Patent ID: 7956308

Claim:
A method of controlling a piston-cylinder assembly for a welding apparatus, said piston-cylinder assembly including a plurality of pressure chambers, wherein said piston-cylinder assembly is operatively coupled to at least one welding electrode of said welding apparatus so as to effect movement of said electrode in a pre-stroke and weld motion, comprising: for pre-stroke motion using a first valve to connect a first fluid pressure to a first port of the piston-cylinder assembly, said first port in communication with a first pressure chamber so as to effect the pre-stroke motion of the electrode; for weld stroke motion using a second valve to connect a second fluid pressure to a second port of the piston-cylinder assembly, said second port in communication with a second pressure chamber so as to effect the weld stroke motion of the electrode, wherein the second fluid pressure is different from the first fluid pressure; upon connecting the first fluid pressure to the first port, using a third valve to simultaneously connect a third fluid pressure to a third port of the piston-cylinder assembly, said third port in communication with a third pressure chamber, wherein a pneumatic timer is coupled to the third valve, and the third valve is operative to connect the first pressure to the third pressure chamber when the pre-stroke motion is complete, and the pneumatic timer controls a time period that fluid pressure is dumped from the third pressure chamber during the pre-stroke motion; upon completion of the pre-stroke motion of the electrode, connecting the first fluid pressure to the third port of the piston-cylinder assembly; and dumping fluid pressure from the third pressure chamber when a clamping force applied by the electrodes on a work piece satisfies a predetermined threshold.