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.
One of the ports on prior art systems is typically provided on a barrel that extends between the ends of the cylinder. The barrel port is a costly feature in that it requires a two-piece tubular wall with a separator arranged between the walls. The multiple components of the barrel must incorporate several seals. As an alternative to using the separator, the port may be welded to a singular tubular wall, which requires subsequent machining of the inner surface of the barrel. Further, the welding process is difficult to control.
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.