Abstract:
A multiple arm weld includes at least two weld gun arms where one arm retracts away from mating work pieces while at least one other arm remains operational, thus allowing a multiple arm weld gun to act as a single arm weld gun. Once one or more weld gun arms is retracted, the remaining weld gun arms may be repositioned with respect to the work pieces in a space not previously accessible to the multiple arm weld gun before retraction of an arm.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application Serial Nos. 60/166,510, and 60/166,449, both filed Nov. 19, 1999, the contents of which are incorporated by reference in their entirety. Applicants also note the existence of U.S. patent application Ser. No. 09/321,931, filed May 28, 1999, now U.S. Pat. No. 6,359,249 in turn claiming priority from U.S. Provisional Patent Application Serial No. 60/095,385 filed Aug. 5, 1998, and now expired; U.S. patent application Ser. No. 09/557,896, filed Apr. 21, 2000, now abandoned and claiming priority from U.S. Pat. No. 5,742,022, filed Apr. 19, 1995, and from U.S. Pat. No. 6,066,824, filed Apr. 20, 1998, all commonly owned with this application and incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electric welding assemblies having multiple weld gun arms for producing multiple simultaneous welds in a single pass. In particular, the present invention relates to assemblies where one arm of a multiple arm weld gun is retractable. 
     BACKGROUND OF THE INVENTION 
     Resistance welding utilizes the flow of electricity to permanently join two or more overlapping metallic work pieces to one another. Typically, the metallic work pieces are placed between two opposing electrode tips, which are on the jaws of a weld gun arm. The electrodes are then forced together until their tips contact the outer surfaces of the work pieces at a pressure sufficient to sandwich the work pieces and ensure an adequate electrical contact between the electrode tips and the work pieces. An electrical current is induced to flow from one electrode tip to the other electrode tip by way of the sandwiched work pieces. The work pieces act as conductors in the resulting electrical circuit, and resistance to the flow of electrical current at the interfaces between the metals generates heat. The affected metal of each work pieces selectively becomes molten, and interacts with molten metal of an adjacent work pieces to form a weld nugget that permanently bonds the work pieces together at the point of electrode tip contact. 
     A number of factors relate to the creation of a weld nugget, including the force and area of contact between the electrode tips and the work pieces, the level of current flow, the length of time the current flow lasts, degree of work pieces imperfection, and even the condition of the electrode tips themselves. 
     Weld guns used in manufacturing processes typically are required to make multiple consecutive welds on a given work pieces. In such a situation, devices exist for moving the work pieces between individual welds, moving the weld gun between individual welds, or both. For example, the electric welding gun may cycle through various locations, i.e. between an operational position with a work pieces and a resting position. The work pieces may be placed on a moving platform that manipulates the work pieces for a welding operation with a movable weld gun. After the work pieces is manipulated, the weld gun may move toward the work pieces to perform a weld cycle, after which the weld gun moves away from the work pieces to allow movement of the piece and manipulation of the next piece to be welded. In some applications, the weld gun must make a significant number of consecutive welds before further manipulation of the work pieces. In such applications, the amount of time required to move the weld gun to make the consecutive welds becomes a rate limiting step. 
     It is known to mount multiple weld gun arms to a single weld gun to decrease the amount of time required to make a significant number of consecutive welds. For example, simply adding one additional gun arm to a weld gun such that both arms are capable of simultaneous welding operation cuts the time required for performing a series of multiple consecutive welds nearly in half. Multiple arm weld guns, usually in the form of dual arm weld guns, have the advantage of being able to make several welds at one time, which decreases the cycle period of the weld gun assembly. In current multiple arm weld gun systems, artificial intelligence controls the weld gun arm position process during a weld cycle by first operationally orienting the multiple arm weld gun to the work pieces. The process includes creating welds by closing electrode tips of the weld guns about the work pieces, creating welds, reorienting the multiple arm weld gun with respect to the work pieces, and creating additional welds. Multiple arm weld guns are thus able to complete more than one weld at once, depending upon the number of weld arms on the weld gun, thereby shortening the period of time it takes to complete all the welds on a work pieces. As a result, the weld cycle period is shortened, i.e. the period of time from the beginning of one work pieces to the beginning of the next work pieces is decreased. 
     However, conventional multiple arm weld guns have a significant disadvantage due to their increased size over single arm assemblies, which are required to accommodate multiple arms on a weld gun. Specifically, currently known multiple arm weld guns are not suitable for welding many types of work pieces, because the multiple arm weld gun cannot make welds in spatially restricted locations of mated work pieces if the weld gun has difficulty gaining access to the work pieces where the weld is required. 
     One solution to the problem of the too-large-multiple arm-weld-gun is to use a second, single arm weld gun that can be accommodated in the spatially restricted space. However, this provides unsatisfactory results because of the added inefficiencies of using two weld guns. The purpose of multiple arm weld guns is to reduce the number of required weld guns, not to increase the number. The added, single arm weld gun would be similarly expensive to design, maintain, and operate as the multiple arm weld gun. In addition, the introduction of a second separate weld gun would unduly increase the cycle period because one weld gun would have to be moved out of the way in order of the other weld gun to move into its welding position. 
     Accordingly, there is a need to provide an improved electric welding system that minimizes or eliminates one or more the problems set forth above. 
     SUMMARY OF THE INVENTION 
     A multiple arm weld gun is provided wherein one or more weld gun arms on the multiple arm weld gun are able to retract away from a work pieces while at least one other arm remains operational, thus allowing a multiple arm weld gun to act as a single arm weld gun. As a result, once one or more weld gun arms are retracted, the remaining weld gun arms may be repositioned with respect to the work pieces in a space not previously accessible to the multiple arm weld gun before retraction of an arm. While all weld gun arms may be retractable, preferably only at least one arm is not retractable. The ability to retract all but one of the weld gun arms effectively overcomes the problem of multiple arm weld guns that are too large, without requiring the use of a separate single arm weld gun. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is perspective view of a dual arm weld gun. 
     FIG. 2 is a side view of a dual arm weld gun. 
     FIG. 3 is a side view of a second embodiment. 
     FIG. 4 is a side view of a third embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In all weld guns, at least one actuator is required per weld gun arm to provide the force necessary to make a weld stroke, which includes opening and closing the jaws of the weld gun arm at the spot of the weld on an engaged work pieces and providing the necessary compressive force to achieve a tight electrical contact between the electrode and the work pieces. For example, in a dual arm weld gun, at least two actuators are required, i.e. one for each weld gun arm. Any known actuators may be used, as well as any known toggle link and actuator combination. U.S. application Ser. No. 09/715,343, filed Nov., 17, 2000[Attorney docket number 65012-0063] depicts various actuators in combination with links and pivotable members, and is incorporated herein by reference in its entirety. In the present invention, the actuator also provides the force necessary to rotate a weld gun arm to a retracted position from an extended position. 
     Referring now to FIGS. 1 and 2, a dual arm weld gun  10  includes a first C-shaped weld gun arm  11  having a generally C-shaped fixed jaw  12  in combination with a second C-shaped weld gun arm  13  to form a multiple weld gun arm. It should be understood that more than two weld gun arms may be interconnected to form a multiple arm weld gun. Additionally, while the below description is directed primarily to only first C-shaped weld gun arm  11 , it should be understood that the description may apply to one or more arms of a multiple arm weld gun. 
     Fixed jaw  12  of first weld gun  11  includes a first end  14 , a second end  16  and an electrode tip  18  at a distal end of the first end  14 . Weld gun arm  11  also includes a moveable jaw  20  having a first end  22 , a second end  24 , an electrode tip  26  at a distal end of the first end  22 , and a guide  28  that is preferably linear. Linear guide structure  28  includes a guide rail  29  received in a bracket  31 , which ensures that during normal welding operation, movable jaw  20  moves only along a fixed, preset path. 
     Weld gun arm  11  further comprises an actuator  30  having an actuator shaft  32  connected to movable jaw  20 . In FIGS. 1 and 2, shaft  32  is fixedly connected to movable jaw  20  through connector  40 . The size and shape of connector  40  may vary as necessary to prevent unwanted interaction between shaft  32  of the actuator  30  and guide  28 . In a preferred construction, actuator  30  is a linear actuator capable of moving only in a fore and aft direction during normal operation. As such, the interconnection of shaft  32  with connector  40  and of connector  40  to movable jaw  20  limits movement of the movable jaw  20  to only a fixed, preset path that is preferably linear during normal operation. 
     Structurally, in the embodiment shown in FIGS. 1 and 2, fixed jaw  11  is connected to at least one mounting bracket  34  at the second end  16  of the fixed jaw  11 . Preferably, bracket  31  of the linear guide structure  28  is also connected to mounting bracket  34  at a locking joint that allows rotational movement, such as by a clutch plate  36 . Preferably, clutch plate  36  is mounted on bracket  34  having a central axis of rotation  38  defined through the center of the clutch plate  36 . As best seen in FIG. 2, both fixed and movable jaws  11 ,  20  are operably connected to the clutch plate  36  for rotational movement about axis  38 . In particular, second ends  16  of fixed jaw  11  enclose portions of linear guide  28  attached to the movable jaw  20 . The connector  40  also serves to operably interconnect actuator  30  with fixed and movable jaws  11 ,  20 . As a result, the entire weld gun arm, including fixed and movable jaws  11 ,  20  as well as the actuator  30  and actuator shaft  32 , is functionally connected to clutch plate  36  on bracket  34 . 
     Under normal operation, clutch plate  34  is locked against rotation about axis  38 . As a result, a work pieces may be positioned between electrodes  18 ,  26 . Movable electrode  26  on movable jaw  20  is cycled on a fixed, preset path, preferably linear, by operation of actuator  30 . In particular, actuator  30  cycles actuator shaft  32  fore and aft as necessary to accomplish welding. Since shaft  32  is operably connected to movable jaw  20 , movable jaw  20  likewise cycles fore and aft in response to movement of shaft  32 . Thus, as actuator  30  extends the actuator shaft  32  in a first direction, the distance between the electrode tips  18 ,  26  decreases until the electrode tips are in an engaged position in contact with the work pieces (not shown). Electricity is passed between the electrode tips  18 ,  26  and through the work pieces to create the weld nugget. This is the weld stroke. After the weld has been completed, the actuator  30  withdraws the actuator shaft  32  in a second direction to release the work pieces and return the weld gun  10  to a disengaged position, depicted in FIGS. 1 and 2. The normal weld stroke may be repeated as necessary until such time that a weld is needed in a spatially restricted portion of the work pieces. Then a retraction stroke takes place, as described below, to retract weld gun  10  to allow the remaining unencumbered weld gun arm to continue normal operation creating additional welds on the work pieces. 
     As necessary, the actuator  30  may withdraw the actuator shaft  32  in the disengaged direction beyond the disengaged position to provide the force necessary to rotate the weld gun arm  11  including jaws  12  and  20  to a retracted position. As noted above, all structural components of the weld gun arm  10  are operably connected to the clutch plate  36 , which is normally locked against rotation. 
     During a retraction stroke, the clutch plate  36  disengages, thereby allowing rotation of weld gun  10  about axis  38 , while actuator  30  provides the force necessary to rotate the gun. Clutch plate  36  may be electrically engaged and spring disengaged, or vice versa, or may use any suitable engagement and disengagement mechanism. When retraction is desired, actuator  30  withdraws actuator shaft  32  beyond the disengaged position. As actuator shaft  32  is withdrawn further toward a retracted position, not only does the actuator create a linear force along its path of movement, it also causes a downward force to be exerted on the jaws  12 ,  20 . Because the actuator  30  and the plate  36  are offset from one another and the main force of the actuator does not pass through the axis  38 , the downward force exerted on jaws  12 ,  20  causes rotation the plate  36 , thereby causing the weld gun  10  to rotate in a downwardly direction, as shown in phantom in FIG.  2 . As it rotates downwardly, the weld gun arm  11  is moved to a retracted position such that the arm  11  will not interfere with the remaining arm  13  as it performs additional welds in a confined space on mating work pieces. In this way, a dual arm weld gun may quickly, easily and reversibly be transformed into a single weld gun arm, or into a weld gun utilizing less than all of its weld gun arms if there are more than two arms. 
     In an alternative embodiment, bracket  34  further includes a cam track  50  formed at a point on bracket  34  adjacent actuator  30  or actuator shaft  32 . A cam follower  52  is affixed along the longitudinal length of the actuator shaft  32  and is slidably engaged in the cam track  50 . Cam track  50  includes a predetermined cam surface  54  along which cam follower  52  slides. In general, the cam track  50  includes two portions, but any conventional design may be employed. A first portion  56  of the cam track  50  is preferably linear and parallel to the normal position of both linear guide  28  and actuator shaft  32 . During normal operation of the actuator, cam follower  52  resides only in the first portion  56  of cam track  50 , which defines motion between engaged and disengaged positions. A second portion  58  of the cam track  50  is preferably arcuately shaped to define a path of travel for gun  10  during a retraction operation, as described more fully below. As actuator shaft is withdrawn further toward and through the retracted position, cam follower  52  moves within cam track  50  from the generally linear first portion  56  to the arcuate second portion  58 . Arcuate second portion  58  of cam track  50  is designed and shaped to cause gun  10  to rotate about axis  38 . In particular, as actuator shaft  32  is withdrawn so that cam follower  52  engages second portion  58  of the cam track  50 , all portions of weld gun  10  that are operationally connected to clutch plate  36  rotate with clutch plate  36  about axis  38 . In FIG. 1, the second portion  58  of cam track  50  angles upwardly with respect to actuator shaft  32 , thereby causing the assembly to rotate in a downwardly direction, as shown in phantom in FIG.  2 . 
     The weld gun  10  of FIGS. 1 and 2 utilizes a linear actuator to translate linear motion into rotation of the weld gun  10  about a rotational joint. However, other conventional types of pivoting or rotational joints are also suitable, such as joints that facilitate a linear sliding motion or a corkscrew motion. 
     Preferably, a single stage actuator or motor is used to provide the force and movement required for both the weld stroke and the retraction stroke by working in combination with the locking joint. Alternately, a two stage actuator or motor may be used to effect both the weld stroke and the retraction stroke. A strategically placed stop or appropriately designed cam track may also be utilized to facilitate a retraction stroke using a two stage actuator. 
     A retractable weld gun utilizing a two stage actuator is shown in FIG. 3. A caliper-type weld gun arm  110  is shown, representing one arm of a multiple arm weld gun. Weld gun arm  110  includes a fixed jaw  112  having a first end  114  and a second end  116 . Fixed jaw first end  114  terminates in electrode tip  118 . The weld gun arm  110  further includes a moveable jaw  120  having a first end  122  inwardly directed towards fixed jaw first end  114 , a second end  124 . Movable jaw first end  122  likewise terminates in an electrode tip  126  in a position opposed to tip  118 . Fixed and movable jaws are rotationally interconnected at a connection point  134  such that opposed electrodes  118 ,  126  may rotationally move toward and away from each other during a weld stroke. Fixed jaw  112  is further connected to a mounting bracket  138  at a selectively lockable second connection point  136 . Second connection point  136  usually acts as a rigid connection point that selectively prevents rotation of fixed jaw  112  about the second connection point. However, if further retraction of gun  110  is required, second connection point  136  may be unlocked to act as a rotational connection similar to first connection point  134 , thereby allowing fixed jaw  112  to rotate about second connection point  136 , as described further below. 
     Weld gun arm  110  further includes an actuator  130 , preferably mounted on bracket  138 , having an actuator shaft  132 . Actuator shaft  132  is rotationally connected at a third connection point  141  to the second end  124  of the moveable jaw  120 . A stop  140  is connected to the second end  116  of the fixed jaw  112 . 
     Under normal operation, actuator  130  extends shaft  132 , forcing movable jaw  120  to rotate about first connection point  134 , thereby decreasing the distance between tips  118 ,  126  until the electrode tips are in contact with the work pieces (not shown) in an engaged position. Electricity is passed between the electrode tips  118 ,  126  and through the work pieces to create the weld nugget. This is the weld stroke. After the weld has been completed, the actuator  130  withdraws the actuator shaft  132  to a disengaged position to release the mated work pieces so that arm  110  or the work pieces may be repositioned with respect to the other. This weld stroke may be repeated until such time that a weld is needed in a spatially restricted portion of the work pieces. Then a retraction stroke takes place, as described below, after which, the remaining weld gun arm or arms may create additional welds on the work pieces. 
     To retract a weld gun arm, second connection point  136  selectively disengages from a rigid connection to a rotatable connection. Selective engagement and disengagement of second connection point  136  may be accomplished by any conventional means. Under normal operation, the maximum withdrawal of actuator shaft  132 , and by association, the maximum distance between electrode tips  118 ,  126 , is limited by stop  140 . However, by selectively disengaging second connection point  136 , continued withdrawal of the actuator shaft  132  by the actuator  130  forces the combined fixed jaw  112  and moveable jaw  120  to rotate as a single unit about both first connection point  134  and second connection point  136 . The linear motion of the actuator  130  is thereby translated into rotational motion of the jaws  112 ,  120 , causing both jaws to rotate upwardly with respect to bracket  138  in FIG.  3 . The rotation about the connections  134 ,  136  effectively swings the weld gun arm away from the work pieces so that additional welds may be made on a spatially restricted portion of the work pieces. 
     The cam track/cam follower mechanism used in FIGS. 1 and 3 may also be adapted to bayonet style weld guns. In such a configuration, a cam follower is fixedly attached to a moveable jaw of bayonet style weld gun arm, while a cam track is located on a fixed jaw of the weld gun arm. In a first portion, the cam track is straight and guides the movable jaw along a fixed, preset preferably linear path between a disengaged and an engaged position. A second portion of the cam track is preferably angled away from the first portion, causing the movable jaw to retract in response to action of the cam follower within the cam track. 
     A bayonet style weld gun  210  is shown in FIG.  4 . As above, gun arm  210  is only one arm of a multiple arm weld gun. The weld gun arm  210  comprises a generally C-shaped fixed jaw  212  having a first end  214  terminating in an electrode tip  218  and a second end  216 . Fixed jaw  212  is pivotally mounted at connection point  236  to a mounting bracket  238 . The weld gun arm  210  further includes a moveable jaw  220  having a first end  222  and a second end  224 . Movable jaw first end  222  terminates in an electrode tip  226  such that tips  218 ,  226  are arranged in an opposed manner. Movable jaw  220  further includes a cam follower  228  mounted on a bracket  229  as necessary such that the cam follower slidably engages a cam track  234  located on fixed jaw  212 . An actuator  230 , moving along a fixed, preset path, and preferably a linear actuator having an actuator shaft  232 , is connected to the movable jaw second end  224  to impart fore and aft motion to the movable jaw  220 . 
     In operation, as actuator  230  extends the actuator shaft  232 , movable jaw  220  moves generally linearly towards fixed jaw first end  214 . Movable jaw  220  is guided in its motion through the action of cam follower  228  sliding within cam track  234 . As shaft  232  moves from a disengaged to an engaged position, the distance between the electrode tips  218 ,  126  decreases until the electrode tips are in contact with the work pieces (not shown). Electricity is passed between the electrode tips  218 ,  226  and through the work pieces to create the weld nugget. This is the weld stroke. After the weld has been completed, the actuator  230  withdraws the actuator shaft  232  to release the work pieces and return the weld gun arm  210  to a disengaged position. This weld stroke may be repeated until such time that a weld is needed in a spatially restricted portion of the work pieces. Then a retraction stroke takes place, as described below, after which, any remaining weld gun arms may create additional welds on the mated work pieces. 
     As noted above, cam track  234  has two portions. A first portion  240  is generally linear and parallel to the motion of the actuator  230 , thereby describing a fixed, preset path corresponding to normal operation. The cam follower  228  slides in this portion of the cam track  234  without significantly moving the fixed jaw  212  because the fixed length cam follower  228  is moving parallel to the motion of the actuator  230 . A second portion  242  of the cam track  234  angles toward the moveable jaw  220  to cause the fixed jaw  212  to pivot about connection  232  as the fixed jaw  212  is drawn toward the movable jaw  222 . Thus, the motion of the fixed jaw  212  during the retraction stroke is defined by the shape of the cam track  234 . 
     Although certain preferred embodiment of the present invention have been described, the invention is not limited to the illustration described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention. A person of ordinary skill in the art will realize that certain modifications and variation will come within the teachings of this invention and that such modifications and variations will come within its spirit and the scope as defined by the claims.