Abstract:
A power tool capable of performing vibration damping action in working operation, without an increase in size. The working tool includes a motor, a housing in which an internal mechanism driven by the motor is stored, a tool bit disposed on one end of the housing, a hand grip continuously connected to the other end of the housing, and a dynamic damper. The dynamic damper is disposed by utilizing a space between the housing and the internal mechanism so that the damping direction of the dynamic damper faces the longitudinal direction of the tool bit.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a technique for reducing vibration in a reciprocating power tool, such as a hammer and a hammer drill, which linearly drives a tool bit. 
     2. Background of the Invention 
     Japanese non-examined laid-open Patent Publication No. 52-109673 discloses an electric hammer having a vibration reducing device. In the known electric hammer, a vibration proof chamber is integrally formed with a body housing (and a motor housing) in a region on the lower side of the body housing and forward of the motor housing. A dynamic vibration reducer is disposed within the vibration proof chamber. 
     In the above-mentioned known electric hammer, the vibration proof chamber that houses the dynamic vibration reducer is provided in the housing in order to provide an additional function of reducing vibration in working operation. As a result, however, the electric hammer increases in size. 
     SUMMARY 
     Object of the Invention 
     It is, accordingly, an object of the present invention to provide an effective technique for reducing vibration in working operation, while avoiding size increase of a power tool. 
     Subject Matter of the Invention 
     The above-described object is achieved by the features of claimed invention. The invention provides a power tool which includes a motor, an internal mechanism driven by the motor, a housing that houses the motor and the internal mechanism, a tool bit disposed in one end of the housing and driven by the internal mechanism in its longitudinal direction to thereby perform a predetermined operation, a handgrip connected to the other end of the housing, and a dynamic vibration reducer including a weight and an elastic element. The elastic element is disposed between the weight and the housing and adapted to apply a biasing force to the weight. The weight reciprocates in the longitudinal direction of the tool bit against the biasing force of the elastic element. By the reciprocating movement of the weight, the dynamic vibration reducer reduces vibration which is caused in the housing in the longitudinal direction of the tool bit in the working operation. 
     The “power tool” may particularly includes power tools, such as a hammer, a hammer drill, a jigsaw and a reciprocating saw, in which a tool bit performs a working operation on a workpiece by reciprocating. When the power tool is a hammer or a hammer drill, the “internal mechanism” according to this invention comprises a motion converting mechanism that converts the rotating output of the motor to linear motion and drives the tool bit in its longitudinal direction, and a power transmitting mechanism that appropriately reduces the speed of the rotating output of the motor and transmits the rotating output as rotation to the tool bit. 
     In the present invention, the dynamic vibration reducer is disposed in the power tool by utilizing a space within the housing and/or the handgrip. Therefore, the dynamic vibration reducer can perform a vibration reducing action in working operation, while avoiding size increase of the power tool. Further, the dynamic vibration reducer can be protected from an outside impact, for example, in the event of drop of the power tool. The manner in which the dynamic vibration reducer is “disposed by utilizing a space between the housing and the internal mechanism” includes not only the manner in which the dynamic vibration reducer is disposed by utilizing the space as-is, but also the manner in which it is disposed by utilizing the space changed in shape. 
     The present invention will be more apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side view showing a hammer drill according to an embodiment of the invention, with an outer housing and an inner housing shown in section; 
         FIG. 1B  is a side view showing a hammer drill according to another embodiment of the invention, with an outer housing and an inner housing shown in section; 
         FIG. 2A  is a side view of the hammer drill, with the outer housing shown in section according to an embodiment of the invention; 
         FIG. 2B  is a side view of the hammer drill, with the outer housing shown in section according to another embodiment of the invention; 
         FIG. 2C  is a side view of the hammer drill, with the outer housing shown in section according to another embodiment of the invention; 
         FIG. 2D  is a side view of the hammer drill, with the outer housing shown in section according to another embodiment of the invention; 
         FIG. 2E  is a side view of the hammer drill, with the outer housing shown in section according to another embodiment of the invention; 
         FIG. 2F  is a side view of the hammer drill, with the outer housing shown in section according to another embodiment of the invention; 
         FIG. 3  is a plan view of the hammer drill, with the outer housing shown in section; 
         FIG. 4  is a plan view of the hammer drill, with the outer housing shown in section; 
         FIG. 5  is a rear view of the hammer drill, with the outer housing shown in section; 
         FIG. 6  is a sectional view taken along line A-A in  FIG. 1A ; and 
         FIG. 7  is a sectional view taken along line B-B in  FIG. 1B . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Representative embodiments of the present invention will now be described with reference to  FIGS. 1A to 7 . In each embodiment, an electric hammer drill will be explained as a representative example of a power tool according to the present invention. Each of the embodiments features a dynamic vibration reducer disposed in a space within a housing or a handgrip. Before a detailed explanation of placement of the dynamic vibration reducer, the configuration of the hammer drill will be briefly described with reference to  FIG. 1A . The hammer drill  101  mainly includes a body  103 , a hammer bit  119  detachably coupled to the tip end region (on the left side as viewed in  FIG. 1A ) of the body  103  via a tool holder  137 , and a handgrip  102  connected to a region of the body  103  on the opposite side of the hammer bit  119 . The body  103 , the hammer bit  119  and the handgrip  102  are features that correspond to the “housing”, the “tool bit” and the “handgrip”, respectively, according to the present invention. 
     The body  103  of the hammer drill  101  mainly includes a motor housing  105 , a crank housing  107 , and an inner housing  109  that is housed within the motor housing  105  and the crank housing  107 . The motor housing  105  and the crank housing  107  are features that correspond to the “outer housing” according to this invention, and the inner housing  109  corresponds to the “inner housing”. The motor housing  105  is located on the lower part of the handgrip  102  toward the front and houses a driving motor  111 . The driving motor  111  is a feature that corresponds to the “motor” according to this invention. 
     In the present embodiments, for the sake of convenience of explanation, in the state of use in which the user holds the handgrip  102 , the side of the hammer bit  119  is taken as the front side and the side of the handgrip  102  as the rear side. Further, the side of the driving motor  111  is taken as the lower side and the opposite side as the upper side; the vertical direction and the horizontal direction which are perpendicular to the longitudinal direction are taken as the vertical direction and the lateral direction, respectively. 
     The crank housing  107  is located on the upper part of the handgrip  102  toward the front and butt-joined to the motor housing  105  from above. The crank housing  107  houses the inner housing  109  together with the motor housing  105 . The inner housing  109  houses a cylinder  141 , a motion converting mechanism  113 , and a gear-type power transmitting mechanism  117 . The cylinder  141  houses a striking element  115  that is driven to apply a striking force to the hammer bit  119  in its longitudinal direction. The motion converting mechanism  113  comprises a crank mechanism and converts the rotating output of the driving motor  111  to linear motion and then drives the striking element  115  via an air spring. The power transmitting mechanism  117  transmits the rotating output of the driving motor  111  as rotation to the hammer bit  119  via a tool holder  137 . Further, the inner housing  109  includes an upper housing  109   a  and a lower housing  109   b . The upper housing  109   a  houses the entire cylinder  141  and most of the motion converting mechanism  113  and power transmitting mechanism  117 , while the lower housing  109   b  houses the rest of the motion converting mechanism  113  and power transmitting mechanism  117 . The motion converting mechanism  113 , the striking element  115  and the power transmitting mechanism  117  are features that correspond to the “internal mechanism” according to this invention. 
     The motion converting mechanism  113  appropriately converts the rotating output of the driving motor  111  to linear motion and then transmits it to the striking element  115 . As a result, an impact force is generated in the longitudinal direction of the hammer bit  119  via the striking element  115 . The striking element  115  includes a striker  115   a  and an intermediate element in the form of an impact bolt (not shown). The striker  115   a  is driven by the sliding movement of a piston  113   a  of the motion converting mechanism  113  via the action of air spring within the cylinder  141 . Further, the power transmitting mechanism  117  appropriately reduces the speed of the rotating output of the driving motor  111  and transmits the rotating output as rotation to the hammer bit  119 . Thus, the hammer bit  119  is caused to rotate in its circumferential direction. The hammer drill  101  can be switched by appropriate operation of the user between a hammer mode in which a working operation is performed on a workpiece by applying only a striking force to the hammer bit  119  in the longitudinal direction, and a hammer drill mode in which a working operation is performed on a workpiece by applying an longitudinal striking force and a circumferential rotating force to the hammer bit  119 . 
     The hammering operation in which a striking force is applied to the hammer bit  119  in the longitudinal direction by the motion converting mechanism  113  and the striking element  115 , and the hammer-drill operation in which a rotating force is applied to the hammer bit  119  in the circumferential direction by the power transmitting mechanism  117  in addition to the striking force in the longitudinal direction are known in the art. Also, the mode change between the hammer mode and the hammer drill mode is known in the art. These known techniques are not directly related to this invention and therefore will not be described in further detail. 
     The hammer bit  119  moves in the longitudinal direction on the axis of the cylinder  141 . Further, the driving motor  111  is disposed such that the axis of an output shaft  111   a  is perpendicular to the axis of the cylinder  141 . The inner housing  109  is disposed above the driving motor  111 . 
     The handgrip  102  includes a grip  102   a  to be held by the user and an upper and a lower connecting portions  102   b ,  102   c  that connect the grip  102   a  to the rear end of the body  103 . The grip  102   a  vertically extends and is opposed to the rear end of the body  103  with a predetermined spacing. In this state, the grip  102   a  is detachably connected to the rear end of the body  103  via the upper and lower connecting portions  102   b ,  102   c.    
     A dynamic vibration reducer  151  is provided in the hammer drill  101  in order to reduce vibration which is caused in the hammer drill  101 , particularly in the longitudinal direction of the hammer bit  119 , during hammering or hammer-drill operation. The dynamic vibration reducer  151  is shown as an example in  FIGS. 2A-2F  and  3  in sectional view. The dynamic vibration reducer  151  mainly includes a box-like (or cylindrical) vibration reducer body  153 , a weight  155  and biasing springs  157  disposed on the front and rear sides of the weight  155 . The weight  155  is disposed within the vibration reducer body  153  and can move in the longitudinal direction of the vibration reducer body  153 . The biasing spring  157  is a feature that corresponds to the “elastic element” according to the present invention. The biasing spring  157  applies a spring force to the weight  155  when the weight  155  moves in the longitudinal direction of the vibration reducer body  153 . 
     Placement of the dynamic vibration reducer  151  will now be explained with respect to each embodiment. 
     First Embodiment 
     In the first embodiment, as shown in  FIGS. 2A and 3 , the dynamic vibration reducer  151  is disposed by utilizing a space in the upper region inside the body  103 , or more specifically, a space  201  existing between the inner wall surface of the upper region of the crank housing  107  and the outer wall surface of the upper region of an upper housing  109   a  of the inner housing  109 . The dynamic vibration reducer  151  is disposed in the space  201  such that the direction of movement of the weight  155  or the vibration reducing direction coincides with the longitudinal direction of the hammer bit  119 . The space  201  is dimensioned to be larger in the horizontal directions (the longitudinal and lateral directions) than in the vertical direction (the direction of the height). Therefore, in this embodiment, the dynamic vibration reducer  151  has a shape conforming to the space  201 . Specifically, as shown in sectional view, the vibration reducer body  153  has a box-like shape short in the vertical direction and long in the longitudinal direction. Further, projections  159  are formed on the right and left sides of the weight  155  in the middle in the longitudinal direction. The biasing springs  157  are disposed between the projections  159  and the front end and the rear end of the vibration reducer body  153 . Thus, the amount of travel of the weight  155  can be maximized while the longitudinal length of the vibration reducer body  153  can be minimized. Further, the movement of the weight  155  can be stabilized. 
     Thus, in the first embodiment, the dynamic vibration reducer  151  is disposed by utilizing the space  201  existing within the body  103 . As a result, vibration caused in working operation of the hammer drill  101  can be reduced by the vibration reducing action of the dynamic vibration reducer  151 , while size increase of the body  103  can be avoided. Further, by placement of the dynamic vibration reducer  151  within the body  103 , the dynamic vibration reducer  151  can be protected from an outside impact in the event of drop of the hammer drill  101 . 
     As shown in  FIG. 2A , generally, a center of gravity G of the hammer drill  101  is located below the axis of the cylinder  141  and slightly forward of the axis of the driving motor  111 . Therefore, when, like this embodiment, the dynamic vibration reducer  151  is disposed within the space  201  existing between the inner wall surface of the upper region of the crank housing  107  and the outer wall surface of the upper region of the upper housing  109   a  of the inner housing  109 , the dynamic vibration reducer  151  is disposed on the side of the axis of the cylinder  141  which is opposite to the center of gravity G of the hammer drill  101 . Thus, the center of gravity G of the hammer drill  101  is located closer to the axis of the cylinder  141 , which is effective in lessening or preventing vibration in the vertical direction. Further, the dynamic vibration reducer  151  disposed in the space  201  is located relatively near to the axis of the cylinder  141 , so that it can perform an effective vibration reducing action against vibration in working operation using the hammer drill  101 . 
     Second Embodiment 
     In the second representative embodiment, as shown in  FIGS. 2B and 5 , a dynamic vibration reducer  213  is disposed by utilizing a space in the side regions toward the upper portion within the body  103 , or more specifically, right and left spaces  211  existing between the right and left inner wall surfaces of the side regions of the crank housing  107  and the right and left outer wall surfaces of the side regions of the upper housing  109   a . The spaces  211  correspond to the lower region of the cylinder  141  and extend in a direction parallel to the axis of the cylinder  141  or the longitudinal direction of the cylinder  141 . Therefore, in this case, as shown by dashed lines in  FIGS. 2B and 5 , the dynamic vibration reducer  213  has a cylindrical shape and is disposed such that the direction of movement of the weight or the vibration reducing direction coincides with the longitudinal direction of the hammer bit  119 . The dynamic vibration reducer  213  is the same as the first embodiment in the construction, except for the shape, including a body, a weight and biasing springs, which are not shown. 
     According to the second embodiment, in which the dynamic vibration reducer  213  is placed in the right and left spaces  211  existing between the right and left inner wall surfaces of the side region of the crank housing  107  and the right and left outer wall surfaces of the side region of the upper housing  109   a , like the first embodiment, the dynamic vibration reducer  213  can perform the vibration reducing action in working operation of the hammer drill  101 , while avoiding size increase of the body  103 . Further, the dynamic vibration reducer  213  can be protected from an outside impact in the event of drop of the hammer drill  101 . Especially in the second embodiment, the dynamic vibration reducer  213  is disposed in a side recess  109   c  of the upper housing  109   a , so that the amount of protrusion of the dynamic vibration reducer  213  from the side of the upper housing  109   a  can be lessened. Therefore, high protection can be provided against an outside impact. The upper housing  109   a  is shaped to minimize the clearance between the mechanism component parts within the upper housing  109   a  and the inner wall surface of the upper housing  109   a . To this end, the side recess  109   c  is formed in the upper housing  109   a . Specifically, due to the positional relationship between the cylinder  141  and a driving gear of the motion converting mechanism  113  or the power transmitting mechanism  117  which is located below the cylinder  141 , the side recess  109   c  is defined as a recess formed in the side surface of the upper housing  109   a  and extending in the axial direction of the cylinder  141 . The side recess  109   c  is a feature that corresponds to the “recess” according to this invention. 
     Further, in the second embodiment, the dynamic vibration reducer  213  is placed very close to the center of gravity G of the hammer drill  101  as described above. Therefore, even with a provision of the dynamic vibration reducer  213  in this position, the hammer drill  101  can be held in good balance of weight in the vertical and horizontal directions perpendicular to the longitudinal direction of the hammer bit  119 , so that generation of vibration in these vertical and horizontal directions can be effectively lessened or prevented. Moreover, the dynamic vibration reducer  213  is placed relatively close to the axis of the cylinder  141 , so that it can perform an effective vibration reducing function against vibration input in working operation of the hammer drill  101 . 
     As shown in  FIGS. 2B and 5 , the hammer drill  101  having the driving motor  111  includes a cooling fan  121  for cooling the driving motor  111 . When the cooling fan  121  is rotated, cooling air is taken in through inlets  125  of a cover  123  that covers the rear surface of the body  103 . The cooling air is then led upward within the motor housing  105  and cools the driving motor  111 . Thereafter, the cooling air is discharged to the outside through an outlet  105   a  formed in the bottom of the motor housing  105 . Such a flow of the cooling air can be relatively easily guided into the region of the dynamic vibration reducer  213 . Thus, according to the second embodiment, the dynamic vibration reducer  213  can be advantageously cooled by utilizing the cooling air for the driving motor  111 . 
     Further, in the hammer drill  101 , when the motion converting mechanism  113  in the inner housing  109  is driven, the pressure within a crank chamber  127  (see  FIGS. 1A and 1B ) which comprises a hermetic space surrounded by the inner housing  109  fluctuates (by linear movement of the piston  113   a  within the cylinder  141  shown in  FIGS. 1A AND 1B ). By utilizing the pressure fluctuations, a forced vibration method may be used in which a weight is positively driven by introducing the fluctuating pressure into the body of the dynamic vibration reducer  213 . In this case, according to the second embodiment, with the construction in which the dynamic vibration reducer  213  is placed adjacent to the inner housing  109  that houses the motion converting mechanism  113 , the fluctuating pressure in the crank chamber  127  can be readily introduced into the dynamic vibration reducer  213 . Further, when, for example, the motion converting mechanism  113  comprises a crank mechanism as shown in  FIGS. 1A AND 1B , the construction for forced vibration of a weight of the dynamic vibration reducer  213  can be readily provided by providing an eccentric portion in the crank shaft. Specifically, the eccentric rotation of the eccentric portion is converted into linear motion and inputted as a driving force of the weight in the dynamic vibration reducer  213 , so that the weight is forced vibrated. 
     Third Embodiment 
     In the third representative embodiment, as shown in  FIGS. 2C and 5 , a dynamic vibration reducer  223  is disposed by utilizing a space in the side regions within the body  103 , or more specifically, a space  221  existing between one axial end (upper end) of the driving motor  111  and the bottom portion of the lower housing  107   b  and extending along the axis of the cylinder  141  (in the longitudinal direction of the hammer bit  119 ). The space  221  extends in a direction parallel to the axis of the cylinder  141 , or in the longitudinal direction. Therefore, in this case, as shown by dashed line in  FIGS. 2C and 5 , the dynamic vibration reducer  223  has a cylindrical shape and is disposed such that the direction of movement of the weight or the vibration reducing direction coincides with the longitudinal direction of the hammer bit  119 . The dynamic vibration reducer  213  is the same as the first embodiment in the construction, except for the shape, including a body, a weight and biasing springs, which are not shown. 
     According to the third embodiment, in which the dynamic vibration reducer  223  is placed in the space  221  existing between one axial end (upper end) of the driving motor  111  and the lower housing  107   b , like the first and second embodiments, the dynamic vibration reducer  223  can perform the vibration reducing action in working operation of the hammer drill  101 , while avoiding size increase of the body  103 . Further, the dynamic vibration reducer  223  can be protected from an outside impact in the event of drop of the hammer drill  101 . 
     In the third embodiment, the dynamic vibration reducer  223  is located close to the center of gravity G of the hammer drill  101  like the second embodiment and adjacent to the driving motor  111 . Therefore, like the second embodiment, even with a provision of the dynamic vibration reducer  223  in this position, the hammer drill  101  can be held in good balance of weight in the vertical and horizontal directions perpendicular to the longitudinal direction of the hammer bit  119 . Moreover, a further cooling effect can be obtained especially because the dynamic vibration reducer  223  is located in the passage of the cooling air for cooling the driving motor  111 . Further, although the dynamic vibration reducer  223  is located at a slight more distance from the crank chamber  127  compared with the second embodiment, the forced vibration method can be relatively easily realized in which a weight is positively driven by introducing the fluctuating pressure of the crank chamber into the dynamic vibration reducer  223 . 
     Fourth Embodiment 
     In the fourth representative embodiment, as shown in  FIGS. 2D and 4 , a dynamic vibration reducer  233  is disposed by utilizing a space existing in the right and left side upper regions within the body  103 , or more specifically, a space  231  existing between the right and left inner wall surfaces of the side regions of the crank housing  107  and the right and left outer wall surfaces of the side regions of the upper housing  109   a  of the inner housing  109 . The space  231  is relatively limited in lateral width due to the narrow clearance between the inner wall surfaces of the crank housing  107  and the outer wall surfaces of the upper housing  109   a , but it is relatively wide in the longitudinal and vertical directions. Therefore, in this embodiment, the dynamic vibration reducer  233  has a shape conforming to the space  231 . Specifically, as shown by dashed line in  FIGS. 2D and 4 , the dynamic vibration reducer  233  has a box-like shape short in the lateral direction and long in the longitudinal and vertical directions and is disposed such that the direction of movement of the weight or the vibration reducing direction coincides with the longitudinal direction of the hammer bit  119 . The dynamic vibration reducer  233  is the same as the first embodiment in the construction, except for the shape, including a body, a weight and biasing springs, which are not shown. 
     According to the fourth embodiment, in which the dynamic vibration reducer  233  is placed in the space  231  existing between the right and left inner wall surfaces of the side regions of the crank housing  107  and the right and left outer wall surfaces of the side regions of the upper housing  109   a  of the inner housing  109 , like the above-described embodiments, the dynamic vibration reducer  233  can perform the vibration reducing action in working operation of the hammer drill  101 , while avoiding size increase of the body  103 . Further, the dynamic vibration reducer  233  can be protected from an outside impact in the event of drop of the hammer drill  101 . Especially, the dynamic vibration reducer  233  of the fourth embodiment occupies generally the entirety of the space  231  existing between the inner wall surfaces of the side regions of the crank housing  107  and the outer wall surfaces of the side regions of the upper housing  109   a . The dynamic vibration reducer  233  in the space  231  is located closest to the axis of the cylinder  141  among the above-described embodiments, so that it can perform a more effective vibration reducing action against vibration input in working operation of the hammer drill  101 . 
     Fifth Embodiment 
     In the fifth representative embodiment, as shown in  FIGS. 1A and 6 , a dynamic vibration reducer  243  is disposed in a space existing inside the body  103 , or more specifically, in the crank chamber  127  which comprises a hermetic space within the inner housing  109  that houses the motion converting mechanism  113  and the power transmitting mechanism  117 . More specifically, as shown by dotted line in  FIG. 1A , the dynamic vibration reducer  243  is disposed in the vicinity of the joint between the upper housing  109   a  and the lower housing  109   b  of the inner housing  109  by utilizing a space  241  existing between the inner wall surface of the inner housing  109  and the motion converting mechanism  113  and power transmitting mechanism  117  within the inner housing  109 . The dynamic vibration reducer  243  is disposed such that the vibration reducing direction coincides with the longitudinal direction of the hammer bit  119 . 
     In order to dispose the dynamic vibration reducer  243  in the space  241 , as shown in  FIG. 6  in sectional view, a body  245  of the dynamic vibration reducer  243  is formed into an oval (elliptical) shape in plan view which conforms to the shape of the inner wall surface of the upper housing  109   a  of the inner housing  109 . A weight  247  is disposed within the vibration reducer body  245  and has a generally horseshoe-like shape in plan view. The weight  247  is disposed for sliding contact with a crank shaft  113   b  of the motion converting mechanism  113  and a gear shaft  117   a  of the power transmitting mechanism  117  in such a manner as to pinch them from the both sides. Thus, the weight  247  can move in the longitudinal direction (in the axial direction of the cylinder  141 ). Specifically, the crank shaft  113   b  and the gear shaft  117   a  are utilized as a member for guiding the movement of the weight  247  in the longitudinal direction. Projections  248  are formed on the right and left sides of the weight  247 , and the biasing springs  249  are disposed on the opposed sides of the projections  248 . Specifically, the biasing springs  249  connect the weight  247  to the vibration reducer body  243 . When the weight  247  moves in the longitudinal direction of the vibration reducer body  243  (in the axial direction of the cylinder  141 ), the biasing springs  249  apply a spring force to the weight  247  in the opposite direction. 
     According to the fifth embodiment, in which the dynamic vibration reducer  243  is placed in the space  241  existing within the inner housing  109 , like the above-described embodiments, the dynamic vibration reducer  243  can perform the vibration reducing action in working operation of the hammer drill  101 , while avoiding size increase of the body  103 . Further, the dynamic vibration reducer  243  can be protected from an outside impact in the event of drop of the hammer drill  101 . 
     Further, in the fifth embodiment, the dynamic vibration reducer  243  is placed very close to the center of gravity G of the hammer drill  101  as described above. Therefore, even with a provision of the dynamic vibration reducer  243  in such a position, as explained in the second embodiment, the hammer drill  101  can be held in good balance of weight in the vertical and horizontal directions perpendicular to the longitudinal direction of the hammer bit  119 , so that generation of vibration in these vertical and horizontal directions can be effectively lessened or prevented. Moreover, the dynamic vibration reducer  243  is placed relatively close to the axis of the cylinder  141 , so that it can effectively perform a vibration reducing function against vibration caused in the axial direction of the cylinder  141  in working operation of the hammer drill  101 . Further, the space surrounded by the inner housing  109  forms the crank chamber  127 . Thus, with the construction in which the dynamic vibration reducer  243  is disposed within the crank chamber  127 , when the forced vibration method is used in which the weight  247  of the dynamic vibration reducer  243  is forced to vibrate by utilizing the pressure fluctuations of the crank chamber  127 , the crank chamber  127  can be readily connected to the space of the body  245  of the dynamic vibration reducer  243 . 
     Sixth Embodiment 
     In the sixth representative embodiment, as shown in  FIGS. 1B and 7 , a dynamic vibration reducer  253  is placed by utilizing a space existing inside the body  103 , or more specifically, a space  251  existing in the upper portion of the motor housing  105 . Therefore, the sixth embodiment can be referred to as a modification of the second embodiment. In the sixth embodiment, as shown by dotted line in  FIG. 1B , the dynamic vibration reducer  243  is disposed by utilizing the space  251  between the upper end of the rotor  111   b  of the driving motor  111  and the underside of the lower housing  109   b  of the inner housing  109 . To this end, as shown in  FIG. 7 , a body  255  of the dynamic vibration reducer  253  is formed into an oval (elliptical) shape in sectional plan view, and a weight  257  is formed into a generally elliptical ring-like shape in plan view. The weight  257  is disposed for sliding contact with bearing receivers  131   a  and  133   a  in such a manner as to pinch them from the both sides and can move in the longitudinal direction (in the axial direction of the cylinder  141 ). The bearing receiver  131   a  receives a bearing  131  that rotatably supports the output shaft  111   a  of the driving motor  111 , and the bearing receiver  133   a  receives a bearing  133  that rotatably supports the gear shaft  117   a  of the motion converting mechanism  117 . The bearing receivers  131   a  and  133   a  are also utilized as a member for guiding the movement of the weight  257  in the longitudinal direction. Further, projections  258  are formed on the right and left sides of the weight  257 , and the biasing springs  259  are disposed on the opposed sides of the projections  258 . Specifically, the biasing springs  259  connect the weight  257  to the vibration reducer body  253 . When the weight  257  moves in the longitudinal direction of the vibration reducer body  253  (in the axial direction of the cylinder  141 ), the biasing springs  259  apply a spring force to the weight  257  in the opposite direction. 
     According to the sixth embodiment, in which the dynamic vibration reducer  253  is placed in the space  251  existing within the motor housing  105 , like the above-described embodiments, the dynamic vibration reducer  253  can perform the vibration reducing action in the working operation of the hammer drill  101 , while avoiding size increase of the body  103 . Further, the dynamic vibration reducer  253  can be protected from an outside impact in the event of drop of the hammer drill  101 . 
     Further, in the sixth embodiment, the dynamic vibration reducer  253  is placed close to the center of gravity G of the hammer drill  101  as described above. Therefore, even with a provision of the dynamic vibration reducer  243  in such a position, as explained in the second embodiment, the hammer drill  101  can be held in good balance of weight in the vertical and horizontal directions perpendicular to the longitudinal direction of the hammer bit  119 , so that generation of vibration in these vertical and horizontal directions can be effectively lessened or prevented. Further, the lower position of the lower housing  109   b  is very close to the crank chamber  127 . Therefore, when the method of causing forced vibration of the dynamic vibration reducer  253  is applied, the fluctuating pressure in the crank chamber  127  can be readily introduced into the dynamic vibration reducer  253 . Moreover, the construction for causing forced vibration of the weight  257  can be readily provided by providing an eccentric portion in the crank shaft  113   b  of the motion converting mechanism  113 . Specifically, the eccentric rotation of the eccentric portion is converted into linear motion and inputted as a driving force of the weight  257  in the dynamic vibration reducer  253 , so that the weight  257  is forced vibrated. 
     Seventh Embodiment 
     In the seventh representative embodiment, as shown in  FIGS. 2E to 4 , a dynamic vibration reducer  263  is disposed by utilizing a space existing inside the handgrip  102 . As described above, the handgrip  102  includes a grip  102   a  to be held by the user and an upper and a lower connecting portions  102   b ,  102   c  that connect the grip  102   a  to the body  103 . The upper connecting portion  102   b  is hollow and extends to the body  103 . In the seventh embodiment, a dynamic vibration reducer  263  is disposed in a space  261  existing within the upper connecting portion  102   b  and extending in the longitudinal direction (in the axial direction of the cylinder  141 ). As shown by dotted line in  FIGS. 2E to 4 , the dynamic vibration reducer  263  has a rectangular shape elongated in the longitudinal direction. The dynamic vibration reducer  263  is the same as the first embodiment in the construction, except for the shape, including a body, a weight and biasing springs, which are not shown. 
     According to the seventh embodiment, in which the dynamic vibration reducer  263  is disposed in the space  261  existing inside the handgrip  102 , like the above-described embodiments, the dynamic vibration reducer  263  can perform the vibration reducing action in working operation of the hammer drill  101 , while avoiding size increase of the body  103 . Further, the dynamic vibration reducer  263  can be protected from an outside impact in the event of drop of the hammer drill  101 . Especially in the seventh embodiment, the dynamic vibration reducer  263  is disposed in the space  261  of the upper connecting portion  102   b  of the handgrip  102 , which is located relatively close to the axis of the cylinder  141 . Therefore, the vibration reducing function of the dynamic vibration reducer  263  can be effectively performed against vibration in the axial direction of the cylinder in working operation of the hammer drill  101 . 
     Generally, in the case of the hammer drill  101  in which the axis of the driving motor  111  is generally perpendicular to the axis of the cylinder  141 , the handgrip  102  is designed to be detachable from the rear end of the body  103 . Therefore, when, like this embodiment, the dynamic vibration reducer  263  is disposed in the space  261  of the connecting portion  102   b  of the handgrip  102 , the dynamic vibration reducer  263  can be mounted in the handgrip  102  not only in the manufacturing process, but also as a retrofit at the request of a purchaser. 
     Eighth Embodiment 
     In the eighth representative embodiment, like the seventh embodiment, a dynamic vibration reducer  273  is disposed by utilizing a space existing inside the handgrip  102 . Specifically, as shown by dotted line in  FIG. 2F , the dynamic vibration reducer  273  is disposed by utilizing a space  271  existing within the lower connecting portion  102   c  of the handgrip  102 . Like the above-described space  261  of the upper connecting portion  102   b , the space  271  of the lower connecting portion  102   c  extends in the longitudinal direction (in the axial direction of the cylinder  141 ). Therefore, as shown by dotted line in  FIG. 2F , the dynamic vibration reducer  273  has a rectangular shape elongated in the longitudinal direction. The dynamic vibration reducer  273  is the same as the first embodiment in the construction, except for the shape, including a body, a weight and biasing springs, which are not shown. 
     According to the eighth embodiment, in which the dynamic vibration reducer  273  is disposed in the space  271  existing inside the handgrip  102 , like the above-described embodiments, the dynamic vibration reducer  273  can perform the vibration reducing action in working operation of the hammer drill  101 , while avoiding size increase of the body  103 . Further, the dynamic vibration reducer  273  can be protected from an outside impact in the event of drop of the hammer drill  101 . Further, if the handgrip  102  is designed to be detachable from the body  103 , like the seventh embodiment, the dynamic vibration reducer  273  can be mounted in the handgrip  102  not only in the manufacturing process, but also as a retrofit at the request of a purchaser. 
     In the above-described embodiments, an electric hammer drill has been described as a representative example of the power tool. However, other than the hammer drill, this invention can not only be applied, for example, to an electric hammer in which the hammer bit  119  performs only a hammering movement, but to any power tool, such as a reciprocating saw and a jigsaw, in which a working operation is performed on a workpiece by reciprocating movement of the tool bit.