Patent Publication Number: US-6218827-B1

Title: Rolling bearing unit having an improved structure for preventing foreign material from entering the unit

Description:
This is a continuation of U.S. Pat. No. 5,814,984 (U.S. application Ser. No. 08/697,347) filed Aug. 22, 1996, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a rolling bearing unit having a rotating speed detector for rotatably supporting a wheel of an automobile by a suspension, and the rolling bearing unit having a rotating speed detector is used for detecting a rotating speed of the wheel. 
     A rolling bearing unit having a rotating speed detector is used for rotatably supporting a wheel of an automobile by a suspension and also used for controlling an anti-lock brake system (ABB) or a traction control system (TCS) by detecting a rotating speed of the wheel with the detector. Conventionally, various types of rolling bearing units having rotating speed detectors are well known. Each rotating speed detector incorporated into the rolling bearing unit includes a tone wheel, which is an element to be detected, rotated in conjunction with a wheel, and a sensor to output an output signal, the frequency of which changes in proportion to a rotating speed of the tone wheel. For example, a rolling bearing unit having a rotating speed detector shown in FIGS. 19 and 20 is described in the Technical Report No. 94-16051 by HATSUMEI KYOKAI, which is referred to as the first conventional example, hereinafter. 
     A hub  1  includes an inner race assembly. A flange  2  for fixing a wheel is formed on an outer circumferential surface of the outside end portion of the hub  1 . The outside is defined as a side located outside of a vehicle with respect to the width direction of the vehicle when the rolling bearing is assembled into the vehicle. An inner raceway  3   a  and a step portion  4  are formed on an outer circumferential surface of the middle portion of the hub  1 . Further, an inner raceway  3   b  is formed on an outer circumferential surface of the hub  1 . In this way, an inner race  5  composing the inner race assembly in conjunction with the hub  1  is supported while an outer end surface of the inner race  5  is pushed against the step portion  4 . In this connection, instead of directly forming the inner raceway  3   a  on the outer circumferential surface of the hub  1 , another inner race (not shown) different from the hub  1  may be provided, and the inner race and the above inner race  5  may be outwardly engaged with the hub  1  so as to be fixed. 
     A male screw portion  6  is formed in a portion close to the inside end of the hub  1 . The inside is defined as a side located inside of a vehicle with respect to the width direction of the vehicle when the rolling bearing is assembled into the vehicle. When a nut  7  is fastened to the male screw  6 , the inner race  5  is fixed in a predetermined portion on the outer circumferential surface of the hub  1 , so that the inner race assembly can be composed. An attaching portion  9  for fixing the outer race  8  to the suspension is formed on an outer circumferential surface of an outer race  8  arranged around the hub  1 . Outer raceways  10   a ,  10   b  are formed on an inner circumferential surface of the outer race  8 . The outer raceways  10   a ,  10   b  are respectively opposed to the inner raceways  3   a ,  3   b . A plurality of rolling elements  11 ,  11  are installed between the inner raceways  3   a ,  3   b  and the outer raceways  10   a ,  10   b . Due to the above arrangement, the hub  1  is capable of rotating inside the outer race  8 . In this connection, in the example shown in FIGS. 19 and 20, balls are used for the rolling elements. However, in the case of a rolling bearing unit used for an automobile to which a heavy load is given, tapered rollers are used as the rolling elements in some cases. A seal ring  12  is fitted between the inner circumferential surface of the outside end portion of the outer race  8  and the outer circumferential surface of the hub  1 . Since the seal ring  12  is arranged between the inner circumferential surface of the outer ring  8  and the outer circumferential surface of the hub  1 , an outside and opening of the space in which the plurality of rolling elements are arranged is closed by the seal ring  12 . 
     A base end portion of the tone wheel  13 , that is, a left end portion of the tone wheel  13  shown in FIGS. 19,  20  is engaged with and fixed to and inside end portion of the inner race  5  which is a little distant from the inner raceway  3   b . The entire tone wheel  13  is formed from a magnetic metal sheet such as a steel sheet into an annular shape (a short cylindrical shape). The tone wheel  13  is composed of a small diameter portion  14  and a large diameter portion  15  which are continuously connected to each other by a step portion  16 , in which the small diameter portion  14  and the large diameter portion  15  are formed concentrically. The large diameter portion  15  of the tone wheel  13  is outwardly engaged with the outer circumferential surface of the end portion of the inner race  5 , and the step portion  15  comes into contact with an end edge of the inner race  5 . Under the above condition, the tone wheel  13  is supported by and fixed to the inner race  5 . Therefore, the small diameter portion  14  is supported concentrically with the inner race  5 . A plurality of through-holes  17  are formed in the small diameter portion  14  of the tone wheel  13  to serve as cutout portions on the rotational side. These through-holes  17  are formed in the circumferential direction at regular intervals. Accordingly, the magnetic characteristic of the tone wheel  13  is alternately changed in the circumferential direction at regular intervals. Shapes of the through-holes  17  are the same. Each through-hole  17  is formed into a rectangle, the long side of which is disposed in the axial direction, that is, the transverse direction in FIGS. 19 and 20. 
     An inside end opening of the outer race  8  is closed by a cover  18  formed into a cylindrical shape having a bottom from a metal sheet such as a stainless steel sheet or an aluminum alloy sheet by means of drawing. An annular synthetic resin member  21  which embeds an annular sensor  20  is fitted on an inner circumferential side of the cylindrical portion  19  composing the cover  18 . The annular sensor  20  includes: a permanent magnet  22 ; a stator  23  composed of a magnetic member such as a steel sheet; and a coil  24 . When these members  22 ,  23 ,  24  are embedded in the above synthetic resin  21 , they are entirely formed into an annular shape. 
     In the above components of the sensor  20 , the permanent magnet  22  is formed into an annular shape (ring shape) and magnetized in the diametrical direction. An inner circumferential surface of the permanent magnet  22  is opposed to a base end portion of the small diameter portion  14  of the tone wheel  13  while a minute clearance  25  is maintained between them. An outer circumferential surface of the portion of the tone wheel  13  in which the through-hole  17  is not formed is opposed to a base and portion of the small diameter portion  14  of the tone wheel  13  while a minute clearance is maintained between them. The stator  23  is entirely formed into an annular shape, the section of which is a substantial J-shape. An inner circumferential surface of the end portion of the outer diameter side cylindrical portion  26  composing the stator  23  is arranged close to or contacted with an outer circumferential surface of the permanent magnet  22 . An inner circumferential surface of the inner diameter side cylindrical portion  27  composing the stator  23  is opposed to a portion of the tone wheel  13  in which the plurality of through-holes are formed. In the inner diameter side cylindrical portion  27 , a plurality of cutout portions  28 , which are stationary side cutout portions, are formed at regular intervals (central angle pitch), which are the same as those of the through-holes  17 , in the circumferential direction of the inner diameter side cylindrical portion  27 . Accordingly, the inner diameter side cylindrical portion  27  is formed into a comb-shape. 
     The coil  24  is formed into an annular shape in such a manner that a wire is wound around a bobbin made of nonmagnetic material. The coil  24  is arranged in a portion of the inner circumferential side  26  of the outer diameter side cylindrical portion  26  composing the stator  23 . An electro-motive force generated in the coil  24  is taken out from a connector  30 , which is a signal taking means, protruding from an outer surface of the cover  18 . 
     In the rolling bearing unit having a rotating speed detector composed in the above manner, when the tone wheel  13  is rotated together with the hub  1 , the magnetic flux density in the stator  23  opposed to the tone wheel  13  is changed, and the voltage generated in the coil  24  is changed at the frequency proportional to the rotating speed of the hub  1 . The principle in which the voltage generated in the coil  24  is changed in accordance with a change in the magnetic flux flowing in the stator  23  is the same as the principle of the conventionally well known sensor used for detecting a rotating speed. The reason why the density of magnetic flux flowing in the stator  23  is changed in accordance with the rotation of the tone wheel  13  is described as follows. 
     Intervals of the plurality of through-holes  17  provided in the tone wheel  13  are the same as those of the cutout portions  28  provided in the stator  23 . Accordingly, when the tone wheel  13  is rotated, each through hole  17  is opposed to each cutout portion  28  all over the circumference at one moment. At this moment when each through-hole  17  is opposed to each cutout portion  28 , a pillar portion, which is a magnetic body, provided between the through-holes  17  adjacent to each other is opposed to a tongue member, which is a magnetic body, provided between the cutout portions  28  adjacent to each other. Under the condition that each pillar portion, which is a magnetic body, is opposed to each tongue member, which is also a magnetic body, a magnetic flux of high density flows between the tone wheel  13  and the stator  23 . 
     On the other hand, when the phase of the through-hole  17  deviates from the phase of the cutout portion  28  by half the interval, the density of magnetic flux flowing between the tone wheel  13  and the stator  23  is lowered. In the above condition, the through-hole  17  provided in the tone wheel  13  is opposed to the tongue member, and at the same time the cutout portion  28  provided in the stator  23  is opposed to the pillar portion. A relatively large space between the tone wheel  13  and the stator  23  is formed all over the circumference under the above condition in which the pillar portion is opposed to the cutout portion  28  and the tongue member is opposed to the through-hole  17 . Under this condition, the density of magnetic flux flowing between both members  13  and  23  is lowered. As a result, a voltage generated in the coil  24  is changed in proportion to the rotating speed of the hub  1 . Due to the above action, the sensor  20  changes an output voltage generated in the coil  24  at the frequency proportional to the rotating speed of the hub  1 . 
     However, in the rolling bearing unit having a rotating speed detector according to the first conventional example, the following problems may be encountered. In the first conventional example shown in FIGS. 19 and 20, the synthetic resin  21  in which the sensor  20  is embedded and the synthetic resin composing the connector  30  are disposed on both sides of the cover  18  made of a metallic sheet. Accordingly, the manufacturing work is complicated, and the cost is raised. That is, the rolling bearing unit having a rotating speed detector shown in FIGS. 19,  20  is manufactured as follows. First, the sensor  20  is embedded in the synthetic resin  21 , and then the synthetic resin  21  is provided inside the cover  18 . After that, under the condition that the cover  18  is set in the mold, injection molding is conducted to form the connector  30 . Therefore, it is necessary to conduct the injection molding process twice. Alternatively, it is necessary to provide an adhesion process in which the synthetic resin  21  and the connector  30 , which have been made by means of injection molding, are made to adhere to each other. As a result, the manufacturing cost is raised. 
     FIG. 21 is a view showing a rolling bearing unit having a rotating speed detector according to the second conventional example disclosed in Unexamined Japanese Patent Publication No. 63-166601. 
     The rotating wheel is composed of a hub  1  and an inner race  2 . A flange  3  for fixing a wheel is formed in the outside end portion of the hub  1 . An inner raceway  4   a  is formed in an outer circumferential surface of the middle portion of the hub  1 . An inner raceway  4   b  is formed on an outer circumferential surface of the inner race  2 . The inner race  2  is outwardly engaged with an outer circumferential surface of the middle portion of the hub  1 . A male screw portion  5  is formed on an outer circumferential surface of the inside end portion of the hub  1 . The male screw portion  5  is screwed to a nut  6 . When the nut  6  is screwed to the male screw portion  5 , an inside end surface of the inner race  2  is pushed, so that the inner race  2  can be fixed at a predetermined position on the outer circumferential surface of the hub  1 . 
     A flange-shaped attaching portion  8  for supporting a suspension (not shown) is formed on an outer circumferential surface of the outer race  7  which is a stationary wheel. Two rows of outer raceways  9   a ,  9   b  are formed on an inner circumferential surface of the outer race  7 . A plurality of rolling elements  10 ,  10  are formed between the outer raceways  9   a ,  9   b  and the inner raceways  4   a ,  4   b . In this arrangement, the hub  1  is rotatably supported inside the outer race  7  supported by the suspension via the attaching portion  8 . 
     The nut  6  includes a disk-shaped rotor  11 . An irregular portion  12  is formed on the inside surface of the rotor  11 . By the action of the irregular portion  12 , the nut  6  generates a pulse-like output in a sensor  14  described later in accordance with the rotation of the hub  1 . That is, the nut  6  functions as a tone wheel. A cover  13  is engaged with the inside opening of the outer race  7 . The sensor is fixed to the cover  13 , and an outside end surface of the sensor  14  is opposed to the irregular portion  12 . 
     In the rolling bearing unit having a rotating speed detector described above, a wheel fixed to the flange portion  3  arranged in the outside end portion of the hub  1  can be rotatably supported by the suspension which holds the outer race  7 . When the rotor  11  integrated with the nut  6  screwed to the inside end portion of the hub  1  is rotated in accordance with the rotation of the wheel, an output of the sensor  14  opposed to the irregular portion on the inside surface of the rotor  11  is changed. The frequency of a change in the output of the sensor  14  is proportional to the rotating speed of the wheel. Accordingly, when the output signal of the sensor  14  is inputted into a controller not shown in the drawing, it is possible to fince the rotating speed of the wheel. Therefore, ABS and TCS can be appropriately controlled. 
     In the conventional rolling bearing unit having a rotating speed detector composed as described above, an outside opening of the cover  13  which holds the sensor  14  with respect to the outer race  7  is tightly fitted into the inside opening of the outer race  7  in order to prevent the cover  13  from being carelessly disconnected from the outer race  7  by the vibration caused when the automobile is running. 
     However, when the cover  13  is tightly fitted into the outer race  7  as described above, it becomes difficult to separate both members  7 ,  13  from each other. When both members  7 ,  13  are forcibly separated from each other with a tool such as a driver, a strong force is given to the cover  13 , and the cover  13  is deformed. As a result, it is impossible to use the cover  13  again. For this reason, when either the rolling bearing portion or the sensor  14  is out of order after the rolling bearing unit has been incorporated into an automobile and used over a long period of time, the entire rolling bearing unit having a rotating speed detector must be also replaced, that is, a portion which is in good order must be replaced. This causes a raise in the maintenance cost. From the viewpoint of economizing resources, such a case is not preferable. 
     In view of the above circumstances, in order to detach the cover  13  from the outer race  7  easily, Unexamined Japanese Utility Model Publication No. 5-14634 discloses a rolling bearing unit having a rotating speed detector as shown in FIG. 22, which is referred to the third conventional example. In the rolling bearing unit having a rotating speed detector, a flange portion  3  for fixing a wheel is formed on an outer circumferential surface of the outside end portion of the hub  1  composing a rotating wheel in conjunction with the inner race  2 . An inner raceway  4   a  and a step portion  15  are formed on an outer circumferential surface of the middle portion of the hub  1 . In the inner race  2  having the inner raceway  4   b  which is formed on the outer circumferential surface of the hub  1 , an outside end surface of the inner race  2  is confronted with the step portion  15  so that the inner race  2  is outwardly supported to the outer circumferential surface of the hub  1 . 
     A male screw portion  5  is formed on an outer circumferential surface of the inside end portion of the hub  1 . When a nut  6  is fastened to the male screw portion  6 , the inner race  2  is fixed in a predetermined portion on the outer circumferential surface of the hub  1 . An irregular portion  16  is formed on an outer circumferential surface of the nut  6 . Therefore, the nut  6  functions as a tone wheel for detecting a rotating speed of the nut  6 . An attaching portion  8  for fixing the outer race  7  to the suspension is formed on an outer circumferential surface of the outer race  7 . A pair of outer raceways  9   a ,  9   b  are formed on an inner circumferential surface of the outer race  7  and are respectively opposed to the inner raceways  4   a ,  4   b . A plurality of rolling elements  10 ,  10  are installed between a pair of inner raceways  4   a ,  4   b  and a pair of outer raceways  9   a ,  9   b  so that the hub  1  can be freely rotated inside the outer race  7 . A seal ring  17  is fitted between the inner circumferential surface on the outside end portion of the outer race  7  and an outer race circumferential surface of the hub  1 . Further, a seal ring  17  is fitted between the inner circumferential surface on the inside end portion of the outer race  7  and an outer circumferential surface of the inner race  2 . That is, the seal ring  17  exists between the inner circumferential surface of the outer race  7  and the outer circumferential surface of the hub  1 , and also the eal ring  17  exists between the inner circumferential surface of the outer race  7  and an outer circumferential surface of the inner race  2 . The seal rings  17  close openings on both side end portions of the space in which the plurality of rolling elements are accommodated. 
     A portion of the opening of the inside end portion (the right end portion in FIG. 22) of the outer race  7  is closed by a cover  18 . The entire cover  18  is formed into an annular shape by press-forming a metallic sheet. The shape of the cover  18  is described as follows. In order to insert a portion of a constant velocity joint inside the cover  18  in the diametric direction, an outer periphery of the base plate portion  19  formed into an annular shape is bent at a right angle to the outside (to the left in FIG.  22 ), so that a cylindrical vertical wall  20  can be formed. Further, an annular step portion  21  is formed on an outer circumferential periphery of the opening of the outside end (the left end in FIG. 22) of the vertical wall  20  so that the annular step portion  21  is capable of being freely confronted with the inside end surface  7   a  of the outer race  7 . 
     When an outer circumferential edge of the step portion  21  is bent outwardly at a right angle, an engaging cylindrical portion  22  capable of engaging with the inside end portion of the outer race  7  is formed. When an outside end opening of the engaging cylindrical portion  22  is bent outwardly in the diametric direction by an angle of 180, an engaging portion  23 , the size in the diametric direction of which is large, is formed. The cover  18  is fixed to the outer race  7  in such a manner that the engaging cylindrical portion  22  is outwardly engaged with the inside end portion of the outer race  7 , and at the same time the step portion  21  is confronted with the inside end surface  7   a  of the outer race  7 . The engaging strength of the engaging cylindrical portion  22  and the inside end portion of the outer race  7  is determined to be a sufficiently high value so that the outer race  7  and the cover  18  can not be disconnected from each other by the vibration caused when the automobile is running. For example, an electromagnetic sensor  14  is fitted inside the cover  18  and an output of the sensor  14  is sent to a controller (not shown) via a lead wire  24 . 
     Substantially in the same manner as that of the rolling bearing unit having a rotating speed detector shown in FIG. 21, by the above rolling bearing unit having a rotating speed detector, the wheel is rotatably held with respect to the suspension, and the rotating speed of the wheel fixed to the flange portion  3  of the hub  1  can be detected. Especially, when it becomes necessary to disconnect the cover  18  from the outer race  7  for the purpose of maintenance in the arrangement shown in FIG. 22, as shown by a chain line in FIG. 22, an inner circumferential edge of a tool  25  such as a hand press or a pulley extractor having an engaging portion of split structure is engaged with an edge of the engaging portion  23  formed in the outer circumferential edge of the outside end opening of the cover  18 . When the tool  25  is displaced in a direction in which the tool  25  is separated from the attaching portion  8 , that is, when the tool  25  is displaced to the right in FIG. 22, the cover  18  is separated from the outer race  7 , so that the cover  18  is disconnected from the outer race  7 . 
     In the conventional structure shown in FIG. 22, the cover  18  is formed from a metallic sheet by means of plastic working. On the other hand, when the cover is made of synthetic resin and the sensor is embedded in a portion of the cover in the process of injection molding, it is possible to reduce the manufacturing cost of the rolling bearing unit having a rotating speed detector. This structure is conventionally known, in which the sensor is embedded in a portion of the cover made of synthetic resin. For example, European Patent Publication No. EP0557931-A1 discloses the above arrangement. This arrangement shown in FIG. 23, which is defined as the fourth conventional example, is described as follows. A sensor  20   a  is embedded in the cover  18   a  made of synthetic resin which closes an inside end opening of the outer race  8 . Onto an outer circumferential surface of the opening end portion of the cover  18   a , a sleeve  31  is fixed, which is formed from a metallic sheet such as a steel sheet having a sufficiently high rigidity, and the section of the sleeve  31  is an L-shape and the entire shape of the sleeve  31  is formed into an annular shape. 
     When the sleeve  31  is set in a cavity of the mold in the process of injection molding of the cover  18   a , it can be embedded in the above synthetic resin. The above cover  18   a  is fixed to the outer race  8  when the above sleeve  31  is inwardly engaged with an inside end opening of the outer race  8 . Compared with the arrangement shown in FIGS. 19 and 20 into which the cover formed from a metallic sheet is incorporated, this arrangement into which the above cover  18   a  made of synthetic resin is incorporated is advantageous in that the number of components can be reduced so that the combining work can be simplified and the manufacturing cost of the bearing unit can be lowered, and further the cover made of synthetic resin can be made lighter than the cover made of metal. 
     However, even in the rolling bearing unit having a rotating speed detector shown in FIG. 23, the following problems to be solved may be encountered. When an automobile is running in a rainy day, muddy water is splashed on the cover  18   a  and the outer race  8 , and further in the case of washing an automobile, water is sprayed on the cover  18   a  and the outer race  8  with high pressure by a washing machine. In this case, both muddy water and washing water are referred to as muddy water hereinafter. When the muddy water enters the cover  18   a  and the outer race  8 , not only the durability of the bearing unit is deteriorated but also the reliability of the rotating speed detector is affected. 
     For example, in the conventional arrangement shown in FIG. 23, muddy water enters the cover  18   a  and the outer race  8  through the following two passages denoted by (1) and (2). 
     (1) A first minute clearance formed between an outer circumferential surface of the sleeve  31  and an inner circumferential surface of the inside end portion of the outer race  8 . 
     (2) A second minute clearance formed between an inner circumferential surface of the sleeve  31  and an outer circumferential surface of the cover  18   a  made of synthetic resin. 
     The first minute clearance is made by minute irregularities which inevitably exist on the outer circumferential surface of the sleeve  31 . That is, it is inevitable that minute irregularities of several tens μm exist on the surface of the sleeve  31  formed from a metallic sheet such as a stainless steel sheet. For this reason, even when the sleeve  31  is press-fitted into the inside end opening of the outer race  8 , the above first minute clearance is made, and there is a possibility that muddy water enters the bearing unit through the first minute clearance. 
     The above second minute clearance is made by a difference between the thermal expansion coefficient of a metal composing the sleeve  31  and the thermal expansion coefficient of synthetic resin composing the cover  18   a  . There is a possibility that muddy water enters the bearing unit through the second minute clearance described above. 
     In the case of the arrangement of the fourth conventional example shown in FIG. 23, it is difficult to assemble a bearing unit without using an exclusive press. Therefore, in a garage having no exclusive press, it is impossible to replace the sensor  20   a  attached to the cover  18   a . As a result, when the rolling bearing unit having a rotating speed detector is inspected and repaired, the cost is raised. When the cover  18   a  is pulled out from the outer race  8 , due to a high frictional force acting between the outer circumferential surface of the end portion of the outer race  8 , an end edge (a left end edge in FIG. 23) of the sleeve  31  strongly pushes an outer circumferential portion  52  close to the fore end portion of the synthetic resin composing the cover  18   a . As a result, a base of the portion  52  made of synthetic resin is given a shearing force, and there is a high possibility that the portion  52  is damaged. When the portion  52  is damaged, the cover  18   a  and the sensor  20   a  embedded in the cover  18   a  can not be used again, which raises the cost of inspection and maintenance. 
     In order to prevent the shearing force from being given to the outer circumferential portion  52  close to the fore and portion when the cover  18   a  is pulled out from the outer race  8 , it can be considered to increase an outer diameter of the flange portion  53  formed in the base portion (the right end portion of FIG. 23) of the sleeve  31 . When the outer diameter of the flange portion  53  is increased and an outer circumferential portion of the flange portion  53  is protruded from the outer circumferential surface of the cover  18   a  so that a tool can be hooked at this protruding portion, it is possible to prevent the occurrence of a shearing force which may cause a damage of the portion  52 . However, when the outer diameter of the flange portion  53  is simply increased, muddy water sprayed onto the outer side of the flange portion  53  in a rainy day or in the process of washing an automobile tends to enter the inside of the outer race  8  through a contact surface of the sleeve  31  with the outer race  8 . Therefore, it is not appropriate to adopt the above countermeasure. 
     In the rolling bearing unit having a rotating speed detector shown in FIG. 23 into which the cover  18   a  made of synthetic resin is incorporated, it is possible to reduce the initial cost. On the other hand, it is difficult to remove the cover  18   a  from the outer race  8  in the arrangement of maintenance and replacement of parts. When the cover  18   a  is forcibly removed from the outer race  8  in the arrangement shown in FIG. 23, there is a possibility that the end portion  51  of the cover  18   a  is damaged due to a frictional force that acts between the outer circumferential surface of the sleeve  31  and the inner circumferential surface of the outer ring  8 . When a portion of the sleeve  31  and a portion of the cover  18   a  are outwardly protruded from the outer circumferential surface of the end portion of the outer race  8  in the diametric direction, it is possible to remove the cover  18   a  from the outer race  8  without damaging the cover  18   a . However, when the above means is adopted for removing the cover  18   a , the outer diameter of the cover  18   a  is increased, so that the cover  18   a  tends to interfere with other parts. Accordingly, the degree of freedom is deteriorated in the design of an automobile. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide a rolling bearing unit having a rotating speed detector in which the rolling bearing unit includes a cover made of synthetic resin and a single seal ring which is attached to the cover and which is capable of preventing the entrance of muddy water into the cover and the outer race through the first and the second clearance. 
     A second object of the present invention is to provide a rolling bearing unit having a rotating speed detector in which the rolling bearing unit includes a cover having a main body made of synthetic resin and an engaging portion which is made of metal and engaged with an outer race so that the cover is capable of ensuring a sufficiently high sealing property. 
     A third object of the present invention is to provide a rolling bearing unit having a rotating speed detector in which the rolling bearing unit includes a cover that can be easily detached from a stationary race such as an outer race without particularly increasing the outer diameter of the cover made of synthetic resin. 
     The rolling bearing unit having a rotating speed detector, according to the first object of the present invention, includes: an outer race on the inner circumferential surface of which an outer raceway is provided in the same manner as the conventional rolling bearing unit having a rotating speed detector, the outer race being not rotated when the rolling bearing unit is used; an inner race, the outer circumferential surface of which is opposed to the above inner circumferential surface of the outer race, so that an inner raceway is formed on the outer circumferential surface of the inner race; a plurality of rolling elements arranged between the outer raceway and the inner raceway so that the rolling elements can be freely rolled; a tone wheel fixed to the inner race, the characteristic in the circumferential direction of which is alternately changed at regular intervals; a cover made of synthetic resin fixed to in inside end opening of the outer race; a sensor opposed to the tone wheel while the sensor is embedded in synthetic resin composing the cover; and a cylindrical sleeve made of metal fixed onto a circumferential surface of the opening of the cover, the cover being fixed to the outer race when one circumferential surface of the sleeve is engaged with a circumferential surface of the end portion of the outer race. 
     A minute clearance formed on an engaging surface between the sleeve and the outer race is defined as a first minute clearance. A minute clearance formed in a contact portion of the sleeve with the synthetic resin composing the cover is defined as a second minute clearance. A minute clearance formed between the outer race and the cover is defined as a third minute clearance. In this case, the first minute clearance and the third minute clearance, which are arranged in series to each other, make a first leaking passage, and the second minute clearance and the third minute clearance, which are arranged in series to each other, make a second leaking passage. 
     In the rolling bearing unit having a rotating speed detector according to the first object of the present invention, there is provided one piece of seal ring between the cover and the outer race at a position located in series to both the first and the second leaking passage. In order to arrange one piece of seal ring such as an O-ring, an annular flat packing and a ring-shaped seal member at a position located in series to both the first and the second leaking passage, for example, an end surface of the cover is not covered with the sleeve, and synthetic resin is exposed onto the end surface, and the seal ring is arranged between the end surface and the end surface of the outer race. 
     The rolling bearing unit having a rotating speed detector, according to the first object of the present invention, includes: an element to be detected, concentrically fixed to one portion of the inner race, the element having the characteristic which is alternately changed at regular intervals in the circumferential direction; a cover engaged with and fixed to an opening end portion of the outer race; a sensor held by a portion of the cover and opposed to the element to be detected so as to detect a rotating speed of the element; and a signal taking means for taking out a detection signal from the sensor. The above cover includes: a main body made of synthetic resin in which the sensor is embedded, the main body covering an opening end portion of the outer race; and a sleeve formed from a metallic sheet into an annular shape, the section of which is an L-shape, the sleeve being embedded in a portion of the main body close to the outer circumference. The sleeve includes: a cylindrical portion, the outer diameter of which is determined so that the sleeve can be freely fixed to an opening end portion of the outer race; and a flange portion bent at an end edge of the cylindrical portion in the diameter direction. The flange portion is combined with the main body under the condition that the entire flange portion and a portion of the cylindrical portion close to the flange portion are embedded in a portion of the main body close to the outer circumference. 
     A portion of the cylindrical portion close to its fore end is inwardly engaged with the opening end portion of the outer race, and under the condition that one side of the main body is confronted with an end surface of the outer race, a groove or a plurality of grooves capable of engaging with a tool are formed between one side of the main body and an end surface of the outer race. An outer circumferential edge portion of the flange portion is located at a position outside the bottom surface of the groove or the plurality of grooves in the diametric direction. A seal ring made of elastic material is attached to a portion where one side of the main body and the end surface of the outer ring are confronted with each other, so that the confronting portion can be tightly sealed. 
     When the rolling bearing unit having a rotating speed detector according to the present invention composed in the above manner holds a suspension of a wheel, or when the rolling bearing unit having a rotating speed detector detects a rotating speed of a wheel, the operation is the same as that of the conventional example described before. According to the rolling bearing unit having a rotating speed detector, the entrance of foreign objects such as muddy water into the cover can be prevented by one piece of seal ring. Therefore, it is possible to realize a highly reliable arrangement of the rolling bearing unit at a low manufacturing cost. 
     According to the present invention, the cover can be attached to and detached from the unit without damaging a main body made of synthetic resin composing the cover. Further, it is possible to positively prevent muddy water from entering the inside of the outer race. 
     In the rolling bearing unit having a rotating speed detector according to the second object of the present invention, the cover includes a main body made of synthetic resin, and a sleeve made of a metallic sheet. The sensor is embedded in the main body. The main body is composed in such a manner that a cylindrical portion capable of being freely confronted with an end surface of the outer race is formed on one side of a sheet portion, the size of which is sufficiently large to cover an opening end portion of the outer race. Concerning the sleeve, a section of the sleeve is formed into a crank-shape in which a small diameter portion and a large diameter portion are continuously connected with each other by a step portion, and the entire sleeve is formed into an annular shape. The small diameter portion of the sleeve, which is a base portion, is embedded in the cylindrical portion. A seal ring is fitted inside the annular groove surrounded by the three surfaces of the inner diameter surface close to the base end of the large diameter portion, the outer circumferential surface close to the fore end of the cylindrical portion, and one side of the step portion. Under the condition that the fore end portion of the large diameter portion is outwardly engaged with the opening end portion of the outer race, the seal ring is elastically interposed between the opening end surface of the outer race and one surface of the step portion. In this way, a confronting portion of the opening end surface of the outer race and the fore end surface of the main cylindrical portion composing the cover is tightly sealed. 
     The rolling bearing unit having a rotating speed detector, according to the third object of the present invention, includes: a stationary race, on the stationary side circumferential surface of which a stationary side raceway is provided, the stationary race being not rotated when the rolling bearing unit is used; a rotating race, the rotating side circumferential surface of which is opposed to the above stationary side circumferential surface of the stationary race, so that a rotating side raceway is formed on the outer circumferential surface of the rotating side race; a plurality of rolling elements arranged between the stationary side raceway and the rotating side raceway so that the rolling elements can be freely rolled; a tone wheel fixed to the rotating race, the characteristic in the circumferential direction of which is alternately changed at regular intervals; a closing plate to close at least a portion of the end opening of the stationary race; a cover made of synthetic resin provided on one side of the closing plate, the cover having an engaging cylindrical portion; and a sensor opposed to the tone wheel under the condition that the sensor is embedded in a portion of the cover. The cover is fixed to the stationary race when the engaging cylindrical portion is engaged with the circumferential surface of the end portion of the stationary race. 
     In the rolling bearing unit having a rotating speed detector according to the third object of the present invention, in the flange portion protruding in the diametric direction from the circumferential surface of the engaging cylindrical portion in the circumferential end portion of the cover, a plurality of confronting portions and cutout portions are alternately formed in the circumferential direction. When the confronting portion confronts an end surface of the stationary race in the axial direction, the cover can be positioned with respect to the stationary race. The cutout portions can provide a clearance between the end surface of the stationary race and the engaging cylindrical portion, into which a tool is inserted to pull out the engaging cylindrical portion from the stationary race. 
     When the rolling bearing unit having a rotating speed detector according to the present invention composed in the above manner holds a suspension of a wheel, or when the rolling bearing unit having a rotating speed detector detects a rotating speed of a wheel, the operation is the same as that of the conventional example described before. 
     According to the rolling bearing unit having a rotating speed detector in the present invention, it is possible to reduce the manufacturing cost when the cover is made of synthetic resin. Further, even when the outer diameter of the cover is not extended, the cover can be easily detached from the stationary race. Furthermore, the cover and the sensor embedded in the cover can be positively positioned with respect to the stationary race. 
     When the cover is detached from the stationary race, the engaging cylindrical portion of the cover is pulled out from the stationary race under the condition that an end portion of the tool is engaged with the plurality of cutout portions. On the other hand, when the confronting portion arranged at a position away from each cutout portion in the circumferential direction confronts an end surface of the stationary race, the cover and the sensor embedded in the cover can be positioned with respect to the stationary race. 
     Since the cover is positioned by each confronting portion and the cutout portions for engaging with the end portion of the tool are intermittently formed in the circumferential direction, the size of the cutout portion in the radial direction can be extended without extending the outer diameter of the cover. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing the first embodiment of the present invention; 
     FIG. 2 is an enlarged view of the right portion of FIG. 1; 
     FIG. 3 is an enlarged view of the portion III of FIG. 2; 
     FIG. 4 is an enlarged view of the right portion of the second embodiment of the present; 
     FIG. 5 is an enlarged view of the portion V of FIG. 4; 
     FIG. 6 is a cross-sectional view of the third embodiment of the present invention; 
     FIG. 7 is a cross-sectional view of the fourth embodiment of the present invention; 
     FIG. 8 is a cross-sectional view of the fifth embodiment of the present invention; 
     FIG. 9 is a cross-sectional view of the sixth embodiment of the present invention; 
     FIG. 10 is an enlarged cross-sectional view corresponding to the portion X of FIG. 9, for explaining a passage of intruding muddy water. 
     FIG. 11 is a view taken in the direction of arrow XI in FIG. 9; 
     FIG. 12 is a view showing an unpreferable example in the same manner as FIG. 10; 
     FIG. 13 is a cross-sectional view showing the seventh embodiment of the present invention; 
     FIG. 14 is an enlarged view of the right end portion of FIG. 13; 
     FIG. 15 is an enlarged view of the upper portion of FIG. 14; 
     FIG. 16 is a view showing an unpreferably example in the same manner as FIG. 15; 
     FIG. 17 is a cross-sectional view showing the eighth embodiment of the present invention; 
     FIG. 18 is a perspective view of the cover used in the eighth embodiment; 
     FIG. 19 is a cross-sectional view showing the first conventional example; 
     FIG. 20 is an enlarged view of the right portion of FIG. 19; 
     FIG. 21 is a cross-sectional view showing the second conventional example; 
     FIG. 22 is a cross-sectional view showing the third conventional example; and 
     FIG. 23 is a partially cross-sectional view showing the fourth conventional example. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIGS. 1 and 2 are views showing the first embodiment of the present invention. The characteristic of the rolling bearing unit having a rotating speed detector according to the present invention is a sealing structure of the engaging portion in which the cover  18   b  supporting the sensor  20   b  composing the rotating detector is engaged with the outer race  8  composing the rolling bearing unit. The structure of the rolling bearing unit is the same as that of the first conventional example shown in FIG.  19 . Therefore, concerning the rolling bearing unit, like reference characters are used to indicate like parts in the drawings, and overlapping explanations are omitted or simplified here. The characteristic of the present invention will be mainly explained below. 
     A cover  18   b  made of synthetic resin is attached to an inside end opening of the outer race  8 , so that the inside end opening of the outer race  8  is closed by the cover  18   b . An inside end portion of the sensor  20   b  is embedded in an outside surface of the cover  18   b . An outside half portion of the sensor  20   b , which is formed into an annular shape, protrudes outside from the outside surface of the cover  18   b . A connector  30  to take out a detection signal from the sensor  20   b  is integrally formed on an inside surface of the cover  18   b . A protrusion  32  is formed on the outside surface of the cover  18   b  between the outer circumferential edge the center of the cover  18   b  in the diametric direction. The protrusion  32  is formed to be annular. The inside end portion of the sensor  20   b  is embedded in the central portion of the protrusion  32  in the diametric direction. 
     A sleeve  31  is formed on an outer circumferential surface of the protrusion  32 . The sleeve  31  is made from a metallic sheet such as a stainless steel sheet into a annular shape, the section of which is an L-shape. The sleeve  31  includes: a cylindrical portion  33 ; and a flange portion  34  which is bent at an inside edge of the cylindrical portion  33  outwardly in the diametric direction. The cylindrical portion  33  is provided along the outer circumferential surface of the protrusion  32  and protrudes outside from a fore end edge (outside edge) of the protrusion  32 . The flange portion  34  is embedded in an outer circumferential edge portion of the cover  18   b , that is the flange portion  34  is embedded in a central portion with respect to the thickness direction of the cover  18   b , and the portion is located outwardly in the diametric direction of the protrusion  32 . An outer circumferential edge of the flange portion  34  does not extend to an outer circumferential edge of the cover  18   b , so that the outer circumferential edge of the flange portion  34  is embedded in the cover  18   b.    
     The cover  18   b  provided with the sleeve  31  described above is engaged with and fixed to an inner circumferential edge of the inside end opening of the outer race  8 . A chamfered portion is formed in the inner circumferential edge of the inside end opening of the outer race  8 . When the inside and opening of the outer race  8  is closed by the cover  18   b , the sleeve  31  is inwardly engaged with the inside end portion of the outer race  8 , and a flange portion  44  which is formed on the outer cylindrical surface of the cover  18   b  is contacted with the outer race  8 . An annular space  36  having a triangular section is formed under the above condition. The annular space  36  is surrounded by the three surfaces of the outside surface of the cover  18   b  contacted with the outer race  8  at the portion  44 , the inner circumferential surface of the middle portion of the cylindrical portion  33 , and the chamfered portion  35 . In the rolling bearing unit having a rotating speed detector according to the present invention, an O-ring  37  serving as a seal ring is fitted in the space  36  under the condition that the O-ring  37  is elastically compressed. 
     Since the O-ring  37  is provided in the space  36  formed in the above manner under the condition that the O-ring  37  is elastically compressed, the intrusion of muddy water into the cover  18   b  and the outer race  8  can be positively prevented. That is, as shown by arrows A of solid lines in FIG. 3, muddy water existing around the cover  18   b  and the outer race  8  enters a portion of the space  36  close to the outer circumference through a third minute clearance which is formed between the inside end surface of the outer race  8  and the outside surface of the cover  18   b . However, after muddy water has entered the space  36  on the outer circumferential side in this way, it is stopped by the O-ring  37 . Therefore, muddy water does not enter the space  36  on the inner circumferential side. In this case, a first minute clearance is formed on an engaging surface between the inner circumferential surface of the sleeve  31  and the inside outer circumferential surface of the outer race  8 , and a second minute clearance is formed in a contact portion between the sleeve  31  and the synthetic resin portion composing the cover  18   b . Both one end of the first minute clearance and one end of the second minute clearance communicate with a portion of the space  36  close to the inner circumference. Accordingly, muddy water that has intruded from the third minute clearance into a portion in the space  36  close to the outer circumference does not intrude into the first minute clearance and the second one. 
     Therefore, the O-ring  37  is arranged in series to the first leakage passage (from arrow A to arrow B in FIG. 3) composed of the third minute clearance and the first one. Also, the O-ring  37  is arranged in series to the second leakage passage (from arrow A to arrow C) composed of the third minute clearance and the second one. Thus, the intrusion of muddy water into the outer race  8  and the cover  18   b  can be prevented by one piece of O-ring  37 . That is, no muddy water flows into the first leakage passage and the second one as shown by arrows A, B and C in FIG.  3 . 
     In the first embodiment, when a relative velocity between the opposed surfaces of the sensor  20   b  and the tone wheel  13   a  is increased and at the same time a magnetic resistance between both members  20   b ,  13   a  is simultaneously changed at two positions, a change in the output of the sensor  20   b  is greatly increased. The sensor  20   b  includes an annular permanent magnet  22   a  magnetized in the axial direction (the transverse direction in FIGS.  1  and  2 ). An outside end surface (the left end surface in FIGS. 1 and 2) of the permanent magnet  22   a  is contacted with a base end portion (inside diametric end portion) of the stator  38 , and an outer circumferential surface of a fore end portion (outside diametral end portion) of the stator  38  is opposed to an inner circumferential surface of the middle portion of the large diameter portion  15   a  composing the tone wheel  13   a  while a minute clearance is formed between the two surfaces. An inside end surface (the right end side in FIGS. 1 and 2) of the permanent magnet  22   a  in the axial direction is contacted with a base end portion (inside diametral end portion) of the second stator  39 . An outer circumferential surface of a fore end portion (outside diametral end portion) of the second stator  39  is opposed to an inner circumferential surface of the inside end portion of the large diameter portion  15   a  in the axial direction while a minute clearance is formed between the two surfaces. 
     Cutout portions  40 ,  40  are formed in the inner half portion of the large diameter portion  15   a  composing the tone wheel  13   a , and cutout portions  28 ,  28  are respectively formed in the fore end portions of the first stator  38  and the second one  39 . Due to the foregoing, each portion is formed into a comb-tooth-shape. Of course, the pitches (the central angle pitches) of the cutout portions  40 ,  28  are the same. The phases of the cutout portions  28 ,  28  composed in the first stator  38  and the second one  39  are the same. Therefore, the through-holes  41 ,  41  formed in the first stator  38  and the second one  39  are engaged with the protrusions  42 ,  42  formed on the bobbin  29   a  around which a lead wire to compose the coil  24   a  is wound. The coil  24   a  is arranged in a portion surrounded by the permanent magnet  22   a , the first stator  38  and the second stator  39 . By a change in the density of magnetic flux flowing in these members  22   a ,  38 ,  39 , a voltage is generated which changes at a frequency proportional to the rotating speed of the tone wheel  13   a.    
     Due to the above arrangement, in accordance with the rotation of the tone wheel  13   a , a resistance to the flow of magnetic flow is changed not only in a portion where the fore end portion of the first stator  38  is opposed to the large diameter portion  15   a  but also in a portion where the fore end portion of the second stator  39  is opposed to the large diameter portion  15   a . Accordingly, a change in the magnetic flux density caused by the rotation of the tone wheel  13   a  is increased, and a high output can be provided by the sensor  20   b . In the first embodiment, the sensor  20   b  is arranged on the inner diameter side of the large diameter portion  15   a  composing the tone wheel  13   a , and an inner circumferential surface of the large diameter portion  15   a  is opposed to an outer circumferential surface of the sensor  20   b . Therefore, the relative velocity between both circumferential surfaces can be increased more than the relative velocity of the conventional structure described before. When the relative velocity between both circumferential surfaces is increased, the output of the sensor  20   b  can be increased. In this connection, the structure of the rotating speed detector described above is not the main point of the present invention. Concerning the structure of the rotating speed detector, various type of structure including the conventional structure shown in FIGS. 19 and 20 may be adopted when the present invention is executed. 
     Next, FIGS. 4 and 5 are views showing an arrangement of the second embodiment of the present invention. In the second embodiment, the flange portion  34  of the sleeve  31  is embedded in a portion on the outside surface of the cover  18   b  in such a manner that the flange portion  34  is arranged on the same surface as the outside surface of the cover  18   b . All over the circumference, an engaging groove  43  is formed in the outside surface of the cover  18   b  which is closer to the outer circumferential edge of the cover  18   b  than the flange portion  34 . An O-ring  37  is fitted in the engaging groove  43 . The O-ring  37  is elastically contacted with the inside end surface of the outer race  8 . Also, in the second embodiment, as shown by arrow A of a solid line in FIG. 5, muddy water that has intruded into the third minute clearance is stopped by the O-ring  37 . Accordingly, it is possible to prevent the muddy water from flowing into the first minute clearance and the second minute clearance as shown by arrows B and C of broken lines in FIG.  5 . 
     FIG. 6 is an arrangement view of the third embodiment of the present invention. The characteristic of the rolling bearing unit having a rotating speed detector according to the third embodiment is a structure in which the cover  18   b  supporting the sensor  20   b  composing the rotating detector is fixed to the outer race  8  composing the rolling bearing unit. The structure of the rolling bearing unit is the same as that of the first conventional example shown in FIG.  19 . Therefore, concerning the rolling bearing unit, like reference characters are used to indicate like parts in the drawings, and overlapping explanations are omitted or simplified here. The characteristic of the present invention will be mainly explained below. 
     When a cover  18   b  composed of a main body  54  made of synthetic resin and a sleeve  55  formed from a metallic sheet is attached to an opening end portion (the right end portion of FIGS. 6 and 7) of the outer race  8 , the inside end opening of the outer race  8  is closed. The main body  54  includes a disk portion  56  integrally formed from synthetic resin. The sensor  20   b  is embedded on the outside surface of the disk portion  56 . The sensor  20   b , which is formed into an annular shape, protrudes from the outer side surface of the disk portion  56 . A connector  30  for taking out a detection signal from the sensor  20   b  is mounted on the inside surface of the disk portion  56 . The connector  30  is formed integrally with the main body  54 . A protrusion  57  which protrudes from the outside surface of the disk portion  56 , composing the main body  54 , is between the outer circumferential edge of the disk portion  56  and the center in the diametric direction. The protrusion  57  is formed to be annular. The sensor  20   b  is embedded in the protrusion  57 . 
     The sleeve  55  is formed into an annular shape, the section of which is an L-shape, from a metallic sheet of high rigidity such as a steel sheet or a stainless steel sheet by means of burring conducted by a press. The sleeve  55  includes: a cylindrical portion  58 , the diameter of which is formed to be a value so that the cylindrical portion  58  can be press-fitted into the opening end portion of the outer race  8 ; and a flange portion  59  bent at the inside edge of the cylindrical portion  58  outwardly in the diametric direction. In order to manufacture the above sleeve  55  from an inexpensive steel sheet by means of burring at low manufacturing cost, length L 55  of the sleeve  55  is restricted to be not more than 0.2 of diameter D 58  of the above cylindrical portion  58  (L 55 ≦0.2 D 58 ). When length L 55  and diameter D 58  are restricted as described above, it is possible to manufacture the sleeve  55  without using the method of deep drawing, the manufacturing cost of which is high. Of course, when an increase in the manufacturing cost is allowed, the third embodiment of the present invention is not restricted by the above restriction of the size. 
     When the main body  54  is made in conjunction with the connector  30  by means of injection molding, the sleeve  55  is embedded in a portion of the main body  54  close to the outer circumferential portion except for a fore end portion of the cylindrical portion  58  (the portion close to the left end in FIG.  6 ). When the sleeve  55  is embedded in the main body  54  together with the sensor  20   b , both members  55 ,  20   b  are concentrically set in the forming mold. When injection molding is conducted to make the main body  54  in the above condition, the entire flange portion  59  and a portion of the cylindrical portion  58  close to the base end (a portion close to the flange portion  59  located on the right in FIG. 6) of sleeve  55  are embedded in a portion of the main body  54  close to the outer circumference. 
     A step portion  60  is formed all over the circumference in the outer circumferential portion on the outside surface of the main body  54 . Due to the existence of the step portion  60 , under the condition that the fore end portion of the cylindrical portion  58  exposed from the outside surface of the main body  54  is inwardly engaged with the inside end opening of the outer race  8  and also under the condition that the outer circumferential portion on the outside surface of the main body  54  is confronted with the inside end surface of the outer race  8 , a groove  61  capable of engaging with a tool can be formed all over the circumference between the outer circumferential portion on the outside surface of the main body  54  and the inside end surface of the outer race  8 . When the size of the step portion  60  is restricted, the diameter D 61  of the bottom portion of the groove  61  is made to be smaller than the outer diameter D 59  of the flange portion  59  (D 61 &lt;D 59 ). Accordingly, the outer circumferential edge portion of the flange portion  59  is located at an outward position of the bottom of the groove  61  in the diametric direction. The width W 61  of the groove  61  is restricted so that an end portion of the tool used to remove the cover  18   b  from the outer race  8  can be positively inserted into the groove  61 . 
     A seal ring (O-ring)  62  is made of elastic material such as rubber and elastomer in a portion where the outer circumferential portion on the outside surface of the primary body  54  is confronted with the inside end surface of the outer race  8 . An engaging groove  63  is formed in the outer circumferential portion on the outside surface of the main body  54  which is a little inward with respect to the step portion  60  in the diametric direction. As described above, under the condition that the fore end portion of the cylindrical portion  58  is inwardly engage with the inside end opening of the outer race  8  and also under the condition that the outer circumferential portion on the outside surface of the main body  54  is confronted with the inside end surface of the outer ring  8 , the seal ring  62  is elastically compressed in the engaging groove  63 , so that the seal ring  62  can be elastically contacted with the surfaces surrounding the seal ring  62  all over the circumference. Under the above condition, the seal ring  62  seals the portion where the outer circumferential portion on the outside surface of the main body  54  is confronted with the inside end surface of the outer race  8 . 
     Even when the sleeve  55  is embedded in the main body  54  in the process of injection molding of the main body  54 , it is inevitable that a minute clearance is formed between a metallic sheet composing the sleeve  55  and a member of synthetic resin composing the main body  54 . In this case, there is a possibility that muddy water intrudes into the unit through the minute clearance. However, in the structure of the present invention, the flange portion  59  composing the sleeve  55  is embedded in synthetic resin. Therefore, a boundary between the metallic plate and synthetic resin is not exposed to the outside on which muddy water is existing. Further, an opening end of this boundary is located at a position inward with respect to the seal ring  62  in the diametric direction. Therefore, this boundary is separate from the outside by the seal ring  62 . Accordingly, there is no possibility that muddy water intrudes into the outer race  8  through the boundary. 
     In the rolling bearing unit having a rotating speed detector according to the third embodiment of the present invention provided with the cover  18   b  including the main body  54  and the sleeve  55  described above, the cylindrical portion  58  of the sleeve  55  is inwardly press-fitted into the inside end opening of the outer race  8 . The seal ring  62  is previously attached into the engaging groove  63 . The cylindrical portion  58  is pushed into the outer race  8  until the outer circumferential portion on the outside surface of the main body  54  confronts the inside end surface of the outer race  8 , so that the seal ring  62  can be elastically compressed in the engaging groove  63 . In this way, assembling work of the cover  18   b  to the outer race  8  is completed. The cover  18   b  in which the sensor  20   b  is embedded in this way is positively fixed to the outer race  8 . 
     When the cover  18   b  is fixed to the inside end opening of the outer race  8 , a fore end surface of the pushing jig is set at an outer circumferential portion on the inside surface of the main body  54 . Accordingly, when the cover  18   b  is pushed to the outer race  8 , a compressive stress is given to a portion of synthetic resin composing the main body  54  located inside the inside surface of the flange portion  59 . When a tensile stress is applied to synthetic resin composing the main body  54  (especially at low temperature), the main body  54  tends to be damaged, that is, cracks tend to occur in the main body  54 . However, when a compressive stress is applied, synthetic resin composing the main body  54  is less susceptible to damage. Accordingly, when the cover  18   b  is pushed to the inside opening of the outer race  8  so that the cover  18   b  can be fixed, there is no possibility that the main body  34  is damaged by stress. 
     When the cover  18   b  is detached from the inside end opening of the outer race  8 , a fore end portion of the pulling jig is engaged with the groove  61  and pulled inside. In this case, in order to make the point of application locate at a position inward with respect to the outer circumferential edge of the flange portion  59  in the diametric direction, the fore end portion of the pulling jig is inserted into an inner portion of the groove  61 . When the pulling jig is pulled inside under the above condition, while a portion of the synthetic resin composing the main body  54  located outside with respect to the outside surface of the flange portion  59  is being given a compressive stress, the cylindrical portion  58  is pulled out from the inside end opening of the outer race  8 . Consequently, the cover  18   b  including the main body  54  is not damaged in the process of removing the cover  18   b.    
     FIG. 8 is a view showing an arrangement of the fourth embodiment of the present invention. In the fourth embodiment, the step portion  60  composing the groove  61  to be engaged with the fore end portion of the pulling jig is not located at the outer circumferential portion on the outside surface of the main body  54 , but it is formed at a half portion on the outer circumference on the inside end surface of the outer race  8 . Other points and arrangements of the fourth embodiment are the same as those of the third embodiment described before. 
     FIG. 8 is a view showing an arrangement of the fifth embodiment of the present invention. In the arrangement of the fifth embodiment, the engaging groove  63  (shown in FIGS. 6 and 7) to be engaged with the seal ring  62  is omitted. Instead of that, the seal ring  62  is elastically held by the three portions of a chamfered portion  67  formed on the inner circumferential edge of the outer race  8 , an outer circumferential surface of the middle portion of the cylindrical portion  58  composing the sleeve  55 , and an outer circumferential portion on the outside surface of the main body  54 . Other arrangements and functions are the same as those of the third embodiment described before. 
     FIGS. 9 to  11  are views showing an arrangement of the sixth embodiment of the present invention. The rolling bearing unit having a rotating speed detector according to the sixth embodiment has a structure wherein the portion of the cover  18   b  which supports the sensor  20   b , composing the rotating speed detector, is fixed to the outer race  8 . The structure of the rolling bearing unit is the same as that of the first conventional example shown in FIG.  19 . Therefore, the overlapping explanations of the rolling bearing unit are omitted or simplified here, and the characteristic portions of the present invention will be explained below. 
     The above main body  54  includes: a disk portion  56  integrally made of synthetic resin; and a cylindrical portion  58  protruding from an outer circumferential portion on the outside surface of the disk portion  56 . 
     A protrusion  57  is formed on the outside surface of the disk portion  56  composing the main body  54 , and in a portion inward with respect to the cylindrical portion  58  in the diametric direction. The protrusion  57  is formed to be annular. The sensor  20   b  is embedded in the protrusion  57  as shown by the hatching in FIG. 9. A cylindrical clearance  69  is formed between an outer circumferential surface of the sensor  20   b  embedded in the protrusion  57  and an inner circumferential surface of the disk portion  56 . The cylindrical clearance  69  has the thickness which is uniform all over the circumference. Into the clearance  69 , a portion of the tone wheel  13   a  described later can be freely inserted. 
     The sleeve  55  is formed into an annular shape, the section of which is a crank-shape, from a metallic sheet of high rigidity such as a steel sheet or a stainless steel sheet by means of burring conducted by a press or by means of deep drawing. The shape of the sleeve  55  is formed as follows. An outside end edge of the small diameter portion  70  and an inside end edge of the large diameter portion  71  are continuously connected with each other by a step portion  72 , and when an inside end edge of the small diameter portion  70  is inwardly bent in the diametric direction, a bent engaging edge  73  is formed. The small diameter portion  70  of the sleeve  55  is embedded in a portion close to the base portion of the cylindrical portion  58 . Under the condition that the small diameter portion  70  is embedded in the cylindrical portion  58 , the bent end edge  73  enhances the joining strength of the small diameter portion  70  with respect to the cylindrical portion  58 . 
     When the main body  54  is made by means of injection molding together with the connector  30 , the sleeve  55  is embedded in an outer circumferential portion of the main body  54 , except for a large diameter portion  71  and the step portion  72  of the sleeve  55 . When the sleeve  55  is embedded in the main body  54  together with the sensor  20   b , both members  55 ,  20   b  are set in a forming mold concentrically to each other. When the main body  54  is made by means of injection molding under the above condition, the small diameter portion  70  and the bent engaging edge  73 , which compose the sleeve  55 , are embedded in the cylindrical portion  58  composing the main body  54 . 
     When an outer diameter of the fore end portion (the outside end portion) of the cylindrical portion  58  is made to be smaller than an outer diameter of the base end portion (the inside end portion), a step portion  74  is formed on the outer circumferential surface of the fore end portion of the cylindrical portion  58 . Under the condition that the small diameter portion  70  is embedded in the cylindrical portion  58 , one face (the inside surface) of the step portion  72  of the sleeve  55  comes into contact with an inner end surface of the step portion  74 . A base end portion of the large diameter portion  71  of the sleeve  55  is located around the step portion  74 . Accordingly, in the fore end portion of the cylindrical portion  58 , a groove  75  is formed all over the circumference which is surrounded by the three portions of an inner circumferential surface of the base end portion of the large diameter portion  71  composing the sleeve  55 , an outside surface of the step portion  72 , and an outer circumferential surface of the step portion  74  formed in the fore end portion of the cylindrical portion  58 . A seal ring such as an O-ring  76  is fitted in the groove  75 . A diameter (thickness) of the section of the O-ring in a free condition is larger than the depth of the groove  75 . An inner diameter of the O-ring  76  in a free condition is a little smaller than the diameter of the outer circumferential surface of the step portion  74  which composes an inner circumferential side surface of the groove  45 . Accordingly, when the O-ring  76  is incorporated into the groove  75 , the inner circumferential edge of the O-ring  76  elastically comes into contact with the outer circumferential surface of the step portion  74  all over the circumference. 
     Further, the front half portion of the large diameter portion  71  protrudes to the front as compared with the fore end surface (the outside end surface) of the cylindrical portion  58 . An inner diameter of the large diameter portion  71  in a free condition is a little smaller than the outer diameter of the inside end portion of the outer race  8  to which the cover  18   b  is fixed. In order to fix the cover  18   b  to the inside end portion of the outer race  8 , an outside end edge of the large diameter portion  71  is aligned with the outer circumferential edge of the inside end portion of the outer race  8 , and an outer circumferential portion on the inside surface of the main body  54  is pushed to the outer race  8 . By this pushing work, the large diameter portion  71  is outwardly engaged with the outer race  8  while the diameter is being elastically expanded. This engaging work is continued until the fore end surface of the cylindrical portion  58  confronts the inside end surface of the outer race  8 . Under the condition that the fore end surface of the cylindrical portion  58  confronts the inside end surface of the outer race  8 , the O-ring is elastically compressed between the fore end surface and the inside end surface. The front half portion of the large diameter portion  71  is outwardly press-fitted to the inside end portion of the outer race  8 . 
     A groove  77  is formed on an outer circumferential surface of the middle portion of the cylindrical portion all over the circumference or alternatively intermittently in the circumferential direction. When the rotating speed detector is overhauled, in order to remove the cover  18   b  from the outer race  8 , a fore end portion of the tool is engaged with the groove  77 . In this connection, the groove  77  may be formed all over the circumference, however, when the groove  77  is formed intermittently in the circumferential direction, an intensity of the compressive force given to the outer circumferential edge portion of the main body  54  can be reduced when the outer diametric portion of the main body  54  is pushed to the outer race  8  so as to attach the cover  18   b  to the outer race  8 . 
     A screw hole  78  is formed in the attaching portion  9  on the outer circumferential surface of the outer race  8 . Under the condition that the cover  18   b  is attached to the outer race  8 , an arcuate cutout portion  79  is formed at a position on the outer circumferential edge of the main body  54  which coincides with the screw hole  78 , as shown in FIG.  11 . The cutout portion  79  is formed for preventing the occurrence of interference between an impact wrench to fasten a bolt to the screw hole  78  and the outer circumferential edge portion of the main body  54 . 
     Even when the sleeve is embedded in the main body  54  in the process of injection molding of the main body  54 , it is inevitable that a minute clearance is formed between a metallic sheet composing the sleeve  55  and synthetic resin composing the cylindrical portion  58  of the main body  54 . In this case, there is a possibility that muddy water intrudes into the unit through the minute clearance. In the arrangement of the sixth embodiment of the present invention, even when muddy water attempts to intrude into the unit through a boundary between the metallic sheet and synthetic resin, the muddy water is stopped by the O-ring  76 , so that the intrusion of muddy water into the outer race  8  can be prevented. Passages of the muddy water intruding into the outer race  8  through the attaching portion of the cover  18   b  are shown by arrows A—A and B—B in FIG.  10 . In this case, the passage shown by arrow A—A is formed on an engaging surface formed by the inner circumferential surface of the large diameter portion  71  and the outer circumferential surface of the outer race  8 , and the passage shown by arrow B—B is formed by the minute clearance described above. In the arrangement of the present invention, no muddy water intrudes into the outer race  8  through these passages because it is blocked by the O-ring  76 . 
     On the other hand, when the groove  75   a  engaging with the O-ring  76  is formed in the sleeve  55   a  itself by means of deep drawing as shown in FIG. 12, it is impossible to prevent muddy water from intruding into the outer race  8  through a portion corresponding to the above minute clearance. According to the arrangement shown in FIG. 12, the manufacturing cost of the sleeve  55   a  is raised. Further, since the inner diameter of the sleeve  55   a  is reduced, it becomes difficult to increase the diameters of the sensor  20   b  and the tone wheel  13   a , which will be described later, for the necessity of preventing the interference with the sleeve  55   a . When it becomes difficult to increase the diameters, it also becomes difficult to increase an output of the sensor  20   b.    
     In the rolling bearing unit having a rotating speed detector according to the sixth embodiment of the present invention in which the cover  18   b  including the main body  54  and the sleeve  55  are provided, the front half portion of the large diameter portion  71  of the sleeve  55  is press-fitted to the inside end opening of the outer race  8  as described before. In this way, the front half portion of the large diameter portion  71  is inwardly engaged with the inside end opening of the outer race  8 . In this case, the O-ring  76  is previously attached into the groove  75 . The outer diameter portion  71  is outwardly inserted into the outer race  8  until the fore end surface of the cylindrical portion  58  confronts the inside end surface of the outer race  8 . Under the condition that the O-ring  76  is elastically compressed between the fore end surface and the inside end surface, the assembling work of the cover  18   b  to the outer race  8  is completed. Under the above condition, the cover  18   b  in which the sensor  20   b  is embedded can be positively fixed to the outer race  8 . 
     In this connection, the structure of the rotating speed detector described in the first to sixth embodiment is not the main point of the present invention. Concerning the structure of the rotating speed detector, various type of structure including the first to the fourth conventional example shown in FIGS. 19 and 23 may be adopted when the present invention is executed. For example, it is possible to adopt an arrangement in which the rotating speed is magnetically detected. Further, it is possible to adopt an arrangement in which the rotating speed is optically detected. In the case where the rotating speed is optically detected, a piece of metallic sheet on which a large number of slits are formed in the circumferential direction at regular intervals is used as an element to be detected, and a light emitting element and a light receiving element are used as a sensor. In the case of an arrangement in which the rotating speed is magnetically detected, it is possible to use a combination in which a tone wheel made of a permanent magnet and an active type sensor into which an MR element or a Hall element is incorporated are combined. Further, the engaging portion which engaging the fore end portion of the pulling jig is not necessarily served as a groove which is to be formed all over the circumference, but a plurality of grooves may be formed in the circumferential direction. It is preferable that three or more grooves are formed in the circumferential direction. 
     FIGS. 13 to  16  are views showing an arrangement of the seventh embodiment of the present invention. The characteristic of the rolling bearing unit having a rotating speed detector according to the seventh embodiment is an structure of the portion where the cover  18   b  supporting the sensor  20   b  composing the rotating speed detector is fixed to the inside end opening of the outer race  8 . The structure of the rolling bearing unit is the same as that of the second conventional example shown in FIG.  21 . Therefore, like reference characters are used to indicate like parts in various views, and the overlapping explanations of the rolling bearing unit are omitted or simplified here. The characteristic portions of the present invention will be explained below. 
     A cover  18   b  made of synthetic resin is attached to the inside end opening of the outer race  8  which is a stationary race so as to close the inside end opening of the outer race  8 . The cover  18   b  includes: a disk-shaped closing plate  128  which closes the overall inside end opening of the outer race  8 ; and an engaging cylindrical portion  129  provided in the outer circumferential portion on the outside surface of the closing plate  128 . In the seventh embodiment, a sleeve  130  made of a metallic sheet is embedded in the base portion (the right end in FIGS. 13 to  16 ) of the engaging cylindrical portion  129  and also embedded in the outer circumferential end portion of the closing plate  128 . The sleeve  130  is made of a stainless steel sheet or a mild steel sheet coated with an anticorrosion layer of chrome by means of plating. The sleeve  130  is formed into an annular shape, the section of which is an L-shape. The sleeve  130  includes: a cylindrical portion  131 ; and a flange portion  132  which is outwardly bent at the inside end edge of the cylindrical portion  131  in the diametric direction. A fore end portion of the cylindrical portion  131 , that is, a left end portion of the cylindrical portion  131  in FIGS. 13 to  16  is exposed to an outer circumferential surface of the base end portion of the engaging cylindrical portion  129 . The flange portion  132  is embedded in the outer circumferential edge portion of the cover  18   b  which is located outside of the engaging cylindrical portion  129  in the diametric direction. The flange portion  132  is embedded in the middle of the portion with respect to the thickness direction. In this connection, an outer circumferential edge of the flange portion  132  does not reach the outer circumferential edge of the cover  18   b , that is, the outer circumferential edge of the flange portion  132  is embedded in the cover  18   b.    
     A plurality of cutout portions  133  are intermittently formed in the circumferential direction on the outside surface of the outer circumferential edge portion of the cover  18   b  existing outwardly with respect to the engaging cylindrical portion  129  in the diametric direction. An outer circumferential surface of the cylindrical portion  131  and an outside surface of the flange portion  132 , which correspond to cutout portions  133 , are not covered with the synthetic resin composing the cover  18   b  but exposed to cutout portions  133 . A confronting portion  134  is formed in an outer circumferential edge portion of the closing plate  128  of the cover  18   b  which exists outwardly with respect to the engaging cylindrical portion  129  in the diametric direction. The confronting portion  134  is formed on the outside surface of the above outer circumferential edge portion of the closing plate  128 . A front end surface of the confronting portion  134  (the left end surface shown in FIGS. 13 to  16 ) is capable of confronting the inside end surface  107   a  of the outer race  8 . In order to close the inside end opening of the outer race  8 , the engaging cylindrical portion  129  is engaged with the inside end portion of the outer race  8  by the cover  18   b , and under the condition that each confronting portion  134  confronts the inside end surface  107   a  of the outer race  8 , a positional relation between the sensor  20   b  described later and the tone wheel  135  outwardly fixed to the inside end portion of the inner race  5  is restricted in a predetermined manner. In this connection, the tone wheel  135  is formed into an annular shape, the section of which is shown in FIGS. 13 and 14, from a sheet made of magnetic metal such as mild steel. 
     An engaging groove  136  is formed on an outer circumferential surface of the fore end portion of the engaging cylindrical portion  129 , and in a portion on the outer circumferential surface which is not covered with the cylindrical portion  131  of the sleeve  130 . The engaging groove  136  is formed all over the circumference of the engaging cylindrical portion  129 . An O-ring  137  is fitted in the engaging groove  136 . An outer circumferential edge of the O-ring  137  elastically comes into contact with the inner circumferential surface of the end portion of the outer race  8 , so that a space formed between the outer circumferential surface of the engaging cylindrical portion  129  and the inner circumferential surface of the outer race  8  can be sealed. An outer diameter of the cylindrical portion  131  in a free condition is a little larger than the inner diameter of the inside end portion of the outer race  8 , so that the sleeve  130  can be press-fitted into the inside end portion of the outer race  8 . When the O-ring  137  is attached in the engaging groove  136  and before the O-ring is engaged with the inside end portion of the outer race  8 , an outer circumferential portion of the O-ring  137  outwardly protrudes from the cylindrical portion  131  in the diametric direction. 
     A sensor  20   b  is fitted on an outside surface of the closing plate  128  composing the cover  18   b  in a portion close to the center in the diametric direction with respect to the engaging cylindrical portion  129 . An inside end portion of the sensor  20   b  is embedded in the closing plate  128 . The sensor  20   b  is entirely formed into an annular shape. An outside half portion of the sensor  20   b  protrudes from the outside surface of the closing plate  128  composing the cover  18   b , and an outer circumferential surface, which functions as a detecting section, is exposed from the synthetic resin composing the cover  18   b . A connector  138  for taking out a detection signal from the sensor  20   b  is formed on an inside surface of the closing plate  128 . The connector  138  is formed being integrated with the cover  18   b.    
     According to the rolling bearing unit having a rotating speed detector in the seventh embodiment arranged as described above, when the cover  18   b  is made of synthetic resin, it is possible to reduce the manufacturing cost. Further, even when the outer diameter of the cover  18   b  is not particularly extended, the cover  18   b  can be easily detached from the outer race  8  which is a stationary race. Moreover, the cover  18   b  and the sensor  20   b  embedded in the cover  18   b  can be positively positioned with respect to the outer race  7 . 
     When the cover  18   b  is detached from the outer race  8 , the fore end portion of an appropriate tool is engaged with a plurality of cutout portions  133  formed on the outside surface of the outer circumferential portion of the closing plate  128 . Since the cutout portions  133  reach the outer circumferential surface of the cylindrical portion  131  composing the sleeve  130 , even if the diameter R 128  (shown in FIG. 14) of the closing plate  128  is not particularly extended, the depth D 133  (shown in FIGS. 14 and 15) of each cutout portion  133  is sufficiently extended, so that a sufficiently large space in which the fore end portion of the tool is engaged with each cutout portion  133  can be ensured. When the tool is given a force in a direction so that the cover  18   b  can be separated from the outer race  8  under the condition that the fore end portion of the tool is sufficiently inserted into the cutout portions  133 , the engaging groove  129  of the cover  18   b  can be pulled out from the outer race  7 , and the cover  18   b  supporting the sensor  20  can be detached from the outer race  8 . In this case, a pulling force given to the cover  18   b  by the fore end portion of the tool is supported by the flange  132  composing the sleeve  130 . Accordingly, a portion of the synthetic resin composing the cover  18   b  is not given a high intensity of stress. Consequently, there is no possibility that the cover  18   b  is damaged when the cover  18   b  is detached from the outer race  8 . 
     Under the condition that the cover  18   b  is engaged with the inside end opening of the outer race  8 , when a plurality of confronting portions  134  formed at a position away from the cutout portions  133  in the circumferential direction confront the inside end surface  107   a  of the outer race  8 , the cover  18   b  and the sensor  20   b  embedded in the cover  18   b  can be positioned with respect to the outer race  8 . When the engaging cylindrical portion  129  is pushed into the inside end opening of the outer race  8  so as to attach the cover  18   b  to the inside end opening of the outer race  8 , a fore end surface of an appropriate pushing shaft is made to confront a pushing surface  149  formed in an outer circumferential portion on the inside surface of the closing plate  128  which corresponds to each confronting portion  134 . 
     In the rolling bearing unit having a rotating speed detector according to the seventh embodiment of the present invention, the confronting portions  134  used for positioning the cover  18   b  and the cutout portions  133  used for engaging the fore end portion of the tool are alternately intermittently formed in the circumferential direction. Since the confronting portions  134  and the cutout portions  133  are alternately intermittently formed in the circumferential direction, it is possible to extend the size of a clearance in the radial direction necessary for inserting the fore end portion of the tool can be extended without extending the outer diameter of the cover  18   b . In this case, the size of a clearance in the radial direction necessary for inserting the fore end portion of the tool is the same as the depth D 133  of the cutout portion  133 . As a result, while the interference of the cover  18   b  with other members is prevented, the cover  18   b  can be detached from the outer race  8  without causing a damage in the cover  18   b.    
     On the other hand, in the arrangement shown in FIG. 16, the cutout portions  133   a  to be engaged with the fore end portion of the tool are provided all over the circumference of the cover  18   b , and on the inner diameter side of the cutout portions  133   a , the confronting portions  134   a  to confront the inside end surface  107   a  of the outer race  8  are also provided all over the circumference of the cover  18   b . According to this arrangement shown in FIG. 16, it impossible to provide the same effect as that of the present invention explained before. The reason is described as follows. When the depth D 133a  of the cutout portion  133   a  is extended in the arrangement shown in FIG. 16 so as to ensure a space in which the fore end portion of the tool is engaged, the outer diameter of the cover  18   b  is extended. Therefore, the cover  18   b  tends to interfere with other members. Due to the foregoing, the degree of freedom is deteriorated in the design of an automobile. Accordingly, the arrangement shown in FIG. 16 is unpreferable as described before. 
     FIGS. 17 and 18 are views showing an arrangement of the eighth embodiment of the present invention. Like reference characters are used to indicate like parts in various views, therefore, overlapping explanations of the roller bearing unit are omitted. In the eighth embodiment, the sleeve  130   a  for ensuring the engaging strength of the cover  18   c  with the outer race  8  is formed into a simple cylindrical shape. The sleeve  130   a  is embedded in and supported by a fore end portion of the engaging cylindrical portion  129  formed on the cover  18   c . An O-ring  137  is fitted between an outer circumferential surface of the engaging cylindrical portion  129  and an inner circumferential surface of the inside end of the outer race  8 . The O-ring  137  is located at a position which is closer to the inside end opening of the outer race  8  than the sleeve  130   a . Accordingly, when the engaging cylindrical portion  129  is inwardly engaged with the inside end portion of the outer race  8 , there is no possibility that rain water intrudes into the sleeve  130   a . Consequently, it is not necessary to use a member made of expensive material such as a stainless steel sheet or a steel sheet coated with an anticorrosion layer, for the above sleeve  130   a . Even if such a member made of expensive material is not used, there is no possibility that both the sleeve  130   a  and the outer race  8  are corroded. Therefore, the manufacturing cost can be reduced by reducing the material cost. 
     In the eighth embodiment of the present invention, the sleeve  130   a  is formed into a simple cylindrical shape, and the sleeve  130   a  is embedded in and supported by the fore end portion of the engaging cylindrical portion  129 . Due to the foregoing, when the engaging cylindrical portion  129  is pulled out from the outer race  8  so as to separate the cover  18   c  from the outer race  8 , a relatively high intensity of stress is given to a portion of the synthetic resin composing the cover  18   c  close to the outer circumferential portion of the closing plate  128   a . According to the eighth embodiment, reinforcing ribs  150 ,  150  are formed on the inside surface of the closing plate  128   a  and on the opposite side to the cutout portions  133 ,  133  which are to be engaged with the fore end portion of the tool. When the reinforcing ribs  150 ,  150  are formed in this way, a stress given to the rigid resin composing the outer circumferential portion of the closing plate  128   a  in the process of pulling can be sufficiently received. Further, according to the eighth embodiment, when the engaging cylindrical portion  129  is pulled out from the inside of the outer race  8 , there is a possibility that the engaging cylindrical portion  129  is not damaged at its end portion by the frictional force between the outer circumference of the sleeve  130   a  and the inner circumference of the outer race  8 . The end portion of the engaging cylindrical portion  129  is secured to have the strength enough to avoid the possibility in accordance with the frictional force. Other arrangements and functions are the same as those of the seventh embodiment described before. Therefore, like reference characters are used to indicate like parts in various views, and the overlapping explanations of the rolling bearing unit are omitted or simplified here. 
     Since the rolling bearing unit of the present invention is arranged and functions as described above, the number of parts can be reduced and the manufacturing cost can be reduced. Further, it is possible to provide a rotating speed detector of high durability and reliability. 
     Manufacturing work of the rolling bearing unit of the present invention can be simplified and the manufacturing cost can be reduced. Further, it is not necessary to use a specific apparatus for the inspection and maintenance of the rolling bearing unit of the present invention. Without using a specific apparatus, the rolling bearing unit can be overhauled and assembled without damaging the components. Accordingly, it is possible to provide a rolling bearing unit having a rotating speed detector, the maintenance cost of which is low. 
     Since the rolling bearing unit having a rotating speed detector according to the present invention is arranged and functions as described above, it can be made compact and the interference of the rolling bearing unit with other members seldom occurs. Therefore, the degree of freedom can be ensured in the design of an automobile. Accordingly, the manufacturing cost can be reduced by suppressing a quantity of material to be used, and further the weight of the rolling bearing unit can be decreased.