Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
1997-12-04
2001-03-20
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
Reexamination Certificate
active
06204468
ABSTRACT:
This invention concerns a stress-controlled resistance welding apparatus, used notably in automobile manufacturing for the assembly of vehicle bodies.
Already known, particularly through documents FR-A-2,585,976 and FR-A-88/09915, are clamp-type resistance welding machines containing two electrode-holding arms capable of moving in relation to one another and of clasping between the electrodes the sheets to be welded; at least one of the arms being connected to drive means, such as a screw-nut system operated by an electric motor run by a microcomputer unit designed, notably, to control the spacing of the electrodes and the stress exerted on the sheets. These resistance welding machines also present systems of electric current supply of the electrodes by means, notably, of welding transformers forming voltage reducers operated from control units capable of passing the current through the electrodes in predetermined cycles.
Such known machines must, in order to secure correct welding of the sheets and to maintain that welding quality at a constant level in the course of time, perfectly control the clamping stress applied by the electrodes on the sheets.
This adjustment of clamping stress of the electrodes is customarily applied according to the aforesaid prior art by following two distinct approaches:
either in open loop with the aid of electronic control of the electric motor, which makes it possible to manage the intensity of the supply current of the motor and, therefore, the torque of the latter, that method assuming that the clamping stress directly depends on the current supplying the motor, the output of the screw-nut system being known and presumed constant;
or else in closed loop by using a stress sensor which can be included in the mechanics of the welding apparatus between the motor and the screw or, better yet, in application of the stress.
The method of open-loop adjustment has the disadvantage of being imprecise, for it does not make it possible to control the stress that the electrodes really exert on the sheets; as for the method of closed-loop adjustment, it has the disadvantage of being expensive to use, necessitating an added sensor and its connector, which moreover can prove to be a source of failure of the resistance welding apparatus.
The object of this invention is, therefore, to remedy those disadvantages by offering a resistance welding apparatus making possible a regulation of stress which at the same time is extremely precise and simple to use.
The resistance welding apparatus according to the invention is of the type containing two welding electrodes capable of moving in relation to one another and of clasping between them the sheets to be welded, at least one of those electrodes being connected for that purpose to drive means formed by a screw-nut mechanism operated by an electric motor, those drive means being connected to electronic command means capable of controlling the force with which the electrodes clasp the sheets to be welded by managing the supply current of the electric motor.
According to the invention, the resistance welding apparatus is characterized in that the screw-nut mechanism transmitting the force of the motor to the electrodes is reversible and of high output and in that the electronic command means control the clamping stress of the electrodes through a closed-loop servomechanism across the image of the supply current running through the electric motor.
According to another characteristics of the resistance welding apparatus, subject of the invention, the helix angle of the thread of the screw of the screw-nut mechanism is greater than six degrees.
According to another characteristic of the resistance welding apparatus, subject of the invention, the helix angle of the thread of the screw of the screw-nut mechanism ranges between six and ten degrees.
According to another characteristic of the resistance welding apparatus, subject of the invention, the microcomputer constituting the electronic command means drives the current supplying the electric motor through a current generator, which relays to it via a specific link the image of the current supplying the electric motor.
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English abstract for Japanese document No. 60-196276.*
DVS—Berichte Band 143, 1992, Elektromotorische Widerstandsschwelbzange, N. Bahier et al., pp. 143-159 (no translation).
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Regie Nationale des Usines Renault
Shaw Clifford C.
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