Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2000-11-17
2003-06-03
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S086410, C219S086330, C219S086310
Reexamination Certificate
active
06573470
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to dissipation of heat generated at the electrode tips and in the arms of electric spot welding guns. More particularly, the invention relates to construction of electric welding guns to provide for heat removal during welding operations without requiring a continuously supplied cooling source.
BACKGROUND OF THE INVENTION
Resistance spot welding utilizes the flow of electricity to permanently join two or more overlapping metallic workpieces to one another. Typically, the metallic workpieces are placed between two opposing electrodes of a spot welding system gun assembly. The electrodes are then forced together until their tips contact the outer surfaces of the workpieces at a pressure sufficient to sandwich the workpieces and ensure an adequate electrical contact between the electrodes and the workpieces. Then an electrical current is induced to flow from one electrode tip to the other electrode tip by way of the sandwiched workpieces. The workpieces 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 workpiece selectively becomes molten, and interacts with molten metal of an adjacent workpiece to form a weld nugget that permanently bonds the workpieces together at the point of electrode tip contact. Additional heat is generated due to contact resistance between the electrode tips and the work piece, as well as by joule heating in the arms themselves. The heat generated must be dissipated to avoid thermal overload and subsequent gun malfunction in production applications.
A number of factors relate to the creation of a weld nugget, including the force and area of contact between the electrode tips and workpieces, the level of current flow, the length of time that the current flow lasts, degree of workpiece imperfection, and even the condition of the electrode tips themselves.
The prior art teaches the importance of creating an adequate weld nugget. Therefore, spot welding systems are over-configured to generate a weld nugget even if there are significant workpiece imperfections by having high force, current levels, and current application times. Yet, many resulting welds are still imperfect. Therefore, typically, somewhere on the order of approximately one quarter of all welds in a workpiece are added to insure adequate structural integrity.
Further, such overcompensation for possible workpiece imperfection results in significantly higher deformation (mushrooming) of the electrode tips at the point of contact between the tips and the mating workpieces. If the electrode tips are inadequately cooled, the electrodes experience excessive tip wear, deformation, tip sticking and even tip melting, all of which contribute to poor weld quality and increased equipment maintenance. The generation of significant heat at the electrode tips also results in significant heat built up in the welding control unit, transformer, and secondary (i.e., high current) cable disposed between the electrodes and the transformer.
Moreover, the application of continuous significant electrode force upon the sandwiched workpieces requires the use of significant sources of compressed air. The compressed air provides for the actuation of various air cylinders to position the welding gun electrodes with respect to the workpieces to be sandwiched therebetween and to generate force.
The use of complex air and water cooling systems with their multitude of hoses and corresponding pipes, valves, and the like, in combination with the controllers and supply mechanisms, greatly increases manufacturing expense. It has been estimated that somewhere on the order of approximately one quarter to one half of the total cost of a spot welding system can be attributed to the use of air and water-cooling circulation systems. It is known to provide alternatives to air-actuated cylinders to position the weld gun, thereby eliminating come of the hoses. However, motor actuated or electrically actuated weld guns produce additional heat that must be removed from the weld gun assembly.
To complicate matters, typical spot welding systems must be custom designed, built, and tested, requiring the services of numerous skilled trades. Following initial construction and testing, the verified welding systems are then torn-down, transported, and rebuilt at a manufacturing facility where they will actually be used. Such intermediate steps significantly increase the time lag and cost in providing a complete electric weld system. Moreover, both the design and testing facility, as well as the final manufacturing facility, must make significant capital and continuous monetary investments in air and water-cooling circulation systems, customizing them for each individual spot welding system.
Nor is the complexity and cost limited to manufacturing of a spot welding system itself. The ongoing maintenance problems of requiring significant water-cooling and air circulation systems are extensive. It has been estimated that on the order of eighty percent of the down time of a typical spot welding system may be attributed to the host of air hoses, and feed and return cooling water hoses in combination with the corresponding pipes, valves, and other components.
There are additional costs to requiring complex water and air supply circulation systems. Each spot welding system becomes unique. Each length of hose, each bend in a pipe or conduit, and each selected placement for various cooling water fittings is necessarily tailored to the particular welding system. The kinematics of the host of hoses (pejoratively referred to as “spaghetti”) cannot be accurately predicted or modeled. Accordingly, the robot movements in each robotic work cell must be inputted on-site, step-by-step, to ensure that hoses do not become entangled. To further exacerbate the problem, the resulting “window” in which a robot arm may move to reach, for example, a weld point, is significantly reduced, again due to the proliferation of the compressed air and water hoses and associated components. Thus, the time to program a robot arm is extensive. Even after programming, the resulting process time to process workpieces is often significantly increased by having a small movement window.
A multitude of factors goes into the design of a spot welding gun. For a given force that needs to be exerted by a weld gun, the factors that enter into the design of the weld gun include the strength of the actuator necessary to effect the weld, the speed with which the actuator can close the arms of the weld gun, the force that the actuator can exert on those arms at a speed commensurate with the desired output of the device, and the speed with which the device can create any particular weld and then move on to the next weld to be performed. The speed with which the weld gun can dissipate the heat generated during the welding process is a contributor to this final factor. Should the weld gun become overheated, the weld tips can become damaged or the weld gun can get out of alignment due to warping. Materials being welded can also be more susceptible to high temperatures generated during the welding process.
A weld gun designer traditionally had only two options available in keeping the temperatures of the welding apparatus within tolerances. The first is to extend the cycle times such that the apparatus has time to cool, thereby keeping the apparatus temperature down. The second is to provide a structure whereby the apparatus is cooled either continually or cyclically during the manufacturing process. As noted above, such a cooling mechanism, however, adds drastically to the cost of the apparatus and to the cost of operation, and the extra accoutrements that go with such an apparatus can hinder the flexibility and mobility of the apparatus.
Accordingly, an all-electric resistance welding system is desired that eliminates the need for extensive water fittings or other continuously available cooling apparatuses to simplify the
Baysore John K.
Brown Ronald C.
Henderson, III James Bronce
Korshurba Gregory J.
Mason Arthur C.
DCT, Inc.
Elve M. Alexandra
Rader & Fishman & Grauer, PLLC
LandOfFree
Weld gun heat removal does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Weld gun heat removal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Weld gun heat removal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3110928