Patent Publication Number: US-6666366-B2

Title: Portable, internal combustion-engined, setting tool

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention related to a portable, internal combustion-engined tool, in particular, to a setting tool for driving in fastening elements, and including a drive piston having a piston rod for driving a fastening element, a combustion chamber located adjacent to the piston and in which a fuel gas mixture is ignited for generating pressure for driving the piston, at least one outlet valve provided in the combustion chamber for removing flue gases; a locking/unlocking device for closing and opening the at least one outlet valve, respectively; sensing means for retaining the locking/unlocking device in a locking position and having a member that is pressed against a circumference of the piston rod for retaining the locking/unlocking device in its locking position and that is displaced, after the piston reaches its initial position, into a path of the piston rod to enable displacement of the locking/unlocking device from the locking position into an unlocking position. 
     2. Description of the Prior Art 
     In the setting tools of the type described above, the drive energy is obtained by combustion of a fuel gas mixture, which drive energy is transmitted by the drive piston to a setting element. An ignitable fuel gas mixture is fed into the combustion chamber when the setting tool is pressed against an object into which a fastening element is to be driven in. Upon actuation of a trigger, an electrical spark is generated. The electrical spark ignites the fuel gas mixture, starting a combustion process that generates energy for driving the drive piston which adjoins the combustion chamber. At the end of its displacement in a setting direction, the drive piston passes past a plurality of outlet openings through which fuel gases can partially exit. After completion of a setting process, piston returns to its initial position as a result of underpressure created in the combustion chamber by cooling of the residual gases therein. During the time the piston returns to its initial position, the combustion chamber should remain sealed from the surrounding environment. The inlet/outlet valve, through which a fresh air is admitted into the combustion chamber, should only open after the completion of the return stroke of the drive piston. The time, which is necessary for returning of the drive piston to its initial position increases, with increase of the temperatures in the setting tool. In addition, high-energy tools require a large expansion volume which again leads to increase of the time necessary for returning the piston to its initial position. 
     It has been proposed to control the opening of the inlet/outlet valve dependent on the position of the piston, so that the valve would open only in a predetermined position of the piston or when the piston reaches its initial position. This permitted to prevent a premature opening of the inlet/outlet valve(s) and a resulting faulty positioning of the piston due to heating or other unfavorable conditions influencing the thermal feedback that provides for the return of the piston to its initial position. With this valve control, the position of the piston is determined, e.g., with a mechanical probe which is pressed against the piston. 
     Nevertheless, there is still a danger that the drive piston would not reach its initial position because of an increased friction which can be caused, e.g., by accumulation of dirt. Furthermore, there can exist operational conditions which would not provide for a reliable return of the drive piston into its initial position by a thermal feedback, e.g., at an increased heating of the tool. 
     In case the piston occupies a faulty position, the user has to have a possibility to bring the tool into its setting or initial position. This can be done by a manual displacement of the piston which, of course, is a serious drawback as it requires time and efforts. Alternatively, the setting tool can be equipped with a special device to that end. Such a device, e.g., is described in a German Publication DE-100 32 310. 
     DE-100 32 310 discloses a portable, internal combustion-engined setting tool for driving in fastening elements and including a drive piston, a combustion chamber having at least one outlet valve and in which a fuel gas mixture is ignited for generating pressure for driving the piston, and a locking/unlocking device for closing and opening the at least one outlet valve, respectively; sensing means for retaining the locking/unlocking device in a locking position and having a member that is pressed against a circumference of the piston rod for retaining the locking/unlocking device in the locking position and that is displaced, after the piston reaches its initial position, into a path of the piston rod to enable displacement of the locking/unlocking device from the locking position into an unlocking position. 
     In this type of setting tool, one or more mechanical elements, which provide for operation of the locking/unlocking device, are arranged in a plane in which the central axis of the piston rod is located, with a mechanical probe forming a pressure contact with the piston rod. Dependent on the position of the piston, these mechanical elements occupy different positions. If the piston has not returned to its initial position, these mechanical elements are deflected, keeping the inlet/outlet valve closed by retaining the locking/unlocking device in its locking position. 
     If the existing faulty position of the piston, which resulted, e.g., because of an increased friction caused by a dirt contamination, should be eliminated, with the conventional locking/unlocking device being used, the device is forcefully displaced into its unlocking position, whereby the inlet/outlet valve is open. This unlocking position is retained despite the probe being pressed against the piston rod and applying a force to the locking/unlocking device acting in the locking direction of the device. The drawback of this consists in that upon occurrence of the next setting process, when the setting tool is pressed against the object, the inlet/outlet valve self-locks again as a result of the probe being pressed against the piston rod. This means that the setting tool remains in its setting condition after it is lifted off the object. Thereby, a possibility of idle setting stroke exists that presents a security risk. 
     Accordingly, an object of the present invention is to provide a setting tool of the type discussed above and having a high safety standard and, in particular, with which a possibility of an idle setting stroke is eliminated. 
     SUMMARY OF THE INVENTION 
     This and other objects of the present invention, which will become apparent hereinafter, are achieved, by providing, in a setting tool of the type described above, means for positioning the member relative to the piston rod, when the piston is not in its initial position, so that the member does not apply to the locking/unlocking device a force acting in a locking direction of the locking/unlocking device. 
     The inventive setting tool has, in comparison with conventional tools, an additional degree of freedom which permits to manually displace the member, which contacts the piston rod, out of a plane of its normal movement, normal direction of movement, to provide for displacement of the locking/unlocking device into its unlocking position. Thereby, upon being displaced into the unlocking position, the locking/unlocking device is not subjected to any force that would bias the device to its locking position. As a result, upon the setting tool being pressed against an object, no self-locking of the inlet/outlet valve takes place as no locking force is applied by the sensing means. Thus, upon lifting of the setting tool of the object, the tool is not in the setting condition anymore, and an idle setting stroke cannot take place. 
     According to the present invention, at an incomplete return stroke of the piston, the sensing device does not apply to the locking/unlocking device a pressure force acting in the locking direction of the locking/unlocking device. The release position of the sensing means results from the sidewise position of the probe stylus when viewed in the sensing or press-on radial direction of the prove stylus. The same object is achieved when the sensing device (the probe holder) is pivoted in the other, opposite direction from its normal position. In the sidewise position of the probe stylus relative to the piston rod, the probe stylus does not transmit any pressure to the locking/unlocking device, and the locking/unlocking device is not biased into its locking direction. 
     According to a further advantageous embodiment of the present invention, there is provided preload means for biasing the probe stylus into engagement with the piston rod from the sidewise position of the probe stylus. The preload means does not have any force component that would bias the locking/unlocking device to its locking position. After the piston rod returns completely to its initial position, the probe stylus can again be returned into the path of the piston rod by the preload means for performing its usual task, namely, sensing the position of the piston rod. The preload means can comprise a suitable spring element. 
     According to another advantageous embodiment of the present invention, the locking/unlocking device can comprise a locking lever, with the sensing device being pivotally supported on the locking lever. With the locking lever extending parallel to the longitudinal axis of the piston rod in a spaced relationship to the piston rod and pivotable in the plane of the piston rod axis, the sensing device can be pivotally supported at the free end of the lever on a pivot axle secured to the lever. The pivot axle can lie in the pivot plane of the lever and be arranged so that the axle would extend transverse or substantially transverse to the piston rod axis. In this way, the sensing device can be pivoted out of the plane of the locking lever so that the sensing device or the probe stylus is located sidewise of the piston rod, without being supported by the piston rod. In this position, the sensing device does not transmits any force to the locking/unlocking device. 
     For pivoting the sensing device from its normal operational position, an actuation element is provided which can be manually operated. The actuation element pivots the sensing device against the biasing force of the preload means discussed above. 
     The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to is construction and its mode of 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 an axial cross-sectional view of an internal combustion-engined setting tool according to the present invention, a combustion chamber of which is separated in several combustion chamber sections, in a completely collapsed condition of the combustion chamber sections; 
     FIG. 2 an axial cross-sectional view of the setting tool shown in FIG. 1 in an expanded condition of the combustion chamber sections; 
     FIG. 3 an axial cross-sectional view of the setting tool shown in FIG. 1 in a press-on condition of the setting tool, with an ignited combustion chamber; 
     FIG. 4 an axial cross-sectional view of the setting tool shown in FIG. 1 in a condition of return of the tool piston to its initial position; 
     FIG. 5 an axial cross-sectional view of the setting tool shown in FIG. 1 in which the tool piston has returned to its initial position; 
     FIG. 6 an axial cross-sectional view of the setting tool shown in FIG. 1 in the tool unlocked condition; 
     FIG. 7 a  a partial axial cross-sectional view of the setting tool shown in FIG. 1 in the tool original position; 
     FIG. 7 b  a partial cross-sectional view of the setting tool shown in FIG. 1, which the cross-sectional view being taken perpendicular to the tool axis; 
     FIG. 8 a  a cross-sectional view similar to that of FIG. 7 a  and in which the tool piston has not yet returned to its initial position and the locking/unlocking device is in its locking position; 
     FIG. 8 b  a cross-sectional view similar to that of FIG. 7 b  and in which the tool piston has not yet returned to its initial position and the locking/unlocking device is in its locking position; 
     FIG. 9 a  a cross-sectional view similar to that of FIG. 8 a  and in which the tool piston has not yet returned to its initial position and the locking/unlocking device is in its unlocking position; and 
     FIG. 9 b  a cross-sectional view similar to that of FIG. 8 b  and in which the tool piston has not yet returned to its initial position and the locking/unlocking device is in its unlocking position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a cross-sectional view of the combustion-engined tool according to the present invention in the region of its combustion chamber. As shown in FIG. 1, a combustion chamber  1  of an inventive combustion-engined tool, in particular, of a setting tool, which is shown in FIG. 1, has a cylindrical shape and includes a cylindrical wall  2  and a ring-shaped bottom  3   a,    3   b  adjoining the cylindrical wall  2 . In the center of the bottom  3   a,    3   b,  there is provided an opening  4   a,    4   b.  A guide cylinder  5 , which has a cylindrical wall  6  and a bottom  7 , adjoins the opening  4   a,    4   b  in the bottom  3   a,    3   b  of the combustion chamber  1 . A piston  8  is slidably displaceably arranged in the guide cylinder  5  for displacement in the longitudinal direction of the guide cylinder  5 . The piston  8  consists of a piston plate  9  facing the combustion chamber  1  and a piston rod  10  extending from the center of the piston plate  9 . The piston rod  10  projects through an opening  11  formed in the bottom  7  of the guide cylinder  5 . 
     FIG. 1 shows a non-operational condition of the setting tool in which the piston  8  is in its rearward off-position. The side of the piston plate  9  adjacent to the bottom  3  of the combustion chamber  1  is located closely adjacent to the bottom  3 , with the piston rod  10  projecting only slightly beyond the bottom  7  of the guide cylinder  5 . The piston plate  9  has a reduced size in a direction toward the combustion chamber  1 , forming a step. The section of the piston plate  9 , which has a reduced diameter, extends into the opening  4   b,  with the section, which has a larger diameter being located in the opening  4   a.  Thus, the section of the piston plate  9 , which has a larger diameter, impacts the bottom plate  3   b  that, thus, serves as a stop for the piston plate  9  when the piston  8  is located in its initial position. On the outer circumference of the piston plate  9 , sealing rings (not shown) can be provided for sealing the space on opposite sides of the piston plate  9 . Inside of the combustion chamber  1 , there is provided a cylindrical plate  14  further to be called a movable combustion chamber wall or movable wall. The plane of the movable wall  14  extends transverse to the longitudinal direction of the tool. The movable wall  14  is displaceable in the longitudinal direction of the combustion chamber  1 . For separating the chambers on opposite sides of the movable wall  14 , an annular sealing is provided on the circumference of the movable wall. The movable wall  14  has a central opening  16 , with an annular seal provided in the wall forming the opening  16 . 
     Between the movable wall  14  and the annular bottom  3   a,    3   b  of the combustion chamber  1 , there is provided a separation plate  18 . The separation plate  18  likewise has a circular shape and an outer diameter corresponding to the inner diameter of the combustion chamber  1 . The side of the separation plate  18  adjacent to the movable wall  14  is provided with a cylindrical lug  19  that projects through the central opening  16  in the movable wall  14  and the length of which exceeds the thickness of the movable wall  14  in several times. The seal provided in the opening  16  engages the outer circumference of the cylindrical lug  19 . At its free end, the cylindrical lug  19  is provided with a shoulder  20  the outer diameter of which exceeds the inner diameter of the opening  16  in the movable wall  14 . The opening  16  is formed by a lug  17  which is connected with the movable wall  14  and surrounds the lug  19 . The free end of the lug  17  faces the shoulder  20  and is spaced therefrom in the position shown in FIG.  1 . An end wall  2   a,  which is connected with the cylindrical wall  2  serves as a stop for the lug  19  and thereby determines the position of the lug  19  and of the separation plate  18 , which is connected with the lug  19 , in the expanded condition of the combustion chamber  1 . 
     In the off position of the setting tool, which is shown in FIG. 1, the separation plate  18  lies on the bottom wall  3   b,  and the movable wall  14  lies on the separation plate  18 . In this position of the setting tool, the combustion chamber is completely collapsed. When the setting tool is pressed against an object (not shown) in which a fastening element should be driven in, as it would be explained in detail further below, the movable wall  14  is lifted off the separation plate  18 , becoming spaced from the separation plate  18  or the bottom wall  3   b.  After sometime, the movable wall  14  entrains the separation plate  18  with the shoulder  20 , with the movable wall  14  and the separation plate  18  forming a so-called fore-chamber. The fore-chamber is a section of the combustion chamber  1  and is designated with a reference numeral  21  (FIG.  2 ). When the movable wall  14  is lifted further, both the movable wall  14  and the separation plate  18  are displaced parallel to each other, with formation between the separation plate  18  and the bottom wall  3   b  or the piston plate  9  of a further combustion chamber section that is called a main chamber. The main chamber is designated with a reference numeral  22  (FIG.  2 ). FIG. 2 shows both the fore-and main chambers  21 ,  22  in their completely expanded condition, with the shoulder  20  of the lug  19  engaging the end wall  2   a.    
     For displacing the movable wall  14 , there are provided several, e.g., three drive rods  23  uniformly distributed along the circumference of the movable wall  14  and fixedly connected therewith. Only one of the drive rods  23  is shown in FIG.  1 . The drive rods  23  extend parallel to the axis of the combustion chamber  1  and outside of the cylindrical wall  6  of the guide cylinder  5 . The drive rods  23  extend through openings  24 , respectively, formed in the separation plate  18  and through corresponding openings  25   a,    25   b  formed in the bottom  3   a,    3   b  of the combustion chamber  1 . The openings  25   a,    25   b  simultaneously function as valve openings and have, in the region of the opening  25   a,  a conical profile. The drive rods  23  and the movable wall  14  can be connected with each other, e.g., by screws. The free ends of the drive rods  23  are connected with each other by a drive ring  28  that is arranged concentrically with the axis of the combustion chamber  1  and surrounds the guide cylinder  5 . The drive ring  28  can likewise be connected with the drive rods  23  by screws. Respective compression springs  27  are provided between the drive ring  28  and a fixed shoulder  26  provided on the guide cylinder  5 , with the drive rods  23  extending through the shoulder  26 . The compression springs  27 , by acting on the drive ring  28 , tend to pull the movable wall  14  toward the bottom wall  3   b.    
     As it has already been mentioned above, there are provided in the region of the bottom  3   a,    3   b,  openings  25   a,    25   b  which serves as valve openings, of which openings  25   a  have a conical profile. In each of the openings  25   a,  a valve tappet  32  sealingly extends. In the open condition of the openings  25   a,    25   b,  the valve tappets  32  are located outside of the combustion chamber  1  or beneath the bottom wall  3   a.  In this position, the valve tappets  32  are blocked by a shoulder  33   a  fixedly secured on the guide cylinder  5 . Compression springs  33   b,  which are provided between edges of the respective openings  25   a  and the valve tappets  32 , press the valve tappets  32  against the shoulder  33   a.  When the drive ring  28  is pushed in a direction toward the bottom wall  3   a,  shoulders  33 , which are provided on the drive rods  23 , entrain the respective valve tappets  32 , carrying them against the pressure applied by the springs  33   b  into the respective openings  25   a,  closing respective valves which are generally designated by a reference numeral  34 . The valve  34  are inlet/outlet valves. It should be understood that a single valve  34  can be used. 
     The shoulders  33 , which are provided on respective drive rods  23  are displaced, during the displacement of the drive rods  23 , through respective openings  33   c  formed in the shoulder  33   a.  The shoulders  26  and  33   a  can be formed as separate projections. 
     The separation plate  18  have a plurality of circumferentially arranged openings  38  which can be equidistantly spaced from the axis of the combustion chamber  1 . At the lower end of the guide cylinder  5 , there are provided, in the wall  6 , a plurality of outlet openings  29  for venting air out of the guide cylinder  5  when the piston  8  moves toward the bottom  7  of the guide cylinder  5 . At the lower end of the guide cylinder  5 , there is provided damping means  40  for damping the movement of the piston  8 . When the piston  8  passes past the openings  39 , flue gases can flow out there through. 
     In the cylindrical wall  2  of the combustion chamber  2 , there are provided two, axially spaced from each other, radial openings  41 ,  42 . Through the openings  41 ,  42  a liquefied fuel gas can flow into the yet not completely expanded, fore- and main chambers  21 ,  22 . Appropriate metering valves can be connected with the radial openings  41 ,  42 . Thereby, a predetermined a mount of a fuel gas mixture can be provided in the fore- and main chambers  21 ,  22 . 
     FIG. 2 shows, as it has already been mentioned above, the setting tool with a completely expanded combustion chamber  1 ,i,e., with the fore-and main chambers  21 ,  22  in their expanded condition. The respecting displacement positions of the movable wall  14  and the separation plate  18  are defined by the position of the valve tappets  32  in the openings  25   a.  When the valve tappet  32  are completely inserted into the openings  25   a,  they prevent, due to their engagement with the shoulders  33  on the drive rods  23 , further displacements of the drive of the drive rids  23  and, thereby, of the movable wall  14  which is fixedly connected with the drive rods  23 . The position of the separation plate  18  is defined by engagement of the lug  19  with the end wall  2   a.  It is noted that the valve tappets  32  has a conically formed section complementary to the conical profile of the openings  25   a.    
     At the end of the cylindrical lug  19  adjacent to the separation plate  18 , there is provided a cage  51  in which an ignition device  52  is received. The ignition device  52  generates an electrical spark for igniting the fuel gas mixture in the forechamber section  21 . The ignition device  52  is located in the central region of the cage  51  the circumference of which is provided with a plurality of openings  53  through which a laminar flame front can exit from the cage  51  into the forechamber  21 . 
     As shown in FIG. 1, sidewise of the guide cylinder  5 , there is provided a locking/unlocking device  54  connected with a sensing device  55  for determining the displacement position of the piston  8  or the piston rod  10 . 
     The locking/unlocking device  54  serves for locking the drive ring  28  and thereby the inlet locking valves  34 . To this end, the locking device  54  has a locking lever  56  that extends parallel to the longitudinal extent of the guide cylinder  5  at a small distance therefrom. The locking lever  56  is pivotally supported, at its rear end, on the outer side of the bottom wall  3   a.  To this end, a support bracket  57  is provided on the outside of the bottom wall  3   a.  At its opposite end, the locking lever  56  extends through and opening  58  formed in the drive ring  28  and carries the sensing device  55  at this end. The sensing device extends into the path of movement of the piston rod  10 . The free end of the sensing device  55  is located immediately beneath the end surface  10   a  of the piston rod  10  when the piston  8  is in its initial position shown in FIG.  1 . The locking lever  56  can be stamped out, e.g., of a sufficiently rigid metal sheet. At its side remote from the wall  6  of the guide cylinder  5 , the locking lever  56  has a locking edge  59  which is adapted to engage the drive ring  28  from beneath when the drive ring  28  is pushed sufficiently far in the direction toward the bottom wall  3   a.  The width of the locking lever  56 , starting from the support bracket  57 , is relatively small, with the width increasing to from the locking edge  59 . The locking lever  56  is pivoted about the support bracket  57  in a direction toward the guide cylinder  5  by a compression spring  60  provided on a side of the locking lever  56  remote from the guide cylinder  5 . Upon the locking lever  56  being pivoted toward the guide cylinder  5 , the locking edge  59  becomes disengaged from the edge of the opening  58 , with the sensing device  55  having its free end located in the path of the piston rod  10 . 
     As shown in FIG. 1, the sensing device  55  has a probe holder  65  pivotable about a pivot axle  66  provided at the free end of the locking lever  56 . The probe holder  65  carries, at its end section adjacent to the piston rod  10 , a probe stylus  67  pivotable about an axle  68  which is carried by the probe holder  65 . A compression spring  69  is provided between and end of the probe stylus  67  remote from the piston rod  10  and a section of the probe holder  65  adjacent to the locking lever  56 . The Compression spring  69  is supported against the probe holder  65  and tends to pivot the probe stylus  67  about the axle  68  in a clockwise direction. The pivotal movement of the probe stylus  67  in the clockwise direction is limited by a stop (not shown). 
     FIG. 1 shows a position of the setting rod in which the probe stylus  67  occupies a position in which its free tip projects into the path of the piston rod  10 . The probe stylus  67  pivots in the opposite direction against a biasing force of the compression spring  69  to move out of the path of the piston rod  10 . In this position, the probe stylus is pressed against the piston rod  10  radially. 
     In the embodiment shown in the drawings, the locking lever  56 , which pivots about the axle of the support bracket  57 , pivots in a plane in which the central axis of the piston rod  10  is located. The axle  66  is likewise located in this plane. However, the axle  66  extends transverse to the central axis of the piston rod  10 . The axle  68 , about which the probe stylus  67  pivots, extends transverse to the pivot plane of the locking lever  56 . 
     An actuation element  70  (not shown in FIG. 1) provides for manual adjustment of the probe holder  65  about the pivot axle  66 , as it will be discussed in detail further below with reference to FIGS. 7 a - 9   b.  By pivoting the probe holder  65  about the axle  66 , the probe stylus  67  can be pivoted out of the plane of the drawing forward or backward, with the free end of the probe stylus  67  being located sidewise of the piston rod  10  in a spaced relationship thereto. The probe stylus  67  can be pressed against the piston rod  10  radially by appropriate pressure means (not shown). In the pivoted position of the probe holder  65  or the probe stylus  67 , out of the plane of the drawing, no force is applied by the piston rod  10 , via the probe stylus  67  and the probe holder  65 , to the locking lever  56 . The non-shown pressure means provides for a return pivotal movement of the probe holder  65  into a position in which the position of the piston rod  10  can be determined. The probe holder  65  or the probe stylus  67  are pivoted back into the initial position shown in FIG. 1 against a stop (not shown). 
     Sidewise of the guide cylinder  5 , there is provided a trigger  61  which is pivotally supported on the outer side of the guide cylinder  5 , on the wall  6 . For a pivotal support of the trigger  61 , there is provided a support bracket  62 , with the trigger  61  being pivoted about the support bracket  62  in a direction toward the bottom  3   a,    3   b  against a biasing force of a compression spring  63 . The actuation section of the trigger  61  is located outside of the locking lever  56 . In the region of the support bracket  62 , the trigger  61  is formed integrally with a projection  64  extending in a direction toward the bottom  7  of the guide cylinder  5 . When the trigger  66  is pivoted about the support bracket  62  counter-clockwise, the projection  64  pivoted with the trigger  61 , acts on the edge of the locking lever  56  facing the guide cylinder  5 . The projection  64  displaces the locking lever  56  against the biasing force of the spring  60 , pivoting the locking lever  56  about the support bracket  57  counterclockwise. 
     The operation of the inventive setting tool will now be described with reference to FIGS. 2-6 in which the same elements as in FIG. 1 are designated with the same reference numerals as in FIG.  1 . 
     FIG. 2 shows a condition in which the setting tool is pressed with its front tip against an object in which a fastening element is to be driven. By a mechanism (not shown) provided at the front end to the tool, the drive ring  28  is displaced in a direction toward the combustion chamber  1 , and the drive rods  23  expand the fore-and main chambers  21 ,  22  and close the inlet/outlet valves  34 . Shortly before a complete expansion of the fore-and main chambers  21 ,  22 , a liquefied fuel gas is injected thereinto through the radial openings  41 ,  42 . In its press-on end position, the drive ring  28  is located in front of the locking edge  59 , however, the locking edge  59  cannot yet engage the drive ring  28  from behind as the trigger  61  has not yet been actuated. Therefore, the free end of the probe stylus  67  is still located in the path of the piston rod  10 , i.e., in front of the end surface  10   a  of the piston rod  10 . 
     FIG. 3 shows the position of the setting toll in which the trigger  61  is actuated, i.e., is pivoted counterclockwise about the support bracket  62  against the biasing force of the spring  63 . In this position of the trigger  61 , the projection  64  pivots the locking lever  56  about the support bracket  57  also counterclockwise, and the locking edge engages the drive ring  28  from beneath. Simultaneously with the pivotal movement of the locking lever  56 , the probe stylus  67  moves out of the displacement path of the piston rod  10 . At the end of the pivotal movement of the trigger  61  and after the probe stylus  67  has moved out of the displacement path of the piston rod  10 , an ignition of the fuel gas mixture, which is present in the force-and main chambers  21 ,  23  is effected with the ignition device  52  which generates an ignition spark. A fuel mixture, which was previously formed in the chambers  21 ,  22 , e.g., an air-fuel gas mixture, starts to burn laminary in the forechamber  21 , with a flame front propagating radially, with a comparatively slow speed, in a direction toward the openings  38 . The flame front displaces the unconsumed air-fuel gas mixture ahead of it. The mixture enter through the openings  38  into the main chamber  22 , creating there turbulence and pre-compression. As the flame front reaches the openings  38 , the flame penetrates therethrough, due to the small cross-section of the openings  38 , in a form of flame jets into the main chamber  22 , creating there a further turbulence. The thoroughly intermixed air-fuel gas mixture in the main chamber  22  ignites over the entire surface of he flame jets. The mixture bums with high speed which leads to a sharp increase of pressure in the main chamber  22 . 
     This pressure acts on the piston plate, and the piston  8 , which moves with a high speed toward the bottom  7  of the guide cylinder  5 , forcing the air from the guide cylinder  5  out through the openings  39 . Upon the piston plate  9  passing the openings,  39 , the exhaust gas is discharged therethrough. The piston rod  10  movable in the direction shown with arrow effects setting of a fastening element in the object against which the setting tool is pressed. Shortly after the ignition of the fuel gas mixture, the trigger  66  can be released. During the movement of the piston rod  10  in a direction shown with the arrow in FIG. 3, the locking lever  56 , together with the sensing devise  55 , is pressed by the compression spring  60  in a direction toward the guide cylinder  5 . However, the locking edge  59  does not disengage from the drive ring  28  as the free end of the probe stylus  67  is pressed against the circumference of the piston rod  10 , which makes a pivotal movement of the locking lever  56  about the support bracket  57  in the clockwise direction impossible. Thereby, the drive ring  28  retains its position, and the inlet/outlet valves  34  remain closed and the fore-and main chambers  21 ,  22  are in their expanded condition. 
     FIG. 4 shows a condition of the setting tool after completion of the setting process or completion of the combustion of the air-fuel gas mixture in the main chamber  22 , when the piston  8  is displaced to its initial position, as a result of thermal feedback produced by cooling of the fuel gases which remain in the combustion chamber  1  and the guide cylinder  5 . As a result of cooling of the fuel gases, and underpressure is created behind the piston  8  which provides for return of the piston  8  into its initial position. The probe stylus  67  slides along the circumferential surface of the piston rod  10 , so that the drive ring  28  remains locked by the locking edge  59 . The valves  34  remain also closed, as the piston  8  has not yet reached its initial position. 
     FIG. 5 shows a condition of the setting tool in which the piston  8  has reached its initial position. In this position of the piston  8 , the openings  4   a,    4   b  are completely closed by the piston plate  9 . The free end of the piston rod  10   a  is so far retracted into the guide cylinder  5  that its end surface  10   a  is located above the region of the free end of the probe styles  67 . In this position of the piston  8 , the compression spring  60  pivots the locking lever  56 , together with the sensing device  55 , clockwise, and the locking edge  59  becomes disengaged from the drive ring  28 . 
     In the following step (not shown in the drawings the compression springs  57  bias the drive ring  28  away from the bottom  3   a,    3   b,  together with the drive rods  23 . With the displacement of the shoulders  33 , which are provided on drive roads  23 , away from the bottom  3   a,    3   b,  the valve tappets  32  move out of the openings  25   a  under the action of the compression springs  33   b.  As the drive rods  23  move away toward the front end of the setting tool, the drive rods  23  pull the movable wall  14  and, thereafter, the separation plate  18 , toward the bottom  3   a,    3   b,  so that the combustion chamber  1  or the fore-and main chambers  21 ,  22  collapse. The flue gases are expelled through the inlet/outlet valves  34  which also admit fresh air into the combustion chamber  1  when the movable wall  14  and the separation plate move away from the bottom  3   a,    3   b.    
     Below, with reference to FIG. 4, it would be described what takes place when the piston  8  after the setting process does not reach its initial position, i.e., the piston  8  occupies a faulty position which is shown in FIG.  4 . 
     In the case the piston  8  does not reach its initial position, the probe holder  65  is manually pivoted about the axle  66  with the actuation element  70  shown in FIGS. 7 b,    8   b,    9   b.  The operational direction of the actuation element  70  in FIG. 4 extends transverse to the plane of the drawing. Assume, e.g., that the actuation element  70  pivots the probe holder  65  in FIG. 4 about the axle  66  backward, i.e., into the plane of the drawing. As the piston rod  10  has a circular cross-section, together with the pivotal movement of the probe holder  65  about the axle  66 , the probe stylus  67  would also pivot clockwise about the axle  68  until it engages a stop (not shown) provided on the probe holder  65  and occupies its end position shown in FIG.  1 . In the condition shown in FIG. 4, the free end of the probe stylus  67  is located sidewise of the piston rod  10 . This means that the probe stylus  67  is not any more supported radially against the piston rod  10 , so that no force can be transmitted by the probe stylus  67  to the locking lever  56  for retaining the locking lever  56  in its locking position. Rather, the spring  60  biases the locking lever  56  to its unlocking position. As a result, the drive ring  28  can move past the locking edge  59 , opening the inlet/outlet valves  34 . Though the piston  8  has not reached its initial position, the setting tool is ready for the next setting process. Upon the initiation of the next setting process, by pressing the setting tool against an object into which a fastening element is to be driven in, the drive ring  28  is again displaced into the position shown in FIG. 4, however, no force acts on the locking lever  56  in the locking direction of the locking lever  56 . Rather, the spring  60  retains the locking lever  56  in its unlocking position, i.e., in the position shown in FIG.  2 . The steps, which were discussed with reference to FIGS. 2-3, can be repeated again in order to bring the piston  8  into its initial position. 
     An interim idle setting stroke is not any more possible. This is because starting from the condition shown in FIG.  4  and with the probe holder  65  being pivoted out, upon the initiation of the next setting step by pressing the setting tool against an object, no positive locking of the locking lever  56  takes place as the sensing device  55  is in a non-operative position. If in this condition, the setting tool is taken away from the object, without the ignition taken place, which can be caused by actuation of the trigger, the drive ring  28  immediately moves past the locking edge  59 , which insures that no setting stroke can take place after the setting tool has been taken off the object. 
     FIGS. 7 a - 9   b  show in somewhat more detail the sensing device  55 . The elements, which are the same as in FIGS. 1-6, are designated with the same reference numerals and would not be discussed further. 
     The condition shown in FIGS. 7 a - 7   b  corresponds to the condition shown in FIG.  1 . The setting tool is in its off position, and the actuation element  70  in FIG. 7 b  is not in its deflected position. The actuation element  70  is formed as pivot lever pivotal about an axle  71  in a plane that extends transverse to the longitudinal direction of the piston rod  10 . The axle  71 , however, extends parallel to the longitudinal direction of the piston rod  10 . The actuation element  70  pivots the probe holder  65  about the axle  66 . 
     FIGS. 8 a,    8   b  show the condition of the setting tool after it has been pressed against an object and the ignition has been initiated. This condition corresponds to the condition shown in FIG.  4 . The probe stylus  67  applies pressure radially from below to the piston rod  10  and transmits the pressure force to the locking lever  56 , retaining the locking lever  56  in its locking condition. The locking edge  59  engages from beneath the drive ring  28 . 
     In order to bring the locking/unlocking device  54  or the locking lever  56  in the unlocking position, the actuation element  70  is pivoted about the axle  71 , whereby the probe holder  65  is pivoted about the axle  66 , as shown in FIGS. 9 a - 9   b.  As a result, the probe stylus  67  is located sidewise of the piston rod  10  and cannot transmit any pressure to the locking lever  56 . The probe stylus  67  does not anymore apply any pressure to the piston rod  10 . In this position of the probe stylus  67 , the spring  60  biases the locking lever  56  to its unlocking position, and the drive ring  28  can be displaced toward the front end of the setting tool, whereby the inlet/outlet valves  34  are open. After the completion of a setting process, as a rule, the piston  8 , together with the piston rod  10 , returns to its initial position. In this condition of the piston  8 , the piston rod  10  does not interfere with the probe stylus  67 , and the probe stylus  67  can again be pivoted form behind into the plane of the drawing, which is effected with preload means  72  (FIG.  96 ). The preload means  72  is formed as a compression spring. In this position, the probe stylus  67  is located in the path of the piston rod  10 , occupying the position shown in FIGS. 7 a - 7   b,  extending radially with respect to the piston road  10 . 
     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 to 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 of variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.