Patent Publication Number: US-2011061983-A1

Title: Rotation transmission device

Description:
TECHNICAL FIELD 
     This invention relates to a rotation transmission device for selectively transmitting and not transmitting power. 
     BACKGROUND ART 
     Patent document 1 discloses a conventional rotation transmission device mounted on an FR (front-engine rear-drive)-based 4-wheel drive vehicle for selectively transmitting and not transmitting driving force to the front wheels as auxiliary drive wheels. 
     The rotation transmission device disclosed in Patent document 1 includes a two-way clutch disposed between a large-diameter portion formed on an input member and an outer race provided around the large-diameter portion, and an electromagnetic clutch provided in juxtaposition with the two-way clutch for selectively engaging and disengaging the two-way clutch. When the two-way clutch is engaged, the input member is coupled to the output member and torque is transmitted between the input member and the output member. 
     The two-way clutch comprises a cylindrical surface formed on the inner periphery of the outer race, cam surfaces formed on the outer periphery of the large-diameter portion of the input member and defining, in cooperation with the cylindrical surface, wedge-shaped spaces having narrow circumferential ends, and engaging elements in the form of rollers disposed between the respective cam surfaces and the cylindrical surface. When a retainer retaining the engaging elements rotates relative to the input member, the engaging elements are adapted to engage the cylindrical surface and the cam surfaces. A switch spring is mounted between the input member and the retainer to bias the retainer toward neutral position where the engaging elements disengage from the cylindrical surface and the cam surfaces. 
     The electromagnetic clutch comprises an armature rotationally fixed to but axially movable relative to the retainer, a rotor axially facing the armature, an electromagnet axially facing the rotor, and a separation spring biasing the armature away from the rotor. When the electromagnet is energized, the armature is pulled to the rotor, so that the armature, which is now coupled to the outer race, and the input member rotate relative to each other, which in turn brings the engaging elements into engagement with the cylindrical surface and the cam surfaces. 
     In this two-way clutch, since each roller is moved from the neutral position, where the roller is located in the wide portion of the wedge-shaped space, and wedged into a narrow end of the wedge-shaped space by rotating the input member and the retainer relative to each other, there exists a large play in the rotational direction. 
     With torque being transmitted between the outer race and the input member in one direction, in order to change the direction in which the torque is transmitted, the retainer has to be turned from the position where each roller is wedged in one of the narrow ends of the wedge-shaped space to the position where the roller is wedged into the other narrow ends of the wedge-shaped space. Thus, it was difficult to sufficiently quickly change the direction in which torque is transmitted. 
     In order to solve these problems, Patent document  2  discloses a two-way roller clutch in which a plurality of rollers are non-equidistantly arranged such that one of any adjacent pair of rollers are located at one circumferential end of the corresponding wedge-shaped space while the other of the adjacent pair is located at the other circumferential end of the corresponding wedge-shaped space. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent document 1: JP Patent Publication 2005-249003A
 
Patent document 2: JP Patent Publication 2003-262238A
 
     SUMMARY OF THE INVENTION 
     Object of the Invention 
     With the two-way roller clutch disclosed in Patent document 2, although play in the rotational direction decreases, there still remains play in the rotational direction. Also, because the clearances between the rollers and the outer ring cylindrical surface and between the rollers and the inner race cam surfaces are small, the rollers may erroneously engage while the two-way clutch is idling. Thus, reliability of operation is low during idling. 
     While torque is being transmitted between the outer race and the inner race, only half of the plurality of rollers are in engagement, while the remaining half of the rollers are not. Thus, torque capacity is small. 
     An object of the present invention is to provide a rotation transmission device which has a minimum play in the rotational direction, which is reliable by preventing the rollers from erroneously engaging during idling, and which has high torque capacity. 
     Means to Achieve the Object 
     In order to achieve this object, the present invention provides a rotation transmission device comprising an outer race having a closed end provided with an output shaft, an input shaft, an inner race mounted on the input shaft and in the outer race, the outer race and the inner race being rotatable relative to each other, wherein a cylindrical surface is formed on one of an inner periphery of the outer race and an outer periphery of the inner race, and a plurality of circumferentially spaced apart cam surfaces are formed on the other of the inner periphery of the outer race and the outer periphery of the inner race, the cylindrical surface and each of the cam surfaces defining a wedge-shaped space therebetween which narrows toward circumferential ends thereof, a control retainer and a rotary retainer rotatably mounted between the outer race and the inner race, wherein the control retainer comprises a flange and a plurality of pillars formed on a radially outer portion the flange, wherein the rotary retainer has the same shape as the control retainer, wherein the flanges of the respective retainers axially face each other, and wherein the flange of the rotary retainer faces one side surface of the inner race, with the pillars of one of the retainers disposed between the respective circumferentially adjacent pillars of the other of the retainers, thereby defining pockets between the respective circumferentially adjacent pillars of the respective retainers, the pockets facing the respective cam surfaces, a plurality of opposed pairs of rollers, each pair being received in one of the pockets, presser members received in the respective pockets and biasing the respective pairs of rollers away from each other while pressing the rollers against the outer periphery of the inner race, torque cams provided between opposed surfaces of the flange of the control retainer and the flange of the rotary retainer that are configured to rotate the retainers relative to each other in a direction in which a circumferential width of the pockets decreases when the control retainer moves in a direction in which the distance between the flange of the control retainer and the flange of the rotary retainer decreases, a retaining plate fixed to another side surface of the inner race and having a plurality of anti-rotation pieces on an outer periphery thereof for supporting the respective pillars of the retainers, thereby keeping the respective opposed pairs of rollers in neutral position, when the control retainer and the rotary retainer rotate relative to each other in the direction in which the circumferential width of the pockets decreases, and an actuator mounted on a torque transmission shaft connected to the inner race for axially moving the control retainer. 
     With this rotation transmission device, when the control retainer is moved by the actuator in the direction in which its flange moves toward the flange of the rotary retainer, the control retainer and the rotary retainer are rotated relative to each other in the direction in which the circumferential width of the pockets decreases by the action of the torque cams, so that the opposed pairs of rollers are pushed toward each other by the respective pillars of the control retainer and the rotary retainer, and disengage. 
     Thus, even when the inner race is rotating, its rotation is not transmitted to the outer race and the inner race idles. While the inner race is idling, the opposed pairs of rollers are prevented from being moved into narrow portions of the respective wedge-shaped spaces by the pillars of the control retainer and the rotary retainer. Also, since the opposed pairs of rollers are always pressed against the outer periphery of the inner race by the presser members, the rollers are never moved radially outwardly under centrifugal force. 
     If the rollers move radially outwardly under centrifugal force, they may contact the inner periphery of the outer race, which could in turn move the rollers into engaged position due to dragging torque acting on the rollers. But because the presser members prevent radially outward movement of the rollers, there will be no erroneous engagement of the rollers. 
     The presser members may each comprise a leaf spring bent in the shape of the letter W. Otherwise, the presser members may each comprise a cylindrical member, a pair of presser elements slidably supported by respective ends of the cylindrical member and having, respectively, inclined roller pressing surfaces facing the respective ones of each opposed pair of rollers, and a coil spring biasing the pair of presser elements against the respective ones of each opposed pair of rollers. 
     A single presser member may be provided between each opposed pair of rollers. Or alternatively, a plurality of such presser members may be arranged in a plurality of rows in the longitudinal direction of the rollers, between each opposed pair of rollers. With the latter arrangement, it is possible to prevent skew of the rollers. 
     With the inner race idling, when the actuator is actuated and the control flange is moved axially in the direction in which its flange moves away from the flange of the rotary retainer, the control retainer and the rotary retainer rotate relative to each other in the direction in which the circumferential width of the pockets increases under the biasing force of the presser members. This causes the opposed pairs of rollers to instantly wedge into the respective narrow portions of the wedge-shaped spaces. Thus torque in one direction is transmitted between the inner and outer races through one of each opposed pair of rollers, and toque in the opposite direction is transmitted through the other of each opposed pair of rollers. 
     The torque cams of the rotation transmission device according to this invention may each comprise an opposed pair of cam grooves formed in the respective opposed surfaces of the flange of the control retainer and the flange of the rotary retainer and circumferentially spaced from the cam grooves of the other toque cams, the cam grooves having a depth that decreases toward circumferential ends thereof, and a ball fitted in the opposed pair of cam grooves, the ball of each torque cam being configured to roll from shallow portions toward deep portions of the respective opposed pair of cam grooves, thereby rotating the retainers relative to each other in the direction in which the circumferential width of the pockets decreases, when the control retainer moves in the direction in which the distance between the flanges of the respective retainers decreases. 
     With this arrangement, when the control retainer is moved in the direction in which the flange of the control retainer moves toward the flange of the rotary retainer, the ball of each torque cam rolls from shallow to deep portions of the respective opposed pair of cam grooves, so that the control retainer and the rotary retainer rotate relative to each other in the direction in which the circumferential width of the pockets decreases. 
     In this arrangement, when the control retainer and the rotary retainer rotate relative to each other in the direction in which the circumferential width of the pockets increases, the ball of each torque cam rolls toward shallow portions of the respective opposed pair of cam grooves. At this time, if the control retainer and the rotary retainer rotate relative to each other with their axes inclined to each other, the distances between the cam grooves of the respective torque cams differ from each other, so that loads applied to the respective balls also differ from each other. In this state, any ball to which load is scarcely or not at all applied may circumferentially come out of the cam grooves from their shallow portions. 
     If this happens, the two-way roller clutch loses its function and the rotation transmission device cannot be reliably operated. 
     By mounting an elastic member between opposed surfaces of the flange of the rotary retainer and the inner race for biasing the flange of the rotary retainer toward the flange of the control retainer, the control retainer and the rotary retainer are always kept coaxial with each other. 
     Thus, loads are uniformly applied to the respective balls, which prevents separation of the balls while the control retainer and the rotary retainer are rotating relative to each other, which in turn allows normal operation of the two-way roller clutch at all times. 
     Further, by providing spherical stopper surfaces at the shallow ends of the cam grooves so as to extend along the outer periphery of the ball, it is possible to more reliably prevent separation of the ball. 
     A thrust needle bearing may be mounted between opposed surfaces of the elastic member and the inner race. With this arrangement, the rotary retainer can be smoothly rotated relative to the inner race, so that the two-way clutch can be operated more smoothly. 
     The actuator of the rotation transmission device according to this invention may be an electromagnetic clutch comprising an armature fixedly coupled to the pillars of the control retainer and slidably fitted on the outer periphery of the torque transmission shaft, a rotor supported by the torque transmission shaft and axially facing the armature, and an electromagnet axially facing the rotor and configured to pull the armature to the rotor when energized. 
     Alternatively, the actuator may be an electromagnetic clutch comprising an armature fixedly coupled to the pillars of the control retainer and slidably fitted on the outer periphery of the torque transmission shaft, a rotor supported by the torque transmission shaft and axially facing the armature, a permanent magnet for pulling the armature to the rotor against the biasing force of the presser members, and an electromagnet axially facing the rotor and configured to reduce the magnetic force of the permanent magnet to a level lower than the biasing force of the presser members. 
     When the electromagnetic coil is energized or deenergized after power has been transmitted between the inner race and the outer race in order to disengage the rollers, if there remains torque between the inner race and the outer race, the residual torque may prevent disengagement of the rollers. This makes it impossible to determine whether the rollers are in engagement or engagement only from the fact that the electromagnetic clutch is energized or deenergized. 
     Thus, in order to determine whether the rollers are actually disengaged, the rotation transmission device may further include a first rotation sensor assembly provided around the input shaft for detecting the rotation of the input shaft, and a second rotation sensor assembly provided around the output shaft for detecting the rotation of the output shaft. 
     With this arrangement, when the electromagnet is energized and deenergized and the rollers are supposed to be disengaged, if the rollers are actually not deenergized due to residual torque, since the input shaft and the output shaft rotate at the same speed, the first rotation sensor assembly and the second rotation sensor assembly generate identical rotation signals. 
     On the other hand, if the rollers are actually disengaged, since only the input shaft keeps rotating while the output shaft stops, a rotation signal is generated from the first rotation sensor assembly, while no rotation signal is generated from the second rotation sensor assembly. 
     Thus, it is possible to determine whether the rollers are actually disengaged based on whether there is a difference in rotation between the rotation signal generated from the first rotation sensor assembly and the rotation signal generated from the second rotation sensor assembly. 
     The rotation transmission device may include a first bearing rotatably supporting the input shaft and carrying the first rotation sensor assembly and a second bearing rotatably supporting the output shaft and carrying the second rotation sensor assembly. With this arrangement, it is possible to mount the first rotation sensor assembly and the second rotation sensor assembly simultaneously when mounting the first bearing and the second bearing. Thus, the rotation transmission device can be assembled easily. 
     Each of the first rotation sensor assembly and the second rotation sensor assembly may comprise a magnetic encoder and a Hall IC for detecting changes in magnetic field due to rotation of the magnetic encoder and generating a digital signal. 
     Alternatively, in order to determine whether the rollers are disengaged, a rotation sensor assembly may be provided between the outer race and the inner race for detecting relative rotation between the outer race and the inner race. 
     With this arrangement, when the rollers are supposed to be disengaged due to energization or deenergization of the electromagnet, if the rollers are actually not disengaged due to residual torque, the input shaft and the output shaft rotate at the same speed, so that no rotation signal is generated from the rotation sensor. 
     On the other hand, if the rollers are actually disengaged, a rotation signal is generated from the rotation sensor because the input shaft and the output shaft rotate relative to each other. Thus, depending on whether a signal is being generated from the rotation sensor, it is possible to reliably determine whether or not the rollers have been disengaged. 
     The rotation transmission device may include a bearing supporting the outer race and the inner race so as to be rotatable relative to each other and carrying the rotation sensor assembly. With this arrangement, it is possible to mount the rotation sensor assembly simultaneously when mounting the bearing. Thus, the rotation transmission device can be assembled easily. 
     In order to determine whether the rollers are disengaged, a gap sensor may be provided for detecting the size of the gap between the armature and the rotor. 
     With this arrangement, when the rollers are supposed to be disengaged due to energization or deenergization of the electromagnet, if the rollers are actually not disengaged due to residual torque, a large gap exists between the rotor and the armature, so that no signal is generated from the gap sensor. If the rollers are disengaged, the gap between the rotor and the armature disappears, or only a small gap remains therebetween. Thus, a signal is generated from the gap sensor. 
     Thus, depending on whether a signal is being generated from the gap sensor, it is possible to determine whether or not the rollers have been disengaged. 
     The size of the gap between the armature and the rotor is inversely proportional to the magnetic attraction force of the electromagnetic clutch. The magnetic attraction force of the electromagnetic clutch is proportional to the magnetic flux. Thus, it is possible to determine the size of the gap between the armature and the rotor from changes in magnetic flux. 
     A magnetic flux is ordinarily detectable using a search coil. Thus, a search coil can be used as the gap sensor. In particular, the search coil may be mounted in the electromagnet so that when the armature is pulled to the rotor and the rollers are disengaged, a predetermined electric current is generated from the search coil. With this arrangement, it is possible to determine whether the rollers are disengaged based on the intensity of the current generated from the search coil. 
     ADVANTAGES OF THE INVENTION 
     According to the present invention, when the control retainer is moved in the direction in which the flange of the control retainer moves away from the flange of the rotary flange, the control retainer and the rotary retainer rotate relative to each other in the direction in which the circumferential width of the pockets increases under the biasing force of the presser members, so that the opposed pairs of rollers instantly wedge into the respective narrow ends of the wedge-shaped spaces. This minimizes play in the rotation direction of the rotation transmission device. 
     Since the opposed pairs of rollers are biased away from each other, while being pressed against the outer periphery of the inner race, by the presser members, the rollers never erroneously engage while the two-way roller clutch is idling. This improves reliability of the operation during idling, and minimizes idling torque. 
     Since torque is transmitted between the outer race and the inner race through as many rollers as the number of the cam surfaces, the rotation transmission device has a large torque capacity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional front view of a rotation transmission device embodying the present invention. 
         FIG. 2(I)  is a sectional view taken along line II-II of  FIG. 1 ; and FIG.  2 (II) is a sectional view showing the state in which rollers are in disengagement. 
         FIG. 3  is a partial plan view of a retainer of a two-way roller clutch. 
         FIG. 4  is a sectional view taken along line IV-IV of  FIG. 1 . 
         FIG. 5(I)  is a plan view of a torque cam while in engagement; FIG.  5 (II) is its plan view while in disengagement; and FIG.  5 (III) is a partial enlarged sectional view of  FIG. 5(I) . 
         FIG. 6  is a sectional view of a different presser member. 
         FIG. 7  is a sectional view of a bearing with a rotation sensor assembly. 
         FIG. 8  is a sectional view of a different means for determining whether the rollers are disengaged. 
         FIG. 9  is a sectional view of a still different means for determining whether the rollers are disengaged. 
         FIG. 10  is a vertical sectional front view of a different electromagnetic clutch as an actuator. 
         FIG. 11  is a sectional view of different cam surfaces. 
     
    
    
     BEST MODE FOR EMBODYING THE INVENTION 
     Now the embodiment of the present invention is described with reference to the drawings.  FIG. 1  shows a rotation transmission device embodying the present invention. As shown, the rotation transmission device includes a two-way roller clutch  10 . 
     The two-way roller clutch  10  includes an outer race  11  and an inner race  12  mounted inside the outer race  11 . The inner race  12  has a boss portion  12   a  on which a bearing  13  is fitted. Through the bearing  13 , the outer race  11  and the inner race  12  are rotatable relative to each other. 
     The outer race  11  has a closed end which is formed with an output shaft  14 . An input shaft  15  as a torque transmission shaft has one end thereof inserted in the inner race  12 . The portion of the input shaft  15  inserted in the inner race  11  is formed with serrations  16  through which the inner race  12  and the input shaft  15  are rotationally fixed to each other. 
     As shown in FIGS.  2 (I) and  2 (II), the outer race  11  has a cylindrical surface  17  on its inner periphery, while the inner race  11  has on its outer periphery a plurality of circumferentially equidistantly spaced apart flat cam surfaces  18  each defining a wedge-shaped space which narrows toward both circumferential ends, in cooperation with the cylindrical surface  17 . 
     A control retainer  19 A and a rotary retainer  19 B are mounted between the outer race  11  and the inner race  12 . As shown in  FIGS. 1 and 3 , the control retainer  19 A comprises a flange  20  and as many pillars  21  as the number of the cam surfaces  18  provided on the radially outer portion of the flange  20  so as to be circumferentially equidistantly spaced apart from each other. Similarly, the rotary retainer  19 B comprises a flange  22  and as many pillars  23  as the number of the cam surfaces  19  provided on the radially outer portion of the flange  22  so as to circumferentially equidistantly spaced apart from each other. 
     The rotary retainer  19 B has its flange  22  fitted on the boss portion  12   a  of the inner race  12  and its pillars  23  disposed between the cylindrical surface  17  and the respective cam surfaces  18 , with the flange  22  facing one side surface of the inner race  12 . 
     The control retainer  19 A has its flange  20  fitted on the boss portion  12   a  of the inner race  12  so as to axially face the flange  22  of the rotary retainer  19 B, and its pillars  21  disposed between the respective adjacent pillars  23  of the rotary retainer  19 B. 
     With the retainers  19 A and  19 B mounted in position in this manner, as shown in  FIGS. 2(I) and 3 , a pocket  24  is defined between each pillar  21  of the control retainer  19 A and the corresponding pillar  23  of the rotary retainer  19 B. The pockets  24  radially face the respective cam surfaces  18  of the inner race  12  and each accommodate an opposed pair of rollers  25  and a presser member  26  biasing the pair of rollers  25  away from each other while pressing them against the cam surface  18  of the inner race  12 . 
     The presser member  26  of the embodiment is a leaf spring bent in the shape of the letter W and arranged such that the rollers  25  are obliquely pressed toward the respective circumferential ends of the cam surface  18  by its bent pieces at both ends, respectively. 
     A single presser member  26  is arranged so as to press the longitudinal central portion of each roller  25 . But instead, a plurality of such presser members  26  may be arranged in a plurality of rows in the longitudinal direction of the rollers  25  to prevent skew of the rollers  25 . 
     As shown in  FIG. 1 , the rotary retainer  19 B is rotatable about the boss portion  12   a  of the inner race  12 . Between the flange  22  of the rotary retainer  19 B and the one side surface of the inner race  12 , a thrust needle bearing  27  and an elastic member  28  for biasing the flange  22  of the rotary retainer  19 B toward the flange  20  of the control retainer  19 A are mounted. 
     The elastic member  28  is a coil spring coaxial with the inner race  12 . But instead, a plurality of spring members may be used that are arranged along an imaginary circle of which the center is located on the axis of the inner race  12 . 
     The control retainer  19 A is rotatable about the boss portion  12   a  of the inner member  12 , and is axially movable. 
     As shown in  FIG. 5(I) , torque cams  40  are provided between the flange  20  of the control retainer  19 A and the flange  22  of the rotary retainer  19 B. Each torque cam  40  comprises an opposed pair of cam grooves  41  and  42  which each gradually shallow from the deepest circumferential central portion toward the circumferential ends, and a ball  43  disposed between one and the other circumferential ends of the respective cam grooves  41  and  42 . 
     The cam grooves  41  and  42  shown are arcuate ones. But instead, V-shaped grooves may be used. As shown in FIG.  5 (III), the cam grooves  41  and  42  have, at their circumferential ends, spherical stopper surfaces  44  extending along the outer periphery of the ball  43 . 
     When the control retainer  19 A moves axially in the direction in which its flange  20  moves toward the flange  22  of the rotary flange  22  of the rotary retainer  19 B, the ball  43  of each torque cam  40  rolls toward the deepest portions of the cam grooves  41  and  42  as shown in FIG.  5 (II), thus allowing the control retainer  19 A and the rotary retainer  19 B to rotate relative to each other in the direction in which the circumferential width of the pockets  24  decreases. 
     As shown in  FIGS. 1 ,  3  and  4 , a retaining plate  45  is fixed to the other side surface of the inner race  12 . The retaining plate  45  is an annular plate having a plurality of anti-rotation pieces  46  formed on the radially outer surface thereof and located in the respective pockets  24  defined between the pillars  21  of the control retainer  19 A and the pillars  23  of the rotary retainer  19 B. 
     When the control retainer  19 A and the rotary retainer  19 B rotate relative to each other in the direction in which the circumferential width of the pockets  24  decreases, the pillars  21  of the control retainer  19 A and the pillars  23  of the rotary retainer  19 B are supported by the respective side edges of the plurality of anti-rotation pieces  46 , so that the opposed pairs of rollers  25  are kept in neutral position. 
     As shown in  FIG. 1 , on one axial side of the two-way roller clutch  10 , an electromagnetic clutch  50  as an actuator for axially moving the control solenoid  19 A is provided. 
     The electromagnetic clutch  50  comprises an armature  51  axially facing the end surfaces of the pillars  21  of the control retainer  19 A, a rotor  52  axially facing the armature  51 , and an electromagnet  53  axially facing the rotor  52 . 
     The armature  51  is fitted on and rotatably supported by the input shaft  15 , and is fixedly coupled to the pillars  21  of the control retainer  19 A by tightening bolts  54  threaded into the end surfaces of the respective pillars  21 . 
     The rotor  52  is fitted on the input shaft  15  so as to be axially held in position by a shoulder  15   a  formed on the outer periphery of the input shaft  15  and a snap ring  55  fitted on the outer periphery of the input shaft  15 . The rotor  52  is also rotationally fixed to the input shaft  15 . 
     The electromagnet  53  comprises an electromagnetic coil  53   a  and a core  53   b  supporting the electromagnetic coil  53   a.  The core  53   b  is supported by a stationary member, not shown. 
     Now the operation of the rotation transmission device of the embodiment is described.  FIG. 1  shows the state in which the electromagnetic coil  53   a  of the electromagnet  53  is not energized. Thus in  FIG. 1 , the armature  51  is separated from the rotor  52 . Further in this state, the two-way roller clutch  10  is in engagement, i.e. as shown in  FIG. 2(I) , the opposed pairs of rollers  25  of the two-way roller clutch  10  are in engagement with the cylindrical surface  17  of the outer race  11  and the respective cam surfaces  18  of the inner race  12 . 
     With the two-way roller clutch  10  in engagement, when the electromagnetic coil  53   a  is energized, the armature  51  is moved axially and pulled to the rotor  52  under magnetic attraction force that acts on the armature  51 . 
     Since the armature  51  is fixedly coupled to the pillars  21  of the control retainer  19 A, when the armature  51  is moved axially, the control retainer  19 A is moved in the direction in which its flange  20  moves toward the flange  22  of the rotary retainer  19 B. 
     In this state, as shown in FIG.  5 (II), the ball  43  of each torque cam rolls toward the deepest portions of the cam grooves  41  and  42 , and thus the control retainer  19 A and the rotary retainer  19 B rotate relative to each other in the direction in which the circumferential width of the pockets  24  decreases. Thus, as shown in  FIG. 3 , each opposed pair of rollers  25  are pushed by the pillar  21  of the control retainer  19 A and the pillar  23  of the rotary retainer  19 B, respectively, and disengage as shown in FIG.  2 (II). The two-way roller clutch  10  thus disengages. 
     With the two-way roller clutch  10  disengaged, when torque is applied to the input shaft  15  and the inner race  12  is rotated in one direction, the anti-rotation pieces  46  of the retaining plate  45  press the pillars  21  of the control retainer  19 A or the pillars  23  of the rotary retainer  19 B, thus rotating the control retainer  19 A and the rotary retainer  19 B together with the inner race  12 . In this state, since the opposed pair of rollers  25  are kept in disengaged neutral position, the rotation of the inner race  12  is not transmitted to the outer race  11  and the inner race  12  rotates alone. 
     In this way, when the control retainer  19 A is moved in the direction in which its flange  20  moves toward the flange  22  of the rotary retainer  19 B, the opposed pairs of rollers  25  are pushed by the respective pillars  21  and  23  of the control retainer  19 A and the rotary retainer  19 B, and disengage. In this state, since the opposed pairs of rollers  25  are prevented from being moved into the narrow portions of the respective wedge-shaped spaces by the pillars  21  and  23  of the control retainer  19 A and the rotary retainer  19 B, the rollers  25  never erroneously engage while the two-way roller clutch  10  is idling. 
     Since each opposed pair of rollers  25  are always pressed against the cam surface  18  of the inner race  12  by the presser member  26  comprising a leaf spring in the shape of the letter W, the rollers  25  never move radially outwardly under centrifugal force. 
     If the rollers  25  should move radially outwardly under centrifugal force, the rollers  25  may contact the cylindrical surface  17  of the outer race  11 , which could result in dragging torque acting on the rollers  25 , thereby moving the rollers to engaged position. But in the arrangement of the present invention, since the presser members  26  prevent radially outward movement of the rollers, the rollers  25  never erroneously engage. 
     Since the rollers  25  rotate without contacting the cylindrical surface  17  of the outer race  25 , the rollers never increase idling torque. 
     When the control retainer  19 A and the rotary retainer  19 B rotate relative to each other in the direction in which the circumferential width of the pockets  24  decreases, the pillars  21  of the control retainer  19 A and the pillars  23  of the rotary retainer  19 B abut the respective side edges of the anti-rotation pieces  46  of the retaining plate  45 , thereby restricting the distance of the relative rotation. 
     This prevents the presser members  26  from being compressed more than necessary, thus preventing fatigue breakage of the presser members even after they are repeatedly expanded and compressed. 
     With the inner race  12  idling, when the electromagnetic coil  53   a  is deenergized, the attraction force applied to the armature  51  disappears, so that the armature  51  becomes rotatable, and the control retainer  19 A and the rotary retainer  19 B rotate relative to each other in the direction in which the circumferential width of the pockets  24  increases. Thus, the opposed pairs of rollers  25  instantly wedge into the respective narrow portions of the wedge-shaped spaces, and torque is transmitted between the inner race  12  and the outer race  11  in one direction through one of each opposed pair of rollers  25 . 
     When the input shaft  15  is stopped in this state, and is rotated in the opposite direction, the rotation of the inner race  12  is transmitted to the outer race  11  through the other of each opposed pair of rollers  25 . 
     Since by deenergizing the electromagnetic coil  53   a,  the control retainer  19 A and the rotary retainer  19 B rotate relative to each other in the direction in which the circumferential width of the pockets  24  increases, and the opposed pairs of rollers  25  instantly wedge into the respective narrow portions of the wedge-shaped spaces, it is possible to instantly transmit the rotation of the inner race  12  to the outer race  11  while minimizing play in the rotational direction. 
     Since torque is transmitted from the inner race  12  to the outer race  11  through as many rollers  25  as the number of the cam surfaces  18 , it is possible to transmit large torque from the inner race  12  to the outer race  11 . 
     When the control retainer  19 A and the rotary retainer  19 B rotates relative to each other in the direction in which the circumferential width of the pockets  24  increases, the ball  43  of each torque cam rolls toward shallow ends of the respective opposed pair of cam grooves  41  and  42 , as shown in  FIG. 5(I) . 
     At this time, if the control retainer  19 A and the rotary retainer  19 B rotate relative to each other with their axes inclined to each other, the distances between the cam grooves  41  and  42  of the respective torque cams differ from each other, so that loads applied to the respective balls  43  also differ from each other. In this state, any ball  43  to which load is scarcely or not at all applied may circumferentially come out of the cam grooves  41  and  42  from their shallow portions. If this happens, the two-way roller clutch  10  does not reliably operate any more. 
     But in the arrangement of the present invention, since the elastic member  28  is mounted between the opposed surfaces of the flange  22  of the rotary retainer  19 B and the inner race  12  to bias the flange  22  of the rotary retainer  19 B toward the flange  20  of the control retainer  19 A, the control retainer  19 A and the rotary retainer  19 B are always kept coaxial with each other. 
     Thus, loads are uniformly applied to the respective balls  43 , which prevents separation of the balls  43  while the control retainer  19 A and the rotary retainer  19 B are rotating relative to each other, which in turn allows normal operation of the two-way roller clutch  10  at all times. 
     As shown in FIG.  5 (III), by providing the spherical stopper surfaces at the shallow ends of the cam grooves  41  and  42  so as to extend along the outer periphery of the ball  43 , it is possible to more reliably prevent separation of the ball  43 . 
     The presser member  26  shown in  FIG. 2  comprises a leaf spring in the shape of the letter W. But the presser member  26  is not limited thereto.  FIG. 6  shows a different presser member  26 , which comprises a cylindrical member  29 , a pair of presser elements  30  each having a pin  31  slidably inserted in one end of the cylindrical member  29 , and a coil spring  33  biasing the presser elements  30  in the directions to protrude from the cylindrical member  29 . The presser elements  30  each have a roller pressing surface  32  in the form of an inclined surface that presses the corresponding roller  25  toward the circumferential end of the cam surface  18  of the inner race  12 . 
     When the electromagnetic coil  53   a  is energized in order to disengage the rollers  25  by attracting the armature  51 , if there remains torque between the inner race  12  and the outer race  11 , the residual torque may prevent disengagement of the rollers  25 . 
     This makes it impossible to determine whether the rollers  25  are in engagement or engagement only from the fact that the electromagnetic coil  53   a  of the electromagnetic clutch  50  is energized or deenergized. 
     In order to reliably determine whether or not the rollers are disengaged, in  FIG. 1 , the input shaft  15  is rotatably supported by a first bearing  61  carrying a first rotation sensor assembly S 1 , and the output shaft  14  is rotatably supported by a second bearing  62  carrying a second rotation sensor assembly S 2 . 
     As shown in  FIG. 7 , each of the sensor assemblies S 1  and S 2  comprises a magnetic encoder  64  mounted to the rotary bearing race  63  of the first bearing  61  or the second bearing  62 , and a magnetic sensor  66  mounted to the stationary bearing race  65  of the first bearing  61  or the second bearing  62  for generating a rotation signal due to changes in magnetic flux generated from the magnetic encoder  64  when the encoder  64  rotates. 
     The magnetic sensor  66  of the embodiment is a Hall IC. 
     By rotatably supporting the input shaft  15  with the first bearing  61  carrying the first rotation sensor assembly S 1  and rotatably supporting the output shaft  14  with the second bearing  62  carrying the second rotation sensor assembly S 2 , when the electromagnetic coil  53   a  is energized and thus the rollers  25  are supposed to be disengaged, if the rollers  25  are actually not disengaged due to residual torque, the input shaft  15  and the output shaft  14  rotate at the same speed, so that identical rotation signals are generated from the magnetic sensor  66  of the first rotation sensor assembly S 1  and the magnetic sensor  66  of the second rotation sensor assembly S 2 . 
     On the other hand, if the rollers  25  are actually disengaged, since only the input shaft  15  keeps rotating while the output shaft  14  stops, a rotation signal is generated from the magnetic sensor  66  of the first rotation sensor assembly S 1 , while no rotation signal is generated from the magnetic sensor  66  of the second rotation sensor assembly S 2 . 
     Thus, depending on whether there is a difference between the rotation signal generated from the magnetic sensor  66  of the first rotation sensor assembly S 1  and the rotation signal generated from the magnetic sensor  66  of the second rotation sensor assembly S 2 , it is possible to reliably determine whether or not the rollers  25  have been disengaged. 
     In  FIG. 1 , rotations of the input shaft  15  and the output shaft  14  are detected by rotation sensor assemblies mounted to the respective bearings. But instead, the rotations of the input shaft  14  and the output shaft  15  may be detected using encoders mounted to the input shaft  15  and the output shaft  14 , respectively, and magnetic sensors provided around the respective encoders. 
     As shown in  FIG. 1 , by using the bearings each carrying a sensor assembly to detect the rotations of the input shaft and the output shaft, it is possible to mount the first rotation sensor assembly and the second rotation sensor assembly simultaneously when mounting the first bearing  61  and the second bearing  62 . Thus, the rotation transmission device can be assembled easily. 
       FIG. 8  shows a different determining means for determining whether or not the rollers  25  have been disengaged when the electromagnetic coil  53   a  is energized and the armature  51  is pulled to the rotor  52 , as shown in  FIG. 1 . This means comprises a bearing  13  supporting the outer race  11  and the inner race  12  so as to be rotatable relative to each other. This bearing  13  is the bearing with a rotation sensor assembly shown in  FIG. 7 . Thus, when the outer race  11  and the inner race  12  rotate relative to each other, a relative rotation signal is generated from the magnetic sensor  66  of the rotation sensor. 
     In this arrangement, when the electromagnetic coil  53   a  is energized and the rollers  25  tend to disengage, if the rollers  25  do not actually disengage, no relative rotation signal is generated from the magnetic sensor  66  because the input shaft  15  and the output shaft  14  are rotating at the same speed in this state. 
     On the other hand, if the rollers are actually disengaged, since the input shaft  15  and the output shaft  14  rotate relative to each other, a relative rotation signal is generated from the magnetic sensor  66 . Thus, depending on whether a relative rotation signal is being generated from the magnetic sensor  66 , it is possible to reliably determine whether or not the rollers  25  have been disengaged. 
     In the arrangement of  FIG. 8 , since the magnetic sensor  66  rotates in unison with the outer race  11 , a rotation signal is read from the magnetic sensor  66  using a slip ring. In  FIG. 8 , a bearing carrying a rotation sensor assembly is used to determine whether the rollers  25  are disengaged. But instead, in order to determine whether the rollers  25  are disengaged, an encoder may be mounted to the radially outer surface of the inner race  12  and a magnetic sensor may be mounted to the radially inner surface of the outer race  11 . 
     In the rotation transmission device of  FIG. 1 , when the electromagnetic coil  53   a  is energized to disengage the rollers  25 , due to a magnetic flux a that flows through the armature  51 , rotor  52  and core  53   b , as shown in  FIG. 9 , a magnetic attraction force acts on the armature  51 , thus pulling the armature  51  to the rotor  52 . Thus, a gap g between the armature  51  and the rotor  52  is supposed to disappears, and the rollers  25  are supposed to disengage. 
     But if the rollers  25  are actually not disengaged in this state, a large gap g remains between the rotor  52  and the armature  51 . 
     Thus, it is possible to determine whether the rollers  25  have been disengaged by measuring the size of the gap g between the armature  51  and the rotor  52 . 
     The size of the gap g between the armature  51  and the rotor  51  is inversely proportional to the magnetic attraction force of the electromagnetic clutch  50 . The magnetic attraction force of the electromagnetic clutch  50  is proportional to the magnetic flux. Thus, it is possible to determine the size of the gap g between the armature  51  and the rotor  52  from changes in magnetic flux. 
     A magnetic flux is ordinarily detectable using a search coil. In the arrangement of  FIG. 9 , a search coil  67  is mounted in the core  53   b.  The search coil  67  generates a large electric current when the electromagnetic coil  53   a  is energized and the magnetic flux changes as a result of the armature  51  being pulled to the rotor  52 . 
     Thus, it is possible to determine whether the rollers  25  are disengaged depending on the intensity of the current generated from the search coil  67  mounted in the core  53   b.    
       FIG. 10  shows a different electromagnetic clutch  50  as an actuator. This electromagnetic clutch  50  differs from the electromagnetic clutch  50  shown in  FIG. 1  in that arcuate slits  71  are formed in the surface of the rotor  51  facing the armature  51  and permanent magnets  72  are received in the respective slits  71 . Elements identical or corresponding to the electromagnetic clutch  50  of  FIG. 1  are denoted by identical numerals and their description is omitted. 
     With this electromagnetic clutch  50 , while the electromagnetic coil  53   a  of the electromagnet  53  is not energized, the armature  51  is pulled toward the rotor  52  under the magnetic force of the permanent magnets  72 . When the electromagnetic coil  53   a  is energized, the magnetic force of the permanent magnets  72  is reduced to a level lower than the biasing force of the presser members  26  disposed between the respective opposed pairs of rollers  25 , so that the armature  51  moves away from the rotor  52  under the biasing force of the presser members  26 . 
     When the armature  51  is moved by energizing and deenergizing the electromagnet  53 , the control retainer  19 A, which is fixedly coupled to the armature  51 , is axially moved. When the control retainer  19 A is moved in the direction in which its flange  20  moves toward the flange  22  of the rotary retainer  19 B, the control retainer  19 A and the rotary retainer  19 B rotate relative to each other in the direction in which the circumferential width of the pockets  24  decreases under the action of the torque cams  40 . As a result, the opposed pairs of rollers  25  are pushed by the respective pillars  21  and  23  of the control retainer  19 A and the rotary retainer  19 B and disengage. 
     When the control retainer  19 A is moved in the direction in which its flange  20  moves away from the flange  22  of the rotary retainer  19 B, the control retainer  19 A and the rotary retainer  19 B rotate relative to each other in the direction in which the circumferential width of the pockets  24  increases under the biasing force of the presser members  26 . As a result, the opposed pairs of rollers  25  instantly wedge into the respective narrow ends of the wedge-shaped spaces. 
     In the arrangement of  FIG. 2 , cam surfaces  18  comprising flat surfaces are formed on the inner race  12 . But different cam surfaces  18  may be used. For example, cam surfaces  18  shown in  FIG. 11  may be used, which each comprise two inclined surfaces  18   a  and  18   b  inclined in opposite directions to each other. In this case, each opposed pair of rollers  25  are mounted in the corresponding pocket  24  such that one of the rollers  25  faces the inclined surface  18   a  and the other faces the other inclined surface  18   b.    
     In the arrangement of  FIG. 2 , the cylindrical surface  17  is formed on the inner periphery of the outer race  11  and the cam surfaces  18  are formed on the outer periphery of the inner race  12 . But instead, the cam surfaces may be formed on the inner periphery of the outer race  11  and the cylindrical surface may be formed on the outer periphery of the inner race  12 . 
     DESCRIPTION OF THE NUMERALS 
     
         
           10 . Two-way roller clutch 
           11 . Outer race 
           12 . Inner race 
           14 . Output shaft 
           15 . Input shaft (Torque transmission shaft) 
           17 . Cylindrical surface 
           18 . Cam surface 
           19 A. Control retainer 
           19 B. Rotary retainer 
           20 . Flange 
           21 . Pillar 
           22 . Flange 
           23 . Pillar 
           24 . Pocket 
           25 . Roller 
           26 . Presser member 
           27 . Thrust needle bearing 
           28 . Elastic member 
           29 . Cylindrical member 
           30 . Presser element 
           32 . Roller pressing surface 
           33 . Coil spring 
           40 . Torque cam 
           41 . Cam groove 
           42 . Cam groove 
           43 . Ball 
           44 . Stopper surface 
           45 . Retaining plate 
           46 . Anti-rotation piece 
           50 . Electromagnetic clutch (Actuator) 
           51 . Armature 
           52 . Rotor 
           53 . Electromagnet 
           61 . First bearing 
           62 . Second bearing 
           64 . Electromagnetic encoder 
           66 . Magnetic sensor 
           67 . Search coil 
           71 . Slit 
           72 . Permanent magnet 
         S 1 . First rotation sensor assembly 
         S 2 . Second rotation sensor assembly