Patent Publication Number: US-7588097-B2

Title: Power impact tool

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a technique for reducing vibration in a power impact tool that linearly drives a tool bit in its longitudinal direction by a swinging mechanism 
   2. Description of the Related Art 
   A technique for reducing or alleviating vibration caused in an electric hammer drill with a swinging mechanism is disclosed in EP1000712. According to the known art, the swinging mechanism includes a swinging ring swinging in the axial direction of a rotating shaft by rotation of the rotating shaft driven by a motor. A tool bit is linearly driven by a tool driving mechanism connected to an upper end region of the swinging ring. In a vibration reducing mechanism in this known technique, a counter weight is connected to the lower end region in a position shifted about 180° in the circumferential direction from the connection between the swinging ring and the tool driving mechanism. The counter weight linearly moves by the swinging movement of the swinging ring and thereby reduces vibration caused during the operation. 
   The counter weight is disposed in a lower region apart from the swinging ring. Therefore, the vertical distance between the path of travel of the counter weight and the axis of the hammer bit is widened. As a result, when the tool driving mechanism and the counter weight are driven by the swinging ring, unnecessary vibration is caused by a couple around the horizontal axis that intersects with the axis of the rotating shaft. Further, because the counter weight linearly moves by the swinging movement of the swinging ring, loss of a striking energy of the tool bit may be caused by resistance of the sliding area. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the invention to provide a technique for further improving the vibration reducing performance in a power impact tool that linearly drives a tool bit by using a swinging mechanism. 
   Above described object is achieved by a claimed invention. According to the invention, a representative power impact tool performs a predetermined operation on a workpiece by striking movement of a tool bit in its axial direction. The power impact tool includes a motor, a rotating shaft, a swinging member and a tool driving mechanism. The rotating shaft is disposed parallel to the axial direction of the tool bit and rotationally driven by the motor. The swinging member is supported by the rotating shaft and caused to swing in the axial direction of the rotating shaft by rotation of the rotating shaft. The tool driving mechanism is connected to an upper end region of the swinging member in the vertical direction that intersects with the axis of the rotating shaft. The tool driving mechanism is caused to linearly move in the axial direction of the tool bit by the swinging movement of the swinging member and linearly drives the tool bit. 
   According to the invention, a counter weight that reduces vibration caused in the axial direction of the tool bit during the operation is provided. The counter weight is disposed in a region higher than a lower end region of the swinging member in the vertical direction that intersects with the axis of the rotating shaft. Further, a lower end of the counter weight is connected to the lower end region of the swinging member. The counter weight extends upward from the connection between the counter weight and the swinging member and has a pivot point in the extending end portion. When the swinging member swings, the counter weight is driven by the swinging member and caused to rotate in the axial direction of the tool bit, thereby reducing vibration caused in the axial direction of the tool bit. 
   The manner of “higher than a lower end region” according to the invention may typically be defined by a state in which the center of gravity of the counter weight is located in a region higher than the lower end region of the swinging member. For example, the counter weight may be disposed between the lower end region and the upper end region of the swinging member, the counter weight may extend in a region lower than the lower end region of the swinging member, or the counter weight may end in a region higher than the upper end region of the swinging member. 
   The counter weight according to the invention may preferably be configured to be disposed on the outside of the swinging member in such a manner as to avoid interface with the swinging member. Preferably, the counter weight may generally U-shaped having an open top. 
   The counter weight is disposed in a region higher than the lower end region of the swinging member and connected to the lower end region of the swinging member. With this construction, the counter weight located nearer to the axis of the tool bit can be driven by the swinging member. Further, the vibration reducing function of the counter weight can be performed in an optimum manner by adjusting the timing at which the swinging member drives the counter weight so as to correspond to the timing of vibration caused during the operation. According to the invention, the counter weight is moved in a position nearer to the axis of the tool bit, so that unnecessary vibration by couple force can be reduced. 
   Further, according to the invention, because the counter weight rotates, the sliding resistance can be reduced and energy loss can be avoided or reduced. Further, compared with the known construction in which the counter weight is designed to linearly move, the supporting structure of the counterweight can be made simpler. 
   As another aspect of the invention, the pivot point of the counter weight may be located at a position higher than the axis of the tool bit. By such construction, the vertical displacement during rotation of the counter weight can be reduced. As a result, the occurrence of unnecessary vertical vibration can be reduced. 
   As another aspect of the invention, the counter weight may include a connecting part connected to the swinging member and extending upward and a weight part seeing as vibration reducing weight. Further, the connecting part and the weight part may be provided as separate members and thereafter integrally formed with each other. Therefore, in manufacturing the counter weight the shapes and configurations of the connecting part and the weight part can be properly set based on individual functions. Specifically, the connecting part can be easily formed as a thin plate member, for example, by sheet metal processing, and the weight part can also be easily formed into a block, for example, as a casting. As a result, the manufacturing cost can be reduced. 
   Further, while the weight required to reduce vibration is ensured on the weight part side, the connecting part can be made thinner, for example, by sheet metal processing. Thus, the counter weight can be reduced in weight as a whole, and the mass of the component parts other than the weight part can be reduced in weight. Therefore, the occurrence of unnecessary vibration by the movement of the counter weight can be reduced. 
   As another aspect of the invention, the connecting part may include right and left arms with respect to the longitudinal axis of the tool to extend upward from the lower end connected to the swinging member and past the side of the swinging member. The lateral distance between the extending end portions of the arms can be changed by elastic deformation of the arms. Further, the pivot point may include a stem that extends in a direction that intersects with the extending direction of the arms and a hole that is fitted onto the stem for relative rotation. One of the stem and the hole may be formed in the extending end portion of each of the arms, and the stem and the hole are engaged with each other by utilizing a movement of changing the distance between the arms by-deformation of the arms. 
   According to such construction, the stem and the hole are engaged with each other by utilizing a movement of changing the distance between the arms by deformation of the arms. 
   As another aspect of the invention, the power impact tool may further include a dynamic vibration reducer that reduces vibration caused during the operation of the tool bit. The dynamic vibration reducer may include a weight that is allowed to reciprocate in the axial direction of the tool bit with a biasing force of an elastic element being applied to the weight. The counter weight drives the weight of the dynamic vibration reducer via the elastic element when the counter weight rotates. With both the vibration reducing functions of the counter weight and the dynamic vibration reducer, a further higher vibration reducing effect can be obtained. Further, with the construction in which the weight of the dynamic vibration reducer is driven by utilizing rotation of the counter weight driven by the swinging member, it is not necessary to additionally provide a driving mechanism specifically designed for driving the weigh, so that simplification in structure can be realized. 
   Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view, partly in section, schematically showing an entire electric hammer drill according to a fist representative embodiment of the invention. 
       FIG. 2  is a side view showing an internal mechanism within a gear housing. 
       FIG. 3  is a bottom view also showing the internal mechanism with the gear housing. 
       FIG. 4  is a sectional view showing a vibration reducing mechanism part. 
       FIG. 5  is a side view showing an internal mechanism within the gear housing according to a second representative embodiment of the invention. 
       FIG. 6  is an external view of the vibration reducing mechanism part. 
       FIG. 7  is a sectional view of the vibration reducing mechanism part. 
       FIG. 8  is a side view showing an internal mechanism within the gear housing according to a third representative embodiment of the invention. 
       FIG. 9  is a bottom view also showing the internal mechanism within the gear housing, with a dynamic vibration reducer shown in section. 
       FIG. 10  is a sectional view of the vibration reducing mechanism part. 
       FIG. 11  is an external view of the vibration reducing mechanism part, with the dynamic vibration reducer shown in section. 
       FIG. 12  is a view for explaining forcible excitation of the dynamic vibration reducer, with a biasing spring shown under maximum pressure. 
       FIG. 13  is a view for explaining forcible excitation of the dynamic vibration reducer, with the biasing spring shown under medium pressure. 
       FIG. 14  is a view for explaining forcible excitation of the dynamic vibration reducer, with the biasing spring shown under no pressure. 
       FIG. 15  is a side view showing an internal mechanism within the gear housing according to a fourth representative embodiment of the invention. 
       FIG. 16  is a sectional view of the vibration reducing mechanism part. 
       FIG. 17  is a sectional view of the vibration reducing mechanism part, showing the assembling procedure of a counter weight. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide and manufacture improved power impact tools and method for using such power impact tools and devices utilize therein. Representative examples of the present invention, which examples utilized many of these additional features and method steps in conjunction, will now be described in detail with reference to the drawings. This detailed description is merely intended to teach a person skilled in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed within the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe some representative examples of the invention, which detailed description will now be given with reference to the accompanying drawings. 
   First Representative Embodiment 
   First representative embodiment of the present invention will now be described with reference to  FIGS. 1 to 4 . As shown in  FIG. 1 , an electric hammer drill  101  as a representative embodiment of the power impact tool according to the present invention comprises a body  103  and a hammer bit  119  detachably coupled to the tip end region of the body  103  via a tool holder  137 . The hammer bit  119  is a feature that corresponds to the “tool bit” according to the present invention. 
   The body  103  includes a motor housing  105 , a gear housing  107  and a handgrip  109 . The motor housing  105  houses a driving motor  111 . The gear housing  107  houses a motion converting mechanism  113 , a power transmitting mechanism  114  and a striking mechanism  115 . The driving motor  111  is a feature that corresponds to the “motor” according to the present invention. The rotating output of the driving motor  111  is appropriately converted into linear motion via the motion converting mechanism  113  and transmitted to the striking element  115 . Then, an impact force is generated in the axial direction of the hammer bit  119  via the striking mechanism  115 . Further, the speed of the rotating output of the driving motor  111  is appropriately reduced by the power transmitting mechanism  114  and then transmitted to the hammer bit  119 . As a result, the hammer bit  119  is caused to rotate in the circumferential direction. The driving motor  111  is started by depressing a trigger  109   a  disposed on the handgrip  109 . In the description hereinafter, the side of the hammer bit  119  is taken as the front side, and the side of the handgrip  109  as the rear side. 
   The motion converting mechanism  113  includes a driving gear  121  that is rotated in a vertical plane by the driving motor  111 , a driven gear  123  that engages with the driving gear  121 , a rotating element  127  that rotates together with the driven gear  123  via an intermediate shaft  125 , a swinging ring  129  that is caused to swing in the axial direction of the hammer bit  119  by rotation of the rotating element  127 , and a cylindrical piston  141  that is caused to reciprocate by swinging movement of the swinging ring  129 . The intermediate shaft  125  and the swinging ring  129  are features that correspond to the “rotating shaft” and the “swinging member”, respectively, according to the present invention. The intermediate shaft  125  is disposed parallel (horizontally) to the axial direction of the hammer bit  219 . The outer surface of the rotating element.  127  fitted onto the intermediate shaft  125  is inclined at a predetermined angle with respect to the axis of the intermediate shaft  125 . The swinging ring  129  is supported on the inclined outer surface of the rotating element  127  via a bearing  126  such that it can rotate with respect to the rotating element  127 . When the rotating element  127  rotates, the swinging ring  129  is caused to swing in the axial direction of the hammer bit  119  and in a direction that intersects with this axial direction. The rotating element  127  and the swinging ring  129  rotatably supported on the rotating element  127  via the bearing  126  form a swinging mechanism. 
   Further, a swinging rod  128  is formed in the upper end region of the swinging ring  129  and extends upward (in the radial direction) from the swinging ring  129 . The swinging rod  128  is loosely fitted in an engaging member  124  that is formed in the rear end portion of the cylindrical piston  141 . The cylindrical piston  141  is slidably disposed within a cylinder  135  and driven by the swinging movement (a component in the axial direction of the hammer bit  119 ) of the swinging ring  129  so that it reciprocates along the cylinder  135 . 
   The striking mechanism  115  includes a striker  143  and an impact bolt  145 . The striker  143  is slidably disposed within the bore of the cylindrical piston  141 . The impact bolt  145  is slidably disposed within the tool holder  137  and is adapted to transmit the kinetic energy of the striker  143  to the hammer bit  119 . The striker  143  is driven by the action of an air spring caused within an air chamber  141   a  of the cylindrical piston  141  by means of sliding movement of the piston  141 . Then, the striker  143  collides with (strikes) the impact bolt  145  slidably disposed within the tool holder  137  and transmits the striking force to the hammer bit  119  via the impact bolt  145 . The cylindrical piston  141 , the striker  143  and the impact bolt  145  are features that correspond to the “tool driving mechanism” according to the inventor. 
   The power transmitting mechanism  114  includes a first transmission gear  131  that is caused to rotate in a vertical pane by the driving motor  111  via the driving gear  121  and the intermediate shaft  125 , a second transmission gear  133  that engages with the first transmission gear  131 , a cylinder  135  that is cased to rotate together with the second transmission gear  133 . The rotation driving force of the cylinder  135  is transmitted to the tool holder  137  and fit to the hammer bit  119  supported by the tool holder  137 . 
   A vibration reducing mechanism  151  will now be described with reference to  FIGS. 2 to 4 . The vibration reducing mechanism  151  is provided to reduce impulsive and cyclic vibration caused in the axial direction of the hammer bit  119  dig processing operation using the hammer drill  101 .  FIGS. 2 and 3  show an internal mechanism disposed within the gear housing  107 .  FIG. 2  is a side view and  FIG. 3  is a bottom view. Further,  FIG. 4  is a sectional view showing a vibration reducing mechanism part. The vibration reducing mechanism  151  of this embodiment includes a counter weight  153  which is driven by the swinging ring  129 . The counter weight  153  is a feature that corresponds to the “counter weight” according to the invention. 
   As shown in  FIG. 4 , the counter weight  153  is generally U-shaped having an open top, as viewed from the front or the back of the hammer drill  101 . The counterweight  153  is disposed on the outside of the swinging ring  129  in such a manner as to cover generally the lower half of the swinging ring  129 . The counter weight  153  has a generally rectangular lower end portion  153   a  (the bottom of the U shape) (see  FIG. 3 ) as viewed from under the hammer drill  101 . Right and left elongate arms  153   b  extend upward from the lower end portion  153   a.  The weights of the lower end portion  153   a  and the arms  153   b  are set such that the center of gravity of the counter weight  153  is located above the lower end region of the swinging ring  129 . The arms  153   b  of the counter weight  153  extend to about the same level as a horizontal plane including the axis of the intermediate shaft  125 . A stem  153   c  is formed on the extending end of each of the arms  153   b  and protrudes generally horizontally outward. The stem  153   c  is rotatably supported by a front support plate (not shown) on the gear housing  107  and a rear support plate  107   b  (see  FIGS. 2 and 3 ) fixedly disposed on an inner housing  107   a  of the gear housing  107 . Specifically, the counter weight  153  is supported in a suspended manner by the front and rear support plates  107   b  which are butted to each other. Thus, the counter weight  153  can rotate on the stem  153   c  in the axial direction of the hammer bit  119 . 
   A cylindrical protrusion  129   a  is provided in the lower end region of the swinging ring  129  or in a position shifted about 180° in the circumferential direction from the connection between the swinging ring  129  and the cylindrical piston  141 . Correspondingly, an engagement hole  153   d  is formed in the lower end portion  153   a  of the counter weight  153 . The protrusion  129   a  of the swinging ring  129  is loosely engaged in the engagement hole  153   d  for free relative movement. Therefore, when the swinging ring  129  swings, the counter weight  153  is driven by the swinging movement (a component of movement in the axial direction of the hammer bit  119 ) of the swinging ring  129  and is caused to rotate in a direction opposite to the direction of the reciprocating movement of the cylindrical piston  141 . Further, a clearance is provided between the inner surface of the counter weight  153  and the outer surface of the swinging ring  129  such that the counter weight  153  can rotate without interfering with the swinging ring  129 . 
   Operation of the hammer drill  101  of the first embodiment constructed as described above will now be explained. When the driving motor  111  (shown in  FIG. 1 ) is driven, the rotating output of the driving motor  111  causes the driving gear  121  to rotate in a vertical plane. When the driving gear  121  rotates, the rotating element  127  is caused to rotate in a vertical plane via the driven gear  123  that engages with the driving gear  121  and the intermediate shaft  125 . Then, the swinging ring  129  and the swinging rod  128  swing, and the cylindrical piston  141  is caused to linearly slide by the swinging movement of the swinging rod  128 . By the action of the air spring function within the air chamber  141   a  of the cylindrical piston  141  as a result of this sliding movement of the cylindrical piston  141 , the striker  143  reciprocates within the cylindrical piston  141 . At this time, the striker  143  collides with the impact bolt  145  and transmits the kinetic energy caused by the collision to the hammer bit  119 . 
   When the first transmission gear  131  is caused to rotate together with the intermediate shaft  125 , the cylinder  135  is caused to rotate in a vertical plane via the second transmission gear  133  that engages with the first transmission gear  131 , which in turn causes the tool holder  137  and the hammer bit  119  held by the tool holder  137  to rotate together with the cylinder  135 . Thus, the hammer bit  119  performs a hammering movement in the axial direction and a drilling movement in the circumferential direction, so that the processing operation (drilling operation) is performed on the workpiece. 
   The hammer drill  101  can be switched not only to hammer drill mode in which the hammer bit  119  performs a hammering movement and a drilling movement in the circumferential direction, but to drilling mode in which the hammer bit  119  performs only a drilling movement or to hammering mode in which the hammer bit  119  performs only a hammering movement. 
   In the above-described processing operation, the counter weight  153  reduces impulsive and cyclic vibration caused in the axial direction of the hammer bit  119 . The counter weight  153  is connected to the swinging ring  129  in a position shifted about 180° from the connection between the swinging ring  129  and the cylindrical piston  141  in the circumferential direction. Therefore, when the cylindrical piston  141  slides within the cylinder  135  toward the striker  143 , the counter weight  153  rotates in a direction opposite to the sliding direction of the striker  143 . Specifically, according to this embodiment, when the cylindrical piston  141  linearly moves toward the striker  143 , and the hammer bit  119  is caused to perform a striking movement via the striker  143  and the impact bolt  145 , the counter weight  153  rotates on the stem  153   c  in the axial direction of the hammer bit  119  and in a direction opposite to the cylindrical piston  141 . In this manner, vibration cause in the hammer drill  101  in the axial direction of the hammer bit  119  can be reduced. 
   According to this embodiment, the counter weight  153  is disposed in a region higher than the lower end region of the swinging ring  129  and with this construction, the center of gravity of the counter weight  153  can be located nearer to the axis of the hammer bit  119  compared with the known art. As a result, unnecessary vibration can be reduced which may be caused by a couple around the horizontal axis that intersects with the axis of the intermediate shaft  125  when the cylindrical piston  141  and the counter weight  153  are driven by the swinging ring  129  in opposite directions. 
   Further, according to this embodiment, the counter weight  153  rotates in the axial direction of the hammer bit  119  on the stems  153   c  on the extending ends of the upwardly extending arms  153 . The counter weight  153  is thus caused to rotate by the swinging movement of the swinging ring  129 . Therefore, the sliding resistance of the sliding area can be reduced, so that loss of the driving force of sting the hammer bit  119  can be avoided or reduced. Further, the structure of supporting the counter weight  153  is formed by the stems  153   c  and the front and rear support plates  107   b  that rotatably support the stems  153   c.  Thus, the structure of supporting the counter weight  153  can be made simpler, compared with the construction in which the counter weight  153  reciprocates. 
   Further, in this embodiment the structure of connecting the counter weight  153  and the swinging ring  129  is realized by the construction in which the protrusion  129   a  of the swinging ring  129  is loosely engaged in the engagement hole  153   d  for free relative movement. Therefore, the lateral swinging movement of the swinging ring  129 , or the swinging movement (shown by the arrow in  FIG. 3 ) of the swinging ring  129  on the vertical axis perpendicular to the axis of the intermediate shaft  125  is not transmitted to the counter weight  153 . Therefore, unnecessary vibration can be prevented from being caused around the vertical axis by driving of the counter weight  153 . 
   Second Representative Embodiment 
   Now, the vibration reducing mechanism  151  according to a second representative embodiment of the present invention is explained with reference to  FIGS. 5 to 7 .  FIG. 5  shows an internal mechanism disposed within the gear housing  107 .  FIG. 6  is an external view of the vibration reducing mechanism part, and  FIG. 7  is a sectional view of the vibration reducing mechanism part. Like in the fist embodiment, the vibration reducing mechanism  151  of the second embodiment also includes a counter weight  163  which is driven by the swinging ring  129 . The pivot point of the counter weight  163  is located at a higher position than in the first embodiment. Except this point, the second embodiment has the same construction as the first embodiment. Components or elements in the second embodiment which are substantially identical to those in the first embodiment are given like numerals as in the first embodiment and will not be described. The counter weight  163  is a feature that corresponds to the “counter weight” according to the present invention. 
   As shown in  FIGS. 6 and 7 , the counter weight  163  is generally U-shaped having an open top, as viewed from the front or the back of the hammer drill  101 . The counter weight  163  is disposed on the outside of the swinging ring  129 . The counter weight  163  is connected to the swinging ring  129  at a lower end portion  163   a  (the bottom of the U shape) of the counter weight  163  via the protrusion  129   a  of the swinging ring  129  and an engagement hole  163   d.  Right and left arms  163   b  extend upward from the lower end portion  163   a.    
   The arms  163   b  of the counter weight  163  extend upward to a position higher than the axis of the intermediate shaft  125  and firer to a position slightly higher than the axis of the hammer bit  119 . A stem  163   c  is founts on the extending end of each of the arms  163   b  and protrudes generally horizontally outward. The stem  163   c  is rotatably supported by a front support plate (not shown) on the gear housing  107  and a rear support plate  107   b  disposed on the inner housing  107   a  of the gear housing  107 . Further, a weight concentration part  163   e  for concentrating the weight is provided genially in the middle of the arms  163   b  of the counter weight  163  in the extending direction. With this weight concentration part  163   e,  the center of gravity of the counter weight  163  is located nearer to the axis of the hanker bit  119  than that of the counter weight  153  of the fist embodiment. 
   According to this embodiment, like the first embodiment, in the processing operation, the counter weight  163  serves to reduce impulsive and cyclic vibration caused in the axial direction of the hammer bit  119 . The counter weight  163  is connected to the swinging ring  129  in a position shifted about 180° from the connection between the swinging ring  129  and the cylindrical piston  141  in the circumferential direction. Therefore, when the cylindrical piston  141  slides within the cylinder  135  toward the striker  143 , the counter weight  163  rotates in a direction opposite to the sliding direction of the striker  143 . Specifically, according to this embodiment, when the cylindrical piston  141  linearly moves toward the striker  143 , and the hammer bit  119  is caused to perform a striking movement via the striker  143  and the impact bolt  145 , the counter weight  163  rotates on the stem  163   c  in a direction opposite to the cylindrical piston  141  in the longitudinal direction of the hammer bit  119 . In this manner, vibration caused in the hammer drill  101  in the axial direction of the hammer bit  119  can be reduced. 
   In this embodiment, as described above, the weight concentration part  163   e  is provided on the arms  163   b  of the counter weight  163 , so that the center of gravity of the counter weight  163  is located nearer to the same level as a horizontal plane including the axis of the hammer bit  119 . As a result, unnecessary vibration can be reduced which may be caused by a couple around the horizontal axis that intersects with the axis of the intermediate shaft  125  when the cylindrical piston  141  and the counter weight  163  are driven by the swinging ring  129  in opposite directions. 
   When the counter weight  163  rotates on the stem  163   c  in the axial direction of the hammer bit  119 , the counter weight  163  moves by a displacement X in the vertical direction that intersects with the axial direction of the hammer bit  119 . In such a case, because the pivot point of the counter weight  163  is located at a higher position than the axis of the hammer bit  119 , the vertical displacement X of the rotating counter weight  163  can be reduced. Therefore, the occurrence of unnecessary vibration by the vertical displacement can be reduced. 
   Third Representative Embodiment 
   Third representative embodiment of the present invention is now explained with reference to  FIGS. 8 to 14 . The vibration reducing mechanism  151  according to this embodiment uses the counter weight  153  and a dynamic vibration reducer  171  together.  FIGS. 8 and 9  show an internal mechanism disposed within the gear housing  107 , with the dynamic vibration reducer  171  shown in section. As shown in  FIGS. 8 and 9 , the dynamic vibration reducers  171  are disposed within the gear housing  107 . The dynamic vibration reducers  171  are disposed on the right and left sides of the axis of the hammer bit  119  in the side region of the gear housing  107  of the hammer drill  101  (see  FIG. 9 ). The right and left dynamic vibration reducers  171  have the same construction. Further,  FIG. 10  is a sectional view of the vibration reducing mechanism part, and  FIG. 11  is an external view of the vibration reducing mechanism part (with the dynamic vibration reduces  171  shown in section).  FIGS. 12 to 14  show the construction and movement of the dynamic vibration reducer  171  in detail. However, in  FIGS. 12 to 14 , the counter weight  153  is not shown except the stem  153   c.    
   In this embodiment, the dynamic vibration reducer  171  includes a cylindrical body  172  that extends in the axial direction of the hammer bit  119 , a vibration-reducing weight  173  disposed within the cylindrical body  172 , and biasing springs  177  disposed on the front and rear sides of the weight  173 . Each of the biasing springs  177  is a feature that corresponds to the “elastic element” according to the present invention. The biasing springs  177  exert a spring force on the weight  173  toward each other when the weight  173  moves in the longitudinal direction of the cylindrical body  172  (in the axial direction of the hammer bit  119 ). Further, an actuation chamber  176  is defined on the both sides of the weight  173  within the cylindrical body  172  of the dynamic vibration reducer  171 . The actuation chamber  176  communicates with the outside of the dynamic vibration reducer  171  via a vent  172   a  (see  FIGS. 12 to 14 ) formed through the wall of the cylindrical body  172  or via a vent  155   a  (see  FIGS. 12 to 14 ) formed through a slider  155  which will be described below. Thus, the actuation chamber  176  is normally in communication with the outside so that air can freely flow in and out. Therefore, the air flow doe not interfere with the reciprocating movement of the weight  173 . 
   The counter weight  153  not only has a function of reducing vibration, but also inputs an excitation force in order to actively drive and forcibly excite the weight  173  of the dynamic vibration reducer  171 . Specifically, in addition to the construction described in the first embodiment, an operating piece  153   e  is provided on the protruding end of each of the stems  153   c  of the counter weight  153  and rotates together with the associated stem  153   c.  The operating piece  153   e  protrudes forward, and the protruding end of the operating piece  153   e  is in contact with the back of the slider  155  which is slidably disposed within the cylindrical body  172  of the dynamic vibration reducer  171 . The slider  155  supports one end of one of the biasing springs  177 . Therefore, when the counter weight  153  rotates toter with the stem  153   c,  the operating piece  153   e  rotates together with the associated stem  153   c,  and the protruding end of tie operating piece  153   e  moves the slider  155  in a direction of pressing the biasing spring  177 . Further, the counter weight  153  has the same construction as in the first embodiment, and is therefore given the same numeral and will not be described. 
   Further, the slider  155  has a cylindrical shape elongated in the direction of movement and having a closed end in the direction of movement. Therefore, the slider  155  can have a wider sliding contact area without increasing the longitudinal length of the cylindrical body  172 . Thus, the movement of the slider  155  in the longitudinal direction can be stabilized. 
   In the third embodiment constructed as described above, in the processing operation, not only the counter weight  153  serves to reduce impulsive and cyclic vibration caused in the axial direction of the hammer bit  119  like in the first embodiment, but also the dynamic vibration reducer  171  disposed in the body  103  has a vibration reducing function. Specifically, the weight  173  and the biasing springs  177  serve as vibration reducing elements in the dynamic vibration reducer  171  and cooperate to passively reduce vibration of the body  103  of the hammer drill  101  on which a predetermined external force (vibration) is exerted. In this manner, vibration of the hammer drill  101  can be effectively reduced. 
   Further, when the hammer drill  101  is driven, the cylindrical piston  141  linearly moves toward the striker  143  by swinging movement of the swinging ring  129 , and the hammer bit  119  is caused to perform a striking movement via the striker  143  and the impact bolt  145 . At this time, like in the first embodiment, the counter weight  153  rotates on the stem  153   c  in a direction opposite to the cylindrical piston  141  in the axial direction of the hammer bit  119 . In this manner, vibration caused in the hammer drill  101  in the axial direction of the hammer bit  119  can be reduced. 
   Further, when the counter weight  153  rotates on the stems  153   c  in the axial direction of the hammer bit  119 , as shown in  FIGS. 12 to 14 , the operating piece  153   e  on the counter weight  153  vertically rotates. When the operating piece  153   e  rotates in one direction (downward in this embodiment), the operating piece  153   e  linearly moves the slider  155  of the dynamic vibration reducer  171  and presses the biasing spring  177 , which in turn moves the weight  173  in the direction of pressing the biasing spring  177 . Specifically, the weight  173  can be actively driven and forcibly excited. Therefore, the dynamic vibration reducer  171  can be steadily operated regardless of the magnitude of vibration which acts upon the hammer drill  101 . As a result, the hammer drill  101  can ensure a sufficient vibration reducing function by actively driving the weight  173  even when, for example, a user performs a hammering operation or a hammer drill option while applying a strong pressing force to the hammer drill  101 , or even in such operating conditions in which, although vibration reduction is highly required, the vibration magnitude inputted to the dynamic vibration reducer  171  may be reduced due to the pressing force so that the dynamic vibration reducer  171  cannot sufficiently function. 
   As described above, according to this embodiment, the counter weigh  153  and the dynamic vibration reducer  171  are used in combination. Therefore, with both the vibration reducing functions of the counter weigh  153  and the dynamic vibration reducer  171 , a further higher vibration reducing effect can be obtained. 
   Particularly in this embodiment, the operating piece  153   e  is disposed on the counter weight  153  provided for vibration reduction, and the operating piece  153   e  drives the slider  155  and inputs an excitation force to the dynamic vibration reducer  171 . With this construction, it is not necessary to additionally provide an operating mechanism specifically designed as a means for inputting the excitation force, so that simplification in structure can be attained. 
   Fourth Representative Embodiment 
   The vibration reducing mechanism  151  according to a fourth representative embodiment of the present invention is now explained with reference to  FIGS. 15 to 17 .  FIG. 15  shows an internal mechanism disposed within the gear housing  107 .  FIGS. 16 and 17  are sectional views of the vibration reducing mechanism part.  FIG. 17  shows the assembling procedure of the vibration reducing mechanism part. Like in the first and second embodiments, the vibration reducing mechanism  151  of the fourth embodiment also includes a counter weight  183  which is driven by the swinging ring  129 . Except for the counter weight  183 , the fourth embodiment has the same construction as the first embodiment. Components or elements in the fourth embodiment which are substantially identical to those in the first embodiment are given like numerals as in the first embodiment and will not be described. The counter weight  183  is a feature that corresponds to the “counter weight” according to the present invention. 
   As shown in  FIG. 16 , the counterweight  183  includes right and left arms  183   b  and right and left weight concentration parts  183   e.  A lower end portion  183   a  of the counter weight  183  is connected to the swinging ring  129 , and in this state, the arms  183   b  extend upward. The weight concentration parts  183   e  are provided on the arms  183   b  and serve as a vibration reducing weight. The counter weight  163  is generally U-shaped as viewed from the front or the back of the hammer drill  101 . In this embodiment, the arms  183   b  and the weight concentration parts  183   e  are formed as separate members. The arms  183   b  and the weight concentration parts  183   e  are features that correspond to the “connecting part” and the “weight part”, respectively, according to the present invention. 
   A circular engagement hole  183   d  is firmed in the lower end portion  183   a  of the alms  183   b.  The protrusion  129   a  extends downward from the lower end region of the swinging ring  129  and is loosely engaged in the engagement hole  183   d  for free relative movement. Thus, the arms  183   b  are connected to the swinging ring  129 . Further, the arms  183   b  extend upward past the side of the swinging rig  129  and to a position slightly higher than the axis of the hammer bit  119 . A circular stem hole  183   c  is formed through the extending end portion of each of the arms  183   b.  The stem holes  183   c  are rotatably engaged with stems (bosses)  106   d  of a weight supporting portion  107   c  formed on the inner housing  107   a.  Thus, the counter weight  183  can rotate on the stems  106   d  in the axial direction of the hammer bit  119 . The stems  106   d  and the stem holes  183   c  are features that correspond to the “stem” and the “hole”, respectively, according to the present invention. 
   The arms  183   b  are shaped into a predetermined fom, or generally U-shaped having the engagement hole  183   a  in the lower end portion  183   a,  the stem holes  183   c  in the extending end portions of the arms, and a plurality of weight mounting holes  183   f  generally in the middle of the arms in the extending diction, by sheet metal processing such as cutting, bending and hole making. The distance between the opposed extending end portions of the arms  183   b  can be changed by elastic deformation of the arms  183   b.  Therefore, assembly of the counter weight  183  to the weight supporting portion  107   c  of the inner housing  107   a,  or engagement of the stem holes  183   c  of the arms  183   b  with the stems  106   d  of the weight supporting portion  107   c  can be achieved by utilizing deformation of the arms  183   b  as shown in  FIG. 17 . The weight concentration parts  183   e  are shaped, for example, into a rectangular block by casting and fastened to the arms  183   b  using fastening means such as rivets  185  through the weight mounting holes  183   f  in the arms  183   b.    
   According to the fourth embodiment constructed as described above, in hammering operation using the hammer drill  101 , the counter weight  183  performs a function to reduce impulsive and cyclic vibration caused in the axial direction of the hammer bit  119 . Thus, the same vibration-reducing effect can be obtained with the vibration reducing mechanism  151  as in the first and second embodiments. 
   According to the fourth embodiment, the arms  183   b  and the weight concentration parts  183   e  are formed as separate members. Therefore, in manufacturing the counter weight  183 , the shapes and configurations of the arms  183   b  and the weight concentration parts  183   e  can be properly set individually in consideration of individual functions. 
   The arms  183   b  to transmit the movement of the swinging ring  129  to the counter weight  183  is formed by sheet metal processing, so that the arms  183   b  can be made thinner and thus lighter in weight while ensuing the strength required to transmit the movement of the swinging ring  129 . As for the weight concentration parts  183   e,  the weight required to reduce vibration caused during operation can be readily ensured. As a result, the vibration reducing effect can be optimized while the counterweight  183  is reduced in weight as a whole. Further, by mass reduction of the component parts other than the weight concentration parts  183   e,  unnecessary vibration can be reduced which may be caused by movement of the counter weight  183 . Further, the manufacturing cost of the counter weight  183  can be reduced with the arms  183   b  made of sheet metal. 
   Further, according to the fourth embodiment, the arms  183   b  can be assembled to the stems  106   d  of the weight supporting portion  107   c  on the body side by utilizing deformation of the arms  183   b.  Specifically, a biasing force is applied to the arms  183   b  in a direction that widens the distance between the opposed arms  183   b,  and the stem holes  183   c  are aligned to the stems  107   c.  Thereafter, the force is released, so that the stem holes  183   c  can be fitted onto the stems  106   d.  Thus, the assembling operation can be easily performed. Further, with the construction in which the counter weight  183  is assembled by utilizing deformation of the arms  183   b,  the counter weight  183  as a whole can be made compact. Further, the arms  183   b  forming the stem holes  183   c  need not have a two-part structure having front and rear section&amp;. Thus, simplification in structure can be attained. 
   Further, in the above-described embodiments, the swinging ring  129  of the swinging mechanism is described as being supported for relative rotation at a predetermined inclination angle by the intermediate shaft  125  and caused to swing in the axial direction of the intermediate shaft  125  when the intermediate shaft  125  rotates. However, the construction of the swinging mechanism is not limited to this. Specifically, the swinging ring  129  may be mounted such that it is inclined at a predetermined angle with respect to the axis of the intermediate shaft and rotates together with the intermediate shaft. Thus, the swinging mechanism may be constructed such that the swinging ring is caused to swing in the axial direction while rotating together with the intermediate shaft when the intermediate shaft rotates. Further, in the above-described embodiments, the hammer drill  101  is described as an representative example of the power impact toot but the present invention can be applied not only to the hammer drill  101  but also to a hammer which performs only hammering operation. 
   Further, in the fourth embodiment, the stem holes  183  may be formed on the arm support portion  107   c  side, and the stems  106   d  on the arms  183   b  side. 
   Description of Numerals 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
               101 
               hammer drill (power impact tool) 
             
             
                 
               103 
               body 
             
             
                 
               105 
               motor housing 
             
             
                 
               107 
               gear housing 
             
             
                 
               107a 
               inner housing 
             
             
                 
               107b 
               support plate 
             
             
                 
               107c 
               arm supporting portion 
             
             
                 
               107d 
               stem 
             
             
                 
               109 
               handgrip 
             
             
                 
               109a 
               trigger 
             
             
                 
               111 
               driving motor 
             
             
                 
               113 
               motion converting mechanism 
             
             
                 
               114 
               power transmitting mechanism 
             
             
                 
               115 
               striking mechanism 
             
             
                 
               119 
               hammer bit (tool bit) 
             
             
                 
               121 
               driving gear 
             
             
                 
               123 
               driven gear 
             
             
                 
               124 
               engaging member 
             
             
                 
               125 
               intermediate shaft (rotating shaft) 
             
             
                 
               126 
               bearing 
             
             
                 
               127 
               rotating element 
             
             
                 
               128 
               swinging rod 
             
             
                 
               129 
               swinging ring (swinging member) 
             
             
                 
               129a 
               protrusion 
             
             
                 
               131 
               first transmission gear 
             
             
                 
               133 
               second transmission gear 
             
             
                 
               135 
               cylinder 
             
             
                 
               137 
               tool holder 
             
             
                 
               141 
               cylindrical piston 
             
             
                 
               141a 
               air chamber 
             
             
                 
               143 
               striker 
             
             
                 
               145 
               impact bolt 
             
             
                 
               151 
               vibration reducing mechanism 
             
             
                 
               153 
               counter weight 
             
             
                 
               153a 
               lower end portion 
             
             
                 
               153b 
               arm 
             
             
                 
               153c 
               stem (pivot point) 
             
             
                 
               153d 
               engagement hole 
             
             
                 
               153e 
               operating piece 
             
             
                 
               155 
               slider 
             
             
                 
               155a 
               vent 
             
             
                 
               163 
               counter weight 
             
             
                 
               163a 
               lower end portion 
             
             
                 
               163b 
               arm 
             
             
                 
               163c 
               stem (pivot point) 
             
             
                 
               163d 
               engagement hole 
             
             
                 
               163e 
               weight concentration part 
             
             
                 
               171 
               dynamic vibration reducer 
             
             
                 
               172 
               cylindrical body 
             
             
                 
               172a 
               vent 
             
             
                 
               173 
               weight 
             
             
                 
               176 
               actuation chamber 
             
             
                 
               177 
               biasing spring (elastic element) 
             
             
                 
               183 
               counter weight 
             
             
                 
               183a 
               lower end portion 
             
             
                 
               183b 
               arm (connecting part) 
             
             
                 
               183c 
               stem hole (hole) 
             
             
                 
               183d 
               engagement hole 
             
             
                 
               183e 
               weight concentration part (weight part) 
             
             
                 
               183f 
               weight mounting hole 
             
             
                 
               185 
               rivet