Patent Publication Number: US-8985236-B2

Title: Handheld power tool

Description:
This application claims the priority of German Patent Document No. 10 2010 040 173.0, filed Sep. 2, 2010, the disclosure of which is expressly incorporated by reference herein. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to a handheld power tool. 
     The inventive handheld power tool has a drive oscillating along a working axis and has a vibration damper. The vibration damper has a mass element suspended in a spring mechanism. The spring mechanism acts in a first direction parallel to the working axis with a first spring stiffness and it acts with a second spring stiffness in a second direction opposite the first direction. The first spring stiffness is different from the second spring stiffness. 
     The handheld power tool, for example, a handheld power tool having a pneumatic striking mechanism, exerts a return blow periodically on the user. The amplitude thereof may be diminished by the vibration damper, but a vibration damper having an asymmetrical design can produce a greater damping effect with the handheld power tool. The spring stiffness may have a discontinuity or a very drastic change relative to the basic position. The discontinuity leads to a highly non-harmonious movement of the mass element and non-harmonious forces, which may be more suitable for damping the machine housing. 
     According to one embodiment, the first spring stiffness amounts to between five and ten times the second spring stiffness. The ratio of the spring stiffness values may be used to adjust the damping of the vibration damper to the rebound behavior of the handheld power tool. The greater the ratio, the shorter and greater is the acceleration of the mass element by the stiffer side. 
     According to one embodiment, the mass element in the basic position is in contact with the spring. In the basic position the mass element may be arranged between two prestressed springs. According to one embodiment, the two prestressed springs are fixedly connected to the mass element. Because of the fixed connection, this results in low losses in the springs due to plastic deformation or due to friction. 
     According to one embodiment, the mass element is attached to a bending spring which is arranged at an inclination to the working direction. The bending spring is relaxed when the mass element is in the basic position. 
     The following description illustrates the invention on the basis of exemplary embodiments and figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embodiment of a handheld power tool in accordance with the principles of the present invention; 
         FIG. 2  illustrates a vibration damper of the handheld power tool of  FIG. 1  in accordance with the principles of the present invention; and 
         FIGS. 3 and 4  illustrate alternative embodiments of a vibration damper in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The same elements or those having the same function are indicated by the same reference numerals in the figures, unless otherwise indicated. 
       FIG. 1  shows as one embodiment a drill hammer  1 . The drill hammer  1  has a tool receptacle  2  to receive a boring tool  3 . A striking mechanism  4  of the drill hammer  1  periodically strikes the boring tool  3  inserted into the tool receptacle  2  along a working axis  5  and thereby drives it into the substrate. Meanwhile, a rotary drive  6  can rotate the boring tool  3  around the working axis  5 . 
     The striking mechanism  4  and the rotary drive  6  may be driven by a shared motor  7 , for example, an electric motor. A machine housing  8  surrounds the striking mechanism  4 , the rotary drive  6  and the motor  7 , which is optionally shared. 
     The striking mechanism  4  is a pneumatic striking mechanism, for example. An exciter  9  and a beater  10  are movably guided in the pneumatic striking mechanism  4  along the working axis  5 . The exciter  9  is coupled to the motor  7  via an eccentric cam  11  or a wobbling finger and forced to execute a periodic linear movement. A pneumatic spring formed by a pneumatic chamber  12  between the exciter  9  and the beater  10  couples a movement of the beater  10  to the movement of the exciter  9 . The beater  10  may directly strike a rear end of the boring tool  3  or may transmit a portion of its pulse to the boring tool  3  indirectly via an essentially stationary intermediate beater  13 . 
     The tool receptacle  2  has a sleeve  14 , for example, into which the boring tool  3  can be inserted. One or more locking elements  15 , e.g., spheres, protrude into the sleeve  14  and engage in longitudinally closed grooves on the boring tool  3 . The boring tool  3  may slide along the working axis  5  according to the length of its grooves in the tool receptacle  2 . The rotary drive  6  rotates the sleeve  14  around the working axis  5 . 
     The user can guide the drilling hammer  1  by hand by a handle  17 . The handle  17  is attached to a side of the machine housing  8  facing away from the tool receptacle  2 . A longitudinal axis  18  of the handle  17  runs obliquely or at a right angle to the working axis  5 . The drill hammer  1  is in mirror symmetry with a plane of symmetry (corresponding to the plane of the drawing), for example, which is spanned by the working axis  5  and a longitudinal axis  18  of the handle  17 . An axis perpendicular to the plane of symmetry is hereinafter referred to as the x axis. The y axis is perpendicular to the x axis and to the working axis  5 . 
     The striking mechanism  4 , which operates periodically, induces vibrations or oscillations in the machine housing  8 . Spring mechanisms  20 ,  21  of the handle  17  on the machine housing  8  partially suppress a transmission of the vibrations to the handle  17  to reduce the physiological burden on the user. 
     A further reduction in the burden for the user is achieved by a vibration damper  30  which is arranged in the machine housing  8 . The vibration damper  30  has a mass element  31 , which is connected by a spring mechanism  32  to the machine housing  8 . The vibrating machine housing  8  excites the mass element  31  of the vibration damper  30  to also vibrate. The system comprising the mass element  31  and the spring mechanism  32  is coordinated with a natural frequency, which is somewhat greater than the excitation frequency due to the machine housing  8 , i.e., the rate of repetition of the striking mechanism  4 . The vibration damper  30  cannot entirely follow the vibration of the machine housing  8  and is stabilized in phase opposition. The deviation in the natural frequency from the excitation frequency is preferably low, for example, less than 10%, which achieves an efficient energy transfer between the machine housing  8  and the vibration damper  30 . 
       FIG. 2  shows in detail an embodiment of the vibration damper  30 . The vibration damper  30  has a housing  33  in which the mass element  31  is mounted along an axis of vibration  34 . An exemplary bearing  35  includes round rods  36  which are fastened parallel to the axis of vibration  34  from the housing  33 . The mass element  31  has longitudinal bores  37  or longitudinal grooves running through the round rods  36 . The bearing  35  is preferably of low friction. Other embodiments of linear bearings, e.g., with rolling bodies, may also be used. 
     The mass element  31  may be shifted from a basic position  38  (shown in  FIG. 2 ) along the axis of vibration  34  into a first direction  39  to a first end  40  of the vibration damper  30  and along the axis of vibration  34  into an opposite second direction  41  to a second end  42  of the vibration damper  30 . The spring mechanism  32  produces a restoring force on the mass element  31  as soon as it is deflected out of the basic position  38 . The spring mechanism  32  is designed to be asymmetrical with the basic position  38 . In the example shown here, the basic position  38  coincides with a geometric center of the spring mechanism  32  or of the vibration damper  30  and thus the spring mechanism  32  is asymmetrical with a plane  43  which is perpendicular to the working axis  5  and runs through the geometric center of the spring mechanism  32 . A greater restoring force acts on the mass element  31  when it is deflected out of the basic position  38  by a stroke in the first direction  39  than when the mass element  31  is deflected out of the basic position  38  by an identical stroke in the opposite second direction  41 . 
     The exemplary spring mechanism  32  has first springs  44 , second springs  45  and a third spring  46 . The first springs  44  are attached to the first end  40  of the housing  33  and to the mass element  31 , for example, by clamping elements  47 ,  48  (only labeled with respect to second springs  45 ). The first springs  44  return the mass element  31  in the second direction  41  when it is deflected out of the basic position  38  in the first direction. The second springs  45  are attached to the second end  42  of the housing  33  and to the mass element  31 . The mass element  31  is returned in the first direction by the second springs  45  when it is deflected out of the basic position  38  in the second direction. The first springs  44  and the second springs  45  may be designed identically, for example, with the same length and the same spring stiffness. The first springs  44  and the second springs  45  may be prestressed when the mass element  31  is in the basic position  38 . In addition, the first springs  44  and the second springs  45  may also be prestressed when the mass element  31  is maximally deflected into the one direction or the other  39 ,  41 . 
     The third spring  46  is arranged on only one side of the mass element  31 , for example, between the first end  40  of the housing  33  and the mass element  31 . The third spring  46  is fixedly connected to the housing  33  but is only in contact with the mass element  31  in its basic position  38 . When the mass element  31  is moved from the basic position  38  into the first direction  39 , the third spring  46  is compressed. With a movement in the second direction  41 , the third spring  46  is released from the mass element  31  as soon as it crosses over the basic position  38 . Alternatively the third spring  46  is fixedly connected to the mass element  31  and is released from a seat  49  on the housing  33 . The length of the third spring  46  is equal to the distance of the mass element  31  to the seat  49 . The third spring  46  is without prestress when the mass element  31  is in the basic position  38 . 
     The spring stiffness of the spring mechanism  32  on the first side  50  of the mass element  31 , i.e., in the first direction  39 , may be selected to be five to ten times larger than the spring stiffness of the spring mechanism  32  on the second side  51 . In the example shown here with two first springs  44  and a third spring  46  on the first side  50  and two second springs  45  on the second side  50 , the third spring  46  may be selected with a stiffness three to eight times greater than that of the same first and second springs  45 ,  42 . 
     With the drill hammer  1  presented here, the vibration damper  30  is arranged with the first direction  39  pointing at the tool  3 , i.e., in the direction of impact  25 . When the beater  10  strikes the tool  3  and drives the latter into the substrate, this yields a short recoil of a high amplitude, which is better coupled to the stiffer side of the vibration damper  30 . A second rebound, which is weaker but longer-acting at the same time, is obtained when the beater  10  is repelled by the exciter  9  via the air cushion. This softer rebound is better coupled to the softer side of the vibration damper  30 . 
     The springs  44 ,  45 , and  46  are helical springs made of steel, for example. The first springs  44  and the second springs  45  may be arranged coaxially with the round rods  36 . 
     In another embodiment the spring mechanism  32  may be embodied with only one spring on each side  50 ,  51  of the mass element  31 , where the springs  45 ,  46  have a different spring stiffness. The softer spring  45  is preferably prestressed to the extent that it is in contact with the mass element  31  in any position of the latter. The harder spring  46  is released from the mass element  31  when the latter moves out of the basic position opposite the softer spring  45 . 
     The axis of vibration  34  is inclined parallel to or at an angle of less than 5 degrees to the working axis  5  of the handheld power tool  1 . 
       FIGS. 3 and 4  illustrate another embodiment. The spring mechanism  32  has a bending spring  60 , e.g., a plate spring which is aligned perpendicular to the axis of vibration  34 . The bending spring  60  is attached at one end  61  to a seat  62  in the housing  33  of the vibration damper. On the other end  64  the mass element  31  is attached. The mass element  31  oscillates along the axis of vibration  34 , whereupon the bending spring  60  is bent along its longitudinal extent. A basic position  38  of the mass element  31  is obtained with the bending spring  60  relaxed and unbent. 
     A helical spring  65  is arranged parallel to the axis of vibration  34  on one side of the mass element  31 . The helical spring  65  touches the mass element  31  when it is in the basic position. In a deflection of the mass element  31  into the first direction  39  the helical spring  65  is compressed. The restoring forces of the bending spring  60  and the helical spring  65  act on the mass element  31 . With a deflection of the mass element  31  in the opposite second direction  41  ( FIG. 4 ) the mass element  31  is released from the helical spring  65 . Only the restoring force of the bending spring  60  acts on the mass element  31 . 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.