Abstract:
An optical device includes an optical system including a plurality of lens groups, at least one lens group being a movable lens group moving in an optical axis direction, and a movable frame to move the movable lens group. The movable frame includes a driving source, a drive screw, a lens supporting frame, and maintaining an integrated state relative to the movable lens group, a plurality of arms disposed in a position to face the drive screw and being openable and closable relative to the drive screw with a single opening and closing axis as a support axis, the plurality of arms including a first arm and a second arm, and a biasing spring configured to apply a biasing force to the first arm in a direction in which the first arm approaches the drive screw, wherein the lens supporting frame is further configured to support the biasing spring.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of U.S. patent application Ser. No. 14/280,977, filed May 19, 2014, which claims priority from Japanese Patent Application No. 2013-108483, filed on May 23, 2013, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
     Field of the Invention 
       [0002]    The present invention relates to a lead screw device, a lens driver, a lens barrel, and a camera using the lead screw device. 
       Description of the Related Art 
       [0003]    A lead screw device is used as a lens driver which drives a lens of a camera in an optical axis direction. The lead screw device includes a lead screw which rotates about an axis and a nut member which is threadably mounted on the lead screw, and moves in a straight line along with the rotation of the lead screw about an axis. The lead screw device is configured to hold a lens in a lens holder connected to the nut member. A focus adjuster described in JP3766379B, for example, uses such a lead screw device. 
         [0004]    In this lead screw device, it is necessary to threadably mount the nut member on the lead screw from one end portion of the lead screw in the case of mounting the nut member on the lead screw. However, in the lead screw device described in JP3766379B, both ends of the lead screw in the axis direction are supported by a U-shaped supporting member. The nut member cannot be therefore assembled to the lead screw. For this reason, it is necessary to assemble the lead screw after assembling the nut member to the lead screw in advance or to provide a special supporting structure of the lead screw such that the nut member can be assembled after assembling the lead screw. Such requirements for the assembling order or the special structure trigger an increase in the manufacturing cost of a lens driver. 
         [0005]    As a method of solving the above problem, it is considered to use a half nut described in JP2009-80248A as the nut member. This half nut is a nut member in which a circular nut member is cut in a radial direction. The half nut can be laterally threadably mounted on the lead screw, namely, the lead screw can be threadably mounted on the lead screw from the radial direction even in a state in which the lead screw is assembled. Therefore, the above-described requirements for the assembling order and the special structure are unnecessary, and an increase in the manufacturing cost of the lens driver can be avoided. 
         [0006]    With the configuration using the half nut as described in JP2009-80248A, the half nut is threadably mounted on the lead screw only from one side of the lead screw in the radial direction of the lead screw. Thus, the strength of the half nut when threadably mounted on the lead screw is reduced. For this reason, tooth skipping of the half nut easily occurs when an external force in the axis direction of the lead screw is applied to the half nut. More specifically, the half nut moves over the thread of the lead screw by the external force, so that the half nut moves in the axis direction of the lead screw. In order to prevent the tooth skipping, if the half nut is pressed to the lead screw in a radial direction so as to have strong contact with the lead screw, the stress due to the external force applied to the half nut is concentrated in the position where the half nut is threadably mounted on the lead screw, namely, the thread portions of both of the half nut and the lead screw. For this reason, the thread portions may be damaged due to the stress. In a lens driver, which drives the lens group on a subject side among a plurality of lens groups disposed in a lens barrel, a photographer may touch the lens group, and the external force may be applied to the lens group. Therefore, it is required to control or prevent the tooth skipping or damage on the thread portion due to such an external force. 
       SUMMARY 
       [0007]    It is, therefore, an object of the present invention to provide a lead screw device which can control tooth skipping and prevent damage on a thread portion. Another object of the present invention is to provide a lens driver, lens barrel, and camera including the lead screw device which prevents the tooth skipping and damage on the thread portion. 
         [0008]    To attain the above object, one embodiment of the present invention provides a lead screw device, including: a lead screw including a thread portion; and a driven body which engages with the thread portion, and moves in an axis direction of the lead screw along with rotation of the lead screw about an axis, the driven body including a pair of driven members disposed to sandwich the lead screw in a radial direction, wherein the pair of driven members is biased in a direction which sandwiches the lead screw in the radial direction, and a tooth portion provided in each of the driven members engages with the thread portion of the lead screw by a biasing force. 
         [0009]    One embodiment of the present invention also provides a lead screw device, including: a lead screw; and a driven body which engages with the lead screw and moves in an axis direction of the lead screw along with rotation of the lead screw about an axis, the driven body including a pair of driven members disposed to sandwich the lead screw in parallel with an axis center of the lead screw, wherein the pair of driven members is biased in a direction coming close to the axis center, and moves in a direction away from the axis center against the biasing force. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings are included to provide further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate an embodiment of the invention and, together with the specification, serve to explain the principle of the invention. 
           [0011]      FIG. 1  is a sectional view illustrating an entire configuration of a lens barrel according to an embodiment of the present invention. 
           [0012]      FIG. 2  is an external perspective view illustrating a lens cylinder according to the embodiment of the present invention. 
           [0013]      FIG. 3  is an exploded perspective view illustrating the lens cylinder according to the embodiment of the present invention. 
           [0014]      FIG. 4  is an external perspective view illustrating a driven body of a lead screw device or a lens driver according to the embodiment of the present invention. 
           [0015]      FIG. 5  is an exploded perspective view illustrating a substantial section of the driven body. 
           [0016]      FIG. 6A  is a front view illustrating the assembled driven body. 
           [0017]      FIG. 6B  is a sectional view illustrating the substantial section of the driven body. 
           [0018]      FIG. 7  is an external perspective view illustrating the substantial section of the lens driver. 
           [0019]      FIGS. 8A, 8B  are a perspective view and a sectional view, respectively, illustrating the substantial section of the lens driver in which a part of the substantial section is cut. 
           [0020]      FIGS. 9A to 9C  are schematic sectional views describing the engagement of a rack tooth and a thread portion. 
           [0021]      FIG. 10  is an exploded perspective view illustrating the substantial section of the driven body including a tooth-skipping detector. 
           [0022]      FIG. 11  is a front view illustrating the driven body including the tooth-skipping detector in which a part of the driven body is cut. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Next, an embodiment of the present invention will be described with reference to the drawings. This embodiment is an example in which the present invention is applied to a lens driver provided in a lens barrel of a camera.  FIG. 1  illustrates an embodiment in which a lens barrel of a camera according to the present invention is integrally incorporated into a camera body CB as a lens unit LU.  FIG. 1  is a sectional view illustrating a camera in shooting which is cut along the optical axis of a lens. Referring to  FIG. 1 , sections associated with the present invention will be described. In the lens unit LU, a rotation barrel  2  is held in the inner circumferential surface of a fixed barrel  1  fixed in the camera body CB illustrated by the dashed line in  FIG. 1 . A straight movement barrel  3  and a lens cylinder  4  are held inside the rotation barrel  2 . The straight movement barrel  3  and the lens cylinder  4  are helicoid-coupled to the rotation barrel  2 . The rotation barrel  2  rotates about a lens optical axis Lx in shooting by a driving force of an extending motor which is not illustrated in  FIG. 1 . The straight movement barrel  3  and the lens cylinder  4  straightly move forward (toward a subject, hereinafter, the subject side is referred to as a front and the opposite side is referred to as a back in the front and back direction) by the rotation of the rotation barrel  2 , so as to extend from the housed position in the camera body CB. The straight movement barrel  3  and the lens cylinder  4  are separately helicoid-coupled to the rotation barrel  2 , so that the extending amount of each barrel and cylinder differs. A lens barrier  31 , which protects the inside of the lens unit LU by opening an opening provided in the front surface of the straight movement barrel  3  in shooting and closing the opening in non-shooting, is provided in the front end portion of the straight movement barrel  3 . 
         [0024]    The lens cylinder  4  includes a motor frame  5  disposed in the back side of the lens optical axis direction and a lens-holding frame  6  which is disposed in front of the motor frame  5 . A lens driver  7  of the present invention moves the lens-holding frame  6  relative to the motor frame  5  in the lens optical axis direction for focusing. When the straight movement barrel  3  is extended by the rotation of the rotation barrel  2 , the motor frame  5  is also extended to a predetermined position. The lens-holding frame  6  includes a first group lens L 1 , shutter S, and second group lens L 2 , and is supported to be movable relative to the motor frame  5  in the lens optical axis direction, and the relative movement of the lens-holding frame  6  in the lens optical axis direction is controlled by the lens driver  7 . 
         [0025]      FIG. 2  is a perspective view illustrating the lens cylinder  4  as seen from the left anterior side.  FIG. 3  is a perspective view illustrating the separated motor frame  5  and the lens-holding frame  6  constituting the lens cylinder  4 . The motor frame  5  includes an approximate circular shape, and a plurality of supporting pieces  51  for holding the lens-holding frame  6 . The supporting pieces  51  are provided in two positions in the lower circumference surface of the motor frame  5 , and the supporting pieces  51  project forward. The motor frame  5  also includes a thin cylindrical main axis  52  which is a guide for moving the lens-holing frame  6  in the lens optical axis direction. The main axis  52  is provided in a part of the upper circumferential surface, and the main axis  52  projects in the lens optical axis direction. 
         [0026]    On the other hand, the lens-holding frame  6  is formed as a short cylindrically-shaped container. The first lens group L 1 , the second lens group L 2 , and the shutter S which are not illustrated in  FIGS. 2, 3  are held in the circular opening provided in the central position of the lens-holding frame  6 . The lens-holding frame  6  is disposed in front of the motor frame  5 , and the lower circumferential surface of the lens-holding frame  6  is supported by the two supporting pieces  51  of the motor frame  5 . A narrow bracket  61  is provided in a part of the circumference of the lens-holding frame  6 , and the narrow bracket  61  extends backward. The main axis  52  of the motor frame  5  is inserted into a main axis hole  611  which opens in the bracket  61  in the lens optical axis direction, so that the lens-holding frame  6  can be relatively movable in the lens optical axis direction while maintaining the coaxial state relative to the motor frame  5 . In this embodiment, the focusing in the lens unit LU is performed by the movement of the lens-holding frame  6  in the lens optical axis direction. 
         [0027]    The lens driver  7  of the present invention includes a lead screw device which reciprocates the lens-holding frame  6  in the lens optical axis direction relative to the motor frame  5 . Namely, as illustrated in  FIGS. 2, 3 , a driving motor  8  of a stepping motor is fastened to the motor frame  5  by a fastening plate  81 . A lead screw  82  having a predetermined length in the lens optical axis direction is coaxially connected to the not-illustrated rotation output axis of the driving motor  8 . A thread portion  83  having a triangular or trapezoidal shape (in this case, trapezoidal shape) in section in an axis center direction is formed in the circumferential surface of the lead screw  82 . A part of the fastening plate  81  is formed to project forward into an L-shape as an axis-supporting piece  84 , in order to ensure the stability of the lead screw  82  in an axis center direction during the rotation, and the leading end portion of the lead screw  82  is supported by the leading end of the axis-supporting piece  84 . With this axis supporting structure, the lead screw  82  stably rotates about an axis center parallel to a lens optical axis Lx while the driving motor  8  rotates. 
         [0028]    On the other hand, the driven body  9  which is engagable with the thread portion  83  of the lead screw  82  is integrally supported by a part of the circumference of the lens-holding frame  6  facing the lead screw  82  in the lens optical axis direction. When the lens-holding frame  6  is assembled to the motor frame  5 , the driven body  9  engages with the thread portion  83  of the lead screw  82 . When the lead screw  82  rotates by the spiral structure of the thread portion  83  of the lead screw  82 , the driven body  9  reciprocates in a straight line in an axis direction, namely, the lens optical axis Lx direction. The lens-holding frame  6  which supports the driven body  9  moves in the lens optical axis direction together with the driven body  9  while being guided by the main axis  52  of the motor frame  5 . 
         [0029]    As illustrated in the external perspective view in  FIG. 4  and the exploded perspective view of the substantial section in  FIG. 5 , the driven body  9  includes a pair of rack members  91  (first driven member),  92  (second driven member), which is supported by one rack axis  93 , so as to individually rotate about the rack axis  93 . The first rack member  91  located in the lower side in  FIGS. 4, 5  includes a base end portion  911  through which the rack axis  93  is inserted, a middle portion  912 , and a leading end portion  913 . The middle portion  912  extends in an approximate straight line in the horizontal direction from the base end portion  911 , and the leading end portion  913  has an inverted L-shape. A first rack tooth  94  (first tooth portion) which can engage with the thread portion  83  of the lead screw  82  is integrally formed on the upper surface of the middle portion  912 . The second rack member  92  located in the upper side in  FIGS. 4, 5  has a length in the horizontal direction, which is shorter than that of the first rack member  91 . The second rack member  92  includes a base end portion  921  through which the rack axis  93  is inserted and a leading end portion  922  extending from the base end portion  921 . A second rack tooth  95  (second tooth portion) which can engage with the thread portion  83  of the lead screw  82  is integrally formed in the lower surface of the leading end portion  922 . The first rack tooth  94  and the second rack tooth  95  are disposed to face each other. Both of the first rack tooth  94  and the second rack tooth  95  engage with the thread portion  83  of the lead screw  82  while sandwiching the thread portion  83  therebetween in a radial direction. In this embodiment, each of the first rack tooth  94  and the second rack tooth  95  includes three teeth arranged in parallel, and each of the three teeth has a triangular shape or a trapezoidal shape (in this case, trapezoidal shape) in section in accordance with the shape of the thread portion  83  of the lead screw  82 . 
         [0030]    A cylindrical boss  923  is integrally formed in the base end portion  921  of the second rack member  92 . The rack axis  93  is inserted through the boss  923 . An approximate U-shaped axis supporter  914  having a measurement corresponding to a length of the base end portion  921  and the boss  923  of the second rack member  92  is integrally formed in the base end portion  911  of the first rack member  91 . Both ends of the rack axis  93  inserted through the base end portion  921  and the boss  923  of the second rack member  92  are supported by the axis supporter  914 . The first and second rack members  91 ,  92  thereby individually rotate about the rack axis  93 . In addition, a second biasing spring  97  (second biasing member) is wound around the boss  923 . The details thereof will be described later. 
         [0031]    The driven body  9  with the above-described configuration is assembled to the upper portion of the lens-holding frame  6  as illustrated in  FIGS. 2, 3 .  FIG. 6A  is a front view illustrating the assembled driven body  9 .  FIG. 6B  is a sectional view along B-B line. Both ends of the rack axis  93  project from the axis supporter  914  of the first rack member  91 . Both of the projecting ends of the rack axis  93  are supported by a pair of ribs  62  illustrated by the dashed line in  FIGS. 6A, 6B . A pair of ribs  62  is provided in a part of a circumference of a circumferential surface  63  of the lens-holding frame  6 , and projects in an external diameter direction, so as to face to each other in the lens optical axis direction. The first rack member  91  and the second rack member  92  thereby rotate about the rack axis  93  in a radial direction on a surface perpendicular to the lens optical axis Lx on the lens-holding frame  6 . 
         [0032]    When the first and second rack members  91 ,  92  are supported by the lens-holding frame  6  as described above, the lower surface of the leading end portion  913  of the first rack member  91  is disposed to face a part of the circumferential surface  63  of the lens-holding frame  6 . A first biasing spring  96  (first biasing member) made of a compression coil spring is arranged between the lower surface and the circumferential surface in an approximate radial direction. The first rack member  91  rotates about the rack axis  93  in the outer diameter direction of the lens-holding frame  6  by the biasing force of the first biasing spring  96 . The second biasing spring  97  made of a compression torsion spring is wound around the boss  923  of the second rack member  92 . Both ends of the second biasing spring  97  are locked to the first rack member  91  and the second rack member  92 , respectively, so that the second rack member  92  rotates in the direction in which the leading end portion  922  comes close to the first rack member  91 , namely, in the inner diameter direction of the lens-holding frame  6 . However, the first rack member  91  has contact with the second rack member  92  in a part of the facing surfaces of the base end portions  911 ,  921 . These contact surfaces therefore operate as position-controlling surfaces  915 ,  925  (position controller). The rack tooth  94  of the first rack member  91  and the rack tooth  95  of the second rack member  92  are also controlled by the biasing force of the second biasing spring  97 , so that they do not come close to each other over a required interval. The second biasing spring  97  is arranged between the second rack member  92  and the first rack member  91  in a state compressed in the axis direction of the rack axis  93 . With this configuration, the end surface T of the axis supporter  914  of the first rack member  91  has contact with the inner surface of the rib  62  of the lens-holding frame  6  by an elastic recovery force. By the contact force in the end surface T, the first rack member  91  is integrated with the lens-holding frame  6 , more specifically, the driven body  9  is integrated with the lens-holding frame  6 , and the first rack member  91 , namely, the driven body  9  and the lens-holding frame  6  move together in the lens optical axis direction. 
         [0033]    The driven body  9  with the above-described configuration is assembled to the lead screw  82 , so as to constitute the lens driver  7 . In this case, the main axis  52  of the motor frame  5  is inserted through a main axis hole  611  of the lens-holding frame  6 , so that the lens-holding frame  6  is coaxially assembled in the front surface position of the motor frame  5 . In this case, the driven body  9  moves forward from the lateral side of the lead screw  82 , and the lead screw  82  is sandwiched between the first rack member  91  and the second rack member  92 .  FIG. 7  is a perspective view illustrating a substantial section in this state. Namely, the first rack member  91  rotates in the inner diameter direction of the lens-holding frame  6  against the biasing force of the first biasing spring  96 , and the second rack member  92  rotates in the outer diameter direction of the lens-holding frame  6  against the biasing force of the second biasing spring  97 . With this configuration, the interval between the facing rack members  91 ,  92  is increased, so that the lead screw  82  can be sandwiched between the rack members  91 ,  92 . 
         [0034]    With this sandwiched state, as illustrated in the perspective view and sectional view in  FIGS. 8A, 8B  in which a part of the configuration is cut, the first rack tooth  94  of the first rack member  91  engages with the thread portion  83  of the lead screw  82  from the inner diameter side of the lens-holding frame  6 , and the second rack tooth  95  of the second rack member  92  engages with the thread portion  83  from the outer diameter side of the lens-holding frame  6  by the biasing forces of the first and second biasing springs  96 ,  97 . More specifically, although both ends of the lead screw  82  are supported by the bracket  84  of the motor frame  5 , the first and second rack members  91 ,  92  of the driven body  9  supported by the lens-holding frame  6  engage with the thread portion  83  of the lead screw  82 . Therefore, the assembling orders of the lens driver  7  are not limited, and such a configuration is advantageous for simplifying the manufacturing and reducing the manufacturing costs. 
         [0035]    In a state in which the driven body  9  is assembled to the lead screw  82 , the first biasing spring  96  applies the biasing force to the leading end portion  913  of the first rack member  91 , and the first rack tooth  94  is arranged in the intermediate portion  912  of the first rack member  91 . The engagement force when the first rack tooth  94  engages with the thread portion  83  of the lead screw  82  therefore becomes larger than the biasing force of the first biasing spring  96  by this principle, so that the stable and preferable engagement with the thread portion  83  of the lead screw  82  can be obtained. The second biasing spring  97  is wound around the rack axis  93 . The second biasing spring  97  can be therefore assembled at the same time as the first rack member  91  and the second rack member  92  are assembled. The assembling operation can be thus simplified, and the entire driven body  9  can be downsized. As described above, the first rack tooth  94  engages with the thread portion  83  by a desired spring force with the first biasing spring  96 , and the second rack tooth  95  engages with the thread portion  83  by a desired spring force with the second biasing spring  97 . 
         [0036]    Accordingly, in the lens driver  7  assembled as described above, the lead screw  82  rotates upon the rotation of the driving motor  8 , and the first rack tooth  94  and the second rack tooth  95  engaging with the thread portion  83  of the lead screw  82  move along the thread portion  83  in the axis direction of the lead screw  82 . The driven body  9  made of the first rack member  91  having the first rack tooth  94  and the second rack member  92  having the second rack tooth  95  and the lens-holding frame  6  which supports the driven body  9  move in the lens optical axis direction, so that the focusing in the lens unit LU can be performed as described above. 
         [0037]    In this case, in the lens unit according to the present embodiment, the lens barrier illustrated in  FIG. 1  opens in shooting. The lens cylinder  4  is exposed through the front opening of the straight movement barrel  3 . The external force in the lens optical axis direction is applied to the lens-holding frame  6  located in the front side when a photographer&#39;s hand or the like has contact with the lens cylinder  4 . The external force applied to the lens-holding frame  6  is transferred to the first and second rack teeth  94 ,  95  through the first and second rack members  91 ,  92  of the driven body  9 . The stress associated with the external force is generated among the first rack tooth  94 , the second rack tooth  95 , and the thread portion  83  of the lead screw  82  engaging with these teeth. The first and second rack teeth  94 ,  95  disengage from the thread portion  83  due to the stress, so that so-called tooth skipping may occur. When the tooth skipping does not occur, the first rack tooth  94 , the second rack tooth  95 , or the screw section  83  may be damaged due to the stress. 
         [0038]    In contrast, in the present embodiment, the first rack tooth  94  elastically engages with the thread portion  83  by the biasing force of the first biasing spring  96 , and the second rack tooth  95  elastically engages with the thread portion  83  by the biasing force of the second biasing spring  97 . Therefore, when the stress among the rack teeth  94 ,  95  and the thread portion  83  is increased, the stress in a radial direction is generated by the wedge effect in the tapered surfaces in which each of the rack teeth  94 ,  95  has contact with the thread portion  83 . The first and second rack teeth  94 ,  95  move in the outer diameter direction of the thread portion  83  by the stress against the biasing force of each of the first and second biasing springs  96 ,  97 . The stress is absorbed or reduced by the movement in the outer diameter direction, so that the rack teeth  94 ,  95  are prevented from being damaged. In this case, both of the rack teeth  94 ,  95  are maintained in the engagement state with the thread portion  83 , so that the tooth skipping does not occur. If the stress in each of the rack teeth  94 ,  95  is further increased upon an increase in the external force, the facing interval between the rack teeth  94 ,  95  is further increased, so that the rack teeth  94 ,  95  disengage from the thread portion  83 , and tooth skipping occurs. However, each of the rack teeth  94 ,  95  is reliably prevented from being damaged. 
         [0039]    In this embodiment, in order to improve the effect of preventing the damage on such a rack tooth and the effect of controlling the tooth skipping, the driven body  9  is configured as follows. More specifically, although the second rack member  92  is biased in a direction close to the first rack member  91  by the biasing force of the second biasing spring  97  as described above, the rotation angle (hereinafter sometimes referred to as a nip angle) of the second rack member  92  relative to the first rack member  91  is controlled by the contact between the position-controlling surfaces ( FIG. 6 )  915 ,  924  provided in the facing surfaces of the first rack member  91  and the second rack member  92 . Namely, both of the rack members  91 ,  92  have contact with each other by the position-controlling surfaces  915 ,  924 , so that the nip angle between the first rack member  91  and the second rack member  92 , i.e., the interval between the facing first rack tooth  94  and the second rack tooth  95  is controlled. An interval L 1  between the facing first rack tooth  94  and the second rack tooth  95  (in this case, the interval between the leading ends of the rack teeth  94 ,  95 ) is set to be larger than an interval L 2  by a predetermined allowance L 0  when both of the rack teeth  94 ,  95  engage with the thread portion  83  in a normal state. This allowance L 0  is a measurement smaller than a height H from the normal engagement position in the thread portion  83  to the screw outer diameter. In addition, L 3  is a measurement of the external diameter of the thread portion  83 . 
         [0040]    The first and second biasing springs  96 ,  97  are designed such that the biasing force when the first rack tooth  94  engages with the thread portion  83  by the first biasing spring  96  becomes larger than the biasing force when the second rack tooth  95  engages with the thread portion  83  by the second biasing spring  97 . More specifically, the biasing force of the first biasing spring  96  is set to be a biasing force which can maintain a state in which the first rack tooth  94  biases the thread portion  83  even if the gravity compresses the first biasing spring  96  according to a change in the position of the lens unit LU. Moreover, it is similar to the biasing force of the second biasing spring  97 . However, the biasing force of the first biasing spring  96  and the biasing force of the second biasing spring  97  are set to be biasing forces, respectively, such that each section is not damaged due to the impact generated by the engagement between the rack teeth  94 ,  95  and the thread portion  83  when the first rack tooth  94  and the second rack tooth  95  sandwich therebetween the thread section  83  in a radial direction by the biasing forces of the first biasing spring  96  and the second biasing spring  97 . For this reason, the biasing force of one biasing spring, in this case, the second biasing spring  97  is set to be smaller so as to avoid the generation of the above-described damage even if the biasing forces of both biasing springs  96 ,  97  are synergized while setting the minimum biasing forces of the first and second biasing springs  96 ,  97  to the above-described biasing force. As a result, in this embodiment, the biasing force of the first rack tooth  94  is set to be approximately twice the biasing force of the second rack tooth  95 . 
         [0041]    With this design, the first rack tooth  94  engages with the thread portion  83  in a normal state by the biasing force of the first biasing spring  96 , as illustrated in  FIG. 9A , when the external force is not generated in the driven body  9 , i.e., the first and second rack teeth  94 ,  95  through the lens-holding frame  6 . However, the second rack tooth  95  is separated from the screw section  83  in an outer diameter direction by the allowance L 0  by the control with the position control surfaces  915 ,  924 . Accordingly, when the lead screw  82  rotates by the driving motor  8  in this state, the driven body  9  straightly moves to the lead screw  82  while maintaining the engagement between the first rack tooth  94  and the thread portion  83 . 
         [0042]    On the other hand, if the external force is applied to the driven body  9 , as illustrated in  FIG. 9B , the stress in the outer diameter direction is generated by the wedge effect in the contact surfaces of the first rack tooth  94  and the thread portion  83 . The first rack tooth  94  moves by the stress in the outer diameter direction of the thread section  83  as the arrow M against the biasing force of the first biasing spring  96 . In this case, the first rack tooth  94  does not move in the outer diameter direction with the external force smaller than the biasing force of the first biasing spring  96 , so that the stable engagement state is ensured. When the stress larger than the biasing force of the first biasing spring  96  is generated based on an increase in the external force, the first rack tooth  94  moves in an outer diameter direction, but the second rack tooth  95  integrally moves in the arrow M direction following the first rack tooth  94  by the biasing force of the second biasing spring  97 . Therefore, the second rack tooth  95  moves in the inner diameter direction of the thread portion  83 , and has contact with the thread portion  83 . The first rack tooth  94  and the second rack tooth  95  thereby engage with the thread portion  83 , so that the engagement relationship among both of the rack teeth  94 ,  95  and the thread portion  83  is improved compared to the engagement in which the first rack tooth  94  only engages with the thread portion  83 . The external force is received by the engagement among the first and second rack teeth  94 ,  95  and the thread portion  83 , and thus, the rack teeth  94 ,  95  are prevented from being damaged by the external force. The tooth skipping in each rack tooth  94 ,  95  can be prevented because the engagement among both of the rack teeth  94 ,  95  and the thread portion  83  is maintained. 
         [0043]    If the external force is further increased from this state, as illustrated in  FIG. 9C , the stress in a radial direction in the engagement surfaces of the thread portion  83  and the first and second rack teeth  94 ,  95  is increased. Both of rack teeth  94 ,  95  move in an outer diameter direction by this stress, namely, the first rack tooth  94  moves in the arrow M direction, and the second rack tooth  95  moves in the arrow N direction, so that the interval between the facing rack teeth  94 ,  95  is increased to the measurement L 1   x . With this state, both of the rack teeth  94 ,  95  slightly engage with the thread portion  83 . The tooth skipping does not occur if the external force is removed from this state, and the normal state illustrated in  FIG. 9A  is again obtained by each of the biasing forces of the first and second biasing springs  96 ,  95 . However, if the interval L 1   x  between the rack teeth  94 ,  95  becomes larger than the outer diameter L 3  of the thread portion  83 , as illustrated in  FIG. 9C , upon a further increase in the external force, both of the rack teeth  94 ,  95  disengage from the thread portion  83 , so that the tooth skipping is generated. Owing to the tooth skipping, an increased stress in the engagement surfaces of both of rack teeth  94 ,  95  and the thread portion  83  is released, and the damage on the rack teeth  94 ,  95  or the damage on the thread portion  83  by the stress is prevented in advance. Although the tooth skipping occurs, the lens driver  7  can be prevented from being damaged and the lens cylinder  4  or the lens unit LU can be prevented from being damaged. 
         [0044]    When the damage on the rack teeth  94 ,  95  and the thread portion  83  is prevented by generating the tooth skipping if a large external force is applied as described above, the damage on the lens driver  7  can be prevented in advance. However, a problem is created in the driving control of the lens-holding frame  6 , in this case, the focusing control because the positions of the rack teeth  94 ,  95  relative to the lead screw  82 , namely, the position of the driven body  9  in the lens optical axis direction is changed, and the position of the lens-holding frame  6  in the lens optical axis direction is changed relative to the rotation angle of the driving motor  8 . With respect to such a problem, a lens position detector which detects a position of the lens-holding frame  6  or the lens cylinder  4  in the lens optical axis direction, for example, a photosensor is disposed in the lens unit LU, and the focusing control is reset when the lens-holding frame  6  is detected by the lens position detector, so that the influence due to the tooth skipping can be resolved, and the subsequent focusing can be appropriately performed. In general, the lens cylinder  4  is housed on the camera body CB side in response to turning off the main switch of the camera. The position of the lens is detected by the lens position detector in the housing, so that the focusing control is reset. 
         [0045]    However, since the influence due to the tooth skipping remains until the position detector detects the lens-holding frame, there may be a problem in focusing in this period. Consequently, in the present embodiment, a tooth-skipping detector is disposed, and the focusing is reset just after the tooth skipping is detected.  FIG. 10  is an exploded perspective view illustrating one example of the tooth-skipping detector. Contact pieces  916 ,  925  are disposed in the position control surfaces  915 ,  924  of the first and second rack members  91 ,  92 , respectively. In this case, as illustrated in the front view of  FIG. 11  in which a part of the configuration is cut, concave portions  917 ,  926  are formed in the position control surfaces  915 ,  924 , respectively, and the contact pieces  916 ,  925  made of a conductive member are buried in the concave portions, respectively. The first contact piece  916  of the first rack member  91  is formed by a simple plate piece, and the second contact piece  925  of the second rack member  92  is curved in a thickness direction, so as to elastically deform in the thickness direction. The first contact piece  916  and the second contact piece  925  have contact with each other in a conduction state when at least one of the first rack tooth  94  and the second rack tooth  95  engages with the thread portion  83 , as illustrated in  FIGS. 9A, 9B . 
         [0046]    On the other hand, when both of the first rack tooth  94  and the second rack tooth  95  do not engage with the thread portion  83 , namely, the tooth skipping occurs due to the interval L 1   x  between the first rack tooth  94  and the second rack tooth  95 , which is larger than the external diameter measurement L 3  of the thread portion  83 , as illustrated in  FIG. 9C , the position control surfaces  915 ,  924  are separated, and the first contact piece  916  and the second contact piece  925  are separated from each other over the elastic deformation amount of the second contact piece  925 , so as to be a non-conduction state. Not-shown lead wires are connected to the first and second contact pieces  916 ,  925 , and the lead wires are connected to a not-shown tooth-skipping detection circuit. The tooth-skipping detection circuit is configured to simply detect the connection state of both of the lead wires. 
         [0047]    In the tooth-skipping detector, if the tooth skipping occurs in accordance with the movement of the first and second rack teeth  94 ,  95  in the outer diameter direction of the thread portion  83  due to a large external force applied to the lens-holding frame  6 , the nip angle between the first and second rack members  91 ,  92  is increased, and the position-controlling surfaces  915 ,  924  are separated by a predetermined measurement or more, so that the first and second contact pieces  916 ,  925  are separated to be in a non-conduction state. The tooth-skipping detection circuit is configured to detect the occurrence of the tooth skipping by detecting the non-conduction of the contact pieces  916 ,  925 , and immediately reset the focusing control similar to that of the lens position detector upon the detection of the tooth skipping. Focusing can be therefore appropriately performed regardless of the occurrence of the tooth skipping. 
         [0048]    The teeth skipping detector is configured to detect possible tooth skipping of the rack teeth  94 ,  95  relative to the thread portion  83 . With this configuration, the tooth skipping can be detected by detecting a situation in which an external force or impact applies to the lens-holding frame  6  or the driven body  9  by predetermined amount or more. For this reason, an acceleration sensor can be integrally provided in the rack member  91  or  92 , for example, and the tooth skipping can be detected when the output detected by the acceleration sensor becomes a previously set predetermined output or more. When a space to dispose the acceleration sensor in the rack member cannot be ensured, the acceleration sensor can be disposed in the lens cylinder  4 , namely, a part of the motor frame  5  or the lens-holding frame  6 . 
         [0049]    In this embodiment, an example is described in which the driven body  9 , which moves in a straight line in the axis direction of the lead screw  82  by the engagement with the thread portion  83  of the lead screw  82 , is made of the first rack member  91  having the rack tooth  94  and the second rack member  92  having the rack tooth  95 . However, the driven member in the present invention is not limited thereto, and the driven member can be a pair of members which is mounted on the lead screw from a direction sandwiching the lead screw in a radial direction and engages with the thread portion of the lead screw. For example, the driven member can be first and second driven members each integrally having a circular arc nut made of a part of a circumference of a circular nut member which can be threadably mounted on the thread portion  83  of the lead screw  82 . 
         [0050]    In the present invention, the configurations of the first and second biasing springs  96 ,  97  are not limited to the present embodiment as long as the first biasing spring  96  biases the first rack member  91  toward the lead screw  82 , and the second biasing spring  97  biases the second rack member  92  toward the first rack member  91 . The first biasing spring  96  can be a torsion spring which biases the first rack member  91  in the radial direction of the lead screw  82  relative to the lens-holding frame  6 . The second biasing spring  97  can be a tension coil spring which is mounted between the first and second rack members  91 ,  92  and biases the second rack member  92  toward the first rack member  91 . In this case, the second biasing spring  97  is mounted between the second rack member  92  and the lens-holding frame  6 , and biases the second rack member  92  toward the first rack member  91 . 
         [0051]    Although the embodiment of the present invention has been described above, the present invention is not limited thereto. It should be appreciated that variations may be made in the embodiment described by persons skilled in the art without departing from the scope of the present invention. The present invention is not limited to the lead screw device as the lens driver in the embodiment of the present invention as long as it has a configuration in which the driven body engaging with the thread portion of the lead screw moves in a straight line in an axis center direction upon the rotation of the lead screw about an axis. Moreover, the lens driver of the present invention is not limited to the lens driver for focusing as described in the embodiment as long as it has a configuration which moves the lens disposed in the lens barrel in a lens optical axis direction. The lens driver of the present invention can be applied as a lens driver for zooming, which changes a lens focusing distance. Furthermore, the lens barrel of the present invention is not limited to the lens unit which is integrally provided in the camera body as described in the embodiment. The lens barrel of the present invention can be applied as a lens barrel for an exchangeable lens of a lens-exchangeable camera. The camera of the present invention is not limited to a camera which images a still image as described in the embodiment. The camera of the present invention can be applied to a camera which images a moving image. 
         [0052]    The present invention is adopted not only to a lead screw device but also to a lens driver including the lead screw device, a lens barrel, and a camera including the lens driver. 
         [0053]    According to the embodiment of the present invention, in the lead screw device, a pair of the driven members which is disposed to sandwich the lead screw in a radial direction is biased in a direction sandwiching the lead screw, and the tooth portion provided in each driven member engages with the thread portion of the lead screw by the biasing force. Each of the driven members therefore stably engages with the thread portion by the biasing force. A pair of driven members moves in the external diameter direction of the lead screw against the biasing forces when an external force is applied to the driven body, so that the driven members can be prevented from being damaged. In particular, a pair of the driven members includes the first driven member and the second driven member which are connected to be rotatable about an axis. The first driven member is biased in a direction rotating toward the lead screw by the first biasing member, and the second driven member is biased in a direction rotating toward the first driven member by the second biasing member. With this configuration, the first driven member appropriately engages with the thread portion by adjusting the biasing force of the first biasing member. The second driven member which is controlled in a predetermined angle position relative to the first driven member is maintained in an engagement state having a predetermined positional relationship relative to the thread portion by adjusting the biasing force of the second biasing member. On the other hand, when an external force is applied, the second driven member disengages from the thread portion, so that the tooth skipping easily occurs. 
         [0054]    According to the embodiment of the present invention, in the lead screw device, the first and second driven members include the position controller which controls the second driven member such that the tooth portion of the second driven member is located inside the external diameter of the thread portion of the lead screw without engaging with the thread portion of the lead screw when the tooth portion of the first driven member engages with the thread portion of the lead screw. With this configuration, when an external force is applied to the driven body, the engagement state by the first driven member is ensured, and when the external force is further increased, the first and second driven members engage with the thread portion. The operation of the lead screw device is therefore ensured, and damage on each driven member and the tooth skipping are prevented. Furthermore, when the external force is unusually increased, the tooth portion of each driven member disengages from the thread portion against the biasing force of the first and second biasing members, and rotates to the external diameter position. Thus, damage on each driven member can be prevented by generating the tooth skipping. 
         [0055]    According to the embodiment of the present invention, the lens driver includes the lead screw device. The lead screw is attached to the motor frame provided with the driving motor, and is rotatable about an axis by the driving motor. The driven body is attached to the lens-holding frame holding a lens, and moves the lens-holding frame in the lens optical axis direction along the rotation of the lead screw about an axis. With this configuration, even if an external force generated when a photographer&#39;s hand has contact with the lens-holding frame is applied, damage on the lens driver is prevented while accurately moving the lens-holding frame. 
         [0056]    According to the embodiment of the present invention, the lens driver includes the lens position detector which detects the movement of the lens in the lens optical axis direction. The lens position detector controls the lens position controller to an initial position when the lens position detector detects the lens position. With this configuration, the lens position can be accurately controlled when the tooth skipping occurs in the lens driver. In particular, the lens driver includes the tooth-skipping detector which detects the disengagement of the driven body from the thread portion of the lead screw. The lens position controller is controlled to the initial position when the tooth-skipping detector detects the lens position. With this configuration, the lens position can be accurately controlled by immediately performing initial setting when the tooth skipping occurs. 
         [0057]    According to the embodiment of the present invention, in the lens barrel and the camera including the lens barrel, highly accurate focusing, zooming, or the like can be performed by the accurate lens position control which is obtained in the above-described lens driver.