Patent Publication Number: US-2022226881-A1

Title: Power tool

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Japanese patent application No. 2021-005484 filed on Jan. 18, 2021, the contents of which are hereby fully incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a power tool. 
     BACKGROUND 
     In some known power tools, a motor is rotatable in two directions, that is, in a normal direction and in a reverse direction. Such known power tools may be configured to perform different actions according to whether the motor rotates in the normal direction or in the reverse direction. For example, Japanese Unexamined Patent Application Publication No. 2018-103257 discloses a fastening tool that is configured to swage a fastener by moving a screw shaft rearward when the motor rotates in the normal direction and to return the screw shaft forward to an initial position when the motor rotates in the reverse direction. 
     SUMMARY 
     In power tools like the above-described fastening tool that performs different actions according to the rotation direction of the motor, required rotation speed and output torque may differ, depending on each action. 
     Accordingly, it is one, non-limiting object of the present disclosure to provide an improvement in a power tool that performs different actions according to a rotation direction of a motor. 
     One, non-limiting aspect of the present disclosure herein provides a power tool that includes a motor and a gear speed reducer. The motor has a motor shaft that is rotatable in two directions, that is, in a normal direction and in a reverse direction. The gear speed reducer is operably coupled to the motor shaft. The gear speed reducer is configured such that a reduction ratio of the gear speed reducer is changed in response to a change of a rotation direction of the motor shaft. 
     According to this aspect, the reduction ratio of the gear speed reducer, and thus the rotation speed (output speed) of an output shaft of the gear speed reducer and torque outputted from the gear speed reducer (output torque of the gear speed reducer) can be changed, according to whether the motor shaft rotates in the normal direction or in the reverse direction. Thus, the power tool can selectively perform two actions that are different in required speed and torque simply by changing the rotation direction of the motor without need for controlling the rotation speed of the motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a fastening tool. 
         FIG. 2  is a sectional view of a speed reducer. 
         FIG. 3  is a perspective, exploded view showing a carrier of a first stage, a sun gear and an internal gear of a second stage, and a reduction-ratio change mechanism. 
         FIG. 4  is a sectional view taken along line IV-IV in  FIG. 2 , for illustrating an action of a lock mechanism in normal driving of a motor. 
         FIG. 5  is a sectional view taken along line V-V in  FIG. 4 . 
         FIG. 6  is a sectional view taken along line VI-VI in  FIG. 4 . 
         FIG. 7  is a sectional view corresponding to FIG. for illustrating an action of the lock mechanism in reverse driving of the motor. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In one non-limiting embodiment according to the present disclosure, the gear speed reducer may include at least one planetary gear mechanism. The at least one planetary gear mechanism may each include a sun gear, an internal gear, a carrier and a plurality of planetary gears. The gear speed reducer may be configured such that the reduction ratio is changed in response to a change of the number of the at least one planetary gear mechanism that functions effectively. According to this embodiment, the gear speed reducer can be made small in size and can provide a large reduction ratio by utilizing the planetary gear mechanism, compared with a gear speed reducer utilizing a combination of spur gears or other gears. Further, the gear speed reducer can rationally change the reduction ratio by changing the number of the at least one planetary gear mechanism that function effectively (i.e. the number of effective stages of the at least one planetary gear mechanism). 
     In addition or in the alternative to the preceding embodiment, the power tool may further include a one-way clutch and a lock mechanism that is operably coupled to the one-way clutch. The one-way clutch may be disposed in a transmission path from the motor shaft to the sun gear. The one-way clutch may be configured to permit rotation of the sun gear relative to the one-way clutch while the motor shaft rotates in a first direction. Also, the one-way clutch may be configured to rotate integrally with the sun gear while the motor shaft rotates in a second direction. The first direction here is one of the normal direction and the reverse direction, and the second direction is the other of the normal direction and the reverse direction. The lock mechanism may be configured to lock the internal gear to be non-rotatable while the one-way clutch permits the relative rotation of the sun gear. Also, the lock mechanism may be configured to cause the internal gear to rotate integrally with the sun gear while the one-way clutch rotates integrally with the sun gear. 
     According to this embodiment, the one-way clutch and the lock mechanism are configured to cooperate together to cause the at least one planetary gear mechanism to effectively function while the motor shaft rotates in the first direction, and to disable the function of the planetary gear mechanism while the motor shaft rotates in the second direction. Particularly, the one-way clutch automatically performs different actions according to the rotation direction of the motor shaft, and thus can efficiently change the action of the lock mechanism in response to the change of the rotation direction of the motor shaft. Owing to such a rational structure, the power tool according to this embodiment can perform an action for which relatively low-speed rotation and/or relatively large-torque output is required while the motor shaft rotates in the first direction, and can perform an action for which relatively high-speed rotation and/or relatively small-torque output is required while the motor shaft rotates in the second direction. 
     In addition or in the alternative to the preceding embodiments, the gear speed reducer may include a plurality of planetary gear mechanisms arranged in multiple stages. The lock mechanism may be configured to act on the internal gear of a planetary gear mechanism at a second stage or a further downstream stage among the plurality of planetary gear mechanisms. According to this embodiment, the lock mechanism can act on the internal gear in the second or further downstream stage after being decelerated at a first stage, so that load on the lock mechanism can be reduced and thus the durability of the lock mechanism can be improved. 
     In addition or in the alternative to the preceding embodiments, the power tool may further include a movable member that is operably coupled to the gear speed reducer and that is configured to move in response to driving of the motor. The power tool may be configured to operate in a cycle, which includes a first stroke in which the movable member moves in a prescribed direction and a second stroke in which the movable member moves in a direction opposite to the prescribed direction. The rotation direction of the motor shaft may be changed in response to a change between the first stroke and the second stroke. According to this embodiment, the power tool can output required torque at required speed in each of the first stroke and the second stroke simply by changing the rotation direction of the motor. 
     In addition or in the alternative to the preceding embodiments, the power tool may be a fastening tool configured to fasten workpieces via a fastener. The fastening tool is a typical example of the power tool that is configured to perform different actions between the first stroke and the second stroke in a fastening operation. According to this embodiment, the fastening tool is provided that can efficiently perform the fastening operation. 
     In addition or in the alternative to the preceding embodiments, the movable member may be configured to grip a portion of the fastener. The movable member may be configured to move in the prescribed direction from an initial position relative to the workpieces while pulling the fastener in the first stroke. Also, the movable member may be configured to return to the initial position in the direction opposite to the prescribed direction without pulling the fastener in the second stroke. The reduction ratio in the first stroke may be larger than the reduction ratio in the second stroke. According to this embodiment, the fastening tool can output a relatively large torque in the first stroke in which the movable member pulls the fastener, and can efficiently return to the initial position at relatively high speed in the second stroke in which the movable member returns to the initial position without pulling the fastener. 
     In addition or in the alternative to the preceding embodiments, the power tool may further include a feed screw mechanism that is arranged between the gear speed reducer and the movable member in a transmission path. Further, the feed screw mechanism may be configured to convert rotation of an output shaft of the gear speed reducer into linear motion of the movable member. According to this embodiment, the feed screw mechanism can perform efficient conversion of relatively large torque into linear motion. 
     In addition or in the alternative to the preceding embodiments, when the motor shaft rotates in one of the normal direction and the reverse direction, the reduction ratio may be at least 2.5 times larger than when the motor shaft rotates in the other of the normal direction and the reverse direction. According to this embodiment, the power tool can perform two actions that are relatively greatly different in speed and torque according to the rotation direction of the motor. 
     In addition or in the alternative to the preceding embodiments, the power tool may further include a control device that is configured to control operation of the power tool. The control device may be configured to change the rotation direction of the motor in response to recognizing a prescribed event. According to this embodiment, the control device can automatically change the rotation direction of the motor upon recognizing a prescribed event, so that the reduction ratio can be changed properly and efficiently. 
     A fastening tool  1  according to a representative, non-limiting embodiment of the present disclosure is now described in detail, with reference to the drawings. The fastening tool  1  of this embodiment is configured to fasten workpieces with a fastener  8 . The fastener  8  is a known fastener (specifically, a multi-piece swage type fastener) including a pin  81  and a collar  85 . 
     The general structure of the fastening tool  1  is now described. 
     As shown in  FIG. 1 , an outer shell of the fastening tool  1  is mainly formed by a body housing  11 , a nose  13 , a handle  15  and a battery housing  17 . The body housing (also referred to as a tool body)  11  has a rectangular box-like shape as a whole and extends along a prescribed driving axis A 1 . The body housing  11  houses a motor  2  and a driving mechanism  3 . The nose  13  protrudes along the driving axis A 1  from one end portion of the body housing  11  in its longitudinal direction. The handle  15  protrudes in a direction that intersects (specifically, in a direction substantially orthogonal to) the driving axis A 1  from a central portion of the body housing  11  in its longitudinal direction. The handle  15  has a trigger  151  configured to be depressed by a user. The battery housing  17  is connected to a protruding end of the handle  15 . A rechargeable battery (or batteries)  182  may be removably received by the battery housing  17 . 
     When the user engages the fastener  8  with a front end portion of the nose  13  and depresses the trigger  151 , the motor  2  is driven and the pin  81  is pulled in its axial direction relative to the collar  85  and workpieces W, so that the workpieces W are fastened by the fastener  8 . 
     In the following description, for convenience of explanation, as for the direction of the fastening tool  1 , an extending direction of the driving axis A 1  (or a longitudinal axis of the body housing  11 ) is defined as a front-rear direction of the fastening tool  1 . In the front-rear direction, the side on which the nose  13  is located is defined as a front side and the opposite side is defined as a rear side. Further, a direction that is orthogonal to the driving axis A 1  and that generally corresponds to the extending direction of a longitudinal axis of the handle  15  is defined as an up-down direction. In the up-down direction, a protruding-end side (the battery housing  17  side) of the handle  15  is defined as a lower side, and a base-end side (the body housing  11  side) of the handle  15  is defined as an upper side. A direction that is orthogonal both to the front-rear direction and the up-down direction is defined as a left-right direction. 
     The detailed structure of the fastening tool  1  is now described. 
     First, the structures/elements/components disposed within the body housing  11  are described. As shown in  FIG. 1 , the body housing  11  mainly houses the motor  2  and the driving mechanism  3 , which is configured to be driven by the motor  2 . 
     The motor  2  is housed in a lower rear end portion of the body housing  11 . In this embodiment, a brushless direct current (DC) motor is employed as the motor  2 . The motor  2  includes a stator  21 , a rotor  22  and a motor shaft  23  that rotates integrally with the rotor  22 . The motor  2  is arranged such that a rotational axis A 2  of the motor shaft  23  extends in parallel to the driving axis A 1  (i.e. in the front-rear direction) below (specifically, directly below) the driving axis A 1 . A front end portion of the motor shaft  23  protrudes into a gear case  40  of the speed reducer  4 . In this embodiment, the rotor  22  and the motor shaft  23  can selectively rotate in two directions. Specifically, the rotor  22  and the motor shaft  23  can selectively rotate in a normal direction and in a reverse direction. In this embodiment, the normal direction corresponds to a direction of moving a screw shaft  56  and a pin-gripping part  63  (described below) rearward. The reverse direction corresponds to a direction of moving the screw shaft  56  and the pin-gripping part  63  forward. Driving the motor  2  to rotate in the normal direction is hereinafter also referred to as normal driving, and driving the motor  2  to rotate in the reverse direction is also referred to as reverse driving. 
     The driving mechanism  3  is now described. The driving mechanism  3  is configured to move the pin-gripping part  63  (described below) along the driving axis A 1  in the front-rear direction relative to the anvil  61  by power of the motor  2 . In this embodiment, the driving mechanism  3  includes a speed reducer  4 , a nut-driving gear  311  provided on a first intermediate shaft  31 , an idler gear  331  provided on a second intermediate shaft  33 , and a ball-screw mechanism  5 . The structures of these components/mechanisms are now described in this order. 
     The speed reducer  4  is disposed coaxially with the motor  2  in front of the motor  2  within the body housing  11 . The speed reducer  4  of this embodiment is a speed reducer that utilizes planetary gear mechanisms (epicyclic gearing, epicyclic gear system). The speed reducer  4  is configured to decelerate rotation of the motor shaft  23  and increase torque, according to its reduction ratio (speed reduction ratio), and output the torque to the first intermediate shaft  31 . In this embodiment, the speed reducer  4  is structured as a multi-stage planetary gear reducer. More specifically, as shown in  FIG. 2 , the speed reducer  4  includes the gear case  40  and three stages (sets) of planetary gear mechanisms  41 ,  42 ,  43  housed in the gear case  40 . The gear case  40  is non-rotatably supported by the body housing  11 . 
     The first-stage (input-side) planetary gear mechanism  41  includes a sun gear  411 , an internal gear (also referred to as a ring gear)  412 , a carrier  415 , and a plurality of planetary gears  418 . 
     The sun gear  411  is fixed onto (around) a front end portion of the motor shaft  23 . In this embodiment, the motor shaft  23  serves as an input shaft for the speed reducer  4 . The internal gear  412  is fixedly held within the gear case  40 . Thus, the internal gear  412  is not substantially movable in the front-rear direction and is not substantially rotatable around the rotational axis A 2 , relative to the gear case  40 . The planetary gears  418  are supported by the carrier  415  and engaged with the sun gear  411  and the internal gear  412 . The carrier  415  has a shaft  416  extending forward along the rotational axis A 2 . The carrier  415  (the shaft  416 ) rotates in the same direction as the motor shaft  23  when the motor  2  is driven. 
     The second-stage planetary gear mechanism  42  includes a sun gear  421 , an internal gear (also referred to as a ring gear)  422 , a carrier  425 , and a plurality of planetary gears  428 . 
     The sun gear  421  is fixed onto (around) a front end portion of the shaft  416  of the first-stage carrier  415 . Thus, the sun gear  421  rotates integrally with the carrier  415  in the same direction as the motor shaft  23  when the motor  2  is driven. The internal gear  422  is fitted in the gear case  40 . Four projections  423  are formed on a rear end of the internal gear  422  and protrude rearward. The projections  423  are arranged substantially at equal intervals in a circumferential direction of the internal gear  422 . The internal gear  422  is not substantially movable in the front-rear direction, but is selectively rotatable around the rotational axis A 2 , relative to the gear case  40 . Whether the internal gear  422  is rotatable or non-rotatable is changed by a reduction-ratio change mechanism  7 , depending on the rotation direction of the motor  2 . The reduction-ratio change mechanism  7  will be described in detail below. The planetary gears  428  are supported by the carrier  425  and engaged with the sun gear  421  and the internal gear  422 . The carrier  425  has a shaft  426  extending forward along the rotational axis A 2 . 
     The third-stage (final-stage, output-side) planetary gear mechanism  43  includes a sun gear  431 , an internal gear (also referred to as a ring gear)  432 , a carrier  435 , and a plurality of planetary gears  438 . 
     The sun gear  431  is fixed onto (around) a front end portion of the shaft  426  of the second-stage carrier  425 . Like the first-stage internal gear  412 , the internal gear  432  is fixedly held within the gear case  40 . The planetary gears  438  are supported by the carrier  435  and engaged with the sun gear  431  and the internal gear  432 . The carrier  435  has a shaft  436  extending forward along the rotational axis A 2 . The shaft  436  of the third-stage (final-stage) planetary gear mechanism  43  serves as a final output shaft of the speed reducer  4 . 
     As shown in  FIG. 1 , the first intermediate shaft  31  is arranged coaxially with the motor shaft  23  and the speed reducer  4  within the body housing  11 , and extends forward from the speed reducer  4 . The first intermediate shaft  31  is connected to the shaft  436  (see  FIG. 2 ) of the third-stage carrier  435  of the speed reducer  4 . The first intermediate shaft  31  is supported rotatably around the rotational axis A 2  by two bearings supported by the body housing  11 , and configured to rotate integrally with the carrier  435 . The nut-driving gear  311  is integrally formed on (around) an outer periphery of the first intermediate shaft  31 . 
     The second intermediate shaft  33  extends in parallel to the first intermediate shaft  31  above (specifically, directly above) the first intermediate shaft  31 . The idler gear  331  is supported by the second intermediate shaft  33  via a bearing. The idler gear  331  is thus rotatable around an axis of the second intermediate shaft  33 . The idler gear  331  is engaged with the nut-driving gear  311  and a driven gear  511  of a nut  51  (described below), but does not affect the rotation speed ratio between the nut-driving gear  311  and the driven gear  511 . 
     The ball-screw mechanism  5  is a known mechanism that mainly includes the nut  51  and the screw shaft  56 . In this embodiment, the ball-screw mechanism  5  is configured to convert rotation of the nut  51  into linear motion of the screw shaft  56  and to linearly move the pin-gripping part  63  described below. The ball-screw mechanism  5  is an example of a feed screw mechanism and is capable of efficiently converting relatively large torque into linear motion. 
     The nut  51  is supported to be substantially immovable in the front-rear direction and rotatable around the driving axis A 1 , relative to the body housing  11 . The nut  51  has a hollow cylindrical shape and has a driven gear  511  integrally formed on (around) its outer periphery. The nut  51  is supported by a pair of radial bearings supported by the body housing  11  in front of and behind the driven gear  511 . The nut-driving gear  311  and the driven gear  511  form a speed-reducing gearing. 
     The screw shaft  56  is engaged with the nut  51  so as to be substantially non-rotatable around the driving axis A 1  and to be movable along the driving axis A 1  in the front-rear direction, relative to the body housing  11 . More specifically, the screw shaft  56  has an elongate shape, and is inserted through the nut  51  to extend along the driving axis A 1 . Although not shown in detail, spiral grooves are respectively formed on an inner peripheral surface of the nut  51  and on an outer peripheral surface of the screw shaft  56 . Multiple balls are rollably disposed within a track defined by these spiral grooves. The screw shaft  56  is engaged with the nut  51  via these balls. Although not shown in detail, two arms are provided on a rear end portion of the screw shaft  56  and extend from the screw shaft  56  to the left and right. Each of the arms rotatably supports a roller. The rollers are respectively engaged with guide grooves of roller guides fixed to the body housing  11 . Each of the rollers can roll along the guide groove in the front-rear direction while being restricted in movement in the up-down direction. 
     Owing to this structure, when the nut  51  is rotated around the driving axis A 1 , the screw shaft  56  linearly moves in the front-rear direction relative to the nut  51  and the body housing  11 . 
     An extension shaft  561  is coaxially connected and fixed to the rear end portion of the screw shaft  56 . The extension shaft  561  is thus integrated with the screw shaft  56 . The screw shaft  56  and the extension shaft  561  integrated with each other are hereinafter also collectively referred to as a driving shaft  560 . The driving shaft  560  has a through hole extending therethrough along the driving axis A 1 . A container  115  is removably attached to a rear end portion of the body housing  115 . The container  115  is provided to store a portion (hereinafter referred to as a pintail) of a shaft part of the pin  81  that is separated from the fastener  8 . The pintail separated from the fastener  8  may reach the container  115  through the through hole of the driving shaft  560  and may be stored in the container  115 . 
     The nose  13  is now described. As shown in  FIG. 1 , the nose  13  mainly includes the anvil  61  and the pin-gripping part  63 . The anvil  61  is configured to abut on (engage with) the collar  85  of the fastener  8 . The anvil  61  is connected to the body housing  11  via a connecting member  62 . The pin-gripping part  63  is configured to grip the pin  81  of the fastener  8 . The pin-gripping part  63  is held to be movable along the driving axis A 1  in the front-rear direction relative to the anvil  61 . The structures of the anvil  61  and the pin-gripping part  63  are known and therefore described only briefly here. 
     The anvil  61  has a hollow cylindrical shape as a whole and has a bore extending along the driving axis A 1 . The pin-gripping part  63  is held coaxially with the anvil  61  within the bore so as to be slidable within the bore. A front end portion of the bore has a smaller diameter than the other portion of the bore and is configured to abut on (engage with) the collar  85 . Although not shown in detail, the pin-gripping part  63  has a plurality of claws (or jaws) which are configured to grip the shaft part of the pin  81 . The pin-gripping part  63  is configured such that the gripping force of the claws increases as the pin-gripping part  63  moves rearward from an initial position relative to the anvil  61 . A rear end portion of the pin-gripping part  63  is connected to a front end portion of the screw shaft  56  via a connecting member  64 . The connecting member  64  has a through hole that extends therethrough along the driving axis A 1  and communicates with the through hole of the driving shaft  560 . 
     The handle  15  is now described. As shown in  FIG. 1 , the handle  15  has an elongate tubular shape. The handle  15  extends contiguously downward from a lower end of a central portion of the body housing  11  in the front-rear direction. The handle  15  is a portion to be held (gripped) by the user. The trigger  151  is provided on an upper end portion of the handle  15  and configured to be depressed by the user. A switch  152  is housed within the handle  15 . The switch  152  is normally kept OFF, and turned ON in response to depressing manipulation of the trigger  151 . The switch  152  is electrically connected to a controller  170  via wires (not shown). When turned ON, the switch  152  outputs an ON signal to the controller  170 . 
     The battery housing  17  is now described. As shown in  FIG. 1 , the battery housing  17  is a hollow body having an inverted U-shape which is relatively long in the front-rear direction. The controller  170  is housed in the battery housing  17 . The controller  170  includes a control circuit  171 , which is configured to control operation of the fastening tool  1 . In this embodiment, the control circuit  171  is formed by a microcomputer including a CPU, a ROM and a RAM. Although not shown in detail, the control circuit  171  is mounted on a circuit board housed in a case, together with a driving circuit for the motor  2  etc. 
     Two battery-mounting parts  181  are provided in a lower end portion of the battery housing  17 . Each of the battery-mounting parts  181  is configured to removably receive the battery  182 . Thus, in this embodiment, two batteries  182  can be mounted on (connected to) the fastening tool  1 . The battery  182  is a rechargeable power source for supplying power to the motor  2  and various other parts of the fastening tool  1 . The battery  182  may also be referred to as a battery pack. The structures of the battery-mounting part  181  and the battery  182  are well known and not therefore described here. 
     The reduction-ratio change mechanism  7  is now described. As described above, the reduction-ratio change mechanism  7  is configured to selectively permit (allow) or prevent rotation of the internal gear  422  of the second-stage planetary gear mechanism  42  of the speed reducer  4 , depending on the rotation direction of the motor  2 . When the state of the internal gear  422  is switched between a rotatable state, in which rotation of the internal gear  422  is permitted, and a non-rotatable state, in which rotation of the internal gear  422  is prevented (prohibited), the number of effective stages of the speed reducer  4  (i.e., the number of the planetary gear mechanisms that function effectively) and thus the reduction ratio of the speed reducer  4  are changed. 
     As shown in  FIGS. 2 and 3 , the reduction-ratio change mechanism  7  includes a one-way clutch  70  and a lock mechanism  71 . 
     The one-way clutch  70  is a clutch having a mechanism that transmits rotation in only one direction and idles in the opposite direction. In this embodiment, the one-way clutch  70  is a general-purpose one-way clutch having a well-known structure, in which a plurality of rollers are respectively biased by springs and supported within a cylindrical sleeve. The one-way clutch  70  is mounted onto (around) the shaft  416  of the first-stage carrier  415 . When the motor shaft  23  and the shaft  416  rotate in the normal direction, the one-way clutch  70  idles relative to the shaft  416 . In other words, when the motor shaft  23  and the shaft  416  rotate in the normal direction, the one-way clutch  70  does not rotate integrally with the carrier  415  and thus does not transmit rotation. On the other hand, when the motor shaft  23  and the shaft  416  rotate in the reverse direction, the one-way clutch  70  rotates integrally with the shaft  416 . In other words, when the motor shaft  23  and the shaft  416  rotate in the reverse direction, the one-way clutch  70  is locked to the shaft  416  and rotates integrally with the shaft  416 , and thus transmits rotation. 
     The lock mechanism  71  is configured to switch whether to permit rotation of the second-stage internal gear  422  according to whether the one-way clutch  70  idles relative to the first-stage carrier  415  (the shaft  416 ) or rotates integrally with the first-stage carrier  415 . 
     The detailed structure of the lock mechanism  71  is now described. As shown in  FIGS. 2 to 6 , the lock mechanism  71  includes a retainer  72 , two rollers  73 , a lock sleeve  74  and a lock cam  75 . 
     The retainer  72  is configured to retain the rollers  73  so as to be movable relative to the retainer  72  in a circumferential direction around the rotational axis A 2 . The retainer  72  includes a hollow cylindrical part  721 , a base part  723  and four projections  725 . The cylindrical part  721  extends in the front-rear direction along the rotational axis A 2  and forms a central portion of the retainer  72 . The base part  723  is an annular part protruding radially outward from a rear end portion of the cylindrical part  721 . The projections  725  are circular arc walls arranged substantially at equal intervals along an outer edge portion of the base part  723  and extending forward from the outer edge portion of the base part  723 . A space is formed (defined) between the cylindrical part  721  and the projections  725  in the radial direction of the retainer  72 . A front end of each of the projections  725  is located rearward of a front end of the cylindrical part  721  (i.e. the projection  725  is shorter than the cylindrical part  721  in the front-rear direction). 
     The cylindrical part  721  of the retainer  72  is press-fitted and fixed onto (around) a sleeve of the one-way clutch  70 . Thus, the retainer  72  rotates integrally with the one-way clutch  70 . Therefore, the retainer  72  is selectively rotatable relative to the first-stage carrier  415 . Specifically, when the motor shaft  23  and the shaft  416  rotate in the normal direction, the retainer  72 , which is integral with the one-way clutch  70 , idles relative to the shaft  416 . In other words, when the motor shaft  23  and the shaft  416  rotate in the normal direction, the retainer  72  does not rotate together with the shaft  416 . On the other hand, when the motor shaft  23  and the shaft  416  rotate in the reverse direction, the retainer  72 , which is integral with the one-way clutch  70 , rotates integrally with the shaft  416 . 
     Each of the rollers  73  is a solid cylindrical member (pin). The roller  73  has a substantially uniform diameter that is smaller than the distance between the adjacent two projections  725  of the retainer  72  and larger than the thickness of the projections  725  in the radial direction. The length of the roller  73  is about the same as the length of protrusion of the projections  725  protruding from the front surface of the base part  723  of the retainer  72 . The two rollers  73  are disposed in two diametrically opposed ones of four spaces defined between the projections  725  of the retainer  72  and extend in the front-rear direction. 
     The lock sleeve  74  is a generally hollow cylindrical member. The lock sleeve  74  is fitted coaxially with the speed reducer  4  in front of the first-stage internal gear  412  within the gear case  40 . A plurality of projections  741  protrude radially outward from an outer peripheral surface of the lock sleeve  74 . The projections  741  each extend from a front end to a rear end of the lock sleeve  74  in the front-rear direction. The projections  741  are respectively engaged with grooves  401  (see  FIG. 4 ) that are formed on an inner peripheral surface of the gear case  40  and that extend in the front-rear direction. Thus, the lock sleeve  74  is held so as not to rotate relative to the gear case  40 . 
     The lock sleeve  74  is disposed around (radially outward of) the retainer  72 . A front end of the lock sleeve  74  is located forward of the front ends of the projections  725  and the rollers  73 . A rear end of the lock sleeve  74  is located rearward of a rear end of the retainer  72 . Thus, the projections  725  of the retainer  72  and the rollers  73  are entirely disposed within (radially inward of) the lock sleeve  74 . The inner diameter of the lock sleeve  74  is set to be substantially equal to or slightly larger than the outer diameter of the base part  723  of the retainer  72 . The retainer  72  is selectively rotatable relative to the lock sleeve  74 . 
     The lock cam  75  is operably coupled to the retainer  72  and configured to be selectively rotated by the retainer  72 . Further, the lock cam  75  is connected to the second-stage internal gear  422  and configured to rotate integrally with the internal gear  422  around the rotational axis A 2  relative to the gear case  40 . The lock cam  75  is basically a tubular member that has a through hole having a circular section and extending along the rotational axis A 2 . The lock cam  75  includes a base part  751 , a flange part  753  and a cam part  755 . 
     The base part  751  is a disc-like portion and forms a front half of the lock cam  75 . The flange part  753  protrudes radially outward from an outer peripheral surface of the base part  751 . The outer diameter of the flange part  753  is about the same as the outer diameter of the second-stage internal gear  422 . The flange part  753  has four recesses  754  (see  FIG. 3 ) recessed radially inward from an outer edge of the flange part  753 . The recesses  754  are arranged substantially at equal intervals in a circumferential direction of the flange part  753 . The recesses  754  are shaped to conform to the projections  423  of the internal gear  422  and always engaged with the projections  423 . The lock cam  75  is thus connected to the internal gear  422  for integral rotation by engagement between the recesses  754  and the projections  423 . 
     The cam part  755  protrudes rearward from a rear surface of the base part  751  and forms a rear half of the lock cam  75 . The cam part  755  has two projections  756  and two flat parts  757 . The projections  756  are disposed at diametrically opposed positions across the rotational axis A 2  and protrude radially outward from an outer peripheral surface of the cam part  755 . The two flat parts  757  are respectively formed in intermediate positions between the two projections  756  in a circumferential direction of the cam part  755 . Portions of the outer peripheral surface of the cam part  755  between the projections  756  and the flat parts  757  are curved surfaces that correspond to an outer peripheral surface of a cylinder. The flat parts  757  are portions of the outer peripheral surface of the cam part  755 , and are diametrically opposed to each other across the rotational axis A 2 . The flat parts  757  extend in parallel to each other and in parallel to the rotational axis A 2 . 
     The distance between the flat part  757  and an inner peripheral surface of the lock sleeve  74  in the radial direction is maximum at the center of the flat part  757 . This distance (maximum distance) is set to be slightly larger than the diameter of the roller  73 . The radial distance between the flat part  757  and the inner peripheral surface of the lock sleeve  74  gradually decreases toward edges of the flat part  757  in the circumferential direction. The radial distance between the edge of the flat part  757  and the inner peripheral surface of the lock sleeve  74  is set to be smaller than the diameter of the roller  73 . 
     The lock cam  75  having the above-described structure is fitted around the cylindrical part  721  of the retainer  72  from the front. The two projections  756  of the lock cam  75  are arranged in two of the four spaces defined between the projections  725  of the retainer  72  in the circumferential direction (specifically, in two spaces in which the rollers  73  are not disposed). The remaining portion of the cam part  755  other than the projections  756  is disposed in the space defined between the cylindrical part  721  and the projections  725  of the retainer  72  in the radial direction. The rollers  73  are each disposed between the flat parts  757  of the cam part  755  of the lock cam  75  and the inner peripheral surface of the lock sleeve  74  in the radial direction. Further, the rollers  73  are each disposed between the rear surface of the base part  751  of the lock cam  75  and the front surface of the base part  723  of the retainer  72  in the front-rear direction. 
     Operation of the reduction-ratio change mechanism  7  (the one-way clutch  70  and the lock mechanism  71 ) is now described. 
     First, the operation in normal driving of the motor  2  is described. 
     When the motor shaft  23  rotates in the normal direction, the first-stage carrier  415  (the shaft  416 ) and the second-stage sun gear  421  also rotate in the normal direction. At this time, as described above, the one-way clutch  70  idles relative to the shaft  416  and thus does not transmit rotation to the retainer  72 . Therefore, the retainer  72  does not rotate actively. 
     The second-stage sun gear  421  rotates the second-stage planetary gears  428  engaged therewith. The second-stage planetary gears  428 , which are also engaged with the second-stage internal gear  422 , rotate the second-stage internal gear  422  in the reverse direction relative to the gear case  40 . At this time, the lock cam  75  also rotates in the reverse direction (in the direction of arrows in  FIG. 4 ). Accordingly, each of the rollers  73  relatively moves toward the edge of the flat part  757  in the circumferential direction. 
     As shown in  FIG. 4 , before the projections  756  of the lock cam  75  abut on the projections  725  of the retainer  72 , each of the rollers  73  is held like a wedge between the flat part  757  and the inner peripheral surface of the lock sleeve  74  at a position between the center and the edge of the flat part  757 . This position of the roller  73  relative to the lock sleeve  74  and the lock cam  75  is hereinafter also referred to as a locking position. Thus, the lock cam  75  is locked to the lock sleeve  74  via the rollers  73  and prevented from rotating relative to the gear case  40 . When the lock cam  75  is locked, the internal gear  422  cannot rotate relative to the gear case  40 . Accordingly, from then on, the planetary gears  428  revolve around the sun gear  421  while rotating, and the carrier  425  rotates in the normal direction. 
     As described above, when the motor shaft  23  rotates in the normal direction and the first-stage carrier  415  and the second-stage sun gear  421  rotate relative to the one-way clutch  70 , the lock mechanism  71  locks the second-stage internal gear  422  such that the internal gear  422  is non-rotatable. As a result, the lock mechanism  71  causes the second-stage planetary gear mechanism  42  to function effectively. Therefore, the number of effective stages of the speed reducer  4  is three, when the motor shaft  23  rotates in the normal direction. 
     Next, the operation in reverse driving of the motor  2  is described. 
     When the motor shaft  23  rotates in the reverse direction, the first-stage carrier  415  (the shaft  416 ) and the second-stage sun gear  421  also rotate in the reverse direction. At this time, as described above, the one-way clutch  70  is locked to the shaft  416  and transmits rotation of the shaft  416  to the retainer  72 . Therefore, the retainer  72  also rotates in the reverse direction (in the direction of arrows shown in  FIG. 7 ). 
     As shown in  FIG. 7 , two of the projections  725  of the retainer  72  respectively abut and push the projections  756  of the lock cam  75  in the reverse direction. At the approximately same time, the other two projections  725  respectively abut and push the rollers  73  in the reverse direction. Each of the two projections  725  thus moves the corresponding roller  73  up to a position where the roller  73  is disengaged from between the flat part  757  and the inner peripheral surface of the lock sleeve  74  (a position substantially corresponding to the center of the flat part  757 , in this embodiment). This position of the roller  73  relative to the lock sleeve  74  and the lock cam  75  is hereinafter also referred to as an unlocking position. In the unlocking position, the roller  73  is loosely disposed between the flat part  757  and the inner peripheral surface of the lock sleeve  74 , so that the lock cam  75  can rotate relative to the lock sleeve  74 . Thus, rotation of the retainer  72  is transmitted to the lock cam  75  and the lock cam  75  rotates integrally with the retainer  72  in the reverse direction. As a result, the second-stage internal gear  422  rotates integrally with the first-stage carrier  415  and the second-stage sun gear  421  in the reverse direction. 
     At this time, the second-stage sun gear  421  attempts to rotate the second-stage planetary gears  428 . The second-stage planetary gears  428 , however, cannot rotate (on their respective axes) since the sun gear  421  rotates integrally with the second-stage internal gear  422 . As a result, the second-stage carrier  425  rotates integrally with the sun gear  421  and the internal gear  422  in the reverse direction. The carrier  425  rotates at the same speed as the sun gear  421  (the first-stage carrier  415 ). 
     As described above, when the motor shaft  23  rotates in the reverse direction and the one-way clutch  70  rotates integrally with the first-stage carrier  415  and the second-stage sun gear  421 , the lock mechanism  71  causes the second-stage internal gear  422  to rotate at the same speed in the same direction as the sun gear  421 . As a result, the lock mechanism  71  disables the functions (the speed reducing function and the torque increasing function) of the second-stage planetary gear mechanism  42 . Therefore, the number of effective stages of the speed reducer  4  is two, when the motor shaft  23  rotates in the reverse direction. 
     As described above, the number of effective stages of the speed reducer  4  is smaller when the motor shaft  23  rotates in the reverse direction than in the normal direction. Accordingly, the reduction ratio of the speed reducer  4  is smaller when the motor shaft  23  rotates in the reverse direction than in the normal direction. Thus, the rotation speed (the output speed of the speed reducer  4 ) of the shaft  436  (the final output shaft of the speed reducer  4 ) of the third-stage carrier  435  is higher when the motor shaft  23  rotates in the reverse direction than in the normal direction. Further, the torque outputted from the speed reducer  4  (output torque of the speed reducer  4 ) is larger when the motor shaft  23  rotates in the normal direction than in the reverse direction. In this embodiment, the speed reducer  4  is configured such that, in normal rotation of the motor shaft  23 , the reduction ratio is at least 2.5 times larger than in reverse rotation of the motor shaft  23 . 
     Operation of the fastening tool  1  in performing an operation (hereinafter referred to as a fastening operation) of fastening workpieces W by using the fastener  8  is now described. In the fastening operation, the screw shaft  56  and the pin-gripping part  63  perform one cycle of actions including a first stroke (or initial/forward stroke) and a second stroke (or return/reverse stroke). During the first stroke, the screw shaft  56  and the pin-gripping part  63  move rearward from their initial positions. During the second stroke, the screw shaft  56  and the pin-gripping part  63  move forward to their initial positions. 
     As shown in  FIG. 1 , in an initial state in which the trigger  151  is not depressed, the screw shaft  56  (i.e. the driving shaft  560 ) and the pin-gripping part  63  are located in their initial positions (foremost positions). The user temporarily fixes the fastener  8  to the workpieces W such that a front end portion (the claws) of the pin-gripping part  63  loosely grips the shaft part of the pin  81 . When the trigger  151  is manually depressed by the user and the switch  152  is turned ON, the control circuit  171  of the controller  170  starts normal driving of the motor  2  in response to an ON signal from the switch  152 . Thus, the first stroke is started. 
     As described above, in response to the start of rotation of the motor shaft  23  in the normal direction, the reduction-ratio change mechanism  7  locks the second-stage internal gear  422 , so that three stages of the speed reducer  4  are effective. Accordingly, the shaft  436  of the speed reducer  4  rotates at relatively low speed and outputs relatively large torque. The torque is transmitted to the nut  51  via the nut-driving gear  311 , the idler gear  331  and the driven gear  511  while being further increased. In response to rotation of the nut  51 , the screw shaft  56  and the pin-gripping part  63  move rearward relative to the body housing  11  and the nut  51 . The shaft part of the pin  81  is firmly gripped by the pin-gripping part  63  and pulled rearward relative to the collar  85  and the workpieces W. 
     The collar  85  is then deformed and swaged onto the shaft part of the pin  81 , and the workpieces W are clamped between the head of the pin  81  and the collar  85 . Subsequently, a portion (pintail) of the shaft part of the pin  81  is torn off and separated from the fastener  8 . Thus, the operation of fastening the workpieces W is completed. The control circuit  171  stops normal driving of the motor  2  when the screw shaft  56  and the pin-gripping part  63  reach respective predetermined stop positions or when the user releases the trigger  151  to turn off the switch  152 . This completes the first stroke. Although not described and shown in detail, the control circuit  171  can determine whether the screw shaft  56  and the pin-gripping part  63  have reached their stop positions, for example, based on detection results of a position detecting device (such as a Hall sensor, an optical sensor and a contact switch). 
     When the trigger  151  is released by the user and the switch  152  is turned OFF, the control circuit  171  starts reverse driving of the motor  2 . Thus, the second stroke is started. 
     When the motor shaft  23  rotates in the reverse direction, as described above, the reduction-ratio change mechanism  7  rotates the second-stage internal gear  422  integrally with the sun gear  421 , so that two stages of the speed reducer  4  are effective (the number of effective stages of the speed reducer  4  is changed to two). Accordingly, the shaft  436  of the speed reducer  4  rotates at higher speed than in the first stroke and outputs lower torque than in the first stroke. The torque is transmitted to the nut  51  via the nut-driving gear  311 , the idler gear  331  and the driven gear  511 . The nut  51  rotates in a direction opposite to that in the first stroke, so that the screw shaft  56  and the pin-gripping part  63  are moved forward relative to the body housing  11  and the nut  51 . The control circuit  171  stops reverse driving of the motor  2  when the screw shaft  56  and the pin-gripping part  63  reach their respective initial positions. This completes the second stroke. The control circuit  171  can determine whether the screw shaft  56  and the pin-gripping part  63  have reached their initial positions, for example, based on detection results of a position detecting device, in the same manner as when determining whether they have reached their stop positions. 
     As described above, the motor  2  (the motor shaft  23 ) of the fastening tool  1  according to this embodiment can selectively rotate in two directions, that is, in the normal direction and in the reverse direction. Further, the speed reducer  4  is configured such that the reduction ratio is changed in response to the change of the rotation direction of the motor shaft  23 . The output speed and the output torque of the speed reducer  4  and thus the moving speed and the pulling force of the pin-gripping part  63  for pulling the pin  81  are changed according to whether the motor shaft  23  rotates in the normal direction or in the reverse direction. Therefore, the fastening tool  1  can perform the two actions, which are different in the moving speed and the pulling force of the pin-gripping part  63  for pulling the pin  81 , according to the rotation direction of the motor shaft  23 . 
     Since the reduction-ratio change mechanism  7  changes the reduction ratio, the control circuit  171  need not control the rotation speed of the motor  2 . This allows the control circuit  171  to drive the motor  2  with high efficiency at all times. Particularly, in this embodiment, the control circuit  171  automatically changes the driving mode of the motor  2  (the rotation direction of the motor shaft  23 ) upon recognizing a particular event (specifically, a change in the depressing state of the trigger  151 , that is, ON/OFF switching of the switch  152 ). Thus, the reduction ratio can be changed properly and efficiently in response to the change of the rotation direction of the motor shaft  23 . 
     The fastening tool  1  is a typical example of a power tool that is configured to perform different actions between the first stroke and the second stroke in the fastening operation. In the first stroke, the pin-gripping part  63  moves rearward from the initial position while pulling the pin  81 , and in the second stroke, the pin-gripping part  63  moves forward to the initial position without pulling the pin  81 . The reduction ratio in the first stroke is larger than that in the second stroke. Therefore, the fastening tool  1  can output a relatively large torque in the first stroke, in which a relatively large force is required to pull the pin  81 . In addition, the fastening tool  1  can efficiently return the pin-gripping part  63  to the initial position at relatively high speed in the second stroke, in which a large force is not particularly required. Thus, the fastening tool  1  can efficiently perform the fastening operation. 
     Particularly, in this embodiment, in normal rotation of the motor shaft  23 , the reduction ratio is at least 2.5 times larger than in reverse rotation of the motor shaft  23 , so that the moving speed and the pulling force of the pin-gripping part  63  for pulling the pin  81  can be made relatively significantly different between the first stroke and second stroke. 
     Further, in this embodiment, the speed reducer  4  is a planetary gear reducer including three stages (sets) of the planetary gear mechanisms  41 ,  42 ,  43 . The planetary gear reducer is small in size and capable of providing a large reduction ratio, compared with a gear speed reducer including a train of spur gears or other gears. In this embodiment, the speed reducer  4 , which is a multi-stage planetary gear reducer, can provide a particularly large reduction ratio. Further, the reduction ratio of the speed reducer  4  can be rationally changed by changing the number of the planetary gear mechanisms  41 ,  42 ,  43  that function effectively (i.e., the number of the effective stages). 
     In this embodiment, the one-way clutch  70  and the lock mechanism  71  are configured to cooperate, in normal driving of the motor  2 , to cause the second-stage planetary gear mechanism  42  to effectively function, and in reverse driving of the motor  2 , to disable the function of the second-stage planetary gear mechanism  42 . Particularly, the one-way clutch  70  automatically performs different actions according to the rotation direction of the motor shaft  23 . Therefore, the one-way clutch  70  can efficiently change the action of the lock mechanism  71  in response to the change of the rotation direction of the motor shaft  23 . Further, the lock mechanism  71  is configured such that the rollers  73  lock and unlock the lock cam  75  by moving between the locking position and the unlocking position in the circumferential direction. Owing to this configuration, the lock mechanism  71  can be made compact in the front-rear direction and the radial direction. Furthermore, the rollers  73  can exhibit the wedge effect by slight movement in the circumferential direction and thereby reliably lock the lock cam  75  and thus the internal gear  422 . 
     Further, the lock mechanism  71  acts on the sun gear  421  and the internal gear  422  in the second-stage planetary gear mechanism  42 , in which the rotation speed is lower than in the first-stage planetary gear mechanism  42  and the torque is smaller than in the third-stage planetary gear mechanism  42 . This configuration can reduce load on the lock mechanism  71  and thus improve the durability of the lock mechanism  71 . 
     Correspondences between the features of the above-described embodiment and the features of the disclosure are as follows. However, the features of the above-described embodiment are merely exemplary and do not limit the features of the present disclosure. 
     The fastening tool  1  is an example of a “power tool”. The motor  2  and the motor shaft  23  are examples of a “motor” and a “motor shaft”, respectively. The speed reducer  4  is an example of a “gear speed reducer”. Each of the planetary gear mechanisms  41 ,  42 ,  43  is an example of a “planetary gear mechanism”. Each of the sun gears  411 ,  421 ,  431  is an example of a “sun gear”. Each of the internal gears  412 ,  422 ,  432  is an example of an “internal gear”. Each of the carriers  415 ,  425 ,  435  is an example of a “carrier”. Each of the planetary gears  418 ,  428 ,  438  is an example of a “planetary gear”. The one-way clutch  70  is an example of a “one-way clutch”. The lock mechanism  71  is an example of a “lock mechanism”. The pin-gripping part  63  is an example of a “movable member”. The fastener  8  is an example of the “fastener”. The ball-screw mechanism  5  is an example of a “feed screw mechanism”. The control circuit  171  of the controller  170  is an example of a “control device”. 
     The above-described embodiment is a mere example and the power tool according to the present disclosure is not limited to the fastening tool  1  of the above-described embodiment. For example, the following modifications may be made. At least one of these modifications can be employed in combination with any of the fastening tool  1  of the above-described embodiment and the claimed features. 
     For example, a brushed motor may be employed as the motor  2 , in place of the brushless motor. The motor  2  may be driven by power supplied not from the battery  182  but from an external AC power source. 
     In the driving mechanism  3 , another type of feed screw mechanism that includes a nut having a female thread on its inner periphery and a screw shaft having a male thread on its outer periphery and directly engaged with the nut may be employed in place of the ball-screw mechanism  5 . The ball-screw mechanism  5  may be configured such that the screw shaft  56  is restricted in movement in the front-rear direction and supported to be rotatable around the driving axis A 1 , while the nut  51  moves in the front-rear direction along with rotation of the screw shaft  56 . In this case, the pin-gripping part  63  may be directly or indirectly connected to the nut  51 , which serves as the final output shaft. The idler gear  331  disposed between the nut-driving gear  311  of the first intermediate shaft  31  and the driven gear  511  of the nut  51  may be omitted, and the nut-driving gear  311  and the driven gear  511  may be directly engaged with each other, or a different gear may be disposed therebetween. 
     The number of the stages of the speed reducer  4  (i.e. the number of the planetary gear mechanisms included in the speed reducer  4 ) and the structures of the planetary gear mechanisms  41 ,  42 ,  43  may be appropriately changed. For example, the planetary speed reducer  4  may include a single planetary gear mechanism, or two or four or more planetary gear mechanisms. In an embodiment that employs a single planetary gear mechanism, the number of effective stages may be switched between zero and one, in response to a change of the rotation direction of the motor  2 . The number of effective stages may be changed by axial movement of any one of the internal gears  412 ,  422 ,  432 . Further, a gear speed reducer including a gear train (a train of spur gears, helical gears, bevel gears or other similar gears) other than a planetary gear mechanism may be employed in place of the speed reducer  4 . In such a modification, the reduction ratio can be changed, for example, by selectively engaging a specific gear, which is slidably arranged, with one of two gears having a different number of teeth. 
     The reduction-ratio change mechanism  7  may be appropriately changed as long as it can operate in response to a change of the rotation direction of the motor shaft  23  and switch the reduction ratio of the speed reducer  4 . For example, the reduction-ratio change mechanism  7  may be configured to move any one of the internal gears  412 ,  422 ,  432  of the speed reducer  4  in the axial direction by using a gear train that is operably coupled to the motor shaft  23  and the speed reducer  4  (or the gear speed reducer of the above-described modifications). 
     The one-way clutch  70  may be changed to a one-way clutch having any other structure (such as a one-way clutch including balls). The shape, arrangement and number of components of the lock mechanism  71  may also be appropriately changed. For example, three or more rollers  73  may be provided. The number of the projections  756  of the lock cam  75  and the number of the projections  725  of the retainer  72  may also be changed. The lock sleeve  74  may be omitted, and the rollers  73  may be disposed between the inner periphery of the gear case  40  and the flat parts  757  of the lock cam  75  so as to be movable between the locking position and the unlocking position. Further, the lock cam  75  and the internal gear  422  may be integrally formed as a single (inseparable) member. 
     In the above-described embodiment, the control circuit  171  is formed by a microcomputer including a CPU. However, the control circuit  171  may be formed, for example, by a programmable logic device such as an ASIC (Application Specific Integrated Circuit) and an FPGA (Field Programmable Gate Array). A plurality of control circuits may be provided to control driving of the motor  2 . Further, the control circuit  171  may switch the driving mode of the motor  2  in response to not only the above-described event, but, for example, manipulation of a manipulation part (such as a push button switch and a touch screen) that is separately provided from the trigger  151 . 
     The fastening tool  1  may be configured to fasten the workpieces W by using a fastener of a different type from the fastener  8  of the above-described embodiment (such as a blind rivet and a multi-piece swage type fastener of a shaft-retaining type). The fastening tool  1  may be compatible with plural kinds of fasteners by replacing the anvil  61  and the pin-gripping part  63 . 
     In the above-described embodiment, the fastening tool  1  is described as an example of the power tool of the present disclosure, but the present disclosure may also be applied to other power tools that perform different actions according to the rotation direction of the motor. For example, the power tool may be embodied as pruning shears (pruning scissors, clippers) having a fixed blade and a movable blade that is configured to pivot on a prescribed axis between a closed position and an open position relative to the fixed blade. 
     Such pruning shears may operate in a cycle that includes a first stroke in which the movable blade pivots from the closed position to the open position and a second stroke in which the movable blade returns from the open position to the closed position while cutting a twig/branch of a plant. It is therefore preferable that the movable blade promptly moves to the open position in the first stroke and exhibits a relatively large cutting force in the second stroke. Accordingly, the pruning shears may be configured such that the rotation direction of the motor is changed in response to a change between the first stroke and the second stroke and such that the reduction ratio of the speed reducer in the second stroke is changed to be larger than that in the first stroke. 
     The power tool is not limited to a power tool that operates, like the fastening tool  1  and the above-described pruning shears, in a cycle with a first stroke in which a movable member moves in a prescribed direction and a second stroke in which the movable member moves in a direction opposite to the prescribed direction. For example, the power tool may be embodied as a rotary tool that rotationally drives an output shaft around a driving axis with a tool accessory removably coupled to the output shaft. In the rotary tool, the rotation direction of the output shaft and the tool accessory is reversed in response to a change of the rotation direction of the motor. Therefore, the rotary tool can be configured to perform different actions according to the rotation direction of the tool accessory by changing the reduction ratio of the speed reducer in response to a change of the rotation direction of the motor. 
     Further, in view of the nature of the present disclosure and the above-described embodiment and its modifications, the following features are provided. At least one of the following features can be employed in combination with at least one of the above-described embodiment, its modifications and the claimed features. 
     (Aspect 1) 
     The gear speed reducer includes three stages of planetary gear mechanisms, a second-stage sun gear is fixed onto a shaft of a first-stage carrier in the three stages of the planetary gear mechanisms, and 
     the one-way clutch is mounted onto the shaft of the first-stage carrier. 
     (Aspect 2) 
     The power tool further includes a housing that houses the motor and the gear speed reducer, 
     the internal gear is selectively rotatable around a first axis relative to the housing, 
     the lock mechanism includes:
         a tubular lock sleeve that is non-rotatable around the first axis relative to the housing;   a lock cam that is connected to the internal gear, that is selectively rotatable integrally with the internal gear around the first axis relative to the lock sleeve, and that is at least partially arranged radially inside the lock sleeve;   a retainer that is at least partially arranged radially inside the lock sleeve, and that is selectively rotatable integrally with the one-way clutch around the first axis relative to the lock sleeve; and   at least one roller that is retained between the lock sleeve and the lock cam in the radial direction by the retainer, that is configured to selectively move between a locking position and an unlocking position in a circumferential direction around the first axis relative to the lock sleeve and the lock cam, the roller at the locking position being held between the lock sleeve and the lock cam to non-rotatably lock the lock cam relative to the lock sleeve, the roller at the unlocking position being loosely disposed between the lock sleeve and the lock cam to permit rotation of the lock cam relative to the lock sleeve,       

     in response to rotation of the motor shaft in the first direction, the at least one roller relatively moves to the locking position as the internal gear and the lock cam are rotated around the first axis via the planetary gears in response to rotation of the sun gear, and thereby non-rotatably locks the internal gear via the lock cam, and 
     in response to rotation of the motor shaft in the second direction, the retainer rotates integrally with the one-way clutch and rotates the internal gear integrally with the sun gear via the lock cam. 
     The body housing  11 , the lock sleeve  74 , the lock cam  75 , the retainer  72  and the roller  73  are examples of a “housing”, a “lock sleeve”, a “lock cam”, a “retainer” and a “roller”, respectively. 
     (Aspect 3) 
     The at least one roller is configured to non-rotatably lock the lock cam by a wedge effect in response to the at least one roller being placed at the locking position. 
     DESCRIPTION OF THE NUMERALS 
       1 : fastening tool,  11 : body housing,  115 : container,  13 : nose,  15 : handle,  151 : trigger,  152 : switch,  17 : battery housing,  170 : controller,  171 : control circuit,  181 : battery-mounting part,  182 : battery,  2 : motor,  21 : stator,  22 : rotor,  23 : motor shaft,  3 : driving mechanism,  31 : first intermediate shaft,  311 : nut-driving gear,  33 : second intermediate shaft,  331 : idler gear,  4 : speed reducer,  40 : gear case,  401 : groove,  41 ,  42 ,  43 : planetary gear mechanism,  411 ,  421 ,  431 : sun gear,  412 ,  422 ,  432 : internal gear,  423 : projection,  415 ,  425 ,  435 : carrier,  416 ,  426 ,  436 : shaft,  418 ,  428 ,  438 : planetary gear,  5 : ball-screw mechanism,  51 : nut,  511 : driven gear,  56 : screw shaft,  560 : driving shaft,  561 : extension shaft,  61 : anvil,  62 : connecting member,  63 : pin-gripping part,  64 : connecting member,  7 : reduction-ratio change mechanism,  70 : one-way clutch,  71 : lock mechanism,  72 : retainer,  721 : cylindrical part,  723 : base part,  725 : projection,  73 : roller,  74 : lock sleeve,  741 : projection,  75 : lock cam,  751 : base part,  753 : flange part,  754 : recess,  755 : cam part,  756 : projection,  757 : flat part,  8 : fastener,  81 : pin,  85 : collar, A 1 : driving axis, A 2 : rotational axis, W: workpiece