Patent Publication Number: US-7712547-B2

Title: Electric hammer

Description:
FIELD OF THE INVENTION 
   The present invention relates to a technique for reducing vibration of an electric hammer that performs a hammering operation on a workpiece. 
   BACKGROUND OF THE INVENTION 
   Japanese laid-open patent publication No. 2004-299036 discloses an electric hammer having a dynamic vibration reducer which forms a vibration reducing mechanism. In this hammer, a weight of the dynamic vibration reducer is actively driven by utilizing the pressure within the crank chamber, so that vibration caused during hammering operation can be reduced. 
   Further, Japanese laid-open patent publication No. 2004-216484 discloses an electric hammer having a counter weight which forms a vibration reducing mechanism. In this hammer, the counter weight is driven via a crank mechanism that converts the rotating output of the electric motor into linear motion, and it serves to reduce vibration caused in the hammer during hammering operation. However, further device improvement is desired in both of these known vibration reducing techniques. 
   DISCLOSURE OF THE INVENTION 
   Problems to be Solved by the Invention 
   Accordingly, it is an object of the present invention to provide a technique that contributes to further improvement of the vibration reducing function in an electric hammer. 
   Means for Solving the Problems 
   In order to solve the above-described problem, the present invention provides an electric hammer including an electric hammer body, a hammer bit that is coupled to the body and performs a hammering operation in contact with a workpiece, a driving motor that is housed within the body, a striker that is housed within the body and driven by the driving motor to apply a striking force to the hammer bit, and a vibration reducing mechanism that is linearly driven in an axial direction of the hammer bit and generates vibration, thereby reducing vibration caused in the body. 
   In the electric hammer according to the invention, first mode and second mode are provided. In a first mode, under loaded driving conditions in which a load acts on the hammer bit from the workpiece side by the hammering operation, the vibration reducing mechanism optimizes vibration reduction by generating vibration corresponding to vibration caused in the body. In a second mode, under unloaded driving conditions in which the driving motor is energized and the hammering operation is not performed, while no load acts on the hammer bit from the workpiece side, the vibration reducing mechanism optimizes vibration reduction by generating vibration corresponding to vibration caused in the body. Preferably, by changing at least one or more of the amplitude, frequency and phase of the vibration reducing mechanism, the vibration reducing mechanism may generate optimum vibration for canceling out the vibration caused in the electric hammer and thereby optimizes the vibration reduction of the electric hammer. 
   According to the invention, the amount of drive of the vibration reducing mechanism differs according to whether under the loaded driving conditions in which vibration reduction is highly required or under the unloaded driving condition in which vibration reduction is less required. Specifically, the amount of drive to be provided to the vibration reducing mechanism is changed such that, under the loaded driving conditions, the vibration reducing mechanism generates vibration corresponding to vibration caused under the loaded driving conditions, while, under the unloaded driving conditions, the vibration reducing mechanism generates vibration corresponding to vibration caused under the unloaded driving conditions. In this manner, suitable vibration reducing effects can be obtained under each of the loaded and unloaded driving conditions. For example, when a dynamic vibration reducer is used as the vibration reducing mechanism, it is preferable that the frequency of the dynamic vibration reducer is set to be in the region of the maximum stroke of the striker which strikes the hammer bit. In this case, the frequency of the weight of the dynamic vibration reducer may preferably be generally equal to this natural frequency. 
   During hammering operation, the load conditions of the hammer bit based on an external force acting on the hammer bit from the workpiece side may preferably be detected by the magnitude of the load current of the driving motor, and the vibration reducing mechanism may be controlled according to the detected load conditions. As a result, the structure can be simplified compared with the known method of detecting the load conditions of the hammer bit by using a mechanical detecting mechanism. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional side view schematically showing an entire electric hammer according to a first embodiment of the invention. 
       FIG. 2  is a sectional partial view showing a counter weight driving mechanism and a stroke changing mechanism. 
       FIG. 3  is a plan view showing the counter weight driving mechanism and the stroke changing mechanism, in the state of the maximum stroke of the counter weight. 
       FIG. 4  is a plan view showing the counter weight driving mechanism and the stroke changing mechanism, in the state of the minimum stroke of the counter weight. 
       FIG. 5  is a sectional view taken along line V-V in  FIG. 4 . 
       FIG. 6  is a view taken from the direction of arrow VI. 
       FIG. 7  is a schematic view illustrating the setting conditions of the counter weight driving mechanism. 
       FIG. 8  is a schematic view illustrating a path of movement of a counter weight driving pin when a stroke changing gear is locked in a certain position and a carrier is rotated. 
       FIG. 9  is a schematic view illustrating a path of movement of the counter weight driving pin when the stroke changing gear is locked in a certain position and the carrier is rotated. 
       FIG. 10  is a view showing a dynamic vibration reducer having a vibration means according to a second embodiment. 
       FIG. 11  is a sectional side view schematically showing an entire electric hammer according to a third embodiment of the invention. 
       FIG. 12  is a sectional plan view showing an essential part of the electric hammer according to the third embodiment, with a piston located in right dead center. 
       FIG. 13  is a sectional plan view showing the essential part of the electric hammer according to the third embodiment, with the piston located in left dead center. 
       FIG. 14  is a view illustrating the vibration reducing effect of the dynamic vibration reducer during hammering operation. 
   

   REPRESENTATIVE EMBODIMENTS OF THE INVENTION 
   First Representative Embodiment of the Invention 
   An electric hammer (hereinafter referred to as hammer) according to a first representative embodiment of the present invention will now be described with reference to the drawings.  FIG. 1  shows an entire hammer  101  according to this embodiment. The hammer  101  according to this embodiment includes a hammer body  103  having a motor housing  105 , a gear housing  107  and a handgrip  111 . A hammer bit  113  is coupled to the tip end (the left end region as viewed in  FIG. 1 ) of the hammer body  103  via a hammer bit mounting chuck  109 . 
   The motor housing  105  houses a driving motor  121 . The gear housing  107  houses a crank mechanism  131 , an air cylinder mechanism  133  and a striking force transmitting mechanism  135 . A tool holder  137  for holding the hammer bit  113  is disposed on the end (left end as viewed in  FIG. 1 ) of the striking force transmitting mechanism  135  within the gear housing  107 . The crank mechanism  131  in the gear housing  107  converts the rotating output of an output shaft  123  of the driving motor  121  into linear motion and transmits the motion to the hammer bit  113 . As a result, the hammer bit  113  is caused to perform a hammering operation. The tool holder  137  holds the hammer bit  113  in such a manner that the hammer bit  113  can reciprocate with respect to the tool holder  137  in its longitudinal direction and is prevented from rotating in its circumferential direction with respect to the tool holder  137 . 
   The crank mechanism  131  is disposed right below a housing cap  108  within the gear housing  107  and includes a speed change gear  141 , a gear shaft  143 , a gear shaft support bearing  145  and a crank pin  147 . The speed change gear  141  engages with a gear part  125  of the output shaft  123  of the driving motor  121 . The gear shaft  143  rotates together with the speed change gear  141 . The gear shaft support bearing  145  rotatably supports the gear shaft  143 . The crank pin  147  is integrally formed with the speed change gear  141  in a position displaced a predetermined distance from the center of rotation of the gear shaft  143 . The crank pin  147  is connected to one end of a crank arm  159 . The other end of the crank arm  159  is connected to a driver in the form a piston  163  via a connecting pin  161 . The piston  163  is disposed within a bore of a cylinder  165  that forms the air cylinder mechanism  133 . The piston  163  slides within the cylinder  165  so as to linearly drive the striker  134  by the action of an air spring of an air spring chamber  165   a . As a result, the piston  163  generates impact loads upon the hammer bit  113  via an intermediate element in the form of an impact bolt  136 . The striker  134  and the impact bolt  136  form the striking force transmitting mechanism  135 . The striker  134  is a feature that corresponds to the “striker” in the present invention. 
     FIGS. 2 to 4  show a counter weight driving mechanism  173  and a stroke changing mechanism  185 . The counter weight driving mechanism  173  drives a counter weight  171  that serves to reduce vibration when the hammer bit  113  is driven. The stroke changing mechanism  185  serves to change the linear stroke of the counter weight  171 .  FIG. 2  is a sectional partial view, and  FIGS. 3 and 4  are plan views. The counter weight  171  is a feature that corresponds to the “vibration reducing mechanism” in this invention, and the counter weight driving mechanism  173  and the stroke changing mechanism  185  are features that correspond to the “power transmitting mechanism” in this invention. The counter weight  171  is disposed above the housing cap  108  and can be moved linearly in the axial direction of the hammer bit  113 . The counter weight  171  has a guide slot  171   b  extending in the axial direction of the hammer bit  113 . A plurality of (two in this embodiment) guide pins  172  extend through the guide slot  171   b  and guide the counter weight  171  to move linearly in the axial direction of the hammer bit  113 . The guide pins  172  are fixedly mounted to the housing cap  108 . 
   The counter weight driving mechanism  173  is disposed between the crank mechanism  131  and the counter weight  171  and serves to cause the counter weight  171  to reciprocate in a direction opposite to the reciprocating direction of the striker  134 . The counter weight driving mechanism  173  includes an internal gear  175 , a planetary gear  179 , a carrier  181  and a counter weight driving pin  183 . The planetary gear  179  engages with internal teeth  175   a  of the internal gear  175  via a plurality of (three in this embodiment) idle gears  177 . The carrier  181  rotatably supports the planetary gear  179  and the idle gears  177 . The counter weight driving pin  183  is integrally formed with the planetary gear  179  in a position displaced a predetermined distance from the center of rotation of the planetary gear  179  with respect to the carrier  181 . The counter weight driving pin  183  is a feature that corresponds to the “power transmitting part” in this invention. 
   The carrier  181  is rotatably supported by the housing cap  108  via a carrier support bearing  182 . An engagement recess  181   a  is formed in the underside of the carrier  181  and engages with a top pin part  147   a  of the crank pin  147  of the crank mechanism  131  (see  FIG. 1 ). Thus, when the crank pin  147  rotates, the carrier  181  is caused to rotate around an axis parallel to the axis of rotation of the speed change gear  141 . The planetary gear  179  has a shaft  179   a  that is rotatably supported by the carrier  181 . Each of the idle gears  177  has a shaft  177   a  that is press-fitted into the carrier  181 , and the idle gear  177  is rotatably supported by the shaft  177   a . The internal gear  175  is rotatably supported by the housing cap  108  and is normally prevented from rotating by the stroke changing mechanism  185 . 
   The counter weight driving pin  183  is slidably fitted in a slot  171   a  that is formed in the counter weight  171  and extends linearly in a direction perpendicular to the axial direction of the hammer bit  113 . When the carrier  181  is rotated by the crank pin  147  in the state in which the rotation of the internal gear  175  is prevented, the planetary gear  179  that engages with the internal gear  175  via the idle gears  177  revolves around the center of rotation of the internal gear  175  while rotating around the shaft  179   a . At this time, the counter weight  117  is caused to reciprocate by components of motion of the counter weight driving pin  183  in the axial direction of the hammer bit  113 . Thus, the counter weight  171  reciprocates in a direction generally opposite to the reciprocating direction of the striker  134  that is driven by the crank mechanism  131  via the air cylinder mechanism  133 . 
   The stroke changing mechanism  185  for the counter weight  171  will now be explained with reference to  FIGS. 2 to 6 .  FIG. 5  is a sectional view taken along line V-V in  FIG. 4 .  FIG. 6  is a view taken from the direction of arrow VI. The stroke changing mechanism  185  can change the rotation prevented position of the internal gear  175  so that the stroke of the counter weight driving pin  183  in the axial direction of the hammer bit  113  and thus the linear stroke of the counter weight  171  in the axial direction of the hammer bit  113  can be changed. Thus, the stroke changing mechanism  185  forms a stroke control mechanism of the counter weight  171 . The internal gear  175  has external teeth  175   b  on its outer peripheral surface. In the following description, the internal gear  175  is referred to as externally-toothed internal gear  175 . 
   The stroke changing mechanism  185  includes a stroke changing gear  189  that engages with the external teeth  175   b  of the externally-toothed internal gear  175  via an intermediate gear  187  at all times, a worm wheel  191  that rotates together with the stroke changing gear  189 , a worm gear  193  that engages with the worm wheel  191  at all times, and an auxiliary motor  195  that drives the worm gear  193 . Specifically, the stroke changing mechanism  185  is powered from the auxiliary motor  195  and rotates the externally-toothed internal gear  175 . As shown in  FIG. 5 , a magnet  199  is installed in the stroke changing gear  189 . A first sensor  197  and a second sensor  198  for detecting the magnet  199  are disposed on the housing cap  108  and arranged with a phase difference of 180° around the center of rotation of the stroke changing gear  189 . The first sensor  197  and the second sensor  198  are provided to detect a rotation prevented position of the externally-toothed internal gear  175  and output respective positioning signals for positioning the counter weight driving pin  183  in predetermined respective positions. Specifically, when the first sensor  197  detects the magnet  199 , the first sensor  197  outputs a signal for positioning the counter weight driving pin  183  in a position (shown in  FIG. 3 ) for loaded driving. When the second sensor  198  detects the magnet  199 , the second sensor  198  outputs a signal for positioning the counter weight driving pin  183  in a position (shown in  FIG. 4 ) for unloaded driving. The auxiliary motor is then stopped according to this signal. Thus, the stroke changing gear  189  is locked for every 180° rotation. The first and the second sensors  197 ,  198  and the magnet  199  are features that correspond to the “positioning means” according to this invention. 
   The load current of the driving motor  121  that drives the hammer bit  113  increases under loaded driving conditions in which the hammer bit  113  is subjected to a load caused by a hammering operation (external force or reaction force that is inputted from the workpiece side to the hammer bit  113  during hammering operation), while it decreases under unloaded driving conditions in which the hammer bit  113  is not subjected to a load caused by a hammering operation. In consideration of this phenomenon, in this embodiment, a motor controller  122  (motor control circuit, see  FIG. 1 ) for controlling the drive of the driving motor  121  detects the driving conditions, loaded or unloaded, by change (increase or decrease) of the load current of the driving motor  121 . Based on this detection result, a driving signal is outputted to the auxiliary motor  195 . Specifically, in the driving state of the hammer  101 , when the load current of the driving motor  121  exceeds a threshold value, it is determined that it has been shifted from the unloaded driving conditions to the loaded driving conditions. On the other hand, when the load current of the driving motor  121  decreases below the threshold value, it is determined that it has been shifted from the loaded driving conditions to the unloaded driving conditions. In the both cases, respective driving signals are outputted to the auxiliary motor  195 . 
   The once started auxiliary motor  195  is stopped according to the detection signal which the first sensor  197  or the second sensor  198  outputs when it detects the magnet  199 . As a result, after started, the stroke changing gear  189  is rotated 180° and then stopped and locked in that position. The motor controller  122  (motor control circuit) for controlling the drive of the driving motor  121  detects change of the load current of the driving motor  121 . Based on this detection result, a driving signal is outputted to the auxiliary motor  195 . Further, the worm gear  193  is designed to have a small lead angle such that the worm gear  193  is provided with a reverse rotation preventing function of preventing it from being caused to rotate from the worm wheel  191  side. Thus, the internal gear  175  is held in the rotation prevented state when the auxiliary motor  195  is in the stopped state. The rotation prevented state corresponds to the “rest state” according to this invention. 
   The hammer  101  according to this embodiment is constructed as described above. Specifically, in the hammer  101 , the stroke of the counter weight driving pin  183  in the axial direction of the hammer bit can be changed by changing the rotation prevented position of the externally-toothed internal gear  175 . With this construction, the linear stroke of the counter weight  171 , which is driven by the counter weight driving pin  183 , in the axial direction of the hammer bit  113  can be changed. The principle will now be explained. 
   In this embodiment, the number of the teeth of the planetary gear  179  is chosen to be half of the number of the internal teeth  175   a  of the externally-toothed internal gear  175 . In other words, the planetary gear  179  turns two turns on its center while revolving one turn around the center of the externally-toothed internal gear  175 . Further, the number of the teeth of the stroke changing gear  189  is chosen to be half of the number of the external teeth  175   b  of the internal gear  175 . As schematically shown in  FIG. 7 , the distance between the axis of rotation of the carrier  181  and the axis of rotation of the planetary gear  179  is designated by r 1 , and the distance between the axis of rotation of the planetary gear  179  and the axis of rotation of the counter weight driving pin  183  is designated by r 2 . 
   When the stroke changing gear  189  (and thus the externally-toothed internal gear  175 ) is locked in a certain position and the carrier  181  is rotated, as schematically shown in  FIG. 8 , the counter weight driving pin  183  moves along an elliptic path having a major axis of (r 1 +r 2 ) and a minor axis of (r 1 −r 2 ). When (r 1 −r 2 )=0, the stroke of the counter weight driving pin  183  in the direction of the minor axis is zero. When the above locked position of the stroke changing gear  189  is rotated 180°, the counter weight driving pin  183  moves along an elliptic path shown in  FIG. 9 , which path is obtained by rotating the path in  FIG. 8  by 90°. Specifically, when the stroke changing gear  189  is locked for every 180° rotation, the path of the counter weight driving pin  183  can be switched between the states shown in  FIGS. 8 and 9 . Therefore, if the counter weight  171  is mounted onto the counter weight driving pin  183 , the linear stroke of the counter weight  171  in the axial direction of the hammer bit can be switched between the longer stroke of {2×(r 1 +r 2 )} and the shorter stroke of {2×(r 1 −r 2 )}. 
   In this embodiment, as shown in  FIG. 3 , when the planetary gear  179  is located in the rear end region (or the front end region) of the internal gear  175  in the axial direction of the hammer bit, the counter weight driving pin  183  is located in the nearest position to the point of proximity of the planetary gear  179  to the internal gear  175 . Further, as shown in  FIG. 4 , when the planetary gear  179  is located in the rear end region (or the front end region) of the internal gear  175  in the axial direction of the hammer bit  113 , the counter weight driving pin  183  is located in the remotest position from the point of proximity of the planetary gear  179  to the internal gear  175 . In the state shown in  FIG. 3 , the first sensor  197  detects the magnet  199  and locks the stroke changing gear  189 . In the state shown in  FIG. 4 , the second sensor  198  detects the magnet  199  and locks the stroke changing gear  189 . Specifically, rotation of the stroke changing gear  189  is prevented with a phase difference of 180° according to the detection of the magnet  199  by the first sensor  197  and the second sensor  198 . Thus, the internal gear  175  which has the external teeth  175   b  twice as many as the teeth of the stroke changing gear  189  is prevented from rotating with the phase difference of 90° between its rotation prevented positions. 
   Operation and usage of the hammer  101  will now be explained. When the driving motor  121  is driven, the piston  163  is caused to reciprocate within the bore of the cylinder  165  via the output shaft  123 , the speed change gear  141 , the crank pin  147 , the crank arm  159  and the connecting pin  161 . At this time, under the loaded driving conditions in which the hammer bit  113  is pressed against the workpiece, the hammer bit  113  is driven linearly in its axial direction via the air cylinder mechanism  131  and the striking force transmitting mechanism  135 . Specifically, when the piston  163  slides toward the hammer bit  113 , which causes an air spring action of the air spring chamber  165   a  that is defined between the piston  163  and the striker  134 , the striker  134  is caused to reciprocate in the same direction within the cylinder  165  by the air spring action and collides with the impact bolt  136 . The kinetic energy (striking force) of the striker  134  which is caused by the collision is transmitted to the hammer bit  113 . Thus, the hammer bit  113  slidingly reciprocates within the tool holder  137  and performs a hammering operation on the workpiece. Large vibration is caused in the hammer  101  in the axial direction of the hammer bit  113  during the loaded driving conditions. Therefore, reduction of such vibration is highly desired. 
   Under unloaded driving conditions in which the hammer bit  113  is not pressed against the workpiece, an idle hammering preventing mechanism is actuated. Specifically, the air spring chamber  165   a  communicates with the outside via a vent hole, so that air within the air spring chamber  165   a  is not compressed. The idle hammering preventing mechanism is known and will not be specifically described below. Thus, the striker  134  is not driven. Therefore, vibration is caused in the hammer  101  in the axial direction of the hammer bit  113  mainly by reciprocating movement of the piston  163 . Such vibration is smaller than under the loaded driving conditions and less desired to be reduced. 
   When the driving motor  121  is shifted, for example, from the unloaded driving conditions to the loaded driving conditions, the load on the driving motor  121  increases, and thus the load current of the driving motor  121  increases. When the load current exceeds a threshold value, a driving signal is outputted to the auxiliary motor  195 , and the auxiliary motor  195  is driven. Then the stroke changing gear  189  is rotated via the worm gear  193  and the worm wheel  191 . When the stroke changing gear  189  is rotated 180° and the first sensor  197  detects the magnet  199 , the auxiliary motor  195  is stopped according to the detection signal. By the 180° rotation of the stroke changing gear  189 , the externally-toothed internal gear  175  is rotated 90° via an intermediate gear  187 . Then the planetary gear  179  is shifted from the state shown in  FIG. 4  to the state shown in  FIG. 3 . When the planetary gear  179  is located in the rear end region (or the front end region) of the externally-toothed internal gear  175  in the axial direction of the hammer bit  113 , the counter weight driving pin  183  is located in the nearest position to the point of proximity of the planetary gear  179  to the internal gear  175 . In this state, when the counter weight driving pin  183  revolves while rotating, the counter weight driving pin  183  has a longer stroke in the axial direction of the hammer bit as schematically shown in  FIG. 8 . By utilizing the stroke of the counter weight driving pin  183 , the counter weight  171  is driven in the axial direction of the hammer bit  113  and in a direction opposite to the reciprocating direction of the striker  134 . In this manner, the counter weight  171  can efficiently reduce vibration during hammering operation of the hammer bit  113 . 
   On the other hand, when the driving motor  121  is shifted from the loaded driving conditions to the unloaded driving conditions, the load on the driving motor  121  decreases, and thus the load current of the driving motor  121  decreases below the threshold value. As a result, a driving signal is outputted to the auxiliary motor  195 , and the auxiliary motor  195  is driven. Then the stroke changing gear  189  is rotated 180° and the second sensor  197  detects the magnet  199 . At this time, the auxiliary motor  195  is stopped according to the detection signal. By the 180° rotation of the stroke changing gear  189 , the externally-toothed internal gear  175  is rotated 90° via the intermediate gear  187 . Then the planetary gear  179  is shifted from the state shown in  FIG. 3  to the state shown in  FIG. 4 . When the planetary gear  179  is located in the rear end region (or the front end region) of the internal gear  175  in the axial direction of the hammer bit  113 , the counter weight driving pin  183  is located in the remotest position from the point of proximity of the planetary gear  179  to the internal gear  175 . In this state, when the counter weight driving pin  183  revolves while rotating, the counter weight driving pin  183  has a shorter stroke in the axial direction of the hammer bit as schematically shown in  FIG. 9 . In this case, when r 1 −r 2 =0 in  FIG. 9 , the apparent stroke of the counter weight driving pin  183 , which is located in the remotest position from the point of proximity of the planetary gear  179  to the internal gear  175 , is zero in the axial direction of the hammer bit even though the planetary gear  179  revolves. 
   As a result, under unloaded driving conditions, even if the planetary gear  179  revolves around the center of rotation of the externally-toothed internal gear  175 , the counter weight driving pin  183  does not move in the axial direction of the hammer bit. In other words, under unloaded driving conditions in which vibration reduction is less desired, even though the driving motor  121  is driven and the planetary gear  179  revolves around the center of rotation of the internal gear  175 , the counter weight driving pin  183  does not drive the counter weight  171  in the longitudinal direction of the hammer  101 . Therefore, undesired vibration can be prevented from being caused when the counter weight  171  is driven. The linear stroke of the counter weight  171  was described above as zero, but the counter weight  171  may be driven with a linear stroke corresponding to the magnitude of the vibration caused when the piston  163  is driven. 
   As described above, according to this embodiment, the load current of the driving motor  121  is electrically detected under the loaded and unloaded driving conditions, and the linear stroke of the counter weight  171  is controlled based on the detection. Therefore, compared with the known method of detecting loaded and unloaded driving conditions by using a mechanical detecting mechanism and changing the linear stroke of the counter weight  171  based on the detection, the vibration reducing control system can be simplified. 
   As described above, according to this embodiment, the load current of the driving motor  121  is electrically detected under the loaded and unloaded driving conditions, and the linear stroke of the counter weight  171  is controlled based on the detection. Therefore, compared with the known method of detecting loaded and unloaded driving conditions by using a mechanical detecting mechanism and changing the linear stroke of the counter weight  171  based on the detection, the vibration reducing control system can be simplified. 
   Further, in this embodiment, under the loaded and unloaded driving conditions, respective vibration reductions for the loaded driving conditions and the unloaded driving conditions are performed by changing the linear stroke of the counter weight  171 . In place of the construction in which the linear stroke of the counter weight  171  is changed, the number of linear strokes of the counterweight  171  may be changed. Specifically, under the loaded driving conditions, the driving motor  121  may be driven at a predetermined number of revolutions, so that the counter weight  171  is driven with a predetermined number of linear strokes corresponding to vibration under the loaded driving conditions. While, under the unloaded driving conditions, the driving motor  121  may be driven at a lower speed than under the loaded driving condition, so that the counter weight  171  is driven with a lower number of linear strokes than under the loaded driving conditions. Alternative to this construction, only the number of linear strokes of the counter weight  171  may be reduced, for example, via a speed reducing means, without changing the number of revolutions of the driving motor  121 , so that the counter weight  171  is driven with a lower number of linear strokes than under the loaded driving conditions. 
   Second Representative Embodiment of the Invention 
   A second representative embodiment of the present invention will now be described with reference to  FIG. 10 . In the second embodiment, a dynamic vibration reducer  211  is used in place of the counter weight  171  as a vibration reducing mechanism. As to other elements, the second representative embodiment has the same construction as the above-described first embodiment except for a mechanism for driving the counter weight  171  and a mechanism for changing the linear stroke of the counter weight  171 . 
   The dynamic vibration reducer  211  mainly includes a cylindrical body  213  that is disposed adjacent to the hammer body  103 , a weight  215  that is made of iron (magnetic material) and disposed within the cylindrical body  213 , and biasing springs  217  that are disposed on the right and left sides of the weight  215 . The biasing springs  217  are features that correspond to the “elastic element” according to this invention. The biasing springs  217  exert a spring force on the weight  215  in a direction toward each other when the weight  215  moves in the axial direction of the cylindrical body  213  (in the axial direction of the hammer bit  113 ). A first actuation chamber  219  and a second actuation chamber  221  are defined on the both sides of the weight  215  within the cylindrical body  213 . 
   The dynamic vibration reducer  211  according to this invention includes a solenoid  223  as a forcible vibration means for forcibly causing vibration in the dynamic vibration reducer  211  by actively driving the weight  215 . In this specification, forcibly causing vibration in the dynamic vibration reducer  211  is referred to as forced vibration. The solenoid  223  mainly includes a frame  225  that is disposed on the axial end of the outer periphery of the cylindrical body  213 , a solenoid coil  227  in the frame  225 , and a weight  215  that corresponds to a movable core. The solenoid  223  applies a voltage to the solenoid coil  227  and thus supplies solenoid current. The solenoid  223  attracts the weight  215  against the biasing force of the biasing spring  217  and thus actively drives the weight  215 . As a result, the dynamic vibration reducer  211  generates vibration. In this case, the frequency of vibration generated by the dynamic vibration reducer  211  is appropriately adjusted by changing the frequencies of energization and de-energization of the solenoid coil  227 , or by changing the operating cycle of the solenoid  223 . Further, the amplitude of vibration generated by the dynamic vibration reducer  211  is appropriately adjusted by changing the value of current to be passed to the solenoid coil  227 . Moreover, the phase of vibration generated by the dynamic vibration reducer  211  is appropriately adjusted by changing the timing of operation for passing the current to the solenoid  227 . 
   During the hammering operation, when the load current of the driving motor  121  is larger than the threshold value, it is determined that it is under the loaded driving conditions in which the hammer bit  113  is subjected to a load caused by the hammering operation. At this time, the solenoid coil  227  is controlled such that the dynamic vibration reducer  211  generates vibration corresponding to the vibration caused in the axial direction of the hammer bit under the loaded driving conditions. On the other hand, when the load current of the driving motor  121  is smaller than the threshold value, it is determined that it is under the unloaded driving conditions in which the hammer bit  113  is not subjected to a load caused by the hammering operation. At this time, the solenoid coil  227  is controlled such that the dynamic vibration reducer  211  generates smaller vibration than under the loaded driving conditions. Otherwise, the solenoid coil  227  is kept in the de-energized state, so that the weight  215  is not actively driven. 
   With the above-described construction, under loaded driving conditions in which vibration reduction is highly desired, the solenoid  223  forcibly vibrates the dynamic vibration reducer  211  such that the dynamic vibration reducer  211  generates vibration corresponding to the magnitude of vibration caused in the hammer body  103 . In this manner, the dynamic vibration reducer  211  can reduce vibration under loaded driving conditions. On the other hand, under unloaded driving conditions in which vibration reduction is less desired, the solenoid  223  forcibly vibrates the dynamic vibration reducer  211  such that the dynamic vibration reducer  211  generates vibration corresponding to the magnitude of vibration caused in the hammer body  103 . Or the counter weight  215  serves as a passive dynamic vibration reducer  211  which is driven with an external force of vibration of the hammer body  103 . In this manner, the dynamic vibration reducer  211  can reduce vibration under unloaded driving conditions. The mode in which the dynamic vibration reducer  211  optimizes vibration reduction under loaded driving conditions corresponds to the “first mode”, and the mode in which the dynamic vibration reducer  211  optimizes vibration reduction under unloaded driving conditions corresponds to the “second mode”, according to this invention. 
   According to this invention, the solenoid  223  is controlled based on the detection of the load current of the driving motor  121 , so that the dynamic vibration reducer  211  can be operated in respective appropriate manners for the loaded driving conditions and the unloaded driving conditions. Therefore, like in the first embodiment, a simpler vibration reducing control system can be realized. Further, the degree of freedom of installation location of the dynamic vibration reducer  211  can be increased by using the solenoid  223  as a means for forcibly vibrating the dynamic vibration reducer  211 . 
   Third Representative Embodiment of the Invention 
   A third representative embodiment of the present invention will now be described with reference to  FIGS. 11 to 14 .  FIG. 11  is a sectional side view showing the entire construction of a hammer  301  according to this embodiment.  FIGS. 12 and 13  are sectional plan views showing an essential part of the hammer  301 .  FIG. 14  is a view illustrating a vibration reducing effect of the dynamic vibration reducer when the hammer is driven. 
   The hammer  301  according to this embodiment includes a hammer body  303  having a motor housing  305 , a gear housing  307  and a handgrip  311 . A hammer bit  313  is coupled to the tip end (the left end region as viewed in the drawings) of the hammer body  303  via a hammer bit mounting chuck  309 . 
   The motor housing  305  houses a driving motor  321 . The gear housing  307  houses a crank mechanism  331 , an air cylinder mechanism  333  and a striking force transmitting mechanism  335 . A tool holder  337  for holding the hammer bit  313  is disposed on the end (left end as viewed in  FIG. 11 ) of the striking force transmitting mechanism  335  within the gear housing  307 . The crank mechanism  331  in the gear housing  307  appropriately converts the rotating output of an output shaft  323  of the driving motor  321  into linear motion and transmits the motion to the hammer bit  313 . As a result, the hammer bit  313  is caused to perform a hammering operation. The tool holder  337  holds the hammer bit  313  in such a manner that the hammer bit  313  can reciprocate with respect to the tool holder  337  in its longitudinal direction and is prevented from rotating in its circumferential direction with respect to the tool holder  337 . The crank mechanism  331  is a feature that corresponds to the “motion converting mechanism” according to this invention. 
   The crank mechanism  331  includes a speed change gear  341 , a gear shaft  133 , a gear shaft support bearing  345  and a crank pin  347 . The speed change gear  341  engages with a gear part  325  of the output shaft  323  of the driving motor  321 . The gear shaft  143  rotates together with the speed change gear  341 . The gear shaft support bearing  345  rotatably supports the gear shaft  343 . The crank pin  347  is integrally formed with the speed change gear  341  in a position displaced a predetermined distance from the center of rotation of the gear shaft  343 . The crank pin  347  is connected to one end of a crank arm  359 . The other end of the crank arm  359  is connected to a driver in the form a piston  363  via a connecting pin  361 . The piston  163  is disposed within a bore of a cylinder  365  that forms the air cylinder mechanism  333 . The speed change gear  341 , the crank pin  347  and the crank arm  359  are disposed within a crank chamber  367 . The crank chamber  367  is a feature that corresponds to the “motion converting mechanism chamber” according to this invention. The crank chamber  367  is prevented from communication with the outside by a sealing structure which is not shown. The effective capacity of the crank chamber  367  periodically increases or decreases according to the movement of the piston  363  which is moved within the cylinder  365  via the crank arm  359 . The piston  363  slides within the cylinder  365  so as to linearly drive the striker  334  by the action of an air spring of an air spring chamber  365   a . As a result, the piston  363  generates impact loads upon the hammer bit  313  via an intermediate element in the form of an impact bolt  336 . The striker  334  and the impact bolt  336  form the striking force transmitting mechanism  335 . The striker  334  is a feature that corresponds to the “striker” in the present invention. 
   As shown in  FIGS. 12 and 13 , the hammer  301  according to this embodiment has a dynamic vibration reducer  371 . The dynamic vibration reducer  371  is a feature that corresponds to the “vibration reducing mechanism” according to this invention. The dynamic vibration reducer  371  mainly includes a cylindrical body  373  that is disposed adjacent to the hammer body  303 , a weight  375  that is disposed within the cylindrical body  373 , and biasing springs  377  that are disposed on the right and left sides of the weight  375 . The biasing springs  377  are features that correspond to the “elastic element” according to this invention. The biasing springs  377  exert a spring force on the weight  375  in a direction toward each other when the weight  375  moves in the axial direction of the cylindrical body  373  (in the axial direction of the hammer bit). A first actuation chamber  379  and a second actuation chamber  381  are defined on the both sides of the weight  375  within the cylindrical body  373 . The first actuation chamber  379  communicates with the crank chamber  367  via a first communication part  383  at all times. 
   When the hammer  301  is driven, the piston  363  linearly moves within the cylinder  365 , so that the capacity of the crank chamber  363  which is sealed against the atmosphere changes. For example, when the piston  363  moves from the left dead center position shown in  FIG. 13  to the right dead center position shown in  FIG. 12 , the capacity of the crank chamber  363  increases, so that the pressure within the crank chamber  363  decreases. Such pressure fluctuations are transmitted to the first actuation chamber  379  of the dynamic vibration reducer  371  via the first communication part  383 . Therefore, when the capacity of the crank chamber  367  decreases and thus the pressure of the crank chamber  367  increases, the weight  375  is acted upon by a force in the direction of the arrow shown in  FIG. 12 . On the other hand, when the capacity of the crank chamber  367  increases and thus the pressure of the crank chamber  367  decreases, the weight  375  is acted upon by a force in the direction of the arrow shown in  FIG. 13 . Specifically, when the hammer  301  is driven, the dynamic vibration reducer  371  actively drives the weight  375  by pressure fluctuations transmitted from the crank chamber  367  and thereby forcibly vibrates the dynamic vibration reducer  371 . In the following description, forcibly vibrating the dynamic vibration reducer  371  is referred to as forced vibration. The pressure transmitted to the first actuation chamber  379  forcibly vibrates the dynamic vibration reducer  371  and forms the forcible vibration means for the dynamic vibration reducer  371 . Specifically, the pressure provides the dynamic vibration reducer  371  with a driving force of forcibly vibrating the dynamic vibration reducer  371 . 
   As described in the first embodiment, the load current of the driving motor  321  that drives the hammer bit  313  increases under loaded driving conditions in which the hammer bit  313  is subjected to a load caused by a hammering operation (external force or reaction force that is inputted from the workpiece side to the hammer bit  313  during hammering operation), while it decreases under unloaded driving conditions in which the hammer bit  313  is not subjected to a load caused by a hammering operation. In consideration of this technical aspect, a motor controller  322  (motor control circuit, see  FIG. 11 ) for controlling the drive of the driving motor  121  detects change of the load current of the driving motor  321 . Based on this detection result, the number of revolutions of the driving motor  321  is controlled. Specifically, in the driving state of the hammer  301 , when the load current of the driving motor  321  exceeds a threshold value, it is determined that it has been shifted from the unloaded driving conditions to the loaded driving conditions. At this time, the driving motor  321  is controlled to be driven at a predetermined high number of revolutions. On the other hand, when the load current of the driving motor  121  decreases below the threshold value, it is determined that it has been shifted from the loaded driving conditions to the unloaded driving conditions. At this time, the driving motor  321  is controlled to be driven at a lower number of revolutions than under the loaded driving conditions. 
   Operation and usage of the hammer  301  having the above-described construction will now be explained. When the driving motor  321  is driven, the piston  363  is caused to reciprocate within the bore of the cylinder  365  via the output shaft  323 , the speed change gear  341 , the crank pin  347 , the crank arm  359  and the connecting pin  361 . At this time, under the loaded driving conditions in which the hammer bit  313  is pressed against the workpiece, the hammer bit  313  is driven linearly in its axial direction via the air cylinder mechanism  331  and the striking force transmitting mechanism  335 . Specifically, when the piston  363  slides toward the hammer bit  313 , which causes an air spring action of the air spring chamber  365   a  that is defined between the piston  363  and the striker  334 , the striker  334  is caused to reciprocate in the same direction within the cylinder  365  by the air spring action and collides with the impact bolt  336 . The kinetic energy (striking force) of the striker  334  which is caused by the collision is transmitted to the hammer bit  313 . Thus, the hammer bit  313  slidingly reciprocates within the tool holder  337  and performs a hammering operation on the workpiece. 
   The dynamic vibration reducer  371  disposed in the hammer body  303  serves to reduce impulsive and cyclic vibration caused when the hammer bit  313  is driven as mentioned above. Specifically, the weight  375  and the biasing springs  377  which serve as vibration reducing elements in the dynamic vibration reducer  371  cooperate to passively reduce vibration of the hammer body  303  on which a predetermined external force (vibration) is exerted. At the same time, the dynamic vibration reducer  371  also acts as an active vibration reducing mechanism by forced vibration or by actively driving the weight  375  by utilizing the pressure fluctuations of the crank chamber  367 . Thus, vibration caused in the hammer body  303  can be effectively alleviated or reduced during hammering operation. 
   Specifically, when the hammer  301  is driven and the piston  363  linearly moves within the cylinder  365 , the capacity of the crank chamber  367  changes and thus the pressure within the crank chamber  367  increases or decreases. Such pressure fluctuations of the crank chamber  367  are transmitted to the first actuation chamber  379  of the dynamic vibration reducer  371  via the first communication part  383 . Therefore, when the pressure of the first actuation chamber  379  increases, the weight  375  is acted upon by a force in the direction of the arrow shown in  FIG. 12 . On the other hand, when the pressure of the first actuation chamber  379  decreases, the weight  375  is acted upon by a force in the direction of the arrow shown in  FIG. 13 . Specifically, when the hammer  301  is driven, the weight  375  of the dynamic vibration reducer  371  is actively driven by pressure fluctuations transmitted from the crank chamber  367 . 
   At this time, when the weight  375  linearly moves within the cylindrical body  373 , the outside air is introduced into or discharged from the second actuation chamber  381  through a second communication part  385  formed in the second actuation chamber  381 . With this construction, when the weight  375  moves, expansion (adiabatic expansion) or compression (adiabatic compression) of the inner space of the second actuation chamber  381  can be effectively prevented which will be caused if air communication with the outside is interrupted. 
   Under the loaded driving conditions in which the hammer bit  313  is subjected to a load caused by a hammering operation, as described above, the driving motor  321  is driven at a predetermined high number of revolutions. The dynamic vibration reducer  371  is configured to effectively reduce vibration caused in the hammer body  303  in the axial direction of the hammer bit under the loaded driving conditions. For example, it is configured such that the vibration generated by the dynamic vibration reducer  371  by forced vibration corresponds in magnitude to vibration caused in the axial direction of the hammer bit under the loaded driving conditions and such that the vibrations are caused in opposite phase. Further, the natural frequency of the dynamic vibration reducer  371  is set to be in the region of the maximum stroke of the striker  334  which strikes the hammer bit  313  under the loaded driving conditions. Thus, the dynamic vibration reducer  371  can effectively reduce vibration under the loaded driving conditions. 
   In the hammer  301  having the above-described construction, in this embodiment, under the unloaded driving conditions in which the hammer bit  313  is not subjected to a load caused by a hammering operation, the number of revolutions of the driving motor  321  is reduced below that under the loaded driving conditions, so that the vibration generated by the dynamic vibration reducer  371  is also reduced. Under the unloaded driving conditions, the striker  334  and the hammer bit  313  are not driven by the idle hammering preventing mechanism (which is a known technique and will not be described) of the hammer  301 . Therefore, under the unloaded driving conditions, vibration in the axial direction of the hammer bit is mainly caused by reciprocating movement of the piston  363 . Such vibration is smaller than under the loaded driving conditions and the phase changes. In this embodiment, the number of revolutions of the driving motor  321  is reduced under the unloaded driving conditions. With this arrangement, vibration generated by the dynamic vibration reducer  371  is reduced, and the frequency of this vibration is displaced from the natural frequency of the dynamic vibration reducer  371 . Further, the phase is changed. In this manner, the vibration reducing effect under the unloaded driving conditions can be enhanced. 
   The vibration reducing effect of the dynamic vibration reducer  371  during hammer driving is now explained with reference to  FIG. 14 .  FIG. 14  shows the results of an experiment on vibration in the axial direction of the hammer bit. This experiment was conducted, with the dynamic vibration reducer  371  installed in the hammer  301 , both in the operating and non-operating conditions of the dynamic vibration reducer  371 , both under the loaded and unloaded driving conditions. In order to keep the total weight of the hammer  301  constant so as to keep the experimental conditions unchanged, the experiment was conducted, with the dynamic vibration reducer  371  installed in the hammer  301 , both in the operating and non-operating conditions of the dynamic vibration reducer  371 . In  FIG. 14 , vibrations of the hammer body  303  during operation of the dynamic vibration reducer  371  (vibration after vibration reduction) are plotted by circles. Specifically, in this case, vibrations under the loaded and unloaded driving conditions are plotted by solid circles and outline circles, respectively. Further, vibrations of the hammer body  303  during non-operation of the dynamic vibration reducer  371  are plotted by rhombuses. Specifically, in this case, vibrations under the loaded and unloaded driving conditions are plotted by solid rhombuses and outline rhombuses, respectively. 
   According to the experimental results, when the dynamic vibration reducer  371  is in the non-operating condition, under the loaded driving conditions, vibration caused in the hammer body  303  in the axial direction of the hammer bit by driving of the hammer  301  gradually increases with increase of the number of strokes. Under the unloaded driving conditions, such vibration increases with increase of the number of strokes at a lower increase rate than under the loaded driving conditions. On the other hand, when the dynamic vibration reducer  371  is in the operating condition, under the loaded driving conditions, vibration caused in the hammer body  303  in the axial direction of the hammer bit by driving of the hammer  301  gradually decreases with increase of the number of strokes and thereafter increases from a certain point. Under the unloaded driving conditions, such vibration decreases with increase of the number of strokes and thereafter increases from a certain point. As clearly seen from the results of the experiment in the operating conditions of the dynamic vibration reducer  371 , optimum vibration reducing effect under the loaded driving conditions is exerted when the number of strokes is around a region shown by A in the drawing, while optimum vibration reducing effect under the unloaded driving conditions is exerted when the number of strokes is around a region shown by B in the drawing. Therefore, under the loaded driving conditions, optimum vibration reduction by the dynamic vibration reducer  371  can be realized by driving the driving motor  213  at such a number of revolutions that the number of strokes is around the region A. Under the unloaded driving conditions, optimum vibration reduction by the dynamic vibration reducer  371  can be realized by driving the driving motor  213  at such a number of revolutions that the number of strokes is around the region B. 
   According to this embodiment, the loaded or unloaded driving conditions during hammering operation are detected by change of the load current of the driving motor  321 . Then the pressure for driving the weight  375 , or the amount of drive to be provided to the dynamic vibration reducer  371  is changed between loaded driving mode in which the dynamic vibration reducer  371  optimizes the vibration reducing effect by generating vibration corresponding to vibration caused under the loaded driving conditions, and unloaded driving mode in which the dynamic vibration reducer  371  optimizes the vibration reducing effect by generating vibration corresponding to vibration caused under the unloaded driving conditions. With this construction, optimum vibration reducing effect of the dynamic vibration reducer  371  can be obtained both under the loaded and unloaded driving conditions. The loaded driving mode and the unloaded driving mode are features that correspond to the “first mode” and the “second mode”, respectively, according to this invention. 
   DESCRIPTION OF NUMERALS 
   
       
         101  electric hammer 
         103  hammer body 
         105  motor housing 
         107  gear housing 
         108  housing cap 
         109  hammer bit mounting chuck 
         111  handgrip 
         113  hammer bit 
         121  driving motor 
         123  output shaft 
         125  output shaft gear part 
         131  crank mechanism 
         133  air cylinder mechanism 
         134  striker 
         135  striking force transmitting mechanism 
         136  impact bolt 
         137  tool holder 
         141  speed change gear 
         143  gear shaft 
         145  gear shaft support bearing 
         147  crank pin 
         147   a  top pin part 
         159  crank arm 
         161  connecting pin 
         163  piston (driver) 
         165  cylinder 
         165   a  air spring chamber 
         171  counter weight (vibration reducing mechanism) 
         171   a  slot 
         171   b  guide slot 
         172  guide pin 
         173  counter weight driving mechanism (power transmitting mechanism) 
         175  externally-toothed internal gear 
         175   a  internal teeth 
         175   b  external teeth 
         177  idle gear 
         177   a  shaft 
         179  planetary gear 
         179   a  shaft 
         181  carrier 
         181   a  engagement recess 
         182  carrier support bearing 
         183  counter weight driving pin (power transmitting part) 
         185  stroke changing mechanism (power transmitting mechanism) 
         187  intermediate gear 
         189  stroke changing mechanism 
         191  wormwheel 
         193  worm gear 
         195  auxiliary motor 
         197  first sensor 
         198  second sensor 
         199  magnet 
         211  dynamic vibration reducer (vibration reducing mechanism) 
         213  cylindrical body (body) 
         215  weight 
         217  biasing spring (elastic element) 
         219  first actuation chamber 
         221  second actuation chamber 
         223  solenoid 
         225  frame 
         227  solenoid coil 
         301  electric hammer 
         303  hammer body 
         305  motor housing 
         307  gear housing 
         308  housing cap 
         309  hammer bit mounting chuck 
         311  handgrip 
         313  hammer bit 
         321  driving motor 
         323  output shaft 
         325  output shaft gear part 
         331  crank mechanism (motion converting mechanism) 
         333  air cylinder mechanism 
         334  striker 
         335  striking force transmitting mechanism 
         336  impact bolt 
         337  tool holder 
         341  speed change gear 
         343  gear shaft 
         345  gear shaft support bearing 
         347  crank pin 
         347   a  top pin part 
         359  crank arm 
         361  connecting pin 
         363  piston (driver) 
         365  cylinder 
         365   a  air spring chamber 
         367  crank chamber (motion converting mechanism chamber) 
         371  dynamic vibration reducer (vibration reducing mechanism) 
         373  cylindrical body (body) 
         375  weight 
         377  biasing spring (elastic element) 
         379  first actuation chamber 
         381  second actuation chamber 
         383  first communication part 
         385  second communication part