Abstract:
A hand-held machine tool having a linear drive for moving a tool along a working axis, e.g. a motor-driven pneumatic driving tool, is provided. At least two dampers are provided in the hand-held machine tool for damping vibrations along the working axis. A resonant excitation of a vibration of the first damper along the working axis occurs at a first resonance frequency, which differs from a second resonance frequency of the second damper for the vibration along the working axis.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority to German Patent Application DE 10 2010 043 810.3, filed Nov. 12, 2010, and entitled “Handwerkzeugmaschine” (“Hand-Held Machine Tool”), the entire content of which is incorporated herein by reference. 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    [Not Applicable] 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    [Not Applicable] 
       BACKGROUND OF THE INVENTION 
       [0004]    The present invention relates to a hand-held machine tool with a damper. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    A hand-held machine tool according to aspects of the present invention has a linear drive for moving a tool along a working axis, e.g. a motor-driven, pneumatic striking tool. At least two dampers are provided in the hand-held machine tool for damping vibrations along the working axis. A resonant excitation of a vibration of the first damper along the working axis occurs at a first resonance frequency, which differs from a second resonance frequency of the second damper for the vibration along the working axis. The two resonance frequencies may differ in a range of about 2% to 5%, i.e., the first frequency is 1.02 to 1.05 times greater than the second resonance frequency. 
         [0006]    One embodiment provides that each of the two dampers has a pendulum arm and an inertial mass. The inertial mass is fastened to a housing of the hand-held machine tool elastically by means of the pendulum arm. The end of the pendulum arm at a distance from the inertial mass forms a bearing point in order to execute a rotary motion by the pendulum arm guided by the inertial mass. The deflection preferably remains small, e.g., less than about 30 degrees around the bearing point, whereby the motion of the inertial mass is approximately perceived as being along the working axis. To counteract a deflection out of the plane of rotation, the pendulum arm or the bearing is designed so it is very stiff, which results in very high resonance frequencies. These high resonance frequencies should be at least on an order of magnitude or ten times higher than the resonance frequencies for an excitation along the working axis in order not to allow excitation. 
         [0007]    In certain embodiments, an adaptation of the resonance frequencies of the two dampers can occur using the length of the pendulum arms, which can differ in a range from about 4% to 10%. In this case, the length indicates the distance of the inertial mass center of gravity up to the bearing point on the housing. Alternatively or additionally, the masses of the inertial masses can differ by about 4% to 10%. 
         [0008]    One embodiment provides that the pendulum arm of a first of the two dampers is arranged parallel to a pendulum arm of a second of the two dampers. The pendulum arms can be mounted at least about 70 degrees with respect to the working axle. The pendulum arms can be designed as leaf springs. The leaf springs can be connected by a rib at the end at a distance from the inertial masses. In one embodiment, the two leaf springs are produced as stamped parts. The inertial masses can be slipped onto the pendulum arm. 
         [0009]    One embodiment provides that a periodicity with which the linear drive moves the tool along the working axis lies between the resonance frequencies of the two dampers. The tool is typically non-harmonic, i.e. not clearly sinusoidal motion. Therefore the term periodicity or repetition rate appears more suitable for indicating how frequently the tool moves back and forth in a time standard. The periodicity, like a frequency, is measured in Hertz. If a frequency is used in the application to describe a non-harmonic motion, this indicates the base frequency. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0010]    The following description explains the invention using exemplary embodiments and figures. In the figures, 
           [0011]      FIG. 1  shows a hand-held machine tool formed in accordance with an embodiment of the present invention. 
           [0012]      FIG. 2  shows a cross section through a damper of  FIG. 1 . 
           [0013]      FIG. 3  shows an excitation spectrum of the damper of  FIG. 2 . 
       
    
    
       [0014]    Elements that are the same or have the same function are indicated with the same reference numbers in the figures, unless otherwise indicated. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  shows a hammer drill  1  schematically. The hammer drill  1  has a tool holding fixture  2 , in which a boring tool  3  can be used. A motor  4  forms a primary drive of the hammer drill  1 , which drives a striking tool  5  and an output shaft  6 . A user can guide the hammer drill  1  using a handle  7  and put the hammer drill  1  in operation using a system switch  8 . In operation, the hammer drill  1  turns the boring tool  3  continuously around a working axis  9  and in this process can drive the boring tool  3  into a substrate along the working axis  9 . 
         [0016]    The striking tool  5  is, for example, a pneumatic striking tool  5 . An exciter  10  and a striker  11  are guided in the striking tool  5  along the working axis  9 . The exciter  10  is linked to the motor  4  by a cam  12  or a finger and forced into a periodic linear motion. A pneumatic spring formed by a pneumatic chamber  13  between exciter  10  and striker  11  couples a motion of the striker  11  to the motion of the exciter  10 . The striker  11  can strike directly at the back end of the boring tool  3  or transfer part of its pulse to the boring tool  3  by way of an essentially resting intermediate striking  14 . The striking tool  5 , and preferably the other drive components, is arranged inside a machine housing  15 . 
         [0017]    Within the machine housing  15 , a first damper  20  and a second damper  21  are mounted. In the side view in  FIG. 1 , the first damper  20  covers the second damper  21 . The cross section in the plane II-II through the two dampers  20 ,  21  is shown in  FIG. 2 . 
         [0018]    The first damper  20  has a first inertial mass  22  that is connected by way of a leaf spring  23  to a rigid bearing point  24  on the housing  15 . The leaf spring  23  is, in rest position, arranged at an angle  25  of at least about  70  degrees with respect to the working axis  9 . A motion of the machine housing  15  along the working axis  9  can excite the inertial mass  22  to the same type of motion along the working axis  9 . Because of the guide of the inertial mass  22  by the leaf spring  23 , the inertial mass  22  follows a curved path  26 . The deflections of the inertial mass  22  are small compared to a length  27  of the leaf spring  23 , whereby the motion can be assumed to be approximately parallel to the working axis  9 . The length  27  of the leaf spring  23  is measured from the fastening  24  to the center of gravity of the first inertial mass  22 . By using a restoring force, the leaf spring  23  counteracts a deflection of the inertial mass  22  from its rest position. The restoring spring force, the length  27  of the leaf spring  23  and the mass of the inertial mass  22  determine a resonance frequency of the first damper  20 . 
         [0019]    The leaf spring  23  has a lower stiffness along the working axis  9  compared to the directions perpendicular to the working axis  9 . An excitation of the leaf spring  23  perpendicular to the working axis  9  is thus only possible at very high frequencies. 
         [0020]    The second damper  21  is structured generally the same as the first damper  20 . A second inertial mass  28  is connected by way of a second leaf spring  29  to the machine housing  15 . The second leaf spring  29  is preferably mounted parallel to the first leaf spring  23  and also, in rest position, tipped by at least about 70 degrees with respect to the working axis  9 . The two leaf springs  22 ,  29  preferably have the same spring constant and thickness; in contrast a length  30  of the second leaf spring  29  is about 4% to 10% longer than the length  27  of the first leaf spring  22 . A mass of the second inertial mass  28  is approximately the same as the mass of the first inertial mass  22 . The different lengths  30 ,  29  cause an about 2% to 5% lower resonance frequency of the second damper  21 . In another embodiment, the inertial masses  22 ,  28  have masses that are different by about 4% to 10%. 
         [0021]    The leaf springs  22 ,  29  can be produced as stamped sheet metal. The two leaf springs  22 ,  29  can connect via a bridge  31 . 
         [0022]      FIG. 3  shows the behavior of the two dampers  20 ,  21  for various excitation frequencies f; the deflection, standardized to the maximum deflection A (amplitude) of the inertial masses  22 ,  28  along the working axis  9 , is entered over the Y axis. The curve  32  indicates the excitation spectrum for the first damper  20 ; curve  33  shows the excitation spectrum for the second damper  21 . 
         [0023]    The two dampers  20 ,  21  are tuned to each other. The tuning of the resonance frequency  34  of the first damper  20  is greater than the resonance frequency  35  of the second damper  21 . An excitation of a damper with frequencies greater than its resonance frequency can lead to a build-up of the damper in the hand-held machine tool  1  and, instead of a desired damping of vibrations causes an increase in the vibrations. This actually contradicts the use of a second damper with another frequency for damping vibrations along working axis  9 . However it was found that when the two dampers  20 ,  21  are only somewhat tuned to each other, these couple with each other and the lower-frequency damper  21  still does not build up if the excitation frequency f through the linear drive  5  lies between the resonance frequencies  35 ,  34  of the two dampers  20 ,  21 . The resonance frequency  34  of the first damper  20  should lie within a frequency band  36 , within which the excitation spectrum  32  of the second damper  21  drops to no more than about one-fourth (shaded area), and preferably to no more than about one-half of the maximum amplitude. The two dampers  20 ,  21  then couple strongly with each other. In total, a broader resonance results for the entire system of the two dampers  20 ,  21 . The coupling of the two dampers  20 ,  21  can be further increased by the elastic bridge  31  between the leaf springs  29 ,  22 . The resonance frequencies  34 ,  35  are preferably adjusted using the pendulum arms  23 ,  29  and the inertial masses  22 ,  28  in such a way that a periodicity of the linear drive  5  lies between the resonance frequencies  34 ,  35 . 
         [0024]    The dampers  20 ,  21  can also be used in a compass saw or a saber saw.