Abstract:
A safety valve for compressed air-operated consumers, including a valve housing having bores, and two direction control valves connected in parallel, each of the control valves having a working piston and a valve head connected to the piston. Two pilot control valves are operatively connected to the directional control valves so that each of the direction control valves is switchable by one of the pilot control valves. Each of the two valve heads is guided in a respective one of the bores in the valve housing. The housing has two cross-channels arranged to connect crosswise to each other. A first one of the pilot valves has a valve seat connected via a first pilot channel in the housing to one of the cross-channels which proceeds from the bore of one of the valve heads. A second of the pilot valves has a valve seat connected via a second pilot channel to another of the cross-channels which proceeds from the bore of another of the valve heads. A compressed-air switching element is provided between two corresponding points at each of the two directional control valves so that the safety valve can be blocked when pressures at the two points are different and turned back on again only by external compressed air-operated actuation of at least one of the two switching elements.

Description:
PRIORITY CLAIM 
     This is a U.S. national stage of application No. PCT/DE00/03376, filed on Sep. 28, 2000. Priority is claimed on that application and on the following application: 
     Country: Germany, application No.: 199 49 874.1, Filed: Oct. 15, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention pertains to a safety valve for a consumer operated by compressed air. 
     A safety valve of this type is described, for example in DE 3,005,547 C2 and in DE 196-22,198 A1. 
     Safety valves of this type are used, for example, to actuate the clutches and brakes of presses. In the case of a switching malfunction in which the two directional control valves assume different switching positions, the feed in these types of safety valves is blocked, and the line leading to the consumer is vented, so that no residual pressure remains in this line. To determine a switching malfunction of this kind and to turn off the system, it is provided in the safety valve described in DE 3,005,547 C2 that the control air for the pilot valves is taken not directly from the feed line but rather from cross-channels, which connect the two bores in which the valve disks are guided to each other in a crosswise manner. As a result, it is possible for the safety valve to monitor itself dynamically at all times. 
     SUMMARY OF THE INVENTION 
     The task of the invention is to improve a safety valve of the general type in question in such a way that, after the malfunction has been corrected, the valve can be reset or turned back on easily, preferably without the need for electric switching elements. 
     The task is accomplished by a safety valve for compressed air-operated consumers with two directional control valves connected in parallel, each with its own working piston and its own valve head connected to the piston. Each of the directional control valves is switchable by its own pilot control valve. Each of the two valve heads is guided in a bore in a valve housing. The bores are connected crosswise to each other by two cross channels. The valve seat of the first pilot valve is connected via a pilot channel to one of the cross channels which proceeds from the bore of the valve head. The valve seat of the second pilot valve is connected via a second pilot channel to the other cross channel which proceeds from the bore of the other valve head. A compressed-air switching element is provided between two corresponding points at each of the two directional control valves, by means of which switching elements the safety valve can be blocked when the pressure at these two points are different and turn back on again only by the external compressed air-operated actuation of at least one of the two switching elements. The present invention offers not only the advantage that the safety valve can be blocked when a problem occurs in one of the two directional control valves but also advantage that the valve can be turned back on again by external actuation of the switching element, which can be done by the use of a key-operated switch, i.e., manually, or by shutting off the system pressure completely. 
     Thus, for example, it is especially advantageous that each switching element has a device for dynamically monitoring the pressure differences in at least two pressure lines, each of these devices being provided with a piston-cylinder unit, which, when there is a difference between the pressures in the two pressure lines, connects the inlet of one of the two directional control valve to the atmosphere. Connecting the inlet of one of the two directional control valves to the atmosphere has the result of preventing the unintentional and unwanted re-actuation of the safety valve. 
     There is preferably a switching element in each of the pilot channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the invention are the objects of the following description and of the drawing: 
     FIG. 1 is a sectional diagram of a safety valve according to the invention in the rest position; 
     FIG. 2 shows a safety valve in the switched position; 
     FIG. 3 shows the safety valve in an incorrectly switched position; 
     FIG. 4 shows the safety valve in the rest position after the occurrence of a fault and the storage of that fault in memory; 
     FIG. 5 shows the resetting of the fault in the safety valve according to FIGS. 1-4; 
     FIG. 6 shows a device, used in a safety valve according to the invention to monitor pressure differences in two pressure lines, in the rest position; 
     FIG. 7 shows the device illustrated in FIG. 6 in the memory position; 
     FIG. 8 shows a mechanically activated valve in a rest position; and 
     FIG. 9 shows the valve of FIG. 8 in an open position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A safety valve  10 , shown in FIGS. 1-5, comprises a housing  12 , in which two directional control valves, connected in parallel, are installed. Each of the two directional control valves has a working piston  16   a  and  16   b  and a valve head  18   a ,  18   b , rigidly connected to the piston. The housing  12  has a feed port  20  for compressed air, a return port  22 , and a consumer port  24 . Each directional control valve has its own electromagnetically actuated pilot valve  26   a ,  26   b  with valve seats  28   a ,  28   b  and vent openings  30   a ,  30   b . The piston-shaped valve heads  18   a ,  18   b  are guided in bores  36   a ,  36   b  in the housing  12  with the result that the valve seats  34   a ,  34   b  can be opened and closed. The valve seats  32   a ,  32   b  are opened and closed by the working pistons  16   a ,  16   b.    
     The two valve heads  18   a ,  18   b  have transverse bores  38 , which open out into a ring-shaped channel  42   a ,  42   b.    
     In the housing  12 , furthermore, two channels  46   a ,  46   b , called “cross-channels” in the following, are provided, which connect the two bores  36   a ,  36   b  to each other. From these cross-channels  46   a ,  46   b , pilot channels  48   a ,  48   b  branch off, which lead ultimately via the storage chambers  60   a ,  60   b  and devices for the dynamic monitoring of pressure differences, to be described in greater detail further below and referred in the following in brief as “memory valves”  70   a ,  70   b , to the valve seats  28   a ,  28   b  of the pilot valves  26   a ,  26   b . From the pilot valves, furthermore, channels  50   a ,  50   b  lead to the working pistons  16   a ,  16   b , i.e., to their working chambers  14   a ,  14   b.    
     The memory valves  70   a ,  770   b  are connected in such a way that the memory valve  70   a  assigned to the one directional control valve or to one of the two directional control valves connected in parallel has a first input line  71   a , which is connected to an electromagnetically actuated valve  72   a  and also to the second input line  73   b  of the other memory valve  70   b . This memory valve  70   a  also has a second input line  73   a , which is connected to the memory chamber  60   b . The memory valve  70   b  assigned to the other directional control valve has a first input line  71   b , which is connected in turn to an electromagnetically actuated valve  72   b  and also to the second input line  73   a  of the other memory valve  70   a , whereas its second input line  73   b  is connected to the first input line  71   a  of the memory valve  70   a . The memory valves  70   a ,  70   b  are thus also connected via the memory chambers  60   a ,  60   b  to the parallel directional control valves in a crosswise manner by the channels  48   a  and  48   b.    
     The way in which the memory valves  70   a ,  70   b  operate is explained first on the basis of FIGS. 6 and 7, so that the way in which the safety valve itself operates can be better understood. 
     The memory valve for the dynamic monitoring of pressure differences and for their storage in memory, shown in FIGS. 6 and 7, comprises a housing  200  with a port  207 , which is connected to the second input line  73   a ,  73   b , and a port  211 , which is connected to the first input line  71   a ,  71   b . In the housing  200 , a piston  202  is free to slide in an opening in the housing  200  against the restoring force of restoring springs  205 . 
     The memory valve also comprises a port  208  and a port  210 , where the port  208 , depending on the position of the piston  202 , is connected either to the port  207  or to the port  210 , which is itself connected to the atmosphere. In the case of the safety valve shown in FIGS. 1-5, the port  208  is connected to the pilot valves  26   a ,  26   b , whereas the port  210  is connected to the return line  22  or to the atmosphere. 
     On an end facing away from the restoring springs  205 , the piston  202  is sealed off by a membrane  201 , which is mounted in the housing  200  and forms an effective pressure surface A 1 , by means of which the piston  202  can be subjected to the pressure which prevails at the port  211 . 
     On the side of the piston  202  facing the springs  205 , the effective pressure surface is formed by the pressure surface A 2  of the piston  202 , the surface on this side being larger. A side of the piston facing away from the springs cooperates with a seat  209 . In addition, the pressure surface can also be formed, for example, by a piston with a sealing element, which, for example, can be in the form of a lip ring. 
     The elastic force produced by the restoring springs  205  holds the piston  202  in the rest position shown in FIG.  6 . For this purpose, the piston  202  has sealing surfaces  203  on the side facing away from the restoring springs  205 , which surfaces cooperate with the valve seat  209 . 
     On the side facing the restoring springs  205 , furthermore, the piston  202  has a sealing surface  204 , which cooperates with the valve seat  206 . 
     The memory valve stores a signal whenever there is pressure at port  211  and simultaneously an absence of pressure at port  207  or whenever there is a pressure difference here, as will be described in greater detail below. 
     If the two ports  207  and  211  are both under pressure, the piston  202  remains in its rest position as shown in FIG.  6 . The elastic force of the restoring springs  205  and the force being exerted on the surface A 2 , this being the force produced by the fluid pressure at port  207  and acting on the valve seat  209 , results in a force which holds the piston  202  in the rest position against the force produced by the fluid pressure acting on the membrane  201 . It is obvious that the two effective surfaces A 1 , A 2  and the spring constants will be selected so that, when the pressures at the ports  207  and  211  are the same, the effective surface A 2  minus the effective surface A 1  results in a force sufficient to hold the piston  202  in the rest position. In this rest position, the fluid pressure present at the port  208  is the same as that at the port  207 . The port  210 , as mentioned above, is connected to the atmosphere or to the return line  22 . 
     The memory function of the memory valve operates as follows. When the fluid pressure falls at the port  207  and thus also falls at port  208 —where the pressure difference at which a memory process is initiated can be determined by the interaction of the surfaces A 1  and A 2  and the elastic force of the springs  205 —then the only force which remains to keep the piston  202  in this rest position is the sum of the elastic force of the springs and whatever resultant force is still present at ports  207 ,  208 , which is now lower than the pressure at port  211 . But because the force present at the membrane  201  is greater than the elastic force of the springs plus whatever force is still present at ports  207 ,  208 , which is now less than that at the port  211 , port  211  continuing to be under the full system pressure, the piston  202  moves downward toward the valve seat  206 , and the surface A 3  assumes the memory position shown in FIG.  7 . 
     When now a fluid pressure is again present at the ports  207  and  208  or when the pressure starts to increase there again, only a weak upward-directed force will be developed, this force being equal to the product “seat surface A 3  of the valve seat  206  times the fluid pressure”. The size of the seat surface A 3  of the valve seat  206  is calculated in such a way that this force, together with the force produced by the restoring springs  205 , is not sufficient to move the piston  202  against the downward-directed force, which is equal to the product “fluid pressure times membrane surface A 1 ”. The equalization of the pressure between port  207  and port  211  does not lead to the resetting or “deletion” of the memory status of the device and the return to the rest position. The pressure difference which has occurred remains, so to speak, “stored” by the position of the piston, which now connects the port  208  via the port  210  to the atmosphere. 
     A resetting of the memory valve, that is, a “delete” function, can be achieved only by venting the space above the membrane  201  through the port  211 , so that the springs  205  and the fluid pressure present at the seat surface A 3  are sufficient to move the piston  202  back into its starting or rest position. When this happens, the stored signal is deleted. This venting can be accomplished via the electromagnetically actuated reset valves  72   a ,  72   b , as will be described below in conjunction with FIGS. 1-5. 
     In FIGS. 1-5, identical elements are designated by the same reference symbols. 
     The safety valve functions in the following way. FIG. 1 shows the safety valve in its rest position. In the rest position, the pilot valves  26   a ,  26   b  are closed, and the working chambers  14   a ,  14   b  of the working pistons  16   a ,  16   b  are vented via the channels  50   a ,  50   b  and the vent openings  30   a ,  30   b  of the pilot valves  26   a ,  26   b . The valve heads  18   a ,  18   b  are thus pushed by the compression springs  80   a ,  80   b  (and the pressure medium) against the seats  34   a ,  34   b , which are thus closed. The valve seats  32   a ,  32   b  of the working pistons are open, so that the consumer port  24  is vented to the return port  22 . When the pilot valves  26   a ,  26   b  are now switched, then, as shown in FIG. 2, their valve seats  28   a ,  28   b  are opened, and their vent openings  30   a ,  30   b  are closed. The volume of the pilot channels  48   a ,  48   b , of the storage chambers  60   a ,  60   b , of the second connecting lines  73   a ,  73   b , and of the connecting lines  75   a ,  75   b  of the memory valves  70   a ,  70   b  is large enough to ensure that the compressed air present in the pilot channels  48   a ,  48   b , in the storage chamber  60   a ,  60   b , in the second connecting lines  73   a ,  73   b , and in the connecting lines  75   a ,  75   b , which air enters the working chambers  14   a ,  14   b  of the working pistons  16   a ,  16   b  via the valve seats  28   a ,  28   b  and the channels  50   a ,  50   b , is sufficient to switch over the working pistons  16   a ,  16   b , so that they occupy the position shown in FIG. 2, in which the valve seats  32   a ,  32   b  of the working pistons are closed and the valve seats  34   a ,  34   b  of the valve heads  18   a ,  18   b  are opened. 
     The compressed air flows now from the feed line  20  into the hollow-bored valve heads  18   a ,  18   b , through the transverse bores  38 , and into the ring-shaped channels  42   a ,  42   b . From there, the air flows through the cross-channels  46   a ,  46   b  into the ring-shaped channels  40   a ,  40   b , and from these through the valve seats  34   a ,  34   b  to the consumer via the consumer port  24 . Simultaneously, compressed air flows from the cross-channels  46   a ,  46   b  and the ring-shaped channels  40   a ,  40   b  into the pilot channels  48   a ,  48   b , from there to the storage chambers  60   a ,  60   b , and then onward to the memory valves  70   a ,  70   b , where the compressed air exerts the same pressure in both the inlet lines  71   a ,  71   b  and the inlet lines  73   a ,  73   b  of the memory valves  70   a ,  70   b . In this state, the memory valves  70   a ,  70   b  are in the above-described rest position, so that the valve seats  28   a ,  28   b  are filled with compressed air at the full feed air pressure, so that the working pistons  16   a ,  16   b  of the parallel-connected valves are kept in the switched position via the lines  50   a ,  50   b . When the pilot valves  26   a ,  26   b  are now switched back again and their valve seats  28   a ,  28   b  are closed, the vent openings  30   a ,  30   b  open simultaneously, and the working chambers  14   a ,  14   b  of the working pistons  16   a ,  16   b  are vented via the vent openings  30   a ,  30   b  of the pilot valves and the channels  50   a ,  50   b.    
     The two directional control valves then switch back into the rest position shown in FIG. 1, because compressed air is no longer acting on the working pistons  16   a ,  16   b , and therefore the springs  80  are able to push the valve heads  18   a ,  18   b  back onto their valve seats  34   a ,  34   b . The pilot channels  48   a ,  48   b , however, are filled with compressed air at the full feed pressure, so that, when another switch is initiated, this pressure is able to move the working pistons  16   a ,  16   b  back into the position shown in FIG.  2 . 
     In the case of the malfunction shown in FIG. 3, it is assumed that the magnet of the pilot valve  26   a  is excited, whereas the magnet of the pilot valve  26   b  is not. Thus the valve seat  28   a  is open, but the valve seat  28   b  is closed. Compressed air is able to act on the working piston  16   a  via the pilot channel  48   b , the second inlet line  73   a , the memory valve  70   a , the line  75   a , the valve seat  28   a , and the channel  50   a , whereas the working piston  16   b  as shown in FIG. 3 is not switched over. Thus the valve seat  34   b  is closed, but the valve seat  34   a  remains open. In this position, however, it is impossible for any compressed air to flow to the valve seat  34   a , because the cross-channel  46   a  leading to the valve seat  34   a  is blocked by the piston-shaped valve head  18   b . The consumer port  24  is vented via the open valve seat  32   b  to the return line  22 . It is impossible for any pressure to build up at the consumer port  24 . 
     Although some compressed air enters the cross-channel  46   b  via the valve head  18   a , the transverse bore  38 , and the ring channel  42   a , it cannot continue to flow onward from there, because the valve head  18   b  is in the closed position. The full feed pressure, however, continues to act on the working piston  16   a  via the pilot channel  48   b , which is connected to the cross-channel  46   b.    
     The pilot channel  48   a , however, is vented via the ring channel  40   a  and the open valve seats  34   a ,  32   b  to the return line  22 , so that no pressure can build up in the pilot channel  48   a . Any pressure which may already be present decreases to the return pressure, e.g., atmospheric me pressure. 
     Because of the way in which the memory valves  70   a ,  70   b  are connected, the memory valve  70   a  in this situation is in its rest position, i.e., the position in which the line  73   a  is connected via the line  75   a  to the valve seat  28   a  of the solenoid valve  26   a , so that the pressurized fluid is able act on the valve seat  28   a  via the pilot line  48   b  and the storage chamber  60   b . The memory valve  70   b , however, has been switched over into its memory position, because the pressures in the two lines  71   b  and  73   b  are not the same. Whereas there is an absence of pressure in line  73   b , because this line is connected via the memory chamber  60   a  and the pilot channel  48   a  to the return line  22  and is thus vented, the line  71   b  is connected via the storage chamber  60   b  and the lines  48   b ,  46   b , the ring channel  42   a , and the transverse bore  38  to the feed port  20  for compressed air. In this case, the port  210  (compare FIGS. 6 and 7) of the memory valve  70   b  is connected via the line  74   b  to the atmosphere. If the pilot valve  26   b  were now to be switched over by some means or other so as to open the valve seat  28   b , it would still be impossible for the working piston  16   b  to be switched, because the channel  50   b  is connected to the line  74   b  via the channel  75   b  and the memory valve  70   b , which remains in the memory position. 
     Even if the working position  16   a  were to move back to its rest position, as illustrated in FIG. 4, the malfunction would remain in memory, because the memory valve  70   b  would still remain in the position shown in FIG. 3, even though the pressures in the lines  71   b  and  73   b  at the memory valve  70   b  would be equalized. It is possible for the safety valve to be actuated again only after the memory valve  70   b  has been reset through actuation of the reset valve  72 , as shown in FIG. 5, that is, by venting the line  71   b , which allows the memory valve to return to its rest position. This resetting can, for example, be accomplished by a key-operated switch or by some other type of manual actuation. 
     The device can also be reset by shutting off the system pressure. If this approach is taken, it would be possible, for example, to omit the solenoid valve or the mechanically actuated valve actuated by, for example, a key-operated switch.