Abstract:
A percussive mechanism, which is provided in the form of an, e.g. pneumatic spring percussive mechanism, comprises an electrodynamic linear drive, a drive piston, which can be reciprocally moved inside a percussive mechanism housing by the linear drive, and a percussive piston. An additional hollow space is provided in front of and/or behind the drive piston and can be isolated at least in part from the surrounding area so that a pneumatic spring can be created in the additional hollow space. The pneumatic spring slows the drive piston at its returning points and facilitates a returning motion without loading the electrodynamic linear drive.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a percussion mechanism having an electrodynamic linear drive. 
     2. Description of the Related Art 
     Drilling and/or striking hammers (designated “hammers” hereinafter) are standardly driven by electric motors in which a rotor rotates a drive shaft. The rotational movement is converted into an oscillating linear movement that is communicated to a drive element in a percussion mechanism. Here, as a percussion mechanism in particular a pneumatic spring mechanism is suitable in which a drive piston that acts as a drive element is moved back and forth. 
     From DE 102 04 861 A1, a pneumatic spring percussion mechanism is known for a hammer in which a drive piston is capable of being driven by an electrodynamic linear drive. The drive piston is coupled to a runner of the linear drive, so that the linear back-and-forth movement of the runner is transmitted to the drive piston. The movement of the drive piston is in turn transmitted (as is standard in pneumatic spring percussion mechanisms) via an air spring to a percussion piston that strikes the end of a tool or strikes an intermediately situated header in a known manner. 
     In a linear electromagnetic drive system of this sort, the runner and the drive piston coupled thereto must be braked when they reach their extreme positions in order to enable a change in the direction of movement. Only in this way is an oscillating percussive operation possible. During the braking, it, is possible to feed part of the kinetic energy back into an intermediate circuit as electrical energy. However, in the coils of the stator that surrounds the runner, heat loss occurs that has an adverse effect on the efficiency of the percussion system. In addition, the lost heat must be conducted away using a suitable cooling device. 
     It is therefore advantageous to intermediately store the kinetic energy of the drive unit made up of the runner and the drive piston in a spring, so that after the reversal of the direction of movement this energy is available for the counter-movement, and supports the electromagnetic drive force of the linear drive. 
     From each of EP 0 718 075 A1 and DE 24 19 164 A1, an electrodynamic drive is known for a percussion mechanism in which a return movement of a percussion piston is received by a mechanical helical spring acting as an end stop. When the percussion piston moves forward again, the helical spring releases the stored energy and thus supports the forward or percussive movement. The described percussion mechanisms are however not pneumatic spring percussion mechanisms, and do not have any separation between a drive piston and a percussion piston. 
     In addition, helical springs have the disadvantage that they can break due to the high impact speeds. Also, significant vibration noise results. Moreover, if the helical spring is too weak, given a correspondingly high impact speed of the percussion piston the spring can bottom out, which can result in damage to the percussion mechanism. 
     DE 27 28 485 A1 indicates an electromagnetically operated percussion device in which a shaped piece that acts as a percussion tool is surrounded by a plunger that can be moved cyclically in the impact direction by an electromagnet. At the rear end of the percussion tool, a piston is provided that operates against a pneumatic damper. 
     From U.S. Pat. No. 1,467,677, an electric hammer is known in which a piston is actuated in alternating fashion by two electromagnets and is moved back and forth in this way. At one end of the piston there is situated a hardened steel tip that strikes a percussion tool. On the opposite side of the piston, an air spring is provided whose strength can be adjusted by opening and closing air ducts. 
     OBJECT OF THE INVENTION 
     The underlying object of the present invention is to indicate a percussion mechanism having an electrodynamic linear drive in which an electromagnetic drive force that is used to reverse the direction of movement of a linearly moved drive unit is supported without having to accept the disadvantages associated with other types of percussion mechanisms. 
     According to the present invention, this object is achieved by a percussion. A percussion mechanism according to the present invention has an electrodynamic linear drive, a drive element that can be moved back and forth in a percussion mechanism housing by the linear drive, a percussion element that strikes a tool, and a coupling, or a coupling device, that is effective between the drive element and the percussion element, via which the movement of the drive element is capable of being transmitted to the percussion element. According to the present invention, the percussion mechanism is characterized in that, seen in the direction of impact, a reversing hollow space is effectively provided before and/or after the drive element, and in that the reversing hollow space is capable of being separated at least at times from the surrounding environment, in such a way that in the reversing hollow space is capable of being separated at least at times from the surrounding environment, in such a way that in the reversing hollow space it is possible to produce a reversing air spring that acts against the drive element and/or against the percussion element. 
     Correspondingly, according to the present invention it is provided that an air spring can be produced in front of and/or behind the drive element during the operation of the percussion mechanism. This reversing air spring, as it is called, is charged, or “tensioned” or compressed, by the movement of the drive element when the drive element moves in, the direction of the air spring or of the reversing hollow space that accommodates the air spring. When there is a reversal of the linear movement of the drive element, the air pressure, then prevailing in the reversing air spring exerts a force on the drive element that supports the reversal of the direction of movement and accelerates the drive element in the opposite direction. 
     It is not absolutely necessary for the reversing air spring to actually be spatially situated axially in front of or behind the drive element. The actual location of the reversing air spring situated in the reversing hollow space is, rather, arbitrary. However, what is important is that the action of the force of the reversing air spring be capable of being transmitted to the drive element (or percussion element), or, conversely, that the charging of the reversing air spring by the drive element (percussion element) be possible. 
     Air spring systems have proven their usefulness in percussion mechanisms, and have a very high degree of reliability. If designed properly, they also have a high degree of efficiency. A complete compression of the air spring, and thus an impact stress on the solid-body components that are moved relative to one another and that form the hollow space, can be avoided due to the progressivity of the spring characteristic (especially in the end area). The constructive length of the reversing air spring can correspondingly be made shorter than is the case in linear metal spring systems (helical springs). In addition, air springs produce less sound. In a particularly advantageous specific embodiment of the present invention, the drive element is connected to a runner of the linear drive and forms an integrated drive unit with the runner. In particular, it is advantageous if the drive element bears the runner or is essentially completely formed by the runner, so that the runner simultaneously takes over the function of the drive element. 
     The linear motor can be a switched reluctance motor (SR motor), and has in the area of movement of the runner a plurality of drive coils (stators) that are connected in a manner corresponding to the desired movement of the drive element. It is to be noted that in the context of the present invention an electrodynamic drive (e.g. in the form of a single electromagnetic coil) acting as a drive coil for the drive element is also regarded as a linear motor. The backward movement of the drive element can then take place for example exclusively via a reversing air spring that can be produced in a reversing hollow space that is present in front of the drive element. 
     In a specific embodiment, the coupling device is formed by a stop that is effective between the drive element and the percussion element. Via the stop, the drive movement of the drive element can be transmitted directly to the percussion element. A variant is possible in which the coupling device is formed by two stops that move the percussion element back and forth corresponding to the movement of the drive element. 
     Preferably, the coupling device is formed as an elastic, in particular spring-elastic, element that is effective in at least one direction between the drive element and the percussion element. In this way, it is possible to reduce the noise emission and mechanical stresses on the relevant components. As an elastic element, a coupling air spring (explained in more detail below) can be used. Alternatively, the above-described stops can be supplemented by an elastic element or provided with an elastic layer in order to deploy a spring-elastic effect. 
     In a preferred specific embodiment, the reversing hollow space is situated at one end of the drive element, between the drive element and the percussion mechanism housing, in particular between the drive unit and the percussion mechanism housing. The reversing hollow space can correspondingly also be situated at one end of the runner coupled to the drive element. The situation at one end makes it possible for the reversing air spring that can be produced in the reversing hollow space to act immediately on the drive unit and thus on the drive element. 
     It is particularly advantageous that the reversing air spring that can be produced in the reversing hollow space counteracts at least at times a movement of the drive element. In this way, the drive element can compress or charge the reversing air spring during its movement. After a reversal of the direction of movement of the drive element, the reversing air spring releases its stored energy and supports the counter-movement of the drive element. 
     Advantageously, the reversing air spring that can be produced in the reversing hollow space counteracts the movement of the drive element at least shortly before a reversal of direction of the drive element. In this way, the reversing air spring contributes to a braking of the drive element shortly before its reversal of direction. Depending on the dimensioning of the linear drive and of the reversing air spring, in some circumstances it is even possible in this way for a return movement of the drive element to be brought about solely by the reversing air spring, while the linear drive is switched off. Likewise, it is possible for the linear drive to control the return movement of the drive element with only low power. If necessary, for this purpose a sensor mechanism is to be provided that constantly determines the precise location of the drive element or of the runner and in this way monitors the action of the reversing air spring. With the aid of the sensor mechanism and a corresponding control unit, the linear drive can be controlled in such a way that the drive element and the runner follow a prespecified course of movement. 
     In a particularly preferred specific embodiment of the present invention, the reversing hollow space is a “first” hollow space that is provided in front of the drive element, a part of the percussive element passing through the first hollow space. 
     It is particularly advantageous if, alternatively or in addition to the first hollow space, a reversing hollow space is provided as a “second” hollow space behind the drive element, and, when there is a return movement, opposite to the direction of impact, of the drive element, the reversing air spring capable of being produced in the second hollow space is effective at least over a movement path of the drive element of greater than 30%, in particular greater than 50%, of the overall path of the return movement of the drive element. 
     Whereas above it was defined that a reversing hollow space is situated in front of the drive element as the “first hollow space,” the reversing hollow space behind the drive element is designated the “second hollow space.” These differing designations are intended only for clarification, and do not have any further meaning with respect to the functioning of the device. Both the first hollow space in front of the drive element and also the second hollow space behind the drive element act as “reversing hollow spaces” for accommodating a reversing air, spring that supports the respective reversal of direction of the drive element and the corresponding acceleration in the opposite direction. The first and the second hollow space can be provided in the percussion mechanism alternatively or together. 
     The relatively elongated effectiveness of the reversing air spring in the second hollow space means that the reversing air spring situated behind the drive element builds up over as long a path as possible, so that the drive unit has to exert force against this reversing air spring over almost its entire return path in order to compress this spring. While during the forward movement of the drive unit in the direction of impact it is sought to transmit as large a portion as possible of the drive energy to the percussion element, so that this portion of drive energy is available as impact energy, during the return movement of the drive unit there is a certain excess of energy, because during the return movement no impact is to be carried out. This excess of energy can now be used to charge the reversing air spring behind the drive element over as long a path as possible. The energy stored in the reversing air spring is then available for the next forward movement, and supports the effect of the linear drive for impact production. In this way, the linear drive can be made weaker, so that the power loss to be applied in the stator coils is also reduced. 
     The drive force produced by the coils is proportional to the current flowing through them, while the power loss in the coils is proportional to the square of the current. The impact or percussion energy is proportional to the product of the force times the path. If the path of the drive element is lengthened, the force that is to be produced by the linear drive, i.e. the stator coils, can be reduced in order to obtain the same energy effect. This increases the efficiency. Even if the air spring itself produces losses, the overall balance is positive compared to an electrical intermediate storage of the electrical braking energy in an intermediate circuit. 
     Preferably, there is provided a ventilation opening that can be closed at times between the reversing hollow space and the surrounding environment. Via the ventilation opening, it is possible to equalize the air between the reversing air spring in the reversing hollow space and the surrounding environment in order to compensate gap losses that necessarily occur during the compression phases. 
     Preferably, the ventilation opening is provided in the percussion mechanism housing in an area past which the drive element or drive unit travels during a percussion cycle. The opening and closing of the ventilation opening can in this way be immediately taken over by the drive element or drive unit itself, without requiring an additional control mechanism. 
     Correspondingly, it is particularly advantageous if the ventilation opening is capable of being opened or closed during a percussion cycle depending on the position of the drive element and/or of the drive unit. 
     A specific embodiment is particularly advantageous in which the percussion mechanism is realized as a pneumatic spring percussion mechanism. For this purpose, the drive element is fashioned as a drive piston and the percussion element is fashioned as a percussion piston. The coupling device is formed by a coupling air spring that acts in a coupling hollow space between the drive piston and the percussion piston. The coupling air spring ensures the transfer of energy from the drive piston to the percussion piston, and is responsible in a known manner for the designation “pneumatic spring percussion mechanism.” Pneumatic spring percussion mechanisms are known from the prior art in many embodiments. However, according to the present invention what is new is the possibility of braking the drive piston and/or the percussion piston using the additional reversing air spring. The coupling air spring can also be regarded as a main air spring, because a significant part of the impact energy is transmitted by it. 
     In a particularly advantageous specific embodiment of the present invention, the drive piston essentially encloses the percussion piston. The percussion piston has a piston head, and, relative to the forward-directed direction of impact, the coupling hollow space having the coupling air spring for transmitting the impact energy to the percussion piston is situated behind the piston head. In front of the piston head, another hollow space for a return air spring is formed between the drive piston and the percussion piston. Such a hollow piston percussion mechanism having a double-action air spring is known. Accordingly, the drive piston has a hollow cavity in which the percussion piston can move back and forth. The return air spring ensures a controlled return movement of the percussion piston after the impact. In this way, the percussion piston is connected in an entrained manner to the movement of the drive piston in its return movement as well. 
     In order to permit formation of the hollow space for the return air spring in front of the piston head, it is necessary that the drive piston enclose the percussion piston not only in the rear area, i.e. in the area of the main air spring, but also in the front area in front of the piston head. Only a shaft of the percussion piston extending from the piston head can be led out from the drive piston. 
     In a preferred specific embodiment, the reversing air spring acts only against the drive piston, and not against the percussion piston. In this way, the percussion piston is freely movable and receives all of its kinetic energy via the coupling to the drive piston. 
     In another specific embodiment of the present invention, however, the reversing air spring additionally acts at least in a direction of movement of the percussion piston, or may even act only against the percussion piston. In this variant, in particular during its return movement the percussion piston can run against the reversing air spring and charge it, so that the reversing air spring, which is not coupled to the drive piston, supports the subsequent forward movement of the percussion piston. 
     In such a specific embodiment, it can be advantageous if the percussion piston is connected with a positive fit to a reversing piston, so that the reversing piston acts against the reversing air spring. It is then possible to situate the reversing air spring at a location remote from the percussion piston. 
     In another specific embodiment of the present invention, the reversing air spring acts at least at times axially against the drive element or against the percussion element, the reversing hollow space being provided in an area that is not situated axially to the drive element. For this reason, a transfer device is provided with which the drive element can be coupled non-positively to the reversing air spring formed in the reversing hollow space. The reversing hollow space can in this way be situated for example laterally next to the drive element or in another area of the percussion mechanism or of the hammer driven thereby. 
     This specific embodiment enables the free situation of the reversing air spring at a location at which there is suitable space for it. Thus, the reversing hollow space with the reversing air spring can for example be situated, next to the drive element. 
     These and additional advantages and features of the present invention are explained in more detail below on the basis of examples, with the aid of the accompanying Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic view of a section through a percussion mechanism, realized as a pneumatic spring percussion mechanism, according to a first specific embodiment of the present invention, having a drive unit in the extreme rear position; 
         FIG. 2  shows the pneumatic spring percussion mechanism of  FIG. 1  with the drive unit in the center position; 
         FIG. 3  shows the pneumatic spring percussion mechanism of  FIG. 1  with the drive unit in the extreme front position; 
         FIG. 4  shows a schematic view of a section through a percussion mechanism, realized as a pneumatic spring percussion mechanism, according to a second specific embodiment of the present invention, having a drive unit in the extreme rear position; 
         FIG. 5  shows the pneumatic spring percussion mechanism of  FIG. 4  with the drive unit in the center position; 
         FIG. 6  shows the pneumatic spring percussion mechanism of  FIG. 4  with the drive unit in the extreme front position; 
         FIG. 7  shows a schematic view of a section through a percussion mechanism according to a third specific embodiment of the present invention; and 
         FIG. 8  shows a schematic representation of a section through a percussion mechanism according to a fourth specific embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 1 to 3  and  4  to  6  show two different specific embodiments of the percussion mechanism according to the present invention, realized as a pneumatic spring percussion mechanism, in a highly simplified schematic representation. In particular, known components such as electrical terminals and sensors are omitted because they do not relate to the present invention. The percussion mechanism according to the present invention can be used particularly advantageously in a drilling and/or striking hammer. Here, various types of percussion mechanism can be realized, of which in particular pneumatic spring percussion mechanisms are particularly suitable. 
       FIGS. 1 to 3  show a first specific embodiment of the present invention having a pneumatic spring percussion mechanism driven by an electrodynamic linear drive. Here, a drive unit (explained in more detail below) is shown in the representation in  FIG. 1  in an extreme upper/rear position; in  FIG. 2  is shown in a center position and in  FIG. 3  it is shown in an extreme lower/front position. 
     The pneumatic spring percussion mechanism has a drive piston  1  that surrounds a piston head  2  of a percussion piston  3 . A shaft  4  of percussion piston  3  extends through a front side of drive piston  1  into a percussion piston guide  5 , and in its frontmost position can strike a tool end  6 , as is shown in  FIG. 3 . Instead of tool end  6 , in a known manner an intermediate header can also be provided. 
     Between drive piston  1  and percussion piston  3  there is formed a first hollow space  7 , in which a main pneumatic spring  8  acts. When there is a forward movement of drive piston  1 , which is capable of axial back-and-forth movement in a percussion mechanism housing  9 , a pressure builds up in main pneumatic spring  8  that drives percussion piston  3  forward, so that it can finally strike against tool end  6 . 
     When there is a return movement of drive piston  1 , a partial vacuum arises in main pneumatic spring  8  that suctions back percussion piston  3  with its piston head  2 . The return movement of percussion piston  3  is also supported by the impact reaction at tool end  6 . In addition, seen in the direction of impact, in front of piston head  2  a return pneumatic spring  10  is formed in another hollow space, and this return spring acts during the return movement of drive piston  1 . It also supports the return movement of percussion piston  3 . 
     In order to compensate air losses in pneumatic springs  8 ,  10 , a plurality of air compensation pockets  11  are provided on the inner wall of drive piston  1 . Their functioning is known from the prior art, so that a more detailed description is not necessary here. Instead of air compensation pockets  11 , other air ducts are also known that enable ventilation of pneumatic springs  8 ,  10  in order to enable the compensation of air losses caused by compression. 
     The oscillating linear back-and-forth movement of drive piston  1  is brought about by an electrodynamic linear drive. For this purpose, drive piston  1  is coupled to a runner  12  of the linear drive. Runner  12  can be formed by a plurality of electrical sheets layered one over the other, and is moved back and forth by alternating magnetic fields produced by a stator  13  of the linear drive. The functioning of such a linear drive is known and is described for example in DE 102 04 861 A1. The linear motor can be for example a reluctance motor having an externally situated stator. 
     Runner  12  and drive piston  1  than a one-piece drive unit. 
     In front of drive piston  1 , an additional, second hollow space  14  is formed between drive piston  1  and percussion mechanism housing  9 ; in the positions shown in  FIGS. 1 and 2 , this hollow space  14  is connected to the surrounding environment via ventilation openings  15 . 
     In the position of the drive unit shown in  FIG. 3 , runner  12  has moved drive piston  1  forward far enough that drive piston  1  has moved past ventilation openings  15 . This causes ventilation openings  15  to be sealed, so that second hollow space  14  is separated from the surrounding environment. Correspondingly, an air spring forms in second hollow space  14  that acts against drive piston  1  and brakes its movement in the forward or impact direction. 
     So that the pneumatic spring can be produced in second hollow space  14  in a suitable manner, and in particular does not act against percussion piston  3 , which is supposed to strike tool end  6  in as unhindered a manner as possible, drive piston  1  forms a piston surface  16  at its front side. Piston surface  16  compresses the pneumatic spring in second hollow space  14 . 
     Depending on the dimensioning, it is possible for stator  13  to be switched currentless at the time at which ventilation opening  15  is closed by drive piston  1 . The braking of the drive unit made up of drive piston  1  and runner  12  then takes place exclusively through the pneumatic spring in second hollow space  14 . Because the compressed pneumatic spring then has a tendency to decompress, it additionally presses the drive unit back against the direction of impact. Then, as needed, stator  13  can again be excited in order to support the return movement. 
     The air spring in second hollow space  14  should be positioned or dimensioned in such a way that the drive unit is caught at the lower reverse point before percussion piston  3  strikes tool end  6 . 
     Corresponding to the air spring in second hollow space  14 , on the opposite side, behind drive piston  1  or behind the overall drive unit, there is formed a third hollow space  17  between drive piston  1 , or the drive unit, and percussion mechanism housing  9 . Percussion mechanism housing  9  is however shown only schematically in the Figures. Of course, percussion mechanism housing  9  can be assembled from various components, or can have a construction differing from that shown in the Figures. 
     In the positions shown in  FIGS. 2 and 3 , third hollow space  17  stands in communicating connection to the surrounding environment via ventilation openings  18 . 
     In contrast, in the position shown in  FIG. 1  the drive unit has, passed over ventilation openings  18  and thus closed them. Correspondingly, third hollow space  17  is separated from the surrounding environment, so that an air spring can build up in this hollow space, as is shown in particular in  FIG. 1 . This air spring brakes the movement of the drive unit during its return stroke. Depending on the dimensioning, the air spring in third hollow space  17  can be strong enough to completely brake the return stroke and to convert it into a counter-movement, namely a movement in the impact direction. Here as well, stator  13 , in a manner similar to the functioning of the air spring in second hollow space  14 , can be switched off, or switched on only as needed. 
     The air spring in third hollow space  17  should be made as long as possible so that it is compressed over a longer movement path of the drive unit. During the return stroke of the drive unit, in comparison to the impact stroke, relatively little energy is required, which can then be stored in the air spring in third hollow space  17 . The stored energy is subsequently available during the forward movement of drive piston  1  in order to move this piston against percussion piston  3 . The energy stored in the air spring of third hollow space  17  thus supports the linear drive, which can then either correspondingly be dimensioned more weakly, or together with which a significantly higher impact energy can be achieved. 
       FIGS. 4 to 6  show a second specific embodiment of the present invention which differs from the first specific embodiment shown in  FIGS. 1 to 3  with respect to the construction of the electrodynamic linear drive. Identical components are designated by identical reference characters.  FIG. 4  shows the drive unit in an extreme upper/rear position,  FIG. 5  shows it in a center position, and  FIG. 6  shows it in an extreme lower/front position. 
     Such a linear drive can be realized for example by a magnetic motor. 
     Drive piston  1  has a runner  19  in the form of two sword-shaped or disk-shaped extensions  20 . Rare earth magnets  21  are fastened to extensions  20 , and these magnets can each be moved back and forth in a stator  22 . 
     Alternatively, in another specific embodiment (not shown) of the present invention, runner  19  can be provided with an annular extension that can be moved in an annular stator. 
     Behind drive piston  1 , in cooperation with percussion mechanism housing  9  a third hollow space  23  is formed in which an air spring  40  can be produced. As explained above, the concept “percussion mechanism housing”  9  is to be understood broadly. What is important is that in cooperation with drive piston  1  or the drive unit made up of drive piston  1  and runner  19 , a hollow space can be produced in which an air spring  40  can form. 
     In runner  19 , a ventilation opening  24  is formed that, in the position shown in  FIG. 5 , covers a ventilation opening  25  present in percussion mechanism housing  9 , so that air can flow from the surrounding environment into third hollow space  23 , in order to restore the air previously lost during the compression of the air spring  40 . In the positions shown in  FIGS. 4 and 6 , ventilation openings  24  and  25  are not positioned one over the other, so that third hollow space  23  is separated from the surrounding environment. 
     The cooperation of drive piston  1  and percussion piston  3 , as well as the functioning of second hollow space  14 , corresponds to the first specific embodiment, so that the description thereof is not repeated here. 
       FIG. 7  shows a schematic section through a third specific embodiment of the present invention. In contrast to the pneumatic spring percussion mechanisms described above on the basis of  FIGS. 1 to 6 , the third specific embodiment according to  FIG. 7  relates to a percussion mechanism in which the energy for the percussion movement cannot be transmitted by an air spring. Correspondingly, this percussion mechanism cannot be designated a pneumatic spring percussion mechanism. 
     The percussion mechanism is driven by an electrodynamic linear drive, in a manner similar to the above-described pneumatic spring percussion mechanisms. It has a drive unit  30  that combines the functions of a drive element and a runner of the linear drive. Drive unit  30  is shown only schematically in  FIG. 7 . Thus, for example the construction of the runner is not shown in detail. However, the details described above relating to runner  12  ( FIG. 1 ) or runner  19  ( FIG. 4 ) hold here as well. 
     Analogously to the above description, drive unit  30  is capable of being moved back and forth in a tube-shaped percussion mechanism housing  9 , the movement being brought about by stator  13 . 
     Drive unit  30  has a sleeve-shaped construction, and has in its interior a hollow area in which percussion piston  3 , which forms a percussion element, is capable of being moved back and forth. Percussion piston  3  then strikes the tool (not shown in  FIG. 7 ) in a known manner. 
     In order to transfer the movement of drive unit  3  to percussion piston  3 , a coupling device is provided. The coupling device has a catch  31 , carried by percussion piston  3 , in particular by piston head  2  of percussion piston  3 , that can be moved back and forth in recesses of drive unit  30  in the working direction of the percussion mechanism. Catch  31  can for example be formed by a cross-bolt that passes through piston head  2  of percussion piston  3 , as is shown in  FIG. 7 . 
     The recesses in drive unit  30  are formed by two longitudinal grooves  32  that extend axially and that pass through the wall of hollow cylindrical drive unit  30 . 
     On the front sides of longitudinal grooves  32 , lower stops  33  and upper stops  34  are formed that limit the longitudinal motion of catch  31  in longitudinal grooves  32 . 
     When there is a back-and-forth movement of drive unit  30 , percussion piston  3  is thus coercively guided by the respective stops  33 ,  34 , as well as by catch  31 . Given a forward movement of drive unit  30  (downward in  FIG. 7 ) in the direction of the tool (working direction), upper stops  34  press catch  31  with percussion piston  3  downward, such that percussion piston  3  should be able to fly free shortly before contacting the tool or the intermediately situated header, in order to avoid damaging effects on drive unit  30  and catch  31 . In the subsequent return movement of drive unit  30 , lower stops  33  come into contact with catch  31  and draw back percussion piston  3 , which is also driven back by the tool, in the direction opposite the working direction. The working cycle then repeats in that drive unit  30 , with upper stops  34 , again accelerates percussion piston  3  against the tool. 
     In this specific embodiment, the coupling device is thus not formed by an air spring, but rather by longitudinal grooves  32 , stops  33 ,  34 , and catch  31 . Of course, the described design serves only for explanation. Numerous other possibilities will be recognized by those skilled in the art for the transfer of the movement of drive unit  30  to percussion piston  3 . 
       FIG. 8  shows, in a schematic representation, a section through a percussion mechanism according to a fourth specific embodiment of the present invention. 
     Here, the basic design of the percussion mechanism is identical to that of the percussion mechanism according to  FIG. 7 . In addition, piston head  2  of percussion piston  3  is coupled with a positive fit to a reversing piston  36  via a piston rod  35 . Reversing piston  36  is capable of being moved back and forth in a reversing cylinder  37 , which is for example part of percussion mechanism housing  9 , in a manner corresponding to the movement of percussion piston  3 . 
     Reversing piston  36  and reversing cylinder  37  enclose a reversing hollow space  38  in which a reversing air spring  39  is formed. 
     Similar to the manner in which, in the first specific embodiment shown in  FIGS. 1 to 3 , the reversing air spring in reversing hollow space  17  brakes a return movement of drive piston  1  depicted there, and later supports a forward movement, the reversing air spring  39  shown in  FIG. 7  is tensioned when there is a return movement of percussion piston  3 , so that this air spring can subsequently support a forward movement of percussion piston  3 . 
     The compensation of air losses of reversing air spring  39  takes place in a manner similar to that in the above-described specific embodiments, so that a detailed description can be omitted here. 
     For reversing air spring  39  as well, it can be particularly useful if it is charged over a longer movement path of percussion piston  3 . In the fourth specific embodiment shown in  FIG. 8 , the compressing of reversing air spring  39  takes place in a particularly reliable fashion, because the entrained movement of percussion piston  3  is achieved through the positive coupling, brought about by the coupling device, between drive unit  30  and percussion piston  3 . 
     The present invention makes it possible to increase the degree of efficiency of a linearly driven electrodynamic percussion mechanism. Through the intermediate storage of energy in the air springs, a more uniform electrical power consumption with low load peaks can be achieved. Moreover, impact-type loads on the hammer housing at the reverse points of the drive unit are avoided. The percussion mechanism according to the present invention can achieve greater demolition performance with a simultaneous reduction in hand-arm vibrations.