Abstract:
The invention relates to a pneumatic percussive tool for a paving breaker and/or a hammer drill comprising a drive piston which is displaceable both backwards and forwards by a crankshaft and arranged in a percussion piston which is displaceable both backwards and forwards. A hollow chamber is connected to a compensating chamber by an idling air channel and embodied between the drive piston and the percussion piston in order to receive a pneumatic spring. A valve is arranged in the idling air channel, the opening and closing position thereof depending on the rotational speed of the crankshaft. If the rotational speed of the crankshaft falls below a predetermined value, the valve opens the connection between the hollow chamber and the compensating chamber so that a pneumatic spring can no longer be to embodied in the hollow chamber and the pneumatic percussion tool is placed in a idling position.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a pneumatic spring hammer mechanism or pneumatic percussive tool for a percussive hammer or drill hammer, according to the preamble of patent claim  1 . 
   2. Description of the Related Art 
   Pneumatic spring hammer mechanisms have long been known in various specific embodiments in which a drive piston is moved back and forth by a crankshaft. In front of the drive piston there is situated a percussion piston that can likewise be moved back and forth, so that between the drive piston and the percussion piston there is formed a hollow space that accommodates an air spring. This air spring, which acts as an air cushion, transmits the movement of the driven drive piston to the percussion piston, which thus follows the movement of the drive piston with a chronological delay. Finally, the percussion piston impacts a shaft of a tool, or of an intermediately connected die, and transfers its impact energy to the tool. 
   Pneumatic spring hammer mechanisms of this sort have proven effective in practice, both in hammers having electromotor drives and in hammers that operate using an internal-combustion engine. 
   In particular, the internal-combustion-engine-operated hammers have a centrifugal force coupling between a motor shaft and the crankshaft that decouples the hammer mechanism drive when the internal-combustion engine is rotating at idling speed. In this way, on the one hand an unproblematic idling operation of the engine is ensured, and on the other hand an easier starting of the engine is enabled. Such a centrifugal force coupling is technically costly, requires constructive space, and finally results in a relatively expensive, heavy hammer that is subject to wear. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   The underlying object of the invention is to provide a pneumatic spring hammer mechanism in which the advantages of the transmission and interruption of torque via the centrifugal force coupling are retained, without having to accept the named disadvantages. 
   According to the present invention, this object is achieved by a pneumatic spring hammer mechanism according to patent claim  1 . Advantageous developments of the present invention are defined in the dependent claims. 
   In the pneumatic spring hammer mechanism according to the present invention, an idling air channel is provided via which the hollow space, situated between the drive piston and the percussion piston, that accommodates an air spring during percussive operation can be connected with a compensating chamber. The compensating chamber can for example be the crank chamber, in which the crankshaft rotates in order to drive the drive piston. Alternatively, the compensating chamber can also be the area surrounding the pneumatic spring hammer mechanism; if this is the case it should be ensured that dirt, dust, moisture, etc., cannot penetrate into the hollow space via the compensating chamber and the idling air channel. 
   In addition, according to the present invention there is situated in the idling air channel a valve whose open and closed positions are dependent on the frequency of movement of the drive piston. In this way, it is achieved according to the present invention that the switching off of the pneumatic spring hammer mechanism, i.e., the transition from percussive operation to idling operation, takes place not in the known manner (i.e., mechanically via a centrifugal coupling), but rather via a movement-frequency-controlled interruption of the suction action of the hammer mechanism. 
   In an advantageous specific embodiment of the present invention, the valve can be opened if the frequency of movement of the drive piston falls below a predetermined value, so that via the idling air channel a communicating connection arises between the hollow space and the compensating chamber. In the idling operation resulting therefrom, an air spring can no longer build up in the hollow space even if the drive piston continues to execute an oscillating motion; the result is that the percussion piston, which during percussive operation is driven by the air spring, now can be neither driven forward nor suctioned backward. A reliable interruption of the percussive operation is thus ensured. 
   The frequency of movement of the drive piston is a parameter that, as far as the present invention is concerned, is equivalent to a series of further parameters. These include, in particular, the rotational speed of a crankshaft or wobble shaft that drives the drive piston, as well as the rotational speed of the drive motor that drives a drive mechanism. The rotational speed of the drive motor can in turn be determined for example through the ignition frequency, i.e., the ignition if the drive motor is an internal-combustion engine. In the case of an electric motor, the rotational speed can be determined on the basis of the power consumption. Because the drive motor is always connected with the drive piston via the drive mechanism, the movement behavior of one of these elements can be used to determine the movement behavior of the other elements as well. Due to the mostly positively-locked energy transmission from the drive motor to the drive piston, there is a linear relation between the individual movement parameters. 
   The same holds in corresponding fashion for other drive designs. If the drive piston is driven e.g. via a linear motor, this offers the expert further possibilities for determining the frequency of movement of the drive piston on the basis of various operating parameters. Correspondingly, a sensor device for acquiring the frequency of movement of the drive piston is to be formed in such a way that it can also acquire a movement parameter that cannot be allocated directly to the drive piston. Accordingly, the sensor device is for example capable of determining the frequency of movement of the drive piston by acquiring the rotational speed of the crankshaft that drives the drive piston. 
   In a preferred specific embodiment, the valve can thus be opened and closed dependent on a signal of a rotational speed sensor that acquires the rotational speed of the crankshaft. 
   Here, a movable centrifugal weight situated on the crankshaft is also to be seen as a rotational speed sensor in a broader sense, via which the valve can be controlled, whereby the valve can be opened and closed dependent on a position of the centrifugal weight. In another specific embodiment of the present invention, the rotational speed sensor is used for the electrical or electronic acquisition of the rotational speed of the crankshaft. Its signal is to be supplied to the valve, e.g., an electromagnetic valve, in a suitable fashion. 
   In a particularly advantageous specific embodiment of the present invention, an additional idling device is provided with which, independent of the frequency of movement of the drive piston or of the speed of rotation of the crankshaft, the hollow space can be brought into communicating connection with the compensating chamber, if the percussion piston moves into a forward axial position that acts as an idling position, through the sliding of a tool impacted by this piston out of a housing of the percussive and/or drill hammer. In this way, it is possible for the pneumatic spring hammer mechanism to move into an idling state independently of the above-described movement-frequency-dependent or rotational-speed dependent idling operation. In this pneumatic spring hammer mechanism, there are therefore two possibilities for achieving idling operation: on the one hand, the idling operation is set automatically if the frequency of movement of the drive piston or the rotational speed of the crankshaft falls below the predetermined value. On the other hand, independently thereof, the pneumatic spring hammer mechanism also enters idling operation if the operator lifts the tool off the stone that is to be processed, and the tool can correspondingly slide out of the housing of the hammer to some extent. 
   The pneumatic spring hammer mechanism according to the present invention can be realized for various design principles, e.g. in what is called a hollow-hammer hammer mechanism, in which the drive piston moves in a hollow region of the percussion piston, or in a hollow piston hammer mechanism, in which the drive piston has a hollow area in which the percussion piston is accommodated so as to be able to move axially, or in a tube hammer mechanism, in which the percussion piston and the drive piston have essentially the same diameter, and are guided in common in a hammer mechanism tube. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other advantages and features of the present invention are explained in more detail below on the basis of an exemplary embodiment, with the aid of the accompanying Figures. 
       FIG. 1  shows a schematic section of a pneumatic spring hammer mechanism according to the present invention, realized as a “hollow-hammer hammer mechanism,” in percussive operation; 
       FIG. 2  shows the pneumatic spring hammer mechanism of  FIG. 1 , in idling operation; 
       FIG. 3  shows a pneumatic spring hammer mechanism according to the present invention, realized as a “hollow-piston hammer mechanism”; 
       FIG. 4  shows a pneumatic spring hammer mechanism according to the present invention, realized as a “tube hammer mechanism”; and 
       FIG. 5  shows a pneumatic spring hammer mechanism according to the present invention, realized as a “hollow-piston hammer mechanism,” and having an electronically controlled valve. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a section through a pneumatic spring hammer mechanism according to the present invention, in percussive operation. 
   Via a connecting rod  2 , a crankshaft  1  drives a drive piston  3  back and forth axially. Drive piston  3  is housed in a percussion piston  4 , which in turn can be moved back and forth axially inside a tube-shaped hammer mechanism housing  5 . 
   Such a pneumatic spring hammer mechanism is also designated a hollow-hammer hammer mechanism, and is known. 
   In percussive operation, drive piston  3  moves back and forth inside percussion piston  4 , which causes an air spring to build up in a hollow space  6  formed between drive piston  3  and percussion piston  4 . When drive piston  3  moves forward (to the left in FIG.  1 ), the air in hollow space  6  is compressed. The energy of the air compressed as an air spring is emitted to percussion piston  4  and likewise drives it forward, against a shaft  7  (shown schematically) of a chiseling tool. Instead of shaft  7 , a rivet header, which is not depicted but is known, can also be impacted by percussion piston  4 . 
   After the impact has taken place, drive piston  3  has already begun its travel back, due to the rotational movement of crankshaft  1 , and suctions percussion piston  4 , which is rebounding from shaft  7 , further backward, until drive piston  3  finally goes into forward motion again, and, through the buildup of a pressure in the air spring, the percussion cycle begins again. 
   Via an idling opening  8 , an annular groove  9  formed on the inside of hammer mechanism housing  5 , and a channel  10 , hollow space  6  is connected with a crank chamber  11  that acts as a compensating chamber. Crank chamber  11  essentially circumscribes the space in which crankshaft  1  can move with connecting rod  2  and drive piston  3 . 
   Idling operation opening  8 , annular groove  9 , and channel  10  together form an idling air channel. 
   At the crank-chamber end of channel  10 , there is situated a valve  12  that is coupled in one piece with a centrifugal weight  13 . Together with centrifugal weight  13 , valve  12  can be moved radially, with reference to crankshaft  1 , in a guide  16 , against the action of a spring  15  that is supported against a stop  14 . 
     FIG. 1  shows the percussive operation of the pneumatic spring hammer mechanism, in which crankshaft  1  is driven with the operating rotational speed of an internal-combustion engine (not shown), or, if a transmission is situated between the internal-combustion engine and crankshaft  1 , with a rotational speed corresponding to the operating rotational speed. Due to the centrifugal force acting on centrifugal weight  13  and, if warranted, also on valve  12 , valve  12 , together with centrifugal weight  13 , is held in guide  16  in the position shown in  FIG. 1 , radially outward against the action of spring  15 . Channel  10  is closed by valve  12 . 
     FIG. 2  shows the same pneumatic spring hammer mechanism as is shown in  FIG. 1 , but in idling operation. 
   Idling operation is a state in which the internal-combustion engine (not shown) rotates not at the operating rotational speed, but with a lower rotational speed, in particular the idling rotational speed. 
   Due to the reduced rotational speed of crankshaft  1 , spring  15  is now strong enough to press centrifugal weight  13  with valve  12  radially inward into guide  16 . In this way, valve  12  moves into an open position in which an opening  17  is aligned with channel  10 , and opens this channel. 
   In this way, there arises a communicating connection between hollow space  6  and crankshaft chamber  11 , via idling opening  8 , annular groove  9 , and channel  10 . As a consequence, no air pressure, and, correspondingly, no air spring, can build up in idling hollow space  6 . Although drive piston  3  continues to move back and forth in percussion piston  4 , percussion piston  4  remains in its idling position, because pneumatic forces no longer act on it due to the lack of changes in air pressure. The pneumatic spring hammer mechanism is thus reliably in idling operation. 
   The action of spring  15  is overcome only when the rotational speed of the motor, and therewith the rotational speed of crankshaft  1 , is increased, so that centrifugal weight  13 , with valve  12 , slides radially outward and closes opening  17 . In this way, hollow space  6  is separated from crank chamber  11 , and an air spring can again build up in hollow space  6 . 
   Strictly speaking, centrifugal weight  13  also acts as a rotational speed sensor, because it detects a change in rotational speed through the shifting of its radial position. The shifting of the radial position is in turn to be evaluated as a signal dependent on which valve  12 , which is connected in one piece with centrifugal weight  13 , is opened or closed. 
   In the above-described specific embodiment, the rotational speed of crankshaft  1  is the criterion according to which the frequency of movement of drive piston  3  is determined. However, instead of the rotational speed of the crankshaft, the rotational speed of the drive motor (not shown) could also be acquired through centrifugal weight  13 . 
   Instead of crankshaft  1 , it is possible to bring drive piston  3  into oscillating back-and-forth movement using other drive mechanisms, such as for example a wobble plate. 
   It should additionally be mentioned that an additional idling device is formed by idling opening  8 , an opening  18 , and a channel  19 . Via idling opening  8 , opening  18 , and a channel  19 , a further communicating connection between hollow space  6  and crank chamber  11  can be produced independently of the above-described connection via annular groove  9  and channel  10 . The manner of functioning of the additional idling device is explained below on the basis of FIG.  3 . 
     FIGS. 3 and 4  show other constructive designs for pneumatic spring hammer mechanisms according to the present invention, in which the manner of functioning of valve  12  and centrifugal weight  13 , dependent on the rotational speed of the crankshaft, is comparable with the pneumatic spring hammer mechanism according to  FIGS. 1 and 2 . Therefore, only the essential differences are explained. 
     FIG. 3  shows, in schematic section, a pneumatic spring hammer mechanism according to the present invention, also designated a hollow-piston hammer mechanism, in which a drive piston  20  having a hollow construction is moved back and forth by connecting rod  2 . 
   In the interior of the hollow space of drive piston  20 , a massive percussion piston  21  can likewise be moved back and forth axially. 
   In the cylindrical side wall of drive piston  20 , a plurality of idling openings  22  are provided via which a hollow space  23  formed between drive piston  20  and percussion piston  21  can be connected with an opening  24  and with channel  10 . 
   Channel  10  leads to crank chamber  11 , which acts as a compensating chamber; valve  12  with centrifugal weight  13  is switched between these in the manner described above. 
   Idling openings  22  are situated axially to one another, so that a constant connection between hollow space  23  and opening  24  is ensured in every axial position of drive piston  20 . 
   In addition, an opening  25  is provided that leads to an additional channel  26 , which likewise stands in communicating connection with crank chamber  11 . Opening  25  can pass over into idling openings  27  provided in drive piston  20 . 
   In this way, an additional idling device is realized with which, independently of the above-described idling device that is dependent on the crankshaft rotational speed, hollow space  23  can be brought into communicating connection with crank chamber  11  when percussion piston  21  moves into its furthest forward axial position (not shown in FIG.  3 ). This axial position, also designated the idling position, is possible whenever the operator lifts the chisel from the stone to be processed, so that shaft  7  slides out of the housing of the hammer somewhat. A rear edge  28  of percussion piston  21  then passes over opening  25 , and clears a connection between hollow space  23  and opening  25 . In this case, hollow space  23  is brought into communicating connection with crank chamber  11 , so that no air spring can build up in hollow space  23 . This idling device enables an idling operating state to be achieved independently of the movement-frequency-dependent idling operation or of the rotational speed of crankshaft  1 , and is thus a supplementary feature. 
     FIG. 4  shows a variant, also designated a tube hammer mechanism, of the pneumatic spring hammer mechanism according to the present invention. 
   A drive piston  30 , driven by crankshaft  1  and connecting rod  2 , can be moved back and forth axially in a housing part that is also designated hammer mechanism tube  31 . 
   In the same hammer mechanism tube  31 , a massive percussion piston  32 , having essentially the same diameter as drive piston  30 , is likewise situated so as to be capable of axial movement. 
   Between drive piston  30  and percussion piston  32 , a hollow space  33  is formed that accommodates an air spring for driving percussion piston  32 . Via an opening  34  and channel  10 , hollow space  33  can be brought into communicating connection with crank chamber  11 ; at the end of channel  10 , valve  12  with centrifugal weight  13  is situated in the manner described above. 
   Here as well, the communicating connection between hollow space  33  and crank chamber  11  can be controlled dependent on the rotational speed of crankshaft  1 . 
   In addition, an opening  35  that leads to an additional channel  36  and thus to crank chamber  11  is provided as an additional idling device. 
   As has already been described in connection with  FIG. 3 , this additional idling device makes it possible for the hammer mechanism to move into an idling operating state even when percussion piston  32  has reached its furthest forward position (not shown in  FIG. 4 ) after the lifting of the chisel from the stone to be processed and the corresponding sliding of shaft  7  out of the housing. In this case, a rear edge  37  of percussion piston  32  slides over opening  35  and clears a connection between hollow space  33  and channel  36 . 
   The additional idling device has the consequence that the hammer mechanism can also move into the idling operating state independently of the rotational speed of the motor or of the crankshaft. 
   After the tool has been placed again onto the stone to be processed, and shaft  7  has correspondingly moved into the interior of the housing, percussion piston  32  is pushed into the position shown in  FIG. 4 , whereby the connection between hollow space  33  and channel  36  or crank chamber  11  is closed. As long as crankshaft  1  is at operating rotational speed, and, correspondingly, opening  17  on valve  12  is closed, percussive operation can begin again. 
     FIG. 5  shows a pneumatic spring hammer mechanism that operates according to the same principle as does the pneumatic spring hammer mechanism of  FIG. 3. A  repeated description of the effective mechanical connections is therefore omitted. 
   However, in place of valve numeral  12  and centrifugal weight  13 , a rotational speed sensor  40  is here situated in the vicinity of crankshaft  1 . Rotational speed sensor  40  is used to acquire the rotational speed of crankshaft  1 . It can operate according to various known principles, e.g. magnetically, optically, inductively, etc. 
   Rotational speed sensor  40  supplies a signal to a control unit (not shown) that controls an electromagnetic valve  41 , situated in a channel  42  that connects hollow space  23  with crank chamber  11 , dependent on the determined rotational speed value. As long as the rotational speed of crankshaft  1  is greater than a predetermined value, valve  41  is closed, and interrupts a connection between hollow space  23  and crankshaft  11 . Valve  41  is for example a 2/2-way valve having a valve body that can be pivoted electromagnetically between two positions. If, however, the rotational speed of crankshaft  1  is less than the predetermined value, the control unit opens valve  41 , so that a communicating connection between hollow space  23  and crank chamber  11  results via channel  42 . 
   The electronic solution shown in  FIG. 5  has, in relation to the mechanical solution shown in connection with  FIGS. 1  to  4 , the advantage that the “dead spaces,” i.e., the space present between hollow spaces  6 ,  23 , and  33  and valve  12 ,  41 , are smaller due to the smaller spacing. This enables a better suctioning back in percussive operation. 
   In all the specific embodiments of the present invention presented here, it has been specified that the hollow space between the drive piston and the percussion piston is to be brought into connection with the crank chamber, which acts as a compensating chamber. As an alternative to this, it is also possible for other hollow spaces that guarantee a certain freedom from dust and therefore a certain cleanness to act as a compensating chamber in a percussive hammer and/or drill hammer. In principle, it would also the possible to bring the hollow space into connection with the area surrounding the percussive hammer and/or drill hammer. However, in this case it would be necessary to use an air filter to ensure that no dirt can penetrate into the hollow space. 
   Besides the already-described mechanical and electromagnetic valves  12 ,  41 , other known valve types, such as for example piezovalves, can be used. 
   In the above specification, it is assumed that the rotational speed of crankshaft  1  is identical with the rotational speed of the internal-combustion engine. Of course, variants are also possible in which a gear mechanism is connected between the motor and crankshaft  1  in order to convert the rotational speed. Here, the predetermined rotational speed value of crankshaft  1 , used as a boundary value for the opening and closing of the valve, is usefully adapted correspondingly to the idling rotational speed of the internal-combustion engine. 
   Instead of the determination of the crankshaft rotational speed by the sensor device, it is also possible to acquire the frequency of movement of the drive piston directly, for example via a proximity sensor, or also to acquire the movement of connecting rod  2 . In addition, there are numerous possibilities for determining the frequency of movement of the drive piston on the basis of the rotational speed of the drive motor, its ignition frequency, or, in the case of an electric motor, the electrical drive frequency or its power consumption.