Patent Publication Number: US-10759036-B2

Title: Power tool

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. application Ser. No. 14/835,228 filed Aug. 25, 2015, which is based on and claims priority under 35 U.S.C. 119 from Japanese Patent Application No. 2014-173252 filed on Aug. 27, 2014. The contents of the above application are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a power tool that rotationally drives a tool accessory. 
     BACKGROUND ART 
     Japanese laid-open patent publication No. H09-011148 discloses a screw tightening tool that performs a screw tightening operation by driving a tool bit coupled to an output shaft member. In this screw tightening tool, when performing a screw tightening operation, a motor rotationally drives the output shaft member with a screw attached to a tip of the tool bit and pressed against a workpiece. 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the above-described screw tightening tool, when an intervening member is held between a driving shaft member and the output shaft member, rotation of the motor is transmitted to the output shaft member via the intervening member. Thus, the tool bit is driven and perform a screw tightening operation. Further, when the holding of the intervening member is released, transmission of rotation of the motor is interrupted and the screw tightening operation is completed. In this screw tightening tool, the holding of the intervening member is released by utilizing a biasing force of a spring. Accordingly, it is an object of the present invention to provide a novel technique for releasing an intervening member in a driving mechanism of a power tool of the type in which rotation of the motor is transmitted to a tool accessory driving shaft by holding the intervening member. 
     In order to solve the above-described problem, according to a preferred aspect of the present invention, a power tool is provided which performs a prescribed operation by rotationally driving a tool accessory detachably coupled to a front end region of the power tool. The power tool includes a motor and a driving mechanism that is driven by the motor and rotationally drives the tool accessory. The driving mechanism includes a tool accessory driving shaft to which the tool accessory is coupled, a driving member that is coaxially disposed with the tool accessory driving shaft and rotationally driven by the motor, and an intervening member that is disposed between the tool accessory driving shaft and the driving member and transmits rotation of the driving member to the tool accessory driving shaft when held between the tool accessory driving shaft and the driving member, while interrupting the transmission of rotation of the driving member to the tool accessory driving shaft when released from the holding. The intervening member suitably has a cylindrical, conical, spherical or prismatic shape or a pyramid shape. The power tool further has a releasing mechanism that is driven by the motor and releases the holding. The intervening member may be held either between the tool accessory driving shaft and the driving member, or between a holding member disposed between the tool accessory driving shaft and the driving member and the tool accessory driving shaft or the driving member. Typically, by the wedge effect of the intervening member held therebetween, the driving member and the tool accessory driving shaft are integrated. Preferably, at least one of components for forming the driving mechanism forms the releasing mechanism. 
     According to this invention, with the structure having the releasing mechanism which is driven by the motor and releases the holding of the intervening member, compared with a structure, for example, in which a biasing force of a spring is utilized to release the holding of the intervening member, the holding of the intervening member can be more reliably released. 
     According to a further aspect of the power tool of the present invention, the tool accessory driving shaft is configured to be movable between a first position close to the front end region and a second position away from the front end region in an axial direction of the tool accessory driving shaft. When the tool accessory driving shaft is located in the second position, the intervening member is held between the tool accessory driving shaft and the driving member in a prescribed holding position and transmits rotation of the driving member to the tool accessory driving shaft. When the tool accessory driving shaft is located in the first position, the intervening member is disposed in a holding disabled position which is different from the holding position and in which the intervening member is not held between the tool accessory driving shaft and the driving member, so that the transmission of rotation of the driving member to the tool accessory driving shaft is interrupted. 
     According to this aspect, the intervening member is moved between the holding position and the holding disabled position according to the position of the tool accessory driving shaft in the axial direction. Therefore, the power tool is provided to perform an operation, for example, by pressing the tool accessory against a workpiece so as to move the tool accessory driving shaft in the axial direction. 
     According to a further aspect of the power tool of the present invention, the releasing mechanism moves the tool accessory driving shaft from the second position to the first position in the axial direction of the tool accessory driving shaft, and at the same time, moves the intervening member from the holding position to the holding disabled position in a circumferential direction around an axis of the tool accessory driving shaft. 
     Typically, the releasing mechanism includes a first element and a second element that can get into contact with the first element. One of the first element and the second element has an inclined surface inclined at a prescribed angle with respect to the axial direction of the tool accessory driving shaft, and the other element has a contact part that can get into contact with the inclined surface. 
     The intervening member is moved from the holding position to the holding disabled position by the releasing mechanism. Specifically, in a state in which the tool accessory driving shaft is located in the second position and the intervening member is held between the tool accessory driving shaft and the driving member in the holding position, when the first element and the second element move with respect to each other in the circumferential direction of the tool accessory driving shaft by rotation of the motor, the contact part slides in contact with the inclined surface and the first and second elements move with respect to each other in the axial direction of the tool accessory driving shaft. Thus, the releasing mechanism moves the tool accessory driving shaft from the second position to the first position, and at the same time, moves the intervening member from the holding position to the holding disabled position. 
     Further, in a state in which the tool accessory driving shaft is located in the first position and the intervening member is not held between the tool accessory driving shaft and the driving member in the holding disabled position, when the first element and the second element move with respect to each other in the axial direction of the tool accessory driving shaft by movement of the tool accessory driving shaft from the first position to the second position, the contact part slides in contact with the inclined surface and the first and second elements move with respect to each other in the circumferential direction of the tool accessory driving shaft. Thus, the releasing mechanism moves the intervening member from the holding disabled position to the holding position. As a result, the intervening member is held between the tool accessory driving shaft and the driving member in the holding position. Therefore, the releasing mechanism also serves to cause the intervening member to be held between the tool accessory driving shaft and the driving member. 
     According to a further aspect of the power tool of the present invention, the second element is configured as a retainer that retains the intervening member in the holding position and the holding disabled position and rotates together with the tool accessory driving shaft with the intervening member held in the holding position. The retainer is configured as part of the driving mechanism. Therefore, a component of the driving mechanism is also utilized for the releasing mechanism, so that the number of parts of the power tool is reduced. 
     According to a further aspect of the power tool of the present invention, the tool accessory driving shaft includes a tool accessory holding shaft that holds the tool accessory, and a first holding member that can hold the intervening member between the first holding member and the driving member and rotates together with the tool accessory holding shaft while holding the intervening member therebetween. The first element is formed by the first holding member. The first holding member is configured as part of the driving mechanism. Therefore, a component of the driving mechanism is also utilized for the releasing mechanism, so that the number of parts of the power tool is reduced. 
     According to a further aspect of the power tool of the present invention, the tool accessory driving shaft further includes a second holding member. When the intervening member is held between the first holding member and the driving member, normal rotation of the motor is transmitted to the tool accessory holding shaft. When the intervening member is held between the second holding member and the driving member, reverse rotation of the motor is transmitted to the tool accessory holding shaft. Therefore, whether the tool accessory is rotationally driven in the normal direction or in the reverse direction, the operation is performed. This aspect is useful for the power tool such as a screw tightening tool. 
     According to a further aspect of the power tool of the present invention, the retainer is configured as a ring-like member that is coaxially disposed with the tool accessory driving shaft. The second holding member is disposed inward of the outer periphery of the retainer in a radial direction of the retainer. Thus, the second holding member is disposed inside the retainer, so that the power tool can be reduced in size in the radial direction of the retainer. 
     According to a further aspect of the power tool of the present invention, the retainer is configured as a ring-like member that is coaxially disposed with the tool accessory driving shaft, and has a retaining part that retains the intervening member at a prescribed distance away from a rotation axis of the tool accessory driving shaft in a radial direction of the retainer. The first holding member has the inclined surface which is formed in a region at the prescribed distance away from the rotation axis of the tool accessory driving shaft in the radial direction of the retainer and configured to correspond to the retaining part, and a holding part which is formed on the rotation axis side with respect to the inclined surface and can hold the intervening member between the holding part and the driving member. With this structure, the first holding member does not protrude from the outer periphery of the retainer in the radial direction of the retainer, so that the power tool can be reduced in size in the radial direction of the retainer. 
     According to a further aspect of the power tool of the present invention, the intervening member is formed by a plurality of rollers extending in the axial direction of the tool accessory driving shaft. The retaining part of the retainer is provided between the rollers and has the contact part configured as a second inclined surface which is inclined in the axial direction of the tool accessory driving shaft at the same angle as the first inclined surface. With this structure, the area of contact between the first and second inclined surfaces increases, so that the retainer and the first holding member can be smoothly moved with respect to each other in the axial direction and the circumferential direction. 
     According to a further aspect of the power tool of the present invention, the power tool further has a biasing member that always biases the tool accessory driving shaft toward the front end region. When releasing the holding of the intervening member, the releasing mechanism can utilize not only the relative movement of the first and second elements in the axial direction and the circumferential direction, but also the biasing force of the biasing member. 
     According to a further aspect of the power tool of the present invention, the driving member is cylindrically shaped. The power tool further has a biased member that is biased in the radial direction of the driving member by the biasing member so as to get in contact with an inner circumferential surface of the driving member. Typically, the biased member is formed by a ball which can move in the radial direction and the circumferential direction of the driving member. When the motor is rotated reversely, the biased member moves the intervening member in the circumferential direction of the driving member by utilizing rotation of the driving member such that the intervening member is held between the driving member and the second holding member. 
     Effect of the Invention 
     According to the present invention, a novel technique for releasing an intervening member is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view schematically showing the overall structure of a screwdriver according to a first embodiment of the present invention. 
         FIG. 2  is a bottom view of a part of the screwdriver. 
         FIG. 3  is a sectional view of the screwdriver. 
         FIG. 4  is a sectional view taken along line IV-IV in  FIG. 2 . 
         FIG. 5  is a sectional view taken along line V-V in  FIG. 1 . 
         FIG. 6  is a side view of a driving mechanism. 
         FIG. 7  is a perspective view of the driving mechanism. 
         FIG. 8  is a perspective view of a retainer 
         FIG. 9  is a perspective view of a lock sleeve. 
         FIG. 10  is a sectional view taken along line X-X in  FIG. 6 . 
         FIG. 11  is a sectional view taken along line XI-XI in  FIG. 6 . 
         FIG. 12  is a sectional view taken along line XII-XII in  FIG. 6 . 
         FIG. 13  is a perspective view of a stopper. 
         FIG. 14  is an exploded view of the stopper. 
         FIG. 15  is a sectional view taken along line XV-XV in  FIG. 4 . 
         FIG. 16  is a sectional view corresponding to  FIG. 4  and showing a state during screw tightening operation. 
         FIG. 17  is a side view corresponding to  FIG. 6  and showing the state during screw tightening operation. 
         FIG. 18  is a sectional view taken along line XVIII-XVIII in  FIG. 17 . 
         FIG. 19  is a sectional view taken along line XIX-XIX in  FIG. 17 . 
         FIG. 20  is a sectional view taken along line XX-XX in  FIG. 16 . 
         FIG. 21  is a sectional view corresponding to  FIG. 12  and showing a state during screw removing operation. 
         FIG. 22  is a sectional view corresponding to  FIG. 11  and showing the state during screw removing operation. 
         FIG. 23  is a sectional view corresponding to  FIG. 15  and showing a state in which a spindle is prevented from rotating in a screw tightening direction by a stopper. 
         FIG. 24  is an enlarged partial view of an oil seal. 
         FIG. 25  is a sectional view taken along line XXV-XXV in  FIG. 2 . 
         FIG. 26  is an exploded view of a stopper according to a second embodiment of the invention. 
         FIG. 27  is a sectional view showing the stopper during screw removing operation. 
     
    
    
     DETAILED REPRESENTATIVE EMBODIMENT FOR PERFORMING THE INVENTION 
     First Embodiment 
     A first embodiment of the present invention is now described with reference to  FIGS. 1 to 25 . As shown in  FIG. 1 , as a representative embodiment of a power tool according to the present invention, a screwdriver  100  which performs a prescribed operation on a workpiece such as a gypsum board is described. The screwdriver  100  mainly includes a body  101  and a handle  107 . A tool bit  119  is detachably coupled to a front end region of the body  101 . For the sake of convenience of explanation, the tool bit  119  side (the right as viewed in  FIG. 1 ) is defined as the front of the screwdriver  100  and the handle  107  side (the left as viewed in  FIG. 1 ) is defined as the rear of the screwdriver  100 , in the axial direction of the tool bit  119  (the horizontal direction as viewed in  FIG. 1 ). Further, the upper side in  FIG. 1  is defined as the upper side of the screwdriver  100  and the lower side in  FIG. 1  is defined as the lower side of the screwdriver  100 , in the vertical direction in  FIG. 1 . 
     As shown in  FIGS. 1 to 3 , the body  101  mainly includes a main housing  103 , a front housing  104  and a locator  105 . The main housing  103  mainly houses a motor  110 , and the front housing  104  is mounted to the front of the main housing  103  and houses a driving mechanism  120 . The driving mechanism  120  is an example embodiment that corresponds to the “driving mechanism” according to the present invention. 
     As shown in  FIG. 3 , a partition wall  103   a  for demarcating the inside of the main housing  103  from the inside of the front housing  104  is formed on the front end of the main housing  103  and extends in the vertical direction. An output shaft  111  of the motor  110  is rotatably supported by a bearing  111   a  held by the partition wall  103   a  and a bearing  111   b  held by a rear portion of the main housing  103 . The output shaft  111  is disposed in parallel to the axial direction of the tool bit  119  (a spindle  160 ). The locator  105  is mounted to cover the front housing  104  in a front end region of the front housing  104 . The tool bit  119  is detachably coupled to the driving mechanism  120  such that a tip of the tool bit  119  protrudes forward from the locator  105  in the front end region of the body  101 . The locator  105  can move in the axial direction of the tool bit  119  with respect to the front housing  104  and fixed in a predetermined position selected in the axial direction. Thus, the amount of protrusion of the tool bit  119  from the locator  105 , or the screwing depth is appropriately set. 
     The handle  107  is connected to the rear of the body  101  (the main housing  103 ). The handle  107  has a trigger  107   a  and a changeover switch  107   b . When the trigger  107   a  is operated, electric current is supplied from outside via a power cable  109  and the motor  110  is driven. Further, the direction of rotation of the output shaft  111  of the motor  110  is switched by operating the changeover switch  107   b . Specifically, the output shaft  111  is driven in a selected direction of either one of normal rotation and reverse rotation. The motor  110  is an example embodiment that corresponds to the “motor” according to the present invention. 
     (Driving Mechanism) 
     As shown in  FIGS. 3 to 12 , the driving mechanism  120  mainly includes a driving gear  125 , a retainer  130 , a roller  140 , a lock sleeve  145 , a spring receiver  150 , a coil spring  155  and a spindle  160 . 
     (Driving Gear) 
     As shown in  FIGS. 4 and 5 , the driving gear  125  is coaxially disposed with the spindle  160  which holds the tool bit  119 . The driving gear  125  has a generally cup-like shape open to the front and having a bottom wall  126  and a side wall  127 . A through hole is formed through the center of the bottom wall  126 , and a rear shaft part  162  of the spindle  160  is inserted through the through hole. The side wall  127  defines a cylindrical internal space inside. The internal space of the driving gear  125  houses the retainer  130 , the roller  140 , the lock sleeve  145  and the coil spring  155 . Gear teeth  128  are provided on the outer periphery of the side wall  127  and engage with gear teeth  112  formed in the output shaft  111  of the motor  110 . The driving gear  125  is rotatably supported on the body  101  (the partition wall  103   a ) by a needle bearing  121  provided on the rear of the bottom wall  126 . 
     (Retainer) 
     As shown in  FIGS. 4 to 8 and 9 to 12 , the retainer  130  has a generally cup-like shape and is coaxially disposed with the driving gear  125 . The retainer  130  has a base  131  facing the bottom wall  126  of the driving gear  125 , and a first side wall  132  and a second side wall  133  facing the side wall  127  of the driving gear  125 . In  FIGS. 6 and 7 , the driving gear  125  is not shown. 
     As shown in  FIGS. 4, 8 and 12 , the base  131  has a through hole through which the rear shaft part  162  of the spindle  160  is inserted. As shown in  FIG. 12 , the through hole of the base  131  is an engagement hole  131   a  having a prescribed length in the circumferential direction of the spindle  160 . As shown in  FIG. 4 , a groove  162   a  is formed in the rear shaft part  162  of the spindle  160  and extends in the axial direction of the spindle  160 . As shown in  FIG. 12 , the groove  162   a  has a semi-circular section, in a direction perpendicular to the axial direction of the spindle  160 , corresponding to a cylindrical shape of an engagement pin  139  for connecting the spindle  160  and the retainer  130 . Therefore, within the range of the engagement hole  131   a  having a prescribed length in the circumferential direction of the spindle  160 , the spindle  160  and the retainer  130  can rotate with respect to each other in engagement with the engagement pin  139 . 
     As shown in  FIGS. 4 to 8 and 11 , the first and second side walls  132 ,  133  extend forward from the base  131  in the axial direction of the retainer  130 . Each of a pair of such first side walls  132  and each of a pair of such second side walls  133  are respectively arranged to face the other across a center axis of the retainer  130 . In other words, the second side walls  133  are arranged between the two first side walls  132 . As shown in  FIG. 11 , a roller retaining part  134  for retaining a roller  140  is formed as a prescribed space between the first and second side walls  132 ,  133  in the circumferential direction of the retainer  130 . Thus, the retainer  130  retains four rollers  140  between the first and second side walls  132 ,  133 . 
     Further, as shown in  FIGS. 6 to 8 , the second side wall  133  has an inclined part  133   a  in the form of an inclined surface formed on its front end portion and inclined with respect to the rotation axis of the spindle  160  (the center axis of the retainer  130 ). The inclined parts  133   a  of the two second side walls  133  are formed in a point symmetry with respect to the center axis of the retainer  130 . In other words, the two inclined parts  133   a  are configured as a lead surface extending along the circumferential direction of the retainer  130  and inclined at the same angle with respect to a contour line (outer periphery) of the retainer  130  in a cross section orthogonal to the axial direction of the retainer  130 . Specifically, the two inclined parts  133   a  are configured and arranged in a double spiral shape. 
     Further, as shown in  FIGS. 5, 6 and 12 , the base  131  has ball retaining parts  131   b  formed in regions corresponding to the two second side walls  133 . The two ball retaining parts  131   b  are formed in a point symmetry with respect to the center axis of the retainer  130 . The ball retaining parts  131   b  are formed each as a groove having a depth greater than the thickness of the second side walls  133  in a radial direction of the retainer  130 . Thus, as shown in  FIG. 5 , the outside and inside of the retainer  130  communicate with each other through the ball retaining parts  131   b  in the radial direction of the retainer  130 . 
     As shown in  FIG. 12 , the ball retaining part  131   b  is configured such that its groove depth, or distance from the outer peripheral surface of the retainer  130  toward a center of the retainer  130  in the radial direction of the retainer  130  in the ball retaining part  131   b , gradually decreases along the circumferential direction of the retainer  130 . Specifically, the groove depth of the ball retaining part  131   b  is set to gradually decrease in the clockwise direction (in a direction B) shown by arrow B in  FIG. 12 . A ball  153  is disposed in each of the ball retaining parts  131   b . When the ball  153  is located in a shallow region of the ball retaining part  131   b , the ball  153  is disposed in contact with both the bottom (wall on the center side) of the ball retaining part  131   b  and the inner circumferential surface of the driving gear  125 . On the other hand, when the ball  153  is located in a deep region of the ball retaining part  131   b , the ball  153  gets in contact with only at least one of the bottom of the ball retaining part  131   b  and the inner circumferential surface of the driving gear  125 . 
     (Lock Sleeve) 
     As shown in  FIGS. 4 to 7, 9 and 10 , the lock sleeve  145  has a generally hexagonal shape having a hollow part inside. The lock sleeve  145  is disposed coaxially with the retainer  130  and the driving gear  125  in front of the retainer  130 . The lock sleeve  145  is arranged such that its front end can get in contact with a rear end of a front shaft part  161  of the spindle  160 . The lock sleeve  145  has four roller engagement parts  146  for engagement with the rollers  140  and two retainer engagement parts  147  for engagement with the second side walls  133  of the retainer  130 , corresponding to the six sides of the hexagon of the lock sleeve  145 . 
     As shown in  FIG. 10 , the roller engagement parts  146  are formed by four flat surfaces parallel to the rotation axis of the spindle  160  (the center axis of the lock sleeve  145 ). The two opposite surfaces of the roller engagement parts  146  are parallel to each other. The roller engagement parts  146  are configured to engage (contact) with the rollers  140  inside the first and second side walls  132 ,  133  in the radial direction of the retainer  130 . 
     Further, as shown in  FIG. 10 , the retainer engagement parts  147  are formed in a region at substantially the same distance as the radius of the retainer  130  away from the center axis of the lock sleeve  145  in the radial direction of the retainer  130 . As shown in  FIGS. 6, 7 and 9 , the inclined part  147   a  is formed on the rear end of the retainer engagement part  147  in the axial direction by an inclined surface inclined with respect to the rotation axis of the spindle  160  (the center axis of the lock sleeve  145 ). The inclined part  147   a  is formed to correspond to the inclined part  133   a  of the second side wall  133 . Specifically, the inclined part  147   a  can engage (contact) with the inclined part  133   a . Therefore, like the inclined parts  133   a , the two inclined parts  147   a  are formed in point symmetry with respect to the center axis of the lock sleeve  145 . In other words, the two inclined parts  147   a  are configured as a lead surface extending along the circumferential direction around the axis of the lock sleeve  145  and inclined at the same angle with respect to a contour line (outer periphery) of the retainer engagement part  147  in a cross section orthogonal to the axial direction of the lock sleeve  145 . Specifically, the two inclined parts  147   a  are configured to form a shape of a double helix. 
     (Spring Receiver) 
     As shown in  FIGS. 4, 5 and 11 , the spring receiver  150  has a generally orthogonal section and is disposed inside the retainer  130 . The spring receiver  150  is disposed between the base  131  of the retainer  130  and the lock sleeve  145  in the axial direction. The spring receiver  150  has a through hole through which the spindle  160  is inserted. The engagement pin  139  disposed in the groove  162   a  of the rear shaft part  162  engages with a semi-circular engagement hole  150   a  of the spring receiver  150 , so that the spring receiver  150  is connected to the spindle  160  so as to rotate together with the spindle  160 . The spring receiver  150  has four roller engagement parts  151  on the outer periphery, corresponding to four of the six sides of the hexagon of the spring receiver  150 . The roller engagement parts  151  are configured as flat surfaces parallel to the rotation axis of the spindle  160 . 
     Further, as shown in  FIG. 5 , a ball contact part  152  is formed in a rear surface of the spring receiver  150 . The ball contact part  152  gets in contact with a region of the ball  153  which is located toward the rotation axis of the retainer  130  (the rotation axis of the spindle  160 ) with respect to the center of the ball  153  in the radial direction of the retainer  130  and acts to push out the ball  153  outward in the radial direction of the spindle  160 . The ball contact part  152  may be configured as an inclined surface inclined with respect to the axial direction of the spindle  160 . The ball  153  pushed outward in the radial direction of the spindle  160  gets in contact with the inner circumferential surface of the driving gear  125 . Therefore, the ball  153  moves within the ball retaining part  131   b  by rotation of the driving gear  125 . 
     (Spindle) 
     As shown in  FIGS. 4 to 7 and 10 to 12 , the spindle  160  is a generally cylindrical, elongate member made of metal and is disposed to be movable in the axial direction of the spindle  160  (the axial direction of the tool bit  119 ). The spindle  160  mainly includes the front shaft part  161  and the rear shaft part  162  integrally connected to the front shaft part  161 . The tool bit  119  is detachable coupled to the front shaft part  161 . A leaf spring is held on the front shaft part  161  and biases a ball. The tool bit  119  engages with the ball and is thereby held by the front shaft part  161 . The front shaft part  161  is rotatably supported by a front bearing  122  held by the front housing  104 . Further, as shown in  FIGS. 4 and 5 , an oil seal  181  is disposed between the front housing  104  and the front shaft part  161  in front of the front bearing  122  which supports the front shaft part  161 . 
     The rear shaft part  162  is coaxially connected to the front shaft part  161 . The rear end of the rear shaft part  162  is supported so as to be rotatable and slidable in the axial direction with respect to a cylinder-like rear end bearing  165  provided in the partition wall  103   a  of the main housing  103 . The rear end bearing  165  is configured as an oilless bearing. Thus, the spindle  160  is supported by the front bearing  122  and the rear end bearing  165 . The rear shaft part  162  is inserted through the driving gear  125 , the retainer  130  and the lock sleeve  145 , and the rear end of the rear shaft part  162  protrudes rearward from the driving gear  125 . The rear shaft part  162  has the groove  162   a  extending in the direction of the rotation axis of the spindle  160 . When the rear end of the groove  162   a  is engaged with the engagement pin  139 , the spindle  160  is prevented from moving forward in the axial direction. Further, the engagement pin  139  gets into contact with the rear end of the coil spring  155  and is prevented from moving forward in the axial direction of the spindle  160 . 
     The rear shaft part  162  has a hollow part  163  open to a rear end surface of the rear shaft part  162  and extending inside the spindle  160  in the axial direction. Thus, the hollow part  163  communicates with the inside of the rear end bearing  165 . Further, the rear shaft part  162  has a communication hole  164  formed through the rear shaft part  162  in the radial direction so as to provide communication between the hollow part  163  and the inside of the front housing  104 . Thus, the inside of the front housing  104  and the inside of the rear end bearing  165  communicate with each other through the hollow part  163 . With such a structure, when the spindle  160  moves rearward as shown in  FIG. 16 , compression of air inside the rear end bearing  165  is prevented. In other words, by provision of the communication hole  164 , air inside the rear end bearing  165  is not compressed, so that rearward movement of the spindle  160  is not hindered. 
     Further, as shown in  FIGS. 4 and 5 , the front shaft part  161  has a rear end portion having a large-diameter part  166  which can engage with a stopper  170  and a small-diameter part  167  which cannot engage with the stopper  170 . As shown in  FIG. 15 , the large-diameter part  166  has a circular arc part  166   a  having a diameter D 1  and a width across flat part  166   b  having a width W. As shown in  FIG. 20 , the small-diameter part  167  has a circular shape having a diameter D 2 . The diameter D 2  is equal to the width W of the width across flat part  166   b.    
     (Coil Spring) 
     As shown in  FIGS. 4 and 5 , the coil spring  155  is coaxially disposed with the spindle  160  such that the spindle  160  is inserted therethrough. A front region of the coil spring  155  is disposed within the hollow part of the lock sleeve  145  and the front end of the coil spring  155  is held in contact with the lock sleeve  145 . Further, the rear end of the coil spring  155  is held in contact with the front surface of the spring receiver  150 . Thus, the coil spring  155  biases the lock sleeve  145  and the spindle  160  forward. The lock sleeve  145  biased forward biases the spindle  160  and gets into contact with the stopper  170  so that the lock sleeve  145  is prevented from moving further forward. Further, the coil spring  155  biases the spring receiver  150 , the ball  153 , the retainer  130  and the driving gear  125  rearward. As shown in  FIG. 5 , the ball  153  is biased by the coil spring  155 , pushed outward in the radial direction of the retainer  130  via the ball contact part  152  of the spring receiver  150  and brought into contact with the inner circumferential surface of the driving gear  125 . The ball  153  and the coil spring  155  are example embodiments that correspond to the “biased member” and the “biasing member”, respectively, according to this invention. 
     (Stopper) 
     As shown in  FIGS. 4 and 5 , the stopper  170  forms a rotation preventing mechanism which prevents the spindle  160  from rotating in a prescribed direction when the spindle  160  is located in a front position. The stopper  170  is ring-shaped and fitted onto the spindle  160  such that the spindle  160  is inserted therethrough. The stopper  170  is fixed to the front housing  104  by an O-ring  180  which is disposed between the stopper  170  and the front housing  104 . As shown in  FIGS. 13 to 15 , the stopper  170  mainly includes a ball retaining ring  171 , a push ring  173 , a ball  175  and a leaf spring  177 . 
     As shown in  FIGS. 13 and 14 , the ball retaining ring  171  is a metal ring-like member and retains the ball  175  which can engage with the spindle  160 . As shown in  FIGS. 14 and 15 , the ball retaining ring  171  has two retaining grooves  172  extending along the circumferential direction. Each of the retaining grooves  172  retains the ball  175  such that the ball  175  can move in the circumferential direction of the ball retaining ring  171 . The retaining groove  172  is configured such that its groove depth, or distance from the inner circumferential surface of the ball retaining ring  171  to the bottom of the retaining groove  172  in the radial direction of the ball retaining ring  171  gradually increases along the circumferential direction of the ball retaining ring  171 . Specifically, the depth (length in the radial direction) of the retaining groove  172  is set to gradually increase in a direction B (screw removing direction) shown by arrow B in  FIG. 15 . Further, a pocket-like region  172   a  is formed in a front portion of the retaining groove  172  in the direction B. When the ball  175  moves in the direction B within the retaining groove  172 , the ball  175  abuts on a wall. The wall extends in a prescribed radial direction of the ball retaining ring  171  and forms the pocket-like region  172   a  in the retaining groove  172 . Therefore, when the ball  175  is located in a position (the pocket-like region  172   a ) shown in  FIG. 15 , the ball  175  is held in the pocket-like region  172   a  and thereby prevented from moving toward the center of the ball retaining ring  171  in the radial direction (toward the rotation axis of the spindle  160 ). 
     A generally C-shaped leaf spring  177  is disposed on the inner circumferential part of the ball retaining ring  171 . As shown in  FIG. 14 , the leaf spring  177  has two through holes  177   a  formed corresponding to the two retaining grooves  172 . Part of each ball  175  protrudes toward the center of the ball retaining ring  171  through the through hole  177   a . Specifically, the diameter of the ball  174  is larger than the depth of the retaining groove  172 . Therefore, the leaf spring  177  serves as a dropping-out prevention member for preventing the ball  175  from dropping out of the retaining groove  172  to the center of the ball retaining ring  171  in the radial direction of the ball retaining ring  171 . With this structure, the amount of protrusion of the ball  175  from the through hole  177   a  of the leaf spring  177  varies according to the position of the ball  175  within the retaining groove  172 . 
     Further, as shown in  FIGS. 13 and 14 , the leaf spring  177  has an engagement hole  177   b  which engages with a ball  176  held by the ball retaining ring  171 . With the leaf spring  177  fitted on the inner circumferential part of the ball retaining ring  171 , the ball  176  is fitted in the ball retaining ring  171  by utilizing elastic deformation of the leaf spring  177 , and thereafter, the balls  175  are fitted in the retaining grooves  172 . In this manner, the ball retaining ring  171 , the balls  175  and the leaf spring  177  are integrally assembled together. The retaining grooves  172  are open to the rear and the balls  175  are put into the retaining grooves  172  from behind the ball retaining ring  171 . The ball  176  is fitted into the ball retaining ring  171  from front and engaged with the engagement hole  177   b  of the leaf spring  177 . The ball retaining ring  171  has a retaining hole for retaining the ball  176  so as to prevent the ball  176  engaged with the leaf spring  177  from moving rearward of the ball retaining ring  171 . The balls  175  and the ball  176  which are prevented from moving forward and rearward, respectively, in engagement with the ball retaining ring  171 , and the leaf spring  177  are associated with each other to form an assembly of the components of the stopper  170 . As a result, the stopper  170  can be easily mounted to the front housing  104 . 
     As shown in  FIGS. 13 and 14 , the push ring  173  is a ring-like member having a smaller diameter than the ball retaining ring  171  and disposed coaxially with the ball retaining ring  171  within the ball retaining ring  171 . The front end surface of the push ring  173  is held in contact with the balls  175  held by the ball retaining ring  171 . The push ring  173  can rotate with respect to the ball retaining ring  171 . The rear end surface of the push ring  173  can come in contact with or separate from a shoulder part of the lock sleeve  145  which is offset rearward from the front end surface of the lock sleeve  145 . Specifically, as shown in  FIG. 5 , when the lock sleeve  145  is biased by the coil spring  155  and located in a front position, the lock sleeve  145  comes in contact with the push ring  173 . On the other hand, as shown in  FIG. 16 , when the lock sleeve  145  is located in a rear position, the lock sleeve  145  separates from the push ring  173 . 
     In the above-described stopper  170 , when the lock sleeve  145  located in the front position in contact with the push ring  173  is rotated, the push ring  173  comes in contact with the ball  175  and the ball  175  moves within the retaining groove  172 . Thus, the amount of protrusion of the ball  175  from the retaining groove  172  varies in the radial direction of the ball retaining ring  171 . Specifically, when the ball  175  moves in the direction A (screw tightening direction) as shown in  FIG. 20 , the amount of protrusion of the ball  175  from the leaf spring  177  increases. On the other hand, when the ball  175  moves in the direction B (screw removing direction) as shown in  FIG. 15 , the amount of protrusion of the ball  175  from the leaf spring  177  decreases. Specifically, when the ball  175  is moved in the circumferential direction of the spindle  160 , the ball  175  moves between a position away from the center axis of the spindle  160  as shown in  FIG. 15  (also referred to as a separate position) and a position closer to the center axis of the spindle  160  as shown in  FIG. 20  (also referred to as a proximity position). 
     When the ball  175  is located in the separate position, the ball  175  does not engage with the large-diameter part  166  and the small-diameter part  167  of the spindle  160 . Thus, in the separate position, the ball  175  cannot engage with the spindle  160 . Therefore, the separate position may also be referred to as an unengageable position. When the ball  175  is located in the proximity position, the ball  175  can engage with the large-diameter part  166  of the spindle  160 . Specifically, when the ball  175  is located in the proximity position and the spindle  160  is located in the front position where the large-diameter part  166  of the spindle  160  faces the ball  175 , the ball  175  engages with the spindle  160 . On the other hand, when the spindle  160  is located in the rear position where the small-diameter part  167  of the spindle  160  faces the ball  175 , the ball  175  does not engage with the spindle  160 . Therefore, in the proximity position, the ball  175  can engage with the spindle  160 . Therefore, the proximity position may also be referred to as an engageable position. The ball  175  is switched from the unengageable position to the engageable position by movement of the ball  175  in the screw tightening direction, while the ball  175  is switched from the engageable position to the unengageable position by movement of the ball  175  in the screw removing direction. 
     (Operation of Screwdriver) 
     In the screwdriver  100  having the above-described structure, the motor  110  is driven when the trigger  107   a  is operated. The driving gear  125  is rotationally driven by rotation of the output shaft  111  of the motor  110 . When the rotation of the driving gear  125  is transmitted to the spindle  160 , the tool bit  119  held by the spindle  160  is rotated and performs a prescribed operation (screw tightening operation or screw removing operation). The spindle  160  is an example embodiment that corresponds to the “tool accessory holding shaft” according to this embodiment. 
     (Screw Tightening Operation) 
     When performing a screw tightening operation, a screw (not shown) on the tip of the tool bit  119  is pressed against a workpiece. At this time, the spindle  160  is moved from the front position shown in  FIG. 4  to the rear position shown in  FIG. 16 . The front position and the rear position are example embodiments that correspond to the “first position” and the “second position”, respectively, according to this invention. By this movement of the spindle  160 , the lock sleeve  145  is rotated with respect to the retainer  130  and the rollers  140  are held between the driving gear  125  and the lock sleeve  145 . As a result, the driving gear  125  and the lock sleeve  145  rotate together by the wedge effect of the rollers  140 , so that the rotation of the output shaft  111  of the motor  110  is transmitted to the spindle  160  via the driving mechanism  120 . Thus, the spindle  160  is rotationally driven and the tool bit  119  performs a screw tightening operation. The roller  140  is an example embodiment that corresponds to the “intervening member” according to this embodiment. The lock sleeve  145  and the spindle  160  form the “tool accessory driving shaft” according to this invention. 
     Specifically, when the spindle  160  is located in the front position as shown in  FIG. 4 , the driving gear  125  is rotationally driven in the direction A in  FIGS. 10 to 12  if the output shaft  111  of the motor  110  is rotated in a prescribed direction (hereinafter referred to as normal direction). At this time, the rollers  140  are not held between the driving gear  125  and the lock sleeve  145 , so that the rotation of the driving gear  125  is not transmitted to the lock sleeve  145 . Further, as shown in  FIG. 12 , by the rotation of the driving gear  125  in the direction A, the ball  153  comes in contact with the inner circumferential surface of the driving gear  125  and moves in the direction A within the ball retaining part  131   b . At this time, however, the depth (length in the radial direction) of the ball retaining part  131   b  is deep enough that the ball  153  is not held between the ball retaining part  131   b  and the driving gear  125 . Specifically, the ball  153  is loosely held within the ball retaining part  131   b , so that the rotation of the driving gear  125  is not transmitted to the retainer  130 . This state is also referred to as an idling state. 
     When the screw held on the tip of the tool bit is pressed against the workpiece in the idling state, the spindle  160  is moved from the front position shown in  FIG. 4  to the rear position shown in  FIG. 16  against the biasing force of the coil spring  155 . As a result, the lock spring  145  is pushed rearward by the rear end of the front shaft part  161  of the spindle  160 , and the retainer engagement part  147  of the lock sleeve  145  comes in contact with the second side wall  133  of the retainer  130 . Specifically, as shown in  FIG. 17 , the inclined part  147   a  of the retainer engagement part  147  and the inclined part  133   a  of the second side wall  133  come in contact with each other. The inclined part  147   a  moves along the inclined part  133   a , so that the lock sleeve  145  moves rearward and rotates around the axis of the retainer  130 . Specifically, as shown in  FIG. 18 , the lock sleeve  145  rotates at a prescribed angle in the direction B around the rotation axis of the spindle  160  with respect to the retainer  130 . As a result, the distance between the roller engagement part  146  of the lock sleeve  145  and the inner circumferential surface of the driving gear  125  is shortened, so that the roller  140  is held between the roller engagement part  146  and the inner circumferential surface of the driving gear  125 . Thus, the roller  140  acts as a wedge and the driving gear  125  and the lock sleeve  145  are integrated via the roller  140 . The position (shown in  FIG. 18 ) of the roller  140  which is held between the lock sleeve  145  and the driving gear  125  is also referred to as a rotation transmitting position. Therefore, the neutral position (shown in  FIG. 10 ) of the roller  140  which is not held between the lock sleeve  145  and the driving gear  125  is also referred to as a rotation transmission disabled position. The lock sleeve  145  is an example embodiment that corresponds to the “first holding member” according to this invention. 
     At this time, as shown in  FIG. 17 , the inclined part  147   a  of the lock sleeve  145  is in contact with the inclined part  133   a  of the retainer  130 . Therefore, as shown in  FIG. 18 , when the driving gear  125  is rotationally driven in the direction A by the output shaft  111  of the motor  110 , the lock sleeve  145  integrated with the driving gear  125  is rotated. Thus, the inclined part  147   a  of the lock sleeve  145  presses the retainer  130  in the direction A and rotates the retainer  130  in the direction A. 
     As shown in  FIG. 19 , the retainer  130  rotated in the direction A rotates the spindle  160  in the direction A (screw tightening direction) via the engagement pin  139  which is engaged with the engagement hole  131   a  of the retainer  130 . As a result, a screw tightening operation is performed by the tool bit  119  held by the spindle  160 . Further, when the spindle  160  is located in the rear position as shown in  FIG. 16 , the small-diameter part  167  of the spindle  160  faces the ball  175  of the stopper  170 . The ball  175  does not engage with the small-diameter part  167 , so that rotation of the spindle  160  in the screw tightening direction is not hindered. 
     When the screw is screwed into the workpiece, the whole screwdriver  100  moves forward along with the movement of the screw, and the front surface of the locator  105  comes in contact with the workpiece. Thereafter, when the screw is further screwed into the workpiece, the spindle  160  holding the tool bit  119  moves forward in the screwdriver  100  with respect to the locator  105  (the front housing  104 ). Specifically, the spindle  160  is allowed to move from the rear position shown in  FIG. 16  to the front position shown in  FIG. 4 . In other words, the spindle  160  is pressed until the locator  105  comes in contact with the workpiece, so that the spindle  160  and the locator  105  are prevented from moving with respect to each other in the direction of the rotation axis of the spindle  160 . 
     The biasing force of the coil spring  155  acts forward upon the spindle  160  via the lock sleeve  145 . Further, the lock sleeve  145  presses the retainer  130  and moves (rotates) the retainer  130  around the rotation axis of the spindle  160 , so that the lock sleeve  145  receives reaction force from the retainer  130 . Specifically, the inclined part  147   a  of the lock sleeve  145  and the inclined part  133   a  of the retainer  130  which are inclined with respect to the rotation axis of the spindle  160  are in contact with each other, so that the lock sleeve  145  receives reaction force in the direction of the rotation axis of the spindle  160  and reaction force around the rotation axis. The inclined part  147   a  is an example embodiment that corresponds to the “second inclined surface” and the “contact part” according to this invention. The inclined part  133   a  is an example embodiment that corresponds to the “inclined surface” according to this invention. 
     Therefore, during screw tightening operation, when the spindle  160  is allowed to move from the rear position to the front position after the locator  105  gets in contact with the workpiece, the lock sleeve  145  is moved forward from the position shown in  FIG. 16  by the resultant (force in the direction of the rotation axis of the spindle  160 ) of the biasing force of the coil spring  155  and the reaction force from the retainer  130 . Specifically, this resultant force exceeds the friction force between the rollers  140  and the lock sleeve  145 . In other words, solely the biasing force of the coil spring  155  does not exceed the friction force between the rollers  140  and the lock sleeve  145 , but the resultant of the biasing force of the coil spring  155  and the reaction force from the retainer  130  exceeds the friction force between the rollers  140  and the lock sleeve  145 . Therefore, the lock sleeve  145  is not moved forward solely by the biasing force of the coil spring  155 , but by the resultant of the biasing force of the coil spring  155  and the reaction force from the retainer  130 . As a result, the lock sleeve  145  and the retainer  130  are separated from each other in the direction of the rotation axis of the spindle  160  and a clearance is formed between the lock sleeve  145  and the retainer  130 . Thus, the lock sleeve  145  shown in  FIG. 18  is rotated in the direction A with respect to the driving gear  125 , so that the rollers  140  are released or disengaged from between the driving gear  125  and the lock sleeve  145 . Specifically, the wedge action of the rollers  140  is released. Therefore, transmission of rotation from the driving gear  125  to the spindle  160  is interrupted so that the screw tightening operation is completed. The lock sleeve  145  is an example embodiment that corresponds to the “first element” according to this invention. 
     (Screw Removing Operation) 
     In a screw removing operation of removing a screw screwed into a workpiece, the screw is reversely rotated by the screwdriver  100  (the tool bit  119 ) to remove the screw from the workpiece. In the screw removing operation, it is not rational to press the tool bit  119  against the screw. Therefore, in the screw removing operation by the screwdriver  100 , the tool bit  119  is driven by the motor  110  without being pressed. Specifically, the spindle  160  is located in the front position while the spindle  160  (the tool bit  119 ) is reversely rotated. 
     Specifically, as shown in  FIG. 1 , in the screw removing operation, the changeover switch  107   b  is switched such that the output shaft  111  of the motor  110  rotates in a direction opposite to the normal direction (hereinafter referred to as a reverse direction). Further, an LED  107   c  is provided in the vicinity of the changeover switch  107   b . When the rotation direction of the output shaft  111  is switched to the reverse direction and the trigger  107   a  is operated, the LED  107   c  emits light. Specifically, the LED  107   c  informs the user that a screw removing operation is performed. When the output shaft  111  of the motor  110  rotates in the reverse direction, the driving gear  125  is rotated in the direction B in  FIGS. 10 to 12 . At this time, since the rollers  140  are not held between the lock sleeve  145  and the driving gear  125 , the rotation of the driving gear  125  is not transmitted to the lock sleeve  145 . 
     By the rotation of the driving gear  125  in the direction B, the ball  153  shown in  FIG. 12  moves in the direction B in contact with the inner circumferential surface of the driving gear  125  within the ball retaining part  131   b  and placed in a position shown in  FIG. 21 . Since the depth (length in the radial direction) of the ball retaining part  131   b  decreases in the direction B, the ball  153  is held between the ball retaining part  131   b  and the driving gear  125  by moving in the direction B within the ball retaining part  131   b . As a result, the ball  153  acts as a wedge, so that the driving gear  125  and the retainer  130  are integrated via the ball  153 . 
     As shown in  FIG. 21 , the engagement hole  131   a  of the retainer  130  has a prescribed length in the circumferential direction of the spindle  160  so that the spindle  160  and the retainer  130  are allowed to rotate with respect to each other. Further, as shown in  FIG. 22 , rotation of the spring receiver  150  with respect to the spindle  160  is prevented via the engagement pin  139 . Therefore, when the retainer  130  is rotated in the direction B together with the driving gear  125 , the retainer  130  rotates in the direction B with respect to the spindle  160  and the spring receiver  150 , and thus the rollers  140  held by the retainer  130  are moved in the direction B. As a result, the rollers  140  are held between the spring receiver  150  and the driving gear  125  and act as a wedge, so that the driving gear  125 , the spring receiver  150  and the spindle  160  are integrated via the rollers  140 . Therefore, the spindle  160  is rotated in the direction B (screw removing direction) by rotation of the driving gear  125  in the direction B. As a result, a screw removing operation is performed by the tool bit  119  held by the spindle  160 . The spring receiver  150  is an example embodiment that corresponds to the “second holding member” according to this invention. 
     In the above-described screwdriver  100 , a screw tightening operation is performed when the spindle  160  is located in the rear position. Screws are mounted to the tip of the tool bit  119  one by one when performing the screw tightening operation. Therefore, when the spindle  160  is located in the front position in which the spindle  160  is not rotationally driven, it is preferred that the spindle  160  is securely stopped. Specifically, in an idling state, it is preferred that the spindle  160  is stopped or not moved around the rotation axis of the spindle  160 . Therefore, in this embodiment, the stopper  170  is provided to prevent the spindle  160  from unintentionally rotating in the screw tightening direction when the spindle  160  is located in the front position. Further, the ball retaining ring  171  of the stopper  170  is fixed to the front housing  104  by the O-ring  180 , so that rotation of the stopper  170  is prevented. 
     Specifically, as shown in  FIG. 4 , when the spindle  160  is located in the front position, the large-diameter part  166  of the spindle  160  faces the ball  175  of the stopper  170 . At this time, even if the driving gear  125  is rotationally driven in the direction A, the lock sleeve  145  and the spindle  160  are not normally rotated. If the lock sleeve  145  is rotated for any reason, however, the ball  175  is moved within the retaining groove  172  in the direction A as shown in  FIG. 20  via the push ring  173  by rotation of the driving gear  125  in the direction A (screw tightening direction), since the lock sleeve  145  is biased by the coil spring  155  and held in contact with the push ring  173 . The depth (length in the radial direction) of the retaining groove  172  decreases in the direction A. When the ball  175  comes to a position shown in  FIG. 20 , the amount of protrusion of the ball  175  from the leaf spring  177  reaches its maximum. As a result, the large-diameter part  166  of the spindle  160  comes in contact with the ball  175 , so that rotation of the spindle  160  in the direction A is prevented. Particularly, as shown in  FIG. 23 , the ball  175  engages with the width across flat part  166   b  of the large-diameter part  166 , so that rotation of the spindle  160  in the direction A is prevented. 
     A screw removing operation is performed without pressing the tool bit held by the spindle  160  against a workpiece. Specifically, the screw removing operation is performed with the spindle  160  located in the front position. As shown in  FIG. 4 , when the spindle  160  is located in the front position, the large-diameter part  166  of the spindle  160  faces the ball  175  of the stopper  170 . At this time, when the driving gear  125  is rotationally driven in the direction B, the ball  175  is moved within the retaining groove  172  in the direction B as shown in  FIG. 15  via the push ring  173  by rotation of the driving gear  125  in the direction B (screw removing direction), since the lock sleeve  145  is biased by the coil spring  155  and held in contact with the push ring  173 . The depth (length in the radial direction) of the retaining groove  172  increases in the direction B. When the ball  175  comes to a position shown in  FIG. 15 , the large-diameter part  166  of the spindle  160  does not come in contact with the ball  175 . The stopper  170  serves to allow the spindle  160  to rotate in the screw removing direction when the spindle  160  is located in the front position. Therefore, in the screw removing operation, rotation of the spindle  160  in the direction B (screw removing direction) is not blocked by the ball  175 . 
     Further, in the above-described screwdriver  100 , as shown in  FIG. 4 , lubricant (not shown) such as grease is provided within the front housing  104  so as to smoothly drive the driving mechanism  120 . Further, in order to prevent the lubricant from leaking out from the front of the front housing  104 , the oil seal  181  is disposed between the outer periphery of the front shaft part  161  of the spindle  160  and the front housing  104  in a front region of the front housing  104 . Thus, the front housing  104  is hermetically formed. 
     As shown in  FIG. 24 , the oil seal  181  has a ring-like shape and has a base  181   a  which is mounted on the front housing  104  and a lip  181   b  which is held in contact with the spindle  160 . Particularly, the base  181   a  which is held in contact with the inner circumferential surface of the front housing  104  is made of elastomer. The front housing  104  has a large-diameter part  104   c  formed in the front end and having a slightly larger diameter than the outer diameter of the oil seal  181 . Further, the outer diameter of the oil seal  181  is slightly larger than the inner diameter of the front housing  104 . Further, the front housing  104  has an upper recess  104   a  and a lower recess  104   b  in the inner circumferential surface. Particularly, a plurality of the recesses  104   a ,  104   b  are formed on the same circumference. The recesses may be configured as a single recess which is continuously formed in the circumferential direction, or a projection may be provided in place of the recess. 
     The above-described oil seal  181  is fitted into the front housing  104  from the front by elastic deformation of the outer periphery of the oil seal  181 . At this time, the oil seal  181  is moved (inserted) along the large-diameter part  104   c  of the front housing  104 . Specifically, the large-diameter part  104   c  serves as a guide when mounting the oil seal  181 . Further, the base  181   a  of the oil seal  181  is engaged with the recesses  104   a ,  104   b  by elastic deformation, so that the oil seal  181  is securely fixed and prevented from coming off the front housing  104 . Thus, the recesses  104   a ,  104   b  serve as a stopper for the oil seal  181 . Further, the oil seal  181  is press fitted into the front housing  104  by elastic deformation and thereby prevented from rotating in the circumferential direction. The rotation of the oil seal  181  in the circumferential direction is further effectively prevented by the plurality of recesses  104   a ,  104   b  of the front housing  104  in the circumferential direction. The driving mechanism  120  is assembled into the front housing  104  having the oil seal  181  mounted thereto. Specifically, the driving mechanism  120  is disposed within the front housing  104  such that the spindle  160  extends through the oil seal  181 . By this arrangement, the oil seal  181  is arranged to seal a gap between the spindle  160  and the front housing  104 . Further, the lip  181   b  formed in the inner circumferential part of the oil seal  181  is always held in contact with the spindle  160  so as to prevent lubricant from leaking out from the front of the front housing  104 . 
     On the rear of the front housing  104 , as shown in  FIG. 25 , the bearing  111   a  serves to prevent lubricant from leaking out from between the output shaft  111  of the motor  110  and the partition wall  103   a . Further, an air passage  190  is formed through the partition wall  103   a  to provide communication between the inside and the outside of the front housing  104 . When the screwdriver  100  is driven, heat is generated within the front housing  104  by driving of the driving mechanism  120 , so that air pressure within the front housing  104  increases. Particularly, in the screwdriver  100  of a small type, the front housing  104  has a small capacity, so that the fluctuation of air pressure within the front housing  104  is large. Therefore, the air passage  190  is formed in the partition wall  103   a  to release the pressure of the front housing  104  to the outside and thereby prevent increase of air pressure within the front housing  104 . Specifically, the front housing  104  and the main housing  103  communicate with each other via the air passage  190 . The air passage  190  is disposed behind the driving gear  125  and above the output shaft  111  (not shown in  FIG. 25 ) of the motor  110 . Further, as shown in  FIGS. 1 and 3 , the main housing  103  has an external communication part  106  formed by communication holes for providing communication between the inside of the main housing  103  and the outside of the screwdriver  100 . 
     As shown in  FIG. 25 , in order to allow communication between the inside and the outside of the front housing  104  and prevent leakage of lubricant, the air passage  190  is provided and formed by a hollow part of a cylindrical (chimney-shaped) passage formation part  191  which extends forward from the vertically extending partition wall  103   a . The air passage  190  is provided. Specifically, the passage formation part  191  has a passage opening  191   a  formed on the front end at a prescribed distance to the front from a front surface  103   f  of the partition wall  103   a  (on the front housing  104  side). With this structure, lubricant is prevented from flowing to the passage opening  191   a  along the partition wall  103   a . The passage opening  191   a  is arranged close to the driving gear  125  and on the side of the rotation axis of the driving gear  125  with respect to the outer periphery of the driving gear  125  in the radial direction of the driving gear  125 . When centrifugal force is generated by rotation of the driving gear  125 , lubricant sticking to the driving gear  125  is moved outward in the radial direction of the driving gear  125 . Therefore, lubricant can be prevented from entering the air passage  190  through the passage opening  191   a . Such a structure having the air passage  190  can prevent leakage of lubricant from the front housing  104  and increase of air pressure within the front housing  104 . 
     Further, an oil filter  195  is disposed in the partition wall  103   a  in preparation for leakage of lubricant through the air passage  190  having the above-described structure. The oil filter  195  is formed of a liquid absorbing material such as felt and sponge. The oil filter  195  is disposed in the rear of the partition wall  103   a  and at the rear of the air passage  190 . Specifically, the oil filter  195  is held by the partition wall  103   a . Therefore, air within the front housing  104  is led into the main housing  103  through the air passage  190  and the oil filter  195 . 
     Second Embodiment 
     A second embodiment of the present invention is now described with reference to  FIGS. 26 and 27 . The second embodiment is different from the first embodiment in the shape of the retaining groove formed in the ball retaining ring of the stopper  170 . Therefore, components other than the retaining groove are given like numerals as in the first embodiment, and they are not described. 
     In the first embodiment, as shown in  FIG. 15 , the pocket-like region  172   a  is formed in the retaining groove  172 , but in the second embodiment, as shown in  FIG. 27 , a radial movement allowable region  272   a  is formed in a retaining groove  272  in place of the pocket-like region  172   a . The ball  175  gets in contact with a wall of the retaining groove  272  when the ball  175  moves in the direction B. The wall extends along a prescribed tangent of the inner periphery of a ball retaining ring  271 , so that the radial movement allowable region  272   a  is formed in the retaining groove  272 . Therefore, when the ball  175  is located in a position shown in  FIG. 27  (the radial movement allowable region  272   a ), the ball  175  is allowed to move toward the center of the ball retaining ring  271  in the radial direction (the rotation axis of the spindle  160 ). 
     In the screw removing operation, the ball  175  is moved in the direction B within the retaining groove  272  in contact with the push ring  173  by rotation of the push ring  173  in the direction B and disposed in the radial movement allowable region  272   a . The ball  175  disposed in the radial movement allowable region  272   a  is further moved toward the center of the ball retaining ring  271  in the radial direction (radially inward) by rotation of the push ring  173  in the direction B. Then, the ball  175  collides with the large-diameter part  166  of the spindle  160  rotating in the screw removing direction (the direction B), so that the ball  175  is moved radially outward within the radial movement allowable region  272   a . Thereafter, the ball  175  is moved again toward the center of the ball retaining ring  271  in the radial direction (radially inward) by rotation of the push ring  173  in the direction B. Specifically, during the screw removing operation, the ball  175  periodically moves radially outward and inward within the radial movement allowable region  272   a.    
     As a result, the ball  175  periodically collides with the large-diameter part  166  of the spindle  160  and generates collision noise. The ball  175  forms a rotation direction informing device which informs a user of rotation of the spindle  160  in the screw removing direction by the collision noise. Specifically, when the spindle  160  is located in the front position, the stopper  170  prevents the spindle  160  from rotating in the screw tightening direction and allows the spindle  160  to rotate in the screw removing direction, and also serves to inform the user of rotation of the spindle  160  in the screw removing direction. Therefore, prior to the screw removing operation, the user can easily confirm the rotation direction (screw removing direction) of the spindle  160 . Therefore, in the second embodiment, it is not necessary to provide the LED  107   c.    
     According to the above-described first and second embodiments, in screw tightening operation, when the spindle  160  is moved to the rear position by pressing, the inclined part  147   a  of the lock sleeve  145  and the inclined part  133   a  of the retainer  130  engage with each other and thereby move the rollers  140  with respect to the lock sleeve  145  in the circumferential direction of the retainer  130 . Specifically, the rollers  140  are moved from the rotation transmission disabled position to the rotation transmission position in the circumferential direction of the retainer  130 . Therefore, the movement of the spindle  160  in the axial direction of the spindle  160  is converted into the movement of the rollers  140  in the circumferential direction of the retainer  130  (the spindle  160 ). In this manner, the position of the rollers  140  can be rationally switched according to the screw tightening operation. 
     Further, according to the first and second embodiments, by using the rollers  140 , rotation of the output shaft  111  of the motor  110  can be reliably transmitted to the spindle  160  by the wedge effect of the rollers  140  held between the driving gear  125  and the lock sleeve  145 . 
     Further, according to the first and second embodiments, in the screw tightening operation, the rollers  140  are released from (the holding between) the driving gear  125  and the lock sleeve  145  as the screw (the spindle  160 ) moves. Particularly, by the resultant of the biasing force of the coil spring  155  in the axial direction of the spindle  160  and the reaction force that the lock sleeve  145  receives from the retainer  130  in the axial direction of the spindle  160  when the lock sleeve  145  rotates the retainer  130 , the rollers  140  are released from the holding between the driving gear  125  and the lock sleeve  145 . In order to release the rollers  140  solely by the biasing force of the coil spring  155 , a larger biasing force of the coil spring  155  is required. By also utilizing the reaction force that the lock sleeve  145  receives from the retainer  130 , however, the rollers  140  can be reliably released and transmission of rotation by the driving mechanism  120  is interrupted. Further, by utilizing the reaction force from the retainer  130  as well, the coil spring  155  having a smaller spring constant can be used. 
     Further, according to the first and second embodiments, in the idling state, the stopper  170  prevents rotation of the spindle  160  in the screw tightening direction. As a result, the spindle  160  can be reliably prevented from being caused to unintentionally rotate, for example, by lubricant solidified within the front housing  104 . Therefore, in screw tightening operation, the spindle  160  is rotationally driven only when the spindle  160  is pressed. Further, in screw removing operation, since the stopper  170  allows the spindle  160  to rotate in the screw removing direction, the spindle  160  is rotationally driven without need of pressing the spindle  160 . Thus, the spindle  160  is rationally driven according to the operation mode. 
     Further, according to the first and second embodiments, by providing the oil seal  181  in the front part of the front housing  104 , lubricant is prevented from leaking out from the front of the front housing  104 . The oil seal  181  is prevented from coming off in the axial direction of the spindle  160  by elastic deformation of the base  181   a  of the oil seal  181  and also prevented from rotating in the circumferential direction when the spindle  160  rotates. In other words, the oil seal  181  is securely fixed in the axial direction and the circumferential direction of the spindle  160 . Further, by providing the recesses  104   a ,  104   b  in the front housing  104 , movement of the oil seal  181  in the axial direction and the circumferential direction of the spindle  160  is more effectively prevented. This fixation of the oil seal  181  is particularly useful with respect to the spindle  160  which rotates around its axis and moves in the axial direction. 
     Further, according to the first and second embodiments, increase of air pressure within the front housing  104  is prevented by the air passage  190 . Further, lubricant leaking through the air passage  190  is reliably caught by the oil filter  195  and prevented from leaking to the outside of the screwdriver  100 . In a structure in which the output shaft  111  of the motor  110  is arranged in parallel to the axial direction of the spindle  160  (the tool bit  119 ), the motor  110  is disposed behind the driving mechanism  120  in consideration of the position of the center of gravity of the screwdriver  100 . Therefore, a free space is created behind the driving mechanism  120  above the motor  110 . The air passage  190  and the oil filter  195  are arranged in such a space, so that components of the screwdriver  100  are rationally arranged. 
     In the above-described embodiments, the LED  107   c  informs the user by illuminating that a screw removing operation is performed. The informing structure is not limited to this. For example, the LED  107   c  may flash, or emit light in different colors to inform that a screw removing operation is performed. Further, as the rotation direction informing device, an actuator which generates vibration and noise may be provided. Further, the rotation direction informing device may inform the user not only of rotation of the spindle  160 ,  360  (the tool bit  119 ) in the screw removing direction in a screw removing operation, but of rotation of the spindle  160 ,  360  (the tool bit  119 ) in the screw tightening direction in a screw tightening operation. 
     Further, in the above-described embodiments, in order to release the holding of the rollers  140  between the driving gear  125  and the lock sleeve  145 , the lock sleeve  145  is moved forward by mechanical contact between the inclined parts  133   a ,  147   a  in cooperation with the biasing force of the coil spring  155 . The lock sleeve  145  may however be moved forward in other methods. Specifically, the lock sleeve  145  may be moved forward solely by contact between the inclined parts  133   a ,  147   a  of which inclination angles are appropriately set. Further, for example, in addition to the inclined parts  133   a ,  147   a , a releasing means may be provided to detect contact of the locator  105  with the workpiece during screw tightening operation and move the lock sleeve  145  forward so as to release the holding of the rollers  140  between the driving gear  125  and the lock sleeve  145 . Further, only either one of the inclined parts  133   a ,  147   a  may be provided. 
     Further, in the above-described embodiments, the driving member or the driving gear  125  has a cylindrical inside shape and the driven member or the lock sleeve  145  has a prismatic outside shape, but they may be shaped otherwise. The driving member may have a prismatic inside shape and the driven member may have a cylindrical outside shape 
     Further, in the above-described embodiments, the releasing mechanism for releasing the rollers  140  is provided to move the lock sleeve  145  and the retainer  130  in the axial direction and the circumferential direction with respect to each other by utilizing the inclined parts  133   a ,  147   a , but the releasing mechanism is not limited to this. For example, in addition to the driving mechanism  120 , a driving device may be provided to move the retainer  130  from the holding position to the holding disabled position by movement of the retainer  130  with respect to the driving gear  125 . In this case, the driving device is driven by the motor  110  and serves as a releasing mechanism. Further, the timing when the driving device releases the rollers  140  is appropriately set by controlling the timing when the driving device is driven by the motor  110 . 
     Further, in the above-described embodiments, the inclined parts  133   a ,  147   a  are provided, but, for example, either one of the inclined parts may be provided. In this case, in the other member having no inclined part, a contact part is formed to slide in contact with the inclined part. 
     In view of the nature of the present invention, a screw tightening tool according to this invention may have the following features. Each feature may be used alone or in combination with others, or in combination with the claimed invention. 
     (Aspect 1) 
     The intervening member is held between the tool accessory driving shaft and the driving member and thereby exhibits a wedge effect so that the driving member, the intervening member and the tool accessory driving shaft rotate together by the wedge effect. 
     (Aspect 2) 
     The driving member has a cylindrical inside shape and the tool accessory driving shaft has a generally prismatic outside shape, as viewed in a cross section perpendicular to the rotation axis of the tool accessory. 
     (Aspect 3) 
     The first holding member has a generally prismatic outside shape, as viewed in a section perpendicular to the rotation axis of the tool accessory. 
     (Aspect 4) 
     The releasing mechanism releases the intervening member by utilizing relative movement of the first element and the second element in the axial direction of the tool accessory driving shaft and in the circumferential direction around the axial direction, which movement is caused by sliding of the inclined surface formed on one of the first element and the second element and the contact part formed on the other element with respect to each other. 
     (Aspect 5) 
     The releasing mechanism releases the intervening member by utilizing relative movement of the first element and the second element in the axial direction of the tool accessory driving shaft and in the circumferential direction around the axial direction and the biasing force of the biasing member. 
     (Aspect 6) 
     The second holding member has a second holding part which can hold the intervening member between the second holding member and the driving member. 
     Correspondences Between the Features of the Embodiments and the Features of the Invention 
     Correspondences between the features of the embodiments and the features of the invention are as follows. The above-described embodiments are representative examples for embodying the present invention, and the present invention is not limited to the structures that have been described as the representative embodiments. 
     The screwdriver  100  is an example embodiment that corresponds to the “screw tightening tool” according to the present invention. 
     The motor  110  is an example embodiment that corresponds to the “motor” according to the present invention. 
     The driving mechanism  120  is an example embodiment that corresponds to the “driving mechanism” according to the present invention. 
     The driving gear  125  is an example embodiment that corresponds to the “driving member” according to the present invention. 
     The spindle  160  is an example embodiment that corresponds to the “tool accessory driving shaft” according to the present invention. 
     The spindle  160  is an example embodiment that corresponds to the “tool accessory holding shaft” according to the present invention. 
     The lock sleeve  145  is an example embodiment that corresponds to the “tool accessory driving shaft” according to the present invention. 
     The lock sleeve  145  is an example embodiment that corresponds to the “first element” according to the present invention. 
     The lock sleeve  145  is an example embodiment that corresponds to the “first holding member” according to the present invention. 
     The inclined part  147   a  is an example embodiment that corresponds to the “second inclined surface” according to the present invention. 
     The inclined part  147   a  is an example embodiment that corresponds to the “contact part” according to the present invention. 
     The roller engagement part  146  is an example embodiment that corresponds to the “holding part” according to the present invention. 
     The retainer  130  is an example embodiment that corresponds to the “second element” according to the present invention. 
     The inclined part  133   a  is an example embodiment that corresponds to the “inclined surface” according to the present invention. 
     The second side wall  133  is an example embodiment that corresponds to the “retaining part” according to the present invention. 
     The roller  140  is an example embodiment that corresponds to the “intervening member” according to the present invention. 
     The spring receiver  150  is an example embodiment that corresponds to the “second holding member” according to the present invention. 
     The ball  153  is an example embodiment that corresponds to the “biased member” according to the present invention. 
     The coil spring  155  is an example embodiment that corresponds to the “biasing member” according to the present invention. 
     DESCRIPTION OF NUMERALS 
     
         
           100  screwdriver 
           101  body 
           103  main housing 
           103   a  partition wall 
           103   f  front surface 
           104  front housing 
           104   a  recess 
           104   b  recess 
           104   c  large-diameter part 
           105  locator 
           106  external communication part 
           107  handle 
           107   a  trigger 
           107   b  changeover switch 
           107   c  LED 
           109  power cable 
           110  motor 
           111  output shaft 
           112  gear teeth 
           119  tool bit 
           120  driving mechanism 
           121  needle bearing 
           122  front bearing 
           123  rear bearing 
           125  driving gear 
           126  bottom wall 
           127  side wall 
           128  gear teeth 
           130  retainer 
           131  base 
           131   a  engagement hole 
           131   b  ball retaining part 
           132  first side wall 
           133  second side wall 
           134  roller retaining part 
           139  engagement pin 
           140  roller 
           145  lock sleeve 
           146  roller engagement part 
           147  retainer engagement part 
           147   a  inclined part 
           150  spring receiver 
           150   a  engagement hole 
           151  roller engagement part 
           152  ball contact part 
           153  ball 
           155  coil spring 
           160  spindle 
           161  front shaft part 
           162  rear shaft part 
           162   a  groove 
           163  hollow part 
           164  communication hole 
           165  rear end bearing 
           166  large-diameter part 
           166   b  width across flat part 
           167  small-diameter part 
           170  stopper 
           171  ball retaining ring 
           172  retaining groove 
           172   a  pocket-like region 
           173  push ring 
           175  ball 
           176  ball 
           177  leaf spring 
           177   a  through hole 
           177   b  engagement hole 
           180  O-ring 
           181  oil seal 
           181   a  base 
           181   b  lip 
           190  air passage 
           191  passage formation part 
           191   a  passage opening 
           195  oil filter 
           271  ball retaining ring 
           272  retaining groove 
           272   a  radial movement allowable region