Abstract:
A device for cooling welding caps of a welding apparatus includes a cooling line ( 10 ). A cooling agent is kept under pressure during removal of the welding caps. An expansion device ( 16 ) including an expansion chamber ( 34 ) is connected to the cooling line ( 10 ).

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to a device for cooling welding caps of a welding apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002]     In robotic welding technology, there are used welding tongs with replaceable welding caps. The wear of the welding caps can be noticeably retarded by a good cooling. For this purpose cooling systems are provided which include cooling lines. In the cooling lines water is used as a cooling agent, to which glycol and further additives have been added, for instance. During replacement of the welding caps the cooling line is automatically sealed up in the region of the welding caps in order to prevent any escape of the pressurized cooling agent. As it is not possible to immediately seal up the cooling line despite the use of quick-acting stop valves, an undesired squirting of cooling agent will occur due to the pressure which continues to be present in the sealed-up portion of the cooling line.  
         [0003]     European Patent Application EP 0 433 586 A shows s device for cooling welding caps of a welding apparatus comprising an exhaust system. Part of the cooling line is shut and completely discharged. This technique is disadvantageous because upon refill it is unavoidable that undesired air gets into the cooling line.  
         [0004]     It is an object of the invention to provide a device for cooling welding caps, which avoids a loss of cooling agent to a large extent.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     According to the invention, a device for cooling welding caps of a welding apparatus comprises a cooling line in which a cooling agent is kept under pressure during removal of the welding caps. The device further comprises an expansion device connected to the cooling pipe, the expansion device including an expansion chamber is. The invention is based on the finding that the squirting of cooling agent when closing the valves is caused by the high pressure in the cooling pipe. During a replacement of the welding caps, the expansion chamber therefore provides the possibility of an immediate pressure reduction so that no highly pressurized cooling agent can escape. Compared with cooling devices which shut and discharge a part of the cooling line, the device according to the invention works according to a different principle. By its expansion the expansion chamber provides for a pressure equalization. The expansion chamber only gets filled until the overpressure is relieved. The cooling line remains filled with cooling agent.  
         [0006]     In the preferred embodiment of the invention the expansion chamber is in direct flow connection with the cooling agent, i.e. the cooling agent can be relieved from pressure directly by flowing into the expansion chamber.  
         [0007]     It is preferred that the expansion chamber is provided in a housing and is delimited by a face associated to a piston which is movable in the housing. Thus, the volume of the expansion chamber can be easily expanded by a displacement of the piston.  
         [0008]     In order to dampen the movement of the piston in the expansion direction, the piston is adapted to be acted upon with a back pressure.  
         [0009]     For adjusting the back pressure to the specific conditions, in particular to the volume which is enclosed in the sealed-up cooling line portion, preferably a pneumatic pilot valve and a pressure limiting device for adjusting the back pressure are provided. Standard components may be used for the pilot valve and the pressure limiting device.  
         [0010]     A piston with a seal separating the expansion chamber on side of the piston from a back pressure chamber arranged on the other side of the pistion has proven to be of particular advantage. With this, the pneumatic section of the device (pilot valve, pressure limiting device, back pressure chamber) is kept separate from the fluidic section of the device (cooling line, expansion chamber) and a mixing of the two media will not occur.  
         [0011]     A further development of the invention provides for that the piston is sealed up with respect to the housing by at least two surrounding, axially spaced seals. An intermediate space is formed between the two seals, the housing having an aperture in the region of the intermediate space. This special construction permits a check of the sealing between the pneumatic and fluidic sections of the device. In case there is a leakage at the seal facing the fluidic section, for instance, cooling agent would enter the intermediate space and exit through the aperture. In the other case, i.e. if the seal facing the pneumatic section has a defect, then it would be air which escapes through the aperture. Thus, by connecting a suitable detector to the aperture it is possible to continuously monitor the seal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a side view of a device according to the invention; and  
         [0013]      FIG. 2  is a basic block diagram of the device of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]      FIG. 1  illustrates a cooling line  10  of a cooling system for the welding caps of a robotic welding tong. The cooling line  10  has a downwardly pointing screw thread connection  12  to which an expansion cylinder  14  is screwed. The expansion cylinder  14  is part of an expansion device  16 . The expansion device further includes a pilot valve  18  which is designed as a solenoid valve and has a female connector  20 , as well as a pressure limiting device in the form of a restrictor  22 . The restrictor  22  can be integrated in the pilot valve  18 , like in the illustrated embodiment.  
         [0015]     The basic construction of the expansion device  16  will be apparent in particular from  FIG. 2 . The expansion cylinder  14  has an essentially cylindrical housing  24  with two connections  26 ,  28  at the end faces. The first connection  26  is connected with the cooling line  10 , and the second connection  28  is connected with the restrictor  22 . The pilot valve  18  is connected to a compressed air pipe  30  and is driven electrically.  
         [0016]     The interior of the expansion cylinder  14  is divided into an expansion chamber  34  and a back pressure chamber  36  by an axially movable piston  32 . At their sides facing each other, the two chambers  34 ,  36  are delimited by the axially movable end faces  32   a  and  32   b  of the piston  32 , respectively. Towards the housing  24 , the piston  32  is sealed up by two seal rings  38 ,  40  so that the pneumatic section of the expansion device  16  underneath the seal  40  is separated from the fluidic section of the expansion device above the seal  38 .  
         [0017]     The piston has a neck  32   c  formed between the two seals  38 ,  40 , whereby an annular intermediate space  42  is formed. The position of the intermediate space  42  varies with the movement of the piston  32 . Towards outside, the intermediate space  42  is limited by the wall of the housing  24 , with a housing aperture  44  (only shown in  FIG. 2 ) being provided in the region of the intermediate space.  42 . The axial dimension of the intermediate space  42  and the arrangement of the housing aperture  44  are chosen such that the housing aperture  44  is in flow connection with the intermediate space  42  in any piston position which is possible, during normal use of the expansion device.  
         [0018]     The functioning of the device according to the invention will be explained in the following. During a replacement of the welding caps of the robotic welding tong, the cooling line  10  is automatically sealed up by quick-acting stop valves. Thereby the cooling agent in the cooling line  10  is enclosed under pressure. The pressure, however, will be reduced immediately because the cooling agent can flow into the expansion chamber  34  of the expansion cylinder  14  and move the piston  32  under enlargement of the expansion chamber  34 . The pressure in the back pressure chamber  36 , which is adjusted by means of the pilot valve  18  and the restrictor  22 , is chosen such that the movement of the piston will be damped at a suitable rate. It is due to the immediate pressure reduction that a squirting of cooling agent in the region of the quick-acting stop valves will not occur.  
         [0019]     After having replaced the welding caps, the piston  32  will be displaced again towards the cooling line  10 , in fact by the back pressure which is made available by the pressure pipe  30  and is suitably adjusted by the pilot valve  18  and the restrictor  22 . Thereby the expansion chamber  34  becomes smaller again and is able to expand anew during a further weld cap replacement.  
         [0020]     A particular advantage of the described device is the isolation of media, i.e. the compressed air for providing the back pressure for the piston  32  does not mix with the cooling agent during use of the device.  
         [0021]     In order to ensure a failure-free operation of the device according to the invention, the expansion cylinder  14  is disposed underneath the cooling line  10  and arranged so as to stand upright.  
         [0022]     The housing aperture  44 , which is in flow connection with the intermediate space  42 , serves for a checking of the seals  38 ,  40 . In case a seal  38  does not work properly, cooling agent enters the intermediate space  42  and escapes from it through the housing aperture  44 . Thus, the determination of such an escape of cooling agent is a direct evidence of a leakage of the seal  38 . On the other hand, any escape of compressed air is a sign of a defect seal  40 . In both cases, however, it is not necessary to immediately replace the seals when the medium escaped from the housing aperture  44  is retained, because the other seal normally is still intact and can perform the sealing function on its own.