Abstract:
A clutch mechanism is provided between a driving rotor and a driven rotor. The clutch mechanism selects from a state for transmitting rotational force and a state for discontinuing the transmission of rotational force. The holder permits the rolling. A holder supports rolling bodies such that the rolling bodies are switched between a transmitting position and a disconnecting position. When held at the transmitting position, the rolling bodies are engaged with both of the driving rotor and the driven rotor so that rotational force of the driving rotor is transmitted to the driven rotor. When held at the disconnecting position, the rolling bodies are disengaged from the driving rotor so that the transmission of rotational force from the driven rotor to the driving rotor is discontinued. When external rotational force is applied to the driven rotor, the holder sets the rolling bodies to the disconnecting position.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an actuator with clutch mechanism, more particularly, to an actuator that is used in a power-assisted vehicle door and in a drum rotating mechanism of a photocopier. 
     Some vehicles have power-assisted doors with actuators. The actuator applies assisting force to a vehicle door when a user opens or closes the door. For example, Japanese Laid-Open Patent Publication No. 6-328940 discloses such a power-assisted door. The apparatus of the publication includes an actuator located in the panel of a door. The actuator has an electric motor as a drive source. The motor has a gear fixed to the rotary shaft. The gear is coupled to a worm gear via a reduction gear. The worm gear is coupled to a slider. The slider is coupled to the vehicle body with an assisting force applying member and brackets. Rotation of the motor is converted into reciprocation of the slider by the worm gear. The reciprocation of the slider is, in turn, converted into opening and closing motion of the door. When predetermined conditions are satisfied, for example, when an operation switch is turned on, the force of the actuator (assisting force) permits the door to be opened or closed with a small force applied by the user. 
     The number of gears and the lead angle of the worm gear are determined such that the rotary shaft of the motor is rotated by force applied by the door. Therefore, when the predetermined conditions are not satisfied, or when the actuator is not working, the door can be opened and closed manually. However, when the door is opened or closed manually, the worm gear is rotated by the force applied from the door. In other words, the force is applied to the output side of the apparatus. Thus, a great force is required for opening and closing the door manually. 
     An electromagnetic clutch may be used for transmitting rotational force from the worm gear (driving member) to the gear of the door (driven member) and for prohibiting the worm gear from receiving rotational force from the gear of the door. If the electromagnetic clutch does not transmit rotational force from the gear of the door to the worm gear when the door is opened or closed manually, the worm gear does not receive any load. This permits the door to be opened and closed with a small force. However, the electromagnetic clutch increases the size of the apparatus and increases the cost. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a clutch mechanism, an actuator with clutch mechanism, and a power-assisted door using the actuator that mechanically transmit force from a driving member to a driven member, permits the driven member to rotate without applying load to the driving member, and reduces the size and the costs. 
     To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a clutch mechanism is provided between a driving rotor and a driven rotor. The clutch mechanism selects from a state for transmitting rotational force of the driving rotor to the driven rotor and a state for discontinuing the transmission of rotational force generated from the driven rotor to the driving rotor. The clutch mechanism includes a plurality of rolling bodies and a holder. The rolling bodies are located between the driving rotor and the driven rotor. The holder holds the rolling bodies and permits the rolling bodies to roll. The holder supports the rolling bodies such that the rolling bodies are switched between a transmitting position and a disconnecting position. When held at the transmitting position, the rolling bodies are engaged with both of the driving rotor and the driven rotor so that rotational force of the driving rotor is transmitted to the driven rotor. When held at the disconnecting position, the rolling bodies are disengaged from the driving rotor so that the transmission of rotational force from the driving rotor to the driven rotor is discontinued. When external rotational force is applied to the driven rotor, the holder sets the rolling bodies to the disconnecting position. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view illustrating an actuator according to a first embodiment of the present invention; 
     FIG. 2 is an enlarged cross-sectional view illustrating the actuator of FIG. 1; 
     FIG.  3 ( a ) is a schematic view showing the operation of the clutch mechanism of FIG. 1; 
     FIG.  3 ( b ) is an enlarged view of FIG.  3 ( a ); 
     FIG.  4 ( a ) is a schematic view showing the operation of the clutch mechanism of FIG. 1; 
     FIG.  4 ( b ) is an enlarged view of FIG.  4 ( a ); 
     FIG.  5 ( a ) is a cross-sectional view illustrating a rotation device according to a second embodiment of the present invention; 
     FIG.  5 ( b ) is an enlarged cross-sectional view illustrating the rotation device of FIG.  5 ( a ); 
     FIG.  6 ( a ) is a schematic view showing the operation of the clutch mechanism of the rotation device shown in FIG.  5 ( a ); 
     FIG.  6 ( b ) is an enlarged view of FIG.  6 ( a ); 
     FIG.  7 ( a ) is a schematic view showing the operation of the clutch mechanism of the rotation device shown in FIG.  5 ( a ); and 
     FIG.  7 ( b ) is an enlarged view of FIG.  7 ( a ). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An actuator  1  used in a power-assisted vehicle door according a first embodiment of the present invention will now be described with reference to FIGS. 1 to  4 ( b ). 
     The actuator  1  is located in the door. The cross-sectional view of FIG. 1 is a view taken perpendicular to the extending direction of the actuator  1 . The actuator  1  includes a rack  2 , which moves linearly along the extending direction of the actuator  1 . The distal end of the rack  2  is coupled to the vehicle body at a part displaced from the axis of the hinge by which the door is supported. That is, the actuator  1  moves the rack  2  linearly to apply assisting force to the door when the door is opened or closed. 
     The actuator  1  includes a direct-current motor  3 , a gear housing  4 , and a clutch mechanism  5 . The direct-current motor  3  is cylindrical and extends along the extending direction of the actuator  1 . The motor  3  has a worm  6  protruding from one side. The motor  3  is fixed to the gear housing  4  with the worm  6  accommodated in a worm container  4   a  of the gear housing  4 . The worm  6  rotates integrally with the rotary shaft (not shown) of the motor  3  when the motor  3  is running. The motor  3  is connected to a controller (not shown). 
     The gear housing  4  accommodates the worm  6 , a driving rotor, a clutch mechanism  5 , a driven rotor, a large diameter gear  9 , and a pinion  10 . In this embodiment, the driving rotor is a worm wheel  7 , and the driven rotor is a small diameter gear  8 . The gear housing  4  also accommodates a part of the rack  2 . 
     Specifically, a pair of first shaft receptacles  4   b ,  4   c  is formed in the gear housing  4 . The shaft receptacles  4   b ,  4   c  are located in the vicinity of the worm container  4   a  and are arranged in the direction of the thickness of the gear housing  4  (lateral direction as viewed in FIG.  2 ). The shaft receptacles  4   b ,  4   c  face each other. Slide bearings  11   a ,  11   b  are fixed to the shaft receptacles  4   b ,  4   c , respectively. 
     The worm wheel  7 , the clutch mechanism  5 , and the small diameter gear  8  are arranged in this order and coupled to one another. A first supporting shaft  12  extends from the worm wheel  7  (from the left side of the worm wheel  7  as viewed in FIG.  2 ). The first supporting shaft  12  is supported by the slide bearing  11   a . A second supporting shaft  13  extends from a side of the small diameter gear  8  (from the right side as viewed in FIG.  2 ). The second supporting shaft  13  is supported by the slide bearing  11   b . The worm wheel  7 , the clutch mechanism  5 , and the small diameter gear  8  are held in the gear housing  4 . The worm wheel  7  is engaged with the worm  6 . 
     Second shaft receptacles  4   d ,  4   e  are formed in the gear housing  4 . The second shaft receptacles  4   d ,  4   e  are displaced from the first shaft receptacles  4   b ,  4   c  and are arranged in the direction of thickness of the gear housing  4  (in the lateral direction as viewed in FIG.  1 ). The second shaft receptacles  4   d ,  4   e  face each other. Slide bearings  14   a ,  14   b  are fixed to the shaft receptacles  4   d ,  4   e , respectively. 
     The large diameter gear  9  and pinion  10  are fixed to the supporting shaft  15 . One end of the supporting shaft  15  (left end as viewed in FIG. 1, or the end closer to the pinion  10 ), is supported by the slide bearing  14   a . The other end of the supporting shaft  15  (right end as viewed in FIG. 1, or the end close to the large diameter gear  9 ) is supported by the slide bearing  14   b . The large diameter gear  9  and the pinion  10  are supported by the gear housing  4  and rotate integrally. The large diameter gear  9  is engaged with the small diameter gear  8 . 
     A rail  16  is located in the gear housing  4 . The rail  16  extends along the extending direction of the actuator  1 . The rail  16  is fixed to a part in the vicinity of the pinion  10  and is parallel to the worm  6 . A through hole  4   f  is formed in the gear housing  4  to communicate the interior of the gear housing  4  with the exterior. The through hole  4   f  is aligned with the rail  16 . 
     The rack  2  is supported by the rail  16  to move linearly along the rail  16  such that the distal end protrudes from the through hole  4   f . The rack  2  is engaged with the pinion  10 . The distal end of the rack  2  is coupled to the vehicle body as described above. 
     The clutch mechanism  5  will now be described with reference to FIGS. 2,  3 ( a ),  3 ( b ),  4 ( a ), and  4 ( b ). The clutch mechanism  5  transmits rotational force of the worm wheel  7  to the small diameter gear  8 . However, the clutch mechanism  5  permits the small diameter gear  8  to rotate without transmitting rotational force of the small diameter gear  8  to the worm wheel  7 . 
     The clutch mechanism  5  includes a collar  21 , a clutch shaft  22 , three rolling bodies, a rotation limiting member, and three retainers. In this embodiment, the rolling bodies are rollers  23 , the rotation limiting member is a wheel-shaped roller holder  24 , and the retainers are calipers  25 . 
     The collar  21  includes a substantially cylindrical portion  21   a  and a disk portion  21   b . The disk portion  21   b  extends radially inward from an end of the cylindrical portion  21   a . The collar  21  is fitted to a protruding portion  8   a  protruding from one end of the small diameter gear  8  (the left end as viewed in FIG. 2) to rotate integrally with the small diameter gear  8 . A sensor magnet M is fixed to the circumferential surface of the cylindrical portion  21   a  of the collar  21 . The sensor magnet M faces a sensor substrate S, which is fixed to the gear housing  4 . 
     As shown in FIG.  3 ( a ), the outer diameter of the distal end of the clutch shaft  22  (the right end as viewed in FIG. 2) is smaller than the inner diameter of the cylindrical portion  21   a . Three cut-off surfaces  22   a  are formed in the circumference of the clutch shaft  22 . In this embodiment, the cut-off surfaces  22   a  are formed by linearly cutting parts of the circumference of the clutch shaft  22 . The three cut-off surfaces  22   a  are arranged in equal angular intervals. The proximal end (the left end as viewed in FIG. 2) of the clutch shaft  22  is fixed to the worm wheel  7  to rotate integrally with the worm wheel  7 . The distal end of the clutch shaft  22  is located inside the cylindrical portion  21   a . The distance between each cut-off surface  22   a  and the inner surface of the cylindrical portion  21   a  varies in the circumferential direction. Specifically, the distance between each cut-off surface  22   a  and the inner surface of the cylindrical portion  21   a  is shorter in the side sections  22   b  of the cut-off surface  22   a  (see FIG.  3 ( b )) and is longer in the center section  22   c  of the cut-off surface  22   a.    
     A holding recess  22   d  is formed in the center of the distal end of the clutch shaft  22  (the right end as viewed in FIG.  2 ). A ball B is received by the holding recess  22   d . The ball B partly projects from the holding recess  22   d  and contacts the protruding portion  8   a  of the small diameter gear  8 . This permits the small diameter gear  8  to smoothly rotate relative to the clutch shaft  22 . 
     Each roller  23  is cylindrical and the diameter is shorter than the distance between the inner surface of the cylindrical portion  21   a  and the center section  22   c  of each cut-off surface  22   a  as shown in FIGS.  3 ( a ),  3 ( b ),  4 ( a ), and  4 ( b ). The diameter of each roller  23  is longer than the distance between the inner surface of the cylindrical portion  21   a  and the side sections  22   b  of each cut-off surface  22   a . Each roller  23  is located between one of the cut-off surfaces  22   a  and the inner surface of the cylindrical portion  21   a.    
     The roller holder  24  includes a substantially cylindrical portion  24   a , a disk portion  24   b , and a large diameter portion  24   c , and an engaging portion. The disk portion  24   b  extends radially outward from one end of the cylindrical portion  24   a  (the left end as viewed in FIG.  2 ). The large diameter portion  24   c  extends axially toward the other end of the cylindrical portion  24   a  (rightward as viewed in FIG.  2 ). In this embodiment, the engaging portion is an annular portion  24   d , which extends radially outward from the large diameter portion  24   c.    
     The outer diameter of the cylindrical portion  24   a  is slightly smaller than the inner diameter of the cylindrical portion  21   a . The inner diameter of the cylindrical portion  24   a  is slightly larger than the outer diameter of the distal end of the clutch shaft  22 . The thickness of the cylindrical portion  24   a  is smaller than the diameter of each roller  23  (see FIG.  3 ( b )). Three receptacles  24   e  are formed in the other end of the cylindrical portion  24   a . The receptacles  24   e  receive and hold the rollers  23  and are arranged in equal angular intervals. Each receptacle  24   e  has a pair of arcuate inner walls in the circumferential direction of the cylindrical portion  24   a . The radius of curvature of the arcuate inner walls in each receptacle  24   e  is slightly greater than the radius of the rollers  23 . Each receptacle  24   e  holds one of the rollers  23  and permits the roller  23  to slightly move in the axial direction. The distances among the three rollers  23  are constant. The roller holder  24  is arranged such that the cylindrical portion  24   a  is inserted into the cylindrical portion  21   a , and the annular portion  24   d  is located outside of the cylindrical portion  21   a  and extends radially outward. 
     As shown in FIG. 2, each caliper  25  is screwed to the gear housing  4  and has a first and a second holding member. In this embodiment, the first and second holding members are first and second shoes  25   a ,  25   b . The calipers  25  are arranged about the roller holder  24  at equal angular intervals. Only one of the supporting members  25  is shown in FIG.  2 . Each caliper  25  holds the annular portion  24   d  with the shoes  25   a ,  25   b  by applying a predetermined pressure. That is, each caliper  25  holds the annular portion  24   d  with a predetermined force and permits the annular portion  24   d  to rotate when a force greater than a predetermined value is applied. The predetermined value is significantly smaller than the rotational force of the worm wheel  7  generated by the direct-current motor  3 . 
     When predetermined conditions are satisfied, for example, when a switch in the vicinity of the door is turned on or when external force applied to the door (manipulating force) is equal to or greater than a predetermined level, the controller (not shown) of the power-assisted door supplies direct current to the direct-current motor  3 , thereby rotating the worm  6 . 
     After rotating the worm  6  with the motor  3 , the controller supplies direct current to the motor  3  to rotate the worm  6  in the opposite direction by a predetermined amount, thereby reversing the rotation of the worm  6  by a predetermined amount. 
     The operation of the actuator  1  will now be described. 
     When predetermined conditions are satisfied, for example, when a user manipulates a switch in the vicinity of the door handle before opening or closing the door or when force applied to the door (manipulation force) is greater than a predetermined level, the controller supplies driving voltage to the direct-current motor  3 . 
     Then, the worm  6  rotates with the rotary shaft of the motor  3 . Accordingly, the worm wheel  7  and the clutch shaft  22  are rotated. At this time, the clutch shaft  22  is slightly rotated counterclockwise as shown in FIGS.  4 ( a ) and  4 ( b ), which moves each roller  23  to one of the side sections  22   b  of the corresponding cut-off surface  22   a . The roller  23  is thus tightly held between the side section  22   b  and the cylindrical portion  21   a . The rollers  23  are moved to transmitting positions. When the clutch shaft  22  is further rotated, each roller  23  receives force that further presses the roller  23  against the corresponding side section  22   b , and the rotational force of the clutch shaft  22  is transmitted to the cylindrical portion  21   a  through the rollers  23 , which rotates the collar  21  and the small diameter gear  8 . The roller holder  24  is held by the caliper  25  with a predetermined force. However, since the rotational force of the worm wheel  7  generated by the direct-current motor  3  is significantly greater than the predetermined force, the roller holder  24  rotates integrally with the rollers  23 . 
     Then, the large diameter gear  9  and the pinion  10  are rotated by rotation of the small diameter gear  8 . Rotation of the pinion  10  is converted into linear motion of the rack  2 . The rack  2 , in turn, applies assisting force to the door in the opening or closing direction. In this manner, the power-assisted door uses the force of the actuator  1  (assisting force) to permit the door to be opened or closed with a small force. 
     The controller first rotates the worm  6  with the rotary shaft of the motor  3  and then provides the motor  3  with a direct current that rotates the motor in the opposite direction by a predetermined amount. Therefore, the worm  6  is rotated in the opposite direction for a predetermined amount. The amount by which the worm  6  is rotated in the opposite direction is determined such that each roller  23  is moved from the position in FIGS.  4 ( a ),  4 ( b ), or transmitting position, to the position in FIGS.  3 ( a ),  3 ( b ), or disconnecting position. In FIGS.  4 ( a ),  4 ( b ), each roller  23  is held between a side section  22   b  of the corresponding cut-off surface  22   a  and the cylindrical portion  21   a . In FIGS.  3 ( a ),  3 ( b ), each roller  23  faces the center section  22   c  of the corresponding cut-off surface  22   a . Therefore, after the motor  3  is actuated, each roller  23  is positioned at the disconnecting position shown in FIGS.  3 ( a ), ( b ) and faces the center section  22   c  of the corresponding cut-off surface  22   a.    
     If the predetermined conditions are not satisfied when the user opens or closes the door, for example, if the switch in the vicinity of the door is not manipulated or if the external force applied to the door is less than the predetermined level, the controller does not supply driving voltage to the direct-current motor  3 . 
     If the user manually opens or closes the door in this state, the rack  2  is moved linearly, which, in turn, rotates the pinion  10  and the large diameter gear  9 . 
     Rotation of the large diameter gear  9  rotates the small diameter gear  8  and the collar  21 . Since the roller holder  24  is held by the calipers  25  with a predetermined force, each roller  23  is not moved to the side sections  22   b , or in the direction to engage with the cut-off surface  22   a  even if the roller  23  slightly contacts the rotating cylindrical portion  21   a  (even if the roller  23  chatters and contacts the cylindrical portion  21   a ). Therefore, rotational force of the collar  21  is not transmitted to the clutch shaft  22 , and the clutch shaft  22  does not receive load. In this manner, if the door is opened or closed manually when the predetermined conditions are not satisfied, the door can be opened or closed with a small force. 
     The characteristic advantages of the actuator  1  having the clutch mechanism  5  will now be discussed. 
     (1) The clutch mechanism  5  transmits rotational force from the worm wheel  7  to the small diameter gear  8 . The clutch mechanism  5  also permits the small diameter gear  8  to rotate without transmitting rotational force from the small diameter gear  8  to the worm wheel  7 . Therefore, compared to an actuator having an electromagnetic clutch, the actuator  1  reduces the size and the costs. 
     (2) The clutch shaft  22  has the three cut-off surfaces  22   a , which are arranged at equal angular intervals. Each roller  23  corresponds one of the cut-off surfaces  22   a . When the worm wheel  7  and the clutch shaft  22  are rotated, rotational force of the clutch shaft  22  is transmitted to the cylindrical portion  21   a  through three paths. The rotational force of the clutch shaft  22  is thus transmitted to the cylindrical portion  21   a  in a well-balanced manner, which improves durability of the members. 
     (3) The roller holder  24  has the annular portion  24   d , which is held by the first and second shoes  25   a ,  25   b  of the caliper  25  by a predetermined pressure. Therefore, even if the roller holder  24  is rotated, the roller holder  24  is always held with a simple structure. 
     (4) After the rotary shaft of the motor  3  (the worm  6 ) is rotated, the direct-current motor  3  is rotated in the opposite direction so that each roller  23  is moved to a position corresponding to the center section  22   c  of the corresponding cut-off surface  22   a . Thus, when the door is opened or closed manually and the small diameter gear  8  is rotated, the rollers  23  are prevented from being held between the sides  22   b  of the cut-off surfaces  22   a  and the cylindrical portion  21   a.    
     A rotation device  31  for rotating a drum  32  of a photocopier according to second embodiment of the present invention will now be described with reference to FIGS.  5 ( a ) to  7 ( b ). 
     The rotation device  31  includes the drum  32 , a stator housing  33 , a rotor housing  34 , a standing-wave type (bolt-clamped Langevin type) ultrasonic motor  35 , a clutch mechanism C, and a reduction mechanism  36 . 
     The drum  32  is substantially cylindrical and rotatably supported in the casing of the photocopier (not shown). A tooth ring  37  is fixed to the inner surface of the drum  32 . 
     The stator housing  33  is substantially cylindrical and has a distal thick portion  33   a . The distal thick portion  33   a  is formed at the distal end of the stator housing  33  (top end as viewed in FIG.  5 ). The diameter of the distal thick portion  33   a  is larger than that of the rest of the stator housing  33 . Five threaded holes  33   b  are formed in the distal thick portion  33   a  (only one is shown in FIG.  5 ). Each threaded hole  33   b  extends from the distal end toward the proximal end of the stator housing  33 . An external projecting portion  33   c  is formed at the proximal portion (lower end as viewed in FIG. 5) of the stator housing  33 . The external projecting portion  33   c  extends radially outward. A cylindrical portion  33   d  extends from the periphery of the projecting portion  33   c . Threaded holes  33   e  are formed in the projecting portion  33   c . The threaded holes  33   e  extend axially and are located close to the cylindrical portion  33   d . The projecting portion  33   c  and the cylindrical portion  33   d  are coupled to the casing (not shown) of the photocopier such that the stator housing  33  is substantially accommodated in the drum  32 . 
     The rotor housing  34  includes a cylindrical portion  34   a , an outer portion  34   b , and an annular inner projection  34   c . The outer diameter of the cylindrical portion  34   a  is substantially equal to the inner diameter of the stator hosing  33 . The outer portion  34   b  extends radially outward from the distal end (upper end as viewed in FIG. 5) of the cylindrical portion  34   a . The inner projection  34   c  projects radially inward from the axial center of the cylindrical portion  34   a . Through holes  34   d  are formed in the outer portion  34   b . Each through hole  34   d  corresponds to one of the threaded holes  33   b  on the stator housing  33 . A screw head receiving portion  34   e  is formed in the distal portion of each through hole  34   d . The screw head receiving portion  34   e  has an enlarged inner diameter. Threaded holes  34   f  (only one is shown in FIG. 5) are formed in the outer portion  34   b . The threaded holes  34   f  extend in the axial direction. 
     The rotor housing  34  is secured to the stator housing  33  by screws N 1 , which are received by the through holes  34   d  and threaded to the threaded holes  33   b . The head of each screw N 1  is received by the head receiving portion  34   e.    
     Two ball bearings  38 ,  39  are fitted in the cylindrical portion  34   a  of the rotor housing  34 . The ball bearing  38  is inserted from the side corresponding to the cylindrical member  34   a  such that the outer ring of the ball bearing  38  contacts the inner projection  34   c . The ball bearing  39  is inserted from the side corresponding to the proximal end of the cylindrical portion  34   a  such that the outer ring of the ball bearing  39  contacts the inner projection  34   c.    
     The ball bearings  38 ,  39  support a driving rotor, which is a motor rotary shaft  40 . A disk  41  is fixed to the distal end of the motor rotary shaft  40  with a nut  42 . A cylindrical column shaped rotor  43  is fixed to the proximal end of the motor rotary shaft  40 . A disk spring  45  is located between the proximal surface of the disk  41  and the inner ring of the ball bearing  38  with a washer  44 . A disk spring  47  is located between the distal surface of the rotor  43  and the inner ring of the ball bearing  39  with a washer  46 . The disk springs  45 ,  47  are arranged in a compressed state. The motor rotary shaft  40 , the disk  41 , and the rotor  43  are supported to be axially movable in a predetermined range. The disk springs  45 ,  47  hold the motor rotary shaft  40 , the disk  41 , and the rotor  43  at the middle position in the predetermined range. 
     A stator  51  is fixed to the stator housing  33 . The stator  51  and the rotor  43  form the ultrasonic motor  35 . 
     The stator  51  includes an upper metal block  52 , a lower metal block  53 , first and second piezoelectric elements  54 ,  55 , first to third electrode plates  56  to  58 , a bolt  59 , and an insulation collar  60 . 
     The upper and lower metal blocks  52 ,  53  are made of conductive metal. In this embodiment, the metal blocks  52 ,  53  are made of aluminum alloy. The upper metal block  52  is substantially cylindrical. The inner diameter of the upper portion of the upper metal block  52  is enlarged to form a horn  52   a . The horn  52   a  is used for vibrating the upper end surface of the upper metal block  52 . A threaded hole is formed in the inner surface of the upper metal block  52  at a part except for the horn  52   a . A thin film (not shown) of frictional material is formed on the upper end surface of the upper metal block  52 . 
     The lower metal block  53  is substantially cylindrical and has the same inner and outer diameters as those of the upper metal block  52 . An annular outer projection  53   a  is formed in the axial center of the lower metal block  53 . Through holes  53   b  are formed in the peripheral portion of the outer projection  53   a . Each through hole  53   b  corresponds to one of the threaded holes  33   e  of the stator housing  33 . Slits (recesses) are formed in the circumference of the upper portion of the lower metal block  53  at a part above the outer projection  53   a . The slits generate torsional vibration based on excited vertical vibration. The slits are inclined relative to the axial direction. A threaded hole is formed in the inner surface of the lower metal block  53 . 
     The first and second piezoelectric elements  54 ,  55  are shaped like disks and have a through hole in the center. 
     The first to third electrode plates  56  to  58  are shaped like disks and have a through hole in the center. 
     The bolt  59  is shaped like a cylindrical column with the threaded circumference. The bolt  59  is threaded to the threaded holes of the upper and lower metal blocks  52 ,  53 . 
     The insulation collar  60  is a cylinder made with electrical insulating material. 
     The lower metal block  53 , the third electrode plate  58 , the second piezoelectric element  55 , the second electrode plate  57 , the first piezoelectric element  54 , the first electrode plate  56 , and the upper metal block  52  are arranged in this order and secured to one another by the bolt  59  threaded to the threaded holes of the upper and lower metal blocks  52 ,  53 . The first and second piezoelectric elements  54 ,  55  are arranged such that the polarization direction of the first piezoelectric element  54  is opposite to that of the second element  55 . The insulation collar  60  is located between the outer surface of the bolt  59  and the first and second piezoelectric elements  54 ,  55 , and between the bolt  59  and the first to third electrode plates  56  to  58 . 
     The stator  51  is fixed to the stator housing  33  by screws N 2 . Specifically, each screw N 2  is inserted into one of the through holes  53   b  of the lower metal block  53  and threaded with the corresponding threaded hole  33   e  of the stator housing  33 . The upper end surface of the stator  51  urges the proximal surface of the rotor  43  toward the distal side. Since the rotor  43  is held at the predetermined position by the disk springs  45 ,  47 , the rotor  43  is pressed against the upper end surface of the stator  51 . The first to third electrode plates  56  to  58  are electrically connected to a controller (not shown) located outside of the stator housing  33  by conducting wires (not shown). 
     The reduction mechanism  36  is coupled to the motor rotary shaft  40  via the clutch mechanism C. The reduction mechanism  36  is a planetary gear train and includes the tooth ring  37  fixed to the drum  32 , first and second supporting members  61 ,  62 , coupler pins  63 , a driven rotor, and planetary gears  65 . In this embodiment, the driven rotor is a sun gear shaft  64 . 
     The first supporting member  61  includes an annular portion  61   a , a thin portion  61   b  axially extending from the inner periphery of the annular portion  61   a , a thick portion  61   c  extending axially from the distal end of the thin portion  61   b . The thick portion  61   c  is thick so that the inner diameter is less than that of the thin portion  61   b . Axially extending through holes  61   d  are formed in the annular portion  61   a . Each through hole  61   d  corresponds one of the threaded portions  34   f  of the rotor housing. Two recesses  61   e  are formed in the inner surface of the thick portion  61   c . The recesses  61   e  extend axially from the distal end toward the proximal end and are spaced apart by one hundred eighty degrees. A substantially cylindrical engaging member  61   f  is fitted to the recesses  61   e  in the axial direction. At the proximal end of the engaging member  61   f  (the lower end as viewed in FIG.  5 ( b )), a holding recess  61   g  is formed in the axial center of the first supporting member  61  (see FIG.  5 ( b )). In this embodiment, the engaging members  61   f , in which the holding recesses  61   g  are formed, and the first supporting member  61  form retainers and holding members, which are part of the clutch mechanism C. 
     The first supporting member  61  is fixed to the rotor housing  34  by screws N 3 , which are inserted into the through holes  61   d  and threaded to the threaded holes  34   f . The inner surface of the drum  32  is coupled to the outer surface of the thick portion  61   c  by the ball bearing  66 . That is, the thick portion  61   c  rotatably supports part of the drum  32 , more specifically, part that is between the tooth ring  37  and the axially proximal end (middle and lower sections as viewed in FIG.  5 ( a )), with the ball bearing  66 . 
     The second supporting member  62  is substantially cylindrical. Two pin receptacles  62   a  are formed in the proximal portion of the second supporting member  62 . The pin receptacles  62   a  extend from the proximal end toward the distal end and are spaced apart by one hundred eighty degrees. 
     The second supporting member  62  is fixed to the engaging member  61   f  of the first supporting member  61  with coupler pins  63 . Specifically, one end of each coupler pin  63  is fitted into the center hole of the corresponding engaging member  61   f , to which the first supporting member  61  is engaged. The other end of the coupler pin  63  is engaged with the corresponding pin receptacle  62   a  of the second supporting member  62 . Accordingly, the first and second supporting members  61 ,  62  are coupled to each other. The inner surface of the drum  32  is coupled to the outer surface of the second supporting member  62  with the ball bearing  67 . 
     That is, the second supporting member  62  rotatably supports part of the drum  32 , more specifically, part between the tooth ring  37  and the distal end (the middle and upper sections as viewed in FIG.  5 ( a )), with the ball bearing  67 . The ball bearing  68  is fixed to the inner surface of the second supporting member  62 . The sun gear shaft  64  is rotatably supported by the ball bearing  68 . At the proximal end of the sun gear shaft  64  (the lower end as viewed in FIG.  5 ), a substantially cylindrical collar  64   a  is formed (see FIG.  5 ( b )). The proximal end of the collar  64   a  is open. The collar  64   a  forms part of the clutch mechanism C. The sun gear  64   b  is formed in the middle portion of the shaft  64 , more specifically, in a portion between the collar  64   a  and the ball bearing  68 . 
     One of the planetary gears  65  is rotatably supported at the middle portion of each coupler pin  63 . The planetary gears  65  are engaged with the sun gear  64   b  and with the tooth ring  37 . 
     The clutch mechanism C will now be described with reference to FIGS.  5 ( a ) to  7 ( b ). 
     The clutch mechanism C transmits rotational force from the motor rotary shaft  40  to the sun gear shaft  64 . The clutch mechanism C also permits sun gear shaft  64  to rotate without transmitting rotational force from the sun gear shaft  64  to the motor rotary shaft  40 . 
     Specifically, the clutch mechanism C includes the collar  64   a , a clutch shaft  71 , three rolling bodies (only one is shown in FIGS.  5 ( a ) and  5 ( b )), and a rotational limiting member, a retainer. In this embodiment, the rolling bodies are rollers  72 , the rotational limiting member is a cylindrical roller holder  73 , and the retainer includes the first supporting member  61  and the engaging member  61   f.    
     As shown in FIGS.  6 ( a ),  6 ( b ), the outer diameter of the distal end of the clutch shaft  71  (the upper end as viewed in FIG.  5 ( a )) is smaller than the inner diameter of the collar  64   a . Three cut-off surfaces  71   a  are formed in the circumference of the clutch shaft  71 . In this embodiment, the cut-off surfaces  71   a  are formed by linearly cutting parts of the circumference of the clutch shaft  71 . The three cut-off surfaces  71   a  are arranged at equal angular intervals. The proximal portion (the middle and lower sections lower as viewed in FIGS.  5 ( a ),  5 ( b )) is fixed to the distal end of the motor rotary shaft  40  (the middle and upper sections as viewed in FIGS.  5 ( a ),  5 ( b )) to rotate integrally with the motor rotary shaft  40 . The distal end of the clutch shaft  71  is located inside the collar  64   a . Therefore, the distance between each cut-off surface  71   a  and the inner surface of the collar  64   a  varies in the circumferential direction. Also, the distance between each cut-off surface  71   a  and the inner surface of the collar  64   a  is shorter in the side sections  71   b  of the cut-off surface  71   a  (see FIG.  6 ( b )) and is longer in the center section  71   c  of the displacement of the cut-off surface  71   a.    
     A holding recess  71   d  is formed in the center of the distal end of the clutch shaft  71  (the upper end as viewed in FIG.  5 ( a )). A ball B is received by the holding recess  71   d . The ball B partly projects from the holding recess  71   d  and contacts the sun gear shaft  64 . This permits the sun gear shaft  64  to smoothly rotate relative to the clutch shaft  71 . 
     Each roller  72  is cylindrical and the diameter is shorter than the distance between the inner surface of the collar  64   a  and the center section  71   c  of each cut-off surface  71   a  as shown in FIGS.  6 ( a ),  6 ( b ). The diameter of each roller  72  is longer than the distance between the inner surface of the collar  64   a  and the side sections  71   b  of each cut-off surface  71   a . Each roller  72  is located between one of the cut-off surfaces  71   a  and the inner surface of the collar  64   a.    
     The roller holder  73  includes a cylindrical portion  73   a  and an engaging portion. In this embodiment, the engaged portion is an annular portion  73   b , which extends radially outward from one end of the cylindrical portion  73   a.    
     The outer diameter of the cylindrical portion  73   a  is slightly smaller than the inner diameter of the collar  64   a . The inner diameter of the cylindrical portion  73   a  is slightly larger than the outer diameter of the distal end of the clutch shaft  71 . The thickness of the cylindrical portion  73   a  is less than the diameter of each roller  72  (see FIG.  6 ( b )). Three receptacles  73   c  are formed in the other end (the middle and upper sections as viewed in FIG.  5 ( a )) of the cylindrical portion  73   a . The receptacles  73   c  receive and hold the rollers  72  and are arranged in equal angular intervals. Each receptacle  73   c  has a pair of arcuate inner walls in the circumferential direction. The radius of curvature of the receptacles  73   c  is slightly greater than the radius of the rollers  72 . Each receptacle  73   c  holds one of the rollers  72  and permits the roller  72  to slightly move in the axial direction. The distances among the three rollers  72  are constant. The roller holder  73  is arranged such that the cylindrical portion  73   a  is inserted into the collar  64   a , and the annular portion  73   b  is located outside of the collar  64   a  and extends radially outward. 
     The first supporting member  61  and the engaging member  61   f  form the holding members and the retainer. The fist supporting member  61  and the engaging member  61   f  hold the annular portion  73   b  of the roller holder  73  by applying a predetermined pressure. That is, the first supporting member  61  and the engaging member  61   f  hold the annular portion  73   b  with the predetermined force and permits the annular portion  73   b  to rotate when a force greater than a predetermined value is applied. The predetermined value is significantly smaller than the rotational force of the motor rotary shaft  40  generated by the ultrasonic motor  35 . 
     The controller (not shown) of the rotation device  31  applies high frequency voltage to the first to third electrode plate  56  to  58  in accordance with manipulation of the photocopier, thereby rotating the rotor  43 . 
     After rotating the rotor  43  in one direction, the controller supplies high frequency voltage to rotate the rotor  43  in the opposite direction by a predetermined amount. 
     The operation of the above photocopier will now be described. 
     When high frequency voltage is applied to the first to third electrode plates  56  to  58  based on manipulation of the photocopier, the first and second piezoelectric elements  54 ,  54  are axially vibrated. Then, the slits (not shown) of the stator  51  covert the axial vibration into torsional vibration. Then, a compound vibration of the torsional vibration and the axial vibration is produced in the stator  51  (in the upper surface of the upper metal block  52 ). The rotor  43  is rotated by lifting force of the axial vibration component and thrust of the torsional vibration component. The motor rotary shaft  40  and the clutch shaft  71  are rotated with the rotor  43 . 
     At this time, if the clutch shaft  71  is slightly rotated counterclockwise as shown in FIG.  7 ( a ), each roller  72  is moved to one of the side sections  71   b  of the corresponding cut-off surface  71   a . The roller  72  is thus tightly held between the side section  71   b  and the collar  64   a . When the clutch shaft  71  is further rotated, each roller  72  receives force that further presses the roller  72  against the corresponding side section  71   b , and the rotational force of the clutch shaft  71  is transmitted to the collar  64   a  through the rollers  72 , which rotates the sun gear shaft  64 . The roller holder  73  holding the rollers  72  is held by the first supporting member  61  and the engaging member  61   f  with a predetermined force. However, since the rotational force of the motor rotary shaft  40  based on the force of the ultrasonic motor  35  is significantly greater than the predetermined force, the roller holder  73  rotates integrally with the rollers  72 . 
     Rotation of the sun gear  64   b  of the sun gear shaft  64  causes each planetary gear  65  to rotate about on its axis. Accordingly, the drum  32  rotates at a rate less than that of the rotation of the rotor  43 . In this manner, the drum  32  is rotated based on manipulation of the photocopier. Since the ultrasonic motor  35  operates quietly and is highly responsive, the rotation device  31  operates quietly and is highly responsive. 
     The controller first rotates the rotor  43  and then provides the rotor  43  with high frequency voltage that rotates the rotor  43  in the opposite direction by a predetermined amount. Therefore, the rotor  43  is rotated in the opposite direction for a predetermined amount. The amount by which the rotor  43  is rotated in the opposite direction is determined such that each roller  72  is moved from the position in FIGS.  7 ( a ),  7 ( b ) at which the roller  72  is tightly held between the side section  71   b  of the cut-off surface  71   a  and the collar  64   a , to the position in FIGS.  6 ( a ),  6 ( b ), at which the roller  72  is located at the center section  71   c  of the cut-off surface  71   a . Therefore, after the rotation device  31  (the ultrasonic motor  35 ) is actuated, each roller  72  is positioned at a position corresponding to the center section  71   c  of the cutoff surface  71   a.    
     When a sheet of paper is stuck in the photocopier and supply of high frequency voltage to the stator  51  is stopped, drum  32  is rotated manually to remove the stuck sheet. At this time, each planetary gear  65  rotates on its axis. This rotates the sun gear shaft  64  (the collar  64   a ). Since the roller holder  73  holding the rollers  72  is held by the first supporting member  61  and the engaging member  61   f  with the predetermined force, each roller  72  is not moved to the side sections  71   b , or in the direction to engage with the cut-off surface  71   a , even if the roller  72  slightly contacts the rotating collar  64   a  (even if the roller  72  chatters and contacts the collar  64   a ). Therefore, rotational force of the sun gear shaft  64  (the collar  64   a ) is not transmitted to the clutch shaft  71 , and the clutch shaft  71  does not receive load. In this manner, the rotor  43  does not apply load against the manual rotation of the drum, that is, regardless of the self-holding force of the ultrasonic motor  35 , the drum  32  can be manually rotated. This facilitates removal of the stuck sheet. 
     The characteristic advantages of the photocopier having the clutch mechanism C will now be described. 
     (1) The clutch mechanism C mechanically transmits rotational force from the motor rotary shaft  40  to the sun gear shaft  64 . The clutch mechanism C also permits the sun gear shaft  64  to rotate without transmitting rotational force from the sun gear shaft  64  to the motor rotary shaft  40 , or applying the load of the motor  40  to the sun gear shaft  64 . Therefore, compared to a case where an electromagnetic clutch is used, the size and the costs of the apparatus are reduced. 
     (2) The clutch shaft  71  has the three cut-off surfaces  71   a , which are arranged at equal angular intervals. Each roller  72  corresponds to one of the cut-off surfaces  71   a . Therefore, when the motor rotary shaft  40  and the clutch shaft  71  are rotated, rotational force of the shaft  71  is transmitted to the collar  64   a  through three paths. The rotational force of the shaft  71  is thus transmitted to the collar  64   a  in a well-balanced manner, which improves durability of the members. 
     (3) The roller holder  73  has the annular portion  73   b , which is held by the first supporting member  61  and the engaging member  61   f  with a predetermined pressure. Therefore, when the roller holder  73  is rotated, the roller holder  73  is always held with a simple structure. 
     (4) After the rotor  43  is rotated, the ultrasonic motor  35  is rotated in the opposite direction by a predetermined amount so that each roller  72  is moved to a position corresponding to the center section  71   c  of the corresponding cut-off surface  71   a . Thus, when the drum  32  is manually rotated and the sun gear shaft  64  is rotated, the rollers  72  are prevented from being held between the sides  71   b  of the cut-off surfaces  71   a  and the collar  64   a.    
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     In the illustrated embodiments, the cut-off surfaces  22   a ,  71   a  of the displacement shafts  22 ,  71  are formed flat by linearly cutting off parts of the shafts  22 ,  71 . However, the shapes of the cut-off surfaces  22   a    71   a  may be changed as long as the distances between the surfaces  22   a ,  71   a  and the cylindrical portion  21   a ,  64   a  change in the circumferential direction. For example, the cut-off surfaces  22   a ,  71   a  may be arcuate so that the distance from the cylindrical portion  21   a ,  64   a  changes gradually compared to the illustrated embodiments. In this case, the same advantages as the illustrated embodiments are obtained. 
     In the illustrated embodiments, the number of the cut-off surfaces  22   a ,  71   a  is three. However, the number may be four or more. Even if the number of the surfaces  22   a ,  71   a  are increased, the same advantages as the illustrated embodiments will be obtained. Also, the number of the surfaces  22   a ,  71   a  may be two. In this case, the advantages (1), (3), (4) of the first embodiment or the advantages (1), (3), (4) of the second embodiment will be obtained. 
     In the first embodiment, the number of the caliper  25  (the first and second shoes  25   a ,  25   b ) may be changed. In the second embodiment, the number of the engaging members  61   f  (retainers) may, be changed. If the number of the engaging members  61   f  is changed, the number of the recesses  61   e  must be changed accordingly. This modification has the same advantages as the illustrated embodiments. 
     In the illustrated embodiments, the roller holders  24 ,  73  have the annular portions  24   d ,  73   b . The annular portion  24   d  is held by the first and second shoes  25   a ,  25   b  of the caliper  25  with a predetermined pressure. The annular portion  73   b  is held by the first supporting member  61  and the engaging member  61   f  with a predetermined pressure. The structure of the roller holders  24 ,  73  and the caliper  25  (the first and second shoes  25   a ,  25   b  or the first supporting member  61  and the engaging member  61   f ) may be changed. 
     For example, the roller holders  24 ,  73  may be engaged with the rollers  23 ,  72  to rotate integrally with the rollers  23 ,  72 , and a separate rotation limiting member having an engaging portion may be fitted about the cylindrical portion  21   a ,  64   a . The caliper  25  (the first and second shoes  25   a ,  25   b  or the first supporting member  61  and the engaging member  61   f ) may be replaced with another retainer that holds the engaging portion with a predetermined holding force and permits the engaged portion to be rotated when a force that is greater than the predetermined holding force is applied to the engaging portion. For example, the caliper  25  (the first and second shoes  25   a ,  25   b ) may be replaced with a retainer that is fixed to the gear housing  4  and holds the annular portion  24   d  with a predetermined holding force by pressing the annular portion  24   d  from a radially outward position. This modification has the advantages (1), (2), (4) of the first embodiment and the advantages (1), (2), (4) of the second embodiments. 
     In the illustrated embodiments, the rollers  23 ,  72  may be replaced with spherical bodies. This modification has the same advantages as the illustrated embodiments. 
     In the second embodiment, the motor is a standing-wave type ultrasonic motor  35 . However, other types of motor, for example, a progressive wave type (flat disk type) ultrasonic motor, may be used as long as the motor has a clutch mechanism that permits the drum  32  to rotate without transmitting rotational force from the drum  32  to the motor. In this case, the shapes of the stator housing  33  and the rotor housing  34  must be changed accordingly. 
     In the second embodiment, the rotor  43  and the clutch mechanism C are coupled to the drum  32  by the reduction mechanism  36 . However the rotor  43  and the clutch mechanism C may be directly coupled to the drum  32  without a reduction mechanism. In this case, the shape of the rotor  43  and the clutch mechanism C may be changed. This modification has the same advantages as the illustrated embodiments. Further, since the reduction mechanism is omitted, the modification reduces the number of the parts. 
     In the illustrated embodiments, the clutch mechanisms  5 , C are used in the power-assisted door and the rotation device  31  of the photocopier. However, the clutch mechanisms  5 , C may be used in other types of apparatuses. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.