Abstract:
Depending on the working and application conditions, it may be desirable to equip fluid-operated percussion devices with a mechanism that protects against no-load strokes, particularly in the interest of avoiding undesired stress. In the present case, this protection mechanism is configured such that the control unit that influences the movements of the percussion piston is blocked by a safety element in the form of a multiple-position valve, and the percussion device is halted if the percussion piston overshoots the extended position occurring in normal operation by a defined distance, thereby assuming a no-load-stroke position. The safety element ( 21 ) that acts on the control unit ( 5 ) of the percussion device ( 1 ) is automatically controlled, notably such that it is not switched to be active until some time after the startup of the percussion device ( 1 ) acted on by the working pressure. The percussion device ( 1 ) can therefore start up without being influenced by the safety element ( 21 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the priority of German Patent Application No. 101 23 202.0 filed May 12, 2001, which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a method for protecting a fluid-operated percussion device against no-load strokes, having a percussion piston that moves inside a work cylinder and impacts a tool, the piston having two piston surfaces of different sizes, of which the smaller surface, being active in the direction of the return stroke, is permanently connected to a pressure line that is subjected to the working pressure, while the larger piston surface, being active in the direction of the work stroke, is alternately connected via a control valve to the pressure line and a pressure-relieved return line; a control unit that includes a distributing regulator, which moves inside the control valve and has two regulator surfaces that differ in size and are active in opposite directions of movement, with the smaller of the surfaces, which acts on the distributing regulator in the direction of the return-stroke position of the regulator, being permanently connected to the pressure line, and the larger regulator surface being connected only alternately and temporarily to the pressure line and the return line via a circumferential groove disposed between the piston surfaces; a no-load-stroke port that opens into the interior of the work cylinder, with the port first being opened toward the interior by the front piston collar of the percussion piston with the smaller piston surface after the percussion piston has overshot the normal impact position by an established distance in the work-stroke direction until it has assumed a no-load-stroke position; and a safety element, which is disposed upstream of the no-load-stroke port, and can be switched between two end positions—the inoperative position and the active position—and is connected on the intake side to the pressure line and serves in exerting the working pressure that originates from the safety element onto the no-load-stroke port in the active position, or, in the inoperative position, serves in breaking the connection between the pressure line and the no-load-stroke port, in which instance the working pressure exerted on the no-load-stroke port in the active position blocks the distributing regulator in the work-stroke position by way of the circumferential groove, should the percussion piston attain the no-load-stroke position.  
           [0003]    The invention further relates to an apparatus that is suited for executing the method.  
           [0004]    Depending on the working and application conditions, it may be desirable to equip fluid-operated percussion devices with a mechanism that protects against no-load strokes, particularly in the interest of avoiding an undesired stress or resulting damage. The Japanese published, non-examined patent application Hei-10-80878 of Mar. 31, 1998, proposes a solution relating to a hydraulic percussion device.  
           [0005]    German Patent Application 100 13 270.7 of Mar. 17, 2000, proposes to equip a fluid-operated percussion device of the generic type mentioned at the outset with a manually-operated no-load-stroke protection mechanism in the form of a switchable safety element such that the percussion piston is shut down—independently of other control-related circumstances—should it reach a defined extended position in the direction of the work stroke.  
         SUMMARY OF THE INVENTION  
         [0006]    It is the object of the invention to build upon the solution presented in the cited German patent application by providing a method and an apparatus that permit the no-load-stroke protection mechanism to be activated automatically, regardless of the manipulation of the device by an operator. The method and the apparatus are intended to be embodied such that the percussion device will not be shut down upon startup when it is subjected to the working pressure, but always can at least start up.  
           [0007]    The object is accomplished according to a first aspect of the invention by a method of protecting a fluid-generated percussion devices of the type originally defined wherein the safety element is transferred from its inoperative position into its active position some time after the startup of the percussion device subjected to the working pressure—counter to the effect of a resetting mechanism that acts on the safety element. As a consequence of the operating mode of the percussion device, a larger activation force that counteracts the resetting action is generated continuously, or at least temporarily at recurring time intervals. This ensures that the safety element is always switched to be active due to the effect of the activation force after the percussion device has started up, in case the percussion piston moves in the work-stroke direction and may be able to reach the no-load-stroke position. As already mentioned, the subject of the invention can be embodied such that the safety element is either held continuously in the active position it has assumed once after the startup of the percussion device, or it is transferred again into its active position, at least in repeating intervals.  
           [0008]    According to features of the invention, the method can be executed such that the safety element is transferred into its active position over the course of the first work cycle of the percussion piston or over the course of the first return-stroke movement of the percussion piston after the percussion device has started up. The work cycle encompasses the work-stroke and return-stroke movements, or the return-stroke and work-stroke movements, of the percussion piston. According to a teaching of the invention, the activation force acting on the safety element can be generated by the working pressure building up temporarily in a percussion-device line that is periodically subjected to pressure during the operation of the percussion device. Furthermore, the method can be embodied such that the safety element is temporarily transferred into its active position within the time frame, and maintained in this position while the percussion-device line is subjected to the working pressure.  
           [0009]    The activation force acting on the safety element can be built up by the working pressure present in the no-load-stroke port. The safety element can, however, also be transferred into its active position if the larger piston surface of the percussion piston or the larger regulator surface of the distributing regulator is subjected to the working pressure for the first time after the percussion device has started up.  
           [0010]    The method can also be embodied such that the safety element is gradually transferred into its active position, as a function of the operating period, after the percussion device has started up. For this to happen, a delay element can cause the safety element to execute a partial switching stroke per time unit in the direction of its active position, with the partial switching stroke being smaller than the switching stroke with which the safety element is transferred out of the inoperative position into the active position.  
           [0011]    As an alternative, the control surface of the safety element can be supplied with a limited control volume, as a function of pressure fluctuations occurring periodically during the operation of the percussion device; consequently, the safety element is transferred in increments into the active position.  
           [0012]    In a particularly simple embodiment of the subject of the invention, the limited control volume can be created by a percussion-device line equipped with a throttle element, which is periodically subjected to the working pressure during the operation of the percussion device. In this regard, the following lines in particular are considered as a percussion-device line: the alternating-pressure line, by way of which the larger piston surface of the percussion piston is temporarily exposed to the working pressure; the reversing line, by way of which the control valve of the percussion device can be switched into the work-stroke position; the short-stroke line, in the event that the percussion device is embodied to be switched between a long-stroke and a short-stroke mode; and the no-load-stroke line, whose mouth or port opens into the interior of the work cylinder.  
           [0013]    In accordance with the invention, the limited control volume can also be created by a pump that conveys a constant volume for each work cycle during the operation of the percussion device. Also in this embodiment, the control surface, over which the safety element moves counter to the effect of the resetting mechanism, is only supplied with a limited control volume in numerous consecutive intervals. The control volume pushes the safety element in increments in the direction of its active position with each conveying action.  
           [0014]    The above-mentioned conveying action can particularly be initiated when the pump is driven by a percussion-device line that is periodically subjected to the working pressure during the operation of the percussion device.  
           [0015]    Within the scope of the invention, the aforementioned pump can also be replaced by other types of metering devices. In particular, it is possible to interpose a metering valve for acting on the control surface of the safety element, with the valve only briefly supplying a limited volume as a function of the change in certain pressure conditions that occurs repeatedly over time. As already mentioned, a percussion-device line that is periodically subjected to the working pressure can switch the metering valve between the blocking and flowing positions.  
           [0016]    A suitable switching of the safety element can ensure that, after being transferred into the active position, the element is held in this position as long as the percussion device is being subjected to the working pressure.  
           [0017]    The object of the invention is also accomplished according to a second aspect of the invention by an apparatus intended for executing the method that has the following features:  
           [0018]    The safety element is an automatically-controlled two-position valve equipped with a resetting mechanism, and has a control surface that influences the mechanism&#39;s position. This surface can be acted upon, by way of a signal line, by a pressure level that forms a control signal such that the two-position valve is transferred from its inoperative position to its active position some time after the percussion device has started up, and counter to the effect of the resetting mechanism. Moreover, the two-position valve is embodied such that it maintains its active position, attained with the control signal, at least in temporary, repeating intervals.  
           [0019]    If the apparatus is configured such that the two-position valve only temporarily maintains its active position—depending on the change in certain pressure conditions that occurs periodically in the operating mode of the percussion device —this naturally stipulates that the safety element must at least assume its active position during the work-stroke movement of the percussion piston, which can prevent no-load piston strokes anticipated to occur during this time. This is ensured in that the control valve is held securely in the work-stroke position by a corresponding exertion of pressure onto its relevant regulator surface; the control valve therefore can no longer be switched into its return-stroke position, and thus also cannot initiate the return-stroke movement of the percussion piston.  
           [0020]    In a modification of the apparatus according to the invention, the signal line is connected to a percussion-device line, which is periodically subjected to the working pressure during the operation of the percussion device. The change in the pressure level occurring in repeating intervals in the relevant percussion-device line represents a signal that either causes the safety element to be transferred into its active position, or at least initiates the transfer.  
           [0021]    The no-load-stroke line having the no-load-stroke port that opens into the interior of the work cylinder is particularly considered as a percussion-device line, as is the alternating-pressure line, by way of which the larger piston surface of the percussion piston is temporarily subjected to the working pressure, and the reversing line, by way of which the larger regulator surface of the distributing regulator that constitutes the control unit, is temporarily subjected to the working pressure.  
           [0022]    If the percussion device has a pilot control that cooperates with the control unit, and a short-stroke line that is connected to the pilot control and the interior of the work cylinder, the signal line can also be connected to the short-stroke line within the scope of the inventive teaching.  
           [0023]    A common feature of the above-mentioned embodiments of the apparatus is that a control signal that influences the safety element is triggered at different times after the percussion device has started up.  
           [0024]    As an alternative to one of the above embodiments, the subject of the invention can also be configured such that the signal line itself is connected to the rear cylinder segment—independently of the alternating-pressure line—by way of which the larger piston surface of the percussion piston is temporarily subjected to the working pressure.  
           [0025]    In another type of embodiment of the apparatus, the signal line is connected to the interior of the work cylinder such that the line is subjected to the working pressure by way of the front cylinder segment in the event that the percussion piston—seen in the direction of the return stroke—assumes a position outside of its normal impact position. In this case, the change in pressure conditions in the work cylinder interior that occurs during the work cycles of the percussion piston is utilized to influence the position of the safety element.  
           [0026]    The opening or port for the signal line into the interior of the work cylinder can be disposed at the level of the no-load-stroke port, or when seen in the direction of the return stroke of the percussion piston, can also be disposed in front of the no-load-stroke port. A crucial point in this connection is that the mouth of the signal line into the work-cylinder interior must be blocked by the percussion piston no later than when the piston has reached the impact position.  
           [0027]    Moreover, the mouth or opening of the signal line into the interior of the work cylinder should be disposed in front of the mouth oar opening of the reversing line into the work-cylinder interior, when seen in the return-stroke direction of the percussion piston—but, in any event, at the level of this mouth.  
           [0028]    If the percussion device can be switched between a long-stroke and a short-stroke mode, the mouth or opening of the signal line into the work-cylinder interior can be located in the region that is limited by the mouth of the short-stroke bore on one side and the mouth of the reversing line into the work-cylinder interior on the other side.  
           [0029]    In principle, the apparatus can also be embodied such that the safety element is gradually transferred into its active position after the percussion device has started up (through the exertion of the working pressure). This can be effected particularly in that the section of the signal line that is connected so as to permit a flow to the control surface of the safety element is connected to the signal source that acts on it such that a limited control volume is at least intermittently- or, alternatively, continuously- supplied to the control surface after the percussion device has started up. This control volume causes the control surface either to execute a partial switching stroke per time unit, or advance in increments in the direction of the active position.  
           [0030]    In this regard, embodiments that include a percussion-device line or a signal line that is connected to the interior of the work cylinder can be embodied correspondingly, i.e., the segment of the signal line that is connected to the control surface so as to permit a flow has a throttle element that acts as a delay element.  
           [0031]    Because the pressure level in the relevant percussion-device lines and in the interior of the work cylinder changes periodically, the control surface of the safety element only advances in stages or steps in the direction of the active position under the additional influence of the throttle element.  
           [0032]    In an embodiment having a continuous supply of a limited control volume, the segment of the signal line that is connected to the control surface so as to permit a flow is connected to the pressure line with the interposing of a throttle element that acts as a delay element. With the effect of the throttle element, which can be embodied as a throttle or baffle, the control surface is continuously supplied with a limited control volume per time unit; consequently, the safety element reaches its active position after a certain length of time.  
           [0033]    The apparatus according to the invention can also be modified such that the segment of the signal line that is connected to the control surface of the safety element so as to permit a flow is equipped with a spring-loaded check valve, which blocks the signal line in the direction of the percussion-device line, or in the direction of the interior of the work cylinder. In this way, an undesired change in the pressure level in the percussion-device line or in the interior of the work cylinder can be barred from influencing the control surface of the safety element and changing the position of the element in a disadvantageous manner.  
           [0034]    An apparatus similar to the switching mechanism of the safety element can be attained through the connection of a pump to the segment of the signal line that is connected to the control surface so as to permit a flow. This pump is driven such that it conveys a constant volume to the control surface per work cycle during the operation of the percussion device, which volume transfers the safety element into its active position in increments.  
           [0035]    For switching the safety element into its inoperative position without a significant delay after the percussion device has been shut off, it should be ensured that the pressure level acting on the control surface can be suitably reduced. This can be effected in that the segment of the signal line that is connected to the control surface so as to permit a flow is additionally connected to the pressure line via a discharge line equipped with a spring-loaded check valve; in this instance, the check valve blocks the pressure line in the direction of the signal line.  
           [0036]    If, after the percussion device has been shut off, the pressure level in the pressure line drops, the control surface can expel fluid into the discharge line and into the pressure line under the effect of the resetting mechanism when the check valve is open. Provided that the pressure line is subjected to the working pressure, the check valve assumes its blocking position, so the discharge line has no effect in the direction of the control surface of the safety element. As an alternative, the segment of the signal line that is connected to the control surface so as to permit a flow can additionally be connected to a throttle line, which is maintained in a pressure-relieved state through its connection to a throttle element disposed inside it. The throttle line can be pressure-relieved by being connected to the return line of the percussion device that terminates into the tank. After the percussion device has been shut down, the resetting mechanism of the safety element also influences the volume enclosed in front of the control surface, as described above, which volume can be carried off externally through the throttle line.  
           [0037]    In an especially simple embodiment of the apparatus, the safety element is configured as an automatically-controlled 2/2-way valve. As an alternative, the safety element can also be configured as an automatically-controlled 3/2-way valve whose intake side is only connected to the pressure line, and whose discharge side is connected to the no-load-stroke mouth and to the signal line, with only the control surface being connected to the signal line in the inoperative position, and with the no-load-stroke mouth and the signal line that is connected to the control surface being subjected to the working pressure in the active position.  
           [0038]    In a further embodiment of the apparatus according to the invention, the safety element is configured as an automatically-controlled 4/2-way valve whose intake side is connected to the pressure line and the signal line, and whose discharge side is connected to the no-load-stroke mouth and to an extension of the signal line, the extension being connected to the control surface so as to permit a flow. Furthermore, the 4/2-way valve is configured such that, in the inoperative position, the connection between the pressure line and the no-load-stroke mouth is broken, while the signal line and its extension are connected to one another.  
           [0039]    Finally, in the active position, the no-load-stroke port and the extension are subjected to the working pressure by way of the pressure line, and the signal line is blocked in the direction of the 4/2-way valve.  
           [0040]    In the last mentioned embodiment, the safety element maintains the active position it has assumed once—through the appropriate effect on its control surface after switching—because in this position, the working pressure present in the pressure line is simultaneously exerted onto the control surface.  
           [0041]    The invention is described in detail below by way of a plurality of exemplary embodiments that are illustrated in the drawing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]    [0042]FIG. 1 is a circuit diagram of a percussion device embodied in accordance with the invention, with automatic stroke switching.  
         [0043]    [0043]FIG. 2 shows the circuit diagram of a percussion device that has no pilot control (corresponding to FIG. 1) that cooperates with the control unit.  
         [0044]    [0044]FIG. 3 shows the circuit diagram of a percussion device having a reversing valve that can be operated arbitrarily for influencing the stroke of the percussion piston.  
         [0045]    [0045]FIG. 4 illustrates a safety element in the form of a 2/2-way valve, shown schematically in a partial section.  
         [0046]    [0046]FIG. 5 shows the circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element whose signal line is connected to the alternating-pressure line of the percussion device.  
         [0047]    [0047]FIG. 6 illustrates a different embodiment of a safety element in the form of a 2/2-way valve, shown schematically in a partial section.  
         [0048]    [0048]FIG. 7 shows the circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety device in the form of a 3/2-way valve whose signal line is connected to the alternating-pressure line of the percussion device.  
         [0049]    [0049]FIG. 9 shows the circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 3/2-way valve whose signal line is connected to the short-stroke line of the percussion device.  
         [0050]    [0050]FIG. 10 is a circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 3/2-way valve whose signal line is connected to the interior of the work cylinder outside of the no-load-stroke mouth.  
         [0051]    [0051]FIG. 11 is a circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 3/2-way valve whose signal line is connected to the pressure line with the interposing of a throttle element, and additionally has a discharge line with a check valve.  
         [0052]    [0052]FIG. 12 is a the circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 3/2-way valve whose signal line is connected to the pressure line with the interposing of a throttle element, and additionally includes a throttle line equipped with a throttle element.  
         [0053]    [0053]FIG. 13 is a circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 3/2-way valve whose signal line is connected to the alternating pressure line of the percussion device with the interposing of a check valve and a throttle element, and additionally has a discharge line with a check valve.  
         [0054]    [0054]FIG. 14 is a circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 3/2-way valve whose signal line is connected to the no-load-stroke line with the interposing of a throttle element and a check valve, and additionally has a discharge line with a check valve.  
         [0055]    [0055]FIG. 15 shows a modification of the circuit diagram of a percussion device according to FIG. 14, with the signal line being connected to the interior of the work cylinder behind the no-load-stroke mouth—seen in the return-stoke direction of the percussion piston.  
         [0056]    [0056]FIG. 16 is a circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 3/2-way valve whose signal line is connected to a pump and has a discharge line with a check valve.  
         [0057]    [0057]FIG. 17 shows a variation of a two-position valve corresponding in function to a 3/2-way valve, schematically and in a partial section.  
         [0058]    [0058]FIG. 18 is a the circuit diagram of a percussion device having an automatic stroke-switching mechanism and a safety element in the form of a 4/2-way valve and a signal line that is connected to the interior of the work cylinder outside of the no-load-stroke mouth.  
         [0059]    [0059]FIG. 19 shows a variation of a two-position valve corresponding in function to a 4/2-way valve, schematically and in a partial section.  
         [0060]    [0060]FIGS. 20 a - c  show switching arrangements of a 2/2-way valve having a signal line or additional discharge mechanism that is connected to the no-load-stroke line via a check valve or a throttle element.  
         [0061]    [0061]FIGS. 21 a - c  show switching arrangements of a 2/2-way valve having a signal line that is independent of the no-load-stroke line, or an additional relief mechanism in the form of a check valve or a throttle element.  
         [0062]    [0062]FIGS. 22 a - c  switching arrangements of a 3/2-way valve, with or without an additional relief mechanism in the form of a check valve or a throttle element.  
         [0063]    [0063]FIGS. 23 a - c  show switching arrangements of a 4/2-way valve, with or without an additional relief mechanism in the form of a check valve or a throttle element. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0064]    Referring now to FIG. 1, there is shown a percussion device, generally represented by  1 , that has an automatic stroke-switching mechanism, and, in addition to the lines and drive and control elements to be described below, a work cylinder  2 , in which a percussion piston  3  is held to move back and forth. The piston has two circumferential piston collars  3   a  and  3   b,  which are disposed inside the work cylinder  2  and are separated from one another by a circumferential groove  3   c.    
         [0065]    Together with the work cylinder  2 , the outward-oriented piston surfaces A 1  and A 2  of the piston collar  3   b  and  3   a,  respectively, limit a front and a rear cylinder segment  2   a  and  2   b,  with the piston surface A 1  being smaller than the piston surface A 2 . Outside of the work cylinder  2 , the percussion piston  3  changes over to a piston point  3   d,  which is located opposite a tool in the form of a chisel  4 . A stop  4   a  limits the chisel&#39;s play in the direction of the percussion piston  3 . The movement of the percussion piston  3  in the direction of the work stroke is indicated by the arrow  3   e.    
         [0066]    The described illustration depicts the percussion device at the point at which the percussion piston  3  impacts the chisel  4 . Normal operation is assumed here, that is, the chisel  4  does not penetrate the material to be reduced, and the percussion piston  3  accordingly assumes the provided, normal impact position.  
         [0067]    The control mechanism for switching the movement of the percussion piston  3  comprises a distributing regulator  5   a,  which can move inside a control valve  5  and whose smaller regulator surface S 1  is continuously subjected to the working pressure (system pressure) via a resetting line  6 . An energy source in the form of a hydraulic pump  7  generates this working or system pressure.  
         [0068]    The smaller piston surface A 1  is also continuously subjected to the working pressure via a pressure line  8  that is connected to the resetting line  6 . Relative to the work cylinder  2 , the opening or port  8   a  of the pressure line  8  is disposed such that it always lies outside of the piston collar  3   b  and thus inside the front cylinder segment  2   b.    
         [0069]    A reversing line  9  connects the larger regulator surface S 2  of the distributing regulator  5   a  to the interior of the work cylinder  2  such that, in the illustrated state, the port  9   a  of the line  9  is connected via the circumferential groove  3   c  to a pressure-relieved return line  10 . The port  9   a  of the reversing line  9  and the port  10   a  of the return line are thus located opposite one another—seen in the longitudinal direction of the percussion piston  3 —with a spacing in the axial direction of the piston  3  that is smaller than the axial length of the circumferential groove  3   c.    
         [0070]    A control line  11  connects the control valve  5  to the pressure line  8 , while an outlet line  12  and a tank  12   a  connect the valve  5  to the return line  10 . Moreover, an alternating-pressure line  13  connects the control valve  5  to the rear cylinder segment  2   a,  by way of which the larger piston surface A 2  can be subjected to the working pressure, if necessary. The control valve  5  can assume two valve positions, namely the illustrated (right) return-stroke position, in which the alternating-pressure line  13  and the outlet line  12  relieve the pressure on the larger piston surface A 2 , and the (left) work-stroke position, in which the working pressure is exerted onto the rear cylinder segment  2   a  by way of the pressure line  8 , the control line  11  connected thereto and the alternating-pressure line  13 . When the control valve is in this position, the percussion piston  3  executes a work stroke in the direction of the arrow  3   e,  counter to the resetting force originating from the smaller piston surface A 1 .  
         [0071]    The percussion device  1  is further equipped with a pilot control in the form of a pilot or control valve  14 , which can assume either the illustrated (upper) blocking position or a (lower) open position.  
         [0072]    Two surfaces, namely the smaller displacement surface V 1  and the larger displacement surface V 2 , influence the position of the control valve  14 . A pilot line  15  connects the larger surface to the interior of the work cylinder  2 , with the opening or port  15   a  of the line  15  being behind the port  9   a  of the reversing line  9 , seen in the direction of the work stroke (arrow  3   e ). The pilot line  15  is in turn connected on the discharge side to the pilot valve  14  via a pilot branch line  15   b  equipped with a baffle or choke  16 .  
         [0073]    A pilot resetting line  17   a  connects the smaller displacement surface V 1  to the pressure line  8 , and provides a path for the continuous exertion of the working pressure. The pilot valve  14  accordingly seeks to assume the open position (not shown) under the effect of the resetting force acting on the displacement surface V 1 .  
         [0074]    On the intake side, a short-stroke line  18  connects the control valve by the port  18   a  to the interior of the work cylinder  2 , while a pilot pressure line  17  connects the valve to the pressure line  8 . The port  18   a  of the short-stroke line  18  is disposed behind the port  15   a  of the pilot line  15 , again seen in the direction of the work stroke (arrow  3   e ). As already mentioned, on the discharge side, the pilot valve  14  is connected via the pilot branch line  15   b  to the pilot line  15 , and is connected on the other side to the reversing line  9  for the control valve  5  by way of an additional line  19 .  
         [0075]    As can be seen in the schematic illustration, in the (upper) blocking position of the pilot valve  14 , the pilot pressure line  17  is connected by way of the pilot branch line  15   b  to the pilot line  15 , and thereby generates the adjusting force that may act in the direction of the blocking position, namely over the displacement surface V 2 . In the illustrated blocking position, the short-stroke line  18  and the additional line  19  are blocked in the direction of the pilot valve  14 .  
         [0076]    The (lower) open position of the pilot valve  14  is characterized in that the short-stroke line  18  is simultaneously connected to the pilot branch line  15   b  and the additional line  19 , and the pilot pressure line  17  is blocked.  
         [0077]    Depending on the position of the percussion piston  3  relative to the port  18   a,  either the pressure conditions in the lines  15 ,  15   b,    19  and  18  or only the pressure conditions in the lines  15 ,  15   b  and  19  match. The latter scenario is the case if, as shown, the piston collar  3   b  blocks the port  18   a  of the short-stroke line against the interior of the work cylinder  2 .  
         [0078]    In the long-stroke mode, the percussion device operates as follows:  
         [0079]    After the control valve  5  has been switched into the (left) work-stroke position, the percussion piston begins to move in the direction of the work stroke (arrow  3   e ) after the upper reversal point has been reached. The pilot valve  14  assumes the illustrated blocking position, and is held in this blocking position by the pilot pressure line  17  (because the working pressure is exerted onto both displacement surfaces V 1  and V 2 ).  
         [0080]    When the percussion piston impacts the chisel  4 , the reversing line  9  is relieved of pressure by way of the circumferential groove  3   c  and the return line  10 . Consequently, the regulating distributor  5   a  of the control valve  5  switches into the illustrated return-stroke position, under the effect of the resetting force originating from the smaller control surface S 1 , thus initiating the return stroke of the percussion piston  3 . If the chisel  4  does not penetrate the material to be reduced, the percussion piston  3  does not leave its provided, normal impact plane, so the port  15   a  of the pilot line  15  remains blocked by the piston collar  3   b.  The percussion piston  3  continues its return stroke until the reversing line  9  is connected by its port  9   a  and the front cylinder segment  2   b  to the pressure line  8 .  
         [0081]    Accordingly, the working pressure is exerted on the larger control surface S 2 . The regulating distributor  5   a  is therefore transferred into the (left) work-stroke position, so the regulator connects the rear cylinder segment  2   a  to the pressure line  8  by way of the control line  11  and initiates a new work stroke.  
         [0082]    If the position of the impact plane shifts in the direction of the work stroke (arrow  3   e ) during the operation of the percussion device, the following processes take place:  
         [0083]    After the control valve  5  has been switched into the work-stroke position and the pilot valve  14  has been switched into the blocking position, the percussion piston  3  first executes a work stroke. If the chisel  4  penetrates the material to be reduced, the percussion piston  3  also leaves its normal impact plane and follows the chisel  4 . As a result of this shift, the port or opening  15   a  of the pilot line  15 , which was initially blocked by the piston collar  3   b,  is now opened and pressure-relieved by way of the connection to the return line  10  produced by the annular groove  3   c.  Accordingly, the pilot valve  14  switches from its blocking position into the open position, which establishes a connection between the short-stroke line  18  and the additional line  19 , which is in turn relieved of pressure by way of the reversing line  9 , the annular groove  3   c  and the return line  10 . This relief of pressure also causes the control valve  5  to switch into the return-stroke position, after which the percussion piston starts its return-stroke movement.  
         [0084]    After a shorter stroke, the so-called short stroke, has been executed, the port  18   a  of the short-stroke line  18  is opened and connected to the pressure line  8  by way of the front cylinder segment  2   b.  By way of the short-stroke line  18 , which is subjected to the working pressure, the lines  15   b  and  15  and the lines  19  and  9  are therefore subjected to pressure, with the interposing of the pilot valve  14 . Consequently, before reaching the maximum possible stroke, the control valve  5  is switched into the (left) work-stroke position, and the work stroke is initiated again. At the same time, the larger displacement surface V 2  of the pilot valve  14 , which is subjected to the working pressure, effects the displacement of the valve into the illustrated blocking position, counter to the resetting force originating from the smaller displacement surface V 2 .  
         [0085]    The described embodiment thus permits a reaction to the characteristics or behavior of the material to be reduced with each individual impact of the percussion piston. If the tool penetrates the material to be reduced, the percussion piston only executes a small stroke, so the energy of the individual impact is low. If the tool does not penetrate the material to be reduced, a large stroke utilizing the corresponding maximum individual-impact energy is executed.  
         [0086]    Because no-load strokes of the percussion piston  3 , and thus an unfavorable stressing of the percussion device, are unavoidable due to working conditions—despite the described automatic stroke switching—the percussion device is further equipped with a no-load-stroke protection mechanism, which switches automatically.  
         [0087]    For this purpose, the interior of the work cylinder  2  additionally has a no-load-stroke port  20   a,  which, with the interposing of a no-load-stroke line  20 , is disposed downstream of a switchable safety element in the form of an automatically-controlled 2/2-way valve  21 . The intake side of this valve  21  is connected to the pressure line  8  by way of an intermediate line  22 , and is therefore continuously subjected to the working pressure.  
         [0088]    The 2/2-way valve can be switched between two end positions, namely the (left) inoperative position and the (right) active position, counter to the effect of a resetting mechanism  23  in the form of a spring element, and has a control surface  21   a,  which influences its position. Via a signal line  24  that is connected to the no-load-stroke line  20 , this surface  21   a  can be subjected to a pressure level that forms a control signal such that the 2/2-way valve  21  is transferred from the illustrated inoperative position into its active position some time after the percussion device  1  has started up.  
         [0089]    Whereas, in the illustrated inoperative position, the connection between the intermediate line  22  (and thus the pressure line  8 ) and the no-load-stroke line  20  is broken, in the (right) active position, the working pressure originating from the pressure line  8  is also present in the signal line  24 , with the interposing of the no-load-stroke line  20 , so the 2/2-way valve  21  maintains the active position once it has been assumed, due to the effect of the activation force originating from its control surface  21   a.    
         [0090]    As evidenced by the representation in FIG. 1, the no-load-stroke port  20   a  is separate from the port  18   a  of the short-stroke line  18 . Accordingly, the no-load-stroke protection mechanism can become effective regardless of the conditions in the long-stroke or short-stroke mode, should the percussion piston  3  have overshot the normal impact position by a specified amount to assume a no-load-stroke position.  
         [0091]    As mentioned above, the percussion piston  3  assumes the normal impact position in the drawing. Here, the front piston collar  3   b  having the smaller piston surface A 1  closes the no-load-stroke port  20   a  in the direction of the interior of the work cylinder  2 .  
         [0092]    If the percussion piston has extended so far in the work-stroke direction (arrow  3   e ) that the front piston collar  3   b  no longer closes the no-load-stroke port  20   a,  the working pressure exerted on the port can act on the larger regulator surface S 2  of the distributing regulator  5   a,  with the interposing of the annular groove  3   c  and the control line  9 , so the control unit  5  is held in the (left) work-stroke position provided during the work stroke. To ensure that a sufficiently high pressure is present in the control line  9  during this time—despite an existing connection between the annular groove  3   c  and the return line  10 —the return line  10  has a correspondingly selected discharge resistance, which is indicated by a throttle unit  10   b.  In other words, the control unit  5  is prevented from switching out of the work-stroke position into the (right) return-stroke position under the aforementioned conditions, with the effect of the working pressure originating from the no-load-stroke port  20   a,  thereby halting the percussion piston  3 .  
         [0093]    The no-load-stroke protection mechanism can easily be switched to be inactive as follows: The 2/2-way valve  21  is transferred into its (left) inoperative position, namely through the cutoff of the working pressure originating from the pressure line  8 . As a result of this cutoff procedure, the pressure level acting on the control surface  21   a  drops, and the 2/2-way valve  21  switches into the illustrated inoperative position under the effect of the spring element  23 .  
         [0094]    Of course, the 2/2-way valve  21  can also be reset hydraulically. That is a hydraulic force that acts in the direction of the inoperative position is generated while the working pressure is exerted onto the percussion device.  
         [0095]    Instead of the above-described embodiment according to FIG. 1, the no-load-stroke protection mechanism (2/2-way valve  21 ) can also be used in connection with other types of percussion devices, such as the embodiments of the percussion device shown in FIGS. 2 and 3.  
         [0096]    The embodiment according to FIG. 2 differs from the embodiment according to FIG. 1 in that there is no automatic pilot device in the form of a pilot valve  14 . Accordingly, in this embodiment, the lines  15 ,  15   b,    17 ,  17   a,    18  and  19 , as well as the ports  15   a  and  18   a  and the baffle  16  associated with the line  15   b,  are omitted. Also in this embodiment, the distributing regulator  5   a  of the control valve  5  either assumes the illustrated return-stroke position (right) or the work-stroke position (left), depending on the pressure conditions in the reversing line  9 .  
         [0097]    If the 2/2-way valve is in the (right) active position, the control line  9  is subjected to the working pressure by way of the no-load-stroke line  20  as the piston collar  3   b  passes in the direction of the work stroke (arrow  3   e ), so the control unit  5  is held in the work-stroke position it assumed during the work stroke (as already explained in connection with FIG. 1). If the return line  10  has an appropriately selected discharge resistance (throttle unit  10   b ), this assures the buildup of a sufficiently high pressure in the control line  9 .  
         [0098]    In the embodiment according to FIG. 3, a reversing valve  14 A is associated with the control unit  5 . This valve  14   a  can arbitrarily (preferably remotely) be moved between two end positions, namely the illustrated blocking position and an open position. A short-stroke line  18  and the port  18   a  connect the reversing valve  14 A to the interior of the work cylinder  2 . An additional line  19  connects the valve to the reversing line  9 .  
         [0099]    In the illustrated blocking position, the reversing valve  14 A exerts no influence on the position of the distributing regulator  5   a  of the control unit  5 . In contrast, if the reversing valve  14 A assumes the (lower) open position, depending on the position of the percussion piston  3  inside the work cylinder  2 , a connection can be established between the cylinder interior and the reversing line  9 , which may shift the distributing regulator  5   a  into the (left) work-stroke position.  
         [0100]    As soon as the piston collar  3   b  opens the port  18   a  of the short-stroke line  18  during the return-stroke movement of the percussion piston  3 , the working pressure acts on the port  18   a  by way of the front cylinder segment  2   b,  so the distributing regulator  5  can be displaced to the right due to the effect of the larger regulator surface S 2 , which is now under pressure, and thus prematurely initiates a new movement of the percussion piston  3  in the direction of the work stroke (arrow  3   e ). The reversing valve  14 A thus allows for arbitrarily influencing the function of the percussion device  1  such that the device may operate temporarily in the short-stroke mode.  
         [0101]    In this embodiment as well, the effect of the no-load-stroke protection mechanism (2/2-way valve  21 ) is independent of the position of the reversing valve  14 A.  
         [0102]    The effect of the 2/2-way valve  21  assuming the (right) active position allows a sufficiently high pressure to be exerted onto the reversing line  9  after the piston collar  3   b  of the percussion piston  3  moving in the work-stroke direction has opened the port  20   a  of the no-load-stroke line  20 . The pressure conditions that then dominate can prevent the distributing regulator  5   a  from assuming the (illustrated) return-stroke position, so the percussion device  1  is halted.  
         [0103]    As can be seen in the aforementioned embodiments, the 2/2-way valve  21  that forms the safety element is initially subjected to the working pressure when the percussion device  1  starts up, and assumes the illustrated (left) inoperative position due to the effect of its resetting mechanism  23 . It is thus switched to be ineffective. Some time after the percussion device has started up, the 2/2-way valve  21  is transferred into its active position when the front piston collar  3   b  opens the no-load-stroke port  20   a  over the course of the return-stroke movement of the percussion piston  3 , and connects the port  20   a  to the pressure line  8  via the front cylinder segment  2   b.  After the switch into the (right) active position, this position is maintained as long as the lines  8 ,  22 ,  20  and  24  are subjected to the working pressure.  
         [0104]    The 2/2-way valve  21 ′ illustrated in FIG. 4 has a hollow cylinder  26  inside a housing  25 , the cylinder being provided with a plurality of transverse bores  26   a  and grooves  26   b.  The hollow cylinder is fixed relative to the housing  25  by a threaded pin  27  that is screwed to the housing, and is sealed against the environment.  
         [0105]    The transverse bores  26   a  and grooves  26   b  connect the hollow cylinder  26  to the intermediate line  22  subjected to the working pressure, or to a pressure-relieved leakage line  28 . The latter can also be connected to the return line  10 . A valve piston  29 , which is supported on the right side against the prestressed spring element  23  acting as a resetting mechanism, is guided to move inside the hollow cylinder  26 , and, due to the effect of the spring, rests with its control surface  21   a  against the housing  25  in the illustrated inoperative position.  
         [0106]    On the left side of the drawing, the valve piston  29  has a center bore  29   a,  which cooperates with the no-load-stroke line  20  and transverse bores  29   b  connected to this bore. Accordingly, the 2/2-way valve  21 ′ differs from the embodiments according to FIGS. 1 through 3 in that the control surface  21   a  is connected directly to the no-load-stroke line  20 , so the signal line  24  shown in FIGS. 1 through 3 is omitted.  
         [0107]    If the working pressure is present in the no-load-stroke line  20 , and thus at the control surface  21   a,  the valve piston  29  is displaced—counter to the resetting force originating from the spring element  23 —to the right in the drawing, so the bores  29   a,    29   b  and  26   a  establish a connection between the lines  20  and  22 . The 2/2-way valve  21 ′ thus assumes its active position explained in connection with FIGS. 1 through 3. However, the 2/2-way valve  21 ′ can easily be switched back into its inoperative position if the pressure level present in the no-load-stroke line  20  drops due to the shutoff of the percussion device.  
         [0108]    The embodiment illustrated in FIG. 5 differs from the embodiment according to FIG. 1 in that the 2/2-way valve  21  serving as a safety element is connected via a signal line  30 , which acts on the control surface  21   a,  to the alternating-pressure line  13 . This line is connected in turn to the rear cylinder segment  2   a,  and subjects the larger piston surface A 2  to the working pressure as long as the control valve  5  does not assume the (left) work-stroke position, not shown, and a connection is therefore present between the lines  13  and  8 .  
         [0109]    In this embodiment, some time after the percussion device  1  has started up, the 2/2-way valve  21  is transferred for the first time into its (right) active position, not shown, as soon as the control valve  5  has assumed its (left) work-stroke position and the signal line  30  is likewise subjected to the working pressure via the alternating-pressure line  13 . Accordingly, the 2/2-way valve  21  is displaced to the left in the drawing, due to the effect of the activation force exerted onto the control surface  21   a.  Consequently, a connection is established between the lines  22  and  20 . If no additional measures are taken, the 2/2-way valve  21  is basically only held in its active position as long as the working pressure is present in the alternating-pressure line  13 .  
         [0110]    The function of the 2/2-way valve  21  as a no-load-stroke protection mechanism is not compromised, however, because the valve is at least always switched to be active if the percussion piston  3  is driven in the direction of the work stroke and no-load strokes could occur.  
         [0111]    The suitable structuring of the 2/2-way valve  21  can ensure that the valve maintains the active position once it has been assumed—even if the pressure exerted onto the signal line  30  during the return-stroke movement of the percussion piston  3  is insufficient, until the working pressure is again present in the alternating-pressure line  13  and initiates the activation force by way of the control surface  21   a.    
         [0112]    [0112]FIG. 6 illustrates an especially simple embodiment of the 2/2-way valve  21  that was explained in conjunction with embodiment of FIG. 5.  
         [0113]    Here, a valve piston  32 , which is provided with an circumferential annular groove  32   a,  is guided to move longitudinally inside a housing  31 , and is supported on the left side against a spring element  23  that is likewise disposed in the housing  31 . The region occupied by the spring element  23  is again kept relieved from pressure by a leakage line  28 .  
         [0114]    In the illustrated inoperative position, the valve piston  32  rests with its control surface  21   a,  which faces the signal line  30 , against the housing  31 . The housing has two spaced bores  31   a  and  31   b,  which are connected to the intermediate line  22  or the no-load-stroke line  20  (see, for example, FIG. 1). With respect to the valve piston  32 , the bore  31   a  is disposed such that it is connected to the annular groove  32   a  in the illustrated inoperative position, while the valve piston blocks the bore  31   b.    
         [0115]    If the working pressure is present in the signal line  30 , the valve piston  32  is displaced to the left into the active position, counter to the resetting effect of the spring element  23 , so that the annular groove  32   a  effects a connection between the bores  31   a  and  31   b —and thus between the lines  22  and  20  (see, for example, FIG. 5). The 2/2-way valve  21  shown in FIG. 6 is thus switched to be effective at least as long as the working pressure is present in the signal line  30 , thereby driving the percussion piston  3  in the direction of the work stroke (arrow  3   e ).  
         [0116]    In the embodiment of the invention shown in FIG. 7, the safety element is embodied as a 3/2-way valve  33 , and is connected via a signal line  34  that acts on the control surface  33   a  to the above-described alternating-pressure line  13 .  
         [0117]    On the intake side, the 3/2-way valve is connected to the intermediate line  22 , which in turn changes over to the pressure line  8 . On the discharge side, the 3/2-way valve  33  is connected to the no-load-stroke line  20 , and to the signal line  34  via a guidance part  34   c.  The signal line  34  is connected to a throttle line  35 , in addition to the 3/2-way valve  33 . With the interposition of a throttle element  35   a,  the throttle line  35  changes over to the return line  10 . Seen from the alternating-pressure line  13 , the connecting point  34   a  between the lines  34  and  35  is downstream of a spring-loaded check valve  34   b,  which blocks the signal line  34  in the direction of the alternating-pressure line  13 .  
         [0118]    In the illustrated (left) inoperative position, the connection between the lines  22  and  20  is broken. That is, only the control surface  33   a  is connected to the signal line  34 . In the (right) active position of the 3/2-way valve  33 , the no-load-stroke line  20  and the signal line  34  are connected to the intermediate line  22  by way of their guidance part  34   c.    
         [0119]    If the alternating-pressure line  13  is subjected to the working pressure, the activation force acting on the control surface  33   a  of the 3/2-way valve  33  can transfer the valve into the (right) active position, counter to the resetting action of the spring element  23 . As a result, the lines  20 ,  34   c  are correspondingly acted upon, and the 3/2-way valve  33  is thus held in the active position it has initially assumed, regardless of subsequent changes in the pressure level in the alternating-pressure line  13 . The check valve  34   b  therefore blocks the signal line  34  in the direction of the alternating-pressure line  13 , while the throttle element  35   a  maintains the present pressure level in the in the guidance part  34   c  and at the control surface  33   a.    
         [0120]    If the percussion device  1  is shut down through the cutoff of the working pressure, the throttle element  35   a  can relieve the guidance part or line  34   c  and the control surface  33   a  of pressure, so the 3/2-way valve  33  is switched back into its illustrated inoperative position.  
         [0121]    The discussed 3/2-way valve  33  is thus modified and connected such that, through the maintaining of the working pressure, the valve is transferred into its active position as a function of the pressure level in the signal line  34 , and maintains this position as long as the percussion device  1  is in the operating mode.  
         [0122]    Some time after the percussion device has started up, the no-load-stroke protection mechanism is activated through the exertion of the working pressure onto the alternating-pressure line  13  after the control valve  5  has been switched into the (left) work-stroke position, and thus drives the percussion piston  3  in the direction of the work stroke (arrow  3   e ). Unlike in the above-described embodiment, the function of the 3/2-way valve  33  can be dependent on the connection of the signal line to the spring-loaded check valve  34   b  for the connection to other percussion-device lines. In the embodiment according to FIG. 8, the signal line  34  is connected to the reversing line  9 , thereby influencing the position of the control valve  5 . In this instance, the 3/2-way valve  33  switches into the (right) active position if the reversing line  9  is subjected to the working pressure, and the control valve  5  is therefore transferred into its (left) work-stroke position for initiating the work-stroke movement of the percussion piston  3 .  
         [0123]    In the embodiment according to FIG. 9, the signal line  34  is connected to the spring-loaded check valve  34   b,  then connected to the short-stroke line  18 . Accordingly, the 3/2-way valve  33  is transferred into its (right) active position as soon as the working pressure is exerted on the short-stroke line  18  for the first time.  
         [0124]    The illustration according to FIG. 10 depicts an embodiment of the subject of the invention in which the signal line  34  associated with the 3/2-way valve  33  is connected to the interior of the work cylinder  2  by way of an additional bore  34   d.  The additional bore  34   d  is disposed such that it terminates into the interior in front of the no-load-stroke port  20   a,  seen in the direction of the return stroke of the percussion piston  3 . A spring-loaded check valve  34   e  is disposed downstream of the guidance part  34   c  of the signal line  34 , in the direction of the additional bore  34   d,  and blocks in the direction of the additional bore  24   d.    
         [0125]    Adjoining the segment of the signal line  34  that allows the control surface  33   a  to be acted upon, the signal line  34  changes over to the above-described throttle line  35  at the connecting point  34   a.  The check valve  34   e  serves in blocking the upstream segments of the signal line  34  against the interior of the work cylinder  2  in the event that the 3/2-way valve  33  has been transferred into the (right) active position and the working pressure is present in the guidance part  34   c  and at the control surface  33   a.  The aforementioned position of the additional bore  34   d  causes the 3/2-way valve  33  to switch into the active position before the front piston collar  3   b  opens the no-load-stroke mouth  20   a  of the no-load-stroke line  20 .  
         [0126]    Unlike in the above-described embodiments, the subject of the invention can also be embodied such that the segment of the signal line  34  connected to the control surface  33   a  of the 3/2-way valve  33  so as to permit a flow is connected to a signal source that either continuously or intermittently supplies a limited control volume to the control surface per time unit, or in increments, after the percussion device  1  has started up. Under the influence of this volume, the 3/2-way valve  33  is transferred into the active position some time after the percussion device has started up.  
         [0127]    For this purpose, in the embodiment illustrated in FIG. 11, the signal line  34  is connected, via a branch line  34   f  that is equipped with a throttle element  36 , to the intermediate line  34   f  subjected to the working pressure. The connecting point  34   a  between the signal line  34  and the branch line  34   f  is also connected to a discharge line  37 , which is likewise connected to the intermediate line  22  and is equipped with a spring-loaded check valve  37   a.  The latter blocks the discharge line  37  in the direction of the signal line  34  and the branch line  34   f.    
         [0128]    After the percussion device  1  has started up, the signal line  34  is supplied with a limited control volume per time unit, as effected by the throttle element  36 . Consequently, the control volume acting on the control surface  33   a  increases continuously, and gradually transfers the 3/2-way valve  33  into the (right) active position. The valve  33  maintains this position as long as the percussion device  1  is in the operating mode, i.e., is subjected to the working pressure.  
         [0129]    The discharge line  37  serves in relieving the pressure in the signal line as soon as possible after the working pressure has been cut off: In the process, the control surface  33   a  expels fluid in the direction of the intermediate line  22 , due to the effect of the resetting force originating from the spring element  23 , so the 3/2-way valve  33  can re-assume the illustrated inoperative position.  
         [0130]    Unlike in the embodiment according to FIG. 11, the subject of the invention can also be embodied such that the discharge line  37  and the check valve  37   a  are omitted. In this case, the control surface  33   a  can expel fluid in the direction of the branch line  34   f  and the intermediate line  22  in a corresponding manner, so the 3/2-way valve  33  can switch again into its illustrated inoperative position.  
         [0131]    The embodiment according to FIG. 12 corresponds in function to the embodiment according to FIG. 11, except that the connecting point  34   a  between the control line  34   c  and the branch line  34   f  is connected to a throttle line  38 , which is likewise equipped with a throttle element  38   a  and terminates in turn into the return line  10 . The throttle element  36  of the branch line  34   f  has a larger flow cross-section than the throttle element  38   a  of the throttle line  38 . Accordingly, a limited control volume is supplied to the control surface  33   a  per time unit by way of the signal line  34 . This corresponds to the difference between the volume introduced through the throttle element  36  and the volume carried off by way of the throttle element  38   a.    
         [0132]    This embodiment also permits the gradual buildup of a control volume in the signal line  34 , following the connection to the throttle element  36 , as a function of the on period of the percussion device  1 . This volume, which continuously displaces the control surface  33   a  per time unit, and thus finally switches the 3/2-way valve  33  counter to the effect of the spring element  23 , into the (right) active position, which the valve maintains as long as the working pressure exerted onto the percussion device is present in the intermediate line  22 .  
         [0133]    As shown in FIG. 13, the percussion device  1  can also be configured such that the segment of the signal line  34  that is connected to the control surface  33   a  so as to permit a flow is connected to a percussion-device line, with the interposition of a throttle element  39 . Depending on the operating mode of the percussion device, the working pressure is only present temporarily in this line. Consequently, the control surface  33   a  is displaced in increments, starting from the illustrated inoperative position, over the course of numerous consecutive movement cycles of the percussion piston  3  until the 3/2-way valve  33  has reached the (right) active position.  
         [0134]    In the embodiment discussed here, the signal line is connected to the alternating-pressure line  13  that is temporarily subjected to the working pressure, and has, in addition to the throttle element  39 , a spring-loaded check valve  40 , which blocks in the direction of the alternating-pressure line  13 . Regardless of the relative position of the components  39  and  40 , the discharge line  37  that was explained above in connection with FIG. 11, and terminates into the intermediate line  22 , is disposed downstream of the check valve  40 . The connecting point between the lines  34  and  37  is, again, represented by  34   a.    
         [0135]    In the above-described embodiment, the control surface  33   a  is supplied with a limited control volume as long as the working pressure is present in the alternating-pressure line  13 . Here, the check valve  37   a  of the discharge line  37  assumes the blocking position due to the effect of the working pressure dominating in the intermediate line  22 . If a sufficient control volume is supplied to the control surface  33   a  over the course of numerous consecutive work cycles, the 3/2-way valve  33  ultimately switches into the (right) active position. As a result, the no-load-stroke line  20  and the signal line  34  are also subjected to the working pressure, and the check valve  40  blocks in the direction of the alternating-pressure line  13 . Accordingly, the 3/2-way valve  33  then remains in the active position it has assumed once as long as the working pressure is present in the intermediate line  22 .  
         [0136]    After the working pressure has been cut off, the 3/2-way valve  33  is relieved of pressure by way of the discharge line  37 . As a result, the 3/2-way valve  33  is returned to the illustrated (left) inoperative position due to the effect of the spring element  23 .  
         [0137]    The latter embodiment can be modified, provided that there are no other changes, such that the signal line  34  is connected to the reversing line  9 , corresponding to FIG. 8, or- borrowing from FIG. 9- to the short-stroke line  18 .  
         [0138]    These percussion-device lines are also only temporarily subjected to the working pressure, and can be used, in cooperation with the correspondingly-equipped lines  34  and  37 , for building up the control volume that is required for displacing the control surface  33   a,  and thus for switching the 3/2-way valve  33 , in stages over the course of numerous consecutive movement cycles of the percussion piston  3 .  
         [0139]    As an alternative, the subject of the invention can also be embodied such that the signal line  34  is connected directly to the rear cylinder segment  2   a,  that is, without the interposition of the alternating-pressure line  13 .  
         [0140]    [0140]FIG. 14 illustrates a further variation of the subject of the invention, in which the volume required for switching the 3/2-way valve  33  can be supplied to the control surface  33   a  in increments over the course of numerous consecutive movement cycles.  
         [0141]    Seen from the connection to the no-load-stroke line  20  and in the direction of the connecting point  34   a  to the discharge line  37 , the signal line  34  connected to the no-load-stroke line  20  is likewise equipped with a throttle element  39  and a spring-loaded check valve  40 . Accordingly, a limited control volume is only briefly supplied to the control surface  33   a  of the 3/2-way valve  33  in the event that the front piston collar  3   b  opens the no-load-stroke port  20   a  over the course of the return-stroke movement of the percussion piston  3 , thereby subjecting the port to the working pressure via the front cylinder segment  2   b.  The check valve  37   a  blocks the discharge line  37  in the direction of the connecting point  34   a  and the signal line  34  as long as the working pressure also dominates in the intermediate line  22 .  
         [0142]    After the 3/2-way valve  33  has been switched into the (right) active position, the guidance part or line  34   c,  in addition to being connected to the no-load-stroke line  20 , is connected to the pressure line  8 , with the interposition of the intermediate line  22 , so the check valve  40  blocks the signal line  34  in the direction of the no-load-stroke line  20 .  
         [0143]    The embodiment according to FIG. 15 differs from the above-described embodiment in that the signal line  34  is connected to the interior of the work cylinder  2 , separately from the no-load-stroke mouth  20   a  via a port  34   c.  Relative to the work cylinder  2 , the port  34   g  of the signal line is disposed between the port  18   a  of the short-stroke line  18  and the port  9   a  of the reversing line  9 , seen in the return-stroke direction of the percussion piston  3 . Also in this case, the control volume acting on the control surface  33   a  is increased incrementally as soon as the front piston collar  3   b  opens the port  34   g  over the course of the return-stroke movement of the percussion piston  3 , thereby exposing the port  34   g  to the working pressure present in the front cylinder segment  2   b.    
         [0144]    Unlike the embodiments according to FIGS. 13 through 15, the control volume required for switching the 3/2-way valve  33  can also be built up in stages by the pump  41  illustrated in FIG. 16. On the intake side, the pump  41  is connected via a suction line  42  to the pressure-relieved outlet line  12 . On the discharge side, the pump  41  is connected to the discharge line  37  and the signal line  34 .  
         [0145]    For the purpose of creating a limited control volume, the pump  41  is connected via a drive line  43  to the reversing line  9 . Accordingly, the pump  41  is only driven if the reversing line  9  is subjected to the working pressure, and supplies the control surface  33   a  with a constant volume per work cycle of the percussion piston  1 .  
         [0146]    To avoid an undesired stress or undesired operating state, the pump  41  is equipped internally with a check valve, not shown, which prevents a reflux counter to the pumping direction.  
         [0147]    Within the scope of the invention, the drive line  43  of the pump  41  can connect the pump  41  to a different percussion-device line that is only temporarily subjected to the working pressure. Drawing from the variations shown in FIGS. 7 and 9, the drive line  43  of the pump  41  can particularly be connected to either the alternating-pressure line  13  (FIG. 7) or the short-stroke line  18  (FIG. 9).  
         [0148]    Furthermore, within the scope of the solution according to the invention, the pump  41  can also be replaced by a metering valve that is controlled by a suitable percussion-device line, especially the percussion-device lines  9 ,  13  or  18 , and supplies only a limited control volume to the control surface  33   a  in intervals. Unlike in the connections of the pump  41 , the metering valve is connected to the intermediate line  22  on the intake side.  
         [0149]    [0149]FIG. 17 illustrates an embodiment variation of a two-position valve  33 ′ serving as a safety element. This valve has a valve piston  45  that is guided to move in a housing  44  and has a spring element  23  which serves as a resetting mechanism and a leakage line  28  (see FIG. 6). The housing  44  additionally has three radial bores  44   a,    44   b  and  44   c  disposed therein. The bores  44   a  and  44   c  are connected to the intermediate line  22 , while the bore  44   b  is connected to the no-load-stroke line  20 . Via the signal line  34 , the control surface  33   a  can be subjected to pressure on the side of the valve piston  45  that is located opposite the spring element  23 . The bore  44   a  is connected to an annular groove  45   a  in the valve piston  45 . The piston  45  is also provided with a center bore  45   b,  which originates in the control surface  33   a  and likewise changes over to a (shorter) annular groove  45   c  in the valve piston  45 . The center bore  45   b  represents the partial line  34   c  shown in the circuit diagram.  
         [0150]    In the illustrated inoperative position of the two-position valve  33 ′, the control surface  33   a  is supported against the housing  44 , in the direction of the signal line  34 , due to the effect of the spring element  23 . The valve piston  45  or its annular groove  45   a  blocks the bores  44   a  through  44   c.  If the working pressure in the signal line  34  acts on the control surface  33   a,  the valve piston  45  is displaced inside the housing  44 , counter to the effect of the spring element  23 , to the left and into the active position. In this position, the annular groove  45   a  connects the bores  44   a  and  44   b,  while the annular groove  45   c  connects the control surface  33   a  and the bore  44   c  via the center bore  45   b.  At the same time, the no-load-stroke line  20  is subjected to the working pressure via the intermediate line  22 , and the control surface  33   a  is subjected to the working pressure via the partial line  34   c.  The two-position valve  33 ′ thus maintains the active position once it has been assumed. The discussion of the embodiments equipped with the 3/2-way valve  33  serves in explaining the further details.  
         [0151]    The safety element can also be configured as a 4/2-way valve  46 , as can be seen in FIG. 18. In such case, the intake side of the aforementioned safety element  46  is connected to the intermediate line  22 , and to a signal line  47 , which is in turn connected via its port  47   a  to the interior of the work cylinder  2 . As already explained in connection with FIG. 15, the port  47   a  is physically separate from the no-load-stroke port  20   a  of the no-load-stroke line  20 , and assumes a position between the port  18   a  of the short-stroke line  18  and the port  9   a  of the reversing line  9  relative to the work cylinder  2 .  
         [0152]    On the discharge side, the 4/2-way valve  46  is connected to the aforementioned no-load-stroke line  20 , and to the guidance part  47   b  of the signal line  47 , by way of which the control surface  46   a  of the 4/2-way valve can also be subjected to the pressure level dominating in the guidance part  47   b.    
         [0153]    At a connecting point  47   c,  the guidance part  47   b  changes over to the above-described discharge line  37 , which is connected to the intermediate line  22  and is equipped with a spring-loaded check valve  37   a  that blocks in the direction of the connecting point  47   c.    
         [0154]    In the drawing, the 4/2-way valve  46  assumes the inoperative position due to the effect of the resetting force originating from its spring element  23 . In this position, the connection between the lines  22  and  20  is broken, while a connection is present between the port  47   a,  the guidance part  47   b  and the control surface  46   a.  In the opposite active position, the lines  20  and  47   b  and the control surface  46   a  are simultaneously subjected to the working pressure via the intermediate line  22 , while the connection between the port  47   a  and the guidance part  47   b  is blocked.  
         [0155]    If, after the percussion device has started up, the front piston collar  3   b  has opened the port  47   a  during the return-stroke movement of the percussion piston  3 , the signal line  47  is connected via its port  47   a,  the front cylinder segment  2   b  and the intermediate line  22  to the pressure line  8 . Consequently, the working pressure also acts on the control surface  46   a  via the guidance part  47   b,  and the 4/2-way valve  41  is transferred into the (right) active position, counter to the force of the spring element  23 . The valve maintains this position as long as the working pressure is present in the intermediate line  22 .  
         [0156]    The 4/2-way valve  46  can be switched back into its illustrated inoperative position after the working pressure has been cut off. In this instance, the guidance part  47   b  and the control surface  46   a  are relieved of pressure by way of the check valve  37   a,  in the direction of the intermediate line  22 .  
         [0157]    Of course, within the scope of the invention, the 4/2-way valve  46  can also cooperate with a signal line whose port  47   a  assumes a different position relative to the work cylinder, or—independently of the interior of the work cylinder  2 —is connected to a suitable percussion-device line, particularly the alternating-pressure line  13 , the reversing line  9  or the short-stroke line  18 . In this regard, refer to the discussion of FIGS. 7 through 9 or FIG. 13.  
         [0158]    As is apparent in FIG. 19, which relates to a variation of the 4/2-way valve, the multiple-position valve  46 ′ has a valve piston  49  that is guided to move in a housing  48 . The piston is supported on the left side against the aforementioned spring element  23 , which is maintained in a pressure-relieved state by the leakage line  28 .  
         [0159]    In the longitudinal direction of the valve piston  49 , the housing  48  has four radial bores, which are adjacent to one another and terminate into the housing, namely a bore  48   a  which is connected to the intermediate line  22 , a bore  48   b,  which is connected to the no-load-stroke line  20 , a bore  48   c,  which is also connected to the intermediate line  22 , and a bore  48   d,  which is connected to the port  47   a.  The bore  48   a  terminates into an annular groove  49   a  disposed on the valve piston  49 . On the side opposite the spring element  23 , the piston  49  is provided with a center bore  49   b,  which originates from the control surface  46   a  and changes over into a further annular groove  49   c  provided on the valve piston.  
         [0160]    As already mentioned, the control surface  46   a  can be subjected to pressure via the bore  48   d  connected to the signal line  47 , and continuously or intermittently displaced in the direction of the spring element  23 .  
         [0161]    In the illustrated inoperative position of the two-position valve  46 ′, the valve piston  49  blocks the bores  48   a  through  48   c,  while the bore  48   d  is connected to the control surface  46   a  via the annular groove  49   c  and the center bore  49   b,  and the discharge line  37 . The latter is blocked by the check valve  37   a  (see FIG. 18). If the working pressure originating from the port  47   a  (see FIG. 18) is present in the bore  48   d,  the pressure also acts on the control surface  46   a.  Consequently, the valve piston moves to the left, thus switching the two-position valve  46 ′ into its active position. In this position, the bore  48   a,  the annular groove  49   a  and the bore  48   b  connect the lines  22  and  20  to one another. The working pressure now acts continuously on the control surface  46   a  via the bore  48   c,  the annular groove  49   c  and the center bore  49   b,  so the two-position valve  46 ′ maintains the active position once it has been assumed. This switching state is maintained as long as the branch line  22  (see FIG. 18) is exposed to the working pressure via the pressure line  8  in the operating mode of the percussion device  1 . If the pressure in the branch line is relieved, the control surface  46   a  can expel fluid through the discharge line  37 , so the two-position valve  46 ′ switches into the inoperative position.  
         [0162]    [0162]FIGS. 20 a  through  23   c  illustrate different switching arrangements of the two-position valves that serve as a safety element and can be used in the solution in accordance with the invention.  
         [0163]    FIGS.  20 - 23   b  and  c  relate to embodiments that additionally permit pressure relief for the segment of the signal line by way of which the respective control surface can be acted upon. The pressure relief is effected either by means of a discharge line that is connected, with the interposing of a spring-loaded check valve, to the pressure line that is subjected to the working pressure, or with a throttle line that is kept in a pressure-relieved state through its connection to a throttle element.  
         [0164]    In the switching arrangements according to FIGS. 20 a  through  c,  the safety element is embodied as a 2/2-way valve—as shown, for example, in FIG. 1, and is controlled by a signal line  24 , which is connected to the no-load-stroke line  20  and acts on the control surface  21   a.    
         [0165]    The switching arrangements according to FIGS. 21 a  through  c  relate to embodiments that employ a 2/2-way valve  21 , that, as shown in FIG. 5, for example, have a signal line  30  that is independent of the no-load-stroke line  20 .  
         [0166]    In the switching arrangements according to FIGS. 22 a  through  c,  the safety element is embodied as a 3/2-way valve  33 , as shown, for example, in FIG. 7, whose signal line  34  has an additional guidance part  34   c  and can be connected to different regions of the work cylinder or to different percussion-device lines.  
         [0167]    [0167]FIGS. 23 a  through  c  relate to switching arrangements having a 4/2-way valve  46 , which is illustrated in FIG. 18, and a signal line  47 , which also acts temporarily on the control surface  46   a  of the 4/2-way valve with the interposition of a guidance line part  47   b.    
         [0168]    On the intake side, the two-position valves  21 ,  33  or  46  are always connected to at least the intermediate line  22  that conveys the working pressure and is connected to the pressure line  8 .  
         [0169]    [0169]FIG. 13, for example, also illustrates the pressure relief of the safety element by a discharge line (FIGS. 20 b,    21   b,    22   b  and  23   b ).  
         [0170]    Because the safety element is equipped with an additional throttle line, FIG. 7 is used as a reference here. The throttle line preferably is connected to the pressure-relieved return line of the percussion device following its connection to the associated throttle element (as shown in FIG. 7).  
         [0171]    The notable advantage attained with the invention is that the percussion device is automatically protected against no-load strokes, with a very low technical outlay, with the safety element being embodied such that the percussion device can start up without an actively-switched no-load-stroke protection mechanism.  
         [0172]    The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.