Patent Publication Number: US-7718916-B2

Title: Low impact spot welding cylinder using dual pistons

Description:
The application claims priority to U.S. Provisional Application No. 60/635,086 filed Dec. 10, 2004. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a weld gun used in spot welding, and more particularly, the invention relates to either a single or double piston welding cylinder for the weld gun generating a low impact force when the weld gun engages the workpiece to perform a spot weld. 
   A typical weld gun used in spot welding includes opposing arms each having an electrode that applies current to a workpiece to generate a weld nugget, for example, between two sheets of metal. The electrodes include malleable welding caps typically manufactured from a copper alloy. Repeated impact force between the welding caps and the workpiece plastically deforms the welding caps thereby increasing the wear of the welding caps and reducing the service life, which increases operating cost. 
   Impact force between the welding caps and workpiece also generates workpiece distortion, which can have detrimental effects on the welding process and quality of the weld. 
   Several prior art weld cylinders have endeavored to reduce the impact force between the welding caps and workpiece to extend the life of welding caps and reduce workpiece distortion. One approach in the prior art has been to slow the advance of the electrodes toward one another by slowing the movement of the pneumatic cylinder. However, while this approach reduces the impact force it also increases the cycle time for a spot weld, which is undesirable. Another drawback is that typically the greater the retract stroke length, the narrower the low impact region becomes relative to the stroke. This is problematic in that the internal components of the weld cylinder must be customized depending upon the application. A more versatile weld cylinder design would provide common components for a wider variety of applications. 
   Another approach in the prior art is to utilize external devices or peripheral components such as valves, regulators, restrictors, and/or electrical switches to control the pressure, timing, and rate that the air is supplied to the pneumatic cylinder thereby controlling the impact force. However, the addition of these externals devices may be difficult to integrate with existing weld guns and is also costly since additional components must be added to the welding system. It is desirable to use the current industry pneumatic actuating systems so that the inventive weld cylinder may be used with current systems. For example, in four weld port systems, the ports are typically pressurized in pairs to achieve the three different weld cylinder stroke positions. 
   Therefore, what is needed is a pneumatic cylinder that reduces impact force between the welding caps and the workpiece but that does not require additional, costly design features or external devices and increases in cycle time. 
   SUMMARY OF THE INVENTION AND ADVANTAGES 
   The present provides a weld cylinder having a dual or single piston arrangement. The cylinder has a movable retract piston assembly with the piston arranged within the retract piston assembly. The piston supports a rod that is movable between home, intermediate, work, and fully advanced work positions. The rod moves rapidly from the home position to the intermediate position. However, the rod moves more slowly from the intermediate position to the work position to reduce the impact force. A cushion valve supported by the retract piston assembly and a middle separator block cooperate to increase the rate at which the rod moves from the work position to the advanced position so that weld force increases rapidly to minimize increases in cycle time. 
   A cushion chamber is pressurized using weld-return air. An isolator valve opens when it engages the middle separator block during the advance of the retract piston assembly. When the isolator moves from closed position to open position, the fluid in the cushion chamber is permitted to slowly escape when moving from the intermediate position to the work position through the vented weld-return port. Thus, the isolator maintains pressure in the cushion chamber until retract-forward position is reached and the isolator is opened. As a result, pressure within the cushion chamber is preserved regardless of the retract stroke of the particular cylinder in which the present invention is being used. 
   The cushion valve is a differential pressure valve that is exposed to the pressure within the cushion chamber and weld-forward air pressure. As the pressure in the cushion chamber falls as the chamber is exhausted and the weld-forward air pressure rises when moving from the intermediate position to the work position, the cushion valve will open. The opened cushion valve enables the cushion chamber to be exhausted even more rapidly so that the weld force can build quickly. 
   Accordingly, the present invention provides a pneumatic cylinder that reduces initial impact force between the welding caps and the workpiece, but that does not require additional, costly external devices and increases in cycle time. 
   These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an inventive weld gun. 
       FIG. 2  is a cross-sectional view of a dual piston weld cylinder shown in  FIG. 1  in the home position. 
       FIG. 3  is an enlarged view of the rearward flange of the retract piston assembly shown in  FIG. 2 . 
       FIG. 4  is a cross-sectional view of the dual piston weld cylinder shown in  FIG. 1  in the intermediate position. 
       FIG. 5  is an enlarged view of the rearward flange engaging the middle separator block, as shown in  FIG. 4 . 
       FIG. 6  is a cross-sectional view of the dual piston weld cylinder shown in  FIG. 1  in the work position. 
       FIG. 7  is an enlarged view of the rearward flange engaging the middle separator block with the cushion valve open, as shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An inventive weld gun  10  including an inventive pneumatic cylinder  18  is shown in  FIG. 1 . The pneumatic cylinder  18  is a four port arrangement (P 1 -P 4 ) that can be easily integrated into existing weld guns utilizing four ports. That is, the plumbing for prior art weld guns having four ports can be used with the inventive pneumatic cylinder  18  without modification to the welding system. The ports P 1 -P 4  are fluidly connected to a compressed air source  22 . The ports P 1 -P 4  are connected to the compressed air source  22  though valves that control the timing of the air signals provided to the pneumatic cylinder  18  by selectively opening and closing the valves. 
   The weld gun  10  includes opposing arms  12  that each include an electrode  14 . The electrodes  14  include welding caps  16  that are typically manufactured from a malleable copper alloy. The welding caps  16  engage a workpiece (not shown) to apply a welding current to the workpiece to generate a weld nugget, as is well known in the art. The arms  12  are typically supported by the weld gun  10  and interconnected to one another at various pivot points. A rod  20  of the pneumatic cylinder  18  is typically connected to one of the arms  12  to actuate the electrodes  14  and welding cap  16  between electrode positions corresponding to home (fully returned), intermediate, and work (caps  16  and workpiece engaged) positions. Current is applied to the electrodes  14  when in the work position using a current source  24  that is electrically connected to the electrodes  14 . 
   Port P 1  provides weld-forward air when pressurized. Port P 2  provides weld-return air when pressurized. Port P 3  provides retract-forward air when pressurized, and port P 4  provides retract-return air when pressurized. Typically, the ports are pressurized in pairs to achieve a desired position with the other, non-pressurized ports vented to atmosphere. In the example shown, the weld-return and retract-return ports, P 2  and P 4 , are pressurized to obtain the home position. The weld-return and retract-forward ports, P 2  and P 3 , are pressurized to obtain the intermediate position. The weld-forward and retract-forward ports, P 1  and P 3 , are pressurized to obtain the work position. The fully advanced work position is the travel limit of the work position. The rod  20  may be actuated between the above three positions or directly from the home position to the work position by actuating the desired valves. It should be understood that the work position is not necessarily a discrete position, but rather, may be any distance along which the rod  20  moves between the intermediate position and the fully advanced work position. The various positions and the condition at the ports is expressed in the following table: 
   
     
       
         
             
             
             
             
             
           
             
                 
                 
             
             
                 
                 
                 
               Retract- 
               Retract- 
             
             
                 
               Weld-Forward 
               Weld-Return 
               Forward 
               Return 
             
             
                 
               (P1) 
               (P2) 
               (P3) 
               (P4) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Home 
               vented 
               pressurized 
               vented 
               pressurized 
             
             
               Intermediate 
               vented 
               pressurized 
               pressurized 
               vented 
             
             
               Work 
               pressurized 
               vented 
               pressurized 
               vented 
             
             
                 
             
          
         
       
     
   
   The pneumatic cylinder  18  includes a cylinder body  26  that has a pair of barrels  34   a  and  34   b  with a separator  30  arranged between the barrels  34   a  and  34   b . A port adapter  38  is arranged on the separator  30  to provide the P 2  port. An end cap  32  is arranged at one end of the cylinder body  26 , and a front block  28  is arranged at the other end of the cylinder body  26 . A middle separator block  31  is arranged between the barrel  34   b  and another barrel  35  that is adjacent the front block  28 . Fasteners  36  secure the components of the cylinder body  26  together so the pneumatic cylinder  18  can be pressurized at ports P 1 -P 4  without leakage from the cylinder body  26 . It should be understood that the ports may be arranged on the cylinder other than described and shown. For example, the ports may be repositioned to provide desired packaging of the weld cylinder. 
   The multiple component cylinder body  26  and its configuration described above are exemplary of a four/port pneumatic cylinder having a dual piston arrangement. However, it should be understood that other configurations may be used and still fall within the scope of the present invention. For example, a single cylinder may also incorporate the present invention. 
   In the example embodiment of the inventive pneumatic cylinder  18 , three cylinder positions are generated.  FIGS. 2 and 3  depict a home position of the pneumatic cylinder  18 .  FIGS. 4 and 5  show an intermediate position of the pneumatic cylinder  18 , and  FIGS. 6 and 7  depict a work position of the pneumatic cylinder  18 . The work or weld position may lie anywhere between the intermediate position and the fully advanced position depending on the mechanical system and the workpiece. The weld position may be varied during the welding process to ensure the weld force is maintained as the electrodes deflect under mechanical load, account for electrode wear, and account for workpiece distortion during the welding process. In the fully advanced position, a weld flange  64  is in close proximity to or engages a rearward flange  44 . 
   The present invention pneumatic cylinder  18  moves the rod  20  quickly from the home position ( FIG. 2 ) to the intermediate position ( FIG. 4 ) in which the welding caps  16  are in close proximity to the workpiece. However, the rod  20  moves more slowly from the intermediate position ( FIG. 4 ) to the work position ( FIG. 6 ) in which the welding caps  16  sufficiently forcefully engage the workpiece. The rod  20  moves forward an additional distance to increase the weld force so that current can be applied to produce a weld nugget. The slower movement from the intermediate position ( FIG. 4 ) to the work position ( FIG. 6 ) ensures that the impact force between the welding caps  16  and the workpiece is minimized. However, the preceding, more rapid movement of the rod  20  ensures that cycle time is not unduly compromised. The present invention reduces the impact force of the caps engaging the workpiece while not significantly increasing cycle times. Further, the invention decouples the retract stroke length of a cylinder from low impact performance. That is, the invention can meet desired low impact targets using the same components for cylinders having various retract stroke lengths. 
   Referring to  FIG. 2 , ports P 1  and P 3  are provided by the end cap  32 , and port P 2  is provided by the separator  30 . Port P 4  is provided by the front block  28 , which also slideably supports the rod  20  with bushing  59 . A retract piston assembly  40  is slideably supported by the barrels  34   a  and  34   b . The retract piston assembly  40  includes a barrel  46  having forward and rearward flanges  42  and  44  secured at opposite ends using retaining ribbons  48 . The retract piston assembly  40  separates a cavity  50  provided by the barrels  34   a  and  34   b  into first and second chambers  52  and  54 . The retract piston assembly  40  is shown in a retract-return position in  FIG. 2  and a retract-forward position in  FIGS. 4 and 6 . 
   The retract piston assembly  40  includes a weld flange  64 , which is secured to an end of the rod  20  opposite an end  60  of the rod  20 . The end  60  is secured to one of the arms  12 . The weld flange  64  is arranged within the retract piston assembly  40  between the forward and rearward flanges  42  and  44  in a cavity  66  that is separated by the weld flange  64  into third and fourth chambers  68  and  70 . The fourth chamber  70  acts as a cushion chamber to reduce the impact between the caps and the workpiece. 
   The rod  20  includes two rod portions  20   a  and  20   b  secured about another weld flange  62 . The weld flanges  62  and  64  are shown in a weld-return position in  FIGS. 2 and 4  and moving toward a weld-forward position in  FIG. 6 . The work position depicted in  FIG. 6  includes a range of positions once the rod  20  begins to move. The rod  20  continues to advance a distance as the pressure builds to allow the weld-force pressure to build quickly. 
   A wear band  67  is arranged on the forward flange  42  to keep the retract piston assembly  40  centered within the barrels  34   a  and  34   b . An annular space  77  is provided between the barrels  34  and  46 . The forward and rearward flanges  42  and  44  each carry a seal  76  that provides a seal between the barrels  34   a  and  34   b  and the retract piston assembly  40 . A passage  74  in the rearward flange  44  provides a fluid connection between one side of the cushion valve  78  and the weld-return port P 2 . 
   The weld flange  62  is arranged in a cavity  90  between the middle separator block  31  and the front block  28 . The weld flange  62  divides the cavity into chambers  92  and  94 . Chamber  94  is in fluid communication with the retract-return port P 4 , and chamber  92  is in fluid communication with the weld-forward port P 1 . A hole  99  in the rod portion  20   a  fluidly connects the chamber  92  to the third chamber  68  so that pressurized fluid from the weld-forward port P 1  will act on both weld flanges  62  and  64 . 
   Referring to  FIG. 3 , the inventive pneumatic cylinder  18  includes a cushion or differential pressure valve  78  supported by the rearward flange  44  of the retract piston assembly  40 . A seal  82  is arranged between the cushion valve  78  and the rearward flange  44 . 
   An isolator  80  is arranged concentrically with and inside of the cushion valve  78 . The isolator  80  is sealed against the cushion valve  78  and rearward flange  44  with seals  79 . A seal  88  is arranged between the isolator  80  and the rod  20 . A spring  81  acts against a retainer  83  that is secured to the isolator  80  to bias the isolator  80  to the closed position shown in  FIGS. 2 and 3 . The pressure in the cushion chamber  70  and spring  81  apply sufficient force to the isolator  80  to maintain the isolator  80  in the closed position. The spring  81  is optional since the second chamber  54  is not pressurized. 
   With continuing reference to  FIGS. 2 and 3 , which depict the home position, the cushion chamber  70  has already been pressurized by weld-return air subsequent to welding the workpiece. Specifically, from the work position shown in  FIG. 6 , the weld-return port P 2  is actuated, i.e. pressurized, (along with the retract-return port P 4 ) to obtain the home position of  FIG. 2 . The retract-return port P 4  provides pressurized air to chamber  94  to retain the retract piston assembly  40  in the retracted position. 
   The intermediate position is shown in  FIGS. 4 and 5  and is achieved by pressurizing the weld-return and retract-forward ports P 2  and P 3 . As the first chamber  52  is pressurized, the second chamber  54  is exhausted through Vent. The isolator  80  engages the middle separator block  31  moving the isolator  80  to the open position, best seen in  FIG. 5 , and compressing spring  81 . An opening  122  is created between the cushion valve  78  and the isolator  80  to fluidly connect the weld-return port P 2  to the cushion chamber  70  through the opening  122  and a hole  124  in the cushion valve  78 . In the intermediate position, the weld-return port P 2  is still pressurized. As a result, the pressure in the cushion chamber  70  is “topped off” prior to actuating the cylinder to the work position. 
   There is a space  126  between the cushion valve  78  and the rearward flange  44 . The isolator  80  includes an aperture  128  that fluidly connects the space  126  to an annular passage  130  arranged between the isolator  80  and the rod portion  20   a . A passage  112  arranged between the rod portion  20   a  and the middle separator block  31  fluidly connects the weld-forward port P 1  to the space  126  via the annular passage  130  and aperture  128 . The weld-forward port P 1  is vented in the intermediate position so that atmospheric pressure acts on the rear of the cushion valve  78 . With the cushion chamber  70  pressurized, the cushion valve  78  is maintained in the closed position. 
   The cylinder is actuated from the intermediate position ( FIGS. 4 and 5 ) to the work position shown in  FIGS. 6 and 7  by pressurizing the weld-forward port P 1  and the retract-forward port P 3 . The weld-return port P 2  is vented while the cushion chamber  70  and weld-return port P 2  are still fluidly connected to one another. 
   Referring to  FIGS. 6 and 7 , the pressurized fluid enters the third chamber  68  through the hole  99  in the rod portion  20   a  that is fluidly connected to the weld-forward port P 1 . As the pressure rises in the third chamber  68 , the weld flange  64  advances, but initially with the cushion valve  78  and isolator  80  in the positions shown in  FIG. 5 . 
   With reference to  FIG. 5 , as the weld flange  64  advances (along with weld flange  62 ), the air from cushion chamber  70  is exhausted slowly through hole  124  out weld-return port P 2  to cushion the contact between the weld caps and the workpiece. The pressure in cushion chamber  70  finally drops to a level at which pressure from the weld-forward port P 1  in space  126  overcomes the pressure in cushion chamber  70  and spring  81  to open the cushion valve  78 , as shown in  FIG. 7 . The cushion valve  78  engages the isolator  80  in the open position. In the open position, the air in the cushion chamber  70  is permitted to exhaust rapidly through annular opening  132  out the weld-return port P 2  so that welding force can rise rapidly. 
   The cylinder is actuated to the intermediate position by pressurizing the cushion chamber  70  with air from the weld-return port P 2 . To move to the home position, the retract-return port P 4  is additionally actuated. Ports are pressurized and vented in accordance with the table above. In both positions, the weld flange  64  is moved to the returned position. Once the cushion chamber  70  is filled, the pressure in the cushion chamber  70  will close the cushion valve  78 . A “topping up” of pressure in the cushion chamber  70  continues as fluid enters through the hole  124  and past annular opening  122 , which is shown in  FIG. 5 . 
   Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.