Patent Publication Number: US-2021187722-A1

Title: Power tool

Description:
TECHNICAL FIELD 
     The present invention relates to a power tool 
     BACKGROUND ART 
     Conventionally, there has been known a power tool including a motor having a rotation shaft, a power transmission portion configured to receive rotational force of the rotation shaft and to transmit driving force based on the rotational force, and a driven portion configured to be driven by receiving the driving force. For example, a typical example of this kind of power tools is a saber saw (see Patent Literature 1) is used to cut a wood, steel, and a pipe (as a workpiece to be cut). 
     The saber saw described in Patent Literature 1 includes a motor having a rotation shaft, a motion converting portion (power transmission portion) configured to convert rotational force of the rotation shaft into reciprocating driving force and to transmit the reciprocating driving force, and a plunger (driven portion) configured to perform reciprocating movement by receiving the reciprocating driving force. A blade (saw blade) as an end bit is attachable to and detachable from the plunger. In the saber saw, the motor is driven to cause the plunger having the blade attached thereto to reciprocally move, and the workpiece is cut by the reciprocating blade. 
     CITATION LIST 
     Patent Literature 
     [PTL1] Japanese Patent Application Publication No. 2013-180382 
     SUMMARY OF INVENTION 
     Technical Problem 
     During cutting work using the above-described saber saw, the plunger having the blade attached thereto may be locked when the blade gets stuck in the workpiece. When this lock of the plunger occurs, a strong impact is applied from the blade (end bit) to the motion converting portion (power transmission portion) through the plunger (driven portion), which may lead to deformation or damage to components (such as gears) constituting the motion converting portion. 
     In view of the foregoing, it is an object of the present invention to provide a power tool in which the impact applied to the power transmission portion can be mitigated to thereby suppress the components constituting the power transmission portion from being deformed or damaged. 
     Solution to Problem 
     In order to attain the above object, the present invention provides a power tool including a motor having a rotation shaft portion rotatable about a rotation axis, a housing accommodating therein the motor, a power transmission portion configured to receive a rotation force of the rotation shaft portion and to transmit a driving force based on the rotation force, and a driven portion configured to be driven by receiving the transmitted driving force, the rotation shaft portion being supported by the housing so as to be movable relative to the housing in an axial direction of the rotation axis. 
     With this structure, an impact applied to the rotation shaft portion and the power transmission portion can be mitigated by the movement in the axial direction of the rotation shaft portion. Accordingly, the rotation shaft portion and the components constituting the power transmission portion can have improved durability against the impact, and thus the rotation shaft portion and the components constituting the power transmission portion can be suppressed from being deformed and damaged. Further, according to the above-described structure, the improvement on the durability of the rotation shaft portion and the components constituting the power transmission portion against the impact can be achieved at low cost and with a compact structure, in comparison with a structure in which a clutch mechanism or the like for mitigating the impact applied to the rotation shaft portion and the power transmission portion is provided on the power transmission path. 
     In the above-described structure, it is preferable that the power tool further includes a shock absorbing portion including an elastic body configured to be elastically deformed by movement of the rotation shaft portion in the axial direction. 
     With this structure, the impact applied to the rotation shaft portion and the power transmission portion can further be effectively mitigated by virtue of the elastic deformation of the elastic body. 
     In the above-described structure, it is preferable: that the power tool further includes a bearing supporting the rotation shaft portion such that the rotation shaft portion is rotatable about the rotation axis; that the bearing is supported by the housing so as to be movable integrally with the rotation shaft portion in the axial direction; that the shock absorbing portion and the bearing are arrayed in the axial direction; and that the shock absorbing portion is in contact with the bearing. 
     This structure can effectively mitigate the impact applied to the rotation shaft portion and the power transmission portion. 
     In the above-described structure, it is preferable: that the shock absorbing portion further includes a plate interposed between the bearing and the elastic body; that the bearing includes an outer race and an inner race which are rotatable relative to each other; that the rotation shaft portion is fixed to the inner race; that the elastic body urges the plate toward the outer race; and that the plate is in contact with the outer race and is away from the inner race. 
     With this structure, since the plate is positioned between the elastic body and the bearing, the elastic body expanded at the time of compression never contacts with the inner race of the bearing. Hence, smooth relative rotation between the inner race and the outer race can be ensured even during the compression of the elastic body. 
     In the above-described structure, it is preferable: that the power tool includes a second bearing supported by the housing and supporting, in cooperation with the bearing, the rotation shaft portion such that the rotation shaft portion is rotatable about the rotation axis; and that the elastic body is positioned between the bearing and the second bearing in the axial direction. 
     With this structure, a space between conventional two bearings used to support a rotation shaft portion can be effectively utilized, whereby an increase in size of the power tool can be suppressed. 
     In the above-described structure, it is preferable: that the rotation shaft portion includes a rotation shaft extending in the axial direction, and a gear provided at the rotation shaft; and that the power transmission portion includes a bevel gear in meshing engagement with the gear. 
     With this structure, the impact applied to the power transmission portion can be efficiently converted into an impact applied to the rotation shaft in the axial direction (i.e., into a thrust force applied to the rotation shaft). Hence, the impact directed in the axial direction can be effectively mitigated by the movement in the axial direction of the rotation shaft. Accordingly, the impact applied to the rotation shaft portion and the power transmission portion can further be effectively mitigated. 
     In the above-described structure, it is preferable: that the power tool includes a shaft reciprocally movable in the front-rear direction by the bevel gear; and that the elastic body is disposed at a position overlapping with a rear end of the shaft in the front-rear direction when the shaft is at a rearmost position. 
     In the above-described structure, it is preferable: that the power tool further includes an orbital mechanism configured to change inclination of the shaft according to a position in the front-rear direction of the shaft; and that the orbital mechanism include a sleeve portion support the shaft such that the shaft is slidably movable. 
     In the above-described structure, it is preferable that the elastic body is disposed at a position overlapping with a rear end of the sleeve portion in the front-rear direction. 
     In the above-described structure, it is preferable that the motor is a brushless motor. 
     With this structure, a structure in which the impact applied to the rotation shaft portion and the power transmission portion is mitigated by moving the rotation shaft portion in the axial direction, can be suitably achieved. 
     Advantageous Effects of Invention 
     According to the present invention, there can be provided a power tool in which the impact applied to the power transmission portion can be mitigated to thereby suppress the components constituting the power transmission portion from being deformed or damaged. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an internal structure of a saber saw according to an embodiment of the present invention. 
         FIG. 2  is a partial cross-sectional view illustrating a brushless motor, a motor case, and a rear portion of a gear housing in the saber saw according to the embodiment. 
         FIG. 3  is an enlarged partial cross-sectional view illustrating a rear bearing support portion and a rear bearing in the saber saw according to the embodiment. 
         FIG. 4  is an enlarged partial cross-sectional view illustrating a front bearing support portion, a front bearing, and a shock absorbing portion in the saber saw according to the embodiment. 
         FIG. 5  is a cross-sectional view taken along the line A-A in  FIG. 2 , and illustrating the front bearing support portion and an elastic body of the shock absorbing portion in the saber saw according to the embodiment. 
         FIG. 6  is a partial cross-sectional view illustrating the internal structure of the saber saw according to the embodiment, and particularly illustrating a state where smooth cutting work is being performed. 
         FIG. 7  is a partial cross-sectional view illustrating the internal structure of the saber saw according to the embodiment, and particularly illustrating a state where the elastic body is compressed due to the occurrence of locking of a driven portion. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a power tool according to one embodiment of the present invention will be described with reference to  FIGS. 1 through 7 . In the following description, “up”, “down”, “front”, “rear”, “right”, and “left” indicated by arrows in the drawings define the upward direction, downward direction, frontward direction, rearward direction, rightward direction, and leftward direction, respectively. 
     The saber saw  1  illustrated in  FIG. 1  is an electrically powered reciprocating tool for cutting a workpiece to be cut such as a wood, a steel, or a pipe. As illustrated in  FIG. 1 , the saber saw  1  includes a housing  2 , a brushless motor  3  having a rotation shaft portion  31 , a sensor board  4 , a fan  5 , a power transmission portion  6 , a driven portion  7  to which a blade B for cutting the workpiece to be cut is detachably attachable, a rear bearing  8 , a front bearing  9 , a shock absorbing portion  10 , a power supply circuit (not illustrated), and a controller (not illustrated). In the saber saw  1 , the brushless motor  3  is employed as a drive source, the driven portion  7  having the blade B attached is reciprocally moved by rotation of the brushless motor  3 , and cutting work is performed using the reciprocating blade B. 
     The housing  2  constitutes the outer shell of the saber saw  1 , and includes a handle housing  21 , a motor case  22 , and a gear case  23 . 
     The handle housing  21  is made of resin, and constitutes the rear portion of the housing  2 . The handle housing  21  is connected to a rear portion of the motor case  22 , and supports the motor case  22 . The handle housing  21  includes a grip portion  211 , a first connecting portion  212 , a second connecting portion  213 , and a motor support portion  214 . 
     The grip portion  211  is a portion that an operator can grip, and has a generally hollow cylindrical shape extending in the up-down direction. The grip portion  211  has a lower end portion from which a power cord  211 A extends, the power cord being connectable to an external power source (for example, a commercial AC power source). The grip portion  211  has an upper portion provided with a trigger switch  211 B which can be manually operated for controlling start and stop of the brushless motor  3 . 
     The first connecting portion  212  connects the lower portion of the grip portion  211  and a rear lower portion of the motor support portion  214 . The first connecting portion  212  accommodates therein a choke coil  212 A and other components which constitute the power supply circuit. The second connecting portion  213  connects the upper portion of the grip portion  211  and a rear upper portion of the motor support portion  214 . 
     The motor support portion  214  is positioned frontward of the grip portion  211 . The motor support portion  214  is connected to the rear portion of the motor case  22  and supports the motor case  22 . The motor support portion  214  includes a right wall and left wall. Each of the right wall and the left wall of the motor support portion  214  has a lower portion formed with a plurality of air inlet holes (not illustrated). Further, the motor support portion  214  accommodates therein a board accommodating portion  214 A and a smoothing capacitor  214 B which constitutes the power supply circuit. 
     The board accommodating portion  214 A is a bottomed box shaped container opening frontward, and accommodates therein a circuit board on which the controller and a part of the power supply circuit are mounted. The power supply circuit is a circuit for supplying electric power of the external power source to the brushless motor  3  through the power cord  211 A. The power supply circuit includes: a noise filter circuit including the choke coil  212 A; a rectification smoothing circuit including the smoothing capacitor  214 B; and an inverter circuit. The controller includes a microcomputer having a CPU, a ROM, and a RAM, and is configured to control the inverter circuit to perform rotation control (driving control) of the brushless motor  3 . 
     The motor case  22  is a member which is integrally molded and made of resin. The motor case  22  has a bottomed hollow cylindrical shape extending in the front-rear direction and opening frontward. As illustrated in  FIG. 2 , the motor case  22  includes a rear wall portion  221  and a sleeve portion  222 , and accommodates therein the brushless motor  3 , the sensor board  4 , and the fan  5 . 
     The rear wall portion  221  constitutes the rear portion of the motor case  22 , and includes a rear bearing support portion  223  and a connection wall  224 . The rear bearing support portion  223  has a bottomed generally hollow cylindrical shape extending in the front-rear direction and opening frontward. The rear bearing support portion  223  supports the rear bearing  8 . Details of the rear bearing support portion  223  and the rear bearing  8  will be described later. 
     The connection wall  224  extends outwardly in the radial direction of the rear bearing support portion  223  from the front end portion of the rear bearing support portion  223  (i.e., from the front end portion of a hollow cylindrical wall  223 A described later). The connection wall  224  connects the front end portion of the rear bearing support portion  223  and the rear end portion of the sleeve portion  222 . The connection wall  224  is formed with a plurality of communication holes  221   a . The plurality of communication holes  221   a  penetrates the connection wall  224  in the front-rear direction and allows the interior of the motor case  22  to be communicated with the interior of the motor support portion  214  (the interior of the handle housing  21 ). 
     The sleeve portion  222  has a generally hollow cylindrical shape extending frontward from the peripheral end portion of the connection wall  224  of the rear wall portion  221 . The sleeve portion  222  has a front open end closed by the gear case  23 . The front end portion of the sleeve portion  222  is formed with a plurality of discharge holes  2   a . The plurality of discharge holes  2   a  penetrates the sleeve portion  222  in the up-down direction and allows the interior of the motor case  22  to be communicated with the outside of the housing  2 . 
     Turning back to  FIG. 1 , the gear case  23  is made of metal and connected to the front portion of the motor case  22 , and extends in the front-rear direction. The gear case  23  accommodates therein the power transmission portion  6  and the driven portion  7 , and includes a rear wall portion  231 . 
     As illustrated in  FIG. 2 , the rear wall portion  231  constitutes the rear portion of the gear case  23 , and closes the open end of the sleeve portion  222  of the motor case  22 . The rear wall portion  231  includes a front bearing support portion  232  and a facing wall  233 . The front bearing support portion  232  supports the front bearing  9  and the shock absorbing portion  10 . Details of the front bearing support portion  232 , the front bearing  9 , and the shock absorbing portion  10  will be described later. 
     The facing wall  233  faces the sleeve portion  222  of the motor case  22  in the front-rear direction, and is connected to the rear end portion of the front bearing support portion  232  (i.e., the rear end portion of a connection wall  236  described later). 
     The brushless motor  3  is accommodated in the motor case  22 , and includes the rotation shaft portion  31 , a rotor  32 , and a stator  33 . The brushless motor  3  is a three-phase brushless DC motor, and is a drive source of the saber saw  1  (that is, a drive source for driving the driven portion  7 ). The brushless motor  3  is an example of a “motor” of the present invention. 
     The rotation shaft portion  31  is supported by the housing  2  (the motor case  22  and the gear case  23 ) through the rear bearing  8  and the front bearing  9  such that the rotation shaft portion  31  is rotatable about a rotation axis A 1  extending in the front-rear direction and movable relative to the housing  2  in the front-rear direction (i.e., in the axial direction of the rotation axis A 1 ). The rotation shaft portion  31  includes a rotation shaft  311  and a pinion gear  312 . The rotation axis A 1  is an example of a “rotation axis” in the present invention. 
     The rotation shaft  311  has a generally solid cylindrical shape extending in the front-rear direction. The rotation shaft  311  is supported by the rear bearing  8  and the front bearing  9  so as to be rotatable about the rotation axis A 1 . Specifically, the rear end portion of the rotation shaft  311  is supported by the rear bearing  8 , and the front end portion of the rotation shaft  311  is supported by the front bearing  9 . In the present embodiment, the rotation shaft  311  (the rotation shaft portion  31 ) is configured to rotate in the counterclockwise direction in a front view when the brushless motor  3  is driven. 
     The pinion gear  312  is a spiral bevel gear whose tooth trace is left-twisted. The pinion gear  312  is provided at the front end portion of the rotation shaft  311  integrally with the rotation shaft  311 . The pinion gear  312  rotates integrally and coaxially with the rotation shaft  311 . The pinion gear  312  is an example of a “gear” in the present invention. 
     The rotor  32  includes permanent magnets, and is fixed to the rotation shaft  311  so as to rotate coaxially and integrally with the rotation shaft  311 . A sensor magnet  32 A having an annular shape from a rear view is provided at the rear end of the rotor  32  so as to rotate coaxially and integrally with the rotor  32 . 
     The stator  33  has a generally hollow cylindrical shape extending in the front-rear direction, and includes three star-connected stator coils. Each of the upper and lower portions of the outer peripheral portion of the stator  33  is fixed to the motor case  22  by a bolt  33 A. Hence, the stator  33  is fixed to the housing  2  in a state where the stator  33  is accommodated in the motor case  22 . 
     The sensor board  4  has an annular shape in a front view, and is provided rearward of the stator  33 . Three Hall elements (not illustrated) are mounted on the sensor board  4  for detecting the rotational position of the sensor magnet  32 A (i.e., the rotational position of the rotor  32 ). 
     The three Hall elements are mounted on the front surface of the sensor board  4 , and are disposed to be arrayed at approximately 60 degrees of intervals in the circumferential direction of the rotation shaft  311 . Each of the three Hall elements is connected to the controller through a signal wire, and is configured to output, to the controller, signals used for detecting the rotational position of the sensor magnet  32 A. Note that, the controller detects the rotational position of the rotor  32  by detecting the rotational position of the sensor magnet  32 A on the basis of signals outputted from each of the three Hall elements, and controls the inverter circuit on the basis of the detection results to rotate the rotor  32  and the rotation shaft portion  31  in a predetermined rotational direction. 
     The fan  5  is a centrifugal fan, and is positioned frontward of the stator  33  of the brushless motor  3 . The fan  5  is fixed to the rotation shaft  311  so as to rotate integrally and coaxially with the rotation shaft  311 . The fan  5  is configured to generate cooling air flows that flow in the housing  2  from the plurality of inlet holes to the plurality of exhaust holes  2   a . The cooling air flows cool the brushless motor  3 , the inverter circuit, the rectification circuit, and other components. 
     As illustrated in  FIG. 1 , the power transmission portion  6  includes a power transmission gear  61 , a pin  62 , and a pin guide  63 . The power transmission portion  6  is a mechanism configured to receive the rotational force of the rotation shaft portion  31  (i.e., the rotation shaft  311 ) and transmit the driving force based on the rotational force to the driven portion  7 . Specifically, the power transmission portion  6  converts rotation of the rotation shaft portion  31  into reciprocating motion in the front-rear direction, and transmit the reciprocating motion to the driven portion  7 . 
     The power transmission gear  61  is a spiral bevel gear whose tooth trace is right-twisted, and is disposed at a lower portion of the interior of the gear case  32  and frontward of the front bearing support portion  232 . The power transmission gear  61  is supported by the gear case  23  so as to be rotatable about a rotation axis A 2  extending perpendicular to the rotation axis A 1  (that is, extending in the up-down direction). The power transmission gear  61  is in meshing engagement with the pinion gear  312 . As described above, according to the present embodiment, the power transmission gear  61  is the spiral gear whose tooth trace is right-twisted, and the pinion gear  312  in meshing engagement with the power transmission gear  61  is the spiral gear whose tooth trace is left-twisted. Further, the rotation shaft portion  31  rotates in the counterclockwise direction in a front view when the brushless motor  3  is driven. Hence, during driving of the brushless motor  3  (during rotation of the rotation shaft portion  31 ), a slight thrust force directed rearward (i.e., directed in a direction away from the power transmission gear  61 ) is constantly imparted on the pinion gear  312 . The power transmission gear  61  is an example of a “bevel gear” in the present invention. 
     The pin  62  has a generally solid cylindrical shape extending in the up-down direction. The pin  62  is fixed to the power transmission gear  61  by force-fitting, and is positioned away from the rotation axis A 2  in a plan view. The upper portion of the pin  62  protrudes upward from the upper surface of the power transmission gear  61 . 
     The pin guide  63  has a generally rectangular parallelepiped shape extending in the left-right direction and is movable in the front-rear direction within the gear case  23 . The pin guide  63  is formed with a pin receiving groove  63   a.    
     The pin receiving groove  63   a  is recessed upward from the lower surface of the pin guide  63 , and extends in the left-right direction. The pin receiving groove  63   a  has a width in the front-rear direction that is slightly greater than a diameter of the pin  62 . The upper end portion of the pin  62  is accommodated in the pin receiving groove  63   a  via a needle bearing. Hence, relative movement in the front-rear direction between the pin guide  63  and the pin  62  is prevented, while relative movement in the left-right movement therebetween is permitted. 
     The driven portion  7  is supported by the gear case  23  so as to be reciprocally movable in the front-rear direction within the gear case  23 . The driven portion  7  is positioned at the opposite side of the motor support portion  214  from the grip portion  211  in the front-rear direction. The driven portion  7  includes a sleeve portion  7   a  extending the front-rear direction, a shaft  71  supported by the sleeve portion  7   a  so as to be slidably movable, and a blade attachment portion  72 . 
     The shaft  71  has a hollow cylindrical shape extending in the front-rear direction. The shaft  71  is supported by the gear case  23  through the sleeve portion  7   a  so as to be reciprocally movable in the front-rear direction. The shaft  71  is fixed to the pin guide  63  so as to reciprocally move in the front-rear direction together with the pin guide  63 . Note that,  FIG. 1  illustrates a state where the shaft  71  is positioned at the rearmost position within a reciprocally movable range of the shaft  71  (i.e., a state where the shaft  71  is at the rear dead center). 
     The blade attachment portion  72  is provided at the front end portion of the shaft  71 . The blade B is attachable to and detachable from the blade attachment portion  72 . 
     Next, the rear bearing support portion  223 , the rear bearing  8 , the front bearing support portion  232 , the front bearing  9 , and the shock absorbing portion  10  will be described in detail while referring to  FIGS. 3 to 5 . In the following description, the circumferential direction of the rotation shaft  311  will be referred to simply as the “circumferential direction”, and the radial direction of the rotation shaft  311  will be referred to simply as the “radial direction”. 
     As illustrated in  FIG. 3 , the rear bearing support portion  223  supports the rear bearing  8  such that the rear bearing  8  is movable in the front-rear direction. The rear bearing support portion  223  includes a sleeve wall  225  and a rear end wall  226 . The sleeve wall  225  has a generally hollow cylindrical shape extending in the front-rear direction. The sleeve wall  225  is formed with a groove  225   a.    
     The groove  225   a  is recessed outwardly in the radial direction from an inner peripheral surface  225 A of the sleeve wall  225 , and extends over the entirety thereof in the circumferential direction. An O ring  225 B is fitted in the groove  225   a . The rear end wall  226  closes the rear open end of the sleeve wall  225 , and has a generally circular shape in a rear view. 
     The rear bearing  8  is a ball bearing including an outer race  81  and an inner race  82 , which are rotatable relative to each other. The rear bearing  8  is force-fitted in the sleeve wall  225  through the O ring  225 B, and a minute gap is provided between the inner peripheral surface  225 A of the sleeve wall  225  and an outer peripheral surface  81 A of the outer race  81 . Hence, the rear bearing  8  is movable relative to the rear bearing support portion  223  in the front-rear direction. The rear end portion of the rotation shaft  311  is force-fitted in and fixed to the inner race  82  of the rear bearing  8  and accordingly, the rotation shaft  311  (the rotation shaft portion  31 ) and the rear bearing  8  move integrally with each other in the front-rear direction. The rear bearing  8  is an example of a “second bearing” in the present invention. 
     As illustrated in  FIGS. 4 and 5 , the front bearing support portion  232  supports the front bearing  9  such that the front bearing  9  is movable in the front-rear direction. The front bearing support portion  232  also supports the shock absorbing portion  10 . The front bearing support portion  232  includes a sleeve wall  234 , a first wall  235 , a connecting wall  236 , a fixed plate  237 , and a second wall  238 . 
     The sleeve wall  234  has a generally hollow cylindrical shape extending in the front-rear direction. As illustrated in  FIG. 5 , the sleeve wall  234  is supported by two ribs  234 E extending in the left-right direction. The sleeve wall  234  includes a thick wall portion  234 A and a thin wall portion  234 B. 
     As illustrated in  FIG. 4 , the thick wall portion  234 A constitutes the front portion of the sleeve wall  234 . The thick wall portion  234 A is formed with a groove  234   a . The groove  234   a  is recessed outwardly in the radial direction from an inner peripheral surface  234 C of the thick wall portion  234 A, and extends over the entirety thereof in the circumferential direction. An O ring  234 D is fitted in the groove  234   a . The thin wall portion  234 B constitutes the rear portion of the sleeve wall  234 . The inner diameter of the thin wall portion  234 B is greater than the inner diameter of the thick wall portion  234 A. 
     The first wall  235  extends outwardly in the radial direction from the rear end portion of the sleeve wall  234  (i.e., the rear end portion of the thin wall portion  234 B). As illustrated in  FIG. 5 , the first wall  235  includes a first protruding portion  235 A protruding upward, a second protruding portion  235 B protruding diagonally rightward and downward, and a third protruding portion  235 C protruding diagonally leftward and downward. The first protruding portion  235 A, the second protruding portion  235 B, and the third protruding portion  235 C are formed with a first thread hole  235   a , a second thread hole  235   b , and a third thread hole  235   c , respectively. 
     As illustrated in  FIG. 4 , the connecting wall  236  extends rearward from the peripheral end portion of the first wall  235 , and connects the first wall  235  and the facing wall  233 . 
     The fixed plate  237  is fixed to the rear surface of the first wall  235 , and includes an annular portion  237 A, a first fixed portion  237 B, a second fixed portion (not illustrated), and a third fixed portion  237 C. 
     The annular portion  237 A has an annular shape in a rear view. The inner peripheral surface of the annular portion  237 A is positioned more inward in the radial direction than the inner peripheral surface of the thin wall portion  234 B. 
     The first fixed portion  237 B protrudes upward from the upper portion of the annular portion  237 A, and is fixed to the rear surface of the first protruding portion  235 A of the first wall  235  by a first bolt  237 D extending through the first thread hole  235   a.    
     The second fixed portion (not illustrated) protrudes diagonally rightward and downward from the right lower portion of the annular portion  237 A, and is fixed to the rear surface of the second protruding portion  235 B of the first wall  235  by a second bolt  237 E extending through the second thread hole  235   b.    
     The third fixed portion  237 C protrudes diagonally leftward and downward from the left lower portion of the annular portion  237 A, and is fixed to the rear surface of the third protruding portion  235 C of the first wall  235  by a third bolt  237 F extending through the third thread hole  235   c.    
     The second wall  238  protrudes inwardly in the radial direction from the front end portion of the sleeve wall  234  (i.e., the front end portion of the thick wall portion  234 A), and has an annular shape in a rear view. 
     The front bearing  9  is a ball bearing including an outer race  91  and an inner race  92 , which are rotatable relative to each other. The front bearing  9  is force-fitted in the thick wall portion  234 A (i.e., the sleeve wall  234 ) through the O ring  234 D, and a minute gap is provided between the inner peripheral surface  234 C of the thick wall portion  234 A and an outer peripheral surface  91 A of the outer race  91 . Hence, the front bearing  9  is movable relative to the front bearing support portion  232  in the front-rear direction. The front end portion of the rotation shaft  311  is force-fitted in and fixed to the inner race  92  of the front bearing  9 , and accordingly, the rotation shaft  311  (i.e., the rotation shaft portion  31 ), the front bearing  9 , and the rear bearing  8  move integrally with one another in the front-rear direction. The front bearing  9  is an example of a “bearing” in the present invention. 
     The shock absorbing portion  10  is a mechanism configured to, when impact is applied to the power transmission portion  6  and the rotation shaft portion  31 , mitigate the applied impact. The shock absorbing portion  10  and the front bearing  9  are disposed so as to be arrayed in the front-rear direction (i.e., the axial direction). The shock absorbing portion  10  includes an annular plate  10 A and an elastic body  10 B. Mitigation of impact by the shock absorbing portion  10  will be described later. 
     The annular plate  10 A is a plate member made of metal and has an annular shape in a front view, and has a predetermined thickness in the front-rear direction. The annular plate  10 A is interposed between the elastic body  10 B and the front bearing  9  in the front-rear direction. The inner diameter of the annular plate  10 A is greater than the outer diameter of the inner race  92  of the front bearing  9 , and is smaller than the inner diameter of the outer race  91 . In other words, the inner peripheral surface of the annular plate  10 A is positioned more outward in the radial direction than the outer peripheral surface of the inner race  92 , and is positioned more inward in the radial direction than the inner peripheral surface of the outer race  91 . Hence, the annular plate  10 A is in contact with the outer race  91  of the front bearing  9 , but is in separation from the inner race  92 . Therefore, the annular plate  10  is not a hindrance to relative rotation of the inner race  92  to the outer race  91 , and accordingly, smooth relative rotation of the inner race  92  to the outer race  91  can be secured. The annular plate  10  is an example of a “plate” in the present invention. 
     The elastic body  10 B is a rubber member elastically deformable, and has a generally hollow cylindrical shape extending in the front-rear direction, as illustrated in  FIGS. 4 and 5 . The elastic body  10 B is interposed between the annular plate  10 A and the annular portion  237 A of the fixed plate  237  in a state where the elastic body  10 B is slightly compressed in the front-rear direction. As illustrated in  FIG. 4 , the front end portion of the elastic body  10 B is in contact with the rear surface of the annular plate  10 A, and the rear end portion of the elastic body  10 B is in contact with the front surface of the annular portion  237 A. 
     The elastic body  10 B urges the annular plate  10 A toward the outer race  91  of the front bearing  9 . In other words, the clastic body  10 B urges the annular plate  10 A, the front bearing  9 , and the rotation shaft portion  31  frontward. Hence, in a case where a force (i.e., a rearward force) for moving the rotation shaft portion  31  rearward (in a direction away from the power transmission gear  61 ) against the urging force of the elastic body  10 B is not imparted on the rotation shaft portion  31  or in a case where the rearward force imparted on the rotation shaft portion  31  is small, the outer race  91  of the front bearing  9  is maintained at a state where the outer race  91  is in contact with the rear surface of the second wall  238  (i.e., at the state illustrated in  FIGS. 4 and 6 ). On the other hand, in a case where a large rearward force is imparted on the rotation shaft portion  31 , the rotation shaft portion  31 , the rear bearing  8 , and the front bearing  9  are integrally moved rearward while compressing the elastic body OB rearward. As a result, the outer race  91  of the front bearing  9  is moved away from the second wall  238 . 
     Further, as viewed in the up-down direction, the elastic body  10 B overlaps with the shaft  71  in a state where the shaft  71  is positioned at the rear dead center. Therefore, additional layout space for disposing the elastic body  10 B in the housing  2  is unnecessary. Thus, the elastic body  10 B can be provided in the housing  2  without increase in the dimension in the front-rear direction of the saber saw  1 . 
     Next, operation in the saber saw  1  will be described. Typically, for performing cutting work with the saber saw  1 , the operator attaches the blade B to the blade attachment portion  72 , and then, he grips the grip portion  211  with his one hand and grips the front portion (i.e., the small diameter portion) of the gear case  23  with another hand. In this state, the operator performs an operation of pulling the trigger switch  211 B with a finger of the hand gripping the grip portion  211 , so that the controller starts to drive the brushless motor  3  and thus the rotation shaft portion  31  and the rotor  32  start to integrally rotate about the rotation axis A 1 . 
     Upon the start of rotation of the rotation shaft portion  31 , the rotation of the rotation shaft portion  31  rotates the power transmission gear  61  that is in meshing engagement with the pinion gear  312 , so that the pin  62  revolves around the rotation axis A 2  of the power transmission gear  61 . Only the motion component in the front-rear direction in the revolving motion is transmitted to the driven portion  7  though the pin guide  63 , so that the pin guide  63 , the driven portion  7 , and the blade B are integrally reciprocally moved in the front-rear direction. Thus, the workpiece can be cut by the reciprocating blade B. 
     Incidentally, the saber saw  1  according to the present embodiment includes a so-called orbital mechanism (a swing mechanism) configured to cause the shaft  71  to swing in the up-down direction during reciprocating movement of the shaft  71 . The orbital mechanism includes a pressure slope surface  61   a , a bearing  71   a , an urging member  7   b , and a swing shaft  7   c.    
     The pressure slope surface  61   a  has an annular shape in a plan view and constitutes the upper surface of an annular wall that is provided on the upper surface of the power transmission gear  61  so as to rotate integrally with the power transmission gear  61 . The annular wall has an annular shape in a plan view centered on the rotation axis A 2  and protrudes upward. The height (the dimension in the up-down dimension) of the annular wall is continuously changed in the circumferential direction of the rotation axis A 2  of the power transmission gear  61 . Thus, the pressure slope surface  61   a  is sloped such that the height of the pressure slope surface  61   a  is continuously changed in the circumferential direction of the rotation axis A 2 . 
     The bearing  71   a  is a ball bearing having an annular shape a in front view, and is fixed to an outer peripheral surface of the rear portion of the sleeve portion  7   a . The bearing  71   a  is positioned rearward of the pin guide  63  in a state where the shaft  71  is positioned at the rear dead center. The bearing  71   a  is engaged with the pressure slope surface  61   a . The height of the portion of the pressure slope surface  61   a  which portion is in engagement with the bearing  71   a  is the highest in a state where the shaft  71  is at the rearmost position (i.e., the rear dead center), and is the lowest in a state where the shaft  71  is at the frontmost position (i.e., the front dead center). Hence, in the state illustrated in  FIG. 1 , the rear end portion of the pressure slope surface  61   a  is the lowest and the front end portion of the pressure slope surface  61   a  is the highest. 
     The urging member  7   b  is a spring extending in the up-down direction, and is provided rearward of the bearing  71   a . The urging member  7   b  presses the rear end portion of the sleeve portion  7   a  downward, so that an urging force directed downward is always imparted on the rear end portion of the sleeve portion  7   a.    
     The swing shaft  7   c  extends in the left-right direction, and is provided at the front lower portion of the sleeve portion  7   a . The swing shaft  7   c  swingably supports the sleeve portion  7   a , so that the sleeve portion  7   a  is swingable about the swing shaft  7   c.    
     When the power transmission gear  61  rotates from the state illustrated in  FIG. 1 , the pressure slope surface  61   a  rotates integrally with the power transmission gear  61 . By the rotation of the pressure slope surface  61   a , the height of the portion of the pressure slope surface  61   a  which portion is in engagement with the bearing  71   a  gradually and continuously increases. As a result, an urging force directed upward (i.e., a force for moving the bearing  71   a  upward) is applied to the bearing  71   a , and at the same time, the urging force directed upward (i.e., a force for moving upward the portion of the sleeve portion  7   a  to which the bearing  71   a  is fixed) is applied through the bearing  71   a  also to the portion of the sleeve portion  7   a  to which the bearing  71   a  is fixed. This urging force causes the sleeve portion  7   a  to swing about the swing shaft  7   c  against the urging force of the urging member  7   b  such that the rear end of the sleeve portion  7   a  moves upward. In accordance with further rotation of the power transmission gear  61 , the shaft  71  moves to the frontmost position (the front dead center), and thus the portion of the pressure slope surface  61   a  which is in engagement with the bearing  71   a  becomes the highest (that is, the bearing  71   a  is engaged with the highest portion of the pressure slope surface  61   a ). 
     In accordance with further rotation of the power transmission gear  61 , the height of the portion of the pressure slope surface  61   a  which portion is in engagement with the bearing  71   a  is gradually lowered, so that the position of the bearing  71   a  is moved downward, and hence, the sleeve portion  7   a  swings about the swing shaft  7   c  such that the rear end of the sleeve portion  7   a  is moved downward. 
     In this way, the angle of the inclination of the shaft  71  is changed through the sleeve portion  7   a  according to the position in the front-rear direction of the shaft  71 , thereby enhancing cutting performance. Incidentally, in the saber saw  1 , for the purpose of attaining the orbital mechanism, the sleeve portion  7   a  is extended to a position rearward of the pin guide  63  in a state where the shaft  71  is at the rearmost position (i.e., the rear dead center) in order for the sleeve portion  7   a  to have both a portion to which the bearing  71   a  is to be fixed and a portion to which the urging force of the urging member  7   b  is to be applied. 
     Next, impact mitigation by the shock absorbing portion  10  in case of application of an impact to the power transmission portion  6  and the rotation shaft portion  31  will be described with reference to  FIGS. 6 and 7 . In the following description, a case where the driven portion  7  (i.e., the shaft  71 ) is locked due to the blade B getting stuck in the workpiece during a cutting work will be described as an example of cases where an impact is imparted on the power transmission portion  6  and the rotation shaft portion  31 . Incidentally, as illustrated in  FIG. 6 , in a state where a cutting work is being smoothly performed (i.e., in a state of no occurrence of locking of the driven portion  7 ), the contact of the outer race  91  of the front bearing  9  with the rear surface of the second wall  238  is maintained by virtue of the urging force of the elastic body  10 B although a thrust force directed rearward is slightly applied to the rotation shaft portion  31 . 
     When the driven portion  7  is locked (that is, at the moment of the occurrence of locking of the driven portion  7 ) due to the blade B getting stuck in the workpiece during a cutting work, the driven portion  7  having been reciprocally moving is suddenly stopped. As a result, reciprocating movement of the pin guide  63 , revolving movement of the pin  62 , and rotation of the power transmission gear  61  are also suddenly stopped. On the other hand, the pinion gear  312  in meshing engagement with the power transmission gear  61  tries to continue rotating (i.e., the rotation shaft portion  31  tries to continue rotating) because the brushless motor  3  continues to be driven. Accordingly, at the time of the occurrence of the locking of the driven portion  7 , an impact is applied to the pin guide  63 , the pin  62 , teeth of the power transmission gear  61 , teeth of the pinion gear  312 , and other parts, and also, the rearward thrust force applied to the rotation shaft portion  31  becomes extremely large. 
     However, when the rearward thrust force applied to the rotation shaft portion  31  becomes extremely large, the rotation shaft portion  31 , the rear bearing  8 , the front bearing  9 , the rotor  32 , and the sensor magnet  32 A are integrally moved rearward, and at the same time, the elastic body  10 B is compressed rearward as illustrated in  FIG. 7 . Therefore, the impact force applied to the rotation shaft portion  31  (mainly, the teeth of the pinion gear  312 ), the power transmission gear  61  (mainly, the teeth of the power transmission gear  61 ), the pin  62 , and the pin guide  63  can be mitigated at the time of the occurrence of the locking of the driven portion  7 . With this configuration, the durability of both the rotation shaft portion  31  and the power transmission portion  6  against the impact can be improved, and breakage and deformation of the rotation shaft portion  31 , the power transmission gear  61 , the pin  62 , and the pin guide  63  can be suppressed. Incidentally, although the elastic body  10 B expands in the up-down direction at the time of the compression as illustrated in  FIG. 7 , the expanded elastic body  10 B can be prevented from contacting with the inner race  92  of the front bearing  9  since the annular plate  10 A having a predetermined thickness in the front-rear direction is provided between the elastic body  10 B and the front bearing  9 . Therefore, smooth rotation of the inner race  92  relative to the outer race  91  can be ensured even during compression of the elastic body  10 B. Incidentally, the thickness of the annular plate  10 A is approximately 2 mm. However, the thickness of the annular plate  10 A is not limited as long as the thickness can provide a strength that can sufficiently withstand a pressure for compressing the elastic body  10 B, and further, the thickness can prevent the expanded elastic body  10 B from contacting with the inner race  92  of the front bearing  9 . 
     Further, at the time of compression of the elastic body  10 B, the sensor magnet  32 A moves rearward integrally with the rotation shaft portion  31  to approach the sensor board  4 . However, the elastic modulus of the elastic body  10 B is designed so that a gap distance L 1  in the front-rear direction between the sensor magnet  32 A and the sensor board  4  will be 2 mm or more at the time that the elastic body OB is compressed. Therefore, the sensor magnet  32 A (a part of the rotor  32 ) can be prevented from coming into collision against the sensor board  4  at the time of compression of the elastic body  10 B (at the time of occurrence of locking of the driven portion  7 ), thereby preventing the three Hall elements from being damaged. Incidentally, according to the present embodiment, the gap distance L 1  in the front-rear direction between the sensor magnet  32 A and the sensor board  4  in the state illustrated in  FIG. 6  is designed to be approximately 3 mm, and the rotation shaft portion  31  is moved rearward by approximately 1 mm at the time of the occurrence of locking of the driven portion  7  (at the time of compression of the elastic body  10 B). Further, a gap distance L 2  in the front-rear direction between the rear bearing  8  and the front surface of the rear end wall  226  in the state illustrated in  FIG. 6  is designed to be approximately 2 mm, which is greater than the distance (approximately 1 mm) by which the rotation shaft portion  31  is moved rearward at the time of compression of the elastic body  10 B. 
     As described above, the saber saw  1  according to the present embodiment of the present invention includes the brushless motor  3  having a rotation shaft portion  31  rotatable about the rotation axis A 1 , the housing  2  accommodating therein the brushless motor  3 , the power transmission portion  6  configured to receive the rotation force of the rotation shaft portion  31  and to transmit a driving force based on the rotation force, and the driven portion  7  configured to be driven by receiving the transmitted driving force. Also, the rotation shaft portion  31  is supported by the housing  2  so as to be movable relative to the housing  2  in the axial direction of the rotation axis A 1 . 
     With the above structure, an impact applied to the rotation shaft portion  31  and the power transmission portion  6  can be mitigated by the movement in the axial direction (rearward in the present embodiment) of the rotation shaft portion  31 . Accordingly, the rotation shaft portion  31  and the components (that is, the power transmission gear  61 , the pin  62 , and the pin guide  63 ) constituting the power transmission portion  6  can have improved durability against the impact, and thus the rotation shaft portion  31  and the components constituting the power transmission portion  6  can be suppressed from being deformed and damaged. Further, according to the above-described structure, the improvement on the durability of both the rotation shaft portion  31  and the components constituting the power transmission portion  6  against the impact can be achieved at low cost and with a compact structure, in comparison with a structure in which a clutch mechanism or the like for mitigating the impact applied to the rotation shaft portion  31  and the power transmission portion  6  is provided on the power transmission path. 
     Further, the saber saw  1  further includes the shock absorbing portion  10  including the elastic body  10 B configured to be elastically deformed (compression in the present embodiment) by the movement of the rotation shaft portion  31  in the axial direction. With this structure, the impact applied to the rotation shaft portion  31  and the power transmission portion  6  can further be effectively mitigated by virtue of the elastic deformation (compression) of the elastic body  10 B. 
     Further, the saber saw  1  further includes the front bearing  9  supporting the rotation shaft portion  31  such that the rotation shaft portion  31  is rotatable about the rotation axis A 1 . the front bearing  9  is supported by the housing  2  so as to be movable integrally with the rotation shaft portion  31  in the axial direction. The shock absorbing portion  10  and the front bearing  9  are arrayed in the axial direction and the shock absorbing portion  10  is in contact with the front bearing  9 . This structure can effectively mitigate the impact applied to the rotation shaft portion  31  and the power transmission portion  6 . 
     Further, the shock absorbing portion  10  of the saber saw  1  further includes the annular plate  10 A interposed between the front bearing  9  and the elastic body  10 B. The front bearing  9  includes the outer race  91  and the inner race  92  which are rotatable relative to each other. Further. The rotation shaft portion  31  is fixed to the inner race  92 , and the elastic body  10 B urges the annular plate  10 A toward the outer race  91 . The annular plate  10 A is in contact with the outer race  91  and is away from the inner race  92 . 
     With this structure, since the annular plate  10 A is provided between the elastic body  10 B and the front bearing  9 , the elastic body  10 B expanded at the time of compression never contacts with the inner race  92  of the front bearing  9 . Hence, smooth relative rotation between the inner race  92  and the outer race  91  can be ensured even during the compression of the elastic body  10 B. 
     Further, the saber saw  1  according to the embodiment further includes a rear bearing  8  supported by the housing  2  and supporting, in cooperation with the front bearing  9 , the rotation shaft portion  31  such that the rotation shaft portion  31  is rotatable about the rotation axis A 1 . The elastic body  10 B is positioned between the front bearing  9  and the rear bearing  8  in the axial direction. With this structure, a space between conventional two bearings used to support a rotation shaft portion can be utilized to dispose the elastic body  10 B therein, whereby the saber saw  1  can be suppressed from being increased in size. Further, in the saber saw  1  according to the embodiment, for the purpose of attaining the orbital mechanism, the sleeve portion  7   a  is extended to a position rearward of the pin guide  63  in a state where the shaft  71  is at the rearmost position in the reciprocating movement of the shaft  71 . With this structure, a dead space is provided at the region within the housing  2  which is downward of the rear end portion of the sleeve portion  7   a  and rearward of the power transmission gear  61 . However, according to the present embodiment, since the elastic body  10 B is disposed in the space, i.e., the region overlapping with the rear end of the sleeve portion  7   a  in the front-rear direction (in other words, the region overlapping with the rear end of the sleeve portion  7   a  as viewed in a direction perpendicular to the rotation axis A 1  (as viewed in the up-down direction in the embodiment)), the dead space can be efficiently utilized to avoid an increase in size of the saber saw  1 . 
     Further, the rotation shaft portion  31  of the saber saw  1  includes the rotation shaft  311  extending in the axial direction, and the pinion gear  312  provided at the rotation shaft  311 . Furthermore, the power transmission portion  6  of the saber saw  1  includes the power transmission gear  61  in meshing engagement with the pinion gear  312 . The power transmission gear  61  is a bevel gear. 
     With this structure, the impact applied to the power transmission portion  6  can be efficiently converted into an impact applied to the rotation shaft  311  in the axial direction (i.e., into a thrust force applied to the rotation shaft  311 ), and the impact in the axial direction can be effectively mitigated by the movement in the axial direction of the rotation shaft  311 . Accordingly, the impact applied to the rotation shaft portion  31  and the power transmission portion  6  can further be effectively mitigated. 
     Further, in the saber saw  1 , the brushless motor  3  is employed as the drive source. Hence, the structure for mitigating the impact applied to the rotation shaft portion  31  and the power transmission portion  6  can be suitably achieved by moving the rotation shaft portion  31  in the axial direction. More specifically, assuming that a rotation shaft portion is designed to be movable in the axial direction thereof in a power tool in which a motor with brush is employed as the drive source of the power tool, contact between the brush and a commutator becomes unstable, which may disrupt driving of the motor may be disrupted. Therefore, in the above-assumed power tool, a structure in which the impact applied to the rotation shaft portion and the power transmission portion can be mitigated by moving the rotation shaft portion in the axial direction cannot be suitably attained. Accordingly, in order to attain the structure for mitigating the impact applied to the rotation shaft portion and the power transmission portion in the power tool employing the motor with brush, that power tool has no choice but to be provide with, for example, an impact mitigating mechanism within a power transmission portion (deceleration mechanism). In contrast, in the saber saw  1  employing the brushless motor  3  in which the brush and the commutator need not be provided, even the configuration in which the rotation shaft portion is movable in the axial direction is less likely to disrupt driving of the brushless motor  3 , and accordingly, there can be suitably achieved the structure in which the impact applied to the rotation shaft portion  31  and the power transmission portion  6  can be mitigated by virtue of the movement in the axial direction of the rotation shaft portion  31 . 
     While the description has been made in detail with reference to the embodiment, it would be apparent that various modifications may be made thereto without departing from the scope of the invention defined in the claims. 
     In the above-described embodiment, there has been exemplified the saber saw  1  including a shaft-intersection type of a gear structure in which the pinion gear  312  provided at the rotation shaft  311  and the power transmission gear  61  in meshing engagement with the pinion gear  312  are both spiral bevel gears. However, the present invention is not limited to this but can be applied to any power tool as long as the power tool includes a gear structure in which a thrust force is imparted on the rotation shaft of the brushless motor. Incidentally, other than the gear structure of the above-described embodiment, examples of the gear structure in which a thrust force is imparted on the rotation shaft of the brushless motor includes, a shaft-intersection type of a gear structure in which a pinion gear provided at the rotation shaft and a power transmission gear in meshing engagement with the pinion gear are both straight bevel gears, and a shaft-parallel type of a gear structure in which a pinion gear provided at the rotation shaft and a power transmission gear in meshing engagement with the pinion gear are both helical gears. 
     Further, in the present embodiment, the rotation shaft  311  and the pinion gear  312  are provided integrally with each other. However, the configurations of the rotation shaft  311  and the pinion gear  312  are not limited as long as the rotation shaft  311  and the pinion gear  312  are integrally rotatable and integrally movable in the front-rear direction. 
     Further, in the present embodiment, a rubber member elastically deformable is employed as the elastic body  10 B. However, any member is available as long as the member can mitigate impact applied to the rotation shaft portion  31  and the power transmission portion  6 . For example, a coil spring, a leaf spring, a disc spring, an elastic body made from metal or resin, and other members are available as the elastic body  10 B. 
     Further, in the present embodiment, the elastic body  10 B is disposed rearward of the front bearing  9 . However, the elastic body  10 B may be disposed rearward of the rear bearing  8 . Although a region where the elastic body can be disposed is limited because the communication holes  221   a  are formed around the rear bearing support portion  223 , the rear bearing support portion  223  can provide higher impact resistance by the combination with the elastic body positioned rearward of the front bearing  9 . Further, in the present embodiment, the pressure slope surface  61   a  of the orbital mechanism is configured to be engaged with the bearing  71   a  fixed to the sleeve portion  7   a . However, the sleeve portion  7   a  may be omitted, and the shaft  71  and the pressure slope surface  61   a  may be configured to directly contact with each other. In this case, an increase in size of the saber saw  1  can be suppressed by disposing the elastic body  10 B such that the elastic body  10 B overlaps with the mar end of the shaft  71  in the front-rear direction (i.e., as viewed in a direction perpendicular to the rotation axis A 1  (as viewed in the up-down direction in the present embodiment)) in a state where the shaft  71  is at the rearmost position (i.e., the rear dead center). 
     REFERENCE SIGNS LIST 
       1 : saber saw,  2 : housing,  3 : brushless motor,  6 ; power transmission portion,  7 : driven portion,  8 : rear bearing,  9 : front bearing,  10 : shock absorbing portion,  10 A: annular plate,  10 B: elastic body,  31 : rotation shaft portion,  312 : pinion gear,  61 : power transmission gear