Abstract:
The invention provides a power transmission device in which the switching mechanism and control operation can be simplified. Different directions are established for the relative rotation in which the first sprag engages the inner race and the outer race, and the relative rotation in which the second sprag engages the inner and outer races, whereby the inner retainer and the outer retainer are caused to move relative to each other about an axial center O by a load imparting device, and either the first sprag or the second sprag can be made to engage the inner race and the outer race. The combined retention of the first sprag and the second sprag by the inner retainer and the outer retainer actuates the load imparting device to allow the power transmission direction to be switched all at once, resulting in a simpler switching mechanism and control operation.

Description:
This application is a national stage of International Application No. PCT/JP2010/064479 filed Aug. 26, 2010, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a power transmission device capable of switching the transmitting direction of power, and relates more specifically to a power transmission device capable of simplifying a switching mechanism and control. 
     BACKGROUND ART 
     As a power transmission device constituted to include a pair of one-way clutches and capable of switching the transmitting direction of power (power transmission device), for example, one disclosed in Japanese Application Publication No. H10-323257 is known. In the power transmission device disclosed in this publication, relative rotation of an outer ring and a shaft is locked by engaging elements constituting a one-way clutch. The locked state of the relative rotation is released by operating an operation lever and moving an engaging/disengaging operation member that holds the engaging elements, and is switched to an idling state. 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, according to the technology disclosed in Patent Literature 1, the engaging/disengaging operation member and the operation lever were arranged for each one-way clutch, and two engaging/disengaging operation members were arranged side by side between the outer ring and the shaft. Therefore, the operation levers were arranged for respective engaging/disengaging operation members, and in switching the locked state and idling state, it was necessary to operate the operation levers individually and to move the engaging/disengaging operation members individually for respective one-way clutches. When the operation levers were to be operated individually, it was necessary that a pair of one-way clutches did not cause double meshing, and there was a problem that the switching mechanism and control of the operation levers became complicated. 
     The present invention has been made in order to address the problems described above, and its object is to provide a power transmission device capable of simplifying the switching mechanism and control. 
     Solution to Problem and Advantageous Effects of Invention 
     According to the power transmission device of the first aspect, portions on the inner ring side of first sprags and second sprags are retained by an inner retainer, and portions on the outer ring side of the first sprags and the second sprags are retained by an outer retainer. Since the direction of relative rotation of the inner ring and the outer ring with which the first sprags engage with the inner ring and the outer ring and the direction of relative rotation of the inner ring and the outer ring with which the second sprags engage with the inner ring and the outer ring are different from each other, by relatively moving the inner retainer and the outer retainer around the axis by a load application device, the sprags engageable with the inner ring and the outer ring can be switched from the first sprags to the second sprags, and can be switched from the second sprags to the first sprags reversely. 
     Since the inner retainer and the outer retainer retain the first sprags and the second sprags together and the direction of relative rotation of the inner ring and the like with which the first sprags engage and the direction of relative rotation of the inner ring and the like with which the second sprags engage are different from each, other as described above, by relatively moving the inner retainer and the outer retainer around the axis by the load application device, the transmitting direction of the power can be switched together. Thus, there is an effect of simplifying the switching mechanism and control. 
     According to the power transmission device of the second aspect, when a load is applied by the load application device, the inner retainer and the outer retainer are relatively moved around the axis and engagement of at least one of the outer peripheral surface of the inner ring and the inner peripheral surface of the outer ring and the engaging surfaces of the first sprags and the second sprags is released, the first sprags or the second sprags are retained by the inner retainer and the outer retainer so that at least one of the outer peripheral surface of the inner ring and the inner peripheral surface of the outer ring and the engaging surfaces are put in brought into a non-contacting state. As a result, sliding of the engaging surfaces of the sprags over the outer peripheral surface of the inner ring and the inner peripheral surface of the outer ring can be prevented. Thus, in addition to the effect of the first aspect, it is capable of suppressing the dragging torque caused on the engaging surfaces of the sprags. Also, because sliding of the engaging surfaces of the sprags over the outer peripheral surface and the inner peripheral surface is prevented, it is capable of suppressing occurrence of wear, heat generation and the like. 
     Also, since the first sprags or the second sprags whose engagement has been released are retained in a non-contacting state with respect to the outer peripheral surface of the inner ring and the inner peripheral surface of the outer ring, the first sprags or the second sprags can be prevented from unintentionally engaging with the inner ring and the outer ring. Thus, there is an effect of securing surety of switching of the power transmission by relative movement of the inner retainer and the outer retainer. 
     According to the power transmission device of the third aspect, with respect to the first sprags or the second sprags, when the engaging surfaces of one of the first sprags and the second sprags are engaged with the outer peripheral surface of the inner ring and the inner peripheral surface of the outer ring, the engaging surfaces of the other of the first sprags or the second sprags and at least one of the outer peripheral surface of the inner ring and the inner peripheral surface of the outer ring are brought into a non-contacting state. Therefore, when one group of the first sprags and the second sprags is engaged with the inner ring and the outer ring, the other group of the first sprags or the second sprags can be prevented from unintentionally engaging with the inner ring and the outer ring. Thus, in addition to the effect of the first and second aspects, it is possible to achieve an effect that double locking in which both of the first sprags and the second sprags engage with the inner ring and the outer ring can be securely prevented. 
     According to the power transmission device of the fourth aspect, since at least one of the inner retainer and the outer retainer is energized by a first energizing member and the inner retainer and the outer retainer are energized to the opposite direction of the direction of relative movement of the inner retainer and the outer retainer by the load application device, when the load application device is not driven, the relative position of the inner retainer and the outer retainer is restricted by the energizing force generated by the first energizing member. In this connection, because the load application device has to be operated only when the relative position of the inner retainer and the outer retainer is to be changed, the time for driving the load application device can be reduced, and, in addition to the effect of any of the first to third aspects, there is an effect that the energy consumption required for driving the load application device can be suppressed. 
     According to the power transmission device of the fifth aspect, the inner retainer or the outer retainer includes a first retaining section and a second retaining section separated from each other in the axial direction, the first sprags are retained by the first retaining section, and the second sprags are retained by the second retaining section. Also, the first retaining section and the second retaining section are constituted so as to be capable of relatively moving in the circumferential direction, and the first retaining section and the second retaining section are energized in one direction in the circumferential direction by a second energizing member. As a result, by the energizing force of the second energizing member, a first surface and a second surface formed in the first retaining section and the second retaining section abut upon each other, and relative movement of the first retaining section and the second retaining section to one circumferential direction is restricted. As a result, the first retaining section and the second retaining section can be integrally moved by the energizing force of the second energizing member, the first sprags retained by the first retaining section and the second sprags retained by the second retaining section can be tilted by applying a load by the load application device. 
     Here, when one group of the first sprags and the second sprags engage with the inner ring and the outer ring by relative movement in the circumferential direction of the inner retainer and the outer retainer, the other group of the first sprags or the second sprags are retained by the inner retainer and the outer retainer, and engagement with the inner ring and the like has been released. When the inner retainer and the outer retainer are formed into one piece respectively, if one group of the first sprags and the second sprags engaging with the inner ring and the outer ring is tilted so as to be engaged more strongly, the inner retainer and the outer retainer are pushed by the tilting, and the inner retainer and the outer retainer relatively move further. Then, the other group of the first sprags or the second sprags may come off from the inner retainer and the outer retainer, and the inner retainer and the outer retainer may be damaged. 
     On the other hand, according to the power transmission of the fifth aspect, since the first retaining section and the second retaining section are constituted so as to be capable of relatively moving in the circumferential direction, when one group of the first sprags and the second sprags engaging with the inner ring and the outer ring is tilted so as to be engaged more strongly, only one of the first retaining section and the second retaining section relatively moves with respect to the other of the first retaining section or the second retaining section by the tilting. As a result, the other of the first retaining section and the second retaining section can be prevented from being affected, and in addition to the effect of any of the first to fourth aspects, there is an effect that the other group of the first sprags or the second sprags can be prevented from coming off from the inner retainer and the outer retainer and the inner retainer and the outer retainer can be prevented from being damaged. 
     According to the power transmission device of the sixth aspect, when engagement of at least one of the outer peripheral surface of the inner ring and the inner peripheral surface of the outer ring and the engaging surfaces of the first sprags and the second sprags is released by relative movement of the inner retainer and the outer retainer and the first sprags and the second sprags are tilted, the first sprags or the second sprags abut upon each other, and the first sprags and the second sprags are held by the inner retainer and the outer retainer. As a result, further relative movement of the inner retainer and the outer retainer retaining the first sprags and the second sprags is also restricted. Accordingly, it is not required to arrange a positioning member and the like restricting the relative movement amount of the inner retainer and the outer retainer, and in addition to the effect of any of the first to fifth aspects, there is an effect that the constitution of the device can be simplified. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic drawing of a vehicle on which a power transmission device in an embodiment of the present invention is mounted. 
         FIG. 2  is an axial cross-sectional view of the power transmission device. 
         FIG. 3  is a peripheral cross-sectional view of the power transmission device taken from line of  FIG. 2 . 
         FIG. 4  ( a ) is a set of exploded perspective views of a part of the power transmission device, ( b ) is a schematic drawing showing the positional relation of a first cam groove and a second cam groove and an engaging element, and ( c ) is a schematic drawing showing the relation of the reciprocating motion of the engaging element and the relative movement of the first cam groove and the second cam groove. 
         FIG. 5  ( a ) is a perspective view of an essential part of the first retaining section and the second retaining section whose relative movement is restricted, ( b ) is a peripheral cross-sectional view of the first retaining section, and ( c ) is a peripheral cross-sectional view of the second retaining section. 
         FIG. 6  ( a ) is a perspective view of an essential part of the first retaining section and the second retaining section that are relatively moved, ( b ) is a peripheral cross-sectional view of the first retaining section, and ( c ) is a peripheral cross-sectional view of the second retaining section. 
         FIG. 7  ( a ) is a perspective view of an essential part of the first retaining section and the second retaining section whose relative movement is restricted, ( b ) is a peripheral cross-sectional view of the first retaining section, and ( c ) is a peripheral cross-sectional view of the second retaining section. 
         FIG. 8  ( a ) is a perspective view of an essential part of the first retaining section and the second retaining section that are relatively moved, ( b ) is a peripheral cross-sectional view of the first retaining section, and ( c ) is a peripheral cross-sectional view of the second retaining section. 
         FIG. 9  ( a ) is a schematic drawing schematically showing the inner structure of the power transmission device during normal traveling, and ( b ) is a schematic drawing schematically showing the inner structure of the power transmission device in shifting up. 
         FIG. 10  ( a ) is a schematic drawing schematically showing the inner structure of the power transmission device during coast traveling, and ( b ) is a schematic drawing schematically showing the inner structure of the power transmission device in shifting down. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Below, preferred embodiments of the present invention will be described referring to the attached drawings. First, a constitution of a power transmission device  1  in an embodiment of the present invention will be described referring to  FIG. 1  to  FIG. 4 .  FIG. 1  is a schematic drawing of a vehicle  100  on which the power transmission device  1  in an embodiment of the present invention is mounted. 
     As shown in  FIG. 1 , the vehicle  100  mainly includes front wheels  101  and rear wheels  102 , an engine  103  and a motor  104  as a power source, the power transmission device  1  and a transmission  106  transmitting the power of the engine  103  and the motor  104  to the rear wheels  102 , and is constituted so that the rear wheels  102  can be driven by the power of the engine  103  and the motor  104 . A main clutch  105  is disposed in a power transmission route from the engine  103  and the motor  104  to the power transmission device  1 , and is a device for connecting and disconnecting the power transmission route. The transmission  106  is a device for outputting the power of the engine  103  and the motor  104  at a predetermined transmission gear ratio, and output of the transmission  106  is transmitted to the rear wheels  102  via a differential gear  107 . 
     Next, the detailed constitution of the power transmission device  1  will be described referring to  FIG. 2  to  FIG. 4 .  FIG. 2  is an axial cross-sectional view of the power transmission device  1 , and  FIG. 3  is a peripheral cross-sectional view of the power transmission device  1  taken from line of  FIG. 2 . Also, in  FIG. 3 , in order to simplify the drawing, illustration of second sprags  5  (on the back side of the paper surface of  FIG. 3 ) juxtaposed with first sprags  4  is omitted. 
     As shown in  FIG. 2 , the power transmission device  1  is constituted to mainly include an inner ring  2 , an outer ring  3  surrounding the outer periphery of the inner ring  2 , and the first sprags  4  and the second sprags  5  to switch transmission of the power of the outer ring  3  and the inner ring  2 . 
     The inner ring  2  functions to transmit the drive force from the engine  103  (refer to  FIG. 1 ) and the motor  104  to the rear wheels  102 , has an outer peripheral surface  2   a  with a circular cross-sectional shape as shown in  FIG. 2  and  FIG. 3 , and is constituted to be rotatable around the axis O. Also, in the present embodiment, the inner ring  2  is formed into a generally annular shape, and is supported by a center shaft  8  formed into a cylindrical shape via a roller bearing B 1 . In the inner ring  2 , a gear  2   b  rotating around the axis O is extended in the axial direction (to the right in  FIG. 2 ), therefore rotation can be transmitted to the inner ring  2  via the gear  2   b , and rotation of the inner ring  2  is outputted via the gear  2   b.    
     The outer ring  3  functions to transmit the drive force to the rear wheels  102  (refer to  FIG. 1 ) along with the inner ring  2 , has an inner peripheral surface  3   a  of a circular cross-sectional shape opposing the outer peripheral surface  2   a  of the inner ring  2 , and is constituted to be rotatable around the axis O similarly to the inner ring  2 . The outer ring  3  in the present embodiment is formed into a generally annular shape, is supported by a case C via a ball bearing B 2 , and is supported by the center shaft  8  via a roller bearing B 3  as shown in  FIG. 2 . 
     The first sprag  4  functions to restrict relative rotation of the inner ring  2  and the outer ring  3 , has engaging surfaces  4   a ,  4   b  (refer to  FIG. 3 ) respectively contacting the outer peripheral surface  2   a  and the inner peripheral surface  3   a , and a plurality of which are disposed at a regular interval in the circumferential direction in an accommodation space g between the outer peripheral surface  2   a  and the inner peripheral surface  3   a . The first sprag  4  is constituted so that the engaging surfaces  4   a ,  4   b  can engage with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  by relative rotation of the inner ring  2  and the outer ring  3  in one direction. 
     The second sprag  5  functions to restrict relative rotation of the inner ring  2  and the outer ring  3  along with the first sprag  4 , has engaging surfaces  5   a ,  5   b  (refer to  FIG. 5 ) respectively contacting the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and a plurality of which are disposed at a regular interval in the circumferential direction along with the first sprags  4  in the accommodation space g between the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 . The second sprag  5  is constituted so that the engaging surfaces  5   a ,  5   b  can engage with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  by relative rotation of the inner ring  2  and the outer ring  3  in the other direction. 
     An inner retainer  6  is a member of a cylindrical shape in which pockets  6   a ,  6   b  (refer to  FIG. 4  ( a )) are penetratingly formed by plural numbers in the circumferential direction, retains portions on the inner ring  2  side of the first sprags  4  inserted to the pockets  6   a  and the second sprags  5  inserted to the pockets  6   b , and is supported by the axis O side of a first disk section  9 . The first disk section  9  is a member formed into a disk shape crossing the axis O direction, and is disposed on the outer periphery side of the inner ring  2  outside the accommodation space g. 
     A first rotating section  10  has a cylindrical shape coaxial with the inner ring  2 , is extended from the outer periphery of the first disk section  9  in the direction parallel to the axis O, and is constituted to be rotatable integrally with the inner retainer  6 . In the present embodiment, the first rotating section  10  is disposed so as to oppose the outer peripheral surface  2   a  of the inner ring  2 . Thus, the length of the inner ring  2  and the first rotating section  10  in the axis O direction can be shortened, and the power transmission device  1  can be made compact. 
     An outer retainer  7  is a member of a cylindrical shape in which pockets  7   c ,  7   d  (refer to  FIG. 4  ( a )) are penetratingly formed by plural numbers in the circumferential direction, retains portions on the outer ring  3  side of the first sprags  4  inserted to the pockets  7   c  and the second sprags  5  inserted to the pockets  7   d , and is supported by the axis O side of a second disk section  11  that is disposed so as to face the first disk section  9 . The second disk section  11  is a member formed into a disk shape crossing the axis O direction, and is disposed outside the accommodation space g. 
     A second rotating section  12  has a cylindrical shape coaxial with the inner ring  2 , is positioned between the inner peripheral surface  3   a  of the outer ring  3  and the first rotating section  10 , is extended from the outer periphery of the second disk section  11  in the direction parallel to the axis O, and is constituted to be rotatable integrally with the outer retainer  7 . In the first rotating section  10  and the second rotating section  12 , a first cam groove  10   a  and a second cam groove  12   a  are penetratingly bored respectively in the peripheral surface along the axis O direction. In the present embodiment, the first rotating section  10  and the second rotating section  12  have portions overlapping with each other along the axis O direction, and the first cam groove  10   a  and the second cam groove  12   a  are formed in the overlapping portion. 
     A load application device  20  is constituted to include an actuator  21  and a reciprocating section  22 . The actuator  21  is a power source generating the load applied to the inner retainer  6  and the outer retainer  7 , and is fixed to the case C. The reciprocating section  22  is a portion transmitting the power of the actuator  21  to the inner retainer  6  and the outer retainer  7 , is disposed in parallel with the axis O, and is reciprocated in parallel with the axis O by driving the actuator  21 . 
     An annular member  13  is disposed along the peripheral surface  10   b  of the first rotating section  10 , includes a first engaging section  13   a  formed into a shape of a continuous groove in the circumferential direction of the outer peripheral surface, and is constituted to be rotatable along the peripheral surface  10   b  of the first rotating section  10 . The first engaging section  13   a  is a portion with which a second engaging section  22   a  projected toward the axis from the distal end of the reciprocating section  22  that is formed into an arm shape is engaged to be slidable in the circumferential direction. The annular member  13  includes an engaging element  14  projected toward the axis O. The engaging element  14  passes through the first cam groove  10   a  penetratingly bored in the first rotating section  10 , and is engaged with the second cam groove  12   a  penetratingly bored in the second rotating section  12 . In the present embodiment, a groove  3   c  is formed on an outer peripheral surface  3   b  of the outer ring  3  in parallel with the axis O, and the engaging element  14  is engaged with the groove  3   c  at the distal end thereof while passing through the first cam groove  10   a  and the second cam groove  12   a.    
     Since the engaging element  14  engages with the groove  3   c , the first cam groove  10   a  and the second cam groove  12   a  as described above, when the outer ring  3  rotates around the axis O, the engaging element  14  engaging with the groove  3   c  formed on the outer peripheral surface  3   b  of the outer ring  3  and the annular member  13  are rotated around the axis O. In response to the rotation, the first rotating section  10  and the second rotating section  12  in which the engaging element  14  passes through the first cam groove  10   a  and the second cam groove  12   a  are also rotated around the axis O. At the same time, the inner retainer  6  and the outer retainer  7  integrated to the first rotating section  10  and the second rotating section  12  as well as the first sprags  4  and the second sprags  5  retained by them are rotated in the circumferential direction. 
     Next, the power transmission device  1  will be described in more detail referring to  FIG. 4 .  FIG. 4  ( a ) is an exploded perspective view of a part of the power transmission device  1 ,  FIG. 4  ( b ) is a schematic drawing showing the positional relation of the first cam groove  10   a , the second cam groove  12   a  and the engaging element  14 , and  FIG. 4  ( c ) is a schematic drawing showing the relation of the reciprocating motion of the engaging element  14  and the relative movement of the first cam groove  10   a  and the second cam groove  12   a.    
     As shown in  FIG. 4  ( a ), the first cam groove  10   a  and the second cam groove  12   a  are formed on the peripheral surfaces of the first rotating section  10  and the second rotating section  12  in shapes different from each other. More specifically, as shown in  FIG. 4  ( b ), the first cam groove  10   a  and the second cam groove  12   a  cross a straight line L parallel to the axis O, and are formed so as to depart from each other as they depart (downward in  FIG. 4  ( b )) from the actuator  21  (refer to  FIG. 2 ). Thus, when the actuator  21  is driven and the reciprocating section  22  is moved to the direction departing from the actuator  21  (to the right in  FIG. 4  ( a )), the second engaging section  22   a  (refer to  FIG. 2 ) engages with the first engaging section  13   a , the annular member  13  is moved to the direction departing from the actuator  21  along the peripheral surface  10   b  of the first rotating section  10 , and the engaging element  14  projectingly arranged on the annular member  13  moves to the direction departing from the actuator  21  through the first cam groove  10   a  and the second cam groove  12   a.    
     Since the first cam groove  10   a  and the second cam groove  12   a  diagonally cross a straight line L that is parallel to the axis O and are formed so as to be apart from each other as they depart from the actuator  21  (refer to  FIG. 2 ) as described above, when the engaging element  14  moves through the first cam groove  10   a  and the second cam groove  12   a  to the direction departing from the actuator  21  (downward in  FIG. 4  ( b )), the first rotating section  10  and the second rotating section  12  are relatively moved in the circumferential direction around the axis O (the arrow direction shown in  FIG. 4  ( c )) as shown in  FIG. 4  ( c ). When the actuator  21  is driven from this state and the engaging element  14  moves through the first cam groove  10   a  and the second cam groove  12   a  to the direction approaching the actuator  21  (upward in  FIG. 4  ( c )), the first rotating section  10  and the second rotating section  12  are relatively moved to the opposite direction (the direction opposite of the arrow direction shown in  FIG. 4  ( c )). Since the inner retainer  6  and the outer retainer  7  have been connected to the first rotating section  10  and the second rotating section  12 , the inner retainer  6  and the outer retainer  7  are relatively moved accompanying the relative movement of the first rotating section  10  and the second rotating section  12 . 
     Description will be made returning to  FIG. 2 . In the present embodiment, a first energizing member  15  (refer to  FIG. 2  and  FIG. 4  ( a )) is provided which energizes the first disk section  9  and the second disk section  11  to the circumferential direction around the axis O. The first energizing member  15  is constituted of a twisting coil spring of an annular shape, is disposed between the first disk section  9  and the second disk section  11  along the first disk section  9  and the second disk section  11 , and both ends thereof are locked to the first disk section  9  and the second disk section  11  respectively. Also, the first rotating section  10  and the second rotating section  12  are set so as to be energized by the first energizing member  15  to the opposite direction of the relative movement direction when the engaging element  14  moves to the direction departing from the actuator  21 . 
     Thus, when the engaging element  14  is positioned near the actuator  21 , even when the actuator  21  is not driven, the first rotating section  10  and the second rotating section  12  can be maintained at a predetermined position by the energizing force of the first energizing member  15 . Since the load application device  20  only has to be driven when the first rotating section  10  and the second rotating section  12  are relatively moved from this position, energy consumption required for driving the load application device  20  can be suppressed. 
     Also, as shown in  FIG. 4  ( a ), the inner retainer  6  is fixed with the distal end  6   c  thereof being inserted to a locking hole  9   a  formed on the axis side of the first disk section  9 . Thus, assembling of the inner retainer  6  can be facilitated. Also, the outer retainer  7  is constituted to include a first retaining section  7   a  retaining the first sprags  4 , and a second retaining section  7   b  separated from the first retaining section  7   a  in the axial direction and retaining the second sprags  5 . In the first retaining section  7   a  and the second retaining section  7   b , the pockets  7   c ,  7   d  are arrayed at equal intervals in the circumferential direction, and the first sprags  4  and the second sprags  5  are inserted to the pockets  7   c ,  7   d.    
     Next, the constitution and motion of the first retaining section  7   a  and the second retaining section  7   b  will be described referring to  FIG. 5  to  FIG. 8 . First, the constitution of the first retaining section  7   a  and the second retaining section  7   b  will be described referring to  FIG. 5  ( a ).  FIG. 5  ( a ) is a perspective view of an essential part of the first retaining section  7   a  and the second retaining section  7   b  whose relative movement is restricted. Also, in  FIG. 5  to  FIG. 8 , illustration of a part of the first sprags  4  and the second sprags  5  retained by the first retaining section  7   a  and the second retaining section  7   b  is omitted, and the drawing is simplified. 
     As shown in  FIG. 5 , in the first retaining section  7   a , a projection  7   e  is arranged so as to project toward the second retaining section  7   b  on the surface that opposes the second retaining section  7   b . In the projection  7   e , a first surface  7   e   1  is formed on one side in the circumferential direction, and a third surface  7   e   2  is formed on the other side. In the second retaining section  7   b , a recess  7   g  receiving the projection  7   e  is formed on the surface that opposes the first retaining section  7   a . The length in the circumferential direction of the recess  7   g  is set to be longer than the length in the circumferential direction of the projection  7   e . In the recess  7   g , a second surface  7   g   1  capable of abutting upon the first surface  7   e   1  is formed on one side in the circumferential direction, and a fourth surface  7   g   2  capable of abutting upon the third surface  7   e   2  is formed on the other side. Although the first retaining section  7   a  and the second retaining section  7   b  can move relatively within a range the projection  7   e  can move inside the recess  7   g , by abutment of the second surface  7   g   1  of the recess  7   g  upon the first surface  7   e   1  of the projection  7   e , relative movement of the first retaining section  7   a  and the second retaining section  7   b  to one circumferential direction is restricted. Also, by abutment of the fourth surface  7   g   2  of the recess  7   g  upon the third surface  7   e   2  of the projection  7   e , relative movement of the first retaining section  7   a  and the second retaining section  7   b  to the other circumferential direction is restricted. 
     Locking sections  7   f ,  7   h  are formed each of the first retaining section  7   a  and the second retaining section  7   b , and are portions to which both ends of a second energizing member (not shown) are locked. In the present embodiment, the locking sections  7   f ,  7   h  are formed penetratingly from the inner peripheral surface to the outer peripheral surface of the first retaining section  7   a  and the second retaining section  7   b , and the second energizing member is formed of a twisting coil spring. In the second energizing member, both ends are locked to the locking sections  7   f ,  7   h , and annular portions are disposed between the first sprags  4  and the second sprags  5  along the inner peripheral surface of the outer retainer  7 . As a result, the second energizing member energizes the first retaining section  7   a  and the second retaining section  7   b  to one side in the circumferential direction so that the first surface  7   e   1  of the first retaining section  7   a  abuts upon the second surface  7   g   1  of the second retaining section  7   b . When a load is applied to the second retaining section  7   b  by the load application device  20  (refer to  FIG. 2 ) via the second rotating section  12  (refer to  FIG. 4  ( a )) and the second disk section  11 , the first retaining section  7   a  follows the second retaining section  7   b  by the energizing force of the second energizing member (not shown), and the first retaining section  7   a  and the second retaining section  7   b  can be moved integrally as the outer retainer  7 . 
     Next, the motion of the first sprags  4  and the second sprags  5  when the inner retainer  6  and the outer retainer  7  are relatively moved to one circumferential direction will be described referring to  FIG. 5  ( b ) and  FIG. 5  ( c ).  FIG. 5  ( b ) is a peripheral cross-sectional view of the first retaining section  7   a , and  FIG. 5  ( c ) is a peripheral cross-sectional view of the second retaining section  7   b . Also, in the present embodiment, it is to be understood that the inner retainer  6  and the outer retainer  7  are relatively moved to the position shown in  FIG. 5  by the energizing force of the first energizing member  15  (refer to  FIG. 2  and  FIG. 4  ( a )). 
     As shown in  FIG. 5  ( b ), a load of the arrow B direction is applied to each of the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) by the first energizing member  15  described above, the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) are relatively moved, and the first sprags  4  are tilted, the first sprags  4  being inserted to the pockets  6   a ,  7   c  of the inner retainer  6  and the first retaining section  7   a . Thus, the engaging surfaces  4   a ,  4   b  of the first sprags  4  can be made to contact the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and the inner ring  2  and the outer ring  3  can be made a state engageable with the first sprags  4 . Then, the first sprags  4  are positioned at a predetermined interval in the circumferential direction between the outer peripheral surface  2   a  and the inner peripheral surface  3   a  opposing to each other without abutment of an abutting section  4   c  arranged so as to project toward the first sprag  4  positioned adjacently. 
     Next, the motion of the first sprags  4  and the second sprags  5  when the inner retainer  6  and the outer retainer  7  are relatively moved to one circumferential direction will be described referring to  FIG. 5  ( b ) and  FIG. 5  ( c ).  FIG. 5  ( b ) is a peripheral cross-sectional view of the first retaining section  7   a , and  FIG. 5  ( c ) is a peripheral cross-sectional view of the second retaining section  7   b . Also, in the present embodiment, it is assumed that the inner retainer  6  and the outer retainer  7  are relatively moved to the position shown in  FIG. 5  by the energizing force of the first energizing member  15  (refer to  FIG. 2  and  FIG. 4  ( a )). 
     As shown in  FIG. 5  ( b ), a load of the arrow B direction is applied to each of the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) by the first energizing member  15  described above, the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) are relatively moved, and the first sprags  4  inserted in the pockets  6   a ,  7   c  are tilted. Thus, the engaging surfaces  4   a ,  4   b  of the first sprags  4  contact the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and the inner ring  2  and the outer ring  3  become engageable with the first sprags  4 . In this situation, the first sprags  4  are positioned at a predetermined interval in the circumferential direction between the outer peripheral surface  2   a  and the inner peripheral surface  3   a  opposing to each other without abutment of an abutting section  4   c  arranged so as to project toward the first sprag  4  positioned adjacently. 
     Also, since the second retaining section  7   b  moves integrally with the first retaining section  7   a  as described above, when a load of the arrow B direction is applied to each of the inner retainer  6  and the outer retainer  7  (the second retaining section  7   b ) as shown in  FIG. 5  ( b ), the inner retainer  6  and the outer retainer  7  (the second retaining section  7   b ) are relatively moved, and the second sprags  5  inserted in the pockets  6   a ,  7   d  are tilted. Thus, at least one of the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  and the engaging surfaces  5   a ,  5   b  of the second sprags  5  are put in brought into a non-contacting state. As a result, the inner ring  2  and the outer ring  3  and the second sprags  5  become unable to engage with each other. 
     When power is transmitted to the inner ring  2  or the outer ring  3  in a state shown in  FIG. 5  ( b ) and the inner ring  2  rotates clockwise of  FIG. 5  or the outer ring  3  rotates counterclockwise of  FIG. 5 , the engaging surfaces  4   a ,  4   b  of the first sprags  4  engage with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and the power is transmitted via the first sprags  4 . 
     On the other hand, in the second sprags  5 , since at least one of the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  and the engaging surfaces  5   a ,  5   b  of the second sprags  5  are brought into a non-contacting state, the engaging surfaces  5   a ,  5   b  of the second sprags  5  are prevented from sliding against the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 . Thus, generation of the dragging torque in the engaging surfaces  5   a ,  5   b  of the second sprags  5  can be prevented. Also, since the engaging surfaces  5   a ,  5   b  of the second sprags  5  are prevented from sliding against the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , occurrence of wear, heat generation and the like can be suppressed. 
     Also, in a state the first sprags  4  engage with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  (refer to  FIG. 5  ( b )), the relative position of the inner retainer  6  and the outer retainer  7  is determined, and at this time, the engaging surfaces  5   a  of the second sprags  5  are brought into a non-contacting state with respect to the outer peripheral surface  2   a  of the inner ring  2  as shown in  FIG. 5  ( c ). As a result, unintentional engagement of the second sprags  5  with the inner ring  2  and the outer ring  3  can be prevented when the first sprags  4  are engaged with the inner ring  2  and the outer ring  3 . Thus, double locking in which both of the first sprags  4  and the second sprags  5  engage with the inner ring  2  and the outer ring  3  can be securely prevented. 
     Also, when the torque transmitted via the first sprags  4  increases and the inner retainer  6  and the outer retainer  7  are relatively moved to the arrow B direction, as shown in  FIG. 5  ( c ), in the second sprag  5 , an abutting section  5   c  abuts upon the second sprag  5  positioned adjacently, and further tilting is restricted. As a result, the inner retainer  6  and the outer retainer  7  (the second retaining section  7   b ) go into a state of holding the second sprags  5 , and further relative movement of the inner retainer  6  and the outer retainer  7  (the second retaining section  7   b ) is restricted. Accordingly, it is not required to provide a positioning member and the like that restricts the relative movement amount of the inner retainer  6  and the outer retainer  7 , and the constitution of the device can be simplified. 
     Next, the motion of the first retaining section  7   a  and the second retaining section  7   b  when the first sprags  4  strongly engage with the inner ring  2  and the outer ring  3  will be described referring to  FIG. 6 .  FIG. 6  ( a ) is a perspective view of an essential part of the first retaining section  7   a  and the second retaining section  7   b  relatively moved,  FIG. 6  ( b ) is a peripheral cross-sectional view of the first retaining section  7   a , and  FIG. 6  ( c ) is a peripheral cross-sectional view of the second retaining section  7   b.    
     When the inner ring  2  is rotated clockwise of  FIG. 5  or the outer ring  3  is rotated counterclockwise of  FIG. 5  by a large torque in a state shown in  FIG. 5 , as shown in  FIG. 6  ( b ), the engaging surfaces  4   a ,  4   b  of the first sprags  4  are strongly engaged with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and the first sprags  4  are largely tilted. Then, the first retaining section  7   a  is pushed by the tilting and is displaced in the circumferential direction (counterclockwise of  FIG. 6 ). On the other hand, the second sprags  5  cannot engage with the inner ring  2  and the outer ring  3  with rotation of the inner ring  2  clockwise of  FIG. 5  or with rotation of the outer ring  3  counterclockwise of  FIG. 5 . Therefore, the second retaining section  7   b  is not displaced in the circumferential direction since it is not affected by the second sprags  5 . Accordingly, the displacement in the circumferential direction of the first retaining section  7   a  relative to the inner retainer  6  is larger than the displacement in the circumferential direction of the second retaining section  7   b  relative to the inner retainer  6 . 
     Here, when the first retaining section  7   a  and the second retaining section  7   b  (the outer retainer  7 ) are formed integrally as a rigid body, the second retaining section  7   b  is displaced in the circumferential direction in response to the displacement of the first retaining section  7   a . Then, the second sprags  5  may come off from the outer retainer  7 , and the inner retainer  6  and the outer retainer  7  may be damaged. 
     However, since the first retaining section  7   a  and the second retaining section  7   b  are separated from each other in the axial direction and are constituted so as to be relatively movable in the circumferential direction, when the first sprags  4  tilt and the first retaining section  7   a  is pushed, only the first retaining section  7   a  relatively moves with respect to the second retaining section  7   b  until the third surface  7   e   2  (refer to  FIG. 6  ( a )) abuts upon the fourth surface  7   g   2 . As a result, the second retaining section  7   b  (refer to  FIG. 6  ( c )) can be prevented from being affected by the displacement of the first retaining section  7   a , and coming off of the second sprags  5  from the pockets  7   d  of the second retaining section  7   b  and damage of the inner retainer  6  and the second retaining section  7   b  can be prevented. 
     Next, the motion of the first sprags  4  and the second sprags  5  when the inner retainer  6  and the outer retainer  7  are relatively moved to the other circumferential direction (opposite direction of  FIG. 5 ) will be described referring to  FIG. 7 .  FIG. 7  ( a ) is a perspective view of an essential part of the first retaining section  7   a  and the second retaining section  7   b  whose relative movement is restricted,  FIG. 7  ( b ) is a peripheral cross-sectional view of the first retaining section  7   a , and  FIG. 7  ( c ) is a peripheral cross-sectional view of the second retaining section  7   b.    
     When the load application device  20  (refer to  FIG. 2 ) is operated and a load of the arrow R direction is applied to the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) respectively as shown in  FIG. 7  ( b ) resisting the first energizing member  15  (refer to  FIG. 2  and  FIG. 4  ( a )), the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) are relatively moved, and the first sprags  4  inserted to the pockets  6   a ,  7   c  of the inner retainer  6  and the first retaining section  7   a  are tilted. Thus, at least one of the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  and the engaging surfaces  4   a ,  4   b  of the first sprags  4  are put in a non-contacting state. As a result, the inner ring  2  and the outer ring  3  and the first sprags  4  come to be not engageable with each other. 
     Also, since the second retaining section  7   b  moves integrally with the first retaining section  7   a  as described above, when a load of the arrow R direction is applied to each of the inner retainer  6  and the outer retainer  7  (the second retaining section  7   b ) (refer to  FIG. 7  ( c )) by the load application device  20  (refer to  FIG. 2 ), the inner retainer  6  and the outer retainer  7  (the second retaining section  7   b ) are relatively moved, and, as a result, the second sprags  5  inserted to the pockets  6   a ,  7   c  of the inner retainer  6  and the second retaining section  7   b  are tilted. Thus, the engaging surfaces  5   a ,  5   b  of the second sprags  5  contact the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and the inner ring  2  and the outer ring  3  become engageable with the second sprags  5 . In this situation, the second sprags  5  are positioned at a predetermined interval in the circumferential direction between the outer peripheral surface  2   a  and the inner peripheral surface  3   a  opposing to each other without abutment of an abutting section  5   c  arranged so as to project toward the second sprag  5  positioned adjacently. 
     When power is transmitted to the inner ring  2  or the outer ring  3  in a state shown in  FIG. 7  ( c ) and the outer ring  3  rotates clockwise of  FIG. 7  or the inner ring  2  rotates counterclockwise of  FIG. 7 , the engaging surfaces  5   a ,  5   b  of the second sprags  5  engage with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and the power is transmitted via the second sprags  5 . 
     On the other hand, as shown in  FIG. 7  ( b ), in the first sprags  4 , since at least one of the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  and the engaging surfaces  4   a ,  4   b  of the first sprags  4  are brought into a non-contacting state, the engaging surfaces  4   a ,  4   b  of the first sprags  4  are prevented from sliding against the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 . Thus, generation of the dragging torque in the engaging surfaces  4   a ,  4   b  of the first sprags  4  can be suppressed. Also, since the engaging surfaces  4   a ,  4   b  of the first sprags  4  are prevented from sliding against the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , occurrence of wear, heat generation and the like can be suppressed. 
     Also, in a state where the second sprags  5  engage with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  (refer to  FIG. 7  ( c )), the relative position of the inner retainer  6  and the outer retainer  7  is determined, and at this time, the engaging surfaces  4   a  of the first sprags  4  are brought into a non-contacting state with respect to the outer peripheral surface  2   a  of the inner ring  2  as shown in  FIG. 7  ( b ). As a result, unintentional engagement of the first sprags  4  with the inner ring  2  and the outer ring  3  can be prevented when the second sprags  5  are engaged with the inner ring  2  and the outer ring  3 . Thus, double locking in which both of the first sprags  4  and the second sprags  5  engage with the inner ring  2  and the outer ring  3  can be securely prevented. 
     Also, when the torque transmitted via the second sprags  5  increases and the inner retainer  6  and the outer retainer  7  relatively move to the arrow R direction, as shown in  FIG. 7  ( b ), in the first sprag  4 , the abutting section  4   c  abuts upon the first sprag  4  positioned adjacently, and further tilting is restricted. As a result, the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) are brought into a state of holding the first sprags  4 , and further relative movement of the inner retainer  6  and the outer retainer  7  (the first retaining section  7   a ) is restricted. Accordingly, it is not required to arrange a positioning member and the like that restricts the amount of relative movement of the inner retainer  6  and the outer retainer  7 , thus, the constitution of the device can be simplified. 
     Next, the motion of the first retaining section  7   a  and the second retaining section  7   b  when the second sprags  5  strongly engage with the inner ring  2  and the outer ring  3  will be described referring to  FIG. 8 .  FIG. 8  ( a ) is a perspective view of an essential part of the first retaining section  7   a  and the second retaining section  7   b  relatively moved,  FIG. 8  ( b ) is a peripheral cross-sectional view of the first retaining section  7   a , and  FIG. 8  ( c ) is a peripheral cross-sectional view of the second retaining section  7   b.    
     When the inner ring  2  is rotated counterclockwise of  FIG. 7  or the outer ring  3  is rotated clockwise of  FIG. 7  by a large torque in a state shown in  FIG. 7 , as shown in  FIG. 8  ( c ), the engaging surfaces  5   a ,  5   b  of the second sprags  5  are strongly engaged with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and the second sprags  5  are largely tilted. Then, the second retaining section  7   b  is pushed by the tilting and is displaced in the circumferential direction (clockwise of  FIG. 8 ). On the other hand, the first sprags  4  cannot engage with the inner ring  2  and the outer ring  3  with rotation of the inner ring  2  counterclockwise of  FIG. 7  or with rotation of the outer ring  3  clockwise of  FIG. 7 . Therefore, the first retaining section  7   a  is not displaced in the circumferential direction since it is not affected by the first sprags  4 . Accordingly, the displacement in the circumferential direction of the second retaining section  7   b  relative to the inner retainer  6  is larger than the displacement in the circumferential direction of the first retaining section  7   a  relative to the inner retainer  6 . 
     However, since the first retaining section  7   a  and the second retaining section  7   b  are separated from each other in the axial direction and are constituted so as to be relatively movable in the circumferential direction as described above, when the second sprags  5  tilt and the second retaining section  7   b  is pushed, only the second retaining section  7   b  relatively moves with respect to the first retaining section  7   a  until the fourth surface  7   g   2  (refer to  FIG. 8  ( a )) abuts upon the third surface  7   e   2 . As a result, the first retaining section  7   a  (refer to  FIG. 8  ( b )) can be prevented from being affected by the displacement of the second retaining section  7   b , and coming off of the first sprags  4  from the pockets  7   c  of the first retaining section  7   a  and damage of the inner retainer  6  and the outer retainer  7  can be prevented. 
     Next, the motion of the power transmission device  1  mounted on the vehicle  100  (refer to  FIG. 1 ) will be described referring to  FIG. 9  and  FIG. 10 . In the vehicle  100 , when the main clutch  105  is connected, the power of the engine  103  and the motor  104  is inputted from the outer ring  3  of the power transmission device  1 , is outputted from the inner ring  2  to the transmission  106  via the first sprags  4  or the second sprags  5 , and is transmitted to the rear wheels  102 . Also, in  FIG. 9  and  FIG. 10 , in order to facilitate understanding, the transmission route of the power is shown by the arrow P, and the rotational direction and the rotational speed of the inner ring  2  and the outer ring  3  are shown by the direction and length of the arrow T. Further, the direction of the load applied to the inner retainer  6  and the outer retainer  7  by the first energizing member  15  (refer to  FIG. 2  and  FIG. 4  ( a )) of the power transmission device  1  is shown by the arrow B, and the direction of the load applied to the inner retainer  6  and the outer retainer  7  by operation of the load application device  20  (refer to  FIG. 2 ) is shown by the arrow R. Furthermore, the direction of the relative rotation of the inner ring  2  and the outer ring  3  with which the first sprags  4  and the second sprags  5  engage with the inner ring  2  and the outer ring  3  which is the rotational direction of the inner ring  2  in the relative rotation with respect to the outer ring  3  as viewed from the outer ring  3  side is shown by the direction of the arrow Ri, and the direction of the relative rotation of the inner ring  2  and the outer ring  3  with which the first sprags  4  and the second sprags  5  engage with the inner ring  2  and the outer ring  3  which is the rotational direction of the outer ring  3  in the relative rotation with respect to the inner ring  2  as viewed from the inner ring  2  side is shown by the direction of the arrow Ro. 
     The motion of the power transmission device  1  when the vehicle  100  travels normally will be described first, and the motion of the power transmission device  1  in shifting up will be described next.  FIG. 9  ( a ) is a schematic drawing schematically showing the inner structure of the power transmission device  1  during normal traveling, and  FIG. 9  ( b ) is a schematic drawing schematically showing the inner structure of the power transmission device  1  in shifting up. In normal traveling and shifting up, the power application device  20  (refer to  FIG. 2 ) is not operated. Thus, as shown in  FIG. 9  ( a ), a load of the arrow B direction is applied to the inner retainer  6  and the outer retainer  7  by the first energizing member  15  (refer to  FIG. 2  and  FIG. 4  ( a )). As a result, the first sprags  4  contact the inner ring  2  and the outer ring  3  by the inner retainer  6  and the outer retainer  7  relatively moved, and the second sprags  5  are put in a non-contacting state with respect to at least one of the inner ring  2  and the outer ring  3 . 
     In this state, the power is transmitted to the outer ring  3  of the power transmission device  1  from the engine  103  (refer to  FIG. 1 ) and the motor  104  via the main clutch  105 . The rotational direction of the outer ring  3  to which the power has been transmitted (the arrow T direction) is to be the same direction of the direction of the relative rotation of the outer ring  3  (the arrow Ro direction) when the first sprags  4  engage with the inner ring  2  and the outer ring  3  (hereinafter referred to as “lock direction”), and is to be the opposite direction of the direction of the relative rotation of the outer ring  3  (the arrow Ro direction) when the second sprags  5  engage with the inner ring  2  and the outer ring  3  (hereinafter referred to as “free direction”). As a result, the power transmitted to outer ring  3  is transmitted to the inner ring  2  via the first sprags  4 . The power transmitted to the inner ring  2  drives the rear wheels  102  via the transmission  106  (refer to  FIG. 1 ). 
     Next, in shifting up, it is preferable to lower the rotational speed of the outer ring  3  that is on the engine  103  side in order to reduce the shift shock. In the power transmission device  1 , the rotational speed of the outer ring  3  (the engine  103  side) can be lowered using a known means while the main clutch  105  (refer to  FIG. 1 ) is connected. That is, as shown in  FIG. 9  ( b ), when the rotational speed of the outer ring  3  (the arrow T) is made slower than the rotational speed of the inner ring  2  (the arrow T), the rotational direction of the inner ring  2  is the free direction (opposite of the arrow Ri direction) in the first sprags  4  and is the lock direction (the arrow Ri direction) in the second sprags  5  as viewed from the outer ring  3  side in the relative rotation with respect to the outer ring  3 . However, because the second sprags  5  are in a non-contacting state with respect to at least one of the inner ring  2  and the outer ring  3 , the power is not transmitted via the second sprags  5 . Therefore, the rotational speed of the outer ring  3  (the engine  103  side) can be lowered without affecting the rotational speed of the inner ring  2  (the rear wheels  102  side). Thus, since a series of operations can be executed while the main clutch  105  (refer to  FIG. 1 ) is connected, the shift operation time can be shortened. 
     Similarly, in the coast travel (inertial movement) in which the accelerator pedal is opened, as shown in  FIG. 9  ( b ), drop of the rotational speed of the inner ring  2  (the rear wheels  102  side) affected by the inertia of the engine  103  and the like can be prevented. Thus, comfortable coast traveling can be achieved without deceleration of the vehicle  100  (refer to  FIG. 1 ) by engine brake. 
     On the other hand, it is also possible to apply the engine brake to the vehicle  100 . Referring to  FIG. 10  ( a ), the motion of the power transmission device  1  when the vehicle  100  executes coast traveling will be described, and the motion of the power transmission device  1  in shifting down the transmission  106  (refer to  FIG. 1 ) will be described next.  FIG. 10  ( a ) is a schematic drawing schematically showing the inner structure of the power transmission device  1  during coast traveling, and  FIG. 10  ( b ) is a schematic drawing schematically showing the inner structure of the power transmission device  1  in shifting down. 
     In the coast traveling in which the accelerator pedal (not shown) of the vehicle  100  (refer to  FIG. 1 ) is opened and the engine brake is applied, in a state where the main clutch  105  is connected, the load application device  20  (refer to  FIG. 2 ) of the power transmission device  1  is operated, a load of the arrow R direction is applied to the inner retainer  6  and the outer retainer  7  as shown in  FIG. 10  ( a ), and the inner retainer  6  and the outer retainer  7  are relatively moved. As a result, the second sprags  5  contact the inner ring  2  and the outer ring  3  by the inner retainer  6  and the outer retainer  7  which are relatively moved, and the first sprags  4  are put in a non-contacting state with respect to at least one of the inner ring  2  and the outer ring  3 . 
     When the vehicle  100  (refer to  FIG. 1 ) executes coast traveling, while the power is not inputted from the outer ring (the engine  103  side) to the inner ring  2 , the power is inputted from the rear wheels  102  that are rotating to the inner ring  2 . The rotational direction of the inner ring  2  is counterclockwise of  FIG. 10  (the arrow T direction) similarly to  FIG. 9 , is therefore the free direction (opposite of the arrow Ri direction) in the first sprags  4  as viewed from the outer ring  3  side in relative rotation with respect to the outer ring  3 , and is the lock direction (the arrow Ri direction) in the second sprags  5  as shown in  FIG. 10  ( a ). As a result, the power transmitted to the inner ring  2  is transmitted to the outer ring  3  via the second sprags  2 . The power transmitted to the outer ring  3  is transmitted to the engine  103  and the like via the main clutch  105  (refer to  FIG. 1 ). As a result, the rotational speed of the outer ring  3  and the inner ring  2  drops due to the inertia of the engine  103  and the like, which leads to drop of the rotational speed of the rear wheels  102 . That is, the engine brake is applied. 
     Next, in shifting down the vehicle  100  (refer to  FIG. 1 ), it is preferable to increase the rotational speed of the outer ring  3  (the engine  103  side) in order to reduce the shift shock. In the power transmission device  1 , the rotational speed of the outer ring  3  (the engine  103  side) can be increased using a known means while the main clutch  105  is connected. That is, as shown in  FIG. 10  ( b ), when the rotational speed of the outer ring  3  is made faster than the rotational speed of the inner ring  2 , the rotational direction of the outer ring  3  is the lock direction (the arrow Ro direction) in the first sprags  4  and is the free direction (opposite of the arrow Ro direction) in the second sprags  5  as viewed from the inner ring  2  side in relative rotation with respect to the inner ring  2 . However, since the first sprags  4  are in a non-contacting state with respect to at least one of the inner ring  2  and the outer ring  3  due to operation of the load application device  20  (refer to  FIG. 2 ), the power is not transmitted via the first sprags  4 . Therefore, the rotational speed of the outer ring  3  (the engine  103  side) can be increased without affecting the rotational speed of the inner ring  2  (the rear wheels  102  side). Thus, since a series of operations can be executed while the main clutch  105  (refer to  FIG. 1 ) is connected, the shift operation time can be shortened. 
     Although the present invention has been described above based on the embodiments, the present invention is not limited to the embodiments described above by any means, and it can be easily surmised that a variety of improvements and modifications are possible without departing from the objects of the present invention. For example, the figures and the shapes adopted in the embodiments described above are only an example, and it is a matter of course that other figures and shapes can be employed. 
     Although the case in which the power transmission device  1  is mounted on the vehicle  100  has been described in the embodiments, the present invention is not necessarily limited to it, and it is a matter of course that the power transmission device  1  can be incorporated into a traveling apparatus for other vehicles (locomotive, passenger vehicle, cargo vehicle, special vehicle and the like) and a power transmission device of a working apparatus, machine tool and the like for example. Also, when the power transmission device  1  is mounted on the vehicle  100 , it is a matter of course that the power source of the vehicle  100  can be at least one of the engine  103  and the motor  104 . 
     In the embodiments described above, although the case has been described in which the inner retainer  6  and the outer retainer  7  are relatively moved so that the first sprags  4  are engaged with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  by the first energizing member  15 , the present invention is not necessarily limited to it, and it is also possible that the inner retainer  6  and the outer retainer  7  are relatively moved so that the second sprags  5  are engaged with the inner ring  2  and the outer ring  3  by reversing the direction of the energizing force applied by the first energizing member  15 . Further, it is also possible that the rotational movement direction of the inner retainer  6  and the outer retainer  7  is switched alternately by omitting the first energizing member  15  and operating the load application device  20 . 
     In the embodiments described above, although the case has been described in which the outer retainer  7  is divided into the first retaining section  7   a  and the second retaining section  7   b , the present invention is not necessarily limited to it, and it is a matter of course that the outer retainer  7  can be one-piece and the inner retainer  6  can be divided into two of a first retaining section and a second retaining section. 
     In the embodiments described above, although the case was described in which the projection  7   e  was formed in the first retaining section  7   a  (refer to  FIG. 5  ( a )) and the recess  7   g  receiving the projection  7   e  was formed in the second retaining section  7   b , the present invention is not necessarily limited to it, and it is a matter of course that the projection  7   e  can be formed in the second retaining section  7   b  and the recess  7   g  receiving the projection  7   e  can be formed in the first retaining section  7   a.    
     In the embodiments described above, although the case has been described in which the second energizing member (not shown) energizing the first retaining section  7   a  (refer to  FIG. 5  ( a )) and the second retaining section  7   b  is formed of a twisting coil spring and is disposed between the inner peripheral surface of the outer retainer  7  and the outer peripheral surface of the inner retainer  6 , the present invention is not necessarily limited to it. For example, it is also possible that the second energizing member is formed of an elastic body such as a synthetic rubber and the like and a compression spring, and the second energizing member is disposed between the third surface  7   e   2  and the fourth surface  7   g   2 . In this case also, the first retaining section  7   a  and the second retaining section  7   b  can be energized so that the first surface  7   e   1  and the second surface  7   g   1  abut upon each other while the third surface  7   e   2  and the fourth surface  7   g   2  are made depart from each other. 
     In the embodiments described above, although the case has been described in which the first cam groove  10   a  and the second cam groove  12   a  formed in the first rotating section  10  (refer to  FIG. 4  ( a )) and the second rotating section  12  diagonally cross the straight line L parallel to the axis O, the present invention is not necessarily limited to it, and it is also possible that one of the first cam groove  10   a  and the second cam groove  12   a  is formed in parallel with the axis O, and the other is made diagonally cross the straight line L parallel to the axis O. In this case also, the inner retainer  6  and the outer retainer  7  can be relatively moved in response to the reciprocating motion of the engaging element  14 . 
     Although the description was omitted in the embodiments described above, it is possible that an energizing member such as a ribbon spring, garter spring and the like (a third energizing member) is disposed between the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  opposing to each other, and the first sprags  4  and the second sprags  5  are energized so that the respective engaging surfaces  4   a ,  4   b ,  5   a ,  5   b  of the first sprags  4  and the second sprags  5  can engage with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 . When the third energizing member is not arranged, by reducing the dimensional tolerance of the pockets  6   a ,  7   c ,  7   d  of the inner retainer  6  and the outer retainer  7 , the first sprags  4  and the second sprags  5  and improving the dimensional accuracy, the first sprags  4  and the second sprags  5  can be made to contact the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 . However, by arranging the third energizing member, even when the dimensional tolerances of them are large, the engaging surfaces  4   a ,  4   b ,  5   a ,  5   b  of the first sprags  4  and the second sprags  5  can be made to securely contact the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  by the energizing force of the third energizing member, and the certainty of the power transmission can be improved. 
     Also, by reversing the direction of energization by the third energizing member, the first sprags  4  and the second sprags  5  can be energized so that the respective engaging surfaces  4   a ,  4   b ,  5   a ,  5   b  of the first sprags  4  and the second sprags  5  depart from the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 . Thus, the first sprags  4  and the second sprags  5  whose engagement with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  has been released can be prevented from unintentionally contacting the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 , and transmission of power can be securely blocked. 
     In the embodiments described above, the case has been described in which the engaging surfaces  4   a ,  5   a  of the first sprags  4  or the second sprags  5  are brought into a non-contacting state with respect to the outer peripheral surface  2   a  of the inner ring  2  when one group of the first sprags  4  and the second sprags  5  is engaged with the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3  (refer to  FIG. 5  ( c ),  FIG. 7  ( b )). However, the present invention is not necessarily limited to it, and other configurations are also possible depending on the shape of the first sprags  4  and the second sprags  5  and the like. As other configurations, there are cases that the engaging surfaces  4   b ,  5   b  of the first sprags  4  or the second sprags  5  are brought into a non-contacting state with respect to the inner peripheral surface  3   a  of the outer ring  3 , and that the engaging surfaces  4   a ,  4   b ,  5   a ,  5   b  of the first sprags  4  and the second sprags  5  are brought into a non-contacting state with respect to both of the outer peripheral surface  2   a  of the inner ring  2  and the inner peripheral surface  3   a  of the outer ring  3 . In these cases also, double locking in which both of the first sprags  4  and the second sprags  5  engage with the inner ring  2  and the outer ring  3  can be securely prevented. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  . . . power transmission device 
               2  . . . inner ring 
               2   a  . . . outer peripheral surface 
               3  . . . outer ring 
               3   a  . . . inner peripheral surface 
               4  . . . first sprag 
               4   a ,  4   b  . . . engaging surface 
               5  . . . second sprag 
               5   a ,  5   b  . . . engaging surface 
               6  . . . inner retainer 
               7  . . . outer retainer 
               7   a  . . . first retaining section 
               7   b  . . . second retaining section 
               7   e   1  . . . first surface 
               7   g   1  . . . second surface 
               15  . . . first energizing member 
               20  . . . load application device 
             O . . . axis