Abstract:
A rolling bearing unit with a rotation speed sensor comprising a stationary member, a rotatable member having a second raceway, a plurality of rolling members rotatably provided between the first raceway and the second raceway, an encoder fixed to the rotatable member and to be concentric with the rotational axis thereof and having a detected portion the characteristics of which are adapted to alternately change, and a sensor fixed to the stationary member to detect changes in the magnetic characteristics of the encoder to produce signals as the rotatable member rotates, and the sensor having a detecting portion which is opposed to the detected portion of the encoder with a clearance therebetween at a circumferential location where the change in the clearance is minimized when subjected to a load from outside.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rolling-bearing unit with rotation speed sensor which is used for rotatably supporting an automobile wheel with respect to the suspension, as well as for detecting the rotation speed of the wheel. 
     BACKGROUND OF THE INVENTION 
     A rolling-bearing unit is used for supporting an automobile wheel so that it rotates freely with respect to the suspension. Moreover, in order to be able to control an anti-lock-brake system (ABS) or traction-control system (TCS), it is necessary to detect the rotation speed of the aforementioned wheel. In order to do this, a rolling-bearing unit with rotation speed sensor, in which a rotation speed detection device has been built into the rolling-bearing unit described above, has been widely used in recent years to support the wheel such that it rotates freely with respect to suspension, as well as to detect the rotation speed of the wheel. 
     FIGS. 1 and 2 show one example of a prior art rotation speed sensor, as disclosed in Japanese Patent Publication No. Toku Kai Hei 8-296634, that is used for the purpose described above. This rolling-bearing unit with rotation speed sensor comprises a stationary race or outer race  1  which does not rotate during use, a rotatable ring or hub  2  which is supported on the inside of this outer race  1  and which rotate during use, an encoder  3  which is fixed at one location on this hub  2 , and a sensor  4  which is supported by the outer race  1  to detect the rotation speed of the encoder  3 . In other words, stationary raceways or outer ring raceways  5  in double rows are formed around the inner peripheral surface, or stationary peripheral surface, of this outer race  1 . Moreover, the hub  2  has a pair of inner races  7  which are fixedly fitted around the outer peripheral surface of the main hub body  6 . 
     Rotating raceways or inner ring raceways  8   a,    8   b  are formed around the rotatable peripheral surfaces or outer peripheral surfaces of both of these inner races  7 , respectively. Multiple rolling members  9  are rotatably supported by a cage  10  in each row between the inner ring raceways  8   a,    8   b  and outer ring raceways  5 , so that they support the hub  2  so as to rotate freely inside the outer race  1 . 
     Moreover, on the axially outer end of the main hub body  6  (end of the outside in the direction of width when installed in the automobile, left end in FIG. 1) in the area that sticks out in the axial direction from the axially outer end of the outer race  1 , there is a flange  11  for attaching to the wheel. Also, on the axially inner end of the outer race  1  (end on the center side in the direction of width when installed in the automobile, right end in FIG. 1) there is an installation section  12  for attaching the outer race  1  to the suspension. Furthermore, the space between the opening on the axially outer end of the outer race  1  and the outer peripheral surface around the mid portion of the hub  2  is covered by a seal ring  13 . 
     In order to install a rotation speed sensor in this kind of rolling-bearing unit, the encoder  3  is fitted around a portion closer to the axially inner end of the main hub body  6  that protrudes axially inward from both of the inner races  7 . This encoder  3  is formed into a circular ring shape from a magnetic metal plate such as steel and provided with a detected portion  14  on the axially inner face (the right face in FIG. 1) at a portion closer to the outer periphery thereof. This encoder  3  is fitted around a portion closer to the axially inner end of the main hub body  6  and is held in place between a nut  15  that screws to the axially inner end of the main hub body  6 , and the surface on the axially inner end face of the inside inner race  7 . 
     The detected section  14  is formed with recesses and lands around in the circumferential direction, and is shaped with a gear-like shape, and the magnetic characteristics of this detected section  14  change alternately at evenly spaced intervals around in the circumferential direction. 
     Furthermore, a cylindrical-shaped cover  16  with a bottom fits into the opening on the axially inner end of the outer race  1  to cover the opening on the axially inner end of the outer race  1 . This cover  16  is made from plastically worked sheet metal, and comprised of a cylindrical fitting section  17  that freely fits inside the opening on the axially inner end of the outer race  1 , and a cover plate section  18  that covers the opening on this axially inner end of the cylindrical fitting section  17 . A sensor  4  is supported at a radially outer portion in this cover plate section  18 , and the tip end face (left end face in FIG. 1) of the detection section  19  of this sensor  4  faces the axially inner face of the detected section  14  of the encoder  3  in the axial direction through a small clearance, for example 0.5 mm. 
     In the case of the rolling-bearing unit with rotation speed sensor described above, the wheel that is attached to the flange  11  formed on the axially outer end of the hub  2  is able to rotate freely with respect to the suspension that supports the outer race  1 . Moreover, as the encoder  3  that is fitted around the axially inner end of the hub  2  rotates with the rotation of the wheel, the lands and recesses formed on the detected section  14  alternately passes by the end surface of the detection section  19  of the sensor  4 . As a result, the density of the magnetic flux that flows through the sensor  4  changes, and thus the output of the sensor  4  changes. 
     The frequency of the changing output of the sensor  4  is proportional to rotation speed of the wheel. Therefore, if the output from the sensor  4  is sent to a controller (not shown in the figure), it is possible to adequately control the ABS or TCS. 
     In order to secure the reliability in detecting the rotation speed of the wheel by a rolling-bearing unit with rotation speed sensor that functions as described above, it is necessary that the dimension of the gap between the tip end face of the detection section  19  of the sensor  4  and the end surface of the detected section  14  of the encoder  3  be stable. On the other hand, the components that make up the rolling-bearing unit deform elastically as the automobile operates. Particularly, as the automobile is turned fast, the amount of elastic deformation of the components increases due to the moment load that is applied to the hub  2  (by the turning acceleration) from the wheel by way of the flange  11 . Also, due to the increase of the amount of this elastic deformation, the dimension of small clearance changes. This kind of change in dimension causes the output of the sensor  4  to change, and thus becomes a possible cause of loss of reliability of the rotation speed detection. 
     Therefore, in the case of the invention disclosed in Patent Publication No. Toku Kai He 8-296634, the sensor  4  is placed on a horizontal plane that passes through the center axis of the hub  2 , so that with the changes in dimensions of the small clearance kept to a minimum, the reliability of the detected rotation speed is maintained regardless of elastic deformation of the components. 
     In the case of the invention disclosed in Japanese Patent Publication No. Toku Kai Hei 8-296634, only part of the elastic deformation of the components due to moment load applied to the hub  2  when the automobile turns fast is taken under consideration. Therefore, it is actually impossible to stabilize the dimension of the small clearance between the tip end face of the detection section  19  of the sensor  4  and the end surface of the detected section  14  of the encoder  3 . In other words, due to the moment load, the displacement occurs in the rolling-bearing unit such that the center axis of the outer race  1  does not coincide with the center axis of the hub  2 , and in addition, displacement that the outer race  1  and the hub  2  shift with reference to each other in the axial direction also occurs. The invention disclosed in Japanese Patent Publication No. Toku Kai Hei 8-296634 only takes into consideration the displacement that the center axis of the outer race  1  does not coincide with the center axis of the hub  2 . Therefore, even though the sensor  4  is actually located on the horizontal plane that passes through the center axis of the hub  2 , it is not possible to stabilize the dimension of the small clearance, and thus does not contribute much to maintaining the reliability of the detected rotation speed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a rolling-bearing unit with rotation speed sensor, taking these problems into consideration, wherein the dimension of the small clearance between the detection section of the sensor and the detected section of the encoder is stabilized to maintain the reliability of the rotation speed detection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view showing a conventional structure of the rolling bearing unit with rotation speed sensor. 
     FIG. 2 is a partly cut-away, cross sectional view taken along the line II—II in FIG. 1 with sensor removed. 
     FIG. 3 is a cross sectional view taken along the line III—O—III in FIG. 4 to show an example of the embodiments of the rolling bearing-unit in the present invention. 
     FIG. 4 is a view taken from the right side in FIG.  3 . 
     FIG. 5 is a cross sectional view taken along the line V—O—V in FIG. 6 to show a second example of the embodiments of the rolling bearing unit in the present invention. 
     FIG. 6 is a view taken from the right side in FIG.  5 . 
     FIG. 7 is a cross sectional view taken along the line VII—O—VII in FIG. 8 to show a third example of the embodiments of the rolling bearing unit in the present invention. 
     FIG. 8 is a cross sectional view taken along the line VIII—O—VIII to show a major section. 
     FIG.  9 (A) is a diagram to explain the displacement of minute clearance when subjected to a relatively small moment load in the + direction with reference to the rigidity of the rolling bearing unit. 
     FIG.  9 (B) is a diagram to explain the displacement of minute clearance when subjected to a relatively small moment load in the − direction with reference to the rigidity of the rolling bearing unit. 
     FIG.  10 (A) is a diagram to explain the displacement of minute clearance which subjected to a relatively large moment load in the + direction with reference to the rigidity of the rolling bearing unit. 
     FIG.  10 (B) is a diagram to explain the displacement of minute clearance when subjected to a relatively large moment load in the − direction with reference to the rigidity of the rolling bearing unit. 
     FIG. 11 is a cross sectional view similar to FIG. 7 to show a fourth example of the embodiments of the present invention. 
     FIG. 12 is a cross sectional view similar to FIG. 7 to show a fifth example of the embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The rolling-bearing unit with rotation speed sensor in one embodiment of this invention, similar to the prior art rolling-bearing unit with rotation speed sensor described above, comprises a stationary race that does not rotate during use and has stationary raceways around its stationary peripheral surface, a rotating race that rotates during use and has rotating raceways on its rotating peripheral surface which face the stationary peripheral surface, multiple rolling members that are located between the stationary raceways and rotating raceways, an encoder that is fixed to the rotating race such that it is concentric with the rotating race and has a circular ring-shaped detected section which has magnetic characteristics that alternate and change at equal intervals around in the circumferential direction, and a sensor that has a detection section and is supported by the non-rotating section such that the detection section faces part of the detected section of the encoder and whose output signal changes as the characteristics of the detected section changes. 
     Particularly, in the rolling-bearing unit with rotation speed sensor of this embodiment of the present invention, the detected portion of the encoder radially faces the detection section of the sensor. And, the sensor is located at a portion which nearly coincides with an imaginary plane that passes through the center axes of both the stationary race and rotating race and extends in the horizontal direction. 
     The rolling-bearing unit with rotation speed sensor of this embodiment constructed as described above, rotatably supports the wheel with respect to the suspension, as well as detects the rotation speed of the wheel, which is similar to that of the prior construction mentioned above. 
     Particularly, in the case of the rolling-bearing with rotation speed sensor of this invention, it is possible to keep the change in dimension of the small clearance in the radial direction that exists between the detected section of the encoder and the detection section of the sensor to a minimum, even when the center axis of the stationary member and the center axis of the rotating member do not coincide with each other and at the same time the stationary member and the rotating member shift in the axial direction due to the moment load applied as the automobile is turned fast. 
     Moreover, even when displacement occurs in the vertical direction between the stationary member and rotating member due to a load applied in the vertical direction, it is possible to keep change in the dimension of the small clearance to a minimum. As a result, the output of the sensor is stabilized, regardless of the elastic deformation in the components members of the rolling bearing unit due to a load that is applied from outside, thus making it possible to improve the reliability of the rotation speed detection. 
     FIGS. 3 and 4 show a first example of the embodiments of the invention. The upward direction in FIG. 4 corresponds to the upward vertical direction with the unit installed in the automobile, and the downward direction in FIG. 4 corresponds to the downward vertical direction with the unit installed in the automobile. The hub  2 , which is the rotating member, comprises the main hub body  6  and inner race  7  securely connected to the main hub body  6 . A double row of inner ring raceways  8   a,    8   b,  which are second raceways, are formed around the outer peripheral surface of the hub body  2 . 
     There is a flange  11  for attaching to the wheel around the outer peripheral surface on the axially outer end (left end in FIG. 3) of the main hub body  6 . The inner ring raceway  8   a  provided on the axially outer side on the outer peripheral surface of the mid portion of the main hub body  6 , with respect to the double row of inner ring raceways  8   a,    8   b.  And, a small-diameter step section  20  is provided on the axially inner end (right end in FIG. 3) of the main hub body  6 . 
     The inner race  7  fits around this step section  20 , and by crimping the axially inner end of the main hub body  6  outward in the radial direction, the inner race  7  is fixed to the axially inner end of the main hub body  6 . The inner raceway  8   b  is formed around the outer peripheral surface of the inner race  7 . Several rolling members  9  are located in the space between these inner ring raceways  8   a,    8   b  and the first raceway or outer ring raceways  5  that are formed around the inner peripheral surface of the stationary member or stationary outer race  1 , and they rotatably support the hub  2  on the inside of the outer race  1 . In the example shown in the drawings, balls are used as the rolling members  9 , however, in the case of a rolling-bearing unit for a heavy automobile, tapered rollers may also be used as these rolling members  9 . 
     Moreover, the based end (left end in FIG. 3) of an encoder  3  fits tightly around the shoulder section that is formed on the axially inner end of the inner race  7  in the section that is separated from the inner ring raceway  8   b.  This encoder  3  is formed from magnetic metal plate such as carbon steel, and it is formed with a large diameter section  25  at the base and a small diameter section  26  on the tip end half which are connected to each other by a step section  27 . Of these two sections, the small-diameter section  26  is formed with through holes  28  in slit shape that are long in the axial direction (left and right direction in FIG. 3) and these through holes  28  are evenly spaced around in the circumferential direction. This small-diameter section  26  has a generally cage like shape, and its magnetic characteristics are changed alternately at equal intervals around in the circumferential direction. The step section  27  comes in contact with the outer peripheral edge of the axially inner end of the inner race  7  to securely position in the axial direction the encoder  3  with reference to the inner race  7 . 
     Also, the small clearance between the opening on the axially outer end of the outer race  1  and the outer peripheral surface around the mid portion of the hub  2  is covered by a seal ring  13 . On the other hand, the opening on the axially inner end (right end in FIG. 3) of the outer race  1  is covered by a cover  16 . This cover  16  is made of synthetic resin and is formed by injection molding and comprises a cylindrical shaped main body  21  with a bottom, and a fitting tube  22  that is connected to the opening of the main body  21 . This fitting tube  22  is made by plastically deforming an anti-corrosive metal plate such as stainless steel, and is generally ring shaped with an L-shaped cross section and comprises a cylindrical fitting section  23  and an outward facing brim section  24  that is bent outward in the radial direction from the base end edge (right end edge in FIG. 3) of the cylindrical fitting section  23 . By molding the outward facing brim section  24  at the same time when performing injection molding of the main body  21 , this fitting tube  22  is joined with the opening portion of the main body  21 . By tightly fitting the cylindrical fitting section  23  of the fitting tube  22  in the axially inner end of the outer race  1 , this cover  16  covers the opening on the axially inner end of the outer race  1 . 
     Moreover, in a part of the bottom plate  29  of the main body  21  of the cover  16 , a cylinder section  30  is formed which sticks axially inward of the bottom plate  29  in the part which coincides with the imaginary plane that passes through the center axes of the outer race  1  and hub  2  and extends in the horizontal direction. Furthermore, an insert hole  31  is formed on the inside of this cylinder section  30  in the axial direction of outer race  1  for communication between the inside end surface of this cylinder section  30  and the outside surface of the bottom plate  29 . 
     The tip end portion of a sensor unit  33 , which comprises a synthetic resin holder  32  and a sensor embedded in the holder  32 , is inserted inside this insert hole  31 . With this sensor unit  33  inserted into the insert hole  31  as described above, the side surface on the radially inner side (lower surface in FIG. 3) of the tip end of the sensor unit  33  faces the detected portion that is the outer peripheral surface around the small-diameter section  26  of the encoder  3  through a small clearance  34 . 
     In this embodiment, in order that the sensor unit  33  can be installed or removed easily and quickly inside the cover  16 , there is a connection spring  36 , that is formed by bending elastic and anti-corrosive wire material such as stainless steel, located between the cylinder section  30  and an anchoring brim section  35  formed in the base end (right end in FIG. 3) of the holder  32 . This connection spring  36  holds the anchoring brim section  35  against the end surface of the opening of the cylinder section  30 . This part is not the main part of this invention, so a detailed explanation is omitted. 
     The rolling-bearing unit with rotation speed sensor of this invention constructed as described above, supports the wheel such that it can rotate freely with respect to the suspension, as well as detects the rotation speed of the wheel, which is similar to that of the previously known rolling-bearing unit with rotation speed sensor. In other words, when it is installed in an automobile, the outer race  1  is attached to the suspension device by the installation fixture  12  that is attached to the outer peripheral surface of this outer race  1 . Moreover, the wheel is attached to the flange  11  that is formed around the outer peripheral surface on the axially outer end of the hub  2 . 
     In this state, the hub  2  rotates together with the wheel, and as the encoder  3 , which is supported by the hub  2 , rotates, the through holes  28  formed in the smaller diameter portion  26  and the magnetic column sections, located between the circumferentially adjacent through holes  28  alternately pass the area near the detection section of the sensor of the sensor unit  33 . This sensor, for example, comprises a permanent magnet, a stator made of magnetic material for conducting the magnetic flux corning from the permanent magnet, a magnetic detection element, such as a Hall effect element or magneto resistive element, the magnetic characteristics of which change according to the amount of magnetic flux passing it, and an IC for converting the changes in characteristics detected by the magnetic detection element. The amount of magnetic flux flowing through the magnetic detection element of the sensor changes as the encoder  3  rotates, so that output of the sensor changes as well. The frequency of the changing sensor output is proportional to the rotation speed of the wheel. Therefore, by sending this sensor output to a controller (not shown in the drawings), it is possible to adequately control the ABS or TCS. 
     Particularly, in the case of the rolling-bearing unit with rotation speed sensor of this invention, it is possible to keep to a minimum the change in dimension of the small clearance  34  in the radial direction that exists between the outer peripheral surface of the encoder  3  and the detection section of the sensor, even when the center axis of the outer race  1  and the center axis of the hub  2  do not coincide with each other while the outer race  1  and the hub  2  shift from each other in the axial direction due to the moment load applied when the automobile is turned fast. 
     In other words, both of these center axes stay nearly on the same vertical plane even when the center axis of the outer race  1  and the center axis of the hub  2  do not coincide with each other. Accordingly, by locating the sensor unit  33 , which contains the sensor, in an area that coincides with an imaginary plane that runs in the horizontal direction through the center axes of the outer race  1  and the hub  2 , there is hardly none if any effect on the change of dimension of the small clearance  34  due to the displacement of these center axes. 
     Moreover, there is hardly none if any effect on the change in dimension of the small clearance  34  due to the displacement in the axial direction of the outer race  1  and the hub  2 . 
     Furthermore, when a load in the vertical direction is applied due to the weight of the automobile, the center axis of the outer race  1  and the center axis of the hub  2  do not coincide with each other, however, even in this case, since both of these axes shift from each other in the vertical direction along the same vertical plane, there is no effect on the change in dimension of the small clearance  34 . 
     As a result, the output of the sensor is stabilized, regardless of elastic deformation of the components, or in other words the outer race  1 , hub  2  and rolling members  9 , of the rolling-bearing unit due to an externally applied load, and thus it is possible to improve the reliability of the rotation speed detection. 
     Next, FIGS. 5 and 6 show a second example of the embodiments of the invention. The upward direction in FIG. 6 corresponds to the upward vertical direction with the unit installed in the automobile, and the downward direction corresponds to the downward vertical direction with the unit installed in the automobile. In this embodiment, a seal ring  13   a  is fixedly fitted into the axially inner end of the stationary member or outer race  1 , such that the edge of the seal lip of the seal ring  13   a  comes in sliding contact with the outer peripheral surface of the encoder  3  so as to cover the space between the outer peripheral surface on the axially inner end of the inner race  7  and the inner peripheral surface on the axially inner end of the outer race  1 . 
     Moreover, an installation hole  38  is formed on part of the knuckle  37  of the suspension which affixes the outer race  1 , such that the part coincides with an imaginary plane that runs horizontally through the center axes of the outer race  1  and hub  2 . 
     The sensor unit  33  which holds the sensor is inserted in the installation hole  38 , and the detection section of the sensor faces the outer peripheral surface of the encoder  3  through a small clearance  34  therebetween. In this state, the sensor unit  33  is fastened to the knuckle  37  by a screw  39 . 
     The other construction and function are substantially the same as that of first embodiment described above. 
     In both the first and second embodiments of this invention, this invention was applied to a rolling-bearing unit with rotation speed sensor that is used for supporting to the suspension device the non-driven wheel of an automobile (rear wheel for the FF vehicle, and front wheel for the FR and RR vehicles). However, this invention can also be applied to a rolling-bearing unit with rotation speed sensor used for supporting to the suspension device the driven wheel of an automobile (front wheel for the FF vehicle, and read wheel for the FR and RR vehicles). 
     Moreover, it is also possible to apply this invention to a rolling-bearing unit with rotation speed sensor in which the inner-race-like member does not rotate and the outer-race-like member rotates. 
     Furthermore, it is also possible to apply this invention to a rolling-bearing unit with rotation speed sensor in which the encoder is installed in the mid portion in the axial direction of the rotating member, and the sensor is installed in the mid portion in the axial direction of the stationary member. 
     The rolling-bearing unit with rotation speed sensor in another feature of this invention, similar to the prior art rolling-bearing unit with rotation speed sensor described above, comprises a stationary race that does not rotate during use and has stationary raceways around its stationary peripheral surface, a rotating race that rotates during use and has rotating raceways on its rotating peripheral surface which face the stationary peripheral surface, multiple rolling members that are located between the stationary raceways and rotating raceways, an encoder that is fixed to the rotating race such that it is concentric with the rotating race and has a circular ring-shaped detected section which has characteristics that alternate and change at equal intervals around in the circumferential direction, and a sensor that has a detection section and is supported by the non-rotating section such that the detection section axially faces part of the detected section of the encoder and whose output signal changes as the characteristics of the detected section changes. 
     Particularly, in the rolling-bearing unit with rotation speed sensor of this feature of the present invention, the detection section of the sensor with reference to the circumferential direction of the stationary race and rotating race is located in an area that coincides with an imaginary plane that runs in the horizontal direction through the center axis of the outer race  1  and hub  2 . 
     FIGS. 7 and 8 show a third example of the embodiments of the invention. The hub  2 , which is the rotating member, comprises the main hub body  6  and the inner race  7  connected to the main body  6 . Around the outer peripheral surface of the main hub body  6 , there is a flange  11  for attaching to the wheel on the axially outer end (left end in FIG. 7) thereof. A double row of second raceways or inner ring raceways  8   a,    8   b  is formed around the outer peripheral surface of the hub  2 . 
     With the inner ring raceways  8   a,    8   b  which are formed around the outer surface of the hub body  2 , the inner ring raceway  8   a  on the axially outer side is formed in the mid portion of the main hub body  6 . A small-diameter step section  20  is formed on the axially inner end (right end in FIG. 7) of the main hub body  6 . The inner race  7  fits around this step section  20 , and by crimping the axially inner end of the main hub body  6  outward in the radial direction, the inner race  7  is fixed to the axially inner end of the main hub body  6 . The inner raceway  8   b  is formed around the outer peripheral surface of the inner race  7 . Several rolling members  9  are located in the space between these inner raceways  8   a,    8   b  and the first raceways or outer ring raceways  5  that are formed around the inner peripheral surface of the stationary member or outer race  1 , so that rotatably support the hub  2  on the inside of the outer race  1 . 
     In the example shown in the drawings, balls are used as the rolling members  9 , however, in the case of a roller-bearing unit for a heavy automobile, tapered rollers may also be used as these rolling members  9 . 
     Moreover, the clearance between the opening portion on the axially outer end of the outer race  1  and the outer peripheral surface in the center of the hub  2  is covered by a seal ring  13 , and the clearance between the opening portion on the axially inner end of the outer race  1  and the outer peripheral surface on the axially inner end of the inner race  7  is covered by a combination seal ring  121 . 
     This combination seal ring  121  contains a metal ring  122  that is fitted around and fastened to the axially inner end of the inner race  7 , and an encoder is mounted to the axially inner surface of the metal ring  122 . This encoder  123  is made from a permanent magnet and is entirely circular ring shaped, and magnetized in the axial direction (left and right direction in FIG.  7 ). The magnetic direction alternates at equal intervals around in the circumferential direction. Moreover, the S pole and N pole are arranged alternately at equal intervals on the axially inner surface, or detected area, of the encoder  123 . 
     Also, an outward flange shaped installation section  12  is formed around the outer peripheral surface on the axially inner end of the outer race  1  and it is fastened to a knuckle  37  which is the non-rotating part of the suspension device. Moreover, there is an installation hole  38  on part of this knuckle  37  in which a holder  32 , that holds the sensor  4 , in inserted, and the detection section of this sensor  4  faces the inside surface of the encoder  123  with a small clearance  127  of about 0.5 mm in the axial direction between them. In this state, the holder  32  is fastened to the knuckle  37  by a screw  39 . 
     Particularly in the rolling bearing unit with rotation speed sensor of this feature of the present invention, the installation position of the sensor  4  is regulated by its relationship to the rigidity of the rolling bearing unit which comprises the outer race  1 , hub  2  and rolling members  9 , as described by conditions {circle around (1)} and {circle around (2)} below. 
     If an automobile, that is installed with this rolling bearing unit with rotation speed sensor, turns such that the rolling bearing unit with rotation speed sensor is located on the outside, then if the acceleration due to the turning of the automobile is assumed to be + 1 G, the relative displacement in the axial direction between the outer race  1  and hub  2  is taken to be “δ a1 ”. 
     Also, if the automobile turns such that this rolling bearing unit with rotation speed sensor is located on the inside, then if the acceleration due to the turning of the automobile is assumed to be − 1 G, the relative displacement in the axial direction between the outer race  1  and hub  2  is taken to be “δ a2 ”. 
     Moreover, when an acceleration of + 1 G is assumed, the angle of inclination (radians) between the center axis of the outer race  1  and the center axis of the hub  2  is taken to be “θ 1 ”. 
     On the other hand, when an acceleration of − 1 G is assumed, the angle of inclination (radians) between the center axis of the outer race  1  and the center axis of the hub  2  is taken to be “0 2 ”. 
     Furthermore, the radius of the detected section of the encoder  123  is taken to be “r”. The radius “r” of this detected section is the distance from the center axis of the hub  2  to the center in the width direction (radial direction) of the part on the axially inner surface of the encoder  123  that the sensor  4  faces. 
     Under these conditions: 
     {circle around (1)} If δ a1 ≧r·θ 1  (at the same time δ a2 ≧r·0 2 ), then the installation position of the sensor  4  with respect to the circumferential direction of the outer race  1  and hub  2  is higher than the center axes of the outer race  1  and hub  2 , and nearly coincides with a vertical line that passes through these center axes. In other words, as shown in FIG. 8, the angle of intersection (radians) “φ” between the dotted chain line “X” that represents the horizontal line, and the dotted chain line “Y” that represents the center axis of the installation hole  38  is taken to be π/2. 
     On the other hand: 
     {circle around (2)} If δ a1 ≦r·θ 1  and δ a2 ≦r·θ 2 , then with the installation position of the sensor  4  with respect to the circumferential direction of the outer race  1  and hub  2 , the angle of intersection (radians) “φ” between the angles of intersection between the dotted chain line “Y” that represents the center axis of the installation hole  38  and the dotted chain line “X” that represents the horizontal line, is given between φ 1  and φ 2  which are given by the equations {circle around (1)} and {circle around (2)} below. 
     
       
         φ 1 =sin −1 (δ a1 /r·θ 1 )  {circle around (1)} 
       
     
     
       
         φ 2 =sin −1 (δ a2 /r·θ 2 )  {circle around (2)} 
       
     
     When actually determining the installation position of the sensor  4 , it is not necessary that the above conditions {circle around (1)} and {circle around (2)} be strictly adhered to. Even if the position is shifted 115 degrees from these conditions {circle around (1)} and {circle around (2)}, the dimension of the small clearance  127  will not change so as to become a problem. 
     With the rolling bearing unit with rotation speed sensor of this embodiment, constructed as described above, the wheel is rotatably supported by the suspension device, and the rotation speed of this wheel is detected in the same way as previously known rolling bearing units with rotation speed sensors. In other words, the hub  2  rotates as the wheel rotates, and as the encoder  123  that is supported by this hub  2  rotates, the S poles and N poles alternately pass by the detection section of the sensor  4 . As a result, the direction that the magnetic flux flows in this sensor  4  alternates, and thus the output of the sensor  4  changes. The frequency of this changing output of the sensor  4  is proportional to the rotation speed of the wheel. Therefore, if the output from the sensor  4  is sent to a controller (not shown in the drawings), it is possible to adequately control an ABS or TCS. 
     Particularly, in the case of the rolling bearing unit with rotation speed sensor of this invention, it is possible to keep to a minimum the change in size in the axial direction of the small clearance  127  that exists between the inside surface of the encoder  123  and the detection section of the sensor  4 , even when the center axis of the outer race  1  does not coincide with the center axis of the hub  2  due to the moment load applied when the automobile is turning fast, as well as when the outer race  1  and hub  2  shift from each other in the axial direction. As a result, it is possible to stabilize the output of the sensor  4  and improve the reliability of the detected rotation speed, regardless of elastic deformation of the components of the rolling bearing unit, that is the outer race  1 , hub  2  and rolling members  9 , due to the moment load. 
     Next, FIGS. 9 and 10 are used to explain the reason why it is possible to keep to a minimum the change in the size of the small clearance  127  by satisfying the aforementioned conditions {circle around (1)} and {circle around (2)}, even when elastic deformation of the components of the rolling bearing unit occurs. Of these FIGS. 9 and 10, FIGS.  9 (A) and  9 (B) are diagrams which explain the reason why it is possible to keep to a minimum the change in size of the small clearance  127  when conditions {circle around (1)} (δ a1 ≧r ·θ 1 ) is satisfied. 
     First, referring to the codes used in FIG. 7, FIGS.  9 (A) and  9 (B) are used to explain the case where δ a1 ≧r·θ 1  while at the same time δ a2 ≧r·θ 2 . When an automobile turns fast, there is elastic deformation of the outer race  1 , hub  2  and rolling members  9  due to the moment load that is applied to the hub  2  from the wheel by way of the flange  11  when turning acceleration occurs. 
     Also, the size of the small clearance  127  that exists between the detected section of the encoder  123 , which is supported by the hub  2 , and the detection section of the sensor  4 , which is supported by the knuckle  37 , changes. For example, if a moment load + 1 G is applied to the hub  2 , then, as shown in FIG. 9 (A), the center axis of the outer race  1  and the center axis of the hub  2  shift from each other by the angle θ 1  in the vertical plane, while at the same time, the outer race  1  and hub  2  shift from each other in the axial direction an amount δ a1 . This angle, θ 1 , and axial shift, δ a1 , can be found from an equation which is related to the previously known bearing rigidity of the double-row rolling bearing. 
     As the center axis of the outer race  1  and the center axis of the hub  2  shift from each other by an angle, θ 1 , the size of the small clearance  127  tends to increase by the amount r·θ 1  at the top position of the small clearance  127 , and to decrease by the amount of r·θ 1  at the bottom position. 
     On the other hand, as the outer race  1  and hub  2  shift from each other in the axial direction by the amount δ a1 , the size of the small clearance  127  tends to decrease all the way around by the amount δ a1 . 
     If the moment load is applied, the size of the small clearance  127  becomes a combination of the displacement due to the shift in center axes, and the shift in the axial direction. Moreover, when δ a1 ≧r·θ 1 , the amount of displacement of the small clearance  127  is, δ a1 r·θ 1  at the top of the small clearance  127 , and δ a1 +r·θ   1  at the bottom position, and δ a1  at the center in the vertical direction. 
     On the other hand, if a moment load of − 1 G is applied to the hub  2 , as shown in FIG.  9 (B), the center axis of the outer race  1  and the center axis of the hub  2  shift from each other by the angle, θ 2 , in a vertical plane, while at the same time, the outer race  1  and hub  2  shift from each other by an amount δ a2  in the opposite axial direction to the case when a + 1 G moment load was applied. 
     As the center axis of the outer race  1  and the center axis of the hub  2  shift from each other by the angle, θ 2 , the size of the small clearance  127  tends to decrease at the top position of the small clearance  127  by the amount of r·θ 2 , and to increase by the amount of r·θ 2  at the bottom position. 
     On the other hand, as the outer race  1  and hub  2  shift from each other in the axial direction by the amount δ a2 , the size of the small clearance  127  tends to decrease all the way around by the amount δ a2 . 
     If δ a2 ≧r·0 2 , the amount of displacement of the small clearance  127  is, δ a2 −r·0 2  at the top of the small clearance  127 , and δ a2 −1r·0 2  at the bottom. Also, it is δ a2  at the center in the vertical direction. Therefore, when δ a1 ≧r ·θ 1 , while at the same time δ a2 ≧r·θ 2 , then it can be seen that the amount of displacement of the small clearance  127  is smallest at the top position. 
     Next, FIGS.  10 (A) and  10 (B) will be sued to explain the case when δ a1 ≦r ·δ a1 , while at the same time, δ a2 ≦r·0 2 . First, when a moment load of + 1 G is applied to the hub  2 , as shown in FIG.  10 (A), the amount of displacement of the small clearance  127  becomes δ a1 −r·0 1  at the top of this small clearance  127 , and δ a1 +r·θ 1  at the bottom. 
     However, when δ a1 ≦r·0 1 , there are two places in the circumferential direction where the displacement of the small clearance  127  is zero because the displacement that occurs as the center axes of the outer race  1  and hub  2  shift from each other and the displacement, δ a1 , in the axial direction cancel each other. 
     If the distance from the center (where the center of the radius r of the detected section of the encoder  123 =center axes of the outer race  1  and hub  2 ) of this small clearance  127  to this point in the vertical direction is taken to be r 1 , and the angle of intersection, where the line that connects this point with the center crosses the horizontal line, is taken to be φ 1 , 
     then r 1 −·δ a1 /0 1 , and φ 1 −sin −1 (r 1 /r)=sin −1 (δ a1 r·0 2 ). 
     If the detection section of the sensor  4  faces the detected section of the encoder  123  at the point where these conditions are satisfied, then there is no change in the size of the small clearance  127  regardless of whether or not a moment load of + 1 G is applied while the automobile is running. 
     Next, when a moment load of − 1 G is applied to hub  2 , as shown in FIG. 10 (B), the amount of displacement of the small clearance  127  becomes δ a2 −r·θ 2  at the top of the small clearance  127 , and δ a2 +r·0 2  at the bottom. Also, at the point where the distance in the vertical direction from the center of the small clearance  127  is r 2 , the displacement due to the shift of the center axes of the outer race  1  and hub  2  from each other and the displacement, δ a2 , in the axial direction, cancel each other, so that the displacement of the small clearance  127  is zero. When the angle of intersection, where the line that connects the point where the displacement is zero with the center, crosses with the horizontal line is taken to be φ 2 , then φ 2 −sin −1 (r 2 /r)=sin −1(δ   a2 r·θ 2 ). 
     At the point where this condition is satisfied, if the detection section of the sensor  4  faces the detected section of the encoder  123 , then there is no change in the size of the small clearance  127 , regardless of whether or not a moment load of − 1 G is applied when the automobile is running. 
     The moment load that is applied while the automobile is running, is applied in either the positive or negative corresponding to the direction the automobile is moving, so if the installation position of the sensor  4  with respect to the circumferential direction of the outer race  1  and hub  2  is placed between φ 1  and φ 2  {or desirably at a position where the angle (radiam) with respect to the horizontal line is (φ 1 +φ 2 )/2}, then it is possible to improve the reliability of the rotation speed of the wheel detected by the sensor  4 , regardless of what direction the moment load is applied. 
     The reason that an acceleration of  1 G (horizontal acceleration) was used as the condition for restricting the installation position of sensor  4  was that it corresponds to the maximum acceleration that is applied with a normal automobile (passenger automobile). In other words, keeping the displacement of the small clearance  127  to minimum under conditions when the displacement of the small clearance is most severe was considered. 
     FIG. 11 shows a fourth example of the embodiments of the present invention, where the opening at the axially inner end (right end in FIG. 11) of the stationary member or outer race  1  is covered by the cover  16 , which is comprised of a main body  21  in a bottomed cylindrical shape, formed by injection molding of synthetic resin, and a fitting tube  22  connected to the opening portion of the main body  21 . 
     The fitting tube  22  is produced by plastic-working a corrosion-resistant metal plate such as stainless steel, and formed in a generally annular shape with an L-shaped cross section, and comprised of a cylindrical fitting portion  23  and an inwardly facing brim portion  132  bent radially inward from the base end edge (right end edge in FIG. 11) of the cylindrical fitting portion  23 . 
     The fitting tube  22  is connected to the opening portion of the main body  21  by molding the inwardly facing brim portion  132  upon injection molding the main body  21 . The cover  16  constructed as described above closes the opening at the axially inner end of the outer race  1  with the fitting tube  22  the cylindrical fitting portion  23  of which is fitted on and fixed to the axially inner end of the outer race  1 . 
     On the portion of the bottom plate  29  of the main body  21  of the cover  16 , which faces the axially inner side face of the encoder  123  fixedly fitted onto the axially inner end of the inner race  7  of the rotatable member or hub  2 , a cylindrical portion  30  is formed to project in the axially inner direction from the bottom plate portion  29 . Formed on the inside of the cylindrical portion  30  in the axial direction (left and right directions in FIG. 11) of the outer race  1  is an insert hole  31  which communicates the axially inner end surface of the cylindrical portion  30  with the outside surface of the bottom plate portion  29 . 
     Inserted into the insert hole  31  is the tip end portion of the sensor unit  33  which has a sensor embedded in the holder  32  made of synthetic resin. With the sensor unit  33  inserted into the insert hole  31 , the tip end face of the sensor unit  33  faces the axially inner side surface of the encoder  123 , that is the detected portion, with the small clearance  127  axially between them. 
     In order to mount and remove the sensor unit  33  to and from the cover  16  in an easy and instant manner, in this example, a connection spring  36  is provided between the cylindrical portion  30  and the anchoring brim portion  35  at the base end (right end in FIG. 11) of the holder  32 . The connection spring  36  is made of a corrosion-resistant and resilient wire member such as stainless spring steel and formed through a bending process. 
     The anchoring brim portion  35  is retained to the end face of the opening portion of the cylindrical portion  30  by the connection spring  36 . Incidentally, this section is not important for the present invention, and detail is omitted. The mount portion of the sensor unit  33  with respect to the cover  16  is controlled as in the first example previously mentioned. 
     FIG. 12 shows a fifth example of the embodiments in the present invention. In this example, the present invention is applied to the rolling bearing unit with rotation speed sensor for supporting to the suspension the driven wheel (front wheels of the FF vehicle, rear wheels of the FR vehicle and RR vehicle, and the whole wheels of the 4WD vehicle) of the automobile, while in the third and fourth examples, the present invention is applied to the rolling bearing unit with rotation speed sensor for supporting to the suspension the non-driven wheel (rear wheels of the FF vehicle, and front wheels of the FR vehicle and RR vehicle) of the automobile. 
     Accordingly, in the present example, the rotatable member or hub  2  is formed in a cylindrical shape, and female spline section  139  is formed on the inner peripheral surface of the hub  2 . Then, a drive shaft (not shown) with a male spline section on its outer peripheral surface can be inserted into the female spline section  139 . 
     That the encoder  123  is mounted to the combination seal ring  121  to close the space between the inner peripheral surface at the inner end of the stationary member or outer race  1  and the outer peripheral surface at the inner end of the inner race  7  of the hub  2 , that the sensor  4  is fixedly mounted to the knuckle  37  for fixedly supporting the outer race  1 , and the mount position of the sensor  4  is controlled are similar to the first embodiment previously described. The present invention is characterized in that the size of the small clearance between the detected portion of the encoder and the detecting portion of the sensor is prevented from changing as the automobile rapidly turns. The structures of the rolling bearing unit, encoder and sensor can be of any conventional type and not limited to the ones illustrated. For example, the present invention can be applied to the rolling bearing unit with the stationary inner race and rotatable outer race. 
     The rolling-bearing unit with rotation speed sensor of this invention, is constructed and functions as described above, and is effective in stabilizing the sensor output and improving the reliability in detecting the rotation speed of the wheel.