Abstract:
A power transmission apparatus for an electric bending endoscope includes an actuating member switchable between a connection position to bring a clutch mechanism into a connection state and a release position to bring the clutch mechanism into a release state, the actuating member being interlocked with the clutch mechanism, an electric drive mechanism being interlocked with the actuating member, and a manual drive mechanism including an operation member manually switchable to at least one of a connection drive position to bring the actuating member into the connection position and a release drive position to bring the actuating member into the release position, and a selective actuation transmission mechanism provided between the operation member and the actuating member and to transmit the actuation of the operation member to the actuating member and absorb the actuation of the actuating member without transmitting the actuation to the operation member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2007-160502, filed Jun. 18, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power transmission apparatus for an electric bending endoscope whose bending portion is to be electrically operated to be bent. 
     2. Description of the Related Art 
     In an electric bending endoscope, a bending portion to be operated to be bent is provided at the distal end portion of an elongate insertion portion to be inserted into a body cavity, and an operation portion to be held and operated by an operator is coupled to the proximal end portion of the insertion portion. The operation portion is provided with a bending switch and includes a motor and an angle mechanism therein, and an angle wire extending out of the angle mechanism is inserted through the insertion portion, and coupled to the distal end portion of the bending portion. When the bending switch is operated, the motor actuate the angle mechanism to move the angle wire back and forth, and so the bending portion is operated to be bent. Here, a clutch mechanism is provided between the motor and the angle mechanism. When the clutch mechanism is operated, the motor is separated from the angle mechanism, and the angle mechanism is free, and so the bending portion becomes linear easily. As such a clutch mechanism, Jpn. Pat. Appln. KOKAI Publication No. 5-95896 has disclosed a clutch mechanism to be actuated by manually operating an operation lever, and Jpn. Pat. Appln. KOKAI Publication No. 2003-275168 has disclosed a clutch mechanism to be actuated by an electric drive mechanism. 
     BRIEF SUMMARY OF THE INVENTION 
     In an aspect of the present invention, a power transmission apparatus for an electric bending endoscope includes: a power transmission mechanism to transmit power; a clutch mechanism switchable between a connection state to permit the transmission of the power by the power transmission mechanism and a release state not to permit the transmission of the power by the power transmission mechanism; an actuating member switchable between a connection position to bring the clutch mechanism into the connection state and a release position to bring the clutch mechanism into the release state, the actuating member being interlocked with the clutch mechanism; an electric drive mechanism electrically switchable between a connection drive state to bring the actuating member into the connection position and a release drive state to bring the actuating member into the release position, the electric drive mechanism being interlocked with the actuating member; and a manual drive mechanism including an operation member manually switchable to at least one of a connection drive position to bring the actuating member into the connection position and a release drive position to bring the actuating member into the release position, and a selective actuation transmission mechanism provided between the operation member and the actuating member and to transmit the actuation of the operation member to the actuating member and absorb the actuation of the actuating member without transmitting the actuation to the operation member. 
     In an aspect of the present invention, an electric bending endoscope includes a power transmission apparatus, the power transmission apparatus including: a power transmission mechanism to transmit power; a clutch mechanism switchable between a connection state to permit the transmission of the power by the power transmission mechanism and a release state not to permit the transmission of the power by the power transmission mechanism; an actuating member switchable between a connection position to bring the clutch mechanism into the connection state and a release position to bring the clutch mechanism into the release state, the actuating member being interlocked with the clutch mechanism; an electric drive mechanism electrically switchable between a connection drive state to bring the actuating member into the connection position and a release drive state to bring the actuating member into the release position, the electric drive mechanism being interlocked with the actuating member; and a manual drive mechanism including an operation member manually switchable to at least one of a connection drive position to bring the actuating member into the connection position and a release drive position to bring the actuating member into the release position, and a selective actuation transmission mechanism provided between the operation member and the actuating member and to transmit the actuation of the operation member to the actuating member and absorb the actuation of the actuating member without transmitting the actuation to the operation member. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a perspective view showing an endoscope system in a first embodiment of the present invention; 
         FIG. 2  is a schematic diagram showing a power transmission apparatus in the first embodiment of the present invention; 
         FIG. 3  is a schematic diagram showing the power transmission apparatus in the first embodiment of the present invention along the III-III line in  FIG. 2 ; 
         FIG. 4A  is a schematic diagram showing a clutch mechanism in a connection state in the first embodiment of the present invention; 
         FIG. 4B  is a schematic diagram showing the clutch mechanism in a release state in the first embodiment of the present invention; 
         FIG. 5  is a perspective view showing a pair of clutch mechanisms in the first embodiment of the present invention; 
         FIG. 6  is a side view showing an electric drive mechanism in the first embodiment of the present invention; 
         FIG. 7  is a top view showing the electric drive mechanism in the first embodiment of the present invention; 
         FIG. 8  is a side view showing a selective actuation transmission mechanism of a manual drive mechanism in the first embodiment of the present invention; 
         FIG. 9  is an exploded perspective view showing a manual lever of the manual drive mechanism in the first embodiment of the present invention; 
         FIG. 10  is a sectional view showing the manual lever of the manual drive mechanism in the first embodiment of the present invention; 
         FIG. 11  is a perspective view showing a motor unit in the first embodiment of the present invention; 
         FIG. 12A  is a schematic diagram showing the connection state of the clutch mechanism by the electric drive mechanism in the power transmission apparatus in the first embodiment of the present invention; 
         FIG. 12B  is a schematic diagram showing the release state of the clutch mechanism by the electric drive mechanism in the power transmission apparatus in the first embodiment of the present invention; 
         FIG. 13A  is a schematic diagram showing the state of the clutch mechanism before released by the manual drive mechanism in the power transmission apparatus in the first embodiment of the present invention; 
         FIG. 13B  is a schematic diagram showing the release state of the clutch mechanism by the manual drive mechanism in the power transmission apparatus in the first embodiment of the present invention; 
         FIG. 13C  is a schematic diagram showing the state of the clutch mechanism before connected by the manual drive mechanism in the power transmission apparatus in the first embodiment of the present invention; 
         FIG. 13D  is a schematic diagram showing the connection state of the clutch mechanism by the manual drive mechanism in the power transmission apparatus in the first embodiment of the present invention; 
         FIG. 14  is a side view showing a power transmission mechanism in a second embodiment of the present invention; and 
         FIG. 15  is a sectional view showing a power transmission mechanism in the second embodiment of the present invention along the XV-XV line in  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will hereinafter be described with reference to the drawings. 
       FIGS. 1 to 13D  show a first embodiment of the present invention. 
     The schematic configuration of an endoscope system is explained with reference to  FIG. 1 . 
     An electric bending endoscope  20  (hereinafter simply referred to as an endoscope  20 ) of the endoscope system includes an elongate insertion portion  22  to be inserted into a body cavity. A bending portion  24  to be bent in four directions, that is, in up, down, left and right directions is provided at the distal end portion of the insertion portion  22 , and an insertion and removal portion  26  is provided at the proximal end portion of the insertion portion  22 . Here, the insertion and removal portion  26  includes an angle mechanism therein, and an angle wire extending out of the angle mechanism is inserted through the insertion portion  22 , and coupled to the distal end portion of the bending portion  24 . Further, the insertion and removal portion  26  is removably inserted into a motor unit  28 , and a drive apparatus for actuating the angle mechanism is provided in the motor unit  28 . As described later, a clutch mechanism is provided in a power transmission apparatus of the drive apparatus. The motor unit  28  is held by a holding apparatus  30  such that the motor unit  28  is movable and fixable, and rotatable about its central axis. Moreover, the motor unit  28  is connected to a video processor  34  via a universal cord  32 , and an operation portion  38  to be held and operated by an operator is connected to the video processor  34  via an electric cord  36 . The operation portion  38  is provided with a bending switch  40  and a changeover switch  42 . When the bending switch  40  is operated, the angle mechanism is actuated by the drive apparatus of the motor unit  28 , the angle wire is moved back and forth, and so the bending portion  24  is bent. When the changeover switch  42  is operated, the clutch mechanism of the power transmission apparatus of the drive apparatus is switched. 
     The drive apparatuses  44   a ,  44   b  of the motor unit  28  are explained with reference to  FIGS. 2 to 11 . 
     Referring to  FIG. 9 , in the motor unit  28 , a rectangular cylindrical frame  60  extends along the central axis of the motor unit  28 , and the first and second drive apparatuses  44   a ,  44   b  are provided outside two opposite sidewalls of the frame  60 , respectively. It is to be noted that in the present specification, the first drive apparatus and its components are indicated with a reference mark Xa, while the second drive apparatus and its components are indicated with a reference mark Xb. One of the drive apparatuses is used for up-down direction bending operation, and the other drive apparatus is used for left-right direction bending operation. The two drive apparatuses  44   a ,  44   b  include the same configuration, and are arranged in rotational symmetry with respect to the central axis of the motor unit  28 . 
     A power transmission apparatus  46   a ,  46   b  of the drive apparatus  44   a ,  44   b  is formed of a power transmission mechanism  48   a ,  48   b , a clutch mechanism  50   a ,  50   b , a drive pin  52   a ,  52   b  as actuating member, a electric drive mechanism  54   a ,  54   b , and a manual drive mechanism  56   a ,  56   b , which will be sequentially explained below. 
     The power transmission mechanism  48   a ,  48   b  is explained with reference to  FIGS. 2 and 3 . 
     A drive shaft of motor  58   a ,  58   b  is connected to an output shaft  86   a ,  86   b  at a reduction ratio via a gear train in a gear unit  59   a ,  59   b . Here, a fixing gear  80   a ,  80   b  as an annular gear are interposed between the gear train. The fixing gear  80   a ,  80   b  is switchable between a fixing state unrotatable on its central axis and a fixing-released state rotatable. In the case where the fixing gear  80   a ,  80   b  is in the fixing state, when the drive shaft of the motor  58   a ,  58   b  is rotated, the gear train is sequentially rotated and so the output shaft  86   a ,  86   b  is rotated at a reduced rotation velocity. On the other hand, in the case where the fixing gear  80   a ,  80   b  is in the fixing-released state, even when the drive shaft of the motor  58   a ,  58   b  are rotated, the gear train idle and rotation torque is not transmitted to the output shaft  86   a ,  86   b.    
     The clutch mechanism  50   a ,  50   b  of the power transmission mechanism  48   a ,  48   b  is explained with reference to  FIGS. 2 to 5 . 
     The outer peripheral portion of the fixing gear  80   a ,  80   b  form spur gear, and external teeth are formed on the fixing gear  80   a ,  80   b . Cam  88   a ,  88   b  is provided axially outside the fixing gear  80   a ,  80   b  and rotatable about a rotational axis coaxial with the central axis of the fixing gear  80   a ,  80   b  between a connection position and a release position. Cam grooves  90   a ,  90   b  extend in rotational symmetry at both ends of the cam  88   a ,  88   b . A cam pin  92   a ,  92   b  are slidably inserted into the cam groove  90   a ,  90   b , and project from limitation member  94   a ,  94   b . The limitation member  94   a ,  94   b  is unrotatable in a circumferential direction of the central axis of the fixing gear  80   a ,  80   b  and slidable in a radial direction thereof with respect to the fixing gear  80   a ,  80   b . By the interaction between the cam groove  90   a ,  90   b  and the cam pin  92   a ,  92   b , the limitation member  94   a ,  94   b  is disposed at a radially inward connection position when the cam  88   a ,  88   b  is disposed at the connection position while the limitation member  94   a ,  94   b  is disposed at a radially outward release position when the cam  88   a ,  88   b  is disposed at the release position. Teeth to gear with the external teeth of the fixing gear  80   a ,  80   b  are formed in the limitation member  94   a ,  94   b . When the limitation member  94   a ,  94   b  is at the connection position, the teeth of the limitation member  94   a ,  94   b  gear with the external teeth of the fixing gear  80   a ,  80   b , and the fixing gear  80   a ,  80   b  is at the unrotatable fixed state by the circumferentially unrotatable limitation member  94   a ,  94   b . On the other hand, when the limitation member  94   a ,  94   b  is at the release position, the teeth of the limitation member  94   a ,  94   b  is separated from the external teeth of the fixing gear  80   a ,  80   b , and the fixing gear  80   a ,  80   b  is at the rotatable fixing-released state. When the fixing gear  80   a ,  80   b  is in the fixing state, the transmission of power by the power transmission mechanism  48   a ,  48   b  is possible. This is the connection state of the clutch mechanism  50   a ,  50   b  (see  FIG. 4A ). When the fixing gear  80   a ,  80   b  is in the fixing-released state, the respective gear idles, and so the transmission of power by the power transmission mechanism  48   a ,  48   b  is impossible. This is the release state of the clutch mechanism  50   a ,  50   b  (see  FIG. 4B ). 
     The first cam  88   a  of the first clutch mechanism  50   a  and the second cam  88   b  of the second clutch mechanism  50   b  are coupled to each other by a coupling beam  96  as a coupling mechanism extending to traverse the frame  60 . Owing to the coupling beam  96 , the second cam  88   b  is also disposed at the connection position when the first cam  88   a  is disposed at the connection position, and the second cam  88   b  is also disposed at the release position when the first cam  88   a  is disposed at the release position. Thus, the first and second clutch mechanisms  50   a ,  50   b  are interlocked with each other. 
     The drive pin  52   a ,  52   b  as the actuating member to be interlocked with the clutch mechanism  50   a ,  50   b  is explained with reference to  FIGS. 4A to 5 . 
     The drive pin  52   a ,  52   b  and limitation pin  98   a ,  98   b  protrude axially outwardly from the cam  88   a ,  88   b , and are arranged in symmetry with respect to the rotational axis on the center line of the cam  88   a ,  88   b . The first drive pin  52   a  is disposed in proximity to the coupling beam  96  in the first cam  88   a , while the second drive pin  52   b  is disposed separately from the coupling beam  96  in the second cam  88   b . The drive pin  52   a ,  52   b  and the clutch mechanism  50   a ,  50   b  are interlocked with each other. That is, the drive pin  52   a ,  52   b  is switched between a connection position and a release position so that the clutch mechanism  50   a ,  50   b  is switched between the connection state and the release state, and the clutch mechanism  50   a ,  50   b  is switched between the connection state and the release state so that the drive pin  52   a ,  52   b  is switched between the connection position and the release position. 
     The electric drive mechanism  54   a ,  54   b  for electrically actuating the clutch mechanism  50   a ,  50   b  is explained with reference to  FIGS. 5 to 7 . 
     In the electric drive mechanism  54   a ,  54   b , a solenoid  100   a ,  100   b  is arranged on the proximal side of the clutch mechanism  50   a ,  50   b  in the direction of the central axis of the motor unit  28 . One end of sliding member  104   a ,  104   b  is coupled to an iron core  102   a ,  102   b  of the solenoid  100   a ,  100   b , and the iron core  102   a ,  102   b  and the sliding member  104   a ,  104   b  is movable back and forth along guide pin  106   a ,  106   b . At the other end of the sliding member  104   a ,  104   b , the drive pin  52   a ,  52   b  is coupled to the sliding member  104   a ,  104   b  rotatably about its central axis. The direction of the back-and-forth movement of the iron core  102   a ,  102   b  and the sliding member  104   a ,  104   b  and the rotating direction of the drive pin  52   a ,  52   b  substantially coincide with each other and are substantially parallel with the direction of the central axis of the motor unit  28 . When the iron core  102   a ,  102   b  and the sliding member  104   a ,  104   b  is moved back and forth by the solenoid  100   a ,  100   b , the drive pin  52   a ,  52   b  is rotated and so the clutch mechanism  50   a ,  50   b  is actuated. 
     The solenoid  100   a ,  100   b  take an attraction state and a reset state, and can only be actuated from the attraction state to the reset state, and the solenoid  100   a ,  100   b  is a self-holding type such that the attraction state is a hold state and the reset state is an open state when electricity is turned off. When the solenoid  100   a ,  100   b  is in the attraction state or the reset state, the sliding member  104   a ,  104   b  is disposed at a backward position or a forward position. Here, the first drive pin  52   a  and the second drive pin  52   b  are disposed in rotational symmetry to each other with respect to a common rotational axis, and so the second solenoid  100   b  is in the reset state or attraction state when the first solenoid  100   a  is in the attraction state or the reset state so that the second sliding member  104   b  is disposed at the forward position or backward position when the first sliding member  104   a  is at the backward position or forward position. 
     When the second solenoid  100   b  is held in the attraction state, the second sliding member  104   b  is held at the backward position, the second drive pin  52   b  is held at the connection position, the second clutch mechanism  50   b  is held in the connection state, and the first clutch mechanism  50   a  is held in the connection state via the coupling beam  96 . This is the connection drive state of the electric drive mechanism  54   a ,  54   b . On the other hand, when the first solenoid  100   a  is in the attraction state, the first sliding member  104   a  is held at the backward position, the first drive pin  52   a  is held at the release position, the clutch mechanism  50   a  is held at the release state, and the second clutch mechanism  50   b  is held in the release state via the coupling beam  96 . This is the release drive state of the electric drive mechanism  54   a ,  54   b.    
     The manual drive mechanism  56   a ,  56   b  for manually actuating the clutch mechanism  50   a ,  50   b  is explained with reference to  FIGS. 8 to 11 . 
     Referring to  FIG. 8 , a selector  108   a ,  108   b  as a selection member are provided axially outside the cam  88   a ,  88   b . The selector  108   a ,  108   b  is rotatable about a rotational axis coaxial with the rotational axis of the cam  88   a ,  88   b  with reference to a neutral position between a connection position and a release position. A pair of play portions  110   a ,  110   b  in the shape of notched grooves is formed in the selector  108   a ,  108   b . When the selector  108   a ,  108   b  is at the neutral position, the drive pin  52   a ,  52   b  and the limitation pin  98   a ,  98   b  of the cam  88   a ,  88   b  are rotatable in the pair of play portions  110   a ,  110   b  without interfering with the selector  108   a ,  108   b . When the selector  108   a ,  108   b  rotates from the neutral position, the drive pin  52   a ,  52   b  and the limitation pin  98   a ,  98   b  are rotated by the selectors  108   a ,  108   b , and the drive pin  52   a ,  52   b  is disposed at the connection position When the selector  108   a ,  108   b  is at the connection position and the drive pin  52   a ,  52   b  is disposed at the release position When the selector  108   a ,  108   b  is at the release position. An urging member  112   a ,  112   b  is provided side by side with the selector  108   a ,  108   b . The urging member  112   a ,  112   b  urge the selector  108   a ,  108   b  by the elastic force of an elastic member  114   a ,  114   b  and thus hold the selector  108   a ,  108   b  at the neutral position. Moreover, an engaging portion  116   a ,  116   b  is formed at the end of the urging member  112   a ,  112   b  on the side of the selector  108   a ,  108   b  and the engaging portion  116   a ,  116   b  engages with and hold the selector  108   a ,  108   b  when the selector  108   a ,  108   b  is disposed at the release position. 
     Referring to  FIGS. 9 and 10 , a manual lever  118   a ,  118   b  as an operation member is coupled to the selector  108   a ,  108   b . That is, a lever shaft  120   a ,  120   b  is axially outwardly coupled to the selector  108   a ,  108   b  by screws coaxially with the selector  108   a ,  108   b . An annular shaft guide  122   a ,  122   b  is fitted coaxially outside the lever shaft  120   a ,  120   b . Here, an annular lever bearing  124   a ,  124   b  is provided coaxially with the lever shaft  120   a ,  120   b  in a housing  117  of the motor unit  28 , and the shaft guide  122   a ,  122   b  is fitted coaxially into the lever bearing  124   a ,  124   b . O-rings  125   a ,  125   b  are interposed between the lever shaft  120   a ,  120   b  and the shaft guide  122   a ,  122   b  and between the shaft guide  122   a ,  122   b  and the lever bearing  124   a ,  124   b , respectively. A clamp member  126   a ,  126   b  is fitted outside the lever shaft  120   a ,  120   b  axially outside the shaft guide  122   a ,  122   b . The proximal end portion of a lever member  128   a ,  128   b  is coupled by a screw to the lever shaft  120   a ,  120   b  and the clamp member  126   a ,  126   b , and the lever member  128   a ,  128   b  extend perpendicularly to the lever shaft  120   a ,  120   b . A lever cover  130   a ,  130   b  covers the lever member  128   a ,  128   b . In addition, a claw portion  132   a ,  132   b  is formed at the proximal end portion of the lever cover  130   a ,  130   b , and the claw portion  132   a ,  132   b  are engaged with the clamp member  126   a ,  126   b  and so the proximal end portion of the lever cover  130   a ,  130   b  is fixed to the clamp member  126   a ,  126   b . On the other hand, the terminal end of the lever cover  130   a ,  130   b  is fixed to the terminal end of the lever member  128   a ,  128   b  by a screw. 
     The manual lever  118   a ,  118   b  is rotatable with reference to a neutral position between a connection drive position and a release drive position. When the manual lever  118   a ,  118   b  is disposed at the connection position, release position or neutral position, the selector  108   a ,  108   b  is disposed at the connection position, release position or neutral position. 
     Referring to  FIG. 11 , the pair of manual levers  118   a ,  118   b  is arranged in rotational symmetry with respect to the central axis of the motor unit  28  on both sides of the motor unit  28 . 
     Next, the actuation of a selective actuation transmission mechanism of the power transmission apparatus  46   a ,  46   b  will be described. 
     Referring to  FIGS. 12A and 12B , the switching of the first and second clutch mechanisms  50   a ,  50   b  by the electric drive mechanism  54   a ,  54   b  are explained. 
     When the manual lever  118   a ,  118   b  is not operated, the manual lever  118   a ,  118   b  and the selector  108   a ,  108   b  are held at the neutral position by the urging member  112   a ,  112   b.    
     When the changeover switch  42  of the operation portion  38  is operated and so a signal for switching from the connection state to the release state is input to the electric drive mechanism  54   a ,  54   b , the second solenoid  100   b  is switched from the attraction state to the reset state. As a result, from the state shown in  FIG. 12A  to the state shown in  FIG. 12B , the second sliding member  104   b  is switched from the backward position to the forward position, the second drive pin  52   b  is switched from the connection position to the release position, the second clutch mechanism  50   b  is switched from the connection state to the release state, the first clutch mechanism  50   a  is switched from the connection state to the release state via the coupling beam  96 , the first drive pin  52   a  is switched from the connection position to the release position, the first sliding member  104   a  is switched from the forward position to the backward position, and the first solenoid  100   a  is switched from the reset state to the attraction state. 
     On the other hand, when the changeover switch  42  of the operation portion  38  is operated and so a signal for switching from the release state to the connection state is input to the electric drive mechanism  54   a ,  54   b , the first solenoid  100   a  is switched from the attraction state to the reset state. As a result, from the state shown in  FIG. 12B  to the state shown in  FIG. 12A , the first sliding member  104   a  is switched from the backward position to the forward position, the first drive pin  52   a  is switched from the release position to the connection position, the first clutch mechanism  50   a  is switched from the release state to the connection state, the second clutch mechanism  50   b  is switched from the release state to the connection state via the coupling beam  96 , the second drive pin  52   b  is switched from the release position to the connection position, the second sliding member  104   b  is switched from the backward position to the forward position, and the second solenoid  100   b  is switched from the reset state to the attraction state. 
     In any one of the switching operations, the selector  108   a ,  108   b  is not actuated by the actuation of the drive pin  52   a ,  52   b  owing the function of play formed between the drive pin  52   a ,  52   b  and the selector  108   a ,  108   b , and the actuation is not transmitted from the drive pin  52   a ,  52   b  to the manual lever  118   a ,  118   b  via the selector  108   a ,  108   b . That is, the actuation is not transmitted from the electric drive mechanism  54   a ,  54   b  to the manual lever  118   a ,  118   b.    
     Referring to  FIGS. 13A to 13D , the switching of the first and second clutch mechanisms  50   a ,  50   b  by the manual drive mechanism  56   a ,  56   b  are explained. 
     In the case where the first and second clutch mechanisms  50   a ,  50   b  are in the connection state, when the second manual lever  118   b , for example, is switched from the neutral position to the release position, from the state shown in  FIG. 13A  to the state shown in  FIG. 13B , the second selector  108   b  is switched from the neutral position to the release position and the second drive pin  52   b  is switched from the connection position to the release position by the second selector  108   b . Due to the switching of the second drive pin  52   b , the second clutch mechanism  50   b  is switched from the connection state to the release state, the second sliding member  104   b  is switched from the backward position to the forward position, and the second solenoid  100   b  is switched from the attraction state to the reset state. Here, as the second solenoid  100   b  is a self-holding type such that the attraction state is held, a sense of click is produced in the operation of the second manual lever  118   b . Further, owing to the coupling beam  96 , the first clutch mechanism  50   a  is switched from the connection state to the release state, and the first drive pin  52   a  is switched from the connection position to the release position. Although the first sliding member  104   a  is switched from the forward position to the backward position and the first solenoid  100   a  is switched from the reset state to the attraction state by the switching of the first drive pin  52   a , the first selector  108   a  is at the neutral position and is thus not actuated by the first drive pin  52   a , and so the first manual lever  118   a  is not actuated and held at the neutral position. In addition, as shown in  FIG. 13B  in a magnified form, when the second selector  108   b  is switched from the neutral position to the release position, the second selector  108   b  is engaged with and held by the second engaging portion  116   b  of the second urging member  112   b , and so the second manual lever  118   b  is held at the release position. Therefore, the state of the second clutch mechanism  50   b  can be judged from the position of the second manual lever  118   b . The actuation described above concerns a case where the second manual lever  118   b  is switched, and also holds true with a case where the first manual lever  118   a  is switched. 
     In the case where the first and second clutch mechanisms  50   a ,  50   b  are in the release state, when the second manual lever  118   b , for example, is switched from the neutral position to the connection position, from the state shown in  FIG. 13C  to the state shown in  FIG. 13D , the second selector  108   b  is switched from the neutral position to the connection position and the second drive pin  52   b  is switched from the release position to the connection position by the second selector  108   b . Due to the switching of the second drive pin  52   b , the second clutch mechanism  50   b  is switched from the release state to the connection state, the second sliding member  104   b  is switched from the forward position to the backward position, and the second solenoid  100   b  is switched from the reset state to the attraction state. Further, owing to the coupling beam  96 , the first clutch mechanism  50   a  is switched from the release state to the connection state, and the first drive pin  52   a  is switched from the release position to the connection position. Although the first sliding member  104   a  is switched from the backward position to the forward position and the first solenoid  100   a  is switched from the attraction state to the reset state by the switching of the first drive pin  52   a , the first selector  108   a  is at the neutral position and is thus not actuated by the first drive pin  52   a , and so the first manual lever  118   a  is not actuated and held at the neutral position. Here, as the first solenoid  100   a  is a self-holding type such that the attraction state is held, a sense of click is produced in the operation of the second manual lever  118   b . In addition, when the operation of the second manual lever  118   b  is released, the second selector  108   b  is reset to the neutral position by the second urging member  112   b , and the second manual lever  118   b  is reset to the neutral position. The actuation described above concerns a case where the second manual lever  118   b  is switched, and also holds true with a case where the first manual lever  118   a  is switched. 
     In any one of the switching operations, the actuation is transmitted from the manual lever  118   a ,  118   b  to the drive pin  52   a ,  52   b  via the selector  108   a ,  108   b , and the electric drive mechanism  54   a ,  54   b  is actuated by the actuation of the drive pin  52   a ,  52   b . That is, the actuation is transmitted from the manual lever  118   a ,  118   b  to the electric drive mechanism  54   a ,  54   b.    
     Therefore, the power transmission apparatus  46   a ,  46   b  in the present embodiment includes the following effects. 
     In the power transmission apparatus  46   a ,  46   b  in the present embodiment, even when the drive pin  52   a ,  52   b  is actuated by the electric drive mechanism  54   a ,  54   b , the actuation is not transmitted from the drive pin  52   a ,  52   b  to the manual lever  118   a ,  118   b  via the selector  108   a ,  108   b  owing to the play provided between the drive pin  52   a ,  52   b  and the selector  108   a ,  108   b  as long as the manual lever  118   a ,  118   b  are disposed at the neutral position. Thus, the actuation is not transmitted from the electric drive mechanism  54   a ,  54   b  to the manual lever  118   a ,  118   b , and so output necessary for the electric drive mechanism  54   a ,  54   b  is reduced as compared with the case where the manual lever  118   a ,  118   b  is actuated by the actuation of the electric drive mechanism  54   a ,  54   b , thereby enabling a size reduction of the electric drive mechanism  54   a ,  54   b  and the power transmission apparatus  46   a ,  46   b . Moreover, the first and second clutch mechanisms  50   a ,  50   b  can be switched in conjunction with each other. 
     Since the solenoid  100   a ,  100   b  are only actuated from the attraction state to the reset state, the configuration of the solenoid  100   a ,  100   b  is simplified, and the solenoid  100   a ,  100   b  can be reduced in size. Moreover, it is not necessary to turn on electricity to hold the solenoid  100   a ,  100   b  in the attraction state, and so the power consumption of the solenoid  100   a ,  100   b  can be reduced. 
     Since the pair of drive apparatuses  44   a ,  44   b  is arranged in rotational symmetry with respect to the central axis of the motor unit  28 , the center of gravity of the pair of drive apparatuses  44   a ,  44   b  is located on the central axis of the motor unit  28 , and so the motor unit  28  is easily operated when rotated about its central axis. 
     As the claw portion  132   a ,  132   b  of the lever cover  130   a ,  130   b  is engaged with and the proximal end portion of the lever cover  130   a ,  130   b  is fixed to the clamp member  126   a ,  126   b , there is no need for a screw to fix the proximal end portion of the lever cover  130   a ,  130   b , and so the number of parts can be reduced. 
     When the motor unit  28  is assembled, as the manual lever  118   a ,  118   b  is attached after the housing  117  is attached and the manual lever  118   a ,  118   b  can be disposed outside the housing  117 , and the width W of the housing  117  between the pair of manual levers  118   a ,  118   b  can be reduced and so the motor unit  28  can be reduced in size. 
     The two clutch mechanisms  50   a ,  50   b  can be switched by operation of one of the manual levers  118   a ,  118   b  and the pair of manual levers  118   a ,  118   b  is arranged in rotational symmetry with respect to the central axis of the motor unit  28 , and so it is possible to easily access the manual levers  118   a ,  118   b  even when the motor unit  28  is rotated about its central axis. 
       FIGS. 14 and 15  show a second embodiment of the present invention. 
     The present embodiment is only different from the first embodiment in the configuration of the manual drive mechanism  56   a.    
     The manual drive mechanism  56   a  is only provided in the first power transmission apparatus  46   a . The play portion  110   a  in the shape of a long hole extends in the sliding member  104   a  as both the actuating member and the selection member in the direction of the back-and-forth movement of the sliding member  104   a . An operation pin  134   a  is inserted through the play portion  110   a  perpendicularly to the direction of the back-and-forth movement of the sliding member  104   a . One end of an operation rod  136   a  is coupled to the operation pin  134   a , and the operation rod  136   a  extends in the direction of the back-and-forth movement of the sliding member  104   a , while the other end of the operation rod  136   a  can be manually operated. A manual rod  138   a  as the operation member composed of the operation pin  134   a  and the operation rod  136   a  can be switched between a forward side connection position and a backward side release position with reference to a neutral position, and is held at the neutral position by the elastic member  114   a . When the manual rod  138   a  is at the neutral position, the operation pin  134   a  is slidable in the play portion  11   a  without disturbing the back-and-forth movement of the sliding member  104   a . When the manual rod  138   a  is disposed at the forward side connection position or the backward side release position, the sliding member  104   a  is disposed at the forward position or the backward position. 
     Next, the actuation of a selective actuation transmission mechanism of the power transmission apparatus  46   a ,  46   b  will be described. 
     When the first clutch mechanism  50   a  is switched by the electric drive mechanism  54   a , the actuation is not transmitted from the sliding member  104   a  to the manual rod  138   a  owing the function of play formed between the sliding member  104   a  and the operation pin  134   a . That is, the actuation is not transmitted from the electric drive mechanism  54   a  to the manual rod  138   a.    
     The switching of the first clutch mechanism  50   a  by the manual drive mechanism  56   a  is explained. 
     In the case where the first clutch mechanism  50   a  is in the connection state and the sliding member  104   a  is at the forward position, the manual rod  138   a  is pulled and switched from the forward side connection position to the backward side release position. As a result, the sliding member  104   a  is switched from the forward position to the backward position, the first drive pin  52   a  is switched from the connection position to the release position, the first clutch mechanism  50   a  is switched from the connection position to the release position, and the first solenoid  100   a  is switched from the reset state to the attraction state. When the operation of the manual rod  138   a  is released, the manual rod  138   a  is reset to the neutral position. 
     In the case where the first clutch mechanism  50   a  is in the release state and the first sliding member  104   a  is at the backward position, the manual rod  138   a  is pushed in and switched from the backward side release position to the forward side connection position. As a result, the sliding member  104   a  is switched from the backward position to the forward position, the first drive pin  52   a  is switched from the release position to the connection position, the first clutch mechanism  50   a  is switched from the release position to the connection position, and the first solenoid  100   a  is switched from the attraction state to the reset state. When the operation of the manual rod  138   a  is released, the manual rod  138   a  is reset to the neutral position. 
     In any one of the switching operations, the actuation of the manual rod  138   a  is transmitted to the sliding member  104   a , and the electric drive mechanism  54   a  is actuated. That is, the actuation is transmitted from the manual rod  138   a  to the electric drive mechanism  54   a.    
     Therefore, the power transmission apparatus  46   a  in the present embodiment includes the following effect. 
     In the power transmission apparatus  46   a  in the present embodiment, even when the sliding member  104   a  is actuated by the electric drive mechanism  54   a , the actuation is not transmitted from the sliding member  104   a  to the manual rod  138   a  owing to the play provided between the sliding member  104   a  and the manual rod  138   a  as long as the manual rod  138   a  is disposed at the neutral position. 
     While the clutch mechanism is switched between the connection state and the release state by the manual drive mechanism in the embodiments described above, the clutch mechanism may be switched by the manual drive mechanism only from the connection state to the release state or from the release state to the connection state. For example, the switching from the connection state to the release state may be only carried out by the manual drive mechanism, and the clutch mechanism may be switched by the electric drive mechanism at normal times, while the clutch mechanism may be switched to the release state by the manual drive mechanism in an emergency. 
     Other advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.