Patent Publication Number: US-8113297-B2

Title: Power tool with ergonomic handle

Description:
CROSS-REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 11/347,363, filed on Feb. 6, 2006 now abandoned, which claims priority to Japanese patent application no. 2005-25286 filed on Feb. 10, 2005. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to techniques for designing and constructing a handgrip of a power tool. 
     2. Description of the Related Art 
     Japanese Pre-Grant Patent Publication No. 2002-254341 discloses a power tool, in which a tool bit is driven by an electric motor. This power tool includes a body, a tool bit mounted on a tip end portion of the body, an electric motor housed within the body to drive the tool bit, and a handgrip that extends from an upper end of the grip proximal to the body down to the distal end of the grip in a direction that crosses the axial direction of the tool bit. 
     Improved techniques for reducing the load on the user&#39;s fingers are desired for a wide variety of hand-held power tools. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present teachings to provide effective techniques or designs capable of reducing the load on the user&#39;s fingers when holding a power tool. 
     In one aspect of the present teachings, a representative power tool may include a body, a tool bit mounted on a tip end portion of the body, a driving mechanism housed within the body and configured to drive the tool bit, and a handgrip that extends from the end of the handgrip closest or proximal to the body to the distal end of the grip in a direction that crosses the axial direction of the tool bit. The term “power tool” broadly includes electric-, pneumatic- or gas powered tools used, e.g., for tightening (e.g. various kinds of screws), cutting, grinding, polishing, nailing, riveting, drilling or other similar operations. Further, the power tool includes a holding optimization region that is arranged on the handgrip and is shaped to match shape of the user&#39;s fingers when the user is holding or gripping the handgrip. In embodiments, in which the handgrip is shaped or configured to match the shape of the user&#39;s fingers as much as possible when the user is holding the handgrip, the amount of force or load that the user&#39;s fingers are required to apply to the handgrip in order to hold the power tool can be optimized. 
     Optionally, the holding optimization region may include at least one of first to fourth portions. The first portion is disposed on the rearward end surface of the distal end region of the grip and is configured such that a normal extending from the rearward end surface crosses the axis of the tool bit forward of the handgrip. The first portion may be formed only on the grip distal end. In the alternative, a plurality of such first portions may be formed within a predetermined region between the grip distal end and the end of the grip proximal to the body. The user can operate the power tool while evenly pressing the first portion of the handgrip in the direction towards the tip end of the tool. Such a handgrip configuration reduces fatigue and pain in the user&#39;s hand during extended power tool operations. 
     The second portion is disposed on a front surface of the trigger such that a normal extending from the front surface of the trigger crosses the axis of the tool bit forward of the handgrip. 
     The third portion is a region of the handgrip having a plurality of oval cross-sections taken in a direction parallel to the axial direction of the tool bit. A first oval cross-section has a maximum diameter portion, wherein both its major axis and minor axis are a maximum, while a second oval cross-section has a minimum diameter portion, wherein both its major axis and minor axis are a minimum. The maximum diameter portion may be disposed in an intermediate region of the handgrip between the end of the grip proximal to the body and the grip distal end, whereas the minimum diameter portion may be disposed nearer to the grip distal end than the maximum diameter portion. In this case, the major and minor axes gradually decrease from the intermediate region towards the grip distal end. Consequently, the maximum diameter portion may be disposed in the intermediate region of the handgrip, while the minimum diameter portion may be disposed nearer to the grip distal end than the maximum diameter portion. In such an embodiment, the holding force of the entire palm can be effectively utilized when holding the handgrip. 
     The fourth portion of the handgrip is configured such that an imaginary S-shaped connecting line continuously and vertically connects vertexes of the plurality of oval cross-sections on a lateral side surface of the handgrip, such that an upper end of the S-shaped line is directed toward a rearward end of the end of the grip proximal to the body and a lower end of the S-shaped line is directed toward a front end of the grip distal end. In such an embodiment, the vertexes (convex portions) of the grip lateral side surface snugly fit into the hollow (concave) portion formed by the palm when the user holds the handgrip. As a result, the force of the user&#39;s fingers can be efficiently applied to the handgrip. 
     Preferably, the S-shaped connecting line, which continuously and vertically connects the vertexes on the lateral side surface of the handgrip, may extend substantially along a heart line or a head line of the user&#39;s palm while the handgrip is being held. This configuration enables the hollow (concave) portion of the user&#39;s palm to conform, in particular, to the heart line or the head line on the user&#39;s palm when holding the handgrip. 
     Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a lateral side view showing an impact driver  100  according to an embodiment of the invention. 
         FIG. 2  is a rear view of the impact driver  100  shown in  FIG. 1  as viewed from the right side in  FIG. 1 . 
         FIG. 3  shows a battery  140  of the impact driver  100  shown in  FIG. 1  in the attached state and in the detached state. 
         FIG. 4  is a side view of a representative handgrip  130 . 
         FIG. 5  is a cross-sectional view of the handgrip  130  taken along line A-A in  FIG. 4   
         FIG. 6  is a cross-sectional view of the handgrip  130  taken along line B-B in  FIG. 4 . 
         FIG. 7  is a cross-sectional view of the handgrip  130  taken along line C-C in  FIG. 4 . 
         FIG. 8  is a cross-sectional view of the handgrip  130  taken along line D-D in  FIG. 4 . 
         FIG. 9  is a cross-sectional view of the handgrip  130  taken along line E-E in  FIG. 4 . 
         FIG. 10  schematically shows the surface contour of the representative handgrip  130 . 
         FIG. 11  shows the shape of the fingers and the palm in the state of holding the handgrip, wherein the handgrip is omitted for illustration purposes. 
         FIG. 12  shows the distribution of skin shearing stress on the web between the thumb and the forefinger as a result of a mechanical simulation analysis on the representative handgrip  130  and on a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide and manufacture improved power tools and method for using such power 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. 
     A representative embodiment of the “power tool” of the present invention will now be described with reference to the drawings. In this embodiment, an electric (battery-powered) impact driver  100  is described as a representative example of the power tool. 
       FIGS. 1 and 2  show external views of an impact driver  100  according to the representative embodiment.  FIG. 1  is a side view of the impact driver  100  and  FIG. 2  is a rear view of the impact driver  100  shown in  FIG. 1  as viewed from the right side in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , the impact driver  100  includes a body  101 , a driver bit  110 , a driving (electric) motor  120 , a handgrip  130  and a battery  140 . The driver bit  110  is removably mounted on a tip end portion of the body  101  and is configured to tighten various screws. The driving motor  120  is housed within the body  101 . 
     The body  101  includes a motor housing  103  and a gear housing  105 . The body  101  is an example of the “body” according to the present invention. The body  101 , however, may also be referred to as the “body” together with the handgrip  130 . 
     The motor housing  103  contains the electric driving motor  120 . The driver bit  110  projects from the end of the gear housing  105  and is driven by the driving motor  120 . The driving motor  120  is an example that corresponds to the “electric motor” according to this invention. The driver bit  110  is an element driven by the driving motor  120  and is an example that corresponds to the “tool bit mounted on the tip end portion of the body” according to this invention. 
     Although not explicitly shown, the gear housing  105  contains a speed reducing mechanism for appropriately reducing the rotational speed of an output shaft of the driving motor  120 , a spindle that is rotated by the speed reducing mechanism, a hammer that is rotated by the spindle via balls, and an anvil that is rotated by the hammer. The end of the anvil protrudes from the end of the gear housing  105 . The driver bit  110  is detachably mounted in this protruding end of the anvil. 
     The handgrip  130  is a handle held by the user to perform a power tool operation or to carry the power tool. When the user holds the power tool, the user&#39;s hand applies a holding (grip) force onto the handgrip. The representative handgrip  130  extends from a grip proximal end  130   a  disposed at the underside of the body  101  to a grip distal end  130   b  in a direction that crosses the axial direction of the driver bit  110  (i.e. it crosses a line that is coaxial with the rotational axis of the driver bit  110 ). A trigger  131  for actuating a power switch (not shown) of the driving motor  120  is provided on the front portion of the handgrip  130 . The trigger  131  is operated by the user to start and stop the driving motor  120 . 
     Further, the body  101  has a casing or housing made of a hard material (a hard synthetic resin material or another similar material). A cushion made of a soft material (a soft synthetic resin material, rubber material or another similar material), which is softer than the hard material, is further provided around the casing. The cushion is indicated with diagonal shading in  FIG. 1  and, for example, includes a lateral side contact portion  107 , a rear contact portion  109 , a grip front contact portion  132 , a grip rear contact portion  133  and a connecting portion  134 . The lateral side contact portion  107  is formed on both lateral surfaces of the body  101  and the rear contact portion  109  is formed on the rearward end surface of the body  101 . The grip front contact portion  132  is formed on the front and lateral surfaces of the handgrip  130  and the grip rear contact portion  133  is formed on the rearward end surface of the handgrip  130 . The connecting portion  134  connects the lateral side contact portion  107 , the grip front contact portion  132  and the grip rear contact portion  133 . By providing the cushion in such a design, the impact driver  100  can provide the user, who is holding the handgrip  130  during a power tool operation, with a soft grip feel as well as a novel appearance. 
     The battery  140  is removably attached to a battery holder disposed on the distal end portion (lower end portion) of the handgrip  130 . The battery  140  has a plurality of cylindrical cells (rechargeable batteries), which are not shown, as the power source for supplying current to the driving motor  120 . The cells are contained within the housing of the battery  140  and are arranged horizontally. Instead of this construction, one or more of the cylindrical cells may also be arranged in an inverted vertical position within the housing. Further, various kinds of boards and wiring that connect the driving motor  120  to the battery  140  are accommodated within the internal space of the handgrip  130 . 
       FIG. 3  shows the battery  140  of the impact driver  100  shown in  FIG. 1  both in the attached state and in the detached state. As shown in  FIG. 3 , the housing of the cylindrical cells of the battery  140  does not project upwardly, either partial or entirely, from the upper surface of the battery. Thus, in the battery attached state, the housing of the cylindrical cells is external to the grip region and is disposed below the grip distal end  130   b . In other words, the housing of the cylindrical cells is not accommodated within the internal space of the handgrip  130 . This construction is thus different from a plug-in type battery, in which the housing of the cylindrical cells projects upwardly from the upper surface of the battery and the housing of the cylindrical cells is accommodated either partially or entirely within the internal space of the handgrip when the battery is attached to the power tool. In the present embodiment, the housing of the cylindrical cells of the battery  140  is external to the grip region and the battery  140  is configured as a so-called slide-type battery pack, in which the battery is attached and detached by sliding the battery  140  relative to the power tool  100 . Therefore, the battery  140  can be detached by sliding the battery  140  from the attached position in the sliding direction (a direction that is generally parallel to the axial direction of the driver bit  110 ). After being detached from the handgrip  130 , the battery  140  can be recharged by connecting it to a battery charger (not illustrated). 
     In the impact driver  100  having the above-described construction, when the user holds the handgrip  130  and depresses the trigger  131  to actuate a power switch, the driving motor  120  is driven. The driver bit  110  is then rotated via the speed reducing mechanism, the spindle, the hammer and the anvil and performs a screw-tightening operation. The operating principle of the impact driver  100  is known in the art and therefore will not be described in further detail. 
     The impact driver  100  may be operated by forwardly pressing a horizontally-extending driver bit  110 , by upwardly or downwardly pressing a vertically-extending driver bit  110 , or by upwardly or downwardly pressing an obliquely-extending driver bit  110 . 
     In the following, the construction and operation of the representative handgrip  130  will be explained in further detail with reference to  FIGS. 4 to 12 . 
     In the above-described representative embodiment with the slide-type battery  140 , the size and design of the internal space within the handgrip  130  is not restricted by having to internally accommodate the housing of the cylindrical cells of the battery  140 . Therefore, this construction is advantageous, because it provides a greater degree of design freedom with respect to the configuration or contour of the handgrip  130 . On the other hand, in a power tool designed to internally accommodate a plug-in type removable battery, the configuration or contour of the housing of the cylindrical cells accommodated within the internal space of the handgrip necessarily influences the configuration and contour of the handgrip. However, by utilizing a construction with a slide-type battery as in the representative embodiment, the configuration or contour of the housing of the cylindrical cells does not influence the configuration or contour of the handgrip, so that a greater degree of design freedom with respect to the configuration and contour of the handgrip can be provided. 
       FIG. 4  shows a lateral side view of the handgrip  130 , in which a first grip region  135  is defined as the region where the web between the thumb and the forefinger is positioned when holding the handgrip  130 . A second grip region  136  is defined as the region where the middle finger is positioned when holding the handgrip. A third grip region  137  is defined as an intermediate region where the middle finger or the ring (fourth) finger is positioned when holding the handgrip. A fourth grip region  138  is defined as the region where the ring (fourth) finger is positioned when holding the handgrip. A fifth grip region  139  is defined as the region where the little (pinky) finger is positioned when holding the handgrip. Particularly, the second to fifth grip regions  136 - 139  are preferably disposed, for example, within the range of 47.0 mm±2% from the grip distal end  130   b  toward the grip proximal end  130   a.    
     With respect to the cross-sectional configuration of the handgrip  130  of  FIG. 4 ,  FIGS. 5 to 9  shows cross-sectional views taken along line A-A, line B-B, line C-C, line D-D and line E-E respectively shown in  FIG. 4 . In this embodiment, these cross-sections are parallel to a lengthwise direction of the battery  140 . 
     As shown in  FIG. 5 , the cross section of the handgrip  130  in the first grip region  135  is configured to be oval. For example, a major axis a 1  is defined within the range of 53.6 mm±2% in the fore-and-aft direction of the handgrip, and a minor axis b 1  is defined within the range of 31.2 mm±2% in the sidewise or lateral direction of the handgrip. 
     As shown in  FIG. 6 , the cross section of the handgrip  130  in the second grip region  136  is configured to be oval. For example, a major axis a 2  is defined within the range of 46.0 mm±2% in the fore-and-aft direction of the handgrip, and a minor axis b 2  is defined within the range of 34.5 mm±2% in the sidewise or lateral direction of the handgrip. 
     As shown in  FIG. 7 , the cross section of the handgrip  130  in the third grip region  137  is configured to be oval. For example, a major axis a 3  is defined within the range of 45.4 mm±2% in the fore-and-aft direction of the handgrip, and a minor axis b 3  is defined within the range of 33.7 mm±2% in the sidewise or lateral direction of the handgrip. 
     As shown in  FIG. 8 , the cross section of the handgrip  130  in the fourth grip region  138  is configured to be oval. For example, a major axis a 4  is defined within the range of 43.3 mm±2% in the fore-and-aft direction of the handgrip, and a minor axis b 4  is defined within the range of 32.0 mm±2% in the sidewise or lateral direction of the handgrip. 
     As shown in  FIG. 9 , the cross section of the handgrip  130  in the fifth grip region  139  is configured to be oval. For example, a major axis a 5  is defined within the range of 38.7 mm±2% in the fore-and-aft direction of the handgrip, and a minor axis b 5  is defined within the range of 29.4 mm±2% in the sidewise or lateral direction of the handgrip. 
     As shown in  FIGS. 6 to 9 , between the second grip region  136  and the grip distal end  130   b , the grip diameter (major and minor axes) is at a maximum in the second and third grip regions  136  and  137 . The grip diameter (major and minor axes) of the handgrip  130  gradually reduces toward the grip distal end  130   b  and reaches a minimum in the fifth grip region  139 . Although not shown, similar to the grip diameter (major and minor axes), the cross-sectional area and the perimeter of the handgrip are also at a maximum in the second and third grip regions  136  and  137 , then gradually reduce toward the grip distal end  130   b  and ultimately reach a minimum in the fifth grip region  139 . 
     Although the grip diameter (major and minor axes) of the handgrip is described in this embodiment with respect to the cross sections extending along the longitudinal direction of the battery, as another example, the grip diameter (major and minor axes) of each portion of the handgrip can be appropriately defined with consideration of the position and orientation of the cross sections, errors and tolerances. 
     When the user holds the handgrip, the holding force of the entire palm can be effectively utilized if the little finger can securely grip the handgrip. Giving consideration to this fact, the handgrip is configured such that the grip size (at least the grip diameter) is at a maximum in the region that is held by the middle finger or the ring finger and is at a minimum in the region that is held by the little finger. In this case, the second or the third grip region  136  or  137  may be examples that correspond to the “maximum diameter portion”, and the fifth grip region  139  may correspond to the “minimum diameter portion”. Further, the position of the maximum diameter portion (the region where the grip diameter is a maximum) may substantially coincide, such as in this embodiment, or may not necessarily coincide with the position of the maximum cross-section portion (the region where the cross-sectional area is a maximum) or the position of the maximum perimeter portion (the region where the grip perimeter is a maximum). Likewise, the position of the minimum diameter portion (the region where the grip diameter is a minimum) may substantially coincide, or may not necessarily coincide with the position of the minimum cross-section portion (the region where the cross-sectional area is a minimum) or the position of the minimum perimeter portion (the region where the grip perimeter is a minimum). 
     Further, although a cross-sectional configuration is not explicitly shown, the region X (shown in  FIG. 4 ), which is closer to the grip proximal end  130   a  than the second grip region  136 , is configured to have a smaller grip diameter, a smaller perimeter and a smaller cross-sectional area than the second grip region  136 . The portion, against which the holding force of the hand is substantially exerted when the user holds the power tool by hand, is initially thinner on the side of the grip proximal end  130   a , then becomes thicker and finally becomes thinner again towards the grip distal end  130   b.    
       FIG. 10  schematically shows the surface configuration of the handgrip  130  according to the representative embodiment. The handgrip  130  shown in  FIG. 10  is configured such that a normal L 1  extending from the rearward end surface of the region of the grip distal end  130   b  (i.e. a line perpendicular to a tangent L 2  on the rearward end surface) crosses the axis of the driver bit  110  forward of the handgrip  130 . In other words, the normal L 1  (straight line) that extends upward and to the left as viewed in  FIG. 10  crosses the axis (not explicitly represented in the drawings) of the substantially horizontally-extending driver bit  110  forward of the handgrip  130 . A normal, which is similar to the normal L 1  that crosses the axis of the driver bit  110  forward of the handgrip  130 , may be defined at least one point in the region of the grip distal end  130   b . For example, such a normal may be defined only on the grip distal end  130   b . In the alternative, a plurality of such normals may be defined within a predetermined region between the grip distal end  130   b  and the grip proximal end  130   a . In this embodiment, the normal L 1  is arranged based on a “human body data analysis”, which will be described below. 
     Further, in the handgrip  130  shown in  FIG. 10 , a curved line L 3  is defined by an imaginary line that continuously and vertically connects the vertexes of the respective oval cross-sections on the lateral side surface of the grip  130 , or as a connecting line that continuously connects, for example, a vertex P (B) in the second grip region  136 , a vertex P (C) in the third grip region  137 , a vertex P (D) in the fourth grip region  138  and a vertex P (E) in the fifth grip region  139 . As shown in  FIG. 10 , the curved line L 3  extends in the shape of the letter S such that the upper end of the line is directed toward the rearward end of the grip proximal end  130   a  and the lower end of the line is directed toward the front or forward end of the grip distal end  130   b . In this case, when the connecting line that connects vertexes on one side or lateral surface of the grip extends in the shape of the letter S, another connecting line that connects vertexes on the other side or lateral surface of the grip is a mirror image of said connecting line (in the shape of the letter S). The curved line L 3  is an example that corresponds to the “connecting line”. In this embodiment, the curved line L 3  or an extending line L 4  that extends along the curved line L 3  is arranged based on the “human body data analysis”, which will be described below. 
     Further, the handgrip  130  shown in  FIG. 10  is configured such that a normal L 7  extending from the front surface of the trigger  131  (i.e. a straight line perpendicular to a tangent L 8  on the front surface of the trigger  131 ) crosses the axis of the driver bit  110  forward of the handgrip. In other words, the normal L 7  (straight line), which extends upward and to the left as viewed in  FIG. 10 , crosses the axis (not shown) of the substantially horizontally-extending driver bit  110  forward of the handgrip. The front surface of the trigger  131  comprises a contact region that is depressed by the user&#39;s forefinger. A normal, such as the normal L 7  that crosses the axis of the driver bit  110  forward of the handgrip, may be formed at one or more points on the front surface of the trigger  131 . In this embodiment, the normal L 7  is arranged based on the “human body data analysis”, which will be described below. 
     The configuration or contour of the handgrip  130  is designed based on the above-mentioned human body data analysis, a sensory evaluation analysis and a mechanical simulation analysis that will now be described. As a result of verification by an evaluation that used the mechanical simulation analysis and the sensory evaluation analysis, the representative handgrip  130  was found to reduce the load on the user&#39;s fingers and enabled comfortable long operating times. 
     In order to examine the shape of the fingers and palm of users holding the handgrip, an investigation of the shape of the fingers and palm was conducted on 30 adult Japanese men, ages 20 to 40. As shown in  FIG. 11 , the vertical distance d 1  from the lower side of the hand to the web averaged 82 mm and the horizontal distance d 2  from the base of the hand to the forefinger averaged 181 mm. Further, a first angle θ 1  defined between a line L 5  extending along the straightened forefinger and a horizontal line averaged 10°, and a second angle θ 2  defined between a line L 6  extending generally vertically along a hollow (concave) portion of the palm and a vertical line averaged 15°. The line L 6  can be defined as a line that extends substantially alongside the heart line or the head line on the palm. 
     In this embodiment, the handgrip  130  is configured such that the above-mentioned normal L 1  (related to the grip configuration) extending from the rearward end surface of the grip extends alongside or parallel to the line L 5 . With this configuration, the pressing force exerted on the rearward end surface of the handgrip  130  can act evenly across the entire handgrip  130 . 
     Further, in this embodiment, the handgrip is configured such that the above-mentioned extending line L 4  or the curved line L 3  (related to the grip configuration) on the grip side or lateral surface extends along or parallel to the line L 6 . In other words, considering the fact that a hollow (concave) portion is formed in the palm while it is holding the grip, in particular along the heart line and the head line, the handgrip is configured such that the vertexes (convex portions) of the grip side surface extend along or are parallel to this hollow portion of the palm and are fitted into the hollow (concave) portion. With this configuration, the user can hold the handgrip evenly using the entire palm. 
     Further, in this embodiment, the handgrip  130  is configured such that the above-mentioned normal L 7  (related to the grip configuration) extending from the front surface of the trigger  131  extends along the line L 5 . In other words, the position and orientation of the trigger  131  are defined based on the first angle θ 1  shown in  FIG. 11  so that the user can easily depress the trigger  131  with the forefinger and the force of the forefinger can be easily exerted onto the trigger. 
     A sensory evaluation analysis was conducted on the handgrip  130 , as well as various known handgrips, using questionnaires given to sampling subjects who were chosen in a manner similar to the above-described human body data analysis. The questionnaires included questions concerning the feel of the hold, which the subject experienced (whether it fit or conformed to the hand well) while actually holding the handgrip. The results showed that the subjects preferred the handgrip according to the representative embodiment, which is configured to have a substantially thin grip diameter over its entirety, is configured such that the grip diameter gradually decreases from the portion for the middle finger towards the portion for the little finger via the grip portion for the ring finger, and is also configured such that the entire palm evenly contacts the surface of the handgrip. 
     Further, a mechanical simulation analysis was conducted on the handgrip  130  of this embodiment as well as comparative examples. In this analysis, a grasping force distribution on the handgrip surface of each of the handgrips was obtained, and pressure and skin shearing stress on each part of the fingers and palm (the web, the part between the forefinger to the little finger, and the palm) were calculated based on the grasping force distribution. As a result, by using the handgrip  130  of this embodiment, favorable values could be realized with respect to the pressure and skin shearing stress on each part of the fingers and palm. 
     In this connection,  FIG. 12  shows the distribution of skin shearing stress on the web as determined by the mechanical simulation analysis on the handgrip  130  for the representative embodiment and a comparative example. A handgrip having the configuration shown by a phantom line in  FIG. 12  was used as the comparative example. As shown in  FIG. 12 , as compared to the comparative example, the skin shearing stress of the fingers and palm on the handgrip is remarkably reduced by using the handgrip of the representative embodiment. Thus, the handgrip of the representative embodiment may have a configuration which can prevent the user from suffering pain in the web portion of the hand. 
     As mentioned above, the handgrip  130  can take some of the load off the user&#39;s fingers. Thus, a handgrip configuration can be realized which is easy to hold, reduces user fatigue and causes less pain in the user&#39;s hand (particularly in the web) during a power tool operation. 
     More specifically, in the embodiment in which the normal L 1  extending from the rearward end surface of the region of the grip distal end  130   b  crosses the axis of the driver bit  110  forward of the handgrip, the pressing force exerted on the rearward end surface of the handgrip  130  can act evenly over the entire handgrip  130  when the user performs a power tool operation, in which the hand holding the grip moves the power tool in its forward direction. Thus, the user can perform the power tool operation while his/her hand evenly presses the handgrip  130  in the direction towards the tip end of the tool. Therefore, a handgrip configuration can be realized that reduces fatigue and causes less pain in the user&#39;s hand during the power tool operation. Especially with the embodiment in which the normal L 1  extends along the line L 5 , a handgrip configuration can be realized in which the pressing force generated by the user&#39;s hand pressing the handgrip  130  is readily transmitted to the axis of the driver bit  110 . Such a configuration of the handgrip  130  is particularly advantageous in power tools, which the user may operate for many hours while pressing the tool bit in various directions. 
     Further, according to the representative embodiment, the grip dimensions (grip diameter, grip cross-sectional area, grip perimeter) gradually decrease from the region intended to be gripped by the middle finger or the ring finger, towards the grip distal end region and is minimal, in particular, in the region intended to be gripped by the little finger. With this configuration or contour, the holding force of the entire palm can be effectively utilized. As to the grip dimensions (grip diameter, grip cross-sectional area, grip perimeter), optimum values can be selected according to variations in size of the fingers and palm by race, sex or age. For example, the grip dimensions of handgrips designed specifically for Europeans and Americans can be scaled up (grip diameter, grip cross-sectional area, grip perimeter) to about 106 to 110% or preferably about 108% of those designed for Asians, while maintaining the same basic grip performance. 
     Further, in the configuration in which a curved line L 3  continuously and vertically connects vertexes on the lateral side surface of the grip and extends in the shape of the letter S substantially along the heart line or the head line of the palm holding the grip, the vertexes (convex portions) of the lateral side surfaces of the grip snugly fit into the hollow (concave) portion of the palm, so that a handgrip having an excellent fit can be realized. 
     Further, in this embodiment, because the normal L 7  extending from the front surface of the trigger  131  extends along the direction that the forefinger extends when straightened from its grip holding position, the user can easily depress the trigger  131  with the forefinger and can easily exert the force of the forefinger on the trigger. 
     Further, by using a battery  140  that has its cylindrical cell housing disposed outside the grip region, a greater degree of design freedom is provided with respect to the configuration and contour of the handgrip  130 . This is effective for realizing a handgrip that is advantageously configured to reduce the load on the user&#39;s fingers. 
     This invention is not limited to an impact driver  100 , and can also be applied to various other power tools which are used for cutting, grinding, polishing, nailing, riveting or drilling. In this regard, the power tool may be configured such that the tool bit is driven by a driving motor that is powered by an AC power source or a battery, or is driven by air or gas pressure. 
     It is explicitly stated that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure as well as for the purpose of restricting the claimed invention independent of the composition of the features in the embodiments and/or the claims. It is explicitly stated that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure as well as for the purpose of restricting the claimed invention, in particular as limits of value ranges. 
     REFERENCE NUMBER LIST 
     
         
           100  impact driver (power tool) 
           101  body 
           103  motor housing 
           105  gear housing 
           107  side contact portion 
           109  rearward end contact portion 
           110  driver bit (tool bit) 
           120  driving motor 
           130  handgrip 
           130   a  grip proximal end 
           130   b  grip distal end 
           131  trigger 
           132  grip front contact portion 
           133  grip rear contact portion 
           134  connecting portion 
           135  first grip region 
           136  second grip region 
           137  third grip region 
           138  fourth grip region 
           139  fifth grip region 
           140  battery