Abstract:
An impact tool is provided that can efficiently reduce the vibration resulting from the striker and that does not lead to a larger design even with the use of a counterweight mechanism. A counterweight mechanism is provided in a motion converter housing, facing the handle. The counterweight mechanism is positioned between a center of gravity of the impact tool and the grip of the handle and is positioned above the control substrate. The counterweight mechanism is equipped with a first support, a second support, a weight support member, and a counterweight.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an impact tool, and more specifically to an impact tool having a vibration control mechanism. 
   2. Description of the Related Art 
   Conventionally, electrical power tools having vibration control mechanisms have been proposed. For example, Japanese Patent Application Publication No. 2004-299036 discloses an electrical power tool including a casing that has a handle, a motor housing, and a gear housing connected with one another. An electrical motor is accommodated in the motor housing. The gear housing has a motion conversion housing, a vibration control housing, and an impact housing. A motion conversion mechanism that converts a rotation motion of the electrical motor into a reciprocation motion is provided in the motion conversion housing. A cylinder extending a direction perpendicular to the rotation axis of the electrical motor is provided in the impact housing. A tool support portion is provided on the front side of the cylinder and is capable of attaching or detaching a working tool. 
   A piston is provided in the cylinder and is slidably provided along the inner periphery of the cylinder. The piston reciprocates along the inner periphery of the cylinder by the motion conversion mechanism. A striking member is provided in the front section of the cylinder and is slidably provided along the inner periphery of the cylinder. An air chamber is formed in the cylinder between the piston and the striking member. An intermediate member is provided in the front side of the striking member and is slidably provided back-and-forth within the cylinder. The working tool mentioned above is positioned at the front side of the intermediate element. 
   The vibration control housing is provided on the side of the impact housing and communicates with the impact housing by way of an air channel. A space formed by the piston, the cylinder, the impact housing, the counterweight, and the vibration control housing is formed as a sealed space. A counterweight and two springs are provided in the vibration control housing. The counterweight is capable of moving a reciprocation motion parallel to the reciprocation motion of the piston. The two springs are positioned at the ends of the counterweight. 
   The rotational driving force of the electrical motor is transmitted to the motion conversion mechanism, and the motion conversion mechanism moves the piston in the cylinder in the reciprocation motion. The reciprocation motion of the piston repeatedly increases and decreases the pressure of the air in the air chamber, thereby applying an impact force to the striking member. The striking member moves forward and collides with the rear end of the intermediate member, thereby applying the impact force to the working tool. The workpiece is fractured by the impact force applied to the working tool. 
   During the operation of the electrical power tool, when the piston moves forward, the counterweight moves rearward because the space formed by the piston, the cylinder, the impact housing, the counterweight, and the vibration control housing is a sealed space. Conversely, when the piston moves rearward, the counterweight moves forward. Thus, in this structure, the counterweight reciprocates in conjunction with the reciprocation motion of the piston. 
   However, in the electrical power tool described above, when the counterweight vibrates, the friction between the two springs and the vibration control housing prevents the counterweight from vibrating efficiently. Thus, the vibration caused by the striking member cannot be reduced efficiently. The vibration control housing is provided on the side of the impact housing, the electrical power tool, thereby leading to as increased size in the electrical power tool. 
   SUMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide an impact tool that is capable of efficiently reducing the vibration resulting from the striking member and that does not lead to an increased size even with the use of a counterweight mechanism. 
   The above and other objects of the present invention can be attained by an impact tool including: a housing, a motor, a reciprocating motion converter, a tool bit, a handle, and a counterweight mechanism. The motor has a rotation shaft, is accommodated in the housing and generates a rotational drive force when powered. The reciprocating motion converter is configured to convert the rotational drive force of the motor to a reciprocating motion reciprocating in directions perpendicular to the rotation shaft of the motor. The tool bit is attached to one end portion of the housing and driven by the reciprocating motion of the reciprocating motion converter. The handle is positioned at another end portion of the housing. The counterweight mechanism is operable to reduce vibrations generated attendant to the reciprocating motion. Such counterweight mechanism is disposed in a section of the housing, the section being in confrontation with the handle. 
   In the impact tool thus arranged, the counterweight mechanism is disposed in the section of the housing and the section in which the counterweight mechanism is disposed is in confrontation with the handle. Thus, the size of the impact tool does not increase even with the presence of the counterweight mechanism. 
   It is preferable to further include a control substrate configured to control the rotational drive force generated by the motor wherein the control substrate is disposed in confrontation with the section in which the counterweight mechanism is disposed. 
   With such an arrangement, an open space in the housing facing the substrate can be used efficiently, and the size of the impact tool does not increase even with the presence of the counterweight mechanism. 
   It is also preferable to include a transfer shaft interposed between the rotation shaft of the motor and the reciprocating motion converter. The transfer shaft transfers the rotational drive force generated by the motor to the reciprocating motion converter, wherein the counterweight mechanism is disposed between the transfer shaft and the handle. 
   It is preferable that the counterweight mechanism include a counterweight and a counterweight support member supporting the counterweight, and wherein a center of gravity of the counterweight is positioned further toward the reciprocating motion converter than a line passing through the center of gravity and extending in parallel to the directions of the reciprocating motion. 
   Preferably, the handle has a grip. The counterweight is disposed between the grip and a center of gravity of the impact tool. Thus, it is possible to reduce the rotational moment centered around the center of gravity G resulting from the reciprocating motion of the reciprocating motion converter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
       FIG. 1  is a cross-sectional view showing an impact tool according to a first embodiment of the present invention; 
       FIG. 2  is a rear-view of a counterweight mechanism of the impact tool according to the first embodiment of the present invention; 
       FIG. 3  is a cross-sectional view showing an impact tool according to a second embodiment of the present invention; 
       FIG. 4  is an exploded view showing a counterweight mechanism of the impact tool according to the second embodiment of the present invention; 
       FIG. 5  is an enlarged view showing the counterweight mechanism of the impact tool according to the second embodiment of the present invention; 
       FIG. 6  is a cross-sectional view showing an impact tool according to a third embodiment of the present invention; 
       FIG. 7  is a cross-sectional view showing an impact tool according to a fourth embodiment of the present invention; 
       FIG. 8  is a cross-sectional view showing an impact tool according to a fifth embodiment of the present invention; 
       FIG. 9  is a cross-sectional view showing an impact tool according to a sixth embodiment of the present invention; 
       FIG. 10  is a cross-sectional view showing the impact tool taken along a line X-X in  FIG. 9 ; and 
       FIG. 11  is a cross-sectional view showing the counterweight mechanism of the impact tool according to a modification of the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An impact tool according to a first embodiment of the present invention will be described while referring to  FIGS. 1 and 2 . In  FIG. 1 , the left side will be described as the front side of the impact tool  1  and the right side will be described as the back side of the impact tool  1 . The impact tool  1  includes a casing having a handle  10 , a motor housing  20 , and a gear housing  30  connected with one another. 
   A power cable  11  is attached to the handle  10 . The handle  10  houses a switch mechanism  12 . A trigger  13  that can be manipulated by the user is mechanically connected to the switch mechanism  12 . The switch mechanism  12  is connected to an external power source (not shown) through the power cable  11 . By operating the trigger  13 , an electrical motor  21  described later can be connected to and disconnected from the external power source. Also, the handle  10  includes a grip  14  that is gripped by the user when the impact tool  1  is used. 
   The motor housing  20  is positioned at a lower front side of the handle  10 . The electrical motor  21  is accommodated in the motor housing  20 . The electrical motor  21  includes an output shaft  22  that outputs a driving force of the electrical motor. A pinion gear  23  is provided on the end of the output shaft  22  and is positioned in the gear housing  30 . A control unit  24  for controlling a rotation speed of the electrical motor  21  is located on the motor housing  20  behind the electrical motor  21 . 
   The gear housing  30  includes a motion conversion housing  31  and a hammer housing  32 . The motion conversion housing  31  is positioned above the motor housing  20  and a rear end of the motion conversion housing  31  is connected to the handle  10 . The hammer housing  32  is positioned above the motor housing  20 . 
   A crank shaft  34  that extends parallel to the output shaft  22  is rotatably supported on the rear side of the pinion gear  23  in the motion conversion housing  31 . A first gear  35  that meshingly engaged with the pinion gear  23  is coaxially fixed to the lower end of the crank shaft  34 . A motion conversion mechanism  36  is provided at the upper side of the crank shaft  34 . The motion conversion mechanism  36  includes a crank weight  37 , a crank pin  38 , and a connecting rod  39 . The crank weight  37  is fixed to the upper end of the crank shaft  34 . The crank pin  38  is fixed to the end portion of the crank weight  37 . The crank pin  38  is inserted into the rear end of the connecting rod  39 . 
   A rotation transmission shaft  51  extending parallel to the output shaft  22  is rotatably supported on the front side of the pinion gear  23  in the motion conversion housing  31 . A second gear  52  that meshingly engaged with the pinion gear  23  is coaxially fixed to the lower end of a rotation transmission shaft  51 . A first bevel gear  51 A is coaxially fixed to the upper end of the rotation transmission shaft  51 . 
   A cylinder  40  extending in a direction perpendicular to the output shaft  22  is provided in the hammer housing  32 . The center axis of the cylinder  40  and the rotation axis of the output shaft  22  are positioned on a same plane. The rear end of the cylinder  40  opposes the electrical motor  21  in the axial direction of the output shaft  22 . A piston  43  is provided in the cylinder  40  and is slidably provided along the inner periphery of the cylinder  40 . The piston  43  reciprocates in the axial direction of the cylinder  40 . The piston  43  includes a piston pin  43 A that inserted into the front end of the connecting rod  39 . A striking member  44  is provided in the front section of the cylinder  40  and is slidably provided along the inner periphery of the cylinder  40  in the axial direction thereof. An air chamber  45  is formed among the cylinder  40 , the piston  43 , and the hammer  44 . 
   A rotating cylinder  50  is rotatably supported in the hammer housing  32 . The rotating cylinder  50  surrounds the front section of the outer perimeter of the cylinder  40 . The rotating cylinder  50  extends forward of the cylinder  40 , and a tool support portion  15  is provided at the end of the rotating cylinder  50  and is capable of attaching or detaching a working tool (not shown) . A second bevel gear  50 A that meshingly engaged with the first bevel gear  51 A is provided on the rear end portion of the rotating cylinder  50 . The center axis of the rotating cylinder  50  and the rotation axis of the output shaft  22  are positioned on a same plane. Also, an intermediate member  46  is provided in the front side of the striking member  44  and is slidably provided against the rotating cylinder  50 . The intermediate member  46  reciprocates in the axial direction of the rotating cylinder  50 . 
   A counterweight mechanism  70  is provided in the motion conversion housing  31  and in opposition to the handle  10 . The counterweight mechanism  70  is positioned between a center of gravity G of the impact tool  1  and the grip  14  of the handle  10  and is positioned above the control unit  24 . The counterweight mechanism  70  will be described while referring to  FIGS. 1 and 2 . The counterweight mechanism  70  includes a pair of support members  71 , a pair of support members  72 , a counterweight holding member  73 , and a counterweight  74 . The support members  71  and  72  are positioned on a plane perpendicular to the reciprocating direction of the piston  43 . The support members  71  oppose the support members  72  on the plane. The pair of support members  71  is made from rubber and is fixed to the upper section of the motion conversion housing  31 . The pair of support members  72  is made from steel roller and is fixed to the motion conversion housing  31 . 
   The counterweight holding member  73  is made from a leaf spring. The upper end portion of the counterweight holding member  73  has an L-shaped, is positioned between the pair of support members  71  and is supported by the support members  71  with line contacts. Since the pair of support members  71  is made from rubber, the upper end portion of the counterweight holding member  73  is supported by the support members  71  while being capable of moving up and down with respect to the support members  71 . The lower end portion of the counterweight holding member  73  is positioned between the pair of support members  72  and is supported by the support members  72  with line contacts. Since the pair of support members  72  is made from the steel roller, the lower end portion of the counterweight holding member  73  is supported by the support members  72  while being capable of moving up and down with respect to the support members  72 . The counterweight  74  is fixed roughly in the vertical center of the counterweight holding member  73  using a bolt  75 . The counterweight  74  is doubly supported at its both ends by the counterweight holding member  73 . As shown in  FIG. 2 , the counterweight  74  includes a base  74 A and two legs  74 B. The base  74 A extends in a direction perpendicular to the extending direction of the counterweight holding member  73  and is fixed to the counterweight holding member  73 . Each of the two legs  74 B is connected to the ends of the base  74 A and extends along and is separated from the counterweight holding member  73 . Hence, the counterweight  74  has an H-shaped. 
   Next, the operation of the impact tool  1  according to the first embodiment will be described. The working tool (not shown) is pressed against a workpiece (not shown) with the handle  10  gripped by the user. Next, the trigger  12  is pulled to supply power to and rotate the electrical motor  21 . This rotation driving force is transmitted to the crank shaft  34  by way of the pinion gear  23  and the first gear  35 . The rotation of the crank shaft  34  is converted into reciprocation motion of the piston  43  in the cylinder  40  by the motion converter mechanism  36  (the crank weight  37 , the crank pin  38 , and the connecting rod  39 ). The reciprocation motion of the piston  43  leads to repeated increments and decrements the pressure of the air in the air chamber  45 , thereby causing a reciprocation motion of the striking member  44 . The striking member  44  moves forward and collides with the rear end of the intermediate member  46 , thereby applying an impact force to the working tool (not shown). 
   Also, the rotation driving force of the electrical motor  21  is transmitted to the pinion gear  23 , the second gear  52 , and the rotation transmission shaft  51 . The rotation of the rotation transmission shaft  51  is transmitted to the rotating cylinder  50  by way of the first bevel gear  51 A and the second bevel gear  50 A, resulting in rotation of the rotating cylinder  50 . The rotation of the rotating cylinder  50  applies a rotation force to the working tool (not shown). The workpiece (not shown) is fractured by the rotation force and the impact force described above applied to the working tool (not shown). 
   During the operation of the impact tool  1  described above, a vibration with a roughly constant frequency resulting from the reciprocation motion of the striking member  44  is generated in the impact tool  1 . The vibration is transmitted to the support members  71  and  72  by way of the motion conversion housing  31 . The vibration transmitted to the support members  71  and  72  is transmitted to the counterweight holding member  73  and the counterweight  74 , leading to the counterweight  74  vibrating in a direction that the piston  43  reciprocates. The vibration of the impact tool  1  can be reduced by the vibration of the counterweight  74 , thereby improving the operation of the impact tool  1 . 
   More specifically, the vibration of a frequency band having a constant width centering on a resonance frequency is reduced by the vibration of the counterweight  74 . The resonance frequency is determined by the counterweight  74  and the counterweight holding member  73  which is a leaf spring. The resonance frequency is set up to be roughly identical to the frequency of the vibration generated by the impact of the impact tool  1 . A resonance frequency (resonance point) f is f=1/(2 π)((k 1 +k 2 )/m) 1/2 , where the spring constants of the counterweight holding member  73  made from the leaf spring are k 1  (the spring constant of the counterweight holding member  73  positioned higher than the counterweight  74 ), k 2  (the spring constant of the counterweight holding member  73  positioned lower than the counterweight  74 ), and the mass of the counterweight  74  is m. Practically, the actual resonance frequency band will be slightly wider and slightly lower than the theoretical resonance frequency band due to the influence of damping and the like. Thus, the resonance point determined from the above equation is set to be slightly higher than the vibration frequency of the impact tool  1 . 
   Since the counterweight  74  is doubly supported on both ends by the counterweight holding member  73  as described above, rotation moment that would be generated with a cantilevered counterweight can be prevented. Also, the ends of the counterweight holding member  73  are movably supported with respect to the support members  71  and  72 . Hence, no friction is generated between the motion conversion housing  31  and the counterweight  74  and the counterweight holding member  73  made from the leaf spring. Accordingly, the counterweight holding member  73  and the counterweight  74  can be vibrated smoothly in the same directions as the directions for the reciprocation motion of the piston  43 . Thus, the vibration of the impact tool  1  caused by the reciprocation motion of the striking member  44  can be efficiently reduced, thereby improving the operation of the impact tool  1 . Also, since the upper end of the counterweight holding member  73  is the L-shaped, the counterweight holding member  73  can be prevented from slipping out from the support members  71 . Furthermore, the counterweight  74  is the H-shaped. As a result, the length of the counterweight holding member  73  needed to obtain a desired resonance frequency can be reduced, thereby providing a compact overall size for the counterweight mechanism  70 . 
   Since the counterweight mechanism  70  is positioned above the control unit  24  and is disposed in opposition to the handle  10 , the open space above the control unit  24  can be used effectively and enlargement of the impact tool  1  by providing the counterweight mechanism  70  can be prevented. The counterweight mechanism  70  is positioned between the grip  14  and the center of gravity G of the impact tool  1 . Therefore, the rotation moment centering on the center of gravity G caused by the reciprocation motion of the piston  43  can be reduced. Also, since springs supporting the counterweight  74  are not placed at ends of the counterweight  74  in the directions of the reciprocation motion of the piston  43 , as in conventional impact tools, frication between the housing, and the springs and the counterweight  74  can be prevented. Thus, the vibration of the counterweight  74  can be stabilized and efficiently absorbed. 
   Next, an electrical power tool according to a second embodiment of the present invention will be described while referring to  FIGS. 3 through 5 . The electrical power tool of the present invention is applied to an impact tool  101 . Like parts and components that are the same as those of the first embodiment will be assigned the same reference numerals to avoid duplicating descriptions, and only different aspects will be described. The impact tool  101  according to the second embodiment does not include the rotating cylinder  50  and the control unit  24  used in the impact tool  1  of the first embodiment. Therefore, no rotation is applied to the working tool during the operation of the impact tool  1 , and the electrical motor  21  rotates at a fixed speed. 
   As in the impact tool  1  of the first embodiment, a counterweight mechanism  170  is provided in the motion conversion housing  31  and is disposed in opposition to the handle  10 . The counterweight mechanism  170  includes a support member  171 , a pair of support members  172 , a counterweight holding member  173 , and a counterweight  174 . The support member  171  will be described while referring to  FIGS. 4 and 5 . The support member  171  includes a bolt  171 A, a washer  171 B, and a spacer  171 C. The pair of support members  172  is made from rubber. The counterweight holding member  173  is made from a leaf spring and is formed with a bolt insertion hole  173   a . The upper end portion of the counterweight holding member  173  is fixed to the motion conversion housing  31  by inserting the bolt  171 A through the washer  171 B, the spacer  171 C, and the bolt insertion hole  173   a . The lower end portion of the counterweight holding member  173  is positioned between the pair of the support members  172  and is supported by the support members  172  with line contacts. Since the support members  172  is made from rubber, the lower end portion of the counterweight holding member  173  is supported by the support members  172  while being capable of moving up and down with respect to the support members  172 . The counterweight  174  is fixed roughly in the vertical center of the counterweight holding member  173 . 
   The counterweight mechanism  170  of the second embodiment also can be efficiently reduced the vibration of the impact tool  101  caused by the reciprocation motion of the striking member  44 . Also, as described above, the counterweight mechanism  170  includes the bolt  171 A, the washer  171 B, and the spacer  171 C. Thus, by adjusting the tightness of the bolt  171 A, the load applied to the upper end portion of the counterweight holding member  173  can be controlled. Hence, the vibration of the counterweight holding member  173  and the counterweight  174  can be controlled and the resonance frequency of the counterweight mechanism  170  can be adjusted. Other advantages of the impact tool  101  are similar to the advantages of the impact tool  1  according to the first embodiment. 
   Next, an electrical power tool according to a third embodiment of the present invention will be described while referring to  FIG. 6 . The electrical power tool of the present invention is applied to an impact tool  201 . Like parts and components that are the same as those of the first embodiment will be assigned the same reference numerals to avoid duplicating descriptions, and only different aspects will be described. 
   A counterweight mechanism  270  is provided in the motion conversion housing  31  and is disposed in opposition to the handle  10 . The counterweight mechanism  270  is positioned above the control unit  24  and is also positioned above a line that passes through the center of gravity G of the impact tool  201  and that extends parallel to the directions of the reciprocation motion of the piston  43 . The counterweight mechanism  270  includes a pair of support members  271 , a pair of support members  272 , a counterweight holding member  273 , and a counterweight  274 . The pair of support members  271  is made from rubber and is fixed to the upper section of the motion conversion housing  31 . The pair of support members  272  is also made from rubber and is fixed to the motion conversion housing  31 . 
   The counterweight holding member  273  is made from a leaf spring. The upper end portion of the counterweight holding member  273  is positioned between the pair of support members  271  and is supported by the support members  271  with line contacts. Since the pair of support members  271  is made from rubber, the upper end portion of the counterweight holding member  273  is supported by the support members  271  while being capable of moving up and down with respect to the support members  271 . The lower end of the counterweight holding member  273  is positioned between the pair of support members  272  and is supported by the support members  272  with line contact. Since the pair of support members  272  is made from rubber, the lower end portion of the counterweight holding member  273  is supported by the support members  272  while being capable of moving up and down with respect to the support members  272 . Thus, the counterweight  274  is doubly supported on both ends by the counterweight holding member  273 . The counterweight  274  is fixed to roughly in the vertical center of the counterweight holding member  273 . 
   The counterweight mechanism  270  according to the third embodiment also can be efficiently reduced the vibration of the impact tool  201  caused by the reciprocation motion of the striking member  44 . Also, as described above, the counterweight mechanism  270  is positioned above the line that passes through the center of gravity G of the impact tool  201  and that extends parallel to the directions of the reciprocation motion of the piston  43 . Therefore, the rotation moment centering on the center of gravity G caused by the reciprocation motion of the piston  43  can be reduced. Other advantages of the impact tool  201  are similar to the advantages of the impact tool  1  of the first embodiment. 
   Next, an electrical power tool according to a fourth embodiment of the present invention will be described while referring to  FIG. 7 . The electrical power tool of the present invention is applied to an impact tool  301 . Like parts and components that are the same as those of the first embodiment will be assigned the same reference numerals to avoid duplicating descriptions, and only different aspects will be described. 
   The crank shaft  34  is positioned at the front side of the pinion gear  23 . A third gear  34 A is coaxially fixed to the crank shaft  34  on the lower side of the first gear  35 . The rotation transmission shaft  51  is positioned at the front side of the crank shaft  34 . The second gear  52  is meshingly engaged with the third gear  34 A. The rotation of the electrical motor  21  is transmitted to the rotation transmission shaft  51  by way of the pinion gear  23 , the first gear  35 , the third gear  34 A, and the second gear  52 . The rotation of the rotation transmission shaft  51  is transmitted to the rotating cylinder  50  by way of the first bevel gear  51 A and the second bevel gear  50 A, resulting in rotation of the rotating cylinder  50 . The rotation of the rotating cylinder  50  applies a rotation force to a working tool (not shown). 
   A counterweight mechanism  370  is provided in a space above the electrical motor  21 . The space is created by positioning the crank shaft  34  on the front side of the pinion gear  23 . The counterweight mechanism  370  includes a support member  371 , a support member  372 , a counterweight holding member  373 , and a counterweight  374 . The support members  371  and  372  have a U-shaped, and the opening of the support member  371  opposes the opening of the support member  372  with each other. The counterweight holding member  373  is made from a leaf spring, and each end thereof is inserted into the openings of the support members  371  and  372 , respectively. The counterweight holding member  373  is supported by the support members  371  and  372  with line contacts. The counterweight  374  is fixed to roughly in the vertical center of the counterweight holding member  373 . Thus, the counterweight  374  is doubly supported on both ends by the counterweight holding member  373 . 
   The counterweight mechanism  370  according to fourth embodiment also can be efficiently reduced the vibration of the impact tool  301  caused by the reciprocation motion of the striking member  44 . Also, as described above, the counterweight mechanism  370  is positioned in a space above the electrical motor  21  created by positioning the crank shaft  34  on the front side of the pinion gear  23 . Accordingly, the open space above the electrical motor  21  can be used efficiently and enlargement of the impact tool  301  by providing the counterweight mechanism  370  can be prevented. Other advantages of the impact tool  301  are similar to the advantages of the impact tool  1  according to the first embodiment. 
   Next, an electrical power tool according to a fifth embodiment of the present invention will be described while referring to  FIG. 8 . The electrical power tool of the present invention is applied to an impact tool  401 . Like parts and components that are the same as those of the first embodiment will be assigned the same reference numerals to avoid duplicating descriptions, and only different aspects will be described. 
   A counterweight mechanism  470  is provided above the control unit  24  and is disposed in opposition to the handle  10 . The counterweight mechanism  470  includes two support members  471 , four springs  473 , and two counterweights  474 . The two support members  471  extend parallel to the directions of the reciprocation motion of the piston  43  and are fixed to the motion conversion housing  31 . Each of the two counterweights  474  is slidably supported by the support members  471 , respectively. Each of the four springs  473  is positioned on each ends of the counterweights  474  and is interposed between the counterweights  474  and the motion conversion housing  31 . 
   The counterweight mechanism  470  according to this embodiment also can be reduced efficiently the vibration of the impact tool  401 , which is caused by the reciprocation motion of the striking member  44 , by the vibration of the counterweights  474 . Other advantages of the impact tool  401  are similar to the advantages of the impact tool  1  according to the first embodiment. 
   Next, an electrical power tool according to a sixth embodiment of the present invention will be described while referring to  FIGS. 9 and 10 . The electrical power tool of the present invention is applied to an impact tool  501 . The impact tool  501  includes a casing having the handle  10 , the motor housing  20 , a weight housing  60 , and a gear housing  80 . 
   The power cable  11  is attached to the handle  10 . The handle  10  houses the switch mechanism  12 . The trigger  13  that can be manipulated by the user is mechanically connected to the switch mechanism  12 . The switch mechanism  12  is connected to an external power source (not shown) through power cable  11 . By operating the trigger  13 , the switch mechanism  12  can be connected to and disconnected from the external power source. 
   The motor housing  20  is provided on the front side of the handle  10 . The handle  10  and the motor housing  20  are formed integrally from plastic. The electrical motor  21  is accommodated in the motor housing  20 . The electrical motor  21  includes the output shaft  22  and outputs rotational drive force. 
   The weight housing  60  is located on the front side of the motor housing  20  and is made from resin. The weight housing  60  includes a first weight housing  60 A opposing the motor housing  20  and a second weight housing  60 B opposing the gear housing  80 . A first intermediate shaft  61  is provided in the weight housing  60  and extends in a direction that the output shaft  22  extends. The first intermediate shaft  61  is rotatably support by bearings  62  and  63 . The rear end portion of the first intermediate shaft  61  is connected to the output shaft  22 . The front end portion of the first intermediate shaft  61  is positioned in the gear housing  80  and is provided with a fourth gear  61 A. 
   A counterweight mechanism  570  is provided in the weight housing  60 . As shown in  FIG. 10 , which is a cross-sectional view taken along the X-X line in  FIG. 9 , the counterweight mechanism  570  includes support members  571  and  572 , a pair of counterweight holding members  573 , a counterweight  574 , and a bolt  575 . The support members  571  and  572  are provided at the upper and lower end portions of the second weight housing  60 B, respectively. The pair of counterweight holding members  573  is made from leaf springs. As shown in  FIG. 9 , the upper and lower end portions of the counterweight holding members  573  have roughly an L-shaped, and each of the distal ends of the upper and lower end portions of the counterweight holding members  573  is positioned in each of recesses  60   c  formed in the second weight housing  60 B, respectively. The upper end portion of the counterweight holding members  573  is supported by the support member  571 , and the lower end portion of the counterweight holding members  573  is supported by the support member  572 . 
   The counterweight  574  has a roughly circular cross-section and is formed with a shaft insertion hole  574   a  formed at the center thereof. The counterweight  574  is fixed to the counterweight holding members  573  by bolts  575 . Hence, the counterweight  574  is doubly supported on its both ends by the pair of counterweight holding members  573 . The first intermediate shaft  61  is inserted through the shaft insertion hole  574   a.    
   The gear housing  80  is located on the front side of the second weight housing  60 B and is made from resin. A metal partition member  80 A is disposed in the gear housing  80  and partitions the gear housing  80  and the weight housing  60 . The gear housing  80  and the partition member  80 A forms a decelerating chamber  80   a , which is a mechanism chamber accommodating a rotation transmission mechanism described later. A second intermediate shaft  82  is rotatably supported on the gear housing  80  and the partition member  80 A via a bearings  82 B and  82 C, and extends parallel to the output shaft  22 . A side handle  16  is provided near the tool support portion  15  of the gear housing  80 , described later. 
   A fifth gear  81  meshingly engaged with the fourth gear  61 A is coaxially fixed to the second intermediate shaft  82  on the electrical motor  21  side thereof. A gear  82 A is formed on the front end portion of the second intermediate shaft  82  to be meshingly engaged with a sixth gear  83 , described later. A cylinder  84  is provided above the second intermediate shaft  82  in the gear housing  80 . The cylinder  84  extends parallel to the second intermediate shaft  82  and is rotatably supported on the partition member  80 A. The sixth gear  83  is fixed to the outer periphery of the cylinder  84  and is meshingly engaged with the gear  82 A described above so that the cylinder  84  can rotate around its central axial. 
   The tool support portion  15  mentioned above is provided on the front side of the cylinder  84 , and a working tool (not shown) is capable of attaching to or detaching from the tool support portion  15 . A clutch  86  is splined to the intermediate section of the second intermediate shaft  82 . The clutch  86  is urged by a spring toward the electrical motor  21 . The clutch  86  can be switched by means of a change lever  87  positioned below the gear housing  80  between a hammer drill mode (the position shown in  FIG. 9 ) and a drill mode (with the clutch  86  moved toward the front) . A motion converter  90  that converts rotational motion into reciprocation motion is rotatably provided on the outer periphery of the second intermediate shaft  82  on the electrical motor  21  side of the clutch  86 . The motion converter  90  has an arm  90 A that is capable of reciprocating back-and-forth the impact tool  501  as a result of the rotation of the second intermediate shaft  82 . 
   When the clutch  86  is switched to the hammer drill mode using the change lever  87 , the clutch  86  engages the second intermediate shaft  82  with the motion converter  90 . The motion converter  90  is connected to and work with a piston  92  provided in the cylinder  84  through a piston pin  91 . The piston  92  is slidably mounted in the cylinder  84  and is capable of a reciprocation motion parallel to the second intermediate shaft  82 . A striking member  93  is provided in the piston  92  and is slidably provided along the inner periphery of the cylinder  84 . An air chamber  94  is formed among the cylinder  84 , the piston  92 , and the striking member  93 . An intermediate member  95  is supported in the cylinder  84  on the opposite side of the striking member  93  from the air chamber  94 . The intermediate member  95  is slidably provided against the cylinder  84  along the direction of the motion of the piston  92 . A working tool (not shown) is positioned on the opposite side of the intermediate member  95  from the striking member  93 . Hence, the striking member  93  strikes the working tool (not shown) through the intermediate member  95 . 
   Rotation output of the motor  21  is transmitted to the second intermediate shaft  82  by way of the first intermediate shaft  61 , the fourth gear  61 A, and the fifth gear  81 . The rotation of the second intermediate shaft  82  is transmitted to the cylinder  84  by way of the meshing between the gear  82 A and the sixth gear  83  mounted to the outer periphery of the cylinder  84 . When the clutch  86  is in the hammer drill mode by operating the change lever  87 , the clutch  86  is connected to the motion converter  90 . Hence, the rotational driving force of the second intermediate shaft  82  is transmitted to the motion converter  90  through the clutch  86 . The rotational driving force is converted to the reciprocation motion of the piston  92  on the motion converter  90  by way of the piston pin  91 . The reciprocation motion of the piston  92  causes the pressure of the air inside the air chamber  94  formed between the striking member  93  and the piston  92  to repeatedly increase and decrease, thereby causing a reciprocation motion of the striking member  93 . When the striking member  93  moves forward and collides with the rear end of the intermediate member  95 , the impact force is applied to the working tool (not shown) through the intermediate element  95 . In this manner, the rotational force and the impact force are simultaneously applied to the working tool (not shown) in the hammer drill mode. 
   If the clutch  86  is in the drill mode, the clutch  86  disengages the connection between the second intermediate shaft  82  and the motion converter  90 , and only the rotational driving force of the second intermediate shaft  82  is transmitted to the cylinder  84  through the gear  82 A and the sixth gear  83 . Accordingly, only rotational force is applied to the working tool (not shown). 
   When the impact tool  501  according to sixth embodiment is operated, a vibration having a roughly constant frequency is generated in the impact tool  501  due to the reciprocation motion of the striking member  93 . The vibration is transmitted to the support members  571  and  572  by way of the second weight housing  60 B. The vibration transmitted to the support members  571  and  572  is transmitted to the counterweight holding members  573  and the counterweight  574 , and the counterweight  574  vibrates in the same directions as the directions of the reciprocation motion of the piston  92 . The vibration of the impact tool  501  can be reduced by the vibration of the counterweight  574 , thereby improving the operation of the impact tool  501 . 
   Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention. For example, The pair of support members  72  of the impact tool  1  according to the first embodiment is made from steel roller, but the present invention is not limited to the steel roller. Any component having good sliding properties, e.g., an oil-impregnated metal, can be used. 
   In the second embodiment, it would also be possible as shown in  FIG. 11  for the shape of the counterweight  174  to be, when the impact tool  101  is seen from the side, an “H” shape formed from: a base  174 A extending in a direction perpendicular to the direction in which the weight support member  173  extends and secured to the weight support member  173 ; and two legs  174 B extended from the ends of the base  174 A, extending on either side of but separated from the weight support member  173 . As a result, the length of the weight support member  173  needed to obtain a desired resonance frequency can be reduced, making it possible to provide a compact overall design for the counterweight unit.