Patent Publication Number: US-8991517-B2

Title: Reaction force cushioning mechanism for an impact tool

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an impact tool for performing a linear hammering operation on a workpiece, and more particularly to a technique for cushioning a reaction force during hammering operation. 
     2. Description of the Related Art 
     Hammering operation by an impact tool is performed with a hammer bit being pressed against a workpiece by application of user&#39;s forward pressing force to a tool body. At this time, the hammer bit is pushed to the tool body side (rearward) and an impact bolt is retracted together with the hammer bit and comes in contact with a tool body side component. 
     By such contact, the tool body is positioned with respect to the workpiece. In this state, when the hammer bit performs a striking movement, the hammer bit is caused to rebound by receiving a reaction force from the workpiece and the reaction force is transmitted to the tool body. Therefore, a reaction force cushioning mechanism for cushioning the striking reaction force is provided in prior art impact tools. For example, Japanese non-examined laid-open Patent Publication No. 2008-279587 discloses such an impact tool. 
     In the known impact tool, however, further improvement is desired to realize size reduction. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an effective technique for realizing size reduction while providing an effect of cushioning a striking reaction force caused during operation, in an impact tool. 
     In order to solve the above-described problem, in a preferred embodiment according to the invention, an impact tool performs a predetermined operation on a workpiece at least by an axial linear movement of a tool bit which is mounted in a front end region of a tool body. The impact tool includes a reaction force transmitting member, a first elastic member and a second elastic member. The reaction force transmitting member is arranged to be movable in an axial direction of the tool bit and moves rearward by receiving a striking reaction force which is caused when the tool bit strikes the workpiece. The first elastic member biases the reaction force transmitting member forward. The second elastic member is pushed by the reaction force transmitting member and compressively deforms, thereby cushioning the striking reaction force, when the reaction force transmitting member moves rearward by receiving the striking reaction force. The “predetermined operation” in this invention suitably includes not only a hammering operation in which the tool bit performs only striking movement in its axial direction, but a hammer drill operation in which it performs striking movement in its axial direction and a rotation around its axis, The “first and second elastic members” in this invention typically comprise a compression coil spring, but suitably include rubber. 
     According to the preferred embodiment of the invention, an initial load of the first elastic member is set to be smaller than an initial load of the second elastic member. In operation, when a user presses the tool bit against the workpiece, the reaction force transmitting member is pushed by the tool bit and compresses the first elastic member, while it comes in contact with the second elastic member in an incompressible state, so that it is placed in a predetermined working position in the longitudinal direction. When the reaction force transmitting member receives the striking reaction force in the working position, the reaction force transmitting member moves rearward in the axial direction of the tool bit and compressively deforms the second elastic member, thereby cushioning the striking reaction force. The first and second elastic members are arranged in tandem in the axial direction of the tool bit. The “initial load” here refers to a load which is applied to the first and second elastic members in the direction of compression in advance and under which the elastic members are mounted. In this case, the initial load of the second elastic member is set to be larger than the user&#39;s normal pressing force of pressing the tool bit against the workpiece. 
     According to this invention, in prior to operation, when the tool bit is pressed against the workpiece and moved rearward, the reaction force transmitting member is pushed by the tool bit and compresses the first elastic member, and also comes in contact with the second elastic member in an incompressible state, so that the reaction force transmitting member is placed in a predetermined working position in the longitudinal direction. Thus, the tool body is positioned with respect to the workpiece. In this state, when the tool bit strikes the workpiece and receives the reaction force, the striking reaction force is transmitted from the tool bit to the reaction force transmitting member and the reaction force transmitting member is moved rearward. When moved rearward, the reaction force transmitting member pushes the second elastic member and compressively deforms it. As a result, the striking reaction force is cushioned, so that low-vibration impact tool can be realized. 
     According to this invention, with the construction in which the first and second elastic members are arranged in tandem in the axial direction of the tool bit, compared with the construction in which they are arranged in parallel, the size can be reduced in a direction (radial direction) transverse to the axial direction of the tool bit. 
     According to a further embodiment of the impact tool of the invention, the impact tool further includes a striking element that linearly moves to linearly drive the tool bit, and a cylinder that houses the striking element. Further, the cylinder receives a force acting upon the second elastic member. 
     According to this invention, with the construction in which the cylinder receives a force acting upon the second elastic member, the second elastic member can be held in noncontact with the housing which forms the tool body. Specifically, with the construction in which the second elastic member is mounted to the cylinder, the second elastic member can be first mounted to the cylinder and then mounted to the housing. Therefore, compared with a construction in which the second elastic member is directly mounted to the housing, mounting of the second elastic member can be facilitated, so that ease of mounting can be enhanced. 
     According to a further embodiment of the impact tool of the invention, the impact tool further includes a striking element that linearly moves to linearly drive the tool bit, and a cylinder that houses the striking element, and the reaction force transmitting member comprises a cylindrical member. Further, the cylindrical member and the first elastic member are arranged in parallel such that the first elastic member is disposed inward of the cylindrical member in a radial direction of the cylinder, in a predetermined region on the cylinder in the axial direction of the tool bit. 
     In a construction in which the cylindrical member in the form of the reaction force transmitting member is fitted on the cylinder, the cylinder and the cylindrical member are provided with respective air vents for air supply and exhaust which provide communication between a cylinder inner space formed in front of the striking element and the outside. In this case, it must be constructed such that the air vent of the cylinder and the air vent of the cylindrical member are normally aligned with each other. In this invention, however, with the construction in which the first elastic member is disposed between the cylinder and the cylindrical member, a clearance for installing the first elastic member is provided between the cylinder and the cylindrical member, so that the air vent of the cylinder and the air vent of the cylindrical member communicate with each other through the clearance. Therefore, an additional structure for aligning the air vent of the cylinder and the air vent of the cylindrical member can be dispensed with. 
     According to a further embodiment of the impact tool of the invention, the impact tool further includes a striking element that linearly moves to linearly drive the tool bit, and a cylinder that houses the striking element. The reaction force transmitting member comprises a cylindrical member that is slidably fitted on the cylinder. Further, the cylindrical member has a passage that provides communication between a cylinder inner space formed in front of the striking element and the outside, and a nonreturn valve that allows air flow from the cylinder inner space to the outside through the passage and blocks air flow in the opposite direction. When the tool bit is pressed against the workpiece by the user and the cylindrical member is placed in a predetermined working position, the passage is closed by the cylinder so that the nonreturn valve is deactivated, and when the tool bit pressed against the workpiece is released and the cylindrical member is moved forward to an initial position by the biasing force of the first elastic member, the cylinder no longer closes the passage so that the nonreturn valve is allowed to activate. 
     According to this invention, when the tool bit is not pressed against the workpiece, the nonreturn valve is allowed to activate. In this state, when the striking element moves forward, air within the cylinder inner space in front of the striking element is discharged to the outside through the passage and the nonreturn valve. Thereafter, when the striking element is going to move rearward, the nonreturn valve blocks inflow of outside air into the cylinder inner space, so that negative pressure is caused in the cylinder inner space. As a result, the striking element is held in the forward position, so that idle driving is prevented. On the other hand, during actual operation in which the impact tool performs an operation with the tool bit being pressed against the workpiece, the nonreturn valve is deactivated. Therefore, unnecessary movement of the nonreturn valve can be reduced, so that durability of the nonreturn valve can be improved. 
     According to this invention, an effective technique for realizing size reduction while providing an effect of cushioning a striking reaction force caused during operation, is provided in an impact tool. Other objects, features and advantages of the invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional side view schematically showing an entire hammer drill according to an embodiment of this invention. 
         FIG. 2  is an enlarged sectional view showing an essential part of the hammer drill, under unloaded conditions in which a hammer bit is not pressed against a workpiece. 
         FIG. 3  is an enlarged sectional view showing the essential part of the hammer drill, under loaded conditions in which the hammer bit is pressed against a workpiece. 
         FIG. 4  is an enlarged sectional view showing a slide sleeve mechanism part and a reaction force cushioning mechanism part. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide and manufacture improved impact tools and method for using such impact tools and devices utilized therein. Representative examples of the present invention, which examples utilized many of these additional features and method steps in conjunction, will now be described in detail with reference to the drawings. This detailed description is merely intended to teach a person skilled in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed within the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe some representative examples of the invention, which detailed description will now be given with reference to the accompanying drawings. 
     An embodiment of the invention is now described with reference to  FIGS. 1 to 4 . In this embodiment, an electric hammer drill is explained as a representative embodiment of an impact tool according to the invention. As shown in  FIG. 1 , a hammer drill  101  of this embodiment mainly includes a body  103  that forms an outer shell of the hammer drill  101 , a hammer bit  119  (see  FIGS. 2 and 3 ) detachably coupled to a tip end region (on the left as viewed in  FIG. 1 ) of the body  103  via a tool holder  137 , and a handgrip  109  that is connected to the body  103  on the side opposite the hammer bit  119  and designed to be held by a user. The body  103  and the hammer bit  119  are features that correspond to the “tool body” and the “tool bit”, respectively, according to the invention. The hammer bit  119  is held by the tool holder  137  such that it is allowed to reciprocate with respect to the tool holder  137  in its axial direction and prevented from rotating with respect to the tool holder  137  in its circumferential direction. In the present embodiment, for the sake of convenience of explanation, the side of the hammer bit  119  is taken as the front and the side of the handgrip  109  as the rear. 
     The body  103  includes a motor housing  105  that houses a driving motor  111 , and a gear housing  107  that includes a barrel  106  and houses a motion converting mechanism  113 , a striking mechanism  115  and a power transmitting mechanism  117 . The driving motor  111  is disposed such that its axis of rotation runs in a vertical direction substantially perpendicular to the longitudinal direction of the body  103  (the axial direction of the hammer bit  119 ). Rotating power of the driving motor  111  is appropriately converted into linear motion by the motion converting mechanism  113  and then transmitted to the striking mechanism  115 . As a result, an impact force is generated in the axial direction of the hammer bit  119  via the striking mechanism  115 . The motion converting mechanism  113  and the striking mechanism  115  form a striking mechanism part. Further, the speed of the rotating power of the driving motor  111  is appropriately reduced by the power transmitting mechanism  117  and then transmitted to the hammer bit  119  via the tool holder  137 , so that the hammer bit  119  is caused to rotate in its circumferential direction. The driving motor  111  is driven when a user depresses a trigger  109   a  disposed on the handgrip  109 . 
     The motion converting mechanism  113  mainly includes a crank mechanism. The crank mechanism is constructed such that a driving element in the form of a piston  129  forming a final movable member of the crank mechanism linearly moves in the axial direction of the hammer bit within a cylinder  141  when the crank mechanism is rotationally driven by the driving motor  111 . The power transmitting mechanism  117  mainly includes a gear speed reducing mechanism comprising a plurality of gears. The power transmitting mechanism  117  transmits the rotating force of the driving motor  111  to the tool holder  137 , so that the tool holder  137  is caused to rotate in a vertical plane and thus the hammer bit  119  held by the tool holder  137  rotates, The constructions of the motion converting mechanism  113  and the power transmitting mechanism  117  are well-known in the art and therefore they are not described in further detail. 
     As shown in  FIGS. 2 and 3 , the striking mechanism  115  includes a striking element in the form of a striker  143  that is slidably disposed within the bore of the cylinder  141 , and an intermediate element in the form of an impact bolt  145  that is slidably disposed within the tool holder  137  and transmits the kinetic energy of the striker  143  to the hammer bit  119 . An air chamber  141   a  is defined between the piston  129  and the striker  143  within the cylinder  141 . The striker  143  is driven via the action of an air spring (pressure fluctuations) of the air chamber  141   a  of the cylinder  141  which is caused by sliding movement of the piston  129 . The striker  143  then collides with (strikes) the intermediate element in the form of the impact bolt  145  that is slidably disposed within the tool holder  137  and transmits the striking force to the hammer bit  119  via the impact bolt  145 . The impact bolt  145  and the hammer bit  119  form a hammer actuating member. Further, the cylinder  141  is housed within the barrel  106  of the gear housing  107  and held by a front end region of the gear housing  107 . 
     In the hammer drill  101  constructed as described above, when the driving motor  111  is driven, a striking force is applied to the hammer bit  119  in the axial direction from the motion converting mechanism  113  via the striking mechanism  115 , and a rotating force is applied to the hammer bit  119  in the circumferential direction via the power transmitting mechanism  117 . Thus, the hammer bit  119  held by a bit holding device  104  performs a hammering movement in the axial direction and a drilling movement in the circumferential direction, so that a hammer drill operation (drilling) is performed on a workpiece (concrete) which is not shown. Further, the hammer drill  101  can be appropriately switched between mode of hammer drill operation by hammering movement and drilling movement in the circumferential direction as described above and mode of hammering operation in which only a striking force in the axial direction is applied to the hammer bit  119 . However, this is not directly related to the invention, and therefore its detailed description is omitted. 
     In the hammer drill  101 , during operation, when the hammer bit  119  is pressed against the workpiece by the user&#39;s pressing force applied forward to the body  103 , the impact bolt  145  is pushed rearward (toward the piston  129 ) together with the hammer bit  119  and comes into contact with a body-side member. As a result, the body  103  is positioned with respect to the workpiece. In this embodiment, such positioning is effected by a compression coil spring  171  for cushioning a reaction force, via a positioning member  151  and a slide sleeve  161  for prevention of idle driving. The slide sleeve  161  and the compression coil spring  171  are features that correspond to the “reaction force transmitting member” and the “second elastic member”, respectively, according to this invention. 
     The positioning member  151  is a unit part including a rubber ring  153 , a front-side hard metal washer  155  joined to the axial front side of the rubber ring  153 , and a rear-side hard metal washer  157  joined to the axial rear side of the rubber ring  153 . The positioning member  151  is loosely fitted onto a small-diameter portion  145   b  of the impact bolt  145 . The impact bolt  145  has a stepped, cylindrical form having a large-diameter portion  145   a  that is slidably fitted in the cylindrical portion of the tool holder  137  and a small-diameter portion  145   b  formed on the rear side of the large-diameter portion  145   a , The impact bolt  145  has a tapered portion  145   c  formed between the outer circumferential surface of the large-diameter portion  145   a  and the outer circumferential surface of the small-diameter portion  145   b.    
     The slide sleeve  161  is a cylindrical member having a stepped bore formed by a small-diameter front portion and a large-diameter rear portion in the longitudinal direction. The bore small-diameter region of the slide sleeve  161  is fitted on a front end outer surface of the cylinder  141  and can slide in the axial direction of the hammer bit. A predetermined clearance C is provided between a bore large-diameter region of the slide sleeve  161  and an outer surface region of the cylinder. A sleeve biasing spring (coil spring)  163  is disposed in the clearance C. The sleeve biasing spring  163  constantly biases the slide sleeve  161  forward, and an axial rear end of the sleeve biasing spring  163  is held in contact with a retaining ring  164  fixed on the outer surface of the cylinder  141 , and an axial front end of the sleeve biasing spring  163  is held in contact with a stepped part  161   a  between the bore large-diameter region and the bore small-diameter region of the slide sleeve  161 . Thus, a front end of the slide sleeve  161  biased forward by the sleeve biasing spring  163  is held in contact with the rear metal washer  157  of the positioning member  151 . The sleeve biasing spring  163  is a feature that corresponds to the “first elastic member” according to this invention. 
     The compression coil spring  171  for cushioning a reaction force is mounted on the cylinder  141  via front and rear spring receiving rings  173 ,  175 . The front spring receiving ring  173  is fitted on the cylinder  141  and held in contact with a rear surface of the retaining ring  164  by the spring force of the compression coil spring  171 , so that the front spring receiving ring  173  is prevented from moving further forward. The rear spring receiving ring  175  is fitted on the cylinder  141  and held in contact with a stepped part  141   c  formed on the outer surface of the cylinder  141 , so that the rear spring receiving ring  175  is prevented from moving further rearward. The compression coil spring  171  is elastically mounted in a pre-compressed state between the front spring receiving ring  173  and the rear spring receiving ring  175 . At this time, the initial load of the compression coil spring  171  is set to be larger than the pressing force of an ordinary user pressing the hammer bit  119  against the workpiece. Further, the above-described sleeve biasing spring  163  is also mounted in a pre-compressed state, but its initial load is smaller than the compression coil spring  171 . In this embodiment, the initial load of the compression coil spring  171  is set to be 20 to 30 kgf, and the initial load of the sleeve biasing spring  163  is set to be 3 to 5 kgf. Further, the front spring receiving ring  173  has a larger diameter than the retaining ring  164 , and an outer region of the front spring receiving ring  173  juts radially outward of the retaining ring  164 . 
     Under unloaded conditions in which the hammer bit  119  is not pressed against the workpiece, as shown in  FIGS. 2 and 4 , the slide sleeve  161  is moved forward to a front end position by the biasing force of the sleeve biasing spring  163 . This front end position is defined as an initial position. In this initial position, the rear end surface of the slide sleeve  161  is not in contact with the front spring receiving ring  173  for the reaction-force cushioning compression coil spring  171 . When the hammer bit  119  is pressed against the workpiece and moved rearward, the slide sleeve  161  is pushed rearward together with the hammer bit  119 , the impact bolt  145  and the positioning member  151 , and the rear end surface of the slide sleeve  161  comes into contact with the front surface of the outer region of the front spring receiving ring  173 . Therefore, the user&#39;s pressing force of pressing the hammer bit  119  against the workpiece is received by the compression coil spring  171  and further by the cylinder  141  via the rear spring receiving ring  175 . Thus, the body  103  is positioned with respect to the workpiece. Specifically, in this embodiment, when the user presses the hammer bit  119  against the workpiece, the body  103  is positioned by the compression coil spring  171  via the positioning member  151  and the slide sleeve  161 . The position at which the rear end surface of the slide sleeve  161  contacts the front spring receiving ring  173  corresponds to the “predetermined working position” according to this invention. Further, with the construction that the initial load of the compression coil spring  171  is larger than the user&#39;s pressing force of pressing the hammer bit  119  against the workpiece, the compression coil spring  171  is not compressed by the user&#39;s pressing force when the body  103  is positioned. This state corresponds to the “incompressible state” in this invention. 
     The air chamber  141   a  for driving the striker  143  by the action of air spring communicates with the outside via a first air vent  165  which is formed in the cylinder  141  for prevention of idle driving. Under unloaded conditions in which the hammer bit  119  is not pressed against the workpiece, or when the impact bolt  145  is not pushed in rearward (rightward as viewed in  FIGS. 2 and 4 ), the striker  143  is allowed to move to a front position to open the first air vent  165 . On the other hand, under loaded conditions in which the hammer bit  119  is pressed against the workpiece, the impact bolt  145  is retracted and thus the striker  143  is pushed by the impact bolt  145  and moves to a rear position to close the first air vent  165  (see  FIG. 3 ). 
     Thus, the first air vent  165  of the air chamber  141   a  is opened and closed by the striker  143 . The action of the air spring is disabled when the first air vent  165  is opened, while it is enabled when the first air vent  165  is closed. 
     A closed front air chamber  141   b  is formed in front of the striker  143  on the side opposite the air chamber  141   a  and surrounded by the striker  143 , the cylinder  141 , the slide sleeve  161 , the positioning member  151  and the impact bolt  145 . The front air chamber  141   b  communicates with the outside via the second air vent  166  which is formed in the cylinder  141  for air supply and exhaust and via the third air vent  167  which is formed in the slide sleeve  161 . Opening and closing of the second air vent  166  for air supply and exhaust are controlled by the position of the striker  143 . Specifically, during operation of the hammer drill  101 , when the striker  143  is situated rearward of a predetermined reference position (substantially near to the impact bolt  145 ), the front air chamber  141   b  communicates with the outside via the second air vent  166  and the third air vent  167 , so that air supply and exhaust of the front air chamber  141   b  are allowed. On the other hand, when the striker  143  is moved forward past the reference position, the communication between the front air chamber  141   b  and the outside is interrupted, so that the air supply and exhaust of the front air chamber  141   b  are prohibited. As a result, the movement of the striker  143  is delayed with respect to the movement of the piston  129 . Further, the second air vent  166  and the third air vent  167  communicate with each other through the clearance C between the outer surface of the cylinder  141  and the bore large-diameter region of the slide sleeve  161 . 
     Further, a fourth air vent  168  and an O-ring  169  are provided in the front end region (bore small-diameter region) of the slide sleeve  161 . The fourth air vent  168  is provided for prevention of idle driving and provides communication between the inside and outside of the front air chamber  141   b . The O-ring  169  closes the fourth air vent  168  from the outer surface of the slide sleeve  161 . The O-ring  169  allows air flow from the front air chamber  141   b  to the outside through the fourth air vent  168  and blocks air flow in the opposite direction. The fourth air vent  168  is formed in a position such that it faces the front air chamber  141   b  under unloaded conditions in which the hammer bit  119  is not pressed against the workpiece, while it is closed by the outer surface of the cylinder  141  when the slide sleeve  161  is moved rearward against the biasing force of the sleeve biasing spring  163  under loaded conditions in which the hammer bit  119  is pressed against the workpiece. The front air chamber  141   b , the fourth air vent  168  and the O-ring  169  are features that correspond to the “cylinder inner space”, the “passage” and the “nonreturn valve”, respectively, according to this invention. 
     Operation of the hammer drill  101  constructed as described above is now explained. When the driving motor  111  is driven, the piston  129  of the crank mechanism which forms the motion converting mechanism  113  is caused to linearly slide within the cylinder  141 . At this time, under unloaded conditions in which the hammer bit  119  is not pressed against the workpiece, as shown in  FIG. 2 , the impact bolt  145  is in the forward position. As a result, the striker  143  is moved to its forward position to open the first air vent  165 . Further, under the unloaded conditions, the slide sleeve  161  is pushed forward by the sleeve biasing spring  163  and the fourth air vent  168  faces the front air chamber  141   b . Therefore, when the striker  143  is moved forward past the position of the second air vent  166 , air within the front air chamber  141   b  is discharged to the outside through the fourth air vent  168  and the O-ring  169 , In this state, when the piston  129  moves rearward, outside air is led into the air chamber  141   a  through the first air vent  165 , but in the front air chamber  141   b , the fourth air vent  168  is closed by the O-ring  169 , so that outside air is not led into the front air chamber  141   b . Therefore, the striker  143  is held in the forward position without being sucked up toward the piston  129  by negative pressure caused in the front air chamber  141   b . Thereafter, even if the piston  129  is driven, the hammer bit  119  is prevented from idle driving. 
     On the other hand, under loaded conditions in which the hammer bit  119  is pressed against the workpiece, as shown in  FIG. 3 , the impact bolt  145  is pushed rearward together with the hammer bit  119  and in turn pushes the positioning member  151  and the slide sleeve  161  against the biasing force of the sleeve biasing spring  163 . Then the rear end surface of the slide sleeve  161  comes in contact with the front surface of the outer region of the front spring receiving ring  173  for the compression coil spring  171 . Thus, the body  103  is positioned with respect to the workpiece. In this state, the striker  143  is pushed rearward by the impact bolt  145  and closes the first air vent  165 . When the piston  129  is moved forward in this state, the striker  143  moves linearly forward within the cylinder  141  and collides with (strikes) the impact bolt  145  by the action of the air spring function of the air chamber  141   a . The kinetic energy of the striker  143  which is caused by the collision with the impact bolt  145  is transmitted to the hammer bit  119 . Thus, the hammer bit  119  performs an operation on the workpiece by striking movement in its axial direction. Further, after collision with the impact bolt  145 , the striker  143  is moved rearward by a rebound caused by striking the impact bolt  145 , and by a sucking force (negative pressure) caused in the air chamber  141   a  by rearward movement of the piston  129 . Thereafter, the above-described movement is repeated. 
     During the above-described operation, when the hammer bit  119  performs striking movement on the workpiece and the hammer bit  119  is caused to rebound by the reaction force from the workpiece, a force caused by this rebound, or striking reaction force moves the hammer bit  119 , the impact bolt  145 , the positioning member  151  and the slide sleeve  161  rearward and elastically deforms (compresses) the compression coil spring  171 . Specifically, the striking reaction force caused by rebound of the hammer bit  119  is efficiently cushioned by elastic deformation of the compression coil spring  171 , so that transmission of the reaction force to the body  103  is reduced. At this time, a flange part  161   b  which extends radially inward from the slide sleeve  161  faces the front end surface of the cylinder  141  with a predetermined clearance therebetween and can come into contact with it, so that the maximum retracted position of the slide sleeve  161  is defined. Therefore, the reaction force cushioning action of the compression coil spring  171  is effected within the range of the above-mentioned clearance. 
     As described above, according to this embodiment, by provision of the mechanism of cushioning the striking reaction force from the hammer bit  119  by the compression coil spring  171  via the slide sleeve  161  for prevention of idle driving, an idle driving prevention effect and a vibration reducing effect can be obtained. 
     Further, according to this embodiment, the compression coil spring  171  is mounted on the cylinder  141  via the front and rear spring receiving rings  173 ,  175 . Therefore, the cylinder  141  and the compression coil spring  171  are assembled into one piece, so that the cylinder  141  and the compression coil spring  171  can be mounted and removed from the gear housing  107  as one piece. 
     Thus, ease of mounting or repairing can be enhanced. 
     Further, in this embodiment, during operation in which the hammer bit  119  is pressed against the workpiece and the slide sleeve  161  is pushed rearward, the fourth air vent  168  is situated in a position to face the outer surface of the cylinder  141  and closed by the outer surface of the cylinder  141 . Specifically, during actual operation in which the hammer drill  101  performs an operation, the nonreturn valve in the form of the O-ring  169  is held at a standstill (deactivated). With this construction, unnecessary movement of the O-ring  169  can be reduced during actual operation, so that durability of the O-ring  169  can be improved. 
     Further, according to this embodiment, the clearance C is provided between the outer surface of the cylinder  141  and the inner surface of the slide sleeve  161 , and the second air vent  166  of the cylinder  161  and the third air vent  167  of the slide sleeve  161  communicate with each other through the clearance C. With this construction, reliability of air supply and exhaust can be enhanced without need of taking measures to align the second air vent  166  and the third air vent  167 . Further, with the construction in which the sleeve biasing spring  163  is arranged in parallel within the clearance C provided between the outer surface of the cylinder  141  and the inner surface of the slide sleeve  161 , size increase of the body  103  in the longitudinal direction can be avoided. 
     Further, according to this embodiment, with the construction in which the sleeve biasing spring  163  and the compression coil spring  171  are arranged in tandem, compared with a construction in which they are arranged in parallel, the size of the body  103  can be reduced in the radial direction. Further, with the construction in which the outside diameter of the slide sleeve  161  is substantially equal to the outside diameter of the compression coil spring  171 , although the slide sleeve  161  and the sleeve biasing spring  163  are arranged in parallel, size increase of the body  103  in the radial direction can be avoided. 
     In the above-described embodiment, as a representative example of the impact tool, the hammer drill  101  was described in which the hammer bit  119  can be switched between mode of hammering operation by hammering movement of the hammer bit  119  and mode of hammer drill operation by hammering movement in the axial direction and drilling movement in the circumferential direction. However, the invention can also be applied to an electric hammer in which the hammer bit  119  performs only hammering movement in its axial direction. 
     According to the aspect of the invention, following features can be provided.
     (1)   

     “The impact tool as defined in any one of claims  1  to  4 , wherein the cylinder includes a front spring receiving ring that is prevented from moving forward and a rear spring receiving ring that is prevented from moving rearward, and the second elastic member comprises a compression coil spring and is elastically disposed in a pre-compressed state between the front spring receiving ring and the rear spring receiving ring.”
     (2)   

     “The impact tool as defined in (1), wherein the cylinder includes a retaining ring which is held in contact with the front spring receiving ring and prevents the front spring receiving ring from moving forward, while receiving a rear end of the first elastic member, and the front spring receiving ring has a larger diameter than the retaining ring, and when the user presses the tool bit against the workpiece, a rear end surface of the reaction force transmitting member contacts a front surface of an outer region of the front spring receiving ring.” 
     DESCRIPTION OF NUMERALS 
     
         
           101  hammer drill (impact tool) 
           103  body 
           105  motor housing 
           106  barrel 
           107  gear housing 
           109  handgrip 
           109   a  trigger 
           111  driving motor 
           113  motion converting mechanism 
           115  striking mechanism 
           117  power transmitting mechanism 
           119  hammer bit (tool bit) 
           129  piston 
           137  tool holder 
           141  cylinder 
           141   a  air chamber 
           141   b  front air chamber (cylinder inner space) 
           141   c  stepped part 
           143  striker (striking element) 
           145  impact bolt (intermediate element) 
           145   a  large-diameter portion 
           145   b  small-diameter portion 
           145   c  tapered portion 
           151  positioning member 
           153  rubber ring 
           155  front metal washer 
           157  rear metal washer 
           161  slide sleeve (reaction force transmitting member) 
           161   a  stepped part 
           161   b  flange part 
           163  sleeve biasing spring (first elastic member) 
           164  retaining ring 
           165  first air vent 
           166  second air vent 
           167  third air vent 
           168  fourth air vent (passage) 
           169  O-ring (nonreturn valve) 
           171  compression coil spring (second elastic member) 
           173  front spring receiving ring 
           175  rear spring receiving ring 
         C clearance