BACKGROUND OF THE INVENTION  
   1. Field of the Invention 
   The present invention relates to a rotational driving apparatus and a camera using the rotational driving apparatus, and more particularly to a rotational driving apparatus to be used as a rotational driving source for a lens device in which a lens is moved forward and backward along the optical axis by a cam mechanism, and a camera using the rotational driving apparatus. 
   2. Description of the Related Art 
   In the case where rotation of a motor is to be decelerated at once, for example, a worm gear is usually used. A worm gear consists of a worm and a worm wheel. In the case where rotation of a motor is to be decelerated at once, a worm is previously attached to an output shaft of the motor, and rotation of the motor is transmitted to a worm wheel which meshes with the worm, thereby immediately decelerating the rotation. 
   Usually, a worm is attached to an output shaft of the motor by press fitting. There is a possibility that the press fitting is loosened as a result of a long term use and the worm moves on the output shaft to slip off from the output shaft. 
   In JP-A-2001-309610, therefore, the tip end of a worm which is press fitted to an output shaft of a motor is pressed by a pressing member such as a spring, thereby preventing the worm from slipping off. 
   In the worm supporting structure disclosed in JP-A-2001-309610, however, the pressing force which is applied to the worm by the pressing member cannot be adjusted, and hence there is a drawback that, when the components have a dimensional error, an adequate pressing force cannot be applied to the worm. As a result, the structure has drawbacks that an excessive pressing force is applied to the worm to produce a friction loss, and that, when the pressing force is insufficient, backlash occurs in the worm. 
   SUMMARY OF THE INVENTION  
   The invention has been conducted in view of such circumstances. It is an object of the invention to provide a rotational driving apparatus which can stably perform a driving operation, and a camera using the rotational driving apparatus. 
   (1) In order to attain the object, the invention provides a rotational driving apparatus which comprises a motor, a worm that is pressingly fitted to an output shaft of the motor, and a worm wheel that meshes with the worm, so as to output rotation of the motor from the worm wheel via the worm, wherein the rotational driving apparatus comprises: a support member that is placed with forming a gap with respect to a tip end of the worm, the support member comprising a through hole positioned on a same axis as the worm; a pressing member that is inserted into the through hole, and protrudes from a tip end of the through hole to butt against a tip end face of the worm; a plug member that is fitted into the through hole to close the through hole, and in which a degree of a fitting of the plug member into the through hole is adjustable; and an urging member that is interposed between the pressing member and the plug member to urge the pressing member toward the worm. 
   According to the invention, the tip end face of the worm is axially pressed by the pressing member which is urged by the urging member. In the urging member which urges the pressing member, the urging force can be adjusted by changing the degree of the fitting of the plug member. Therefore, an adequate pressing force can be applied to the worm. 
   (2) In order to attain the object, the invention provides a rotational driving apparatus wherein, in the apparatus of (1), the pressing member is formed into a spherical shape. 
   According to the invention, the pressing member is formed into a spherical shape. As a result, the pressing member makes point contact with the tip end face of the worm, and hence the friction loss can be suppressed to a minimum level. 
   (3) In order to attain the object, the invention provides a rotational driving apparatus wherein, in the apparatus of (1) or (2), the plug member is a male thread which is screwed with a female thread portion formed in an inner periphery of the through hole. 
   According to the invention, the plug member is configured by a male thread, and the degree of the fitting into the through hole is adjusted by changing the fastening position or the thread length. 
   (4) In order to attain the object, the invention provides a rotational driving apparatus wherein, in the apparatus of (2) or (3), the tip end face of the worm comprises a recess having a hemispherical shape. 
   According to the invention, the tip end face of the worm comprises a recess having a hemispherical shape. Therefore, the pressing member which has a spherical shape can be always caused to butt against the center of the worm, and hence a stable pressing force can be applied without causing eccentricity. 
   (5) In order to attain the object, the invention provides a lens; and a cam mechanism that operates by receiving a rotational driving force from a rotational driving apparatus of (1), (2), (3), or (4), wherein the lens is moved forward and backward along an optical axis by the cam mechanism. 
   According to the invention, the rotational driving apparatus of (1), (2), (3), or (4) is used as a rotational driving source for the lens device in which a lens is moved forward and backward along the optical axis by a cam mechanism. In a lens device in which a lens is moved by a cam mechanism, the operating power is changed at the inflection point of a cam. When the rotational driving apparatus of (1), (2), (3), or (4) is used as a rotational driving source, however, abnormal noises and vibrations are suppressed, so that stable driving is enabled. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view showing the external configuration of a camera using the rotational driving apparatus of the invention; 
       FIG. 2  is a view schematically showing the configuration of a zoom driving portion for an imaging lens; 
       FIG. 3  is a view showing the configuration of a worm supporting mechanism; 
       FIG. 4  is a view showing another embodiment of the worm supporting mechanism; and 
       FIG. 5  is a view showing a further embodiment of the worm supporting mechanism. 
   


   DETAILED DESCRIPTION OF THE INVENTION  
   Hereinafter, the best mode for carrying out the rotational driving apparatus of the invention, and a camera using the rotational driving apparatus will be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a perspective view showing the external configuration of a camera using the rotational driving apparatus of the invention. The camera  10  is a film camera which uses a 135 film. An imaging lens  14 , a finder window  16 , a strobe flash  18 , and the like are disposed on the front face of the camera body  12 . A shutter release button  20  is disposed on the upper face of the camera body  12 , and a power switch, a zoom button, and the like are disposed on the back face of the camera body  12  which is not shown. 
   The imaging lens  14  is configured by a collapsible type zoom lens. When the power supply of the camera  10  is turned ON, the lens advances from the front face of the camera body  12 , and then stops at the wide-angle end. When the power supply of the camera  10  is turned OFF, the lens is housed in the camera body  12 . When the zoom button is operated, the imaging lens  14  protruding from the camera body  12  is zoom-driven so that the focal length is changed. 
     FIG. 2  is a view schematically showing the configuration of the zoom driving portion for the imaging lens  14 . In the imaging lens  14 , when an operating gear  24  formed on the outer periphery of a lens barrel  22  is rotated, the lens barrel  22  is caused to extend or contract along the optical axis by the function of a cam mechanism which is not shown, whereby the focal length is changed. The operating gear  24  is rotated by a zoom motor  26 . 
   The zoom motor  26  is fixed to a camera body frame which is not shown. A worm  28  is attached to an output shaft  26 A of the motor. A worm wheel  30  meshes with the worm  28 , and rotation of the zoom motor  26  is transmitted from the worm wheel  30  to the operating gear  24  via a reduction gear train  32 . 
   The worm  28  is formed into a hollow shape, and attached to the output shaft  26 A of the zoom motor  26  by pressingly fitting the output shaft  26 A into the hollow portion. 
   The worm wheel  30  is rotatably supported by a shaft  34  formed on the camera body frame which is not shown. A small-diameter output gear  36  is formed coaxially with the upper face of the worm wheel  30  in an integral manner. 
   The reduction gear train  32  is configured by first to sixth gears  32 A to  32 F. The first gear  32 A is rotatably supported by a shaft  40  formed on the camera body frame which is not shown, and meshes with the output gear  36 . The second gear  32 B having a small diameter is formed coaxially with the lower face of the first gear  32 A in an integral manner. 
   The third gear  32 C is rotatably supported by a shaft  42  formed on the camera body frame which is not shown, and meshes with the second gear  32 B. The fourth gear  32 D having a small diameter is formed coaxially with the upper face of the third gear  32 C in an integral manner. 
   The fifth gear  32 E is rotatably supported by a shaft  44  formed on the camera body frame which is not shown, and meshes with the fourth gear  32 D. 
   The sixth gear  32 F is rotatably supported by a shaft  46  formed on the camera body frame which is not shown, and meshes with the fifth gear  32 E, and also with the operating gear  24 . 
   Rotation of the output shaft  26 A of the zoom motor  26  is transmitted from the worm  28  to the worm wheel  30 , and then from the worm wheel  30  to the operating gear  24  via the reduction gear train  32 , whereby the imaging lens  14  is driven. 
   As described above, the attachment of the worm  28  to the output shaft  26 A of the zoom motor  26  is realized by press fitting onto the output shaft  26 A. Consequently, there is a possibility that the fitting is loosened as a result of a long term use and the worm slips off from the output shaft  26 A. In order to prevent the worm  28  from being loosened, a worm supporting mechanism  50  which axially presses the worm  28  to support it is disposed in a tip end portion of the worm  28 . 
     FIG. 3  is a view showing the configuration of the worm supporting mechanism  50 . As shown in the figure, the worm supporting mechanism  50  is configured by a worm supporting mechanism body  52 , a rigid ball  54 , a screw  56 , and a coil spring  58 . 
   The worm supporting mechanism body  52  is formed into a block-like shape, and placed with forming a predetermined gap with respect to the tip end face of the worm  28  which is press fitted onto the output shaft  26 A. The worm supporting mechanism body  52  is fixed to the camera body frame which is not shown. A through hole  60  is formed in the body so as to be coaxial with the worm  28 . 
   The rigid ball  54  is inserted into the through hole  60  formed in the worm supporting mechanism body  52 , and protrudes from the tip end face of the through hole  60  to butt against the tip end face of the worm  28 . 
   A female thread portion  60 A is formed in the inner periphery of a basal end portion of the through hole  60 . The screw  56  is screwed with the female thread portion  60 A of the through hole  60 . 
   The coil spring  58  is interposed between the rigid ball  54  and the screw  56 . The rigid ball  54  is urged by the coil spring  58  to axially press the tip end face of the worm  28 . 
   When the screw  56  is deeply fastened to the female thread portion  60 A, the coil spring  58  exerts a large urging force, and, when the screw  56  is shallowly fastened, the coil spring  58  exerts a small urging force. Namely, the degree of the urging force exerted by the coil spring  58  can be adjusted in accordance with the fastening position of the screw  56 . As a result, it is possible to adjust the pressing force exerted by the rigid ball  54 . 
   In the thus configured zoom driving portion for the imaging lens  14  of the camera  10  of the embodiment, when the zoom motor  26  is driven to rotate the output shaft  26 A, the rotation of the output shaft  26 A is transmitted from the worm  28  to the worm wheel  30 , and then from the worm wheel  30  to the operating gear  24  via the reduction gear train  32 . As a result, the imaging lens  14  is driven, and the lens barrel  22  is caused to extend or contract along the optical axis by the function of the cam mechanism which is not shown, whereby the focal length is changed. 
   In this case, the worm  28  is always axially pressed by the rigid ball  54  of the worm supporting mechanism  50 , and hence is not loosened, so that stable rotation can be always ensured. 
   The degree of the pressing force which is applied form the rigid ball  54  to the worm  28  can be adjusted by the fastening position of the screw  56 . Even when the accuracies of the components are dispersed, therefore, a pressing force which is optimum for each product can be always applied, and stable operation can be ensured. 
   Since the component which presses the tip end face of the worm  28  is the rigid ball  54 , the rigid ball  54  makes point contact with the worm  28 . Therefore, the friction loss can be suppressed to a minimum level, so that the driving operation can be efficiently performed. 
   In the embodiment, the component which presses the tip end face of the worm  28  is the rigid ball  54 . However, the component which presses the tip end face of the worm  28  is not restricted to this. When a friction loss is considered, however, it is preferable to employ a configuration where the member makes point contact with the tip end face of the worm  28  as in the embodiment. As shown in  FIG. 4 , for example, a pressing member  55  in which a tip end portion has a conical shape may press the tip end face of the worm  28 . 
   In the case where the component which presses the tip end face of the worm  28  is the rigid ball  54 , the tip end face  28 A of the worm  28  may be formed into a hemispherical shape as shown in  FIG. 5 . According to the configuration, the rigid ball  54  is always caused to press the center of the worm  28  by the centripetal action of the tip end face, whereby stable rotation of the worm  28  can be always ensured. 
   In the embodiment, the screw  56  is used as the plug member which closes the basal end portion of the through hole  60  formed in the worm supporting mechanism body  52 . The plug member is requested only to have a configuration in which the degree of fitting (fitting depth) into the through hole  60  is adjustable, and is not restricted to the screw  56 . 
   In the embodiment, the degree of fitting into the through hole  60  is adjusted by the fastening position of the screw  56 . Alternatively, the degree of fitting into the through hole  60  may be adjusted by using screws of different lengths. 
   In the above, the embodiment in which the invention is applied to a zoom driving portion of a camera has been exemplarily described. The application of the invention is not restricted to this, and the invention can be similarly applied to other machines. 
   In the rotational driving apparatus of the invention, and a camera using the rotational driving apparatus, it is possible to stably perform a driving operation. 
   The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth.
A rotational driving apparatus with a motor, a worm pressingly fitted to an output shaft of the motor, and a worm wheel meshing with the worm, is provided to output rotation of the motor from the worm wheel via the worm. The apparatus includes support, pressing, plug, and urging members. The support member is placed forming a gap with respect to a tip end of the worm, and includes a through hole positioned on a same axis as the worm. The pressing member is inserted into the through hole, and protrudes from a tip end of the through hole to butt against a tip end face of the worm. The plug member is fitted into the through hole to close the through hole, in which its fitting degree into the through hole is adjustable. The urging member is interposed between the pressing member and the plug member to urge the pressing member toward the worm.