefficient of thermal expansion higher than that of structural parts of the bearing.  
         [0014]     According to the present invention, it is preferable that at least one of a load sensor, a vehicle speed sensor and a temperature sensor may be incorporated in the preload varying means. This makes it possible to always obtain an optimum bearing preload by applying a predetermined voltage to the preload varying means with timely computing the preload based on real time informations for determining whether the vehicle is now on straight or cornering run.  
         [0015]     In another embodiment of the present invention, there is provided a bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, said hub wheel formed on its outer periphery with one of the inner raceway surfaces of the double row rolling bearing and with an axially extending cylindrical portion from the inner raceway surface, an inner member including a separate inner ring press fitted on the axially extending portion, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: said outer member comprises an outer ring member formed on its inner periphery with one of the outer raceway surfaces and a separate outer ring formed on its inner periphery with the other of the outer raceway surfaces, and an electrostrictive element arranged at an abutting portion between said outer ring member and the outer ring can be extended or contracted by applying a predetermined voltage to the electrostrictive element based on an output signal from a sensor.  
         [0016]     According to this embodiment of the present invention, since an electrostrictive element arranged at an abutting portion between said outer ring member and the outer ring can be extended or contracted by applying a predetermined voltage to the electrostrictive element based on an output signal from sensors such as a steering angle sensor, a vehicle speed sensor etc, it is possible to suppress the inclination of the brake rotor by increasing the bearing preload when the bearing rigidity is required during cornering of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the vehicle is running on a straight road, it is possible to reduce the rotational torque and the friction loss of the bearing by reducing the bearing preload and thus to improve the fuel consumption.  
         [0017]     If the electrostrictive element is a laminated piezo element, the assembly of the bearing apparatus can be simplified and control of the preload can be achieved by a relatively exact and easy manner.  
         [0018]     In the other embodiment of the present invention, there is also provided a bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, said hub wheel formed on its outer periphery with one of the inner raceway surfaces of the double row rolling bearing and with an axially extending cylindrical portion from the inner raceway surface, an inner member including a separate inner ring press fitted on the axially extending portion, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: one end of the hub wheel is provided with a securing portion, a magnetostrictive element is arranged between the securing portion and the inner ring, an electromagnetic coil is arranged at the end of the outer member oppositely to the magnetostrictive element, and the magnetostrictive element can be extended or contracted by applying a magnetic field from the electromagnetic coil based on an output signal from a load sensor arranged between the outer and inner members.  
         [0019]     In such a structure, when a large load such as a momentum load is applied to the bearing during cornering of vehicle, a predetermined magnetic field is applied to the magnetostrictive element in accordance with the applied load and an optimum preload is set in accordance with increase of bearing preload due to extension of the magnetostrictive element. Thus it is possible to suppress the inclination of the brake rotor by increasing the bearing preload when the bearing rigidity is required during cornering of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the load applied to the bearing is not fluctuated as in a case of vehicle running on a straight road, it is possible to reduce the rotational torque of the bearing thus to improve the fuel consumption since no magnetic field is applied to the magnetostrictive element and thus the initial preload is maintained.  
         [0020]     Preferably the load sensor may comprise a magnetostrictive portion to be detected (i.e. magnetostrictive detected portion) made of Fe—Al alloy and a detecting coil arranged oppositely to the magnetostrictive detected portion. This structure enables to achieve an exact load detection without being influenced by lubricant oil such as grease.  
         [0021]     A temperature sensor may be arranged within the double row rolling bearing, and the application of voltage may be switched off to stop the preload varying function when the output signal from the temperature sensor exceeds a predetermined threshold level. This structure enables to prevent a trouble caused by abnormal temperature rise and thus the safety of vehicle.  
         [0022]     The initial preload may be set by an output signal from the electrostrictive element or the magnetostrictive element. This structure enables the preload control during assembly of the bearing apparatus easier than the preload control of the prior art since the assembling work can be completed at a point in which a predetermined preload is obtained with the assembling work being carried out with watching an output signal from the electrostrictive element  15  forming the preload varying means.  
       EFFECT OF THE INVENTION  
       [0023]     One embodiment of the present invention is a bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, an inner member including said hub wheel and formed therewith double row inner raceway surfaces each arranged opposite to each of said double row outer surfaces, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: there are arranged a separate outer or inner ring on at least one of said outer and inner members, and a preload varying means arranged at an abutting portion between said outer and inner members for varying a preload applied to the bearing, said preload varying means can be extended or contracted based on an output signal from a detecting sensor for detecting the running condition of vehicle. This makes it possible to suppress the inclination of the brake rotor by increasing the bearing preload when the bearing rigidity is required during cornering of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the vehicle is running on a straight road, it is possible to reduce the rotational torque and the friction loss of the bearing by reducing the bearing preload and thus to improve the fuel consumption.  
         [0024]     Another embodiment of the present invention is a bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, said hub wheel formed on its outer periphery with one of the inner raceway surfaces of the double row rolling bearing and with an axially extending cylindrical portion from the inner raceway surface, an inner member including a separate inner ring press fitted on the axially extending portion, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: said outer member comprises an outer ring member formed on its inner periphery with one of the outer raceway surfaces and a separate outer ring formed on its inner periphery with the other of the outer raceway surfaces, and an electrostrictive element arranged at an abutting portion between said outer ring member and the outer ring can be extended or contracted by applying a predetermined voltage to the electrostrictive element based on an output signal from a sensor. This makes it possible to suppress the inclination of the brake rotor by increasing the bearing preload when the bearing rigidity is required during cornering of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the vehicle is running on a straight road, it is possible to reduce the rotational torque and the friction loss of the bearing by reducing the bearing preload and thus to improve the fuel consumption.  
         [0025]     The other embodiment of the present invention is a bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, said hub wheel formed on its outer periphery with one of the inner raceway surfaces of the double row rolling bearing and with an axially extending cylindrical portion from the inner raceway surface, an inner member including a separate inner ring press fitted on the axially extending portion, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: one end of the hub wheel is provided with a securing portion, a magnetostrictive element is arranged between the securing portion and the inner ring, an electromagnetic coil is arranged at the end of the outer memberoppositely to the magnetostrictive element, and the magnetostrictive element can be extended or contracted by applying a magnetic field from the electromagnetic coil based on an output signal from a load sensor arranged between the outer and inner members. In such a structure, when a large load such as a momentum load is applied to the bearing during cornering of vehicle, a predetermined magnetic field is applied to the magnetostrictive element in accordance with the applied load and an optimum preload is set in accordance with increase of bearing preload due to extension of the magnetostrictive element. Thus it is possible to suppress the inclination of the brake rotor by increasing the bearing preload when the bearing rigidity is required during cornering of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the load applied to the bearing is not fluctuated as in a case of vehicle running on a straight road, it is possible to reduce the rotational torque of the bearing thus to improve the fuel consumption since no magnetic field is applied to the magnetostrictive element and thus the initial preload is maintained.  
       BEST MODE FOR CARRYING OUT THE INVENTION  
       [0026]     A bearing apparatus for a wheel of vehicle comprising a hub wheel integrally formed therewith a wheel mounting flange on the periphery at one end thereof, and a double row rolling bearing, the double row rolling bearing comprising an outer member integrally formed therewith a body mounting flange on the periphery thereof and also formed therewith double row outer raceway surfaces, said hub wheel formed on its outer periphery with one of the inner raceway surfaces of the double row rolling bearing and with an axially extending cylindrical portion from the inner raceway surface, an inner member including a separate inner ring press fitted on the axially extending portion, and double row rolling elements freely rotatably contained between said double row outer and second raceway surfaces, said double row rolling bearing being adapted to be applied a predetermined preload characterized in that: said outer member comprises an outer ring member formed on its inner periphery with one of the outer raceway surfaces and a separate outer ring formed on its inner periphery with the other of the outer raceway surfaces, and an electrostrictive element comprising laminated piezo element arranged at an abutting portion between said outer ring member and the outer ring can be extended or contracted by applying a predetermined voltage to the piezo element based on an output signal from a sensor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein:  
         [0028]      FIG. 1  is a longitudinal section view showing a first embodiment of the bearing apparatus for a wheel of the present invention;  
         [0029]      FIG. 2  is a longitudinal section view showing a second embodiment of the bearing apparatus for a wheel of the present invention;  
         [0030]      FIG. 3  is a flow chart showing a method for controlling the preload of the bearing apparatus for a wheel of the present invention;  
         [0031]      FIG. 4  is a longitudinal section view showing a third embodiment of the bearing apparatus for a wheel of the present invention;  
         [0032]      FIG. 5  is a longitudinal section view showing a fourth embodiment of the bearing apparatus for a wheel of the present invention;  
         [0033]      FIG. 6  is a longitudinal section view showing a fifth embodiment of the bearing apparatus for a wheel of the present invention;  
         [0034]      FIG. 7  is a graph showing a wave of an output signal from the electrostrictive element of the present invention;  
         [0035]      FIG. 8  is a longitudinal section view showing a preload varying bearing unit of the prior art;  
         [0036]      FIG. 9  is a graph showing a relation between the bearing preload and the rotational torque;  
         [0037]      FIG. 10  is a graph showing a relation between the bearing preload and the inclination angle of the bearing;  
         [0038]      FIG. 11  is a longitudinal section view of the bearing apparatus for a wheel of the prior art; and  
         [0039]      FIG. 12  is a longitudinal section view showing an example of a power transmitting apparatus in which a bearing apparatus for a wheel of vehicle is incorporated. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0040]     Preferred embodiments of the present invention will be described with reference to the accompanied drawings.  FIG. 1  shows a first embodiment of a bearing apparatus for a wheel of vehicle of the present invention.  
         [0041]     The bearing apparatus for a wheel of vehicle is that used for a driven wheel and comprises a hub wheel  1  and a double row rolling bearing  2 . In the description below, a term “outboard side” (left hand side in drawings) of the apparatus denotes a side which is positioned outside of the vehicle body and a term “inboard side” (right hand side in drawings) of the apparatus denotes a side which is positioned inside of the body when the bearing apparatus is mounted on the vehicle body.  
         [0042]     The hub wheel  1  is integrally formed with a wheel mounting flange  3  at an end of the outboard side, an outboard side inner raceway surface  1   a  of a double row rolling bearing  2  on the outer peripheral surface thereof, and a cylindrical portion  1   b  axially extending from the inner raceway surface  1   a . Hub bolts  4  for securing the wheel on the flange  3  are equidistantly arranged along the periphery of the flange  3 . The outer peripheral surface of the hub wheel  1  is formed with a hardened layer having a surface hardness 54˜64 HRC in a region from the inner raceway surface  1   a  to the axially extending portion  1   b . The heat treatment is preferably carried out by high frequency induction hardening suitable for a local heating and for easily setting a depth of the hardened layer. An end of the axially extending portion  1   b  is remained as non-hardened portion less than 25 HRC for forming a caulked portion  5  plastically deformed radially outward.  
         [0043]     A separate inner ring  6  is press fitted onto the axially extending portion  1   b  of the hub wheel  1  and secured thereto by the caulked portion  5 . An inner raceway surface  6   a  of the inboard side is formed on the outer peripheral surface of the inner ring  6  and forms together with the inner raceway surface  1   a  double row inner raceway surfaces  1   a  and  6   a.    
         [0044]     The double row rolling bearing  2  comprises an outer member  7 , an inner member  8  and double row rolling elements  11  and  11 . The outer member  7  comprises an outer ring member  9  integrally formed on its outer peripheral surface with a body mounting flange  9   b  for mounting the bearing apparatus on the body (not shown) of vehicle and on its inner peripheral surface an outer raceway surface  9   a , and a separate outer ring  10  press fitted into the outer ring member  9  at its inboard side and formed on its inner peripheral surface an outer raceway surface  10   a . The inner member  8  denotes the hub wheel  1  and the inner ring  6 . The double row rolling elements  11  and  11  are contained between the double row outer raceway surfaces  9   a  and  10   a  and the double row inner raceway surfaces  1   a  and  6   a  and rollably retained therein by cages  12  and  12 . A hardened layer having a surface hardness 54˜64 HRC in a region from the outer raceway surface  9   a  to the inboard side end is formed on the inner peripheral surface of the outer ring member  9 . The heat treatment is preferably carried out by high frequency induction hardening suitable for a local heating and for easily setting a depth of the hardened layer. Seals  13  and  14  are arranged at the ends of the double row rolling bearing  2  to prevent leak of grease contained within the bearing  2  as well as ingress of rain water or dusts. The illustrated ball rolling elements  11  and  11  may be replaced for example by conical rolling elements.  
         [0045]     There is arranged at an abutted region between the outer ring member  9  and outer ring  10  a ring shaped electrostrictive element  15  formed by laminated piezo elements etc to which a voltage control apparatus (not shown) is connected via a wire cable  16  for controlling the voltage applied to the electrostrictive element  15 . In this embodiment, a predetermined voltage is applied to the electrostrictive element  15  during a cornering run of a vehicle in accordance with a steering angle and a vehicle speed etc based on output signals from a steering angle sensor and a vehicle speed sensor etc (not shown) in order to extend the electrostrictive element  15  to increase the bearing preload and to set an optimum preload. On the contrary, when the vehicle runs on a straight road, no voltage is applied to the electrostrictive element  15  and the initial preload is maintained. Accordingly, it is possible to suppress the inclination of the brake rotor (not shown) by increasing the bearing preload when the bearing rigidity is required during cornering of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the vehicle is running on a straight road, it is possible to reduce the rotational torque and the friction loss of the bearing by reducing the bearing preload and thus to improve the fuel consumption. According to this embodiment, since the bearing preload is set at light preload so as to meet the running condition on a straight road which is very longer than run in cornering, the life of bearing can be further improved.  
       Second Embodiment  
       [0046]      FIG. 2  is a longitudinal view showing a second embodiment of the bearing apparatus for a wheel of the present invention. Same reference numerals are used in this embodiment for designating same parts having same functions used in the first embodiment.  
         [0047]     This bearing apparatus for a wheel of vehicle comprises the hub wheel  1  and a double row rolling bearing  17  and this double row rolling bearing  17  comprises an outer member  18 , an inner member  19  and the double row rolling elements  11  and  11 . The outer member  18  is integrally formed on its outer peripheral surface with a body mounting flange  9   b  for mounting the bearing apparatus on the body (not shown) of vehicle and on its inner peripheral surface double row outer raceway surfaces  9   a  and  9   a.    
         [0048]     A separate inner ring  6  is press fitted onto the axially extending portion  1   b  of the hub wheel  1  and secured thereto by the caulked portion  5 . The double row rolling elements  11  and  11  are contained between the double row outer raceway surfaces  9   a  and  9   a  and the double row inner raceway surfaces  1   a  and  6   a  and rollably retained therein by cages  12  and  12 . A hardened layer having a surface hardness 54˜64 HRC in a region from the outer raceway surface  9   a  to the press fitted portion of seals  13  and  14  is formed on the inner peripheral surface of the outer member  18 . The heat treatment is preferably carried out by high frequency induction hardening suitable for a local heating and for easily setting a depth of the hardened layer.  
         [0049]     A ring shaped magnetostrictive element  20  is interposed between an end  6   b  of the inner ring  6  and the caulked portion  5 . On the other hand, an electromagnetic coil  21  is mounted on the inboard side end face of the outer member  18  oppositely to the magnetostrictive element  20 . The magnetostrictive element  20  extends and increases the preload when the magnetic field is applied to the magnetostrictive element  20  by the electromagnetic coil  21 . Herein, the inner member  19  denotes the hub wheel  1 , the inner ring  6  and the magnetostrictive element  20 .  
         [0050]     According to the present embodiment, a load sensor  22  is arranged between the outer member  18  and the hub wheel  1 . The load sensor  22  comprises a portion to be detected (i.e. detected portion)  22   a  made of magnetostrictive material such as Fe—Al alloy and mounted on the hub wheel  1  and a detecting coil  22   b  mounted on the outer member  18 . The load sensor  22  can detect a variation of magnetostriction caused by increase and decrease of load applied to the apparatus as variation of magnetic resistance detected by the detecting coil  22   b . Accordingly, when a large load such as a momentum load is applied to the apparatus during cornering run on a curved road, a predetermined magnetic field is applied to the magnetostrictive element  20  by the electromagnetic coil  21  in accordance with the load and thus the magnetostrictive element  20  is extended to increase the bearing preload and to set an optimum preload. On the contrary, when a vehicle runs on a straight road, no magnetic field is applied to the magnetostrictive element  20  and thus the initial preload is maintained. Accordingly, it is possible to suppress the inclination of the brake rotor by increasing the bearing preload when the bearing rigidity is required during cornering run of vehicle and thus to ensure safety driving without any irregular motion. On the contrary, when the vehicle is running on a straight road, it is possible to reduce the rotational torque and the friction loss of the bearing by reducing the bearing preload and thus to improve the fuel consumption.  
         [0051]     Then a method for controlling the preload will be described with reference to a flow chart shown in  FIG. 3 . As previously described, the bearing apparatus of the present invention is provided with the preload varying means including the electrostrictive element  15  or magnetostrictive element  20  and electromagnetic coil  21 , the load sensor  22 , the vehicle speed sensor, temperature sensor etc for detecting the running condition of vehicle. Obtaining various real time informations as to the steering angle, the vehicle speed and the load acting on the bearing, a predetermined voltage is applied to the preload varying means so as to achieve a desired optimum preload by momentarily computing during travel of vehicle. During which, the voltage application is made “OFF” when the output of the temperature sensor mounted within the bearing exceeds a threshold in order to stop the preload varying function. Thus, it is possible to previously prevent any trouble which would be otherwise caused by abnormal temperature rise and thus to ensure the safety of vehicle.  
         [0052]     Although the preload varying means comprising the electrostrictive element  15  and that comprising the magnetostrictive element  20  and the electromagnetic coil  21  are illustrated as preferable embodiments, it is possible for example to use shape memory alloy in stead of them. In this case, if setting two conditions of the shape memory at higher or lower than the transformation point, two axial lengths condition of the shape memory alloy can be obtained. Thus it is possible to extend the shape memory alloy in the cornering travel condition of vehicle and not to extend it in the straight travel condition of vehicle. It is possible to use either a contact method or a non-contact method via electric power supply to a electromagnetic coil as a heating means for the shape memory alloy. In addition it is also possible to use aluminum alloy or plastics having a high coefficient of thermal expansion in place of shape memory alloy. Furthermore, the output signals of the steering angle sensor, the load sensor etc may be transmitted outside via a wireless manner.  
       Third Embodiment  
       [0053]      FIG. 4  is a longitudinal view showing a third embodiment of the bearing apparatus for a wheel of the present invention. Same reference numerals are used in this embodiment for designating same parts having same functions used in the second embodiment since this embodiment is different from the second embodiment only in its preload applying structure.  
         [0054]     This bearing apparatus for a wheel of vehicle comprises a hub wheel  23  and a double row rolling bearing  24  and this double row rolling bearing  24  comprises an outer member  18 , an inner member  25  and the double row rolling elements  11  and  11 . The outer member  18  is integrally formed on its outer peripheral surface with a body mounting flange  9   b  for mounting the bearing apparatus on the body (not shown) of vehicle and on its inner peripheral surface double row outer raceway surfaces  9   a  and  9   a . A separate inner ring  6  is press fitted onto the axially extending portion  1   b  of the hub wheel  23 . The double row rolling elements  11  and  11  are contained between the double row outer raceway surfaces  9   a  and  9   a  and the double row inner raceway surfaces  1   a  and  6   a  and rollably retained therein by cages  12  and  12 .  
         [0055]     The hub wheel  23  is formed as having a hollow structure and a securing member  26  is inserted into a central bore of the hub wheel  23 . The securing member  26  has a flange portion  26   a  at its one end and a shaft portion  26   b  on which a male thread  26   c  is formed. A ring shaped magnetostrictive element  20  is interposed between the end of larger diameter of the inner ring  6  and the flange portion  26   a . The inner ring  6  is axially immovably secured relative to the hub wheel  24  by fastening a nut  27  on the male thread  26   c  at a predetermined fastening torque. On the contrary, an electromagnetic coil  21  is mounted on the inboard side end face of the outer member  18  oppositely to the magnetostrictive element  20 . The magnetostrictive element  20  extends to increase the preload when the magnetic field is applied to the magnetostrictive element  20  by the electromagnetic coil  21 . Herein, the inner member  25  denotes the hub wheel  23 , the securing member  26 , the nut  27 , the inner ring  6  and the magnetostrictive element  20 .  
       Fourth Embodiment  
       [0056]      FIG. 5  is a longitudinal view showing a fourth embodiment of the bearing apparatus for a wheel of the present invention. This embodiment is a modification of the third embodiment and thus same reference numerals are used in this embodiment for designating same parts having same functions used in the third embodiment.  
         [0057]     This bearing apparatus for a wheel of vehicle comprises a hub wheel  28  and a double row rolling bearing  29  and this double row rolling bearing  29  comprises the outer member  18 , an inner member  30  and the double row rolling elements  11  and  11 . The hub wheel  28  has the axially extending portion extending from the inner raceway surface  1   a  and a shaft portion  28   a  on which a male thread  28   b  is formed. The separate inner ring  6  is press fitted onto the axially extending portion  1   b  of the hub wheel  28  and immovably secured relative to the hub wheel  28  by the fastening nut  27  via a ring shaped securing member  31 .  
         [0058]     A ring shaped magnetostrictive element  20  is interposed between the end  6   b  of larger diameter of the inner ring  6  and the securing member  31 . On the contrary, the electromagnetic coil  21  is mounted on the inboard side end face of the outer member  18  oppositely to the magnetostrictive element  20 . The magnetostrictive element  20  extends to increase the preload when the magnetic field is applied to the magnetostrictive element  20  by the electromagnetic coil  21 . Herein, the inner member  30  denotes the hub wheel  28 , the securing member  31 , the nut  27 , the inner ring  6  and the magnetostrictive element  20 .  
       Fifth Embodiment  
       [0059]      FIG. 6  is a longitudinal view showing a fifth embodiment of the bearing apparatus for a wheel of the present invention. Same reference numerals are used in this embodiment for designating same parts having same functions used in the third embodiment ( FIG. 4 ) since this embodiment is modification of the third embodiment and different from the third embodiment only in a manner of securing the securing member.  
         [0060]     This bearing apparatus for a wheel of vehicle comprises a hub wheel  32  and a double row rolling bearing  33  and this double row rolling bearing  33  comprises the outer member  18 , an inner member  34  and the double row rolling elements  11  and  11 . The hub wheel  32  is formed as having a hollow structure and a securing member  35  is inserted into a central bore of the hub wheel  32 . The securing member  35  has a hollow structure and is formed with the flange portion  26   a , the shaft portion  26   b , and a fitting portion  36  at an end of the shaft portion  26   b . A ring shaped magnetostrictive element  20  is interposed between the end  6   b  of larger diameter of the inner ring  6  and the flange portion  26   a.    
         [0061]     An irregular portion  32   a  is formed on the inner peripheral surface of the hub wheel  32  and a hardened layer having a surface hardness 54˜64 HRC is formed thereon. The heat treatment is preferably carried out by high frequency induction hardening suitable for a local heating and for easily setting a depth of the hardened layer. The irregular portion  32   a  may be formed as having a knurled crisscross pattern.  
         [0062]     The hub wheel  32  and the securing member  35  are integrally united by expanding and plastically deforming the fitting portion  36  using a suitable means such as a mandrel so that the irregular portion  32   a  of the hub wheel  32  bites into material of the fitting portion  36  of the securing member  35 . Thus the inner ring  6  is axially immovably secured relative to the hub wheel  32 . The fitting portion  36  is remained as non-hardened portion less than 24 HRC and it is preferable to set the difference of surface hardness between the fitting portion  36  and the irregular portion  32   a  more than 30 HRC. This enables the irregular portion  32   a  to easily and deeply bite into the fitting portion  36  and also to plastically unite them without causing deformation of tips of the irregular portion  32   a . Herein, the inner member  34  denotes the hub wheel  32 , the securing member  35 , the inner ring  6  and the magnetostrictive element  20 .  
         [0063]     Then the preload controlling method during assembly of the bearing apparatus for a wheel of vehicle of the present invention will be described with reference to the embodiment of  FIG. 1 .  
         [0064]     In assembly of the bearing apparatus, an output signal from the electrostrictive element  15  during fixation of the inner ring  6  relative to the hub wheel by the caulked portion  5  has a wave configuration shown in  FIG. 7 . In this wave configuration, a left hand portion with respect to the peak value (a hatched portion in  FIG. 7 ) shows a plus portion in the preload variation and a right hand portion shows a discharge of electric charge in the electrostrictive element  15 . Since an integral value of the left hand portion corresponds to the preload, it is possible exactly set the suitable initial preload by controlling its integral amount. Thus, since the assembly of the bearing apparatus can be carried out with watching the output signal from the electrostrictive element  15  forming the preload varying means, it is possible to complete the assembling work at a time in which the predetermined preload is obtained and also possible to more exactly and easily control the preload than the conventional torque control manner.  
         [0065]     Similar preload control can be carried out in the case of using the magnetostrictive element  20  in the second embodiment ( FIG. 2 ). In this magnetostrictive element  20 , it is possible to easily detect the variation of the preload as a variation of the magnetic permeability, i.e. as a voltage variation exhibiting a linear relation in the magnetic coil  21 . Accordingly, since the assembling work can be carried out, similarly to the case of the electrostrictive element  15 , with watching the output signal from the magnetostrictive element  20 , the preload control can be exactly and easily carried out.  
       APPLICABILITY IN INDUSTRY  
       [0066]     The bearing apparatus for a wheel of vehicle of the present invention can be applied to any kind of structure in which a predetermined preload is adapted to be applied to the double row rolling bearing.  
         [0067]     The present invention has been described with reference to the preferred embodiments. Obviously, modifications and alternations will occur to those of ordinary skill in the art upon reading and understanding the preceding detailed description. It is intended that the present invention be construed as including all such alternations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof.