Abstract:
A setting tool for driving in fastening elements includes a drive piston ( 13 ) displaceable in a piston guide ( 11 ) between its initial ( 32 ) and setting positions, and device for returning the drive piston ( 13 ) in its initial position upon completion of a setting process and having an electromotive drive ( 30 ) and a transmission ( 20 ) for transmitting a motive actuating torque to the drive piston ( 13 ).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a setting tool for driving in fastening elements and including a piston guide, a drive piston displaceable in the piston guide between its initial position and its setting position in which the drive piston drives in a fastening element, and a device for returning the drive piston to its initial position after completion of a setting process. 
   2. Description of the Prior Art 
   Setting tools of the type described above can be operated with solid, gaseous, or liquid fuels or with compressed air. In combustion-operated setting tools, the setting or drive piston is driven by combustion gases. The setting piston drives a fastening element in a constructional component. 
   Germani Publication DE 197 55 730 A1 discloses a setting tool having a piston guide in which a drive piston is displaceable. The piston guide itself is also axially displaceable in the tool housing. After completion of a setting process, the drive piston returns to its initial position. In a first step, the drive piston is displaced in a direction opposite the setting direction by an elastic element that was preloaded during the setting process. A further displacement of the piston takes places in a second step during a new press-on process when the bolt guide is displaced in the direction opposite the setting direction relative to the piston guide, entraining the drive piston therewith. 
   During the return stroke, it can happen that the drive piston would not completely return into its initial position. This is a serious drawback, in particular when during the following setting process, e.g, a nail, a bolt, etc., having a greater length should be driven in a constructional component. Further, the elastic return element is subjected to wear and, therefore, the periods between maintenance are reduced. Still further, return of the piston by the press-process requires application of a greater press-on force by the tool user. 
   Accordingly, an object of the present invention is to provide a setting tool of the type described above in which the drawbacks of conventional setting tools, e.g., those of DE 197 55 730 A1, are eliminated. 
   SUMMARY OF THE INVENTION 
   These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a setting tool, the drive piston returning device of which includes an electromotive drive and a transmission for transmitting a motive actuation torque to the drive piston. The foregoing novel features of the present invention insure that the drive piston returns into its initial position reliably and without a need to apply an increased press-on force by the tool user. Further, the return of the drive piston in its initial position can be effected continuously and not only stepwise. 
   Advantageously, the electromotive drive is formed as a d.c. motor. The advantage of a d.c. motor consists in what it can generate a multiple of the return force at an abnormally high resistance to the return movement. 
   Advantageously, the transmission includes at least one friction wheel engaging the drive piston shaft. With one or several friction wheel(s), the return movement is transmitted to the piston shaft, in an ideal case, by frictional forces. The one or several friction wheel(s) can also assume the function of a piston holder. 
   Advantageously, the transmission includes a gear unit which is formed in particular as a reducing gear and has its driving side connected with the electromotive drive, and its driven side connected with the at lease one friction wheel. The advantage of the combination friction wheel-gear unit consists in that the gear unit is not damaged by a rapid displacement of the drive piston during the setting process. 
   It is further advantageous when the one or several friction wheel(s) is/are elastically biased in a direction toward the piston drive shaft. With the friction wheel(s) being biased against the drive piston shaft, an elastic press-on force applied to the drive piston shaft provides for a better holding of the shaft and for compensation of any vibrations. 
   Advantageously, the one or several friction wheel(s) is/are provided with a support(s) acting in a setting direction and having a counter lock(s) for the drive piston. This insures sliding, during the setting process, of the drive piston shaft relative to the friction wheel(s) which remain or are held stationary. The displacement of the drive piston in the setting direction does not result in actuation of the gear unit or the electromotive drive, which substantially increases their service life. 
   By the provision of a free-running gear between the gear unit and the friction wheel(s), an action of the drive piston acceleration on the gear unit and the rotor of the electromotive drive during recoil, i.e., rapid return displacement of the drive piston, is prevented. 
   Advantageously, in the piston guide, there is provided a switch for detecting presence of the drive piston in its initial position. With the provision of the switch, the piston can be returned only over a required return path, which permits to minimize the amount of energy necessary for returning the drive piston to its initial position. The switch reacts very rapidly to the presence of the drive piston in its initial position, which provides for a greater setting frequency. Preferably, the switch is formed as a magnetoresistive sensor. 
   Advantageously, there is provided an adjustment element having a plurality of driving power positions, and a control unit for controlling the electromotive drive and operatively connected with the adjustment element. The control unit actuates the electromotive drive in accordance with a driving power position selected by a setting tool operator. Thereby, the drive piston is displaced in a setting direction by an amount that provides for driving the drive piston with a predetermined, by the tool operator, driving power. At a maximum driving power, the drive piston is located in its initial position. When the drive piston is to be driven with a smaller driving power, it is displaced, i.e., its initial position is displaced, in the setting direction by a corresponding amount. Thus, the provision of a power selector and a control unit for the drive piston return device permits to regulate the driving power or energy with which the drive piston is driven in the setting direction. 
   The novel features of the present invention, which are considered as characteristics for the invention, are set forth in the appended claims. The invention itself, however both as to its construction and its mode operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show: 
       FIG. 1  a side, partially cross-sectional view of a setting tool according to the present invention; 
       FIG. 2  a cross-sectional view along line II—II in  FIG. 1 ; 
       FIG. 3  a cross-sectional view of Section III in  FIG. 2  at an increase, in comparison with  FIG. 2 , scale; and 
       FIG. 4  a cross-sectional view along IV—IV in FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-4  show a powder charge-operated embodiment of a setting tool according to the present invention. The setting tool  10 , which is shown in the drawings, has a housing  16  with a handle  17  provided thereon. An actuation switch  18  for actuating the tool  10  is provided on the handle  17 . In the interior of the housing  16 , there is provided a piston guide  11  in which a drive piston  13  is axially displaceable. In  FIG. 1 , the drive piston  13  is shown in its initial position  32  in which it is completely located in the piston guide  11 . The drive piston  13  has a piston shaft  14  a setting direction end  15  of which drives a fastening element  50  such as a bolt, nail or the like in a constructional component (not shown). 
   At the setting direction end of the piston guide  11 , there is arranged a bolt guide  12  which includes a cylindrical hollow space through which the drive piston  13  is displaceable. 
   The setting direction is shown with arrow  40 . Before start of a setting process, a fastening element  50  is brought into the bolt guide  12  from a magazine  19  arranged in a region of the bolt guide  12 . 
   After a setting process ends, the drive piston  13  should again be brought into its initial position  32  to again establish the operational readiness of the setting tool  10 . To this end, there are provided in the setting tool  10  an electromotive drive  30 , which is formed, in the embodiment shown in the drawings, as a d.c. motor, and a transmission means  20 . To provide for electrical power supply to the electromotive drive  30  and to other electrical consumers, the setting tool  10  is equipped with a power source such as, e.g., a battery, and accumulator, or a mains connector. The transmission means  20  includes a gear unit  26 , e.g., a reducing gear unit which can be flange-mounted on the electromotive drive  30 . The gear unit  26  and the electromotive drive  30  are received in a receptacle  29  associated with the magazine  29 . Naturally, the gear unit  26  and the electromotive drive  30  can be arranged in another location of the setting tool  10 . The gear unit  26  is connected with a power take-off  28 . 1  downstream of which, a free-running gear  27  is arranged. An axle  28 . 3 , which is supported in a bearing means with a counter lock  25 , is attached to the free-running gear  27 . A frictional wheel  21  is secured on the axle  28 . 3 . The frictional wheel  21  has an inner cylindrical elastic member  23  and an outer wheel or a wheel rim  24  mounted on the cylindrical elastic member  23 . The wheel rim  24  can be formed, e.g., as a friction roller. A second friction wheel  22 , identical with the first friction wheel  21 , is mounted on the second axle  28 . 2 . The second axle  28 . 2  is not actively driven but serves as a counter support for the piston shaft  14  which is supported on its opposite side by an actively driven friction wheel  21 . The second or a further friction wheel can also be actively driven via the elastic member  23 , the friction wheels  21 ,  22  apply an elastic pressure to the piston shaft  14 . 
   At the end of the piston guide  11 , which faces in a direction opposite the setting direction  40 , there is provided switch means  31 , such as, e.g., a magnetoresistive sensor. With the switch means  31 , the electromotive drive  30  can be actuated when the drive piston  13  has left its initial position  32  (with a time delay, if necessary), and deactuated when the drive piston  13  returns to its initial position  32 . 
   Upon initiation of a setting process by the user of the setting tool  10 , the drive piston  13  is displaced forward in the setting direction  40  under action of expanding reaction gases. The friction wheels  21 ,  22  do not rotate by the piston shaft  14  but are statically retained by the bearing means with the counter lock  25 . Thus, the forward moving piston shaft  14  of the drive piston  13  slides through the friction wheels  21 ,  22 . 
   When the drive piston is not driven, it is held in its initial position  32  by frictional forces applied by the friction wheels  21 ,  22 . 
   After the setting process ends, the drive piston  13  recoils very rapidly to its initial position  32 . The free-running gear  27 , which is arranged between the friction wheel  21  and the gear unit  26 , prevents acceleration of the electromotive drive  30  by the rebounding drive piston  13 . This prevents any damage of the rotor of the electromotive drive  30 . 
   After the setting process ends, the switch means  31  actuates, with a time delay, if necessary, the electrical moor means  30  when the switch means  31  does not detect presence of the drive piston  13  in its initial position  32 . The rotational movement of the electromotive drive  30  is transmitted to the piston shaft  14  via the gear unit  26 , the axle  28 ,  3 , the free-running gear  27 , and the friction wheel  21 . The torque applied by the piston shaft  14  provides for return of the drive piston  14  to the its initial position  32 . 
     FIGS. 1-2  show an electrical control unit  33  for controlling the electromotive drive  30 . The electrical control unit  33  can control the drive energy of the setting tool  10 . The adjustment of the drive or setting energy is effected with adjustment means  34  which is formed as a selector switch. E.g., upon the drive piston  13  reaching its initial position  32 , the switch means  31  initializes the control unit  33  which, in turn, actuates the electromotive drive  30 . The electromotive drive  30  displaces the drive piston  13  in the setting direction  40  by an amount corresponding to a power stage of the adjusting means  34  which was selected by the tool operator. With the displacement of the drive piston  13  in the setting direction  40 , a greater combustion volume is provided, and acceleration path of the drive piston  13  becomes reduced, with both resulting in reduction of the setting energy. The maximum power stage of the adjusting means  34  corresponds to the initial position  42  of the drive piston  13 . 
   The electromotive drive  30  and the gear unit  26  are, advantageously, sealed against dust and moisture in order to insure their extended service life and perfect functioning. 
   The drawings do not show electrical conductors and connections (e.g., a connection of the power source with the switch means, or a power source electromotive drive or switch means electromotive drive) between separate electrical components. It should be understood that such connection exits to enable functioning of the tool. 
   It is also possible to combine the described above piston return device with a per se known, gas piston return device. 
   Though the present invention was shown and described with references to the preferred embodiment, such is merely illustrative of the present invention and is not to be construed as a limitation thereof, and various modifications of the present invention will be apparent to those skilled in the art. It is, therefore, not intended that the present invention be limited to the disclosed embodiment or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.